The Origin of Marching Modulus of Silica-Filled Tire Tread Compounds

Abstract

Silica-reinforced S-SBR/BR tire tread compounds often show characteristic vulcanization profiles that do not exhibit a distinct maximum in the cure curve nor a plateau profile within acceptable time scales (marching modulus). In such a situation, it is difficult to determine the optimum curing time, and as a consequence, the physical properties of the rubber compounds may vary. Previous studies stated that the curing behavior of silica-filled rubber compounds is related to the degree of filler dispersion, the silanization, and the filler–polymer coupling reaction, as well as to the donation of free sulfur from the silane coupling agent. Such results imply that these are the key factors for minimization of the marching modulus. Various silane coupling agents with different sulfur ranks and functionalities were mixed at varied silanization temperatures. The correlation between these factors and their effect on the marching modulus intensity (MMI) were investigated. The MMI was monitored by measuring the vulcanization rheograms using a rubber process analyzer at small (approximately 7%) and large (approximately 42%) strains to discriminate the effects of filler–filler and filler–polymer interactions on the marching modulus of the silica-filled rubber compounds. Both factors have an intricate influence on the marching modulus, determined by the degree of filler–filler interaction and the coupling agent.

The Effect of Silanization Temperature and Time on the Marching Modulus of Silica-Filled Tire Tread Compounds

“…According to Mihara et al [12], it is possible to observe the flocculation of silica particles by monitoring the change of storage modulus (G') at low strain under isothermal conditions. The present results can best be fitted with Equation (1) or (2) [13,29]:…”

“…Therefore, only the filler-polymer interaction was taken into account in the CR. The torque levels at different times were recorded, and then the CR was calculated following Equation 3, based on the same concept as Equation (1) [13,29]:…”

“…• 0.5 • (~7% of strain) and 1.667 Hz of frequency: ASTM D5289-95 [30] conditions; • 3 • (~42% of strain) and 1.667 Hz of frequency [13].…”

“…Mihara et al [12] and Jin et al [13] reported that the silanization temperature strongly affects the degree of silanization and the formation of chemically bound rubber. A too low temperature results in higher remaining filler-filler interaction and consequently in silica re-agglomeration.…”

“…The consequence of an incomplete silanization is a marching modulus during the vulcanization stage of such compounds [13]. Marching modulus during vulcametry is a problem often observed in silica-filled solution styrene-butadiene rubber/butadiene rubber (S-SBR/BR) tire tread compounds.…”