Mechanical Properties of PA6/MMT Polymer ... - ScienceDirect

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ScienceDirect Procedia Engineering 96 (2014) 75 – 80

Modelling of Mechanical and Mechatronic Systems MMaMS 2014

Mechanical properties of PA6/MMT polymer nanocomposites and prediction based on content of nanofiller Branislav Duleba a* , Emil Spišák a, František Greškovič a a

Technical University of Kosice, Faculty of Mechanical Engineering, 74 Masiarska St., 040 01 Kosice, Slovakia

Abstract The possibility of creating new polymers with desired properties is now harder, so there is great place for modification of known thermoplastic with addition of fillers. At conventionally used fillers such as talc, calcium carbonate or glass fibers is normal degree of filling up to 40%. New nanocomposite fillers can improve all kinds of properties with small degree of filling. This study is aimed on improvement of basic mechanical properties of well-known polyamide 6 (PA6) thermoplastic with addition of three different types of nanofillers based on montmorillonite clay. Two fillers – Cloisite 30B and Cloisite 93A were organically modified, while filler Cloisite Na+ was in its pure form. © 2014The TheAuthors. Authors. Published by Elsevier Ltd. © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of organizing committee of the Modelling of Mechanical and Mechatronic Systems MMaMS (http://creativecommons.org/licenses/by-nc-nd/3.0/). 2014. Peer-review under responsibility of organizing committee of the Modelling of Mechanical and Mechatronic Systems MMaMS 2014 Keywords: polymer composites; nanocomposites; Cloisite filler; montmorillonite; PNC

1. Polymer composites and nanocomposites The development of nano-particle reinforced polymer composites is presently seen as one of the most promising approaches in the field of future engineering applications. The unique properties of at least some of these nanoparticles (e.g., MMT, CNTs) and the possibility of combining them with conventional reinforcements (e.g., carbon-, glass- or aramid-fibres), has led to an intense research in the field of nanocomposites. [1] One widely used nanosized filler fot polymers is clay, especially montmorillonite (MMT). Inside of the multi-layered structure of MMT the sodium ions can be replaced by alkyl ammonium ions, which improve intercalation and exfoliation during preparation.

* Corresponding author. Tel.: +421-55-602-3549; E-mail address: [email protected]

1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of organizing committee of the Modelling of Mechanical and Mechatronic Systems MMaMS 2014 doi:10.1016/j.proeng.2014.12.100

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Branislav Duleba et al. / Procedia Engineering 96 (2014) 75 – 80

Nomenclature A11 Hamaker constant of unmodified montmorillonite [J] A22 Hamaker constant of polymer [J] A33 Hamaker constant of organic modifier [J] Wa adhesion work [J/m2] d Wa adhesion work due to dispersion forces [J/m2] h Wa adhesion work based on hydrogen bonds [J/m2] d spacing between two particles [nm] 1.1. Polymer composites and nanocomposites It has been more than 50 years since physicist Richard Feynman delivered his lecture titled “There is plenty of room at the bottom” to the American Physical Society at Caltech meeting. Feynman considered the possibility of direct manipulation of individual atoms as a more powerful form of synthetic chemistry than those used at the time. The talk is considered to be a seminal event in the history of nanotechnology, as it inspired the conceptual beginnings of the field decades later. From then on, the technological and scientific mastery of the nanometric scale has continually become stronger due to new research tools and theoretical and experimental developments. In this scenario, the worldwide nanotechnology market, in the next five years, is expected to be on the order of one trillion dollars. [2] A polymer composite is made by the combination of a polymer and synthetic or natural inorganic filler. Fillers are employed to improve the desired properties of the polymer or simply reduce the cost. Nowadays, the application of polymer composites as engineering materials has become state of the art. Polymer composites with improved mechanical, thermal, barrier and fire retardancy properties are widely used in very large quantities in variety of applications. However by the application of conventional fillers such as talc, calcium carbonate, fibers, etc., it often requires to use a large amount of filler in the polymer matrix to have significant improvements in the composite properties which may result to some other undesired properties such as brittleness or loss of opacity. [3, 4] A polymer nanocomposite is defined as a composite material in which at least one dimensions of at least one component is in the nanometer size scale (< 100 nm). In recent years the characterization and control of structures at the nanoscale have been studied, investigated and exploited by the learning from the natural surroundings. Consequently the nanocomposite technology has emerged as an efficient and powerful strategy to upgrade the structural and functional properties of synthetic polymers. This is the new nanocomposite science, so referred recently in nanotechnology, and was started by the Toyota report on the superior improvement in the properties of nylon-6 by the preparation of exfoliated nylon-6/clay nanocomposites. This new material developed for timing belts, that only had 4.2 wt. %, had an increase of 40 % in the rupture tension, 68 % in the Young modulus and 126 % in the flexural modulus as well as an increase in the heat distortion temperature from 65 ºC to 152 ºC in comparison with pure polymer. From then on, several companies introduced thermoplastic nanocomposites, such as polyamide and polypropylene, in automotive applications [5 - 8]. 1.2. Montmorillonite nanoclay Nanoclays are clay minerals optimized for use in clay nanocomposites– multi-functional material systems with several property enhancements targeted for a particular application. Polymer-clay nanocomposites are an especially well-researched class of such materials. Nanoclays are a broad class of naturally occurring inorganic minerals, of which plate-like montmorillonite is the most commonly used in materials applications. Montmorillonite consists of ~ 1 nm thick aluminosilicate layers surface-substituted with metal cations and stacked in ~ 10 μm-sized multilayer stacks. The stacks can be dispersed in a polymer matrix to form polymer-clay nanocomposite. Within the nanocomposite, individual nm-thick clay layers are fully separated to form plate-like nanoparticles with very high (nm × μm) aspect ratio. Even at low nanoclay loading (a few weight %), the entire nanocomposite consists of interfacial polymer, with majority of polymer chains residing in close contact with the clay surface. This can dramatically alter properties of a nanocomposite compared to the pure polymer. Potential benefits include increased

