Patent Publication Number: US-2007100062-A1

Title: Process for the manufacture of fluoroelastomers having bromine or lodine atom cure sites

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of U.S. Provisional Application No. 60/731,381 filed Oct. 28, 2005. 
    
    
     FIELD OF THE INVENTION  
      This invention relates to a process for the manufacture of fluoroelastomers having bromine, iodine or both bromine and iodine atom cure sites wherein the comonomer or chain transfer agent containing said bromine or iodine atoms is added to the polymerization reactor in the form of an aqueous emulsion.  
     BACKGROUND OF THE INVENTION  
      Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been widely employed for sealing materials, containers, and hoses. Examples of fluoroelastomers include copolymers comprising units of vinylidene fluoride (VF 2 ) and units of at least one other fluorine-containing monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF) and fluoroethers such as a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether) (PMVE), perfluoro(ethyl vinyl ether), and perfluoro(propyl vinyl ether). Other examples of fluoroelastomers include copolymers of tetrafluoroethylene with a perfluoro(alkyl vinyl ether) such as perfluoro(methyl vinyl ether). Ethylene (E) and propylene (P) are non-fluorinated monomers that are also often used to prepare fluoroelastomers.  
      In order to develop the physical properties necessary for most end use applications, fluoroelastomers must be crosslinked. A preferred curing system for many end uses is the combination of an organic peroxide and a multifunctional unsaturated coagent. The coagent forms crosslinks by reacting with cure sites on the backbone of the fluoroelastomer polymer chain. A preferred cure site is a bromine or an iodine atom bonded to a carbon atom on the fluoroelastomer chain.  
      Fluoroelastomers are typically prepared by free radical emulsion polymerization. One method of introducing iodine or bromine cure sites into the fluoroelastomer is by conducting the polymerization in the pp presence of a chain transfer agent containing iodine or bromine. In this manner an iodine or bromine atom is attached to the resulting fluoroelastomer at one or more terminal points. Such chain transfer agents generally have the structure RX n , where R may be a C 1 -C 3  hydrocarbon, a C 1 -C 6  fluorohydrocarbon, a C 1 -C 6  chlorofluorohydrocarbon or a C 2 -C 8  perfluorocarbon, X is iodine or bromine, and n=1 or 2 (U.S. Pat. Nos. 4,243,770 and 4,973,633).  
      Another common method of introducing iodine or bromine cure sites onto a fluoroelastomer polymer chain is by copolymerizing a minor amount of an iodine or bromine-containing fluoroolefin or fluorovinyl ether cure site monomer with other monomers (e.g. VF 2 , HFP, TFE, PAVE, P, E, etc.). In this manner, cure sites may be randomly distributed along the resulting polymer chain.  
      A general problem with the use of either iodine- or bromine-containing cure sites or comonomers is related to the generally high specific gravity of these compounds. For example, at 25° C., the specific gravity of methylene iodide (CH 2 I 2 ) is 3.33, the specific gravity of methylene bromide (CH 2 Br 2 ) is 2.51, the specific gravity of 1,4-diiodooctafluorobutane is 2.48, and the specific gravity of 1,6-diiodododecafluorohexane is 2.35. When materials of such high specific gravity are added to a polymerization reactor, they tend to settle to the bottom of the reactor. Because these materials settle to the reactor bottom, they may be incorporated unevenly into polymer chains such that a portion of the chains has an abnormally high amount of cure site while other portions have an abnormally low amount of cure site. This will result in undesirable variability in the end use performance of products made from fluoroelastomers. Another consequence of the high specific gravity is that a portion of the iodine- or bromine-containing compound may not even become incorporated into the polymer, resulting in reduced efficiency of the iodine- or bromine-containing compound, and the need for additional waste treatment facilities to capture any residual iodine- or bromine-containing cure site or chain transfer agent. Yet another consequence of the high specific gravity is poor viscosity control of the resulting fluoroelastomer polymer, because the iodine- or bromine-containing chain transfer agent does not become evenly incorporated into the polymer chains.  
      Another problem with the use of many iodine- or bromine-containing cure site monomers or chain transfer agents is their lack of solubility in water. This low solubility can cause variable incorporation of these compounds in emulsion polymerization processes ( J. Appl. Polym. Sci.  51, 21 (1994)). In order to achieve complete incorporation of a poorly soluble chain transfer agent into a polymer, the overall polymerization rate may need to be decreased, which leads to an inefficient use of polymerization reactors.  
      To solve the problems of high specific gravity and low aqueous solubility, it has been proposed to dissolve the iodine- or bromine-containing cure site monomer or chain transfer agent in an organic solvent and inject this solution into the polymerization reactor (U.S. Pat. Nos. 4,973,633 and 5,284,920). However, this approach is unsatisfactory because it requires additional waste treatment facilities to remove the solvent from either the polymer or the waste water. In addition, it is difficult to find satisfactory solvents that do not interfere with the polymerization reaction and slow it down.  
      Another proposed solution is to incorporate an iodine-containing chain transfer agent into a spontaneously generated fluorinated microemulsion (U.S. Pat. No. 5,585,449). However, this method has the drawback that the fluorinated oils that comprise the microemulsion are retained in the polymer. These can be detected (e.g. by headspace GC-MS), and may adversely affect adhesion to metals and mill behavior, as well as food contact status.  
     SUMMARY OF THE INVENTION  
      In one aspect, the present invention provides a process for the manufacture of fluoroelastomers comprising the use of a mechanically-generated emulsion of an iodine- or bromine-containing chain transfer agent and/or an iodine- or bromine-containing cure site monomer in which the average droplet size of the resulting emulsion is less than 50 microns, and which is substantially free of any organic solvent or oil, and which may optionally contain a surfactant.  
      In another aspect, the present invention provides a process for preparing a fluoroelastomer having bromine, iodine or both bromine and iodine cure sites. The process comprises:  
      (A) charging a reactor with a quantity of an aqueous solution;  
      (B) feeding to said reactor a quantity of an initial monomer mixture to form a reaction medium, said initial monomer mixture comprising i) a first monomer, said first monomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene, and ii) one or more additional copolymerizable monomers, different from said first monomer, wherein said additional monomer is selected from the group consisting of fluorine-containing olefins, fluorine-containing ethers, propylene, ethylene and mixtures thereof;  
      (C) feeding to said reactor at least one aqueous emulsion comprising a cure site source selected from the group consisting of i) an iodine-containing cure site monomer, ii) a bromine-containing cure site monomer, iii) an iodine-containing chain transfer agent, and iv) a bromine-containing chain transfer agent; wherein said emulsion has a droplet size of less than 50 microns; and  
      (D) polymerizing said monomers in the presence of a free radical initiator to form a fluoroelastomer having cure sites. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention is directed to an emulsion polymerization process for manufacturing fluoroelastomers that contain bromine, iodine or both bromine and iodine atom cure sites.  
      The fluoroelastomers prepared by the process of this invention comprise copolymerized units of a first monomer which may be vinylidene fluoride (VF 2 ) or tetrafluoroethylene (TFE) and one or more additional monomers, different from said first monomer, selected from the group consisting of fluorine-containing olefins, fluorine-containing ethers, propylene, ethylene and mixtures thereof. The level of copolymerized units of first monomer present in fluoroelastomers prepared by the process of this invention is no more than 85 mole percent, based on total number of moles of all copolymerized monomers incorporated in the fluoroelastomer.  
      Examples of fluorine-containing olefins copolymerizable with the first monomer include vinylidene fluoride, hexafluoropropylene (HFP), tetrafluoroethylene (TFE), 1,2,3,3,03-pentafluoropropene (1-H PFP), chlorotrifluoroethylene (CTFE) and vinyl fluoride.  
      Examples of fluorine-containing ethers that may be employed in the present invention include perfluoro(alkyl vinyl ethers), perfluoro(alkyl alkenyl ethers) and perfluoro(alkoxy alkenylethers).  
      Perfluoro(alkyl vinyl ethers) (PAVE) suitable for use as monomers include those of the formula
 
