ROHSTOFFE UND ANWENDUNGENRAW MATERIALS AND APPLICATIONS

Ageing Effect on Dynamic andMechanical Propertiesof NR/Cel II Nanocomposites

Natural rubber � Cel II � Nanocompo-sites � Ageing

The influence of ageing caused byheating on nanocomposites preparedby co-coagulation of natural rubberlatex and Cel xanthate mixtures wasstudied in this work. For comparisonpurposes, carbon black natural rubbervulcanizates were also prepared. Theamount of Cel II (Cel II) varied from 0 to30 phr, while carbon black from 0 to35 phr. The different composites wereanalyzed as for crosslink density, dy-namic and mechanical properties, be-fore and after ageing.

A. F. Martins, L. L.Y. Visconte,

R. H. Schuster, F. Boller, Hannover and

Regina C.R. Nunes,

Rio de Janeiro (Brazil)

Corresponding author:

Regina Ceila Reis Nunes

Federal University

Macromolecular Institut

bl. J. Ilha do Fundao

21945-970 Rio de Janeiro Brazil

Einfluss der Alterung auf dyna-mische und mechanische Ei-genschaften von NR/Cel II Na-nocomposite

Naturkautschuk � Cel II � Nanocom-posite � Alterung

Der Einfluss der Hitzealterung aufNanocomposite, die durch Co-Ko-agulation von Naturkautschuk-La-tex und Cellulosexantat hergestelltwurden, ist untersucht worden. ZumVergleich der Eigenschaften wurdenrußgefullte Vulkanisate herangezo-gen. Die Verbundwerkstoffe wur-den hinsichtlich ihrer Vernetzungs-dichte und der mechanischen Ei-genschaften vor und nach der Alte-rung untersucht.

Nanocomposites are a new class of com-posite, constituted of particle-filled poly-mers for which the dispersed particleshave at least one dimension in the nano-meter range. One can distinguish three ty-pes of nanocomposites, depending onhow many dimensions of the dispersedparticles are in the nanometer range [1].When the three dimensions are in the or-der of nanometers, we are dealing withisodimensional nanoparticles, such asspherical silica nanoparticles obtained byin situ sol-gel methods, but also can inclu-de semiconductor nanoclusters andothers. When two dimensions are in thenanometer scale and the third one is larger,forming an elongated structure, we consi-der nanotubes or whiskers as, for example,carbon nanotubes or Cel whiskers whichhave been extensively studied as reinfor-cing nanofillers yielding materials with ex-ceptional properties. The third type of na-nocomposites is characterized by only onedimension in the nanometer range. In thiscase the filler is present in the form ofsheets of one to a few nanometer thickto hundreds to thousands nanometerslong.Because of the small size of the structuralunit (particle, grain or phase) and the highsurface-to-volume ratio, nanostructuredmaterials exhibit unique behavior compa-red to conventional materials with mi-cron-scale structures [2].In the area of polymers (especially elasto-mers) one is always searching for improvedmechanical properties to fit certain specificapplications. Reinforcement of elastomersis normally achieved by adding nano-sca-led fillers, such as carbon black and silica[3]. However, it is difficult to control themorphological structure of the resultingmaterial, particularly the degree of filler di-spersion. In situ generation of inorganic fil-ler, typically silica, through the sol-gel pro-cess provides an interesting method toovercome these problems [4].In the special case of the use of Cel as re-inforcing agent, the efficiency is related tothe nature of Cel itself and in particular to

its crystallinity, which in turn is dictated byits molecular structure [5].As elastomeric systems are important inthe field of industrial materials, they are of-ten used in many critical service and appli-cations. As a result, the estimation of theageing behavior of an elastomer is a mat-ter of great concern from the technologicalpoint of view [6].A fruitful approach to lifetime prediction ofany organic material is a molecular under-standing of the chemical processes invol-ved in the course of ageing. In polymer-ba-sed materials, chain scissions, crosslinkingreactions and oxidation. The two mainageing processes in rubbers are oxidationand ozonolysis. Oxidation ageing generallyoccurs quite slowly at ambient temperatu-res, but may have serious consequences asthe temperature is increased. Oxidation ofrubbers can involve the reaction of free ra-dicals present in the rubber with molecularoxygen. The free radicals can be formed bythe decomposition of hydroperoxides,which are present in the rubber in minuteamounts after processing. These reactionsthen lead to chain scission and/or additio-nal crosslinking, depending on the type ofrubber. For instance, oxidation of naturalrubber is initially dominated by chain scis-sion, which causes the rubber to soften.This softening is followed by an increasein crosslinking, which then leads to harde-ning. Both processes result in a weaker,brittle polymer. At ambient temperaturesthe main ageing mechanism involves thepresence of ozone from the atmosphere.The reaction with ozone leads to chain

