Abstract:
Heterogeneous copolymers of vinylidene fluoride and chlorotrifluoroethylene are obtained by copolymerization in suspension or emulsion in two stages. In a first stage the vinylidene fluoride and the chlorotrifluoroethylene are copolymerized, and in a second stage the polymerization of vinvylidene fluoride is continued after discontinuance of the introduction of chlorotrifluoroethylene.

Description:
This is a division of application Ser. No. 146,814 filed Jan. 22, 1988, now U.S. Pat. No. 4,851,479. 
    
    
     FIELD OF THE INVENTION 
     This invention is directed to a heterogeneous copolymer of vinylidene fluoride (VF 2 ) and chlorotrifluoroethylene (C 2  F 3  Cl) comprised of nodules of vinylidene fluoride elastomer and chlorotrifluoroethylene dispersed in a matrix formed by a quasi polymer of vinylidene fluoride The copolymer is obtained by copolymerization of VF 2  AND C 2  F 3  Cl so as to obtain an elastomer on which the VF 2  is polymerized. 
     BACKGROUND OF THE INVENTION 
     Poly vinylidene fluoride (PVDF) is a polymer known for its excellent properties, particularly mechanical firmness and firmness in respect to chemical agents. However, it has the disadvantage of lacking suppleness and hence, this disadvantage limits its use in applications where this property is required, such as in tubular materials that must be rolled up. 
     To overcome the disadvantage, it has been proposed to copolymerize the VF 2  with C 2  F 3  Cl, as, for example, has been described in Japanese Patent Application 51-149392/76. According to this patent, the copolymerization is carried out by uniformly adding the C 2  F 3  Cl to the reaction medium containing the entire amount of the VF 2 . In this type of copolymerization the speed at which the C 2  F 3  Cl is introduced to the VF 2 , taking into consideration the differences of speed of reaction between the two monomers, that of the VF 2  being slower than that of the C 2  F 3  Cl, is adjusted so as to obtain a homogeneous copolymer of the type (--CF 2  --CH 2  --CF 2  --CH 2  --CF 2  --CFCl--) n . 
     The homogeneous copolymers thus obtained have a high degree of flexibility, however, their melting point decreases as the number of --CF 2  --CFCl groups increases in the macromolecular chain. In addition, when 15% by weight of C 2  F 3  Cl in the copolymer is exceeded, elastomers are formed, which in homogeneous structure, are known as products which have weak resistance to rupture and to discharge threshold, but of high tensile strength. 
     SUMMARY OF THE INVENTION 
     A copolymer of vinylidene fluoride and chlorotrifluoroethylene is provided wherein the heterogeneous structure of the particles is comprised of nodules of a copolymeric homogeneous elastomer of vinylidene fluoride and chlorotrifluoroethylene dispersed in a matrix and chemically bonded thereto, the matrix being formed of a quasi-polymer of vinylidene fluoride. 
     The copolymer is obtained by copolymerization in suspension or emulsion in two stages, one of which involves the copolymerization of vinylidene fluoride and chlorotrifluoroethylene and the other the polymerization of vinylidene fluoride. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a micrograph (25,000 magnification) of a known homogeneous copolymer having a total weight of C 2  F 3  Cl of 8%; and 
     FIG. 2 is a micrograph (25,000 magnification) of a heterogeneous copolymer having a total weight of C 2  F 3  Cl of 8% and which has been prepared in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The copolymer according to the invention has the characteristic of possessing at the same time a good flexibility and a melting temperature approximate to that of the PVDF. As has already been indicated, the copolymer according to the invention is a copolymer of VF 2  and C 2  F 3  Cl wherein the heterogeneous structure of the particles is comprised of nodules of homogeneous copolymeric elastomer of VF 2  and C 2  F 3  Cl dispersed in a matrix formed by a quasipolymer of VF 2 . While being dispersed in the matrix, the nodules are closely bonded by the chemical bonds due to the copolymerization of the monomers. 
