Air-permeable membrane and microphone

Abstract

To provide an air-permeable membrane having high heat resistance, high water pressure resistance and sufficient air permeability.SOLUTION: A gas-permeable membrane 1 according to an embodiment includes a polytetrafluoroethylene vesicular film 2 and a fluororesin-containing gas-permeable member layer 5 laminated on the polytetrafluoroethylene vesicular film 2. The gas-permeable membrane 1 has a water entry pressure of 50kPa or higher after being subjected to a heat treatment at a temperature of 260 °C for 10 minutes.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a breathable membrane and a microphone. [Background technology]

[0002] The housings of smartphones, mobile phones, and the like have openings for acousto-electrical transducers such as microphones and speakers. Such openings are often fitted with water-impermeable breathable membranes to prevent water and dust from entering the housing. Porous polytetrafluoroethylene (PTFE) films, for example, are used as breathable membranes.

[0003] The microphone manufacturing process includes, for example, attaching a PTFE porous film as a breathable membrane to a microelectromechanical systems (MEMS) microphone, which is passed through a solder reflow oven to attach the breathable membrane.

[0004] Due to their intended use, microphones are required to have a certain degree of breathability. However, since PTFE porous films are produced by uniaxial or biaxial stretching, they shrink in high-temperature environments such as reflow ovens. Shrunk PTFE porous films may not have sufficient breathability for use in microphones.

[0005] A gas sensor, for example, includes a catalyst that promotes a reaction of the gas to be detected, and detects the gas based on the presence or absence of the reaction. The gas-permeable membrane described above can be used to allow gas to pass through the detection unit equipped with the catalyst and to prevent water and dust from entering the detection unit. The catalyst may be baked onto the gas-permeable membrane, but the membrane may be deformed by being exposed to high temperatures (e.g., 280°C) during the baking process, which can cause problems during processing.

[0006] In order to suppress shrinkage of a PTFE porous film, Patent Document 1 describes a method for producing a porous PTFE membrane, which includes an annealing step in which the porous fluororesin membrane is kept in a shape-fixed state and is kept at a temperature that is lower than the melting point of the fluororesin and within a 30°C difference from the melting point for 1 to 20 hours.

[0007] Furthermore, Patent Document 2 describes coating a metal net with a layer of polyether ether ketone (PEEK) and laminating a layer of porous polytetrafluoroethylene onto the coated metal net.

[0008] Patent Document 3 describes a laminate of uniaxially stretched PTFE and fluorine-coated glass cloth. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2015 / 002002 [Patent Document 2] Special Publication No. 10-513119 [Patent Document 3] Japanese Patent Application Laid-Open No. 2017-71131 Summary of the Invention [Problem to be solved by the invention]

[0010] The porous PTFE membrane described in Patent Document 1 has a single-layer structure consisting of only PTFE membrane. Therefore, it suffers from the problem of considerable shrinkage in high-temperature environments. Furthermore, the laminated product described in Patent Document 2 uses a metal mesh, which is not easy to process, such as by cutting or punching. Furthermore, the metal mesh coated with a PEEK layer is very hard. Therefore, this metal mesh is not very flexible and is prone to cracking even with slight bending. Furthermore, the chemical resistance of PEEK is inferior to that of PTFE. Patent Document 3 describes breathable membranes as having high water resistance in addition to heat resistance, but there is room for improvement in terms of waterproofing. Generally, breathability and water pressure resistance are in a trade-off relationship, and they cannot be immediately achieved together. This trade-off is particularly evident in uniaxially stretched products. Although Patent Document 3 also mentions biaxial stretching, it only evaluates breathable membranes manufactured using uniaxial stretching alone and does not describe water pressure resistance.

[0011] The present invention has been made in view of the above circumstances, and has an object to provide a breathable membrane that has high heat resistance and waterproof properties and sufficient breathability, and a microphone equipped with this breathable membrane. [Means for solving the problem]

[0012] According to a first aspect of the present invention, there is provided a breathable membrane comprising a porous polytetrafluoroethylene film and a fluororesin-containing breathable material layer laminated on the porous polytetrafluoroethylene film, wherein the breathable membrane has a water pressure resistance of 50 kPa or more after being heat-treated at 260°C for 10 minutes.

