The heart of leather biodegradability – Part 3 of 4
Updated: Nov 26, 2021
In Part 3 of the Heart of Leather Biodegradability we looked at the challenges that bacteria and fungi present in the tanning environment. In this Part 3, we turn the tables and start to ask whether this can be used to understand and enhance biodegradability while maintaining ultimate leather quality.
Interestingly, an age-old problem for leather – degradation due to acid hydrolysis – may be one of the keys to improved biodegradability. If the leather can be manufactured with an innate ability to release acidity at the end-of-life, then biodegradability times may be shortened. Sulphur dioxide, sulfuric acid, sulphates, nitrogen dioxide, are often implicated in the breakdown of leathers due to the hydrolytic effects mentioned above.
Some fatliquors and tanning agents can develop radicals or acid by-products over time and these can be used at end-of-life to create disintegration centres that will help with material breakdown. Practitioners who manufacture archival quality leather do everything in their power to prevent these forces. A scan of the deterioration of leather literature shows conservationists and archaeologists spending lifetimes understanding the breakdown of leather in the presence of acid/alkali and heat. It is to these sources that research should focus in determining future strategies for biodegradable leathers.
Research, development, and industry support should be given to the scientists that have built up knowledge and experience in understanding how to prevent these damages in archival relics, because if the industry knows how to prevent it, then logic tells that the industry should be able to exploit it, commercially, in the proper way.
One of the fundamental aims of tanning is to provide resistance to the material when trying to prevent the deleterious effects of heat in the manufacture or use of leather. The gelatine industry is highly familiar with how the collagen of hides and skins, can be partly denatured allowing its disintegration, solubilisation, and secondary use in other industrial applications. Again, it is to these scientists (and their literature) that the industry should turn to, when looking to improved mechanisms of leather deterioration. Figure 3 shows a tanned leather that has low heat resistance (also known as shrinkage temperature) which allows the proteins to partially unravel when heat is applied. A lack of chemical stability between and in the collagen, units allow for easy excitement, energy absorption, and bond breaking of the supramolecular protein.
Figure 3. A low shrinkage temperature tannage, with hints of gelatinisation.
The introduction of tanning agents, the way these agents interact with the collagen, and the collagen supramolecular water are pivotal in predicting how the leather will respond to a heat agent. In general, heat can be used for destruction, rather effectively – especially in combination with an enzymatic helper. Preparation of hydrolysates of collagen (or other proteins) usually occur with a heat or acid/alkali pre-treatment, followed by enzymatic digestion. In principle, this is the method for composting – 55°C thermophilic bacterial digestion in the presence of oxygen. The heat denatures the protein, allowing the bacterial enzymes at the appropriate pH (8. to degrade the leather as much as possible.
Introducing a tanning material into the collagen – and specifically the quantity of that tanning material plays a major role in the biodegradability of the material. In principle, the quantity of active substance that is introduced will shield the collagen to differing degrees. A normal glutaraldehyde tannage (24% active) added at 1-3% will have 0.24-0.72% tanning agent of wet weight (assuming 100% uptake). Chromium salts (26% Cr2O3) added at 5% (wet weight) will give about 3% Cr2O3 on dry basis (assuming 100% uptake). Vegetable tannins added at 20-30% of wet weight, can have nearly half of those tanning agents fixed inside the collagen. The amount of tanning material may seem more important rather than the type of chemical for the tannage.
In the retanning, dyeing, and fatliquoring the amount of material placed inside will interfere with the bacterial infection, the ability of the degradative agents (e.g., enzymes) to access the collagen. The chemistry of these retanning agents is also important, not because these directly interfere with the biodegradation (by shielding the collagen), but they can also persist in the degradation product. Petroleum-based or inert plastic type retanning agents are often left in the compost (as micro-plastics) in the compost. ISO 20200 looks at disintegration. ISO technical committee 61 is currently looking at the environmental aspects of plastics, including microplastics, see Figure 4. This committee may address microplastics in the disintegrated product of ISO 20200, but the leather industry could have its own method to detect these fragments. The plastic content of finishes would also need to be assessed.
Figure 4. The presence of plastics less than 1 mm, also known as microplastics, in the environment.
A lesser understood, but vitally important part of the biodegradability of leathers revolves around products that have been included in the chemistry (or deliberately added to the leather) to prevent the growth of mould. These include the fungicide added in the wet-end, in pickle/tan, or final tan wash. The other major source of fungicides is to prevent biochemistry added in post tanning or finishing, such as protein fillers, pigments, fillers, or protein binders. These products will rapidly biodegrade on the shelf or will breakdown in the leathers to produce malodour and degradation products. These fungicides are capable of being general micro-biocides and can be toxic to composting organisms at end-of-life.
It is still early days, due to limited research thus far, but it is easy to understand how these additives can give rise to problems if the product enters its degradation phase. Increases in protection from fungi and bacteria during the leather products working life will cause problems at its end-of-life. Leather scientists will need to know how much long-term impact their protections are creating.