By A. Clare
Vegetable tanning agents (vegtans) were used for thousands of years until 1885 when the discovery of the versatile nature of basic chromium sulfate subsequently decreased their application. The use of chromium salts dominate in the tanning industry today; but how long this will continue is an open question.
Vegtans are water-soluble, phenolic compounds, extracted from plants. Vegtans form a complex with collagen through hydrogen bonding of their hydroxyl groups with the peptide bonds – thus functioning the same as other tanning agents by improving the protein stability.
Degradation of vegetable tanned leathers:
All tanning agents are susceptible to three types of degradation: chemical, oxidative, and microbial (biodegradation), which can occur alone or in combination. Chemical and oxidative degradation occurs through the degradation of collagen on a purely chemistry-based agency. Alternatively, during biodegradation, biochemical agents break the material down to produce biologically useful nutrients which are then assimilated. In the case of vegetable-tanned leathers, degradation is characterised by the collagen fibre structure severely disintegrating - often through chemical means.
Chemical degradation:
The extent of chemical degradation is influenced by the type of vegetable tannin used. Leathers can be tanned using either hydrolysable or condensed vegetable tannins. Hydrolysable tannins are very susceptible to chemical degradation caused by acid or alkali hydrolysis – or through the action of bacteria (looking for the sugar content of the tannins).
Acid hydrolysis can be caused by air pollution as leather absorbs pollutants such as sulfur dioxide and nitrogen dioxide. Vegetable tannins act as a sink for sulfur dioxide, causing the bonds within the collagen structure to be hydrolysed. The charge of the collagen side chains is altered, and the structural integrity of the collagen is reduced, resulting in cleavage of the peptide chains. At pH 3, the collagen becomes weak and the structure becomes severely damaged at pH 2, which lowers the hydrothermal stability. At high pH, alkali hydrolysis occurs. Alkaline hydrolysis strips the vegetable tannins from the collagen structure, causing the leather to crack and darken in colour.
Oxidative degradation:
Condensed vegetable tannins are less prone to chemical degradation, but they are susceptible to oxidation. Oxidative degradation results from environmental factors, such as temperature (thermal degradation) and light (photooxidation). Temperature and light can degrade the leather structure by cleaving the collagen peptide and modifying the amino acid side chains.
The phenolic compounds of the vegetable tannins make these tanning agents susceptible to photooxidation. Under prolonged irradiation, vegetable-tanned leathers initially darken as quinones form on the phenolic structure. This darkening reaction is subsequently followed by a lightening reaction as the collagen structure weakens and degrades.
Leather experiences thermal degradation when exposed to heat. The wet shrinkage temperature (i.e., the temperature at which collagen loses its structure and proteins are denatured) for new vegetable-tanned leathers is between 75 °C (hydrolysable tannins) and 85 °C (condensed tannins). In dry vegetable-tanned leathers an initial water-removal stage occurs between 25–150 °C. The main decomposition stage then begins at 150 °C when proteins begin to decompose (burn). This is followed by a rapid loss in mass between 200–600 °C.
Biodegradation:
Microbial degradation occurs when heterotrophic organisms (such as bacteria and fungi) decompose organic matter and assimilate biologically available nutrients. Biodegradation alters the physical and chemical composition of the collagen and results in a decrease in mass.
The phenolic nature of vegetable tannins exerts some antimicrobial effect by damaging bacterial cell walls and affecting cellular contents, thereby preventing growth and protease activity. Vegetable-tanned leathers can, however, be degraded by fungi, which can easily break down aromatics.
The filaments of the fungi, known as hyphae, can connect air-filled pores and penetrate through solid material in terrestrial soil environments. Hyphae can also transport nutrients to areas of metabolic activity, facilitating biomass recycling and playing an important part in the decomposition of organic matter.
Fungi can thrive on vegetable-tanned leathers, which are rich in carbohydrates, fats, and proteins - but like all vegetable materials the breakdown is slow. These compounds are vital nutrients which promote the germination of fungal spores, thus further stimulating growth. Fungal growth results in loss of protein material, which reduces the physical and mechanical properties of the collagen and causes the leather to vary in colour.
Conclusion:
In a terrestrial environment, many, if not all these processes have the potential to degrade vegetable-tanned leathers, albeit slowly. Chemical degradation can occur in the form of acid hydrolysis as leather absorbs large quantities of sulfur- and nitrogen-dioxide when exposed to polluted air. Similarly, photooxidation is a common observation for vegetable-tanned leathers and, this exposure to light massively limits their application.
Although vegetable-tanned leathers struggle to be degraded by bacteria, the high diversity of fungi in terrestrial environments provides an opportunity for the break down and assimilation of products during biodegradation.
But what happens to vegetable-tanned leather when it enters a different ecosystem with different environmental conditions, such as the ocean? How well will vegetable-tanned leather break down and is there a potential for biodegradation? Check back next week to find out how vegetable tanned leather will break down in the marine environment.
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