Process for producing multilayer polytetrafluoroethylene porous membrane

A process for producing a multilayer polytetrafluoroethylene porous membrane comprising at least two layers having different average pore diameters is disclosed, the process comprising the steps of: filling the inside of a cylinder of an extruding mold distinctively with at least two kinds of polytetrafluoroethylene fine powders with each of which a liquid lubricant has been mixed; subsequently paste-extruding the powders to obtain a multilayer extrudate, which is then optionally rolled; and then stretching the extrudate or the rolled extrudate at least monoaxially after the liquid lubricant is removed therefrom or without removing the liquid lubricant.

FIELD OF THE INVENTION 
The present invention relates to a process for producing a multilayer 
polytetrafluoroethylene (hereinafter referred to as "PTFE") porous 
membrane. More particularly, it relates to a process for producing a 
multilayer PTFE porous membrane in which all the layers have been united 
tightly and which comprises at least two layers having different average 
pore diameters. 
BACKGROUND OF THE INVENTION 
PTFE is plastics having excellent heat and chemical resistance, and porous 
membranes made of PTFE are extensively utilized as filter media for 
corrosive gases and liquids, permeable membranes for electrolysis, and 
battery separators. Their use as a filter medium in the precision 
filtration of various gases and liquids used in the semiconductor industry 
has become an extremely important application thereof. 
In order that a porous membrane be an excellent filter medium, the pore 
diameter distribution of the membrane should be sharp and, when a fluid is 
allowed to permeate through the membrane pores at a certain pressure, the 
amount of the fluid passing through the membrane per unit time should be 
large. It has conventionally been known that the smaller the membrane 
thickness, the higher the permeation rate for a fluid, when porosity and 
pore diameter are constant. However, porous membranes having smaller 
membrane thicknesses may suffer deformation due to pressure exerted 
thereon during filtration and, as a result, the pore diameters are changed 
or, in some cases, the membranes are broken to be unable to function as a 
filter medium. In addition, the handling properties of such thin porous 
membranes are so poor that they are apt to be damaged when processed into 
filter modules or fixed to filter holders. 
For the purpose of eliminating these problems, several multilayer PTFE 
porous membranes have been proposed which comprise a filter layer having 
small pore diameters and a support layer having larger pore diameters than 
the filter layer. Conventional processes for producing such membranes 
include, for example, (1) a process in which one or more PTFE porous 
structures having smaller pore diameters and one or more PTFE porous 
structures having larger pore diameters are superposed on each other in an 
unsintered state and then press-bonded, and the resulting film is sintered 
at a temperature not lower than the melting point of PTFE to obtain a 
multilayer PTFE porous membrane (as described in JP-A-54-97686), and (2) a 
process in which an unsintered film is stretched between a roll revolving 
at a low speed and a roll revolving at a high speed, while a temperature 
gradient is being created in the direction of the thickness of the thin 
film and, at the same time, a compressive force is being applied in that 
direction, thereby to obtain a porous membrane in which its obverse side 
and reverse side have different pore diameters (as described in 
JP-B-63-48562). (The term "JP-A" and "JP-B" as used herein mean an 
"unexamined published Japanese patent application" and an "examined 
Japanese patent publication", respectively.) 
Further, although intended for producing a filter medium not for precision 
filtration but for the separation and enrichment of mixed isotopic gases, 
a conventional method for manufacturing a microporous permeable membrane 
include (3) a process in which one or more PTFE thin films in which a 
liquid pore-forming agent has been incorporated and one or more other PTFE 
thin films in which a liquid pore-forming agent has been incorporated are 
superposed on each other, the resulting assemblage is rolled to bond the 
thin films with each other, and then the liquid pore-forming agents are 
extracted with a low molecular weight liquid to form pores, thereby 
obtaining a multilayer PTFE porous membrane comprising at least two layers 
having different average pore diameters (as described in JP-B-55-22504). 
