Polyester, film, and metal-clad laminate

Abstract

Provided are: a polyester that has excellent solubility to a general-purpose amide-based solvent and excellent low dielectric characteristics in a high frequency range; a film obtained by molding the polyester; and a metal-clad laminate including the film.　The present invention uses a polyester which includes a structural unit (A) such as an aromatic oxycarbonyl unit and is linked together or the like by a group derived from a methylol group-containing compound (B) introduced in the main chain of the structural unit (A). The methylol group-containing compound (B) is a compound or the like having at least one methylol group linked to an aromatic ring having an electron-donating substituent. The structural unit (A) includes a specific amount of a structural unit (C) that has at least two polymerizable functional groups selected from the group consisting of a carboxy group and a hydroxy group, and is derived from an aromatic hydrocarbon monomer having a bending structure.

Description

Polyester, film, and metal-clad laminates

[0001] The present invention relates to polyester, a film formed from the polyester, and a metal-clad laminate comprising the film.

[0002] In recent years, liquid crystal polyester has been developing liquid crystal polyester films suitable for circuit boards, surface-mount electronic components, etc., due to its excellent high-frequency characteristics, low dielectric properties, low water absorption, and heat resistance (see Patent Documents 1 and 2).

[0003] As described above, liquid crystal polyester films have excellent properties, but liquid crystal polyester films manufactured by extrusion molding from liquid crystal polyester have a problem in that they have low mechanical strength due to their high anisotropy. Therefore, in order to obtain a liquid crystal polyester film with low anisotropy, Patent Document 3 discloses a method for producing a liquid crystal polyester film by casting, in which a liquid crystal polyester composition is prepared by dissolving a polyesteramide having constituent units derived from an aromatic amine in N-methylpyrrolidone.

[0004] Japanese Patent Publication No. 2004-175995, Japanese Patent Publication No. 2002-359145, Japanese Patent Publication No. 2009-280831

[0005] However, as described in the comparative example in Patent Document 3, liquid crystal polyesters that do not have constituent units derived from aromatic amines do not dissolve at all in N-methylpyrrolidone. Furthermore, the liquid crystal polyester film disclosed in Patent Document 3 has the problem of not being able to maintain low dielectric properties in the high frequency range exceeding 25 GHz.

[0006] The object of the present invention is to provide a polyester with excellent solubility in general-purpose amide solvents and low dielectric properties in the high-frequency range, a film formed from the polyester, and a metal-clad laminate equipped with the film.

[0007] As a result of diligent research to solve the above problems, the present inventors have found that a polyester containing structural units derived from aromatic hydrocarbon monomers having a bent structure, wherein the polyester is linked by groups derived from a methylol group-containing compound introduced into its main chain, exhibits excellent solubility in common amide solvents, and have completed the present invention.

[0008] In other words, aspects of the present invention relate to the following polyesters, films, and metal-clad laminates.

[0009] [1] A polyester comprising at least one structural unit (A) selected from the group consisting of aromatic oxycarbonyl units, aromatic dioxy units, and aromatic dicarboxyl units, wherein the polyester is linked by groups derived from a methylol group-containing compound (B) introduced into the main chain of the structural unit (A), or the main chain of the structural unit (A) has groups derived from the methylol group-containing compound (B), wherein the methylol group-containing compound (B) is a compound having at least one methylol group linked to an aromatic ring having an electron-donating substituent, or a compound having at least one methylol group linked to a nitrogen atom, and the methylol group may be alkyl etherified, wherein the structural unit (A) comprises a structural unit (C) derived from an aromatic hydrocarbon monomer having a bent structure and having at least two polymerizable functional groups selected from the group consisting of carboxyl groups and hydroxyl groups, and the aromatic hydrocarbon monomer is A polyester wherein a monomer (i) is formed in which aromatic hydrocarbon skeletons having at least one polymerizable functional group are linked via a divalent group that can rotate intramolecularly around a single bond, or a monomer (ii) in which an aromatic hydrocarbon skeleton has at least two polymerizable functional groups, wherein if monomer (ii) has two polymerizable functional groups, the monomer (ii-1) is formed in which the two polymerizable functional groups are bonded to the aromatic hydrocarbon skeleton at non-opposite positions, wherein the aromatic hydrocarbon skeleton may have substituents other than the polymerizable functional groups, and the content of methylol groups in groups derived from the methylol group-containing compound (B) relative to 100 mol% of the constituent unit (A) is greater than 2 mol% and less than or equal to 12.0 mol%. [2] The polyester according to [1], wherein the methylol group-containing compound (B) has at least one structure selected from the group consisting of a urea structure, an amide structure, a triazine structure, and a phenol structure. [3] The polyester according to [1] or [2], wherein the content of the constituent unit (C) is 15 to 70 mol% relative to 100 mol% of the total of the constituent units (A).[4] The polyester according to any one of [1] to [3], wherein the divalent group of monomer (i) is at least one selected from the group consisting of an ether group, a sulfide group, a sulfoxide group, a carbonyl group, an oxyalkylene group, and an alkylene group. [5] The polyester according to any one of [1] to [4], wherein the two polymerizable functional groups of the monomer (ii-1) are bonded to positions selected from the group consisting of the o- and m- positions of the benzene skeleton, the 1 and 2, 1 and 3, 1 and 6, 1 and 7, 1 and 8, 2 and 3, 2 and 7 positions of the naphthalene skeleton, the 1 and 2, 1 and 3, 1 and 6, 1 and 7, 1 and 8, 1 and 9, 2 and 3, 2 and 6, 2 and 7, 2 and 9, 2 and 10 positions of the anthracene skeleton, and the 2 and 2', 2 and 3', and 3 and 3' positions of the biphenyl skeleton. [6] The polyester according to any one of [1] to [5], wherein the constituent unit (A) contains 20 to 80 mol% of a structure derived from a monomer having a naphthalene skeleton, based on 100 mol% of the total constituent unit (A). [7] The polyester according to any one of [1] to [6], wherein the polyester is of the fully aromatic type. [8] The polyester according to any one of [1] to [7], wherein the dielectric loss tangent at 28 GHz is 0.0050 or less. [9] A film formed from the polyester according to any one of [1] to [8].

