Clay Nanocomposite

Polymer-Plastics Technology and Engineering

ISSN: 0360-2559 (Print) 1525-6111 (Online) Journal homepage: http://www.tandfonline.com/loi/lpte20

Technical Relevance of Epoxy/Clay Nanocomposite with Organically Modified Montmorillonite: A Review Sagheer Gul, Ayesha Kausar, Bakhtiar Muhammad & Saira Jabeen To cite this article: Sagheer Gul, Ayesha Kausar, Bakhtiar Muhammad & Saira Jabeen (2016) Technical Relevance of Epoxy/Clay Nanocomposite with Organically Modified Montmorillonite: A Review, Polymer-Plastics Technology and Engineering, 55:13, 1393-1415, DOI: 10.1080/03602559.2016.1163593 To link to this article: http://dx.doi.org/10.1080/03602559.2016.1163593

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Date: 04 February 2017, At: 03:20

POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING 2016, VOL. 55, NO. 13, 1393–1415 http://dx.doi.org/10.1080/03602559.2016.1163593

Technical Relevance of Epoxy/Clay Nanocomposite with Organically Modified Montmorillonite: A Review Sagheer Gula,b, Ayesha Kausara, Bakhtiar Muhammadb, and Saira Jabeena,b a

Nanosciences Division, National Center for Physics, Quaid-i-Azam University Campus, Islamabad, Pakistan; bDepartment of Chemistry, Hazara University, Mansehra, Pakistan ABSTRACT

KEYWORDS

This review covers almost all known categories of compounds used for nanoclay surface modification with special emphasis on organic modification of layered silicate montmorillonite. Commonly used organic modifiers include quaternary ammonium ions, quaternary phosphonium ions, and amino acids. Dispersion of organomodified nanoclays in epoxy is particularly focused upon in this article. Epoxy-based materials are used as convenient matrices for montmorillonite dispersion since years due to superior properties of resulting polymeric nanocomposites, such as mechanical strength, electrical conductivity, flammability, and thermal stability. Owing to their high performance epoxy nanocomposites have endless applications in aerospace, automotives, construction, electrical, adhesives, and coating industries.

Aerospace; epoxy; montmorillonite; organic modifiers; strength

GRAPHICAL ABSTRACT

Introduction In order to achieve better material properties, naturally available nanoclays are surface-modified. Surface modification of nanoclays has got significant importance during the last years because of their use in the production of polymer clay nanocomposites[1]. These nanocomposites reveal better material properties as compared to pristine polymers. Surface modification of nanoclays with some organic surfactant is necessary to increase the compatibility between the polymer and the clay[2].

The better compatibility between the matrix and the filler may lead to enhanced physical properties. Nanoclays has layered structure possessing small intercalary cations that can easily be replaced by organic cations (Figure 1). Common modifiers used for the organic modification of nanoclays are quaternary ammonium cations[3], quaternary phosponium cations[4], diamines[5], and amino acids[6]. Main issue after surface modification of nanoclay is its thermal stability specially when dealing with the melt-compounding technique for material synthesis. The organic chains may result in

CONTACT Ayesha Kausar [email protected] Nanosciences Division, National Center for Physics, Quaid-i-Azam University Campus, Islamabad 44000, Pakistan. Color versions of one or more of the figures in this article can be found online on at www.tandfonline.com/lpte. © 2016 Taylor & Francis

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Figure 1. Nanaoclay structure.

organic modification of nanoclays by either surface modification or intercalations. They are prone to high temperatures and results in degradation[7]. However, several studies have revealed that the thermal stability of phosphonium organoclays was superior to that of ammonium organoclays[8,9]. In naturally occurring nanoclays, the high cation exchange capacity (CEC) of sodium bentonite makes it an ideal host for surface modification[10]. Such organomodified nanoclays are dispersed into the polymer matrix where they act as reinforcing agent. Consequently material properties of the resultant nanocomposites are improved. Sometimes conventional alkyl ammonium cations are not suitable for surface modifications. In case of nonpolar polymers such as polyolefins, they do not lead to exfoliated nanocomposites. This is because of the absence of any positive interaction between the filler and the nonpolar matrices. The only reason for compatibility of alkylammonium cations–modified nanoclays with nonpolar polymers is the higher basal-plane spacing of the filler that results in its delamination like peels of an onion during shearing with the polymer. Achievement of such very high basal-plane spacing using commonly available surface modifiers is again not an easy task. It needs to practice some specific methods or special surface modifiers that can lead to a higher concentration of organic cations in the clay interlayers. Thus, higher basal-plane spacing is achieved due to reduced forces of attraction[11]. For this purpose, either long-chainlength surface modifiers are introduced or some polymerization reactions on the filler surface are used to graft polymer chains. The organic modification of layered silicates is completed through a very simple

Figure 2. Ion exchange mechanism.

ion-exchange reaction as shown in Figure 2. Aforementioned surface modification of nanoclay is necessary before the introduction into a polymer matrix. These organomodified nanoclays have been used with a number of polymer matrices for production of polymeric nanocomposites. Among these epoxy-based polymer matrices have acquired tremendous importance during the past decade due to their excellent mechanical, thermal, and chemical properties (Figure 3).[12,13] Epoxy resins reinforced with nanoparticles, especially layered silicates, have been used in many applications such as electronics, adhesives, and coatings due to the possibility of improving several properties (stiffness, strength, fire resistance, dimensional stability, and shrinkage) even at low filler loadings[14]. Modified nanoclays with variety of surfactants have been dispersed in epoxy resins in order to acquire desired properties for the nanocomposites. In this regard many authors have investigated an improvement of properties in epoxy/clay nanocomposites incorporated with alkylammonium-modified nanoclays[15]. Nanoclay particles possessing higher aspect ratio of about 1-nm to 10– 100-nm dimensions are found to be successfully incorporated in the epoxy matrix with the production of nanocomposites with improved bulk material properties. The common nanoclays used for production of epoxy/clay nanocomposites are typically montmorillonite (MMT). However, the use of saponite and fluorohectorite forms of clay, which have a structure different than MMT, is also not limited. Nanoclays possess properties such as ion-exchange capacity and surface activity usually with small inorganic ions like sodium in gallery providing surface for intercalation.

POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING

Figure 3. Epoxy nanocomposites features.

Moreover, these cations are replaceable with many organic compounds[16].

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and polystyrene (PS)/clay nanocomposites. Mechanical properties like temperature dependence of the storage modulus of these nanohybrids were investigated (Figure 4)[20]. Octadecylammonium cation was used to modify sodium montmorillonite (Na-MMT) clay for the preparation of polylactide (PLA)/layered silicate nanocomposites through melt extrusion technique. The PLA/layered silicate nanocomposite presented extraordinary improvement of material properties in both solid and melt states as compared to the pure polymer matrix without clay[21]. Commercially available octadecyl ammonium ion-modified Na-MMT nanoclay was incorporated in high-functionality epoxy resins. The morphology, thermal relaxations, and mechanical properties of organomodified nanoclay-based nanocomposites were studied[22]. Structure of different types

