Liquid crystal polyester, film, and metal-clad laminate

Abstract

Provided are: a liquid crystal polyester which gives a molded film having a small coefficient of linear expansion in the thickness direction and a small difference in the coefficient of linear expansion between the in-plane direction and the thickness direction without impairing excellent low dielectric characteristics in a high frequency region; a film obtained by molding the liquid crystal polyester; and a metal-clad laminate comprising the film.　This liquid crystal polyester comprises a linear polyester containing at least one structural unit (A) selected from the group consisting of an aromatic oxycarbonyl unit, an aromatic dioxy unit and an aromatic dicarboxy unit, the linear polyester being linked by a crosslinkable group (B) introduced into the main chain of the structural unit (A), wherein the crosslinkable group (B) is a group derived from a compound having at least two methylol groups linked to an aromatic ring having an electron-donating substituent, or a group derived from a compound having at least two methylol groups linked to a nitrogen atom, the methylol group may be alkyl etherified, and the content of the crosslinkable group (B) with respect to 100 mol% of the structural unit (A) is more than 0 mol% and 4.0 mol% or less.

Description

Liquid crystal polyester, film, and metal-clad laminate

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

[0002] In electronic devices that require high-speed transmission and reception of large volumes of data, the frequency of electrical signals is increasing, but as the frequency of the electrical signals used increases, transmission loss increases. To reduce this transmission loss, the substrate material that constitutes the base material of the wiring board is required to have a low dielectric constant and dielectric loss tangent.

[0003] Circuit board materials using liquid crystal polyester have been attracting attention in recent years as materials for flexible printed circuit boards (FPCs) and rigid circuit boards used in electronic devices that use high frequency bands, such as antenna modules for smartphones for fifth-generation mobile communication systems (5G), wireless communication base stations, and automotive millimeter-wave radar, due to their excellent high-frequency characteristics, low dielectric properties, and low water absorption.

[0004] As a circuit board material using such liquid crystal polyester, for example, Patent Document 1 discloses a liquid crystal polymer film formed from a blend of liquid crystal polyester and a thermoplastic resin.

[0005] Japanese Patent Application Laid-Open No. 2004-175995

[0006] For rigid substrate applications requiring multilayer lamination, there is a strong demand for a reduction in the linear expansion coefficient of the material in the thickness direction. However, while the technology of Patent Document 1 succeeds in suppressing the linear expansion coefficient of the film in the in-plane direction to 5 to 25 ppm / K, the linear expansion coefficient in the thickness direction is 270 ppm / K or less, resulting in a large difference in the thermal expansion coefficient between the in-plane direction and the thickness direction. In other words, the film disclosed in Patent Document 1 has high anisotropy of the material, which raises concerns about a significant decrease in dimensional accuracy during processing of circuit boards, for example. Furthermore, the incorporation of a large amount of thermoplastic resin such as polyarylate raises concerns about deterioration in heat resistance and dielectric properties.

[0007] The object of the present invention is to provide a liquid crystal polyester that can give a molded film having a small linear expansion coefficient in the thickness direction and a small difference in the linear expansion coefficient between the in-plane direction and the thickness direction without impairing the excellent low dielectric properties in the high frequency range, a film molded from the liquid crystal polyester, and a metal-clad laminate plate comprising the film.

[0008] As a result of extensive research aimed at solving the above problems, the present inventors have discovered that a liquid crystal polyester in which linear polyesters are linked by crosslinkable groups derived from a specific compound having two or more methylol groups can give a molded film that has a small linear expansion coefficient in the thickness direction and a small difference in the linear expansion coefficient between the in-plane direction and the thickness direction, without impairing the excellent low dielectric properties in the high frequency range, and have thus completed the present invention.

[0009] That is, aspects of the present invention relate to the following liquid crystal polyester, film, and metal-clad laminate.

