Non-linear stress relaxation behaviour of crosslinked polyester resins in simple extension

Abstract

A series of crosslinked polyester resins have been prepared by curing an orthophthalic unsaturated resin containing different amounts of either of two flexibilizers, one bifunctional and the other monofunctional. The samples thus obtained show a variety of morphologies and mechanical properties over a wide range. Stress relaxation measurements in simple extension at room temperature were made at various strains in the region of the non-linear behaviour. It has been found that the relaxation curves can be superimposed by a shift along the logarithmic time axis, with shift factors that are in a good linear relation to the initial stress, and with slope and intercept values characteristic of every material. This property makes it possible to determine, at any strain, relaxation master curves covering a notable time range, and clearly to single out the stress upper bound of the linear behaviour range as the intercept on the stress axis.