EP2260115A1

EP2260115A1 - Process for pre-tanning and tanning skins with organic tanning agent

Info

Publication number: EP2260115A1
Application number: EP09727033A
Authority: EP
Inventors: Gareth Collins; Chris Jones; Kelvin Pritchard
Original assignee: Rhodia Operations SAS
Current assignee: Rhodia Operations SAS
Priority date: 2008-04-01
Filing date: 2009-03-25
Publication date: 2010-12-15
Also published as: WO2009121769A1; AU2009231404B2; CN102016078A; GB0805878D0; AU2009231404A1; NZ602485A; CN102016078B; AR071121A1; GB2458913A

Abstract

The invention relates to a process for pre-tanning skins comprising the steps of liming and washing the skins; deliming, bating and washing the skins; adjusting the pH, if required; pre-tanning the skins; and adjusting the pH, if required; wherein the step d) of pre-tanning the skins involves treating the skins with one or more organic tanning agent; and one or more stabilising agent, wherein the skins are treated with the stabilising agent after being treated with the tanning agent; application of the process to to methods of tanning, in particular methods of pre-tanning and tanning that do not require a pickling step.

Description

PROCESS FOR PRE-TANNING AND TANNING SKINS WITH ORGANIC TANNING AGENT

The present invention relates to methods of tanning, in particular methods of pre-tanning and tanning that do not require a pickling step.

Tanning is a process for the preservation of skins, by which is meant the collagen-containing integuments of vertebrates including mammals (e.g. cows, pigs, deer, goats, sheep, seals, antelope, mink, stoats and camels), fish, (e.g. sharks), reptiles (e.g. snakes, lizards and crocodiles), and birds (e.g. ostrich). Skins comprise a layer of collagen, and tanning entails reacting the collagen with a cross linking, or tanning, agent to cross link reactive sites within the collagen molecule. The product of the cross linking is leather, which is substantially less susceptible than skin to bacterial degradation.

A consequence of the cross linking is an increase in the minimum temperature at which the wet leather tends to shrink. This shrink temperature is often used as an indication of the degree of tanning.

The collagen layer of the skin is typically separated from fats, connective tissue and other subcutaneous protein, and optionally from the outer keratinous layer, by a combination of chemical and physical steps. The former may include liming, bating and pickling. The skin is then subjected to treatment in one or more stages with various pre-tanning or tanning agents selected to give the desired end properties.

In the liming process the skin is treated with liming chemicals which destroy or loosen the hair and epidermis to leave a hairless skin. The skin is then de-limed to remove the liming chemicals.

The de-limed skin is then bated, which uses enzymes to open up pores in the skin and aid tanning. The next stage in the tanning process is usually the pickling stage, wherein the skins are treated with an acid to prepare them for tanning. Traditionally, a pickling treatment involves the use of a mineral acid, (e.g. hydrochloric acid or sulphuric acid), or an organic acid (e.g. formic acid) or a mixture thereof (e.g. a mixture of sulphuric and formic acids) together with the addition of a salt (e.g. sodium chloride) to prevent swelling of the skins which would otherwise occur when the collagen is exposed to an acid medium.

The pickling process typically takes the pH of a delimed and bated skin from its initial pH, of about 8, down to a pH of about 3.

When a tanning agent is added to the skin at a low pH, such as pH3, it will penetrate the hide or skin, but will only be activated when the pH of the hide or skin is raised (by adding a basifying agent). As the pH is raised, the tanning agent is activated and cross-links with the skin so that tanning is achieved.

Such pickling treatments not only lengthen the overall tanning process, but also have an environmental impact in that they result in the production of tannery effluents which must be disposed of, commonly having a high salt concentration. They also place a loading on the tannery in terms of the acid and salt required.

Accordingly, there is a need to reduce the environmental impacts from tanning processes. There is also a need to reduce the processing times involved in tanning processes, and consequently making the overall leather production process more efficient.

