EP0615550B1 - Process for the treatment of skins, leathers or collagen-containing sheet materials using a pressurized dense fluid - Google Patents

Abstract

PCT No. PCT/FR93/00960 Sec. 371 Date Jun. 2, 1994 Sec. 102(e) Date Jun. 2, 1994 PCT Filed Oct. 1, 1993 PCT Pub. No. WO94/08054 PCT Pub. Date Apr. 14, 1994.A process for the treatment of products constituted by hides and skins with a view towards their transformation into leather, and for the treatment of leather which involves contacting the product with a dense, pressurized fluid, such as CO2, in the supercritical state or pressurized liquid, which optionally contains one or more active substances and a specified pressure Pi and temperature Ti, in order to provide degreasing of the skins if pure dense fluid is used, or to impregnate the products with active substances such as tanning, stiffening, dyeing or waterproofing agents. The products are subsesquently brought to atmospheric pressure to eliminate the dense fluid therefrom.

Description

The present invention relates to a process for treating skins, leathers or sheet materials containing collagen, at different stages of the transformation of skins into leathers, and at different stages of finishing these products.

The different stages of transformation of skins into leather are in particular the degreasing which consists in removing a part of the fats from the skin to facilitate its transformation into leather, and the tanning which transforms the raw putrescible skin into resistant and rot-proof leather; this last operation is considered to be the most important in the chain of transformation of skins into leathers.

The different stages of finishing can be retanning, which is an operation to reinforce the mechanical strength of leathers, dyeing and waterproofing for waterproofing.

In the current treatment processes for hides and skins, these different stages of treatment use liquid solvents or aqueous solutions loaded with active principles, which leads to a large quantity of effluents which are harmful to the environment. In addition, standards for the use of liquid solvents are becoming increasingly stringent, and some of them will be banned in the short term.

For example, the quantity of mainly aqueous effluents currently discharged by the leather industry worldwide is around 400 to 500 million m ³ per year, or almost 40 million equivalent "daily inhabitants".

The present invention specifically relates to a method of treating skins, hides or sheet materials containing collagen, which makes it possible to avoid the use of solvents harmful, penalizing for the environment, and to significantly limit the volumes of effluent produced during these treatments.

The process of the invention applies in particular to the degreasing and tanning of hides and to the finishing treatments (retanning, strengthening of the structure, dyeing and water repellency) of hides, leathers and other sheet materials containing collagen .

• a) mettre en contact les peaux avec un fluide dense capable de dissoudre les graisses, sous une pression Pi et une température Ti, la pression Pi étant au moins égale à la pression critique Pc du fluide et la température Ti étant telle que le fluide a une densité suffisante pour assurer la solubilisation partielle des graisses, et
• b) ramener les peaux à la pression atmosphérique pour éliminer le fluide dense sous forme de gaz.

Also, the subject of the invention is a method of degreasing the skins with a view to their transformation into leathers, which comprises the following steps:

• a) bringing the skins into contact with a dense fluid capable of dissolving fat, under a pressure Pi and a temperature Ti, the pressure Pi being at least equal to the critical pressure Pc of the fluid and the temperature Ti being such that the fluid a a sufficient density to ensure the partial solubilization of greases, and
• b) bring the skins to atmospheric pressure to remove the dense fluid in the form of gas.

Advantageously, in step a) of this process, the dense fluid is circulated continuously at the pressure Pi and at the temperature Ti in a treatment enclosure containing the skins, the dense fluid is purified at the outlet of the treatment enclosure in order to separate the dissolved fat therefrom and the purified dense fluid is recycled at the inlet of the enclosure.

The dense fluid can be purified by varying its pressure and / or its temperature in order to eliminate the dissolved products in liquid form. To recycle the dense fluid, it is then necessary to bring it back under the pressure conditions Pi and temperature Ti chosen.

In step b), the circulation of the dense fluid under pressure is interrupted and the enclosure is brought back to atmospheric pressure.

In this Drocédé, the dense fluid under pressure is used as a solvent to extract fats of a lipidic nature, but one operates in such a way that the extraction of fats is not total to keep the skin supple.

