tanning

Tanning is the process of making leather, which does not easily decompose, from the skins of animals, which do. Often this uses tannin, an acidic chemical compound. Coloring may occur during tanning. A tannery is the term for a place where these skins are processed. Tanning leather involves a process which permanently alters the protein structure of skin so that it cannot ever return to rawhide. Making rawhide does not require the use of tannin and is made simply by removing the flesh and fat and then the hair by way of soaking in an aqueous solution (often called liming when using lime and water or bucking when using wood ash (lye) and water), then scraping over a beam with a somewhat dull knife, and then leaving to dry, usually stretched on a frame so that it dries flat. The two aforementioned solutions for removing the hair also act to clean the fiber network of the skin and therefore allow penetration and action of the tanning agent.
The English word for tanning is from medieval Latin tannāre, deriv. of tannum (oak bark), related to Old High German tanna meaning oak or fir (related to modern Tannenbaum). This refers to use of the bark of oaks (the original source of tannin) in some kinds of hide preservation. [1]
In ancient history, tanning was considered a noxious or "odiferous trade" and relegated to the outskirts of town, amongst the poor. Indeed, tanning by ancient methods is so foul smelling that tanneries are still isolated from those towns today where the old methods are used. Ancient civilizations used leather for waterskins, bags, harnesses, boats, armor, quivers, scabbards, boots and sandals. Tanning was being carried out by the South Asian inhabitants of Mehrgarh between 7000–3300 BC.[2] Around 2500 BC, the Sumerians began using leather, affixed by copper studs, on chariot wheels.
Skins typically arrived at the tannery dried stiff and dirty with soil and gore. First, the ancient tanners would soak the skins in water to clean and soften them. Then they would pound and scour the skin to remove any remaining flesh and fat. Next, the tanner needed to remove the hair fibers from the skin. This was done by either soaking the skin in urine, painting it with an alkaline lime mixture, or simply letting the skin putrefy for several months then dipping it in a salt solution. After the hair fibers were loosened, the tanners scraped them off with a knife.
Once the hair was removed, the tanners would bate the material by pounding dung into the skin or soaking the skin in a solution of animal brains. Among the kinds of dung commonly used were that of dogs or pigeons. Sometimes the dung was mixed with water in a large vat, and the prepared skins were kneaded in the dung water until they became supple, but not too soft. The ancient tanner might use his bare feet to knead the skins in the dung water, and the kneading could last two or three hours.
It was this combination of urine, animal feces and decaying flesh that made ancient tanneries so odiferous.
Children employed as dung gatherers were a common sight in ancient cities. Also common were "piss-pots" located on street corners, where human urine could be collected for use in tanneries or by washerwomen. In some variations of the process, cedar oil, alum or tannin were applied to the skin as a tanning agent. As the skin was stretched, it would lose moisture and absorb the agent.
Leftover leather would be turned into glue. Tanners would place scraps of hides in a vat of water and let them deteriorate for months. The mixture would then be placed over a fire to boil off the water to produce hide glue.
Variations of these methods are still used by do-it-yourself outdoorsmen to tan hides. The use of brains and the idea that each animal (except buffalo) has just enough brains for the tanning process have led to the saying "Every animal has just enough brains to preserve its own hide, dead or alive".[citation needed]
[edit] Modern methods


