Radio wave-transmissive sheet and radio wave-transmissive structure

Abstract

Provided are a radio wave-transmissive sheet and a radio wave-transmissive structure that, by being configured to have good radio wave transmissivity and to be heat-resistant, experience little change in relative permittivity and can suppress effects on radio wave transmissivity, even in actual use environments.　A radio wave-transmissive sheet 10 comprises a foam 11 having a radio wave incidence surface 11A and a radio wave radiation surface 11B that is the surface on the opposite side from the radio wave incidence surface 11A. The foam 11 is a crosslinked body containing a thermoplastic resin, or the foam 11 contains 50 mass% or more of a resin with a softening point of 83°C or greater with respect to the whole resin. The density of the radio wave incidence surface 11A side of the foam 11 is 0.04 g / cm3 to 0.25 g / cm3, and the dielectric loss tangent (tanδ) of the foam 11 at 76-81 GHz is 0.015 or less.

Description

Radio wave transparent sheet and radio wave transparent structure

[0001] The present invention relates to a radio wave transparent sheet and a radio wave transparent structure.

[0002] Detection technologies using high-frequency radio waves are being adopted for applications requiring high-density information detection and long-distance detection. For example, there is increasing demand for millimeter-wave radar, which uses high-frequency millimeter-wave bands as detection technology for vehicles that need to detect long-distance distances between themselves and objects moving at high speeds.

[0003] In millimeter-wave radar, when millimeter waves pass through a forward object such as a cover member, the millimeter waves are reflected by the forward object and thermally lost, which can reduce the amount of millimeter waves that penetrate to the outside. Furthermore, because millimeter waves become detectable when they bounce off the target object and pass through the forward object such as a cover member again, the loss due to the transparency of the forward object has a square effect, and therefore, radio wave-transmitting materials are required for forward objects such as cover members (see, for example, Patent Document 1).

[0004] Special Publication No. 2023-5220331

[0005] As radio wave permeable materials to be used for frontal objects such as cover members, materials containing air and having a low dielectric constant are generally considered, but appropriate materials have not yet been defined because radio wave permeability involves a complex intertwining of properties such as thickness, dielectric constant, and dielectric loss. In addition, actual millimeter wave radars often reach high temperatures due to heat in the vehicle environment and their own heat generation, and with general foams, the amount of air contained changes at high temperatures, which can affect radio wave permeability.

[0006] The present invention has been made to solve such problems, and aims to provide a radio wave transparent sheet and a radio wave transparent structure that have good radio wave transparency and heat resistance, so that even in actual usage environments there is little change in dielectric constant and the impact on radio wave transparency can be suppressed.

[0007] The present invention has been made to solve the above-mentioned problems, and the gist of the present invention is as follows: [1] A radio wave transparent sheet made of a foam having a radio wave incident surface and a radio wave emitting surface which is the surface opposite to the radio wave incident surface, wherein the foam is a crosslinked body containing a thermoplastic resin, or the foam contains 50 mass % or more of a resin with a softening point of 83°C or higher relative to the entire resin, and the density of the radio wave incident surface side of the foam is 0.04 g / cm ３ 0.25g / cm or more ３ or less, and the foam has a dielectric loss tangent (tan δ) of 0.015 or less at 76 to 81 GHz. [2] The radio wave transmission sheet according to [1], wherein the rate of thickness reduction in a heat resistance test in an 80°C environment is 15% or less. [3] The radio wave transmission sheet according to [1] or [2], wherein the foam has a gel fraction of 10 to 55%. [4] The radio wave transmission sheet according to any one of [1] to [3], wherein the foam has a thickness of 0.8 mm or less. [5] The radio wave transmission sheet according to any one of [1] to [4], wherein the foam has a relative dielectric constant of 1.05 to 3.1 at 76 to 81 GHz. [6] The radio wave transmission sheet according to any one of [1] to [5], wherein the foam has an average cell diameter in the thickness direction (ZD direction) of 15 to 200 μm. [7] The radio wave transmission sheet according to claim 1, wherein the foam has a closed cell ratio of 80% or more. [8] The radio wave transmission sheet according to any one of [1] to [7], wherein the foam is a multilayered body, and at least two foam layers constituting the foam have different densities. [9] The density of each foam layer constituting the foam is 0.04 g / cm ３ 0.25g / cm or more ３

[10] The radio wave transmitting sheet according to [8], wherein the density (a) of the foam layer on the radio wave incident surface side is 0.04 g / cm or less, and the relationship between the density (a) of the foam layer on the radio wave incident surface side and the density (b) of the foam layer on the radio wave emitting surface side is b / a > 1.0. ３ 0.15g / cm or more ３The radio wave transmission sheet according to [8] or [9], wherein the dielectric constant at 76 to 81 GHz of the foam layer on the radio wave incident surface side is 1.05 to 3.0, the dielectric constant at 76 to 81 GHz of the foam layer on the radio wave emitting surface side is 1.10 to 3.0, and the dielectric constant at 76 to 81 GHz of the foam layer on the radio wave incident surface side is lower than the dielectric constant at 76 to 81 GHz of the foam layer on the radio wave emitting surface side.

[12] The radio wave transmission sheet according to any of [1] to

[11] , wherein a pressure-sensitive adhesive layer is provided on the foam.

[13] The radio wave transmission sheet according to

[12] , wherein the thickness of the pressure-sensitive adhesive layer is 0.01 mm to 0.25 m.

[14] The radio wave transmission sheet according to

[12] or

[13] , wherein the dielectric constant of the pressure-sensitive adhesive layer is 1.5 to 3.0.

[15] The radio wave transparent sheet according to any one of

[12] to

[14] , wherein the pressure-sensitive adhesive layer has a dielectric loss tangent (tan δ) of 0.035 or less at 76 to 81 GHz.

[16] A radio wave transparent structure comprising the radio wave transparent sheet according to any one of [1] to

[15] and a resin sheet.

[17] The radio wave transparent structure according to

[16] , wherein the resin sheet has a modulus of elasticity of 1,500 MPa or more.

[18] The radio wave transparent structure according to

[16] or

[17] , wherein the radio wave transparent sheet and the resin sheet are joined by fusion bonding.

[0008] According to the present invention, by having a configuration that improves radio wave transmittance and is heat resistant, it is possible to provide a radio wave transmittance sheet and a radio wave transmittance structure that can suppress the effect on radio wave transmittance by causing little change in dielectric constant even in actual usage environments.

[0009] FIG. 1 is a schematic cross-sectional view (part 1) of a radio wave transmission sheet according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view (part 2) of a radio wave transmission sheet according to an embodiment of the present invention. FIG. 3 is a schematic cross-sectional view (part 3) of a radio wave transmission sheet according to an embodiment of the present invention. FIG. 4 is a schematic cross-sectional view (part 4) of a radio wave transmission sheet according to an embodiment of the present invention. FIG. 5 is a schematic cross-sectional view (part 5) of a radio wave transmission structure according to an embodiment of the present invention. FIG. 1 is a schematic cross-sectional view (part 1) of a radio wave transmission structure according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view (part 2) of a radio wave transmission structure according to an embodiment of the present invention.

[0010] [Radio Wave Transmitting Sheet] As shown in Fig. 1, the radio wave transmitting sheet 10 of the present invention comprises a foam 11 having a radio wave incident surface 11A and a radio wave emitting surface 11B, which is the surface opposite the radio wave incident surface 11A. The radio wave incident surface 11A and the radio wave emitting surface 11B are the main surfaces of the sheet-like foam 11. In the foam 11, the radio wave incident surface 11A is one surface onto which an outgoing radio wave L1, such as a millimeter wave, of a high-frequency radio wave emitted from a radar device 20 is incident, and the radio wave emitting surface 11B is the other surface from which the outgoing radio wave L1 is emitted. However, in the radio wave transmitting sheet 10, the radio wave emitting surface 11B is also one surface onto which a reflected radio wave L2 emitted from the radar device 20 and reflected by an object 30 is incident, and the radio wave incident surface 11A is also the other surface from which the reflected radio wave L2 is emitted.

[0011] The radio wave transmission sheet 10 of the present invention may have a multi-layer foam as shown in Fig. 2. In Fig. 2, the foam is multi-layered, and the foam layer 11 １ , 11 ２ Although the foam has two layers, it is not limited to two layers and may have three or more layers. When the foam has multiple layers, it is preferable that the densities of at least two foam layers constituting the foam are different from each other, and it is preferable that the densities of the foam layers are different from each other. Furthermore, when the foam has multiple layers, it is preferable that the foam layer in the foam 11 has two layers.