     The elastomer that constitutes the nodules is a homogeneous copolymer of the type (--CF 2  --CH 2  --CF 2  --CH 2  --CF 2  --CFCl--) n  wherein the --CF 2  --CFCl group of the C 2  F 3  Cl employed represents by weight from about 15 to about 70%, preferably from about 23 to about 45% of the elastomer. 
     This elastomer is dispersed in a matrix constituted by a quasi polymer of VF 2 . This matrix is called quasi polymer, for, as it is explained in the preparation process below, the matrix closely bonded to the nodule is constituted, at the interface bonding the quasi polymer to the nodule, of a copolymer of VF 2  and C 2  F 3  Cl of which the C 2  F 3  Cl diminishes as the surface of the particle is approached, the copolymer being progressively and rapidly replaced by almost pure PVDF. The presence of almost pure PVDF in the quasi polymer is due to the essentially total elimination in the reaction medium of C 2  F 3  Cl after preparing the elastomer that represents the nucleus. It is evidently recommended to have a minimum of C 2  F 3  Cl in this quasi polymer which must be ideally comprised of essentially all PVDF. 
     This quasi polymer of PVDF usually represents, by weight, from about 45% to about 90% of the final heterogeneous copolymer. The total amount of C 2  F 3  Cl in the heterogeneous copolymer preferably represents from about 5% to about 15% by weight. 
     The original structure of the copolymer can be confirmed by electronic microscopy. The confirmation method consists in revealing the elastomer nodules by a selective solvent, preferably methylethylketone. The copolymer, preferably in the form of granules for facilitating the manipulation, is coated by a resin cut in fine laminae on the order of less than 100 nanometers by a microtome. The laminae are soaked for about twelve hours in the solvent at room temperature. The solvent selectively dissolves the elastomer of VF 2  --C 2  F 3  Cl. 
     FIGS. 2 and 2 are micrographs (enlarged 25,000 times) corresponding respectively to a homogeneous copolymer of the prior art having a total 8% by weight of C 2  F 3  Cl and to a heterogeneous copolymer according to the invention having a structure of the products. According to FIG. 2, it is observed that there exists nodules coated with polymer represented by holes of a dimension from about 0.1 --about 0.4 micron corresponding to the place occupied by the elastomer prior to its dissolution. On the other hand, FIG. 1 shows only a perfectly homogeneous film. Nuclear magnetic resonance (NMR), analysis of the dry extract of the selective solvent, indicated that it is really an elastomer of VF 2  -C 2  F 3  Cl containing most of the --CF 2  --CFCl-- groups of the whole of the copolymer. 
     When the melting temperature of the PVDF, measured by differential thermal analysis, is on the order of 160-169° C., the melting temperature of the heterogeneous copolymers according to the invention is now on the order of from 160-169° C., that is, within limits approximate to that of the PVDF while possessing a flexion module that can be lowered to less than 50% of that of the PVDF. 
     By way of comparison between a homogeneous copolymer and a copolymer according to the invention, the respective melting points are 153 and 168° C. for a total content by weight of C 2  F 3  Cl of 8% and 144 and 164° C. for a total content by weight of C 2  F 3  Cl of 12%. 
     The copolymer according to the present invention can be obtained by copolymerization in emulsion or suspension, C 2  F 3  Cl and VF 2 , the principle here being the following: in a first stage there is copolymerized in the VF 2  the C 2  F 3  Cl that is progressively introduced in the medium in a proportion sufficient for obtaining a homogeneous elastomer, then, in a second stage, the polymerization reaction of the VF 2  is continued, stopping the supply of C 2  F 3  Cl and eventually adding VF 2  if the whole of the VF 2  used has not been initially introduced in the reactor. The essence of the copolymerization process is that in the stage of preparation of the elastomer, the proportion by weight of C 2  F 3  Cl used in relation to the copolymerizable VF 2  should be at least equal to 15% and at most preferably on the order of 70%. 