[0010] [1] A liquid crystal polyester comprising a linear polyester containing at least one structural unit (A) selected from the group consisting of an aromatic oxycarbonyl unit, an aromatic dioxy unit, and an aromatic dicarboxy unit, and linked to the main chain of the structural unit (A) by a crosslinkable group (B) introduced into the main chain of the structural unit (A), wherein the crosslinkable group (B) is a group derived from a compound having at least two methylol groups linked to an aromatic ring having an electron-donating substituent, or a group derived from a compound having at least two methylol groups linked to a nitrogen atom, and the methylol groups may be alkyl-etherified, and the content of the crosslinkable group (B) relative to 100 mol% of the structural unit (A) is more than 0 mol% and 4.0 mol% or less. [2] The liquid crystal polyester according to [1], wherein the crosslinkable group (B) has at least one structure selected from the group consisting of an acetylene diurea structure, a triazine structure, and a phenol structure. [3] The liquid crystal polyester according to [1] or [2], wherein the linear polyester is fully aromatic. [4] The liquid crystal polyester according to any one of [1] to [3], wherein the linear polyester has a type II structure. [5] The liquid crystal polyester according to any one of [1] to [4], wherein the structural unit (A) contains a structure derived from a monomer having a naphthalene skeleton in an amount of 20 to 80 mol %, relative to a total of 100 mol % of the structural units (A). [6] The liquid crystal polyester according to any one of [1] to [5], which gives a molded film having a glass transition temperature (Tg) of 125°C or higher, as determined from the peak top temperature of the loss tangent (tan δ) obtained by dynamic mechanical analysis (DMA). [7] The liquid crystal polyester according to any one of [1] to [5], wherein the linear expansion coefficients in the thickness direction and the plane direction of the film are each α Ｚ (ppm / ℃) and α ＸＹ (ppm / °C), α ＸＹ is 55 ppm / °C or less, and α Ｚ / α ＸＹ [8] The liquid crystal polyester according to any one of [1] to [6], which provides a formed film having a linear expansion coefficient α in the thickness direction of 8.0 or less. ｚ[9] A film obtained by molding the liquid crystalline polyester according to any one of [1] to [8].

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

[0011] According to the present invention, it is possible to provide a liquid crystal polyester that can produce a molded film having a small linear expansion coefficient in the thickness direction and a small difference in the linear expansion coefficient between the in-plane direction and the thickness direction without impairing the excellent low dielectric properties in the high frequency range, a film molded from the liquid crystal polyester, and a metal-clad laminate plate comprising the film.

[0012] This is an example of a flow curve when measuring the outflow start temperature.

[0013] Liquid Crystal Polyester In the liquid crystal polyester of this embodiment, linear polyesters containing the structural unit (A) described below are linked by a crosslinkable group (B) introduced into the main chain of the structural unit (A). The crosslinkable group (B) is a group derived from a compound having at least two methylol groups linked to an aromatic ring having an electron-donating substituent, or a group derived from a compound having at least two methylol groups linked to a nitrogen atom, and the methylol groups may be alkyl-etherified. The content of the crosslinkable group (B) relative to 100 mol% of the structural unit (A) is more than 0 mol% and 4.0 mol% or less. The liquid crystal polyester, in which the linear polyesters containing the structural unit (A) are linked by the crosslinkable group (B) introduced into the main chain of the structural unit (A), provides a molded film with a small linear expansion coefficient in the thickness direction and a small difference in the linear expansion coefficient between the in-plane direction and the thickness direction, without impairing the excellent low dielectric properties in the high frequency range. The linear polyester containing the structural unit (A) contained in the liquid crystal polyester and the crosslinkable group (B) will be described below.

[0014] <Linear Polyester Comprising Structural Unit (A)> The linear polyester comprises the structural unit (A). Therefore, the linear polyester causes the liquid crystalline polyester to exhibit liquid crystallinity. Liquid crystallinity means that the liquid crystalline polyester exhibits an anisotropic molten phase upon heating.

[0015] [Structural Unit (A)] The structural unit (A) is at least one selected from the group consisting of an aromatic oxycarbonyl unit, an aromatic dioxy unit, and an aromatic dicarboxy unit. Of these structural units, it is preferable that at least an aromatic oxycarbonyl unit is included.

[0016] The aromatic oxycarbonyl unit, aromatic dioxy unit, and aromatic dicarboxy unit constituting the structural unit (A) are represented by the following formulas (A-1), (A-2), and (A-3), respectively.

[0017] 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. Preferred examples of the aromatic skeleton include a benzene skeleton and a naphthalene skeleton.

[0018] The structural unit (A) preferably contains a structure derived from a monomer having a naphthalene skeleton in an amount of 20 to 80 mol %, more preferably 30 to 80 mol %, and even more preferably 40 to 80 mol %, relative to a total of 100 mol % of the structural unit (A).

[0019] The structural unit (A) preferably contains a structure derived from a monomer having a benzene skeleton in an amount of 20 to 80 mol %, more preferably 20 to 70 mol %, and even more preferably 20 to 60 mol %, relative to a total of 100 mol % of the structural unit (A).

