EP0210481A2 - Method of deliming hides and use of sulphurous acid cyclic esters as deliming agents - Google Patents

Abstract

The invention relates in particular to a time-saving and efficient method for deliming skins with a cyclic ester of sulfurous acid with an aliphatic 1,2-diol and the use of these esters as deliming agents.

Description

The invention relates in particular to a time-saving and efficient method for deliming skins with cyclic esters of sulfurous acid with an aliphatic 1,2-diol and the use of these esters as 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. Then the material has to be freed from the liming chemicals, especially the lime, in preparation for further processing, that is to say "decalcified". This is usually done with acids or acid salts or, for example according to DE-C-28 25 081, with esters hydrolyzing under the decalcification conditions.

The usual conditions are liquor lengths of 0 to 300, preferably 0 to 200%, based on the weight of the pelts. Temperatures from 5 to 38, preferably 25 to 35 ° C, pH values in the liquor at the end of the decalcification from 6 to 8.7 and decalcification times from 1 to 6, preferably 1 to 3 hours.

The serious disadvantages of acidic decalcification processes are based on the fact that pH values below 5 can occur at the start of decalcification, which means that protein substances that have gone into solution can be precipitated with other substances during liming and hydrogen sulfide escapes in such quantities that H₂S often escapes through the decalcification liquors - Concentrations of 2000 ppm and more can be measured.

From EP-PS 59 909 it is known that 5- or 6-membered cyclic carbonates of a polyhydric aliphatic alcohol, e.g. Ethylene carbonate or 1,2-propylene carbonate to be used as a decalcifying agent, the pH of which should not fall below 7.5 when used and the development of gaseous hydrogen sulfide is largely avoided.

A disadvantage of this process is that the cyclic carbonates hydrolyze relatively slowly in the deliming process, so that in the case of thicknesses of around 4.5 mm, times of at least 2 hours are required for complete deliming. These times are sometimes too long for practice.

If you want unsplit bare layers with a gap thickness of more than 4.5 mm, e.g. 5 to 7 mm, descale, descaling times are so long that the use of cyclic carbonates becomes inefficient.

Under the process conditions described in EP-PS 59 909, less gaseous hydrogen sulfide escapes from the decalcifying liquor compared to the usual acidic decalcifying agents or compared to the esters mentioned in DE-C-28 25 081, but it is disadvantageous that this decalcifies at the end is solved. If this liquor is combined with acidic wastewater, which happens more frequently in practice, then high amounts of gaseous hydrogen sulfide are produced, which can lead to severe annoyance and even danger. In addition, noticeable hydrogen sulfide is also released from the decalcified pelt material in the subsequent pick process by the acid supply.

The object of the invention is to provide a decalcifying agent for a rational decalcification process in which the disadvantages mentioned are avoided.

in der R¹ ein Wasserstoffatom oder einen gesättigten Alkylrest mit 1 bis 4 C-Atomen, der gegebenenfalls durch ein Chloratom substituiert ist, und R² ein Wasserstoffatom oder einen Methylrest bedeuten, zum Entkälken von Häuten sowie ein Verfahren zum Entkälken von Häuten unter an sich üblichen Entkälkungsbedingungen hinsichtlich Flottenlänge, Temperatur und pH-Wert der Flotte, bei dem man als wirksames Agens eine Verbindung der Formel (I) einsetzt.The invention relates to the use of compounds of the formula I.

in which R¹ is a hydrogen atom or a saturated alkyl radical having 1 to 4 carbon atoms, which is optionally substituted by a chlorine atom, and R² is a hydrogen atom or a methyl radical, for deliming hides and a process for deliming hides under conventional deliming conditions with regard to liquor length, temperature and pH of the liquor, in which a compound of the formula (I) is used as the active agent.

The decalcification according to the invention takes place completely within 30 minutes to 4 hours, ie, surprisingly, the decalcification times can frequently be halved by the invention compared to the cyclic carbonates described. As a rule, descaling is completed within 60 to 90 minutes in the case of split beef bare bones with a split thickness of 4 to 4.5 mm. In addition, it was surprisingly found that despite a lower pH value at the end of the descaling process, no higher concentration of gaseous hydrogen sulfide was found is, although this would have been expected at a somewhat lower pH range.

In addition, it was surprisingly found that when using the new decalcifying agent, in the pickling process following the decalcification, in contrast to the processes described in EP-PS 59 909, no hydrogen sulfide is released from the decalcified pelts when the acid is supplied. Moreover, in contrast to the cyclic carbonates, only about 1/4 to 1/5 of the amount of hydrogen sulfide is found in the decalcifying liquor when using the decalcifying agent according to the invention.

Alkyl radicals having 1 to 4 carbon atoms are to be understood as meaning saturated, straight-chain or branched aliphatic radicals which are optionally substituted by a chlorine atom. Of these, methyl and ethyl are particularly preferred.

The particulars to be emphasized for R¹ and R² are hydrogen and methyl.

The compounds of formula (I) are cyclic esters of sulfurous acid with an aliphatic 1,2-diol. The compounds of the formula I are known or can be prepared in a manner known per se, for example by reacting sulfur dioxide with a corresponding epoxide.

Examples of compounds of the formula I which may be mentioned are: ethylene glycol sulfite, propylene glycol sulfite, 1,2-butylene glycol sulfite, isobutylene glycol sulfite and the cyclic ester of epichlorohydrin and sulfur dioxide.

Ethylene glycol sulfite and propylene glycol sulfite are mentioned as preferred.

