Polymer-clay Nanocomposites, Preparations and ...

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REVIEW ARTICLE

Polymer-clay Nanocomposites, Preparations and Current Applications: A Review Farman Alia,*, Hayat Ullaha, Zarshad Alia, Fazal Rahima, Fahad Khana and Zia Ur Rehmana a

Department of Chemistry, Hazara University, Mansehra- 21300, Pakistan

A R T I C L E H I S T O R Y Received: February 12, 2016 Revised: April 18, 2016 Accepted: May 17, 2016 DOI: 10.2174/240546150166616062508 0118

Abstract: Polymer-clay nanocomposites (PCN) are the most important nanomaterials of the current decade with wide range of applications. Montmorillonite, vermiculite, sepiolite, laponite, bentonite and attapulgite are the main classes of clay used as reinPlease provide forcement in polymer nanocomposites. Clay nanocomposites show characteristic feacorresponding author(s) tures of thermal stability, flame retardancy, barrier and anticorrosive properties. This photograph size should be 4" x 4" inches work comprehensively review current developments and applications of polymerclay nanocomposites in automotive, sporting goods, coating technology, packaging, insulation, building construction, electrochemical, biomedical and environmental. This work also aims to describe the importance of polymer clay nanocomposites in various fields, especially for environmental applications. Polymer clay based nanocomposites also have the potential to decontaminate and remediate aqueous systems, therefore purification and remediation of contaminated soil and air with the help of clay based nanocomposites have also been discussed.

Keywords: Polymer-clay nanocomposites, synthesis, characterization, properties, applications. 1. INTRODUCTION Nanomaterials are essential building blocks for nanotechnology. These building blocks include carbon based components, semiconductors, metal oxides and metals. With structural feature in the nano domain, the nanomaterials can be found in the form of thin films, multilayer, clusters and crystalline materials with dimensionality of 0, 1, 2 and 3. These materials are crystalline and amorphous alloys, metals, oxides, nitride, semiconductors, ceramics, thin films and multilayer nanocrystalline materials [1, 2]. Their dimensional classification is given in (Table 1). New family of nanocomposite materials consists of nano dimension nano filler in support of intermingled organic frame work, producing new materials for heterogeneous catalysis, polymeric pharmaceuticals, analytical and environmental applications [3-11]. Materials obtained from sol gel method at nanometer level were recognized in the beginning of 1980s. Roy komarneni et al. for the first time used the term nanocomposites in their research [12, 13]. A common term “nanocomposite” is more appropriate word for the materials which contain two (or more) components in nanometer size and differ in structure and composition. A search on subject of nanocomposites on the web of science in March 2012 showed only one reference published *Address correspondence to this author at the Department of Chemistry, Hazara University, Mansehra- 21300, Pakistan; Tel: +92 997414136; Fax: +92-997-530045; E-mail: [email protected] 2405-4615/16 $58.00+.00

in 1986 for the first time. Nowadays this term is widely used and has increased exponentially and by the end of 2011cumulatively reached to 54,500 hits. Accepted name for heterogeneous material nanocomposites has at least one component domain ranging from few angstrom to various nanometers [14]. Individual components properties can be significantly modified by some inorganic impurity and in some cases spectacular properties arise from synergism developed therein. The use of this new term in the 1980s seeded the infatuation for these materials in the next two decades. 1.1. Introduction To Clay Nanocomposites Tremendous and productive advancement has been achieved in the field of nano composites by designing clay based polymeric nanocomposites (CPN) [15, 16]. Appreciable enhancement in the properties of many polymers occurred by good dispersion and distribution of small amount (less than 5%) of clay mineral [17]. In modern literature, exfoliation and delaminating terms are interchangeably used. There is no clear distinction in terminology used while dealing with clay minerals and therefore proper distinction should be made [19]. In the revised definition used today, the term exfoliation is assigned for disintegration of larger aggregates into minor particles while delamination denotes separation method for individual particle layers. Delamination in limiting case is represented by exfoliated nanocomposites where in polymer random dispersion is resulted by separation of individual layers [18]. © 2016 Bentham Science Publishers

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Table 1.

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Dimensional classification of nanomaterial [1, 2].

Dimension

Example of Nanomaterial

0D

Nanoparticles, colloids, nanoclusters, nanodots

1D

Nanowires, nanobelts, nanorods, nanotubes

2D

Quantum wells, membranes, super lattices

3D

Filamentary composites, nanocomposites, hybrids, cellular materials, porous materials, block polymers, nanocrystal arrays

Fig. (1). The three idealized representations of (micro or nano) composites [18].

