of Doctor of Philosophy (Ph.D) in Polymer Science, Federal University of ... Rotimi Sadiku, Department of Polymer Technology, Tshwane University of Technology,. (TUT), South ...... thermal coefficients of linear expansion for filler and matrix (Fried, 2000). This ...... http://sinnott.mse.ufl.edu/Syllabus_abet_3010_2007_v02.pdf.
Synthesis, Preparation and Characterization of Nanoporous Core-Shell-Clay Epoxy Composites
By
Iheaturu, Nnamdi Chibuike B.Eng., M.Sc., (FUTO), EMMS (UA Portugal; AAU Denmark & TUHH Germany)
Reg. No.: 20064771568
A thesis submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy (Ph.D) in Polymer Science, Federal University of Technology, Owerri, Nigeria
October, 2014
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Certification This is to certify that this research titled, “Synthesis, Preparation and Characterization of Nanoporous Core-Shell-Clay Epoxy Composites”, was carried out by Nnamdi Chibuike Iheaturu, with registration number 20064771568.
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Prof. M. E. Enyiegbulam
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(Principal Supervisor)
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Engr. Dr. I. C. Madufor (Co-Supervisor)
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Prof. I. M. Mejeha (Co-Supervisor)
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Engr. Dr. Martin Obidiegwu
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Date
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(Head of Department)
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Engr. Prof. E. E. Anyanwu
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(Dean, SEET)
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Engr. Prof. (Mrs.) K. B. Oyoh (Dean, Postgraduate School)
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Prof. B. T. Nwufo (External Examiner)
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Date
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Date
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Dedication This Ph.D thesis is dedicated to my late father, Sir A. M. Iheaturu, who supported me throughout the period of my studies abroad, and to late Prof. Celestine O. E. Onwuliri (FAS, KSJI), the 5th substantive Vice Chancellor of the Federal University of Technology, Owerri, Nigeria, who approved my study leave to Europe at that time. Also, this work is dedicated to my lovely wife, Mrs. Chioma Joan Iheaturu, nee Onwuliri, who stood by me when the storm raged.
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Acknowledgements I am grateful to Prof. Mannu Ejemmanu Enyiegbulam (FPIN) Project Supervisor; Engr. Dr. Innocent Chimezie Madufor (MPIN) Co-supervisor, of the Department of Polymer & Textile Engineering; Prof. I. M. Mejeha (KSM), co-supervisor, of the Department of Physics, School of Science (SOSC), Federal University of Technology, Owerri (FUTO), Nigeria; Engr. Prof. E. E. Anyanwu, Dean, School of Engineering & Engineering Technology (SEET); Engr. Dr. Martin Obidiegwu, Head of Department, Polymer & Textile Engineering, Federal University of Technology, Owerri, Imo State, Nigeria; Prof. Okoro Ogbobe, FPIN; Prof. Chukwuma B. C. Ohanuzue, FPIN; Prof. Anthony Iheonye; Prof. Isaac Igwe and Dr. Godwin N. Onyeagoro. My colleagues are also acknowledged; Engr. Dr. Henry Obasi, Bibiana Aharanwa, Ikechukwu Anyanwu, Engr. Dr. Innocent Arukalam, Mrs. Catherine ChikeOnyegbula, Innocent Ochiagha, Engr. Ezeamaku Uche, Engr. Simeon Nwanonenyi, Francis Onuoha and Miss Vincentia Ezeh, all of the Department of Polymer & Textile Engineering, and Dr. Chijioke S. Nwobodo, Head of Department of Materials & Metallurgical Engineering, School of Engineering & Engineering Technology (SEET), Federal University of Technology, Owerri, Nigeria. I am equally grateful to my AAU thesis supervisor, Jens Chr. Rauhe, Ph.D., my EMMS Project Supervisor, Lars Rosgaard Jensen, Ph.D., who are both Associate Professors at Institut f. Maskintecknik, Aalborg Universitet, (AAU), Denmark; Prof. Pyrz Ryszard of AAU, Denmark and Laboratory Technologist, Thomas Quaade; Prof. Dr. Ana Barros-Timmons and Assoc. Prof. Florinda Mendes da Costa, of the Departments of Chemistry and Physics respectively, Universidade de Aveiro, Portugal; Prof. Emmanuel Rotimi Sadiku, Department of Polymer Technology, Tshwane University of Technology, (TUT), South Africa, Mr. Bernard Oboarekpe, Well Engineering Manager; Mr. Thompson Ukomah, Well Engineering Department; and to all the staff of Well Engineering, The Shell Petroleum Development Co. Of Nig. Ltd., (SPDC), Port Harcourt, Rivers State, Nigeria. Also to Mr. Theo Ihenacho and Mr. Nwoke Adinigwe of Specialty Drilling Fluids (SDF) Port Harcourt, Nigeria; Laboratory Manager and all staff of FUGRO Consultants, Port Harcourt, Nigeria; Sir. A. N. C. Onwuliri (KSM) of Adamac Petroleum, Port Harcourt, Nigeria; Mr. Etekamba Okon Willie, Principal Engineer Specialist, Load Carrying Systems, VESTAS R & D, Hedeager 44, Århus North, Denmark. I am equally very grateful. These people mentioned were of immense help at various stages of this project. Finally and in a special way, I am very grateful to the immediate past Vice Chancellor of Federal University of Technology, Owerri, Imo State, Nigeria, Late Prof. Celestine O. E.
v Onwuliri, for granting me study fellowship to enable me undertake part of this study abroad.
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Abstract This project presents core-shell-clay epoxy filled organic-inorganic nanocomposite wherein a new hybrid material was synthesized by solvothermal synthesis and acid activation of natural clays from spent drilling mud. Literature shows smectites from natural origin as having micropores and spongy morphology. Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDX) and X-ray Diffraction (XRD) analysis present an insoluble nanoporous hybrid aluminosilicate-hydrocalcite clay material which interlayer spacing was found to be in nanodimension. Interlayer distance was measured as 0.37nm. Fourier Transform Infrared (FT-IR) spectroscopy clearly revealed effect of calcination and acid treatment on the clay material. With FT-IR results, vibrational bands between 3200cm-1 and 3600cm-1, attributed to -OH- stretching in composite fly ash, and that at 1085cm-1 to siliceous (Si-O-Si) linkages of SiO4 tetrahedra, and having associated the peaks at 1010cm-1 to 1090cm-1 to Si-O-Si linkages, it is clear that high temperature treatment of spent oil base drilling mud and acid activation of thermally desorbed ash, basically composed of calcium aluminosilicates from EDX and XRD results, are in agreement with the results of Fen et al., 2006. Therefore, solvothermal treatment of spent oil base drilling mud solids and acid activation of thermally desorbed ash may have produced a zeolitic nanoporous core clay material. In order to form aminosilane shell, 3-Aminopropyltriethoxysilane (3-APTES), was hydrolysed and polycondensed on hybrid aluminosilicate-hydrocalcite core before incorporation in phenol-free epoxy matrix. Electrophoresis measurement show that due to poor dispersion of core clay material during solvothermal treatment / calcination, the clay rather than being exfoliated remained agglomerated within epoxy matrix due to the strong hold of aluminosilicate nano-bridges. Fractography was carried out after the materials were fractured at liquid nitrogen temperature of -14°C. Fractured surface analysis show a more diffuse stress concentration for SOBM filled composites compared to acid treated TDU ash filled composites which exhibited decohesive rupture. Material mechanical properties were determined and modelled with Schrager’s equation.
Keywords: 3-Aminopropyltriethoxosilane (3-APTES), Core-Shell-Clay, Epoxy, Fractography, Nanocomposites, Ormocal, Spent oil base drilling mud (SOBM), Thermally desorbed ash
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Table of Contents Certification ...................................................................................................................... ii Dedication ....................................................................................................................... iii Acknowledgements ......................................................................................................... iv Abstract ........................................................................................................................... vi Table of Contents ........................................................................................................... vii List of Figures................................................................................................................... xi List of Tables .................................................................................................................. xix List of Abbreviations ...................................................................................................... xxi Chapter One ..................................................................................................................... 1 Introduction...................................................................................................................... 1 1.1 Background Information .................................................................................................................. 1 1.2 Statement of the Problem ................................................................................................................. 4 1.3 Project Objectives ................................................................................................................................ 5 1.4 Justification for this Project ............................................................................................................. 6 1.5 Scope of Work........................................................................................................................................ 8
Chapter Two ................................................................................................................... 10 Literature Review ........................................................................................................... 10 2.1 Core-Shell Nanoparticles ............................................................................................................... 10 2.2 Core-Shell Morphology ................................................................................................................... 11 2.3 Clay Minerals and Nanoporous Clay Structure ..................................................................... 12 2.3.1 Structure of Some Layered Silicates (Phyllosilicates) ............................................... 14 2.4 Clay Treatment and Surface Modification of Core-Clay Nanostructures ................... 21 2.5 Alkyl Ammonium Exchanged Core-Clay Nanostructures ................................................. 23 2.6 Fatty Acids and Organo-silane Core-Clay Treatment......................................................... 25 2.7 Nanoporous Core Clays by Thermal Treatment................................................................... 28 2.7.1 Pillared Interlayered Clays (PILC)..................................................................................... 29 2.7.2 Zeolites.......................................................................................................................................... 31 2.8 Nanoporous Evolution and Porous Clays Morphology ..................................................... 32 2.8.1 Effect of Calcination on Hydrocalcites and Mixed Oxide Clays – The Specific Case of Spent Oil Base Drilling Mud................................................................................................. 33 2.8.2 Acid Activation of Core-Clay Nanostructures ............................................................... 35 2.9 Preparation of Core-Shell Nanomaterials ............................................................................... 38 2.9.1 Conventional Methods............................................................................................................ 39
viii 2.9.2 Advanced Synthetic Methods .............................................................................................. 40 2.10 Particle Characteristics and Size Analysis ........................................................................... 47 2.10.1 Granulometry and Morphology of Clay Particles ..................................................... 47 2.10.2 Particle Size Analysis............................................................................................................ 48 2.11 Colloidal Processing and Interfacial Phenomena.............................................................. 48 2.11.1 Concept of Flocculation, Agglomeration and Coagulation .................................... 49 2.11.2 Colloidal Stability and DVLO Theory ............................................................................. 51 2.11.3 Creating a Stable Colloidal System ................................................................................. 55 2.11.4 Interfacial Characterization and the Electronic Double Layer............................ 57 2.11.5 Acid-Base Interactions on Clay Particle Surfaces ..................................................... 59 2.12 Zeta Potential Measurements - Electrophoresis ............................................................... 61 2.13 Mechanism of Formation of Coloured Clay Organic Complexes ................................. 65 2.13.1 Charge Transfer Reaction At Crystal Edges ................................................................ 67 2.14 Chemistry of Organosilane Clay Treatment ........................................................................ 68 2.14.1 Chemistry of Epoxy – Aminosilane Treated Clay...................................................... 69 2.15 Fourier Transform Infrared (FTIR) Spectroscopy Characterization of Clays and Clay Minerals.................................................................................................................................................. 71 2.16 Epoxy Resin Technology ............................................................................................................. 76 2.16.1 Epoxy Resin Synthesis ......................................................................................................... 77 1.16.2 Epoxy resin curing................................................................................................................. 79 2.17 Epoxy – Clay Nanocomposites .................................................................................................. 81 2.17.1 Clay Dispersion Morphologies in polymers ................................................................ 82 2.17.1.1 Intercalation ......................................................................................................................... 82 2.17.1.2 Exfoliation ............................................................................................................................. 82 2.17.1.3 Immiscible Agglomerates ............................................................................................... 83 2.18 Previous Works on Epoxy-Clay Nanocomposites ............................................................. 84 2.19 Morphology and Characterization of Epoxy-Clay Nanocomposites .......................... 90 2.20 Mechanical Properties and Theoretical Considerations of Ultimate Strength and Hardness of Core-Shell Clays Particulate Filled Nanocomposites ........................................... 91 2.20.1 The Schrager Model .............................................................................................................. 95 2.20.2 Halpin-Tsai Model ................................................................................................................. 96 2.20.3 Bela Pukanszky Model ......................................................................................................... 99 2.20.4 Hardness Property of Polymer Clay Nanocomposites ......................................... 100 2.21 Damage Mechanics, Microstructure and Fractography ............................................... 101 2.21.1 Modes of Fracture................................................................................................................ 105 2.21.2 Fractography by Microscopy Technique.................................................................... 111
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Chapter Three............................................................................................................... 114 Materials and Methods ................................................................................................ 114 3.1 Clay Materials ................................................................................................................................... 114 3.2 Epoxy Resin ....................................................................................................................................... 116 3.3 Other Chemicals and Reagents .................................................................................................. 118 3.4 Experimental Methods.................................................................................................................. 119 3.4.1 Method for Silane Coupling ................................................................................................ 120 3.4.2 Zeta Potential Measurement of Clay Particles ............................................................ 121 3.4.3 Method for Epoxy Composite Moulding........................................................................ 122 3.4.4 FT-IR Characterization of Core-Shell Clays.................................................................. 125 3.4.5 SEM / EDX Analysis of Clay Samples .............................................................................. 127 3.4.6 X-Ray Diffraction (XRD) Analysis .................................................................................... 131 3.4.7 Tensile Testing of Epoxy Composites – ASTM D 638 .............................................. 134 3.4.8 Hardness Testing of Epoxy Composites – ASTM D 2240 ....................................... 135 3.4.9 Epoxy Composites Fracture and Fractography.......................................................... 137
Chapter Four................................................................................................................. 138 Results and Discussions ................................................................................................ 138 4.1 Particle Size Distribution of SOBM Clay Particles.............................................................. 138 4.1.1 Particle Size Analysis of Acid Treated TDU Ash ......................................................... 138 4.1.2 Particle Size Analysis of SOBM core clay ...................................................................... 141 4.2 Micrographs of Synthesized SOBM Clay Samples.............................................................. 143 4.2.1 Micrograph of Sonicated Synthesized SOBM clay ..................................................... 145 4.2.2 Micrographs of Aminosilane Coupled SOBM Clay .................................................... 147 4.3 Microscopy of TDU Ash Samples .............................................................................................. 149 4.3.1 Micrograph of Aminosilane Coupled Acid Activated TDU ash ............................. 151 4.4 XRD of Powder Samples ............................................................................................................... 152 4.5 Electron Dispersive X-ray Spectroscopy (EDX).................................................................. 154 4.5.1 EDX Analysis of Synthesized SOBM clay ....................................................................... 154 4.5.2 EDX Analysis of Thermally Desorbed Clay Sample (TDU Ash)............................ 158 4.6 Two-Point Analysis of Core Filler Samples by EDX .......................................................... 163 4.6.1 Two-Point Analysis of SOBM Filler ................................................................................. 164 4.6.2 Two-Points Analysis of TDU Ash...................................................................................... 165 4.6.3 Reason for Colour Change................................................................................................... 166 4.7 Results of Electrophoresis........................................................................................................... 166 4.8 FT-IR Results of Acid Treated TDU Ash ................................................................................. 168
x 4.9 FT-IR Results of Synthesized SOBM Core Clay.................................................................... 172 4.10 FT-IR Results of Epoxy Clay Nanocomposites .................................................................. 177 4.10.1 FT-IR Spectra of SOBM Core-Shell Filled Epoxy...................................................... 179 4.10.2 FT-IR Spectra of TDU Ash Core-Shell Filled Epoxy ................................................ 182 4.11 Results of Tensile Testing of Samples (ASTM D-638) ................................................... 185 4.11.1 Effect of Silane Coupled (Core-Shell) Fillers on Mechanical Properties of Epoxy Composites ................................................................................................................................. 189 4.12 Determination of Core Filler Material Shape Factor...................................................... 192 4.13 Schrager’s Equation Modelling of Epoxy Composites Ultimate Strength ............. 195 4.14 Results of Shore D Hardness Testing (ASTM D - 2240)................................................ 197 4.15 Fractography of Epoxy Clay Nanocomposites .................................................................. 198 4.15.1Fractography of 3-APTES coupled TDU ash filled epoxy nanocomposites ...... 198 4.15.2Fractography of 3-APTES coupled nanoporous SOBM filled epoxy nanocomposites ..................................................................................................................................... 201
Chapter Five ................................................................................................................. 206 Conclusions and Recommendations ............................................................................ 206 5.1 Conclusions........................................................................................................................................ 206 5.1.1 Specific Inferences ................................................................................................................. 207 5.2 Contributions to Knowledge ...................................................................................................... 209 5.3 Areas of Application of “SOBM filler 1” .................................................................................. 211 5.4 Research Trust ................................................................................................................................. 211 5.5 Recommendations .......................................................................................................................... 212
References .................................................................................................................... 214 Appendix....................................................................................................................... 237
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List of Figures Figure 1. 1: Process of crude oil drilling and oil drill cuttings recovery from a rig site in the Niger Delta area of Nigeria ............................................................................................................................... 7 Figure 1. 2: Mobile Thermal Desorption Unit (M-TDU) at Shell, Port Harcourt, Nigeria ......... 7 Figure 1. 3: Process flow chart of the M-TDU............................................................................................ 8 Figure 2. 1: Schematic representation of core-shell morphology (a.) with 1/8th quartet cutoff, (b.) transverse face of one half cut-off ................................................................................................ 11 Figure 2. 2: Layer structure of montmorillonite (MMT) clay, a member of the smectite group .................................................................................................................................................................................. 15 Figure 2. 3: Structure of sodium montmorillonite, courtesy of Southern Clay Products, Inc., USA .......................................................................................................................................................................... 15 Figure 2. 4 : Scanning Electron Micrographs (SEM) of (a.) dioctahedral smectite and (b.) trioctahedral smectite (Source: Clay Mineral Society, UK) ................................................................ 16 Figure 2. 5: Structure of kaolin clay .......................................................................................................... 18 Figure 2. 6: Scanning Electron Micrographs (SEM) of different kaolin books (a.) Kaolinite 1 and (b.) Kaolinite plates (Source: Clay Mineral Society, UK) ............................................................. 18 Figure 2. 7: Structure of layered double hydroxide (LDH) ................................................................ 19 Figure 2. 8: Crystallographic pattern of monohydro calcite clay (Swainson, 2008).............. 20 Figure 2. 9: Evolution process for porous aluminosilicate clay templates during calcination / solvothermal synthesis.................................................................................................................................. 29 Figure 2. 10: Schematic representation of stages in raw clays pillaring process by thermal decomposition ..................................................................................................................................................... 30 Figure 2. 11: FI-IR spectra of (a) acid-treated Na-MMT and acid-catalyzed sol–gel-modified clays and (b) pristine Na-MMT and non-catalyzed sol–gel-modified clays with different TEOS/clay ratios (Zhongzhong et al., 2008) ............................................................................................ 38 Figure 2. 12: Structure of mesoporous M41S materials: (a) MCM-41 (2D hexagonal, space group p6mm), (b) MCM-48 (cubic, space group Ia3-d) and (c) MCM-50 (lamellar, space group p2) (Hoffmann et al., 2006)............................................................................................................... 39 Figure 2. 13: Inorganic Fe2O4@Ag and Fe2O4@Au core-shell nanoparticles synthesized by sol gel technique (Zhichnan et al., 2007)................................................................................................... 41
xii Figure 2. 14: Grafting (postsynthetic functionalization) for organic modification of mesoporous pure silica phases with terminal organosilanes of the type (R'O)3SiR, R=organic functional group (Hoffmann et al., 2006) ................................................................................................. 42 Figure 2. 15: General synthetic pathway to PMOs constructed from bissilylated organic bridging units, R=organic bridge (Hoffmann et al., 2006) ................................................................ 42 Figure 2. 16: Surfactant templating on an alkoxysilane substrate, wherein the resulting material contains the organic moiety as an integral part of the wall (Kickelbick, 2004) ...... 43 Figure 2. 17: Scheme of the main synthesis routes to mesoporous materials: Precipitation (A), True Liquid Crystal Templating, TLCT (B), Evaporation-Induced Self-Assembly, EISA (C) and Exotemplating (D) (Soler-Illia & Azzaroni, 2011). ....................................................................... 46 Figure 2. 18: Scheme showing flocculation and coagulation in a colloidal system (Malvern, 2011) ...................................................................................................................................................................... 51 Figure 2. 19: Schematic diagram showing the variation of free energy with particle separation according to DVLO theory (Malvern, 2011) ...................................................................... 54 Figure 2. 20: Schematic diagram showing the variation of free energy with particle separation at high salt concentrations showing the possibility of a secondary minimum according to DVLO theory (Ganguli & Chatterjee, 1997) .................................................................. 55 Figure 2. 21: Representation of steric and electrostatic stabilization.......................................... 56 Figure 2. 22: Negatively charged clay particle attracting positive charges (Malvern, 2011) .................................................................................................................................................................................. 58 Figure 2. 23: Negatively charged clay particle attracting positive charges (Ganguli & Chatterjee, 1997) ............................................................................................................................................... 59 Figure 2. 24: Reaction chemistry of particle interaction with acidic or basic environment (Ganguli & Chatterjee, 1997)......................................................................................................................... 60 Figure 2. 25: Clay particle interactions with the head group of a surfactant. Paths (a), (b) and (c), are possible synthetic pathway in acidic, basic, or neutral media respectively (Hoffmann et al., 2006).................................................................................................................................... 61 Figure 2. 26: Zeta potential scheme of a colloidal system showing; the isoelectric point (IEP), upper and lower regions for a stable colloidal system and region for unstable colloidal system (Malvern, 2011) ................................................................................................................................... 62 Figure 2. 27: Optical configuration of the Zetasizer for zeta potential measurement (Malvern, 2011) .................................................................................................................................................. 63 Figure 2. 28: Zeta potential of alumina-10ppm polyethylene glycol (PEG) system as a function of pH, in the absence and presence of different concentrations of ammonium poly methacrylate (APMA) (Saravanan & Subramanian, 2012) ............................................................... 64
