Process for producing granular powder of modified polytetrafluoroethylene

To provide a process capable of preparing a modified PTFE granular powder, in which the ganular powder having excellent handling property, i.e. powder flowability and apparent density is prepared without using a finely pulverized powder and a molded article obtained from the granular powder has excellent physical properties, and a process capable of preparing a modified PTFE granular powder, in which the ganular powder is granulated with only water without using an organic liquid and further a molded article having a break down voltage equal to that in the case of using a finely pulverized powder of unmodified PTFE can be obtained. The granulation is carried out by stirring, in an aqueous medium, a powder of perfluoro(vinyl ether)-modified polytetrafluoroethylene prepared by suspension polymerization and having an average particle size of 100 to 300 .mu.m.

TECHNICAL FIELD 
The present invention relates to a process for preparing a granular powder 
of modified polytetrafluoroethylene. 
The present invention particularly relates to a process for preparing a 
granular powder of modified polytetrafluoroethylene by granulating a 
powder of perfluoro(vinyl ether)-modified polytetrafluoroethylene as a 
molding powder which is obtained by suspension polymerization 
substantially in the absence of an emulsifying agent. 
BACKGROUND ART 
Hitherto many proposals with respect to a process for preparing a granular 
powder by granulating a polytetrafluoroethylene (PTFE) powder have been 
made. For example, JP-B-44-22619 discloses a process for stirring and 
granulating a PTFE powder in an aqueous medium of 30.degree. to 
150.degree. C. containing a water-insoluble organic liquid having a 
boiling point of 30.degree. to 150.degree. C. 
In order to improve the process disclosed in the above-mentioned patent 
publication, JP-B-5 7-15128 discloses a process in which an equipment 
having mechanism for pulverizing a PTFE powder is employed. 
As a process for granulating by using water only, for example, JP-B-43-8611 
discloses a process for granulating in which a PTFE powder is stirred in 
water of 40.degree. to 90.degree. C., JP-B-47-3187 discloses a process for 
granulating in which a PTFE powder is stirred in an aqueous medium of not 
less than 40.degree. C., and further JP-A-3-259926 discloses a process for 
granulating in which a PTFE powder is stirred in an aqueous medium in 
combination use of pulverizing mechanism. 
Any of the processes disclosed in the above-mentioned patent publications 
use a so-called finely pulverized PTFE powder having an average particle 
size of less than 100 .mu.m. The reason why the finely pulverized powder 
is used is that when a coarsely pulverized powder is used, a molded 
article obtained therefrom is poor in physical properties, for example, 
tensile strength. 
It is possible to pulverize a PTFE powder to an average particle size of 
about 100 .mu.m with a coarsely pulverizing machine, but when pulverizing 
to an average particle size of less than 100 .mu.m, another finely 
pulverizing step is separately necessary. 
However, the finely pulverizing machine is expensive, and demands a larger 
energy consumption because machine size is large. Further there are 
required a pneumatic equipment for feeding the PTFE powder to the finely 
pulverizing machine, an auxiliary equipment for pneumatic line, a bag 
filter for collecting the obtained finely pulverized powder and the like. 
For the mentioned reasons, it is strongly required that the granulation is 
conducted by using a coarsely pulverized powder having an average particle 
size of not less than 100 .mu.m as a PTFE powder to produce a PTFE 
granular powder giving a molded article having good physical properties. 
As a granulation process with a coarsely pulverized powder, there is no 
process but a process disclosed in, for example, JP-A-3-259925, which is a 
process for granulating in which a PTFE powder of 420 .mu.m particle size 
is stirred in an aqueous medium of 60.degree. to 100.degree. C. in 
combination use of pulverizing mechanism. 
Even by the process disclosed in the above-mentioned patent publication, 
the obtained PTFE granular powder does not sufficiently satisfy physical 
properties of a molded article such as tensile strength, and for 
commercial production, a further step such as a gelling and pulverizing 
step is required. Also since a molded article obtained by molding the 
granular powder has a low break down voltage, it cannot be used for 
applications requiring a high break down voltage. Further as mentioned 
above, that process requires the pulverizing mechanism. 
The present inventors have studied, from various aspects, conventional 
processes for preparation of a PTFE granular powder by using a finely 
pulverized powder of tetrafluoroethylene homopolymer, and as a result, 
have found that use of modified PTFE, particularly perfluoro(vinyl 
ether)-modified PTFE enables a granulation process of a coarsely 
pulverized powder to give an excellent flowability, that is, handling 
property and a good apparent density, to a granular powder and enables a 
granulation process in which only water is used. The thus obtained 
modified PTFE granular powder gives a molded article having an excellent 
break down voltage. 
An object of the present invention is to provide a process for preparing, 
without using a finely pulverized powder, a modified PTFE granular powder 
being excellent in handling property, i.e. flowability and in apparent 
density and giving a molded article having excellent physical properties. 
Another object of the present invention is to provide a process in which 
granulation is carried out without using an organic liquid, but with water 
only. 
Further object of the present invention is to provide a process for 
preparing a modified PTFE granular powder which gives a molded article 
having break down voltage equal to that of a molded article obtained from 
finely pulverized powder of unmodified PTFE, even if the modified PTFE 
granular powder is obtained from a coarsely pulverized powder. 
