WO2005071696A1

WO2005071696A1 - Process for deposition of conductive polymer coatings in supercritical carbon dioxide

Info

Publication number: WO2005071696A1
Application number: PCT/FI2005/000053
Authority: WO
Inventors: Jani Pelto; Samu Aalto; Antero Laitinen
Original assignee: Valtion Teknillinen Tutkimuskeskus
Priority date: 2004-01-27
Filing date: 2005-01-27
Publication date: 2005-08-04
Also published as: EP1774539A1; FI116626B; FI20040114A0; FI20040114A

Abstract

A method for forming an electrically conductive polymeric surface on a solid polymeric substrate (S) comprises the following successive steps: 1) treatment of the solid polymeric substrate (S) in a pressure reactor (1) with a first supercritical or liquid carbon dioxide phase containing a monomer to cause the monomer to enter the structure of the polymeric substrate,2) removal of the first supercritical or liquid carbon dioxide phase from the reactor (1), together with possible residues of the monomer 3) feeding of a second supercritical or liquid carbon dioxide phase containing an oxidative agent into the reactor (1) into contact with the substrate (S) that has remained in the reactor, and 4) performing in-situ oxidative polymerization of the monomer in the polymeric substrate with the help of the oxidative agent to form an electrically conductive polymer surface on the polymeric substrate (S), 5) removal of the second supercritical or liquid carbon dioxide phase from the reactor.

Description

PROCESS FOR DEPOSITION OF CONDUCTIVE POLYMER COATINGS IN SUPERCRITICAL CARBON DIOXIDE

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to conductive polymeric composites. In particular, the present invention relates to methods of improving electrical conductivity of surface layer of thermoplastic polymeric articles.

2. Brief summary of the Related art

Supercritical phase of carbon dioxide (scC0₂) is present at temperatures above +31 °C and pressures above 7,3 MPa. Supercritical C0₂ is a solvent for many organic, particularly nonpolar molecules and is capable of reversibly swelling various polymers i.e. plasticizing effect. These well-known properties enable scC0₂ to act as a carrier phase for transport for small molecules into a polymer matrix. The most important factors affecting the efficacy of the material transport are the solubility of the particular small molecules in scC0₂ and diffusion kinetics of scC0₂ inside the matrix material. Solubility of a molecule in scC0₂ can be enhanced by use of a proper co-solvent such as ethyl or methyl alcohols, acetone or water. The main advantages of liquid or supercritical C0₂ over the most commonly used organic solvents are its nonflammability, nontoxicity, low price and tunable solvent power.

Many techniques exist for rendering otherwise insulating polymeric articles or their surfaces electrically conductive. A common method is to mix conductive fillers with the polymer. The bulk conductivity thus achieved depends on the filler used, but the surface conductivity is typically less than expected from the bulk properties of the composite material. In addition, the mechanical properties and the process ability of the matrix polymers are usually weakened.

Another approach, where the surface conductivity is essential, is use of conductive coatings. These include solid metallic films and solvent-based coatings containing conductive phase and a polymeric binder. One challenging issue associated with most coatings is their adhesion to their substrate. Other issues rise in form of corrosion of metallic coating films and deposited layers and, in certain cases, the emissivity and reflectance of electromagnetic radiation.

Inherently conductive polymers, such as polyaniline, polypyrrole and polythiophenes are relatively new class of electrically conductive materials. In their conductive form they are usually insoluble and non-fusible materials. Therefore, to achieve highly conductive and mechanically satisfactory composites with insulating materials, solution processing in organic solvents and in-situ polymerization of the conductive phase is usually applied.

US Patent 6214260 is one example of in-situ polymerization for preparing a conductive elastomeric foam. First a pre-formed polyurethane foam is treated with a solution of an oxidant in a supercritical fluid solvent to swell the foam and allow substantially even distribution of the oxidant in the wall and struts of the foam. Thereafter the foam is treated with a polymer precursor vapour or a polymer precursor solution in a supercritical fluid solvent.

As described in the example 2 of the patent, the polyurethane foam is first swelled with iron (III) trifluoromethane sulfonate in supercritical carbon dioxide solution, whereafter the foam so impregnated is subjected to a pyrrole solution in supercritical carbon dioxide to bring about the polymerization to polypyrrole.