[10] A metal-clad laminate in which a metal layer is laminated on at least one main surface of the film according to [9].

[0010] According to the present invention, it is possible to provide a polyester with excellent solubility in general-purpose amide solvents and low dielectric properties in the high-frequency range, a film formed from the polyester, and a metal-clad laminate equipped with the film.

[0011] ≪Polyester≫ The polyester of this embodiment is a polyester containing a structural unit (A) described later, linked by groups derived from a methylol group-containing compound (B) introduced into the main chain of structural unit (A), or the main chain of structural unit (A) has groups derived from the methylol group-containing compound (B). The methylol group-containing compound (B) is a compound having at least one methylol group linked to an aromatic ring having an electron-donating substituent, or a compound having at least one methylol group linked to a nitrogen atom, and the methylol group may be alkyl etherified. The structural unit (A) contains a structural unit (C) derived from an aromatic hydrocarbon monomer having a bent structure and having at least two polymerizable functional groups selected from the group consisting of carboxyl groups and hydroxyl groups. The above aromatic hydrocarbon monomer is a monomer (i) in which aromatic hydrocarbon skeletons having at least one polymerizable functional group are linked via divalent groups that can rotate intramolecularly around a single bond, or a monomer (ii) in which the aromatic hydrocarbon skeleton has at least two polymerizable functional groups, provided that if monomer (ii) has two polymerizable functional groups, then monomer (ii-1) is in which the two polymerizable functional groups are bonded to the aromatic hydrocarbon skeleton at non-opposite positions. However, the above aromatic hydrocarbon skeleton may have substituents other than the above polymerizable functional groups. The content of methylol groups in groups derived from the methylol group-containing compound (B) relative to 100 mol% of the above constituent unit (A) is greater than 2 mol% and less than or equal to 12.0 mol%.

[0012] The polyester of this embodiment is a polyester containing a structural unit (C) derived from an aromatic hydrocarbon monomer having a flexible structure, and the polyester is linked by groups derived from a methylol group-containing compound (B) introduced into its main chain, thereby exhibiting excellent solubility in general amide solvents and low dielectric properties in the high-frequency range. The structural units (A), (C), and groups derived from the methylol group-containing compound (B) contained in the polyester will be described below.

[0013] <Constituent Unit (A)> Constituent unit (A) is at least one selected from the group consisting of aromatic oxycarbonyl units, aromatic dioxy units, and aromatic dicarboxyl units. It is preferable that at least aromatic oxycarbonyl units are included among these constituent units.

[0014] The aromatic oxycarbonyl unit, aromatic dioxy unit, and aromatic dicarboxyl unit that constitute the constituent unit (A) are represented by the following formulas (A-1), (A-2), and (A-3), respectively.

[0015] In the above formulas (A-1), (A-2), and (A-3), R represents an aromatic skeleton. R is derived from a monomer having an aromatic skeleton. As the aromatic skeleton, for example, a benzene skeleton and a naphthalene skeleton are preferred.

[0016] The constituent unit (A) preferably contains 20 to 80 mol% of a structure derived from a monomer having a naphthalene skeleton, more preferably 30 to 80 mol%, and even more preferably 40 to 80 mol%, based on 100 mol% of the total constituent unit (A).

[0017] The constituent unit (A) preferably contains 20 to 80 mol% of a structure derived from a monomer having a benzene skeleton, more preferably 20 to 70 mol%, and even more preferably 20 to 60 mol%, relative to 100 mol% of the total constituent unit (A).

[0018] (Aromatic Oxycarbonyl Units) Aromatic oxycarbonyl units are derived from aromatic hydroxycarboxylic acids as monomers. Examples of aromatic hydroxycarboxylic acids that give or derive aromatic oxycarbonyl units include 4-hydroxybenzoic acid, 3-hydroxybenzoic acid, 2-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, 6-hydroxy-1-naphthoic acid, 7-hydroxy-2-naphthoic acid, 3-hydroxy-2-naphthoic acid, 7-hydroxy-1-naphthoic acid, 4'-hydroxyphenyl-4-benzoic acid, 3'-hydroxyphenyl-4-benzoic acid, 4'-hydroxyphenyl-3-benzoic acid, and alkyl, alkoxy, or halogen-substituted derivatives thereof. Among these, 4-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid, or a combination thereof is preferred from the viewpoint of easily adjusting the heat resistance and dielectric properties of the resulting polyester. Ester-forming derivatives such as ester derivatives of aromatic hydroxycarboxylic acids and acid halides can also be suitably used in the same way as aromatic hydroxycarboxylic acids.

