Influence of various factors on chromium (VI) formation by photo-ageing

The results of earlier studies have shown that it is possible to systematic ally obtain skins with undetectable hexavalent chromium only by careful was hing and suitable adjustment of the final pH to 3.5-3.7 with formic acid.(1 ) thereby obviating the need for specific reducing or anti-oxidant products . Exposure of a skin without an initial chromium (VI) content to UV light lea ds to the progressive appearance of this element. This study seeks to gain a better understanding of the products and processes that affect chromium ( VI) formation in leather exposed to light. The influence of the light (daylight and UV light of low energy (366nm)) on the chromium (VI) formation in leather subjected to various processes was studied. The relatively unexpected results showed that the effect of daylig ht, even through a common glass filter, was in general as much or more mark ed than that obtained in the laboratory with UVA lamps of 366 nm. By contrast, the reproducibility of the tests performed with artificial UV light was much higher than that obtained with natural light which is subjec ted to seasonal, timetable and climatological variations. Four independent tests performed on different days on acrylic polymer retanned skins exposed to UVA light yielded average results similar to those obtained with natura l light, in the order of 21 ppm of chromium (VI). Nevertheless, the precisi on obtained with artificial light was almost four times higher than that ob tained with natural light. Given this greater reproducibility, photo-ageing tests were performed with artificial light. The influence of different ret anning processes on the formation of chromium (VI) was studied. The results obtained with three vegetable extracts and seven synthetic retanning agent s are presented. The application of liposoluble antioxidants and the influe nce of a pigmented finish were studied. As expected, less chromium (VI) was formed in the pigmented skins. The average content showed a reduction of 5 0% with the highest concentration of pigment applied. Of the formulations studied in this work, those that provided the greatest resistance to hexavalent chromium formation were retanning processes with v egetable extracts, which even with only 1% extract on shaved weight achieve d better results than the liposoluble antioxidants. Using 4% of extract, ch romium (VI) was undetectable in skins in any of the conditions of photoagei ng applied.