JPS6348562B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] This invention relates to a method for producing a porous thin film material made of the tetrafluoroethylene resin of the present invention. The present invention relates to a method for manufacturing a thin film material with asymmetric pore size. [Prior art] Regarding the manufacturing method of porous tetrafluoroethylene resin material, Japanese Patent Publication No. 42-13560 and Japanese Patent Publication No. 18991-1973
etc. are publicly known. In these methods, tetrafluoroethylene resin containing a liquid lubricant is formed into a sheet, rod, tube, etc. by extrusion rolling or a method including both, and then stretched in an unsintered state in at least one direction and heated to approximately 327°C or higher. It is characterized by heating. Although the porous materials obtained by these methods vary somewhat depending on the stretching ratio, temperature and speed during stretching, etc., they have a fibrous structure consisting of nodes interconnected by small fibers. The spaces surrounded by these fibers and nodules correspond to porous pores. Generally, by increasing the stretching ratio, the fiber length is increased and the nodule size is decreased.
The percentage of porosity or porosity increases. [Structure of the Invention] The present invention relates to improvements in these methods, and involves forming a tetrafluoroethylene resin containing a liquid lubricant into a thin film by a paste method, removing the liquid lubricant, and then forming the tetrafluoroethylene resin into a thin film. The present invention relates to a method for producing an asymmetric pore membrane, which is continuously stretched using rotating rolls at a temperature below the melting point, and at this time creating a temperature difference between a low-speed rotating roll and a high-speed rotating roll. All conventional tetrafluoroethylene resin membranes are stretched under constant temperature conditions, and therefore, all methods are for producing membranes with symmetric pores, and there is no known method for producing membranes with asymmetric pores as in the present invention. On the other hand, most reverse osmosis membranes and ultrafiltration membranes, such as cellulose ester membranes, have asymmetric pore sizes, and are known to have a nonuniform structure in which the pore sizes on the front and back surfaces differ by 10 or 100 times or more. Recently, methods for producing asymmetric pore membranes using materials such as aromatic polyamide and acrylonitrile have become known, but these production methods require dissolving the resin, so tetrafluoroethylene resin is not suitable for use with tetrafluoroethylene resin. Not applicable at all. This is because there is no solvent that can dissolve the tetrafluoroethylene resin. Industrial applications of porous thin films require the ability to accurately separate, concentrate, and divide different components, as well as be capable of mass processing. The narrower the pore size distribution, the more accurate the separation and division, but in order to increase the throughput in a given area and time, the number of pores must be dramatically increased or the thickness of the thin film material must be made as thin as possible. A large increase in the number of holes that would be required to do so would be extremely difficult within the framework of specific manufacturing conditions, and a rapid decrease in thickness would also impair mechanical strength, making this a drawback that would not be a practical solution. had. Cellulose ester membranes with asymmetric pore diameters have been developed as a technology to overcome these drawbacks, and although reverse osmosis membranes, which are typically used for seawater desalination, have found little economic advantage with conventional symmetric pore membranes, First, asymmetric pore membranes have been found to be economical because they have a sufficient throughput, and have been put into practical use. As described above, the asymmetric pore membrane can claim economic superiority by increasing the throughput while having the same separating and dividing functions as compared to the conventional symmetric pore membrane. The asymmetric pore diameter thin film material of the present invention is composed of nodules interconnected by extremely small fibers, and the entire fiber structure including the length, thickness and nodule shape of the fibers is the same between the front and back sides of the thin film material. As a result, the thin film material is characterized by an asymmetry in pore size defined by the different fiber lengths and thicknesses. The term "front" and "back" as used herein is determined by manufacturing conditions and arbitrarily refers to both surfaces of a single sheet of thin film material, and it does not matter which surface is defined as the "front". Let's say that the one with the smaller pore diameter is the front surface, and the one with the larger pore diameter is the back surface. For applications such as separation, it is an efficient method to flow the solution from the side with small pores. Fiber length is defined as the distance between nodes, and when the fiber is in contact with another node between the nodes, it represents the shortest distance between the nodes. Therefore, only the length of the fibers in the porous space can be defined as the length. Furthermore, the average fiber length can be calculated as a weighted average of the individual lengths. The difference in length of fibers on both sides running through this porous space is very large, sometimes 5 times, often 50 times.
It has more than doubled. The length of the fibers on the back side defined here can be clearly determined, for example, from 1μ to 100μ when viewed with a scanning electron microscope photograph of about 1000 times magnification, but the fiber length on the front side is often so small that it cannot be determined, for example, from 0.1μ to 10μ. It will be about. Figure 1 is a scanning electron micrograph of the back side magnified 1000 times, and Figure 2 is a scanning electron micrograph of the front side at the same magnification. The length of the fiber 1 on the surface of the figure corresponds to less than 1μ as long as the porous space is included in the background, and as a result, the ratio of the fiber lengths on both the surfaces of Figures 1 and 2 is at least 15 to 30 times. It's doubled. In the case of uniaxial stretching, the shape of the nodes 2 becomes individually elongated on the back side of FIG.
Although the long axes of the nodules 2 are oriented perpendicular to the stretching direction, on the surface shown in Figure 2, individual nodules are no longer visible but are connected, and the entire surface changes to look like the surface of a tatami mat. I can see that it is. The front and back sides are not all in the state shown in Figures 1 and 2, and this is just an example. For example, although the average fiber length on the front and back sides is the same, the nodule shape on the back side is individually elongated, whereas on the front side, the short axis of the nodule is the same as the back side, but the long axis of the nodule is much longer. As we progressed further, we observed that there were no independent nodules on the surface, and all the nodules were connected in a flying geese pattern. Of course, the back surface remains an independent nodule even in this state. When all the nodules on the surface are connected, the average fiber length connecting the nodules becomes shorter on the front side than on the back side, and one of the states in which this has progressed to an extreme is shown in Figures 1 and 2. This seems to indicate the situation. One issue here is how much the fiber tissue layers on the front and back sides account for the total thickness of the thin film material. For purposes such as separation and division, it is desirable that the surface layer with small pores be as thin as possible. On the other hand, in cases where strength is required under high pressure or in applications where ultra-precise division is required, it is desirable to have a certain level of surface layer thickness. As described above, changes in the fiber structure in the thickness direction of the thin film material are also a factor in the asymmetric pore size, but the object of the present invention has at least a different fiber structure on both surfaces, so a surface layer with a small pore size is possible. This includes those that are very thin, those that are quite thick, and those that have as thin a back layer as possible. As the tetrafluoroethylene resin used in the present invention, any resin can be used as long as it is compatible with a paste processing method called fine powder. This resin powder is uniformly mixed with a liquid lubricant, pre-compression molded, and formed into a thin film by extrusion, rolling, or a method including both. The liquid lubricant is then removed by evaporation or extraction. The process up to this step is a conventional paste processing method, which is a well-known method for manufacturing sealing materials. The thin film is then stretched in at least one direction, in most cases in the direction of extrusion or rolling, using a pair of rolls with different rotation ratios. Perform while heating. This heating method can also air-heat the drawing space using a furnace, but
