JPS61171505A - Preparation of asymmetric pore size membrane material - Google Patents

Abstract

PURPOSE:To make the structure of a front surface and that of a back surface non-uniform, by forming an ethylenetetrafluoride resin containing a liquid wetting agent into a membrane by a paste method and removing the liquid wetting agent before stretching the formed membrane by using a low speed rotary roll and a high speed rotary roll between which temp. difference is provided. CONSTITUTION:An ethylenetetrafluoride resin fine powder is uniformly mixed with a liquid wetting agent while the resulting mixture is subjected to preparatory compression molding and subsequently extruded and rolled to be molded into a membrane. Subsequently, the liquid wetting agent is removed by evaporation or extraction and the molded membrane is stretched to the rolled direction at temp. equal to or less than the m.p. of the resin by a pair of rolls different in a rotary ratio. At this time, the temp. of the low speed rotary roll or high speed rotary roll is made higher by at least 50 deg.C or more than that of a furnace. The temp. of the high speed rotary roll is pref. set to at least 250 deg.C or more. After stretching, sintering is performed at 327 deg.C or more.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a porous thin film material made of a tetrafluoroethylene resin, and in particular, the present invention relates to a method for producing a porous thin film material made of a polytetrafluoroethylene resin. This invention relates to a method for manufacturing a thin film material with asymmetric pore diameters having a different non-uniform structure on the back surface.

[Conventional technology]

Regarding the manufacturing method of porous tetrafluoroethylene resin material,
Special Publication No. 42-13560 and Special Publication No. 51-18991
etc. are publicly known. In these methods, a 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 at a temperature of about 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 space surrounded by the nodules and the nodules coincide with zero porous pores.

In general, increasing the stretching rate increases the length of the fibers, decreases the nodule size, and increases the percent porosity.

[Structure of the invention]

The present invention relates to improvements to these methods, in which a tetrafluoroethylene resin containing a liquid lubricant is formed into a thin film by a paste method, the liquid lubricant is removed, and the liquid lubricant is removed at a temperature below the melting point of the tetrafluoroethylene resin. The present invention relates to a method for producing an asymmetric pore membrane, which is continuously stretched using rotating rolls, and is characterized in that a temperature difference is created 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 ultrapolar membranes, typified by cellulose ester membranes, are membranes with asymmetric pore diameters, and are known to have a nonuniform structure in which the pore diameters on the front and back surfaces differ by 10 or 100 times or more.

Recently, methods for manufacturing asymmetric pore membranes using materials such as aromatic polyimide and acrylonitrile have become publicly known, but these manufacturing methods require dissolving the resin, so tetrafluoroethylene resin cannot be used. Not applicable at all. This is because there is no solvent that can dissolve the tetrafluoroethylene resin.

In industrial applications of porous thin films, different components are
It is required to have the ability to perform concentration, division, etc. with high precision, and to be able to process large quantities at the same time.

The narrower the distribution of hole diameters, the more accurate the division into individual units.1
However, in order to increase the throughput in a given area and time, it is necessary to dramatically increase the number of holes or to reduce the thickness of the thin film material as much as possible. is extremely difficult within the framework of specific manufacturing conditions;
In addition, a rapid decrease in thickness also deteriorates mechanical strength, which has the disadvantage that it cannot be used as a practical means. Cellulose ester membranes with asymmetric pore diameters have been developed as a technology to overcome these drawbacks, and reverse osmosis membranes, 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. In this way, the asymmetric pore membrane can claim economic superiority by increasing the throughput while having the same door-to-door and dividing functions as compared to the conventional symmetric pore membrane.

The asymmetric pore diameter thin film material of the present invention consists 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 difference in fiber length and thickness. The front and back sides here are determined by manufacturing conditions and arbitrarily refer to both surfaces of a single thin film material, and it does not matter which surface is defined as the front, but if the pore size The smaller pore size is the front surface, and the larger pore size is the back surface. For applications such as door-to-door distribution, an efficient method is to flow the solution from the surface 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 that are exposed against the background of the porous space can be defined.

