PROCESS FOR PREPARING VINYLIDENE FLUORIDE HOMOPOLYMERS AND ITS COPOLYMERS

Abstract: A process for preparing vinylidene fluoride homopolym­ ers of low molecular weight and its copolymers by carrying out the polymerization in the presence of a radical initiator at a temperature from 20° to 160°C in a solvent of general formula R-COO-R₁, wherein R is hydrogen or an alkyl radical having up to 4 carbon atoms and R₁ is an alkyl radical having up to 5 carbon atoms, wherein R and R₁ may optionally contain groups OH or OR₂ wherein R₂ is the same as R₁.

Description: This invention relates to a process for preparing vinylidene fluoride homopolymers having low molecular weight and its copolymers by polymerization in solution. As is known, the most common processes for preparing fluoropolymers, and in particular fluoroelastomeric copolymers of vinylidene fluoride, utilize the copolymerization in emulsion technique in an aqueous medium at high pressures, as is described by Kirk Othmer, Encyclopaedia of Chemical Technology, vol. 8, page 504 and following (1979). According to this technique, the copolymerization is carried out in the presence of radical initiators, either organic or inorganic, usually ammonium persulphate or potassium persulphate, in the presence of emulsifiers and of a chain regulator such as for example carbon tetrachloride. At the end of the copolymerization, the latexes are coagulated by addition of salts or of acids or of a combination thereof. These processes exhibit, however, the drawback of the presence in the product so obtained of inorganic residues due to the initiator and in particular due to the mineral salts or acids utilized for the coagulation. Such inorganic residues, even if present in very low amounts, affect adversely the chemical-physical properties of the fluoropolymer or of the fluoroelastomer in many applications. U.S. Patent No. 3,069,401 describes a polymerization process for preparing elastomeric copolymers of vinylidene fluoride and hexafluoropropene by means of radical initiators and in the presence of a chain transfer compound selected from aliphatic compounds of the type of alcohols, ketones and carboxylic acid esters. The copolymerization reaction is conducted in bulk, with transfer/monomer molar ratios ranging from 1/2 to 1/40. This second method provides copolymers having only a low molecular weight, generally ranging from 1200 to 8400, and containing remarkable amounts of the chain transfer compound, from 0.9% to 7% by weight. Such amounts of chain transfer compound in the copolymers affect adversely the chemical and physical properties of the copolymers in many applications. Vinylidene fluoride homopolymers are usually prepared in the art by radical polymerization of the monomer in aqueous emulsion or in suspension, in the presence of chlorofluorocarbons as solvent. The homopolymer can also be prepared by using radiation as has been described in G.B. Patent No. 1,188,889. By operating according to these methods homopolymers are obtained having a high viscosimetric molecular weight, generally ranging from 300,000 to 1,000,000. In addition the thus obtained homopolymers contain large amounts of chain transfer compounds similar to those indicated above for the copolymers. When the polymerization is carried out in fluorocarbon solvent the amounts of solvent which remain in the homopolymers is very high, generally ranging from 5% to 10% by weight. We have now surprisingly found that it is possible to remove the drawbacks described hereinabove if the homopolymerization of vinylidene fluoride or its copolymerization with hexafluoropropene or tetrafluoroethylene or mixtures thereof, is carried out in a particular class of solvents. In particular the amounts of solvent residues in the (co)polymers are very low, in general lower than 0.3% by weight. The present invention provides a process for preparing vinylidene fluoride homopolymers having an average viscosimetric molecular weight of from 1,000 to 20,000, and copolymers of vinylidene fluoride with hexafluoropropene and/or tetrafluoroethylene of a molecular weight corresponding to intrinsic viscosity values of from 0.02 to higher than 0.5 dl/g, which comprises carrying out the (co)polymerization in the presence of a radical initiator at a temperature from 20° to 160°C, in a