The action of anthraquinone sensitizers in the photo-oxidative degradation of low density polyethylene: Mechanical evidence of dark processes

Low density polyethylene (LDPE) films containing three quinoid sensitizers, anthraquinone (I), 2-butylanthraquinone (II) and 2-octanoyloxyanthraquinone (III), have been exposed to the uv light of a fluorescent tube. The kinetics of the photo-oxidative degradation have been monitored by measuring infra-red spectra and three mechanical characteristics derived from stress-strain traces, viz. strain at break, Young's modulus and yield stress. During the first stage of the process, the greater solubility of (II) and (III) in LDPE as compared to (I) was clearly manifested in both the mechanical and spectral data. The sensitizers are consumed at the very beginning of the exposure, but once the degradation starts it continues at a higher rate around introduced or intrinsic centers. The increase of yield stress values after a long-term dark storage which followed the irradiation period is ascribed to a slow continuous crystallization of the degraded polymer. No dark storage effect has been found with samples not previously exposed to uv light.