In process (1) above, sintering of unsintered stretched superposed films at 
a temperature not lower than the melting point of the PTFE powders gives a 
fusion-bonded united film, as disclosed in JP-A-51-30277. When unsintered 
sheets or films made from PTFE fine powders are lapped and then sintered, 
the respective layers are fusion-bonded with each other to give a united 
shape, and this technique has conventionally been known as, for example, a 
manufacturing method for PTFE-lapped electrical cables and PTFE-lapped 
tubes or pipes. Therefore, the method of superposing stretched porous 
structures with different pore diameters on each other and sintering the 
assemblage at a temperature not lower than the melting point of the PTFE 
has been quite common in the art. Process (1) above is disadvantageous in 
that it necessitates a step of separately forming two or more sheets or 
films having different porosities and the subsequent sintering step, which 
should be performed while the sheets or films superposed on each other are 
being pressed together. Furthermore, in order to industrially produce 
films with extremely small thicknesses or low strengths by such a 
laminating technique, expensive facilities and a high degree of skill are 
required so as to avoid occurrence of wrinkling, breakage, etc. in the 
process. 
Process (2) above is disadvantageous in that the stretching, which is 
conducted between rolls, is limited to monoaxial stretching and biaxial 
stretching cannot be used in this method. 
Process (3) above is characterized in that a membrane comprising two or 
more layers having different average pore diameters is obtained not 
through stretching, but by varying the packing densities of 
emulsion-polymerized PTFE powders having different primary particle sizes 
and shapes and also by use of pore-forming agents of different kinds. 
However, it should be noted that the pores in this membrane are mere 
spaces among emulsion-polymerized PTFE particles, that is, the unsintered 
film obtained from emulsion-polymerized PTFE by a paste-processing 
technique has a structure which nearly is the closest packing of the PTFE 
primary particles. Illustratively state, the primary particles have 
specific gravities of from 2.1 to 2.3 and the processed film has a bulk 
specific gravity of from 1.5 to 1.6 in the case where an ordinary 
petroleum solvent or the like has been used for shaping the film, and the 
difference between the specific gravities is ascribable to pores, which 
are spaces among the polymer particles. Such a membrane has a poor filter 
performance, i.e., very poor fluid permeability, and also has a very low 
strength compared with sintered membranes. If the unsintered multilayer 
membrane is sintered in order to increase its strength, it becomes 
non-porous to be unusable as a filter medium for fluids in the 
semiconductor industry. 
It has been proposed to obtain a multilayer porous membrane by a method in 
which rolled PTFE sheets containing a lubricant are superposed on each 
other, and the resulting assemblage is further rolled to a smaller 
thickness and then stretched (as described in JP-A-57-131236). The porous 
membrane obtained by this process, however, consists of layers that do not 
differ in porosity from each other at all, although it has high 
inter-layer bonding strength. JP-B-56-17216 discloses a process for 
producing a single-layer PTFE porous membrane having a high tensile 
strength. Conventionally, the size of small pores has been controlled by 
stretching and amorphous-lock, especially by changing the temperature, the 
drawing rate per unit time, and the draw ratio. 
On the other hand, unsymmetrical membranes consisting of an extremely thin 
filter layer and a support layer which is thicker and has larger pore 
diameters than the filter layer are manufactured from cellulose acetate or 
polysulfone. However, since such unsymmetrical membranes are obtained by 
wet coagulation processes, the membrane material is required to be soluble 
in the solvent used and, hence, this method has not been applicable to 
PTFE, which is not soluble in any ordinary solvent at all. 
SUMMARY OF THE INVENTION 
The present inventors have conducted intensive studies in order to 
eliminate the above-described problems of the conventional techniques. As 
a result, a method has been developed for producing a multilayer PTFE 
porous membrane which comprises a filter layer having a small average pore 
diameter and a support layer having a larger average pore diameter than 
the filter layer, and in which all the layers have been bonded to the 
adjacent layer(s) completely throughout the interface(s). By this method, 
an extremely thin filter layer can be formed. 
That is, the present inventors have surprisingly found that by stretching a 
multilayer structure consisting of layers of two or more kinds of PTFE 
fine powders having different average molecular weights, a multilayer 
porous membrane can be obtained easily in which the layers have different 
pore diameters despite the same stretching conditions and which is free of 
interlaminar peeling. 
Accordingly, an object of the present invention is to provide a process for 
producing a multilayer PTFE porous membrane free from the above-described 
prior art problems and having excellent permeability to various kinds of 
gases and liquids. 
Other objects and effects of the present invention will be apparent from 
the following description. 
