JP7510317B2 - Irregular particles containing an amic acid oligomer having an imide group and a method for producing the same - Google Patents

Description

The present invention relates to irregular particles containing an amide acid oligomer having an imide group and a method for producing the same.

Aromatic polyimides have excellent moldability, the highest level of heat resistance among polymers, and excellent mechanical and electrical properties, and are therefore used as materials (heat-resistant composite materials) in a wide range of fields, including aircraft, space industry equipment, electrical and electronic equipment, general industrial applications, and vehicle engine (peripheral) components. Therefore, there is a demand for a method for producing polyimide, which is a matrix resin, inexpensively, easily, and in a short time.

As a method for producing polyimide, Patent Document 1 describes a method for producing polyimide powder in which a polyamic acid solution is heated to precipitate polyimide particles from the solvent, and polyimide powder is directly collected from the solvent. Patent Document 2 describes a method for producing polyimide powder by reacting an aromatic tetracarboxylic acid component with an aromatic diamine component in a reaction solvent to precipitate fine particles, and then continuing the reaction while distilling off water. Patent Document 3 describes a method for producing terminal-modified imide oligomer powder by adding an NMP solution of terminal-modified imide oligomer to ion-exchanged water, filtering the precipitated solids to obtain granular terminal-modified imide oligomer, and then pulverizing the terminal-modified imide oligomer.

A method is also known in which a polyimide molded body is obtained directly by obtaining a polyamic acid powder and molding it, without isolating the polyimide powder. Patent Document 4 describes a method for producing a polyimide molded body, in which polyamic acid is polymerized in a specific solvent, powder particles are removed from the suspension, and the powder particles are heated and molded into a polyamic acid molded body, which is then imidized.

It is also known that polyamic acid powder can be used as an adhesive. Patent documents 5 and 6 describe that after preparing a polyamic acid solution, polyamic acid powder can be produced by a number of methods and used to bond cold-rolled steel.

Japanese Patent Application Laid-Open No. 7-33875 Japanese Patent Application Laid-Open No. 11-302380 WO 2018/180930 A1 JP 7-118388 A JP 6-16811 A JP 6-1856 A

However, in the techniques described in Patent Documents 1 and 2, polyamic acid is heated in a solution to prepare a polyimide solution, and then the polyimide particles are precipitated, collected, washed, and dried to produce powdered polyimide. In the technique described in Patent Document 3, imide oligomer in a solution is precipitated (deposited) in a poor solvent, and then collected, washed, and dried to produce powdered imide oligomer. In the technique described in Patent Document 4, a suspension of polyamic acid is prepared in a specific solvent, and the polyamic acid is collected, washed, and dried to produce powdered polyamic acid. In the techniques described in Patent Documents 5 and 6, in addition to the above-mentioned methods, a solution of polyamic acid is prepared, and then the solvent is volatilized to produce polyamic acid powder.

Therefore, the techniques described in Patent Documents 1 to 4 above require many steps, are complicated, and require a long time (several hours) for production, so there is room for further improvement from the perspective of producing polyimide cheaply, easily, and in a short time. In addition, the techniques described in Patent Documents 5 and 6 above for producing polyamic acid powder by volatilizing the solvent from a polyamic acid solution only exemplify combinations of polyamic acids with very limited compositions and very limited types of solvents, so its versatility is completely unknown, and the use of the resulting polyamic acid is limited to adhesives, so its application to polyamic acids suitable for obtaining molded bodies is completely unknown. In addition, the use of a high-boiling point aprotic solvent raises safety concerns, and its application to other solvents is completely unknown.

One aspect of the present invention aims to provide a method for producing irregular particles (P) containing an amic acid oligomer having an imide group in a cheaper, simpler, and shorter time than conventional techniques, and to provide the irregular particles (P) containing the amic acid oligomer.

As a result of intensive research into solving the above problems, the inventors discovered that irregular particles containing an amic acid oligomer (P) having imide groups can be produced inexpensively, easily, and in an extremely short time (e.g., several minutes) by melt processing a solution of an amic acid oligomer, and that the resulting amic acid oligomer (P) having imide groups has novel physical properties and characteristics that have never been seen before, and thus completed the present invention.

That is, one embodiment of the present invention includes the following configuration.
[1] A part of the amic acid units in the amic acid oligomer (A) represented by the following general formula (2) is converted into an imide ring, and the amic acid oligomer (P) has an imide group with an imidization rate of 1 to 80% (wherein α to β represents α or more and β or less; the same applies hereinafter),
Irregular particles having an average particle size of 100 to 500 μm when dispersed in water.

(In the general formula (2), _R1 and _R2 represent divalent aromatic diamine residues which may be the same or different from each other, _R3 and _R4 represent tetravalent aromatic tetracarboxylic acid residues which may be the same or different from each other, one of _R5 and _R6 is a hydrogen atom and the other is a phenyl group, m and n satisfy the relationships 1≦m, 0≦n, 1≦m+n≦20 and 0.05≦m/(m+n)≦1, and the arrangement of the repeating units may be either block or random.)
[2] The irregular particles according to [1], containing 20% by weight or less of a solvent.
[3] Irregular particles according to [2], wherein the solvent is a mixed solvent of an alcohol-based solvent and an ether-based solvent, a single solvent of a hydroxyether-based solvent, a mixed solvent of an alcohol-based solvent and a hydroxyether-based solvent, a mixed solvent of an ether-based solvent and a hydroxyether-based solvent, or a mixed solvent of an alcohol-based solvent, an ether-based solvent and a hydroxyether-based solvent, and the boiling point (the boiling point of each single solvent in the case of a mixed solvent) is 130°C or less.
[4] Irregular particles according to [3], wherein the solvent is a mixed solvent of methanol and 1,3-dioxolane.
[5] A method for producing irregular particles according to [1], comprising a step of melt-processing a solution of an imide group-containing amic acid oligomer (P) having an imidization rate of 0 to 1%, in which a part of the amic acid units in the amic acid oligomer (A) represented by the following general formula (2) is converted into imide rings:

(In the general formula (2), _R1 and _R2 represent divalent aromatic diamine residues which may be the same or different from each other, _R3 and _R4 represent tetravalent aromatic tetracarboxylic acid residues which may be the same or different from each other, one of _R5 and _R6 is a hydrogen atom and the other is a phenyl group, m and n satisfy the relationships 1≦m, 0≦n, 1≦m+n≦20 and 0.05≦m/(m+n)≦1, and the arrangement of the repeating units may be either block or random.)
[6] The method for producing irregular particles according to [5], wherein the melt processing is carried out at a temperature of 250° C. or less.
[7] The method for producing irregular particles according to [6], in which an extruder is used during melt processing.
[8] The method for producing irregular particles according to [7], wherein the residence time in the extruder is 1 to 30 minutes.

According to one aspect of the present invention, there is no need to precipitate (precipitate) the imide-containing amic acid oligomer (P) in a poor solvent or to grind it. Therefore, the present invention has the effect of providing a manufacturing method that can produce amorphous particles containing an imide-containing amic acid oligomer (P) that has excellent handleability, is inexpensive, simple, and takes an extremely short time (several minutes) compared to conventional techniques, and can provide amorphous particles containing an imide-containing amic acid oligomer (P) that have never before been produced with new physical properties and characteristics.

This is an example of irregular particles observed with an optical microscope.

The following describes in detail the embodiments of the present invention. However, the present invention is not limited to these, and various modifications are possible within the scope described. The technical scope of the present invention also includes embodiments obtained by appropriately combining the technical means disclosed in the different embodiments and examples. In this specification, unless otherwise specified, "A to B" indicating a numerical range means "A or more (including A and larger than A) or B or less (including B and smaller than B)." In addition, "weight" and "mass" are used as synonyms.

[1. Irregular particles containing an amic acid oligomer (P) having an imide group]
In one embodiment of the present invention, the irregular particles containing the amic acid oligomer (P) having an imide group are irregular particles, and each particle has sharp edges (it is not spherical). Whether a particle is an "irregular particle" can be easily determined by observing the particle using, for example, an optical microscope.

The irregular particles containing the amic acid oligomer (P) having an imide group of the present invention contain the amic acid oligomer (P) having an imide group as an essential component, and may further contain the imide oligomer (B), the solvent (C), and other components as optional components. Among these, it is preferable to contain either the imide oligomer (B) or the solvent (C), and it is more preferable to contain the imide oligomer (B) and the solvent (C).
The amic acid oligomer (P) having an imide group and the imide oligomer (B) do not have to be chemically related to each other, but it is preferable that they are chemically related to each other. In other words, it is preferable that the amic acid oligomer (P) having an imide group is a precursor of the imide oligomer (B), and it is preferable that the imide oligomer (B) is generated from the amic acid oligomer (P) having an imide group by a chemical reaction (imidization).

The amic acid oligomer (P) having an imide group is a product in which a part of the amic acid units in the amic acid oligomer (A) represented by the above general formula (2) is converted to an imide ring. That is, the amic acid oligomer (A) is different from the amic acid oligomer (P) having an imide group.
The ratio of imide rings converted from amic acid units can be expressed as the imidization ratio.
The imidization ratio of the amic acid oligomer (P) having an imide group can be calculated by measuring the peak area derived from aromatic ¹ H and the peak area derived from residual amide ¹ H by ¹ H-NMR. The imidization ratio (Rp) of the amic acid oligomer (P) having an imide group of the present invention is 1 to 80%, preferably 3 to 60%, and more preferably 5 to 40%.

