Model study of the leather degradation by oxidation and hydrolysis

Abstract

Abstract Many objects of culture heritage, comprised of leather, need to receive the right treatment to be restored and to elongate their lifespan. Determination of the degradation degree and even better the type of the degradation is a crucial knowledge for the way of subsequent conservation, exhibition and long-term storage. Collagen based materials are very sensitive to the deterioration and undergo, mainly hydrolysis and oxidation. Namely acid hydrolysis and photooxidation are the most often causes of the disintegration of leather. Contrary to the leather investigation, a few studies dedicated to parchment described some typical features of hydrolysis, oxidation and gelatinization observed applying Attenuated Total Reflection Fourier-Transform Infrared spectroscopy (ATR-FTIR) which is widely used in collagen degradation type research. Except of the collagen secondary structure, followed by IR spectroscopy, we determined the shrinkage temperature of the collagen substrate by Micro Hot Table method (MHT) to reach the degradation level. In this paper, artificially degraded leather samples as a theoretical representative of cultural heritage objects were examined. We discuss the use of both techniques (IR and MHT) as potential methods for fast assessment of oxidation and hydrolysis of vegetable tanned leathers and degradation level. New samples of leather tanned by various vegetable tannins were artificially degraded under controlled conditions. We simulated the photooxidation by means of the Xenon arc lamp exposure, oxidation using the soaking in hydrogen peroxide, acid hydrolysis by the soaking in hydrochloric acid and alkaline hydrolysis using the soaking in natrium hydroxide. ATR-FTIR spectra of reference and tested samples were compared. Oxidation causes increase of the distance between amide I (AI) and amide II (AII) wavenumbers (Δν) above 100 cm−1 and the intensity ratio between AI and AII bands (AI/AII) above 1.6. The AI/AII ratio depends on the type of hydrolysis. The increase above 1.8 proves acid hydrolysis while the decrease under 1 demonstrate alkaline hydrolysis. MHT results are not so obvious. Generally, mainly the hydrolysis causes the decrease of the temperatures. We have found out that knowledge of the whole shrinkage interval is important and provides more appropriate information about the leather disintegration.

Keywords

• Leather
• ATR-FTIR
• MHT
• Sparse PCA