xiii Figure 2. 29: Scheme showing the transformation of benzidine into its coloured radicalcation forms activated by clays (Theng, 1971) ....................................................................................... 67 Figure 2. 30: L-R; Aqueous solutions of: cobalt (II) nitrate, Co(NO3)2 (red); potassium dichromate, K2Cr2O7 (orange); potassium chromate, K2CrO4 (yellow); nickel (II) chloride, NiCl2 (green); copper (II) sulfate, CuSO4 (blue); potassium permanganate, KMnO4 (violet) . 68 Figure 2. 31: Hydrolysis and condensation reaction in clay organosilane coupling ............... 69 Figure 2. 32: Hydrolysis of aminosilane (A), Condensation reaction of amino hydroxo silane with clay-OH groups (B), suggested silanization reaction with epoxy matrix (C) ..................... 70 Figure 2. 33: FTIR spectra obtained as potassium bromide pellet of (a) silica (b) calcite clays (Reig et al., 2002) .................................................................................................................................... 72 Figure 2. 34: Reaction scheme of epichlorohydrin with bisphenol A and sodium hydroxide. The product of the reaction is diglycidyl ether of bisphenol A, DGEBA. Epoxide end group is indicated in circle. Where, n = 2-30 for high molecular mass epoxy, n = 3.7 average for low molecular mass epoxy) .................................................................................................................................... 77 Figure 2. 35: Structure of commercially available epoxy-phenol Novalac (EPN), where R = H; and epoxy-cresol Novalac (ECN), where R = CH3 .............................................................................. 78 Figure 2. 36: Structures of epoxy resins derived from multifunctional aromatic glycidyl amine resins such as triglycidyl-p-aminophenol (ERL 0510 by CIBA-GEIGY), and tetraglycidyl-4,4-diaminodiphenylmethane (TGDDM), (Araldite MY 720 by CIBA-GEIGY) ... 78 Figure 2. 37: Structure of diglycidyl ester of hexahydrophthalic acid .......................................... 79 Figure 2. 38: Epoxy resin cure with a tertiary amine as curing agent in several ring-opening homo-polymerization reactions to form 3D crosslink network ........................................................ 80 Figure 2. 39: Epoxy resin reaction with; (1) Primary amine, (2) Secondary amine, (3) Tertiary amine .................................................................................................................................................... 80 Figure 2. 40: Illustration of different states of dispersion of organoclays in polymers with corresponding WAXS and TEM images (Paul & Robeson, 2008)...................................................... 84 Figure 2. 41: Stress-strain curve of polymers showing 5 different regions of the curve, (A) elastic region, (B) Yield (intrinsic a and extrinsic), (C) Strain softening, (D) Cold drawing or strain hardening, (E) Failure (Callister, 2007) ....................................................................................... 92 Figure 2. 42: Schematic illustration of deformation mechanisms and stress-strain curves showing the effect on lamellae orientation (Cotterell, Chia, & Hbaieb, 2007; Zhou & Wilkes, 1998) ...................................................................................................................................................................... 93 Figure 2. 43: Microscopic observation of Polystyrene at 22°C in; (A) compression with a polarized light optical microscope and (B) tension with a transmission electron microscope (TEM) showing crazes and fibrils = 20nm (Papanicolaou & Bakos, 1992) .................................. 96
xiv Figure 2. 44: Schematic representation of toughening mechanisms proposed for thermoplastic-modified epoxied; (1) Particle bridging, (2) Crack pinning, (3) Crack path deflection, (4) Particle yielding-induced shear banding, (5) Particle yielding, (6) Microcracking (Pearson & Yee, 1993) ................................................................................................................. 102 Figure 2. 45: Effect of direction of maximum stress (σmax) on the shapes of dimples formed by microvoid coalescenceInfluence of direction of maximum stress ( max) on the shapes of dimples formed by microvoid coalescence. (a) In tension, equiaxed dimples are formed on both fracture surfaces. (b) In shear, elongated dimples point in opposite directions on matching fracture surfaces. (c) In tensile tearing, elongated dimples point toward fracture origin on matching fracture surfaces (Victor Kerlins, 1987) ......................................................... 106 Figure 2. 46: Formation of elongated dimples under tear and shear loading conditions. (a) Tear fracture (b) Shear fracture (Kerlins & Philips, 1987). ........................................................... 107 Figure 2. 47: Schematic of cleavage fracture formation showing the effect of sub-grain boundaries (a) Tilt boundary (b) Twist boundary (Kerlins & Philips, 1987) .......................... 108 Figure 2. 48: Types of cleavage fractures (a) Twist boundary, cleavage steps, and river patterns in a material that was fractured by impact (b) Tongues (arrows) on the surface of a material that fractured by cleavage (Victor Kerlins, 1987) ........................................................... 108 Figure 2. 49: Mechanism of fatigue crack propagation by alternate slip at the crack tip. Sketches are simplified to clarify the basic concepts. (a) Crack opening and crack tip blunting by slip on alternate slip planes with increasing tensile stress. (b) Crack closure and crack tip re-sharpening by partial slip reversal on alternate slip planes with increasing compressive stress (C. A. Zapffe & Clogg (Jnr.), 1944) ........................................................................ 110 Figure 2. 50: Saw-tooth and groove-type fatigue fracture profiles. Arrows show crack propagation direction (a) Distinct saw-tooth profile (aluminum alloy) (b) Poorly formed saw-tooth profile (steel) (c) Groove-type profile (aluminum alloy).............................................. 110 Figure 2. 51: Schematic illustration of decohesive rupture along grain boundaries. (a) Decohesion along grain boundaries of equiaxed grains. (b) Decohesion through a weak grain-boundary phase. (c) Decohesion along grain boundaries of elongated grains (Kerlins & Philips, 1987; J L McCall, 1972)................................................................................................................... 111 Figure 3. 1: (L-R) Core clay materials by solvothermal synthesis / calcination (SOBM filler 1) and acid activation (TDU ash) ............................................................................................................... 116 Figure 3. 2: Structural formula of 3-Aminopropyl triethoxysilane - APTES ............................. 118 Figure 3. 3: Outlay of experimental design ........................................................................................... 119 Figure 3. 4: The processes of acid activation, sieving, ultrasonication, silane coupling and oven drying ......................................................................................................................................................... 121 Figure 3. 5: DELSA 440x electrophoresis apparatus ......................................................................... 122
xv Figure 3. 6: Mixing of 3-aminopropyltriethoxosilane (APTES) coupled (a.) SOBM clay, and (b.) TDU ash samples, with epoxy matrix using a high speed mechanical mixer ..................... 123 Figure 3. 7: Process of casting epoxy composites; (a.) silicon resin mixing, (b.) silicon mould making, and (c.) epoxy composites casting ............................................................................................ 123 Figure 3. 8: Dog bone epoxy clay composites (a.) Core clay filled composites (b.) Acid activated TDU Ash filled composites ......................................................................................................... 124 Figure 3. 9: Dimensions of dog-bone sample. All measurements are in mm ............................ 124 Figure 3. 10: VARIAN FTIR Pike MIRacle ATR ..................................................................................... 126 Figure 3. 11: Characteristic frequencies associated with vibrational modes of simple molecules in FT-IR ........................................................................................................................................... 127 Figure 3. 12: FT-IR fingerprints of simple molecules (Wikipedia, 2010) ................................... 127 Figure 3. 13: Scheme of the Scanning Electron Microscope (SEM).............................................. 129 Figure 3. 14: Picture of Electron Microscope (SEM) at the Ceramics and Glass department, Universidade de Aveiro, (UA) Portugal .................................................................................................... 131 Figure 3. 15: XRD machine showing 3 very important parts ......................................................... 134 Figure 3. 16: ZWICK ROELL Z100 tensile testing machine at Institut f. Maskintechnik, Aalborg Universitet, (AAU) Denmark ....................................................................................................... 135 Figure 3. 17: Zwick durometer from Zwick Co. Einsingen Uber Ulm, Germany ...................... 136 Figure 3. 18: Dimensions of type B / D indentor of a durometer (ASTM D 2240) .................. 136 Figure 4. 1: Particle size distribution of TDU ash ............................................................................... 139 Figure 4. 2: Cumulative frequency plot showing particle size distribution of TDU ash........ 140 Figure 4. 3: Particle size distribution of SOBM core clay sample .................................................. 141 Figure 4. 4: Cummulative frequency plot for particle distribution of SOBM core clays ....... 142 Figure 4. 5: Calcined spent oil base mud (SOBM) clay. Marked zone indicate frozen melt region ................................................................................................................................................................... 143 Figure 4. 6: Synthesized spent oil base mud (SOBM) clay ............................................................... 144 Figure 4. 7: Sonicated and synthesized SOBM clay ............................................................................ 145 Figure 4. 8: SEM measurement of pore dimensions at different points on the SOBM clay sample .................................................................................................................................................................. 146
xvi Figure 4. 9: Micrograph of aminosilane coupled SOBM clay .......................................................... 148 Figure 4. 10: Micrograph of commercially available Cloisite® clay. Cloisite is a sodium montmorrillonite (Na-MMT) clay .............................................................................................................. 149 Figure 4. 11: Untreated thermally desorbed (TDU) ash .................................................................. 149 Figure 4. 12: Acid activated thermally desorbed (TDU) ash .......................................................... 150 Figure 4. 13: Micrograph of aminosilane coupled acid-activated TDU ash ............................. 151 Figure 4. 14: XRD characterization showing mineralogy of calcined particles; silica, barites, andradite and wollastonite .......................................................................................................................... 152 Figure 4. 15: X-ray diffraction result of synthesized SOBM powder showing plot of intensity in a.u. against diffraction angle, 2θ........................................................................................................... 153 Figure 4. 16: EDX spectra of synthesized SOBM clay filler .............................................................. 155 Figure 4. 17: EDX spectra of sonicated SOBM clay filler .................................................................. 156 Figure 4. 18: EDX spectra of aminosilane coupled SOBM clay sample ....................................... 157 Figure 4. 19: EDX spectra of thermally desorbed (TDU) clay sample ......................................... 159 Figure 4. 20: EDX of acid activated thermally desorbed clay (TDU ash) ................................... 160 Figure 4. 21: EDX of Silane Coupled thermally desorbed clay ....................................................... 162 Figure 4. 22: Two-point analysis of synthesized SOBM clay ........................................................... 164 Figure 4. 23: Two-point analysis of thermally desorbed clay (TDU Ash) .................................. 165 Figure 4. 24: Electrophoretic measurements of synthesized SOBM and thermally desorbed clay samples ....................................................................................................................................................... 167 Figure 4. 25: FT-IR spectra of thermally desorbed (TDU) ash sample in transmittance mode ................................................................................................................................................................................ 168 Figure 4. 26: FT-IR spectra of thermally desorbed (TDU) acid activated clay in absorbance mode ..................................................................................................................................................................... 172 Figure 4. 27: FT-IR spectra of synthesized spent oil base drilling mud (SOBM) core clay .. 173 Figure 4. 28: FT-IR spectra of synthesized, sonicated and aminosilane coupled spent oil base drilling mud (SOBM) core clay in transmittance mode ..................................................................... 174 Figure 4. 29: FT-IR spectra of synthesized, sonicated and aminosilane coupled spent oil base drilling mud (SOBM) core clay in absorbance mode........................................................................... 174
xvii Figure 4. 30: FT-IR spectra of unfilled epoxy matrix ......................................................................... 178 Figure 4. 31: FT-IR spectra of 0.8% 3-APTES coupled SOBM core clay filler in epoxy.......... 179 Figure 4. 32: FT-IR spectra of 1.6% 3-APTES coupled SOBM core clay filler in epoxy.......... 180 Figure 4. 33: FT-IR spectra of 4% 3-APTES coupled SOBM core clay filler in epoxy ............. 180 Figure 4. 34: FT-IR spectra of 8% 3-APTES coupled SOBM core clay filler in epoxy ............. 181 Figure 4. 35: FT-IR spectra of 0.8% 3-APTES coupled acid activated TDU ash in epoxy ..... 182 Figure 4. 36: FT-IR spectra of 1.6% 3-APTES coupled acid activated TDU ash in epoxy ..... 183 Figure 4. 37: FT-IR spectra of 4% 3-APTES coupled acid activated TDU ash in Epoxy ........ 183 Figure 4. 38: FT-IR spectra of 8% 3-APTES coupled acid activated TDU ash in epoxy ........ 184 Figure 4. 39: Comparative Stress-Strain behaviour of 3-APTES coupled TDU ash filled epoxy composites .......................................................................................................................................................... 185 Figure 4. 40: Comparative Stress-Strain behaviour of 3-APTES coupled SOBM core clay filled epoxy composites .................................................................................................................................. 187 Figure 4. 41: Comparative plot showing the effect of 3-APTES coupled SOBM core clay filler content on the mechanical properties of epoxy composites ............................................................. 190 Figure 4. 42: Comparative plot showing the effect of 3-APTES coupled TDU ash core clay filler content on the mechanical properties of epoxy composites .................................................. 191 Figure 4. 43: Composite ultimate strength and 3-APTES coupled SOBM core clay filler volume fraction plotted against the shape factor, r. ........................................................................... 192 Figure 4. 44: Composite ultimate strength and 3-APTES coupled TDU ash filler volume fraction plotted against shape factor, r. .................................................................................................. 194 Figure 4. 45: Modelling of epoxy composite ultimate strength using Schrager’s equation 196 Figure 4. 46: SEM micrograph of fractured surface of unfilled epoxy composite surface ... 198 Figure 4. 47: SEM micrograph of fractured epoxy composite surface filled with 0.8wt% of 3APTES coupled TDU ash filler ...................................................................................................................... 199 Figure 4. 48: SEM micrograph of fractured epoxy composite surface filled with 1.6wt% of 3APTES coupled TDU ash filler ...................................................................................................................... 199 Figure 4. 49: SEM micrograph of fractured epoxy composite surface filled with 4wt% of 3APTES coupled TDU ash filler ...................................................................................................................... 200
xviii Figure 4. 50: SEM micrograph of fractured epoxy composite surface filled with 8wt% of 3APTES coupled TDU ash filler ...................................................................................................................... 200 Figure 4. 51: SEM micrograph of fractured epoxy composite surface filled with 0.8wt% of 3APTES coupled SOBM core-shell clay filler ............................................................................................. 202 Figure 4. 52: SEM micrograph of fractured epoxy composite surface filled with 1.6wt% of 3APTES coupled SOBM core-shell clay filler ............................................................................................. 202 Figure 4. 53: SEM micrograph of fractured epoxy composite surface filled with 4wt% of 3APTES coupled SOBM core-shell clay filler ............................................................................................. 203 Figure 4. 54: SEM micrograph of fractured epoxy composite surface filled with 8wt% of 3APTES coupled SOBM core-shell clay filler ............................................................................................. 204
xix
List of Tables Table 2. 1: Clay mineral classification ...................................................................................................... 13 Table 2. 2: Showing some unsaturated and saturated fatty acids, their molecular formulae and their molecular masses (Wikipedia, 2012) ...................................................................................... 26 Table 2. 3: Particle shape characterization and associated values ............................................... 48 Table 2. 4: Colloidal particles nature and behaviour associated to zeta potential values (Malvern, 2011) ................................................................................................................................................. 65 Table 2. 5: FT-IR band assignments for montmorillonite clay (Tyagi et al., 2006).................. 74 Table 2. 6: Showing vibrational band assignments of some clay obtained by FT-IR technique (Steudel et al., 2009a, 2009b)........................................................................................................................ 75 Table 2. 7: Ductile and brittle response of solids ................................................................................ 103 Table 3. 1: Composition of clay samples used for this study ........................................................... 115 Table 3. 2: Handling characteristics of PRO-SET epoxy resin ........................................................ 117 Table 3. 3: Physical properties of cured/hardener (135/229) @ room temperature ........... 118 Table 3. 4: Epoxy composites formulation ............................................................................................ 124 Table 3. 5: FT-IR parameters with VARIAN Pike MIRacle ATR...................................................... 126 Table 4. 1: Results of TDU ash dry sieving ............................................................................................. 138 Table 4. 2: Particle size analysis for TDU ash using the cumulative frequency table............ 139 Table 4. 3: Cumulative frequency and mass fraction of particle size distribution of SOBM core clay sample ............................................................................................................................................... 141 Table 4. 4: Point-to-point pore size measurements on synthesized SOBM clay ....................... 147 Table 4. 5: Peak properties of X-ray diffraction experiment .......................................................... 154 Table 4. 6: Elemental composition of synthesized SOBM clay filler ............................................. 155 Table 4. 7: Elemental composition of sonicated SOBM clay sample ............................................ 157 Table 4. 8: Elemental composition of aminosilane coupled SOBM clay sample ...................... 158 Table 4. 9: Elemental composition of thermally desorbed (TDU) clay sample ........................ 159
xx Table 4. 10: Elemental composition of acid activated thermally desorbed clay (TDU ash)161 Table 4. 11: Elemental composition of silane coupled thermally desorbed clay ..................... 163 Table 4. 12: Two-point analysis of synthesized SOBM clay filler .................................................. 164 Table 4. 13: Two-point analysis of untreated thermally desorbed clay (TDU ash)................ 165 Table 4. 14: Wavenumbers and assignments of vibrational bands for TDU ash .................... 169 Table 4. 16: Mechanical properties of 3-APTES coupled TDU ash epoxy composites ........... 186 Table 4. 17: Mechanical properties of 3-APTES coupled-SOBM core clay epoxy composites ................................................................................................................................................................................ 188 Table 4. 18: Shape factor, r, based on tested composite ultimate strength of 3-APTES coupled SOBM core clay epoxy composite .............................................................................................. 192 Table 4. 19: Shape factor, r, based on tested composite ultimate strength of 3-APTES coupled TDU ash epoxy composite............................................................................................................. 194 Table 4. 20: Shore D hardness for 3-APTES coupled TDU ash filled epoxy composites ........ 197 Table 4. 21: Shore D hardness for 3-APTES coupled SOBM core clay filled epoxy composites ................................................................................................................................................................................ 197
xxi
List of Abbreviations
S/No.
Abbreviation
Meaning
1.
1D
1 Dimensional
2.
2D
2 Dimensional
3.
3-APTES
3-Aminopropyltriethoxysilane
4.
3D
3 Dimensional
5.
ACS
American Chemical Society
6.
AEM
Acoustic Emmission Microscopy
7.
AFaM
Atomic Force Acoustic Microscopy
8.
AFM
Atomic Force Microscopy
9.
AIPEA
Association Internationale pour I’Etude des Argiles
10.
APMA
Ammonium Poly(methacrylate)
11.
ASTM
American Society for Testing and Materials
12.
ATR
Attenuated Total Reflectance
13.
BET
Brunauer Emmett Teller
14.
Ca-MMT
Calcium-Monmorrillonite
15.
CEC
Cation Exchange Capacity
16.
CMS
Clay Mineral Society
17.
DDS
4,4 diaminodiphenyl sulphone
18.
DGEBA
Diglycidyl ether of Bisphenol A
19.
DMAPP
Β-dimethyl-aminopropionone
20.
DPR
Department of Petroleum Resources
21.
DSC
Differential Scanning Calorimetry
22.
DVLO
Derjaguin, Verwey, Landau and Overbeek
23.
EDX
Electron Dispersive X-ray Spectroscopy
24.
ESA
Envelope Surface Area Analyzer
25.
FEM
Finite Element Modelling
26.
FGCMs
Functionally Graded Composite Materials
27.
F-TDU
Fixed Thermal Desorption Unit
28.
FT-IR
Fourier Transform Infrared Spectroscopy
xxii
29.
FT-Raman
Fourier Transform-Raman Spectroscopy
30.
GNHEA
Glycine-n-hexylester amine
31.
HDTMA-Br
Hexadecyltrimethylammonium Bromide
32.
IEP
Isoelectric Point
33.
IPN
Interpenetrating Network
34.
IR
Infrared Spectroscopy
35.
IZA
International Zeolites Association
36.
JNC
Joint Nomenclature Committee
37.
LDH
Layered Double Hydroxide
38.
MMT
Monmorrillonite
39.
M-TDU
Mobile-Thermal Desorbtion Unit
40.
Na-MMT
Sodium-Monmorrillonite
41.
NPDEA
N-phenyldiethanolamine
42.
O/G
Oil and Grease
43.
ORMOCAL
Organically Modified Calcites
44.
ORMOCERS
Organically Modified Ceramics
45.
ORMOSILS
Organically Modified Silicates
46.
PEG
Polyethylene Glycol
47.
PILC
Pillared Interlayered Clays
48.
PS
Polystyrene
49.
PZC
Point of Zero Charge
50.
RAMAN
Raman Spectroscopy
51.
SAXS
Small angle x-ray scattering
52.
SDF
Specialty Drilling Fluids
53.
SEM
Scanning Electron Microscopy
54.
SOBM
Spent Oil Base Drilling Mud
55.
STM
Scanning Tunnelling Microscopy
56.
TDU
Thermally Desorbed Ash
57.
TEM
Transmission Electron Microscopy
58.
TGA
Thermogravimetric Analysis
59.
TGDDM
Tetraglycidyl-4,4’-diaminodiphenyl methane
60.
TPH
Total Petroleum Hydrocarbon
xxiii
61.
WAXS
Wide angle x-ray scattering
62.
XRD
X-Ray Diffraction
1
Chapter One Introduction 1.1
Background Information
Core-shell nanoparticles may consist of inorganic or organic core coated with a thin layer of inorganic or organic shell. The inorganic core may be natural or synthetic nanoclays including smectite, bentonite, silica, wollastonite, mica, barite, titania, zirconia, calcite, ferrite, alumina, hectorite, layered double hydroxides or a mixture of oxide clays. The shell may be inorganic, organic or polymers with functionalities. They may exhibit combination of qualities which neither is inherent in the core nor the shell. Core-shell nanoparticles and procedures for obtaining them have been elucidated (Bala et al., 2007; Hu et al., 2009; Marini et al., 2008; Zhang et al., 2006). They have been used to fabricate functionally graded composite materials (FGCMs) wherein there may exist gradual micro structural transitions from either ceramic to metal and vice versa, ceramic to polymer and vice versa, or metal to polymer and vice versa (Krawczak, 2010). Achieving these gradual transitional behaviours or “smart changes” in composite materials, has been attributed largely to secondary phase inclusions, distributed non-uniformly in their host matrix (Krawczak, 2010). It is also possible that between the host matrix and the particulate inclusions, there could be a switch of functions as a result of the particle microstructural appearance, shape and size (Gupta, 2007; Icardi & Ferrero, 2009). It has been reported that core-shell nanoparticles have been able to impact this character in nanocomposites, hence have found applications in polymer nanocomposites (Paul & Robeson, 2008; Stevens, 2003; Utracki, 2008), coatings & adhesives, greases and lubricants (Choudhury et al.,
2
2008), cosmetics and drug delivery vehicles, therapeutics, diagnostics and imaging (Parveen et al., 2012; Patel et al., 2006), optics, electronics, electrical energy storage, biomedical assays, catalysis, industrial sieves and environmental applications (Polizos et al., 2010; Rozenberg & Tenne, 2008; Suh et al., 2009). Therefore, functionally graded polymer nanocomposites, may be described as engineering materials comprising two or more dissimilar materials brought together for the sole aim of improving overall properties / performance of the final material, designed in such a way that there is a gradual transition from the hard nano-phase ceramic core filler, via a medium soft interphase shell, to the matrix. Hence, core-shell nanoporous fillers embedded in a thermoplastic or thermoset matrix. It is interesting that such materials have been designed and applied in various ways due to their enhanced character. Long before now, naturally occurring composites have existed from the creation of the world to date. Bone which is a composite of collagen and hydroxyappatite, wood; a composite of lignin and cellulose fibre, Snail (Gastropod) shell; a composite of calcium carbonate precipitated into an organic matrix known as conchiolin, silk made of a gel core encased by a solid structure of aligned organonanoparticles. The ancient Egyptians used a combination of mud and straw for bricks. In each of these combinations, a composite is seen to compose of matrix and reinforcement. However, the properties of components of these materials have been seen to be complimentary rather than individualistic (Callister, 2007). The matrix gives adhesion, while the reinforcement gives strength and dimensional stability to the composite. However, in trying to mimic the characteristics of the natural composites, design engineers investigated the
3
criteria necessary to obtain composite materials of same form with naturally occurring ones. Understanding what role matrix and reinforcements play, characteristics of matrix and reinforcement, and how they can be engineered to effectively improve composite performance, became a major research question. Nature of matrix, dimensions of reinforcement, nature of bonding between matrix and reinforcement (interface) and the surrounding area between reinforcement and matrix (interphase), play predominant role in performance of composites during application (Liu, 2007). However, application of core-shell-nanoparticles in polymer nanocomposites which initially was intended just for reinforcement, structural and thermal stability, has been used to induce electrical energy storage character (Polizos et al., 2010), fluorescence, colour and hue to the composites (Nair et al., 2003). Attempts in previous study (Iheaturu, 1999) to cast spent oil base, water base and pseudo oil base drilling mud systems into building blocks using blast furnace slag, show that increased oil content inhibits solidification of slag-mud-mix cement. This collaborates with the work of Minocha and co. in 2003 (Minocha et al., 2003). Whereas reduced oil content for oil base and pseudo oil base mud systems gave somewhat good products, water base mud gave the best block material for experiments carried out after 48hrs exposure under sunlight. Potential hazards in mud systems disposed of to the environment were identified as oil (especially from oil base drilling fluids and those based on mineral oils of the isomerized olefins), salts and soluble trace elements as Zinc (Zn), Aluminium (Al), Calcium (Ca), Iron (Fe), Lead (Pb), Copper (Cu), Cadmium (Cd), Nickel (Ni), Mercury (Hg), Arsenic (As), Barium (Ba) and other trace elements. In view of the foregoing, mud
4
solids (clay) were recovered from samples of spent oil base drilling mud waste collected from various rig sites in 2004, sieved, treated and calcined. Physiochemical properties of the calcined samples indicate the presence of about 94.3% CaO, 1.45% silica, 2% Fe2O3 and trace elements of Al2O3, K2O, Na2O and MgO. As a result of calcination and solvothermal treatment, at a temperature of 900°C, an egg yolk yellow powdery substance was obtained and the solids were seen to be highly insoluble in both organic and inorganic solvents (Enyiegbulam et al., 2011). SiO2(s)+Mg2++yCa2++K++NaCl(aq)+Na2O+Al2+(aq)+2Fe2+(aq)+3C2O42-(aq)
Ca2yNa2x-1KMgAl2Fe2(C2O4)3.xH2O(s)+SiO2(s) @ 900ºC
However, above 900°C and up to 1200°C gave a fused yellow ceramic material (Iheaturu, 2006). The material having been processed into an insoluble powdery solid and sieved into various sizes of between 63m and 425m, was complexed into unsaturated polyester composite material and tested for mechanical properties (Iheaturu, 2006). Results showed improved mechanical strength for composites filled with 63m sized particles. Such a novel material has been proposed to serve in making beautiful decorative laminates, composite structures for satellites and aerospace applications (Enyiegbulam et al., 2007), where high solar radiations exist in a vacuum called space. 1.2
Statement of the Problem
The use of water base, synthetic, oil base and pseudo oil base drilling mud systems have tremendous impacts on the environment if not properly handled and disposed. An appraisal of spent drilling mud disposal techniques practiced in the
5
oil industry in Nigeria have showed that solid content of spent drilling mud, which comprises of various clay deposits and mixed oxides, heavy metals and sand, added or picked up from the formation during drilling, has been a major source of worry to industry operators in Nigeria, more so when industry regulators have stipulated stringent rules and procedures for their disposal. In Nigeria, which is one of the major oil producers in the world, and perhaps in some oil producing developing countries, thermal desorption of the spent mud has been adopted as the industry’s best practices in spent mud disposal. Thermal desorption is the extraction of oil and water from spent mud sludge and their reuse in mud formulation leaving behind enormous mass of solid content (mixed oxides, sand, clays and clay deposits) lying as waste at dump sites. In spite of the existing disposal technique, utilization or the reuse of the solid content remains a major problem in the oil and gas industry. Their conversions into core clay nano filler materials for the polymer industry in Nigeria, from pre-thermal desorption or post-thermal desorption, via high temperature treatment (calcination) or acid activation respectively, may lead to a drastic reduction in waste pile-up, on the one hand, while on the other, may be a major source of revenue for both the oil industry and the polymer processing industries in Nigeria. 1.3
Project Objectives
The main objective of this study is to synthesize organically modified core-shellclay filler from spent drilling mud waste and to apply the material as reinforcement in epoxy matrix. Specific objectives of this project therefore are itemized as follows;
6
(i.)
