EP0306474A1 - Method of post-liming skins and slit hides - Google Patents

Abstract

The process described is carried out with an aqueous alkaline liquor containing formanidine sulfinic acid (thiourea dioxide) in combination with calcium hydroxide and/or alkalis and, if necessary, swelling inhibitors.

Description

The invention relates to a process for the post-washing of pelts or crevices by using an aqueous alkaline liquor in which formamidine sulfinic acid is used in combination with calcium hydroxide and / or alkalis and, if appropriate, with agents which inhibit swelling.

Hides and skins are sometimes processed as so-called "green" skins in the tanneries immediately after slaughter. In general, however, they are first preserved by adding salt or disinfectants to prevent rotting. The salted and dried raw skins are freed from dirt, manure, salt and preservatives by the tanneries and returned to their original condition.

In the subsequent dehairing process, a method is then selected which is based on the desired leather quality and the question of hair retention or hair destruction.

In the case of skins with valuable hair or wool, a shrinkage is carried out from the meat side by spreading a lime-sodium sulphide slurry on the meat side. To achieve a particularly smooth grain (surface) and for very sensitive skins, e.g. Calves, the Schwödebrei is spread on the hair side. In order to achieve a skin disruption, it is generally necessary to carry out a post-scrubbing in these shredding processes.

The most common methods, however, are the liming, in which the hair is completely destroyed or only loosened to the extent that it can be easily removed mechanically by mechanical stripping. Combinations of sodium sulfide / sodium hydrogen sulfide and lime in aqueous solution are used in the liming. Other liming processes use sodium chlorite (DE-AS 1 276 286) or mixtures of organic nitrogen and Sulfur compounds, such as dimethylamine, dimethylamine sulfate, 1,3-diaminopropane, 2-amino-2-methylpropanol or / and N-methylaminoethanol, ethanolamines, diethanolamine, N-butylamine, triethylamine, dibutylamine, urea, formamide, dimethylformamide, acrylic and methacrylamide, Isothiuronium compounds, hydrazine, ethanethiol, propanethiol, thioglycolic acid and their salts, thiourea dioxide and cystine hydrochloride (DE-PS 23 01 591, DE-OS 20 53 016, DE-OS 33 32 748, AT-PS 381 952).

Enzymatic hair loosening and dehairing processes are carried out with fungal or bacterial proteinases alone or in combination with amines or amine-releasing compounds and optionally with reducing organic sulfur compounds (DE-OS 34 29 047).

Modern methods are known for reducing the wastewater load, which work to preserve the hair and are sulfide-free or low in sulfide.

In the classic hair-preserving liming with 0.5% sodium sulfide and lime overnight in a barrel or mixer, the hair can be separated as an easily filterable mass. With the Sirolime process, the hair should at least largely fall out and can be removed and filtered off by suitable rinsing devices. In the Darmstadt continuous process, the Reutlingen vaccination process and the lactic acid process, the hair must be removed mechanically after exposure to the chemicals.

These can be sold as valuable long-term fertilizers and do not burden the sewage treatment plants in a dissolved form.

Excessive swelling of the skin is feared in all liming processes because the result of the depilation is worsened. The swelling traps the hair roots and prevents them from attacking the liming chemicals. In addition to the hair or wool, the epidermis (epidermis) must also be present when liming from the horny layer, grain layer and slime layer. More recent descriptions therefore divide into a hair detachment (liming) and into an open skin (post-pulping), see LEATHER AND SKIN MARKET, No. 30, 38th year, October 24, 1986, page 14, between which the process steps of the meat removal and Columns.

Hair-preserving liming methods, the oxidation liming, but also enzymatic processes and the swamps require a so-called post-liming. The fiber structure of the dermis (collagen) should swell in the post-scrubber and be prepared for the absorption of the tannins. The task in the post-liming is therefore completely contrary to that of the liming. While the chemicals are only supposed to act on the surface in the Schweden and liming processes, they have to penetrate into the interior of the raw skin in the post-liming. This special processing step of the post-scrubber (skin disruption) must be carried out if, e.g. Pig or wild skins processed and wants to produce chamois, clothing, glove or lining leather. The post-scrubber is usually carried out as a pure limestone scrubber with calcium hydroxide (hydrated lime) or with slaked lime (lime porridge). By adding reducing substances, such as. B. sodium sulfide or sodium sulfhydrate, hair root remnants and base hair should be stained. The aim is to remove unwanted proteins that adversely affect leather quality. Undesirable protein substances are albumins, globulins, melanins, keratins, procollagen, tropocollagen and mucopolysaccharides, and are referred to in technical terms as Grund or Gneist.

