Process for the preparation of low-viscosity fluorinated...

Details Process for the preparation of low-viscosity fluorinated... Process for the preparation of low-viscosity fluorinated...

1999-06-04

2001-04-03

Wu, David (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S204000, C526S254000, C526S217000, C526S255000, C526S206000, C526S247000

Reexamination Certificate

active

06211314

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a new process for the preparation of low-viscosity fluorinated rubbers.
BACKGROUND OF THE INVENTION
In the rubber industry, there is a general demand for better processability of the rubbers employed. This concerns the flow properties in particular. The lower the viscosity of the crude rubber, the simpler the processing technology, the greater the productivity and the lower the waste. The aspects mentioned are of great relevance to fluorinated rubbers in particular, since these are expensive rubbers which cannot be processed without reservation on the injection moulding machines of the rubber industry.
The process of emulsion polymerization which has hitherto been customary on an industrial scale is unsuitable for the preparation of low-viscosity fluorinated rubbers, such has have been described e.g. in DE-A 196 40 972.1, since on the one hand the need to use ionic inorganic initiators leads to ionic end groups in the polymer, which increase the viscosity of the polymer, and on the other hand large amounts of molecular weight regulators are necessary, and these as a rule lead to a drastic reduction in the reactor productivity because of side reactions with the initiator in the aqueous phase and termination reactions.
Of the non-aqueous processes, polymerizations in the pure liquefied fluorine monomer have proved to be unfavourable, since the polymers formed are usually not soluble therein and they possess poor swelling properties. A reproducible polymerization procedure with good transfer of heat and material and therefore acceptable reactor productivity is possible to just as little an extent by this route.
In contrast, fluorine monomers can be readily polymerized in the presence of certain fluorine-containing solvents, see e.g. U.S. Pat. No. 4,243,770, DE-A 196 40 972.1. U.S. Pat. No. 5,182,342 describes the use of fluorinated hydrocarbons as the polymerization medium, which meet certain criteria in respect of the F/H ratio and the position of the hydrogens. With all compounds of this type which contain hydrogen and optionally also additionally chlorine, there is always the problem that these can undergo transfer and/or termination reactions. In the case where defined end groups are to be introduced by the molecular weight regulator, however, no additional transfer reactions should start from the solvent under the reaction conditions chosen. This applies in particular to the preparation of low-viscosity rubbers which contain covalently bonded iodine on the terminal carbon atoms due to the use of diiodo-organic chain transfer agents. In DE-A 196 40 972.1, this is achieved by the use of 1,1,2-trichlorotrifluoroethane as the polymerization medium. Compounds of this type (fluorochlorocarbons), however, have a considerable ozone-depleting potential. For this reason, their use in industry is already banned in many industrial countries.
The object of the present invention was therefore to discover a practicable process for the preparation of low-viscosity fluorinated rubbers which proceeds without the use of ozone-depleting compounds.
SUMMARY OF THE INVENTION
It has now been found that at least one compound of the formula (I) or (II)
wherein R
1
represents a fluorine atom or a perfluoroalkyl radical having 1-4 C atoms and R
2
represents a perfluoroalkyl radical having 1-4 C atoms and n=4 or 5,
are suitable media for the preparation of low-viscosity fluorinated rubbers in respect of the criteria discussed above.
DETAILED DESCRIPTION OF THE INVENTION
The present invention therefore provides a process for the preparation of low-viscosity fluorinated rubbers, in which at least one fluorine monomer is polymerized by free radicals in the presence of one or more compounds of the formula (I) or (II)
wherein R
1
