AU1779000A

AU1779000A - Method for preparing halogenated copolymers, resulting halogenated polymers and use thereof

Info

Publication number: AU1779000A
Application number: AU17790/00A
Authority: AU
Inventors: Josselin Bobet; Bernard Guenaire; Yves Vanderveken
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1998-12-02
Filing date: 1999-11-30
Publication date: 2000-06-19
Also published as: MXPA01005500A; BR9915887A; CA2353881A1; BE1013023A3; EP1141053A1; JP2002531604A; CN1334829A; WO2000032660A1

Description

WO 00/32660 PCT/EP99/09371 Process for preparing halogenated copolymers, halogenated copolymers obtained and use thereof The invention relates to a process for 5 preparing halogenated copolymers, to the halogenated copolymers obtained, to their use for preparing extruded articles and to the extruded articles obtained. Generally, vinylidene chloride copolymers are 10 prepared by an aqueous-dispersion polymerization process in which all of the initiator is introduced at the start of the polymerization. This process usually requires relatively high polymerization temperatures, which has the consequence of producing copolymers of 15 low heat stability. Moreover, vinylidene chloride copolymers are generally characterized by a relatively high melting point, which involves very high working temperatures. These halogenated copolymers moreover have a relatively 20 low heat stability and limited flexibility. The relatively low heat stability which characterizes them causes the formation of large deposits of degraded materials on the die used during the implementation. One subject of the present invention is a 25 process for preparing halogenated copolymers which does not have the drawbacks presented by the processes of the prior art. A subject of the present invention is also halogenated copolymers which do not have the drawbacks 30 presented by the copolymers of the prior art. A subject of the invention is also the use of these halogenated copolymers. A subject of the invention is also the articles obtained from the copolymers according to the 35 invention. To this end, the invention relates firstly to a process for preparing halogenated copolymers by - 2 copolymerization of at least two monomers, in which the copolymerization is carried out in aqueous dispersion using an initiator, a fraction of which is introduced into the polymerization charge and the other fraction 5 is introduced subsequently. For the purposes of the present invention, the expression "fraction introduced into the polymerization charge" is intended to denote that a fraction of the total amount of the initiator is introduced from the 10 start of the polymerization with the ingredients usually introduced at that time. For the purposes of the present invention, the expression "fraction introduced subsequently" is intended to denote that a fraction of the total amount 15 of the initiator is introduced a certain time after the start of the polymerization. The time at which the subsequent introduction of the initiator begins is usually the time at which the initiation of the polymerization by the initiator 20 fraction introduced into the polymerization charge is complete. Usually, the subsequent introduction of the initiator is started between 10 and 60 minutes after the start of the polymerization. Generally, the subsequent introduction of the 25 initiator is started at least 10 minutes, preferably at least 15 minutes, particularly preferably at least 20 minutes and most particularly preferably at least 25 minutes after the start of the polymerization. Usually, the subsequent introduction of the initiator 30 is started not more than 60 minutes, preferably not more than 50 minutes, particularly preferably not more than 45 minutes and most particularly preferably not more than 40 minutes after the start of the polymerization. 35 Generally, the fraction introduced subsequently may be introduced totally in a single portion or in fractions or continuously. It is preferably introduced continuously.