The present invention provides a process for producing a multilayer 
polytetrafluoroethylene porous membrane comprising at least two layers 
having different average pore diameters, the process comprising the steps 
of: filling the inside of a cylinder of an extruding mold distinctively 
with at least two kinds of polytetrafluoroethylene fine powders with each 
of which a liquid lubricant has been mixed; subsequently paste-extruding 
the powders to obtain a multilayer extrudate, which is then optionally 
rolled; and then stretching the extrudate or the rolled extrudate at least 
monoaxially after the liquid lubricant is removed therefrom or without 
removing the liquid lubricant.

DETAILED DESCRIPTION OF THE INVENTION 
The process of the present invention for producing a multilayer PTFE porous 
membrane comprises the following steps. 
(1) Paste-Extrusion Step: 
This step may be performed according to the paste-extrusion method 
conventionally known as a technique for manufacturing PTFE unsintered 
articles. However, this step is characterized in that prior to extrusion, 
a multilayer preform 7 is obtained, for example, in a manner such as that 
illustrated in FIG. 1. As FIG. 1 (e) shows, this multilayer preform 7, for 
example, consists of a first layer 4, a second layer 5, and a third layer 
6 made of three PTFE fine powders 1, 2, 3, respectively (the Figure shows 
an example of a preform of a three-layer flat structure, but the preform 
prepared in the present invention is not limited thereto). Each of the 
layers 4 to 6 is formed from a wetted powder obtained by adding a liquid 
lubricant, such as solvent naphtha or white oil, to a fine powder prepared 
by the coagulation of an aqueous dispersion of emulsion-polymerized PTFE 
having an average primary particle diameter of from 0.2 to 0.4 .mu.m. The 
amount of the liquid lubricant to be used is varied depending on its kind, 
forming conditions, etc. Generally, however, the liquid lubricant is used 
in an amount of from 20 to 35 parts by weight per 100 parts by weight of 
the fine powder. A colorant or the like may further be added to the fine 
powder. The preform 7 is prepared as follows. First, as shown in FIG. 1 
(a), a PTFE fine powder 1 for obtaining a first layer 4 is placed in a 
box-shaped mold 8 in such a manner that the powder 1 is spread over a 
bottom force 9 to form a layer of the powder 1. Subsequently, as shown in 
FIG. 1 (b), a top force 10 is pressed against the powder in the direction 
indicated by arrow 11. Thus, the powder is compressed to form the first 
layer 4. 
The top force 10 is then removed, and a PTFE fine powder 2 for forming a 
second layer 5 is placed in the mold 8 as shown in FIG. 1 (c). This powder 
2 is compressed with the top force 10 in the same manner as in FIG. 1 (b) 
described above, to form the second layer 5 on the first layer 4 as shown 
in FIG. 1 (d). Thereafter, a PTFE fine powder 3 for forming a third layer 
6 is placed in the mold 8 as shown in FIG. 1 (d) and then compressed with 
the top force 10. 
Thus, the multilayer preform 7 is finally obtained which comprises the 
first layer 4, the second layer 5, and the third layer 6 as shown in FIG. 
1 (e), and which has been shaped so as to fit almost tightly into the 
inside of the cylinder 12 of a paste-extruding mold shown in FIG. 2. 
This preform 7 is put in the cylinder 12 of a paste-extrusion apparatus 
shown in FIG. 2, and then pushed with a ram 14. This cylinder 12 of the 
mold shown in FIG. 2, for example, has a rectangular section measuring 50 
mm.times.100 mm, in the direction perpendicular to the axis, and narrows 
at one end thereof at an outlet part 13 of the mold to form a nozzle 
having an orifice measuring 50 mm.times.5 mm. 
By pushing the preform 7 through the nozzle orifice, the first layer 4, the 
second layer 5, and the third layer 6 are completely united to form a 
paste-extruded sheet 15 in which each layer has a uniform thickness. It 
was ascertained by a stereomicroscopic examination that the relative 
thickness of each of the layers constituting this paste-extruded sheet 15 
was the same as that for the multilayer preform used. As described above, 
by forming the preform 7 beforehand, it has become possible to easily 
produce even a laminate having a very thin and low-strength layer; 
production of such a laminate has been difficult with the conventional 
techniques. 
