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Clay Nanolayer Reinforcement ofcis-1,4-Polyisoprene andEpoxidized Natural Rubber

YEN T. VU,1 JAMES E. MARK,1 LY H. PHAM,2 MARTIN ENGELHARDT3

1 Department of Chemistry, The University of Cincinnati, Cincinnati, Ohio 45221-0172

2 Institute of Chemistry, Nghiado, Tuliem, Hanoi, Vietnam

3 Yokohama Tire Corporation, Salem, Virginia 24153

Received 15 September 2000; accepted 27 December 2000

ABSTRACT: Conditions were established for dispersing clay nanolayers into both cis-1,4-polyisoprene (synthetic) natural rubber (NR) and epoxidized natural rubbers (ENR)having 25 or 50 mol % epoxide. The clay was a sodium montmorillonite and was usedas a pristine layered silicate or as organically modified layered silicates to make thegalleries more hydrophobic and thus more compatible with the elastomers. The chem-ical modifications were carried out using an ion-exchange reaction with alkyl ammo-nium cations. Incorporation of the clays into the elastomers was achieved by mixing thecomponents themselves in a standard internal blender or by mixing dispersions of themin toluene or methyl ethyl ketone. X-ray diffraction results indicated intercalation ofNR and ENR into the silicate interlayers, followed by exfoliation of the silicate layersinto the elastomer matrices. Of primary interest was the effect of the intercalated andexfoliated clays on the mechanical properties of the elastomers. The reinforcing effectsobtained were found to depend strongly on the extent of the dispersion of the silicate

layers into the rubber matrices. 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82:13911403, 2001

Key words: elastomers; clay reinforcement; nanocomposites; modulus; tensilestrength; elongation at break

INTRODUCTION

A variety of clays are now being used to obtainunusual nanocomposites, by exploiting the abilityof the clay silicate layers to disperse into polymermatrices at the nanoscale level. The resulting clay

polymer nanocomposites exhibit properties verydifferent from their conventional counterparts.Some of the advantages of nanocomposites areimprovements in the mechanical properties, heat

stability, flame retardance, gas-barrier proper-ties, abrasion resistance, and solvent resistance.These improvements can be achieved at remark-ably low clay contents.

The first such clay-based composites were syn-thesized by a Toyota research group1,2 and involved

a polyamide as the polymeric phase. This work wasexpanded to the point that now there are numerousnanocomposites prepared using either clays or avariety of related layered materials. These include awide variety of other polymers, including polysty-rene,35 poly(ethylene oxide),4,6 polypropylene,511

polydimethylsiloxane,12,13 polycaprolactone,14 poly-urethanes,15,16 methyl methacrylate copolymers,17

styreneacrylonitrile copolymers,18 the polyisobuty-

Correspondence to: J. E. Mark (markje@email.uc.edu).Contract grant sponsor: National Science Foundation; con-

tract grant number: INT-900165.Journal of Applied Polymer Science, Vol. 82, 13911403 (2001) 2001 John Wiley & Sons, Inc.

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lenebromomethylstyrene copolymer,19 unsatur-ated polyesters,20 polyimides,21,22 epoxy resins,2326

polyaniline,27 polyhydroxybutyrate,28 nitrile rub-ber,29,30 and a high-performance poly(biphenylether triphenylphosphate) polymer.31 There hasalso been interest in both theory and simulations

addressing the preparation and properties of thesematerials.3239

The crystal structures of clays are unusuallyinteresting in their own right.40 For example, theclay montmorillonite consists of a dioctahedralaluminum sheet sandwiched between two silicatetrahedral sheets in a layer structure that is 1nm thick. Stacking of the layers leads to a regularvan der Waals space between the layers called aninterlayer or gallery. The primary particles them-selves consist of 1020 layers in a coplanar ori-entation (usually referred to as the tactoid forma-tion). Clay particles can associate through face-

to-face, face-to-edge, and edge-to-edge bonding toform aggregates and agglomerates. Clay has theability to undergo extensive interlayer expansionor swelling, exposing a large active surface area,permitting guest molecules to enter into the gal-leries. Interlayer cations such as Na, Ca, andK exist on the internal surfaces, but can be ex-changed with alkyl ammonium cations to givesurfaces that are less ionic or polar.40 Such organ-ically modified galleries are more easily pene-trated by polymers (either in the molten state orin solution) or by monomers that are subse-quently polymerized in situ. These entering guest

molecules can either simply increase the dis-tances between the still-parallel layers in an in-tercalation process or randomly disperse the sep-arate sheets entirely in an exfoliation. Eithertype of dispersion occurs at the molecular leveland the resulting materials are therefore justifi-ably called nanocomposites.