- 3 The period of time over which the subsequent introduction of the initiator takes place generally ranges from 60 to 600 minutes. Generally, the period of time over which the 5 subsequent introduction of the initiator takes place is at least 60 minutes, preferably at least 100 minutes and particularly preferably at least 150 minutes. Generally, the period of time over which the subsequent introduction of the initiator takes place is not more 10 than 600 minutes, preferably not more than 500 minutes and particularly preferably not more than 400 minutes. Advantageously, an activator is added to the initiator. Generally, the total amount of the activator is 15 introduced into the polymerization charge. A fraction of the total amount of the activator is usually introduced into the polymerization charge and the other fraction is introduced subsequently. In general, the ratio between the amount of 20 activator and the amount of initiator introduced into the polymerization charge ranges from 1 to 10. In general, the ratio between the amount of activator and the amount of initiator introduced into the polymerization charge is greater than or equal to 25 1, preferably greater than or equal to 2 and particularly preferably greater than or equal to 3. In general, the ratio between the amount of activator and the amount of initiator introduced into the polymerization charge is less than or equal to 10, 30 preferably less than or equal to 9.5 and particularly preferably less than or equal to 9. In general, the ratio between the amount of activator and the amount of initiator introduced subsequently ranges from 0 to 10. 35 In general, the ratio between the amount of activator and the amount of initiator introduced subsequently is greater than or equal to 0, preferably greater than or equal to 1 and particularly preferably greater than or equal to 2. In general, the ratio - 4 between the amount of activator and the amount of initiator introduced subsequently is less than or equal to 10, preferably less than or equal to 9 and particularly preferably less than or equal to 8. 5 In general, the weight ratio between the fraction of the activator and of the initiator introduced into the polymerization charge and the total amount of activator and of initiator introduced during the polymerization ranges from 10% to 80%. 10 In general, the weight ratio between the fraction of the activator and of the initiator introduced into the polymerization charge and the total amount of activator and of initiator introduced during the polymerization is less than or equal to 80%, 15 preferably less than or equal to 60% and particularly preferably less than or equal to 50%. In general, the weight ratio between the fraction of the activator and of the initiator introduced into the polymerization charge and the total 20 amount of activator and of initiator introduced during the polymerization is greater than or equal to 10%, preferably greater than or equal to 15% and particularly preferably greater than or equal to 20%. In general, the polymerization temperature is 25 less than or equal to 60 0 C, preferably less than or equal to 55OC and particularly preferably less than or equal to 50 0 C. In general, the polymerization time is less than or equal to 16 hours, preferably less than or 30 equal to 14 hours, particularly preferably less than or equal to 12 hours and most particularly preferably less than or equal to 10 hours. Advantageously, all the monomers participating in the copolymerization are introduced at the start of 35 the polymerization into the polymerization charge. The process for preparing halogenated copolymers according to the invention is suitable when the main monomer is a halogenated monomer, particularly suitable when the main monomer is a monomer containing - 5 chlorine and most particularly suitable when the main monomer is vinylidene chloride. For the purposes of the present invention, the expression "main monomer" is intended to denote the 5 monomer which is present in the resulting copolymers in a proportion of at least 50% by weight. For the purposes of the present invention, the expression "halogenated copolymers" is intended to denote the copolymers obtained by aqueous-dispersion 10 free-radical polymerization of a halogenated monomer, which is the main monomer, with one or more monomers copolymerizable therewith. For the purposes of the present invention, the expression "halogenated monomer" is intended to denote 15 any free-radical-polymerizable monomer containing terminal olefinic unsaturation and substituted with at least one halogen atom. Preferably, these monomers are chosen from substituted ethylene and propylene derivatives and contain only two or three carbon atoms, 20 respectively.' As non-limiting examples of such monomers, mention may be made of vinyl chloride, vinylidene chloride, vinyl bromide, vinylidene bromide, vinyl fluoride, vinylidene fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene and 25 hexafluoropropylene. Among the monomers that are copolymerizable with the halogenated monomer, mention may be made, in a non-limiting manner, of halogenated monomers of different nature, vinyl esters such as, for example, 30 vinyl acetate, vinyl ethers, acrylic acids, esters and amides, methacrylic acids, esters and amides, styrene, styrene derivatives, butadiene, olefins such as, for example, ethylene and propylene, itaconic acid and maleic anhydride. 35 The process for preparing halogenated copolymers according to the invention is particularly suitable for use in preparing copolymers containing chlorine.

