Process for producing fluorinated copolymer, fluorinated copolymer, crosslinkable composition containing same and sealant

Vinylidene fluoride and chlorotrifluoroethylene are copolymerized in the presence of a compound represented by the general formula I: EQU I.sub.n Br.sub.m R (I) wherein R represents a fluorohydrocarbon group, a chlorofluorohydrocarbon group, a chlorohydrocarbon group or a hydrocarbon group, and n is 0, 1 or 2 and m is 0, 1 or 2 provided that n+m.gtoreq.2. This provides a process for producing a fluorinated copolymer which enables providing a fluorinated copolymer crosslinking product having mechanical properties satisfactory for practical use as sealants even if no inorganic additive is added thereto.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a process for producing a fluorinated
 copolymer and a fluorinated copolymer, and more particularly relates to a
 process for producing a fluorinated copolymer which enables providing a
 crosslinking product having excellent mechanical properties even if no
 inorganic additive is added thereto. Furthermore, the present invention
 relates to a crosslinkable composition containing the fluorinated
 copolymer and a sealant obtained by crosslinking the crosslinkable
 composition.
 2. Prior Art
 Fluorinated copolymers are polymeric materials having excellent heat and
 chemical resistances. In particular, fluorinated copolymer elastomers are
 used as materials for forming sealants such as O-rings, packings oil seals
 and gaskets and hoses which are required to possess heat and oil
 resistances.
 However, the mixing of inorganic fillers such as carbon black and silica
 into the fluorinated copolymer elastomer has been inevitable for imparting
 mechanical properties such as hardness and strength and compression set
 resistance characteristics that are satisfactory for practical use as
 sealants to vulcanizates of fluorinated copolymer elastomers.
 On the other hand, with respect to the sealants for use in the
 semiconductor industry, medical materials, food industry and the like
 among those required to possess heat and chemical resistances, the mixing
 of various inorganic additives such as an inorganic filler, an acid
 receptive agent, a vulcanization accelerator and a colorant thereinto is
 not desirable from the viewpoint that it is not desirable to contaminate
 products and product materials brought into contact with the sealants.
 For coping with this dilemma, a process for producing a fluorinated resin
 which does not require crosslinking, can be easily molded and is flexible
 has been proposed as disclosed in Japanese Patent Laid-open Publication
 No. 58(1983)-206615. The "flexible fluorinated resin" described in this
 published specification comprises fluorinated elastomeric segments and
 fluorinated crystalline segments and provides molding with satisfactory
 mechanical properties even if no inorganic filler is added thereto.
 However, this fluorinated resin is thermoplastic and therefore has a
 drawback in that it cannot be employed at temperatures over the melting
 point of the crystalline segments (155 to 160.degree. C.). Further, with
 respect to the particular process, the production of the fluorinated resin
 requires a plurality of burdensome steps such as emulsion polymerization,
 salting out/water washing, drying, solvent washing, drying, graft
 (solution) polymerization and solvent separation/drying. Thus, this
 process can hardly be stated as a desirable one from the viewpoint of
 cost.
 Moreover, fluorinated thermoplastic elastomers having desirable mechanical
 properties are disclosed in Japanese Patent Laid-open Publication No.
 53(1978)-3495 and Japanese Patent Publication No. 61(1986)-49327. However,
 the compression set resistance thereof which is important in the use as
 sealants is not on a satisfactory level at all. Further, these fluorinated
 thermoplastic elastomers are thermoplastic and therefore have a drawback
 in that the use temperature thereof is restricted by the melting point of
 the crystalline segments thereof.
 On the other hand, for example, Japanese Patent Publication Nos.
 2(1990)-36365, 5(1993)-18329 and 1(1989) -56659 and Japanese Patent
 Laid-open Publication No. 59(1984)-40066 describe that these fluorinated
 thermoplastic elastomers can be crosslinked by radiation and peroxides to
 thereby enable enhancing the compression set and crushing resistances
 although the recyclability of the fluorinated thermoplastic elastomers is
 sacrificed. However, without exception, the production of fluorinated
 thermoplastic elastomers as base materials is performed by multi-stage
 block polymerization, so that a cost increase as compared with the common
 one-stage polymerization is inevitable.
 In Japanese Patent Laid-open Publication No. 7(1995)-11086, the applicant
 disclosed a fluorinated elastomer composition comprising a copolymer
 obtained by copolymerizing ethylidene fluoride and chlorotrifluoroethylene
 and a salicylaldimino copper complex as a crosslinking agent. Although
 this composition has realized desirable vulcanization rate and vulcanizate
 properties, it is essential for this composition to contain a metal
 complex as a crosslinking agent and to contain a divalent metal oxide or
 hydroxide as an acid receptive agent with the result that there has been a
 difficulty in the application thereof to the above sealants for use in the
 semiconductor industry, medical materials, food industry and the like, in
 which the addition of inorganic additives is not suitable. Further, the
 thus obtained fluorinated elastomer composition is not satisfactory in
 compression set, so that the application thereof to sealants, especially
 O-rings, cannot necessarily be stated as being satisfactory.
