Process for producing fluoroelastomers

Details Process for producing fluoroelastomers Process for producing fluoroelastomers

2001-03-16

2002-02-19

Zitomer, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S080000, C526S247000, C526S249000, C526S250000, C526S255000

Reexamination Certificate

active

06348552

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to a novel process for the production of a fluoroelastomer; more particularly, it pertains to a suspension polymerization process for the production of a fluoroelastomer comprising copolymerized units of vinylidene fluoride, units of at least one other fluorinated major monomer and units of at least one cure site monomer and wherein said fluoroelastomer has substantially no ionic endgroups.
BACKGROUND OF THE INVENTION
Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely 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 copolymerizable fluorine-containing major monomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE), vinyl fluoride (VF), and a perfluoro(alkyl vinyl ether) (PAVE). Specific examples of PAVE include perfluoro(methyl vinyl ether), perfluoro(ethyl vinyl ether) and perfluoro(propyl vinyl ether).
In order to develop the physical properties necessary for some end use applications, fluoroelastomers must be crosslinked. Typical curatives for promoting crosslinking include polyamines, polyols and the combination of an organic peroxide and a multifunctional unsaturated coagent. All these compounds form crosslinks by reacting with a cure site on the fluoroelastomer polymer chain. Examples of cure sites include a double bond, or a labile hydrogen, bromine, iodine, or chlorine atom. A common method of introducing a cure site into a fluoroelastomer made by continuous emulsion polymerization is to continuously add a minor amount of a copolymerizable cure site monomer, along with the major monomers (e.g. VF
2
, HFP, TFE, PAVE, etc.) to the polymerization reactor. In this manner, cure sites are randomly distributed along the resulting fluoroelastomer polymer chain. Suitable cure site monomers include bromine- or iodine-containing olefins, and bromine- or iodine-containing unsaturated ethers, non-conjugated dienes and 2-hydropentafluoropropylene (2-HPFP). Alternatively, or in addition to cure site monomers, cure sites may be introduced into the fluoroelastomer by conducting the polymerization in the presence of a chain transfer agent containing iodine, bromine or both. In this manner, a bromine or iodine atom is attached to the resulting fluoroelastomer polymer chain at one or both ends. Such chain transfer agents typically have the formula RI
n
, RBr
n
or RBrI, where R may be a C
1
-C
3
hydrocarbon, a C
1
-C
6
fluorohydrocarbon or chlorofluorohydrocarbon, or a C
2
-C
8
perfluorocarbon, and n is 1 or 2.
Production of such fluoroelastomers by emulsion and solution polymerization methods is well known in the art; see for example U.S. Pat. No. 4,214,060. Generally, fluoroelastomers are produced in an emulsion polymerization process wherein a water-soluble polymerization initiator and a relatively large amount of surfactant are employed. The resulting fluoroelastomer leaves the reactor in the form of a latex which must be degassed (i.e. freed from unreacted monomers), coagulated, filtered and washed. Emulsion processes suffer from several disadvantages including production of polymers having high Mooney viscosity, which tends to make it difficult to process these materials (i.e. mixing, extruding, molding) into cured articles, due to the presence of ionic endgroups on the fluoroelastomer polymer chains. Another disadvantage is that the polymer products contain impurities from retained surfactants, coagulants, buffers and defoamers. milliequivalents of ionic endgroups per kg fluoroelastomer. Ionic (or ionizable) endgroups include, but are not limited to, sulfate, sulfonate, sulfonic acid, carboxyl and carboxylate endgroups.
In particular, the present invention is directed to a suspension process for producing a fluoroelastomer having a selected molar ratio of copolymerized monomer units, said fluoroelastomer comprising copolymerized units of vinylidene fluoride major monomer, at least one other copolymerizable fluorinated major monomer, and at least one cure site monomer, comprising the steps of:
(A) charging a reactor with a quantity of an aqueous medium comprising a suspension stabilizer, said suspension stabilizer being present in said aqueous medium at a concentration of 0.001 to 3 parts by weight per 100 parts of said aqueous medium; said quantity of aqueous medium being such that a sufficient vapor space is left in said reactor for receiving gaseous monomer;
(B) charging the vapor space in said reactor with an initial quantity of a gaseous monomer mixture comprising vinylidene fluoride major monomer and at least one other fluorinated major monomer; and continuously mixing said aqueous medium and said monomer mixture to form a dispersion;
(C) initiating polymerization of said monomers at a temperature of 45° C. to 70° C. by adding to said dispersion an oil soluble organic peroxide polymerization initiator in an amount of 0.001 to 5 parts by weight per 100 parts of said aqueous medium, said initiator being added as a solution consisting essentially of 0.1 to 75 wt. % of an oil soluble organic peroxide in a water-soluble hydrocarbon solvent; and
(D) incrementally feeding to said reactor, during polymerization, so as to maintain a constant pressure in said reactor, said major monomers and at least one
On the other hand, in a suspension polymerization process, polymerization is carried out by dispersing one or more monomers, or an organic solvent with monomer dissolved therein, in water and using an oil-soluble organic peroxide. No surfactant or buffer is required and fluoroelastomer is produced in the form of polymer particles which may be directly filtered, i.e. without the need for coagulation, and then washed, thus producing a cleaner polymer than that resulting from an emulsion process. Also, the fluoroelastomer polymer chains are substantially free of ionic endgroups so that the Mooney viscosity is relatively low and the polymer has improved processability compared to polymer produced by an emulsion process (U.S. Pat. Nos. 3,801,552, 4,985,520 and 5,824,755).
A disadvantage of suspension polymerization processes disclosed in the prior art is that it is difficult to incorporate a cure site monomer uniformly into the polymer because polymerization rate and polymer molecular weight increase throughout the reaction period. Many cure site monomers, if present in excess, greatly hinder the polymerization reaction, so that the desired polymerization rate and polymer molecular weight can not be attained in suspension polymerization processes of the prior art.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides a suspension polymerization process for the production of fluoroelastomers having uniformly distributed copolymerized units of one or more cure site monomers. The fluoroelastomers are characterized by having molecular weights sufficiently high to permit processing and curing using conventional techniques.
A further aspect of the invention relates to production of fluoroelastomer products which are substantially free of ionic endgroups. Such fluoroelastomers have lower Mooney viscosities than fluoroelastomers of similar comonomer composition and molecular weight produced from an emulsion polymerization process. By “substantially no ionic endgroups” is meant fewer than 1 cure site monomer, said major monomers and said cure site monomer being fed to the reactor in said selected molar ratio until a fluoroelastomer product having a number average molecular weight of between 50,000 to 2,000,000 daltons is obtained.
Optionally, a chain transfer agent may be added near the beginning of the polymerization process and additional quantities may be introduced throughout the process.
Another embodiment of this invention is the fluoroelastomer produced by the above process of this invention. Such a fluoroelastomer may be distinguished from a fluoroelastomer made by a different polymerization process i

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