Modified polyphenylene ether resin foam sheet

Abstract

The present invention provides a foamed sheet material, which is a modified polyphenylene ether resin foamed sheet material with good formability and processability. It has very low relative permittivity and dielectric loss factor at high frequencies, and is endowed with self-extinguishing or flame-retardant properties, and has excellent surface properties. The modified polyphenylene ether resin foamed sheet of the present invention is made by foaming modified polyphenylene ether resin containing flame retardant, and has a relative permittivity of 1.10 to 2.00 and a dielectric loss factor (tanδ) of 0.5 × 10⁻⁶. -3~2.5×10 -3 range; and the modified polyphenylene ether resin foamed sheet of the present invention is made by foaming a modified polyphenylene ether resin foamed resin containing both halogen-based flame retardants and phosphorus-based flame retardants with specific weight average molecular weights, and the average bubble diameter and bubble number density of the bubbles in the modified polyphenylene ether resin foamed sheet are within the specified range.

Description

Modified polyphenylene ether resin foamed sheet The present invention relates to a modified polyphenylene ether resin foamed sheet. The industry requires a resin material that can also be used as a housing, electrical insulation material, sealing material, protective material, or substrate for high-frequency electromagnetic wave control components for electrical and electronic parts. Since polyphenylene ether (hereinafter sometimes simply referred to as PPE) resin has good flame retardancy, thermal stability, and heat resistance to water, and also has excellent electrical properties, it is widely used in the field of electronic devices and the like. However, since the melt fluidity of polyphenylene ether resin is extremely low, it is used by alloying with other resins during molding. Among them, since polystyrene is completely compatible with polyphenylene ether, it can be alloyed without a compatibilizer and sold in the form of "modified polyphenylene ether resin". Since the material after alloying with this polystyrene is easy to mold, has excellent balance of heat resistance and mechanical strength, and is also easy to perform flame retardant treatment, it is widely used. Here, regarding polystyrene, for convenience, it is sometimes described as PS in the following text of the specification. Especially in the electrical and electronic fields, the heat resistance, flame retardancy, dimensional stability, etc. of alloys with polystyrene-based resins are evaluated and used for flyback transformer boxes, coil bobbins, relay sockets, adapter boxes, switches, etc. However, when this modified polyphenylene ether resin is foamed, although it has more excellent flame retardancy than common foams, for the reasons described later, it still does not meet the standard of V-1 or above in the UL standard flame retardancy test. Utilizing the relatively high properties of the modified polyphenylene ether resin, it can also be used as a foam. For example, a foam formed by the bead foaming method as described in Patent Document 1 is known. Although the resin foam of Patent Document 1 has excellent flame retardancy and antistatic properties for use in electronic devices or automotive components, it does not specifically disclose bubble structures such as bubble diameter or bubble density, or dielectric properties and surface properties. Also, Patent Document 2 is known to disclose a method for obtaining a modified polyphenylene ether resin foam containing a phosphate ester compound and having high heat resistance and flame retardancy, but similarly does not specifically disclose bubble structures or dielectric properties and surface properties. Also, Patent Document 3 is known to disclose a method for obtaining a hollow foamed blow molded body, but the foamed blow molded body of Patent Document 3 cannot obtain a foamed sheet with a thickness thicker than that of the present invention. The purpose of these foams is not to control the bubble structure of the foam to obtain a foam with excellent dielectric properties and surface properties. Furthermore, Patent Document 4 discloses a foam for a substrate, which is obtained by mixing a resin having at least one of a crystal melting temperature, a glass transition temperature, and a liquid crystal transition temperature of 260°C or higher and an amorphous resin such as a modified polyphenylene ether having a glass transition temperature of 230°C or lower at a specified ratio and foaming it by batch foaming, and the bubble diameter is 0.3 to 1.5 μm. However, the relative dielectric constant of this foam is 2.6 to 2.9, and the relative dielectric constant is greater than that of the present invention. The reason is that in the invention of Patent Document 4, the foaming ratio of the resin is small, and it is impossible to introduce fine bubbles at a high density to make the relative dielectric constant smaller. That is, to reduce the relative dielectric constant, it is necessary to increase the bubble density while maintaining fine bubbles. In addition, Patent Document 5 discloses a laminated foam in which the auxiliary layer on the back surface is a foam. Although the relative dielectric constant in the frequency bands of 79 GHz and 29 GHz is described in this invention, the relative dielectric constant is in the range of 2.28 to 2.73, and the dielectric loss factor tanδ is in the range of 0.002 to 0.0077. The bubble diameter is 44 to 103 μm, and the foaming ratio is 9.2 to 30.8 times. In the case of Patent Document 5, the bubble diameter exceeds 30 μm, the foaming ratio is also high, and the relative dielectric constant also exceeds 2.0, and the dielectric characteristics are hardly excellent. Although Patent Document 6 describes the use of various bubble nucleating agents and the production of a foam with a bubble diameter of 10 μm or more and 20 μm or less by the batch method, the invention of this document requires a bubble nucleating agent, is not related to a modified PPE foam, and the purpose is not to improve the dielectric characteristics. Patent Document 7 discloses a PPS (Polyphenylene Sulfide) resin foam using batch foaming and adding a crosslinking agent, and the problem thereof is to provide a polyphenylene sulfide foam and a manufacturing method thereof that not only maintain the basic characteristics of the PPS resin but also have excellent secondary formability. Regarding this PPS resin foam, it is described that the average bubble diameter can be made 20 μm or less. According to Patent Documents 6 and 7, when a fine bubble structure is obtained by using batch foaming, in order to control the bubble structure, a bubble nucleating agent or a crosslinking agent may sometimes be used as needed. On the other hand, since the above-mentioned high flame retardancy and low dielectric characteristics of the modified polyphenylene ether resin are relatively balanced, it is particularly expected to be used in high-frequency related components, and it is expected to apply the foam of the modified polyphenylene ether resin to, for example, terahertz wave components that are expected to expand in the future. However, if the same low dielectric characteristics are to be exhibited for terahertz waves, considering the losses such as reflection and scattering during the transmission of radio waves in the terahertz frequency band, the foam requires fine bubbles of at least 30 μm or less, and preferably 10 μm or less. [Prior Art Documents] [Patent Documents] [Patent Document 1] Japanese Patent No. 5642521 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2017-155196 [Patent Document 3] Japanese Patent No. 6106420 [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2008-303247 [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2021-136540 [Patent Document 6] Japanese Patent No. 3138488 [Patent Document 7] Japanese Patent No. 5809895 [Problems to be Solved by the Invention] The bubble diameter is not described in either of Patent Documents 1 and 2 above. Generally, it is difficult to suppress bubble growth by the methods shown in these documents. Therefore, it is impossible to make the surface properties excellent while making the bubble diameter thinner. A bead foam molded body is used in Patent Document 1, and the content assuming the same method is also described in Patent Document 2. However, in the case of bead foaming, it is difficult to form a sheet. Even if a sheet is formed, since there are interfaces between particles, it is not advantageous, for example, when considering bonding to a copper foil. When a modified polyphenylene ether resin is foamed, the contact area with air increases, the combustion speed increases, and the flame retardancy decreases. Therefore, it is usually difficult to satisfy the vertical flame retardancy required in the fields related to high frequencies. Therefore, although the fine foam sheet using a modified polyphenylene ether resin is more excellent in flame retardancy than common foam resin sheets, it still does not satisfy the standard of V-1 or higher in the flame retardancy test according to the UL standard. Since the number of contact points between air and resin increases in the foam compared to the unfoamed body, the flame retardancy significantly decreases. The portion of the resin amount that the foam lacks compared to the unfoamed resin is easily softened by the combustion heat, and as a result, resin indentation easily occurs during combustion. On the other hand, if the amount of flame retardant added to the foam is increased to improve the flame retardancy, the flame retardant usually causes foaming hindrance. Therefore, a foam having a target bubble diameter or density cannot be obtained, and at the same time, the surface properties of the foam deteriorate depending on the situation. Therefore, it is very difficult to obtain a foam of a foam sheet that is a modified polyphenylene ether resin foam sheet and that not only satisfies the formability for processing into a molded body but also satisfies easy and complex and fine foaming, and improves weight reduction and flame retardancy and has excellent surface properties. Therefore, an object of the present invention is to obtain a foam sheet that is a modified polyphenylene ether resin foam sheet, has good formability, has a very low relative dielectric constant and dielectric loss factor at high frequencies, and has self-extinguishing properties or flame retardancy and excellent surface properties. [Technical Means for Solving the