PROCESSES FOR MAKING LEATHER

Description The present invention is directed to processes for making leather, wherein Methoxy-3,4-Dihy- dro-2H-pyran (MOP) is used as a tanning agent.

Tanning is the process to preserve animal skin against bacterial or fungal degradation through chemical crosslinking of the collagen fibers in the skin. Currently, the major tanning agent used is chromium (III) sulfate. Problem with this tanning agent is the potential formation of toxic chromium (VI) in the later production process or use of leather.

Tanning is an early step within the leather making process, followed by splitting or shaving, filling or re-tanning with polycondensates, polymers or vegetable tannins, and fatliquors, by dyeing and finally by finishing with films of polyacrylates or polyurethanes to preserve the leather on the surface. All modifications of the tanning step have to be compatible with the later steps in leather making.

A widely accepted proof for successful tanning is the shrinkage temperature (Ts). Shrinkage temperatures Ts given herein are determined according to DIN 53336 (1977-01 ). Green cattle hide shrinks irreversibly when heated above 65°C. (M. Fein, E. M. Filachione US Patent 2.941.859, 1959). With chromium (III) tanning a shrinkage temperature of up to 100°C can be reached (H. Spahrkas, H. Schmidt, Das Leder 10, 145, 1959). Ideally, shrinkage temperatures of leather are at least 75 or 75 °C.

An alternative is the use of glutaraldehyde as tanning agent, which forms irreversible covalent crosslinking with collagen. With glutaraldehyde tanning shrinkage temperature of up to 80°C can be accomplished. Typical shrinkage temperatures range from 72 - 75°C (C. Gabagnou, F. Dopert, D. Herta, M. Mess, J. T. Pelckmans, J. Christner, J. Fennen, World Leather, December 2015). Glutaraldehyde is toxic, but since it reacts with the collagen covalently to form a new crosslinked collagen, the toxicity diminishes in the tanning process. However, the problem of the toxicity of glutaraldehyde as a product remains in transportation, storage, and handling prior to tanning. Another issue with glutaraldehyde in tanning is the formation of yellowing of the resulting leathers during the tanning process.

Industrially, glutaraldehyde is made from the acidic hydrolysis of alkoxy-3,4-dihydro-2H-pyrans (US 2,546,018, EP 66 224). DE 42 19 419 describes the preparation of tanning agents starting from alkoxy-3,4-dihydro-2H-pyran and alcohols. WO 97/21839 reveals the co-polymerization of alkoxy-3,4-dihydro-2H-pyrans and ethylenically unsaturated monocarboxylic acids leading to tanning agents. US 2 941 859 analyzes the tanning with glutaraldehyde and mentions that ethoxy-3,4-dihydro- 2H-pyran may be used as a tanning agent.

It was an objective of the present invention to provide less toxic tanning agents and processes for making leather that are easy and fast to carry out show good yellowing properties.

This objective was solved by processes for making leather, wherein Methoxy-3,4-Dihydro-2H- pyran (MOP) is used as a tanning agent.

MOP has the following structural formula

The use of MOP as a tanning agent turned to yield leather excellent shrinkage temperatures and yellowing properties.

Surprisingly, it was also found that the use of MOP has leads to better and faster tanning effect than for example ethoxy-3,4-dihydro-2H-pyran.

In one embodiment MOP is used as a pure substance.

In one embodiment, MOP is used in a solution in solvent L said solvent L being an organic solvent having a miscibility with water of at least 20% (i.e. mixtures of said solvent with water can comprise at least 20 wt% of said solvent without showing turbidity at room temperature).

Suitable solvents L include ethylene or propylene carbonate, alkoxylates, especially ethoxylates or propoxylates of C2 to C18 alcohols and / or alkoxylates of carboxylic acids, especially ethoxylates of C6 to C20 carboxylic acids.

In another embodiment, MOP is used in a formulation F, formulation F being an aqueous formulation.

In one embodiment, formulation F is an aqueous formulation.

