Rubber Toughening of Polylactic Acid (PLA) with Poly(butylene adipate-coterephthalate) (PBAT): Mechanical Properties, Fracture Mechanics and Analysis of Ductile-to-Brittle Behavior while Varying Temperature and Test Speed

The present work fits into the stream of study and analysis of biobased and/or biodegradable materials, considering the need to improve their properties from a technological point of view to allow their wider use in final products. In this paper a blend of an extrusion grade poly(lactic acid) (PLA) and a percentage of a particular type of high molecular weight poly(butylene adipate-co-terephathalate) (PBAT) was investigated. The blend was such that the PBAT resulted in a dispersed phase in the PLA matrix. Tensile properties, Charpy impact strength, energy absorbed by the sample during three-point bending tests, melt flow index, fracture behavior and morphology were determined to characterize PLA/PBAT blends with the aim of correlating the composition to processability and final properties. Furthermore, analytical models were applied to check if variations of the mechanical properties with an increase of the rubber content were in agreement with existing predictive theories. To evaluate the fracture behavior, starting from the Load Separation Criterion, “material key curves” of the materials, a support for a comparative study (macroscopic scale) of the plastic deformational mechanisms involved in the fracture process was achieved. The influence of the deformation rate and the temperature on the toughness of the PLA/PBAT was investigated. In particular, the ductile-to-brittle transition temperature, at a fixed test speed was estimated and a theoretical model, which is useful to predict the absorbed energy by varying the deformation rate, was proposed. This last model was validated by comparing the resulting analytical equation with the experimental data obtained through three-point bending and impact tests at different velocities.