Organically modified clays Alkylammonium cations-modified nanoclays With the advent of studies on nanoclays different routes were practiced to modify the layered silicates[17]. However, the most convenient and straightforward route to modify the nanoclays was through cation exchange method. The very initial production of modified MMTs developed in 1990s used quaternary alkylammonium salts[3]. Various molecular environments have been adopted by these alkylammonium cations within the layered silicate structure of nanoclays. Afterward several types of alkylammonium cations with different structures with respect to chain length and tacity were used and are still in practice for organic modification. Nanaoclay modified with onium ions with alkyl carbon chain length up to eight carbons has been incorporated in epoxy. The interfacial affects on the reinforcement properties of polymer/organoclay nanocomposites have been studied[18]. MMT nanoclay has been modified with stearyl amine using the cation-exchange reaction to increase the compatibility of hydrophilic nanoclay with polypropylene (PP)[19]. Stearyl amine-modified nanoclay was successfully dispersed in the polymer matrix using melt-compounding technique. The resultant nanocomposites showed a considerable increase in dynamic storage moduli compared to pure PP. Smectite nanoclays were modified using quaternary ammonium cations like oligo (oxypropylene) diethyl methyl ammonium chloride [(C2H5)2(CH3)N1(O–iPr)25]Cl2 and methyl trioctyl-ammonium chloride, [CH3(C8H17)3 N1]Cl2. These organomodified clays were dispersed through in situ polymerization technique for the production of polymethylmethacrylate (PMMA)/clay

Figure 4. TEM micrographs of thin section of nanocomposites: (a) PMMA/methyl-trioctil-ammonium chloride (STN), (b) PMMA/ SPN, and (c) PS/oligo(oxypropylene)- diethyl-methyl-ammonium chloride (SPN). The arrows in panel (a) indicate the oriented collections of STN silicate layers[20]. Note: TEM, transmission electron microscopy; PMMA, polymethylmethacrylate; PS, polystyrene. © Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.

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of epoxy resins used for the incorporation of nanoclays is given in Figure 5. Commercially modified Na-MMT with stearyl ammonium ions was used for the preparation of nylon 6/clay nanocomposites using melt-compounding technique. The nanocomposites produced showed high strength, modulus, and heat distortion temperature compared with neat nylon 6[23]. A schematic figure depicting the compounding process in nylon/clay nanocomposite preparation is shown in Figure 6. Naturally occurring MMT modified with bis (2-hydroxyethyl) quaternary ammonium salt was incorporated in amine-cured epoxy polymer. The processing and chemical characterization of the nanocomposite was investigated[24]. Na-MMT modified with quaternary ammonium ion with formula [C4H9Nþ (CH2CH2OH)3Br ] was incorporated in styrene butadiene rubber to produce nanocomposites with better flame-retardant properties compared with pristine polymers[25]. Cloisite is a commercially available layered silicate. It was organically modified by quaternary ammonium cations such as dimethyl benzyl hydrogenated tallow ammonium salt, dimethyl dehydrogenated tallow ammonium salt, and methyl tallow bis-2-hydroxyethyl ammonium salt. The nanoclay was successfully

Figure 5. Epoxy resins and hardener as used for the nanocomposite synthesis.

dispersed in high-impact PS matrix for the preparation of polymer nanocomposites with better thermal, gas barrier, and tensile properties[26]. Tetrabutyl ammonium chloride, N-acetyl-N,N,N-trimethyl ammonium bromide, and hexadecyl trimethyl ammonium chloride intercalating agents were used to modify bentonite clay. The combined effect of ultrasound and nanoclay on adsorption of phenol was studied[27]. Long-chain quaternary alkylammonium-modified nanoclay was dispersed in styrene-(ethylene-co-butylene)-styrene triblock copolymer matrix to investigate the morphological, mechanical, and thermomechanical properties of resultant nanocomposites[28]. A number of quaternary alky ammonium salts like tetramethyl ammonium bromide, hexadecyl trimethyl ammonium bromide, benzyl triethyl ammonium bromide, octadecyl trimethyl ammonium bromide, and dodecyl trimethyl ammonium bromide were used for the organic modification of raw Na-MMT clay. These organoclays were then incorporated in the epoxy blends and were concluded as perfect reinforcing agents for material synthesis[29]. Six different types of quaternary ammonium salts were used to modify Brazilian clay from the state of Paraiba (north Brazil). Six modifiers used were diestearildimethylammonium chloride (DEDMA), dialquildimethylammonium chloride (DADMA), ditallowalkyldimethylammonium chloride (DTADMA), hexadecyltrimethylammonium chloride (HDTMA), alquildimethylbenzylammonium chloride (ADMBA), and fettalkyldimethylhydroxiethylammonium chloride (FADMHEA). These organomodified nanoclays were dispersed in polyolefinic systems through melt method to study the mechanical and thermal properties of the resultant nanocomposites[30]. Na-MMT was modified with quaternary ammonium cations like dimethyl methylbenzyl octadecyl (DMPO), methyl hydroxy ethyl octadecyl (MHEO), dimethyl dioctadecyl (DMDO), and trimethyl octadecyl (TMO) cation-exchange reaction. These organomodified nanoclays were incorporated in polylactide (PL), and the morphology of the resultant nanhybrids was investigated[31]. Special type of quaternary ammonium cations containing imidazolium ions like 1-Methyl 3(4-vinylbenzyl) imidazoliumchloride (VBIMCl), 1-Dodecyl-3-(4-vinylbenzyl) imidazolium chloride (VBIMCl), and 1-Hexyl-3 (4-vinylbenzyl) imidazoliumchloride (VBIMCl) were used to modify MMT clay. These organomodified nanoclays were successfully dispersed in PS matrix using in situ polymerization technique. PS/organoclay nanocomposites showed greater thermal stability compared to pure PS[32]. Thermal decomposition behavior and morphology (Figure 7) of the nanohybrid were investigated using conventional techniques[33]. A concise thermogravimetric analysis

POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING

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Figure 6. Schematic figure depicting the compounding process for preparing the NCH-CS using the clay slurry.

(TGA) data of phenoxy/clay nanocomposites is given in Table 1. Nanokaolinite clay commercially organomodified with an amino silane group was used to prepare polypropylene and high-density polyethylene/nanokaolinite clay composites through melt-intercalation method. These nanocomposites revealed better thermomechanical properties compared to pristine polymers[34]. Commercially modified nanoclay cloisite was used in the preparation of thermoplastic polyurethane (PU)/clay-based nanocomposites. Their morphological, thermal, chemical, and physical properties were examined[35]. Summary of nanoclays modified using quaternary ammonium cations is shown in Table 2. Alkylphosphonium cation-modified nanoclays A fewer studies and applications of quaternary phosphonium surfactants are available in the literature compared with quaternary ammonium surfactants. The

reason for lesser attention to the phosphonium surfactants is due to their lesser stability under a variety of conditions and complicated synthesis methods[36]. Nevertheless, phosphonium cations are successful counterparts in place of alkylammonium cations where processing of the polymers at elevated temperature is practiced. This is because the thermal stability of phosphonium organoclays is superior to that of ammonium organoclays[8,9]. Moreover, phosphonium salts have the ability to undergo a wide range of reactions. As a consequence, they behave differently relative to their ammonium counterparts because of the greater steric tolerance of phosphorus atom in phosphonium salts as compared to nitrogen of ammonium ion. Furthermore, its low-lying d-orbitals participate in the processes of making and breaking new chemical bonds[37]. These phosphonium surfactants are mainly composed of short alkyl chains, sometimes benzene rings, and usually a long alkyl chain[38]. Nanoclays

Figure 7. Low- (left) and high-magnification (right) TEM micrographs of Ph/VMT-ETO nanocomposite with 10 wt% clay[33]. Note: TEM, transmission electron microscopy. © Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.

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Table 1.

TGA data of phenoxyl and its nanocomposites in nitrogen atmosphere[33]. 2°C/min

Sample Phenoxy Ph/VMT-ETO 2 wt% Ph/Cloisite 30B 2 wt% Ph/VMT-ETO 5 wt% Ph/Cloisite 30 B 5 wt% Ph/VMT-ETO 10 wt% Ph/Cloisite 30 B 10 wt%

T5 (°C) 348 327 352 312 330 305 318

5°C/min T50 (°C) 389 392 399 378 394 349 369

T5 (°C) 372 339 372 327 351 322 351

10°C/min T50 (°C) 410 407 421 389 416 357 416

T5 (°C) 395 359 381 355 359 337 353

T50 (°C) 429 422 429 419 422 392 412

20°C/min T5 (°C) 396 373 403 351 384 357 370

T50 (°C) 440 440 454 411 451 391 432

© Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.