[0012] (Density) The density of the foam 11 on the radio wave incident surface 11A side is 0.04 g / cm ３ 0.25g / cm or more ３ The density of the foam 11 on the radio wave incident surface 11A side is 0.04 g / cm ３ If the density of the foam 11 on the radio wave incident surface 11A side is less than 0.04 g / cm, it is not possible to impart an appropriate mechanical strength to the foam 11, and the thickness may decrease or the dielectric constant may change in a thermal environment, which may affect the radio wave transmittance. ３ It is preferable that the density of the foam 11 on the radio wave incident surface 11A side is 0.25 g / cm or more because mechanical strength can be obtained. ３If the density of the foam 11 on the radio wave incident surface 11A side is greater than 0.25 g / cm, the radio wave reflectivity at the radio wave incident surface 11A increases, and the radio wave transmittance decreases. ３ If the foam is a single layer, the density of the foam on the radio wave incident surface 11A side refers to the density of the single-layer foam. If the foam is a multi-layer foam, the density of the foam on the radio wave incident surface 11A side refers to the density of the foam layer arranged on the radio wave incident surface 11A side, and it is preferable that the density of the foam layer arranged closest to the radio wave incident surface 11A side among the multiple foam layers is within the above range. If the foam is a single layer, from the viewpoint of improving both mechanical strength and radio wave transmittance in a balanced manner, the density of the foam 11 on the radio wave incident surface 11A side is 0.08 g / cm ３ 0.25g / cm or more ３ It is preferable that the density is 0.10 g / cm or less. ３ 0.25g / cm or more ３ More preferably, it is 0.12 g / cm or less. ３ 0.25g / cm or more ３ Less than 0.04g / cm ３ 0.25g / cm or more ３ When the foam is multi-layered, from the viewpoint of improving both the mechanical strength and the radio wave transmittance in a well-balanced manner, the density of each foam layer is preferably 0.04 g / cm or less. ３ 0.50g / cm or more ３ It is preferable that the concentration is 0.04 g / cm or less. ３ 0.25g / cm or more ３ It is preferable that the density is 0.05 g / cm or less. ３ 0.25g / cm or more ３ More preferably, it is 0.06 g / cm or less. ３ 0.23g / cm or more ３ It is more preferable that the density of the foam in this specification means the apparent density, and is a value measured in accordance with JIS K 7222 (2005).

[0013] In the radio wave transmission sheet 10 of the present invention, when the foam is multilayered, the relationship between the density (a) of the foam layer on the radio wave incident surface 11A side and the density (b) of the foam layer on the radio wave emitting surface 11B side is preferably b / a>1.0, more preferably b / a≧2.0, and even more preferably b / a≧3.0. By ensuring that the relationship between the density (a) of the foam layer on the radio wave incident surface 11A side and the density (b) of the foam layer on the radio wave emitting surface 11B side satisfies the above range, an appropriate density relationship can be achieved, thereby improving radio wave transmission. Furthermore, the relationship between the density (a) of the foam layer on the radio wave incident surface 11A side and the density (b) of the foam layer on the radio wave emitting surface 11B side is not particularly limited, but is preferably b / a≦25, more preferably b / a≦20, and even more preferably b / a≦15, from the viewpoint of the mechanical strength of the foam. Furthermore, in the case of a multilayered structure, the density (a) of each foam layer on the radio wave incident surface side is 0.04 g / cm ３ 0.15g / cm or more ３ It is preferable that the density is 0.043 g / cm or less. ３ 0.15g / cm or more ３ It is preferable that the density is 0.046 g / cm or less. ３ 0.15g / cm or more ３ In the case of a multi-layer structure, the density (b) of the foam layer on the radio wave emitting surface side is preferably 0.08 g / cm or less from the viewpoint of the mechanical strength of the foam and the electromagnetic wave transmittance. ３ 1.5g / cm or more ３ It is preferable that the density is 0.16 g / cm or less. ３ 0.8g / cm or more ３ More preferably, it is 0.17 g / cm or less. ３ 0.6g / cm or more ３ More preferably, it is 0.18 g / cm or less. ３ 0.4g / cm or more ３It is even more preferable that the density (b) is not more than 1 / 2 of the foam layers. When the number of foam layers is three or more, it is preferable that the density (b) of the foam layer that is arranged closest to the radio wave emitting surface among the plurality of foam layers is within the above range. Furthermore, when the number of foam layers is three or more, from the viewpoint of radio wave transmittance, it is sufficient that the densities of the foam layers gradually increase from the radio wave incident surface side to the radio wave emitting surface side, that is, it is preferable that the foam layer that is arranged closest to the radio wave emitting surface among the plurality of foam layers has the highest density.

[0014] (Dielectric Dissipation Factor) The foam of the radio wave transmission sheet has a dielectric dissipation factor (tanδ) of 0.015 or less at 76 to 81 GHz, calculated from the amount of air. If the dielectric dissipation factor (tanδ) of the foam at 76 to 81 GHz exceeds 0.015, heat loss increases when high-frequency radio waves pass through the foam, resulting in reduced radio wave transmission. From the above viewpoints, the dielectric dissipation factor (tanδ) of the foam at 76 to 81 GHz is preferably 0.012 or less, more preferably 0.010 or less, and even more preferably 0.009 or less. The lower limit of the dielectric dissipation factor (tanδ) of the foam at 76 to 81 GHz is not particularly limited as long as it is 0 or more, but may be, for example, 0.0001 or more, or 0.0002 or more. In addition, when the foam of the radio wave transmission sheet 10 of the present invention is a multilayered foam, the dielectric loss tangent (tan δ) at 76 to 81 GHz calculated from the air content of the foam refers to the dielectric loss tangent (tan δ) at 76 to 81 GHz of each foam layer constituting the multilayered foamed layer. In this specification, the dielectric loss tangent (tan δ) at 76 to 81 GHz calculated from the air content of the foam can be measured by the method described in the Examples below. The dielectric loss tangent (tan δ) of the foam can be adjusted by the type of resin, the apparent density of the foam, the additives contained in the foam, etc.

[0015] (Thickness reduction rate in heat resistance test in 80°C environment) The thickness reduction rate of the radio wave transmission sheet in a heat resistance test in an 80°C environment is preferably 15% or less, more preferably 12% or less, and even more preferably 9% or less. When the thickness reduction rate of the radio wave transmission sheet in a heat resistance test in an 80°C environment is below the above upper limit, no change in radio wave transmittance occurs in the actual use environment, and performance is stable. The lower the thickness reduction rate, the better, for example, 0% or more. When the foam of the radio wave transmission sheet 10 of the present invention is a single layer, the thickness reduction rate of the radio wave transmission sheet in a heat resistance test in an 80°C environment refers to the thickness reduction rate of the single layer foam. Furthermore, when the foam of the radio wave transmission sheet 10 of the present invention is a multilayer, it refers to the thickness reduction rate when the heat resistance test is performed on the entire foam consisting of the multilayer foam layers. Furthermore, when an adhesive layer is provided as described below, the thickness reduction rate refers to the thickness reduction rate of the radio wave transmission sheet including the adhesive layer. The thickness reduction rate of the radio wave transmitting sheet in the heat resistance test in an 80°C environment in this specification can be measured by the method described in the examples below.

[0016] (Gel Fraction) The gel fraction of the foam is preferably 10 to 65%, more preferably 10 to 55%, and even more preferably 15 to 53%. When the gel fraction of the foam is within the above range, appropriate heat resistance and density can be achieved, and stable radio wave transmission can be maintained. When the foam of the radio wave transmission sheet 10 of the present invention is a single layer, the gel fraction of the foam refers to the gel fraction of the single layer foam. When the foam of the radio wave transmission sheet 10 of the present invention is a multilayer foam, the gel fraction of the foam refers to the gel fraction of each foam layer, and it is preferable that all of the gel fractions of each foam layer satisfy the above range. The gel fraction of the foam in this specification can be measured by the method described in the Examples below.

[0017] (Thickness) The thickness of the foam is preferably 0.8 mm or less, more preferably 0.7 mm or less, and even more preferably 0.6 mm or less. Having a foam thickness equal to or less than the above upper limit results in good radio wave transmission. When the foam of the radio wave transmission sheet 10 of the present invention is a single layer, the thickness of the foam refers to the thickness of the single layer foam. When the foam of the radio wave transmission sheet 10 of the present invention is a multilayer foam, the thickness of the foam refers to the total thickness obtained by adding up the thicknesses of the individual foam layers. When the foam is a multilayer foam, the thickness of each foam layer is not particularly limited, but is preferably 0.1 to 0.7 mm, more preferably 0.15 to 0.65 mm, and even more preferably 0.2 to 0.6 mm. The thickness of the foam in this specification can be measured by the method described in the Examples below.

[0018] (Relative dielectric constant) The relative dielectric constant of the foam at 76 to 81 GHz is preferably 1.05 to 3.1, preferably 1.1 to 2.0, and more preferably 1.3 to 1.7. When the relative dielectric constant of the foam at 76 to 81 GHz is within the above range, radio wave transmission is improved. When the foam of the radio wave transmission sheet 10 of the present invention is a single layer, the relative dielectric constant of the foam at 76 to 81 GHz refers to the relative dielectric constant of the single layer foam at 76 to 81 GHz. When the foam of the radio wave transmission sheet 10 of the present invention is a multilayer, the relative dielectric constant of the foam at 76 to 81 GHz refers to the relative dielectric constant of each foam layer at 76 to 81 GHz, and it is preferable that all of the relative dielectric constants of each foam layer at 76 to 81 GHz satisfy the above range. Note that the relative dielectric constant of the foam at 76 to 81 GHz in this specification can be measured by the method described in the Examples below.

