JP2024063946A - Composite film - Google Patents

Abstract

To provide a composite film in which lowering of ventilation performance caused by attachment of oil is suppressed.SOLUTION: A composite film has a fluororesin porous film, and an oil absorption layer laminated on the fluororesin porous film. The oil absorption layer contains a lipophilic material and is porous.SELECTED DRAWING: Figure 1

Description

Related to composite membranes.

Various devices are provided with vents for adjusting internal pressure and ventilation. In addition, openings are provided for functional reasons in parts of mobile phones and other devices that emit or recognize voice, such as speakers and microphones. To prevent moisture and other foreign matter from entering the device through the vents and openings, a breathable and waterproof membrane is attached. For example, water-repellent nonwoven fabrics and meshes, as well as porous membranes of fluororesins such as polytetrafluoroethylene (PTFE; tetrafluoroethylene resin) are used as the breathable membrane. From the viewpoint of high breathability and water resistance, PTFE porous membranes are often used as water-resistant and breathable membranes. For example, Patent Document 1 (JP Patent Publication 2005-334758 A) describes a ventilation filter equipped with a water-repellent resin porous membrane containing fluororesins such as PTFE.

There are situations where a breathable and waterproof membrane is required even in an environment where oil may adhere to the membrane. However, a normal porous membrane that has not been specially treated, such as a typical PTFE porous membrane, is not oil-resistant even if it has water repellency, and its breathability is significantly reduced when exposed to oil. Various studies have been conducted on means to prevent deterioration of breathability due to adhesion of oil. For example, Patent Document 1 describes applying a liquid-repellent treatment such as a water-repellent treatment or an oil-repellent treatment to a breathable filter as necessary. In addition, Patent Document 2 (JP Patent Publication 7-126428 A) and Patent Document 3 (JP Patent Publication 2014-31412 A) describe preventing the adverse effects caused by adhesion of oil or surfactant to the breathable membrane by imparting oil repellency to a porous membrane such as PTFE.

One method for preventing deterioration of breathability due to the adhesion of foreign matter is to prevent direct exposure of the breathable membrane. For example, Patent Document 4 (JP 2014-102970 A) describes a breathable member to be attached to the opening of a housing, which includes an inner member made of elastomer to which a waterproof breathable membrane is attached and an outer member made of resin attached to the inner member so as to cover the waterproof breathable membrane. The exposed surfaces of the inner member and the outer member are treated with a liquid repellent treatment, which prevents oil and cleaning liquid from entering the inside of the housing. In addition, since the outer member covers the waterproof breathable membrane, the risk of breathability being hindered by the accumulation of sand, mud, etc. on the surface of the waterproof breathable membrane from the outside of the housing can be reduced. Patent Document 5 (JP 2014-191902 A) describes a breathable member to be attached to the opening of a housing, which includes a breathable membrane, a support that supports the breathable membrane and is fixed to the opening of the housing, and a cover member attached to the support so as to cover the breathable membrane. The support has an oil-repellent portion formed around the periphery of the breathable membrane, so not only can the cover member block oil from adhering to the breathable membrane from the front, but it can also prevent oil from adhering to the breathable membrane even if oil enters the ventilation space between the cover member and the support through an opening on the side of the cover member.

Oil that may adhere to the breathable membrane is not limited to oils of external origin; oil contained inside the housing may also impair the breathability of the breathable membrane. For example, the housing of an automobile part may contain liquids such as oil, and oil mist and other liquid floating matter that may be generated when the pressure inside the housing is low may adhere to the breathable membrane from the inside.