However, the above-described method does not confer conductivity to the substrate to a sufficient degree. Furthermore, this order of adding reactants tend to leave impurities in the polymer matrix that may deteriorate the polymeric structure of the substrate after the second step. So that a final product of high purity is obtained, a washing step with methanol is required.

Tang M. et al. in European Polymer Journal 39 (2003) pp. 143-149 describe the use of supercritical carbon dioxide as solvent for the monomer (pyrrole) to impregnate an insulating host polymer (polystyrene) with the monomer before the polymerization. The polymerization is carried out by doping the impregnated host polymer with an oxidizing agent by soaking it in an aqueous or acetonitrile solution of FeCI₃.

However, there is no such method available by which conductive polymer could be deposited or a polymer surface with a good production rate and effectively without harmful wastes, making it possible to treat insulating polymers in industrial scale for various applications where materials with a conductive surface is needed.

A major drawback in processes utilizing in-situ polymerization technique is the large amount of organic waste produced. A partial solution to this problem has been suggested by the above reference Tang et. al, who used supercritical carbon dioxide in impregnation of polystyrene with pyrrole. Pyrrole was subsequently polymerized in aqueous- or acetonitrile oxidant baths. Still, a large amount of inorganic (ferric chloride) oxidant solution, contaminated by polymerization residues, was produced.

SUMMARY OF THE INVENTION

The above discussed and other shortcomings of the prior art are alleviated by the method of the present invention, wherein a polymeric article is first treated with scC0₂ rich phase containing a monomer (process step 1) and subsequently with scC0₂ rich phase containing a oxidant capable of polymerizing said monomer to form intrinsically conductive polymer on an insulating polymer (process step 2). For the most part the polymerization reaction takes place in the surface- or the near-surface layer of the matrix containing monomer and swollen with scC0₂. Polymerization residues such as unreacted monomer and oxidant can be extracted from the matrix by successive treatment with pure scC0₂ or mixture of scC0₂ and a proper co-solvent.

Instead of supercritical C0₂, high pressure liquid C0₂ near the supercritical state can also be used. This is a feasible alternative especially in the oxidation step (process step 2).

It is advantageous to conduct the oxidation with the supercritical C0₂ or high pressure liquid C0₂ as solvent for the oxidant in the second step after the impregnation by monomer in the same type of solvent. The oxidant reacts immediately after it has been absorbed in the monomer-impregnated surface. The inner structure of the substrate (matrix polymer) remains thus intact and is not affected by the oxidant, for example in the case of textile fibres. Further, it has been proved experimentally that this order of different process steps have considerable positive effect on the conductivity of the final product.

When the different process steps one after the other are performed on material placed within a pressure reactor, the material needs no transport between the steps, and all solvent phases, each in a suitable composition, can be led one after the other to the reactor and, after the required residence time, from the reactor.

The use of organic solvents is minimized since only carbon dioxide and less than 10 vol-%, preferably less than 1 vol-% of proper co- solvent is used in each process step. Suitable co-solvent are, but are not restricted to lower alcohols, acetone and water. A great deal of residual substances from the reaction (polymer residues, oligomerized monomers, surplus polymer detached from the surface, unreacted oxidant, reduced oxidant, various derivatives of oxidant) remain in the solvent phase(s) taken from the reactor, especially in the co-solvent phase after the separation of the carbon dioxide. A complete solution to the waste problem of prior art is to eliminate the inorganic aqueous or the organic solvent based bath by using carbon dioxide as the only major solvent in the process.

Suitable monomers are aniline, pyrrole, thiophenes and their substituted derivatives that can be polymerized using chemical oxidative polymerization.

Thickness of the conductive layer can be adjusted by varying the treatment time, monomer and oxidant concentrations, pressure and temperature of the scC0₂-phase and the depressurizing rate between process steps 1 and 2.

The resulting surface structure of the composite will consist of essentially interpenetrating networks of the matrix- and conductive phases. The surface layer will be well adherent and show superior electrical and mechanical properties.

The above discussed features of the present invention will be understood by those skilled in the art from the detailed description and the examples.