[0020] (Aromatic Oxycarbonyl Unit) The aromatic oxycarbonyl unit is derived from an aromatic hydroxycarboxylic acid as a monomer. Examples of aromatic hydroxycarboxylic acids that provide or derive the aromatic oxycarbonyl unit 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, dielectric properties, and melting point of the resulting liquid crystal polyester. Ester derivatives of aromatic hydroxycarboxylic acids, acid halides and other ester-forming derivatives can also be suitably used in the same manner as aromatic hydroxycarboxylic acids.

[0021] (Aromatic Dioxy Unit) The aromatic dioxy unit is derived from an aromatic diol as a monomer. Examples of aromatic diols that provide or derive the aromatic dioxy unit include 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 ester-forming derivatives thereof, such as alkyl-, alkoxy-, or halogen-substituted products, and acylated products thereof. Among these, hydroquinone, 2,6-dihydroxynaphthalene, or a combination thereof is preferred.

[0022] (Aromatic Dicarboxy Unit) The aromatic dicarboxy unit is derived from an aromatic dicarboxylic acid as a monomer. Examples of aromatic dicarboxylic acids that provide or derive the aromatic dicarboxy unit 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.

[0023] The linear polyester is preferably a wholly aromatic polyester from the viewpoint of heat resistance and flame retardancy.

[0024] Linear polyesters are conveniently classified into liquid crystalline polyesters having a type I structure, a type II structure, and a type III structure, in descending order of heat resistance. In this specification, melting point is used as an index of heat resistance. Type I structure refers to liquid crystalline polyesters having a melting point of 340°C or higher. Type II structure refers to liquid crystalline polyesters having a melting point of 260°C or higher and lower than 330°C. Type III structure refers to liquid crystalline polyesters having a melting point lower than 240°C. The linear polyester may have any of a type I structure, a type II structure, or a type III structure, but is preferably a type II structure from the viewpoints of heat resistance, dielectric loss tangent, and melt moldability.

[0025] <Crosslinkable Group (B)> As described above, the crosslinkable group (B) is a group derived from a compound having at least two methylol groups linked to an aromatic ring having an electron-donating substituent (hereinafter also referred to as a "crosslinkable group (B-I)"), or a group derived from a compound having at least two methylol groups linked to a nitrogen atom (hereinafter also referred to as a "crosslinkable group (B-II)").

[0026] [Crosslinkable Group (B-I)] Examples of the aromatic ring in the crosslinkable group (B-I) include a benzene ring, a fluorene ring, a naphthalene ring, an anthracene ring, a phenanthrene ring, a biphenyl ring, and an aromatic heterocycle in which a portion of the carbon atoms constituting these aromatic rings are substituted with heteroatoms. Examples of the heteroatom in the aromatic heterocycle include an oxygen atom, a sulfur atom, and a nitrogen atom. Of these, a benzene ring is preferred.

[0027] The aromatic rings may be the same or different aromatic rings among the aromatic rings described above, and may be linked to each other by a single bond or via a divalent linking group. In such an embodiment, benzene rings are preferably linked to each other via a divalent linking group. Examples of the divalent linking group include a divalent saturated hydrocarbon group and a divalent oxygen atom-containing group.

[0028] 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, for example, —CH ２ -, -CH ２ CH ２ -, -C((CH ３ ) ２ )-, -CH(C6H5)-, etc. Among these, -CH ２ -, -C((CH ３ ) ２ )- is preferred.

[0029] 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 non-hydrocarbon oxygen atom-containing groups and alkylene groups. These combinations may also include sulfonyl groups (-SO ２ -) may be linked.

[0030] Examples of the electron-donating substituent on the aromatic ring 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. Of these, a hydroxy group is preferred.

[0031] The number of methylol groups in the crosslinkable group (BI) is preferably 2 or more and 12 or less, and more preferably 2 or more and 8 or less.

[0032] A specific example of the crosslinkable group (B-I) is preferably a group derived from a compound having a benzene ring with a hydroxy group as an electron-donating substituent and at least two methylol groups linked to the benzene ring. In this specification, such a crosslinkable group (B-I) may be referred to as a "crosslinkable group (B-I) having a phenol structure."

[0033] [Crosslinkable Group (B-II)] In the crosslinkable group (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, or a nitrogen atom in a nitrogen-containing substituent of a compound having a cyclic skeleton. The cyclic skeleton may be a carbocyclic skeleton in which all atoms constituting the ring are carbon, or 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.

