US3533724A - Substantially dry syntan or waste sulfite liquor pretannage of vegetable tanned or syntanned leather - Google Patents

Description

United States Patent ()1 fice 3,533,724 Patented Oct. 13, 1970 SUBSTANTIALLY DRY SYNTAN R WASTE SUL- FITE LIQUOR PRETANNAGE 0F VEGETABLE TANNED 0R SYNTANNED LEATHER Wolfhard Luck, Leverkusen, Bruno Zinz, Cologne-Flittard, and Helmut Schmid, Leverkusen, Germany, assignors to Farbenfabriken Bayer Aktiengesellschaft, Leverkusen, Germany, a corporation of Germany No Drawing. Filed Feb. 8, 1966, Ser. No. 525,833 Claims priority, applicatigg 1(grmany, Feb. 10, 1965,

3 Int. Cl. C14c 3/18 US. Cl. 8-94.24 Claims ABSTRACT OF THE DISCLOSURE Process for the rapid tanning of leather by pretanning pickled pelts with synthetic tanning agents followed by final tanning with vegetable or synthetic tanning agents wherein the pretanning step is accomplished by employing 5-15% by weight of the pelts of auxiliary tanning agents or sulfite waste liquor or a mixture of both, in the absence of liquor, until the entire cross section of the pelt is tanned through, and then rinsing with water before the final tanning step.

Rapid tanning processes for the production of medium weight and heavy leather Which is tanned with vegetable tannins or syntans are known from US. Pat. Nos. 3,253; 879 and 3,254,937; in these processes the hides which are pretanned 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 tannins. A disadvantage of these rapid tanning processes is the olive-tinted colour of the leather caused by the pre-tanning with chromiumcontaining pre-tanning agents. Furthermore, a content of chromium is rejected by the processing 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 the storability. Finally, it has been found that pretanning 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 difiiculties 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 tanning 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 this purpose, the addition of neutral salts, e.g. sodium chloride, is necessary, to prevent an undesirable swelling of the pelts. To eliminate the often disadvantageous influence 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, eg 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.

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 prepared in the lime house in the usual manner are acidified in known manner by a treatment with acids in the presence of salts, then pre-tanned without a liquor with about 5l5%, referred to the pelt weight, of auxiliary syntans and/ or sulphite waste liquor (in the following also called pre-tanning agents), possibly together with about 50% exchange syntans, referred to the amount of pretanning agents employed, until the whole cross-section of the pelt is tanned through, subsequently rinsed and finally tanned in known manner with vegetable tannins and/or syntans.

The term auxiliary syntans, as distinct from exchange syntans, comprises those syntans of aromatic nature which have themselves no good tanning action, or none at all (for definition of tanning adjuvants and exchange syntans see e.g. A. Kuntzel, Das Leder, 6 (1955), pages 200-207, particularly page 202, paragraph 1). The auxiliary syntans chiefly comprise the group of condensation products from naphthalene-sulphonic acids with formaldehyde as well as phenolic novolaks which are comparatively strongly sulphonated. Such more strongly sulphonated nolalaks contain e.g. on average, about 0.8-1.0 sulphonic acid groups linked to One phenyl radical (regarding 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 condensation 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, 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 neutralizing l g. dry substance of the tanning agent.

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 p-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 chiefly 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 process of the present invention it is also possible to proceed in such a manner that the pelts which have been acidified in known manner with acids in the presence of salts are first treated with a pre-tanning agent which is a condensation product from naphthalene-sulphonic acid with formaldehyde and/or a sulphite waste liquor not yet adjusted to an acid number of at least 30, 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 vegetable tannins and/or syntans.

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.

Pre-tanning is carried out without a liquor and continued until the pelts are completely tanned through. About -15% pre-tanning agent, referred to the pelt weight are sufiicient for this purpose; about 23-10% pretanning agent are preferably used. Depending on the pelt material, the pre-tanning is completed after 2-4 hours. The final tanning with vegetable tannins and/or syntans is carried out by known methods.

