Estimation of Resin Reinforcement in Tire Inter-Belt Wedge Compounds

摘要

This report is the fifth in a series of scientific reports used to guide the agencys development of a laboratory-based accelerated service life test for light vehicle tires (i.e. tire aging test). This paper focuses on the modulus results (akin to hardness) in the inter-belt wedge rubber component of the tire. The wedge compounds of five of the six tire models retrieved from service in Phoenix, AZ, were found to increase progressively in modulus with increasing amounts of service. However, one tire type (Type H light truck tires) decreased in modulus with increasing amounts of service. It has been reported in scientific literature that the rubber compounds in passenger tires tend to increase in modulus during service due to oxidation. However, when optional resins are added to the rubber compounds to promote adhesion and impart reinforcement, they form a network structure with elevated modulus values. This resin network structure is quickly broken down by mechanical strain during rolling service. At sufficiently high levels of resin content, this could lead to a net mechanical softening of the rubber compound during service despite the ongoing hardening effects of oxidation. To validate this hypothesis, the resin content in wedge compounds of 24 different tire models was estimated by comparing their softening in double strain sweep testing to model compounds produced with known levels of resin. Bead apex rubber compounds from five tire models were also tested as a control since they were likely to contain high levels of resin. Of the six models collected in the NHTSA Phoenix study, only the wedge compound of Type H light truck tire model was estimated to contain a high level of resin. The wedges of twelve models, including four of the NHTSA Phoenix models, were estimated to contain low levels of resin. The wedges of the remaining eleven tire models were estimated to contain no resin. These results indicate that matching the modulus properties of wedge (and belt-coat) compounds of laboratory-aged tires to in-service tires will require a combination of both oxidative aging and mechanical strain.