There are essentially no reports on the use ofnanolayered silicates in natural rubber (NR)itself or in its synthetic analog cis-1,4-polyiso-prene. The effects of the clays on any propertiesof these rubber composites is unexplored, eventhough one effect could well be the reduction ofair permeability in rubber tires. For these rea-sons, the present investigation involved intro-ducing nanolayers of a montmorillonite clayinto both nonpolar synthetic NR (cis-1,4 poly-isoprene) and NR modified by epoxidation. Theepoxidized natural rubber (ENR) has epoxygroups that are randomly distributed along thechain backbones, which gives it increased polar-ity and higher glass transition temperatures

(Tgs). The increased polarity is of particularinterest in the present context since it fre-quently facilitates reinforcement by fillers with-out the need for coupling agents.4143 This in-vestigation focused specifically on clay compos-ites in which NR or ENR was introduced by

mixing solutions of the elastomer with the or-ganically modified clays, to obtain both interca-lated and exfoliated materials.

EXPERIMENTAL

Materials

Synthetic NR, cis-1,4-polyisoprene (Natsyn 2200),was supplied by the Goodyear Tire and Rubber Co.(Akron, OH). Commercial-grade ENRs having 25 or50 mol % epoxy groups (ENR25 and ENR50) were

provided by Guthrie Latex, Inc. (Tuscon, AZ). Areference filler material, precipitated silica (Hi-Sil233), was provided by the Harwick Co. (Akron, OH).The clay used in this work was sodium montmoril-lonite (NaMt), and both pristine samples and or-ganically modified versions were supplied by South-ern Clay Products (Gonzales, TX). The organic mod-ifications were carried out using cation-exchangereactions between NaMt and the alkyl ammoniumcations which are described in detail in Table I. Theorganically modified clays used in this work werebis(2-hydoxyethyl)methyl tallow ammonium mont-morillonite (C1), dimethyl dihydrogenated tallowammonium montmorillonite (C2), and dimethyl hy-drogenated tallow (2-ethylhexyl)ammonium mont-morillonite (C3). These organic modifications in-creased the interlayer distances of the layered sili-cates from the original value of 11 (as provided bythe supplier) to 23 for C1 to the ranges 18.319.5 for C2 and 28.630.2 for C3.

Preparation of Samples

The clay loading used in polymerclay compositesis usually less than 10 wt %. In this investigation,the amounts of clay or organically modified clayswere 10, 20, and 30 parts per hundred (phr) ofrubber. Mixing of pristine clay (NaMt) or silicawith rubbers was accomplished using a Bra-bender mixer at a temperature setting of 100oC.The mixing of the organically modified clays withENR50 or Natsyn, however, was facilitated by theuse of solvents. Specifically, ENR50 rubber wasdissolved in methyl ethyl ketone (MEK) and syn-thetic NR (Natsyn) was dissolved in toluene. The

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modified clays were swollen in toluene and thenmixed with either of the elastomer solutions withvigorous stirring. The solvents in the resultingdispersions were then evaporated and the sam-ples dried under a vacuum. The resulting clay orsilica-filled rubbers were mixed with the additivesand then with sulfur in a Brabender mixer at atemperature not exceeding 60oC. The formula-tions are described in detail in Table II. The vul-canizations were carried out in a standard hotpress at 150oC for 30 min.

Characterization Techniques

Wide-angle X-ray diffraction (XRD) was used tostudy the nature and extent of the dispersions ofthe clays in the filled samples.44 The XRD pat-terns were obtained using an X-pert diffractom-eter (Phillips), at the wave length CuK 1.54 ,with the scattering shown as a function of thescattering angle. Dynamic mechanical analyses(DMA) were carried out on the uncured com-pounds using an RPA2000 mechanical spectrom-eter (Alpha Technologies, Akron, OH).45 Tensile

properties were obtained using an Instron Tester(Instron, Acton, MA) on dog-bone-shaped sampleshaving a length of 33 mm, a width of 5.7 mm, anda thickness of 2 mm. The strain rate was 500mm/min.