An example of a means for preventing the oil mist floating in the housing from adhering to the ventilation membrane is the ventilation member described in Patent Document 6 (JP Patent Publication 2014-191903 A). The ventilation member disclosed in Patent Document 6 includes a support body having a through hole that is attached to the opening of the housing and connects the inside and outside of the housing, a filter member that blocks the through hole on the inside side of the housing, and a ventilation membrane that blocks the through hole on the outside side of the housing. The through hole includes a first ventilation path and a second ventilation path connected to the side of the first ventilation path, one end of the first ventilation path is connected to an opening on the inside side of the housing, and one end of the second ventilation path is connected to an opening on the outside side. The through hole further includes a bag-shaped path formed to extend in a bag shape from the other end of the first ventilation path. When the oil mist in the housing is ventilated to the outside, it passes through the filter member to become granular oil, and adheres to the inner circumferential surface of the first ventilation path and the inner circumferential surface of the bag-shaped path. This makes it difficult for the oil mist to move to the second ventilation path, reducing adhesion of oil mist suspended inside the housing to the ventilation membrane.

As another example, Patent Document 7 (JP Patent Publication 2017-204620A) describes a ventilation member that includes a breathable membrane, a collection membrane that is installed without contacting the breathable membrane, and a mounting member that holds them and is attached to the opening of the housing or to another member. When the pressure inside the housing becomes greater than the external pressure, the air inside the housing is released to the outside through the ventilation member, and liquid floating particles such as oil mist that move to the outside at that time are collected by the collection membrane provided in front of the breathable membrane. Therefore, the liquid floating particles are prevented from reaching the breathable membrane. When the pressure inside the housing becomes negative relative to the external pressure, gas flows from the breathable membrane to the collection membrane, causing the liquid floating particles collected by the collection membrane to coarsen, liquefy, and fall off. In addition, the liquid attached to the collection membrane falls under its own weight. In order to promote liquid separation from the collection membrane, not only the breathable membrane but also the collection membrane can be subjected to a liquid repellent treatment such as a water repellent treatment or an oil repellent treatment. In other words, by including a collection membrane, the ventilation member performs the same function as a breather cap equipped with a structure to prevent oil splash leakage.

Simply imparting oil repellency to a porous membrane may result in a significant decrease in breathability when the membrane is exposed to oil. Ventilation components equipped with a material that protects the breathable membrane from exposure are larger in size than membranes alone (including composite membranes), which places constraints on equipment design. Furthermore, although such ventilation components can avoid direct exposure to oil, if placed in an oil mist environment, adhesion to the membrane may not be completely eliminated, and the membrane may be exposed to oil. Regardless of the method used, it is difficult to avoid a decrease in functionality when oil reaches the breathable membrane.

JP 2005-334758 A Japanese Patent Application Laid-Open No. 7-126428 JP 2014-31412 A JP 2014-102970 A JP 2014-191902 A JP 2014-191903 A JP 2017-204620 A

The aim is to provide a composite membrane that suppresses the decrease in breathability caused by oil adhesion.

A composite membrane is provided that includes a fluororesin porous membrane and an oil-absorbing layer laminated with the fluororesin porous membrane. The oil-absorbing layer contains an oleophilic material and is porous.

The composite membrane of the above configuration suppresses the decrease in breathability caused by the adhesion of oil, so that the composite membrane can maintain breathability when used as a breathable membrane in an environment where oil adheres. In addition, the composite membrane has a simple and compact structure, so it can also meet the demand for space saving.

FIG. 1 is a cross-sectional view illustrating an example of a composite membrane. FIG. 2 is a cross-sectional view conceptually illustrating the function of the composite membrane shown in FIG. 1 . FIG. 4 is a cross-sectional view illustrating another example of a composite membrane. FIG. 13 is a cross-sectional view showing a schematic modification of the composite membrane. FIG. 11 is a cross-sectional view illustrating another modified example of the composite membrane. FIG. 1 is a cross-sectional view illustrating an example of a conventional breathable membrane. FIG. 7 is a cross-sectional view conceptually illustrating a problem with the breathable membrane shown in FIG. 6 . FIG. 11 is a cross-sectional view conceptually illustrating a problem in another example of a conventional breathable membrane.

