implementation thereof

The present invention relates to a process for preparing halogenated polymers and to a device for the implementation thereof.

It is known to prepare halogenated polymers in a medium comprising supercritical carbon dioxide (see for example : "Polymerization of vinyl chloride in supercritical carbon dioxide", Ge Li, et al, Polymer Preprints, 2001, 42(2), page 817).

Document US 6258910 Bl describes, in particular, the polymerization of vinyl chloride in supercritical carbon dioxide.

In the known processes, high working pressures are generally used, in order to simultaneously ensure the temperature and the density that are necessary for polymerization reactions, and a moderate solids content is used that guarantees free flow of the polymer suspension towards the outlet of the reactor.

Liquid or supercritical carbon dioxide (also referred to more simply as "L/SC-C0 ₂ " in the present description) constitutes an advantageous medium for the polymerization of halogenated monomers, in particular vinyl and vinylidene halides. Besides the fact that L/SC-CCb is readily accessible, non-flammable, non-toxic and inexpensive, it is chemically inert during the synthesis. SC-C0 ₂ also constitutes a good solvent for these monomers and a poor solvent for the synthesized polymers. The synthesized polymer is therefore recovered by precipitation in the form of particles, often extremely fine particles, from which the residual monomer is easy to remove. Furthermore, considering the absence of compounds which are generally present in the conventional aqueous polymerization medium of most halogenated monomers (dispersants, surfactants, emulsifiers, etc.), the polymer obtained in L/SC-C0 ₂ is not contaminated by residues of these compounds and has a very high purity.

Finally, the use of L/SC-C0 ₂ as main constituent of the polymerization medium makes it possible to avoid the conventional use of large amounts of demineralized water and, as a consequence, the consumption of large amounts of steam (or other forms of energy) for drying the polymer.

The use of L/SC-C0 ₂ as main constituent of the polymerization medium of halogenated monomers has however one drawback linked to the need to remove the heat released by the polymerization, which involves the use of a heat-transfer fluid (water) for cooling the polymerization reactor. Since the heat capacity of the halogenated polymers is low, the amount of sensible heat absorbed is also low and the amount of heat-transfer fluid needed to control the temperature of the reactor is very high, which is detrimental to the economics of the process.

The objective of the present invention is to overcome this drawback by proposing, mainly, a novel continuous process for preparing halogenated polymers in a medium comprising L/SC-C0 ₂ .

Consequently, the invention relates to a continuous process for preparing a halogenated polymer comprising a step of polymerization of at least one halogenated monomer in a medium comprising carbon dioxide in the liquid or supercritical state (L/SC-C0 ₂ ), in at least one pressurized reactor, said process being characterized in that it comprises at least :

- (1) an expansion of the suspension resulting from the reactor ;

- (2) a separation of the expanded suspension resulting from the reactor into a first stream, containing the solid phase comprising the halogenated polymer and into a liquid, gaseous or supercritical second stream, which is a mixture of CO? and of monomer not converted to polymer ; and the recovery of this first stream and of this second stream ;

- (3) an optional reheating, applied to the expanded suspension, before or after its separation into first and second streams, with optional recovery of the cold energy generated by the expansion ;

- (4) a recompression of the second stream and the optional cooling thereof in order to recycle it to the reactor feed, with a fresh monomer make-up and, possibly, a C0 ₂ make-up ;

- (5) a final step of degassing the halogenated polymer, optionally in

combination with a continuous technique of forming the polymer into granules ;

said process being characterized also by the fact that the recompressed second stream is recycled to the reactor feed at a plurality of points.

The expression "continuous process" is understood to mean, in the present description, a process in which the feeding with carbon dioxide, with monomers, with initiators and optional additives and the drawing-off of the contents of the reactor are carried out continuously. Preferably, the continuous process according to the invention is such that the control of the feeding operations, of the drawing-off and of the other polymerization conditions ensures stable operating conditions in the reactor.

In the present description, the terms "monomer", "polymer", "initiator", "additive", "reactor", "expansion", "recompression", "reheating", "first stream" and "second stream" are used without distinction in the singular and in the plural.

