Thermal characterization of leathers tanned by metal salts and vegetable tannins

In this work, the thermal properties of vegetable (mimosa), metal salt (chromium) and mixed (chromiummimosa and aluminum-myrobalan) tanned bovine leathers were studied to understand the thermal decomposition processes with the aim of possible utilization of various leather wastes by thermal conversion. Thermogravimetry/mass spectrometry (TG/MS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) measurements were carried out to compare the thermal behavior and reveal interactions between the leather and various tannins. Bovine parchment was used as an untanned reference material. The tanning agent ratios were estimated by determination of ash, nitrogen and protein contents of the samples. It was found that ash yield is a good indicator for the inorganic content, while the protein content is inversely proportional to the total tannin content in leathers. The thermal decomposition of the samples as a function of the temperature was performed using TG/MS. The differences between the samples are clearly recognizable on the thermogravimetric curves and the mass spectrometric evolution profiles of the products. Among the tanning agents, chromium had the largest impact on the thermal decomposition process. Therefore, additional TG/MS experiments were carried out on chromium-treated parchment samples and the catalytic effect of chromium was demonstrated on the evolution curves of several products. It was established that hydrolysis of the amide bonds of the polypeptide chain was retarded resulting in less ammonia formation, and higher yield of water. On the other hand, higher amounts of hydrogen cyanide and other nitriles were released in the presence of chromium sulfate. The decomposing chromium sulfate released oxygen beside sulfur dioxide, which dehydrogenated certain amino acid residues leading to the enhanced formation of nitriles and water. The volatile compounds of the decomposition were determined by Py-GC/MS at 400 and 600 degrees C. The diketopiperazines (DKPs) dominated the pyrograms at both temperatures, but the ratio of gaseous and other products of smaller molecular mass was increased at higher pyrolysis temperature and in the presence of metal tanning agents. The chromium-treated parchment samples were also studied by Py-GC/MS. The relationship between the chromium content and the formation of certain decomposition products has been confirmed. Principal component analysis (PCA) was performed using the PyGC/MS data of leathers and parchment. The PCA results show clear separation between the chromiumtanned, vegetable-tanned and mix-tanned leathers as well as parchment.