Environmental stress cracking of poly(vinylidene fluoride) and welds in alkaline solutions

Abstract

A study has been carried out to investigate the resistance of poly(vinylidene fluoride) (PVDF) to high pH aqueous sodium hydroxide solutions. In addition to surveying the conditions that caused environmental stress cracking (ESC) in test samples machined from injection moulded plaques, interest centred on regions of formed components that might contain high residual stresses, in particular vibration-welded samples. The first series of experiments involved subjecting standard test samples to fixed strains in three-point bend testing rigs and immersing them in highly alkaline environments. As expected, higher strains caused more problems for the material, with a cut off limit at about 5% strain, below which no measurable effects were observed in any environment within the timescale (63 days) of the experiments. Even above 5% strain, PVDF was found to be resistant to NaOH solutions of pH 12.5 and 13 although significant effects were seen as the pH was increased to 13.5, 14 and 14.39. Under these high strain, high pH conditions, the damage was manifest as discolouration of the samples due to dehydrofluorination. It was found that the time between immersion in the solution and the observation of initial discolouration was the most useful parameter for characterising the relative severity of environments. Tests on welded samples indicated that the flash from the welding process contained high residual strains (certainly >5%), but tensile residual stresses in the bulk material at the weld line were relatively small. Weld strength was about 90% of that measured for the bulk material but failure at welds was relatively brittle.