The polymerization step of the process according to the invention is carried out in a medium that is advantageously inert with respect to the halogenated monomers, comprising carbon dioxide in the liquid state (L-CO?) or supercritical state (SC-CO? ). In the present description, the term "medium" is understood to mean the contents of the reactor with the exception of the halogenated monomer introduced and the halogenated polymer formed. The medium in which the polymerization step of the process according to the invention is carried out may comprise other solvents of the halogenated monomers. These other solvents are generally chosen from those which do not generate an excessive amount of chain transfer reactions. Among these solvents, mention may be made of C ₂ - C ₈ hydrocarbons, Q - C ₈ alcohols, methylene chloride, toluene, cyclohexane, etc. The polymerization step of the process according to the invention is

advantageously carried out in a medium comprising at least 50 % by volume, preferably at least 80 % by volume and, more particularly, at least 95 % by volume of carbon dioxide (CO?) in the supercritical state. It is very particularly preferred to carry out the polymerization step of the process according to the invention in a medium constituted of C0 ₂ in the supercritical state.

In the present description, the term "supercritical" is understood in its conventional thermodynamic sense. For pure C0 ₂ , it denotes conditions above its critical temperature and critical pressure (31.1 °C ; 7.38 MPa)

("critical point"). If the C0 ₂ is in the supercritical state, its compression will lead only to an increase in its density without the appearance of a liquid phase.

Advantageously, the temperature in the reactor is at least -60°C, preferably at least 0°C. Advantageously, the temperature is at most 200°C, preferably at most 150°C. In the preferred case of the use of SC-C0 ₂ , the temperature in the reactor is advantageously at least 32°C, preferably at least 40°C. Still in this case, the temperature is advantageously at most 100°C, preferably at most 85°C.

Advantageously, the pressure in the reactor is at least 0.5 MPa, preferably at least 4 MPa. Advantageously, the pressure is at most 100 MPa, preferably at most 50 MPa. In the preferred case of the use of SC-C0 ₂ , the pressure in the reactor is advantageously at least 4.5 MPa, preferably at least 8 MPa. Still in this case, the pressure is advantageously at most 40 MPa, preferably at most 25 MPa.

Advantageously, the density of the medium in the reactor is at least 500 kg/m ³ , preferably at least 600 kg/m ³ . Advantageously, the density of the

3 3 medium in the reactor is at most 1200 kg/m , preferably at most 1000 kg/m .

The expression "polymerization of at least one halogenated monomer" is understood to mean both the homopolymerization of a halogenated monomer and its copolymerization with one or more other ethylenically unsaturated monomers, for the purpose of obtaining halogenated polymers.

The expression "halogenated polymer" is understood to mean both homopolymers and copolymers of halogenated monomers. Among these, mention in particular may be made of the following : homopolymers of halogenated monomers, such as vinyl chloride and vinylidene chloride ; vinyl fluoride and vinylidene fluoride ; trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene and hexafluoropropylene ; fluoroacrylates ; fluorinated vinyl ethers, for example perfluorinated vinyl ethers bearing perfluoroalkyl groups containing 1 to 6 carbon atoms. Mention may also be made of the copolymers that these halogenated monomers form together and the copolymers of one or more of these halogenated monomers with another ethylenically unsaturated monomer such as olefins, for example ethylene, propylene ; styrene derivatives and styrene ; halogenated olefins ; vinyl ethers ; vinyl esters such as for example vinyl acetate ; acrylic acids, esters, nitriles and amides and methacrylic acids, esters, nitriles and amides.

Preferably, the process according to the invention is characterized in that the halogenated polymers obtained are chlorine-containing polymers.

The expression "chlorine-containing polymer" is understood to mean both homopolymers and copolymers of chlorine-containing monomers. Among these, mention in particular may be made of the following : homopolymers of chlorinated monomers, such as chloroolefins, for example vinyl chloride and vinylidene chloride ; chloroacrylates and chlorinated vinyl ethers, for example perchlorinated vinyl ethers bearing perchloroalkyl groups containing 1 to 6 carbon atoms. Mention may also be made of the copolymers that these chlorine-containing monomers form together, such as for example copolymers of vinylidene chloride with another chlorinated monomer as defined above, and copolymers of one or more of the abovementioned chlorine-containing monomers with another ethylenically unsaturated monomer such as olefins, for example ethylene, propylene ; styrene derivatives and styrene ; halogenated olefins ; vinyl ethers ; vinyl esters such as for example vinyl acetate ; acrylic acids, esters, nitriles and amides and methacrylic acids, esters, nitriles and amides.

Particularly preferably, the process according to the invention is characterized in that the halogenated polymer obtained is a vinyl chloride polymer.