Table 2.

Alkylammonium cations–modified nanoclays.

Nanoclay Montmorillonite

Surfactant Polyether amine

Dispersion matrix Epoxy (Ex)

Montmorillonite

Stearyl amine

Polypropylene (PP)

Smectite

(i) Polymethylmeth-acrylate (PMMA) (ii) Polystyrene (PS)

Montmorillonite

(i) Oligo (oxypropylene) diethyl methyl ammonium chloride and (ii) Methyl trioctyle-ammonium chloride Octadecylammonium cation

Montmorillonite

Octadecyl ammonium ion

Epoxy (Ex)

Montmorillonite

Stearylammonium ions

Nylon 6

Montmorillonite

bis(2-hydroxyethyl)ammine

Epoxy (Ex)

Montmorillonite

N,N,N-tri(2-hydroxyethyl) N-butyl ammonium bromide. Dimethyl benzyl hydrogenated tallow ammonium salt

Styrene butadiene rubber (SBR) Polystyrene (PS)

Cloisite Bentonite

Montmorillonite Montmorillonite

Brazilian clay

(i) Tetrabutyl ammonium chloride (ii) N-acetyl-N,N,N trimethyl ammonium bromide (iii) hexadecyl trimethyl ammonium chloride Long-chain alkyl ammonium cation (i) Tetramethyl ammonium bromide (ii) Hexadecyl trimethyl ammonium bromide (iii) Benzyl triethyl ammonium bromide (iv) Octadecyl trimethyl ammonium bromide (v) Dodecyl trimethyl ammonium bromide (i) Diestearil dimethylammonium chloride (ii) Dialquil-dimethylammonium chloride (iii) Ditallowalkyldimethyl-ammonium chloride (iv) Hexadecyltrimethyl-ammonium chloride (v) Alquildimethylben-zylammonium chloride (vi) Fettalkyldimethylhydroxiethylammonium chloride

Study/properties Investigation of interfacial effects on reinforcement properties of polymer/organoclay nanocomposites Increase in dynamic storage moduli of nanocomposites compared to pure PP Temperature dependence of storage modulus of nanohybrids

References [18]

Improvement of materials properties in both solid and melt states as compared to pure polymer matrix Investigation of morphology, thermal relaxations, and mechanical properties of organomodified nanoclay composites Nanocomposites had high strength, modulus, and heat distortion temperature compared with neat nylon 6 Investigation of processing and chemical characterization Study of flame-retardant properties of nanocomposites formed Nanocomposites with better thermal, gas barrier, and tensile properties compared to pristine polymers Study of combined effect of ultrasound and nanoclay on adsorption of phenol

[21]

Styrene (ethylene-cobutylene)-styrene triblock copolymer Epoxy (Ex)

Investigation of morphological, mechanical, and thermomechanical properties of composites Organomodified nanoclay found as the best reinforcing agent

[28]

Polyolefinic systems

Composites were studied for their mechanical and thermal properties

[30]

Polylactide (PLA)

Phenol solution

[19] [20]

[22]

[23]

[24] [25] [26] [27]

[29]

(Continued)

POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING

Table 2.

Continued.

Nanoclay Montmorillonite

Montmorillonite

(i) Montmorillonite (ii) Vermiculite Nanokaolinite Cloisite

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Surfactant (i) (ii) (iii) (iv) (i)

Dimethyl methylbenzile octadecyl Methyl hydroxy ethyl octadecyl Dimethyl dioctadecyl Trimethyl octadecyl cation 1-Methyl 3(4vinylbenzyl) imidazoliumchloride (ii) 1-Dodecyl-3-(4-vinylbenzyl) imidazolium chloride (iii) 1-Hexyl-3 (4vinylbenzyl) imidazoliumchloride (i) Methyl, tallow, bis(2-hydroxyethyl) ammonium chloride (ii) Oleyl bis(2-hydroxyethyl) methyl ammonium chloride Amino silane group Commercially modified

Dispersion matrix

Study/properties

References

Poly lactide (PL)

Investigation of morphology of the resultant nanohybrids

[31]

Polystyrene (PS)

Nanocomposites with greater thermal stability compared to pure PS were prepared

[32]

Phenoxy resin

Thermal decomposition behavior of phenoxy/clay nanohybrids

[33]

(i) Polypropylene (ii) High-density polyethylene

Nanocomposites with better thermomechanical properties compared to pristine polymers Morphological, thermal, and chemical properties

[34]

Polyurethane (PU)

modified with phosphonium surfactants exhibit comparatively higher thermal stability than ammonium surfactant-modified organoclays. Moreover, these phosphonium compounds are investigated to enhance flame retardancy in the resultant nanocomposites[39]. Tetraoctyl phosphonium bromide and benzyltriphenyl phosphonium chloride were used to modify bentonite nanoclay using typical ion-exchange reaction. Such organomodified nanoclays were used to prepare polyamide 66 (PA 66) nanocomposites. Their thermal and mechanical properties were investigated[38]. Hexadecyltributylphosphonium cation was used to modify saponite nanoclay. Quaternary phosphonium-modified nanoclay was successfully dispersed in the polybutylene succinate (PBS) polymer matrix through melt-intercalation technique to prepare polymeric nanocomposites. The structure, material properties, melt rheological pattern, and biodegradability were investigated[40]. Synthetic fluoromica somasif nanoclay was successfully organomodified with tributylhexadecylphosphonium bromide surfactant. This nanoclay was introduced in PS by pressing them together at high temperature for the preparation of polymer/clay nanocomposites. Heterogeneity of the phosphonium surfactant layer in organically modified silicates and morphology of nanocomposites were studied[41]. Hexadecyl triphenyl phosphonium bromide was used to modify MMT nanoclay. Organically modified nanoclay was used to prepare polymeric nanocomposites of polystyreneacrylonitrile (SAN) through melt extrusion in a twin-screw extruder. The nanocomposites produced were studied for their thermal and mechanical properties[42]. Decyltriphenylphosphonium bromide and hexadecyl triphenylphosphonium bromide surfactants were used to modify MMT nanoclay. The

[35]

resultant nanoclay was used to prepare polybutylene terephthalate (PBT) polymer nanocomposites using melt intercalation. Thermal behavior of organoclay was studied using TGA, and mechanical properties of the composites were studied using differential scanning calorimetry (DSC)[43]. Trihexyltetradecylphosphonium chloride and tetraoctylphosphonium bromide surfactants were used to modify MMT and hectorite nanoclays by a new process using supercritical carbon dioxide gas CO2. These organomodified nanoclays were incorporated in polyamide 6 matrix for nanocomposites preparation in order to investigate their morphological and flame-retardant properties[44]. Diphosphonium-modified MMTs were prepared through typical ion-exchange reaction using para, meta, and ortho-bis(triphenyl phosphonium methylene)-benzene-dichloride. Removal of Telon dye from aqueous media was investigated[45]. Octadecyltriphenylphosphonium salt was used to modify MMT utilizing supercritical carbon dioxide liquid in place of conventional cation-exchange reaction. The properties of phosphonium cation-modified nanoclays through cation-exchange method and those modified using supercritical carbon dioxide were compared [46] . Purified Na-MMT was modified with tetraphenylphosphonium chloride quaternary ammonium cations as surface modifiers by typical ion-exchange reaction. The thermal stability of organoclays modified with different surfactants was analyzed[47]. Quaternary phosphonium salt tributyl hexadecyl phosphonium bromide was utilized in order to modify bentonite nanoclay. The organomodiifed bentonite nanoclay was used in the synthesis of polypropylene/ maleic anhydride-grafted polypropylene/organoclay nanocomposites. The mechanical properties of

