JP6157778B1 - Totally aromatic polyester and method for producing the same - Google Patents

Abstract

Provided is a wholly aromatic polyester that is sufficiently compatible with low melting point and heat resistance and excellent in hue. The wholly aromatic polyester according to the present invention comprises the following constituent units (I) to (IV) as essential constituent components, and the constituent units (I) are 61 to 68 mol%, constituent units with respect to all constituent units. 7 to 14 mol% of (II), 5.5 to 9 mol% of structural unit (III), 16 to 19.5 mol% of structural unit (IV), and the total of structural units (II) and (III) The ratio of the structural unit (III) to is 0.30 to 0.48, has an ester bond or a combination of an ester bond and a ketone bond in the molecule, and the ketone with respect to the sum of the ester bond and the ketone bond The amount of bonds is 0 to 0.18 mol%, and exhibits optical anisotropy when melted.

Description

  The present invention relates to a wholly aromatic polyester and a method for producing the same.

  What is currently marketed as a fully aromatic polyester is based on 4-hydroxybenzoic acid. However, since the homopolymer of 4-hydroxybenzoic acid has a melting point higher than the decomposition point, it is necessary to lower the melting point by copolymerizing various components.

  For example, wholly aromatic polyesters using 1,4-phenylenedicarboxylic acid, 1,4-dihydroxybenzene, 4,4'-dihydroxybiphenyl, and the like as copolymerization components are known. However, the melting point of this wholly aromatic polyester is 350 ° C. or higher, which is too high for melt processing with a general-purpose apparatus.

  In addition, various methods have been tried to lower the melting point of such wholly aromatic polyesters to a temperature at which they can be processed by general-purpose melt processing equipment. However, there is a problem that the heat resistance of the wholly aromatic polyester, represented by mechanical strength at a high temperature (near the melting point), cannot be maintained while lowering the melting point to some extent.

  In order to solve these problems, Patent Document 1 discloses a copolymer obtained by combining 4-hydroxybenzoic acid with 1,4-phenylenedicarboxylic acid, 1,3-phenylenedicarboxylic acid, and 4,4′-dihydroxybiphenyl. Polyester has been proposed.

Japanese Patent Publication No.57-24407

  However, conventional wholly aromatic polyesters are insufficient in achieving both low melting point and heat resistance. Further, the wholly aromatic polyester is required to be excellent in hue so as to have a good appearance.

  In view of the above problems, an object of the present invention is to provide a wholly aromatic polyester excellent in hue and having both a low melting point and heat resistance, and a method for producing the same.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, it has been found that the above-mentioned problems can be solved by a wholly aromatic polyester composed of specific structural units, the content of each structural unit being in a specific range, and the amount of ketone bond being in a specific range. It came to be completed. More specifically, the present invention provides the following.

(1) As an essential constituent, it is composed of the following structural units (I) to (IV),
Content of structural unit (I) is 61-68 mol% with respect to all the structural units,
The content of the structural unit (II) is 7 to 14 mol% with respect to all the structural units,
Content of structural unit (III) is 5.5-9 mol% with respect to all the structural units,
The content of the structural unit (IV) is 16 to 19.5 mol% with respect to all the structural units,
The ratio of the structural unit (III) to the total of the structural unit (II) and the structural unit (III) is 0.30 to 0.48,
It has an ester bond or a combination of an ester bond and a ketone bond in the molecule, and the amount of the ketone bond relative to the total of the ester bond and the ketone bond is 0 to 0.18 mol%. A wholly aromatic polyester showing anisotropy.

  (2) The wholly aromatic polyester according to (1), which has a melting point of 320 to 340 ° C.

(3) The wholly aromatic polyester according to (1) or (2), wherein the difference between the melting point and the deflection temperature under load is 85 ° C. or less,
The deflection temperature under load is obtained by melt-kneading 60% by mass of the wholly aromatic polyester and 40% by mass of milled fiber having an average fiber diameter of 11 μm and an average fiber length of 75 μm at the melting point of the wholly aromatic polyester + 20 ° C. Wholly aromatic polyester measured in the state of the polyester resin composition obtained.

