US3521990A - Process for the rapid tanning of leather - Google Patents

Description

United States Patent 3,521,990 PROCESS FOR THE RAPID TANNING OF LEATHER Helmut Schmid and Wolfhard Luck, Leverkusen, Bruno Zinz, Cologne-Flittard, Ernst Komarek, Leverkusen, and Ernst Loderhose, Stuttgart, Germany, assignors to Farbenfabriken Bayer Aktiengesellschaft, Leverkusen, Germany, a corporation of Germany No Drawing. Filed Feb. 8, 1966, Ser. No. 525,839 Claims priority, application Germany, Feb. 26, 1965,

Int. Cl. C14c 3/28, 1/08 US. Cl. 8-94.17 12 Claims ABSTRACT OF THE DISCLOSURE Process for tanning leather without chromium tanning agents, in which pelts from the lime-house are acidified without substantial amounts of water with a mixture of (1) a mineral acid or an organic sulfonic acid and (2) ammonium salt alone or in combination with a carboxylic acid, a salt of a sulfurous acid, or a water-soluble inorganic alkali metal salt, until the pH is less than 4; followed by pretanning the acidified pelts without substantial amounts of water with 5-15 by weight of the pelt of an auxiliary syntan or sulfite waste liquor, until the crosssection of the pelt is tanned completely through; and final tanning of the leathers with vegetable tannin or syntan.

Rapid tanning processes for the production of medium weight and heavy leather which is tanned with vegetable or synthetic tannins are known from U 5. Pat. Nos. 3,253,879 and 3,254,937; in these processes the hides which are pre-tanned with chromium-containing tanning agents are finally tanned with vegetable tannins and syntans in powdered form without a liquor in the drum within a short time. Compared with other known rapid tanning processes using concentrated tanning liquors, these tanning processes without a liquor have the substantial advantage that no residual liquors remain after tanning and that a very high acceleration of the tanning is achieved, due to the very high initial concentration of the tanning agents. A disadvantage of these rapid tanning processes is the olive-tinted colour of the leather caused by the pretanning with chromium-containing pre-tanning Iagents. Furthermore, a content of chromium is rejected by the manufacturing industry for various types of leather. The mineral acids liberated by hydrolysis of the chromium tanning agents can lead to a high acidity of the leather, which has a detrimental effect on their storability. Finally, it has been found that pre-tanning with chromium-containing tanning agents normally results in a smaller leather surface than the conventional pre-tanning with syntans. In many cases this difference disappears in the course of the final tanning with vegetable tannins and/ or syntans, but in other cases a smaller leather surface is obtained.

Attempts have therefore been made to attain the advantage of a rapid tanning without a liquor while eliminating the pre-tanning with chromium-containing tanning agents. However, it has hitherto not been possible to resolve the main difficulties of such a tanning, i.e. to prevent with certainty a case-hardening and as far as possible the formation of a drawn grain, on an industrial scale in a satisfactory manner without pre-tanning with chromiumcontaining tanning agents.

In a number of rapid tlanning processes for the production of vegetable-tanned leather, in which the final tanning is carried out with a liquor, a pre-treatment of the delimed pelts with acids in the form of a pickle is common use. If mineral acids or lower fatty acids are used for the purpose, the (additional of neutral salts, e.g. sodium "ice chloride, is necessary to prevent an undesirable swelling of the pelts. To eliminate the often disadvantageous infiuence of the neutral salts on the final tanning With vegetable tannins and on the properties of the leather, it has already been proposed to use organic acids which have no swelling action, e.g. naphthalene-sulphonic acids, for acidification of the pelts. When these acids are employed, the addition of salts is unnecessary.

The acidification of the pelts requires a specific amount of acid, which is comparatively high for the organic acids without swelling action, because of their high equivalent weight. In comparison with mineral acids, such as sulphuric acid, these organic acids are furthermore obtainable only at a relatively high price, and their practical use is therefore restricted for economic reasons. When naphthalene-sulphonic acids are used as acidification agents, it must also be taken into account that these acids which also have a strong corrosive effect on metals, are highly hygroscopic and available in the form of pastes or blocks which are difficult to handle. It is therefore expedient to dissolve these acids in water prior to application. For this reason, they are not very suitable for the acidification of pelts without a liquor in the drum.

A rapid tanning process for the production of vegetabletanned leather has now been found, which is carried out after previous acidification of the pelts with elimination of the influence of the neutral salts used for this purpose on the final tanning, and without the use of chromiumcontaining pre-tanning agents. This new process is the object of the present invention.

