Mechanistic Evaluation of Silane-spacer Length on Dynamic and Tribological Behavior of SBR-Modified Silica Rubber Composite

Hypothesis: Substitution of carbon black by surface-modified silica in tire tread compound formulation often brings a lower friction coefficient and inadequate vehicle safety. Through modifying polymer-filler interactions, silane chain length is capable of altering viscoelastic properties. The connection between tribological properties and viscoelastic dissipation can be regarded as an important factor to control the frictional behavior of tire tread compounds. It has always been speculated that silane chain length dictates the properties of silane-treated silica filled rubbers through two possible reinforcing mechanisms namely: entropic interaction and/or mechanical engagement. In this contribution, the existence and severity of each mechanism is realized by excluding mechanical contributions of reinforcement from that of entropic interactions by surface-energy theories.
Two aliphatic silanes of short (trimethoxy(propyl)silane) and long chain (hexadecyl trimethoxysilane) with spacer length of 3 and 16 carbons are grafted onto the silica surface. The surface energy of the resulting powders is controlled by controlling the density of silane grafting. The rubber matrix constitutes a solution styrene butadiene rubber (S-SBR) and the compounds have been prepared by means of an internal mixer and a two-roll mill. The surface characteristics of silica as well as the morphological, mechanical and tribological properties of the resulting rubber composites are characterized and compared with a conventional bi-functional silane commonly used for this tread compound.
Findings: For systems in which the surface energy and thus energetic contributions of polymer-filler interaction are controlled to be equivalent for both short- and long-chain silane treated silica, no variations either in dynamic or tribological properties are detected, indicating that no mechanical engagement associated with interlocking of long chain silanes is available. At the same time, entropic interactions played a significant role in final dynamic and tribological properties of the composites. It is also observed that friction coefficient is correlated with loss modulus of the compound better than the loss factor.