Modified polyphenylene ether resin foamed sheet

Abstract

[Problem] To obtain a modified polyphenylene ether resin foamed sheet, with said foamed sheet having good formability and an exceptionally low relative permittivity and dielectric loss tangent at high frequencies, being imparted with a self-extinguishing property or a flame-retardant property, and excelling in surface characteristics. [Solution] Provided are: a modified polyphenylene ether resin foamed sheet obtained by foaming a modified polyphenylene ether resin including a flame retardant, with said modified polyphenylene ether resin foamed sheet having a relative permittivity of 1.10 to 2.00, and a dielectric loss tangent (tanδ) of 0.5×10-3 to 2.5×10-3; and a modified polyphenylene ether resin foamed sheet obtained by foaming a modified polyphenylene ether resin foamed resin including specified weight-average molecular weights of both a halogen-based flame retardant and a phosphorus-based flame retardant, with the average bubble diameter and the bubble number density of bubbles in the modified polyphenylene ether resin foamed sheet being within prescribed ranges.

Description

Modified polyphenylene ether resin foam sheet

[0001] The present invention relates to a modified polyphenylene ether resin foam sheet.

[0002] There is a demand for resin materials that can be used as housings, electrical insulating materials, sealing materials, protective materials, or substrates for high-frequency electromagnetic wave control components used as members of electric and electronic parts.

[0003] Polyphenylene ether (hereinafter sometimes abbreviated as PPE) resins are widely used in fields such as electronic devices due to their excellent flame retardancy, thermal stability, hot water resistance, and electrical properties. However, due to extremely low melt fluidity, polyphenylene ether resins are alloyed with other resins when molded. Polystyrene, in particular, is completely compatible with polyphenylene ether, and is therefore alloyed without the need for a compatibilizer and commercially available as "modified polyphenylene ether resin." This polystyrene alloy material is widely used because it is easy to mold, has an excellent balance of heat resistance and mechanical strength, and is easily flame-retardant. Hereinafter, for simplicity, polystyrene will sometimes be referred to as PS in this specification.

[0004] In particular, in the electrical and electronic fields, alloys with polystyrene resins are highly rated for their heat resistance, flame retardancy, dimensional stability, etc., and are used in flyback transformer cases, coil bobbins, relay sockets, adapter cases, switches, etc. However, even though foaming this modified polyphenylene ether resin has superior flame retardancy compared to general foams, for reasons that will be described later, it does not meet the V-1 or higher standard in the UL flame retardancy test.

[0005] Modified polyphenylene ether resins are also used as foams, taking advantage of their excellent properties. For example, foams produced by a bead expansion method are known, as disclosed in Patent Document 1. The resin foam in Patent Document 1 is a resin foam with excellent flame retardancy and antistatic properties for use in electronic devices and automotive components, but there is no specific disclosure regarding the cell structure (e.g., cell diameter and cell density), dielectric properties, or surface properties. Patent Document 2 is known as 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 there is no specific disclosure regarding the cell structure, dielectric properties, or surface properties. Patent Document 3 is known as a method for obtaining hollow foamed blow-molded articles, but the foamed blow-molded articles in Patent Document 3 are thicker than those of the present invention and do not produce foam sheets. These foams are not intended to obtain foams with excellent dielectric properties and surface properties by controlling the cell structure of the foam.

[0006] Furthermore, Patent Document 4 discloses a foam for substrates with a cell diameter of 0.3 to 1.5 μm, which is produced by batch foaming a mixture of a resin having at least one of a crystalline melting temperature, glass transition temperature, and liquid crystal transition temperature of 260°C or higher and an amorphous resin such as modified polyphenylene ether having a glass transition temperature of 230°C or lower. However, this foam has a relative dielectric constant of 2.6 to 2.9, which is higher than that of the present invention. This is because the resin expansion ratio in this invention is low, and it is not possible to further reduce the relative dielectric constant by introducing fine bubbles at a high density. In other words, to lower the relative dielectric constant, it is necessary to increase the cell density while maintaining the fine bubbles.

[0007] Furthermore, Patent Document 5 discloses a laminated foam in which the auxiliary layer on the back side is a foam. This invention describes the relative dielectric constant in the 79 GHz and 29 GHz frequency bands, with the relative dielectric constant being 2.28 to 2.73 and the dielectric loss tangent (tanδ) being in the range of 0.002 to 0.0077. The cell diameter is 44 to 103 μm and the expansion ratio is 9.2 to 30.8 times. In the case of Patent Document 5, the cell diameter exceeds 30 μm, the expansion ratio is high, and the relative dielectric constant exceeds 2.0, so the dielectric properties cannot be said to be excellent. Patent Document 6 describes the use of various bubble nucleating agents to produce foams with bubble diameters of 10 μm or more and 20 μm or less by a batch process. However, the invention described in this document requires a bubble nucleating agent, is not related to modified PPE foams, and is not intended to improve dielectric properties. Patent Document 7 discloses a PPS resin foam containing a crosslinking agent produced by batch foaming, and its object is to provide a polyphenylene sulfide foam that maintains the basic properties of PPS resin while exhibiting excellent post-formability, and a method for producing the same. It is also disclosed that the PPS resin foam can have an average cell diameter of 20 μm or less. As can be seen from Patent Documents 6 and 7, when a fine cell structure is obtained by batch foaming, a cell nucleating agent or a crosslinking agent may be used as needed to control the cell structure.

[0008] On the other hand, since modified polyphenylene ether resins have a good balance between the high flame retardancy and low dielectric properties mentioned above, they are expected to be used particularly in high-frequency components, and foams thereof are expected to be applied to terahertz wave components, for example, which are expected to expand in the future. However, in order to exhibit similar low dielectric properties for terahertz waves, taking into account losses such as reflection and scattering during transmission of radio waves in the terahertz band, the foam needs to have fine cells of at least 30 μm or less, and preferably 10 μm or less.

[0009] Japanese Patent No. 5642521, Japanese Patent Application Publication No. 2017-155196, Japanese Patent No. 6106420, Japanese Patent Application Publication No. 2008-303247, Japanese Patent Application Publication No. 2021-136540, Japanese Patent No. 3138488, Japanese Patent No. 5809895

[0010] Although neither the above-mentioned Literature 1 nor Literature 2 describes the bubble size, the methods described in these literatures generally make it difficult to suppress bubble growth, and therefore it is not possible to simultaneously reduce the bubble size and improve the surface properties. Literature 1 uses a bead foam molding, and Literature 2 describes a similar method, but with bead foam, it is difficult to create a sheet shape, and even if it is possible, there is an interface between the particles, which is disadvantageous when considering bonding to copper foil, for example.

[0011] When modified polyphenylene ether resin is foamed, the contact area with air increases, the burning rate increases, and the flame retardancy decreases, making it difficult to satisfy the vertical flame retardancy performance generally required in high-frequency fields. Therefore, even though a finely foamed sheet using modified polyphenylene ether resin has superior flame retardancy compared to general foamed resin sheets, it does not meet the V-1 or higher standard in the UL flame retardancy test.

[0012] Compared to unfoamed resin, foamed resin has more contact points with the air, resulting in significantly lower flame retardancy. Compared to unfoamed resin, foamed resin has a lower resin content, which means it is more likely to soften due to the heat of combustion, resulting in resin dripping during combustion. On the other hand, if the amount of flame retardant added to foamed resin is increased to improve flame retardancy, the flame retardant generally inhibits foaming, making it impossible to obtain foam with the desired cell size and density, and in some cases, the surface quality of the foam may deteriorate.

[0013] Therefore, it has been extremely difficult to obtain a modified polyphenylene ether resin foamed sheet that not only has good moldability into molded articles but also facilitates complex and fine foaming, and simultaneously satisfies the requirements of lightweight foam, improved flame retardancy, and excellent surface properties. Therefore, an object of the present invention is to obtain a modified polyphenylene ether resin foamed sheet that has good moldability, extremely low relative dielectric constant and dielectric dissipation factor at high frequencies, self-extinguishing properties or flame retardancy, and excellent surface properties.