The present invention provides, in a first aspect, a process for pre-tanning skins comprising the steps of: a) liming and washing the skins; b) deliming, bating and washing the skins; c) adjusting the pH, if required; d) pre-tanning the skins; and e) adjusting the pH, if required; wherein the step d) of pre-tanning the skins involves treating the skins with: i. one or more organic tanning agent; and ii. one or more stabilising agent.

The pre-tanning process of the present invention does not require the use of a pickling treatment. Accordingly, in a preferred embodiment, the process does not include a pickling stage.

The omission of the pickling stage removes the need for the use of acids (which can be dangerous to handle) and salts (which can cause effluent problems). Furthermore it reduces the time and labour required to complete the tanning process. There are also cost reductions as fewer chemicals are used.

Additionally, the present process minimizes pH fluctuations through the process, and so maintains the natural grain quality. By allowing a smooth, continuous decrease in pH from de-liming onwards, excessive looseness in the skins and loss of mechanical strength (tensile and tear) is avoided. In particular, this permits efficient processing of "casualty cows" and new season pelts which have loose skins.

The pre-tanning process may be used to treat any suitable skins that require pre-tanning. In one embodiment, it is used to treat ovine skins, such as white lamb skins. The liming step a) may be carried out using conventional liming agents, such as sulfides, followed by washing with water.

The de-liming and bating step b) may be carried out using conventional de- liming agents, such as ammonium chloride, and bating enzymes, followed by washing with water.

When the pH is adjusted in optional step c), preferably it is decreased. The pH may be adjusted in step c) with the use of any acid, for example an organic acid, such as formic acid, lactic, acid, oxalic acid, citric acid, malic acid, or acetic acid, or an inorganic acid, such as sulphuric acid, nitric acid or hydrochloric acid.

Preferably when the pH is adjusted in optional step c) it is adjusted to 7 or less, more preferably 6 or less, most preferably 5 or less, e.g. 4.5 or less, such as from 2 to 4.

However, the pH adjustment is not essential in the present pre-tanning process. It is possible for the steps to be carried out at a high pH, of higher than 7, such as 8 or higher, due to the removal of the pickle stage. Accordingly, the pre-tanning step d) may be carried out directly after the de-liming step b).

When the pH is adjusted in optional step e), preferably it is increased. The pH may be adjusted in step e) with the use of any base, for example a weak base, such as sodium bicarbonate, potassium bicarbonate, sodium carbonate or potassium carbonate.

When the pH is adjusted in optional step e), preferably it is adjusted to 4 or more, more preferably 4.5 or more, most preferably 5 or more, such as from 5 to 7. The pre-tanning step d) may use any organic tanning agent. Accordingly, the agent may be any organic molecule capable of crosslinking proteins and imparting a degree of hydro lytic stability to collagen.

In one embodiment, the pre-tanning step d) may use a tanning agent which is selected from tetrakis (hydroxymethyl) phosphonium salts, condensates, or derivatives thereof, oxazolidines (including Oxazolidine A and Oxazolidine E), and aldehydes (including glyoxal, formaldehyde, glutaraldehyde and modified glutaraldehyde). A single product may be used or a combination of two or more such products may be used.

As the skilled man would appreciate, the term "modified glutaraldehyde" refers to glutaraldehyde that has been chemically reacted or physically blended with other chemicals to reduce/control its reactivity. This process typically involves the use of chemicals such as alcohols, glycols, polyglycols and/or aldehydes. Several commercially available "modified glutaraldehyde" products are known in the art.

In one embodiment, the pre-tanning step d) uses a tetrakis (hydroxymethyl) phosphonium salt, condensate or derivative thereof as the tanning agent.

