Heat Stable Tetrafluoroethylene-Perfluoro(alkyl Vinyl Ether) Copolymers

This invention relates to tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymers (TFE/FAVE copolymers) in which heat unstable carbinol end-groups (--CH.sub.2 OH groups) present have been protected against heat degradation.

It had earlier been discovered that when tetrafluoroethylene (TFE) and perfluoro(alkyl vinyl ethers) (FAVE) are copolymerized in the presence of organic halocarbon solvents certain properties of the resulting copolymer are enhanced when alcohols, such as methanol, are added to the reaction mixture. The properties enhanced include reduced die swell on extrusion and improved flex life. The advantages of using such alcohols are described in U.S. Pat. No. 3,642,742.

The alcohol acts as a chain transfer agent, i.e., it stops chain propagation by reacting with free radical end groups of the growing polymer chain. This reaction of alcohol with free radical ends results in a polymer that has carbinol, i.e., --CH.sub.2 OH, end groups. While these carbinol groups are relatively stable, it has been found that TFE/FAVE copolymers sometimes need to be annealed, i.e., heated to high temperatures after molding into molded articles, in order to impart good dimensional stability to the molded articles. However, the high temperatures used during annealing in an ambient atmosphere cause the carbinol, --CH.sub.2 OH, end groups to decompose to acid fluoride, --COF, groups. These acid fluoride end groups then hydrolyze to carboxyl, --COOH, groups with evolution of hydrogen fluoride (HF). HF is a reactive, corrosive acid and its presence causes difficulties in moled articles derived from TFE/FAVE copolymers that contain the original --CH.sub.2 OH end groups. In addition, the --COOH groups degrade on fabrication of the polymer to produce gases (CO.sub.2) that produce undesirable bubbles in molded articles.

The raw polymer, as made, contains some --COF and --COOH end-groups and these decompose also, along with the --COF and --COOH groups formed from decomposition of the --CH.sub.2 OH end-groups.

In an effort to reduce the number of --COF and --COOH end-groups, it has now been discovered that those groups present in the raw polymer, as made, can be converted to amide end-groups. It has also now been discovered that this conversion to amide groups of the original --COF and --COOH end-groups surprisingly imparts thermal stability to the --CH.sub.2 OH end-groups and retards their decomposition. Accordingly, this invention is directed to a means for stabilizing the carbinol (--CH.sub.2 OH) end groups to retard their decomposition to --COF groups and, in turn, the breakdown of the --COF groups to form corrosive HF and undesirable --COOH end groups. The stabilizing means is not completely known or understood, but it has now been found that when TFE/FAVE copolymers containing --CH.sub.2 OH, --COF and --COOH end groups (which are conveniently made in the presence of halocarbon solvents and an alcohol) are treated with nitrogen-containing compounds such as ammonia or ammonia-producing compounds such as ammonium salts or organic amines, the resulting material has enhanced propensity against heat-degradation of the carbinol groups. These nitrogen-containing compounds react with --COF and --COOH end-groups present in the polymer to form amide end groups, i.e., --CONH.sub.2 groups.

The copolymer to be protected from thermal degradation is conveniently made by reacting tetrafluoroethylene (TFE) and at least one perfluoro(alkyl vinyl ether) (FAVE) containing 3 to 10 carbon atoms, i.e. R.sub.f O--CF.dbd.CF.sub.2 where R.sub.f is perfluoroalkyl of 1-8 carbon atoms in the presence of a halocarbon solvent and a 1-3 carbon alcohol present as a transfer agent. The resulting copolymer contains --CH.sub.2 OH, --COOH, and --COF end groups. Typically, --CH.sub.2 OH end-groups will be present in an amount of at least 15 groups, preferably at least 20, more preferably at least 30 and most preferably at least 40 groups, per 10.sup.6 carbon atoms; and --COF and --COOH groups will be present in a combined amount of at least 15 groups, preferably at least 30, and more preferably at least 40 groups, per 10.sup.6 carbon atoms. The copolymers generally will contain 0.5 to 20% (by weight) FAVE units and generally will have a melt viscosity of between 1.times.10.sup.4 poise and 35.times.10.sup.4 poise at 372.degree. C. The preparation of these copolymers is described in U.S. Pat. No. 3,642,742. Suitable solvents include chlorofluoroalkanes of 1-4 carbons and preferable of 1-2 carbons. Suitable alcohol chain transfer agents include alcohols of 1-3 carbon atoms, e.g., isopropanol, propanol, ethanol and preferably methanol. In addition, the copolymers can contain minor amounts, e.g. up to 15% by weight, of other copolymerizable comonomer units, e.g. hexafluoropropylene units.

To treat these copolymers with nitrogen compounds to convert the --COF and --COOH groups to amide groups, the copolymer is simply contacted with the nitrogen compound. Ammonia vapor is the preferred nitrogen compound and it can be simply passed into a closed container containing the copolymer for a desired amount of time or can be passed over or through a bed of the copolymer for a desired amount of time. The copolymer can be agitated during contact if desired. The form of the copolymer is not important; it can be in flake, cube, pellet or article form. The atmosphere in the container can be 100% NH.sub.3 or can be as little as 0.1% by volume NH.sub.3 (with the remainder being air or inert gases). In dilute form, 1-30% by volume is preferred and 10-30% most preferred. Contact time is long enough to achieve the desired degree of conversion to amide end groups, usually at least 50%, and preferably about 100%. The time is usually between 1/2 to 24 hours, with 1 to 71/2 hours preferred and 2 to 6 hours most preferred. Temperature and pressure are not critical. Pressure is usually atmospheric, for convenience. Temperature is usually room temperature, 20.degree.-30.degree. C., but can be between 0.degree. and 100.degree. C. Usually when --COOH groups are to be converted to amide groups, higher temperatures are used.

