CA1099464A

CA1099464A - Asymmetric porous film materials and process for producing same

Info

Publication number: CA1099464A
Application number: CA285,877A
Authority: CA
Inventors: Koichi Okita
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1976-09-03
Filing date: 1977-08-31
Publication date: 1981-04-21
Also published as: FR2363351B1; GB1586224A; FR2363351A1; JPS5330668A; DE2739705C3; US4277429A; DE2739705A1; JPS603842B2; DE2739705B2; US4248924A

Abstract

ABSTRACT OF THE DISCLOSURE
Porous film materials composed of polytetrafluoro-ethylene, having an asymmetric structure and suitable for industrial use in mass transfer operations is disclosed, as is a process for producing the materials by stretching the film using a pair of revolving rolls which are heated to a temperature lower than the melting point of the film. Stretching is produced by revolving the two rolls at different speeds which also tends to produce a compressive force in the thickness of the film. The rolls are heated to different temperatures to produce a tempera-ture difference of preferably 50C or more. The combination of the compressive force and the temperature gradient in the thickness of the film is believed to produce the asymmetric structure in the resulting porous film materials which are characterized by a fiber structure in the front surface which is different from that in the back surface.

Description

1~9946~

The present invention relates to porous film materials composed of polytetrafluoroethylene and particularly to asymmetric porous film materials having an unhomogeneous structure that the fiber structure of the porous polytetrafluoroethylene in the front surface is different from that in the back surface and a process for producing the same.
Processes for producing porous polytetrafluoroethylene materials have been described in Japanese patent Publications 13560/67 and 18991/76. These processes are characterized by molding a tetrafluoroethylene resin containing a liquid lubricat-ing agent by extruding or rolling or by both to form sheets, rods or tubes and heating thereafter to about 327C or more with stretching in at least one direction under an unsintered state.
The porous materials produced by these processes have a fiber structure comprising nodules linked by fine filaments, but the state of the nodules and filaments varies somewhat depending on the stretching ratio, the stretching temperature or the speed at which the materials are stretched. The space surrounded with the filaments and the nodules corresponds with a pore. In general, by increasing the stretching ratio, the length of the filamen*s increases and the size of the nodules decreases and, consequently, the porosity increases.
SUMMARY OF THE INVE~TION
The present invention relates to an improvement of these processes and, particularly, it is characterized by the fiber structure, including the length and thickness o the fila- -ments in the porous polytetrafluoroethylene film and the state of the nodules linked by these filaments, is unhomogeneous, wherein the fiber structure in the front surface is different from that in .

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1 the back surface and, consequently, it provides films wherein the pore size in the front surface is different from that in the back surface, namely, asymmetric porous films. Generally, films wherein the pore size in the front surface is equal to that in the back surface are called symmetric porous films The prior porous poly-tetrafluoroethylene films are such symmetric porous films.

Further, the present invention relates to a process for producing asymmetric porous films which is characterized by molding a polytetrafluoroethylene resin containing a liquid lubricating agent by a paste method to form a film, removing the liquid lubricating agent and stretching continuously by revolving rolls heated to a temperature lower than the melting point o the polytetrafluoroethylene, wherein a temperature difference is provided between the low-speed roll and the high-speed roll.

BRIBF DESCRIPTION OF THE DRAWING
'' In the drawing:
Fig. 1 is a scanning type electron photomicro-graph of the back of a film according to the invention magnified to 1000 times; and Fig. 2 is a scanning type electron photo-m}crograph of the surface of a film according to the invention also magnified to 1000 times.

DETAILED DESCRIPTION OF THE INVENTION

In the prior processes for producing polytetra-fluoroethylene films, stretching is carried out at a fixed temperature, by which symmetric porous films are always obtained.

Accordingly, a process for producing asymmetric porous films as in the present invention has not been known.