An example of a breathable and waterproof membrane is shown in FIG. 6. FIG. 6 is a cross-sectional view that shows a schematic diagram of an example of a conventional breathable membrane. The illustrated breathable membrane 10 is a composite product of a laminated resin porous membrane 13 and a porous support 14. The resin porous membrane 13 may be, for example, a porous membrane made of a fluororesin, specifically, a porous membrane made of polytetrafluoroethylene (PTFE; polytetrafluoroethylene) resin. The porous support 14 may be, for example, a nonwoven fabric, and is included to improve the durability and handling properties of the breathable membrane 10. Even the resin porous membrane 13 alone, omitting the porous support 14, has the function of a breathable and waterproof membrane.

The resin porous membrane 13 and the porous support 14 contained in the breathable membrane 10 are both breathable, and the breathable membrane 10 exhibits breathability in the thickness direction. On the other hand, because the pores in the resin porous membrane 13 are small, liquids such as water cannot penetrate unless high pressure is applied. Therefore, as shown in the figure, gas molecules 20 can pass through the breathable membrane 10 in either direction, whereas water droplets 21 are repelled by the resin porous membrane 13. Therefore, it is possible to prevent moisture from entering from the resin porous membrane 13 side (e.g., the outside of the device) while maintaining ventilation and sound permeability between the front and back sides of the breathable membrane 10.

Water does not penetrate the resin porous membrane 13, but oil does. As shown in Figure 7, when oil droplets 22 adhere to the resin porous membrane 13, the oil 23 penetrates into the membrane and blocks the pores. This impairs the breathability of the breathable membrane 10.

In order to prevent oil from penetrating the resin porous membrane 13, an air-permeable membrane is used that has a porous membrane that has been treated with an oil repellent agent, for example. Another example of such a conventional air-permeable membrane is shown in FIG. 8. The air-permeable membrane 10 in the example shown in FIG. 8 is equipped with an oil-repellent resin porous membrane 15 instead of the resin porous membrane 13 that has not been treated with an oil repellent agent, unlike the examples shown in FIGS. 6 and 7. The oil-repellent resin porous membrane 15 is, for example, a porous membrane similar to the resin porous membrane 13 that has been treated with an oil repellent agent, to give it oil repellency. Although the oil-repellent resin porous membrane 15 can repel small amounts of oil and low-viscosity oil, when oil droplets 22, such as oil mist of high-viscosity oil, adhere to the membrane, the oil droplets 22 form an oil film 24 on the surface of the oil-repellent resin porous membrane 15. The oil film 24 prevents air from passing through, impairing the breathability of the air-permeable membrane 10.

The following describes the embodiments with reference to the drawings as appropriate. Note that common components throughout the embodiments are given the same reference numerals, and duplicate explanations will be omitted. Also, each figure is a schematic diagram to explain the embodiments and facilitate their understanding, and the shapes, dimensions, ratios, etc. may differ from the actual devices in some places, but these can be modified as appropriate by taking into account the following explanations and known technologies.

The composite membrane according to the embodiment of the invention comprises a fluororesin porous membrane and an oil absorbing layer laminated with the fluororesin porous membrane. The oil absorbing layer contains an oleophilic material. The oil absorbing layer is porous.

The fluororesin porous membrane is obtained, for example, by subjecting a fluororesin film or a fluororesin sheet to a stretching process to make it porous. The fluororesin porous membrane contains at least one fluororesin selected from the group consisting of polytetrafluoroethylene (PTFE), modified polytetrafluoroethylene, perfluoroalkoxyalkane (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), perfluoroethylenepropene copolymer (FEP), and polyvinylidene fluoride (PVDF). It is preferable that the fluororesin porous membrane contains PTFE.

A specific example of a fluororesin porous membrane is a PTFE expanded porous film. A PTFE expanded porous film is a porous membrane made up of nodes and fibrils made of PTFE. When the film is expanded to make it porous, the fibrils are pulled out from the nodes roughly along the direction of expansion. A node refers to an area where the polymer fibers are not stretched and the PTFE material is aggregated. A fibril refers to the polymer fibers that exist between the nodes and are oriented along the direction of expansion. The fine and dense presence of these fibers results in a porous membrane with countless small pores, which allows the membrane to exhibit high water resistance.

