composites: part bsite.icce-nano.org/clients/iccenanoorg/hui pub/2013 epoxy... · 2013. 3. 11. ·...

Composites: Part B xxx (2012) xxx–xxx

Contents lists available at SciVerse ScienceDirect

Composites: Part B

journal homepage: www.elsevier .com/locate /composi tesb

Epoxy clay nanocomposites – processing, properties and applications: A review

Asif Abdul Azeez a, Kyong Yop Rhee a,⇑, Soo Jin Park b,⇑, David Hui a,c

a Department of Mechanical Engineering, College of Engineering, Kyung Hee University, Yongin 446-701, Republic of Koreab Department of Chemistry, Inha University, 253, Nam-gu, Incheon 402-751, Republic of Koreac Department of Mechanical Engineering, University of New Orleans, LA 70148, United States

a r t i c l e i n f o

Article history:Received 2 February 2012Accepted 12 April 2012Available online xxxx

Keywords:A. Polymer–matrix composites (PMCs)A. Nano-structuresB. Mechanical properties

1359-8368/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.compositesb.2012.04.012

⇑ Corresponding authors. Tel.: +82 31 201 2565; fax:E-mail addresses: [email protected] (K.Y. Rhee), sj

Please cite this article in press as: Azeez AA et(2012), http://dx.doi.org/10.1016/j.compositesb

a b s t r a c t

The review renders a short background on the research work carried out on epoxy clay nanocomposites.Clays are one of the ideal nano reinforcements for polymers because of their high intercalation chemistryand aspect ratio. Epoxy clay nanocomposites are finding vast applications in various industries like aero-space, defense, automobile, etc. The physical and chemical properties of the epoxy systems are influencedby the processing techniques, clay modifier and curing agents used for the preparation of nanocompos-ites. The clay morphology (intercalation/exfoliation) of the nanocomposites is also depended on theabove parameters. So the emphasis of the present work is to highlight these parameters on morphologyand the final mechanical, thermal and barrier properties of the nanocomposites. The proposed applica-tions of the epoxy clay nanocomposites are also discussed.

� 2012 Elsevier Ltd. All rights reserved.

1. Epoxy resin

The term ‘epoxy resin’ refers to both the prepolymer and itscured resin/hardener system. The former is a low molecular weightoligomer that contains one or more epoxy groups per molecule(more than one unit per molecule is required if the resultantmaterial is to be crosslinked). The characteristic group, a three-membered ring known as the epoxy, epoxide, oxirane, glycidyl orethoxyline group as shown in Fig. 1 is highly strained and thereforevery reactive. Epoxy resins can be cross-linked through a polymer-ization reaction with a hardener at room temperature or atelevated temperatures (latent reaction). Curing agents used forroom temperature cure are usually aliphatic amines, and for hightemperature, aromatic amines and acid anhydrides are used.Polyfunctional amines, polybasic carboxylic acids, mercaptansand inorganic hardeners are also used as specialized curingagents. In general, the high temperature cured resin systemshave improved properties, such as higher glass transition temper-ature, strength and stiffness, compared to those cured at roomtemperature [1–5].

Among the thermoset materials, epoxy resins shows specialchemical characteristics such as absence of byproducts or volatilesduring curing reactions, low shrinkage up on curing, curing over awide temperature range and the control of degree of cross-linking.Depending on the chemical structure of the curing agents and cur-ing conditions, the properties of cured epoxy resins will vary. Epoxyresins are versatile with excellent chemical and heat resistance,

ll rights reserved.

+82 31 202 6693 (K.Y. Rhee)[email protected] (S.J. Park).

al. Epoxy clay nanocomposite.2012.04.012

high adhesive strength, good impact resistance, high strength andhardness, and high electrical insulation [3–5].

2. Polymer nanocomposites

Polymer nanocomposites have attracted great interest, both inindustry and in academics, because they exhibit remarkableimprovement in material properties compared with virgin polymeror conventional micro and macro composites [6–20]. Conventionalcomposites usually require a high content (>10%) of the inorganicfillers to impart the desired mechanical properties. Such high fillerlevels increase their density of the product and can cause the dete-rioration in properties through interfacial incompatibility betweenthe filler and the organic material. Besides, processability worsensas filler content increases. In contrast, nanocomposites showenhanced thermo mechanical properties even with a small amountof layered silicate (65%). Polymer nanocomposites are generallydefined as the combination of polymer matrix and fillers that haveat least one dimension in nanometer range. The nano fillers canbe one-dimensional (layered minerals such as clay) [6–13],two-dimensional (like carbon nanotube, nanowires, nanofibers,cellulose whiskers, etc.) [14–16], and three-dimensional (sphericalparticles include silica nanoparticles, nanowhiskers, etc.) [17–20].Polymer nanocomposites are known for its outstanding mechanicalproperties like high elastic modulus [6–11,21–24], increasedstrength [6–11,25], barrier resistance [26–30], flame retardancy[31–36], etc. with very small addition (65 wt.%) of nano particles.This is due to the very large surface area of interaction betweenpolymer matrix and nano filler. Among the different nanofillers,special attention has been paid to clays in the field of

s – processing, properties and applications: A review. Composites: Part B

http://dx.doi.org/10.1016/j.compositesb.2012.04.012

mailto:[email protected]

mailto:[email protected]


http://www.sciencedirect.com/science/journal/13598368

http://www.elsevier.com/locate/compositesb


Fig. 1. Epoxy or oxirane group.

2 A.A. Azeez et al. / Composites: Part B xxx (2012) xxx–xxx

nanocomposites. Clays (Layered silicates) are found to be one of theideal nano reinforcements for polymers, because of its high interca-lation chemistry, high aspect ratio, ease of availability and low cost.

3. Clays

Clays are hydrous silicates or alumino silicates and are funda-mentally containing silicon, aluminum or magnesium, oxygenand hydroxyl with various associated cations. These ions and OHgroups are organized into two dimensional structures as sheets.Clay minerals are also called layered silicates or phyllo silicates be-cause their structural frame work is basically composed of 1 nmthick silicate layers comprising silica and alumina sheets joined to-gether in various proportions and stacked on top of each other incertain way with a variable interlayer distance. The clay mineralscan be classified into three different types based on the condensa-tion ratio of silica to alumina sheet [12,13].

3.1. 1:1 Type

It consists of one octahydral alumina sheet and one tetrahydralsilica sheet condensed in 1:1 ratio, called as dimorphic or twosheet minerals. The sheet does not bear any charge due to the ab-sence of isomorphic substitution in both silica and alumina sheet.Layers are held together by hydrogen bonding between hydroxylgroup in octahedral sheets and oxygen in tetrahedral sheets. Thespace between the layers is occupied by water molecules, e.g., Kao-linite, Perlite, Hallosite, etc.

3.2. 2:1 Type

In 2:1, trimorphic or three sheet minerals, an alumina sheet issandwiched between two silica sheets. This type of clay belongsto smectite family. Stacking of these layers create a Vander Waalsgap between clay layers. Isomorphic substitution of Al3+ with Fe2+,Mg2+, Li+ in the octahedron sheets and/or Si4+ with Al3+ in the tet-rahedron sheets gives each layer an overall negative charge, whichis counterbalanced by exchangeable metal cations such as Na+,Ca2+, Mg2+, Fe2+, and Li+ residing in the interlayer space, e.g., mont-morillonite (MMT), hectorite, saponite, flouro hectorite, laponite,flouromica (somasif).

3.3. 2:2 Type

The layer structure of 2:2 type, known as tetramorphic of foursheet minerals, is formed by the alternate condensation of silicatetrahedron sheets and alumina or magnesium octahedron sheets,e.g., Chlorite.

4. Montmorillonite

Among the different types of clay minerals, montmorillonite isthe most commonly used for the preparation of polymer clay nano-composites [6–13]. Montmorillonite owes special attention amongthe smectite group due to its ability to show extensive inter layerexpansion or swelling, because of its peculiar structure as shown inFig. 2.

Please cite this article in press as: Azeez AA et al. Epoxy clay nanocomposite(2012), http://dx.doi.org/10.1016/j.compositesb.2012.04.012

The crystal structure of montmorillonite consists of layersformed by sandwiching an edge shared octahedral sheet of alu-mina between two silica tetrahedral sheets, so that the apical oxy-gen atoms of the tetrahedral sheets are all shared with theoctahedral sheet. Isomorphous substitution of aluminum for sili-con in the tetrahedral sheet and iron or magnesium for aluminumin the octahedral sheet provides an overall negative charge to thecrystal lattice. As the surface between the layers is negativelycharged it attracts cations such as Fe2+, Ca2+ or Na+. They form apositively charged layer between the negatively charged surfacesof the clay layers. Such layers extend continuously in the planeconstituted by the x and y axes and are stacked up in the z axis,forming the whole crystal structure. The silicate layers of MMTare planar, stiff about 1 nm in thickness with high aspect ratioand large active surface area (700–800 m2/g). These layers organizethemselves in a parallel way to form stacks with a regular van derWaals gap in between them, called as interlayer or gallery. Thesum of the single layer thickness and the interlayer is called d-spacing or basal spacing. The total quantity of the absorbed cations(K+, Na+, Ca2+, and Mg2+) in the inter gallery of clay layers at a PH

value of 7 is referred as the cation exchange capacity (CEC) of clayminerals. It is measured by the unit mill mol/100 g of layered sili-cate (or MMT). Higher the value of negative charge, the stronger isthe capacity for hydration, swelling and dispersion.

