How to avoid Cr (VI) formation in leathers?

5 March 2001

Hexavalent chromium formation caused by ageing has been widely investigated with many papers concentrating on how to solve the problem by eliminating or reducing the likelihood of oxidation of Cr(III) to Cr(VI)1-7. The oxidation of chromium in leather is believed to occur via an auto-oxidation route. Initially, oxygen reacts with double bonds in the different auxiliaries used in the post tannage processes giving rise to the formation of unstable and highly reactive peroxide molecules. These radicals can react with other double bonds initiating a free radical chain reaction. Formation of such free radicals can favour conditions for the oxidation of chromium (III) to chromium (VI). These reactions are catalysed by heat and UV radiation. The mechanism is shown in figure 1. To avoid the formation of Cr (VI) on leathers after ageing the use of vegetable tannins during retanning, alone or in combinations with other retanning agents, are reported to be the most effective. At least 2%, based on the shaved weight, of mimosa or tara can reduce Cr (VI) formation. However, in the case of garment or upholstery leathers such an addition adversely influences the final characteristics of the finished leather. This can be worse for doubleface production. In this work, factors influencing Cr (VI) formation and methods to manage these factors were investigated. Different parameters have been investigated in order to reveal the characteristics of a leather process that can reduce the formation of hexavalent chromium to below detectable limits. These include: * the influence of free Cr (III) retanning of wet-blue and research on how to avoid/reduce the formation of Cr (VI) due to the presence of free chromium * the influence of the neutralisation pH and research into high neutralisation levels * problems due to fatliquors * the influence of retanning agents according to their structure * the effect of final washing in a reductive salt bath * a new approach using a preserving product concept, which prevents hexavalent chromium formation when the leather is exposed to ageing. The addition of this product does not affect the final handle of the leather Experimental The work consisted of two parts. In the first phase different materials were screened by reacting each with a solution containing 25ppm Cr (VI) for 30 min and determining the resultant chromium (VI) concentration. 25 ppm Cr (VI) was chosen as a representative concentration that could have been formed in leather when it is exposed to heat and/or UV radiation. In the second phase, chrome tanned pieces were treated in experimental drums for each step of the post-tanning process. In each trial group, treated leathers were tested against a non treated reference. Leathers were analysed for their Cr (VI) content according to IUF188. A new product, Garmin CR6, was researched in an effort to avoid changing the final characteristics of the leather while preventing Cr (VI) formation. Leather trials were carried out using Turkish origin wet-blue, shaved to 1.4-1.6 mm. In the case of the fatliquoring trials, 6% active matter fatliquor, based on shaved weight, was added. Leathers were fatliquored using the same sulfited fishoil. All leathers were dried using a vacuum dryer (3min, 60°C) or naturally air dried. Leather samples were exposed to ageing conditions in accordance with the work of LGR4. For each analysis, 15 cm² pieces were exposed to heat at 80°C for 24h. Cr (VI) analyses were carried out after exposure. During the ageing operation, it was observed that the results were highly dependent on the air circulation and on how they were positioned in the oven. Therefore, all ageing operations were realized using the same oven, the same air circulation and the same position in the oven. This problem could have been caused by the factors discussed at the VGCT Aachen meeting7,9. UV ageing operations were carried out at the Igualada Leather Institute in Spain. Chrome analyses were carried out in accordance with DIN 53314 at the Cognis laboratories. Some parallel tests were sent to LGR and the results were compared with the Cognis ones for confirmation. No big differences were observed. As the test method is not suitable for coloured leathers, all trials were realised on non-coloured leathers. Results and discussion Post-tanning processes have been investigated to determine which factors affect hexavalent chromium formation on aged leathers. These processes were: chrome retanning, neutralisation, retanning, fatliquoring, acid fixation and final washing. 