TANNING AGENT

Abstract: A tanning agent comprising a composition of a mixed complex of aluminium (III) and titanium (IV) ions masked with a poly(hydroxy)­ monocarboxylic acid and a basic salt of an alkaline earth metal and a process for the pre-tanning, tanning and re-tanning of hides and skins using the tanning agent, alone or in conjunction with other tanning agents.

Description: This specification describes a composition suitable for use as a tanning agent in the preparation of leather and to a tanning process using the composition. The most commonly used and effective commercial tanning agents are salts of chromium (III). However, these suffer two serious drawbacks (i) that chromium imparts a substantial blue coloration to the tanned leather which causes problems with the preparation of pale-shade or undyed leathers and (ii) there are doubts about the toxicity of chromium (III) which make its presence undesirable in industrial effluents. Various alternative tanning agents, which overcome one or both of these problems, have been proposed but none of the alternative agents is as effective as chromoium (III) salts in protecting the leather against hydrolysis. A recently proposed alternative tanning agent, comprising a composition of a mixed complex of aluminium (III) and titanium (IV) ions masked with a poly(hydroxy)monocarboxylic acid, is described in GB 2,165,859A. Although this tanning agent is non-toxic and does not colour the leather, its tanning performance is pH sensitive and a number of pH adjustments are required before and during tanning. Furthermore, the tanned leather requires aging for 24 hours or more before it is suitable for use. It has now been found that the performance of, and ease of tanning with, this masked Al/Ti complex can be improved by the presence of certain salts. According to the present invention there is provided a tanning agent comprising a composition of a mixed complex of aluminium (III) and titatium (IV) ions masked with a poly(hydroxy)monocarboxylic acid characterised in that the composition contains from 2.5% to 45% of a basic salt of an alkaline earth metal (expressed as weight of metal oxide) based upon the total weight of the composition. The preparation and masking of the mixed complex is described in GB 2,165,859A (see particularly page 1 line 63 to page 2 line 33 and Examples 1 to 3). However, the present invention allows the employment of a broader range of Al:Ti ratios than are disclosed in the aforementioned patent. In the present tanning agent, the ratio of aluminium ions to titanium ions, expressed as Al₂O₃ and TiO₂ respectively, may conveniently vary from 0.8: 1 to 8: 1 and more preferably from 0.8: 1 to 4: 1; an especially preferred ratio is 2:1. The combined alumiunium and titanium content (expressed as Al₂ O₃ + TiO₂) of the agent is preferably from 10% to 30% by weight. The aluminium and titanium ions are preferably present as salts with a strong acid, especially as sulphates. The poly(hydroxy)monocarboxylic acid, which may be in the form of a water-soluble salt, especially with an alkali metal, is preferably of the type described in UK 2,165,859 A, especially an acid derivable from monosaccharides, such as a poly(hydroxy)hexanoic or poly(hydroxy)heptanoic acid, e.g. the heptonic acid and glucoheptanoic acid and the hexonic acids, gluconic acid, galactonic acid and mannonic acid. Any basic salt of an alkaline earth metal, i.e. one having a pH > 7 in aqueous solution, is suitable but preferred salts are those of calcium, magnesium and barium, such as carbonates, sulphites, thiosulphates etc., and especially the naturally occurring complex carbonates of magnesium and calcium, such as dolomite. According to a second feature of the present invention there is provided a process for the tanning of animal hides or skins, hereinafter referred to as "hides", which comprises treating the hides with a composition of a mixed complex of aluminium (III) and titatium (IV) ions masked with a poly(hydroxy)monocarboxylic acid characterised in that the composition contains from 2.5% to 45% of a basic salt of an alkaline earth metal (expressed as weight of metal oxide) based upon the total weight of the