[0013] According to a second aspect of the present invention, there is provided a microphone equipped with the breathable membrane according to the first aspect. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a breathable film that is highly heat-resistant and waterproof and has sufficient breathability, and a microphone equipped with this breathable film. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a cross-sectional view schematically showing an example of a gas-permeable membrane according to the first embodiment. [Figure 2] FIG. 3 is an enlarged plan view schematically showing a fluororesin-containing breathable material layer having a mesh-like weave. [Figure 3] FIG. 3 is a cross-sectional view schematically showing an example of a fluororesin layer having a two-layer structure. [Figure 4] Photographs of heat-treated test pieces of breathable membranes according to Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0016] (First embodiment) A first embodiment of the present invention will be described below. The breathable membrane according to this embodiment includes a porous polytetrafluoroethylene (PTFE) film and a fluororesin-containing breathable material layer laminated on the porous polytetrafluoroethylene film. Even after heat treatment at 260°C for 10 minutes, the breathable membrane has a water pressure resistance of 50 kPa or more.

[0017] A PTFE porous film has a porous structure composed of amorphous node portions and fibril portions drawn out from the node portions. Such a porous structure can be finer than the structures of other breathable materials such as nonwoven fabrics. Therefore, a breathable membrane containing a PTFE porous film can have high water resistance.

[0018] However, since the PTFE porous film is produced by a method including a stretching step, it contains residual stress. This residual stress acts to return the PTFE porous film to its pre-stretched state when used in a high-temperature environment. In other words, a single PTFE porous film will shrink in a high-temperature environment.

[0019] Furthermore, even if uniaxially stretched products manufactured by stretching in only one direction can achieve water resistance, their water pressure resistance is limited to the extent achieved by the water-repellent function of the PTFE resin. For example, the water pressure resistance of a laminate film formed by laminating a uniaxially stretched product with a backing material is limited to approximately 30 kPa to 50 kPa. Therefore, it is difficult to say that PTFE porous films obtained by only uniaxial stretching and laminated products using such films are waterproof. In addition, there is a trade-off between breathability and water pressure resistance, and this trade-off is particularly pronounced in uniaxially stretched products, making it impossible to achieve both breathability and water pressure resistance. Specifically, increasing breathability drastically reduces water pressure resistance.

[0020] The breathable membrane according to this embodiment is obtained by laminating a fluororesin-containing breathable material layer on the PTFE porous film. The PTFE porous film may be, for example, a biaxially stretched product obtained by biaxially stretching an extrusion-molded PTFE product to make it porous. Unlike uniaxially stretched products, biaxially stretched PTFE membranes have an excellent balance between breathability and water pressure resistance. In other words, by using a biaxially stretched membrane for the PTFE porous film, both high breathability and high water pressure resistance can be achieved.

[0021] Specifically, biaxially stretched membranes can impart a water pressure resistance of 100 kPa to 400 kPa while maintaining high breathability. The desired water pressure resistance can be obtained by adjusting the molecular weight of the PTFE resin, the stretching ratio in each of the longitudinal direction (machine direction: MD) and transverse direction (vertical direction: TD), and the processing temperature. As the water pressure resistance can be controlled through a variety of designs, it is possible to achieve high water pressure resistance without sacrificing breathability. Preferably, a PTFE porous film is used that can impart a water pressure resistance of 300 kPa or more, more preferably 350 kPa or more, to the breathable membrane.

[0022] Furthermore, the fluororesin-containing breathable material layer can suppress thermal shrinkage of the PTFE porous film. Therefore, the breathable membrane can have high dimensional stability and exhibits little change in water pressure resistance even when used under high-temperature conditions such as 260°C or 300°C. Specifically, the water pressure resistance of the breathable membrane after heat treatment at 260°C for 10 minutes is 50 kPa or more. Preferably, the water pressure resistance of the breathable membrane after heat treatment at 260°C for 10 minutes is 200 kPa or more, and more preferably, the water pressure resistance after this heat treatment is 300 kPa or more. A breathable membrane having a water pressure resistance of 300 kPa or more after heat treatment at 300°C for 10 minutes is even more preferred. For example, the change in water pressure resistance when the breathable membrane is heat treated at 260°C for 10 minutes may be less than 15% in absolute value, and the change in water pressure when the breathable membrane is heat treated at 300°C for 10 minutes may be less than 20% in absolute value.

[0023] In addition, this breathable membrane exhibits high dimensional stability against heat. For example, the heat shrinkage of the breathable membrane in both one direction and the direction perpendicular to the one direction when heat-treated for 10 minutes at a temperature of 200°C may be less than 3%. A breathable membrane in which the heat shrinkage in each direction when heat-treated for 10 minutes at a temperature of 300°C is less than 3% is more preferable. Specifically, the one direction and the perpendicular direction referred to here refer to the MD direction and the TD direction, which correspond to the respective stretching directions in the biaxial stretching when the PTFE is made porous. The method for measuring the heat shrinkage will be described later.

