Surface lignin removal on coir fibers by plasma treatment for improved adhesion in thermoplastic starch composites

The high lignin to cellulose ratio of coir fibers results in low compatibility between these fibers and natural polymers like starch, leading to poor mechanical properties in the composites. Plasma treatment using either air or oxygen proved to be an effective in removing the lignin rich amorphous layer on coir fibers, as it was clearly observed by SEM. The ratio of the FTIR signal related to lignin (1508cm^-1) and cellulose (1317cm^-1) decreases 10 times for air plasma treated fibers and 20 times for oxygen plasma treated samples. Composites of plasma treated short coir fibers and thermoplastic starch presented considerable increase in mechanical properties in comparison to composites made with untreated fibers. Tensile strength increased by up to 300% and elastic modulus improved by a factor of nearly 20 times, which was associated with enhanced fiber-matrix adhesion after plasma treatment with oxygen for 7.2min at 80W power.