Influência da extrusão no stress cracking de filmes de poliestireno.

Abstract

The environmental stress cracking (ESC) is among the major factors that affect the lifetime of polymer products, being responsible for about 25% of failures in service. The phenomenon occurs when a combination of mechanical stress and an aggressive chemical agent is present. This phenomenon may occur in both amorphous and crystalline polymers. However, amorphous polymers are more susceptible to ESC. Among the amorphous polymers, stand out the polystyrene (PS), a glassy polymer highly sensitive to high radiation, impact and ESC. However, polymers may have their properties increased by many manufacturing processes, such as the extrusion, which may induce a preferential molecular orientation, which may directly affect the product properties. Therefore, in the current work, the influence of extrusion in ESC of polystyrene films a dependence to the chains stretching direction was investigated. The films were produced in a plane films extruder with two different parameters and controlled thickness, then the samples were cut in parallel and perpendicular direction to the preferential orientation induced in the extrusion process. Afterwards, the films were mechanically evaluated by tensile testing, in two deformation rates, in the presence and absence of butanol. The samples surfaces, after the tensile test, were inspected by light and electron microscopy. The results indicated that butanol caused a substantial modification in polystyrene, both in parallel and perpendicular directions, with a drastic decrease in mechanical properties. The samples with parallel direction were more stable, even in the presence of butanol. The morphology and structure of crazes and cracks in the two directions were quite different. In perpendicular direction, the crazes were deeper, making these films more susceptible to the stress cracking effects.