To convert drill cuttings waste into novel polymer (thermoset / thermoplastic) core filler material by solvothermal synthesis and acid activation.
(ii.)
To convert as-synthesized organophobic core filler to organophilic core – shell filler using amino silane coupling as shell on organophobic core clay.
(iii.)
To reinforce phenol-free epoxy matrix with the novel filler at various volume fractions.
(iv.)
To characterize novel fillers and composite materials for properties using some advanced characterization techniques.
1.4
Justification for this Project
Spent drilling mud has been defined (Iheaturu, 2008). The material which before now lie as waste at rig sites, sea beds, oceans and our farmlands will be better put to use and its energy harnessed and changed to serve as reinforcement to a more or less inert and yet friendly 21st century material – Polymerics. This has been earlier proposed (Iheaturu, 2007). Interestingly, guidelines for effective disposal of spent mud and drill cuttings has been documented by the Department of Petroleum Resources, DPR, Federal Ministry of Petroleum Resources, Port Harcourt, Nigeria (DPR, 2004). In the light of this, the present situation in the oil and gas industry in Nigeria is the deployment of mobile thermal desorption unit (M-TDU) at various rig sites in the Niger Delta and fixed thermal desorption unit (F-TDU) at Onne near Port Harcourt, Rivers State, Nigeria. The thermal desorption unit, (TDU), based on vaporisation of oil and water by heat generated via mechanical agitation and in-direct heating of the drum was built by THERMTECH AS, Norway (Thermtech, 2004). From Figures 1.1, 1.2 and 1.3, by-products of the thermal desorption units, include; oil, water and solids (clay). Oil and water are
7
recovered and reused while solid content (clays, cuttings and mud rock) remain a source of concern to operators in the oil and gas industry. Therefore, disposal of solid by-product as a result of thermal desorption of oil base drilling mud remains a major problem in the oil and gas industry.
Figure 1. 1: Process of crude oil drilling and oil drill cuttings recovery from a rig site in the Niger Delta area of Nigeria
Figure 1. 2: Mobile Thermal Desorption Unit (M-TDU) at Shell, Port Harcourt, Nigeria
8
Non - Condensable Gas
Cooling water
Drilling waste
Vapours
Feed Feed hopper
Proc ess Mill
Drive unit
Cyc lone
Steam con denser
Oil con denser
Hydraulic Feed pump
Recovered Water
Recovered oil Recovered solids Screw conveyor
Figure 1. 3: Process flow chart of the M-TDU
1.5
Scope of Work
The general scope of this work has been published earlier (Enyiegbulam et al., 2011). However, spent oil base drill cuttings will be sourced from the oil rig sites in Nigeria. In addition to the steps taken by Enyiegbulam et al, 2011, thermally desorbed ash from thermal desorption unit at SDF facility at Onne, Rivers State, Nigeria, will be activated by acid activation. However, attempts will be made in order to reduce iron fillings content in ash material by magnetic separation before acid activation. Furthermore, the composite materials produced from this research may possess good dimensional stability, mechanical and thermal properties which could make them the preferred material for harsh weather.
9
This project involves the following: (i.)
Collection of recovered spent oil base and synthetic drilling mud waste
(ii.)
Recovery of mud solids (clays) by incineration (Jones, 2010)
(iii.)
Calcination / Solvothermal synthesis of recovered oil base drill cuttings
(iv.)
Acid activation of thermally desorbed ash
(v.)
Ultra Sonication of products from (iii.) and (iv.)
(vi.)
Core clay functionalization with aminosilane
(vii.)
Determination of functionalization by Fourier Transform Infrared Spectroscopy
(viii.)
Incorporation of core-shell clays into epoxy matrix
(ix.)
Composite curing
(x.)
Testing of composite materials for mechanical properties, hardness
(xi.)
Physical modelling using Schrager’s equation
(xii.)
Fractography of epoxy core-shell clay filled composites
(xiii.)
Applications of core-shell filled epoxy composite material
10
Chapter Two Literature Review 2.1
Core-Shell Nanoparticles
Nanoparticles with core-shell structure represent a new class of materials gaining tremendous interest due to their controllable properties. As their name imply, core-shell nanoparticles have an inner particle which size is less than 100nm in one dimension and coated with a thin layer of another material for a specific purpose. The main objective usually, is to obtain somewhat different material that could impart the desired character to the new material. The necessity to shift to core-shell materials is the enhancement of nanoscale particle – matrix interphase. Clay nanoparticles exhibit this nano effect when complexed in a polymer matrix to form polymer nanocomposites (Kornmann et al., 1999). This is because clay platelets comprising of nanosheets in the dimension of 0.72 to 0.1nm thick have high surface area for interaction with surrounding matrix, making it possible for improvement of overall composite performance. However, clay nanoparticles flocculate, agglomerate and cluster with themselves, making them difficult to work with. They are also hydrophilic and organophobic. This is because, apart from Van der Waal’s forces of attraction, there exists surface ions on the clay platelets (Bergaya & Lagaly, 2006), making them polar compounds. By taking into account these nanoparticles behaviours, a suitable organic coating or “soft shell” on the clay particles, may be applied in order to prevent agglomeration, to foster deflocculation and to enhance organic interactions (Wang et al., 2007). Further functionalization imparts surface active end groups and in some cases long chain
11
organic chains or ions, that make for their miscibility and reaction with organic matrix (Garea et al., 2010). 2.2
Core-Shell Morphology
A simple explanation of the core-shell appearance maybe seen in AVOCADO PEAR (botanical name; “persea americana”), a native South and Central American tree which fruit consists of a seed covered by a yellowish green flesh and a leathery green skin. The fruit may be pear-shaped, egg shaped or spherical. Now, consider that the seed is in the dimension of 100nm or less, coated with a thin film of organic or inorganic nature. Figure 2.1a, presents a core shell nanoparticle sliced into 8 quartets, with 1/8 quartet removed, giving insight into the egg yolk yellow nanoparticle inside, otherwise the core. Figure 2.1b, presents the sliced face of a core shell nanoparticles which has been sliced into 2 halves. The shell thickness maybe in nano dimension which may be organic with amphoteric character or inorganic thin film of a metal or metal oxide.
S h e ll t h ic k n e s s in n m
C o re
(a.)
S h e ll
S h e ll t h ic k n e s s in n m
C o re
(b.)
S h e ll
Figure 2. 1: Schematic representation of core-shell morphology (a.) with 1/8th quartet cutoff, (b.) transverse face of one half cut-off
12
Consider also that the core or shell maybe porous, hence nanoporous core or nanoporous shell. Consider also that the individual difference in the material are subdued and the core-shell combination act as one nanoparticle as a result of surface active end-groups linking the two phases together and also surrounding the entire shell. The picture as presented, explains in a nutshell the core-shell morphology. 2.3
Clay Minerals and Nanoporous Clay Structure
The term clays and clay minerals have been defined by the Joint Nomenclature Committees (JNC) of the Association Internationale pour l´Etude des Argiles (AIPEA) and the Clay Minerals Society (CMS) (Bergaya & Lagaly, 2006). From their definition, clay minerals may be either natural or synthetic phyllosilicate or nonphyllosilicate with various amounts of Al, Si, O2, H2, Fe, Mg, alkaline metals, alkaline earths metals and surface cations. Table 2.1 shows clay mineral classification, their chemical formula and crystal structure.
13
Table 2. 1: Clay mineral classification Clay type
1:1 Clays (TO)
Name / Classification
Crystal structure / Unit cell
Cation Exchange Capacity
Kaolin Group (General formula: Al2Si2O5(OH)4) (1:1)
Kaolinite Dickite Halloysite(Al2Si2O5(OH)4.2H2O) Nacrite
Triclinic
Dioctahedral
Montmorrilonites (MMT) - Bentonite (Na,Ca)0.33(Al,Mg)2(Si4O10)(OH)2.nH2O Nontrollinites (CaO0.5,Na)0.3Fe3+2(Si,Al)4O10(OH)2.nH2O
Monoclinic
Trioctahedral (1:2) Smectite group
2:1 Clays
(TOT)
Saponite (Ca,Na)0.33(Mg,Fe+2)3(Si,Al)4O10(OH)2.4H2O Sepiolite Mg4Si6O15(OH)2.6H2O Attapulgite (Mg,Al)2Si4O10(OH).4(H2O) Hectorite Na0.4Mg2.7Li0.3Si4O10(OH)2 Vermiculite (Mg,Al)2Si4O10(OH).4(H2O)
Illites (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]
92.6 – 120
86.6
Monoclinic 120 150
Monoclinic
Mica
Chlorites
Clinoclore(Mg5Al)(AlSi3)O10(OH)8 Chamosite(Fe5Al)(AlSi3)O10(OH)8 Nimite (Ni5Al)(AlSi3)O10(OH)8 Pennantite(Mn,Al)6(Si,Al)4O10(OH)8
Monoclinic
All phyllosilicates are built on 1 octahedral (O) sheet, sandwiched by 2 tetrahedral (T) sheets in a 1:1 (TO) or 2:1 (TOT) arrangement. With this arrangement, they are seen as a stack of sheets with anisotropic properties. The sheets could be negatively or positively charged or without surface charges (uncharged as in talc).
14
The surface charges are exchangeable and may determine colloidal properties and nature of interaction between clay platelets, water and the environment. In the context of this work, nanoparticles refer to a stack of clay mineral nanosheets, while a collection of these nanoparticles are described as aggregates. 2.3.1 2.3.1.1
Structure of Some Layered Silicates (Phyllosilicates) Smectite Clays
Smectite clays are composed of 1 octahedral sheet (O), sandwiched by 2 tetrahedral sheets (T) as shown in Figure 2.2. The octahedral sheets contain divalent or trivalent cations (Al3+, Mg2+, Fe3+), which are surrounded by 8 oxygen atoms in the octahedral structure, while the tetrahedral sheet contains 1 silicon atom surrounded by 4 oxygen atoms. In the specific case of montmorillonites (MMTs), the crystal lattice of 2:1 phyllosilicate consists of an octahedral sheet of alumina sandwiched by 2 silica tetrahedral sheets whereby the epical oxygen atom of the tetrahedral also belongs to the octahedral as shown in Figures 2.2 and 2.3. Each silicate sheets which is approximately 0.72nm in thickness, is characterized by lateral dimensions varying from 300Å to a few microns depending on the silicate particle, clay source and method of preparation (Ray & Okamoto, 2003). For example clays prepared by milling may have dimensions between 0.1 – 1.0µm giving them very high aspect ratio up to 1000 (Pavlidou & Papaspyrides, 2008a). The silicate sheets are held together by weak Van der Waal’s forces which make it easy for them to swell in water or organic compounds. Furthermore, isomorphic substitution within the layers whereby Al3+ is replaced by Mg2+ or Fe2+, or Mg2+replaced by Li+, generates anions that are counterbalanced by loosely bound
15
alkali and alkaline earth cations (Na+, K+ and Ca2+) situated in the interlayer space. The ability to exhibit this character in smectite is regarded as cation exchange capacity (CEC) expressed in mequiv/100gm. (A.)
Na+
+
Na
Na+
+
Na
Na+
Interlayer spacing
Tetrahedral sheet Silicate layer (Approx. 0.72nm)
Octahedral sheet
Basal spacing (Approx. 1nm)
Tetrahedral sheet
(C.)
(B.)
(A.) Montmorrillonite clay showing layered TOT structure where one Oxygen atom is in the octahedral sheet & 3D representation of (B) tetrahedral sheet (C) octahedral sheet Oxygen atom
Silicon atom
Al3+, Mg2+, Fe3+ ion
NB: Interlayer cations could be Ca 2+
Figure 2. 2: Layer structure of montmorillonite (MMT) clay, a member of the smectite group
Figure 2. 3: Structure of sodium montmorillonite, courtesy of Southern Clay Products, Inc., USA
16
Bergaya and Lagaly, 2006, accounts that all phyllosilicate mineral clays, have porous structures which differ in size and shape due to the nature and arrangement of the sheets. Figure 2.4 (a.) and (b.) are scanning electron micrographs of dioctahedral and trioctahedral smectite clays showing spongy appearance.
(a.) Dioctahedral smectite from Yucca Mountain, Nevada, drill hole UE25a#1 and a depth of 1296.2 feet. Courtesy of Steve Chipera, Los Alamos National Laboratory.
(b.) Trioctahedral Smectite 1. Ecocene marine sediments, 518m below sea floor, CRP drill core 3, Victoria Land basin, Antarctica. Courtesy of Anthony Prestas, Boston University, USA.
Figure 2. 4 : Scanning Electron Micrographs (SEM) of (a.) dioctahedral smectite and (b.) trioctahedral smectite (Source: Clay Mineral Society, UK)
The SEM images show clearly a porous clay structure, like a sponge (Figure 2.4a) and a honey comb (Figure 2.4b). The pores are seen to be less than 10µm. This physical structure may be the reason for swelling in water. It may also explain why ionic organic compounds or long chains are able to penetrate and lock-up into a solid 3D structure. 2.3.1.2
Kaolinite Clays
Previously, several works have been published to elucidate kaolinite clay mineralogy and structure (Bailey, 1964; Bear, 1965; Cruz, 1980). Kaolin clay
17
minerals are composed of an orderly arrangement of thin sheets of layered silicates usually bound together by counter ions and electrostatic forces of attraction. These thin sheets are also referred to a “kaolin plates”. Their “platy morphology” is that of tetrahedral and octahedral thin sheets, in which case silicon is surrounded by 4 oxygen atoms in the tetrahedral sheets in the form of SiO2, while aluminium is surrounded by 6 oxygen atoms in the octahedral sheets in the form of AlO2(OH)4 (Komori & Kuroda, 2000). The dioctahedral structure or 1:1 (TO) structure consists of a repetition of 1 tetrahedral and 1 octahedral sheet in which case the epical oxygen atoms are shared. Figure 2.5 and Figure 2.6 present layer structure of kaolin clay and micrographs showing the “kaolin plates” respectively. Each kaolin plate is approximately 0.7nm thick (Brigatti et al., 2006). It is reported that neither cations nor anions are present in the interlayer spacing of kaolinites because the interlayer region is surrounded by OH- groups of the octahedral sheets on one side and O2- groups of the tetrahedral sheets on the other.
18
(A.)
O2-
O2-
O2-
O2-
OH-
OH-
OH-
OH-
Interlayer spacing
Basal spacing (Approx. 1nm)
Tetrahedral sheet Silicate layer (Approx. 0.7nm)
Octahedral sheet
O2(B.)
O2-
O2-
O2-
(C.)
(A.) Kaolinite clay showing layered TO structure where oxygen atoms are shared & 3D representation of (B) tetrahedral sheet (C) octahedral sheet Oxygen atom
Silicon atom
Al3+, Mg2+, Fe3+ ion
NB: There are no interlayer cations but OH - and O 2- anions on either side
Figure 2. 5: Structure of kaolin clay
(a.) Kaolinite 1. Well crystallized kaolinite from (b.) Kaolinite plates. Thin idiomorphic platelets the Keokuk Geode, USA. Courtesy of M. Roe, from a of kaolinite from Hirschau, South East Germany. sample in the Macauley Institute collection. Courtesy of Frank Friedrich, Forschungszentrum, Karlsruhe, Germany.
Figure 2. 6: Scanning Electron Micrographs (SEM) of different kaolin books (a.) Kaolinite 1 and (b.) Kaolinite plates (Source: Clay Mineral Society, UK)
19
The SEM images in Figures 2.6(a.) and (b.), show clearly plate-like clay structure, where several clay plates are stacked together in layers. 2.3.1.3
Layered Double Hydroxides (LDHs)
These are anionic layered clays represented by the general formula; [MII+1xMIII+x(OH)2]x+(Xn-)x/n.yH2O,
where MII is a divalent cation; Ca2+, Mg2+, Mn2+, Fe2+,
Co2+, Ni2+, Cu2+ or Zn2+, and n a trivalent cation; Al3+ or Fe3+. Xn- is an n-valent anion. Simply put, LDHs have a layered crystal structure composed of positively charged hydroxide layers,[MII+1-xMIII+x(OH)2]q+, counterbalanced by anions (Cl, Br or NO3) within the interlayer spacing and water molecules [(Xn-)x/n.yH2O](Utracki et al., 2007). Pure phases have been shown to have x which represent a portion of trivalent metal cations in the range of 0.2 – 0.33. It is also possible that a monovalent cation like Li+, may replace the divalent cation, in which case LiAl2(OH)6]X∙yH2O. X represents a generic anion and the value of y is normally found to be between 0.5 – 4. A representation of LDHs is shown in Figure 2.7. They are also referred to as hydrotalcites (Mg-Al-LDH).
Figure 2. 7: Structure of layered double hydroxide (LDH)
Layered double hydroxides (LDHs) exhibit anion-exchange properties in aqueous solutions. The interlayer anions can be replaced via intercalation reactions with a
20
number anionic compounds ranging from simple carbonates (CO32-), through benzoates or succinates to complex biomolecules like DNA. This unique property of layered double hydroxides can also be used in the removal of ions in solutions. At temperatures of approximately 300 – 500°C, layered double hydroxides (LDHs) are decomposed to form mixed oxides of MII and MIII metals. 2.3.1.4
Structure of Hydrocalcites
Hydrocalcites (CaCO3.xH2O) are the hydrous form of calcium and magnesium carbonate mineral. It is a trigonal mineral. Though till date not much have been published on hydrocalcites, considerable deposits of the mineral have been found in Arctic Ikka Fjord, South West of Greenland (Dahl & Buchardt, 2006) and in the South Australia (Swainson, 2008). Its structure and phase composition have been x-rayed (Swainson, 2008). The crystal structure of mono hydrocalcite is presented in Figure 2.6. Calcium (Ca) atoms are represented as blue balls, Oxygen (O) atoms as red balls, and the carbonate (-CO32-) anion and water (H2O) molecules are shown as bonded entities.
Figure 2. 8: Crystallographic pattern of monohydro calcite clay (Swainson, 2008)
21
2.4
Clay Treatment and Surface Modification of Core-Clay Nanostructures
Clay particles are hydrophilic. And so, in order to induce organic-shell hydrophobic end groups on the core clay particles, they may be modified by any of the following ways (Bergaya & Lagaly, 2001);
[i.]
Adsorption:- which describes the process by which cationic species of other substances bond to the negative electrostatic charges on the outer surface of clay bodies. This ability is enhanced by the surface ionic density otherwise known as cation exchange capacity (CEC) of clay particles.
[ii.]
Ion exchange with inorganic cations and cationic complexes:- This is made possible by the displacement of surface electrostatic charges by surrounding inorganic and cationic species. When the CEC of clay particles is high, ion exchange with surrounding cations also increases.
[iii.]
Ion exchange with organic cations:- This is possible when source of surrounding cations is organic or polymeric. Deng and co-workers had reported a case where they studied adsorption of anionic, non-ionic and cationic polyacrylamides on smectite, illite and kaolinite (Deng et al., 2006). The anionic polyacrylamides promoted clay dispersion while the cationic polyacrylamides caused the clay particles to flocculate, to the extent that, some trapped clay particles became inaccessible to polyacrylamide cations.
[iv.]
Binding of inorganic and organic anions, edge to edge. This leads to clay flocculation.
[v.]
Grafting of organic compounds:- In this case, organic ligand having hydrophilic and hydrophobic end groups are attached to clay surfaces with
22
the help of compatibilizer such as the use of alkylammonium cations (Nakas & Kaynak, 2009; Xidas & Triantafyllidis, 2009a, 2010). Also polymeric ligands can be grafted covalently unto surfaces of inorganic nanoparticles via hydroxyl groups on the particles (Kango et al., 2013). [vi.]
Acid activation; involves treating Ca-MMTs (bentonite) with inorganic acid (Valenzuela-Diaz & Santos, 2001).
[vii.]
Pillaring by different types of poly (hydroxo metal) cations (Hutson et al., 1998; Ishii & Shinohara, 2005; Kloprogge et al., 2005; Pergher et al., 1999).
[viii.] Inter lamellar or inter particle and inter particle polymerization [ix.]