[0014] 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 has a relative dielectric constant of 1.10 to 2.00 and a dielectric loss tangent (tanδ) of 0.5×10 －３ ~2.5 x 10 －３ The modified polyphenylene ether resin foam sheet of the present invention may have an arithmetic mean surface roughness Ra of 0.50 μm or less. Since the flame retardancy can be improved even when a predetermined amount of only a phosphorus-based flame retardant is contained as the flame retardant, the modified polyphenylene ether resin foam sheet may contain only a phosphorus-based flame retardant as the flame retardant. The modified polyphenylene ether resin foam sheet may contain, as the flame retardant, both a halogen-based flame retardant and a phosphorus-based flame retardant, each having a weight-average molecular weight of 1,000 to 1,000,000. As described above, by using appropriate amounts of a phosphorus-based flame retardant and a predetermined halogen-based flame retardant, foaming inhibition can be prevented, as described below, and flame retardancy can be further improved to a level satisfying the UL standard. The modified polyphenylene ether resin foam sheet has a relative dielectric constant of 1.20 to 1.57 and a dielectric loss tangent (tan δ) of 1.0×10 －３ ~2.1 × 10 －３ may be in the range of

[0015] The modified polyphenylene ether resin foam sheet of the present invention is a modified polyphenylene ether resin foam sheet obtained by foaming a modified polyphenylene ether resin containing, as a flame retardant, a halogen-based flame retardant and a phosphorus-based flame retardant, each having a weight-average molecular weight of 1,000 or more, and the modified polyphenylene ether resin foam sheet has bubbles having an average bubble diameter of 1 to 30 μm and a bubble number density of 1.8×10 ４ ~9 x 10 ８ pieces / mm ３As described above, the cell structure of the modified polyphenylene ether resin foam sheet may be such that the average cell diameter is set to be fine cells in the range of 1 to 30 μm, and the cell number density is set to be 1.8×10 ４ ~9 x 10 ８ pieces / mm ３ By setting the range to the range, it is possible to obtain dielectric properties such as the relative permittivity and the dielectric loss tangent tanδ within the above range. In addition, the modified polyphenylene ether resin foamed sheet has bubbles having an average bubble diameter of 1 to 22 μm and a bubble number density of 2.0×10 ４ ~8 x 10 ８ pieces / mm ３ In this way, the dielectric properties and surface properties of the foamed sheet can be further improved by making the average cell diameter finer and increasing the lower limit of the cell number density. Regarding flame retardancy, even when only an inorganic flame retardant such as a phosphorus-based flame retardant is added, the addition of the flame retardant can improve flame retardancy and impart self-extinguishing properties during combustion. However, increasing the amount of flame retardant added inhibits foamability, making it difficult to expect further improvements in flame retardancy. Even when a specific halogen-based flame retardant is added to a phosphorus-based flame retardant as a flame retardant, unless a polymeric flame retardant with the specified molecular weight is used and the composition range of the flame retardant is adjusted to a specified range, it is not possible to maintain the same average cell diameter and cell density as when no flame retardant is added, without affecting the foaming state and satisfying the V-0 or V-1 standard in the UL94 flame retardancy test. Here, the objective of the present invention is to improve flame retardancy without causing foaming inhibition, specifically, firstly, to impart self-extinguishing properties to a foam even if the foam is flammable by adding a flame retardant, thereby reducing the flammability, and secondly, to obtain a foam whose flame retardancy has been improved to a level that satisfies V-0 or V-1 in the UL94 flame retardancy test.

[0016]

[0006] In other manufacturing methods such as bead foaming, injection foaming, and extrusion foaming (crosslinking and non-crosslinking foaming), it is usually difficult to achieve a fine average cell diameter and a cell density within the specified range, as described above, and as a result, it is difficult to obtain the above-described cell structure. Even if a material having the above-described cell structure and dielectric properties can be obtained by a manufacturing method other than the fine foaming process of the present invention, the foam produced by such a foaming process has a wide cell size distribution, making it difficult to maintain excellent surface properties of the foamed sheet, and therefore it is difficult to maintain the surface roughness Ra (arithmetic mean surface roughness) of the foamed sheet within the range of 0.50 μm or less.

[0017] The modified polyphenylene ether resin foam sheet of the present invention may have an arithmetic mean surface roughness Ra of 0.50 μm or less, as described above. Furthermore, considering the lower limit of the arithmetic mean surface roughness Ra of the surface of the modified polyphenylene ether resin foam sheet, the high-frequency characteristics such as the skin effect can be further improved by limiting the arithmetic mean surface roughness Ra to a range of 0.08 to 0.50 μm, preferably 0.10 to 0.40 μm, and more preferably 0.10 to 0.30 μm. Therefore, such a modified polyphenylene ether resin foam sheet is more preferable for use as a substrate for high-frequency circuits, etc.

[0018] The average thickness of the modified polyphenylene ether resin foam sheet can be 0.05 mm to 2.0 mm, and can also be 0.1 mm to 2.0 mm. The average thickness of the modified polyphenylene ether resin foam sheet of the present invention can be reduced by using a batch foaming process in which the molding process and the foaming process are separated to foam the product, compared to bead foaming, extrusion cross-linking foaming, extrusion gas foaming, etc., and by independently controlling the impregnation and foaming of the foaming gas. Therefore, the upper limit of the foam sheet thickness is 2 mm, and the lower limit must be at least 50 μm (0.05 mm) because skin layers are formed on the top and bottom of the sheet even when the sheet thickness is reduced. In this case, roll rolling may be performed as needed after the micro-foaming process is completed to adjust the thickness and surface properties, etc.

[0019] The modified polyphenylene ether resin foam sheet is preferably a modified polyphenylene ether resin foam sheet whose flame retardancy is evaluated by a test in accordance with the UL-94 vertical method (20 mm vertical combustion test) of the U.S. UL standard, and the test result satisfies the V-0 and V-1 standards. Materials in which a flame retardant is added to a base resin within the range of the present invention satisfy the V-0 and V-1 standards of UL-94 below, but materials in which no flame retardant is added or in which the amount of flame retardant added is below the range of the present invention cannot satisfy the V-0 and V-1 standards of the UL-94 vertical method of the U.S. UL standard in terms of flame retardancy.

[0020] The modified polyphenylene ether resin foam sheet may be used for housings for electric / electronic parts, electrical insulating materials, sealing materials, protective materials, or substrates for high-frequency electromagnetic wave control members.

[0021] (Regarding Mixing of Modified Polyphenylene Ether Resin and Polystyrene Resin) Here, the reason for mixing polystyrene resin with polyphenylene ether resin is to improve processability, since polyphenylene ether resin has excellent heat resistance and strength but poor processability and cannot be processed. Here, the reason why the amount of polyphenylene ether resin is set to 40% by mass or more and 80% by mass or less is that if the amount of polyphenylene ether resin is less than 40% by mass, the desired heat resistance, strength, etc. cannot be obtained, and the reason why it is set to 80% by mass or less is that if the amount of polyphenylene ether resin is more than 80% by mass, the processability is reduced. Furthermore, the amount of polystyrene resin is usually the remainder, so it is 20% by mass or more and 60% by mass or less, and it is a well-known fact that polystyrene resin can be mixed with polyphenylene ether resin and that it can be mixed in the above ratio. By adding a flame retardant to the modified polyphenylene ether resin foam sheet, it is possible to impart self-extinguishing properties and reduce flammability, or to obtain a polyphenylene foam sheet with improved flame retardancy and excellent dielectric properties and surface properties. Ultimately, by simultaneously adding a halogen-based flame retardant and a phosphorus-based flame retardant, it is possible to further improve the flame retardancy and satisfy the V-0 or V-1 standard in the UL94 flame retardancy test. This is because a synergistic effect is obtained by using both in combination.