The pre-tanning step d) may use any tetrakis (hydroxymethyl) phosphonium salt as tanning agent. In principal, any water soluble THP salt with an anion which does not interact adversely with THP or other components of the tannage, or with the leather may be used. It is preferred that anions which are oxidising agents for THP should be substantially absent. Preferably, a tetrakis(hydroxymethyl)phosphonium salt of formula THPX, wherein X is chloride, sulphate, bromide, iodide, phosphate, acetate, oxalate, citrate, borate, chlorate, lactate, nitrate, fluoride, carbonate or formate is used.

The pre-tanning step d) may use any tetrakis (hydroxymethyl) phosphonium condensate as tanning agent. THP condensates are water soluble or sparingly water soluble copolymers of THP with organic nitrogen compounds, such as urea or an amine. In one embodiment, the condensate is a copolymer of THP with urea, a C1-C20 alkylamine, dicyandiamide, thiourea or guanidine. The molar ratio of THP to nitrogen comound may be, for example, 2: 1 or higher, such as 3 : 1 or higher, preferably 4: 1 or higher, such as 5 : 1 or higher, for instance from 5 : 1 to 7: 1 molar THP:nitrogen compound.

THP condensates may contain two or more phosphorus atoms, so long as the phosphorus compound is water soluble to a concentration of at least 0.5 g/1 at 25O⁰C. Such phosphorus compounds contain a total of at least two hydroxymethyl groups, usually at least one per phosphorus atom, and preferably at least two hydroxymethyl groups per phosphorus atom. In the THP condensate the group or groups joining the phosphorus atoms together may be of the formula -R-, -R-O-, -R-O-R-, -R-NH-R or -R-R"-R where R is an alkylene group of 1 to 4 carbon atoms and R" is the residue formed by removal of two hydrogen atoms, bonded to nitrogen, from a di or polyamide or an amine or di or polyamine, such as urea, a C1-C20 alkylamine, dicyandiamide, thiourea or guanidine. Such compounds with two or more, e.g. three, hydroxyalkyl groups per phosphorus atom may be made by self condensation of THP salts with a compound of general formula R"H2 such as urea, or a C1-C20 alkylamine, e.g. by heating at 40 to 1200⁰C.

A THP condensate may be formed in situ by adding THP or a THP salt and a suitable nitrogen containing compound, e.g. guanidine or dicyandiamide, to the tannage.

The pre-tanning step d) may use any THP derivative as tanning agent. In this regard, derivatives of THP or THP salts where one or more of the hydroxymethyl groups is replaced may be mentioned. Preferably, only one of the hydroxymethyl groups is replaced. The THP salt from which the derivative is obtained may be one of those THP salts mentioned above.

The replaced hydroxymethyl group may be replaced with an organic substituent group which does not react chemically with collagen. The replacement group may, for example, be a C 1-20 alkyl, alkenyl, aryl, aralkyl, alkaryl, polyalkyleneoxy, alkylpolyalkyleneoxy or polyalkyleneoxy alkyl group. In particular, the replacement group may be a branched or straight chain alkyl group, aryl group or aromatic group. These groups may in particular be C1-C18 groups, such as C2-C16 groups, e.g. C3-C12 groups, for example C3, C4, C5, C6, C7 or C8 groups. The replacement groups may optionally contain one or more functional groups, for example the replacement groups may optionally contain one or more functional groups independently selected from sulphonate, carboxylate and phosphonate.

Most preferably, tetrakis (hydroxymethyl) phosphonium sulphate (THPS), tetrakis (hydroxymethyl) phosphonium chloride (THPC) or tetrakis (hydroxymethyl) phosphonium phosphate (THPP) is used as the tanning agent in step d).

The amount of tanning agent (e.g. tetrakis (hydroxymethyl) phosphonium salt, condensate or derivative) used, as a percentage by weight based on the weight of the skin to be treated, is preferably from 0.25 to 10%, more preferably from 0.5 to 5%, such as from 1 to 4%, e.g. from 1.5 to 3%.