In one preferred embodiment, formulation F is an aqueous solution.

In one preferred embodiment, formulation F is an aqueous emulsion. The term "aqueous" shall imply that the solvent of such formulations comprise at least 50% by weight of water, preferably at least 70 % by weight, more preferably at least 90 % by weight. The term "solution" shall imply that MOP is complete dissolved in the aqueous solvent.

The term "emulsion" shall imply that formulation F is a multiple phase, preferably two-phase system that comprises an aqueous phase and a liquid oil phase, MOP being at least partly present in the oil phase.

In one preferred embodiment formulation F comprises

a) 10 to 80 parts by weight of MOP,

b) 20 to 90 parts by weight of at least one surfactant S,

c) 0 to 10 parts by weight of at least one thickener T, and

d) Water.

Preferably the pH of such aqueous formulations F is from 5 to 9.

In one embodiment formulation F is an aqueous emulsion and comprises

a) 10 to 80 parts by weight of MOP,

b) 20 to 90 parts by weight of at least one surfactant S,

c) 0 to 10 parts by weight of at least one thickener T, and

d) Water,

Wherein the pH of such aqueous formulations F is preferably from 5 to 9. Preferably, aqueous formulations F comprises 10 to 50 % by weight of MOP, based on the formulation.

Preferably, aqueous formulations F comprises 30 to 70 % by weight of water, based on the formulation.

Said at least one surfactant S is preferably a nonionic surfactant.

More preferably, surfactant S is selected from fatty alcohols, alkoxylates, especially ethoxylates of C6 to C30 monools, esters of C6 to C20 carboxylic acids or sulfited or sulfated esters of C6 to C20 carboxylic acids and polyalkylene oxides, alkoxylates of polyols, glycerol alkyl esters, block copolymers of polyalkyleneoxides, alkyl ethers of polyols, C6 to C20 carboxylic acid amides, N- alkoxylates of C6 to C20 carboxylic acid amides.

Sulfited or sulfated esters of C6 to C20 carboxylic acids shall mean the addition products of sulfuric acid or sulfurous acid to ethylenically unsaturated double bonds in esters, especially triglycerides, of C6 to C20 unsaturated carboxylic acids.

Especially preferred surfactants S include esters of C6 to C20 carboxylic acids or sulfited or sulfated esters of C6 to C20 carboxylic acids.

Thickeners are in principle known to skilled persons. A thickener is a substance which can in- crease the viscosity of a liquid without substantially changing its other properties. Preferably, thickeners T are selected from polyacrylic acid and or polyacrylic esters that show an increase in viscosity upon increase of pH (i.e. with decreasing acidity).

In one embodiment, formulation F comprises

a) 10 to 80 parts of MOP,

b) 19 to 89 parts of at least one solubilizer SL,

c) 1 to 10 part of at least one thickener T,

d) Water,

Wherein the pH of said formulation is preferably from 5 to 9.

A solubilizer is this context a compound that is miscible with water and in which MOP has a solubility of at least 100 g/l, preferably an unlimited miscibility or solubility. Through the inclusion of solubilizer SL in formulation F, MOP can be provided in an aqueous solution of emulsion. In one embodiment, formulation F is an aqueous solution and comprises

a) 10 to 80 parts of MOP,

b) 19 to 89 parts of at least one solubilizer SL,

c) 1 to 10 part of at least one thickener T,

d) Water,

Wherein the pH of said formulation is preferably from 5 to 9.

In one embodiment, formulation F is an aqueous emulsion and comprises

a) 10 to 80 parts of MOP,

b) 19 to 89 parts of at least one solubilizer SL,

c) 1 to 10 part of at least one thickener T,

d) Water,

Wherein the pH of said formulation is preferably from 5 to 9.

Suitable solubilizers SL are different from surfactants S and include polyols like glycol, glycerol, sorbitol and their alkoxylates.

