JPH09302121A - Method for producing polytetrafluoroethylene porous membrane - Google Patents

Abstract

(57) [Abstract] [Problem] PTF with high performance and small variation in pressure loss.
(EN) Provided is a production method capable of industrially easily producing a porous E membrane. A liquid lubricant is added to PTFE fine powder and mixed, and the mixture is molded into a sheet in an unfired state by at least one of an extrusion method and a rolling method.
The liquid lubricant is removed by at least one of a heating method and an extraction method. This sheet-shaped molded body is heated at a temperature of 1
Stretched in the longitudinal direction at a stretching ratio of 2 to 30 times at a temperature of 50 ° C. or more and less than 250 ° C., having an endothermic peak in the temperature region of 345 ± 5 ° C. on the crystal melting curve by a differential scanning calorimeter, and crystal conversion Sheet PTFs with a rate of 0.1 to 0.85
An E molded body is manufactured. This is heat-treated with the longitudinal dimension fixed, and then stretched in the width direction to obtain P
A TFE porous membrane is manufactured.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to polytetrafluoroethylene (hereinafter,
The present invention relates to a method for producing a PTFE porous membrane, which enables easy production of a porous membrane (referred to as "PTFE").

[0002]

2. Description of the Related Art PTFE porous membranes are used in various fields as filter media for filters. In particular, an extremely thin PTFE porous membrane having a structure composed of only fibers without substantially large-scale nodules is useful as a filter medium for an air filter used in the field of semiconductor manufacturing or the like that requires a severe cleaning environment. Things.

For such a useful and high-performance PTFE porous membrane, for example, a sheet-shaped PTFE semi-baked body is prepared (Japanese Patent Laid-Open No. 59-152825) and biaxially stretched to make it porous. Can be manufactured by the following method (Japanese Patent Application Laid-Open No. 3-221541 or 5-202217).
Issue).

[0004]

However, in this manufacturing method, it is difficult to set the conditions for realizing the "semi-baked state", which is convenient for the subsequent stretching. Especially, the temperature conditions are very narrow, It was very difficult to achieve such a state industrially. Therefore, the useful and high-performance PTFE porous membrane could not be efficiently produced. In addition, the obtained PTFE porous membrane had a large variation in pressure loss, and there was a problem in quality.

The present invention has been made in view of the above circumstances, and is a high-performance PTFE with a small variation in pressure loss.
An object of the present invention is to provide a production method capable of producing a porous membrane efficiently and industrially.

[0006]

In order to achieve the above object, the method for producing PTFE of the present invention is such that a liquid lubricant is added to PTFE fine powder and mixed, and the mixture is subjected to at least one of an extrusion method and a rolling method. The sheet-shaped molded product was molded into a sheet in an unfired state by one of two methods, and the sheet-shaped molded product was stretched in the longitudinal direction at a temperature condition lower than the melting point of the PTFE sintered product, and 345 ± 5 on the crystal melting curve by a differential scanning calorimeter. It has an endothermic peak in the temperature range of 0 ° C and a crystal conversion rate of 0.1 to 0.
A sheet-shaped PTFE molded body of No. 85 is manufactured, and this is heat-treated under a temperature condition lower than the melting point of the PTFE fired body with the dimension in the longitudinal direction fixed, and then stretched in the width direction.

One of the features of the production method of the present invention is the development of a technique capable of easily producing a sheet-shaped PTFE molded product, which is a semi-finished product of a PTFE porous membrane, under conditions that allow easy setting. That is, in the production method of the present invention, the sheet-shaped PTFE is manufactured by a very simple and easy condition setting step of uniaxially stretching the unfired sheet-shaped molded body in the longitudinal direction.
Manufactures molded products (semi-finished products). Then, this is an extremely simple process of heat-treating this under a temperature condition lower than the melting point of the PTFE fired body in a state where the dimension in the longitudinal direction is fixed, and then stretching in the width direction. It is possible to easily manufacture an extremely thin PTFE porous membrane having a structure composed of only fibers. Another feature is that it is possible to manufacture a high-quality PTFE porous membrane with a small variation in pressure loss by the heat treatment in which the dimension in the longitudinal direction is fixed.

In the method for producing a PTFE porous membrane of the present invention, after being stretched in the width direction, heat treatment (baking) may be carried out under a temperature condition equal to or higher than the melting point of the PTFE fired body. Calcination improves the dimensional stability and strength of the PTFE porous membrane.

