Characterization of historical leather bookbindings by various thermal methods (TG/MS, Py-GC/MS, and micro-DSC) and FTIR-ATR spectroscopy

The goal of this study was to obtain information about the thermal stability, composition and molecular structure of four historical leather bookbindings dated between the 17th and 19th century and to identify correlations between their thermal, structural and chemical properties and deterioration mechanisms by using thermal analysis and decomposition methods (TG/MS, micro-DSC, Py-GC/MS), and infrared spectroscopy (FTIR-ATR). The TG/MS study revealed that the maximum thermal decomposition rate of the historical samples depended on the deterioration degree of leather. Micro-DSC was applied to determine the collagen population profile, i.e., “leather-like”, “parchment-like”, or “gelatin-like” behavior, which characterizes the deterioration pattern of the samples. The historical samples have higher amount of adsorbed water compared with the modern leather indicating their higher polarity. The significantly higher yield of carbon dioxide during the pyrolysis of historical leathers demonstrates that leather underwent various oxidation processes over time. The smaller yield of the pyrolysis products attributed to vegetable tannins detected in the chromatograms of historical leathers was related to the degradation of tannins (i.e., de-tanning), while the higher amount of 2-pyrrolidone and succinimide confirmed the oxidation of proline and hydroxyproline amino acid residues in the historical leathers. Leather de-tanning and collagen gelatinization were clearly evidenced by both micro-DSC and FTIR-ATR.