Process for purifying perfluoro(propylvinylether)

Pure perfluoro(propylvinylether) is obtained by extractive distillation of crude perfluoro(propylvinylether) containing a hydrogen fluoride adduct of perfluoro(propylvinylether), which is obtained by gas phase or liquid phase thermal decomposition of perfluoro(2-propoxypropionyl)fluoride, in the presence of a ketone having a boiling point of 100.degree. C. of higher.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process for purifying 
perfluoro(propylvinylether) and more particularly to a process for 
purifying perfluoro(propylvinylether) by effectively separating 
perfluoro(propylvinylether) from the hydrogen fluoride adduct of 
perfluoro(propylvinylether). 
2. Description of the Prior Art 
Perfluoro(propylvinylether) (FPVE) is synthesized from 
perfluoro(2-propoxypropionyl) fluoride by gas phase thermal decomposition 
or by liquid phase thermal decomposition according to the following 
reaction equation (U.S. Pat. Nos. 3,132,123, 3,250,808 and 3,321,532): 
EQU C.sub.3 F.sub.7 OCF(CF.sub.3)COF.fwdarw.C.sub.3 F.sub.7 OCF.dbd.CF.sub.2 
The gas phase thermal decomposition is carried out by contacting the raw 
material perfluoro(2-propoxypropionyl) fluoride with an alkali metal 
compound such as sodium carbonate in a reaction zone kept at a high 
temperature such as 300.degree. to 600.degree. C. to thermally decompose 
the raw material. On the other hand, the liquid phase thermal 
decomposition is carried out by adding the raw material and an alkali 
metal compound such as sodium carbonate to a polar organic solvent such as 
ethyleneglycol, stirring the mixture at room temperature, thereby 
converting the raw material to an alkali metal salt thereof, and then 
heating the mixture to a temperature of 110.degree. to 130.degree. C. to 
thermally decompose the alkali metal. 
In any of these thermal decomposition procedures, about 5 to about 10% by 
weight of heptafluoropropyl-1,2,2,2-tetrafluoroethylether C.sub.3 F.sub.7 
OCFHCF.sub.3 is by-produced as a hydrogen fluoride adduct (FPVE.HF) of the 
product (FPVE) to inevitably contaminate the product (FPVE). Accordingly, 
European Patent No. 0260773 proposes to control formation of the FPVE.HF 
by-product by simultaneously using diethyleneglycol dimethylether and 
dimethylformamide as solvents in the liquid phase thermal decomposition 
procedure, but its effect has been found not satisfactory. 
Most of the by-products can be readily separated and removed by ordinary 
washing and distillation, but FPVE.HF has a boiling point of 42.degree. 
C., which is quite near that of FPVE, i.e. 35.5.degree. C. and thus is 
hard to separate by the ordinary distillation. It may be also possible to 
convert FPVE.HF to any appropriate derivative and then separate the 
derivative, but the procedure including the chemical conversion is also 
hard to carry out and thus is not commercially practicable. 
SUMMARY OF THE INVENTION 
It is preferable from a commercial viewpoint to use an appropriate solvent 
to extract any one of FPVE and its HF adduct. An object of the present 
invention is to provide a process for purifying FPVE by effectively 
separating FPVE from a HF adduct of FPVE by extractive distillation with a 
specific solvent. 
DETAILED DESCRIPTION OF THE INVENTION 
The object of the present invention can be attained by subjecting crude 
perfluoro(propylvinylether) containing a hydrogen fluoride adduct of 
perfluoro(propylvinylether) to extractive distillation in the presence of 
a ketone having a boiling point of 100.degree. C. or higher, thereby 
purifying perfluoro(propylvinylether). 
That is, when a mixture of FPVE and FPVE.HF, usually crude FPVE obtained by 
the thermal decomposition procedure is subjected to extractive 
distillation in the presence of a ketone having a boiling point of 
100.degree. C. or higher, FPVE and FPVE.HF can be effectively separated 
from each other. 
