Natural fiber composites water absorption

The slightest amount of water can significantly affect the mechanical properties of natural fiber-reinforced composites. Water absorption in a fibrous composite is dependent on many factors such as temperature, fiber loading, orientation of fibers, permeability of fibers, surface protection, area of the exposed surfaces, diffusivity, void content, hydrophilicity of the individual components, etc. 

Table 7.10 somewhat positions typical natural fibers such as jute and kenaf vs. glass fiber, when used at equal level in polypropylene. Tensile, flexural, and impact properties, along with water absorption and specific gravities were measured on injection molded samples. As can be seen, natural fibers composites are quite challenging the glass fibers-filled composite with however a significant difference in moisture sensitivity. 

Water absorption by natural fibers, such as cellulose causes the formation of mtemal forces capable of rotating fibers around their axis. This twisting motion introduces stress into the structure of composite. ... 

M. Gnatowski. Water absorption by wood-plastic composites in exterior exposure. 8th International Conference on Woodfiber-Plastic Composites (and Other Natural Fibers), Madison, WI, May 23-25, 2005. 

F. Yehia, S. Law, M.T. Kortschot. Water absorption in wood fiber reinforced polyethylene. In Progress in Wo odflbre-Plastic Composites, Conference Proceedings, Canadian Natural Composites Council, University of Toronto, Toronto, Canada, 2002. 

Water Absorption in Natural Fiber Plastic Composites... 

The neat polyester resin showed very low water uptake due to the three dimensionally cross-linked network structure after curing. The -OH group in the chain end of polyester and oxygen of the ester linkage influences the formation of hydrogen bonds. However it absorbs 0.05 mol% of water due to the presence of micro cracks and also due to the hydrophihc nature of polyester. The fiber-reinforced composites absorb water very rapidly at the initial stage and later a saturation level is attained, and there is no further increase in water absorption. As the flber content increased the water absorption also increased due to the hydrophilic nature of the fiber [101]. 

Another chemical modification studied was poly(vinyl alcohol) (PVA). The modification was conducted in water hyacinth fiber before mixing with low-density polyethylene and natural rubber to form a LDPE/NR/WHF composite. The tensile strength and modulus, melting temperature and water absorption resistance of the LDPE/NR/ WHF composites are reported to increase after PVA modification. However, the elongation at break decreased after the modification. Lower value of interparticle spacing was also found in PVA-modified WHF composites, enhancing the interparticle interaction between WHF and the matrix. 

Natural fiber/PP-composites absorb moisture from the environment. This moisture absorption does not lead to a change in width or length but to an alteration in thickness (about 1% at 1% water absorption). The tensile and flexural properties depend also on the moisture content. So the effects of water uptake are only partly reversible. The relative... 

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