Biologically degradable leather

Details Biologically degradable leather Biologically degradable leather

1999-08-23

2001-07-03

Gupta, Yogendra N. (Department: 1751)

Bleaching and dyeing; fluid treatment and chemical modification

Treatment of hides, skins, feathers and animal tissues

Tanning

C008S436000, C008S437000, C428S473000

Reexamination Certificate

active

06254644

ABSTRACT:

The invention relates to biodegradable leather and to a process for its preparation.
Tanning converts animal skins, with crosslinking of the collagen, into leather. One of the most important features of the leather is the increased shrinkage temperature compared with untanned skins, i.e. the enhanced hot-water resistance, and the white appearance (not transparent) after drying.
The method of tanning still predominant today is chrome tanning, in which covalent bonds having a crosslinking effect are formed with the carboxyl groups of the collagen using chromium(III) compounds under the influence of OH ions. By contrast, the hydrogen bonds to the amide groups of the collagen that are obtainable using polyfunctional vegetable tanning agents are much weaker, which is evident inter alia in an only moderately increased shrinkage temperature. Aliphatic aldehydes too, such as glutaraldehyde, for example, which lead to crosslinking by way of primary amino groups of the collagen, have been recommended as tanning agents (U.S. Pat. No. 2,941,859).
The use of aliphatic diisocyanates such as hexamethylene diisocyanate (DE-C 72,981) has, for toxicological reasons, not become established.
The use of bisulphite-blocked aliphatic, cycloaliphatic or aromatic diisocyanates such as hexamethylene diisocyanate, isophorone diisocyanate and tolylene diisocyanate as tanning agents, as is recommended in U.S. Pat. No. 2,923,594 and 4,413,997, does indeed lead to leathers which are light in colour and in the case of the aliphatic isocyanates are even light-fast, but the tanning liquors are not pH-stable.
On the part of the automotive industry there is an increasing desire for leathers which on the one hand are free from heavy metals and on the other hand can be disposed of without problems, for example by composting. Since leather, according to a definition from Stather and Pauligk, is “skin which no longer rots” (Ges. Abh. des deutschen Lederinstituts Freiburg 17 (1962), p. 37), it would appear that there is a preprogrammed conflict of interests here: compostable leather would not correspond to the said definition, and it would have to be assumed that such “leathers” would also lack some of the properties which make them suitable for everyday use. This implies that tanning, inter alia, is also to be understood as a method of preservation.
According to the guidelines of the German state working committee on waste (LAGA 10), (conventionally prepared) leather is among those substances which must not be disposed of in composting plants since they are not biodegradable and owing to- their heavy metal content represent a hazard to the humus and the ground water.
The search for biodegradable tanning agents of natural origin is under way (see for example H. Oertel, G. Reich, L. Meyer, E. Lange “Vegetabilische Gerbstoffe nach Ma&bgr;—ein Beitrag zur Sicherung der Rohstoffgrundlage der Lederindustrie” [Vegetable tanning agents to order—a contribution to securing the raw materials base in the leather industry], Das Leder 1994, pp. 188 to 198). However, even tanning agents of natural origin are not eo ipso biodegradable, since they are produced by the plant to protect against attack by microbes and fungi and against being eaten; cf. J. B. Harborne, Ökologische Biochemie, Heidelberg 1995, p. 170.
Meeting the requirements placed on biodegradable leather appeared to be barely possible:
On the one hand, the customer of course expects even biodegradable leathers to have complete suitability for use; on the other hand, the preservation of such leathers gives rise to problems. Leather is a hydrophilic material having a very large internal surface area, and therefore constitutes an ideal nutrient base for bacteria and fungi. During their growth, microbes frequently release enzymes which damage the material. For reasons of hygiene as well, microbes and fungi which grow in the leather are undesirable.