The expression "vinyl chloride polymer" is understood to mean both homopolymers of vinyl chloride and its copolymers with other ethylenically unsaturated monomers, whether these are halogenated (chloroolefins, for example vinylidene chloride ; chloroacrylates ; chlorinated vinyl ethers such as for example perchlorinated vinyl ethers bearing perchloroalkyl groups containing 1 to 6 carbon atoms) or not halogenated (olefins such as for example ethylene, propylene ; styrene derivatives and styrene ; vinyl ethers ; vinyl esters such as for example vinyl acetate ; acrylic acids, esters, nitriles and amides ; methacrylic acids, esters, nitriles and amides). Vinyl chloride homopolymers and

copolymers of vinyl chloride with a halogenated or non-halogenated comonomer advantageously containing at least 50 %, preferably at least 60 %, particularly preferably at least 70 % and very particularly preferably at least 85 % by weight of monomeric units derived from vinyl chloride are particularly preferred. Vinyl chloride homopolymers and copolymers of vinyl chloride with vinyl acetate are very particularly preferred. Vinyl chloride homopolymers are truly very particularly preferred.

The polymerization step of the process according to the invention may be carried out according to any technique compatible with the medium comprising L/SC-C0 ₂ in which this step is conducted. Thus, the polymerization of the halogenated monomer may be carried out by virtue of the action of radical initiators, of ionizing radiation, of Friedel-Crafts type catalysts, of Ziegler type catalysts, etc. Preferably, the polymerization step is carried out by radical means and customarily involves the use of one or more initiators, the nature, number and concentration of which may also be chosen according to the requirements. As initiators useful within the context of the present invention, it is possible to use any appropriate radical initiator, in particular an organic radical initiator. Therefore, the polymerization step is preferably carried out in the presence of an organic radical initiator. The use of a support or of a solvent can also be envisaged for promoting the dispersion of the initiator in the polymerization medium. As supports that can be envisaged for this purpose, mention may be made of silicas, silica gels, alumina and other metal oxides, such as titanium oxide. As solvent, mention may for example be made of toluene, alcohols such as methanol, ethanol and isopropanol and hydrocarbons such as pentane, hexane and cyclohexane.

The initiator that can be used according to the invention, may be chosen for example from the following : peroxides, such as dimethyl peroxydicarbonate, diethyl peroxydicarbonate, di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, diacetyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, di(2-ethylhexyl) peroxydicarbonate, /-butyl peroxyisopropylcarbonate, /-butyl peroxy-n-decanoate, /-butyl peroxyacetate, 1, 1 ,3,3-tetramethylbutyl

peroxymethoxyacetate, /-butyl peroxypivalate, t-amyl peroxypivalate, di-/-butyl peroxide, 2,2-bis(2,2-dimethylpropanolperoxy)-4-methylpentane, diacyl peroxides, such as diacetyl peroxide, di-n-propionyl peroxide, diisobutyryl peroxide, dibenzoyl peroxide, diisobutanoyl peroxide, dioctanoyl peroxide, dilauroyl peroxide, dicumyl peroxide, di-t-amyl peroxide, /-butyl per-2- ethylhexanoate, f-butyl peroxymaleate, cumyl peroxyneodecanoate and t-amyl peroxyneodecanoate, cumene hydroperoxide, pinane hydroperoxide and p- menthane hydroperoxide ; nitriles, such as 2,2'-azobis(methoxy-2,4- dimethylvaleronitrile) and 2,2'-azobis(2,4-dimethylvaleronitrile) ; and similar compounds.

Preferred initiators are peroxides with a short half-life, for example diacyl peroxides, such as diacetyl peroxide, di-n-propionyl peroxide and diisobutyryl peroxide ; and also 1,1,3,3-tetramethylbutyl peroxymethoxyacetate, 2,2-bis(2,2- -dimethylpropanolperoxy)-4-methylpentane, diisobutanoyl peroxide, cumyl peroxyneodecanoate and t-amyl peroxyneodecanoate.

The concentration of initiators for the polymerization step is

advantageously between 0.001 % and 2 % by weight relative to the total weight of the polymerization medium.

The polymerization step of the process according to the present invention may be optionally carried out in the presence of one or more surfactants or one or more dispersants. Any suitable surfactant or dispersant known to those skilled in the art may be employed.

The polymerization step of the process according to the invention may optionally be carried out in the presence of additives other than the

abovementioned additives (surfactants, dispersants) for improving the implementation of the process and/or the characteristics of the resulting polymer. Examples of other additives are chain transfer agents, anti-crust agents, antistatic agents and cosolvents.