[0022] (Regarding blends of modified polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant) When the total content of polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant is defined as 100% by mass, the blending amount of polyphenylene ether resin relative to the entire blend, taking into account strength, heat resistance, and processability, is approximately the same as in the case of a blend of modified polyphenylene ether resin and polystyrene resin when no phosphorus-based flame retardant is contained, that is, 40% by mass or more and 80% by mass or less. Therefore, the combined blending amount of both polystyrene and phosphorus-based flame retardant is 20% by mass to 60% by mass, approximately the same as in the case of a blend of only modified polyphenylene ether resin and polystyrene resin. Here, when the combined content of polystyrene and phosphorus-based flame retardant is at the lower limit, the lower limits of polystyrene and phosphorus-based flame retardant, respectively, are 15% by mass and 5% by mass. Furthermore, when the combined content of polystyrene and phosphorus-based flame retardant is at its upper limit, the respective upper limits for polystyrene and phosphorus-based flame retardant are 40% by mass and 20% by mass. In this case, the contents of polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant can be 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 phosphorus-based flame retardant may be appropriately selected within the above ranges so that the total amount of polyphenylene ether resin is 100% by mass. The reasons for setting the upper and lower limits for the blending amounts of polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant are described below. The reason why the blending amount of the polyphenylene ether resin is set 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 desired heat resistance, strength, etc. cannot be obtained, and the reason why it is set to 80% by mass or less is that if it exceeds 80% by mass, processability will be reduced, similar to the case of mixing only modified polyphenylene ether resin and polystyrene resin. Here, the reason why the blending amount of the polystyrene resin is set to 15% by mass or more and 40% by mass or less is that if it is less than the lower limit, processability will be insufficient, and if it exceeds the upper limit of 40% by mass, the content of the polyphenylene ether resin will be insufficient, resulting in insufficient performance such as heat resistance and strength.Furthermore, when the total amount of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant used in the modified polyphenylene ether resin foam sheet is taken as 100% by mass, the phosphorus-based flame retardant can be 5% by mass or more and 20% by mass or less. The reason for limiting the amount of phosphorus-based flame retardant to 5% by mass or more and 20% by mass or less is that if the amount is less than the lower limit, char formation during combustion (a carbonized layer that forms a three-dimensional structure via a solid phase during carbonization) is insufficient, resulting in a lack of interaction with the halogen-based flame retardant described below, and the desired flame retardancy cannot be achieved even when the flame retardant is added. If the amount exceeds the upper limit, foamability decreases. The reason for specifying the total amount of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant as 100% by mass, rather than the total amount of the polyphenylene ether resin and polystyrene resin, is that these materials are typically commercially available as blends and are used. In this application, Asahi Kasei's Zylon 340Z and 540Z were used as these materials.

[0023] The flame retardant used in the modified polyphenylene ether resin sheet may be a halogen-based flame retardant as an oligomer or polymer-based flame retardant in addition to a phosphorus-based flame retardant. A specific halogen-based flame retardant may be added in an amount of 5 to 40 parts by mass per 100 parts by mass of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant. The composition of the base resin contains 5 to 20% by mass of the phosphorus-based flame retardant per 100% by mass of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant. A preferred embodiment of the flame retardant for the modified polyphenylene ether resin foam sheet is that the phosphorus-based flame retardant accounts for 5% to 20% by mass, based on 100 parts by mass of the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant. The flame retardant contains a halogen-based flame retardant having a weight-average molecular weight of 1,000 to 1,000,000, in an amount of 5 to 40 parts by mass, based on 100 parts by mass of the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant. Consequently, the flame retardant contains both a phosphorus-based flame retardant and a halogen-based flame retardant. The blending amounts are preferably as described above. By adding the halogen-based flame retardant within the above range, high flame retardancy can be achieved while maintaining foamability by combining the char formation caused by the phosphorus-based flame retardant with the radical trapping and oxygen blocking effects of the halogen-based flame retardant. As a result, a foam sheet that satisfies the UL94 V-0 and V-1 standards can be obtained, as described below.

[0024] The flame retardant used in the modified polyphenylene ether resin foam sheet preferably has a weight-average molecular weight of 1,000 or more, more preferably 1,000 to 1,000,000. In this case, the flame retardant may be either a brominated carbonate oligomer or a brominated polystyrene. By adjusting the molecular weight of the halogen-based flame retardant to the above-mentioned lower limit or higher, a good resin foam sheet without foaming inhibition can be obtained. The use of such a modified polyphenylene ether resin foam sheet can impart flame retardancy to the modified polyphenylene ether resin foam sheet. In this case, a weight-average molecular weight of 5,000 or more is preferred because the flame retardant acts more stably. It is more preferred that the average molecular weight of the flame retardant be 5,000 to 50,000. 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. The reason for this is that if the molecular weight exceeds 1,000,000, it becomes difficult to suppress cell growth and obtain the desired expansion ratio and cell density.

[0025] According to the present invention, it is possible to obtain a modified polyphenylene ether resin foam sheet that has good moldability, very low dielectric constant and dielectric loss tangent at high frequencies, high flame retardancy, and excellent surface properties. As a result, it is possible to provide a low-dielectric modified polyphenylene ether resin foam sheet that has excellent surface properties, self-extinguishing properties, and flame retardancy and is usable for housings for electric and electronic components, electrical insulating materials, sealing materials, protective materials, high-frequency substrates, or substrates for electromagnetic wave control members.

[0026] The modified polyphenylene ether resin foam sheet of the present invention contains a flame retardant. The flame retardant may be a phosphorus-based flame retardant or a combination of a phosphorus-based flame retardant and a specific halogen-based flame retardant. The modified polyphenylene ether resin foam sheet of the present invention has a relative dielectric constant of 1.10 to 2.00 and a dielectric loss tangent (tanδ) of 0.5×10 －３ ~2.5 x 10 －３Alternatively, another embodiment of the present invention is a modified polyphenylene ether resin foamed sheet obtained by foaming a modified polyphenylene ether resin containing, as a flame retardant, a halogen-based flame retardant and a phosphorus-based flame retardant, each having a weight-average molecular weight of 1,000 to 1,000,000, wherein the average bubble diameter of the bubbles present is 1 to 30 μm and the bubble number density is 1.8×10 ４ ~9 x 10 ８ pieces / mm ３ First, a preferred embodiment of the resin composition of the present invention is a modified polyphenylene ether resin containing polystyrene, which will be described. Furthermore, the component composition of the phosphorus-based flame retardant and the specific halogen-based flame retardant will be described, followed by the density of the resin, the cell diameter, the manufacturing method, and the physical properties of the foamed sheet.

[0027] (Polyphenylene Ether Resin) Examples of the polyphenylene ether resin used in the present invention include a homopolymer comprising a repeating unit represented by the following general formula (1) and a copolymer containing a repeating unit represented by the following general 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 may be the same or different from each other. In addition, two adjacent groups may be linked to each other to form a ring.]

[0028] R in formula (1) １ and R ４ R is 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. Of these, a methyl group and an ethyl group are preferred. ２ and R ３is 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. Of these, a hydrogen atom is preferred. The polyphenylene ethers may be used singly or in combination of two or more types.

[0029] The molecular weight of the polyphenylene ether is not particularly limited, but preferably has a weight-average molecular weight of 10,000 to 30,000. In the present invention, "modified" refers to, for example, the above-mentioned polyphenylene ether being alloyed with the polystyrene described in the next section, or having a specific functional group substituted at a specific site. In the modified polyphenylene ether resin foam sheet of the present invention, it is preferable to use a modified polyphenylene ether resin obtained by mixing the above-mentioned polyphenylene ether and polystyrene to form an alloy. In this case, the mixing ratio of polyphenylene ether to polystyrene is not particularly limited, but the polyphenylene ether is preferably 40 to 80% by mass, and more preferably 50 to 70% by mass, of the total ratio of both polyphenylene ethers and the phosphorus-based flame retardant, taken as 100% by mass.

[0030] (Polystyrene) In this specification, examples of polystyrene resins include homopolymers of styrene-based monomers such as styrene, α-methylstyrene, vinyltoluene, chlorostyrene, ethylstyrene, i-propylstyrene, dimethylstyrene, and bromostyrene, or copolymers thereof; copolymers of a styrene-based monomer and a vinyl monomer polymerizable therewith, containing a styrene-based monomer as the main component; copolymers of a styrene-based monomer and a rubber component such as butadiene; homopolymers of styrene-based monomers or copolymers thereof; or mixtures or polymers of a copolymer of a styrene-based monomer and a vinyl monomer and a diene-based rubber polymer, which are so-called high impact polystyrenes, but are not limited thereto.

[0031] Examples of homopolymer polystyrene resins include polystyrene, poly-α-methylstyrene, and polychlorostyrene. Examples of copolymer polystyrene resins 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, and ethylvinylbenzene-divinylbenzene copolymer, as well as terpolymers such as ABS and butadiene-acrylonitrile-α-methylbenzene copolymer, but are not limited thereto.

[0032] Also included are graft copolymers such as styrene-grafted polyethylene, styrene-grafted ethylene-vinyl acetate copolymer, (styrene-acrylic acid) grafted polyethylene, styrene-grafted polyamide, etc. These may be used alone or in combination of two or more.