- 6 For the purposes of the present invention, the expression "copolymers containing chlorine" is intended to denote the copolymers obtained by aqueous-dispersion free-radical polymerization of a monomer containing 5 chlorine, which is the main monomer, such as, for example, vinyl chloride and vinylidene chloride, with one or more monomers that are copolymerizable therewith. Among the monomers that are copolymerizable 10 with the monomer containing chlorine, mention may be made, in a non-limiting manner, of monomers containing chlorine of different nature, vinyl esters such as, for example, vinyl acetate, vinyl ethers, acrylic acids, esters and amides, methacrylic acids, esters and 15 amides, styrene, styrene derivatives, butadiene, olefins such as, for example, ethylene and propylene, itaconic acid and maleic anhydride. The process for preparing halogenated copolymers according to the invention is most 20 particularly suitable for use in preparing vinylidene chloride copolymers. For the purpose of the present invention, the expression "vinylidene chloride copolymers" is intended to denote copolymers of vinylidene chloride, which is 25 the main monomer, with one or more monomers that are copolymerizable therewith. Among the monomers that are copolymerizable with vinylidene chloride, mention may be made, in a non-limiting manner, of vinyl chloride, vinyl esters 30 such as, for example, vinyl acetate, vinyl ethers, acrylic acids, esters and amides, methacrylic acids, esters and amides, styrene, styrene derivatives, butadiene, olefins such as, for example, ethylene and propylene, itaconic acid and maleic anhydride. 35 The process for preparing halogenated copolymers according to the invention is most particularly suitable for use when the monomers are vinylidene chloride, vinyl chloride and at least one - 7 (meth)acrylic monomer corresponding to the general formula:

CH

2 =CRiR 2 in which R, is chosen from hydrogen and a methyl 5 radical and R 2 is chosen from a -CN radical and a

-CO-OR

3 radical in which R 3 is chosen from hydrogen, alkyl radicals containing from 1 to 18 carbon atoms, alkoxyalkyl radicals containing a total of from 1 to 10 carbon atoms and radicals -NR 4

R

5 in which R 4 and R 5 are 10 chosen from hydrogen and an alkyl radical containing from 1 to 10 carbon atoms. For the purposes of the present invention, the expression "at least one (meth)acrylic monomer" is intended to denote that the vinylidene chloride 15 copolymers may contain one or more (meth)acrylic monomer(s), denoted hereinbelow as (meth)acrylic monomer. The (meth)acrylic monomer is preferably chosen from acrylic and methacrylic esters containing from 1 20 to 8 carbon atoms and particularly preferably from acrylic and methacrylic esters containing from 1 to 6 carbon atoms. Examples of such acrylic and methacrylic esters that are particularly preferred are methyl acrylate, 25 methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, t-butyl acrylate t-butyl methacrylate, n-pentyl acrylate, n-pentyl 30 methacrylate, isoamyl acrylate, isoamyl methacrylate, n-hexyl acrylate, n-hexyl methacrylate, 2-methylpentyl acrylate and 2-methylpentyl methacrylate. The (meth)acrylic monomer is most particularly preferably selected from acrylic and methacrylic esters 35 containing from 1 to 4 carbon atoms. Examples of such acrylic and methacrylic esters that are most particularly preferred are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, - 8 n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, t-butyl acrylate and t-butyl methacrylate. In this case, vinylidene chloride is the main 5 monomer. The vinylidene chloride is thus generally present in the resulting copolymers in a proportion of at least 50% by weight. In general, the amount of vinylidene chloride in the vinylidene chloride copolymers ranges from 50% 10 to 95% by weight, preferably from 60% to 95% by weight and particularly preferably from 70% to 95% by weight. In general, the amount of vinyl chloride in the vinylidene chloride copolymers ranges from 3% to 50% by weight, preferably from 3% to 40% by weight and 15 particularly preferably from 4.5% to 30% by weight. In general, the amount of the (meth)acrylic monomer in the vinylidene chloride copolymers ranges from 0.5% to 20% by weight, preferably from 1% to 10% by weight and particularly preferably from 1.5% to 4.5% 20 by weight. Preferably, the total amount of vinylidene chloride, of vinyl chloride and of (meth)acrylic monomer is introduced at the start of the polymerization into the polymerization charge. 25 For the purposes of the present invention, the expression "aqueous-dispersion copolymerization" is intended to denote free-radical copolymerization in aqueous dispersion and also free-radical copolymerization in aqueous emulsion and 30 copolymerization in aqueous microsuspension. For the purposes of the present invention, the expression "aqueous-suspension free-radical copolymerization" is intended to denote any free radical copolymerization process which is carried out 35 in aqueous medium in the presence of oleosoluble free radical initiators and dispersants. For the purposes of the present invention, the expression "aqueous-emulsion free-radical copolymerization" is intended to denote any free- - 9 radical copolymerization process which is carried out in aqueous medium in the presence of water-soluble free-radical initiators and emulsifiers. For the purposes of the present invention, the 5 expression "copolymerization in aqueous micro suspension", also referred to as homogenized aqueous dispersion, is intended to denote any free-radical copolymerization process in which oleosoluble initiators are used and an emulsion of droplets of 10 monomers is prepared by means of vigorous mechanical stirring in the presence of emulsifiers. The process according to the invention is particularly suitable for preparing halogenated copolymers by aqueous-emulsion copolymerization. Thus, 15 the copolymerization is carried out using emulsifiers present in amounts that are known to those skilled in the art. Examples of emulsifiers which may be mentioned include anionic emulsifiers and nonionic emulsifiers. 20 Among the anionic emulsifiers which may be mentioned, in a non-limiting manner, are paraffin sulphonates, alkyl sulphates, alkyl sulphonates, alkylaryl mono- or disulphonates and alkyl sulphosuccinates. Among the nonionic emulsifiers which may be mentioned, in a non 25 limiting manner, are alkyl- or alkylarylethoxylated derivatives. Water-soluble initiators which may be mentioned include water-soluble peroxides such as ammonium or alkali metal persulphates, hydrogen peroxide, 30 perborates and t-butyl hydroperoxide. Activators which may be mentioned include erythorbic acid alone or as a mixture with ferric nitrate, ammonium or alkali metal sulphites or metabisulphites, hydrazine, hydroxylamine, thiols, 35 sodium thiosulphate and iron or copper salts. Preferably, the activator is chosen from erythorbic acid and mixtures of erythorbic acid with ferric nitrate, and the initiator is hydrogen peroxide.

