Thermal degradation of polyacrylic acid in dilute aqueous solution

The thermal degradation of polyacrylic acid of molecular weight (M-w) 5660 (PAA 5K) has been studied between 180 and 260 degreesC, representing the heat conditions prevailing at the high pressure heaters (185 degreesC) and steam generators (260 degreesC) of the steam-condensate cycle of CANDU nuclear power plants. The degradation does not follow a simple first-order kinetics but probably a rather complex mechanism that includes decarboxylation and another process that attacks the integrity of the polymer chain. The degradation seems to be rapid at first with up to 10% lost in the first hour required to reach the set temperature in the pressure vessel and a 50% loss after approximately 5 h at 260 degreesC. The major thermal decomposition products of PAA are identified as carbon dioxide in the gas phase, while in the liquid phase the degradation primarily yields carbonate/bicarbonate, with the presence of low-molecular-weight organic acids, including acetate and to a lesser extent formate. Also, the size exclusion chromatography (SEC) analysis tends to indicate the production of smaller polymer chains, which are likely to have fewer charged carboxyl groups and possibly unsaturated double bonds. These results suggest that, along with the decarboxylation mentioned in the literature, another degradation mechanism that would attack the integrity of the polymer chain is likely taking place, especially at a high pH level. The acetate and formate seem to play an intermediate role since their concentration levels have been shown to increase at first but later decrease to a lower level during the course of our tests. (C) 2002 Elsevier Science Ltd. All rights reserved.