Process for the production of polytetrafluoroethylene porous membranes

A process for the production of PTFE porous membranes is provided wherein a PTFE preform is prepared by compression molding a PTFE molding powder of 1-90 .mu.m obtained by suspension polymerization of tetrafluoroethylene, the preform is sintered at a temperature above 327.degree. C., followed by skiving the heated preform into a film form. The thus-obtained film is heat treated at a temperature above 327.degree. C., the heat treated film is slowly cooled at a cooling rate of less than 70.degree. C./hr so as to adjust the crystallinity of PTFE in the sintered film to 60-75% and the film is stretched either uniaxially or biaxially to 1.3 to 6.5 times while heating at a temperature of 100.degree.-320.degree. C.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to a process for the production of 
polytetrafluoroethylene porous membranes. More particularly, the invention 
is concerned with a process for the production of porous membranes which 
are produced from the starting molding powder of polytetrafluoroethylene 
obtained by suspension polymerization, which porous membranes are 
excellent in strength and have pores which are nearly round in shape and 
relatively uniform in size, and, moreover, which are also excellent in 
porosity. 
TECHNICAL BACKGROUND OF THE INVENTION AND PROBLEMS ASSOCIATED THEREWITH 
Because of their excellent chemical resistance, thermal resistance and 
mechanical properties, polytetrafluoroethylene resins (hereinafter 
abbreviated sometimes to PTFE) are used in various fields. For instance, 
porous membranes made of PTFE are widely used, utilizing such properties 
as referred to above, as filters for corrosive materials or high 
temperature substances, and also used as diaphragms for electrolytic 
baths, fuel cells, etc. 
In recent years, moreover, with the developments in the semi-conductor 
industry and in molecular biology, porous membranes having pores which are 
of nearly round and uniform in pore diameter are in demand for the purpose 
of removing very minute impurities from gases or liquids. PTFE porous 
membranes as characterized above have come to attract keen interests of 
various fields of industry. 
For the production of porous membranes from PTFE resins, there has 
heretofore been adopted a process in which PTFE finely divided particles, 
present as fine powders having an average particle diameter of 0.1-0.4 
.mu.m obtained by emulsion polymerization of tetrafluoroethylene, is 
incorporated into a liquid lubricant, and the mixture is compression 
molded to prepare it to a preform. The preform is then processed into a 
film form by an extruding or rolling technique or combination thereof, the 
liquid lubricant is removed therefrom, and the PTFE membrane obtained is 
monoaxially or biaxially stretched under the usual heating conditions. For 
instance, Japanese Patent Publication No. 42794/1978 discloses a process 
for the production of PTFE porous materials, wherein the sintered PTFE 
membrane is heat treated at a temperature above 327.degree. C., followed 
by slow cooling, so that crystallinity of PTFE in the membrane becomes 80% 
or higher, and the membrane, thus treated, is stretched at a temperature 
of 25.degree.-260.degree. C. monoaxially at a draw ratio of 1.5-4 times. 
However, the process referred to above involved difficulties in that 
because the starting material used is a PTFE fine powder, the PTFE 
membrane prepared therefrom is liable to be fibrillated at the time the 
membrane is stretched, and hence it is difficult to produce PTFE membranes 
having pores which are nearly in a round shape and a predetermined pore 
diameter. That is, the pores formed in the PTFE porous membranes obtained 
by this process were of a long elliptical form having a marked difference 
between the major axis and the minor axis thereof, and hence it was 
difficult to obtain pores nearly of a round shape even when the PTFE 
membranes were biaxially stretched. Furthermore, this process involved 
such problems that the mechanical strength of the porous membranes 
obtained thereby cannot be said to be sufficient. 
We have conducted researched in an effort to solve the above-mentioned 
problems and have eventually accomplished the invention on the basis of 
the following facts. That is, it has been discovered that (a) in a process 
for production PTFE porous membrane, it is better to use a molding power 
of PTFE obtained by suspension polymerization of tetrafluoroethylene than 
the use of finely divided particles of PTFE known as the aforesaid fine 
powder obtained by emulsion polymerization of tetrafluoroethylene. A 
further discovery is that (b) in a process for stretching PTFE in a film 
form under heating conditions, by virture of the addition thereto of such 
an operation that the PTFE in a film form is previously heated at a 
temperature above 327.degree. C. and then slowly cooled at a cooling rate 
of less than 70.degree. C./hr so as to adjust the crystallinity of PTFE to 
60-75%, a PTFE porous membrane having greatly improved pore 
characteristics and mechanical strength as well as an excellent porosity 
can be obtained. 
