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Research ArticleSilica-Reinforced Natural Rubber: Synergistic Effects byAddition of Small Amounts of Secondary Fillers toSilica-Reinforced Natural Rubber Tire Tread Compounds

S. Sattayanurak,1,2 J. W. M. Noordermeer ,1 K. Sahakaro ,1,2 W. Kaewsakul,1

W. K. Dierkes ,1 and A. Blume1

1Elastomer Technology and Engineering, Department of Mechanics of Solids Surfaces & Systems,Faculty of Engineering Technology, University of Twente, Netherlands2Department of Rubber Technology and Polymer Science, Faculty of Science and Technology,Prince of Songkla University, Pattani Campus, 'ailand

Correspondence should be addressed to J. W. M. Noordermeer; [email protected]

Received 4 October 2018; Accepted 3 December 2018; Published 3 February 2019

Academic Editor: Federica Bondioli

Copyright © 2019 S. Sattayanurak et al. 0is is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

Modern fuel-saving tire treads are commonly reinforced by silica due to the fact that this leads to lower rolling resistance andhigher wet grip compared to carbon black-filled alternatives. 0e introduction of secondary fillers into the silica-reinforced treadcompounds, often named hybrid fillers, may have the potential to improve tire performance further. In the present work, twosecondary fillers organoclay nanofiller and N134 carbon black were added to silica-based natural rubber compounds at aproportion of silica/secondary filler of 45/10 phr. 0e compounds were prepared with variable mixing temperatures based on themixing procedure commonly in use for silica-filled NR systems. 0e results of Mooney viscosity, Payne effect, cure behavior, andmechanical properties imply that the silica hydrophobation and coupling reaction of the silane coupling agent with silica andelastomer are significantly influenced by organoclay due to an effect of its modifier: an organic ammonium derivative. 0is has aneffect on scorch safety and cure rate. 0e compounds where carbon black was added as a secondary filler do not show thisbehavior. 0ey give inferior filler dispersion compared to the pure silica-filled compound, attributed to an inappropriate highmixing temperature and the high specific surface area of the carbon black used. 0e dynamic properties indicate that there is apotential to improve wet traction and rolling resistance of a tire tread when using organoclay as secondary filler, while thecombination of carbon black in silica-filled NR does not change these properties.

1. Introduction

Tire compounds generally contain about 30% by weight ofactive fillers like carbon black and silica [1]. Carbon black isconventionally used as reinforcing filler for tire treadcompounds because it can effectively improve mechanicalproperties such as tensile strength, modulus, tear strength,flex fatigue, and abrasion resistance as well as providinggood skid resistance.0e reinforcing effect of carbon black isnormally controlled by particle size, structure, specificsurface area, and surface chemistry [2]. Replacement ofcarbon black with silica in tire tread compounds, after the

patent of Michelin [3], offers tires with lower rolling re-sistance and higher wet grip [4], so less fuel consumptionand better safety. 0e other compound properties also canimprove by using silica such as tensile strength, tear strength,heat build-up, cut-, chip-, and chunking-resistance [5].However, the use of silica in rubber compounds has someshortcomings such as incompatibility with nonpolar tirerubbers, poor dispersion and distribution in the rubbermatrix, and poor filler-rubber interaction [6]. In order toovercome those limitations, sulfur-containing silane cou-pling agents such as bis(3-triethoxysilylpropyl)tetrasulfide(TESPT) are commonly applied in such compounds. 0e

HindawiAdvances in Materials Science and EngineeringVolume 2019, Article ID 5891051, 8 pageshttps://doi.org/10.1155/2019/5891051

mailto:[email protected]://orcid.org/0000-0002-1478-9896http://orcid.org/0000-0002-2908-7213http://orcid.org/0000-0002-2606-4534https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2019/5891051

mixing of silica with silane in rubber involves a chemicalreaction, so called silanization, and the extent of this reactionhas a dramatic effect on the resulting compound properties[7, 8]. Due to the nature of TESPTas sulfur donor, to ensurescorch safety during processing, bis(3-triethoxysilylpropyl)disulfide (TESPD) may be used [9]. Moreover, epoxidizednatural rubber (ENR) has been also used as a compatibilizerin silica-reinforced NR tire tread compounds, resulting in asubstantial improvement in the properties when comparedto a compound without ENR, but somewhat less than withusing a silane coupling agent [10].

