I n managing the possible formation of CrVI, fatliquors are generally known as a class of chemicals that are able to cause chromium III (CrIII) to oxidise towards CrVI.
This article discusses the results of a study where fatliquors of varying raw material constitutions have been compared. The research has been based on the understanding of lipids and their oxidation characteristics; whereas the standard determination of CrVI has been compared with the oxidation susceptibility of leathers treated with different fatliquors.
In recent decades, several researchers studied the potential factors involved in CrVI formation on leather. Today, practical guidelines are available for tanners to prevent CrVI formation in the leathers. Besides a good regulation of the process conditions in the bath – for instance, maintaining a low pH after fixation (3.5–4.0) – the use of fatliquors is also mentioned as one of the factors playing a role in the formation of CrVI. For example, sulphited or sulphated oils are fatliquors based on natural oils and their triglycerides.
A certain part of the fatty acid fragments of these oils contain unsaturated carbon-carbon bonds, which can play a role in the formation of CrVI. During ageing of the leather, these fragments are able to generate reactive oxygen species (ROS), resulting from lipid oxidation. In most instances, the oxidation is a free radical chain reaction that can be described in terms of initiation, propagation and termination processes.
The ROS could, under certain conditions, oxidise CrIII to CrVI. Production, storage and use of leathers are determining factors in this. Recently, the European Commission issued a new regulation to establish restrictions on CrVI in leather articles and articles that contain leather parts. It restricts the presence of CrVI in leather articles with skin contact at concentrations equal to or greater than 3mg/kg (0.0003% by weight).The restriction indicates that, to end up with safe leathers, there must be full control of the leather-making process.
In the food and feed industry, the formation of ROS by fats, oils and lipids has a direct influence on the quality characteristics, such as flavour, colour, texture and storage stability. The oxidation susceptibility of these products can be studied with Oxipres. With this apparatus, an accelerated shelf-life test can be performed, based on a reaction of the product with oxygen under standardised conditions. To protect food products against oxidation, several types of antioxidants can be added. The antioxidant (AO) prevents the formation of ROS and the subsequent oxidation of the ingredients. It was of interest to see if the Oxipres could be of help to study the oxidation susceptibility of leather. Therefore, a series of fish and vegetable oils containing fatliquors were formulated, based on sulphited or sulphated oils.
Also, a synthetic oil fatliquor was taken into account, containing a mono-unsaturated fat alcohol-based (oleylalcohol) material. Furthermore, the fatliquors were formulated with and without the addition of an antioxidant. The leathers prepared with the fatliquors were tested for their CrVI content and for their oxidation characteristics with the Oxipres.
Parameters
In this study, chrome-tanned bovine leathers of 1.4–1.6mm were used. The leathers were processed according to the recipe in Table 1 ‘Leather processing recipe (see opposite).
A series of fatliquors were mixed, containing sulphited or sulphated fish or vegetable oils. A synthetic oil fatliquor was produced, containing an oleylalcohol-based emulsifier. The fatliquors were formulated with and without the inclusion of an antioxidant (Table 4, Oxipes and CrVI values above).
CrVI determinations were performed at external testing institutes according to method EN ISO 17075 (IUC 18).
Oxipres measurements were performed on 50g leather samples, cut into pieces. Test conditions were 140 hours at 80°C, starting at 5–6 bar of oxygen pressure.
Results and discussion
In order to test the importance of the leather-processing conditions, chrome-tanned leathers were prepared with representative commercially available fatliquors. Some of the leathers were processed with and without a fixation step with formic acid, ending with a pH of 3.6–4.0 and 7.7–8.0, respectively. Also a piece of non-fatliquored chrome-tanned leather was taken into account as the blank reference.
The same series of leather was analysed for CrVI content by two different test institutes, according to the EN ISO 17075 method. The results clearly showed the importance of fixation (low pH) to keep the CrVI levels low. Under slight alkaline conditions, increased amounts of CrVI are found (also in the blank reference leathers). Furthermore, the results show that for the same leathers, different values of CrVI are measured by the different test institutes. It is also shown that fatliquors are available in the market that are not formulated sufficiently to be used safely on chrome-tanned leather.
The formation of CrVI from CrIII on leather takes time and is favoured by the combination of heat, light and oxygen. From the CrVI values in Table 3 (see above) it can be observed that on the freshly prepared leathers no CrVI was detected. After storing the leathers for 24 hours at 80°C, CrVI is generated, even when the leather is processed without a fatliquor. To mimic ageing of leather, the EN ISO 17075 method includes a leather ageing step, storing the leathers for 24 hours at 80°C.
The results from the leather ageing test indicate that, at higher temperatures, oxidation processes occur on the leather matrix. The leathers studied with the Oxipres show a similar phenomenon. The leathers were exposed to a concentrated oxygen atmosphere (5–6bar) at a high temperature (80°C) for 140–160 hours. In the case of the fatliquored leathers where no antioxidant is used in the formulation, a fast drop in oxygen pressure was observed, indicating a severe oxidation of the leathers. This might show that the initiation and propagation stage in the formation of ROS directly starts at these conditions.
After several hours of oxidation, the oxygen pressure-decrease curve becomes less steep. Here, the oxygen consumption becomes lower, entering the termination phase of radical formation. At this point, the major part of the oxidation-sensitive compounds of the leather is oxidised. The addition of an antioxidant to the fatliquors has a clear effect on the oxidation processes on the leathers.
For the sulphated oils, a clear induction period of the oxidation process can be determined. The induction period is the time needed in hours after which a fast oxidation starts – reflecting the initiation period of ROS formation. For the sulphated oils, it takes about 66–67 hours at 80°C to start the oxidation. This means that for these leathers the oxidation is prevented for a certain period by the antioxidant. After that, reactive oxygen species are formed, initiating the oxidation process on the leather, followed by rapid oxidation (propagation).
Further investigation
For the sulphited oils and the synthetic oil, the addition of antioxidant also results in a change of the oxidation process on the leather. These are slightly different from that of the sulphated oils, showing that the oxidation processes are also slowed down by the antioxidant. In the cases where the antioxidant is present, but no induction point was found, additional research will be needed; for instance, looking at the antioxidant level or the oxidation temperatures.
In this way, the Oxipres could be a useful piece of equipment to study oxidation processes on leather, helping in the formulation of robust fatliquors. Besides the oxidation curves, the CrVI content of the leathers were also determined (see Table 4).
The Oxipres could be a valuable tool to study the oxidation susceptibility of leather. It was shown that the addition of antioxidant into the fatliquor formulations alter the oxidation processes on the leathers, hindering the formation of reactive oxygen species and the subsequent fat oxidation. By proper antioxidant protection of fatliquors, leathers can be produced with CrVI values below 3ppm, independently from the fatliquor type used; for example, fish, vegetable or synthetic oil-based.