[0034] 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 in which these monocycloalkanes are linked or fused; or compounds in which some of the carbon atoms constituting these monocycloalkanes or polycyclic cycloalkanes are substituted with nitrogen atoms. Specific examples of compounds having a cyclic skeleton are preferably triazine ring-containing compounds and acetylenediurea structure (glycoluril structure)-containing compounds.

[0035] Nitrogen-containing substituents include, for example, —NH ２ and a secondary amino group represented by -NHR. R represents a monovalent substituent. Examples of the monovalent substituent include an alkyl group, an aryl group, or an alkoxy group. In the crosslinkable group (B-II), the methylol group is linked to the nitrogen atom by substituting for a hydrogen atom in the amino group or secondary amino group.

[0036] Specific examples of the crosslinkable group (B-II) include a group derived from a compound having a triazine ring and having at least two methylol groups linked to nitrogen atoms in a nitrogen-containing substituent bonded to a carbon atom constituting the triazine ring (hereinafter, such a crosslinkable group (B-II) may be referred to as a "crosslinkable group (B-II) having a triazine structure"), and a group derived from a compound having an acetylene diurea structure and having at least two methylol groups linked to nitrogen atoms constituting the structure (hereinafter, such a crosslinkable group (B-II) may be referred to as a "crosslinkable group (B-II) having an acetylene diurea structure").

[0037] The methylol group in the crosslinkable group (B) may be alkyl-etherified with an alkyl group, such as a methyl group or an ethyl group.

[0038] The upper limit of the content of the crosslinkable group (B) relative to 100 mol% of the structural unit (A) is 4.0 mol% or less, preferably 3.5 mol% or less, and more preferably 3.0 mol% or less, from the viewpoint of providing a molded film having a small linear expansion coefficient in the thickness direction and a small difference in the linear expansion coefficient between the in-plane direction and the thickness direction without impairing the excellent low dielectric properties in the high frequency range. The lower limit of the content of the crosslinkable group (B) relative to 100 mol% of the structural unit (A) is not particularly limited as long as it is more than 0 mol%, and is preferably 0.1 mol% or more, and more preferably 0.2 mol% or more.

[0039] <<Method for producing liquid crystal polyester>> The liquid crystal polyester is typically produced by polycondensing a monomer mixture containing an acylated product of a monomer having a phenolic hydroxyl group. The polycondensation method is not particularly limited, and the liquid crystal polyester can be obtained by, for example, a melt acidolysis method, a slurry polymerization method, or the like.

[0040] The melt acidolysis method is a preferred method for producing liquid crystal polymers in terms of cost and production time. This method involves first heating the monomers to melt them, followed by a polycondensation reaction to obtain a molten polymer. Vacuum may be applied to facilitate removal of volatile by-products (e.g., acetic acid, water, etc.) produced in the final stage of condensation.

[0041] Slurry polymerization is a process in which monomers are reacted in the presence of a heat exchange fluid, resulting in a solid product suspended in the heat exchange medium.

[0042] The acylated product of a monomer having a phenolic hydroxyl group may be synthesized in the reaction system by adding an acylating agent such as acetic anhydride or acetic acid during the synthesis of the liquid crystal polyester, or may be synthesized by separate acylation.

[0043] When a monomer having a phenolic hydroxyl group is acylated in the reaction system, the acylating agent is preferably used in an amount of at least 1.1 times the total amount of phenolic hydroxyl groups on the monomer.

[0044] The polymerization reaction temperature is 200 to 400°C, preferably 250 to 350°C, and is preferably carried out under atmospheric pressure and / or reduced pressure.

[0045] The polycondensation is preferably carried out in the presence of a catalyst, examples of which include metal compounds such as magnesium acetate, stannous acetate, tetrabutyl titanate, lead acetate, sodium acetate, potassium acetate, and antimony trioxide, and nitrogen-containing heterocyclic compounds such as 1-methylimidazole.

[0046] The amount of the catalyst used is not particularly limited, and is preferably 0.1 parts by mass or less per 100 parts by mass of the monomer mixture. The liquid crystal polyester obtained by such a polycondensation reaction is usually discharged in a molten state from a reaction vessel and then processed into a powder, pellets, or flakes.

