JPH1180397A - Polytetrafluoroethylene porous film and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a polytetrafluoroethylene (hereinafter referred to as PTFE) porous film having both high air permeability and moderate water resistance by improving a method for orienting the PTFE sheet. SOLUTION: An unbaked PTFE sheet in a continuous length is preoriented at a lower temperature than the melting point of the PTFE in the longitudinal direction of the sheet at 2-20 times orientation ratio. The preoriented PTFE sheet is then oriented at a temperature of the melting point or above in the same direction as that in the preorientation so as to provide 192-216 times total orientation ratio and simultaneously baked. The baked PTFE sheet is then oriented in the width direction at 3-6 times orientation ratio to thereby afford a PTFE porous film having 180-380 cm/sec air permeability (Frazier number) and >=10 cm water resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a polytetrafluoroethylene porous membrane suitable as a filter medium and a method for producing the same.

[0002]

2. Description of the Related Art Polytetrafluoroethylene (hereinafter, referred to as polytetrafluoroethylene)
“PTFE”. ) Is excellent in heat resistance, water repellency, release property, solvent resistance, etc., and its porous membrane is used as a filter, etc.
It is used in various fields, including the field of semiconductor manufacturing where a severe clean environment is required. When a PTFE porous membrane is used as a filter medium, if the permeability of the membrane is low, a load is applied to the filter portion, and the loss (pressure loss) of the filtration medium flow rate increases. In order to compensate for this loss, it is necessary to increase the driving power at the time of filtration and to take a means for forcibly sending the filtration medium into the filter. As a result, energy consumption may be increased and the filter may be damaged. And other problems.

As another application of the porous PTFE membrane, there is a waterproof / dustproof / ventilating material used for an automobile lamp, a mobile communication device, and the like. A large ventilation area is required for inflow and outflow in a certain period of time, which hinders the design of the housing.

[0004] In view of the above, there is a demand for a PTFE porous membrane having as high air permeability as possible.

[0005]

However, the conventional PTF
The permeability of the E porous membrane is 177.8 cm / Fragile number.
Since the second (Japanese Patent Publication No. 8-32791) was the limit, it was difficult to sufficiently reduce the pressure loss during filtration when used as a filter. In addition, PTF
When the E porous membrane is used as a waterproof breathable material, it is required to have high permeability and simple waterproofness. However, the breathability and the waterproofness tend to be incompatible with each other. It was difficult.

[0006] It is an object of the present invention to provide a PTFE porous membrane having both high air permeability and moderate waterproofness, and a method for producing such a PTFE porous membrane.

[0007]

In order to achieve the above object, the porous PTFE membrane of the present invention has a Frazier number of 18
0 to 380 cm / sec, and the water resistance is 10 cm or more. PTFE having such a configuration
The porous membrane has an advantage that, when used as a filtration filter, filtration can be rapidly performed with small energy because of its excellent air permeability. In addition, since it has moderate water resistance in addition to high air permeability, it is also useful as a waterproof ventilation material. The number of the Frazier is determined by JIS L
It is a value measured by 1096 (Fragile tester method), and the water resistance is a value measured by JIS L1092 (low water pressure method).

[0008] Further, in the PTFE porous membrane,
Preferably, the PF value is 20 or more. The PF value
Is a scale for evaluating the characteristics of filter media.
Air through the porous PTFE membrane at a flow rate of 5.3 cm / sec.
Pressure loss L [mmH _TwoO] and a particle diameter of 0.
From the collection efficiency of aerosol of 3 to 0.5 μm E [%]
This is a value calculated by the following equation (1). However,
The collection efficiency is determined by a method according to JIS K3803.
The value to be measured.

PF value = −log (1−E / 100) / L × 100 (1)

[0010] The PTFE porous membrane having such a structure is composed of P
Since the F value is large, that is, the particle collection efficiency is high and the pressure loss is small, it is particularly suitable as a filter medium.

