JPH07233274A - Production of porous film - Google Patents

Abstract

PURPOSE:To obtain a porous film with a specified pore size by casting a dispersion contg. a PTFE (polytetrafluoroethylene) powder and fine depolymerizable or thermally decomposable polymer particles to form a film. depolymerizing or thermally decomposing the particles to form pores in the film, and repeating the abode process including casting and pore-forming steps. CONSTITUTION:A film is formed by casting a dispersion contg. a PTFE powder and fine depolymerizable and/or thermally decomposable polymer particles, removing the dispersion medium from the resulting cast, and heating the cast to melt the PTFE powder to bind the particles to each other. The particles in the film are then depolymrized and/or thermally decomposed to give a porous fluororesin film. The obtd. porous film is further subjected to the above process comprising the casting, solvent removal, film-forming, and pore-forming steps at least once, thus giving the target porous film.

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 film made of polytetrafluoroethylene (hereinafter referred to as "PTFE"), and more specifically to depolymerization and / or thermal decomposition of a fine particle polymer. To use.

[0002]

2. Description of the Related Art PTFE is excellent in various properties such as heat resistance, chemical resistance, and electrical properties, and its porous film is a selective permeable membrane for removing a specific substance from a fluid, a filter, a sensor, a waterproof permeable membrane. Widely used for wet materials.

As a method for producing a PTFE porous film, a mixture obtained by adding a liquid lubricant such as naphtha to PTFE powder is extruded, rolled into a film, then the liquid lubricant is removed, and then stretched. There is known a method of making the material porous. The method for producing a fluororesin porous film by this stretching is described in, for example, Japanese Patent Publication No. 42-13560.
Japanese Patent Publication No. 51-18991 and Japanese Patent No. 3,953,566.

Further, Japanese Patent Publication No. 1-502166 discloses a film formed by adding pore-forming powder such as calcium carbonate powder to PTFE powder, and two or more films having different particle diameters of calcium carbonate. Film is prepared, these are superposed and heated to a temperature not lower than the melting point of PTFE to be sintered and integrated, and then the pore-forming powder is extracted and removed from two or more porous layers having different pore sizes. A method of making the film is disclosed.

[0005]

The above-mentioned stretching method has an advantage that a porous film can be efficiently produced, but on the other hand, since the pore size of fine pores is determined by the stretching conditions, a film having a predetermined pore size is obtained. In order to obtain a stable product at all times, it is necessary to pay close attention to the manufacturing conditions. For example, the stretching temperature and the stretching rate must be strictly controlled, and even if the PTFE powders are of the same grade, the physical property values are inevitable to change depending on the production lot. In some cases, the stretching conditions also had to be changed.

Further, the fluororesin porous film obtained by the stretching method has a large number of fine fibers oriented along the stretching direction, and is connected by these fibers and oriented in a direction orthogonal to the stretching direction. It has a fine structure composed of a number of nodes.
The micropores of this porous film are spaces between fibers, but the formation of fibers by stretching is likely to be irregular, and therefore the pore size is also likely to be nonuniform, which is not always suitable as a selective permeable membrane or the like. Absent.

Further, in the case of the stretching method, it is quite difficult to obtain a porous film having different pore sizes and porosities on the front and back surfaces.

On the other hand, in the case of the extraction method, since the pore diameter of the obtained porous film corresponds to the particle diameter of the pore-forming powder, it is possible to easily form two or more porous layers having different pore diameters. Since it takes a long time to extract and remove the powder for use, not only the production efficiency is poor, but also the powder is apt to remain.

Therefore, an object of the present invention is to provide a method for easily producing a porous film having a predetermined pore size, and further to easily prepare a porous film having different pore sizes and porosities on both front and back surfaces. It is also an object of the present invention to provide a method for manufacturing the same.

[0010]

In order to solve the above problems of the prior art, the present inventor has conducted earnest research and as a result, can achieve the object by utilizing depolymerization and / or thermal decomposition of fine particle polymers. The present invention has been completed and the present invention has been completed.