The polymerization step of the process according to the invention has an exothermic character most of the time and may be carried out in any type of suitable continuous reactor, as long as it withstands the high temperatures and pressures needed for carrying out the process. Use may be made of a single reactor or of several reactors connected in series. The reactors that can be used may be chosen for example from continuous mechanically stirred tank reactors and tubular reactors. In the case of continuous mechanically stirred tank reactors, advantageously several thereof are used, connected in series. In the case of tubular reactors, advantageously a single one thereof is used. These reactors may advantageously be equipped with a heating system and/or with a cooling system for controlling the temperature.

Tubular reactors are preferred for carrying out the process according to the invention and, among the latter, tubular reactors with piston flow ("plug-flow" tubular reactors).

The expression "tubular reactor" is understood to mean a reactor comprising an elongated tube, inside which the reaction medium circulates, the heat exchange necessary for thermal control taking place across the wall of the tube, for example by means of a jacket inside which a heat-transfer fluid (water) circulates, generally counter-current. The elongated tube may be constituted of a plurality of straight segments joined together by 180° bends, together forming a compact assembly.

The polymerization step of the process according to the invention is advantageously carried out so that the average residence time of the reaction medium in the reactor is between 5 and 60 minutes, preferably between 10 and 35 minutes. Frequently, from 20 to 90 % and, more often, from 40 to 80 % by weight of the monomer are polymerized in the reactor. In other words, frequently, from 10 to 80 %, more often from 20 to 60 % by weight of monomer are not converted to polymer. This unconverted monomer should, preferably, be recycled for the economics of the process. The degree of conversion of the monomer to polymer is in particular governed by the choice of the temperature and pressure and by the choice of the nature and concentration of the initiator.

The reactor is advantageously fed at least with L/SC-C0 ₂ , with monomer and with initiator, to which one or more of the abovementioned additives are optionally added. The respective amounts of C0 ₂ and monomer introduced into the reactor are such that the weight ratio of the L/SC-C0 ₂ to the liquid monomer(s) is generally between 0.5/1 and 100/1, preferably between 0.6/1 and 30/1. The best results are obtained when the [C0 ₂ /monomer] weight ratio is between 0.6/1 and 5/1, very particularly between 0.8/1 and 3/1.

The order in which the L/SC-C0 ₂ , the monomer and the initiator are introduced into the reactor is not particularly critical. Advantageously, a portion of the monomer, drawn off from a storage container then compressed to the pressure at which the polymerization step is carried out, is combined with the mixture of C0 ₂ and of recycled monomer (monomer not converted to polymer), before addition of the initiator to the mixture thus formed. Usually, a

predominant proportion, or even almost all, of the CO? circulates in a closed loop in the device in which the process according to the invention is carried out and is therefore recycled to the reactor as a mixture with the monomer not converted to polymer (that is to say not polymerized in the reactor).

In the process according to the invention, there are therefore

advantageously two sources for feeding the reactor with monomer :

- the first source comprises the fresh monomer, drawn off from the storage container then compressed, as mentioned above ;

- the second source comprises the monomer, not converted to polymer, which is recycled.

This second source for feeding the reactor with monomer may itself comprise several streams, preferably two streams. Indeed, the process according to the invention comprises the recompression (4) of the mixture of monomer not converted to polymer and of CO? (second stream) advantageously carried out in several steps, preferably in two steps such as (see below) the expansion (1) of the suspension resulting from the reactor and its separation (2). Hence, it is possible for a monomer-rich liquid to be condensed to an intermediate pressure and separated from a C0 ₂ -rich vapour phase, and for the resulting streams to then be recycled separately to the reactor feed.

For the implementation of the process according to the invention, the reactor is fed with a recompressed and recycled mixture, in the liquid, gaseous or supercritical state, of monomer not converted to polymer and of C0 ₂ (second stream) at a plurality of points. Indeed, it has been observed that if the reactor is fed with recycled mixture in a delayed manner, by injecting said mixture at several different points, following one another along the entire length of the reactor, downstream of the fresh monomer feed point, this makes it possible to contribute to the control of the temperature of the polymerization and thus to reduce the amount of heat-transfer fluid necessary for this control. As a corollary and for the same purpose, it is possible to inject into the reactor, in the same staggered manner, the portions of initiator necessary for the

polymerization. In this case, initiators will preferably be chosen that have a relatively short half-life so that no residues of active initiator remain on leaving the reactor. The organic radical initiator is thus preferably also introduced at a plurality of points of the reactor.

The number of points through which the reactor is fed with the mixture of monomer not converted to polymer and of C(¾ and, possibly, of the initiator, is not critical and depends on the geometry of the reactor. The reactor is advantageously fed with the second stream, and preferably also with initiator, at a number of points between 2 and 20, preferably between 3 and 10 and particularly preferably between 4 and 8.