From the viewpoint of obtaining the stretchability and the above-mentioned predetermined physical properties, the stretching conditions in the longitudinal direction are preferably a temperature of 150 ° C. or higher and lower than 250 ° C. and a stretching ratio of 2 to 30 times.

Since the variation in pressure loss of the obtained PTFE porous membrane can be reduced more effectively, the temperature of the heat treatment performed with the lengthwise dimension fixed is
It is preferably 270 ° C. or higher and lower than 327 ° C.

Further, the stretching temperature in the width direction is preferably 40 to 100 ° C. in order to reduce the variation in the stretchability and the pressure loss of the obtained PTFE porous membrane.

[0012]

Next, the present invention will be specifically described. In the method for producing a PTFE porous membrane of the present invention, first, a sheet-shaped PTFE molded body is manufactured, and this is heat-treated at a temperature condition lower than the melting point of the PTFE fired body in a state where the dimension in the longitudinal direction is fixed, and then in the width direction. It is a manufacturing method in which the film is stretched.

The above-mentioned sheet-shaped PTFE molded article is PTFE
It is manufactured using fine powder and liquid lubricant. The PTFE fine powder is not particularly limited, and commercially available products can be used. For example, Polyflon F-104 (manufactured by Daikin Industries, Ltd.), Fluon C
Examples include D-123 (manufactured by Asahi / ICI Fluoropolymers) and Teflon 6J (manufactured by Mitsui / Dupont Fluorochemicals).

Further, the liquid lubricant can wet the surface of PTFE, and after the sheet-shaped molded body is obtained, evaporation,
There is no particular limitation as long as it can be removed by a method such as extraction. For example, in addition to liquid paraffin, naphtha, white oil, hydrocarbon oils such as toluene and xylene, alcohols, ketones, esters, and a mixture of two or more of these.

The amount of the liquid lubricant added to the PTFE fine powder is appropriately adjusted depending on the types of the PTFE fine powder and the liquid lubricant, and the molding method for obtaining the sheet-shaped molded product. The liquid lubricant is about 5 to 50 parts by weight based on parts by weight.

Then, using this PTFE fine powder and a liquid lubricant, a sheet-shaped PTFE molded body is obtained.
It is manufactured as follows. That is, first, the PT
Mix FE fine powder and liquid lubricant.

Then, the mixture is formed into a sheet by at least one of the extrusion method and the rolling method.
For example, after the mixture is extruded into a rod shape, a method of forming a rolled sheet by a pair of rolls, a method of extruding a sheet into a sheet and a method of extruding a sheet into a rolled sheet by a roll, etc. Can be given. Such a sheet-shaped molded product usually has a thickness of 0.05 to 0.5.
mm.

The sheet-shaped molded product thus obtained is then stretched to make it porous, but before that, the liquid lubricant is usually removed. This removal
It is carried out by a heating method, an extraction method or a combination thereof.

In this way, a sheet-shaped PTFE molded product having the desired physical properties can be obtained. That is, this sheet-shaped PTFE molded product has an endothermic peak in the temperature region of 345 ± 5 ° C. on the crystal melting curve by a differential scanning calorimeter, has a crystal conversion rate of 0.1 to 0.85, and is porous. And the specific gravity is 1.4 or less.

This sheet-shaped PTFE molded article is a conventional sheet-shaped PTFE semi-baked article (JP-A-59-1) in the following points.
52825). First, it is obtained by stretching below the melting point, has a specific gravity of 1.4 or less, and is formed porous. In addition, it is difficult to measure the crystallinity of this sheet-shaped PTFE molded product. This is the sheet-like PTF of the present invention
This is because the E molded body has a large orientation by stretching and the crystallinity cannot be accurately measured by X-ray diffraction.

The endothermic peak and the crystal conversion rate on the crystal dissolution curve by the differential scanning calorimeter (DSC) are as follows.
As in JP-A-59-152825, it is defined by the peak temperature measured by a differential scanning calorimeter (DSC) and the heat of crystal fusion.