[0033] The polystyrene resin preferably has a weight-average molecular weight of 180,000 to 500,000. In this specification, the weight-average molecular weight refers to the weight-average molecular weight determined by gel permeation chromatography (GPC) measurement, using a calibration curve (prepared using the peak molecular weight of the standard polystyrene) obtained from measurements of commercially available standard polystyrene. The carrier and column may be selected depending on the subject being measured. An example of a carrier is tetrahydrofuran (THF). In this specification, unless otherwise specified, the molecular weight of an oligomer or polymer refers to that determined by the above method. When the total of the polyphenylene ether resin, polystyrene resin, and phosphorus-based flame retardant is taken as 100% by mass, the polystyrene content is preferably 15% by mass to 40% by mass, more preferably 15% by mass to 37% by mass, and particularly preferably 25% by mass to 35% by mass.

[0034] (Phosphorus-Based Flame Retardant) The modified polyphenylene ether resin foam sheet of the present invention preferably further contains a phosphorus-based flame retardant, which not only exhibits self-extinguishing properties but also provides a further enhanced flame retardancy through interaction with the halogen-based flame retardant component described below.

[0035] The phosphorus-based flame retardant is preferably a phosphate ester compound. The phosphate ester compound is not particularly limited, but is preferably one that has the effect of being a flame retardant. Furthermore, the phosphate ester compound is preferably, for example, bisphenol A bis(diphenyl phosphate) or an aromatic condensed phosphate ester compound. The phosphate ester compound is not limited to the following, but examples thereof include phosphate ester compounds represented by the following general formula (2) and condensates thereof. [In the formula, R １１ , R １２ , R １３ , and R １４are each independently 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, and even more preferably 1 to 3), a cycloalkyl group (preferably having 3 to 24 carbon atoms, more preferably 3 to 12, and even more preferably 3 to 6), an aryl-substituted alkyl group (preferably having 7 to 22 carbon atoms, more preferably 7 to 18, and even more preferably 7 to 10), an aryl group (preferably having 6 to 22 carbon atoms, more preferably 6 to 18, and even more preferably 6 to 10), a halogen-substituted aryl group (preferably having 6 to 22 carbon atoms, more preferably 6 to 18, and even more preferably 6 to 10), and an alkyl-substituted aryl group (preferably having 7 to 22 carbon atoms, more preferably 7 to 18, and even more preferably 7 to 10), and may be the same or different. X represents an arylene group (preferably having 6 to 22 carbon atoms, more preferably 6 to 18, and even more preferably 6 to 10). m is an integer of 0 or more.

[0039] When the phosphate ester compound is a mixture of phosphate esters and / or condensates thereof, each having a different m, m represents the average value thereof. When m = 0, the compound of general formula (2) represents a phosphoric acid or phosphate ester monomer. The phosphate ester compound may be used alone or in combination of two or more.

[0036] Representative phosphate ester monomers include, but are not limited to, triphenyl phosphate, tricresyl phosphate, trixylenyl phosphate, and the like.

[0037] The phosphorus-based flame retardant is preferably a condensed phosphate ester compound, and more preferably a condensed phosphate ester compound in which m in formula (2) is equal to or greater than 1. The upper limit of m may be determined appropriately, and is, for example, 5 or less.

[0038] From the viewpoint of the flame retardancy and heat resistance exhibited when kneaded into the resin composition of the present embodiment, the phosphorus-based flame retardant is １１ , the above R １２ , the above R １３ and the above R １４Preferably, at least one of the groups is an aryl group, a halogen-substituted aryl group, or an alkyl-substituted aryl group, and more preferably, all of the groups are aryl groups, halogen-substituted aryl groups, or alkyl-substituted aryl groups. From the same viewpoint, preferred aryl groups, halogen-substituted aryl groups, and alkyl-substituted aryl groups are phenyl groups, xylenyl groups, cresyl groups, or halogenated derivatives thereof.

[0039] The arylene group-containing group represented by X is preferably a residue in which two hydroxyl groups have been eliminated from phenylene, resorcinol, hydroquinone, bisphenol A, bisphenol F, biphenol, or a halogenated derivative thereof.

[0040] Examples of the phosphate ester compound include, but are not limited to, resorcinol-bisphenyl phosphate compounds, bisphenol A-polyphenyl phosphate compounds, and bisphenol A-polycresyl phosphate compounds.

[0041] The condensed phosphate ester compounds include compounds of the following formulae (3-1), (3-2), (4-1), and (4-2). In formulas (3-1), (3-2), (4-1), and (4-2), Q １ , Q ２ , Q ３ and Q ４ are each a substituent, each independently representing an alkyl group having 1 to 6 carbon atoms; R ７ and R ８ Each of n1 and n2 independently represents an integer of 0 to 2, and each of m1, m2, m3, and m4 independently represents an integer of 0 to 3. １ Although the above are not present in formulas (3-1) and (4-1), they may be read as definitions of the groups present in each formula.

[0042] In the condensed phosphate esters represented by formulae (3-1), (3-2), (4-1), and (4-2), n is an integer of 1 or more, preferably an integer of 1 to 3.

[0043] Among these, in formulas (3-1), (3-2), (4-1), and (4-2), R ７ and R ８ is a methyl group, and Q is a condensed phosphate ester １ , Q ２ , Q ３ , Q ４ are all methyl groups, n1 and n2 are 0, and m1, m2, m3, and m4 are each independently an integer of 0 to 3, with n being preferably in the range of 1 to 3. Among these, those containing 50 mass% or more of a phosphate ester in which n is 1 are particularly preferred. These flame retardants can also be commercially available products, such as those manufactured by Daihachi Chemical Industry Co., Ltd. under the trade names "CR-741," "CR733S," and "PX-200." Specific examples of the phosphate ester include bisphenol A bisdiphenyl phosphate and resorcinol bisdichil phosphate.

[0044] For the compounds of formula (3-1), (3-2), (4-1), and (4-2), reference can be made to those disclosed in JP-A-2010-123933.

[0045] Specific examples of phosphorus-based flame retardants include compounds represented by the following formula (5) or (6).

[0046] From the viewpoint of obtaining a resin composition having even more excellent foamability, heat resistance, and flame retardancy, the content of the phosphorus-based flame retardant in the modified polyphenylene ether resin foam sheet of the present embodiment is preferably 5% by mass or more and 20% by mass or less, and more preferably 7% by mass or more and 15% by mass or less, when the total of the polyphenylene ether resin, the polystyrene resin, and the phosphorus-based flame retardant is taken as 100% by mass.

[0047] (Halogen-Based Flame Retardant) The modified polyphenylene ether resin foam sheet of the present invention preferably contains a specific halogen-based flame retardant as a constituent resin material. The halogen atom contained in the halogen-based flame retardant is not particularly limited, but is preferably bromine or chlorine, and more preferably bromine. The halogen-based flame retardant is preferably a brominated carbonate oligomer or brominated polystyrene. The halogen-based flame retardant is preferably a compound having a bisphenol skeleton, and specifically, preferably a compound (brominated carbonate oligomer) having the following structural formula (Formula (7)):

[0048] R ２１ and R ２２ represents a halogen atom or an alkyl group. The halogen atom is preferably a bromine atom. The alkyl group is preferably an alkyl group having 1 to 12 carbon atoms, 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 represents a hydrogen atom or an alkyl group having 1 to 3 carbon atoms. z and v represent integers of 0 to 5. o is a natural number arbitrarily determined depending on the molecular weight, and is preferably 4 to 15.

[0049] Specific examples of halogen-based flame retardants include compounds of the following formula (7-1) or (7-2). n is 4 to 15 (molecular weight: about 3,000 to about 10,000) n is 4 to 15. (Molecular weight: about 2,500 to about 9,000)

[0050] In 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): where x is a natural number from 500 to 10,000, and y is a natural number from 1 to 5.

[0051] The molecular weight of the halogen-based flame retardant is important from the perspective of maintaining the foamability of the resin. An oligomer or polymer with a weight-average molecular weight of 1,000 to 1,000,000 is preferred, with 5,000 to 800,000 (50,000 for oligomers) being preferred. A molecular weight below the lower limit is recognized as a foreign substance during foaming, inducing gas diffusion and inhibiting foamability. A molecular weight above the upper limit inhibits cell growth, making it impossible to obtain a foam with the desired density. However, for brominated carbonate oligomers, the above range is preferred, while for polymers with a high degree of polymerization, such as brominated polystyrene, the upper limit is higher, preferably 1,000,000 or less. In this specification, "molecular weight" refers to "weight-average molecular weight" unless otherwise specified. The method for measuring weight-average molecular weight is as described above.