In order to achieve great progress in clay based nanocomposites, clay is chemically modified to enhance compatibility with chosen polymer matrix. Commonly, it can be done by various ways such as through Halloysite nanotubes, resorcinol di (phenyl phosphate) (RDP) adsorption, easy to implement and far safer than quaternary ammonium chloride salts [20-22]. Megharaj et.al. Synthesized modified clay sorbents and used for the uptake of chemical compounds. These modified clay sorbents particularly used for the sorption of both hydrophobic and anionic compounds [23]. 1.1.1. Characteristic Features of Polymer-clay Nanocomposites (CPN) It is desired to get a nanocomposite with properties that overwhelmed the shortcomings of polymers while keeping the intrinsic benefits of polymer matrix intact. Due to low price, accessibility, high aspect ratio as well as required nanostructures, clays can offer better properties at very low loadings while keeping in hand the original beneficial properties of polymer.

1.1.1.2. Mechanical Properties of CPN As reinforcing agents the important achievement of fillers is its application in polymer for improved mechanical properties [24]. As higher module mechanism of strengthening is based on greater resistance of rigid filler materials. Due to rigid structure of clay layers and its high aspect ratio of layered silicates has been recognized for increasing modulus and stiffness of polymer matrix very effective under well dispersed condition [25]. The mechanical strength of CPN can also be increased by the involvement of adhesion of polymer chains by strong physio sorption forces to rigid clay layers that can become a part of materials showing high modulus. Though any enhancement in the polymer-clay interfacial contact leads to the wellstress transfer in the nanocomposite. The enhancement of clay interfacial adhesion properties and polymer surface chains modification is done by use of suitable polar compatibilizers that causes enhancement of mechanical properties of nanocomposites [26]. The adverse effects were observed on modulus due to higher

Polymer-clay Nanocomposites, Preparations and Current Applications

loading of compatibilizers owing to plasticization causing lower molecular weight [27]. Due to exfoliation/intercalation organically modified clay filler increase both stiffness and modulus of nanocomposites. Overall any factor that helps in the interlinking of polymer molecule with in clay gallery and as a result causes more interfacial and exfoliation interactions, result in the enhancement of modulus. Total exfoliation of clay layers is not easy to be attained and within different thickness in polymer matrix there is variety of platelet (depending on number of layer stack together). Effect of incomplete exfoliation on nanocomposite properties have analytically formulated by Fornes and Paul [28]. It has been reported that increase in filler volume increases tensile modulus in nanocomposites [29]. Threshold limit value of clay loading causes leveling off in Young’s modulus, which is due to partial intercalated/exfoliated structure formation [30, 31]. The elongation at break properties of CPN depends on interfacial interaction of clay/polymer system. An increase/decrease of elongation at break has been reported in literature for clay/polymer nanocomposite [32, 33]. Clay/ polymer nanocomposite strength impact has been studied and comparedwith pure polymer system. It has been reported that impact strength increases by addition of very low fraction clay loading such as 0.1wt % [34, 35]. Organically modified clay filler addition was studied for compatibilized polyolefines. It has been reported that impact resistance decreases with addition of clay materials [36]. Studies of dynamic mechanical analysis showed that storage modulus and glass transition temperatures are enhanced by addition of clay nano fillers [37-40]. Organically modified and well exfoliated montmorillonite based maleic anhydride nanocomposites with grafted elastomer of maleic anhydride as reinforcing filler showed enhanced mechanical properties as compared to conventional compounds containing a mixture of carbon black, CaCO3 and inert fillers [41]. 1.1.1.3. Thermal Properties of CPN Thermal stabilities of clay based nanocomposites were studied and compared with pure polymer under inert and oxidative environments. These studies showed that such nanocomposites are thermally more stable than pure polymer system [42-45]. It has been reported that formation of layered carbonaceous char helped degradation of clay nanocomposite [46]. The temperature at which the organic polymers are degraded into volatile compounds,the clay minerals remain stable at that temperature. In TGA experiment the clay content of nanocomposite remain as residue [47]. Polymethylmetacrylate (PMMA)/montmollinirite (MMT) based nanocomposite was the first thermally stable nanocomposites reported by blumstein [48]. TGA analysis showed that PMMA interlinked with Na-MMT have 40-50Chigher decomposition temperature than pristine polymer. 1.1.1.4. Flame Retardancy of CPN Due to enormous use of polymers and their applications in daily life, it is desired to reduce the ignition and burning potential of these materials. Usually for the controlling of ignition and burning chemical additives are added as flame retardants. Usually a large number of phosphorous as well as halogen based compounds have flame retardancy effects