According to the new process the pelts which have been soaked and depilated in the lime house in the usual manner are acidified in one operation with a mixture of (a) mineral acids and/or organic sul-phonic acids and (b) ammonium salts, without a liquor, optionally with the addition of carboxylic acids, especially aliphatic carboxylic acids, and/or salts of sulphurous acid and/or other watersoluble inorganic alkali metal salts. The acids are used in such an amount that at the end of the treatment the pelts have a pH value below 4, preferably of 3.0 to 3.6, throughout their cross-section. The acidified pelts are then pretanned without a liquor with about 515%, referred to the pelt weight, of auxiliary syntans and/or sulphite Waste liquor, optionally together with up to 50%, referred to the amount of pre-tanning agents employed, of exchange syntans, until the whole cross-section of the pelts is tanned through. The pelts are subsequently rinsed, if desired, and the final tanning is then performed in known manner with syntans and/ or vegetable tannins.

The new process enables vegetable leather to be produced continuously in one and the same drum, starting from the alkaline pelt up to the finally tanned leather. It has proved especially advantageous that the pelts which have been acidified and pre-tanned according to the invention can be finally tanned without a liquor with syntans and/or vegetable tannins. The risk of case-hardening or of an undesirable strong formation of drawn grain does not arise, even when working on an industrial scale.

In comparison with the acidification with a liquor, the acidification of the pelts without a liquor enables the necessary amount of salt to be reduced, since only a definite concentration of salt is necessary to prevent a swelling of the pelts, and this, obviously, requires a smaller amount of salt in a system without liquor than in a liquor, i.e. a dilute system. Moreover, the acidification without a liquor proceeds more rapidly than the usual acidification with a liquor. In many cases, scudding of the pelts can be dispensed with, when the acidification has been performed without a liquor.

A preferred method of carrying out the new process consists in that the pelts are acidified with (a) mineral acids and (b) ammonium salts in the presence of aliphatic carboxylic acids and salts of sulphurous acid without a liquor, and then pre-tanned and re-tanned in the stated manner.

The concurrent use of ammonium salts and carboxylic acids leads to a more uniform and faster acidification of the pelts. The addition of salts of sulphurous acid removes the hydrogen sulphide liberated by the acidification of the alkaline pelts.

Ammonium salts to be used according to the invention are, inter alia, water-soluble ammonium salts of inorganic acids, such as ammonium sulphate and ammonium chloride, or water-soluble ammonium salts of organic acids, such as ammonium acetate, ammonium formate and ammonium phthalate. Mixtures of such ammonium salts are also frequently used with advantage.

Carboxylic acids which are especially suitable to be used according to the invention are those of the aliphatic series, such as formic acid, acetic acid, propionic acid, adipic acid, glutaric acid, oxalic acid and hydroxy-carboxylic acids, such as lactic acid, as well as citric acid. Examples of the aromatic series are sulphophthalic acid and sulphosalicylic acid. Mixtures of carboxylic acids may also be used.

As has already been mentioned, organic sulphonic acids without swelling action, especially naphthalene-sulphonic acids, can also be used or concurrently used for the acidification. The use of acids without swelling action obviates the risk of an acid swelling by the inadvertent access of water to a liquor-free acidification.

For the acidification of the pelts there may be used naphthalene-mnosulphonic acids as well as naphthalenedisulphonic acids and also technical mixtures containing naphthalene-diand trisulphonic acids, such as are obtained as by-products in the preparation of naphthalenedisulphonic acids.

The auxiliary syntans to be used according to the invention as pre-tanning agents, as distinct from exchange syntans, comprise those tanning agents of aromatic nature which have themselves no good tanning action, or none at all (for definition of auxiliary syntans and exchange syntans cf. e.g. Kuntzel, Das Leder 6 (1955), pages 200-207, especially page 202, paragraph 1). The auxiliary syntans mainly comprise the group of condensation products obtained from naphthalene-sulphonic acids with formaldehyde, as well as phenolic novolakes which are comparatively strongly sulphonated. Such more strongly sulphonated novolakes contain e.g., on average, about 0.8- 1.0 sulphonic acid groups linked to a phenyl radical (with regard to the term novolaks, see B. K. Hultzsch, Chemie der Phenolharze (1950), page 107).

Auxiliary syntans based on naphthalene-sulphonic acid/ formaldehyde condensation products are especially suitable for the new process. In particular, those condenssation products from naphthalene-sulphonic acids and formaldehyde have proved to be advantageous, which are adjusted to an acid number of at least 30, preferably 50- 120, by means of inorganic or organic acids, and which may also contain 5-20% of their weight of water-soluble inorganic neutral salts. By the term acid number there is to be understood the consumption of mg KOH for neutralising 1 g. dry substance of the tannin.

The condensation products from naphthalene-sulphonic acids and formaldehyde to be used as pre-tanning agents can be replaced completely or, more advantageously, in part by sulphite waste liquor.