 With respect to the production of vinylidene
 fluoride/chlorotrifluoroethylene copolymers, U.S. Pat. No. 2,752,332
 includes Examples in which vinylidene fluoride and chlorotrifluoroethylene
 are charged together in a molar ratio of 20/80 to 30/70 and suspension
 polymerized at low temperature, for example, -20.degree. C. for 18 hr in
 the presence of a redox initiator composed of a persulfate salt, acid
 sodium sulfite and a polyvalent metal salt to thereby obtain copolymers
 whose component ratio (vinylidene fluoride/chlorotrifluoroethylene ratio)
 is in the range of 24/76 to 25/75 with a yield of 14 to 27% by weight.
 Also, the mechanical properties of copolymer whose vinylidene
 fluoride/chlorotrifluoroethylene ratio is 25/75 among the above copolymers
 are evaluated in the '332 patent. The elongation at break thereof is
 extremely low to thereby render the application to sealants difficult.
 Further, U.S. Pat. No. 2,770,606 discloses Examples in which vinylidene
 fluoride and chlorotrifluoroethylene are charged together into a reactor
 in a molar ratio of 50/50 to 75/25 and suspension polymerized at 25 to
 35.degree. C. in the presence of an initiator composed only of a
 persulfate salt or composed of a persulfate salt, acid sodium sulfite and
 a polyvalent metal salt to thereby obtain vinylidene
 fluoride/chlorotrifluoroethylene copolymers. In particular, Example 4 of
 the '606 patent describes that a rubbery copolymer whose vinylidene
 fluoride/chlorotrifluoroethylene ratio is 75/25 is obtained at a
 conversion of 70% by carrying out the reaction at 25 to 35.degree. C. for
 21 hr in the presence of potassium persulfate as an initiator. However, as
 a result of the follow-up test by the inventors, it was found that the
 polymerization reaction minimally proceeded at reaction temperature of 25
 to 35.degree. C. in the absence of a reducing agent. In the '606 patent,
 it is only described that the obtained various copolymers are available in
 the formation of coating solutions, and there is no description as to the
 properties which are important in the use as sealants, for example,
 physical properties such as glass transition temperature and melting point
 and mechanical properties of moldings.
 Still further, U.S. Pat. No. 2,738,343 includes Comparative Examples in
 which rubbery fluorinated copolymers whose respective vinylidene
 fluoride/chlorotrifluoroethylene ratios are 34.4/65.6 and 56.5/43.5 are
 obtained. However, there is no description as to the elastomeric
 properties of these copolymers, and the polymerization reaction without
 exception requires a prolonged period of time, for example, 24 to 168 hr.
 Therefore, it can hardly be stated that a practical productive process is
 disclosed.
 The inventors have conducted extensive and intensive studies with a view
 toward resolving these problems. As a result, it has been found that the
 copolymer obtained by copolymerizing vinylidene fluoride and
 chlorotrifluoroethylene in the presence of a specified compound of the
 formula I.sub.n Br.sub.m R has excellent mechanical properties and is
 advantageous in that the production thereof can be effected without the
 need to conduct complex operations such as multi-stage block
 polymerization which must be carried out in the production of the above
 fluorinated copolymers.
 The present invention has been made with a view toward solving the above
 problems. An object of the present invention is to provide a process for
 producing a fluorinated copolymer which can impart mechanical properties
 satisfactory for practical use as sealants even if no inorganic additive
 is added thereto. Further, other objects of the present invention are to
 provide a crosslinkable composition enabling crosslinking of this
 fluorinated copolymer and a crosslinked material obtained from the
 crosslinkable composition.
 SUMMARY OF THE INVENTION
 The process for producing a fluorinated copolymer according to the present
 invention, intended to overcome the above problems, comprises
 copolymerizing vinylidene fluoride and chlorotrifluoroethylene in the
 presence of a compound represented by the general formula I:
EQU I.sub.n Br.sub.m R (I)
 wherein R represents a fluorohydrocarbon group, a chlorofluorohydrocarbon
 group, a chlorohydrocarbon group or a hydrocarbon group, and n is 0, 1 or
 2 and m is 0, 1 or 2 provided that (n+m).gtoreq.2.
 The fluorinated copolymer of the present invention is produced by the above
 process, which is rubbery at room temperature and has a melting point.
 The crosslinkable composition of the present invention comprises the above
 fluorinated copolymer and a peroxide crosslinking agent.
 The sealant of the present invention comprises a crosslinked material
 produced by crosslinking of the above crosslinkable composition.
 DETAILED DESCRIPTION OF THE INVENTION
 The process for producing a fluorinated copolymer, fluorinated copolymer
 produced by the process, crosslinkable composition comprising the
 fluorinated copolymer and sealant produced from the crosslinkable
 composition, according to the present invention, will be described below.