[0024] The heat shrinkage rate is preferably close to 0%, and the lower limit thereof may be 0%. The heat shrinkage rate may be, for example, 2.0% or less, or even 0.5% or less. A heat shrinkage rate of 2.0% or less is preferable because it can suppress variations in air permeability and water pressure resistance due to heating.

[0025] The fluororesin-containing breathable material layer can have breathability to the extent that it does not inhibit the breathability of the PTFE porous film, and therefore, a breathable membrane in which a fluororesin-containing breathable material is laminated on a PTFE porous film can have sufficient breathability.

[0026] As described above, the breathable film of this embodiment can have high heat resistance and water pressure resistance, and can have sufficient breathability.

[0027] Hereinafter, the present embodiment will be described with reference to the drawings. Note that throughout the drawings, components that perform the same or similar functions are designated by the same reference numerals, and redundant description will be omitted.

[0028] FIG. 1 is a cross-sectional view schematically showing an example of a gas-permeable membrane according to this embodiment. As shown in Fig. 1, the breathable membrane 1 includes a PTFE porous film 2 and a fluororesin-containing breathable material layer 5. The fluororesin-containing breathable material layer 5 is composed of, for example, an air-permeable support material 3 and a fluororesin layer 4. The fluororesin layer 4 is partially impregnated into the air-permeable support material 3, and is thereby supported on the air-permeable support material 3.

[0029] The PTFE porous film 2 is substantially made of PTFE. The PTFE porous film 2 may be baked. If the thickness of the PTFE porous film 2 is excessively small, the water resistance may be insufficient, handling may be poor during processing, and the breathable membrane may be prone to thermal shrinkage in high-temperature environments. If the thickness of the PTFE porous film 2 is excessively large, the breathability may be insufficient. In addition, in this case, the shrinkage force of the fluororesin layer 4 is strong compared to the rigidity of the breathable support material 3, so the breathable membrane may warp due to heat treatment.

[0030] In the illustrated example, the fluororesin-containing breathable material layer 5 is laminated on only one side of the PTFE porous film 2. The fluororesin-containing breathable material layer 5 may be laminated on both sides of the PTFE porous film 2. When laminated on only one side, the area where the fluororesin layer 4 blocks the pores of the PTFE porous film 2 can be reduced, making it easier to ensure sufficient breathability. When laminated on both sides, the breathable film 1 is less likely to warp even in a high-temperature environment.

[0031] The fluororesin-containing breathable material layer 5 includes a breathable support material 3. This breathable support material 3 includes, for example, one or more types of fiber selected from the group consisting of glass cloth, aramid cloth, and metal fiber cloth. The breathable support material 3 may be made of one or more types of fiber selected from the group consisting of glass cloth and aramid cloth. When the breathable support material 3 is made of glass cloth, it has excellent dimensional stability, cuttability during processing, and flexibility, stable punching processability, and excellent chemical resistance.

[0032] The weave of the fluororesin-containing breathable material layer 5 may be either mesh-like or non-mesh-like. Here, mesh-like refers to a case where the opening ratio ε (%), which represents the open area per unit area, is in the range of 10% to 70%. The opening ratio ε (%) is expressed by the formula ε=A 2 / (A+d) 2 × 100, where d represents the wire diameter of the flattened woven fabric, and A represents the opening size. The opening size will be explained below with reference to the drawings.

[0033] Fig. 2 is an enlarged plan view schematically showing a fluororesin-containing breathable material layer having a mesh-like weave. In Fig. 2, d indicates the wire diameter of the flattened woven fabric, and A indicates the mesh size. When the wire diameter d is d = 0.6 mm and the mesh size A is A = 0.9 mm, the opening ratio ε (%) is 36.

[0034] The opening rate ε of the fluororesin-containing breathable material layer 5 is, for example, in the range of 1% to 70%. When the opening rate ε is within this range, even when the fluororesin-containing breathable material layer 5 is laminated on the PTFE porous film 2, the breathability of the PTFE porous film 2 is not impaired, which is preferable.