In other words, in the amic acid oligomer (P) having an imide group, the molar numbers of amic acid units and imide ring units in one molecule have the following relationship:
0.01≦(molar number of imide ring units)/(molar number of imide ring units+molar number of amic acid units)≦0.80
Preferably, in the amic acid oligomer (P) having an imide group, the molar numbers of amic acid units and imide ring units in one molecule have the following relationship:
0.03≦(molar number of imide ring units)/(molar number of imide ring units+molar number of amic acid units)≦0.60
More preferably, in the amic acid oligomer (P) having an imide group, the molar numbers of amic acid units and imide ring units in one molecule satisfy the following relationship:
0.05≦(molar number of imide ring units)/(molar number of imide ring units+molar number of amic acid units)≦0.40

In the present invention, it is necessary for the imide group-containing amic acid oligomer (P) to have an imidization rate (Rp) in the specific range as described above in order to achieve the effects of the present invention. If the imidization rate (Rp) is less than 1%, the fluidity may be too high, resulting in problems such as dripping and bubbles. If the imidization rate (Rp) is more than 80%, the fluidity may be too low, resulting in poor moldability. In addition, in the present invention, it is believed that the imide group contained in the imide group-containing amic acid oligomer (P) contributes to providing a molded product with excellent mechanical properties, such as elongation at break.

The amide acid oligomer (P) can ensure molding fluidity even if the imidization rate (Rp) is more than 60% and less than 80%, but if higher molding fluidity is required, the imidization rate (Rp) is preferably 60% or less, and more preferably 40% or less.

In addition, if the imidization rate (Rp) is low, for example when attempting to obtain a thick molded body by simultaneously performing imidization and molding, the water generated by imidization may be difficult to remove and may result in air bubbles, so it is preferable for the imidization rate (Rp) to be 3% or more, and more preferably 5% or more.

The imidization rate (Rp) must be set appropriately depending on the properties and molding conditions of the molded body. For example, when attempting to obtain a thick molded body by simultaneously performing imidization and molding, the imidization rate (Rp) is preferably 5% or more and less than 80%, and more preferably 10% or more and less than 80%. For example, when attempting to obtain a molded body with excellent breaking elongation in its mechanical properties, the imidization rate (Rp) is preferably 5% or more and less than 50%, and more preferably 10% or more and less than 40%.

The imidization rate (Rp) of the imide group-containing amic acid oligomer (P) can be controlled by controlling the temperature and time of melt processing. It can also be adjusted by blending multiple types of imide group-containing amic acid oligomers (P) with different imide groups. The method for measuring the imidization rate will be described in detail in the Examples.

When the amorphous particles of the present invention containing the imide group-containing amic acid oligomer (P) also contain the imide oligomer (B), it is preferable that (P) is 50 wt% or more and less than 100 wt%, with the total of the imide group-containing amic acid oligomer (P) and the imide oligomer (B) being 100 wt%.

In addition, it is preferable that the imidization rate (Rpb) taking into consideration all imide units and amic acid units contained in the amic acid oligomer (P) having an imide group and the imide oligomer (B) is 1 to 80%. In other words, in the irregular particles containing the amic acid oligomer (P) having an imide group and the imide oligomer (B), it is preferable that the molar number of all the amic acid units and the imide ring units satisfy the following relationship:
0.01≦(molar number of imide ring units)/(molar number of imide ring units+molar number of amic acid units)≦0.80

When chemically corresponding amic acid oligomer (P) and imide oligomer (B) coexist, the amic acid oligomer (P) softens and flows at a lower temperature than the imide oligomer (B), so molding flowability can be ensured even if the amount of amic acid oligomer is relatively small (for example, the imidization rate (Rpb) taking into account all imide units and amic acid units in the amic acid oligomer (P) having imide groups and the imide oligomer (B) is in the range of 50% to 80%). However, when the imidization rate (Rpb) exceeds 80%, moldability may be poor. In addition, when the imidization rate (Rpb) is low, for example, when a thick molded body is to be obtained by simultaneously performing imidization and molding, the water generated by imidization may not escape easily and may become bubbles, so it is necessary to set it appropriately depending on the properties of the molded body and the molding conditions. For example, when attempting to obtain a thick molded body by simultaneously performing imidization and molding, the imidization rate (Rpb) is preferably 5% or more and less than 80%, and more preferably 10% or more and less than 80%. Also, for example, when attempting to obtain a molded body with excellent breaking elongation in its mechanical properties, the imidization rate (Rpb) is preferably 5% or more and less than 50%, and more preferably 10% or more and less than 40%.

In one embodiment of the present invention, the amorphous particles containing an amic acid oligomer (P) having an imide group may be a mixture of amic acid oligomers having imide groups with different molecular weights. Also, the amorphous particles may be a mixture of amic acid oligomers having imide groups with different monomer compositions. In these cases, it is preferable that imide oligomers (B1, B2...) that chemically correspond to the amic acid oligomers (P1, P2...) having each imide group are included.

The molecular weight (weight average molecular weight) of the soluble polymer components (e.g., the amic acid oligomer (P) having an imide group and the imide oligomer (B)) contained in the amorphous particles containing the amic acid oligomer (P) having an imide group can be confirmed by GPC. The above molecular weight is preferably 1000 to 100,000, more preferably 3000 to 50,000, and even more preferably 5000 to 30,000. The molecular weight of the amic acid oligomer (P) having an imide group can be controlled by controlling the temperature and time of melt processing. The measurement conditions for the molecular weight are set to be optimal according to the structure of the oligomer in the amorphous particles containing the amic acid oligomer (P) having an imide group to be measured by GPC.

The average particle size of the amorphous particles containing the imide group-containing amic acid oligomer (P) can be measured by dispersing the amorphous particles containing the imide group-containing amic acid oligomer (P) in water. The average particle size is preferably within the range of 10 to 500 μm, more preferably within the range of 10 to 300 μm, and even more preferably within the range of 20 to 100 μm. The particle size distribution (D50) is preferably within the range of 100 to 500 μm. The method for measuring the average particle size will be described in detail in the Examples.

The minimum melt viscosity of the irregular particles containing the imide group-containing amic acid oligomer (P) in one embodiment of the present invention is preferably 20,000 Pa·sec or less, more preferably 10,000 Pa·sec or less, even more preferably 5,000 Pa·sec or less, and particularly preferably 3,000 Pa·sec or less. The minimum melt viscosity of the irregular particles containing the imide group-containing amic acid oligomer (P) in one embodiment of the present invention is preferably 1 to 20,000 Pa·sec, but is not limited to this range. If the minimum melt viscosity is within the above range, the irregular particles containing the imide group-containing amic acid oligomer (P) in one embodiment of the present invention are more excellent in moldability. In this specification, the minimum melt viscosity refers to a value measured by the method described in the Examples below.

In one embodiment of the present invention, the melt viscosity at 280°C of the amorphous particles containing the imide group-containing amic acid oligomer (P) is preferably 200 to 1,000,000 Pa·sec, more preferably 200 to 800,000 Pa·sec, and even more preferably 200 to 500,000 Pa·sec. If the melt viscosity at 280°C exceeds 1,000,000 Pa·sec, the particles tend to be difficult to flow. Therefore, when producing a fiber-reinforced composite material, it is difficult for the imide group-containing amic acid oligomer (P) to be impregnated between the fibers, and it tends to be difficult to obtain a fiber-reinforced composite material in which defects such as voids or unimpregnated parts are reduced or eliminated. In addition, if the melt viscosity at 280°C is less than 200 Pa·sec, the resin tends to flow too easily, making it difficult to produce a prepreg, and as a result, it may be difficult to ensure the drapeability of the prepreg. In this specification, melt viscosity at 280°C refers to the value measured by the method described in the Examples below.

In one embodiment of the present invention, the amic acid oligomer (P) having an imide group is an amic acid oligomer (A) represented by the following general formula (2) in which some of the amic acid units are converted to imide rings, and the imidization rate (Rp) is 1 to 80%.

[2. Amic acid oligomer (P) having an imide group]
Hereinafter, in this specification, unless otherwise specified, the term "amic acid oligomer" is used as a synonym for "terminally modified amic acid oligomer."
The amic acid oligomer (P) having an imide group of the present invention is an amic acid oligomer having an imide group, in which a part of the amic acid units in the amic acid oligomer (A) represented by the following general formula (2) is converted to imide rings, and has an imidization rate (Rp) of 1 to 80%.

(In the general formula (2), _R1 and _R2 represent divalent aromatic diamine residues which may be the same or different from each other, _R3 and _R4 represent tetravalent aromatic tetracarboxylic acid residues which may be the same or different from each other, one of _R5 and _R6 is a hydrogen atom and the other is a phenyl group, m and n satisfy the relationships 1≦m, 0≦n, 1≦m+n≦20 and 0.05≦m/(m+n)≦1, and the arrangement of the repeating units may be either block or random.)