According to the invention, the suspension resulting from the reactor, which is generally in the form of polymer particles dispersed in the liquid, gaseous or supercritical mixture of C0 ₂ and of monomer not converted to polymer (dispersing phase), is subjected to an expansion (1) and to a

separation (2).

As an optional variant, the expansion (1) to which the suspension resulting from the reactor is subjected and the separation (2) of the expanded suspension resulting from the reactor into first and second streams may be advantageously carried out in several steps at successively decreasing pressures. Preferably, the expansion (1) and the separation (2) may be carried out in two steps ; (as regards the expansion (1), these two steps are also referred to as "first expansion" and "second expansion" in the present description).

This approach makes it possible to advantageously reduce the amount of energy necessary for the recompression of the mixture of CO? and of monomer not converted to polymer to be recycled. The suspension resulting from the reactor may then firstly be advantageously brought, via a first expansion, back to a pressure intermediate between that existing in the reactor and atmospheric pressure. This intermediate pressure must be low enough so that the density of the liquid, gaseous or supercritical mixture of CO? and of monomer not converted to polymer decreases substantially, by simple decompression in the supercritical state, or by vaporization. The expression "substantial reduction in the density of the liquid, gaseous or supercritical mixture of C0 ₂ and of monomer not converted to polymer" is understood, in the present description, to mean a reduction in this density advantageously by at least a factor of 3, preferably by at least a factor of 5. This intermediate pressure is advantageously below the critical pressure, preferably between 2 and 20 times lower, more particularly between 5 and 16 times lower and, very particularly preferably, between 7 and 13 times lower than that existing in the reactor. This first expansion of the suspension resulting from the reactor may be carried out in any known device, such as for example, an automatic valve.

In the case of this optional variant, the second expansion of the suspension resulting from the reactor may preferably be carried out on the, optionally reheated, solid phase of this suspension, in order to bring it from the intermediate pressure, achieved after the first expansion, to atmospheric pressure. The separation (2) of the halogenated polymer contained in the solid phase of the expanded suspension resulting from the reactor (first stream) may be carried out by any known means for separating a solid and a gas. Preferably, considering the generally very fine particle size of the polymer obtained according to the process of the invention, filtration means, preferably bag filters, are used to carry out this separation. It is preferred to carry out an at least partial separation of the solid phase of the suspension after each expansion, with concomitant discharge, for recycling to the reactor, of the dispersing phase comprising the C0 ₂ and the monomer not converted to polymer.

It may prove advantageous and even necessary to recycle the monomer, not converted to polymer, which would possibly be condensed on the solid portion of the suspension (the synthesized polymer) during the expansion (1).

Said suspension may be, for this purpose, reheated, with optional recovery of the cold energy generated by the expansion, which prevents the partial condensation of the mixture of C0 ₂ with the unconverted monomer, which constitutes the dispersing phase of the suspension. This reheating advantageously simplifies the subsequent separation of the dispersed solid phase of the suspension, essentially constituted of the synthesized polymer.

The optional reheating (3) (and the optional recovery of the cold energy generated by the expansion) is applied to the expanded suspension before or after its separation into first and second streams. It may be applied to the first stream resulting from the separation of the expanded suspension, containing the solid phase that comprises the halogenated polymer. It may also be applied after each constituent step of the expansion (1). Preferably, however, the optional reheating is carried out after the first expansion of the suspension resulting from the reactor.

This reheating may take place in any suitable device but it is preferred to carry it out during an operation that simultaneously involves the optional cooling of the mixture, recycled to the reactor, of C0 ₂ and of monomer not converted to polymer. This cooling may advantageously be carried out by virtue of the recovery, preferably simultaneous recovery, of the cold energy generated during the expansion of the suspension resulting from the reactor. In this case, the device suitable for the reheating is preferably a cross-flow heat exchanger providing heat exchange between, on the one hand, the expanded suspension resulting from the reactor which travels through it in one direction and, on the other hand, the mixture of C0 ₂ and of monomer not converted to polymer which travels through it in the other direction.

The solid halogenated polymer obtained according to the process of the invention is generally collected in the form of submicron-sized particles.

