Sustainable Leather Processing: pH-Driven in Situ Self-Assembly of Biomass-Based Tanning Agents from Tea Polyphenols

To mitigate chromium pollution in leather manufacturing, a novel biomass-based chrome-free tanning agent (polyphenol-cyanuric chloride derivative, PCC) was developed through molecular modification of tea polyphenol (TP), a renewable agricultural byproduct, with cyanuric chloride (CC). Leveraging the pH-responsive phenolic hydroxyl groups of TP, PCC demonstrates a phase-adaptive tanning mechanism: it initially permeates as nanoscale particles (100–200 nm) under mild acidic conditions (pH 4.5–6.0) and then undergoes HCl-triggered supramolecular self-assembly into micron-scale networks (2000 nm) via π–π stacking under strong acidity (pH < 3.0). This biphasic behavior eliminates salt (NaCl) requirements while enabling complete collagen cross-linking, as evidenced by a significantly increased shrinkage temperature (Ts) of 80.7 ± 1.2 °C, which is comparable to commercially available glutaraldehyde tanning agents. The TP-derived architecture enhances leather fullness and resistance to yellowing (Δb* < 1.5 after UV aging), which is superior to aldehyde-tanned leather (Δb* > 4.0). This study establishes a sustainable strategy for converting agricultural residues into high-performance tanning materials, offering a viable solution for eco-friendly leather production.