[0011] The filter medium comprising the porous PTFE membrane of the present invention in which a reinforcing material having air permeability is laminated on at least one surface has a small pressure loss at the time of filtration, and thus has excellent filtration efficiency. In addition, in order to realize more excellent filtration efficiency,
In the above-mentioned filter media, air is passed at a flow rate of 5.3 cm.
/0.5mm pressure loss when permeating per second
It is preferably H ₂ O and the PF value is 20 or more.

In order to achieve the above object, a method for producing a porous PTFE membrane according to the present invention comprises the steps of: preliminarily stretching a green sheet of PTFE at a temperature lower than the melting point of PTFE; Stretching in the same direction as the total stretching ratio is 192 to 216 times,
Stretching the film in a direction perpendicular to the above direction so that the stretching ratio becomes 3 to 6 times.

[0013] With such a structure, that is, by performing preliminary stretching at a temperature lower than the melting point in advance and then stretching at a temperature higher than the melting point, stretching at a high magnification becomes possible.
The air permeability of the porous membrane can be significantly improved. In addition, since the baking of PTFE proceeds at the same time as the stretching is performed at a temperature equal to or higher than the melting point, a PTFE porous membrane having high mechanical strength can be efficiently produced. Further, the stretching ratio is set to 192 to 216 times in the direction of pre-stretching, and 3 to 6 times in the direction orthogonal to the direction of pre-stretching. If it exceeds, the water resistance is remarkably reduced. By setting such a stretching ratio, high air permeability can be realized while securing appropriate waterproofness. In addition, the total stretching ratio is
It is a stretching ratio represented by the product of the ratio of the preliminary stretching and the ratio of the stretching after the preliminary stretching.

In the method for producing a porous PTFE membrane, the preliminary stretching ratio is preferably 2 to 20 times.
By adopting such a configuration, in the subsequent stretching,
Stretching to a desired magnification can be reliably performed without fear of breaking the sheet.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The unfired PTFE sheet used in the present invention is obtained by mixing unfired PTFE powder and an extrusion aid,
This mixture is mixed with at least one selected from extrusion and rolling.
It is obtained by molding into a sheet by two means.

The PTFE powder that can be used is not particularly limited, but PTFE fine powder is preferably used. As the PTFE fine powder, commercially available products such as Polyflon F-104 (manufactured by Daikin Industries, Ltd.) can be used.

The extrusion aid is not particularly limited as long as it can wet the PTFE surface and can be removed by a method such as evaporation or extraction after PTFE molding. Specifically, liquid paraffin, naphtha, hydrocarbon oils such as white oil, toluene, aromatic hydrocarbons such as xylene,
Alcohols, ketones, esters, and mixtures of two or more selected from these can be used. The addition amount of the extrusion aid is suitably from 20 to 30 parts by weight, preferably from 22 to 30 parts by weight, per 100 parts by weight of the PTFE powder.
27 parts by weight.

The above-mentioned PTFE powder and the above-mentioned extrusion aid are mixed, and this mixture is extruded by a paste extruder or rolled by a calender roll or the like, or formed into a sheet by a method combining the both, and the PTFE is not calcined. Get a sheet. The film thickness of this green sheet is 0.2 to
0.7 mm is appropriate, and preferably 0.2 mm.
To 0.5 mm.

Next, the unfired PTFE sheet is stretched and made porous, and it is preferable to remove the extrusion aid by a heating method, an extraction method, or a combination thereof before stretching.

The stretching is performed in two axial directions as described below. First, preliminary stretching is performed. By this pre-stretching, a high stretching ratio can be realized without fear of breaking the sheet in the subsequent main stretching. The pre-stretching is performed at a temperature lower than the melting point of PTFE (about 327 ° C), preferably at 120 to 320 ° C. Stretching can be performed by a conventional method such as roll stretching or tenter stretching. Pre-stretching is usually performed in the sheet forming (extrusion) direction, that is, in the longitudinal direction of a long sheet, and the stretching ratio is 2 to 2.
Make it 0 times. If it is less than 2 times, the sheet may be broken before reaching a desired stretching ratio in the subsequent stretching,
If it exceeds 0 times, the sheet may break or tear during pre-stretching. The pre-stretched sheet usually has a thickness of 0.1 to 0.5 mm, preferably 0.1 to 0.2 mm, and a porosity of 40 to 85%, preferably 50 to 65%.

