A Systematic Review of Alkali-Treated Polylactic Acid Biocomposites Relevant to Pineapple Leaf and Water Hyacinth Hybrid Design

The increasing demand for sustainable materials has accelerated the development of biodegradable polymer composites. Polylactic acid (PLA)-based natural fiber composites have attracted significant attention due to their renewable characteristics; however, their performance remains limited by weak fiber–matrix compatibility, moisture sensitivity, and inconsistent interfacial behavior. Alkali treatment has been widely applied to modify lignocellulosic fibers by removing non-cellulosic components and improving surface characteristics, although its effectiveness remains dependent on treatment conditions and fiber properties. This systematic review evaluates the influence of alkali treatment on PLA-based natural fiber composites and assesses the available evidence supporting future pineapple leaf fiber (PALF) and water hyacinth fiber (WHF) hybrid composite development. The review followed the PRISMA 2020 framework using Scopus-indexed articles published between 2020 and 2026. A total of 336 records were identified, with 295 studies retained for evidence synthesis and classified into five categories (E1–E5) to distinguish direct evidence from transferable knowledge. The results indicate that no direct PLA/PALF-WHF hybrid composite study has been reported. Alkali treatment generally improves fiber surface modification and interfacial interaction; however, the resulting performance is highly condition-dependent, influenced by alkali severity, fiber characteristics, loading ratio, and processing route. WHF remains insufficiently characterized in PLA-based systems due to hydrophilicity and compatibility challenges. Therefore, future studies should focus on controlled PLA/PALF-WHF hybrid experiments, optimized treatment parameters, and multi-response performance evaluation.