WO2001009392A1 - Leather tanning - Google Patents

Abstract

Skins are tanned by (1) impregnating them with a THP salt at a pH below that required to cause substantial conversion of the salt to THP (e.g. below 4); (2) contacting the impregnated skins with a nitrogenous compound such as urea or melamine which is capable of copolymerising with THP; and (3) raising the pH sufficiently to effect substantial conversion of the THP salt to THP (e.g. 4.5 to 7).

Description

LEATHER TANNING

The present invention relates to a novel process for tanning leather.

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 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, pickling and/or degreasing.

The skin is then subjected to treatment in one or more stages with various tanning agents selected to give the desired end properties.

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 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. 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 tannages.

For centuries the production of leather was based on the vegetable tannages 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 form of basic chromium sulphate, which produces a blue grey leather with high shrink temperatures. However mineral tannages in general, and chrome tannages in particular are under pressure on environmental grounds. Syntans are less environmentally harmful than mineral tanning agents.

Tetrakis (hydroxymethyl) phosphonium salts which will be referred to herein as "THP salts" have long been used as fire-retardants for textiles and have been applied to the keratinous (fur) side of skins for this purpose. The salts may be applied directly to the fabric or in the form of precondensates which are water soluble or sparingly water soluble copolymers of THP with organic nitrogen compounds such as urea or an amine and which are referred to herein as "THP condensates". THP salts have also been known as possible ingredients of tanning liquors for more than forty years. US 2 732 278 describes tanning with THP chloride (THPC). US 2 992 879 recommended a combination of THPC and a phenol such as resorcinol speculating that the two react together in situ to form an effective tanning agent when the pH is raised. In fact THP salts do copolymerise with phenols such as resorcinol (see, for example, Textile Research Journal, December 1982, P743). US 3 104 151 describes the use of such THPC phenol copolymers as pretannages for leather in which the main tannage is vegetable or mineral. GB 2 287 953 describes the use of THP salts as cross linkers in conjunction with melamine formaldehyde or urea formaldehyde prepolymers, in order to form a copolymeric tanning agent in situ in the tanning liquor. EP 0 559 867 describes the use of phosphonium salts such as THP sulphate

(THPS) on raw or cured skin prior to tanning e.g. in acid degreasing. EP 0 681 030 describes the use of THPS as a cross linker for casein finishes applied to leather after tanning. GB 2 314 342 describes the use of hydroxyalkyl phosphines and phosphonium salts as tanning agents in conjunction with aromatic anionic syntans.

THP salts are stable under acidic conditions in the absence of air or oxidising agents. At pH above 3 and in the absence of oxidising agents they are gradually converted to the parent base, tris(hydroxymethyl)phosphine herein referred to as THP. Conversion is rapid and substantially complete between pH of about 4 and 6. Above pH 7, or in the presence of oxidising agents THP salts or THP are converted to tris(hydroxymethyl)phosphine oxide (THPO), conversion being rapid and substantially complete at pH above about 10, e.g. 12. It has been stated, e.g. in US 2 993 744, that THPO is the effective tanning agent in THP based tannages.

Contrary to statements in the art, THPO is not effective as a tanning agent for leather, and THP salts are also ineffective as tannages. THP on its own is an effective main tanning agent which is usually most effective when formed in situ by first impregnating the leather with a THP salt., in the substantial absence of monomers or prepolymers which react or copolymerise with THP and raising the pH above 4 and preferably above 5.

In addition to THP, compounds of the formula R P(CH OH) , referred to herein as "THP analogues", where R is an organic group which does not react chemically with collagen, such as a Cι_-20 alkyl, alkenyl, aryl, aralkyl, alkaryl, polyalkyleneoxy, alkylpolyalkyleneoxy or polyalkyleneoxy alkyl group are highly effective tanning agents.

For convenience "THP" will be used herein, where the context permits, to refer generically to THP, THP condensates and THP analogues, which are active tanning agents and "THP salts" will be used to include the salts of THP condensates and THP analogues which are formed at low pH and are essentially inactive as tanning agents. 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 which soften and lubricate the finished leather.

A problem with THP is that it is highly reactive and tends to give a tight, hard leather, which is difficult to lubricate with fat liquor or to retan.

By mixing THP with various modifying agents a softer, fuller leather can be obtained. For instance it has been proposed to use THP with nitrogenous copolymerisable compounds such as urea, amines, dicyandiamide or melamine. (See for example EP O 808 908).

However when the THP is used in conjunction with such modifying agents the shrink temperature is often undesirably low. Moreover it has not been found possible to raise the shrink temperature appreciably by retanning with THP and/or syntans.

There is a need for a process of tanning that can provide the advantages of THP and can achieve full, soft leather with high shrink temperatures.