- 10 Preferably, the activator is chosen from sodium sulphite or metabisulphite and the initiator is chosen from ammonium or alkali metal persulphates. The invention also relates to halogenated 5 copolymers which have a melting point, determined by differential thermal analysis, of less than or equal to 140 0 C and a relative viscosity, measured at a concentration of 10 g/l in tetrahydrofuran, of greater than or equal to 1.30. 10 The melting point determined by differential thermal analysis of the copolymers according to the invention is generally less than or equal to 140 0 C, preferably less than or equal to 1300C and particularly preferably less than or equal to 125 0 C. 15 The relative viscosity of the copolymers according to the invention, measured at a concentration of 10 g/l in tetrahydrofuran, is generally greater than or equal to 1.30 and preferably greater than or equal to 1.35. 20 Advantageously, the mixtures of the copolymers according to the invention with 4% by weight of epoxidized soybean oil also have a heat stability, measured on a blender, at a temperature 20 0 C above the melting point, which is greater than or equal to 25 16 minutes, preferably greater than or equal to 20 minutes and particularly preferably greater than or equal to 24 minutes. For the purposes of the present invention, the expression "halogenated copolymers" is intended to 30 denote the copolymers obtained by aqueous-dispersion free-radical polymerization of a halogenated monomer, referred to as the main halogenated monomer, with one or more monomers that are copolymerizable therewith. For the purposes of the present invention, the 35 expression "main halogenated monomer" is intended to denote the halogenated monomer which is present in the resulting halogenated copolymers in a proportion of at least 50% by weight.