solvent selected from carboxylic acid esters of general formula: R - COO - R₁ (I) in which R is a hydrogen atom or an alkyl radical containing from 1 to 4 carbon atoms and R₁ is an alkyl radical containing from 1 to 5 carbon atoms. R and/or R₁ may optionally contain, as substituents, a hydroxyl group OH or an alkoxy group OR₂ wherein R₂ has the same meaning as R₁. The solvent/(co)polymer weight ratio is preferably in the range from 4/1 to 10/1 at the end of the polymerization reaction. Preferred radical initiators are selected from organic peroxide-type free-radical formers and azo catalysts, suitably as described in Encyclopaedia of Polymer Science and Technology, vol. 3, page 27, 1969 such as azo-bis-alkylnitriles, in particular 2,2'-azo-bis-isobutyronitrile and 2,2'-azo-bis-2-methylbutyronitrile. Radical initiators which are suitable for the copolymerization in solution reaction of the present invention may be those commonly utilized in the technique of the art, such as the organic peroxides cited in Kirk Othmer, Encyclopaedia of Chemical Technology, Vol. 8, pages 504 and following (1979). Examples of useful organic peroxides are benzoyl peroxide, dicumyl peroxide, bis-(4-t.butylcyclohexyl)-peroxy-dicarbonate and di-t.butyl peroxide. In particular, there are preferred the radical initiators active at temperatures from 20 to 50°C, such as bis-(4-t.butylcyclohexyl)-peroxy-dicarbonate, if the copolymer must have a high molecular weight; or radical initiators active at higher temperatures, for example from 120 to 160°C, such as di-t.butyl-peroxide, if the copolymer must have a lower molecular weight. The copolymers obtained according to the process of the invention have molecular weights varying over a wide range corresponding to intrinsic viscosity values from 0.02 dl/g up to values higher than 0.5 dl/g. Preferred solvents for the copolymers are the esters of formic, acetic, and propionic acids with methyl, ethyl and t.butyl alcohol. Since the ester is also a chain transfer compound, if it is desirable to obtain high molecular weight copolymers, then it is advisable to use esters which contain only methyl groups, while the presence of -CH₂ - groups of alcoholic origin results in a higher transferring action and so in lower molecular weights. Solvents which promote the obtainment of high molecular weight copolymers are methyl formate and acetate or t.butyl formate and acetate; ethyl acetate may be used as a solvent which promotes the obtainment of low molecular weights. From H.F. Mark, Encyclopaedia of Polymer Science and Technology, vol. 3, page 605 (1965) it is also known that the chain transfer action which leads to the obtainment of lower molecular weights is enhanced if it is operated at higher temperatures; temperatures of at least 120°C are preferred when a low molecular weight product is to be obtained, while temperatures of not more than 50°C are preferred for obtaining high molecular weight copolymers. The process of the present invention is particularly suitable for obtaining copolymers containing from 40 to 95% by moles of vinylidene fluoride, from 0 to 45% by moles of hexafluoropropene and from 0 to 30% by moles of tetrafluoroethylene. More preferably the copolymers contain from 50 to 85% moles of vinylidene fluoride, from 15 to 45% moles of hexafluoropropene and from 0 to 25% moles of tetrafluoroethylene. By operating under the copolymerization conditions in order to obtain the last-mentioned copolymers, the copolymers remain dissolved in the solvent during the copolymerization. The obtaining of a copolymer dissolved in the solvent allows some advantages since it can be directly used in the vulcanization step as indicated below. We have furthermore found that, in the homopolymerization of vinylidene fluoride, with the working conditions shown above, a homopolymer having a very low molecular weight of from 1,000 to 20,000 is obtained, these data being referred to the average viscosimetric molecular weight. For obtaining vinylidene fluoride homopolymers having viscosimetric molecular weights from 1000 to 5000, preferred conditions consist in operating at a temperature from 130° to 150°C with an initiator consisting of di-t.butylperoxide in an amount from 1 to 4 g per 100 g of vinylidene fluoride, in the presence of methyl acetate in an amount 4-40 