A recent study on silica-reinforced natural rubber (NR)tire tread compounds has shown that, under optimal con-ditions and formulation, silica can effectively reinforce NR asshown by mechanical and dynamic properties [8]. However,there remains a question regarding the performance of suchtire treads in terms of abrasion or wear resistance. 0eabrasion mechanism of rubber vulcanizates is very complexinvolving multiple factors, and fillers have a major influenceon this property [11]. 0e filler particle size, structure,surface activity, and filler-rubber interaction all have animpact on the abrasion performance. Nowadays, in additionto carbon black and silica, there is a new generation ofreinforcing fillers available, especially nanofillers that can bealso applied for tire compounds. One of the most widelystudied nanofillers is organo-modified clay or shortlyorganoclay (OC) which has been modified by cation ex-change with ammonium salts or phosphonium salts in orderto obtain the hydrophobic structure that is compatible withnonpolar rubbers [12, 13]. OC-elastomer composites withlow nanofiller contents (usually 10 phr or less) show im-proved mechanical properties, thermal stability, gas per-meability, and wear resistance [14].

To reinforce the rubber, either a single or dual fillersystem is utilized. A combination of silica and carbon blackattracts interests to combine the advantages of each filler inthe rubber compounds. Investigations on silica and carbonblack reinforced NR-based truck tread compounds dem-onstrated that, when N220 carbon black (N220 CB) wasprogressively replaced by TESPT-modified silica, the com-plete replacement of carbon black by the TESPT-silicaresulted in 30% improvement in the rolling resistancewith little change in the treadwear index (abrasion re-sistance) and wet traction [15]. 0e retraction behavior ofNR vulcanizates as influenced by silica-N220 CB mixedfillers in the presence and absence of a silane coupling agentwas studied [16], in which the vulcanizates containing thesilane coupling agent showed faster recovery rates comparedto those without containing the silane coupling agent, due tothe polymer-filler interactions, crosslinks between therubber and silica, and increased crosslink densities. Withincreasing the silica proportion, the vulcanizates with andwithout the silane showed a decrease in hardness andmodulus. Based on the mechanical properties of NR rein-forced with various ratios of N330 CB/silica hybrid fillers, itwas reported that vulcanizates containing 20 and 30 phr ofsilica with a total mixed filler content of 50 phr exhibited thebest overall mechanical properties [17]. 0e partial sub-stitution of CB with silica/silane improved tear strength and

crack growth but had a negative effect on abrasion resistance[18]. Composites of solution-SBR rubber/silica/CB showedbetter filler dispersion, lower Payne effect, and a synergisticeffect in mechanical properties compared to the compoundwith a single filler. In this case, the SiO2/CB ratio of 20/50showed the best overall properties in which a good balanceof rolling resistance, wet skid resistance, and wear resistancewas obtained. With increasing silica content, the optimumcure time was prolonged and the surface and volume re-sistivity was increased [19].0e addition of small amounts ofsilica into SSBR/N330 CB compounds, such as by the use of aCB/silica ratio of 45/5 phr, decreased the filler clusterbranching and increased the reinforcement efficiency. 0edepressed filler networking and more homogeneous fillerdispersion resulted in better abrasion resistance, lowerrolling resistance, and better wet skid resistance. However,when the silica proportion was high, filler cluster branchingincreased quickly and deteriorated the properties [20]. 0euse of semi-reinforcing furnace (SRF) CB/silica hybrid fillerin nitrile rubber compounds showed that the replacement ofcarbon black with silica decreased the material’s stiffness,tensile strength, compressive strength, tear strength, andmodulus but increased elongation at break and reboundresilience [21].