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scission and the formation of polymericperoxides, which can also increase therate of oxidative ageing [7].As those processes are related to substan-tial modifications of the macromolecularbackbone, substantial damage in mecha-nical properties are expected, even atlow rate of conversion (less than 0.1%)[8]. The extent of degradation is very diffi-cult to be predict because reactions andthe rate at which a rubber degrades willdepend onmany factors such as the opera-ting temperature, chemical environment,loading conditions and type of rubber [8].In this work crosslink density, dynamic andmechanical properties of natural rubber(NR) and Cel II (Cel II) nanocomposites, be-fore and after ageing, were studied. Thesystems were prepared by co-coagulationof natural latex and Cel xanthate mixtures,so that filler content varied from 0 to30 phr. Through transmission electron mi-croscopy (TEM) these composites werefound to contain Cel II dispersed at the na-noscale range. Carbon black filled naturalrubber samples were also prepared by theconventional mixing. The properties of thevulcanisates were investigated before andafter ageing.

Experimental

Natural rubber and Cel II nanocompositeswere prepared by co-coagulation, as de-scribed elsewhere [9]. Mixing was carried

out on a Berstorff two-roll mill at 55 8C.The formulation used followed ASTM D3184. For comparison carbon black fillednatural rubber mixes were prepared follo-wing the same conditions as for Cel II. Theloading of carbon black N 762 was variedfrom 0 to 35 phr.The cure parameters were determined ac-cording to ASTM D 2084 on an OscillatingDisk Rheometer, Monsanto model 100S(ODR), operating at 140 8C and 38 arc. Vul-canization was carried out in an electricallyheated hydraulic press at 140 8C, The curetimes were previously determined in therheometer. From the resulting vulcanizedsheets, samples for the mechanical testswere cut.Stress-strength measurements were car-ried out on an Instron Universal Machine,model 1101, according to ASTM D 412, atroom temperature and crosshead speed of500 mm/min. Hardness was determinedaccording to ASTM D 2240.The crosslink density was determined fromequilibrium swelling in heptane at roomtemperature by using the Flory-Rehnerequation [10]. Small specimens(2.0 � 2.0 � 0.2 cm) dried to constantweight were allowed to swell in thedark, in sealed bottles until no furtherswelling occurred. The volume of imbibedheptane was calculated from the differen-ce between the weights of swollen anddeswollen samples.

Dynamic mechanical thermal analysis werecarried out according to ASTM D 4065 in aRheometric Scientific, model MK III, DMTAanalyzer. As mode of deformation, singlecantilever bending was used at a heatingrate of 2 8C/min; a frequency of 1 Hz inthe temperature range � 80 to 40 8C.The phase morphology was examined byTransmission Electron Microscop (TEM)using a Zeiss EM 902-machine. Thin secti-ons (max. 100 nm) were prepared with theultramicrotome “Ultracut FC 4 E”, (Cam-bridge Instruments) at 153–173 K and de-posited on non-treated 400 mesh coppergrids.Composites were submitted to acceleratedageing in an air-circulating oven at 70 8Cfor 72 hours. After ageing, crosslink densi-ty, dynamic and mechanical propertieswere determined.