[0035] The fluororesin layer 4 supported on the breathable support material 3 may have a single-layer structure or a multi-layer structure in which two or more layers are laminated. The fluororesin layer 4 may block some of the pores of the breathable support material 3, but desirably does not block all of the pores. By supporting the fluororesin layer 4 on the breathable support material 3, it becomes possible to integrate the fluororesin-containing breathable material layer 5 and the PTFE porous film 2. That is, the fluororesin layer 4 plays a role in integrating the fluororesin-containing breathable material layer 5 and the PTFE porous film 2. Because the PTFE porous film 2 and the fluororesin layer 4 have good compatibility, peeling is unlikely to occur at the interface between the fluororesin-containing breathable material layer 5 and the PTFE porous film 2.

[0036] Furthermore, the fluororesin is exposed on the outermost surface of both the fluororesin-containing breathable material layer 5 and the PTFE porous film 2. Therefore, the breathable membrane 1 formed by integrating these has excellent chemical resistance.

[0037] The fluororesin layer 4 contains at least one fluororesin selected from the group consisting of PTFE, perfluoroalkoxyalkane (PFA), and perfluoroethylenepropene copolymer (FEP). The fluororesin layer 4 may be made of at least one selected from the group consisting of PTFE, PFA, and FEP.

[0038] Fig. 3 is a cross-sectional view that schematically shows a case where the fluororesin layer 4 has a two-layer structure. In Fig. 3, the fluororesin layer 4 includes a first fluororesin layer 41 and a second fluororesin layer 42. The first fluororesin layer 41 is formed on both sides of the breathable support material 3. The second fluororesin layer 42 is formed on both first fluororesin layers.

[0039] The first fluororesin layer 41 contains PTFE. The first fluororesin layer 41 may further contain a fluororesin other than PTFE. The second fluororesin layer 42 contains a melt-soluble fluororesin such as PFA. The second fluororesin layer 42 may further contain PTFE.

[0040] Here, as an example, a case will be described in which the first fluororesin layer 41 is made of PTFE and the second fluororesin layer 42 is made of PFA. In this case, the fluororesin-containing breathable material layer 5 and the PTFE porous film 2 can be easily integrated. This is because PFA is supported on the outermost surface of the fluororesin layer 4. In addition, this case is preferable because the raw material cost is lower than when the fluororesin layer 4 is made only of a melt-soluble fluororesin.

[0041] The thickness of the first fluororesin layer 41 is, for example, in the range of 50 μm to 500 μm. The thickness of the second fluororesin layer 42 is, for example, in the range of 0.5 μm to 3 μm, and preferably in the range of 1 μm to 2 μm.

[0042] When the thicknesses of the first fluororesin layer 41 and the second fluororesin layer 42 are within the above ranges, the amount of fluororesin carried in the second fluororesin layer 42 is relatively small. Therefore, even if the above integration is performed, the fluororesin contained in the second fluororesin layer 42 is unlikely to block the pores of the PTFE porous film 2.

[0043] The Gurley air permeability (air flow resistance) of the PTFE porous film 2 alone is, for example, 30 seconds or less, preferably 5 seconds or less, and more preferably 2 seconds or less. When the PTFE porous film 2 is used as an air permeable membrane equipped in a microphone, the Gurley air permeability is preferably 2 seconds or less. As long as the above-mentioned water pressure resistance is maintained, it is preferable that the Gurley air permeability value of the PTFE porous film 2 is low. The method for measuring the Gurley air permeability will be described later.

[0044] The breathability of the single fluororesin-containing breathable material layer 5 is very high. Therefore, the Gurley air permeability of the single fluororesin-containing breathable material layer 5 is less than 1 second, and the measurement error is large, making the measurement unreliable.

[0045] The Gurley air permeability (air flow resistance) of the breathable membrane 1 is, for example, 30 seconds or less, preferably 4 seconds or less. The Gurley air permeability of the breathable membrane 1 preferably does not change even when subjected to heat treatment at a temperature of 260°C for 10 minutes, and more preferably does not change even when subjected to heat treatment at a temperature of 300°C for 10 minutes. For example, the absolute value of the change in Gurley air permeability when heat treated at 260°C for 10 minutes preferably remains 20% or less, and even when heat treated at 300°C for 10 minutes preferably remains 25% or less. The Gurley air permeability of the breathable membrane 1 after such heat treatment is, for example, 30 seconds or less, preferably 5 seconds or less, and more preferably 4 seconds or less. When the Gurley air permeability of the breathable membrane 1 after this heat treatment is 5 seconds or less, the breathability of the breathable membrane 1 is sufficiently high, and the breathable membrane 1 can be suitably used for microphones, gas sensors, etc.

