A Highly Compatible Polylysine-Based Chrome-Free Tanning Agent: Achieving the Double Benefit of Superior Adsorption to Anionic Materials and Ecological Leather Manufacturing

The existing biomass-based chrome-free tanning agents mainly cross-link with amino groups of collagen fibers, which results in a relatively low isoelectric point of tanned leathers and correspondingly poor absorption rates to anionic materials in postprocessing. This study employs glycerol triglycidyl ether (GTE) as a modifier and leverages the structural advantages of polylysine (PLL) to successfully synthesize a novel epoxy-modified polylysine-based chromium-free tanning agent (EPL). This new tanning agent contains abundant terminal epoxy and amino groups, and its isoelectric point is increased to 7.9. When the ratio of n(PLL)/n(GTE) is 1:9, the shrinkage temperature of resultant EPL-3-tanned leather reaches 93 °C, and the thickening rate is 87%, significantly improving the mechanical properties of leather. Most importantly, the absorption rates of EPL-3-tanned leather to anionic dyes and fatliquors reach 98.6% and 92.7%, which are 33.1% and 39.9% higher than those of F-90-tanned leather and 16.4% and 31.1% higher than those of TWS-tanned leather. The strengthening effect is achieved by cross-linking between epoxy groups and amino groups of collagen fibers. In the meantime, the amino groups of EPL-3 can effectively compensate for the positive charge of tanned leather, enhancing the absorption rate of anionic materials. The self-prepared EPL-3 can be used in postprocessing of traditional chrome tanning and exhibits excellent environmental compatibility, making it a promising alternative to traditional chrome tanning agents. This study offers new insight for the development of green and sustainable tanning agents and lays a solid foundation for the broader application of chrome-free tanning technologies in the production of ecological leather.