All tests were repeated several times on differ-ent samples, using identical procedures to test forreproducibility. The results were essentially in-distinguishable for the XRD and DMA tests, andnone of the differences in the tensile propertiesexceeded 5%.

RESULTS AND DISCUSSION

Morphologies from the XRD Results

These XRD results bear on the regular arrange-ments of silicate layers in both pristine or in-

tercalated forms and the irregular arrange-ments in the case of complete exfoliation of thelayers.40 Figure 1 shows the X-ray diffractionpatterns for the melt-blended compounds of ep-oxidized rubbers and pristine clay (NaMt).44

As usual, the 2 values along the abscissa canbe converted to layer spacing values d throughthe Bragg relationship (l 2d sin , where l isthe wavelength of the radiation). In both theENR25- and ENR50-filled elastomers, peakswere observed in the two angular ranges corre-sponding to spacings of 3032 and 1214 ,respectively. This indicates that mechanicalmixing above the melt facilitated the intercala-tion of ENR into the galleries of the clay. Whenthe NaMt loading was small (10 phr), the in-terlayer distances in ENR25 and ENR50 werearound 31.7 and 14.0 and 32.7 and 13.0 ,respectively. For 30 phr clay NaMt loading,the interlayer distances for ENR25 and ENR50were 30.8 and 11.3 and 31.9 and 11.9 ,respectively.

Table I Some Characteristics of the Clay and Its Modifications

The Clay and Its OrganicModifications Organic Modifiersa

Losses onIgnition (%)

NaMt 6C1 (CH3)N

(R)(CH2CH2OH)2 32

C2 (CH3)2N

(Rh)CH(CH)(C4H9) 34C3 (CH3)N

(Rh)2CH(CH)(C4H9) 43

a R represents the tallow (65% C18, 30% C16, and 5% C14), and Rh represents the same tallowafter hydrogenation.

Table II Compound Formulations

Ingredient I II III

ENR25 100 ENR50 100 Natsyn 100

Sulfur 1 1 1Stearic acid 2 2 2Zinc oxide 5 5 5Calcium stearate 2 2 MBTa 1CBSb 1 1 Fillersc

a Mecaptobenzothiazole.b N-Cyclohexyl-2-benzothiazolesulfenamide.cVarious fillers were employed, as described in the text.

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The extent of intercalation depends on thepolymer transport processes through the ag-glomerate micropores, the agglomerate size, thediffusion of the elastomer chains within the sil-icate galleries, and the nature of the elastomeritself. For example, more intercalation occurs inthe case of ENR50, which has a higher polarity.This intercalation of ENRs into the clay galler-ies occurred for a portion of the clay filler wherethe rubbery polymer was exposed to the silicate

layers, and this increased the interlayer dis-tance from 11 to 3032 . Some additionalswelling occurred in other regions, but this ap-

parently increased the interlayer distance byonly 1 or 2 . Although the extent of intercala-tion is thought to be independent of the fillerconcentration, in most cases, usually less than10% w/w, increased clay loadings from 10 to 30phr led to less intercalation of the polymer intothe galleries of the silicate layers, which meanssmaller interlayer distances.

More pronounced intercalation was achievedby using solutions for the mixing of the elas-

tomers and clays. The organically modified clayis able to swell in organic solvents due to itslong alkyl chain modification. In the case of the

Figure 2 X-ray diffraction patterns for compounds of ENR50 with organo-modifiedclay prepared by solution mixing.

Figure 1 X-ray diffraction patterns for compounds of ENR with NaMt prepared byblending the components in a Banbury mixer. Here and throughout, the first number inthe designation specifies the mol percent epoxy groups in the elastomer, and the secondnumber specifies the parts per hundred (phr) of the dispersed phase.

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ENR50 compounds with 10 phr organicallymodified clays, no peaks were observed in itsXRD pattern for the C1 compound, as shown inFigure 2. This indicates that sufficient rubberypolymer was introduced into these galleries sothat the layers were exfoliated (dispersed intorandom arrangements). For the compound with10 phr C2, the interlayer distance was in-creased by 12 , specifically from 18.319.5 to3032 , as shown in Figure 3. The differencein filler dispersion of the two clays presumablyresulted from the different organic modifiers.Organically modified clay C1 would be expectedto have better compatibility with ENR50 than

does C2, since the alkyl ammonium chains of C1contain polar groups.