Problems] The modified polyphenylene ether resin foam sheet of the present invention is a sheet obtained by foaming a modified polyphenylene ether resin containing a flame retardant, and its relative dielectric constant is 1.10 to 2.00, and the dielectric loss factor (tanδ) is 0.5×10 -3 ~2.5×10 -3 in the range. The modified polyphenylene ether resin foam sheet of the present invention can be made into the following modified polyphenylene ether resin foam sheet, that is, the arithmetic mean surface roughness Ra of its surface is 0.50 μm or less. Here, even if only a specified amount of a phosphorus-based flame retardant is included as the flame retardant, an improvement effect in flame retardancy is confirmed. Therefore, the above-mentioned modified polyphenylene ether resin foam sheet can also be made into a modified polyphenylene ether resin foam sheet containing only a phosphorus-based flame retardant as the above-mentioned flame retardant. The above-mentioned modified polyphenylene ether resin foam sheet can also be the following modified polyphenylene ether resin foam sheet, that is, containing both a halogen-based flame retardant with a weight average molecular weight of 1,000 or more and 1,000,000 or less and a phosphorus-based flame retardant as the above-mentioned flame retardant. As described above, if an appropriate amount of a phosphorus-based flame retardant and a specified halogen-based flame retardant are used, as described later, foaming hindrance can be prevented, and the flame retardancy can be further improved to a level that satisfies the UL standard. Also, the relative dielectric constant of the above-mentioned modified polyphenylene ether resin foam sheet can be 1.20 to 1.57, and the dielectric loss factor (tanδ) can be 1.0×10 -3 ~2.1×10 -3 in the range. The modified polyphenylene ether resin foam sheet of the present invention can also be the following modified polyphenylene ether resin foam sheet, characterized in that: it is formed by foaming a modified polyphenylene ether resin containing a halogen-based flame retardant with a weight average molecular weight of 1,000 or more and a phosphorus-based flame retardant as the flame retardant, and the average bubble diameter of the bubbles of the above-mentioned modified polyphenylene ether resin foam sheet is 1 to 30 μm, and the bubble number density is 1.8×10 4 ~9×10 8 pieces / mm 3 . As described above, regarding the bubble structure of the modified polyphenylene ether resin foam sheet, by making it into fine bubbles with an average bubble diameter in the range of 1 to 30 μm and making the bubble number density 1.8×10 4 ~9×10 8 pieces / mm 3 in the range, dielectric properties such as the relative dielectric constant and the dielectric loss factor tanδ in the above range can be obtained as a result. Also, in the above-mentioned modified polyphenylene ether resin foam sheet, the average bubble diameter of the bubbles of the above-mentioned modified polyphenylene ether resin foam sheet can be 1 to 22 μm, and the bubble number density can also be 2.0×10 4~8×10 8 pieces / mm 3 In this way, by making the average bubble diameter finer, the lower limit value of the bubble number density is increased, and thus the dielectric properties or surface properties of the foamed sheet can be further improved. Regarding the flame retardancy, when only an inorganic flame retardant such as a phosphorus-based flame retardant is added, the addition of the flame retardant can improve the flame retardancy and impart self-extinguishing properties during combustion. However, if the addition amount of the flame retardant increases, the foamability will be hindered, and thus it is difficult to foresee the effect of further improving the flame retardancy. Even if a specific halogen-based flame retardant is added to the phosphorus-based flame retardant as a flame retardant, unless a polymer-based flame retardant with the specified molecular weight is used and the composition range of the flame retardant is not adjusted to the specified range, it is impossible to meet the V-0 and V-1 standards in the UL94 standard flame retardancy test without affecting the foaming state, and it is impossible to maintain the same average bubble diameter and bubble density as in the case of not adding the flame retardant. Here, regarding the flame retardancy, the purpose is to improve the flame retardancy without causing foaming hindrance. Specifically, the purpose is to obtain the following foam. First, by adding a flame retardant, although it is flammable, it is given self-extinguishing properties to reduce flammability. Further, second, the flame retardancy level is raised to a level that meets the V-0 and V-1 standards in the UL94 standard flame retardancy test. Here, in other manufacturing methods such as bead foaming, injection foaming, and extrusion foaming (cross-linked foaming, non-cross-linked foaming), it is difficult to make the average bubble diameter as fine as above and make the bubble density within the specified range. As a result, it is difficult to obtain the above bubble structure. Also, even if a material having the above bubble structure and dielectric properties is successfully obtained using a manufacturing method other than the microcellular foaming process of the present invention, when manufacturing using other foaming processes, the bubble diameter distribution of the foam is large, and it is difficult to keep the surface properties of the foamed sheet in an excellent state. Therefore, it is difficult to suppress the surface roughness Ra (arithmetic mean surface roughness) of the foamed sheet to a range of 0.50 μm or less. As described above, the arithmetic mean surface roughness Ra of the modified polyphenylene ether resin foamed sheet of the present invention can also be 0.50 μm or less. Also, when considering the lower limit value of the arithmetic mean surface roughness Ra of the modified polyphenylene ether resin foamed sheet in this way, by suppressing the arithmetic mean surface roughness Ra to a range of 0.08 to 0.50 μm, more preferably 0.10 to 0.40 μm, and even more preferably 0.10 to 0.30 μm, the high-frequency characteristics such as the skin effect can be further improved. Therefore, it is more preferably used for substrate applications such as high-frequency circuits. The average plate thickness of the above-mentioned modified polyphenylene ether resin foamed sheet is a modified polyphenylene ether resin foamed sheet with an average plate thickness of 0.05 mm to 2.0 mm capable of manufacturing the sheet, and the average plate thickness of the sheet can also be 0.1 mm to 2.0 mm. Since the average plate thickness of the modified polyphenylene ether resin foamed sheet of the present invention uses a batch foaming process that separates the forming process and the foaming process to foam the product, and further enables independent control of the impregnation and foaming of the foaming gas, it is considered that the thickness of the foamed sheet can be made thinner. Therefore, the upper limit of the foamed sheet is 2 mm. Regarding the lower limit value, even if the sheet thickness becomes thinner, a surface layer is formed on the upper and lower sides of the sheet, so the lower limit value of the foamed sheet needs to be at least 50 μm (0.05 mm). At this time, after the microcellular foaming process is completed, rolling can also be performed as needed to adjust the plate thickness and surface properties, etc. Using the above-mentioned modified polyphenylene ether resin foamed sheet, a test is conducted according to the UL-94 vertical method (20 mm vertical combustion test) of the American UL standard, and the flame retardancy is evaluated. Regarding the test results, the above-mentioned modified polyphenylene ether resin foamed sheet is preferably a modified polyphenylene ether resin foamed sheet that meets the V-0 and V-1 standards. Materials with a flame retardant added within the scope of the present invention in the base resin meet the following V-0 and V-1 standards of UL-94, but when no flame retardant is added or the addition amount of the flame retardant does not meet the scope of the present invention, the flame retardancy cannot meet the V-0 and V-1 of the UL-94 vertical method of the American UL standard. The above-mentioned modified polyphenylene ether resin foamed sheet can also be a modified polyphenylene ether resin foamed sheet as follows, which is used for the housing of electrical and electronic parts, electrical insulation materials, sealing materials, protective materials, or substrates for high-frequency electromagnetic wave control components. (Regarding the mixing of modified polyphenylene ether resin and polystyrene resin) Here, the reason for mixing polystyrene resin in polyphenylene ether resin is that although polyphenylene ether resin has excellent heat resistance and strength, its processability is low and it cannot be processed, so its processability needs to be improved. Here, the reason for setting the blending amount of polyphenylene ether resin to 40% by mass or more and 80% by mass or less is that if the blending amount of polyphenylene ether resin is less than 40% by mass, the target heat resistance, strength, etc. cannot be obtained. The reason for setting it to 80% by mass or less is that if it exceeds 80% by mass, the processability becomes low. Also, usually, the blending amount of polystyrene resin is the remaining amount of polyphenylene ether resin. Therefore, the blending amount of polystyrene resin is 20% by mass or more and 60% by mass or less, and it is a well-known fact that polystyrene resin is mixed in polyphenylene ether resin and mixed at the above ratio. By adding a flame retardant to the above-mentioned modified polyphenylene ether resin foamed sheet, a polystyrene foamed sheet with self-extinguishing property, reduced flammability, or improved flame retardancy, excellent dielectric properties and surface properties can be obtained. Finally, by adding a halogen-based flame retardant and a phosphorus-based flame retardant at the same time, the flame retardancy can be further improved, and it meets the V-0 and V-1 standards in the UL94 standard flame retardancy test. The reason is that a synergistic effect is obtained by using the two in combination. (Regarding the mixture of modified polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant) When the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant is specified as 100% by mass, considering strength, heat resistance, or processability, the blending amount of the polyphenylene ether resin in the mixture as a whole is almost