CF 2 ═CFO(R f′ O) n (R f″ O) m R f   (I)
 
 where R f′  and R f″ are different linear or branched perfluoroalkylene groups of 2-6 carbon atoms, m and n are independently 0-10, and R f  is a perfluoroalkyl group of 1-6 carbon atoms. 
 
      A preferred class of perfluoro(alkyl vinyl ethers) includes compositions of the formula
 
CF 2 ═CFO(CF 2 CFXO) n R f   (II)
          where X is F or CF 3 , n is 0-5, and R f  is a perfluoroalkyl group of 1-6 carbon atoms. 
 
 A most preferred class of perfluoro(alkyl vinyl ethers) includes those ethers wherein n is 0 or 1 and R f  contains 1-3 carbon atoms. Examples of such perfluorinated ethers include perfluoro(methyl vinyl ether) (PMVE) and perfluoro(propyl vinyl ether) (PPVE). Other useful monomers include compounds of the formula
 
CF 2 ═CFO[(CF 2 ) m CF 2 CFZO] n R f   (III)
 
 where R f  is a perfluoroalkyl group having 1-6 carbon atoms, m=0 or 1, n=0-5, and Z=F or CF 3 . 
 
 Preferred members of this class are those in which R f  is C 3 F 7 , m=0, and n=1. 
       

      Additional perfluoro(alkyl vinyl ether) monomers include compounds of the formula
 
CF 2 ═CFO[(CF 2 CF{CF 3 }O) n (CF 2 CF 2 CF 2 O) m (CF 2 ) p ]C x F 2x+1   (IV)
          where m and n independently=0-10, p=0-3, and x=1-5. 
 
 Preferred members of this class include compounds where n=0-1, m=0-1, and x=1. 
       

      Other examples of useful perfluoro(alkyl vinyl ethers) include
 
CF 2 ═CFOCF 2 CF(CF 3 )O(CF 2 O) m C n F 2n+1   (V)
 
 where n=1-5, m=1-3, and where, preferably, n=1. 
 
      Perfluoro(alkyl alkenyl ethers) suitable for use as monomers include those of the formula VI
 
R f O(CF 2 ) n CF═CF 2   (VI)
 
 where R f  is a perfluorinated linear or branched aliphatic group containing 1-20, preferably 1-10, and most preferably 1-4 carbon atoms and n is an integer between 1 and 4. Specific examples include perfluoro(propoxyallyl ether) and perfluoro(propoxybutenyl ether). 
 
