Effect of relative humidity on the mechanical properties of poly(1,4‐butylene terephthalate)

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Abstract
Three grades of poly(1,4‐butylene terephthalate) (PBT) were aged up to three years at 100, 75, 50, and 11% relative humidity and temperatures of 66–93°C. The decrease in mechanical properties, caused by hydrolysis, occurs rapidly at the higher temperatures and relative humidities and progressively slows as the temperature and/or humidity are decreased. Equations for making life‐cycle predictions at any combination of temperature and humidity were derived from Arrhenius plots. If a 50% loss in tensile strength constitutes failure, then the PBT examined should be expected to last only three to four years at 50°C and 100% relative humidity. Reducing the humidity level to 50% extends the useful life at this temperature to 10–20 years. Predictions based on the tensile strength half‐life should not be used where toughness or impact properties are important because PBT embrittles long before the tensile strength half‐life is reached.