The Role of Coupling Agents in the Mechanical and Thermal Properties of Polypropylene/Wood Flour Composites

Abstract

This work is a collaborative effort between academia and industry to promote the development of new sustainable and profitable materials for manufacturing products. Incorporating wood flour particles (WF) in polypropylene (PP) grants environmental advantages in developing products that use renewable resources to manufacture PP/WF composites using the melt intercalation process. However, the interaction between a hydrophilic strengthening phase (wood flour) with a nonpolar polymer matrix (PP) is poor, resulting in deficient mechanical performance. This investigation details the use of graft and masterbatch coupling agents to evaluate their effects on mechanical parameters. The low compatibility between the constituents favors increasing the composites’ thermal properties because the reinforcing phase acts as a nucleating agent. PP showed typical mechanical behavior, with a marked necking and a wide deformation capacity of approximately 180%. The mechanical behavior of the PP/WF composites revealed an elastic region followed by a termination after their yield point, shortening the stress–strain curves and reducing their ductility at strain values of approximately 2–4%. Graft coupling agents have better intermolecular performance with PP than masterbatch coupling agents. The modulus of elasticity of the composites increased to around 82% relative to PP. Processing methods influenced the thermal properties of the composites. The melt-blending process promoted molecular orientation, while injection molding erased the thermomechanical history of the extruded pellets. The melting temperature was similar in the composites, so there was no evidence of thermal degradation. The results showed that the coupling agents favor the crystallinity of the PP over tensile strength. SEM observations showed insufficient adhesion between the WF and PP, which promotes a reduction in stress transfer during tensile testing. The WF particles act as fillers that increase the stiffness and reduce the ductility of composites.