EP0061420B2 - Method of retanning leather with acrylic oligomers - Google Patents

Description

The present invention relates to a process for retanning leather with acrylic-based oligomers.

From US-A-4 150 944 a method for retanning leather is known, in which a reaction product is used as the tanning agent, which is obtained by reacting one of e.g. B. diphenyl ether, phenol and oleum previously formed sulfonation with z. B. dimethylol urea or formaldehyde is available.

From DE-A-2 127 959 a process for retanning leather is also known, in which polyampholytic acrylic polymers are used as the tanning agent which contain (a) acidic carboxyl groups, (b) basic amino groups, (c) resulting from e.g. B. acrylic monomer-derived linear, carbon-containing polymer structure and (c) have an isoelectric point in the pH range between 2.5 and 4.5, in particular copolymers and terpolymers of methacrylic acid and terpolymers of methacrylic acid and quaternary ammonium salts such as. B. dimethylaminoethyl methacrylate hydrochloride and optionally acrylates such. B. methyl methacrylate and ethyl acrylate can be used, these copolymers and terpolymers have been prepared in the absence of sulfites or hydrogen sulfites.

Furthermore, US Pat. No. 3,945,792 describes a process for the aftertreatment of tanned leather with fillers which are prepared from homo- or copolymeric acrylic acids which have been prepared in the absence of sulfites or hydrogen sulfites, e.g. B. consist of a copolymer of acrylic acid and ethyl acrylate, such fillers are mandatory in a mixture with protein glue.

Finally, FR-A-1311 602 also discloses a process for retanning leather, in which the tanning agent is ammonium or amine salts of polymers of acrylic or methacrylic acid, alkyl esters and, if appropriate, other comonomers, such as, for. As acrylonitrile or methacrylonitrile, which have also been produced in the absence of sulfites or hydrogen sulfites, in particular ammonium salts of copolymers of methacrylic acid and ethyl acrylate can be used.

it has now been found that, for retanning leather, instead of the reaction product according to US Pat. No. 4,150,944, a particularly easily accessible oligomer with at least equivalent tanning activity can advantageously be used and instead of the acrylic compounds according to DE-A-2 127 959, US -A-3 945 792 and FR-A-1 311 602, an acrylic-based oligomer with an average molecular weight of at most 14,000 can also be used for this purpose, whereby a leather is obtained which has good light fastness, a compact grain and a has a soft grip.

(4)-SO₃M₁ und gegebenenfalls

aufweist worin X₁-CN, -COOR₁, -OOCR₂ oder -CONHR₃, Y₁-COOH oder -COOM₂, Y₂-CONH₂, -CH₂OH, -OCH₃ oder -OC₂H₅, Z₁, Z₂ und Z₃ je Wasserstoff, Methyl oder Aethyl, M₁ und M₂ je ein Amin-, Ammonium- oder Alkalimetall-Kation und R₁, R₂ und R₃ je Alkyl, Hydroxyalkyl oder Alkoxyalkyl mit insgesamt höchstens 8 Kohlenstoffatomen bedeuten, und das behandelte Leder fettet und trocknet, wobei vor oder nach dem Fetten das behandelte Leder gegebenenfalls noch gefärbt wird.The present invention now relates to a process for retanning chrome-tanned, optionally dyed leather, in which the leather is treated with an aqueous solution containing an acrylic compound as the tanning agent, which is characterized in that an oligomer is used as the tanning agent average molecular weight is at most 14,000, the oligomer being produced using sulfites or hydrogen sulfites and structural elements of the formulas

(4) -SO₃M₁ and optionally

has wherein X₁-CN, -COOR₁, -OOCR₂ or -CONHR₃, Y₁-COOH or -COOM₂, Y₂-CONH₂, -CH₂OH, -OCH₃ or -OC₂H₅, Z₁, Z₂ and Z₃ each hydrogen, methyl or ethyl, M₁ and M₂ each represent an amine, ammonium or alkali metal cation and R₁, R₂ and R₃ each denote alkyl, hydroxyalkyl or alkoxyalkyl with a total of at most 8 carbon atoms, and the treated leather greases and dries, the treated leather possibly being colored before or after the greasing becomes.