1.Influence of chrome retanning To achieve an homogeneous and even colour, leathers are often retanned with chrome salts. To observe how this operation could affect Cr (VI) formation, two wet-blue sides, a left and a right, from two different whole wet-blue hides were chrome retanned. They were treated with 4% (on shaved weight) of 33% basic chromium sulfate. The other wet-blue sides were processed without chrome retanning in order to see a comparison between the two treatments. All leathers were thereafter fatliquored as described before and finished by acidifying with formic acid to a final pH 3.8-4.0. The resultant Cr6+concentrations are shown in figure 2. It is clearly seen that chrome retanning leads to very high Cr (VI) concentrations while leathers without chromium retanning show negative results. This result suggests that the presence of free Cr (III) could favour the formation of Cr (VI) since a large amount of the main reactant in the oxidation reaction remains unfixed. The quantity of unfixed Cr (III) was obviously increased by the chrome retanning process. Therefore, measures have to be taken during chrome retanning. The best idea is to bind this free chrome through an accurate neutralisation. To ensure a reduction in the levels of free Cr (III), leathers were washed with water, 0.5% formic acid (based on the shaved weight) or with a suitable complex active surfactant, in this particular case with 0.5% of Perbon CC, in a float of 150% water for 30 min. At the end of the treatment leathers were thoroughly washed with water for 15 min. The results are reported in figure 3. From these trials, the use of a suitable complex active surfactant to remove the free Cr (III) achieves a reduction in the formation of Cr (VI). However, hydrophobic leather production presents a problem, since the use of a surfactant leads to an increase in the hydrophilicity of the final article. In this case, a slight basification of the fixation chrome used after the hydrophobing fatliquor, can lead to lower Cr (VI) values as is shown in figure 4. 2. Effect of neutralisation The effect of the neutralisation was also widely investigated and the results have been reported on different occasions4,6. It was clearly demonstrated that high pH values increase the formation of Cr (VI). Therefore, the addition of reductive products, ie a reductive neutralisation, has been suggested. A classical neutralisation was compared with a reductive one. The screening tests were carried out by reacting 2% of each neutralising agent with the Cr (VI) solution for 30 min at ambient temperature. At the end of the time the Cr (VI) concentrations were analysed to determine the effect of the materials. The results are reported in figure 5. Materials such as Na2S2O5 , Na2SO3 can be distinguished for their high reductive capacity. Neutralising agent 1, microcrystalline aluminosilicate and sodium formate had no effect on the reduction of Cr (VI). Neutralising agent 2 and the naphthalene based auxiliary syntan showed only a limited effect. In the second phase, the reductive effect of the materials were tested on leather. Neutralisations were carried out in a 150% float, adding 2% on the wet-blue weight of each product. They were drummed for 60min, until an homogeneous neutralisation to the cut was achieved. Results can be seen in figure 6. In this case, Cr (VI) formation was hindered to some extent by the materials which gave negative results in the screening tests. On the other hand the best reducing products, Na2S2O5 and Na2SO3, produced a very limited reduction in Cr (VI) levels in the leather trials. This can be explained by the fact that the reductive salts, which are not substantive to the collagen, were washed out during successive washings. Therefore, when the leather was exposed to the ageing circumstances no reductive product was present in the leather network. From these data, it can be concluded that products able to bind on the leather framework can have a positive impact on Cr (VI) formation. 3. Fatliquors: protected fatliquor concept It is well known that one of the most important operations in the whole leathermaking process is fatliquoring. However, fatliquoring favours the conditions most likely to produce Cr (VI) formation. This is especially so with sulfited fish oils1,4,6 but it has been observed that other types of fatliquoring agents, depending on their chemical structure, can lead to oxidation conditions for Cr (III). In this study, substances most suitable for hindering or eliminating Cr (VI) formation have been investigated. With this in mind, fatliquors were produced using a new concept of 'free radical capturing'. The effect of a protected fatliquor was investigated by applying it to two different fatliquors, the first with an iodine Index of approximately 85 against the same non protected fatliquor and the second with a lower iodine index of approximately 40 against its non protected traditional version. The results are plotted in figure 7. They reveal the necessity for fatliquoring with protected fatliquors to hinder Cr (VI) formation. 