One of the consequences of tanning is that the biological breakdown of protein is prevented. The biodegradation of tanned product has therefore appeared to date to be a contradiction in terms.
From DE-A 4 422 246 it is known that leather can be biologically degraded with thermophilic microorganisms in the presence of oxygen. Inorganic constituents are obtained in this case as the chromium(III) oxide or chromium(III) salt and can be passed to a recovery stage. This process constitutes a decisive step in the direction of the biodegradability of leather. However, it sets requirements which are often difficult to fulfil in terms of the microorganisms which can be used and of the inorganic constituents.
It has now been surprisingly found that it is possible without the use of chromium, titanium, iron, aluminium and zirconium tanning agents to obtain biodegradable and yet serviceable leathers if low-salt pretanning or tarming is carried out with a reactive organic tanning agent and then substantially or completely biodegradable products are used for the subsequent processes.
The invention therefore provides a process for preparing leather, according to which
I. Pelts are (pre)tanned with
a) aldehydes or
b) bisulphite-blocked polyisocyanates,
II. if desired, the resulting product is (re)tanned with
a) polyaspartic acid, its salts and/or its anhydrides and/or
b) polyaspartamides,
III.
a) the resulting product is bottomed with polyurethane and auxiliaries based on natural substances,
b) a finish of polyurethane and/or polyesteramide is applied, and
IV. the leather thus dressed is aftertreated, if desired, with a leather preservative.
Tanning agents and assistants selected within the context of the invention are preferably products which in accordance with DIN 54 900, Part 3 (draft) are at least 30, preferably at least 50 and, in particular, at least 60% by weight biodegradable. With preference, at least 60% by weight of the sum of all tanning agents and assistants employed is biodegradable.
Preferred aldehydes I a) comprise formaldehyde, acrolein, crotonaldehyde, glyoxal, glutardialdehyde and aldehydes obtainable by the oxidation of fats—i.e. compounds and mixtures as described, for example, in F. Stather, “Gerbereichemie und Gerbereitechnologie”, Akademie Verlag Berlin 1967, p. 477 ff.
Preferred “bisulphite-blocked polyisocyanates” I b) are the products of reaction of
A. organic polyisocyanate,
B. based on isocyanate equivalent of A, from 0 to 0.4 equivalents of polyether alcohol with incorporated polyalkylene oxide units (the equivalents are based on the hydroxyl groups of the polyether alcohol), from 40 to 100, preferably from 50 to 100 mol-% of the polyalkylene oxide units consisting of polyethylene oxide units with a sequence length of from 5 to 70, preferably from 6 to 60 and, in particular, 7 to 40,
C. if desired, other NCO-reactive components, and
D. ammonium or alkali metal bisulphites or disulphites.
The reaction products I b).above can be obtained from the intermediates, themselves obtainable from A, B and, if desired, C, having NCO contents of from 3 to 50, preferably from 5 to 45 and, in particular, from 20 to 45% by weight (based on intermediate) by subsequent blocking of the free isocyanate groups. The product I b) then contain—calculated as sodium salt and based on solids—from 9.7 to 78, preferably from 14 to 74 and, in particular, from 46.5 to 74% by weight of carbamoylsulphonate groups.
Suitable organic polyisocyanates A) are aliphatic, cycloaliphatic, araliphatic, aromatic or heterocyclic polyisocyanates as are described, for example, by W. Siefken in Liebigs Annalen der Chemie 562, pages 75 to 136.
Preferred polyisocyanates A) are compounds of the formula Q(NCO)
n
having a mean molecular weight of below 800, in which n is a number which is at least 2, preferably from 2 to 4, Q is an aliphatic C
4
-C
12
-hydrocarbon radical, a cycloaliphatic C
6
-C
15
-hydrocarbon radical, an araliphatic C
7
-C
15
-hydrocarbon radical or a heterocyclic C
2
-C
12
radical having 1 to 3 heteroatoms from the series oxygen, sulphur and nitrogen, for example (i) diisocyanates such as ethylene diisocyanate, 1,4-tetramethylene diis

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