' ~; `' 1~399464 1 On the other hand, it has been known that reverse osmosis or ultrafiltration membranes represented by cellulose ester films are almost asymmetric porous films having an unhomo-geneous structure wherein the pore size in the front surface is tenfold to more than hundredfold different from that in the back surface.
Recently, processes for producing asymmetric porous films using aromatic polyamides or acrylonitrile have been described. However, these processes cannot be utilized for poly-tetrafluoroethylene because dissolution of resins is essentialin these processes. This is because no solvent to dissolve polytetrafluoroethylene is known.
Industrial use of the porous films in mass transfer operations, requires, that the films function to correctly carry out the filtration, condensation and separation of different components. Though the narrower the distribution of the pore size is, the more the filtration or the separation can correctly ~ -carried out, it is necessary, in o~der to increase the amount treated per unit area and unit time, to greatly increase the 20 number of pores or to decrease the thickness of the film material -to as thin as possible. To greatly increase the number of pores is very difficult under the specific conditions for production.
Further, a rapid decrease in the thickness of the film material is not a practical solution because it causes a deterioràtion in the mechanical strength of the film. As techniques for over-coming such defects, asymmetric porous cellulose ester films have been developed, and they have been practically used as reverse osmosis membranes for desalting of sea water and other uses, while prior symmetric porous films are not economically advanta-geous in these applications. As described above, asymmetric ' ' - ', , ' ~ ' ~ :
.

i~9464 1 porous films are economically superior to prior asymmetric porous films because the former have a larger treating capability in the same functions of filtration or separation.
The asymmetric porous film materials of the present invention are composed of nodules linked to one another by very fine filaments, wherein the fiber structure, including the length and thickness of the filaments and the state of the nodules, in the front surfac~ is different from that in the back surface.
Accordingly, as the pore size is determined by the fiber structure these film materials have the characteristic that the pore size is asymmetric corresponding with the asymmetry or difference in length or thickness of the filaments between the front surface and the back surface. The term "front surface"
referred to above indicates one of the two surfaces of the film material with the term "back surface" used to describe the other surface of the film. Though either of the surfaces may be called the "front surface", the surface having a smaller pore size is usually called the "front surface" and the surface having a --larger pore size is usually called the "back surface" in this specification. In order to use the film materials for filtration, it is effective to permeate a solution from the face having the - smaller pore size.
The filament length is defined as the length of the filament between one nodule and the next nodule along the length of the filament such that if the filament contacts one nodule between two other nodules, the filament length is the shortest distance along the filament between the nodules. Therefore, only the lengths of the filaments which form the perimeter of a pore are used to define the dimensions of the pore. Further, average ilament length can be calculated as a weighted mean of each r, ' 1~99464 1 filament length. The filament length is very different between the two surfaces and the difference is 5 times even in the case of having a small difference and it is often 50 times or more.
Filament length in the back surface as defined in this specification can be clearly discriminated by means of a scanning type electron microscope at a magnification of lOOO as, for example, 1 u to 100 u or so, while filament length in the front surface is as short as, for example, 0.1 u to lO u which can not be discriminated sometimes.
Referring to the drawings, the length of a filament in the back surface as shown in Fig. 1 is 15 u to 30 u, while the length of a filament in the front surface as shown in Fig. 2 corresponds to less than 1 u. As will be observed, the ratio of filament length in the two surfaces shown in Fig. l and Fig. 2 is at least 15 times to 30 times.
In the case of a uniaxial stretching, the nodules in the back surface, as shown in Fig. 1, are generally separate from and independent to other nodules and are long and slender, the -longer axis of the nodule oriented perpendicularly to the stretching direction. On the other hand! in the front surface shown in Fig. 2, independent nodules are not observed, and the nodules are linked to one another such that the whole surface appears as if it were a mat.
The front surface and the back surface of the film do not always exist as shown in Fig. l and Fig. 2. Thus, these figures show only an example. For example, there is a case where the average filament length in the front surface is not different from that in the back surface but the nodules in the back surface are independent and separate from one another as well as long and slender, wherein the long axis of the nodules in the front ,, s -~.' 1(~99464 1 surface is by far longer than the long axis of the nodules in the back surface, although the shorter axis of the nodules is the same in the front surface as in the back surface. Further, a case has also been recognized where nodules do not exist indepen-dently or separately in the front surface and all the nodules are linked linearly to one another. In this case, however, the nodules in the back surface are independent of one another. When all nodules in the front surface link one another, the average filament length is shorter than that in the back surface. One extreme example of such a case is that shown in Fig. l and Fig. 2.
The relative ratios of each of the thickness of the fiber structure in the layer forming the front surface and the thickness of the fiber structure in the layer forming back sur-face to the overall thickness of the film material may vary. For use in filtration or separation, it is preferred that the front surface layer have a small pore size and be as thin as possible.
On the other hand, in applications requiring high strength such as when operating under a high pressure or for superprecise separation, it is preferred for the front surface layer to have a certain minimum thickness. Since the variation of the fiber structure in directions normal to the surfaces of the film material is a factor causing the asymmetric port size, and the objects of the present invention which include film materials having two surfaces, each with a different fiber structure~ the present invention includes those film materials characterized by a front surface layer having a small pore size and which layer is as thin as possible, as well as those film materials characteriz-ed by having a fairly large overall thickness and those characterized by a back surface layer which is as thin as possible.