(2) Rolling Step: 
In this step, which may be performed if required and necessary, the 
paste-extruded sheet may be rolled according to an ordinary rolling method 
as follows. 
The sheet obtained in paste-extrusion step (1) is cut into a proper length. 
The cut sheet is rolled by means of pressure rolls in a direction along or 
across the extruding direction, thereby to obtain a multilayer film having 
a thickness of, for example, 100 .mu.m. 
Thereafter, the liquid lubricant may be or may not be removed from the 
multilayer film. The removal of the liquid lubricant can be conducted by 
extraction and/or drying (for example, heat-drying in an oven at 
250.degree. C. for 20 seconds). Thus, a multilayer PTFE unsintered film is 
obtained. The removal of the liquid lubricant may be carried out after the 
subsequent stretching step. 
In the above-described paste-extrusion step (1) and rolling step (2) if 
any, the PTFE preform receives shearing force to partly change into 
fibers. Due to the fiber formation, the paste-extruded sheet or the rolled 
film can have a moderate strength and elongation, which are properties 
needed for the subsequent stretching step. 
All procedures in the above two steps are performed at temperatures not 
higher than about 327.degree. C., which is the melting point of sintered 
PTFE, and in general, performed at around room temperature. 
(3) Stretching Step: 
The multilayer unsintered film obtained through the above-described 
paste-extrusion step (1) and rolling step (2) if any is stretched at least 
monoaxially. 
In this step, the multilayer unsintered film is stretched in an unsintered 
state. The stretching is generally carried out between rolls revolving at 
different speeds or by means of a tenter in an oven. The stretching 
temperature is preferably not higher than the melting point of sintered 
PTFE. The stretching may be performed either monoaxially or biaxially, and 
the draw ratio may be determined according to use of the membrane being 
produced. 
(A) In the case of monoaxial stretching, the multilayer unsintered film is 
stretched in a direction parallel with or perpendicular to the extruding 
direction. 
(B) In the case of biaxial stretching, the multilayer unsintered film is 
first stretched in the same manner as (A) above, and subsequently further 
stretched in a direction perpendicular to the first stretching. 
Through the stretching, each layer in the multilayer unsintered film comes 
to be of a porous structure in which micropores are present uniformly 
throughout the layer. Thus a multilayer PTFE porous membrane in which each 
layer has micropores is finally obtained. 
If required and necessary, the multilayer porous membrane thus obtained may 
be heated at a temperature not lower than the melting point of sintered 
PTFE, or at a temperature not lower than the stretching temperature. Due 
to this heating, the multilayer porous membrane is made to undergo no 
dimensional change and to have an enhanced mechanical strength. 
The average pore diameters of the layers in the multilayer porous membrane 
are determined by the kind of the PTFE fine powders 1, 2, 3, etc. used to 
constitute respective layers and by incorporation of other ingredient(s) 
thereinto. Illustratively stated, in order that a multilayer porous 
membrane comprising two or more layers having different average pore 
diameters be obtained according to the present invention, it is important 
that the two or more layers should be made respectively from at least two 
kinds of PTFE fine powders 1, 2, 3 etc. 
One factor that can make one of the PTFE fine powders 1, 2, 3, etc. 
different from one or more of the other fine powders is average molecular 
weight. 
Generally, in a multilayer PTFE porous membrane obtained from a combination 
of a PTFE fine powder having an average molecular weight of 6,000,000 or 
more and a PTFE fine powder having an average molecular weight less than 
6,000,000, the layer made from the PTFE fine powder having an average 
molecular weight of 6,000,000 or more has a smaller average pore diameter 
than the layer made from the PTFE fine powder having an average molecular 
weight less than 6,000,000. The combination of a PTFE fine powder having 
an average molecular weight of from 3,500,000 to 6,000,000 and that having 
an average molecular weight of from 6,000,000 to 10,000,000 is preferably 
used. It is preferable in such a case that the average molecular weight 
difference between the two powders be 1,000,000 or more, and the larger 
the average molecular weight difference, the more preferred. 
The term "PTFE" herein includes not only homopolymers of 
tetrafluoroethylene but also copolymers of tetrafluoroethylene and not 
more than 2% by weight, preferably not more than 1% by weight, of other 
monomer(s) copolymerizable therewith, e.g., trifluorochloroethylene, 
hexafluoropropylene, perfluoroalkyl vinyl ether, etc. 
