WO2019226641A1

WO2019226641A1 - Varnish of polyimide having high heat resistance and excellent mechanical strength

Info

Publication number: WO2019226641A1
Application number: PCT/US2019/033300
Authority: WO
Inventors: Masahiko Miyauchi
Original assignee: Kaneka Americas Holding, Inc.
Priority date: 2018-05-21
Filing date: 2019-05-21
Publication date: 2019-11-28
Also published as: WO2019226641A9

Abstract

A varnish and a polyimide resin composition, a prepreg, and a fiber-reinforced laminate prepared therefrom are disclosed herein. In an embodiment, the varnish includes components (A) to (D), wherein components (A), (B), and (C) are present at a solid content of more than 65 %, wherein component (A) is an aromatic tetracarboxylic acid diester represented by General Formula (1) below and present in an amount of 1 to 500 parts by weight, wherein component (B) is 2-phenyl-4,4'-diaminodiphenyl ether and present in an amount of 1 to 450 parts by weight, wherein component (C) is a 4-(2- phenylethynyl)phthalic acid monoester represented by General Formula (2) and present in an amount of 1 to 400 parts by weight, and wherein the component (D) is an organic solvent present in an amount of 100 parts by weight, based on the total weight of the varnish.

Description

VARNISH OF POLYIMIDE HAVING HIGH HEAT RESISTANCE

AND EXCELLENT MECHANICAL STRENGTH

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of the filing date of U.S. Provisional

Application No. 62/674,298, filed May 21, 2018, entitled VARNISH OF POLYIMIDE MONOMERS AND POLYIMIDES HAVING HIGH HEAT RESISTANCE AND EXCELLENT MECHANICAL STRENGTH, the disclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention relates to a polyimide resin composition and a prepreg and a fiber- reinforced laminate including the composition, and specifically relates to member materials usable in various fields requiring excellent moldability and high heat resistance, including aircraft and apparatuses for the aerospace industry.

BACKGROUND ART

[0003] Aromatic polyimides are categorized into the highest level of heat resistance among polymers, also have excellent mechanical, electric, and other characteristics, and thus have been used as materials in various fields including the aerospace and the electronics.

[0004] Common aromatic polyimides are unsuitable to use particularly for melt molding or as the matrix resin of fiber-reinforced composite materials due to the poor processability.

[0005] To use the aromatic polyimide as the matrix resin for fiber-reinforced composite materials, thermal addition reactive polyimides are typically used as follows: the polyimides still having a low molecular weight is impregnated into fibers; and then the polyimides is crosslinked and cured in the final process. PMR-15 (PMR: in-situ polymerization of monomer reactants) is exemplified as one of the representative examples of polyimide resins previously developed for fiber-reinforced composite materials. As illustrated in FIG. 1A and FIG. IB, a cross section microscopic view of PMR-15 demonstrates breakdown due to thermal cycle exposure. (Owens, G.A. et ah, Composites Science and Technology 1998, 33, 177-190). Further, as illustrated in FIG. 2, the interlaminar shear strength (ILSS) decreases by about 50% after 1600 thermal cycles in PMR-15. (see Wilson, D., 18^th International SAMPE Technical Conference, 1996, 242-253). A review of PMR-15, and its limitations can be found in Wilson, D., British Polymer Journal, 1988, 20, 405-416. Among the limitations of PMR-15 are reliable methods of quality control, prepreg batch to batch variability, microcracking of carbon fiber reinforced materials during thermal cycling, health and safety hazards, and high temperature cracking of carbon fiber reinforced materials. Methods of the prior art require multi-steps to prepare the highly concentrated varnishes with diamines via the isolation of diester and monoester after esterification of dianhydride and monoanhydride, respectively or by further addition of other alcohols. Because alcohol with less polarity or a longer alkyl group needed to be used for the esterification of anhydrides in comparison with the one as a solvent of varnish in order to prepare highly concentrated varnishes with an excellent solution storage stability. SUMMARY OF THE INVENTION

[0006] The inventor of the present invention has intensively studied in order to solve the problems, and consequently have completed the present invention.

[0007] The present invention has an object to provide a varnish with more than 65 % solid content in an organic solvent with a boiling point of 150°C or less at 1 atmosphere. The varnish has an excellent solution storage stability and can be easily converted to modified imide oligomer with good moldability such as low melt viscosity, a solid imide resin composition including the terminally modified imide oligomer, and a cured product, a prepreg, an imide prepreg, and a fiber-reinforced composite material that are produced by using the solid imide resin composition and have high thermal and mechanical characteristics such as heat resistance, elastic modulus, tensile strength, and elongation.

[0008] The present invention provides a varnish including components (A) to (D). The components (A), (B), and (C) are dissolved in the varnish at a solid content of more than 65 %;

the component (A) is an aromatic tetracarboxylic acid diester represented by General Formula (1) and is contained in an amount of 1 to 500 parts by weight;

the component (B) is 2-phenyl-4,4'-diaminodiphenyl ether and is contained in an amount of 1 to 450 parts by weight;

the component (C) is a 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2) and is contained in an amount of 1 to 400 parts by weight; and

the component (D) is an organic solvent having a boiling point of 150°C or less at 1 atmosphere or a mixture of two or more of the organic solvents which includes methanol, ethanol, 1- propanol, and 2-propanol and is contained in an amount of 100 parts by weight.

[General Formula (1)]

[0009] In the formula, R_j is an aromatic tetracarboxylic acid diester residue; R₂ and R₃ are the same or different and are an aliphatic organic group or an aromatic organic group; R₂ and R₃ are located in a cis configuration or a trans configuration; and the compound is optionally a single isomer or a mixture of two isomers.

[General Formula (2)]

[0010] In the formula, R4 and R₅ are a hydrogen atom, an aliphatic organic group, or an aromatic organic group; and one of R and R₅ is an aliphatic organic group or an aromatic organic group.

[0011] The aliphatic organic group represented by R₂ and R₃ in General Formula (1) is an organic group having an aliphatic chain, and the aromatic organic group is an organic group having an aromatic ring.

[0012] The aromatic tetracarboxylic acid diester residue represented by Ri in General Formula

(1) is a tetravalent aromatic organic group formed by removing four carboxyl groups from an aromatic tetracarboxylic acid.

[0013] In General Formula (1), the aromatic tetracarboxylic acid diester residue represented by

Ri is preferably a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid or a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid.

[0014] In General Formula (1), Ri may be a combination of two or more of a tetravalent aromatic tetracarboxylic acid diester represented by a tetravalent residue of a 1,2,4,5- benzenetetracarboxylic acid, an aromatic tetracarboxylic acid diester represented by a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid, and an aromatic tetracarboxylic acid diester represented by a tetravalent residue of a bis(3,4-carboxyphenyl) ether.

[0015] The aliphatic organic group represented by R or R₅ in General Formula (2) is an organic group having an aliphatic chain, and the aromatic organic group is an organic group having an aromatic ring.

[0016] In the varnish, 2-phenyl-4,4'-diaminodiphenyl ether and two or more of divalent aromatic diamines may be used in combination.

[0017] The present invention also provides a solid imide resin composition represented by

General Formula (3). The solid imide resin composition is produced by heating the varnish to remove the organic solvent.

[General Formula (3)]

[0018] In the formula, R₆ and R₇ are each a hydrogen atom or a phenyl group; one of R₆ and R₇ is a phenyl group; R₈ and R₉ are the same or different and are a divalent aromatic diamine residue; R₁₀ and Rn are the same or different and are a tetravalent aromatic tetracarboxylic acid residue; m and n satisfy relations of m > 1, n > 0, 1 < m + n < 10, and 0.05 < m/(m + n) < 1; and repeating units are optionally arranged in a block sequence or a random sequence. [0019] The present invention further provides a molded article of a polymerized imide resin composition, in which the polymerized imide resin composition is obtained by heating the solid imide resin composition in a molten state.

[0020] In the molded article, the imide resin composition has a glass transition temperature (Tg) of 300°C or more, more preferably 330°C or more, and even more preferably 350°C or more.

[0021] The present invention also provides a film obtained from the molded article of the imide resin composition. The film preferably has a tensile elongation at break of 10% or more, more preferably 15% or more, and even more preferably 20% or more.

[0022] The present invention further provides a prepreg including the varnish and fibers into which the varnish is impregnated. The present invention provides both a wet prepreg that contains a solvent and a dry prepreg from which a solvent is substantially completely removed.

[0023] The present invention also provides an imide prepreg obtained by further heating the prepreg. The present invention provides both a semidried imide wet prepreg that partially contains a solvent and an imide dry prepreg from which a solvent is substantially completely removed.

[0024] The present invention also provides a fiber-reinforced composite material obtained by stacking the prepregs, the imide prepregs, or a combination of the prepregs and the imide prepregs and thermally curing the stacked prepregs. The fiber-reinforced composite material preferably has a Tg of 300°C or more and more preferably 330°C or more.

[0025] The present invention also provides a fiber-reinforced composite material showing no generation of microcracks inside and maintaining greater than about 70 %, preferably about more than 80 %, and even more preferably 90 % initial interlaminar shear strength (ILSS) or short beam shear (SBS) strength as measured at room temperature (25 °C), and about 200 to about 250 °C, preferably about 232 °C after thermal cycling in the temperature range between about -60 and about 250 °C, preferably between about -54 and 232 °C, preferably for 500 cycles or more.

[0026] The present invention further provides a method of producing the varnish. The method includes heating an aromatic tetracarboxylic anhydride and 4-(2-phenylethynyl)phthalic anhydride in a state the aromatic tetracarboxylic anhydride and the 4-(2-phenylethynyl)phthalic anhydride are dissolved in an organic solvent having a boiling point of 150°C or less at 1 atmosphere, to prepare an aromatic tetracarboxylic acid diester represented by General Formula (1); preparing a solution of an organic solvent having a boiling point of 150°C or less at 1 atmosphere, which contains the aromatic tetracarboxylic acid diester represented by General Formula (1) and a 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2); adding a diamine including 2 -phenyl-4, 4'- diaminodiphenyl ether to the solution; and uniformly dissolving, in the solution, the diamine including 2- phenyl-4,4'-diaminodiphenyl ether.

ADVANTAGEOUS EFFECTS OF INVENTION

[0027] The present invention can provide the varnish having excellent solubility at a solid content of more than 65 wt % and long-term storage stability because of the effect of 2 -phenyl-4, 4'- diaminodiphenyl ether which has an asymmetric chemical structure. [0028] The present invention can also provide the solid imide resin composition having excellent melt flowability at high temperature and molding processability, by heating the varnish to give a particular terminally modified imide oligomer component having 2-phenyl-4,4'-diaminodiphenyl ether.

[0029] The present invention can provide the imide resin molded article having both high heat resistance and excellent breaking elongation, by further heating the solid imide resin composition to polymerize the terminally modified imide oligomer component.