The use of a dense fluid under pressure makes it possible to obtain this partial extraction of greases because the solvent and dehydrating properties of the dense fluid can be modulated judiciously by adjusting the pressure Pi and the temperature Ti so as to adjust the density of the dense fluid and obtain the desired rate of defatting of the skins.

The process of the invention therefore makes it possible to perform the degreasing of the skins under good conditions.

• a) mettre en contact les peaux, les cuirs ou les matériaux en feuilles avec un fluide dense contenant au moins une substance active pour la transformation ou la finition des peaux, des cuirs ou des matériaux en feuilles, sous une pression Pi au moins égale à la pression critique Pc du fluide et à une température Ti telle que le fluide a une densité suffisante pour imprégner les peaux, les cuirs ou les matériaux en feuilles de la (des) substance(s) active(s), et
• b) ramener les peaux, les cuirs ou les matériaux en feuilles à la pression atmosphérique pour éliminer le fluide dense sous forme de gaz.

The invention also relates to a process for treating products made up of skins, leathers or sheet materials. containing collagen for processing into leather or finishing which includes the following steps

• a) bringing the skins, leathers or sheet materials into contact with a dense fluid containing at least one active substance for the transformation or the finishing of the skins, leathers or sheet materials, under a pressure Pi at least equal to the critical pressure Pc of the fluid and at a temperature Ti such that the fluid has a density sufficient to impregnate the skins, the leathers or the sheet materials with the active substance (s), and
• b) bring the skins, leathers or sheet materials to atmospheric pressure to remove the dense fluid in the form of gas.

In this treatment process, step a) is advantageously carried out by continuously circulating the dense fluid containing the active substance (s) at the pressure Pi and at the temperature Ti in a treatment chamber containing skins, leathers or sheet materials, by treating the dense fluid at the outlet of the treatment chamber to readjust its content of active substance (s) to the desired value, and thereby recycling the dense fluid treated in the treatment enclosure.

The treatment of the dense fluid leaving the enclosure in order to readjust its content of active substance (s) to the desired value can be carried out in gas-liquid or liquid-liquid contactors such as exchange columns with against the current.

Step b) can be carried out as above by interrupting the circulation of the fluid dense in the enclosure and reducing it to atmospheric pressure.

In some cases, it may be advantageous to rinse the products treated in the enclosure with pure dense fluid, before performing step a) of impregnation with the active substance and / or before step b) decompression.

In addition, depending on the nature of the active substance, step a) can be carried out statically (soaking) or dynamically, that is to say with continuous circulation of the dense fluid containing the active substance.

According to the invention, the expression “dense fluid” means a fluid under a pressure P greater than the critical pressure Pc of this fluid, which is preferably under temperature conditions Ti close to the critical temperature Tc of the fluid, this temperature and this pressure being chosen to give the fluid either a high extraction power with respect to grease, or a high dissolution or transport power with respect to the active substance or substances used.

Advantageously, the dense fluid used is in the form of gas at pressure and at room temperature.

This dense fluid can be chosen, for example, from carbon dioxide, sulfur hexafluoride, nitrous oxide, ammonia and light alkanes, having for example from 2 to 5 carbon atoms.

Preferably, carbon dioxide is used because it is non-toxic, non-flammable, not very reactive and inexpensive.

Furthermore, these supercritical conditions are very accessible since its critical pressure and temperature are 7.3 MPa and 31 ° C respectively.

According to the invention, the dense fluid can be in the subcritical or supercritical state.

Subcritical fluid is understood to mean a fluid at a temperature T lower than the critical temperature Tc of the fluid, which in the process of the invention corresponds to the liquid state, since the pressure Pi of the fluid is always greater than the pressure critic Pc.

By supercritical fluid is meant a fluid whose temperature T is greater than the critical temperature, which, in the process of the invention, corresponds to the supercritical state since the pressure Pi of the fluid is always greater than the critical pressure.

In the appended FIG. 1, the state diagram (in dashed lines) of pressure (in MPa) - temperature (in ° C.) of carbon dioxide CO _{2 has been shown} . Thus, the critical point C of CO ₂ corresponds to a critical temperature Tc of 31 ° C and to a critical pressure Pc of 7.3 MPa.

The supercritical fluid in the supercritical state according to the invention corresponds to the hatched area A.

