ULTRA-VIOLET DEGRADATION OF POLYPROPYLENE .2. RESIDUAL STRENGTH AND FAILURE MODE IN RELATION TO THE DEGRADED SURFACE-LAYER

In a previous paper the degradation profile and the thickness of the degraded layer of artificially ultra-violet (u.v.) degraded specimens were described. This paper deals with the effects of the embrittled surface layer on the mechanical properties of degraded polypropylene (PP). The fracture energy of artificially degraded PP specimens was measured and compared to non-degraded specimens with an equivalent notch. Deformation velocity, test temperature, specimen type, thickness and processing conditions were varied. Fracture energies and fracture surfaces of degraded and notched specimens were compared. The results show that, depending on the condition of the embrittled layer and test conditions, the failure properties of the degraded material go through three stages (I, II, III). For short degradation times, there is no effect of degradation (I). For intermediate degradation times, the fracture energy decreases because defects form in the degraded layer (II). The occurrence of the so-called 'crack speed effect' causes an especially drastic reduction of strength. For long degradation times, a regeneration may occur caused by multiple cracking of the degraded layer (III). With respect to mechanical properties it appears better to have a thin severely degraded layer than a thicker, less severely degraded layer.