These endothermic peaks and crystal conversion are measured as follows, as in JP-A-59-152825. First, 10.0 ± 0.1 mg of the sheet-shaped PTFE molded product of the present invention is weighed to obtain a cut sample. Since the heat denaturation of PTFE proceeds from the surface of the sheet to the inside, when the sample is collected, the denaturation degree in the sheet thickness direction should be included in average. In the same manner, 10.0 ± 0.1 mg of a sample of the unfired PTFE body is prepared. Then, using these samples, a crystal melting curve is obtained as follows.

That is, a sample of the PTFE unfired body was DS
C, and the heat of fusion of the unfired PTFE body and the heat of fusion of the fired PTFE body are measured by the following steps (1) to (3).

(1) The sample was heated to 277 ° C. at a heating rate of 160 ° C./min and then heated to 27 ° C. at a heating rate of 10 ° C./min.
Heat from 7 ° C to 360 ° C. The position of the endothermic peak appearing in the crystal melting curve recorded in this heating step is defined as "melting point of PTFE unsintered body" or "melting point of PTFE fine powder".

(2) Immediately after heating to 360 ° C., the sample is cooled to 277 ° C. at a cooling rate of 80 ° C./min. (3)
The sample is heated again to 360 ° C at a heating rate of 10 ° C / min. The endothermic peak that appears in this heating step is defined as the "melting point of the PTFE fired body".

The heats of fusion of the PTFE non-fired body and the PTFE fired body are proportional to the area between the endothermic curve and the baseline. The base line is a straight line drawn from the point of 307 ° C. on the DSC chart so as to be in contact with the base at the right end of the endothermic curve.

Next, the crystal melting curve of the sheet-shaped PTFE molded product is recorded according to the above step (1). Then, the crystal conversion rate is calculated by the following formula (Equation 1).

[0028]

(Equation 1)

Crystal conversion rate = (S ₁ −S ₃ ) / (S ₁ −S ₂ ) In the above formula (Equation 1), S ₁ is the area of the endothermic curve of the PTFE unfired body, and S ₂ is the PTFE fired body. The area of the endothermic curve, S ₃ is the area of the endothermic curve of the sheet-shaped PTFE molded product of the present invention.

The crystal conversion of the sheet-shaped PTFE molding of the present invention is 0.1 to 0.85, preferably 0.15.
Is about 0.70. The surface of the sheet-shaped PTFE molded product of the present invention is porous, and this can be examined by, for example, observation with a scanning electron microscope or a method using a marker ink shown below.

That is, first, after the marker ink is applied to the surface of the sheet-shaped PTFE molded body, the surface is wiped off with a cloth impregnated with a solvent such as toluene. If the ink is wiped off, it is nonporous, and if the ink remains, it is porous.

Further, in the present invention, "specific gravity" means "apparent specific gravity" obtained by dividing weight by apparent volume. Next, the sheet-shaped PTFE molded body is heat-treated at a temperature lower than the melting point of the PTFE fired body while fixing the dimension in the longitudinal direction. By this heat treatment, variations in pressure loss of the obtained PTFE porous membrane are suppressed and high quality is obtained. The reason for this is not clear, but because the non-uniformity of the residual stress generated by the stretching in the longitudinal direction in the previous step is eliminated,
The present inventors speculate.

The temperature of the heat treatment is usually 270 ° C. or higher and 3
It is less than 27 ° C. The heat treatment time is preferably longer, but is usually 30 seconds to 5 minutes, preferably 1 to 5 minutes. Then, the target PTFE porous membrane is obtained by stretching the sheet-shaped PTFE molded body after the heat treatment in the width direction.

The stretching temperature is preferably 40 to 100 ° C. The draw ratio is not particularly limited, but is usually about 4-6.
It is twice. In this way, a high-performance and high-quality PTFE porous membrane useful as a filter material for an air filter can be obtained. The thickness of this PTFE porous membrane is usually
It is 0.5 to 300 μm.

This PTFE porous membrane may be further heat-treated (baked) in order to increase strength and obtain dimensional stability. The heat treatment is usually performed by fixing the dimensions above the melting point of the PTFE fired body.

The PTFE porous membrane thus obtained may be inferior in workability such as pleats required as a filter medium when used alone, and therefore other low pressure loss porous materials are usually laminated as a reinforcing material. To be done.

As the reinforcing material, for example, a non-woven fabric, a woven fabric, a mesh, a porous membrane such as a net can be used. As a mode of lamination, a reinforcing material may be laminated on one side or both sides of the PTFE porous membrane according to the present invention, or a reinforcing material may be sandwiched with the PTFE porous membrane according to the present invention. The laminating method can also be selected from known methods such as heat bonding and bonding using an adhesive.

