Fatliquor influence on ageing and chrome VI formation20 August 2007
The term 'ageing' refers to the combination of manifestations, both physical and organoleptic, that occur in leather after manufacture, particularly over time. R Segura1 found that one of these manifestations is yellowing, which may arise from the fatliquors used and/or synthetic retanning agents. In the case of fatliquors, the best test is that of temperature, while retanning agents are more likely to be affected by ultraviolet light.
A Püntener2 reported that, whereas ultraviolet light mainly attacks aromatic structures (syntans), temperature affects aliphatic chains, primarily in the unsaturated bonds of the fatliquors. The presence of atmospheric oxygen is necessary for these oxidation reactions to take place, as it has been shown that the process does not occur in a nitrogen atmosphere3. The oxidation reactions that occur, widely described in the literature,1,2,3,4 produce yellowing of the leather, as well as a decrease in shrinkage temperature, poor odour,1,5 hardening,1 possible self-destruction of the leather during the drying stages5,6 due to autoflammability and, lastly, the possible formation of hexavalent chromium. There are a number of different literature references that discuss hexavalent chromium formation2,3,4,7,8,9,10,11 in which a number of factors are reported to influence the formation of this substance, ranging from those related to application (neutralisation, retanning, fatliquoring, pH values etc) to those involved in testing (heat sources, temperature, humidity, light or lamp type etc). It is important to emphasise that, in practice, the type of article in which hexavalent chromium is more likely to appear have a high proportion of fatliquor and are also stored for long periods in the 'crust' state, such as doubleface, suede for clothing, and gloves. In these articles, it is not possible to apply rules to prevent the formation of hexavalent chromium9, such as the inclusion of vegetable extracts, since these articles do not allow it technically. On the other hand, they are those with the most stringent requirements in terms of standards (values of less than 3 ppm). Therefore, the choice of suitable fatliquors is crucial to avoid all the previously described phenomena. The literature data can be organised into the following sections: 1.1 Manifestations of the oxidation process * Organoleptic: * Yellowing * Poor odour * Surface area shrinkage * Hardening * Physical and chemical: * Detanning (decrease in shrinkage temperature) * Autoflammability (destruction of the leather) * Hexavalent chromium formation 1.2 What causes oxidation? * Light: a radiant energy from 320-400nm, with a value quantified by the product of the constant (h) multiplied by the wavelength (v), acting on the leather surface exposed to this radiation. * Heat: calorific energy produced by an increase in temperature that affects the entire piece of leather. 1.3 When do these manifestations appear? * Yellowing: during the storage and use of the article * Detanning: during storage * Shrinkage of the surface area: after drying and during storage * Autoflammability during leather piling at the dryer outlet * Poor odour during the storage and use of the article * Formation of hexavalent chromium during drying and storage 2. Aim of the study The purpose of the study was to evaluate the influence of different types of fatliquors on the above parameters and to reproduce these manifestations, separately analysing the action of light and temperature as well as relating these values to the oxidising capacity of the fatliquoring, as defined by its iodine number. 3. Experimental method 3.1 Products used 1. Sulfited lecithin LS 2. Sulfited fish + lecithin PSL 3. Bisulfited fish PB 4. Sulfited maleic ester EMS 5. Special sulfited ester ESL 6. Sulfited triglyceride TS 7. Sulfited ester + lecithin EL 8. Sulfited fish PS 9. Sulfited natural + synthetic NS 10. Sulfated triglyceride TA 11. Sulfated natural VS 12. Phosphoric ester FS 13. Sulfated derivatives DS 14. Sulfochlorinated paraffin SC 15. Polymer fatliquor PO 16. Sulfited lanolin LA 17. Sulfited vegetable NE 18. Sulfited ester ES 3.2. Substrate used The tests were done using wet-blue hides, shaved to 1.2mm and tanned with a chromium salt of 33% basicity, adjusted to pH 3.9, with a masking product. The hide was rounded and cut into 20 pieces, each measuring 20 x 15 cm. 3.3 Process applied (see Table 1a). 3.4 Mechanical operations * Allow to rest on the horse, with the pieces separated by plastic * Dry-strain, with air at 45ºC * Dampen for 24 hours at 25ºC and 65% relative humidity. 