The oligomers which have structural elements of the formulas (1), (2) and (4) are known per se and, for. As described in US-A-3 646 099 as conductive and surface active compounds. The oligomers which, in addition to the structural elements (1) and (2) additionally contain the structural element of the formula (3), are known from US Pat. No. 2,893,977 as a component of resin compositions with good adhesive properties.

Although in the two cited patents the oligomers are prepared in the presence of sulfites or hydrogen sulfites, they have structural elements of the formula (4) in one case and not in the other. Based on analytical studies contain oligomers which, after the end of known production in the presence of sulfites or hydrogen sulfites, by adding strong acids, e.g. As hydrochloric acid, are precipitated from their aqueous solutions, no or only traces of structural elements of the formula (4). It must therefore be assumed that sulfite or hydrogen sulfite after the preparation of the oligomers in aqueous solution has ended, is present exclusively or at least mainly as a mixture component and not as a structural element of the oligomer in the form of the formula (4). Since oligomers reprecipitated with hydrochloric acid, when used as tanning agents, give the same results as non-reprecipitated oligomers, the presence or absence of structural elements of the formula (4) is not a critical feature of the oligomers used according to the invention. For the retanning process according to the invention, however, the oligomers used do not have to be reprecipitated before they are used.

An essential feature of the oligomers which can be used according to the invention, in addition to the presence of the structural elements of the formulas (1), (2), if appropriate (3) and if appropriate (4), is the relatively low average molecular weight, which is at most 14,000, in particular 3,000 to 12,000, is preferably 3,000 to 10,000, in particular 4,000 to 9,000. These relatively low molecular weights of the oligomers used according to the invention compared to high molecular weight polymers which, for. B. find use in the plastics industry for the production of synthetic fibers are caused by the use of sulfites or hydrogen sulfites in the oligomer production.

Because of their average molecular weights, preferred oligomers which are used as tanning agents in the process according to the invention each contain 2 to 75, preferably 5 to 40, in particular 10 to 25 structural elements of the formulas (1) and (2) and 0 to 25, preferably 0 to 10 , in particular 0 to 5 structural units of the formula (3).

aufweisen, worin X₂-CN, -COOR₄, -OOCR₅ oder -CONHR₅, R₄ alkyl, Hydroxyalkyl oder Methoxyalkyl mit insgesamt höchstens 4 Kohlenstoffatomen, R₅ Methyl oder Aethyl und R₆ Alkyl mit 1 bis 8 Kohlenstoffatomen oder Hydroxyalkyl mit 1 oder 2 Kohlenstoffatomen bedeuten, und Z₁ die angegebenen Bedeutungen hat, und insbesondere Strukturelemente der Formel

aufweisen, worin X₃-CN, -COOCH₃, -COOC₂H₅, -COOC₂H₄OH, -COOC₃H₆OH, -COOC₂H₄OCH₃, -OOCCH₃, -OOCC₂H₅, -CONHCH₃, -CONHC₂H₅, -CONHCH₂OH, -CONHC₃H₇, -CONHC₈H₁₇ bedeutet und Z₁ die angegebenen Bedeutungen hat.Above all, with regard to their use as tanning agents, oligomers come into consideration which, in addition to the structural elements of the formulas (2), optionally (3) and optionally (4) structural elements of the formula

have wherein X₂-CN, -COOR₄, -OOCR₅ or -CONHR₅, R₄ alkyl, hydroxyalkyl or methoxyalkyl with a total of at most 4 carbon atoms, R₅ methyl or ethyl and R₆ alkyl with 1 to 8 carbon atoms or hydroxyalkyl with 1 or 2 carbon atoms, and Z₁ has the meanings given, and in particular structural elements of the formula