the same as in the case without the phosphorus-based flame retardant material, that is, in the case of mixing the modified polyphenylene ether resin and the polystyrene resin, and is 40% by mass or more and 80% by mass or less. Therefore, the total blending amount of both the polystyrene and the phosphorus-based flame retardant is 20% by mass to 60% by mass, which is almost the same as in the case of mixing only the modified polyphenylene ether resin and the polystyrene resin. Here, when the total content of the polystyrene and the phosphorus-based flame retardant material takes the lower limit value, the lower limit values of the polystyrene and the phosphorus-based flame retardant material are set to 15% by mass and 5% by mass, respectively. Also, when the total content of the polystyrene and the phosphorus-based flame retardant material takes the upper limit value, the upper limit values of the polystyrene and the phosphorus-based flame retardant material are set to 40% by mass and 20% by mass, respectively. The contents of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant at this time can be set to 40% by mass or more and 80% by mass or less, 15% by mass or more and 40% by mass or less, and 5% by mass or more and 20% by mass or less, respectively. In this case, the polystyrene resin and the phosphorus-based flame retardant can be appropriately selected as long as the total of the polystyrene resin and the phosphorus-based flame retardant within the above range is 100% by mass relative to the specified amount of the polyphenylene ether resin. Hereinafter, the reasons for setting the upper and lower limits of the blending amounts of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant are described. The reason for setting the blending amount of the polyphenylene ether resin to 40% by mass or more and 80% by mass or less is that if the blending amount of the polyphenylene ether resin is less than 40% by mass, the target heat resistance or strength cannot be obtained, and the reason for setting it to 80% by mass or less is that if it exceeds 80% by mass, the processability becomes low, and this reason is the same as in the case of mixing only the modified polyphenylene ether resin and the polystyrene resin. Here, the reason for setting the blending amount of the above polystyrene resin to 15% by mass or more and 40% by mass or less is that if it does not reach the lower limit value, the processability is insufficient, and if it exceeds the upper limit value of 40% by mass, the content of the polyphenylene ether resin is insufficient, and properties such as heat resistance and strength are lacking. Also, when the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant used in the above modified polyphenylene ether resin foamed sheet is set to 100% by mass, 5% by mass or more and 20% by mass or less of the phosphorus-based flame retardant can be contained. Also, the reason for setting the blending amount of the phosphorus-based flame retardant to 5% by mass or more and 20% by mass or less is that if it does not reach the above lower limit value, the formation of char (forming a three-dimensional structure carbonized layer via the solid phase during carbonization) during combustion is insufficient, the interaction with the halogen-based flame retardant described later cannot be obtained, and even if this phosphorus-based flame retardant is added, the required flame retardant performance cannot be ensured, and if it exceeds the above upper limit value, the foamability decreases.The reason for setting the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant to 100% by mass instead of setting the total of the polyphenylene ether resin and polystyrene resin to 100% by mass is that they are usually blended and sold and used together. In this application, Zylon 340Z and 540Z manufactured by Asahi Kasei are used as these materials. As the flame retardant used in the above-mentioned modified polyphenylene ether resin sheet, in addition to the phosphorus-based flame retardant, a halogen-based flame retardant can also be used as an oligomer or a polymer-based flame retardant. Regarding a specific halogen-based flame retardant, an aspect of adding 5 parts by mass or more and 40 parts by mass or less relative to 100 parts by mass of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant can be exemplified. Here, regarding the composition of the base resin, 5% by mass or more and 20% by mass or less of the phosphorus-based flame retardant is contained relative to 100% by mass of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant. As a preferred aspect of the flame retardant of the above-mentioned modified polyphenylene ether resin foamed sheet, for example, when the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant is set to 100% by mass, 5% by mass or more and 20% by mass or less of the phosphorus-based flame retardant is contained, and relative to 100 parts by mass of the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant, further 5 parts by mass or more and 40 parts by mass or less of a halogen-based flame retardant having a weight average molecular weight of 1,000 or more and 1,000,000 or less is contained. As a result, the flame retardant uses a phosphorus-based flame retardant and a halogen-based flame retardant. Its blending amount is preferably as described above. By adding the halogen-based flame retardant within the above range, foaming property is ensured, and the carbonization formation caused by the phosphorus-based flame retardant is combined with the free radical capture and oxygen barrier effects caused by the halogen-based flame retardant, and higher flame retardancy can be exhibited. As described below, a foamed sheet meeting the V-0 and V-1 standards of the UL94 standard can be obtained. The weight-average molecular weight of the flame retardant used in the above-mentioned modified polyphenylene ether resin foamed sheet is preferably 1,000 or more, more preferably 1,000 to 1,000,000. At this time, the flame retardant can be either a brominated carbonate oligomer or a brominated polystyrene. By making the molecular weight of the halogen-based flame retardant 1,000 or more, a good resin foamed sheet without foaming hindrance can be obtained. By using such a modified polyphenylene ether resin foamed sheet, the modified polyphenylene ether resin foamed sheet can be made flame retardant. At this time, if the weight-average molecular weight of the flame retardant is 5,000 or more, it further stably exerts its function, so it is preferred. The average molecular weight of the above-mentioned flame retardant is more preferably 5,000 or more and 50,000 or less. In the case of a brominated carbonate oligomer, it is preferably within the above range. On the other hand, in the case of a polymer with a high degree of polymerization such as brominated polystyrene, the upper limit is higher, preferably 1,000,000 or less. The reason is that if the molecular weight exceeds 1,000,000, the growth of bubbles is instead inhibited, and it is difficult to obtain the required foaming ratio and bubble density. [Effects of the Invention] According to the present invention, a foamed sheet can be obtained, which is a modified polyphenylene ether resin foamed sheet, has good formability, very low relative dielectric constant and dielectric loss factor at high frequencies, and has high flame retardancy and excellent surface properties. As a result, a low-dielectric modified polyphenylene ether resin foamed sheet can be provided, which can be used for housings for electrical and electronic parts, electrical insulation materials, sealing materials, protective materials, substrates for high frequencies, or substrates for electromagnetic wave control components, and has excellent surface properties and excellent self-extinguishing or flame retardancy. The modified polyphenylene ether resin foamed sheet of the present invention contains a flame retardant. Examples of the flame retardant include: a phosphorus-based flame retardant, or a composition containing a phosphorus-based flame retardant and a specific halogen-based flame retardant. The relative dielectric constant of the modified polyphenylene ether resin foamed sheet of the present invention is 1.10 to 2.00, and the dielectric loss factor (tanδ) is 0.5×10 -3 ~2.5×10 -3 In the range. Alternatively, as another configuration aspect of the present invention, the following modified polyphenylene ether resin foamed sheet can be exemplified: a modified polyphenylene ether resin foamed sheet obtained by foaming a modified polyphenylene ether resin containing a halogen-based flame retardant having a weight-average molecular weight of 1,000 or more and 1,000,000 or less and a phosphorus-based flame retardant as a flame retardant, and the average bubble diameter of the bubbles present in the modified polyphenylene ether resin foamed sheet is 1 to 30 μm, and the bubble number density is 1.8×10 4 ~9×10 8 pieces / mm 3First, as the above resin composition of the preferred embodiment of the present invention, a modified polyphenylene ether resin containing polystyrene can be exemplified, and this aspect will be described. Furthermore, the component composition of the phosphorus-based flame retardant and a specific halogen-based flame retardant will be described, and then the density of the resin, the bubble diameter, the manufacturing method, and the physical properties of the foamed sheet will be described. (Polyphenylene ether resin) As the polyphenylene ether resin used in the present invention, examples include: a homopolymer composed of repeating units represented by the following general formula (1), and a copolymer containing repeating units represented by the following general formula (1). [Chemical formula 1] [In the formula, R 1 , R 2 , R 3 , and R 4 are each independently a monovalent group selected from the group consisting of a hydrogen atom, a halogen atom, an alkyl group having 1 to 7 carbon atoms, a phenyl group, a haloalkyl group, an aminoalkyl group, and a hydrocarbonoxy group, and they may be the same or different from each other; also, two adjacent groups may be bonded to each other to form a ring] R in formula (1) 1 and R 4 are more preferably a linear or branched alkyl group having 1 to 6 carbon atoms, and more preferably an alkyl group having 1 to 3 carbon atoms. Among them, methyl and ethyl are preferred. R 2 and R 3 are more preferably a hydrogen atom or a linear or branched alkyl group having 1 to 6 carbon atoms, and more preferably a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. Among them, a hydrogen atom is preferred. The