Adhesion chemistry of plant-based cellulosic fibers and polymeric matrices in composites: a review

Plant-based cellulosic fibers are often considered a preferred material choice for polymer composites due to growing environmental concerns. The hierarchical macro- and micro-structures of the reinforcing fibers determine the characteristics of fiber-matrix interfaces and performance of the composite. The non-aligned spatial orientation of cellulosic fibers (when used as discontinuous fibers in composites), variability in the fiber length and diameter, and heterogeneity in the chemical composition exert complex effects on the adhesion behavior in composites. Additionally, cellulosic fiber polymer composites exhibit limited interfacial compatibility with polymer matrices due to the hydrophilicity of the reinforcing materials. The review herein presents the dominant adhesion mechanisms of mechanical interlocking, chemical bonding, physical adhesion, and weak boundary layers and their impact on composite properties. Furthermore, this review studies the relationship between the cellulosic fiber structure-morphology-topography and adhesion mechanisms to address and counteract adhesion problems. For a given fiber diameter, an increase in the fiber length (up to a threshold value) increases the composite properties due to enhancement in adhesion properties. An increase in the fiber length enhances the mechanical interlocking within composites and is beneficial until it induces a curling effect. However, interfacial adhesion in composites decreases with the increase in the fiber or yarn twist due to the increase in compactness, decrease in porosity, and possible reorientation of the twisted fibers or yarns from the loading direction. The percentage cellulose content primarily determines the interface properties, while the non-cellulosic components, such as hemicellulose and lignin, contribute to the formation of weak boundary layers, adversely affecting the fiber-matrix adhesion behavior.