SOUSA, A. R.; http://lattes.cnpq.br/9897600291568251; SOUSA, Alexandre Rangel de.
Résumé:
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.