In one embodiment of the present invention, the divalent aromatic diamine residue represented by R ₁ and R ₂ in the general formula (2) refers to an aromatic organic group present between two amino groups of an aromatic diamine. In addition, in one embodiment of the present invention, the tetravalent aromatic tetracarboxylic acid residue represented by R ₃ and R ₄ in the general formula (2) refers to an aromatic organic group present between four carbonyl groups of 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 30 carbon atoms, more preferably an organic group having 4 to 18 carbon atoms, and even more preferably an organic group having 4 to 12 carbon atoms. In addition, the aromatic organic group is preferably a group containing carbon and hydrogen having 6 to 30 carbon atoms, more preferably a group containing carbon and hydrogen having 6 to 18 carbon atoms, and even more preferably a group containing carbon and hydrogen having 6 to 12 carbon atoms. The divalent aromatic diamine residues represented by R ₁ and R ₂ may be the same as each other or different from each other. The divalent aromatic diamine residues represented by _R1 and _R2 may be different aromatic diamine residues for each repeating unit.

The divalent aromatic diamine residue represented by _R1 and _R2 is more preferably at least one selected from the group consisting of 2-phenyl-4,4'-diaminodiphenyl ether (Ph-ODA), 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, 1,3-diaminobenzene, and 4-phenoxy-1,3-diaminobenzene.

The amic acid oligomer (A) represented by the general formula (2) is, for example, an amic acid oligomer using 2-phenyl-4,4'-diaminodiphenyl ether, and is more preferably synthesized by the following method.
(i) one or more tetracarboxylic acids selected from the group consisting of 1,2,4,5-benzenetetracarboxylic acids (particularly, their dianhydrides), 3,3',4,4'-biphenyltetracarboxylic acids (particularly, their dianhydrides), and bis(3,4-carboxyphenyl)ethers (particularly, their dianhydrides);
(ii) aromatic diamines including 2-phenyl-4,4'-diaminodiphenyl ether;
(iii) 4-(2-phenylethynyl)phthalic anhydride (PEPA), which is an unsaturated acid anhydride for introducing an unsaturated terminal group into the amic acid oligomer;
and the like are charged in amounts such that the total amount of dicarboxylic acid groups and the total amount of primary amino groups are approximately equal, and reacted in the presence (or absence) of an organic solvent.

When multiple dicarboxylic acid groups are adjacent, the number of dicarboxylic acid groups is calculated by dividing the number of carboxyl groups by 2. For example, the structure "COOH-COOH-COOH-COOH" contains two dicarboxylic acid groups.

In the thus synthesized amic acid oligomer (A), R ₃ and R ₄ are each independently selected from tetravalent aromatic tetracarboxylic acid residues derived from one or more of the above aromatic tetracarboxylic acids, and may be the same or different. In addition, in the case of 1<m and 1<n (m and n are integers), R ₃ (or R ₄ ) of each repeating unit may be the same or different. Furthermore, in the case of 1<m, one or both of a repeating unit in which R ₅ is a phenyl group and R ₆ is a hydrogen atom and a repeating unit in which R ₅ is a hydrogen atom and R ₆ is a phenyl group may be optionally included.

In the general formula (2), m and n satisfy the relationships 1≦m, 0≦n, 1≦m+n≦20, and 0.05≦m/(m+n)≦1.
The amic acid oligomer (P) having an imide group in one embodiment of the present invention may be 1≦m≦5, 0<n≦5, 1<m+n≦10, or 0.5≦m/(m+n)<1. Furthermore, the amic acid oligomer (P) having an imide group in one embodiment of the present invention is more preferably 4≦m+n, and even more preferably 5≦m+n. When m and n satisfy the above relationship, the amic acid oligomer (P) having an imide group in one embodiment of the present invention has better storage stability in solution, and the polyimide obtained by curing has better heat resistance and mechanical strength. Note that m and n can be easily adjusted by controlling the types and molar ratios of the tetracarboxylic acids and aromatic diamines used as raw materials.

In this specification, "the arrangement of the repeating units may be either block or random" means that the repeating unit portion may be a block polymer or a random polymer.

The above-mentioned amic acid oligomer (P) having an imide group is more preferably an amic acid oligomer having an amide bond in the main chain, (i) having an unsaturated end group capable of addition polymerization derived from 4-(2-phenylethynyl)phthalic anhydride at the end (more preferably both ends), (ii) satisfying the relationship 1≦m+n≦20, and (iii) being a solid at room temperature (e.g., 23°C).

Examples of the 1,2,4,5-benzenetetracarboxylic acids include 1,2,4,5-benzenetetracarboxylic acid, 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), and acid derivatives of 1,2,4,5-benzenetetracarboxylic acid (esters, salts, etc.). Of these, 1,2,4,5-benzenetetracarboxylic dianhydride is more preferred.

Examples of the 3,3',4,4'-biphenyltetracarboxylic acids include 3,3',4,4'-biphenyltetracarboxylic acid, 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA), and acid derivatives of 3,3',4,4'-biphenyltetracarboxylic acid (esters, salts, etc.). Of these, 3,3',4,4'-biphenyltetracarboxylic dianhydride is more preferred.

Examples of the bis(3,4-carboxyphenyl) ethers include bis(3,4-carboxyphenyl) ether, bis(3,4-carboxyphenyl) ether dianhydride (s-ODPA), and acid derivatives of bis(3,4-carboxyphenyl) ether (esters, salts, etc.). Among these, bis(3,4-carboxyphenyl) ether dianhydride is more preferred.

In one embodiment of the present invention, _R3 and/or _R4 may have a structure derived from an aromatic tetracarboxylic acid other than 1,2,4,5-benzenetetracarboxylic acids, 3,3',4,4'-biphenyltetracarboxylic acids, and bis(3,4-carboxyphenyl)ethers. In other words, the amic acid oligomer (P) having the imide group may be synthesized by adding an aromatic tetracarboxylic acid other than 1,2,4,5-benzenetetracarboxylic acids, 3,3',4,4'-biphenyltetracarboxylic acids, and bis(3,4-carboxyphenyl)ethers to the raw material (using it as a monomer). Examples of such other aromatic tetracarboxylic acids 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, 1,2,3,4-benzenetetracarboxylic dianhydride, etc. Only one type of other aromatic tetracarboxylic acid may be used, or two or more types may be used in combination.

In addition, the above-mentioned amide acid oligomer (P) having an imide group may be synthesized by adding other aromatic diamines other than 2-phenyl-4,4'-diaminodiphenyl ether to the raw materials (using them as monomers). Examples of such other aromatic diamines include 1,4-diaminobenzene, 1,3-diaminobenzene, 1,2-diaminobenzene, 2,6-diethyl-1,3-diaminobenzene, 4,6-diethyl-2-methyl-1,3-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, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 1,4-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, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, and the like. Among the above, 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, and 1,3-diaminobenzene are more preferred as the other aromatic diamines. Only one type of the other aromatic diamines may be used, or two or more types may be used in combination.

In particular, in applications requiring mechanical strength, it is preferable to use (i) 2-phenyl-4,4'-diaminodiphenyl ether and (ii) other aromatic diamines in combination in the synthesis of the above-mentioned amic acid oligomer. In this case, the amount of the other aromatic diamines used is preferably more than 0 and not more than 50 mol%, more preferably more than 0 and not more than 25 mol%, and even more preferably more than 0 and not more than 10 mol%, based on the total amount of the aromatic diamines being 100 mol%. The other aromatic diamines used in this case are preferably one or more selected from 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, and 1,3-diaminobenzene.

Similarly, in applications requiring mechanical strength, in general formula (1), it is preferable that 0.50≦m/(m+n)<1, it is more preferable that 0.75≦m/(m+n)<1, it is even more preferable that 0.90≦m/(m+n)<1, and it is even more preferable that 0.90≦m/(m+n)≦0.95.

Such amide acid oligomers are easy to handle, have high solubility, and also have excellent mechanical properties.

As the unsaturated acid anhydride for terminal modification (end capping), it is preferable to use 4-(2-phenylethynyl)phthalic anhydride. When synthesizing the above-mentioned amide acid oligomer, it is preferable to use 4-(2-phenylethynyl)phthalic anhydride in a range of 5 to 200 mol%, and more preferably in a range of 5 to 150 mol%, assuming the total amount of aromatic tetracarboxylic acids to be 100 mol%.

[3. Imide Oligomer (B)]
The irregular particles containing the amic acid oligomer (P) having an imide group in one embodiment of the present invention may contain an imide oligomer (B) represented by the following general formula (1) as an optional component. It is preferable that the imide oligomer (B) and the amic acid oligomer (P) having an imide group or the amic acid oligomer (A) are chemically compatible. In other words, it is preferable that the imide oligomer (B) is generated by a chemical reaction (imidization) from the amic acid oligomer (P) having an imide group or the amic acid oligomer (A).

Here, the imide oligomer (B) is different from the amic acid oligomer (P) having an imide group. That is, the imide oligomer (B) may have some of the imide units in the imide oligomer (B) represented by the following general formula (1) converted to amic acid units, and has an imidization rate (Rb) of more than 80% and not more than 100%. Preferably, the imidization rate is 90% or more and not more than 100%.

