JP2533229B2 - Polytetrafluoroethylene porous body and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a polytetrafluoroethylene (hereinafter abbreviated as “PTFE”) porous body used as a membrane filter, a membrane for batteries, a material for coating electric wires, and the like. Specifically, it relates to a PTFE porous body having a small pore size and excellent in permeability and dimensional stability, and a method for producing the same.

[Conventional technology]

PTFE porous materials are used in a wide range of fields such as membranes for fuel cells, membrane filters, electric wires, analyzers and artificial blood vessels. By the way, in recent years, in the fields of microfiltration filters, supports for high-performance membranes, air filters, etc., there has been a demand for a porous PTFE body having a small pore size and excellent in permeability and dimensional stability.

Conventionally, as a method for producing a PTFE porous body, for example, (1) a method in which an unsintered molded body obtained by PTFE paste extrusion is stretched at a temperature below the melting point and then sintered (Japanese Patent Publication No. 42-13560). (Gazette), (2) Sintered PTFE
After gradually cooling the molded body to enhance crystallization, the draw ratio is 1.5 to 4
Double uniaxial stretching (Japanese Patent Publication No. 53-42794),
(3) The difference between the unsintered molded body obtained by the paste extrusion of PTFE fine powder at a temperature below the melting point of the powder of the fine powder and above the melting point of the molded product (sintered body) obtained from the fine powder. Method of causing no change in the endothermic peak of the fine powder on the crystal melting diagram in the scanning calorimeter, and after heat treatment so that the specific gravity of the molded body is 2.0 or more, a method of stretching at a temperature not higher than the melting point of the powder (JP-A-58-145735), (4)
A method in which a molded body obtained by paste extrusion of PTFE fine powder having a number average molecular weight of 1,000,000 or less is heat treated after sintering to increase the crystallinity and then stretched in at least one axial direction (JP-A-64- Japanese Patent No. 78823) is known.

However, the method (1) of stretching an unsintered molded body has a limit in obtaining a porous body having a small pore size and excellent permeability. In the method of stretching the sintered product of (2), since the stretching ratio cannot be made high, even if the pore diameter is small, the porosity is low and only a membrane having low permeability can be obtained. In the method of stretching after the heat treatment of (3), although a relatively small pore size is easily obtained, it is still insufficient and the heat resistance is not sufficient. In the method of stretching a sintered PTFE body having a number average molecular weight of 1,000,000 or less in (4), a relatively small porosity and a relatively high porosity can be obtained. Is not enough.

As described above, the conventional technique is insufficient in obtaining a PTFE porous body having a small pore size, a high porosity, and good heat resistance. Further, the conventional PTFE porous body has a large shrinkage rate (solvent shrinkage rate) after being immersed in a solvent, and, for example, when it is used as a filtering material for filtering an organic solvent vapor or is washed with an organic solvent. However, there is a problem that the gas flow rate is reduced due to contraction in the thickness direction.

[Problems to be Solved by the Invention]

An object of the present invention is to provide a PTFE porous body having a small pore size, a high porosity, and excellent permeability.

Another object of the present invention is to provide a PTFE porous body having a small heat shrinkage and good heat resistance (heat stability).

Further, an object of the present invention is to provide a PTFE porous body having a small solvent shrinkage ratio and excellent dimensional stability.

As a result of intensive studies to overcome the problems of the prior art, the present inventors have found that unsintered compacts obtained by paste extrusion of PTFE fine powder can be obtained by a differential scanning calorimeter (DSC) on a crystal melting diagram. The heat treatment is performed so that a peak occurs between the endothermic peak position of fine powder (around 347 ° C) and the endothermic peak position of the sintered body (around 327 ° C), and then the product is stretched in at least one direction to achieve the above-mentioned purpose. It has been found that

In the method described in JP-A-58-145735 described above, the molded product is heat-treated and then stretched, which is the endothermic peak (347 ° C) of the fine powder in the DSC chart.
It is a heat treatment that does not cause a change in (near), and is a process for stretching a substantially unsintered molded product, and the resulting porous body is insufficient in terms of small pore size and has a heat shrinkage. Rate and solvent shrinkage are relatively large, and heat resistance and dimensional stability are not sufficient.