The naphthalene-sulphonic acid/formaldehyde condensation products to be used as pre-tanning agents are preferably those which can be prepared by known methods from technical naphthalene-sulphonation mixtures substantially containing B-naphthalene-sulphonic acid. The molar ratio of naphthalene-sulphonic acid to formaldehyde is advantageously about 1:06 to 1 mol. When the reaction is completed, the condensation products are normally converted into their alkali metal or ammonium salts and then dried. They contain alkali metal or ammonium sulphate originating from the sulphuric acid usually employed in excess for the sulphonation of naphthalene. It is also possible to use products freed from these neutral salts.

The term sulphite waste liquor is known to mean technical mixtures which contain, as the main component, lignin-sulphonic acid in the form of its salts, especially the sodium salts.

For adjusting the acid number of the pre-tanning agents preferably to be used to at least 30, aliphatic carboxylic acids are chiefiy suitable, such as acetic acid, formic acid, glutaric acid or adipic acid. Liquid as well as solid acids can be admixed to the naphthalene-sulphonic acid condensation products or to the sulphite waste liquor.

According to a variant of the new process it is also possible to proceed in such a manner that the pelts which have been acidified according to the invention are first treated with a pre-tanning agent which is a condensation product from naphthalene-sulphonic acids with formaldehyde, and/ or a sulphite waste liquor, not yet adjusted to an acid number of at least 30, possibly together with the stated proportion of exchange tannin, that an acid number of at least 30, referred to the amount of pre-tanning agent employed, is adjusted by means of inorganic or organic acids before or during this treatment with such a neutral or more weakly acidic pre-tanning agent, and that the final tanning is subsequently carried out in the usual manner, as mentioned above, with syntans and/or vegetable tannins.

Examples of water-soluble inorganic neutral salts are, inter alia, sodium sulphate, sodium chloride, potassium sulphate, potassium chloride, ammonium sulphate, ammonium chloride, and other known water-soluble inorganic salts of neutral reaction.

By definition, exchange syntans may be used together with the auxiliary syntans for pre-tanning. However, the proportion of exchange syntans should normally not exceed about 50% of the total amount of pre-tanning agents.

Pretanning is carried out without a liquor and continued until the pelts are completely tanned through. About 5-15% pre-tanning agent, preferred to the pelt weight, are sufiicient for this purpose; about 810% pretanning agent are preferably used. Depending on the pelt material, the pre-tanning is completed after 2-4 hours. The final tanning with syntans and/or vegetable tannins is carried out by known methods. The final tanning is preferably carried out without a liquor.

The term acidification or tanning without a liquor in the meaning of the present invention comprises those processes in which the agents for acidification or tanning are applied to the pelt material or leather in the wet state, but without water or substantial amounts of water. Every alkaline swollen pelt yields water during acidification. As a rule, this amounts to about 15-25%, referred to the pelt weight, that is to say that during the acidification without a liquor according to the invention a corresponding amount of liquor is formed. This liquor can be discharged before pre-tanning. However, pre-tanning in the Presence of this liquor separated during acidification is also possible within the scope of the present invention Without disadvantage. When working without a liquor, the products are normally used in powdered form, but the use of solid products, for example, in block form, or of highly concentrated extracts, is also possible. Acidification or tanning without a liquor is mainly carried out by drumming the pelts or leather with the acidification or tanning agents in a rotating drum.

It is generally known to use syntans in a liquor for the pre-tanning of hides. If 5-15 referred to the pelt weight, are used, the pelts are tanned only superficially and a complete tanning through is not achieved. If the concentration of tanning agent is increased While the liquor ratio remains the same, i.e. if the amount of tanning agent, referred to the pelt weight, is increased, a tanning through can be achieved. In this case, however,

the pelt absorbs more than 5-15 tanning agent, and the tanning process takes considerably more time than the pre-tanning according to the new process. Furthermore, the amount of tanning agent absorbed and the amount of tanning agent required to achieve the tanning through are the greater the more adstringent the tanning agent. If fairly large amounts of tanning agent are absorbed, the character of the leather is already determined to a great extent by the pre-tanning agent, and this process can no longer be regarded as a pre-tanning in the strict meaning of the term. In contradistinction thereto, the pretanning without a liquor according to the invention makes it possible to achieve a complete tanning through of the pelt with the amounts of pre-tanning agents which are customarily used for pre-tanning and which otherwise lead only to a superficial tanning. The character of the leather can thus be determined as desired by the type of final tanning. The pelts pre-tanned according to the invention without a liquor are substantially smoother than those obtained after pre-tanning with the same tanning agents in a liquor. The pre-tanning according to the invention also leads to a faster and more even tanning through in the subsequent final tanning with syntans and/or vegetable tannins.

Surprisingly, the salts introduced into the system with the alkaline pelts, in particular the calcium salts, have no detrimental effect on the pre-tanning.