Tanneries of Marrakech


Two men pressing the leather near the end of the tanning process in an American tannery. circa: 1976
The first stage is the preparation for tanning. The second stage is the actual tanning and other chemical treatment. The third stage, known as retanning, applies retanning agents and dyes to the material to provide the physical strength and properties desired depending on the end product. The fourth and final stage, known as finishing, is used to apply finishing material to the surface or finish the surface without the application of any chemicals if so desired.
Preparing hides begins by curing them with salt. Curing is employed to prevent putrifaction of the protein substance (collagen) from bacterial infection during the time lag that might occur from procuring the hide to when it is processed. Curing removes excess water from the hides and skins using a difference in osmotic pressure. The moisture content of hides and skins get greatly reduced. In wet-salting, the hides are heavily salted, then pressed into packs for about 30 days. In brine-curing the hides are agitated in a salt water bath for about 16 hours. Generally speaking, methods employed for curing make the chance of bacterial growth greatly unfavorable. Curing can also be done by preserving the hides and skins at a very low temperature.
In a process known as soaking, the hides are then soaked in clean water to remove the salt and increase the moisture so that the hide or skin can be further treated.
[edit] Liming process of hides and skins
After soaking, the soaked hides and skins are taken for the next operation where these are treated with milk of lime (a basic agent) with or without the addition of "sharpening agents" (disulfide reducing agents) like sodium sulfide, cyanides, amines etc. The objective of this operation are mainly to:
• Remove the hairs, nails and other keratinous matters
• Remove some of the interfibrillary soluble proteins like mucins
• Swell up and split up the fibers to the desired extent
• Remove the natural grease and fats to some extent
• Bring the collagen in the hide to a proper condition for satisfactory tannage
The weakening of hair is dependent on the breakdown of the disulfide link of the amino acid called cystine, which is the characteristic of the keratin class of protein which gives strength to hair and wools (keratin typically makes up 90% of the dry weight of hair). The hydrogen atoms supplied by the sharpening agent reduce the cystine molecular link to cysteine, and the covalent disulfide bond links are ruptured. This weakens the keratin. To some extent, the weakening produced by sharpening also contributes to "unhairing," as the hairs break down.
The isoelectric point of the collagen in the hide (this is a tissue strengthening protein unrelated to keratin) is also shifted to around 4.7 due to liming, which is an acidic type of tannage.
Unhairing agents used during liming are:
• Sodium sulfide
• Sodium hydroxide
• Sodium hydrosulfite
• Arsenic sulfide
• Calcium hydrosulfide
• Dimethyl amine
• Sodium sulphydrate
The majority of hair is then removed using a machine, with remaining hair being removed by hand using a dull knife, a process known as scudding. Depending on the end use of the leather, hides may be treated with enzymes to soften them in a process called "bating." But before bating, the pH of the collagen is brought down to a lower level so that enzymes may act on it. This process is known as "deliming."
Once bating is complete, the hides and skins are treated with a mixture of common (table) salt and sulphuric acid, in case a mineral tanning is to be done. This is done to bring down the pH of collagen to a very low level so as to facilitate the penetration of mineral tanning agent into the substance. This process is known as "pickling." The common salt (sodium chloride) penetrates the hide twice as fast as the acid and checks the ill effect of sudden drop of pH.
Tanning can be performed with either vegetable or mineral methods. Before tanning, the skins are unhaired, degreased, desalted and soaked in water over a period of 6 hours to 2 days. To prevent damage of the skin by bacterial growth during the soaking period, biocides, such as pentachlorophenol, are used.
Vegetable tanning uses tannin (this is the origin of the name of the process). The tannins (a class of polyphenol astringent chemical) occur naturally in the bark and leaves of many plants. Tannins bind to the collagen proteins in the hide and coat them causing them to become less water-soluble, and more resistant to bacterial attack. The process also causes the hide to become more flexible. The primary barks used in modern times are chestnut, oak, tanoak, hemlock, quebracho, mangrove, wattle (acacia; see catechu), and myrobalan. Hides are stretched on frames and immersed for several weeks in vats of increasing concentrations of tannin. Vegetable tanned hide is flexible and is used for luggage and furniture.
Mineral tanning
Chrome tanning:
Tanner`s golden rule
Colloid-chemical:
Hydrophilic –pretanning with small particles
Hydrophobic-final tanning with large particles
Or tannage
Start with low astringency and low propensity for bonding,
Finish with high astringency and high propensity for bonding
Or according to the countercurrent principle
Fresh tan / spent tan
The basicity of chrome tannage should be in a range of about 50-60% in order to achieve the best possible tang effect and sufficient resistance to boiling and also to attain a good bonding to skin fibers and a high degree of exhaustion of the tan liquor this is done by means of products with an alkaline reaction during or at the end of tanning process.