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mechanical strength, decreased gas permeability, superior flame-resistance, and even enhanced transparency when dispersed nanoclay plates suppress polymer crystallization. [9 - 12] Idealized structure montmorillonite clay (pre-organic modification) – Fig. 1, showing two tetrahedral-site sheets fused to an octahedral-site sheet. Red spheres are oxygen atoms and grey spheres are Si, Al, or Mg atoms. Purple spheres represent Na or K ions. SEM image of refined MMT is shown on Fig. 2. Since the aspect ratio of exfoliated MMT is p = 100 to 2000, the specific surface is in the order of 800 m2/g. Thus, a small amount of anisometric particles leads to large effects. On a molecular level, the surface energy of clay particles is high. As a result, adsorbed molecules have a tendency to be strongly bonded in the layer adjacent to the clay surface. This engenders a solid-like behavior of the 2–3 nm thick surface layer and progressive reduction of viscosity with distance to the bulk liquid viscosity at about 15 nm.

Fig. 1 Idealized structure of montmorillonite

Fig.2 SEM image of refined MMT

2. Compatibility between the polymer matrix and organically modified clay Interactions between clay and polymer can be evaluated by estimating the effective Hamaker constant of clay/polymer system. Due to the fact that the chemical structure of organic modifiers Cloisite are known, Hamaker constants can be calculated using the Group Contribution method [13], [14], [15]. Values of the Hamaker constants for organic modifiers are calculated in the range from 5.3 x10 -20J to 6.0x10-20J. Hamaker constant is higher for amides (about 12x10 -20J), as they have functional polar groups that are capable to form hydrogen bonds. Effective Hamaker constant for the composite composed of a polyamide matrix and organically modified clay is still positive (about 0.1 × 10 -20J). At polyethylene is the Hamaker constant close to zero or negative, polytetrafluoroethylene always have negative values of Hamaker constant. Hamaker constant values for organic modifiers were derived using study of methods of groups according Carrey [13] and the data of surface tension according to studies of Jasper [14]. The non-polar polymer matrix has a negative Hamaker constant, which leads to poor dispersion and formation of tactoides. This suggests that the organically modified clays can be better dispersed in the polyamide matrix. Effective Hamaker constant for the system of modified clay and polymer matrix (A123) is given by Eq. 1. ‫ܣ‬ଵଶଷ ൌ ൫ඥ‫ܣ‬ଵଵ െ ඥ‫ܣ‬ଷଷ ൯Ǥ ൫ඥ‫ܣ‬ଶଶ െ ඥ‫ܣ‬ଷଷ ൯

(1)

The general conclusion based on the study of microparticulate theory at filled polymers is that the strength of the composite material may be maximized where interfacial adhesion between the surface of the filler and the polymer matrix is optimized. The work of adhesion Wa can be quantified to determine the interfacial bond strength of clay and filler surface the polymer matrix and is given in Eq. 2. ܹ௔ ൌ ܹ௔ௗ ൅ ܹ௔௛

(2)

In the case of polymers, such as polyethylene and polystyrene, which do not have the ability to form hydrogen

