(Meth)acrylic resin composition for millimeter wave radar cover, millimeter wave radar cover, millimeter wave radar device, and vehicle

Abstract

To provide a (meth)acrylic resin composition for millimeter wave radar which is excellent in millimeter wave permeability and weather resistance.SOLUTION: A (meth)acrylic resin composition for millimeter wave radar cover contains a (meth)acrylic polymer (P) containing 80-100 mass% of a methyl methacrylate-derived repeating unit, in which a content ratio of the (meth)acrylic polymer (P) is 90 mass% or more with respect to 100 mass% of the total mass of the (meth)acrylic resin composition, and a molding obtained by molding the (meth)acrylic resin composition has a dielectric constant at a measurement frequency when measured at frequencies of 70-90 GHz of 2.80 or less and dielectric loss tangent when measured at frequencies of 70-90 GHz of 0.006 or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a (meth)acrylic resin composition for millimeter-wave radar, which has excellent millimeter-wave transmittance and weather resistance, a millimeter-wave radar cover including the composition and which stores or protects a millimeter-wave radar unit that transmits or receives millimeter waves, and a millimeter-wave radar device and a vehicle including the millimeter-wave radar cover. [Background technology]

[0002] Efforts are underway to install millimeter wave radars in vehicles such as cars and motorcycles to ensure vehicle safety and enable autonomous driving, as well as to install millimeter wave radars on roads and intersections to alleviate traffic congestion. A millimeter-wave radar transmits electromagnetic waves (millimeter waves) in the frequency band of 30 GHz to 300 GHz, receives the reflected waves when the transmitted millimeter waves hit an object and are reflected back, and uses the difference between the transmitted and received waves to detect the presence of an obstacle, measure the distance, relative speed, and positional relationship (direction) between the vehicle in front and the vehicle itself, or measure the volume of vehicle traffic on roads and intersections.

[0003] A millimeter-wave radar typically comprises an antenna for transmitting or receiving millimeter waves, a millimeter-wave radar unit incorporating a control circuit, and a housing to house these, with a millimeter-wave radar cover (radome) attached to the front of the antenna to protect the antenna surface. Of these, the material that constitutes the millimeter wave radar cover is a resin molded body from the standpoint of vehicle weight reduction and safety, such as preventing scattering when broken.

[0004] In order for millimeter-wave radar to detect the distance and direction of an object with high accuracy, the resin material that makes up the millimeter-wave radar cover is required to have excellent millimeter-wave permeability, so that the emitted millimeter waves and the reflected waves are not attenuated by absorption or reflection when they pass through the millimeter-wave radar cover. It is known that the higher the relative dielectric constant of a resin material through which millimeter waves pass, the higher the reflectivity of the millimeter waves, resulting in attenuation of the millimeter waves. It is also known that the higher the dielectric loss tangent of a resin material through which millimeter waves pass, the greater the proportion of millimeter waves that are converted into heat when passing through, resulting in attenuation of the millimeter waves (for example, Patent Document 3, etc.). In other words, in order to improve millimeter wave transmission, a resin material with a low relative dielectric constant and dielectric loss tangent is required. In addition, from the viewpoint of design that gives a sense of luxury to a vehicle equipped with a millimeter-wave radar, the resin material constituting the millimeter-wave radar cover is required to have transparency in applications that do not include colorants such as dyes and pigments, and is required to clearly display the color tone of the colorant in applications that include a colorant. When the millimeter-wave radar cover is installed in a position that is likely to be exposed to direct sunlight outdoors, a resin material with insufficient weather resistance tends to reduce the transparency of the material itself, and the clarity of the color tone of the colorant tends to be impaired even in applications that include a colorant. In other words, the resin material constituting the millimeter-wave radar cover is required to have excellent weather resistance.

[0005] Resin materials used for millimeter wave radar covers are disclosed in the following Patent Documents 1 to 7. Patent Document 1 exemplifies a millimeter wave radar cover made of a resin having a relative dielectric constant of 3.0 or less, such as polycarbonate, syndiotactic polystyrene, polypropylene, or acrylonitrile-butadiene-styrene (ABS) resin. Patent Document 2 discloses a millimeter wave radar cover formed from cyclopolyolefin as a thermoplastic resin having a dielectric loss tangent of 0.0005 or less and a relative dielectric constant of 3 or less. Patent Document 3 discloses a millimeter wave radar cover made of a resin composition containing polycarbonate resin and alumina particles. Patent Document 4 discloses a millimeter wave radar cover made of a resin composition containing a polyphenylene ether resin and styrene-based resin alumina particles. Patent Document 5 discloses a millimeter wave radar cover made of a resin composition containing a polybutylene terephthalate resin and a cyclic olefin resin having a glass transition temperature of 100° C. or higher. Patent Document 6 discloses a millimeter wave radar cover formed from a thermoplastic resin composition containing (A) a rubber polymer reinforced resin and (B) a polyolefin resin. Patent Document 7 discloses a millimeter wave radar cover formed from a thermoplastic resin composition containing a rubber-polymer-reinforced vinyl resin having a polymer portion derived from an ethylene-α-olefin rubber having an ethylene unit content of 50 to 95 mass % and a vinyl resin portion. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2004-312696 A [Patent Document 2] JP 2007-13722 A [Patent Document 3] JP 2011-71881 A [Patent Document 4] JP 2013-102512 A [Patent Document 5] JP 2013-43942 A [Patent Document 6] JP 2016-121307 A [Patent Document 7] JP 2016-121334 A Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, the resin material of the millimeter wave radar cover is required to have excellent millimeter wave transmittance and weather resistance. However, the millimeter wave radar cover of Patent Document 1 has insufficient millimeter wave transmittance because the resin composition has a low relative dielectric constant but a high dielectric loss tangent. The millimeter wave radar covers of Patent Documents 2 to 7 are intended to improve millimeter wave transmittance by using a resin composition having a low relative dielectric constant and a low dielectric loss tangent, but have insufficient weather resistance.