The pre-tanning step d) may use any stabilising agent. The stabilising agent is a compound that will oxidise the phosphorus from the tanning agent from the +3 to the +5 oxidation state. In particular, the stabilising agent is a compound that will convert both phosphorus that has reacted with the skin and unreacted phosphorus from the +3 to the +5 oxidation state. The stabilising agent may suitably be selected from oxidising agents such as peroxide, perborate, perchlorate, percarbonate and permanganate compounds; in particular Group I peroxide, perborate, perchlorate, percarbonate and permanganate compounds. For example, hydrogen peroxide may be mentioned, and alkali metal salts of peroxide, perborate, perchlorate, percarbonate and permanganate, such as sodium or potassium salts thereof, e.g. sodium percarbonate and sodium perborate.

Preferred stabilising agents are peroxides and perborates.

The amount of stabilising agent used, as a percentage by weight based on the weight of the skin to be treated, may be from 0.1 to 10%, preferably from 0.2 to 5%, such as from 0.3 to 2%, more preferably from 0.4 to 1%, most preferably from 0.5 to 0.8%, e.g. from 0.6 to 0.7%.

The tanning agent and stabilising agent may be used simultaneously, sequentially or separately. Preferably, the skin is treated with stabilising agent after it has been treated with tanning agent, i.e. the tanning agent and the stabilising agent are added sequentially or separately, with the tanning agent being added first. Alternatively, the tanning agent and stabilising agent may be added together.

The pre-tanning step d) optionally includes treating the skin with iii. one or more masking agent.

Any masking agent may be used in this optional treatment. Organic masking agents for use in tanning are well known in the art. Water soluble compounds are preferred. Generally it is preferred to use masking agents that do not contain nitrogen. Typical masking agents are organic compounds containing at least one carboxyl group and preferably one or more hydroxyl groups, including carboxylic or hydroxycarboxylic acids, and water soluble salts thereof, such as alkali metal salts thereof. Specific masking agents that may be mentioned are alkali metal salts of acetic, citric or tartaric acids, and salts of a polyhydroxy monocarboxylic acid.

Other masking agents that can be used are aliphatic polyhydroxy or polypyrolidone compounds, in particular aliphatic polyhydroxy or polypyrolidone compounds comprising at least three adjacent, geminal or vicinal groups:

where R is hydrogen or an alkyl group, e.g. a C1-C20 alkyl group, and X is a hydroxyl or pyrolidone group. Preferably, the polyhydroxy or polypyrolidone compound has a molecular weight less than 200,000, more preferably less than 150,000, most preferably less than 100,000, e.g. less than 75,000, such as less than 50,000.

For example, the masking agent may be glycerol, sorbitol, trimethylol propane, pentaerythritol, 1 ,2, 3-trihydroxy butane, 1 ,2,3,4-tetrahydroxy butane, 1 ,2, 3-trihydroxy pentane, 2,3,4-trihydroxy pentane, tetrahydroxy pentane, pentahydroxy pentane, water soluble carbohydrates including mono and disaccharides and soluble starches and alginates. Examples of suitable carbohydrates include sucrose, glucose, maltose, lactose, dextrose, fructose, xylose, arabinose, mannose, ribose andrhamnose and dialdehyde starch. The compound may also be a derivative of a carbohydrate, including alkyl glycosides, such as C1-C25 alkyl glucoside or polyglucoside, or ascorbic, mannonic or gluconic acids, or alginates or sorbitol or mannitol or C1-C25 acyl esters of either sucrose or sorbitan.

The amount of masking agent used, as a percentage by weight based on the weight of the skin to be treated, is preferably from 0.25 to 8%, more preferably from 0.5 to 5%, such as from 1 to 4%, e.g. from 1.5 to 3%.

Where a masking agent is used, the masking agent and tanning agent may be added simultaneously, sequentially or separately. In one embodiment, they are added simultaneously. In another embodiment, they are added sequentially, with the masking agent being added either immediately before or immediately after the tanning agent.

The pre-tanning step d) may optionally also include treating the skins with: iv. one or more acid.

