Process for producing fluoroelastomers

Details Process for producing fluoroelastomers Process for producing fluoroelastomers

2001-07-18

2003-01-28

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...

C526S247000, C526S249000, C526S250000, C526S255000

Reexamination Certificate

active

06512063

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to a novel emulsion polymerization process for the production of fluoroelastomers wherein a certain class of hydrocarbon sulfonate anionic surfactant is used as the dispersing agent.
BACKGROUND OF THE INVENTION
Fluoroelastomers having excellent heat resistance, oil resistance, and chemical resistance have been used widely for sealing materials, containers and hoses.
Production of such fluoroelastomers by emulsion and solution polymerization methods is well known in the art; see for example U.S. Pat. Nos. 4,214,060 and 4,281,092. 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 surfactant most often used for such processes has been ammonium perfluorooctanoate (C-8). Fluoroelastomers prepared in such processes leave the reactor in the form of a dispersion.
While C-8 works very well as a surfactant in the polymerization process, it is relatively expensive, and its future commercial availability is uncertain. Thus, it would be desirable to find other surfactants effective for use in the emulsion polymerization of fluoroelastomers.
Efforts to replace C-8 have focussed on other expensive anionic fluorosurfactants such as 1) F—(CF
2
CF
2
)
n
CH
2
CH
2
—OSO
3
M, where n is an integer from 2-8, or mixtures thereof, and M is an alkali metal cation, hydrogen ion or ammonium ion (U.S. Pat. No. 4,524,197); 2) F—(CF
2
CF
2
)
n
CH
2
CH
2
—SO
3
M, where n is an integer from 2-8, or mixtures thereof, and M is an alkali metal cation, hydrogen ion or ammonium ion (U.S. Pat. No. 4,380,618); and 3) C
6
F
13
CH
2
CH
2
SO
3
M wherein M is a cation having a valence of 1 (U.S. Pat. No. 5,688,884). Such fluorinated surfactants are effective in forming stable dispersions of highly fluorinated fluoroelastomer latex particles during the emulsion polymerization process because of the highly fluorinated structure of the surfactants.
Hydrocarbon sulfonate surfactants such as sodium dodecylbenzenesulfonate and sodium sulfosuccinate have been used in the emulsion polymerization of tetrafluoroethylene/propylene copolymers (G. Kostov et al. in God. Vissh. Kim.-Tekhnol. Inst., 29(2), 316-320 (1983)). These copolymers contain a large amount of propylene units and have a relatively low amount of fluorine, i.e. less than 58 weight percent fluorine. It is believed that hydrocarbon sulfonate surfactants can be successfully employed in the emulsion polymerization of tetrafluoroethylene/propylene copolymers because of the compatibility between the hydrocarbon surfactant and the high level of propylene units present in the copolymer. Such surfactants would be unlikely to satisfactorily stabilize highly fluorinated (i.e. having at least 58 wt. % fluorine) fluoroelastomer latex particles.
SUMMARY OF THE INVENTION
Surprisingly, it has been found that certain hydrocarbon sulfonate surfactants may be used to manufacture highly fluorinated fluoroelastomers. One aspect of the present invention provides an emulsion polymerization process for the production of fluoroelastomers, said fluoroelastomers having at least 58 weight percent fluorine, comprising:
(A) charging a reactor with a quantity of an aqueous solution comprising a surfactant of the formula CH
3
—(CH
2
)
n
—SO
3
M where n is an integer from 6 to 17, or mixtures thereof, and M is a cation having a valence of 1;
(B) charging the reactor with a quantity of a monomer mixture to form a reaction medium, said monomer mixture comprising i) from 25 to 70 weight percent, based on total weight of the monomer mixture, of a first monomer, said first monomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene, and ii) between 75 and 30 weight percent, based on total weight of the monomer mixture, of 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 vinyl ethers, hydrocarbon olefins and mixtures thereof; and
(C) polymerizing said monomers in the presence of a free radical initiator to form a fluoroelastomer dispersion while maintaining said reaction medium at a pH between 1 and 7, at a pressure between 0.5 and 10 MPa, and at a temperature between 25° C. and 130° C.
In other aspects of the invention, the surfactant employed in (A) may be substituted by CH
3
—(CH
2
)
n
—C
6
H
4
—SO
3
M or CH
3
—(CH
2
)
n
—CH═CHCH
2
—SO
3
M, wherein n and M are as defined above.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to an emulsion polymerization process for producing a fluoroelastomer. By “fluoroelastomer” is meant an amorphous elastomeric fluoropolymer. The fluoropolymer may be partially fluorinated or perfluorinated, so long as it contains at least 58 percent by weight fluorine, preferably at least 64 wt. % fluorine. Fluoroelastomers made by the process of this invention contain between 25 to 70 weight percent, based on the weight of the fluoroelastomer, of copolymerized units of a first monomer which may be vinylidene fluoride (VF
2
) or tetrafluoroethylene (TFE). The remaining units in the fluoroelastomers are comprised of one or more additional copolymerized monomers, different from said first monomer, selected from the group consisting of fluorine-containing olefins, fluorine-containing vinyl ethers, hydrocarbon olefins and mixtures thereof.
According to the present invention, fluorine-containing olefins copolymerizable with the first monomer include, but are not limited to, vinylidine fluoride, hexafluoropropylene (HFP), tetrafluoroethylene (TFE), 1,2,3,3,3-pentafluoropropene (1-HPFP), chlorotrifluoroethylene (CTFE) and vinyl fluoride.
The fluorine-containing vinyl ethers employed in the present invention include, but are not limited to perfluoro(alkyl vinyl) ethers. 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.
If copolymerized units of PAVE are present in fluoroelastomers prepared by the process of the invention, the PAVE 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.
Hydrocarbon olefins useful in the fluoroelastomers prepared by the

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