Optimizing the Performance of Natural Fiber Reinforced Plastics Composites: Influence of Combined Optimization Paths on Microstructure and Mechanical Properties

This study investigated the combination of two different optimization paths (phase compatibilization and manufacturing process optimization) on the microstructure (fiber dimensions, matrix crystallinity and matrix molecular properties) and mechanical properties (flexural and tensile moduli, impact strength, and tensile stress at yield) of flax fiber/postconsumer recycled plastic composites. A two-step optimization methodology was adopted. First, the material composition was optimized by phase compatibilization using maleic anhydride grafted polypropylene (MAPP) and maleic anhydride grafted ethylene octene metallocene copolymer (EO-g-MAH) as additives. Then, the manufacturing process (extrusion followed by injection) was optimized in terms of temperature profile and screw speed for extrusion, as well as barrel temperature profile, mold temperature, injection speed, injection pressure, injection time and back pressure for injection. The results showed that, besides good fiber-matrix interfacial adhesion, the combination of both optimization paths promoted an optimum balance between components degradation and composite homogeneity (good fiber and additives dispersion in the matrix), leading to better mechanical properties. It is shown that this optimization procedure was able to improve all the mechanical properties of the composites, as well as being effective in terms of performance and costs.