Conventional additives may optionally be added thereto. Among these additives, mention may be made of :

stabilisateurs#stabilisateursconventional stabilizers derived from calcium, tin or zinc for example ;

plastifiantsplasticizers, such as adipates and organophosphates for example ; pigmentsinorganic and organic pigments ;

chargesfillers, such as calcium carbonate, talc and dolomite for example ;

lubrifiantslubricants, such as organic waxes, fatty acids and alcohols, esters or metal salts for example ;

igmfugeantsflame retardants, such as metal oxides for example.

The solid halogenated polymer obtained is advantageously degassed by conventional means in order to remove the residual traces of unconverted monomer. In general, the solid halogenated polymer obtained contains, depending on the effectiveness and number of expansions of the suspension resulting from the reactor, between 0.2 and 5 % by weight, preferably between 0.5 and 3 % by weight of unconverted monomer. Among the conventional means for removing residual traces of unconverted monomer (also referred to as "degassing" in the present description), mention may be made of placing it under reduced pressure, preferably placing it under vacuum and purging by means of a carrier gas or of an inert vapour. In the present description, the terms "to degas" and "degassing" are used to define the passage of the unconverted monomer, present in dissolved, absorbed, adsorbed or occluded form in the solid halogenated polymer, to the gaseous state.

According to one variant of the process of the invention, it may be advantageous to combine, continuously, the preparation of the halogenated polymer and its degassing and its forming into granules, in order to obtain a ready-to-use polymer. The forming of the halogenated polymer into granules is generally carried out in an extruder. The term "extruder" is understood to mean any continuous device comprising at least one feed zone and, at its outlet, a discharge zone preceded by a compression zone, the latter forcing the molten mass to pass through the discharge zone. The discharge zone is advantageously followed by a granulating device that gives the extruded polymer its final form. Advantageously, use is made of known extruders based on the work of a single screw or of several screws, preferably twin screws which, in the latter case, may operate in a co-rotating or counter-rotating manner (same direction of rotation or opposite directions of rotation). Counter-rotating twin-screw extruders are preferred because they prevent the excessive shear forces which could lead to degradation of the polymer.

The extruder that can be used for carrying out the process of the invention according to this variant is arranged so that it advantageously comprises at least one feed zone, one melting or gelling zone, at least one degassing zone, optionally one zone for introducing additives, and one discharge zone. The order in which the zones between the feed and the discharge are sequenced may be different from the order in which they are mentioned above. Each of these zones has a very specific function and is at a very specific temperature. The extruder preferably comprises at least one degassing zone that itself advantageously comprises at least one degassing vent. At least one degassing vent

advantageously communicates with a means for creating the vacuum, such as for example a vacuum pump, in order to remove the residual unconverted monomer contained in the halogenated polymer. Preferably, the extruder comprises at least two degassing zones, the first of which is maintained at a pressure close to atmospheric pressure, and the second operates at reduced pressure.

The second liquid, gaseous or supercritical stream, which is a mixture of C0 ₂ and of monomer not converted to polymer, resulting from the

separation (2) of the expanded suspension resulting from the reactor, is (step (4)) recompressed, optionally cooled, then recycled to the feed of the reactor where the polymerization step of the process according to the invention takes place.

Optionally, the monomer not converted to polymer, resulting from the degassing of the halogenated polymer, may also be recompressed, cooled, then recycled to the reactor. For this purpose, it may advantageously be recycled separately to the reactor or, preferably, be combined with the second stream in order to produce a supplementary mixture of monomer not converted to polymer and of CO?. In the case where the degassing technique releases the monomer at several different sources (such as for example in an extruder with several degassing zones), it is up to those skilled in the art to determine the practical advantage of recycling each source of monomer.

The recompression (4) of the mixture of C0 ₂ and of monomer not converted to polymer (second stream) may be carried out in several steps.

Preferably, in at least one of these steps, the recompression (4) is carried out at a pressure and at a temperature which are suitable for ensuring the condensation thereof into the liquid phase. The pressure applied to the mixture during its recompression during this step is, for example, between atmospheric pressure and 10 MPa, preferably between atmospheric pressure and 5 MPa.

The cooling of the mixture of C0 ₂ and of monomer not converted to polymer (second stream) is optional insofar as the recompression (4) may be carried out under isothermal conditions (for example in a liquid ring pump) or under conditions that involve a release of heat energy (resulting from the conversion of the work carried out for example by a conventional compressor). For practical reasons, in particular when the recompression (4) is not carried out under isothermal conditions, it is however preferred to carry out the cooling of the second stream. This cooling may be carried out in any suitable device, such as one or more condensers and/or one or more heat exchangers, but it is preferred to carry it out, at least in part, simultaneously with the reheating of the expanded suspension resulting from the reactor, by virtue of the recovery of the cold energy generated during the expansion of said suspension. In this case, the device suitable for the cooling of the second stream is the cross-flow heat exchanger, the function of which was explained above.