The fluororesin porous membrane may have an average pore size of, for example, 0.01 μm or more and 10 μm or less. A fluororesin porous membrane with an average pore size in this range has a good balance between breathability and water resistance.

The fluororesin porous membrane may have a thickness of, for example, 0.01 mm or more and 0.5 mm or less. A thickness of 0.01 mm or more ensures water resistance. From the viewpoint of freedom of design of equipment in which the composite membrane is installed as a ventilation membrane, a thinner membrane is preferable.

The fluororesin porous membrane may be, for example, a porous membrane made of fluororesin. Alternatively, the fluororesin porous membrane may be a porous membrane made of fluororesin that has been given oil repellency by, for example, applying an oil repellent treatment. An example of the oil repellent treatment is a method of applying an oil repellent agent containing a polymer having a perfluoroalkyl group to cover the surface of the porous membrane with a coating of the oil repellent agent.

The oil absorbing layer is a porous layer containing an oil-philic material. Examples of the oil-philic material include resin materials that exhibit oil-philicity, such as polypropylene (PP), polyethylene (PE), polystyrene (PS), and urethane foam. It is desirable for the oil absorbing layer to have higher breathability than the fluororesin porous membrane. It is also desirable for the oil absorbing layer to have a form with a large surface area. For example, the oil absorbing layer may be a nonwoven fabric. A specific example of the oil absorbing layer is a PP nonwoven fabric. In addition to the nonwoven fabric, the oil absorbing layer may further contain a self-swelling oil absorbing polymer or a gelling oil absorbing polymer arranged on a part of the surface.

The oil absorbing layer is included as a surface of the composite membrane to which oil can adhere. For example, in applications where oil can adhere to both the front and back main surfaces of the composite membrane, the oil absorbing layer can be arranged on the outermost surface on both sides. By providing an oil absorbing layer that is oleophilic and has a large surface area on the outer surface side, the oil adhering to the surface of the composite membrane can be adsorbed by the oil absorbing layer. As a result, some of the pores in the oil absorbing layer are filled with oil, but the pores in the composite membrane are not blocked three-dimensionally, so that the breathability is not impaired. When the amount of adsorbed oil increases and the pores in the oil absorbing layer are filled and approach saturation, the breathability decreases, but until then the breathability path is not obstructed and breathability can be maintained. In the case of an oil absorbing layer in which a self-swelling oil absorbing polymer or a gelling oil absorbing polymer is partially arranged on the oil adsorption surface, the oil can be fixed and the breathability path can be prevented from being filled, so that the decrease in breathability in the oil absorbing layer can be suppressed. In other words, by providing an oil absorbing layer on the outermost surface of the composite membrane, the exposure of the fluororesin porous membrane itself to oil can be prevented, and thus the decrease in breathability can be prevented.

In addition, the oil absorbing layer also functions as a physical protective layer for the fluororesin porous membrane. The presence of the oil absorbing layer outside the fluororesin porous membrane makes it possible to prevent damage to the fluororesin porous membrane from external elements. This makes it possible to avoid damage to the fluororesin porous membrane and loss of its waterproofing.

The oil absorbing layer may have a thickness of, for example, 1.0 mm or more and 5.0 mm or less. It is more preferable that the thickness of the oil absorbing layer is 1.2 mm or more and 3.0 mm or less. By having a thickness of 1.0 mm or more, a ventilation path can be secured when oil adheres. Furthermore, the thicker the oil absorbing layer, the greater the amount of oil that can adhere before the pores of the oil absorbing layer are filled with oil and saturated. From the viewpoint of the degree of freedom in designing the equipment in which the composite membrane is installed as a ventilation membrane, a thinner thickness is preferable.

