Tanning leather

Details Tanning leather Tanning leather

2000-04-14

2003-04-01

Gupta, Yogendra N. (Department: 1751)

Bleaching and dyeing; fluid treatment and chemical modification

Treatment of hides, skins, feathers and animal tissues

Treatment of untanned skins or hides

C008S09419C, C008S094110, C008S094130, C008S094150, C008S094170, C008S094180, C008S094330, C106S015050

Reexamination Certificate

active

06540790

ABSTRACT:

The present invention relates to a novel process for tanning leather using phosphonium salts. 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 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. 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 but typically give an off white or yellowish leather. Formaldehyde and difunctional aldehydes present a health hazard and are unpleasant to handle. Existing tanning processes cannot readily provide a good quality, pure white leather suitable for use as such or for dyeing, at least using environmentally acceptable and economically viable ingredients.
An object of the invention is to provide an environmentally acceptable tanning process which can produce white leather with high shrink temperatures e.g. above 80 and preferably above 85°. A further object is to produce a leather which is readily dyed, to give clear, light shades.
Tetrakis(hydroxymethyl)phosphonium salts which will be referred to generically 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. Such copolymers are referred to herein as THP condensates. THP condensates may contain 2 or more phosphorus atoms, so long as the phosphorus compound is water soluble to a concentration of at least 0.5 g/l at 25° C. Such phosphorus compounds contain a total of at least 2 hydroxymethyl groups, usually at least one per phosphorus atom, and preferably at least 2 hydroxymethyl groups per phosphorus atom. 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 C
1-20
alkylamine, dicyandiamide, thiourea or guanidine. Such compounds with 2 or more, e.g. 3, hydroxyalkyl groups per phosphorus atom may be made by self condensation of THP salts with a compound of general formula R″H
2
such as urea, or a C
1 to 20
alkylamine, e.g. by heating at 40 to 120° C. THP salts have also been known as possible ingredients of tanning liquors for more than thirty six years. U.S. Pat. No. 2,992,879 referred to THP chloride (THPC) as an unsatisfactory tanning agent on its own, and recommended a combination of THPC and a phenol such as resorcinol speculating that the two react together 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). U.S. Pat. No. 3,104,151 describes the use of such THPC phenol copolymers as light 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 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 for casein finishes applied to leather after tanning.
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 that THPO is an effective tanning agent.
We have now discovered that contrary to statements in the art, THPO is not an effective main tanning agent for leather, and that THP salts are also ineffective as main tannages. However we have found that THP and THP condensates are both highly effective main tanning agents. THP is 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. THP provides a particularly effective tanning system, which is ecologically friendly. We have found that THP gives leather which is especially suitable for dyeing. THP is especially effective in combination with syntans. In the absence of other tanning agents, especially vegetable tannages, mineral tannages, aldehydes and phenols the use of THP, alone or with syntans, and especially resin syntans, results in a particularly white leather.
The invention provides a method of tanning which comprises reacting collagen with THP or a THP condensate in a main tannage or retannage.
According to a preferred embodiment this invention provides a method of tanning in which skins which have been subjected to partial tanning in a pretanning operation are contacted with a THP salt in the substantial absence of a monomer or prepolymer capable of reacting or copolymerising therewith or of an oxidising agent capable of oxidising THP to THPO and at a pH below 4, and the pH is raised above 4, preferably above 4.5, more pr

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