[0047] The pellets of the liquid crystalline polyester are preferably obtained by melt-kneading a powder of the liquid crystalline polyester to form strands. The temperature during melt-kneading is not particularly limited, but from the viewpoint of stable strand formation, the temperature is preferably from −10° C. to +30° C., more preferably from −5° C. to +20° C., the temperature at which the liquid crystalline polyester powder starts to flow.

[0048] The liquid crystal polyester in pellet, flake, or powder form may be heated under reduced pressure, vacuum, or an inert gas atmosphere such as nitrogen or helium to cause a polymerization reaction in a solid state and increase the molecular weight. By performing the heat treatment in the solid state, improvement in heat resistance, etc. can be expected.

[0049] Whether the liquid crystalline polyester obtained by the above method exhibits liquid crystallinity can be evaluated by checking the presence or absence of optical anisotropy when melted by a conventional polarization inspection method using crossed polarizers.

[0050] (Other additives) If necessary, inorganic filler can be blended with the above-mentioned liquid crystal polyester to be used as liquid crystal polyester composition.As inorganic filler, calcium carbonate, talc, clay, silica, magnesium carbonate, barium sulfate, titanium oxide, alumina, montmorillonite, gypsum, glass flake, glass fiber, milled glass fiber, carbon fiber, alumina fiber, silica alumina fiber, aluminum borate whisker and potassium titanate fiber etc.The inorganic filler can be used alone or in combination of two or more.

[0051] The amount of these inorganic fillers used is determined appropriately depending on the application of the liquid crystal polyester, within a range that does not impair the low dielectric properties of the liquid crystal polyester. For example, when a film is formed using the liquid crystal polyester composition, the upper limit of the amount of inorganic fillers used is determined within a range that does not significantly impair the mechanical strength of the film.

[0052] The liquid crystal polyester composition may further contain various additives, as needed, 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 alone or in combination of two or more.

[0053] <<Uses>> The liquid crystal polyester or the liquid crystal polyester composition (hereinafter also simply referred to as "liquid crystal polyester, etc.") can be processed into a molded product, film, fiber, or the like by a known molding method such as press molding, extrusion molding, injection molding, compression molding, or blow molding, by heating it at a temperature equal to or higher than the melting point.

[0054] Among the above-mentioned applications, for example, when pellets of the above-mentioned liquid crystal polyester or the like are molded to obtain a film, the temperature during molding is not particularly limited, but from the viewpoint of obtaining a molded film having a small difference in linear expansion coefficient between the in-plane direction and the thickness direction without impairing the excellent low dielectric properties in the high frequency range, the temperature is preferably not lower than the flow-out start temperature of the powder of the liquid crystal polyester or the like but not higher than the flow-out start temperature + 30°C, and more preferably not lower than the flow-out start temperature of the powder of the liquid crystal polyester or the like + 5°C but not higher than the flow-out start temperature + 20°C.

[0055] <Film Properties> (Glass Transition Temperature) The glass transition temperature (Tg) of a film obtained by molding a liquid crystalline polyester or the like is preferably 125° C. or higher, more preferably 140° C. or higher. The melting point of the film can be measured by a method reading it from the peak top temperature of the loss tangent (tan δ) obtained by dynamic mechanical analysis (DMA).

[0056] (Dielectric Constant) The dielectric constant at a frequency of 28 GHz of a film obtained by molding a liquid crystal polyester or the like is preferably 8.0 or less, more preferably 6.0 or less, even more preferably 5.0 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.

[0057] (Dielectric loss tangent) The dielectric loss tangent at a frequency of 28 GHz of a film obtained by molding a liquid crystal polyester or the like is preferably 0.0030 or less, more preferably 0.0028 or less, even more preferably 0.0027 or less, and particularly preferably 0.0025 or less. The dielectric loss tangent of the resin film can be measured by the method described later in the examples.

[0058] (Linear expansion coefficient) The linear expansion coefficient in the planar direction of a film obtained by molding a liquid crystal polyester or the like (hereinafter referred to as "XY-CTE" or "α ＸＹ The value of the linear expansion coefficient in the thickness direction of the film (hereinafter referred to as "Z-CTE" or "α") is not particularly limited, but is preferably 55 ppm / °C or less, more preferably 50 ppm / °C or less, and particularly preferably 45 ppm / °C or less. Ｚ The value of the CTE (also referred to as "CTE") is not particularly limited, and is preferably 150 ppm / °C or less, more preferably 120 ppm / °C or less, even more preferably 100 ppm / °C or less, and particularly preferably 90 ppm / °C or less. XY-CTE and Z-CTE can be evaluated by the method described later in the examples.