[0019] (Aromatic Dioxy Units) Aromatic dioxy units are derived from aromatic diols as monomers. Aromatic diols that give or derive aromatic dioxy units include, for example, hydroquinone, resorcinol, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 1,4-dihydroxynaphthalene, 1,5-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 3,3'-dihydroxybiphenyl, 3,4'-dihydroxybiphenyl, 4,4'-dihydroxybiphenyl, 4,4'-dihydroxydiphenyl ether, 2,2'-dihydroxybinaphthyl, and their alkyl, alkoxy, or halogen-substituted derivatives, as well as ester-forming derivatives such as their acylids. Among these, hydroquinone, 2,6-dihydroxynaphthalene, or a combination thereof is preferred.

[0020] (Aromatic Dicarboxyl Units) Aromatic dicarboxyl units are derived from aromatic dicarboxylic acids as monomers. Examples of aromatic dicarboxylic acids that give or derive aromatic dicarboxyl units include terephthalic acid, isophthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 1,6-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 2,7-naphthalenedicarboxylic acid, 4,4'-dicarboxybiphenyl, 3,4'-dicarboxybiphenyl, 4,4'-dicarboxydiphenyl ether, 4,4'-dicarboxyterphenyl, and alkyl, alkoxy, or halogen-substituted derivatives thereof. Among these, terephthalic acid, 2,6-naphthalenedicarboxylic acid, or a combination thereof is preferred.

[0021] <Constituent Unit (C)> Constituent unit (C) is a specific embodiment of constituent unit (A) described above. As described above, constituent unit (C) is derived from an aromatic hydrocarbon monomer having a bent structure and having at least two polymerizable functional groups selected from the group consisting of carboxyl groups and hydroxyl groups. It is presumed that the polyester of this embodiment, by containing constituent unit (C), has its high structural regularity and high molecular orientation disrupted, and as a result exhibits excellent solubility in general-purpose amide solvents.

[0022] The aromatic hydrocarbon monomer having a bent structure is monomer (i) or monomer (ii) described below.

[0023] (Monomer (i)) As described above, monomer (i) is formed by linking aromatic hydrocarbon skeletons, each having at least one of the polymerizable functional groups, via divalent groups that can rotate intramolecularly around a single bond.

[0024] Examples of the above aromatic hydrocarbon skeletons include the benzene skeleton, fluorene skeleton, naphthalene skeleton, anthracene skeleton, biphenyl skeleton, terphenyl skeleton, and phenanthrene skeleton. Among these, the benzene skeleton and naphthalene skeleton are preferred.

[0025] The above-mentioned divalent group is preferably at least one selected from the group consisting of an ether group, a sulfide group, a sulfoxide group, a carbonyl group, an oxyalkylene group, and an alkylene group.

[0026] In monomer (i), the number of polymerizable functional groups on each aromatic hydrocarbon skeleton is preferably one to three, more preferably one to two, and even more preferably one.

[0027] Examples of monomer (i) include 4,4'-dicarboxydiphenyl ether, 4,4'-dihydroxydiphenyl ether, 4,4'-dicarboxydiphenyl sulfone, 4,4'-dihydroxydiphenyl sulfone, benzophenone-4,4'-dicarboxylic acid, and benzophenone-4,4'-diol.

[0028] (Monomer (ii)) As described above, monomer (ii) is a monomer in which the aromatic hydrocarbon skeleton has at least two of the polymerizable functional groups. However, if monomer (ii) has two of the polymerizable functional groups, it is monomer (ii-1) in which the two polymerizable functional groups are bonded to the aromatic hydrocarbon skeleton at non-opposite positions.

[0029] Examples of the above aromatic hydrocarbon skeletons include the benzene skeleton, fluorene skeleton, naphthalene skeleton, anthracene skeleton, biphenyl skeleton, terphenyl skeleton, and phenanthrene skeleton. Among these, the benzene skeleton and naphthalene skeleton are preferred.

[0030] In monomer (ii), the number of polymerizable functional groups on the aromatic hydrocarbon skeleton is preferably two to four, more preferably two to three, and even more preferably two.

[0031] The two polymerizable functional groups of the monomer (ii-1) are preferably bonded to positions selected from the group consisting of, for example, the o- and m- positions of the benzene skeleton, the 1- and 2-, 1- and 3-, 1- and 6-, 1- and 7-, 1- and 8-, 2- and 3-, 2- and 7- of the naphthalene skeleton, the 1- and 2-, 1- and 3-, 1- and 6-, 1- and 7-, 1- and 8-, 1- and 9-, 2- and 3-, 2- and 6-, 2- and 7-, 2- and 9-, 2- and 10- of the anthracene skeleton, and the 2- and 2'-, 2- and 3'-, and 3- and 3'- of the biphenyl skeleton.

[0032] Examples of monomers (ii) include isophthalic acid and resorcinol.

[0033] The aromatic hydrocarbon skeleton contained in the aromatic hydrocarbon monomer that provides or induces the constituent unit (C) may have substituents other than the polymerizable functional group, as described above. Examples of such substituents include alkyl groups having 1 to 8 carbon atoms, halogen groups, allyl groups, and so on.

[0034] The content of constituent unit (C) is preferably 15 to 70 mol%, more preferably 15 to 50 mol%, and even more preferably 15 to 40 mol%, relative to 100 mol% of the total of constituent units (A). The total of constituent units (A) refers to the sum of constituent units (A) that do not fall under constituent unit (C) and constituent units (C).