Clays with long alkyl-chain modifications (C2and C3) were chosen for reinforcement of syn-thetic NR (Natsyn). The interlayer distance oforganically modified clay C3 was in a range28.630.2 , as shown in Figure 4. A broad peakaround 35.036.0 was observed in the XRDpatterns for the compound with 10 phr C3. Thisindicates that the intercalation of Natsyn intothe organically modified clay was pronounced,with some of the clay possibly being exfoliatedinto the elastomer matrix. Extensive intercala-tion was also observed for the C2 compound at

Figure 3 X-ray diffraction patterns for compounds of ENR50 with organo-modifiedclay C2.

Figure 4 X-ray diffraction patterns for compounds of synthetic NR, Natsyn, withorgano-modified clay C3.

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10 phr, as can be seen in Figure 5. When theorganically modified clays were present to theextent of 20 phr in the Natsyn elastomer, theinterlayer distance was 33.234.2 for the C3compound. The peak corresponding to 11.711.9 may be attributed to an unmodified part ofthe C3. For organically modified clay C2, Nat-syn increased the interlayer distance from18.319.5 to 30.131.6 , as shown in Figure 5,indicating that the amount of the intercalatedpolymer is greater for C2 than that for C3.

Thus, there is intercalation of rubber (ENR

or Natsyn) into the interlayers of either thepristine clay or the organically modified clays,by mixing above the melt or by solvent, withcorresponding increases in the interlayer dis-tances. Extensive intercalation was obtainedfor the ENR50 and Natsyn compounds at 10 phrloading of organo-modified clays.

Dynamic Mechanical Properties

Relationship to the Morphologies of the Materials

These results focus on the strain dependence ofthe elastic storage modulus G and tan (which isthe ratio of loss modulus G to G). Since thedynamic tests were run on uncured compounds,the storage modulus is determined by polymer-to-filler interactions, hydrodynamic reinforcement,and filler secondary structures.46,47 For a conven-tional particulate filler such as silica or carbonblack, the interactions between the elastomer and

the filler occur on the surfaces of the fillers, asillustrated in Figure 6(a). Bound rubber is madeup of elastomeric chains adsorbed onto the sur-face to the extent that they cannot be extracted bya solvent and can exhibit significantly elevatedglass transition temperatures (Tgs). This is oneaspect of fillerelastomer behavior that indicates

Figure 5 X-ray diffraction patterns for compounds of synthetic NR, Natsyn, withorgano-modified clay C2.

Figure 6 Schematic showing (a) adsorption of polymer onto conventional fillers(bound rubber), (b) intercalation of polymer into silicate layers, and (c) extensiveintercalation or exfoliation of silicate nanolayers into a rubber matrix, giving a verylarge surface area from a small amount of the clay filler.

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particularly strong interactions.4850 In thepresent context, elastomer chains intercalatedinto the layered structures, as illustrated in Fig-ure 6(b), could be considered bound rubber, withanalogous changes in the Tg. Polymerfiller inter-actions also include (i) interactions of intercalatedpolymer chains with surface layers of pristineclay filler in the case of NaMt and (ii) secondary

interactions with the long alkyl groups in the caseof the organically modified clays. This is illus-trated in Figure 7.

Hydrodynamic reinforcement is frequentlypresent in the case of conventional particulates, andarises from increase in the viscosity in the case of aliquid or increase in the modulus in the case of apolymer matrix, upon introduction of a filler. Hy-drodynamic reinforcement is a function of the filler-volume fraction and the shape factor of the fillerparticles.47,48,51,52 The shape factor, as described byGuth,51 is the ratio of the longest dimension of theparticle to the shortest. Thus, the shape factorwould be very large for those silicate layers exfoli-ated into a polymer matrix. An effective volume

fraction (instead of theoretical volume fraction)which takes into account the polymer chains immo-bilized by adhesion and occlusion has been used todescribe the effect of an active filler on elastomerproperties.53 The effective immobilization of thesechains is thought to make them more a part of thefiller rather than of the polymer. Thus, hydrody-namic reinforcement is largely dependent on the

filler surface area, surface energy, and aggregatestuctures.47,48,51,52,54The intercalation of a polymerinto silicate interlayers increases the active surfacearea of the filler. In addition, polymer layers con-fined between silicate layers are immobilized; thus,intercalation also increases the effective volume.The increased effective volume of modified claysincludes the modifier and the amount of the inter-calated polymer.

