Ageing of polypropylene: processes and consequences

Based on information available in the literature and new investigations, the ageing behaviour of polypropylene at room temperature and in annealing was studied. Two effects have to be discerned: changes in the amorphous and/or mesomorphic regions of the material at room temperature, which lead to an increase in density and modulus in connection with embrittlement on the one hand, and relaxation- and recrystallization-processes at higher temperatures, which positively influence heat deflection temperature and impact strength alongside with modulus and density on the other hand. Use of elevated temperatures to accelerate ageing for the determination of long-term properties is therefore not possible in the case of polypropylene.