In the case where the PTFE constituting a fine powder is a homopolymer, its 
average molecular weight (Mn) can be calculated from the value of the 
specific gravity (S.G.) of the PTFE fine powder particle, using the 
following equation. 
EQU log.sub.10 Mn=28.04-9.790.times.S.G. 
In the case where the PTFE constituting a fine powder is a copolymer, 
however, there are cases where the average molecular weight value 
calculated using the above equation does not agree with the actual average 
molecular weight. Therefore, PTFE fine powders to be combined with each 
other are not substantially limited in average molecular weight and those 
having average molecular weights outside the above-specified range may be 
used, as long as mono-layer films separately made from the respective PTFE 
fine powders give, when stretched under the same conditions, mono-layer 
porous membranes having different average pore diameters. 
Another factor that can make one of the PTFE fine powders 1, 2, 3, etc. 
different from one or more of the other fine powders is the presence of a 
non-fiber-forming material, i.e., the case in which at least one of the 
PTFE fine powders 1, 2, 3, etc. contains a non-fiber-forming material. 
In general, PTFE fine powder particles have the property of readily forming 
fibers during the paste-extrusion step, rolling step, stretching step, 
etc., where shear stress is exerted on the powder particles being treated. 
On the other hand, particles of a low molecular weight PTFE polymer and 
particles of a polymer such as PFA (tetrafluoroethylene-perfluoroalkyl 
vinyl ether copolymer), FEP (tetrafluoroethylene-hexafluoropropylene 
copolymer), or the like do never form fibers in the above processing 
steps. For this reason, a layer of fine powder containing a 
non-fiber-forming material, such as the polymer particles mentioned above, 
forms a smaller number of fibers through the above-described steps and, as 
a result, gives a stretched layer having a larger average pore diameter, 
while a stretched layer made from PTFE fine powder only has a smaller 
average pore diameter. The non-fiber-forming polymer particles do not 
readily fall off the layer since they have been incorporated in 
interlocked fibers formed from the fine powder. However, in order to 
completely prevent the non-fiber-forming polymer particles from falling 
off the final porous membrane, it is effective to heat the membrane at a 
temperature not lower than the melting point of the polymer particles 
thereby to fusion-bond the polymer particles to the fibers. 
The amount of the non-fiber-forming polymer particles mixed with a fine 
powder is generally from 5 to 120 parts by weight, preferably from 20 to 
100 parts by weight, per 100 parts by weight of the PTFE fine powder. If 
the incorporated amount thereof is less than 5 parts by weight, no effect 
is produced by the incorporation thereof. If the amount thereof is larger 
than 120 parts by weight, there is a problem that the resulting multilayer 
porous membrane has impaired strength. 
The non-fiber-forming material is not limited to fluoroplastics such as 
those described above. Other materials that can be used as the 
non-fiber-forming material to produce the above-described effect include 
inorganic materials such as carbon, graphite, titanium oxide, iron oxide, 
silica, glass fibers, and other inorganic particles such as glass beads, 
and organic materials such as particles of organic polymers including a 
polyimide, polyamideimide, polyphenylene sulfide, aromatic polyester, 
polyetheretherketone, and the like. 
The particle diameter of the non-fiber-forming material is generally from 
0.03 to 20 .mu.m, and preferably from 1.0 to 10 .mu.m. If it is smaller 
than 0.03 .mu.m, the effect of the addition thereof tends to be 
insufficient, and if it is larger than 20 .mu.m, the molding property of 
the PTFE fine powder tends to be deteriorated. 
The particle diameter of the PTFE fine powder used in the present invention 
is not particularly limited, and is preferably from 200 to 1,000 .mu.m, 
and more preferably from 450 to 600 .mu.m. 
As described hereinabove, the process of the present invention can provide 
a multilayer PTFE porous membrane in which all the layers have been united 
tightly and which comprises at least two layers having different average 
pore diameters, only by the ordinary steps of PTFE paste extrusion, 
rolling if any, and stretching. This process is characterized in that at 
least two kinds of PTFE fine powders 1, 2, 3, etc. are used to form the 
respective layers in the multilayer porous membrane, and that the process 
does not necessitate the troublesome step of superposing films on each 
other. 