The effect of the filler secondary structure, or thefiller-networking effect, arises from the tendency offiller aggregates to form agglomerates or chainlikefiller networks, especially at high filler-volume frac-tions. Such interparticular networks are usuallybroken down under applied dynamic strains. The

Figure 8 Strain dependence of G at 50oC and 5 Hz for ENR50 compounds with 10phr of various fillers. DSA is the dynamic strain amplitude.

Figure 7 (a) Polymerfiller interactions for an intercalated polar elastomerorgano-modified clay system; (b) polymerfiller interactions for an intercalated nonpolar elas-tomerorgano-modified clay system.

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breakdown of the filler network caused by increas-ing the strain amplitude leads to a decrease in themodulus, generallytermed the Payne effect.55 Thefiller-networking effect depends on fillerfiller in-teractions, aggregate spacings, and fillerpolymerinteractions.47,54,56 Silica has a strong Payne effectdue to its significant fillerfiller interactions andweak polymerfiller interactions.47,5457 The or-ganic modification and the intercalation change thechemistry and structure of the clay particles. Thus,these changes have effects on the filler secondarystructure.

Storage Moduli

The present results were put into context by someresults on the silica (Hi-Sil) reference material.

Figure 8 shows the effect of the dynamic strainamplitude (DSA) on the storage modulus for com-pounds of ENR50 at 10 phr filler loading. Al-though the change in storage modulus values wasnot profound at this small filler loading, someeffects of the filler structures and morphologies onthe storage modulus were observed. The higherstorage modulus of the compound ENR50 withNaMt in comparison to that of compound withHi-Sil possibly results from a greater hydrody-namic reinforcement due to the intercalation ofthe polymer into the galleries of the NaMt. Withan identical NaMt loading, the storage modulusof ENR50 was greater than that of ENR25.44 Thisresult can be attributed to stronger polymerfillerinteractions in the more polar ENR50. The stor-

Figure 9 Strain dependence of G at 50oC and 5 Hz for ENR50 and (*) ENR25compounds with 30 phr filler.

Figure 10 Strain dependence of G at 50oC and 5 Hz for NR compounds with differentorgano-modified clays.

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age modulus also shows the expected increasewith increasing clay-filler loading.44

In the case of the organically modified clays, itshould be noted that the filler loadings cited in-clude the organic modification layers. Their rela-tive contributions can be gauged from the ignitionlosses given in Table I. Although the filler-net-working effect (decrease of G with increasingDSA) was not strong at 10 phr loading, some fillernetwork may still exist, especially for the C2 com-pound. The intercalation of the rubbery polymerinto the galleries of C2 expanded the clay parti-

cles. The expansion of the filler particles in-creased the filler volume so that more filler net-working occurred. However, at the same fillerloading, the storage modulus of the compoundwith C1 was rather constant with increasingDSA. This indicates that a filler-networking effectdid not exist. As seen in the XRD pattern of the 10phr C1 compound (Fig. 2), the elastomer was ex-tensively intercalated and it disrupted the regu-lar stacked-layer structure of the clay. When theprimary clay particles were dispersed into indi-vidual layers as illustrated in Figure 6(c), theaggregates and agglomerates that were formedfrom the primary particles disappeared. The ex-tensive intercalation or exfoliation of C1 in-creased the filler active surface while it broke upthe filler secondary structure. In addition, thestorage modulus of the 10 phr C1 compound atlarge-strain deformations is higher than that ob-tained using silica, because of a greater hydrody-namic reinforcement from an increase in the fillersurface area due to exfoliation. The fact that C1

gave a higher storage modulus than did C2 wasdue to its larger active surface area and also tothe stronger polymerfiller interactions arisingfrom the two OH groups in its organic chains. Forsuch organically modified clay fillers, the interac-tions depend on the nature of the elastomer ma-trix and on the chain length and structure of theorganic modifiers. Besides interacting with thesilicate layers themselves, a polymer would inter-act with the organic modifiers and a polar poly-mer can have stronger interactions with a modi-

fier that has polar groups. This stronger polymerfiller effect was more evident at higher loadingsas can be seen in Figure 9. The storage modulus ofthe compound with C1 was larger than that of thecompound with C2 over the range of the dynamicstrains investigated.

