PALF–Hemp Hybrid Reinforced PLA Composites: A Systematic Review of Fibre Ratio, Interfacial Modification, and Mechanical Performance

Abstract

Natural-fibre-reinforced poly(lactic acid) (PLA) composites are investigated as sustainable alternatives to petroleum-based materials; however, the combined effects of pineapple leaf fibre (PALF), hemp fibre, hybrid ratio, interfacial modification, and processing remain insufficiently consolidated. This systematic review evaluates PALF–hemp hybrid-reinforced PLA composites, focusing on fibre ratio, total loading, alkali and silane treatments, tensile, flexural and impact performance, and fracture morphology. A Scopus search using a reproducible TITLE-ABS-KEY Boolean strategy identified 3,061 records. Following filtering, title–abstract screening, eligibility assessment, and hierarchical evidence classification, 30 studies were retained, comprising direct PALF–hemp/PLA evidence and related PALF-, hemp-, and hybrid-PLA systems. The findings indicate that mechanical performance is governed by a coupled composition–processing–interface–fracture–property relationship rather than fibre addition alone. The only direct PALF–hemp/PLA study reported improvements of 73.50% in tensile strength, 60.00% in flexural strength, and 58.40% in impact resistance following optimisation of fibre content, moulding temperature, and pressure. Supporting studies demonstrated that fibre length, orientation, surface treatment, matrix continuity, crystallinity, and void content also influence mechanical performance. Alkali and silane treatments generally improved interfacial adhesion and reduced water uptake, although moisture exposure could still cause substantial stiffness losses. Fracture observations associated effective interfaces with fibre rupture, matrix adhesion, crack deflection, and controlled pull-out, whereas weaker interfaces exhibited debonding, interfacial gaps, agglomeration, and void-assisted cracking. However, only one study directly investigated PALF–hemp/PLA composites, limiting the available evidence for establishing a generalisable optimum PALF ratio. Consequently, any apparent optimum remains specific to the reported formulation and processing conditions and requires independent experimental validation. Future research should distinguish fibre ratio from total loading, optimise each fibre component systematically, standardise processing and conditioning conditions, and validate multi-response performance through microscopy, physicochemical characterisation, durability testing, sensitivity analysis, and confirmation experiments.