have, in which X₃-CN, -COOCH₃, -COOC₂H₅, -COOC₂H₄OH, -COOC₃H₆OH, -COOC₂H₄OCH₃, -OOCCH₃, -OOCC₂H₅, -CONHCH₃, -CONHC₂H₅, -CONHCH₂OH, -CONHC₃H₇, -CONHC₈₁ has the meanings given and is CONHC₈H₁

gegebenenfalls

und

auf, worin X₄-CN, -COOCH₃, -COOC₂H₅, -COOC₂H₄OH, -COOC₃H₆OH, -OOCCH₃, -OOCC₂H₅, -CONHCH₃, -CONHC₂H₅, -CONHC₃H₇ und -CONHC₈H₁₇, Y₃-COOH oder -COOM₄, Y₄-CONH₂, -CH₂OH oder -OCH₃, M₃ und M₄ je ein Natrium-, Kalium- oder Ammoniumkation und Z₄, Z₅ und Z₆ je Wasserstoff oder Methyl bedeuten.Structural elements of the formulas have structural elements which are particularly suitable as tanning agents

possibly

and

on, in which X₄-CN, -COOCH₃, -COOC₂H₅, -COOC₂H₄OH, -COOC₃H₆OH, -OOCCH₃, -OOCC₂H₅, -CONHCH₃, -CONHC₂H₅, -CONHC₃H₇ and -CONHC₈H₁ Y, Y₃-COOH or -COOM₂, Y CON-CONOH or -OCH₃, M₃ and M₄ each represent a sodium, potassium or ammonium cation and Z₄, Z₅ and Z₆ each represent hydrogen or methyl.

auf.Oligomers, which are of primary interest as tanning agents, have structural elements of the formulas

on.

worin X₁, Y₁, Y₂, Z₁, Z₂ und Z₃ jeweils die angebenen Bedeutungen haben, stets in Gegenwart der Verbindungen der Formeln $$\text{SO₃(M₁)₂}$$

oder $$\text{HSO₃M₁}$$

worin M₁ die angegebenen Bedeutungen hat, in wässrigem Medium und gegebenenfalls in Gegenwart eines Polymerisations-Katalysators bei höchstens 70 °C umsetzt, und bei dieser Temperatur das wässrige Reaktionsgemisch mit einer Base der Formel $$\text{M₂OH,}$$

worin M₂ die angegebenen Bedeutungen hat, auf einen pH-Wert von 5 bis 7 einstellt.The preparation of the known oligomers is carried out according to methods known per se, which, for. B. in US-A-3 646 099 and US-A-2 893 977 are described by using the compounds of the formulas

wherein X₁, Y₁, Y₂, Z₁, Z₂ and Z₃ each have the meanings given, always in the presence of the compounds of the formulas $$\text{SO₃ (M₁) ₂}$$

or $$\text{HSO₃M₁}$$

wherein M₁ has the meanings given, in an aqueous medium and optionally in the presence of a polymerization catalyst at a maximum of 70 ° C, and at this temperature the aqueous reaction mixture with a base of the formula $$\text{M₂OH,}$$

wherein M₂ has the meanings given, to a pH of 5 to 7.

