Hydrophobic leather characteristics

At the XXV IULTCS Science in Leather Congress in 1999, a commercially available polysiloxane was assessed for imparting hydrophobic qualities to leather. The effects of vacuum drying time and quebracho vegetable tannin extract content, used in the retanning process, on the resistance to water penetration and on water vapour permeability were examined. In addition, the effect of capping with chromium or zirconium on the number of Maeser flexes to water penetration is described.

Introduction

In modern high performance leather upper materials, properties such as the level of waterproofness, water uptake and breathability are important¹. As part of a project on the production of hydrophobic leather for boot upper material, a series of experiments were conducted to determine the effect of a number of variables on the final leather produced. The influence on the number of Maeser flexes to water penetration and water vapour permeability was studied on:

1) the vacuum drying time

2) the level of quebracho vegetable tannin in the retannage of chrome tanned leather

3) the short term ageing of the crust leather

4) the effect of capping with chromium and zirconium.

In addition, the distribution of silicon and zirconium (in zirconium capped leather) throughout the thickness of the leather was also determined.

Experimental

Outlined in Figure 1 is the process used for the production of hydrophobic leather. The starting material used for processing was commercially available wet-blue suitable for water resistant leather production. The important criteria in wet-blue for production of water resistant leather have been identified by a number of authors^2-6.

Sampling regime

Samples for water vapour permeability measurements, determination of the number of Maeser flexes to water penetration and percentage water uptake were taken from leather positions shown in Figure 2. Samples were conditioned at 20°C and at 65% relative humidity for at least 24 hours prior to all measurements. Unless otherwise indicated, samples were tested 2-3 weeks after production.

Water vapour permeability was determined according to Satra method PM 47. Results were determined with the grain surface as well as the suede surface facing the water vapour.

Dynamic resistance to water penetration

Dynamic resistance to water penetration was calculated using the Satra Maeser tester, Satra method PM 34. Results were determined for grain surface as well as the suede surface flexing in water.

Percentage water uptake was calculated as the difference in mass from the start of the experiment until water first penetrated through the sample.

X-ray mapping

Data were acquired using a Phillips field emission scanning electron microscope (XL30 FEG) attached to LinkISIS X-ray analysis system from Oxford Instruments Pvt Ltd using an accelerating voltage of 30keV.

For silicon mapping, the K-line was used at 1.74keV and for zirconium mapping the L-line was used at 2.219keV.

Effect of vacuum drying time

Chrome capped leather sides, produced according to the procedure outlined in Figure 1, were vacuum dried for 4, 10 and 18 minutes at 60°C and 25mm Hg vacuum.

Effect of quebracho content

Chrome capped leather sides, produced according to the procedure outlined in Figure 1, were vacuum dried for four minutes at 60°C and 25mm Hg vacuum. The amount of quebracho plus syntan was kept to a constant value. All other chemical quantities were kept constant.

Effect of ageing

Chrome capped leather sides, produced according to the procedure outlined in Figure 1, were vacuum dried for four minutes at 60°C and 25mm Hg vacuum. The samples were first tested one month after production and then tested again two months after production. During ageing, the samples were stored at room temperature.

Effect of metal capping

Leather sides were produced according to the process in Figure 1. Sides were capped either with chromium sulfate or with zirconium sulfate. The sides were vacuum dried for four minutes at 60°C and 25mm Hg vacuum.

In Figure 2, grain (G) and suede (S) indicate where samples were taken for water vapour permeability (Satra PM 47) and dynamic resistance to water penetration using the Satra Maeser method (Satra PM 34). Water vapour permeability and Maeser testing were carried out on adjacent samples with G and S surfaces exposed to the water vapour and water respectively.

Results and discussion

a) vacuum drying time

The results obtained for Maeser flexes versus vacuum drying time at 60°C and 25mm Hg vacuum for both grain and suede surfaces in the water on a butt sample are shown in Figure 3. The numbers of Maeser flexes were found to decrease with increasing vacuum drying time at 60°C ie increasing the vacuum drying time decreased leather hydrophobicity. The same trend was observed for both grain and suede surfaces in the water. This decreasing hydrophobicity with increasing vacuum drying time was also observed for samples from the neck and belly.

Water vapour permeability on the other hand was not affected by the vacuum drying time with values ranging from 5.2-7.3 mgcm-2hr-1. There was no observable difference between the permeability of grain or suede surfaces exposed to water vapour.

b) quebrachro content

Quebracho was added to the retannage in amounts ranging from 0-12% in 2% increments. Results obtained from butt samples with grain in the water or grain surface in contact with water vapour are shown in Figure 4. For the 0% quebracho sample, water had not penetrated after 144,000 Maeser flexes. Clearly, increasing the amount of quebracho significantly decreases the number of Maeser flexes before water penetration. Water vapour permeability, on the other hand, was not affected by the amount of quebracho in the retannage, for the range 0-12%.

c) ageing

Ageing of the leather results in higher Maeser values before water penetration. The first with the grain surface in the water and the second with the suede surface in the water, tested one month after processing and retested two months after processing.

For the same leather piece with grain in the water, the number of Maeser flexes increased from under 50,000 flexes before water penetrated to over 380,000 flexes. For the sample with the suede surface in the water, the flexes increased from 125,000 flexes before water penetrated (one month after processing) to over 380,000 flexes (two months after processing). In both cases, water had not penetrated after 380,000 flexes and the amount of water absorbed by the leather was less than 10% for samples which had been aged for two months.

d) metal capping

Generally, for leather produced with 33% basic chromium sulfate as the capping reagent, there was little difference in the number of Maeser flexes to water penetration between leather samples with the grain or the suede surface in the water.

For zirconium sulfate capped leathers, samples with the suede surface in the water gave higher Maeser results than samples with the grain surface in the water.

In an attempt to gain an understanding of these results, the distribution of silicon and zirconium in the leather was analysed by elemental X-ray mapping.

Comparisons of the silicon distribution for the two leather samples show that in both cases it is about the same. However, the distribution of zirconium is uneven.

The zirconium has accumulated in the grain layer and approximately halfway through the corium towards the grain layer.

There appears to be a distinctly lower zirconium concentration at the grain-corium boundary. There is a band below the grain (approximately 1/3 to 1/2 of the corium layer) where the zirconium concentration is lower than elsewhere.

The large differences in Maeser readings may be related to the distribution of zirconium in the corium and grain layers.

Conclusion

The number of Maeser flexes to water penetration decreases with increasing vacuum drying time and increasing quebracho vegetable tannin content.

Ageing of leather can increase the number of Maeser flexes to water penetration. Water vapour permeability, on the other hand, is not affected by vacuum drying time or quebracho content.

Zirconium capped leathers, unlike chromium capped leathers, show better Maeser flexes when the suede side rather than the grain side is immersed in the water.