[0030] The present invention can provide the imide prepreg having excellent preservability and handling properties and achieving excellent adhesion properties between prepregs, by infiltrating the varnish into fibers.

[0031] The prepreg or the imide prepreg of the present invention can readily yield a high quality fiber-reinforced composite material having excellent heat resistance and mechanical characteristics and containing no large voids in the material because the organic solvent having a low boiling point used in the varnish is readily removed from a composite material prepared by stacking the prepregs or the imide prepregs, in a step of thermoforming the composite material.

[0032] The fiber-reinforced composite material shows an excellent thermal cycle resistance because of both high heat resistance and excellent breaking elongation of the molded imide resin involved inside.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Fig. 1A is a cross sectional microscope observation of a prior art fabric composite prior to thermal cycle exposure.

[0034] Fig. IB is a cross sectional microscope observation of a prior art fabric composite after

2000 thermal cycles.

[0035] Fig. 2 is a plot of interlaminar shear strength (IFSS) strength of a prior art fabric composite as a function of thermal cycling.

[0036] Fig. 3 is a graph representing the short beam strength (SBS) strength of fiber composites after thermal cycling exposure in accordance with embodiments of Example 13.

[0037] Figs. 4A-4B are a cross sectional microscope observation of fiber composites before and after thermal cycle exposure in accordance with embodiments of Example 13.

DESCRIPTION OF EMBODIMENTS

Varnish

[0038] A varnish of the present invention is characterized by including the following components (A) to (D), and the components (A), (B), and (C) are dissolved in the varnish at a solid content of more than 65 %.

[0039] The component (A) is an aromatic tetracarboxylic acid diester represented by General

Formula (1) and is contained in an amount of 1 to 500 parts by weight;

the component (B) is 2-phenyl-4,4'-diaminodiphenyl ether and is contained in an amount of 1 to 450 parts by weight; the component (C) is a 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2) and is contained in an amount of 1 to 400 parts by weight; and

[General Formula (1)]

[0040] In the formula, Ri is an aromatic tetracarboxylic acid diester residue; R₂ and R₃ are the same or different and are an aliphatic organic group or an aromatic organic group; R₂ and R₃ are located in a cis configuration or a trans configuration; and the compound is optionally a single isomer or a mixture of two isomers.

[General Formula (2)]

[0041] In the formula, R₄ and R₅ are a hydrogen atom, an aliphatic organic group, or an aromatic organic group; and one of R₄ and R₅ is an aliphatic organic group or an aromatic organic group Component (Aj

[0042] As a component of the varnish of the present invention, the aromatic tetracarboxylic acid diester represented by General Formula (1) is used. The aromatic tetracarboxylic acid diester represented by General Formula (1) is a component that reacts with the components (B) and (C) to form a part of the skeleton of the terminally modified imide oligomer represented by General Formula (3).

[0043] The aromatic tetracarboxylic acid constituting the aromatic tetracarboxylic acid diester residue represented by Rl in General Formula (1) is exemplified by tetravalent residues of 1, 2,4,5- benzenetetracarboxylic acids, tetravalent residues of 3,3',4,4'-biphenyltetracarboxylic acids, and tetravalent residues of bis(3,4-carboxyphenyl) ethers.

[0044] In the aromatic tetracarboxylic acid diesters included in the varnish, specifically, the aromatic tetracarboxylic acid diester residue represented by Ri in General Formula (1) is preferably a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid, a tetravalent residue of a 3, 3', 4,4'- biphenyltetracarboxylic acid, or a tetravalent residue of a bis(3,4-carboxyphenyl) ether, and more preferably a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid or a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid because a molded article of an imide resin composition can achieve a high glass transition temperature (Tg), long-term thermal stability, and anti-oxidation stability at high temperature.

[0045] In the aromatic tetracarboxylic acid diesters included in the varnish, in addition to the above, preferred examples of the combination of tetravalent aromatic tetracarboxylic acids include a combination partially containing a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid and containing a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid as the remainder; a combination partially containing a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid and containing a tetravalent residue of a bis(3,4-carboxyphenyl) ether as the remainder; and a combination partially containing a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid and containing a tetravalent residue of a bis(3,4-carboxyphenyl) ether as the remainder.

[0046] The aliphatic organic group or the aromatic organic group represented by R₂ and R₃ in

General Formula (1) is preferably an organic group having 1 to 12 carbon atoms, more preferably an organic group having 1 to 9 carbon atoms, and even more preferably an organic group having 1 to 6 carbon atoms because an alcohol component that is generated and eliminated by amic acid formation reaction with a diamine by heat preferably has a low boiling point in order to be immediately volatilized and removed during production of the imide resin composition or molding of the composite material.

[0047] The aromatic tetracarboxylic acid diester represented by General Formula (1) is basically a 1,2,4,5-benzenetetracarboxylic acid diester, a 3,3',4,4'-biphenyltetracarboxylic acid diester, a combination of them, a 1,2,4,5-benzenetetracarboxylic acid diester that is partially replaced with a diester of bis(3,4-carboxyphenyl) ether, or a 3,3',4,4'-biphenyltetracarboxylic acid diester that is partially replaced with a diester of bis(3,4-carboxyphenyl) ether. A 1,2,4,5-benzenetetracarboxylic acid diester, a 3,3',4,4'-biphenyltetracarboxylic acid diester, or a diester of bis(3,4-carboxyphenyl) ether may be partially replaced with an additional aromatic tetracarboxylic acid as long as the advantageous effects of the invention are achieved.

[0048] Examples of the additional aromatic tetracarboxylic acid include 3, 3', 4,4'- benzophenonetetracarboxylic dianhydride (BTDA), 2,3,3',4'-biphenyltetracarboxylic dianhydride (a- BPDA), 2,2',3,3'-biphenyltetracarboxylic dianhydride (i-BPDA), 2,2-bis(3,4-dicarboxyphenyl)methane dianhydride, bis(3,4-carboxyphenyl) ether dianhydride, and 1,2,3,4-benzenetetracarboxylic dianhydride. The additional aromatic tetracarboxylic acids may be used singly or in combination of two or more of them.

[0049] The aromatic tetracarboxylic acid diester represented by Formula (1) includes 1,2,4,5- benzenetetracarboxylic acid dimethyl ester, 1,2,4,5-benzenetetracarboxylic acid diethyl ester, 1,2,4,5- benzenetetracarboxylic acid dipropyl ester, 1,2,4,5-benzenetetracarboxylic acid diisopropyl ester, 1,2,4,5- benzenetetracarboxylic acid dibutyl ester, and other isomers of these compounds in terms of two ester groups, but is not necessarily limited to them. Two ester groups are not necessarily the same.

[0050] Among them, 1,2,4,5-benzenetetracarboxylic acid dimethyl ester and 1,2,4,5- benzenetetracarboxylic acid diethyl ester are preferred because a resin after thermal curing can achieve a high glass transition temperature. Component (BZ

[0051] As a component of the varnish of the present invention, 2-phenyl-4,4'-diaminodiphenyl ether is used. The use of the component allows the terminally modified imide oligomer represented by General Formula (3) to have the skeleton derived from the 2-phenyl-4,4'-diaminodiphenyl ether in the molecule. In the present invention, the 2-phenyl-4,4'-diaminodiphenyl ether may be partially replaced with an additional aromatic diamine.

[0052] Examples of the additional aromatic diamine include 1,4-diaminobenzene, 1,3- diaminobenzene, 1,2-diaminobenzene, 2, 6-diethyl- 1,3-diaminobenzene, 4,6-diethyl-2-methyl-l,3- diaminobenzene, 3, 5-diethyltoluene-2, 6-diamine, 4,4'-diaminodiphenyl ether (4,4'-ODA), 3,4'- diaminodiphenyl ether (3,4'-ODA), 3,3'-diaminodiphenyl ether, 3,3'-diaminobenzophenone, 4,4'- diaminobenzophenone, 3,3'-diaminodiphenylmethane, 4,4'-diaminodiphenylmethane, bis(2,6-diethyl-4- aminophenyl)methane, 4,4'-methylene-bis(2,6-diethylaniline), bis(2-ethyl-6-methyl-4- aminophenyl)methane, 4,4'-methylene-bis(2-ethyl-6-methylaniline), 2,2-bis(3-aminophenyl)propane, 2,2- bis(4-aminophenyl)propane, l,3-bis(3-aminophenoxy)benzene, l,4-bis(4-aminophenoxy /benzene, 1,4- bis(3-aminophenoxy)benzene, benzidine, 3,3'-dimethylbenzidine, 2,2-bis(4-aminophenoxy)propane, 2,2- bis(3-aminophenoxy /propane, 2,2-bis[4'-(4"-aminophenoxy/phenyl]hexafluoropropane, 9,9-bis(4- aminophenyl/fluorene, and 9,9-bis(4-(4-aminophenoxy/phenyl/fluorene. These diamines may be used singly or in combination of two or more of them. The aromatic diamine compound is particularly preferably 9,9-bis(4-aminophenyl/fluorene, 9,9-bis(4-(4-aminophenoxy/phenyl/fluorene, or 1,3- diaminobenzene. For applications requiring higher heat resistance and mechanical strength, the aromatic diamine compounds are preferably copolymerized, and the copolymer is used in an amount of 0 to 50% by mole, preferably 0 to 25% by mole, and more preferably 0 to 10% by mole relative to the total amount of diamines. The diamine for copolymerization is particularly preferably 9,9-bis(4-aminophenyl/fluorene. This allows an imide oligomer obtained by heating to exhibit excellent moldability and allows a cured product after thermal curing to exhibit high heat resistance and excellent mechanical characteristics, achieving excellent effects. Needless to say, a copolymer is not necessarily used in the present invention depending on an application.

Component (CZ

[0053] As a component of the varnish of the present invention, the 4-(2-phenylethynyl/phthalic acid monoester represented by General Formula (2/ is used. The 4-(2-phenylethynyl/phthalic acid monoester represented by General Formula (2/ is a component that reacts with the components (A) and (B) to form a part of the skeleton of the imide resin composition represented by General Formula (3/ described later.

[0054] The aliphatic organic group or the aromatic organic group represented by R4 or R5 in

General Formula (2/ is preferably an organic group having 1 to 12 carbon atoms, more preferably an organic group having 1 to 9 carbon atoms, and even more preferably an organic group having 1 to 6 carbon atoms because an alcohol component that is generated and eliminated by amic acid formation reaction with a diamine by heat preferably has a low boiling point in order to be immediately volatilized and removed during production of the imide resin composition or molding of the composite material.

[0055] Examples of the 4-(2-phenylethynyl)phthalic acid monoester represented by General

Formula (2) include, but are not necessarily limited to, 4-(2-phenylethynyl)phthalic acid monoethyl ester, 4-(2-phenylethynyl)phthalic acid monomethyl ester, 4-(2-phenylethynyl)phthalic acid monopropyl ester, 4-(2-phenylethynyl)phthalic acid monoisopropyl ester, and 4-(2-phenylethynyl)phthalic acid monobutyl ester.