The mixture of C0 ₂ and of monomer not converted to polymer, and also, optionally, the monomer not converted to polymer resulting from the degassing operation, which are advantageously compressed in the supercritical state, or compressed and recondensed in liquid phase, are then recycled to the reactor feed as described above.

This recycling may advantageously comprise one or more purification steps. These may be based on any technique known to those skilled in the art, including partial condensation, settling, adsorption and the combined use thereof.

According to another aspect, the invention also relates to a device, comprising at least one pressurized reactor for the polymerization of at least one halogenated monomer in a medium comprising carbon dioxide in the liquid (L-C0 ₂ ) or supercritical (SC-C0 ₂ ) state. This device can be especially used for the implementation of the process for the continuous preparation of a

halogenated polymer according to the invention. The features and preferences defined above for the process according to the invention advantageously also apply to the device according to the invention.

This device comprises at least, besides the pressurized reactor :

- one storage container for the fresh halogenated monomer ;

- means for compressing the halogenated monomer to the pressure at which the polymerization step is carried out ;

- means for feeding the reactor with monomer not converted to polymer, and with a mixture of this unconverted monomer and C0 ₂ , which are compressed in the liquid, gaseous or supercritical state and recycled ; these means are arranged so as to inject these compressed and recycled fluids at the fresh monomer feed point but also at several different points, downstream of the latter, which follow one another along the entire length of the reactor ;

- means for expanding the suspension resulting from the reactor comprising the synthesized polymer in the liquid, gaseous or supercritical mixture of C0 ₂ and of monomer not converted to polymer ; advantageously, these means may be arranged so that the expansion undergone by the suspension resulting from the reactor is carried out in several steps at successively decreasing pressures ; preferably, this expansion is carried out in two steps : the first step may advantageously consist of an expansion to a pressure intermediate between that existing in the reactor and atmospheric pressure, which is low enough so that the density of the mixture of C0 ₂ and of monomer not converted to polymer decreases substantially, by simple decompression in the supercritical state, or by vaporization ; the second step may itself be carried out in two stages, the first stage resulting in a pressure intermediate between that obtained after the first step and atmospheric pressure, and which is low enough so that the density of the dispersing phase decreases substantially. Regardless of the number of intermediate expansions, the pressure downstream of the last expansion is advantageously between 0.09 MPa and 0.15 MPa.

- optional means for reheating the suspension resulting from the reactor after its expansion, advantageously after the first expansion ;

- optional means for recovering the cold energy generated by the expansion of the suspension resulting from the reactor ; in this optional variant, this reheating and this recovery of cold energy are usually carried out

simultaneously in a cross-flow heat exchanger providing heat exchange between, on the one hand, the expanded suspension, resulting from the reactor, which travels in one direction and, on the other hand, the mixture of C0 ₂ and of monomer not converted to polymer which travels in the other direction ;

- means for separating the expanded suspension resulting from the reactor into a first stream, containing the solid phase comprising the halogenated polymer and into a liquid, gaseous or supercritical second stream ;

- means for recovering this first stream and this second stream ;

- means for recompressing the second stream, which is a mixture of C0 ₂ and of monomer not converted to polymer, and, optionally, the monomer resulting from the degassing of the polymer, completely or partially ;

- optional means for cooling the second stream ;

- means for recycling the second stream to the reactor feed, with a fresh monomer make-up and, possibly, a C0 ₂ make-up ;

- means for degassing the halogenated polymer, optionally in combination with continuous means of forming the polymer into granules ; preferably these means take the form of a counter-rotating twin-screw extruder ; particularly preferably, the extruder comprises at least one degassing zone that itself comprises at least one degassing vent that communicates with a means for creating the vacuum, such as for example a vacuum pump, in order to remove the residual unconverted monomer contained in the halogenated polymer. The device according to the invention is preferably characterized in that the means for feeding the reactor with monomer not converted to polymer, and with a mixture of this unconverted monomer and C0 ₂ , are arranged so that said mixture is injected at 3 to 10 different points, following one another along the entire length of the reactor, downstream of the fresh monomer feed point. The continuous preparation process and the device according to the invention advantageously make it possible to obtain, more economically than via conventional polymerization processes in an aqueous medium, halogenated polymers that are dry, ready-to-use and of high purity, considering the absence of compounds which are generally present in the conventional aqueous

polymerization medium of most halogenated monomers.