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without effecting quality and properties of pristine polymer. On the other hand environmental contamination of halogenated compounds leads to halogen free flame retardants [49]. High level loading of some inorganic compounds caused unwanted effects such as durability, quality of the final product and high cost [50, 51]. Survey on clay/polymer nanocomposites showed that along with enhancement in mechanical and physical properties clay minerals also show some degree of flame retardancy [52]. 1.1.1.5. Anticorrosive Properties of CPN To prevent the corrosion of metals, polymers are widely used as anticorrosive coating agents. Polymeric coatings act as physical barriers against diffusion of aggressive species to metal surface. However, longer time contact with aggressive species of most polymer show some degree of permeations. To improve the barrier effect of polymeric coating different techniques have been used. One of the best method is preparation of polymer based nanocomposite or composite coating by addition of proper fillers to polymer coating matrix. Addition of layered silicates showed excellent enhancement of anticorrosive barrier effect of polymer coatings by increasing length of diffusion pathways for aggressive species. Due to its platelet structure and high aspect ratio of clay materials are in well dispersed state that decreases permeability of polymer coating films by diffusion pathways. Anticorrosive coatings on metals of organically modified and unmodified clays are used in polyimide and epoxy, polypyrrole, polyacryltes, polysulfone, polyaniline, polystyrene, poly(styrene co-acrylonitrle) nanocomposites polymeric materials have been investigated [53, 54]. 1.2. Polymer Clay Nano Composites (CPN) Preparation Methods In polymer matrix dispersion of clay mineral is like a chemical reaction, which is possible only when its free energy and enthalpy tend to decrease. Unfavorable change in entropy occurs (i.e. entropy increase) as configurationally entropy is lost by polymeric chains if they are partially restrained between clay mineral layers. Enthalpy effect must be complimentary to overcome this effect, that is interaction between clay mineral and polymer interaction have to be exothermic. On more quantitative basic considerations were set by estimation based on polymer chains lattice modelwhich find out change in entropy [55, 56]. 1.2.1. Direct Polymer Intercalation in Clay Minerals Due to existence of less or more restricted layer charges, the pristine clay mineral shows hydrophilic nature due to adsorbed water molecules. Dispersion by aqueous or by direct solid ball milling method clay mineral based nanocomposites of water soluble biopolymer and hydrophilic nature such as pectin can be prepared [57]. By using non aqueous solvent e.g.N-diemethyl acetamide polymer can be directly interlinked in pristine clay mineral such as montmorillonite (MMt)-sulfonated poly(ether ether ketone) (SPEEK) nanocomposites [58]. 1.2.2. Polymer Intercalation in Organoclays Direct chemical modification of layers is a rarely used strategy reported for clay minerals even in partial fluorina-


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Fig. (2). Different strategies for CPN synthesis: Rectangles, clay layers, wobbly lines, non-polar polymer chains, small open circles, polar groups (A) direct interlinking of hydrophilic polymers into pristine clay mineral, (B) interlinking of a polar polymers into organoclays (clays pre-expanded by surfactants such as alkyl ammonium), (C) pre-compatibilization of polymer through grafting of polar moieties for intercalation into pristine clay minerals, (D) one-pot synthesis. (E) In situ polymerization, (F) polymer-templated clay mineral synthesis [80].

tion (OH groups of the layers substituted by fluoride ions) allowing more lipophilic and Polymer-Polymer interlinking [59]. One of the general methods used for this purposes is the chemical modification of clay surfaces. Here we cannot review extensive literature on organically modified clays [60]. De Paiva and his coworkersrevealed that half of the research work published in past was on organo-modified clays for CPN preparations [61]. Good interactions between polymer and organo clays of nanocomposites made less use of solvent and lead to green methods such as extrusion or melt interlinking. Montmorillonite (MMT)-polyethylene oxide (PEO)nanocomposites were prepared by melt process and a wide range of nonpolar/organoclays containing polymers were reported [62]. The mixture of the two components was heated above the softening temperature of polymers [63]. Nano-oxide coated adsorbents were prepared by coating hydrotalcite containing composite materials or mixed with hydroxides metal layered such as Ca-Al, Zn-Cr, Mg-Al, Zn-Al type that have layered positive charge from that stable gel is synthesized by addition of swellable clay, e.g., montmorillonite etc. in different hydrotacite. With help of cordierite/mullite honey comb monolithic supports the gel is dipcoated. With the help of these dip coated monoliths are used to synthesize aluminosilicate supported nano-oxide coats over honey comb ceramic substances that help in adsorption of H2S and SO2 in He flow in various flow rates from 400 to 1000 C [64].

1.2.3. Polymer Modification by Using Compatibilizers Clay minerals are made compatible with polymer matrix by organic modification of the clay surface. An alternate approach can also be adopted and the polymer is made compatible with clay mineral. In this case a hydrophilic group is grafted on the polymer surface and thus polar moieties such as acrylic acid, linear alcoholic chains or maleic anhydrideare introduced into polymer chainsfor CPN preparation [61]. The polymer surface is modified with a suitable compatilizer such as maleic anhydride [65] or cellulose acetate [66]. More ionizable group onto polymer chains would be compatible that could protonate the clay mineral through ion exchange e.g. polystyrene(PS-NH3+)[67]. 1.2.4. One-pot’ Synthesis of CPN Since there are commercially available organoclays, commercial products are not synthesized according to CPN synthesis requirement and industrialist want to synthesize these products from the beginning. In this case one pot synthesis is feasible substitute for the preparation of CPN. In a single step operation all the components of CPN namely surfactant, polymer and clay mineral are added together. This was an example of preparation of CPN with NR [68], polyethylene (PE) [69, 70], or ethylene vinyl acetate (EVA) [71, 72]. In final product no changes were observed in nanocomposites prepared from earlier synthesized organoclays.


