Thermo‐oxidative and photo‐oxidative aging of polypropylene under simultaneous tensile stress

Abstract The effect of uniaxial tensile stress at constant load in the range of 0 to 6.86 MN/m 2 upon the behavior of stabilized and unstabilized types of isotactic polypropylene was investigated in the course of thermo‐oxidative aging at 80° to 130°C and of photo‐oxidative aging in the Xenotest 450 apparatus at 45° and relative humidity of 65%. From kinetic evaluation of the temperature dependence of weight changes of unstabilized polypropylene during thermo‐oxidative aging, it was found that the weight losses of unstressed and tension‐stressed specimens obey the kinetic equations for a reaction of the first order. The degradation of stressed specimens, however, proceeds at a considerably higher rate as compared with the unstressed state and is marked by a decrease in apparent activation energy as well as by an increased rate of crack development. The cracks develop in the direction perpendicular to the tensile stress and have the shape of hollow funnels widening outward. The tensile stressing of stabilized types of polypropylene in thermo‐oxidative and photo‐oxidative environments causes an accelerated embrittlement of the polymer, manifested particularly by a marked decrease of elongation at break. This accelerated embrittlement goes on under the action of tensile stresses exceeding the so‐called safe stress, i.e., the stress below which no acceleration of aging (as measured by the relative decrease in the elongation at break of the stressed and unstressed specimens), can be detected. The values of this safe tensile stress decrease with the temperature and lie well below the tensile strength of the polymer. In the range of tensile stresses exceeding the safe stress, the dependence of the logarithm of time of a 50% decrease of the elongation at break on the applied tensile stress was found to be linear. The findings are in agreement with Zhurkov's kinetic concept of mechanical deterioration of polymers, where the effect of the applied mechanical stress is superimposed on the thermal fluctuation and leads to the scission of chemical bonds and to the formation of cracks in the polymer.