Usually uses chromium in the form of basic chromium sulfate. It is employed after picking. Once the desired level of penetration of chrome into the substance is achieved, the pH of the material is raised again to facilitate the process. This is known as "basification". In the raw state chrome tanned skins are blue and therefore referred to as "wet blue." Chrome tanning is faster than vegetable tanning (less than a day for this part of the process) and produces a stretchable leather which is excellent for use in handbags and garments.
High-extraction chrome tanning process
United States Patent 4919680
Crome tanning of pickled animal hides with a tanning agent comprising a reaction product of a basic chromium (III) sulfate and aliphatic C4 -C6 dicarboxylic acids or their salts is improved by preparing the reaction product in an aqueous solution from basic chromium (III) sulfate and 0.2 to 0.8 mol of the aliphatic dicarboxylic acids per mol chromium oxide of the basic chromium sulfate with subsequent adjustment of the theoretical basicity to 0 to 50% with alkali metal hydroxide or carbonate, the reaction product being added to a hide pickling liquor in the form of an aqueous solution having a chromium oxide content of at least 5% or in powder form in a quantity of 0.9 to 1.5% chromium oxide, based on hide weight, with a liquor volume less than 100%, based on hide weight, the final pH value being above 4.0 and the final temperature above 40° C.
US Patent References:
Tanning with carboxylic acid carrying ester, urethane and/or amide group
Traubel et al. - November, 1978 - 4126413

Tanning skins using polycarboxylic acid partial esters
Bockelman et al. - July, 1978 - 4101271

Tanning of hides
Backer et al. - August, 1977 - 4042321


Inventors:
Wehling, Bernhard (Bergisch-Gladbach, DE)
Makowka, Bernd (Bergisch-Gladbach, DE)
Klein Deceased., Heinz-gunter (late of Bergisch-Gladbach, DE)
Klein, Heir Gertrud (Cologne, DE)
Rosentreter, Helga (Leverkusen, DE)
Application Number:
07/231233
Publication Date:
04/24/1990
Filing Date:
08/11/1988
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Export Citation:
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Assignee:
Bayer Aktiengesellschaft (Leverkusen, DE)
Primary Class:
8/94.270
Other Classes:
8/94.260, 8/94.250
International Classes:
C14C3/06; C14C3/00; C14C3/06
Field of Search:
8/94.26, 8/94.27, 252/8.57
Foreign References:
DE3636002 April, 1988 89/427
FR2271290 May, 1975
Primary Examiner:
Lieberman, Paul
Assistant Examiner:
Mcnally, John F.
Attorney, Agent or Firm:
Connolly, And Hutz
Claims:
What is claimed is:

1. In an improved process for the chrome tanning of pickled hides with a tanning agent comprising a reaction product of a basic chromium (III) sulfate and aliphatic C4 -C6 dicarboxylic acids or salts thereof,
the improvement comprises preparing said reaction product in an aqueous solution from basic chromium (III) sulfate and 0.2 to 0.8 mol of the aliphatic dicarboxylic acids per mol chromium oxide of the basic chromium sulfate to 50° to 100° C. over a period of 30 to 180 minutes with subsequent adjustment of the theoretical basicity to 0 to 50% with alkali metal hydroxide or carbonate,
the reaction product being added to a hide pickling liquor in the form of an aqueous solution having a chromium oxide content of at least 5% or in powder form in a quantity of 0.9 to 1.5% chromium oxide, based on hide weight, the liquor volume comprising less than 100%, based on hide weight, the final pH value being above 4.0 and the final temperature above 40° C.


2. A process as claimed in claim 1 wherein the reaction product is added to the hide pickling solution in the form of a spray-dried product.

3. A process as claimed in claim 1 wherein said reaction product is in spray-dried form in admixture with acid-binding agents.

4. A process as claimed in claim 3 wherein the acid-binding agent comprises dolomite.

5. A process as claimed in claim 3 wherein the acid-binding agent comprises low-reactivity magnesium oxide.

6. A process as claimed in claim 1 wherein said C4 -C6 dicarboxylic acids comprise a technical mixture of glutaric, succinic and adipic acid.


