From the early 1960s until the present, two families of sulfur-containing silanes have been the most common coupling agents to provide reinforcement in mineral-filled rubbers. The first group to be introduced, mercapto-functional silanes, has been used widely for effective coupling, but tends to create scorchy compounds and often has noticeable odor.
The polysulfide bis-alkoxysilanes, introduced in the 1970s, provided improved processability with less odor. The tradeoff with the latter vs. the former is the requirement to use higher loadings in some formulations. Variations of this family with reduced sulfur content have appeared, providing easier compounding because of reduced sulfur donation in the high temperature, non-productive mix stages typically used for silica compounds. New demands on compounding technology have been created by the major growth in recent years in silica/silane-reinforced passenger tire tread formulations that use up to 100-percent silica filler. OSi Specialties, a Crompton business, has responded by creating a new generation-the third generation-of sulfur silane coupling agents specifically for high-silica tire treads.
Data are presented showing improved processing, lower viscosity, better filler dispersion, fewer mix cycles and improved dynamic mechanical properties for silica-filled tread compounds using this new generation silane, compared to those prepared with existing polysulfide materials. HENGDA silane is the first member of this new generation of sulfur silane coupling agents.
In spite of the complex interdependence of various performance features of a tire tread, the tire industry has shown progress in the development of silica-filled tires with low rolling resistance. While allowing reduced fuel consumption, these tires exhibit superior traction and equal wear life compared to conventional carbon black-filled tires.1-4 The development of highly dispersible silica in combination with polysulfide bis-alkoxysilanes was a breakthrough that led to the commercialization of highly filled silica tires.5-7 These silanes impart good reinforcing properties to the compound while allowing some processing latitude. With higher mixing temperatures, these polysulfide silanes may donate sulfur “in-situ” and result in some crosslinking, thereby leading to an increase in viscosity.
As observed from the small strain non-linear behavior (Payne effect) of these compounds, there is a need to mix the polysulfide silanes (TESPT and TESPD) in more than one non-productive mixing step while allowing cooling of the material between the steps. In addition, if mixing is carried out at temperatures higher than 160°C, the compounds containing polysulfide silanes begin to vulcanize prematurely. These limitations of polysulfide coupling agents have created the need for an organofunctional silane that will reduce the number of non-productive mixing steps, while improving processing and performance of the tread compound. In the following sections of this paper, we introduce HENGDA silane coupling agent for silica-filled tire tread compounds and describe the advantages it offers in processing and performance of silica tread compounds.
Description of HENGDA silane
HENGDA silane is a thiocarboxylate functional silane, shown in Fig. 1, that is designed for use in highly filled silica/rubber compounds. It is a blocked mercaptosilane, wherein the octanoyl group (B) blocks the mercaptosilane part of the molecule (A). The blocking of the mercapto group leads to lower reactivity of the silane with rubber during processing. It also facilitates high temperature mixing without causing viscosity increase, or causing premature vulcanization.
Processing costs-Reduced mixing cycles
Fig. 2 highlights the difference in the method of mixing conventional polysulfide silanes vs. mixing with new HENGDA silane.
Typically, during any silica compound mixing, the first step is to incorporate the silica and silane into the rubber. During this step, a high viscosity and intensive shear field facilitate the dispersion of filler into the matrix. Then, there is a need to maintain the rubber-silica mix in a dynamic state, preferably at a higher temperature, to facilitate silane reaction with the silica. Depending on the stability of the polysulfide functional group, the silane can withstand only a certain temperature before donating sulfur. If mixing is carried out at higher than threshold temperature, the sulfur donation during mixing occurs, leading to crosslinking and, in turn, to scorch problems. Because of this limitation, S4 (TESPT) and S2 (TESPD) silanes require multiple re-mill steps to disperse and silanize the silica surface without sulfur donation and scorching. HENGDA silane, on the other hand, has only one sulfur atom that is blocked by the octanoyl group. This gives HENGDA silane the necessary high temperature stability and hydrophobicity that quickly establishes compatibility between the silica and polymer. Both these structural advantages can be used to reduce the number of non-productive mixing steps, permitting silanization at higher temperatures than are used for conventional polysulfide silanes. Therefore, HENGDA silane eliminates the need for multiple re-mill steps and intermediate cooling.