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resultant polymeric nanocomposites were found better as compared to pristine polymers[48]. Quaternary phosphonium salts (1,3-Dioxolan-2-ylmethyl) triphenylphosphonium bromide and dodecyltriphenylphosphonium bromide were used to modify MMT organically through cation-exchange reaction. Modified organoclay was incorporated in polyamide 6 (PA 6) polymer matrix for the preparation polymeric nanocomposites. Nanocomposites were studied for their flame-retardant properties compared to pristine polymers[49]. o-Xylylenebis(triphenylphosphonium bromide) was used in order to modify MMT. Characterization of organically modified nanoclay was carried out utilizing various techniques like TGA, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscope (SEM)[50]. Summary of nanoclays modified using quaternary phosphonium cations is shown in Table 3. Amino acids-modified nanoclays Due to rigorous studies carried out in relation to the human impact on the environment, scientists have been developing numerous solutions to produce new environment-friendly materials. In this regard positively charged amino acids used as biosurfactants for modification of nanoclays are found to be more successful in the preparation of biodegradable polymers. These biosurfactants are similar to conventional alkylammonium cations surfactants, and comparatively amino acid biosurfactants have important advantages such as biodegradability and low toxicity over conventional surfactants[51]. A synthetic amino acid ω-Aminododecanoic acid (ADA) was used to modify synthetic layered silicate for the dispersion of organo-nanoclay in polyamide 12 (PA12) polymer matrix. Purpose of producing polymer/clay nanocomposites was to examine their morphology and investigate the role of nanofiller during the deformation processes (Figure 8)[52]. Three different unnatural amino acids were used to modify Na-MMT nanoclay. The amino acids used were (�)-2-aminopimelic acid, 5-aminovaleric acid, and DL-2-aminocaprylic acid. The purpose of modification was to design intercalated clay structures that could be used for bone biomaterial applications[53]. Cloisite nanoclay was modified by a natural L-methionine amino acid using typical ionexchange reaction. The organomodified cloisite was used to prepare poly(vinyl alcohol) (PVA)/organonanoclay nanocomposite films by solution-casting method to investigate their thermal and optical clarity properties[54]. L-isoleucine amino acid was used to

modify cloisite nanoclay to disperse in PVA polymer matrix using solution-casting method. The nanocomposite films were prepared in order to investigate their biodegradability[51]. The 5-aminovaleric acid was used as an organic modifier for sodium MMT nanoclay. The modified nanoclay was used to mineralize hydroxyapatite (HAP). This HAP interacts with organically modified nanoclay through the aminovaleric acid. The study provided a framework for selection of biomaterials used in tissue engineering[55]. Summary of nanoclays modified using different amino acids is shown in Table 4. Modified nanoclays with other type of organic compounds Apart from the aforementioned typical organic surfactants used for surface modification of nanoclays, some other organic compounds bring fruitful results when nanoclays are modified with them. One class of such nontypical surfactants are nonionic surfactants used to modify bentonites. These are actually linear-chain alcohol ethoxylates with properties such as low toxicity and biodegradability, commonly used as detergents and remediation of polluted soils. Bentonites modified with these nonionic surfactants possess better chemical stability and higher adsorption capacity[56]. Sodium MMT was modified with metanil yellow dye (p-phenylamino-azo-benzene-3-benzene sodium sulfonate) to study kinetics and adsorption of intercalation. Metanil dye easily replaced sodium cations from the interlayers increasing basal spacing[57]. Another interesting approach in modifying layered silicates was practiced using crown-ethers. Sodium and potassium MMTs were organomodified and incorporated in polystyrene matrix through in situ polymerization for the preparation of polymeric nanocomposites. Organic modification of nanoclays with crown-ethers resulted in an increased basal spacing from 1.5 to 1.9 nm[58]. A thermally stable cationic surfactant imadazolium salt was used to modify MMT organically. Such modified nanoclay was dispersed in polystyrene to produce polymeric nanocomposites with better thermal stability than the conventional alkylammonium-modified clay/polymer nanocomposites[59]. A similar organic modification of MMT was also reported by Award et al.[60]. Heterocyclic specie 2-aminopyrimidine was used to modify MMT and sepiolite nanoclays organically. The compound was well-intercalated in MMT by increasing basal spacing of interlayers, while the same molecules were just adsorbed at the surface of sepiolite nanoclay[61]. Maleic anhydride and pentaerythritol were used as organic modifiers for MMT nanoclay to prepare polyethylene

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Hexadecyl triphenyl phosphonium bromide (i) Decyltriphenylphosph-onium bromide (ii) Hexadecyl triphenyl-phosphonium bromide (i) Trihexyltetradecyl-phosphonium chloride (ii) Tetraoctylphos-phonium bromide bis(triphenyl phosphonium methylene)-benzene-dichloride

Octadecyltriphenylphosphonium salt

Tetraphenylphosphonium chloride

Tributyl hexadecyl phosphonium bromide

(i) (1,3-Dioxolan-2-ylmethyl)triphenylphosphonium bromide (ii) Dodecyltriphenyl-phosphonium bromide O-xylylenebis (triphenylphosphonium bromide)

Montmorillonite Montmorillonite

Montmorillonite

Montmorillonite

Bentonite

Montmorillonite

Montmorillonite

Montmorillonite Hectorite Montmorillonite

Tributylhexadecylphosphonium bromide

Synthetic fluoromica Somasif nanoclay

Surfactant (i) Tetraoctyl phosphonium bromide (ii) Benzyltriphenyl phosphonium chloride Hexadecyltributylphosphonium cation

Alkylphosphonium cations–modified nanoclays.

Saponite nanoclay

Nanoclays Bentonite

Table 3.

Nil

Polyamide 6 (PA 6)

Polypropylene (PP)

Nil

Nil

Nil

Polyamide 6 (PA 6)

Polystyrene-Acrylonitrile (SAN) Polybutylene terepthalate (PBT)

Polystyrene (PS)

Polybutylene succinate (PBS)

Dispersion matrix Polyamide 66 (PA 66)

Characterization of organically modified nanoclay was carried out

Morphological and flame-retardant properties of nanocomposites Investigation for removal of Telon dye from aqueous media Comparison of properties of modified clays prepared by two different methods Thermal stability of organoclays modified with different surfactants Mechanical properties of resultant polymeric nanocomposites Flame-retardant properties of nanocomposites

Structure, material properties, melt rheological pattern, and biodegradability of nanocomposites Heterogeneity of the phosphonium surfactant layer in organically modified silicates and morphology of nanocomposites Thermal and mechanical properties of nanocomposites Thermal behavior of organoclay

Study/properties Thermal and mechanical properties were studied

[50]

[49]

[48]

[47]

[46]

[45]

[44]

[42] [43]

[41]

[40]

References [38]

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Figure 8. Characteristic deformation structures in higher magnification taken from in situ tensile tests under the electron microscope. Tensile direction is in the arrow direction. (a) Deformation structure in case of layered silicates stacked perpendicular to the external load, (b) Case of stacked silicate layers orientated with a certain angle to the applied stress, and (c) Stacked silicate layers that are orientated parallel to the applied stress[52]. © Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.

Table 4.

Amino acids modified nanoclays.