     Even though for the sake of convenience, it may be preferable to form the nodules of elastomer of VF 2  --C 2  F 3  Cl prior to the PVDF matrix, it is not ruled out to polymerize in a first stage the VF 2 , then, at a chosen degree of homopolymerization, to introduce for the second stage the C 2  F 3  Cl to form the nodule of elastomer. 
     In the copolymerization of the C 2  F 3  Cl and the VF 2 , the ratio of relative reactivity of each monomer is quite different and the consumption of C 2  F 3  Cl has been found to be much more rapid than that of VF 2 . That is why in order to obtain in this stage of copolymerization a homogeneous compound, it is indispensable to introduce the C 2  F 3  Cl progressively into the reaction medium. 
     This progressive introduction can be effected by any means known either continuously or discontinuously by programmed successive additions as a function of the reactivity speed of the reaction medium and so as to preserve the homogeneity of said medium. It is recommended to form particles of elastomer of from 0.05 to 0.5 micron and more preferably from 0.1 to 0.2 micron. 
     The two stages of the process are described below in more detail: 
     In the usual conditions of the polymerization in emulsion of the VF 2 , the work is carried out in a rector, while stirring, in aqueous medium containing an emulsifier selected among the customary emulsifiers. The polymerization reaction is started by an initiator of hydrosoluble free radicals usually selected from among the alkaline persulfates. The reaction temperature is generally comprised between about 60 and about 100° C. for pressures of from about 60 to about 100 bars. The VF 2  can be initially all introduced into the reactor, or only a portion of the charge can be introduced and the remaining VF 2  being added progressively in the course of polymerization in order to maintain constant the pressure in the reactor. In the copolymerization stage the C 2  F 3  Cl is progressively introduced taking into consideration the difference of reaction speed of the VF 2 . It is possible simultaneously to introduce the VF 2  and the C 2  F 3  Cl, but in this case the delivery of C 2  F 3  Cl has to be regulated in a manner such that the formation of C 2  F 3  Cl patterns be controlled in relation to the formation of VF 2  patterns. In this copolymerization stage, it is not essential to prepare a strictly homogeneous elastomeric copolymer, but it is necessary that in its composition the content by weight of C 2  F 3  Cl be at least 15%. 
     In the copolymerization stage, there is formed from 10 to 55% by weight of the total of the product formed in the two stages. The copolymerization stage is deemed terminated after the introduction of C 2  F 3  Cl is discontinued. 
     In the preferred case where the first stage consists in copolymerizing the VF 2  and C 2  F 3  Cl for forming the nodules of elastomers, there is produced in the second stage, after discontinuing the introduction of C 2  F 3  Cl, a progressive exhaustion of the amount of C 2  F 3  Cl. The effect of this exhaustion is to coat the particles of latex of elastomer with layers of quasi polymer of VF 2  increasingly poor in C 2  F 3  Cl, the final layers being essentially the homopolymer of VF 2 . The conditions of polymerization of the VF 2  in the stage of formation of the quasi polymer may be different from those of the stage of formation of the elastomer insofar as these conditions correspond to the hypercritical state of the VF 2 . This state of formation of the quasi polymer is customarily carried out at a temperature comprised between 50-90° under a pressure of from 50 to 90 bars. 
     As in the classical polymerizations, the molecular weights of the products of each one of the stages are adjusted by means of transfer agents such as ketone, esters with from C 3  to C 8 , chlorinated or brominated derivatives and others. 
     The latex of heterogeneous copolymer finally obtained is recovered and transformed under the conditions customary for polymerizations in emulsion. 
     The heterogeneous polymer according to the invention can also be obtained by the process of polymerization in suspension. 
     Under the usual conditions of the polymerization in suspension, the work is carried out in a reactor, while stirring, in aqueous medium in the presence of a protective colloid and an organosoluble starting agent or initiator. 