 In the process of the present invention, vinylidene fluoride and
 chlorotrifluoroethylene; together with another copolymerizable monomer
 according to necessity, are copolymerized in the presence of a compound
 represented by the general formula I.sub.n Br.sub.m R (wherein R
 represents a fluorohydrocarbon group, a chlorofluorohydrocarbon group, a
 chlorohydrocarbon group or a hydrocarbon group, and n is 0, 1 or 2 and m
 is 0, 1 or 2 provided that n+m.gtoreq.2). The thus obtained fluorinated
 copolymer has a melting point and is rubbery at room temperature.
 With respect to the composition of the thus obtained fluorinated copolymer,
 the molar ratio of vinylidene fluoride/chlorotrifluoroethylene is in the
 range of 31/69 to 85/15, preferably 40/60 to 80/20, and more preferably
 60/40 to 75/25. When the molar ratio is in these ranges, the obtained
 fluorinated copolymer has a desirable balance of chemical resistance and
 rubber elasticity.
 For imparting desirable properties to the fluorinated copolymer, a
 copolymerizable monomer, such as tetrafluoroethylene, hexafluoropropene,
 perfluoro(methyl vinyl ether), perfluoro(propyl vinyl ether), vinyl
 fluoride, ethylene or propylene, can be copolymerized with vinylidene
 fluoride and chlorotrifluoroethylene in an amount such that the objective
 of the present invention is not departed from.
 Although the copolymerization reaction can be effected by any of the common
 polymerization methods such as the emulsion, suspension, solution and bulk
 polymerization methods, it is preferred to employ the emulsion
 polymerization method from the viewpoint of an increase of polymerization
 degree and a reduction of cost.
 In the emulsion polymerization of a fluorinated monomer, a water soluble
 inorganic peroxide such as ammonium persulfate is generally used as an
 initiator. In the present invention, it is preferred to use I.sub.n
 Br.sub.m R in combination with this water soluble inorganic peroxide such
 as ammonium persulfate in the copolymerization reaction. Further, use is
 made of an emulsifier such as ammonium perfluorooctanoate, ammonium
 perfluoropentanoate, ammonium perfluorononanoate or a mixture thereof. Of
 these, ammonium perfluorooctanoate is preferred. In the present invention,
 the use of the above water soluble inorganic peroxide such as ammonium
 persulfate in combination with reducing agents customarily employed in the
 emulsion polymerization is not favorable because there is the danger of
 inhibiting the action of the I.sub.n Br.sub.m R.
 The polymerization reaction is performed under conditions such that the
 polymerization pressure is in the range of 0 to 10 MPa.cndot.G, preferably
 0 to 5 MPa.cndot.G, and such that the polymerization temperature is in the
 range of 0 to 90.degree. C., preferably 50 to 80.degree. C. In that
 polymerization, an electrolyte substance having buffering activity, such
 as NaHPO.sub.4, NaH.sub.2 PO.sub.4 or KH.sub.2 PO.sub.4, may be added so
 as to regulate the pH value of the polymerization system.
 With respect to the supply of monomers to a reactor in the copolymerization
 reaction, it is preferred that whole amounts of the I.sub.n Br.sub.m R
 compound described below, vinylidene fluoride and chlorotrifluoroethylene,
 together with the above other copolymerizable monomer used according to
 necessity, be charged together in the reactor before the initiation of
 polymerization. The thus obtained fluorinated copolymer is rubbery at room
 temperature and has a melting point.
 As apparent from the above, the process for producing a fluorinated
 copolymer according to the present invention is performed in the presence
 of a compound represented by the general formula I.sub.n Br.sub.m R which
 has a chain transfer activity.
 In the general formula, R represents a hydrocarbon group, a
 fluorohydrocarbon group, a chlorohydrocarbon group or a
 chlorofluorohydrocarbon group.
 The hydrocarbon group is, for example, selected from among saturated
 aliphatic hydrocarbon groups each having 2 to 10 carbon atoms, such as
 ethyl, propyl, butyl, pentyl, hexyl and octyl; unsaturated aliphatic
 hydrocarbon groups each having 2 to 10 carbon atoms, such as vinyl,
 propenyl, butenyl and pentenyl; saturated or unsaturated alicyclic
 hydrocarbon groups each having 4 to 10 carbon atoms, such as cyclobutyl
 and cyclohexyl; aromatic hydrocarbon groups such as phenyl; and
 substituted hydrocarbon groups obtained by substituting these hydrocarbon
 groups with, for example, an aryl group such as phenyl or an alkoxy group
 such as methoxy, ethoxy, propoxy, allyloxy or vinyloxy.
 The fluorohydrocarbon group is, for example, selected from among groups
 obtained by partly or entirely substituting a hydrogen atom of the above
 hydrocarbon groups with fluorine.
 The chlorohydrocarbon group is, for example, selected from among groups
 obtained by partly or entirely substituting a hydrogen atom of the above
 hydrocarbon groups with chlorine.
