Flexural Stiffness of MAPP-Coupled PP–Henequen Composites: Micromechanical Modeling of Structure–Property Relationships

Abstract

The flexural behavior and intrinsic mechanical properties were studied for polypropylene (PP) composites reinforced with 20 to 50 wt.% henequen strands, using maleic anhydride grafted polypropylene (MAPP) as compatibilizer. The maximum strain at break of the fibers was estimated by regression-based extrapolation (ε = 8%), which enabled calculation of the microfibrillar angle (MFA = 22.8°) using a modified Satyanarayana approach. Together with cellulose content and crystallinity, the resulting structural parameter (δ = 1.72) confirmed the suitability of henequen strands as reinforcement for thermoplastic composites. The composite flexural modulus exhibited a linear increase with fiber volume fraction (R² = 0.99), rising from 1.15 GPa for neat PP to 3.75 GPa at 50 wt.% fiber content, while the strain at break decreased consistently with increasing reinforcement content. Predictions obtained using the Hirsch model and the modified rule of mixtures, with efficiency factors (η_e) ranging from 0.45 to 0.55, showed good agreement with experimental results. Furthermore, three micromechanical models, Hirsch, Tsai–Pagano, and Nielsen, yielded consistent intrinsic flexural moduli for the henequen strands, in the range of 16 to 17 GPa. These results highlight the reinforcing potential of henequen strands and reveal their effectiveness in increasing the flexural stiffness of polypropylene-based composites.