GB2394229A - Leather tanning - Google Patents

Abstract

A method of tanning a delimed and bated skin or hide, comprising the steps of adding the delimed and bated skin or hide to a float and keeping the skin or hide in the float until the pH of the float equilibrates to that of the skin or hide; adding a tanning agent to the float at the equilibrated pH; continuing the tanning process without externally adjusting the pH to achieve a tanned skin or hide. The tanning agent may particularly be a phosphonium or phosphine compound.

Description

LEATHER TANNING

This invention relates to a novel method for tanning leather and in particular to a method of tanning which does not include pickling of the 5 skin or hide.

Tanning is a process for the preservation of skins or hides, by which is meant the collagen-containing integuments of vertebrates including mammals (e. g. cows, pigs, deer, goats, sheep, seals, antelope, mink, 10 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 15 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 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 chemical steps generally include liming, bating, pickling and decreasing.

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

30 The delimed skin or hide is then bated, which uses enzymes to open up pores in the skin or hide and aid tanning. Some skins or hides must be

degreased after the bating step and this is typically performed by means of one or more solvents.

The next stage in the tanning process is usually the pickling stage, 5 wherein the skins or hides are treated with an acid to prepare them for tanning. The pickling process typically takes the pH of a delimed and bated skin or hide from its initial pH of 8 to 8.3 down to a pH of about 3.0. Typically this reduction of pH is achieved by adding an acid, such as sulphuric acid, in a sodium chloride solution, to the skin or hide.

When a tanning agent is added to the skin or hide 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 hide 15 or skin so that tanning is achieved.

The skin or hide can be treated with a number of different tanning agents, according to the desired properties of the finished leather.

20 The main types of tannage are:- vegetable tannage, based on tannin as the active cross linking agent; mineral tannage using various polyvalent metal salts, especially salts of chromium, aluminium, iron, or zirconium; and synthetic tanning agents, referred to as "syntans". Syntans include replacement syntans, which are active tanning agents capable of tanning 25 leather when used as the sole tannage e.g. by reacting with collagen at two or more sites to form cross-links, and auxiliary syntans which are added to other tannages to modify the character of the leather but which are not in themselves active tanning agents. Auxiliary syntans are absorbed by the leather or may react with collagen at one site only.

30 Syntans include various polymers and copolymers, such as those obtained by condensing formaldehyde with, for example, phenols and/or aryl

sulphonates, and acrylate, methacrylate, acrylamide and/or acrylonitrile homopolymers and copolymers. Formaldehyde itself, and dialdehydes such as glutaraldehyde, are also used in tanning, usually in combination with other tonnages.

For centuries the production of leather was based on the vegetable tonnages, which produce the characteristic brown colour traditionally associated with leather. One of the first mineral tannages was alum, but currently the most widely used tanning agent is chrome, usually in the 10 form of basic chromium sulphate, which produces a blue-grey leather with a high shrink temperature. However, mineral tonnages in general, and chrome tannages in particular, are becoming less acceptable on environmental grounds.

15 The steps required to produce leather, including the pickling and degreasing which usually precede tanning, remove most of the natural oils and fats from leather. These are normally at least partially replaced after tanning by fat liquoring, which entails contacting the leather with an aqueous emulsion of oils and fats to soften and lubricate the finished 20 leather.

It has hitherto been the opinion of those skilled in the art that, if the pH of the skin or hide were not lowered by pickling prior to the addition of the tanning agent, the tanning agent would react immediately with the 25 surface of the skin or hide. There would accordingly be no penetration of the skin or hide and only the surface would be tanned.

However, the present applicants have found that it is possible to produce tanned leather in a way which enables the pickling stage to be omitted.

Accordingly, the present invention provides a method of tanning a delimed and bated skin or hide, the method comprises the steps of: (i) adding the delimed and bated skin or hide to a float and keeping the skin or hide in the float until the pH of the float equilibrates to 5 that of the skin or hide; (ii) adding a tanning agent to the float at the equilibrated pH; (iii) continuing the tanning process without externally adjusting the pH to achieve a tanned hide or skin.

10 The method does not require the skin or hide to be pickled, the tanning agent acting directly on the delimed and bated skin.