[0059] (Anisotropy of Linear Expansion) The anisotropy of linear expansion of a film obtained by molding a liquid crystal polyester or the like is expressed by the linear expansion coefficients in the thickness direction and the plane direction of the film, respectively, as α Ｚ (ppm / ℃) and α ＸＹ (ppm / °C), α Ｚ / α ＸＹ It is evaluated using the index α Ｚ / α ＸＹ is preferably 8.0 or less, more preferably 7.0 or less, and even more preferably 6.0 or less. The anisotropy of linear expansion can be evaluated by the method described later in the examples.

[0060] A film obtained by molding a liquid crystal polyester or the like has a small linear expansion coefficient in the thickness direction and a small difference in the linear expansion coefficient between the in-plane direction and the thickness direction without impairing its excellent low dielectric properties in the high frequency range. Therefore, such a film can be suitably used as a film constituting a metal-clad laminate. As a metal-clad laminate, it is preferable that a metal layer is laminated on at least one main surface of the film.

[0061] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0062] [Examples 1 to 11, Comparative Examples 1 to 6] (Materials Used) In the examples and comparative examples, the following A1 and A2 were used as the monomer A that provides the structural unit (A): A1: 4-hydroxybenzoic acid A2: 6-hydroxy-2-naphthoic acid

[0063] In the examples and comparative examples, the following compounds B1 to B7 were used as compounds B that provide the crosslinkable group (B). Of these, B1 to B5 are compounds that provide the crosslinkable group (B) in the embodiments of the present invention, and B6 and B7 are comparative compounds. B1: Tetramethylolacetylene diurea B2: Tetramethoxymethylacetylene diurea B3: Hexamethoxymethylmelamine B4: 2,6-dihydroxymethyl-4-methylphenol B5: 2,2'-bis(4-hydroxy-3,5-dihydroxymethylphenyl)propane B6: 4,4'-bis(methoxymethyl)benzene B7: Tetramethylolmethane (pentaerythritol)

[0064]

[0065]

[0066] Example 1 Compounds A1, A2, and B1 described below were charged into a reaction vessel equipped with a stirrer with a torque meter, a distillation tube, and a reflux condenser, and acetic anhydride in an amount 1.1 times the moles of hydroxyl groups (moles) of all the charged compounds and a catalytic amount of 1-methylimidazole were further added, followed by deacetylation polymerization under the following conditions: A1: 27.6 g (50.0 mol%) A2: 37.6 g (50.0 mol%) B1: 0.524 g (0.5 mol%)

[0067] The inside of the reaction vessel was thoroughly purged with nitrogen gas, heated to 145 ° C while stirring, and maintained at the same temperature for 30 minutes. Thereafter, the temperature was quickly raised to 185 ° C and maintained at the same temperature for 30 minutes. Thereafter, the temperature was raised to 260 ° C over 20 minutes while distilling off the by-product acetic acid, and maintained for 2 hours and 30 minutes. Thereafter, the pressure inside the reaction vessel was reduced to 7.5 torr over 10 minutes and maintained for 1 hour. Thereafter, the temperature was quickly raised to 265 ° C while maintaining the vacuum and maintained for 30 minutes. Thereafter, the temperature was quickly raised to 270 ° C while maintaining the vacuum and maintained for 30 minutes. Thereafter, the temperature was quickly raised to 275 ° C while maintaining the vacuum, and when the torque reached a predetermined value, the reaction was deemed to be complete, and the contents were removed. The obtained solid was allowed to cool to room temperature, pulverized in a pulverizer, washed with methanol, and dried. Thereafter, the mixture was maintained at 300° C. for 2 hours in a nitrogen gas atmosphere to allow the polymerization reaction to proceed in the solid phase, and the resulting solid was then pulverized again in a pulverizer to obtain a powder of the liquid crystal polyester of Example 1.

[0068] (Examples 2 to 11, Comparative Examples 1 to 6) Liquid crystal polyester powders of Examples 2 to 11 and Comparative Examples 1 to 6 were obtained in the same manner as in Example 1, except that the monomers that provide the structural unit (A) and the compounds that can provide the crosslinkable group (B) were used in the proportions shown in Tables 1 and 2 (each numerical value represents mol %).

