Dynamic Mechanical Properties And Thermal Conductivity Of Emulsion-polymerized Styrene-butadiene Rubber Composites Used For High Performance Tire Tread

In modern society, with increasingly developed highway transportation, energy and environmental issueshave become increasingly prominent. It is urgent for tire industry to produce "green tires" which haveproperties of good abrasion and wet skid resistance, low rolling resistance and heat generation.Emulsion-polymerization styrene-butadiene rubber (ESBR) used in tire tread has an advantage in highabrasion resistance and good wet skid resistance. But, because of the limitation of its own chain structure,the heat generation of ESBR is considerably high, and also its thermal conductivity is relatively low. So,traditional ESBR composites can not meet the requirements of "green tires". In this work, ESBR, liquidisoprene rubber (LIR) with different molecular structure and functional fillers were choosen to blendmechanically. ESBR composites with high thermal conductivity, good wet skid resistance, low heatbuild-up and low rolling resistance properties were prepared successfully. The dynamic mechanicalproperties, thermal conductivity and other properties of ESBR composites filled with LIR and functionalfillers were studied systematically. The effects of many factors on thermal conductivity of ESBRcomposites were revealed by finite element method (FEM).ESBR composites filled with thermally conductive fillers were prepared. The thermal conductivity ofESBR vulcanizates filled with alumina (Al2O3) or zinc oxide (ZnO), increased nearly linearly withincreasing filler loading when the filler loading exceeded20phr. The ESBR vulcanizates filled with carbonnanotubes (CNTs) have the highest thermal conductivity at a given filler loading in comparison with otherESBR vulcanizates. When SiC of a single particle size was used, thermal conductivity of ESBR vulcanizatefilled with large particle sizes of SiC was higher than that of ESBR vulcanizates filled with small particlesizes of SiC at the same filler loading. At a given loading of100phr, the ESBR vulcanizate filled with twokinds of different particle sizes of SiC of1–3and5–11μm at the mass ratio of1:1has the highest thermalconductivity of0.661W/(m·K), and relatively good mechanical properties. At the filler loading of100phr,the ESBR vulcanizates filled with colloidal graphite (CG) or AlN had significantly increased thermalconductivity, which were1.22W.(m.K)-1and0.707W.(m.K)-1, respectively. But their mechanicalproperties became poor. The ESBR vulcanizate filled with acetylene black at the loading of30phr, had arelatively high thermal conductivity and good mechanical properties.The experimental results are analyzedusing Maxwell model, Geometric mean model and Agari’s equation. The thermal conductivity of the ESBRvulcanizates filled with Al2O3, ZnO or CNTs could be well predicted by optimized parameters usingAgari’s equation for a polymer composite filled with mixtures of particles. Storage modulus (E′) of theESBR vulcanizates filld with acetylene black or CNTs increased significantly. At CNTs loading of3phr,the ESBR vulcanizate had both improved dynamic mechanical properties and enhanced thermalconductivity. Small particle size of SiC or CG had the best effect on the enhancement of E′of the ESBRvulcanizates, but they could not lead to the decrease of tan δ at60oC.LIR was used as a modifier for ESBR composites and its effect on the resultant properties of ESBRcomposites was investigated. After the introduction of LIR, the dispersion of filler particles was improved,Payne effect was reduced, the bound rubber content in the ESBR compounds increased, the crosslinkdensity of the ESBR composites increased, and also the polymer-filler interactions were strengthened. Theintroduction of LIR led to ESBR vulcanizates having higher loss factor (tan δ) in the temperature range-30to0C, and lower tan δ in the range60to80C. The addition of LIR-403(3phr) led to a7.5%increase intan δ from-30to0C, and a24.9%decrease in tan δ from60to80C. An increase of the loading ofvulcanization system (VS) could promote the decrease of tan δ at60oC of ESBR/LIR vulcanizates. Theincrease of the loading of VS and the introduction of LIR-403had synergistic effect in improving