1.2.5. In Situ Polymerization of CPN The additional elementary process may be required for special molecular modification to force polymer pre-existing chain to intermingle between the interlayer spaces of clay minerals. The CPN prepared by in situ polymerization of monomers previously interlinked in interlayer space. Asmontmorillonite (MMt)-polyaniline (PAN) nanocomposite was first synthesized for conducting studies [73]. Organomodifiedmontmorillonite (MMt)-polylactic acid (PLA) [74], montmorillonite (MMt) - polycaprolactone (PCL) [75, 76] are usually synthesized in this way. Ring opening polymerization is used in case of polylactone. Other systems are clay mineral-polycarbonate (PC), clay mineral-polystyrene (PS), clay mineral-polyurethane (PU) [77], and a large number of clay mineral-copolymers are synthesized [78]. 1.2.6. Polymer-Templated Clay Mineral Polymer templated clay minerals can also be synthesized from small inorganic clay mineral precursors to which desired polymer (hydrophilic) is added. For the formation of hectorite-polymer nanocomposites polymer containing silicate gels are hydrothermally crystallized in this way [79]. 1.2.7. Melt Intercalation Method for CPN Synthesis In melt intercalation method, at molding temperature the polymer matrix is mixed with clay. Within polymer matrix for dispersion of clay layers the other conventional methods such as injection and extrusion molding are used. For preparation of thermoplastic nanocomposites this method is an effective tool [81]. In the polymer chains the clay layer are exfoliated or intercalated. In order to enhance the compatibility and their exfoliation the clays are organically modified and polymer chains are surface modified with more polar functional groups. For preparation of nanocomposites melt intercalation is popular method in industry in which no solvent is used [30]. Roasted aluminosilicates e.g. haloloysite are dispersed in to the polymer system for producing polymer nano-composite. Using haloloysite of an aluminosilicate produces uniform dispersion and adding it to polymer in melt mixing system form nano-composites [82, 83]. 1.3. Interaction Mechanisms of Layered Silicates with Polymer Chains 1.3.1. Exfoliated versus Intercalated Nanocomposites Reinforcements of nanometric character of these materials (clay) have made new advancement in the field of CPN. Some doubt still exists as whether fully exfoliated clay mineral particles of CPN really exists or not in these systems. Isolated clay mineral layers may not be a true representative of polymer matrix usually shown in electron micrographs because of partially underlined effects of more or less conscious selection of these material. 1.3.2. Characterization Techniques Used for CPN Investigations 1.3.2.1. Structural Information by Transmission, Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) In the field of CPN especially in elastomers these two techniques are systematically applied [84]. For example


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nanocomposites contain PU together with cloisite and laponite, cloisite bind preferentially with soft segments and laponite was observe for hard ones [85]. The stipulation of multi-scaled characterization of CPN have been several time outlined and as well as for other polymer sciences leave question mark, addressed by combination of XRD, Atomic force microscopy (AFM) and transmission electron microscopy (TEM) or by combination of X-ray, small angle x-ray diffraction (SAXS including USAXS and wide angle x-ray diffraction WAXS) technique [86, 87]. For controlling the polymer processing method rheology is commonly applied, as a technique to judge dispersion degree of clay mineral, corresponding to other structural CPN characterization technique [88]. 1.3.2.2. Molecular-Level Information by Vibrational Spectroscopies IR spectra allows (i) follow class if in situ polymerization [89], occasionally extensively different from free polymerization [90], or (ii) polymer bands identification and those of clay mineral in final CPN [91]. The confirmation of precise interactions e.g. in 2% cloisite high density polyethylene (HDPE) nanocomposites band envelop of clay mineral contain many bands of inorganic matrix in region of 950-1200 cm-1 was analyzed carefully. The coherent evolution during polymer process was showed by out of plane bending mode. This was recognized for interaction start of clay mineral and polymer but which type of interaction are involved remains ambiguous [92]. Change in SidO band on CPN formation was also reported [93]. In polyvinyl alcohol (PVOH)-starch, addition of clay mineral causes shifting of polymer bands, especially OH stretching bands was observed [94]. Due to Hbond interactions in organo-MMt-PP nanocomposites shift CH3 deformation bands [95]. In MMt-caprolacton was reported for H-bonding [96]. 1.3.2.3. Molecular-Level Information by Solid-State Nuclear Magnetic Resonance (NMR) NMR active nuclei in the polymers are 1H and 13C. In MMt-starch small changes were observed in 13C spectra of polymer chains [97] and MMT-PCL [173] by comparison with pristine polymer. Caramel-clay mineral nanocomposites 13 C spectra were also recorded [98]. To find out degree of polymerization of caprolacton1H-NMR was used [99]. Vanderhart and his coworker showed stabilization of g over crystallites in nylon-Mt due to presence of clay mineral by 13C and 1H-NMR [100, 101]. From 15N NMR same conclusion was drawn [102]. In special classes of nanocomposites other informative nuclei are also available e.g. in polymer chain some functional group have limited mobility showed by 19F spectra of organo-MT/Nafion, preferentially indicate the interlinking with clay mineral particles [103]. Reinhodt and his coworkers use naturally occurring interlayer cations such as 23Na and 7Li for the interlayer hydration in Mt-nanocomposites characterization and revealed that polymer chains are not completed by interlayer cations [104]. Hetero nuclear correlation (HETCOR) estimate the distance between two active nuclei of NMR. In block/hectorite PS-PEO [105, 106], identified by 1H 29Si HECTOR the PEO protons improvement in close area to Si of clay mineral particles and hypothesize that correspond to intercalated PEO segment as expected polarity