[0019] In the radio wave transmission sheet 10 of the present invention, when the foam is multi-layered, the relative dielectric constant of the foam layer on the radio wave incident surface 11A side at 76 to 81 GHz is preferably 1.05 to 3.0, more preferably 1.06 to 1.5, and even more preferably 1.07 to 1.35. Furthermore, when the foam is multi-layered, the relative dielectric constant of the foam layer on the radio wave incident surface 11A side at 76 to 81 GHz is preferably lower than the relative dielectric constant of the foam layer on the radio wave emitting surface 11B side at 76 to 81 GHz. Note that when the foam has three or more foam layers, the relative dielectric constant of the foam layer located closest to the radio wave incident surface among the multiple foam layers should be within the above range. In the radio wave transmission sheet 10 of the present invention, when the foam is multi-layered, the relative dielectric constant of the foam layer on the radio wave emitting surface 11B side at 76 to 81 GHz is preferably 1.10 to 3.0, more preferably 1.2 to 2.0, and even more preferably 1.25 to 1.7. In addition, when the number of foam layers is three or more, it is preferable that the relative permittivity of the foam layer that is arranged closest to the radio wave emitting surface among the plurality of foam layers is within the above range. When the relative permittivity of the foam layer laminated on the radio wave emitting surface 11B at 76 to 81 GHz satisfies the above condition, the dielectric constant inside the foam can be appropriately changed in stages, reflection of high-frequency radio waves can be prevented, and radio wave transmittance can be improved.

[0020] (Bubble Diameter) The average bubble diameter in the thickness direction (ZD direction) of the foam is preferably 15 to 200 μm, more preferably 30 to 190 μm, and even more preferably 45 to 180 μm. By having the average bubble diameter in the thickness direction (ZD direction) of the foam within the above range, interference between the emitted radio waves L1 and the reflected radio waves L2 at the foam's bubble film thickness can be suppressed, resulting in good radio wave transmittance. Furthermore, the average bubble diameter in the MD direction of the foam is preferably 50 to 300 μm, more preferably 60 to 290 μm, and even more preferably 70 to 280 μm. Furthermore, the average bubble diameter in the TD direction of the foam is preferably 60 to 350 μm, more preferably 70 to 340 μm, and even more preferably 80 to 330 μm. In this specification, MD direction (machine direction) is an abbreviation for Machine Direction and refers to the direction of flow of the composition during application, etc., as described below, and is the longitudinal direction of the sheet. Furthermore, the TD direction (vertical direction) is an abbreviation for Transverse Direction and is the direction perpendicular to the MD direction. When the foam of the radio wave transmission sheet 10 of the present invention is a single layer, the average bubble diameter of the foam refers to the average bubble diameter of the single-layer foam. When the foam of the radio wave transmission sheet 10 of the present invention is a multi-layer foam, the average bubble diameter of the foam refers to the average bubble diameter of each foam layer. It is preferable that at least the average bubble diameter of the foam layer on the radio wave incident surface side meets the above requirements, but it is preferable that all of the average bubble diameters of each foam layer meet the above range. The average bubble diameter of the foam in this specification can be measured by the method described in the examples below.

[0021] (Closed Cell Ratio) The closed cell ratio of the foam is preferably 80% or more, more preferably 85% or more, and even more preferably 90% or more. When the closed cell ratio of the foam is within the above range, the heat shrinkage resistance of the foam can be suppressed, and radio wave transmittance can be improved. When the foam of the radio wave transmission sheet 10 of the present invention is a single layer, the closed cell ratio of the foam refers to the closed cell ratio of the single layer foam. When the foam of the radio wave transmission sheet 10 of the present invention is a multilayer foam, the closed cell ratio of the foam refers to the closed cell ratio of each foam layer, and it is preferable that the closed cell ratio of all of the foam layers satisfy the above range. The closed cell ratio of the foam in this specification can be measured by the method described in the Examples below.

[0022] <Foam Layer> The foam in the radio wave transmission sheet of the present invention has a configuration (first configuration) of being a crosslinked body containing a thermoplastic resin. Alternatively, the foam in the radio wave transmission sheet of the present invention has a configuration (second configuration) of containing 50 mass% or more of a resin having a softening point of 83°C or higher, based on the total mass of the resin contained in the foam. When the foam in the radio wave transmission sheet of the present invention has either of the above configurations, it can be made to have heat resistance, and even in an actual use environment, there is little change in the dielectric constant, so that the effect on radio wave transmission can be suppressed. From the viewpoint of heat resistance, it is preferable that the foam have both the above first configuration and second configuration.

[0023] (First Component) Examples of the thermoplastic resin constituting the foam as the first component include olefin-based resins, acrylic-based resins, polyurethane-based resins, and thermoplastic elastomers. These may be used alone or in combination of two or more. The foam is preferably a crosslinked body containing an olefin-based resin.

[0024] Specific examples of olefin resins include polyethylene resins, polypropylene resins, and ethylene-vinyl acetate copolymers. Among these, from the viewpoint of heat resistance, at least one selected from polyethylene resins and polypropylene resins is preferred, and polypropylene resins are more preferred. Examples of polyethylene resins include polyethylene resins polymerized with a polymerization catalyst such as a Ziegler-Natta compound, a metallocene catalyst, or a chromium oxide compound, and preferably polyethylene resins polymerized with a metallocene catalyst are used.

[0025] Examples of metallocene catalysts include compounds such as bis(cyclopentadienyl) metal complexes, which have a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, examples include compounds in which one or more cyclopentadienyl rings or analogs thereof are present as ligands on a tetravalent transition metal such as titanium, zirconium, nickel, palladium, hafnium, or platinum. Such metallocene catalysts have uniform active site properties, with each active site having the same activity. Polymers synthesized using metallocene catalysts have high uniformity in molecular weight, molecular weight distribution, composition, and composition distribution, and therefore, when a sheet containing a polymer synthesized using a metallocene catalyst is crosslinked, the crosslinking proceeds uniformly. A uniformly crosslinked sheet is foamed uniformly, making it easier to stabilize its physical properties. Furthermore, the foam can be stretched uniformly, allowing for a uniform foam thickness.

[0026] Examples of the ligand include a cyclopentadienyl ring and an indenyl ring. These cyclic compounds may be substituted with a hydrocarbon group, a substituted hydrocarbon group, or a hydrocarbon-substituted metalloid group. Examples of hydrocarbon groups include a methyl group, an ethyl group, various propyl groups, various butyl groups, various amyl groups, various hexyl groups, 2-ethylhexyl groups, various heptyl groups, various octyl groups, various nonyl groups, various decyl groups, various cetyl groups, and a phenyl group. The term "various" refers to various isomers including n-, sec-, tert-, and iso-. Alternatively, a cyclic compound polymerized as an oligomer may be used as the ligand. In addition to π-electron unsaturated compounds, monovalent anionic ligands such as chlorine and bromine, or divalent anionic chelating ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, arylamides, phosphides, arylphosphides, and the like may also be used.

[0027] Examples of metallocene catalysts containing a tetravalent transition metal or ligand include cyclopentadienyltitanium tris(dimethylamide), methylcyclopentadienyltitanium tris(dimethylamide), bis(cyclopentadienyl)titanium dichloride, and dimethylsilyltetramethylcyclopentadienyl-t-butylamido zirconium dichloride. Metallocene catalysts, when combined with a specific cocatalyst (promoter), function as a catalyst during the polymerization of various olefins. Specific cocatalysts include methylaluminoxane (MAO) and boron-based compounds. The ratio of the cocatalyst to the metallocene catalyst is preferably 100,000 to 1,000,000 moles, and more preferably 50 to 5,000 moles.

[0028] The polyethylene resin is low-density polyethylene (density: 0.930 g / cm ３ less than 0.930 g / cm ３ 0.942g / cm or more ３ less than 0.942 g / cm ３ and linear low-density polyethylene.

[0029] Of the above polyethylene resins, low-density polyethylene and linear low-density polyethylene are preferred, and linear low-density polyethylene is more preferred. The linear low-density polyethylene is more preferably a linear low-density polyethylene obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more, based on the total amount of monomers) with a small amount of an α-olefin as needed. Specific examples of α-olefins include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Of these, α-olefins having 4 to 10 carbon atoms are preferred. The density of the polyethylene resin, for example, the linear low-density polyethylene described above, is 0.870 to 0.925 g / cm from the viewpoint of flexibility. ３ is preferably 0.890 to 0.925 g / cm ３ More preferably, 0.910 to 0.925 g / cm ３ As the polyethylene resin, a plurality of polyethylene resins may be used, and a polyethylene resin having a density outside the above range may be added.

[0030] The ethylene-vinyl acetate copolymer used as the olefin resin is, for example, an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene.

