Process for tanning hides

Description

[0001] This invention relates to a process for tanning hides. More particularly, the invention relates to a process for tanning hides for vamps and peltry, by means of ammonium titanyl sulphate.

[0002] As is known ammonium titanyl sulphate can be utilized as a tanning agent for hides. It has been proposed to subject delimed hides to a pretreatment with an activating substance, for example phthalic anhydride; tanning follows, with the ammonium titanyl sulphate, and alkaline substances are then added to the bath, and tanning can be completed with synthetic organic tanning agents. This method gives rise to several drawbacks. First of all, phthalic anhydride causes in the hides an acid swelling, which is irreversible and renders the tanned hide brittle. Furthermore, during the alkalinisation of the bath, the ammonium titanyl sulphate tends to precipitate in the form of titanium hydroxide, which adversely affects the qualitative constancy of the article and gives rise to losses of the tanning agent.

[0003] The present invention aims to provide a process for tanning hides, particularly for vamps and peltry by using ammonium titanyl sulphate, which process avoids acid swelling of the hides, thus imparting to the tanned leather a higher tearing resistance.

[0004] The invention also aims to provide a process in which the ammonium titanyl sulphate is practically fully absorbed by the hides, with consequent both economical and ecological advantages, as practically no titanium compounds are discharged along with the residual waters.

[0005] A further aim of the invention is to provide a process capable of producing a tanned hide having a high compactness (hide of closed texture), with good stretch recovery (firm hide) and free from blowings (loose grain).

[0006] The present invention provides a process for tanning hides, in particular a process for the tanning of hides for vamps and peltry by means of ammonium titanyl sulphate. This process is characterized in that delimed hides are treated, in the presence of water, with ammonium sulphate and naphthalene sulphonic acid; ammonium titanyl sulphate is added to the bath, while maintaining the temperature at not more than 40°C; a complexing agent for tetravalent titanium ion is optionally added, maintaining the temperature at not more than 40°C; the bath is gradually made alkaline by repeatedly adding (adding batchwise) sodium sulphite and hexamethylenetetramine until there is obtained a pH value in the hide of from 4.0 to 5.0, while maintaining the temperature at not more than 40°C: the hides are removed from the bath and are allowed to rest; the hides are pressed, sheared (shaved) and washed with water; the hides are subjected to a deacidification treatment with an alkaline substance, in the presence of water and optionally in the presence of a complexing agent for tetravalent titanium ion, this treatment being continued until there is obtained a pH value in the hide of from 5.0'to 6.5; the hides are washed with water; tanning is completed, in the presence of water, by means of one or more synthetic tannins and/or a polymeric material for the tanning completion; the bath is discharged and the hides are dyed and greased according to conventional techniques. The tetravalent titanium ion complexing agent is added during at least one of the two steps referred to hereinbefore.

[0007] It has been found, in accordance with the present invention, that the pretreatment of the hides with naphthalene sulphonic acid and ammonium sulphate improves the properties of the hides subsequently tanned with the ammonium titanyl sulphate. It has been also found that the use of a complexing agent for the tetravalent titanium ion, after the addition step of the ammonium titanyl sulphate or during the deacidification step or during both such steps, improves the quality of the tanned hides, imparting a better softness to them. It has been also found that the gradual, batchwise addition of sodium sulphite and hexamethylenetetramine, during the alkalinisation step, results in a progressive completion of the tetravalent titanium penetration and absorption by the hides, whilst simultaneously reducing the titanium content of the bath. It has been further found that, under such conditions, the ammonium titanyl sulphate, which remains in the bath in decreasing amounts as the pH value of the bath increases, leads to no precipitation of Ti(.OH)₄. Furthermore,, the final titanium content of the bath is very low, and does not normally exceed about 0.3 g/liter (calculated as TiO₂), so that tanning agent losses do not practically occur and the discharging of the bath does not practically give rise to ecological problems deriving from the presence of titanium.

