GB2057497A - Tanning Process - Google Patents

Abstract

Skins or hides are tanned by drum tanning with a vegetable tan and simultaneously or thereafter tanning with an aluminium compound. Mimosa extract, condensed or catechol tanning and wattle tanning may be used as the vegetable tanning agent and aluminum sulphate as the aluminium compound.

Description

SPECIFICATION Tanning Process The present invention relates to a tanning process and more particularly relates to a process for the tanning of hides and skins which includes a tanning stage using a soluble aluminium salt.

One of the most common tanning processes in use of the tanning industry to yield leather from hides and skins is the well-known process of chrome tanning which process involves treating the hide or skin to be processed with a salt of chrominum, for example chromium sulphate. The chrome tanned leather so obtained is useful, for example, for shoe and boot uppers and also garment leather.

Although chrome tanning produces leather of good quality, it has the disadvantage of being undesirable from an environmental view-point because salts of chromium in paricular hexavalent chromium are toxic, and effluent from tanning plants using such a process may as a consequence contain toxic waste.

It has now been found that hides and skins may be tanned to yield leather of at least equivalent quality to that obtained by chrome tanning, but without any associated toxicity problems, by means of a drum vegetable tanning process with subsequent or simultaneous tanning with a soluble aluminium salt. The conbination of drum tanning with a vegetable tan and tanning with a soluble aluminium salt which is utilized according to the invention is a novel concept and the various aspects thereof described herein incorporate this inventive concept with an aluminium containing tanning agent in particular a water soluble aluminium salt.

In a preferred aspect of the invention the vegetable tanning in the drum is followed further tanning with the soluble aluninium salt. However they may be carried out simultaneously. One may also form a mixture of the vegetable tanning agent and the aluminium salt and use this mixture as the tanning agent. The aluminium salt may form a complex with the vegetable tanning agent and the mixture will then contain such complex. This complex corresponds to that which appears to be formed in situ when tanning with the two tanning agents separately, either simultaneously or consecutively as in the preferred aspect of the invention referred to above.

For optimum results the preferred embodiment is carried out under certain pH conditions as described more fully herein.

In the preferred embodiment of the invention the hide or skin is first subjected to a vegetable tanning process in a drum this process being known and described for example in U.K. Patent No.

1,394,684. According to this process a pickled and if desired delimed hide or skin which optionally has been degreased is conditioned in the presence of water with an inorganic sulphate and/or a syntan and .then subjected to vegetable tanning process in the presence of the added sulphate or syntan. Preferred sulphates used for the conditioning are sodium and magnesium sulphate, of which anyhydrous sodium sulphate being particularly preferred.

Preferred tannins for use in the vegetable tanning stage are condensed or catechol tannins, wattle tannins and most preferably mimosa extract. Tanning with mimosa extract is particularly preferred as the vegetable tanning agent according to the invention; the preferred water soluble aluminium salt for use therewith is aluminium sulphate.

The preliminary stages of treatment of the hide or skin up to the stage at which a hide or skin cured with a mimosa tanning agent are described below.

Soaking: A normal soaking for the type of stock being worked should be given bearing in mind that efficient soaking is essential as for any successful tannage. The addition to the soak of 0.5% to 1.0% degreasant on weight of hide can be beneficial if the hides tend to be greasy.

Liming: Liming can be conventional, either a 2 day drum liming before fleshing or a 1 day liming followed by fleshing and a 1 day reliming in a drum being suggested. The actual process employed is largely immaterial provided that structure of the hide is well opened up to facilitate the subsequent penetration of tan. During the drum liming the process should be intermittent-running say 5 minutes in every hour-with the drum not left idle overnight, which can cause uneven liming and severe creasing of the bottom hides in the drum. Good clean fleshing is essential particularly to remove fat deposits and flesh tissue both of which can retard tan penetration. If the fleshing operation is carried out midway through the liming stage, the advantage is gained that the fibre structure tends to be mechanically opened up.

(All percentages which follow are based on the limed pelt weight) Wash: 200% float of water Drum for 1 hour Loose lime is washed off the pelt reducing the amount of expensive deliming chemicals used in the next stage.

Deliming: Any deliming method is satisfactory, provided that it is capable of bringing the pelt to a point where the whole cross section of the most compact area of the hide is between a pH of 6.5 and 7.0.

