Objective Chrome tanning is currently the widely utilized tanning technique for leather manufacture, while the environmental concerns to chrome discharge during the chrome tanning process leads to the development of chrome-free tanning technique becoming a necessity for realizing environmentally benign and clean manufacture of leather. Conventionally, non-chrome metal tanning agents and organic tanning agents were employed to replace chrome tanning agent for the cross-linkage of rawhide to achieve chrome-free tanning. Unfortunately, these conventional chrome-free tanning agents fail to provide the chrome-free leather with high thermal stability comparable to that of chrome-tanned leather. To address these issues, it is essentially important to develop a brand-new chrome-free tanning strategy that is capable of converting rawhide to leather without the necessity to form cross-linkage with rawhide. Based on the amphiphilic non-covalent interactions between surfactants and collagen fibers (CFs), the present investigation explored the feasibility to prepare tanning agent-free leather by a novel surfactant-aided drying strategy.

Methods Tween 80, a typical non-ionic surfactant, was employed as the steric hindrance medium to accomplish its efficient infiltration and distribution among CFs. The Tween 80-water mixture with the content of Tween 80 at 40% was used to infiltrate rawhide. After that, toggling drying was employed to dehydrate the Tween 80-infiltrated rawhide. Based on the reversibility of non-covalent interactions between Tween 80 and CFs, Tween 80 was further removed through solvent elution using ethyl acetate. In this way, the novel tanning agent-free leather was prepared, which featured high fiber dispersity, high thermal stability and excellent mechanical properties. Furthermore, a novel superhydrophobic tanning agent-free leather was prepared through a hydrophobic treatment for improving the practical utilization performances.

Results Our experimental results demonstrated that by the regulation of liquid crystal behaviors, Tween 80 successfully served as the effective steric hindrance medium for the preparation of the tanning agent-free leather. Polarizing microscope observations showed that liquid crystal structures were formed when the content of Tween 80 were 50%, 60% and 70% in the Tween 80-water mixture. In contrast, the liquid crystal formation of Tween 80 was suppressed when the content of Tween 80 was controlled at 40% in the Tween 80-water mixture, thus allowing Tween 80 to achieve efficient infiltration and high dispersity in the hierarchical fiber network of CFs. In subsequent drying process, Tween 80 efficiently solved the sticking problem of CFs by providing effective steric hindrance, thus enabling the preparation of the tanning agent-free leather. The as-prepared tanning agent-free leather exhibited the high porosity of 48.95%, the low dry heat shrinkage rate of merely 11.00%, the tensile strength up to 32.17 MPa and the tear strength as high as 146.8 N·mm⁻¹. For the prepared superhydrophobic tanning agent-free leather, the static water contact angles of the grain side and flesh side reached 151.2° and 154.3°, respectively, accompanied by outstanding water repellency and self-cleaning capability.

Conclusion The utilization of Tween 80 as steric hindrance medium to suppress the sticking of CFs during the drying of rawhide was a novel strategy for realizing tanning agent-free leather making. The prepared tanning agent-free leather featured with high porosity, thermal stability and outstanding mechanical properties. Moreover, a hydrophobic treatment was capable of providing the tanning agent-free leather with superhydrophobicity on both grain side and flesh side to provide outstanding water repellency and self-cleaning capability. In summary, our investigation demonstrated a new conceptual tanning agent-free leather making strategy that does not rely on the use of tanning agent to cross-link CFs for accomplishing leather making, which radically resolved the environmental problem associated with chrome discharge by the conventional chrome tanning, thus providing a promising alternative for realizing the clean manufacture of environmentally benign and eco-friendly leather.