The subcritical fluid in the liquid state according to the invention corresponds to the hatched area B. According to the pressure and temperature conditions chosen in areas A and B, the density of the fluid and therefore its power can be adjusted. solvent vis-à-vis the fat to dissolve or its capacity for dissolution and transportability of the active substances used, while further optimizing its capacity for dissolution so that it is not penalizing vis-à-vis the products to be treated.

In Figure 1, the solid isomass lines are shown (in solid lines) (in kg / cm ³ ) of CO ₂ .

Thus, it can be seen that in the case of carbon dioxide, the densities obtained under pressure are around 1 kg / cm ³ for the liquid state and adjustable from 0.2 to 1 kg / cm ³ for the supercritical state, ie a solvent power equivalent to that of traditional liquid solvents for the densest states; moreover, the fact that this solvent power is adjustable by pressure and / or temperature, can be judiciously used in the purification and impregnation operations with the active substance (s), during the recycling of dense fluid.

Preferably, according to the invention, a dense fluid is used in the supercritical state. In this state, the dense fluid also has viscosities ten times lower and diffusion coefficients 10 to 100 times greater than in the liquid state, which constitutes an important advantage for the kinetics of extraction and for the operations. impregnation.

According to the invention, the pressure Pi can be chosen, for example in the range from 6 to 5OMPa, and the temperature Ti can be, for example from 10 to 300 ° C.

In certain cases, it is also possible to improve the diffusion of the dense fluid in the products to be treated by periodically varying the pressure of the dense fluid from ΔP around Pi.

The pressure variation ΔP can represent around 10% of Pi, for example over a period of 5 to 50s.

When the process of the invention is used to carry out treatments for transforming skins into leather or finishing treatments, the active substances used are chosen according to the objective to be reached.

Generally, these active substances are organic materials or organo-mineral complexes, and the dense fluid used is also chosen according to the active substance so as to dissolve this substance or to be able to ensure its transport on the products to be treated in the pregnant.

In the transformation of skins into leather or in the modifications of the appearance of leathers of natural origin or sheet materials containing collagen, the most important impregnation operations are the filling or reinforcement by tanning and / or retanning , coloring and water repellency.

Also, the active substance is advantageously chosen from tanning agents, reinforcing agents, coloring agents, water-repellent agents and the precursors of these agents.

By way of example, the tanning or reinforcing agents can be aldehyde compounds such as formaldehyde and glutaraldehyde; condensation products of the acrylate-diisocyanate, methylol-urea, or methylol-melamine type, of the telechelic oligomer type or of the precursors of these products, for example polyethylene glycols (PEG) and polytetraethylene glycols (PTEG) or methylene glycols (PTMG ) modified in the case of telechelic oligomers; phenol, naphthol, naphthalene or dihydroxyphenylsulfone products; vegetable tannins for example based on pyrogallics or catechism, or their precursors, tannins of chromium, of aluminum, of zirconium, titanium and iron.

By way of example of coloring agents which can be used, mention may be made of nitrated and nitrosated compounds, such as mono and polyazo dyes and with metal complexes; stilbenic derivatives; derivatives of diphenyl and triphenylmethane; oxazine, thiazine and azine compounds; pyridine, quinoline and acridine; phthalocyanic compounds; anthraquinone compounds, sulfur dyes; and their precursors.

By way of example of water-repellent agents which can be used in the invention, mention may be made of chromium and fluorinated fatty acid complexes, chromium and aluminum alkylphosphate, imidoacetic acid derivatives, phosphoric acid esters, polysiloxane resins, fluorocarbon compounds or their precursors.

According to the invention, the dense fluid can also comprise an additive to modify the solvent or dehydrating power of the dense fluid, or to facilitate the dissolution or the transport of the active substance (s) in the dense fluid. .

Thus, when the active substance is not directly soluble in the dense fluid used, it can be dissolved in an additive acting as an auxiliary solvent, the whole being soluble or transported in the dense fluid.

By way of example of additives which can be used, mention may be made of water and alcohols such as ethanol, methanol, isopropyl alcohol, etc.

