Chrome tanning process

Abstract

The invention concerns a process for tanning hide to obtain leather. The general process for obtaining the intermediary wet blue stage comprises the following steps: a) a picking step with acid and salt, followed by b) a tanning step with chromium salt, followed by c) a basification step. The invention is characterized in that between step b) and c), a re-acidification step with organic acids is added. In particular the organic acids are selected from glutaric acid (GA), 2-methyl glutaric acid (MGA), succinic acid, ethyl succinic acid (ESA), adipic acid (AA), maleic anhydride, fumaric anhydride, tricarboxylic acids, hydroxycarboxylic acids, and mixture thereof. This invention enables an increase of the up-taking of the re-tanning products and provides leather with improved mechanical properties.

Claims

1. A process for tanning hides, comprising: a) immerging the hides in a bath with acid and salt, followed by b) immerging the hides in a bath with chromium salt, and subsequently c) immerging the hides in a bath with a basifying agent; further comprising, between step b) and step c), re-acidifying the bath of step b) with organic acids, wherein the organic acids comprise a mixture of 2-methyl glutaric acid, 2-ethyl succinic acid, and adipic acid, and wherein, after re-acidifying the bath of step b) of the process and prior to performing step c) of the process, the pH of the bath is between 2.6 and 3.5.

2. The process according to claim 1, wherein the amount of organic acids used in re-acidifying the bath of step b) of the process comprises between 0.25% and 10%, based on the weight of the hides.

3. The process according to claim 2, wherein the amount of organic acids used in re-acidifying the bath of step b) of the process comprises between 0.5% and 5%, based on the weight of the hides.

4. The process according to claim 3, wherein the amount of organic acids used in re-acidifying the bath of step b) of the process comprises between 0.7% and 3%, based on the weight of the hides.

5. The process according to claim 1, wherein the chromium salt comprises a chromium ion, the hides have a hide cross-section, and step c) of the process is performed after the chromium ion has crossed the hide cross-section.

6. The process according to claim 5, re-acidifying the bath of step b) of the process is performed between 10 and 25 hours after step b).

7. The process according to claim 1, wherein the pH of the bath of step c) of the process is between 3.6 and 4.2.

8. The process according to claim 1, wherein the bath of step a) of the process comprises, based on the weight of the hides: between 25 and 75 weight % of water, between 3 and 12 weight % of NaCl, and between 0.1 and 3 weight % of H.sub.2SO.sub.4.

9. The process according to claim 1, wherein, after re-acidifying the bath of step b) of the process and prior to performing step c) of the process, the pH of the bath is between 2.6 and 3.

10. The process according to claim 1, wherein the organic acids comprise a mixture of: at least 70 weight % of 2-methyl glutaric acid; greater than 0 and at most 30 weight % of 2-ethyl succinic acid; and greater than 0 and at most 15 weight % of adipic acid.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) Advantageous characteristics of the process can be found in the sub-claims and below.

(2) In one preferred embodiment of the process according to the invention, the organic acids used in the re-acidification step comprise: between 70 and 100 weight % of MGA; between 0 and 30 weight % of ESA; and furthermore between 0 and 15 weight % of AA.

(3) In another preferred embodiment of the process according to the invention, the organic acids used in the re-acidification step comprise a mixture of adipic, glutaric and succinic acid.

(4) In this case, the organic acids composition used in the re-acidification step advantageously comprise: between 10 and 85 weight % of adipic acid, between 10 and 70 weight % of glutaric acid, and between 3 and 30 weight % of succinic acid.

(5) In the process according to the invention, the amount of organic acids used in the re-acidification step is preferably comprised between 0.25% and 10% of the weight of the hide to be treated, more preferably between 0.5% and 5% of the weight of the hide to be treated and in particular between 0.7% and 3% of the weight of the hide to be treated.

(6) It is particularly preferred that the pH obtained after the re-acidification step is comprised between 2.6 and 3.5, preferably between 2.6 and 3.

(7) In the process according to the invention, the basifying step c) is preferably performed after the chrome has crossed the hide cross-section.

(8) The re-acidification step is advantageously performed during a time comprised between 10 and 25 hours, in order to enable the chrome to cross the hide cross-section.

(9) The pH obtained after the basifying step c) is preferably comprised between 3.6 and 4.2.

(10) In the process according to the invention, the bath of the pickling step a) comprises, with regards to the weight of the hides to be treated: between 25 and 75 weight % of water between 3 and 12 weight % of NaCl, and between 0.1 and 3 weight % of H.sub.2SO.sub.4.

