High purity fluoropolymers

Details High purity fluoropolymers High purity fluoropolymers

2001-05-23

2004-02-17

Wilson, Donald R. (Department: 1713)

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

Synthetic resins

Treating polymer containing material or treating a solid...

C525S486000, C525S487000, C525S490000

Reexamination Certificate

active

06693164

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to novel high purity fluoropolymers and methods for their production.
BACKGROUND OF THE INVENTION
Fluoropolymers are widely used in industrial applications because of their unique combination of chemical stability and high temperature properties. Certain applications, such as in the semi-conductor industry, demand the highest levels of stability and purity.
The process of polymerizing fluoropolymers results in end groups on the polymer chain ends which are determined by the type of polymerization initiator. Some of these end groups are reactive under certain conditions. Thus, they are a source of instability, albeit at a low level. Reduction of these end groups through fluorination is known and discussed in U.S. Pat. No. 4,743,658, EP 457 255, GB 1,210,794 and DE 1,901,872.
Extractable ions are contained in fluoropolymers as a result of current practices for the production of these materials. The level of these impurities may be increased as a result of the corrosion potential of the unstable end groups discussed above. When these unstable end groups come in contact with metal articles, such as process equipment or containers, impurities can result.
A need still exists for fluoropolymers with none of or very low levels of these impurities and for improved methods for their reduction and/or removal.
SUMMARY OF THE INVENTION
The present invention provides novel fluoropolymers comprising low levels of extractable ions and improved methods of producing such materials.
In one aspect, the present invention relates to a method for the production of a high purity fluoropolymer comprising the steps of:
a) providing a fluoropolymer having extractable ions, and
b) contacting the fluoropolymer with an aqueous acid medium for a time sufficient to remove the extractable ions from the fluoropolymer.
In a further aspect, the invention relates to a method for the production of a high-purity fluoropolymer wherein the fluoropolymer has been contacted with the aqueous acid medium for a time sufficient to reduce the level of the extractable ions in the fluoropolymer to less than 0.05 parts per million (ppm) extractable ion, preferably less than 0.02 ppm. These extractable ions are predominantly iron.
This invention is preferably used when the fluoropolymer is a thermoplastic and more preferably used when the fluoropolymer is perfluorinated, i.e. a fluoropolymer derived from perfluorinated monomers.
The present invention also pertains to a method comprising the further step of removing unstable end groups from perfluorinated polymers, particularly when such perfluoropolymers are thermoplastic. Fluorination of the fluoropolymer is a useful method of removing unstable end groups. A useful method of fluorination comprises contacting the fluoropolymer with a fluorine-containing gas. A preferred method of fluorination occurs in an essentially stationary bed. The fluorination process is preferably carried out at a temperature range of between 50° C. and the onset of melting.
By use of such methods, the number of unstable end groups in the fluoropolymer may be reduced to less than 30 per 10
6
carbon atoms. Preferably, the methods are used to reduce the number of unstable end groups to less than 5 per 10
6
carbon atoms and more preferably to reduce the unstable end groups to less than 1 per 10
6
carbon atoms and still more preferably to reduce the unstable end groups to zero per 10
6
carbon atoms.
The present invention further relates to a fluoropolymer comprising less than 0.05 ppm extractable metal ions, preferably less than 0.02 ppm. More preferably, the fluoropolymer also comprises less than 30 unstable end groups per 10
6
carbon atoms. Such fluoropolymers are particularly useful in the semi-conductor industry.
The invention also relates, in a preferred combination, to a method for the production of a high purity perfluorinated thermoplastic polymer essentially free from extractable ions comprising the steps of:
a) providing a perfluorinated thermoplastic polymer in agglomerate form,
b) drying the agglomerate to remove residual moisture,
c) fluorinating the dried agglomerate in an essentially stationary bed at a temperature between 50° C. and the onset of melting of the agglomerate,
d) removing the fluorination media from the agglomerate,
e) pelletizing the fluorinated agglomerate, and
f) removing any extractable ions by contacting the pelletized agglomerate with an aqueous acid medium.
DESCRIPTION OF THE INVENTION
The invention pertains to a method for the production of high-purity fluoropolymers, whereby the agglomerate is first fluorinated preferably in an essentially stationary bed. The fluorinated agglomerate is then melt pelletized. The pellets are then subjected to an extraction process by which extractable ions are removed in an aqueous acid medium. The high-purity products obtained in this way are well-suited for purposes demanding high purity, in particular for the semiconductor industry.
In the following, preferred versions of the invention are explained in more detail.
Typical perfluorothermoplastics are semi-crystalline copolymers made up primarily of units of tetrafluoroethylene (TFE) and of perfluoroalkylvinyl ethers such as perfluoro-(n-propyl-vinyl) ether (PPVE) or perfluorinated olefins such as hexafluoropropylene (HFP). Copolymers made of TFE and PPVE are commercially available under the designation “PFA”, and copolymers of TFE with HFP are available as “FEP”. PFA is extensively described in
Modern Fluoropolymers
, John Wiley & Sons, 1997, p. 223 ff., and FEP in Kirk-Othmer,
Encyclopedia of Chemical Technology
, John Wiley & Sons, Fourth Edition, Volume 11 (1994), p. 644. In conjunction with this, copolymers such as PFA and FEP can contain additional perfluorinated comonomers. In this regard, perfluorinated thermoplastics is understood to mean that the resin contains no hydrogen except in the end groups.
As a result of the current practice of production and processing of the resins, metal impurities are unavoidable. This invention describes a method for minimizing and removing these impurities. Because the concentration of iron impurities is generally higher than that of other heavy metals by as much as a factor of 10 or more, iron is considered to be the primary impurity and will be the impurity used to track performance. The product of the invention will exhibit a low level of extractable ions, preferably less than 0.05 ppm of extractable ions and of that, less than 0.02 ppm iron. In accordance with the invention, the extraction is carried out in an aqueous acid medium, advantageously using a volatile acid such as formic acid, hydrofluoric acid, hydrochloric acid or, preferably, nitric acid. The acid used should be of a high purity, such as an analytical grade. The use of hydrochloric acid may lead to corrosion, however. The pH value of the extraction solution is preferably below 6. Thus, if the melt pellets still contain hydrofluoric acid from the production of the polymer, it might not be necessary to add additional acid to the extraction solution.
It is known that the purity of fluorinated plastics can be analytically determined by means of extraction, e.g., through the use of aqueous nitric acid. This is not known as a method of producing fluoropolymers, but only a means of finding and quantifying the impurities, after which the extracted sample is discarded. One aspect of the invention therefore pertains to a method for removing extractable ions by contacting the fluoropolymer with an acidic aqueous extraction solution.
The purification of fluoropolymers, fluorothermoplastics, for example, in the form of a granulate or a formed object by means of extraction using aqueous solutions which contain an inorganic peroxide such as hydrogen peroxide and a complexing agent without acid groups such as triethanolamine, is known from EP A-652 283. In addition, the treatment of fluoropolymer melt pellets with aqueous ammonia for the suppression of corrosion of the metal vessels being used is described in U.S. Pat. No. 5,861,464.

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