Enhanced Mechanical Properties of Epoxy/Microfibril Cellulose Composites via GPTMS Modification: Optimization and Microstructural Analysis

This study investigates the effect of 3-glycidoxypropyltrimethoxysilane (GPTMS) concentration on the mechanical, interfacial, and fracture behavior of epoxy/microfibrillated cellulose (MFC) composites derived from oil palm empty fruit bunch (OPEFB). GPTMS was incorporated at 1, 3, and 5 Phr to improve compatibility between hydrophilic MFC and the hydrophobic epoxy matrix. Mechanical testing revealed that GPTMS concentration significantly influenced composite performance in a concentration-dependent manner, with 1 Phr GPTMS providing the most balanced reinforcement. At this concentration, tensile strength increased by 14.5% from 32.88 ± 3.61 MPa to 37.65 ± 1.42 MPa, while flexural strength improved by 5.55% from 70.24 ± 5.30 MPa to 74.14 ± 4.10 MPa compared with the unmodified composite. Tensile modulus also increased from 2.07 ± 0.06 GPa to 2.21 ± 0.16 GPa, accompanied by improved flexural modulus from 2.39 ± 0.12 GPa to 2.47 ± 0.21 GPa. SEM analysis revealed that the optimized formulation promoted more uniform MFC dispersion, improved interfacial integrity, reduced void formation, and enhanced fracture resistance through tortuous crack propagation, localized radial crack branching, and matrix tearing. In contrast, higher GPTMS concentrations (3 and 5 Phr) reduced mechanical efficiency, with flexural strength declining to 65.27 ± 5.33 MPa and 66.16 ± 4.23 MPa, respectively, due to increased fiber pull-out, interfacial heterogeneity, and more continuous crack propagation. FTIR analysis suggested possible silane-related interfacial modifications consistent with GPTMS incorporation, although these findings are interpreted as supportive rather than definitive evidence of grafting. Overall, the results demonstrate that moderate GPTMS incorporation (1 Phr) is the optimum strategy for enhancing epoxy/MFC composite performance, offering a practical pathway for developing sustainable lightweight bio-based composites with balanced strength, stiffness, and fracture resistance. This research contributes to SDG 12 (Responsible Consumption and Production) by promoting sustainable utilization of oil palm biomass waste for advanced engineering materials.