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 15 problems). Furthermore it reduces the time required to complete the tanning process. There is also no Gasification required once the tanning agent has been added. gasification has hitherto been required to activate the tanning agent added at a low pH. Omission of pickling and basification reduces the time and labour required to complete the tanning 20 process. There are also cost reductions as fewer chemicals are used.

One further benefit of tanning delimed hides in the manner described is the achievement of a smooth, continuous decrease in pH from deliming to retannage, which avoids excessive looseness in the skins and presents the 25 skins in a form suitable for retannage without extra pH modifications.

Preferably, the skin or hide is washed in cold water before being added to the float, this deactivates the bate by removing any chemicals/enzymes remaining from the bating process. The low temperature of the wash 30 takes the bating enzyme out of its optimal temperature range for activity, thereby deactivating it.

When the delimed and bated skin or hide is added to the float, it typically has a pH of about 8.3. When the skin or hide has been kept in the float, the equilibrated pH of the float and skin is about 8.0.

The float may be between 50 and 200% water, and is preferably 100% water, however this may vary considerably depending on the process vessel and the skin loading.

10 The tanning agent is added to the equilibrated float, typically at an offer concentration of between about 1 to 8% by wet weight of skin or hide (based upon a 40% active solid THPX derivative being offered).

Alternatively, the tanning agent may be added at between about 2 and 6% by wet weight of skin or hide. Preferably, the tanning agent is added at a 15 concentration of about 4% by wet weight of skin or hide.

The tanning may be carried out at a cold or ambient temperature.

Alternatively, more elevated temperatures up to 35 C could increase the speed of reaction.

When the tanning agent is added to the float containing the hide or skin it is activated over a period of time and consequently the pH in the system reduces. This natural pH reduction acts as a controlling mechanism for the tannage enabling the product to penetrate before significant grain 25 tanning occurs. The pH at the end of tanning usually lies in the region of pH 5.0 - 5.5 without external pH control, enabling a smooth progression into the retanning process.

Preferably, a pretannage is not required.

Preferably a fat liquor and/or syntan and/or resin may be added during the tannage to enhance the appearance and handle characteristics of the resulting wet white.

5 Preferably, the tanning agent is a phosphonium compound having the general formula (1): [Yn P ( CH2OHl-n]x (I) 10 wherein n is a positive integer of from 1 to 4; X is an anion; and Y is an organic residue comprising a hydrophilic group.

The hydrophilic group, for example, may be selected from unsaturated or saturated, aromatic or aliphatic, derivatives of C, to C,O carboxylic acids, 15 phosphoric acids, sulphonic acids, acid phosphates, monohydric or polyhydric alcohols.

Suitably, Y may be selected from C, to C,O groups containing polyethylene glycol and/or polypropylene glycol moieties.

General formula (I) shows group "Y" in its unionised form. There will be analogous formulae, readily apparent to those skilled in the art, which represent the ionised forms.

25 The anion X may be, for example, chloride or sulphate. Alternatively, X may represent any other anion (which results in the product of formula (I) being water-soluble) including for example, bromide, iodide, phosphate, acetate, oxalate, citrate, borate, chlorate, nitrate, fluoride, carbonate and formate.

Alternatively, the tanning agent may be a phosphine compound having the general formula (II): 5 Yn P (CH2oH)3-n (II) wherein n is a positive integer of from l to 3; and Y is as described for formula (I). Such phosphines can typically be generated by action of a base on a phosphonium compound of formula (I).

Phosphonium compounds of formula (I) or phosphine compounds of formula (II) may be prepared by the reaction of a tetrakis(hydroxyorgano) phosphonium salt (THP+ salt), a base and an organic compound including a hydrophilic group, for example, an unsaturated or saturated, aromatic 15 or aliphatic, Cat to C,O carboxylic acid, phosphoric acid, sulphonic acid, acid phosphate or monohydric or polyhydric alcohol.

In particular, the THP+ salt and the base may be reacted with an unsaturated carboxylic acid, or an ester or salt of an unsaturated 20 carboxylic acid.

Alternatively, phosphonium or phosphine compounds may be prepared by reacting a tetrakis(hydroxyorgano) phosphonium salt (THIS salt) and an organic compound including a hydrophilic group, for example, an 25 unsaturated or saturated, aromatic or aliphatic, C, to C,O carboxylic acid, phosphoric acid, sulphonic acid, acid phosphate or monohydric or polyhydric alcohol - without the addition of a base.

