Purification of perfluorosulfonyl fluoride perfluorovinyl ethers by thermal decomposition of unstable isomers

This application is a continuation-in-part of Ser. No. 44,111 filed June 8, 1970, now abandoned, by the same inventor.

This invention relates to copolymerization of tetrafluoroethylene with perfluorosulfonyl fluoride perfluorovinyl ethers, and the preparation of these perfluorovinyl ethers in substantially pure form.

Copolymers of tetrafluoroethylene and perfluorosulfonyl fluoride fluorovinyl ethers have outstanding properties. The inert properties of polytetrafluoroethylene are combined with melt fabricability. It has now been found that those properties can be improved by removing certain isomers associated with the monomeric perfluorovinyl ether, which isomers may act as chain transfer agents resulting in low molecular weight copolymer.

The present invention also relates to a process for polymerizing tetrafluoroethylene with primary isomers of perfluorosulfonyl fluoride perfluorovinyl ethers containing less than 0.3 percent by weight and preferably less than 0.1 percent by weight secondary and tertiary isomers. The invention also relates to a process for producing primary isomers substantially free of either or both secondary or tertiary isomers and a method for separating primary isomer from the secondary and tertiary isomers. The monomeric isomers have the formula ROCF=CF 2 where R is a sulfonyl fluoride-containing fluorinated alkyl radical.

The process of separating these isomers is carried out by selective destruction of the secondary and tertiary isomer present by heating the mixture between the temperatures of 250°-350° C. Each of the isomers will be affected to some degree. However, it is found that either or both secondary and tertiary isomers will rearrange almost completely into a form which can be easily removed by distillation or extraction.

This process for purifying substantially pure primary isomer of compounds having the formula ROCF=CF 2 where R has the same meaning as defined above, the other isomers having the same number of fluorine and oxygen atoms per molecule as the primary isomer, is characterized by heating a mixture consisting essentially of said isomers at a temperature of between 250°-350° C. and recovering the primary isomer. Preferably, the temperature is at least 300° C.

The carbon atom of R bonded to the oxygen is also bonded only to atoms from the class consisting of carbon and fluorine. By way of example, the primary isomer is perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonyl fluoride (PSEPVE) and there is also present in the mixture the isomer, perfluoro-3,6-dioxa-5-methyl-7-octene-sulfonyl fluoride (iso-PSEPVE).

This invention also comprises a process for copolymerizing tetrafluoroethylene and substantially pure primary isomers of compounds having the formula ROCF=CF 2 where R has the same meaning as defined above, said process comprising pyrolyzing a mixture of isomers at a temperature of 250°-350° C. and separating substantially pure primary isomer and copolymerizing said primary isomer with tetrafluoroethylene at temperatures of 30° C. to 85° C. and pressures of 15 psi to 1,000 psi in a nonaqueous solvent, and 50°-110° C. in an aqueous system at pressures of 1-500 psig.

Typical copolymerization methods for polymerizing tetrafluoroethylene and vinyl ethers are described in U.S. Pat. Nos. 3,282,875 and 3,132,123.

Preferably, the isomer to be copolymerized with tetrafluoroethylene is perfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride.

This invention also comprises the process of preparing the primary vinyl ether by pyrolyzing a compound of the group consisting of the corresponding acid fluoride [ROCF(CF 3 )COF] or the alkali metal salt [ROCF(CF 3 )COOAlkali metal] of the said fluoride at a temperature of 75° to 600° and subsequently separating the primary isomer from the other isomers formed therewith by heating the mixture substantially free of acid halide at a temperature of between 250°-350° C. and recovering the primary isomer. As illustrated in Example 1, the pyrolysis step in which the ether is formed is typically conducted at a temperature of less than 250° C. and usually less than 200° C., in which case the subsequent heating step (250°-300° C.) involves a heating up of the pyrolysis mixture.

R is a fluorocarbon group which contains a sulfonyl fluoride group, -- SO 2 F, and may optionally contain one or more groups such as ether groups or other functional groups. R has at least 3 carbon atoms. While for purposes of the separation of isomers, no upper limit is necessary on the number of carbons in R for practical purposes, compounds with an R group of 15 or less carbon atoms are more likely to be used.