Results and discussion

The mechanical properties before ageingare presented in Figures 1–3. From the re-sults of tensile strength it can be observedthat, compared to unfilled NR, the highestvalue of the stress at break (Fig. 1) is achie-ved by the composite containing 15 phr ofCel II, while the elongation at break (Fig. 2)decreases more accentuately as the fillercontent increases above 15 phr. Bothgroups of results indicate an optimum fillercontent for good tensile properties at

Fig. 1. Tensile strength of NR/Cel II before and after ageing Fig. 2. Elongation at break of NR/Cel II before and after ageing

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15 phr. It is interesting to note that the pre-sence of Cel II in amounts up to 15 phr re-inforces NR, without changing the elonga-tion at break (Fig. 2).Concerning hardness (Fig. 3), the increa-sing incorporation of Cel II into NR causesa continuous increase in this property.As a practical criterion for the stiffness ofrubber vulcanizates the modulus at300% (r300) was investigated. The resultsof r300 obtained for NR/Cel II compositesbefore ageing (Fig. 4) correlate well withhardness data.In comparison to the mechanical proper-ties demonstrated before ageing, the ten-sile strength (Fig. 1) and strain at rupture(Fig. 2) of the unfilled NR decrease, whilethe NR/Cel II composites exhibit an in-crease in tensile strength and a decreasein elongation at failure. It is interestingto emphasize that the composite contai-ning 15 phr of Cel II was the one to presentthe best ageing resistance. The drop inelongation at break after ageing is causedmainly by the weakening of the matrix as aresult of the main chain scission. The cross-link maturation reactions, which result inthe conversion of polysulphidic linkagesinto di- and monosulphidic ones, alsomight be contributing towards the de-crease in strain at break [11].Hardness data after ageing are in accordwith the tensile strength results afterageing, up to a filler content of 15 phr.

Above this critical filler concentration,the effects of degradation become moreevident.The modulus at 300% of the aged samp-les are shown in Figure 4. The benefit ofincorporating Cel II is clearly seen by a largeincrease in this parameter. Again, the un-filled sample reveal the detrimental ageingeffects. The decrease in the modulus valuefor NR gum upon ageing is due to the ge-neral weakening of the matrix resultingfrom the extensive main chain scission.The results of the investigation of ageingon network structure are shown in Fig. 5.Before ageing, the presence of Cel II indu-ces a significant increase in the apparentcrosslink density. Up to 15 phr there isan increase in crosslinking density obser-ved. However, as more Cel II is added,the crosslink density increases significantly.Both groups of values suggest a strongrubber-filler interaction.After ageing, a dramatic increase in thecrosslink density is observed for both theunfilled NR and the NR/Cel II composites.The most pronounced effect is shown byunfilled NR. This results from both oxidati-ve crosslinking and the post-curing reacti-ons [11]. For most applications crosslinkdensity must be sufficient to give the rub-ber mechanical integrity so that it can bearloads and present deformation recovery.However, the crosslink density shouldnot be so high as to make the polymer

chains immobilized, which then wouldlead to a hard, brittle rubber [7].Figures 6 and 7 show tan delta and the ela-stic modulus vs temperature for NR/Cel IIvulcanizates before ageing. As a characte-ristic feature of the unfilled system are thetwo preeminent relaxations: the first,around � 50 8C, which corresponds tothe glass transition temperature (Tg) ofthe NR and the other one, between� 25 and� 20 8C, related to its crystalliza-tion temperature (Tc).It is known that vulcanized NR also under-goes crystallization. The degree to which itoccurs depends upon factors such as thelevel of strain in the rubber, temperatureand the nature of the crosslinking or curingsystem. If a high-sulfur curing system isused, enough sulfur will react with NR toreduce the regularity of the molecules.Molecular irregularity reduces the capabili-ty of this rubber vulcanizate to crystallize.Under appropriate conditions, however,crosslinked NR will crystallize [12].From Figure 6, it can also be observed thatin all NR/Cel II vulcanizates NR forms supra-molecular crystaline organisations, as indi-cated by the melting signal (Tc) which doesnot vary significatively with the filler incor-poration. Thus, Cel II does not prevent cry-stallization of natural rubber probably dueto its very fine dispersion. The crescent in-corporation of Cel II narrows tan deltapeaks which is an indication of good di-

Fig. 3. Hardness of NR/Cel II before and after ageing Fig. 4. Modulus at 300% of NR/Cel II before and after ageing