      Perfluoro(alkoxy alkenyl ethers) differ from perfluoro(alkyl alkenyl ethers) in that R f  in formula VI contains at least one oxygen atom in the aliphatic chain. A specific example includes, but is not limited to perfluoro(methoxyethoxyallyl ether).  
      If copolymerized units of a fluorine-containing ether are present in the fluoroelastomers of the invention, the ether unit content generally ranges from 25 to 75 weight percent, based on the total weight of the fluoroelastomer. If perfluoro(methyl vinyl) ether is used, then the fluoroelastomer preferably contains between 30 and 55 wt. % copolymerized PMVE units.  
      Fluoroelastomers prepared by the process of this invention also contain cure sites suitable for organic peroxide induced crosslinking. The source of the cure sites may be i) a copolymerizable cure site monomer containing bromine or iodine, ii) a bromine or iodine-containing chain transfer agent, or iii) both i) and ii). The level of bromine or iodine atoms incorporated into the fluoroelastomers is between 0.03 and 1.5 mole percent, based on the total number of moles of copolymerized monomers in the fluoroelastomers. If the fluoroelastomer contains both iodine- or bromine-containing cure site monomers and iodine- or bromine-containing endgroups (resulting from chain transfer agents), the level of iodine or bromine atoms may be in the range of 0.03 to 1.5 mole percent from each of the cure site sources (i.e. from the cure site monomer and from the chain transfer agent), for a total of 0.06 to 3 mole percent iodine or bromine cure sites.  
      Bromine atom containing cure site monomers may contain other halogens, preferably fluorine. Examples of brominated olefin cure site monomers are bromotrifluoroethylene; 4-bromo-3,3,4,4-tetrafluorobutene-1 (BTFB); and others such as vinyl bromide, 1- bromo-2,2-difluoroethylene; perfluoroallyl bromide; 4-bromo-1,1,2-trifluorobutene-1; 4-bromo-1,1,3,3,4,4,-hexafluorobutene; 4-bromo-3-chloro-1 ,1,3,4,4-pentafluorobutene; 6-bromo-5,5,6,6-tetrafluorohexene; 4-bromoperfluorobutene-1 and 3,3-difluoroallyl bromide. Brominated vinyl ether cure site monomers useful in the invention include 2-bromo-perfluoroethyl perfluorovinyl ether and fluorinated compounds of the class CF 2 Br—R f —O—CF═CF 2  (R f  is a perfluoroalkylene group), such as CF 2 ═CFOCF 2 CF 2 CF 2 OCF 2 CF 2 Br; CF 2 BrCF 2 O—CF═CF 2 , and fluorovinyl ethers of the class ROCF═CFBr or ROCBr═CF 2  (where R is a lower alkyl group or fluoroalkyl group) such as CH 3 OCF═CFBr or CF 3 CH 2 OCF═CFBr.  
      Suitable iodine atom containing cure site monomers include iodinated olefins of the formula: CHR═CH-Z-CH 2 CHR—I, wherein R is —H or —CH 3 ; Z is a C 1 -C 18  (per)fluoroalkylene radical, linear or branched, optionally containing one or more ether oxygen atoms, or a (per)fluoropolyoxyalkylene radical as disclosed in U.S. Pat. No. 5,674,959. Other examples of useful iodinated cure site monomers are unsaturated ethers of the formula: I(CH 2 CF 2 CF 2 ) n OCF═CF 2  and ICH 2 CF 2 O[CF(CF 3 )CF 2 O] n CF═CF 2 , and the like, wherein n=1-3, such as disclosed in U.S. Pat. No. 5,717,036. In addition, suitable iodinated cure site monomers including iodoethylene, 4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB); 3-chloro4-iodo-3,4,4-trifluorobutene; 2-iodo-1,1,2,2-tetrafluoro-1-(vinyloxy)ethane; 2-iodo-1-(perfluorovinyloxy)-1,1,-2,2-tetrafluoroethylene; 1,1,2,3,3,3-hexafluoro-2-iodo-1-(perfluorovinyloxy)propane; 2-iodoethyl vinyl ether; 3,3,4,5,5,5-hexafluoro4-iodopentene; and iodotrifluoroethylene are disclosed in U.S. Pat. No. 4,694,045. Allyl iodide and 2-iodo-perfluoroethyl perfluorovinyl ether are also useful cure site monomers.  
      If a bromine or iodine containing cure site monomer is employed in the process of the invention, it is introduced to the reactor in the form of an aqueous emulsion (described hereinafter).  
      In addition to, or instead of a cure site monomer, iodine or bromine atom containing endgroups may optionally be present at one or both of the fluoroelastomer polymer chain ends as a result of the use of bromine or iodine atom containing chain transfer or molecular weight regulating agents during preparation of the fluoroelastomers. The chain transfer agent is typically of the formula RX n , where R may be a C 1 -C 3  hydrocarbon, a C 1 -C 6  fluorohydrocarbon, a C 1 -C 6  chlorofluorohydrocarbon or a C 2 -C 8  perfluorocarbon, X is iodine or bromine, and n=1 or 2 (U.S. Pat. Nos. 3,707,529 and 4,243,770). Such agents include those of formula CH 2 X 2  where X is I or Br; X(CF 2 ) n Y where X is I or Br, Y is I or Br (preferably both X and Y are I) and n is an integer between 3 and 10.  
      Specific examples include methylene iodide; 1,3-diiodoperfluoropropane; 1,4-diiodoperfluorobutane; 1,6-diiodoperfluorohexane; 1,8-diiodoperfluorooctane; 1,10-diiodoperfluorodecane; and 1-iodo-nonafluorobutane. Other chain transfer agents such as those of formula RBr n I m  (R is as defined above; n and m each are 1 or 2) may also be used. Particularly preferred are diiodinated perfluoroalkane chain transfer agents and mixtures thereof.  
      If a bromine or iodine containing chain transfer agent is employed in the process of the invention, it is introduced to the reactor in the form of an aqueous emulsion. If both a cure site monomer and a chain transfer agent are employed for introducing cure sites onto the fluoroelastomer, the cure site monomer and chain transfer agent are typically in separate aqueous emulsions that can be added to the reactor at different times and at different rates.  
      Aqueous emulsions of cure site monomer or chain transfer agent are typically prepared by high shear mechanical mixing. An aqueous phase, which optionally contains a surfactant, is contacted with an organic phase, which contains either a cure site monomer or a chain transfer agent or a mixture of both, in the presence of a device that generates a high shear field. The shear field may be generated by a device with moving or rotating parts such as a homogenizer or rotor/stator combination. Alternatively, the high shear field may be generated by devices with essentially no moving parts such as a static mixer or micromixer. Emulsions may be prepared separately away from the reactor, temporarily stored until needed, and then transferred into the polymerization reactor. Alternatively, the emulsion may be prepared in-line whereby the aqueous phase and the organic phase are simultaneously fed into a device that creates the emulsion and immediately transfers it into the reactor with essentially no temporary storage. The resulting aqueous emulsion has a mean droplet size of 50 microns or less, preferably 20 microns or less.  