[0052] The blending amount of the halogen-based flame retardant is not particularly limited, but when the total amount of the polyphenylene ether resin, the polystyrene resin, and the phosphorus-based flame retardant is 100 parts by mass, the blending amount of 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.

[0053] <Resin Density and Expansion Ratio> The density of the resin constituting the modified polyphenylene ether resin foam sheet of the present invention is 0.60 g / cm ３ ~0.10 g / cm ３ and preferably 0.50 g / cm ３ ~0.15g / cm ３ More preferably, it is 0.30 g / cm ３ ~0.15g / cm ３ It is more preferable that the density of the resin is within the above range. The density of the resin is an index that indicates the degree of foaming, and in this respect, it is a variable that is common to the foaming ratio. In the resin of the present invention, the foaming ratio is preferably 2 to 12 times, more preferably 2 to 10 times, or even 2 to 7 times. A resin density (expansion ratio) within the above range means that there are many air bubbles present in the foam sheet, which approaches the dielectric constant of air, thereby achieving a low dielectric constant.

[0054] <Average Cell Diameter> In the modified polyphenylene ether resin foamed sheet of the present invention, the average cell diameter of the bubbles present in the resin constituting the sheet must satisfy the range of 1 to 30 μm. Here, the lower limit of the average cell diameter is 1.0 μm or more, and more preferably greater than 1.5 μm. The upper limit is 30 μm or less, preferably 28 μm or less, more preferably 26 μm or less, even more preferably 22 μm or less, and even more preferably 18 μm or less. Furthermore, the average cell diameter is more preferably 12 μm or less, desirably 10 μm or less, and particularly preferably 8 μm or less. In the foamed sheet of the present invention, if the average cell diameter is small as described above, advantages include excellent moldability, favorable realization of a low relative dielectric constant and a low dielectric loss tangent, and the realization of surface properties such as low surface roughness.

[0055] <Cell Number Density> Here, the dielectric constant, which is a dielectric property, is better when the average cell diameter of the foamed sheet is smaller. However, when the expansion ratio is low, the cell density in the cross section of the resin foam sheet is low, resulting in a higher proportion of the resin component. This increases the probability of bubble-free areas around bubbles and also increases the thickness of the cell walls. Therefore, simply reducing the cell diameter is not sufficient to reduce the dielectric constant. This increases loss due to bubble-free areas and cell walls. Therefore, it is preferable to reduce the cell diameter of the foamed sheet and simultaneously maintain a relatively high cell number density in the cross section of the foamed sheet. That is, to improve the dielectric properties (dielectric constant and tan δ) of the foamed sheet of the present invention, it is necessary not only to set the average cell diameter of the foam to a predetermined range of 1 to 30 μm, preferably 1 to 22 μm, but also to set a high cell number density. From this perspective, the cell number density of the foamed sheet of the present invention is 1.8 × 10 ４ ~9.0 x 10 ８ pieces / mm ３ , or 2.0 × 10 ４ ~9.0 x 10 ８ pieces / mm ３ is 1.0 × 10 5 ~9.0 x 10 ８ pieces / mm ３ is preferable, and 1.0 × 10 ６~9.0 x 10 ８ pieces / mm ３ More preferably, it is 3.0 × 10 ６ ~7.5 x 10 ８ pieces / mm ３ It is more preferable that:

[0056] <Method for Producing Foamed Sheet (Fine Foaming Process)> The fine foaming process is a batch process in which a high-pressure inert gas (e.g., nitrogen gas or carbon dioxide gas) serving as a foaming agent is dissolved in a solid resin sheet at room temperature in a high-pressure vessel until the gas reaches a saturated dissolution amount. The gas in the high-pressure vessel is then depressurized or heated to induce phase separation due to thermodynamic instability of the gas contained in the resin, thereby foaming the resin. A specific example of the process includes the following three steps. The method for producing a polyphenylene ether resin foamed sheet of this embodiment includes a step of incorporating a non-reactive gas into the resin under pressure (hereinafter referred to as the "first step"). Next, the resulting thermoplastic resin is heated under no pressure in a temperature range above the crystallization peak temperature and below the melting point peak temperature of the resin measured by a differential scanning calorimeter to foam the resin (hereinafter referred to as the "second step"). Finally, the resulting thermoplastic resin is cooled (hereinafter referred to as the "third step"). The average cell diameter of the foamed sheet can be adjusted by the holding time and temperature in the second step.

[0057] <Surface Roughness> When AC current flows through a conductor, the higher the frequency of the current, the more difficult it is to pass through the center of the conductor, resulting in a skin effect that causes the current to flow only through the surface layer. Regarding the relationship between surface roughness and transmission loss in high-frequency copper circuits, if the surface roughness is greater than the skin depth, the signal transmission path becomes longer, increasing signal loss. Conversely, if the surface roughness is smaller than the skin depth, the transmission path becomes shorter, reducing signal loss. Therefore, a lower surface roughness for the foam sheet is preferable. However, particularly in high-frequency bands such as the GHz and THz bands, the surface properties of the foam sheet may affect the surface properties of the circuit material, thereby affecting loss. Furthermore, for resin foam sheets expected to be used as circuit board materials, considering the future trend toward even higher precision in circuits, it is necessary to reduce the surface roughness of the foam sheet, taking into account the influence of the surface properties of the board surface on the surface properties of the laminated circuits. Here, the average surface roughness of the sheet is important, and therefore the arithmetic mean surface roughness Ra is used as the surface roughness in the present invention. From this perspective, the surface roughness Ra of the polyphenylene ether resin foam sheet is 0.10 to 0.50 μm, preferably 0.10 to 0.35 μm, more preferably 0.10 to 0.30 μm, and even more preferably 0.10 to 0.25 μm. Thus, the excellent surface roughness of the resin foam sheet of the present invention is due to the fact that the gas penetration step and the foaming step are separated in the micro-foaming process, resulting in small cell diameters and a stable cell diameter distribution. This facilitates stable surface properties, and the arithmetic mean surface roughness Ra, an index of surface roughness, can be kept low by incorporating the above-mentioned roll processing.

[0058] <Dielectric Constant, Dielectric Loss Tangent> The foamed sheet has a low dielectric constant and a low dielectric loss tangent, and therefore can exhibit favorable performance when used in housings for electric and electronic components, electrical insulating materials, sealing materials, protective materials, high-frequency substrates, or substrates for electromagnetic wave control members. From this viewpoint, the dielectric constant of the foamed sheet is preferably 1.10 to 2.00, more preferably 1.10 to 1.60, even more preferably 1.10 to 1.50, and particularly preferably 1.10 to 1.35. The dielectric loss tangent (tan δ) is 0.5×10 －３ ~2.5 x 10 －３is preferably 0.7 × 10 －３ ~2.2 × 10 －３ More preferably, it is 1.0 × 10 －３ ~2.0 x 10 －３ It is particularly preferable that －３ ~1.8 x 10 －３ It is more desirable to set the value to 1.0 × 10 －３ ~1.5 x 10 －３ The relative dielectric constant is a value measured by the method described in the examples.

[0059] (Measurement Method) The expansion ratio and foam density were measured as follows. <Method for Measuring Expansion Ratio> The expansion ratio is an apparent ratio commonly used for resin foams. This apparent ratio can be calculated by cutting a specimen having dimensions of 10 cm x 10 cm from a foamed resin sheet, measuring its weight (W1 [g]), then measuring the thickness at the four corners and at the center of the specimen (using a measuring machine conforming to JIS K6767), and using the average value (T [cm]) of the five points, according to the following formula (1). Apparent ratio (expansion ratio) = 10 x 10 x T / W1 (1)

[0060] <Method for measuring foam density> The density of the resin foam sheet was measured in accordance with JIS K7222-1999, "Foamed plastics and rubber - Method for measuring apparent density." Test specimens were cut into 10 cm x 10 cm pieces from resin foam sheets that had been conditioned at 23°C ± 2°C for 72 hours or more after molding. Specifically, the foam density was calculated using the following formula (2). The unit is g / cm. ３ [Equation 1] ρ ａ = {(m + ma)} / V ... (2) m is the mass of the test piece (g), V is the volume of the test piece (cm ３ ), ma: mass of replaced air (g). Note that ma: mass of replaced air is negligible due to the low expansion ratio, so calculations were made ignoring this. Measurements were performed five times, and the average value was calculated from the measurement results of all test pieces, and this was used as the density.