According to the process of the present invention, the filter layer, which 
has the smallest average pore diameter and determines permeability to 
gases and liquids, can be made to have a very thin thickness. Therefore, 
the multilayer PTFE porous membrane obtained by the process of the present 
invention is useful as a high-permeability filter medium for precision 
filtration and, further, there is no fear of interlaminar peeling during 
use since all the layers have been completely united. 
In the case where the multilayer PTFE porous membrane produced by the 
process of the present invention is a flat membrane, it is useful as a 
filter medium for the precision filtration of liquids and gases, a battery 
separator, a permeable membrane for electrolysis, an electrical insulating 
material, or the like. In the case where the multilayer porous membrane is 
a tubular membrane, it is useful as a hollow fiber filter medium for 
liquids and gases, a material for producing artificial organs, such as 
artificial blood vessels and artificial lungs, endoscope tubes, etc. 
The present invention will be explained in more detail by reference to the 
following Examples and Comparative Example, but the Examples should not be 
construed to be limiting the scope of the present invention. 
In the examples, various properties were measured by the following methods. 
(1) Membrane Thickness: 
The membrane thickness was measured with a membrane thickness meter (model 
"1D-110MH", manufactured by Mitsutoyo Co., Ltd., Japan). 
(2) Porosity: 
The pores in the membrane to be evaluated are filled with pure water by the 
ethanol displacement method and the weight W (g) of this water-impregnated 
membrane was measured. 
Further, the absolute dry weight W.sub.O (g) and volume V (cm.sup.3) of the 
membrane were measured. From these measured values, the porosity was 
calculated using the following equation. 
EQU Porosity=(W-W.sub.O).times.100/V (%) 
(3) Gas Permeability: 
The porous membrane to be evaluated was cut into a disk having a diameter 
of 25 mm, and this disk was fixed to a filter holder having an effective 
permeation area of 2.15 cm.sup.2. One side of the resulting filter was 
exposed to a pressurized nitrogen gas of 0.639 bar and the amount of the 
nitrogen gas passing through the membrane was measured with a mass flow 
meter. 
From the thus-measured value, permeation rate (unit; l/cm.sup.2 
.multidot.hr) was calculated which was the amount of the gas that passed 
through the membrane per square centimeter (cm.sup.2) of the effective 
permeation area per hour. 
(4) Average Pore Diameter: 
The mean flow pore diameter (MFP) measured by "Coulter Porometer" 
(manufactured by Coulter Electronics Co., U.S.A.) was regarded as the 
average pore diameter. From the following model experiment, it was 
ascertained that the thus-measured average pore diameter of the multilayer 
porous membrane of the present invention was substantially in agreement 
with the average pore diameter of the layer in the multilayer porous 
membrane that had he smallest average pore diameter. 
Model Experiment 
Two kinds of single-layer PTFE porous membranes were prepared which were 
porous membrane A having an average pore diameter as measured by "Coulter 
Porometer" of 0.20 .mu.m and a thickness of 47 .mu.m and porous membrane B 
having an average pore diameter as measured by "Coulter Porometer" of 0.98 
.mu.m and a thickness of 69 .mu.m. Then, porous membrane A was just 
superposed on porous membrane B to give a two-layer porous membrane. On 
the other hand, one porous membrane A, as an intermediate layer, was 
sandwiched between two porous membranes B to give three-layer porous 
membrane. The thus-obtained two multilayer porous membranes were examined 
for average pore diameter with "Coulter Porometer". As a result, the 
average pore diameter of the former membrane was 0.19 .mu.m and that of 
the latter was 0.18 .mu.m, these average pore diameter values being 
substantially in agreement with the average pore diameter of porous 
membrane A. 
In the following Examples and Comparative Example, three kinds of PTFE fine 
powders specified below were used. 
______________________________________ 
PTFE Average Non-fiber-forming 
fine powder molecular weight 
material 
______________________________________ 
1 5,100,000 -- 
2 7,200,000 -- 
3 5,100,000* Polymer particles 
of low molecular 
weight PTFE 
______________________________________ 
Note: *Excepting the low molecular weight PTFE 
The above PTFE fine powders 1 to 3 each has an average primary particle 
diameter of from about 0.2 to 0.4 .mu.m and has been obtained by the 
coagulation of an aqueous dispersion of emulsion-polymerized PTFE. 