Component (D)

[0056] The organic solvent used for the preparation of the varnish is a solvent having a boiling point of 150°C or less at 1 atmosphere and is preferably a solvent having a boiling point of 100°C or less in order to be immediately volatilized and removed during synthesis of the imide oligomer by heat. In some embodiments, the boiling point is 90°C or less. In some embodiments, the boiling point is 80°C or less. In some embodiments, the boiling point is 70°C or less.

[0057] In some embodiments, the organic solvent include methanol having a boiling point of about 65°C, ethanol having a boiling point of about 78°C, 2-propanol having a boiling point of about 82°C, and 1 -propanol having a boiling point of about 97°C. These organic solvents may be used singly or as a mixture of two or more of them.

Amount

[0058] In order to produce a prepreg in which the varnish is sufficiently impregnated into monofilaments of fibers and to produce a fiber-reinforced composite material exhibiting higher heat resistance, the amount of the aromatic tetracarboxylic acid diester represented by General Formula (1) included in the varnish is 1 to 500 parts by weight, preferably 20 to 280 parts by weight, and more preferably 40 to 200 parts by weight relative to 100 parts by weight of the organic solvent.

[0059] From the same viewpoint as the above, the amount of the 2-phenyl-4,4'-diaminodiphenyl ether is 1 to 450 parts by weight, preferably 40 to 400 parts by weight, and more preferably 40 to 280 parts by weight relative to 100 parts by weight of the organic solvent.

[0060] From the same viewpoint as the above, the amount of the 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2) is 1 to 400 parts by weight, preferably 5 to 100 parts by weight, and more preferably 10 to 80 parts by weight relative to 100 parts by weight of the organic solvent.

Production method

[0061] In the varnish, each of the aromatic tetracarboxylic acid diester represented by General

Formula (1), the 2-phenyl-4,4'-diaminodiphenyl ether, and the 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2) is in a dissolved state in the organic solvent. Here, the dissolved state means the condition in which each component is substantially uniformly dissolved in an organic solvent to such an extent that each component is not visually observed and the components are present without reacting with each other. The condition in which each component is present can be observed by the method described in Example 1. [0062] The varnish of the present invention can be obtained by mixing the aromatic tetracarboxylic acid diester represented by General Formula (1), the aromatic diamine including 2- phenyl-4,4'-diaminodiphenyl ether, and the 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2) in the organic solvent in such a manner that the total amount of ester groups is substantially the same as the total amount of primary amino groups while the amount of each component is adjusted within the above parts by weight.

[0063] In particular, in order to form a sufficient amount of the imide resin composition during a molding step and to produce a prepreg in which the imide resin composition is in close contact with fibers for the production of a fiber-reinforced composite material exhibiting excellent mechanical strength, the aromatic tetracarboxylic acid diester represented by General Formula (1), the aromatic diamine including 2-phenyl-4,4'-diaminodiphenyl ether, and the 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2) are preferably uniformly dissolved in the organic solvent in the varnish in a total solid content concentration of greater than 65% by weight or more at 25°C, and more preferably 70 % by weight or more, and even more preferably 80 % by weight or more. In some embodiments, the total solid content concentration is between 70% and 95% by weight. In some embodiments, the total solid concentration is between 70% and 90% by weight. In some embodiments, the total solid concentration is between 70% and 85% by weight. In some embodiments, the total solid concentration is between 70% and 80% by weight. In some embodiments, the total solid concentration is between 75% and 90% by weight. In some embodiments, the total solid concentration is between 75% and 85% by weight. In some embodiments, the total solid concentration is between 75% and 80% by weight.

Examples of the production method

[0064] For example, the varnish of the present invention can be produced by using the components in such a manner that the total molar amount of ester groups of one or two or more of the aromatic tetracarboxylic acid diester compounds and the 4-(2-phenylethynyl)phthalic acid monoester is substantially the same as the total molar amount of amino groups of the aromatic diamine including 2- phenyl-4,4'-diaminodiphenyl ether, and stirring the components in the organic solvent preferably at a temperature of 75 °C or more and particularly preferably 85 °C or more to uniformly dissolve the components.

[0065] The varnish of the present invention can also be obtained through successive steps including a step of using an dianhydride of an aromatic tetracarboxylic acid or anhydride of 4-(2- phenylethynyl )phthalic acid or the mixtures as the starting materials and carrying out esterification using an alcohol as the reaction solution. For example, one or two or more of the aromatic tetracarboxylic anhydrides or 4-(2-phenylethynyl)phthalic anhydride or the mixture are heated, refluxed, and stirred in an alcohol solvent at a temperature of 100°C or less, particularly 80°C or less, for 3 hours or more, particularly preferably 6 hours and more and the fully esterified aromatic tetracarboxylic acid diester represented by General Formula (1) or fully esterified 4-(2-phenylethynyl)phthalic acid monoester by General Formula (2) or the mixture are synthesized. If needed, the solvent is removed, and the aromatic tetracarboxylic acid diester represented by General Formula (1) or 4-(2-phenylethynyl)phthalic acid monoester by General Formula (2) or the mixture are isolated. The aromatic diamines including 2- phenyl-4,4'-diaminodiphenyl ether are used in such a manner that the total molar amount of ester groups of all the components is substantially the same as the total molar amount of amino groups, and the components are stirred in an organic solvent preferably at a temperature of 90 °C or less, particularly preferably 80 °C or less, to be uniformly dissolved, yielding the varnish.

[0066] The varnish prepared as above may be concentrated by partially volatilizing the organic solvent used or may be diluted by freshly adding the organic solvent, if the solid content concentration is required to be adjusted. Alternatively, by completely volatilizing the organic solvent used, a solid material composition of the varnish in which the components are uniformly mixed may be isolated. The isolated material composition can be dissolved in an organic solvent to give the varnish once again, as needed. The varnish or the solid material composition undergoes no reaction of forming a terminally modified amic acid oligomer“having an amide-acid bond” (also called amic acid oligomer) when stored at room temperature or a temperature equal to or lower than the room temperature, and can be stably stored for a long period of time.

[0067] A preferred method for producing the varnish of the present invention is exemplified by a method including a step of synthesizing an aromatic tetracarboxylic acid diester (A) represented by General Formula (1) and a 4-(2-phenylethynyl)phthalic acid monoester (C) represented by General Formula (2) and a step of preparing a varnish by adding an aromatic diamine (B) including 2 -phenyl-4, 4'- diaminodiphenyl ether.

[0068] First, in the step of synthesizing an aromatic tetracarboxylic acid diester (A) represented by General Formula (1) and a 4-(2-phenylethynyl)phthalic acid monoester (C) represented by General Formula (2), the aromatic tetracarboxylic anhydride and 4-(2-phenylethynyl)phthalic anhydride are added to an organic solvent having a boiling point of 150°C or less at 1 atmosphere, a solution or a suspension in which the respective added components are uniformly dissolved is heated at a reaction temperature of about 30 to 100°C and stirred for about 1 to 360 minutes, and then is cooled to give a solution in which all the aromatic tetracarboxylic acid diester (A) represented by General Formula (1) and the 4-(2- phenylethynyl)phthalic acid monoester (C) represented by General Formula (2) are uniformly dissolved in the organic solvent or a solution in a suspension state in which the aromatic tetracarboxylic acid diester (A) represented by General Formula (1) and the 4-(2-phenylethynyl)phthalic acid monoester (C) represented by General Formula (2) are partially dissolved in the organic solvent. At the time, the organic solvent can be partially or completely volatilized to concentrate the solution, if needed.

[0069] In the step of preparing a varnish, the aromatic diamine (B) including 2 -phenyl-4, 4'- diaminodiphenyl ether is added to the solution in the alcohol solution above, and the resulting solution in the alcohol solvent is stirred at a reaction temperature of about 70 to 85°C for about 30 to 360 minutes, yielding a solution (varnish) in the alcohol solvent in which all the components are uniformly dissolved.

[0070] The varnish of the present invention may have any solution viscosity as long as the advantageous effects of the invention are achieved, but the solution viscosity is preferably 500 centi poise or more, more preferably 1000 centipoise or more, and even more preferably 3,000 centi poise or more, at 25°C. The solution viscosity is determined by the method described in examples.

Imide resin composition

[0071] The varnish is heated to react the component (A), the component (B), and the component

(C) with each other, giving a terminally modified amic acid oligomer. Next, the amic acid oligomer is dehydrated and cyclized, yielding an imide resin composition containing the terminally modified imide oligomer represented by General Formula (3) having a 4-(2-phenylethynyl)phthalic acid residue at terminals.

[0072] The dehydration and cyclization method of the amic acid oligomer is exemplified by a method of adding an imidizing agent at a temperature of about 0 to 140°C and a method of heating the oligomer at a temperature of 140 to 275°C.

[0073] In the obtained terminal-modified imide oligomer, it is desirable that the thermally reactive substituent at each terminal do not cause polymerization reaction. The obtained imide resin composition may be in a varnish form in which the composition is dissolved in an organic solvent, a semidried paste form, or a completely dried solid form. In particular, the completely dried solid form can have excellent melt flowability at high temperature and excellent molding processability.

[General Formula (3)]

[0074] In the formula, R₆ and R₇ are a hydrogen atom or a phenyl group; one of R₆ and R₇ is a phenyl group; R₈ and R₉ are the same or different and are a divalent aromatic diamine residue; Ri₀ and Ri i are the same or different and are a tetravalent aromatic tetracarboxylic acid residue; m and n satisfy relations of m > 1, n > 0, 1 < m + n < 10, and 0.05 < m/(m + n) < 1; and repeating units are optionally arranged in a block sequence or a random sequence.

[0075] The aromatic diamine residue represented by R₈ and R₉ in General Formula (3) means a divalent aromatic organic group formed by removing two amino groups from an aromatic diamine. The aromatic tetracarboxylic acid residue means a tetravalent aromatic organic group formed by removing four carboxyl groups from an aromatic tetracarboxylic acid. Here, the aromatic organic group is an organic group having an aromatic ring. The aromatic organic group is preferably an organic group having 4 to 40 carbon atoms, more preferably an organic group having 4 to 30 carbon atoms, and even more preferably an organic group having 4 to 20 carbon atoms.

[0076] The aromatic tetracarboxylic acid constituting the tetravalent aromatic tetracarboxylic acid residue represented by Ri₀ and Rn in General Formula (3) is preferably a 1, 2,4,5- benzenetetracarboxylic acid, a 3,3',4,4'-biphenyltetracarboxylic acid, or a bis(3,4-carboxyphenyl) ether, and specifically preferably 1,2,4,5-benzenetetracarboxylic dianhydride and 3, 3', 4,4'- biphenyltetracarboxylic dianhydride.

[0077] In the terminally modified imide oligomer in the imide resin composition of the present invention, it is preferable that some of the“m” pieces of R_j0 and the“n” pieces of Rn in General Formula (3) be a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid, and the remainder of them be a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid.