represents a fluorine atom or a perfluoroalkyl radical having 1-4 C atoms and R
2
represents a perfluoroalkyl radical having 1-4 C atoms and n=4 or 5,
and, optionally, a molecular weight regulator.
Monomers which can be employed in the context of the invention are fluorinated, optionally substituted ethylenes which, in addition to fluorine, can contain hydrogen and/or chlorine, such as e.g. vinylidene fluoride, tetrafluoroethylene and chlorotrifluoroethylene, fluorinated 1-alkenes having 2-8 carbon atoms, such as e.g. hexafluoropropene, 3,3,3-trifluoropropene, chloropentafluoropropene, hexafluoroisobutene and/or perfluorinated vinyl ethers of the formula CF
2
=CF—O—X, where X=C
1
-C
3
-perfluoroalkyl or —(CF
2
—CFY—O)
n
-RF, wherein n=1-4, Y=F or CF
3
and RF=C1-C
3
-perfluoroalkyl.
The combination of vinylidene fluoride, hexafluoropropene and, optionally, tetrafluoroethylene and/or perfluorinated vinyl ethers, such as e.g. perfluoro-(methyl vinyl ether), is particularly preferred.
In addition, the use of copolymerizable bromine-containing monomers, such as e.g. bromotrifluoroethylene, 4-bromo-3,3,4,4-tetrafluorobut-1-ene, as described in U.S. Pat. No. 4,035,565, or 1-bromo-2,2-difluoroethylene, is also possible for the preparation of peroxidically crosslinkable fluorinated rubbers.
The free-radical polymerization is preferably carried out in the presence of at least one initiator.
In each case organic or fluoro-organic dialkyl peroxides, diacyl peroxides, dialkyl peroxydicarbonates, alkyl peresters and/or perketals are preferably employed as the initiator, e.g. tert-butylperoxypivalate, tert-butylperoxy-2-ethyl-hexanoate, dicyclohexylperoxydicarbonate, bis(trifluoroacetylperoxide) or the peroxide of hexafluoropropeneoxide-dimer {CF
3
CF
2
CF
2
OCF(CF
3
)COO}
2
. The nature and the amount to be employed depend on the particular reaction temperature. The half-lives of the peroxide to be chosen are preferably between 30 and 500 min. Correspondingly, amounts of between 0.05 and 1.0 parts by wt. peroxide per 100 parts by wt. monomers to be reacted are preferably required.
The molecular weights and therefore the viscosities of the target products can be determined via the amount of initiator or by addition of one or more molecular weight regulators. Molecular weight regulators which are preferably employed are compounds of the formula (III)
R
3
Br
n
I
m
  (III)
where m and n=0 to 2 and m+n=1 or 2,
wherein R
3
can be an aliphatic hydrocarbon, fluorohydrocarbon, fluorochlorohydrocarbon or fluorocarbon radical having 1-8 carbon atoms. 1,2-Dibromo-1-chlorotrifluoroethane and/or 1-bromo-2-iodo-tetrafluoroethane are preferred as the compound of the formula (III). Diiodo-organic compounds in which the two iodine atoms are bonded to one carbon atom or to different carbon atoms are preferred. Hydrocarbon or fluorocarbon compounds having one or 4 carbon atoms, the iodine being on the terminal carbon atoms, are particularly preferred. Diiodomethane and/or 1,4-diiodo-perfluorobutane are especially preferred. The amount of diiodo-organic compound is preferably 0.3 to 3.0 parts by wt. iodine per 100 parts by wt. polymerized fluorine monomer.
In the process according to the invention, the free-radical polymerization is preferably carried out in the presence of at least one fluoro-organic dialkyl peroxide, diacyl peroxide, dialkyl peroxydicarbonate, alkyl perester and/or perketal as an initiator, in combination with a molecular weight regulator of the formula (III) in which R
3
is a hydrocarbon radical having 1-8 carbon atoms.
It is also preferable to employ, as the initiator, an organic peroxide which does not contain fluorine, in combination with a molecular weight regulator of the formula (III) in which R
3
is a fluorohydrocarbon, fluorochlorohydrocarbon or fluorocarbon radical having 1-8 carbon atoms.
For the preferred case of polymerization in the presence of a diiodo-organic molecular weight regulator, it has proved advantageous for either the molecular weight regulator or the initiator to be fluorinated.
In a particularly preferred embodiment of the process according to the invention, an organic peroxide which does not contain fluorine is empl

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