Compatibilization strategies for PLA biocomposites: a comparative study between extrusion-injection and dry blending-compression molding

In this work, PLA biocomposites were prepared by two different processing methods to evaluate two coupling agents: maleic anhydride grafted PLA (MA-g-PLA) and glycidyl methacrylate grafted PLA (GMA-g-PLA). Coupling agents were directly incorporated during extrusion before injection molding and through fiber surface treatment before dry-blending (DB) and compression molding. The biocomposites morphology was more homogeneous and with fewer voids using both coupling agents due to a better adhesion. The processing method influenced the coupling efficiency and determined the biocomposites properties. Tensile strength decreased with the agave fibers incorporation to a larger extent in DB-compression molding (from 47 to 34 MPa) than in extrusion-injection (from 55 to 51 MPa) due to the high level of compaction and dispersion achieved with high shear rate. Tensile strength was improved with both surface treatments in DB-compressed samples, while in the extrusion-injected materials, this property increased up to 67 and 59 MPa with GMA-g-PLA and MA-g-PLA, being even higher than the neat PLA. However, the excess of the coupling agent could affect its stiffness. The hydrophobic coating deposited on the surface reduced fibers hydrophilicity, and water diffusivity decreased due to a more compact internal structure, resulting in water absorption lower than 15% in all cases.