[0061] <Method for measuring bubble diameter> The bubble diameter was determined in accordance with ASTM D3576-77. An SEM photograph of the longitudinal cross section of the sheet was taken, and straight lines were drawn horizontally and vertically on the SEM photograph, and the lengths t of the chords of the bubbles crossed by the straight lines were averaged. The magnification of the photograph, M, was substituted into the following formula to determine the average bubble diameter d (d = t / (0.616 × M)). The average bubble diameter was determined by measuring SEM photographs of three different fields of view and averaging the results.

[0062] <Method for measuring cell number density> An SEM photograph of the longitudinal section of a polyphenylene ether resin foam sheet was taken, and five regions of 100 μm × 100 μm were randomly selected on the SEM photograph, and the number of cells present in each region was counted. ２ By calculating the number of bubbles per 1 mm based on each area, ３ The number of bubbles per 1 mm was calculated based on each of the five regions. ２ The average number of bubbles per square centimeter was calculated, and the bubble density ( ２ The obtained 1 mm ２ The number of bubbles per square meter is multiplied by (3 / 2) to obtain a square meter of 1 mm based on each area. ３ The number of bubbles per 1 mm was calculated based on each of the five regions. ３ The average number of bubbles per square centimeter was calculated, and the bubble density ( ３ When measuring the bubble diameter and bubble number density, it is desirable to prepare a plurality of SEM photographs (preferably three or more) and calculate the average value of the values ​​calculated for each SEM photograph.

[0063] <Measurement of relative permittivity and dielectric loss tangent> The relative permittivity and dielectric loss tangent were measured using a cavity resonator perturbation method, with the measurement frequency set to 2 GHz. ｒThe complex dielectric constant in the planar direction of the polyphenylene ether resin foam sheet can be measured by a cavity resonator method, where tanδ is the dielectric loss tangent and tanδ is the dielectric constant. In the present invention, the dielectric constant was measured by the cavity resonator method at a resonance frequency of 2 GHz using a sample having a length of 78 mm, a width of 2.4 mm, and a thickness of 1 mm. The measurement equipment used in this measurement can be, for example, a CP461 cavity resonator manufactured by Kanto Electronics Application Development Co., Ltd., and an E8361A network analyzer manufactured by Agilent Technologies.

[0064] <Surface Roughness Measurement> To evaluate the overall surface quality of the modified polyphenylene ether foam sheet, the surface quality of the foam sheet of the present invention was evaluated by measuring Ra rather than Rz, which is significantly affected by factors such as maximum irregularities. The arithmetic mean roughness (Ra) was determined in accordance with JIS B0601:2013. A Handysurf E-30A (manufactured by Tokyo Seimitsu Co., Ltd.) was used for this measurement. The reference length for each measurement was 2.5 mm. In the present invention, a surface roughness Ra of, for example, 1.0 μm or less means that the average roughness of the five measurements obtained by randomly determining five measurement lines in the machine direction on the surface of the modified PPE foam sheet and measuring the arithmetic mean roughness of each measurement line is 1.0 μm or less, and the average roughness of the five measurements obtained by randomly determining five measurement lines in the transverse direction is 1.0 μm or less. In other words, the surface roughness in the present invention is evaluated as the average of both the MD and TD directions. Here, the reason why the average of both directions is used to take into account the influence of the surface roughness in not only the MD direction but also the TD direction as the surface characteristics is that during the production of the sheet material, the sheet is passed through rolls, and at that time, in addition to the surface pressing effect of the rolls, tension is applied to the sheet to stretch it in the MD direction, so that the surface quality in the MD direction is often better than that in the TD direction.

[0065] <Flame Retardancy Test> The flame retardancy test was set to two levels: whether the flame retardancy can be improved even slightly by the flame retardant, and an oxygen index evaluation based on the flammability test for plastics based on JIS K 7201-2:2021, which is an evaluation of flammability, self-extinguishing property in flammability, flame retardancy, etc.; and whether the vertical flame retardancy test in the UL94 flame retardancy test satisfies V-0 or V-1.

[0066] [Flammability Test for Plastics Based on JIS K7201-2] The minimum oxygen volume fraction in an oxygen and nitrogen mixture at 23°C ± 2°C at which a material can sustain flaming combustion under specified test conditions is expressed as a percentage (%). The test is conducted using a test specimen mounted on a specified test specimen holder in a transparent cylinder through which a laminar upward flow of an oxygen and nitrogen mixture is placed. The test is conducted at room temperature after conditioning. The top-end ignition procedure involves contacting the top end of the test specimen with a flame for a maximum of 30 seconds, during which the flame is removed every 5 seconds to check whether the specimen is burning. This method is used to prevent a gradual increase in the temperature of the test specimen, as an increase in temperature generally reduces the oxygen index. Other details are omitted, but the test was conducted based on the above JIS standard.

[0067] [UL94 Flame Retardancy Test: Vertical Flame Retardancy Test] In accordance with UL94 (a standard established by Under Writers Laboratories Inc., USA), five test pieces, each 125 mm long and 13.0 mm wide, were prepared using the obtained sheet and subjected to a vertical flame retardancy test. In the test, each test piece was attached vertically to a clamp and exposed to a 20 mm flame for 10 seconds twice, and the resulting combustion behavior was evaluated as V-0, V-1, or V-2. The flame retardancy standards for V-0, V-1, and V-2 are as follows:

[0068] [Evaluation criteria for oxygen index in flammability tests for plastics based on JIS K 7201-2] The oxygen index is generally considered to be as follows: 22 or less: Flammable and burns 23 to 27: Burns but self-extinguishing 27 or more: Flame retardant

[0069] [Flame retardancy standards for V-0, V-1, V-2] V-0: The duration of flaming combustion was within 10 seconds for both the first and second tests, the sum of the duration of flaming combustion and flameless combustion time for the second test was within 30 seconds, the total duration of flaming combustion for the five test pieces was within 50 seconds, no sample burned up to the position of the fixing clamp, and no cotton ignition was caused by falling burning particles. V-1: The duration of flaming combustion was within 30 seconds for both the first and second tests, the sum of the duration of flaming combustion and flameless combustion time for the second test was within 60 seconds, the total duration of flaming combustion for the five test pieces was within 250 seconds, no sample burned up to the position of the fixing clamp, and no cotton ignition was caused by falling burning particles. V-2: The duration of flaming combustion was within 30 seconds in both the first and second tests, the sum of the duration of flaming combustion and the flameless combustion time in the second test was within 60 seconds, the total flaming combustion time of the five test pieces was within 250 seconds, no sample burned up to the position of the fixing clamp, and cotton ignition due to falling burning material.In addition, samples that did not pass the vertical flame retardancy test and did not fall into any of the above V-0, V-1, or V-2 were marked as non-compliant (×).

[0070] Modified polyphenylene ether resins used Modified polyphenylene ether (hereinafter referred to as m-PPE): ZYLON 340Z manufactured by Asahi Kasei Corporation A composition containing polyphenylene ether containing poly 2,6-dimethyl-1,4-phenyloxide, polystyrene, and a phosphorus-based flame retardant (a phosphate ester compound of the formula (5) or (6) above). Here, the polystyrene is high-impact polystyrene. Modified polyphenylene ether (hereinafter referred to as m-PPE): ZYLON 540Z manufactured by Asahi Kasei Corporation A composition containing polyphenylene ether containing poly 2,6-dimethyl-1,4-phenyloxide, polystyrene, and a phosphorus-based flame retardant (a phosphate ester compound of the formula (5) or (6) above). Here, the polystyrene is high-impact polystyrene.