When ammonia-forming ammonium salts or organic amines are used, the copolymer is ordinarily melted in contact with the nitrogen compound. A convenient means is to extrude the molten copolymer and the nitrogen compound. Among the nitrogen compounds that can be employed in this manner are

The amount of the nitrogen compound present in this melt contact can be 0.01-0.5% by weight of copolymer. The melt temperatures can be 320.degree.-370.degree. C., and the contact times can be 2-30 minutes.

Regardless of the nitrogen compound used, the resulting copolymer is one in which --COF and --COOH end groups present originally have been converted to --CONH.sub.2 groups. Thus the treated copolymer contain less than half the --COF and --COOH groups originally present and preferably will contain substantially no such groups. Conversely, the treated copolymer will contain an amount of --CONH.sub.2 groups approximately equal to or greater than the combined number of --COF and --COOH groups. Ordinarily there will be at least 20 --CONH.sub.2 groups present per 10.sup.6 carbon atoms, and sometimes 30 or 40 or more such groups.

The hydrolytic instability of raw TFE/FAVE copolymers is easily demonstrated by immersing the copolymer in water and measuring the amount of fluoride ions extracted into the water. Typically, raw copolymer will liberate 20 ppm or more F.sup.- ions based on weight of copolymer, whereas the treated copolymer liberates less than 5 ppm F.sup.-, and preferably less than 1.

The type and quantity of certain end groups in the polymer were determined by the infrared spectrum obtained on compression molded films of about 10 mils (0.25 mm) thickness. The end groups of interest and absorbance frequencies are shown below:

The quantitative measurement of the number of these groups was obtained by the measurement of the extinction coefficients of each of these groups from model compounds and transferring these coefficients to the measurements obtained on the polymer. The end groups are expressed as the number per one million carbon atoms in the polymer.

The melt viscosity is measured according to ASTM D-1238-52T modified by using a cylinder, orifice and piston tip made of a corrosion-resistant alloy (Haynes Stellite 19), by charging a 5.0 g sample to the 9.53 mm inside diameter cylinder which is maintained at 372.degree. C..+-.1.degree. C., and by extruding the sample 5 minutes after charging through a 2.10 mm diameter, 8.00 mm long square-edged orifice under a load (piston plus weight) of 5000 g. (This corresponds to a shear stress of 0.457 kg/cm.sup.2). The melt viscosity in poises is calculated as 53150 divided by the observable extrusion rate in grams per minute.

The tetrafluoroethylene/perfluoropropyl vinyl ether copolymer (PPVE) used in these examples was prepared in accordance with U.S. Pat. No. 3,642,742 (Carlson). This patent describes a process for preparing TFE/PPVE copolymers in F-113 solvent, with 3-P initiator, using methanol as a chain transfer agent.

A 10-20 g sample of the copolymer is treated as follows: one part (by weight) of sample was mixed with one part of H.sub.2 O:methanol (50/50) and one part total ionic strength activity buffer (cyclohexylene dinitrilo tetraacetic acid) (TISAB, Orion, Cat. No. 94-09-09A). Fluoride ion concentrations were measured after initial aqueous contact and after 18-24 hours contact at ambient temperatures (22.degree.-25.degree. C.) using the fluoride specific ion electrode by direct measurement as described in Orion publication number IM94, 96-09/7721.

Extrusion pelletized cubes (approx. 1600 lbs, about 2-3 mm in size) of TFE/PPVE Copolymer (of 3.4.times.10.sup.4 poise melt viscosity at 372.degree. C. and containing about 3.2 wt% PPVE) were charged to a 32 cu. ft. container (made of polyethylene) and fitted with a gas inlet tube at the bottom and vapor outlet port on the top. The vapor space/cube volume ratio was 1.8. Anhydrous ammonia (approximately 1.5 lbs) was introduced into the vessel through the gas inlet tube over a 15 minute period. The inlet and outlet tubes were then capped and the polymer cubes were allowed to stand in contact with the dilute ammonia atmosphere for a 24 hour period at 22.degree.-25.degree. C. The vessel outlet port was then fitted to an exhaust system while nitrogen was blown through the inlet tube to displace the NH.sub.3 treatment vapor. As shown below, infrared analysis of the polymer end groups before and after ammonia vapor treatment show that the carbonyl fluoride groups were converted to amide groups.

The polymer was tested for fluoride ion extractability both before and after NH.sub.3 treatment. Results were as follows:

Stabilization of the carbinol end groups to heat aging was demonstrated as follows: Samples of the polymers described above, before and after ammonia treatment, were placed in a circulating air oven at 295.degree. C. At the various time intervals shown below, portions of the resin were removed from the oven and analyzed by infrared for end group content. As shown by the results summarized below, the untreated (unamidated) resin steadily lost --CH.sub.2 OH end groups and showed an increase in --COF groups as a result of heat aging. The treated resin (amidated) showed essentially no change in end group content under the test conditions.