     The starting agents most commonly used are the alkyl percarbonates with from C 2  to C 6  and the tertiary butyl perpivalate. 
     The protective colloid is chosen among those known and used in polymerization of this kind. By way of example there are the polyvinyl alcohols, the methyl celluloses, the hydroxypropyl celluloses and the like. 
     The polymerization temperature is comprised between about 30 and about 90° C. and the pressure between 50 and 100 bars. 
     Although all of the VF 2  can be introduced initially in the reactor, it may be preferable to introduce initially only part of the charge, the remaining VF 2  being added progressively in the course of polymerization so as to maintain a constant pressure within the reactor. In the copolymerization stage the C 2  F 3  Cl is progressively introduced, considering the difference of speed of reaction with the VF 2 . It is possible simultaneously to introduce the VF 2  and the C 2  F 3  Cl, but in this case the delivery of C 2  F 3  Cl has to be regulated in a manner such that the formation of C 2  F 3  Cl patterns be controlled in relation to the formation of VF 2  patterns. In this copolymerization stage, it is not indispensable to form a strictly homogeneous elastomeric copolymer, but it is necessary that in the composition thereof the content by weight of C 2  F 3  Cl be at least 15%. 
     In the copolymerization stage there is formed from 10 to 55% by weight of the total copolymer formed in the two stages. 
     The copolymerization stage is deemed terminated after the introduction of C 2  F 3  Cl is halted. 
     The polymerization conditions of the VF 2  in the stage of formation of the quasi polymer can be different from those of the stage of formation of the elastomer insofar as these conditions correspond to the hypercritical state of the VF 2 . Usually this stage of formation of the quasi polymer is carried out at a temperature comprised between 50-90° C. under a pressure of from 50 to 90 bars. 
     As in the classical polymerizations, the molecular weights of the products of each one of the stages are adjusted by means of a transfer agent such as ketone, an ester with from C 3  to C 8 , chlorinated or brominated derivatives and others. After polymerization, the polymer obtained in suspension is filtered, dried and granulated. 
     According to the above descriptions, the process of preparation of heterogeneous copolymer of VF 2  and C 2  F 3  Cl in emulsion or suspension is characterized in that the VF 2  and the C 2  F 3  Cl are copolymerized in one stage by progressively introducing the latter in the reaction medium so as to form a homogeneous elastomer and in another stage a matrix of PVDF is formed by polymerizing in situ VF 2 , the copolymerization stage being preferably the first. 
     The thermoplastic polymeric products obtained according to these processes contain, based on the monomer, from 95 to 85% by weight of VF 2  wherein from 6 to 50 % of this monomer VF 2  is contained in the elastomer VF 2  --C 2  F 3  Cl, the rest forming the matrix of quasi polymer. 
     The examples that follow illustrate the invention without limiting it. 
     EXAMPLE 1 (Comparative) 
     In a 60 liter autoclave provided with a stirrer and a heating system, there are introduced 40 1 of de-ionized water, 50 g emulsifier C 6  F 13  C 2  H 4  SO 3  K, 4 g sodium pyrophosphate, 4 g potassium persulfate and 300 g paraffin with a melting point of from 54-56° C. The temperature is regulated at 80° C. and the reactor is placed under vacuum of 700 mm Hg. There are then introduced 50 g acetone, 450 g C 2  F 3  Cl and there is allowed a VF 2  pressure of 90 bars. The stirring is started and the polymerization immediately lowers the pressure When the decrease of pressure reaches 3 bars, the pressure is increased to 90 bars by addition of VF 2 . There is then introduced in the reaction medium 0.090 g C 2  F 3  Cl per gram of VF 2  that has been introduced to cause the pressure to rise from 87  to 90 bars The VF 2  is introduced at a speed of 3 kg/h. After having introduced 6 kg of VF 2 , there are added 400 g acetone and 1 g potassium persulfate After having introduced 18 kg of VF 2  and 1,620 kg of C 2  F 3  Cl, the reactor is degasified. There is obtained a latex with 37% dry extract, of homogeneous composition by microscopy, with a weight rate of 8% of C 2  F 3  Cl measured by RMN analysis of the extraction with methylethyl ketone. 