On the other hand, in the method of the present invention, the molded body is stretched after being made into a kind of semi-sintered body by heat treatment,
The resulting porous body has a small pore size and high permeability,
Moreover, it has good heat resistance and dimensional stability.

The present invention has been completed based on these findings.

[Means for solving the problem]

Thus, according to the present invention, the unsintered molded body obtained by paste extrusion of polytetrafluoroethylene fine powder, the endothermic peak position of the fine powder on the crystal melting diagram by a differential scanning calorimeter, and the endotherm of the sintered body. Provided is a method for producing a polytetrafluoroethylene porous body, which comprises performing heat treatment so that at least one peak is generated between the peak position and stretching in at least one direction.

Further, according to the present invention, there is provided a polytetrafluoroethylene porous body obtained by the above production method.

 Hereinafter, the present invention will be described in detail.

(Polytetrafluoroethylene) PTFE used in the present invention is a fine powder,
Usually, the number average molecular weight is 500,000 or more, preferably 2 to 200
One hundred thousand is used.

(Paste Extrusion) In the present invention, the paste extrusion is conventionally performed by using unsintered PTFE.
According to the paste extrusion method known as a method for manufacturing a molded body.

In the paste extrusion method, usually, 15 to 40 parts by weight, preferably 20 to 30 parts by weight of a liquid lubricant is mixed with 100 parts by weight of PTFE and extrusion molding is performed.

As the liquid lubricant, various kinds of lubricants that have been conventionally used in the paste extrusion method can be used. Specific examples include solvent naphtha, petroleum-based solvents such as white oil, hydrocarbon oils, toluols, ketones, esters, silicone oils, fluorocarbon oils, and polymers such as polyisobutylene and polyisoprene dissolved in these solvents. It may be a solution, a mixture of two or more thereof, water or an aqueous solution containing a surfactant, and the like.

Molding by paste extrusion is carried out by molding a mixture containing PTFE fine powder and a liquid lubricant into a predetermined shape at a temperature not higher than the sintering temperature of PTFE (327 ° C or lower), usually around room temperature. Preforming may be performed prior to paste extrusion. Therefore, in general, the mixture is preformed at a pressure of, for example, about 1 to 50 kg / cm ² (pressure preforming), and then extruded by a paste extruder, or rolled by a calender roll, or rolled after being extruded. A molded body having a predetermined shape is manufactured by the above method.

The shape of the molded body includes a sheet, a tube, a rod, a strip, a film and the like, and a thin sheet can be obtained by rolling. The shape of the molded body is not particularly limited as long as it can be stretched after the heat treatment described later.

The liquid lubricant is removed from the molded body by extruding, melting or heating and evaporating the molded body by paste extrusion. When using a liquid lubricant having a relatively high boiling point such as silicone oil or fluorocarbon, removal by extraction is preferable.

In addition to the liquid lubricant, other substances can be included depending on the purpose. For example, pigments for coloring, carbon black, graphite, silica powder, asbestos powder, glass powder, glass fiber, silicates for improving wear resistance, preventing cold flow and facilitating generation of pores. Inorganic fillers such as and carbonates, metal powder, metal oxide powder,
Metal sulfide powder or the like can be added.

In addition, heating, extraction, to help create a porous structure,
Substances that can be removed or decomposed by dissolution, such as ammonium chloride, sodium chloride, other plastics,
Rubber or the like can be blended in the form of powder or solution.

(Heat Treatment) The unsintered molded body obtained by paste extrusion is then subjected to heat treatment.

PTFE fine powder is a crystal melting diagram by TSC (DSC
On the chart, it has an endothermic peak near 347 ° C (usually 347 ° C ± 2 ° C). This endothermic peak also appears in an unsintered molded product obtained by paste extrusion of PTFE fine powder. This is called the melting point of fine powder or the endothermic peak position of fine powder (near 347 ° C) on the DSC chart.

This endothermic peak around 347 ° C usually accompanies a shoulder or other low peak around 338 ° C (see chart (A) in Fig. 1), but depending on the type of fine powder, the shoulder or other Some have no peaks.