One particular method is as follows:- 100% float of water 2% Ammonium sulphate Drum for 2 hour Add a suitable bate according to manufacturers recommendations, if desired.

Drum 21 to 1 hour Add 1.5% sodium bisulphate powder Drum until deliming is complete in the thickest parts tested with, for example, a Universal Indicator.

Drain: Drain as much float as possible from the drum.

Wash: 100% float of water Drum 2 hour Drain: Drain as much float as possible from the drum Pickle: The well drained drum should contain approximately 30% float.

To the float remaining in the drum; Add 6% salt or sufficient salt to give 120 B6 after drumming for 2 hour Add separately equal parts of formic acid (85%) and sulphuric acid (98%). The formic acid must be added first.

E.g. (Add 0.5% formic acid, drum 2 hour, add 0.5% sulphuric acid) Both acids must be previously diluted by pouring one part of the strong acid into nine parts of water before addition. Sufficient acid should be used to give a pH through the pelt section of 3.2-3.3 at the end of the pickling operation. A cut section should be checked with Universal Indicator.

Drum + hour for each millimetre of substance in the thickest part of the pelt being processed, typically 2-3 hours.

Drain: Drain as much float as possible from the drum Sulphate Conditioning: Add 10% anhydrous sodium sulphate Drum approximately - hour for each millimetre of substance in the thickest part of the pelt being processed, typically 3-4 hours.

The temperature during conditioning should not be allowed to fall below 250C but should not exceed 38"C.

The final pH of the section should be between 3.6--3.8.

Penetration can be gauged visually by examination of the cut section which should be homogenous throughout.

Vegetable Tannage: Add to drum 1221 15% Spray Dried Minosa Powder, the percentage used being dependent upon substance.

Drum 3 hours.

Add 50% water Continue drumming for 2 hour per millimetre of substance, typically 2-3 hours but drumming should be continued until 100% penetration is achieved.

The tan will now have been fixed and excess salt can be washed out.

Wash: Wash until removal of sulphate is complete Samm: Samm the stock to remove surplus moisture and set out lightly.

At this stage one has a mimosa-cured hide or skin suitable for a further tanning with an aluminium salt according to the invention. It is important that at this stage the hide or skin should have a pH of 3.6 to 3.8;as indicated it may be dried but this is not necessary. The treatment with aluminium sulphate The treatment with aluminium sulphate is preferably carried out in a drum, although a pit process (which is however considerable slower) may also be used.

The aluminium sulphate treatment is performed with the following sequential steps 1. Wet back (optional) Re-wet hides or skins, if dry; 2. Treat with a solution of aluminium sulphate; 3. Basification; 4. Neutralization (Optional; 5. Fatliquoring (Optional; 6. Dyeing (Optional) The wet back stage is preferably carried out with 100% water, based on the weight of the hide or skin which is being vegetable tanned, at 400 C. Advantageously the wet backed hide or skin is then horsed up over night.

After wetting back the hides or skins are ready for treatment with the aluminium sulphate. The amount of aluminium sulphate used should be sufficient to give 2% Al203 in the crust leather, and it has been found that the most practical amount of aluminium sulphate used is 3% Al203,that is, 18 to 19% Ai2(SO4)3. 1 6H20 based on the weight of dry vegetable tanned hide or skin. Greater amounts of aluminium sulphate may be used where greater uptake of aluminium sulphate is required, although the amount indicated is generally preferred. The amount of aluminium retained in the skin or hide after the further tanning will depend inter alia on the anount of mimosa extract used to tan the skin or hide in the first stage.The use of a spray-dried wattle extract at a concentration of 1 5% based on the weight of the split lime weight of the hides or skins is the most practical level. The pH during the further tanning with aluminium sulphate should correspond to that of the skin or hide cured with mimosa which as indicated above is of the order of 3.6 to 3.8 because at this pH maximum penetration of the aluminium sulphate throughout the skin or hide takes place and the aluminium sulphate complexs with the mimosa tan already present and bound to the collagen. At a higher pH precipitation of a complex between the aluminium sulphate and the mimosa tan takes place which is undesirable at this stage.

However by using a so-calied masking agent such as for example an acetate preferably hydrated sodium acetate one may operate at a pH of up to 4 without precipitation. Preferably this is carried out by forming a solution of the appropriate amount of aluminium sulphate as is demanded by the required properties of the end product with 10% of sodium acetate .3H2O, based on the weight of aluminium sulphate used, in 250%, of warm water.