As described above, the production method of the PTFE porous membrane of the present invention is a production method that is very easy in terms of conditions and can be industrially sufficiently implemented, and further, the variation in the pressure loss of the obtained PTFE porous membrane is suppressed. Can be made smaller. The PTFE porous membrane produced by the production method of the present invention is useful as a filter material for an air filter because it has high performance and high quality, but in addition to this, it is also used as a breathing filter for precision electronic equipment such as a hard disk drive. is there.

[0039]

Next, an embodiment will be described. The pressure loss and the collection efficiency in the examples were measured by the methods described below, and the endothermic peak, crystal conversion, surface porosity and specific gravity were measured by the methods described above.

[Pressure Loss] The sample PTFE porous membrane was set in a circular holder having an effective area of 100 cm ² , and
A pressure gauge (manometer) measures the pressure loss when a pressure difference is applied to the inlet side and the outlet side (front side and back side of the PTFE porous membrane) of the holder and the air permeation flow rate of the sample is adjusted to 5.3 cm / sec. It was measured at. The measurement was performed at 50 points for each sample.

[Collection efficiency] Using the same device as the pressure loss measurement, the air permeation flow rate was set to 5.3 cm / sec, and polydisperse dioctyl phthalate (DOP) was added to a particle size of 0.1 to 0.1.
Flow 5 μm particles so that the density is about 10 ⁷ / L, measure the downstream concentration with a particle counter,
The collection efficiency (%) was calculated by the following formula (Equation 2). The measurement is
It performed 50 places for one sample.

[0042]

(Equation 2)

Collection efficiency (%) = {1- (downstream concentration / upstream concentration)} × 100 (however, the particles to be measured are in the range of 0.1 to 0.15 μm) (Example 1) PTFE fine powder (Full on CD
-123, 100 parts by weight of Asahi ICI Fluoropolymers Co.) was uniformly mixed with 30 parts by weight of a liquid lubricant (liquid paraffin), and this mixture was preformed under the condition of 20 kg / cm ² , It is extruded into a rod, and the rod is passed between a pair of metal rolling rolls to form a long film (sheet-like molded body) having a thickness of 0.2 mm.
I got Next, the liquid lubricant was removed from the sheet-shaped molded body by an extraction method using trichlene, and then wound around the tubular core body in a roll shape.

This sheet-shaped molded product was uniaxially stretched in the longitudinal direction by a roll stretching method, and the temperature and the stretching ratio at the time of stretching were changed to prepare five kinds of sheet-shaped PTFE molded products (NO. 5) was produced. The table shows DS
The endothermic peak (° C.) measured at C, the crystal conversion rate and the specific gravity are also shown together.

[0045]

[Table 1]

Next, the NO. 4 sheet PTFE
The molded body (porous, specific gravity 0.48) was subjected to heat treatment for 2 minutes while fixing the dimension in the longitudinal direction and changing the temperature. The heat treatment temperature at this time is shown in Table 2 below.

[0047]

[Table 2]

Next, the NO. 6 sheet P
The TFE molded product was stretched in the width direction at a magnification of 20 times using a tenter, and the temperature was changed in this stretching (3 types).
Then, three kinds of PTFE porous membranes shown in Table 3 below were produced. Then, the pressure loss and the collection efficiency of these PTFE porous membranes were examined. The results are shown in Table 3 below. In the table, the stretching temperature is also shown. Moreover, the pressure loss and the collection efficiency showed the minimum value and the maximum value.

[0049]

[Table 3]

From Table 3 above, NO. The sheet-shaped PTFE molded body of No. 4 was subjected to a predetermined heat treatment with the dimensions fixed in the longitudinal direction (NO. 6), and a predetermined widthwise stretching to obtain the obtained NO. All of the PTFE porous membranes 9 to 11 had low pressure loss, high collection efficiency, and small variations in pressure loss. From these results, it can be said that according to the production method of the present invention, a high-performance PTFE porous membrane can be easily produced industrially, and variations in pressure loss of the obtained PTFE porous membrane can be reduced.

(Example 2) No. 1 in Table 3 above. 10 PTs
The FE porous membrane was heat-treated at 345 ° C. for 15 seconds with the dimensions fixed, to obtain a target PTFE porous membrane. For this, pressure loss and collection efficiency were examined in the same manner as in Example 1.
The results are shown in Table 4 below.