DISCLOSURE OF THE INVENTION 
The present invention relates to a modified polytetrafluoroethylene 
granular powder prepared through granulation by stirring, in an aqueous 
medium, a powder of perfluoro(vinyl ether)-modified 
polytetrafluoroethylene obtained by suspension polymerization and having 
an average particle size of 100 to 300 .mu.m. 
Also in the present invention, it is preferable to carry out the 
above-mentioned granulation under a pressure of 0 to 5 kg/cm.sup.2 G in an 
aqueous medium of 50.degree. to 160.degree. C. in the absence of an 
organic liquid. 
Also in the present invention, it is preferable to carry out the 
above-mentioned granulation under a pressure of 0 to 5 kg/cm.sup.2 G in an 
aqueous medium of 10.degree. to 120.degree. C. in the presence of an 
organic liquid. 
Also in the present invention, it is preferable that an average particle 
size of the powder of perfluoro(vinyl ether)-modified 
polytetrafluoroethylene obtained by suspension polymerization is from 100 
to 150 .mu.m and that a break down voltage of a molded article obtained by 
molding the granular powder prepared by granulation with stirring is not 
less than 3.5 kV. 
Also in the present invention, it is preferable that the above-mentioned 
modified polytetrafluoroethylene is a copolymer prepared by copolymerizing 
99.0 to 99.999% by mole of tetrafluoroethylene (TFE) with 1.0 to 0.001% by 
mole of perfluoro(alkyl vinyl ether) or perfluoro(alkoxyalkyl vinyl ether) 
represented by the formula (I): 
EQU CF.sub.2 .dbd.CF--OR.sub.f (I) 
wherein R.sub.f is a perfluoroalkyl group having 1 to 10 carbon atoms, a 
perfluoro(alkoxyalkyl) group having 4 to 9 carbon atoms, an organic 
residue represented by the formula (II): 
##STR1## 
wherein m is 0 or an integer of 1 to 4, or an organic residue represented 
by the formula (III); 
##STR2## 
wherein n is an integer of 1 to 4. 
Also in the present invention, it is preferable to carry out the 
above-mentioned granulation in the presence of polytetrafluoroethylene 
aqueous dispersion obtained by emulsion polymerization in an amount of 0.1 
to 10% by weight based on perfluoro(vinyl ether)-modified 
polytetrafluoroethylene powder. 
Also in the present invention, it is preferable that 
polytetrafluoroethylene in the polytetrafluoroethylene aqueous dispersion 
is a tetrafluoroethylene homopolymer or a modified polytetrafluoroethylene 
obtained by copolymerizing 98.0 to 99.999% by weight of 
tetrafluoroethylene with 2.0 to 0.001% by weight of other 
fluorine-containing monomer.

BEST MODE FOR CARRYING OUT THE INVENTION 
In the present invention, the above-mentioned perfluoro(vinyl 
ether)-modified PTFE may be a copolymer obtained by copolymerizing, for 
example, 99.0 to 99.999% by mole, preferably 99.9 to 99.99% by mole of TFE 
with 1.0 to 0.001% by mole, preferably 0.1 to 0.01% by mole of a compound 
represented by the formula (I): 
EQU CF.sub.2 .dbd.CF--OR.sub.f (I), 
and as a copolymerization method, usual suspension polymerization may be 
used. 
When the percentage of TFE is in the above-mentioned range, there is 
obtained effect of exhibiting excellent tensile strength, elongation and 
crack resistance. Also when the percentage of the compound represented by 
the formula (I) is in the above-mentioned range, there is obtained effect 
of exhibiting excellent creep resistance. 
In the formula (I), R.sub.f represents a perfluoroalkyl group having 1 to 
10 carbon atoms, a perfluoro(alkoxyalkyl) group having 4 to 9 carbon 
atoms, an organic residue represented by the formula (II) or an organic 
residue represented by the formula (III). 
The number of carbon atoms of the perfluoroalkyl group is from 1 to 10, 
preferably from 1 to 5. When the number of carbon atoms is in the above 
range, effect of exhibiting excellent creep resistance can be obtained 
with maintaining not-melt-processable characteristic. 
Examples of the perfluoroalkyl group are, for instance, perfluoromethyl, 
perfluoroethyl, perfluoropropyl, perfluorobutyl, perfluoropentyl, 
perfluorohexyl and the like. From the viewpoints of creep resistance and 
cost of monomer, perfluoropropyl is preferable. 
In the present invention, there are used primary particles having an 
average particle size of 100 to 300 .mu.m and obtained by coarsely 
pulverizing a powder of the perfluoro(vinyl ether)-modified PTFE in the 
presence of water or in dry state, for example, with a pulverizing machine 
such as a hammer mill, a grinder having a rotor with blades, a jet mill or 
an impact mill. 
When the average particle size of the primary particles is in the above 
range, there can be obtained excellent handling property, i.e. flowability 
and excellent apparent density of the powder and excellent physical 
properties of the obtained molded article. Particularly when the average 
particle size of the primary particles is in the range of 100 to 150 
.mu.m, there can be obtained an effect of giving excellent break down 
voltage to the obtained molded article. 
As the aqueous medium used in the present invention, water is usually used. 