Description:
The present invention relates to an improved process for high-extraction chrome tanning of animal hides.
BACKGROUND OF THE INVENTION
Efforts to improve the environmental situation around leather factories are very much concerned with reducing the chromium content of the wastewater. In addition to the recovery of unused chromium from residual tanning and retanning liquors, high-extraction chrome tanning processes have been adopted with a view to achieving this objective (K. Faber, Bibliothek des Leders, Vol. 3, pages 158 et seq, UmschauVerlag, Frankfurt/Main, 1st Edition, 1985). In Faber excesses of chromium which are not actually required and which always have to be circulated are not used for tanning as they are in recycling processes. Instead, only that quantity of chromium which is needed to establish the desired chromium content in the leather is used for tanning. Accordingly, the residual tanning and retaining liquors contain only small quantities of chromium.
To improve the extraction of chromium, it has been proposed, inter alia, to use so-called crosslinking aliphatic dicarboxylic acids, such as for example succinic acid, adipic acid and glutaric acid, for pickling or for chrome tanning (see for example Das Leder 23 (1972), 174 et seq; Das Leder 28 (1977), 155 et seq; Leder- und Hautemarkt 30 (1978), 132 et seq).
The aliphatic dicarboxylic acids are added to the chrome tanning liquor after the chrome tanning material in the form of their disodium salts. Although the extraction of chromium is distinctly improved in relation to a conventional tanning process, the quality of the leather can nevertheless be adversely affected (cf. K. Faber, loc. cit., page 83). The distribution of chromium and dyeability are impaired, particularly with thick or unskived hides. Another negative factor is that, in the described process, the penetration of the chrome tanning agent through the skin cross-section is impeded by the presence of the dicarboxylic acids (Leder- und Hautemarkt 30 (1978) 140).
To improve the situation, it is recommended to work at final pH values in the tanning liquor below 4.0 and at final temperatures of around 40° C. However, this only provides for extraction levels of up to 95% (Das Leder 28 (1977), 157).
In addition, it is recommended to add the dicarboxylic acid salts in several portions. However, this is an additional complication of the chrome tanning process.
The difficulties described above are eliminated by the process described in German 2,424,301, in which tanning mixtures consisting of chromium(III) salts, acid-binding agents and aliphatic C 4 -C 6 dicarboxylic acids or salts thereof are initially used together with conventional chrome tanning agents for chrome tanning. To obtain high extraction, at least 1.6 mol dicarboxylic acid has to be used per mol chromium oxide in the tanning mixture for full tanning.
In addition, it is known that solutions of chromium(III) salts and sodium adipate can be used for the chrome tanning of calf skins (Journal of the International Society of Leather Trades Chemists 27 (1943), 83 et seq). In this case, however, more than 1.5% chromium oxide (based on pickle weight, corresponds to considerably more than 1.5% chromium oxide, based on skin weight) has to be used to obtain an adequate tanning effect (boil dressing). The chrome liquors used have a chromium oxide content of at most only 2.8% and are therefore unsuitable for use on a commercial scale. Where more than 1.5% chromium oxide (based on pickle weight) is available, the extraction of chromium is distinctly in excess of 1.0 g chromium oxide/l.
Accordingly, in all hitherto known high-extraction chrome tanning processes where aliphatic dicarboxylic acids are co-used, the total quantity of chrome tanning material and the dicarboxylic acids always have to be added to the liquor in unreacted form in at least two portions.
BRIEF DESCRIPTION OF THE INVENTION