of two type of segment the PS units were not intercalated. WISE (wide separation) is an additional technique sensitive to molecular groups’ mobility [107], confirm PEO chains mobility were indeed reduced. 1.4. Clay-Polymer Nanocomposites (CPN) Applications There are four main category of properties that are fire retardancy (cable industry, etc.), mechanical (automotive industry, etc.), physical and optical (batteries, electronic industry) and barrier (bottle, packaging and film industry, etc.). With modified-Laponite Nafion nanocomposite membrane were studied for proton conductivity [108] and for (PEMFC) membrane fuel cells were studied for proton exchange studies.In biomaterials, as core in tennis balls and particularly in biomimetic materials CPN and employed in it [109]. Inorganic/organic polymer fine particles are dispersed in aqueous solution and are used for making an ink jet recording chemicals, as paper making chemicals, cosmetics and in medical appliances [110]. 1.4.1. Clay-Polymer Nano (CPN) Composite in Automotive Field The use of CPN in automotive field can be summarized as (i) vehicle of lighter weight saving fuel and reduce CO2 emission (ii) greater safety (iii) comfort increasement and (iv) better drivability. These aims are achieved in vehicle by introduction of nanofillers in polymer nanocomposites. 1.4.2. CPN in Timing Belt Cover Timing belt is an internal combustion engine that controls the timing of engines valve. Before 1990s Toyota col-

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laboration with Ube for camry cars firstly commercializes CPN in automotive application [112]. CPN was prepared from exfoliated MMt modified with o-trimethylammonium undecanoic acid and Nylon-6. 4.2% mass of MMt showed incredible advances in mechanical properties. Strength was increased by more than 50% and modulus was doubled. Deformation temperature was increased by 80C as compared to pristine polymer [113, 114]. Ube reported that CPN with 5 times more resistant gasoline permeation can be achieved with 2% of organo-clay than unloaded PA. Ecobesta was developed as trade name for product line with following structure co-extrusion of a multilayer nanocomposite made by PA12/adhesive/PA6/66 nanocomposites based on PA6. It was claimed that multi-layer structure gave better adhesive property, recyclability, high speed extrusion, cost reduction and barrier properties [115]. 1.4.3. CPN in Engine Cover Engine cover called bonnet (UK) or hood (USA) is a part that allow an access to engine. Unitika Co. use Nylon-6 nanocomposites functionalized materials for engine covers on Mitsubishi GDI engine in the same year when Toyota developed CPN [116]. CPN was produced by injection molding and was claimed mass reduction up to 20% with excellent surface finishing. 1.4.4. CPN in Step-assists, Doors, Centre Bridges, Sail Panels and Box-rail Protectors The teamwork of general motors, Southern Clay Products and Basell (Lyondell Basell industries) in 2002 applied reinforced TPO by organo-clay at 3% mass, for outer surface

Fig. (3). Automotive field prominent applications of clay–polymer nanocomposites [111].


Table 2.


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Parts of a car and polymer matrices for CPN applications [111]. Part

Polymer Matrix

Engine cover, Timing belt cover

Nylon-6

Engine cover

Nylon-6

Step-assist, doors, center bridge, sail panel, seat backs, and box-rail protector

PP, TPO

Rear floor

Thermoset (+glass fiber)

Tyre Tread

SBR, NR, BR

Inner liner

IIR, NR

Internal compounds (base)