composition. The tanning process is conveniently performed in the pickling liquor with from 1% to 20%, more preferably from 3% to 10% by weight of the composition based on the total weight of hides, by gentle tumbling for periods of from 6 to 20 hours at or around ambient temperature and without any pH adjustment. Hides which have been tanned using the present composition can be subsequently re-tanned even after they have been dried which means that hides can be lightly tanned ("pre-tanned") and dried for protection, immediately after they have been removed from the animal, and subsequently stored and/or transported to a new location, in a dry condition, at a reduced weight and with reduced risk of bacterial spoilage. The pre-tanned hides can then be re-wetted and re-tanned, to give full leathers, in accordance with local practice. The present tanning agent can also be used in conjunction with chrome tanning agents, such as chromium salts, for the tanning of hides in order to reduce the quantity of chromium salts, and thus to alleviate the effects of the aforementioned problems associated with the use of chromium salts alone in the tanning of hides. The use of the present agent allows reductions in the usage of chromium salts of up to 70% compared with normal usage without any significant reduction in the resistance of the tanned leather to hydrolysis. In this way it is possible to obtain leathers which are resistant to shrinkage in boiling water and yet are only very faintly coloured. Furthermore, the liquor at the end of the tanning process is virtually exhausted of chromium salts and therefore poses a less serious problem for disposal. When a chrome tanning agent is used in conjunction with the present tanning composition, the amount of chromium, expressed as Cr₂O₃, can vary with in wide limits, up to 500%, with respect to the sum of the aluminium and titanium (expressed as Al₂O₃ and TiO₂) but is preferably from 10% to 300% and more preferably from 35% to 250%, by weight. It is possible to obtain a leather having a shrinkage temperature in excess of 90°C by the use of less than 1% by weight of a chrome tanning agent (expressed as Cr₂O₃ with respect to the hides) in conjunction with about 0.5% by weight of the present tanning composition (expressed as Al₂O₃ + TiO₂ with respect to the hides) and by increasing the proportion of the latter to from 1-2%, more preferably 1.5-2.0%, the shrinkage temperature can be raised to in excess of 95°C. Where a lower shrinkage temperature can be tolerated the proportion of chromium can be reduced further with proportional benefits in a reduced colour of the leather and reduced chromium content of the shavings and offcuts and the tanning liquor at the end of the process. The hides can be treated concurrently with the present composition and the chrome tanning agent or they can be used consecutively in either order. It has been found that the pre-tanned or fully tanned leathers resulting from the use of the present agent and process are plumper (and retain their plumpness during subsequent treatments) than conventionally tanned or pre-tanned leathers which means that splitting and shaving can be performed more accurately and with less waste of leather. This increased plumpness improves the quality of the leather, which has better "plastic metal elasticity" i.e. shape retaining characteristics. This means that hides can be converted into quality leathers having a better "feel" than they can by the use of conventional agents and processes. Because the present agent and process give rise to improvements in the quality of leather and the quality can be judged more accurately at the pre-tanning stage, the tanner can have greater freedom and confidence in the selection of tanned and pre-tanned leathers for the later and expensive finishing stages which are so vital to the production of top quality leathers. Because the present invention allows a significant reduction (up to 70%) in the quantity of chromium required to achieve a fully tanned leather (with shrinkage temperature > 95°C) the blue coloration associated with conventional chrome tanning of leather is reduced significantly. It has been found that this permits the more