JP7479680B2 - Waterproof breathable filter and method for manufacturing porous film - Google Patents

Description

The present invention relates to a porous film and a method for manufacturing a porous film.

Waterproof and breathable filters are used in automotive electrical components, office automation equipment, home appliances, medical equipment, and the like to eliminate the pressure difference between the outside and the inside of a housing that houses electronic components and control boards. This waterproof and breathable filter is attached to the housing so as to cover an opening in the housing, ensuring ventilation while providing dust protection and waterproofing. PTFE porous film is often used as a waterproof and breathable filter.

PTFE porous films that are integrated with nonwoven fabric and have fine air holes are available on the market, but heat resistance is sometimes required for waterproof breathable filters in automobile parts and some sensors. Since nonwoven fabric often has problems with heat resistance, there is a demand from the market for films that omit the nonwoven fabric. There is also a demand for a single PTFE porous film that omits the nonwoven fabric and can be directly bonded to a housing or the like.

When adhering a standalone PTFE porous film to a housing or the like, the PTFE porous film is generally laminated by attaching it to an adhesive tape with a release paper made of a PET base material, and then the laminate is punched out into the desired shape and the porous film is peeled off from the release paper for use. However, a standalone PTFE porous film that is not integrated with a nonwoven fabric has the problem of lacking stiffness and poor handling.

JP 2018-204006 A JP 2011-177986 A JP 2019-041407 A

The present invention aims to provide a porous film that is stiff and has excellent breathability, and a method for manufacturing the porous film.

According to a first aspect of the present invention, there is provided a waterproof breathable filter made of a single porous film containing polytetrafluoroethylene , the bending resistance of the porous film being within the range of 0.5 mN cm to 20 mN cm, and the Gurley air permeability of the porous film being within the range of 1 sec to 100 sec.

According to a second aspect of the present invention, a method for producing a porous film is provided, which includes preparing an unsintered polytetrafluoroethylene tape, uniaxially stretching the polytetrafluoroethylene tape to obtain an unsintered polytetrafluoroethylene sheet, passing the unsintered polytetrafluoroethylene sheet between a first roll pair and a second roll pair while heat-treating the polytetrafluoroethylene sheet at 50°C-330°C to shrink the polytetrafluoroethylene sheet, and sintering the polytetrafluoroethylene sheet after shrinkage in an environment of 340°C-420°C, in which the rotation speed of the first roll pair is higher than that of the second roll.

The present invention makes it possible to provide a porous film that is stiff and has excellent breathability, as well as a method for manufacturing the porous film.

FIG. 1 is a cross-sectional view illustrating an example of a porous film according to an embodiment.

Conventionally, in order to obtain a PTFE porous film with strong stiffness, for example, it is known to increase the thickness of the porous film. In general, a thick unsintered PTFE sheet is stretched only in one axial direction to make it porous, thereby obtaining a porous film with a large thickness. However, increasing the thickness of the porous film tends to reduce the breathability. A porous film with reduced breathability may not be suitable as a waterproof breathable filter. Therefore, it is possible to increase the stretching ratio to improve breathability, but this will reduce the film thickness and stiffness. In other words, there is a problem in that it is difficult to achieve both stiffness and excellent breathability in a porous film.

The inventors have succeeded in obtaining a porous film that is strong and has excellent breathability by stretching an unsintered PTFE sheet, shrinking the porous film as a step prior to firing (sintering), and then firing the film. A detailed method for producing the porous film according to the embodiment will be described later.

The porous film according to the embodiment contains polytetrafluoroethylene (PTFE), has a bending resistance in the range of 0.5 mN·cm-20 mN·cm, and a Gurley air permeability in the range of 1 second-100 seconds. A porous film whose bending resistance is within the above numerical range and whose Gurley air permeability is also within the above numerical range has a strong stiffness and excellent breathability.

The porous film may consist essentially of PTFE. The porous film may contain at least one additive selected from the group consisting of a liquid repellent material, a hydrophilic material, a conductive material, a coloring material, an antistatic material, and an antibacterial material.