Advantageously, the thus formed solution is left overnight before use.

When the treatment with aluminium sulphate solution, which is preferably carried out in a drum for 2 hours is complete, the contents of the drum are basified preferably to a final pH of up to 4.0 with a basic reagent such as soda ash, which is preferably added in stages in analogous fashion to that employed in standard chrome tanning. This step is necessary even when a masking agent has been used as described above. It has been found that increase in basicity results in an increase aluminium uptake shrinkage temperature and also aluminium utilization. Most preferably the contents of the drum are basified to a pH of 3.9. After this stage, the treated hides or skin are possible horsed up over night and then rinsed with an excess of water to remove any free aluminium.

After the above stages the hides or skins which have an acid reaction may then be neutralized to provide adequate boil proofness if this property is required, with a weak basic solution, for example sodium bicarbonate solution to a pH of preferably 4.5. Preferably this neutralization stage is carried out in more than one step. It should be noted that it is not necessary to age the hide or skin before this neutralizaltion.

Having been subjected to this optional neutralization stage the hide or skin may then be optionally fatliquored and dyed.

The leather prepared by the process of the present invention shows no marked decrease in strength or ageing and its other properties are comparable to those of leathers prepared by conventional chrome tanning.

Although in the preferred embodiment of the invention the hide or skin after conventional conditioning is tanned sequentially with a vegetable tanning agent (preferably mimosa extract) and with an aluminium salt (preferably aluminium sulphate) it is possible to treat the conditioned hide or skin with these two tanning agents at the same time either by adding the vegetable tanning agent and a solution of the aluminium salt simultaneously, preferably under such conditions that the tanning liquor has a pH of 3.6 to 3.8, followed by conventional basification as described above, or by preparing a mixture of the vegetable tanning agent and the aluminium salt and treating the skin or hide with such mixture.In a particular embodiment of the method of operation a mimosa extract may be reacted with a solution of the aluminium salt to form a complex between the mimosa extract and the aluminium sulphate salt and this mixture may then be spray dried to produce a material which can be stored for later use.

The present invention is further illustrated by the following Examples.

Example 1 Trial 1: Mimosa-cured sides were prepared with different amounts of spray dried mimosa extract viz. 10, 1 5, 20 and 25% based on the lime split weight (split to give a final leather substance of about 2.0 mm), according to procedure described earlier herein. The mimosa cured sides were cut up into smaller samples and sorted so that on further tanning with aluminium sulphate samples of the different mimosa cured sides were available. Different levels of aluminium sulphate were used for further tanning equivalent to 0.7, 1.1, 1.4, 2.0, 2.4,2.8 and 3.2% Awl203 based on the dry mimosa cure weight.

A solution of the aluminium sulphate was preprepared the day before use by dissolving the appropriate amount of aluminium sulphate Al2(SO4)3.1 6H20 and sodium acetate 3H20 (10% based on the amount of aluminium sulphate used) were dissolved in hot water (250% based on the amount of aluminium sulphate used) and left overnight.

The mimosa cured pieces were wet back and further tanned as follows: (percentage based on dry mimosa cured weight) Wet Back 1000/o water (400C) Horse up overnight.

Further tanning 4.5,7.9 12.5, 15, 17.5, 20% Al2(S04)3.1 6H20 (containing 1.6% Awl203) which is equivalent to 0.7, 1.1, 1.4, 2.0, 2.4, 2.8, 3.2% Al203 prepared as above with sodium acetate.

350% water (400C) drum 2 hours Basified to 33% basicity with appropriate amounts of soda ash dissolved in water and added in three lots, 30 mins, apart 2 hours Final pH 3.6-3.7.

Horse up overnight Rinse 350% water 5 mins.

Neutralize 350% water (350C) 1% sodium bicarbonate 30 Mins.

1% sodium bicarbonate 30 Mins.

pH approx. 4.5 Fat Liquor 350% water (500C) 4% Trupon DX 2% Truponol GT 30 Mins.

Slick out, and toggle dry.

Samples of the leathers were then analysed for aluminium. The results are given in Table 1.

Trial 2: Samples of the various Mimosa Cured sides were further tanned with 17.5%, Al2(SO4)3. 1 6H20 (i.e. 2.8% Awl203) in a similar manner to that described above, except that the basification was varied so that final basicities of 33, 42, and 50% were obtained. The final pH values were 3.7, 3.8 and 3.9 respectively. The results are given in Table 2.

