Water sorption and hydrolytic stability of polycarbonates

Abstract

AbstractThe hydrolytic stability of a new commercial polycarbonate (Calibre 300, Dow Chemical USA) was investigated and compared with that of other commercial polycarbonates. The tests were conducted between 56% and 95% relative humidity (R. H.) at 100°C. Also performed were water immersion tests at 80 and 100°C. The water diffusivity was found to be 8.7 × 10−7 cm2/s at 100°C with an activation energy of 7.9 kcal/mole. These values are similar to other glassy polymers. The equilibrium water sorption, C∞, was found to increase with temperature and R.H. The isotherm at 100°C was determined to be: C∞ = 0.005945 [R.H.]. When samples immersed in a water bath at 100°C were transferred into room‐temperature water, visible aqueous microcavities were formed due to the condition of super‐saturation, and under stress may become crack initiation sites. For the polycarbonate investigated here, it was found that the decrease in weight‐average molecular weight (M̄)w was a first‐order process under a constant R.H. and temperature, and that hydrolytic embrittlement, i. e., (M̄)w <34,000, was reached after ca. 188, 143, 99, and 66 days under 56%, 73%, 87%, and 95% R.H., respectively, at 100°C. A comparison with reported hydrolytic stability data for other polycarbonates showed large differences in their stability which are believed to be due to the extent of end‐group capping (over 95% in Calibre 300) and resin purity: both phenolic end‐groups and some additives (i.e., fire retardants, thermal stabilizers) are known to accelerate hydrolytic degradation.