The stiffness of a porous film can be evaluated by the bending resistance of the film. The bending resistance of the porous film is preferably in the range of 0.7 mN·cm-20 mN·cm. A porous film with a bending resistance in the range of 0.5 mN·cm-20 mN·cm has excellent handling properties. For example, when peeling the porous film from an adhesive tape applied as a support, the porous film does not follow the support, so there is an advantage that it can be easily peeled off. The upper limit of the bending resistance may be 10 mN·cm, 5 mN·cm, or 3 mN·cm.

The bending resistance of the porous film can be measured in accordance with JIS L 1913:2010. The bending resistance can be measured, for example, by using a cantilever type flexibility tester manufactured by Daiei Kagaku Seiki Seisakusho Co., Ltd., or a device having equivalent functions.

The breathability of a porous film can be evaluated by the level of its Gurley air permeability. The Gurley air permeability of a porous film is preferably in the range of 1 second to 50 seconds. If the Gurley air permeability is within this range, a higher internal pressure adjustment function can be obtained. As an example, in applications where the porous film is required to be waterproof, a lower Gurley air permeability value is preferable as long as sufficient waterproofness is maintained. When a porous film is used for internal pressure adjustment purposes, the Gurley air permeability is preferably 50 seconds or less. The Gurley air permeability may be 10 seconds or less.

The Gurley air permeability of a porous film can be measured in accordance with JIS P 8117:2009. The device used to measure the Gurley air permeability can be, for example, an automatic Gurley densometer manufactured by Yasuda Seiki Seisakusho Co., Ltd., or a device with equivalent functions.

The manufacturing method of the porous film according to the embodiment will be described later, but the porous film produced by the method including stretching an unsintered PTFE sheet, shrinking it, and then firing it is characterized by being strong even though it is thin in thickness. When a shear force is applied to an unsintered PTFE sheet by stretching, multiple fibrils are pulled out from multiple nodes (knots) to form a network structure. In other words, in the PTFE sheet after stretching, multiple nodes are connected by multiple fibrils.

Then, by subjecting the stretched PTFE sheet to a shrinking process, the distance between the multiple nodes can be reduced without a significant decrease in breathability. As a result, a porous film with a stronger stiffness can be obtained, even at a similar film thickness, compared to when the PTFE sheet is not shrunk.

The thickness of the porous film according to the embodiment is, for example, in the range of 50 μm to 500 μm, and preferably in the range of 150 μm to 400 μm. If the thickness is within this range, it is possible to achieve both breathability and stiffness.

The porous film according to the embodiment can be used, for example, in applications (internal pressure adjustment) to adjust internal pressure fluctuations caused by temperature changes, etc., inside various housings of electronic devices, etc.

Figure 1 is a cross-sectional view showing an example of a porous film according to this embodiment.

Figure 1 shows a porous film 1. The porous film 1 has a mesh structure composed of nodes and fibrils. The porous film 1 includes numerous air holes 2 that exist as gaps in the mesh structure. Although Figure 1 shows a case where the air holes 2 are independent pores, the air holes 2 mainly exist as interconnected pores. The air holes 2 may include independent pores.

Next, an example of a method for producing a porous film according to an embodiment will be described.

A fine powder containing PTFE resin is prepared, and 100 parts by mass of this powder is mixed with, for example, 20-30 parts by mass of hydrocarbon oil as an auxiliary. For example, fine powder with an average particle size of 250 μm-800 μm can be used. The average particle size refers to D50, which is the particle size that corresponds to 50% of the cumulative number of particles in the particle size distribution measured based on the laser diffraction scattering method.

After stirring the mixture until it becomes homogeneous, the paste is extruded and preformed, for example, into a rod shape. The resulting preform is rolled, for example, using a metal rolling roll. The auxiliary agent is then removed to obtain an unsintered PTFE tape. The thickness of the unsintered PTFE tape is, for example, in the range of 50 μm to 600 μm.

In this specification and claims, "unbaked" means that baking has not been performed in a temperature environment above the melting point of the PTFE resin.

Unsintered PTFE tape is uniaxially stretched in the direction in which it was previously rolled, i.e., in the machine direction (MD), at a stretch ratio of 1.5 to 30 times in a temperature environment below the melting point to obtain an unsintered PTFE sheet. If the stretch ratio is lowered, the bending resistance of the resulting porous film tends to be higher and the Gurley air permeability value tends to be higher. On the other hand, if the stretch ratio is higher, the bending resistance of the resulting porous film tends to be lower and the Gurley air permeability value tends to be lower. The temperature during uniaxial stretching is, for example, in the range of 50°C to 320°C. Stretching at a temperature below the melting point draws out fibrils between the nodes. As a result, a mesh structure is formed in the PTFE tape, and a porous, unsintered PTFE sheet can be obtained. The thickness of the resulting PTFE sheet is, for example, in the range of 50 μm to 500 μm.