That is, according to the present invention, a PTFE powder and a dispersion liquid containing a depolymerized and / or thermally decomposed fine particle polymer are cast, then the dispersion medium of the dispersion liquid is removed, and then heated to obtain a PTFE powder. By melting the powders to bind the powders to each other to form a film, and then depolymerizing and / or thermally decomposing the fine particle polymer to make the film porous, and further disperse the dispersion liquid on the formed porous layer. The present invention relates to a method for producing a porous film made of PTFE, characterized in that a series of steps of casting, removal of a dispersion solvent, film formation and film porosification are performed at least once.

As the PTFE powder in the present invention, a powder obtained by homopolymerizing tetrafluoroethylene is usually used, but if desired, a copolymer powder of tetrafluoroethylene and another monomer can also be used. Examples of such other monomer include ethylene, hexafluoropropylene, perfluoroalkyl vinyl ether and the like. The content of the other monomer in this copolymer is usually 2% by weight or less. The particle size of this PTFE powder is not particularly limited, but is usually about 0.1 to 0.5 μm.
Is. The particle size of the fluororesin powder is preferably smaller than that of the fine particle polymer described below.

On the other hand, as the fine particle polymer, one which can be depolymerized and / or thermally decomposed is used. As the polymer capable of depolymerization and / or thermal decomposition, polymethylmethacrylate, polyα-methylstyrene, polyisobutylene, polyoxymethylene, polystyrene, polyisobutylmethacrylate, etc. are already known. Fine particles of these polymers can be used. Of these, polymethylmethacrylate, poly-α-methylstyrene, polyoxymethylene, and polyisobutylmethacrylate are depolymerized by heating to be vaporized, and polystyrene and polyisobutylene are depolymerized and thermally decomposed by heating to be vaporized. This polymer is preferably an uncrosslinked polymer, but a crosslinked polymer can also be used.
However, in the present invention, as the fine particle polymer, one having a depolymerization temperature and a thermal decomposition temperature higher than the melting point of PTFE is selected and used.

The particle size of the fine particle polymer determines the pore size of the porous film, and when spherical particles are used, those having a diameter of about 0.2 to 50 μm are usually used. In many cases, a fine particle polymer having a uniform particle size is used, but in some cases, a small particle size and a large particle size may be mixed and used at an appropriate ratio. Further, in addition to the spherical shape, an arbitrary shape such as a columnar shape, a spindle shape, or a square bar shape can be used.

In the present invention, first, a dispersion liquid containing PTFE powder and a fine particle polymer which is depolymerized and / or thermally decomposed at a temperature higher than the melting point of PTFE is cast. The dispersion used here can be prepared, for example, by mixing the fine particle polymer in the dispersion of the PTFE powder having a concentration of about 30 to 60% by weight, or in the state of the dispersion. As the dispersion medium, water is usually used, but an organic solvent which does not dissolve the fine particle polymer may be used. In this dispersion, the mixing ratio of the PTFE powder and the fine particle polymer is determined according to the porosity of the target porous film, the particle size of the fine particle polymer, etc. The proportion is about 0.1 to 60% by weight. A surfactant may be added to the dispersion liquid in order to improve the dispersibility of the PTFE powder or the fine particle polymer.

The dispersion is cast by heat-resistant plastics such as polyimide and polyetheretherketone, metals,
This can be carried out by using a substrate of an arbitrary shape made of a heat-resistant material such as ceramics, immersing the substrate in the dispersion liquid, and applying the dispersion liquid to the surface of the substrate by spray coating or brush coating. The casting thickness of the dispersion liquid on the substrate is preferably not more than 3 times the particle size of the fine particle polymer, more preferably not more than the particle size of the polymer.

After casting the dispersion liquid on the substrate in this way,
The dispersion medium of the dispersion is removed. This removal can be performed by a heating method, a decompression method, or the like. When the heating method is adopted,
The temperature is set to be lower than the melting point of the fluororesin powder and lower than the depolymerization temperature and the thermal decomposition temperature of the fine particle polymer.

After removing the dispersion medium as described above, P
The PTFE powder is melted by heating it to a temperature not lower than the melting point of TFE and lower than the depolymerization temperature and the thermal decomposition temperature of the fine particle polymer to bond the PTFE powders to each other to form a film containing the fine particle polymer. The heating time required for film formation may vary depending on the temperature and the like, but is usually about 30 seconds to 5 minutes.

