Application of amphoteric polymers in the process of leather post-tanning

Novel Biomass-Based Polymeric Dyes: Preparation and Performance Assessment in the Dyeing of Biomass-Derived Aldehyde-Tanned Leather

High-performance chrome-free leather production is currently one of the most concerning needs to warrant the sustainable development of the leather industry due to the serious chrome pollution. Driven by these research challenges, this work explores using biobased polymeric dyes (BPDs) based on dialdehyde starch and reactive small-molecule dye (reactive red 180, RD-180) as novel dyeing agents for leather tanned using a chrome-free, biomass-derived aldehyde tanning agent (BAT). FTIR, ¹H NMR, XPS, and UV-visible spectrometry analyses indicated that a Schiff base structure was generated between the aldehyde group of dialdehyde starch (DST) and the amino group of RD-180, resulting in the successful load of RD-180 on DST to produce BPD. The BPD could first penetrate the BAT-tanned leather efficiently and then be deposited on the leather matrix, thus exhibiting a high uptake ratio. Compared with the crust leathers prepared using a conventional anionic dye (CAD), dyeing, and RD-180 dyeing, the BPD-dyed crust leather not only had better coloring uniformity and fastness but it also showed a higher tensile strength, elongation at break, and fullness. These data suggest that BPD has the potential to be used as a novel sustainable polymeric dye for the high-performance dyeing of organically tanned chrome-free leather, which is paramount to ensuring and promoting the sustainable development of the leather industry.

Self-driven directional dehydration enabled eco-friendly manufacture of chrome-free leather

Manufacture of eco-friendly chrome-free leather is of great significance for realizing sustainable development of leather industry. Conventional tanning theory believes that it is impossible to convert raw hide to leather without the utilization of cross-linking agent (e.g., chrome salts) among collagen fibers in raw hide. Here, we developed a brand-new leather manufacture strategy that relied on the composite dehydration media enabled self-driven directional dehydration mechanism to accomplish chrome-free leather manufacture for the first time, rather than followed the classic cross-linking mechanism that has been obeyed for more than one century in leather industry. We demonstrated that the essence of leather making is to regulate the water content in raw hide rather than to form cross-linkage among collagen fibers. The composite dehydration media comprised of anhydrous ethanol and molecular sieves (3A activated zeolite powder) successfully guaranteed continuous self-driven directional dehydration of raw hide by establishing stable water concentration gradient between raw hide and ethanol, which significantly increased the dispersity of collagen fibers in raw hide (with the water content reduced from 56.07% to 5.20%), thus obtaining chrome-free leather that is more ecological than chrome-tanned leather due to the elimination of any tanning agent. The as-prepared chrome-free leather exhibited outstanding tear force (174.86 N), tensile strength (24.56 N mm−2), elongation at break (53.28%) and dry-thermal stability, superior to chrome-tanned leather. Notably, the used composite dehydration media was recyclable for chrome-free leather manufacture, therefore facilitating an environmentally benign leather manufacture process. Our investigations are expected to open up a new conceptual leather making strategy that is applicable for realizing substantial manufacture of eco-friendly leather.

Graphical abstract

Effect of cationic monomer structure on the aggregation behavior of amphoteric acrylic polymer around isoelectric point

Amphoteric polymer can be used as retanning agent in leather manufacture. It is particularly useful in chrome-free tanning systems since it can regulate the charge properties of chrome-free leather and enhance the fixation of anionic post-tanning chemicals in leather. However, the aggregation and precipitation of amphoteric polymer retanning agents around the isoelectric point (pI) hinder their wide application. Herein, we synthesized five amphoteric acrylic polymers (AAPs) by free radical copolymerization with acrylic acid and five different cationic acrylic monomers. The effect of cationic monomer structure on the aggregation behavior of AAPs was investigated. The aggregation of AAPs in aqueous solution showed pH and concentration dependence. Light scattering analysis showed that Poly (AA-co-MAPTAC) and Poly (AA-co-DMAPMA) were in the shape of coiled linear flexible chains with small particle size (Rg 7.6 nm and 14.8 nm, respectively) near the pI. However, Poly (AA-co-DAC), Poly (AA-co-DMC) and Poly (AA-co-DMAEMA) were in the shape of hollow spheres and exhibited serious aggregation. Quantum chemical calculations suggested that the amide groups in the cationic monomers MAPTAC and DMAPMA enhanced the nucleophilicity of AAPs. Thus the corresponding AAPs could carry a large number of cationic charges to slow their aggregation when the pH just climbed over the pI. The results are expected to provide theoretical reference for the synthesis and widespread application of AAPs.