As long as the bending resistance and Gurley air permeability are within the above-mentioned ranges, additional stretching in the TD direction perpendicular to the MD direction may be performed. However, if stretching is performed in two axial directions, that is, in the MD and TD directions, the thickness may become too small, resulting in a loss of stiffness. Therefore, in general, when producing a porous film to be used in applications requiring stiffness, it is preferable to stretch only uniaxially.

Here, if the process following uniaxial stretching involves baking at a temperature above the melting point, the final porous film tends to have a low bending resistance, i.e., a low stiffness, which is not preferred.

To obtain the porous film according to the embodiment, a shrinking step is carried out to shrink the PTFE sheet as a step following the uniaxial stretching. The shrinking step is carried out, for example, by passing the unsintered PTFE sheet between two pairs of rolls. Both of the two pairs of rolls can be heat rolls. The unsintered PTFE sheet passes through the first heat roll pair and then the second heat roll pair. At this time, the PTFE sheet passing between the first heat roll pair and the second heat roll pair is slackened. In other words, the tension applied to the PTFE sheet passing between the two heat roll pairs is set to be weaker than when the PTFE sheet does not slacken. This setting can be achieved, for example, by making the rotation speed of the first heat roll pair higher than that of the second heat roll pair. By increasing the rotation speed of the first heat roll pair compared to the rotation speed of the second heat roll pair, the delivery speed of the PTFE sheet from the first heat roll pair can be increased compared to the recovery speed of the PTFE sheet by the second heat roll pair.

In this state, the PTFE sheet is subjected to a heat treatment for 0.1 to 300 seconds in an environment of, for example, 50°C to 330°C, to perform the shrinking process. The shrinking process may be performed by heat treating the unsintered PTFE sheet while it is left stationary. The shrinking process releases the stress in the PTFE sheet caused by uniaxial stretching, and the distance between the nodes is reduced. As a result, the mesh structure that existed in a sparse state in the PTFE sheet can be made into a relatively dense mesh structure. This can increase the bending resistance of the finally obtained porous film. On the other hand, the air permeability of the PTFE sheet is not significantly reduced. In other words, the Gurley air permeability is not excessively increased. The thickness of the PTFE sheet after the shrinking process is, for example, in the range of 50 μm to 500 μm.

The PTFE sheet that has been subjected to the shrinking process is then baked at a temperature equal to or higher than its melting point. This baking is carried out, for example, in an environment of 350°C-420°C for 0.1-600 seconds. This baking sinters the PTFE resin, resulting in a porous film. Note that even if the shrinking process is carried out on the sintered porous film, the distance between the nodes hardly changes, so it is difficult to process the sintered porous film to strengthen the stiffness of the porous film itself.

[Example]
Examples will be described below, but the embodiments are not limited to the examples described below.

Example 1
100 parts by mass of PTFE fine powder with an average particle size of 500 μm was uniformly mixed with 25 parts by mass of hydrocarbon oil as an auxiliary agent. The mixture was paste-extruded and preformed into a rod shape. The preform was passed through a pair of metal rolling rolls and further dried to remove the auxiliary agent, and a rectangular unsintered tape with a thickness of 350 μm and a width of 200 mm was obtained.

Next, this unsintered tape was uniaxially stretched at a temperature of 250°C in the direction in which it was previously rolled, i.e., the MD direction, by a factor of 5 using a roll stretching machine. The thickness of the PTFE sheet obtained after uniaxial stretching was 340 μm.

After that, the shrinking process was carried out as described below. In the shrinking process, two pairs of heat rolls were prepared, and the PTFE sheet obtained by uniaxial stretching was transported so that it passed between these pairs of heat rolls. At this time, the PTFE sheet was transported in a state in which it was deflected between the two pairs of heat rolls. Both pairs of heat rolls were set to a temperature of 250°C, and the PTFE sheet was transported so that it passed through these pairs of heat rolls in 30 seconds, completing the shrinking process. The thickness of the PTFE sheet after the shrinking process was 350 μm.