Next, in order to enlarge the pores formed by the preliminary stretching, the film is stretched in the same direction as the preliminary stretching at a temperature not lower than the melting point of PTFE. The stretching temperature is a temperature higher than the melting point, that is, 32
The temperature is not particularly limited as long as it is 7 ° C. or higher,
~ 380 ° C is preferred. This stretching can also be performed by roll stretching, tenter stretching, or the like, but it is preferable to stretch the sheet by arranging both ends in the stretching direction of the sheet outside the heating region. The total draw ratio in the longitudinal direction of the sheet is 19
Adjust so that it becomes 2 to 216 times. Total stretch ratio of 19
If it is less than 2 times, the expansion of the pores is insufficient and the desired air permeability cannot be obtained, and if it exceeds 216 times, the sheet breaks or pinholes are generated, and the water resistance is significantly reduced.

The stretching at this high magnification increases the porosity and significantly improves the permeability of the membrane. In addition, the stretching is P
Since the sintering is performed at a temperature equal to or higher than the melting point of TFE, sintering of PTFE proceeds simultaneously, and the mechanical strength of the sheet is improved. The PTFE fired sheet obtained by this operation has a thickness of usually 0.005 to 0.02 mm, preferably 0.005 to 0.02 mm.
The porosity is 90 to 98%, preferably 95 to 98%.

In order to further expand the pores, the fired PTFE sheet is stretched in a direction perpendicular to the direction of the preliminary stretching, usually in the width direction of the long sheet. In the stretching in the width direction, a conventional stretching method can be adopted. Stretching in the width direction is usually performed at a temperature lower than the melting point, preferably 20 to 15
It is carried out at 0 ° C, more preferably at 110 to 140 ° C. The stretching ratio is 3 times or more, preferably 3 to 6 times, more preferably 4 times.
Up to 6 times. If it is less than 3 times, sufficient air permeability cannot be obtained,
On the other hand, if it exceeds 6 times, the sheet may be broken or a pinhole may be generated. Further, in order to surely enlarge the pores, it is preferable to perform stretching in the width direction while regulating the size in the longitudinal direction.

By the above-described manufacturing method, a porous PTFE membrane having an air permeability (Fragile number) of 180 to 380 cm / sec and a water resistance of 10 to 35 cm can be produced. Of particular note is the 180cm Fragile number
Per second / second or more.
When a TFE porous membrane was used as a filter medium,
Energy consumption is small and filtration efficiency is improved. In addition, a value of 10 cm or more is secured for the water resistance,
It is also useful as a waterproof ventilation material.

The porous membrane preferably has a pressure loss of 0.3 to 1.0 mmH ₂ O, a collection efficiency of an aerosol having a particle diameter of 0.3 μm of 30 to 60%, and a PF value of 20%. That is all. The thickness is 0.002 to 0.01 m
m, the porosity is preferably 95 to 99%.

When this porous PTFE membrane is used as a filter medium, it is laminated with a reinforcing material having air permeability in order to improve handleability and mechanical strength. At this time, the reinforcing material only needs to be laminated on at least one surface of the PTFE porous membrane, and the number of layers of the reinforcing material and the PTFE porous membrane is not particularly limited.

The reinforcing material is not particularly limited as long as it has air permeability, and nets, nonwoven fabrics, woven fabrics and other porous membranes can be used. Also,
The reinforcing material has a thickness of 0.1 to 0.3 mm and a pressure loss of 0.2 to
0.5mmH ₂ O, is preferably a basis weight of 10 to 100 g / m ^2.