5. Organic modification of clay

Generally clays are hydrophilic in nature. In order to make com-patible with organic polymers, the surface of the clay mineralsshould be modified to organophilic prior to its use. Organic cationssuch as an ammonium ion or phosphonium ion are the commonlyused organic modifiers for clay minerals [6–11,37,38]. Modificationinvolves the exchange of interlayer inorganic cations with organiconium salts. The organic modification causes the expansion of theinterlayer space and thereby increases the d spacing to certain ex-tent (normally over 2 nm). Thus the organic modification favorsthe diffusion of polymer or its precursor into the interlayer space.Fig. 3 shows the schematic representation of the organic modifica-tion of clay. Alkyl ammonium ions are most popular since they caneasily be exchanged with the ions situated between the layers.Depending on the layer charge density of the clay, the alkyl ammo-nium ions may adopt different structures between the clay layers.Alkyl ammonium ions reduce the electrostatic interactions be-tween the silicate layers thus facilitate diffusion of the polymerinto the galleries. In general, the longer the surfactant chain length,the more will be the d spacing of the clay layers [9].

6. Structure of polymer clay nanocomposites

Depending on the nature of the components, processing condi-tion and strength of the interfacial interactions between polymerand layered silicates (modified or unmodified), either conventionalcomposites or nanocomposites can be formed as shown in Fig. 4[6–11,24]. In a conventional composite, the polymer cannot diffusebetween the clay layers and the clay particles exist in their originalaggregated state. Properties of these composites are similar to themicro particle filled composites. An improvement in modulus isnormally achieved in conventional clay composite but this rein-forcement benefit is usually accompanied with a deficiency inother properties such as strength or toughness. If there is a favor-able condition for the mixing of clay minerals with polymer, twoextremes of nanocomposites are formed, i.e., intercalated and exfo-liated nanocomposites. In intercalated nanocomposites, the claylayers retain the well ordered multi structure of alternating poly-meric and clay layers with a d spacing of 20–30 A�. On the other



Fig. 2. Structure of montmorillonite.

Fig. 3. Organic modification of clay.

A.A. Azeez et al. / Composites: Part B xxx (2012) xxx–xxx 3

hand, in exfoliated nanocomposites, the individual clay layers arewell separated and randomly distributed in the continuous poly-mer matrix with a d spacing of more than 50 A�. The intercalationand exfoliation of the clay layers in the polymer matrix can beidentified through wide angle X-ray diffraction (WAXD) and Trans-mission Electron Microscopy (TEM). The characteristic WAXD ofunmodified clay (Cloisite Na), organically modified clay (Cloisite30B and clay 3) and organically modified clays swelled by epoxy

Fig. 4. Types of polymer clay


resin is shown in Fig. 5 [39]. Here it is clear that the d spacing in-creases with the organic modification of clay and it further in-creases when it swelled in epoxy resin. After curing, the epoxyclay nanocomposites can give either intercalated or exfoliatedstructure, where the d spacing will be increased further for interca-lated structure or the peak corresponding to the d spacing in theWAXD will be disappeared in exfoliated structures as shown inthe case of epoxy-5 wt.% Cloisite 30B (clay 1) mixtures cured atdifferent temperatures (Fig. 6) [39]. Lingaiah et al. suggested thatair plasma etching followed by scanning electron microscopy(SEM) imaging is a promising technique for visualizing the exfoli-ation and dispersion of inorganic nanofillers like clay and CNT inpolymer nanocomposites [40].

7. Preparation of nanocomposites

There are several methods to prepare clay based polymer nano-composites. These include in situ polymerization, melt intercala-tion and solution casting.

nanocomposites [9,24].



Fig. 5. The characteristic WAXD of (a) unmodified clay (Cloisite Na), (b and c)organically modified clay (Cloisite 30B (b) and clay 3 (c)) and (d) organicallymodified clays swelled by epoxy resin (Epon 828) [78].

Fig. 6. WAXD patterns showing (a) intercalated and (b) exfoliated clay structuresobtained by curing Epon 828–5 wt.% clay 1(Cloisite 30B) mixtures using JeffamineD230 respectively at 50 �C for 5 h and at 100 �C for 2 h [78].


7.1. In situ polymerization

In this method, the liquid monomers or prepolymers (epoxyresin) are intercalated into clay layers and polymerizes withinthe clay layers resulting the expansion of the interlayer distance(d spacing). Polymerization can be initiated by heat or a suitableinitiator. Most of the exfoliated nanocomposites are produced bythis method because it provides to select suitable reagents andpolymerization routes resulting a good affinity between clay andpolymer. In situ polymersiation technique has been used for thepreparation of nanocomposites based on polyamide (PA) [30],poly(e-caprolactone) [41], polystyrene (PS) [42], polyolefien (PPand PE) [43–45] polyethylene terephthalate (PET) [46], epoxy[6,10].

7.2. Melt intercalation

The melt intercalation involves the blending of clay with thepolymer matrix in molten state. If the layer surfaces have sufficientaffinity with the polymer, the polymer can diffuse between theclay layers and form either an intercalated or an exfoliated nano-composite. Melt intercalation technique is used for the preparationof nanocomposites based on polyamide such as nylon 6 [47] andnylon 66 (PA66) [48], and polyethylene terephthalate (PET) [49].This method is more economical and simpler than other methods.The melt intercalation process has become increasingly popularbecause of its great potential for application with rapid processing


methods such as injection molding [50] and twin screw extrusion[51]. Melt blending technique is more efficient and produces nano-composites with improved mechanical properties when it is pro-cessed under the aid of super critical carbon dioxide [52].

7.3. Solution casting

In the solution method, polymer clay nanocomposites areprepared by using a suitable solvent such as water, acetone, andchloroform, in which the polymer is soluble and the clay is dispers-ible. When the polymer solution and the clay-dispersed solutionare mixed, the polymer chains will be intercalated between theclay layers by replacing the solvent molecules. Intercalated poly-mers will remain in the clay layers upon the removal of solvent.It is reported that the increase in entropy by desorption of solventmolecules is the driving force for the intercalation of polymer fromsolution [53]. Water soluble polymers such as poly (ethyleneoxide) [54], and poly(ethylene vinyl alcohol) [55] have been inter-calated between the clay layers by this method. Nanocompositesbased on cellulose [56], high-density polyethylene [57], polyimide[29], etc. have been synthesized by this method using non-aqueoussolvents. The major advantage of this method is that it offers thepossibility to synthesize intercalated nanocomposites based onpolymers with low or even without polarity.

8. Epoxy clay nanocomposites

Among polymer layered silicate (clay) nanocomposites, epoxybased systems has been reported in detail, due to the ease ofprocessing as well as its versatile applications in various fields.The structure and properties of epoxy clay nanocomposites areinfluenced by the curing agents, clay modifier and processingmethod.

9. Curing agents

In epoxy clay systems, there have been used amine and anhy-dride based curing agents, which rendered different propertiesand morphology to the epoxy systems. It was found that whenanhydride was used as curing agent, an exfoliated morphologywhile diamino diphenyl methane (DDM) gave an intercalated mor-phology, since anhydride is a liquid and can easily diffuse into theclay gallery unlike DDM which is a solid [58]. Similar observationswere made by Xu et al. [59] for diethylenetriamine and tung oilanhydride. Kornman et al. [60] found that aliphatic amine curedepoxy produced an exfoliated morphology compared to cyclo ali-phatic amine cured epoxy because of the higher reactivity of for-mer. The diffusion rate and reactivity of the curing agent alsoinfluence the exfoliation of clay. Chin et al. [61] reported that anexfoliated nanocomposites is formed when the DGEBA epoxy withoctadecyl ammonium modified MMT is cured with less than stoi-chiometric amount of meta-phenylene diamine (MPDA) or withautopolymerization without curing agent. On other hand an inter-calated nanostructure is formed when it is cured with equimolar ofhigher concentration of MPDA. Extragallery cross linking has beendominated in the case of higher concentration of curing agent,resulting in intercalated nanocomposites. Kong et al. [62] studiedthe exfoliation behavior of epoxy clay nanocomposites varyingthe electro negativities of the aromatic diamine curing agent andcuring temperatures. Epoxy clay nanocomposites based on DGE-BA/PDA and DGEBA/MDA systems produced an intercalated struc-ture due to the high reactivity of the curing agents which inducesfaster gelation in the extragallery region, while DGEBA/DDS systemgave exfoliated structure due to the low reactivity of the curingagent thereby slowed down the extragalley gelation and provides




enough time for intragallery polymerization. The TEM picturesshowing the exfoliation and intercalation of DGEBA-MDA systemand DGEBA-DDS system are shown in Fig. 7a and b, respectively[62].

McIntyre et al. [63] has reported that the storage modulus andTg of the nanocomposites were increased with the addition of Cloi-site 30B for DGEBA epoxy cured by tryethylenetetramine (TETA),DDS and DDM while a lowering of Tg was observed for TGDDM/DDS system. Among the DGEBA system, DDS cured system showedimproved Tg and modulus compared to other curing agents. Allcomposites have not shown any significant improvement in thethermal stability by the addition of clay particles. Zilg et al. [64]found higher toughness for an intercalated structure and highermodulus (stiffness) for completely exfoliated structure for nano-composites based on anhydride cured DGEBA epoxy.