[0008] Therefore, an object of the present invention is to provide a millimeter wave radar cover having excellent millimeter wave transmittance and weather resistance, a (meth)acrylic resin composition for a millimeter wave radar cover for providing this millimeter wave radar cover, and a millimeter wave radar device equipped with this millimeter wave radar cover. [Means for solving the problem]

[0009] The above problems are solved by the present invention described below. The first gist of the present invention is a (meth)acrylic resin composition for a millimeter wave radar cover, which contains a (meth)acrylic polymer (P) containing 80 mass% or more of repeating units derived from methyl methacrylate, wherein the content of the (meth)acrylic polymer (P) is 90 mass% or more relative to 100 mass% of the total mass of the (meth)acrylic resin composition, and a molded article obtained by molding the (meth)acrylic resin composition has a dielectric constant of 2.80 or less at a measurement frequency of 70 to 90 GHz and a dielectric loss tangent of 0.006 or less at a measurement frequency of 70 to 90 GHz. A second gist of the present invention is a millimeter wave radar cover comprising the (meth)acrylic resin composition for millimeter wave radar. A third gist of the present invention is a millimeter-wave radar device including a millimeter-wave radar unit mounted on a substrate, and a millimeter-wave radar cover provided on a front side of the millimeter-wave radar unit so as to cover the front side of the millimeter-wave radar unit, wherein the millimeter-wave radar cover includes the (meth)acrylic resin composition for millimeter-wave radar. A fourth aspect of the present invention is a vehicle equipped with the millimeter wave radar device. A fifth aspect of the present invention is a vehicle equipped with a millimeter wave radar device for detecting an object in front of the vehicle, the millimeter-wave radar device includes a millimeter-wave radar unit mounted on a substrate, and a millimeter-wave radar cover provided on a front side of the millimeter-wave radar unit so as to cover the front side of the millimeter-wave radar unit, the millimeter-wave radar cover includes the (meth)acrylic resin composition for millimeter-wave radar, and the millimeter-wave radar cover constitutes a front grille or emblem of the vehicle. Effect of the Invention

[0010] The present invention can provide a millimeter wave radar cover having excellent millimeter wave transmittance and weather resistance, a (meth)acrylic resin composition for a millimeter wave radar cover for providing this millimeter wave radar cover, and a millimeter wave radar device equipped with this millimeter wave radar cover. [Brief description of the drawings]

[0011] [Figure 1] 1 is a schematic cross-sectional view showing an example of a millimeter wave radar cover and a millimeter wave radar device according to the present invention. [Diagram 2] FIG. 2 is a schematic cross-sectional view showing another example of the millimeter wave radar cover and the millimeter wave radar device of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention will be described in detail below. In the present invention, "(meth)acrylate" means at least one selected from "acrylate" and "methacrylate", and "(meth)acrylic acid" means at least one selected from "acrylic acid" and "methacrylic acid". In the present invention, "monomer" means an unpolymerized compound, and "repeating unit" means a unit derived from a monomer formed by polymerization of the monomer. The repeating unit may be a unit formed directly by a polymerization reaction, or may be a unit in which a part of the unit is converted into a different structure by treating the polymer. In the present invention, "mass %" indicates the content ratio of a specific component contained in a total amount of 100 mass %. Unless otherwise specified, in this specification, a numerical range expressed with "~" means a range that includes the numerical values ​​before and after "~" as the lower and upper limits, and "A~B" means A or more and B or less.

[0013] <Millimeter wave radar cover> The millimeter wave radar cover of the present invention is a radar cover containing the (meth)acrylic resin composition for millimeter wave radar covers described below. A "radar cover" is also called a "radome" (or simply "radome") or a "sensor cover", and is a component used in either or both of the radar dome (radome) 2 or cover 3, as shown in Figure 1. The millimeter wave radar cover of the present invention has excellent millimeter wave transmittance and weather resistance, and is therefore suitable for applications requiring excellent design and long-term durability, such as millimeter wave radar covers for millimeter wave radar devices mounted on vehicles such as automobiles and motorcycles, and on moving objects such as railways, ships, aircraft, etc. In particular, in vehicles such as automobiles and motorcycles, when a millimeter wave radar is mounted, the millimeter wave radar cover of the present invention can also be used as an exterior member for a vehicle such as a front grille or emblem, or as a part of the exterior member for a vehicle.

