**4. Impact of surface treatment of natural fiber on moisture absorption**

Numerous drawbacks of NFr include decreased strength, decreased interfacial adhesion, poor moisture absorption resistance, and reduced durability and dimensional stability. To overcome these constraints, different methods for NFr modification have been proposed [31]. Four different approaches are used for treating NFr, physical processes, chemical methods, biological methods, and nano enhance photocatalysis treatment. These revised techniques have varying degrees of efficacy in enhancing the mechanical properties of fibers, adhesion between matrix and fiber,

and improving various characteristics of the end products [32]. However, moisture absorption leads to debonding between NFr and the matrix, because of the high susceptibility of NFr to moisture, leading to a significant decrease in the mechanical characteristics of the NFr composite [27]. This is because NFr in nature is polar and hydrophilic due to the existence of non-cellulosic components (i.e., pectin, lignin, and hemicelluloses), because it makes moisture absorption possible through the availability of functional groups such as hydroxyl (OH) and carboxylic acid [33]. Low fiber bonding with a polymer matrix generates empty areas surrounding organic composite fibers, resulting in increased moisture absorption. Fiber modification through alkalization will minimize moisture content. Potassium hydroxide or sodium hydroxide is frequently employed to remove open groups of hydroxyls that bind to water molecules [22]. Additionally, it removes hemicelluloses and reduces the capacity of fiber to absorb moisture. This combination results in the formation of chemical and hydrogen bonds, which retards the degradation of the NFr-matrix interface caused by humidity [34].

There are certain limitations to utilizing the physical, chemical, and biological treatments, such as obtaining optimal chemical concentrations to avoid additional delignification of the NFr because greater concentrations might weaken and deteriorate the NFr, as reported by Alnaid et al. [35] and Dahham et al. [36]. The chemical compounds that are employed in treating NFr will continue to damage the fibers after exposure to weathering if they are not thoroughly cleaned after-treatment process, eventually leading to severe fiber swelling and disintegration. Furthermore, the NFr composites treated with these techniques have a low UV resistance, resulting in UV deterioration of composite components, such condition contributes to more interlaminar delamination and more brittleness of the composites when placed under high loads [23].
