Application of infrared spectroscopy (FTIR and NIR) in vegetable tanning process control

The tanning industry is currently influenced by the globalization of international markets. Competitiveness among industries has increased substantially. This therefore results in the need to further control the tanning processes in order to adjust each process thus preventing potential manufacturing errors. To avoid compromising production, controls must be performed in the least possible time. In the case of vegetable-tanned leather the official method for determining the ‘degree of tannage’ or ‘combined tannins’ parameters is not directly from a single standardized test method. The procedure to obtain these parameters involves performing several independent analyses and these present several drawbacks. The process is slow, involves the use of pollutants and the reproducibility of results depends on the experience of the analysts who perform the test. Consequently, the determination is infeasible in some cases. Our work aims to develop a new analytical method for determining the tanning degree and the parameters associated with its calculation (combined tannins, hide substance and leather substance) of leathers tanned with vegetable extracts in order to solve this problem. We prepared a wide group of leathers for this purpose, tanned with a mixture of vegetable extracts (mimosa and quebracho) and having different values of the degree of tannage. Initially, the spectra of leather samples were recorded by Fourier-Transform Mid-Infrared Spectroscopy (FTIR) and Attenuated Total Reflectance cell (ATR). They were also recorded with Near Infrared Reflectance Spectroscopy (NIR) using a fibre optic module. The subsequent mathematical treatment of spectral signals by a Partial Least Squares Regression (PLSR) chemometric method enabled the calculation of the parameters studied. The analytical method proposed has several advantages over the traditional method. It enables direct leather analysis, without any manipulation, in a very short time and without using chemicals. Finally, this method allows systematic, fast and easy process control at a very low cost. Good results were obtained with both spectroscopic techniques used, when compared with those obtained by the traditional analysis