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bonds with the clay-based filler, only the dispersion forces are responsible for the interfacial adhesion. The work of adhesion between two adjacent particles on the basis of dispersion forces can be expressed as in Eq. 3. ܹ௔ௗ ൌ െ

஺

(3)

ଵଶగௗ మ

The value of d, particle limit distance is usually expressed as value from 0.165 to 0.185 nm [16]. In the case of PA6 polymer, the improvement of mechanical properties can be attributed to a strong interaction between the matrix, organic modifier and layers of clay, through the formation of hydrogen bonds. Binding energy of N-H hydrogen bonds is in the range 12-20kJ/mol. Specific surface area of exfoliated MMT clay is 700-786 m2/g and cation exchange capacity is from 0.92 to 0.95 meq/g. Organic modifier in the case of the filler Cloisite 93A does not have any functional groups which would form hydrogen bonds. This means that the work of adhesion arises only from the dispersion forces. The work of adhesion of hydrogen bonds to the unmodified clay can be determined by different methods. The string length of PA6 between two anchorages of hydrogen chains is 1.67 nm. At surface of 1m2 can be located 3,58x1017 hydrogen bonds (Avogadro’s constant), the energy of bonds per unit area of one square meter is the 1,19x10-5 kJ/m2. Calculation of adhesion work between surface and montmorillonite clay are listed in the Table 1, with usage of Hamaker constant calculations. Table 1. Calculated values of adhesion work Hamaker constant A123 [(10-20) J]

Wad [(10-2) J/m2]

Wah [(10-2) J/m2]

Wa [(10-2) J/m2]

PA6/Cloisite 30B (A)

0,35

0,28

5,00

5,28

PA6/Cloisite 93A (B)

0,44

0,35

-

0,35

PA6/Cloisite Na+ (C)

9,70

7,70

1,19

8,89

These results demonstrate that a significant proportion of the total adhesion work of PA6/Cloisite 30B comes from hydrogen bonds, which are absent in the PA6/Cloisite 93A system. The total adhesion work is highest in PA6/Cloisite Na+ system, which is unmodified clay. Major proportion of the total adhesion work in this system comes from the adhesion of dispersion forces. Layers in the unmodified clays are close together – 1 nm and adhesion between them is strong. As a result, the unmodified clay exfoliation is not supported within the polymer matrix and the specific surface area is small. For this reason, the total adhesive force between the clay and the polymer is low. However, in the case that nonmodified clay can be exfoliated in some way, it would be possible to prepare nanocomposites with the highest enhanced characteristics. 3. Preparation and testing of polymer/clay nanocomposites A concentrate containing 10% nanofiller (fillers Cloisite 30B, Cloisite 93A, Cloisite Na+) in the PA6 polymer matrix was prepared at the Institute of Engineering Polymeric Materials and Colors in Torun, Poland (Instytut Inżynierii Materiałów Polimerowych i Barwników, Toruń). The processing of materials was performed in a Buhler twin-screw extruder BTSK 20/40D. Samples for tensile tests were molded by an ARBURG Allrounder 370S injection molding machine. Measured values of tensile strength and impact strength are showed on Fig. 3 and Fig. 4.

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PA-C2

PA-C3

PA-C4

PA-C5

PA-C6

PA-C3

PA-C4

PA-C5

PA-C6

PA-C1

PA-C2

PA-B6

PA-B5

PA-B4

PA-B3

PA-B2

PA-B1

PA-A6

PA-A5

PA-A4

PA-A3

PA-A2

PA

100 90 80 70 60 50 40 30 20 10 0 PA-A1

Tensile strength σm[MPa]


Fig.3 Tensile strength of tested samples

Impact strenght aCN [kJ.m-2]