2 to 75, in particular 5 to 40 and in particular 10 to 25 moles of the starting monomers (5), (6) and optionally (7) per mole or per equivalent of the starting compounds of the formulas (8) or (9) are preferably used. submitted, usually first component (5) and then component (6) and optionally (7) are used in order to avoid homopolymerization of the generally more reactive component (6), then small amounts of the monomers (e.g. B. 1/20 to 1/5 of the total amount of starting compounds of the formulas (5) to (9) of a polymerization catalyst, such as azoisobutylronitrile or preferably peroxides, such as benzoyl peroxide or in particular ammonium peroxodisulfate (ie the compound of the formula (NH₄) ₂S₂O₈) of the aqueous solution of the starting compounds added in portions, for example in 8 to 12 portions, or metered in continuously over the course of 5 to 8 hours, an exothermic polymerization reaction occurring s The reaction mixture is cooled so that the temperature does not exceed 70 ° C, especially 40 ° C. At preferred temperatures of 50 to 55 ° C., the polymerization is generally complete 4 to 6 hours after the catalyst has been added. After the polymerization has ended, the reaction mixture is adjusted to an appropriate pH of about 5 to 7, in particular 6.1 to 6.3, by adding the compound of the formula (10) with cooling so that the temperature indicated above is not exceeded. whereby an aqueous, gel-like solution of the copolymer is obtained. After the addition of the compound of formula (10), the reaction mixture can, if necessary, for the complete removal of any unpolymerized starting compounds, in particular monomers of formula (5), for 6 to 10 hours under reduced pressure (-10 to -1 bar) at 60 can be kept up to 80 ° C.

So z. B. for the preparation of a terpolymer having structural elements of the formulas (1.2), (2.1), (3.1) and optionally (4.1), sodium, potassium or ammonium sulfite or hydrogen sulfite as examples of specific representatives of the starting compounds of formula (8) or (9), acrylonitrile, methacrylonitrile, Aethylacrylnitril, methyl acrylate, ethyl acrylate, methyl methacrylate, ethyl methacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, Methoxyäthylacrylat, vinyl acetate, vinyl propionate, N-methyl acrylamide, N-Aethylacrylamid, N-methylolacrylamide, N-isopropylacrylamide or N -Isooctylacrylamid as examples of specific representatives of the monomer Formula (5), acrylic acid, methacrylic acid or α-ethyl acrylic acid as examples of specific representatives of the monomer of formula (6) and acrylamide, methacrylamide, vinyl methyl ether or allyl alcohol as examples of specific representatives of the monomer of formula (7) mixed polymerized with one another and then with sodium hydroxide, potassium hydroxide or neutralized ammonia as examples of specific representatives of the base of formula (10).

When retanning chrome-tanned leather, it is expedient to proceed according to methods known per se by treating the leather with an aqueous solution which contains at least one oligomer of the type indicated above, rinsing the tanned material in this way if necessary, and then generally greasing it and dries. If desired, a coloring can be carried out before or after the oiling, but preferably before the oiling, provided the leather to be retanned is not already colored. As a rule, 100 to 200, preferably 150 to 200 parts by weight of water and 2 to 50, in particular 5 to 10 parts by weight, calculated on dry substance, of at least one of the oligomers of the type mentioned are used per 100 parts by weight of leather.

Preferred aqueous solutions for carrying out the retanning process, however, preferably contain 1 to 10, in particular 2 to 5, percent by weight of the oligomer as the tanning agent.

Generally, the leather is treated with the aqueous solution containing the oligomer for 1 to 4 hours at 10 ° C to 50 ° C, preferably 10 ° C to 30 ° C. If a coloring of the uncoloured leather with commercially available leather dyes, e.g. B. acid or metal complex dyes is made, the dye is expediently used immediately after retanning by adding the dye of the type specified in the tanning liquor, so that in this case the rinsing is generally not necessary. However, if the leather is not dyed, it is advantageous to the leather after retanning a short, z. B. 15 to 30 minutes of rinsing with water at 10 ° C to 50 ° C, preferably 10 ° C to 30 ° C. The retanned, optionally dyed leather is then washed with a customary, preferably lightfast, fatliquor based on e.g. B. greased sulfonated sperm oil or claw oil. This fat is z. B. at 30 ° C to 80 ° C for about 30 to 90 minutes. After drying at about 30 ° C to 80 ° C, a leather is obtained which has excellent lightfastness, a fine, compact, smooth grain and in particular a soft feel. With undyed leather, a very light leather can also be obtained. The strong bleaching effect achieved during retanning is particularly advantageous on uncoloured chrome leather. In addition, the oligomers used as tanning agents according to the invention have excellent compatibility with other commercial tanning agents, e.g. B. vegetable tanning agents or synthetic retanning agents, so that the latter can be used together with the oligomers.