1(~99~64 1 As for the polytetrafluoroethylene used in the present invention, all resins can be utilized if they are resins called fine powders suitable for the paste method. This resin powder is uniformly mixed with the liquid lubricating agent and the mixture is then molded by compression. The molded mixture is then extruded and/or rolled to form a film. The liquid lubricating agent is then removed by evaporation or extraction.
These steps are called the paste method in the prior art and are known as a process for producing sealing materials.
Then, the film is stretched or expanded in at least one direction. For example, the film may be stretched by a pair of rolls having a different-revolving ratio in the direction of rolling. In this case, the stretching is carried out by heating the film to a temperature of less than about 327C which is less than the melting point of the polytetrafluoroethylene. Though the heating may be carried out by air-heating a stretching space by means of a furnace, it is convenient to directly heat the rolls. Hitherto, heating uniformly to the required temperature - has been employed. However, according to the present invention~
it is preferred for the temperature of the high-speed roll to be higher than that of the low-speed roll, and it is particularly - preferred for the temperature difference between the high-speed rolI and the low-speed roll to be at least 50C. Though a temperature difference of 30 to 49C causes change in the fiber structure, such change becomes remarkablewhere the temperature difference is 50C or more.
It has been found that by keeping the temperature of the high-speed~roll to at least 250C but lower than the melting point of the polytetrafluoroethylene, the front surface and the back surface of the film material each have a different fiber , ~
,. ... .

1~9~9~64 1 structure including average filament length, and the desired asymmetric pore size can be effectively obtained.
The reason why the fiber structure changes as described above is believed to be as follows.
The film heated to a temperature of the low-speed roll is first of all stretched due to the different revolving speeds of the rolls. Tension is necessary in order to stretch the film drawn by the rolls, and this tension is caused by transmission of the revolving power of the rolls at fulcrums where the film contacts the rolls. Since the contact of the film becomes an arc, the tension is caused in the stretching direction of the film while a compressive force corresponding to the tension is caused in the thickness direction. At the same time, one surface of the film in contact with the high-speed roll is heated to the higher temperature to cause a temperature gradient in the thickness direction of the film. It is believed that both the temperature gradient in the thickness direction of the film and the compressive force are factors which cause the change of fiber structure. The nodules forming the fiber structure and average filament length not only depend upon the revolving ratio of the rolls, but also are affected by factors such as the thickness of the film used, the strength of the film before stretching and the residual amount of the liquid lubricating agent. However, in cases where a temperature gradient is present but the compressive force is insufficient or in cases where a temperature gradient is not present though the compressi~e force is sufficient, the change of fiber structure in the front surface and the back surface of the film does not occur.
The revolving ratio of the rolls, the tangential surface speed of the low-speed roll, the diameter of the rolls, . :
:

