A Spider-like Polyphenol–Metal Dendrimer Chrome-Free Tanning Agent for Sustainable Leather Manufacturing

To solve the problem of chrome pollution in the leather industry from the source, based on the large end-group functionality and strong designability of the dendrimer, a series of phenolic hydroxyl-terminated dendrimers (Gx-GA) were synthesized. Then, Gx-GA complexed with Al³⁺ to construct different generations of chrome-free tanning agents (Gx-GA-Al) with polyphenol–metal network (MPN) structures. It can take advantage of its branched structure to form multipoint and multiform cross-linking with leather collagen fibers, thus giving leather excellent physical and chemical properties. The results showed that the shrinkage temperature (T_s) of leather tanned with the first generation of chrome-free tanning agents (G1-GA-Al) reached 87.2 °C, which was 17.6 °C higher than that of 8% aluminum-tanned leather. The tensile strength and elongation at break increased by 26.5 and 60.8%, and the fastness to washing was also greater than 99%. The antibacterial rates against Staphylococcus aureus and Escherichia coli reached 99.9 and 98.7%, respectively. Moreover, leather tanned by Gx-GA-Al also had a good thickening effect, fullness, and softness. The degradation rate of Gx-GA-Al-tanned leather under natural conditions was much higher than that of chrome-tanned leather. Treated tanning wastewater was used in agricultural irrigation, and results showed that the growth of crops was accelerated. Therefore, the construction of Gx-GA-Al avoided the pollution of chrome from the source and endowed leather with better tanning properties. This study provides a new strategy for sustainable leather manufacturing.