Melamine-Based Polymeric Crosslinker for Cleaner Leather Production

Development of masked silica tanning system for sustainable leather production

Amid mounting pressure on the long-term recyclability of chromium in tanned leather and the associated environmental hazards, the quest for an alternative, cleaner tanning system has gained tremendous momentum. In this context, our study explores the remarkable potential of silicates as a versatile platform for skin/hide tanning, circumventing the inherent risks and ecological threats posed by chromium exposure and leaching. We present an alternative approach of using a silica-based tanning system, employing a Taguchi model, to optimize a masked silica (MaSil) tanning product/process for achieving effective collagen stabilization. Our results demonstrate the significant advancements made in hydrothermal denaturation temperature, reaching an impressive 79 °C through precise Taguchi parameters-5% SiO2, masked with 0.3 mole of citrate salt, and a tanning process fixation pH of 4.5. Notably, the mechanical strength analysis reveals compliance with the stringent upper leather recommendation standards, validating the practicality and quality of MaSil crust leather. Moreover, our research highlights the unprecedented environmental benefits of the first reported application of Taguchi's approach to the MaSil tanning system. The developed tanning system remarkably reduces total dissolved solids (TDS), biological oxygen demand (BOD), chemical oxygen demand (COD), and overall water load by 68.4%, 25.4%, 59.5%, and 33.7%, respectively, heralding a promising era of water and environmental sustainability in the leather sector. This study holds the potential to transform leather production, wherein the envisioned future on the use of the Taguchi model and optimized MaSil tanning system could find a place in shaping a cleaner, greener, and more sustainable leather industry.

Sustainable metal-free leather manufacture via synergistic effects of triazine derivative and vegetable tannins

Restrictions on heavy metals, especially chromium, have encouraged alternative tanning systems that can reduce environmental and human health risks from conventional chrome-based tanning. In this work, metal-free combination tanning was developed by using vegetable tannins and a triazine-based syntan containing active chlorine groups (SACC). Specifically, the relationship between leather performance (e.g., hydrothermal stability and organoleptic properties) and technical protocols (e.g., types and dose of tannins) was systematically established. The optimized protocol involving a unique procedure (i.e., 10% SACC pre-tanning, shaving, and 25% wattle tanning) endowed the leather with high shrinkage temperature (~ 92 °C) and met the Chinese standards for shoe upper leather (QB/T 1873-2010). Our method not only produces zero chrome-containing solid wastes, but also uses ~ 75% less tannin for leather manufacture. The excellent leather performance was ascribed to the synergistic effects, where SACC and wattle diffused into collagen fibrils and may bind to collagen via covalent, hydrogen and ionic bonding, locking the hierarchical structure of collagen from microfibrils to fiber bundles. Moreover, we summarized these findings and proposed a diffusion-binding-locking mechanism, providing new insights for current tanning theory. Together with the biodegradable spent tanning liquor, this approach will underpin the development of sustainable leather manufacture.

Graphical Abstract

Imparting Fire Retardancy and Smoke Suppression to Leather during Tanning Processes

Leather is considered a luxury good when used in seating and upholstery. To improve safety, flame retardancy in leather is usually achieved through various finishing processes such as spray or roller coating. These treatments require processing steps that cost time and are labor-intensive. One avenue to achieving flame retardancy in leather is to add flame retardants during the tanning process. However, the influence on flame retardancy exerted by specific intumescent additives specifically added during leather tanning has yet to be investigated. This work explores the roles played by intumescent additive compounds in flame retarding leather when they are added during tanning instead of applied as a coating. Via a systematic investigation of various compound mixtures, the flame retardant effects in the condensed and the gas phases are elucidated. The results show a strong impact of melamine in the gas phase and of polyphosphates in the condensed phase. Their impact was quantified in fire and smoke analysis, showing a 14% reduction in the peak of heat release rate, strongly reduced burning lengths, and a 20% reduction in total smoke release compared to nontreated leather. These results illuminate the key role played by specific compounds in the flame retardancy of leather, particularly when they are added specifically during the tanning process instead of being applied as a coating. This method has great potential to reduce processing steps, lower costs, and improve material safety.

A new-fangled horizon in leather process to sidestep toxic chrome and formaldehyde using hyperbranched polymer

A novel chrome-free tanning and formaldehyde-free post tanning process with PEG-melamine base hyperbranched polymer by complexing aluminum (Al³⁺) present in aluminum sulfate for eco-friendly tanning applications. The hyperbranched polymers PEGM-400-C-Al and PEGM-600-C-Al were synthesized and characterized by FT-IR, NMR, UV, and XRD. The molecular weight of polymers was assessed by GPC and subjected to the leather process. The processed crust leathers were analyzed for physical characteristics by tensile strength, tear strength, elongation, and quality assessments by hand evaluation by experts. FE-SEM analyzed collagen fibers and fiber splitting of goat skin, and COD, BOD, and total solid in spent liquor were analyzed and compared. The highlighting feature of hyperbranched polymers is (a) Improved shrinkage temperature (Ts) (85 ± 1 °C), (b) Improved physical-mechanical properties (c) Better BOD, COD, and total solids over the aluminum sulfate tanning process. This study confirmed that hyperbranched polymer is effective for tanning and post-tanning leather, which obviates the need to use toxic chromium and formaldehyde for tanning leathers.