[0071] Flame retardant (1): Fireguard FG-8500 (compound of formula (7-1)) manufactured by Teijin Limited

[0072] Flame retardant (2): Fireguard FG-7500 (compound of formula (7-2)) manufactured by Teijin Limited

[0073] Flame retardant (3): SAYTEX HP7010G manufactured by Albemarle Japan Co., Ltd. (a compound represented by the chemical formula (8), where x and y are approximately 2,000 and 2.7, respectively; molecular weight approximately 700,000)

[0074] Flame retardant (4) FR-1410 (formula (a) below) manufactured by ICL Japan Co., Ltd. Molecular weight: 971.2

[0075] Flame retardant (5) SAYTEX 8010 manufactured by Albemarle Japan Co., Ltd. Same compound as the above chemical formula (a) (different manufacturer)

[0076] First, a total of 12 types of test material were used in the tests, as follows. <Test Material Used in the Test> Test materials 1 to 8 were based on modified polyphenylene ether (hereinafter referred to as m-PPE): Zylon 540Z manufactured by Asahi Kasei Corporation, and each was blended with a predetermined amount of flame retardant. Here, test materials 1 to 3 were blended with modified PPE Zylon 540Z and 20 parts, 30 parts, and 35 parts, respectively, of Teijin Limited's Fireguard FG-8500 (flame retardant (1)). Similarly, test materials 4 and 5 were blended with modified PPE Zylon 540Z and 20 parts and 30 parts, respectively, of Teijin Limited's Fireguard FG-7500 (flame retardant (2)). Furthermore, Test Materials 6, 7, and 8 similarly contained 20, 30, and 40 parts, respectively, of SAYTEX HP7010G (flame retardant (3) manufactured by Albemarle Japan Co., Ltd.) blended with ZYLON 540Z (modified PPE). Test Materials 1 to 8 were based on ZYLON 540Z (modified PPE), to which three different flame retardants with specific molecular weights were further added in predetermined amounts. Test Materials 9 and 10 were foam sheets made from a material consisting solely of modified polyphenylene ether (modified PPE): ZYLON 340Z (manufactured by Asahi Kasei Corporation), and a material consisting solely of modified polyphenylene ether (hereinafter referred to as modified PPE): ZYLON 540Z (manufactured by Asahi Kasei Corporation), with different product numbers. Test Materials 9 and 10 contained no flame retardants other than the phosphorus-based flame retardants originally contained in ZYLON 340Z and ZYLON 540Z. Test materials 11 and 12 are a foam sheet made of a material obtained by adding 20 parts of FR-1410 manufactured by ICL Japan as a flame retardant (4) to modified polyphenylene ether (modified PPE): Zylon 340Z manufactured by Asahi Kasei Corporation, and a foam sheet made of a material obtained by adding 20 parts of SAYTEX 8010 manufactured by Albemarle Japan Co., Ltd. as a 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 were both made using Zylon 340Z manufactured by Asahi Kasei Corporation, and test materials 10 and 12 were made using Zylon 540Z manufactured by Asahi Kasei Corporation. Both Zylon 540Z and Zylon 340Z are blends of PPE and PS. Considering the difference in glass transition temperature Tg between PPE and PS, it is thought that the greater the blended amount of PS relative to PPE, the lower the Tg will be. Therefore, it is thought that Zylon 340Z has a higher PS content than Zylon 540Z.

[0077] <Production of Test Material Foam Sheets> A roll of foam sheet made from the test materials shown in Table 1 was obtained, placed in a pressure vessel, and pressurized with carbon dioxide gas to 5.2 MPa. The carbon dioxide gas was allowed to penetrate the resin film for 24 hours. The roll was then removed from the pressure vessel and continuously fed into a hot-air circulating foaming oven set at 160°C for a predetermined period of time to foam the PPE foam sheets shown in Table 1. Test materials 11 and 12 experienced foaming inhibition, resulting in little foaming and failure to produce the desired foam. Test materials 7 and 8 were foamed at a foaming temperature of 170°C. Test materials 9 and 10 were foamed at a foaming temperature of 180°C.

[0078] <Test Results for Evaluating the Properties of Test Material Foam Sheets> Table 1 shows the results of the property tests using test materials 1 to 12 as test materials.

[0079] <Foaming Characteristics and Cell Structure Characteristics of Modified PPE Foam Sheets> The foaming characteristics of the test materials were evaluated based on the expansion ratio and density. Except for test materials 11 and 12, test materials 1 to 8 and test materials 9 and 10 had proper expansion ratios within the range of 2 to 10, specifically, 2.8 to 6.8, and the foam densities were within the range of 0.10 to 0.50, specifically, 0.17 to 0.41. In contrast, test materials 11 and 12 had an expansion ratio of only 1.1 due to the inhibition of expansion by the addition of a flame retardant, and the density hardly changed due to expansion, ranging from 1.07 to 1.08, making it impossible to obtain the desired foamed sheet. The evaluation parameters used to evaluate the cell structure of the foam were the cell diameter (average cell diameter) and cell number density. The test results showed that the average cell diameter was within the range of 1 to 22 μm, specifically, 1.7 to 21.2 μm, and the cell number density was within the range of 10 μm. 4 The above was the goal, but 2.0 x 10 4 ~7.2 x 10 ８ pieces / cm ３ The modified resin foam sheet of the present invention had the desired average cell diameter and cell number density.

[0080] <Dielectric properties of modified PPE foam sheets> Test results for the dielectric properties of test materials 1 to 8, such as the relative permittivity and dielectric loss tangent (tan δ), were obtained using a network analyzer with a cavity resonator perturbation method. 4 The dielectric constants of test materials 1 to 8 were 1.20 to 1.57, and the dielectric loss tangent tanδ was 1.0×10 －３ ~1.5 x 10 －３ The relative permittivity of test material 8 and test material 9 is both 1.20, and the dielectric loss tangent tanδ is 1.0×10 －３ ~2.1 × 10 －３The dielectric constant and dielectric loss tangent were both excellent, regardless of whether or not a polymer flame retardant was added. Furthermore, test materials 1 to 10 had an average cell diameter in the range of 1.7 to 21.2 μm, which is 22 μm or less. Therefore, not only are the dielectric properties good in the gigahertz band, but the comparison of wavelength and average cell diameter suggests that there is also little reflection and diffusion loss when light passes through a foamed sheet. Test materials 11 and 12 were not evaluated for their dielectric properties because foaming inhibition occurred and no foamed sheet was obtained.

[0081] <Surface Properties of Modified PPE Foam Sheets> The surface properties of modified PPE sheets are shown in Table 1, where the average values ​​of Ra were measured five times in each of the MD and TD directions of the foam surface for Test Materials 1 to 8 and Test Materials 9 and 10, excluding Test Materials 11 and 12, for which no properly expanded foam sheets were obtained. The results of measuring the arithmetic mean surface roughness Ra are shown as surface roughness. Although the surface roughness of Test Materials 3 and 8 was high, even including Test Materials 3 and 8, the arithmetic mean surface roughness Ra was in the range of 0.10 to 0.30 μm, which is 0.5 μm or less. With the exception of Test Materials 3 and 8, the difference in arithmetic mean roughness Ra between materials was small, with Ra ranging from 0.10 to 0.14 μm, satisfying the requirement of 0.20 μm or less. The arithmetic mean surface roughness Ra of the present invention is considered to have superior surface properties compared to the surface roughness obtained by bead foaming, etc., even when the upper limit of Ra is 0.5 μm or less. Here, for test materials 11 and 12, foaming inhibition occurred and foam sheets could not be obtained, so evaluation of surface properties was not performed.

[0082] <Flame Retardancy of Modified PPE Foam Sheet> Table 1 shows the evaluation results of the flame retardancy test for each test material. In the table, the oxygen index is represented as OI value. The results of the vertical flame retardancy test in accordance with UL94 showed that the modified PPE foam sheets of test materials 1 to 8, which were modified PPE foam sheets to which predetermined amounts of a phosphorus-based flame retardant and a polymer-based flame retardant were added, had oxygen indices in the range of 27.4 to 32.5, exceeding the oxygen index of 27, which is the flame retardancy standard, and were confirmed to satisfy the UL94 V-0 and V-1 standards. In contrast, Test Material 9, which was made of a Zylon 340Z foam sheet containing only a phosphorus-based flame retardant added to a modified PPE foam sheet, and Test Material 10, which was made of a Zylon 540Z foam sheet containing only a phosphorus-based flame retardant added, did not experience foaming inhibition due to the flame retardant added, and although an improvement in the oxygen index was observed, the oxygen indexes were 24.8 and 25.4, and compared to Test Materials 1 to 8, they had self-extinguishing properties but lacked flame retardancy, failing to meet any of the UL-94 V-0 to V-1 standards. Furthermore, Test Materials 1 to 8 were considered to have flame retardancy because their oxygen indexes exceeded 27. Here, in the case of only a phosphorus-based flame retardant, the flame retardant content was low and no polymer-based flame retardant was not included. Therefore, compared to modified PPE containing non-flame-grade flame retardants, the flame retardancy was improved due to the addition of a flame retardant, but the UL-94 V-0 to V-1 standards were not met. For reference, although not specifically mentioned in the examples, it has been confirmed that when test materials 8 and 9 are subjected to a UL flame retardancy test in an unfoamed state, rather than being foamed, they satisfy the V-0 standard.

