Efeito combinado de fotodegradação e stress cracking no poliestireno.

Abstract

The mechanisms of photodegradation and stress cracking of polymeric materials have been vastly investigated on independent ways, with a lot of reports at technical literature. Although under service conditions these two kinds of degradation can occur under combined way and/or simultaneously, no work about this possibility was found. The aim of this work was to investigate the influence of photodegradation at the stress cracking. Injection moulded samples of polystyrene were previously exposed to the ultraviolet radiation for different times and then submitted to butanol a stress cracking agent, under several levels of tensile loads and temperatures. Mechanical analyses were done using stress relaxations and tensile tests at different strain rates. The molar mass was determined by size exclusion chromatography; the moulded and fracture surfaces of the samples after mechanical tests were analyzed by optical and scanning electron microscopy; and spectroscopy at ultravilotet and visible. The results indicated that the photodegradation caused reduction in tensile properties with the progress of the exposure time up to 40 days. After 60 days there was a partial recovery in tensile properties. The virgin (not exposed to UV radiation) polystyrene was relatively inert to butanol exposure at room temperature (23°C), without external stress. Despite the small liquid absorption at room temperature, at long exposure times (140 days), the moulded surfaces of samples that previously received direct incidence on UV radiation for 30 days became highly cracked. The polystyrene after exposure to butanol at 60°C became plasticized. The evaluation of the influence of the photodegradation at the stress cracking using stress relaxation test showed different behaviour between the virgin and photodegraded polystyrene, with higher stress relaxation rates for the last. The tensile strength of samples previously submitted to stress under the presence of butanol dropped drastically comparing to samples tested without the liquid. A complex relation between photodegradation, contact time between the polymer and liquid, strain rate at tensile tests and mechanical properties was identified. The more photodegraded the polymer, the more intense was the stress cracking.