DETAILED DESCRIPTION OF THE INVENTION

Various, otherwise electrically insulating thermoplastic and thermosetting polymers are used in applications where electrical conductivity in needed. Applications areas most commonly encountered are electrostatic discharge (ESD) -protection and electromagnetic interference (EMI) suppression. Conductivity requirements are quite different in these two application areas: ESD- protection is believed to be optimal with materials possessing surface resistivity of about 10⁶-10⁸ ohms whereas bulk resistivity less than 10^"1 ohm*cm is usually required in EMI suppression. Inherently conductive polymers have shown potential in both areas. In many cases only the surface conductivity is essential and various coating methods are available to fill the surface conductivity requirement. Motivation for using various coating methods is based on (i) savings in conductive material, which has usually high price compared to the matrix polymer, and more importantly, (ii) intact critical material properties of the matrix polymer. Such critical properties are mechanical performance and processability.

A good example of such a product is a textile fabric. Fabrics are weaved or knitted from fibers that have been spun and treated to highly tuned performance. A non-woven fabric is one further example of a textile fabric.

Melt blending conductive materials, for example intrinsically conductive polyaniline salt within fiber-forming polymers, for example polyesters and polyamides usually do not yield satisfactory mechanical properties. This is due to extremely rigid nature of the conductive polymer inself and the lack of compatibity between the phases in microscopic level. In another approach, fibers are drawn from solution containing the fiber forming polymer and the conductive polymer. Much finer microstructures are formed but the process involves handling of large amounts of volatile organic solvent.

Coating on common textile fibers (polyesters, polyamides) have been recognized to the most feasible way to produce fabrics for ESD-, and in some case also EMI -protection. But it is a very challenging task to deposit a durable coating of a rigid conductor onto flexible substrate fiber. Sprayed, dipped and brushed coatings of various kind (possibly reactive) have been utilized but they have limited durability and tend to be very expensive. In addition, a considerable portion of the expensive coating is wasted in the process. Solution to the durability problem is to use in-situ polymerization onto the fiber surface using the above-described subsequent process steps 1 and 2. since the resulting surface structure of the composite will consist of essentially interpenetrating networks of the matrix- and conductive phases. The fibers so treated can be in form of filaments, yarns, or large-surface textile structures formed of these filaments or yarns.

In the present invention the in-situ polymerization is carried in the process described in the Example 1 below and in Figure 1. In the general example 1 , a roll of polyester textile fabric is being treated. The invention is by no means restricted in processing fibers or fabrics. One suitable substrate is a flexible sheet-like material not made by textile technology, such as a belt or the like. This material can be of elastomeric material or can have elastomer on its surface that is to be treated in the ways described above. The optimum processing time (typically 15-120 min) depends on the process conditions (T,p) and the concentrations of the reactants (monomer, oxidant).

Example 1. General description of the process.

1. A roll of polyester fabric S is sealed in a pressure vessel 1 (polymerization reactor). The roll is held static during the processing.

Process step 1 :

2. Liquid carbon dioxide is pumped in from a C0₂ tank 2 into the pressure vessel through a heater 10 and a C0₂ pump 9 and stabilized in the temperature of +35°C and pressure of 9 Mpa (supercritical state) in the pressure vessel 1. Liquid pyrrole monomer (0.01 vol-%) is pumped from a monomer tank 3 to the vessel 1 through a monomer pump 11. A low-power mixer 8 in the pressure vessel 1 maintains circulation of the fluid phase.

3. Supercritical carbon dioxide diffuses into the polyester fabric. Monomer is able to enter the scC0₂-swollen fibers of the fabric. Conditions are maintained for approximately one hour. 4. The fluid contents of the pressure vessel 1 (carbon dioxide, excess monomer) are let to flow through a heater 12 into the recovery system (separator 4) in lower hydrostatic pressure. After half an hour the positive pressure in the vessel 1 has dropped to approximately 0,1 MPa. As the pressure of the supercritical phase is lowered and gaseous C0₂ -phase appears, monomer is separated as liquid in the separator 4. C0₂ is again liquefied and stored, preferably circulated back to form a new supercritical solvent phase. In fig. 1 , the C0₂ is led back to the C0₂ tank 2 through the return line 5 from the separator 4. The return line 5 comprises a condenser 13, from where the C0₂ is returned back to the tank 2 as a liquid. Monomer can be led to an intermediate tank 6 from the bottom of the separator 4 and used again in process step 1.

Process step 2:

5. The pressure vessel 1 is filled with a mixture of supercritical or liquid carbon dioxide and an oxidant, which is pumped from an oxidant tank 7, for example by the same pump 11 that was used in pumping the monomer. A proper co-solvent can be used to render the oxidant soluble in C0₂, in which case the co-solvent can be pumped from the oxidant tank 7 together with the dissolved oxidant. Organic acid can be added to adjust the pH- value of the mixture and to act as a dopant.