94~4 1 the thickness and the strength of the film are related to the tension and, therefore, they become factors for controlling the compressive force. Here, the revolving ratio of the rolls means the ratio of the-tangential surface speeds of the rolls when the diameter of each roll is different. On the other hand, the revolving rolls have a temperature difference of preferably 50C
or more. Further, the temperature of the high-speed roll is set at 250C or more by which it is possible to provide a temperature distribution in the thickness direction of the film.
tO After the conclusion of the stretching step, the film is sintered at a temperature of about 327C or more.
Further, it is possible to carry out the stretching step twice or more. It is only necessary that the rolls have a temperature difference in at least one stretching step and the temperature difference is preferably 50C or more. It is not critical to the practice of the invention whether the first stretching step is carried out without a temperature difference or the final stretching step is carried out without a temperature difference. But in any case, by carrying out at least one stretching step using rolls with a temperature difference, the fiber structure of the film becomes asymmetric. Under some con-ditions where it is difficult to obtain the asymmetric state, it is preferred to carry out two or more stretching steps with a temperature difference. On the other hand, in order to ensure that the film does not become more asymmetric than is required, it may be preferred that the final stretching step be without a temperature difference.
The degree of difficulty of making the film asymmetric is affected by the thickness of the film, the strength of the film, the temperature of the rolls, the temperature difference _g_ ,~ .

1~99~64 1 and the revolving ratio of the rolls. In general, the asymmetric state is more easily obtained if the thickness of the film is thinner, the strength is higher, the temperature of the rolls is higher, the temperature difference is larger and the revolving ratio is larger.
Determination of the asymmetric state of the film is easily carried out using the above described photomicrographs.
Further, the degree of the asymmetric state can be measured by the distribution of pore size or a measurement of the bubble 10 point (maximum pore size) according to the ASTM F316-70 method - or a measurement of the porosity according to the ASTM D276-72 method. If the bubble point is measured by applying pressure to both of the front surface and the back surface of the film, the - difference between both measurement values becomes large with -9a-: .
.

99~64 ( 1 an increase in the degree of the asymmetric state.
In the following, examples are described in order to illustrate the present invention in detail.

After uniformly mixing 50 kg of Polyflon F-103, tradename of a polytetrafluoroethylene produced by Daikin Kogyo Co., Ltd.
with 11.5 kg of white oil (Smoyl P-55, produced by Muramatsu Petroleum Co.), the mixture was first molded by compression to O form cubes 300 mm on a side. The molded mixture was then extruded from a die orifice of 12 x 300 mm2~ and the resulting sheet was rolled using a calender roll to form a long film having a thickness of 0.3mm. After removing the white oil with trichloro-ethylene, the thickness was 0.32 mm, the specific gravity was 1.~5, the tensile strength in the lengthwise direction after rolling was 1.3 kg/mm2 and that in the crosswise direction was 0.25 kg/mm2.
The film was stretched using a pair of 120 mm rolls capable of being heated to 330C under conditions that the 20 distance of stretching the film was 8.5 mm, the revolving ratio of the rolls was 1 : 9, the surface speed of the low-speed roll was 2 m/min, the temperature of the low-speed roll was 130C and the temperature of the high-speed roll was varied as shown in Table 1.
Finally, the film was sintered at a temperature of about 327C or more. The characteristics of the-resulting film are shown in Table 1.

~ 946~ ( Experi- Temper- Porosity (~) Tensile Streng-th Thickness (mm) ment No. ature of in lengthwise high- Before After direction Before After speed sinter- sinter- (kg/mm) sinter sinter-r(ool) ing ing Before After ing ing sinter- sinter-ing ing 1 280 79 74 2.45 4.30 0.12 0.09

2 230 82 75 1.85 4.26 0.12 0.08

3 180 ~2 77 1.45 4.80 0.11 0.08

4 130 84 78 0.99 4.05 0.10 0.09 It is observed that though the porosity before sintering decreases as the temperature of the high-speed roll is increased, the tensile strength becomes large. Here, the porosity is that calculated from a measurement of the specific gravity.

-Stretching was carried out under the same conditions as in Example 1 except that the revolving ratio of the rolls was 1 : 12 and the surface speed of the low-speed roll was 25 cm/min.
The characteristic values obtained are shown in Table 2.
The relationship of the porosity and the tensile strength to the temperature of the high-speed roll have the same tendency as the results in Example 1, but the tensile strength in the length-wise direction is higher than the case of Example 1.