[0069] [Flow Start Temperature] The flow start temperature of the liquid crystalline polyester powder obtained in each Example and Comparative Example was measured according to the following method. The flow start temperature was measured using a Shimadzu CFT500D high-temperature flow tester. 2 g of liquid crystalline polyester powder was filled and the resin heated at a heating rate of 5.0°C / min was extruded from a die with a diameter of 1.0 mm and a length of 10 mm under a load of 10 MPa. The flow curve was shown in Figure 1, and the temperature at which the resin extrusion piston began to descend was calculated as the flow start temperature (Tfb). The results are shown in Tables 1 and 2.

[0070] [Preparation of Resin Film] A liquid crystal polyester resin film was prepared using a tabletop mini press (MP-2F manufactured by Toyo Seiki Seisakusho, Ltd.). Specifically, the liquid crystal polyester powder obtained in each example and comparative example was placed in a 0.31 mm thick brass spacer, sandwiched between two stainless steel plates, and heat-pressed at 10 MPa and 360°C for 3 minutes to form a film, producing a resin film. The thickness of the obtained film was measured with a micrometer. The appearance of the prepared film was visually inspected for the presence or absence of bubbles.

[0071] [Evaluation of Resin Film] The dielectric loss tangent, linear expansion coefficient in the plane direction, linear expansion coefficient in the thickness direction, anisotropy of linear expansion, and glass transition temperature of each obtained film were measured according to the following methods. The results are shown in Tables 1 and 2.

[0072] <Dielectric Constant (Dk) and Dielectric Loss Tangent (Df)> The dielectric loss tangent was measured using a network analyzer (KEYSIGHT N5222B) and a split cylinder resonator (EM Labs CR-728). A sample measuring 3.5 cm x 5.0 cm was cut out from each film, and after 24 hours of humidity conditioning in a 23°C / 50% RH environment, the dielectric constant (Dk) and dielectric loss tangent (Df) were measured. The measurements were performed at 28 GHz.

[0073] <Linear expansion coefficient in the plane direction (XY-CTE)> The linear expansion coefficient in the plane direction was measured by a tensile load method using a thermomechanical analyzer (TMA4000SA manufactured by NESZSCH). A sample of 5 mm x 10 mm was cut out from each film, and heated to 80 to 150°C at a heating rate of 10°C / min under a nitrogen gas flow, then cooled to 30°C at a heating rate of 20°C / min, and further heated at 10°C / min. In the chart obtained by this heating, the linear expansion coefficient at 25°C (T １ ) is the sample length in L １ , glass transition temperature T ２ The sample length at (the value obtained by the differential scanning calorimetry) is L ２ , the reference length is L ０ When this is the case, the value α expressed by the following formula (1) ＸＹ is the coefficient of linear expansion in the plane direction (XY-CTE).

[0074]

[0075] <Coefficient of linear expansion in the thickness direction (Z-CTE)> The coefficient of linear expansion in the thickness direction was measured by a compressive load method using a thermomechanical analyzer (TMA8310 manufactured by Rigaku Corporation). A 5 mm x 5 mm, 1 mm thick sample was cut out from each film, heated to 80 to 150°C at a heating rate of 10°C / min under a nitrogen gas flow, cooled to 25°C at a cooling rate of 8°C / min, and further heated at 10°C / min. The chart obtained by heating the sample at 25°C (T １ ) is the sample length in L １ , glass transition temperature T ２ The sample length at (the value obtained by the differential scanning calorimetry) is L ２ , the reference length is L ０ When this is the case, the value α expressed by the following formula (2) Ｚ is the coefficient of linear expansion in the thickness direction (Z-CTE).

[0076]

[0077] <Anisotropy of Linear Expansion> The anisotropy of linear expansion was measured by the linear expansion coefficient (α ＸＹ , α Ｚ ) and α Ｚ α ＸＹ Divided by (α Ｚ / αＸＹ The closer this value is to 1, the smaller the anisotropy of linear expansion is.

[0078] <Glass transition temperature (tan δ (Tg))> The glass transition temperature of the film was measured using a dynamic viscoelasticity measuring device (DMS8100 manufactured by TA Instruments). The peak top temperature of tan δ was calculated as the glass transition temperature (tan δ (Tg)). Sample measurement range: width 5 mm Measurement temperature range: 25°C to 300°C Heating rate: 5°C / min Strain amplitude: 10 μm Measurement frequency: 10 Hz

[0079]

[0080]

[0081] The results in Tables 1 and 2 show that the resin films of Examples 1 to 11 have a smaller linear expansion coefficient in the thickness direction and a smaller difference in the linear expansion coefficient between the in-plane direction and the thickness direction, without compromising their excellent low dielectric properties in the high frequency range, compared to the resin films of Comparative Examples 1 to 6.