6. Chemical oxidative polymerization begins at the surface of the fibers. The reaction is allowed to continue for at least 15 minutes or until desired yield is attained without over-oxidation of the product. The optimum polymerization time depends on the polymerizable monomer and oxidant concentrations used. Quenching of the reaction takes places in the following stages.

7. Fluid contents of the vessel (carbon dioxide, excess oxidant, co-solvent, organic acid, residual substances from the reaction) are let flow into the recovery system (separator 4) in lower hydrostatic pressure. As the pressure of the supercritical phase is lowered, gaseous C0₂ -phase separates from the liquids and solids. All the components are recovered. Gaseous C0₂ can be circulated back in the same way as above. The residual oxidant remains dissolved in the co-solvent and can be used again in the solution for the new process step 2.

Extraction:

8. Supercritical or liquid carbon dioxide is pumped from the C0₂ tank 2 into the pressure vessel 1. Polymerization reaction is terminated and excess reactants are extracted from the fabric S.

The present invention can be utilized to many polymeric substrates. Supercritical carbon dioxide has been shown to swell most thermoplastics at temperatures close to their glass transition temperature. Monomer affinity of the substrates varies strongly with the chemical functionality available and can therefore be adjusted by slight chemical (or physico-chemical) modification. The substrates can be rigid or flexible, partially crosslinked or elastomeric.

Possible substrates polymers include but are not restricted to:

- thermoplastic polyesters such as polyethylene terephtalate (PET), polybutylene terephtalate (PBT) and polytrimethylene terephtalate (PTT).

- polyamides such as PA 6, PA 6.6

- polycarbonate (PC)

- polymethylmetacrylate (PMMA)

- polyolefins such as polymethylpentene (PMP), polyethylene (PE), polypropylene, ethylenepropylene-diene (EPDM) their copolymers, functionalized analogues and their blends

- polyvinylchloride (PVC)

- cyclo-olefin copolymer (COC) and -polymer (COP) - styrenic copolymers and their blends with other thermoplastics; styrene-ethylene-butadiene-styrene copolymer (SEBS), acrylonitrile-butadiene-styrene copolymer (ABS)

- polyurethanes

Suitable monomers are aniline, pyrrole, thiophene, and their substituted derivatives that can be polymerized using chemical oxidative polymerization. These substituted derivatives include but are not limited to methylpyrrole, methoxyaniline, 2,2'-bithiophene, 3- alkylthiophene and 3,4-ethylenedioxythiophene.

Other monomers than pyrrole can thus be used to make the corresponding conductive polymer by polymerization. It is commonly known that aniline, pyrrole and thiophene and their above-mentioned derivatives can be polymerized using oxidants well-known to scientific literature. The feasibility of the monomer is limited only by its solubility to carbon dioxide, which is sufficient for all above- mentioned monomers, because they are not needed in high concentrations. If needed, the solubility can be enhanced by a co- solvent.

Suitable oxidants for the above mentioned monomers are well known in the scientific literature and include but are not restricted to peroxides, peroxidesulfates and ferric salts. Specific Fe(lll) -organic salts, especially salts of sulfonic acids, are preferred as oxidants since their counter-ions act at the same time as efficient and stable dopants in intrinsically conductive polyanilines, polypyrroles and polythiophenes.

Particularly useful are Fe(lll)p-toluenesulfonate and Fe(lll)trifluoromethanesulfonate, which has increased solubility in supercritical fluid solvents with ethanol co-solvent, as described in the above-mentioned US patent 6214260.

The second supercritical or liquid carbon dioxide phase may contain both the oxidant and a separate dopant, if it is necessary to increase the conductivity of the polymerized polymer to a desired level. Various peroxides and peroxidesulfates can be used together with dopant acids in the process step 1 to produce the conductive form of the polymer.

Especially organic sulfonic acids such as dodecyl benzene sulfonic acid (DBSA), para-toluenesulfonic acid (PTSA) and camphor sulfonic acid (CSA) are used to protonate the intrinsically conductive polymer to its conductive form and to enhance the electrical stability and mechanical properties of the conductive polymer phase. These dopants can be used together with the oxidant in the second supercritical or liquid C0₂ phase.