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Experi- Temper- Porosity (~) Tensile Strength subble Point ment No. ature of (kg/cm2) (kg/cm2) high Before After speed roll sinter- sinter- ~efore After Front Back (C) ing ing sinter- sinter- Surf- Surf-ing ing ace ace 320 ~4 76 2.8 5.6 0.86 0.69 6 300 81 75 3.6 6.0 0.85 7 280 82 75 2.8 5.5 0.81 -8 230 80 75 2.3 5.4 0.50 10 9 180 80 71 2.0 5.8 0.56 130 72 69 2.2 5.3 0.77 0.75 -' The bubble point shows a pressure at which the initial air bubble passes through the film wetted by alcohol, which is in - inverse proportion to the pore si'ze of the film. The higher the ~' bubble point is, the smaller the pore size is, while the lower the bubble point is, the larger the pore size is.
In order to confirm the difference of fiber structure between the front surface and the back surface of the film, the bubble point measured by applying air pressure to the front surface and,the bubble point measured by applying air pressure to the back surface were compared. A remarkable difference is observed in Experiment No. 5. In Experiment No. 10, they coincide with each other in a range of experimental error. Where the bubble point of the front surface coincides with that of the back surface of the film is a symmetric porous film. A larger difference in the bubble point means more advancement toward the asymmetric porous state.

Stretching was carried out in the same manner as in Example 1 except that the revolving ratio of the rolls was 1:12, "
the surface speed of the low-speed roll was 25 cm/min, the distance ~9946gL

1 of stretching the film was 15.5 mm, the temperature of the high-speed roll was 300C and the temperature of the low-speed roll was varied. The measured results are shown in Table 3.

TAsLE 3 Experi- Temper- Porosity Bubble ~oint Permeation time ment No. ature of (~) (kg/cm ) (second) low-speed Front Back Front Back roll (C) Surface Surface Surface Surface 11 50 77 0.47 0.44 20.2 19.2 12 80 74 0.79 0.73 39.5 38.7 13 130 72 0.81 0.68 45.1 31.0 14 150 73 0.83 0.71 41.5 30.7 200 79 0.80 0.74 35.5 28.6 16 250 86 0.20 0.19 8.7 7.2 17 300 84 0.28 0.27 9.2 8.9 Here, the measured values are those after sintering.
The permeation time means the time necessary for passing 100 ml of isopropanol at a pressure difference of 70 cmHg through an effective area of 40mm~. The characteristic values in the front surface and the back surface of the film are particularly remark-able-in Experiment Nos. 12-15.

In order to examine the effect caused by the contact time between the high-speed roll and the film, the following experiments were carried out in the same manner as in Example 1.
The diameter of the low-speed roll was 120 mm, the temperature thereof was 100C and the surface speed thereof was 240 cm/min. The distance of stretching the film was 15.5 mm.
The temperature of the high-speed roll was 300C and the diameter thereof was 120 mm, 80 mm or 40 mm, but the surface speed ..~,~

.

~-` 1(399~64 1 Of the roll was set such that the stretching ratio caused by the revolving ratio was 800%. The contact time of the stretched film with the high-speed roll was almost proportional to the diameter of ~ g -- the roll. The measured results~are shown in Table 4.

Experi- Diameter Porosity Bubble Point Permeation time ment No. of high- (%)(kg/cm2) (second) speed Front Back Front Back .roll (mm) Surface Surface Surface Surface 18 40 680.98 0.89 34.8 30.5 19 80 671.08 0.95 43.2 41.6 120 661.12 1.05 51.0 47.8 As the diameter of the high-speed roll increases the bubble point becomes high and, consequently, the pore size becomes small. However, the porosity increases as the diameter of the roll decreases and the permeation time becomes short. The characteristic: values between the front surface and the back surface are clearly different, and the difference -of fiber structure between them is remarkable.

~0 In order to examine the effect when the stretching step is performed twice, the same procedure as in ~xample 1 was carried out. The first stretching step was carried out under conditions that the surface speed of the low-speed roll was 1 m/min, the temperature of the low-speed roll was 200C, the temperature of the high-speed roll was 300C, the distance of stretching the film was 15.5 mm and the revolving ratio of the rolls was 1 : 2.
Then the second stretching step was carried out under the same conditions as the first stretching step except that the revolving ratio of the rolls was var;ed as shown in Table 5. The measured results after sintering are shown in Table 5.