[0035] <Groups derived from methylol group-containing compound (B)> The embodiments of the groups derived from the methylol group-containing compound (B) present in the polyester of this embodiment include an embodiment in which the polyester containing the above-mentioned structural unit (A) is linked by groups derived from the methylol group-containing compound (B) introduced into the main chain of the above-mentioned structural unit (A), and an embodiment in which the main chain of the above-mentioned structural unit (A) has groups derived from the methylol group-containing compound (B). In the former embodiment, the groups derived from the methylol group-containing compound (B) function as crosslinkable groups, and in the latter embodiment, the groups derived from the methylol group-containing compound (B) function as non-crosslinkable groups.

[0036] The polyester of the present embodiment contains groups derived from the methylol group-containing compound (B), which disrupts the high structural regularity and high molecular orientation of the polyester, and as a result, the polyester is presumed to exhibit excellent solubility in general-purpose amide solvents.

[0037] As described above, the compound that provides or derives a group derived from the methylol group-containing compound (B) is a compound having at least one methylol group linked to an aromatic ring having an electron-donating substituent (hereinafter also referred to as "compound (B-I)"), or a compound having at least one methylol group linked to a nitrogen atom (hereinafter also referred to as "compound (B-II)").

[0038] (Compound (B-I)) Examples of the aromatic ring in compound (B-I) include a benzene ring, a fluorene ring, a naphthalene ring, an anthracene ring, a phenanthrene ring, a biphenyl ring, and aromatic heterocycles in which some of the carbon atoms constituting these aromatic rings are substituted with hetero atoms. Examples of the hetero atom in the aromatic heterocycle include an oxygen atom, a sulfur atom, and a nitrogen atom. Among these, a benzene ring is preferred.

[0039] Regarding the above aromatic rings, among the aforementioned aromatic rings, two identical or different aromatic rings may be linked to each other via a single bond or a divalent linking group. As such an embodiment, an embodiment in which two benzene rings are linked via a divalent linking group is preferred. Examples of the divalent linking group include a divalent saturated hydrocarbon group and a divalent oxygen atom-containing group.

[0040] The divalent saturated hydrocarbon group may be linear, branched, or cyclic. The divalent saturated hydrocarbon group preferably has 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms. Specific examples of the divalent saturated hydrocarbon group include -CH ２ -, -CH ２ CH ２ -, -C((CH ３ ) ２ )-, -CH(C6H5)- and the like. Among these, -CH ２ -, -C((CH ３ ) ２) is preferred.

[0041] Examples of the divalent oxygen atom-containing group include non-hydrocarbon oxygen atom-containing groups such as an ether bond (-O-), an ester bond (-C(=O)-O-), an oxycarbonyl group (-O-C(=O)-), an amide bond (-C(=O)-NH-), a carbonyl group (-C(=O)-), and a carbonate bond (-O-C(=O)-O-); and combinations of a non-hydrocarbon oxygen atom-containing group and an alkylene group. A sulfonyl group (-SO ２ -) may be linked to this combination.

[0042] Examples of the electron-donating substituent that the aromatic ring has include a hydroxy group, an alkoxy group having 1 to 8 carbon atoms, an alkylthio group having 1 to 8 carbon atoms, a linear or branched mono-substituted amino group having 1 to 4 carbon atoms, and a linear or branched di-substituted amino group having 1 to 4 carbon atoms. Among these, a hydroxy group is preferred.

[0043] The number of methylol groups contained in compound (BI) is preferably 1 or more and 12 or less, more preferably 1 or more and 8 or less.

[0044] As a specific example of compound (B-I), a compound having a benzene ring having a hydroxy group as an electron-donating substituent and having at least one methylol group linked to the benzene ring is preferred. Hereinafter, such a compound may sometimes be referred to as a "compound having a phenol structure".

[0045] (Compound (B-II)) In compound (B-II), the nitrogen atom linked to the methylol group is preferably a nitrogen atom constituting a ring in a compound having a cyclic skeleton, a nitrogen atom in a nitrogen-containing substituent of a compound having a cyclic skeleton, or a nitrogen atom constituting the amide bond in a compound having an amide bond. The cyclic skeleton may be a carbocyclic skeleton in which all atoms constituting the ring are carbon, or may be a heterocyclic skeleton containing atoms other than carbon. The cyclic skeleton may be an aromatic ring skeleton or an alicyclic skeleton. The cyclic skeleton may be monocyclic or polycyclic.

[0046] Examples of compounds having a cyclic skeleton include aromatic compounds such as benzene, fluorene, naphthalene, anthracene, phenanthrene, and biphenyl; or aromatic heterocyclic compounds in which some of the carbon atoms constituting these aromatic compounds are substituted with nitrogen atoms; monocycloalkanes having 5 to 15 carbon atoms; or polycyclic cycloalkanes formed by linking or fused monocycloalkanes; or compounds in which some of the carbon atoms constituting these monocycloalkanes or polycyclic cycloalkanes are substituted with nitrogen atoms. Preferred specific examples of compounds having a cyclic skeleton include triazine ring-containing compounds and acetylenediurea structure (glycoluryl structure)-containing compounds.

[0047] Examples of nitrogen-containing substituents include -NH ２ Examples include amino groups represented by and secondary amino groups represented by -NHR. R represents a monovalent substituent. Examples of monovalent substituents include alkyl groups, aryl groups, or alkoxy groups. In the crosslinkable group (B-II), the methylol group is linked to the nitrogen atom by substituting a hydrogen atom in these amino groups or secondary amino groups.