The general descaling conditions with the agent to be used according to the invention correspond to the usual ones in practice:

Ashes are used in the usual way, for example cattle flanks and skins of small animals such as sheep and goats. As a rule, beef skins cleaved from the liming with a thickness of 2.5 to 4.5 mm, for example, which have been washed after liming, for example by milling with water, are used. Uncleaved pelts, for example, have a thickness of approximately 5 to 7 mm. Small animal skins, such as those of sheep and goats, thicknesses of 1 to 3 mm.

For the decalcification according to the invention, liquor lengths are from 0 to 300%, based on the weight of the nakedness. Temperatures of 5 to 38 ° C and pH values within a range of 6 to 8.7 are appropriate. The preferred areas are:

Fleet lengths from 0 to 200%, temperatures from 25 to 32 ° C and pH values in the range from 6 to 7.5.

The compounds of the formula I to be used according to the invention are expediently used in an amount of 0.5 to 3, preferably 1 to 2%, based on the pelt weight.

The compounds of formula I are liquid and are advantageously added directly or in the form of an aqueous solution. Emulsification or dispersion of insufficiently water-soluble compounds of the formula I can be carried out in a conventional manner with cationic, anionic or nonionic emulsifiers. It is in no way problematic.

While the descaling process according to European patent 59 909 with a cyclic carbonate does not fall below a pH value of 7.5 and the hydrolysis stops at a pH value in the range from 7.8 to 7.5 (see page 3 , Lines 4 ff), with the decalcifying agent according to the invention a complete decalcification as the end point reaches a pH of 6 to 7, without this resulting in a higher H₂S concentration.

The decalcifying agents according to the invention can also be used in mixtures with other customary deliming agents, for example the cyclic carbonates mentioned above, in particular ethylene and propylene carbonate, esters of formic acid, boric acid, ammonium salts or ammonium sulfate.

If mixtures of different decalcifying agents are used, decalcifications together with ethylene, propylene carbonate or ethylene glycol formate are preferred. Mixtures of a cyclic sulfite of the formula I and one of the cyclic carbonates mentioned above are advantageously used, in which 10 to 50% by weight of the cyclic sulfite of the formula I have been replaced by a cyclic carbonate. These mixtures are also expediently used in an amount of 0.5 to 3, preferably 1 to 2%, based on the weight of the nakedness.

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

example 1

110 parts of ashed beef flanks with a gap thickness of 4 to 4.5 mm are first washed with 330 parts of water at 35 ° C. for 15 minutes by being drummed in a tanning drum. The wash liquor is then discarded. 110 parts of water at 35 ° C. and then 2.2 parts of propylene glycol sulfite are added to the washed pelts in the same barrel. After 5 minutes of flexing, a pH of 6.9 is measured in the liquor. 30 minutes after adding the descaling agent, the liquor has a pH of 7.2, the degree of descaling is 60%. 30 minutes later, the pH was 7.1 and the pelts were 75% decalcified. The descaling is complete after a further 30 minutes, the final pH is 7.

Comparative Example 1

In parallel to Example 1, 2.2 parts of propylene carbonate are used as descaling agents under the same conditions. After 5 minutes, the liquor shows a pH of 8.1 with a degree of descaling of 10%. Decalcification is complete after 3 hours and the final pH is 8.4.

The following example or comparative examples are also carried out according to example 1. The overall results are summarized in Table 1.

Example 3

Analogously to Example 1, 1100 parts of ashed beef flanks with a gap thickness of 5 to 5.5 mm are cooled to 1100 parts of aqueous liquor by adding 22 parts of ethylene glycol sulfite. The pH of the liquor is initially 6.6 after 3 hours 6.1. A hydrogen sulfide concentration of 400 ppm is measured above the liquor in the gas space of the tanning drum.

Comparative Example 3A

In parallel to Example 3, the deliming is carried out with 1100 parts of beef flanks of the same gap strength in 1100 parts of aqueous liquor by adding 22 parts of the ethylene glycol formate mentioned in Example 1 in DE-C-28 25 081. The initial pH is 6.7, after 3 hours, well over 2000 ppm of hydrogen sulfide are measured in the gas space above the liquor at pH 6.4.

Comparative Example 3B

In parallel to Example 3 and Comparative Example 3A, descaling is carried out with 22 parts of ethylene carbonate. The initial pH is 7.6. Decalcification is complete after 3 hours. The final pH is 7.5; 800 ppm of hydrogen sulfide are measured above the fleet.

The sulfide content is determined in the residual liming liquor obtained according to Example 3 and Comparative Examples 3A and 3B.

The decalcified pelts obtained according to Example 3 and Comparative Examples 3A and 3B are washed for 2 minutes in 5000 parts of water at 25 ° C. and then in tanning vats with 1000 parts of pickling liquor (mixture of formic acid, sulfuric acid and sodium chloride) of pH 0.5 25 Pissed for minutes. The pickle liquor shows a pH of 2.7 at the end. The concentration of hydrogen sulfide is then determined in the gas space above the pickling liquor.

The entire results of the hydrogen sulfide determination in the descaling and pickling process are summarized in Table 2.

The numerical values show that during the descaling process itself, the hydrogen sulfide values are significantly reduced both in the protruding gas space as well as in the remaining liquor and even in the pickling process.

Claims (3)

1. Use of compounds of formula I.

in which R¹ is a hydrogen atom or a saturated alkyl radical having 1 to 4 C atoms, which is optionally substituted by a chlorine atom, and R² is a hydrogen atom or a methyl radical, for decalcifying skins. 2. Use of ethylene glycol sulfite or propylene glycol sulfite according to claim 1 for decalcifying skins. 3. A process for descaling skins that have been ashed in a customary manner under customary deliming conditions with regard to liquor length, temperature and pH of the liquor, characterized in that a compound of the formula I is used as the deliming agent.