(4) A method for producing a wholly aromatic polyester exhibiting optical anisotropy when melted,
In the method, 4-hydroxybenzoic acid and 4,4′-dihydroxybiphenyl are acylated with a fatty acid anhydride in the presence of a fatty acid metal salt, and 1,4-phenylenedicarboxylic acid and 1,3-phenylenedicarboxylic acid are esterified. Including the step of exchanging,
For all monomers consisting of 4-hydroxybenzoic acid, 1,4-phenylenedicarboxylic acid, 1,3-phenylenedicarboxylic acid, and 4,4′-dihydroxybiphenyl,
The amount of 4-hydroxybenzoic acid used is 61 to 68 mol%,
The amount of 1,4-phenylene dicarboxylic acid used is 7 to 14 mol%,
The amount of 1,3-phenylenedicarboxylic acid used is 5.5-9 mol%,
The amount of 4,4′-dihydroxybiphenyl used is 16 to 19.5 mol%.
And
The ratio of the amount of 1,3-phenylenedicarboxylic acid used to the total amount of 1,4-phenylenedicarboxylic acid and 1,3-phenylenedicarboxylic acid used is 0.30 to 0.48,
The method whose usage-amount of the said fatty acid anhydride is 1.02-1.04 times the sum total hydroxyl equivalent of 4-hydroxybenzoic acid and 4,4'- dihydroxybiphenyl.

  (5) The method according to (4), wherein the fatty acid metal salt is an acetic acid metal salt and the fatty acid anhydride is acetic anhydride.

  (6) The total number of moles of 1,4-phenylene dicarboxylic acid and 1,3-phenylene dicarboxylic acid is 1 to 1.06 times the number of moles of 4,4′-dihydroxybiphenyl, or 4,4 The method according to (5), wherein the number of moles of '-dihydroxybiphenyl is 1 to 1.06 times the total number of moles of 1,4-phenylenedicarboxylic acid and 1,3-phenylenedicarboxylic acid.

  According to the present invention, the wholly aromatic polyester of the present invention, which is composed of specific structural units and exhibits optical anisotropy when melted, has both a low melting point and sufficient heat resistance, and is excellent in hue. .

  The wholly aromatic polyester of the present invention does not have a very high molding processing temperature, and thus can be injection molded, extruded, compressed, etc. without using a molding machine having a special structure.

  As described above, the wholly aromatic polyester of the present invention is excellent in moldability and can be molded using various molding machines. As a result, it can be easily processed into various three-dimensional molded products, fibers, films and the like. For this reason, molded products such as connectors, CPU sockets, relay switch parts, bobbins, actuators, noise reduction filter cases, or heat fixing rolls for OA equipment, which are suitable uses of the wholly aromatic polyester of the present invention, can be easily obtained. .

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Totally aromatic polyester>
The wholly aromatic polyester of the present invention comprises the following structural unit (I), the following structural unit (II), the following structural unit (III), and the following structural unit (IV).

  The structural unit (I) is derived from 4-hydroxybenzoic acid (hereinafter also referred to as “HBA”). The wholly aromatic polyester of the present invention contains 61 to 68 mol% of the structural unit (I) with respect to all the structural units. When the content of the structural unit (I) is less than 61 mol% or exceeds 68 mol%, at least one of lowering the melting point and heat resistance tends to be insufficient.

  The structural unit (II) is derived from 1,4-phenylenedicarboxylic acid (hereinafter also referred to as “TA”). The wholly aromatic polyester of the present invention contains 7 to 14 mol%, preferably 9.5 to 11.5 mol% of the structural unit (II) with respect to all the structural units. If the content of the structural unit (II) is less than 7 mol% or exceeds 14 mol%, at least one of lowering the melting point and heat resistance tends to be insufficient.