NR

step assist of GM Safari and Chevrolet Astro vans. In 2003 and 2004 models of cars was CPN was first commercialization as exterior auto parts. Later in 2004 in Chevrolet Impalas General Motors, a highest volume of the car doors were made from CPN [117]. CPN in Seat Backs From Forte PP nanocomposites the structural seat backs of Honda Acura TL 2004 were made replacing glass filled PP by Noble polymer, avoiding problem such as visual defects, wrapping and processing difficulties. CPN have improved surface quality, superior mechanical properties, recyclability and lower density as 0.928g/cm3. Recently clay mineral PP nanocomposites were developed by Honda Acura for structural seat backs. 1.4.5. CPN for Rear Floor Nano-filledsheet molding compounds (SMC) were developed in 2009 by Molded Fiber Glass Companies (MFG, Ashtabula, Ohio). To thermoset resin organo-clay was added obtaining about density of 1.5g/cm3 with improved fuel efficiency much lower than conventional microsphere filled SMCs. GM applied these composites materials for Chervrolet Corvette Coupe, Vorvette ZO6 and Pontiac Solace. 1.4.6. CPN in Tyres, Tyre Treads, Tyre Innerliners and Tyre Curing Bladders The car tyre is the most abundant rubber composite. Pneumatic tyre can be defined as toroidal high performance composites material which have flexible membranous characteristics, under pressure have gases and load carrying, cushioning and road handling properties. Recently with radial structure tyre run at 90 angle to crush circumference with carcass cords that extend transversely from bead to bead. To reduce radial growth several layers are placed above carcass plies and under thread providing steering responding and directional stability. CPN incorporation in tyre compounds was designed for achieving the following performances and properties (i) reduction in dissipation of energy of compounds (ii) weight reduction (iii) heat buildup reduction with subsequent less tyre failures (iv) balance extension of so called magic triangle of tyre tread performances, rolling resistance, wear, traction (v) electrical conductivity (vi) air retention enhancement and (vii) colorability.

Through reduction in weight and in rolling resistance in cars are required to give key input to sustainable development of tyres. Introduction of silica as reinforcing filler for tyre especially of tyre tread compounds caused a breakthrough in reduction of rolling resistance. For the longer service life and better safety, lower dissipation of energy is useful for lower heat buildup inside compounds. Nanofillers such as clay minerals both organically modified and pristine were expected for the promotion of reduction of heat build-up as they are used in small amount with respect to silica. 1.4.7. CPN in Base Compounds In patent literature OC were reported to be effective ingredients for achieving an outstanding improvement of dynamic-mechanical properties in base compounds [118-122]. Pirelli Tyre announced in 2007 the use of a base compound containing an OC in P Zero tyres designed for high and ultrahigh performances. Higher stiffness, better handling/comfort trade-off, no decay, and dynamic modulus stability with temperature were claimed. Compared to aramide reinforced fibers, OC gave better isotropic behavior and better ultimate properties that are in equal performance in lateral and longitudinal directions. 1.4.8. CPN in Rubber Automotive Compounds other than Tyres Many automotive parts are made of rubber compounds such as fuel systems, under-hood, hood bump stops, engine bay rubbers, power train, chassis and underbody, front and rear bumper bars, front and rear mudguards, front and rear screen seals, door seals, and trims. Few data have been published on OC applications in these automotive parts.Cationic mediator and clay comprised nanocomposites have cationic and hydrophobic unit. With the help of cationic mediator the clay is intercalated or exfoliated. Such types of nanocomposites have enhanced and well balanced properties e.g. cure properties or mechanical properties, gas permeability etc. that can be practically useful in formulating materials such as tyre and rubber products etc. [123]. 1.4.9. Barrier Coating Technology The application of CPN in the field of sports balls was aimed at improving the pressure retention without impairing


other properties, namely, the feel, rebound and reproducibility of performances. The core of the technology [124, 125] was the application of a barrier coating containing highly exfoliated clay mineral particles. This technology is based on three main aspects: (i) the use of highly exfoliated clay mineral particles with a large aspect ratio. Vermiculite particles among thickness about 1–3nm and crosssectional dimensions of 1–30mm were selected. Coating was made with a mixture of aqueous vermiculite dispersion with an aqueous dispersion of butyl rubber. A large number of exfoliated particles was found in the relatively thin (about 10–30mm) coating layer; (ii) the control of the clay mineral–polymer interface, obtained by modifying the clay mineral surface and by the addition of neutral siloxane-based surfactants; and (iii) after drying and coating the clay mineral particles have good dispersion. Transmission electron microscopy revealed welldispersed particles of 1-2nm thickness and 200-400nm diameter with wavy or curved morphology, as well as micrometre-sized particles. An aspect ratio of about 100–400 was estimated. The diffusion coefficients was reduced by two orders of magnitude, as determined by applying butyl rubbers latex (rubber particle of about1mm in diameter) containing 20–30mass% vermiculite on PPO-coated Anapo receramic disc [126]. Air defense products (inmat.com, 2012) were commercialized by In Mat Company by applying coating layers on elastomeric substrates via spray or dip coating processes. The product used for sporting applications was Air defense 2000, having a solid content of 12.7–13.3mass%, pH 6.5–7.5, shelf life of 48 months (manufacture data), permeability of 2.5–3.5cm3 mm/m2 day atm. at 23˚C (on a PP substrate), and a strain to first damage at 25C >25%. The reduction of gas permeability with respect to the bare butyl rubber was up to 300 times. A product with a lower permeability, 1–1.5cm3mm/ m2day atm. is also commercialized. 1.4.10. Applications in Footwear OC were tested in TPU elastomer for shoe soles to influence the friction and wear characteristics. Various organocations were tested (octadecyl ammonium and bis(2hydroxyethyl) alloylmethylammonium) at a relatively low level (about 5mass%). A lower friction coefficient and higher resistance to wear were observed for TPU composites reinforced by the OC [127]. 1.4.11. Applications of CPN in Packaging Mitsubishi and Nanocor produce high-barrier CPN with the trade name Imperm® N based on polyamide. In a threelayer PET bottle, a 100-fold reduction of oxygen transmission rate was reported compared to virgin PET. The stiffness of CPN was doubled while maintaining gloss and clarity of the co-polyamide film. Permeation of gasoline, methanol, and organic solvents was limited. Films and thermoformed containers (for potato chips, ketchup and cheeses) were also prepared. PET bottles with this Imperm nanocomposite are commercially used in European countries for alcoholic beverages and beer. Alcoa CSI, Crawfordsville, applied multilayer CPN as barrier liner materials for enclosure applications. Co-extruded caps for juice, beer, and soft carbonated drinks were prepared from layers of clay mineral–Nylon-6 nanocomposite plus 1-2 EVA layers with oxygen scaven-

