Tetrafluoroethylene copolymer and process for preparing the same

A copolymer of tetrafluoroethylene and a perfluoro(alkyl vinyl ether) having the formula (I): EQU CF.sub.2 .dbd.CF--O--Rf (I) wherein Rf is a perfluoroalkyl group having 3 or 4 carbon atoms; and said copolymer having 1 to 10% by weight of the perfluoro(alkyl vinyl ether) units, having 7 to 20, per 10.sup.6 carbon atoms, of terminal groups --CONH.sub.2, having substantially no --CH.sub.2 OH and no --COF, and having a melt viscosity at 380.degree. C. of 0.1.times.10.sup.4 to 100.times.10.sup.4 poise and a preparation process thereof. The tetrafluoroethylene copolymer is thermally stable and has excellent powder properties. Molded articles thereform have a few bubbles and excellent dimensional stability on heating.

BACKGROUND OF THE INVENTION 
The present invention relates to a melt-processable tetrafluoroethylene 
copolymer having excellent processability and thermal stability, and more 
particularly to a thermally stable tetrafluoroethylene copolymer which is 
obtained by stabilizing thermally unstable groups existing at the 
copolymer ends, and a process for preparing the same. 
Copolymers prepared from tetrafluoroethylene and perfluoro(alkyl vinyl 
ether) (hereinafter referred to as "PFA") have been well known as a 
melt-processable fluorocarbon resin, and have been widely used for various 
uses, for instance, as starting materials for molded articles such as 
tubes, pipes, joints, containers and coating materials for electric wire, 
as coating materials, as lining materials, as starting materials for 
roto-molded articles such as hollow articles, and the like. 
As to PFA, more or less amount of group --COF is inevitably formed at the 
polymer ends due to the polymerization mechanism of PFA. Also, in case of 
emulsion polymerization using a polymerization initiator such as ammonium 
persulfate (APS), groups --COOH are produced at the polymer ends, or in 
case of using methanol as a molecular weight controlling agent, groups 
--CH.sub.2 OH or groups --COOCH.sub.3 are formed at the polymer ends. 
Since these terminal groups are thermally unstable, it has been known that 
the bubble formation during molding or the generation of 
fluorine-containing acids is caused from these terminal groups, thus 
resulting in failure of molding or corrosion of a mold of a molding 
machine. 
Under the circumstances, a technique to stabilize these thermally unstable 
terminal groups has hitherto been studied and some have been proposed. For 
instance, Japanese Unexamined Patent Publication No. 61-98709 discloses a 
process for preparing a thermally stable PFA which comprises contacting 
PFA with ammonia gas or a nitrogen compound capable of producing ammonia 
for a time sufficient to convert at least 50% of terminal groups --COF and 
--COOH into groups --CONH.sub.2. 
Also, Japanese Unexamined Patent Publication No. 62-104822 discloses a 
method wherein PFA having more than 6, per 10.sup.6 carbon atoms, of 
terminal groups --CF.sub.2 CH.sub.2 OH, --CONH.sub.2 or --COF is contacted 
with a fluorine-containing gas under temperature, time and pressure 
conditions sufficient to decrease the number of the terminal groups to 
less than 6 per 10.sup.6 carbon atoms. 
However, when PFA prepared through the process for stabilizing the 
thermally unstable terminal groups according to the above-mentioned 
publications is subjected to the roto-molding, the obtained molded article 
is greatly shrunk by heating or is poor in dimensional stability on 
heating. Also, according to the fluorinating method disclosed in Japanese 
Unexamined Patent Publication No. 62-104822, it is required to conduct the 
reaction at a high temperature for many hours in order to heighten the 
conversion for fluorination. If the severe reaction conditions can be 
lightened even a little, such a reaction is advantageous from the 
viewpoints of the economy and the safety. 
An object of the present invention is to provide a thermally stable PFA 
having excellent powder properties, which can provide a molded article 
having no bubbles and excellent dimensional stability. 
A further object of the present invention is to provide a preparation 
process of the thermally stable PFA as mentioned above. 
These and other objects of the present invention will become apparent from 
the description hereinafter. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, there is provided a copolymer of 
tetrafluoroethylene and a perfluoro(alkyl vinyl ether) having the formula 
(I): 
EQU CF.sub.2 .dbd.CF--O--Rf (I) 
wherein Rf is a perfluoroalkyl group having 3 or 4 carbon atoms; 
the copolymer having 1 to 10% by weight of the perfluoro(alkyl vinyl ether) 
units, having 7 to 20, per 10.sup.6 carbon atoms, of terminal groups 
--CONH.sub.2, 
having substantially no --CH.sub.2 OH and no --COF, and having a melt 
viscosity at 380.degree. C. of 0.1.times.10.sup.4 to 100.times.10.sup.4 
poise. 