  The structural unit (III) is derived from 1,3-phenylenedicarboxylic acid (hereinafter also referred to as “IA”). The wholly aromatic polyester of the present invention contains 5.5 to 9 mol%, preferably 6.5 to 8 mol% of the structural unit (III) with respect to all the structural units. When the content of the structural unit (III) is less than 5.5 mol% or exceeds 9 mol%, at least one of lowering the melting point and heat resistance tends to be insufficient.

  The structural unit (IV) is derived from 4,4′-dihydroxybiphenyl (hereinafter also referred to as “BP”). The wholly aromatic polyester of the present invention contains 16 to 19.5 mol% of the structural unit (IV) with respect to the total structural unit. When the content of the structural unit (IV) is less than 16 mol% or exceeds 19.5 mol%, at least one of lowering the melting point and heat resistance tends to be insufficient.

  In the wholly aromatic polyester of the present invention, the ratio of the structural unit (III) to the total of the structural unit (II) and the structural unit (III) is 0.30 to 0.48 mol%. When the ratio is less than 0.30 mol% or exceeds 0.48 mol%, at least one of lowering the melting point and heat resistance tends to be insufficient.

One of the factors affecting all physical properties of wholly aromatic polyesters is a ketone bond formed by a side reaction during polymerization.
In the wholly aromatic polyester of the present invention, the amount of the ketone bond relative to the total of the ester bond and the ketone bond is 0 to 0.18 mol%. When the amount of the ketone bond exceeds 0.18 mol%, the hue tends to be lowered.

  As described above, the wholly aromatic polyester of the present invention contains a specific amount of each of the specific structural units (I) to (IV) with respect to the total structural units, and the structural unit (II) and the structural units. Since the ratio of the structural unit (III) to the total of (III) is adjusted to a specific range, and the amount of ketone bond is adjusted to a specific range, both low melting point and heat resistance are compatible. Is sufficient and has excellent hue.

  As an index representing the heat resistance, a difference between a melting point and a deflection temperature under load (hereinafter also referred to as “DTUL”) can be given. If this difference is 85 ° C. or less, the heat resistance tends to increase, which is preferable. DTUL is a polyester resin obtained by melt-kneading 60% by mass of the wholly aromatic polyester and 40% by mass of milled fiber having an average fiber diameter of 11 μm and an average fiber length of 75 μm at the melting point of the wholly aromatic polyester + 20 ° C. It is a value measured in the state of the composition, and can be measured according to ISO75-1,2.

  Subsequently, the manufacturing method of the wholly aromatic polyester of this invention is demonstrated. The wholly aromatic polyester of the present invention is polymerized using a direct polymerization method or a transesterification method. In the polymerization, a melt polymerization method, a solution polymerization method, a slurry polymerization method, a solid phase polymerization method, or the like is used.

  In the present invention, at the time of polymerization, an acylating agent for the polymerization monomer or a monomer having terminal activated as an acid chloride derivative can be used. Examples of the acylating agent include fatty acid anhydrides such as acetic anhydride.

In the polymerization, various catalysts can be used. Typical examples include dialkyl tin oxide, diaryl tin oxide, titanium dioxide, alkoxy titanium silicates, titanium alcoholates, fatty acid metal salts, BF ₃ Lewis acid salts such as are mentioned, and fatty acid metal salts are preferred. The amount of the catalyst used is generally about 0.001 to 1% by weight, particularly about 0.003 to 0.2% by weight, based on the total weight of the monomers.

  Moreover, when performing solution polymerization or slurry polymerization, as a solvent, liquid paraffin, a high heat resistant synthetic oil, an inert mineral oil, etc. are used.

  The reaction conditions are, for example, a reaction temperature of 200 to 380 ° C. and a final ultimate pressure of 0.1 to 760 Torr (that is, 13 to 101,080 Pa). Particularly in the melt reaction, for example, the reaction temperature is 260 to 380 ° C., preferably 300 to 360 ° C., the final ultimate pressure is 1 to 100 Torr (that is, 133 to 13,300 Pa), preferably 1 to 50 Torr (that is, 133 to 6,670 Pa). ).