[0031] Examples of polypropylene resins include homopolypropylene and propylene-α-olefin copolymers containing 50% by mass or more of propylene. These may be used alone or in combination of two or more. Specific examples of the α-olefins constituting the propylene-α-olefin copolymers include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Among these, α-olefins having 6 to 12 carbon atoms are preferred.

[0032] Examples of the thermoplastic elastomer include olefin-based thermoplastic elastomers, styrene-based thermoplastic elastomers, vinyl chloride-based thermoplastic elastomers, polyurethane-based thermoplastic elastomers, polyester-based thermoplastic elastomers, and polyamide-based thermoplastic elastomers. These thermoplastic elastomers may be used alone or in combination of two or more. Among these, olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers are preferred, and styrene-based thermoplastic elastomers are more preferred.

[0033] Examples of olefin-based thermoplastic elastomers include blend-type, dynamically crosslinked-type, and polymerized-type elastomers. More specifically, examples include thermoplastic elastomers that use a thermoplastic crystalline polyolefin such as polypropylene or polyethylene for the hard segment and fully vulcanized or partially vulcanized rubber for the soft segment. Examples of the thermoplastic crystalline polyolefin include a homopolymer of an α-olefin having 1 to 4 carbon atoms or a copolymer of two or more α-olefins, with polyethylene or polypropylene being preferred. Examples of the soft segment component include butyl rubber, halobutyl rubber, EPDM, EPM, acrylonitrile / butadiene rubber, NBR, and natural rubber, with EPDM being preferred.

[0034] Further, olefin-based thermoplastic elastomers include block copolymer types. Block copolymer types include those having a crystalline block and a soft segment block, and more specifically, crystalline olefin block-ethylene-butylene copolymer-crystalline olefin block copolymer (CEBC) is exemplified. In CEBC, the crystalline olefin block is preferably a crystalline ethylene block, and examples of commercially available CEBCs include "DYNARON 6200P" manufactured by JSR Corporation. Examples of olefin-based thermoplastic elastomers include "Absortomer EP1001."

[0035] Examples of the styrene-based thermoplastic elastomer include block copolymers having a styrene polymer or copolymer block and a conjugated diene compound polymer or copolymer block. Examples of the conjugated diene compound include isoprene and butadiene. The styrene-based thermoplastic elastomer used in the present invention may or may not be hydrogenated. When hydrogenation is performed, the hydrogenation can be performed by a known method.

[0036] The styrene-based thermoplastic elastomer is usually a block copolymer, and examples thereof include styrene-isoprene block copolymer, styrene-isoprene-styrene block copolymer, styrene-butadiene block copolymer, styrene-butadiene-styrene block copolymer, styrene-ethylene / butylene-styrene block copolymer (SEBS), styrene-ethylene / propylene-styrene block copolymer (SEPS), styrene-ethylene / butylene block copolymer (SEB), styrene-ethylene / propylene block copolymer (SEP), styrene-ethylene / butylene-crystalline olefin block copolymer (SEBC), etc. As the above-mentioned styrene-based thermoplastic elastomer, a block copolymer is preferred, and among these, SEBS and SEBC are more preferred.

[0037] Commercially available styrene-based thermoplastic elastomers include those manufactured by JSR Corporation under the trade name "DYNARON 8600P" (styrene content: 15% by mass), "DYNARON 4600P" (styrene content: 20% by mass), and "DYNARON 1321P" (styrene content: 10% by mass), those manufactured by Kuraray Co., Ltd. under the trade name "HYBRAR 7311," and those manufactured by Asahi Kasei Corporation under the trade name "S.O.E. (registered trademark) S1609."

[0038] (Second Configuration) The foam as the second configuration contains 50% by mass or more of a resin with a softening point of 83°C or higher relative to the total resin content of the foam. Unless the resin with a softening point of 83°C or higher is less than 50% by mass relative to the total resin content and is not crosslinked as described above, the foam will not have sufficient heat resistance, resulting in a large change in dielectric constant in actual use environments and affecting radio wave transmittance. From the above perspective, the resin with a softening point of 83°C or higher is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more relative to the total resin content of the foam. In this specification, the term "softening point" refers to the temperature at which a substance, such as a resin, softens and begins to deform due to an increase in temperature. Typically, the temperature at which a substance becomes completely liquid when its temperature is increased is called the melting point. However, substances such as resins gradually soften and reach a molten state without exhibiting a clear melting point, making it difficult to identify a clear change in state. Therefore, the term "softening point" is sometimes used to distinguish it from the melting point. The softening point here can be represented by the Vicat softening temperature measured in accordance with Method A of JIS K7206:2016 “Plastics-Thermoplastics-Determination of Vicat Softening Temperature (VST)”.

[0039] Examples of resins having a softening point of 83°C or higher that constitute the foam as the second component include olefin resins, acrylic resins, polyurethane resins, and thermoplastic elastomers. These may be used alone or in combination of two or more. The olefin resins, acrylic resins, polyurethane resins, and thermoplastic elastomers described above in the first component can be used. The softening point of the resin having a softening point of 83°C or higher is preferably 97°C or higher, more preferably 105°C or higher. Polyethylene resins and polypropylene resins are preferred as resins having a softening point of 83°C or higher, and polypropylene resins are more preferred from the viewpoint of heat resistance.

[0040] The resin constituting the foam preferably contains one selected from the group consisting of polyethylene resin and polypropylene resin in an amount of 60 to 100 mass %, more preferably 70 to 100 mass %, and even more preferably 80 to 100 mass %, based on the total amount of resin contained in the foam. The one selected from the group consisting of polyethylene resin and polypropylene resin may have a softening point of 83°C or higher.

[0041] When a foam contains one resin selected from the group consisting of polyethylene resin and polypropylene resin, a resin other than these resins (other resins) may be used in combination. The other resins are not particularly limited, but include resins with a softening point of less than 83°C. Specifically, preferred examples include EVA, olefin-based thermoplastic elastomers, and styrene-based thermoplastic elastomers, with EVA being more preferred. The content of the other resins is preferably 40% by mass or less, more preferably 30% by mass or less, and even more preferably 20% by mass or less, based on the total resin content of the foam. Furthermore, it is more preferred that the one resin selected from the group consisting of polyethylene resin and polypropylene resin includes at least polypropylene resin. The use of polypropylene resin facilitates improved heat resistance. The content of polypropylene resin is preferably 30 to 100% by mass, more preferably 50 to 90% by mass, and even more preferably 60 to 85% by mass, based on the total resin content of the foam. When the foam is composed of multiple layers, the resin content and content of the entire foam should be as described above. However, it is also preferred that the resin content and content of each foam layer be within the above ranges.

[0042] The foam is preferably produced by foaming a foamable composition containing the resin described above. The foamable composition for forming the foam preferably contains a foaming agent, particularly a thermally decomposable foaming agent. The thermally decomposable foaming agent may be an organic foaming agent or an inorganic foaming agent. Examples of organic foaming agents include azo compounds such as azodicarbonamide, azodicarboxylic acid metal salts (e.g., barium azodicarboxylate), and azobisisobutyronitrile; nitroso compounds such as N,N'-dinitrosopentamethylenetetramine; hydrazine derivatives such as hydrazodicarbonamide, 4,4'-oxybis(benzenesulfonylhydrazide), and toluenesulfonylhydrazide; and semicarbazide compounds such as toluenesulfonylsemicarbazide. Examples of inorganic foaming agents include ammonium carbonate, sodium carbonate, ammonium bicarbonate, sodium bicarbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate. Among these, from the viewpoint of obtaining fine bubbles and from the viewpoints of economy and safety, azo compounds are preferred, and azodicarbonamide is more preferred. One type of thermal decomposition type foaming agent may be used alone, or two or more types may be used in combination.

[0043] The amount of foaming agent in the foamable composition for forming a foam is preferably 1 to 15 parts by mass, more preferably 1.5 to 10 parts by mass, and even more preferably 2 to 5 parts by mass, per 100 parts by mass of the resin. By adjusting the amount of foaming agent to be equal to or greater than the above-mentioned lower limit, it is possible to impart a certain level of flexibility to the foam. Furthermore, by adjusting the amount of foaming agent to be equal to or less than the above-mentioned upper limit, it is possible to prevent excessive foaming and improve the mechanical strength of the foam.

[0044] (Additives) The foam (i.e., the foamable composition) in the radio wave transmission sheet of the present invention may contain additives as long as the object of the present invention is not impaired. Examples of various additives that can be added to the foam include foaming aids, antioxidants, and crosslinking aids. Examples of various additives that can be added to the foam include dielectric loss tangent adjusters such as glass fiber and calcium carbonate, from the viewpoint of adjusting the dielectric loss tangent of the foam. Only one type of additive may be used, or two or more types may be used in combination. In addition to the additives mentioned above, the additives may contain additives commonly used in foams, such as heat stabilizers, colorants, flame retardants, antistatic agents, and fillers, as necessary.

[0045] The foamable composition for forming a foam may contain a foaming aid as an additive. Examples of the foaming aid include zinc oxide and urea. The content of the foaming aid is preferably 0.1 to 8 parts by mass, more preferably 0.15 to 4 parts by mass, and even more preferably 0.2 to 2 parts by mass, per 100 parts by mass of the resin.