Phosphonium or phosphine compounds may also be prepared by reacting a 30 THP+ salt and a base with an alkyl halide which contains at least one

reactive halogen and at least one group which imparts hydrophilic character. Alternatively, the phosphonium or phosphine compounds may be prepared 5 by reacting a THP' salt with an alkyl halide which contains at least one reactive halogen and at least one group which imparts hydrophilic character - without addition of a base.

Preferably, the alkyl halide comprises one to ten carbon atoms. The 10 group which imparts hydrophilic character is preferably derived from an unsaturated or saturated, aromatic or aliphatic, C' to C,O carboxylic acid, phosphoric acid, sulphonic acid, acid phosphate or monohydric or polyhydric alcohol. Alternatively, the group imparting hydrophilic character may be selected from C, to C,O groups containing polyethylene 15 glycol and/or polypropylene glycol moieties.

The tetrakis(hydroxyorgano)phosphonium salt (THP. salt) is preferably a tetrakis(hydroxymethyl)phosphonium salt of formula THPX, where X is an anion as described above with reference to formula (I). The THP+ salt 20 may be, for example, tetrakis(hydroxymethyl)phosphonium chloride (THPC), sulphate (THPS), acetate (THPA), oxalate (THPO) or phosphate (THPP), although any water soluble THP+ salt can be used.

Preferably, the conditions of the reaction to produce the phosphonium or 25 phosphine compounds are optimised to encourage mono, rather than di or tri, substitution of the THP.

Preferably an unsaturated carboxylic acid, preferably a mono-carboxylic acid such as acrylic acid, may be reacted with a THP+ salt in the presence 30 of a base. In some cases, the addition of a free base is not essential.

Other unsaturated mono-carboxylic acids which may be used to make a compound of formula (1) and/or formula (II) include methacrylic acid, crotonic acid, isocrotonic acid, angelic acid and tiglic acid.

5 Alternatively, the unsaturated carboxylic acid may be a di-carboxylic acid such as maleic acid, fumaric acid, citraconic acid, mesaconic acid, itaconic acid or glutaconic acid.

Alternatively an alkyl halide, such as 2-bromopropionic acid, may be 10 reacted with a THE salt in the presence of a base.

There are many alkyl halides, which contain a hydrophilic group, which could be used. These will be readily apparent to those skilled in the art.

15 Preferred examples of compounds of formula (I) and/or formula (II) include the mono-substituted and all-substituted adducts of THPX and acrylic acid or of THPX and 2-bromopropionic acid.

Compounds of formula (II) may be formed when a hydroxymethyl group 20 is removed from a phosphonium compound of formula (I). Removal of a hydroxymethyl group can occur due to hydrolysis of a phosphonium compound. Hydrolysis can arise due to the addition of water or an increase in pH.

25 The compounds of formula (I) and/or formula (II) may also be the reaction product of a tetrakis(hydroxyorgano) phosphonium salt, a base and vinylindene- l, l - diphosphonic acid (VDPA) or a salt or an ester of VDPA.

Phosphonium compounds produced by this reaction may have the general formula (III): M203P 5 \CH-CH2 P. CH2OH X- (III)

M2O3P /

n 4-n wherein: 10 M is hydrogen, an alkali metal, an alkaline earth metal, a polyvalent metal, ammonium or a quaternised amine radical and each M may be the same or different; X is an anion having the same significance as in formula (I); n is a number having a positive average value of up to 4. The number n 15 need not be an integer.

Examples of polyvalent metals in formula (111) above include transition metals such as copper, chromium, iron, titanium, or zirconium, as well as other metals such as aluminium, while examples of quaternised amine 20 radicals include salts derived from neutralization with amines e.g. triethanolamine and quaternary ammonium bases e.g. tetrabutyl ammonium hydroxide.

Phosphonium compounds of formula (III) may be readily oxidised, for 25 example, by peroxides or atmospheric oxygen, to the corresponding phosphine oxide, with the loss of formaldehyde and a proton. Phosphine oxides of this type are believed to be novel.