above polyphenylene ether can be used alone in one kind, or two or more kinds can be used in combination. The molecular weight of the polyphenylene ether is not particularly limited, and the weight average molecular weight is preferably 10,000 or more and 30,000 or less. Furthermore, in the present invention, "modification" means, for example, alloying the above polyphenylene ether with polystyrene listed in the following items, or a specified functional group is substituted at a specific site. In the foamed sheet of the modified polyphenylene ether resin of the present invention, it is preferable to use a modified polyphenylene ether resin obtained by mixing and alloying the above polyphenylene ether and polystyrene. At this time, the mixing ratio of the polyphenylene ether and polystyrene is not particularly limited. When the ratio of both the polyphenylene ether and polystyrene and the phosphorus-based flame retardant is set to 100% by mass, it is preferably set to 40% by mass or more and 80% by mass or less for the polyphenylene ether, and more preferably set to 50% by mass or more and 70% by mass or less. (Polystyrene) In this specification, as polystyrene-based resins, examples include: homopolymers of styrene-based monomers such as styrene, α-methylstyrene, vinyltoluene, chlorostyrene, ethylstyrene, isopropylstyrene, dimethylstyrene, bromostyrene, etc., or copolymers thereof; copolymers with styrene-based monomers as the main component and vinyl monomers capable of polymerizing with styrene-based monomers; copolymers of styrene-based monomers and rubber components such as butadiene, homopolymers of styrene-based monomers or copolymers thereof, or mixtures or polymers of copolymers of styrene-based monomers and vinyl monomers and diene-based rubbery polymers, such as so-called impact-resistant polystyrene, etc., but are not limited thereto. As polystyrene-based resins as homopolymers, examples include: polystyrene, poly-α-methylstyrene, polychlorostyrene, etc.; as polystyrene-based resins as copolymers, examples include: styrene-butadiene copolymer, styrene-acrylonitrile copolymer, styrene-maleic acid copolymer, styrene-maleic anhydride copolymer, styrene-maleimide copolymer, styrene-N-phenylmaleimide copolymer, styrene-N-alkylmaleimide copolymer, styrene-N-alkyl-substituted phenylmaleimide copolymer, styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, styrene-methyl acrylate copolymer, styrene-methyl methacrylate copolymer, styrene-n-alkyl acrylate copolymer, styrene-n-alkyl methacrylate copolymer, ethylvinylbenzene-divinylbenzene copolymer. In addition, examples also include: terpolymers such as ABS (Acrylonitrile Butadiene Styrene), butadiene-acrylonitrile-α-methylbenzene copolymer, etc., but are not limited thereto. Furthermore, graft copolymers are also included, for example: styrene-grafted polyethylene, styrene-grafted ethylene-vinyl acetate copolymer, (styrene-acrylic acid)-grafted polyethylene, styrene-grafted polyamide, etc. These can be used alone as one kind, or two or more kinds can be used in combination. As a polystyrene resin, those having a weight average molecular weight of 180,000 to 500,000 are preferred. Further, in this specification, the weight average molecular weight means the weight average molecular weight determined by gel permeation chromatography (GPC) using a calibration curve (prepared using the peak molecular weight of commercially available standard polystyrene) of the peak molecular weight of the chromatogram obtained by the measurement of commercially available standard polystyrene. Regarding the carrier and the column, they may be selected according to the analyte to be measured. As the carrier, for example, tetrahydrofuran (THF) can be mentioned. Further, in this specification, when referring to the molecular weight of an oligomer or a polymer, unless otherwise specified, it means the one determined by the above method. When the total of the polyphenylene ether resin, the polystyrene resin, and the phosphorus-based flame retardant is set to 100% by mass, the polystyrene is preferably 15% by mass or more and 40% by mass or less, more preferably 15% by mass or more and 37% by mass or less, and particularly preferably 25% by mass or more and 35% by mass or less. (Phosphorus-based flame retardant) The modified polyphenylene ether resin foamed sheet of the present invention preferably further contains a phosphorus-based flame retardant. When the phosphorus-based flame retardant is contained, while exhibiting self-extinguishing properties, a higher flame retardancy improvement effect can be obtained by the interaction with the halogen-based flame retardant component described later. As the phosphorus-based flame retardant, a phosphate ester compound is preferred. The phosphate ester compound is not particularly limited, and those having an effect as a flame retardant are preferred. Further, the above phosphate ester compound is preferably bisphenol A bis(diphenyl phosphate) or an aromatic condensed phosphate compound, for example. As the above phosphate ester compound, for example, the phosphate ester compound represented by the following general formula (2), its condensate, etc. can be mentioned, but it is not limited thereto. [Chemical formula 2] [In the formula, R 11 、R 12 、R 13 、and R 14Independently represent a monovalent group selected from the group consisting of a hydrogen atom, a halogen atom, an alkyl group (preferably having 1 to 12 carbon atoms, more preferably 1 to 6 carbon atoms, still more preferably 1 to 3 carbon atoms), a cycloalkyl group (preferably having 3 to 24 carbon atoms, more preferably 3 to 12 carbon atoms, still more preferably 3 to 6 carbon atoms), an aryl-substituted alkyl group (preferably having 7 to 22 carbon atoms, more preferably 7 to 18 carbon atoms, still more preferably 7 to 10 carbon atoms), an aryl group (preferably having 6 to 22 carbon atoms, more preferably 6 to 18 carbon atoms, still more preferably 6 to 10 carbon atoms), a halogen-substituted aryl group (preferably having 6 to 22 carbon atoms, more preferably 6 to 18 carbon atoms, still more preferably 6 to 10 carbon atoms), and an alkyl-substituted aryl group (preferably having 7 to 22 carbon atoms, more preferably 7 to 18 carbon atoms, still more preferably 7 to 10 carbon atoms), and they may be the same or different from each other; X represents a group containing an arylene group (preferably having 6 to 22 carbon atoms, more preferably 6 to 18 carbon atoms, still more preferably 6 to 10 carbon atoms); m is an integer of 0 or more. Further, when the phosphate compound is a mixture of phosphates and / or their condensates with different m values as described above, m represents the average value thereof. When m = 0, the compound of formula (2) represents phosphoric acid or a phosphate monomer. The above phosphate compounds can be used alone or in combination of two or more. As representative phosphate monomers, for example, triphenyl phosphate, tricresyl phosphate, tris(xylene) phosphate, etc. can be cited, but not limited thereto. The phosphorus-based flame retardant is preferably a condensed phosphate compound, more preferably a condensed phosphate compound with m of 1 or more in formula (2). The upper limit of m can be appropriately defined, for example, exemplified as 5 or less. Regarding the phosphorus-based flame retardant, from the viewpoint of the flame retardancy and heat resistance exhibited when it is kneaded into the resin composition of the present embodiment, it is preferably the above R 11 、the above R 12 、the above R 13 and the above R 14 Among them, at least one is an aryl group, a halogen-substituted aryl group, or an alkyl-substituted aryl group, more preferably the above R 11 、the above R 12 、the above R 13 and the above R 14 Are all aryl groups, halogen-substituted aryl groups, or alkyl-substituted aryl groups. Also, from the same viewpoint, as preferred aryl groups, halogen-substituted aryl groups, and alkyl-substituted aryl groups, phenyl, xylenyl, tolyl, or their halogenated derivatives are preferred. As the arylene-containing group of the above X, a phenylene group, a residue obtained by removing two hydroxyl groups from resorcinol, hydroquinone, bisphenol A, bisphenol F, biphenol, or their halogenated derivatives is preferred. Examples of the above phosphate ester compounds include, but are not limited to, resorcinol-diphenyl phosphate compounds, bisphenol A-polyphenyl phosphate compounds, bisphenol A-polytolyl phosphate compounds, etc. Examples of the condensed phosphate ester compounds include the compounds of the following formulas (3-1), (3-2), (4-1), and (4-2). [Chemical formula 3] [Chemical formula 4] [Chemical formula 5] [Chemical formula 6] In formulas (3-1), (3-2), (4-1), and formula (4-2), Q 1 , Q 2 , Q 3 and Q 4 are each a substituent and each independently represents an alkyl group having 1 to 6 carbon atoms, and R 7 and R 8 each independently represents a hydrogen atom or a methyl group. n1 and n2 each independently represent an integer of 0 to 2, and m1, m2, m3, and m4 each independently represent an integer of 0 to 3. Furthermore, in formulas (3-1) and (4-1), there is no Q 1 etc., but it is understood that the provisions of the groups such as Q 1 etc. exist respectively. The same applies hereinafter. In the condensed phosphate esters represented by formulas (3-1), (3-2), (4-1), and formula (4-2), n is an integer of 1 or more, and preferably an integer of 1 to 3. Among these, in formulas (3-1), (3-2), (4-1), and formula (4-2), preferably, the condensed phosphate esters in which R 7 and R 8 are methyl groups, and Q 1 , Q 2 , Q 3 , Q 4All are methyl groups, n1 and n2 are 0, m1, m2, m3 and m4 are independently integers from 0 to 3 of condensed phosphate esters, and the range of n is 1 to 3. Among these, particularly preferably, the phosphate ester with n being 1 accounts for 50% by mass or more in the composition. These flame retardants can also be commercially available products. For example, commercially available products such as those with the trade names "CR-741", "CR733S", and "PX-200" manufactured by Daihachi Chemical Co., Ltd. can be cited. As the phosphate ester, specifically, bisphenol A bis(diphenyl phosphate) and resorcinol bis(xylenyl phosphate) are particularly preferred. Regarding the compounds of formula (3-1), (3-2), (4-1), and formula (4-2), reference can be made to those disclosed in Japanese Patent Application Laid-Open No. 2010-123933. As specific examples of the phosphorus-based flame retardant, compounds represented by the following formula (5) or (6) can be cited. [Chemical formula 