When using such an additive acting as an auxiliary solvent, one can no longer speak of direct solubility of the active substance in dense fluid, but of indirect solubility or transportability of the active substance and its solvent by the dense fluid under pressure. Indeed, the solvent contains the active substance and the whole is dissolved or transported by the dense fluid under pressure.

In this case, the use of dense fluid under pressure makes it possible to significantly reduce the amount of liquid solvent in contact with the treated products.

The additive used can also be water, which makes it possible to modulate the drying power of the dense fluid. This possibility of regulating the water content of the treated products by modulating the drying power of the dense fluid constitutes a significant improvement in the chain of transformation of skins into leathers.

In the process of the invention, the active substance (s) can be included in the treated products, either by capillary action or direct absorption in these products when they have a particular affinity for the active substance, either by reaction of the active substance with compounds of the treated product, or by means of an appropriate treatment which transforms the active substance into a form retained by the product to be treated. In this case, the active substance may consist of a precursor of a tanning, reinforcing, coloring or water repellency agent.

Treatments capable of modifying the active substance so that it is retained in the treated product can consist of chemical, physicochemical, photochemical treatment, irradiation, or even heat treatment.

These treatments can be carried out during or after the operation of bringing the products into contact with the dense fluid.

The method of the invention is very advantageous for the treatment of skins, leathers, and other sheet products containing collagen, since it can be implemented at different stages of the processing and finishing chain of these products. When the process of the invention is used in different stages of this chain, pressure and temperature conditions suitable for the treatment carried out are used in each stage, and these conditions may be the same or different when passing from one stage treatment to another. Furthermore, the same dense fluid can be used in all stages or different dense fluids.

• la figure 1 déjà décrite, représente (en traits mixtes) le diagramme d'état du CO₂ et (en traits pleins) les droites d'isomasse volumique du CO₂, et
• la figure 2 représente une installation de traitement pour mettre en oeuvre le procédé de l'invention.

Other characteristics and advantages of the invention will appear better on reading the description which follows given of course by way of illustration and not limitation with reference to the appended drawing in which:

• FIG. 1, already described, shows (in dashed lines) the CO ₂ state diagram and (in solid lines) the lines of CO ₂ volume isomass, and
• FIG. 2 represents a treatment installation for implementing the method of the invention.

In FIG. 2, it can be seen that the installation includes a treatment enclosure (1) in which the products (2) to be treated, such as skins, leathers or sheet products containing collagen, can be placed on a support. appropriate to promote the flow of dense fluid in the products.

The dense fluid is introduced into the enclosure (1) by an inlet pipe (3) coming from a storage container (4) after having been brought to the pressure Pi desired by the compressor (5) and to the temperature Ti required by the heat exchanger (7). This dense fluid was optionally charged with active substance (s) and / or additive (s) in contactors such as contactors (9) and (11).

The contactor (9) in which circulates for example water or an additive such as an alcohol through the pipes (13) and (15), can be connected by the pipe (17) to the inlet pipe (3 ) dense fluid in the enclosure (1). In the contactor (9), the dense fluid introduced by the pipe (17) is evacuated by the pipe (19) then recycled in the inlet pipe (3), either by the pipe (21), or by the pipe ( 23). The lines 17, 19, 21 and 23 and the pipe (3) are provided with suitable valves, not shown in the drawing, to allow the dense fluid to follow the desired circuit before it enters the enclosure (1).

Similarly, the contactor (11) which is traversed for example by an active substance introduced by a pipe (25) and evacuated by a pipe (27), can be connected to the inlet pipe (3) by a pipe (29 ). The dense fluid loaded with active substance leaving the contactor (11) via the pipe (31) is then recycled into the inlet pipe (3) through the pipe (23). As before, the lines (29) and (31) have suitable valves.

At the outlet of the enclosure (1), the dense fluid which is discharged into the outlet pipe (33) can be recycled without pressure loss other than the pressure losses, at the inlet of the enclosure (1) by the line (34), the circulator (35), the heat exchanger (7) and possibly the contactors (9) and (11).

However, when the treatment carried out in the enclosure is a degreasing treatment, the dense fluid is preferably recycled in the enclosure after having purified it from the dissolved products that it contains.