[0046] The foamable composition for forming a foam may contain an antioxidant as an additive. Examples of antioxidants include phosphorus-based antioxidants, phenol-based antioxidants, sulfur-based antioxidants, and amine-based antioxidants. The antioxidant may be a mixture of the above-listed antioxidants, depending on the purpose. The content of the antioxidant is preferably 0.1 to 8 parts by mass, more preferably 0.15 to 4 parts by mass, and even more preferably 0.2 to 2 parts by mass, per 100 parts by mass of the resin.

[0047] The foamable composition for forming a foam may contain a crosslinking aid as an additive. Examples of crosslinking aids that can be used include polyfunctional monomers. More specifically, examples of crosslinking aids include trifunctional (meth)acrylate compounds such as trimethylolpropane trimethacrylate and trimethylolpropane triacrylate; compounds having three functional groups in one molecule such as trimellitic acid triallyl ester, 1,2,4-benzenetricarboxylic acid triallyl ester, and triallyl isocyanurate; bifunctional (meth)acrylate compounds such as 1,6-hexanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, and neopentyl glycol dimethacrylate; compounds having two functional groups in one molecule such as divinylbenzene, diallyl phthalate, diallyl terephthalate, and diallyl isophthalate; ethylvinylbenzene, lauryl methacrylate, and stearyl methacrylate. These crosslinking aids can be used alone or in combination of two or more. The content of the crosslinking aid is preferably 1.0 to 6.0 parts by mass, more preferably 1.8 to 4.9 parts by mass, and even more preferably 2.5 to 3.8 parts by mass, based on 100 parts by mass of the resin. When the foam has multiple layers, it is preferable that each foam layer is formed by foaming the foamable composition.

[0048] <Adhesive Layer> The radio wave transmitting sheet 10 of the present invention may be composed of a foam 11 alone, or may have a configuration in which an adhesive layer 12 is provided on the foam 11, as shown in FIG. 3 . The adhesive layer 12 is preferably configured to be laminated on the radio wave emitting surface 11B side of the foam 11. The adhesive layer 12 may be an adhesive layer made of a known adhesive. The adhesive that constitutes the adhesive layer is not particularly limited, and examples that can be used include acrylic adhesives, urethane adhesives, and rubber adhesives. Furthermore, a release sheet such as release paper may be further attached to the surface of the adhesive layer.

[0049] The adhesive composition for forming the adhesive may contain various additives, such as a tackifier resin, in addition to a main adhesive component such as an acrylic resin, a urethane resin, or a rubber component. Examples of tackifier resins include rosin resins, terpene resins, phenolic resins, terpene-phenolic resins, xylene resins, coumarone resins, ketone resins, and modified resins thereof. Terpene-phenolic resins are resins having a terpene structure and a phenol structure. One type of tackifier resin may be used alone, or two or more types may be used in combination. The content of the tackifier resin in the adhesive composition is preferably 10 to 200 parts by mass, more preferably 20 to 150 parts by mass, and even more preferably 30 to 125 parts by mass per 100 parts by mass of the main adhesive component.

[0050] The pressure-sensitive adhesive layer preferably contains bubbles. The pressure-sensitive adhesive layer contains bubbles, which can reduce the dielectric constant and also reduce the dielectric loss tangent of the radio wave transmission sheet even when the pressure-sensitive adhesive layer is present. The pressure-sensitive adhesive layer may contain bubbles by incorporating expanded particles into the pressure-sensitive adhesive composition and expanding the expanded particles. Alternatively, pre-expanded hollow expanded particles may be blended into the pressure-sensitive adhesive composition.

[0051] Examples of the expanded particles include the above-mentioned thermally decomposable foaming agents and thermally expandable microparticles. The thermally expandable microparticles are preferably thermally expandable microcapsules. Examples of the thermally expandable microcapsules include microcapsules whose shell resin is a copolymer primarily composed of a thermoplastic resin, such as acrylonitrile, methacrylonitrile, or vinylidene chloride, and which encapsulate a low-boiling substance, such as a hydrocarbon having approximately 3 to 8 carbon atoms. The thermally expandable microcapsules are preferably pre-expanded hollow particles and blended into the pressure-sensitive adhesive composition. The expanded particles may be added as needed to adjust the dielectric constant and dielectric loss tangent of the pressure-sensitive adhesive layer. The pressure-sensitive adhesive composition may also contain various additives conventionally used in pressure-sensitive adhesives, such as fillers, plasticizers, softeners, pigments, dyes, polymerization initiators, flame retardants, and thickeners.

[0052] The thickness of the pressure-sensitive adhesive layer 12 is preferably 0.01 mm to 0.25 mm, more preferably 0.025 mm to 0.20 mm, and even more preferably 0.04 mm to 0.15 mm. When the thickness of the pressure-sensitive adhesive layer is within the above range, the pressure-sensitive adhesive function is maintained without adversely affecting the radio wave transparency of the radio wave transmission sheet 10.

[0053] The relative dielectric constant of the pressure-sensitive adhesive layer 12 is preferably 1.5 to 3.0, more preferably 1.6 to 2.9, and even more preferably 1.7 to 2.8. When the relative dielectric constant of the pressure-sensitive adhesive layer is within the above range, the radio wave transparency of the radio wave transmission sheet 10 is not adversely affected.

[0054] The dielectric dissipation factor (tan δ) of the pressure-sensitive adhesive layer 12 at 76 to 81 GHz is preferably 0.035 or less, more preferably 0.030 or less, even more preferably 0.025 or less, and even more preferably 0.020 or less. The lower limit of the dielectric dissipation factor (tan δ) of the pressure-sensitive adhesive layer 12 at 76 to 81 GHz is not particularly limited as long as it is 0 or more, and may be, for example, 0.0001 or more, or 0.0002 or more. When the dielectric dissipation factor (tan δ) of the pressure-sensitive adhesive layer 12 at 76 to 81 GHz is within the above range, the radio wave transmittance is good. The dielectric dissipation factor (tan δ) of the pressure-sensitive adhesive layer 12 at 76 to 81 GHz in this specification can be measured by the method described in the Examples below.

[0055] <Method for manufacturing radio wave transmission sheet> (Method for manufacturing foam) The foam as the first component in the radio wave transmission sheet of the present invention can be manufactured by a known method, but specifically, it is industrially advantageous to manufacture it by the procedure shown in the following steps (1) to (3). In particular, when a polyolefin resin is used as the resin, it is preferable to manufacture it by the procedure shown in the following steps (1) to (3). Step (1): A step of supplying each component of a foamable composition containing a resin and a foaming agent to a kneading device and kneading them, and then obtaining a foamable composition. Step (2): A step of irradiating the foamable composition obtained in step (1) with ionizing radiation to crosslink it. Step (3): A step of heating the foamable composition crosslinked in step (2) to a temperature equal to or higher than the decomposition temperature of the foaming agent to foam it, thereby obtaining a foam.

[0056] In the step (1), the foamable composition is kneaded using a kneader such as a Banbury mixer or a pressure kneader, and then continuously extruded using an extruder, a calender, conveyor belt casting, or the like, to produce, for example, a sheet-like foamable composition.

[0057] Examples of the ionizing radiation used in step (2) include α-rays, β-rays, γ-rays, and electron beams, with electron beams being preferred. The dose of ionizing radiation may be any dose that can achieve a desired degree of crosslinking, and is preferably 0.5 to 12 Mrad, more preferably 1.0 to 9 Mrad, and even more preferably 1.5 to 7 Mrad.

[0058] In step (3), the temperature at which the foamable composition is heated to foam depends on the decomposition temperature of the thermally decomposable foaming agent used as the foaming agent, but is usually 140 to 300° C., preferably 150 to 260° C. In step (3), the foam may be stretched in either or both of MD and TD after or while foaming.

[0059] In the case where the foam is a multilayer foam as described above, the foamable compositions for forming the respective foam layers in step (1) may be laminated by co-extrusion. However, a multilayer foam may also be formed by laminating a plurality of foams prepared by the above method.

[0060] When the radio wave transmitting sheet has a pressure-sensitive adhesive layer, the pressure-sensitive adhesive layer can be obtained by applying a pressure-sensitive adhesive composition to one side of the foam produced as described above and drying it as necessary. Alternatively, the pressure-sensitive adhesive layer can be obtained by transferring a pressure-sensitive adhesive layer previously formed on a release film or the like onto the foam.

[0061] [Radio wave transparent structure] As shown in Figures 4 and 5, the radio wave transparent structure 40 of the present invention includes the radio wave transparent sheet 10 described above and a resin sheet 41. As shown in Figures 6 and 7, the resin sheet 41 can be configured to be adhered to the foam 11 by an adhesive layer 12. The adhesive layer 12 can be the same as that described above. The resin sheet 41 is preferably configured to be laminated on the radio wave emitting surface 11B side of the foam 11. The resin sheet 41 is a member that can transmit the emitted radio waves L1 and the reflected radio waves L2, and corresponds to, for example, a cover member attached to the radar device 20 or a bumper component member of a vehicle.