4. Influence of retanning auxiliaries and their characteristics The effects of different retanning agents, syntans based on acrilic resins, condensation resins, aldehydes, proteinic resins and substitute synthetic tannins as well as different vegetable tannins such as mimosa, tara, chesnut, quebracho and valonea were examined. Screening experiments were done using the same conditions as described above, with 2% of each retanning auxiliary. Results of these screening tests are shown on the figure 8. The results of the first trial group reveal that synthetic tanning auxiliaries have no effect on the reduction of Cr (VI), while vegetable tannins show a good effect, with pyrogallic moieties having a stronger effect than catechol groups. In the scale up trials, all the synthetics were investigated on leather while only one vegetable tannin, mimosa, was chosen because this is the most recommended vegetable tannin to avoid Cr (VI) formation. In each case, 2% of matter was used in a 100% float and drummed for 45 min. In both trial groups the unretanned leather was treated as a reference. The retanned leathers were analysed in terms of their Cr (VI) contents after they have been exposed to ageing. The results are plotted in figure 9. Analysing the results of these two trials, it can be seen that synthetic retanning auxiliaries have no reducing or hindering effect on Cr (VI) formation, while vegetable tannins show a remarkable effect. Vegetable tannins, especially pyrogallic ones, can remain bound to the fibre and, hence, reduce Cr (VI). 5.Influence of final washing on Cr (VI) formation During the screening experiments, it was observed that salts which have a high reductive power were washed out during the post-tanning operations, as described before. Consequently, such a product can not remain in the leather framework and, therefore, connot show their reducing powers, since Cr (VI) is formed due to ageing conditions. Referring to the data collected, the effect of sodium metabisulfite has been investigated. In the final washing trials, two sides from two different leathers were used. In the first group, leathers were washed as usual and in a second drum the other halves were washed for 30 min with 2% Na2S2O5 just after the acid fixation. Samples from both leathers were thereafter aged and analyzed for their Cr (VI) content. The results are shown in figure10. 6. Garmin CR6 The main scope of this study has been the search for a specific product which has no effect on the final characteristics of the leather but reduces Cr(VI) levels to undetectable concentrations. After testing several substances, Cognis have developed a very specific product fulfilling the above criteria. Cr (VI) contents are comparable with those obtained through pyrogallic vegetable tannins but without showing the inconvenient property changes associated with the latter. The effect of using this product is shown in figure 11. Conclusion By extrapolating the results from this research, the elimination or avoidance of Cr (VI) formation in leathers exposed to heat and /or UV radiation is a global concept rather than a one stop remedy. All measures during the post-tanning operations have to be considered together and the overall process has to be controlled in order to achieve satisfactory results. These measures can be summarised as follows: * Avoid free Cr (III). So, avoid chrome retanning if possible. If chrome retanning is indispensable to the process then wash the retanned leather with a suitable complex active surfactant * Neutralisation is one of the most important steps regarding Cr (VI) fomation. Final neutralisation pH and neutralising agent play a great role in the oxidation phenomenon. Homogeneous neutralisation to pH levels not higher than 5 is recommended. Using a 'reductive' neutralisation agent will also reduce Cr (VI) formation * Fatliquoring has an essential effect on the formation of Cr (VI). Thus, protected fatliquors are suitable to reduce the potential of the oxidation reaction. This is independent of their composition * Synthetic retanning agents do not have a significant effect on Cr (VI) formation but vegetable tannins show a very good reducing effect, especially the pyrogallic based ones. However, it should be taken into consideration that they can affect other characteristics of the leather.This is more likely in the case of garment and doubleface articles * A final washing with reducing salts, especicially with Na2S2O5 , decreases the likelihood of Cr (VI) formation * The use of Garmin CR6 helps avoid Cr (VI) formation and without altering the final characteristics of any article. A combination of the above mentioned remedies is the safest way to reduce the presence of hexavalent chromium. * In the case of hydrophobic leathers, a moderate basification of the chromium sulfate used for the fixation of special hydrophobing fatliquor is indispensable, the recommended pH being 4.0-4.2

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