Examples 1 to 3

Production of the Wet Blue Intermediary Stage

Example 1

(11) 100 kg of hide for which liming, unhairing and fleshing process are completed, is washed, in a tanning drum with the same weight of water for 10 minutes.

(12) For all examples, all ingredients added thereafter are in weight pourcentage of the initial hide weight.

(13) After the washing the water is discharged and the hides are submitted to deliming and bating with the use of ammonium sulfate and a commercial deliming agent based on di-carboxylic acids (Ex: Rhodiaeco Descal SD commercialized by Rhodia Poliamida e Especialidades Ltda.). After all, bating process takes place, adding 0.08% of a standard commercial proteolytic enzyme and the drum is run for 1 hour (pH=8.0). At the end of these operations the cross-section of the hide does not show a pink color with phenolphthalein indicator.

(14) The hides are washed twice with 100% water, based on hide weight, and the washings are discharged.

Pickling Step

(15) With relation to the hide weight, it is added into the drum, 50% of water, 6% of sodium chloride (B (Baum scale) 6 to 7) and 0.4% of a commercial bleaching (run 15 minutes). 0.6% of 85% formic acid diluted in water (1 to 10) is added (run 30 minutes) followed by the addition of 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes), 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes) followed by the addition of 0.4% of sulfuric acid diluted in water (1 to 15) and the drum is run for an additional 3 hours. After this time the pH of the bath is around 2 to 3.

Tanning Step

(16) At this point a 3% of commercial chromium sulphate salt (ex: basic chromium sulphate, which presents 33% alkalinity and 25 to 26% chrome III oxide) is added (run 30 minutes). A second addition of 3% of chromium sulphate salt is added and the drum is run for additional 30 minutes followed by the addition of 0.75% of a mixture of 2-methyl glutaric acid (MGA), 2-ethyl succinic acid (ESA) and adipic acid (AA). A pH of 2.6 to 2.8 is then obtained. After 17 hours, the chromium has completely crossed the hide cross-section.

Basifying Step

(17) After this time, 20% of water is added and 0.35% of a commercial basifying agent (ex: magnesium oxide) (run 90 minutes). A second portion of 0.23% of magnesium oxide is added and the drum is run for an additional 90 minutes. A pH of 3.6 to 4 is then obtained. The water bath is heated from 35 C. to 50 C. during 5 hours, the drum is drained, the wet blue is washed with 100% water (based on pelt weight), drained and washed again with 80% water.

(18) The obtained Wet Blue was submitted to the analysis of total chromium content, chromium in layers and evaluation by Scanning Electron Microscope (EDS) (Table 1 and FIG. 1a).

Example 2

(19) 500 kg of hide for which liming, unhairing and fleshing process are completed, is washed in a tanning drum with 100% water (based on pelt weight) for 10 minutes. After that, the hides were submitted to deliming and batting according to example 1.

(20) With relation to the hide weight, it is added into the drum, 50% of water, 6% of sodium chloride (B 6 to 7) and 0.4% of a commercial bleaching (run 15 minutes). 0.6% of 85% formic acid diluted in water (1 to 10) is added (run 30 minutes) followed by the addition of 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes), 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes) followed by the addition of 0.4% of sulfuric acid diluted in water (1 to 15) and the drum is run for an additional 3 hours. After this time the pH of the bath is around 2 to 3.

(21) At this point a 3% of commercial chromium sulphate salt (ex: basic chromium sulphate, which presents 33% alkalinity and 25 to 26% chrome III oxide) is added (run 30 minutes). A second addition of 3% of chromium sulphate salt is added and the drum is run for additional 30 minutes followed by the addition of 2.5% of an aqueous solution of a mixture of 2-methyl glutaric acid (MGA), 2-ethyl succinic acid (ESA) and adipic acid (AA). A pH of 2.6 to 2.8 is then obtained. After 17 hours, the chromium has completely crossed the hide cross-section.

(22) After this time, 20% of water is added and 0.35% of a commercial basifying agent (ex: magnesium oxide) (run 90 minutes). A second portion of 0.23% of magnesium oxide is added and the drum is run for an additional 90 minutes. The water bath is heated from 35 C. to 50 C. during 5 hours, the drum is drained, the wet blue is washed with 100% water (based on pelt weight), drained and washed again with 80% water.

Example 3 (Comparative Example)

(23) 100 kg of hide for which liming, unhairing and fleshing process are completed, is washed in the tanning drum with 100% water (based on pelt weight) for 10 minutes. After that, the hides were submitted to deliming and batting according to example 1.

