Tanning of hides and skins

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General description

In the tanning process, the collagen fibre is stabilised by the tanning agents, such that the hide is no longer susceptible to putrefaction or rotting. In this process, the collagen fibres are stabilised by the cross-linking action of the tanning agents. After tanning, the hides or skins are not subject to putrefaction, their dimensional stability, resistance to mechanical action, and heat resistance increase [3, Andres 1995] [2, HMIP 1995].

There is a wide variety of tanning methods and materials and the choice depends chiefly on the properties required in the finished leather, the cost of the materials, the plant available, and the type of raw material.

The majority of tanning agents fall into one of the following groups:

• mineral tannages
• vegetable tannins
• syntans
• aldehydes
• oil tannage.

The most commonly used tanning agent is basic chromium sulphate (Cr(OH)SO4). A high proportion (80 – 90 %) of all the leather produced today is tanned using chromium(III) salts. Hexavalent chromium (chromium(VI)), in contrast, is not used in the tanning process and has no tanning effect. The possible formation of chromate in leather during its manufacture (see Section 1.4) depends on synergetic effects of several components. The raising of the pH during the neutralisation of wet blue facilitates the oxidation of chromium(III) to chromium(VI)). The drying of leather and its intermediate products may also create favourable conditions for the formation of chromium(VI). Fatty acids in the fatliquor may have an especially important influence (unsaturated fats may promote chromium oxidation).

In order to avoid the formation of chromium(VI), the precautions given below can be taken.

• The use of a reducing agent as an auxiliary during the neutralisation of wet blue.
• Avoidance of the use of ammonia as a wetting back agent for crust leather before dyeing.
• The use of vegetable retanning agents for chromium-tanned leather or the addition of a small proportion of a vegetable tanning agent (e.g. 0.25 – 4 % depending on the agent) in the retanning process step. Vegetable tannins derived from Tara are particularly effective in this role.
• Choosing a fatliquoring agent that does not contain simple or multiple unsaturated free or esterified fatty acids.
• The use of fatliquors with antioxidants.
• Degreasing skins with a high content of natural grease (sheep and pig skins) before tanning.

[ 85, Hauber and Knödler 2008 ] [ 159, Rydin 2002 ].

There are several types of vegetable tanning systems, and the types of leathers produced with each system do not have characteristics comparable to chrome tanned leathers, e.g. resistance to high temperature and flexibility. Conversely some of the qualities of vegetable tanned leathers, e.g. tooling, burnishing can only be found in this type of leather.

Emerging technologies

Use of vegetable tannins derived from grape seeds

Description

Degreased and crushed grape seed is sulphited using sodium metabisulphite under temperature and pressure producing the solubilisation of tannin molecules. In these conditions 11.4 % of grape seed is solubilised. It is concentrated, and then preserved with biocide.

Achieved environmental benefits

Waste grape seed could provide a sustainable source of vegetable tannins, beyond the current capacity of managed plantations of tropical trees. A beneficial use is made of a waste product of another industry.

Operational data

Adoption of the technique has not occurred, due to problems in the supply of grape seeds.

Applicability

Certain types of vegetable tanning can be carried out using grape seed tannins.

Economics

If developed this technique offers the possibility of stable material costs for vegetable tannins, due to European sourcing.

Driving force for implementation

Should the use of vegetable tanning increase, a sustainable source of vegetable tannins would be required.

Status of development

The extraction and concentration have been carried out at pilot plant scale. The material produced has been tested for leather production. Reference literature Life Tannins project [130, Life 2008].

Use of a cross-linking agent derived from olive waste

Description

Olive leaves which are collected when the olives are harvested are separated at the processing factories and form a waste. Olive leaves also arise as an agricultural waste from the pruning of olive trees. An aqueous extract is made from the leaves and then concentrated.

Achieved environmental benefits

Waste from the processing of olives could provide a sustainable source of non-chromium tanning agents. A beneficial use is made of a waste product of another industry.

Operational data

The material can be used in existing drums.

Economics

If developed this technique offers the possibility of stable material costs, due to European sourcing.

Driving force for implementation

Should the use of non-chromium tanning increase, a sustainable source of tanning agents would be required.

Status of development Laboratory and pilot scale trials have been carried out.

Reference literature [ 154, Marx et al. 2011 ].

Source: Joint Research Centre, Best Available Techniques (BAT) Reference Document for the Tanning of Hides and Skins, 2013

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