 The chlorofluorohydrocarbon group is, for example, selected from among
 groups obtained by partly or entirely substituting a hydrogen atom of the
 above hydrocarbon groups with fluorine and chlorine.
 In the general formula I, is 0, 1 or 2 and m is 0, 1 or 2 provided that
 (n+m).gtoreq.2.
 Hereinafter, the compounds of the general formula wherein both of n and m
 are not 0 are referred to as "iodobrominated compounds". The compounds of
 the general formula wherein n is not 0 and m is 0 are referred to as
 "iodinated compounds". The compounds of the general formula wherein n is 0
 and m is not 0 are referred to as "brominated compounds".
 The iodobrominated compounds may have a chain structure or cyclic structure
 and may be aromatic.
 Examples of the iodobrominated compounds having a chain structure or cyclic
 structure include 1-bromo-2-iodoperfluoroethane,
 1-bromo-3-iodoperfluoropropane, 1-bromo-4-iodoperfluorobutane,
 2-bromo-3-iodoperfluorobutane, 1-bromo-2-iodoperfluoro(2-methylpropane),
 monobromomonoiodoperfluorocyclobutane, monobromomonoiodoperfluoropentane,
 monobromomonoiodoperfluoro-n-octane,
 monobromomonoiodoperfluorocyclohexane,
 1-bromo-1-iodo-2-chloroperfluoroethane,
 1-bromo-2-iodo-2-chloroperfluoroethane,
 1-iodo-2-bromo-2-chloroperfluoroethane, 1,1-dibromo-2-iodoperfluoroethane,
 1,2-dibromo-2-iodoperfluoroethane, 1,2-diiodo-2-bromoperfluoroethane,
 1-bromo-2-iodo-1,2,2-trifluoroethane,
 1-iodo-2-bromo-1,2,2-trifluoroethane, 1-bromo-2-iodo-1,1-difluoroethane,
 1-iodo-2-bromo-1,1-difluoroethane, 1-bromo-2-iodo-1-fluoroethane,
 1-iodo-2-bromo-1-fluoroethane,
 1-bromo-2-iodo-1,1,3,3,3-pentafluoropropane,
 1-iodo-2-bromo-1,1,3,3,3-pentafluoropropane,
 1-bromo-2-iodo-3,3,4,4,4-pentafluorobutane,
 1-iodo-2-bromo-3,3,4,4,4-pentafluorobutane,
 1,4-dibromo-2-iodoperfluorobutane, 2,4-dibromo-1-iodoperfluorobutane,
 1,4-diiodo-2-bromoperfluorobutane,
 1,4-dibromo-2-iodo-3,3,4,4-tetraflucrobutane,
 1,4-diiodo-2-bromo-3,3,4,4-tetrafluorobutane,
 1,1-dibromo-2,4-diiodoperfluorobutane, 1-bromo-2-iodo-1-chloroethane,
 1-iodo-2-bromo-1-chloroethane, 1-bromo-2-iodo-2-chloroethane,
 1-bromo-2-iodo-1,1-dichloroethane, 1,3-dibromo-2-iodoperfluoropropane,
 2,3-dibromo-2-iodoperfluoropropane, 1,3-diiodo-2-bromoperfluoropropane,
 1-bromo-2-iodoethane, 1-bromo-2-iodopropane, 1-iodo-2-bromopropane,
 1-bromo-2-iodobutane, 1-iodo-2-bromobutane,
 1-brcmo-2-iodo-2-trifluoromethyl-3,3,3-trifluoropropane,
 1-iodo-2-bromo-2-trifluoromethyl-3,3,3-trifluoropropane,
 1-bromo-2-iodo-2-phenylperfluoroethane,
 1-iodo-2-bromo-2-phenylperfluoroethane,
 3-bromo-4-iodoperfluorobutene-1,3-iodo-4-bromoperfluorobutene-1,1-bromo-4-
 iodoperfluorobutene-1,1-iodo-4-bromoperfluorobutene-1,3-bromo-4-iodo-3,4,4-
 trifluorobutene-1,4-bromo-3-iodo-3,4,4-trifluorobutene-1,3-bromo-4-iodo-1,1
 ,2-trifluorobutene-1,4-bromo-5-iodoperfluoropentene-1,4-iodo-5-bromoperfluo
 ropentene-1,4-bromo-5-iodo-1,1,2-trifluoropentene-1,4-iodo-5-bromo-1,2,2-tr
 ifluoropentene-1,1-bromo-2-iodoperfluoroethyl perfluoromethyl ether,
 1-bromo-2-iodoperfluoroethyl perfluoroethyl ether,
 1-bromo-2-iodoperfluoroethyl perfluoropropyl ether,
 2-bromo-3-iodoperfluoropropyl perfluorovinyl ether,
 1-bromo-2-iodoperfluoroethyl perfluorovinyl ether,
 1-bromo-2-iodoperfluoroethyl perfluoroallyl ether,
 1-bromo-2-iodoperfluoroethyl methyl ether, 1-iodo-2-bromoperfluoroethyl
 ethyl ether, 1-iodo-2-bromoethyl ethyl ether and 1-bromo-2-iodoethyl
 2'-chloroethyl ether.
