Environmental stress cracking resistance. Behaviour of polycarbonate in different chemicals by determination of the time-dependence of stress at constant strains

Abstract

The mechanism of Environmental Stress Cracking (ESC) of amorphous polycarbonate in isopropyl alcohol, ethylene glycol monomethyl ether and methanol were studied. The three point bending method and the variation of stress with the immersion time at constant strain were used to evaluate the chemical attack. The results show that the variation of the stress with the immersion time can be a very good indicator of how a polymer will behave in specified environment and strain conditions after exposure to various chemicals. The resistance of the polymer was monitored as a function of time at different given strain-values and different initial stresses. The time to ESC was defined as the point where the resistance of a polymer immersed in a given solvent deviates from the equivalent measure of the polymer sample in air. The results showed that the critical strain of polycarbonate is 1.21±0.06% when immersed in isopropyl alcohol, 1.74±0.12% when immersed in methanol, and 0.29% when immersed in ethylene glycol monomethyl ether. (The 1.74% value of methanol is lower than the value given in the literature). The results show that all three liquids give massive reductions in the strength of the polycarbonate. This is especially true in the polycarbonate-ethylene glycol monomethyl ether combination, where the specimens showed complete failure after about 22 s. The images obtained with optical microscopy showed that the cracking propagates from the edge to the center of the specimen and reaches typically about 5 μm below the surface. The diffusion rates of the different solvent molecules in the polymer were further determined by absorption measurements in an attempt to establish a correlation with the environmental stress cracking resistance. The results showed that methanol has the fastest diffusion rate, followed by ethylene glycol monomethyl ether. Polycarbonate immersed in isopropyl alcohol showed very limited weight gain within the given time of monitoring.