Thickeners T useful in combination with solubilizers SL are the same as those useful in combination with surfactants S, as described above. Preferably, formulations F comprising at least one solubilizer SL comprise 30 to 70 % by weight of water, based on the formulation.

In one embodiment, formulation F is a solution in a solvent L, said solvent L being an organic solvent having a miscibility with water of at least 20%. In one embodiment, formulation F is a solution comprising 10 to 80 % by weight of MOP and at least one solvent L, wherein solvent L is selected from ethylene carbonate or propylene carbonate, alkoxylates, especially ethoxylates or propoxylates, of C2 to C18 alcohols and / or alkox- ylates of carboxylic acids, especially ethoxylates of C6 to C20 carboxylic acids.

In one embodiment, formulation F comprises at least one solvent L in combination with water. Preferably, formulation comprises 20 to 70 % by weight of at least one solvent L and 5 to 50 % by weight of water.

In one embodiment, formulation F comprises ethylene carbonate in combination with water. Preferably, formulation comprises 20 to 70 % by weight of ethylene carbonate and 5 to 50 % by weight of water.

In one embodiment, formulation F comprises propylene carbonate in combination with water. Preferably, formulation comprises 20 to 70 % by weight of propylene carbonate and 5 to 50 % by weight of water.

Preferably, MOP or formulations F are uses as tanning agents in leather making in the tanning or the pretanning step.

Normally formulations F are prepared and stored at a pH from 5 to 9, preferably 6 to 8.5.

For the application of formulation F to the animal skin, the pH of the application medium ("float") is normally adjusted to a value from 2 to 4.

In one embodiment, processes for making according to the invention comprise the following steps: A) Adjusting the pH of the application medium to a value of 2 to 4,

B) applying the application medium obtained in step A) comprising formulation F to an animal skin in the tanning or pretanning step.

Another aspect of the invention is leather obtained in processes according to the invention.

Another aspect of the invention are shoes, car interiors, garments or accessories comprising leather obtained in processes according to the invention.

Processes according to the invention are easy and economical to carry out. They make use of compounds that have a low toxicity and that are chromium free and are thus environmentally friendly and easy to handle in terms of working hygiene.

Processes according to the invention are allow for fast chromium free tannage of animal skins. Processes according to the invention yield chromium free leather with excellent yellowing prop- erties. Processes according to the invention yield chromium free leather with high shrinking temperatures.

Processes according to the invention yield chromium free leather with excellent haptical proper- ties like tackiness and softness.

Experiments

Ethoxy-3,4-dihydro-2H-pyran was purchased from TCI Europe N.V. with a purity of greater than 97 percent.

Methoxy-3,4-dihydro-2H-pyran was purchased from BASF and had a purity of more than 95%. Shrinkage temperatures were determined according to DIN 53336:1977-01 . In order to compare methoxy- and ethoxy-3,4-dihydro-2H-pyran on a one to one basis, their molecular weight was taken into account: (1 14,14 g/mol), ethoxy-3,4-dihydro-2H-pyran (128,17 g / mol), glutaraldehyde (100,12 g/mol). Consequently, the amount of ethoxy-3,4-dihydro-2H-pyran with a 10% higher molecular weight was raised 10% compared to methoxy-3,4-dihydro-2H-py- ran, which again was 10% higher that glutaraldehyde. This provided the fact that the tanning ex- periments had the same molar ratio of aldehyde functionalities, being either released from both alkoxy-3,4-dihydro-2H-pyrans via acidic hydrolysis or being already present in the glutaraldehyde.

Formulation 1

In a bottle with magnet stirring device 30 g of an ethoxylated Ci2-alcohol bearing a number average of 7 units of ethyleneoxy units was mixed with 30 g methoxy-3,4-dihydro-2H-pyran (MOP) at room temperature. Stirring was continued for 1 h. A clear solution without turbidity was obtained.