[0014] In the millimeter wave radar cover of the present invention, a protective layer containing a synthetic resin containing acrylonitrile units and styrene units is provided on the front surface side, thereby imparting paintability, rigidity, processability, or a high-quality texture to the millimeter wave radar cover even without painting. Examples of the synthetic resin forming the protective layer include acrylonitrile-styrene resin (AS resin), acrylonitrile-ethylene-propylene-diene-styrene resin (AES resin), and acrylonitrile-butadiene-styrene resin (ABS resin).

[0015] Furthermore, in the millimeter wave radar cover of the present invention, the design can be improved by providing a metal film formed by vapor deposition of indium, aluminum, etc. between the protective layer and the surface layer of the millimeter wave radar cover of the present invention.

[0016] <(Meth)acrylic resin composition for millimeter wave radar cover> The (meth)acrylic resin composition for a millimeter wave radar cover is a constituent component of the millimeter wave radar cover of the present invention.

[0017] The (meth)acrylic resin composition for a millimeter wave radar cover of the present invention (hereinafter referred to as "the (meth)acrylic resin composition of the present invention") is a resin composition containing the (meth)acrylic polymer (P) described below. In the (meth)acrylic resin composition of the present invention, the lower limit of the content of the (meth)acrylic polymer (P) is 90 mass% or more relative to 100 mass% of the total mass of the (meth)acrylic resin composition, from the viewpoint of fully exhibiting the millimeter wave transmittance and weather resistance of the (meth)acrylic polymer (P). The upper limit of the content of the (meth)acrylic polymer (P) is not particularly limited, and may be 100% by mass.

[0018] The upper limit of the dielectric constant (εr) at a measurement frequency of 70 to 90 GHz of the molded article obtained by molding the (meth)acrylic resin composition of the present invention is 2.800 or less from the viewpoint of obtaining good millimeter wave transmittance of the millimeter wave radar cover. It is more preferably 2.700 or less. On the other hand, the lower limit of the dielectric constant is not particularly limited, but is preferably 2.400 or more, more preferably 2.500 or more from the viewpoint of eliminating the influence of disturbances due to electromagnetic waves other than millimeter waves around the millimeter wave radar device. The above upper and lower limits can be combined arbitrarily. Alternatively, the dielectric constant (εr) of a molded article obtained by molding the (meth)acrylic resin composition of the present invention at a measurement frequency of 70 to 90 GHz is preferably 2.400 or more and 2.700 or less, more preferably 2.500 or more and 2.700 or less.

[0019] The upper limit of the dielectric loss tangent (tan δ) at a measurement frequency of 70 to 90 GHz of the molded article molded from the (meth)acrylic resin composition of the present invention is 0.006 or less from the viewpoint of good millimeter wave transmittance of the millimeter wave radar cover. It is preferably 0.005 or less. The lower limit of the dielectric loss tangent is not particularly limited, but is preferably 0.001 or more, more preferably 0.002 or more, from the viewpoint of good maintenance of strength and processability of the molded article. The above upper and lower limits can be arbitrarily combined. Alternatively, the dielectric loss tangent (tan δ) of the molded article obtained by molding the (meth)acrylic resin composition of the present invention at a measurement frequency of 70 to 90 GHz is preferably 0.001 or more and 0.005 or less, more preferably 0.002 or more and 0.005 or less. The dielectric constant and dielectric loss tangent of the molded article obtained by molding the (meth)acrylic resin composition of the present invention can be controlled by appropriately selecting the copolymerization ratio of MMA and the alkyl (meth)acrylate (M) in the (meth)acrylic polymer (P) described later, or, when the alkyl (meth)acrylate (M) is used, the type of the alkyl (meth)acrylate. The method for measuring the dielectric constant (εr) and the dielectric tangent (tan δ) will be described later.

[0020] The lower limit of the refractive index of the (meth)acrylic resin composition of the present invention is preferably 1.40 or more as measured by sodium d line (589 nm) from the viewpoint of improving the gloss of the millimeter wave radar cover. On the other hand, the upper limit of the refractive index is preferably 1.55 or less from the viewpoint of suppressing glare of the millimeter wave radar cover and improving the appearance. The refractive index of the (meth)acrylic resin composition can be controlled by the copolymerization ratio of methyl methacrylate and alkyl (meth)acrylate (M) in the (meth)acrylic polymer (P) described later, or by appropriately selecting the type of alkyl (meth)acrylate (M) when used.

[0021] <(Meth)acrylic polymer (P)> The (meth)acrylic polymer (P) is one of the components of the (meth)acrylic resin composition of the present invention. Since the (meth)acrylic resin composition of the present invention contains the (meth)acrylic polymer (P), the millimeter wave radar cover obtained has good millimeter wave transmittance and weather resistance. The (meth)acrylic polymer (P) in the present invention is a (meth)acrylic polymer containing 80 mass % or more of repeating units derived from methyl methacrylate (hereinafter referred to as "MMA units").