      Optionally a surfactant may be employed to help in stabilizing the emulsions. Specific examples of suitable surfactants include alkyl sulfonates such as sodium octyl sulfonate and sodium dodecylsulfonate, alkyl sulfates such as sodium lauryl sulfate and sodium decyl sulfate, alkyl carboxylates such as sodium caprylate and sodium stearate, nonionic surfactants such as nonylphenolpoly(ethylene oxide) and alkylpoly(ethylene oxide), perfluorinated carboxylic acids such as perfluorohexylethylsulfonic acid, perfluorooctanoic acid and their salts, partially fluorinated sulfonic acids such as tridecafluorohexylethyl sulfonic acid and its salts, and partially fluorinated carboxylic acids such as 3,3,4,4-tetrahydroundecafluorooctanoic acid and its salts.  
      Surfactant, if present, is typically at the level of 0.05 to 5% by weight in the aqueous phase. The amount of surfactant used in the emulsion will depend on the specific requirements of the process and product. Generally, higher levels of surfactant increase the stability of the emulsion, but introduce more potential impurities into the final fluoroelastomer that may be deleterious to end use performance. If insufficient surfactant is used, the resulting emulsion will display insufficient stability as evidenced by droplet coalescence and formation of a separate organic phase that is visible to the naked eye. When the emulsion is temporarily stored, by e.g. placing in a storage tank, longer emulsion stability is required than if the emulsion is fed directly into the polymerization reactor. Once a separate organic phase is created, the benefits conferred by this invention are lost.  
      The emulsion polymerization process of this invention may be operated either in semi-batch or continuous fashion. In a semi-batch process, a gaseous monomer mixture of a desired composition (initial monomer charge) is introduced into a reactor which contains an aqueous solution. The aqueous solution may optionally comprise a surfactant emulsifying agent such as a fluorosurfactant (e.g. ammonium perfluorooctanoate, Zonyl® FS-62 (available from DuPont) or Foraface® 1033D (available from DuPont)), or a hydrocarbon surfactant (e.g. sodium dodecyl sulfonate). Optionally, the aqueous solution may also contain inorganic salts such as a pH buffer (e.g. a phosphate or acetate buffer for controlling the pH of the polymerization reaction). Instead of a buffer, a base, such as NaOH may be used to control pH. Generally, pH is controlled to between 1 and 10 (preferably 3-7), depending upon the type of fluoroelastomer being made. Alternatively, or additionally, pH buffer or base may be added to the reactor at various times throughout the polymerization reaction, either alone or in combination with other ingredients such as polymerization initiator, an aqueous emulsion of liquid cure site monomer or an aqueous emulsion of chain transfer agent. Also optionally, the initial aqueous solution may contain a water-soluble inorganic peroxide polymerization initiator such as ammonium persulfate (or other persulfate salt), or the combination of an inorganic peroxide and a reducing agent such as the combination of ammonium persulfate and sodium sulfite.  
      The initial monomer charge contains a quantity of a first monomer of either TFE or VF 2  and one or more additional monomers which are different from the first monomer. The amount of monomer mixture contained in the initial charge is set so as to result in a reactor pressure between 0.5 and 10 MPa (preferably between 0.5 and 3.5 MPa). In the initial gaseous monomer charge, the relative amount of each monomer is dictated by reaction kinetics and is set so as to result in a fluoroelastomer having the desired ratio of copolymerized monomer units (i.e. very slow reacting monomers must be present in a higher amount relative to the other monomers than is desired in the composition of the fluoroelastomer to be produced).  
      The monomer mixture is dispersed in the aqueous medium and, optionally, an aqueous emulsion of chain transfer agent may also be introduced at this point while the reaction mixture is agitated, typically by mechanical stirring. Alternatively, if employed in the process of the invention, the aqueous emulsion of chain transfer agent may be introduced at any time up to the point when all of the incremental monomer mixture has been fed to the reactor. The entire amount of chain transfer agent may be added at one time, or addition may be spread out over time, up to the point when 100% of the incremental monomer mixture has been added to the reactor. Most preferably, the chain transfer agent aqueous emulsion is introduced to the reactor before polymerization begins, or shortly thereafter, and the entire amount of chain transfer agent is fed to the reactor by the time that 5 wt. % of the total amount of incremental monomer mixture has been fed to the reactor.  
      The temperature of the semi-batch reaction mixture is maintained in the range of 25° C.-130° C., preferably 30° C.-90° C. Polymerization begins when the initiator either thermally decomposes or reacts with reducing agent and the resulting radicals react with dispersed monomer.  
      Additional quantities of the gaseous monomers (referred to herein as incremental monomer mixture feed) are added at a controlled rate throughout the polymerization in order to maintain a constant reactor pressure at a controlled temperature. The relative ratio of gaseous monomers contained in the incremental monomer mixture feed is set to be approximately the same as the desired ratio of copolymerized monomer units in the resulting fluoroelastomer. If employed in the process of the invention, additional chain transfer agent aqueous emulsion may also, optionally, be continued to be added to the reactor at any point during this stage of the polymerization. Additional surfactant and polymerization initiator may also be fed to the reactor during this stage. The amount of polymer formed is approximately equal to the cumulative amount of incremental monomer mixture feed. One skilled in the art will recognize that the molar ratio of monomers in the incremental gaseous monomer mixture feed is not necessarily exactly the same as that of the desired copolymerized monomer unit composition in the resulting fluoroelastomer because the composition of the initial charge may not be exactly that required for the desired final fluoroelastomer composition, or because a portion of the monomers in the incremental monomer mixture feed may dissolve into the polymer particles already formed, without reacting.  