Formulation 2

In a bottle with magnet stirring device 40 g of a 70 wt% concentrated sorbitol solution in water was mixed with 40 g methoxy-3,4-dihydro-2H-pyran and 1 1 1 g of water. 6 g of a thickener on polyacrylic basis (40% solution in water) was added at a pH of 4. Three ml. of a 1 N NaOH solution were added and a viscous dispersion formed. The pH of the mixture was 7. Stirring was continued for 1 h. The mixture obtained was stable over several days at 4°C and 23°C and did not show phase separation or turbidity. Tanning Process 1 :

In a 3.5 L tanning drum 100 g of water and 7 g of sodium chloride were treated with 0.8 g of formic acid (98%). The resulting float had a pH of 2.8. 100 g of picked cattle pelt was added and the tanning drum was turned for one hour. The tanning agent given in table 1 was added and agitated for 1 h. The pH was 2.8. The tanning was continued for 16 h. A sample was cut off the leather to measure the shrinkage temperatures. Afterwards the tanning process was continued for another 3 h. Results are given in table 1 and 2.

Table 1 : Results of the tanning experiments between methoxy-3,4-dihydro-2H-pyran and eth- oxy-3 ,4-dihyd ro-2 H-py ra n

The results indicate that methoxy-3,4-dihydro-2H-pyran is the better tanning agent compared to ethoxy-3,4-dihydro-2H-pyran as shown by differences in shrinkage temperature of about 4°C under the same condition, as shown in table 1 .

Further we observed that the increase of the amount of ethoxy-3,4-dihydro-2H-pyran from 1 ,9% to 2.3% in tanning process 1 did improve the shrinkage temperature to the extent that 1.7 % methoxy-3,4-dihydro-2H-pyran imposed on pelt, as can be seen in table 1 . Tanning Process 2:

In a 3.5 L tanning drum 100 g of water and 7 g of sodium chloride were treated with 1.6 g of formic acid (98%). The resulting float had a pH of 2.5. 100 g of picked pelt was added and the tanning drum was turned for one hour. The tanning agent given in table 3 were added and agitated for one hour. The pH of the float was 2.5. Glutaraldehyde was added as an aqueous 50 % by weight solution. Tanning was continued for 17 hours. A sample was cut off the leather to determine shrinkage temperatures. Afterwards the pH was raised through addition of 3 g NaHCC>3 within one hour. The pH reached 4 and the tanning process was continued for another one hour. A sample was cut off the leather to determine shrinkage temperatures after 20 hours. Through addition of another 2 g NaHCC>3 within one hour; the pH reached 4.6. A sample was cut off the leather to obtain shrinkage temperatures after 24 hours. Results are given in table 3.

Tanning Process 3:

In a 3.5 L tanning drum 100 g of water and 7 g of sodium chloride were mixed with 2 g of formic acid (98%). The resulting float had a pH of 2.3. 100 g of picked pelt was added and the tanning drum was turned for one hour. The tanning agent given in table 3 was added and agitated for one hour: pH = 2.3. Glutaraldehyde was added as an aqueous 50 % by weight solution. Tan- ning was continued for 7 hours. A sample was cut off the leather to obtain shrinkage temperatures. Afterwards the pH was raised through addition of 5 g NaHCC>3 within one hour. The pH reached 4.5 and the tanning process was continued for another 8 hours. A sample was cut off the leather to obtain shrinkage temperatures after 17 hours. Results in table 3. Tanning Process 4:

In a 3.5 L tanning drum 100 g of water and 7 g of sodium chloride were treated with 2 g of formic acid (98%). The resulting float had a pH of 2.3. 100 g of picked pelt was added and the tanning drum was turned for one hour. The tanning agent in formulation 2 was added and agitated for one hour: pH = 2.3. Glutaraldehyde was added as an aqueous 50 % by weight solution. Tanning was continued for 3 hours. A sample was cut off the leather to obtain shrinkage temperatures. Afterwards the pH was raised through addition of 5 g NaHCC>3 within one hour. The pH reached 4.5 and the tanning process was continued for another three hours. A sample was cut off the leather to obtain shrinkage temperatures after 8 hours. Results are given in table 3.