Water is not necessarily one having high purity. However if inorganic or 
organic impurities are mixed in water, they remain in the obtained 
modified PTFE granular powder, which causes undesirable coloring on the 
molded article and lowering of break down voltage. Therefore, it is 
necessary to eliminate such impurities previously from the aqueous medium. 
The aqueous medium is used in an amount of 150 to 5000% (% by weight, 
hereinafter the same) on the basis of the powder of perfluoro(vinyl 
ether)-modified PTFE. After addition of the powder into the aqueous 
medium, stirring for granulation is carried out. An amount of the aqueous 
medium is much enough to give flowability to the aqueous medium containing 
the powder of perfluoro(vinyl ether)-modified PTFE. Excess amount is 
disadvantageous from economical point of view. On the contrary, an 
insufficient amount cannot make the granulating operations smooth. 
However, as far as the aqueous medium containing the perfluoro(vinyl 
ether)-modified PTFE is in the completely flowable state, there is no 
problem even if the amount of the aqueous medium increases or decreases 
somewhat. 
In the present invention, in order to granulate the powder of 
perfluoro(vinyl ether)-modified PTFE by stirring it in the aqueous medium, 
there are, for example, a method of granulating in which the powder is 
stirred in the aqueous medium of 10.degree. to 120.degree. C. in the 
presence of an organic liquid (hereinafter may be referred to as "aqueous 
granulation method"), a hot water granulation method in which granulation 
is carried out by stirring in the aqueous medium of 50.degree. to 
160.degree. C. in the absence of an organic liquid, and other methods. 
From a point that it is not necessary to use expensive organic liquid, the 
hot water granulation method is preferable. 
As the organic liquid used in the present invention, there may be used, for 
example, a water-insoluble organic liquid having a surface tension at 
25.degree. C. of not more than about 40 dyne/cm. As the examples thereof, 
there can be used aliphatic hydrocarbons such as pentane and dodecane; 
aromatic hydrocarbons such as benzene, toluene and xylene; halogenated 
hydrocarbons such as tetrachloroethylene, trichloroethylene, chloroform, 
chlorobenzene, trichlorotrifluoroethane, monofluorotrichloromethane and 
difluorotetrachloroethane; and the like. Among them, halogenated 
hydrocarbons are preferable. Particularly preferable are chlorinated 
hydrocarbons and fluorochlorinated hydrocarbons such as 
1,1,1-trichloroethane, 1,1-dichloro-2,2,3,3,3-pentafluoropropane, 
1,3-dichloro-1,1,2,2,3-pentafluoropropane, 
1,1-dichloro-2,2,2-trifluoroethane and 1,1-dichloro-1-fluoroethane. This 
is because those organic liquids are inflammable and satisfy requirements 
for flon restriction. Those organic liquids may be used solely or in 
combination of two or more thereof. 
It is preferable that the amount of the organic liquid is from 5 to 100%, 
more preferably from 10 to 20% on the basis of the aqueous medium from the 
viewpoints of an average particle size and apparent density of the 
granular powder granulated. 
In the present invention, in the case of the hot water granulation method 
where the granulation is carried out by stirring the powder of 
perfluoro(vinyl ether)-modified PTFE in the aqueous medium in the absence 
of the organic liquid, the stirring may be carried out at a temperature 
within the range of 50.degree. to 160.degree. C., preferably 50.degree. to 
120.degree. C., more preferably 90.degree. to 120.degree. C. 
In that case, the temperature condition is very important. By maintaining a 
temperature of the slurry in the above-mentioned range, there can be 
easily obtained the modified PTFE granular powder having a desired 
particle size, a large apparent density, excellent powder flowability, 
uniform particle size, dense property and excellent electrical property. 
As an equipment which can be used in the process of the present invention, 
there may be used one equipped with a conventional stirring mechanism, 
and, for example, an equipment as disclosed in JP-A-3-259926 can be used. 
In the present invention, an equipment with a pulverizing mechanism is not 
necessarily required. 
In the process of the present invention, in addition to the PTFE powder, 
there can be used a PTFE aqueous dispersion obtained by emulsion 
polymerization and having an average particle size of 0.05 to 0.5 .mu.m 
and a solid content of 10 to 60%. The use of the PTFE aqueous dispersion 
makes it possible to prevent production of a finely divided powder of the 
perfluoro(vinyl ether)-modified polytetrafluoroethylene at the time of the 
granulation. 
When using the PTFE aqueous dispersion, PTFE in the dispersion covers the 
finely divided powder and then the finely divided powder disappear. 
Therefore, hysical properties of the granular powder are not lowered and 
handling property, i.e. flowability of the granular powder is enhanced. 
An amount of the PTFE aqueous dispersion is from 0.1 to 10% by weight, more 
preferably from 1 to 3% by weight on the basis of the perfluoro(vinyl 
ether)-modified polytetrafluoroethylene powder. It is suitable to add the 
PTFE aqueous dispersion before the perfluoro(vinyl ether)-modified 
polytetrafluoroethylene powder is added to water. 
PTFE in the PTFE aqueous dispersion includes not only tetrafluoroethylene 
homopolymer but also a copolymer such as modified PTFE obtained by 
copolymerizing 98.0 to 99.999% by weight of tetrafluoroethylene with 2.0 
to 0.001% by weight of other fluorine-containing monomer such as 
chlorotrifluoroethylene, hexafluoropropene or perfluoro(alkyl vinyl 
ether). 