OBJECTS OF THE INVENTION 
The present invention is intended to solve the problems as mentioned 
previously, and an object thereof is to provide a process for the 
production of PTFE porous membranes which have pores having nearly a round 
shape, the pore diameter of which can be controlled uniformly to a 
predetermined size. A further object of the invention is to provide a 
process for the production of PTFE porous membranes which are excellent in 
mechanical strength as well as in porosity. 
An essential feature of the processes for the production of PTFE porous 
membranes in accordance with the invention resides in the following: A 
PTFE preform is prepared by the compression molding of a PTFE molding 
powder having an average particle diameter of 1-90 .mu.m obtained by 
suspension polymerization of tetrafluoroethylene; the preform is heated at 
a temperature above 327.degree. C. followed by skiving the heated preform 
into a film form; the obtained film is heat treated at a temperature above 
327.degree. C.; the heated film is slowly cooled at a cooling rate of less 
than 70.degree. C./hr so as to adjust the crystallinity of PTFE in the 
heated film to 60-75%, and the film is stretched either monoaxially or 
biaxially to 1.3 to 6.5 times while heating at a temperature of 
100.degree.-320.degree. C. 
DETAILED DISCLOSURE OF THE INVENTION 
The PTFE used in the production of PTFE porous membranes in accordance with 
the present invention is a PTFE molding powder having an average particle 
diameter of 1-900 .mu.m, preferably 10-50 .mu.m, obtained by suspension 
polymerization of tetrafluoroethylene. 
The PTFE molding powder as referred to above is preformed in a metal mold 
or the like at a molding pressure of 10-30 MPa, whereupon a PTFE preform 
is obtained. Subsequently, this preform is sintered at a temperature of 
above 327.degree. C., preferably 340.degree.-380.degree. C., and then 
slowly cooled to obtain a cylindrical molding ordinarily. This cylindrical 
molding is skived by means of a skiver into a PTFE film of about 0.05-0.2 
mm in thickness. The PTFE film thus obtained is used as a preferable film 
in the process of the invention. 
Though the PTFE film is obtained by skiving the heated PTFE preform, PTFE 
film obtained by procedures other than the skiving technique can also be 
used in the present invention. In short, PTFE film used in the invention 
may be any film, provided it is one obtained by the compression molding of 
PTFE molding powder of an average particle diameter of 1-900 .mu.m 
obtained by suspension polymerization of tetrafluoroethylene, followed by 
heating. 
The thus obtained PTFE film is heat treated again at a temperature above 
327.degree. C., preferably 350.degree.-390.degree. C., and the heat 
treated film is slowly cooled at a cooling rate of less than 70.degree. 
C./hr so as to adjust the crystallinity of PTFE in the film to 60-75%. The 
cooling rate used in that case is desirably 10.degree. C./hr or less, 
preferably about 5.degree. C./hr. By virtue of subjecting the heat treated 
PTFE film to this slow cooling treatment, it becomes possible that a pore 
diameter of pores of PTFE porous membrane obtained later in the subsequent 
step can be controlled to a predetermined largeness, the pores obtained 
can be made nearly round in shape, and the porosity of the PTFE porous 
membrane obtained can be enhanced. Thus, the porous membrane obtained 
according to the present invention is excellent in mechanical strength. 
Furthermore, the PTFE film thus slowly cooled is excellent in stability 
when it is stretched, as evidenced by the fact that breakage or pinholing 
is less liable to occur at the time of streching said PTFE film. 
Where PTFE in a molten state is cooled at a cooling rate greater than 
70.degree. C./hr, the crystallinity of PTFE becomes less than 55%. If the 
crystallinity of PTFE is less than 55%, the PTFE porous membrane obtained 
therefrom is only that which is more or less low in porosity, though it is 
possible to control the pore diameter of the pores of said membrane to a 
predetermined largeness and make the pores nearly round in shape. 
The PTFE film is subjected at least once to such a slow cooling treatment 
in the process of the invention. That is, the PTFE film may be subjected 
twice or more to this slow cooling treatment. 