[0022] When the (meth)acrylic polymer (P) contains 80% by mass or more of MMA units, the molded article obtained by molding the (meth)acrylic resin composition of the present invention has a dielectric constant of 2.800 or less at a measurement frequency of 70 to 90 GHz and a dielectric dissipation factor of 0.006 or less at a measurement frequency of 70 to 90 GHz, and therefore the millimeter wave transmittance of the obtained millimeter wave radar cover is good. Furthermore, when the (meth)acrylic polymer (P) contains 80 mass % or more of MMA units, the weather resistance of the obtained millimeter wave radar cover becomes good.

[0023] That is, the (meth)acrylic polymer (P) is a homopolymer of methyl methacrylate (MMA) or a copolymer containing 80% by mass or more and less than 100% by mass of MMA units based on the total mass of the (meth)acrylic polymer (P). When transparency is required for the millimeter wave radar cover, the (meth)acrylic polymer (P) is preferably a homopolymer of MMA.

[0024] The (meth)acrylic polymer (P) may be a homopolymer of MMA, from the viewpoint of improving the transparency of the obtained millimeter wave radar cover. Alternatively, from the viewpoint of improving the millimeter wave transmittance and weather resistance of the obtained millimeter wave radar cover, the (meth)acrylic polymer (P) preferably contains 80% by mass or more and 100% by mass or less of MMA units and more than 0% by mass and less than 20% by mass of repeating units derived from an alkyl (meth)acrylate (M) (excluding methyl methacrylate) having an alkyl group having 1 to 8 carbon atoms on a side chain (hereinafter referred to as "alkyl (meth)acrylate units"), more preferably contains 85% by mass or more and 99.5% by mass or less of MMA units and 0.5% by mass or more and 15% by mass or less of alkyl (meth)acrylate (M) units, and even more preferably contains 90% by mass or more and 98% by mass or less of MMA units and 2% by mass or more and 10% by mass or less of alkyl (meth)acrylate (M) units.

[0025] The alkyl (meth)acrylate (M) is not particularly limited as long as it is a monomer copolymerizable with methyl methacrylate, and examples thereof include methyl acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, iso-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl (meth)acrylate, n-hexyl (meth)acrylate, cyclohexyl (meth)acrylate, n-octyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate. These alkyl (meth)acrylates (M) may be used alone or in combination of two or more. The type and copolymerization ratio of these alkyl (meth)acrylates (M) may be selected so that the molded article obtained by molding the (meth)acrylic resin composition of the present invention has a dielectric constant of 2.800 or less at a measurement frequency of 70 to 90 GHz and a dielectric tangent of 0.006 or less at a measurement frequency of 70 to 90 GHz.

[0026] Among the alkyl (meth)acrylates (M), (meth)acrylate compounds other than methyl methacrylate are preferred since they are less likely to impair the inherent performance of the (meth)acrylic resin. Methyl acrylate, ethyl acrylate, and n-butyl acrylate are more preferred, and methyl acrylate and ethyl acrylate are even more preferred, since they provide good thermal decomposition resistance, weather resistance, and heat moldability to the resin molded product obtained by molding the (meth)acrylic resin composition of the present invention.

[0027] The (meth)acrylic polymer (P) can contain repeating units derived from monomers other than the alkyl (meth)acrylate (M) as long as the performance of the millimeter wave radar cover of the present invention is not impaired. The other monomer is not particularly limited as long as it is a monomer copolymerizable with methyl methacrylate and the alkyl (meth)acrylate (M), and examples thereof include the following a) to h). a) (meth)acrylate compounds such as dodecyl (meth)acrylate, tridecyl (meth)acrylate, stearyl (meth)acrylate, phenyl (meth)acrylate, benzyl (meth)acrylate, phenoxyethyl (meth)acrylate, isobornyl (meth)acrylate, glycidyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, norbornyl (meth)acrylate, adamantyl (meth)acrylate, dicyclopentenyl (meth)acrylate, and dicyclopentanyl (meth)acrylate; b) (meth)acrylic acid; c) (meth)acrylonitrile; d) (meth)acrylamide compounds such as (meth)acrylamide, N-dimethyl(meth)acrylamide, N-diethyl(meth)acrylamide, N-butyl(meth)acrylamide, dimethylaminopropyl(meth)acrylamide, N-methylol(meth)acrylamide, N-methoxymethyl(meth)acrylamide, N-butoxymethyl(meth)acrylamide, hydroxyethyl(meth)acrylamide, and methylenebis(meth)acrylamide; e) aromatic vinyl compounds such as styrene and α-methylstyrene; f) vinyl ether compounds such as vinyl methyl ether, vinyl ethyl ether, and 2-hydroxyethyl vinyl ether; g) vinyl carboxylate compounds such as vinyl acetate and vinyl butyrate; h) Olefin compounds such as ethylene, propylene, butene, and isobutene. These other monomers may be used alone or in combination of two or more.