As the aqueous granulation method of the present invention, there can be 
employed, for example, method mentioned below. 
A 10-liter stainless steel granulation tank equipped with a stirrer is 
charged with 3 to 7 liters of deionized water as the aqueous medium and 
further 400 to 1,500 ml of the above-mentioned organic liquid. To the 
mixture is added 1 to 2 kg of a powder of perfluoro(vinyl ether)-modified 
PTFE (water content: 10 to 20%) which was coarsely wet-pulverized 
previously to 100 to 300 .mu.m. Then with stirring at 400 to 800 rpm, a 
temperature in the system is kept in the range of 10.degree. to 
120.degree. C. The stirring is continued at around the mentioned 
temperature for 0.5 to 2 hours to complete the granulation. 
Then the stirring is stopped, and the wet modified PTFE granular powder 
obtained by filtration employing a 150 mesh sieve is dried at 120.degree. 
to 180.degree. C. for 20 to 10 hours in an electric oven to give 1 to 2 kg 
of a modified PTFE granular powder of the present invention. 
Physical properties of the obtained modified PTFE granular powder and 
molded article are, for example, as follows. 
Average particle size: 200 to 1,000 .mu.m 
Apparent density: 0.6 to 0.95 
Flowability: 4 to 8 times (21B method) 
Break down voltage: 1.5 to 7 (kV/0.1 mm) 
Tensile strength: 200 to 450 (kg/cm.sup.2 G) 
Elongation: 200 to 400 (%) 
The modified PTFE granular powder obtained according to the above-mentioned 
process can be prepared without using a finely pulverized powder and has 
excellent handling property, i.e. flowability and apparent density. The 
obtained molded article is excellent in physical properties. The modified 
PTFE granular powder can be suitably used as a molding material for, for 
example, packing, gasket, lining sheet of storage tank for chemical 
liquids, and the like. 
Also in the present invention, the aqueous granulation can be carried out 
under pressure. From the viewpoint of equipment cost required for a 
pressure resistant structure of the granulation tank, the granulation 
pressure is from 0 to 5 kg/cm.sup.2 G, preferably from 0 to 2 kg/cm.sup.2 
G, more preferably 0 kg/cm.sup.2 G. 
In the hot water granulation method of the present invention, there can be 
employed the same procedures as in the aqueous granulation method except 
that the organic liquid is not used and the temperature in the system is 
adjusted to 50.degree. to 160.degree. C. 
Physical properties of the modified PTFE granular powder obtained through 
the hot water granulation method and molded article are, for example, as 
follows. 
Average particle size: 200 to 1,000 .mu.m 
Apparent density: 0.6 to 0.9 
Flowability: 4 to 8 times (21B method) 
Break down voltage: 1.5 to 7 (kV/0.1 mm) 
Tensile strength: 200 to 450 (kg/cm.sup.2 G) 
Elongation: 200 to 400 (%) 
The modified PTFE granular powder obtained according to the hot water 
granulation method can be prepared by granulation with water only without 
using an organic liquid and without a finely pulverized powder, and has 
excellent handling property, i.e. flowability. The molded article obtained 
is therefore excellent in physical properties. The modified PTFE granular 
powder can be suitably used as the same molding material as mentioned 
above. 
Also in the present invention, the hot water granulation can be carried out 
under pressure. From the viewpoint of equipment cost required for a 
pressure resistant structure of the granulation tank, the granulation 
pressure is from 0 to 5 kg/cm.sup.2 G, preferably from 0 to 2 kg/cm.sup.2 
G, more preferably 0 kg/cm.sup.2 G. 
Also in the present invention, by adjusting an average particle size of the 
perfluoro(vinyl ether)-modified PTFE powder to 100 to 150 .mu.m, break 
down voltage of the obtained molded article becomes not less than 3.5 kV. 
EXAMPLE 
The present invention is then explained by means of Experimental Examples, 
and is not limited thereto. 
Test methods in Experimental Examples (physical properties of powders and 
molded articles) are mentioned below. 
Apparent density: Measured according to JIS K 6891--5.3. Particle size 
after coarsely pulverizing (Particle size of primary particles) 
Dry laser method: Measured under a pressure of 3 bar by using a laser 
diffraction type particle size distribution meter Herosystem available 
from Nippon Denshi Kabushiki Kaisha. 
Flowability: Measured according to the method disclosed in JP-A-3-259925. 
Namely as an equipment for measuring the flowability, there is used one 
shown in FIG. 3 of the mentioned patent publication and comprising a 
support base (42) and upper and lower hoppers (31) and (32), the both of 
which are aligned on their center lines and supported on the support base 
(42). The upper hopper (31) has an inlet (33) of a 74 mm diameter and an 
outlet (34) of a 12 mm diameter. The height from the inlet (33) to the 
outlet (34) is 123 mm. The outlet (34) is provided with a partition plate 
(35), and thereby the powder in the hopper can be kept therein or dropped 
optionally. The lower hopper (32) has an inlet (36) of a 76 mm diameter 
and an outlet (37) of a 12 mm diameter. The height from the inlet (36) to 
the outlet (37) is 120 mm. The outlet (37) is provided with a partition 
plate (38) like the upper hopper. The distance between the partition 
plates of the upper and lower hoppers is adjusted to 15 cm. In FIG. 3, 
numerals (39) and (40) indicate outlet covers of the upper and lower 
hoppers, respectively. Numeral (41) is a vessel for receiving the dropped 
powder. 