      If a copolymerizable cure site monomer is employed in the process of the invention, a stream of cure site monomer aqueous emulsion is fed to the reactor at a rate so as to result in the entire amount of cure site monomer emulsion being fed to the reactor by the time that 99 wt. % of the incremental monomer mixture has been fed. Total polymerization times in the range of from 2 to 30 hours are typically employed in this semi-batch polymerization process.  
      The continuous emulsion polymerization process of this invention differs from the semi-batch process in the following manner. The reactor is completely filled with aqueous solution so that there is no vapor space. Gaseous monomers and solutions of other ingredients such as water-soluble monomers, aqueous emulsions of chain transfer agents, buffer, bases, polymerization initiator, surfactant, etc., are fed to the reactor in separate streams at a constant rate. Feed rates are controlled so that the average polymer residence time in the reactor is generally between 0.2 to 4 hours. Short residence times are employed for reactive monomers, whereas less reactive monomers such as perfluoro(alkyl vinyl) ethers require more time. The temperature of the continuous process reaction mixture is maintained in the range of 25° C.-130° C., preferably 80° C.-120° C.  
      In the process of this invention, the polymerization temperature is maintained in the range of 25°-130° C. If the temperature is below 25° C., the rate of polymerization is too slow for efficient reaction on a commercial scale, while if the temperature is above 130° C., the reactor pressure required in order to maintain polymerization is too high to be practical.  
      The polymerization pressure is controlled in the range of 0.5 to 10 MPa, preferably 1 to 6.2 MPa. In a semi-batch process, the desired polymerization pressure is initially achieved by adjusting the amount of gaseous monomers in the initial charge, and after the reaction is initiated, the pressure is adjusted by controlling the incremental gaseous monomer feed. In a continuous process, pressure is adjusted by means of a back-pressure regulator in the dispersion effluent line. The polymerization pressure is set in the above range because if it is below 1 MPa, the monomer concentration in the polymerization reaction system is too low to obtain a satisfactory reaction rate. In addition, the molecular weight does not increase sufficiently. If the pressure is above 10 MPa, the cost of the required high pressure equipment is very high.  
      The amount of fluoroelastomer copolymer formed is approximately equal to the amount of incremental feed charged, and is in the range of 10-30 parts by weight of copolymer per 100 parts by weight of aqueous medium, preferably in the range of 20-25 parts by weight of the copolymer. The degree of copolymer formation is set in the above range because if it is less than 10 parts by weight, productivity is undesirably low, while if it is above 30 parts by weight, the solids content becomes too high for satisfactory stirring.  
      Water-soluble peroxides which may be used to initiate polymerization in this invention include, for example, the ammonium, sodium or potassium salts of hydrogen persulfate. In a redox-type initiation, a reducing agent such as sodium sulfite, is present in addition to the peroxide. These water-soluble peroxides may be used alone or as a mixture of two or more types. The amount to be used is selected generally in the range of 0.01 to 0.4 parts by weight per 100 parts by weight of polymer, preferably 0.05 to 0.3. During polymerization some of the fluoroelastomer polymer chain ends are capped with fragments generated by the decomposition of these peroxides.  
      The resulting fluoroelastomer emulsion, prepared by either semi-batch or continuous processes, may be isolated, filtered, washed and dried by conventional techniques employed in the fluoroelastomer manufacturing industry.  
      In addition to cure site, preferred fluoroelastomers of this invention comprise copolymerized units of i) vinylidene fluoride and hexafluoropropylene; ii) vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene; iii) vinylidene fluoride, tetrafluoroethylene and perfluoro(methyl vinyl ether); and iv) tetrafluoroethylene and perfluoro(methyl vinyl ether).  
      Fluoroelastomers prepared by the process of this invention can be crosslinked (i.e. vulcanized or cured) by an organic peroxide. Curable fluoroelastomer compositions comprise a) a fluoroelastomer prepared by the process of this invention (as defined above), b) an organic peroxide, and c) a coagent. Preferably, the compositions also contain an acid acceptor such as a divalent metal hydroxide, a divalent metal oxide, a strongly basic (i.e. pKa&gt;10) organic amine such as ProtonSponge® (available from Aldrich), or a combination of any of the latter. Examples of divalent metal oxides and hydroxides include CaO, Ca(OH) 2  and MgO.  