[0008] The ammonium titanyl sulphate (NH₄)₂TiO(SO₄)₂.H₂O and a method of preparing the same are described in French patent No. 2,042,206. Another preparation method is described in Italian patent application No. 20571 A/83. The ammonium titanyl sulphate utilized in the present invention is not subjected to the stabilizing treatment described in French patent No. 2.042,206.

[0009] The process according to the invention may be suitably carried out in the following manner.

[0010] The naphthalene sulphonic acid utilized in the first step of the process of the invention may be the alpha-naphthalene sulphonic acid or beta-naphthalene sulphonic acid, or mixtures thereof. Preferably, from 1 to 4% of naphthalene sulphonic acid and from 2 to 4% of ammonium sulphate are added, with respect to the pelt weight of the hides. After the addition of water and of both reagents, the tanning drum is generally rotated until the reagents have passed through the hide section before adding the ammonium titanyl sulphate. With respect to the hide pelt weight, 1 to 4% of ammonium titanyl sulphate, calculated as TiO₂, are preferably used, more preferably from 2% to 4% thereof. If no complexing agent is added, the tanning drum is rotated, after addition of ammonium titanyl sulphate, for a time generally ranging from 3 to 7 hours.

[0011] The tetravalent titanium ion complexing agent is preferably an alkali metal citrate or tartrate, for example of sodium or of potassium. As already explained hereinbefore, it can be added either after the ammonium titanyl sulphate or during the deacidification step or in both steps.

[0012] The total amount of alkali metal citrate utilized in one step or in both steps is preferably from 0.05 to 0.4 mole per mole of TiO₂ contained in the ammonium titanyl sulphate; in the case of the alkali metal tartrate, such amount is preferably from 0.07 to 0.6 mole per mole of Ti0₂. When the complexing agent is used in both steps, about 50 - 80% of such amounts are preferably added after the ammonium titanyl sulphate, the remaining amount being added during the deacidification step.

[0013] When the complexing agent is added during the tanning step with the ammonium titanyl sulphate, it is usually added when the tanning agent has substantially passed through the hide section, after about 2-4 hours. After addition of the complexing agent, the drum is usually rotated for 1 to 3 hours.

[0014] The alkalinisation step follows. Sodium sulphite and hexamethylenetetramine are gradually added batchwise to the bath, until there is obtained a pH value in the hide of from 4.0 to 5.0. Preferably, such pH is from 4.0 to 4.5. Usually, the addition of sodium sulphite and of hexamethylenetetramine occurs in 3-4 batches, for example at constant time intervals totally lasting about 6 hours. The sodium sulphite: hexamethylenetetramine weight ratio is usually about 1 : 1.

[0015] At this stage the hides are removed from the bath and allowed to rest, after which they are pressed, sheared (shaved), and washed with water. There then follows the deacidification step with an alkaline substance.

[0016] The alkali metal substance used in the deacidification step is preferably sodium bicarbonate or sodium sulphite. Deacidification is continued until there is obtained, in the hide, a pH value of from 5.0 to 6.5, preferably from 5.0 to 6.0. When a tetravalent titanium ion complexing agent is used during the deacidification step, it is usually added together with the alkaline substance.

[0017] The hides are then washed with water, after which there follows the step of completing the tanning with one or more synthetic tannins and/or a suitable polymeric material.

[0018] The synthetic tannin is preferably selected from phenol and formaldehyde sulphonated condensates and from naphthalene and formaldehyde sulphonated condensates. The polymeric material for completing the tanning is preferably selected from polyurethane polymers, acrylic polymers and styrene-maleic polymers.

[0019] The bath is then discharged and the hides are dyed and greased according to conventional techniques.

[0020] A detailed and preferred embodiment of the process of the present invention is described hereinbelow.

[0021] During the first step, the hides, previously subjected to the usual liming, fleshing and deliming treatments, are introduced into a drum. All the amounts referred to below are indicated in per cent by weight with respect to the pelt weight of the hides, unless otherwise specified.