times the weight of the vinylidene fluoride. The polyvinylidene fluorides obtained by the process according to the invention are particularly utilizable as plasticizers for paints based on vinylidene polyfluoride and as additives of fluorinated elastomers based on vinylidene fluoride. According to another embodiment of the present invention, vinylidene fluoride homopolymers are obtained in solution, by carrying out the polymerization reaction in methyl or ethyl acetate, methyl and ethyl formate as a solvent; the solution can be directly used as such to prepare films and coatings as described in Encyclopaedia of Polymer Science and Technology, vol. 14, page 606, 1969. Otherwise the homopolymer can be recovered by known methods such as evaporation and the final product contains amounts of solvent less than 0.2% by weight. The copolymer of vinylidene fluoride obtained at the end of the copolymerization in solution in its own solvent can be used as such or it can be recovered by means of usual operations such as precipitation and washing in water or in an organic liquid. If the copolymer is to be isolated by precipitation in water, then the copolymerization should be preferably conducted in a solvent exhibiting a sufficient solubility in water, such as methyl formate, ethyl formate or methyl acetate, the solubilities of which in water at 20°C are about 23, 9 and 24% by weight, respectively. Conversely, if it is desired to directly utilize the polymeric solution, then it is advantageous to use less water-soluble and less volatile solvents, such as butyl acetate or t.butyl acetate. In the last case the solution can be additioned with a conventional vulcanization ingredient such as Tecnocin A, a Schiff base prepared from cinnamic aldehyde and hexamethylene diamine, magnesium and/or lead oxides, calcium hydroxide, etc., and vulcanization is directly accomplished by heating and evaporating of the solution. If the copolymer is to be utilized in the solid state, then the solution is precipitated in water or in another liquid, as soluble as possible in the ester of the polymerization reaction, for example hexane or cyclohexane; said liquid is preferably employed in an amount from 4 to 20 times greater than the reaction solution volume. The copolymer is filtered and the last traces of ester are removed by heating to a temperature higher than 50°C or, if the precipitation is effected in water, by stripping with steam. The resulting copolymer, after drying in a ventilated oven, contains solvent amounts lower than 0.3% by weight. The invention will be further described with reference to the following illustrative Examples. Example 1 1 liter of methyl acetate and 5 g of bis-4-t.butyl-cyclohexylperoxy-dicarbonate (Perkadox 16)® were charged, at 15°C, into a 3-1 autoclave equipped with a mechanical stirrer. Under an autogenous pressure there were introduced 50 g of hexafluoropropene. Subsequently, by means of a compressor, a gaseous mixture having the following molar composition: vinylidene fluoride (VDF) 80% hexafluoropropene (PFP) 20% was introduced from a gasholder, until a pressure of 5 atm was reached. By heating to 45°C, the pressure increased to 8 atm. When the pressure began to decrease, the feeding of the same gaseous mixture contained in the gasholder was resumed, in order to maintain a constant pressure of 8 atm. in the autoclave. After 20 minutes, 3 moles of monomers were consumed. Stirring was stopped, the autoclave was cooled down, the residual gases were discharged and the solution was collected. The copolymer was precipitated by pouring the solution into demineralized water, washing it with water and drying at 60°C under vacuum. The product exhibited an intrinsic viscosity of 0.25 dl/g in methylethylketone (MEK) at 30°C and had a composition corresponding to that of the monomeric mixture; the methyl acetate content in the product was lower than 0.3% by weight. Example 2 There was followed the procedure as described in Example 1, but with 60 g of hexafluoropropene, and introducing from the gasholder a mixture consisting of 90 mole % of VDF and 10 mole % of PFP, at a pressure of 13 atm.; after a 67-minute reaction, 4 moles of monomer were consumed. The copolymer, which was isolated as in Example 1, has an intrinsic viscosity of 0.39 dl/g in DMF at 30°C and a composition