Dehydroxylation and calcination: - involves subjecting clay particle to very high temperatures for prolonged hours in order to effect thermal decomposition or phase transitions other than melting. It also removes water of crystallization drive off volatile organic matter like CO2, oil and grease.
[x.]
Delamination and re-aggregation of smectite clay minerals as a result of intercalation (He et al., 2006).
[xi.]
Physical treatments such as lyophilisation, ultrasound and acetylene plasma on hydrophilic surfaces of laponite clay mineral in order to make them compatible with polyethylene (Celini et al., 2007).
Amongst the above mentioned methods of clay surface modification, processes of acid activation, dehydroxylation, calcination or high temperature treatment on calcites, hydrothermal and solvothermal synthesis, are explained in this literature. The main reason for organoclay synthesis and modification is to introduce micro /
23
nanopores to clay bodies in the case of calcined spent oil base drilling mud (Betega de Paiva et al., 2008). It should be noted that calcination refers to thermal treatment given to calcite clays. However, such high temperature treatment may be given to mixed oxide clays in an oil medium, hence solvothermal synthesis, or in water medium, referred to as hydrothermal synthesis (Kumar et al., 2009). While acid treatment was carried out on ash from thermal desorption unit in order to activate clay platelets present in the bulk powder. 2.5
Alkyl Ammonium Exchanged Core-Clay Nanostructures
Clays have been seen to be hydrophilic as they have high affinity for water. In most cases as in bentonite clays, hydration levels may be as high as 45%. As a matter of fact, hydrophilic stack-like layered silicates have a high surface energy, making wettability and dispersion in organic compounds difficult. Clays are therefore immiscible in organic solutions and polymers. In order to achieve high degree of polymer-clay interaction, it is important that clay surfaces are modified to make them organophilic (Pavlidou & Papaspyrides, 2008a) and thermodynamically acceptable to organic molecules, whereby Gibb’s free energy of mixing is higher than zero (ΔGmix> 0). Interestingly, exchangeable cations impart adsorbent character to clays. Lagaly had reported that adsorbent properties of clays has been known since 7000BC (Lagaly, 1984)a. A practical example given by Lagaly et al, is the suction ability of swellable clays facilitated by clay adsorption character for water molecules (Lagaly et al., 2006a). On the other hand, it is also possible to reverse clay hydration by displacing such water molecules with polar organic species like alkylammonium ions (Le Pluart et al., 2002; Xidas & Triantafyllidis, 2009b, 2010). The use of long chain alkyl ammonium salts have proved as one
24
effective way of clay surface modification for polymer intercalation (Bergaya & Lagaly, 2001). This process of ion exchange with alkylammonium salt of the type NH3R+, NH2R2+, NHR3+, or NR4, have been elucidated (Betega de Paiva et al., 2008). Furthermore, Lagaly et al, 2006b, explains that displacement reactions of H2O molecules within clay interlayers are driven by hydrogen bonds, ion-dipole interactions, co-ordination bonds, acid-base reactions, charge transfer and Van der Waal’s forces. Grafting reactions enabled by covalently bonded reactive surface groups and organic species, help to hydrophobize clay particles. Whereas clay surfaces provide the silanol and aluminol groups, hydroxyl groups within the interlayer spaces of clay platelets provide displaceable H+ ions as clay inter-planar spacing is expanded. A stepwise process of gradual solvation / swelling of clay platelets, displacement / ion exchange, complexation and grafting, have been reported (Lagaly et al., 2006b), even though the whole process is not as easy as it seems. In trying to find a way of attaching organic ligands to clay minerals, Tiwari et al, 2008, synthesized novel organo-MMT using β-dimethyl-aminopropionone (DMAPP), N-phenyl-2,2-iminodiethanol (N-phenyldiethanolamine) NPDEA and glycine-n-hexylester amine (GNHEA) (Tiwari et al., 2008), while, different NaMMT
clays
modified
with
polymeric
Al/Fe
species,
hexadecyl-
trimethylammonium (HDTMA) surfactant and a complex of polymeric Al/FeHDTMA have been comparatively reported (Jiang & Zeng, 2003). Generally, inorganic contaminants like copper, tend to be adsorbed by the polymeric Fe/Al modified clays while organic impurities like phenols are preferentially adsorbed by surfactant (HDTMA) modified clays. The specific surface properties of the clays might have played a dominant role in giving this character. In all, Jiang & Zeng,
25
2003, found that metal : surfactant : clay ratio as well as surface area of their clay samples, affects adsorption performance. However, it should be noted that heavy metals and their oxides may also exist in thermally desorbed ash. Their findings were in agreement with the work of Yilmaz and Yapar, 2004, wherein as the surface area of bentonite (Na-MMT) clays determined by Brunaur Emmett and Teller (BET) measurements increased from an indeterminate value to as much as 10m2/g (Yilmaz & Yapar, 2004), the hydrogen potential (pH) of the bentonite surfaces also increased their adsorption potential. The mechanism of adsorption was described by two steps, viz; by hydrogen bonds to the water molecules of the inorganic interlayer cations, and by van der waals attraction to the alkyl chains. Until 2008, a general overview of different organoclays obtained via treatment with quaternary ammonium salts, aminosilanes or biomolecules like proteins and fatty acids, and their various routes of preparation, mainly by; cation exchange and solid state reactions, and their applications have been published until 2008 (Betega de Paiva et al., 2008). Whereas the use of amino / alkoxysilanes is not in doubt (Khoeini et al., 2010; Witucki, 1993), that of vegetable oils and long chain fatty acid compounds as compatibilizer with mineral fillers, is still a subject of further investigation. 2.6
Fatty Acids and Organo-silane Core-Clay Treatment
Fatty acids are long chain organic molecules having a carboxylic end functional group (-COOH) attached to a saturated or unsaturated aliphatic chain. Organic acids or unsaturated fatty acids have been used to modify mineral inorganic filler surfaces (Karoussia et al., 2008). Such treatment is similar to using inorganic acids for clays like precipitated calcium carbonates and layered silicates. Examples of
26
unsaturated and saturated fatty acids used for filler modification are given in Table 2.2. Table 2. 2: Showing some unsaturated and saturated fatty acids, their molecular formulae and their molecular masss (Wikipedia, 2012) Molecular Fatty acid Molecular formula mass (g)
Unsaturated fatty acids Myristoleic acid Palmitoleic acid Sapienic acid Oleic acids Elaidic acid Vaccenic acid Linoleic acid Linoelaidic acid α-Linoleic acid
CH3(CH2)3CH=CH(CH2)7COOH CH3(CH2)5CH=CH(CH2)7COOH CH3(CH2)8CH=CH(CH2)4COOH CH3(CH2)7CH=CH(CH2)7COOH CH3(CH2)7CH=CH(CH2)7COOH CH3(CH2)5CH=CH(CH2)9COOH CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH CH3CH2CH=CHCH2CH=CHCH2CH=CH(CH2)7COOH
Caprylic acid Capric acid Lauric acid Myristic acid Palmitic acid Stearic acid Arachidic acid Behenic acid Lignoceric acid
CH3(CH2)6COOH CH3(CH2)8COOH CH3(CH2)10COOH CH3(CH2)12COOH CH3(CH2)14COOH CH3(CH2)16COOH CH3(CH2)18COOH CH3(CH2)20COOH CH3(CH2)22COOH
Saturated fatty acids
226 254 254 282 282 282 280 280 278 144 172 200 228 256 284 312 340 368
Reasons for inorganic filler modification with fatty acids include; [i.]
To enable easy dispersion of filler agglomerates in polymer matrix systems
[ii.]
To render the filler surfaces hydrophobic. By so doing, filler materials remain stable during storage without absorbing water.
[iii.]
Fatty acids are effective in assisting compatibility between polar inorganic fillers and non-polar polymer matrices. This assist in the improved dispersion, reduced melt viscosity, and reduction in matrix degradation due to high shear rate.
27
[iv.]
Lubrication in polymer processing machines. For instance, stearic acids serve as lubricants in PVC pipe processing when using calcite (CaCO3) fillers.
However, in recent times, organosilanes; alkoxysilanes and aminosilanes, have also become increasingly important and a natural fit for treating surfaces of inorganic fillers to make them compatible with polymers. Such alkoxysilanes as methyltrimethoxysilane,
CH3-Si(OCH3)3;
propyltrimethoxysilane,
[C3H7-
Si(OCH3)3]; t-Butyltrimethoxysilane [(CH3)3C-Si(OCH3)3]; and aminosilanes as amino
propyltriethoxysilane
[H2NC3H6-Si(OC2H5)3],
methacryloxypropyltrimethoxy silane [H2C=CH(CH3)-C(O)OC3H6-Si(OCH3)3 ], have been effectively used on glass beads, quartz, sand, talc, mica, clay and wollastonite (Lu et al., 2004). Inorganic filler containing metal hydroxyl groups such as magnesium hydroxide, iron oxide, copper oxide and tin oxide maybe reactive to a lesser extent, but often benefit from silane treatment. Silane coupling agents give poor bonding to carbon black, graphite and calcium carbonate. Reasons for silane coupling on clay fillers may include; [v.]
Improving adhesion through coupling reaction between clay mineral and polymer
[vi.]
Improving wet-out of mineral by the polymer
[vii.] Improving electrical properties [viii.] Improving mechanical properties Inorganic fillers are treated with either neat silane or a solution of silane in water and / or alcohol. With neat silane, adsorbed water on the filler surface is often
28
enough to hydrolyze the alkoxysilane and simultaneously bond the silane to the filler surface (Dow Corning, 2009).
2.7
Nanoporous Core Clays by Thermal Treatment
Thermal treatment of clay nanostructures involves subjecting clay nano-sheets to temperature variations and vapour pressure. Macro and meso pores arise from particle-to-particle interactions, while micro and nanopores occur in interlayer spaces of pillared phyllosilicates whereby the lamellar nano-sheets structure of the clay has been joined at different points within the clay lamellae (Heller-Kallai, 2006). During calcination and solvothermal synthesis, at very high temperatures above dehydroxylation, physiochemical changes occur depending on clay mineral. For instance, on high temperature treatment, montmorillonite produces arcillite, a porous clay material. Heller-Kallai went further to observe that dehydroxylation destroys the layer structure of trioctahedral 2:1 (TOT) clay type, whereas those of their dioctahedral counterparts are preserved. However, in some clays like kaolinites, there is a complete loss of crystallographic pattern which is detected by a broad XRD pattern (Katsuki et al., 1996). Thus, the general idea in thermal treatment is to produce a bulk material with macro, meso, micro or nano pores. This morphological appearance provide for channels and cavities for sorption of sorbate ions and interpenetrating polymer chains. This situation also results during alkaline thermal treatment of oil base clays from spent drilling mud. Enyiegbulam et al, 2007, had reported a case of porous inorganic material obtained from calcined spent oil base drilling mud powder at 900°C, which had initial pH of 8.5, total retained petroleum hydrocarbons of 2.76% and chloride
29
content of 35,000g/l. It is believed that oil within the clay agglomerates generated micro pore regions while hydrated regions whose loss of water takes place earlier during high temperature treatment, generated the aluminosilicate bridges as presented in Figure 2.9 (Enyiegbulam et al., 2007).
400°C
Na,Ca-MMT 2:1 clay sheet
Colloidal oil & H2O droplets
900°C
Oil droplet Growing aluminosilicate pillar
Porous Ca/MMT clay structure (Core clay)
Figure 2. 9: Evolution process for porous aluminosilicate clay templates during calcination / solvothermal synthesis
2.7.1
Pillared Interlayered Clays (PILC)
Pillared Interlayered Clays (PILC) or simply pillared clay minerals are 2 dimensional porous materials synthesized by high temperature treatment (Schoonheydt et al.,
1999). Synthesis of PILC was pioneered by Barrer and
McLeod who obtained microporous materials by replacing the interlayer exchangeable cations in smectites with tetra alkyl ammonium ions (Barrer, 1986). Such microporous clays have been applied majorly in the cracking of petroleum hydrocarbons (catalysis). Other applications are in environmental protection, molecular sieves, selective adsorbents, thermal insulators, electrochemical and optical devices. PILC are synthesized as organic or inorganic depending on the nature of interlayer cations. Whereas organic pillared clay minerals are unstable at very high temperatures (>250˚C) making them unable to retain permanent porosity, inorganic intercalated clays are stable at very high temperatures and retain their porosity (Sawhney, 1968).
30
2.7.1.1
Preparation of PILC
The first account on the preparation of an inorganic pillared clay mineral was given by Brindley and Sempels in 1977. The procedure consists of exchanging the Na+ ions in pristine smectite with oligomeric hydroxyl aluminium cations which are then converted by heat treatment in Al-oxides. By propping the smectite layers permanently apart, the Al-oxide species act as “pillars” in the interlayer space. Pillaring can be achieved in dilute or concentrated media (Bergaya et al., 2006). Industrially, the process has always been carried out batch-wise. However, scaleup studies for the preparation of pillared layered clays at 1kg per batch level has been published by Kaloidas (Kaloidas, 1995), schematically outlined in Figure 2.10.
Pillering
Thermal decomposition
Dehydroxylation
Raw clays
Figure 2. 10: Schematic representation of stages in raw clays pillaring process by thermal decomposition
31
2.7.2
Zeolites
Zeolites (Na12[Si12Al12O48].nH2O), occur naturally (Galli & Gottardi, 1985) or are synthesized into a 3 dimensional porous inorganic materials (Kevan, 2004). They are characterized by an aluminosilicate tetrahedral framework, ion exchangeable large cation and loosely held water molecules. Typical large cations are the alkali and alkaline earth metals such as Na+, K+, Ca2+, Sr2+ and Ba2+. The large cations, coordinated by a framework of oxygen and water molecules reside in large cavities in the crystal structure. These cavities permit the passage of liquids, gases and organic molecules. Thus, dehydrated zeolites adsorb such liquids as ammonia, alcohol and H2S2 instead of H2O. Crystalline zeolites are usually white but maybe pink, brown, red, yellow or green (Trindade, 2011). Specific gravity is between 2 – 2.5 because of its open structure. Though, zeolites have been in existence naturally, synthetic mesoporous zeolites have been produced via template synthesis route (Tao et al., 2004), and hydrothermal method (Byrappa & Yoshimura, 2001a; Chen et al., 2011; Cundy & Cox, 2005; Okubo & Wakihara, 2005; Wakihara, 2011). Furthermore, an organic template approach using sol-gel processing has been reported, (Cao et al., 1996), wherein micro porous amorphous silica of narrow size distribution comparable to zeolite ZSM-5. The membranes so synthesized had ideal gas separation factors, at least two folds magnitude larger than those expected for Knudsen diffusion separation mechanism. Other successes recorded include synthesis of silica molecular sieves by sol gel technique (Diniz Da Costa, 2002; Nair et al., 1997). In same vein, a precipitation technique involving hydrolysis reaction of silicon alkoxide in ethanol used to produce sub-micron sized silica sphere zeolites with narrow size distribution and 40% porosity, has been published (Vacassy et al., 2000). The
32
presence of glycerol in the reaction affected the precipitation mechanism resulting in a larger mean particle size. In each of these synthetic routes, zeolites with various morphologies and pore sizes investigated with transmission electron microscopes (TEM) (Diaz & Mayoral, 2011), have been produced and have been associated with various nomenclatures. However, atlas of zeolite framework types and nomenclatures has been published by the International Zeolites Association (IZA) (Baerlocher et al., 2007). Also, crystal structures of natural zeolites have also been reported (Armbruster & Gunter, 2001). Applications of zeolites include gas sensors, catalysts, templates for molecular synthesis, molecular sieves and membranes, biological applications, filler materials and microfiltration (Schmidt, 2001). 2.8
Nanoporous Evolution and Porous Clays Morphology
Nano pores are achieved as Na+ and K+ ions act as templates for aluminosilicates to polymerize. By so doing, they form a zeolitic material with nano-bridges and pores in the bulk structure. Such a material, synthesized in sea water using Nabentonite has shown high selectivity for NH4+ ions (Bojemueller et al., 2001; Huang et al., 2009; Ruiz et al., 1997; Smith & Oyanedel-craver, 2006). In the case of calcium bentonite, Yuksel and others investigated the effect of thermal treatment on
physiochemical
properties
of
some
calcium
rich
bentonite
and
montmorrilonite clays, (Yuksel et al., 2006). They reported that changes in d(001) spacing and the deformation of the crystal structure of calcium rich montmorillonites (Ca-M) depends largely on high temperature treatment of the clay by dehydration, dehydroxylation, recrystallization, shrinkage and possibly fracture into smaller nanosize units. For calcium rich bentonite, (Ca-B), activation
33
energies for dehydration and dehydroxylation calculated from thermogravimetric data was 33KJmol-1 and 59KJmol-1 respectively. From X-ray diffraction data, they showed that maximum porosity was achieved at 500°C, while dehydration was irreversible without any changes in crystal structure. Also specific surface area and specific micropore / mesopore volume calculated from adsorption and desorption data respectively, show a zigzag variation with increasing temperature up to 700°C. 2.8.1
Effect of Calcination on Hydrocalcites and Mixed Oxide Clays – The Specific Case of Spent Oil Base Drilling Mud
Generally, calcination is an exothermic process. Various articles have been published to explain calcination mechanisms. Calcination has been reported as a process (Irfan & Gulsen, 2001), which give rise to changes in physical and structural properties of inorganic reactants with carbonates (Dogu, 1981). The limit of calcination is seen when the mixture of solids melt and the ions become highly mobile leading to fused ceramic solid on cooling. In the course of calcination, formation of precursors as metal carbonates, hydroxides and nitrates, enhances reaction rates at some temperature ranges, whereas the existence of bentonite / barites and traces of iron, magnesium, manganese, potassium oxides may be seen as impurities. The crystallites of these precursors disintegrate with the loss of gaseous species and heat, breaking down the material to yield fine, very reactive nanoparticles. This realignment of ions as a result of diffusion, gives rise to decomposition of the initial bulk material and the production of entirely new material different from the original. It is already known that increasing the temperature of a solid state reaction normally lowers the activation energy as the rates of diffusion of the various ions increase (Trindade, 2011).
34
Calcination is not just for the purpose of driving out structural water and burning volatile organic matter within a porous inorganic network, but also for the consolidation of the silicate framework via thermal condensation (Bagshaw & Bruce, 2008). Calcination is usually associated with high temperature treatment of calcium based clay compounds. In the case of calcium carbonate (CaCO3), as temperature is increased, CO2 is released as gas, leaving a solid residue of CaO (Parker, 1994); CaCO3(s)→ CaO(s) + CO2(g) ∆H = +182.1KJmol-1 @ 1200°C In the case of hydro-dolomite; CaSO4.2H2O(s) + heat → CaSO4.½H2O(s) + 11⁄2H2O(g)∆H = +90KJmol-1@120°C In the specific case of spent oil base drilling mud which has been defined in an earlier study by Iheaturu, 2008 and seen to have more than 50% solid content comprising of 55.65% clay content per 100mls comprising with 95.01% calcium (Iheaturu, 2006). The clay minerals may contain more than one metal in addition to oxygen; binary, ternary and quaternary oxides; carbonates and nitrates. This is the case of limestone, a metal carbonate and hydro dolomite picked up in drilling operations in the Niger Delta of Nigeria (Akpokodje, 1989). Alongside limestone, bentonite and barites, other components of spent drilling mud may include salts; NaCl / KCl, soda ash (Na2CO3) which acts as fluxes during heating. It is expected that at the end of the two processes, spent oil base drilling mud could be converted to amorphous porous egg yolk yellow ash / powder, flue gas and heat. Earlier on, Theng, 1971, published a review of mechanisms of formation
35
of coloured clay-organic complexes, wherein the pH of the system, the nature of the solvent and that of the exchangeable cation, influence the intensity, quality and rate of colour development (Theng, 1971). Inorganic components form 99% of the resultant solids which may take the form of porous agglomerates, porous zeolitic material, and fused ceramics / metallic melts. Reaction chemistry of calcined spent oil base mud solids have been reported (Enyiegbulam et al., 2011);
SiO2(s)+Mg2++yCa2++K++NaCl(aq)+Na2O+Al2+(aq)+2Fe2+(aq)+3C2O42-(aq)
@ 900°C Ca2yNa2x-1KMgAl2Fe2(C2O4)3.xH2O(s)+SiO2(s) Highly insoluble hydrocalcites Where x is the number of water molecules within clay interlayer, while y is valence of metal ion. The formation of spinels within the inorganic powder with such chemistry may not be totally out of place. Thermal treatment could also lead to immobilization of heavy metals and their oxides. Investigations, reveals that thermal treatment of clay / fly ash admixtures, leads to immobilization of heavy metals (Cr, Cu, Pb and Zn) with the formation of stable powder composed of Silicon (Si) and Aluminum (Al) (Chaturvedi et al., 2007). 2.8.2
Acid Activation of Core-Clay Nanostructures
This refers to subjecting clay nanostructures to acid treatment. Organic acid, for example acetic acid (CH3COOH), inorganic acid, for example hydrochloric acid (HCl), or tetraoxosulphate VI acid (H2SO4), may be used for acid activation of the clays. The main objective is to obtain partly dissolved material of increased
36
surface area, porosity, adsorption capacity and surface acidity. Though from the industrial point of view, the term “acid-activated clays” was reserved for acid treated bentonite clays, usually Ca2+-bentonite. When Ca2+-bentonite is treated with inorganic acids, divalent calcium ions are replaced with monovalent hydrogen ions, while ferric, ferrous, aluminium and magnesium ions are leached out. Auto transformation of H+ exchanged clays is initiated when protons from the acid replace exchangeable cations and then begins to attack the clay layers. Exchange reaction is fast and the quantity of exchanged protons is independent of smectite type (Komadel & Madejová, 2006). By so doing, smectite layers are altered and disintegrated as a result of leaching of octahedral sites, with surface area and porosity increasing. This process results in the production of clays suitable for a range of bleaching or decolourizing applications (Valenzuela-Diaz & Santos, 2001). Most recently, Steudel A. et al, 2009, elaborated the differences in dissolution mechanisms of swellable and non-swellable clay minerals. While swellable clays (Steudel et al., 2009a) allow interlayer and edge attack, nonswellable clays (Steudel et al., 2009b) only allow edge attack. They went further to report that acid treatment causes erosion of the octahedral layer, while the SiO4 and SiO3OH groups of the tetrahedral layer remain intact (Steudel et al., 2009a). However, in the case of ash from thermally desorbed spent drilling mud which contains significant amounts of iron fillings, it is expected that during sulphuric acid (H2SO4) treatment, H2SO4 would react with iron (Fe) to give iron sulphate and hydrogen gas thus; Fe(s) + H2SO4(aq) → H2(g) + FeSO4(aq)
37
Depending on the extent of acid activation, the resulting powder material is characterized by some unaltered smectite layers and amorphous 3 dimensional cross-linked silica. The filtrate would contain leached ions depending on chemical composition of the clay mineral and type of acid used. 2.8.2.1
Characterizing Acid Activated Clays by Fourier Transform Infrared (FT-IR) Spectroscopy
Fourier transform infrared (FT-IR) spectroscopy has been reported to be a very useful technique in the characterization of acid activated clay minerals (Komadel & Madejová, 2006; Madejová et al., 2009; Madejova, 2003). The technique has been used to characterize both swellable and non-swellable clays (Pentrák et al., 2010; Steudel et al., 2009a, 2009b). In the FT-IR spectra of acid activated clays, pure silicate (O-Si-O) band usually appear between 850cm-1 and 1100cm-1, and shifts to higher wavenumbers on substitution of Al or Fe in the clay mineral lattice as a result of acid erosion of octahedral 2:1 clay lattice. Silicate bands indicate the content of silicate phases in clay samples (Madejova, 2003). Acid activation of clay mineral is therefore inferred when the major silicate bands, which usually appear between 850 to 1100cm-1, shifts to higher wavenumbers. The movement of these bands largely depends on the extent of acid attack on the clays with time. Madejova, 2003, obtained IR spectra depicting a gradual shift of silicate (O-Si-O) bands from 1030cm-1 to 1100cm-1 after 8hours treatment with hydrochloric acid. Furthermore, FT-IR spectra shown in Figure 2.11, presents a gradual movement of O-Si-O band of acid treated sodium montmorillonite (Na-MMT) and acid catalysed sol-gel modified clays, from 847cm-1 to 961cm-1 (Zhongzhong et al., 2008).