 These iodobrominated compounds can be produced by appropriate conventional
 processes. For example, monobromomonoiodofluoroolefins can be obtained by
 reacting fluoroolefins with iodine bromide.
 Examples of suitable aromatic iodobrominated compounds include substituted
 benzenes such as 1-iodo-2-bromobenzene, 1-iodo-3-bromobenzene,
 1-iodo-4-bromobenzene, 3,5-dibromo-1-iodobenzene,
 3,5-diiodo-1-bromobenzene, 1-(2-iodoethyl)-4-(2-bromoethyl)benzene,
 1-(2-iodoethyl)-3-(2-bromoethyl)benzene,
 1-(2-iodoethyl)-4-(2-bromoethyl)benzene,
 3,5-bis(2-bromoethyl)-1-(2-iodoethyl)benzene,
 3,5-bis(2-iodoethyl)-1-(2-bromoethyl)benzene,
 1-(3-iodopropyl)-2-(3-bromopropyl)benzene,
 1-(3-iodopropyl)-3-(3-bromopropyl)benzene,
 1-(3-iodopropyl)-4-(3-bromopropyl)benzene,
 3,5-bis(3-bromopropyl)-1-(3-iodopropyl)benzene,
 1-(4-iodobutyl)-3-(4-bromobutyl)benzene,
 1-(4-iodobutyl)-4-(4-bromobutyl)benzene,
 3,5-bis(4-iodobutyl)-1-(4-bromobutyl)benzene,
 1-(2-iodoethyl)-3-(3-bromopropyl)benzene,
 1-(3-iodopropyl)-3-(4-bromobutyl)benzene,
 3,5-bis(3-bromopropyl)-1-(2-iodoethyl)benzene,
 1-iodo-3-(2-bromoethyl)benzene, 1-iodo-3-(3-bromopropyl)benzene,
 1,3-diiodo-5-(2-bromoethyl)benzene, 1,3-diiodo-5-(3-bromopropyl)benzene,
 1-bromo-3-(2-iodoethyl)benzene, 1-bromo-3-(3-iodopropyl)benzene,
 1,3-dibromo-5-(2-iodoethyl)benzene and
 1,3-dibromo-5-(3-iodopropyl)benzene; and substituted perfluorobenzenes
 such as 1-iodo-2-bromoperfluorobenzene, 1-iodo-3-bromoperflucrobenzene,
 1-iodo-4-bromoperfluorobenzene, 3,5-dibromo-1-iodoperfluorobenzene and
 3,5-diiodo-1-bromoperfluorobenzene.
 Examples of suitable iodinated compounds include 1,2-diiodoperfluoroethane,
 1,3-diiodoperfluoropropane, 1,4-diiodoperfluorobutane,
 1,6-diiodoperfluorohexane and 1,8-diiodoperfluorooctane. Of these,
 1,4-diiodoperfluorobutane is preferred.
 Examples of suitable brominated compounds include saturated aliphatic
 compounds each having 2 to 10 carbon atoms, such as
 1,2-dibromo-1-fluoroethane, 1,2-dibromo-1,1-difluoroethane,
 1,2-dibromo-1,1,2-trifluoroethane, 1,2-dibromo-1-chlorotrifluoroethane,
 2,3-dibromo-1,1,1-trifluoropropane, 1,2-dibromohexafluoropropane,
 1,2-dibromoperfluorobutane, 1,4-dibromoperfluorobutane,
 1,4-dibromo-2-chloro-1,1,2-trifluorobutane and 1,6-dibromoperfluorohexane;
 unsaturated aliphatic compounds each having 2 to 10 carbon atoms, such as
 2-bromo-1,1-difluoroethylene, 1,1-dibromodifluoroethylene,
 bromotrifluoroethylene, 2-bromo-3,3,3-trifluoropropene,
 4-bromo-1,1,2-trifluorobutene-1 and
 4-bromo-3-chloro-3,4,4-trifluorobutene-1; and aromatic compounds, such as
 1,2-dibromo-3,5-difluorobenzene, 1,2-dibromo-4,5-difluorobenzene,
 1,4-dibromo-2,5-difluorobenzene, 2,4-dibromo-1-fluorobenzene,
 1,3-dibromo-5-fluorobenzene, 1,4-dibromo-2-fluorobenzene,
 1,2-dibromoperfluorobenzene, 1,3-dibromoperfluorobenzene and
 1,4-dibromoperfluorobenzene.