Then, the shrunk PTFE sheet was baked at a temperature of 380°C for 30 seconds to obtain the porous film of Example 1. The thickness of this porous film was 320 μm.

The bending resistance of the porous film of Example 1 was measured according to JIS L 1913:2010 and was 0.7 mN cm. In addition, the Gurley air permeability of the porous film was measured according to JIS P 8117:2009 and was 2.4 seconds.

(Comparative Example 1)
Except for omitting the shrinking step, a porous film was produced in the same manner as in Example 1. The thickness of this porous film was 320 μm.

The bending resistance of the porous film of Comparative Example 1 was measured according to JIS L 1913:2010 and found to be 0.2 mN cm. In addition, the Gurley air permeability of the porous film was measured according to JIS P 8117:2009 and found to be 2.2 seconds.

Comparing the porous film of Example 1 with the porous film of Comparative Example 1, it can be seen that the thicknesses are almost the same and there is not much change in the Gurley air permeability, but the bending resistance of Example 1 is significantly improved. This is thought to be because the mesh structure of the porous film is tightened by the shrinkage process, shortening the distance between the nodes. In other words, it can be seen that Example 1 provides a porous film that is both stiff and has excellent breathability.

The present invention is not limited to the above-mentioned embodiment, and can be modified in various ways in the implementation stage without departing from the gist of the invention. In addition, each embodiment may be implemented in combination as appropriate as possible, and in that case, the combined effect can be obtained. Furthermore, the above-mentioned embodiment includes inventions at various stages, and various inventions can be extracted by appropriate combinations of the disclosed multiple constituent elements. For example, even if some constituent elements are deleted from all the constituent elements shown in the embodiment, if the problem described in the "Problem to be Solved by the Invention" column can be solved and the effect described in the "Effect of the Invention" column can be obtained, the configuration from which the constituent elements are deleted can be extracted as the invention.
The invention as originally claimed in the present application is set forth below.
[1] Contains polytetrafluoroethylene,
The bending resistance is within the range of 0.5 mN cm to 20 mN cm,
A porous film having a Gurley air permeability in the range of 1 sec to 100 sec.
[2] The porous film according to [1], having a thickness in the range of 50 μm to 500 μm.
[3] Preparing an unsintered polytetrafluoroethylene tape;
uniaxially stretching the polytetrafluoroethylene tape to obtain an unsintered polytetrafluoroethylene sheet;
The unsintered polytetrafluoroethylene sheet is passed between a first roll pair and a second roll pair and heat-treated at 50° C.-330° C. to shrink the polytetrafluoroethylene sheet;
The polytetrafluoroethylene sheet after shrinkage is baked in an environment of 340°C to 440°C.
Including,
A method for producing a porous film, wherein the rotation speed of the first roll pair is higher than the rotation speed of the second roll pair.
[4] The method for producing a porous film according to [3], wherein the uniaxial stretching is carried out at a stretching ratio of 1.5 to 30.

1...porous film, 2...ventilation hole.

Claims (4)

A waterproof breathable filter made of a porous film containing polytetrafluoroethylene,
The bending resistance of the porous film is within the range of 0.5 mN cm to 20 mN cm;
The waterproof breathable filter has a Gurley air permeability of the porous film within the range of 1 sec to 100 sec. 2. The waterproof air-permeable filter according to claim 1, wherein the porous film has a thickness in the range of 50 μm to 500 μm. Providing an unsintered polytetrafluoroethylene tape;
uniaxially stretching the polytetrafluoroethylene tape to obtain an unsintered polytetrafluoroethylene sheet;
The unsintered polytetrafluoroethylene sheet is passed between a first roll pair and a second roll pair and heat-treated at 50° C.-330° C. to shrink the polytetrafluoroethylene sheet;
and baking the polytetrafluoroethylene sheet after shrinkage in an environment of 340°C to 440°C.
A method for producing a porous film, wherein the rotation speed of the first roll pair is higher than the rotation speed of the second roll pair. The method for producing a porous film according to claim 3, wherein the uniaxial stretching is performed at a stretching ratio of 1.5 to 30 times.