     The results of the measurements made on this product are given in the table following Example 5. 
     Example 2 
     In a 60 liter autoclave, there are introduced 40 1 de-ionized water, 40 g emulsifier of the formula C 6  F 13  C 2  H 4  SO 3  K, 4 g sodium pyrophosphate, 4 g persulfate and 500 g paraffin having a melting point between 54 and 56° C. The temperature is adjusted at 80° C. and the reactor is placed under vacuum of 700 mm mercury. 
     There are then introduced 50 g acetone, 500 g C 2  F 3  Cl and there is allowed a VF 2  pressure of 90 bars. 
     The start of the stirring marks the beginning of the polymerization and causes the pressure to lower. When the pressure lowers to 3 bars, VF 2  is introduced in a uniform manner to raise the pressure and keep it at 90 bars. 
     There is then introduced the C 2  F 3  Cl at a rate of 0.3 g C 2  F 3  Cl per gram of VF 2  introduced to raise the pressure and keep it at 90 bars in the reactor. After an addition of 6 kg VF 2  for raising the pressure and keeping it at 90 bars and of 1.8 kg C 2  F 3  Cl, the supply of C 2  F 3  Cl is discontinued and there are added 400 g acetone and 1 g potassium persulfate, then 12 kg VF 2  at a speed that allows keeping the pressure at 90 bars. The speed at which the VF 2  is introduced is on the order of 3 kg/h during and after the addition of the C 2  F 3  Cl. The reactor is degasified, and there is obtained a latex having 36% dry extract, of heterogeneous composition by microscopy, containing a total amount of 8% by weight of C 2  F 3  Cl. The copolymer obtained contains  35% by weight of nodules which themselves contain about 22.8% by weight of patterns originating from the C 2  F 3  Cl measured by RMN analysis of the extraction with methylethyl ketone. 
     The results of the measurements made on this product are given in the table following Example 5. 
     Example 3 
     In a 60 liter autoclave, there are introduced 40 1 de-ionized water, 40 g emulsifier of the formula C 6  F 13  C 2  H 4  SO 3  K, 4 g sodium pyrophosphate, 4 g persulfate and 500 g paraffin having a melting point between 54 and 56° C. The temperature is adjusted at 80° C. and the reactor is placed under vacuum of 700 mm mercury. There are then introduced 50 g acetone, 1100 g C 2  F 3  Cl and there is allowed a VF 2  pressure of 90 bars. The start of the stirring marks the beginning of the polymerization and causes the pressure to lower. When the pressure lowers to 3 bars, VF 2  is introduced uniformly to raise the pressure and keep it at 90 bars. The C 2  F 3  Cl is then introduced at the rate of 0.44 g C 2  F 3  Cl per gram of VF 2  that has been introduced to raise the pressure and keep it at 90 bars in the reactor. After having added 4.10 kg of VF 2  to raise the pressure and keep it at 90 bars in the reactor and 1.84 kg of C 2  F 3  Cl, the delivery of C 2  F 3  Cl is discontinued, and there are added 400 g acetone, 1 g potassium persulfate and then 8.4 kg VF 2  at a speed allowing the pressure to be kept at 90 bars. The speed at which the VF 2  is introduced is on the order of 3kg/h while C 2  F 3  Cl is being added and thereafter. The reactor is degasified and there is recovered a latex having 37% of dry extract, of heterogenous composition by microscopy and containing a total amount of 14% by weight C 2  F 3  Cl. The copolymer obtained contains 45% by weight of nodules which themselves contain about 31% by weight of patterns originating from C 2  F 3  Cl measured by RMN analysis of the extraction with methylethyl ketone. 
     The results of the measurements made on this product are given in the table following Example 5. 
