All Green Processing Technology of Multifunctional Kappa-Carrageenan-Based Chrome-Free Tanning Agent Toward Efficient and Sustainable Leather Processing

Abstract

Leather plays a significant role in daily life due to its exceptional permeability, mechanical strength, and durability. However, traditional tanning processes not only lead to chromium pollution but also promote bacterial growth and yellowing. This study aims to develop an all green processing technology of multifunctional chromium-free tanning agent (OKC-EGDE) based on kappa-carrageenan (KC), in which natural plant-derived KC was pretreated by a green H₂O₂/Cu²⁺ oxidation system, followed by cross-linking modification with ethylene glycol diglycidyl ether (EGDE). The aldehyde (–CHO) and carboxyl (–COOH) groups introduced during the oxidation process significantly enhance the antimicrobial properties of OKC-EGDE. During tanning, these aldehyde and epoxy groups bind with amino and carboxyl groups on collagen fibers, leading to significant improvements in the mechanical properties of the tanned leather. Characterization results from FTIR, ¹H NMR, and XRD analyses indicate that the epoxy value of OKC-EGDE is 0.37 mol/100 g, the oxidation value is 71%. Compared to traditional commercial chromium-free tanning agents (TWS and F-90), leather tanned with OKC-EGDE exhibits superior mechanical properties (tensile strength: 17.5 MPa, elongation at break: 38.7%, tear strength: 55.6 N/mm), thermal stability, yellowing resistance, and biocompatibility. Meanwhile, the OKC-EGDE has high antimicrobial rate of 99% against both Escherichia coli and Staphylococcus aureus. The degradation of tanning wastewater and life cycle analysis confirm that OKC-EGDE-tanned leather achieves full-process environmental sustainability. This study demonstrates the significant application potential of natural plant polysaccharides and provides a new approach for sustainable and clean leather production.