[0022] Into the drum there are added :

- water: generally from 80 to 140%,

- ammonium sulphate at 20-21% of nitrogen: generally from 2 to 4%,

- naphthalene sulphonic acid: generally from 1 to 4%.

[0023] During this step, the temperature generally ranges from 20 to 25°C. The drum is generally rotated until the reagents have passed through the hide section, which requires generally from 2 to 4 hours. At the end of this step, the pH of the bath generally ranges from 4.0 to 4.5.

[0024] Thus, from 2 to 4% of ammonium titanyl sulphate, calculated as TiO₂, is generally introduced into the bath. After introduction of the tanning agent, if no complexing agent of the tetravalent titanium ion is added, the drum is usually rotated during 3 to 7 hours. If the complexing agent is added, the drum is generally rotated, before introducing said agent, until the tanning agent has passed through the hide section, which takes generally 2 to 4 hours; when the complexing agent is added, in the case of alkali metal citrate, from 0.05 to 0.3 mole of citrate for each mole of TiO₂ is generally used at this stage. After the complexing agent has been added, the drum is generally rotated during 1 to 3 hours.

[0025] Then the alkalinisation step is started. To such purpose,

- Na₂so_{3 :} generally from 2 to 4%, and

- hexamethylenetetramine : generally from 2 to 4% are usually introduced into the bath in 3-4 times.

[0026] The alkalinisation step terminates when the pH, in the whole section of the hide, is from 4.0 to 4.5. This generally takes 6 to 8 hours. At the end of the alkalinisation step, the bath is practically free from tetravalent titanium: its maximum residual concentration does not exceed 0.3 g of TiO₂ per liter.

[0027] During the whole tanning operation with ammonium titanyl sulphate, the temperature must not exceed 40°C.

[0028] The hides are taken out from the bath and are allowed to rest during a period of time generally varying from 24 to 28 hours. The hides are then pressed, sheared and subsequently washed with about 300% of water at 30-35°C.

[0029] From now on the per cent by weight amounts refer to the weight of the sheared hides, considered as equal to 100. To accomplish the deacidification, there are introduced into the drum :

- water : generally 150 to 300%,

- NAHCO₃ or Na₂SO₃ ; generally 1 to 3%,

- optionally : an alkali metal citrate or tartrate.

[0030] Usually, from 1 to 4% of sodium citrate.is utilized if it was not used in the tanning step with the ammonium titanyl sulphate, and from 0.5 to 3% if it was already employed in said tanning step.

[0031] The deacidification treatment is generally carried on until the hide, in its entire section, has reached a pH value of from 5.0 to 6.0; this generally takes from 1 to 3 hours.

[0032] Subsequently the bath is discharged and the hides are washed with about 300% of water at 30-35°C.

[0033] Then the complementary tanning step begins.

[0034] To such purpose there are introduced into the drum:

- generally from 150 to 200% of water at 25-30°C,

- one or more synthetic tannins and/or a polymeric material to complete the tanning, in the following amounts :

- generally from 6 to 12% of synthetic tannin or of synthetic tannins (if they are used alone),

- generally from 3 to 8% of polymeric material to complete the tanning (if used alone),

- generally from 4 to 8% of synthetic tannin or synthetic tannins and from 3 to 5% of polymeric material (if simultaneously used).

[0035] The drum is generally rotated until the reagent or the reagents has/have passed through the section of the hide; this generally takes from 2 to 3 hours.

[0036] The bath is discharged and the hides are dyed and greased according to conventional techniques.

[0037] The hides tanned by the process of the present invention exhibit a good tearing resistance. They are very compact, with good stretch recovery and free from blowings.

[0038] The invention will be further described with reference to the following illustrative Examples.

Example 1

[0039] The delimed hides of 3 calves (pelt weight: 30 kg) were placed into a tanning vessel (rotative drum). The hides were treated with 120% of water at 25°C, 3% of (NH₄)₂SO₄ and 3% of commercial-grade naphthalene sulphonic acid. After a 3-hour rotation, the pH in the whole section of the hides was equal to 4.5.