The ketone for use in the present invention includes, for example, 
2-pentanone, 3-pentanone, 2-hexanone, methylisobutylketone, 2-heptanone, 
4-heptanone, diisobutylketone, acetonylacetone, mesityl oxide, phorone, 
isophorone, cyclohexanone, methylcyclohexanone, acetophenone, etc., and 
particularly preferable are methylisobutylketone, 2-hexanone, 2-heptanone, 
acetonylacetone and cyclohexanone. 
The solvent is used in an amount of about 0.5 to about 50 parts by weight, 
preferably about 1 to about 10 parts by weight on the basis of one part by 
weight of FPVE contained in the crude FPVE. When a ketone is used having a 
boiling point lower than 100.degree. C., the ketone forms an azeotropic 
mixture with FPVE and thus is not suitable for the extractive 
distillation. 
Extractive distillation is usually carried out in a set of an extractive 
distillation column and an extracting solvent recovery column. That is, a 
mixture of FPVE-FPVE.HF as crude FPVE is fed to the extractive 
distillation column at the lower level position and an extracting solvent 
is fed thereto at a higher level position to make countercurrent contact 
of the mixture with the extracting solvent. The component (FPVE) hard to 
dissolve in the extracting solvent is withdrawn from the extractive 
distillation column at the column top, whereas the component (FPVE.HF) 
easy to dissolve in the extracting solvent goes toward the column bottom 
together with the extracting solvent. 
The column bottoms is led to the extracting solvent recovery column 
operated at a higher temperature and/or under a lower pressure than the 
temperature and pressure of the extractive distillation column, and the 
component (FPVE.HF) dissolved in the extracting solvent is stripped off 
and withdrawn from the extracting solvent recovery column at the column 
top, while recycling the extracting solvent to the extractive distillation 
column. The recycle rate of the extracting solvent (feed rate) can be 
determined in advance from the working curve prepared for the individual 
extracting solvents. 
When the crude FPVE containing FPVE.HF is subjected to extractive 
distillation in the presence of a ketone having a boiling point of 
100.degree. C. or higher, the ketone works to control the evaporation of 
FPVE.HF and enhance the relative volatity of FPVE, that is, to separate 
FPVE and FPVE.HF from each other by distillation. 
FPVE purified to a high degree by the extractive distillation can be 
effectively used in the copolymerization reaction with 
tetrafluoroethylene. It is known that PFA resin, i.e. the copolymer of 
FPVE and tetrafluoroethylene, can largely improve the moldability of 
tetrafluoroethylene homopolymer (PTFE resin) that has distinguished heat 
resistance and solvent resistance, but has a problem in the moldability. 
When FPVE to be used as a comonomer in the copolymerization reaction 
contains a HF adduct of FPVE, the HF adduct acts as a chain transfer agent 
to give a low molecular weight copolymer. By completely removing the HF 
adduct, desired PFA resin can be obtained.

The present invention will be explained in detail below, referring to 
Examples. 
PREFERRED EMBODIMENTS OF THE INVENTION 
Example 1 
1,280 g of a mixture of FPVE-FPVE.HF in a ratio by weight of 93. 1:6.9 and 
250 g of methylisobutylketone were charged into a 2-1 vessel provided with 
a vacuum jacket distillation column (diameter: 1.5 cm; height: 140 cm) 
with a fractionation head filled with a packing material (Helipak packing 
No. 1). 
The vessel was heated to boil the liquid content and at the same time 1,060 
g of methylisobutylketone was continuously fed to the distillation column 
at a level of 100 cm from the bottom of the column at a feed rate of about 
1.1 g/min. and so controlled that the total could be always refluxed. 
After the column temperature approached the equilibrium state, 275 of 
initial fraction was withdrawn from the column in a reflux ratio of 60:2 
(refluxing: 60 seconds and withdrawal: 2 seconds) over 4 hours, and then 
at the time when the column top temperature was stabilized to the same 
temperature as the boiling point of FPVE, 776 g of main fraction was 
withdrawn in a reflux ratio of 40:2 over 12 hours. The main fraction was 
assayed by gas chromatography and it was found that FPVE.HF was 0.028% by 
weight and methylisobutylketone was below the detection limit. 