10. Clay modifier

Generally based on the organic modifier used in the modifica-tion clay, the structure and properties of the nanocomposites var-ies considerably. Intercalated nanocomposite is generally producedwith quaternary and tertiary alkyl ammonium surfactants due tolow bronsted acidity of the surfactants. The fixed layer separationof clay layers is unable to provide optimum level of reinforcement.Exfoliated nanocomposite is produced with primary and secondaryalkyl ammonium surfactants or quaternary surfactants containinghydroxyl groups due to the high bronsted acidity of the surfactants.In exfoliated nanocomposites, the clay layers will be sufficientlyseparated and randomly oriented to allow full interfacial bondingwith matrix resin to improve the properties of nanocomposites.In the case of primary and secondary ammonium modified clay,the rate of intragallery polymerization occurs at a faster rate com-pared to extragallery polymerization leading to an exfoliated struc-ture while the extragallery polymerization is in faster rate in thecase of tertiary and quaternary ammonium modified clay resultsan intercalated morphology. It is also reported that as the chainlength of alkyl ammonium (clay modifier) increases, the morphol-ogy changes from intercalation to exfoliation morphology [65].Similarly Wang et al. [66] also reported that the curing speed ofthe intra and extra layer epoxy amine reaction is the key factorfor the synthesis of exfoliated epoxy clay nanocomposites.

Park et al. [39] investigated a mechanism of nanoclay exfolia-tion in epoxy clay nanocomposites. The complete exfoliation ofclay layers from the intercalated tactoids can be produced if theratio of shear modulus to complex viscosity is maintained at or

Fig. 7. Transmission electron micrographs of (a) DGEBA-MDA-C18 clay (10 phr)and (b) DGEBA-DDS-C18 clay (10phr). Scale bar indicates 20 nm [68].


above 2–4 1/s, such that elastic forces inside the galleries outweighthe viscous forces offered by the extragallery epoxy. So the elasticforce exerted by the cross-linking epoxy molecules inside the claygalleries pushed out the outermost clay layers from the tactoidsagainst the opposing forces arising from electrostatic and van derWaals attraction and shear, the latter due to motion of clay sheetsduring exfoliation as shown Fig. 8. Contrary to earlier reports[65,66], they [39] found that the faster intragallery polymerization,though accelerated the exfoliation process, was not necessary forexfoliation. It was also observed that clays containing hydroxylatedquaternary ammonium ions and quaternary ammonium ions withno polar functional groups produced exfoliated structures equallyeasily, provided the ratio of storage modulus to complex viscositywas maintained above 2–4 1/s. Higher curing temperature as wellas the presence of organically modified clay particles acceleratedthe formation of gels, and the gel time presented an upper boundof time available for exfoliation.

Pluart et al. [67] demonstrated that adequate compatibilisationof montmorillonite followed by swelling of clay galleries by epoxywas necessary to obtain intercalated/exfoliated nanocomposites. Inanother study [68], they correlated the morphology and mechani-cal properties of epoxy clay nanocomposites and observed animprovement in stiffness for exfoliated nanocomposites. However,a very interesting stiffness/toughness balance was shown by theintercalated nanostructures without lowering their Tg. Lakshmi etal. [69] investigated the thermal stability and structural character-istics of different epoxy clay nanocomposites using hexadecylammonium and phosphonium clay and DDS as hardener. Theammonium modified clay epoxy system showed appreciablemechanical and glass transition temperature properties whilephosphonium modified clay epoxy system exhibited highest ther-mal resistance properties compared with unmodified epoxy sys-tems. Similarly, the higher thermal resistance properties of thealkyl phosphonium clay epoxy nanocomposites were reported byWei et al. [70].

Influence of clay surface modification on the structure of theepoxy clay nanocomposites using a hardener of polyoxypropylen-ediamine have been investigated by Ryznarova et al. [71]. Theyobserved that the difference between the curing rates inside andoutside clay gallery was crucial for achieving intercalation/exfolia-tion of the prepared nanocomposites. Protonated and functional-ized clay modifiers catalyzed the intragallery polymerization ofepoxy resulted in the reduction in gelation times, gradual increasein d spacing during curing and high degree of dispersion leading toincreased elongation at break and toughness. In contrast, nonfunctionalized alkyl ammonium ions were unable to catalyze theintragallery polymerization resulted in the faster extragallery poly-merization leading to only partially intercalated nanocomposites.

Kornman et al. [72] also studied the influence of the nature ofthe clay on the structure of epoxy clay nanocomposites. Organicmodified montmorillonite clay with a low CEC showed an exfoli-ated structure compared to clay with high CEC, which showed anintercalated structure during the swelling of clay in the epoxy resinfor 24 h prior to curing. This is due to the low amount of organicmodifier (octadecyl ammonium chloride) present in the formerclay provides more space for DGEBA molecules. So the self poly-merization of epoxy can occur in large extent and causes the diffu-sion of new DGEBA molecule between the clay layers leading to theexfoliation of clay. Triantafyllidis et al. [73] reported that the epoxyclay nanocomposites prepared by the incorporation of homoionicorganic clays exchanged with relatively short chain di- or tri-amines and mixed-ion organic/inorganic clays partially exchanged(35%) with long chain diamines modified by di- or tri-amines re-sulted in intercalated structures with improved young’s modulusand storage modulus. On the other hand, homoionic organic claysexchanged with long chain diamines and triamines resulted in



Fig. 8. Schematic illustration of the intercalated state and exfoliation process showing the forces acting on a pair of clay layers (a) organically modified clay, (b) epoxyintercalated state, and (c) forces acting on a two-particle tactoid [76].


exfoliated nanocomposites, but with compromised mechanicalproperties especially reduced Tg due to the plasticizing effect ofthe long chain amine modifiers.

Ha et al. modified the nanoclay using 3-amino propyl triethoxysilane and studied the wear [74], tensile [75] and fracture [76]properties of silane-treated clay/epoxy nanocomposites. The frac-ture, tensile and wear properties of the nanocomposites increasedfor the silane treated clay, due to the good dispersion of clay inepoxy as well as the improved interfacial adhesion between epoxyand clay layers. They also investigated the temperature effects onthe tensile and fracture properties of silane treated epoxy claynanocomposites [77]. Recently, Xu et al. [78] reported the effectof curing reaction and thermal properties of the epoxy claynanocmposites made of different polymerically-modified clays.They found an exfoliated morphology by XRD for (polystyrene-co-acrylic acid) PSAA modified clay–epoxy with improved thermalproperties due to the reaction occurred between epoxy groups andcarboxylic acid in the inter-gallery of PSAA which could facilitate toclay dispersion in the epoxy matrix.

11. Processing method

Processing methods influence the clay morphology. The usualprocessing methods to disperse the clay layers in epoxy matrixare mechanical stirring, ultrasound sonication [79,80], high shearmixing [81,82], ball milling [83], etc. Lam et al. [79] reported that10 min ultrasonication result an optimum micro-hardness at4 wt.% clay containing epoxy nanocomposite. Zunjarro et al. [80]reported that high speed shear mixing yielded better mechanicalproperties compared to ultrasonication, even though both methodsgave exfoliated morphology. Yasmin et al. [81] found that epoxyclay nanocomposites processed by three roll mill is efficient inachieving high levels of exfoliation and dispersion of clay particleswithin a short period of time. The modulus of the nanocomposites


found to increase with increase in clay concentration. However, thetensile strength was decreased with addition of clay particles com-pared to pure epoxy due to the occasional occurrence of nano tomicro sized voids in the microstructure. They observed that effec-tive degassing during processing will enhance the tensile strengthof the resultant nanocomposites. Chen et al. [84] prepared a fullyexfoliated layered silicate epoxy nanocomposites by the combina-tion of high shear mixing and ultrasonication in the presence ofacetone. Lu et al. [83] observed a decrease in impact strengthand flexural strength for 4,40-diamino diphenyl sulfone (DDS)cured epoxy clay nanocomposites processed by mechanical stirreras well as high speed emulsifying and homogenizing mixer(HEHM) where as processing by HEHM followed by ball milling im-proved the mechanical properties. It was reported that epoxy–DDS–clay nanocomposites processed by high pressure mixingmethod showed dramatic increase in fracture toughness comparedto direct mixing method [85]. However, the glass transitiontemperature decreases as the clay content increases. Liu et al. stud-ied [86,87] the effect of mixing method to improve the dispersionof clay in epoxy and observed a significant improvement in frac-ture toughness at 1 wt.% of clay loading for tetraglycidyl diaminodiphenyl methane (TGDDM) epoxy-DDS-clay nanocomposites syn-thesized with high pressure mixing method compared to directmixing method. The Tg of the nanocomposites is found to decreasewith increase in clay content. The effect of temperature, speed andtime at the pre-mixing step during dispersion on intercalation andexfoliation of clay in the epoxy resin have been studied by Ngo etal. [88]. Even though the above premixing parameters have not anysignificant effect on the intercalation of organoclay at the pre-mix-ing step, they have a positive effect on the intercalation/exfoliationof nanoclay at the curing step.

The effect of processing variables on the mechanical propertiesof clay/epoxy nanocomposites produced in a centrifuge has beenstudied by varying processing conditions such as centrifuge rotor




speed and curing temperature, including different types of clay[89]. As the amount of clay is increased, the elastic modulus ofepoxy–clay nanocomposites increased up to a maximum of 6% clay.The tensile strength and energy to failure continually increasewith clay quantity all the way up to 10%. The amount of increasein a particular property depends on the type of clay used and thesurface treatment of that clay. For the rotor speed, the slowerspeeds were found to be the best, while the higher speeds causedthe properties to deteriorate with the critical speed being around3000 rpm. After 3000 rpm the higher speeds caused the clay layersto actually break rather than separate, making the layers ineffec-tive as nanocomposite fillers. The curing temperature againshowed best results at lower temperature, but with higher temper-atures causing specimen brittleness and deterioration of its prop-erties. The optimum temperature range was about 80–100 �C.