Prior to measuring the flowability, about 200 g of powder is allowed to 
stand for not less than four hours in a room, the temperature of which is 
adjusted to 23.5.degree. to 24.5.degree. C., and then sieved with a 10 
mesh sieve (sieve opening: 1,680 .mu.m). The measurement of the 
flowability is carried out at the same temperature. 
(I) At first, immediately after the upper hopper (31) is charged with just 
a cup of powder by using a 30 cc cup, the partition plate (35) is pulled 
out to drop the powder into the lower hopper. When the powder does not 
drop, the powder is stuck with a wire. After the powder has dropped 
completely into the lower hopper (32), the dropped powder is allowed to 
stand for 15.+-.2 seconds, and then the partition plate (38) of the lower 
hopper is pulled out to see whether the powder is dropped or not from the 
outlet (37). When the powder is dropped completely within eight seconds, 
the powder is estimated to have been dropped as required. 
(II) The same steps as above are repeated three times to see if the powder 
is dropped as required. In case where the powder is dropped satisfactorily 
twice or more, the flowability of the powder is estimated to be "Good." In 
case where the powder is never dropped, the flowability of the powder is 
estimated to be "Not good." In case where in three series of the dropping 
test, the powder has been dropped only one time, the dropping test is 
further conducted twice, and when the two series of the dropping test are 
both satisfactory, the flowability is estimated to be "Good." In other 
cases, the flowability is estimated to be "Not good." 
(III) With respect to the powder estimated to be "Good," the upper hopper 
is charged with two cups of powder by using the same 30 cc cup, and the 
dropping test of the powder is conducted in the same manner as above. When 
as a result, the flowability is estimated to be "Good," the number of cups 
filled with the powder is increased successively and the dropping test is 
continued until the flowability is estimated to be "Not good." The 
dropping test is conducted up to eight cups at most. The powder having 
flowed out from the lower hopper in the previous dropping test may be 
re-used. 
(IV) The larger the amount of the PTFE powder is, the more difficult to 
drop. 
The number of cups when the flowability is estimated to be "Not good" is 
substracted by 1, and the obtained value is taken as "Flowability" of the 
powder. Average particle size of granular powder: 
Standard sieves of 10, 20, 32, 48 and 60 meshes (inch mesh) are placed in 
that order from the top, and PTFE granular powder is put on the 10 mesh 
sieve. The sieves are vibrated to drop smaller particles downward through 
each sieve in order. Then after the ratio of the powder remaining on each 
sieve is obtained by %, accumulated percentages (ordinate) of each 
remaining powder to the openings of each sieve (abscissa) are plotted on 
the logarithmic probability paper, and those points are connected with a 
line. The particle size, the proportion of which is 50% on that line, is 
obtained and is regarded as an average particle size. Particle size 
distribution: 
The particle size distribution is a proportion in weight of the particles 
having a diameter 0.7 to 1.3 times the average particle size on the basis 
of the whole particles, and is calculated by multiplying the average 
particle size by 0.7 or 1.3. The obtained values are plotted on the 
accumulated weight percentage curve, and thus the weight percentage is 
obtained. Tensile strength (hereinafter may be referred to as "TS") and 
elongation (hereinafter may be referred to as "EL"): 
A die having an inner diameter of 50 mm is charged with a powder of 210 g 
and a pressure is gradually applied thereto over about 30 seconds until 
the final pressure reaches about 300 kg/cm.sup.2. Further that pressure is 
kept for 5 minutes to give a pre-molded article. The pre-molded article is 
taken out of the die mold and heated up to 365.degree. C. in an electric 
oven (atmosphere: air) at a rate of 50.degree. C./hour. After that 
temperature is kept for 5.5 hours, the pre-molded article is cooled to a 
room temperature at a rate of 50.degree. C./hour to obtain a sintered 
article having a cylindrical form. The sintered article is cut along its 
side to give a 0.5 mm thick band-like sheet which is then punched with a 
JIS dumbbel No. 3 to give a sample. The sample is stretched at a 
stretching rate of 200 mm/minute with an autograph having a gross weight 
of 500 kg according to JIS K 6891-58, and then a stress at break of the 
sample and elongation are measured. 
Break down voltage (hereinafter may be referred to as "B. D. V."): 
A block of a molded article which is produced in the same manner as for 
measurement of the tensile strength and elongation is cut to give a 0.1 mm 
thick tape. The break down voltage is measured by using the obtained tape 
according to JIS K 6891. 
Surface roughness: Measured according to JIS B 0601. 
Experimental Example 1 
(1) Base Powder 
A perfluoro(vinyl ether)-modified PTFE powder (Polyflon M-111 which is a 
modified PTFE available from Daikin Industries, Ltd. and copolymerized 
with 0.1% by mole of perfluoro(propyl vinyl ether)) in the form of reactor 
powder was used as a base powder. Physical properties of the base powder 
and molded article were measured according to the above-mentioned test 
methods. The results are shown in Table 1. 