It is a decisive advantage of the new process that it is possible to remove the salts from the pre-tanned pelts by simple rinsing. The influence of the salts still contained in the pelts on the subsequent final tanning can thus be eliminated to a higher or lesser degree depending on the extent of rinsing.

A particularly advantageous method of carrying out the new process consists in that the pelts are acidified without a liquor with (A) mineral acids and (B) salts of naphthalene-sulphonic acids. The salts of naphthalenesulphonic acids can be used for this purpose in the form of the ammonium salts, whereby the condition for (b) of the above definition is fulfilled; in addition, alkali metal salts of naphthalene-sulphonic acids and/or other ammonium salts and/or carboxylic acids and/or salts of sulphurous acid may be present. Alternatively, alkali metal salts of naphthalene-sulphonic acids may be used as salts of naphthalene-sulphonic acids, instead of the ammonium salts, and in this case ammonium salts other than those of naphthalene-sulphonic acids must also be present according to the minimum condition stated above for (b); obviously, the concurrent use of further additives, such as carboxylic acids and/or salts of sulphurous acid is again possible. This method of carrying out the new process leads to the same good results as are obtained with the use of free naphthalene-sulphonic acids. In particular, the acceleration of the tanning through in the final tanning with syntans and/or vegetable tannins and the brightening of the leather colour achieved with such mixtures are the same as are obtained with free naphthalene-sulphonic acids. This was not to be foreseen, since it has hitherto generally been assumed that only the free acids which have no swelling action have these advantageous effects when used as a salt-free pickle. In comparison with the use of free naphthalene-sulphonic acids, the leather obtained with mixtures of salt of naphthalenesulphonic acids with mineral acids have a smoother grain. Compared with free naphthalene-sulphonic acids, the salts thereof have the advantage of being considerably more convenient to handle.

It is particularly advantageous to mix the salts of naphthalene-sulphonic acids with sodium and/or potassium bisulphate and to use the mixture as a powdered agent for the acidification of pelts. Equally advantageous is the use of spray-dried alkali metal or ammonium salts of those technical mixtures which contain naphthalenedisulphonic acids and possibly naphthalene-trisulphonic acids, such as are obtained as byproducts in the preparation of naphthalene-disulphonic acids, in admixture with bisulphates as powdered agents for the acidification of pelts.

The process according to the invention is particularly suitable for the production of welting leather, split hides, insole leather, sole leather splits or sole leather.

In the following examples which are given for the purpose of illustrating the invention, the parts by weight stand for kilograms, unless otherwise stated. The percentages are referred to the weight of the pelts.

EXAMPLE 1 300 kilograms of shoulders of alkaline cow pelts (splitting substance 3.5-4 mm.) which are suitable for the production of welting leather and have been soaked and delimed in the lime house in the usual manner, but have not been scudded, are first washed with 500% (percentages referred to pelt weight) water and then rinsed at 25 to 30 C. for 5 minutes. After discharging the rinsing liquor, the pelts are drummed without the addition of water in the drum for about 2-3 hours with 5% of the powdered acidification agent described below, 0.3% sodium metabisulphite, 1% of 85% formic acid, and 0.5% acetic acid. When 'bromocresol green indicator is applied at the end of the treatment, the pelts exhibit a yellowgreen colour throughout their cross-section. The alkaline swollen pelt yields water during the treatment (about 20-25%). The liquid accumulating in the drum has a pH value of 3.5-3.7.

The powdered acidification agent is a mixture of 50 parts by weight sodium bisulphate, 30 parts by weight ammonium sulphate and 20 parts by weight ammonium salts of naphthalene-diand -trisulphonic acids.

The acidified pelts are then further drummed in the same drum for about 3 hours with 10% of the pre-tanning agent described below. The pelts which are completely penetrated by the pre-tanning agent are then rinsed with water at 30-35 C. for 10 minutes.

The pre-tanning agent employed is a powdered mixture of 50 parts by weight of a condensation product, present in the form of the sodium salt, which has been obtained from a technical naphthalene-sulphonation mixture sub stantially containing [i-naphthalene-sulphonic acid with 0.65 mol formaldehyde per mol naphthalene-sulphonic acid, 25 parts by weight purified delimed sulphite waste liquor (sodium salt) and 15 parts 'by Weight of a condensation product prepared by known methods from 1.5 mol S-naphthalene-sulphonic acid, 1 mol dihydroxy-diphenylsulphone and about 1.25 mol formaldehyde, as well as 10 parts by weight glutaric acid.

After discharging the rinsing liquor, the pre-tanned and rinsed leather is drummed without a liquor in the drum for about 1 hour with 15% pure tannin of a powdered sulphited quebracho extract and 1% sulphonated fish oil. 5% of a pure tannin of a powdered mimosa extract are then added and drumming is continued for 2-4 hours. The finally tanned leather is bleached and finished in the usual manner. A smooth leather is thus obtained, which has no drawn grain.