Also, in accordance with the present invention, there is provided a process 
for preparing the tetrafluoroethylene copolymer as mentioned above which 
comprises: 
contacting PFA containing 1 to 10% by weight of units of a perfluoro(alkyl 
vinyl ether) with fluorine gas to give a PFA having 7 to 40, per 10.sup.6 
carbon atoms, of the total number of terminal groups --COF and terminal 
groups --COOH, and 
contacting the resulting PFA with ammonia gas or a gaseous 
nitrogen-containing compound capable of forming ammonia gas to convert the 
groups --COF into groups --CONH.sub.2. 
The obtained PFA can solve the above-mentioned problems. 
DETAILED DESCRIPTION 
The thermally unstable groups existing in PFA at the polymer ends, namely, 
--CH.sub.2 OH, --COOH and --COOCH.sub.3 are contacted with fluorine gas to 
finally convert into --CF.sub.3 through --COF. In the course of conversion 
of the terminal groups containing oxygen, a state wherein only --COF and 
--COOH exist arises. In the present invention, the contact of the PFA with 
fluorine gas is stopped at the time when only --COF and --COOH exist and 
the total number of the groups COF and the groups --COOH is from 7 to 40, 
preferably from 8 to 30, per 10.sup.6 carbon atoms. Accordingly, the 
treating conditions by fluorine gas such as time, temperature and pressure 
conditions can be lightened, that is, the process of the present invention 
is advantageous in both economy and safety. 
Subsequently, the resulting PFA is contacted with ammonia gas or a gaseous 
nitrogen-containing compound capable of forming ammonia gas. By the 
contact with ammonia or the nitrogen-containing compound, the groups COF 
are converted into --CONH.sub.2, so the obtained PFA has substantially no 
--COF and no --CH.sub.2 OH at the polymer ends. The expression "PFA has 
substantially no --COF and no --CH.sub.2 OH" means that not only PFA 
having completely no --COF and no --CH.sub.2 OH but also PFA having not 
more than 2, per 10.sup.6 carbon atoms, of the groups --COF and --CH.sub.2 
OH are included. Further, the number of the terminal groups --CONH.sub.2 
in the obtained PFA is adjusted to 7 to 20, preferably from 7 to 15, per 
10.sup.6 carbon atoms. The obtained PFA can contain fluorine ion in the 
state of ammonium fluoride. 
As apparent from Examples as mentioned below, when the number of the 
terminal groups --CONH.sub.2 is within the range of from 7 to 20 per 
10.sup.6 carbon atoms, the bubble formation can be remarkably decreased 
during molding. When the number of the terminal groups --CONH.sub.2 is 
less than 7 per 10.sup.6 carbon atoms, the molded articles therefrom are 
remarkably shrunk. 
The PFA used in the present invention is a copolymer of tetrafluoroethylene 
and a perfluoro(alkyl vinyl ether) having the formula (I): 
EQU CF.sub.2 .dbd.CF--O--Rf (I) 
wherein Rf is a perfluoroalkyl group having 3 or 4 carbon atoms, which has 
1 to 10% by weight of units of the perfluoro(alkyl vinyl ether) (I). As 
the perfluoro (alkyl vinyl ether), there are preferable the perfluoro 
(alkyl vinyl ether) having the formula (I) wherein Rf is the 
perfluoroalkyl group having 3 carbon atoms and the perfluoro(alkyl vinyl 
ether) having the formula (I) wherein Rf is the perfluoroalkyl group 
having 4 carbon atoms, concretely, perfluoro(propyl vinyl ether) and 
perfluoro(butyl vinyl ether). The perfluoro(alkyl vinyl ether) can be used 
alone or as an admixture thereof. The PFE can be in the state of a powder, 
pellets, flakes, and the like. 
The preparation process of the present invention comprises the first step 
wherein the PFA containing the thermally unstable groups at the polymer 
ends is treated with fluorine gas and the second step wherein the 
resulting PFA is treated with ammonia gas or the gaseous 
nitrogen-containing compound capable of forming ammonia gas. 