As the material of the reinforcing material, for example, polyethylene, polypropylene, polyethylene terephthalate,
Examples thereof include polyamide, polyimide, fluororesin, and glass, and these can be used alone or in combination. In particular, it is preferable to use a composite material composed of two or more components having different melting points.
It is preferable to use a laminate of the above components or a core-sheath composite fiber composed of a component having a higher melting point than the sheath component. With such a composite material,
Shrinkage due to heat during lamination with the PTFE porous membrane is unlikely to occur.

The method of laminating the porous PTFE membrane and the reinforcing material is not particularly limited. However, when the reinforcing material is a thermoplastic material, the melting point is not lower than the melting point when the reinforcing material is a thermoplastic material. A method of heat welding at a temperature or the like can be employed.

Since such a filter medium is composed of a porous PTFE porous membrane as a component, the pressure loss is small and the filtration efficiency is excellent. The filter medium preferably has a pressure loss of 0.5 to 1.5 mmH.
_It has characteristics of ₂ O and PF value of 20 or more.

[0031]

The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

The number of Frazier, water resistance and collection efficiency of the porous PTFE membrane and the filter medium prepared in the following Examples and Comparative Examples were measured according to the above JIS, and the pressure loss was measured according to the method described in JIS B 9927. so,
The wind speed was measured at 5.3 cm / sec. Further, the PF value was calculated from the measured collection efficiency and pressure loss according to the formula (1).

(Example 1) 20 parts by weight of a hydrocarbon oil (trade name "Isoper M", manufactured by Esso Petroleum) was added to 100 parts by weight of PTFE fine powder (trade name "Polyflon F-104U", manufactured by Daikin Industries). The uniformly mixed mixture was formed into a tape by paste extrusion, and the formed body was passed between a pair of metal rolling rolls to form a film having a thickness of 0.2 mm.
mm and a width of 370 mm were obtained. This green tape was pre-drawn at 320 ° C. twice in the longitudinal direction of the tape. Next, the pre-stretched unfired tape was heated to 380 ° C.
In the heated region, the product of the preliminary stretching and the stretching ratio is 2
The tape was stretched in the longitudinal direction of the tape at a stretch ratio of 16 times to obtain a baked uniaxially stretched tape. Next, the uniaxially stretched tape was stretched four times in the width direction of the tape at a temperature of 130 ° C. using a tenter to obtain a PTFE porous membrane.

(Example 2) A porous PTFE membrane was obtained in the same manner as in Example 1 except that the stretching in the width direction of the tape was performed at 25 ° C so that the stretching ratio was 3 times.

Comparative Example 1 An unfired PTFE tape obtained by the same material and operation as in Example 1 was pre-drawn at 100 ° C. 2.4 times in the width direction of the tape. The pre-stretched green tape was stretched 19.6 times in the longitudinal direction of the tape in a region heated to 250 ° C. Next, the tape was stretched 7.4 times in the width direction of the tape, and then heated and fired at 360 ° C. while restraining the obtained biaxially stretched tape to obtain a porous PTFE membrane.

The P obtained in Examples 1 and 2 and Comparative Example 1
For the TFE porous membrane, the thickness, the number of Frazier, the water resistance, the collection efficiency of a 0.3 μm aerosol, and the pressure loss were measured. Further, the PF value was determined from the collection efficiency and the pressure loss. Table 1 shows the results.

[0037]

[Table 1]

Example 3 In the same manner as in Example 1 except that the stretching ratio was changed, the 19 types of PTs shown in Table 2 below were used.
FE porous membranes (Nos. 1 to 19) were prepared, and the number of Frazier, water resistance, collection efficiency of 0.3 μm aerosol, and pressure loss were measured. Further, the PF value was determined from the collection efficiency and the pressure loss. Table 2 shows the results.

[0039]

[Table 2]

As is clear from the results shown in Table 2, when the total stretching ratio in the longitudinal direction is in the range of 192 to 216 times and the stretching ratio in the width direction is in the range of 3 to 6 times, the Frazier number and the water resistance of the present invention are reduced. It was in the range and it was confirmed that it was preferable.