  The reaction can be started by charging all the raw material monomers (HBA, TA, IA, and BP), the acylating agent, and the catalyst in the same reaction vessel (one-stage system), or the hydroxyl groups of the raw material monomers HBA and BP. Can be acylated with an acylating agent and then reacted with carboxyl groups of TA and IA (two-stage system).

  The melt polymerization is performed after the inside of the reaction system reaches a predetermined temperature, and after starting the pressure reduction to a predetermined degree of pressure reduction. After the torque of the stirrer reaches a predetermined value, an inert gas is introduced, and the total aromatic polyester is discharged from the reaction system by changing from a reduced pressure state to a normal pressure to a predetermined pressure state.

  The wholly aromatic polyester produced by the above polymerization method can be further increased in molecular weight by solid-state polymerization that is heated at normal pressure or reduced pressure in an inert gas. The preferable conditions for the solid state polymerization reaction are a reaction temperature of 230 to 330 ° C., preferably 250 to 320 ° C., and a final ultimate pressure of 10 to 760 Torr (ie 1,330 to 101,080 Pa).

In the process for producing a wholly aromatic polyester of the present invention, 1,4-phenylenedicarboxylic acid and 1 are prepared by acylating 4-hydroxybenzoic acid and 4,4′-dihydroxybiphenyl with a fatty acid anhydride in the presence of a fatty acid metal salt. Preferably comprising a step of transesterification with 3,3-phenylenedicarboxylic acid,
For all monomers consisting of 4-hydroxybenzoic acid, 1,4-phenylenedicarboxylic acid, 1,3-phenylenedicarboxylic acid, and 4,4′-dihydroxybiphenyl,
The amount of 4-hydroxybenzoic acid used is 61 to 68 mol%,
The amount of 1,4-phenylene dicarboxylic acid used is 7 to 14 mol%,
The amount of 1,3-phenylenedicarboxylic acid used is 5.5-9 mol%,
The amount of 4,4′-dihydroxybiphenyl used is 16 to 19.5 mol%.
It is preferable that
The ratio of the amount of 1,3-phenylenedicarboxylic acid used relative to the total amount of 1,4-phenylenedicarboxylic acid and 1,3-phenylenedicarboxylic acid is preferably 0.30 to 0.48,
The amount of the fatty acid anhydride used is preferably 1.02 to 1.04 times the total hydroxyl equivalent of 4-hydroxybenzoic acid and 4,4′-dihydroxybiphenyl. More preferably, the fatty acid metal salt is an acetic acid metal salt and the fatty acid anhydride is acetic anhydride. The total number of moles of 1,4-phenylene dicarboxylic acid and 1,3-phenylene dicarboxylic acid is 1 to 1.06 times the number of moles of 4,4′-dihydroxybiphenyl, or 4,4 The number of moles of '-dihydroxybiphenyl is preferably 1 to 1.06 times the total number of moles of 1,4-phenylenedicarboxylic acid and 1,3-phenylenedicarboxylic acid.

  Next, the properties of the wholly aromatic polyester will be described. The wholly aromatic polyester of the present invention exhibits optical anisotropy when melted. An optical anisotropy when melted means that the wholly aromatic polyester of the present invention is a liquid crystalline polymer.

  In the present invention, the fact that the wholly aromatic polyester is a liquid crystalline polymer is an indispensable element for the wholly aromatic polyester has both thermal stability and easy processability. The wholly aromatic polyesters composed of the structural units (I) to (IV) may not form an anisotropic melt phase depending on the constituent components and the sequence distribution in the polymer. Limited to wholly aromatic polyesters that exhibit optical anisotropy when melted.

  The property of melt anisotropy can be confirmed by a conventional polarization inspection method using an orthogonal polarizer. More specifically, the melting anisotropy can be confirmed by melting a sample placed on a hot stage manufactured by Linkham Co., Ltd. using a polarizing microscope manufactured by Olympus and observing it at a magnification of 150 times in a nitrogen atmosphere. The liquid crystalline polymer is optically anisotropic and transmits light when inserted between crossed polarizers. If the sample is optically anisotropic, for example, polarized light is transmitted even in a molten stationary liquid state.