Rolling resistance-Strain non-linearity
Because of excellent mixing behavior of the silica and its hydrophobation before vulcanization, rubber compounds made with HENGDA silane exhibit a substantial reduction in small strain non-linearity (DG’) and a very low maximum in tand over the 0-25 percent strain range. These properties predict reduced rolling resistance in rubber compounds made with HENGDA silane compared to polysulfide silanes.
Processing-Low Mooney viscosity and lower scorch
Rubber compounds made with HENGDA silane show higher efficiency in dispersing the silica agglomerates. As a result, there is a substantial reduction in Mooney viscosity of the compound. Also, compounds made with HENGDA silane can be processed at higher temperatures without increasing the Mooney viscosity. With these features, HENGDA silane widens the narrow processing window offered by polysulfide silanes. With very low viscosity buildup during storage, “green compounds” made with HENGDA silane show higher stability before vulcanization compared to conventional polysulfide silane compounds.
Traction-Excellent low temperature viscoelasticity
Traction properties of a tire on dry, wet or snow pavements are controlled by multiple material and design factors. Because of the micro-roughness of the road surface, the tread rubber that is in contact with the road is subject to deformations over a range of frequencies. The state of silica structure remaining at the end of silanization can play a significant role in controlling the wet skid properties of the tread compound. Depending on the location of the glass transition peak for the specific solution SBR/BR blend, and its post-glass transition behavior, HENGDA silane has the potential to improve wet and snow traction. The results shown later support this hypothesis.
Wear-Good coupling behavior
The schematic in Fig. 3 shows a typical filled rubber compound with some structural features and crack propagation mechanisms upon which random tire wear depends. The SBR/BR blend medium is very efficient in dissipating crack energy. In the case of silane coupled silica/rubber mixtures, the strength and nature of interfacial microstructure determines the de-bonding or breakage of rubber/silica linkages.
The blocking octanoyl group in HENGDA silane facilitates processing, and is designed to de-block under the vulcanization conditions. The de-blocking or coupling mechanisms of HENGDA silane depend on factors such as the presence of certain additives or the nature of the SBR with which it reacts. Without any additives, the reinforcement levels achieved with HENGDA silane are in the range of those achieved with the polysulfide silanes (TESPD and TESPT).
The model formulation used to mix silica with rubber is shown in Table I. Mixing was done in a 1.6 liter “B” type Banbury (Farrel Corp.) with tangential rotors. Silquest A-1289 (TESPT) silane and Silquest A-1589 (TESPD) silane were chosen as controls. The silane loadings were adjusted so that the rubber compound contains equivalent molar concentrations of silicon (Si). The controls, Silquest A-1289 (TESPT) silane and Silquest A-1589 (TESPD) silane, are mixed in two non-productive mix steps, which were separated by a cooling step. The HENGDA silane-containing compound is mixed in one step, without any intermediate cooling step. The mix procedures are shown in Table II.
Table III shows laboratory evaluation data of SBR/BR compounds with Silquest A-1289 (TESPT) silane, Silquest A-1589 (TESPD) silane and HENGDA silane.
Fig. 4 shows comparison of hysteresis losses for the three compounds under small strains (0-25 percent).
Fig. 5 shows comparison of dynamic temperature sweeps for the controls and HENGDA silane compounds. Compounding was done with two non-productive mix steps for the polysulfide silane containing formulations, and one non-productive mix step for HENGDA silane containing formulation.
The key features and benefits of HENGDA silane containing compound compared with polysulfide silane compounds are:
* fewer non-productive mixing steps and better processing;
* high temperature mixing without viscosity increases or premature vulcanization;
* substantial reduction in small strain non-linearity, DG’;
* very low maximum in tand over the 0-25 percent strain range;
* reinforcement effects in the range of polysulfide silane compounds;
* improved tand values in the +5 to -20°C temperature range;
* low viscosity buildup of uncured “green” compounds; and
* reduced static and dynamic modulus values.