dynamicmechanical properties of ESBR/LIR vulcanizates and in reducing the heat build-up. The temperature rise ofcompression heat build-up test of ESBR/LIR vulcanizates reduced along with the increase of theircrosslinking density. The change in tan δ from60to80C was related to polymer–filler interactions. The characteristic constant of filler–ESBR matrix interaction (m) was calculated. At a given filler volumeofraction, the increase in m in the presence of LIR could be well related to the decrease in tan δ from60to80C. At a relatively low given loading such as3–5phr, ESBR/LIR vulcanizates filled with a mixture of threekinds of LIR have higher E′and loss modulus (E″) than that filled with a single kind of LIR. ESBR/LIRvulcanizates filled with the traditional fillers and modified by LIR overall had better dynamic mechanicalproperties than that filled with fumed silica, or carbon black and silica dual phase filler (CSDPF). A smallamount of LIR had a slight effect on the tensile properties and thermal conductivity of ESBR/LIRvulcanizates.ESBR composites filled with different amounts of fumed silica, CSDPF or ZnO were prepared. At thefiller loading of10phr, fumed silica and ZnO had very little effect on the enhancement of E′of ESBRvulcanizates, whereas CSDPF had some certain effect on the enhancement of E′. In the view of the trend ofthe temperature-dependence of tan δ, ESBR vulcanizates filled with fumed silica or ZnO had betterdynamic mechanical properties than that filled with CSDPF. The ESBR vulcanizate filled with ZnO at theloading of10phr had a relatively low tan δ at60oC, meanwhile its thermal conductivity increased. The tanδ at0oC of ESBR vulcanizates filled with fumed silica reduced with increasing fumed silica loading,however, tan δ at60oC increased with increasing fumed silica loading, and vulcanization time has a littleeffect on dynamic mechanical properties of the ESBR vulcanizates. The tan δ at60oC of ESBRvulcanizates filled with CSDPF increased with increasing CSDPF loading, while the tan δ at0oC decreasedgenerally with increasing CSDPF loading. But with increasing vulcanization time and CSDPF loading, tanδ at0oC suddenly increased. E′at room temperature and E″at-30oC and0oC varied considerably withincreasing CSDPF loading, and vulcanization time had prominent effect on dynamic mechanical propertiesof the ESBR vulcanizates. With the increase of vulcanization time, tan δ at-30oC of ESBR compositesfilled with the traditional fillers had little change, while tan δ at0oC and60oC decreased gradually, andvulcanization time had a slight effect on the tensile properties of ESBR vulcanizates filled with traditionalfillers. The fumed silica loading and vulcanization time had slight effects on100%and300%moduli of theESBR vulcanizates, but they led to the decrease of break elongation and tensile strength. With increasingvulcanization time, break elongation and tensile strength decreased obviously. For CSDPF/ESBRvulcanizates, with increasing CSDPF loading,100%and300%moduli of the ESBR vulcanizates increasedobviously, but the break elongation and tensile strength decreased obviously. With increasing vulcanizationtime, tensile strength decreased obviously.Based on the studies of the chapters above, ESBR/LIR composites, meeting the requirements for highperformance tire tread such as low rolling resistance, low heat generation, good wet skid resistance andhigh thermal conductivity, were prepared by comprehensive controlling measures and using optimallyselected types of fillers and LIR. Dynamic mechanical properties of the ESBR/LIR compounds andvulcanizates, together with thermal conductivity and tensile properties of the vulcanizates were studied.The experimental results showed that when the traditional carbon black N234was replaced with an equalloading of acetylene black, thermal conductivity of the ESBR/LIR composites increased effectively in thecondition of keeping nearly unchanged dynamic mechanical properties. The increase of the amount ofvulcanization system (VS) by20%led to the ESBR/LIR composites having greatly increased tan δ of0.418(the value of Control was0.283) at-30C, and obvious decreased tan δ in the range60to80C. At thesame time, the thermal conductivity of the ESBR/LIR composites increased significantly. Furthermore,with the addition of ZnO (10phr), the prepared ESBR/LIR composite had an obvious increased tan δ at-30C, and a significantly decreased tan δ in the range60to80C, especially for tan δ at80C, a decrease of21.7%, implying an obvious decrease of heat generation. At the same time, the thermal conductivity of theESBR/LIR composites increased by40.0%, up to0.392W.