Put PTFE fine powder or its paste extrusion molded product at a temperature above the melting point of fine powder, usually 350 to 4
When heated in a heating furnace kept at 50 ° C for several minutes to several tens of minutes and sintered, the endothermic peak near 347 ° C on the DSC chart disappears, including the shoulder part, and instead around 327 ° C (usually A relatively low endothermic peak appears at 327 ° C. ± 1 ° C. (see chart (D) in FIG. 1). This temperature is called the melting point of the sintered body or the endothermic peak position of the sintered body (around 327 ° C).

The heat treatment of the present invention is carried out so that at least one peak occurs between the endothermic peak position of fine powder (near 347 ° C.) and the endothermic peak position of the sintered body (near 327 ° C.) on the DSC chart. To do that, PT
It is carried out by heating an unsintered compact obtained by paste extrusion of FE fine powder at a temperature not lower than the melting point (around 327 ° C) of the sintered product, for several seconds to several tens of minutes, and in some cases, for a longer time. The heat treatment is usually 33
It is preferable to heat in a heating furnace maintained at 0 to 450 ° C.

When heat treatment is insufficient, such as when the heat treatment time is short, the endothermic peak at 347 ° C that initially had a shoulder near 338 ° C (chart (A) in Fig. 1) shows a single peak near 347 ° C. An endothermic peak appears (chart (B)).
In the endothermic peak state shown in this chart (B),
Sintering has not progressed and almost behaves like PTFE fine powder. The invention described in JP-A-58-145735 discloses that a molded product is stretched in a state where the endothermic peak (near 347 ° C.) of the fine powder is not changed even by heat treatment.

On the other hand, when the heat treatment was further advanced, as shown in the chart (C) of FIG. 1, the endothermic peak of fine powder (near 347 ° C.) disappeared and the endothermic peak of the sintered body (327) was disappeared.
An endothermic peak like a semi-sintered body appears at a temperature higher than (about ℃). In the chart (C) of FIG. 1, a single endothermic peak appears at 335 ° C. Most of the molded products showing such an endothermic peak position are close to the sintered product, but the sintering is not complete and it is a semi-sintered product. In the present invention,
This semi-sintered body is stretched to form a porous body.

When the heat treatment is further advanced, as described above, the molded body is sintered and exhibits a single endothermic peak near 327 ° C. shown in the chart (D) of FIG. 1.

Even if a molded product having an endothermic peak at the position shown in chart (A) (unsintered), chart (B) (unsintered) and chart (D) (sintered) is stretched, the object of the present invention can be achieved. Can not. Molded product heat-treated so as to have at least one peak between the endothermic peak position of fine powder (near 347 ° C) and the endothermic peak position of the sintered body (near 327 ° C) as shown in chart (C) By stretching the (semi-sintered body), it is possible to obtain a PTFE porous body having a small pore size, a high porosity, and good heat resistance and dimensional stability.

(Stretching) This heat-treated molded body is then stretched in at least one direction.

Stretching can be carried out by mechanically stretching a heat-treated molded body formed into a predetermined shape such as a sheet, a rod or a tube by a usual method. For example, in the case of a sheet, when winding from one roll to another roll, the winding speed is made higher than the feed speed, or the two opposite sides of the sheet are grasped and stretched so as to widen the distance. It can be stretched. Tubes and rods are easy to stretch along their length. Further, various stretching methods such as other-stage stretching, sequential biaxial stretching, and simultaneous biaxial stretching can be adopted.

The stretching temperature is usually below the melting point of the sintered body (327 ° C).
The following), preferably at a temperature of 0 to 300 ° C. Stretching at a low temperature tends to produce a porous body having a relatively large pore size and a high porosity, and stretching at a high temperature tends to produce a dense porous body having a small pore size. Therefore, by combining these conditions, the pore size and porosity can be controlled.

The stretching ratio is 1.5 times (area ratio) or more, but in order to increase the porosity, it is preferably 5 times (area ratio) or more, and preferably 6 to 100 times (area ratio).

In the case of biaxial stretching, it is usually suitable to stretch in each direction from 2 to 10 times (length ratio), and the longitudinal / lateral stretching ratio is appropriately in the range of 1: 5 to 5: 1. The stretching may be carried out in a first stage at a low temperature of about 20 ° C. and then in a second stage under high temperature conditions.