[0028] When the (meth)acrylic polymer (P) contains a repeating unit derived from another monomer, the content ratio of the repeating unit derived from the other monomer in 100 mass% of the (meth)acrylic polymer (P) is preferably more than 0 mass% and not more than 10 mass%, and more preferably more than 0 mass% and not more than 5 mass%, since the content ratio is unlikely to impair the performance of the millimeter wave radar cover. As described above, a homopolymer of MMA that does not contain repeating units derived from other monomers can also be used as the (meth)acrylic polymer (P).

[0029] Examples of the method for producing the (meth)acrylic polymer (P) include bulk polymerization, suspension polymerization, emulsion polymerization, solution polymerization, etc. Among these polymerization methods, the (meth)acrylic polymer (P) is preferably produced by the bulk polymerization or suspension polymerization, more preferably by the bulk polymerization, since these tend to be excellent in productivity.

[0030] The mass average molecular weight of the (meth)acrylic polymer (P) is preferably from 20,000 to 200,000, and more preferably from 50,000 to 150,000. When the mass average molecular weight of the (meth)acrylic polymer (P) is equal to or greater than the lower limit, the molded article tends to have excellent mechanical properties, and when it is equal to or less than the upper limit, the flowability during melt molding tends to be excellent. The lower limit of the mass average molecular weight of the (meth)acrylic polymer (P) is more preferably 50,000 or more. The upper limit of the mass average molecular weight of the (meth)acrylic polymer (P) is more preferably 150,000.

[0031] In this specification, the mass average molecular weight is a value measured by gel permeation chromatography using standard polystyrene as a standard sample.

[0032] <Additives> To the (meth)acrylic resin composition of the present invention, general additives that are typically blended into normal thermoplastic resin compositions, such as flame retardants, flame retardant assistants (anti-dripping agents), phosphorus-based stabilizers, phenolic antioxidants, impact resistance improvers, release agents, ultraviolet absorbing agents, colorants such as dyes and pigments, antistatic agents, antifogging agents, lubricants / antiblocking agents, flow improvers, compatibilizers, plasticizers, dispersants, antibacterial agents, and inorganic fillers other than alumina, may be blended as necessary within the scope of the object of the present invention.

[0033] Furthermore, by blending a colorant such as a dye or pigment into the (meth)acrylic resin composition of the present invention, the millimeter wave radar cover of the present invention can exhibit a desired color tone. The dye or pigment is not particularly limited, and known dyes or pigments can be used. For example, at least one dye or pigment selected from the group consisting of red dyes, yellow dyes, green dyes, blue dyes, and purple dyes, and red pigments, yellow pigments, green pigments, blue pigments, and purple pigments can be used.

[0034] Alternatively, the (meth)acrylic resin composition of the present invention may contain carbon black as a colorant, thereby allowing the millimeter wave radar cover of the present invention to exhibit a deep jet black color. The type of carbon black is not particularly limited, and any known carbon black may be used. In particular, carbon black particles whose particle surfaces have been coated with an appropriate organic compound or the like to improve dispersibility in the (meth)acrylic resin composition are preferred. Furthermore, the millimeter wave radar cover of the present invention may exhibit a deep jet black color by using carbon black in combination with a dye that exhibits a black color tone.

[0035] <Manufacturing method for millimeter wave radar cover> The (meth)acrylic resin composition of the present invention or a molded article obtained by molding a composition containing the (meth)acrylic resin composition of the present invention may be used alone as a millimeter wave radar cover, or may be combined with other members to form a millimeter wave radar cover.

[0036] The method for producing the (meth)acrylic resin composition of the present invention is not particularly limited, and examples thereof include a method in which the (meth)acrylic polymer (P) and additives to be blended as necessary are melt-kneaded together to obtain pellets. The method for obtaining the millimeter wave radar cover of the present invention is not particularly limited, and for example, a method of molding pellets of the (meth)acrylic resin composition of the present invention into the shape of a millimeter wave radar cover using a known molding method such as injection molding, extrusion molding, or pressure molding. The obtained molded body may be further molded using a known molding method such as pressure molding or vacuum molding. Molding conditions such as molding temperature and molding pressure may be appropriately set.

[0037] When formed by the above-mentioned method, the shape of the millimeter wave radar cover of the present invention is not particularly limited and may have curved surfaces, corners, etc. as shown in Fig. 1 or Fig. 2, and may be determined according to the structure and shape of the millimeter wave radar device or antenna module. The millimeter wave radar cover of the present invention may be, for example, the cover 2 of the millimeter wave radar 1 shown in Fig. 1, or it may be a radome 3, or it may be both.

[0038] The thickness of the millimeter wave radar cover of the present invention is not particularly limited, but if it is too thick, the attenuation rate of the millimeter wave will be large, and if it is too thin, the module and antenna inside the millimeter wave radar will be visible, impairing the design, and the strength as a radar cover cannot be sufficiently secured. Usually, the thickness is about 1 to 5 mm.