[0082] [Example 12] [Production of Liquid Crystal Polyester Resin Pellets] 100 parts by mass of the liquid crystal polyester powder obtained in Example 1 was supplied from a hopper port to a twin-screw extruder (25 mmφ, L / D=41, manufactured by Parker Corporation, HK-25D(41D)) set at a cylinder temperature of 285°C and a screw rotation speed of 200 rpm, and melt-kneaded to obtain liquid crystal polyester resin pellets.

[0083] [Flow Start Temperature] The flow start temperature of the liquid crystalline polyester resin pellets obtained in Example 12 was measured in the same manner as described above, except that 1.6 g of the liquid crystalline polyester resin pellets were filled instead of 2 g of the liquid crystalline polyester powder in Example 1. The results are shown in Table 3.

[0084] [Preparation of Resin Film] A resin film was prepared from the liquid crystal polyester resin pellets obtained in Example 12 in the same manner as described above, except that instead of the liquid crystal polyester powder of Example 1 being hot-pressed at a pressure of 10 MPa and 360°C for 3 minutes, the liquid crystal polyester resin pellets were hot-pressed at a pressure of 10 MPa and 290°C for 3 minutes.

[0085] [Evaluation of Resin Film] The dielectric constant, dielectric loss tangent, linear expansion coefficient in the plane direction, linear expansion coefficient in the thickness direction, anisotropy of linear expansion, and glass transition temperature of the obtained resin film were measured in the same manner as described above. The results are shown in Table 3.

[0086]

[0087] In Table 3, for ease of comparison, the results for the liquid crystal polyester resin pellets of Example 12 and the results for the liquid crystal polyester powder of Example 1 are shown together. From Table 3, it can be seen that when the liquid crystal polyester powder of Example 1 is melt-processed at 285°C, which is higher than the flow-initiating temperature of 280°C, strands can be stably formed. Furthermore, when a resin film is produced using the liquid crystal polyester resin pellets of Example 12, the pressing temperature can be lowered from 360°C in Example 1 to 290°C. In addition, the dielectric loss tangent (Df) and α Ｚ / α ＸＹ From the results, it can be seen that the resin film obtained from the liquid crystal polyester resin pellets of Example 12 has a smaller difference in linear expansion coefficient between the in-plane direction and the thickness direction without impairing the excellent low dielectric properties in the high frequency range.

Claims

1. A liquid crystal polyester comprising linear polyesters containing at least one structural unit (A) selected from the group consisting of aromatic oxycarbonyl units, aromatic dioxy units, and aromatic dicarboxy units, and linked by crosslinkable groups (B) introduced into the main chain of the structural unit (A), wherein the crosslinkable groups (B) are groups derived from a compound having at least two methylol groups linked to an aromatic ring having an electron-donating substituent, or groups derived from a compound having at least two methylol groups linked to a nitrogen atom, the methylol groups may be alkyl-etherified, and the content of the crosslinkable groups (B) relative to 100 mol % of the structural unit (A) is greater than 0 mol % and less than 4.0 mol %.

2. The liquid crystal polyester according to claim 1, wherein the crosslinkable group (B) has at least one structure selected from the group consisting of an acetylene diurea structure, a triazine structure, and a phenol structure.

3. The liquid crystal polyester according to claim 1 or 2, wherein the linear polyester is a wholly aromatic polyester.

4. The liquid crystal polyester according to claim 1 or 2, wherein the linear polyester has a type II structure.

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

6. The liquid crystal polyester according to claim 1 or 2, which gives a molded film having a glass transition temperature (Tg) of 125°C or higher, as determined from the peak top temperature of loss tangent (tan δ) obtained by dynamic mechanical analysis (DMA).

7. The linear expansion coefficients in the thickness direction and plane direction of the film are α Ｚ (ppm / ℃) and α ＸＹ (ppm / °C), α ＸＹ is 55 ppm / °C or less, and α Ｚ / α ＸＹ The liquid crystal polyester according to claim 1 or 2, which gives a formed film having a viscosity of 8.0 or less.

8. The linear expansion coefficient α in the thickness direction Ｚ The liquid crystal polyester according to claim 7, wherein the melting point is 150 ppm / ° C. or less.

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

10. A metal-clad laminate comprising the film according to claim 9 and a metal layer laminated on at least one of its main surfaces.

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