[0048] Specific examples of compound (B-II) include compounds derived from compounds having a triazine ring and at least one methylol group linked to a nitrogen atom in a nitrogen-containing substituent bonded to a carbon atom constituting the triazine ring (hereinafter, such compounds may be referred to as "compounds having a triazine structure"), compounds having an acetylenediurea structure, an imidazolidin-2-one structure, or an imidazolidin structure and at least one methylol group linked to a nitrogen atom constituting the structure (hereinafter, such compounds may be referred to as "compounds having a urea structure"), and compounds having at least one methylol group linked to a nitrogen atom constituting a (meth)acrylamide structure (hereinafter, such compounds may be referred to as "compounds having an amide structure").

[0049] The methylol group in the methylol group-containing compound (B) may be alkyl etherified by an alkyl group. Preferred alkyl groups include, for example, a methyl group and an ethyl group.

[0050] The content of methylol groups in the groups derived from the methylol group-containing compound (B) relative to 100 mol% of the above-mentioned constituent unit (A) is preferably more than 2 mol% and 12.0 mol%, more preferably 2.0 mol% to 10.0 mol%, more preferably 2.0 mol% to 8.0 mol%, and even more preferably 2.0 mol% to 6.0 mol%, from the viewpoint of excellent solubility in general-purpose amide solvents and low dielectric properties in the high-frequency range.

[0051] <Structure and Properties of Polyester> (Liquid Crystallinity) The polyester of this embodiment may or may not exhibit liquid crystallinity. The liquid crystallinity of polyester can be evaluated by checking for the presence or absence of optical anisotropy during melting using a conventional polarization inspection method that utilizes orthogonal polarizers.

[0052] (Solvent Solubility) The polyester of this embodiment exhibits excellent solubility in general-purpose amide solvents. Examples of such solvents include N,N'-dimethylformamide, N,N'-dimethylacetamide, tetramethylurea, and N-methylpyrrolidone. In addition to general-purpose amide solvents, the polyester of this embodiment can also be used in lactone solvents such as γ-butyllactone and sulfide solvents such as dimethyl sulfoxide.

[0053] (Structure) The polyester of this embodiment is preferably of the fully aromatic type, from the viewpoint of having excellent solubility in general-purpose amide solvents and low dielectric loss tangent in the high-frequency range.

[0054] ≪Method for Producing Polyester≫ The aforementioned polyester is typically produced by polycondensation of a monomer mixture containing an acylated monomer having a phenolic hydroxyl group. There are no particular restrictions on the polycondensation method; for example, polyester can be obtained by melt acidolysis, slurry polymerization, etc.

[0055] The molten acidolysis method is a preferred method for producing polyester in terms of cost and manufacturing time. This method involves first heating and melting the monomer, and then continuing the polycondensation reaction to obtain a molten polymer. Vacuum may be applied to facilitate the removal of volatile by-products (e.g., acetic acid, water, etc.) produced in the final stage of condensation.

[0056] Slurry polymerization is a method in which monomers are reacted in the presence of a heat exchange fluid, and the solid product is obtained in a suspended state in the heat exchange medium.

[0057] Acylated monomers having phenolic hydroxyl groups may be synthesized in the reaction system during polyester synthesis by adding acyling agents such as acetic anhydride or acetic acid, or they may be synthesized separately through acylation.

[0058] When acylating a monomer having phenolic hydroxyl groups in a reaction system, it is preferable to use an acylating agent in an amount of 1.1 times or more relative to the total amount of phenolic hydroxyl groups in the monomer.

[0059] The polymerization reaction temperature is 200 to 400°C, preferably 250 to 350°C, and is best carried out under atmospheric pressure and / or reduced pressure.

[0060] Polycondensation is preferably carried out in the presence of a catalyst. Examples of catalysts include metal compounds such as magnesium acetate, stannous acetate, tetrabutyl titanate, lead acetate, sodium acetate, potassium acetate, and antimony trioxide, as well as nitrogen-containing heterocyclic compounds such as 1-methylimidazole.

[0061] The amount of catalyst used is not particularly limited, but for example, 0.1 parts by mass or less per 100 parts by mass of the monomer mixture is preferred. The polyester obtained by such a polycondensation reaction is usually discharged from the reaction vessel in a molten state and then processed into powder, pellet, or flake form.

[0062] Polyester pellets are preferably obtained by melt-kneading polyester powder to form strands. The temperature during melt-kneading is not particularly limited, but from the viewpoint of stable strand formation, it is preferably between -10°C and the outflow start temperature of the polyester powder, and more preferably between -5°C and the outflow start temperature, and more preferably between +20°C.

[0063] Polyester in pellet, flake, or powder form may undergo polymerization in a solid state by heating under reduced pressure, vacuum, or in an inert gas atmosphere such as nitrogen or helium, thereby increasing its molecular weight. Solid-phase polymerization can be expected to improve heat resistance and other properties.

[0064] (Other Additives) The aforementioned polyester can be used as a polyester composition by adding inorganic fillers as needed. Examples of inorganic fillers include calcium carbonate, talc, clay, silica, magnesium carbonate, barium sulfate, titanium dioxide, alumina, montmorillonite, gypsum, glass flakes, glass fibers, milled glass fibers, carbon fibers, alumina fibers, silica-alumina fibers, aluminum borate whiskers, and potassium titanate fibers. Inorganic fillers may be used alone or in combination of two or more types.