After the PTFE film is formed in this way, it is heated to a temperature above the depolymerization temperature and / or thermal decomposition temperature of the fine particle polymer. The heating temperature is preferably set lower than the decomposition temperature of PTFE in order to prevent deterioration of the film. The heating time is set according to the temperature, but is usually about 2 to 30 minutes.

By this heating, the fine particle polymer in the PTFE film is depolymerized and / or thermally decomposed, the main chain of the polymer is cleaved, and the monomer or oligomer is vaporized. Then, the portions where the fine particle polymer was present in the PTFE film become fine pores, and the film becomes porous.

In the present invention, after the film is made porous in this way, a dispersion liquid containing PTFE powder and a depolymerized and / or thermally decomposed fine particle polymer is cast on the porous layer, Remove the dispersion medium of this dispersion,
Thereafter, the PTFE powder is melted by heating to bind the powder particles together to form a film, and then the fine particle polymer is depolymerized and / or thermally decomposed to make the film porous. As a result, a porous film composed of the porous layer and the porous layer formed thereon can be obtained. The series of steps of casting the dispersion liquid on the porous layer, removing the dispersion medium, forming a film and making it porous can be performed once or twice or more times. According to the present invention, the porous film obtained is relatively thick.

In the present invention, various types of porous films are obtained by appropriately selecting the fine particle polymer when preparing a dispersion liquid containing the PTFE powder and the fine particle polymer which is depolymerized and / or thermally decomposed. be able to. For example, if (a) a dispersion liquid having the same particle diameter of the fine particle polymer is used consistently, a porous film composed of two or more porous layers having the same pore diameter of each layer is obtained. If two or more kinds of dispersion liquids having different particle diameters are used, a porous film composed of two or more porous layers having different pore diameters (c) has the same particle diameter of the fine particle polymer, and the compounding amount is changed. By using at least one kind of dispersion liquid, a porous film composed of two or more porous layers having different porosities can be obtained.

After forming two or more porous layers in this manner, the porous film can be obtained by peeling from the substrate. Peeling from the substrate can be easily done because PTFE is inherently non-adhesive. Also,
If desired, a porous film with a reinforcing material can be obtained by adhering a suitable reinforcing material to the porous film and then peeling it off.

In the present invention, a porous layer is formed on a substrate, which is peeled off from the substrate and the dispersion is cast on the peeled porous layer, the dispersion medium is removed, a film is formed, and a film is formed. May be made porous. Also in this case, the reinforcing material can be attached to the porous layer before the peeling, or the reinforcing material can be attached after the peeling.

Next, a method for producing a fluororesin porous film according to another embodiment of the present invention will be described. In this manufacturing method, a dispersion liquid containing a PTFE powder and a fine particle polymer that undergoes depolymerization and / or thermal decomposition is cast, then the dispersion medium of the dispersion liquid is removed, and then heated to melt the PTFE powder. By binding the powders together to form a film, a fine particle polymer is depolymerized and / or pyrolyzed to form a porous layer. Further, the dispersion liquid is cast on the formed porous layer, and the dispersion medium is dispersed. It is characterized in that a series of steps of removing and forming the film into a porous film is performed at least once.

In the above-mentioned method, the PTFE film was formed and then the film was made porous. In the method according to this embodiment, while the film is being formed, the fine particle polymer is depolymerized and / or pyrolyzed to make it porous. To convert.

Therefore, the point that a polymer capable of depolymerization and / or thermal decomposition at a temperature equal to or higher than the melting point of the PTFE powder is used as the fine particle polymer, and the heating temperature after removing the dispersion medium is P
It is different from the above-mentioned method in that the temperature is set above the melting point of TFE and above the depolymerization and / or thermal decomposition temperature of the fine particle polymer. However, other than this, the same operation as that described above may be performed.

The thickness, pore size, and porosity of the porous film obtained by the method of the present invention vary depending on various conditions, but usually the thickness is about 5 to 100 μm and the pore size is about 0.5 to 50 μm. The porosity is about 0.1 to 60%.