It is more convenient to heat the rotating roll directly. Conventionally, it has been known to carry out this heating at a uniform and the same temperature, but in the present invention the temperature of the fast rotating roll is higher than the temperature of the slow rotating roll or the furnace, in particular the temperature difference between the two is at least 50°C. It is preferable to set the temperature to ℃ or higher. Even with a temperature difference of 30 to 49°C, changes begin to appear in the fiber structure such as the long axis of the nodule, but the changes become more noticeable when the temperature difference is 50°C or more. Furthermore, even in these cases, setting the high-speed rotating roll at a temperature of at least 250°C or higher and below the melting point of the tetrafluoroethylene resin makes the fiber structure, including the average fiber length, different on the front and back sides of the thin film material, and the pore size The present invention has been completed by discovering that it is effective in making the structure asymmetrical. This change in fiber structure is thought to occur for the following reasons. A thin film heated to a temperature such as a low rotation roll is first stretched according to the rotation ratio of the roll, and when it comes into contact with a high rotation roll, that roll is at a high temperature, so the thin film is reheated from the contact surface of the high rotation roll. A temperature distribution occurs in the thickness direction of the thin film. On the other hand, tension is required to stretch a thin film stretched on rolls having different rotation ratios, and this tension is the result of the rotational force of the rolls being transmitted using the part of the thin film that is in contact with the rolls as a fulcrum. Since the part of the thin film in contact with the rolls forms an arc, tension is applied in the stretching direction of the thin film, and at the same time, a compressive force corresponding to the tension is generated in the thickness direction. Ultimately, temperature distribution in the thickness direction of the thin film and compressive force are considered to be the two factors that bring about changes in the fiber structure. Therefore, the knots and average fiber length that form the fibrous structure not only depend on the roll rotation ratio, low roll speed, and the distance of the thin film being stretched, but also on the thickness of the thin film used and the length of the thin film before stretching. It is also influenced by factors such as strength and amount of liquid lubricant remaining. However, even if a temperature distribution occurs, there is insufficient compressive force, or when there is sufficient compressive force but no temperature distribution, no change occurs in the fiber structure on the front and back surfaces of the thin film. The rotation ratio of the rolls, the low rotational roll speed, the roll diameter, the thickness and strength of the thin film, etc. are related to the tension during stretching and are therefore factors that govern the compressive force. Of course, the roll rotation ratio here should be changed to the circumferential speed ratio of the rolls when a pair of rotating rolls having different roll diameters are targeted. On the other hand, the temperature distribution in the thickness direction of the thin film can be set by providing a temperature difference between the rotating rolls, preferably at 50° C. or higher, and by setting the high-speed rotating roll temperature to at least 250° C. or higher. The thin film that has undergone these stretching processes is sintered at a temperature of approximately 327°C or higher, but even if there is a moment when a temperature distribution occurs in the thickness direction, since no compressive force is applied, the difference between the front and back surfaces of the fiber structure It seems unlikely that the difference would be significant. On the other hand, it is also possible to carry out the stretching process over two or more times, in which case it is necessary to set a temperature difference between the rolls during at least one stretching, and more preferably to set a temperature difference of at least 50°C or more. Become. At this time, it can be arbitrarily selected whether the first stretching is carried out at a constant temperature or the final stretching is carried out at a constant temperature, but in any case, it is necessary to carry out stretching at least once using rolls with a temperature difference like this. It is true that the fiber structure of the thin film becomes asymmetrical due to this. Under conditions where it is difficult for the asymmetry to progress, it is desirable to apply a temperature difference two or more times, and on the other hand, when the asymmetry progresses too much, it is desirable to carry out constant temperature stretching last. The degree of difficulty with which the asymmetry progresses is influenced by the thickness and strength of the thin film, the temperature of the roll, temperature difference, rotation ratio, etc. Generally, the thinner the thin film, the greater the strength, the higher the temperature of the roll, the greater the temperature difference, and the greater the rotation ratio, the easier the asymmetry will progress. Determining whether a thin film has asymmetric pore sizes can be easily done by looking at micrographs, as described above. _On the other hand, the degree of asymmetry can also be determined by measuring the pore size distribution or bubble point (maximum pore diameter) according to the method of ASTM F _316-70 , and also by measuring the porosity according to the method of ASTM D _{276-72 .} I can do it. When the bubble point is determined by applying pressure from both the front and back sides of the thin film, the difference between the constant values on both sides becomes larger as the asymmetry progresses. [Example] Examples are shown below to explain the present invention in more detail. Example 1 After uniformly mixing 50 kg of Polyflon F-103, a tetrafluoroethylene resin manufactured by Daikin Industries, with 11.5 kg of white oil (Sumoil P-55 manufactured by Muramatsu Oil Co., Ltd.)
Compression preformed to 300mm square. This was extruded into a plate shape through a 12×300 mm ² die orifice, and then rolled into a long thin film with a thickness of 0.3 mm using a calendar roll and wound up. After white oil is extracted and removed using trichloride, the thickness is 0.32mm, the specific gravity is 1.65, and the tensile strength after rolling is 1.3Kg/longitudinal.
mm ² and 0.25 Kg/mm ² in the lateral direction. Using a pair of 12mm rolls that can heat up to 330℃,
Thin film distance stretched 8.5 mm, roll rotation ratio 1:9,
Stretching was carried out by setting the rotational speed of the low-speed roll to 2 m/min, the temperature of the low-speed roll to 130°C, and changing the temperature of the high-speed roll as shown in Table 1. The properties of the product sintered at a temperature of about 327°C or higher are also shown in the table below.