Trial 3: Instead of aluminium sulphate being used to further tan the Mimosa Cure, two levels of a commercial basic aluminium tanning agent viz. 9.1 and 13.6% Lutan B(BASF) which is equivalent to 2 and 3% Al203,were used. Soda ash was used to basify the further tanning liquor bath after two hours of further tanning to a final pH of 3.6. Further processing was then the same as described for Trial 1.

The results are given in Table 3.

It may be seen from Table 1 that by further tanning with aluminium sulphate the shrinkage temperatures of the leathers were found to be generally higher with those samples with higher amounts of mimosa extracts. In addition, the uptake of aluminium sulphate was also better in those samples with higher amounts of mimosa extract. Thus it may be seen that the higher the amount of mimosa extract used in the mimosa cure, the better the uptake of aluminium salt in subsequent retannage. This is however, only true up to a certain point, since Table 1 shows that there is only really a difference between 10% mimosa used and 15%; on increasing the amount of mimosa extract further, this results in only a small increase in, for instance, the uptake of aluminium salt.

It may also be seen from Table 1 that increasing the amount of aluminium sulphate offered results in a steady increase in the uptake of aluminium in the shrinkage temperature. The most level of aluminium sulphate used in the further tanning would appear to be around 18 to 19% aluminium sulphate, lower amounts apparently giving less aluminium uptake and too low a shrinkage temperature. More aluminium sulphate may be used but this is not undesirable on economic grounds. The aluminium utilisation in the further tanning is shown in Table 1 to be better at lower levels of aluminium sulphate used. However, this is only a consequence of mass action and practically, sufficient aluminium sulphate needs to be used as aforementioned, to obtain about 2% Awl203 in the final leather.

This will ensure that the leather will stand boiling water.

Table 2 shows that increasing the basicity resulted in an increase in aluminium uptake, shrinkage temperature and aluminium sulphate utilisation. Increasing the basicity results in an increase in pH with more complexation and hence greater uptake of aluminium sulphate. There is of course a limit to this, since too high a pH value in the drum during further tanning will result in precipitation of the aluminium tannate. However, the masking action of acetate prevents this up to pH value of around 4.

As previously mentioned the initial pH during further tanning should be low in order to obtain penetration of the aluminium sulphate. Thereafter basification proceeds in stages (to prevent precipitation of aluminium tannate in the bath) as is normally carried out in standard chrome tanning.

The findings of Trial 1 were also confirmed in Trial 2 as shown in Table 2 viz. that increasing the amount of mimosa extract used for mimosa cure, increased the aluminium sulphate uptake, shrinkage temperature and aluminium sulphate utilisation.

Table 3 shows that using Lutan B in place of aluminium sulphate for retannage resulted in a better uptake of aluminium and hence a better aluminium utilisation, but the shrinkage temperatures were lower relative to those leathers further tanned with equivalent amounts of Awl203. This implies that more aluminium is fixed if Lutan B is used but the shrinkage temperature is not increased as much. This is probably due to the fact that Lutan B is a highly basic aluminium salt, probably made up of aluminium compounds of high molecular weight, hence more deposition of aluminium occurs on further tanning of mimosa cured stock and less complexation.Although leather of good handle can be prepared by retanning mimosa cure with Lutan B, this can be achieved at less expense simply with aluminium sulphate with the further advantage that higher shrinkage temperatures are possible. Table 3 also shows that increasing the mimosa extract used results in an increase in the amount of aluminium fixed.

Table 1 Further tanning with aluminium sulphate of mimosa cure (pre-acetate masked, drum basified 33%) Ts (OC): (Ts=Shrinkage Temperature) % Mimosa used for Mimosa cure %AI2O3offered 10 15 ~ 20 25 ~ Av.

0.7 89 92 90 89 90 1.1 86 93 92 94 91 1.4 91 93 95 92 93 2.0 91 97 94 97 95 2.4 91 97 95 98 95 2.8 92 98 100 100 98 3.2 93 99 106 105 101 Av. 90 96 96 96 95 %AI2Q3 in leather: 0.7 0.66 0.70 0.59 0.51 0.61 1.1 0.74 1.04 0.87 0.87 0.88 1.4 0.98 0.85 1.06 1.08 0.98 2.0 1.29 1.36 1.38 1.36 1.35 2.4 1.30 1.36 1.51 1.51 1.42 2.8 1.43 1.74 1.68 1.78 1.66 3.2 1.63 1.83 2.06 1.91 1.86 Av. 1.15 1.27 1.31 1.29 1.26 Table 1 continued Ts ( C): % Mimosa used for Mimosa cure %A1203 offered 10 15 20 25 Av.