      Organic peroxides suitable for use include 1,1-bis(t-butylperoxy)-3,5,5-trimethylcyclohexane; 1,1-bis(t-butylperoxy)cyclohexane; 2,2-bis(t-butylperoxy)octane; n-butyl4, 4-bis(t-butylperoxy)valerate; 2,2-bis(t-butylperoxy)butane; 2,5-dimethylhexane-2,5-dihydroxyperoxide; di-t-butyl peroxide; t-butylcumyl peroxide; dicumyl peroxide; alpha, alpha′-bis(t-butylperoxy-m-isopropyl)benzene; 2,5-dimethyl-2,5-di(t-butylperoxy)hexane; 2,5-dimethyl-2,5-di(t-butylperoxy)hexene-3; benzoyl peroxide, t-butylperoxybenzene; 2,5-dimethyl-2,5-di(benzoylperoxy)-hexane; t-butylperoxymaleic acid; and t-butylperoxyisopropylcarbonate. Preferred examples of organic peroxides include 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, dicumyl peroxide, and alpha, alpha′-bis(t-butylperoxy-m-isopropyl)benzene. The amount compounded is generally in the range of 0.05-5 parts by weight, preferably in the range of 0.1-3 parts by weight per 100 parts by weight of the fluoroelastomer. This particular range is selected because if the peroxide is present in an amount of less than 0.05 parts by weight, the vulcanization rate is insufficient and causes poor mold release. On the other hand, if the peroxide is present in amounts of greater than 5 parts by weight, the compression set of the cured polymer becomes unacceptably high. In addition, the organic peroxides may be used singly or in combinations of two or more types.  
      Coagents employed in the curable compositions are polyfunctional unsaturated compounds such as triallyl cyanurate, trimethacryl isocyanurate, triallyl isocyanurate, trimethallyl isocyanurate, triacryl formal, triallyl trimellitate, N,N′-m-phenylene bismaleimide, diallyl phthalate, tetraallylterephthalamide, tri(diallylamine)-s-triazine, triallyl phosphite, bis-olefins and N,N-diallylacrylamide. The amount compounded is generally in the range of 0.1-10 parts by weight per 100 parts by weight of the fluoroelastomer. This particular concentration range is selected because if the coagent is present in amounts less than 0.1 part by weight, crosslink density of the cured polymer is unacceptable. On the other hand, if the coagent is present in amounts above 10 parts by weight, it blooms to the surface during molding, resulting in poor mold release characteristics. The preferable range of coagent is 0.2-6 parts by weight per 100 parts fluoroelastomer. The unsaturated compounds may be used singly or as a combination of two or more types.  
      Optionally, other components, for example fillers such as carbon black, Austin black, graphite, thermoplastic fluoropolymer micropowders, silica, clay, diatomaceous earth, talc, wollastonite, calcium carbonate, calcium silicate, calcium fluoride, and barium sulfate; processing aides such as higher fatty acid esters, fatty acid calcium salts, fatty acidamides (e.g. erucamide), low molecular weight polyethylene, silicone oil, silicone grease, stearic acid, sodium stearate, calcium stearate, magnesium stearate, aluminum stearate, and zinc stearate; coloring agents such as titanium white and iron red may be employed as compounding additives in compositions containing fluoroelastomers prepared by the process of this invention. The amount of such filler is generally in the range of 0.1-100 parts by weight, preferably 1-60 parts by weight, per 100 parts by weight of the fluoroelastomer. This range is selected because if the filler is present in amounts of less than 0.1 part by weight, there is little or no effect, while, on the other hand, if greater than 100 parts by weight are used, elasticity is sacrificed. The amount of processing aid compounded is generally less than 10 parts by weight, preferably less than 5 parts by weight, per 100 parts by weight of the fluoroelastomer. If the amount used is above the limit, heat resistance is adversely affected. The amount of a coloring agent compounded is generally less than 50 parts by weight, preferably less than 30 parts by weight per 100 parts by weight of the fluoroelastomer. If greater than 50 parts by weight is used, compression set suffers.  
      The fluoroelastomer, organic peroxide, coagent, and any other ingredients are generally incorporated into curable compositions by means of an internal mixer or rubber mill. The resulting composition may then be shaped (e.g. molded or extruded) and cured. Curing typically takes place at about 150°-200° C. for 1 to 60 minutes. Conventional rubber curing presses, molds, extruders, and the like provided with suitable heating and curing means can be used. Also, for optimum physical properties and dimensional stability, it is preferred to carry out a post curing operation wherein the molded or extruded article is heated in an oven or the like for an additional period of about 14-8 hours, typically from about 180°-275° C., generally in an air atmosphere.  
      The fluoroelastomers prepared by the process of this invention are useful in many industrial applications including seals, wire coatings, tubing and laminates.  
     EXAMPLES  
     Test Methods  
      Mooney viscosity, ML (1+10), was determined according to ASTM D1646 with an L (large) type rotor at 121° C. (unless otherwise noted), using a preheating time of one minute and rotor operation time of 10 minutes.  
      Inherent viscosities were measured at 30° C. Methyl ethyl ketone was employed as solvent (0.1 g polymer in 100 ml solvent) for fluoroelastomers that contained copolymerized units of vinylidene fluoride. A mixed solvent of 60/40/3 volume ratio of heptafluoro-2,2,3-trichlorobutane, perfluoro(α-butyltetrahydrofuran) and ethylene glycol dimethyl ether was used (0.2 g polymer in 100 ml solvent) for fluoroelastomers containing copolymerized units of tetrafluoroethylene and perfluoro(methyl vinyl ether).  
      Iodine content of the polymers was measured by X-ray fluorescence analysis of the isolated, dried polymer.  
      Emulsion droplet size was measured at room temperature with a Coulter LS Particle Size Analyzer and a 61 second analysis time.  
     Emulsion Preparation  
      Two different methods for preparation of perfluoroalkyl diiodide aqueous emulsions were used in the following examples. These preparation methods should not be considered limiting. Other methods for preparing emulsions are known to those skilled in the art. In Method A, a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane, with a flow rate of 1 milliliter per minute, and a 1 wt. % perfluorohexylethylsulfonic acid solution in water, with a flow rate of 10 milliliters per minute, were simultaneously passed through a SIMM-LAS micromixer (manufactured by IMM, Mainz, Germany) to form an emulsion with a mean droplet size of 5.7 microns and with 95% of all droplets less than 15 microns.  
      In Method B, 22.5 milliliters of a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane were added together with 427.5 milliliters of a 1 wt. % perfluorohexylethylsulfonic acid solution in water into a Microfluidics M-110Y microfluidizer. This mixture was passed through the microfluidizer 4 times to generate a 5 volume percent emulsion of the diiodo compounds that had a mean droplet size of 0.18 microns and with 95% of all droplets less than 0.27 microns.  