For this purpose, the dense fluid leaving via the pipe (33) is expanded in the valve (37), then introduced into the enclosure (39) for recovering the dissolved products which are separated from the dense fluid and discharged through the pipe (41 ). After this separation, the dense fluid which is in the gaseous state is brought back to the temperature suitable for recycling by the exchanger (43) and conveyed in the storage container (4) to be recycled in the enclosure (1) to desired temperature and pressure.

In the case of a degreasing treatment, the contactors 9 and 11 are not connected to the supply pipe (3).

In the case of other treatments using, for example, wet CO ₂ with a low water content, it may also be advantageous to recycle the dense fluid through the storage tank (4). In this case, the contactor (9) can be replaced by a contactor (40) supplied with additive by the lines (42) and (44), which is placed on the dense fluid recycling circuit. Thus, an additive, for example water, can be added in small quantities to the fluid, when it is in the gaseous state after expansion in the valve 37.

At the end of treatment, the compressor (5) is stopped and the dense fluid leaving the enclosure (1) is decompressed by the expansion valve (37) to evacuate it in gaseous form as previously in the storage container (4 ) and bring the treated products to atmospheric pressure.

When the treatment pressure is to oscillate around Pi, this is ensured by appropriate regulation of the circulator (35) or of the valve (37) in order to have a pulsed treatment regime.

Finally, the installation can be supplied with treatment fluid through the pipe (45) and drained through the pipe (47).

The following examples are given by way of illustration of the invention.

Example 1 : Degreasing of sheep skins in a pickled state.

In this example, there are 1.2 kg of whole Pickless skins wound around an axis in the treatment enclosure (1) and the degreasing operation is carried out by circulating in the enclosure dry and pure carbon dioxide. in the supercritical state under a pressure of 30 MPa at a temperature of 40 ° C. The treatment is carried out for a period such that 300 kg of carbon dioxide are used for the 1.2 kg of treated skins.

At the end of the operation, the lipid and water content of the treated skins is determined. We aknowledge as well as the lipid content decreased by 40% and that the water content decreased by 17%.

The skins retain a beautiful supple appearance with a few small deposits of mineral salts.

Example 2 : Degreasing of skins in the White Stabilized Wet state (BSH).

In this example, 1.5 kg of whole BSH skins are wound around an axis in the treatment enclosure (1) and the degreasing operation is carried out using dry and pure CO ₂ in the supercritical state under a pressure of 30 MPa, at a temperature of 60 ° C, for a period such that the amount of CO ₂ used is 300 kg.

At the end of the operation, the lipid and water content of the skins is determined. It is found that the lipid content has decreased by 26% and that the water content has decreased by 21%.

The skins retain a beautiful supple appearance with a very slight deposit of mineral salts.

By comparing the results of these two examples, we note that degreasing is more effective on Picklees skins than on BSH skins while the fat content is practically identical at the start in both cases. It is assumed that this is due to the different water content of the skins. Water loss is greater on BSH skins.

The skins treated in Examples 1 and 2 were transformed into finished leather and were compared to skins having undergone the same treatments but for which the degreasing had been carried out in a conventional fashion in tanning by using white spirit associated with an emulsifying agent consisting of a base alkyl-oxyethylenated.

The skins degreased according to the conventional route or according to the route of Examples 1 and 2 were tanned with chromium salt (10% basic sulfate at 33% basicity), then wrung, thickened by stripping; they were then retanned, dyed and fed under the same conditions. During these operations, all the skins were treated in the same material in a single batch.

After drying, corrugating and finishing, there were no significant differences in terms of suppleness, the fineness of the grain, the flower and the feel.

The colors are identical and no stain related to the presence of excess fat has been observed.

Examples 3 to 6 : Impregnation of hides and skins .

In these examples, the process of the invention is used to impregnate skins and leathers by using, as active substance, reinforcing agents constituted by telechelic oligomers of the polyethylene glycol (PEG) type having molecular weights in the range from 200 to 1500. The use of these oligomers is interesting because they are liquid products which can be dissolved in a dense fluid under pressure unlike the high solid polymers generally used for the reinforcement of leather products.

In these examples, the pressure conditions Pi and the temperature conditions Ti are chosen to have good solubility of the active substance (PEG) in the dense fluid consisting of carbon dioxide.