[0040] 3% of TiO₂ (as (NH₄)₂TiO (SO₄)₂. H20 at 20% of Ti0₂) was then added to the bath. After a 3-hour rotation, the hides proved to be fully penetrated. 1.4% of sodium citrate was then added, and the drum was rotated for an additional 2 hours. At the end of this step the temperature was about 32°C.

[0041] 2% of a mixture of sodium sulphite and hexamethylenetetramine, in a weight ratio 1 : 1, was added, and rotation was carried out during 2 hours. A second 2% addition of the alkalinising mixture was effected, and rotation again carried out during 2 hours; the pH of the bath and of the whole section of the hide was equal to 3.8. A third addition of 1% of the alkalinising mixture was then carried out, and rotation carried out for an additional 2 hours; a pH value of 4.4 was reached in the bath and in the entire section of the hide.

[0042] The drum was discharged and the hides were allowed to rest during 48 hours. The hides were pressed, sheared and washed with 300% of water at 30°C, whereafter deacidification was accomplished. To such purpose, the hides were introduced again into the drum (sheared weight: 12 kg), and 200% of water at 35°C, 2% of sodium citrate and 2% of sodium bicarbonate were added. The drum was rotated for 90 minutes; after which the pH value of the bath and of the whole section of the hide was equal to 6.0.

[0043] The bath was discharged, and the hides were washed with 300% of water at 35°C. The complementary tanning step was then commenced. To this purpose, 150% of water at 30°C and 8% of a mixture of synthetic tannins, consisting of a naphthalene-formaldehyde sulphonated polycondensate and of a phenol-formaldehyde sulphonated polycondensate, was added. The drum was rotated during 2 hours. The hides were checked and a complete penetration was ascertained.

[0044] The bath was discharged and the hides were dyed and greased.

[0045] The resulting leathers were compact and relatively soft, not elastic, and did not give rise to blowings. They were suitable for vamps.

Example 2

[0046] The delimed hides of 3 calves (pelt weight: 25 kg) were placed into a tanning drum and were treated as in Example 1 until completion of the tanning step with the ammonium titanyl sulphate.

[0047] 8 kg of sheared hides were then deacidified by addition of 200% of water at 30°C and of 1.8% of sodium sulphite. The drum was rotated for 2 hours. The bath and the whole section of the hides exhibited a pH value of 5.8.

[0048] After discharging of the bath and washing of the hides, the complementary tanning step was carried out. To this purpose, 150% of water at 30°C, 5% of a synthetic tannin consisting of a sulphonated polycondensate of naphthalene and formaldehyde, and 4% of acrylic resin were added.

[0049] After a 3-hour rotation, the bath was discharged and the hides were dyed and greased.

[0050] The resulting leathers were compact and soft and did not give rise to blowings. They were suitable for vamps and soft bag articles.

Example 3

[0051] The delimed hides of 4 calves (pelt weight: 37 kg) were placed into a tanning drum. The hides were treated with 120% of water at 25°C, 2.5% of (NH₄)₂SO₄ and 2.5% of commercial-grade naphthalene sulphonic acid. After a 3-hour rotation the pH of the bath and of the entire section of the hides was equal to 4.3.

[0052] 2.5% of Tio₂ as ammonium titanyl sulphate was added. After a 3-hour rotation, the hides were completely penetrated; the bath temperature was about 32°C. Alkalinisation was then carried out. To such purpose there was added 2.5% of a mixture of sodium sulphite and hexamethylenetetramine (weight ratio = 1:1). Rotation was carried on for 2 hours. Further 2.5% of the alkalinising mixture was added, and a further 2-hour rotation was carried out. After such time the pH of the bath and of the entire section of the hide was equal to 3.6. Further 1.5% of alkalinising mixture was added, and the drum was rotated during an additional hour, thus obtaining a pH of 4.3 in the bath and in the entire section of the hide.