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gersfor plastic bottle. This type of liner was said to outperform other barrier materials at very high humidity (95–96% RH) [128]. CPN based on polymers from renewable sources and nanofillers are usually indicated as bio-nanocomposites. In the field of packaging, particularly food packaging, there is particular interest for biodegradable nanocomposites [129]. Clay minerals were investigated as fillers for thermoplastic starch. Elongation and tensile strength at break were enhanced at 5mass% Na+-Mt. Biodegradable polyester use on large scale (PLA, PHB), for packing is prevented by the elevated cost as well as low performance. Clay mineral–PLA nanocomposites improved the thermal and mechanical properties, allowing the reduction of film thickness. Rather moderate improvements were observed in the case of PHB. CPN of PCL with OC was reported that showed better physical properties. Clay minerals were also introduced in a biodegradable plastic made from corn starch by the Australian bio plastics producer Plastic Technologies. The Plastic R1 sheet had gas barrier performance, clarity, and strength and could be used for flexible and rigid food packaging.Bio-based film is multilayered film having barrier properties have one or more layers. Multilayered packaging film includes bio-based films on one aspect, where polyhydroxyalkonate is the biobased film consisting of graphic image [130]. Clay palygorskite/attapulgite, sepioiite or combination that contained nucleating agent clay form foamed polymeric compositions. These foamed polymeric materials can be used in packaging and insulating materials [131]. 1.4.12. Application of CPN for Wire and Cables The possible use of CPN in wires and cables is based on the improved barrier properties and flame resistance and retardancy obtained by combining clay minerals with traditional halogenated or non-halogenated flame retardants [132134]. Clay minerals could lower the content of flameretardant substances or, at the same level of flame retardants, reduce the degradation of physical and mechanical properties. CPN were prepared using a copolymer of EVA and 28mass% vinyl acetate with well dispersed OC, either natural Mt or dehydrogenated tallow dimethyl ammonium-Mt. Such CPN showed a much improved burn performance compared to the composite with only magnesium hydroxide. Improvements occur in tensile strength and elongation at break [135]. 1.4.13. Application of CPN for Biomedical Applications The biomedical/biotechnological field is characterized by an increasing interest for nanoparticles and nanotechnology. To prepare Nano fiber scaffolds, electro spinning of exfoliated Mt /PLA dispersions was used with subsequent salt leaching and gas foaming. The formed Nano and micro pores allowed cell growth and hosted blood vessels [136]. With CPN based on PU designed for biomedical applications such as in total artificial hearts it act as blood sacs in ventricular assist devices, water vapor permeation was reduced by more than 50% at an OC volume fraction of only 0.02 [137]. 1.4.14. CPN for Environmental Applications An overview of nanoclays organoclays is presented onutilization of nanoclays as strengthening portion in poly-


mer matrices for formation of polymer/layered silicate nanocomposite. They have applications in inks, greases, rheological modifier, paints, as therapeutic agent’s vehicle for drug delivery for controlled relief and nanoclays for water treatment of urban and potable water for making an advanced steps toward green environment. To greater extent a slight addition of nanoclay can modify the properties of polymers like greases and inks etc. layered silicates, interlayer spacing flexibility facilitates therapeutic agents that can be shortly to be released to the damaged cells. Since in intercalated layered materials release of drug is controllable that’s why these new materials in pharmaceutical field had a greater potential as delivery host. Tianping in 2012 stabilize clay by addition of particulate additive with treating fluid for subterranean formation by preventing them from migration or swelling. These particular additive are of different metal/transition metals oxides or post transition metal hydroxide, pyroelectric crystal and/or piezoelectric crystals. Particle size may be at nanometer scale that helps in stabilizing the clay by unique particle charges. Treating fluids can be applied for subterranean formations of hydrocarbons such as in completion fluids, hydraulic fracturing, fluid loss pills and gravel packing fluids. These carrier fluids used in treating fluids may be hydrocarbon, brine, aqueous or alcoholic based [138].