The term tanning without a liquor in the meaning of the present invention comprises those tanning processes in which the tanning agents are used without water or without substantial amounts of water. The pre-tanning agents are normally used in powdered form, but the 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 syntan is increased while the liquor ratio remains the same, i.e. if the amount of syntan, referred to the pelt weight, is increased, a tanning through can be achieved. In this case, however, the pelt absorbs more than 5-15 syntan, and the tanning process takes considerable more time than the pre-tanning according to the new process. Furthermore, the amount of syntan absorbed and the amount of syntan required to achieve the tanning through are the greater the more astringent the syntan. If fairly large amounts of syntan are absorbed, the character of the leather is already determined to a large extent by the pretanning agent, and this process can no longer be regarded as a pre-tanning in the strict meaning of the term. In contra-distinction thereto, the pre-tanning 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. As decisive advantage the new process oifers the possibility of removing the salts which are undesirable for the tanning from the pre-tanned pelts by simple rinsing. 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 tannins in a liquor. The pie-tanning according to the invention also leads to a faster and more even tanning through in the subsequent final tanning with vegetable and optionally synthetic tannins.

It has further been found that the final tanning of the pelts which have been pre-tanned according to the new process and rinsed can advantageously be carried out with vegetable tannins and/or syntans without a liquor. The process according to the invention makes it possible to carry out the final tanning without a liquor also on an industrial scale without the risk of case-hardening or of an undesired strong formation of drawn grain.

The process according to the invention is particularly suitable for the production of welting leather, vachettes, inside sole 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.

EXAMPLE 1 An unsplit cow butt of 8 mm. thickness which has been limed in the lime house in the manner customary for sole leather is divided into pieces of 8 x 30 cm. The pieces are drummed in the drum overnight in 300% water, 2% ammonium sulphate, 1.5% sodium metabisulphite and 0.5% of formic acid (all percentages are referred to the pelt weight). After discharging the liquor (pH value about 6), the completely delimed pelt pieces the cross-section of which is coloured level yellow to yellowgreen by the bromothymol blue indicator, are drummed with water, 7% sodium chloride, 1.5% sulphuric acid (66 Be'.) and 1% calcium formate for about 1 to 2 hours and then left in the liquor overnight (pH value about 3.2 to 3.5). Some of the pelt pieces are then pretanned without a liquor in a drumming machine as described by E. Kornarek and G. Mauthe in Das Leder 12 (1961), pages 285289, particularly page 287, with 10% of the pre-tanning agent described below. The pelt is completely tanned through after a total drumming time of about 5 hours.

As pre-tanning agent there is used a powdered mixture which consists of 40 parts by weight of a condensation product which has been obtained from B-naphthalenesulphonic acid and formaldehyde in a molar ratio of 1:065 and is present in the form of the ammonium salt, 20 parts by Weight of a delimed sulphite waste liquor in the form of the sodium salt, 20 parts by weight of a condensation product which has been obtained from 1.5 mol fl-naphthalenesulphonic acid with 1 mol dihydroxydiphenyl-sulphone and 1.25 mol formaldehyde and is present in the form of the ammonium salt, 10 parts by weight glutaric acid, and 10 parts by weight sodium sulphate.

The vegetable tannin mixture consists of 60 parts by the drum with 400% water and 35% pure tannin of the mixture of vegetable tannins described below. Half of the vegetable tannin is used immediately, the remaining part is added in 3 portions at time intervals of 3 hours.

The vegtable tannin mixture consists of 60 parts by weight pure tannin of chestnut (normal), 20 parts by weight pure tannin of mimosa, 10 parts by weight pure tannin of valonia and 10 parts by weight pure tannin of myrobalams.

Comparable pelt pieces from the same butt are drummed in the drum with the same pre-tanning agent, but in 300% water. When 10% pre-tanning agents are used, only about one fifth of the pelt is tanned from either side after a drumming time of hours. After a drumming time of about 100 hours, one third of the pelts is tanned from the grain side and about one fourth from the flesh side. Even if 40% pre-tanning agents are used in 300% liquor, the pelt is not yet completely tanned through after a tanning time of about 100 hours. If instead of the pre-tanning agent described above, there is used an exchange syntan based on a condensation product obtained from 1.5 mol naphthalene-sulphonic acid with 1 mol dihydroxy-diphenyl-sulphone and 1.3 mol formaldehyde, tanning through cannot be achieved with an amount of if larger amounts are used, the pelt absorbs more than 10-15 namely about 30% syntan. This means that the leather is practically completely tanned and not only pre-tanned.