The oligomers used and the pressure and temperature conditions as well as the solubility of the oligomer in the dense fluid under pressure are given in Table 1 which follows.

In each example, the impregnation of skins and leathers is carried out under satisfactory conditions to obtain their reinforcement by passing 300 kg of dense fluid loaded with oligomer per kg of treated products.

Examples 7 to 9 : Impregnation of hides and skins .

In these examples, the same procedure is followed as in Examples 3 to 6 for impregnating skins and leathers, but products marketed under the name Térathanes which are oligomers of the PTEG or PTMG type, polyurethane precursors, are used as reinforcing product. .

As in the case of polyethylene glycols, these oligomers are liquid products which are more easily dissolved in the dense fluid than the high solid polymers usually used.

The pressure and temperature conditions used in these examples, as well as the nature of the oligomer used and its solubility are given in table 1 appended.

As before, by carrying out the impregnation operation with 300 kg of supercritical carbon dioxide for 1 kg of treated product, satisfactory results are obtained.

It should be noted that the solubility values indicated in table 1 are not the maximum solubility values of the products concerned.

Examples 10 to 13 which follow illustrate the impregnation of an active substance constituted by a nourishing principle in skins and leathers.

Example 10 :

In this example, skins and hides are treated using as nourishing product an oil of animal origin sulfitated at 60% content of active materials marketed under the name Lipoderm-Licker PK.

For this purpose, carbon dioxide is introduced into the treatment enclosure containing the skins and leathers as well as the oil, at a pressure of 25 MPa, at a temperature of 40 ° C. and the operation is carried out under static conditions for 16 hours. .

The skins and leathers are then brought to atmospheric pressure and their fat content is determined.

The results obtained are given in Table 2 which follows. The appearance of the leather is not degraded.

Example 11 :

Skins and hides are treated using as active substance the same nourishing product as in Example 10, but in this case performing the impregnation under dynamic conditions as follows.

Carbon dioxide is passed under a pressure of 25 MPa, at a temperature of 40 ° C, with a flow rate of 15 kg / h in a first autoclave comprising the oil and then in the treatment chamber containing the hides and skins , for 4 hours.

After this treatment, the fat content of the hides and skins is determined. The results obtained are given in Table 2. In view of these results, it is noted that despite the flows of CO ₂ and greater in oil than in Example 10, the fat content is substantially the same.

Example 12 :

• 1) traitement préalable par CO₂ pur des cuirs et des peaux pendant 5 h, et
• 2) imprégnation par CO₂ chargé d'huile pendant 4 h comme dans l'exemple 11.

Skins and hides are treated using the same active substance as in Examples 10 and 11, operating a dynamic regime as in Example 11, but performing the following 2 steps under the same pressure conditions (25 MPa) , temperature (40 ° C) and CO ₂ flow rate (15 kg / h):

• 1) prior treatment with pure CO ₂ of the hides and skins for 5 h, and
• 2) impregnation with CO ₂ loaded with oil for 4 h as in Example 11.

The results obtained are given in Table 2. Thus, it is noted that carrying out the 2 steps makes it possible to improve the fat content of the leathers.

Example 13 :

Skins and hides are treated using as active substance the same nourishing product as in Example 11, but in this case the impregnation is carried out in pulsed dynamic regime by circulating CO ₂ in a first autoclave containing the oil, then in the treatment chamber containing the hides and skins, under the same temperature (40 ° C.) and flow rate (15 kg / h) conditions as in Example 11, by varying the pressure siniusoidally, at a frequency of 10 s, 1 MPa around the average value of 19 MPa.

The results obtained under these conditions after 5 h of treatment are given in Table 2. Thus, it is noted that the pulsed regime is beneficial for the impregnation.

Despite a reduction in the water content of leathers, from 18 to 16%, no degradation phenomenon (cardboard - tarnishing of the flower ...) has been found after test. The variation in humidity of the leathers can be limited by humidifying the CO ₂ beforehand. Water can however modify the impregnation mechanisms and can therefore have a positive as well as a negative aspect.