In the first step, namely the treatment with fluorine gas, the PFA 
containing the thermally unstable groups at the polymer ends is contacted 
with fluorine gas at a temperature of, generally from 150.degree. to 
250.degree. C., preferably from 150.degree. to 200.degree. C. for 1 to 10 
hours, preferably from 2 to 5 hours under a pressure of 1 to 10 
atmospheres. Usually, the reaction is conducted under atmospheric 
pressure. A pure fluorine gas can be used. It is preferably to use a 
diluted fluorine gas with an inert gas containing from 5 to 25% by volume, 
preferably from 7 to 15% by volume, of fluorine gas from the viewpoint of 
the safety. Examples of the inert gases are, for instance, nitrogen gas, 
argon gas, helium gas, and the like. By the first step, namely the 
treatment of PFA with fluorine gas, the PFA having only --COF and --COOH 
at the polymer ends and having 7 to 40, per 10.sup.6 carbon atoms, of the 
total number of the two terminal groups is obtained. Accordingly, the 
preparation process of the present invention can be conducted at a lower 
temperature for a shorter time than that described in Japanese Unexamined 
Patent Publication No. 62-104822 wherein almost all of the terminal groups 
are fluorinated. 
In the second step of the process of the present invention, the obtained 
PFA in the first step is contacted with ammonia gas or the gaseous 
nitrogen-containing compound capable of forming ammonia gas to convert the 
groups --COF into --CONH.sub.2. It is preferable that the PFA is washed 
with an inert gas such as nitrogen gas prior to passing ammonia gas or the 
gaseous compound capable of forming ammonia gas. Both 100% ammonia gas and 
a diluted ammonia gas with an inert gas containing about 5 to 50% by 
volume of ammonia can be used. The time, temperature and pressure 
conditions in the treatment with ammonia or the gaseous 
nitrogen-containing compound are not particularly limited. The treatment 
can be conducted at a temperature of, usually from 0.degree. to 
100.degree. C., preferably at room temperature, for usualy from 0.5 to 5 
hours, preferably from 60 to 90 minutes, under a pressure of, usually 0.5 
to 10 atmospheres, preferably under atmospheric pressure. The reaction of 
the second step can rapidly progress to convert the groups --COF into 
--CONH.sub.2. As a result, the obtained PFA has substantially no --COF and 
no --CH.sub.2 OH and has 7 to 20, per 10.sup.6 carbon atoms, of the groups 
--CONH.sub.2. 
The thus obtained PFA contains 1 to 10% by weight, preferably from 2 to 6% 
by weight, of units of the perfluoro(alkyl vinyl ether), and has a melt 
viscosity at 380.degree. C. of 1.0.times.10.sup.4 to 100.times.10.sup.4 
poise, preferably 2.0.times.10.sup.4 to 30.times.10.sup.4 poise. When 
using the thus obtained PFA, the bubble formation scarcely occurs during 
molding, and the molded articles therefrom have a small shrinkage and 
excellent dimensional stability. 
In the present invention, the kind and the number of the terminal groups, 
and the melt viscosity of PFA are measured as follows: 
ANALYSIS OF THE TERMINAL GROUPS 
A PFA powder is subjected to compression molding at 350.degree. C. for 30 
minutes to give a film having a thickness of 0.25 to 0.3 mm. Infrared 
absorption spectrum analysis of the obtained film is conducted. The kind 
of the terminal group is decided by comparing the obtained results with 
results of an infrared adsorption spectrum analysis concerning a known 
film. Also, the number of the terminal groups is calculated from its 
difference spectrum by using the following equation: 
##EQU1## 
wherein I is an absorbance, K is a correction factor and t is a film 
thickness. Correction factors of the terminal groups are as follows: 
______________________________________ 
Absorption Correction 
Terminal group 
frequency (cm.sup.-1) 
factor 
______________________________________ 
--COF 1883 440 
--COOH 3560 440 
--COOCH.sub.3 1795 400 
--CONH.sub.2 3436 460 
--CH.sub.2 OH 3648 2300 
______________________________________ 
The correction factors are decided from an infrared absorption spectrum 
concerning a model compound, in order to calculate the number, per 
10.sup.6 carbon atoms, of the terminal groups. 
The infrared absorption spectrum analysis is conducted by using a 
Perkin-Elmer-FTIR spectrometer 1760 X and a Perkin-Elmer-7700 professional 
computer which are commercially available from The Perkin-Elmer Corp., and 
the scanning operation is conducted 100 times. 
MEASUREMENT OF MELT VISCOSITY 
The melt viscosity is measured by using a Koka Shiki Flow tester 
commercially available from Kabushiki Kaisha Shimazu Seisakusho. A 
cylinder having an inside diameter of 11.3 mm is charged with the 
copolymer and is kept at a temperature of 380.degree. C. for 5 minutes. 