EXAMPLE 2 In order to produce case leather in one operation from the alkaline delimed pelt to the finally tanned leather, 600 kg. of split hide pelts (splitting substance 3 mm.) Which have been prepared in the lime house in the usual manner are drummed, without previous scudding, in the drum with 5% (referred to the pelt weight) of the powdered mixture described below, 1% of a 40% sodium bisulphite solution and 0.8% of 85 formic acid diluted with water 1:1. After 30 minutes, the pelts are uniformly acidified. The small amount of liquor formed (about 15-20% which has a pH value of 3.6-3.7, is discharged.

The mixture used for acidification consists of parts by Weight sodium 'bisulphate and 50 parts by weight of a technical mixture of about sulphuric acid and about 35% 1,5- and 1,6 naphthalene-disulphonic acid which still contains small amounts of 1,3,6- and 1,3,7- naphthalene-trisulphonic acid and has been neutralised with ammonia and spray-dried.

The acidified pelts are subsequently further drummed in the same drum with 8% of a powdered pre-tanning agent.

The pre-tanning agent consists of 80 parts by weight of a condensation product obtained from a technical naphthalene-sulphonation mixture substantially containing B-nap-hthalene-sulphonic acid with 0.65 'mol formaldehyde per mol naphthalene-sulphonic acid, parts by weight sodium sulphate and 10 parts by Weight adipic acid.

The pelts are evenly tanned through after a drumming time of 2-3 hours and can then be rinsed to remove the salts, if desired.

The pre-tanned pelts are then first drummed without a liquor in the same drum for 1 hour with 10% pure tannin of a powdered sulphited quebracho extract and then finally tanned with the addition of 4% pure tannin of a powdered sweetened chestnut wood extract, 2% pure tannin of the same powdered quebracho extract and 2% of a commercial soft-tanning exchange syntan, as can be prepared from 1 mol 4,4 dihydroxydiphenyl-sulphone, 0.9 mol ,B-naphthalene-sulphonic acid and 1 mol formaldehyde.

The leather is completely tanned after a total tanning time of about 4-5 hours and then fat-liquored and finished in the usual manner. A light-coloured leather of level shade is obtained, which has only a negligible drawn grain, or none at all.

EXAMPLE 3 To produce split hides, pelts which have been prepared in the lime house are acidified as described in Example 2, with the difference that a powdered mixture of 50 parts by Weight potassium bisulphate and 50' parts by weight ammonium sulphate is used for acidification.

Without discharging the small amount of liquor formed during the preceding treatment, the acidified pelts are then drummed with 10% of a powdered mixture of 70 parts by weight of the spray-dried ammonium salt of a condensation product obtained from 1 mol fi-naphthalenesulphonic acid with 1 mol formaldehyde, and 30 parts by weight of a spray-dried resin obtained by condensation of 1 mol 4,4-dihydroxy-diphenyl-sulph-one with 1 mol fl-naphthalene-sulphonic acid and 1 mol formaldehyde, followed by neutralisation with a sodium hydroxide solution, 0.5% acetic acid being added. The pelts are evenly tanned through after about l /2-2 hours. The leather is washed with 300% water at about 25-30 C. for 10 minutes and finally rinsed with water at 30 C. for minutes.

The pre-tanned leather is finally tanned in the usual manner in a series of suspenders with 5 pits of 26 B., using a tanning mixture of sulphited quebracho and mimosa in the proportion of equal parts by weight pure tannin. The leather is completely tanned after a tanning time of about 1 week and then finished in the usual manner. A smooth plump leather is obtained.

EXAMPLE 4 Alkaline split hide pelts (splitting substance 3 mm.) which have been prepared in the lime house in the usual manner are drummed in the drum without a liquor with 1.0% ammonium sulphate and 0.5% sodium metabisulphite. After a running time of 5-10 minutes, 0.8% of 85% formic acid diluted with water 1:3 are added through the hollow axle of the drum, followed by the addition of 0.8% sulphuric acid of 66 B. diluted with water 1:10. The pelts are uniformly acidified after a drumming time of about 1 hour. Instead of sulphuric acid, there may also be used an equivalent amount of a naphthalene-sulphonic acid. After discharging the small amount of liquor formed during acidification, the acidic pelts are drummed in the drum without a liquor for about 2 hours with 10% of the powdered pre-tanning agent described below. The pelts are then evenly tanned through and are rinsed with water at about 30 C. for 10-20 minutes.