PTFE fine powders 1 and 2 were commercially available products and PTFE 
fine powder 3 was prepared in the following manner. 
Preparation Method for PTFE Fine Powder 3 
100 Parts by weight, on a dry basis, of an aqueous dispersion of 
emulsion-polymerized PTFE having an average molecular weight of 5,100,000 
and an average primary particle diameter of from about 0.2 to 0.4 .mu.m 
was mixed with 100 parts by weight, on a dry basis, of an aqueous 
dispersion of low molecular weight PTFE polymer particles (trade name 
"Lublon L-5", particle diameter: 0.1 to 0.4 .mu.m, average molecular 
weight: 300,000 to 600,000, manufactured by Daikin Industries, Ltd., 
Japan) as a non-fiber-forming material. This mixture was stirred in a 
stirring vessel, upon which the two kinds of primary particles were mixed 
uniformly and coagulate to form secondary particles of about 200 to 1,000 
.mu.m. The resulting secondary particles were dried at 150.degree. C. to 
remove the water, thereby obtaining PTFE fine powder 3. 
EXAMPLE 1 
100 Parts by weight of each of PTFE fine powder 1 (average molecular weight 
5,100,000) and PTFE fine powder 2 (average molecular weight 7,200,000) was 
mixed with 23 parts by weight of a liquid lubricant (trade name "Isopar 
M", manufactured by Exxon Co.). In a manner similar to that illustrated in 
FIG. 1, the resulting two kinds of wetted powders were used to prepare a 
multilayer preform in which the ratio of the thickness of one layer to 
that of the other was 1/1. Subsequently, this multilayer preform was put 
in the cylinder 12 of a paste-extruding mold as shown in FIG. 2, and then 
extruded by means of a ram 14 to obtain a sheet. The sheet thus obtained 
was cut into about 100 mm length, and rolled in a direction perpendicular 
to the extruding direction. The rolled sheet was then heat-dried in an 
oven at 250.degree. C. for 20 seconds to remove the liquid lubricant, 
thereby obtaining a multilayer unsintered film having a thickness of 100 
.mu.m. 
Separately, the same multilayer unsintered film as that obtained above was 
prepared in the same manner as above except that one of the two powders 
used had been colored beforehand with a pigment. A section of this 
multilayer film, which section was cutting across the thickness of the 
film, was examined with a stereomicroscope. As a result, it was 
ascertained that the ratio of the thickness of one layer to that of the 
other was 1/1 as similar to the case of the multilayer preform. 
In an oven kept at about 300.degree. C., the multilayer unsintered film 
obtained above was stretched in an unsintered state in a direction same as 
the rolling direction at a stretching rate of 1,000%/sec in a draw ratio 
of 2.5, thereby obtaining a multilayer porous membrane having a thickness 
of 96 .mu.m. 
A scanning electron photomicrograph (magnification: 3,000; hereinafter 
referred to as "SEM photograph") of the surface of that layer of the 
multilayer porous membrane which was made from fine powder 1 is shown in 
FIG. 3, while an SEM photograph of the surface of the layer made from fine 
powder 2 is shown in FIG. 4. From the two photographs, it can be seen that 
in the multilayer porous membrane obtained, the layer made from fine 
powder 1 had a larger average pore diameter and the layer made from fine 
powder 2 had a smaller average pore diameter. 
This multilayer porous membrane had a porosity of 70%, an average pore 
diameter of 0.33 .mu.m, and a gas permeation rate of 66.1 l/cm.sup.2 
.multidot.hr. 
EXAMPLE 2 
Using the same PTFE fine powders 1 and 2 as those used in Example 1, 
extrusion, rolling, and stretching were conducted in the same manner as in 
Example 1 except that the ratio of the thickness of the layer of fine 
powder 1 to that of the layer of fine powder 2 was 4/1. Thus, a multilayer 
porous membrane having a thickness of 95 .mu.m was obtained. As similar to 
Example 1, a comparison between SEM photographs of the surfaces of the two 
layers of the thus-obtained multilayer porous membrane showed that the 
layer made from fine powder 1 had a larger average pore diameter and the 
layer made from fine powder 2 had a smaller average pore diameter. This 
multilayer porous membrane had a porosity of 68%, an average pore diameter 
of 0.34 .mu.m, and a gas permeation rate of 86.1 l/cm.sup.2 .multidot.hr. 