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 reaching 5 times, and often 50 times or more. The length of the fibers on the back side defined here is about 1000 times larger. When viewed in a scanning electron micrograph, the length of the fibers on the back side can be clearly determined, for example, from 1μ to 100μ, but the length of the fibers on the front side is often so small that it cannot be determined, for example, from 0.1μ. 1
It becomes about 0μ.

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 considered as a background, and as a result, the ratio of the fiber length on both the surfaces of FIG. 1 and FIG. 2 is at least 15 to 30 times. It's doubled.

In the case of uniaxial stretching, the shape of the nodules 2 becomes individually elongated on the back side of Figure 1, and the nodules 2
The long axis of the tatami mat is oriented perpendicular to the stretching direction, but on the surface shown in Figure 2, individual nodules are no longer visible but are connected, and the entire surface has changed to look like the surface of a tatami mat. I understand.

The front and back sides are not all in the state shown in Figures 1 and 2; this is just an example.

For example, although the average fiber length on the front and back sides is the same, the shape of the nodules 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 anymore, 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 on the front side becomes shorter than that on the back side, and one of the states where 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 applications such as separation and division, it is desirable that the surface layer with small pore diameters be as thin as possible. On the other hand, in cases where strength is required under high pressure or in applications where ultra-precision T-cutting is required, it is desirable that the surface layer has a certain thickness.
1 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. It includes those that are extremely 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 suitable for paste processing called fine powder can be used. This resin powder is uniformly mixed with liquid lubricant,
Preliminary compression molding is performed and the product is 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 extruded in at least one direction, most often
Alternatively, the thin film is stretched in the rolling direction using a pair of rolls having different rotation ratios, while heating the thin film at a temperature below about 327° C., which is the melting point of the tetrafluoroethylene resin. In this heating method, it is possible to air-heat the drawing space using a furnace, but it is more convenient to directly heat the rotating rolls. Conventionally, it has been known to carry out this heating at the same uniform temperature, but in the present invention, low speed rotating rolls or
Raising the temperature of the high-speed rotating rolls higher than the temperature of the furnace;
In particular, it is preferable to set the temperature difference between the two to at least 50°C or higher. Even with a temperature difference of 30 to 49°C, changes begin to appear in the fiber structure, such as the long axes of nodules, but the changes become 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 will make the fiber structure, including the average fiber length, different on the front and back sides of the thin film material. The present invention was completed by discovering that this method is effective in making the pore diameter 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,
Since the temperature of the roll is high, the thin film is reheated from the contact surface of the high-speed roll, and a temperature distribution occurs in the thickness direction of the thin film.

On the other hand, one roll with a different rotation ratio stretches the stretched thin film.
In order to achieve this, tension is required, and that tension is transmitted through the rotational force of the roll or the part of the thin film that is in contact with the roll as a fulcrum. Since the part of the thin film in contact with the rolls forms an arc, tension acts 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, when there is a temperature distribution but the compressive force is insufficient, or when there is a 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.

After these stretching processes, the thin film is sintered at a temperature of approximately 327°C or higher, but at that time, even though there is a moment when a temperature distribution occurs in the thickness direction, no compressive force is applied, so there is a difference in the fiber structure between the front and back sides. There seems to be little chance of making it larger.

On the other hand, it is also possible to carry out the stretching process two or more times, in which case a temperature difference is applied to the rolls during at least one stretching, more preferably at least 50°C.
It is necessary to set a temperature difference above ・When the temperature is 0, it is possible to arbitrarily choose whether to carry out the first stretching at a constant temperature, or to carry out the final stretching at a constant temperature. It is true that the fiber structure of the thin film is made asymmetric by being stretched at least once using rolls with a temperature difference of .

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.

As mentioned above, determining whether a thin film has asymmetric pore sizes can be easily done using a microscopic photograph. on the other hand!
Pore size distribution or bubble point (maximum pore size) measurements according to the method of STM F3□6-7o, and AST
The degree of asymmetry can also be determined by the measured value of porosity by the method of MD276-72.

When the bubble point is determined by applying pressure from both the front and back surfaces of the thin film, the difference between the two measured values increases as the asymmetry progresses.

〔Example〕

Examples are shown below to explain the present invention in more detail.