[0046] <Method for measuring heat shrinkage> The heat shrinkage rate is measured as follows. The dimensional stability of the breathable film can be evaluated by measuring the heat shrinkage rate. The lower the heat shrinkage rate, the higher the dimensional stability.

[0047] A test specimen was cut to a size of 100 mm x 100 mm, and a marking line was drawn 20 mm from each side along the machine direction (MD) and the transverse direction (TD) to form an 80 x 80 mm rectangle. The test specimen was then heated at a predetermined temperature (e.g., 200°C or 300°C) for 10 minutes using an electric furnace (Espec Corporation, STPH-200). The distance between the marking lines along the machine direction and the transverse direction of the test specimen was then measured using a steel ruler.

[0048] When the PTFE porous film provided in the breathable membrane as the test specimen is produced by uniaxial stretching, the length of the test specimen in the MD direction typically shrinks and the length of the test specimen in the TD direction typically expands due to the heat treatment. On the other hand, when the PTFE porous film provided in the breathable membrane as the test specimen is produced by biaxial stretching, the length of the test specimen in the MD direction typically shrinks and the length of the test specimen in the TD direction also shrinks due to the heat treatment.

[0049] For any distance, if the distance after heat treatment is smaller than before measurement, i.e., if the specimen has shrunk in a certain direction, measure the distance between the gauge lines at the most shrunk portion. Also, for any distance, if the distance after heat treatment is larger than before measurement, i.e., if the specimen has expanded in a certain direction, measure the distance between the gauge lines at the most expanded portion. However, if the specimen has turned up or curled due to a change in shape caused by thermal shrinkage, unfold the specimen so that it is approximately flat before measurement.

[0050] If no shrinkage or expansion in any direction can be visually confirmed, first measure the dimensions at both ends of the gauge line that intersects with the MD (TD) direction (the corners of the rectangle) and at three points near the midpoint of this gauge line. Of these three dimensions, the dimension at the point that has deformed the most from 80 mm before heat treatment is taken as the dimension in the MD (TD) direction (gauge line distance).

[0051] For each of the MD and TD directions, the dimensional change due to heat treatment relative to the dimension before heat treatment (80 mm) is multiplied by 100 to calculate the dimensional change rate due to heat shrinkage (dimensional change rate (%) = [(dimension after heat treatment - dimension before heat treatment) / dimension before heat treatment] x 100). A negative dimensional change rate means that the test piece shrank in that direction, and can be converted to a positive thermal shrinkage rate (thermal shrinkage rate (%) = [dimensional change rate (%)] x -1).

[0052] <Method for measuring Gurley air permeability> The Gurley air permeability is measured using, for example, an automatic Gurley densometer manufactured by Yasuda Seiki Seisakusho Co., Ltd. as an apparatus in accordance with JIS P 8117:2009.

[0053] <Water pressure resistance measurement method> The water pressure resistance is measured in accordance with Method B (high water pressure method) of JIS L 1092:2009 (waterproof test). A mesh dish with a φ3 mm opening specified in JIS K 6404-3:2020 is used as the jig to hold the test specimen.

[0054] Next, a method for producing the gas-permeable membrane according to this embodiment will be described. First, the PTFE porous film 2 is prepared as described below.

[0055] PTFE fine powder is prepared, and 100 parts by mass of this powder is mixed with 20 to 30 parts by mass of hydrocarbon oil. This mixture is stirred to be homogeneous, and then preformed by paste extrusion. The obtained preform is rolled, for example, using a metal rolling roll. In this way, an unsintered tape, which is a precursor of the PTFE porous film 2, is obtained. The shape of the unsintered tape is not particularly limited, but is, for example, rectangular or approximately square.

[0056] This unsintered tape is stretched 3 to 8 times in the direction of the previous rolling, i.e., the longitudinal direction, at a temperature below the melting point. The temperature at this time is, for example, 270°C. Stretching at a temperature below the melting point draws out fibrils between the PTFE nodes. As a result, cavities are formed within the PTFE tape, making it porous.

[0057] Next, the obtained uniaxially stretched product is subjected to width expansion stretching. It is stretched at a magnification of 3 to 10 times in the direction perpendicular to the previous rolling direction (MD direction), i.e., the transverse direction (TD direction). The temperature at this time is, for example, 200°C. Next, it is baked at a temperature above the melting point. The temperature above the melting point is, for example, 380°C. The unbaked PTFE film made porous by the above-mentioned biaxial stretching has poor dimensional stability and may gradually shrink even at room temperature. This shrinkage can be prevented by baking the unbaked PTFE film at a temperature above the melting point and heat setting it.

