Efeito das condições de resfriamento na fotodegradação do polipropileno.

Abstract

Polypropylene is widely used in the fabrication of products for automobiles, electronics and domestic appliances. Howeve, when this polymer is exposed to weathering a change in molecular structure may occur, causing deterioration in mechanical properties and in surface appearance. The absorption of ultraviolet light is closely related to the polymer chemical structure and also to impurities that are present within the material. The result is chain scission reactions that can also lead to oxidative degradation. This process is inhibited by the stabilizers, resulting in products with better performance. The main objective of this dissertation is to analyze the effects of photodegradation in polypropylene samples prepared with different physical structures. Different processing conditions were applied on order to produce samples with different thicknesses and morphologies. The consequences of these procedures on the polymer degradation were estimated by measurements of mechanical properties, DSC and infrared spectroscopy. The effects of two photostabilizers, Tinuvin 327 and Tinuvin 791, were also studied. The results indicated that a variation in cooling conditions during processing lead to spherulites with different sizes, causing a significant influence in mechanical properties and in the degradation behavior. In non-stabilized polypropylene samples produced under slow cooling conditions, very large spherulites were observed causing a much higher deterioration in properties after exposure. This may be due to the scission of tie chain molecules that are in lower number in these samples. On the other hand, stabilized samples when compared to quenched samples, possibly due to segregation of stabilizers to the interspherulitic regions. The properties of thick samples were much better than the thin ones, as a consequence of depth profile of degradation.