[0078] In General Formula (3), if m + n is less than 1, a cured resin may have extremely lower toughness, whereas if m + n is more than 10, a composition may not exhibit excellent melt flowability at high temperature conditions. In General Formula (3) and General Formulae (4) and (5), m/(m + n) is preferably 0.1 or more and 1 or less.

[0079] In General Formula (3) in the imide resin composition of the present invention, it is preferable that the tetravalent aromatic tetracarboxylic acid residue represented by Ri₀ and Rn be a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid, and the terminally modified imide oligomer be the compound represented by General Formula (4).

[General Formula (4)]

[0080] In the formula (4), R₆ and R₇ are a hydrogen atom or a phenyl group; one of R₆ and R₇ is a phenyl group; R₈ and R₉ are the same or different and are a divalent aromatic diamine residue; m and n satisfy relations of m > 1, n > 0, 1 < m + n < 10, and 0.05 < m/(m + n) < 1; and repeating units are optionally arranged in a block sequence or a random sequence.

[0081] In General Formula (3) in the imide resin composition of the present invention, it is preferable that the tetravalent aromatic tetracarboxylic acid residue represented by Ri₀ and Rn be a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid, and the terminally modified imide oligomer be the compound represented by General Formula (5).

[General Formula (5)]

[0082] In the formula (5), R₆ and R₇ are a hydrogen atom or a phenyl group; one of R₆ and R₇ is a phenyl group; R₈ and R₉ are the same or different and are a divalent aromatic diamine residue; m and n satisfy relations of m > 1, n > 0, 1 < m + n < 10, and 0.05 < m/(m + n) < 1; and repeating units are optionally arranged in a block sequence or a random sequence.

[0083] To produce the terminally modified imide oligomer, the varnish is stirred and reacted at a reaction temperature of 30 to 150°C for about 1 to 180 minutes to give the terminally modified amic acid oligomer. The reaction solution is then further stirred at 140 to 275°C for 5 minutes to 24 hours to give the terminally modified imide oligomer and to remove the organic solvent in the reaction solution. If necessary, the reaction solution is cooled to around room temperature, and the terminally modified imide oligomer is crystallized. The crystal is subjected to solid-liquid separation by filtration, for example, giving a solid imide resin composition.

[0084] The solid imide resin composition preferably has a minimum melt viscosity of 10,000

Pa· sec or less, more preferably 5,000 Pa· sec or less, and even more preferably 3,000 Pa· sec or less in order that when the organic solvent in prepregs is removed out of the system under high temperature conditions in a step of molding a fiber-reinforced composite material, the remaining imide oligomer is melted and infiltrated into fibers. The minimum melt viscosity is determined by the method described later.

[0085] The terminally modified imide oligomer included in the imide resin composition of the present invention has substantially no possibility of undergoing hydrolysis, thus causing no viscosity reduction or other deteriorations as compared with amic acid oligomers, and can be stably stored for a long period of time without additives.

[0086] The terminally modified imide oligomer may be mixed with other oligomers having different molecular weights or with thermoplastic polyimides.

[0087] The thermoplastic polyimide is a polyimide that becomes soft by heat, and specifically may be any commercial product without any limitation in terms of type and the like.

Molded article

[0088] The solid imide resin composition can be further heated in a molten state to give a molded article of an imide resin composition, having a higher molecular weight.

[0089] For example, the molded article can be produced by melting the solid imide resin composition at a temperature of 200 to 280°C and thermally curing the molten composition at 280 to 500°C for about 10 minutes to 40 hours. The molded article can also be produced by a single step of heating a varnish applied onto a support at 280 to 500°C for about 10 minutes to 40 hours.

[0090] The molded article preferably has a Tg of 300°C or more, more preferably 330°C or more, and even more preferably 350°C or more, for example, when used as high temperature members around the engines of aircraft.

[0091] Polymerization of the imide resin composition can be observed by the method described in examples. The degree of high molecular weight is not limited specifically.

[0092] The molded article can be molded into a desired shape by a known method. The shape is exemplified by a film shape, a sheet shape, shapes molded into three dimensional shapes such as a rectangular solid shape and a rod-like shape, but is not limited to particular shapes. For example, a molded article molded into a film preferably has a tensile elongation at break of 10% or more, more preferably 15% or more, and even more preferably 20% or more in order to absorb the energy of external impact to reduce damage when the molded article is used as a cured resin molded article or a fiber- reinforced composite material.

[0093] The Tg and the tensile elongation at break are determined by the methods described in examples.

[0094] The molded article of the imide resin composition is preferably colored transparent from the viewpoint of the uniformity of curing reaction and reaction completion.

Prepreg

[0095] The prepreg of the present invention is produced by infiltrating the varnish into fibers.

[0096] The prepreg of the present invention can be obtained as follows, for example.

[0097] For example, the material compositions (A), (B), and (C) are uniformly dissolved at a high concentration of a total amount of greater than 65% by weight or more to give a varnish. If necessary, the varnish is appropriately concentrated or diluted, and then is impregnated into fibers arranged in one direction in a planer shape or a fiber fabric, yielding a wet prepreg. The wet prepreg may be dried by a known method, giving a dry prepreg. The prepreg of the present invention includes the wet prepreg and the dry prepreg.

[0098] In order that a fiber-reinforced composite material produced by using the prepreg exhibits excellent mechanical strength due to the balance between a cured resin and fibers in the fiber- reinforced composite material, the amount of the terminally modified imide oligomer represented by General Formula (3) attached to the fibers is preferably 10 to 60% by weight, more preferably 20 to 50% by weight, and even more preferably 30 to 50% by weight, relative to the total weight of the prepreg.

[0099] In order to easily handle the prepregs at the time of stacking and to prevent the resin from flowing out in a step of molding a composite material at a high temperature to produce a fiber- reinforced composite material exhibiting excellent mechanical strength, the amount of the organic solvent attached to fibers is preferably 1 to 30% by weight, more preferably 5 to 25% by weight, and even more preferably 5 to 20% by weight relative to the total weight of the prepreg.

[0100] Examples of the fibers used in the present invention include inorganic fibers such as carbon fibers, glass fibers, metal fibers, and ceramic fibers; and synthetic organic fibers such as polyamide fibers, polyester fibers, polyolefin fibers, and novoloid fibers. These fibers may be used as a single type or a combination of two or more types. In particular, in order to achieve excellent mechanical characteristics, carbon fibers are desirable. If having a carbon content ranging from 85 to 100% by weight and having a continuous fiber form that at least partially has a graphite structure, any type of carbon fibers can be used without any limitation, and examples of the carbon fibers include polyacrylonitrile (PAN)-based carbon fibers, rayon-based carbon fibers, lignin-based carbon fibers, and pitch-based carbon fibers. Among them, carbon fibers such as PAN-based carbon fibers and pitch-based carbon fibers are preferred because they are generally used and inexpensive and have high strength. Typically, the carbon fibers have been subjected to sizing treatment. Such fibers may be used without any treatment or may be washed with an organic solvent or the like to remove the sizing agent, as necessary. It is preferable that fiber bundles be opened with air, rollers, or other means in advance and the resin or a resin solution be impregnated between single yams of the carbon fibers.

[0101] Imide prepreg

[0102] The imide prepreg of the present invention is produced by further heating the prepreg.

[0103] The imide prepreg of the present invention can be obtained as follows, for example.

[0104] A solution of the wet prepreg or the dry prepreg in an organic solvent is heated at 140 to

275°C for 5 minutes to 24 hours to partially or completely remove the organic solvent, yielding an imide wet prepreg or an imide dry prepreg in which the terminally modified imide oligomer is attached to the fibers.

[0105] The amount of the terminally modified imide oligomer attached to the fibers in the imide wet prepreg is preferably 5 to 50% by weight, more preferably 20 to 50% by weight, and even more preferably 30 to 50% by weight relative to the total weight of the prepreg. The amount of the organic solvent attached to the fibers is preferably 1 to 30% by weight, more preferably 5 to 25% by weight, and even more preferably 5 to 20% by weight relative to the total weight of the prepreg.

[0106] The amount of the terminally modified imide oligomer attached to the fibers in the imide dry prepreg is preferably 20 to 80% by weight, more preferably 20 to 60% by weight, and even more preferably 30 to 50% by weight relative to the total weight of the prepreg.

[0107] The fibers used in the imide prepreg of the present invention may be the same as the fibers used in the above described prepreg. The fiber material constituting the imide prepreg has a structure of a continuous fiber form such as UD (unidirectional) forms, weave forms (plain weave, satin weave, for example), and knit forms, and is not limited to particular forms. The form can be appropriately selected depending on the purpose. The forms may be used singly or in combination of two or more of them.

Fiber-reinforced composite material

[0108] The fiber-reinforced composite material of the present invention can be obtained as follows, for example.

[0109] A predetermined number of the prepregs are stacked and thermally cured with an autoclave, a hot press, or a similar apparatus at a temperature of 80 to 500°C at a pressure of 1 to 1,000 kg/cm² for about 10 minutes to 40 hours, giving a fiber-reinforced composite material. In the present invention, in addition to the use of the prepreg, the imide wet prepregs or the imide dry prepregs may be stacked and thermally cured in the same manner as the above, giving a fiber-reinforced composite material.

[0110] The fiber-reinforced composite material of the present invention obtained as above preferably has a glass transition temperature (Tg) of 300°C or more. The measurement is in accordance with the method described later.

[0111] The present invention also provides a fiber-reinforced composite material showing no generation of microcracks inside and maintain greater than about 70%, preferably about more than 80%, and even more preferably more than 90% initial interlaminar shear strength (ILSS) or short beam shear (SBS) strength measured at room temperature, and about 200 °C to about 250 °C, preferably about 232 °C after thermal cycling in the temperature range between about -60 °C and about 250°C, preferably between about -54 °C and about 232 °C preferably for 500 cycles or more, and more preferably 1000 cycles or more and even more preferably 2000 cycles or more.

[0112] The film-like molded article of the imide resin composition or the imide prepreg may be inserted between a fiber-reinforced composite material and a different material, and the whole may be heated and melted to be integrated, giving a fiber-reinforced composite material structure. Here, the different material is not limited to particular materials and may be any material commonly used in the field. Examples of the material include honeycomb metal materials and sponge-like core materials. EXAMPLES AND COMPARATIVE EXAMPLES

[0113] Some examples will next be described in order to explain the present invention, but are not intended to limit the present invention. The characteristics were determined in the following conditions.

Test methods

Measurement of glass transition temperature (Tg)

[0114] A differential scanning calorimeter (DSC, model: DSC-2010, manufactured by TA

Instruments) was used for measurement under a nitrogen stream at a temperature increase rate of 5°C/min. For film-like products, a dynamic viscoelasticity analyzer (DMA, model: RSA-II, manufactured by Rheometric) was used for measurement at a temperature increase rate of 10°C/min at a frequency of 1 Hz. The intersection of two tangent lines before and after the drop of a storage elastic modulus curve was regarded as the glass transition temperature. For fiber-reinforced composite materials, a dynamic viscoelasticity analyzer (DMA, model: DMA-Q-800, manufactured by TA Instruments) was used for measurement in a cantilever manner at a strain of 0.1% at a frequency of 1 Hz under a nitrogen stream at a temperature increase rate of 3°C/min. The intersection of two tangent lines before and after the drop of a storage elastic modulus curve was regarded as the glass transition temperature.

