Environmentally benign dyeing mechanism of knitted cotton fabric with condensed and hydrolyzable tannin derivatives enriched bio-waste extracts

This study reported a sustainable comparative cotton dyeing mechanism of condensed and hydrolyzable tannin enriched extracts by exploiting watermelon rind (WR) and mango seed kernel (MSK) bio-waste. Both crude dye extracts possess various coloring chromophores such as flavonoid, betacyanin, quercetin, β-carotene together with condensed and hydrolyzable tannin at a different level of concentrations, which were confirmed by several phytochemical screenings, thin layer chromatography, and UV spectroscopy. At optimized reaction condition, the resulted fixation rates of various WRCs (at 60 °C for 60 min) and MSKCs (at 90 °C for 60 min) were found as 25%–75% and 55%–71%, respectively. MSKCs have higher absorbance intensity and proactive anchoring sites than those of WRCs, resulting in three times higher color strength (K/S). For promoting the dye fixation, cotton fabric samples were chelated with different types of metallic salts (Fe^2＋, Al^3＋, Sn²⁺, and Cu²⁺) and 65% and 45% enhanced color strength was found for WRCs (K/S increased from 0.95 to 1.57) and MSKCs (K/S increased from 2.75 to 3.98), respectively. The effects of metal chelation with cellulose chain were estimated in terms of several crystallinity indices, hydrogen bonding configurations, and asymmetric factor and elaborately correlated with the improved dye fixation. In addition, the electrolyte was also added to the dye bath for further improvement of dye exhaustion, thereby granted 12% and 6% higher color depth for WR (K/S increased from 1.57 to 1.76) and MSK dyed fabric (K/S increased from 3.98 to 4.23). Finally, distinguishing colorimetric appearances and excellent colorfastness properties were ensured for different tannin classes.