The oil absorbing layer may have a basis weight of, for example, greater than 100 g/m ² and less than 500 g/m ^2. The basis weight of the oil absorbing layer is preferably 150 g/m ² or more and 300 g/m ² or less. The oil absorbing layer may have a density of, for example, 0.03 g/cm ³ or more and 0.1 g/cm ³ or less. The density of the oil absorbing layer is preferably 0.035 g/cm ³ or more and 0.095 g/cm ³ or less. In consideration of the oil absorption effect of the oil absorbing layer, it is preferable that the basis weight and density of the oil absorbing layer are large. In addition, an oil absorbing layer with a high density has a high strength. On the other hand, in consideration of the weight reduction of the equipment equipped with the composite membrane, it is preferable that the basis weight and density of the oil absorbing layer are small.

The composite membrane may further include a porous support on the main surface of the fluororesin porous membrane opposite the oil absorption layer. The porous support is, for example, a porous sheet or a porous membrane. By including the porous support as a backing material, the handling property of the composite membrane can be improved and the strength of the composite membrane can be further improved. As the porous support, for example, a nonwoven fabric or mesh material made of a resin such as polyethylene terephthalate (PET), polyester, polyamide, aramid resin, polyimide, fluororesin, or ultra-high molecular weight polyethylene can be used. Alternatively, a metal porous support such as a metal porous body or a metal mesh can be used. In addition, a composite fabric agent in which a fiber cloth such as a glass cloth or a metal mesh is coated with a fluororesin such as PTFE can be used as the porous support.

The porous support may have a thickness of, for example, 0.05 mm or more and 0.3 mm or less. The basis weight of the porous support may be 30 g/m ² or more and 100 g/m ² or less. A porous support having a thickness and basis weight in this range can achieve a balance between the thickness of the composite membrane and the effect of the porous support in improving the handleability and strength.

As described above, the fluororesin porous membrane may be provided with oil repellency. By providing the fluororesin porous membrane with oil repellency, when the amount of oil absorbed by the oil absorbing layer increases, it is possible to prevent oil from reaching the fluororesin porous membrane from the oil absorbing layer. Also, instead of providing the fluororesin porous membrane itself with oil repellency, an oil barrier layer can be provided between the fluororesin porous membrane and the oil absorbing layer.

The oil barrier layer contains an oil-repellent material and can block the movement of oil from the oil absorption layer to the fluororesin porous membrane. The oil barrier layer is porous and does not impede ventilation. The oil barrier layer can be, for example, an oil-repellent nonwoven fabric. Examples of the oil-repellent material contained in the oil barrier layer include at least one selected from the group consisting of fluorine-based coating agents such as perfluoroalkyl group-containing polymers, silicon-based coating agents such as methylhydrogenpolysiloxane, wax-based coating agents such as paraffin and polyolefin wax, zirconium salt coating agents such as zirconium fatty acid salts, ethyleneurea-based coating agents such as octadecylethyleneurea, methylolamide-based coating agents such as N-methylolstearic acid amide, pyridinium salt-based coating agents such as stearamide methylpyridinium chloride, and metal soap-based coating agents such as aluminum stearate. Fluorine-based and silicon-based oil-repellent materials are preferred because they can impart heat resistance. A specific form of the oil barrier layer can be, for example, a nonwoven fabric imparted with oil repellency by the oil-repellent coating agent. The form of the oil barrier layer is not limited to this, and any film that is oil repellent and breathable can be used as the oil barrier layer.

The oil barrier layer may have a thickness of, for example, 0.05 mm or more and 0.3 mm or less. The basis weight of the porous support may be 30 g/ ^m2 or more and 100 g/ ^m2 or less. An oil barrier layer having a thickness and basis weight within this range can prevent adhesion of oil to the porous membrane without impeding ventilation, and can prevent a decrease in breathability.

The fluororesin porous membrane and oil absorption layer contained in the composite membrane, as well as the oil barrier layer and porous support that may be optionally contained, all have a membrane or sheet shape. Because the composite membrane is a laminate of these parts, it is not a three-dimensional bulky component, but is compact. Because the composite membrane has such a simple and compact structure, there are no restrictions on the design of the installation location of the composite membrane, and it can be used in a variety of devices. Another advantage of a composite membrane with such a structure is that it can be easily manufactured.