A combination of silica and clay has also been studied.0e physicomechanical properties of silica/China clay-filledNR for a heavy-duty truck tire tread formulation [22]showed the best balance of properties in heat build-up andabrasion resistance at a silica/clay ratio of 60/20. 0e use ofprecipitated silica (PS)/montmorillonite (MMT) inperoxide-cured silicone rubber (SR) nanocompositesresulted in the improved maximum stress compared tounreinforced silicone rubber [23]. 0e use of MMT (Cloisite20A) and silica with Si69 (TESPT) as silane coupling agent inSSBR/BR tread compounds improved tensile strength,elongation, and traction properties at low Si69 content whileenhancing modulus, hardness, wear resistance, dry han-dling, and rolling resistance at high Si69 content [24].Hybrid filler systems consisting of precipitated silica andkaolin modified with a sodium salt of rubber seed oil (SRSO)in NR/BR blends demonstrated that a substitution of 5–10 phr of silica with SRSO-modified kaolin resulted in lowerMooney viscosity, higher cure rate, increased chemicalcrosslink density index, and bound rubber content [25],indicating a higher extent of rubber-rubber and rubber-fillerinteractions. 0is resulted in enhanced mechanical prop-erties such as abrasion resistance, compression set, tensileand tear strength, and elongation at break of the blendvulcanizates. 0e vulcanizate-containing silica/SRSO hybridfiller also showed enhancement of dynamic properties,beneficial for tire tread applications. 0e reduction of fillernetworking in silica-based elastomeric nanocomposites withexfoliated organomontmorillonite resulted in improvedmechanical reinforcement and reduced energy dissipationand thus fuel consumption and carbon footprint [26].

0e present work aims at a synergistic effect of silica withdifferent additional fillers in order to shift tire performancewith respect to wet grip and rolling resistance for fuelsavings, respectively, towards a better abrasion resistance, all

2 Advances in Materials Science and Engineering

characterized by the dynamic mechanical properties of thevulcanized compounds.0e use of a secondary filler that hasdifferent filler characteristics in combination with silica, aso-called hybrid filler system, may lead to better filler dis-persion, filler-rubber interaction, and consequently a betterbalance of performance characteristics.

As to silica-reinforced NR compounds, the dump/discharge temperature after mixing is a crucial parameterthat needs to be controlled as it has a strong influence onboth processing and vulcanizate properties [8]. 0erefore,the influence of initial mixer temperature setting was in-vestigated in this work in order to determine an optimalcondition for the best possible properties of the compounds.

2. Experimental

2.1. Compound Preparation. 0e rubber formulations usedin this study are shown in Table 1. 0e compounds wereprepared using a two-step mixing procedure: the first was toprepare a masterbatch of rubber and fillers, and the secondwas to prepare the final compounds including the curatives.For the first step, an internal mixer Brabender Plasticorder350ml was used, operated at a rotor speed of 60 rpm, fillfactor of 70%, and varied initial temperature settings of 60,80, 100, and 120°C.

0e secondary fillers carbon black or organoclay wereadded together with the first half of silica, the first half ofbis(3-triethoxysilylpropyl)disulfide and diphenyl-guanidine(DPG) secondary accelerator in order to obtain good dis-persion. 0e other halves of silica and TESPD were addedlater on in the first mixing step, together with treated dis-tillate aromatic extract (TDAE) extender oil.0e second stepwas for the addition and mixing of the other half of DPG, N-cyclohexyl-2-benzothiazole sulfenamide (CBS) primary ac-celerator, and sulfur at a rotor speed of 30 rpm, fill factor of70%, and an initial temperature setting of the internal mixerof 70°C. 0e two-step mixing procedure is summarized inFigure 1.

2.2. Sample Characterizations. Mixing data were derivedfrom the real-time monitoring program coupled with theinternal mixer.

2.2.1. Mooney Viscosity, ML (1 + 4) 100°C. It was testedusing a Mooney viscometer (MV200VS, Alpha Technolo-gies) according to ASTM D1646.

2.2.2. Payne Effects. 0e Payne effects or filler-filler in-teractions of still uncured silica-filled NR compounds withcuratives were studied using a Rubber Process Analyzer(RPA2000, Alpha Technologies) at 100°C, frequency 0.5Hz,and varying strains in the range of 0.56 to 100%.

2.2.3. Cure Characteristics. 0e cure properties of thecompounds were studied using the RPA at 150°C, frequency0.83Hz, and 2.79% strain for 30mins.

2.2.4. Tensile Properties. 0e compounds were vulcanized totheir optimum cure times (tc,90) using a Wickert WLP 1600laboratory compression press at 150°C and 100 bars into2mm thick sheets. Type 2 dumbbell test specimens were die-cut from the press-cured sheets, and tensile tests werecarried out with a Zwick tensile tester (model Z1.0/TH1S) ata crosshead speed of 500mm/min according to ASTMD412.