- 11 For the purposes of the present invention, the expression "halogenated monomer" is intended to denote any free-radical-polymerizable monomer containing terminal olefinic unsaturation and substituted with at 5 least one halogen. Preferably, these monomers are chosen from substituted ethylene and propylene derivatives and contain only two or three carbon atoms, respectively. Non-limiting examples of such monomers which may be mentioned include vinyl chloride, 10 vinylidene chloride, vinyl bromide, vinylidene bromide, vinyl fluoride, vinylidene fluoride, trifluoroethylene, tetrafluoroethylene, chlorotrifluoroethylene and hexa fluoropropylene. Among the monomers that are copolymerizable 15 with the halogenated monomer, mention may be made, in a non-limiting manner, of halogenated monomers of different nature, vinyl esters such as, for example, vinyl acetate, vinyl ethers, acrylic acids, esters and amides, methacrylic acids, esters and amides, styrene, 20 styrene derivatives, butadiene, olefins such as, for example, ethylene and propylene, itaconic acid and maleic anhydride. The halogenated copolymers are preferably copolymers containing chlorine and more preferably 25 vinylidene chloride copolymers. For the purposes of the present invention, the expression "copolymers containing chlorine" is intended to denote the copolymers obtained by aqueous-dispersion free-radical polymerization of a monomer containing 30 chlorine such as, for example, vinyl chloride and vinylidene chloride, with one or more monomers that are copolymerizable therewith. The monomer containing chlorine is in this case the main monomer, i.e. the one which is present in the resulting copolymers in a 35 proportion of at least 50% by weight. Among the monomers that are copolymerizable with the monomer containing chlorine, mention may be made, in a non-limiting manner, of monomers containing chlorine of different nature, vinyl esters such as, for - 12 example, vinyl acetate, vinyl ethers, acrylic acids, esters and aides, methacrylic acids, esters and amides, styrene, styrene derivatives, butadiene, olefins such as, for example, ethylene and propylene, 5 itaconic acid and maleic anhydride. For the purposes of the present invention, the expression "vinylidene chloride copolymers" is intended to denote copolymers of vinylidene chloride with one or more monomers that are copolymerizable therewith. In 10 this case, the vinylidene chloride is the main monomer, i.e. the monomer which is present in the resulting copolymers in a proportion of at least 50% by weight. Among the monomers that are copolymerizable with vinylidene chloride, mention may be made, in a 15 non-limiting manner, of vinyl chloride, vinyl esters such as, for example, vinyl acetate, vinyl ethers, acrylic acids, esters and amides, methacrylic acids, esters and amides, styrene, styrene derivatives, butadiene, olefins such as, for example, ethylene and 20 propylene, itaconic acid and maleic anhydride. The copolymers that are particularly preferred are copolymers of vinylidene chloride with vinyl chloride and at least one (meth)acrylic monomer corresponding to the general formula: 25 CH 2 =CRiR 2 in which R, is chosen from hydrogen and a methyl radical and R 2 is chosen from a -CN radical and a

-CO-OR

3 radical in which R 3 is chosen from hydrogen, alkyl radicals containing from 1 to 18 carbon atoms, 30 alkoxyalkyl radicals containing a total of from 1 to 10 carbon atoms and radicals -NR 4

R

5 in which R 4 and R 5 are chosen from hydrogen and an alkyl radical containing from 1 to 10 carbon atoms. The (meth)acrylic monomer is preferably chosen 35 from acrylic and methacrylic esters containing from 1 to 8 carbon atoms and particularly preferably from acrylic and methacrylic esters containing from 1 to 6 carbon atoms.

- 13 Examples of such acrylic and methacrylic esters that are particularly preferred are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, 5 n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, t-butyl acrylate t-butyl methacrylate, n-pentyl acrylate, n-pentyl methacrylate, isoamyl acrylate, isoamyl methacrylate, n-hexyl acrylate, n-hexyl methacrylate, 2-methylpentyl 10 acrylate and 2-methylpentyl methacrylate. The (meth)acrylic monomer is most particularly preferably selected from acrylic and methacrylic esters containing from 1 to 4 carbon atoms. Examples of such acrylic and methacrylic esters 15 that are most particularly preferred are methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutyl methacrylate, t-butyl acrylate and 20 t-butyl methacrylate. In this case, vinylidene chloride is the main monomer. In general, the amount of vinylidene chloride in the vinylidene chloride copolymers ranges from 50% 25 to 95% by weight, preferably from 60% to 95% by weight and particularly preferably from 70% to 95% by weight. In general, the amount of vinyl chloride in the vinylidene chloride copolymers ranges from 3% to 50% by weight, preferably from 3% to 40% by weight and 30 particularly preferably from 4.5% to 30% by weight. In general, the amount of the (meth)acrylic monomer in the vinylidene chloride copolymers ranges from 0.5% to 20% by weight, preferably from 1% to 10% by weight and particularly preferably from 1.5% to 4.5% 35 by weight. The invention also relates to the halogenated copolymers obtained by the process according to the invention.