corresponding to 90 mole % of VDF and 10 mole % of PFP; the methyl acetate content in the product was lower than 0.2% by weight. Example 3 There was followed the procedure as described in Example 1, at 40°C, and using as a solvent 1 liter of t.butyl acetate instead of methyl acetate. After a 60-minute reaction, 4 moles of monomers were consumed. The copolymer, after isolation by precipitation in hexane, exhibited an intrinsic viscosity of about 0.6 dl/g in MEK at 30°C. Example 4 There was followed the procedure as described in Example 2, at 40°C, and using as a solvent 1 liter of methyl formate. After a 90-minute reaction, 4 moles of monomers were consumed. The copolymer had an intrinsic viscosity of 0.7 dl/g in DMF at 30°C. The solution was then poured into 10 liters of demineralized water and the resulting product was filtered and then stripped in 2 liters of water by means of steam. After cooling, the copolymer was filtered and dried in a ventilated oven at 50°C. The solvent content was lower than 0.1%. Example 5 300 cc of a solution prepared as in Example 3 were treated under stirring with 4 g of Tecnocin A, 10 g of lead oxide and 25 g of carbon black. The slurry so obtained was applied as a paint onto an iron surface. By heating to 100°C by means of hot air, solvent evaporation and cross-linking were simultaneously obtained. Example 6 There was followed the procedure as described in Example 1, but using 1 liter of ethyl acetate and 6 g of di-t.butyl peroxide. 40 g of perfluoropropene were introduced by means of an autogenous pressure. From a gasholder and by means of a compressor, a gaseous mixture having the following molar composition: VDF = 65%; PFP = 19%; TFE = 16%, was fed until a pressure of 8 atm was reached. There was carried out heating to 130°C and the pressure increased to 16 atm. The resulting copolymer exhibited an intrinsic viscosity of 0.15 dl/g and a composition corresponding to that of the monomeric mixture. Example 7 Into a 0.3-liter autoclave, equipped with a slide stirrer, there were introduced: methyl acetate 100 cc di-t.butylperoxide 0.25 cc vinylidene fluoride 11.8 g The autoclave content was brought, under stirring, to 130°C in an oil bath, where it was maintained during 30 minutes. During the reaction, the pressure reached a maximum value of 18 atmospheres. A polymeric solution was obtained, from which the polymer (polyvinylidene fluoride) was separated by evaporation of the solvent. In order to characterize the product, it was then dissolved again in methyl acetate, re-precipitated by pouring it into water, and dried at 70°C under vacuum. The amount of polymer obtained was 2.1 g. N.M.R. hydrogen analysis revealed that the average molecular weight of the polymer was 1700; the polymeric chain end groups were found to consist of: -CF₂-CH₂-OCO-CH₃; -CF₂-CH₂-CF₂H; -CH₂-CF₂-CH₃. The polymer melting temperature was from 143 to 145°C. The intrinsic viscosity determined by a Desreux-Bishoff viscosimeter in dimethylacetamide at 25°C was equal to 0.06 dl/g, corresponding to a viscosimetric molecular weight of 4,800 according to the G.J. Welch relation (Polymer, 15, page 429, 1974). Example 8 There was followed the procedure as described in Example 7, except that 0.7 g of Perkadox 16® (bis-4-t.butyl-cyclohexylperoxy-dicarbonate, produced by AKZO) were used as initiator, and that polymerization was conducted at 45°C during 60 minutes, obtaining 3.2 g of a polymer having an average viscosimetric molecular weight of 11,000. The average numerical molecular weight resulting from the data of N.M.R. analysis was about 3,500. Example 9 There was followed the procedure as described in Example 7, but at 150°C during 15 minutes, at a maximum starting pressure of 20 atmospheres. There were obtained 2.2 g of a polymer having a melting temperature of 131°C, an average numerical molecular weight determined by N.M.R. equal to 1,200 and an average viscosimetric molecular weight, determined as above, equal to 3,600. Example 10 There was followed the procedure as described in Example 8, but carrying out the polymerization reaction at 45°C during 90 minutes and by using 0.6 g of Perkadox 16 and 100 ml of ethyl acetate as solvent. There were obtained 2.8 g of a polymer having an average viscosimetric molecular weight, determined as above, equal to 4,000.