As chrome-tanned leather, which can be retanned according to the invention, the so-called “wet blue” leather is particularly suitable, with all types of leather, eg. B. calf, cowhide, goat or sheep leather are considered and the chrome-tanned leather before retanning according to the invention, as usual, with z. B. formates or bicarbonates is neutralized.

The percentages and parts in the examples below are weight units.

Manufacturing regulations for oligomers

Instructions A: A solution of 165.3 parts of acrylic acid (2.29 mol), 122.0 parts of acrylonitrile (2.29 mol) and 39.8 parts of a 40% aqueous sodium bisulfite solution (0.153 mol) in 280 Parts of deionized water are heated to 29 ° C. In this solution, 0.96 part (ie a total of 9.6 parts within 2 1/2 hours) of a 1% aqueous ammonium peroxodisulfate solution is added 10 times every 1/4 hour, the reaction mixture being cooled such that it warms up to a maximum of 34 ° C after each addition. The white emulsion so obtained is then diluted with 96 parts of deionized water. The reaction mixture is then cooled to 20 ° C. and kept at this temperature for 7 hours. After this time, the reaction mixture is kept under reduced pressure and inert nitrogen atmosphere for 2 hours to remove unreacted acrylonitrile fractions. The emulsion is then adjusted to a pH of 6 using 287.3 parts of a 30% strength aqueous sodium hydroxide solution (2.15 mol), the reaction mixture being cooled so that its temperature does not exceed 40.degree elevated. After cooling the reaction mixture to 20 ° C., 1000 parts of a light yellow, gel-like, clear, aqueous solution which contains 34% of an oligomer which contains about 15 structural elements of the formula (1.4), about 15 structural elements of the formula (2.2) and traces of the structural element of the formula -SO₃Na.

Instructions B: The procedure described in Instructions A is followed, but a solution of 185.9 parts of acrylic acid (2.58 mol), 64.7 parts of acrylonitrile (1.22 mol) and 19.8 parts of the 40% sodium bisulfite Solution (0.076 mol) in 69.8 parts of water, which with a total of 19.8 parts of the ammonium peroxodisulfate solution in 10 portions, and after dilution with 96 parts of water, with 344 parts of the sodium hydroxide solution (2.58 mol) is moved. This gives 1000 parts of a light yellow, gel-like, clear, aqueous solution which contains 31% of an oligomer which has about 16 structural elements of the formula (1.4), about 34 structural elements of the formula (2.2) and traces of the structural element of the formula SO₃Na.

Instructions C: The procedure described in Instructions A is followed, but a solution of 185 parts of acrylic acid (2.56 mol), 34 parts of acrylonitrile (0.64 mol) and 55.6 parts of the sodium bisulfite solution (0.214 mol) is used in 257 Parts of water, which are mixed with a total of 10.7 parts of the ammonium peroxodisulfate solution in 10 portions and, after dilution with 116.4 parts of water, with 341.3 parts of the sodium hydroxide solution (2.56 mol). 1000 parts of a light yellow, gel-like, clear, aqueous solution which contains 28% of an oligomer which has about 3 structural elements of the formula (1.4), about 12 structural elements of the formula (2.2) and traces of the structural element of the formula -SO₃Na.

Process D: The procedure is as described in instruction A, but a solution of 138.3 parts of acrylic acid (1.92 mol), 127.3 parts of acrylonitrile (2.4 mol), 34.1 parts of acrylamide (0.48 mol ) and 41.6 parts of the sodium bisulfite solution (0.16 mol) in 298.4 parts of water, which together with a total of 11.4 parts of the ammonium peroxodisulfate solution in 10 portions and, after dilution with 92.9 parts of water, 256 parts of the sodium hydroxide solution (1.92 mol) are added. 1000 parts of a light yellow, gel-like, clear, aqueous solution which contains 34% of an oligomer which contains about 3 structural elements of the formula (3.2), about 15 structural elements of the formula (1.4), about 12 structural elements of the formula (2.2) and traces are obtained of the structural element of the formula -SO₃Na.