These phosphine oxides may be represented by the general formula (IV): / M 20 3P\ \ \l CH-CH2tP(CH2OH) M2O3P in 3-n (IV) 5 wherein M has the same significance as in formula (III); and n is a number having a positive average value of up to 3.

The phosphonium compounds of formula (1) and formula (III), and/or the phosphine compound of formula (II), may be made by the method 10 described in our co-pending application no: PCT/GB2002/003907.

Alternatively, the tanning agent may be a tetrakis(hydroxyorgano) phosphonium salt (THP' salt), for example a tetrakis(hydroxymethyl) phosphonium salt of formula THPX, where X is 15 an anion as described above with reference to formula (1). The THPX may be, for example, tetrakis(hydroxymethyl)phosphonium chloride (THPC), sulphate (THPS), acetate (THPA), oxalate (THPO) or phosphate (THPP).

20 Alternatively, the tanning agent may be a THPX/organic nitrogen compound condensate. Preferably, the organic nitrogen compound is urea or thiourea.

Although it is not intended that the present invention be construed with 25 respect to any particular theory, it is believed that under normal conditions the THP+ salt or THP+/organic nitrogen compound condensate forms the corresponding tris(hydroxyorgano)phosphine, with evolution of

an acid corresponding to the anion of the THPt salt and consequent lowering of the pH.

The present invention also provides a tanned hide or skin produced by the 5 method hereinabove described.

The present invention will be illustrated, merely by way of example, as follows: 10 EXAMPLE I

A tannage was carried out on a delimed and bated full-substance and grainsplit belly hide which had been cold water washed to deactivate the bate. The hide was then added to a loot float and kept until the pH of 15 the hide and float equilibrated to between pH 8.0 to 8.5.

Three floats containing hides were prepared and dosed with a 1:1 adduct of THPC and acrylic acid (40% solids) as the tanning agent. In the first float, the tanning agent was added at an offer of 4% by wet weight of 20 hide. In the second float, the tanning agent was added at an offer of 6% by wet weight of hide, and in the third float, the tanning agent was added at an offer of 8% by wet weight of hide.

Immediately after adding the tanning agent the pH (across the cross 25 section of the hide) was between 7.5 and 8.3, and the shrinkage temperature of the delimed and bated bellies was 58 C. The floats and skins were kept rotating overnight. The skins were then tested for shrinkage temperature, which was shown to have risen to: 67-68 C for full-substance hide dosed at 4% by wet weight.

70 C for full-substance hide dosed at 6% by wet weight.

71-72 C for full-substance hide dosed at 8% by wet weight.

and s 65OC for grain-split samples dosed at 4% by wet weight.

68-69 C for grain-split samples dosed at 6% by wet weight.

72-730 C for grain-split samples dosed at 8% by wet weight.

10 In addition, after the overnight rotation, the pH of the float had lowered to between 4.5 and 5.8 and between 5.2 and 6.0 on the cross section.

This pH is ideal to proceed to retannage without externally adjusting the pH.

Claims (1)

1. A method of tanning a delimed and bated skin or hide, the method comprising the steps of: 5 (i) adding the delimed and bated skin or hide to a float and keeping the skin or hide in the float until the pH of the float equilibrates to that of the skin or hide; (ii) adding a tanning agent to the float at the equilibrated pH; (iii) continuing the tanning process without externally adjusting 10 the pH to achieve a tanned skin or hide.

2. A method according to Claim 1, in which the method does not

include pickling of the skin or hide.

15 3. A method according to Claim 1 or Claim 2, in which the skin or hide is washed in cold water before it is added to the float.

4. A method according to Claim 1, 2 or 3, in which the float is between 50 and 200 % water.

5. A method according to Claim 4 in which the float is 100% water.

6. A method according to any preceding claim in which a fat liquor is added to the float during tannage.

6. A method according to any preceding claim in which a syntan is added to the float during tannage.

6. A method according to any preceding claim in which a resin is 30 added to the float during tannage.

l 9. A method according to any one of the preceding claims, in which the tanning agent is added at an offer of about 1% to about 8% by wet weight of skin or hide for a 40% solids product. l 5 10. A method according to any one of Claims 1 to 8, in which the tanning agent is added at an offer of between about 2% to about 6% by wet weight of skin or hide.