- 14 The invention also relates to the use of the halogenated copolymers according to the invention to prepare extruded articles, for example bioriented or blown monolayer or multilayer barrier films, monolayer 5 or multilayer tubes, monolayer or multilayer sheets and sheets prepared by extrusion layering on polymeric substrates (polyvinyl chloride, polyethylene tere phthalate or polypropylene) or on paper. The invention also relates to extruded articles 10 manufactured with the halogenated copolymers according to the invention. These articles are usually used in the food-wrapping field and in the medical field (for example for pharmaceutical blister packs). The copolymers according to the invention, 15 although being heterogeneous in terms of distribution of vinyl chloride, have the great advantage over the known copolymers of being characterized, unexpectedly, by a melting point which is markedly lower than the known copolymers. By virtue of this property, the 20 copolymers may be used at lower temperatures, which makes it possible to obtain higher yields and to dispense with the addition of stabilizer during the implementation. Moreover, the copolymers according to the invention are advantageously characterized by 25 improved heat stability and improved flexibility compared with the copolymers of the prior art. When the copolymers according to the invention are implemented, no deposition of degraded materials is observed on the die used and products of very acceptable quality are 30 obtained. The copolymers according to the invention also have the advantage of being more compatible with the additives. The examples which follow are intended to illustrate the invention without, however, limiting its 35 scope. Example 1 - Preparation of a halogenated copolymer 21.7 litres of demineralized water are first introduced into a 40-litre reactor fitted with a stirrer of impeller type. 112 g of sodium - 15 dodecylbenzene sulphonate, 11 520 g of vinylidene chloride, 3 840 g of vinyl chloride and 640 g of ethyl acrylate are then introduced, with stirring at 120 rpm. The reactor temperature is then raised to 400C. When 5 the temperature reaches 400C, 0.8 g of hydrogen peroxide and 5.12 g of erythorbic acid are added. Thirty minutes later, 3.864 g of erythorbic acid and 0.96 g of hydrogen peroxide are then added continuously, over a period of about 300 minutes. Five 10 hours thirty minutes after the start of the polymerization, the reactor is returned to atmospheric pressure and placed under vacuum for 3 hours at 45 0 C. It is then cooled to room temperature. The degree of conversion is 91%. 15 41.1 litres of a 0.2 g/l solution of aluminium sulphate are introduced into a 150-litre container fitted with a stirrer of impeller type. The container is then stirred at about 200 rpm and the temperature is adjusted to 10-14 0 C. All of the aqueous dispersion 20 obtained is then added over about 30 minutes along with 5 litres of a solution containing 1.3 g/l of aluminium sulphate. The temperature of the container is then brought to 70 0 C and maintained for 1 hour 30 minutes. The container is then cooled to room temperature. A 25 slurry is thus obtained. This slurry is then dried in two steps. The first step consists of a liquid/solid separation in an Escher Wyss system to form a cake. The second step consists in drying the cake in a fluidized bed of Mtnster type with an inlet-air 30 temperature of about 60 0 C. A halogenated copolymer with a volatile material content of less than 0.3% is obtained. The volatile material content is determined by measuring the loss of mass of the sample after a residence time of 45 minutes in a ventilated oven 35 maintained at 120 0 C. Example 2- Properties of the halogenated copolymer Various properties of the halogenated copolymer obtained in Example 1 were measured. Among these are - 16 the relative viscosity, the melting point, the modulus of elasticity under tension and the heat stability. The relative viscosity is measured using an Ubbelohde viscometer with a constant K of about 0.003, 5 at a temperature of 20 0 C. The solvent used is tetrahydrofuran. The concentration of the solution of the halogenated copolymer in tetrahydrofuran is 10 g/l. The melting point is measured using a Perkin Elmer® differential thermal analysis machine. 