Subsequently, the PTFE film thus slowly cooled is streched either 
uniaxially or biaxially to 1.3 to 6.5 times while heating at a temperature 
of 100.degree.-320.degree. C., preferably 200.degree.-250.degree. C. When 
the temperature of the PTFE film is less than 100.degree. C. at the time 
of stretching thereof, the mechanical strength of the resulting PTFE 
porous membrane is undesirably found insufficient as breakage or the like 
sometimes occurs in the PTFE porous membrane being produced. The streching 
temperature of above 320.degree. C., on the one hand, is not preferable 
since pores which are uniform in diameter are not formed in the PTFE 
porous membrane obtained thereby. 
The draw ratio employed for stretching the PTFE film in that case is 
preferably 1.3 to 6.5 times, the use of a draw ratio exceeding 6.5 times 
is not preferable, since it is liable to cause pinholing in the film or 
breakage of the film at the time of streching thereof. The use of a draw 
ratio less than 1.3 times, on the one hand, is not preferable since no 
minute pores as desired are obtained in the resulting PTFE porous 
membrane. 
In the case of subjecting the PTFE film to a slow cooling treatment, the 
draw ratio employed for streching the film can be raised higher than that 
used in the case of a PTFE film subjected to a quenching treatment and, 
moreover, pores uniform in pore diameter can be obtained in the PTFE 
porous membrane obtained from the slowly cooled PTFE film. This is 
considered ascribable to such facts that the PTFE film subjected to slow 
cooling treatment is higher in crystallinity than the PTFE film subjected 
to quenching treatment, the surface of the PTFE film during slow cooling 
treatment is liable to fibrillation and the fibrillated portions are 
widened preferably in the biaxial direction at the time of streching, 
thereby facilitating the control of the pore diameter of the pores formed 
thereby, and moreover making the pores nearly round in shape. 
The stretching as referred to above is performed in the uniaxial or biaxial 
direction, preferably the biaxial direction. By virture of subjecting the 
PTFE film to biaxial stretching, there is observed such effects that the 
pores of the PTFE porous membrane obtained thereby are found to be nearly 
round in shape. 
PTFE porous membranes obtained in accordance with the present invention 
have a large number of continuous pores and are excellent in porosity, as 
evidenced by the fact that said porous membranes demonstrate large gas 
permeation amounts. The gas permeation amount Q demonstrated by the PTFE 
porous membrane obtained according to the present invention varies 
greatly, depending on the draw ratio and film thickness of the PTFE film 
used, on the gas differential pressure .DELTA.P, etc. For instance, when 
the gas differential pressure .DELTA.P is 0.05 MPa, the gas permeation 
amount Q is about 50-300 ml/min/cm.sup.2. Similarly, the porosity of PTFE 
porous membranes obtained according to the present invention varies 
greatly depending on the draw ratio and film thickness of the PTFE film 
used, and the porosity obtained is usually about 15-35%. 
The pores which are formed in the PTFE membranes obtained in accordance 
with the present invention have a shape which is nearly round, as 
evidenced by the electron microscope photographs thereof. Though the pore 
diameter of these pores varies greatly, depending on the draw ratio of the 
PTFE film used, the pore diameter of the pores formed in the PTFE porous 
membrane was usually about 0.1-0.5 .mu.m when the PTFE film was biaxially 
stretched to 2.times.2 times. The porosity determined in that case was 
about 0.1.times.10.sup.8 to about 3.0.times.10.sup.8 /cm.sup.2. In this 
connection, the pores formed in PTFE porous membranes obtained from the 
PTFE fine powder are only those which are of an elliptical shape with a 
large difference between the major axis and minor axis. 
EFFECTS OF THE INVENTION 
Because the processes for the production of PTFE porous membranes in 
accordance with the present invention are so designed as to use PTFE 
molding powder having an average particle diameter of 1-900 .mu.m obtained 
by suspension polymerization of tetrafluoroethylene, and to subject the 
PTFE film, before streching thereof, to a heat treatment again and to a 
slow cooling treatment so as to adjust the crystallinity of PTFE in the 
PTFE film thus treated, the PTFE porous membranes obtained thereby come to 
possess such effects as mentioned below. 
(a) The present PTFE porous membranes have pores which are nearly round in 
shape and, moreover, the pore diameter of the pores are almost uniform. 