Then the copolymer is extruded through an orifice having an inside 
diameter of 2.1 mm and a length of 8 mm under 7 kg of a piston load, the 
extrusion rate (g/minute) is measured. The melt viscosity is a value 
obtained by dividing 53150 by the extrusion rate (g/minute).

The present invention is more specifically described and explained by means 
of the following Examples, in which all % and parts are by weight unless 
otherwise noted. It is to be understood that the present invention is not 
limited to the Examples, and various changes and modifications may be made 
in the invention without departing from the spirit and scope thereof. 
EXAMPLE 1 
A tetrafluoroethylene/perfluoro(propyl vinyl ether) copolymer (a weight 
ratio: 97/3) was prepared in aqueous suspension polymerization according 
to the method described in Japanese Unexamined Patent Publication No. 
189210/1983. The obtained PFA has --CH.sub.2 OH, --COOCH.sub.3 and --COOH 
as the terminal groups, and the number of the groups --CH.sub.2 OH, 
--COOCH.sub.3 and --COOH are 100, 43 and 2 per 10.sup.6 carbon atoms, 
respectively. 
In a box reaction oven was sealed 15 g of the obtained PFA placed in a 
private tray, and the space in the oven was thoroughly substituted for 
nitrogen gas. A gas mixture of 10% by volume of fluorine gas and nitrogen 
gas was passed through the oven at a flow rate of 1.0 l/minute at 
180.degree. C. for 3 hours under atmospheric pressure. After that, the 
oven was stopped heating and nitrogen gas was passed through the oven 
instead of the gas mixture over about one hour to completely replace 
fluorine gas by nitrogen gas. At this time, the temperature in the oven 
was 30.degree. C. 
As to the obtained PFA, the kind and the number of the terminal groups were 
analyzed. The PFA had 28, per 10.sup.6 carbon atoms, of the groups --COF 
and 3, per 10.sup.6 carbon atoms, of the groups --COOH, and had no 
--CH.sub.2 OH and no --COOCH.sub.3. 
Then, a gas mixture of 50% by volume of ammonia gas and nitrogen gas was 
passed through the reaction oven at a flow rate of 2.0 l/minute at room 
temperature (about 30.degree. C.) for 30 minutes to treat the PFA treated 
with fluorine gas with ammonia gas. Subsequently, nitrogen gas was passed 
thorugh the oven until the outlet gas showed neutrality. Then, the oven 
was opened to the atmosphere and the content was taken out. 
As to the obtained PFA, the kind and the number of the terminal groups were 
analyzed and the melt viscosity was measured. The PFA had 15, per 10.sup.6 
carbon atoms, of the groups --CONH.sub.2 and a melt viscosity of 
7.5.times.10.sup.4 poise. (The results are shown in Table 1 as a sample 
No. 4.) 
The same procedure as above was repeated except that the PFA was treated 
with fluorine gas under conditions as shown in Table 1. The kind and the 
number of the terminal groups, and the melt viscosity were measured in the 
same manner as above. The results are shown in Table 1. 
Also, an output of fluorine ion of the finally obtained PFA is shown in 
Table 1. The output of the fluorine ion was measured as follows: 
FLUORINE ION OUTPUT 
In a polyethylene bottle were thoroughly stirred 10 g of a sample (in the 
state of a pellet, film or powder), 5 ml of water, 5 ml of methanol and 10 
ml of a total ion strength adjustment buffer wherein 500 g of sodium 
chloride, 500 g of acetic acid, 320 g of sodium hydroxide and 5 g of 
sodium citrate.2H.sub.2 O are dissolved in 10 l of ion-excharged water, 
and the mixture is allowed to stand for 24 hours. The mixture is filtered 
and an amount of fluorine ion in the obtained filtrate is measured by 
using a fluorine ion meter (Publication No. 96-90-00) commercially 
available from Orion Corp. 
EXAMPLE 2 
Using each PFA powder (Sample Nos. 1 to 8) obtained in Example 1 and the 
non-treated PFA (Sample No. 9) which was the starting material, the 
roto-molding was conducted. 
A blast-treated roto-molding mold of 3 l for molding a bottle (inside 
diameter: 161 mm) was coated with a mold-release compound, and was charged 
with 600 g of each sample Nos. 1-9. In an oven having a temperature of 
380.degree. C., the roto-molding was conducted (revolution speed: 9.4 rpm, 
autorotation speed: 23 rpm) for one hour. After cooling, the bottle was 
taken out from the mold. The obtained bottle was uniform in a thickness of 
2 mm. 