It has now been found that high extraction chrome tanning with uniform distribution of the chromium throughout the leather can be carried out with only one addition of the total quantity of the chrome tanning material and the dicarboxylic acids providing the pickled hides are chrome-tanned with a reaction product of a basic chromium sulfate and 0.2 to 0.8 mol/mol chromium oxide of the basic chromium sultat of an aliphatic C 4 -C 6 dicarboxylic acid or mixtures thereof.
DETAILED DESCRIPTION OF THE INVENTION
The present high-extraction chrome tanning of pickled animal hides is achieved with a tanning agent comprising a reaction product of a basic chromium (III) sulfate and aliphatic C 4 -C 6 dicarboylic acids or their salts wherein the reaction product is prepared in an aqueous solution from basic chromium (III) sulfate and 0.2 to 0.8 mol of the aliphatic dicarboxylic acids per mol chromium oxide of the basic chromium sulfate with subsequent adjustment of the theoretical basicity to 0 to 50% alkali metal hydroxyde or carbonate, the reaction product being added to hide pickling liquor in the form of an aqueous solution having a chromium oxide content of at least 5% or in powder form in a quantity of 0.9 to 1.5% chromium oxide, based on hide weight, with a float volume less than 100%, based on hide weight, the final pH value being above 4.0 and the final temperature above 40° C. In addition, the reaction product of basic chromium (III) sulfate and dicarboxylic aids may contain monocarboxylic acids, such as formic acid or acetic acid as masking agents.
In one preferred embodiment of the process, the reaction product used in accordance with the invention, referred to hereinafter as a chromium-dicaroxylate complex, is dried and optionally mixed with the usual acid-binding agents.
The process according to the invention is carried out in a liquor volume of less than 100% and preferably 10 to 50% (based on hide weight) at a final pH value above 4.0 and preferably from 4.2 to 4.6 and at a final temperature above 40° C. and preferably from 42° to 50° C. If the chromium-dicarboxylate complex is not used together with acid-binding agents in the chrome tanning liquor, alkalis are used in the usual way for basification to the desired pH value. The term alkali is intended to mean the hydroxides and carbonates of alkali metals such as lithium, sodium, and potassium, as well as ammonia.
The chromium oxide content of the residual chrome tanning liquors obtained in the process according to the invention is below 1 g/l.
The chromium-dicarboxylate complex is prepared by reaction of basic chromium(III) sulfates withe the corresponding aliphatic C 4 -C 6 dicarboxylic acids or mixtures thereof in aqueous solution for 30 to 80 minutes at temperatures of 50° to 100° C. The desired theoretical basicity of 0 to 50% is then adjusted with alkali solution, preferably with soda solution (for calculation of the theoretical basicity, see German 3,519,287).
The chromium-dicarboxylate complex thus obtained may be used in the existing solution for chrome tanning. In one preferred embodiment of the process, the solution is concentrated by evaporation and preferably spray-dried. The powder-form tanning material obtained in this way may be used for chrome tanning either as such or in admixture with acid-binding agents.
Suitable basic chromium(III) sulfates are the reaction products of hexavalent chromium compounds with reducing agents normally used in chrome tanning (cf. K. Faber, loc. cit., page 83). They may additionally contain monocarboxylic acids (for example formic or acetic acid) or salts thereof as masking agents.
The chromium oxide content of the solutions according to the invention should be at least 5% and preferably 10 to 15%.
Aliphatic C 4 -C 6 dicarboxylic acids are, for example, succinic, glutaric, adipic, maleic, fumaric, aspartic and glutamic acid and mixtures thereof. It is preferred to use glutaric acid and adipic acid or mixtures of these acids, optionally together with other dicarboxylic acids.
Suitable acid-binding agents (basifying agents) are, for example, dolomite, alkali carbonates and bicarbonates and magnesium oxides.
The mineral double salt CaCO 3 .MgCO 3 , which has a CaO content of 20 to 40% CaO and preferably 25 to 35% CaO and an MgO content of 10 to 25% and preferably 16 to 24% MgO, is used as dolomite.