18 mg of 10 resin are used and the heating rate is 10OC/minute. The modulus of elasticity under tension is measured according to ISO standard 527 on a 30 gm non bioriented extruded monosheet. To do this, a mixture of the copolymer obtained in Example 1 with 4% by weight 15 of Edenol® D 82 epoxidized soybean oil is extruded using a Brabender® extruder fitted with a 19 mm sheath and a screw having a length-to-diameter ratio of 20, and a 200 x 0.5 mm plate die made of Duranickel® material. A 30 gm sheet is then formed using a 20 Brabender® calender. The heat stability is measured on a mixture of the copolymer obtained in Example 1 with 4% by weight of Edenol® D 82 epoxidized soybean oil. To do this, 960 g of the halogenated copolymer are introduced into 25 a planetary mixer equipped with a K-shaped paddle rotating at a speed of about 60 rpm. When the temperature reaches 35OC, 40 g of Edenol® D 82 epoxidized soybean oil preheated to 55 0 C are introduced. When the temperature reaches 70 0 C, the 30 mixture is unloaded into a stainless-steel tank and is left to cool to room temperature. The heat stability of the mixture was measured in a blender, at a temperature 200C above the melting point of the copolymer. To do this, 95 g of the mixture are introduced into a 35 Brabender® Plasti Corder® type N50 blender with a tank made of Hastelloyo C276 material 60 cm 3 in volume, via a hopper. The spin speed of the blender is 50 rpm. Every 3 minutes, a pellet is taken out to monitor the colour. The torque in Newton.metres is also recorded as a - 17 function of time. The heat stability is determined by a change in the slope of this torque over time. This change in slope generally corresponds to a change in the colour of the pellet. 5 The relative viscosity, melting point, modulus of elasticity under tension and heat stability values measured for the halogenated copolymer obtained in Example 1 are summarized in Table I. Example 3 - Preparation of a halogenated copolymer 10 21.76 litres of demineralized water are first introduced into a 40-litre reactor fitted with a stirrer of impeller type. 112 g of sodium dodecylbenzene sulphonate, 11 520 g of vinylidene chloride, 3 840 g of vinyl chloride and 320 g of ethyl 15 acrylate and 320 g of n-butyl acrylate are then introduced, with stirring at 120 rpm. The reactor temperature is then raised to 40 0 C. When the temperature reaches 40OC, 0.8 g of hydrogen peroxide and 5.12 g of erythorbic acid are added. Thirty minutes 20 later, 4.048 g of erythorbic acid and 1.056 g of hydrogen peroxide are then added continuously, over a period of about 300 minutes. Six hours twenty minutes after the start of the polymerization, the reactor is returned to atmospheric pressure and placed under 25 vacuum for 3 hours at 45OC. It is then cooled to room temperature. The degree of conversion is 89%. 20 litres of a 0.2 g/l solution of aluminium sulphate are introduced into a 150-litre container fitted with a stirrer of impeller type. The container 30 is then stirred at about 200 rpm and the temperature is adjusted to 10-14OC. All of the aqueous dispersion obtained is then added over about 30 minutes along with 0.7 litre of dimineralized water and 5 litres of a solution containing 2.1 g of aluminium sulphate. The 35 temperature of the container is then brought to 70 0 C and maintained for 1 hour 30 minutes. The container is then cooled to room temperature. A slurry is thus obtained. This slurry is then dried in two steps. The first step consists of a liquid/solid separation in an - 18 Escher Wyss system to form a cake. The second step consists in drying the cake in a fluidized bed of Minster type with an inlet-air temperature of about 60 0 C. A halogenated copolymer with a volatile material 5 content of less than 0.3% is obtained. The volatile material content is determined by measuring the loss of mass of the sample after a residence time of 45 minutes in a ventilated oven maintained at 1200C. Example 4 - Properties of the halogenated copolymer 10 The relative viscosity, melting point, modulus of elasticity under tension and heat stability of the halogenated copolymer obtained in Example 3 were measured in the same way as in Example 2. The results of these various measurements are 15 summarized in Table I. TABLE I Example Relative Melting Modulus of Heat viscosity point elasticity stability under (melting tension point + 200C) 2 1.65 117 0 C 286 Mpa > 25 minutes 4 1.66 118oC 296 Mpa > 30 minutes