Wang et al. [90,91] developed a new method, namely slurrycompounding for the preparation of highly exfoliated epoxy claynanocomposites using pristine clay, which involved the transferof clay water suspension to epoxy resin by solvent exchange stepand silane modification step. The important feature of the tech-nique was that very little amount of organic modifiers was enoughto facilitate the exfoliation of clay, in contrast to conventional orga-noclays, which normally contains at least 25–45% of organic sur-factants. The resultant nanocomposites showed improvement infracture toughness, young’s modulus, storage modulus and Tg.The formation of a large number of microcracks and the increasein fracture surface area due to crack deflection are the majortoughening mechanism in the nanocomposites. Wang et al. [92]investigated the effect of clay concentration on the morphologyand properties of epoxy clay nanocomposites prepared by in situpolymerization under mechanical stirring followed by ultrasonica-tion and observed a decrease in layer space along with aggregationof clay particles with increase in clay concentration due to the in-crease in viscosity. Thermal decomposition temperature remainedunchanged and the Tg decreased, whereas the storage modulus in-creased with increase in clay concentration. According to them, theimproved storage modulus is due to the stiff filler reinforcementwith partial exfoliation.

12. Properties of epoxy clay nanocomposites

Epoxy clay nanocomposites show enhanced thermo mechanicalproperties even with a small amount of layered silicate (65%).Improvements comprise higher modulus, increased strength, heatresistance, decreased gas permeability, reduced coefficient of ther-mal expansion and decreased flammability. The main reason forthis improved property in nanocomposites is the large interfacialinteraction between the matrix and layered silicate and also thehigh aspect ratio of the dispersed clay particles.

Fig. 9. Stress–strain behavior of clay nanocomposites [81].

13. Mechanical properties

Mechanical properties of polymer–clay nanocomposites dependon the microstructure in which how the clay layers are dispersed inthe polymer matrix. Generally the well dispersion of the clay par-ticles in the polymer matrix yields enhanced tensile modulus, stor-age modulus and tensile strength. Even though, the tensilestrength and modulus tend to increase with increasing clay con-tent, the increasing trend is more noticeable for the tensile modu-lus. The reinforcing effect of clay layers on the tensile modulus ismainly due to the high modulus and high aspect ratio of the dis-persed clay layers. This will provide large interfacial interaction be-tween clay layers and polymer matrix. Chan et al. [93] hasinvestigated the reinforcing mechanism of the epoxy clay nano-composites, particularly the interaction between nanoclay and sur-


rounding matrix. They observed 34% and 25% increase in Young’smodulus and tensile strength respectively with the incorporationof 5 wt.% of nanoclay to the epoxy matrix, as compared with a pris-tine sample. The reinforcing mechanism is proved to be the exis-tence of interlocking and bridging effects. Nanoclay clusters withthe diameter of 10 nm could enhance the mechanical interlockinginside the composites and thus, breaking up the crack propagation.The formation of boundaries between the nanoclay clusters andepoxy can refine the matrix grains and further improve the flexuralstrength of the composites.

Basara et al. observed 17.2% increase in tensile modulus when7 wt.% of Cloisite 30B was incorporated to epoxy matrix [94].Yasmin et al. [81] reported that the tensile modulus (stiffness)increases with increase in clay loadings as shown in the stress–strain behavior of epoxy clay nanocomposites (Fig. 9). But the ten-sile strength and strain to failure are found to decrease as the claycontent decreases.

A three phase model of epoxy, exfoliated clay layers and nano-layer clusters was developed by Luo and Daniel [95] to characterizeand model the young’s modulus of the nanocomposites. The Mori–Tanaka method was used to evaluate the young’s modulus of thesystem as a function various parameters such as exfoliation ratio,clay layer and clay cluster aspect ratios, d spacing and intragallerymodulus, matrix modulus and matrix poisson’s ratio. Relevant vol-ume ratios of the clays have been calculated from the weight ratio(w), matrix density (qm) and clay particle density (qc). Withoutintercalation or exfoliation, the clay particle volume ratio, Vc canbe calculated as

Vc ¼w=qc

w=qc þ ð1�wÞ=qm

The actual silicate material volume ratio V 0c

V 0c ¼ Vct

do

where t is the layer thickness and do is the initial layer spacing.After processing, resulting in partial exfoliation and intercala-

tion, the volume ratio, Ve of the exfoliated fraction, re of the clayparticle is

Ve ¼ Vcret=do

The volume ratio Vi of the intercalated cluster is

Vi ¼ v 0cð1� reÞdt¼ vcð1� reÞ

ddo

where d is the layer spacing in the intercalated clusters.The effective stiffness tensor C⁄ of the composite is calculated

by Mori–Tanaka’s method




C� ¼ C1 þ V2fðc2 � c1ÞAg

where C1 is the matrix phase stiffness tensor, C2 is the inclusionstiffness tensor, and V2 the inclusion volume ratio. The concentra-tion tensor A is given by

A ¼ AðdilÞ½V1I þ V2fAðdilÞg��1

and

AðdilÞ ¼ ½I þ SC�11 ðC2 � C1Þ��1

where I is the fourth order unit tensor, S is the fourth order Eshel-by’s tensor, and V1 the matrix volume ratio

For comparison purposes, the stiffnesses computed by the Voigtand reuss models are

C�ðVoigtÞ ¼ V1C1 þ V2fC2g;C�ðReussÞ ¼ ½V1C�1

1 þ V2fC�12 g�

�1

Interestingly, the model predictions as shown in the Fig. 10 were ingood agreement with experimental results. From the above model,it was concluded that (1) the composite modulus of the system in-creases with increase in the degree of dispersion, (2) exfoliation ra-tio has a decisive role in the enhancement of modulus, (3) highaspect ratios of the single clay platelet as well as clay clusters aredesirable in stiffness enhancement, and (4) larger d spacing is pre-ferred along with high cluster aspect ratio for large improvement incomposite modulus.

The study on the morphology and mechanical properties ofepoxy system with octadecylammonium ion-modified MMTshowed an improvement in modulus and fracture toughness (KIC)and exhibited a mixed intercalated/exfoliated structure by Beckeret al. [96]. Pluart et al. [68] also reported similar improvements inthe tensile strength and stiffness as well as fracture toughness ofDGEBA resin resulting from clay additions. Zhang et al. [97] found88% and 21% improvement in impact strength and tensile strengthrespectively for epoxy clay nanocomposites having 3 wt.% organo-clay. Ingram et al. [98] observed an increase in storage modulusfor DDS and DDM cured DGEBA epoxy clay nanocomposites. Similarobservations are made by Hussain et al. [99] for aromatic aminecured DGEBF epoxy clay nanocomposites. The storage modulusand fracture toughness were found to increase with increase in clayconcentration for epoxy clay nanocomposites cured by diethyltolu-enediamine [100].

Lin [101] has studied the effect of exfoliated nanoparticles onthe epoxy matrix to mechanical properties using organically trea-ted Na-montmorillonite (Cloisite 30B) and titanium dioxide nano-particles. It is found that the compression strength, fracturestrength and Young’s modulus for both reinforced nanocomposites

Fig. 10. Predicted and experimental results for Young’s modulus of epoxy/claynanocomposite as a function of clay concentration (DER 331 epoxy matrix) [95].


are much higher than that of pure epoxy matrix which is attributedto the vein shape and the ductile dendrite phase occurred in thematrix during the compressive deformation. The friction coeffi-cient and wear coefficient of Cloisite 30B nanocomposites wereeffectively reduced with rising filler content which should beattributed to the improved dispersion of the nano particles. Theyalso assumed that the wear rate of Cloisite 30B nanocompositesmay improve nearly 30% due to its large agglomerated particlewhich will prevent the entrapping of nanoparticle and epoxy deb-ris between two sliding materials.

However, many authors [9,102–104] have reported a reductionin tensile strength, impact strength, fracture toughness as well asthe strain at break of the polymer clay nanocomposites with theaddition of clay particles. There are several reasons to explain thedecrease in properties as clay content increases .One reason isthe stress concentration effect of agglomerated clay particles athigher clay loadings. The agglomeration of clay particles at higherclay concentration also results lower in mechanical properties dueto lowering of filler surface area and lower polymer/clay surfaceinteraction. Another reason is that as clay content increases, theviscosity of the system increases resulting in heterogeneity andnanovoids formation due to the entrapment of air bubbles duringsample preparation. Miyagawa et al. [105,106] have observed a de-crease in impact strength with increase in clay content for anhy-dride and amine cured epoxy nanocomposites.

The durability studies of epoxy montmorillonite clay nanocom-posites under room temperature, in hot and cold conditionsshowed that the mechanical properties were found to decreasewith increase in time. 2 wt.% clay filled epoxy nanocompositesshowed enhancement in properties with relatively less numberof cracks and better interfacial bonding in all conditions over itsneat counterpart [107]. Qi et al. [108] reported 25% improvementin fracture toughness with the incorporation of 5% Cloisite 30B inepoxy matrix.