  Since a nematic liquid crystalline polymer causes a significant decrease in viscosity at the melting point or higher, generally exhibiting liquid crystallinity at the melting point or higher is an index of workability. The melting point (liquid crystallinity expression temperature) is preferably as high as possible from the viewpoint of heat resistance, but it is 320 to 340 ° C. in consideration of thermal degradation during polymer melt processing, heating capability of the molding machine, and the like. Is a preferred guideline. In addition, More preferably, it is 325-335 degreeC.

<Polyester resin composition>
Various fibrous, granular, and plate-like inorganic and organic fillers can be blended in the wholly aromatic polyester of the present invention according to the purpose of use.

  As an inorganic filler mix | blended with the polyester resin composition of this invention, there exist a fibrous form, a granular form, and a plate-shaped thing.

  Silica such as glass fiber, asbestos fiber, silica fiber, silica / alumina fiber, alumina fiber, zirconia fiber, boron nitride fiber, silicon nitride fiber, boron fiber, potassium titanate fiber, wollastonite as fibrous inorganic filler Inorganic fibrous materials such as fibers, magnesium sulfate fibers, aluminum borate fibers, and metal fibrous materials such as stainless steel, aluminum, titanium, copper, and brass. A particularly typical fibrous filler is glass fiber.

  In addition, as the granular inorganic filler, carbon black, graphite, silica, quartz powder, glass beads, milled glass fiber, glass balloon, glass powder, calcium oxalate, aluminum oxalate, kaolin, clay, diatomaceous earth, wollast Silicates such as knight, iron oxide, titanium oxide, zinc oxide, antimony trioxide, metal oxides such as alumina, metal carbonates such as calcium carbonate and magnesium carbonate, metal sulfates such as calcium sulfate and barium sulfate Examples thereof include salts, other ferrites, silicon carbide, silicon nitride, boron nitride, and various metal powders.

  Examples of the plate-like inorganic filler include mica, glass flakes, talc, and various metal foils.

  Examples of organic fillers include heat-resistant high-strength synthetic fibers such as aromatic polyester fibers, liquid crystalline polymer fibers, aromatic polyamides, and polyimide fibers.

  These inorganic and organic fillers can be used alone or in combination of two or more. The combined use of the fibrous inorganic filler and the granular or plate-like inorganic filler is a preferable combination in order to combine mechanical strength, dimensional accuracy, electrical properties, and the like. Particularly preferably, the fibrous filler is glass fiber, and the plate-like filler is mica and talc. It is. By combining glass fiber with mica or talc, the polyester resin composition is particularly prominent in improving the heat distortion temperature and mechanical properties.

  In using these fillers, if necessary, a sizing agent or a surface treatment agent can be used.

  As described above, the polyester resin composition of the present invention contains the wholly aromatic polyester of the present invention, an inorganic or organic filler as an essential component, but other components are included as long as the effects of the present invention are not impaired. It may be included. Here, the other component may be any component, and examples thereof include other resins, antioxidants, stabilizers, pigments, crystal nucleating agents and the like.

  Moreover, the manufacturing method of the polyester resin composition of this invention is not specifically limited, A polyester resin composition can be prepared by a conventionally well-known method.

<Polyester molded product>
The polyester molded article of the present invention is formed by molding the wholly aromatic polyester or polyester resin composition of the present invention. The molding method is not particularly limited, and a general molding method can be employed. Examples of general molding methods include injection molding, extrusion molding, compression molding, blow molding, vacuum molding, foam molding, rotational molding, gas injection molding, and the like.

  A polyester molded product formed by molding the wholly aromatic polyester of the present invention is excellent in heat resistance and toughness. Moreover, since the polyester molded article formed by shape | molding the polyester resin composition of this invention is excellent in heat resistance and toughness and contains an inorganic or organic filler, mechanical strength etc. are further improved.

