EP0059909B1 - Method of deliming hides - Google Patents

Abstract

Conventionally limed hides are delimed for from one to six hours under conventional deliming conditions in respect of liquor length, temperature and pH of the liquor, by a process in which a cyclic carbonate of a polyhydric aliphatic alcohol of 2 to 6 carbon atoms is employed as the active agent.

Description

The invention relates to a process for deliming skins, in which cyclic carbonates of polyhydric aliphatic alcohols are used as the “safety deliming agent”. This avoids the disadvantages associated with overdosing the known deliming agents.

Ashes of the skins for the purpose of depilation and skin disruption in the context of leather production are carried out in an alkaline medium, usually with the help of inorganic or organic sulfides. Lime is usually used as the alkaline agent, usually alone, but occasionally also in a mixture with caustic soda or soda. Subsequently, the liming chemicals, especially the lime, have to be freed from the material in preparation for further processing, that is, «descaled. This usually happens with acids or acid salts, e.g. B. phthalic acid, sulfophthalic acid, formic acid, acetic acid, boric acid, aliphatic dicarboxylic acid mixtures, hydrochloric acid, sulfuric acid and their ammonium salts. The usual conditions are:

• Die zum Entkälken erforderliche Chemikalienmenge hängt stark vom Gehalt der Häute an Kalk und gegebenenfalls anderen basischen Verbindungen, z. B. Natriumsulfid und Natriumhydroxid, ab. Da dieser stets schwankt, ist eine exakte Dosierung der Entkälkungschemikalien in der Praxis kaum möglich. Es kommt daher leicht zu einer Überdosierung. Diese hat zur Folge, daß der pH-Wert zu Beginn der Entkälkung bis auf oder sogar unter den isoelektrischen Punkt des Eiweißes (pH ca. 5) sinkt. Dabei werden beim Äschern in Lösung gegangene Eiweißsubstanzen, gegebenenfalls zusammen mit anderen gelöstem « Schmutz » ausgefällt. Sie schlagen sich auf der Oberfläche des Hautmaterials in schwer entfernbarer Form nieder, machen es unansehnlich und stören bei späteren Verarbeitungsgängen, insbesondere beim Färben. Auch der Griff wird ungünstig beeinflußt. Dieses Problem versuchte man bisher zu lösen, indem man Ammoniumsalze als sogenannte « Sicherheitsentkälkungsmittel verwendete. Da aber der pH-Wert wäßriger Ammoniumsalzlösungen unter 5 liegen kann, bieten auch diese Mittel nicht die erwünschte Sicherheit. Außerdem ist der Einsatz von Ammoniumsalzen auch aus ökologischer Sicht problematisch. Ammoniumsalze werden in Kläranlagen nur unvollständig abgebaut und erhöhen somit den CSB-Wert des Ablaufwassers.

A serious disadvantage of the previously known descaling processes is based on the following circumstances:

• The amount of chemicals required for descaling depends heavily on the skins' content of lime and possibly other basic compounds, e.g. As sodium sulfide and sodium hydroxide. Since this always fluctuates, exact dosing of the decalcifying chemicals is hardly possible in practice. An overdose is therefore easy. As a result, the pH value drops to or even below the isoelectric point of the protein (pH approx. 5) at the beginning of the decalcification. In the process of liming, protein substances that have gone into solution are precipitated, possibly together with other dissolved «dirt». They deposit on the surface of the skin material in a form that is difficult to remove, make it unsightly and interfere with later processing steps, especially when dyeing. The grip is also adversely affected. So far, attempts have been made to solve this problem by using ammonium salts as so-called “safety decalcifying agents. However, since the pH of aqueous ammonium salt solutions can be below 5, these agents also do not offer the desired safety. The use of ammonium salts is also problematic from an ecological point of view. Ammonium salts are only partially broken down in sewage treatment plants and thus increase the COD value of the waste water.

The use of butyrolactone as a deliming agent is known from DE-C-804827. This inner ester hydrolyzes very slowly under the usual descaling conditions. The necessary descaling times are therefore very long. This is probably the reason why butyrolactone has not found its way into practice. Finally, according to DE-C-28 25 081, hydrolyzing esters are used under the descaling conditions. They overcome the above-mentioned disadvantages of conventional decalcifying agents by not dropping the pH below 5 and also being free of ammonium salts. However, they too cannot prevent hydrogen sulfide from being released when they are overdosed. Due to the high toxicity of hydrogen sulfide, this possibility represents a constant latent danger to life for the staff, apart from the unpleasant smell.