The stretched PTFE porous body can be used as it is, but when high dimensional stability is required, the stretched state should be kept under tension such as by fixing both stretched ends to 200 to 3
You may heat-process at the temperature of 00 degreeC for about 1 to 30 minutes. further,
In order to improve dimensional stability, it can be sintered by holding it for several tens of seconds to several tens of minutes in a heating furnace kept at a temperature above the melting point of fine powder (near 347 ° C), for example, 350 to 550 ° C. Good.

The heat-treated porous body can be further stretched, resulting in higher porosity.

(PTFE porous body) The PTFE porous body obtained by the method of the present invention has a shape of a molded body obtained by paste extrusion, for example,
It has various shapes such as a sheet shape and a tube shape, and is characterized by having fine pores and high porosity and excellent permeability.

Moreover, the PTFE porous body of the present invention has a higher degree of heat resistance (low heat shrinkage) and dimensional stability (low solvent shrinkage) than conventional products.

The pore size of the fine pores varies depending on the crystallinity of the PTFE molded product, the stretching ratio, etc., but is usually about 0.01 to 1 μm.
Further, in the method of the present invention, since the draw ratio can be increased, the porosity is 80 to 95% as well as the fine pores.
It can be as high as a degree. As will be defined later, it is possible to evaluate the pore size at the bubble point and the permeability at the Galley second.

Regarding the thickness of the porous film, various ones can be prepared by changing the stretching ratio and the like. For example, a thin film having a thickness of about 10 to 30 μm can be easily obtained.

The PTFE porous body of the present invention is stable to a solvent. The conventional PTFE porous body is isopropanol (hereinafter,
When immersed in a solvent such as IPA) and then restrained and dried, there is a problem that the film shrinks in the thickness direction and becomes thinner. The solvent contraction rate has a strong correlation with the gas flow rate (Galley second: the reciprocal of the gas flow rate). Especially, as the thickness is reduced, the gas flow rate of the PTFE porous body is significantly reduced,
In severe cases, it will be 1/2 to 1/4 of the gas flow rate before immersion in the solvent.

Therefore, when the conventional PTFE porous body is used as a filter material for filtering air and organic solvent vapor, for example, it contracts in the thickness direction over time and the gas flow rate decreases. Also, in order to carry out a large amount of filtration, the filtration device (cartridge), which is processed into pleats to increase the surface area of the filter and is housed in a small container, uses a solvent when performing cleaning. And the gas flow rate decreases.

However, in the PTFE porous body of the present invention, although the solvent shrinkage rate varies depending on the heat treatment condition (semi-sintered condition), the solvent shrinkage rate is significantly smaller than the conventional one, and the deterioration of the gas flow rate is minimized. It can be suppressed. In particular, in the heat treatment, the endothermic peak of the molded product was
The closer to (° C), the closer the solvent shrinkage rate becomes to 0,
Almost no deterioration of gas flow rate. Also, in the heat treatment, when the endothermic peak of the molded body approaches the endothermic peak of the PTFE fine powder, the solvent shrinkage rate increases, but it is at most 30% or less, and the gas flow rate is significantly higher than that of the conventional PTFE porous body. The characteristics such as excellent dimensional stability and permeability of the PTFE porous body of the present invention capable of preventing deterioration can be considered to be due to its microstructure.

Fig. 2 shows the surface of the porous body of the present invention, and Fig. 3 shows the conventional PT.
The scanning electron micrograph (5000 times) of a FE porous body is shown.

As shown in FIG. 3, the structure of the conventional PTFE porous body is composed of a nodule that is a lump of resin, fibers that connect the nodule, and fine pores surrounded by these. However, the PTFE porous body of the present invention shown in FIG. 2 has a structure having almost no knots and essentially only fibers,
This structure makes it difficult for the solvent to shrink,
Therefore, it is considered that the deterioration of the gas flow rate can be suppressed to the minimum.

The reason why the PTFE porous body of the present invention has a structure consisting of only fibers is that the particles of the PTFE fine powder are partially melted, bonded and stretched by heat-treating the molded body until semi-sintering. It is presumed that this is because PTFE has a structure that facilitates fiberization. Further, the fiberization is promoted as the fiber is drawn, and the structure essentially consists of fibers.