  Moreover, since the wholly aromatic polyester and the polyester resin composition of the present invention are excellent in moldability, a polyester molded product having a desired shape can be easily obtained.

  Preferred uses of the polyester molded product of the present invention having the above properties include connectors, CPU sockets, relay switch parts, bobbins, actuators, noise reduction filter cases, heat fixing rolls for OA equipment, and the like.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited to a following example.

<Example 1>
A polymerization vessel equipped with a stirrer, a reflux column, a monomer inlet, a nitrogen inlet, and a decompression / outflow line was charged with the following raw material monomers, fatty acid metal salt catalyst, and acylating agent, and nitrogen substitution was started.
(I) 4-hydroxybenzoic acid 10.9 mol (66 mol%) (HBA)
(II) 1.7 mol (10.3 mol%) of terephthalic acid (TA)
(III) 1.1 mol (6.7 mol%) of isophthalic acid (IA)
(IV) 2.8 mol (17 mol%) of 4,4′-dihydroxybiphenyl (BP)
Potassium acetate catalyst 110mg
1756 g of acetic anhydride (1.04 times the total hydroxyl equivalent of HBA and BP)
After charging the raw materials, the temperature of the reaction system was raised to 140 ° C. and reacted at 140 ° C. for 1 hour. Thereafter, the temperature is further increased to 360 ° C. over 5.5 hours, and then the pressure is reduced to 10 Torr (ie, 1330 Pa) over 20 minutes to melt while distilling acetic acid, excess acetic anhydride, and other low-boiling components. Polymerization was performed. After the stirring torque reached a predetermined value, nitrogen was introduced and the pressure was changed from a reduced pressure state to a normal pressure, and the polymer was discharged from the lower part of the polymerization vessel.

<Evaluation>
About the wholly aromatic polyester of Example 1, evaluation of melting | fusing point, DTUL, a hue (L value), and a ketone bond amount was performed with the following method. The evaluation results are shown in Tables 1 and 2.

[Melting point]
After observing the endothermic peak temperature (Tm1) observed by DSC (manufactured by TA Instruments) at a temperature rising condition of 20 ° C./min from room temperature, the temperature is 2 at (Tm1 + 40) ° C. After being held for a minute, the sample was once cooled to room temperature under a temperature drop condition of 20 ° C./min, and then the temperature of the endothermic peak observed when measured under a temperature rise condition of 20 ° C./min was measured again.

[DTUL]
60% by mass of polymer and 40% by mass of glass fiber (manufactured by Central Glass Co., Ltd., milled fiber, average fiber diameter 11 μm, average fiber length 75 μm), a twin-screw extruder (TEX30α type, manufactured by Nippon Steel) And melt-kneading at a cylinder temperature of the melting point of the polymer + 20 ° C. to obtain polyester resin composition pellets.
The polyester resin composition pellets were molded under the following molding conditions using a molding machine (“SE100DU” manufactured by Sumitomo Heavy Industries, Ltd.) to obtain measurement specimens (4 mm × 10 mm × 80 mm). Using this test piece, the deflection temperature under load was measured by a method based on ISO75-1,2. Note that 1.8 MPa was used as the bending stress. The results are shown in Tables 1 and 2.
〔Molding condition〕
Cylinder temperature: Polymer melting point + 20 ° C
Mold temperature: 80 ℃
Back pressure: 2MPa
Injection speed: 33mm / sec

[Hue (L value)]
The L value of the polymer was measured using a spectral color difference meter (“SE6000” manufactured by Nippon Denshoku Industries Co., Ltd.).

[Ketone binding amount]
The ketone bond amount of the polymer was calculated by a pyrolysis gas chromatography method described in Polymer Degradation and Stability 76 (2002) 85-94. Specifically, the polymer is heated in the presence of tetramethylammonium hydroxide (TMAH) using a pyrolysis device ("PY2020iD" manufactured by Frontier Laboratories) to generate gas by pyrolysis / methylation. It was. This gas is analyzed using gas chromatography ("GC-6890N" manufactured by Agilent Technologies), and the amount of ketone bond is calculated from the ratio of the peak area derived from the ketone bond to the peak area derived from the ester bond. did.