     EXAMPLE 4 
     In 60 a liter autoclave, there are introduced 40 1 de-ionized water, 40 g emulsifier of the formula C 6  F 13  C H 2  SO 4  K, 4 g sodium pyrophosphate, 4 g persulfate and 500 g paraffin having a melting point between 54 and 56° C. The temperature is adjusted at 80° C. and the reactor is placed under vacuum of 700 mm of mercury. There are then introduced 50 g acetone, 600 g C 2  F 3  Cl and a VF 2  pressure of 90 bars is allowed. 
     The start of the stirring marks the beginning of the polymerization and causes the pressure to lower. Then the pressure lowers to 3 bars, VF 2  is uniformly introduced to raise the pressure and keep it at 90 bars. There is then introduced the C 2  F 3  Cl at a rate of 0.3 g C 2  F 3  Cl per gram of VF 2  that has been introduced to raise the pressure and keep it at 90 bars in the reactor. After having added 5.46 kg VF 2  for raising the pressure and keeping it at 90 bars in the reactor and 1.64 kg C 2  F 3  Cl, the delivery of C 2  F 3  Cl is discontinued and there are added 400 g acetone and 1 g potassium persulfate, then 6.82 kg VF 2  at a speed that allows keeping the pressure at 90 bars. The speed at which VF 2  is introduced is on the order of 3 kg/h while the C 2  F 3  Cl is being added and thereafter. The reactor is degasified and there is recovered a latex having 33% of dry extract of heterogeneous composition by microscopy containing a total amount of 12% by weight C 2  F 3  Cl. The copolymer obtained contains 53% by weight of nodules that themselves contain about 22.6% by weight of patterns originating from the C 2  F 3  Cl measured by RMN analysis of the extraction with methylethyl ketone. 
     The results of the measurements made on this product are given in the table following Example 5. 
     EXAMPLE 5 
     In a 60 liter autoclave, there are introduced 40 1 de-ionized water, 40 g emulsifier of the formula C 6  F 13  CH 2  SO 4  K, 4 g sodium pyrophosphate, 4 g persulfate and 500 g paraffin having a melting point between 54 and 56° C. The temperature is adjusted at 80° C. and the reactor is placed under vacuum of 700 mm of mercury. There are then introduced 50 g acetone, 600 g C 2  F 3  Cl and a VF 2  pressure of 90 bars is allowed. 
     The start of the stirring marks the beginning of the polymerization and causes the pressure to lower. When the pressure lowers to 3 bars, VF 2  is uniformly introduced for maintaining the pressure at 90 bars. 
     There is then introduced the C 2  F 3  Cl at a rate of 0.3 g C 2  F 3  Cl per gram of VF 2  that has been introduced for raising the pressure and keeping it at 90 bars in the reactor. After having added 2.74 kg VF 2  for raising the pressure and keeping it at 90 bars in the reactor and 0.82 kg C 2  F 3  Cl, the delivery of C 2  F 3  Cl is discontinued and there are added 400 g acetone, 1 g potassium persulfate, then 8.72 kg VF 2  at a speed that allows keeping the pressure at 90 bars. The speed at which the VF 2  is introduced is on the order of 3 kg/h while the C 2  F 3  Cl is being added and thereafter, the reactor is degasified and there is recovered a dry extract of heterogeneous composition by microscopy containing a total amount of 7.4% by weight of C 2  F 3  Cl. The copolymer contains 32% by weight of nodules that themselves contain about 23% by weight of patterns originating from C 2  F 3  Cl measured by RMN analysis of the extraction with methylethyl ketone. 
     The results of the measurements made on this product are given in the table that follows: 
     
         ______________________________________       Melting Temperature                      MPa       by differential                      Module of ElasticityRef.        thermal analysis                      under flexion______________________________________Example 1   153            800Example 2   168            805Example 3   163            414Example 4   164            530Example 5   168            829Pure PVDF   169            1700(Foraflon 1000 HD)______________________________________