(b) The present PTFE porous membranes are excellent in mechanical strength 
as well as in porosity. 
(c) The present PTFE film is excellent in stability when it is stretched, 
causing no occurrence, or a little, if any, of breakage or pinholing in 
the resulting film. 
Accordingly, it may be said that the PTFE porous membranes according to the 
present invention are excellent in filtering characteristics.

The present invention is illustrated below with reference to examples, but 
it should be construed that the invention is in no way limited to those 
examples. 
EXAMPLE 1 
PTFE molding powder (Polyflon M 12 produced and sold by Daikin Kogyo K.K.) 
having an average particle diameter of 25 .mu.m obtained by suspension 
polymerization of tetrafluoroethylene was premolded in a metal mold at a 
molding pressure of 15 MPa. The preform obtained was then sintered at 
365.degree. C. to prepare a PTFE molding. This PTFE molding was skived 
into a film to prepare a PTFE film of 0.1 mm in thickness. 
The PTFE film obtained was heat treated again in a circulating-air oven at 
370.degree. C., and then subjected to a slow cooling treatment in a 
circulating-air oven at a cooling rate of about 5.degree. C./hr. The PTFE 
film after the slow cooling treatment had a crystallinity of 65% and a 
specific gravity of 2.18. 
The PTFE film, thus subjected to the slow cooling treatment, was biaxially 
stretched at a draw ratio of 2.0.times.2.0 and a temperature of 
250.degree. C. to obtain a PTFE porous membrane. This porous membrane had 
a large number of pores which are nearly round in shape with the largest 
pore diameter of 0.5 .mu.m, and a porosity of about 34%. 
The N.sub.2 gas permeation amount Q of the PTFE porous membrane obtained 
above was measured while varying the gas pressure differential .DELTA.P to 
obtain the results as shown in Table 1. 
EXAMPLE 2 
A PTFE porous membrane was obtained by repeating the same procedure as in 
Example 1, except that the PTFE film was biaxially stretched at a 
temperature of 315.degree. C. 
The PTFE porous membrane obtained had a large number of pores which were 
round in shape with a pore diameter of less than 0.5 .mu.m. This PTFE 
porous membrane had a porosity of 22%. The N.sub.2 gas permeation amount Q 
of this membrane was measured in the same manner as in Example 1 to obtain 
the results as shown in Table 2. 
TABLE 1 
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Pressure differential [MPa] 
Permeation amount Q 
.DELTA.P [ml/min/cm.sup.2 ] 
______________________________________ 
Example 1 
0.05 65 
0.08 110 
0.10 130 
0.15 200 
0.20 270 
0.30 440 
0.40 600 
Example 2 
0.05 8 
0.08 13.5 
0.10 18 
0.15 28 
0.20 36 
0.30 58 
0.40 85 
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COMATIVE EXAMPLE 1 
A PTFE porous membrane was produced in the same procedure as in Example 1, 
except that the PTFE film was not subjected to a heat treatment again and 
a slow cooling treatment. 
On examining the thus obtained PTFE porous membrane by means of an electron 
microscope photograph thereof, the pores formed were not uniform and the 
strength of the membrane was low. Furthermore, this PTFE porous membrane 
was processed, whereupon pinholing occurred in the processed product. 
COMATIVE EXAMPLE 2 
In Example 1, the PTFE file was heated in a furnace at 370.degree. C. for 1 
hour, the film was taken out of the circulating-air oven and quenched at a 
cooling rate of higher than 70.degree. C./hr while allowing to stand in an 
ambient atmosphere. After quenching, the PTFE film had a crystallinity of 
52.5% and a specific gravity of 2.14. 
This film was biaxially stretched at a draw ratio of 2.0.times.2.0 and a 
temperature of 250.degree. C. to obtain a PTFE porous membrane. This 
porous membrane had a large number of pores nearly round in shape with the 
largest pore diameter of 0.5 .mu.m, but the porosity of this membrane was 
27%, which is somewhat low. 
COMATIVE EXAMPLE 3 
With the intention of producing the PTFE porous membrane, the same 
procedure as in Example 1 was repeated except that the PTFE film was 
stretched at a temperature of 340.degree. C. However, no pores were formed 
in the stretched PTFE film, and the desired PTFE porous membrane was not 
obtained.