In other words, in the imide oligomer (B), the molar numbers of the amic acid units and the imide ring units in one molecule have the following relationship:
0.80<(molar number of imide ring units)/(molar number of imide ring units+molar number of amic acid units)≦1.00
Preferably, in the imide oligomer (B), the molar numbers of the amic acid units and the imide ring units in one molecule satisfy the following relationship:
0.90≦(moles of imide ring units)/(moles of imide ring units+moles of amic acid units)≦1.00

In this specification, unless otherwise specified, "imide oligomer" is used as a synonym for "terminally modified imide oligomer."

(In general formula (1), _R1 and _R2 , _R3 and _R4 , _R5 and _R6 , m and n, and the arrangement of repeating units are the same as those in general formula (2).)
In one embodiment of the present invention, the divalent aromatic diamine residues represented by _R1 and _R2 in general formula (1) are the same as those in general formula (2). In addition, in one embodiment of the present invention, the tetravalent aromatic tetracarboxylic acid residues represented by _R3 and _R4 in general formula (1) are the same as those in general formula (2).

As in the case of general formula (2), the divalent aromatic diamine residue represented by _R1 and _R2 is preferably at least one selected from the group consisting of 2-phenyl-4,4'-diaminodiphenyl ether (Ph-ODA), 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, 1,3-diaminobenzene, and 4-phenoxy-1,3-diaminobenzene.

The imide oligomer (B) represented by general formula (1) is, for example, an imide oligomer using 2-phenyl-4,4'-diaminodiphenyl ether, and is preferably derived from the corresponding (A) by imidization.

In the imide oligomer (B) thus derived, _R3 and _R4 are the same as those in the general formula (2).

In general formula (2), m and n are the same as in general formula (2).

The imide oligomer is more preferably an imide oligomer having an imide bond in the main chain, (i) having an addition-polymerizable unsaturated terminal group derived from 4-(2-phenylethynyl)phthalic anhydride at a terminal (more preferably both terminals), (ii) satisfying the relationship 1≦m+n≦20, and (iii) being a solid at room temperature (e.g., 23° C.).

Examples of the 1,2,4,5-benzenetetracarboxylic acids include 1,2,4,5-benzenetetracarboxylic acid, 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA), and acid derivatives of 1,2,4,5-benzenetetracarboxylic acid (esters, salts, etc.). Among these, 1,2,4,5-benzenetetracarboxylic dianhydride is more preferred. When R ₃ and R ₄ are 1,2,4,5-benzenetetracarboxylic acids, the imide oligomer is represented by the following general formula (1-2). In general formula (1-2), the definitions of R ₁ , R ₂ , R ₅ , R ₆ , m, and n are the same as those in general formula (1).

Examples of the 3,3',4,4'-biphenyltetracarboxylic acids include 3,3',4,4'-biphenyltetracarboxylic acid, 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA), and acid derivatives of 3,3',4,4'-biphenyltetracarboxylic acid (esters, salts, etc.). Among these, 3,3',4,4'-biphenyltetracarboxylic dianhydride is more preferred. When R ₃ and R ₄ are 3,3',4,4'-biphenyltetracarboxylic acids, the imide oligomer is represented by the following general formula (1-3). In the general formula (1-3), the definitions of R ₁ , R ₂ , R ₅ , R ₆ , m, and n are the same as those in the general formula (1).

Examples of the bis(3,4-carboxyphenyl) ethers include bis(3,4-carboxyphenyl) ether, bis(3,4-carboxyphenyl) ether dianhydride (s-ODPA), and acid derivatives of bis(3,4-carboxyphenyl) ether (esters, salts, etc.). Among these, bis(3,4-carboxyphenyl) ether dianhydride is more preferred. When R ₃ and R ₄ are bis(3,4-carboxyphenyl) ethers, the imide oligomer is represented by the following general formula (1-4). In general formula (1-4), the definitions of R ₁ , R ₂ , R ₅ , R ₆ , m, and n are the same as those in general formula (1).

In one embodiment of the present invention, _R3 and/or _R4 may have a structure derived from aromatic tetracarboxylic acids other than 1,2,4,5-benzenetetracarboxylic acids, 3,3',4,4'-biphenyltetracarboxylic acids, and bis(3,4-carboxyphenyl)ethers. In other words, the imide oligomer may be synthesized by adding aromatic tetracarboxylic acids other than 1,2,4,5-benzenetetracarboxylic acids, 3,3',4,4'-biphenyltetracarboxylic acids, and bis(3,4-carboxyphenyl)ethers to the raw material (using them as monomers). Examples of such other aromatic tetracarboxylic acids 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, 1,2,3,4-benzenetetracarboxylic dianhydride, etc. Only one type of other aromatic tetracarboxylic acid may be used, or two or more types may be used in combination.

In addition, the amic acid oligomer (A) from which the above imide oligomer (B) is derived may be synthesized by adding other aromatic diamines other than 2-phenyl-4,4'-diaminodiphenyl ether to the raw materials (using them as monomers). Examples of such other aromatic diamines include 1,4-diaminobenzene, 1,3-diaminobenzene, 1,2-diaminobenzene, 2,6-diethyl-1,3-diaminobenzene, 4,6-diethyl-2-methyl-1,3-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, 1,3-bis(4-aminophenoxy)benzene, 1,3-bis(3-aminophenoxy)benzene, 1,4-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, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, and the like. Among the above, 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, and 1,3-diaminobenzene are more preferred as the other aromatic diamines. Only one type of the other aromatic diamines may be used, or two or more types may be used in combination.

In particular, in applications requiring mechanical strength, it is preferable to use (i) 2-phenyl-4,4'-diaminodiphenyl ether and (ii) other aromatic diamines in combination in the synthesis of the above-mentioned amic acid oligomer. In this case, the amount of the other aromatic diamines used is preferably more than 0 and not more than 50 mol%, more preferably more than 0 and not more than 25 mol%, and even more preferably more than 0 and not more than 10 mol%, based on the total amount of the aromatic diamines being 100 mol%. The other aromatic diamines used in this case are preferably one or more selected from 9,9-bis(4-aminophenyl)fluorene, 9,9-bis(4-(4-aminophenoxy)phenyl)fluorene, and 1,3-diaminobenzene.

Similarly, in applications requiring mechanical strength, in general formula (1), it is preferable that 0.50≦m/(m+n)<1, it is more preferable that 0.75≦m/(m+n)<1, it is even more preferable that 0.90≦m/(m+n)<1, and it is even more preferable that 0.90≦m/(m+n)≦0.95.

Such amide acid oligomers are easy to handle, have high solubility, and also have excellent mechanical properties.

As the unsaturated acid anhydride for terminal modification (end capping), it is preferable to use 4-(2-phenylethynyl)phthalic anhydride. When synthesizing the above-mentioned amide acid oligomer, it is preferable to use 4-(2-phenylethynyl)phthalic anhydride in a range of 5 to 200 mol%, and more preferably in a range of 5 to 150 mol%, assuming the total amount of aromatic tetracarboxylic acids to be 100 mol%.

4. Solvent
The particles containing the amic acid oligomer (P) may contain a solvent (C) as an optional component. The content of the solvent (C) (residual solvent) in the particles containing the amic acid oligomer (P) is preferably 20% by weight or less, more preferably 10% by weight or less, and even more preferably 8% by weight or less.
The solvent is not particularly limited, but may be, for example, the solvent used in the synthesis of the amic acid oligomer.

For example, the solvent may include one or more solvents selected from the group consisting of alcohol-based solvents, ether-based solvents, and hydroxyether-based solvents.

Further examples include a mixed solvent of an alcohol solvent and an ether solvent; a single hydroxyether solvent; a mixed solvent of an alcohol solvent and a hydroxyether solvent; a mixed solvent of an ether solvent and a hydroxyether solvent; or a mixed solvent of an alcohol solvent, an ether solvent, and a hydroxyether solvent. In addition, whether these solvents are used as a single solvent or as a mixed solvent, it is more preferable that the boiling point (the boiling point of each single solvent in the case of a mixed solvent) is 130°C or less. Of these, a mixed solvent of methanol and 1,3-dioxolane is more preferable.

The solvent content can be measured by heating particles containing an amic acid oligomer to near the glass transition temperature (Tg) using DSC to soften them and volatilize the solvent that has been incorporated inside. In other words, in this specification, the solvent content (residual solvent) refers to the value calculated from the weight loss rate of particles containing an amic acid oligomer near the glass transition temperature in TGA-DSC.

[5. Other additives]
The amorphous particles containing the amic acid oligomer (P) having an imide group according to one embodiment of the present invention may be mixed with other soluble polyimides or thermoplastic polyimides. The thermoplastic polyimides are polyimides that become soft when heated, and examples of the thermoplastic polyimides include commercially available products, but are not limited thereto.

