Effect of NaOH concentration and immersion time on the mechanical properties of coconut coir–sugar palm hybrid composites

Abstract

Natural fiber–reinforced composites are increasingly developed as sustainable alternatives to synthetic materials, but their performance depends strongly on fiber–matrix interfacial bonding. Alkali treatment can improve this bonding by removing hemicellulose, lignin, and surface impurities; however, the joint optimisation of NaOH concentration and immersion time for coconut coir–sugar palm hybrid composites at multiple fiber loadings has not been quantified with full statistical analysis. This study evaluated four alkali treatments (5% and 10% w/v NaOH for 2 h and 6 h) and an untreated control, applied to coconut coir and sugar palm fibers in equal mass proportion. Twenty single-fiber specimens per condition were tested; treated and untreated fibers were then fabricated into hybrid composites with an unsaturated polyester matrix at three fiber volume fractions (Vf = 0.20, 0.25, 0.30), with five composite specimens per cell tested per ASTM D638. Single-fiber tensile strength rose from 38.26 ± 13.53 MPa (untreated) to 62.30 ± 32.69 MPa under 10% NaOH for 2 h; one-way ANOVA was significant (F(4,95) = 3.11, p = 0.019), and Tukey HSD identified 10% NaOH for 2 h as the only condition differing from the untreated control (p = 0.015). At the composite level, two-way ANOVA showed highly significant main effects of treatment (F(4,60) = 23.36, p < 0.001) and volume fraction (F(2,60) = 75.11, p < 0.001) and no significant interaction (p = 0.063). The highest composite strength of 17.28 ± 2.22 MPa occurred at 10% NaOH for 2 h with Vf = 0.30, against 9.17 ± 1.98 MPa for the untreated control. Prolonged immersion at 10% NaOH for 6 h reduced composite strength to 12.12 ± 2.32 MPa, consistent with cellulose degradation. Within the conditions tested, 10% NaOH for 2 h is the optimal treatment.