Nanoclay Synthetic layered silicate Montmorillonite

Nil

Cloisite

Surfactant ω-Aminododecanoic acid (ADA) (i) (�)-2-aminopimelic acid (ii) 5-aminovaleric acid (iii) DL-2-aminocaprylic- acid L-methionine

Dispersion matrix Polyamide 12 (PA12)

Cloisite

L-isoleucine

Poly(vinyl alcohol) (PVA)

Montmorillonite

5-aminovaleric acid

Hydroxyapatite (HAP)

Poly(vinyl alcohol) (PVA)

terephthalate/clay nanocomposites. The basal spacing of MMT was not disturbed in maleic anhydride-modified nanoclay and increased considerably to 1.45 nm for pentaerythritol-modified MMT[62]. An irreversible cation-exchange reaction was carried out by treating bentonite and sepiolite nanoclays with basic methylene blue. Methylene blue cations were well-adsorbed and successfully replaced inorganic cations from both nanoclays[63]. (3-aminopropyl)-triethoxysilane was utilized to interact with the synthetic flurohectorite and natural montmorillonite nanoclays organically. The grafting of silane modifier resulted in an increased basal spacing of up to 1.45 and 1.77 nm, respectively, proving its intercalation inside layered silicate galleries. However, the arrangement of silane modifier in both nanoclay layers was different. This interlayer arrangement of silane modifier has much importance in the synthesis of polymer/clay nanocomposites and their better properties[64]. Laponite nanoclay was organically modified with organic compounds like γ-metacryloxypropyl dimethyl methoxysilane and trimethoxysilane. The silane molecules were covalently attached to the hydroxyl groups of nanoclays. The

Study/properties Morphological studies of resultant nanocomposites Bone biomaterial applications Study of thermal and optical properties of nanocomposites Synthesis of biodegradable polymer nanocomposites Framework for synthesis of biomaterials used in tissue engineering

References [52] [53] [54] [51] [55]

evidence of surface modification was illustrated by FTIR analysis of the samples[65]. Different aniline halide salts were used to modify MMT organically. Anillium cations replaced the sodium ions from interlayer galleries resulting in an increase of basal spacing up to 2.47 nm for flouride and chloride surfactants and up to 1.48 nm for bromide and iodide surfactants[66]. A hydrophobic and neutral dye Nile Red was adsorbed on the surface of disc-shaped laponite nanoclay. Such organomodified nanoclay could be used as optical probes for drug delivery capacities in case of tumor therapy imaging[67].

Epoxy/modified MMT nanocomposite Epoxy/alkylammonium cations-modified nanoclay nanocomposite Epoxy is a class of thermosetting resins that are widely used in various industries for a number of applications. Their use is much pronounced in paint and coating industries, adhesives, and especially as matrices for the preparation of polymeric nanocomposites due to

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Figure 9. Curing reaction of epoxy.

exceptional properties[68]. The properties of epoxy resins are further enhanced by addition of small amounts of reinforcing agents such as nanofillers. In such reinforced epoxy/nanofiller composites, properties are mostly related to the interfacial chemistry and possible interaction between nanofiller materials and epoxy molecules. Epoxy is a class of thermosetting resin that needs curing by addition of some hardener, usually diamines, for better properties in the resultant nanocomposites (Figure 9). Cured epoxy matrices along with nanofillers results in the enhancement of bulk material properties of polymeric nanocomposites formed. Many types of nanofillers have been developed in order to disperse in epoxy matrices for the preparation of polymeric nanocomposites with enhanced properties. However, among many commercially available nanofillers used as reinforcing agents for epoxy resins, nanoclay turned out to be the one with the ability to enhance mechanical and thermal properties of epoxy (Figure 10)[15]. As aforementioned nanoclays are hydrophilic and polymer molecules are hydrophobic, in order to increase compatibility between the two phases nanoclays are organically modified with some organic surfactants. This portion of the review shall cover such nanoclays modified with alkylammonium cations as surfactants and incorporated in epoxy matrices for the preparation of epoxy/clay nanocomposites. Park et al. used epoxy/ PMMA as polymer dispersion medium for nanoclay particles using melt-extrusion technique. Organically modified nanoclay particles were successfully dispersed in PMMA along with mixtures of aromatic and aliphatic epoxies to produce three-phase polymeric nanocomposites (Figure 11). The mechanical properties of resultant nanocomposites were evaluated and compared with epoxy/nanoclay, PMMA/nanoclay, and PMMA/epoxy composite systems. Mechanical properties like tensile and impact strengths showed significant improvement compared to pure polymer systems (Figure 12)[69]. Thermal dissociation of alkyl ammonium cations used as modifying agents for layered silicate clays was studied, and effects of this dissociation on plasticization of

epoxy networks was investigated. They concluded that at cure temperatures higher than the dissociation temperature the thermal dissociation of alkyl ammonium ions results in the formation of primary amines and excess chloride salt. These amines react readily with

Figure 10. Stress–strain curves of pristine glassy epoxy polymer {EPON 828RS þ D-230 Jeffamine} and of epoxy–clay nanocomposites, (a) with 3 wt% (silicate basis) of various organoclays: (1) pristine EPON 828RS, (2) Naþ-PGW, (3) NaþCloisite, (4) I.30E, (5) I.28E, (6) C10A, (7) C15A, (8) C20A, and (b) with varying loadings of I.30E organoclay: (A) pristine EPON 828RS, (B) 1% I.30E, (C) 3% I.30E, (D) 6% I.30E, and (E) 10% I.30E[15]. © Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.

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Figure 11. SEM images of fractured surfaces of cured PMMA– epoxy–clay composites with the following composition: (a) 80:20:2 and (b) 70:30:3[69]. Note: SEM, scanning electron microscope; PMMA, polymethylmethacrylate. © Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.

Figure 12. Results of thermogravimetric analysis of cured PMMAepoxy-clay composites[69]. Note: PMMA, polymethylmethacrylate. © Elsevier. Reproduced by permission of Elsevier. Permission to reuse must be obtained from the rightsholder.

the epoxy molecules resulting in the formation of linear chains[70]. Wang et al. prepared epoxy/clay nanocomposites using commercially modified nanoclay with a high degree of exfoliation by “slurry compounding” method[71]. This technique was helpful for transferring the dispersed state of nanoclay successfully on the epoxy matrix. The epoxy/clay polymeric nanocomposites exhibited a high degree of clay exfoliation and much better thermomechanical properties compared to pristine epoxy. In another attempt, Wang et al[72]. developed a spectacular approach using solvent to disperse organomodified nanoclay into epoxy matrix. The resultant polymeric nanocomposites showed improvement in storage modulus, Young’s modulus, and fracture toughness due to reinforcing behavior of nanoclay into the polymer matrix. Wang and Qin[73] synthesized epoxy/clay nanocomposites by incorporating MMT organically modified with alkylammonium cation in epoxy matrix through in situ polymerization technique under ultrasonic treatment. It was found that clay layers separated further by gradual increase in the duration of ultrasonic stirring that was illustrated by XRD measurement. Moreover, TGA of the nanocomposites showed that maximum thermal decomposition temperature of nanocomposites increased with the gradual increase in the duration of ultrasonic stirring. Tan et al[74]. used sol–gel intercalation technique for the preparation of epoxy/clay nanocomposites. The polymeric nanocomposite produced through sol–gel intercalation method showed a high degree of clay exfoliation from symmetrical exfoliation to highly abrupt exfoliation with improved thermal stability property. Glaskova and Aniskevich[75] used octadecylamine-modified MMT nanoclay for dispersing in epoxy polymer matrix. The moisture absorption index of polymer composites compared to pure polymer matrix was studied. It was concluded through experimentation that the sorption process in polymer nanocomposites occurs more slowly than in pristine epoxy resin. For a maximum amount of nanoclay content diffusivity reduces approximately half of the diffusivity of pure epoxy resin. Ávila[76] prepared epoxy/clay-based polymer nanocomposites by incorporation of commercially modified nanoclay cloisite using ultrasonication technique. Thermal properties of resultant nanocomposites were studied in detail. Sancaktar and Kuznicki[77] prepared epoxy/clay nanocomposites using two different epoxy systems. MMT modified with quaternary ammonium cation was dispersed in the polymer matrix using solution-casting method. Significant improvement in mechanical properties of resultant polymer nanocomposites like tensile strength, elastic modulus, and yield stress were observed. Thiagarajan et al[78].