448 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004

spersion, as well as lowers tan delta values,a consequence of higher elastic moduli. Inaddition, lower values of Tg for compositescontaining 20 and 30 phr of Cel II, as com-pared to unfilled NR are found, while fillercontents of 10 and 15 phr do not alter theTg of NR. Lower Tg values indicates again agood dispersion of Cel II in the elastomericmatrix.From Figure 7 it can be seen that the cre-scent incorporation of Cel II causes an in-crease in the elastic modulus of NR/Cel IIvulcanizates, more pronounced in the tran-sition and rubbery regions. For the latter, itcan be observed that the increase of theelastic modulus is related to the highercrosslink density of the filled vulcanizatesbefore ageing (Figure 5). The differencebetween the composites containing 10and 15 phr of Cel II is the higher stabilityof the crosslinkings for that one with15 phr of filler throughout the temperatu-re range.A rubber network consists of chemicalcrosslinks, physical entanglements, andloose chain ends. The effective crosslinkdensity contains a contribution due to che-mical crosslinks and another one due tochain entanglements and loose chainends, acting as physical crosslinks in therubber. This is usually estimated by swel-ling measurements, from modulus valuesin the rubbery region or from stress-straindata [13]. It is interesting to note that thecomposites containing 20 and 30 phr of

Cel II present Tg values lower than thatfor the unfilled natural rubber, as alreadymentioned, and higher crosslink densities.Thus, these composites probably have asmall contribution from chemical cross-links, which effectively restrict molecularmotion of the polymeric chains.DMTA data taken after ageing are presen-ted in Figures 8 and 9. The results of tandelta (Figure 8) show that Tg’s were all shif-ted towards lower temperatures, around� 55 8C, when compared to the results be-fore ageing (Figure 6). This corroboratesthe previous statement that the effect cau-sed by degradation is mainly due to chainscission. Another finding is that, afterageing, the composites no longer exhibita crystallization temperature.From Figure 9 it can be observed that, afterageing, the elastic moduli are higher thanthose before ageing (Figure 7), mainly inthe rubbery plateau. In addition, the hig-hest effect of degradation is on compositescontaining 20 and 30 phr of filler, as theypresent E 0 values smaller than the one con-taining 15 phr in the entire range studied,thus corroborating the hardness results de-scribed before. After ageing, the higher va-lues found agree with the results of cross-link density discussed earlier.The properties of carbon black vulcaniza-tes before ageing are presented in Table 1.From the results of stress-strain experi-ments, it can be observed that, as compa-red to unfilled NR, the highest value of the

tensile strength is achieved by the compo-site containing 30 phr of carbon black,while the elongation at break decreasesas the filler content increases. No reinfor-cing effect for NR composites with 10and 20 phr of carbon black N 726 couldbe observed. According to the literature[13], the occurence of a maximum in thetensile strength-loading curves for reinfor-cing blacks in NR depends on the nature ofthe rubber compound and the mixing con-ditions. Sometimes tensile maxima are notobserved where gums have high strength.This is observed in the in this work also.Concerning the reinforcing effect on NR,Cel II was more efficient than carbon blackN 762. In addition, the maximum reinfor-cement could be achieved with an amountof Cel II which was half the amount of car-bon black without any loss in the elonga-tion at break.Data of hardness and modulus 300% forthe carbon black composites are largerthan that for the unfilled one. However,both properties were found to be inferiorin this case than in NR/Cel II nanocompo-sites.The effective crosslink density for NR/CBcomposites increases as the filler contentincreases, passing through a maximumat 30 phr of carbon black. Once again,these results are lower than those forNR/Cel II nanocomposites.The properties of NR/CB composites afterageing are also shown in Table 1. The ten-

Fig. 5. Crosslink density of NR/Cel II before and after ageing Fig. 6. Tan delta versus temperature of NR/Cel II before ageing

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sile strength shows a maximum at 20 phrof filler, hardness is not affected by ageingand modulus 300% decreases for all car-bon black composites studied, except forthat containing 35 phr. These two latterparameters present different behavior af-ter ageing and cannot be correlated.