 In the present invention, when use is made of any of the above
 iodobrominated compounds, the radical cleavage of iodine and bromine is
 readily realized by the action of an organic peroxide radical generator
 during the polymerization reaction. The reactivity of the thus formed
 radicals is so high that the addition/growth reaction of monomer is
 induced. This reaction is terminated by the abstraction of iodine and
 bromine from the iodobrominated compound. As a result, a fluoroelastomer
 having its molecular terminals bonded with iodine and bromine is provided.
 The iodine and bromine atoms bonded with the molecular terminals act as
 crosslinking sites at the time of peroxide vulcanization.
 The copolymerization reaction can also be performed in the presence of the
 above iodobrominated compound, iodinated compound or brominated compound
 together with a brominated or iodinated compound having a radically
 polymerizable unsaturated group.
 The iodinated compound having a radically polymerizable unsaturated group
 is, for example, iodotrifluoroethylene, 1-iodo-2,2-difluoroethylene or
 perfluoro(2-iodoethyl vinyl ether).
 The brominated compound having a radically polymerizable unsaturated group
 can be, for example, selected from among 2-bromo-1,1-difluoroethylene,
 1,1-dibromodifluoroethylene, bromotrifluoroethylene,
 2-bromo-3,3,3-trifluoropropene,
 4-bromo-1,1,2-trifluorobutene-1,4-bromo-3-chloro-3,4,4-trifluorobutene-1
 and perfluoro(2-bromoethyl vinyl ether).
 Although these iodinated compounds, iodobrominated compounds and brominated
 compounds are used either individually or in combination, appropriation
 selection is determined taking into account crosslinking conditions for
 fluorinated copolymer, the reactivity of these compounds, etc.
 Although the molecular weight of the fluorinated copolymer of the present
 invention is determined taking into account the processability,
 moldability and mechanical properties of the fluorinated copolymer, the
 solution viscosity .eta..sub.sp /C as an index for the molecular weight is
 preferably in the range of about 0.3 to 2.0 dl/g, more preferably 0.5 to
 1.3 dl/g.
 For obtaining the fluorinated copolymer having a molecular weight
 corresponding to the above range of solution viscosity, a chain transfer
 agent such as ethyl malonate, acetone or isopropanol can be used during
 the polymerization reaction according to necessity. However, the above
 iodinated compounds, iodobrominated compounds and brominated compounds
 themselves have chain transfer activity, so that the addition of any other
 chain transfer agent is not needed except for special cases.
 Although the fluorinated copolymer of the present invention can be cured by
 any of various conventional crosslinking methods, such as the peroxide
 crosslinking method in which, for example, an organic peroxide is added,
 the polyamine crosslinking method in which use is made of a polyamine
 compound and the irradiation crosslinking method in which irradiation is
 effected by radiation, electron beams and the like, the peroxide
 crosslinking method can be stated as being preferable because the use of
 any inorganic additive is not needed and any special apparatus is not
 required.
 Examples of the organic peroxides suitable for a peroxide vulcanization
 include 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, 2,5-dimethyl-2,5-bis
 (tert-butylperoxy)hexyne-3, benzoyl peroxide, bis(2,4-dichlorobenzoyl)
 peroxide, dicumyl peroxide, di-tert-butyl peroxide, tert-butyl cumyl
 peroxide, tert-butylperoxybenzene,
 1,1-bis(tert-butylperoxy)-3,5,5-trimethylcyclohexane,
 2,5-dimethylhexane-2,5-dihydroxyperoxide,
 .alpha.,.alpha.'-bis(tert-butylperoxy)-p-diisopropylbenzene,
 2,5-dimethyl-2,5-di(benzoylperoxy)hexane and tert-butylperoxyisopropyl
 carbonate.
 In the peroxide vulcanization method in which these organic peroxides are
 employed, a polyfunctional unsaturated compound such as tri (meth) allyl
 isocyanurate, tri(meth)allyl cyanurate, triallyl trimellitate,
 N,N'-m-phenylenebismaleimide, diallyl phthalate,
 tris(diallylamine)-s-triazine, triallyl phosphite, 1,2-polybutadiene,
 ethylene glycol diacrylate or diethylene glycol diacrylate is generally
 used in combination therewith as a co-crosslinking agent.
 In the use of the above compounds in the peroxide vulcanization system, the
 organic peroxide is generally added in an amount of about 0.1 to 10 parts
 by weight, preferably about 0.5 to 5 parts by weight, and the
 co-crosslinking agent is generally added in an amount of about 0.1 to 10
 parts by weight, preferably about 0.5 to 5 parts by weight, per 100 parts
 by weight of the fluorinated copolymer.
 The fluorinated copolymer of the present invention can also be blended and
 co-crosslinked with other peroxide crosslinkable material, such as
 silicone oil, silicone rubber, fluorosilicone rubber, fluorophosphagen
 rubber, ethylene/vinyl acetate copolymer, other ethylene/acrylic ester
 copolymer, ethylene/propylene (/diene) copolymer rubber,
 acrylonitrile/butadiene copolymer rubber and acrylic ester rubber.