(m.K)-1. At CNTs loading of3phr, theESBR/LIR composite had improved dynamic mechanical properties, especially, an obviously increased E′at room temperature as well as an enhanced thermal conductivity. But the tensile properties of theESBR/LIR remained nearly unchanged. When the traditional fillers (N234and silica) were replaced withan equal loading of SiC mixture with two kinds of particle sizes of SiC of1–3and5–11μm at the massratio of1:1together with the introduction of10phr of ZnO, the thermal conductivity of the preparedESBR/LIR composite is relatively high, up to0.405W.(m.K)-1. Tan δ of the ESBR/LIR composite had agreatly increase by109%at-30C, and a significantly decrease by above55%in the temperature range60to80C. But E′of the ESBR/LIR composite at room temperature was considerably low. On the basis ofabove, with the addition of a mixture of acetylene black and N234(3:1, mass) with a total loading of20phrand the increase of the amount of VS by20%, the thermal conductivity of the prepared ESBR/LIRcomposite increased by86.4%, up to0.522W.(m.K)-1. Tan δ of the ESBR/LIR composite increased by over12.5%in the temperature range-30to0C, and decreased by over24.9%in the range60to80C. E′of the ESBR/LIR composite at room temperature and the tensile properties was considerably ideal, implying agood application prospect of the ESBR/LIR composite in high performance tire tread.Two-dimensional (2D) and three-dimensional (3D) finite element method (FEM) models were developedto study the effects of many factors on the thermal conductivity of ESBR composites filled with SiC, Al2O3,ZnO and CNTs. An increase of thermal conductivity of the ESBR composites with increasing aspect ratio(AR) values was predicted by the FEM models. The shape and particle size of SiC had a synergistic effecton the thermal conductivity as predicted by the FEM models. At SiC loadings of100and200phr, thecomposites filled with two kinds of SiC at mass ratio of1:1exhibited the highest thermal conductivity aspredicted by rectangular particle filler (RPF) FEM models (AR of4for SiC1#(1–3μm) and AR of5forSiC2#(5–11μm)). At a given filler loading, thermal conductivity of ESBR composites increased with theincreasing intrinsic thermal conductivity of fillers in a certain different range for different shape of fillers.An increase of thermal conductivity of ESBR composites with increasing Al2O3or ZnO loadings waspredicted by FEM models. The orientation angle (OA) of CNTs and interfacial thermal resistance (ITR)strongly affect the thermal conductivity of CNTs filled ESBR composites as predicted by FEM models.The thermal conductivity anisotropy (TCA) of CNTs also has a prominently effect on thermal conductivityof ESBR composites at relatively small OA of CNTs. The comparison of predicted thermal conductivitiesby FEM models and Agari’s models with the experimental results showed the trends of thermalconductivity predicted by FEM models agreed with the experimental data considerably as well as thosepredicted by Agari’s models. Thermal conductivity of ESBR composites predicted by2D and3D sphericalparticle filler (SPF) FEM models as a function of ZnO and Al2O3loading showed that3D SPF FEM modelsagreed with the experimental results well at low loadings (not higher than20phr), while2D SPF FEMmodels agreed with the experimental results well at high loadings (higher than80phr). The trend andparticular values of thermal conductivity of SiC filled ESBR composites predicted by the RPF FEM models(AR of4for SiC1#and AR of5for SiC2#) agreed with the experimental data considerably. Comparedwith the experimental data, predicted values have the biggest difference of not more than5%. The proposedmodels showed that it would be feasible to fabricate ESBR composites with high thermal conductivity bythe measures (such as controlling appropriate filler OA, reducing ITR, etc.), which are revealed by thepredicted results of the FEM models.