[0039] <Millimeter wave transmittance> Since the millimeter wave radar cover of the present invention has a low relative dielectric constant and dielectric tangent, the attenuation rate of millimeter waves due to reflection and absorption when the transmitted millimeter waves and reflected waves pass through the cover can be kept low, and the millimeter wave transmittance is excellent. When the millimeter wave radar cover of the present invention is formed by molding the above-mentioned (meth)acrylic resin composition, it has a dielectric constant of 2.800 or less at a measurement frequency of 70 to 90 GHz and a dielectric loss tangent of 0.006 or less at a measurement frequency of 70 to 90 GHz, and is excellent in millimeter wave transmittance. More preferably, in the millimeter wave radar cover of the present invention, the dielectric constant is not less than 2.400 and not more than 2.700, and the dielectric loss tangent is not less than 0.001 and not more than 0.005. The dielectric constant and the dielectric tangent of the millimeter wave radar cover of the present invention can be controlled by appropriately selecting the copolymerization ratio of MMA and the alkyl (meth)acrylate (M) in the above-mentioned (meth)acrylic polymer (P) or, when the alkyl (meth)acrylate (M) is used, the type of the alkyl (meth)acrylate. The method for measuring the dielectric constant and the dielectric loss tangent will be described later.

[0040] The millimeter wave radar cover of the present invention is excellent in weather resistance because it is formed from the above-mentioned (meth)acrylic resin composition. Therefore, even if a millimeter wave radar equipped with the millimeter wave radar cover of the present invention is installed outdoors in a position where it is likely to be exposed to direct sunlight, the deterioration of the millimeter wave radar cover can be suppressed, so that the millimeter wave transmittance is not reduced, and the performance as a millimeter wave radar and the design as seen by a viewer can be maintained. As an index of weather resistance, the change in haze (ΔHaze) measured by the method described below can be used.

[0041] <Millimeter wave radar device> An embodiment of the millimeter wave radar device of the present invention is equipped with the above-described millimeter wave radar cover of the present invention, and is applicable to millimeter wave radars of various structures and uses, not limited to the millimeter wave radar shown in Figures 1 and 2. Furthermore, an example of another embodiment of the millimeter-wave radar device of the present invention is a millimeter-wave radar device comprising an antenna module having a transmitting and receiving antenna, an antenna base for fixing the antenna module, and a millimeter-wave radar cover provided on the front side of the antenna module so as to cover the front side of the antenna module, wherein the millimeter-wave radar cover contains the (meth)acrylic resin composition for millimeter-wave radar of the present invention described above. The millimeter wave radar device of the present invention is equipped with the above-mentioned millimeter wave radar cover, so that it can accurately detect vehicles and obstacles around the vehicle. In addition, because the millimeter wave radar cover has excellent weather resistance, even if it is installed in a location that is easily exposed to direct sunlight, such as outdoors, it can maintain its performance as a millimeter wave radar and its design as seen by a viewer.

[0042] <Vehicle> An embodiment of the vehicle of the present invention is a vehicle equipped with the above-mentioned millimeter wave radar device of the present invention. Furthermore, an example of another embodiment of the vehicle of the present invention is a vehicle equipped with a millimeter-wave radar device for detecting an object in front of the vehicle, the millimeter-wave radar device including a millimeter-wave radar unit mounted on a substrate, and a millimeter-wave radar cover provided on the front side of the millimeter-wave radar unit so as to cover the front side of the millimeter-wave radar unit, the millimeter-wave radar cover including the above-mentioned (meth)acrylic resin composition for millimeter-wave radar of the present invention, and the millimeter-wave radar cover constituting a front grille or emblem of the vehicle can be mentioned. Since the vehicle of the present invention is equipped with the above-mentioned millimeter wave radar device, it can accurately detect vehicles and obstacles around the vehicle, ensuring the safety of the vehicle and enabling autonomous driving. In addition, since the millimeter wave radar cover has excellent weather resistance, the performance and design of the millimeter wave radar device can be maintained even if the vehicle is used in a place where it is likely to be exposed to direct sunlight, such as outdoors. EXAMPLES

[0043] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0044] <Evaluation method> The evaluations in the examples and comparative examples were carried out by the following methods.

[0045] (Dielectric constant, dielectric tangent) As indicators of millimeter wave transmittance of the (meth)acrylic resin composition for millimeter wave radar, the dielectric constant and the dielectric loss tangent were measured by the following method. Using a vector network analyzer (Keysight Technologies, device name: PNA N5227A, frequency: 10 MHz to 67 GHz) equipped with a millimeter wave module (Virginia Diodes, Inc, device name: WR12, frequency: 55 GHz to 95 GHz) and a Keycom frequency change method dielectric constant / dielectric loss tangent measurement system, the dielectric constant (εr) and dielectric loss tangent (tanδ) at measurement frequencies of 70 GHz to 90 GHz were measured for test pieces A obtained in the examples and comparative examples in an environment of a temperature of 24°C and a relative humidity of 33%, and the millimeter wave transmittance was evaluated using the following criteria. A: Dielectric constant is 2.8 or less and dielectric tangent is 0.006 or less. B: The dielectric constant exceeded 2.8, or the dielectric tangent exceeded 0.006.