Any suitable acid may be used, for example an organic acid, such as formic acid, lactic, acid, oxalic acid, citric acid, malic acid, or acetic acid; preferably formic acid.

The pre-tanning step d) may optionally also include treating the skins with: v. one or more salt.

Any suitable salt may be used, for example sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, sodium phosphate or potassium phosphate; preferably sodium chloride.

The pre-tanning step d) may optionally also include treating the skins with: vi. one or more complexing agent. Any suitable complexing agent may be used, for example an agent that complexes iron, such as a phosphonate that will complex iron, e.g. an aminomethylene phosphonate.

The process may comprise the further step of: f) piling, grading and packing the skins.

This step is carried out after step d) and optional step e). In contrast, in conventional processes, piling, grading and packing are carried out before the pre-tanning step.

The process may then comprise the further step of: g) shaving the skins.

The process may then comprise the further step of: h) degreasing the skins.

When step h) is carried out, preferably aqueous degreasing is used. In particular, step h) may suitably involve one or more washing steps using water. It may also involve the use of one or more conventional degreasing agents, such as surfactants.

The degreasing step h) may be carried out at lower temperatures than used conventionally. For example, the step h) may be carried out at 3O⁰C or less, such as 25⁰C or less, e.g. 2O⁰C or less. This is advantageous not only in that by reducing the temperature needed for efficient degreasing energy costs are reduced, but also there is reduced shrinkage leading to increased are yield and superior grain quality.

Alternatively, the degreasing step h) may be carried out at more conventional temperatures, such as 45⁰C or less, e.g. 4O⁰C or less, such as 35⁰C or less. Surprisingly, the process of the present invention results in superior degreasing. Lower grease levels and lower levels of free fatty acid residuals are achieved. Additionally, it provides easier & better processing to produce superior quality leather goods.

The process may then comprise the further step of: i) tanning the skins.

When step i) is carried out, it may suitably involve the use of one or more conventional tanning agents, such as mineral tanning agents, synthetic tanning agents, vegetable tanning agents or organic tanning agents.

The present invention provides, in a second aspect, a process for tanning skins comprising the steps of:

A. liming and washing the skins;

B. deliming, bating and washing the skins; and

C. tanning the skins; wherein the step C of tanning the skins involves treating the skins with: i. one or more organic tanning agent; and ii. one or more stabilising agent.

The tanning process of the present invention does not require the use of a pickling treatment. Accordingly, in a preferred embodiment, the process does not include a pickling stage.

The omission of the pickling stage removes the need for the use of acids (which can be dangerous to handle) and salts (which can cause effluent problems). Furthermore it reduces the time and labour required to complete the tanning process. There will be cost decreases in relation to: the reduction in processing time and labour input, and the need for fewer chemicals to be used. Additionally, the present process minimizes pH fluctuations through the process, and so maintains the natural grain quality. By allowing a smooth, continuous decrease in pH from de-liming onwards, excessive looseness in the skins and loss of mechanical strength (tensile and tear) is avoided. In particular, this permits efficient processing of "casualty cows" and new season pelts which have loose skins.

The tanning process may be used to treat any suitable skins that require tanning. In one embodiment, it is used to treat bovine skins.

The preferred features described in relation to steps a) and b) of the method of the first aspect apply equally to the steps A and B of the method of the second aspect.

The preferred features described above in relation to pre-tanning step d) of the method of the first aspect (e.g. in relation to the compounds used and amounts thereof, and in relation to the additional optional compounds that may be used, and amounts thereof) apply equally to the tanning step C of the method of the second aspect.

In the method of the second aspect, the skin may optionally undergo pre- tanning after deliming step B and prior to tanning step C.

If pre-tanning is carried out, then one or more of the optional steps e) to h) as described above in relation to the first aspect may be carried out after pre-tanning and prior to tanning.

Typical measured benefits of the processes of the present invention include: • Up to 35% reduction in processing times

• Up to 80% reduction in salt use • Up to 20% reduction in water use

• Reduced temperatures during degreasing

• Residual grease levels below 5%

• Residual free fatty acid levels below 1%.

