Formation and Failure of Elastomer Networks via Thermal, Mechanical and Surface Characterization

Abstract

Abstract : Systematic study of tank track pads tested on paved, cross-country, and gravel courses was done with respect to the change in thermal, surface and mechanical properties. Experimental measurements were conducted mostly on paved course pads of different service mileage and manufacturers. ESCA was utilized to monitor the oxidative degradation caused by road service. In most cases, oxygen to carbon ratio (0/C) was found to increase with increasing service mileage. Solvent extracted samples show clearly the pad surface contains more oxygen than the bulk. SEM/EDXA identifies not only the nature of failure initiation sites but also the morphology of service-pad surface and bulk. Swelling experiment shows crosslink density changes with service life for thin-surface samples, but no changes for thick samples. The evidence accumulated from LSCA, SC-V and swelling suggests strongly that the oxidative crosslinking reaction takes place primarily at the pad surface. Within 1mm from the surface, the composition is similar to the original bulk. Thermal property changes are experimentally less pronounced in magnitude. Nevertheless, volatile content from TGA, and glass transition temperature (T(sub g) from TMA do correlate quite well with the O/C ratio from ESCA. It seems certain that the oxidation plays a role in restricting the molecular mobility, while the volatile moieties act as plasticizers. Cross-country pads tested show that T(sub g) for crack surfaces is generally higher than at wear sites. This observation supports our model of crack propagation through at embrittled matrix. Statistical field test data indicate the % weight loss per 250 miles of service is 4 to 8% on paved course, 8 to 15% on gravel and 30 to 45% in cross-country service. The visual appearance of failed pads is also quite different in cross-country than in paved course.