Curaua/polypropylene composites: Phase compatibilization and alternative micromechanics modeling

Curaua strands, a natural fiber scarcely explored for reinforcement applications, were investigated as the dispersed phase in polypropylene composites. Although maleic anhydride-grafted polypropylene (MAPP) is widely used as a compatibilizer in natural-fiber composites, the optimal dosage for curaua fibers and the mechanism limiting its effectiveness remain unclear, representing an important research gap for designing high-performance materials. In this study, polypropylene–curaua composites were produced with varying MAPP contents, and the compatibilization behavior was interpreted in relation to the topochemical composition of the fiber surface. Molecular simulation was employed to estimate the surface density of accessible hydroxyl groups, enabling a mechanistic evaluation of the interaction between MAPP and the fiber surface. Tensile strength reached its maximum at a MAPP-to-fiber ratio of 6 wt%, identified as the optimal level due to the limited availability of reactive –OH groups located in the outer layers (rich in pectins, lignin, extractives, and inorganic ions). At this condition, the intrinsic tensile strength of curaua fibers was estimated as 603 MPa using the Hirsch model, while other micromechanical approaches yielded values between 568 and 606 MPa. A complementary linear model based on cellulose content, crystallinity, and microfibril angle predicted an intrinsic tensile strength of 609 MPa, aligning with the upper bound of micromechanical estimations. These results address the aforementioned research gap by identifying the optimal MAPP dosage, discussing the mechanistic reason behind its limitation, and predicting intrinsic tensile strength in cases of strong interfacial compatibilization. • Even without coupling agent, curaua fiber increased the tensile strength of polypropylene-based composites. • Strength increased with MAPP content up to a MAPP-to-fiber percentage of 6 wt%. • The limited availability of –OH groups at the surface is deemed a setback for compatibilization. • The intrinsic tensile strength of curaua fibers was correlated to their microfibril angle and crystalline cellulose content.