9

1.4.17. CPN for Removal of Hazardous Dyes from Water Color and dyes are an integral part of human life. Global production of thousands of different types of dyes is about one million tons annually. A significant part of these synthetic dyes and its by-products are eventually released in to the environment imposing a risk to human health. To reduce and eliminate hazard to living organisms, it is essential to detect and remove these chemical contamination from the environment. Synthetic dyes can be categorized into about 30 groups based on their structure. Some major dye groups are azo, anthraquinone, di- and tri-arylmethane, phthalocyanine, indigoid, oxazine, azine, thiazine, etc. However, the largest synthetic dye group is the acid dyes; anionic compounds, with reactive groups forming covalent bonds. Other common dyes classes are metal complex dyes, basic dyes, direct dyes, disperse dyes, mordant dyes, pigment dyes, anionic dyes, solvent dyes, ingrain dyes, sulfur dyes, etc. 1.4.18. CPN for Soil Cleaning

It is noteworthy that the environmental contaminants are typically measured in parts per billion (ppb) or parts per million (ppm). Further, the contaminant’s toxicity is expressed by a threshold ‘toxic level’ defined for each contaminant by environmental regulatory bodies. For instance, the toxic level for mercury is 0.002 ppm in water whereas for arsenic is 10 ppm in soil. In addition, contaminants are generally found as mixtures, demanding for extreme care in determining the overall toxicity level.

Generally, soil degradation can be classified into: physical, chemical and biological degradation. Physical degradation affects the air and water-holding capacity, permeability, root development and biological activity of soil. The two most significant activities responsible for physical degradation of soil are agriculture and forestry. The chemical degradation has been related to contamination, salinization and acidification, and nutrient depletion [139]. Oil and heavy metals are the primary soil contaminants, while gasoline, metal industries and vehicle service stations are considered as the typical sources of local soil contamination. Biological degradation is concerned with the soil organic matter (SOM) degradation which is in term correlated to the conversion of grassland, forests and natural vegetation to arable land, deep ploughing of arable soils. Further, accumulation of soluble salts in the soils (salinization) also falls in this category.

1.4.15. CPN for Water Cleaning

1.4.19. CPN for Air Cleaning

Water is the most essential and important component of life on earth. Contamination and degradation of aquatic environment by many compounds is one of the major global concerns of our society. In particular, industrial effluents such as organic and inorganic wastes, heavy metal ions, dyes, aromatic compounds, etc. pose considerable risk to drinking water sources. Clay based nanotechnology has the potential for decontamination and remediation of aqueous system, as discussed below.

Clean air is essential for healthy life. However, air often contains significantly high levels of contamination and pollutants that are extremely harmful to human health. The commonly found pollutants in air include particulate pollution, carbon monoxide, ground level ozone, nitrogen oxides, sulfur oxides, and lead. Increasing air pollution has imposed serious risks to human health, plants and wildlife. The health problems associated with air contamination include increased frequency in respiratory symptoms, heart or lung related diseases, and even premature death.Therefore, it is necessary to put significant efforts towards air pollution reduction and remediation.

1.4.16. CPN for Removal of Heavy Metals from Water For instance, Hg enter the water system through various routes such as industrial activities, household, acid rain causing e leaching of soil and has been found to damage to the kidneys, nervous system and vision. Mining and industrial waste, automobile exhaust and incinerator ash are considered as the prime sources of Pb contamination that eventually lead to anemia, kidneys damage, nervous system deterioration, impairment of protein syntheses etc. In addition, the sources of Cd include electroplating, plastic industries, mining, and sewage. Human exposure to Cd appears to have lethal health impacts in terms of provoking cancer, kidney and bone damage, mucous membrane destruction, and even impairment of progesterone and testosterone production.

2. CONCLUSION In the above discussions, it is clearly shown that the clay nanocomposites playavital role in various industries, and inter-disciplinary research has been remarkably a dynamic subject. A lot of work has been made toward clay nanocomposites. Numerou striumphs showed that the clay nanocomposites possess extensive potential applicationsin current developments of automotive field, in timing belt cover, in engine cover, in step-assists, doors, center bridges, sail panels, and box-rail protectors, in seat backs, rear floor, in tyres, in rubber automotive compounds, in sporting goods, in bar-


rier coating technology, packaging, insulation and automotive to household applications, building construction and electrochemical and biomedical applications. Within both academic and industry, polymer-clay nanocomposites have concerned the interest of technologists and scientists. Contaminants of fresh water can be removed by treating industrialized and urban waste water with organoclays in blend with other sorbents like alum and activated carbon. Organoclays established water treatment technology is advanced than any other method where large amounts contaminants containing water is treated. It is expected that this data would give rise to design potential clay nanocomposites with enhanced properties and higher specificity, together with development of novel technology. CONFLICT OF INTEREST

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The author(s) confirm that this article content has no conflict of interest. [22]

ACKNOWLEDGEMENTS Declared none.

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