[0062] Specific examples of resin components constituting the resin sheet 41 include polyesters (PES) such as polyethylene terephthalate (PET), polyethylene furanoate (PEF), polylactic acid (PLA), polytrimethylene terephthalate (PTT), polybutylene terephthalate (PBT), and polybutylene succinate (PBS), polyolefins such as polyethylene (PE), polypropylene (PP), and ethylene-vinyl acetate copolymer (EVA), polyurethane (PU), triacetyl cellulose (TAC), and polyamide (PA). These resin components may be used alone or in combination. The resin sheet 41 is preferably a non-foamed material.

[0063] In order to improve mechanical strength, the resin sheet 41 may contain additives such as fibrous materials and mineral materials in addition to the above resin components. Examples of fibrous materials include inorganic fibers such as glass fiber, carbon fiber, boron fiber, and metal fibers such as copper and iron, as well as organic fibers such as acrylic, PBO, and nylon. Examples of mineral materials include talc and calcium carbonate. One type of fibrous material or mineral material may be used alone, or two or more types may be used in combination. Among these, glass fiber is preferred from the viewpoints of radio wave transparency, mechanical strength, and the like. The content of the fibrous material or mineral material is not particularly limited, but is, for example, 5 to 100 parts by mass, preferably 10 to 80 parts by mass, and more preferably 15 to 50 parts by mass per 100 parts by mass of the resin component.

[0064] (Elastic modulus of resin sheet) The elastic modulus of the resin sheet is preferably 1,500 MPa or more, more preferably 1,550 MPa or more, and even more preferably 1,600 MPa or more. When the elastic modulus of the resin sheet is equal to or greater than the above lower limit, the mechanical strength of the radio wave transparent structure 40 can be improved, and sufficient durability can be obtained in actual use environments. The elastic modulus of the resin sheet is not particularly limited, but is, for example, 20,000 MPa or less, preferably 15,000 MPa or less, and more preferably 10,000 MPa or less. The elastic modulus of the resin sheet is synonymous with the tensile elastic modulus measured in accordance with JIS K 7161.

[0065] (Relative dielectric constant of resin sheet) The relative dielectric constant of the resin sheet at 76 to 81 GHz is preferably 1.5 to 4.0, preferably 1.9 to 3.8, and more preferably 2.4 to 3.4. When the relative dielectric constant of the resin sheet at 76 to 81 GHz is within the above range, radio wave transmittance is improved. Note that the relative dielectric constant of the resin sheet at 76 to 81 GHz in this specification can be measured by the method described in the examples below.

[0066] (Dielectric Loss Tangent of Resin Sheet) The dielectric loss tangent (tan δ) of the resin sheet at 76 to 81 GHz is preferably 0.01 to 0.05, more preferably 0.015 to 0.045, and even more preferably 0.02 to 0.04. When the dielectric loss tangent (tan δ) of the resin sheet at 76 to 81 GHz is within the above range, the radio wave transmittance is good. Note that the dielectric loss tangent (tan δ) of the resin sheet at 76 to 81 GHz in this specification can be measured by the method described in the examples below.

[0067] (Thickness reduction rate of radio wave transparent structure in heat resistance test in 80°C environment) The thickness reduction rate of the radio wave transparent structure in a heat resistance test in an 80°C environment is preferably 10% or less, more preferably 7% or less, and even more preferably 5% or less. When the thickness reduction rate of the radio wave transparent structure in a heat resistance test in an 80°C environment is below the above upper limit value, no change in radio wave transmittance occurs in the actual usage environment and performance is stable. The lower the thickness reduction rate of the radio wave transparent structure in a heat resistance test in an 80°C environment, the better, for example, it may be 0% or more. Note that the thickness reduction rate of the radio wave transparent structure in the heat resistance test in an 80°C environment in this specification can be measured by the method described in the Examples below.

[0068] The radio wave transparent structure 40 of the present invention can be configured such that the above-mentioned pressure-sensitive adhesive layer 12 is omitted and the foam of the radio wave transparent sheet 10 is joined to the resin sheet 41 by fusion bonding. The fusion of the radio wave transparent sheet 10 and the resin sheet 41 can be achieved by applying energy to the radio wave transparent sheet 10 and the resin sheet 41 at the joining portion to melt one of them and then applying pressure to join the radio wave transparent sheet 10 and the resin sheet 41. Examples of means for fusing the radio wave transparent sheet 10 and the resin sheet 41 include heat fusion, high-frequency fusion, and ultrasonic fusion.

[0069] <Method for manufacturing radio wave transparent structure> A method for manufacturing the radio wave transparent structure of the present invention includes manufacturing a radio wave transparent sheet by the radio wave transparent sheet manufacturing method described above and providing a resin sheet on one side of the radio wave transparent sheet. The radio wave transparent sheet and the resin sheet can be joined by applying a pressure-sensitive adhesive, adhesive, etc. to the joining surface between the radio wave transparent sheet and the resin sheet, or by appropriately providing a pressure-sensitive adhesive layer. Another method for joining the radio wave transparent sheet and the resin sheet is to join the radio wave transparent sheet 10 and the resin sheet 41 by fusion bonding.

[0070] [Uses] The radio wave transparent sheet and radio wave transparent structure of the present invention can be used, for example, as a spacer or a component that doubles as a spacer for a millimeter-wave radar mounted on a vehicle such as an automobile, without any particular limitation. Specifically, the radio wave transparent sheet is used by being attached to the back side of a bumper or the inner surface of a radome as a cover member accompanying a millimeter-wave radar. Furthermore, the radio wave transparent structure can be used as a component that constitutes a bumper or a radome as a cover member accompanying a millimeter-wave radar, and the resin sheet provided in the radio wave transparent structure may constitute part of the bumper or part of the radome (for example, the bottom surface of the radome).

[0071] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0072] [Measurement Methods] The methods for measuring and evaluating each physical property are as follows.

[0073] <Thickness of each layer (thickness of each layer after foaming)> Cross sections of the radio wave transparent sheet and the radio wave transparent structure were photographed using a digital microscope (manufactured by Keyence Corporation, product name VHX-900), and the thicknesses of the foam and the resin sheet were measured from the photographed images.

[0074] <Thickness of Multilayer Foam (Total Foam Thickness)> When the foam is multilayered, the total thickness of the foam is defined as the total thickness of the foam.

[0075] <Expansion Ratio> The foam constituting the radio wave permeable sheet and the radio wave permeable structure has an apparent density (g / cm ３ The reciprocal of this was taken as the expansion ratio (times).

[0076] <Density (Apparent Density)> The foam and adhesive layer constituting the radio wave transparent sheet and radio wave transparent structure are measured for density (apparent density) (g / cm 3 ) in accordance with JIS K 7222 (2005). ３ ) was measured.

[0077] <Average bubble diameter> The foam constituting the radio wave transmission sheet and the radio wave transmission structure was cut into 50 mm squares, immersed in liquid nitrogen for 1 minute, and then cut in the thickness direction along each of the MD and TD. A 200x magnified photograph was taken using a digital microscope (Keyence Corporation, product name VHX-900). In the foam in the photographed image, the MD and ZD bubble diameters and the TD and ZD bubble diameters were measured for all bubbles present in the 2 mm long cut surfaces in each of the MD and TD, and this operation was repeated 5 times. The average values ​​of the bubble diameters of all the bubbles in each of the MD and TD were taken as the average bubble diameters of MD and TD, and the average value of the bubble diameters of all the ZDs measured by the above operation was taken as the average bubble diameter of ZD.

[0078] <Gel fraction> A test piece of about 100 mg was taken from the foam constituting the radio wave transmission sheet and the radio wave transmission structure, and the mass C (mg) of the test piece was precisely weighed. Next, this test piece was immersed in 30 cm of xylene at 120°C. 3 After immersion for 24 hours, the insoluble matter on the mesh was filtered through a 200-mesh wire netting and collected. The insoluble matter was dried in vacuum and the mass D (mg) of the insoluble matter was precisely weighed. The gel fraction (mass%) was calculated from the obtained value using the following formula: Gel fraction (mass%) = 100 × (D / C)

[0079] <Closed Cell Ratio> This was measured according to the method of ASTM D2856:1998. Specifically, the measurement was carried out as follows. First, a flat square test piece with a side length of 5 cm was cut out from the foam constituting the radio wave transmission sheet and the radio wave transmission structure. Then, the thickness of the test piece was measured to determine the apparent volume V of the test piece. １ Calculate the weight of the test piece W１ Next, the volume V occupied by the bubbles was measured. ２ was calculated based on the following formula: The density of the matrix resin constituting the test piece was ρ (g / cm ３ The volume occupied by the bubbles V ２ =V １ -W １ Next, the test piece was submerged in distilled water at 23°C to a depth of 100 mm from the water surface, and a pressure of 15 kPa was applied to the test piece for 3 minutes. After that, the pressure was released in the water, and the test piece was left to stand for 1 minute. Then, the test piece was taken out of the water, and the water adhering to the surface of the test piece was removed, and the weight W of the test piece was measured. ２ The open cell rate F and closed cell rate F were measured based on the following formula: ２ The open cell ratio F １ (%) = 100 × (W ２ -W １ ) / V ２ Closed bubble rate F ２ (%) = 100 - F １

[0080] <Relative permittivity> Planar test pieces measuring 10 cm x 10 cm were cut out from the foam (or each foam layer, if multiple foams are used) constituting the radio wave transmission sheet and the radio wave transmission construct, the pressure-sensitive adhesive layer, and the resin sheet. The relative permittivity of the test pieces at frequencies of 76 to 81 GHz was continuously measured by the free space method (S-parameter method). The relative permittivity was calculated from the measured values ​​of the S-parameters obtained at 76 to 81 GHz using a free space evaluation measuring jig (a vector network analyzer (N5290A) manufactured by Keysight Technologies) and a permittivity calculation program (software manufactured by Keysight Technologies).