[0065] The amount of these inorganic fillers used is determined appropriately according to the application of the polyester, as long as it does not impair the low dielectric properties of the polyester. For example, when forming a film using a polyester composition, an upper limit is set on the amount of inorganic filler used, as long as it does not significantly impair the mechanical strength of the film.

[0066] The polyester composition may optionally contain various additives such as organic fillers, antioxidants, heat stabilizers, light stabilizers, flame retardants, lubricants, antistatic agents, colorants, rust inhibitors, crosslinking agents, foaming agents, fluorescent agents, surface smoothing agents, surface gloss improvers, and mold release improvers. These additives may be used individually or in combination of two or more.

[0067] <<Applications>> Polyester or polyester compositions (hereinafter also simply referred to as "polyester, etc.") can be processed into molded articles, films, or fibers by known molding methods such as press molding, extrusion molding, injection molding, compression molding, or blow molding, after being heated to a temperature above its melting point. Furthermore, it is also possible to process polyester into films by casting using a solution obtained by dissolving polyester in an amide-based solvent.

[0068] <Film> The film of this embodiment is obtained by molding the polyester described above. For example, when obtaining a film by molding pellets of the polyester described above, the molding temperature is not particularly limited, but from the viewpoint of not impairing the excellent low dielectric properties in the high frequency range, it is preferable that the temperature is above the outflow start temperature of the polyester powder and below the outflow start temperature + 30°C.

[0069] <Film Properties> (Relative Permittivity) The relative permittivity at a frequency of 28 GHz of a film obtained by molding polyester or the like is preferably 6.0 or less, more preferably 5.0 or less, even more preferably 4.5 or less, and particularly preferably 4.0 or less. The dielectric loss tangent of the resin film can be measured by the method described later in the examples.

[0070] (Dielectric Loss Tangent) The dielectric loss tangent at a frequency of 28 GHz of a film obtained by molding polyester or the like is preferably 0.0050 or less, more preferably 0.0048 or less, and even more preferably 0.0045 or less. In particular, the dielectric loss tangent at a frequency of 28 GHz of a film obtained by casting after preparing a liquid composition by dissolving the aforementioned pelletized, flake-like, or powdered polyester obtained by polycondensation and not solid-phase polymerized in an N-methylpyrrolidone solvent is preferably 0.0035 or less, more preferably 0.0030 or less, and even more preferably 0.0025 or less. The dielectric loss tangent of the resin film can be measured by the method described later in the examples.

[0071] <Metal-clad laminates> Films obtained by molding polyester or the like have a small coefficient of thermal expansion in the thickness direction and a small difference between the coefficient of thermal expansion in the in-plane direction and the thickness direction, without impairing their excellent low dielectric properties in the high-frequency range. For this reason, such films can be suitably used as films constituting metal-clad laminates. In the case of metal-clad laminates, it is preferable that a metal layer is laminated on at least one main surface of the above film.

[0072] The metal layer is not particularly limited, and any metal foil can be used. The metal foil can preferably be one or more selected from, for example, copper or copper alloys, stainless steel or its alloys, nickel or nickel alloys, or aluminum or aluminum alloys. While copper, such as rolled copper and electrolytic copper, is commonly used in general metal laminates, it can also be preferably used in this invention. Furthermore, the metal foil can be selected to have various characteristics such as surface treatment and surface roughness depending on the purpose. In addition, the surface of the metal foil may be coated with a rust-preventive layer, a heat-resistant layer, or an adhesive layer.

[0073] The thickness of the metal layer can be set appropriately depending on the purpose and application, and is not particularly limited, but is preferably 1 μm to 5000 μm, more preferably 2 μm to 2000 μm, and even more preferably 5 μm to 1000 μm.

[0074] The present invention will be described in more detail below with reference to examples. The present invention is not limited to these examples.

[0075] <<Examples of Polyester Synthesis>> [Examples 1-17, Comparative Examples 1-6] (Materials Used) In the examples and comparative examples, the following A1-A6 were used as monomer A that gives the constituent unit (A). However, monomer A does not give the constituent unit (C). A1: 4-hydroxybenzoic acid A2: 6-hydroxy-2-naphthoic acid A3: terephthalic acid A4: hydroquinone A5: 4,4'-biphenyldiol A6: 2,6-naphthalenediol

[0076] In the examples and comparative examples, the following C1 to C3 were used as monomer C that gives the constituent unit (C): C1: Isophthalic acid C2: 4,4'-dicarboxydiphenyl ether C3: 4,4'-dihydroxydiphenyl ether

[0077] In the examples and comparative examples, the following B1 to B4 were used as compound B that gives a group derived from the methylol group-containing compound (B): B1: Tetramethylolacetylene diurea B2: N-methylolacrylamide B3: 2-hydroxybenzyl alcohol B4: 2,6-dimethylol-p-cresol

[0078] In the comparative example, D1 was used as monomer D, which provides a constituent unit derived from an aromatic amine. D1: 4-aminophenol

[0079] (Example 1) The compounds described below were charged into a reaction vessel equipped with a torque meter-equipped stirrer, a distillation tube, and a reflux condenser. Then, 1.1 times the molar amount of acetic anhydride relative to the total amount of hydroxyl groups (moles) of the charged compounds, and a catalytic amount of 1-methylimidazole were added, and deacetic acid polymerization was carried out under the following conditions: A2: 32.6 g (50.0 mol%) A4: 9.5 g (25.0 mol%) C1: 14.4 g (25.0 mol%) B1: 0.918 g (1.0 mol%)

