THE FURTHER INVESTIGATION OF TANNING MECHANISMS OF TYPICAL TANNAGES BY ULTRAVIOLET-VISIBLE AND NEAR INFRARED DIFFUSED REFLECTANCE SPECTROPHOTOMETRY

The ultraviolet-visible and near-infrared diffuse reflection (UV-Vis-NIR DR) spectroscopy was employed for further understanding the mechanisms of three kinds of classical tannages, including aldehyde tanning, vegetable tanning and mineral tanning. The NIR DR spectra of glutaraldehyde tanned leather exhibited a decreased peak intensity of amino group at 1475 nm, suggesting that the main reaction groups of collagen in aldehyde tanning are amino groups. Comparing the NIR DR spectra of pickled pelt and vegetable tanned leather, it was proved that the vegetable tanning is based on the mechanism of multiple hydrogen bonding interaction, where the amino groups, carboxyl groups and peptide chains are involved. As for mineral tannages, the tanning mechanisms of chromium sulfate, zirconium sulfate, ferrum sulfate, titanium sulfate and aluminum sulfate were investigated. Based on UV-Vis-NIR DR analysis in the wavelength range of 200 to 2070 nm, it was confirmed that the main reactions involved in mineral tanning are the chelating interactions of metal ions with both the carboxyl groups and the amino groups of collagen, although the reactivity of metal salts toward carboxyl groups is higher than toward amino groups. The reactivity of metal salts toward carboxyl groups of collagen follows the sequence of chromium salt > zirconium salt > ferrum salt > titanium salt > aluminum salt. However, the reactivity of metal salts to amino groups of collagen is in the sequence of zirconium salt > chromium salt > ferrum salt > titanium salt > aluminum salt. Meanwhile, the peptide chain of collagen was also found to play an important role in fixation of metal ions during tanning process.