[0058] By laminating the PTFE porous film 2 produced by biaxial stretching as described above with a fluororesin-containing breathable material layer 5 as a backing material, a breathable membrane that combines high breathability and high water pressure resistance can be obtained.

[0059] The fluororesin-containing breathable material layer 5 can be produced as follows. First, the breathable support material 3 is immersed in an aqueous dispersion of PTFE microparticles, and after removing it from the dispersion, the solvent is dried and then the material is baked. This procedure is repeated several times to produce the fluororesin layer 4. When the fluororesin layer 4 has a multi-layer structure, for example, after a first fluororesin layer 41 made of PTFE is produced by the above procedure, the breathable support material 3 equipped with this first fluororesin layer 41 is further immersed in an aqueous dispersion of PFA microparticles. Thereafter, the solvent is dried and the material is baked to produce a second fluororesin layer 42 made of PFA.

[0060] When producing the fluororesin layer 4, a fluororesin film can also be used instead of an aqueous dispersion containing fluororesin particles.

[0061] The fluororesin-containing breathable material layer 5 may be a commercially available product.

[0062] The PTFE porous film 2 and the fluororesin-containing breathable material layer 5 prepared above are laminated together using, for example, a roll laminator. This lamination is performed under conditions of a roll temperature of 360°C to 420°C, a linear pressure of 20 N / cm to 60 N / cm, and a roll speed of 1 m / min to 4 m / min. By laminating the PTFE porous film 2 and the fluororesin-containing breathable material layer 5 under these conditions, it is possible to prepare a breathable membrane that maintains a water pressure resistance of 50 kPa or more, desirably 200 kPa or more, and preferably 300 kPa or more, even after heat treatment at 260°C for 10 minutes, and more preferably maintains a water pressure resistance of 300 kPa or more even after heat treatment at 300°C for 10 minutes.

[0063] As described above, the roll temperature is, for example, 360° C. to 420° C., and preferably 380° C. to 400° C. If the roll temperature is lower than 360° C., poor fusion may occur, and if it exceeds 420° C., the PTFE may decompose, which is not preferable.

[0064] As described above, the linear pressure is, for example, 20 N / cm to 60 N / cm, and preferably 30 N / cm to 40 N / cm. If the linear pressure is lower than 20 N / cm, poor fusion may occur, and if it exceeds 60 N / cm, the pores of the PTFE porous film may be significantly crushed, resulting in a decrease in breathability.

[0065] As mentioned above, the roll speed is, for example, 1 m / min to 4 m / min. If the roll speed is lower than 1 m / min, production efficiency is poor, and if it is higher than 4 m / min, poor fusion may occur.

[0066] The breathable membrane according to this embodiment can be manufactured in the manner described above. This breathable membrane includes a PTFE porous film and a fluororesin-containing breathable material layer laminated on the PTFE porous film, and exhibits a water pressure resistance of 50 kPa or more even after heat treatment at a temperature of 260°C for 10 minutes. Therefore, it has high heat resistance and water pressure resistance, and sufficient breathability.

[0067] (Second Embodiment) According to this embodiment, a microphone is provided. This microphone can be manufactured by a known method except that it includes the ventilation film according to the first embodiment. Further, since this microphone includes the ventilation film according to the first embodiment, it has high heat resistance and waterproofness and sufficient air permeability.

[0068] Furthermore, even if the manufacturing process of the microphone includes a process performed in a high-temperature environment such as a soldering flow process, the ventilation film can maintain a high water pressure resistance. Also, even after passing through a process in such a high-temperature environment, the ventilation film is less likely to thermally shrink. Therefore, the microphone after the soldering flow process has sufficient air permeability (frequency characteristics) and waterproofness.

Example

[0069] Hereinafter, examples will be described. In the examples, various measurements were performed by the methods described in the first embodiment.