Instructions E: The procedure described in Instructions A is followed, except that an emulsion of 80 parts of 211 parts of acrylic acid (2.92 mol), 141.5 parts of vinyl acetate (1.64 mol) and 43 parts of the 40% sodium bisulfite solution (0.165 mol) is used ml of water, which is dissolved at 50 ° C with a total of 1.5 parts of ammonium peroxodisulfate in 25 ml of water in 10 portions and, after dilution with 155 parts of water, with 320 parts of the sodium hydroxide solution (2.4 mol). This gives 1,000 parts of a light yellow gel-like aqueous solution containing 40% of an oligomer, which has about 10 structural elements of the formula (1.3), wherein X₄-COOCH₃ and Z₄ are hydrogen, about 18 structural elements of the formula (2.2) and traces of the structural element of Formula -SO₃Na has.

Instructions G: The procedure described in Instructions A is followed, but a solution of 128.4 parts of acrylic acid (1.78 mol), 144.2 parts of methacrylic acid hydroxypropyl ester (1 mol) and 47.4 parts of a 25% strength aqueous potassium hydrogen sulfite solution is used. Solution (0.1 mol) in 100 parts of water with a total of 10 parts of the ammonium peroxodisulfate solution in 10 portions at 60 ° C and, after dilution with 351.5 parts of water, with 218.5 parts of the sodium hydroxide solution (1.63 mol) is added. This gives 1,000 parts of a slightly yellowish, gel-like, clear, aqueous solution which contains 31% of an oligomer which has about 12 structural elements of the formula (1.3), in which X, -COOC₃H₆OH and Z₄ are methyl, about 22 structural elements of the formula (2.2 ) and traces of the structural element of the formula -SO₃Na.

Instructions H: The procedure described in Instructions A is followed, but a solution of 73.6 parts of 2-ethoxyethyl acrylate (0.51 mol), 93.8 parts of acrylic acid (1.30 mol) and 33.6 parts of a 25th %, aqueous potassium hydrogen sulfite solution (0.07 mol) in 60 parts of water, which with 5 parts of the ammonium peroxodisulfate solution in 5 portions at 60 ° C and, after dilution with 565.8 parts of water, with 168, 2 parts of the sodium hydroxide solution (1.26 mol) is added.

1 000 parts of a clear, aqueous solution containing 20% of an oligomer which contains about 7.3 structural elements of the formula (1) in which X₁-COOR₁, R₁ 2-ethoxyethyl and Z₁ are hydrogen, about 18.5 structural elements of the formula ( 2.2) and traces of the structural element of the formula -SO₃Na.

Instructions I: The procedure described in Instructions A is followed, but a solution of 130.7 parts of acrylic acid (1.81 mol), 120.3 parts of acrylonitrile (2.26 mol), 26.3 parts of allyl alcohol (0.40 mol) and 72.6 parts of a 25% aqueous potassium hydrogen sulfite solution (0.15 mol) in 283.5 parts of water, which are combined with a total of 37.8 parts of an ammonium peroxodisulfate solution in 5 portions and, after dilution with 87 parts of water, with 241 , 8 parts of the sodium hydroxide solution (1.81 mol) is added. You get 1,000 parts of a light yellow, gel-like, clear, aqueous solution containing 32% of an oligomer containing about 2.6 structural elements of the formula (3.1), wherein Y₄-CH₂OH and Z₆ are hydrogen, about 15 structural elements of the formula (1.4); about 12 structural elements of the formula (2.2), and traces of the structural element of the formula -SO₃Na.