EXAMPLE 2 150 parts by weight of shoulders of alkaline cow pelts (splitting substance 3.5-4 mm.) which are suitable for the production of welting leather and have been limed in the lime house in the usual manner, are first washed with 500% water (all percentages are referred to the pelt weight) and then rinsed with water at 20-25 C. for 5 minutes. After draining the rinsing liquor, the pelts are delimed with 300% water, 2% ammonium sulphate, 1.5% sodium metabisulphite and 0.3% of 37% hydrochloric acid. After a drumming time of 2 to 3 hours, the

liquor is drained. The pelts are then drummed with 60% water at 20 C., 5% sodium chloride, 1.5% calcium formate and 1.2% sulphuric acid (66 B.) for about 2 hours and left in the pickling liquor overnight. The pickling liquor which has a pH value of about 3.6-3.8 is then drained as far as possible.

Immediately afterwards, the pickled pelts are further drummed for about 3 to 4 hours in the samedrum with the addition of 10% of the pre-tanning agent described below. The pelts are then completely penetrated by the pre-tanning agent. They are subsequently rinsed with water at 30-35" C. for 10 minutes, and the rinsing liquor is drained.

As pre-tanning agent there is used a powdered mixture of 50 parts by weight of a condensation product which has been obtained from a technical naphthalenesulphonation mixture mainly containing fi-naphthalenesulphonic acid with 0.65 mol formaldehyde per mol naphthalene-sulphonic acid and is present in the form of the sodium salt, 25 parts by weight purified delimed sulphite waste liquor (sodium salt) and parts by weight of a condensation product prepared by known methods from 1.5 mol ,B-naphthalene-sulphonic acid, 1 mol dihydroxy-diphenyl-sulphone and about 1.25 mol formaldehyde, as well as 10 parts by weight glutaric acid. Such a condensation product and process for preparing it is found in German Pat. 611,671, corresponding to US. Pat. 1,901,536.

The pretanned and rinsed leather is left in the drum and subsequently drummed for about one hour without a liquor with 15% pure tannin of a powdered sulphited quebracho extract and 1% sulphonated fish oil. 5% pure tannin of a powdered mimosa extract are then added and drumming is continued for 2 to 4 hours. The leather is bleached with a commercial synthetic bleaching tannin and finished in the usual manner. A smooth leather without drawn grain is obtained.

EXAMPLE 3 To produce case leather, 800 parts by weight of split hide pelts (thickness of split 2.5-3 mm.) which have been prepared in the lime house in the usual manner are drummed for I to 1 /2 hours in the drum with 200% water at about 30 C., 1.5% ammonium sulphate and 0.5% sodium metabisulphite (all percentages referred to the pelt weight). The liquor is drained and the pelts are pickled immediately afterwards with 60% water, 5% sodium chloride, 1.2% calcium formate and 0.8% sulphuric acid (66 B.). After a drumming time of about 2 hours, the pelts are left in the liquor overnight. The pickling liquor which has a final pH value of about 3.6-3.8 is drained.

The acidified pelts are subsequently further drummed in the same drum with 8% powdered pre-tanning agent. The pre-tanning agent consists of parts by weight of a condensation product obtained from a technical naphthalene-sulphonation mixture mainly containing fl-naphthalene-sulphonic acid with 0.65 mol formaldehyde per mol naphthalene-sulphonic acid, 10 parts by weight sodium sulphate and 10 parts by weight adipic acid.

The pelts are evenly tanned through after a drumming time of 2 to 3 hours; they are rinsed to removethe salts.

Final tanning is subsequently carried out in the same drum, first with 10% pure tannin of a powdered sulphited quebracho extract (drumming time one hour) and then 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.

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

EXAMPLE 4:

To produce split hides, pelts which have been prepared in the lime house are delimed and pickled as described in Example 3.

The acidic pelts are then drummed without a liquor and with the addition of 0.5% acetic acid, with 10% of a powdered mixture of 50 parts by weight of the spraydried sodium salt of a condensation product obtained from 1 mol fl-naphthalene-sulphonic acid with 1 mol formaldehyde, and 50 parts by weight of a spray-dried resin obtained by condensation of 1 mol 4,4'-dihydro-xy-diphenyl-sulphone with 1 mol B-naphthalene-sulphonic acid and 1 mol formaldehyde, followed by neutralisation with a sodium hydroxide solution. The leather is washed with 300% water at about 30 C. for 10 minutes and then rinsed with water at 30 C. for 10 minutes.