[0070] (Example 1) <Production of PTFE porous film> 100 parts by mass of PTFE fine powder (manufactured by Daikin Industries, Ltd., F106) was uniformly mixed with 28 parts by mass of hydrocarbon oil (manufactured by ExxonMobil, Isopar M). This mixture was paste extruded and preformed into a sheet. This preform was passed between a pair of metal rolling rolls to obtain a rectangular green sheet with a thickness of 0.2 mm and a width of 180 mm. Next, this green sheet was stretched 7 times in the direction of prior rolling, i.e., the longitudinal direction (machine direction: MD), at a temperature below the melting point (270 °C) using a roll stretcher to obtain a uniaxially stretched product in the form of a rectangle with a thickness of 0.18 mm and a width of 170 mm. Both ends of this uniaxially stretched product in the longitudinal direction were gripped by a tenter and stretched 9 times in the width direction (perpendicular direction; TD) at a temperature of 200 °C. Further, heat setting was carried out by firing at a temperature above the melting point (380 °C) while still being gripped by the tenter. The gripped portion was cut off by trimming to obtain a biaxially stretched product with a thickness of 0.04 mm and a width of 830 mm. Next, this biaxially stretched product was slit to adjust the width to 400 mm to obtain a fired PTFE film.

[0071] <Composite of PTFE porous film and fluororesin-containing breathable material> As the fluororesin-containing breathable material, Porous Fabric FGB207-6-1 manufactured by Chukyo Kasei Kogyo Co., Ltd. was prepared. The breathable support material of this breathable material was a plain woven glass cloth, and the fluororesin contained in this breathable material was PTFE. Also, this breathable material was non-mesh, the wire diameter d was about 0.36 mm, the mesh opening A was about 0.07 mm, and the porosity ε (%) was about 2.7%.

[0072] Using a roll laminator, the PTFE porous film and the fluororesin-containing breathable material were overlapped and passed between a hot roll and a rubber roll and nipped to carry out lamination. Thus, a breathable film was produced.

[0073] (Comparative Example 1) A breathable film equivalent to PTFE porous film composite product SEF-501N manufactured by Chukyo Kasei Kogyo Co., Ltd. was produced in the same manner as in Example 1, except that a two-layer PET non-woven fabric (modified PET - PET) was used instead of the fluororesin-containing breathable material.

[0074] (Comparative Example 2) A breathable membrane equivalent to the PTFE porous film composite SEF-501M manufactured by Chukoh Chemical Industry Co., Ltd. was produced in the same manner as in Example 1, except that a PET mesh (a woven fabric made of fibers in which a PET core is covered with a modified PET sheath) was used instead of the fluororesin-containing breathable material.

[0075] (Measurement of heat shrinkage rate, water pressure resistance change rate, and air permeability change rate) The breathable membranes of Example 1, Comparative Example 1, and Comparative Example 2 were each cut to a size of 100 mm x 100 mm, and multiple test specimens were prepared for each example. Some of the test specimens were marked with 80 mm x 80 mm marks in the MD and TD directions and used to measure the thermal shrinkage. Another portion of the test specimens was measured for water pressure resistance. Furthermore, another portion of the test specimens was measured for air permeability using the Gurley test method.

[0076] Next, for each example, some of the test specimens were heat-treated at 100°C for 10 minutes, some at 200°C for 10 minutes, some at 260°C for 10 minutes, and some at 300°C for 10 minutes using an electric furnace. After the heat treatment at each temperature, the heat shrinkage, water pressure resistance, and air permeability of each test specimen were measured. However, the heat shrinkage after the heat treatment at 260°C was not measured. The heat shrinkage and water pressure resistance measurements were performed a total of three times (n = 3), and the air permeability measurements were performed a total of five times (n = 5).

[0077] The heat shrinkage results for each example, i.e., the dimensional stability evaluation results, are shown in Table 1. In Table 1, the "Dimensions (mm)" column indicates the gauge spacing in the MD or TD direction for each test specimen after heat treatment. The "Average" column indicates the average of the above dimensions in the MD and TD directions for each test specimen, measured three times. The "Change (%)" column indicates the percentage change in the average dimension (gage spacing in each direction) after heat treatment at each temperature relative to the average dimension before heat treatment (80 mm in both MD and TD). In the "Change (%)" column, a negative sign (-) indicates that the breathable membrane shrank in the measured direction. The term "melt decomposition" indicates that the PET resin constituting the backing material melted and decomposed. Photographs of some test specimens before and after heat treatment are shown in Figure 4. Note that the gauge lines on the test specimens are highlighted in Figure 4 to improve their visibility.

[0078] Table 2 shows the measurement results of water pressure resistance for each example. Table 3 also shows the measurement results of Gurley air permeability for each example. As in Table 1, the "Water Pressure Resistance (kPa)" and "Air Permeability (sec)" columns show the respective measurement results for each test piece, and the "Average" and "Change Rate (%)" columns are also displayed in the same way as in Table 1 above. However, in the "Change Rate (%)" column, a value with a negative sign (-) indicates a decrease in water pressure resistance or air resistance.