With increase of the filler loading from 10 to 30phr, a strong Payne effect was observed for bothorganically modified clays C1 and C2, but notwith compounds ENR50 or ENR25/NaMt, asshown in Figure 9. This indicates different filler-networking effects, which result from differenteffective volume fractions. As previously men-

tioned, the intercalated polymer ENR50 confinedwithin silicate layers was immobilized and actedas a part of the filler, increasing the volume of theclay particles and the extent of the filler network-ing. While more intensive intercalation wasachieved by solvent mixing (the solvent expandsthe polymer and expands the galleries of the sil-icate layers, facilitating the intercalation), it ispossible that only a small portion of the NaMt

Figure 11 Effect of organo-modified clay loading on the strain dependence of tan at50oC and 10 Hz for ENR50 compounds.

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filler was intercalated upon being mixed in theelastomer melt.

Figure 10 shows the strain dependence of Gfor the nonpolar elastomer, cis-1,4-polyisoprene(Natsyn). The storage modulus increased with in-creasing filler loading, as expected. Overall, thestorage modulus of Natsyn and the cited network-ing effects were much smaller than were those forthe polar epoxidized rubber compounds. To an

extent, the organic modification of the clay mayplay a surface modification role similar to that ofsilica in the case of nonpolar elastomers. Specifi-cally, surface modification greatly reduces fillerfiller interactions, filler-networking effects, andstorage moduli at low strains.54,55 With the samefiller loading, 20 phr, a stronger filler networkingeffect was observed for C3 compounds havinglarger interlayer distances. As usual, the filler-

Figure 12 Strain dependence of tan at 50oC and 10 Hz for NR compounds withvarious organically modified clays.

Table III Tensile Properties

CompoundsModulus at 100%

Strain (MPa)Modulus at 300%

(MPa)Tensile Strength

(MPa)Elongation at

Break (%)

ENR5010 phr Hi-Sil 1.17 3.08 16.3 68310 phr NaMt 1.34 3.48 15.2 66120 phr NaMt 1.87 5.17 16.1 60030 phr NaMt 3.07 7.83 15.2 48310 phr C1 2.15 4.40 12.6 90730 phr C1 5.82 9.20 9.89 39210 phr C2 2.15 4.50 11.8 77830 phr C2 9.89 14.8 274

ENR25No filler 0.10 2.36 13.2 69020 phr NaMt 1.73 4.51 16.1 63530 phr NaMt 2.10 5.50 13.9 538

Natsyn10 phr Hi-Sil 0.55 1.05 5.01 77210 phr C2 1.00 2.32 17.7 95520 phr C2 1.11 2.74 17.0 88910 phr C3 1.12 2.05 12.8 94420 phr C3 1.67 3.40 23.4 917

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networking effect depends on the interlayer dis-tance of the clays, with larger interlayer distancescorresponding to larger active volumes and in-creased filler-network effects.

Values of the Loss Tangent

The quantity tan G/G represents the ratio ofwork converted to heat to work recovered, for agiven work input. With regard to the filler effect,it is thought that tan is the ratio of the portionof the filler structure capable of being brokendown and reconstituted to the portion remainingunchanged during dynamic strains.54,56 The fac-

tor governing tan is thus the state of filler net-working, and, in general, the stronger the fillernetwork, the lower the value of tan . For conven-tional fillers, the hydrodynamic effect on tan iseliminated by forming the quotient of G and G.Above the glass transition temperature of theelastomer, the tan obtained using carbon blackis higher than that using silica due to its weakerfillerfiller interactions and stronger polymerfiller interactions.54,57 The difference in the dy-namic properties (G and tan ) between carbonblack and silica are reduced in polar rubbers sincethe strong filler-networking effect of silica is less-ened due to the increased affinity between thesilica surface and the polar elastomer.58,59