effective coloration of the leather with standard acid dyes. The new tanning agent permit the achievement of brighter deep shades and truer pastel shades. It has also been found that the dyes exhaust more fully into leather tanned with the present composition which, in turn, allows more efficient use of dyes with less waste in the dye bath. It has also been found that any defects in the leather, such as scarring or other surface damage arising from damage to the hides, on the animal or after skinning, are more effectively masked by conventional dyes when the leather is tanned with the present tanning agent and process, in place of conventional tanning processes. This means that the quality of hides can be upgraded by use of the present tanning agent and process. Without prejudice to the scope of the invention, it is believed that the advantages arising from the use of the present agent and process are due, at least in part, to the different structure of the derived leather. Microscopic examination of leathers from the present process shows that they have a more fibrous or textured structure, akin to that of the original hide, in comparison with conventionally chrome tanned leathers. The present invention is further illustrated by the following examples in which all parts and percentages are by weight unless otherwise indicated. Agent A This is a masked aluminium/titanium tanning agent in accordance with GB 2,165,859, comprising a solution of titanium sulphate (20 g/l as TiO₂) and aluminium sulphate (40 g/l as Al₂O₃) masked with sodium glucoheptonate (1 mole/mole of total metal oxides, i.e. Al₂O₃ + TiO₂) and dried to a powder. Example 1 (a) Agent B A complex tanning agent was prepared by mixing 800 parts of Agent A, 100 parts of calcium carbonate powder and 100 parts of magnesium carbonate powder. (b) Tanning Process with Agent B To 100 parts of depilated, lime-treated bulls hide sides, pickled to pH 3, and tumbling in the pickling bath at 18°C were added 5 parts Agent B and 85 parts of water. Tumbling was continued for 4 hours with the tanning drum on slow rotation (3-5 rpmin) followed by 4 hours of alternating 1-hour spells at rest and 1-hour spells at slow rotation. At the end of this period, the temperature of the bath was 37°C and the pH was 3.5. The leathers obtained were very fine-grained and perfectly white, i.e. as the natural colour of the skins, and had a shrinkage temperature in water of 78°C. They were also in a condition for subsequent vegetable, synthetic or mineral (e.g. chrome) tanning operations. Unlike conventional chrome leathers (wet-blues), the leathers could be dried, for storage and/or transportation, and subsequently re-wetted ready for de-tanning and/or re-tanning with chrome or other tanning agents when rewetted in a pickling liquor. Example 2 (a) Agent C A complex tanning agent was prepared by mixing 600 parts of Agent A with 400 parts of powdered dolomite DRB 30 having the following chemical composition: CO₂ 47.42% CaO 31.21% MgO 21.11% Al₂O₃ 0.02% Fe₂O₃ 0.01% Moisture 0.05% pH of the saturated solution: 9.85 (b) Tanning Process with Agent C and a Chromium Tanning Agent To 100 parts of sheepskins, which had been dewoolled, lime-treated and pickled to pH 3.2, tumbling in the pickling bath at 20°C were added 120 parts of water, 12 parts of fine salt, 5 parts of Chrome Tanning Agent 1 (a commercial chromium sulphate comprising 25% chromium as Cr₂O₃ and having a basicity of 33°Sch) and 1.5 parts of Agent C. After 4 hours tumbling, when the pH was 2.5, a further 5 parts Agent C were added and tumbling continued for 7 hours. After standing 1 hour followed by rotation for 10 minutes, the pH rose to 3.8. The leather obtained was very light and fine-grained with a shrinkage temperature of at least 100°C and could be stuffed at once without further operation. At the end of the tanning process the liquors contained less than 0.2 parts/litre of chromium metal. To obtain a leather of equal resistance to shrinkage without the inclusion of the present composition would have required at least 10 parts of Chrome Tanning Agent 1 and the resultant leather would have had a substantial blue coloration. Furthermore the liquor at the end