The porous membrane according to the present invention has fine pores and high porosity, is excellent in liquid and gas permeability, has high uniformity, has a smooth surface, has high mechanical strength, and is non-adhesive. so,
It has low wear and is flexible. further,
Low heat shrinkage and solvent shrinkage, good heat resistance and dimensional stability.

Specifically, the PTFE porous body of the present invention has an IPA bubble point of 1.0 kg / cm ² or more and an IPA solvent shrinkage rate of 30% or less,
Galley seconds are less than 100 seconds. By adjusting heat treatment conditions, etc., the IPA bubble point will be 1.0 to 3.0 kg / cm
² , PTFE porous body with IPA solvent shrinkage of 1.5% or less, Galley second of 20 seconds or less, or IPA bubble point of 3.0 kg / c
Greater than m ² , IPA solvent shrinkage less than 30%, Galley second is 1
A PTFE porous body of 00 seconds or less is obtained. The methods for measuring these physical properties will be described later collectively.

Therefore, the PTFE porous body of the present invention is, for example, a filter material,
Although it has a wide range of applications as a diaphragm, sliding material, non-adhesive material, etc., it makes use of the characteristics of fine pores and high porosity, and is used as a microfiltration filter, high-performance membrane support, air filter, etc. Is suitable as And semiconductors,
It can be used for a wide range of applications in the fields of medicine, biotechnology, etc. such as filtration filters for chemicals, separation membranes for blood components, and diaphragms for artificial lungs.

〔Example〕

Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

<Physical property measuring method> The physical property measuring methods in Examples and Comparative Examples are as follows.

Endothermic peak temperature in DSC: By differential scanning calorimeter (DSC), using about 10 mg of sample,
It was measured at a temperature rising rate of 10 ° C / min.

IPA bubble point: Measured by the method of ASTM-F-316. The larger this value, the smaller the pore size.

IPA flow rate: Measured by the method of ASTM-F-317, the differential pressure is 7
It was set to 0 cmHg. The larger the flow rate, the better the permeability.

Heat shrinkage ratio: Measured after standing at 150 ° C. for 30 minutes.

Solvent Shrinkage: After being dipped in a solvent (IPA), restrained and dried, the shrinkage of the thickness before and after the dipping was determined by the following formula.

T: Thickness before soaking in solvent t: Thickness after soaking in solvent Galley second: The galley second is a differential pressure of 12.4 mmH ₂ O and 1 square inch (6.45c
m ²⁾ is the time required for air flow of 100 cc, AST
It was measured by the method of MD-726. In addition, the galley second indicates a value measured before and after immersion in the solvent (IPA).

[Example 1, Comparative Examples 1 to 3] To 1000 g of PTFE fine powder F-104 (molecular weight of 4 to 5 million) manufactured by Daikin Industries, Ltd., 230 g of white oil as a liquid lubricant was added and uniformly mixed to obtain a pressure of 50 kg / After pre-pressurizing at cm ² , the paste was extruded by a paste extruder and rolled to form a sheet having a thickness of 0.3 mm. This was immersed in trichlorethylene to extract and remove the liquid lubricant.

Then, heat treatment was carried out in a constant temperature bath at 340 ° C. to prepare four samples having different endothermic peak positions on the DSC chart (note that one sample was not heat treated).

Next, each sample was stretched 500% in the uniaxial direction (longitudinal direction) with a roll-type stretching machine heated to approximately 275 ° C, and 500% in the width direction.
It was stretched. Approximately 500 with this stretched sheet stretched
Sintered by heating at 0 ° C for 1 minute.

The results of measuring the physical properties of each of the obtained stretched samples are shown in Table 1.

As is clear from Table 1, the rolled sheet produced by the method of the present invention has a large bubble point, a small pore size, a large flow rate, and excellent permeability. Moreover, the thermal shrinkage is small and the thermal stability is good. Further, the stretched sheet of the present invention is also very excellent in solvent shrinkage, and can stably filter fluid.