The composite membrane according to the embodiment can be manufactured, for example, by preparing a fluororesin porous membrane and an oil absorbing layer, stacking them, and fixing them together by heat fusion, ultrasonic fusion, or by using an adhesive or double-sided tape. When a porous support or oil barrier layer is included, these parts are also stacked and fixed appropriately so that they are arranged as described above.

The fluororesin porous membrane can be produced, for example, by stretching a fluororesin sheet having a non-porous structure (solid structure). The fluororesin sheet having a solid structure that serves as the precursor of the fluororesin porous membrane can be, for example, an extrusion molded product. As an example of extrusion molding, a method for producing a PTFE porous membrane is described below.

The raw materials for PTFE extrusion molded products can be PTFE fine powder and an extrusion aid such as a lubricant. By mixing PTFE fine powder with an extrusion aid such as a lubricant, a paste-like mixture can be obtained. By extruding the mixture containing PTFE fine powder into a sheet shape, a sheet-like extrusion molded product can be obtained.

Examples of extrusion aids include the commonly used solvent naphtha (e.g., registered trademark: Isoper E, manufactured by Exxon Chemical), white oil, and liquid paraffin having 6 to 12 carbon atoms (e.g., registered trademark: Cactus Normal Paraffin N-10, manufactured by Japan Energy Corporation).

The mixture obtained by mixing the PTFE fine powder with the extrusion aid may be aged before being subjected to extrusion molding. The aged mixture may also be compressed to produce a compression molded body (billet). By compressing, air in the PTFE fine powder can be removed, improving the uniformity of the extrusion molded product. The shape of the compression molded body is not particularly limited, but may be, for example, cylindrical.

Next, the PTFE sheet manufactured as a precursor for the fluororesin porous membrane is stretched. The stretching process may be either uniaxial or biaxial. When stretched, polymer fibers are pulled out from the polymer material contained in the precursor and stretched in the stretching direction. In this way, a microstructure containing nodes and fibrils can be formed.

The average pore size of the fluororesin porous membrane can be adjusted by adjusting the stretching ratio during stretching. By increasing the stretching ratio during stretching, a fluororesin porous membrane with a large average pore size can be formed. By decreasing the stretching ratio during stretching, a fluororesin porous membrane with a small average pore size can be formed. The stretching ratio can be adjusted appropriately depending on the desired breathability and waterproofness.

Next, the unsintered fluororesin porous membrane is dried and sintered. Drying is performed, for example, in a drying oven. By drying at a temperature equal to or higher than the boiling point of the petroleum-based extrusion aid used earlier, the aid can be volatilized. Next, the dried porous membrane is heated and sintered at a temperature equal to or higher than the melting point of PTFE, for example, at a temperature equal to or higher than 330°C. Firing can be performed, for example, by fixing the stretched material so that the porous membrane does not shrink. In this way, the fluororesin porous membrane can be produced.

As the oil absorption layer and the oil barrier layer, for example, a porous body such as a nonwoven fabric made of the above-mentioned material can be prepared. As the porous support, for example, a porous body such as a nonwoven fabric or a mesh sheet made of the above-mentioned material can be prepared.

Specific examples of composite membranes according to the embodiments are described below with reference to the drawings.

An example of a composite membrane is shown in Figures 1 and 2. Figure 1 is a cross-sectional view that shows a schematic example of a composite membrane. Figure 2 is a cross-sectional view that conceptually shows the function of this composite membrane.

The composite membrane 1 shown in the figure includes a fluororesin porous membrane 3 and an oil absorbing layer 2 laminated on one of the main surfaces of the fluororesin porous membrane 3. In the example of FIG. 1, the composite membrane 1 further includes a porous support 4 on the main surface of the fluororesin porous membrane 3 opposite the surface on which the oil absorbing layer 2 is provided.