Procedure J: The procedure described in Procedure A is followed, but a solution of 133 parts of acrylic acid (1.84 mol), 149.4 parts of methyl acrylate hydroxypropyl ester (1.03 mol) and 49 parts of a 25% strength aqueous potassium hydrogen sulfite solution (0 , 10 mol) in 98.4 parts of water, which with a total of 12.2 parts of the ammonium peroxodisulfate solution in 10 portions at 40 ° C. and, after dilution with 461.7 parts of water, with 96.3 parts of a 30% aqueous ammonia solution (1.7 mol) is added. This gives 1,000 parts of a yellowish, gel-like, clear, aqueous solution which contains 31% of an oligomer which has about 10 structural elements of the formula (1.3) in which X₄-COOC₃H₆OH and Z₄ are methyl, about 18 structural elements of the formula (2.1), wherein Z₅ is hydrogen, Y₃-COOM₄ and M₄ is an ammonium cation and has traces of the structural element of the formula -SO₃NH₄.

example 1

100 parts of a chrome-tanned calf leather neutralized in the usual way are aftertreated with a solution of 200 parts water and 5 parts, calculated on dry substance, of the oligomer according to regulation A for 1 1/2 hours at 30 ° C. After a short rinse, the post-treated leather is greased as usual with 4 to 6 parts of a lightfast fatliquor based on sulfonated sperm oil and then dried.

You get a leather of light color, excellent lightfastness, with a soft full grip and with a fine grain.

Example 2

100 parts of chrome-tanned sheep leather are mixed with 200 parts of water at 40 ° C. Then 10 parts, calculated on dry substance, of an oligomer according to regulation B are added to the liquor and left to act on the leather at 40 ° C. for 2 hours. After rinsing, the treated leather is greased with about 5 parts of a conventional fatliquor and then dried. You get a full, supple leather with a closed, firmly adhering scars. This also applies when 10 parts of the oligomer according to regulation B are replaced by 10 parts of the oligomer according to regulation C.

Example 3

wird das Leder bei 60 °C während 30 Minuten gefärbt. Nach dem Absäuern mit 1 Teil 85 %iger Ameisensäure während 30 Minuten bei 50 °C werden in das ausgezogene Färbebad 3 Teile sulfatiertes Klauenöl gegeben und das Rindsleder während 60 Minuten bei 60 °C gefettet. Das Leder wird nun ausgereckt und getrocknet. Nach dem Stollen erhält man ein Leder von guter Fülle mit weichem Griff, kompakten, feinem Narben und egaler, intensiver brauner Färbung. Dies trifft auch zu beim Ersatz von 6 Teilen des Terpolymers gemäss Vorschrift D durch 6 Teile des Terpolymers gemäss Vorschrift E, durch 6 Teile des Copolymers gemäss Vorschrift G, durch 6 Teile des Copolymers gemäss Vorschrift H, durch 6 Teile des Terpolymers gemäss Vorschrift I oder durch 6 Teile des Copolymers gemäss Vorschrift J.100 g of chrome-plated, folded cowhide leather is neutralized in a barrel in 200 parts of liquor in a known manner to such an extent that the pH of the leather remains below 4 on average. The bath is then mixed with 6 parts, calculated on dry substance, of the terpolymer according to regulation D and the neutralized leather is treated at 45 ° C. for 2 hours. In a fresh liquor (300 parts) containing 1 part of the dye of the formula

the leather is dyed at 60 ° C for 30 minutes. After acidifying with 1 part of 85% formic acid for 30 minutes at 50 ° C, 3 parts of sulfated claw oil are added to the dyed bath and the cowhide is greased at 60 ° C for 60 minutes. The leather is now stretched out and dried. After the tunnel, you get a leather of good fullness with a soft feel, compact, fine scars and level, intense brown color. This also applies to the replacement of 6 parts of the terpolymer according to regulation D by 6 parts of the terpolymer according to regulation E, by 6 parts of the copolymer according to regulation G, by 6 parts of the copolymer according to regulation H, by 6 parts of the terpolymer according to regulation I or by 6 parts of the copolymer according to regulation J.