11. A method according to any one of Claims 1 to 8, in which the 10 tanning agent is added at an offer of about to by wet weight of skin or hide. 12. A method according to any one of the preceding claims, in which the tanning process is carried out at ambient temperature.

13. A method according to any of Claims 1 to 11, in which the tanning process is carried out at a temperature of up to about 35 C.

14. A method according to any one of the preceding claims, in which 20 the tanning agent is a phosphonium compound having the general formula (I) : [ Yn P ( C04 n]X (I) wherein n is a positive integer of from 1 to 4; X is an anion; and Y is an organic residue comprising a hydrophilic group.

/ 15. A method according to any of Claims 1 to 13, in which the tanning agent is a phosphine compound having the general formula (II): 5 Yn P (CH2oH)3-n (II) wherein n is a positive integer of from 1 to 3; and Y is an organic residue including a hydrophilic group.

10 16. A method according to Claim 14 or Claim 15, in which Y is selected from unsaturated or saturated, aromatic or aliphatic, derivatives of C, to C,O carboxylic acids, phosphoric acids, acid phosphates, sulphonic acids and monohydric or polyhydric alcohols.

15 17. A method according to Claim 14 or Claim 16 when dependent upon Claim 14, in which X is chloride, sulphate, bromide, iodide, phosphate, acetate, oxalate, citrate, borate, chlorate, nitrate, fluoride, carbonate or formate. 20 18. A method according to Claim 14 or Claim 15, in which the phosphonium or phosphine compound is the reaction product of a tetrakis(hydroxyorgano) phosphonium salt with an unsaturated or saturated, aromatic or aliphatic, C, to C,O carboxylic acid, phosphoric acid, sulphonic acid, acid phosphate or monohydric or polydydric 25 alcohol, together with a base.

19. A method according to Claim 14 or Claim 15, in which the phosphonium or phosphine compound is the reaction product of a tetrakis(hydroxyorgano) phosphonium salt and an unsaturated or 30 saturated, aromatic or aliphatic, C, to C,O carboxylic acid, phosphoric

acid, sulphonic acid, acid phosphate or monohydric or polydydric alcohol. 5 20. A method according to Claim 14 or Claim 15, in which the phosphonium or phosphine compound is the reaction product of a tetrakis(hydroxyorgano)phosphonium salt with an alkyl halide containing at least one reactive halogen and at least one group which imparts hydrophilic character, together with a base.

21. A method according to Claim 14 or Claim 15, in which the phosphonium or phosphine compound is the reaction product of a tetrakis(hydroxyorgano) phosphonium salt and an alkyl halide containing at least one reactive halogen and at least one group which imparts 15 hydrophilic character.

22. A method according to any one of Claims 18 to 21, in which the tetrakis(hydroxymethyl) phosphonium salt is tetrakis(hydroxymethyl) phosphonium chloride (THPC), sulphate (THPS) acetate (THPA) or 20 phosphate (THPP).

23. A method according to Claim 18 to Claim 22 when dependent upon Claim 14, in which the unsaturated carboxylic acid is a mono-carboxylic acid. 24. A method according to Claim 23, in which the acid is acrylic acid.

25. A method according to any of Claims 1 to 13, in which the tanning agent is a tetrakis(hydroxyorgano)phosphonium salt (THEA salt).

26. A method according to Claim 25 in which the THP+ salt is a tetrakis(hydroxymethyl)phosphonium salt of formula THPX, wherein X is chloride, sulphate, bromide, iodide, phosphate, acetate, oxalate, citrate, borate, chlorate, nitrate, fluoride, carbonate or formate.

27. A method according to any one of Claims I to 13, in which the tanning agent is a THPX/organic nitrogen compound condensate.

28. A method according to Claim 27 wherein the organic nitrogen 10 compound is urea or thiourea.

29. A method according to any one of the preceding claims, in which one or more retannage agents are added at the end of the tanning process.

15 30. A method according to Claim 29, in which the retanned leather is fat liquored and/or dyed.

31. A method of tanning a delimed and debated skin or hide, i substantially as hereinbefore described with reference to the Example(s).

32. A method according to Claim l, substantially as hereinbefore described. 33. A tanned hide or skin produced by the method of any one of the 25 preceding claims.