(24) With relation to the hide weight, 60% of water, 6% of sodium chloride (B 6 to 7) and 0.4% of a commercial bleaching are added and the drum is run for 15 minutes. 0.6% of 85% formic acid diluted in water (1 to 10) is added (run 30 minutes) followed by the addition of 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes), 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes), followed by the addition of 0.4% of sulfuric acid diluted in water (1 to 15) and the drum is run for an additional 3 hours. After this time the pH of the bath is around 2.7 to 3.

(25) At this point a 3% of commercial chromium sulphate salt (ex: basic chromium sulphate, which presents 33% alkalinity and 25 to 26% chrome III oxide) is added (run 30 minutes). A second addition of 3% of chromium sulphate salt is added and the drum is run for an additional 17 hours after which time the chromium has completely crossed the hide cross-section.

(26) After this time, 20% of water is added and 0.35% of a commercial basifying agent (ex: magnesium oxide) (run 90 minutes). A second portion of 0.27% of magnesium oxide is added and the drum is run for an additional 90 minutes. The water bath is heated from 35 C. to 50 C. during 5 hours, the drum is drained, the wet blue is washed with 100% water (based on pelt weight), drained and washed again with 80% water.

(27) The obtained Wet Blue was submitted to the analysis of the total chromium content, chromium in layers and evaluation by Scanning Electron Microscope (EDS) (Table 1 and FIG. 1b).

Example 4

Production of the Retanned Hides (Crust Step) of Examples 1 to 3

(28) The Wet Blue obtained in the examples 1 and 3 are identified by different marks, combined and submitted to a standard re-tanning process. The Wet Blue hides are placed in the drum, and washed with 200% of water at 30 C. (based on Wet Blue weight) for 30 minutes and the washing is drained.

(29) 150% of water, 2% of sodium formiate and 0.3% of sodium bicarbonate are added and the drum is run for 60 minutes after which time the water bath shows a pH=4.4.

(30) The water bath is drained and 60% of water at 30 C., 2% of a commercial polyacrylate (powder) are added and the drum is run for 60 minutes and the water bath is drained.

(31) 150% of water at 60 C., 2% of commercial sulfited synthetic oil, 2% of commercial sulfated emulsified vegetal oil are added, the drum is run for 45 minutes and the water bath is drained.

(32) 0.3% of 85% formic acid diluted in water (1 to 5) is added (run 20 minutes), the water bath is drained and the re-tanned hides are washed.

(33) The re-tanned hides are left standing for 12 hours, stretched, naturally dried and softened.

(34) The properties of the re-tanned hides are evaluated and compared concerning tensile strength, breaking strength, tear strength, progressive tensile strength, up-taking of re-tanning products based on a square foot weight of crust, light fastness, color appearance and comparative evaluation by Scanning Electron Microscope (EDS) (Tables 2 to 4 and FIG. 2).

Examples 5 and 6

Production of the Wet Blue Intermediary Stage

Example 5

(35) 250 kg of hide for which liming, unhairing and fleshing process are completed, is washed in the tanning drum with 100% of water (based on pelt weight) for 10 minutes. After that, the hides are submitted to deliming and batting according to example 1.

(36) With relation to the hide weight, 50% of water, 6% of sodium chloride (B 6 to 7) and 0.4% of a commercial bleaching are added and the drum is run for 15 minutes. 0.6% of 85% formic acid diluted in water (1 to 10) is added (run for 30 minutes) followed by the addition of 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes), 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes) followed by the addition 0.4% of sulfuric acid diluted in water (1 to 15) and the drum is run for 3 hours. After this time the pH of the bath is around 2 to 3.

(37) At this point a 3% of commercial chromium sulphate salt (ex: basic chromium sulphate, which presents 33% alkalinity and 25 to 26% chrome III oxide) is added (run 30 minutes. A second addition of 3% of chromium sulphate salt is added (run 30 minutes) followed by the addition of 0.70% of Dioro (commercial product from Rhodia Poliamida e Especialidades Ltda. based on a mixture of di-carboxylic acids) and the drum is run for an additional 12 hours after which time the chromium has completely crossed the hide cross-section.

(38) After this time, 20% of water is added and 0.35% of a commercial basifying agent (ex: magnesium oxide) (run 90 minutes). A second portion of 0.35% of magnesium oxide is added and the drum is run for an additional 90 minutes. The water bath is heated from 35 C. to 50 C. during 5 hours, the drum is drained, the wet blue is washed with 100% of water (based on pelt weight), drained and washed again with 80% of water.

(39) The obtained Wet Blue was submitted to the analysis of the total chromium content and chromium in layers (Table 1).

