Effect of Alkali Treatment on the Mechanical and Structural Properties of Pineapple Leaf Fiber for Epoxy Composite Reinforcement

Alkali treatment is widely reported to enhance the performance of natural fiber–reinforced polymer composites, typically attributed to improved fiber–matrix adhesion. However, the extent to which this treatment modifies the internal structure of the fibers─and thereby their intrinsic mechanical properties─has remained insufficiently quantified. This question is particularly relevant for pineapple leaf fiber (PALF), an abundant agricultural residue with a high cellulose content and promising reinforcement potential. X-ray diffraction (XRD) confirmed that the crystalline phase of cellulose I was retained, while the crystalline order increased due to the removal of amorphous components such as hemicellulose and lignin. This structural refinement enhanced the intrinsic stiffness and strength of the fibers: the modulus increased from 18 to 29 GPa and the strength from 390 to 530 MPa, as measured using the impregnated fiber bundle test (IFBT). These results demonstrate that alkali treatment directly reinforces the fiber structure itself, independent of interfacial effects. When 20 wt % of treated PALF was incorporated into epoxy to form unidirectional composites, this fiber-level strengthening translated into substantial increases in flexural modulus (≈200%), flexural strength (≈180%), and notched impact resistance (≈400%) compared with neat epoxy. Overall, the findings clarify that alkali treatment enhances composite performance not only by improving adhesion but, more importantly, by intrinsically strengthening the fiber, offering a more accurate understanding of the mechanism and a scalable approach to high-performance biobased composites.