 The above components can be blended together by means of, for example, a
 roll mill, a kneader or Banbury mixer. The crosslinking of the thus
 obtained blend can be effected by heating the same at about 140 to
 220.degree. C. for about 2 to 60 min. Further, when secondary crosslinking
 is intended, it is preferably performed at about 150 to 250.degree. C. for
 a period of about 1 to 50 hr in air or an inert gas such as nitrogen gas.
 A sealant can be produced by forming the above crosslinkable composition
 into desired configuration and crosslinking the same according to, for
 example, the press crosslinking, oven crosslinking or steam crosslinking
 technique. The obtained sealant is excellent in mechanical properties such
 as hardness, tensile strength and compression set even if no inorganic
 additive is contained therein, so that it can be suitably used in the
 semiconductor industry, medical materials, food industry and the like.
 EFFECT OF THE INVENTION
 The present invention enables providing the crosslinkable fluorinated
 copolymer, which, even if no inorganic additive is contained therein, is
 excellent in not only ordinary-state properties such as hardness and
 tensile strength but also compression set.
 Moreover, the use of the fluorinated copolymer of the present invention
 enables providing the sealant which can be suitably used in the
 semiconductor industry, medical materials, food industry and the like.

EXAMPLES
 The present invention will now be illustrated in greater detail with
 reference to the following Examples, which in no way limit the scope of
 the invention.
 The melting peak temperature (Tm) of the fluorinated copolymer obtained in
 each of the following Examples and Referential Example was measured by the
 use of DSC in accordance with ASTM D 3418-97. The copolymer composition of
 the obtained fluorinated copolymers was determined by means of F-NMR.
 Each of the obtained fluorinated copolymers was blended with the following
 compounds, thereby obtaining fluorinated copolymer compositions. The
 obtained fluorinated copolymer compositions were crosslinked under the
 below described conditions.
 Composition

fluorinated copolymer 100 parts by weight,
 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane 1 part by weight, and
 triallyl isocyanurate 2 parts by weight.
 The above components of fluorinated copolymer composition were milled by
 rolls, and the blend was subjected to press crosslinking (primary
 crosslinking) at 180.degree. C. for 5 min and oven crosslinking (secondary
 crosslinking) at 200.degree. C. for 4 hr. Thus, sheet and O-ring which is
 crosslinked were formed.
 The following measurements of each of the obtained crosslinked products
 were conducted:
 ODR: Vulcanization was effected at 180.degree. C. for 10 min, and the
 minimum torque value (M.sub.L), maximum torque value (M.sub.H) and time
 (Tc90) required for reaching 90% of the maximum torque value were
 measured, by means of oscillating disc rheometer (ASTM-100 type,
 manufactured by Toyo Seiki Co., Ltd.).
 Ordinary-state properties:
 Hardness (Shore A) measured in accordance with ASTM D-2240-81,
 Hardness (IRHD) measured in accordance with ASTM D-2240-81,
 100% modulus measured in accordance with ASTM D-412-83,
 Tensile strength measured in accordance with ASTM D-412-83, and
 Elongation measured in accordance with ASTM D-412-83.
 Low-temperature properties: TR test was performed in accordance with ASTM
 D-1329.
 Compression set: measured with respect to O-ring having 3.5 mm line
 diameter which is compressed to 25% at 200.degree. C. for 70 hr and at
 150.degree. C. for 70 hr.
 Example 1
 20 g of ammonium perfluorooctanoate, 10 g of potassium dihydrogenphosphate
 and 3.8 lit. of deionized water were charged into an autoclave of 10 lit.
 internal volume, and the internal space gas was fully replaced by nitrogen
 gas. Subsequently, 7.65 g of 1,4-diiodoperfluorobutane was introduced
 thereinto under pressure, and 720 g of vinylidene fluoride (VdF) and 540 g
 of chlorotrifluoroethylene (CTFE) (molar ratio: 71/29) were further
 introduced under pressure. The internal temperature was raised to
 70.degree. C.
 Thereafter, an aqueous solution of polymerization initiator prepared by
 dissolving 4 g of ammonium persulfate in 120 ml of deionized water was
 introduced under pressure into the autoclave, and polymerization reaction
 was initiated. Immediately thereafter, the internal pressure of the
 autoclave was decreased. When the internal pressure reached 0.4
 MPa.cndot.G 5 hr later, the unreacted gas within the autoclave was
 immediately expelled and the reaction was terminated.
 The thus obtained latex was subjected to salting out with a 1% aqueous
 solution of calcium chloride. Drying was conducted, and 1134 g of white
 rubbery copolymer was obtained (yield: 90%).
 Copolymer composition: VdF/CTFE=69/31 (mol %), .eta..sub.sp /C [1%
 dimethylformamide (DMF) solution at 35.degree. C.]: 0.75 dl/g, and
 Tm: 137.degree. C.