%Al utilisation: 0.7 94 100 84 73 88 1.1 67 95 79 79 80 1.4 70 61 76 77 71 2.0 65 68 69 68 68 2.4 54 57 63 63 59 2.8 51 62 60 64 59 3.2 51 57 64 60 58 Av. 65 71 71 69 69 Table 2 Further tanning with aluminium sulphate retannage of mimosa cure-different basities (2.8% Al2O3 offered, pre-acetate masked, drum basified) Ts ( C): % Mimosa used for Mimosa cure Basicity (%) 10 15 20 25 Av.

33 (end pH 3.7) 96 98 100 100 99 42 (end pH 3.8) 98 102(108) 106(110) 102 102 50 (end pH 3.9) 98 101 108 109 104 Av. 97 100 105 104 102 %A12O3 in leather: 33 1.43 1.74 1.68 1.78 1.66 42 1.53 1.75(2.22) 1.93(2.25) 1.93 1.78 50 1.82 1.92 2.06 2.03 1.96 Av. 1.59 1.80 1.89 1.91 1.80 % Al utilisation: 33 51 62 60 64 59 42 55 63(69) 69(70) 69 64 50 65 69 74 73 70 Av. 57 65 68 69 64 (values in parenthesis refer to 3.2% Awl 203 offer, average of 6 results) Table 3 Further tanning with Lutan B of mimosa cure Ts ( C); % Mimosa used for Mimosa cure %Al2O3 offered 10 15 20 25 Av.

2 (9.1% Lutan B) 88 91 91 91 90 3 (13.6% Lutan B) 90 92 93 94 92 Av. 89 92 92 93 91 %Al2O3 in leather: 2 1.56 1.60 1.77 1.88 1.70 3 1.99 2.26 2.43 2.61 2.32 Av. 1.78 1.93 2.10 2.25 2.01 Table 3 (cont.) Further tanning with Lutan B of mimosa cure % Mimosa used for Mimosa cure %A/203 offered 10 15 20 25 Av.

% Al utilisation: 2 78 80 89 94 85 3 67 75 81 87 78 Av. 73 78 85 91 82 Example 2 Trial 1: Two split mimosa cured sides of 2.5 mm substance (prepared with 1 5% mimosa extract) were cut into 1 6 rectangular pieces. These were then wet back with 100% water at 400C (based on the dry mimosa cure weight) and drained, 350% water at 400C added and then further tanned with aluminium 18.6% aluminium sulphate .16 H20 (that is 3% Al203 on dry mimosa cure weight), masked with 10% sodium acetate three H20 (based on the weight of aluminium sulphate used). This solution was prepared by dissolving the required amounts of aluminium sulphate and masking agent in hot water and aged overnight.After 2 hours tanning (pH 3.2) the solution was basified slowly over 2 hours with a solution of soda ash to a final basicity of 21% and a pH of 3.5.

The samples were sorted into four lots of four for neutralizing with 1.5,3.0,4.5, and 6% sodium bicarbonate (based on the dry mimosa cure weight) in 350% water at 350C and one hour. The bicarbonate was added in lots of 1.5% so as to avoid high initial pH values. Fat liquoring consisted of 350% water at 500C, 4% Trupon DX and 2% Truponol GT for 30 minutes. The aluminium tanned pieces were slicked out and toggle dried.

Samples of the Mimosa-Al leather were taken for aluminium determination and shrinkage temperature measurements.

Trial 2: In a similar manner to that described above, samples of aluminium tanned mimosa cure were prepared (except that the tan bath was basified to 42% with an end pH of 3.9) prior to neutralizing three lots of nine tanned samples with the following different processes: (a) 350% water at 350C (based on dry mimosa cure wt.) 6% Neosyn N 3% Sodium bicarbonate 2 hours (b) 350% water at 350C 3% Calcium formate 3% sodium bicarbonate 2 hours (c) 350% water at 350C 3% Sodium bicarbonate 2 hours Further processing was then as for Trial 1.