[6. Imidized and cured product]
The 5% weight loss temperature in air of the polyimide resin obtained by imidizing and curing the amorphous particles containing the amic acid oligomer (P) having an imide group in one embodiment of the present invention is preferably 520°C or higher, more preferably 530°C or higher, and even more preferably 535°C or higher. It is considered that the 5% weight loss temperature in air is correlated with the rate of oxidative deterioration when the polyimide resin is used for a long period of time in a high-temperature environment. The higher the 5% weight loss temperature in air, the longer the polyimide resin can be used in a high-temperature environment. In other words, it can be said that the higher the 5% weight loss temperature in air, the better the long-term heat resistance stability. The 5% weight loss temperature in air refers to a value measured by the method described in the examples below.

In one embodiment of the present invention, the amorphous particles containing the imide group-containing amic acid oligomer (P) can be pressed, heated and melted, and imidized and cured to form, for example, a polyimide film. This film has unprecedented new physical properties and characteristics, and has the advantage of being easy to handle and moldable, and can provide a fiber-reinforced composite material that exhibits excellent heat resistance and mechanical properties.

Amorphous particles containing an amide acid oligomer (P) having an imide group can be used in a wide range of fields requiring easy moldability and high heat resistance, including general industrial applications such as aircraft, space industry equipment, and engine (peripheral) parts for vehicles, transport arms, robot arms, roll materials, friction materials, and sliding parts such as bearings. Examples of aircraft parts include engine fan cases, inner frames, moving blades (fan blades, etc.), stationary blades (structural guide vanes (SGVs), etc.), bypass ducts, and various piping. Examples of vehicle parts include brake parts, engine parts (cylinders, motor cases, air boxes, etc.), and energy regeneration system parts.

[7. Method for producing irregular particles containing an amic acid oligomer (P) having an imide group]
The above-mentioned amic acid oligomer (P) having an imide group can be synthesized, for example, by the following method.
1. (i) one or more aromatic tetracarboxylic acids selected from the group consisting of 3,3',4,4'-biphenyltetracarboxylic acids (particularly, this acid dianhydride), 1,2,4,5-benzenetetracarboxylic acids (particularly, this acid dianhydride), and bis(3,4-carboxyphenyl)ethers (particularly, this dianhydride), (ii) aromatic diamines including 2-phenyl-4,4'-diaminodiphenyl ether, and (iii) 4-(2-phenylethynyl)phthalic anhydride are prepared in such amounts that the total amount of dicarboxylic acid groups and the total amount of primary amino groups are approximately equal.
2. Each of the prepared monomers is polymerized in an organic solvent at a reaction temperature of about 100° C. or less (preferably about 80° C. or less) to synthesize an amic acid oligomer (A) in a solution state.
3. The solution of the amic acid oligomer is melt processed at a temperature of 180° C. or less, thereby obtaining irregular particles containing the amic acid oligomer (P) having 4-(2-phenylethynyl)phthalic anhydride residues at the terminals.

As a more preferred specific example of a method for producing the irregular particles containing the above-mentioned amic acid oligomer, the following method can be mentioned.
1. An aromatic diamine containing 2-phenyl-4,4'-diaminodiphenyl ether is dissolved uniformly in an organic solvent.
2. An aromatic tetracarboxylic dianhydride is added to the obtained solution and dissolved uniformly. The aromatic tetracarboxylic dianhydride is one or more selected from 3,3',4,4'-biphenyltetracarboxylic dianhydride, 1,2,4,5-benzenetetracarboxylic dianhydride, and bis(3,4-carboxyphenyl)ether dianhydride.
3. The resulting solution is reacted at a reaction temperature of about 5 to 60° C. with stirring for about 1 to 180 minutes.
4. Add 4-(2-phenylethynyl)phthalic anhydride to the resulting reaction solution and dissolve uniformly.
5. The resulting solution is reacted with stirring for about 1 to 180 minutes at a reaction temperature of about 5 to 60° C. In this manner, an amic acid oligomer (A) in the form of a solution is synthesized.
6. The reaction solution thus obtained is melt-extruded at a temperature of 180° C. or less to cause an imidization reaction and form particles. After melt-extrusion, the particles thus obtained are cooled to about room temperature, if necessary. In this way, irregular particles containing the amic acid oligomer (P) having the imide group are synthesized.

All or some of the steps in the above synthesis are preferably carried out under an inert gas atmosphere (such as nitrogen gas, argon gas, etc.) or under reduced pressure.

Examples of organic solvents used in the synthesis include N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), N,N-diethylacetamide, N-methylcaprolactam, γ-butyrolactone (GBL), cyclohexanone, and a mixed solvent of methanol and 1,3-dioxolane. Only one of these organic solvents may be used, or two or more of them may be used in combination. These organic solvents can be appropriately selected by applying known techniques related to soluble polyimides. Of these, it is more preferable to use a mixed solvent of methanol and 1,3-dioxolane.

That is, in one embodiment of the present invention, the method for producing irregular particles containing an amic acid oligomer (P) having an imide group is a method that includes a step of melt processing a solution of an amic acid oligomer (A) represented by the following general formula (2):

(In formula (2), _R1 and _R2 represent divalent aromatic diamine residues which may be the same or different from each other, _R3 and _R4 represent divalent aromatic tetracarboxylic acid residues which may be the same or different from each other, _R5 and _R6 represent a hydrogen atom or a phenyl group, and one of them is a phenyl group, m and n satisfy the relationships 1≦m, 0≦n, 1≦m+n≦20 and 0.05≦m/(m+n)≦1, and the arrangement of the repeating units may be either block or random.)
The solution of the amic acid oligomer can be produced by reacting aromatic tetracarboxylic acids, aromatic diamines, and an unsaturated acid anhydride for terminal modification in a solvent, as described above. The method for producing the solution of the amic acid oligomer can be a known method and is not particularly limited to the above-mentioned method.

The solvent used to prepare the solution of the amic acid oligomer, i.e., the solvent in which the amic acid oligomer is dissolved, is preferably the solvent used during the reaction. Specifically, for example, amide-based solvents such as N-methyl-2-pyrrolidone (NMP, boiling point 202°C (unless otherwise specified below, the boiling point indicates the temperature under normal pressure)), N,N-dimethylacetamide (DMAc, boiling point 165°C), N,N-diethylacetamide (boiling point 185°C), and N-methylcaprolactam (boiling point 120°C under reduced pressure (25 mmHg)); ester-based solvents such as γ-butyrolactone (GBL, boiling point 204°C); ketone-based solvents such as cyclohexanone (boiling point 156°C); 1,3-diaminopropanediol (1,3-dimethylpropanediol, boiling point 165°C); Examples of the solvent include ether solvents such as hexane (boiling point 105°C), 1,4-dioxane (boiling point 101°C), tetrahydrofuran (boiling point 66°C), 1,3-dioxolane (boiling point 75°C), and 1,2-dimethoxyethane (boiling point 83°C); alcohol solvents such as methanol (boiling point 65°C), ethanol (boiling point 78°C), 1-propanol (boiling point 98°C), and 2-propanol (boiling point 82°C); and hydroxyether solvents such as methoxymethanol (boiling point 90-95°C), ethoxymethanol (boiling point 102°C), 2-methoxyethanol (boiling point 124°C), 2-ethoxyethanol (boiling point 135°C), and 1-methoxy-2-propanol (boiling point 119°C). These solvents may be used alone or in combination of two or more. In addition, a mixed solvent such as a mixed solvent of methanol and 1,3-dioxolane may be used as the solvent.

The boiling point of the solvent (at normal pressure) is preferably 200°C or less, more preferably 150°C or less, even more preferably 130°C or less, and most preferably 100°C or less, in view of the ease of volatilization during melt processing. If the boiling point (at normal pressure) exceeds 200°C, the precipitation of the amic acid oligomer may not proceed smoothly, or the solvent may be difficult to volatilize.

In addition, in view of the ease of precipitation of the imide oligomer during volatilization, the solvent is more preferably an ester solvent, a ketone solvent, an ether solvent, an alcohol solvent, a hydroxyether solvent, or a mixed solvent of these solvents, and even more preferably an ether solvent, an alcohol solvent, a hydroxyether solvent, or a mixed solvent of these solvents and an alcohol solvent. The ether solvent is more preferable because it has two or more oxygen atoms per molecule and has excellent solubility. A solvent that has too high amic acid oligomer dissolving ability may not allow smooth precipitation of the amic acid oligomer or may be difficult to volatilize. A solvent that has too low amic acid oligomer dissolving ability may not be able to obtain an amic acid oligomer solution of an appropriate concentration.

