Structure–Property Relationships in Alkali-Treated Pineapple Leaf Fiber Reinforced PLA Composites: Influence of Fiber Loading and Processing-Induced Effects

Biodegradable natural fiber-reinforced composites are increasingly being explored as sustainable engineering materials. In this study, untreated (UPF) and alkali-treated pineapple leaf fibers (APF) were incorporated into polylactic acid (PLA) at 5–20 wt.% using twin-screw melt blending followed by injection molding. Alkali treatment reduced the average fiber diameter from 138.8 μm to 92.2 μm and modified the fiber chemical composition. Differential scanning calorimetry showed that fiber incorporation promoted heterogeneous nucleation with only minor changes in the glass transition temperature (54–59 °C). Dynamic mechanical analysis indicated improved stiffness, with the 10UPF composite exhibiting the highest storage modulus. The tensile modulus increased by more than 45%; however, tensile and flexural strengths declined at higher fiber loadings and in several APF composites. Void content, water absorption and fracture morphology revealed increased porosity, fiber pull-out and interfacial discontinuities following alkali treatment and processing. The results demonstrate that composite performance is governed by the combined effects of fiber characteristics and processing-induced phenomena, including void formation, fiber agglomeration and fiber damage, rather than by alkali treatment alone, providing mechanistic insight into the structure–property relationships of injection-molded PF/PLA composites.