[0083] In contrast, test materials 11 and 12, which contained a halogenated flame retardant with a molecular weight below 1,000, had higher oxygen indices of 29.3 and 29.5, respectively, compared to the case of phosphorus-based flame retardants alone, due to the increased amount of flame retardant added by adding the halogenated flame retardant in addition to the phosphorus-based flame retardant. Although the flame retardancy met the UL94 V-0 standard, the addition of these flame retardants inhibited the foaming of the modified PPE, preventing the production of a modified PPE foam sheet. In this invention, oligomer or higher polymer flame retardants were used as the halogenated flame retardant. Conventional flame retardants, such as low-molecular-weight flame retardants and metal hydroxide-based flame retardants, do not expand or penetrate the flame retardant at high temperatures due to the foaming gas, which can cause foaming inhibition. However, polymer flame retardants allow carbon dioxide to penetrate the flame retardant, and they also have appropriate viscoelasticity at high temperatures near the foaming temperature, which is believed to enable the formation of a foam with fine cells without causing foaming inhibition.

[0084] <Summary of Test Results> From the above, to obtain the modified PPE foam sheet of the present invention, as shown in Test Materials 9 and 10, a modified PPE resin obtained by adding a predetermined amount of a phosphorus-based flame retardant to a modified PS resin (PPE, PS, etc.) with a predetermined range was used. This resulted in a foam sheet with an appropriate cell structure, dielectric properties, and excellent surface properties. However, because the amount of flame retardant added was small, the flame retardancy was improved and the foam sheet was flammable, but self-extinguishing properties were imparted, but the flame retardancy did not meet the UL standard. Furthermore, adding a low-molecular-weight halogen-based flame retardant, as in Test Materials 11 and 12, to ensure flame retardancy, satisfied the UL standard, but caused foaming inhibition, making it impossible to obtain the desired foam sheet. In contrast, Test Materials 1 to 8, which contain a phosphorus-based flame retardant and a polymeric flame retardant with a weight-average molecular weight of 1,000 to 1,000,000 as the flame retardant, resulted in foam sheets with excellent cell structure, dielectric properties, surface properties, and flame retardancy without foaming inhibition. Furthermore, the modified PPE foam sheet of the present invention not only has excellent dielectric properties, but also a small average cell diameter. Therefore, when used in circuit boards in the gigahertz band, it can reduce not only transmission loss but also reflection and scattering loss. Furthermore, even when a conductor such as copper foil with a circuit formed on it is placed and laminated to the foam sheet, the foam sheet's excellent surface properties minimize the impact on the skin effect of the circuit or conductor of the base foam sheet. The simultaneous satisfaction of the above-mentioned cell structure, cell density, surface properties, and flame retardancy was only possible by combining a batch-type micro-foaming process tailored to the modified PPE resin and a flame retardant containing a phosphorus-based flame retardant and a halogen-based polymer-based flame retardant of at least an oligomer having a predetermined molecular weight. Furthermore, the present invention allows for the production of thin-walled modified PPE foam sheets up to 50 μm thick.

[0085] <Discussion> As can be seen from Table 1 above, the modified polyphenylene ether foam sheets of test materials 1 to 8, which contained halogen-based flame retardants (1) to (3), exhibited excellent flame retardancy (UL94 V-0). On the other hand, the inclusion of the halogen-based flame retardants did not significantly affect the electrical properties (dielectric constant, dielectric loss tangent), and the properties were comparable to those of the modified polyphenylene ether foam sheets of test materials 9 and 10, which contained a predetermined amount of only a phosphorus flame retardant without a halogen-based flame retardant. That is, in the present invention, when producing a modified polyphenylene ether foam using a microcellular foaming method, adding a predetermined amount of only a phosphorus-based flame retardant improves flame retardancy but does not satisfy the UL94 standard. However, by combining a phosphorus-based flame retardant with a halogen-based flame retardant having a predetermined molecular weight, it became possible to obtain a foam sheet that exhibited very low dielectric constant and dielectric loss tangent at high frequencies and high flame retardancy, satisfying the UL94 V-0 and V-1 standards, while maintaining the surface quality of the foam sheet. In this case, a crosslinking agent or a bubble nucleating agent may be used, but it is also advantageous that the above-mentioned modified polyphenylene ether foamed sheet can be obtained without using a crosslinking agent or a bubble nucleating agent. Even when a predetermined amount of only a phosphorus-based flame retardant is added, the flame retardancy is improved, and judging from the oxygen index level, it is considered to have self-extinguishing properties although it is flammable. The foam has excellent cell structure, dielectric properties, and surface properties, so it can be used sufficiently depending on the purpose and form of use.

Claims

1. A modified polyphenylene ether resin foam sheet obtained by foaming a modified polyphenylene ether resin containing a flame retardant, wherein the modified polyphenylene ether resin foam sheet contains, as the flame retardant, both a halogen-based flame retardant and a phosphorus-based flame retardant, each having a weight-average molecular weight of 1,000 or more and 1,000,000 or less, and has a relative dielectric constant of 1.10 to 2.00 and a dielectric dissipation factor (tanδ) of 0.5×10 －３ ~2.5 x 10 －３ The modified polyphenylene ether resin foam sheet is characterized in that the modified polyphenylene ether resin foam sheet has a viscosity of 1000 MPa or more.

2. 2. The modified polyphenylene ether resin foam sheet according to claim 1, wherein the surface of the modified polyphenylene ether resin foam sheet has an arithmetic mean surface roughness Ra of 0.50 μm or less.

3. (delete)

4. (delete)

5. The modified polyphenylene ether resin foam sheet has a relative dielectric constant of 1.20 to 1.57 and a dielectric loss tangent (tanδ) of 1.0×10 －３ ~2.1 × 10 －３ 3. The modified polyphenylene ether resin foam sheet according to claim 1, wherein the modified polyphenylene ether resin foam sheet has a viscosity of 1000 MPa or more.

6. A modified polyphenylene ether resin foamed sheet obtained by foaming a modified polyphenylene ether resin containing a flame retardant, wherein the modified polyphenylene ether resin foamed sheet contains, as the flame retardant, both a halogen-based flame retardant and a phosphorus-based flame retardant, each having a weight-average molecular weight of 1,000 to 1,000,000, and the average bubble diameter of bubbles in the modified polyphenylene ether resin foamed sheet is 1 to 30 μm and the bubble number density is 1.8×10 ４ ~9 x 10 ８ pieces / mm ３ A modified polyphenylene ether resin foam sheet, characterized by:

7. The modified polyphenylene ether resin foam sheet has bubbles having an average bubble diameter of 1 to 22 μm and a bubble number density of 2.0×10 ４ ~8 x 10 ８ pieces / mm ３ 7. The modified polyphenylene ether resin foam sheet according to claim 6, wherein

8. 7. The modified polyphenylene ether resin foam sheet according to claim 1, wherein the average thickness of the modified polyphenylene ether resin foam sheet is 0.05 mm to 2.0 mm.

9. The modified polyphenylene ether resin foam sheet according to claim 1 or 6, wherein a test based on the UL-94 vertical method (20 mm vertical flame test) of the U.S. UL standard is conducted using the modified polyphenylene ether resin foam sheet to evaluate flame retardancy, and the test result satisfies V-0 or V-1 standard.

10. The modified polyphenylene ether resin foam sheet according to claim 1 or 6, which is used for housings for electric and electronic components, electrical insulating materials, sealing materials, protective materials, high-frequency substrates, or substrates for electromagnetic wave control members.

11. A housing for electric / electronic parts, an electric insulating material, a sealing material, a protective material, a high frequency substrate, or a substrate for an electromagnetic wave control member, obtained by using the modified polyphenylene ether resin foamed sheet according to claim 1 or 6.