Claims (20)

1. Process for the degreasing of skins with a view to their transformation into leather, characterized in that it comprises the following stages:

   a) contacting the skins with a dense fluid able to dissolve grease, under a pressure Pi and a temperature Ti, the pressure Pi being at least equal to the critical pressure Pc of the fluid and the temperature Ti being such that the fluid has an adequate density for ensuring the partial solubilization of the grease and

   b) returning the skins to atmospheric pressure in order to eliminate the dense fluid in gaseous form.
2. Process according to claim 1, characterized in that in stage a), there is a continuous circulation of the dense fluid at the pressure Pi and the temperature Ti in a treatment enclosure containing the skins, the dense fluid is purified on leaving the treatment enclosure in order to separate therefrom the dissolved substances and the purified, dense fluid is recycled to the enclosure entrance.
3. Process for the treatment of products constituted by skins, hides or sheet materials containing collagen with a view to their transformation into leather or their finishing, characterized in that it comprises the following stages:

   a) contacting the skins, hides or sheet materials with a dense fluid containing at least one active substance for the transformation or finishing of the skins, hides or sheet materials, under a pressure Pi at least equal to the critical pressure Pc of the fluid and at a temperature Ti such that the fluid has an adequate density for impregnating the skins, hides or sheet materials with the active substance or substances and

   b) returning the skins, hides or sheet materials to atmospheric pressure in order to eliminate the dense fluid in gaseous form.
4. Process according to claim 3, characterized in that in stage a), there is a continuous circulation of the dense fluid containing the active substance or substances at the pressure Pi and the temperature Ti in a treatment enclosure containing the skins, hides or sheet materials, the dense fluid is treated on leaving the treatment enclosure in order to readjust to the desired value its active substance content and the thus treated, dense fluid is recycled to the treatment enclosure.
5. Process according to either of the claims 3 and 4, characterized in that it also comprises a complementary rinsing stage of the skins, hides or sheet materials by means of a pure dense fluid, performed before stage a) and/or before stage b).
6. Process according to either of the claims 2 and 4, characterized in that in stage b), the circulation of the dense, pressurized fluid is interrupted and the enclosure returned to atmospheric pressure.
7. Process according to any one of the claims 1 to 6, characterized in that the dense fluid is chosen from among carbon dioxide, sulphur hexafluoride, nitrous oxide, ammonia and light alkanes.
8. Process according to claim 7, characterized in that the dense fluid is carbon dioxide.
9. Process according to either of the claims 7 and 8, characterized in that the dense fluid is in the supercritical state.
10. Process according to any one of the claims 1 to 9, characterized in that the dense fluid also comprises an additive for modifying the solvent or dehydrating power of the dense fluid, or for facilitating the dissolving or transfer of the active substance or substances into the dense fluid.
11. Process according to claim 10, characterized in that the additive is constituted by water or an alcohol.
12. Process according to any one of the claims 3 to 11, characterized in that the active substance is chosen from among tannin agents, reinforcing agents, dyeing agents, waterproofing agents and precursors of said agents.
13. Process according to any one of the claims 3 to 12, characterized in that the active substance reacts with the products to be treated.
14. Process according to any one of the claims 3 to 12, characterized in that the active substance is such that it can be transformed by an appropriate treatment into a form retained by the products to be treated.
15. Process according to claim 14, characterized in that the treatment is a chemical, physicochemical, photochemical, irradiation or thermal treatment.
16. Process according to any one of the claims 1 to 12, characterized in that during the contacting of the products to be treated with the dense fluid, the pressure of the dense fluid is periodically varied by ΔP around Pi.
17. Process according to claim 16, characterized in that the pressure variation ΔP represents 10% of Pi.
18. Process according to either of the claims 1 and 3, characterized in that the pressure Pi is from 6 to 50 mPa and the temperature Ti from 10 to 300°C.
19. Process according to any one of the claims 1, 3 and 18, characterized in that the pressure Pi and the temperature Ti are chosen so as to adjust the solvent power with respect to grease or the dissolving capacity of the active substance or substances by the dense fluid to the desired value.
20. Process according to either of the claims 3 and 12, characterized in that the active substance is a telechelic oligomer belonging to the group of polyethylene glycols and/or polytetraethylene (or methylene) glycols.

Images