Effect of chrome tanning on the thermal behavior of collagen fibers: A calorimetric and kinetic analysis

To further understand the effect of chrome tanning on the thermal stability of leather, the thermal shrinkage behaviors of native and chrome-tanned collagen fibers were studied by differential scanning calorimetry (DSC) using isoconversional and multivariate non-linear regression (Multivar-NLR) methods. The activation energies of the shrinkage processes were evaluated by both the differential (Friedman) and the integral (Ozawa-Flynn-Wall) isoconversional methods and the Multivar-NLR method was applied to fit the DSC data with several kinetic models. Moreover, the shrinkage behaviors of the chrome-tanned collagen fibers were simulated at different heating rates or temperatures based on the best of the estimated models and the corresponding kinetic parameters determined by Multivar-NLR. Results showed that the chrome-tanned samples had relatively higher shrinkage temperature, enthalpy and effective activation energy than the native ones under the same conditions. The activation energies decreased throughout the shrinkage process, which could be best described by a three-step model (a reversible step followed by an irreversible one); the three-state (native, unfolded and denatured states) shrinkage behaviors of collagen fibers could approximate to a two-state (native and denatured states) process at low heating rates or temperatures. This work might provide a theoretical basis for the design of chrome-free tanning agent from the perspective of the kinetic thermostability of collagen fibers.