Cluster size effect in the hardness and interlaminar shear prop-erties of nanoclay/epoxy composites with different amount ofnanoclay contents were examined [109]. It is found that the mi-cro-hardness of the composites was enhanced with the additionof small amount of nanoclay into the epoxy. However, there wasan optimal limit in which the hardness was dropped by continu-ously increasing the nanoclay content. This might be due to the sizeof the clusters reaching a crucial limit and therefore the reinforcingfunction of the nanoclays decreased. Interlaminar shear testshowed the short beam shear strength of the epoxy decreased afteradding few percents of nanoclay particles. Microscopic observationusing SEM on the fracture surfaces showed that the size of the clus-ters varied with the amount of nanoclays used in the composites. Itis reported that the exfoliation and dispersion of nanofillers innanocomposites can be effectively visualized by air plasma etchingfollowed by scanning electron microscope (SEM) imagingtechnique [40]. The method was applied to study MMT clay in vinylester, MMT/epoxy and carbon nanotube (CNT) in vinyl esternanocomposites. This technique provides microscopic to submicro-scopic 3-D details of the inorganic fillers in a polymer composite.

14. Thermal properties

Polymer clay nanocomposites are known for its high thermalstability and flame retardancy. The improved thermal stability isattributed to the action of clay layers as superior insulator andmass transport barrier to the volatile products generated duringdecomposition as well as assisting in the formation of char afterthermal decomposition [6,110–112]. The slowing down of the es-cape of the volatile products in nanocomposites is because of thelabyrinth effect of the silicate layers in the polymer matrix




[113,114]. The improved thermal stability of the polymer claynanocomposites has been reported for various types of organoclaysand polymer matrices. Zhang et al. [97] reported that the heat dis-tortion temperature and thermal decomposition were heightenedfrom 124 �C and 348 �C to 133 �C and 373 �C, respectively for epoxyclay nanocomposites with 5 wt.% clay compared to pristine epoxymatrix.

Lakshmi et al. [69] compared the thermal stability of unmodi-fied epoxy (UME) systems and clay modified epoxy (CME) systemsby using different epoxy resins such as bisphenol A diglycidyl ether(BDGE), bisphenol A propoxylate diglycidyl ether (BPDG), bisphe-nolAbrominated diglycidyl ether (BBDG) and tetraglycidyl ofdiaminodiphenylmethane (TGDDM) and different clay modifiessuch as hexadecyltrimethylammonium (HDTMA) modified clay(MMT) and hexadecyltriphenylphosphonium(HDTPP) modifiedclay as shown in Fig. 11a and b. The initial decomposition temper-ature (IDT) of the CME is found to be higher than any of the UMEsystems. Among the clays HDTTP modified systems showed highIDT. The improved thermal stability of the clay modified epoxy sys-tems are attributed to barrier action of hard MMT-Clay nanolayers,which protect from volatilizing epoxy polymer chains present inbetween them and also restrict the segmental motion of the poly-mer networks. The enhanced thermal stability of the nanocompos-ites may also be due to the presence of inorganic phases like, SiO2,Al2O3, and MgO in clay particles, high temperature resistant moie-ties like, phenyl units and/or bromine atoms present in the epoxy

Fig. 11. Thermal properties of (a) unmodified epoxy systems (UME) and (b) claymodified epoxy (CME) systems [69].


and the triphenylphosphine unit in the HDTPP-MMT clay. A signif-icant improvement in thermal stability has also been reported onepoxy clay nanocomposites prepared with reactive phosphorouscontaining organo clay (RPC) by the evaluation of activation energyand integral procedural decomposition temperature [115].

Wang et al. [115] has reported large increment in limiting oxy-gen index (LOI) with the incorporation of 5 wt.% of RPC to theepoxy resin indicating the extraordinary improvement in flameretardancy, which is ascribed to the synergistic effect of phospho-nium ions and silicate layers in the clay as well as enhanced sur-face area by the exfoliation of clay layers. The improved flameretardancy of the polymer clay nanocomposites may be attributedto the formation of thermal insulating and low permeable char res-idue of clay layers at the outer surface of the nanocomposite duringcombustion and acts as a protective barrier by reducing the heatand mass transfer between the flame and polymer. The char resi-due of clay layers will also reduce the oxygen uptake and the es-cape of volatile gases produced by polymer degradation [116].Camino et al. [117] reported the mechanism of the improved fireretardany of epoxy clay nanocomposites cured by methyl tetrahy-dropthalein anhydride. This is due to the formation of protectiveskin created by ablative reassembling of the clay layers as wellas the chemical structure of the clay. An improved fire resistancewas also observed in the case of epoxy carbon fiber compositeswith the addition of nanoclay and graphene nanosheets [118].

Gu and Liang [119] reported that thermal stability of the epoxyclay nanocomposites were influenced by the clay loading, structureand nature of the purge gas. The thermal degradation behavior wasfound to be in three steps in air and two steps in nitrogen for the 2and 10 wt.% of clay loading to the epoxy. In addition, the improvedthermal stability was observed for 2 wt.% clay loading due to itsexfoliated structure, while lowest thermal stability observed for10 wt.% systems with an intercalated structure in nanocomposites.Further, among the systems, the 10 wt.% clay incorporated systemshowed better flame retardancy due to high char yield and lowmaximum degradation rate temperature.

Kaya et al. [120,121] reported that the incorporation of unmod-ified clay (MMT) into the epoxy resin did not affect the Tg value,while the addition of 3 wt.% of organically modified clay (OMMT)increased the Tg by about 15 �C due to the better exfoliation of clayin the epoxy matrix. It is also observed that the epoxy clay nano-composites containing OMMT clay particles exhibited better opti-cal transparency than those with MMT. Flame retardancy of theepoxy was increased by the addition of clay particles and the burn-ing rate is decreased by 38% and 58% for MMT and OMMT nano-composites respectively for 10% clay loading.

Many investigators reported that the Tg of the polymer eitherincrease or decrease with the addition of clay particles. The in-crease in Tg is attributed to the slower segmental motion due thepolymer chains being anchored to the surface of the clay. Lu etal. [122] and Miyagawa et al. [123] observed an increase in Tg withthe addition of clay particles to epoxy resin. Liu et al. [100] ob-served a decrease in Tg with increase in clay content. This wasattributed to the plasticizer effect of the clay modifier.

Temperature effects on the fracture behavior and tensile prop-erties of silane-treated clay/epoxy nanocomposites were investi-gated [77]. Tensile tests were performed at �30 �C, 25 �C, 40 �C,and 70 �C. Tensile strength and elastic modulus were greater thanthose of unmodified samples for all temperatures except 70 �C.However, the tensile properties decreased as temperature in-creased. In particular, at 70 �C, the tensile properties were less than10% of the original value at room temperature, independent of sur-face treatment. The fracture and tensile properties of silane-treatedclay/epoxy nanocomposites increased due to good dispersion ofthe clay in epoxy and improvement in interfacial adhesive strengthbetween epoxy and clay layers.




15. Barrier properties

Clay layers in the polymer matrix can act as an effective barrierto the penetrants. The enhanced barrier property of polymer nano-composites is due to the labyrinth or tortuous path (Fig. 12) thatretards the diffusion of gas molecules through the polymer matrix.

Neilson’s equation is proved to be a reliable estimate [124] ofgas permeability of polymer-layered silicate nanocomposite sys-tems. The Neilson’s equation is as follows:

Pn

Pm¼ ð1� /Þ

1þ a/2

� �

where Pn represents the permeability of the resulting nanocompos-ite and Pm represents the permeability of the matrix polymer, / isthe volume fraction of clay platelets, a ¼ L

t, is the aspect ratio ofthe clay layers and L and t are the length and thickness of clay layersrespectively. Due to the mismatch of the experimental values withtheoretical predictions, the tortuosity factor was corrected by themodified Neilson’s model to include the orientation factor as pro-posed by Bharadwaj [27].

According to Bharadwaj, the permeability of a nanocomposite(Pn) containing clay stacks of length (L) (L also being the lengthof the individual clay platelets) and thickness (t) is related to thepermeability of the pure matrix (Pm) by

Pn

Pm¼ ð1� /Þ

1þ L/2t

� �23

� �Sþ 1

2

� �

where / is the volume fraction of clay platelets, S is defined as anorder parameter (S ¼ 1

2 ð3 cos2 h� 1Þ) representing the orientation

Fig. 12. Zigzag diffusion (tortuous) pathway of a gas through clay-based polymernanocomposites [29].

Fig. 13. Order parameter o


of the tactoids in the matrix and h represents the angle betweenthe direction of preferred orientation (n) and the sheet normal (p)unit vectors as shown in Fig. 13. The order parameter (S) can rangefrom 1 to –1/2. If S = 1(h = 0), the tactoids will be aligned perpendic-ular to the gas flow direction (in this case, the model is equivalent toNeilson’s model). When S = 0 (h = 57), the tactoid will be distributedrandomly and if S = �1/2 (h = p/2), the tactoids will be aligned in thegas flow direction. From Bharadwaj’s model, it can be concludedthat both aspect ratio and order parameter of the clay plateletsinfluence the permeability of nanocomposites.

Hydrogen gas permeability has been reduced to 70% for epoxyclay nanocomposites compared to base epoxy resin and observedfivefold decrease in helium leak rate for the filament wound carbonfiber reinforced cryo tanks made with the above nanocomposites[125]. Mittel [126] observed 30% reduction in O2 gas permeabilityfor 3.5 vol.% of organically modified vermiculite clay incorporatedepoxy systems. He also noticed a substantial decrease in water per-meation in the above epoxy clay systems. But for the unmodifiedsystem (Na vermiculite clay),the water permeation is found to in-crease indicating that the polarity of the hydrophilic interlayers issignificantly reduced after the organic modification, but interlayersare still partially polar to attract the molecules of water. Furtherimprovement in barrier property can be achieved by introducingmuch longer alkyl chains as well as increasing the grafting densityin the ammonium ions.