2.2.5. Dynamic Properties. 0e tan delta at −20°C and 0°C ofthe vulcanizates was determined using a dynamic me-chanical analyzer (Metravib DMA) with temperature de-pendence analysis in the tensionmode at a strain of 0.1% andfrequency 10Hz. For the mechanical loss angle tan delta at60°C, the RPA was employed with conditions set at tem-perature at 60°C, strain 3.49%, and varying frequency sweepsin the range of 0.05–33.00Hz. Prior to these measurements,the samples were cured in the same RPA chamber at 150°C totheir optimum cure times before being cooled down to 60°Cand tested.

3. Results and Discussion

Figure 2(a) shows that the dump/discharge temperatureincreases with raising the initial mixer temperature settingfor both secondary fillers. 0e final torque of silica/CBcompounds mixed at various temperatures significantlyincreases and then levels off at 80°C temperature setting,which is attributed to a higher degree of silanization thatresults in a better filler-rubber interaction. On the other side,silica/OC compounds show a slight decrease in final mixingtorque with increasing dump or discharge temperature. 0ismust be due to the effect of the modifying agent, an organicammonium derivative [3], within the organoclay whichremarkably reduces the mutual filler-filler interaction ofsilica, as is also confirmed by the Mooney viscosities andPayne effect results depicted in Figure 2(b).

A higher mixer temperature setting and consequentdump temperature results in a lower Mooney viscosity (at100°C) of the compounds due to increased silanization andconsequent decreased filler-filler interactions. However, thehigher temperature causes also breakdown of NR moleculesby oxidative reactions under shear forces [27], a factor not to

Table 1: Compound formulations used for this study.

IngredientsDosage (phr)

Silica reference Silica/secondary fillerNatural rubber (RSS#3) 100.0 100.0Silica (ULTRASIL 7005) 55.0 45.0Secondary filler∗ — 10.0TESPD 5.0 4.1TDAE oil 8.0 8.0ZnO 3.0 3.0Stearic acid 1.0 1.0TMQ 1.0 1.0DPG 1.1 0.9CBS 1.5 1.5Sulfur 1.5 1.5∗Carbon black N134 (CB) or organoclay Dellite 67G (OC).

Advances in Materials Science and Engineering 3

be overlooked for NR. Organoclay shows a more substantialdrop in Mooney viscosity than carbon black, which must beattributed to influence of the modifier of the clay, as statedbefore.

0e Payne effects of the silica/CB-filled and silica/OC-filled NR compounds decrease with increasing dump tem-perature due to better silanization at higher mixing tem-peratures. 0is clearly confirms the explanation for themixing data and corresponding compound Mooney vis-cosities. 0e Payne effect of the silica/CB-filled NR is higher

than for the silica-filled NR reference compound.0is is dueto the smaller interaggregate distance of the strongly rein-forcing carbon black that gives a higher possibility for theformation of a strong filler-filler network. On the other side,the Payne effects of the silica/OC-filled NR are much lowerthan for the silica-filled NR reference compound, due tolower filler-filler interactions as well as the lower Mooneyviscosities, which may be ascribed again to the clay modifier.

Based on cure curves in Figure 3, the scorch times ofsilica-filled NR compounds with carbon black and

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Figure 1: Two-step mixing procedure for preparing the compounds in this study. (a) First step. (b) Second step.


organoclay as secondary fillers decrease when comparedwith the reference compound with silica alone. It is wellknown that free silanol groups on the silica surface interfere

with vulcanization due to their acidic nature and theirtendency to adsorb vulcanization accelerators, thusretarding the vulcanization [17]. Only about 30% of the

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silanol groups on silica are reacting with the coupling agent,so 70% remains unreacted. 0e role of the secondary ac-celerator, in particular the alkaline DPG, is to shield theseremaining silanol groups. In the present case, theorganoclay-containing compounds show by far the largestdecrease in scorch time, which is again the result of thealkaline modifying agent of the organoclay, an organicammonium derivative [28]. Interestingly, the organoclay-containing compounds also show a considerable decrease intorque increment (Figure 3(b)). 0e main reason must againbe the effect of the modifying agent of the organoclay thatmight play an extra role in further modification of the silicasurface over DPG, leading to a lowered extent of hydrogenbonding between the silica particles and thus reducing Payneeffect or filler-filler interactions as seen in Figure 2(b). 0eseresults mutually confirm each other.