Final tanning is carried out in the usual manner in a set of suspenders with 5 pits of 26 B. A tannin mixture of sulphited quebracho and mimosa in the ratio of equal parts by weight pure tannin is used for this purpose. The leather is completely tanned after a tanning time of about one week and then finished in the usual manner. A pale and smooth leather is obtained.

EXAMPLE 5 Split hide pelts (thickness of split 3 mm.) which have been prepared in the lime house in the usual manner are delimed and pickled as described in Example 3. After discharging the pickling liquor, the acidic pelts are drummed for about 2 hours in the drum, without a liquor, with 10% of the powdered pre-tanning agent described below. The pelts are then evenly tanned through and rinsed with water at about 30 C. for 1 0-20 minutes.

As pre-tanning agent there is used a spray-dried condensation product obtained from a technical naphthalenesulphonation mixture mainly containing naphthalene B- 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 about by the addition of glutaric acid.

The pre-tanned leather is rinsed and subsequently finally EXAMPLE 6 Pelt splits which are suited for the preparation of sole leather splits are delimed in the customary manner and pickled with sulphuric acid, formic acid and rock salt. The pelt pieces the cross-section of which is coloured level yellow-green by the bromocresol green indicator are drummed without liquor with (referred to the weight of the pelt) of a powdered pretanning agent as described below. After a drumming time of about 4 to 5 hours the pelts are evenly tanned through and are then rinsed with water.

The pretanning agent used is a powdered mixture of 50 parts by weight of a spray-dried delimed sulphite waste liquor in form of its sodium salt, and 50% of an exchange syntan. The exchange syntan was prepared by reaction of 1 mol of phenol sulphonic acid with 1.2 mol of urea and 1.7 mol of formaldehyde, after-treatment of the reaction product thus obtained with 0.75 mol of phenol and 0.56 mol of formaldehyde, neutralisation with soda-lye, spraydrying of the mixture and adding adipic acid. Adipic acid is added in an amount sufiicient to adjust the acid number of the powdered mixture to 100. Somewhat more detailed information about the preparation of the exchange syntan used may be taken from, for instance, the British Patent No. 890,150.

After completion of the pretanning and rinsing the final tanning is subsequently carried out in the same drum after draining the liquor. For this purpose one uses without liquor 17% of a powdered sulphited quebracho extract and 7% of powdered tannin of chestnut (all percentages referred to the weight of the material to be treated).

The leathers obtained are finished in the usual manner. One obtains smooth and flexible sole leather.

EXAMPLE 7 In the process of Example 6 the pretanning agent used is replaced by 15% (referred to the weight of pelt) of the following pretanning agents:

The pretanning agent contains 35 parts by weight of a spray-dried neutral condensation product of a technical naphthalene sulphonation and formaldehyde, in form of its sodium salt. The technical naphthalene sulphonation consists essentially of B-naphthalene sulphonic acid. The molar ratio between naphthalene sulphonic acid and formaldehyde amounts to 1:07. The pretanning agent further contains 35 parts by weight of a neutral spray-dried condensation product of 1 mol of fi-naphthalene sulphonic acid, 0.5 mol of o-chlorophenol and 0.87 mol of formaldehyde, in form of its sodium salt. Condensation products of this kind are to be obtained, for instance, according to the processes described in U.S. Pat. No. 2,829,122. The pretanning agent further contains 10 parts by weight of unhydrous oxalic acid and parts by weight of ammonium sulphate or potassium sulphate.

One obtains a very valuable leather.

EXAMPLE 8 A very good sole leather split is likewise obtained if in Example 6 the pretanning agent is replaced by the following pretanning agent.

The pretanning agent contains 10% (referred to the 'weight of the pelt) of a spray-dried neutral condensation product of [i-naphthalene sulphonic acid and formaldehyde (molar ratio 1:065), in form of its ammonium salt and 10% of a sirupy exchange syntan which contains a condensation product of 1 mol of naphthalene sulphonic acid, 1.9 mol of pyrocatechol and 1.46 mol of formaldehyde, neutralised with ammonia and adjusted to an acid number of (referred to the solid content of the exchange syntan) by the addition of acetic acid.