One of the main problems with depilation is due to insufficient hair loosening, with the result that hair roots and base hair remain in places after the liming or the Schwöde. These lead to serious defects on the upper leather, such as a rough surface, uneven dyeings and unevenness in the applied top layers. Incompletely removed gneist and ground cause an uneven absorption of the tannins and irregular staining. The addition of sulfides in the post-scrubber causes "black spots" because black sulfide deposits are formed in contact with iron or copper. These can only be removed from the leather with difficulty and with considerable effort.

The object was therefore to provide a system for the post-scrubber which leads to a complete removal of base hair, base and gneiss and on the other hand avoids the disadvantages of the obligatory use of sulfides and a skin substance breakdown.

It has been found that surprising and unpredictable effects are achieved in the post-scrubber if formamidine sulfinic acid is used according to the invention in aqueous alkaline liquor, formamidine sulfinic acid advantageously being present in an amount of 0.1 to 1.0%, preferably 0.2-0.5%, based on the weight of the nakedness, in combination with calcium hydroxide and / or alkalis and, if appropriate, in the presence of swelling-inhibiting substances.

The pelts obtained are completely free of ground and gneiss, have extremely fine and firm scars and are characterized by exceptional brightness. It could not have been foreseen that, in addition to the elimination of sulfides in the post-scrubber, which is at least partially possible according to the invention, and the associated reduction in environmental pollution, above all the brightness of the nakedness can be increased in this way.

Light nakednesses are very popular because the color tones appear with better opacity and brilliance in the case of weakly covering colors or undyed leathers. For this reason, after tanning, the leather is treated with an oxidation bleach with potassium permanganate / sodium hydrogen sulfite or with hydrogen peroxide or with sodium chlorite pulled. Reduction bleaching with hydrosulfite, sodium dithionite, hydroxymethanesulfinic acid is also common, possibly in combination with optical brighteners. When using the combination according to the invention, bleaching can be dispensed with in any case and an additional processing step can thereby be saved. It was not foreseeable that the additional processing step that would otherwise be required can be saved by the addition of formamidine sulfinic acid to the post-scrubber and a similar increase in brightness is achieved as is described in DE-OS 36 15 816.

The hides and skins are subjected to the process according to the invention after a customary switch, sweat or liming. The usual operational steps such as fleshing, splitting and trimming can also be carried out unchanged.

The optimum effect for the treatment of the bare or gaps in the post-scrubber is in the pH range of the liquor from 8.0 to 13.0, preferably 9.5 to 12.0. The pH generally adjusts itself automatically when the pelts or fissures are returned to the water after the fleshing and fissuring, because chemical residues remaining in the skin diffuse into the water, or when calcium hydroxide is added. The pH can also be adjusted in a manner known per se by adding e.g. Caustic soda, sodium hydroxide solution, magnesium hydroxide, sodium carbonate (soda) or amines or amine-releasing compounds such as dimethylamine, dimethylamine sulfate, 1,3-diaminopropane, 2-amino-2-methylpropanol, N-methylaminoethanol, ethanolamine, diethanolamine, N-butylamine, triethylamine , Dibutylamine or hydrazine.

Salts such as sodium chloride or calcium chloride, potassium rhodanide, potassium oxalate, potassium hydrogen tartrate, but also urea, formamide, dimethylformamide, acrylic and methacrylamide, glycols and dioxane may be mentioned as further possible additives which reduce the tautness.

The amount of water for carrying out the post-washing according to the invention is generally about 100 to 500%, based on the weight of the pelts or gaps used, and can be varied within wide limits without restricting the advantages according to the invention.

The times in the process of the invention are 2 to 24 hours, in special cases in the production of pig or chamois leather several days.

Calcium hydroxide is used in amounts of 0.1 to 5.0%, preferably 0.5 to 2.5%. Calcium hydroxide loosens the structure of the collagen structure through alkaline swelling and dissolves non-collagen proteins. This opens up the skin. The amount of calcium hydroxide is primarily determined by the previous liming and the skin breakdown achieved, and secondly by the desired type of leather (upper, lining, lower, saddler, upholstery, baggage, clothing, belt or glove leather ).