The pre-tanning agent employed is a spray-dried condensation product obtained from a technical naphthalenesulphonation mixture substantially containing B-naphthalene-sulphonic acid, with 0.6 mol formaldehyde per mol naphthalene-sulphonic acid, which is present in the form of the sodium salt and has been adjusted to an acid number of about 110 by the addition of adipic acid.

After rinsing, the pre-tanned leather is finally tanned in the drum with a vegetable tanning liquor of 5 B. The tanning mixture consists of 50 parts by weight pure tannin of sulphited quebracho, 35 parts by weight pure tannin of mimosa and 15 parts by Weight pure soft-tanning exchange syntan, as can be prepared from 1 mol 4,- 4-dihydroxy-diphenyl-sulphone, 0.85 mol B-naphthalenesulphonic acid and 1 mol formaldehyde.

After a tanning time of about 2 days the leather is completely tanned. A smooth leather of gentle feel is obtained after the usual finishing.

a EXAMPLE 5 To produce insole leather, 500 kg. split alkaline pelts (thickness 4-5 mm.) are washed for 10 minutes, rinsed with water at 2530 C. for 5 minutes, and the rinsing liquor is drained. 5% of the agent for acidification of the pelts described below, 1.2% of formic acid, and 0.2% sodium metabisulphite are then added, and drumming is carried out without a liquor for about 1-1 /2 hours. The pelts have a pH value of about 3.2 to 3.4. The small amount of liquor formed by the elimination of water from the alkaline pelts is drained.

The acidification agent used is a powdered mixture of 50 parts by weight sodium or potassium bisulphate, 25% ammonium sulphate and 25% ,B-naphthalene-sulphonic acid sodium salt.

The acidic pelts are drummed without a liquor with 10% of a powdered mixture of 50 parts by weight of the spray-dried sodium salt of a condensation product obtained from 1 mol [R-naphthalene-sulphonic acid with 1.0 mol formaldehyde, and 50 parts by weight of a spraydried resin obtained by condensation of 1 mol 4,4'-dihydroxy-diphenyl-sulphone with 1 mol fi-naphthalene-sulphonic acid and 1.1 mol formaldehyde, followed by neutralisation with a sodium hydroxide solution, 0.5 acetic acid being added. The pelts are evenly tanned through after about 2 hours. The leather is washed with 300% water at about 30 C. for 10 minutes and then rinsed with water at about 30 C. for 10 minutes.

Final tanning is subsequently carried out in the same drum with 20% pure tanning agent in the form of a powdered mixture of 60 parts by Weight of spray-dried sulphited quebracho extract, 20 parts by weight of a spray-dried mimosa extract, and 20 parts by weight of a spray-dried chestnut extract. The tanning agents are added in one portion. The leather is evenly tanned through after drumming time of 3-4 hours. water and 0.5 of a bleaching syntan are added for bleaching, and drumming is continued for a further 10 minutes. The leather is then removed from the drum and finished in the usual manner. A smooth leather of level colour is obtained.

EXAMPLE 6 500 kilograms of alkaline cow butt pelts prepared in the lime house by the methods customary for the production of sole leather are levelled (thickness 4.2 mm.) and drummed for about 1 hour without the addition of water with 3% (referred to the pelt weight) sodium bisulphate, 2% ammonium sulphate, 0.1% sodium metabisulphite, and 0.8% of 85 formic acid. The small amount of liquor formed by the water eliminated from the alkaline swollen pelts has a pH value of 3.5-3.6. The pelts are uniformly acidified. A hydrogen sulphide odour does not occur.

10% of the powdered pre-tanning agent described below are then added and drumming is continued in the drum for about hours. The pelts are then evenly tanned through, and after discharging the small amount of liquor, they are rinsed with water at about 30 C. for 20 minutes.

The pre-tanning agent employed is a spray-dried condensation product obtained from a technical naphthalenesulphonation mixture substantially containing 3-naphthalene-sulphonic acid with 0.65 mol formaldehyde per mol naphthalene-sulphonic acid, which is present in the form of the ammonium salt and has been adjusted to an acid number of 120 by the addition of glutaric acid.

After rinsing, the leather is drummed in the same drum with 8% pure tannin of a sulphited quebracho extract in powdered form. To avoid chafe marks, it is expedient to add with the tanning agent about 0.5% of a sulphited castor oil together with 1% water. After a drumming time of about one hour, there are added 6% pure tannin of quebracho extract, 6% pure tannin of mimosa extract and 3% pure tannin of chestnut wood extract, all in powdered form, and tanning is continued. Depending on the type of raw material, the leather is completely tanned after a total tanning time of 24-30 hours. The leather is then finished in the usual manner. A smooth firm sole leather is obtained, which has a negligible drawn grain, or none at all.