Measurement of minimum melt viscosity

[0115] A rheometer (model: AR2000, manufactured by TA Instruments) was used for measurement with a 25-mm parallel plate at a temperature increase rate of 4°C/min.

Measurement of 5% weight loss temperature

[0116] A thermogravimetric analyzer (TGA, model: SDT-2960, manufactured by TA

Instruments) was used for measurement under a nitrogen stream at a temperature increase rate of 5°C/min.

Elastic modulus measurement, breaking strength measurement, breaking elongation measurement

[0117] A tensilon versatile testing machine (trade name: TENSIFON/UTM-II-20, manufactured by ORIENTEC Co., Ftd.) was used for measurement at room temperature at a tensile speed of 3 mm/min. The test pieces had a film-like shape having a length of 20 mm, a width of 3 mm, and a thickness of 80 to 120 mhi. Measurement of infrared absorption spectrum

[0118] A FT/IR-230S spectrometer manufactured by JASCO Corporation was used for infrared absorption spectrum measurement at room temperature in a measurement range of 400 cm ¹ to 4,000 cm ¹ at an accumulation number of 32.

Measurement of solution viscosity

[0119] An E-type viscometer, model R550 manufactured by Toki Sangyo Co., Ltd., was used for measurement at 23°C.

Ultrasonic defect test

[0120] An ultrasonic defect tester, model SDS7800R manufactured by KRAUTKRAMER, was used for measurement with a 5- to 15-MHz testing probe in water.

Optical microscope observation

[0121] A measuring microscope, STM-MJS manufactured by Olympus Corporation, was used for measurement at a magnification of 50 to 500.

Measurement of interlaminar shear strength

[0122] Measurement was carried out in accordance with ASTM-D2344.

Thermal cycling test

[0123] Composite specimens are placed in a thermal cycling chamber. The composites were thermally cycled between -54 °C and 232 °C with a 15-minute hold at 232 °C and a 10-minute hold at - 54°C. Up to 2000 thermal cycles were performed. Samples were taken out every 400 cycles for microcrack inspection using optical microscope and interlaminar shear strength test.

Example 1

[0124] Into a 2,000-mL four-necked flask equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, 396.98 g (1820 mmol) of 1,2,4,5-tetracarboxylic dianhydride and 225.90 g (910.0 mmol) of 4-(2-phenylethynyl)phthalic anhydride were placed, and 836.38 g (18154 mmol) of ethanol was added. Under a nitrogen stream, stirring of the resulting suspension was started, while the suspension was heated and refluxed in the temperature range between 75 and 80 ° C. After the start of stirring, acid anhydrides were observed to be gradually dissolved in the solvent, and at about 60 minutes after the start of stirring, the compounds were completely dissolved. When the solution was further continuously stirred, some insoluble precipitate was observed in the solvent. At 360 minutes after the start of stirring, to the suspension solution, 628.67 g (2275 mmol) of 2-phenyl-4,4’-diaminodiphenyl ether was added. After the start of stirring in the temperature range between 75 and 80 ° C the start of gradual dissolution of the precipitate in the solvent was observed, and the precipitate was completely uniformly dissolved at 120 minutes after the start of stirring, giving a varnish having a solid content concentration of about 70% by weight, in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent.

[0125] The varnish was placed in a glass petri dish. By heating the varnish in a circulation air oven at an internal temperature of 60° C for 3 hours and further heating the varnish at 250 ° C for 1 hour, amic acid bond formation reaction and imide bond formation reaction were carried out while ethanol was removed, yielding a terminal-modified imide oligomer. The obtained terminally modified imide oligomer was represented by General Formula (4) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ and R₉ were a 2-phenyl-4,4'-diaminodiphenyl ether residue, and m = 4 and n = 0 on average.

[0126] The powdery terminal-modified imide oligomer before curing had a Tg of 210°C from

DSC measurement result and had a minimum melt viscosity of 62 Pa· sec (340°C). Some of the powdery terminal-modified imide oligomer was inserted between two polyimide films (trade name: UPILEX-75S; thickness: 75 mhi; size: 15 cm per side; manufactured by Ube Industries, Ltd.) having excellent surface smoothness. The whole was pressurized at 370°C for 1 hour, then cooled, and released, giving a transparent red-brown film-like cured product (a thickness of 96 mhi). IR spectrum measurement of some of the film-like imide resin composition showed the disappearance of the absorption around 2,210 cm ¹ assigned to the stretching vibration of the triple bond in the phenylethynyl moiety as the terminal group of the terminally modified imide oligomer, and this indicated that the pressurized thermoforming allowed the terminally modified imide oligomer component in the film-like imide resin composition to undergo thermal addition reaction and to be polymerized. The film-like cured product had a Tg of 337°C by DSC measurement, a Tg of 336°C by DMA measurement, and a 5% weight loss temperature of 538°C by TGA. As for the mechanical characteristics by tensile test, the film-like cured product had an elastic modulus of 3.1 GPa, a breaking strength of 143 MPa, and a breaking elongation of 31%.

Example 2

[0127] A varnish having a solid content concentration of about 75 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent was prepared in a same manner with Example 1 other than changing the amount of ethanol initially added to 695.93 g (15106 mmol).

Example 3

[0128] A varnish having a solid content concentration of about 80 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent was prepared in a same manner with Example 1 other than changing the amount of ethanol initially added to 574.36 g (12467 mmol).

Example 4

[0129] A varnish having a solid content concentration of about 85 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent was prepared in a same manner with Example 1 other than changing the amount of ethanol initially added to 467.45 g (10147 mmol).

Example 5

[0130] Into a 2,000-mL four-necked flask equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, 396.98 g (1820 mmol) of 1,2,4,5-tetracarboxylic dianhydride and 225.90 g (910.0 mmol) of 4-(2-phenylethynyl)phthalic anhydride were placed, and 574.36 g (12467 mmol) of ethanol was added. Under a nitrogen stream, stirring of the resulting suspension was started, while the suspension was heated and refluxed in the temperature range between 75 and 80 ° C. After the start of stirring, acid anhydrides were observed to be gradually dissolved in the solvent, and at about 60 minutes after the start of stirring, the compounds were completely dissolved. When the solution was further continuously stirred, some insoluble precipitate was observed in the solvent. At 360 minutes after the start of stirring, to the suspension solution, 565.81 g (2047 mmol) of 2-phenyl-4,4’-diaminodiphenyl ether and 79.27 g (227 mmol) of 9,9-bis(4-aminophenyl)fluorene was added. After the start of stirring in the temperature range between 75 and 80 ° C the start of gradual dissolution of the precipitate in the solvent was observed, and the precipitate was completely uniformly dissolved at 120 minutes after the start of stirring, giving a varnish having a solid content concentration of about 80 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent.

[0131] The varnish was placed in a glass petri dish. By heating the varnish in a circulation air oven at an internal temperature of 60°C for 3 hours and further heating the varnish at 250 °C for 1 hour, amic acid bond formation reaction and imide bond formation reaction were carried out while methanol was removed, yielding a terminally modified imide oligomer. The obtained terminal-modified imide oligomer was represented by General Formula (4) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ was a 2-phenyl-4,4'-diaminodiphenyl ether residue or a 9,9-bis(4-aminophenyl)fluorene residue, R₉ was a 9,9-bis(4-aminophenyl)fluorene residue, and m = 3.6 and n = 0.4 on average.

[0132] The powdery terminal-modified imide oligomer before curing had a Tg of 221°C from

DSC measurement result and had a minimum melt viscosity of 94 Pa· sec (345°C). Some of the powdery terminal-modified imide oligomer was inserted between two polyimide films (trade name: UPILEX-75S; thickness: 75 mhi; size: 15 cm per side; manufactured by Ube Industries, Ltd.) having excellent surface smoothness. The whole was pressurized at 370°C for 1 hour, then cooled, and released, giving a transparent red-brown film-like cured product (a thickness of 80 mhi). IR spectrum measurement of some of the film-like imide resin composition showed the disappearance of the absorption around 2,210 cm ¹ assigned to the stretching vibration of the triple bond in the phenylethynyl moiety as the terminal group of the terminally modified imide oligomer, and this indicated that the pressurized thermoforming allowed the terminally modified imide oligomer component in the film-like imide resin composition to undergo thermal addition reaction and to be polymerized. The film-like cured product had a Tg of 355°C from DSC measurement result, a Tg of 357°C from DMA measurement result, and a 5% weight loss temperature of 537°C by TGA. As for the mechanical characteristics by tensile test, the film-like cured product had an elastic modulus of 3.2 GPa, a breaking strength of 137 MPa, and a breaking elongation of 20%.

[0133] The varnish obtained above was dried under vacuum at room temperature, giving a powdered raw material composition of the terminally modified polyimide resin. The powder was smoothly dissolved in MeOH-d₄ and was subjected to H-NMR measurement in the same manner as in Example 1. The result revealed that all the components included in the raw material composition of the terminally modified polyimide resin formed ion complexes (salts) in the varnish in the methanol solution prepared in the example.

Example 6

[0134] Into a 2,000-mL four-necked flask equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, 396.98 g (1820 mmol) of 1,2,4,5-tetracarboxylic dianhydride and 129.08 g (520.0 mmol) of 4-(2-phenylethynyl)phthalic anhydride were placed, and 745.52 g (16182 mmol) of ethanol was added. Under a nitrogen stream, stirring of the resulting suspension was started, while the suspension was heated and refluxed in the temperature range between 75 and 80 ° C. After the start of stirring, acid anhydrides were observed to be gradually dissolved in the solvent, and at about 60 minutes after the start of stirring, the compounds were completely dissolved. When the solution was further continuously stirred, some insoluble precipitate was observed in the solvent. At 360 minutes after the start of stirring, to the suspension solution, 574.79 g (2080 mmol) of 2-phenyl-4,4’-diaminodiphenyl ether was added. After the start of stirring in the temperature range between 75 and 80 ° C the start of gradual dissolution of the precipitate in the solvent was observed, and the precipitate was completely uniformly dissolved at 120 minutes after the start of stirring, giving a varnish having a solid content concentration of about 70 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent.

[0135] The varnish was placed in a glass petri dish. By heating the varnish in a circulation air oven at an internal temperature of 60°C for 3 hours and further heating the varnish at 250 °C for 1 hour, amic acid bond formation reaction and imide bond formation reaction were carried out while methanol was removed, yielding a terminally modified imide oligomer. The obtained terminal-modified imide oligomer was represented by General Formula (4) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ and R₉ were a 2-phenyl-4,4'-diaminodiphenyl ether residue, and m = 7 and n = 0 on average.