We have now discovered that these objects are achieved when skins are pretreated with a THP salt for a sufficient time to impregnate the skin thoroughly with sufficient of the salt to provide some or all of the THP requirement of the tannage, while maintaining the pH below that at which significant conversion to THP occurs and are subsequently contacted with a nitrogenous compound prior to basification sufficient to convert the THP salt to THP.

The pretreatment does not involve a pretannage since the THP salt is not an active tanning agent, and no significant elevation of shrink temperature is observed until the skins are basified. Our invention comprises: (1) impregnating skins with THP salt whilst maintaining the pH below that required to cause substantial conversion of the salt to THP; (2) contacting the impregnated skins with a nitrogenous compound capable of condensing or copolymerising with THP; and (3) raising the pH sufficiently to effect substantial conversion of the THP salt to THP.

In the first stage the skins are contacted with a THP salt at a pH below 4 and preferably between 0 and 3 e.g. 1 to 2. The skins are maintained in contact with the aqueous THP salt, preferably with agitation and optionally at an elevated temperature below the shrink temperature e.g. 20 to 50°C until the salt has penetrated evenly throughout the skin.

The penetration may be tested by contacting a sample with selenious acid. A pale yellow colour extending across the whole thickness of the skin indicates that the impregnation is complete. Typically the first stage may require from 20 minutes to 5 hours to complete e.g. 30 minutes to 4 hours, preferably 1 to 2 hours.

The salt may be any salt of THP which is water soluble and stable. The nature of the counterion is not important provided that it does not oxidise THP or adversely affect leather. Its choice is therefore determined largely by economic considerations. The cheapest and most commonly available salt of THP is THPS and this is therefore generally preferred but THPC or the corresponding phosphate, phosphite, nitrate, bromide, acetate, formate, borate, citrate or lactate, or any other THP and leather- compatible anion that forms a water soluble salt, may in principle be used.

The salt is preferably the tetrakis(hydroxymethyl)phosphonium salt itself, but may alternatively be a salt of a THP condensate or analogue. THP condensates which are suitable include condensates with urea, melamine, fatty amines dicyandiamide, guanidine and thiourea. The second stage involves contacting the impregnated skin with a nitrogenous compound capable of condensing with THP. Particularly preferred is melamine, however any of the condensable nitrogen compounds listed above may be employed.

Part of the total THP offer is preferably supplied by including a THP salt in the second stage in admixture with the nitrogen compound.

The basification step may be effected with any convenient base. Typically sodium carbonate or bicarbonate, borax or dolomite will be preferred on economic grounds, but potassium bicarbonate or potassium carbonate could also be used. Ammonium or alkali metal hydroxides and ammonium carbonate are preferably not used but would work. Other alkalis, alkaline salts or bases such as sodium silicate, lime or organic bases could be used but are generally not preferred.

Sufficient base is added to raise the pH above 3 and preferably above 4.

High pH is preferably avoided because it converts the phosphine group to phosphine oxide, which is substantially ineffective as a tanning agent. We prefer that solutions for use according to our invention should have a pH less than 10, more preferably less than 9, especially less than 8, typically less than 7.5, most preferably 4.5 to 7. We prefer that the THP contains less than 15%, more preferably less than 10%, e.g. less than 9% of THPO based on the weight thereof. Generally the less THPO present the better.

The THP is preferably used in a total concentration of from 0.01 to 20% by weight based on the total weight of the tanning liquor, more preferably 0.5 to 10%, e.g. 1 to 5%, most preferably 1.5 to 4%. The total offer of the THP in stages 1 and 2 is preferably from 0.3 to 20% by weight based on the weight of wet skin, more preferably 1 to 15%, especially 1.5 to 10%, most preferably 2 to 5%.

It is possible to supply all the THP as the salt in stage 1. Alternatively a proportion of the total offer may be mixed with or added immediately before or after the nitrogen compound in stage 2. The proportion by weight of the total used in stage 1 may thus vary from 1 to 100%, preferably more than 5%, e.g. 10 to 40%.

The nitrogen compound is preferably present at a concentration of from 0.5 to 35% by weight of the tanning liquor, e.g. 1 to 20%, more preferably 2 to 10% especially 3 to 6%. The total proportion of nitrogen compound used is preferably from 1 to 20% by weight based on the wet weight of skins, e.g. 2 to 10% especially 3 to 5%.

The proportion by weight of THP to nitrogen compound may typically be from 1 :10 to 10:1, preferably 1 :5 to 2:1, especially 1 :2 to 1 :1. The total proportion of tannages used is preferably from 2 to 20% active weight based on the wet weight of skins, e.g. 3 to 10%, especially 4 to 8%. The total tannage used preferably comprises more than 80% by weight, more preferably more than 90% by weight, e.g. more than 95% of the THP and nitrogen compound. Where white leather is required, we prefer that the total tannages consist essentially of the THP and nitrogen compound. In particular we prefer the leather is not tanned with vegetable or mineral tannage.