On the other hand, the sheet (Comparative Example 1) obtained by stretching an unsintered sample that has not been heat-treated has insufficient permeability and poor heat resistance. In particular, the solvent shrinkage is as large as 56%, the thickness shrinks to the original 44%, and the dimensional stability is poor. In the sheet (Comparative Example 2) obtained by stretching the sample heat-treated to such an extent that the endothermic peak (near 347 ° C.) of the fine powder was not changed, the bubble point was small and the pore size was relatively large. The heat resistance is also insufficient. The solvent shrinkage rate is as large as 43% and the dimensional stability is poor.

The sheet obtained by stretching the sintered sample (Comparative Example 3) could not be a porous body.

[Example 2] Asahi Glass Co., Ltd. PTFE fine powder CD-123 (molecular weight of about 2
10,000,000) 1000g, 200g of white oil as liquid lubricant
Add to mix evenly, press preform at a pressure of 100 kg / cm ² , extrude with a paste extruder, and roll to 0.3 mm.
It was formed into a thick sheet. This was immersed in trichlorethylene to extract and remove the liquid lubricant.

Then, heat treatment was performed in a constant temperature bath at 400 ° C. to prepare four samples having different endothermic peak positions on the DSC chart. Note that one sample (Comparative Example 4) was not heat-treated.

Next, each sample was stretched 300% in the longitudinal direction and 900% in the width direction with a roll type stretching machine heated to about 300 ° C.

The results of measuring the physical properties of each of the obtained stretched samples are shown in Table 2.

From Table 2 as well, the stretched sheet of the present invention has a large bubble point, a small pore size, a large flow rate, and excellent permeability. In particular, the solvent shrinkage ratio is very excellent and stable with respect to the solvent.

〔The invention's effect〕

According to the present invention, small pore size and high permeability (high porosity)
Thus, it is possible to provide a PTFE porous body having high heat resistance (low heat shrinkage) and dimensional stability (low solvent shrinkage).

The PTFE porous body of the present invention can be used in a wide range of fields as a filter for precision filtration, a support for high-performance membranes, an air filter, and the like.

[Brief description of drawings]

FIG. 1 is a diagram showing endothermic peak positions in a DSC (differential scanning calorimeter) chart of a paste extruded product of PTFE fine powder. Chart (A); endothermic peak of fine powder Chart (B); endothermic peak of heat-treated (unsintered) molded article Chart (C); endothermic peak chart of heat-treated molded article of the present invention (D); of sintered molded article Endothermic peak FIG. 2 is the surface of the porous body of the present invention, and FIG.
Scanning electron micrograph (5000x) of the surface of PTFE porous body
And shows the shape of the PTFE fiber.

Claims (7)

(57) [Claims] Claims: 1. An unsintered molded article obtained by paste extrusion of polytetrafluoroethylene fine powder, the endothermic peak position of the fine powder on the crystal melting diagram by a differential scanning calorimeter, and the endothermic peak position of the sintered article. A method for producing a polytetrafluoroethylene porous body, which comprises subjecting to a heat treatment so that at least one peak is generated between and then stretching in at least one direction. 2. The method for producing a polytetrafluoroethylene porous body according to claim 1, wherein the unsintered molded body is heat-treated and then stretched in at least two directions, and the stretching ratio is 5 times or more in terms of area ratio. 3. The endothermic peak position of the fine powder and the endothermic peak position of the fine powder on the crystal melting diagram by a differential scanning calorimeter of the unsintered compact obtained by paste extrusion of polytetrafluoroethylene fine powder A porous polytetrafluoroethylene body, which is obtained by performing heat treatment so that at least one peak is generated between the two, and then stretching in at least one direction. 4. The polytetrafluoroethylene porous body according to claim 3, which has a structure consisting essentially of polytetrafluoroethylene fibers. 5. IPA bubble point is 1.0kg / cm ² or more, IP
The polytetrafluoroethylene porous body according to claim 3, wherein the A solvent shrinkage is 30% or less and the Galley second is 100 seconds or less. 6. The IPA bubble point is 1.0 to 3.0 kg / cm ² , IPA
The polytetrafluoroethylene porous body according to claim 5, which has a solvent shrinkage of 15% or less and a Galley second of 20 seconds or less. 7. The polytetrafluoroethylene porous body according to claim 5, which has an IPA bubble point of more than 3.0 kg / cm ² , an IPA solvent shrinkage of 30% or less, and a Galley second of 100 seconds or less.