[Table] As the temperature of the high-speed rotating roll increases, the porosity before sintering decreases, but the tensile strength increases, and the temperature distribution and compressive force in the thickness direction of the thin film work effectively. I know that there is. The porosity here is calculated from the measured value of specific gravity. Example 2 Roll rotation ratio is 1:12, low rotation roll speed
When stretched under the same conditions as in Example 1 except that the stretching speed was 25 cm/min, the physical properties shown in Table 2 were obtained. The relationship between the porosity and tensile strength and the high-speed rotating roll temperature shows a similar tendency to the results of Example 1, but the longitudinal tensile strength here is greater than that of Example 1.

[Table] On the other hand, the bubble point represents the pressure at which the first bubble passes through a thin film wetted with alcohol, and is inversely proportional to the pore size of the thin film. After all, the higher the bubble point, the smaller the pore diameter, and the lower the bubble point, the larger the pore diameter. In order to confirm that the fiber structures on the front and back sides of the thin film were different, we calculated the bubble point when air pressure was applied from the front side and the value when pressure was applied from the back side, and it was noticeable in Experiment No. 5. Differences were recognized;
In Experiment No. 10, there is agreement within the experimental error. A film in which the bubble points on the front and back sides match is a membrane with a symmetric pore size, and the larger the difference without matching, the more asymmetric pore size progresses. Example 3 Roll rotation ratio 1:12, low speed roll rotation speed
Table 3 shows the properties of a film that was stretched in a similar manner to Example 1 at 25 cm/min, a thin film stretching distance of 15.5 mm, a high-speed rotating roll temperature of 300°C, and varying the low-speed rotating roll temperature.