Accordingly, the invention further provides, in a third aspect, the use of: one or more organic tanning agent; and one or more stabilising agent in a pre-tanning or tanning process, to remove the need for a pickling stage.

The invention also provides, in a fourth aspect, the use of: one or more organic tanning agent; and one or more stabilising agent in a pre-tanning or tanning process, to reduce the process time.

The invention also provides, in a fifth aspect, the use of: one or more organic tanning agent; and one or more stabilising agent in a pre-tanning or tanning process, to reduce the salt and/or water used in the process.

The invention also provides, in a sixth aspect, the use of: one or more organic tanning agent; and one or more stabilising agent in a pre-tanning or tanning process, to reduce the residual grease levels below 5% and/or to reduce the residual free fatty acid levels below 1%..

In each of these uses, the preferred features of the organic tanning agent and stabilising agent are as described above in relation to the first aspect. In each of these uses, a masking agent may optionally be used in combination with the tanning agent and the stabilising agent. In this regard, the preferred features of the masking agent are as described in relation to the first aspect.

In each of these uses, the tanning agent and stabilising agent may be used simultaneously, sequentially or separately. In particular, the tanning agent may be used prior to the stabilising agent.

The invention will now be further described, in order to illustrate the invention, by the following non-limiting examples.

Example 1

Pre-tanning of wet white lamb skins with aqueous degreasing at low temperatures

Method

Results

Independent analysis of the wet white lamb skin produced gave the following results (as determined using BS 1309 test methodology) :

• 3.9 % grease (target was <5%)

• 0.62% free fatty acids (target was <1%) . Conclusion

The process in accordance with the invention, with no pickling stage and carrying out aqueous degreasing at low temperature, gave good results in terms of grease levels and free fatty acid levels, both being significantly below the target values.

Example 2

Pre-tanning of wet white lamb skins at high pH (straight from de-liming - no pH adjustment)

Method

Results

Analysis of the wet white lamb skin produced gave the following results (as determined using BS 1309 test methodology) :

• 0.43% residual free fatty acid (target was <1%) .

Conclusion

The process in accordance with the invention, with no pickling stage and carrying out pre-tanning at high pH, i.e. avoiding the need for pH adjustment steps before and after pre-tanning and the associated use of acid and base, gave good results in terms of free fatty acid levels, significantly below the target values.

Example 3

Tanning of bovine skins

Method

Step Details

Results

The THPx successfully tanned the skins directly from the delime and bate, without the need for a pickling step.

Claims

1. A process for pre-tanning skins comprising the steps of: a) liming and washing the skins; b) deliming, bating and washing the skins; c) adjusting the pH, if required; d) pre-tanning the skins; and e) adjusting the pH, if required; wherein the step d) of pre-tanning the skins involves treating the skins with: i. one or more organic tanning agent; and ii. one or more stabilising agent, wherein the skins are treated with the stabilising agent after being treated with the tanning agent.

2. The process of claim 1 , wherein the skins treated are ovine skins.

3. The process of claim 1 or claim 2, wherein the pH is adjusted in step c) to 5 or less.

4. The process of claim 1 or claim 2, wherein the pH is not adjusted in step c), with the pre-tanning step d) being carried out directly after the de- liming step b).

5. The process of any one of the preceding claims wherein the tanning agent used is selected from tetrakis (hydroxymethyl) phosphonium salts, condensates, or derivatives thereof; oxazolidines; and aldehydes.

6. The process of claim 5 wherein the tanning agent used is: a tetrakis(hydroxymethyl)phosphonium salt of formula THPX, wherein X is chloride, sulphate, bromide, iodide, phosphate, acetate, oxalate, citrate, borate, chlorate, lactate, nitrate, fluoride, carbonate or formate; or a tetrakis (hydroxymethyl) phosphonium condensate which is a copolymer of THP with urea, a C1-C20 alkylamine, dicyandiamide, thiourea or guanidine.

