Moisture-induced swelling in natural fiber-reinforced composites: A critical review of chemical treatments, hybrid strategies, and environmental durability

This review aims to critically investigate the swelling behavior of natural fiber-reinforced composites (NFRCs), with a focus on identifying key mechanisms of moisture absorption, the influence of fiber–matrix interactions, and the effectiveness of mitigation strategies such as chemical treatments and hybridization. A systematic synthesis of over 120 peer-reviewed articles published between 2015 and 2025 was conducted. These studies were analyzed to extract data on moisture uptake behavior, microstructural evolution, treatment effects, matrix properties, and performance under various environmental conditions. The review identifies cellulose crystallinity, hemicellulose content, and fiber–matrix interfacial bonding as dominant factors influencing swelling. While Fickian diffusion models describe early-stage absorption, long-term exposure often results in non-Fickian behavior due to matrix plasticization and interface degradation. Chemical treatments such as alkali, silane, and acetylation reduce water absorption but may compromise fiber strength if overapplied. Hybridization with hydrophobic fibers and the use of non-polar matrices improve resistance. Moisture-induced performance degradation is strongly influenced by environmental variables such as temperature, humidity cycling, and salt fog. This review consolidates current knowledge on swelling mechanisms in NFRCs and clarifies the trade-offs between moisture resistance and mechanical performance. It also proposes design guidelines and testing protocols for enhancing long-term durability. The findings are applicable to industries seeking sustainable yet durable composite materials, particularly in automotive, construction, marine, and packaging sectors where environmental exposure is a critical concern.