In Figures 1 and 2, the upper side of each figure is the side where the composite membrane 1 may be exposed to oil, such as the outside of a device in which the composite membrane 1 is installed. The composite membrane 1 has an oil absorbing layer 2 as its outermost surface, so it can maintain breathability even if oil adheres to it. Oil droplets 22 that adhere to the composite membrane 1 adhere to and are adsorbed by the oil absorbing layer 2. Even if some of the pores in the oil absorbing layer 2 are blocked by oil, the ventilation path in the composite membrane 1 is three-dimensional, so ventilation in the composite membrane 1 can be ensured.

In the example shown in Figures 1 and 2, the composite membrane 1 is provided with a porous support 4, but the porous support 4 may be omitted.

Figure 3 shows another example of a composite membrane. The composite membrane 1 shown in the figure includes a fluororesin porous membrane 3 and an oil absorbing layer 2 laminated on each of the main surfaces of the fluororesin porous membrane 3. The composite membrane 1 of the example shown in Figure 3 is installed in equipment used in an environment where both sides of the membrane may be exposed to oil. Since the oil absorbing layers 2 are provided on the outermost surfaces of both main surfaces of the composite membrane 1, oil can be adsorbed by the oil absorbing layers 2 regardless of which side the oil adheres to, and ventilation in the composite membrane 1 can be ensured.

Figure 4 shows a modified composite membrane. The composite membrane 1 of this modified example includes a fluororesin porous membrane 3, an oil absorbing layer 2, and an oil barrier layer 5 interposed therebetween. The oil barrier layer 5 and the oil absorbing layer 2 are laminated in sequence on one main surface of the fluororesin porous membrane 3. In the example of Figure 4, the composite membrane 1 further includes a porous support 4 on the main surface of the fluororesin porous membrane 3 opposite the main surface on the oil barrier layer 5 and oil absorbing layer 2 side. The porous support 4 may be omitted.

In the modified example shown in FIG. 4, the oil barrier layer 5 is provided between the oil absorbing layer 2 and the fluororesin porous membrane 3, so that even if the amount of oil adsorbed in the oil absorbing layer 2 increases, the oil does not reach the fluororesin porous membrane 3. Therefore, the composite membrane 1 can tolerate a large amount of oil before a decrease in breathability occurs.

Figure 5 shows another modified example of the composite membrane. The composite membrane 1 shown in Figure 5 includes a fluororesin porous membrane 3, and an oil barrier layer 5 and an oil absorbing layer 2 laminated in sequence on each main surface of the fluororesin porous membrane 3. In this modified example, an oil barrier layer 5 is interposed between the fluororesin porous membrane 3 and the oil absorbing layer 2 on both main surfaces of the fluororesin porous membrane 3, and the amount of oil that can be tolerated before a decrease in air permeability occurs is increased on both sides of the composite membrane 1.

(Sample preparation)
A nonwoven fabric-reinforced PTFE porous film (C-Porous (registered trademark) SEF-501N, manufactured by Chukoh Chemical Industry Co., Ltd.) was prepared by laminating a polyethylene terephthalate (PET) nonwoven fabric as a porous support and a 0.05 mm thick fluororesin porous membrane made of polytetrafluoroethylene (PTFE). The prepared film was cut into a size of 100 mm x 100 mm to obtain Sample No. 1 as a standard for performance evaluation.

A 2 mm thick polypropylene (PP) nonwoven fabric (Tufnel® Oil Blotter® AB-50, manufactured by Mitsui Chemicals, Inc.) was prepared as an oil absorbing layer. A 100 mm x 100 mm piece of nonwoven fabric-reinforced PTFE porous film was cut out in the same way as sample No. 1, and a composite membrane (three-layer sheet) was prepared by laminating the PP nonwoven fabric by heat fusion. Several composite membranes were prepared, and each was designated as sample No. 2-9.

In addition, a nonwoven fabric-reinforced PTFE porous film (two-layer sheet) similar to sample No. 1 was prepared as sample No. 10 for performance comparison.