Example 1 Tetrafluoroethylene resin manufactured by Daikin Industries, Ltd., Polyflon F-
103.50 kg was uniformly mixed with 11.5 kg of white oil (Sumoil P-s 5 manufactured by Muramatsu Oil Co., Ltd.), and then compressed and preformed into a 300 mm square. This is 12
x 300 mm through an orifice, and then using a calender roll to
It was rolled and wound into a thick long thin film.

The thickness after white oil was extracted and removed using trichlorene was 0.32 mm, the specific gravity was 1.65, and the tensile strength after rolling was 1.3 kg/mm2 in the longitudinal direction and 0.2 in the transverse direction.
It showed 5 kg/mm.

Using a pair of 12 mm rolls that can be heated up to 330℃,
Thin film distance stretched 8.5 mm, roll rotation ratio 1:9,
The rotational speed of the low-speed roll was set to 2 m/min, the temperature of the low-speed roll was set to 130°C, and the temperature of the high-speed roll was set to 2 m/min.
It was stretched as shown in the table.

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 no longer 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 is 25c
When stretched under the same conditions as in Example 1 except that the stretching speed was changed to m/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 thicker than in Example.

Table 2 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. In the end, 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 are different, air pressure was applied from the front surface. Experiment A5 was obtained by calculating the value when pressure was applied from the bubble point and the back side.
In Experiment 10, a remarkable difference was observed, and in Experiment 10, there was agreement within the experimental error.

A membrane in which the bubble points on the front and back sides match is a symmetric pore size membrane, whereas they do not match and the difference is large (this clearly indicates that the asymmetric pore size is progressing).

Example 3 Roll rotation ratio is 1:12, rotation speed of low speed roll is 25
Table 3 shows the properties of the film stretched in a similar manner to Example 1 while setting the thin film stretching distance to 300°C and the temperature of the high-speed rotating roll to 300°C and changing the temperature of the low-speed rotating roll. .

Table 3 The measured values here are after sintering, and the liquid permeation time is 4.
The time required for 1OOrnl of isopropyl alcohol to pass through an effective area of Q mmφ with a pressure difference of 7 Q cmHg is displayed.

The characteristic values on the front and back sides of the thin film are from Experiment'AI 2 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 120 mm, the temperature is 100°C, the rotation speed is 240 crrVmin, and the stretching distance of the thin film is 15.
゜5 mm, high speed roll temperature 300℃, diameter 12
The rotational speed was set such that the stretching ratio of the thin film was 800%, although the thickness was changed to 0 mm, 80 mm, and 40 mm. The time that the stretched thin film is in contact with the high-speed rotating rolls varies in proportion to the diameter of the rolls, and these characteristics are shown in Table 4.

Table 4 The larger the diameter of the high-speed rotating roll, the higher the bubble point, resulting in a smaller pore size.

However, the porosity increases as the roll diameter decreases, and the liquid permeation time also decreases. 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 Thin film The effect of the stretching distance was confirmed in the following experiment.

Thin film thickness Q, l mm, low speed roll temperature 130
"C1 rotational speed 25 cm/min, high speed roll temperature 300° C., stretching ratio 100%, and the thin film stretching distance was changed to obtain properties as shown in Table 5.

Table 5: The liquid permeation time of isopropyl alcohol varies by about 3 times depending on whether it is flowed down from the front or back surface, and the difference in fiber structure and texture is clear.

[Brief explanation of drawings]

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 (4)

[Claims] (1) When a tetrafluoroethylene resin containing a liquid lubricant is formed into a thin film by a paste method, the liquid lubricant is removed, and the film is stretched at a temperature below the melting point of the tetrafluoroethylene resin. 1. A method for producing a thin film material with an asymmetric pore size, characterized by stretching with a high-speed rotating roll that is hotter than a low-speed rotating roll, and 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 having different rotation ratios, and at least one of the stretching is performed by creating a temperature difference between the rolls having different rotation ratios. A method for producing asymmetric pore size thin film materials. (3) A method for producing an asymmetric pore diameter thin film material according to claim 1 or 2, characterized in that the stretching is carried out with a temperature difference of at least 50° C. between rolls having different rotation ratios. (4) A patent claim characterized in that among rolls having 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.