<Examples 2-7, Comparative Examples 1-12>
A polymer was obtained in the same manner as in Example 1 except that the type of raw material monomer and the charging ratio (mol%) were as shown in Table 1 or 2. Moreover, the same evaluation as Example 1 was performed. However, when the polymer of Comparative Example 11 was obtained, the potassium acetate catalyst was not used, and the amount of acetic anhydride used was 1.10 times the total hydroxyl equivalent of HBA and BP. The evaluation results are shown in Tables 1 and 2.

Claims (6)

As an essential constituent, it consists of the following structural units (I) to (IV),
Content of structural unit (I) is 61-68 mol% with respect to all the structural units,
The content of the structural unit (II) is 7 to 14 mol% with respect to all the structural units,
Content of structural unit (III) is 5.5-9 mol% with respect to all the structural units,
The content of the structural unit (IV) is 16 to 19.5 mol% with respect to all the structural units,
The ratio of the structural unit (III) to the total of the structural unit (II) and the structural unit (III) is 0.39 to 0.41 ,
It has an ester bond or a combination of an ester bond and a ketone bond in the molecule, and the amount of the ketone bond relative to the total of the ester bond and the ketone bond is 0 to 0.18 mol%. A wholly aromatic polyester showing anisotropy.

  The wholly aromatic polyester according to claim 1, which has a melting point of 320 to 340 ° C. The wholly aromatic polyester according to claim 1 or 2, wherein the difference between the melting point and the deflection temperature under load is 85 ° C or less,
The deflection temperature under load is obtained by melt-kneading 60% by mass of the wholly aromatic polyester and 40% by mass of milled fiber having an average fiber diameter of 11 μm and an average fiber length of 75 μm at the melting point of the wholly aromatic polyester + 20 ° C. Wholly aromatic polyester measured in the state of the polyester resin composition obtained. A method for producing a wholly aromatic polyester exhibiting optical anisotropy when melted,
In the method, 4-hydroxybenzoic acid and 4,4′-dihydroxybiphenyl are acylated with a fatty acid anhydride in the presence of a fatty acid metal salt, and 1,4-phenylenedicarboxylic acid and 1,3-phenylenedicarboxylic acid are esterified. Including the step of exchanging,
For all monomers consisting of 4-hydroxybenzoic acid, 1,4-phenylenedicarboxylic acid, 1,3-phenylenedicarboxylic acid, and 4,4′-dihydroxybiphenyl,
The amount of 4-hydroxybenzoic acid used is 61 to 68 mol%,
The amount of 1,4-phenylene dicarboxylic acid used is 7 to 14 mol%,
The amount of 1,3-phenylenedicarboxylic acid used is 5.5-9 mol%,
The amount of 4,4′-dihydroxybiphenyl used is 16 to 19.5 mol%.
And
The ratio of the amount of 1,3-phenylene dicarboxylic acid used to the total amount of 1,4-phenylene dicarboxylic acid and 1,3-phenylene dicarboxylic acid used is 0.39 to 0.41 ;
The method whose usage-amount of the said fatty acid anhydride is 1.02-1.04 times the sum total hydroxyl equivalent of 4-hydroxybenzoic acid and 4,4'- dihydroxybiphenyl.   The method according to claim 4, wherein the fatty acid metal salt is an acetic acid metal salt and the fatty acid anhydride is acetic anhydride.   The total number of moles of 1,4-phenylene dicarboxylic acid and 1,3-phenylene dicarboxylic acid is 1 to 1.06 times the number of moles of 4,4′-dihydroxybiphenyl, or 4,4′-dihydroxy The method according to claim 5, wherein the number of moles of biphenyl is 1 to 1.06 times the total number of moles of 1,4-phenylenedicarboxylic acid and 1,3-phenylenedicarboxylic acid.