Insights into the Development of Imine-Bond-Stabilized Organic Tanning and a Heteropolymer for the Post-Tanning Process─A Metal-Free Sustainable Leather Process

Organic tanning agents present a huge opportunity for developing tanning agents devoid of metals for sustainable and cleaner leather processing. An ideal tanning agent should be nontoxic, biocompatible, and cost-effective. Herein, we report an aromatic aldehyde-based modified vanillin synthesized by a single-step substitution reaction that can be used in the tanning process. Also, multifunctional polymers with three groups (carboxyl, long-chain hydrocarbon, and quaternary ammonium) are synthesized using hydroxyethyl methacrylate monomers, which can be used in the post-tanning process. The interaction of the modified vanillin derivative with collagen was studied at the molecular and fiber levels. Circular dichroism studies confirm no changes in the triple-helical conformation with increasing concentrations of modified vanillin. The crosslinking mechanism of collagen-modified vanillin by imine bond formation was confirmed through XPS. Further, the tanning input of modified vanillin was optimized, and 10% modified vanillin-tanned leather achieved 70 °C hydrothermal stability. For an integrated post-tanning process, the synthesized multifunctional polymer results in the increased uptake of dye and fatliquor. Further, the experimental leathers showed homogeneous pore size distribution, better fullness, and good mechanical strength. As expected, the pollution load of the experimental organic tanning process is significantly lesser than that of the chromium process. Thus, the present study overlays a greener metal-free leather-processing strategy.