Among the above-mentioned solvents, a mixed solvent of an alcohol-based solvent having excellent solubility for the tetracarboxylic acids and aromatic diamines used as raw materials for the amic acid oligomer and an ether-based solvent having excellent solubility for the tetracarboxylic acids and the obtained amic acid oligomer; a single solvent of a hydroxyether-based solvent having the characteristics of both an alcohol-based solvent and an ether-based solvent; a mixed solvent of an alcohol-based solvent and a hydroxyether-based solvent; a mixed solvent of an ether-based solvent and a hydroxyether-based solvent; or a mixed solvent of an alcohol-based solvent, an ether-based solvent, and a hydroxyether-based solvent; are particularly preferred because they can efficiently polymerize the amic acid oligomer. In consideration of solubility, it is more preferred that the ether-based solvent has two or more oxygen atoms per molecule. In addition, whether these solvents are used as a single solvent or a mixed solvent, it is more preferred that their boiling points (boiling points of each single solvent in the mixed solvent) are 130°C or less. In consideration of volatility, a mixed solvent of an alcohol-based solvent and an ether-based solvent is even more preferred because it forms an azeotropic mixture and volatilizes. Alcohol-based solvents and hydroxyether-based solvents have the advantage that the hydroxyl groups in the molecules stabilize radicals that may be formed from ether-based solvents during autoxidation with oxygen, for example, making it difficult for peroxides to be generated when obtaining an amic acid oligomer. In other words, alcohol-based solvents and hydroxyether-based solvents also act as stabilizers for ether-based solvents. Unlike ether-based solvents, the above-mentioned hydroxyether-based solvents have the function of stabilizing themselves even when used alone. However, they may be used as mixed solvents with alcohol-based solvents in the hope of further stabilization.

Specific examples of mixed solvents of alcohol-based solvents and ether-based solvents include mixed solvents of methanol and 1,3-dioxolane, more preferably mixed solvents with a weight ratio of methanol:1,3-dioxolane of 1:4 to 4:1, and even more preferably mixed solvents with a weight ratio of 1:2 to 2:1.

The concentration of the amic acid oligomer solution is preferably 10 to 90% by weight, more preferably 20 to 70% by weight, and even more preferably 30 to 50% by weight. When the concentration of the amic acid oligomer solution is within the above range, particles containing the amic acid oligomer can be efficiently produced.

In the melt processing step, the amic acid oligomer (A) is melt processed while volatilizing the solvent from the solution of the amic acid oligomer (A) to produce an amic acid oligomer (P) having an imide group.
The melt processing is preferably carried out at a temperature of 180°C or less. Therefore, the set temperature of the processing machine in the melt processing is preferably 80 to 230°C, more preferably 100 to 210°C, and even more preferably 120 to 190°C. If the set temperature of the processing machine is less than 80°C, the solvent tends to volatilize insufficiently. If the set temperature of the processing machine is more than 230°C, the corresponding imide oligomer tends to be excessively produced or an undesired curing reaction tends to be promoted. The imidization reaction can be easily controlled by adjusting the temperature during melt processing.

The melt processing method is not particularly limited, but a simple method is to use an extruder as a processing machine to perform melt extrusion. The extruder that can be used in the melt processing step is any extruder that can heat the amic acid oligomer and extrude it while melt-kneading it, and examples of the extruder that can be used include, but are not limited to, a single-screw extruder and a twin-screw extruder. Among these, a twin-screw extruder is preferred. The extruder may be equipped with a perforated plate at its tip.

It is desirable to provide a temperature gradient from the supply port side to the extrusion port side of the extruder so that melt extrusion can be carried out optimally. Specifically, the temperature on the supply port side is preferably room temperature to 80°C, more preferably room temperature to 50°C, and even more preferably around room temperature. The temperature on the extrusion port side is preferably 80 to 230°C, more preferably 100 to 210°C, and even more preferably 120 to 190°C.

The extruder serves as a desolvation device that volatilizes the solvent from the amic acid oligomer solution, a reactor that performs an imidization reaction (dehydration reaction) until the desired imidization rate is reached if necessary, and a powdering device that turns the resulting amic acid oligomer into particles. Therefore, within the extruder, the evaporation of the solvent (desolvation), the imidization reaction if necessary, and powdering can be carried out continuously from the supply port side to the extrusion port side.

That is, in the method for producing particles containing an amic acid oligomer in one embodiment of the present invention, for example, an extruder is used to heat and melt-knead a solution of the amic acid oligomer, and during the melt-kneading, the solvent and, if necessary, the water produced by the imidization reaction are volatilized while the imidization reaction is carried out, so that even if partial imidization is required, it is not necessary to add an imidization agent or a dehydration agent.

The residence time of the amic acid oligomer in the extruder is preferably 1 to 30 minutes, more preferably 2 to 20 minutes, and even more preferably 3 to 10 minutes. By adjusting the residence time by controlling the shape and rotation speed of the extrusion member such as the screw, the imidization reaction can be easily controlled when necessary. If the residence time is less than 1 minute, the solvent will not be sufficiently removed and the desired imidization rate will not be obtained. If the residence time exceeds 30 minutes, the curing reaction will tend to be accelerated.

In other words, in the method for producing particles containing an amic acid oligomer according to one embodiment of the present invention, the imidization rate of the particles containing an amic acid oligomer obtained by melt extrusion can be easily controlled as required by appropriately adjusting the shape and rotation speed of the extrusion member, and the temperature and residence time during melt extrusion.

The irregular particles containing the amic acid oligomer (P) having imide groups extruded from the extruder are irregular particles. If the particles containing the amic acid oligomer are connected together to form flakes, they can be lightly crushed. The obtained irregular particles containing the amic acid oligomer having imide groups can be subjected to a curing reaction without purification.

In the manufacturing method according to one embodiment of the present invention, amorphous particles containing an amic acid oligomer (P) having an imide group are produced by performing a melt processing step, and since no solvent, imidization agent, or dehydration agent is used during imidization when necessary, there is no need for a recovery step in which the amic acid oligomer in the solution is precipitated (deposited) in a poor solvent, or a post-processing step such as a washing step or a drying step. Therefore, the manufacturing method according to one embodiment of the present invention can produce particles containing an amic acid oligomer inexpensively, easily, and in an extremely short time (several minutes) compared to conventional techniques.

The particles containing the imide group-containing amic acid oligomer (P) obtained by the manufacturing method in one embodiment of the present invention are irregular particles, and each particle has an acute angle (not spherical). Whether the particle is an "irregular particle" can be easily determined by observing it using, for example, an optical microscope. For example, when comparing FIG. 1, which shows an optical microscope observation of the irregular particle containing the imide group-containing amic acid oligomer (P) in one embodiment of the present invention manufactured by the above manufacturing method, with FIG. 2, which shows an optical microscope observation of the particle containing the amic acid oligomer manufactured by a conventional manufacturing method, it is easily understood that the irregular particle containing the imide group-containing amic acid oligomer (P) in one embodiment of the present invention has an acute angle and is an "irregular particle".

8. Prepreg
The amorphous particles containing the amic acid oligomer (P) having an imide group in one embodiment of the present invention can be mixed with reinforcing fibers to form a prepreg. The prepreg uses amorphous particles containing the amic acid oligomer (P) having an imide group in one embodiment of the present invention, and thus the remaining volatile matter such as a solvent is reduced. In addition, a resin composite material (e.g., a carbon fiber reinforced composite material) manufactured using the prepreg has a very advantageous effect of having a glass transition temperature (Tg) equal to or higher than that of a resin alone, since defects such as voids caused by the volatilization and decomposition of volatile matter such as a solvent are reduced (or eliminated). Note that one embodiment of the present invention may include a fiber reinforced laminate obtained by stacking the prepreg and heat curing it.

The amount of residual volatile matter in the prepreg is preferably 20% by weight or less, more preferably 10% by weight or less, and even more preferably 8% by weight or less, based on the weight derived from the amic acid oligomer (P) having imide groups contained in the prepreg. The volatile matter in the prepreg includes the organic solvent used in the production of the amic acid oligomer and the water that is eliminated when the amic acid oligomer is imidized. If the amount of residual volatile matter in the prepreg is within the above range, when a resin composite material (e.g., a carbon fiber reinforced composite material) is produced using the prepreg, defects such as voids caused by the volatilization and decomposition of the solvent, etc. are reduced or eliminated, and a good composite material having a Tg equal to or greater than that of the resin alone is obtained, which is preferable.

In one embodiment of the present invention, even if the molecular weight of the amic acid oligomer is relatively large, a prepreg and a resin composite material can be produced without preparing a suspension of particles containing the amic acid oligomer. Furthermore, the prepreg can maintain its drapeability, and the resulting resin composite material has excellent heat resistance.

In this specification, "prepreg" refers to a resin-reinforced fiber composite in which amorphous particles containing an amic acid oligomer are partially impregnated (semi-impregnated state) into the reinforcing fibers, forming an integrated structure. Because "prepreg" is in a semi-impregnated state, it contains a fiber arrangement that is not impregnated with resin, which means that drapeability is not impaired and it is easy to mold into complex shapes. One form of "prepreg" is often one in which a resin-rich layer is present on the outer surface of the reinforcing fibers.

The above "drapeability" refers to an index showing the degree of flexibility of deformation of a resin-reinforced fiber composite. "Drapability" is an index showing the degree to which a resin-reinforced fiber composite flexibly conforms to the shape of another object such as a mold without destruction or breakage of the reinforcing fibers when deformed along the object. High drapeability makes it easy to mold onto a curved surface, while low drapeability makes it difficult to mold onto a curved surface. Furthermore, low drapeability naturally makes it difficult to mold press-molded products with complex shapes.

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the different embodiments are also included in the technical scope of the present invention.

The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. Note that the pressures in the examples and comparative examples are all actual pressures applied to the samples, and are not the indicated pressures (gauge pressures) of presses, etc.