The leather thus obtainable has excellent properties.

EXAMPLE 9 If in Example 6 the following pretanning agent is used, valuable leathers are likewise obtainable:

As a pretanning agent 10% (referred to the weight of the pelt) of a powdered mixture of 50 parts by weight of a spray-dried neutral condensation product o fB-naphthalene sulphonic acid and formaldehyde (molar ratio 1:1), in form of its ammonium salt, 30 parts by weight of anhydrous sodium bisulphate and 20 parts by weight of ammonium sulphate is employed.

EXAMPLE 10 Pickled pelts, e.g. goat pelts, which are suitable for the production of lining leather are drummed with about 20% (referred to the weight of the pelt) of water of 20 C. for 20 to 30 minutes. The water shall be completely absorbed by the pelts. 7% of a powdered spray-dried neutral condensation product of fi-naphthalene sulphonic acid and formaldehyde, in the form of its sodium salt are then added. After having drummed for some minutes about 1% of sodium acetate (1:10 dissolved in water) are added. The amount of sodium acetate shall be cal culated in such a manner as to adjust the pH of the small amount of liquor which is present in the drum to 3.2 to 3.5. After a drumming time of about 2 hours the skins are tanned through and are then rinsed with water of 20 C. The pickled liquor is now drained and the pre tanned pelts finally tanned without liquor in the same drum with 12% of a pure tannin which is a mixture of 50 parts by weight of pure tannin of sulphited quebracho extract and 25 parts by weight of pure tannin of a chestnut extract and 25 parts by weight of pure tannin of a weakly sulphited quebracho extract. After about 2 hours the final tannage is complete.

The leathers thus obtained are fat-liquored and finished in the usual manner. One obtains smooth leather without drawn grain.

'We claim:

I1. In a process for the rapid tanning of leather in the absence of chromium-containing tanning agents, wherein pelts prepared in the lime-house are acidified by treatment with an acid in the presence of a neutral salt and then the hides in the pickled condition are pretanned with syntans and finally tanned with one or more tanning agents from the group consisting of vegetable tannins and syntans; the improvement which comprises conducting said pretanning step, without substantial amounts of water, with about 5 to 15 based on the weight of the pelts, of a pretanning agent selected from the group consisting of:

(a) auxiliarysyntans,

(b) a mixture of auxiliary syntans and sulfite waste liquor, and

(c) a mixture of sulfite waste liquor with not more than 50% by weight of an exchange syntan based on said sulfite waste liquor, until the entire cross section of the pelt is tanned and thereafter rinsing with water before the final tanning step.

2. Process of claim 1 wherein said final tanning step is carried out without substantial amounts of water.

3. Process of claim 1 wherein said auxiliary syntan employed as a pretanning agent is a condensation product of one or more naphthalene sulfonic acids and formaldehyde.

4. Process of claim 3 wherein said auxiliary syntan employed as a pretanning agent is a condensation product of one or more naphthalene sulfonic acids and forinalde- 9 10 hyde, which product is acidified with at least one inor- OTHER REFERENCES gapic acid or organifi acid in the amolmt suflicient to Berkman: Journal of the American Leather Chemists ad ust the acid number of the pretanning agent to at Association, An Investigation of Synthetic Tanning Maleast terials, August 1947, pp. 409, 415.

5 Journal of the Amer. Leather Chem. Assoc., May 1958,

p. 307, Tanning Compositions.

OFlaherty: Chemistry & Technology of Leather, vol. 2, p. 170.

5. Process of claim 1 wherein said pretanning agent is (c) a mixture of sulfite waste liquor with not more than 50% by weight of an exchange syntan based on said sulfite waste liquor.

References Cited 10 DONALD LEVI, Primary Examiner UNITED STATES PATENTS 2,040,212 5/1936 Orthner et a1 3 -94.24X US 2,129,553 9/1933 Russell et al. 894.24X -s 94.31, 94.13, 94.32

2,955,904 10/1960 Schnoller s 94.21

3,253,379 5/1966 Komaret et a1. 8-94.26 15 3,254,937 6/1966 Komaret et al 8--94.26