The exposure time for the post-scrubber according to the invention, like the amount of calcium hydroxide, depends on the type of leather desired. The removal of ground and gneiss as well as the brightness is reached after at least two hours of exposure and is completed after this time. Longer exposure times and higher amounts of calcium hydroxide only result in better skin disruption and lead to softer leathers.

The examples below explain the process according to the invention without restricting the protection sought to these exemplary embodiments.

Example 1: Production of box calf aniline leather Raw goods:

Veal skins, black and white, weight class 8 - 12 kg

Soft weight:

600 kg

The skins are normally subjected to a dirt and main switch. Then it is fleshed and stretched. For the Schwöde sodium sulfide in warm Pre-dissolved water and adjusted to a density of 1.08 g / cm³ by adding hydrated lime and kaolin. This sweat porridge is applied to the meat side of the veal. After 5 hours the hair is loosened so that it can be easily scraped off.

A main ash washer with 1.3% sodium hydrogen sulfide is then carried out in the barrel.

The pelts are very flat and have remnants of hair roots on the surface. The pigmentation is still visible. The post-treatment was carried out in the following way:

Post-purifier (barrel):

250% water, 27 ° C
2.0% hydrated lime (white lime)
0.5% formamidine sulfinic acid
0.2% sodium hydroxide solution 50%
Move for 30 min to mix at 2 revolutions per minute. Duration of treatment 10 hours. During this time, move for 5 minutes every hour. The pH of the liquor is 12.0. The percentages relate to the soft weight.

After the treatment period, the liquor is drained off and the pelts are rinsed with 250% water, 27 ° C.

The nakedness is swollen flat, completely free of ground and gneiss and very clean. No pigmentation and hair roots can be found. The brightness of the nakedness is particularly striking. The scar is dense and tight.

The further processing steps to aniline box calf leather can be carried out without any process changes. The finished leathers stand out due to their completely uniform coloring and higher brilliance of the color. After the aniline finish, a fine, tightly closed surface structure of the leather can be seen (grain pattern).

Example 2: Deer Manufacture of chamois leather Raw goods:

Deer, New Zealand

Soft weight:

440 kg = 55 pieces

The raw material is soaked in the normal way and subjected to a barrel shrinkage. After the Schwöde is fleshed and the scars are repelled. To achieve complete skin disruption, the following post-scrubber is placed in the reel:
Water: 400%, 25 ° C
0.4% formamidine sulfinic acid
2.5% hydrated lime

The usual addition of 0.5% sodium sulfide is not carried out. The pH of the liquor is 10.5. After a treatment period of 4 days, the bare parts are removed from the reel. The nakedness is white-yellow and slightly swollen. The final pressures from the corrugated rollers after the fleshing and scar rejection are no longer visible. All subsequent processing steps can be carried out without changes. The finished chamois leather has a soft, pleasant feel and is significantly lighter than conventionally re-washed leather.

Example 3: Production of sheep nappa leather Raw goods:

Short-wool, New Zealand sheepskins, salted

Soft weight:

240 kg = 80 pieces

The switch is carried out in the normal way. Then it is fleshed and depilated in a Schwöde with sulfur sodium. The woolen skins are very flat, occasionally still have clumps of hair and are gray-green in color. They are placed in the following post-scrubbers to open up the skin:

Post-purifier (barrel):

500% water
0.6% formamidine sulfinic acid
0.2% sodium hydroxide solution 50%
1% sodium chloride
1.5% hydrated lime

The pH of the liquor is 11.5%. The duration of treatment is 24 hours, with the barrel being moved for 5 minutes per hour. After the post-scrubbing process, the bare skin is slightly swollen and does not decay, as a sign of extensive skin substance breakdown. The nakedness is free of hair roots and hair. Grund and Gneist have been completely removed. The scar is flat, tightly closed and elastic. Further processing is carried out in the normal way. Due to the brightness of the nakedness, oxidation bleaching with potassium permanganate / sodium bisulfite can be saved after tanning. The leathers are soft, completely draft-free and light. The surface is striking due to its pronounced fine porosity and firm grain.