Claims

1. Process for preparing halogenated copolymers by copolymerization of at least two monomers characterized in that the copolymerization is carried out in aqueous dispersion using an initiator, a fraction of which is introduced into the polymerization charge and the other fraction is introduced subsequently.

2. Process for preparing halogenated copolymers according to Claim 1, characterized in that an activator is added to the initiator.

3. Process for preparing halogenated copolymers according to Claim 2, characterized in that the ratio between the amount of activator and the amount of initiator introduced into the polymerization charge ranges from 1 to 10.

4. Process for preparing halogenated copolymers according to either of Claims 2 and 3, characterized in that the ratio between the amount of activator and the amount of initiator introduced subsequently ranges from 0 to 10.

5. Process for preparing halogenated copolymers according to any one of Claims 2 to 4, characterized in that the weight ratio between the fraction of the activator and of the initiator introduced into the polymerization charge and the total amount of activator and of initiator introduced during the polymerization ranges from 10% to 80%.

6. Process for preparing halogenated copolymers according to any one of Claims 1 to 5, characterized in that the polymerization temperature is less than or equal to 60 0 C.

7. Process for preparing halogenated copolymers according to any one of Claims 1 to 6, characterized in that the polymerization time is less than or equal to 16 hours.

8. Process for preparing halogenated copolymers according to any one of Claims 1 to 7, characterized in - 20 that all the monomers are introduced into the polymerization charge.

9. Process for preparing halogenated copolymers according to Claim 8, characterized in that the main monomer is a monomer containing chlorine.

10. Process for preparing halogenated copolymers according to either of Claims 8 and 9, characterized in that the main monomer is vinylidene chloride.

11. Process for preparing halogenated copolymers according to any one of Claims 8 to 10, characterized in that the monomers are vinylidene chloride, vinyl chloride and at least one (meth)acrylic monomer corresponding to the general formula: CH 2 =CRiR 2 in which R 1 is chosen from hydrogen and a methyl radical and R 2 is chosen from a -CN radical and a -CO-OR 3 radical in which R 3 is chosen from hydrogen, alkyl radicals containing from 1 to 18 carbon atoms, alkoxyalkyl radicals containing a total of from 1 to 10 carbon atoms and radicals -NR 4 R 5 in which R 4 and Rs are chosen from hydrogen and an alkyl radical containing from 1 to 10 carbon atoms.

12. Process for preparing halogenated copolymers according to Claim 11, characterized in that the (meth)acrylic monomer is chosen from acrylic and methacrylic esters containing from 1 to 8 carbon atoms.

13. Process for preparing halogenated copolymers according to any one of Claims 1 to 12, characterized in that the copolymerization takes place in aqueous emulsion.

14. Halogenated copolymers, characterized in that they have a melting point, determined by differential thermal analysis, of less than or equal to 140 0 C and a relative viscosity, measured at a concentration of 10 g/l in tetrahydrofuran, of greater than or equal to 1.30.

15. Halogenated copolymers according to Claim 14, characterized in that their mixtures with 4% by weight of epoxidized soybean oil have a heat stability, - 21 measured on a blender, at a temperature 20 0 C above the melting point, which is greater than or equal to 16 minutes.

16. Halogenated copolymers according to either of Claims 14 and 15, characterized in that they are copolymers containing chlorine.

17. Halogenated copolymers according to any one of Claims 14 to 16, characterized in that they are vinylidene chloride copolymers.

18. Halogenated copolymers according to any one of Claims 14 to 17, characterized in that they are copolymers of vinylidene chloride with vinyl chloride and at least one (meth)acrylic monomer corresponding to the general formula: CH 2 =CRiR 2 in which R 1 is chosen from hydrogen and a methyl radical and R 2 is chosen from a -CN radical and a -CO-OR 3 radical in which R 3 is chosen from hydrogen, alkyl radicals containing from 1 to 18 carbon atoms, alkoxyalkyl radicals containing a total of from 1 to 10 carbon atoms and radicals -NR 4 R 5 in which R 4 and R 5 are chosen from hydrogen and an alkyl radical containing from 1 to 10 carbon atoms.

19. Halogenated copolymers according to Claim 18, characterized in that the (meth)acrylic monomer is chosen from acrylic and methacrylic esters containing from 1 to 8 carbon atoms.

20. Use of the halogenated copolymers according to Claims 14 to 19, to prepare extruded articles.

21. Extruded articles, characterized in that they are manufactured with the halogenated copolymers according to any one of Claims 14 to 19.