[0046] (ΔHaze) As an index of weather resistance of the (meth)acrylic resin composition for millimeter wave radar, the difference in haze (ΔHaze) of the test piece before and after the weather resistance test was measured by the following method. For the test pieces B obtained in the examples and comparative examples, a weather resistance test was performed using a Sunshine Weather Meter (model name "S80HBBR", manufactured by Suga Test Instruments Co., Ltd.) under rain conditions of a black panel temperature of 63°C, a test time of 2000 hours, and a cycle of 12 minutes of pure water spray every 60 minutes. Using a spectrophotometer (model name: U-4100, manufactured by Hitachi), the difference in haze (ΔHaze) was measured for the test pieces before and after the weather resistance test in accordance with ISO14782. Weather resistance was judged using the following criteria. A: ΔHaze is less than 5.0 B: ΔHaze is 5.0 or more

[0047] (raw materials) The abbreviations for the compounds used in the examples and comparative examples are as follows. MMA: Methyl methacrylate (product name: Acryester M, manufactured by Mitsubishi Chemical Corporation) MA: Methyl acrylate (Mitsubishi Chemical Corporation) Polymerization initiator (1): 2,2'-azobis-2-methylbutyronitrile (product name: V-59, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) n-OM: n-octyl mercaptan (Tokyo Chemical Industry Co., Ltd.) Acrylic resin (1): (meth)acrylic polymer prepared in [Production Example 1] Polycarbonate resin (1): Polycarbonate resin pellets (product name: Iupilon (registered trademark) H3000, manufactured by Mitsubishi Chemical Corporation)

[0048] [Production Example 1] Production of acrylic resin (1): 900 parts by mass of deionized water, 60 parts by mass of sodium 2-sulfoethyl methacrylate, 10 parts by mass of potassium methacrylate, and 12 parts by mass of MMA were added to a reaction vessel equipped with a reflux condenser whose inside was replaced with nitrogen, and the temperature of the liquid in the reaction vessel was raised to 50° C. while stirring. Then, 0.08 parts by mass of polymerization initiator (1) was added, and the temperature of the liquid in the reaction vessel was raised to 60° C. while stirring, and then MMA was continuously dropped at a rate of 0.24 parts by mass / min over 75 minutes using a dropping pump. Then, the mixture was held for another 6 hours to polymerize, and a dispersant (solid content 10% by mass) was obtained. 2000 parts by mass of deionized water and 4.2 parts by mass of sodium sulfate were added to a reaction vessel equipped with a reflux condenser and a nitrogen gas inlet tube, and then stirred at a stirring speed of 320 rpm for 15 minutes. Then, a mixed solution of 1388.0 parts by mass of MMA (99.0 mol%), 12.0 parts by mass of MA (1.0 mol%), 2.8 parts by mass of polymerization initiator (1), and 4.2 parts by mass of n-OM was added to the reaction vessel and stirred for 5 minutes. Next, 6.72 parts by mass of the dispersant produced in Production Example 1 was added to the reaction vessel, and then stirred to disperse the monomer composition in the reaction vessel in water. Then, the inside of the reaction vessel was replaced with nitrogen. Next, the liquid temperature in the reaction vessel was raised to 75°C, and then the liquid temperature in the reaction vessel was continuously measured and maintained at 75°C until a polymerization heat generation peak was observed. After the polymerization heat generation peak was observed, the liquid temperature in the reaction vessel was raised to 90°C, and then maintained for 60 minutes to carry out polymerization. Thereafter, the mixture in the reaction vessel was filtered, and the filtrate was washed with deionized water and dried at 80°C for 16 hours to obtain a bead-shaped (meth)acrylic polymer, which was named acrylic resin (1). The acrylic resin (1) contained 98% by mass of MMA units and 2% by mass of MA units. The acrylic resin (1) had a glass transition temperature of 105° C. and a weight average molecular weight (Mw) of 100,000 in terms of polystyrene measured by GPC.

[0049] [Example 1] The acrylic resin (1) obtained in Production Example 1 was fed to a twin-screw extruder (model name "PCM45", manufactured by Ikegai Corporation) and kneaded at 250°C to obtain pellets of the acrylic resin (1). The pellets were dried with hot air at 80°C for about 4 hours, then fed into an injection molding machine (model name "FAS-T100D", manufactured by Fanuc Corporation) and injection molded under the conditions of cylinder setting temperature 250°C, mold temperature 60°C, and molding time 60 seconds to obtain test piece A (hemispherical molded product with outer diameter 50 mm and thickness 4.8 mm) for measuring dielectric constant and dielectric tangent, and test piece B (flat plate with thickness 3 mm x width 50 mm x length 50 mm) for evaluating weather resistance. The evaluation results of the obtained test pieces A and B are shown in Table 1.