[0081] <Dielectric Loss Tangent> Planar test pieces measuring 10 cm x 10 cm were cut out from the radio wave transmission sheet and the foam (or each foam layer, if multiple foams are used), the pressure-sensitive adhesive layer, and the resin sheet constituting the radio wave transmission structure. The dielectric loss tangent of the test pieces at frequencies of 76 to 81 GHz was continuously measured by the free space method (S-parameter method). The dielectric loss tangent was calculated from the measured values ​​of the S-parameters obtained at 76 to 81 GHz using a free space evaluation measuring jig (a vector network analyzer (N5290A) manufactured by Keysight Technologies) and a dielectric constant calculation program (software manufactured by Keysight Technologies).

[0082] <Elastic Modulus> The elastic modulus (tensile elastic modulus) of the resin sheet constituting the radio wave transparent structure was measured in accordance with JIS K 7161.

[0083] <Heat Resistance (Thickness Reduction Rate)> For the foams constituting the radio wave transmission sheet and the radio wave transmission structure, the thickness reduction rate of the foam in a heat resistance test in an 80°C environment was measured in accordance with the heat shrinkage conditions of JIS K 6767. Specifically, first, the thickness X of the foam before heating was measured. Then, the foam was left in an 80°C environment for 22 hours, and then left in a 23°C environment for 1 hour, and the thickness Y of the foam after heating was measured. The thickness reduction rate (%) was calculated based on the following formula: Thickness reduction rate (%) = (1 - Y / X) x 100

[0084] <Radio wave transmittance> The radio wave transmittance sheet and the radio wave transmittance structure were cut into planar test pieces measuring 10 cm x 10 cm. The radio wave transmittance of the test pieces at frequencies of 76 to 81 GHz was continuously measured using the free space method (S parameter method). Using a free space evaluation measuring jig (Keysight Technologies Vector Network Analyzer (N5290A)) and a radio wave transmittance calculation program (Keysight Technologies software), the radio wave transmittance (S21) of the S parameter at 76 to 81 GHz was measured, and the radio wave transmittance was calculated from the measured radio wave transmittance. In addition, when the foam was multilayered, the foam 2 side was the radio wave incident surface side.

[0085] <Radio wave reflectivity> The radio wave transmission sheet and the radio wave transmission structure were cut into 10 cm x 10 cm planar test pieces. The radio wave transmission amount of the test pieces at frequencies of 76 to 81 GHz was continuously measured using the free space method (S parameter method). The radio wave reflection amount (S11) of the S parameter at 76 to 81 GHz was measured using a free space evaluation measuring jig (Keysight Technologies Vector Network Analyzer (N5290A)) and a radio wave transmission calculation program (Keysight Technologies software), and the radio wave reflectivity was calculated from the measured radio wave reflection amount. In addition, when the foam was multilayered, the foam 2 side was the radio wave incident surface side.

[0086] <Heat Resistance Evaluation> Heat resistance was evaluated based on the thickness reduction rate (%) calculated for the radio wave transparent sheet and the foam constituting the radio wave transparent structure. The evaluation criteria are as follows: (Evaluation Criteria) A: Thickness reduction rate is 5% or less B: Thickness reduction rate is more than 5% to less than 15% C: Thickness reduction rate is 15% or more

[0087] <Evaluation of Radio Wave Transmittance (Radio Wave Transmittance Sheet)> The radio wave transmittance of the radio wave transmittance sheet was evaluated based on the measured radio wave transmittance (dB). The evaluation criteria are as follows: (Evaluation Criteria) A: -0.20 dB or more B: Less than -0.20 dB to -0.30 dB or more C: -0.30 dB or less

[0088] <Evaluation of Radio Wave Permeability (Radio Wave Permeable Structure)> For the radio wave permeability structure, the radio wave permeability T1 of the radio wave permeability structure (foam + resin sheet) and the radio wave permeability T2 of the resin sheet alone were measured, and T3 was calculated using the formula T1 - T2, and the radio wave permeability was evaluated based on the calculated T3. The evaluation criteria are as follows: (Evaluation Criteria) A: T3 ≧ 0.30 dB B: 0.15 dB < T3 < 0.30 dB C: T3 ≦ 0.15 dB The materials used in each example and comparative example are as follows:

[0089] <Resins> Polyolefin resin (a): Kernel KF283 (manufactured by Japan Polyethylene Corporation, linear low-density polyethylene resin), softening point: 102°C Polyolefin resin (b): LE520H (Japan Polyethylene Corporation, low-density polyethylene resin), softening point: 95°C Polyolefin resin (c): 2022D (Prime Polymer Co., Ltd., linear low-density polyethylene resin), softening point: 97°C Polyolefin resin (d): NUC-3660 (ENEOS NUC Corporation, ethylene-vinyl acetate copolymer), softening point: 78°C Polyolefin resin (e): E-330 (Prime Polymer Co., Ltd., random polypropylene), softening point: 117°C Polyolefin resin (f): S-135 (Prime Polymer Co., Ltd., homopolypropylene), softening point: 125°C Polycarbonate resin (a): HE9119HR (Mitsubishi Engineer-Plastics Corporation, polycarbonate), softening point: 135°C

[0090] <Additives> Foaming agent: thermal decomposition type foaming agent (azodicarbonamide) Foaming assistant: OW-212F (Sakai Chemical Industry Co., Ltd., zinc oxide) Antioxidant: phenolic antioxidant (2,6-di-t-butyl-p-cresol) Antioxidant: two-type mixture of AO-60 (ADEKA Corporation, phenolic antioxidant) and Crack 400 (Ouchi Shinko Chemical Industry Co., Ltd., sulfur-based antioxidant) Crosslinking assistant: TND-46U (Kyoeisha Chemical Co., Ltd., light ester) Adjuster: dielectric dissipation factor adjuster (calcium carbonate) Flame retardant: FCX-210 (Teijin Limited, phosphorus-based flame retardant)

[0091] <Adhesive> Rubber-based adhesive: Quintac 3421 (Zeon Corporation, synthetic rubber) Tackifier resin: YS Resin PX1250 (Yasuhara Chemical Co., Ltd., terpene resin) Expanded particles: EMC-20B (Nippon Phillite Co., Ltd., expanded thermally expandable microcapsules)

[0092] <Resin Sheet> Sheet (a): X5061 (manufactured by Prime Polymer Co., Ltd., polypropylene, molded sheet) Sheet (b): PP-GF20-01 (manufactured by Polyplastics Co., Ltd., fiber-reinforced plastic obtained by blending 20 parts by mass of glass fiber with 100 parts by mass of polypropylene, molded sheet) Sheet (c): TECADUR PBT GF30 (Ensinger GmbH, fiber-reinforced plastic obtained by blending 30 parts by mass of glass fiber with 100 parts by mass of polybutylene terephthalate)

[0093] [Method of Producing Radio Wave Transmitting Sheet] The radio wave transmitting sheets of Examples 1 to 5 and Comparative Examples 1 to 6 were produced as follows. Note that all formulations in the tables are given in parts by mass. Example 1 Using an extruder, a polyolefin resin (a) was mixed with additives such as a foaming agent, a foaming aid, an antioxidant, a crosslinking aid, and others according to the formulations in Table 1 to produce a sheet-like foamable composition. Both sides of the obtained sheet-like foamable composition were irradiated with an electron beam at an acceleration voltage of 500 keV and an intensity of 7.0 Mrad, and the foamable composition was heated to 250°C to foam, and then stretched to a target thickness in a stretching step, thereby obtaining a foam as the radio wave transmitting sheet of Example 1.

[0094] <Examples 2, 4, and 5> Foams serving as radio wave permeable sheets were obtained in the same manner as in Example 1, except that the type of resin used to form the foam, the composition of each component, and the thickness were changed as shown in Table 1, and the amount of electron beam irradiation was adjusted so as to obtain the gel fraction shown in Table 1.