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prepared glass-fiber-reinforced epoxy/nanoclay composites utilizing hand lay-up techniques. Alkylammoniumbased modified clay was incorporated in epoxy polymer matrix. The resultant epoxy/nanoclay composites were subjected to morphological studies. Esteves et al[79]. used organomodified MMT clay for the production of polymeric nanocomposites with epoxy matrix system. The epoxy/clay nanocomposites were investigated for their mechanical and tibological properties. Mohammad[80] prepared epoxy/nanoclay composites to examine their chemical and irradiation resistance. Organomodified nanoclay with alkylammonium cations as surfactant was successfully dispersed in the polymer matrix. It was found that minimum 1 wt% loading of the nanoclay in the epoxy matrix showed chemical resistance in both acidic and basic solutions. Asadi[81] investigated the ability of self-healing repair of cracks in an epoxy/nanoclay nanocomposite by the use of poly[ethylene-co-methacrylic acid] (PEMAA) particles. It was concluded through experimentation that PEMAA acted as an effective self-healing agent and can be used for epoxy/nanoclay nanocomposites. Epoxy/alkyl phosphonium-modified nanoclay nanocomposites As discussed previously, many authors have reported the incorporation of alkyl-ammonium-modified nanoclays for improvement of various properties of epoxy/ clay nanocomposites. However, the poor thermal stability of such polymeric nanocomposites makes the use of ammonium-modified clays in thermoset matrices very limited. Especially when dealing with such nanocomposites using melt-compounding and injectionmolding techniques, where processing temperatures exceed 200°C, nanoclays modified with ammonium ions show degradation[82]. This thermal degradation in consequence can cause polymer degradation along with some other unwanted effects during processing. Compared to alkylammonium-modified nanoclays in the epoxy matrix use of alkyl phosphonium-modified nanoclays increases the thermal stability of final polymeric nanocomposites[83]. These phosphonium salts have the capability to undergo a variety of other reactions in contrast to ammonium ions because of the presence of phosphorous atom that shows steric tolerance. Such thermal stability of polymeric nanocomposites is necessary for the curing and processing of nanocomposite materials for high-performance applications (windmill blades, automotive pieces, and petroleum pipelines). There has been little research and systematic study on epoxy resins incorporated with alkyl phosphonium– exchanged cations nanoclays for the production of

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polymeric nanocomposites with better thermal and mechanical properties[84]. A very recent investigation regarding curing reactions or kinetics of the epoxy resins incorporated with alkyl phosphonium–modified nanoclays have been reported to produce epoxy/clay nanocomposites[85]. In this work commercially available bentonite was modified with tributyl hexadecyl phosphonium bromide to reinforce epoxy resin. A detailed kinetic study on resultant epoxy/clay nanocomposites was performed using FTIR analysis. The modifier was also found responsible to affect the overall yield of the product.

Epoxy/other inorganic nanoparticle– intercalated nanocomposite Epoxy matrix comprises complex molecular structure that provides unoccupied spaces of molecular dimensions. These spaces are somewhat similar to a discontinuity in a continuous phase. The possible reason for these discontinuities may be the presence of an amorphous phase in a somewhat crystalline phase or it may be the presence of physical vacancies. Such effects on the molecular structure of epoxy are also attributed to loose packing of molecules. The idea to produce epoxy nanocomposites is to fill these regions with other particles having superior properties, for which nanoparticles were most suitable due to their tiny particle size and large surface area[86]. TiO2, SiO2, and ZnO nanoparticles were introduced in epoxy matrix in order to study the effects of the irradiation damage of the epoxy resin and its nanocomposites. Nano–TiO2 particles provided better resistance performance under vacuum ultraviolet radiations. Similarly, epoxy nanocomposites compared to pure epoxy system showed low mass loss and low gas extraction with decreasing trend in gas component varieties after irradiation[87]. The effects of nanosilica particles on a variety of parameters, like glass transition temperatures, curing reaction, dielectric behavior, and thermomechanical properties, were assessed using epoxy resin as a polymer matrix. In the resultant nanocomposites, nanosilica particles were found to be responsible for the increase in toughness and strength of the epoxy resin at low nanofiller loadings (up to 3 wt%). This is because at this low composition nanosilica particles were well dispersed without any great aggregations[88]. Epoxy/SiO2 nanocomposite materials were prepared in order to investigate the electrical properties of resultant polymer nanocomposites. The results showed that the electrical features of the nanosilica-incorporated nanocomposites were superior to the pristine specimens[89]. A study on micro-composites filled with microsilica particles and mixture composites

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incorporated with both nanosilica and microsilica particles in epoxy matrices were carried out. The thermal and thermomechanical properties of the resultant nanocomposites were studied using DSC and dynamic mechanical analyzer (DMA). Both the mechanical and thermal properties of the nanocomposites were improved[90]. Nanosilica and microsilica particles were used in the preparation of epoxy micro- and nano-composites to examine partial discharge resistance offered by them. Three systems were analyzed for the purpose: pure epoxy, epoxy microcomposites, and mixture composites containing both micro- and nanoparticles. The partial discharge resistance of microcomposites was found to be better than pure epoxy matrix while this property was much more pronounced in mixture composites[91]. Indium tin oxide nanoparticles were filled in epoxy matrix for the preparation of nanocomposites with highly visible transparent surface to be used for coatings. The nanocomposites were prepared by dispersing polyglycidyl methacrylate Indium tin oxide nanoparticles into a commercially available epoxy matrix system. The epoxy nanocomposites were carefully applied on glass and plastic surfaces as a visibly transparent film that was opaque to both UV and IR radiations[92]. Nanoparticles of calcium carbonate (CaCO3) were introduced in epoxy matrix for the preparation of nanocomposites. The nanocomposites formed were investigated for their interlaminar shear strength. It was shown that the nanoparticles increased the interlaminar shear strength to a highest degree[93]. Thermal and electrical properties of epoxy resin–based matrix silica composites were studied using a modified injection-molding technique. The nanosized silicon oxides, aluminum oxides, and titanium oxides nanoparticles were incorporated in epoxy system. The resultant polymeric nanocomposites exhibited improved electrical and thermal properties compared to the pure systems. For instance, the maximum thermal conductivity was achieved for the composites with 2.0 wt% of titanium oxide[94].

electrical insulation properties[95]. These properties of epoxy resins are enhanced further by addition of nanoparticles that act as reinforcing agents. Among these nanoparticles nanoclays/layered silicates incorporated in epoxy matrix results in high improvement in mechanical, thermal, and barrier properties. Due to the excellent properties of epoxy/clay nanocomposites, they can be used for several specific applications in aerospace, defense, and automotive industries. Apart from these, epoxy/clay nanocomposites are also used in high degree of structural and functional applications such as laminates and composites, sealants, adhesives, tooling, electronics, and construction. Major application of nanoclay-based epoxy nanocomposites are discussed in the subsequent section. Automotives and aircraft industries In the past few years, limiting the fuel consumption in vehicles and controlling carbon dioxide emission from internal combustion engine remained an area of focus in automotive industries. Researchers have come up with different techniques to achieve the goals, but the reduction of vehicle weight seemed to be the most promising solution to this. The objective can be achieved by preparing lightweight structures for automotive applications (Figure 13). Epoxy resins in this regard play a major role in developing materials for the automotive industry[96]. Epoxy/clay nanocomposites prepared by reinforcing high-strength carbon/glass fiber showed the greatest importance for commercial applications in the field of automotive and aircraft industries because of their power to reduce individual component weight with enhanced mechanical properties[97]. Liquid helium storage tanks made from epoxy/clay nanocomposite with carbon fiber reinforcement showed five times lower helium-leak rate than the cryotanks made without clay [98] . The considerable reduction in the leak rate of helium may be attributed to the special alignment of

Applications of epoxy/nanoclay nanocomposite Epoxy resins, a class of thermoset materials, possess special chemical characteristics such as curing reactions, never yields byproducts or volatile compounds, shrinkage factor is too low upon curing, can be cured over a wide temperature range because of which better control on the degree of cross-linking is achieved. Epoxy resins are an excellent choice due to their chemical and heat resistance, good impact resistance, high strength and hardness, high adhesive power, and much better

Figure 13. Reducing weight of vehicles.