[0053] There was subsequently followed the procedure as described in Example 1.

[0054] The resulting leathers were compact, not elastic and did not give rise to blowings. They were suitable for the manufacture of suit-cases and bags.

Claims

1. A process for tanning hides, characterized by treating delimed hides, in the presence of water, with ammonium sulphate and naphthalene sulphonic acid; adding ammonium titanyl sulphate, maintaining the temperature at not more than 40°C; optionally adding a complexing agent for the tetravalent titanium ion, maintaining the temperature at not more than 40°C; gradually making the bath alkaline by adding sodium sulphite and hexamethylenetetramine batchwise until there is obtained a pH in the hide of from 4.0 to 5.0, while maintaining the temperature at not more than 40°C; removing the hides from the bath and allowing them to rest; subjecting the hides to pressing, shearing, and washing with water; subjecting the hides to a deacidification treatment with an alkaline substance, in the presence of water, and optionally in the presence of a complexing agent for the tetravalent titanium ion, such treatment being continued until there is obtained a pH in the hide of from 5.0 to 6.5; washing the hides with water; completing tanning, in the presence of water, with one or more synthetic tannins and/or a polymeric material for tanning completion: and discharging the bath and dyeing and greasing the hides; the tetravalent titanium ion complexing agent being added in at least one of the two steps, in which mention was made of it.

2. A process as claimed in claim 1, characterized by adding 1 to 4% of ammonium titanyl sulphate, calculated as Ti0₂, with respect to the pelt weight of the hides.

3. A process as claimed in claim 1 or 2, characterized in that the tetravalant titanium ion complexing agent is selected from alkali metal citrates and tartrates.

4. A process as claimed in claim 3, characterized in that the amount of alkali metal citrate introduced during the tanning step with the ammonium titanyl sulphate and/or during the deacidification step is in aggregate from 0.05 to 0.4 moles of citrate per mole of TiO₂ contained in the ammonium titanyl sulphate.

5. A process as claimed in claim 3, characterized in that the amount of alkali metal tartrate introduced during the tanning step with the ammonium titanyl sulphate and/or during the deacidification step is in aggregate from 0.07 to 0.6 moles of tartrate per mole of TiO₂ contained in the ammonium titanyl sulphate.

6. A process as claimed in any of claims 1 to 5, characterized in that, when the tetravalent titanium ion complexing agent is added in the tanning step with the ammonium titanyl sulphate, the said complexing agent is added when the ammonium titanyl sulphate has substantially passed through the hide section.

7. A process as claimed in any of claims 1 to 6, characterized in that the naphthalene sulphonic acid is selected from alpha-naphthalene sulphonic acid, beta-naphthalene sulphonic acid and mixtures thereof.

8. A process as claimed in any of claims 1 to 7, characterized by adding from 1 to 4% of naphthalene sulphonic acid and from 2 to 4% of ammonium sulphate, with respect to the pelt weight of the hides.

9. A process as claimed in any of claims 1 to 8, characterized by adding sodium sulphite and hexamethylenetetramine until there is obtained a pH in the hide of from 4.0 to 4.5.

10. A process as claimed in any of claims 1 to 9, characterized in that the alkaline substance utilized in the deacidification step is sodium bicarbonate or sodium sulphite.

11. A process as claimed in any of claims 1 to 10, characterized in that the deacidification step is continued until there is obtained a pH in the hide of from 5.0 to 6.0.

12. A process as claimed in any of claims 1 to 11, characterized in that the synthetic tannin is selected from phenol and formaldehyde sulphonated condensates and from naphthalene and formaldehyde sulphonated condensates.

13. A process as claimed in any of claims 1 to 12, characterized in that the polymeric material for completing the tanning is selected from polyurethane polymers, acrylic polymers and styrene-maleic polymers.

14. Hides for vamps and peltry tanned according to the process as claimed in any of claims 1 to 13.