The effective crosslink density results afterageing are similar for 10 and 20 phr of car-bon black, when compared to those befo-re ageing, and highest at 30 phr. The va-lues of this parameter for the compositeswith 30 and 35 phr of carbon black in-crease upon ageing. These results are in

agreement with the tensile strength ofNR/CB composites after ageing.According to the literature [11] carbonblack causes an increase in the rate ofmain chain scission and crosslink scission,more pronounced when a conventionalvulcanization system is used. It acceleratesthe oxygen uptake of sulphur-cured rubberand the reaction is accompanied by a rapiddegradation of the polymer. The catalysisof oxidation of rubber by carbon blackcould involve the breakage of polysulphi-dic linkages and cyclic sulphides in the net-work, whose concentration is much higherin the conventional vulcanizates, intomono- and disulphidic ones. Because ofthe higher rate of oxygen uptake of car-bon-black filled vulcanizates, penetrationof oxygen to the inner layers of the vulca-nizates will be slower in these layers. Thiscauses the presence of a fairly undegradedinner layer of rubber in the vulcanizate. Du-ring the tensile test most of the appliedstress can be carried by this undegradedlayer. In the case of the unfilled vulcani-zate, the rate of oxygen uptake is muchslower but the rate of penetration of oxy-gen to the inner layer is not hindered as inthe case of the carbon black-filled vulcani-zates. Hence chances for the presence ofan undegreaded inner layer are less. Theresults obtained are in agreement withthis reasoning.

Tab. 1. Properties of NR/CB vulcanizates before and after ageing

Property Carbon black con-tent (phr)

Before ageing After ageing

0 21.5 18.910 20.1 21.1

Tensile Strength 20 20.4 20.7(MPa) 30 23.0 16.4

35 18.0 11.1

0 700 55010 550 550

Elongation at break 20 500 500(%) 30 500 425

35 450 300

0 42 4210 43 41

Hardness 20 47 46(Shore A) 30 52 50

35 52 56

0 2.58 2.1910 4.17 3.89

Modulus at 300% 20 6.83 4.84(MPa) 30 8.63 7.75

35 8.30 11.1

0 2.58 11.30Effective Crosslink density 10 3.51 3.47

20 3.85 3.64(mol/cm3) � 104 30 4.01 4.47

35 3.72 4.37

Fig. 7. Elastic modulus versus temperature of NR/Cel II before ageing Fig. 8. Tan delta versus temperature of NR/Cel II nanocomposites afterageing

450 KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004

Comparing the after ageing results for NR/Cel II and NR/CB composites, it can be con-cluded that Cel II is more efficient in pre-venting a fast degradation and loss of pro-perties of the natural rubber due to its ex-cellent dispersion in the elastomeric matrixand the strong rubber-filler interaction.The nanocomposite character of these NR/Cel II systems was proven by transmissionelectron microscopy and is presented in Fi-

gure 10. Cel II is well dispersed and has ananofibrilar structure. In this work, only thecomposite contaning 15 phr of Cel II isshown, since it is the one to give thebest performance.

Conclusions

Cel II has large influence on mechanicaland dynamic properties of natural rubber

vulcanizates, before and after ageing. Be-fore ageing, this filler participates on cross-linking and reinforces the composites,indicating a good rubber-filler interaction.The best performance was achieved by thecomposite with filler content of 15 phr.After ageing, NR/Cel II composites showedbetter properties and, again, the compo-site with 15 phr of Cel II was the bestone. From these observations, it can besaid that either the ageing conditionsused in this work were not severeenough and/or somehow Cel II protectsnatural rubber from an acCelerated de-gradation.

Acknowledgements

The authors thank the Brazilian fundingagency, Conselho Nacional de Desenvolvi-mento Cientıfico e Tecnologico (CNPq) andFundacao Universitaria Jose Bonifacio(FUJB) for providing the financial support,and Fibra S.A. for supplying Cel xanthate.

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Fig. 9. Elastic modu-lus versus tempera-ture of NR/Cel II na-nocomposites afterageing

Fig. 10. Transmissionelectron microscopyof NR/Cel II nano-composite containing15 phr of Cel II

KGK Kautschuk Gummi Kunststoffe 57. Jahrgang, Nr. 9/2004 451