JPH0532810A - Production of porous body - Google Patents

Abstract

PURPOSE:To facilitate the formation of a thin polytetrafluoroethylene porous body having a small pore diameter and a high porosity. CONSTITUTION:A dispersion of a polytetrafluoroethylene powder is applied to the surface of a substrate made of a heat-resistance material and the dispersing medium is removed. The power is partially fused by heating to form a film, which is released from the substrate and stretched to make it porous.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel method for producing a porous body made of polytetrafluoroethylene (hereinafter referred to as PTFE).

[0002]

2. Description of the Related Art PTFE is useful as an industrial material because it has various properties such as heat resistance, chemical resistance, mechanical properties, and electrical properties, and its porous material can be selected from corrosive substances and high-temperature substances. It is used in a wide range of technical fields such as permeable membranes, filters, sensor membranes, and waterproof / moisture permeable materials.

By the way, as a method for producing a PTFE porous film, for example, a mixture of PTFE powder and a liquid lubricant such as naphtha is extruded and rolled to form a film, and then the liquid lubricant is removed. A method of stretching this film to make it porous (Japanese Patent Publication No. 42-1356).
No. 0, Japanese Patent Publication No. 51-18991, U.S. Pat. No. 3,953,566) or PTFE powder is compression-molded into a block shape, which is then heated to a temperature above the melting point of PTFE and baked to form a film. A method in which the film is cut, and then the film is once heated to a temperature equal to or higher than the melting point of PTFE, then rapidly cooled or uncooled, and then stretched (Japanese Patent Publication
No. 40254 and Japanese Patent Publication No. 3-17859) are known.

[0004]

The porous PTFE film obtained by the former method has a fine pore size of about 0.1 to 3 μm.
m, the thickness is about 50 μm or more, the pore size of the micropores is large, and it is thick, and it is difficult to obtain a thin porous film having a small pore size by this method. On the other hand, the porous film obtained by the latter method has a porosity of at most 30%, and it is difficult to obtain a high porosity product by this method.

While thick PTFE porous films are preferred for certain applications, thin applications may be desired for other applications. For example, when used as a selectively permeable membrane for fluid, if the pore diameter and porosity of the micropores are the same, the amount of fluid permeation per unit time increases as the thickness of the porous film decreases. Is preferable. It is also preferable that the porosity is high for the same reason. Further, in recent years, separation of solids having a minute diameter has also been carried out, and in such applications, appearance of a porous film having a smaller pore diameter is desired.

[0006]

DISCLOSURE OF THE INVENTION As a result of earnest studies for solving the above problems of the prior art, the present inventor has found that PTFE
Using a dispersion liquid of the powder, after removing the dispersion medium after coating it on the substrate, the powders are in a specific state when forming a film, and then by stretching this, the diameter of the micropores is small and thin, Moreover, they have found that a porous body having a high porosity can be obtained, and completed the present invention.

That is, the method for producing a porous body according to the present invention is P
A dispersion of TFE powder is applied on a substrate, and then the dispersion medium is evaporated and removed by heating, and the PTFE powders are partially bound to each other to form a film. The film is peeled from the substrate and then stretched to form a porous film. It is characterized by qualification.

As an unsintered powder of PTFE, a powder obtained by emulsion polymerization or suspension polymerization is known. In the present invention, it has been proved to be preferable to use an unsintered powder obtained by emulsion polymerization. ing. The particle size of the powder is not particularly limited, but is usually about 0.1 to 0.5 μm. The PTFE powder is used by being dispersed in water or an organic solvent so that the concentration thereof is about 40 to 60% by weight.

In the present invention, the term "PTFE" means not only a homopolymer of tetrafluoroethylene,
It is meant to include a copolymer of tetrafluoroethylene and another monomer copolymerizable therewith. Examples of such other monomer include hexafluoropropylene, perfluoroalkyl vinyl ether, chlorotrifluoroethylene and the like. The content of other monomer components in this copolymer is often 2
Although the content is less than or equal to wt%, it does not prevent the content exceeding this.

In the present invention, first, a dispersion of PTFE powder is applied onto a substrate. As the substrate, one made of a heat-resistant plastic such as polyimide or polyether ether ketone, or a heat-resistant material such as metal or ceramic is used, and its shape may be any of sheet, tube, rod and the like. The PTFE dispersion liquid may be applied to the surface of the substrate by a method of immersing the substrate in the dispersion liquid and pulling it up, a method of spray coating the dispersion liquid on the substrate, a brush coating method of the dispersion liquid on the substrate, and the like. In addition, in order to improve the wettability of the PTFE dispersion on the surface of the substrate, a silicone-based or fluorine-based surfactant may be added to the dispersion.
Further, the coating thickness can be adjusted by a measuring bar or the like after coating.

After coating the PTFE dispersion on the substrate in this way, it is heated to evaporate and remove the dispersion medium, and P
The TFE powders are partially bound together to form a film. It is an important point of the present invention to form a film by partially binding the PTFE powders by this step, and the intended purpose can be achieved by such an operation.