(2) Powder Prepared by Hot Water Granulation Method A 10-liter granulation 
tank equipped with a stirrer was charged with 6.7 liter of deionized water 
and 1.5 kg of the mentioned base powder. With stirring at 800 rpm, the 
inside of the system was heated up to around 90.degree. C. over 20 
minutes. In that state, the granulation was carried out over three hours. 
Then the heating and stirring were stopped and a wet powder was obtained 
through filtration by using a 150 mesh sieve. The wet powder was then 
dried at 165.degree. C. in an electric oven for 16 hours to give 1.495 kg 
of a modified PTFE granular powder. With respect to the modified PTFE 
granular powder, physical properties of the powder and molded article were 
measured according to the above-mentioned test methods. The results are 
shown in Table 1. 
(3) Powder Prepared by Aqueous Granulation Method 
A modified PTFE granular powder was prepared in the same manner as in the 
above-mentioned steps for preparing the hot-water-granulated powder except 
that 750 ml of CH.sub.2 Cl.sub.2 was added as an organic liquid after 
charging of the base powder, the inside temperature of the system was 
adjusted to 38.degree. C. and the granulation was carried out for 10 
minutes. With respect to the obtained modified PTFE granular powder, 
physical properties of the powder and molded article were measured by the 
mentioned test methods. The results are shown in Table 1. 
Experimental Examples 2 to 6 
(1) Base Powder 
A base powder was obtained by using a pulverizing machine "JIYU MILL" 
available from Nara Kikai Kabushiki Kaisha and having a motor power of 2.2 
kW and a screen diameter shown in Table 1 and by rotating a hammer at the 
number of rotations shown in Table 1 to coarsely pulverize M-111. With 
respect to the obtained base powder, physical properties of the powder and 
molded article were measured by the mentioned test methods. The results 
are shown in Table 1. 
(2) Powder Prepared by Hot Water Granulation Method 
A modified PTFE granular powder was prepared through the same preparation 
steps as in (2) of Experimental Example 1 except that the base powder 
obtained in each of Experimental Examples 2 to 6 was used. With respect to 
the obtained modified PTFE granular powder, physical properties of the 
powder and molded article were measured by the mentioned test methods. The 
results are shown in Table 1. 
(3) Powder Prepared by Aqueous Granulation Method 
A modified PTFE granular powder was prepared through the same preparation 
steps as in (3) of Experimental Example 1 except that the base powder 
obtained in each of Experimental Examples 2 to 6 was used. With respect to 
the obtained PTFE granular powder, physical properties of the powder and 
molded article were measured by the mentioned test methods. The results 
are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
6 - 
Experimental Example 
1 2 3 4 5 6 
__________________________________________________________________________ 
Base powder 
Kind M-111 M-111 M-111 M-111 M-111 M-111 
Screen diameter (mm) -- 0.5 0.5 0.5 0.3 -- 
Number of rotations (rpm) Reactor 3000 5000 9600 9600 4P 
powder 
Apparent density 0.74 0.53 0.45 0.42 0.40 0.35 
Particle size after coarsely 280 161 152 116 91 30 
pulverizing (.mu.m) 
Dry laser method 
TS (kg/cm.sup.2 G) 218 309 321 334 428 439 
EL (%) 272 347 361 370 398 453 
B.D.V. (mean; kV) 1.5 4.7 7.6 11.5 11.6 12.8 
Surface roughness (.mu.m) 4.5 2.6 2.0 1.8 1.5 1.1 
Powder prepared by hot water granulation method 
Apparent density 0.78 0.78 0.78 0.75 0.69 0.58 
Flowability (21B method; times) 8 5 6 8 2 0 
Flowability (23 method; times) 2 0 1 5 1 0 
Average particle size (.mu.m) 330.0 180.0 300.0 700.0 1220.0 3000.0 
TS (kg/cm.sup.2 G) 216 303 312 
344 430 393 
EL (%) 270 370 375 391 402 415 
B.D.V. (mean; kV) 1.5 2.8 3.8 5.9 5.8 6.2 
Surface roughness (.mu.m) 4.3 2.6 2.7 2.4 2.7 4.2 
Powder prepared by aqueous 
granulation method 
Apparent density 0.78 0.83 0.78 0.75 0.68 0.88 
Flowability (21B method; times) 8 6 8 8 3 8 
Flowability (23 method; times) 2 0 1 6 2 2 
Average particle size (.mu.m) 330.0 410.0 390.0 380.0 300.0 350.0 
TS (kg/cm.sup.2 G) 217 305 315 
345 435 403 
EL (%) 271 365 380 395 405 425 
B.D.V. (mean; kV) 1.5 3.0 4.4 6.9 7.0 12.7 
Surface roughness (.mu.m) 4.4 2.9 2.8 2.2 2.1 2.2 
__________________________________________________________________________ 
Experimental Examples 7 to 9 
(1) Base Powder 
A base powder was prepared in the same preparation steps as in (1) of 
Experimental Example 2 except that Polyflon M-15 (PTFE homopolymer 
available from Daikin Industries, Ltd.) was used as a PTFE powder and the 
number of rotations and screen diameter shown in Table 2 were used. With 
respect to the obtained base powder, physical properties of the powder and 
molded article were measured by the mentioned test methods. The results 
are shown in Table 2. 