38
Figure 2. 11: FI-IR spectra of (a) acid-treated Na-MMT and acid-catalyzed sol–gel-modified clays and (b) pristine Na-MMT and non-catalyzed sol–gel-modified clays with different TEOS/clay ratios (Zhongzhong et al., 2008)
2.9
Preparation of Core-Shell Nanomaterials
Core shell nanomaterials are not trivial in present nanotechnology due to their various areas of application. However, their areas of application require different morphologies for effective performance. Therefore their synthesis and preparation may follow different routes, giving them different morphological properties. In an in-depth review, Hoffmann et al, 2006, writes that there are different routes for synthesizing core-shell nanomaterials, nano or meso organic inorganic hybrids, divided into traditional or conventional methods and advanced
39
synthetic routes (Hoffmann et al., 2006). The conventional routes have been adopted in ceramic powder preparation, while the synthetic routes have been used to synthesize nanostructured, nanoporous and mesoporous materials. Different morphological structures of mesoporous materials; 2D hexagonal space group p6mm, cubic, space group Ia3-d and lamellar space group p2, as reported by Hoffmann et al, are presented in Figure 2.12.
Figure 2. 12: Structure of mesoporous M41S materials: (a) MCM-41 (2D hexagonal, space group p6mm), (b) MCM-48 (cubic, space group Ia3-d) and (c) MCM-50 (lamellar, space group p2) (Hoffmann et al., 2006) Other methods of preparing and processing nano-sized core filler materials include; 2.9.1
Conventional Methods
COMMUNITION involves breaking down bulk particles into nano-size dimension by mechanical means. Most times, a series of crushing, grinding or milling operations, then particle attrition may be required in order to progressively produce micron- to submicron sized powders. In the case of particle attrition, wear from the particle surface as a result of shear action and friction between particles and hard ceramic or metallic grinding media, leads to size reduction of primary particles and generation of secondary fine particles. This way consumes a lot of energy and time. Its disadvantage remains inability to obtain homogenous nanoparticles in terms of chemical properties, shape and size. However, most naturally occurring clay minerals including bentonite, barites, titanium, zirconia,
40
calcium and sodium montmorillonite clays and calcium carbonates, are processed this way. High temperature synthesis involves the application of high temperature to inorganic solids substrates. High temperature synthesis may involve plasma synthesis, flame synthesis (Akurati et al., 2006), spray drying and gas phase synthesis (Binner, 1990; Ganguli & Chatterjee, 1997). 2.9.2 (i.)
Advanced Synthetic Methods Sol gel technique is a wet chemical route for synthesizing nanoparticles,
ceramics and glasses. It involves two basic steps starting from formation of the sol by hydrolysing a metal alkoxide in water and then polycondensation into a gel by controlling pH of the colloidal sol either by addition of a base or acid (Kim & Lin, 2000; Kron et al., 2001; Latthe et al., 2010; Marques et al., 2009; Rao et al., 2005a). With proper quality control, different materials can be fabricated in the form of thin films and coatings, drawn fibre if the gel is made too viscous, monoliths, micro and mesoporous particles, micro-emulsions and aerogels. With controlled pH, temperature and pressure, process may lead to materials with predetermined colours, shapes and sizes (Dimitriev et al., 2008). This chemical route has been used to synthesize organic – inorganic core shell nanoparticles (Du et al., 2009; Marini et al., 2008; Yan et al., 2010). Monodisperse and uniform size silica nanoparticles have also been synthesized using ultra sonication by sol gel process (Rao et al., 2005), wherein silica particles were obtained by hydrolysis of tetraethyl ortho silicate (TEOS) in ethanol followed by polycondensation of siloxane bridges. On the other hand, inorganic-inorganic core shell nanoparticle (Fe3O4@Ag / Fe3O4@Au) has been synthesized by sol gel as shown in Figure 2.13.
41
Figure 2. 13: Inorganic Fe2O4@Ag and Fe2O4@Au core-shell nanoparticles synthesized by sol gel technique (Zhichnan et al., 2007) Calcium carbornate core coated with silica shell, CaCO3@SiO2, have also been published (Zhang & Li, 2004).
(ii. )
Hydrothermal Synthesis defined as any heterogeneous reaction in the
presence of aqueous solvents or mineralizers under high temperature and pressure, usually above room temperature and pressure of 1 atm (Byrappa & Adschiri, 2007; Byrappa & Yoshimura, 2001b). In some cases, hydrothermal synthesis is deployed in order to dissolve and recrystallize (recover) materials that are relatively insoluble in water under ordinary conditions (Stambaugh, 1989). In both sol gel and hydrothermal synthesis, functionalization is achieved by any of 3 ways, viz; by subsequent attachment of organic components onto a pure silica matrix by grafting as presented in Figures 2.14 and 2.15, by simultaneous reaction of condensable inorganic silica species and silylated organic compounds (cocondensation or one-pot synthesis), and by direct synthesis using bissilylated organic precursors that lead to periodic mesoporous organosilicas (PMOs) (Hoffmann et al., 2006; Pires, 2004). PMOs have high thermal stability, disordered
42
pore systems with relatively wide distribution of pore radii. Surface areas may be measured up to 1800m2g-1.
Figure 2. 14: Grafting (postsynthetic functionalization) for organic modification of mesoporous pure silica phases with terminal organosilanes of the type (R'O)3SiR, R=organic functional group (Hoffmann et al., 2006)
Figure 2. 15: General synthetic pathway to PMOs constructed from bissilylated organic bridging units, R=organic bridge (Hoffmann et al., 2006)
43
(iii.) One-Pot Synthesis is a form of direct synthesis of mesoporous catalysts involving the use of structure directing agents (SDAs) and organic precursors in a reaction chamber. One – pot synthesis is also known as co-condensation, reported to have been used to synthesize a good number of organically modified silica phases. Organic functionalities such as alkyl, thiol, amino, cyano/isocyano, vinyl/allyl, organophosphine, alkoxy or aromatic groups have been reported as being incorporated into pore walls of silica frameworks, mesoporous silica, and silica-aluminosilicate composites (Areva et al., 2004; Chamnankid et al., 2011; Gu et al., 2012; Jung et al., 2012; Marini, et al., 2008; Zarabadi-Poor et al., 2011). (iv.) Supramolecular templating enables precise engineering of pore size, shape and connectivity on the mesoscopic scale. As shown in Figure 2.16, such template-based approaches involving the cooperative organization of organicinorganic assemblies into hierarchical morphologies that mimic the intricate structures found so often in nature.
Figure 2. 16: Surfactant templating on an alkoxysilane substrate, wherein the resulting material contains the organic moiety as an integral part of the wall (Kickelbick, 2004) Applications of such a hybrid material can be predicted depending on the functionalities of the nanoporous substrate and that of the alkoxysilane (Kickelbick, 2004).
44
(v.)
Solvothermal synthesis is a facile method for synthesizing nanoparticles.
It involves subjecting a non-aqueous or organic precursor solution in a stainless steel autoclave or kiln to high temperature and pressure (Inoue, 2005; Wikipedia, 2011a). This makes for the precise control of the size distribution, pore size, shape and crystallinity of metal oxide nanoparticles or nanostructures. These characteristics can be altered by changing certain reaction conditions, including reaction temperature, reaction time, solvent type, surfactant type, and precursor type. The main difference between hydrothermal and solvothermal synthesis lies in the nature of initial precursor solution. Aqueous solvent is used for the former, while non-aqueous solution is used in the later process (Feng & Li, 2011). Literature has shown that solvothermal method has been used to synthesis a good number of ceramic nanoparticles (Great & Millennium, 2001; Sato, 2010; Wang et al., 2011; Yoshimura & Suchanek, 1997), polymers (Fang et al., 2009), and nanoparticles with magnetic property (Chen et al., 2008), using combined solvothermal-sol gel method. The magnetic nanoparticles were well-crystallized magnetite
(Fe3O4)
nanoparticles
obtained
through
high
temperature
decomposition of intermediate complexing compound using diethylene glycol (DEG) and diethanolamine (DEA) as solvent and complexing agent respectively. Then a novel egg-yolk-like magnetic silica nanostructure composed of a silica core and another silica shell with magnetic nanoparticles embedded in the boundary was synthesized by a one-step sol-gel technique (Chen et al., 2008). The diameters of the final products were effectively tuned using silica colloidal cores with different diameters. Shell thickness was effectively controlled by changing the quantity of tetraethyl orthosilicate (TEOS).
45
(vi.)
Sonochemical synthesis involves subjecting reactants in a liquid medium
to ultrasound energy in an ultrasonic bath. The realization that reactions can be initialized upon irradiation of a chemical system with high intensity sound or ultrasound energy, generating acoustic cavitations, which grow into microbubbles and eventually implode in the system, gave rise to sonochemical synthesis route (Zhang et al., 2009). Bubble collapse in a liquid produces enormous amounts of energy from the conversion of kinetic energy of the liquid motion into heat leading to short-lived localized hot-spots dispersed within the liquid. Experimental results have shown that such implosive bubbles have temperatures up to 5000K, pressures of roughly 1000atm, and heating and cooling rates above 1010K/sec (Wikipedia, 2011b). If the cavitations are generated in a liquid-particle mixture, such cavitations can produce high velocity inter-particle collisions leading to changes in particle morphology, composition and reactivity. On the other hand, if the cavitations occur near an extended or porous solid surface, micro-bubble collapse is non-spherical and drives high-speed jets of liquid to the surface or through the nanoparticle pores. These high speed jets of liquid and associated short waves can clean the nanopores thereby increasing reaction surface area. This method has been used to synthesize ultrafine particles (Okitsu & Bandow, 1996), zinc oxide nanorods (Zhang et al., 2005), bismuth sulfide nanorods (Wang et al., 2002; Zhu, 2003). However, three classes of sonochemical reactions exist, viz; homogeneous sonochemistry of liquids, heterogeneous sonochemistry of liquid-liquid or solid-liquid systems and lastly, sonocatalysis. (Vii.) Other synthesis routes used to synthesize core shell nanoparticle involves aqueous dispersion polymerization, Percy et al, 2003, (Percy et al., 2003)
46
and batch emulsion polymerization (Whitcombe et al., 1999). A scheme showing other synthesis routes; precipitation, true liquid crystal templating, evaporation induced self-assembly and exo-templating, to mesoporous materials is presented in Figure 2.17.
Figure 2. 17: Scheme of the main synthesis routes to mesoporous materials: Precipitation (A), True Liquid Crystal Templating, TLCT (B), Evaporation-Induced Self-Assembly, EISA (C) and Exotemplating (D) (Soler-Illia & Azzaroni, 2011).
47
2.10 Particle Characteristics and Size Analysis 2.10.1
Granulometry and Morphology of Clay Particles
Granulometry and morphology, which determines particle size and shape respectively, are important quality control parameters in processes involving colloidal dispersion, clays and inorganic nanoparticles. Particle size and shape usually have influence on bulk density, rheology and surface area (Mikli, 2001). Particle size and shape also determines areas of application. Whereas, in abrasives, there is a direct correlation between particle shape and wear rate, in sprays. Hwever, well rounded particles may be what is needed for good flowability from spray nozzle and dispersion. Natural clay granulometry and morphology is not easily controlled as they are produced by natural paedogenesis and weathering over time. Thus, natural clay granulometry can be determined by sieve analysis, light image analysis or laser analysis (Dukhin, 2011). Particle shape parameters such as circularity, convexity and elongation may be used as a “fingerprint” in order to identify and quantify subtle variations in particle shape. Traditionally, such descriptions as “jagged”, “smooth”, “needle” can be used to qualify and quantify nanoparticles while associating them with process variables such as flowability, active area and grinding efficiency. Each parameter is usually normalized between 0 and 1 for quick and easy comparability as shown in Table 2.3.
48
Table 2. 3: Particle shape characterization and associated values Particle Shape Morphology Circularity is a measure of the closeness to a perfect circle, which is sensitive to changes in overall form and surface roughness.
1
0.47
Convexity is a measure of the surface roughness of a particle. Convexity is sensitive to changes in surface roughness but not overall form.
1
1
Elongation is a measure of the ratio of length: width of the particle. Elongation is unaffected by surface roughness i.e. a smooth ellipse has a similar elongation with a spiky ellipse of the same aspect ratio.
0
0.82
2.10.2
Particle Size Analysis
Particle size analysis refers to the use of statistical functions to define the relative size content or distribution of a powder sample (Ganguli & Chatterjee, 1997). However, in order to carry out the exercise of particle size analysis, particle size measurements or simply particle sizing is done following laboratory techniques which separate the various particle sizes in a powder sample into discrete size ranges and or average mean sizes. Such laboratory technique could be sieving, sedimentation, and elutriation, or microscopy techniques, using a coulter counter or dynamic light scattering. Size of clay particles are found to be in the range of 2µm or less for colloids, 2µm to 75µm for silt, and above 74µm for sand (Bergaya & Lagaly, 2006). 2.11 Colloidal Processing and Interfacial Phenomena Colloidal processing involves manipulating and controlling inter-particle forces in powder suspensions in order to remove heterogeneities and to optimize suspension properties. Clays or metal oxides dispersed in a liquid phase possess charged surfaces at an early stage of dispersion due to hydroxylation and
49
transformation of the acquired surface. This is due to variation in solution pH. Knowledge of the nature of surface charges and inter-particle forces can be used to create stable systems whereby weak forces of attraction are broken down to facilitate good mixing of different powders. Large, hard agglomerates can be removed by sedimentation and filtration as phase separation occurs at low volume fractions (Bergström, 2001). 2.11.1
Concept of Flocculation, Agglomeration and Coagulation
Matter exist in 3 forms; solids, liquids and gases. A colloidal system is formed when one of the forms of matter is finely or microscopically dispersed in the other. In such systems, the behaviours of the dispersed medium may differ in various media as a result of nature of interactions that may exist between particles and between particles and the dispersion medium. 2.13.1
Agglomeration and Flocculation
The particles may be attracted to each other if the forces of attraction between the particles are greater than forces of repulsion between them. When this happens, a floc is formed as a secondary particle, and the process is described as flocculation. Therefore, a floc may be described as an agglomerate, which is a small assemblage of primary particles held together by weak forces, forming a “network of inter connective pores”. Forces in agglomerates include; electrostatic forces, Van der Waals forces, liquid bridges, capillary forces, solid bridges, polymer bridges, hydration and solvation forces. Soft agglomerates are formed when the forces of attraction are weak such as Van der Waals forces of attraction. Soft agglomerates can be broken down to primary particles or at least much smaller agglomerates by light mechanical forces or ultra-sonication. But, when agglomerates are formed
50
through solid bridges between particles, they are called “hard” agglomerates or aggregates, if they are very large in size. Hard agglomerates are undesirable in powder processing or polymer compounding as they do not easily disintegrate into primary particles by simple means, rather, they may be broken down by forces sourced from strong mechanical means as milling (ball milling, roller milling), communition, crushing or grinding. 2.13.2
Coagulation
The particles may be aggregated into dense entities, denser than the dispersion medium in a process called coagulation, which usually leads to a phase separation. The concepts of agglomeration, flocculation, segregation and coagulation in a colloidal system are pictorially represented in the scheme presented in Figure 2.18.
51
Flocculation
Sedimentation Stable colloidal system
Coagulation
Flocculation
Sedimentation
Coagulation
Phase separation
Figure 2. 18: Scheme showing flocculation and coagulation in a colloidal system (Malvern, 2011)
2.11.2
Colloidal Stability and DVLO Theory
When two (2) particles approach each other very slowly, the isothermal work done in bringing them from infinity to the position of closest approach will be the same as the repulsive energy, VR. Note that electrostatic force of attraction existing between 2 particles is quite different from Van der Waal’s forces of attraction, which are small, short range, attractive forces between 2 isolated molecules which may or may not possess permanent dipoles. Hamaker, (Hamaker, 1937), proposed
52
that the energy of attraction VA, between 2 equal spheres of radius, α = D/2, situated at a distance, h (which is the closest approach in vacuum) is; 1
𝑉𝐴 = −𝐴
1
2ln(𝑥2 +2𝑥)
( 2 )+( 2 )+ 2 [ 𝑥 +2𝑥 𝑥 +2𝑥+1 𝑥 +2𝑥+1 12
]
2.1
Where; A is the Hamaker constant, and x = h/2a or h/D The Hamaker constant can be expressed as; 𝐴 = (√𝐴2 − √𝐴1 )2
2.2
Where; A1 = Hamaker constant for the liquid medium, and A2 = Hamaker constant for the spherical particles. 2.11.1.1
Derjaguin, Verwey, Landau and Overbeek (DVLO) Theory
The DVLO theory was developed in the 1940’s by four scientists; Derjaguin and Verwey in Russia, Landau and Overbeek in Holland, in order to explain the stability of colloidal systems. The theory presents that the total potential energy, VT, of a stabilized colloidal system depends on the summation of attractive and repulsive forces as well as the forces due to the solvent medium (Horn, 1995).
VT = VA + V R + VS
2.3
Where;
VS = Potential energy due to solvent (almost negligible) VR = Potential energy due to force of repulsion (very prominent) VA = Potential energy due to force of attraction (very prominent) VT = Total potential energy
VA=
−𝑨 𝟏𝟐𝝅𝑫𝟐
2.4
53
A = Hamaker constant, is approximately 10-20Joules and very difficult to determine experimentally. 𝜋 = Solvent permeability D = Particle separation
VR= 𝟐𝝅𝜺𝒐 𝒂𝜻𝟐 𝒆𝒙𝒑−𝑲𝑫
2.5
VR is far more complex α = Particle radius 𝜋 = Solvent permeability
εo = Permittivity of free space or electric constant K = Function of the ionic component D = Particle separation ζ = Zeta potential From Equation 2.5, the DVLO theory proposes that an energy barrier due to net repulsive forces prevents particles approaching one another in a colloidal system from adhering together. But if the particles collide with forces greater than the repulsive force, then that energy barrier will be broken giving the particles tremendous attractive forces that will make them adhere together irreversibly. Therefore, in order to obtain a stable or deflocculated colloidal system, rather than attractive forces, repulsive forces between the particles should be more prominent, thereby suspending the particles in the medium. Figure 2.19, explains the DVLO theory showing that as the particle separation increases, repulsive forces due to the electrostatic double layer reduces while increasing the Van der Waals forces of attraction between them. However, a net energy balances the forces of attraction and repulsion at a certain particle distance.
54
Figure 2. 19: Schematic diagram showing the variation of free energy with particle separation according to DVLO theory (Malvern, 2011)
However, in the case of high alkaline medium with high concentration of salt, the zeta potential is reduced, thereby creating a secondary minimum of net energy where a much weaker and potentially reversible adhesion may exist between particles. This leads to agglomeration and formation of weak flocs which can be broken by mechanical stirring. Figure 2.20, is a schematic diagram showing the variation of free energy with particle separation at high salt concentrations and the possibility of a secondary minimum.
55
Figure 2. 20: Schematic diagram showing the variation of free energy with particle separation at high salt concentrations showing the possibility of a secondary minimum according to DVLO theory (Ganguli & Chatterjee, 1997)
2.11.3
Creating a Stable Colloidal System
There are two ways of creating a stable colloidal system. 1.
Steric repulsion
2.
Electrostatic or charge stabilization
Steric repulsion involves the adsorption of polymer molecules onto a particle surface and preventing the particle surfaces from coming into contact. The amount of polymer molecules adsorbed on the particle surface leads to a coating on the particle of certain thickness which depends on the size and number of molecules adsorbed. The coating is sufficient to weaken Van der Waal’s forces of attraction between particles thereby keeping the particles separated by steric repulsion between the polymer layers. This system requires the addition of suitable polymer
56
in a colloidal dispersion adhering head to tail or tail to head on particle surface. However, it can pose some problems should there be a need for flocculation of the coated particles. Also, there may also be a need to burn out the polymer in a ceramic green body in order to remove defects after sintering. Long chain quaternary ammonium cations [R’-NH4+)4] or charged polyatomic ions and polyacrylic acids, are good candidates for steric repulsion in polymer-clay hybrid systems. Electrostatic or charge stabilization refers to keeping particles separated due to distribution of charged particles in a colloidal system. Natural clays are endowed with electrostatic charges which are balanced by surface charges. A measure of the clay surface charge distribution gives an idea of the cation exchange capacity (CEC). Figure 2.21 is a representation of steric and electrostatic stabilization of two clay particles.
Figure 2. 21: Representation of steric and electrostatic stabilization
57
2.11.4
Interfacial Characterization and the Electronic Double Layer
Most polymers and inorganic materials have charged surfaces making them acidic or basic or even both. Depending on surface charges on clay particles, acid base interactions cause enhancements in wettability, adsorption, charge transfer, adhesion, intercalations and exfoliation. Nature of counter ions attracted or repelled also depends on whether environment is aqueous or non-aqueous. Such acid base interactions and its effect on some clays and metal oxides have been previously studied in various media. For instance Solomon and Murray, 1972, published that the strength of the surface acid sites of kaolinite varies with moisture content (Solomon & Murray, 1972).
Ultimately, interfacial properties of particles give rise to increase or decrease of charge concentration within the vicinity of particle circumference, giving a picture of a layer of progressively changing charge concentration in the solution with the maximum at the particle liquid interface. This immediate surrounding or interfacial region is known as the stern layer. The charge concentration of this layer thus decreases with increasing distance from the interface till it equalizes with the concentration of the bulk solution. Figure 2.22 shows a clay particle with surrounding positive charges around the stern layer. The stern layer can easily shear itself from the bulk liquid and move with the particle in Brownian motion.
58
+ + + + + + + + + ++ + + ++ + + + ++ ++++++ +++ + + + + +++ + + + ++ + + + + ++ ++ + + ++ + ++ + + + + + + ++ + ++ + + + + + + + ++++ ++ + + + + +++ + +++ + + + + + ++ + + + + + + + + +
_
_
_
+ +
+
+
+
+ + +
Particle liquid interface
_
+ +
_
Stern layer
+ _
Figure 2. 22: Negatively charged clay particle attracting positive charges (Malvern, 2011)
The layer next to the stern layer, otherwise called the diffuse layer, is characterized by very low charge concentration. However, both regions constitute the electronic double layer as shown in Figure 2.23. The thickness of this layer increases with increase in counter ion concentration in the solution. The charge concentration (or potential) which decreases to zero at the outer surface of the double layer is called the zeta potential. The pH at which the zeta potential is zero is called the isoelectric point (IEP), while the point of zero charge (PZC) indicates the pH at which the particle attains neutrality or the pH at which the surface cations are counterbalanced by the solution anions. Note that IEP is different from the point of zero charge (PZC).