This fluororesin porous film can be used not only for selective permeable membranes, filters, sensors and waterproof / moisture permeable materials like conventional porous films, but also for medical materials, wound dressings, bandages and plaster casts. Material, mask material,
It can also be used as a hospital gown, sheets, tent, various battery membranes, separators, clothing materials, gas permeable membranes and the like. When it is used for these, it can be partially joined to a breathable material such as a woven fabric or a non-woven fabric in a dot shape, a mesh shape, or a stripe shape.

[0030]

EXAMPLES The present invention will be described in more detail below with reference to examples. In addition, all "parts" indicating the blending amounts of the components are "parts by weight".

Example 1 Aqueous dispersion in which the concentration of PTFE (melting point 327 ° C.) powder having an average particle size of 0.25 μm was 60% by weight (however, 100 parts of PTFE was added to the nonionic surfactant 6).
Part) is prepared.

Separately from this, poly-α-methylstyrene spherical particles having an average particle size of 5 μm (depolymerization temperature 290 ° C.) 10
An aqueous dispersion (concentration of poly-α-methylstyrene spherical particles of 50% by weight) containing 10 parts of a nonionic surfactant is prepared for 0 part.

A dispersion A is obtained by mixing 36 parts of the aqueous dispersion containing poly-α-methylstyrene spherical particles with 100 parts of the aqueous dispersion of the above PTFE powder.

Next, 1 part of nonionic surfactant was added to 100 parts of spherical particles of poly-α-methylstyrene having an average particle size of 2 μm.
Aqueous dispersion containing 0 parts (spherical particle concentration 5
0% by weight) was prepared, and 18 parts of this was mixed with 100 parts of the same aqueous dispersion of PTFE powder as above to obtain a dispersion B.

Then, the dispersion A is spray-coated on one surface of the stainless steel foil so as to have a thickness of 5 μm, and the temperature is 120 ° C.
Water is removed by evaporation by heating at a temperature of 1 minute. Then 40
The mixture is heated to a temperature of 0 ° C. for 30 minutes, and the PTFE powder is melted and the powder particles are bound to each other to form a film, and poly α-methylstyrene is vaporized by depolymerization to make it porous and cooled to room temperature.

Thereafter, this first porous layer (thickness 5 μm
m) is spray-coated so that the thickness of the dispersion B is 2 μm, and heated at a temperature of 120 ° C. for 1 minute to evaporate and remove water. Next, heat to a temperature of 400 ° C. for 30 minutes to remove PTF.
The E powder was melted and the powders were bound to each other to form a film, and poly (α-methylstyrene) was vaporized by depolymerization to form a second porous layer (thickness 2 μm). The porous film was peeled from the.

Observation of both the front and back surfaces of this PTFE porous film with a scanning electron microscope revealed that the surface on the first porous layer side had a pore diameter of 4.6 μm and a porosity of 38%, and the second porous The surface of the layer side has a pore diameter of 1.8 μm and a porosity of 2
It was 3%.

Example 2 100 parts of poly-α-methylstyrene spherical particles having an average particle size of 1 μm (depolymerization temperature of 290 ° C.) and 10 parts of a nonionic surfactant were mixed in an aqueous dispersion (concentration of spherical particles of 50% by weight). To prepare.

Dispersion C is prepared by mixing 30 parts of this poly-α-methylstyrene spherical particle-containing dispersion with 100 parts of the same PTFE powder dispersion as used in Example 1.

On the other hand, separately from this, the same operation as in Example 1 was performed to sequentially form the first and second porous layers on the stainless steel foil, and the dispersion liquid C was formed on the second porous layer to a thickness of To 1 μm by spraying, heating at 120 ° C. for 1 minute to remove water by evaporation, and then heating at 400 ° C. for 30 minutes to melt the PTFE powder and bind the powder to each other. Then, while forming a film, poly-α-methylstyrene was vaporized by depolymerization to form a third porous layer (thickness 1 μm), which was cooled to room temperature and peeled from the stainless steel foil.

This PTFE porous film was composed of three porous layers having different pore sizes and porosities, and the surface on the side of the third porous layer had a pore size of 0.6 μm and a porosity of 35%.

