Mixture of THP and a moderator to tan leather

Details Mixture of THP and a moderator to tan leather Mixture of THP and a moderator to tan leather

2002-02-06

2004-12-07

Einsmann, Margret (Department: 1751)

Bleaching and dyeing; fluid treatment and chemical modification

Treatment of hides, skins, feathers and animal tissues

Tanning

C008S09419C, C008S094200, C008S094240, C008S094250, C008S094260, C008S094270, C008S09410P

Reexamination Certificate

active

06827745

ABSTRACT:

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 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, 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 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 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 and EP 0 808 908 describes the use of THP salts with condensable nitrogen compounds.
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 commonly 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 U.S. Pat. No. 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. Moreover THP used in conjunction with co-condensable monomers or polymers provides complex systems which are difficult to control to obtain consistent results.
WO99/23261 describes the use of THP on its own as an effective main tanning agent. THP 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. WO99/23261 also describes the use of THP condensates as main tanning agents.
In addition to THP, compounds of the formula R P(CH
2
OH)
2
, referred to herein as “THP analogues”, where R is an organic group which does not react chemically with collagen, such as a C
1-20
alkyl, alkenyl, aryl, aralkyl, alkaryl, polyalkyleneoxy, alkylpolyalkyleneoxy or polyalkyleneoxy alkyl group have been referred to in the literature as highly effective tanning agents.
For convenience “THP” will be used herein, where the context permits, to refer generically to THP, THP salts, THP condensates and THP analogues. A major problem when tanning with derivatives of formaldehyde, including formaldehyde condensate syntans and THP is the evolution of formaldehyde during tanning and the presence of formaldehyde residues in the leather which give rise to objectionable odours and may cause a health hazard to process operators.
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. It has been found that leather which has been tanned with THP alone is tight and tends to be relatively hard. It requires high levels of fat liquor to soften it. It would be useful to be able to obtain the advantages of THP, but prepare a fuller, softer and more easily fat liquored leather.
It is often necessary to split, e.g. bovine leather, prior to retanning, in order to obtain the desired thickness. Conventionally tanned leather may split unevenly and the split leather may require substantial shaving to obtain a consistent thickness.
A further problem is swelling of the skin, control of which usually requires the presence of salts such as chloride or sulphate.
One object of the present invention is to provide tannages which exhibit the advantages of THP but provide leather which i

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