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Experi- Revolving Porosity Bubble point Permeation time ment No. ratio of (%) (kg/cm2) (second) rolls Front Back Front ~ack Surface Surface Surface Surface 21 1 : 3 80.5 0.80 0.70 39.8 35.6 22 1 : 4 80.1 0.95 0.90 46.6 39.7 23 1 : 5 78.2 1.06 0.95 61.9 54.7 The first stretching step was carried out under the same conditions as in Example 5 except that the temperature of the low-speed roll and the high-speed roll was 200C. The second stretching step was carried out under conditions that the surface speed of the low-speed roll was 3 m/min, the temperature was 200C, the temperature of the high-speed roll was 300C and the distance of stretching the film was 15.5 mm and the revolving ratio of the rolls was varied as shown in Table 6. The 0~ S~ q measured results~are shown 1~ Table 6.

Experi- Revolving Porosity Bubble ~oint Permeation time 20 ment No. ratio of (%) (kg/cm ) (second) rolls Front Back Front Back Surface Surface Surface Surface 24 1 : 2.5 83.5 0.58 0.49 25.1 22.5 1 . 3 82.1 0.68 0.62 30.1 29.6 26 1 : 3.5 80.5 0.71 0.63 35.1 34.1 27 1 : 4 81.3 0.77 0.73 31.8 30.1 28 1 : 5 78.2 0.81 0.70 37.0 36.0 .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An asymmetric porous film material of polytetrafluoro-ethylene wherein said polytetrafluoroethylene has a fiber struc-ture comprising nodules linked to one another by filaments and wherein the fiber structure is such that the average filament length in one surface of said porous film material is different from the average filament length in the opposite surface of said porous film material.

2. The asymmetric porous film material as set forth in claim 1 wherein the average filament length in said one surface of said porous film material is different from the average filament length in said opposite surface of said porous film material by a factor of at least 5.

3. The asymmetric porous film material as set forth in claim 1 wherein the nodules in said one surface of said porous film material exist in a different state than the nodules in said opposite surface of the porous film material.

4. The asymmetric porous film material as set forth in claim 1, wherein said one surface of said porous film material has a state such that a part of said nodules are linked to one another by being bonded directly to its neighbouring nodules and said opposite surface of said porous film material has a state such that a part of said nodules are bonded to said filament and therefore not directly bonded to its neighbouring nodules.

5. The asymmetric porous film material as set forth in claim 1 wherein the average filament length in said one surface of said porous film material ranges from 1µ to 100 µ and the

Claim 5 continued ....

average filament length in said opposite surface of said porous film material ranges from 0.1µ to 10µ.

6. A process for producing asymmetric porous film materi-als which comprises molding a polytetrafluoroethylene containing a liquid lubricating agent by a paste method to form a film, removing the said liquid lubricating agent, and stretching the film at a temperature lower than the melting point of the poly-tetrafluoroethylene and at least one step of generating a temper-ature difference simultaneously with a compressive force in the thickness direction of the film.

7. A process for producing asymmetric porous film materials as set forth in claim 6, wherein one said step of generating a temperature difference simultaneously with a compressive force is carried out during said stretching the film using rolls having a different revolving ratio and having a temperature difference between each other.

8. A process for producing asymmetric porous film materials as set forth in claim 6, wherein said stretching is carried out at least twice using rolls having a different revolving ratio and wherein in at least one said stretching using rolls having a different revolving ratio, the rolls also have a temperature difference between each other.

9. A process for producing asymmetric porous film materials as set forth in claims 7 and 8, wherein the temperature difference between the rolls is at least 50°C.

10. A process for producing asymmetric porous film materials as set forth in claims 7 and 8, wherein said rolls having a different revolving ratio include a low-speed roll and a

Claim 10 continued ....
high-speed roll, the low-speed roll having a temperature of about 230°C or less and the high-speed roll having a temperature of about 250°C or more but lower than the melting point of the polytetrafluoroethylene.