(2) Hot-water-granulated Powder 
A PTFE granular powder was prepared in the same preparation steps as in (2) 
of Experimental Example 1 except that the base powder obtained in each of 
Experimental Examples 7 to 9 was used. With respect to the obtained PTFE 
granular powder, physical properties of the powder and molded article were 
measured by the mentioned test methods. The results are shown in Table 2. 
(3) Powder Prepared by Aqueous Granulation Method 
A PTFE granular powder was prepared in the same preparation steps as in (3) 
of Experimental Example 1 except that the base powder obtained in each of 
Experimental Examples 7 to 9 was used. With respect to the obtained PTFE 
granular powder, physical properties of the powder and molded article were 
measured by the mentioned test methods. The results are shown in Table 2. 
Experimental Example 10 
(1) Base Powder 
A PTFE powder (PTFE homopolymer "Polyflon M-12" available from Daikin 
Industries, Ltd.) as a reactor powder was used as a base powder, and 
physical properties of the powder were measured by the mentioned test 
methods. The results are shown in Table 2. 
(2) Hot-water-granulated Powder 
A PTFE granular powder was prepared in the same preparation steps as in (2) 
of Experimental Example 1 except that the base powder obtained in this 
Experimental Example was used. With respect to the obtained PTFE granular 
powder, physical properties of the powder and molded article were measured 
by the mentioned test methods. The results are shown in Table 2. 
(3) Powder Prepared by Aqueous Granulation Method 
A PTFE granular powder was prepared in the same preparation steps as in (3) 
of Experimental Example 1 except that the base powder obtained in this 
Experimental Example was used. With respect to the obtained PTFE granular 
powder, physical properties of the powder and molded article were measured 
by the mentioned test methods. The results are shown in Table 2. 
Experimental Examples 11 and 12 
(1) Base Powder 
A base powder was prepared in the same preparation steps as in (1) of 
Experimental Example 2 except that the mentioned Polyflon M-12 was used as 
a PTFE powder and the number of rotations and screen diameter shown in 
Table 2 were used. With respect to the obtained base powder, physical 
properties of the powder and molded article were measured by the mentioned 
test methods. The results are shown in Table 2. 
(2) Hot-water-granulated Powder 
A PTFE granular powder was prepared in the same preparation steps as in (2) 
of Experimental Example 1 except that the base powder obtained in this 
Experimental Example was used. With respect to the obtained PTFE granular 
powder, physical properties of the powder and molded article were measured 
by the mentioned test methods. The results are shown in Table 2. 
(3) Powder Prepared by Aqueous Granulation Method 
A PTFE granular powder was prepared in the same preparation steps as in (3) 
of Experimental Example 1 except that the base powder obtained in each of 
Experimental Examples 11 and 12 was used. With respect to the obtained 
PTFE granular powder, physical properties of the powder and molded article 
were measured by the mentioned test methods. The results are shown in 
Table 2. 
TABLE 2 
__________________________________________________________________________ 
Experimental Example 
7 8 9 10 11 12 
__________________________________________________________________________ 
Base powder 
Kind M-15 M-15 M-15 M-12 M-12 M-12 
Screen diameter (mm) 0.7 0.4 -- -- 0.5 -- 
Number of rotations (rpm) 5000 5000 4P Reactor 5000 4P 
Powder 
Apparent density 0.75 0.53 0.41 0.50 0.36 0.29 
Particle size after coarsely 280 148 41 280 149 40 
pulverizing (.mu.m) 
Dry laser method 
TS (kg/cm.sup.2 G) 170 248 450 320 355 505 
EL (%) 130 150 380 289 330 410 
B.D.V. (mean; kV) 1.5 1.5 10.4 2.9 3.6 12.5 
Surface roughness (.mu.m) 5.4 3.8 1.7 1.6 1.7 0.8 
Powder prepared by hot water granulation method 
Apparent density 0.78 0.78 0.59 0.52 0.52 0.42 
Flowability (21B method; times) 5 6 0 8 7 0 
Flowability (23 method; times) 0 1 0 2 0 0 
Average particle size (.mu.m) 300.0 350.0 2500.0 400.0 750.0 3500.0 
TS (kg/cm.sup.2 G) 168 200 250 
329 356 403 
EL (%) 128 152 180 293 333 380 
B.D.V. (mean; kV) 1.5 1.5 1.5 3.1 3.5 
Surface roughness (.mu.m) 5.2 4.2 5.8 1.5 1.6 3.2 
Powder prepared by aqueous 
granulation method 
Apparent density 0.78 0.82 0.85 0.52 0.59 0.83 
Flowability (21B method; times) 5 7 8 8 8 8 
Flowability (23 method; times) 0 2 0 2 0 3 
Average particle size (.mu.m) 300.0 360.0 380.0 400.0 390.0 330.0 
TS (kg/cm.sup.2 G) 169 205 350 
325 354 450 
EL (%) 129 151 380 295 335 350 
B.D.V. (mean; kV) 1.5 1.5 1.5 3.0 3.6 6.5 
Surface roughness (.mu.m) 5.1 4.1 3.3 1.5 1.6 3.3 
__________________________________________________________________________ 
Relation between the physical properties of the granular powders or molded 
articles obtained in the above Experimental Examples and the particle 
sizes of primary particles is plotted in FIGS. 1 to 6. 