Tanning Tanning agent / Ts in ° Ts in ° Ts in ° Ts in ° Comment Pro¬

Process Formulation C after C afC afC affor sub-secess

4 h ter 8 h ter 17 ter 24 quent time

h h leather

making

process

2 8.5 g formulation 2 66 76 o.k. 24 h

( = 1 .7 % Methoxy- pH = pH =

3,4-dihydro-2H-py- 2.5 4.6

ran)

2 1.5 % Glutaralde64 82 o.k. 24 h

hyde (=3% of a pH = pH =

50% concentrated 2.5 4.6

product)

3 8.5 g formulation 2 57 79 o.k. 17 h

( = 1 .7 % Methoxy- Tanning Tanning agent / Ts in ° Ts in ° Ts in ° Ts in ° Comment ProProcess Formulation C after C afC afC affor sub-secess

4 h ter 8 h ter 17 ter 24 quent time

h h leather

making

process

3,4-dihydro-2H-py- pH = pH =

ran) 2.3 4.5

3 1.5 % Glutaralde- 61 78 o.k. 17 h

hyde (=3% of a pH = pH =

50% concentrated 2.3 4.5

product)

4 8.5 g formulation 2 53 75 o.k. 8 h

( = 1 .7 % Methoxy- pH = pH =

3,4-dihydro-2H-py- 2.3 4.5

ran)

4 1.5 % Glutaralde- 60 78 o.k. 8 h

hyde pH = pH =

(=3% of a 50% 2.3 4.5

concentrated product)

Table 3: Variation in process time; process time tanning refers to tanning only

Tanning Process 5: In a 3.5 L tanning drum 500 g of water and 35 g of sodium chloride were treated with 10 g of formic acid (98%). The resulting float had a pH of 2.4. 500 g of picked pelt was added and the tanning drum was turned for one hour. The tanning agent in formulation 3 was added and agitated for one hour: pH = 2.4. Glutaraldehyde was added as an aqueous 50 % by weight solution. Tanning was continued for 7 hours. A sample was cut off the leather to obtain shrinkage tempera- tures. Afterwards the pH was raised through addition of 5 g NaHCC>3 within one hour. The pH reached 4.3 and the tanning process was continued for another 8 hours. A sample was cut off the leather to obtain shrinkage temperatures after 17 hours. Results are given in table 4. Tanning Tanning agent / Ts in °C after 8 h Ts in °C after 17 h Process time Process Formulation on 500

g pickled pelt

5 35 g formulation 2 53 73 17 h

( = 1.4 % Methoxy- pH = 2,4 pH = 4,3

3,4-dihydro-2H-py- ran)

5 28 g formulation 2 49 71 17 h

( = 1.1 % Methoxy- pH = 2,4 pH = 4,3

3,4-dihydro-2H-py- ran)

Table 4: Variation in concentration of Methoxy-3,4-dihydro-2H-pyran; process time tanning refers to tanning only

Yellowing Properties

Measurements were carried out with tanned leathers before subsequent leather making steps in a spectral photometer (Coloreye 2180 UV, GretagMacbeth). The value for the leather tanned with methoxy-3,4-dihydro-2H-pyran was used as a reference and the value for the leather tanned with glutaraldehyde was compared with that standard. Clearly, leathers tanned with methoxy-3,4-dihydro-2H-pyran were less colored than leathers tanned with glutaraldehyde, as measured with the ΔΕ value, which quantifies the difference in color between two samples, without indications about the direction of color change. The yellowing can be quantified in the same photometer via ΔΒ values. Here positive values indicate more yellowing, as observed for leathers tanned with glutaraldehyde, and depicted in table 5.

Table 5: Results yellowing; the value for the leather tanned with methoxy-3,4-dihydro-2H-pyran was used as a reference and set to zero

Interestingly, this effect was observed for formulated as well as for unformulated methoxy-3,4- dihydro-2H-pyran.