7] Regarding the content of the phosphorus-based flame retardant in the modified polyphenylene ether resin foamed sheet of the present embodiment, from the viewpoint of obtaining a resin composition with more excellent foamability, heat resistance, and flame retardancy, when the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant is set to 100% by mass, it is preferably 5% by mass or more and 20% by mass or less, more preferably 7% by mass or more and 15% by mass or less. (Halogen-based flame retardant) The modified polyphenylene ether resin foamed sheet of the present invention preferably contains a specific halogen-based flame retardant as the resin material constituting it. The halogen atoms contained in the halogen-based flame retardant are not particularly limited, preferably bromine or chlorine, more preferably bromine. The halogen-based flame retardant is preferably a brominated carbonate oligomer or brominated polystyrene. As the halogen-based flame retardant, a compound having a bisphenol skeleton is preferred. Specifically, a compound (brominated carbonate oligomer) having the following structural formula (formula (7)) is preferably included. [Chemical formula 8] R 21 and R 22 represent a halogen atom or an alkyl group. As the halogen atom, a bromine atom is preferred. As the alkyl group, an alkyl group having 1 to 12 carbon atoms is preferred, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an alkyl group having 1 to 3 carbon atoms. R 23 and R 24 represent a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. z and v represent integers from 0 to 5. o is a natural number arbitrarily determined according to the molecular weight, preferably 4 to 15. As specific examples of the halogen-based flame retardant, compounds represented by the following formula (7-1) or (7-2) can be cited. [Chemical formula 9] n is 4 to 15 (molecular weight of about 3,000 to about 10,000). [Chemical formula 10] n is 4 to 15 (molecular weight of about 2,500 to about 9,000). As another embodiment different from the above, the halogen-based flame retardant is preferably a brominated polystyrene having a repeating structural unit represented by the following formula (8). In the formula, x represents a natural number from 500 to 10,000. y represents a natural number from 1 to 5. [Chemical formula 11] From the viewpoint of maintaining the foaming property of the resin, the molecular weight of the halogen-based flame retardant is more important. It is preferably an oligomer or polymer having a weight average molecular weight of 1,000 to 1,000,000, and more preferably an oligomer or polymer having a weight average molecular weight of 5,000 to 800,000 (50,000 in the case of an oligomer). If the molecular weight is less than the above lower limit, it is recognized as a foreign substance during foaming and induces gas diffusion, hindering the foaming property. Also, if the molecular weight exceeds the above upper limit, the growth of bubbles is inhibited and a foam having the required density cannot be obtained. However, in the case of a brominated carbonate oligomer, the above range is preferred. On the other hand, in the case of a polymer with a high degree of polymerization such as brominated polystyrene, the upper limit is higher, preferably 1,000,000 or less. Furthermore, in this specification, when referring to "molecular weight", unless otherwise specified, it means "weight average molecular weight". The measurement method of the weight average molecular weight is as described above. The blending amount of the halogen-based flame retardant is not particularly limited. When the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant is set to 100 parts by mass, the halogen-based flame retardant is preferably 5 parts by mass or more and 40 parts by mass or less, more preferably 10 parts by mass or more and 40 parts by mass or less, and even more preferably 20 parts by mass or more and 40 parts by mass or less. <Density and Foaming Ratio of Resin> The density of the resin constituting the modified polyphenylene ether resin foamed sheet of the present invention is preferably 0.60 g / cm 3 ~0.10 g / cm 3 , more preferably 0.50 g / cm 3 ~0.15 g / cm 3 , even more preferably 0.30 g / cm 3 ~0.15 g / cm 3. The density of the resin is an index for understanding the degree of foaming and is a common variable with the foaming ratio in this regard. In the resin of the present invention, the foaming ratio is preferably 2 to 12 times, more preferably 2 to 10 times, 2 to 7 times. That the density (foaming ratio) of the resin is within the above range means that there are more bubbles in the foamed sheet, and thus the closer it is to the dielectric constant of air, the lower the relative dielectric constant can be achieved. <Average bubble diameter> The average bubble diameter of the bubbles present in the resin constituting the modified polyphenylene ether resin foamed sheet of the present invention needs to satisfy 1 to 30 μm. Here, the lower limit of the average bubble diameter is 1.0 μm or more, more preferably more than 1.5 μm. The upper limit is 30 μm or less, preferably 28 μm or less, more preferably 26 μm or less, further preferably 22 μm or less, further preferably 18 μm or less. Also, the average bubble diameter is further preferably 12 μm or less, more desirably 10 μm or less, and particularly preferably 8 μm or less. The foamed sheet of the present invention has the following advantages, that is, the smaller the average diameter of the bubbles is as described above, the better the formability, the better the low relative dielectric constant and low dielectric loss factor can be achieved, and the surface characteristics with smaller surface roughness can be achieved. <Bubble number density> Here, the smaller the average bubble diameter of the foamed sheet is, the better the relative dielectric constant as a dielectric property is. However, if the foaming ratio is low, the bubble density in the sheet cross-section occupied by the resin foamed sheet is low, and as a result, the ratio of the resin component becomes larger. Therefore, the probability of the existence of a portion without bubbles around the bubbles becomes high, and at the same time, the thickness of the cell wall of the bubbles becomes thick. Therefore, in order to reduce the relative dielectric constant, simply making the bubble diameter smaller is not enough, and the loss generated by the portion without bubbles or the bubble wall will become larger. Therefore, it is preferable to make the bubble diameter of the foamed sheet smaller and at the same time make the bubble number density of the foamed sheet cross-section a certain high value. That is, in order to improve the dielectric properties (relative dielectric constant and tanδ) of the foamed sheet of the present invention, not only the average bubble diameter of the foam is set within the specified range of 1 to 30 μm, more desirably 1 to 22 μm, but also the bubble number density needs to be set higher. From this point of view, the bubble number density of the foamed sheet of the present invention is 1.8×10 4 ~9.0×10 8 pieces / mm 3 、or 2.0×10 4 ~9.0×10 8 pieces / mm 3 ,preferably 1.0×10 5 ~9.0×10 8 cells / mm 3 , more preferably 1.0×10 6 ~9.0×10 8 cells / mm 3 , still more preferably 3.0×10 6 ~7.5×10 8 cells / mm 3 . <Manufacturing Method of Foamed Sheet (Microcellular Foaming Process)> The microcellular foaming process is a batch method, and the process is as described below. That is, first, at room temperature in a high-pressure vessel, a high-pressure inert gas (for example, nitrogen or carbon dioxide) is used as a foaming agent and sufficiently dissolved in a solid resin sheet until the gas saturation solubility is reached. Thereafter, by depressurizing or heating the gas in the high-pressure vessel, phase separation is caused due to the thermodynamic instability of the gas contained in the resin, thereby foaming the resin. As a specific step example, for example, it is a method passing through the following three steps. First, in the manufacturing method of the polyphenylene ether resin foamed sheet of the present embodiment, a step of containing a non-reactive gas in the resin under pressure (hereinafter, referred to as "Step 1") is adopted. Subsequently, a step of foaming the obtained thermoplastic resin by heating it within a temperature range above the crystallization peak temperature and below the melting peak temperature of the resin measured by a differential scanning calorimeter under non-pressure (hereinafter, referred to as "Step 2") is passed through. Finally, a step of cooling the obtained thermoplastic resin (hereinafter, referred to as "Step 3") is adopted. The average cell diameter of the foamed sheet can be adjusted according to the holding time and holding temperature in Step 2. <Surface Roughness> When an alternating current flows through a conductor, there is a skin effect, that is, the higher the frequency of the current, the less likely it is to flow to the central part of the conductor and only flows to the surface layer. Also, regarding the relationship between the surface roughness and transmission loss of a copper circuit for high frequencies, if the unevenness on the surface is greater than the skin depth, the signal transmission path becomes longer and the signal loss increases. Conversely, if the unevenness is less than the skin depth, the transmission is smaller, the transmission path becomes shorter, and the signal loss decreases. Therefore, the lower the surface roughness of the foamed sheet, the better. Especially in high-frequency bands such as the GHz band or THz band, the surface shape of the foamed sheet can affect the surface shape of the circuit material and there is a possibility of affecting the loss. Also, in resin foamed sheets expected to be applied to circuit board materials, etc., considering the higher precision of circuits in the future and the influence of the unevenness on the substrate surface on the surface shape of the laminated circuit, it is necessary to reduce the surface roughness of the foamed sheet. Here, as the surface roughness, since it is important to have uniform unevenness on the sheet surface, the arithmetic mean surface roughness Ra is used as the surface roughness in the present invention. From this point of view, the surface roughness Ra of the polyphenylene ether resin foamed sheet is 0.10 to 0.50 μm, preferably 0.10 to 0.35 μm, more preferably 0.10 to 0.30 μm, and still more preferably 0.10 to 0.25 μm. In this way, the reason why the surface roughness of the resin foamed sheet of the present invention is excellent as the