Ratios of 1:0.50:0.35 for limed weight: damp shaved mimosa cure: dry mimosa cure indicate that float lengths of about 100% based on the limed weights were used in these trials.

The results of aluminium determinations and shrinkage temperatures with the increasing levels of neutralisation are in Table 4.

Table 4 Effect of different levels of neutralisation % sodium carbonate EndpH %awl203 in Ts in neutralisation Value leather ( CJ 1.5 4.1 1.4 97 3 4.8 1.6 99 4.5 6.4 1.6 107 6 7.3 1.5 108 Average from four samples It may be seen from Table 4 that shrinkage temperatures can be improved by neutralising to a higher level. Increased neutralisation will promote complexation and hence crosslinking will be increased as reflected by the increased shrinkage temperature at the same alumina level. It may be seen that sufficient basification is required to give an end pH in the retanning bath of close to four.

Then only a normal neutralisation is necessary to obtain further crosslinking and boil-proof leather.

Sharp increases in pH to high value should be avoided in this system for a number of reasons; precipitation of aluminium as the hydroxide in the retannage bath, and alkaline oxidation of the tannins could take place. Hence basification with for example soda ash takes place in stages and neutralisation should preferably be buffered.

The results in Table 5 show that the presence of an organic buffer salt gives the most rapid penetration of alkali during the neutralisation and the highest shrinkage temperature.

Table 5 Effect of different neutralising processes Calcium Neosyn N Formate + Bicarb.

Bicarb. Bicarb. alone pH of bath after 0.5 hr 5.2 4.8 5.2 1.0 hr 4.8 4.7 4.6 1.5 hr 4.6 4.5 4.4 Penetration with bromo cresolgreen indicator after 0.5 hr slightly nearly slightly pen through pen 1.0 hr nearly through nearly through through 1.5 hr still not through still not through through Ts (OC) of leather 109 110 109 This example shows that if basification is too low, then aluminium utilisation and thermal stability are also low. A low basification can be overcome to a certain extent by increasing the neutralisation but there is the danger of over-neutralising the leather. Hence it is best to basify and neutralise moderately which will ensure that the pH increases gradually in stages. The unbuffered neutralising processes having an uneven distribution of pH through the cross-section is shown in Table 5.This results in lower shrinkage temperature relative to the even distributed buffered samples. Some samples from the unbuffered processes had shrinkage temperatures below 1 000C but on the examination of the test pieces, it was found that only the central portions of the cross-section had shrunk. This indicates that it is important to have an even pH distribution which can be easily obtained with a simple formate/bicarbonate neutralisation. The pH value must still be above about 4.5 (with an alumina content of about 1.52% on a dry basis) in order to obtain a boilproof leather.

Example 3 Trial 1: The effect of ageing aluminium tanned Mimosa Cure before neutralising and fatliquoring was assessed by taking samples from a number of experiments, ageing them for up to two weeks in the wet state and regularly measuring their shrinkage temperatures. The results are given in Table 7. Shrinkage temperatures were also taken before, during and after aluminium tanning viz. after sulphate conditioning, after mimosa curing with 1 5% mimosa extract based on the limed weight, after two hours aluminium tanning with acetate masked aluminium sulphate pre-basified to 25%, after two hours of drum basification to 42% (end pH=3.8), after neutralising to a pH of 5, and after fatliquor and drying (Al203 content 1.9%). The results are given in Table 8.

Trial 2: Six sides were limed in a standard manner, split to about 3mm, delimed and bated, pickled, sulphate conditioned, and mimosa tanned with 15% mimosa extract based on the limed weight. Two sides were dried out giving a weight of about 35% of the limed weight. These were then dry shaved, wet back and retanned with 18.6% aluminium sulphate, 1 6 hydrate (i.e. 3% Awl203 based on the dried out mimosa cure weight), pre-acetate masked and pre-basified to 25%, in 300% water (based on the dry mimosa cure weight) at 400C for one hour, then drum basified 42% with soda ash, (end pH 3.8) for three hours.The sides were horsed up over night and then rinsed for 5 minutes, neutralised with calcium fomate (2%) and sodium bicarbonate (3% added in two lots) for 1.5 hours (end pH 5.0) in 200% water (based on dry mimosa cure weight) at 350C, dyed and fatliquored in 200% water at 500C, 1.5% acid dyestuff for 40 minutes, 8% Trupon DX and 2% Truponol GT was added and drummed for 40 minutes, then 0.75% formic acid (diluted 1:5 with water) was added and drummed for 30 minutes before draining, rinsing and toggle drying the Mimosa Al leather.