Kim et al. [127] reported a gradual decrease in moisture perme-ability with increase in clay content for epoxy clay nanocompos-ites, agreeing the prediction based on tortuous path model.Among the different organically modified clay, he observed bettermoisture barrier properties for nanocomposites in which the clayparticles with larger interlayer distance between the individualclay layers and more uniform distribution. Bagherzadeh et al.[128] found an increase in the barrier and anti-corrosive propertieswith increase in clay loading. Nanocomposite coating with 1 wt.%clay loading showed about 70% reduction in water uptake andthe best anti-corrosive performance of coatings was obtained at3 and 5 wt.% clay concentrations. In another report, the barrier effi-ciency of epoxy/montmorillonite nanocomposite coating to H2Swas substantially increased under high temperature and pressureoil–gas environment compared to pure epoxy coating [129].

16. Applications

Because of the large improvement observed in the mechanical,thermal and barrier properties, epoxy clay nanocomposites can be

f the clay layers [27].




used for many specific applications in aerospace, defense and auto-mobile industries. These composites are also used in high perfor-mance structural and functional applications such as laminatesand composites, adhesives, sealants, tooling, molding, casting,electronics and construction.

Epoxy clay nanocomposites reinforced with a high strength car-bon/glass fiber have the greatest potential for commercial applica-tions within the automotive and aircraft industries [130], due totheir ability to reduce component weight and enhance mechanicalproperties. Filament wound carbon fiber reinforced cryotanksmade with epoxy nanocomposite had a fivefold lower helium leakrate than the corresponding tanks made without clay [125]. Theprominent reduction in leak rate may be due to the flow inducedalignment of the clay layers during processing. Use of these ad-vanced, high barrier composites would eliminate the need for aliner in composite cryotanks, thereby simplifying constructionand reducing propellant leakage. Fatigue crack propagation behav-ior of epoxy composites have great importance in engineeringcomponents that are subjecting to cyclic loading. Khan et al.[131] observed a significant improvement (74%) in fatigue life withthe incorporation of 3 wt.% nanoclay to epoxy CFRP composite .Theenhanced fatigue life of clay-CFRP hybrid composites is due totoughening mechanisms induced by the improved fiber/matrixinterfacial bond and nanoclay induced dimples.

Since epoxy clay nanocomposites provide improved anticorro-sion protection [126], it can find new applications in modern air-craft anticorrosion coatings. Epoxy clay nanocomposites havebeen extensively used for the structural adhesive applications,due to its potential improvement in adhesive properties with prac-ticality and low cost. Recently, it was reported that the bulk adhe-sive strength of the epoxy resin could be improved with theaddition of nanoclay [132].

17. Conclusion

Epoxy nanocomposites are highly versatile polymer systems forthe new era of making lighter structural composites for variousaerospace and automobile applications. The final morphology,physical, chemical and barrier properties of the nanocompositeswere influenced by processing method, clay modifier and curingagents. Epoxy clay nanocomposites showed remarkable improve-ment in tensile, flexural and fracture toughness properties. Thermalstability and barrier properties were significantly improved by theincorporation of clay particles to epoxy systems. CTE and water per-meation of the nanocomposite were reduced considerably.

Acknowledgements

This work was supported by the Center for Science &Technology Research (CSTR) grant funded by the Korea govern-ment (MEST) (CSTR-002-100701-03) and Basic Science ResearchProgram through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology(2010-0023106).

References

[1] Skiest I. Handbook of adhesives. 2nd ed. NewYork: VNR Company; 1978.[2] Gaw KO, Kakimoto M. Polyimide-epoxy composites. Adv Polym Sci

1999;140:109.[3] May CA. Epoxy resins: chemistry and technology. New York: Marcel Dekker;

1973.[4] Mc Adams LV, Gannon JA. Encyclopedia of polymer science and engineering,

2nd ed., vol. 6. Wiley Interscience; 1986.[5] Ellis B. Chemistry and technology of epoxy resins. UK: Blackie Academic and

Professional; 1993.[6] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from

preparation to processing. Prog Polym Sci 2003;28:1539.


[7] Zeng QH, Yu AB, Lu GQ, Paul DR. Clay-based polymer nanocomposites:research and commercial development. J Nanosci Nanotechnol 2005;5:1574.

[8] LeBaron PC, Wang Z, Pinnavaia JT. Polymer-layered silicate nanocomposites:on overview. Appl Clay Sci 1999;15:11.

[9] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites:preparation, properties and uses of a new class of materials. Mater Sci EngR 2000;28:1–63.

[10] Pinnavaia TJ, Beall GW. Polymer clay nanocomposites. John Wiley &Sons, Inc.;2001.

[11] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicatenanocomposites. Prog Polym Sci 2008;33:1119–98.

[12] Ke YC, Stroeve P. Polymer-layered silicate and silica nanocomposites. Netherlands: Elsevier Inc.; 2005.

[13] Theng BKG. Formation and properties of clay-polymer complexes. Amsterdam: Elsevier Scientific publishing company; 1979.

[14] Calvert P. Potential applications of nanotubes. In: Ebbesen TW, editor. Carbonnanotubes. Boca Raton, FL: CRC Press; 1997.

[15] Favier V, Canova GR, Shrivastava SC, Cavaille JY. Mechanical percolation incellulose whisker nanocomposites. Polym Eng Sci 1997;37:1732–9.

[16] Chazeau L, Cavaille JY, Canova G, Dendievel R, Boutherin B. Viscoelasticproperties of plasticized PVC reinforced with cellulose whiskers. J Appl PolymSci 1999;71:1797–808.

[17] Mark JE. Ceramic-reinforced polymers and polymer-modified ceramics.Polym Eng Sci 1996;36:2905–20.

[18] Liu H-Y, Wang G-T, Mai Y-W, Zeng Y. On fracture toughness of nano-particlemodified epoxy. Compos Part B-Eng 2011;42:2170–5.

[19] Von Werne T, Patten TE. Preparation of structurally well-difined polymer-nanoparticle hybrids with controlled/living radical polymerizations. J AmChem Soc 1999;121:7409–10.

[20] Herron N, Thorn DL. Nanoparticles: uses and relationships to molecularcluster compounds. Adv Mater 1998;10:1173–84.

[21] Vaia RA, Price G, Ruth PN, Nguyen HT, Lichtenhan J. Polymer/layered silicatenanocomposites as high performance ablative materials. J Appl Clay Sci1999;15:67–92.

[22] Giannelis EP, Krishnamoorti R, Manias E. Polymer-silicate nanocomposites:model systems for confined polymers and polymer brushes. Adv Polym Sci1999;138:107.

[23] Biswas M, Sinha Ray S. Recent progressive in synthesis and evaluation ofpolymer-montmorillonite nanocomposites. Adv Polym Sci 2001;155:167–221.

[24] Giannelis EP. Polymer layered silicate nanocomposites. Adv Mater1996;8:29–35.

[25] Giannelis EP. Polymer-layered silicate nanocomposites: synthesis, propertiesand applications. Appl Organomet Chem 1998;12:675–80.

[26] Xu R, Manias E, Snyder AJ, Runt J. New biomedical poly(urethane urea) –layered silicate nanocomposites. Macromolecules 2001;34:337–9.

[27] Bharadwaj RK. Modeling the barrier properties of polymer-layered silicatenanocomposites. Macromolecules 2001;34:9189–92.

[28] Messersmith PB, Giannelis EP. Synthesis and barrier properties of poly(e-caprolactone)-layered silicate nanocomposites. J Polym Sci Part A: PolymChem 1995;33:1047–57.

[29] Yano K, Usuki A, Okada A, Kurauchi T, Kamigaito O. Synthesis and propertiesof polyimide-clay hybrid. J Polym Sci Part A: Polym Chem 1993;31:2493–8.

[30] Kojima Y, Usuki A, Kawasumi M, Fukushima Y, Okada A, Kurauchi T, et al.Synthesis of nylon 6-clay hybrid. J Mater Res 1993;8:1179–84.

[31] Laoutid F, Bonnaud L, Alexandre M, Lopez-Cuesta J-M, Dubois Ph. Newprospects in flame retardant polymer materials: from fundamental tonanocomposites. Mater Sci Eng R 2009;63:100–25.

[32] Morgan AB, Wilkie CA. Flame retardant polymer nanocomposites. John Wiley& Sons, Inc.; 2007.

[33] Gilman JW, Kashiwagi T, Lichtenhan JD. SAMPE J 1997;33:40–5.[34] Gilman JW. Flammability and thermal stability studies of polymer layered-

silicate (clay) nanocomposites. Appl Clay Sci 1999;15:31–49.[35] Dabrowski F, Le MB, Bourbigot S, Gilman JW, Kashiwagi T. In: Proceedings of

the Eurofillers’99. Lyon-Villeurbanne, France, 6–9 September 1999 X.[36] Bourbigot S, LeBras M, Dabrowski F, Gilman JW, Kashiwagi T. PA-6 clay

nanocomposite hybrid as char forming agent in intumescent formulations.Fire Mater 2000;24:201–8.