surface quality is that in the microcellular foaming process, since the gas penetration step and the foaming step are separated in this foaming method, the bubble diameter can be made smaller and the bubble diameter distribution can be made more stable. Therefore, the surface shape is easily stabilized, and by combining the roll processing as described above, the arithmetic mean surface roughness Ra as an index of the surface roughness can be suppressed to be lower. <Relative Dielectric Constant, Dielectric Loss Factor> Since the foamed sheet has a low relative dielectric constant and a low dielectric loss factor, when it is used for housings for electrical and electronic parts, electrical insulation materials, sealing materials, protective materials, substrates for high frequencies, or substrates for electromagnetic wave control members, it can exhibit suitable properties. From this point of view, the relative dielectric constant of the foamed sheet is preferably 1.10 to 2.00, more preferably 1.10 to 1.60, still more preferably 1.10 to 1.50, and particularly preferably 1.10 to 1.35. The dielectric loss factor (tanδ) is preferably 0.5×10 -3 ~2.5×10 - 3 , more preferably 0.7×10 -3 ~2.2×10 -3 , particularly preferably 1.0×10 -3 ~2.0×10 -3 , and more preferably 1.0×10 -3 ~1.8×10 -3 , and may also be 1.0×10 -3 ~1.5×10 -3 . Furthermore, the relative dielectric constant is the value measured by the method described in the examples. [Examples] (Measurement method) The measurement of the expansion ratio and the density of the foam is carried out as follows. <Measurement method of expansion ratio> The expansion ratio is the apparent ratio commonly used in the foam of the resin. This apparent ratio can be calculated as follows. That is, a test piece with a size of 10 cm×10 cm is cut out from the foamed resin sheet and weighed (W1[g]). Then, the thicknesses at the corners and the center of the four corners of the test piece are measured (the measuring machine is based on the JIS K6767 method), and the average value (T[cm]) of the five points is used. According to the following formula (1), the expansion ratio can be calculated. Apparent ratio (expansion ratio) = 10×10×T / W1 (1) <Measurement method of foam density> The measurement method of the density of the resin foam sheet is carried out according to JIS K7222-1999 and according to "Foamed plastics and rubbers - Method for the determination of apparent density". After being in a conditioned state for 72 hours or more after molding at 23°C±2°C, a 10 cm×10 cm size is cut out from the obtained resin foam sheet to be used as a test piece. Specifically, the foam density is calculated according to the following formula (2). The unit is set to g / cm 3 . [Equation 1] ρ a ={(m + ma)} / V (2) m is the mass (g) of the test piece, V is the volume (cm 3 ) of the test piece, and ma is the mass (g) of the replaced air. Furthermore, since the expansion ratio is low, the mass of the replaced air of ma can be ignored. Therefore, the calculation is carried out by ignoring ma. Five measurements are carried out, and the average value is calculated based on the measurement results of all the test pieces, and this is used as the density. <Method for Measuring Bubble Diameter> The bubble diameter is determined in accordance with ASTM D3576 - 77. An SEM (Scanning Electron Microscope) photograph of the longitudinal section of the sheet is taken. On the SEM photograph, straight lines are drawn in the horizontal and vertical directions, and the average value of the chord length t of the bubbles crossed by the straight lines is obtained. Taking the magnification of the photograph as M, it is introduced into the following formula to obtain the average bubble diameter d (d = t / (0.616×M)). Regarding the average bubble diameter, SEM photographs are taken for three different fields of view, and their average value is taken as the average bubble diameter. <Method for Measuring Bubble Number Density> An SEM photograph of the longitudinal section of the polyphenylene ether resin foamed sheet is taken. On this SEM photograph, five 100 μm×100 μm regions are randomly selected, and the number of bubbles existing in each region is counted. Based on each count value, the number of bubbles per 1 mm 2 in each region is calculated. Thus, it is taken as the number of bubbles per 1 mm 3 in each region. The average value of the number of bubbles per 1 mm 2 in the five regions is calculated and taken as the bubble density (number / mm 2 ). By multiplying the obtained number of bubbles per 1 mm 2 by (3 / 2), the number of bubbles per 1 mm 3 in each region is calculated. The average value of the number of bubbles per 1 mm 3 in the five regions is calculated and taken as the bubble density (number / mm 3 ). Furthermore, when measuring the bubble diameter or the bubble number density, it is preferably to prepare a plurality of (preferably 3 or more) SEM photographs and take the average value of the values calculated in each SEM photograph. <Measurement of Relative Dielectric Constant and Dielectric Loss Factor> The relative dielectric constant and the dielectric loss factor are measured using the resonant cavity perturbation method with the measurement frequency set to 2 GHz. As the relative dielectric constant ε rRegarding the dielectric loss factor tanδ, the complex dielectric constant in the plane direction of the polyphenylene ether resin foamed sheet can be measured by the cavity resonator method. In the present invention, the relative dielectric constant is measured by the cavity resonator method at a resonance frequency of 2 GHz using a specimen with a length of 78 mm × a width of 2.4 mm × a plate thickness of 1 mm. As the device used for the measurement at this time, for example, as the cavity resonator, CP461 manufactured by Kanto Electronic Application Development Co., Ltd. can be used, and as the network analyzer, E8361A manufactured by Agilent Technologies Inc. can be used. <Measurement of surface roughness> Regarding the evaluation of the surface properties, in order to evaluate the overall surface properties of the modified polyphenylene ether foamed sheet, the surface properties of the foamed sheet of the present invention are evaluated by measuring Ra instead of Rz which is greatly affected by the maximum unevenness and the like. According to JIS B0601:2013, the arithmetic mean roughness of Ra is determined. This measurement is performed using HANDYSURF E-30A (manufactured by Tokyo Seimitsu Co., Ltd.). In each measurement, the reference length is set to 2.5 mm. In the present invention, when the surface roughness Ra is, for example, 1.0 μm or less, it means that the average value of the average roughness in the following two directions is 1.0 μm or less, that is, five measurement lines are randomly determined in the MD direction on the surface of the modified PPE foamed sheet, and the arithmetic mean roughness of the measured values in each measurement line is measured to obtain the average roughness of the five measured values, and five measurement lines are randomly determined in the TD direction, and the arithmetic mean roughness of the measured values in each measurement line is measured to obtain the average roughness of the five measured values. That is, the surface roughness in the present invention evaluates the average value in the two directions of the MD direction and the TD direction. Here, since the surface characteristics are affected not only by the surface roughness in the MD direction but also by the surface roughness in the TD direction, the reason for taking the average value of the two directions is that when manufacturing the sheet, it sometimes passes through rollers, so at this time, in the MD direction of the sheet, in addition to the surface pressing effect generated by the rollers, tension is also applied and it is stretched, so the surface properties in the MD direction are generally better than those in the TD direction. <Flame retardancy test> Regarding the flame retardancy test, the flame retardancy imparting level is set in the following two stages: based on the evaluation of the oxygen index according to the plastic flammability test based on JIS K 7201-2:2021, to evaluate whether the flame retardancy can be slightly improved by the flame retardant, flammability, self-extinguishing property with flammability, flame retardancy, etc.; and in the flame retardancy test of the UL94 standard, it satisfies V-0 and V-1 in the vertical flame retardancy test. [Plastic Flammability Test Based on JIS K 7201-2] It represents the minimum volume fraction of oxygen in a mixture of oxygen and nitrogen at a temperature of 23°C ± 2°C at which the material can sustain flaming combustion under specified test conditions, expressed as a percentage (%). The test is carried out using a test piece mounted on a specified test piece support tool in a transparent cylinder where a laminar flow of a mixture of oxygen and nitrogen flows upward. The test is carried out at room temperature after adjustment. In the procedure of ignition from the upper end, the flame is brought into contact with the upper end of the test piece for up to 30 seconds, and at this time, the flame is withdrawn every 5 seconds to confirm whether the test piece burns. This method is to avoid continuous temperature rise of the test piece because temperature rise usually reduces the oxygen index. In addition, the test is carried out in accordance with the above JIS standard, but some descriptions are omitted. [UL94 Flammability Test: Vertical Flammability Test] Based on UL94 (the standard specified by Underwriters Laboratories Inc. in the United States), five test pieces with a length of 125 mm and a width of 13.0 mm are made using the obtained sheet material, and the vertical flammability test is carried out. During the test, each test piece is vertically mounted on a fixture, and the flame contact is made with a 20 mm flame for 10 seconds twice, and according to its combustion behavior, it is judged as V-0, V-1, or V-2. Here, the flammability criteria for V-0, V-1, and V-2 are as follows. [Evaluation Criteria for Oxygen Index Based on Plastic Flammability Test Based on JIS K 7201-2] Generally, the oxygen index is understood as follows. 22 or less: Flammable and burns. 23 - 27: Burns but has self-extinguishing property. 