[Various test methods for particles containing imide oligomers]
(1) Measurement of glass transition temperature (Tg) and heat generation start temperature Measurement was performed using a Q100 (DSC) device manufactured by TA Instruments. The measurement conditions were 1st run: 40 to 450°C, heating rate 20°C/min, 2nd run: 40 to 500°C, heating rate 20°C/min. The intersection of the two tangents before and after the heat flow (W/g) curve of the 2nd run was lowered was taken as the glass transition temperature (Tg) after curing of the particles containing the imide oligomer. That is, the glass transition temperature (Tg) after curing of the particles (powder) containing the imide oligomer is the Tg of the 2nd run in the DSC measurement. In addition, when an exothermic peak was observed in the heat flow (W/g) curve, the temperature of the exothermic peak was taken as the curing exothermic peak temperature of the particles containing the imide oligomer.

(2) Measurement of imidization rate The imide oligomer was dissolved in deuterated DMF (deuterated N,N-dimethylformamide) to prepare a measurement sample, and the peak area was measured at 30° C. using a proton nuclear magnetic resonance spectrometer (model: AV-400M, manufactured by Bruker Corporation, ¹ H-NMR). The imidization rate (mol %) of the particles containing the imide oligomer was calculated from the peak area derived from aromatic ¹ H with a chemical shift of 7 to 9 ppm and the peak area derived from residual amide ¹ H with a chemical shift of around 11 ppm.

(3) Measurement of particle size distribution and average particle size Particles containing imide oligomers were dispersed in water to prepare a measurement sample, and the volume average particle size distribution of the particles containing imide oligomers was measured using a laser diffraction particle size distribution measuring device (Microtrack, manufactured by Nikkiso Co., Ltd.). The 50% cumulative volume average particle size was taken as the average particle size (μm) of the particles containing imide oligomers.

(4) Tensile Test Using an Autograph AGS-X manufactured by Shimadzu Corporation, a tensile test was performed on the cured film (press molded product) made from the particles containing the imide oligomer at a chuck distance of 20 mm and a TS of 5 mm/min to measure the mechanical properties (tensile modulus (N/mm ² ), breaking stress (N/mm ² ), and breaking strain (%)).

(5) Solubility Test 10 g of N-methyl-2-pyrrolidone (NMP) was added to 0.1 g of particles containing an imide oligomer, and the mixture was stirred at room temperature for 1 hour so that the concentration became 1% by weight. Then, the presence or absence of insoluble matter was visually confirmed, whereby a solubility test was performed.

(6) Particle Shape An appropriate amount of particles containing imide oligomer was taken, and the shape of the aggregated particles was observed using an optical microscope (VHX-5000, manufactured by KEYENCE). In addition, 3.7 g of methanol was placed in a screw tube, and about 0.1 mg of particles containing imide oligomer was added, and then ultrasonic waves were applied for 10 minutes using an ultrasonic cleaning device. Next, the obtained suspension was collected using a Pasteur pipette, and one drop was dropped onto a glass plate, which was then thoroughly dried at room temperature in a draft. The shape of a single particle was then observed using an ultra-deep microscope (manufactured by KEYENCE) by observing the residue after volatilizing the methanol.

(7) Fluidity The fluidity of the particles containing the imide oligomer was evaluated by relative comparison as good (A), slightly good (B), normal (C), usable (just barely usable) (D), or poor (E).

(8) Residual Solvent Ratio About 2 mg of particles containing an amic acid oligomer was weighed out, and the temperature was raised from room temperature to 600° C. at a rate of 5° C./min in a nitrogen stream using a TGA-50 (manufactured by Shimadzu Corporation). The weight loss rate observed near the glass transition temperature (Tg) was taken as the solvent content (residual solvent ratio).

(9) BET Specific Surface Area Particles containing an amic acid oligomer were placed in a TriStar 3000 (manufactured by Shimadzu Corporation) and subjected to pretreatment in air at 120° C. for 1 hour, after which the BET specific surface area was measured in nitrogen at 77 K.

(10) GPC Molecular Weight A solution of particles containing an amic acid oligomer in N-methylpyrrolidone (containing 0.01 M LiBr) was prepared, and 20 μL of the sample was measured using a Prominence (manufactured by Shimadzu Corporation) equipped with a degasser, pump, column oven, UV detector, and refractive index detector, and connected in series to Phenogel (column, manufactured by Phenomenex). Standard polystyrene (manufactured by TOSOH Corporation) was used as a sample for preparing a calibration curve.

[Production Example 1]
After heating the inside of a 7 L autoclave equipped with a thermometer and a stirrer to 45°C with a heater, 1105 g (4.0 mol) of 2-phenyl-4,4'-diaminodiphenyl ether (Ph-ODA) and a mixed solvent of 1650 g of 1,3-dioxolane and 1650 g of methanol (weight ratio 1:1) were added and a nitrogen flow was started. After dissolving the 2-phenyl-4,4'-diaminodiphenyl ether, 232.6 g of 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA) was added to the autoclave three times at intervals (total of 697.8 g (3.2 mol)), and polymerization reaction was carried out at a set temperature of 45°C for 3 hours to obtain an amic acid oligomer. To the reaction solution containing this amic acid oligomer, 397.1 g (1.6 mol) of 4-(2-phenylethynyl)phthalic anhydride (PEPA) was added, and the reaction was continued for 1.5 hours to modify the terminals, thereby obtaining a solution of a terminal-modified amic acid oligomer (solid content (theoretical value)=2200/5500=about 40% by weight).

The obtained terminal-modified amic acid oligomer has n=0, _R1 is represented by a 2-phenyl-4,4'-diaminodiphenyl ether residue, _R3 is represented by a 1,2,4,5-benzenetetracarboxylic dianhydride residue, and m=4 (tetramer) on average in the above general formula (2). The obtained solution of the terminal-modified amic acid oligomer was named varnish (N1). The imidization rate of the terminal-modified amic acid oligomer in the varnish was approximately 0%.

[Production Example 2]
23.43 g (84.8 mmol) of 2-phenyl-4,4'-diaminodiphenyl ether and 82.5 g of N-methyl-2-pyrrolidone (NMP) were placed in a three-necked 300 mL flask equipped with a thermometer and a stirrer. After dissolving the 2-phenyl-4,4'-diaminodiphenyl ether, 3.28 g (9.41 mmol) of 9,9-bis(4-aminophenyl)fluorene was added to the flask and stirred until dissolved. Next, 16.44 g (75.4 mmol) of 1,2,4,5-benzenetetracarboxylic dianhydride was added to the flask, and the mixture was sealed with nitrogen and polymerized at room temperature for 1.5 hours to obtain an amic acid oligomer. To the reaction solution containing this amic acid oligomer, 9.35 g (37.7 mmol) of 4-(2-phenylethynyl)phthalic anhydride and 15 g of NMP were added, and nitrogen was sealed in, followed by reaction at room temperature for 1.5 hours to modify the terminals. Further, a nitrogen inlet tube was attached to the flask, and the mixture was stirred under a nitrogen stream at 200° C. for 5 hours to form imide bonds, followed by cooling to obtain an NMP solution of a terminal-modified imide oligomer (solid content concentration (theoretical value)=52.5/150=approximately 35% by weight).

The obtained terminal-modified imide oligomer has the above general formula (1), in which R ₁ is represented by a 2-phenyl-4,4'-diaminodiphenyl ether residue or a 9,9-bis(4-aminophenyl)fluorene residue, R ₂ is represented by a 9,9-bis(4-aminophenyl)fluorene residue, R ₃ and R ₄ are represented by a 1,2,4,5-benzenetetracarboxylic dianhydride residue, and m=3.6 and n=0.4 on average. The NMP solution of the obtained terminal-modified imide oligomer was named varnish (N2). The imidization rate of the terminal-modified amic acid oligomer in the varnish was approximately 0%.

Example 1
As the twin-screw extruder used in the melt processing step, MPU25TW (manufactured by Technovel Co., Ltd.), which is a twin-screw type with a screw diameter of 25 mmφ and a maximum screw effective length (L/D) of 60, was used. The heater temperature of the cylinder part of this twin-screw extruder was set such that the varnish inlet (C0) was only water-cooled, and the inlet side block (C1) was set at 40 ° C., the upstream intermediate block (C4) was set at 80 ° C., and approximately half of the downstream blocks (C6 to C10, DH) were set at 150 ° C., with a temperature gradient between them. No die was attached to the downstream end of the twin-screw extruder, and the die head was set to open. The screw rotation speed was set to 10 rpm. This rotation speed corresponds to a residence time of about 6 minutes.

When varnish (N1) was fed through the varnish inlet (C0), a yellow powder was discharged from the downstream end of the twin-screw extruder. No blocking was observed in the obtained powder, and it had usable fluidity. In the solubility test, no insoluble matter was confirmed. When this powder was measured using the various test methods described above, the imidization rate was 7.1 mol%, the average particle size was 55 μm (single peak), and the particle shape was irregular. Therefore, the powder was an irregular particle containing an amic acid oligomer.