[0136] The varnish was placed in a glass petri dish. By heating the varnish in a circulation air oven at an internal temperature of 60°C for 3 hours and further heating the varnish at 250°C for 1 hour, amic acid bond formation reaction and imide bond formation reaction were carried out while ethanol was removed, yielding a terminal-modified imide oligomer. The obtained terminal-modified imide oligomer was represented by General Formula (4) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ and R₉ were a 2-phenyl-4,4'-diaminodiphenyl ether residue, and m = 7 and n = 0 on average.

[0137] The powdery terminal-modified imide oligomer before curing had a Tg of 245°C from

DSC measurement result and had a minimum melt viscosity of 400 Pa s (340°C). Some of the powdery terminal-modified imide oligomer was inserted between two polyimide films (trade name: UPIFEX-75S; thickness: 75 mhi; size: 15 cm per side; manufactured by Ube Industries, Ftd.) having excellent surface smoothness. The whole was pressurized at 370°C for 1 hour, then cooled, and released, giving a transparent red-brown film-like cured product (a thickness of 96 mhi). IR spectrum measurement of some of the film-like imide resin composition showed the disappearance of the absorption around 2,210 cm ¹ assigned to the stretching vibration of the triple bond in PEPA as the terminal group of the terminally modified imide oligomer, and this indicated that the pressurized thermoforming allowed the terminally modified imide oligomer component in the film-like imide resin composition to undergo thermal addition reaction and to be polymerized. The film-like cured product had a Tg of 334°C by DSC measurement, a Tg of 335°C by DMA measurement, and a 5% weight loss temperature of 538°C by TGA. As for the mechanical characteristics by tensile test, the film-like cured product had an elastic modulus of 3.1 GPa, a breaking strength of 141 MPa, and a breaking elongation of 35%.

Example 7

[0138] A varnish having a solid content concentration of about 75 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent was prepared in a same manner with Example 6 other than changing the amount of ethanol initially added to 622.87 g (13520 mmol).

Example 8

[0139] A varnish having a solid content concentration of about 80 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent was prepared in a same manner with Example 6 other than changing the amount of ethanol initially added to 515.54 g (11190 mmol).

Example 9

[0140] A varnish having a solid content concentration of about 85 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an ethanol solvent was prepared in a same manner with Example 6 other than changing the amount of ethanol initially added to 419.72 g (9110 mmol).

Example 10

[0141] A varnish having a solid content concentration of about 85 % by weight in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in an methanol solvent was prepared in a same manner with Example 6 other than changing the solvent initially added to methanol.

Comparative Example 1

[0142] Into a 100-mL three-necked flask equipped with a thermometer, a stirrer, and a nitrogen inlet tube, 2.7613 g (10 mmol) of 2-phenyl-4,4'-diaminodiphenyl ether and 10 mL of N-methyl-2- pyrrolidone were added and dissolved. Next, 1.7450 g (8 mmol) of 1,2,4,5-benzenetetracarboxylic dianhydride and 0.8 mL of N-methyl-2-pyrrolidone (boiling point: about 204°C) were added. Under a nitrogen stream, the mixture was polymerized at room temperature for 2.5 hours, then at 60°C for 1.5 hours, and at room temperature for 1 hour, giving an amic acid oligomer. To the reaction solution, 0.9929 g (4 mmol) of 4-(2-phenylethynyl)phthalic anhydride was added. Under a nitrogen stream, the mixture was reacted at room temperature for 12 hours to undergo terminal modification, and subsequently was stirred at 195°C for 5 hours to undergo imide bond formation. [0143] After cooling, the reaction solution was poured into 900 mL of ion-exchanged water, and precipitated powder was collected by filtration. The powder was washed with 80 mL of methanol for 30 minutes, and the powder obtained by filtration was dried under reduced pressure at 130°C for a day, giving a product. The obtained terminal-modified imide oligomer was represented by General Formula (5) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ and R₉ were a 2-phenyl-4,4'-diaminodiphenyl ether residue, and m = 4 and n = 0 on average.

[0144] The powdery terminal-modified imide oligomer before curing had a Tg of 213°C from

DSC measurement result and had a minimum melt viscosity of 150 Pa· sec (343 °C). Some of the powdery terminal-modified imide oligomer was inserted between two polyimide films (trade name: UPILEX-75S; thickness: 75 mhi; size: 15 cm per side; manufactured by Ube Industries, Ltd.) having excellent surface smoothness. The whole was pressurized at 370°C for 1 hour, then cooled, and released, giving a transparent red-brown film-like cured product (a thickness of 100 mhi). IR spectrum measurement of some of the film-like imide resin composition showed the disappearance of the absorption around 2,210 cm ¹ assigned to the stretching vibration of the triple bond in the phenylethynyl moiety as the terminal group of the terminally modified imide oligomer, and this indicated that the pressurized thermoforming allowed the terminally modified imide oligomer component in the film-like imide resin composition to undergo thermal addition reaction and to be polymerized. The obtained film-like cured product had a Tg of 346°C by DSC measurement, a Tg of 343°C by DMA measurement, and a 5% weight loss temperature of 538°C by TGA. As for the mechanical characteristics by tensile test, the film-like cured product had an elastic modulus of 3.2 GPa, a breaking strength of 132 MPa, and a breaking elongation of 16%.

Comparative Example 2

[0145] Into a 100-mL four-necked flask equipped with a thermometer, a stirrer, a nitrogen inlet tube, and a reflux condenser, 4.905 g (17.8 mmol) of 2-phenyl-4,4'-diaminodiphenyl ether and 10 mL of N-methyl-2-pyrrolidone were added and dissolved. Next, 4.007 g (14.2 mmol) of 1, 2,4,5- benzenetetracarboxylic acid dimethyl ester produced in Production Example 1, 1.990 g (7.1 mmol) of 4- (2-phenylethynyl)phthalic acid monoethyl ester produced in Production Example 4, and 0.8 mL of N- methyl-2-pyrrolidone were added. Under a nitrogen stream, the mixture was stirred at 60°C for 3 hours to undergo amic acid bond formation reaction. The mixture was then reacted under a nitrogen stream at 200°C for 5 hours to undergo imide bond formation reaction.

[0146] After cooling, the reaction solution was poured into 900 mL of ion-exchanged water, and precipitated powder was collected by filtration. The powder was washed with 80 mL of methanol for 30 minutes, and the powder obtained by filtration was dried under reduced pressure at 240°C for 5 hours, giving a product. The obtained terminal-modified imide oligomer was represented by General Formula (5) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ and R₉ were a 2-phenyl-4,4'-diaminodiphenyl ether residue, and m = 4 and n = 0 on average.

[0147] The powdery terminal-modified imide oligomer before curing had a Tg of 217°C from

DSC measurement result and had a minimum melt viscosity of 216 Pa· sec (340°C). Some of the powdery terminal-modified imide oligomer was inserted between two polyimide films (trade name: UPILEX-75S; thickness: 75 mih; size: 15 cm per side; manufactured by Ube Industries, Ltd.) having excellent surface smoothness. The whole was pressurized at 370°C for 1 hour, then cooled, and released, giving a transparent red-brown film-like cured product (a thickness of 86 mih). IR spectrum measurement of some of the film-like imide resin composition showed the disappearance of the absorption around 2,210 cm ¹ assigned to the stretching vibration of the triple bond in the phenylethynyl moiety as the terminal group of the terminally modified imide oligomer, and this indicated that the pressurized thermoforming allowed the terminally modified imide oligomer component in the film-like imide resin composition to undergo thermal addition reaction and to be polymerized. The film-like cured product had a Tg of 336°C by DSC measurement, a Tg of 346°C by DMA measurement, and a 5% weight loss temperature of 538°C by TGA. As for the mechanical characteristics by tensile test, the film-like cured product had an elastic modulus of 2.8 GPa, a breaking strength of 128 MPa, and a breaking elongation of 18%.

Comparative Example 3

[0148] Into a 100-mL three-necked flask equipped with a thermometer, a stirrer, and a nitrogen inlet tube, 3.484 g (12.6 mmol) of 2-phenyl-4,4'-diaminodiphenyl ether, 0.488 g (1.4 mmol) of 9,9-bis(4- (4-aminophenoxy)phenyl)fluorene, and 10 mL of N-methyl-2-pyrrolidone were placed and dissolved. Next, 2.619 g (12.0 mmol) of 1,2,4,5-benzenetetracarboxylic dianhydride and 0.8 mL of N-methyl-2- pyrrolidone were added. Under a nitrogen stream, the mixture was polymerized at room temperature for 2.5 hours, then at 60°C for 1.5 hours, and at room temperature for 1 hour, giving an amic acid oligomer. To the reaction solution, 0.993 g (4 mmol) of 4-(2-phenylethynyl)phthalic anhydride was added. Under a nitrogen stream, the mixture was reacted at room temperature for 12 hours to undergo terminal modification and subsequently was stirred at 195°C for 5 hours to undergo imide bond formation.

[0149] After cooling, the reaction solution was poured into 900 mL of ion-exchanged water, and precipitated powder was collected by filtration. The powder was washed with 80 mL of methanol for 30 minutes, and the powder obtained by filtration was dried under reduced pressure at 130°C for a day, giving a product. The obtained terminal-modified imide oligomer was represented by General Formula (5) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ and R₉ were a 2-phenyl-4,4'-diaminodiphenyl ether residue or a 9,9-bis(4-aminophenyl)fluorene residue, and m = 6 and n = 1 on average.

[0150] The powdery terminal-modified imide oligomer before curing had a Tg of 213°C from

DSC measurement result and had a minimum melt viscosity of 9,036 Pa· sec (346 °C). Some of the powdery terminal-modified imide oligomer was inserted between two polyimide films (trade name: UPILEX-75S; thickness: 75 mih; size: 15 cm per side; manufactured by Ube Industries, Ltd.) having excellent surface smoothness. The whole was pressurized at 370°C for 1 hour, then cooled, and released, giving a transparent red-brown film-like cured product (a thickness of 100 mih). IR spectrum measurement of some of the film-like imide resin composition showed the disappearance of the absorption around 2,210 cm ¹ assigned to the stretching vibration of the triple bond in the phenylethynyl moiety as the terminal group of the terminally modified imide oligomer, and this indicated that the pressurized thermoforming allowed the terminally modified imide oligomer component in the film-like imide resin composition to undergo thermal addition reaction and to be polymerized. The obtained film- like cured product had a Tg of 356°C by DSC measurement, a Tg of 356°C by DMA measurement, and a 5% weight loss temperature of 538°C by TGA. As for the mechanical characteristics by tensile test, the film- like cured product had an elastic modulus of 3.2 GPa, a breaking strength of 132 MPa, and a breaking elongation of 15%.

[0151] Each vacuum-dried product of the varnishes obtained in Examples 1 to 10 had excellent solubility in organic solvents such as methanol and ethanol.