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the clay layers during processing. Production of these advanced, promising, and high-barrier composites totally eliminated the need for inner liner in composite storage tanks. As a consequence, construction of these automotive tanks became simple with a marked reduction in propellant leakage. Epoxy/clay composites have got considerable importance in the manufacturing of engineering components for cyclic loading. The fatigue crack propagation behavior of epoxy/clay nanocomposites plays a vital role in the construction of such parts of machinery. For instance, a significant improvement (up to 74%) in fatigue resistance of epoxy/clay nanocomposites was observed with the incorporation of 3 wt% nanoclay[99]. The enhancement in fatigue life of epoxy/ clay composites is generally attributed to toughening mechanisms introduced by the improved epoxy/fiber matrix interfacial bonds and nanoclay-induced depressions. Epoxy/clay nanocomposites possess anticorrosive properties due to which they are used in aircraft anticorrosive coatings[100]. Similarly in the automotive industry epoxy/clay nanocomposites are used as structural glue reducing the cast of welding. Epoxy along with hardener sets in a short time even at high temperature and thus helps in the contact of automotive body parts[101]. During the past few years the use of thermoset polymers like epoxy has gained considerable attention regarding their successful use in the aerospace industry. When coupled with low amount of nanofillers these polymers exhibit excellent mechanical properties for applications where structural soundness and high performance in desired (aerospace industries)[102]. Within the scope of this field, epoxy/clay nanocomposites are used in the preparation of radar-protecting structures referred to as radome due to their high strength, stiffness, and ability to withstand high temperature (Figure 14)[103]. The epoxy-based radomes have a drawback; they are susceptible to environmental degradation due to their moisture-absorbing capability[104]. Epoxy/MMT nanocomposites were prepared in order to improve the radome performance

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and tolerance to environmental degradation factors. The addition of about 2 wt% nanoclay resulted in increase of 16% degradation tolerance of epoxy/clay nanocomposites in relative permittivity by 760 h[105]. Construction Modifying epoxy resins with organoclay particles is an interesting achievement in recent years. For construction purposes high mechanical strength of the material is required. Unfortunately very limited research has been carried out regarding epoxy/clay nanocomposites for high-performance applications. The very initial achievement regarding preparation of high-performance epoxy/clay nanocomposites was made by Kornmann et al[106]. In their work synthetic sodium fluorohectorite nanoclay was modified by cation-exchange reaction with quaternary alkylammonium cation and a number of other similar surfactants. The organomodified nanoclay was introduced in epoxy resin for the production of epoxy/clay nanocomposites. The nanocomposites produced showed high mechanical strength that acted as the basis for materials used in modern construction. Similarly epoxy/clay nanocomposites prepared by Becker et al. have high fracture toughness required in construction[107]. High-performance epoxy/clay nanocomposites were prepared by incorporating novel phosphonium-modified nanoclays. The epoxy/clay nanocomposites showed high mechanical and thermal properties for use in construction and buildings[84]. Epoxy/clay nanocomposites with high toughness for absorbing huge amount of energy during impact fracture and to hold high loads were prepared in order to meet the demands for modern technological requirements in construction. Nanocomposites were prepared using a different technique for attaining compatibility between polymer and nanoclay. Suitable Jeffammine specie was grafted on the interlayers of MMT. Such grafted nanoclay was dispersed in epoxy matrix; as result nanocomposites with improved toughness were obtained without any drawbacks such as reduction of resin modulus[83]. High-functionality epoxy/clay polymeric nanocomposites were prepared by a research group in Australia with much better mechanical and thermal relaxation properties meeting the challenges in modern construction technology. Three different epoxy polymers were incorporated with alkylammonium cation modified nanoclay through in situ intercalation method[108]. Adhesives

Figure 14. Aircraft radome.

Modern adhesives are quickly replacing traditional fasteners and are getting popular in structural designing due

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to their light weigh, low cost, and ease of manipulation. The most widely used adhesives nowadays in modern construction is epoxy resins; they are also used as typical polymer composite matrices for the preparation of nanocomposites. This is because of high mechanical and adhesion characteristics of epoxy resins with good electrical insulating properties, chemical resistance, and ease of processing[109]. A major drawback in the past that limited the use of epoxy resins as structural adhesives was their extraordinary brittleness. To overcome this problem many experimentations were carried out to increase the fracture strength of epoxy resins. An extensive research in this direction concluded that the use of nanoparticles as reinforcing agents, especially nanoclays, can improve the crack resistance and toughness of epoxy resins used for structural adhesion[110]. By the advent of nanotechnology, improvement in the mechanical, thermal, and barrier properties of epoxy adhesives have been achieved. Moreover, the dispersion of nanoparticles like nanofibers, nanotubes, and nanoclays resulted in tremendous improvements in the above-mentioned adhesive properties, especially at low costs. The extensive use of epoxy/clay nanocomposites, especially for the structural adhesive applications, has been investigated by many researchers. Nevertheless, these data are almost equal to nothing as far as the most important property of epoxy resins, that is, adhesion, is concerned[111]. Epoxy/clay nanocomposites are used as adhesives due to their better adhesion properties and low cost. It has been investigated that the bulk adhesive strength of the epoxy resin could be sufficiently improved by the incorporation of nanoclay[112]. Adhesive and rheological behavior of epoxy/clay nanocomposites was studied by incorporating an organically modified MMT into the epoxy resin matrix using ultrasonic treatment. It has been investigated and found that the addition of modified nanoclay into the epoxy matrix results in an improvement of 40–65% adhesion properties while the worst results were obtained with the addition of unmodified nanoclay[113]. Two types of organically modified nanoclays were incorporated into epoxy resin in order to obtain polymer nanocomposite adhesives. Different amounts of nanofillers were added to the epoxy matrix to check the effect of reinforcing agent on properties. It was shown that the addition of proper amount of nanoclay can induce better adhesion properties in epoxy resins with decreased brittleness and increased fracture toughness[114].

Applications of epoxy/nanoparticle nanocomposite Both metallic and inorganic nanofillers can be incorporated into epoxy matrices to achieve excellent mechanical, thermal, and electrical properties. Use of these nanofillers

