Reducing the Heterogeneity in Tensile Strength in the Coir (Cocos nucifera)-Epoxy Composite Through Enhancing Surface Quality and Maintaining Longitudinal Integrity

Abstract

Coir fibers (Cocos nucifera) are strong and sustainable alternative to hazardous synthetic fiber; however, there are certain challenges that has to be dealt with, in order to fully exploit the strength of these fiber. These challenges include hydrophilic and hygroscopic behavior of coir fiber, poor fiber–matrix interface, and the twirling of fibers. In this work, these challenges are resolved in different stages through various surface modification techniques, like alkali treatment for reduced hydrophilicity, silane coating for enhanced interface, and straightening to reduce fiber twirling. FTIR spectroscopy reveals the removal of hemicellulose, lignin, wax, etc. post-alkali treatment, and the following silane treatment show development of new Si–O–Si bonds. Their influence on tensile strength of composite is monitored after every step. Hand-layup methodology is employed for composite fabrication of 25% (w/w) fibers with matrix epoxy LY556/HY951. Developed composite includes untreated coir epoxy (UCE), alkali-treated coir epoxy, alkali-silane coir epoxy, and alkali-silane-straightened coir epoxy (ASSCE). Tensile behavior and thermogravimetric analysis (TGA) were examined. The results show 41% rise in the tensile strength from 29 MPa for UCE specimens to 41 MPa for ASSCE. In addition, there was 70% reduction in the standard deviation for ASSCE specimens. Analysis of variance shows that these modifications produce significant results. TGA test reveals that coir epoxy composites degrade on and above 250 ℃, the untreated coir epoxy shows slightly more thermal stability than alkali-silane composite, followed by alkali treated. The morphology of failed specimens examined under an optical microscope and scanning electron microscope reveals details like fiber cross section, microfibrils, and fiber rupture.