Influence of Pre-Exposition and Pre-Loading on the Environmental Stress Cracking of a PBT/PBA Co-Poly(Ester Ester)

摘要

Previous research showed Environmental Stress Cracking (ESC), i.e. mutual intensification of the destructive effect of loading and environment, of a chemical nature of poly(butylene terephthalate)/poly(butylene adipate) (PBT/PBA) copoly (ester ester) in water and phosphoric acid solution at 353K. The ESC was studied by means of constant-load time-to-failure (TTF) experiments performed on sharply notched tensile specimens. In most practical situations the application of stress and initial contact with a potential ESC fluid do not coincide. It is known that in the case of physical ESC the order of loading as well as immersion and the time inbetween can influence the severity of attack. To investigate if this is also the case with chemical ESC of PBT/PBA in water and phosphoric acid solution, the interaction between environment and deformation is tested by means of exposition or loading before the actual ESC testing (TTF tests in environment). The tests are performed at 2.5 MPa, within the stress range where chemical ESC occurs. Previous research without pre-exposition and pre-loading showed a heavy reduction in the TTF, indicating an acceleration of the ESC, due to the presence of acid. Also a lifetime inversion region is found (longer TTF for higher load) indicating competition between crack growth and blunting mechanisms. These results, in combination with the results of the pre-exposition and pre-loading experiments described in this paper, lead to the hypothesis that early mechanical loading causes crack tip blunting and local strain hardening due to orientation, resulting in a TTF increase. Accordingly, early chemical exposition, especially exposition to acid solution, reduces the favourable effect of blunting and orientation resulting in a decrease of TTF.