For modified clays, tan may involve morecomplicated mechanisms. For ENR50, com-pounds with clay C1 have stronger polymerfiller interactions, but have a lower tan thanthat of the C2 compounds, as shown in Figure11.44 This means that the portion of the net-work structure of intercalated C1 capable of

being broken down and reconstituted is smallerthan that of intercalated C2. In other words, C1behaved more like silica, while C2 behavedmore like carbon black. This may result fromthe difference in fillerfiller interactions due todifferent organic modifications, specifically thefact that C1 was the more polar filler. In addi-tion, since the clay fillers were intercalated bythe rubbery polymer, the strength of the clayfiller networks should also depend on theamount of the intercalated polymer which de-termines the interparticular reinforcement viajoint shells and polymer links.54,60 The strain

dependence of tan

for the 10 phr C1 compoundshows a different behavior, specifically under-going a minimum modulus at high strains. Thisbehavior may be due to the exfoliation of mod-ified clay into the rubber matrix.

For NR, tan also increased with increasingfiller loadings, as expected. Tan of the com-pounds with C3 is larger than that of compoundswith C2 and with Hi-Sil, as shown in Figure 12.The fact that the behavior of C3 was more likethat of carbon black is due to the structure of C3.C3 has more organic content, and its organic mod-ifier has two long alkyl chains, giving it a higher

affinity for nonpolar elastomers.

StressStrain Results in Tension

Some of the tensile properties of the vulcanizatesare shown in Table III.44 Compounds of ENRprepared by mixing pristine clay NaMt into therubber melt could attain tensile properties61 closeto or even higher than those of compounds withprecipitated silica (Hi-Sil). However, organically

Figure 13 Stressstrain curves in tension for ENR50 compounds with various clayfillers.

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modified clay showed different effects. At small

strains, vulcanizates with these clays generallyhad a much greater modulus, as shown in Figure13. This may result from a very high effectivevolume fraction and large hydrodynamic rein-forcement. The increased effective volume withincreasing filler loading led to a drastic decreasein the elongation at break. For both intercalationand exfoliation types of dispersion, the moduli ofthe C1 and C2 compounds at high strains weresmaller than were those for the NaMt com-pounds. One reason for this effect may be theweaker polymer networks. The mixture of the poly-mer with organo-modified clays obtained by solvent

mixing was hardened due to the high active vol-umes, resulting in a poorer sulfur dispersion.

For the synthetic NR (Natsyn), vulcanizateswith organically modified clay fillers had muchbetter tensile properties in comparision to thesilica Hi-Sil, as shown in Figure 14. This resultwas due to better polymerfiller interactions andweaker fillerfiller interactions of the modifiedclays. For 10 phr loading, the C2 compound hadbetter tensile properties than those of C1. Thus,exfoliation had a greater reinforcing effect on theproperties of these rubber vulcanizates.

CONCLUSIONS

Reinforcing effects obtained by the introduction oflayered silicate fillers are characterized for twoimportant types of elastomers. For the nonpolarelastomer, cis-1,4-polyisoprene, organic modifica-tion of the clay filler facilitated the intercalationof the elastomer into the clay galleries. Most im-

portantly, the modification was found to be simi-

lar to the well-known surface modifications of sil-ica in that it reduced fillerfiller interactions, im-proved filler dispersion, and, thus, efficientlyincreased the tensile properties of the vulcani-zates.

In the case of the polar elastomer (ENR), themixing of pristine clay into the elastomer led tointercalation, which increased the active surfacearea of the clay. This reinforcing effect of thepristine clay was found to be comparable to thatof high surface area silica. More intercalation wasachieved with the organically modified clays. Theamount of the intercalated polymer increases the

hydrodynamic reinforcement effects. The in-creased effective volume depended on the inter-layer distance, which is determined by the or-ganic modification and the intercalation of theelastomer. Increasing the effective filler volumethus formed more filler network, leading to a highstorage modulus at small strains.

The authors gratefully acknowledge the financial sup-port provided by the National Science Foundation(Grant INT-900165). The authors would like to thankSouthern Clay Products, the Guthrie Latex Co., theGoodyear Tire & Rubber Co., and the Harwick Co. for

generously providing samples.

REFERENCES

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Figure 14 Stressstrain curves for various formulations for the synthetic NR (Natsyn).

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