[0152] The varnishes obtained in Examples 1 to 10 were allowed to stand in a freezer at -5°C.

After several months, the varnishes were taken out and thawed to room temperature. Observation of the solution states indicated no precipitate or gelation. GPC measurement gave the same GPC curves before and after the frozen storage. These results revealed that the varnishes produced by the present invention have excellent long-term storage stability.

[0153] Each solid imide resin composition obtained by heating the varnishes obtained in

Examples 1 to 10 had a minimum melt viscosity of higher than 300°C, which indicates excellent melt flowability at high temperature and excellent molding processability.

[0154] Each film-like molded article obtained by heating the solid imide resin compositions obtained in Examples 1 to 10 in a molten state to be polymerized had a Tg of higher than 300°C and underwent almost no thermal decomposition even at a high temperature of higher than 500°C. This result reveals that the cured resin moldings have extremely high heat resistance and also have high breaking strength and breaking elongation.

[0155] The varnishes obtained in Examples 1 to 10 included organic solvents having lower boiling points than those included in the varnishes obtained in Comparative Examples 1 to 3. It is thus obvious that such organic solvents can be readily removed out of the system for a short period of time, and a polyimide powder having excellent thermal properties can be simply obtained without any special purification operation (reprecipitation).

Example 11

[0156] Some of the varnish produced in Production Example 3 was impregnated into harness satin fabrics“THORNEL T650-35 8HS” (a fiber basis weight of 380 g/m, made of desized carbon fiber) with dimensions of 30 cm x 30 cm manufactured by Cytec Engineered Materials., giving 20 wet prepregs to which the raw material composition of the terminally modified polyimide resin was attached. The weight contents of the raw material composition of the terminally modified polyimide resin, solvent, carbon fiber were about 40 % by weight, 8 % by weight, 52 % by weight in the obtained wet prepregs, respectively.

Example 12

[0157] By heating three wet prepregs, to which the raw material composition of the terminally modified polyimide resin was attached, produced in Example 11 in a circulation air oven at an internal temperature of 250 °C for 1 hour, amic acid bond formation reaction and imide bond formation reaction were carried out while the alcohol components in the prepregs were removed, giving imide dry prepregs of the terminally modified imide oligomer. The obtained imide dry prepreg had an average terminal- modified imide oligomer content of about 45 % by weight and an average carbon fiber content of about 55 % by weight.

[0158] Visual observation of the appearance of the obtained imide dry prepreg showed that the resin was uniformly attached to the surface and the inside of the carbon fibers, and this indicated the imide prepreg had excellent adhesiveness between the resin and the prepreg.

Example 13

[0159] On a stainless steel plate having dimensions of 30 cm x 30 cm, a polyimide film was placed as a release film, and two set of the stacked wet prepregs of the raw material composition of the terminally modified polyimide resin produced in Example 11 were prepared on the film with lay-up sequences of [0]_x and [0/+45/-45/90]_s, respectively. The lay-up sequence [0]_x indicates a stack of 8 wet prepregs, where the orientation of the fibers between each successive wet prepreg in the stack is in the same direction. The lay-up sequence of [0/+45/-45/90]_sis also a stack of 8 wet prepregs; however, in this sequence the first wet prepreg has fiber orientation defined at 0 degrees, the second wet prepreg has fiber orientation at +45 degrees relative to that of the first wet prepreg, the third wet prepreg has a fiber orientation of -45 degrees relative to that of the first wet prepreg, and the fourth wet prepreg has fiber orientation of 90 degrees relative to that of the first wet prepreg, and then the fifth through eighth wet prepregs have a symmetrically mirrored sequence to the first through fourth, i.e, the fifth through eighth wet prepregs have fiber orientations of 90, -45, +45, and 0, respectively. A polyimide film and a stainless steel plate were further stacked. The whole was heated on a hot press at a temperature increase rate of about 5 °C/min from room temperature to 250 °C and heated at 250 °C for 1 hour. The whole was further heated at a temperature increase rate of about 5°C/min from 250°C to 320°C and heated at 320°C for 10 minutes. Next, the whole was heated under a pressure condition of 1.3 MPa at a temperature increase rate of about 5°C/min to 370°C and was heated under the same pressure condition at 370°C for 1 hour. The appearance observation revealed the production of a good fiber-reinforced composite material laminate having very smooth surfaces and containing the resin that was uniformly infiltrated into the fibers. The obtained laminate had a glass transition temperature (DSC) of 345 °C, a fiber volume fraction (Vf) of 0.55.

[0160] The inside of the laminate was observed by ultrasonic defect test and by cross section observation under an optical microscope, and no void was observed. This indicates that the fiber- reinforced composite material has extremely high quality.

[0161] The DMA measurement and the TGA measurement revealed that the organic solvents having low boiling points, such as ethanol and methanol, contained in the varnishes were completely removed from the obtained fiber-reinforced composite material laminate by the heat treatment.

[0162] The obtained fiber-reinforced composite material laminate had a glass transition temperature of higher than 300°C, which indicates excellent heat resistance, and had an short beam shear (SBS) strength of about 73 MPa determined by short beam shear test at room temperature in a three-point bending manner, which indicates excellent mechanical characteristics, as illustrated in FIG. 3 for both the [0]g and [0/+45/-45/90]_s lay-up sequences.

[0163] The obtained fiber-reinforced composite material laminates were exposed in a thermal cycling condition in the temperature range between -54 °C and 232 °C up to 2000 cycles. The obtained fiber-reinforced composite material laminates showed no generation of microcracks, cracks and delaminations inside after thermal cycles by microscope observation as shown in FIGS. 4A-4B for the [0]₈ and [0/+45/-45/90]_s lay-up sequences, respectively, between 0 to 2000 thermal cycles in increments of 400 cycles.

[0164] The obtained fiber-reinforced composite material laminates also showed a greater than

97 % initial interlaminar shear strength measured at room temperature and 232 °C after 2000 thermal cycles. This indicates that the fiber-reinforced composite material has extremely high resistance for severe environmental durability.

Comparative Example 4

[0165] Into a 100-mL sample bottle, 1.741 g (16.1 mmol) of p-phenylenediamine and 6.8 g

(212.2 mmol) of methanol were placed and completely dissolved, and then 4.000 g (12.9 mmol) of 1,2,4,5-benzenetetracarboxylic acid diethyl ester produced in Production Example 2 and 1.913 g (6.5 mmol) of 4-(2-phenylethynyl)phthalic acid monoethyl ester produced in Production Example 4 were added. The container was purged with nitrogen and sealed, and stirring of the resulting suspension was started at room temperature. After the start of stirring, the start of gradual dissolution of the ester compounds in the solvent was observed, and the compounds were completely uniformly dissolved at 60 minutes after the start of stirring. The solution was further continuously stirred, and then at 24 hours after the start of stirring, the stirring was stopped, giving a varnish in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in methanol.

[0166] The varnish was placed in a glass petri dish. By heating the varnish in a circulation air oven at an internal temperature of 60°C for 3 hours and further heating the varnish at 200°C for 1 hour, amic acid bond formation reaction and imide bond formation reaction were carried out while methanol was removed, yielding a terminally modified imide oligomer. The obtained terminal-modified imide oligomer was represented by General Formula (4) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ and R₉ were a p-phenylenediamine residue, and m = 0 and n = 4 on average.

[0167] The powdery terminal-modified imide oligomer before curing was not melted even when heated at 300°C or higher, and failed to give a cured resin film.

Comparative Example 5

[0168] Into a 100-mL sample bottle, 4.707 g (16.1 mmol) of l,3-bis(4-aminophenoxy)benzene and 6.8 g (212.2 mmol) of methanol were placed and completely dissolved, and then 4.000 g (12.9 mmol) of 1,2,4,5-benzenetetracarboxylic acid diethyl ester produced in Production Example 2 and 1.913 g (6.5 mmol) of 4-(2-phenylethynyl)phthalic acid monoethyl ester produced in Production Example 4 were added. The container was purged with nitrogen and sealed, and stirring of the resulting suspension was started at room temperature. After the start of stirring, the start of gradual dissolution of the ester compounds in the solvent was observed, and the compounds were completely uniformly dissolved at 60 minutes after the start of stirring. The solution was further continuously stirred, and then at 24 hours after the start of stirring, the stirring was stopped, giving a varnish in which the raw material composition of the terminally modified polyimide resin was uniformly dissolved in methanol.

[0169] The varnish was placed in a glass petri dish. By heating the varnish in a circulation air oven at an internal temperature of 60°C for 3 hours and further heating the varnish at 200°C for 1 hour, amic acid bond formation reaction and imide bond formation reaction were carried out while methanol was removed, yielding a terminally modified imide oligomer. The obtained terminal-modified imide oligomer was represented by General Formula (4) in which R₆ and R₇ were a hydrogen atom or a phenyl group, one of R₆ and R₇ was a phenyl group, R₈ and R₉ were a l,3-bis(4-aminophenoxy /benzene residue, and m = 0 and n = 4 on average.

[0170] The powdery terminal-modified imide oligomer before curing was not melted even when heated at 300°C or higher, and failed to give a cured resin film.

Industrial Applicability

[0171] The present invention can provide a varnish having excellent solubility in organic solvents having low boiling points, such as alcohols, and excellent solution storage stability. The terminally modified imide oligomer produced by using the varnish exhibits excellent moldability, and thermal curing of the oligomer enables the production of a cured product having high heat resistance, toughness, and mechanical characteristics.

[0172] When a prepreg or an imide prepreg prepared by infiltrating the varnish into fibers is used to form a composite material, the organic solvent can be completely removed during thermal curing, and this enables simple production of a fiber-reinforced composite material having exceptional mechanical strength and high heat resistance. On this account, such materials are usable in wide variety of fields as member materials requiring easy moldability and high heat resistance, including aircraft and apparatuses for the aerospace industry.