Mixtures containing at least 40% of the total quantity of acid-binding agents used in the form of dolomite and/or low-reactivity magnesium oxides are particularly suitable.
Low-reactivity magnesium oxides are, for example, commercially available products, such as ®TANBASE (a product of Steetley Quarry Products Ltd.) or ®NEUTRIGAN MO (a product of BASF).
The chromium-dicarboxylate complexes according to the invention are also suitable for use in the retanning of chrome leathers. In this case, 0.3 to 1.5% chromium oxide (based on sharing weight) is available.
The value of the process according to the invention lies primarily in the fact that the use of the chromium-dicarboxylate complex according to the invention provides for high extraction levels in chrome tanning coupled with a particularly simple procedure involving very little effort without the quality of the leather being adversely affected. Residual liquors containing less than 1 g chromium oxide/l are obtained for only a single addition of chrome tanning material and dicarboxylic acid. It was not foreseeable that the use of the chromium-dicarboxylate complexes according to the invention would greatly improve tanning or that such high extraction levels would be obtained, despite the small quantity of dicarboxylic acid, based on the chromium available.
The process according to the invention is illustrated by the following Examples in which percentages are percentages by weight unless otherwise indicated.
EXAMPLE 1
In a 1000 liter reactor, 722 kg of a chromium(III) sulfate solution having a basicity of 33.2% (according to Stiasny) and a chromium(III) oxide content of 16.8% are heated to 80° C. 42 kg technical glutaric acid (40% glutaric acid, 30% succinic acid and 30% adipic acid) are added, followed by stirring for 30 minutes. After addition of 100 ml polysiloxane-based foam inhibitor (foam inhibitor E 100, a product of Bayer AG), 347 kg of a 3N soda solution are pumped in over a period of about 120 minutes. The reaction solution is then stirred for 60 minutes at 80° C. and has a theoretical basicity of 40% for a chromium oxide content of 10.9%. The solution is then dried while still hot in a spray dryer (entry temperature: 185° C., exit temperature: 120° C.). 490 kg chromium-dicarboxylic complex containing 24% chromium oxide and 0.4 mol technical glutaric acid per mol chromium oxide and having a theoretical basicity of 40% are obtained.
EXAMPLE 2
In a 1 m 3 stirred reactor, 735 kg of a chromium(III) sulfate solution having a basicity of 33.2% (according to Stiasny) and a chromium oxide content of 15.6% are heated to around 80° C. and reacted with 64 kg technical glutaric acid. After stirring for 30 minutes at 80° C., the chrome tanning solution (basicity 13%, chromium oxide content 15.2%) is dried in a spray dryer (entry temperature: 185° C., exit temperature: 115° C.). 485 kg chromium-dicarboxylate complex tanning material having a theoretical basicity of 13% and a chromium oxide content of 23.5% (molar ratio of chromium oxide to glutaric acid 1:0.6) are obtained.
EXAMPLE 3
In a 1 m 3 stirred reactor, 700 kg of a solution of chromium sulfate having a basicity of 33.2% (according to Stiasny) and a Cr 2 O 3 content of 18% are heated to a temperature of 80° C. 43.8 kg of a mixture consisting of 32% succinic acid, 39% glutaric acid and 29% adipic acid are then dissolved therein, followed by stirring for 30 minutes. The liquid mixture obtained has a theoretical basicity of approximately 20% and a Cr 2 O 3 content of 16.9%; the molar ratio of chromium oxide to dicarboxylic acid is 1:0.4. The quantities used for the preparation of other chromium-dicarboxylate complex tanning materials suitable for use in liquid form are shown in the following Table as Examples 3a-3d (method of preparation as in Example 3).
________________________________________________________ __________________
pbw* pbw* Exam- Molar ratio Cr 2 O 3 : chromium sul- glutaric ple glutaric acid fate sol.** acid Basicity % Cr 2 O 3
________________________________________________________ __________________