59
_
_
+ _ + ++ + ++ +++ + + + _ + + + ++ + + ++ ++ + + + + + + +++ + + + + + + ++ + + + + + +++ ++ + + +++ ++ + ++ + + + + + + + + + + ++ + + + + +++ + + + + + + + + ++ + + Diffuse layer + + + + + _ + + + + + + + + + + +
+
_
Negatively charged clay particle
Potential (mV)
_
+
+
+
Surface potential Stern layer Slipping plane
Zeta potential
Electronic double layer Distance (d), between particles
Figure 2. 23: Negatively charged clay particle attracting positive charges (Ganguli & Chatterjee, 1997)
2.11.5
Acid-Base Interactions on Clay Particle Surfaces
Given that clay particles have acidic or basic sites or even both, making them able to interact to enhance wettability, adsorption, charge-transfer and adhesion, the interaction are still independent of dipole moment and occur only when one material has acid groups which can interact with the basic groups of the other material. The presence of a third component in the form of polymer, solvent, plasticizer, penetrant, colourant or dispersant, can interfere with adsorption, charge-transfer or adhesion if the third component has strong affinity to either or both the clays and itself due to acid-base interactions. For instance, water, a weak acid and a weak base, tends to weaken adhesion of polymers in clay materials but
60
not when the polymer and the clay have strong acid-base interactions. A quantitative approach to predicting the enthalpy of acid-base interactions between polymers and inorganic surfaces has been reported (Fowkes, 1981), based on determining the E and C constants of Drago’s correlation for some polymers (polymethylmethacrylate and chlorinated polyvinylchloride) and inorganic surfaces (iron oxide and silica). Also, kaolinites show considerable chemical activity and particle to particle physical interactions. By investigating acid-base interactions and the properties of kaolinite in non-aqueous media, (Solomon & Murray, 1972), were able to prove that 1% moisture content at the surface of kaolinite clay samples was equivalent to 48% sulphuric (H2SO4) acid, whereas 0% moisture content was equivalent to 90% H2SO4 acid. Therefore where acid- base interactions are involved, the presence of very small amount of moisture usually inhibits the reaction at clay mineral surfaces while completely dry kaolinite clay will promote or catalyse chemical reactions. The reaction chemistry of particle interaction with acid or basic environment is given in Figure 2.24.
Figure 2. 24: Reaction chemistry of particle interaction with acidic or basic environment
(Ganguli & Chatterjee, 1997) igure 2.25 illustrates clay particle interactions with the head group of a surfactant showing possible synthetic pathway in different media; acidic, basic, or neutral.
61
Figure 2. 25: Clay particle interactions with the head group of a surfactant. Paths (a), (b) and (c), are possible synthetic pathway in acidic, basic, or neutral media respectively (Hoffmann et al., 2006) 2.12 Zeta Potential Measurements - Electrophoresis Zeta (ζ) potential is a measure of the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. In order to measure this potential difference, a measure of the magnitude of repulsion and attraction between the dispersed articles needs to be known. Understanding zeta potential helps processors to electrostatically disperse and control colloids in a wide variety of industries including colloidal processing of clays and ceramics, cement, mineral processing, polymer processing, paints, soap, aerosols, brewing, medicine, pharmaceuticals and drugs as well as in water treatment. Zeta potential is a physical quantity used to optimize formulations of suspensions and emulsions. Therefore, the interaction between inorganic core-
62
shell clay nanoparticles in polymer medium can be measured and monitored if zeta potential or potential difference or magnitude of charge in the electrostatic interfacial double layer (DL) between the 2 media is known. This + / - potential difference measured in milli-volts (mV) is taken as an arbitrary value that separates low charged surfaces from highly charged surfaces. Small molecules or nanoparticles are associated with high zeta potential values while bigger molecules are associated with low zeta potential values. Colloids with high positive or negative zeta potential are electrically dispersed or stable while colloids with low positive or negative zeta potential tend to coagulate or flocculate. A dummy zeta potential plot showing regions of stable and unstable colloidal system is shown in Figure 2.26.
Zeta potential (mV)
Isoelectric point (IEP)
Stable colloidal system
Unstable colloidal system
Stable colloidal system
PH
Figure 2. 26: Zeta potential scheme of a colloidal system showing; the isoelectric point (IEP), upper and lower regions for a stable colloidal system and region for unstable colloidal system (Malvern, 2011)
Zeta potential is not measured directly, but can be measured by theoretical or experimental means involving electrophoretic dynamic mobility or simply put
63
electrophoresis measurement with an electrophoresis apparatus as the Malvern Zetasizer. Figure 2.27 shows optical configuration of the zetasizer for zeta potential measurement.
Combining Optics
Reference beam
Compensation Optics
Beam splitter
Scattering beam splitter
Attenuator
Cell
Incident beam
Laser Source
Digital signal processor
Computer display monitor
Figure 2. 27: Optical configuration of the Zetasizer for zeta potential measurement (Malvern, 2011)
The general principle involves inserting two electrodes into a dilute clay suspension and applying direct current voltage to the suspension. The clay particles are attracted towards the electrodes with the opposite charge. Negatively charged clay particles are attracted to positively charged electrode (anode) and vis versa. The velocity of the travelling particles depends on the particle surface charge (or zeta potential) and the applied voltage. Therefore, if the velocity can be
64
measured, zeta potential can be determined using Henry or Smoluchowski equations, which describes the flow of ions dissolved in a liquid with an electric field pulling the ions in some direction. Various plots of zeta potential in mV against hydrogen potential (pH) of alumina-10ppm polyethylene glycol (PEG) system as a function of pH, in the absence and presence of different concentrations of ammonium poly(methacrylate) – APMA, is presented in Figure 2.28 (Saravanan & Subramanian, 2012).
Figure 2. 28: Zeta potential of alumina-10ppm polyethylene glycol (PEG) system as a function of pH, in the absence and presence of different concentrations of ammonium poly methacrylate (APMA) (Saravanan & Subramanian, 2012)
Table 2.4 shows zeta potential values associated with colloidal behaviour and stability predicting nature of colloidal particle and effective characterization apparatus.
65 Table 2. 4: Colloidal particles nature and behaviour associated to zeta potential values (Malvern, 2011) Ζeta (ζ) potential (mV)
Stability behaviour of the colloid
Prediction of nature of colloidal particles
0 to ±5
Rapid coagulation or flocculation
May be very porous, very large, heavy particles at the same time very lowly charged
Streaming potential or streaming current apparatus
±10 to ±30
Incipient instability
May be porous, large or heavy particles.
Streaming potential or streaming current apparatus
±30 to ±40
Moderate stability, moderately dispersed
Usually associated with poly dispersed particles. Proper dispersion may be achieved by agitation or mechanical means.
Electrophoretic Zetasizer
±40 to ±60
Good stability, well dispersed
Usually associated with colloidal microdispersions / microparticles (mono or polydispersed)
Electrophoretic Zetasizer
Greater than ±61
Excellent stability, highly dispersed
Usually associated with colloidal nanoparticles (mono or polydispersed)
Electrophoretic Zetasizer
Apparatus needed for characterization
From Table 2.4, whereas colloids with heavy, porous, coagulated or flocculated particles are associated with low zeta potential values of +/-10 and below, moderate stability may be inferred at +/-30 to +/-40, while good or excellent colloidal stability may be inferred if zeta potential values exceed +/- 40. 2.13 Mechanism of Formation of Coloured Clay Organic Complexes Mechanism of formation of coloured clay complexes have been attributed to either acid – base interactions or oxidation – reduction reactions when organic compounds are adsorbed on clay platelets. The explanation of mechanism of formation of coloured clay-organic complexes ascribes the reason for colour change to be due to charge transfer between clay mineral platelets and adsorbed organic species (Theng, 1971). The active sites on the clay are aluminium
66
exposed at crystal edges and / or transition metal cations in the higher valence state at planar surfaces both of which can act as electron acceptors. The pH of the system, the nature of the solvent, nature of the exchangeable cation and steric factors involving bulky hydrocarbon compounds all play a role in influencing the rate of colour development and the final intensity of the colour produced. Since the activity of a clay mineral can be supressed in an alkaline medium, acid washed clays can act as Brønsted acid sites or proton donors in the reaction medium. On the other hand, oxidation-reduction reactions may make the clays become electron acceptors or Lewis acids from adsorbed species, in which case, there is no prior treatment with acid. Such associations between clays and organic complexes may be described as electron-donor-acceptor complexes. As reported earlier, cation exchanged clays when dried to a low water content of about 5 – 6% of oven dry mass, can also behave as strong Brønsted acids, wherein dissociation of residual water molecules into H+ and OH- within clay platelets, may result in reactive protons on the clay surface due to polarization of exchangeable cations. Apart from the nature of clay, nature of exchangeable cations, clay and moisture content, acidity in this case, is dependent on basicity of the interacting molecules and nature of surrounding solvent. In colour reaction of clays and organic molecules, both Brønsted and Lewis acidity interplay and in most cases, it may be difficult to differentiate which one has overriding influence. Recognizing the importance of crystal edges as electron accepting sites, Solomon et al, 1968, concluded that most colour reactions take place as a result of charge transfer between clay mineral and adsorbed organic species (Solomon et al., 1968).
67
2.13.1
Charge Transfer Reaction At Crystal Edges
The interaction of benzidine with clays as illustrated in Figure 2.29 is a good example of a colour reaction activated by layer lattice silicates. Theng had earlier on given in-depth account of the colour transformation to a blue colour exhibited by certain clays when brought in contact with colourless diamine (Theng, 1971). Further reduction of the blue monovalent radical gives a yellow divalent radical cation (Dodd & Ray, 1960). Therefore, when a series of colour forming organics are reacted with clays under prescribed conditions, for instance, clays constituted primarily of montmorillonite (MMT) may be transformed into yellow colour when reacted with benzidine at a pH of 1 (Mielenz & Kong, 1951). Proton from water polarized by exchangeable cation; for example [Al(H2O)x]3+ --> [Al(OH)(H2O)x-1]2+ + H+
Al-
NH2
NH2
Al Crystal edge
NH2
+H-
-e -e
-H+
Fe3+ Planar surface
Fe2+ NH2
Benzidine Colourless & uncharged base
NH2 Blue monovalent radical-cation
b
NH2 Yellow divalent radical-cation
c
a Figure 2. 29: Scheme showing the transformation of benzidine into its coloured radical-cation forms activated by clays (Theng, 1971) Egg-yolk yellow clays obtained by calcination / solvothermal synthesis of spent oil base drilling mud, may be due to ligand to metal charge transfer transition
68
(LMCT) of transition metal compounds of higher valences present in the clay sample or metal to ligand charge transfer (MLCT) whereby the metal is in a low oxidation state and the ligand is easily reduced. Electrons may jump from a predominantly metal orbital to a predominantly ligand orbital (Wikipedia, 2011c). As shown in Figure 2.30, the yellow colour of potassium chromate (K2CrO4), orange colour of potassium dichromate (K2Cr2O7) and purple colour of potassium permanganate (KMnO4), the blue colour of copper II sulphate (CuSO4), turquoise colour of nickel II chloride (NiCl2)and red for mercuric iodide (HgI2), are largely due to LMCT.
Figure 2. 30: L-R; Aqueous solutions of: cobalt (II) nitrate, Co(NO3)2 (red); potassium dichromate, K2Cr2O7 (orange); potassium chromate, K2CrO4 (yellow); nickel (II) chloride, NiCl2 (green); copper (II) sulfate, CuSO4 (blue); potassium permanganate, KMnO4 (violet) In the case of SOBM filler 1, it is likely that that the presence of heavy metallic elements of chromate and iron may have reacted or displaced aluminium in the alkaline salty medium giving rise to the egg-yolk-yellow appearance of the calcined nanoporous clay particles. 2.14 Chemistry of Organosilane Clay Treatment Silanization of clay particles begins with hydrolysis reaction of alkoxy or aminosilane, followed by polycondensation reaction between hydrolyzed silane molecules and hydrophilic clay mineral surfaces. Clay minerals with silicon and aluminium hydroxyl groups on their surfaces are generally receptive to bonding
69
with alkoxysilanes or aminosilane (Katz & Milewski, 1978; Theng, 1971, 1974). Reason for this is the presence of structural water within the clay lamellae which begins to hydrolyze the silane compound at first contact. The chemistry of clay organosilane treatment is shown in Figure 2.31.
R-Si(OR)3
+ nH2O Hydrolysis
HOH
R-Si(OR)2OH
+
OH
OH
Hydrophilic clay surface
Where; R = -CH3; -C2H5; -C3H7
Condensation R R
O
Si O
R Si
+
O
nROH
Si
O
O O
O
Organo-reactive clay surface
Figure 2. 31: Hydrolysis and condensation reaction in clay organosilane coupling
2.14.1
Chemistry of Epoxy – Aminosilane Treated Clay
After silanization, the clay surfaces become tethered with attached functional groups. However, coupling chemistry between epoxy/aminosilane/clay interface could be understood from explanations given by Dai and Huang, 1999 (Dai & Huang, 1999) in Figures 2.32A and B, while 2.32C suggests reaction between silane treated clays (TDU ash and SOBM clay) with epoxy matrix thus;
70
A. HYDROLYSIS OF AMINOSILANE
(OH)3-Si-CH2CH2CH2NH2 + 3CH3CH2OH
(CH3CH2O)3-Si-CH2CH2CH2NH2
3H2O 3-Aminotriethoxopropylsilane + B.
3-Aminotrihydroxopropylsilane
ethanol
CONDENSATION REACTION BETWEEN AMINOHYDROXOSILANE WITH CLAY-OH GROUPS
n
SOBM or TDU Ash
OH
+ (OH)3-Si-CH2CH2CH2NH2
SOBM or TDU Ash
O-Si-(CH2)3NH2 + 3nH2O
3-Aminotrihydroxopropylsilane C. SUGGESTED REACTION OF SILANE TREATED TDU / SOBM CLAY WITH EPOXIDE END GROUPS n
SOBM or TDU ash
O-Si-(CH2)3NH2 +
OH
OH CH2-CH-CH2-O
O-CH2-CH-CH2 N(CH2)3Si-O O-CH2-CH-CH2
SOBM or TDU ash
O-Si(CH2)3N CH2-CH-CH2-O
OH
OH
Figure 2. 32: Hydrolysis of aminosilane (A), Condensation reaction of amino hydroxo silane with clay-OH groups (B), suggested silanization reaction with epoxy matrix (C) The final result of reaction as presented in Figure 2.32C, show an embedded SOBM core-shell clay / TDU ash within epoxy matrix with amine end groups, -NH2, attaching two epoxy chains via epoxide end group on both sides.
71
2.15 Fourier Transform Infrared (FTIR) Spectroscopy Characterization of Clays and Clay Minerals Fourier transform infrared spectroscopy (FTIR) has been used to study clay materials (Madejová et al., 2009). By subjecting clay minerals to near infrared radiation by transmission or absorption, it is possible to obtain clay mineral information (Bergaya, 2006). Usually, such information is compared with fingerprints of molecular vibrations, stretching, translational, in-plane and out of plane motions of functional units on the clay platelets, thereby making it possible to determine clay structure, composition and structural changes upon modification (Madejova, 2003). On a case by case basis, FT-IR studies on acid treated clays and thermally modified clays have shown that nature of inorganic acid; HCl or H2SO4, acid concentration, cation exchange capacity (CEC), moisture content, particle size distribution, temperature and period / time of acid treatment, affects clay morphology after treatment (Valenzuela-Diaz & Santos, 2001). Studies on structural modification of swellable and non-swellable clay platelets by acid activation earlier reported (Steudel et al., 2009a, 2009b) using FT-IR technique, shows that as protons (H+) from acid penetrate into the tetrahedral and octahedral layers, hydroxyl groups (-OH) are attacked resulting in dehydroxylation of clay platelets. Dehydroxylation is connected with successive release of the octahedral atoms, Al or Mg, followed by changes in OH vibrations. These fundamental vibrational frequencies are captured as transmittance or absorbance intensities at different wavenumbers in infrared spectroscopy. In view of this, FT-IR technique has been used to expose phase transitions in some natural calcite clay minerals (Gunasekaran & Anbalagan, 2008). FT-IR has also been used to identify natural minerals in silica and calcite admixtures wherein spectra
72
obtained as presented in Figure 2.33 are used as fingerprints identities for the clay minerals (Reig et al., 2002).
Figure 2. 33: FTIR spectra obtained as potassium bromide pellet of (a) silica (b) calcite clays (Reig et al., 2002)
Interpreting Figure 2.33, silica is characterized by 550cm-1, 779cm-1, 798cm-1 and 1040cm-1, while calcite clays (calcium carbonate) are characterized by two major peaks at 712cm-1, 875cm-1, and 1400cm-1. A general peak exists at 3660cm-1 signifying hydroxyl group within layers for both silica and calcite clays. However, other peaks appear at 2900cm-1 and 2560cm-1 for silica and calcite respectively
73
indicating impurities of magnesium (Mg), aluminium (Al) and iron (Fe), within the clay
layers.
Furthermore,
Li
et
al.,
2008,
investigated
hexadecyltrimethylammonium (HDTMA) intercalation into rectorite clay, a nonswelling clay mineral (Li et al., 2008). Rectorite (Na,Ca)Al4(Si,Al)8O20(OH)4•2(H2O), described as a mixed clay mineral and made of regular 1:1 stacking of dioctahedral mica-like and MMT-like layer exhibited adsorption behaviour similar to that of MMT. As a result of MMT component in the mixed clay, its absorption behaviour towards cationic surfactants alkyl ammonium like certyl trimethylammonium bromide (CTAB), HDTMA-Br, was similar to that of MMT. FT-IR results showed that a monomer-like intercalation with extensive gauche conformers was formed at surfactant loading less than the cation exchange capacity (CEC) of the mineral. At higher surfactant loading, the CH2 symmetric and anti-symmetric vibrations shifted to lower frequencies, suggesting a more ordered all trans surfactant interlayer configuration. In summary, FT-IR of swellable and non-swellable clays, would indicate out-ofplane bending (v2), asymmetric stretching (v3) and in-plane bending (v4) modes to be very active at different wavenumbers (cm-1) as shown in Tables 2.5 and 2.6 (Steudel et al., 2009a, 2009b; Tyagi et al., 2006). However, upon acid treatment, FT-IR peak intensities would broaden with concentration of inorganic acid and period of treatment as the spectra shifts to higher wave numbers. High temperature treatment also weakens the vibrations of siliceous linkages thereby broadening the intensity spectra of calcined clay samples resulting in low intensities of FT-IR spectra (Bergaya, 2006).
74
Table 2. 5: FT-IR band assignments for montmorillonite clay (Tyagi et al., 2006) Wavenumber (cm−1)
Assignments
529
Si-O bending
692
Quartz
836
AlMgOH bending
875
AlFeOH bending
915
AlAlOH bending
1035
Si-O stretching, in-plane
1113
Si-O stretching, out-of-plane
1639
-OH bending, hydration
3440
H-O-H stretching, hydration
3623
-OH stretching
3697
-OH stretching
75
Table 2. 6: Showing vibrational band assignments of some clay obtained by FT-IR technique (Steudel et al., 2009a, 2009b) Dioctahedral smectite Swellable clays (Steudel et al., 2009b)
Trioctahedral Non-swellable clays (Steudel et al., 2009a) Illite
Kaolinite
Wavenumber (cm-1)
Assignment
Wavenumber (cm-1)
Assignment
Wavenumber (cm-1)
Assignment
470
Si-O-Si vibration Si-O-Mg vibration
431
Si-O bending
428
Si-O bending
524
Si-O-Al vibration (Al octahedral cation)
470
Si-O (In plane) bending association with OH
541
Si-O-Al (Outof-plane) bending (Al in tetrahedral sheet)
644
Inner surface OH vibration
622
842 885 921 1020
1091
R-O-Si with R = Al, Mg, Fe AlMgOH bending AlMgOH bending AlFeOH bending AlAlOH bending Si-O stretching vibration (Inplane) Si-O stretching vibration (Out of plane)
472
538
622 750 823
Si-O (In plane) bending association with OH Si-O-Al (Outof-plane) bending (Al in tetrahedral sheet) Inner surface OH vibration Si-O-Al vibration AlMgOH bending
754 792
Si-O-Al vibration Si-O-Al vibration
877
AlFeOH bending
916
Inner OH deformation
912
AlAlOH bending
935
Inner surface OH deformation
1033
Si-O stretching vibration (Inplane)
1112
Si-O stretching vibration (Out-of-plane)
1030
1101
Si-O stretching vibration (In-plane) Si-O stretching vibration (Out-ofplane)
76
On the other hand, thermally treated fly ash have shown higher intensities in absorbance spectra (Fen et al., 2006). Time range for thermal treatment of the fly ash was from 2 to 6 hours. They had associated the wide vibrational band between 3200 and 3600cm-1, to -OH- stretching in composite fly ash, while that at 1085cm-1 to siliceous (Si-O-Si) linkages of SiO4 tetrahedra. The peaks at 1010cm-1 to 1090cm-1 were associated to cyclic Si-O-Si structure. With their results, Fen et al., 2006, concluded that high temperature treatment of composite fly ash basically composed of calcium aluminosilicates, produced zeolitic porous material. 2.16 Epoxy Resin Technology The first commercial epoxy was produced in 1946 and was widely used in industry as protective coatings, adhesives and bonding applications, moulding, wood laminates, and composites structures (May, 1988). Epoxy resin is a candidate for so many applications because of the ability of the epoxide ring to react with a variety of substrates. Epoxy resin reaction with curing agents gives a three dimensional (3D), insoluble and intractable thermoset polymers. Cured epoxy resins are corrosion resistant, thermally and dimensionally stable. They also have outstanding adhesive strengths, low shrinkage upon cure, good electrical insulating properties, and easy processability (Wikipedia, 2013). Depending on the specific needs for certain physical and mechanical properties, combinations of choices of epoxy resin and curing agents can be formulated to meet specific purposes. However, in terms of adhesive and structural applications, cured epoxy resin is usually amorphous, brittle and notch sensitive (Kinloch, 2003; Riew et al., 1996). As a result, efforts have been made by several researchers to improve toughness properties of epoxy resins (Hourston & Lane, 1992; Iijima et al., 1991; Iijima et al., 1990; Murakami et al., 1992).
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2.16.1
Epoxy Resin Synthesis
Epoxy resins are synthesized by the reaction of excess epichlorohydrin and bisphenol A in the presence of sodium hydroxide. The initial reaction produces pre-polymer, diglycidyl ether of bisphenol A (DGEBA), containing more than one epoxide end-group functionality per molecule (May, 1988). The reaction scheme is shown in Figure 2.34, and is known as the “Taffy process”. Epoxy resin can also be produced by the direct epoxidation of olefins with per-acids.
Epoxide end group
Figure 2. 34: Reaction scheme of epichlorohydrin with bisphenol A and sodium hydroxide. The product of the reaction is diglycidyl ether of bisphenol A, DGEBA. Epoxide end group is indicated in circle. Where, n = 2-30 for high molecular mass epoxy, n = 3.7 average for low molecular mass epoxy)
An epoxide end functional group is a three membered ring molecule containing one oxygen atom and two carbon atoms. While the presence of this functional group defines a molecule as an epoxide, the molecular base to which it is attached can vary, giving various classes of epoxy resins. Commercial epoxy resins contain aliphatic, cycloaliphatic or aromatic backbones. Multifunctional epoxy resins such as aromatic glycidyl ether resins and aromatic glycidyl amine resins are Epoxy Phenol Novalac resins (EPN, where R=H), and Epoxy Cresol Novalac resin (ECN, where R = CH3), which structures are shown in Figure 2.35.