Claims: A process for preparing vinylidene fluoride homopolymers characterized by having an average viscosimetric molecular weight of from 1,000 to 20,000, and copolymers of vinylidene fluoride with hexafluoropropene and/or tetrafluoroethylene of a molecular weight corresponding to intrinsic viscosity values of from 0.02 to higher than 0.5 dl/g, characterized in that the polymerization reaction is carried out in the presence of a radical initiator at a temperature from 20° to 160°C, in a solvent selected from carboxylic acid esters of general formula: R - COO - R₁ in which R is a hydrogen atom or an alkyl radical group containing from 1 to 4 carbon atoms and R₁ is an alkyl radical containing from 1 to 5 carbon atoms, wherein R and/or R₁ may optionally contain, as substituents, an hydroxyl group OH or an alkoxy group OR₂ wherein R₂ is the same as R₁. A process as claimed in claim 1, characterized in that the radical initiator is selected from organic peroxide-type free-radical formers and azo catalysts. A process as claimed in claim 2, characterized in that the radical initiator is an azo-bis-alkylonitrile, preferably an azo-bis-isobutyronitrile. A process as claimed in any of claims 1 to 3, characterized in that the copolymer contains from 40 to 95 mole % of vinylidene fluoride, from 0 to 45 mole % of hexafluoropropene and from 0 to 30 mole % of tetrafluoroethylene. A process as claimed in claim 4, characterized in that the copolymer contains from 50 to 85 mole % of vinylidene fluoride, from 15 to 45 mole % of hexafluoropropene and from 0 to 25 mole % of tetrafluoroethylene. A process as claimed in claim 5, characterized in that a vulcanizing agent is directly added to the copolymer solution. A process as claimed in claim 6, characterised in that the copolymer is vulcanized directly in solution by heating and evaporating the solution. A process as claimed in any of claims 1 to 7, characterized in that the solvent/polymer weight ratio is from 4/1 to 10/1 at the end of the polymerization reaction. A process as claimed in any of claims 1 to 8, characterized in that the polymer is recovered as a solid by means of precipitation in water or in an organic liquid. A process as claimed in any of claims 1 to 9, for preparing vinylidene fluoride copolymers, characterized in that the solvent is an ester of formic acid, acetic acid or propionic acid with methyl alcohol, ethyl alcohol or t.butyl alcohol. A process as claimed in any of claims 1 to 10 for preparing copolymers of a molecular weight corresponding to intrinsic viscosity values of higher than 0.5 dl/g, characterized in that the reaction is conducted in the presence of a solvent selected from methyl formate, methyl acetate, t.butyl formate and t.butyl acetate, at a temperature from 20 to 50°C, with bis-(4-t.butylcyclohexyl)-peroxy-dicarbonate as a radical initiator. A process as claimed in any of claims 1 to 10 for preparing copolymers of a molecular weight corresponding to an intrinsic viscosity of about 0.15 dl/g, characterized in that the reaction is conducted in the presence of ethyl acetate as a solvent, at a temperature from 120 to 160°C, with t.butyl-peroxide as a radical initiator. A process as claimed in any of claims 1 to 9, characterized in that the homopolymerization of vinylidene fluoride is carried out in methyl acetate, ethyl acetate, methyl formate or ethyl formate, as a solvent. A process as claimed in claim 1 for preparing vinylidene fluoride homopolymers characterised by having viscosimetric molecular weights from 1,000 to 5,000, further characterized in that the reaction is carried out at a temperature from 130° to 150°C with an initiator which comprises di-t.butylperoxide in an amount from 1 to 4 g per 100 g of vinylidene fluoride, in the presence of methyl acetate in an amount 4 to 40 times the weight of the vinylidene fluoride. A vinylidene fluoride homopolymer, characterized by having an average viscosimetric molecular weight of from 1,000 to 5,000.

Inventors: BONARDELLI, PIERGIORGIO, CARATTO, ALESSANDRA, MOGGI, GIOVANNI, MONTI, CLAUDIO
Applicants: AUSIMONT S.P.A.
Application year: 1985
Main classification: C08F14
IPC codes: C08F2, C08F14