[0052]

[Table 4]

By firing, the strength and dimensional stability of the PTFE porous membrane were improved. (Example 3) The dimension of the molded body (No. 1, 2, 3, 5) in Table 1 in the longitudinal direction was fixed, and heat treatment was performed at 310 ° C. for 2 minutes,
Then, it is stretched 20 times at a temperature of 90 ° C. with a tenter.
Further, the molded product (No. 6, 8) in Table 2 is stretched 20 times in the width direction at a temperature of 90 ° C. by a tenter. As a result, 6 types of PTFE porous membranes were obtained. Table 5 shows the pressure loss and collection efficiency of these porous membranes.

[0054]

[Table 5]

From the results shown in Table 5, all of the six types of PTFE porous membranes showed high collection efficiency, and the pressure loss was in a practically low range.

[0056]

As described above, according to the method for producing a PTFE porous membrane of the present invention, a liquid lubricant is added to PTFE fine powder and mixed, and this mixture is prepared by at least one of an extrusion method and a rolling method. It is molded into a sheet in the fired state, and the sheet-shaped molded body is expanded in the longitudinal direction with PTFE.
It has an endothermic peak in the temperature region of 345 ± 5 ° C. on the crystal melting curve by a differential scanning calorimeter when stretched under a temperature condition lower than the melting point of the fired body, and the crystal conversion rate is 0.1 to 0.85. A sheet-shaped PTFE molded body is produced, and this is heat-treated under a temperature condition below the melting point of the PTFE fired body in a state where the dimension in the longitudinal direction is fixed, and then stretched in the width direction.

That is, according to the manufacturing method of the present invention, the above-mentioned sheet-shaped PTFE molded body (semi-finished product) can be easily manufactured under the condition that the setting is easy, and this is simply stretched in the width direction after the predetermined heat treatment. Thus, a high-performance PTFE porous membrane can be manufactured. Further, in the method for producing PTFE of the present invention, since the dimension in the longitudinal direction is fixed and the heat treatment is performed at a predetermined temperature after the stretching in the longitudinal direction, variations in pressure loss of the obtained PTFE porous membrane are reduced and high quality is obtained. Will be the one. Therefore, according to the method for producing a PTFE porous membrane of the present invention,
A high-performance and high-quality PTFE porous membrane applicable to an air filter medium or the like can be industrially and efficiently produced.

Claims (6)

[Claims] 1. A liquid lubricant is added to and mixed with polytetrafluoroethylene fine powder, and the mixture is molded into a sheet in an unfired state by at least one of an extrusion method and a rolling method. Was stretched in the longitudinal direction at a temperature condition lower than the melting point of the polytetrafluoroethylene calcined product to obtain 34 on the crystal melting curve by a differential scanning calorimeter.
A sheet-like polytetrafluoroethylene molded product having an endothermic peak in the temperature range of 5 ± 5 ° C. and a crystal conversion rate of 0.1 to 0.85, and having a fixed longitudinal dimension. In the method for producing a polytetrafluoroethylene porous film, the polytetrafluoroethylene porous film is heat-treated at a temperature lower than the melting point of the fired polytetrafluoroethylene, and then stretched in the width direction. 2. The method for producing a polytetrafluoroethylene porous film according to claim 1, wherein after the film is stretched in the width direction, heat treatment is performed under a temperature condition equal to or higher than the melting point of the polytetrafluoroethylene fired body. 3. The longitudinal stretching condition is a temperature of 150 ° C.
A draw ratio of 2 to 30 times at 250 ° C. or higher.
Alternatively, the method for producing a polytetrafluoroethylene porous membrane according to 2 above. 4. The polytetrafluoroethylene porous membrane according to any one of claims 1 to 3, wherein the temperature of the heat treatment performed in a state where the lengthwise dimension is fixed is 270 ° C or higher and lower than 327 ° C. Manufacturing method. 5. The method for producing a polytetrafluoroethylene porous film according to claim 1, wherein the liquid lubricant is removed from the sheet-shaped molded product before stretching in the longitudinal direction. 6. The stretching temperature in the width direction is 40 to 100 ° C.
The method for producing a polytetrafluoroethylene porous membrane according to any one of claims 1 to 5.