Two of the other four mimosa pre-tanned sides were allowed partially to dry out to a weight of about 70% of the limed weight and a moisture content suitable for damp shaving to substance.

Subsequent quantities were based on this damp shaved weight (the dry mimosa cure weight is about 50% of this damp shaved weight, e.g. 9.3% Al2(SO4)3. 16 H20 was used). Further processing was similar to the above method for retanning dry mimosa cure bearing in mind that the damp shaved weight is about twice the dry mimosa cure weight.

The remaining two sides were further aluminium tanned straight through without removing from the drum in which mimosa pretanning took place. Equivalent quantities of chemicals were used as used above for the other four sides but in this case the amounts were based on the split limed weight (the dry mimosa cure weight is about 35% of the limed weight, e.g. 6.5% A12(S04)3.1 6 H20 was used). After aluminium tanning, the sides were partially dried out to a suitable condition for damp shaving to substance. The shaved sides were reweighed and the remaining processing was as for the other four sides but based on this damp shaved weight.

Samples of the mimosa-Al leather were taken for, amongst other things, aluminium determination and shrinkage temperature measurements. The results are given in Table 8.

As may be seen from Table 6 ageing appears to have very little effect on shrinkage temperature.

Although Table 6 shows that horsing up for a day did give a slight improvement, this was not reflected in the shrinkage temperature of the final leather; that is to say that the aged samples gave final leather shrinkage temperatures similar to the unaged samples of about 11 70C. Hence horsing up after aluminium retanning does not appear to be essential.

The change in shrinkage temperature during processing from sulphate conditioned state to the final leather is shown in Table 7. These values again emphasise importance of complexation between the bound mimosa tannin molecules and aluminium ions which increases with increase in pH I giving rise to crosslinking and an increase in shrinkage temperature. The test to show whether boil-proof leather has been prepared, should be applied after neutralisation, and not prior to neutralisation since Table 7 shows at this stage the shrinkage temperature is below 1 000C.

Trial 3: The results of a comparison between (A) further tanning dried out Mimosa Cure, (B) further tanning straight through after damp shaving the mimosa pretanned material, and (C) processing straight through without shaving until after aluminium tanning are given in Table 8. These results indicate that one can process straight through without first preparing the mimosa cure by drying out the pretanned mimosa material. However, it would appear that the mimosa pretanned material should preferably be shaved to substance before retanning since a much lower shrinkage temperature is obtained if shaving is delayed till after neutralisation.

Table 6 Shrinkage temperatures (OC) of aged aluminium tanned Mimosa Cure prior to neutralisation Ageing period (days) Expt 0 1 2 3 4 5 6 7 14 1 92 94 92 94 94 94 2 91 95 96 95 96 3 94 97 98 4 95 96 94 Table 7 Shrinkage temperatures at different stages in manufacture of Mimosa-Al leather Stage Ts(0C) Sulphate condition 62 Mimosa Cure 76 Two hours aluminium tanning (pH 3.3) 88 Two hours drum basification (pH 3.8) 94 Neutralisation (pH 5.0) 118 Mimosa-Al leather 119 Table 8 Comparison of processing Mimosa Cure as opposed to Straight through processes %Al2O3 Process Ts( C) in leather (A) Tanning Mimosa Cure 120 1.8 (B) Tan straight through after shaving 121 1.7 (C) Tan straight through then shave 107 1.6 Example 4 Leather prepared by various embodiments of the process of the present invention were aged for 9 months at 200C and 650Rh and then their physical properties compared to those which they possessed before ageing.

The results are given below.

Table 9 (percentage aged to non-aged in parenthesis) %Al2(SO4)3.16 H20 offered based on dry Mimosa Cure Wt.