EXAMPLE 7 Cow butts prepared in the lime house by the methods customary for the production of sole leather are drummed without addition of water with 1% of ammonium sulphate, 0.2% of sodium meta-bisulphite and 2% of ammonium acetate for 10 minutes. 2.5% of sodium bisulphate and 1.5% of glutaric acid are then added. The drumming is continued until the pelts are evenly acidified and show a pH of 3.5 throughout their cross-section. 10% of the precursory tanning agent described below are then added and the drumming continued for 3 hours. The leathers are left in the drum overnight and the drumming is continued in the next morning for a short time. The pelts thus obtained are evenly tanned through; they are finally rinsed with water.

The precursory tanning agent to be used above consists of 40 parts by weight of a spray-dried neutral condensation product from fi-naphthalene sulphonic acid and formaldehyde (molar ratio of naphthalene sulphonic acidzformaldehyde 1:07), in form of its sodium salt, furthermore parts by weight of ammonium sulphate and 40 parts by weight of the sodium salt of a neutral spray-dried condensation product from 1 mol of ,8- naphthalene sulphonic acid, 0.5 mol. of o-chlorophenol and 0.85 mol of formaldehyde; the last mentioned condensation product can be obtained, for instance according, to the methods described in German patent specification 927,331.

' The leather is then finally tanned with a vegetable tanning agent and finished according to the method described in Example 6. A smooth flexible sole leather is obtained which exhibits a negligible drawn grain or none at all.

EXAMPLE 8 Pelt butts suitable for the preparation of sole leather splits are rinsed with water of 250 C. for 10 minutes. The rinsing liquid is then completely removed and the pelts drummed without liquor with 2% of ammonium chloride, 0.2% of sodium meta-bisulphite and 1% of the sodium salt of sulphophthalic acid. After few minutes 2% of 40% lactic acid diluted with water in the ratio of 1:1 are added through the hollow axle. After further drumming for 5 minutes 0.8% of concentrated sulphuric acid (66 B.) diluted with water in the ratio of 1:20 are added through the hollow axle. The drumming is then continued for 2 to 3 hours. The pelts obtained exhibits a pH of 3.3 to 3.5 through the entire cross-section. Half of the liquid formed during the drumming is removed. 15% of the powdered precursory tanning agent described below are then added and the drumming continued.

The precursory tanning agent to be used above is a powdered mixture of 50 parts by weight of delimed spray-dried sulphite waste liquor, in form of its soduim salt, and 50 parts by weight of a exchange syntan the exchange syntan was prepared by condensation of 1 mol of phenol sulphonic acid with 1.5 mol of urea and 2.1 mol of formaldehyde and after-treatment of the condensation products thus obtained with 0.75 mol of phenol and 0.56 mol of formaldehyde, neutralisation with soda-lye, spraydrying of the neutral mixture and mixing wih glutaric acid; the amount of glutaric acid has to be sufficient to give the powdered mixture an acid number of 120. Exchange syntans of the above kind are, for instance, prepared according to British Patent No. 890,150.

In place of the above mentioned exchange syntan one may alSO use a similar condensation product from 1 mol of phenol sulphonic acid with 1 mol of urea and 1.55 mol of formaldehyde which is after-treated with 0.75 mol of phenol and 0.56 mol of formaldehyde and worked up as described above.

The pretanning is finished as soon as the pelts are evenly tanned through. This can be accomplished after a drumming time of about 3 to 5 hours. The pretanned pelts are then rinsed and the rinsing liquor completely discharged. Without addition of further liquor the pretanned pelts are then finally tanned with 20% of a powdered sulphite quebracho extract and 10% of a powdered chestnut Wood extract. After a drumming time of about 20 hours the leathers obtained are completely tanned. The finishing of the leathers is performed in the customary manner. One obtains smooth and flexible sole leather.

EXAMPLE 9 Pelt butts are treated as indicated in the first part of Example 8 while using, however, for acidifying 5% of a powdered mixture of 50 parts by weight of potassium bisulphate, 20 parts by weight of common salt and 30 parts by weight of the ammonium salt of fi-naphthalene sulphonic acid together with 0.2% of sodium bisulphite, 1% of formic acid, diluted with water in the ratio of 1:2, as well as 0.5% of anhydrous oxalic acid.

The pretanning is continued according to the process described in Example 8 while using 10% of a spray-dried neutral condensation product from fi-naphthalene sulphonic acid and formaldehyde (molar ratio 1:1), in form of its ammonium salt, and 10% of an about 50% syrupy exchange tannin which is prepared according to the process of German patent specification 870,268 by condensation of 1 mol of naphthalene sulphonic acid, 1.9 mol of pyrocatechol and 1.45 mol of formaldehyde.

The leathers are finished as indicated in Example 8. Flexible sole leathers are thus obtained.