The invention was therefore based on the object of developing a deliming process which not only avoids the disadvantages of the usual deliming agents mentioned above, but also the development of hydrogen sulfide with certainty.

This object is achieved in the method according to the invention. It was surprisingly found that of all the esters examined, only the cyclic esters (5- and 6-rings) of polyhydric aliphatic alcohols with 2-6 carbon atoms and carbonic acid are suitable for achieving the above-mentioned object. With them, the pH does not drop below 7.5, even in the case of a gross overdose, so that hydrogen sulfide is not released in dangerous amounts, while it can certainly drop below 7.5 with other esters, with hydrogen sulfide escaping in amounts such that over Descaling liquor often H ₂ S concentrations of 2000 ppm can be measured. Among the cyclic carbonates of polyhydric aliphatic alcohols to be used according to the invention, preference is given to those whose equivalent weight (with respect to the carbonic acid component, not the alcohol component) is below 150, preferably below 100 and in particular below 60. Suitable carbonates are accordingly the carbonic diesters of polyhydric, if appropriate still free hydroxyl groups, of polyhydric, that is to say more than one, preferably 2 to 3, in particular 2 hydroxyl group-containing, aliphatic alcohols having 2 to 6, preferably 2 to 3, carbon atoms, at least 2 hydroxyl groups being linked to one another Are arranged in the 1,3- or preferably 1,2-position, for example the cyclic carbonates of 1,2-butanediol, 2,3-butanediol, neopentyl glycol, glycerol, the dicarbonate of pentaerythritol (spiro compound), preferably the cyclic carbonate from Ethylene glycol and 1,2-propylene glycol.

The carbonates are prepared in a known manner, e.g. B. by reaction of the alcohols with phosgene or preferably of epoxides with carbon dioxide.

The carbonate is hydrolyzed by the limestone contained in the liming so far until a pH value in the range of - depending on the carbonate excess -7.8 to 7.5 is reached. The lime is dissolved as calcium bicarbonate. Then the hydrolysis stops. The pH did not fall below 7.5 in any experiment. This range is optimal, because on the one hand this ensures that no hydrogen sulfide escapes from the solution, and on the other hand the pH is not higher than absolutely necessary to avoid the formation of hydrogen sulfide. Too high pH values are also undesirable with regard to the grain smoothness and the feel of the leather as well as the safety of limescale removal.

The amounts used for the carbonate depend on the content of lime and other basic compounds (e.g. sodium sulfide and sodium hydroxide) in the skin material. They are of the order of 0.8 to 4%, preferably 1 to 2%, based on the weight of the wet hides.

The carbonates are preferably dissolved in water. The emulsification or dispersion of insufficiently water-soluble carbonates can be carried out in the usual way with cationic, anionic or nonionic emulsifiers. It is in no way problematic.

The parts and percentages given in the examples relate to the weight.

example 1

136 parts of ashed beef flanks with a gap thickness of 3.5-4.5 mm were first washed with 408 parts of water at 35 ° C. for 20 minutes by tumbling in a tanning drum. The wash liquor was then drained completely. 3.4 parts of 1,2-propylene carbonate and 0.3 part of a commercially available wetting agent based on an alkyl sulfonate were then added. After drumming for 5 minutes, checking the pH of the new liquor resulting from the swelling of the nakedness showed a value of 8.7. 20 minutes after adding the descaling agent, the liquors showed a pH of 7.9. At that time, the nakedness was about 50% decalcified. After a further 20 minutes the pH was 8.0 and the nakedness was 70% decalcified. 80 minutes later the decalcification was complete and the pH was 8.2.

Comparison test

Example 1 was repeated in exactly the same way, except that a commercially available product based on the ammonium salt of sulfophthalic acid was used as the deliming agent. After 5 minutes of walking, the pH of the decalcifying liquor was measured at 3.2, 15 minutes later it had risen to 7, and then after 40 minutes of walking it adjusted to the final pH of 8.6.

A comparison of the decalcified pelts showed that the pelts of Example 1 were significantly cleaner and therefore lighter than those of the comparative test.

The following examples were carried out in a manner similar to Example 1. 100 parts of bare bark were always decalcified. The test results are summarized in the table.

All of the decalcified nakedness was exceptionally clean and bright.

Claims (1)

1. A process for deliming a conventionally limed hide for from one to six hours under conventional deliming conditions in respect of liquor length, temperature and pH of the liquor, wherein a 5- or 6- membered cyclic carbonate of a polyhydric aliphatic alcohol of 2 to 6 carbon atoms is employed as the active agent.