Heating is carried out by a one-step heating method in which the temperature is higher than the melting point of PTFE for a predetermined time, or the dispersion medium is removed by heating to the evaporation temperature of the dispersion medium, and then the temperature is raised to a temperature higher than the melting point of PTFE and heated for a predetermined time. A multi-step heating method can be adopted. In any heating method, it is necessary to take care so that the PTFE powders are not bound together as a whole beyond the state where they are partially bound.

By the above heating, the dispersion medium is first evaporated and removed, and then the PTFE powders are partially bound to each other at a temperature higher than the melting point of PTFE. Partial binding means a state in which only the surface of the PTFE powder is in a molten (calcined) state (the inside of the powder is unmelted, therefore, unbaked), and the powder particles are bound to each other on this molten surface. Therefore, the film formed by this heating has a mixture of the fired portion (mainly the surface portion) and the non-fired portion (mainly the inside). PT
If the temperature is the same, the degree of partial binding between the FE powders is
Depends on heating time. If the heating time at a temperature equal to or higher than the melting point of PTFE is too long, the entire powder will be melted, and a film in which a fired portion and a non-fired portion are mixed cannot be obtained.

The film formed on the substrate is PTFE
It is not possible to visually determine whether the powders are partially bound or the entire powder is melted. However, the present inventor has also found a method for facilitating this determination, and therefore this is mentioned.

When the PTFE dispersion is applied onto the substrate and then heated, the dispersion medium is first evaporated and removed, as described above.
The PTFE powders are then bound together to form a film. When the PTFE powders are partially bound to each other, this film has a tensile strength of 100 kg / cm ² or more and an elongation of 500% or more in two directions (for example, a longitudinal direction and a transverse direction) orthogonal to each other, and It has been found that the tensile strength and elongation in these two directions are substantially isotropic. Here, "substantially isotropic" means that the value obtained by dividing the tensile strength (or elongation) in one direction by the tensile strength (or elongation) in the other direction by 100 is 90 to 110.
Means to indicate. On the other hand, the film in which the entire PTFE powder is melt-bonded has a decrease in elongation in two directions,
It was also found to be less than 0%, and the powders have different physical property values from the partially bound film. By utilizing such a difference in the physical property value of the film, it is possible to judge the binding state between the PTFE powders in the film formed on the substrate. Specifically, prior to actual manufacturing,
A dispersion of PTFE powder is coated on a substrate, heated to a predetermined temperature (however, above the melting point of PTFE), and samples are taken from the film formed at each elapsed time from the start of heating,
If you work to measure the tensile strength and elongation of each sample,
It is possible to know the permissible time for obtaining a film in which the powders are partially bound to each other, and the time when the powders become bound to each other as a whole, thereby determining an appropriate heating time at the above predetermined heating temperature. You can do it. This heating time may vary depending on various conditions, but usually the temperature is 3
It is about 1 to 30 minutes at 27 to 350 ° C and about 30 seconds or less at 360 ° C.

By the above heating, a film having substantially isotropic mechanical properties is formed on the substrate. In the present invention, coating of the PTFE dispersion on the substrate and film formation by heating are repeated a predetermined number of times. It can also be done to increase the film thickness. When this coating and heating are repeated, PTF in the film formed before that is repeated.
There was concern about the influence on the degree of binding of the E powders to each other, but when actually working, it was found that there was little influence.

After the PTFE film is formed on the substrate in this way, it is peeled off from the substrate. This peeling can be easily performed because the film itself is non-adhesive. Of course, in order to make this work easier, it is possible to subject the surface of the substrate to a release treatment such as coating with a silicone resin in advance. The film peeled from the surface of the substrate has substantially isotropic mechanical properties as described above. The degree of binding between the PTFE powders forming this film is
It can be known by the tensile strength and elongation of the film. That is, the higher the degree of binding, the higher the tensile strength and the smaller the elongation, and the weaker the degree of binding, the lower the tensile strength and the greater the elongation.

The PTFE film is then stretched to make it porous. The stretching may be uniaxial stretching or multiaxial stretching. The stretching temperature and the stretching ratio are not particularly limited, but from the viewpoint of workability, the temperature is preferably about 25 to 260 ° C., and the stretching ratio is preferably about 10 times or less. By this stretching, countless micropores having a pore diameter of about 0.1 μm or less are formed in the film, and a high porosity product having a porosity of about 65% can be obtained. Further, a tubular body or a rod-shaped body is used as a substrate, and a PTFE dispersion is applied to the inner peripheral surface and / or the outer peripheral surface of the substrate and heated to form a PTFE tube-shaped product, which is peeled from the substrate and then stretched. By doing so, it is possible to obtain a tubular porous body.