A primary isomer is one in which the --OCF=CF 2 group is connected to a --CF 2 group in the R group. A secondary isomer is one in which the --OCF=CF 2 group is connected to a > CF-- in the R group. A tertiary isomer is one in which the --OCF=CF 2 group is connected to a ##STR1## (carbon) bearing no fluorine.

By substantially free of secondary or tertiary isomer is meant that the primary isomer has less than 0.3 percent by weight of the secondary or tertiary isomer present. Preferably less than 0.1 percent by weight of said isomer is present. It is commercially desirable to prepare copolymers of tetrafluoroethylene and vinyl ethers from primary isomer of the vinyl ether which is substantially free of members of the class consisting of secondary and tertiary isomer and preferably contains less than 0.1 percent thereof.

Typically, the amount of secondary and tertiary isomer associated with the primary isomer in the production of the vinyl ether is greater than 0.1 percent by weight and most often, above 0.3 percent by weight. Thus, in its most significant embodiment, the process of this invention processes a feed stream containing greater than 0.1 percent by weight of members of the class of secondary and tertiary isomer to a product stream with a concentration of less than 0.1 percent by weight of members of that class. Most typically, the feed stream contains greater than 0.3 percent by weight of members of the class of secondary and tertiary isomers and the product stream contains less than 0.3 percent by weight of members of that class.

Specifically, perfluoro-3,6-dioxa-5-methyl-7-octene-sulfonylfluoride (iso PSEPVE) can be removed from perfluoro-3,6-dioxa-4-methyl-7-octenesulfonylfluoride (PSEPVE) by selective pyrolysis. The perfluoro-3,6-dioxa-5-methyl-7-octenesulfonyl-fluoride is rearranged into the compound perfluoro-5-oxa-3-methyl-7-fluorosulfonylheptanoyl fluoride which then may be easily separated from the desired monomer, perfluoro-3,6-dioxa-4-methyl-7-octenesulfonyl fluoride by extraction with dilute aqueous solutions of sodium bicarbonate or potassium bicarbonate at room temperature.

This process also includes within its scope steps of pyrolysis of the isomer followed by dicarboxylation by known methods, in order to separate secondary and tertiary isomers from the desired primary fluorovinyl ethers.

For determining the relative amounts of PSEPVE and iso-PSEPVE, the following gas chromatographic column is used.

Specifically, the first section consists of a 20 feet × 1/4 inch 20 percent polyperfluorocarbonoxide oil (fluorinated oil) [=Krytox" 143 - trade name Du Pont] on 60/80 mesh "Chromosorb P" trade name for calcined diatomaceous earth of Johns Manville Corp. This column shows iso-PSEPVE as a shoulder on the main PSEPVE peak. Temperature should be about 150° C. and the flow rate should be about 20 ml/min. to provide about 11,000 theoretical plates.

Analytic results based on these chromatographic methods depend in part on the integration of the curves for their accuracy. Unless otherwise specified in the examples which follow, these techniques were used to measure isomer content. In the examples, integration was carried out by weighing the areas of chart paper under the chromatographic curves or graphically. It is estimated that experimental error is of the order of 0.02 percent concentration by weight of isomer in the results reported.

Typical perfluorovinyl ethers which may be separated by the process of this invention include:

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More stable From

Less stable (may be separated)

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CF.sub.3 CF.sub.2 CF.sub.2 CF.sub.2 OCFCF.sub.2 . . . .

##STR2##

##STR3##

##STR4##

CF.sub.3 CF.sub.2 CF.sub.2 CF.sub.2 CF.sub.2 OCFCF.sub.2

##STR5##

##STR6##

##STR7##

##STR8##

##STR9##

##STR10##

##STR11##

CF.sub.3 ClCFClCF.sub.2 OCFCF.sub.2 . . . . .

##STR12##

##STR13##

##STR14##

CF.sub.3 CF.sub.2 CF.sub.2 OCFCF.sub.2 . . . . .

##STR15##

##STR16##

##STR17##

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Vinyl ethers may be prepared by reaction of the appropriate fluorosulfonyl fluoracyl fluoride with hexafluoropropylene oxide followed by decarboxylation in situ of the sodium salt made from the acyl fluoride product. A general procedure for this synthesis is found in U.S. Pat. No. 3,282,875 to form compounds, in terms of primary isomers, of the formula FO 2 SCFR f CF 2 O(CFYCF 2 O) n CF=CF 2 wherein R f is F or perfluoroalkyl having from one to 10 carbon atoms, Y is F or CF 3 and n is an integer of 1 to 3.

