Objective Leather is a hierarchical fiber network material assembled from collagen molecules. Tanning, as a typical heterogeneous reaction process, involves two kinetic processes: mass transfer and crosslinking of tanning agent molecules within the fiber network. However, the mechanism of tanning, regarding mass transfer and crosslinking of tanning agent in the multi-scale structure of leather, is still not well understood. Herein, the effect of tanning degree on the hierarchical structure of leather tanned by dialdehyde carboxymethyl cellulose (DCMC) was investigated. The chrome-free tanning mechanism was expected to be elucidated from the perspective of multi-scale structure evolution of leather.

Methods The tanning degree was adjusted by changing the DCMC dosage from 1% to 9%. The evolution of the hierarchical structure of the tanned leather, ranging from collagen fiber network to intermolecular level, was systematically characterized by multiple techniques. The mass transfer and crosslinking behavior of DCMC in fiber network and fiber levels was characterized by using fluorescence microscopic analysis. The multi-scale morphology and pore structure characteristics of tanned leather were analyzed by scanning electron microscopy and mercury intrusion porosimetry. The effect of tanning degree on the intra-fibril structure was investigated by synchrotron radiation small-angle X-ray scattering (SAXS). In situ SAXS was employed to track the real-time evolution of fibril structure throughout the tanning process. Moreover, amino group reaction rate, shrinkage temperature (T_s), and thermal denaturation temperature (T_d) of the tanned leathers were determined.

Results 1% DCMC could transfer to the intermolecular level of collagen. In situ SAXS revealed that the D-period remained unchanged during the penetration stage of tanning. Then intra-fibril cross-linking was conducted during the basification stage of tanning, which reduced the D-period of collagen fibrils from 64.9 nm to 63.9 nm and increased the fibril diameter from 104.31 nm to 138.41 nm. The tanning effects were further enhanced by dehydration of fibril and Schiff base formation with DCMC during the heating stage of tanning, as evidenced by the significant increase in SAXS peak intensity ratios R_5/3 and R_6/5. When the DCMC dosage increased from 1% to 6%, the degree of cross-linking tended to saturate without structural change at the fibril level, and fluorescence analysis indicated that cross-linking process shifted to fiber and fiber bundle levels. The porosity of leather increased from 62% to 72%, especially with an increase in the proportion of pores with a diameter >10 000 nm. T_s of leather increased from 65.2 ℃ to 76.6 ℃, and T_d of leather increased from 90.0 ℃ to 101.6 ℃. When the DCMC dosage reached 9%, the degree of cross-linking at the fiber-to-fiber bundle level also approached saturation. Single-site binding of excess DCMC occurred mainly at the fiber network level. No significant change was observed in the multi-scale structure and thermal stability of leather.

Conclusion As tanning degree increased, DCMC exhibited a hierarchical crosslinking and tanning behavior on leather, proceeding sequentially from the microscopic level to the macroscopic level. The elucidation of the "stepwise crosslinking" tanning mechanism of DCMC is expected to guide the molecular design and tanning application of biomass-based chrome-free tanning agents, and also provide a reference for improving the understanding of chrome-free tanning theory.