If feasible, the polymer surface obtained in the process of the invention can be aftertreated with a dopant, such as an acid (for example DBSA, CPA, or PTSA) in a separate process step, to bring about the conductivity of the polymer obtained during the process steps 1 and 2, to increase its conductivity to a desired level, or to enhance its electrical or mechanical properties.

Examples 2-5 are given to illustrate the versatility of the process. A each example the weight gain of the sample is reported in grams and percentage. These values include the simultaneous weight loss of the matrix polymer, due to extraction of low molecular weight species. The yield of the process is decscribed by a calculated efficiency C.E. = [polymerized monomer/monomer feed before monomer recovery] in weight.

Example 2.

Polyester (PET) woven fabric (192 g) was sealed in a seven-liter reactor. 10 ml pyrrole was injected to the botton of the vessel and liquid carbon dioxide was pumped into the reactor until equilibrium conditions (T= +40°C, p=9,0 MPa) were established. Conditions were maintained for 45 minutes. Pressure of the vessel was relieved at rate of approximately 1 MPa/min. Excess pyrrole (8,1 m l) was collected from a separator system (FIG.1).

Twelve grams of Fe(lll)trifluoromethanesulfonate in 100 ml ethanol was injected into the vessel. Liquid carbon dioxide was pum ped in and the pressure was raised to 13,1 MPa (7= +40°C). After 2 hours the pressure was relieved and the excess ethanol/oxidant mixture was collected from the separator. The reactor was again pressurized by pure carbon dioxide to extract the monomer and oxidant residues from the fabric. A black, well adherent and uniform polypyrrole trifluoromethane sulfonate coating had deposited onto the fabric.

Surface resistivity of the coating was approximately 5-10² Ω/square measured by two-point probe setup. Weight of the fabric had increased about Am =1 ,15 g (≡0,6%). Neglecting the weight loss of PET, the calculated efficiency C.E. [polymerized monomer/m nomer feed] of the pyrrole polymerization was 3% before the recovery of excess monomer.

Example 3.

Polyamide 6 (Nylon) nonwoven (80 g) was sealed in a seven-liter reactor. 30 ml pyrrole was injected to the botton of the vessel and liquid carbon dioxide was pumped into the reactor until equilibrium conditions (7= +40°C, p=9,7 MPa) were established. Conditions were maintained for 50 minutes. Pressure of the vessel was relieved at rate of approximately 1 MPa/min. Excess pyrrole (22 m l) was collected from a separator system (FIG.1 ).

70 grams of Fe(lll)trifluoromethanesulfonate in 200 ml ethanol was injected into the vessel. Liquid carbon dioxide was pumped in and the pressure was raised to 10,0 MPa (T=+40°C). After 75 minutes the pressure was relieved and the excess ethanol/oxidant mixture was collected from the separator. The reactor was again pressurized by pure carbon dioxide to extract the monomer and oxidant residues from the fabric. A black, well adherent and uniform polypyrrole trifluoromethanesulfonate coating had deposited onto the fabric (p_s = 10² Ω/square, Am =1 ,72 g ≤ 2%, C.E. = 1 ,5%).

Example 4.

The seven-liter reactor was loaded with 178 g PET fabric.

The scheme of the example 1 was repeated by using ethylenedioxy- thiophene (EDOT) as monomer (7 ml EDOT in 20 ml ethanol). Monomer impregnation time and conditions were: t= 45 min, 7= +40°C, p=9,5 MPa. (process step 1)

20 grams of Fe(lll)p-toluenesulfonate in 80 ml ethanol was used as oxidant (process step 2). Polymerization conditions were: t= 105 min, T= +40°C, p=9,7 MPa. A dark blue, well adherent and uniform PEDOT p-toluenesulfonate coating had deposited onto the fabric (p_s = 10⁵ Ω/square, Am =0,23 g = 0,13%, C.E. = 1 ,0%).

Example 5.

The seven-liter reactor was loaded with 50 g clear polycarbonate (Lexan 9034) strips (0,9 mm x 10 mm x 60 mm). Strips were placed in a rack in such a way that all the surfaces where equally and directly exposed to fluid phase.