[0080] The reaction vessel was thoroughly purged with nitrogen gas, heated to 145°C while stirring, and held at that temperature for 30 minutes. Then, the temperature was rapidly increased to 185°C and held at that temperature for 30 minutes. Subsequently, the temperature was increased to 280°C over 20 minutes while distilling off the by-product acetic acid, and held for 2 hours and 30 minutes. Then, the pressure inside the reaction vessel was reduced to 7.5 torr over 10 minutes and held there. After that, the reaction was held at 280°C for 30 minutes while maintaining a vacuum. The reaction was considered complete when the torque reached a predetermined value, and the contents were removed. The obtained solid was allowed to cool to room temperature, then pulverized in a pulverizer, washed with methanol, and dried to obtain polyester powder (1). Subsequently, the mixture was held at 220°C under a nitrogen gas atmosphere for 3 hours to allow the polymerization reaction to proceed in the solid phase, and the obtained solid was pulverized again in a pulverizer to obtain polyester powder (2).

[0081] (Examples 2-10, Comparative Examples 1-5) Polyester powders (1) and (2) of Examples 2-10 and Comparative Examples 1-5 were obtained in the same manner as in Example 1, except that monomer A that gives constituent unit (A), monomer C that gives constituent unit (C), and compound B that gives a group derived from the methylol group-containing compound (B) were used in the proportions shown in Tables 1 and 2 (each value is in mol%).

[0082] (Comparative Example 6) Polyester powders (1) and (2) of Comparative Example 6 were obtained in the same manner as in Example 1, except that monomer A, monomer C, and monomer D, which give constituent units (A), were used in the proportions shown in Table 2 (each value is in mol%).

[0083] ≪Evaluation≫ [NMP Solubility] The solubility of the polyester powder (1) obtained in each example and comparative example was evaluated according to the following method. 0.5 g of each polyester powder was added to 9.5 g of N-methylpyrrolidone (NMP), and the mixture was heated and stirred at 140°C for 2 hours to obtain a 5% solution. 1.0 g of polyester powder was added to 9.0 g of N-methylpyrrolidone, and the mixture was heated and stirred at 140°C for 2 hours to prepare a 10% solution. The presence or absence of insoluble matter in each prepared solution was visually confirmed, and the NMP solubility was evaluated according to the following evaluation criteria. The results are shown in Tables 1 and 2. ◎: No insoluble matter in the 10% solution 〇: No insoluble matter in the 5% solution, but insoluble matter is present in the 10% solution ×: Insoluble matter is present in the 5% solution

[0084] ≪Film Fabrication Example≫ Polyester resin films were fabricated using a desktop mini press (MP-2F, manufactured by Toyo Seiki Seisakusho). Polyester powder (2) obtained in each example and comparative example was placed in a brass spacer with a thickness of 0.13 mm, sandwiched between two stainless steel plates, and the film was formed by heating and pressing at a pressure of 5 MPa for 3 minutes. The thickness of the fabricated film was measured with a micrometer.

[0085] ≪Evaluation≫ [Relative Permittivity (Dk) and Dielectric Loss Tangent (Df)] The dielectric loss tangent of the films obtained in each example and comparative example was measured by the method described below. The results are shown in Tables 1 and 2. The relative permittivity and dielectric loss tangent were measured using a network analyzer (KEYSIGHT N5222B) and a split cylinder resonator (EM Labs CR-728). A sample measuring 3.5 cm × 5.0 cm was cut from the film, and the relative permittivity and dielectric loss tangent were measured after conditioning for 24 hours at 23°C / 50% R.H. The measurements were performed at 28 GHz. The results are shown in Tables 1 and 2.

[0086]

[0087]

[0088] From the results in Tables 1 and 2, the polyesters of Comparative Examples 1 to 5 showed poor solubility in NMP (evaluation: ×), while the polyesters of Examples 1 to 10 showed excellent solubility in NMP (evaluation: ◎ or ○). Furthermore, the films obtained from the polyesters of Examples 1 to 10 showed a lower dielectric loss tangent in the high-frequency range of 28 GHz compared to the film obtained from the polyester of Comparative Example 6. The polyester of Comparative Example 6 corresponds to the polyester disclosed in the aforementioned Patent Document 3.

[0089] (Example 11) Monomer A that gives constituent unit (A), monomer C that gives constituent unit (C), and compound B that gives a group derived from methylol group-containing compound (B) were charged in the proportions shown in Table 3 (each value is in mole percent). Further, 1.1 times the mole amount of acetic anhydride and a catalytic amount of 1-methylimidazole were added relative to the amount of hydroxyl groups (moles) of the total compound charged, and polyester powder (1) was obtained in the same manner as in Example 1. 16 g of the obtained polyester powder (1) was added to 91 g of N-methylpyrrolidone, and the mixture was stirred in an oil bath at 140°C for 3 hours to prepare a liquid composition.

[0090] A copper foil (Fukuda Metal Foil & Powder Industry Co., Ltd., CF-T49A-HD2, 12 μm thick) was placed as a support in an automatic coating machine (manufactured by Tester Industry Co., Ltd.), and the film applicator with a micrometer was set to "750 μm". After applying the liquid composition to the set film thickness, it was dried in a hot air dryer at 150°C and 200°C to create a laminate. The resulting laminate was treated at 230°C under a nitrogen atmosphere for 3 hours. This resulted in a resin-coated copper foil in which a polyester film was formed on the copper foil.

