Investigação da falha prematura por stress cracking em PET - efeito de carga nanoparticulada e uso da técnica de emissão acústica.

Abstract

The objective of this study is to investigate the effect of nanoparticulate fillers in the failure of poly (ethylene terephthalate) (PET) by stress cracking (ESC), and to evaluate the use of acoustic emission (AE) technique on the analysis of this failure. A sodic montmorillonite clay (Cloisite Na+) was used as a filler and PET and composites were exposed sodium hydroxide (NaOH) aqueous solutions in different concentrations as ESC agents. The tests were conducted under tensile and stress relaxation. The results showed that the amount of NaOH solution on the sample surface influences on the ESC damage, specifically in cracks formation and their propagation. The chemical attack, which occurs simultaneously to the ESC effects, is also a function of the solution quantity and the cracks propagation speed, which in turn is correlated to the stress concentration caused by nanofillers in the hybrid PET/clay. Higher filler amount and less interaction with the polymer causes higher stress concentration. The interaction with the polymer, on the other hand, depends on the clay morphology, and this affects in a different way the mechanism of ESC failure. Higher state of aggregation and ordering of clay lamellae induces a greater stress concentration and crack propagation rate with little or no visible surface cracking. The resistance to stress cracking in tension is affected to the same degree, regardless of the amount of filler. In some situations the hybrid PET/clay was more resistant to ESC, suggesting that it may act as a barrier to aggressive fluid. The AE technique proved to be efficient in the analysis of ESC, and can detect more accurately the changes in the material and to correlate with the failure.