This powder was then preheated in a hot press heated to 325°C. After confirming that it was melted, pressure was gradually applied and the air was removed by repeatedly pumping. The powder was then heated while applying a predetermined pressure, and after reaching 370°C, it was heated and pressurized for one hour to obtain a cured film (press molded product). A tensile test was carried out to measure the mechanical properties (tensile modulus, breaking stress, and breaking strain), which showed good properties. The results are summarized in Table 1.

Example 2
The heater temperature of the cylinder part of the twin screw extruder was set such that the varnish inlet (C0) was only water-cooled, the inlet side block (C1) was 40°C, the upstream intermediate block (C4) was 80°C, and approximately half of the downstream blocks (C6-C10, DH) were 150°C, with a temperature gradient between them. The screw rotation speed was set to 30 rpm. This rotation speed corresponds to a residence time of about 3 minutes. The varnish (N1) was introduced from the varnish inlet (C0) under the same conditions as in Example 1, except for the above.

When varnish (N1) was fed through the varnish inlet (C0), a yellow powder was discharged from the downstream end of the twin-screw extruder. No blocking was observed in the obtained powder, and it had usable fluidity. In the solubility test, no insoluble matter was confirmed. When this powder was measured using the various test methods described above, the imidization rate was 20.0 mol%, the average particle size was 48 μm (single peak), and the particle shape was irregular. Therefore, the powder was an irregular particle containing an amic acid oligomer.

The mechanical properties of the cured film obtained from this powder under the same conditions as in Example 1 were measured by tensile testing, and the properties were good. The results are summarized in Table 1.

Comparative Example 1
The heater temperature of the cylinder part of the twin screw extruder was set such that the varnish inlet (C0) was only water-cooled, the inlet side block (C1) was 40°C, the upstream intermediate block (C4) was 80°C, and approximately half of the downstream blocks (C6-C10, DH) were 200°C, with a temperature gradient between them. The screw rotation speed was set to 30 rpm. This rotation speed corresponds to a residence time of about 3 minutes. The varnish (N1) was introduced from the varnish inlet (C0) under the same conditions as in Example 1, except for the above.

When varnish (N1) was fed through the varnish inlet (C0), a yellow powder was discharged from the downstream end of the twin-screw extruder. The resulting powder showed no blocking and had normal fluidity. No insoluble matter was found in the solubility test. When this powder was measured using the various test methods described above, the imidization rate was 91.8 mol%, the average particle size was 54 μm (single peak with a shoulder), and the particle shape was irregular. Therefore, the powder was not an irregular particle containing the amide acid oligomer of the present invention. The results are summarized in Table 1.

Comparative Example 2
Without using a twin-screw extruder, varnish (N1) was poured into a glass container, and left to stand in a vacuum oven set at 45° C. for 18 hours, and then further left to stand in a hot air oven set at 250° C. for 1 hour to obtain a yellow solid. In other words, no powder was obtained.

This solid was cut out and measured using the various test methods described above, and the imidization rate was found to be 99.9%. In the solubility test, no insoluble matter was found. In addition, a cured film was obtained from this solid under the same conditions as in Example 1, and the mechanical properties were measured by a tensile test. The results are summarized in Table 1.

Comparative Example 3
100 g of varnish (N2) was added to 720 g of methanol, and the precipitated powder was filtered off. The obtained powder was washed with 480 g of methanol for 30 minutes five times, and then the powder was filtered off. The obtained powder was dried under reduced pressure at 120° C. for one day to obtain a granular terminal-modified imide oligomer. The obtained granular terminal-modified imide oligomer was then pulverized with a hammer mill to obtain a terminal-modified imide oligomer powder.

The obtained powder showed no blocking and had usable fluidity. In the solubility test, no insoluble matter was found. When this powder was measured using the various test methods described above, the imidization rate was 98.2 mol%, the average particle size was 71 μm (single peak), and the particle shape was amorphous. In addition, when the mechanical properties of a cured film obtained from this powder under the same conditions as in Example 1 were measured by a tensile test, the properties were good. The results are summarized in Table 1.

[Discussion of the results]
Comparing Example 1 and Example 2, it can be seen that by setting the heater temperature of the extruder (which refers to the set temperature of roughly half of the downstream blocks (C7 to C10); the same applies below) to 150°C, it is possible to reduce the imidization rate of the resulting powder (irregular particles containing amic acid oligomer) to roughly 20 mol% or less, and the powder flowability is not impaired. It can also be seen that even if the heater temperature of the extruder is the same, the imidization rate can be controlled by controlling the screw rotation speed.

Among these, the physical properties of the cured product shown in Example 2 show that the breaking strain is particularly large, and that the mechanical properties are excellent. Although the reason for this is unclear, it is speculated that the imide oligomer, which is a partially imidized component, is difficult to melt and therefore forms domains, while the amic acid oligomer is easy to melt and therefore forms a matrix, and that the crosslinking can also vary in density, resulting in a flexible and resilient structure, which improves the breaking elongation without significantly deteriorating the elastic modulus. In other words, from the viewpoint of mechanical properties, it is speculated that there is an optimum point for the imidization rate.

In addition, when comparing Examples 1 and 2 with Comparative Example 1, it can be seen that by increasing the heater temperature of the extruder to 200°C, the imidization rate is significantly improved to exceed 80%, which is the upper limit in the present invention, and particles containing amic acid oligomers cannot be obtained.

Comparing the physical properties of the cured film made from the amorphous particles containing amic acid oligomer obtained in Examples 1 and 2 with the physical properties of the cured film made from the varnish obtained in Comparative Example 2, it is found that Example 1 and Comparative Example 2 show similar physical properties. However, Comparative Example 2 takes more than 19 hours to obtain a solid from the varnish (N1), which is poor in efficiency. Moreover, Comparative Example 2 requires the laborious process of cutting out the obtained solid and then processing it into a cured film, which makes it poor in processability and handleability. In contrast, Examples 1 and 2 are found to be excellent in efficiency, processability, and handleability.

In Comparative Example 3, a terminally modified imide oligomer powder was obtained using a conventional manufacturing method (grinding method). Comparing the physical properties of the cured film obtained in Examples 1 and 2 with those of the cured film obtained in Comparative Example 3, it can be seen that Comparative Example 3 has less elongation and harder physical properties. In addition, in Comparative Example 3, it takes a long time and a large amount of methanol to obtain a powder by reprecipitation from the varnish (N2), making it less efficient. Moreover, in Comparative Example 3, it requires the labor of pulverizing with a hammer mill (grinding machine) to obtain a powder, making it less easy to process and handle. In contrast, it can be seen that Examples 1 and 2 are excellent in efficiency, processability, and handleability.

The present invention has excellent moldability, heat resistance, and excellent mechanical and electrical properties, and can therefore be used as a material (heat-resistant composite material) in a wide range of fields, including aircraft, space industry equipment, electrical and electronic equipment, general industrial applications, and vehicle engine (peripheral) components.

Claims (8)

The present invention relates to irregular particles having an average particle size of 100 to 500 μm as measured by dispersing the particles in water, the average particle size being ...

(In the general formula (2), _R1 and _R2 represent divalent aromatic diamine residues which may be the same or different from each other, _R3 and _R4 represent tetravalent aromatic tetracarboxylic acid residues which may be the same or different from each other, one of _R5 and _R6 is a hydrogen atom and the other is a phenyl group, m and n satisfy the relationships 1≦m, 0≦n, 1≦m+n≦20 and 0.05≦m/(m+n)≦1, and the arrangement of the repeating units may be either block or random.) The irregular particles according to claim 1, containing 20% by weight or less of a solvent. The irregular particles according to claim 2, wherein the solvent is a mixed solvent of an alcohol-based solvent and an ether-based solvent, a single solvent of a hydroxyether-based solvent, a mixed solvent of an alcohol-based solvent and a hydroxyether-based solvent, a mixed solvent of an ether-based solvent and a hydroxyether-based solvent, or a mixed solvent of an alcohol-based solvent, an ether-based solvent and a hydroxyether-based solvent, and the boiling point (the boiling point of each single solvent in the mixed solvent) is 130°C or lower. The irregular particles according to claim 3, wherein the solvent is a mixed solvent of methanol and 1,3-dioxolane. 2. A method for producing irregular particles according to claim 1, comprising the step of melt-processing a solution of an imide group-containing amic acid oligomer (P) having an imidization rate of 0 to 1%, in which a part of the amic acid units in the amic acid oligomer (A) represented by the following general formula (2) is converted into imide rings:

(In the general formula (2), _R1 and _R2 represent divalent aromatic diamine residues which may be the same or different from each other, _R3 and _R4 represent tetravalent aromatic tetracarboxylic acid residues which may be the same or different from each other, one of _R5 and _R6 is a hydrogen atom and the other is a phenyl group, m and n satisfy the relationships 1≦m, 0≦n, 1≦m+n≦20 and 0.05≦m/(m+n)≦1, and the arrangement of the repeating units may be either block or random.) The method for producing irregular particles according to claim 5, wherein the melt processing is carried out at a temperature of 250°C or less. The method for producing irregular particles according to claim 6, in which an extruder is used during melt processing. 8. The method for producing irregular particles according to claim 7, wherein the residence time in the extruder is from 1 to 30 minutes.