(evaluation)
For each composite film of sample No. 3-9, engine oil (EVOLUTION 900 RACING 1 manufactured by elf) was sprayed in mist form over the entire surface from the oil absorbing layer side. For the single film of sample No. 10, engine oil was sprayed in mist form over the entire surface from one main surface side. Engine oil was sprayed onto each sample in the amount shown in Table 1 below. Sample No. 1 was used as a control for the evaluation standard, and was not treated in any way. Furthermore, engine oil was not applied to sample No. 2 either.

The air permeability of each sample coated with engine oil as described above was measured using the Gurley air permeability test in accordance with JIS P8117:2009. The air permeability of samples No. 1 and 2 was also measured. More specifically, the time (Gurley seconds) required for a specified volume of air to permeate the sample was calculated as the air permeability (air resistance). The shorter the time required for air to permeate, the higher the air permeability. The air permeability obtained for sample No. 1 was used as the standard, and the increase in air resistance for each of the other samples was calculated in comparison (increase in air permeability resistance for sample No. X = [(air permeability of sample No. X - air permeability of sample No. 1) / air permeability of sample No. 1] x 100%).

The amount of engine oil applied to each sample and the evaluation results are summarized in Table 1 below. The amount of engine oil applied is shown as the amount applied to the entire sample and the amount applied per area. The evaluation results show the air permeability measured by the air permeability test and the rate of increase in air resistance compared to sample No. 1. However, for sample No. 10, the test was not completed even after 20 minutes had passed since the start of the air permeability test, so it was determined that there was no air permeability.

As shown by the results of Sample No. 2-3 in Table 1, no decrease in breathability was observed when a small amount of engine oil, about 200 mg, was applied. As shown by the results of Sample No. 4-9, a decrease in breathability was observed from an application amount of 400 mg, and as the amount of engine oil applied to the composite film increased, the breathability decreased. However, in Sample No. 10, which was a reinforced PTFE film alone, the breathability was almost completely lost even with the application of 100 mg of engine oil, while in the composite film with an oil absorption layer, as shown by Sample No. 9, even when 5,000 mg of engine oil was applied, the breathability was not lost, although a decrease in performance was observed.

As described above, by laminating a porous oil-absorbing layer containing an oil-philic material onto a fluororesin porous membrane, a composite membrane can be obtained in which the decrease in breathability due to oil adsorption is suppressed.

The present invention is not limited to the above-described embodiment, and various modifications can be made in the implementation stage without departing from the gist of the invention. The embodiments may be implemented in appropriate combination, in which case the combined effects can be obtained. Furthermore, the above-described embodiment includes various inventions, and various inventions can be extracted by combinations selected from the multiple constituent elements disclosed. For example, if the problem can be solved and an effect can be obtained even if some constituent elements are deleted from all the constituent elements shown in the embodiment, the configuration from which these constituent elements are deleted can be extracted as an invention.

1...composite membrane, 2...oil absorption layer, 3...fluororesin porous membrane, 4...porous support, 5...oil barrier layer, 10...breathable membrane, 13...resin porous membrane, 14...porous support, 15...oil-repellent treated resin porous membrane, 20...gas molecules, 21...water droplets, 22...oil droplets, 23...oil, 24...oil film.

Claims (5)

A fluororesin porous membrane;
A composite membrane comprising the fluororesin porous membrane and an oil-absorbing layer which contains an oil-philic material and is porous and laminated with the fluororesin porous membrane. The composite membrane according to claim 1, wherein the fluororesin porous membrane contains polytetrafluoroethylene. The composite membrane according to claim 1 or 2, wherein the oil-absorbing layer is a polypropylene nonwoven fabric. The composite membrane according to claim 1 or 2, further comprising a porous oil barrier layer containing an oil-repellent material between the fluororesin porous membrane and the oil-absorbing layer. The composite membrane according to claim 1 or 2, further comprising a porous support on the main surface of the fluororesin porous membrane opposite the oil absorbing layer.