Figure 4 shows the mechanical properties of the cor-responding vulcanizates. At various dump temperatures, thetensile strength, elongation at break, 100% and 300%moduliof NR filled with silica/CB (Figure 4(a)) slightly increaseuntil 160°C and then decrease when the dump temperaturesurpasses 166°C. Natural rubber degradation at highertemperatures can be the primary reason that causes thatdrop in tensile strength. On the other hand, the tensilestrength and elongation at break of NR filled with silica/OC

remain almost on the same levels. 0e moduli at 100% and300% strain of NR filled with silica/OC (Figure 4(b)) increasewith raising dump temperature until 151°C and then slightlydecrease. 0is is again due to degradation of NR at highertemperatures.0emoduli are significantly lower than for thereference silica and silica/CB compounds, which corre-sponds with the much lower torque differences seen in thecure curves (Figure 3) for these compounds. However, in-terestingly the tensile strengths and elongations at break donot differ much from those for the reference silica and silica/OC compounds.

Figure 5 shows the DMA results of tan delta at −20°C and0°C for silica-filled NR with the hybrid fillers.0e tan delta at−20°C to +20°C measured on laboratory scale is oftenconsidered to predict the wet traction of a tire equipped witha tread based on the same compound [28], respectively, thevalue at −20°C as a first indication of wear resistance.0e tandelta at −20°C of the silica/CB-filled NR as well as the silica/OC-filled NR rises with increasing dump temperature. 0isis again due to the better silanization at higher dumptemperatures as also demonstrated in the lower Payne effectsin Figure 2(b). 0e tan delta at −20 and 0°C of the silica/OCfilled with NR is substantially higher than for the silica/CB aswell as for the pure silica compounds. 0is indicates thatthere is a potential to improve wet traction and wear

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performance of a tire tread by using a small amount oforganoclay in combination with silica.

0e tan delta at 60°C of the vulcanizates indicates theenergy loss by hysteresis under dynamic deformation, re-lated to tire rolling resistance, commonly used to indicaterolling resistance on the laboratory scale [29]. Whencomparing the results of tan delta at 60°C of the referencesilica-filled NR, silica/CB-filled NR, and silica/OC-filled NRat the given mixer dump temperatures, the results for thesilica/CB compounds are comparable with the referencesilica-filled NR. So, the replacement of a little silica by carbonblack would not change the level of rolling resistance of atire. On the other hand, the organoclay gives a significantlylower tan delta at 60°C compared to the reference silica-compound, pointing at a large decrease/improvement of therolling resistance for tires with some organoclay applied nextto silica.

4. Conclusions

(i) An increased dump temperature for silica-filled NRcompounds with the secondary fillers leads to abetter silanization reaction between the silica andthe coupling agent TESPD, but NR degradation canoccur at too high mixing temperatures.

(ii) 0e silica/CB-filled NR shows a higher cure rate andtensile moduli, while maintaining similar ultimatetensile properties and tan delta at 60°C compared toa pure silica-filled system.

(iii) 0e silica/OC-filled NR shows a lower Payne effect,higher cure rate, lower moduli combined withsimilar ultimate tensile properties to silica or silica/CB, and higher tan delta at 20°C and 0°C, indicatingbetter wear and wet grip performance, respectively,and a lower tan delta at 60°C compared to the silica-filled system, indicative of reduced rollingresistance.

(iv) 0e use of hybrid fillers, small amounts of secondaryfillers as carbon black, and in particular, organoclaynext to silica in NR therefore offers the potential offurther improvement in wear resistance, wet trac-tion, and rolling resistance of tire treads.

Data Availability

All data are available from the researcher, with a backup atthe chair of Elastomer Technology and Engineering, De-partment of Mechanics of Solids, Surfaces and Systems,Faculty of Engineering Technology, University of Twente,the Netherlands.

Conflicts of Interest

0e authors declare that they have no conflicts of interest.

Acknowledgments

0e authors gratefully acknowledge the Dutch NaturalRubber Foundation (Rubber Stichting, the Netherlands),

Apollo Tyres Global R&D B.V. (the Netherlands), and theGraduate School of Prince of Songkla University (0ailand)for financial support.

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