Time and temperature effects on the tensile yield properties of polypropylene

Tensile tests were made on polypropylene films as a function of aging temperature from 80 to 130 degrees C at a strain rate of 5 cm min(-1). Polypropylene films aged at 60 and 100 degrees C and at time intervals up to 180 min were also stretched at the same strain rate. The yield stress and initial modulus were found to be linear functions of temperature, extrapolating to a zero value close to the thermodynamic melting point of the polymer (170 degrees C). The work of yield, the plastic and yield strains also decreased with increase in aging temperature but the elastic strain increased. The plastic strain, yield strain, yield stress, and initial modulus for the 60 degrees C aged film had larger values than the corresponding values for the 100 degrees C aged film at equivalent time intervals and all properties decreased with increasing log time of aging. These decreases in properties were explained in terms of decrease in the density (crystallinity) of aged PP films. (C) 1997 John Wiley & Sons, Inc.