SANTOS, Z. I. G.; SANTOS, Zora Ionara Gama dos.
Resumo:
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.