78
Figure 2. 35: Structure of commercially available epoxy-phenol Novalac (EPN), where R = H; and epoxy-cresol Novalac (ECN), where R = CH3
EPN and ECN are prepared from excess epichlorohydrin and phenol-formaldehyde or o-cresol-formaldehyde resins. The multi functionality of these epoxy resins when used as composite matrices leads to high crosslink density, improved thermal stability and chemical resistance. Epoxy resins derived from multifunctional aromatic glycidyl amine resins such as triglycidyl-p-aminophenol (ERL 0510 by CIBA-GEIGY), and tetraglycidyl-4,4-diaminodiphenylmethane (TGDDM) as shown in Figure 2.36, (Araldite MY 720 by CIBA-GEIGY), also have excellent high temperature properties.
Figure 2. 36: Structures of epoxy resins derived from multifunctional aromatic glycidyl amine resins such as triglycidyl-p-aminophenol (ERL 0510 by CIBA-GEIGY), and tetraglycidyl-4,4diaminodiphenylmethane (TGDDM), (Araldite MY 720 by CIBA-GEIGY)
Other commercially available epoxies are glycidyl ester prepared from cycloaliphatic carboxylic acid (CY 184 by CIBA-GEIGY), with structure shown in Figure 2.37, and cycloaliphatic epoxy resins based on the epoxidation of cyclo
79
olefins using basic inorganic such as NaOH or Na2CO3 as catalyst. This process is known as the “advancement” or “fusion” process.
Figure 2. 37: Structure of diglycidyl ester of hexahydrophthalic acid
1.16.2
Epoxy resin curing
Epoxy resins reaction with curing agents or hardeners gives a three dimensional insoluble and infusible network. A wide variety of curing agents are available. The choice of curing agents depends on the required physical and chemical properties, processing methods and curing conditions. This is because the choice of curing agent can substantially influence the cure chemistry, cure rate, crosslink density, morphology and fracture behavior of epoxy nanocomposites. Epoxy resins can be cured with either catalytic or co-reactive curing agents (Lee & Neville, 1967). Catalytic curing agents; function as initiators for epoxy ring-opening homopolymerization. These catalytic curing agents may be Lewis bases such as tertiary amines or Lewis acids such as boron triflouride monoethylamine and can be used for homopolymerization as accelerators, or supplemental curing agents for other curing agents such as amines or anhydrides (Rhodes, 1991). Figure 2.38, shows reaction scheme. In the application of solvent-free or high solid content epoxies as surface coatings, curing agents are cationically photoinitiated.
80
Figure 2. 38: Epoxy resin cure with a tertiary amine as curing agent in several ring-opening homo-polymerization reactions to form 3D crosslink network
Co-reactive curing agents; exist where epoxide ring reacts with compounds with activated hydrogen atoms such as alcohols, amines, and carboxylic acids. Primary and secondary amines are the most widely used for co-reactive cure of epoxies. A primary amine reacts with an epoxide group to produce a secondary amine and a secondary alcohol. The secondary amine can further react with an epoxide group to form a tertiary amine. This scheme is presented in Figure 2.39. Aliphatic amines cure epoxy resins at room temperature, while aromatic amines are less basic and require high temperatures or accelerators in order to cure.
Figure 2. 39: Epoxy resin reaction with; (1) Primary amine, (2) Secondary amine, (3) Tertiary amine
81
Other commercially important curing agents are formaldehyde resins, acid anhydrides based on cycloaliphatic structures, carboxylic acids, mecaptans, and isocyanates.
Melamine-formaldehyde,
urea-formaldehyde
and
phenol-
formaldehyde resins react with hydroxyl groups of high molecular mass epoxy resins to form interpenetrating 3D crosslinks. Generally, epoxy-anhydride systems have low viscosity, long pot life and low exothermic heats of reaction. This type of epoxy resins make them preferred choice in structural adhesives and epoxy matrix filled with nanoporous core-shell clay fillers. In their cured state, they have good mechanical and electrical properties, more so when they are cured with fillers (Goodman, 1986).
2.17 Epoxy – Clay Nanocomposites Epoxy materials have been filled with natural and synthetic inorganic compounds in order to improve material properties. Conventional filler compounds exist in different forms and shapes (Wypych, 1999), viz; particulate - calcium carbonate; sheets - clays / layered-silicates; fibres - fibre glass and asbestos; plate like - mica. Interestingly, these compounds have been used because of their nanosize advantage. But among all the compounds, clays and layered silicate compounds have been the most widely investigated probably because the starting materials are easily available, cheap and their intercalation chemistry have been elucidated (Annabi-Bergaya, 2008). But to obtain materials with good qualities depends on properties of individual components on the one hand and the composites preparation methods on the other. Such composites preparation parameters may include volume fraction of clay filler, degree of clay dispersion, coupling, and level
82
of phase separation or interaction between components (Lan et al., 1994). Clay dispersion is difficult, but possibility of achieving high degree of dispersion / exfoliation has been demonstrated (Lan et al., 1995; Park & Jana, 2003). 2.17.1
Clay Dispersion Morphologies in polymers
Three (3) dispersion morphologies exist in literature (Pavlidou & Papaspyrides, 2008a). They are intercalation, exfoliation and immiscibility of clay platelets in polymer matrices. 2.17.1.1 Intercalation This involves making the host polymer chains penetrate into the layers of the clay galleries, expanding the nano-sheets to a great degree while maintaining the silicate nano-sheet memory in registry. In such a case, the book arrangement of the clay nano-sheets is not totally destroyed. The clays are not dispersed, but remain swollen with polymer macromolecules holding back each silicate nano-sheets in place resulting in minimal expansion. 2.17.1.2 Exfoliation This involves making the silicate layers to expand to an extent that each silicate nano-sheet finds its way, clinging to a group of polymer macromolecules in an independent manner. This situation is made possible because, expansion is so large that the forces of agglomeration between layers are totally broken making it impossible for clay nano-sheets to remain in book-like form. There is a total loss of stack-like registry and association between clay nano-sheets. Though this is the presumed situation for exfoliation to exist, but it is generally accepted that exfoliation has been achieved, when inter planar spacing between clay nano sheets and organosilicate layers are in the order of 10 to 30Å (1 to 3nm),
83
otherwise, the nano clays remain intercalated. Factors influencing exfoliation and physical properties enhancement in epoxy clay nanocomposites have been published, (Ingram et al., 2007), which includes choice of curing agent, higher cure temperatures, the use of sonication and higher levels of clay moisture. 2.17.1.3 Immiscible Agglomerates The third would be a situation where silicate nanostructures do not exhibit any of the above situations of intercalation nor exfoliation. Then, it is said that the nano platelets are immiscible in polymer matrix. In this case, the polymer and layered silicate are completely separated, where no polymer chain has penetrated inbetween the gallery of the layered silicate.
An illustration of different states of dispersion of organoclays in polymers with corresponding wide angle x-ray spectroscopy (WAXS) and TEM images showing morphological appearance (Paul & Robeson, 2008), are presented in Figure 2.40.
84
Figure 2. 40: Illustration of different states of dispersion of organoclays in polymers with corresponding WAXS and TEM images (Paul & Robeson, 2008)
However, pinning each nanosheet to each other in a network, to obtain a more or less nanoporous nanoclay filler material, would be interesting. Through such nanopores, polymer macromolecules can find their way, get entangled and lock-up, to form a 3D interpenetrating network (IPN) of some sort with the host polymer matrix. This may be termed a hybrid network with the nanosheets registry still intact. In that case, they are held strongly by silicate nano-bridges at intermittent points within the bulk material. 2.18 Previous Works on Epoxy-Clay Nanocomposites So many articles have been published on preparation, structure, morphology and properties of polymer-clay nanocomposites. Amongst several published articles
85
are; nanocomposites of polymers and inorganic particles (Caseri, 2006; Jeon & Baek, 2010; Pomogailo, 2000), epoxy-clay nanocomposites (Kornmann et al., 1999; Lan et al., 1994), preparation and properties of epoxy-clay nanocomposites (Khanbabaei et al., 2007). In same way, the complexation of organo-treated clays; montmorillonites (MMTs), layered silicates or clay admixtures, to various epoxy matrices,
characterization
procedures
and
the
determination
of
final
nanocomposites properties for application advice have been elucidated (Julia & Popall, 2005; Sanchez et al., 2005; Velmurugan & Mohan, 2008). Other publications
include
organoclay-modified
high
performance
epoxy
nanocomposites (Liu et al., 2005), layered clay epoxy composites (Caseri, 2006), synthesis of polymer mesoporous silicate nanocomposites (Wei et al., 2010a) and epoxy polymer reinforcement with porous synthetic smectite clay giving composites with good mechanical properties above or below the glass transition temperature (Tg), with or without organic modifier (Wei et al., 2010b; Xue & Pinnavaia, 2008). Furthermore, Dai et al, 2005, published study on morphology and mechanical properties of high functional epoxy based clay nanocomposites (Dai et al., 2005), while in 2006, Naderi et al, reported organic–inorganic hybrids wherein an organic substrate was filled with inorganic core-shell powders (Naderi et al., 2006; Velmurugan & Mohan, 2008), rubbery and glassy epoxy polymer were reinforced with meso-structured and organo-silica with wormhole framework structures (Jiao et al., 2008, 2009) and then in 2008, Nguyen and Berg reported novel core-shell (dendrimer) epoxy tougheners (Nguyen & Berg, 2008). In the area of fracture analysis of epoxy nanocomposites, Yao et al, 2008, worked on macro/microscopic fracture characterizations of SiO2/epoxy nanocomposites (Yao et al., 2008). Apart from impact of nanoparticle inclusion in a substrate
86
polymer matrix on structural and mechanical properties, such nanoparticles have also impacted on catalytic, optical (Nguyen et al., 2009), dielectric, electrical (Maity & Biswas, 2006) and magnetic properties of the new material (Li et al., 2010; Polizos et al., 2010). However, in all the work so far, synthesis and method of composite preparation have varied leading to varied nanocomposites morphologies, degrees of clay particle dispersion and final nanocomposites properties. However, work on clay – matrix interphase, clay nanoparticle–epoxy matrix bonding ability, improved clay intercalation, exfoliation or dispersion, continues to be major source of concern to researchers, suggesting various ways of surface modification of nanoclays, functionalization or clay surface treatment and cure (Mansoori et al., 2010; Miyagawa et al., 2004a; Zaman et al., 2011). Miyagawa et al, 2004, reported the processing of epoxy clay nanocomposites composed of amine-cured diglycidyl ether of bisphenol A (DGEBA) and reinforced with organo-MMT clays (Miyagawa et al., 2004a). They followed a novel facile route of sonication of the constituent materials in a solvent, then solvent extraction to generate composites with homogeneous dispersions of the nano clays in the glassy epoxy network before complexing in epoxy matrix. Fourier Transform
Infrared
Spectroscopy
(FT-IR),
Fourier
Transform
Raman
Spectroscopy (FT-Raman) confirmed that the solvent used to treat their clay samples during composites preparation did not in any way affect composite chemical structure. Transmission electron microscopy (TEM), wide angle x-ray scattering (WAXS) and small angle x-ray scattering (SAXS) confirmed that the nano clays were effectively dispersed, and were intercalated and exfoliated respectively. In furtherance to their earlier investigation, Miyagawa et al, 2004, uncovered the thermo physical and mechanical properties (Miyagawa et al.,
87
2004b) of their epoxy clay nanocomposites obtained earlier (Miyagawa et al., 2004a). They however discovered that the glass transition temperature (Tg) of the epoxy clay nanocomposites increased linearly with volume fraction of organoclay filler, while the storage modulus at 100°C, above the Tg, also increased tremendously with 10wt% organoclay filler. They tried to use two models; Tandon-Weng and Halpin-Tsai models, to explain the reinforcing effect of their organoclay in the nanocomposites. Generally, aminosilane treatment of clay platelets leads to improved intercalation and interfacial bonding between clay and epoxy matrix which in turn leads to improved mechanical and dynamic properties (Choi et al., 2009), improved fracture toughness and fatigue strain energy (Sung Rok Ha & Rhee, 2008), as well as improved wear behaviour in as-synthesized epoxy nanocomposites, (Ha et al., 2008; Rok & Yop, 2008). In an attempt to produce nanocomposites for aerospace applications, layered silicate fillers were crosslinked in aerospace grade epoxy matrix (Chen et al., 2003). This was achieved after treating the clays with Jeffamine D400, n-decylamine, nhexadecylamine and n-octadecylamine in hydrochloric acid (HCl). In same vein, MMT clays were modified before introducing them in epoxy matrix at different concentrations (Ha et al., 2007). Though, the work of Ha and others as reported, yielded a 55% exfoliation from the basal interlayer spacing obtained from X-ray diffraction, the clay particles were not effectively dispersed in any cationic surfactant. They had employed mechanical stirring to disintegrate the Na-MMT agglomerates
in
100ml
of
solvent
at
25°C
before
adding
3-
aminopropyltriethoxysilane (3 APTES). Comparatively, tensile strength and elastic modulus of surface modified Na-MMT clays which they obtained increased with clay loading, while for unmodified clays, tensile strength was not affected by clay
88
loading. Similarly, clay treatment with 3-aminopropyltriethoxysilane (3-APTES), leads to high residual strength (Sung Rok Ha & Rhee, 2008), as a result of high degree of intercalation and interfacial bonding with epoxy matrix. This was collaborated by the work of Rok and Yop (Rok & Yop, 2008). Kinloch and Taylor produced micro and nanocomposites after various modifiers were used on mica clay particles to obtain exfoliated, intercalated and particulate composite morphologies with as much as 150% improvement in fracture toughness (Kinloch & Taylor, 2006). Similarly, the influence of clay organophilic treatment on reactivity, morphology and fracture properties of epoxy-MMT nanocomposites has been investigated (Pluart et al, 2005). The organoclays were made with benzyl dimethyl tallow alkyl ammonium MMT and octadecylammonium ion modified MMT. Their results confirm the reinforcing effect of the organoclays since stresses and strains at break increased simultaneously leading to improved stiffness and fracture toughness (KIC). However, they found it difficult to link increase in mechanical property values to nano-filled epoxy composite morphology. This was as a result of contradictions in nanoscale dispersion observed in epoxy composite morphology. In other words, modified clay platelets were not properly dispersed. Rather than getting lower values in modulus of elasticity, strain at break and fracture energy at low dispersion, higher values were obtained. High performance epoxy-layered
silicate
nanocomposites
based
on
tetraglycidyl-4,4’-
diaminodiphenyl methane (TGDDM) resin, cured with 4, 4 diaminodiphenyl sulphone (DDS) have been synthesized (Bucknall & Gilbert, 1989; Kornmann et al., 2002). While, Bucknall and Gilbert, 1989, saw that polyetherimide (PEI) forms a separate phase when dissolved in TGDDM based resin with 30p.h.r DDS as curing agent, Kornmann et al., 2002, found that with just 4.2% silicate content, fracture
89
toughness (KIC) and Young’s modulus improved by 112% and 28% respectively, while Tg reduced to 257°C. This is in comparison with earlier reported improvement of fracture toughness of epoxy matrix with polyetherimide as toughening agent at 10% loading (Lazzeri & Bucknall, 1993). It may be interesting to observe that, a reduction of resin volume fraction with high loading of filler material up to 20% maybe traded off with ductility and toughness. The nanocomposite material inadvertently becomes more and more brittle with higher loading. More so, processability becomes difficult by increasing wear rate of processing machinery. But in contrast, nano-particulate calcites and silica filled epoxies possess significant improvements in dimensional stability (Tjong, 2006). However, the quest for innate mechanical properties for thermoset based nanocomposites based on vinyl esters, unsaturated polyesters and epoxies resin matrices has led to recent interests in mesoporous silica nano fillers (Ji et al., 2003; Luo et al., 2007). Wei et al, 2010, accounts that due to their ordered structure, high surface area and ease of functionalization of their nanopores, they tend to have profound intimacy with polymer matrices when compared with nonporous fillers (Wei et al., 2010b).
On how nanoparticles affect the thermal stability of polymer-clay nanocomposites, Bikiaris and Chrissafis inquest, provides answers to the question, “Can nanoparticles enhance the thermal stability of nanocomposites?”, (Bikiaris, 2011; Chrissafis & Bikiaris, 2011).
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2.19 Morphology and Characterization of Epoxy-Clay Nanocomposites Epoxy-nanoporous clay nanocomposites present an interesting innovation in the field of epoxy nanocomposites as they offer advantages of superior mechanical properties, gas permeability, thermal expansion coefficient and flame retardancy, compared to pristine polymers. Therefore, characterizing epoxy-nanoporous clay composites may involve but not limited to the following, i.
Internal structure investigation; Wide angle X-ray Diffraction (WAXS), Small angle X-ray Diffraction (SAXS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) (Miyagawa et al., 2006)
ii.
Performance properties by mechanical testing involving tensile, E-modulus, hardness, impact and compression (Anthoulis & Kontou, 2008; Chen & Evans, 2008; Manjunatha et al., 2010; Radford, 1971; Rupp et al., 2005).
iii.
Fractured surface morphology by Optical microscopy, SEM, AFM (atomic force microscopy) and TEM, (Ha et al., 2008; Ha et al., 2007; Nakamura et al., 1991; Wetzel et al., 2006; Zhang et al., 2008; Zhao & Luo, 2008; Zunjarrao & Singh, 2006). Furthermore, fractured surfaces of epoxy nanocomposites subjected to three-point bending until fracture, have been investigated using atomic force acoustic microscopy (AFaM) (Preghenella et al., 2006).
iv.
Filler dispersion and filler / matrix interphase characterization by XRD, TEM (Chiu et al., 2007; Choi et al., 2009; Kornmann et al., 2005; Vaia et al., 1996; Zheng, 2003), Fourier Transform Infrared Spectroscopy (FT-IR), Fourier Transform Raman Spectroscopy (FT-Raman), (Choi et al., 2009).
v.
Thermal properties by Thermogravimetric Analysis (TGA), Differential Scanning Analysis (DSC), (Meneghetti & Qutubuddin, 2006; Sarathi et al., 2007).
91
vi.
Nanoporous filler porosity characterization by microscopy (TEM / SEM) (Diaz & Mayoral, 2011), XRD, porosity and nitrogen adsorption by Brunauer Emmett and Teller (BET) method (Diaz & Mayoral, 2011).
vii.
Nanocomposites performance property prediction by mathematical, macro / micro mechanical modelling and finite element modelling / analysis (FEM) (Hbaieb et al., 2007; Luo, 2003; Mesbah et al., 2009; Pisano et al., 2012)
2.20 Mechanical Properties and Theoretical Considerations of Ultimate Strength and Hardness of Core-Shell Clays Particulate Filled Nanocomposites One of the major objectives of introducing reinforcements in a polymer is to improve on mechanical strength of the more or less weak and soft matrix (Callister, 2007). Such mechanical properties are strongly influenced by the size, shape, type of reinforcement - fibre reinforcement or particulate filler (Vollenberg & Heikens, 1986), concentration and dispersion of fibre / filler as well as extent of interfacial adhesion between filler and matrix (Rothon, 2003). All in all, enhancement of overall mechanical properties is achieved by effective load transfer and sharing between matrix and reinforcing agent – fibre or particulate filler, which is central to the understanding of the mechanical behaviour of any nanocomposite material. Two important properties are used to characterize mechanical properties of nanocomposite materials, viz; stiffness and toughness. Stiffness property is the resistance to plastic deformation while toughness may be described as the ability of a composite material to absorb energy and plastically deform without fracturing, or simply, the resistance of a material to fracture when stressed. Both properties are derived quantities. Stiffness is determined by modulus of elasticity while toughness is calculated by the area under the stress-
92
strain curve (Stevens, 1999). An illustration of the stress-strain curve is shown in Figure 2.41. E D A
C B
Figure 2. 41: Stress-strain curve of polymers showing 5 different regions of the curve, (A) elastic region, (B) Yield (intrinsic a and extrinsic), (C) Strain softening, (D) Cold drawing or strain hardening, (E) Failure (Callister, 2007)
Each of the regions shown in Figure 2.41, is accompanied by morphological changes as shown in Figure 2.42, as a result of molecular chain rearrangements, temperature difference, nature of crack propagation and growth (Aklonis, 1981), until eventual failure. That is to say that, increase in stress-strain or engineering stress-engineering strain property, is accompanied by dimensional changes in dumb bell sample giving rise to true stress-true strain relationship (Enyiegbulam & Iheaturu, 2007). In such situations, equations 2.6 and 2.7 are expressions used to calculate the true stress and true strain for composite materials that exhibit dimensional changes in tension. True stress, σT, is given as;
σT = σn(1 + εn)
2.6
93
While True strain, εT is given as;
εT = εlog = ln(1 + εn) Where;
2.7
σn = nominal stress given as F/Ao F = Force, and Ao = Original cross-sectional area of the material
εn = nominal strain given as ∆L/Lo ∆L = extension or change in length as a result of force, and Lo = Original gauge length of the material before extension The true stress-strain values obtained is usually higher than the nominal values. However, morphological changes are determined by polymer type; thermoplastic, thermoset or elastomer.
Figure 2. 42: Schematic illustration of deformation mechanisms and stress-strain curves showing the effect on lamellae orientation (Cotterell, Chia, & Hbaieb, 2007; Zhou & Wilkes, 1998)
94
In general, ultimate strength of a composite material depends on adhesive strength of the matrix-filler interphase, which makes it difficult to be estimated theoretically. This is because, interfacial strength may be affected by the presence of adsorbed water on the filler surface or by thermal stress from a mismatch of thermal coefficients of linear expansion for filler and matrix (Fried, 2000). This was confirmed by Awaji et al, 2002, while reporting on a study involving toughening and strengthening mechanisms in ceramic-based nanocomposites based on Griffiths energy equilibrium and residual stresses around a spherical dispersed particle within a concentric sphere of matrix grain (Awajiet al., 2002a). Their analytical results revealed that the ratio of thermal expansion coefficients of the particle and matrix has a marked effect on residual stresses and the estimated residual stresses. The estimated residual stresses were of sufficient magnitude to generate lattice defects such as dislocations around the particle leading to failure. It should be noted that, polymers have relatively high linear expansion coefficients (60 to 80 x 10-6 per °C for PS) compared to fillers such as silica glass (0.6 x 10-6 per °C) or graphite (7.8 x 10-6 per °C).
Rather than allow the material to fail abruptly in engineering applications, it behoves any engineering design to ensure the right material properties for a given application. How this can be achieved depends on experimental designs and models to estimate material properties during service. Apart from experimental determination of composite strength through stress-strain measurements, modulus of glassy-polymer nanocomposites containing rigid particulate filler may be estimated either by use of any of the continuum mechanics models; Schrager Model, Halpin-Tsai model, or most recently Bela Pukanszky model.