Example 4: Vegetably tanned leather Raw goods:

Bull skin, black colored, Ireland, weight class 40 - 49.5 kg

Soft weight:

570 kg = 13 pieces

The switch and the limescale / sulphide ash washer are carried out in the normal way. Then it is rinsed, fleshed and split. There are many hair roots in the mast folds. The pigmentation (Grund and Gneist) is clearly visible. After splitting, the grain splits are subjected to the following post-treatment:

Barrel:

200% water, 25 ° C
1.8% hydrated lime (white lime)
0.5% formamidine sulfinic acid
0.3% soda calc.
0.2% dimethylamine sulfate

The pH of the liquor is 12.5. The treatment time is 3 hours, with 15 minutes per hour at 5 revolutions per minute. The fleet is then drained off and the columns are further processed in the normal manner.

After treatment in the post-scrubber, the hair roots are completely removed. The gaps are plump, but free of scars and very bright. Pigmented areas are no longer visible and the reason is completely removed. The assortment loss for the production of vegetable tanned leather is greater.

The further processing steps are carried out in the usual way.

The Mimosa-tanned leathers show a very light, level, soft pink color with very fine and tightly closed stigmas. The leather is firm, easy to grip and the strengths are not changed.

Example 5: Production of cowhide leather Raw goods:

Bull skins, black and white, weight class 40 - 49.5 kg

Commitment:

98 kg soft weight = 2 pieces of meat and grain slits

The raw material is soaked in the normal manner and is washed in a low-sulfide liming system, consisting of a commercially available mixture of organic reducing compounds (2%) in combination with hydrated lime (5%) and 0.8% sodium hydrogen sulfide.

After the fleshing and splitting, both the meat and the grain splits are removed from the production quantity of 2 pelts and subjected to the following post-treatment:

a) barrel:

200% water, 30 ° C
0.5% hydrated lime
0.3% formamidine sulfinic acid
0.1% sodium hydroxide solution 50%
0.2% mecaptoacetic acid sodium salt (thioglycolic acid sodium salt)

The pH of the liquor is 9.5. The liquor is agitated for 15 minutes to mix the chemicals and the gaps. Then it is set to automatic by 5 min the drum is moved at 10 revolutions per minute per hour. The total treatment time in the post-treatment is 10 hours.

The other grain columns of the production quantity are subjected to the following standard post-treatment:

b) barrel:

200% water, 30 ° C
0.5% hydrated lime
0.2% sodium hydrogen sulfide
0.2% Mecaptoacetic acid Na salt (thioglycolic acid Na salt).

The pH of the liquor is 11.0. The duration of this post-scrubber is 10 hours. The liquor is agitated for 5 minutes per hour.

A comparison of the gaps shows that in the post-scrubber according to the invention the gaps are very light, completely free of ground and gneiss and completely free of hair roots and fine hair. The conventionally post-washed columns are dark, have a gray-greenish color and get black spots when they come into contact with iron. In some cases, immunized fine hair can be identified.

The wastewater from the post-scrubber according to the invention used according to part a) of this example is free from sulfides and can be neutralized without releasing malodorous hydrogen sulfide. Further processing shows that the gaps re-washed according to a) are significantly lighter both in the wet-blue state and as finished leather. The coloring is more even and brilliant. No differences can be determined in the chemical (ash, chromium content, fat) and physical (strength, water absorption, etc.) values.

Claims (8)

1. A process for the after-washing of hairless bones or gaps, characterized in that an aqueous alkaline liquor is allowed to act on the bare parts or gaps, in which formamidine sulfinic acid (thiourea dioxide) is present in combination with calcium hydroxide and / or alkalis and, if appropriate, with anti-swelling agents. 2. The method according to claim 1, characterized in that one uses calcium hydroxide, sodium hydroxide solution, caustic soda, potassium hydroxide solution, magnesium hydroxide or soda. 3. The method according to claim 1, characterized in that organic amines are used as anti-swelling agents. 4. The method according to claim 3, characterized in that dimethylamine sulfate is used as the organic amine. 5. The method according to any one of claims 1 to 4, characterized in that one or more reducing sulfur compounds are additionally added. 6. The method according to claim 5, characterized in that sodium, ammonium or calcium sulfide is used as the reducing sulfur compound. 7. The method according to claim 5, characterized in that a thiol is used as the reducing sulfur compound. 8. The method according to claim 7, characterized in that one uses as thiol thiol glycolic acid or salts of this acid.