3.5 Evaluations performed 3.5.1 Temperature Oven at 90ºC for 48 hours 3.5.2 Light Xenon ultraviolet lamp (1500 watts), at 280 nm for 72 hours at 40ºC and 30% relative humidity 4. Results and discussion 4.1 temperature resistance 4.1.1 Yellowing Test pieces of 4 x 2 cm were cut and subjected to the temperature tests indicated in Section 3.5.1. A colorimeter was used to measure luminosity values (L*) and tone variations (a*, b*), (Figure 1) before and after exposure. Yellowing was evaluated as the variation of the value of b*(Ab*) corresponding to the yellow-blue axis. Figure 2 shows that the reference (REF), corresponding to the non-fatliquored leather, has a slight variation at Ab*=3, which means that merely exposing a fatliquor-free sample to heat causes a certain amount of yellowing. The highest degree of yellowing was observed with the lecithin compounds (LS, PSL, PB, and EL) and, 'atypically' the sulfited maleic ester (EMS). The lowest degree of yellowing was found in the sulfited natural and synthetic oil (NS), sulfited ester (ES), sulfited lanolin (LA) and greasing polymer (PO). 4.1.2. Detanning Following the temperature tests, the test pieces were soaked for 12 hours in water at 25ºC, excess water was removed by slight pressure with filter paper, and the shrinkage temperature was measured according to the standard IUP-16. Figure 3 shows that in the reference (REF), there is a slight decrease in shrinkage temperature from 100ºC of the original wet-blue to 96ºC after oven processing. The largest decreases in shrinkage temperature are found in fatliquors containing lecithin (PSL, LS and EL), bisulfited fish (PB) as well as in maleic ester (EMS). The smallest decreases are found in greasing polymer (PO), sulfited natural and synthetic oil (NS) and sulfited lanolin (LA). If Figures 2 and 3 are compared, it can be observed that the fatliquors that most yellow (LS, PSL, PB, EMS, EL) are those that give rise to the largest fall in shrinkage temperature and the fatliquors that yellow the least (NS, ES, LA, PO and SC) are those that give rise to the smallest shrinkage temperature fall. The physical-chemical processes giving rise to yellowing and to the fall in shrinkage temperature are very complex. In the first phase, peroxide-type free radicals are formed, which are strongly oxidising and react with the unsaturated bonds of the fatty acids and/or esters, producing decomposition reactions (mechanism 1), acrolein and ketone products that yellow the leather. At the same time, these oxidising radicals attack bonds, especially chromium-collagen bonds, breaking part of these bonds and giving rise to partial detanning, evidenced by the decrease in shrinkage temperature. The mechanism of this detanning could be related to the strong attraction of acrolein and the ketone groups for lysine13 (base of 'oil tanning'), giving rise to chromium displacement and its substitution by acrolein, which produces the typical yellow colour of leather. 4.1.3. Formation of hexavalent chromium The test pieces that underwent temperature testing (90ºC for 48 hours) were conditioned for 24 hours in an atmosphere at 25ºC and 65% relative humidity. To quantify possible hexavalent chromium, the IUC-8 standard was followed. This is based on the reaction with diphenylcarbazide and subsequent colorimetric assay, comparing it with a standard solution. None of the test pieces yielded values higher than 3 ppm for hexavalent chromium. A plot of shrinkage temperature versus quantity of fixed chromium oxide in the leather was obtained by Bowes in 194712 (Figure 4). A 'hysteresis' plot has been added, corresponding to the detanning process, bearing in mind that decreases in shrinkage temperature range from 5ºC to 25ºC (shrinkage temperature of 95ºC to 75ºC) and, from the qualitative point of view, the values of between 3.4116 and 13.661 ppm chromium have been eliminated the tanning function. Where is the chromium that does not appear as hexavalent chromium? The studies performed by A D Covington et al13 show that the shrinkage temperature is not only a function of the fixed chromium but, more importantly, 'how' the chromium is fixed, in such a way that the size of the tanning complex, the degree of masking and, in summary, the form of the chromium collagen bond is a determining factor in thermal water stability. It is our interpretation that, under the conditions of our tests, the oxidation reactions arising are only able to break the chromium collagen bonds partially without oxidising them to hexavalent chromium. 4.1.4.Autoflammability The 'autoflammability' of leather has been evaluated by the 'denaturing' temperature of hide5,6 in which the 1cm2 test pieces of leather previously used for the temperature tests were soaked in water for 12 hours at 25ºC. Excess water was eliminated by gentle pressure with filter paper, placing the test pieces in a differential scanning calorimeter (DSC 821) with a 40 L pan with sealed column. The rest was carried out with a temperature programme from 20ºC to 120ºC and with a gradient of 5ºC/minute. The resulting plot had an inflection point at which the denaturing temperature (Td) of the leather occurred. It can be observed (Figure 5) how lecithin-containing components have the lowest temperatures (PSL, LS, EL). In contrast with expectations, phosphoric ester (FS) and sulphochlorinated paraffin (SC) appear. The remaining fatliquors have values between 92ºC and 97ºC, including the reference sample. 