The porous body obtained by stretching may contract due to the residual stretching stress. When such shrinkage is not desirable, it is preferable to remove the residual stress by subjecting the porous body to heat treatment after stretching, which is heated to a predetermined temperature (for example, a temperature higher than the stretching temperature). The temperature at the time of heat treatment can be set to a temperature equal to or higher than the melting point of PTFE, and in this case, the entire body is fired to form a porous body having high mechanical strength. Of course, when this heating is performed, it is preferable to maintain the stretched state of the porous body (with a means for preventing shrinkage in the stretching direction) in order to prevent the porosity from decreasing.

[0020]

EXAMPLES The present invention will be described in more detail below with reference to examples.

Example 1 An aqueous dispersion (concentration of PTFE powder having an average particle size of 0.2 μm and 6 parts by weight of nonionic surfactant per 100 parts by weight of PTFE) was prepared in which the concentration of unsintered PTFE powder was 50% by weight. Fluorine-based surfactant (Megafax F-142D, manufactured by Dainippon Ink and Chemicals, Inc.)
E is added in an amount of 2 parts by weight with respect to 100 parts by weight.

In this dispersion, a thickness of 125 μm
Dip and pull up the long polyimide film (base) of
The coating thickness is adjusted to 20 μm with a measuring bar. Then, by heating at 100 ° C. for 1 minute and further at 350 ° C. for 2 minutes, water is evaporated and removed, and the PTFE powders are partially bound to each other to form a film. This dip coating and heating were repeated 4 times, and then the PTFE films were peeled off from both sides of the polyimide film. These films have a thickness of 35 μm, and the tensile strength (unit: kg / cm ² ) and elongation (unit:%) in the longitudinal direction and the width direction (direction perpendicular to the longitudinal direction) are as shown in Table 1 below. It was a host. The tensile strength and elongation are JI
It was measured by the method defined in SK 6887. A universal tensile tester (manufactured by Toyo Baldwin, Tensilon UTM-III-100) was used for the measurement.

Next, this film was stretched at a temperature of 50 ° C. in the longitudinal direction and the transverse direction so that the stretching ratio was twice each, and the thickness was 20 μm, the porosity was 53%, and the pore diameter was 0.05 μm.
To obtain a porous film (Sample 1).

The ethanol flow rate and ethanol puddle point of this porous film are shown in Table 2 below. The ethanol flow rate was calculated by using a disk-shaped porous film having a diameter of 45 mm as a filter and measuring the time required for 100 ml of ethanol to permeate with a pressure difference of 23 mmHg. Also, the ethanol pable point is A
It was measured by the method specified in STM F316-70.

Example 2 All operations were performed in the same manner as in Example 1 except that “350 ° C. for 2 minutes” as a heating condition was changed as shown in Table 1. Table 1 also shows the tensile strength and elongation of the PTFE film before stretching.

[0026]

[Table 1]

Table 2 shows the physical properties of the five types of porous films (Samples 2 to 6) thus obtained. The unit of porosity, pore diameter, ethanol flow rate and ethanol pubble point is "%", "μm", "ml / cm ² · m"
in ”and“ kg / cm ² ”.

[0028]

[Table 2]

Example 3 A thickness of 35μ made in the same way as used in Example 1.
The PTFE film of m was drawn at a draw ratio as shown in Table 3 below at a temperature of 50 ° C. to obtain four types of porous films (Samples 7 to 10). Table 3 shows the physical properties of this porous film.
Also described in. The unit of thickness in Table 3 is “μm”, and the unit of other physical properties is the same as in Table 2.

[0030]

[Table 3]

Comparative Example The same procedure as in Example 1 was repeated except that “350 ° C. for 2 minutes” as the heating condition was changed as shown in Table 1.
A variety of porous films (Samples 11 to 13) were obtained. Table 1
Indicates the tensile strength and elongation of the PTFE film before stretching. In addition, Table 2 also shows the physical property values of these porous films.

[0032]

The present invention is constituted as described above, and when the PTFE dispersion is applied onto the substrate and heated, P
Since the TFE powders are partially bound to each other to form a film and then stretched, it is possible to relatively easily manufacture a thin PTFE film having a small pore size.

Claims (3)

[Claims] 1. A dispersion of polytetrafluoroethylene powder is coated on a substrate, and then the dispersion medium is evaporated and removed by heating, and the polytetrafluoroethylene powder is partially bound to each other to form a film. A method for producing a porous body, which comprises peeling a film from a substrate and then stretching the film to make it porous. 2. A dispersion of polytetrafluoroethylene powder is applied onto a substrate, and then the dispersion medium is evaporated and removed by heating, and the polytetrafluoroethylene powder is partially bound to each other to form a film. A method for producing a porous body, which comprises peeling a film from a substrate, stretching it to make it porous, and then heat-treating the porous body while keeping the stretched state. 3. The method for producing a tubular porous body according to claim 1, wherein a tubular or rod-shaped substrate is used, and a dispersion of polytetrafluoroethylene powder is applied to the peripheral surface of the substrate.