Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1990-06-07
1991-09-03
Lipman, Bernard
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
5253262, 5253264, 525379, 525382, 525384, C08F 832
Patent
active
050456059
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for the stabilization of certain fluoropolymers, especially those useful in many high technology applications.
2. Background Art
Many fluoropolymers are known in the art. They include especially various copolymers of two or more comonomers such as, for example, tetrafluoroethylene (TFE), hexafluoropropene (HFP), chlorotrifluoroethylene (CTFE), perfluoro(methyl vinyl ether) (PMVE), perfluoro(propyl vinyl ether) (PPVE), and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD).
Such copolymers may be melt-processible and thus can be fabricated at high temperatures. However, they frequently suffer some deterioration during high temperature processing and thus lose some of their desirable properties such as, for example, freedom from bubbles. This thermal deterioration can be traced to the presence of various labile end groups, such as, e.g., carboxyl (--COOH) and fluorocarbonyl (--COF). The former tends to eliminate carbon-dioxide at high temperatures, while the latter, while more thermally stable, nevertheless tends to hydrolyze in the presence of moisture, which normally cannot be completely avoided, and is converted to carboxyl groups. Such hydrolysis also results in the evolution of hydrofluoric acid which is corrosive to most materials of industrial importance, including many metals, glass and quartz.
The removal of unstable end groups has long been an important part of the technology of perfluorinated melt-processible copolymers of TFE. Schreyer, U.S. Pat. No. 3,085,083, discloses the treatment of such polymers "with water, preferably in the presence of inorganic compounds having a pH of at least 7, such as stable bases, . . . at a temperature of 200.degree.-400.degree. C., and recovering a fluorocarbon polymer having at least half of all the end-groups in the form of difluoromethyl groups" (--CF.sub.2 H). There is no suggestion in this patent, which discloses the use of inorganic treating agents, of the unexpected results achieved by means of this invention which requires the use of secondary or tertiary amines.
Buckmaster et al., U.S. Pat. No. 4,675,380, disclose the fluorination of melt-processible TFE copolymers which have been coagulated by stirring in the presence of a mineral acid and a water-immiscible liquid and then isolated. The total number of unstable end groups was reduced to less than 80 per 10.sup.6 carbon atoms.
U.K. Patent 1,210,794 to Du Pont discloses the fluorination of fluorocarbon copolymers to reduce the number of unstable end groups. The process of that patent, when carried out with some of the copolymers of interest in the present invention, needs to employ a fluorination temperature of at least 225.degree. C. to remove unstable end groups.
Although fluorination of fluoropolymers can be employed to reduce the concentration of multiple bonds and unstable end groups, complete fluorination requires high temperatures, usually above 200.degree. C., to remove substantially all --COF groups. However, if the polymers soften or begin melting at the fluorination temperature, such a process causes agglomeration of polymer particles, which leads to difficulties in their further handling and processing.
Furthermore, high temperature fluorination can cause equipment corrosion, and it is difficult to handle fluorine safely because it is toxic and is a strong oxidizing agent. It would be desirable to be able to remove --COOH and --COF groups from fluoropolymers without the use of fluorine, and preferably without having to heat the polymer above its melting point.
Carbon- and graphite-filled fluorocarbon compositions for electrical applications have been known for some time. They are primarily used in preference to other conductive polymers when chemically active and/or high temperature environments are to be encountered. Applications in which conductive fluorocarbons containing carbon black and/or graphite are used include current-limiting devices, e.g., self-regulating heater cable (U.S. Pat. Nos. 4,318,881, 4,624,
REFERENCES:
patent: 3085083 (1963-04-01), Schreyer
patent: 3686154 (1972-08-01), Khan
patent: 4276214 (1981-06-01), Yoshimura et al.
patent: 4530569 (1985-07-01), Squire
patent: 4675380 (1987-06-01), Buckmaster et al.
patent: 4693553 (1987-09-01), Sasaki et al.
E. I. Du Pont de Nemours and Company
Lipman Bernard
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