[0079] [Table 1]

[0080] [Table 2]

[0081] [Table 3]

[0082] It can be seen from Table 1 that the breathable membrane of Example 1 did not undergo any dimensional change in either the MD or TD direction upon heat treatment at 300°C. In contrast, the breathable membranes of Comparative Examples 1 and 2, which used a PET nonwoven fabric or a PET mesh instead of a fluororesin-containing breathable material layer, underwent heat shrinkage even upon heat treatment at 100°C, and the PET material melted and decomposed upon heat treatment at 300°C, making it impossible to measure the heat shrinkage rate for each.

[0083] Referring to Tables 2 and 3, it can be seen that the breathable membrane of Example 1 hardly deformed due to the heat treatment, and therefore the water pressure resistance and air permeability remained almost unchanged. On the other hand, it can be seen that the breathable membranes of Comparative Examples 1 and 2 were significantly deformed by the heat treatment, and the water pressure resistance and air permeability also changed relatively significantly. Furthermore, it can be seen that melting and decomposition occurred even when the membrane was heated at 260°C.

[0084] The present invention is not limited to the above-described embodiments, and various modifications can be made in the implementation stage without departing from the spirit of the invention. Furthermore, the embodiments may be implemented in appropriate combinations, in which case the combined effects can be obtained. Furthermore, the above-described embodiments include various inventions, and various inventions can be extracted by combining selected elements from the disclosed elements. For example, if the problem can be solved and the desired effect can be obtained even if some elements are deleted from all elements shown in the embodiments, the configuration from which these elements are deleted can be extracted as an invention. [Explanation of symbols]

[0085] 1...breathable membrane, 2...PTFE porous film, 3...breathable support material, 4...fluororesin layer, 5...fluororesin-containing breathable material layer, 41...first fluororesin layer, 42...second fluororesin layer, d...wire diameter, A...mesh size.

Claims

1. a polytetrafluoroethylene porous film; a fluororesin-containing breathable material layer laminated on the polytetrafluoroethylene porous film, A breathable membrane having a water pressure resistance of 50 kPa or more after heat treatment at a temperature of 260°C for 10 minutes.

2. 2. The breathable membrane according to claim 1, which has a water pressure resistance of 200 kPa or more after being heat-treated at a temperature of 260°C for 10 minutes.

3. 2. The breathable membrane according to claim 1, which has a water pressure resistance of 300 kPa or more after heat treatment at a temperature of 260°C for 10 minutes.

4. The breathable membrane according to claim 1, which has a water pressure resistance of 300 kPa or more after being heat-treated at a temperature of 300°C for 10 minutes.

5. 5. The breathable film according to claim 1, wherein the heat shrinkage rate in both the machine direction (MD) and the transverse direction (TD) perpendicular to the machine direction (MD) is less than 3% when heat treated at a temperature of 200°C for 10 minutes.

6. 5. The breathable film according to claim 1, wherein the heat shrinkage rate in both the machine direction (MD) and the transverse direction (TD) perpendicular to the machine direction (MD) is less than 3% when heat treated at a temperature of 300°C for 10 minutes.

7. The breathable membrane according to any one of claims 1 to 4, which has a Gurley air permeability of 5 seconds or less after heat treatment at a temperature of 260°C for 10 minutes.

8. The breathable membrane according to any one of claims 1 to 4, which has a Gurley air permeability of 5 seconds or less after heat treatment at a temperature of 300°C for 10 minutes.

9. 5. The ventilation film according to claim 1, wherein the fluororesin-containing ventilation material layer has an opening rate ε (%), which represents the open area per unit area, in the range of 1% to 70%.

10. 5. The breathable membrane according to claim 1, wherein the fluororesin-containing breathable material layer comprises an air-permeable support material containing one or more types of fibers selected from the group consisting of glass cloth and aramid cloth, and a fluororesin layer supported on the air-permeable support material.

11. The breathable membrane according to claim 10, wherein the fluororesin layer contains at least one fluororesin selected from the group consisting of polytetrafluoroethylene and perfluoroalkoxyalkane.

12. The breathable membrane according to any one of claims 1 to 4, having a Gurley air permeability of 30 seconds or less.

13. The breathable membrane according to any one of claims 1 to 4, having a Gurley air permeability of 4 seconds or less.

14. A microphone comprising the breathable membrane according to any one of claims 1 to 4.

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