JP7464474B2 - Maleimide resin, curable resin composition and cured product thereof - Google Patents

Description

The present invention relates to a new maleimide resin, a curable resin composition using the same, and a cured product thereof, which are suitable for use in electrical and electronic components such as semiconductor encapsulants, printed wiring boards, and build-up laminates, as well as lightweight, high-strength materials such as carbon fiber reinforced plastics and glass fiber reinforced plastics.

In recent years, the laminates that carry electric and electronic components have become more widely and sophisticated in terms of their required characteristics due to the expansion of the fields of use. For example, in the past, semiconductor chips were mainly mounted on metal lead frames, but semiconductor chips with advanced processing capabilities such as CPUs are increasingly being mounted on laminates made of polymer materials. As the speed of elements such as CPUs increases and the clock frequency increases, signal propagation delay and transmission loss become problems, and wiring boards are being required to have low dielectric constants and low dielectric tangents. At the same time, as the speed of elements increases, the chips generate more heat, so there is a need to improve heat resistance. In addition, mobile electronic devices such as mobile phones have become widespread in recent years, and precision electronic devices are now being used and carried in outdoor environments and in close proximity to the human body, so resistance to external environments (especially humid and heat environments) is required.

Furthermore, with the rapid advancement of electronics in the automotive field, precision electronic devices are sometimes placed near the engine, which has created a demand for higher levels of heat and moisture resistance. Safety, such as flame retardancy, is becoming more important in automotive applications and portable devices, but with growing awareness of environmental issues in recent years, the use of halogen-based flame retardants is being avoided, and there is an increasing need to provide flame retardancy without using halogens.

Conventionally, wiring boards using BT resin, a resin that combines a bisphenol A type cyanate ester compound and a bismaleimide compound as described in Patent Document 1, have been widely used as high-performance wiring boards because of their excellent heat resistance, chemical resistance, and electrical properties. However, further improvements are needed in situations where high performance is required as described above.

In addition, in recent years, efforts to reduce the weight of airplanes, automobiles, trains, ships, etc. have been progressing from the perspective of energy conservation, and studies are being conducted to replace metal materials that were previously used with lightweight, high-strength carbon fiber composite materials. For example, the Boeing 787 has been made lighter by increasing the proportion of composite materials, significantly improving fuel efficiency. In the aviation sector, there is also a movement to introduce carbon fiber composite materials into parts around the engine in order to further reduce weight, and naturally a high level of heat resistance is required. In the automotive sector, some cars are equipped with propeller shafts made of composite materials, and there is also a movement to make the bodies of luxury cars out of composite materials.

In the field of carbon fiber composites, composite materials using epoxy resins such as bisphenol A diglycidyl ether and tetraglycidyl diaminodiphenylmethane, and hardeners such as diaminodiphenylmethane and diaminodiphenylsulfone have traditionally been used, but in order to achieve greater weight reduction and heat resistance, it is necessary to broaden the scope of application of composite materials, and maleimide resins are being considered as one possible means of achieving this.

In this situation, most of the maleimide compounds available on the market are bismaleimides, and although their cured products have good heat resistance, they have the disadvantage of being highly hygroscopic.

Japanese Patent Application Laid-Open No. 50-129700 Japanese Patent Application Laid-Open No. 61-000044

The object of the present invention is to provide a new maleimide resin and its cured product that can achieve low moisture absorption, low dielectric properties, and low dielectric loss tangent.

The present inventors have conducted extensive research to solve the above problems and have completed the present invention.
That is, the present invention relates to the following [1] to [7].
[1]
A maleimide resin represented by the following formula (1):

(In formula (1), n is the average number of repetitions, and 1<n<5.)
[2]
The maleimide resin according to the above item [1], wherein n in formula (1) is 1<n<3.
[3]
The maleimide resin according to the above item [1] or [2], wherein the content of the bismaleimide represented by n = 1 in the formula (1) is 98% or less.
[4]
The maleimide resin according to any one of items [1] to [3] above, which is obtained by reacting an amine resin represented by the following formula (2) with maleic acid or maleic anhydride:

(In formula (2), n is the average number of repetitions, and 1<n<5.)
[5]
A curable resin composition comprising the maleimide resin according to any one of items [1] to [4] above.
[6]
A cured product obtained by curing the curable resin composition according to the above item [5].
[7]
An amine resin represented by the following formula (2):

(In formula (2), n is the average number of repetitions, and 1<n<5.)

The cured product obtained from the maleimide resin of the present invention can achieve low moisture absorption, low dielectric properties, and low dielectric tangent.

1 shows a GPC chart of the maleimide resin of Example 1.

The maleimide resin of the present invention is represented by the following formula (1).

(In formula (1), n is the average number of repetitions, and 1<n<5.)

In formula (1), the value of n can be calculated from the number average molecular weight determined by gel permeation chromatography (GPC, detector: RI) of the maleimide resin, but approximately it can be considered to be almost equal to the value of n calculated from the GPC measurement results of the raw material amine resin.

In the present invention, the content of the n=1 component in formula (1) can be determined by gel permeation chromatography (GPC, detector: RI) analysis.

The content of the bismaleimide represented by n=1 in the formula (1) in the maleimide resin of the present invention as determined by GPC analysis (RI) is preferably 98 area% or less, more preferably 20 to 95 area%, even more preferably 30 to 90 area%, and particularly preferably 50 to 90 area%. When the content of the bismaleimide represented by n=1 in formula (1) is 98 area% or less, the heat resistance is good and the solubility is also improved. The lower limit of the bismaleimide represented by n=1 in formula (1) may be 0 area%, but when it is 20 area% or more, the viscosity of the resin is not too high, resulting in good workability.