(Example 4) A nonwoven fabric A having a basis weight of 15 g / m ² made of a fiber having a core-sheath structure in which a sheath made of polyethylene is coated on a core made of polyethylene terephthalate, and a basis 2 made of polyethylene terephthalate fiber
5 g / m ² of nonwoven fabric B at a temperature of 150 ° C. and a laminating speed of 5
m / min and a pressure of 4 kg / cm ² , and a thickness of 25
A reinforcing material having 0 μm and a pressure loss of 0.3 mmH ₂ O was obtained. Next, the P obtained in Example 1 was placed on the nonwoven fabric A side of the reinforcing material.
The TFE porous membranes were overlaid and laminated under the conditions of a temperature of 130 ° C., a laminating speed of 4 m / min, and a pressure of 4 kg / cm ² to obtain a filter medium.

Example 5 A filter medium was obtained in the same manner as in Example 4 except that the porous PTFE membrane obtained in Example 2 was used.

Comparative Example 2 A filter medium was obtained in the same manner as in Example 4 except that the porous PTFE membrane obtained in Comparative Example 1 was used.

With respect to the filter media obtained in Examples 4 and 5 and Comparative Example 2, the thickness, pressure loss, 0.3 to 0.
The collection efficiency and water resistance of an aerosol of 5 μm were measured. Further, a PF value was obtained from the pressure loss and the collection efficiency. Table 3 shows the results.

[0045]

[Table 3]

[0046]

As described above, the porous PTFE membrane of the present invention has a high air permeability of 180 to 380 cm / sec of Frazier number and a high air permeability of 10 to 380 cm / sec.
cm and more than water resistance, it is not only useful as a filtration filter that can perform filtration quickly with small pressure loss and small operating energy,
It is also useful as a waterproof ventilation material.

Further, according to the present invention, after the green sheet of PTFE is pre-stretched at a temperature lower than the melting point of PTFE, the total stretching ratio is 192 to 216 times in the same direction as the pre-stretching at a temperature higher than the melting point. Stretching to become
By including the step of stretching 3 to 6 times in the direction orthogonal to the above direction, a PTFE porous membrane having both high air permeability and appropriate water resistance as described above can be obtained.
In addition, since calcination and stretching are performed at the same time, a PTFE film having high mechanical strength can be obtained efficiently.

Claims (6)

[Claims] 1. A polytetrafluoroethylene porous membrane having a Frazier number of 180 to 380 cm / sec and a water resistance of 10 cm or more. 2. The polytetrafluoroethylene porous membrane according to claim 1, which has a PF value of 20 or more. Where P
The F value is defined as a pressure loss L [m when air is passed through a polytetrafluoroethylene porous membrane at a flow rate of 5.3 cm / sec.
mH ₂ O] and the collection efficiency E [%] of the aerosol having a particle size of 0.3 to 0.5 μm. PF value = −log (1-E / 100) / L × 100 3. A filter medium characterized by laminating a permeable reinforcing material on at least one surface of the polytetrafluoroethylene porous membrane according to claim 1. 4. The method according to claim 3, wherein the pressure loss when passing air at a flow rate of 5.3 cm / sec is 0.5 to 1.5 mmH ₂ O and the PF value is 20 or more. Filter media. 5. An unsintered sheet of polytetrafluoroethylene is pre-stretched at a temperature lower than the melting point of polytetrafluoroethylene, and then has a total stretching ratio of 192 to 192 in the same direction as the direction of pre-stretching at a temperature not lower than the melting point. A method for producing a porous polytetrafluoroethylene membrane, comprising: a step of stretching to 216 times; and a step of stretching to a stretching ratio of 3 to 6 times in a direction orthogonal to the above-mentioned direction. 6. The method for producing a polytetrafluoroethylene porous membrane according to claim 5, wherein the preliminary stretching magnification is 2 to 20 times.