Dialdehyde starch, a sustainable, environment-friendly tanning chemical

Introduction
Tanneries are coming under more and more pressure from an environmental point of view to reduce wastage and to look at using more sustainable tanning processes. Chrome has been considered a major pollutant for many decades, but a suitable alternative has been difficult to find.
Starch is an abundant chemical. It is also biodegradable, inexpensive, and naturally renewable, and it has no risk to health or the environment. Native starch cannot be used directly in the tanning industry due to certain drawbacks. It has a high molecular weight which makes it difficult if not impossible to penetrate the fibre structure of a hide. It is insoluble in water and also has no reactive groups that can bind to the functional groups of collagen in order to stabilize and tan the collagen. If starch is to be used in the tanning industry, it will need to undergo some chemical modification.
Previous research by the authors of this research paper has shown that oxidation of starch with hydrogen peroxide can produce certain smaller chemicals that have some tanning effect on collagen. In this study, the researchers decided to focus on introducing reactive groups onto the starch that could react with the collagen via stable bonds. A literature review showed that it is possible to introduce aldehyde groups onto the starch by periodate (NaIO4) oxidation (Figure 1). These dialdehyde starches that have reactive groups are used in a number of industrial applications such as paper, textile and pharmaceuticals, and should theoretically be able to bind to collagen via the reaction shown in Figure 2.

Figure 1. Dialdehyde starch formation by oxidation with periodate.

Figure 2. Aldehyde reaction with collagen.

This study was carried out to firstly oxidize starch with periodate to introduce dialdehyde groups, and then to characterise the different products obtained to determine their tanning performance.

Experimental
Various different sizes of dialdehyde starch were produced by oxidation of native starch with different molar ratios of periodate. These were then tested for tanning performance as follows:
1. Goatskins were de-pickled to a pH of 5.5.
2. 10% dialdehyde starch was added to the drum in a 100% float.
3. The drum was run for 120’.
4. Another 10% dialdehyde starch was added to the drum and the drum run for a further 120’.
5. The pH was raised to 7.5-7.8 at the end of the tanning process using sodium bicarbonate.
6. After tanning, the skins were retanned and fatliquored.
The tanning effect of the different sizes of dialdehyde starches produced was determined by shrinkage temperature. All leathers were also tested for tensile strength, tear strength and elongation at break.

Results
Tanning of the pelts with different molar sizes of dialdehyde starches resulted in an increase in the shrinkage temperature of between 25 and 29°C compared to the untanned pelt. It was also found that the thickness of the leather produced increased in conjunction with increasing molar ratio of the starch in the dialdehyde produced.
The physical properties of the leather produced from dialdehyde starch are shown in Table 1. 

Table 1. Physical test results of dialdehyde starch tanned leather

The UNIDO recommended physical test results for goatskins are 20N/mm2 for tensile strength and 40N/mm2 for tear strength. It can be seen that the results obtained for the dialdehyde tanned goatskins exceed these specifications.
The leathers also exceeded the UNIDO recommendations for grain cracking, distention, and bursting.
After drying, the leather samples tanned with dialdehyde starch were found to be bone coloured, firm, and had a compact structure similar to vegetable tanned leathers. They did have a slightly rough surface feel.
Scanning electron micrographs (SEM) of the leather tanned with dialdehyde starch showed that the fibril bundles were separated, indicating that the dialdehyde starch was able to have a tanning effect on the goatskins. It was also found that the fibre separation increased with increasing oxidation of the native starch i.e., the larger molecular weight products produced better fibre separation and tanning.

Conclusion
The results of this study have shown that dialdehyde starch has a remarkable tanning effect and can be utilized as a tanning agent for making leather. It has several advantages as a tanning agent and the process produces less swelling, less astringency, no pebbled grain, and loss of area. The tanning process developed does not require a pickling process, and the avoidance of using acids and salts gives it an important advantage from an environmental point of view. The main advantage is that the starch used as the chief chemical in the process is derived from natural renewable resources that are biodegradable and thus has huge potential from a sustainability and environmental point of view.

This article is a summary of the paper “Alternative Tanning Agent for Leather Industry from a Sustainable Source: Dialdehyde Starch by Periodate Oxidation.” Journal of the Society of Leather Technologists and Chemists (JSLTC) Vol 116, 89 - 99 (2021).