7. The process of claim 6 wherein the agent is tetrakis (hydroxymethyl) phosphonium sulphate, tetrakis (hydroxymethyl) phosphonium chloride or tetrakis (hydroxymethyl) phosphonium phosphate.

8. The process of claim 5 wherein the tanning agent used in step d) is a derivative of THP or THP salts where one of the hydroxymethyl groups is replaced.

9. The process of claim 8 wherein the replacement group is a C 1-20 alkyl, alkenyl, aryl, aralkyl, alkaryl, polyalkyleneoxy, alkylpolyalkyleneoxy or polyalkyleneoxy alkyl group.

10. The process of any one of the preceding claims wherein the tanning agent is used in an amount, as a percentage by weight based on the weight of the skin to be treated, of from 0.25 to 10%.

11. The process of any one of the preceding claims wherein the pre- tanning step d) further involves treating the skins with a masking agent.

12. The process of any one of the preceding claims wherein the pre- tanning step d) uses a masking agent in an amount, as a percentage by weight based on the weight of the skin to be treated, of from 0.25 to 8%.

13. The process of any one of the preceding claims wherein the stabilising agent used is an oxidising agent that will oxidise the phosphorus from the tanning agent from the +3 to the +5 oxidation state.

14. The process of any one of the preceding claims wherein the stabilising agent is used in an amount, as a percentage by weight based on the weight of the skin to be treated, of from 0.1 to 1%.

15. The process of any one of the preceding claims wherein the process comprises the further steps, after step d) and optional step e), of: f) piling, grading and packing the skins; g) shaving the skins; and h) degreasing the skins.

16. The process of claim 15, wherein in step h), aqueous degreasing is used.

17. The process of claim 16, wherein step h) is carried out at 3O⁰C or less.

18. The process of any one of the preceding claims wherein the process comprises the further step of: i) tanning the skins.

19. A process for tanning skins comprising the steps of:

A. liming and washing the skins;

B. deliming, bating and washing the skins; and C. tanning the skins; wherein the step C. of tanning the skins involves treating the skins with: i. one or more organic tanning agent; and ii. one or more stabilising agent, wherein the skins are treated with the stabilising agent after being treated with the tanning agent.

20. The process of claim 19, wherein the tanning agent is as defined in any one of claims 5 to 10.

21. The process of claim 19 or 20, wherein the stabilising agent is as defined in claim 13 or 14.

22. The process of any one of claims 19 to 21 , wherein the skins treated are bovine skins.

23. The process of any one of claims 19 to 22 wherein the tanning step C. further involves treating the skins with a masking agent.

24. The process of any one of the preceding claims, which does not include a pickling step.

25. The use of: one or more organic tanning agent; and one or more stabilising agent in a pre-tanning or tanning process for skins, wherein the stabilising agent is used to treat the skins after they have been treated with the tanning agent, to remove the need for a pickling stage.

26. The use of: - one or more organic tanning agent; and one or more stabilising agent in a pre-tanning or tanning process for skins, wherein the stabilising agent is used to treat the skins after they have been treated with the tanning agent, to reduce the process time.

27. The use of: one or more organic tanning agent; and one or more stabilising agent in a pre-tanning or tanning process for skins, wherein the stabilising agent is used to treat the skins after they have been treated with the tanning agent, to reduce the salt and/or water used in the process.

28. The use of: one or more organic tanning agent; and - one or more stabilising agent in a pre-tanning or tanning process for skins, wherein the stabilising agent is used to treat the skins after they have been treated with the tanning agent, to reduce the residual grease levels below 5% and/or to reduce the residual free fatty acid levels below 1%.

29. The use of any one of claims 25 to 28, wherein the tanning agent is as defined in any one of claims 5 to 10.

30. The use of any one of claims 25 to 29, wherein the stabilising agent is as defined in claim 13 or 14.