[0095] Example 3 Using a co-extruder, polyolefin resin (b) and additives such as a blowing agent, a foaming aid, and an antioxidant were blended according to the formulation in Table 1 to prepare a foamable composition for Foam 1, and polyolefin resin (b), a blowing agent, a foaming aid, and an antioxidant were blended according to the formulation in Table 1 to prepare a foamable composition for Foam 2. These foamable compositions were extruded by co-extrusion to obtain a laminate sheet in which the foamable compositions for Foam 1 and the foamable compositions for Foam 2 were laminated. Both surfaces of the obtained sheet-like laminate sheet were irradiated with an electron beam at an acceleration voltage of 500 keV and an intensity of 5.2 Mrad, and the foamable composition was heated to 250°C to foam, and then stretched to a target thickness in a stretching step to obtain a multilayer foam as the radio wave transparent sheet of Example 3.

[0096] Comparative Examples 1 to 6 Foams serving as radio wave transmitting sheets were obtained in the same manner as in Example 1, except that the type of resin used to form the foam, the composition of each component, and the thickness were changed as shown in Table 1, and the amount of electron beam irradiation was adjusted to achieve the gel fraction shown in Table 1. However, in Comparative Example 1, the electron beam irradiation itself was omitted.

[0097]

[0098] [Method for Producing Radio Wave Transparent Structure] The radio wave transparent structures of Examples 6 to 11 and Comparative Examples 7 to 9 were produced as follows. Example 6 A multilayer foam as a radio wave transparent sheet was obtained in the same manner as in Example 3, except that the type of resin used to form the foam, the composition of each component, and the thickness were changed as shown in Table 2, and the electron beam irradiation dose was adjusted to achieve the gel fraction shown in Table 2. Of the resulting multilayer foam, sheet (a) was placed as a resin sheet on the foam 1 side, and heat-sealed at 170°C to obtain a radio wave transparent structure.

[0099] Example 7 A radio wave transparent structure was obtained in the same manner as in Example 6, except that the composition of the components for forming the foam and the thickness were changed as shown in Table 1, and the amount of electron beam irradiation was adjusted so as to obtain the gel fraction shown in Table 2.

[0100] Example 8 A multilayer foam was obtained in the same manner as in Example 6, except that the formulation and thickness of the components used to form the foam were changed as shown in Table 2, and the electron beam irradiation dose was adjusted to achieve the gel fraction shown in Table 2. Next, a rubber-based adhesive and a tackifier resin were dissolved in toluene according to the formulation shown in Table 2, and then foamed particles were added and mixed. The solution was applied to the silicone surface of a release film SP3000-75 (manufactured by Toyo Cross Co., Ltd.) and dried at 110°C for 5 minutes to form a 0.10 mm thick adhesive layer, which was then placed on the radio wave emitting surface of the resulting foam. Next, a resin sheet (a) was attached to the foam via the adhesive layer to obtain a radio wave transparent structure.

[0101] Example 9 A radio wave transparent structure was obtained in the same manner as in Example 8, except that the composition of the components for forming the foam and the thickness were changed as shown in Table 2, the amount of electron beam irradiation was adjusted to obtain the gel fraction shown in Table 2, and sheet (b) was used as the resin sheet.

[0102] Example 10 A radio wave transparent structure was obtained in the same manner as in Example 8, except that the blending of the components for forming the foam and the thickness were changed as shown in Table 2, the electron beam irradiation dose was adjusted to achieve the gel fraction shown in Table 2, the composition of the pressure-sensitive adhesive composition was blended as shown in Table 2, and sheet (c) was used as the resin sheet.

[0103] Example 11 A radio wave transparent structure was obtained in the same manner as in Example 8, except that the blending of the components for forming the foam and the thickness were changed as shown in Table 2, the electron beam irradiation dose was adjusted to achieve the gel fraction shown in Table 2, the composition of the pressure-sensitive adhesive composition was blended as shown in Table 2, and sheet (b) was used as the resin sheet.

[0104] Comparative Example 7 A radio wave transparent structure consisting of only the sheet (a) was produced.

[0105] Comparative Example 8 A radio wave transparent structure consisting of only the sheet (b) was produced.

[0106] Comparative Example 9 A radio wave transparent structure consisting of only the sheet (c) was produced.

[0107]

[0108] [Method of Producing Radio Wave-Transmitting Constructs and Radio Wave-Transmitting Sheets] The radio wave-transmitting constructs of Examples 12 to 16 were produced as follows: All formulations in the tables are given in parts by mass.

[0109] Examples 12 to 14 Radio wave transparent structures were obtained in the same manner as in Example 8, except that the blending of the components for forming the foam and the thickness were changed as shown in Table 3, the amount of electron beam irradiation was adjusted so as to obtain the gel fraction shown in Table 3, and sheet (a) was used as the resin sheet.

[0110] Examples 15 and 16 Foams were obtained in the same manner as in Example 1, except that the type of resin used to form the foam, the composition of each component, and the thickness were changed as shown in Table 3, and the electron beam irradiation dose was adjusted to achieve the gel fraction shown in Table 3. Next, a pressure-sensitive adhesive layer was formed in the same manner as in Example 8, and a foam was obtained as a radio wave transmitting sheet, with the pressure-sensitive adhesive layer disposed on the radio wave emitting surface of the obtained foam.

[0111]

[0112] From the above results, the radio wave transmission sheets and radio wave transmission structures produced in the examples, which satisfied the requirements of the present invention, were good in both radio wave transmission and heat resistance. On the other hand, the radio wave transmission sheets and radio wave transmission structures produced in the comparative examples were not good in both radio wave transmission and heat resistance.

[0113] 10: Radio wave permeable sheet 11: Foam 11A: Radio wave incident surface 11B: Radio wave emitting surface 12: Adhesive layer 20: Radar device 30: Target object 40: Radio wave permeable structure 41: Resin sheet

Claims

1. A radio wave transmitting sheet made of a foam having a radio wave incident surface and a radio wave emitting surface on the opposite side of the radio wave incident surface, wherein the foam is a crosslinked body containing a thermoplastic resin, or the foam contains 50% by mass or more of a resin with a softening point of 83°C or higher relative to the entire resin, and the density of the radio wave incident surface side of the foam is 0.04 g / cm ３ 0.25g / cm or more ３ or less, and the dielectric loss tangent (tanδ) of the foam in the range of 76 to 81 GHz is 0.015 or less.

2. The radio wave transmitting sheet according to claim 1, which shows a thickness reduction rate of 15% or less in a heat resistance test in an 80°C environment.

3. The radio wave transparent sheet according to claim 1, wherein the gel fraction of the foam is 10 to 55%.

4. The radio wave transmitting sheet according to claim 1, wherein the foam has a thickness of 0.8 mm or less.

5. The radio wave transmitting sheet according to claim 1, wherein the foam has a relative dielectric constant of 1.05 to 3.1 at 76 to 81 GHz.

6. The radio wave transmitting sheet according to claim 1, wherein the foam has an average bubble diameter in the thickness direction (ZD direction) of 15 to 200 μm.

7. The radio wave transmitting sheet according to claim 1, wherein the closed cell rate of the foam is 80% or more.

8. The radio wave transparent sheet according to claim 1, wherein the foam is multi-layered, and at least two foam layers constituting the foam have different densities.

9. The density of each foam layer constituting the foam is 0.04 g / cm ３ 0.25g / cm or more ３ The radio wave transmitting sheet according to claim 8, wherein the relationship between the density (a) of the foam layer on the radio wave incident surface side and the density (b) of the foam layer on the radio wave emitting surface side is b / a>1.

0.

10. The density (a) of the foam layer on the radio wave incident surface side is 0.04 g / cm ３ 0.15g / cm or more ３ The radio wave transmission sheet according to claim 8, wherein:

11. The radio wave permeable sheet according to claim 8, wherein the foamed layer on the radio wave incident surface side has a relative dielectric constant of 1.05 to 3.0 at 76 to 81 GHz, the foamed layer on the radio wave emitting surface side has a relative dielectric constant of 1.10 to 3.0 at 76 to 81 GHz, and the relative dielectric constant of the foamed layer on the radio wave incident surface side at 76 to 81 GHz is lower than the relative dielectric constant of the foamed layer on the radio wave emitting surface side at 76 to 81 GHz.

12. The radio wave transparent sheet according to claim 1, further comprising an adhesive layer on the foam.

13. The radio wave transparent sheet according to claim 12, wherein the thickness of the adhesive layer is 0.01 mm to 0.25 mm.

14. The radio wave transparent sheet according to claim 12, wherein the dielectric constant of the adhesive layer is 1.5 to 3.

0.

15. The radio wave transparent sheet according to claim 12, wherein the dielectric loss tangent (tan δ) of the pressure-sensitive adhesive layer in the range of 76 to 81 GHz is 0.035 or less.

16. A radio wave transparent structure comprising the radio wave transparent sheet according to any one of claims 1 to 15 and a resin sheet.

17. The radio wave transparent structure according to claim 16, wherein the elastic modulus of the resin sheet is 1,500 MPa or more.

18. The radio wave transparent structure according to claim 16, wherein the radio wave transparent sheet and the resin sheet are joined by fusion.

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