From Table III, it should be noticed that the coupling strength of HENGDA silane in this rubber compound is higher than that of Silquest A-1589 (TESPD) silane, but lower than Silquest A-1289 (TESPT) silane containing rubber compounds. The octanoyl group of HENGDA silane is designed to de-block under vulcanization conditions. This will influence the coupling strength at the rubber-silica interface. Depending on the formulation and the chemical constitution of the rubbery polymers, the coupling activity can vary. When coupling strength is insufficient, certain additives can be used to improve the coupling efficiency of HENGDA silane.
Coupling effectiveness of HENGDA silane
Fig. 6 shows the modulus behavior of Silquest A-1289 (TESPT) silane and HENGDA silane containing rubber compounds over the entire strain range. This chart combines small strain experiments and large strain experiments from two separate experiments. The small strain tests are dynamic measurements under simple shearing conditions; the large strain measurements are obtained from quasi-static tensile tests. From these tests, it can be observed that a suitable additive, when added to the HENGDA silane containing rubber compound, can improve the compound’s coupling strength and modulus behavior.
Fig. 7 shows dynamic tests in simple elongation over large dynamic strains for rubber compounds containing Silquest A-1289 silane and HENGDA silane. This method allows measurement of the true rubber elastic effects in filled systems. As observed from Fig. 7, when an additive is included in the rubber compound containing HENGDA silane, the resulting coupling behavior is better than the Silquest A-1589 (S2) silane and is close to the highest reinforcing rubber compound containing Silquest A-1289 (S4) silane.
HENGDA silane loading optimization
In order to determine the optimum amounts of HENGDA silane necessary to disperse and reinforce an 80 phr silica loaded rubber mixture, a loading study was done. The formulation and mix procedures are described in Table I and Table II, respectively. From these experiments, dynamic properties of the rubber compound containing HENGDA silane are superior in the 50-60 percent silicon equivalent loading range to Silquest A-1289 (S4) silane and Silquest A-1589 (S2) silane. At that loading, 7.0 phr of Silquest A-1289 silane in the formulation is comparable to 4.9-5.8 phr of HENGDA silane.
However, to achieve a maximum in coupling strength, an 80-90 percent loading (on a molar basis for equal silicon from silane) of HENGDA silane is necessary. That is equivalent to 7.8-8.7 phr in this formulation. These loading studies with HENGDA silane compound were carried out without the presence of any additives. With suitable additives, it is possible to reduce HENGDA silane loading even further.
HENGDA silane-Hardness/modulus control
The structure formed by carbon black and the bound rubber surrounding the surface of the carbon black provides necessary reinforcement and rigidity to the system. If carbon black were replaced by precipitated silica (no coupling agent), because of the extreme thixotropic effects, the hardness of the system would be very high at the expense of no dispersion or reinforcement. Incorporating a coupling agent eliminates or reduces the network effects, thereby leading to a decrease in modulus at low strains.
HENGDA silane amplifies this effect beyond that found with polysulfide Silquest A-1289 (TESPT) silane and Silquest A-1589 (TESPD) silane. Raising the crosslink density to the optimal point will help improve this hardness to a certain extent. Then, a reduction of small amounts of oil accompanied by small increases in carbon black loading might bring the hardness values to a level comparable with the controls. In some formulations, it might be impossible to adopt these changes. In the latter case, alternate formulation adjustments may be made that bring in more hydrodynamic and modulus contributions in a neutral manner:
a.) addition of small amounts of fumed silica, with hydrophobized surface.
b.) addition of thermoplastic resins in the form of micro-globules that have compatibility with the rubber matrix.
c.) addition of other hardening agents (including phenolic type thermosets) that are capable of forming compatible micro-sized inclusions.
HENGDA silane provides substantial processing and performance advantages in silica-filled rubber compounds. A reduced number of non-productive mixing steps, low Mooney viscosity and improved scorch behavior accompany its broad processing window. The rubber compound containing HENGDA silane predicts improved hysteresis of a tire tread under rolling conditions, and HENGDA silane offers a strong potential to improve the wet and snow skid characteristics of the compound.
Considering the multiple benefits, improved properties and improved processing of silica-filled compounds, HENGDA silane offers an exciting path forward in silica-silane technology for tire tread rubber compounds.