4.1.5 Odour Test pieces with a diameter of 8 cm were cut and placed in a tightly sealed glass cell and placed in an oven for 48 hours at 90ºC. The capsules were removed from the oven, opened and the odour tested by three people, who separately scored the samples between 1 (faint odour) and 3 (maximum). A weighted average was taken of the three scores. Figure 6 shows the same tendency as in previous evaluations. Leather fatliquored with lecithin (LS, PSL, EL), fish (PB), and vegetable (NE) compounds gave the strongest odour, whereas the reference (non-fatliquored) leather, the leathers with sulfited natural and synthetic oil (NS), greasing polymer (PO) and sulfited triglyceride (TS) gave no odour. 4.1.6. Surface area Test pieces of 10 x 10 cm were cut and subjected to the temperature test, then conditioned for 24 hours at 25ºC and 65% relative humidity, and measured again. Figure 7 shows that the non-fatliquored reference leather has the largest decrease in surface area (16%). In this type of test, there is a direct relationship between the surface variation and the lubricating capacity of the fatliquor, such that lower fatliquoring capacities produce more shrinkage in area. This is independent from yellowing. Special sulfited ester (ELS) is the fatliquor giving the least shrinkage. 4.1.7. Iodine number Figure 8 shows the values of the iodine numbers of the fatliquors used. If Figures 2, 3 and 8 are compared, a 'trend' toward greater yellowing and lower shrinkage temperature can be observed in fatliquors with a higher iodine number, such as PSl, PSB and PB, whereas lower iodine numbers (SC, ES, PO, FS and LA) also coincide with the best fastness, although there is no direct relationship and there are even fatliquors such as sulfited maleic ester (EMS), with a low iodine number (15) and high yellowing (A b*=7.5) and large drop in shrinkage temperature (79ºC). 4.2. Light fastness 4.2.1. Yellowing Test pieces of 4 x 2 cm were cut and exposed to ultraviolet light under the conditions defined in Section 2.5.2. The values L*, a*, b* were measured by colorimetry, using the part exposed to the light. In Figure 9, the yellowing (A b*) is governed by different criteria from those of the temperature effect and the phenomenon even arises that with some fatliquors A b* is less than in the non-fatliquored reference.The A b* for light fastnesses are approximately half those of temperature fastnesses (Figures 2 and 9). 4.2.2. Detanning The reference (non-fatliquored) sample retains the shrinkage temperature of wet-blue (100ºC). If Figures 9 and 10 are compared, it can be seen that, in general, fatliquors giving a higher degree of yellowing are those that most lower the shrinkage temperature. On the other hand, if Figures 3 and 10 are compared, it can be deduced that the fatliquors behaviour, with respect to variations in shrinkage temperature, is different when the leather is exposed to ultraviolet light rather than temperature and, as a whole, the values of the drops are lower in the light test. 5. Influence of antioxidants A malonic acid ester was used; this product being soluble on fat and having the capacity to react with free radicals. This substance was applied mixed with the fatliquors at a dose of 0.5% by weight of fatliquor product. Three fatliquors were chosen, one of them with good temperature fastness b*=3.4, Ts = 92ºC), eg sulfited lanolin (LA), another of medium fastness (b*=5.2, Ts = 84º C), the sulfited derivative (DS) and a third of low fastness (A b*= 11, Ts = 78º C), sulfited lecithin (LS). Table 1 shows the high effectiveness of the antioxidant in temperature fastness, as yellowing was reduced with the three fatliquors, achieving an improvement of two points (going from 1 to 3) in the case of sulfited lecithin. However, the most revealing data indicating a true chemical reaction with the elimination of free radicals is the non-reduction of the shrinkage temperature, which goes from 78ºC to 97ºC in the case of the LS fatliquor, ie there is no detanning. Concerning fastness to light, the influence of the antioxidant used is slight or null. 6. Conclusions 6.1. The degree of yellowing and the decrease in shrinkage temperature of leathers exposed to the temperature test are governed by the same criteria. 6.2. The iodine number of the fatliquors indicates an oxidation capacity trend (yellowing and drop in shrinkage temperature), but the relation is non-linear. 6.3. In tanned leather and under the conditions applied in this study, hexavalent chromium is not formed in amounts greater than 3 ppm after applying the temperature test. 6.4. The denaturing and odour tests follow similar criteria to those of yellowing and the drop in shrinkage temperature. 6.5. The variation of the surface area, measured before and after the temperature test, is related to the lubricating capacity of the fatliquor rather than its oxidation capacity. 6.6. When the ultraviolet light test is performed, the effects of yellowing and detanning are much less than those resulting from the application of the temperature test. 6.7. The application of an 'antioxidant' significantly decreases the effects of temperature on the leather.