Example 6 (Comparative Example)

(40) 250 kg of hide for which liming, unhairing and fleshing process are completed, is washed in a tanning drum with 100% of water (based on pelt weight) for 10 minutes. After that, the hides are submitted to deliming and batting according to example 1.

(41) With relation to the hide weight, 50% of water, 6% of sodium chloride (B 6 to 7) and 0.4% of a commercial bleaching are added and the drum is run for 15 minutes. 0.6% of 85% formic acid diluted in water (1 to 10) is added (run 30 minutes) followed by the addition of 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes), 0.3% of sulfuric acid diluted in water (1 to 15) (run 15 minutes), followed by the addition of 0.4% sulfuric acid diluted in water (1 to 15) and the drum is run for an additional 3 hours. After this time the pH of the bath is around 2.7 to 3.

(42) At this point a 3% of commercial chromium sulphate salt (ex: basic chromium sulphate, which presents 33% alkalinity and 25 to 26% chrome III oxide) is added (run 30 minutes). A second addition of 3% of chromium sulphate salt is added and the drum rum for an additional 12 hours after which time the chromium has completely crossed the hide cross-section.

(43) After this time, 20% of water is added and 0.35% of a commercial basifying agent (ex: magnesium oxide) (run 90 minutes). A second portion of 0.35% magnesium oxide is added and the drum is run for an additional 90 minutes. The water bath is heated from 35 C. to 50 C. during 5 hours, the drum is drained, the wet blue is washed with 100% of water (based on pelt weight), drained and washed again with 80% of water.

Example 7

Production of the Retanned Hides (Crust Step) of Examples 5 and 6

(44) The Wet Blue obtained in the examples 5 and 6 are identified, combined and submitted to a standard re-tanning process according to example 4.

Results

(45) The properties of the re-tanned hides are evaluated and compared concerning tensile strength, breaking strength, tear strength, progressive tensile strength, up-taking of re-tanning products based on a square foot weight of crust, light fastness and color appearance (Tables 2 to 4).

(46) TABLE-US-00001 TABLE 1 Analysis of chromium in the final Wet Blue 3.5% minimum Reference Value Example 3 Example 1 Example 5 Dermis 4.2 4.0 3.8 Middle 3.5 3.0 2.6 Epidermis 4.3 3.7 3.9 Layers Average 4.0 3.6 3.4

(47) FIG. 1 represent Scanning Electron Microscope (EDS) of Wet Blue samples.

(48) FIG. 1.a is a wet blue according to example 1 of the invention

(49) FIG. 1.b. is a wet blue according to comparative example 3

(50) TABLE-US-00002 TABLE 2 Comparative weight of crust leather Example 1 compared Example 5 compared to Example 3 after to Example 6 after re-tanning re-tanning Crust mass variation +8.2 g +5.8 g (up taking of re-tanning products) Percentage Variation 14% 12%

(51) TABLE-US-00003 TABLE 3 Comparative physical-mechanical properties of crust leather Minimum Example 3 Example 1 Example 5 Measurements Values after re-tanning after re-tanning after re-tanning Tensile Strenght Direction A Breaking Strenght (N) 150 192.1 347.5 143.5 IULTCS IUP6/ISO Tension Strenght 15 to 18 15.8 26 13.5 3376:2011 (N/mm2) Direction B Breaking Strenght (N) 230.6 168.7 176 Tension Strenght 19.8 12.3 16.3 (N/mm2) Progressive Tensile Strenght Tear Strenght 50 68.4 103 84.2 IULTCS 8/ISO 3372-2:2002 (N) Specific strength 49.1 59.2 73.5 84.3 (N/mm) Comparative light fastness Example 3 after Example 1 after Example 5 after re-tanning re-tanning re-tanning Light Fastness 4 >4 >4 NOTES: Xenon Lamp (Method EN ISO 105-B02:2002) Total exposure time: 24 hours, Filter Ultraviolet Irradiance: 445 W/m.sup.2 in 300 to 800 nm

(52) After the test the specimens were kept at least 1 hour in the dark in a conditioned environment at 23+/2 C. and air relative humidity of 50+/5%.

(53) FIG. 2 represent Scanning Electron Microscope (EDS) of Crust samples

(54) FIG. 2.a is a crust from wet blue according to example 1 of the invention

(55) FIG. 2.b. is a crust from wet blue according to comparative example 3

CONCLUSIONS

(56) The above results show the improvement brought with the present invention with regards to the up-taking of retanning products as well as the gain in mass of crust. The invention also enables better physical-mechanical properties of crust and final leather.