of the tanning process would have contained at least 0.6 parts/litre of chromium metal. Example 3 (a) Agent D A mixture was made of 800 parts of Agent A with 100 parts of magnesium carbonate, 50 parts of calcium carbonate and 50 parts of calcium formate. (b) Tanning Process with Agent D and a Chromium Tanning Agent 100 Parts of depilated, lime-treated cowhide sides, (1.7-1.8 mm thick) pickled to pH 3.1, were tumbled in a bath at 18°C with 80 parts of water, 7 parts NaCl, 0.3 parts sulphuric acid and 0.5 parts formic acid for 25 minutes until the pH was 2.6-2.7 To the pickling bath was added 1 part of Agent D and rotation continued for a further 1 hour. Then, with the door closed and the drum rotating, 10 parts of Chrome Tanning Agent 2 was introduced via the hollow axle. Rotation of the bath was continued for 2 hours, when the pH was 2.5 and the temperature 32°C, after which a further 4.5 parts of Agent D was added. After a further 7 hours of rotation, the pH had risen to 4, and the temperature to 40°C. The resultant leather was resistant to boiling water. Analysis of the bath liquor showed that only 6% of the original chromium remained. Chrome Tanning Agent 2 was a liquor or chromium sulphate obtained by reducing NaCr₂O₇ with glucose having a Cr₂O₃ content 0.123g/l and a basicity of 42°Sch (this is a relatively high basicity in solution such that it represents about half of its volume of commercial chromium sulphates in conventional powder form). The leather was then retanned without maturation. by adding 200 parts of water at 30°C and 2.4 parts of degreasing agent for each 100 parts of the original hides and rotating the drum for 30 minutes when the pH of the bath was 3.7-3.8. The liquors were then drained from the bath and replenished with 120 parts of water at 40°C and 6 parts of Agent D. Following rotation for 2 hours, the pH rose to 4.6 and the liquors were again drained. The leather was then stuffed by tumbling for 1 hour with 60 parts water at 60°C, 8 parts emulsifying fat liquor, 0.6 parts raw fat liquor and 0.005 parts of antiseptic when the pH was 4.9. After adding 2.2 parts of formic acid the drum was rotated for 30 minutes when the pH was 4.2. The resultant leather was white and very light, and could be buffed to provide Nubuck, or corrected grain, or used without further treatment as full-grain, white finish. Example 4 (a) Agent E A mixture is prepared of 700 parts of Agent A, 100 parts of calcium carbonate, 100 parts of magnesium carbonate and 100 parts of barium carbonate. (b) Tanning Process with Agent E In a paddle-vat, woolled sheepskins pickled to pH 2.6 were tumbled in a bath consisting of 30 litres per skin of a 6% NaCl solution for 2 hours. The pH of the solution then stabilised at around 2.8, and 24 g of Agent E, per litre of liquor, was added. The paddle was rotated non-stop for 4 hours and then intermittently for 12 hours to obtain a rise in temperature to around 35°C. Then 10 litres of hot water (60°C) per skin was added and rotation continued for a further 4 hours. The bath was then emptied and the skins were rinsed and drained. The tanned skins were resistant to hot water at a temperature of 82°C, and had a flesh-side which was perfectly white and a wool of remarkable whiteness and gloss compared with similar woolled skins which have been chrome-tanned by conventional processes. Example 5 (a) Agent F A mixture was prepared of 800 parts of Agent A, 186 parts of dolomite DRB 30 and 14 parts of sodium sulphite. (b) Tanning Process with Agent F and a Chromium Tanning Agent To 100 parts of calf skins, pickled to pH 2.8 and tumbled in a tanning drum with 60 parts water at 18°C and 10 parts fine salt (NaCl) for 10 minutes, was added 25 parts of Chrome Tanning Agent 3 and rotation continued for 4 hours at slow speed (2-3 rpm). At this point 6 parts of Agent F were added and rotation was continued for a further 4 hours. The resultant leather had an extremely fine and smooth grain, a white colour with a slight tinge of pink, and was shrink-resistant in water at 95°C. Chrome Tanning Agent 3 was an organo-metallic tannin, comprising a chrome salt complexed with aldolised beta-naphthol sulphonic acid (A-BNSA), neutralised with