Resumen de Contribució a l'estudi de la degradació i envelliment de la pell
Leather is a natural material that can be used in different environments and often under aggressive environmental conditions that may cause premature ageing. Since upholstery leather is considered a very high-tech product, a long service life is expected by the costumer. According to different studies carried out on historical/ancient leathers by leather conservation centers and museums, leather also is strongly affected by these three main environmental parameters: temperature, relative humidity, and UV radiation. The aim of this work is to study the effect of the environment exposure, to identify the most important variables affecting this weathering process and to check for interactions among the three factors on the degradation and leather ageing on wet-blue and wet-white upholstery leather, applying the leather to natural weathering, tropical environmental exposure and finally a combination of the three critical factors. The tests were carried out on Spanish pickled cattle hides. Two types of tanned leather were tested: on the one hand, chrome tanned leather and on the other hand, wet-white tanned leather using tetrakishydroxymethylphosphonium sulphate (THPS) and silicate-phenolic synthetic and retanned with protein-amide polymer. Both types of leathers were finished using common industrial formulation. Firstly, the work was undertaken to better understand the effect of degradation of chrome tanned leather and wet-white leather and the physical and chemical processes occurring during long-term exposure to outdoor conditions. The samples were tested before and after 20, 40, 80, 120 and 180 days of exposure. Increasing the levels of the three critical weathering variables, the leathers were exposed to tropical environment using a climatic chamber at 70ºC and 95% relative humidity during 7, 14 and 21 days. Additionally half of the leathers were exposed to UV radiation for 4 days. Finally, the leathers were exposed to accelerating ageing test using a multilevel centralized factorial experimental design and an analysis of variance (ANOVA) with 3 variables and 2 levels (23). The evaluation of the leathers was carried out with physical, fastness, chemical and organoleptic properties of the leathers. With the natural weathering studies, wet-white leather and chrome-tanned leather show a different behavior to leather ageing. The loss in properties in chrome-tanned leather increases steadily until 40 days exposure, and after this point the leather stabilizes and no longer degrades. In wet-white leather, however, there is a progressive loss in properties but stabilization does not occur until a period of exposure of 80-120 days. Tropical environmental exposure causes physical and chemical degradation of both types of tanned leather. In chrome-tanned leather, the loss of all properties occurs progressively over time, in wet-white leather it occurs only after 7 days exposure, and is followed by carbonization of the leathers after 14 days exposure. Additionally, when wet-white samples are exposed for long periods at high relative humidity are attacked by fungi. Resistance and dimensional stability of wet-white leather both in accelerated ageing tests and natural weathering exposure is higher than that of chrome-tanned leather. However, these leathers have the disadvantage of not resisting simultaneous exposure to high temperatures and high relative humidity, that's to say, tropical environmental conditions. Results from experimental designs prove again that chrome-tanned leather and wet-white leather have a different ageing behavior. Whereas chrome-tanned leathers are strongly affected by UV radiation, wet-white leathers are strongly affected by relative humidity. Additionally, no correlation has been found between ageing caused by natural weathering and that caused under controlled conditions in the laboratory.
