The effect of photo-oxidative degradation on fracture in ABS pipe resins

Abstract

This work investigates ultraviolet (UV) degradation and its influence on fracture failure in acrylonitrile–butadiene–styrene (ABS) resins. Preliminary chemiluminescence assessment of coupon specimens indicates that degradation is restricted to the specimen surface, with a relatively rapid depletion of residual stabiliser compared to the bulk of the specimen. Fourier transform infra-red spectral analysis indicates that this surface-specific degradation forms photoproducts with the distortion of absorbance bands (corresponding to the trans CC unsaturation (vinyl) in polybutadiene (PB), and the 1,2-butadiene terminal vinyl C–H band), indicating bond unsaturation. These changes in chemical structure are attributed to cross-linking and chain scission in the PB phase. It is proposed that surface degradation after UV exposure promotes brittle behaviour and may therefore influence the failure mechanism of ABS pressure pipes under static loading. The effect of UV exposure in coupon specimens is compared to that of surface notching, and plane-strain fracture toughness tests are used to obtain an empirical relationship between exposure time and “equivalent” notch depth. By assuming that this relationship is independent of specimen geometry (i.e. degradation is restricted to the surface), a methodology for estimating the lifetime of UV-exposed ABS pipes is proposed. An analysis of DN100 Class 15 ABS pipes predicts that a transition to brittle fracture failure occurs after 137 years. However, it is recommended that the influence of pipe wall thickness on degradation kinetics be quantified.