One particular embodiment of the process of the invention and of a device for the implementation thereof will now be illustrated with reference to the drawing that accompanies the present description. This drawing consists of the appended Figure 1, schematically representing one practical embodiment of the aspects of the invention, without limiting the scope thereof.

According to this embodiment, a tubular reactor C made of carbon steel comprises (not shown in detail or to scale) 6 sections with a length of 300 m constituted of straight segments joined together by 180° bends and jacketed by a jacket, itself also made of several sections, in which a coolant circulates. This reactor is fed, via the monomer pump B which brings it to a pressure of 15 MPa and to a temperature of 50°C, with fresh vinyl chloride (monomer) from the fresh (liquid) monomer storage tank A and with liquid vinyl chloride recycled via the line R ₃ . This reactor is also fed with a recycled mixture of vinyl chloride in SC-C0 ₂ , for 20 % of the volume of this mixture via the line R ₂ and, for the remaining 80 % of the volume, via the 5 injectors shown schematically as Ri, respectively. In order to transport, at relatively constant temperature and at an appropriate velocity, the increasing stream of SC-C0 ₂ and of monomer to be polymerized circulating in the reactor, the segments of the reactor and of their jacketing have a diameter that increases between the inlet and outlet of the reactor.

A solution of a heavy organic peroxide in an aliphatic monoalcohol is also introduced into the reactor (supply not shown) as an initiator, by injecting into the reactor at the same points as the mixture of monomer and of C0 ₂ recycled via the line R ₂ and the injectors R _\ .

The suspension which leaves the reactor firstly undergoes an adiabatic expansion to 1.4 MPa by passing through the automatic expansion valve E. To prevent the monomer not converted to polymer from condensing on the solid portion of the suspension, the latter is reheated in the cross-flow heat

exchanger O, which is travelled through in the other direction by the vaporized mixture V ₄ of monomer not converted to polymer and of C0 ₂ . The solid portion of the suspension thus expanded and reheated, constituted essentially of the synthesized polymer, is then separated in the bag filter Fi, the vaporized portion, comprising the monomer not converted to polymer and the CO?, being discharged as Vi in order to be recycled.

The solid portion of the suspension separated in the bag filter Fi is subjected to a second expansion to 0.1 MPa via the (upstream-downstream) flood gate or lock chamber G and again separated, in the bag filter F ₂ , from the vapour phase generated which is discharged as V ₂ in order to be recycled. The solid polymer (Pol) collected in the form of submicron-sized particles, optionally to which stabilizers and conventional additives (Add) have been added, is introduced, via the hopper H, into a counter-rotating twin-screw extruder I. After melting/plasticization of the polymer in a first zone of the extruder, the residual unconverted monomer contained in the polymer powder (around 1 % by weight) is removed in a first vent at atmospheric pressure and 180°C, then under vacuum at around 133 Pa (absolute) and at a temperature of 180°C in a second vent. The vacuum is created by the vacuum pump K. Finally, ready-to-use granules (GR) of vinyl chloride polymer are obtained. The residual unconverted monomer, degassed at atmospheric pressure, is discharged as V ₃ (vapour phase) in order to be recycled.

The monomer not converted to polymer, discharged as V ₃ (vapour phase) and the mixture of unconverted monomer and of C0 ₂ , discharged as Vi (vapour phase) and V ₂ (vapour phase) are recompressed to 2.5 MPa via the multi-stage compressors Li and L ₂ with cooling via the heat exchanger M. In the

vapour/liquid separator or decanter N, the separation between a liquid phase and a vapour phase comprising the vaporized mixture of unconverted monomer and of C0 ₂ is carried out. The liquid phase comprises a two-phase liquid mixture, of which the upper organic liquid phase (qVg), constituted essentially of monomer, is recycled as R ₃ to the pump B and of which the lower aqueous phase (cp _aq ) (resulting from impurities contained in the additives and initiator solutions) is discharged from the device. The vapour phase comprising the mixture of unconverted monomer and of C0 ₂ , recycled as V ₄ , with optional fresh C0 ₂ make-up via the tank A ₂ , is condensed via the cross-flow heat exchanger O by heat exchange with the suspension resulting from the reactor C that travels through the heat exchanger in the other direction after its adiabatic expansion by passing through the automatic expansion valve E (see above). The condensed mixture of unconverted monomer and of C0 ₂ is then purged of inert gases at P (vapour/liquid separator) and recycled to the reactor as Ri and R ₂ by means of the pump Q, as mentioned above.