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2.20.1
The Schrager Model
Although most results on ultimate strength of particulate reinforced composites have been obtained experimentally, a lot of mathematical relationships and models have been proposed to relate the tensile and ultimate strength of particulate reinforced composites to the ultimate strength of the unfilled matrix. In 1992, a theoretical model for the prediction of tensile strength of particulate filled polymers by determining the influence of adhesion bond between matrix and filler was proposed (Papanicolaou & Bakos, 1992). According to Papanicolaou and Bakos, each particle is divided into an infinite number of coaxial cylinders and by applying Cox’s theory, the mean stress developed in each section of the particle can be calculated. They went further to apply a modified version of the rule of mixtures whereby the degree of adhesion between matrix material and filler particles as well as the degradation of the mechanical behaviour of the matrix due to the existence of the particles were taken into account. Therefore, when tensile strengths of polymer and particles are known, tensile strength of the filled polymer corresponding to specific values of degree of adhesion and matrix degradation can be estimated, they reasoned. However, amongst all the models so far proposed, Schrager’s equation published in 1978 (Schrager, 1978), seems to be the most simplified version of all models, with the assumption of good adhesion between the dispersed filler and matrix. Composite ultimate strength (𝜎𝑢𝑙𝑡𝑖𝑚𝑎𝑡𝑒 ) may be calculated thus;
𝜎𝑢𝑙𝑡𝑖𝑚𝑎𝑡𝑒 = 𝜎𝑚𝑎𝑡𝑟𝑖𝑥 𝑒𝑥𝑝−𝑟∅𝑓𝑖𝑙𝑙𝑒𝑟 Where;
2.8
96
σ is ultimate strength, r is an interfacial factor (typically 2.66 for many composites), θ is filler volume fraction. From equation 2.8, strength of the composite will decrease with decreasing interfacial strength. This however, provides a ceiling value for strength up to 35 – 40% filler volume fraction. In compression, plastic deformation starts inhomogenously at localized points for thermoplastic polymers as shown in Figure 2.43(a), creating random micro-shear bands in the polymer under tension, while thermosets generate crazes at circumferential dimensions of approximately 20nm as presented in Figure 2.43(b)(Papanicolaou & Bakos, 1992).
Figure 2. 43: Microscopic observation of Polystyrene at 22°C in; (A) compression with a polarized light optical microscope and (B) tension with a transmission electron microscope (TEM) showing crazes and fibrils = 20nm (Papanicolaou & Bakos, 1992)
2.20.2
Halpin-Tsai Model
Halpin-Tsai model is a mathematical model used in predicting the mechanical behaviour and elastic modulus of a composite based on the geometry and orientation of the filler and the elastic properties of the filler and matrix. It has
97
been seen that particle characteristics such as shape, impacts directly on mechanical properties of filled polymers (Chow, 1980). In terms of polymer clay nanocomposites, concept of effective particle was introduced (Yung et al., 2006). Effective Assuming filler particles are rigid non porous and of a particular geometry, the modulus of a glassy polymer containing such rigid particulate filler may be estimated thus (Halpin & Kardos, 1976); 𝐸𝑐 𝐸𝑚
=
1+𝐴𝐵∅𝑓 1−𝐵∅𝑓
2.9
Where; A = Shape parameter constant which depends on filler geometry or aspect ratio and loading direction or orientation. 𝐿
𝐴 = 𝑓( ) 𝑙
2.10
B = function of A, relative moduli of filler (Ef / Em), and the filler geometry. B is given as;
𝐵=
(𝐸𝑓 ⁄𝐸𝑚 )−1 (𝐸𝑓 ⁄𝐸𝑚 )+𝐴
2.11
Ec=Tensile or shear or bulk modulus of the composite Em=Modulus of the unreinforced matrix Ef = Modulus of filler Φf _= volume fraction of the filler B = function of A and the relative moduli of the filler (Mf) and matrix
98
For an isotropic composite filled with uniformly dispersed particles, the mechanical properties are independent of direction of acting force. This is in contrast to uni-directionally laid fiber composites, which are anisotropic in character. For anisotropic composites like fiber reinforced composites, with infinite filler length, the maximum modulus is obtained in the orientation direction of the fibers, which is computed with the mixture rule, given in equation 2.8. 𝐸𝑐 = ∅𝑓 𝐸𝑓 + (1 − ∅𝑓 )𝐸𝑚
2.12
As the aspect ratio (A) tends to zero, the Halpin-Tsai equation converges to the inverse rule of mixtures, thus; 1 𝐸𝑐
=
∅𝑓 𝐸𝑓
+
(1− ∅𝑓 ) 𝐸𝑚
2.13
In that case, the maximum modulus of the composite is obtained in the fiber orientation direction. Assumptions A number of assumptions follow this reasoning; i.
That the filler and matrix are linearly elastic, isotropic and firmly bonded
ii.
The filler is perfectly aligned in the direction of loading, asymmetric and uniform in shape and size as published (Giannelis, 1996).
iii.
There exists particle-particle interactions at high filler loading
iv.
Particles are seemingly or nearly spherical
99
2.20.3
Bela Pukanszky Model
However, modelling a core-shell clay filled matrix may present very difficult and complex mathematics, from the irregularity posed by the particle in terms of aspect ratio. A practical route may be to assume very intimate relationship between matrix and core-shell filler particles as a result of interpenetrating polymer networks formed within and around a given particle as a result of large surface area, an inherent character of mesoporous solids. In other words, there is no slippage. This situation agrees with the first assumption, that filler and matrix are linearly elastic, isotropic and firmly bonded. Particle-particle interactions may exist at high volume fraction but not to the extent that filler particles are perfectly aligned in the loading direction, asymmetric and uniform in shape as a result of irregular micro porosity. Therefore, the second assumption is far from being ideal in core-shell filled polymers. An ideal situation is to say there is random distribution and dispersion of the particles in the polymer matrix without alignment. However, in a practical sense, tensile yield stress at very high deformations and high exfoliation may be traded off with high filler volume fraction and low exfoliation. With this in mind, a quantitative estimation of the reinforcing effect of layered silicates in 40 samples of polypropylene nanocomposites was analysed (Szazdi et al., 2006). In a series of publications, (Pukanszky et al., 1988; Pukanszky, 1990; Turcsanyi et al., 1988), a model was developed to describe dependence of the tensile yield stress of particulate filled composites. The model which assumes that an interphase forms spontaneously in the composite, shows that tensile yield stress is proportional to actual composite composition expressed in Equation 2.14 (Szazdi et al., 2006). 𝜎𝑦𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 = 𝜎𝑦𝑚𝑎𝑡𝑟𝑖𝑥 𝐴 exp(𝐵𝜑)
2.14
100
But, by considering the effective load bearing capacity cross-section of the matrix expressed as A in Equation 2.15,
𝐴=
1−𝜑
2.15
1+2.5𝜑
Equation 2.15 can therefore be rewritten as; 1−𝜑
𝜎𝑦𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 = 𝜎𝑦𝑚𝑎𝑡𝑟𝑖𝑥 1+2.5𝜑 exp(𝐵𝜑)
2.16
Where 𝜎𝑦𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑒 and 𝜎𝑦𝑚𝑎𝑡𝑟𝑖𝑥 are tensile yield stress of composite and matrix respectively. 𝜑 is the volume fraction of the filler in the composite, while B is a function of all factors influencing the load carrying capacity of the dispersed filler which includes level of interaction at matrix filler interface and filler surface area and is given in Equation 2.17 as;
𝐵 = (1 + 𝐴𝑓 𝜌𝑓 𝑙)𝑙𝑛
𝜎𝑦𝑖𝑛𝑡𝑒𝑟𝑝ℎ𝑎𝑠𝑒 𝜎𝑦𝑚𝑎𝑡𝑟𝑖𝑥
2.17
Where Af is the specific surface area of the filler (contact surface), ρf is its density, l thickness and σyinterphase is yield stress at interphase. l and σyinterphase largely depend on filler matrix interaction. At zero interaction between matrix and filler therefore, the entire load is carried by the polymer and the load bearing cross-section decreases with increasing filler content. The same correlation goes for tensile strength at very marginal elongation, less than 100%. 2.20.4
Hardness Property of Polymer Clay Nanocomposites
Hardness is the resistance of a material to permanent indentation or deformation when in sudden contact with a load indenter (Gopal, 2011). It is a measure of a materials’ resistance to a localized plastic deformation. The depth of the
101
indentation is measured, which is related to a hardness index number. Lower indentation index number is associated with soft materials, while higher indentation index number is associated with hard materials. Hardness is a relative quantity which is related to the binding forces of molecules and the different components in a bulk material, just as strength and modulus are (Callister, 2007). Correlating hardness and tensile properties of a nanocomposite material gives a direct relationship, whereby both properties increase with an increase in the resistance to elastic and plastic deformation. Specifically, hardness property of layered silicate epoxy nanocomposites has shown significant increment with organoclay filler volume fraction (Salahuddin, 2004). This is because, there has been evidences of improvement in micro-hardness property of in-situ and particulate filled epoxy nanocomposites with reduced inter particle distance at high nano-silica content prepared so far (Bugnicourt et al., 2007; Shen et al., 2006; Xue et al., 2006; Zhang et al., 2006). 2.21 Damage Mechanics, Microstructure and Fractography Materials behavior and Damage mechanics is concerned with mechanicsbased analyses of microstructural defects in solids responsible for changes in their response to loading. Microstructural defects can appear as cracks, voids, slipped regions with a spatial distribution within the volume of a solid. If a solid contains oriented elements in its microstructure such as fillers or fibres, homogeneity is sacrificed to in-homogenous conformational structural differences, giving room to heterogeneous materials. It has been recorded that the theoretical shear strength of a perfect crystal is two to three folds greater than the shear strength of a polycrystalline material containing the same crystals measured experimentally. Thus, overall properties of a material including ultimate strength, elongation at
102
break, yield stress, fracture toughness, creep resistance, stiffness and toughness properties are all structure-sensitive properties. On the other hand, properties such as modulus of elasticity together with density, melting point, specific heat and coefficient of thermal expansion, are structure insensitive properties. On the basis of continuum damage mechanics explained (Krajcinovic, 1989), the elastic parameters of a polycrystalline material depends to a large extent on the type of constituent crystal that makes it a stable mechanical system of atoms held together by interatomic forces. While tough and less brittle materials may be as a result of effective crack bridging mechanisms due to nanoparticle inclusions in an epoxy matrix filled with thermoplastic material as shown in Figure 2.44. From scanning electron micrographs (SEM), nanoparticles, more or less, remain rigid within their host matrix; thermoplastic or thermoset, and have non-linear response to an applied force (Bagheri & Pearson, 2000; Liang & Pearson, 2009).
Figure 2. 44: Schematic representation of toughening mechanisms proposed for thermoplastic-modified epoxied; (1) Particle bridging, (2) Crack pinning, (3) Crack path deflection, (4) Particle yielding-induced shear banding, (5) Particle yielding, (6) Microcracking (Pearson & Yee, 1993)
103
However, the non-linear response of solid particles fillers and ultimately, their mechanical strength, are dependent on the type, distribution, size and orientation of the lattice defects in its structure. Such defects can be classified with respect to their geometry into; i.
Point defects, which arise from vacancies, interstitial and impurity atoms
ii.
Line defects, as a result of dislocations
iii.
Plane defects, as a result of slip planes and cracks
iv.
Volume defects, as a result of the existence of pores and cavities
Whereas elastic deformations result from shear forces acting on a material leading to slippages, inelastic deformation is as a result of twinning, stacking faults, phase changes and diffusional flow. In a broad sense therefore, and based on material response to an applied force, material behaviour is classified into ductile and brittle. Table 2.7 presents different response types associated with ductile and brittle materials. Table 2. 7: Ductile and brittle response of solids Response type
Ductile
Brittle
Mode of microstructural changes
Slip, twinning, diffusional flow
Micro-crack nucleation and growth
Location
Slip plane
Cleavage plane
Cause
Shear stress
Tensile and shear stress
Failure mode
Localization into a shear band, necking
Localization into a macrocrack or fault
Change in elastic parameters
Minimal
Substantial
Residual strain
Substantial
Minimal
History recording parameter
Inelastic strain
Relative loss of integrity (damage)
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However, no material is totally ductile or totally brittle. A material may therefore be classified as being ductile or brittle if the persistence of the collective response types dominates the other during the process of deformation or energy dissipation at a particular point in time. Experimentally, it is possible to determine mode of failure or material behavior by examining the fractured surfaces of the failed material (Zunjarrao & Singh, 2006) by fractography. FRACTOGRAPHY– A word coined from two latin words; “Fractus” meaning “fracture” and “Grapho” meaning “descriptive treatment”, by Carl A. Zapffe, (Zapffe & Clogg Jr., 1945), following his discovery of a means for overcoming the difficulty of bringing the lens of a microscope sufficiently near the jagged surface of a fracture to disclose its details within individual grains. The main objective of fractography is to examine fractured surfaces, analyze fracture features and to attempt to link observed or prominent features like topography of the fractured surface to the cause(s) and / or the basic mechanisms of fracture (McCall, 1972). Composites failure may occur in different ways and for different reasons. However, the main reason for probing into the cause of failure of a composite material is to prevent a reoccurrence of the fracture, from the initial stage, during design. In acceding to this fact therefore, the most important source of information of a failed material is the fractured surface which holds detailed information on the failure history of the material, evidence of loading history, environmental effects and material quality. However, the examination of fractured surfaces and extracting vital information therefrom can only be possible using high resolution electron microscopy techniques; scanning electron microscopy (SEM) and transmission electron microscopy (TEM), or advanced surface characterization techniques as atomic force microscopy (AFM).
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2.21.1
Modes of Fracture
Fracture in engineering materials can take place across the grain (trans or intragranular), or along the grain boundaries (inter-granular). Basically, 4 modes of fracture exist (Kerlins & Philips, 1987); dimple rupture, cleavage, fatigue and decohesive rupture. Each of the four modes has a characteristic fracture surface appearance and a mechanism or mechanisms by which the fracture propagates. [i.]
Dimple rupture results when micro-voids nucleate at regions of localized
strain discontinuity, such as that associated with second-phase particles, inclusions, grain boundaries and dislocation pile ups. As a result of an overload, a material can develop micro-voids rapidly, which grow, coalesce and eventually lead to a fracture. Cup-like depressions known as dimples, on the fractured surface are fingerprints as a result of micro-voids coalescence. The size of a dimple depends on the number and distribution of micro-voids that are nucleated. Figures 2.45 and 2.46 presents different types of dimples formed when micro voids coalesce in a composite material as a result of maximum stress concentrations in; (a.) tension, (b.) shear and (c.) tensile tearing.
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(a.)
(b.)
(c.) Figure 2. 45: Effect of direction of maximum stress (σmax) on the shapes of dimples formed by microvoid coalescenceInfluence of direction of maximum stress ( max) on the shapes of dimples formed by microvoid coalescence. (a) In tension, equiaxed dimples are formed on both fracture surfaces. (b) In shear, elongated dimples point in opposite directions on matching fracture surfaces. (c) In tensile tearing, elongated dimples point toward fracture origin on matching fracture surfaces (Victor Kerlins, 1987)
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Figure 2. 46: Formation of elongated dimples under tear and shear loading conditions. (a) Tear fracture (b) Shear fracture (Kerlins & Philips, 1987).
[ii.]
Cleavage is a low energy fracture that propagates along well-defined low
index crystallographic planes known as cleavage planes. Theoretically, a cleavage fracture should have perfectly matching faces and should be completely flat and featureless. Figures 2.47 and 2.48 are schemes of cleavage fracture showing the effect of sub-grain boundaries.
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Figure 2. 47: Schematic of cleavage fracture formation showing the effect of subgrain boundaries (a) Tilt boundary (b) Twist boundary (Kerlins & Philips, 1987)
Figure 2. 48: Types of cleavage fractures (a) Twist boundary, cleavage steps, and river patterns in a material that was fractured by impact (b) Tongues (arrows) on the surface of a material that fractured by cleavage (Victor Kerlins, 1987)
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[iii.]
Fatigue fracture is as a result of cyclic or repetitive loading. This leads to
crack initiation and growth through most of length and breadth of the material until catastrophic breakage. This type of fracture occurs in three (3) stages, viz; Stage I: Initiation stage, Stage II: Propagation stage, Stage III: catastrophic fracture. In the initiation stage, there may be slip plane cracking of active slip systems along crystallographic planes but changes directions at discontinuities such as grain boundaries, while the growth stage is strongly influenced by microstructure and mean stress concentrations. At large plastic strain amplitudes, fatigue cracks may initiate at grain boundaries. A typical stage I fatigue crack is presented in Figures 2.49 and 2.50. Features at the initial stage normally observed on high-cycle lowstress fractures, are faceted and may not exhibit fatigue striations prominent in stages II and III. The profile of the fatigue fracture can also vary from saw-tooth to groove-type fatigue fracture profiles, depending on the material and state of stress.
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Figure 2. 49: Mechanism of fatigue crack propagation by alternate slip at the crack tip. Sketches are simplified to clarify the basic concepts. (a) Crack opening and crack tip blunting by slip on alternate slip planes with increasing tensile stress. (b) Crack closure and crack tip re-sharpening by partial slip reversal on alternate slip planes with increasing compressive stress (C. A. Zapffe & Clogg (Jnr.), 1944)
Figure 2. 50: Saw-tooth and groove-type fatigue fracture profiles. Arrows show crack propagation direction (a) Distinct saw-tooth profile (aluminum alloy) (b) Poorly formed saw-tooth profile (steel) (c) Groove-type profile (aluminum alloy) [iv.]
Decohesive rupture occurs when the surface of a fractured material
exhibits little or no bulk plastic deformation and does not occur by dimple fracture, cleavage or fatigue fracture. This type of fracture is generally as a result of reactive environment or a unique microstructure. It is however, associated with
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rupture along grain boundaries as shown in Figure 2.51. This is because grain boundaries are low energy spots in any multicomponent material. They form easy paths for diffusion and sites for segregation of such elements as hydrogen, sulphur, phosphorous, antimony, arsenic, carbon, and halide ions such as chlorides, as well as routes of penetration by low melting point metals such as gallium, mercury, cadmium and tin. The presence of such elements at the grain boundary, is capable of reducing to a great extent the cohesive strength of the material at the boundaries and promote decohesive rupture.
Figure 2. 51: Schematic illustration of decohesive rupture along grain boundaries. (a) Decohesion along grain boundaries of equiaxed grains. (b) Decohesion through a weak grain-boundary phase. (c) Decohesion along grain boundaries of elongated grains (Kerlins & Philips, 1987; J L McCall, 1972)
It should be noted that decohesive rupture cannot be caused by any unique fracture process, but by several different mechanisms which may involve the weakening of atomic bonds, the reduction in surface energy required for local deformation, molecular gas pressure, the rupture of protective films and anodic dissolution at active sites, which are associated with hydrogen embrittlement and stress-corrosion cracking (SCC).
2.21.2
Fractography by Microscopy Technique
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Recent advances in electron microscopy – SEM/TEM, acoustic emission microscopy technique (AEMT), atomic force microscopy (AFM), scanning tunnelling microscopy (STM) and a host of other advanced microscopy techniques of non-destructive testing, have revealed that composite materials response to applied force / load largely depends not only on the basic structure of the matrix but also on the type and distribution of defects in the matrix. Heavily cross-linked thermosetting polymers such as epoxies and certain flexible chain thermoplastic polymers which undergo crazing may show brittle behaviour in tension but can undergo significant plastic deformation in compression. Tensile brittleness arise as a result of low fracture toughness, while plastic deformation results from premature fracture precipitated in craze matter, due to acquisition of ill-adhering inorganic particulate inclusions of larger than micron size by the crazes during their growth. However, it has been found that, the presence of nanoparticles in epoxy induces various fracture mechanisms (Wetzel et al., 2006). They highlighted the fracture mechanisms to include crack deflection, plastic deformation and crack pinning or trapping. These fracture mechanisms are mostly prevalent in thermoset composites more than in thermoplastic composites. However, improving on the fracture toughness of epoxy filled composites may involve among other techniques, the inclusion of elastomer nanoparticles or elastomer coated nanoparticles, in the thermoset matrix. A particular attractive mechanism of toughening composites made of epoxies is by a crack-trapping process through the use of a well-adhering set of tough fibres which force the brittle crack to bow around the reinforcing fibres. Decreasing the craze flow stress below the critical threshold strength defined by the size of inclusions of pre-packaged, stressactivated diluent plasticizers, has been suggested (Argon et al., 1994; Argon &
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Cohen, 2003), as one of the effective ways to toughen crazable epoxies. Awaji et al, 2002, obtained results that revealed that the ratio of thermal expansion coefficients of the particle and matrix has a marked effect on residual stresses and the estimated residual stresses were of significant magnitude to generate lattice defects such as dislocations around the particle, while the nano-size particles within a matrix is only able to create dislocations around the particle (Awaji et al., 2002b). In another investigation by Liang and Pearson in 2009, on toughening mechanisms of epoxy-silica nanocomposites (ESNs) filled with micro-size glass spheres, epoxy matrix crosslink density was found to be inversely proportional to composite toughness. Low crosslink density has been reported to increase fracture toughness of ESNs which is as a result of zone shielding mechanism involving matrix plastic deformation (Liang & Pearson, 2009). Another reason may be that, absence of matrix plastic deformation results in brittle fracture in most thermoset composite materials (Papakonstantopoulos et al., 2007; Wachtman, 2009; Wiederhorn, 1984).
In a related fractography investigation on nanoparticle-modified epoxy polymer samples, Johnsen et al, 2007, discountenanced toughening mechanisms such as crack pinning, crack deflection and immobilised polymer chains, rather, microscopy of the samples showed evidence of de-bonding of the nanoparticles and subsequent plastic void growth (Johnsen et al., 2007). Furthermore, crack propagation in epoxy composites and associated fractography have earlier been published (Yamini & Young, 1979).
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Chapter Three Materials and Methods 3.1
Clay Materials
The first sample, spent oil base drilling mud (SOBM) which refers to used drilling mud that has lost its rheological properties and which lie as waste at rig sites, was collected from the waste pits at rig sites in 2 litres polyethylene terephthalate (PET) bottles. The sample which hitherto is formulated with a base fluid – oil or pseudo oil, bentonite, shale and formation clays picked up during drilling, weighting agents, deflocullants, fluid loss reducers, shale control inhibitors, defoamers, surfactants, dispersants, viscosifiers, emulsifiers, and corrosion inhibitors, contained more than 50% clays or solid content in a base fluid – oil as the carrier. Oil content was 31.1 weight %, water 16.9 weight % and solids 52 weight %. Chloride content was 35,000g/l, alkalinity of 8.5 and average specific gravity of solids as 2.52gcm-3. Furthermore, all reagents used were of analytical grade and conformed to the specification of the Committee on Analytical Reagents of the American Chemical Society (ACS). The clay sample was incinerated in open air, to burn off base oil used in its formulation and to recover the solid content. The sample was then sieved in order to break up the clay crumbs into aggregate particles sizes. This helped to loosen the clay particles and prepare the sample for high temperature treatment with its left-over oil base in a laboratory kiln at 950˚C for 2hours, hence solvothermal synthesis. After incineration, oil and grease (O/G) on cuttings was measured as 3,201mgkg-1, while total petroleum hydrocarbon (TPH) content on cuttings was 1.898mgkg-1. This information was however supplied with the clay samples.
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The second material of interest used in this study was obtained from mobile and fixed thermal desorption units at Onne, Rivers State, Port Harcourt, Rivers State, Nigeria. The thermally desorbed ash appeared as aggregate particles with little or no oil and grease content. At the end of solvothermal synthesis (calcination) and thermal desorption, an egg-yolk-yellow core clay powder and a dark brown powder were obtained respectively. Both samples were analysed to have the following composition presented in Table 3.1; Table 3. 1: Composition of clay samples used for this study Composition
Calcined SOBM (Weight %)
TDU Ash (Weight %)
CaCO3
-
5.27
K2O
0.25
0.19
Na2O3
0.52
0.22
CaO
94.26
2.95
MgO
0.87
0.15
Fe2O3
2.02
1.08
BaO
-
0.07
Al2O3
0.62
0.15
PbO
Negligible