[37] Manias E, Touny A, Wu L, Strawhecker K, Lu B, Chung TC. Polypropylene/montmorillonite nanocomposites – review of the synthetic routes andmaterials properties. Chem Mater 2001;13:3516–23.

[38] Zanetti M, Lomakin S, Camino G. Polymer layered silicate nanocomposites.Macromol Mater Eng 2000;279:1–9.

[39] Park JH, Jana SC. Mechanism of exfoliation of nanoclay particles in epoxy-claynanocomposites. Macromolecules 2003;36:2758–68.

[40] Lingaiah S, Sadler R, Ibeh C, Shivakumar K. A method of visualization ofinorganic nanoparticles dispersion in nanocomposites. Compos Part B-Eng2008;39:196–201.

[41] Messersmith PB, Giannelis EP. Polymer-layered silicate nanocomposites:in situ intercalative polymerization of epsilon -caprolactone in layeredsilicates. Chem Mater 1993;5:1064–6.

[42] Akelah A, Moet M. Polymer-clay nanocomposites: free-radical grafting ofpolystyrene on to organophilic montmorillonite interlayers. J Mater Sci1996;31:3589–96.

[43] Jin Y-H, Park H-J, Im S-S, Kwak S-Y, Kwak S. Polyethylene/clay nanocompositeby in situ exfoliation of montmorillonite during Ziegler–Natta polymerizationof ethylene. Macromol Rapid Commun 2002;23:135–40.




[44] Bergman JS, Chen H, Giannelis EP, Thomas MG, Coates GW. Synthesis andcharacterization of polyolefin–silicate nanocomposites: a catalystintercalation and in situ polymerization approach. J Chem Soc ChemCommun 1999;21:2179–80.

[45] Tudor J, Willington L, O’Hare D, Royan B. Intercalation of catalytically activemetal complexes in phyllosilicates and their application as propenepolymerisation catalysts. Chem Commun 1996;17:2031–2.

[46] Ke YC, Long CF, Qi ZN. Crystallization, properties and crystal and nanoscalemorphology of PET-clay nanocomposites. J Appl Polym Sci 1999;71:1139–46.

[47] Lim S-H, Dasari A, Wang G-T, Yu Z-Z, Mai Y-W, Yuan Q, et al. Impact fracturebehaviour of nylon 6-based ternary nanocomposites. Compos Part B-Eng2010;41:67–75.

[48] Timmaraju MV, Gnanamoorthy R, Kannan K. Influence of imbibed moistureand organoclay on tensile and indentation behavior of polyamide 66/hectorite nanocomposites. Compos Part B-Eng 2011;42:466–72.

[49] Wang Y, Gao J, Ma Y, Agarwal. Study on mechanical properties, thermalstability and crystallization behavior of PET/MMT nanocomposites. USCompos Part B-Eng 2006;37:399–407.

[50] Ferreira JAM, Reis PNB, Costa JDM, Richardson BCH, Richardson MOW. Astudy of the mechanical properties on polypropylene enhanced by surfacetreated nanoclays. Compos Part B-Eng 2011;42:1366–72.

[51] Aristéia de Lima J, Pinotti CA, Felisberti MI, Gonçalves MC. Blends and claynanocomposites of cellulose acetate and poly(epichlorohydrin). Compos PartB-Eng 2012;43:2375–81.

[52] Su F-H, Huang H-X, Zhao Y. Microstructure and mechanical properties ofpolypropylene/poly (ethylene-co-octene copolymer)/clay ternarynanocomposites prepared by melt blending using supercritical carbondioxide as a processing aid. Compos Part B-Eng 2011;42:421–428.

[53] Vaia RA, Giannelis EP. Lattice model of polymer melt intercalation inorganically-modified layered silicates. Macromolecules 1997;30:7990–7999.

[54] Aranda P, Ruiz-Hitzky E. Poly(ethylene-oxide)-silicate intercalation materials.Chem Mater 1992;4:1395–403.

[55] Zhao X, Urano K, Ogasawara S. Adsorption of polyethylene glycol fromaqueous solution on montmorillonite clays. Colloid Polym Sci1989;267:899–906.

[56] Delhom CD, White-Ghoorahoo LA, Pang SS. Development andcharacterization of cellulose/clay nanocomposites. Compos Part B-Eng2010;41:475–81.

[57] Jeon HG, Jung HT, Lee SW, Hudson SD. Morphology of polymer/silicatenanocomposites – high density polyethylene and a nitrile copolymer. PolymBull 1998;41:107–13.

[58] Jiankun L, Yuacai K, Zongneng Q, Xiao-su Y. J Polym Sci B: Polym Phys2001;39:115.

[59] Xu W, Bao S, He P. Intercalation and exfoliation behavior of epoxy resin/curing agent/montmorillonite nanocomposite. J Appl Polym Sci 2002;84:842.

[60] Kornmann X, Lindberg H, Berglund LA. Synthesis of epoxy-claynanocomposites. Influence of the nature of the curing agent on structure.Polymer 2001;42:4493–9.

[61] Chin I-J, Thurn-Albrecht T, Kim H-C, Russell TP, Wang J. On exfoliation ofmontmorillonite in epoxy. Polymer 2001;42:5947–52.

[62] Kong D, Park CE. Real time exfoliation behavior of clay layers in epoxy-claynanocomposites. Chem Mater 2003;15:419–24.

[63] McIntyre S, Kaltzakorta I, Liggat JJ, Pethrick RA, Rhoney I. Influence of theepoxy structure on the physical properties of epoxy resin nanocomposites.Ind Eng Chem Res 2005;44:8573–9.

[64] Zilg C, Mulhaupt R, Finter J. Polyurethane nanocomposites containinglaminated anisotropic nanoparticles derived from organophilic layeredsilicates. Macromol Chem Phys 1999;200:661.

[65] Lan T, Kaviratna PD, Pinnavaia TJ. Mechanism of clay tactoid exfoliation inepoxy-clay nanocomposites. Chem Mater 1995;7:2144–50.

[66] Wang Q, Song C, Lin W. Study of the exfoliation process of epoxy-claynanocomposites by different curing agents. J Appl Polym Sci 2003;90:511–7.

[67] Pluart LL, Duchet J, Sautereau H, Halley P, Gerard JF. Rheological properties oforganoclay suspensions in epoxy network precursors. Appl Clay Sci2004;25(3–4):207–19.

[68] Pluart LL, Duchet J, Sautereau. Epoxy/montmorillonite nanocomposites:influence of organophilic treatment on reactivity, morphology and fractureproperties. H Polym 2005;46:12267–78.

[69] Lakshmi MS, Narmadha B, Reddy BSR. Polymer degradation and stability.Polym Degrad Stabil 2008;93:201–13.

[70] Wei X, Rongcai X, Wei-Ping P, Dough H, Bryan K, Loon-Seng T. Thermalstability of quaternary phosphonium modified montmorillonites. ChemMater 2002;14:4837–45.

[71] Ryznarova B, Zelenka J, Lednicky F, Baldrian J. Epoxy-clay nanocomposites:influence of the clay surface modification on structure. J Appl Polym Sci2008;109:1492–7.

[72] Kornmann X, Lindberg H, Berglund LA. Synthesis of epoxy-claynanocomposites: influence of the nature of the clay on structure. Polymer2001;42:1303–10.

[73] Triantafyllidis KS, Xidas PI, Pinnavaia TJ. Alternative synthetic routes to epoxypolymer-clay nanocomposites using organic or mixed-ion clays modified byprotonated di/triamines (Jeffamines). Macromol Symp 2008;267:41–6.

[74] Ha SR, Rhee KY. Effect of surface-modification of clay using 3-aminopropyltriethoxysilane on the wear behavior of clay/epoxy nanocom-posites. Colloids Surf A 2008;322:1–5.


[75] Ha SR, Ryu SH, Park SJ, Rhee KY. Effect of clay surface modification andconcentration on the tensile performance of clay/epoxy nanocomposites. MatSci Eng A 2007;448:264–8.

[76] Ha SR, Rhee KY, Kim HC, Kim JT. Fracture performance of clay/epoxynanocomposites with clay surface-modified using 3-aminopropyltriethoxy-silane. Colloids Surf A 2008;313–314:112–5.

[77] Ha S-R, Rhee K-Y, Park S-J, Lee J-H. Temperature effects on the fracturebehavior and tensile propertiesof silane-treated clay/epoxy nanocomposites.Compos Part B-Eng 2010;41:602–7.

[78] Xu Y, Peng H, Wang X, Su S. Comparative study of different polymerically-modified clays on curing reaction and thermal properties of epoxy resin.Thermochim Acta 2011;516:13–8.

[79] Lam C, Lau K, Cheung H, Ling H. Effect of ultrasound sonication in nanoclayclusters of nanoclay/epoxy composites. Mater Lett 2005;59:1369–72.

[80] Zunjarrao SC, Sriraman R, Singh RP. Effect of processing parameters and clayvolume fraction on the mechanical properties of epoxy–clay nanocomposites.J Mater Sci 2006;41:2219–28.

[81] Yasmin A, Abot JL, Daniel IM. Processing and characterization of clay/epoxynanocomposites by shear mixing. Scr Mater 2003;49:81.

[82] Velumurugan R, Mohan TP. Room temperature processing of epoxy–claynano composites. J Mater Sci 2004;39:7333.