Furthermore, it was found that the vapors in the vessel contained 42% by 
weight of FPVE.HF. 
Example 2 
Extractive distillation of 1,300 g of a mixture of FPVE-FPVE.HF in a ratio 
of 85.2:14.8 by weight was carried out in the same manner as in Example 1, 
while 1,405 g of methylisobutylketone was continuously fed to the 
extractive distillation column at the same feed rate as in Example 1. 
Example 3 
Extractive distillation of 1,270 g of a mixture of FPVE-FPVE.HF was carried 
out in the same manner as in Example 1, except that methylisobutylketone 
was replaced with acetonylacetone. That is, 1,580 g of acetonylacetone was 
continuously fed to the extractive distillation column at a feed rate of 
1.6 g/min. 
Example 4 
Extractive distillation of 1,275 g of a mixture of FPVE-FPVE.HF was carried 
out in the same manner as in Example 1, except that methylisobutylketone 
was replaced with 2-hexanone. That is, 2,100 g of 2-hexanone was fed to 
the extractive distillation column at a feed rate of 2.2 g/min. 
Example 5 
Extractive distillation of 1,270 g of a mixture of FPVE-FPVE.HF was carried 
out in the same manner as in Example 1, except that methylisobutylketone 
was replaced with 2-heptanone. But, 2-heptanone was not directly charged 
into the vessel and total 1,920 g of 2-heptanone was fed to the extractive 
distillation column at a feed rate of 2.0 g/min. 
Example 6 
Extractive distillation of 1,270 g of a mixture of FPVE-FPVE.HF was carried 
out in the same manner as in Example 1, except that methylisobutylketone 
was replaced with cyclohexane. But, cyclohexanone was not charged directly 
into the vessel and total 2,100 g of cyclohexanone was supplied to the 
extractive distillation column at a feed rate of 2.5 g/min. 
Results of the foregoing Examples 2 to 6 are summarized in the following 
Table 1. The solvent in the main fraction was below the detection limit in 
all the Examples. 
TABLE 1 
______________________________________ 
FPVE.HF 
Initial Main in main 
in vapors 
fraction fraction fraction 
in vessel 
Ex. No. (g) (g) (%) (%) 
______________________________________ 
2 248 702 0.033 54 
3 258 783 0.031 45 
4 264 762 0.033 42 
5 235 582 0.029 34 
6 243 572 0.034 36 
______________________________________ 
Comparative Example 1 
Extractive distillation of 1,260 g of a mixture of FPVE-FPVE.HF was carried 
out in the same manner as in Example 1 without using the extracting 
solvent. 
Comparative Example 2 
Extractive distillation was carried out in the same manner as in Example 1, 
except that the same amount of methylethylketone was used in place of 
methylisobutylketone. 
Comparative Example 3 
Extractive distillation of 1,275 g of a mixture of FPVE-FPVE.HF was carried 
out in the same manner as in Example 1, except that the same amount of 
acetonitrile was used in place of methylisobutylketone. 
Comparative Example 4 
Extractive distillation of 1,275 g of a mixture of FPVE-FPVE.HF was carried 
out in the same manner as in Example 1, except that the same amount of 
butyl acetate was used in place of methylisobutylketone. 
Comparative Example 5 
Extractive distillation was carried out in the same manner as in Example 1, 
except that the same amount of n-butanol was used in place of 
methylisobutylketone. 
Results of the foregoing Comparative Examples 1 to 5 are summarized in the 
following Table 2. 
TABLE 2 
______________________________________ 
Initial Main 
Comp. fraction fraction In main fraction 
Ex. No. 
(g) (g) FPVE.HF(%) 
Solvent(%) 
______________________________________ 
1 280 725 1.5 -- 
2 272 730 0.052 7.8 
3 283 710 0.35 1.1 
4 234 728 0.78 below detection 
limit 
5 247 723 1.2 below detection 
limit 
______________________________________