The determination of melt index or flow rate (melt flow) consists of weighing the amount of resin extruded through a designated orifice at a designated temperature under a designated weight and during a fixed period of time. Melt flow of the IEE/PSEPVE copolymer is defined as the weight in grams extruded in 10 minutes through a 0.0825 inch diameter orifice 0.315 inch long at 250° C., when subjected to a load of 500 g.

The water absorptivity is defined as the percent weight increase in weight due to the absorption of water in the copolymer of TFE and PSEPVE after the sulfonyl fluoride groups (SO 2 F) are converted to sulfonic acid groups (SO 3 H).

A sample of polymer is pressed into a 10 mil thick film, the --SO 2 F groups hydrolyzed by dilute sodium hydroxide to --SO 3 Na groups and these acidified with hydrochloric acid to give polymer with sulfonic acid groups. A sample of this polymer, c.a. five-eighths sq. in., is immersed in boiling water for 10-15 minutes, removed and washed at room temperature with water, dilute sulfuric acid and twice more with water. The film is removed from the last water bath, the excess water wiped off and weighed. The dry weight of the film is determined by drying the sample in a vacuum oven at 104°-107° for 30-40 minutes. The water absorptivity is calculated from these two weights.

The equivalent weight of the polymer is defined as that weight of polymer which contains one gram equivalent of --SO 3 H. A sample of polymer is hydrolyzed with aqueous sodium hydroxide to convert all the sulfonyl fluoride groups to --SO 3 Na groups. The latter are converted to --SO 3 H groups by treatment with dilute hydrochloric acid. After all the excess hydrochloric acid has been carefully washed from the polymer and the polymer dried to constant weight, the equivalent weight of the polymer is determined by titration of a weighed polymer sample with standardized sodium hydroxide solution.

The term "melt viscosity" means the apparent melt viscosity as measured at 380° C. (or 300° C.) under a shear stress of 6.5 lbs per square inch. The melt viscosity is determined by using a melt indexer of the type described in ASTM D1238-52-T, modified for corrosion resistance to embody a cylinder, orifice, and piston weighing about 50 grams, made of stellite cobalt-chromium-tungsten alloy. The resin (5.0 grams) is charged to the 0.375 inch I. D. cylinder which is held at 380° C. ± 5° C. (or 300° C. ± 5° C.) and allowed to come to equilibrium temperature during 5 minutes. Then it is extruded through the 0.0825 inch diameter, 0.315 inch long orifice under a piston loading of 5,000 grams. The melt viscosity in poises is calculated as 53,150 divided by the observed extrusion rate in grams per minute.

In the Examples which follow, all parts are by weight unless otherwise indicated.

The primary isomer PPVE and the other isomers incidental to production of the primary isomer according to the method of Example 2 were passed through a tube over sodium carbonate at the temperatures indicated. The average contact time over sodium carbonate was 22 seconds. The feed and product concentrations of the undesirable isomer are listed in addition to the percent product recovery of the desirable isomer.

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% ofA in CF.sub.3 CF.sub.2 CF.sub.2 OCFCF.sub.2

Temp. Feed Product Product Recovery

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250 0.66 0.61 95%

280 0.50 0.23 --

300 0.65 0.10 97%

300 0.37 0.10 90%

300 0.37 0.08 95%

300 0.37 0.10 97%

300 0.37 0.07 93%

300 0.37 0.08 97%

300 0.37 0.15 91%

320 0.26 0.10 88%

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##STR21##

A stainless steel tube four inches in diameter and five feet long was filled with ten pounds of anhydrous sodium carbonate. The tube was heated to 300° C. and the sodium carbonate fluidized. Then 11.5 pounds of PPVE containing 0.65 percent iso-PPVE was fed through the tube in 68 minutes. The PPVE recovered weighed 11.2 pounds and contained 0.10 percent iso-PPVE.

Analogous experiments were performed using potassium carbonate, glass beads or stainless steel mesh in place of the sodium carbonate. However because of ease in handling, heat transfer, etc., the use of carbonate is preferred.