3a 1:0.2 700 21.9 26.5%
17.5
3b 1:0.3 700 32.8 23.2%
17.2
3c 1:0.6 700 65.6 13.2%
16.5
3d 1:0.8 700 87.5 6.5%
16.0
________________________________________________________ __________________
*pbw = parts by weight **composition as in Example 3
EXAMPLE 4
In a 1000 liter reactor, 750 kg of a 33.2% basic chromium(III) sulfate solution having a chromium oxide content of 16.2% are heated to around 90° C. 70 kg adipic acid are then added, followed by stirring with heating for 30 minutes. After addition of a commercial foam inhibitor (E 100, a product of Bayer AG), 343 kg of a 20% soda solution are pumped in over a period of 180 minutes. The reaction solution is then stirred with heating for 60 minutes. It has a theoretical basicity of approximately 40% and a chromium oxide content of 10.4%. The still hot, liquid tanning mixture is then spray-dried (entry temperature: 180° C., exit temperature: 118° C.).
A chromium-dicarboxylate tanning material containing 22.8% chromium oxide and 0.6 mol adipic acid per mol chromium oxide and having a theoretical basicity of approximately 40% is obtained.
EXAMPLE 5
To make shoe upper leather, 100 kg cowhides (unskived) limed in the usual way are first washed for 10 minutes with 150% (based on hide weight) water at 38° C. in a tanning drum (diameter 2 m, width 2.25 m, rotational speed 12 r.p.m.). The liquor is drained off and, after deliming for 60 minutes with 2.5% ammonium sulfate, 0.3% sodium bisulfate and 0.4% formic acid in the absence of liquor, 50% water at 35° C. is added and the hides bated for 50 minutes with 0.5% of a standard commercial bate (1500 trypt. units), pH value of solution 7.5. The cross-section of the hides produces no further red coloration with phenolphthalein. The hides are then washed twice with 150% water at 20° C. and the liquor is drained off to a residual liquor of around 50%. 5 minutes after the addition of 6% sodium chloride, 1.6% sulfuric acid (diluted 1:10) are added and the hides are pickled for 120 minutes (pH 1.8).
6.7% of the tanning mixture described below are added to the pickling solution, followed by tumbling for another 20 hours. The final pH is 4.5 and the final temperature 50° C.
The residual liquor contains 0.6 g chromium oxide/l. On examination after skiving, the chrome leathers show outstanding full tanning. Finishing in the usual way gives upper leathers having a soft, full feel, a fine grain and very even coloring.
The tanning mixture used consists of:
800 parts of the powder-form chromium-dicarboxylic complex prepared in accordance with Example 1 and
174 parts dolomite.
The chromium oxide content is 19.7%.
EXAMPLE 6
7 parts of the tanning mixture described below are added to 100 parts cowhides pretreated as in Example 5, followed by tumbling for 20 hours. The final pH is 4.6 and the final temperature 48° C. The residual liquor contains 0.6 g Cr 2 O 3 /l.
The tanning mixture used, which has a Cr 2 O 3 content of 18.6%, has the following composition:
100 parts of the chromium-dicarboxylate complex tanning material prepared in accordance with Example 2,
5 parts TANBASE (Steetley, Gt. Britain) and
21 parts dolomite.
EXAMPLE 7
7.9 parts of a chromium-dicarboxylate complex solution prepared in accordance with Example 2 are added to 100 parts cowhides pretreated as in Example 5. After 1 hour, the tanning liquor is basified with 1.8% soda (dissolved in a ratio of 1:15) in 1 hour, followed by further tumbling for 15 hours. The final temperature is 46° C. and the final pH value 4.3. The residual liquor contains 0.5 g Cr 2 O 3 /l.
EXAMPLE 8
Corresponding cowhide halves pretreated as in Example 5 up to and including pickling are comparatively chrome-tanned.
100 parts hides of one half A are tumbled for 20 hours with 7.6 parts of the tanning mixture described below. Composition of the tanning mixture containing 17.4% chromium oxide (molar ratio of Cr 2 O 3 to glutaric acid 1:0.4) for A: 500 parts of a powder-form 33% basic chromium(III) sulfate containing 26% chromium oxide,
45 parts techn. glutaric acid,
86 parts sodium bicarbonate,
118 parts dolomite.
The final tanning temperature is 42° C. and the final pH value 4.3. The residual liquor contains 0.6 g Cr 2 O 3 /l.
100 parts cowhides of the corresponding half B are treated for 20 hours with 6.7 parts of the following mixture: Composition of the mixture containing 19.7% Cr 2 O 3 for B: 80 parts of the chromium-dicarboxylate complex prepared in accordance with Example 1 and 18 parts dolomite.
The temperature reached at the end of the tanning process is 42° C. and the final pH value 4.3. The residual liquor has a chromium oxide content of 0.8 g/l.
By comparison with B, the leathers of the corresponding pieces A show much poorer full tanning after skiving.
The dye finishes subsequently applied in the usual way are also distinctly less uniform in the case of A.
EXAMPLE 9
For the production of upper leather, 3000 kg cowhides (skived to approx. 3.5 mm) limed in the usual way are first washed for 10 minutes with 150% (based on hide weight) water at 38° C. in a tanning drum (diameter 3 m, width 3 m, rotational speed 5 and 10 r.p.m.). The liquor is drained off and, after deliming for 30 minutes with 30% water at 35° C. containing 2% ammonium sulfate, 0.2% sodium bisulfate and 0.2% formic acid, the hides are bated for 30 minutes with 0.5% of a standard commercial bate (1500 tryptic units), pH value of the solution 8.3.
