In the past, fillers were primarily used to reduce the overall cost of polymer composites. However, it has since been revealed that many fillers can enhance the toughness and mechanical strength of these composites. Based on their reinforcing efficiency, fillers can be categorized, as reported in Table 1. Factors such as the volume fraction, shape, size of the particles, and interactions between the filler and the matrix significantly influence the physical performance of elastomeric materials. When reinforcing fillers are applied to elastomers, they can simultaneously increase the modulus and other essential performance parameters up to their optimum loading. This enhancement is particularly beneficial across various industries, notably the tire industry, where specific material attributes are essential. The importance of low rolling resistance tires has grown significantly over the decades due to an increasing commitment to environmentally friendly solutions. Tire rolling resistance refers to the energy required for a vehicle to transmit to its tires to maintain a consistent speed over a surface. In essence, it is the effort necessary to keep a tire rolling. The primary factor contributing to rolling resistance is a phenomenon known as hysteresis, which refers to the energy loss that occurs as a tire moves through its footprint. This energy loss must be compensated by the vehicle's engine, leading to additional fuel consumption. Tires with lower rolling resistance can decrease fuel consumption, contributing to the conservation of petroleum resources and reducing CO2 emissions. Therefore, it is crucial to minimize the energy dissipation of rubber within tire components during deformation to reduce rolling resistance. The rubber's energy dissipation is related to the rubber's loss factor (tan δ). Nevertheless, there is a risk that reducing the rubber's loss factor can lower its grip performance, another significant tire property. Wet grip refers to a tire's ability to maintain traction on wet surfaces and is closely associated with factors such as shorter braking distances on wet surfaces, better driving characteristics, and more stable handling under wet conditions. Tires connect vehicles to the road, and they experience deterioration and wear throughout their lifespan due to mechanical wear and chemical aging, affecting their durability and reducing mileage. The behavior of tire wear is fundamental in achieving superior mileage. Resistance to wear and abrasion is termed abrasion resistance.
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For tire tread applications, three key properties—rolling resistance, wet grip performance, and abrasion resistance—form the "magic triangle." It is complicated to simultaneously optimize all three properties (see Fig. 1). Numerous trade-offs exist among these three properties. Replacing carbon black with silica improves wet grip and rolling resistance performance but negatively impacts abrasion resistance. In the temperature range of 50-70°C, rolling resistance correlates directly with tan δ values obtained from dynamic mechanical analysis (DMA); thus, a lower tan δ indicates lower rolling resistance. However, decreasing the tan δ in this temperature range also results in lower tan δ at lower temperatures.
Conversely, the wet grip performance of tires is associated with a higher tan δ at lower temperatures (around -10 to 0°C). Therefore, it is challenging to separate the wet grip and rolling resistance corners of the magic triangle. The abrasion resistance of carbon black materials consistently exceeds that of silica due to differences in hardness at equivalent filler loadings. Silica-filled rubber compounds generally exhibit lower hardness compared to carbon black-filled compounds at the same amount of filler. The addition of silica enhances both rolling resistance and wet grip properties. However, without incorporating silanes, the enhancement of abrasion resistance is unattainable. Silanes chemically bond the silica to the polymer, creating strong linkages between the filler and rubber that prevent the silica from chipping under pressure.
This paper discusses the reinforcement mechanisms of fillers, challenges during silica mixing, and potential solutions. We examine the effects of various silanes, mixing parameters, silane concentration, silanization catalysts, ionic liquids, and other compounding ingredients. This review aims to aid academic and industrial researchers in comprehending the chemistry of silica, silica compounding, and the mixing of silica-filled rubber compounds.
The surface functional group concentration for typical furnace carbon blacks like N220 contains approximately 1-2 COOH groups per nm² or 2-4 OH groups per nm², which is significantly less than silica. The concentration of silanol groups in most precipitated silica types utilized in the tire industry ranges from 4 to 7 per nm². The differences between carbon black and silica are not exclusively related to surface chemistry and energy. The morphology of the particles of these two fillers significantly influences the properties of the vulcanizate. Carbon black aggregates are generally smaller than those of precipitated silica due to lower interparticle interactions. Moreover, the unmodified commercially used grades of precipitated silica possess polar groups, which result in stronger filler-filler interactions in a silica compound compared to a carbon black-filled elastomer compound, which can be illustrated by the Payne effect (strain sweep). As deformation amplitude increases, the complex modulus of a filled, cross-linked elastomer system decreases, a phenomenon known as the Payne effect. The damping peak (Loss Tangent) of the composites increases with greater strain, resulting in a higher value of the Payne effect (G') exhibiting a greater damping peak or hysteresis for the same system. Thus, hysteresis arises from the breakdown of the filler network, and straining disruptions can dissipate energy. Denser rubber networks exhibit lower energy dissipation, or tan δ.
In its rubbery state, the hysteresis of carbon black reinforced materials is greater than that of silica-filled systems. This is primarily due to energy loss during the periodic destruction and regeneration of the filler network. However, as the temperature rises, the hysteresis decreases, similar to polymers without fillers, since the carbon black filler network is easily thermally degraded. In contrast, within silica-filled rubber, rising temperatures also reduce hysteresis, thereby weakening the filler-filler hydrogen bonding connections and increasing the portion of the filler network that can be disrupted and regenerated.
The silica compound exhibits significantly lower tan δ values than the carbon black vulcanizate in the transition zone. For example, Hi-Sil 210 Silica is a commercially available precipitated silica with a surface area of 135 m²/g, a pH of 7, and a density of 240 g/L. Nevertheless, the hysteresis for carbon black remains higher in the rubbery state at temperatures exceeding 20°C, mainly due to energy lost during repeated destruction and reconstruction of the filler network. Tan δ declines rapidly as temperature increases due to reduced filler-filler interactions and filler-polymer interactions. It is noteworthy that the hysteresis of silica-filled rubber increases with temperature, eventually indicating a crossover point with carbon black around 90°C. As temperature rises, the portion of the filler network that can be decomposed and reformed during cyclic deformation increases, weakening filler-filler interactions.
The relationships of silica and its various surface treatments with the vulcanization of silica-filled SBR were examined. This study analyzed how different silica treatments affect the vulcanization of rubber containing silica (SBR). Grafting results in a change in vulcanization kinetics compared to silica-filled compounds with DPG, potentially leading to more homogeneous crosslinking by reducing N-Cyclohexyl-2-Benzothiazole Sulfonamide (CBS) adsorption. Additionally, the use of antioxidant-functionalized silica has shown reinforcing and antioxidation effects in styrene-butadiene rubber. Combining precipitated silica with an antioxidant coupling agent, through the reactions of 3-Glycidyloxypropyl) trimethoxy silane (A-187) and N-phenyl-1,4-phenylenediamine (PPDA), has been found to significantly improve thermal stability and aging properties without negatively impacting SBR/Silica mechanical properties.
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