in appropriate amount sufficiently reduces the brittleness of epoxy to be used in many applications, including in electronics, construction, and automotive industries[115]. These nanofillers possess large surface-to-volume ratio; as a reason their surface becomes chemically active for proper binding with polymer matrix[116]. The incorporation of these nanofillers into the epoxy matrices can effectively improve its properties, such as flexural modulus, tensile modulus, and tensile strength[117]. Aside from nanoclay particles, many other nanosized additives have been introduced in the epoxy matrices for obtaining desired set of properties for modern-day applications. These nanoparticles include ZnO, Ag, Al2O3, TiO2, and carbon nanotubes. Their dispersion in epoxy matrices may cause an additional fracture mechanism that ultimately increases the bulk stiffness, durability, toughness, fire retardancy, and conductivity of epoxy[118]. As incorporation of nanoparticles may lead to an immense improvement in bulk properties of epoxy matrices, epoxy/nanoparticles-based nanocomposites have many applications in the following disciplines. Electronics and electrical industries The increasing demand for substantially high voltage with massive capacity has become an initiative for the preparation of a new electric transfer apparatus that can withstand such drastic high voltage. To accomplish construction of such materials had also led to electrical accidents with insulator breakdowns.[119,120] To overcome such incidents and to meet these demands, epoxy resins incorporated with a variety of nanoparticles have been utilized. Epoxy resin–based nanocomposites by nature are thermosets insulating materials that can be used to insulate power transformer apparatus. These epoxy nanocomposites can effectively withstand high voltages due to their elevated thermal stability and other excellent features like chemical and water resistance[121]. Addition of these nanofillers is very necessary as for as the economic advantage and enhanced mechanical properties are concerned. However, it is evident from research that the electrical properties of epoxy resins are reduced by the addition of these nanoparticles due to the formation of interface between the filler and the matrix. In this regard, several nanoparticles have been developed as insulating materials for the construction of electrical power apparatus[122]. Nanosilica particles were incorporated successfully into the epoxy polymer matrix to increase the insulating properties of polymer to be used in power transformer apparatus and many other electronic devices. Small amount of nanosilica loading up to 0.4 wt% in epoxy matrix produced the nanocomposites with highest electrical properties

POLYMER-PLASTICS TECHNOLOGY AND ENGINEERING

compared to pure epoxy specimens[123]. A significant amount of research has been made in the production of epoxy-based nanocomposites with high dielectric properties at low cost to withstand workload in modern electrical machines[124]. So far, several important dielectric properties of epoxy-based polymeric nanocomposites incorporated with different types of nanoparticles have been studied. The dielectric properties include resistance to partial discharges[125], space charge effect[126], dielectric degradation strength[127], resistance to water teering[128] and electrical teering[129], thermal conductivity[130], and thermal tolerance[131]. Epoxy nanocomposites doped with metal oxide nanoparticles of SiO2 and Al2O3 have been prepared for applications in electrical industries where high dielectric properties of equipment are demanded. Successful dispersion of metal oxide nanoparticles was achieved in the epoxy matrices with production of polymeric nanocomposites having high dielectric constant values and having the potential to be used in electrical industries where premium-grade insulating materials are needed[87]. Protective coatings Metal corrosion is one of the severe problems that has caused substantial economic loses. To protect metals from being corroded, different methods have been utilized with the passage of time. Out of these corrosion-resisting techniques, use of organic substrates has been found to be a more common and cost-effective method. In the recent past, organic nanocoatings have dramatically reduced the risk of metal corrosion. These organic nanocoatings have been found more effective than the conventional microscale filler polymer coatings. The use of graphene and its derivatives such as graphite nanoplatelets have been incorporated into different polymer matrices, including epoxy, and are found to be more effective nanomaterial coatings[132]. These graphite nanoplatelets have excellent mechanical, electrical, and gas-barrier properties that render them useful for production of polymeric nanocomposites used as protective coatings[133]. Epoxy based nanocoatings were prepared by the incorporation of functionalized graphite nanoplaelets. These epoxy nanocomposites showed excellent homogeneity and demonstrated a more effective anticorrosion behavior for carbon steel[134]. Addition of nanoparticles to epoxy matrices increases its barrier performances by decreasing the porosity for harmful species that causes corrosion. The epoxy nanocomposites provide environment-friendly solutions by increasing the durability of coatings because these fine nanoparticles fill the cavities[135]. This helps in bridging the cracks and crack bowing[136]. Nanoparticles-incorporated epoxy

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coatings are more effective against corrosion because these nanoparticles occupy and fill small holes on the surface where they are applied. As a result, the total free volume of coating is reduced and cross-linking density is increased[137]. Furthermore, these nanoparticles in epoxy coatings provide excellent barrier properties that help in corrosion protection and reduce the chances for the coatings to blister[22]. Nanoparticles of SiO2, Zn, and Fe2O3 were incorporated into epoxy matrix to produce nanocoating to be applied on steel substrates. The nanoparticles significantly improved the microstructure of epoxy matrix, enhancing the anticorrosive properties[138–140]. Aerospace and defense The most profitable modern-day aerospace, navigation, and defense industries are in search of materials with high performance and light weight. The field of nanotechnology is enabling scientists to produce materials for defense technologies. To fulfill this need of industries, epoxy nanocomposites provide an excellent set of materials to be used in the above-mentioned applications. Epoxy-based nanocomposites provide lightweight, high-performance materials with excellent chemical resistance and enhanced mechanical and thermal properties for manufacturing of equipment used in defense[141]. Advances in the field of nanotechnology are providing some revolutionary materials for diverse uses in several military applications. These are used in many defense applications to provide them with mobility, stealth, aerodynamics, nanosensors, power generation, resilience, and robustness. Epoxy matrices incorporated with carbon nanotube have been used for the synthesis of aerospace materials[142]. Carbon nanotube–reinforced epoxy matrix composites are extensively used in the structural building of modern spacecrafts. This is because of the unique properties of epoxy nanocomposites that can withstand high vacuum, electromagnetic radiations, and low and high temperatures. They are used for manufacturing spacecraft parts that have to face drastic conditions when the spacecraft is moving in its orbit[143]. However, sometimes these carbon-reinforced epoxy nanocomposites deteriorate when the spacecraft is moving in its orbit[144]. To minimize the degree of such destruction, TiO2 nanoparticles are found to be more effective for the preparation of epoxy nanocomposites used in the structural design of spacecraft[145]. BaTiO3 nanoparticles were successfully dispersed in epoxy matrix to prepare polymer nanocomposites for micro-machined embedded capacitors to be used in electronic devices of special purposes in aerospace industry[146].

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Future potentials of epoxy nanocomposite During the past few years, a special interest has come about in the development of electrical devices with reduced size and high performance. For induction of these features in electrical components, a high integration is required in the passive components of an electrical instrument, for example, capacitors and resistors. Such components occupy about 70% of the total space in the instrument[147]. Out of these passive components capacitors have remained under special focus because of their use in several important electrical instruments where they perform vital function like bypassing, decoupling, timing, and filtering. Moreover, capacitors also play a vital role in reducing the size and cost of electrical devices with improved electrical performance[148]. These passive components of an electrical instrument are being replaced by a new technology called embedded passive technology in which an organic substrate layer buries all passive components forming an integrated circuit. This organic substrate is usually a dielectric material with enhanced properties like epoxy nanocomposites. It is believed that future electronic devices will be mainly based on this emerging embedded-capacitor technology that will definitely improve the performance and functionality of modern electrical devices (Figure 15). An evaluation of epoxy nanocomposites was carried out by Gorur and Iyer for high-voltage insulation materials[149]. Cycloaliphatic epoxy is used for such high-voltage applications because it is characterized by all saturated bonds with a molecular structure for better tracking and erosion resistance. The molecular structure of cycloaliphatic epoxy resin thus presents an excellent partner for outdoor superior performances. The production of epoxy-based nanocomposite filled with carbon nanotube has gained much

Figure 15. Future potentials of epoxy nanocomposites in electrical devices.

attention in the past few years due to noticeable improvement in their electrical and mechanical properties[150]. In short, potential applications of epoxy-based nanocomposites are endless in future due to their extraordinary properties. These potentials include the production of high-performance materials, actuators, conductive adhesives, and sensors[151].

Conclusion Surface modification of nanoclays with organic modifiers is a strong tool for the production of polymeric nanocomposite materials. Amine modification of MMT has emerged as a successful tool for the formation of high-performance epoxy/MMT materials. These nanocomposites reveal tremendous material properties compared to pristine polymers and thus can be used as an alternative to conventional materials like steel and wood for reducing cost and weight. Out of these polymeric nanocomposites, epoxy-based nanocomposites play a vital role in many applications in modern-day materials used in aerospace, automotive, and electrical industries.

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