Mixture of THP and chrome as tanning agents

Details Mixture of THP and chrome as tanning agents Mixture of THP and chrome as tanning agents

2002-01-10

2004-02-03

Einsmann, Margaret (Department: 1751)

Bleaching and dyeing; fluid treatment and chemical modification

Treatment of hides, skins, feathers and animal tissues

Tanning

C008S094250, C008S094260, C008S094330, C008S09410P

Reexamination Certificate

active

06685747

ABSTRACT:

This application is a U.S. National Phase Application under 35 USC 371 of International Application PCT/EP00/00841 (published in English) filed Feb. 2, 2000.
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 untanned 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, acrilamide 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. 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 give rise to effluent problems which may require expensive treatment to avoid environmental damage. Syntans are less environmentally harmful than mineral tanning agents but are typically less cost effective. Formaldehyde and difunctional aldehydes present a health hazard and are unpleasant to handle.
Tannages are usually applied to skins at levels of from about 3% to 20% by weight based on the wet weight of the skins. In the case of chrome tans a typical level for a main tan would be 8%. Where chrome is used in combination tannage, e.g. as a retannage to complete the tanning of leather previously treated with a vegetable tannage or syntan, the concentration may be as low as 4% based on the wet weight of skins. Little or no effective tanning is normally expected at concentrations below 3% tannage.
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 condensates of THP with organic nitrogen compounds such as urea or an amine. Such condensates are referred to herein as THP condensates.
THP condensates may contain 2 or more phosphorus atoms, so long as the condensate is water soluble to a concentration of at least 0.5 g/l at 25° C. Such condensates 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, melamine or guanidine. Such condensates 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.
U.S. Pat. No. 2,992,879 recommended a combination of THP chloride (THPC) and a phenol such as resorcinol to form an effective tanning agent when the pH is raised. THP salts copolymerise with the phenols. U.S. Pat. No. 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 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.
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, or at lower pH in the presence of atmospheric oxygen.
In our copending application PCT/EP98/06837 we have described the use of THP and THP condensates as highly effective main tanning agents. The THP may be 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 an effective tanning system, which is ecologically friendly. We believe that hydroxymethyl phosphine species, other than THP, THPO and THP salts and condensates may be formed in the leather as intermediates during the tanning process when THP or its condensates contact collagen. Where the context permits, references herein to THP should be construed as including references to any such species so formed, or added to the tannage.
A benefit of THP is the tight quality of the leather. Hitherto this desirable characteristic has only been achievable in conjunction with relatively low area yield, that is to say, the total area of usable leather obtained is reduced, as a result of shrinkage. With some mineral tannages area yields are increased relative to the area of the original skin. Area yield is an important economic consideration.
Another characteristic of THP based tannages is that the maximum shrink temperatures that have been achieved, even in combination with syntans, does not match those attainable with mineral tannages. This has limited the use of THP to applications in which hydrothermal stability is not a primary concern.
A particular problem with chrome tannages is the formation of chrome soaps when they react with grease in the skin. These cause unsightly staining of the skins, which staining persists through the proce

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