     Example 1  
      A 41 liter reactor was charged with a water solution containing 17.5 grams perfluorohexylethylsulfonic acid, 12.9 grams disodium phosphate heptahydrate, and 24,969.6 grams deionized water. The reactor was brought to 80° C. and flushed with nitrogen to remove oxygen and then pressurized to 1.38 MPag with a mixture of 43 wt. % vinylidene fluoride, 3 wt. % tetrafluoroethylene, and 54 wt. % perfluoro(methyl vinyl ether). 30.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. % disodium phosphate heptahydrate was added to initiate polymerization. As the reactor pressure dropped, a monomer feed of 55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % per perfluoro(methyl vinyl ether) was added to maintain pressure. After 90 grams of this monomer mixture had been added, an emulsion of a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in a 1% (wt basis) perfluorohexylethylsulfonic acid solution in water, prepared according to Method A (above), was fed into the reactor. After a total of 30.0 grams of the diiodide mixture had been fed to the reactor, the diiodide mixture feed was discontinued and the aqueous 1 wt. % perfluorohexylethylsulfonic solution fed for another minute before also being shut off. Additional initiator solution was added as needed to maintain polymerization. After a total of 8,333 grams of the mixture of 55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % per perfluoro(methyl vinyl ether) had been fed to the reactor, the reaction was stopped and the reactor depressurized. 33,525 grams of a 23.76 wt. % solids latex was obtained. The polymer was isolated by adding aluminum sulfate to the latex, and then dried at 70° C.  
     Example 2  
      A 41 liter reactor was charged with a water solution containing 17.5 grams perfluorohexylethylsulfonic acid, 12.9 grams disodium phosphate heptahydrate, and 24,969.6 grams deionized water. The reactor was brought to 80° C. and flushed with nitrogen to remove oxygen and then pressurized to 1.38 MPag with a mixture of 43 wt. % vinylidene fluoride, 3 wt. % tetrafluoroethylene, and 54 wt. % perfluoro(methyl vinyl ether). 30.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. % disodium phosphate heptahydrate was added to initiate polymerization. As the reactor pressure dropped, a monomer feed of 55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % per perfluoro(methyl vinyl ether) was added to maintain pressure. After 90 grams of this monomer mixture had been added, an emulsion of a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in 1% (wt basis) perfluorohexylethylsulfonic acid solution in water, prepared according to Method A (above) was then fed into the reactor. After a total of 10.0 grams of the diiodide mixture had been fed to the reactor, the diiodide mixture feed was discontinued and the aqueous 1 wt. % perfluorohexylethylsulfonic solution fed for another minute before also being shut off. After 833 grams of monomer mixture had been added, an emulsion of a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in 1% (wt basis) perfluorohexylethylsulfonic acid solution in water, again prepared according to Method A, was fed to the reactor. After an additional 20.0 grams of the diiodide mixture had been fed to the reactor, the diiodide mixture feed was discontinued and the aqueous 1 wt. % perfluorohexylethylsulfonic solution fed for another minute before also being shut off. Additional initiator solution was added, as needed, to maintain polymerization. After a total of 8,333 grams of the mixture of 55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % per perfluoro(methyl vinyl ether) had been fed to the reactor, the reaction was stopped and the reactor depressurized. 33,800 grams of a 25.39 wt. % solids latex was obtained. The polymer was isolated by adding aluminum sulfate to the latex, and then dried at 70° C.  
     Comparative Example 1  
      A 41 liter reactor was charged with a water solution containing 17.5 grams perfluorohexylethylsulfonic acid, 12.9 grams disodium phosphate heptahydrate, and 24,969.6 grams deionized water. The reactor was brought to 80° C. and flushed with nitrogen to remove oxygen and then pressurized to 1.38 MPag with a mixture of 43 wt. % vinylidene fluoride, 3 wt. % tetrafluoroethylene, and 54 wt. % perfluoro(methyl vinyl ether). 30.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. % disodium phosphate heptahydrate was added to initiate polymerization. As the reactor pressure dropped, a monomer feed of 55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % per perfluoro(methyl vinyl ether) was added to maintain pressure. After 90 grams of this monomer mixture had been added, a total of 30.0 grams of a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane was fed neat to the reactor over a 10 minute period. Additional initiator solution was added as needed to maintain polymerization. After a total of 8,333 grams of the mixture of 55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % per perfluoro(methyl vinyl ether) had been fed to the reaction, the reaction was stopped and the reactor depressurized. 33,695 grams of a 25.31 wt. % solids latex was obtained. The polymer was isolated by adding aluminum sulfate to the latex, and then dried at 70° C.  
      Analytical results from these examples are shown in Table I. Each of these examples used 30.0 grams of the mixture of 1,4-diiodooctafluorobutane and 1,6-diiodo dodecafluorohexane. The iodine content of this mixture was 48.6 wt. %. Therefore, each example polymer received 14.58 grams iodine.  
                       TABLE I                                      Example                                 1   2   Comparative 1                                         Grams polymer   7965   8602       8528       Inherent Viscosity   0.71   0.72   0.94       Mooney viscosity   41.2   43.1    79.3       Mol % Iodine   0.12   0.12   0.09       Wt. % Iodine   0.182    0.177   0.140       Grams I in polymer   14.50   15.22    11.93       Iodine yield, %   99   104 1       82                   1 within experimental error 100%             
 