[Table] The measured values here are after sintering, and the liquid permeation time indicates the time required for 100ml of isopropyl alcohol to pass through an effective area of 40mmφ with a pressure difference of 70cmHg. The characteristic values on the front and back surfaces of the thin film are from experiment No. 12 to 15.
This is particularly noticeable. Example 4 The following experiment similar to Example 1 was conducted for the purpose of investigating the effect of the contact time between the high-speed rotating roll and the thin film. The diameter of the low speed roll is 120mm, the temperature is 100℃, the rotation speed is 240cm/min, the stretching distance of the thin film is 15.5mm, the temperature of the high speed roll is 300℃, and the diameter is changed to 120mm, 80mm, and 40mm, but the stretching rate depends on the rotation ratio of the thin film. is 800%
The rotation speed was set so that The time that the stretched thin film is in contact with the high speed rotating rolls varies approximately in proportion to the diameter of the rolls, and these characteristics are shown in Table 4.

[Table] The larger the diameter of the high-speed rotating roll, the higher the bubble point becomes, resulting in a smaller pore size. However, the porosity increases as the roll diameter becomes smaller;
The liquid permeation time has also become shorter, there is a clear difference in the characteristic values between the front and back surfaces, and the difference in fiber structure is remarkable. Example 5 The effect of the stretching distance of the thin film was confirmed in the following experiment. Thin film thickness 0.1mm, low speed roll temperature 130℃,
Rotation speed 25cm/min, high speed roll temperature 300℃,
The properties of the thin films obtained by setting the stretching ratio to 100% and changing the stretching distance of the thin film were as shown in Table 5.

[Table] The liquid permeation time of isopropyl alcohol varies by about three times depending on whether it is flowed down from the front or back surface, and the difference in the fiber structure is clear.

[Brief explanation of the drawing]

FIG. 1 is a scanning electron micrograph of the back side where fibers 1 and nodules 2 can be clearly distinguished, and FIG. 2 is a scanning electron micrograph of a thin film with asymmetric pore sizes on the front side.

Claims (1)

[Scope of Claims] 1. After forming a tetrafluoroethylene resin containing a liquid lubricant into a thin film by a paste method, the liquid lubricant is removed, and when stretching at a temperature below the melting point of the tetrafluoroethylene resin, low speed rotation is performed. 1. A method for producing a thin film material with an asymmetric pore size, characterized by stretching the thin film using a roll and a high-speed rotating roll having a higher temperature than the low-speed rotating roll, thereby simultaneously generating a temperature difference and compressive force in the thickness direction of the thin film. 2. Stretching is performed at least two times using rolls with different rotation ratios, and at least one of the stretching is performed by creating a temperature difference between the rolls with different rotation ratios.
A method for producing a thin film material with asymmetric pore size as described in Section 1. 3. A method for producing an asymmetric pore diameter thin film material according to claim 1 or 2, characterized in that the stretching is performed with a temperature difference of at least 50° C. between rolls having different rotation ratios. 4. Claims characterized in that among rolls with different rotation ratios, the temperature of the low-speed rotating roll is set to 230°C or lower, and the temperature of the high-speed rotating roll is set to at least 250°C or higher and lower than the melting point of the tetrafluoroethylene resin. A method for producing an asymmetric pore diameter thin film material according to item 1 or 2.