Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2001-12-11
2004-01-13
Pezzuto, Helen L. (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, C526S253000, C526S255000
Reexamination Certificate
active
06677414
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the aqueous emulsion polymerization of fluorinated monomers to produce fluoropolymers. In particular, the present invention relates to the aqueous emulsion polymerization involving liquid fluorinated monomers which have a boiling point of at least 50° C.
BACKGROUND OF THE INVENTION
Aqueous emulsion polymerization is a well-known method for making fluoropolymers, i.e. polymers of which the carbons in the backbone of the polymer have fluorine atoms attached to them. Generally, the amount of fluorination of the backbone of fluoropolymers will be at least 40% by weight, preferably at least 50% by weight. Fluoropolymers which have fully fluorinated backbones are called perfluoropolymers.
The fluoropolymers are generally produced by (co)polymerization of gaseous fluorinated monomers, in particular fluorinated olefins such as tetrafluoroethylene (TFE) hexafluoropropylene (HFP), vinyl fluoride, vinylidene fluoride (VDF) or chloro-trifluoroethylene (CTFE).
To obtain certain desired properties, it is known to copolymerise the aforementioned gaseous fluorinated olefins with monomers such as fluoro alkenyl ethers which include fluoro vinyl ethers and fluoro allyl ethers. Fluoro alkenyl ethers that have a high boiling point of 50° C. or more have a low vapor pressure at typical polymerization temperatures of 20° C. to 100° C. Additionally, they have an extremely low water solubility. As a result it has proven difficult to incorporate these liquid comonomers into fluoropolymers at larger levels as may be desired for certain applications. Incorporating large amounts of such comonomers is of particular value for producing elastomers that have a low Tg. When one wants to improve the amount incorporated of such monomers by increasing the polymerization temperature, that also increases the likelihood of undesired chain transfer reactions which impair the possibility of obtaining high molecular weight. On the other hand, polymerization at low temperature reduces the rate of polymerization making the process economically less attractive.
EP 219065 discloses an aqueous free radical emulsion polymerization of perfluoro vinyl ethers of the formula CF
2
═CF—O—(CF
2
CFXO)
m
—R
f
wherein X is F or CF
3
, m is 1 to 5 and R
f
is a C
1
-C
6
perfluoroalkyl group. EP 219065 discloses the use of carboxylate terminated perfluoropolyether emulsifiers for conducting the polymerization. However, in the examples of this EP-patent, fairly large amounts of the emulsifier are used, typically about 10% by weight based on the aqueous phase. Such high emulsifier concentrations usually lead to a so-called micro-emulsion polymerization which typically results in polymer particles of considerably less than 100 nm.
EP 250767 discloses an aqueous free radical emulsion polymerization involving for example perfluoroalkyl perfluorovinyl ether monomers by using a microemulsion of a perfluoropolyether with polyether based emulsifiers. This microemulsion is taught to have droplet sizes of less than 200 nm. The microemulsion is added to the polymerization medium at the beginning of the polymerization. The resulting particle sizes of the polymer particles obtained is very small, e.g. 40 nm, which is evidence that the polymerization occurs via microemulsion polymerization. Thus, the number of polymer particles per liter is increased by an order of magnitude. Therefore, the polymerization rate is considerably increased. However, EP 250767 does not describe pre-emulsification of liquid polymerizable fluorinated monomers like perfluoroalkyl perfluorovinyl ether monomers.
EP 816397 discloses the use of similar microemulsions of perfluoropolyethers with polyether based emulsifiers for the aqueous emulsion polymerization of vinylidene fluoride (VDF). As disclosed, the obtained latex particles have a very small size (apparently well below 100 nm) and likewise an increase of the polymerization rate is found. The polymerization here therefore evidently also occurs via microemulsion polymerization.
U.S. Pat. No. 4,864,006 also discloses the use of perfluoropolyethers microemulsified in perfluoroether-based emulsifiers at the aqueous emulsion polymerization of fluorinated monomers. All of these processes lead to microemulsion polymerizations characterized by an increased polymerization rate due to the observed small particle size, considerably lower than 100 nm.
The perfluoropolyether emulsifiers have the disadvantage that they have high boiling points and are difficult to remove from the polymer resin. Residual emulsifiers can have an adverse effect on the processing properties and the properties of the final product, for example lead to discoloration during work-up or processing, or to bleed-out during long-term use of the final product.
U.S. Pat. No. 5,608,022 teaches the aqueous emulsion polymerization of sulfonyl fluoride or carboxylic acid ester containing fluorinated vinyl ethers wherein the so-called functionalized vinyl ether monomer is pre-emulsified. According to Example 1 of this US-patent, the droplet size of the monomer droplets in the emulsion is 550 nm.
It would be desirable to find an improved aqueous emulsion polymerization process that allows for effective incorporation of large amounts of highly boiling fluorinated monomers. Desirably, the polymerization reaction should proceed at a high speed allowing for a more cost effective production of such fluoropolymers. The process desirably may be run using low emulsifier levels and using emulsifiers that can be readily removed from the resulting polymerization product.
SUMMARY OF THE INVENTION
The present invention provides a method of preparing a fluorinated polymer including repeating units derived from a liquid fluorinated monomer having a boiling point of at least 50° C. and selected from the group consisting of fluorinated olefins, fluorinated allyl ethers and fluorinated vinyl ethers that do not contain hydrolysable groups that upon hydrolysation yield ionic groups, the method comprising the steps of:
pre-emulsifying said liquid fluorinated monomer in water with the aid of a fluorinated emulsifier to obtain an aqueous emulsion of said fluorinated monomer; and
polymerizing a thus obtained emulsified fluorinated monomer.
Preferably the pre-emulsified liquid fluorinated monomer is copolymerized with one or more gaseous fluorinated monomers.
By the term “liquid fluorinated monomer” is meant that the monomer is generally present as a liquid at ambient conditions of temperature and pressure, i.e. at a temperature of 20° C. and a pressure of 1 atm. By the term “pre-emulsified” in connection with the present invention is meant that the fluorinated monomer is emulsified in water with the aid of the fluorinated emulsifier prior to polymerization of the liquid fluorinated monomer. The term aqueous emulsion is to be understood as a liquid emulsified in water that generally has a milky appearance generally having a settling time of at least 1 hour. Such settling time is generally achieved using a fluorinated emulsifier (preferably a non-telogenic emulsifier) other than a fluorinated polyether emulsifier. By the term “boiling point” in connection with the present invention is meant a boiling point under ambient conditions, i.e. at a pressure of about 1 atm. By the term “gaseous” in connection with the present invention is meant that the respective compounds are present as a gas under ambient conditions of temperature and pressure, i.e. at a temperature of about 20° C. and a pressure of about 1 atm. The fluorinated vinyl ether should be free of hydrolysable groups that upon hydrolysation yield ionic groups, in particular the fluorinated vinyl ether should not contain hydrolysable groups such as esters and SO
2
F. The fluorinated vinyl ether may contain substituents like chlorine, bromine and iodine, fluorinated alkyl groups, fluorinated alkoxy groups and fluorinated polyether functions.
It was found that when the liquid fluorinated monomer was emulsified prior to its copolymerization with gaseous fluorinated monomers, the pol
Hintzer Klaus
Löhr Gernot
März Franz
3M Innovative Properties Company
Harts Dean M.
Pezzuto Helen L.
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