EXAMPLE 3 
Using PTFE fine powder 3, which was a mixture of 100 parts by weight of the 
same PTFE fine powder 1 as that used in Example 1 and 100 parts by weight 
of low molecular weight PTFE polymer particles, and also using the same 
PTFE fine powder 2 as that used in Example 1, extrusion, rolling, and 
stretching were conducted in the same manner as in Example 1 except that 
the ratio of the thickness of the layer of fine powder 3 to that of the 
layer of fine powder 2 was 4/1. Thus, a multilayer porous membrane having 
a thickness of 99 .mu.m was obtained. 
An SEM photograph of the surface of that layer of the thus-obtained 
multilayer porous membrane which was made from fine powder 3 is shown in 
FIG. 5, while an SEM photograph of the surface of the layer made from fine 
powder 2 is shown in FIG. 6. From the two photographs, it can be seen that 
in the multilayer porous membrane obtained, the layer made from fine 
powder 3 has a larger average pore diameter and the layer made from fine 
powder 2 has a smaller average pore diameter. 
This multilayer porous membrane had a porosity of 71%, an average pore 
diameter of 0.34 .mu.m, and a gas permeation rate of 110.6 l/cm.sup.2 
.multidot.hr. 
EXAMPLE 4 
Using the same PTFE fine powder 3 as that used in Example 3 and the same 
PTFE fine powder 2 as that used in Example 1, a multilayer preform was 
prepared which had a three-layer structure consisting of one fine powder 2 
layer sandwiched between two fine powder 3 layers, with the ratio of the 
thickness of one of the fine powder 3 layers to that of the fine powder 2 
layer to that of the other fine powder 3 layer being 4/1/4. This preform 
was extruded and rolled in the same manner as in Example 1 to obtain a 
multilayer unsintered film having a thickness of 55 .mu.m. This multilayer 
unsintered film was then stretched in the same manner as in Example 1, 
thereby obtaining a multilayer porous membrane having a thickness of 53 
.mu.m. This multilayer porous membrane had a porosity of 72%, an average 
pore diameter of 0.42 .mu.m, and a gas permeating rate of 853.9 l/cm.sup.2 
.multidot.hr. 
The thickness of the intermediate layer of the above-obtained multilayer 
porous membrane was measured and found to be about 5 .mu.m. Separately, it 
was tried to prepare the same multilayer porous membranes as above by the 
conventional laminating method, but membrane fabrication was so difficult 
that none of the thus-obtained multilayer porous membranes had an 
intermediate layer having a uniform thickness of about 5 .mu.m. 
Further, the multilayer porous membranes obtained in Examples 1 to 4 were 
subjected to a physical breakage test in the following manner: On both 
sides of the porous membrane at the edge thereof, adhesive tapes were 
adhered while the adhesive tapes did not contact with each other. The 
adhesive tape were pulled to be peeled from the porous membrane, and it 
was observed whether or not interlaminar peeling occurred in the porous 
membrane. As a result, no interlaminar peeling was observed in all the 
porous membrane. 
COMATIVE EXAMPLE 
Using PTFE fine powder 2 only as raw fine powder, extrusion, rolling, and 
stretching were conducted in the same manner as in Example 1 to obtain a 
porous membrane having a thickness of 97 .mu.m. 
This porous membrane had a porosity of 70%, an average pore diameter of 
0.32 .mu.m, and a gas permeation rate of 33.0 l/cm.sup.2 .multidot.hr. 
Table 1 summarizes the results of the above Examples and Comparative 
Example. 
TABLE 1 
______________________________________ 
Gas 
Membrane Average pore 
permeation 
thickness 
Porosity diameter rate 
(.mu.m) (%) (.mu.m) (l/cm.sup.2 .multidot. hr) 
______________________________________ 
Example 1 
96 70 0.33 66.1 
Example 2 
95 68 0.34 86.0 
Example 3 
99 71 0.34 110.6 
Example 4 
53 72 0.42 853.9 
Comparative 
97 70 0.32 33.0 
Example 
______________________________________ 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.