Physical ageing of Poly(Lactic acid): Factors and consequences for practice

Injection molded specimens were prepared from poly(lactic acid) (PLA) and their properties were determined as a function of time to study physical ageing. The mechanical testing of specimens showed that properties change rapidly with time. The stiffness of the specimens increases considerably, while their deformability decreases drastically from 250% after injection molding to a few percent after less than a day of ageing. Thermal analysis showed that both relaxation enthalpy and the change in the glass transition temperature (Tg) of the polymer increased with time. Tg decreased with increasing ageing time that could not be explained by the generally accepted approach of decreasing free volume. The analysis of literature data showed that the decrease or increase of Tg depends on the temperature of ageing and on the rate of cooling. Tg decreases at relatively low ageing temperatures, while it increases when ageing temperatures are closer to the Tg of the polymer. Besides the decrease of free volume, the development of internal stresses also plays a role in the determination of the glass transition temperature. Internal stresses result in a decrease of Tg, while decreased free volume leads to an increase. Internal stresses determine deformation and failure mechanism as well; large stresses lead to crazing/cracking and finally to brittle failure, which may hinder the application of PLA in many areas.