Changes in tannage24 October 2005
In my research to gain a PhD, using titanium (III) as a tanning agent proved something of a thankless task, but it did highlight the ways in which the parameters of the tanning bath affected the uptake of the metal and, hence, the shrinkage temperature of the resultant leathers. Some of the results are discussed here. Determination of titanium in leather The first problem to overcome was to find a method for determining the amount of titanium in the leathers produced. Petroselli1 commented that: 'Hydrogen peroxide reacts with titanium to produce a yellow/orange colour. The more intense the colour, the more titanium is present. Presumably, once the process has been established, this reaction could be used to determine the amount of titanium left in solution by using colorimetry.' There is no official method for the determination of titanium in leather. Thus, a method was developed. Samples of leather were digested using the wet oxidation method for chromium analysis, SLC 8 [IUP/8]2. A small amount of chromium (III) sulfate powder (ca 3mg) was added to determine when the oxidation process was complete. This addition converted the solution from a green colour to orange. It is necessary since the titanium solution is colourless and there is no visible colour change. A method for determining the amount of titanium, using hydrogen peroxide, was then employed3. The solution was diluted to 250 cm3. An aliquot was taken, to give a final solution containing 2-25 mg l-1 of titanium and 10 cm3 of hydrogen peroxide (3%[v/v]) was added and diluted to 100 cm3 with 1 mol dm-3 sulfuric acid. A blank of the reagents minus the hydrogen peroxide was used in the reference cell and the absorbance was measured at 410 nm. Calibration curve Potassium titanyl sulfate (3.68g) was weighed into a Kjeldahl flask and to it was added ammonium sulfate (8g) and sulfuric acid (100 cm3, conc). The mixture was refluxed for 10 minutes. After cooling, the solution was poured into distilled water (750 cm3) and diluted to 1 dm3. The concentration of the titanium is 0.5mg cm-1. The graph is shown in Figure 1. The molarity of the titanium (III) solutions was determined using the following method. 5 cm3 of phosphoric acid was added to an aliquot of the titanium (III) solution, which was titrated with potassium dichromate (0.0167 mol dm-3) and the end point was noted by observing the colour change with barium diphenylamine sulfonate indicator. Tanning with titanium (III) sulfate While tanning with titanium (III) salts did not produce the hoped for high shrinkage temperatures, it did give an insight into some of the tanning parameters. Solutions of titanium (III) sulfate were made4 and the following experiments were carried out to determine the parameters of titanium (III) tannage. All experiments were carried out in a water bath under a flow of nitrogen to reduce the amount of air getting to the solution. The process for all tannages is set out in Table 1. 1 Effect of titanium offer on the shrinkage temperature of pickled pelt The offer of titanium salt, added to depickled pelt at pH 4, was varied to determine the optimum shrinkage temperature achievable. The leathers produced were white and the shrinkage temperature was measured using SLP18. The results are shown in Table 2. While the actual values are unremarkable, they do show that as the amount of titanium offered increases the shrinkage temperature increases. This suggests there is some positive interaction with the collagen. However, a 4.5% increase in the amount of titanium offered results in only a 16°C increase in temperature. It would seem that the effect of adding more titanium is to increase steric hindrance around the collagen structure and so increase the entropic effects, which leads to an increase in the shrinkage temperature. 2 Effect of temperature The experiment was repeated at two higher temperatures to see if there was an effect due to a change in temperature. The results are shown in Table 3. There is no discernible pattern in the shrinkage temperatures as a function of the temperature of the processing bath. Shrinkage temperatures for the lowest percentage of titanium (III) sulfate added could not be measured, as the samples showed signs of gelatinisation. This may be a function of the low pH of processing and the temperature. The interaction with the collagen probably does not occur to any great extent until basification takes place. Hence the effect of heating the processing bath during tannage is negated. 3 Effect of heating the bath on the Ts of pickled pelt In chromium (III) tannages the temperature of the bath during tanning is important. Covington5 showed that to achieve the optimum shrinkage temperature the tanning bath should be heated at the end of processing. This was tried with titanium (III) sulfate on the pickled weight, and the tannage was processed for 3 hours at 25°C. The temperature was then increased to 35°C for 2 hours and basified with sodium bicarbonate to pH 4 over 2 hours. The leathers produced were again white and the shrinkage temperature was measured as before. The results are shown in Table 4. There was little effect in increasing the temperature at which the tannage was carried out. If anything, the results are slightly lower than in the first trial, Table 2. This may be due to the titanium complex hydrolysing when the heat is applied. 5 Role of the basifying agent Rusakova6 showed that the basifying agent was an important factor in attaining hydrothermally stable leather. The results of his work suggest that by using hexamethylenetetramine (HMT) the stability of the titanium (IV) tanned leather could be increased by up to 10°C compared with other basification systems, Table 5. The effect compared with magnesium oxide or ammonia shows an enhanced effect. It also highlights how well the HMT works as a basifying agent compared with the sodium bicarbonate basification. 6 Effect of HMT concentration on the hydrothermal stability of the tannage To determine whether the effects of HMT would be beneficial to titanium (III) tannages, the amount of HMT added in basifying was varied to determine the optimum conditions to achieve hydrothermally stable leather, Table 6. Samples of the leathers were then split into three layers and a stratographic analysis of the titanium in each layer determined using the newly developed procedure. Table 7 shows the percentage of titanium in the grain, middle and flesh layers of samples from each concentration of HMT used. The uptake of titanium was determined as a percentage of the total added. The amount of titanium in each layer is a percentage of the total in the leather. The maximum effect of the HMT seems to be around 5% of the compound on the pickled weight. The majority of the titanium seems to be on the flesh and grain surfaces. This may be due to basifying the samples too quickly, or it may be a function of the polymerisation of the tannage, reducing the ability to penetrate the skin. The results show that, compared with the sample which was not basified with HMT (0 in the table), the HMT distributes the titanium through the cross-section more evenly. This is reflected in the increase in shrinkage temperature compared with the non HMT basified tannage. Conclusion As a single tanning agent it seems that titanium (III) has limited use, however, the work did show that tanning bath parameters are important in the uptake and fixation of metal tanning agents.