As to the obtained bottle, the number of bubbles, the shrinkage and the 
volume were measured as follows: 
SHRINKAGE 
An inside diameter, D (mm) of the obtained bottle according to the 
roto-molding is measured, and the shrinkage (%) is calculated by the 
following equation: 
##EQU2## 
VOLUME 
The obtained bottle is filled with pure water, and then a volume of the 
water (ml) is measured by a measuring cylinder. 
THE NUMBER OF BUBBLES 
The bottle is cut longitudinally with a knife, and the number of all 
bubbles on the inside surface of the bottle is counted with the naked eye. 
TABLE 1 
__________________________________________________________________________ 
The number of the terminal 
Treatment groups (per 10.sup.6 carbon atoms) 
with After treatment 
After treatment Melt Fluorine 
fluorine with fluorine 
with ammonium viscosity 
ion The 
Sample 
Temp. 
Time 
gas gas (.times. 10.sup.4 
output 
number 
Shrinkage 
Volume 
No. (.degree.C.) 
(hr) 
--COF 
--COOH 
--CONH.sub.2 
--COF 
--CH.sub.2 OH 
poise) 
(ppm) 
of bubbles 
(%) (ml) 
__________________________________________________________________________ 
1 170 2 57 3 29 0 0 7.6 40.0 &gt;100 1.55 3300 
2 170 3 38 2 19 0 0 7.5 39.8 15 3.42 3120 
3 180 2 40 3 21 0 0 7.5 10.8 &gt;100 3.11 3140 
4 180 3 28 3 15 0 0 7.5 13.0 10 4.35 3090 
5 200 2 25 1 14 0 0 7.4 14.4 3 4.35 3080 
6 200 3 11 0 7 0 0 7.3 2.5 50 4.66 3040 
7 230 2 2 0 2 0 0 7.3 1.6 &gt;100 5.28 3020 
8 230 3 0 0 0 0 0 7.3 1.0 &gt;100 5.28 2990 
9 -- -- -- -- -- -- -- 7.7 0.9 Numberless 
* * 
__________________________________________________________________________ 
(Note) *Impossible to measure 
As apparent from the results in Table 1, the molded articles from the PFA 
having 7 to 2, per 10.sup.6 carbon atoms, of the terminal groups 
--CONH.sub.2 (the samples No. 2, No. 4, No. 5 and No. 6), have a few 
bubbles and have an allowable dimensional stability (shrinkage). Although 
there is not a clear correlation between the output of fluorine ion and 
the bubble formation or shrinkage of the molded article, there is tendency 
that some degree of the output of fluorine ion gives the depression of the 
bubble formation. 
EXAMPLE 3 
Treatment 1 
Using a melt indexer MX-101 (commercially available from Takara Kogyo 
Kabushiki Kaisha), each of the PFA powder obtained in Example 1 (the 
samples No. 4, No. 8 and No. 9) was extruded. That is, the PFA powder was 
allowed to stand at 372.degree. C. for 5 minutes, then was extruded by a 
piston. The obtained strand was cut into pellets having a length of 2 mm. 
As to the pellets, the fluorine ion output and the number of terminal 
groups --COF were measured. 
Treatment 2 
Each of the PFA powders (the samples No. 4, No. 8 and No. 9) was previously 
heat-treated, that is, an aluminum cup was charged with 20 g of the PFA 
powder and was heated in the air at 380.degree. C. for 5 hours. Then the 
extrusion was conducted in the same manner as in Treatment 1, and the 
fluorine ion output and the number of the groups --COF were measured. The 
results are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Sample 
Fluorine ion output (ppm) 
The number of --COF (per 10.sup.6 carbon 
atoms) 
No. Before treatment 
Treatment 1 
Treatment 2 
Before treatment 
Treatment 1 
Treatment 2 
__________________________________________________________________________ 
4 13.8 1.1 11 0 0 6 
8 1.0 3.8 14 0 0 10 
9 0.9 4.9 110 0 0 81 
__________________________________________________________________________ 
From the results as shown in Table 2, it would be understood that even if 
the PFA of the present invention is heated, the fluorine ion output and 
the number of --COF terminal groups are increased only a little, so the 
PFA of the present invention is thermally stable. 
According to the present invention, the thermally stable PFA having 
excellent powder properties can be provided. The molded articles from the 
PFA of the present invention have a few bubbles and have the excellent 
dimensional stability on heating. 
In addition to the elements used in the Examples, other elements can be 
used in the Examples as set forth in the specification and the drawings to 
obtain substantially the same results.