27 or more: Flame-retardant. [Flammability Criteria for V-0, V-1, and V-2] V-0: The duration of flaming combustion for both the first and second times is within 10 seconds. Furthermore, the sum of the duration of flaming combustion and the duration of non-flaming combustion for the second time is within 30 seconds. Furthermore, the sum of the flaming combustion times of the five test pieces is within 50 seconds. No specimen burns to the position of the fixing fixture, and no cotton is ignited by burning debris. V-1: The duration of flaming combustion for both the first and second times is within 30 seconds. Furthermore, the sum of the duration of flaming combustion and the duration of non-flaming combustion for the second time is within 60 seconds. Furthermore, the sum of the flaming combustion times of the five test pieces is within 250 seconds. No specimen burns to the position of the fixing fixture, and no cotton is ignited by burning debris. V-2: The duration of flaming combustion for both the first and second times is within 30 seconds. Furthermore, the sum of the duration of flaming combustion and the duration of non-flaming combustion for the second time is within 60 seconds. Furthermore, the sum of the flaming combustion times of the five test pieces is within 250 seconds. No specimen burns to the position of the fixing fixture, and cotton is ignited by burning debris. Furthermore, in the vertical flammability test, a sample that does not meet any of the above V-0, V-1, and V-2 standards is marked as unqualified (×). - Modified polyphenylene ether resin used: Modified polyphenylene ether (hereinafter referred to as m-PPE): Zylon 340Z manufactured by Asahi Kasei Corporation, which is a composition containing polyphenylene ether containing poly(2,6-dimethyl-1,4-phenylene oxide), polystyrene, and a phosphorus-based flame retardant (the phosphate compound of the above formula (5) or (6)). Here, the polystyrene is impact-resistant polystyrene. Modified polyphenylene ether (hereinafter referred to as m-PPE): Zylon 540Z manufactured by Asahi Kasei Corporation, which is a composition containing polyphenylene ether containing poly(2,6-dimethyl-1,4-phenylene oxide), polystyrene, and a phosphorus-based flame retardant (the phosphate compound of the above formula (5) or (6)). Here, the polystyrene is impact-resistant polystyrene. Flame retardant (1): Fire Guard FG-8500 manufactured by Teijin Limited (the compound of the above formula (7-1)) Flame retardant (2): Fire Guard FG-7500 manufactured by Teijin Limited (the compound of the above formula (7-2)) Flame retardant (3): SAYTEX HP7010G manufactured by Albemarle Japan Co., Ltd. (the compound represented by the above chemical formula (8), where x and y are approximately 2000 and 2.7 respectively, and the molecular weight is approximately 700,000) Flame retardant (4): FR-1410 manufactured by ICL Japan Co., Ltd. (the following formula (a)) [Chemical formula 12] Molecular weight: 971.2 Flame retardant (5): SAYTEX 8010 manufactured by Albemarle Japan Co., Ltd., which is the same compound as the above chemical formula (a) (different manufacturer). First, the following 12 test materials were used in the test. <Test materials for the test> Test materials 1 to 8 are based on modified polyphenylene ether (hereinafter referred to as m-PPE): Zylon 540Z manufactured by Asahi Kasei Corporation, and a specified amount of flame retardant is respectively formulated therein. Here, in test materials 1 to 3, 20 parts, 30 parts, and 35 parts of Fire Guard FG-8500 manufactured by Teijin Limited as flame retardant (1) are respectively formulated relative to Zylon 540Z of modified PPE. Also, in test materials 4 and 5, similarly, 20 parts and 30 parts of Fire Guard FG-7500 manufactured by Teijin Limited as flame retardant (2) are respectively formulated relative to Zylon 540Z of modified PPE. Furthermore, in test materials 6, 7, and 8, similarly, 20 parts, 30 parts, and 40 parts of SAYTEX HP7010G manufactured by Albemarle Japan Co., Ltd. as flame retardant (3) are respectively formulated relative to Zylon 540Z of modified PPE. Here, test materials 1 to 8 are based on Zylon 540Z of modified PPE, and a specified amount of flame retardants with three different specific molecular weights is further added thereto. Test materials 9 and 10 are respectively a foamed sheet made of a material composed only of modified polyphenylene ether (modified PPE): Zylon 340Z manufactured by Asahi Kasei Corporation, and a foamed sheet made of a material with a different product number composed only of modified polyphenylene ether (hereinafter referred to as modified PPE): Zylon 540Z manufactured by Asahi Kasei Corporation. In test materials 9 and 10, no other flame retardants are added except for the phosphorus-based flame retardants originally contained in Zylon 340Z and Zylon 540Z. Test materials 11 and 12 are respectively a foamed sheet made of a material in which 20 parts of FR-1410 manufactured by ICL Japan is added as flame retardant (4) to modified polyphenylene ether (modified PPE): Zylon 340Z manufactured by Asahi Kasei Corporation, and a foamed sheet made of a material in which 20 parts of SAYTEX 8010 manufactured by Albemarle Japan Co., Ltd. is added as flame retardant (5) to modified polyphenylene ether (hereinafter referred to as modified PPE): Zylon 540Z manufactured by Asahi Kasei Corporation. Test materials 9 and 11 both use materials of Zylon 340Z manufactured by Asahi Kasei Corporation, and test materials 10 and 12 use materials of Zylon 540Z manufactured by Asahi Kasei Corporation. Since both Zylon 540Z and Zylon 340Z are PPE blended with PS, considering the difference in the glass transition temperature Tg between PPE and PS, it is considered that the higher the blending amount of PS in PPE, the lower the Tg. Therefore, it is considered that the content of PS in Zylon 340Z is more than that in Zylon 540Z. <Manufacture of Test Material Foamed Sheet> A wound roll of a foamed sheet containing the test materials shown in Table 1 above was obtained. The wound roll was placed in a pressure vessel, and the pressure vessel was pressurized to 5.2 MPa with carbon dioxide, and the carbon dioxide was allowed to permeate into the resin film for 24 hours. Thereafter, the wound roll was taken out from the pressure vessel and foamed by continuously supplying it to a hot air circulation type foaming furnace set at 160 °C and maintaining for a specified time, thereby obtaining each PPE foamed sheet shown in Table 1. At this time, foaming hindrance occurred in Test Materials 11 and 12, and almost no foaming occurred, and the target foam could not be obtained. Furthermore, in Test Materials 7 and 8, the foaming temperature was set at 170 °C for foaming. In Test Materials 9 and 10, the foaming temperature was set at 180 °C for foaming. <Test Results of Characteristic Evaluation of Test Material Foamed Sheet> Table 1 shows the characteristic test results using Test Materials 1 to 12 as test materials. [Table 1] <Foaming Characteristics of Modified PPE Foamed Sheet and Characteristics Related to Bubble Structure> The foaming properties of the test materials were evaluated based on the foaming ratio and density. Since the test materials 1 to 8 and Test Materials 9 and 10 other than Test Materials 11 and 12 were appropriately foamed, the foaming ratio was in the range of 2 to 10 times, specifically in the range of 2.8 to 6.8 times, and the density of the foam was in the range of 0.10 to 0.50, specifically in the range of 0.17 to 0.41. In contrast, due to the foaming hindrance caused by the addition of the flame retardant in Test Materials 11 and 12, the foaming ratio was only 1.1, and there was almost no density change caused by foaming, only 1.07 to 1.08, and the target foamed sheet could not be obtained. As evaluation parameters for evaluating the bubble structure of the foam, the bubble diameter (average bubble diameter) and the bubble number density were used. The test results showed that the average bubble diameter satisfied the range of 1 to 22 μm, specifically satisfied the range of 1.7 to 21.2 μm, and the bubble number density was targeted at 10 4 or more, and actually satisfied the range of 2.0×10 4 to 7.2×10 8 per cm 3 range. The modified resin foamed sheet of the present invention has a specified average bubble diameter and bubble number density. <Dielectric Properties of Modified PPE Foamed Sheets> Using the resonant cavity perturbation method and a network analyzer, the relative dielectric constant and dielectric loss factor (tanδ) of the dielectric properties of Test Specimens 1 to 8 were tested. The results showed that the average bubble diameter satisfied 1 to 22 μm, and the bubble number density satisfied 10 4 or more. Therefore, the dielectric properties were also good. The relative dielectric constants of Test Specimens 1 to 8 were 1.20 to 1.57, and the dielectric loss factor tanδ was 1.0×10 -3 to 1.5×10 -3 . The relative dielectric constants of Test Specimens 8 and 9 were both 1.20, and the dielectric loss factor tanδ was 1.0×10 -3 to 2.1×10 -3 . Whether or not a polymer-based flame retardant was added, the relative dielectric constant and dielectric loss factor were excellent. Also, the average bubble diameters of Test Specimens 1 to 10 were in the range of 1.7 to 21.2 μm below 22 μm. Therefore, not only were the dielectric properties at the gigahertz band expected, but also in terms of the comparison between the wavelength and the average bubble diameter, less reflection and diffusion loss were expected when passing through the foamed sheet. Here, regarding Test Specimens 11 and 12, since foaming was hindered and no foamed sheets were obtained, the evaluation of dielectric properties was not performed. <Surface Properties of Modified PPE Foamed Sheets> Regarding Test Specimens 1 to 8 and Test Specimens 9 and 10 other than Test Specimens 11 and 12 for which normal foaming was not obtained, the average values of the results of 5 Ra measurements each in the MD direction and TD direction on the surface of the foam were shown as the surface properties of the modified PPE sheet. The results of measuring the arithmetic mean surface roughness Ra were shown as the surface roughness. Although the surface roughnesses of Test Specimens 3 and 8 were relatively large, in the case including Test Specimens 3 and 8, the arithmetic mean surface roughness Ra was still in the range of 0.10 to 0.30 μm below 0.5 μm. Except for Test Specimens 3 and 8, the differences in the arithmetic mean roughness Ra between materials were small, and Ra was in the range of 0.10 to 0.14 μm, satisfying below 0.20 μm. Furthermore, regarding the arithmetic mean surface roughness Ra of the present invention, even considering that the upper limit of the relatively large arithmetic mean surface roughness Ra was below 0.5 μm, its surface properties were considered to be superior to the surface roughness obtained