Example 3 An aqueous dispersion of the same PTFE powder as used in Example 1 and poly alpha methyl styrene (average particle size 5μ
m) Prepare an aqueous dispersion. And PTF
For 100 parts of aqueous dispersion of E powder, poly α
Dispersion D is prepared by mixing 3 parts of an aqueous dispersion of methylstyrene. Further, to 100 parts of this PTFE powder aqueous dispersion, 36 parts of the same poly α-methylstyrene aqueous dispersion as used in Example 2 is mixed to prepare a dispersion E.

Then, the dispersion D is spray-coated on one surface of the stainless steel foil so as to have a thickness of 5 μm, and the temperature is 120 ° C.
Water is removed by evaporation by heating at a temperature of 1 minute. Then 40
The mixture is heated to a temperature of 0 ° C. for 30 minutes to melt the PTFE powder and bind the powder to each other to form a film, and poly (α-methylstyrene) is vaporized by depolymerization to make it porous.

Then, the dispersion E is spray-coated to a thickness of 1 μm on the first porous layer (thickness 5 μm) formed by the dispersion D, and heated at a temperature of 120 ° C. for 1 minute. Water is removed by evaporation. Then, the temperature of 400 ℃ 30
After heating for a minute, the PTFE powder is melted and the powders are bound to each other to form a film, while polyα-methylstyrene is vaporized by depolymerization to form a second porous layer (thickness 1 μm) and cooled to room temperature. Then, the porous film was peeled off from the stainless steel foil.

Observation of both the front and back surfaces of this PTFE porous film with a scanning electron microscope revealed that the surface on the first porous layer side had a pore diameter of 4.6 μm and a porosity of 4%,
The surface of the porous layer on the porous layer side has a pore size of 0.7 μm and a porosity of 39.
%Met.

Example 4 The dispersion A prepared in Example 1 is spray-coated on one side of a stainless steel foil so that the thickness is 5 μm, and heated at a temperature of 120 ° C. for 1 minute to remove water by evaporation. Then 400
The mixture is heated at a temperature of 30 ° C. for 30 minutes to melt the PTFE powder and bind the powder to each other to form a film, and poly (α-methylstyrene) is vaporized by depolymerization to make it porous.

Thereafter, the first porous layer (thickness 5 μm
m), spray coating of dispersion A, removal of water by evaporation, and porosification by vaporizing poly-α-methylstyrene by depolymerization while forming a film are performed again under the same conditions as above, and the second porous layer To form. And it cooled to room temperature and peeled the porous film from the stainless steel foil. Both front and back surfaces of this PTFE porous film (thickness 10 μm) were observed with a scanning electron microscope. As a result, both front and back surfaces had a pore diameter of 4.6 μm and a porosity of 38%.

Example 5 The same PTFE aqueous dispersion as used in Example 1 and 100 parts of polystyrene spherical particles having an average particle size of 5 μm (pyrolysis temperature 360 ° C.) were mixed with 10 parts of a nonionic surfactant. The prepared aqueous dispersion (concentration of polystyrene spherical particles: 40% by weight) is prepared.

60 parts of polystyrene aqueous dispersion was mixed with 100 parts of PTFE aqueous dispersion to obtain dispersion F, and 6 parts of polystyrene aqueous dispersion was mixed with 100 parts of PTFE aqueous dispersion and dispersed. Liquid G is obtained.

This dispersion F is spray-coated on one surface of the stainless steel foil so that the thickness is 5 μm, and heated at a temperature of 120 ° C. for 5 minutes to remove water. Next, the PTFE powder is melted by heating at a temperature of 330 ° C. for 10 minutes to bind the powders together to form a film, and then at a temperature of 380 ° C. for 2 minutes.
The polystyrene is pyrolyzed and depolymerized by heating for 0 minutes to make it porous, and then cooled to room temperature.

Then, the first porous layer (thickness 5 μm
m), spray coating of the dispersion G, water removal by evaporation, film formation, and thermal decomposition and depolymerization of polystyrene to make the film porous are conducted under the same conditions as the formation of the first porous layer. To form a porous layer. And
After cooling to room temperature, the porous film was peeled off from the stainless steel foil.