[0091] (Examples 12-14, 16-17) Resin-coated copper foil was obtained in the same manner as in Example 11, except that monomer A, which gives constituent unit (A), monomer C, which gives constituent unit (C), and compound B, which gives a group derived from the methylol group-containing compound (B), were used in the proportions shown in Table 3 (each value is in mole percent).

[0092] (Example 15) A resin-coated copper foil was obtained in the same manner as in Example 11, except that 16 g of the obtained polyester powder (1) was added to 120 g of N-methylpyrrolidone.

[0093] ≪Example of film preparation≫ The obtained resin-coated copper foil was immersed in an aqueous solution of ferric chloride to etch off the copper foil and obtain a single-layer film.

[0094] ≪Evaluation≫ [NMP Solubility] The NMP solubility of the polyester powder (1) obtained in Examples 11 to 17 was evaluated in the same manner as the polyester powder (1) obtained in Example 1, etc. The results are shown in Table 3.

[0095] [Relative permittivity (Dk) and dielectric loss tangent (Df)] The dielectric loss tangent of the films obtained in Examples 11 to 17 was measured using the same method as for the films obtained in Example 1, etc. The results are shown in Table 3.

[0096]

[0097] The films obtained from polyester in Examples 11 to 17 were obtained by casting using polyester powder (1) obtained by polycondensation and not solid-phase polymerized as raw material, while the films obtained from polyester in Examples 1 to 10 were obtained by hot press molding using polyester powder (2) obtained through polycondensation and solid-phase polymerization as raw material. From the results in Tables 1 and 3, the films of Examples 11 to 17 showed a lower dielectric loss tangent in the high-frequency range of 28 GHz compared to the films of Examples 1 to 10.

Claims

1. A polyester comprising at least one structural unit (A) selected from the group consisting of aromatic oxycarbonyl units, aromatic dioxy units, and aromatic dicarboxyl units, wherein the polyester is linked by groups derived from a methylol group-containing compound (B) introduced into the main chain of the structural unit (A), or the main chain of the structural unit (A) has groups derived from the methylol group-containing compound (B), wherein the methylol group-containing compound (B) is a compound having at least one methylol group linked to an aromatic ring having an electron-donating substituent, or a compound having at least one methylol group linked to a nitrogen atom, and the methylol group may be alkyl etherified, wherein the structural unit (A) comprises a structural unit (C) derived from an aromatic hydrocarbon monomer having a bent structure and having at least two polymerizable functional groups selected from the group consisting of carboxyl groups and hydroxyl groups, and the aromatic hydrocarbon monomer is A monomer (i) in which aromatic hydrocarbon skeletons having at least one polymerizable functional group are linked via divalent groups that can rotate intramolecularly around a single bond, or a monomer (ii) in which the aromatic hydrocarbon skeleton has at least two polymerizable functional groups, wherein if monomer (ii) has two polymerizable functional groups, the monomer (ii-1) is such that the two polymerizable functional groups are bonded to the aromatic hydrocarbon skeleton at non-opposite positions, wherein the aromatic hydrocarbon skeleton may have substituents other than the polymerizable functional groups, and the content of methylol groups in groups derived from the methylol group-containing compound (B) relative to 100 mol% of the constituent unit (A) is greater than 2 mol% and less than or equal to 12.0 mol%.

2. The polyester according to claim 1, wherein the methylol group-containing compound (B) has at least one structure selected from the group consisting of a urea structure, an amide structure, a triazine structure, and a phenol structure.

3. The polyester according to claim 1 or 2, wherein the content of the constituent unit (C) is 15 to 70 mol% relative to 100 mol% of the total of the constituent units (A).

4. The polyester according to claim 1 or 2, wherein the divalent group of monomer (i) is at least one selected from the group consisting of an ether group, a sulfide group, a sulfoxide group, a carbonyl group, an oxyalkylene group, and an alkylene group.

5. The polyester according to claim 1 or 2, wherein the two polymerizable functional groups of the monomer (ii-1) are bonded to positions selected from the group consisting of the o- and m- positions of the benzene skeleton, the 1- and 2-, 1- and 3-, 1- and 6-, 1- and 7-, 1- and 8-, 2- and 3-, 2- and 7- of the naphthalene skeleton, the 1- and 2-, 1- and 3-, 1- and 6-, 1- and 7-, 1- and 8-, 1- and 9-, 2- and 3-, 2- and 6-, 2- and 7-, 2- and 9-, 2- and 10- of the anthracene skeleton, and the 2- and 2'-, 2- and 3'-, and 3- and 3'- of the biphenyl skeleton.

6. The polyester according to claim 1 or 2, wherein the constituent unit (A) contains 20 to 80 mol% of a structure derived from a monomer having a naphthalene skeleton, relative to 100 mol% of the total constituent unit (A).

7. The polyester according to claim 1 or 2, wherein the polyester is of the fully aromatic type.

8. The polyester according to claim 1 or 2, wherein the dielectric loss tangent at 28 GHz is 0.0050 or less.

9. A film obtained by molding the polyester according to claim 1 or 2.

10. A metal-clad laminate having a metal layer laminated on at least one main surface of the film according to claim 9.

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