[0173] To summarize, the present disclosure describes a varnish comprising components (A) to

(D), wherein components (A), (B), and (C) are present in the varnish at a solid content of more than 65 %, wherein the component (A) is an aromatic tetracarboxylic acid diester represented by General Formula (1) below and is present in an amount of 1 to 500 parts by weight, based on the total weight of the varnish; wherein the component (B) is 2-phenyl-4,4'-diaminodiphenyl ether and is present in an amount of 1 to 450 parts by weight, based on the total weight of the varnish, wherein the component (C) is a 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2) and is present in an amount of 1 to 400 parts by weight, based on the total weight of the varnish, and wherein the component (D) is an organic solvent having a boiling point of 150°C or less at 1 atmosphere or a mixture of two or more of the organic solvents which includes methanol, ethanol, 1 -propanol, and 2-propanol and is present in an amount of 100 parts by weight, based on the total weight of the varnish,

[General Formula (1)]

wherein Ri is an aromatic tetracarboxylic acid diester residue,

wherein R₂ and R₃ are the same or different, and are an aliphatic organic group or an aromatic organic group, and R₂ and R₃ are located in a cis configuration or a trans configuration, and

wherein the compound (A) is optionally a single isomer or a mixture of two isomers,

[General Formula (2)]

wherein R4 and R₅ are a hydrogen atom, an aliphatic organic group, or an aromatic organic group, and

wherein one of R4 and R₅ is an aliphatic organic group or an aromatic organic group; and/or wherein the aliphatic organic group represented by R₂ and R₃ in General Formula (1) is an organic group having an aliphatic chain, and wherein the aromatic organic group is an organic group having an aromatic ring; and/or

wherein the aromatic tetracarboxylic acid diester residue represented by R_j in General Formula (1) is a tetravalent aromatic organic group formed by removing four carboxyl groups from an aromatic tetracarboxylic acid; and/or

wherein the aromatic tetracarboxylic acid diester residue represented by Ri is a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid or a tetravalent residue of a 3, 3’, 4,4’- biphenyltetracarboxylic acid; and/or

wherein Ri in General Formula (1) is a combination of two or more selected from the group consisting of a tetravalent aromatic tetracarboxylic acid diester represented by a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid, an aromatic tetracarboxylic acid diester represented by a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid, and an aromatic tetracarboxylic acid diester represented by a tetravalent residue of a bis(3,4-carboxyphenyl) ether; and/or

wherein the aliphatic organic group represented by R4 or R₅ in General Formula (2) is an organic group having an aliphatic chain, and wherein the aromatic organic group is an organic group having an aromatic ring; and/or

wherein the component (B) further comprises one or more additional aromatic diamines. [0174] Also described herein is a prepreg including a fiber impregnated by the varnish comprising components (A) to (D); and/or

wherein the aliphatic organic group represented by R₂ and R₃ in General Formula (1) is an organic group having an aliphatic chain, and wherein the aromatic organic group is an organic group having an aromatic ring; and/or

wherein the aromatic tetracarboxylic acid diester residue represented by Ri in General Formula (1) is a tetravalent aromatic organic group formed by removing four carboxyl groups from an aromatic tetracarboxylic acid; and/or

wherein the aliphatic organic group represented by R₄ or R₅ in General Formula (2) is an organic group having an aliphatic chain, and wherein the aromatic organic group is an organic group having an aromatic ring; and/or

wherein the component (B) further comprises one or more additional aromatic diamines.

[0175] Also described herein is an imide prepreg prepared by heating the prepreg; and/or

wherein the imide is represented by General Formula (3),

[General Formula (3)]

wherein R₆ and R₇ are each a hydrogen atom or a phenyl group,

wherein one of R₆ and R₇ is a phenyl group,

wherein R₈ and R₉ are the same or different, and are each a divalent aromatic diamine residue, wherein R₁₀ and R_n are the same or different, and are each a tetravalent aromatic tetracarboxylic acid residue,

wherein m and n satisfy relations of m > 1, n > 0, 1 < m + n < 10, and 0.05 < m/(m + n) < 1, and wherein repeating units of the m-unit and n-unit in General Formula (3) are optionally arranged in a block sequence or a random sequence. [0176] Also described herein is a fiber-reinforced composite material prepared by stacking at least one of a plurality of prepregs, a plurality of imide prepregs, or a combination of prepregs and imide prepregs, and thermally curing the stack, wherein a prepreg including a fiber impregnated by the varnish comprising components (A) to (D), and wherein an imide prepreg is prepared by heating a prepreg; and/or

wherein the fiber-reinforced composite material has a Tg of 300°C or more; and/or

wherein the fiber-reinforced composite material of 330°C or more; and/or

wherein the imide in the imide prepreg is represented by General Formula (3),

[General Formula (3)]

wherein R₆ and R₇ are each a hydrogen atom or a phenyl group,

wherein one of R₆ and R₇ is a phenyl group,

wherein R₈ and R₉ are the same or different, and are each a divalent aromatic diamine residue, wherein Ri₀ and Rn are the same or different, and are each a tetravalent aromatic tetracarboxylic acid residue,

wherein m and n satisfy relations of m > 1, n > 0, 1 < m + n < 10, and 0.05 < m/(m + n) < 1, and wherein repeating units of the m-unit and n-unit in General Formula (3) are optionally arranged in a block sequence or a random sequence.

[0177] Also described herein is a solid imide resin composition prepared by heating the varnish comprising components (A) to (D) to remove the component (D), where the solid imide resin composition is represented by General Formula (3),

[General Formula (3)]

wherein R₆ and R₇ are each a hydrogen atom or a phenyl group,

wherein one of R₆ and R₇ is a phenyl group,

wherein R₈ and R₉ are the same or different, and are each a divalent aromatic diamine residue, wherein Ri₀ and Rn are the same or different, and are each a tetravalent aromatic tetracarboxylic acid residue. wherein m and n satisfy relations of m > 1, n > 0, 1 < m + n < 10, and 0.05 < m/(m + n) < 1, and wherein repeating units of the m-unit and n-unit in General Formula (3) are optionally arranged in a block sequence or a random sequence.

[0178] Also described herein is a molded article of a polymerized imide resin composition prepared by heating the solid imide resin composition in a molten state; and/or

wherein the polymerized imide resin composition has a glass transition temperature (Tg) of 300°C or more; and/or

wherein the polymerized imide resin composition has a glass transition temperature (Tg) of 330°C or more; and/or

wherein the polymerized imide resin composition has a glass transition temperature (Tg) of 350°C or more; and/or

wherein the molded article is in the shape of a film, the film having a tensile elongation at break of 10% or more; and/or

wherein the film having a tensile elongation at break of 15% or more; and/or

wherein the film having a tensile elongation at break of 20% or more.

[0179] Although the disclosure herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present disclosure. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present disclosure as defined by the appended claims.

Claims

TN THE f!TATMS

1. A varnish, comprising:

components (A) to (D),

wherein components (A), (B), and (C) are present in the varnish at a solid content of more than

65 %,

wherein the component (A) is an aromatic tetracarboxylic acid diester represented by General Formula (1) below and is present in an amount of 1 to 500 parts by weight, based on the total weight of the varnish;

wherein the component (B) is 2-phenyl-4,4'-diaminodiphenyl ether and is present in an amount of 1 to 450 parts by weight, based on the total weight of the varnish,

wherein the component (C) is a 4-(2-phenylethynyl)phthalic acid monoester represented by General Formula (2) and is present in an amount of 1 to 400 parts by weight, based on the total weight of the varnish, and

wherein the component (D) is an organic solvent having a boiling point of 150°C or less at 1 atmosphere or a mixture of two or more of the organic solvents which includes methanol, ethanol, 1- propanol, and 2-propanol and is present in an amount of 100 parts by weight, based on the total weight of the varnish,

[General Formula (1)]

wherein Ri is an aromatic tetracarboxylic acid diester residue,

[General Formula (2)]

wherein R₄ and R₅ are a hydrogen atom, an aliphatic organic group, or an aromatic organic group, and

wherein one of R₄ and R₅ is an aliphatic organic group or an aromatic organic group.

2. The varnish of claim 1, wherein the aliphatic organic group represented by R₂ and R₃ in General Formula (1) is an organic group having an aliphatic chain, and wherein the aromatic organic group is an organic group having an aromatic ring.

3. The varnish of claim 1, wherein the aromatic tetracarboxylic acid diester residue represented by Ri in General Formula (1) is a tetravalent aromatic organic group formed by removing four carboxyl groups from an aromatic tetracarboxylic acid.

4. The varnish of claim 3, wherein the aromatic tetracarboxylic acid diester residue represented by Ri is a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid or a tetravalent residue of a 3, 3', 4, 4'-bi phenyl tetracarboxylic acid.

5. The varnish of claim 1, wherein Ri in General Formula (1) is a combination of two or more selected from the group consisting of a tetravalent aromatic tetracarboxylic acid diester represented by a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid, an aromatic tetracarboxylic acid diester represented by a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid, and an aromatic tetracarboxylic acid diester represented by a tetravalent residue of a bis(3,4-carboxyphenyl) ether.

6. The varnish of claim 1, wherein the aliphatic organic group represented by R₄ or R₅ in General Formula (2) is an organic group having an aliphatic chain, and

wherein the aromatic organic group is an organic group having an aromatic ring.

7. The varnish of claim 1, wherein the component (B) further comprises one or more additional aromatic diamines.

8. A prepreg including a fiber impregnated by the varnish of claim 1.

9. The prepreg of claim 8, wherein the aliphatic organic group represented by R₂ and R₃ in General Formula (1) is an organic group having an aliphatic chain, and wherein the aromatic organic group is an organic group having an aromatic ring.

10. The prepreg of claim 8, wherein the aromatic tetracarboxylic acid diester residue represented by Ri in General Formula (1) is a tetravalent aromatic organic group formed by removing four carboxyl groups from an aromatic tetracarboxylic acid.

11. The prepreg of claim 10, wherein the aromatic tetracarboxylic acid diester residue represented by Ri is a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid or a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid.

12. The prepreg of claim 8, wherein Ri in General Formula (1) is a combination of two or more selected from the group consisting of a tetravalent aromatic tetracarboxylic acid diester represented by a tetravalent residue of a 1,2,4,5-benzenetetracarboxylic acid, an aromatic tetracarboxylic acid diester represented by a tetravalent residue of a 3,3',4,4'-biphenyltetracarboxylic acid, and an aromatic tetracarboxylic acid diester represented by a tetravalent residue of a bis(3,4-carboxyphenyl) ether.

13. The prepreg of claim 8, wherein the aliphatic organic group represented by R₄ or R₅ in General Formula (2) is an organic group having an aliphatic chain, and

wherein the aromatic organic group is an organic group having an aromatic ring.

14. The prepreg of claim 8, wherein the component (B) further comprises one or more additional aromatic diamines.

15. An imide prepreg prepared by heating the prepreg of claim 16.

16. The imide prepreg of claim 16, wherein the imide is represented by General Formula (3), [General Formula (3)]

wherein R₆ and R₇ are each a hydrogen atom or a phenyl group,

wherein one of R₆ and R₇ is a phenyl group,

17. A fiber-reinforced composite material prepared by stacking at least one of a plurality of prepregs, a plurality of imide prepregs, or a combination of prepregs and imide prepregs, and thermally curing the stack,

wherein a prepreg including a fiber impregnated by the varnish of claim 1 , and

wherein an imide prepreg is prepared by heating a prepreg.

18. The fiber-reinforced composite material of claim 17, wherein the fiber-reinforced composite material has a Tg of 300°C or more.

19. The fiber-reinforced composite material of claim 17, wherein the fiber-reinforced composite material of 330°C or more.

20. The fiber-reinforced composite material of claim 17, wherein the imide in the imide prepreg is represented by General Formula (3),

[General Formula (3)]

wherein R₆ and R₇ are each a hydrogen atom or a phenyl group,

wherein one of R₆ and R₇ is a phenyl group,