EXAMPLE 10 If proceeding as described in Example 8 but using for acidifying of the pelts a mixture of 4% of fl-naphthalene sulphonic acid, 2% of ammonium sulphate, 1.5% of 85% formic acid, diluted with water in the ratio of 1:2) and furthermore using as precursory tanning agent 20% of an about 50% delimed sulphite waste liquor in form of its soduim salt and 0.6% of acetic acid valuable flexible sole leathers are obtainable.

In the preceding examples the given percentages are to indicate the amount of treating agent or liquor referred to the weight of the pelt, to be treated except for few instances where the percentage is to give the concentration of the treating agent itself (e.g. 85% formic acid, a 40% sodium bisulphite solution, a technical mixture of about 65% sulphuric acid and about 35% 1.5- and 1.6-naphthalene-disulphonic acid).

We claim:

1. In a process for the tanning of leather by pickling of the pelts, pretanning with syntans and final tanning with at least one member selected from the class consisting of vegetable tannins and syntans, the improvement which comprises conducting the entire process in the absence of chromium-containing tanning agents by:

(1) acidifying pelts prepared in the lime house, without substantial amounts of water and without tanning agents, with a mixture of (a) a member selected from mineral acids and naphthalene sulfonic acids, and

(b) an ammonium salt or an ammonium salt mixed with a member selected from the class consisting of a carboxylic acid, a salt of sulfurous acid, and a water-soluble inorganic alkali metal salt other than said salt of a sulfurous acid, such that at the end of the treatment each pelt has a pH below 4 throughout its cross-section;

(2) pretanning the acidified pelts, without substantial amounts of water, with about 5-15 based on the weight of the pelt, of a pretanning agent selected from the class consisting of auxiliary syntans and sulfite Waste liquors until the cross-section of the pelt is completely tanned; and

(3) thereafter subjecting the pretanned leathers to final tanning with a member selected from the class consisting of vegetable tannins and syntans.

2. Process of claim 1 wherein said pH is from 3.0

3. Process of claim 1 wherein said auxiliary syntan is employed in combination with not more than 50%, based on the weight of said auxiliary syntan, of an exchange syntan. I

4. Process of claim 1 wherein said sulfite waste liquor is employed in combination with not more than 50% based on the weight of said sulfite waste liquor, of an exchange syntan.

5. Process of claim 1 wherein the pretanned leather is rinsed with water before the said final tanning.

6. Process of claim 1 which comprises acidifying pelts prepared in the lime house, in the absence of a liquor, with a mixture of (a) a mineral acid and (b) an alkali metal salt of naphthalene sulfonic acid in combination with at least one member selected from the class consisting of (c) an ammonium salt, (d) an ammonium salt in the presence of a carboxylic acid, (e) an ammonium salt in the presence of a salt of sulfurous acid, and (f) an ammonium salt in the presence of a carboxylic acid and. a salt of sulfurous acid.

7. Process of claim 1 which comprises acidifying pelts prepared in the lime house, in the absence of a liquor, with a mixture of (a) a mineral acid and (b) an ammonium salt of a naphthalene sulfonic acid.

8. Process of claim 1 wherein said carboxylic acids are aliphatic carboxylic acids.

9. Process of claim 1 wherein said mixture used in acidifying pelts prepared in the lime house is a powdered mixture of (a) an alkali or ammonium salt of naphthalene sulfonic acid and (b) sodium bisulphate or potassium bisulphate.

10. Process of claim 9 wherein said mixture contains additionally an ammonium salt different from that in said mixture.

11. Process of claim 9 wherein said mixture contains additionally an aliphatic carboxylic acid and a salt of sulfurous acid.

12. Process of claim 1 wherein said pretanning agent is a condensation product of naphthalene sulfonic acid and formaldehyde or such a condensation product in combination with suflicient inorganic acid or organic acid to impart to the condensation product an acid number of at least 30.

References Cited UNITED STATES PATENTS 3,253,879 6/1966 Komarek et al 894.26 3,254,937 6/1966 Komarek et al. 894.26

' FOREIGN PATENTS 170,135 8/ 1904 Germany. 264,302 l/ 1927 Great Britain.

OTHER REFERENCES OFlaherty et al., Chemistry and Technology of Leather, p. 176, vol. 2, pub. 1958 by Reinhold pub. New York City.

Otto, J. American Leather Chemist Association, pp. 298 and 299, May 1958 and p. 371 of June 1964.

Otto, Das Leder, vol. 14, pp. 206-209, November 1963.

Progress in Leather Science 1920-1945, pp. 181, 194- 197, 252-253, 257-259 and 405-407, pub. 1948, by Brit. Lea Mfg. Res. Assoc., London, England.

Progress in Leather Science 1920-1945, p. 262, pub. 1948 by Brit. Lea. Mfg. Res. Assoc., London, England.

DONALD LEVY, Primary Examiner US. Cl. X.R.