Claims (8)

1. A process for retanning chrome-tanned dyed or undyed leather, in which said leather is treated with an aqueous solution that contains an acrylic compound as tanning agent, which process comprises using as tanning agent an oligomer having an average molecular weight of not more than 14,000, and which has been prepared by using sulfites or hydrogen sulfites and which contains structural units of formulae

wherein X₁ is -CN, -COOR₁, -OOCR₂ or -CONHR₃, Y₁ is -COOH or -COOM₂, Y₂ is -CONH₂, -CH₂OH, -OCH₃ or -OC₂H₅, each of Z₁, Z₂ and Z₃ is hydrogen, methyl or ethyl, each of M₁ and M₂ is an amine cation, an ammonium cation or an alkali metal cation, and each of R₁, R₂ and R₃ is alkyl, hydroxyalkyl or alkoxyalkyl containing a total of not more than 8 carbon atoms, fatliquoring the treated leather and drying it, and, if appropriate, additionally dyeing said treated leather before or after it has been fatliquored.

2. A process according to claim 1, which comprises using as tanning agent an oligomer which contains 2 to 75 structural units of each of formulae

and 0 to 25 structural units of formulae

wherein X₁, Y₁, Y₂, Z₁, Z₂ and Z₃ each have the meanings assigned to them in claim 1.

3. A process according to claim 1, which comprises using as tanning agent an oligomer which contains structural units of formulae

wherein X₂ is -CN, -COOR₄, -OOCR₅ or -CONHR₆, R₄ is alkyl, hydroxyalkyl or methoxyalkyl containing altogether at most 4 carbon atoms, R₅ is methyl or ethyl, R₆ is alkyl of 1 to 8 carbon atom or hydroxyalkyl of 1 or 2 carbon atoms, and M₁, Y₁, Y₂, Z₁, Z₂ and Z₃ each have the meanings assigned to them in claim 1.

4. A process according to claim 3, which comprises using as tanning agent an oligomer which contains structural units of formulae

wherein X₃ is -CN, -COOCH₃, -COOC₂H₅, -COOC₂H₄OH, -COOC₃H₆OH, -COOC₂H₄OCH₃, -OOCCH₃, -OOCC₂H₅, -CONHCH₃, -CONHC₂H₅, -CONHCH₂OH, -CONHC₃H₇ or -CONHC₈H₁₇, and M₁, Y₁, Z₁, Z₂ and Z₃ each have the meanings assigned to them in claim 1.

5. A process according to claim 4, which comprises using as tanning agent an oligomer which contains structural units of formulae

wherein X₄ is -CN, -COOCH₃, -COOC₂H₅, -COOC₂H₄OH, -COOC₃H₆OH, -OOCCH₃, -OOCC₂H₅, -CONHCH₃, -CONHC₂H₅, -CONHC₃H₇ and -CONHC₈H₁₇, Y₃ is -COOH or -COOM₄, Y₄ is -CONH₂, -CH₂OH or -OCH₃, each of M₃ and M₄ is a sodium cation, potassium cation or ammonium cation, and each of Z₄, Z₅ and Z₆ is hydrogen or methyl.

6. A process according to claim 5, which comprises using as tanning agent an oligomer which contains structural units of formulae

7. A process according to any one of claims 1 to 6, wherein 2 to 50 parts by weight, based on solids content, of oligomer as tanning agent are used per 100 parts by weight of leather.

8. A process according to any one of claims 1 to 7, wherein the leather is treated for 1 to 4 hours at 10° to 50°C with the aqueous solution containing the oligomer as tanning agent, and either the so treated leather is rinsed, fatliquored and dried, or the so treated leather is dyed, fatliquored and dried.