[83] Lu H, Liang G, Ma X, Zhang B, Chen X. Polym Int 2004;53:1545–53.[84] Chen C, Tolle TB. Fully exfoliated layered silicate epoxy nanocomposites. J

Polym Sci B: Polym Phys 2004;42:3981–6.[85] Liu Z, Erhan SZ, Xu J. Preparation, characterization and mechanical

properties of epoxidized soybean oil/clay nanocomposites. Polymer2005;46:10119–27.

[86] Liu W, Hoa SV, Pugh M. Organoclay-modified high performance epoxynanocomposites. Compos Sci Technol 2005;65:307–16.

[87] Liu W, Hoa SV, Pugh M. Fracture toughness and water uptake of high-performance epoxy/nanoclay nanocomposites. Compos Sci Technol2005;65:2364–73.

[88] Ngo T-D, Ton-That M-T, Hoa SV, Cole KC. Effect of temperature, duration andspeed of pre-mixing on the dispersion of clay/epoxy nanocomposites.Compos Sci Technol 2009;69:1831–40.

[89] Samandari SS, Khatibi AA, Basic D. An experimental study on clay/epoxy.nanocomposites produced in a centrifuge, Compos Part B-Eng 2007;38:102–107.

[90] Wang K, Wang L, Wu J, Chen L, He C. Preparation of highly exfoliated epoxy/clay nanocomposites by ‘‘slurry compounding’’: process and mechanisms.Langmuir 2005;21(8):3613–8.

[91] Wang K, Chen L, Wu J, Toh ML, He C, Yee AF. Epoxy nanocomposites withhighly exfoliated clay: mechanical properties and fracture mechanisms.Macromolecules 2005;38(9):788–800.

[92] Wang J, Kong X, Cheng L, He Y. Influence of clay concentration on themorphology and properties of clay-epoxy nanocomposites prepared by in-situ polymerization under ultrasonication. J Univ Sci Technol Beijing2008;15(3):320–3.

[93] Chan M-L, Lau K-T, Wong T-T, Ho M-P, Hui D. Mechanism of reinforcement ina nanoclay/polymer composite. Compos Part B-Eng 2011;42:1708–12.

[94] Basara C, Yilmazer U, Bayram G. Mechanism of reinforcement in a nanoclay/polymer composite. J Appl Polym Sci 2005;98:1081.

[95] Luo J-J, Daniel IM. Characterization and modeling of mechanical behavior ofpolymer/clay nanocomposites. Comp Sci Technol 2003;63:1607–16.

[96] Becker O, Varley R, Simon G. Morphology, thermal relaxations andmechanical properties of layered silicate nanocomposites based upon high-functionality epoxy resins. Polymer 2002;43:4365.

[97] Zhang K, Wang L, Wang F, Wang G, Li Z. Preparation and characterization ofmodified-clay-reinforced and toughened epoxy-resin nanocomposites. J ApplPolym Sci 2004;91:2649–52.

[98] Ingram S, Rhoney I, Liggat JJ, Hudson NE, Pethrick RA. Some factorsinfluencing exfoliation and physical property enhancement in nanoclayepoxy resins based on diglycidyl ethers of bisphenol A and F. J Appl Polym Sci2007;106:5–19.

[99] Hussain F, Chen J, Hojjati M. Epoxy-silicate nanocomposites: cure monitoringand characterization. Mater Sci Eng A 2007;445–446:467–76.

[100] Liu T, Tjiu WC, Tong Y, He C, Goh SS, Chung T-S. Morphology and fracturebehavior of intercalated epoxy/clay nanocomposites. J Appl Polym Sci2004;94:1236–44.

[101] Lin J-C. Compression and wear behavior of composites filled with variousnanoparticles. Compos Part B-Eng 2077;38:79–85.

[102] Peeterbroeck S, Alexandre M, Jerome R, Dubois Ph. Poly(ethylene-co-vinylacetate)/clay nanocomposites: effect of clay nature and organic modifiers onmorphology, mechanical and thermal properties. Polym Degrad Stabil2005;90:288–94.

[103] Zhao C, Qin H, Gong F, Feng M, Zhang S, Yang M. Mechanical, thermal andflammability properties of polyethylene/clay nanocomposites. Polym DegradStabil 2005;87:183–9.

[104] Goettler LA. Ann Tech Conf Soc Plast Eng 2005:1980–2.[105] Miyagawa H, Drzal LT. The effect of chemical modification on the fracture

toughness of montmorillonite clay/epoxy nanocomposites. J Adhesion SciTechnol 2004;18:1571.

[106] Miyagawa H, Foo KH, Daniel IM, Drzal LT. Mechanical properties and failuresurface morphology of amine-cured epoxy/clay nanocomposites. J ApplPolym Sci 2005;96:281.




[107] Zainuddin S, Hosur MV, Zhou Y, Kumar A, Jeelani S. Durability studies ofmontmorillonite clay filled epoxy composites under different environmentalconditions. Mater Sci Eng A 2009;507(1–2):117–23.

[108] Qi B, Zhang QX, Bannister M, Mai YW. Investigation of the mechanicalproperties of DGEBA-based epoxy resin with nanoclay additives. ComposStruct 2006;75:514.

[109] Lam C-K, Cheung H-Y, Lau K-T, Zhou L-M, Ho M-W, Hui D. Cluster size effectin hardness of nanoclay/epoxy composites. Compos Part B-Eng 2005;36:263–9.

[110] Ray SS, Bousima M, Biodegradable polymers and their layered silicatenanocomposites. In: Greening the 21st century materials world, Progress inMaterials Science, vol. 50; 2005. p. 962–1079.

[111] Becker O, Varley RJ, Simon GP. Thermal stability and water uptake of highperformance epoxy layered silicate nanocomposites. Eur Polym J2004;40:187–95.

[112] Zhu J, Uhl FM, Morgan AB, Wilkie CA. Studies on the mechanism by which theformation of nanocomposites enhances thermal stability. Chem Mater2001;13:4649–54.

[113] Mc Neill LS. Comprehensive polymer science, vol. 6. Oxford: Pergamon Press;1989.

[114] Camino G, Sgobbi R, Colombier S, Scelza C. Investigation of flame retardancyin EVA. Fire Mater 2000;24:85–90.

[115] Wang WS, Chen HS, Wu YW, Tsai TY, Chen-Yang YW. Properties of novelepoxy/clay nanocomposites prepared with a reactive phosphorus-containingorganoclay. Polymer 2008;49:4826–36.

[116] Blumstein A. Polymerization of adsorbed monolayers. II. Thermaldegradation of the inserted polymer. J Polym Sci A 1965;3:2665–73.

[117] Camino G, Tartaglione G, Frache A, Manferti C, Costa G. Thermal andcombustion behaviour of layered silicate–epoxy nanocomposites. PolymDegrad Stabil 2005;90:354–62.

[118] Avila AF, Yoshida MI, Carvalho MGR, Dias EC, Avila Jr, J. An investigation onpost-fire behavior of hybrid nanocomposites under bending loads. ComposPart B-Eng 2010;41:380–7.


[119] Gu A, Liang G. An investigation on post-fire behavior of hybridnanocomposites under bending loads. Poly Degrad Stabil 2003;80:383–91.

[120] Kaya E, Tanoglu M, Okur S. Layered clay/epoxy nanocomposites:thermomechanical, flame retardancy, and optical properties. J Appl PolymSci 2008;109:834–40.

[121] Kaya E, Tanoglu M. In: Proceedings of advancing with composites conference.Italy; 2005. p. 27.

[122] Lu HB, Nutt S. Restricted relaxation in polymer nanocomposites near theglass transition. Macromolecules 2003;36:4010.

[123] Miyagawa H, Rich MJ, Drzal LT. Amine-cured epoxy/clay nanocomposites. I.Processing and chemical characterization. J Polym Sci Part B: Polym Phys2004;42:4384–90.

[124] Neilson LE. Models for the permeability of filled polymer systems. JMacromol Sci Part A 1967;1:929–42.

[125] Campbell SG, Johnston C. Polymer/silicate nanocomposites used tomanufacture gas storage tanks with reduced permeability. NASA Glenn’sResearch and Technology reports; 2004.

[126] Mittel V. Epoxy—vermiculite nanocomposites as gas permeation barrier. JCompos Mater 2008;42:2829–39.

[127] Kim J, Hu C, Woo RSC, Sham M. Moisture barrier characteristics oforganoclay–epoxy nanocomposites. Compos Sci Technol 2005;65:805–13.

[128] Bagherzadeh MR, Mahdavi F. Preparation of epoxy–clay nanocomposite andinvestigation on its anti-corrosive behavior in epoxy coating. Progr OrgCoating 2007;60:117–20.

[129] Wang XF, Hu YC, Li YP, Zhou Q. Preparation of epoxy/montmorillonitenanocomposite coating and its application in the high-temperature oil–gasenvironment with H2S. Adv Mat Res 2010;154–155:508–14.

[130] Njuguna J, Pielichowski K, Alcock JR. Epoxy-based fibre reinforcednanocomposites. Adv Eng Mater 2007;9(10):835–47.

[131] Khan SU, Munir A, Hussain R, Kim J-K. Fatigue damage behaviors of carbonfiber-reinforced epoxy composites containing nanoclay. Compos Sci Technol2010;70:2077–85.

[132] Sancaktar E, Kuznicki J. Nanocomposite adhesives: mechanical behavior withnanoclay. Int J Adhes Adhes 2011;31(5):286–300.



composites: part bsite.icce-nano.org/clients/iccenanoorg/hui pub/2013 epoxy... · 2013. 3. 11. ·...

Documents