FIG. 1 is a graph for explaining relation between the apparent density and 
the particle size of primary particles in Experimental Examples 1 to 6. 
Numeral 1 represents a curve in case of the hot-water-granulated powder, 
numeral 2 represents a curve in case of the base powder and numeral 3 
represents a curve in case of the powder prepared by aqueous granulation 
method. 
As it is clear from FIG. 1, in case of the hot-water-granulated powder, an 
apparent density equal to that of the powder prepared through conventional 
aqueous granulation method can be obtained. 
FIG. 2 is a graph showing relation between the average particle size of the 
granular powder and the particle size of primary particles in Experimental 
Examples 1 to 6. Numeral 1 represents a curve in the case of the 
hot-water-granulated powder, numeral 2 represents a curve in the case of 
the base powder and numeral 3 represents a curve in the case of the powder 
prepared by the aqueous granulation method. 
As it is clear from FIG. 2, in case where the particle size of the primary 
particle is not less than 100.mu.m, the average particle size of the 
hot-water-granulated powder which is equal to that in the case of the 
powder prepared by conventional aqueous granulation method can be 
obtained. 
FIG. 3 is a graph showing relation between the flowability and the particle 
size of primary particles in Experimental Examples 1 to 6. Numeral 1 
represents a curve in the case of the hot-water-granulated powder, numeral 
2 represents a curve in the case of the base powder and numeral 3 
represents a curve in the case of the powder prepared by the aqueous 
granulation method. 
As it is clear from FIG. 3, in case where the particle size of the primary 
particle is not less than 100 .mu.m, the flowability of the 
hot-water-granulated powder which is equal to that in the case of the 
powder prepared by conventional aqueous granulation method can be 
obtained. 
FIG. 4 is a graph showing relation between the break down voltage and the 
particle size of primary particles in Experimental Examples 1 to 12. 
Numeral 1 represents a curve in the case of the hot-water-granulated 
powder, numeral 2 represents a curve in the case of the base powder and 
numeral 3 represents a curve in the case of the powder prepared by the 
aqueous granulation method. Any of the powders are those obtained in 
Experimental Examples 1 to 6. Numeral 4 represents a curve in the case of 
the hot-water-granulated powder of unmodified PTFE which was obtained in 
Experimental Examples 7 to 12. 
As it is clear from FIG. 4, in case where the particle size of the primary 
particle is not less than 100 82 m, the break down voltage of the 
hot-water-granulated powder which is equal to that in the case of the 
powder prepared by conventional aqueous granulation method can be 
obtained. 
FIG. 5 is a graph showing relation between the tensile strength and the 
particle size of primary particles in Experimental Examples 1 to 12. 
Numeral 1 represents a curve in the case of the hot-water-granulated 
powder, numeral 2 represents a curve in the case of the base powder and 
numeral 3 represents a curve in the case of the powder prepared by the 
aqueous granulation method. Any of the powders are those obtained in 
Experimental Examples 1 to 6. Numeral 4 represents a curve in the case of 
the hot-water-granulated powder of unmodified PTFE which was obtained in 
Experimental Examples 7 to 9. 
As it is clear from FIG. 5, the tensile strength of the 
hot-water-granulated powder which is equal to that in the case of the 
powder prepared by conventional aqueous granulation method can be 
obtained. 
FIG. 6 is a graph showing relation between the elongation and the particle 
size of primary particles in Experimental Examples 1 to 12. Numeral 1 
represents a curve in the case of the hot-water-granulated powder, numeral 
2 represents a curve in the case of the base powder and numeral 3 
represents a curve in the case of the powder prepared by the aqueous 
granulation method. Any of the powders are those obtained in Experimental 
Examples 1 to 6. Numeral 4 represents a curve in the case of the 
hot-water-granulated powder of unmodified PTFE which was obtained in 
Experimental Examples 7 to 9. 
As it is clear from FIG. 6, the tensile strength of the 
hot-water-granulated powder which is equal to that in the case of the 
powder prepared by conventional aqueous granulation method can be 
obtained. 
INDUSTRIAL APPLICABILITY 
As is clear from the above-mentioned results, in each of the processes of 
the present invention, the following effects can be obtained. 
In the process for granulating with stirring, in an aqueous medium, a 
powder of perfluoro(vinyl ether)-modified PTFE prepared by suspension 
polymerization and having an average particle size of 100 to 300 .mu.m, 
there can be obtained a modified PTFE granular powder which has excellent 
handling property, i.e. flowability and apparent density of the powder and 
gives a molded article having excellent physical properties. 
Also when the granulation is carried out with stirring in an aqueous medium 
of 50.degree. to 160.degree. C. under a pressure of 0 to 5 kg/cm.sup.2 G 
in the absence of an organic liquid, the same modified PTFE granular 
powder as above can be obtained without using an expensive organic liquid. 
Also when the granulation is carried out with stirring in an aqueous medium 
of 10.degree. to 120.degree. C. under a pressure of 0 to 5 kg/cm.sup.2 G 
in the presence of an organic liquid, the same modified PTFE granular 
powder as above can be obtained. 
Also when the powder of perfluoro(vinyl ether)-modified PTFE powder 
prepared by suspension polymerization and having an average particle size 
of 100 to 150 .mu.m is used, the break down voltage of the molded article 
obtained by molding the granular powder becomes not less than 3.5 kV.