The THP salt in stage 1 is preferably applied in the substantial absence of monomers or prepolymers capable of copolymerising with the THP such as phenol, urea, melamine or their precondensates with formaldehyde. For the purposes of this specification, "the substantial absence of monomers or prepolymers" means less than the minimum that would be capable of reacting or copolymerising with 50% of the THP. more preferably less than the minimum that would be required to react or copolymerise with 20%, e.g. less than 5% by weight based on the weight of THP, most preferably less than 2%, especially less than 1%.

The leather may be retanned using syntans, e.g. by treating the skin after stage 3 with said syntan. Alternatively the syntan (preferably a syntan which does not react with THP under normal tanning conditions) may be added at stage 2.

For the purpose of this specification "syntan" is used to refer to replacement syntans which are synthetic organic compounds capable of reacting with collagen at two or more sites to form cross links and also to auxiliary syntans which do not in themselves contribute substantially to the cross linking but which are physically absorbed by the leather or react at no more than one site so as to modify the physical properties of the leather. For example the term includes any water soluble polymer prepared by copolymerising formaldehyde, which is capable of increasing the shrink resistance of collagen and which comprises at least two units of the formula

M where each M is an aryl group such as a phenyl,

naphthyl or aniline group substituted with one or more hydroxyl and/or sulphate, sulphone or sulphonimide groups or a urea or melamine residue. As used herein the term "syntan" also includes resin syntans which are homopolymers and copolymers of unsaturated carboxylic acids or their salts, esters, amides or nitriles, e.g. acrylic acid, methacrylic acid, acrylamide, acrylonitrile, maleic acid, fumaric acid, itaconic acid, aconitic acid, crotonic acid, isocrotonic acid, citraconic acid, mesaconic acid, angelic acid, tiglic acid and cinnamic acid. The copolymers may also comprise other vinylic comonomers such as styrene. Also included are acetone condensates with, for example sulphones and sulphonamides. Resin syntan may modify such properties as dye levelling, filling, grain appearance, break, grain strength, buffing nap, softness and tensile or tear strength.

The tanned leather may be fat liquored, dyed and/or submitted to other operations conventional in the manufacture of leather.

The invention will be illustrated by the following examples in which percentages unless stated to the contrary are expressed as percentages by weight based on the wet weight of skins.

200 gm unsplit pickled bovine pelt was immersed in 1200ml water, various concentrations of THP salt, either THPS or the sulphate salt of THP urea condensate were applied at pH 3.6 and the mixture rotated for 2 hours. A mixture of THP urea condensate and melamine was then added, while maintaining the pH below 3.5. The mixture was rotated overnight at room temperature and then basified to pH 6.5.

The results are recorded in the following table.

In each case the leather was full and soft compared with leather tanned with 3% THP only. In the absence of THP pretreatment the THP + melamine tannage gave shrink temperatures of only 67°C. These temperatures were not significantly increased by retanning with THP.

Claims

1. A method of tanning which comprises (1) impregnating skins with THP salt whilst maintaining the pH below that required to cause substantial conversion of the salt to THP; (2) contacting the impregnated skins with a nitrogenous compound capable of condensing or copolymerising with THP; and (3) raising the pH sufficiently to effect substantial conversion of the THP salt to THP.

2. A method according to claim 1 wherein the pH in stage 1 is between 0 and 3.

3. A method according to either of claims 1 and 2 wherein the impregation stage (1) is continued for from 30 minutes to four hours.

4. A method according to any foregoing claim wherein the THP salt is a salt of a condensate of THP with urea, melamine, dicyandiamide guanidine, thiourea or fatty amine.

5. A method according to any foregoing claim wherein the nitrogenous compound of step (2) is any of those specified in the claim 4.

6. A method according to any foregoing claim wherein a further amount of the THP salt is included in the stage (2).

7. A method according to any foregoing claim to wherein the basification stage (3) is effected using sodium bicarbonate, borax, and/or sodium carbonate.

8. A method according to any foregoing claim where the pH is raised in stage 3 to a value from 4.5 to 7.

9. A method according to any foregoing claim wherein the total offer of THP in stages (1) and (2) is from 1 to 15% by weight of the wet skins.

10. A method according to any foregoing claim wherein the proportion of the total offer of THP added in stage (1) is from 1 to 100%.

11. A method according to any foregoing claim wherein the proportion of the nitrogen compound used in stage (2) is from 1 to 20% by weight of wet skins.