      The data in Table I show that the use of perfluoroalkyldiiodide emulsion increased iodine incorporation into the polymer to essentially 100% and that the resulting polymer had lower inherent and Mooney viscosities than did a comparative example made with neat perfluoroalkyldiiodide.  
     Example 3  
      A 41 liter reactor was charged with a water solution containing 17.5 grams perfluorohexylethylsulfonic acid, 12.9 grams disodium phosphate heptahydrate, and 24,969.6 grams deionized water. The reactor was brought to 80° C. and flushed with nitrogen to remove oxygen and then pressurized to 1.38 MPag with a mixture of 43 wt. % vinylidene fluoride, 3 wt. % tetrafluoroethylene, and 54 wt % perfluoro(methyl vinyl ether). 30.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. % disodium phosphate heptahydrate was added to initiate polymerization. As the reactor pressure dropped, a monomer feed of 55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % perfluoro(methyl vinyl ether) was added to maintain pressure. After 90 grams of this mixture had been added, a 5 volume % emulsion of a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in 1 wt. % perfluorohexylethylsulfonic solution, prepared as described in Method B (above), was fed at the rate of 25 mL/minute. After 10 minutes this feed was stopped. Additional initiator solution was added as needed to maintain polymerization. After a total of 8,333 grams of the mixture of 55 wt. % vinylidene fluoride, 10 wt. % tetrafluoroethylene, and 35 wt. % perfluoro(methyl vinyl ether) had been fed to the reaction, the reaction was stopped and the reactor depressurized. A 24.95 wt. % solids latex was obtained. The polymer was isolated by adding aluminum sulfate to the latex, and then dried at 70° C. The polymer had a Mooney viscosity of 42 and an inherent viscosity of 0.72.  
     Example 4  
      A 41 liter reactor was charged with a water solution containing 34.5 grams perfluorohexylethylsulfonic acid, 40.0 grams disodium phosphate heptahydrate, and 24,925.5 grams deionized water. The reactor was brought to 80° C. and flushed with nitrogen to remove oxygen and then pressurized to 2.00 MPag with a mixture of 25 wt. % tetrafluoroethylene, and 75 wt % perfluoro(methyl vinyl ether). 40.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. % disodium phosphate heptahydrate was added to initiate polymerization. As the reactor pressure dropped, a monomer feed of 52 wt. % tetrafluoroethylene, and 48 wt. % perfluoro(methyl vinyl ether) was added to maintain pressure. After 45 grams of this mixture had been added, a 9 volume% emulsion of a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in 1 wt. % perfluorohexylethylsulfonic solution, prepared as described in Method A (above), was fed at the rate of 16.5 mL/minute. After 7 minutes this feed was stopped. Additional initiator solution was added as needed to maintain polymerization. After a total of 8,333 grams of the mixture of 52 wt. % tetrafluoroethylene, and 48 wt. % perfluoro(methyl vinyl ether) had been fed to the reaction, the reaction was stopped and the reactor depressurized. A 24.03 wt. % solids latex was obtained. The polymer was isolated by adding aluminum sulfate to the latex, and then dried at 70° C. The polymer had a Mooney viscosity of 69.5.  
     Example 5  
      A 41 liter reactor was charged with a water solution containing 24.7 grams perfluorohexylethylsulfonic acid, 20.0 grams disodium phosphate heptahydrate, and 24,955.3 grams deionized water. The reactor was brought to 80° C. and flushed with nitrogen to remove oxygen and then pressurized to 1.72 MPag with a mixture of 25 wt. % vinylidene fluoride, 2 wt. % tetrafluoroethylene, and 73 wt % hexafluoropropylene. 50.0 grams of a solution of 1 wt. % ammonium persulfate and 5 wt. % disodium phosphate heptahydrate was added to initiate polymerization. As the reactor pressure dropped, a monomer feed of 50 wt. % vinylidene fluoride, 20 wt. % tetrafluoroethylene, and 30 wt. % hexafluoropropylene was added to maintain pressure. After 45 grams of this mixture had been added, a 9 volume % emulsion of a mixture of 1,4-diiodooctafluorobutane and 1,6-diiodododecafluorohexane in 1 wt. % perfluorohexylethylsulfonic solution, prepared as described in Method A (above), was fed at the rate of 16.5 muminute. After 12.5 minutes this feed was stopped. Additional initiator solution was added as needed to maintain polymerization. After a total of 8,333 grams of the mixture of 50 wt. % vinylidene fluoride, 20 wt. % tetrafluoroethylene, and 30 wt. % hexafluoropropylene had been fed to the reaction, the reaction was stopped and the reactor depressurized. A 25.46 wt. % solids latex was obtained. The polymer was isolated by adding aluminum sulfate to the latex, and then dried at 70° C. The polymer had a Mooney viscosity of 21 and an inherent viscosity of 0.55.