Effect of chemical treatments, hybridizations and water absorption on flexural strength of sisal/polyester composites

This study examined the effects of chemical treatments and hybridization on the flexural strength and water absorption-induced degradation of sisal/polyester composites. Sisal fibers were treated with sodium hydroxide and sodium bicarbonate prior to composite fabrication. Hybrid reinforcements included unidirectional glass and carbon fibers, as well as glass and carbon fillers. Various stacking sequences, fiber orientations, and volume fractions were employed. For filler-reinforced composites, four filler contents (2.5 wt%, 5 wt%, 7.5 wt%, and 10 wt%) and two sisal fiber contents (20 wt% and 30 wt%) were tested. Results indicated that chemical treatment notably improved flexural strength, with sodium bicarbonate treatment producing a 25% increase over untreated fibers. Hybridization with unidirectional carbon fibers, especially when positioned in the compressive layer of the laminate, enhanced flexural strength by approximately 140%. Among filler-reinforced composites, those containing 30 wt% sisal fibers combined with 5 wt% carbon fillers showed the highest flexural strength, exceeding filler-free composites by 30.9%. Water absorption reduced flexural strength across all samples; however, degradation was significantly lessened by chemical treatment and hybridization. The composite with 12% carbon fiber volume demonstrated the lowest flexural strength reduction of 2.7%, corresponding to an 84% decrease in degradation compared to untreated samples.