ammonia and re-acidified with acetic acid, of which a solution of 28% dry extract contains: A-BNSA: 62.5g/l Cr₂O₃: 23.0g/l Na₂SO₄: 141.0g/l or: Example 6 Tanning Process with Agent C, Agent F and a Chrome Tanning Agent To 100 parts of sheepskins, dewoolled by sweating, lime-treated and pickled to pH 1.8, tumbled for 30 minutes at 20°C in 200 parts of water, 20 parts of sodium chloride and 3 parts of Agent C, was added 3 parts of Chrome Tanning Agent 4 (a commercial mineral-reduced chromium sulphate with a basicity of 45°Sch). After 30 minutes of rotation the leather was totally impregnated by the chromium and the temperature and pH were 30°C and 2.4, respectively. Then 3 parts of Agent F were added and rotation continued for 8 hours, when the pH and temperature were 3.5 and 40°C, respectively. The tanned skins were resistant to shrinkage in water at 98°C, were pale coloured, and were ready for immediate tanning with vegetable or synthetic tannins. Example 7 (a) Agent G A mixture was prepared of 900 parts of Agent A, 60 parts of Dolomite DRB 30 and 40 parts of calcium formate. (b) Tanning Process with Agent G and a Chrome Tanning Agent 100 parts of calf skins, which had been limed, rinsed, fleshed, trimmed and re-rinsed were deliming and bated with 250 parts of a 0.17% aqueous solution of ammonium sulphate at 35°C followed by, after rotation for 1 hour, 0.8 parts of a pancreatic bate. Rotation was continued for a further 1 hour before draining the skins. The skins were then sequentially pickled and tanned in the same bath as follows. The following solution was added to the bated skins, Sea salt 5.0 parts Sulphuric acid 0.6 parts Formic acid 0.3 parts Water at 20°C 150.0 parts rotation was effected for 3 hours and the skins were left soaking in the bath overnight. The pH at the end of pickling was pH 4. Tanning was then effected by the addition of 1 part of Chromium Tanning Agent 4 (chrome alum) and, after 10 minutes, 4 parts of Agent G and 4 parts of Chromium Tanning Agent 5 (a commercial chrome salt with a basicity of 33° Sch). Rotation was then continued for 9 hours after which the liquor was at pH 4. The leathers were removed and were ready for and subsequent chemical operations. The leathers obtained are resistant to boiling water and are of a light colour and fine grain. Example 8 Tanning Process with Agent D and a Chrome Tanning Agent The procedure of Example 3 was repeated except that Chrome Tanning Agent 2 was replaced by 5 parts of Chrome Tanning Agent 6 (a commercial "self-basifying" chrome salt) and 5 parts water. The resultant leather had the same properties.

Claims: 1. A tanning agent comprising a composition of a mixed complex of aluminium (III) and titanium (IV) ions masked with a poly(hydroxy)monocarboxylic acid characterised in that the composition contains from 2.5 to 45% of a basic salt of an alkaline earth metal (expressed as weight of metal oxide) based on the total weight of the composition. 2. A tanning composition according to Claim 1 wherein the ratio of aluminium ions to titanium ions, expressed as Al₂O₃ and TiO₄, respectively, is from 0.8: 1 to 8: 1. 3. A tanning composition according to Claim 1 or Claim 2 wherein the total amount of aluminium ions and titanium ions, expressed as Al₂O₃ and TiO₂, is from 10% to 30% by weight based on the total weight of the composition. 4. A composition according to any one of Claims 1 to 3 wherein the poly(hydroxy)monocarboxylic acid is glucoheptanoic acid or gluconic acid or the salt thereof with an alkali metal. 5. A composition according to any one of Claims 1 to 4 wherein the basic salt of the alkaline earth metal is a calcium carbonate, magnesium carbonate or a dolomite. 6. A process for the tanning of animal hides which comprises treating the hides with a tanning composition according to any one of Claims 1 to 5. 7. A tanning process according to Claim 6 using from 1% to 20% by weight of the tanning composition. 8. A tanning process according to Claim 6 or Claim 7 in which the hides are treated concurrently or consecutively with a chrome tanning agent.

Inventors: ARBAUD, PAUL GEORGES
Applicants: I.C.I. FRANCOLOR SNC
Application year: 1988
Main classification: C14C3
IPC codes: C14C3