140 120 100 80 60

40 20 PA-C1

PA-B6

PA-B5

PA-B4

PA-B3

PA-B2

PA-B1

PA-A6

PA-A5

PA-A4

PA-A3

PA-A2

PA

PA-A1

0

Fig.4 Impact strength of tested samples

Based on the measurement results, we can conclude that the addition of small amount of nanofiller Cloisite 30B had great effect on the mechanical properties of the final product. The tensile strength after addition of 6% of nanofiller increased by 41.4% to 91.9 MPa. For the material with the addition of 6% of nanofiller Cloisite 93A increase of 33% (86.5 MPa) in tensile strength was measured. Material containing 6% filler Cloisite Na+ tensile strength decreased by 11.4%. Impact strenght increased at all three materials. While at material PA6/Cloisite 93A was the increase minimum, polyamide with addition of 6% of Cloisite Na+ increased its impact strenght from 102,6 kJ.m-2 to 126,4 kJ.m-2. SEM structures of tested materials are shown on Fig. 5 – Fig. 7. From these images we can conclude, that while in the materials PA6/Cloisite 30A (A) and PA6/Cloisite 93A exfoliation took place, in the material with Cloisite Na+ nanofiller the clusters of particles and undigested filler are visible. Unmodified MMT based fillers can be processed by ultrasonic dispersing or are suitable for the modification by ion exchange (organofilization) to obtain the characteristic properties. The physical mixture of polymer and layered silicate does not form nanocomposite. This situation is similar to the mixture of polymers with different compositions, where, in most cases, are separated into separate phases. In miscible polymer-clay system are weak physical interactions between the organic and the inorganic component of the composition, resulting in unsatisfactory mechanical and thermal properties. These mixtures typically respond to conventionally filled polymers.

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Fig. 5 PA6/Cloisite 30B

Fig. 6 PA6/Cloisite 93A

Fig. 7 PA6/Cloisite Na+

In contrast to this, the strong mutual interaction between the polymer and the modified layered silicate lead to dispersion of the silicate particles in the organic phase at the level of nanometres. Consequently nanocomposites exhibit excellent properties as opposed to processed silicate composites, which are formed as microcomposites. These conclusions were also drawn based on the calculation of the first part of article. Conclusion Based on this study we can conclude that addition of only small amount of nanofiller can rapidly change properties of polyamide. At micro-fillers such as talc, calcium carbonate, short- or long glass fibers it is normal to fill polymer with large amount of filler up to 40%. This study proves, that organically modified montmorillonite fillers such as Cloisite 30B and Cloisite 93A can be incorporated to PA6 matrix without modification, while at Cloisite Na+ the exfoliation is not supported within the polymer matrix and the specific surface area is small and for that reason the total adhesive force between the clay and the polymer is low and it is recommended to modify the filler with organic modifier before application. Acknowledgements Article is the result of the Projects implementation: University Science Park TECHNICOM for Innovation Applications Supported by Knowledge Technology, ITMS: 26220220182, supported by the Research & Development Operational Programme funded by the ERDF, project PIRSES-GA-2010-269177 and VEGA 1/0872/14. References [1] F.H. Gojny, M.H.G. Wichmann, U. Köpke, B. Fiedler, K. Schulte, Carbon nanotube-reinforced epoxy-composites, Composites Science and Technology 64, (2004), 2363-2371. [2] P. Anadao, Nanocomposites - New Trends and Developments, Intech, 2012. [3] A. Olad, Polymer/Clay Nanocomposites, Advances in Diverse Industrial Applications of Nanocomposites, ISBN: 978-953-307-202-9, 2011. [4] S. S. Ray, M. Okamoto, Polymer/layered silicate nanocomposites, Progress in Polymer Science 28, (2003), 1539-1641. [5] A. Okada, U.S. Patent 4739007, 1988. [6] M. Kawasumi, U.S. Patent 4810734, 1989. [7] S. Pavlidou, C.D. Papaspyrides, A review on polymer-layered silicate nanocomposite, Progress in Polymer Science, 33 (2008) 1119–1198. [8] Q.T. Nguyen, D.G. Baird, Preparation of polymer-clay nanocomposites, Advances in Polymer Technology, 25 (2006) 270-285. [9] E. P. Giannelis, Polymer Layered Silicate Nanocomposites, Advanced Materials 8 (1996) 29-35. [10] E. A. Vaia, K. D. Jandt, E. Kramer, E. P. Giannelis, Microstructural evolution of melt intercalated polymer-organically modified layered silicates nanocomposites Chem. Mater 8, (1996), 2628-2635. [11] P. C. LeBaron, Z. Wang, T. J. Pinnavaia, Polymer-layered silicate nanocomposites: an overview, Applied Clay Science 15, (1999), 11-29. [12] A. B. Morgan, Nanoclays for Composites, Material Matters 2, (2007), 20-25. [13] J. Vial, A. Carre, Calculation of Hamaker constant and surface energy of polymers by a simple group contribution method, International Journal of Adhesion and Adhesives 11, (1991), 140–143. [14] J. J. Jasper, J. Phys, The Surface Tension of Pure Liquid Compounds, Journal of Physical and Chemical Reference Data 4, (1972), 952-955. [15] A. W. Neumann, S. N. Omenyi, C. J. Van Oss, Negative Hamaker coefficients, Colloid and Polymer Science 257, (1979), 737-744. [16] J. N. Israelachvili, Intermolecular and Surface Forces, London 1992, 704p.