Side-chain Poly[2]pseudorotaxanes containing β-cyclodextrin for more sustainable tanning process

Sustainability of leather lies in how the hide, a sustainable, naturally renewable, raw material, is processed. Tanning chemistry has been a limiting factor for leather sustainability. In this study, a host-guest synthesis strategy was selected to modify one of the most widely used tanning polymer, MIDA DD, and obtain a new hybrid tanning system containing β-cyclodextrin, and leather drastically less impactful on the earth and people. Poly[2]pseudorotaxane Side-Chain Complexes (PSCCs) have been obtained by threading β-cyclodextrin units onto the side-chains of the commercial MIDA DD. The formation of PSCCs in aqueous solution was investigated by using 1D NMR, ATR-FTIR and TGA experiments. The ability of PSCCs to stabilize the collagen matrix was tested at laboratory and industrial pilot scale by micro-DSC, ATR-FTIR and solid-state NMR techniques. The physical and mechanical performance of the obtained crust leather was determined by standard tests used in tanning industry. Side-chain poly[2]pseudorotaxanes showed better tanning performances than the fossil-based MIDA DD, the most effective supramolecular tannins being obtained by mixing β-CD and MIDA DD in ratios close to 1/1 (w/w). The new tanning mixtures allow for significantly reducing both the amount of fossil-based MIDA-DD polymer in the current tanning processes (by 45%) and the free bisphenol content in leather crust (by more than 80%) due to the presence of bio-based β-CD in the composition of the new supramolecular tanning agents. The findings disclosed here pave the way for the CDs’ employment in improving the sustainability of tanning processes.