Silane coupling agents for silica-filled tire-tread compounds: The link between chemistry and performance

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Silanes are commonly used as coupling agents for silica-filled compounds in the rubber industry, mainly for tire-tread compounds. The replacement of carbon black by silica results in an improvement of tire performance in terms of wet grip, abrasion resistance and rolling resistance. The most widely used silanes are bis-(triethoxysilylpropyl)tetrasulfide (TESPT) and the corresponding disulfide (TESPD). The coupling agent reacts first with the silanol groups of the silica filler, forming a hydrophobic shell around the filler particle and improving the compatibility between the filler and the rubber. Secondly, the sulfur moiety reacts with the rubber with formation of a stable network between the filler and the rubber polymer. The different steps of the formation of the filler-rubber network and their influence on the in-rubber properties have been investigated separately. The silanisation efficiency of different types and numbers of alkoxygroups, as well as the effect of the coupling of the polysulfide groups to the rubber polymer were analyzed. It was found that all functionalities have to be combined in a single molecule to provide the well-balanced property profile that is characteristic for silica compounds.

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The silanization of silica with and without acid-base additives were investigated via TG-FTIR (Thermogravimetry coupled with Fourier transform infrared) technology and reaction kinetic simulation. The results showed that two parallel reactions were included in the silanization process of silica: one is the hydrolysis of ethoxy group of TESPT followed by the condensation with the silanol of silica (the activity energy E=57kJ/mol); and the other is the direct condensation between the silanol of silica and the ethoxy group of TESPT (E= 90.4kJ/mol). The hydrolysis of ethoxy group is much easier in comparison with the direct condensation with the silanol. The silanization was effectively promoted by means of adding stearic acid (HST) or diphenyl guanidine (D), and the reaction rate increased with the increase of HST or D in a reasonable range. HST promoted the condensation between ethoxy group and silanol (E=69.5kJ/mol), without affecting the hydrolysis of ethoxy group. However, D promoted both of the two reactions obviously (E=59.5kJ/mol and 70.8kJ/mol, respectively). When HST and D coexisted in the system, the condensation between ethoxy group and silanol was promoted more obviously by the acid-base complex (E=53.5kJ/mol). The two-step parallel reaction model was used to simulate the silanization processes and good fitting results were achieved.
The present study discusses that filler–filler mechanical engagement resulting from the grafted long-chain silanes on the silica surface is indeed a reinforcing mechanism in rubber composites, as already speculated by nonlinear viscoelastic properties in our previous study. The existence and severity of such a phenomenon are assessed purely by isolating the energetic contribution of reinforcement from interfering with filler mechanical engagement in the silica network formation and breakdown processes. In a novel approach, the driving force of fillers to flocculate energetically at elevated temperatures was defined using surface energy theories, and it was adjusted to be similar in two composites having silica treated by short- and long-chain silanes. Filler–filler mechanical engagement was monitored by tracking network formation (filler flocculation) in a matrix of styrene–butadiene rubber and also by conducting various dynamic viscoelastic experiments on liquid paraffin suspensions having short...
We evaluated the significance of mechanical engagement and energetic interaction between a polymer and a filler as two reinforcing mechanisms in SBR composites containing silica modified by short- and long-chain silanes. To exclude mechanical contributions of reinforcement from that of energetic contributions, surface energy of silica particles was systematically adjusted to prepare fillers of identical and diverse surface energies. Having analyzed interactions using a temperature sweep in a small-strain oscillatory test and a uniaxial tension test, results indicated that the chain length of the silane has remarkable influence on energetic filler-filler and filler-polymer interactions, but no detectable difference associated with filler-polymer mechanical engagement was observed from these experiments. However, dynamic strain sweep experiments showed that the rate of breakage of the filler network (Payne effect) is less for the composite having longchain silane compared to that having short-chain silane. It was hypothesized that this behavior could be correlated to mechanical engagements of long-chain silanes existing on the filler structure. © 2016, Rubber Division of the American Chemical Society. All rights reserved.
The presence of physisorbed water during the silanization of silica is well-known for strongly influencing the silane reactivity at the interface. In this work, the reactivity of bis[3-(triethoxysilyl)propyl] tetrasulfide (TESPT), widely used in the rubber industry, on hydrated precipitation silica was investigated by time-resolved operando FTIR spectroscopy and chemometrics. The predominating reaction scheme is elucidated in conditions representative of industrial mixing process at the molecular scale. Based on multivariate curve resolution analysis and a quantitative kinetic model, it is shown that TESPT chemisorption is governed by two competitive reaction routes both producing only ethanol in the gas phase: (i) direct grafting reaction between an ethoxy moiety and a silica surface silanol and (ii) silane hydrolysis followed by co-condensation with a vicinal silane species. Reactions involving silanol–silanol condensation with production of water were not found to be significant. Although several types of water coexist on hydrated silica surface, it is demonstrated that strongly adsorbed monolayer water (Ea = 44 ± 2 kJ mol–1), which represents 5% w/w of the physisorbed water, is of primary importance in the hydrolysis reaction. Its relative surface concentration with respect to the amount of physisorbed silane is strongly correlated to the final ratio between co-condensation and grafting.
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