10 15 20 Tensile Strength: Grain extn. (%) 45 (118) 40(114) 43 (110) Break extn. (%) 47(115) 46(115) 48(113) Force extn. (%) 19.4 (96) 19.8 (98) 18.0 (93) Slit tear: Force (N/mm) 105 (95) 88 (101) 95 (78) Lastometer: Force (N/mm) 154(79) 185(106) 198(124) Distn. (mm) 7.7 (98) 7.6 (107) 7.2 (106) Ts ( C) 88(103) 94 (98) 97 (97) Table 10 (percentage aged to non-aged in parenthesis) %Al2(SO4)3. 16 H20 offered 10 15 20 Tensile Strength: Grain extn. (%) 42 (100) 48 (100) 50 (110) Breakextn. (%) 52(83) 56(93) 50(114) Force (MPa) 21.2 (90) 26.2 (90) 23.5 (98) Slit Tear: Force (N/mm) 103(102) 124(82) 100(99) Lastometer:: Force (N/mm) 166 (145) 270 (100) 292 (89) Distn. (mar) 7.1(102) 7.8 (103) 8.5 (92) Ts( C) 89(100) 91(99) 94(100) Table 11 (percentage aged to non-aged in parenthesis) Method ofpreparation Pre-masked # Pre-masked & Drum masked # pre-basified drum basified drum basified Tensile Strength: Grain extn. (%) 40 (114) 52 (100) 54 (94) Breakextn. (%) 46(115) 55 (98) 57 (95) Force (MPa) 1 9.8 (98) 22.9 (96) 21.6 (98) Table 11 (cent.) Slit Tear: Force(N/mm) 83(101) 97(98) 83(126) Lastometer:: Force (N/mm) 185(106) 219(114) 228 (112) Distn. (mm) 7.6 (107) 8.3 (101) 8.2 (99) Ts (OC) 94 (98) 96 (99) 90 (100) Table 12 (percentage aged to non-aged in parenthesis) % Basicity 33 42 50 50* Tensile Strength: Grain extn. (%) 52 (100) 46 (109) 50 (98) so (108) Breakextn.(%) 55 (98) 55(110) 52 (98) 56 (97) Force (MPa) 22.9 (96) 19.3 (98) 20.7 (93) 18.6 (98) Slit Tear: Force (N/mm) 97 (98) 83 (101) 95 (94) 90 (94) Lastometer: : Force (N/mm) 219(114) 189(92) 214(103) 206 (92) Distn. (mm) 8.3(101) 7.5 (98) 8.2 (107) 7.8 (104) Ts (OC) 95 (99) 94 (101) 93(101) 98(101) The results obtained from non-aged samples were all satisfactory for shoe upper leather and the naturally aged result did not differ significantly from those of the originals. The strengths merely fluctuated slightly up and down indicating a normal variation in physical test data. There was thus no marked decrease in strength on ageing which would have indicated deterioration of the leather prepared according to the process of the present invention.

Claims (14)

Claims

1. A process of tanning skin and hides which comprises subjecting the skin or hide to a drum vegetable tanning process and subsequently or simultaneously tanning it with an aluminium containing tanning agent.

2. A process as claimed in Claim 1 in which the aluminium containing tanning agent is a watersoluble aluminium salt.

3. A process as claimed in Claim 2 wherein the salt is aluminium sulphate.

4. A process as claimed in any of Claims 1 to 3 in which the vegetable tanning agent is mimosa extract.

5. A process as claimed in Claim 4 in which in the vegetable tanning process mimosa cured skins or hides are produced having a pH of 3.6 to 3.8.

6. A process as claimed in Claim 5 in which the aluminium containing tanning agent is aluminiurn sulphate and the tanning with this agent is carried out at pH of not less than 3.6 and not substantially greater than 4.

7. A process as claimed in Claim 6 wherein masking agents are used with the aluminium sulphate and the tanning is carried out at pH of approximately 4.0.

8. A process as claimed in any of Claims 3 to 7 in which the amount of aluminium sulphate used is such as to give approximately 2.0% Al in the crust leather.

9. A process as claimed in Claim 8 in which the amount of aluminium sulphate used is a solution approximately 18 to 19% AI2(S04)3.1 6 H20 based on the weight of the vegetable tanned hide or skin.

10. A modification of the process as claimed in any of Claims 1 to 9 in which the skins of hides are tanned with the vegetable tanning agent and the aluminium tanning agent simultaneously.

11. A process as claimed in Claim 10 in which a pre-prepared mixture of the two tanning agents is utilized.

12. A process as claimed in Claim 1 substantially as herein described.

13. A process as claimed in Claim 1 substantially as herein described with reference to the Examples.

14. Skin and hides which have been tanned by a process as claimed in any of Claims 1 to 13.