 Example 2
 Polymerization reaction and salting out/drying were carried out in the same
 manner as in Example 1, except that 3.44 g of
 1-bromo-2-iodoperfluoroethane and 1.60 g of 2-bromo-1,1-difluoroethylene
 were used in place of 7.65 g of 1,4-diiodoperfluorobutane. As a result,
 1100 g of white rubbery copolymer was obtained (yield: 87%).
 Copolymer composition: VdF/CTFE=70/30 (mol %), .eta..sub.sp /C [1% DMF
 solution at 35.degree. C.]: 0.80 dl/g, and
 Tm: 140.degree. C.
 Referential Example
 20 g of ammonium perfluorooctanoate, 10 g of potassium dihydrogenphosphate
 and 3.8 lit. of deionized water were charged into an autoclave of 10 lit.
 internal volume, and the internal space gas was fully replaced by nitrogen
 gas. Subsequently, 7.65 g of 1,4-diiodoperfluorobutane was introduced
 thereinto under pressure, and 240 g of vinylidene fluoride (VdF) and 180 g
 of chlorotrifluoroethylene (CTFE) were further introduced under pressure.
 The internal temperature was raised to 70.degree. C.
 Thereafter, an aqueous solution of polymerization initiator prepared by
 dissolving 4 g of ammonium persulfate in 150 ml of water was introduced
 under pressure into the autoclave, and polymerization reaction was
 initiated.
 Immediately thereafter, the operation of introducing the gas mixture of the
 above composition until the internal pressure reached 2.17 MPa.cndot.G
 under pressure was repeated until the solid content of the formed latex
 reached 22% by weight. When this solid content was reached, the unreacted
 gas within the autoclave was immediately expelled and the reaction was
 terminated.
 The thus obtained latex was subjected to the same salting out and drying as
 in Example 1. As a result, 1150 g of white rubbery copolymer was obtained.
 Copolymer composition: VdF/CTFE=68/32 (mol %), .eta..sub.sp /C [1% DMF
 solution at 35.degree. C.]: 1.0 dl/g, and
 Tm: not detected.
 Fluorinated copolymer composition was prepared from each of the fluorinated
 copolymers produced in Example 1, Example 2 and Referential Example in the
 aforementioned manner, and crosslinking/forming thereof was carried out.
 The properties of the resultant crosslinked products were evaluated, and
 the results are listed in Table 1.
 TABLE 1
 Referential
 Example 1 Example 2 Example
 [Compsn. of copolymer]
 VdF (mol %) 69 70 68
 CTFE (mol %) 31 30 32
 [Visc. of copolymer
 soln.]
 .eta..sub.sp /C (dl/g, DMF) 0.75 0.80 1.00
 [m.p. of copolymer]
 Tm (.degree. C.) 137 140 not detected
 [ODR]
 M.sub.L (dN.m) 0.9 1.0 0.7
 M.sub.H (dN.m) 10.5 11.8 8.0
 Tc90 (min) 1.18 2.00 1.20
 [Ord.-state properties]
 hardness (Shore A) (pts) 66 68 58
 100% modulus (MPa) 1.9 2.1 1.6
 tensile strength (MPa) 14.0 14.8 12.0
 elongation (%) 390 370 450
 [Low-temp. properties]
 TR-10 (.degree. C.) -9.7 -10.3 -9.2
 TR-70 (.degree. C.) -0.5 -2.2 -1.2
 [Compression set]
 150.degree. C., 70 hr (%) 21 19 34
 200.degree. C., 70 hr (%) 41 38 58
 As apparent from the above results of the Examples and Referential Example,
 as compared with the crosslinked product of fluorinated copolymer obtained
 in Referential Example, the crosslinked products of fluorinated copolymers
 obtained in Examples 1 and 2 in which whole amounts of vinylidene fluoride
 (VdF) and chlorotrifluoroethylene (CTFE) were simultaneously charged prior
 to copolymerization reaction exerted the following effects:
 (1) with respect to the ordinary-state properties, favorable values of
 hardness and tensile strength were exhibited without the use of inorganic
 fillers; and
 (2) the compression set was markedly enhanced to a level such that the use
 as sealants was highly practicable.
 Comparative Example 1
 Polymerization reaction and salting out/drying were carried out in the same
 manner as in Example 1, except that 2.6 g of carbon tetrachloride having a
 chain transfer activity was used in place of 7.65 g of
 1,4-diiodoperfluorobutane. As a result, 1130 g of white rubbery copolymer
 was obtained (yield: 90%).
 Copolymer composition: VdF/CTFE=69/31 (mol %), .eta..sub.sp /C [1% DMF
 solution at 35.degree. C.]: 0.83 dl/g, and
 Tm: 134.degree. C.
 A fluorinated copolymer composition comprising 100 parts by weight of the
 fluorinated copolymer thus obtained, 1 part by weight of
 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane and 2 parts by weight of
 triallyl isocyanurate were milled by rolls.
 The measurement of the ODR of the roll-milled product was carried out, but
 the vulcanization torque was not raised at all.