Biodegradable alginate derivatives obtained by eco-friendly ultrasound technology for more sustainable leather tanning†

This study investigates ten biodegradable re-tanning agents derived from sodium alginate, utilizing various ultrasound technologies and varying key process parameters, including ultrasound frequency, intensity, and treatment duration, at a maintained temperature of 40 °C. The sodium alginate derivatives (SADs) were characterized through viscosimetric analysis, gel permeation chromatography (GPC), proton nuclear magnetic resonance (1H-NMR) spectroscopy, thermogravimetric analysis, and attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy. The findings demonstrate that ultrasound primarily facilitates the cleavage of 1–4 linkages within the polymer chains. The thermal stability of SAD-collagen matrices was influenced by the molecular weight and viscosity of the SADs, along with the intrinsic heterogeneity of the collagen matrix. Subsequently, the most effective SAD for the re-tanning process was evaluated at a pilot scale. The leather produced underwent comprehensive characterization through physical–chemical methods (micro-differential scanning calorimetry, unilateral NMR, ATR-FTIR) and standard tests. Notably, the selected SAD fully replaced existing commercial products, such as acrylic resins and synthetic re-tanning agents, resulting in white leather with a natural softness that maintains the technical performance characteristics, such as tear strength, surface durability, grain distention, and colour fastness, required for high-end footwear and leather goods. Additionally, this approach offers enhanced advantages for both human health and the ecosystem by eliminating the need for dyeing processes.