The cross-section of the hides produces no further red coloration with phenolphthalein. The hides are then washed twice with 150% water at 20° C. and the liquor drained off to a residual liquor of approximately 20%. 5 minutes after the addition of 4% sodium chloride, 0.5% formic acid (diluted in a ratio of 1:5) and, after 10 minutes, 0.8% sulfuric acid (diluted in a ratio of 1:10) are added and the hides pickled for 60 minutes (pH 2.7). 8.1% of the chromium-dicarboxylate complex solution prepared in accordance with Example 3d are pumped into the pickling solution.
After 1 hour, the liquor is basified with 2.2% soda (dissolved in water in a ratio of 1:15) in 6 portions in 1 hour, after which tumbling is continued for 18 hours.
Towards the end of the tanning process, the temperature is 40° C. and the pH value 4.0. The residual liquor has a chromium(III) oxide concentration of 0.4 g/l.
Finishing in the usual way gives full, soft leathers distinguished by an extremely even dye finish.
EXAMPLE 10
6.7 parts of the tanning mixture described below are added to 100 parts cowhides pretreated as in Example 9 (skived to approx. 3.5 mm) in the pickling solution.
The tanning mixture, which has a Cr 2 O 3 content of 21.0%. has the following composition:
100 parts of the chromium-dicarboxylate complex tanning material produced in accordance with Example 4 and
8.5 parts TANBASE.
The final pH value is 4.3 and the final temperature 45° C. The residual liquor contains 0.5 g/l Cr 2 O 3 .
EXAMPLE 11
To make furniture leather, 300 kg cowhides (skived to approx. 2 mm) limed in the usual way are first washed for 10 minutes with 150% (based on hide weight) water at 38° C. in a tanning drum (diameter 3 m, width 3 m, rotational speed 5 and 10 r.p.m.). The liquor is drained off and, after deliming for 40 minutes with 50% water at 35° C. containing 2.5% ammonium chloride, 0.3% sodium bisulfite and 0.2% formic acid, the hides are bated for 45 minutes with 0.7% of a standard commercial bate (1500 tryptic units), pH value of the solution 8.5.
The cross-section of the hides produces no further red coloration with phenolphthalein. The hides are then washed twice with 150% water at 20° C. and the liquor drained off to a residual liquor of approximately 50%. 5 minutes after addition of 6% sodium chloride, 1.1% sulfuric acid (diluted 1:10) is added and the hides pickled for 120 minutes (pH 2.7).
6.3% of the tanning solution described in Example 3 is added to the pickling solution and, after 1 hour, the liquor is basified in 1 hour with 1.4% soda (dissolved in a ratio of 1:15), followed by further tumbling for 15 hours. The final pH is 4.2 and the final temperature 45° C.
The residual liquor contains 0.2 g/l chromium oxide. Finishing in the usual way gives furniture leather having a soft feel and very even dye finishes.
EXAMPLE 12
5.5 parts of the tanning mixture described below are added to 100 parts cowhides (skived to approx. 2 mm) limed, delimed, bated and pickled as in Example 11 in the pickling solution, followed by further tumbling for 18 hours. The final pH value is 4.5 and the final temperature 45° C. The residual liquor has a chromium oxide content of 0.3 g/l. Composition of the powder-form tanning mixture containing 19% Cr 2 O 3 :
750 parts of a chromium-dicarboxylate complex tanning material prepared in accordance with Example 4 and
151 parts dolomite.
EXAMPLE 13
500 kg wet blues (sharing thickness 1.5) are tumbled for 90 minutes in a tanning drum (diameter 2.5 m, width 2 m, rotational speed 12 r.p.m.) with 200% water at 55° C. and 2.5% (based on sharing weight) of the tanning mixture described in Example 5 with a chromium oxide content of 19.7% and then circulated for 5 minutes every hour overnight via an automatic switch mechanism. The chrome retanning liquor has a pH value of 4.6 and contains 0.1 g Cr 2 O 3 /l. The leathers are finished in the usual way for upper leather. Very evenly dyed, grain-stable upper leathers are obtained.
EXAMPLE 14
100 parts cowhides (skived to 3.5 mm) pretreated as in Example 9 are tanned in the pickling solution with 6.75 parts of the following tanning mixture:
500 parts of the chromium-dicarboxylate complex tanning material produced in accordance with Example 1 and 101 parts dolomite.
Cr 2 O 3 content: 20%.
After tumbling for 12 hours, the final temperature was 42° C. for a final pH value of 4.1. The liquor contains 0.9 g Cr 2 O 3 per liter.





Tawing is a method that uses alum and aluminum salts, generally in conjunction with other products such as egg yolk, flour, and other salts. The leather becomes tawed by soaking in a warm potash alum and salts solution, between 20°C and 30°C. The process increases the leather's plyability, stretch, softness, and quality. Adding egg yolk and flour to the standard soaking solution further enhances its fine handling characteristics. Then, the leather is air dried ("crusted") for several weeks, which allows it to stabilize. Tawing is traditionally used on pigskins and goatskins to create whitest colors. However, exposure and aging may cause slight yellowing over time and, if it remains in a wet condition, tawed leather will suffer from decay. Technically, tawing is not tanning.[3]
Depending on the finish desired, the hide may be waxed, rolled, lubricated, injected with oil, split, shaved and, of course, dyed. Suedes, nubucks etc. are finished by raising the nap of the leather by rolling with a rough surface.