by bead foaming, etc. Here, regarding Test Specimens 11 and 12, since foaming was hindered and no foamed sheets were obtained, the evaluation of surface properties was not performed. <Flammability related to modified PPE foamed sheet> The evaluation results of the flammability tests of each test material in Table 1 are shown. In the table, the oxygen index is denoted as the OI value. The results of the vertical flammability test according to UL94 indicate that in the modified PPE foamed sheets of test materials 1 to 8 in which a specified amount of a phosphorus-based flame retardant and a polymer-based flame retardant are added to the modified PPE foamed sheet, the oxygen index is in the range of 27.4 to 32.5, exceeding the flammability standard oxygen index of 27 and meeting the UL94 V-0 and V-1 standards. In contrast, in test material 9 of the Zylon 340Z foamed sheet in which only a phosphorus-based flame retardant is added to the modified PPE foamed sheet, and in test material 10 of the Zylon 540Z foamed sheet in which only a phosphorus-based flame retardant is added in the same way, although there is no foaming hindrance caused by the addition of the flame retardant and the effect of increasing the oxygen index is confirmed, the oxygen indexes are 24.8 and 25.4. Compared with the above test materials 1 to 8, although they have self-extinguishing properties, they lack flammability and do not meet any of the UL94 V-0 to V-1 standards. Also, since the oxygen indexes of test materials 1 to 8 exceed 27, they are considered to have flammability. Here, when only a phosphorus-based flame retardant is added, since the content of the flame retardant is small and no polymer-based flame retardant is contained, compared with the modified PPE without a non-flame-retardant grade flame retardant, the flammability is improved due to the addition of the flame retardant, but it does not meet the UL-94 V-0 to V-1. Here, it should be noted that although not specifically described in the examples, when the UL standard flammability test is performed on test materials 8 and 9 without making them into foams and in an unfoamed state, it is confirmed that the V-0 standard is met. In contrast, in test materials 11 and 12 to which a halogen-based flame retardant with a molecular weight lower than 1,000 is added, since a halogen-based flame retardant is added in addition to the phosphorus-based flame retardant, the amount of the flame retardant increases. Therefore, compared with the case where only a phosphorus-based flame retardant is added, the oxygen indexes are higher, at 29.3 and 29.5, and the flammability is improved. Although the flammability meets the UL94 V-0 standard, the addition of these flame retardants causes foaming hindrance of the modified PPE, and the foaming hindrance results in the failure to obtain the modified PPE foamed sheet. In the present invention, a polymer-based flame retardant with a molecular weight higher than that of an oligomer is used as the halogen-based flame retardant. However, low-molecular-weight flame retardants or metal hydroxide-based flame retardants commonly used as flame retardants do not cause phenomena such as expansion of the flame retardant at high temperatures due to foaming gas and penetration of the foaming gas into the flame retardant. Therefore, this phenomenon becomes the cause of foaming hindrance. However, in the case of a polymer-based flame retardant, carbon dioxide can penetrate into the flame retardant, and furthermore, it has appropriate viscoelasticity at high temperatures near the foaming temperature. Therefore, it is considered that foaming hindrance will not be caused and a foamed body with fine bubbles can be formed. <Summary of Test Results> As described above, in order to obtain the modified PPE foamed sheet of the present invention, as shown in Test Specimens 9 and 10, when a modified PPE resin in which a specified amount of a phosphorus-based flame retardant is added to a modified PS resin of PS within a specified range is added to PPE, although a foamed sheet having an appropriate cell structure, dielectric properties, and excellent surface properties is obtained, since the amount of the flame retardant added is small, although the flame retardancy is improved and self-extinguishing property is successfully imparted even if it is flammable, a foamed sheet having a flame retardancy satisfying the UL standard is not obtained. Further, in order to ensure the flame retardancy, as shown in Test Specimens 11 and 12, a low molecular weight halogen-based flame retardant is added. Although the flame retardancy satisfies the UL standard, foaming hindrance is caused and a specified foamed sheet is not obtained. In contrast, in the case of the foamed sheet of Test Specimens 1 to 8 in which, in addition to adding a phosphorus-based flame retardant, a high molecular weight flame retardant higher than an oligomer having a weight average molecular weight of 1,000 or more and 1,000,000 or less is added as a flame retardant, a foamed sheet having no foaming hindrance, excellent cell structure, dielectric properties, surface properties, and excellent flame retardancy can be obtained. Furthermore, the modified PPE foamed sheet of the present invention not only has excellent dielectric properties, but also has a small average cell diameter of the foamed sheet. Therefore, when it is used for a printed circuit board in the gigahertz band or the like, not only can the transmission loss be suppressed to a low level, but also the reflection and scattering loss can be suppressed to a low level. At the same time, when a conductor such as a copper foil having a circuit formed thereon is placed and bonded to the foamed sheet, since the surface properties of the foamed sheet are excellent, the influence of the substrate foamed sheet on the skin effect of the circuit or the conductor can also be reduced. The present invention has achieved, for the first time, the above requirements for simultaneously satisfying the cell structure, cell density, surface characteristics, and flame retardancy. The method is to use a batch-type microcellular foaming step customized for the modified PPE resin as the foaming step, and a combination of a phosphorus-based flame retardant and a halogen-based high molecular weight flame retardant higher than an oligomer having a specified molecular weight as the flame retardant. Furthermore, according to the present invention, a thin-walled modified PPE foamed sheet having a thickness as low as 50 μm can be obtained. <Research> According to Table 1 above, in the modified polyphenylene ether foamed sheets of Test Specimens 1 to 8, halogen-based flame retardants (1) to (3) were formulated, showing excellent flame retardancy (UL94V-0). On the other hand, although containing the above halogen-based flame retardants, there was no significant impact on the electrical properties (relative dielectric constant, dielectric loss factor). Compared with the modified polyphenylene ether foamed sheets of Test Specimens 9 and 10 that did not contain halogen-based flame retardants but only contained a specified amount of phosphorus flame retardant, the electrical properties were also comparable. That is, in the present invention, when using the microcellular foaming method to produce a modified polyphenylene ether foam, adding only a specified amount of phosphorus-based flame retardant also improved the flame retardancy, but a foam that met the UL94 standard could not be obtained. By combining the phosphorus-based flame retardant with a halogen-based flame retardant having a specified molecular weight, a foamed sheet with very low relative dielectric constant and dielectric loss factor at high frequencies, high flame retardancy, and meeting the V-0 and V-1 performance of the UL94 standard could be obtained while maintaining the surface quality of the foamed sheet. At this time, a crosslinking agent or a bubble nucleating agent could also be used, but it was also an advantage that the above modified polyphenylene ether foamed sheet could be obtained without using a crosslinking agent or a bubble nucleating agent. Although only a specified amount of phosphorus-based flame retardant was added, an improvement effect in flame retardancy was confirmed. Judging from the oxygen index level, it was considered that although it was flammable, it had self-extinguishing properties, and the bubble structure, dielectric properties, and surface properties of the foam were excellent, so it could be fully used according to the usage or form of use.

Claims

1. A modified polyphenylene ether resin foamed sheet, characterized in that: it is made by foaming a modified polyphenylene ether resin containing a flame retardant, having a relative permittivity of 1.10 to 2.00, a dielectric loss factor (tanδ) in the range of 0.5×10⁻³ to 2.5×10⁻³, and containing both a halogen-based flame retardant and a phosphorus-based flame retardant with a weight average molecular weight of 1,000 to 1,000,000 as the flame retardant.

2. The modified polyphenylene ether resin foamed sheet as requested in item 1, wherein the arithmetic mean surface roughness Ra of the modified polyphenylene ether resin foamed sheet is less than 0.50 μm.

3. The modified polyphenylene ether resin foamed sheet as requested in item 1 or 2, wherein the relative permittivity of the modified polyphenylene ether resin foamed sheet is 1.20 to 1.57 and the dielectric loss factor (tanδ) is in the range of 1.0×10-3 to 2.1×10-3.

4. The modified polyphenylene ether resin foam sheet as requested in item 1 has an average thickness of 0.05 mm to 2.0 mm.

5. The modified polyphenylene ether resin foamed sheet as requested in item 1, wherein the modified polyphenylene ether resin foamed sheet is used, and the flame retardancy is evaluated according to the UL-94 vertical method (20 mm vertical burning test) of the US UL standard, and the test results meet the V-0 or V-1 standard.

6. The modified polyphenylene ether resin foamed sheet as claimed in claim 1, wherein the modified polyphenylene ether resin foamed sheet is used for housings of electrical and electronic components, electrical insulation materials, sealing materials, protective materials, substrates for high frequency applications, or substrates for electromagnetic wave control components.

7. A housing, electrical insulation material, sealing material, protective material, high-frequency substrate, or electromagnetic wave control component substrate for electrical and electronic parts, which is obtained by using the modified polyphenylene ether resin foam sheet as claimed in claim 1.