Hydrolytic and photochemical aging studies of a Kevlar®-PBI blend

The focus of this work is the study of the hydrolytic and photochemical aging behavior of a Kevlar (R)-PBI blend fabric. Tensile tests carried out on yarns extracted from this fabric after either irradiation with UV light or exposure to high humidity indicated a continuous decrease of the breaking force with exposure time. ATR-FTIR analyses of photo-chemically aged samples showed evidence of a photo-oxidative reaction initiated by the cleavage of the amide bond of Kevlar. The overlapping of the breaking-force curves that was observed as the irradiance level was increased at constant temperature is believed to be caused by a "screen" effect produced by Photo-Fries products. The fact that at constant temperature the breaking force was unaffected by the variation of the relative humidity suggests that the absorption of water is not the rate-controlling step in the degradation kinetics. ATR-FTIR analyses revealed the presence of a new absorption band ascribed to carboxylic acid end groups produced during the hydrolysis of the amide linkage that occurred after humidity aging. The relative intensity of the COOH band tended to a constant value as exposure times increased, suggesting that in addition to the hydrolysis, a competing recombination reaction takes place during degradation. A kinetic model for the hydrolytic degradation process was formulated and solved. (C) 2011 Elsevier Ltd. All rights reserved.