Mechanical performance of in vitro degraded polylactic acid/hydroxyapatite composites

Polylactic acid (PLA)-based products can be found in a wide range of industrial applications due to their favorable performances. Unfortunately, neat PLA also exhibits some limitations which could be overcome by blending, composites or self-reinforced compounding. On the other hand, the mechanical characterization of PLA during in vitro degradation process has been generally limited to some basic mechanical tests which can be improved by the application of fracture mechanics analysis. The PLA matrix was reinforced (2.5 to 20 wt.%) with hydroxyapatite (HA) and processed as film or fiber in order to be experimentally evaluated. The evolution of mechanical performance during in vitro degradation was characterized by stress-based analysis and fracture mechanics. The composites displayed similar particle size distributions and more frequent agglomeration at large filler content. The film with 2.5% HA showed an enhanced crack resistance followed by a drastic drop at shorter immersion times linked to a premature matrix degradation. Fibers displayed higher tensile performance, compared to films, and faster degradation in accordance with thinner cross section. The performed characterizations indicated hydrolysis of PLA controlled by autocatalytic random scission and marginal end scission. Experimental behavior also showed an energy–stress transition, with the filler volume fraction, to define the most sensitive mechanical parameter. The whole performance during in vitro degradation can be described by three stages: (i) running-in, (ii) steady and (iii) severe. Particularly, the combination of composition, compounding method and evaluated mechanical parameter defines the presence or not of these three different stages along the in vitro degradation process.