This PTFE porous film (thickness 10 μm
The surface of m) was observed by a scanning electron microscope.
The surface of the porous layer on the porous layer side has a pore diameter of 4.6 μm and a porosity of 44.
%, And the surface on the second porous layer side had a pore diameter of 4.6 μm and a porosity of 7%.

Example 6 The same PTFE aqueous dispersion as used in Example 1 and polystyrene spherical particles 10 having an average particle size of 2.0 μm.
An aqueous dispersion (concentration of polystyrene spherical particles of 40
%), And prepare PTFE aqueous dispersion 100
A dispersion H is obtained by mixing 36 parts of polystyrene aqueous dispersion.

The same dispersion F as that used in Example 5 was spray coated on one surface of the stainless steel foil so that the thickness was 5 μm, and heated at 120 ° C. for 5 minutes to remove water by evaporation. Next, heat to a temperature of 330 ° C. for 10 minutes to remove PTF.
After the E powder is melted and the powders are bound to each other to form a film, it is heated at 380 ° C. for 20 minutes to thermally decompose and depolymerize polystyrene to make it porous, and then cooled to room temperature.

Thereafter, the first porous layer (thickness 5 μm
m), spray coating of dispersion H (however, coating thickness is 2 μm), water evaporation removal, film formation, polystyrene pyrolysis and depolymerization to make porous once as described above. A second porous layer is formed. And it cooled to room temperature and peeled the porous film from the stainless steel foil.

This PTFE porous film (thickness 7 μm
When the both sides of m) were observed with a scanning electron microscope,
The surface on the first porous layer side has a pore size of 4.6 μm and the porosity is 44%, and the surface on the second porous layer side has a pore size of 1.8.
μm, and the porosity was 20%.

Example 7 The dispersion H prepared in Example 6 is spray-coated on one surface of a stainless steel foil to a thickness of 2 μm, and heated at 120 ° C. for 5 minutes to remove water by evaporation. Then 330
After heating to a temperature of ℃ for 10 minutes, PTFE powder is melted and the powder particles are bound to each other to form a film, and then heated at a temperature of 380 ° C. for 20 minutes to thermally decompose and depolymerize polystyrene to make it porous, Cool to room temperature.

Then, the first porous layer (thickness 2 μm)
m), spray coating of the dispersion H, evaporation of water, film formation, and thermal decomposition of polystyrene and porosification by depolymerization are performed once under the same conditions as above to form a second porous layer. And it cooled to room temperature and peeled the porous film from the stainless steel foil.

This PTFE porous film (thickness 4 μm
When the both sides of m) were observed with a scanning electron microscope,
Both the front and back surfaces had a pore diameter of 1.8 μm and a porosity of 20%.

[0060]

EFFECTS OF THE INVENTION The present invention is constituted as described above, and since the fine particle polymer is vaporized by depolymerization and / or thermal decomposition to form fine pores, a porous film having a desired diameter can be easily formed. A porous film having different pore sizes and different porosities on the front and back surfaces can be easily produced.

Claims (2)

[Claims] 1. A dispersion liquid containing polytetrafluoroethylene powder and a fine particle polymer that undergoes depolymerization and / or thermal decomposition is cast, then the dispersion medium of the dispersion liquid is removed, and then heated to obtain polytetrafluoroethylene. The fluoroethylene powder is melted to bind the powders to each other to form a film, and then the fine particle polymer is depolymerized and / or thermally decomposed to make the film porous, and further, on the formed porous layer. In the method for producing a porous film, the series of steps of casting the dispersion liquid, removing the dispersion medium, forming a film and making the film porous is performed at least once. 2. A dispersion liquid containing polytetrafluoroethylene powder and a fine particle polymer that undergoes depolymerization and / or thermal decomposition is cast, then the dispersion medium of the dispersion liquid is removed, and then heated to obtain polytetrafluoroethylene. The fluoroethylene powder is melted to bind the powders to each other to form a film, thereby forming a porous layer by depolymerization and / or thermal decomposition of the fine particle polymer, and further forming a dispersion on the formed porous layer. A process for producing a porous film, which comprises performing a series of steps of casting, removing a dispersion solvent, and making the film porous while forming the film at least once.