[0050] [Comparative Example 1] Pellets of polycarbonate resin (1) were dried with hot air at 100°C for about 4 hours, and then fed into an injection molding machine (same as above) and injection molded under conditions of a cylinder set temperature of 280°C, a mold temperature of 80°C, and a molding time of 60 seconds, to obtain test pieces A and B in the same manner as in Example 1. The evaluation results A and B of the obtained test pieces are shown in Table 1.

[0051] [Table 1]

[0052] The millimeter wave radar cover of Example 1 had a well-balanced low dielectric constant and dielectric tangent, and in particular, the low dielectric tangent gave it excellent millimeter wave transmittance and weather resistance. On the other hand, the comparative millimeter wave radar cover of Comparative Example 1 had insufficient millimeter wave transmittance and weather resistance. [Industrial Applicability]

[0053] A millimeter wave radar cover containing the (meth)acrylic resin composition for millimeter wave radar of the present invention has excellent millimeter wave transmittance and weather resistance. Since the millimeter wave radar cover of the present invention has excellent millimeter wave transmittance and weather resistance, it can be suitably used for millimeter wave radar devices mounted on vehicles such as automobiles and motorcycles, as well as moving bodies such as trains, ships, and aircraft. The (meth)acrylic resin composition for millimeter wave radar, the millimeter wave radar cover, and the millimeter wave radar device of the present invention can be used in automobile collision prevention sensors, autonomous driving systems, or traffic information systems (ITS) that aim to alleviate traffic congestion and reduce accidents. [Explanation of symbols]

[0054] 1. Millimeter wave radar equipment 2 Cover 3. Radar Dome (Radome) 4 Millimeter wave radar cover 5 Antenna Module 6 Antenna Base

Claims

1. A (meth)acrylic resin composition for a millimeter wave radar cover, comprising a (meth)acrylic polymer (P) containing 80% by mass or more of a repeating unit derived from methyl methacrylate, The content ratio of the (meth)acrylic polymer (P) is 90% by mass or more with respect to 100% by mass of the total mass of the (meth)acrylic resin composition, The (meth)acrylic polymer (P) is a copolymer containing a repeating unit derived from methyl methacrylate and a repeating unit derived from an alkyl (meth)acrylate (M) (excluding methyl methacrylate) having an alkyl group having 1 to 8 carbon atoms on a side chain, and a molded article obtained by molding the (meth)acrylic resin composition has a relative dielectric constant of 2.800 or less at a measurement frequency of 70 to 90 GHz and a dielectric dissipation factor of 0.006 or less at a measurement frequency of 70 to 90 GHz; The (meth)acrylic resin composition for millimeter wave radar cover, wherein the difference in haze (ΔHaze) of the molded body before and after the following weather resistance test is less than 5.0%. Weather resistance test: A test under rain conditions with a black panel temperature of 63° C., a test time of 2000 hours, and a cycle of 12 minutes of pure water spray every 60 minutes.

2. The (meth)acrylic polymer (P) contains 80% by mass or more and less than 100% by mass of the repeating unit derived from the methyl methacrylate and more than 0% by mass and less than 20% by mass of the repeating unit derived from the alkyl (meth)acrylate (M). The (meth)acrylic resin composition for millimeter wave radar covers according to claim 1.

3. The (meth)acrylic resin composition for a millimeter wave radar cover according to claim 1 or 2, wherein the relative dielectric constant is 2.400 or more and 2.700 or less.

4. The (meth)acrylic resin composition for millimeter wave radar covers according to any one of claims 1 to 3, wherein the dielectric tangent is 0.001 or more and 0.005 or less.

5. The (meth)acrylic resin composition for millimeter wave radar covers according to any one of claims 1 to 4, wherein the refractive index at sodium d line (589 nm) is 1.40 to 1.

55.

6. A millimeter wave radar cover comprising the (meth)acrylic resin composition for millimeter wave radar covers according to any one of claims 1 to 5.

7. The millimeter wave radar cover according to claim 6, which is used as an exterior member for a vehicle.

8. The millimeter wave radar cover according to claim 7, wherein the vehicle exterior member is a front grille or an emblem.

9. A millimeter wave radar device comprising the millimeter wave radar cover according to any one of claims 6 to 8.

10. A millimeter wave radar device comprising: an antenna module having a transmitting / receiving antenna; an antenna base to which the antenna module is fixed; and a millimeter wave radar cover provided on a front side of the antenna module so as to cover the front side of the antenna module, A millimeter wave radar device, wherein the millimeter wave radar cover comprises the (meth)acrylic resin composition for millimeter wave radar covers according to any one of claims 1 to 5.

11. A vehicle comprising the millimeter wave radar device according to claim 9 or 10.

12. A vehicle equipped with a millimeter wave radar device for detecting an object in front of the vehicle, the millimeter wave radar device includes an antenna module having a transmitting / receiving antenna, an antenna base to which the antenna module is fixed, and a millimeter wave radar cover provided on a front side of the antenna module so as to cover the front side of the antenna module; The millimeter wave radar cover comprises the (meth)acrylic resin composition for millimeter wave radar covers according to any one of claims 1 to 5, and the millimeter wave radar cover constitutes a front grille or emblem of the vehicle. Vehicle.

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