The scheme of the example 1 was repeated by using pyrrole as monomer with acetone co-solvent (7 ml pyrrole in 20 ml acetone). Monomer impregnation time and conditions were: f= 80 min, 7= +80°C, p = 14,5 MPa. (process step 1 )

55 grams of Fe(lll)trifluoromethanesulfonate in 200 ml ethanol was used as oxidant (process step 2). Polymerization conditions were: f= 130 min, 7= +80°C, p=12,0 MPa. A black, well adherent and uniform polypyrrole trifluoromethanesulfonate coating had deposited onto the fabric ( _s = 10⁷ Ω/square, Am =0,15 g s 0,3%, C.E. = 0,6%). The invention is suitable for increasing the conductivity of the surface of practically any polymeric material that can be treated under supercritical or liquid conditions of carbon dioxide, by in-situ polymerization on the surface and near the surface of an intrinsically conductive polymer that at least after a possible aftertreatment possesses higher electrical conductivity than the matrix polymer or polymer blend of the substrate. The invention is especially suitable for antistatic treatment of textiles, sheets and belts, but it should be understood that the invention is not limited solely to the above substrates and the above applications.

Claims

1. A method for forming an electrically conductive polymeric surface on a solid polymeric substrate (S), said method comprising the following successive steps:

1) treatment of the solid polymeric substrate (S) in a pressure reactor (1) with a first supercritical or liquid carbon dioxide phase containing a monomer to cause the monomer to enter the structure of the polymeric substrate,

2) removal of the first supecritical or liquid carbon dioxide phase from the reactor (1), together with possible residues of the monomer

3) feeding of a second supercritical or liquid carbon dioxide phase containing an oxidative agent into the reactor (1) into contact with the substrate (S) that has remained in the reactor, and

4) performing in-situ oxidative polymerization of the monomer in the polymeric substrate with the help of the oxidative agent to form an electrically conductive polymer surface on the polymeric substrate (S),

5) removal of the second supercritical or liquid carbon dioxide phase from the reactor.

2. The method according to claim 1 , wherein the method comprises the following step after the removal of the second carbon dioxide phase:

6) extraction of residual reactants from the substrate (S) with a third supercritical or liquid carbon dioxide phase.

3. The method according to claim 2, wherein the extraction is performed by feeding the third carbon dioxide phase into the reactor (1) into contact with the substrate (S) in the reactor.

4. The method according to claim 1 , 2 or 3, wherein the monomer is selected from the group consisting of pyrrole, substituted pyrrole derivative such as methylpyrrole, aniline, substituted aniline derivative such as methoxyaniline, thiophene, and substituted thiophene derivative such as 2,2'-bithiophene, 3-alkylthiophene or 3,4-ethylenedioxythiophene.

5. The method according to any of claims 1 to 4, wherein the oxidative agent is selected from the group consisting of ferric salts, peroxides and peroxidesulfates

6. The method according to claim 5, wherein the ferric salt is an organic salt of sulfonic acid, such as Fe(lll)p-toluenesulfonate or Fe(lll)trifluoromethanesulfonate.

7. The method according to any of the preceding claims, wherein the second carbon dioxide phase contains a co-solvent increasing the solubility of the oxidative agent in said phase, the co-solvent being present preferably in an amount of less than 10 vol-%, more preferably less than 1 vol-%.

8. The method according to any of the claims 2 to 7, wherein the third carbon dioxide phase contains a co-solvent increasing the solubility of the oxidative agent in said phase.

9. The method according to claim 7 or 8, wherein the co-solvent is selected from the group consisting of lower alcohols, acetone and water.

10. The method acording to any of the preceding claims, wherein the polymeric substrate (S) treated comprises polymer fibres, especially synthetic fibres, such as polyamide or polyester fibres.

11. The method according to claim 10, wherein the polymeric substrate (S) treated is in the form of textile having polymer fibres in its structure, such as a woven fabric or a knitted fabric or a non- woven fabric.

12. The method according to any of the preceding claims, wherein the polymeric substrate (S) is a flexible sheet-like material, such as a belt, for example a material having elastomer on its surface.

13. The method according to any of the preceding claims, wherein the first carbon dioxide phase and/or the second carbon dioxide phase is taken out from the pressure reactor (1 ) to a separation stage where the pressure of the carbon dioxide is reduced in order to separate residual reactants from carbon dioxide.

14. The method according to claim 13, wherein the carbon dioxide taken out of the pressure reactor (1) is circulated back to the pressure reactor where it forms a phase in a new cycle of successive steps.