The maleimide resin represented by the formula (1) above can be obtained by subjecting the amine resin represented by the following formula (2) to an addition or dehydration condensation reaction with maleic acid or maleic anhydride (hereinafter also referred to as "maleic anhydride") in the presence of a solvent and a catalyst.

The solvent used in the reaction is water-insoluble, since it is necessary to remove water generated during the reaction from the system. Examples include aromatic solvents such as toluene and xylene, aliphatic solvents such as cyclohexane and n-hexane, ethers such as diethyl ether and diisopropyl ether, ester-based solvents such as ethyl acetate and butyl acetate, and ketone-based solvents such as methyl isobutyl ketone and cyclopentanone, but are not limited to these, and two or more types may be used in combination.

In addition to the water-insoluble solvent, an aprotic polar solvent can also be used in combination. Examples include dimethyl sulfone, dimethyl sulfoxide, dimethylformamide, dimethylacetamide, 1,3-dimethyl-2-imidazolidinone, and N-methyl-2-pyrrolidone, and two or more of these can be used in combination. When using an aprotic polar solvent, it is preferable to use one that has a higher boiling point than the water-insoluble solvent used in combination.

The catalyst used in the reaction is an acid catalyst, and although there is no particular limitation, examples include p-toluenesulfonic acid, hydroxy-p-toluenesulfonic acid, methanesulfonic acid, sulfuric acid, phosphoric acid, etc. The amount of the acid catalyst used is usually 0.1 to 10% by weight, preferably 1 to 5% by weight, based on the aromatic amine resin.

For example, the aromatic amine resin represented by the formula (2) is dissolved in toluene, maleic anhydride is added thereto to generate an amic acid, and then p-toluenesulfonic acid is added, and the reaction is carried out under reflux conditions while removing the generated water from the system.

Alternatively, maleic anhydride is dissolved in toluene, and a toluene solution of the aromatic amine resin represented by formula (2) is added under stirring to generate an amic acid, and then p-toluenesulfonic acid is added to carry out the reaction under reflux conditions while removing the generated water from the system.

Alternatively, maleic anhydride is dissolved in toluene, p-toluenesulfonic acid is added, and the toluene solution of the aromatic amine resin represented by formula (2) is added dropwise under stirring and reflux while the water that forms an azeotropic mixture is removed from the system and the toluene is returned to the system to carry out the reaction (this is the first stage reaction).

In either method, maleic anhydride is usually used in an amount of 1 to 3 equivalents, preferably 1.2 to 2.0 equivalents, relative to the amino groups of the aromatic amine resin represented by formula (2).

In order to reduce the amount of unclosed amic acid, water is added to the reaction solution after the maleimidization reaction listed above, separating it into a resin solution layer and an aqueous layer. Excess maleic acid, maleic anhydride, aprotic polar solvent, catalyst, etc., which are dissolved in the aqueous layer, are removed by liquid separation, and the same operation is repeated to thoroughly remove excess maleic acid, maleic anhydride, aprotic polar solvent, and catalyst. A catalyst is added again to the maleimide resin solution in the organic layer from which the excess maleic acid, maleic anhydride, aprotic polar solvent, and catalyst have been removed, and the dehydration and ring-closing reaction of the remaining amic acid is carried out again under heating and reflux conditions, resulting in a maleimide resin solution with a low acid value (this is the second-stage reaction).

The time for the re-dehydration ring-closing reaction is usually 1 to 5 hours, preferably 1 to 3 hours, and the aforementioned aprotic polar solvent may be added if necessary. After the reaction is completed, the reaction mixture is cooled and repeatedly washed with water until the washing water becomes neutral. Thereafter, the water is removed by azeotropic dehydration under heating and reduced pressure, and the solvent may be distilled off or another solvent may be added to adjust the resin solution to the desired concentration, or the solvent may be completely distilled off to obtain a solid resin.

Next, the curable resin composition of the present invention will be described.
The curable resin composition of the present invention may contain a compound capable of crosslinking with the maleimide resin of the present invention. The compound is not particularly limited as long as it has a functional group (or structure) capable of crosslinking with the maleimide resin, such as an amino group, a cyanate group, a phenolic hydroxyl group, an alcoholic hydroxyl group, an allyl group, a methallyl group, an acryl group, a methacryl group, a vinyl group, or a conjugated diene group. Since an amine compound and a maleimide compound undergo a crosslinking reaction, an aromatic amine resin represented by the formula (2) may be used. Since the maleimide resin is capable of self-polymerization, it may be used alone. In addition, an amine compound other than the aromatic amine resin represented by the formula (2) or a maleimide compound other than the maleimide resin of the present invention represented by the formula (1) may be used in combination.

The content of the maleimide resin in the curable resin composition of the present invention is preferably 10% by weight or more, more preferably 15% by weight or more, and even more preferably 20% by weight. When it is in the above range, the physical properties of the cured product tend to be high in mechanical strength, high peel strength, and high heat resistance.

As the amine compound that can be blended in the curable resin composition of the present invention, conventionally known amine compounds can be used. Specific examples of amine compounds include diethylenetriamine, triethylenetetramine, tetraethylenepentamine, m-xylenediamine, trimethylhexamethylenediamine, 2-methylpentamethylenediamine, diethylaminopropylamine, isophoronediamine, 1,3-bisaminomethylcyclohexane, bis(4-aminocyclohexyl)methane, bis(4-amino-3-methylcyclohexyl)methane, norbornenediamine, 1,2-diaminocyclohexane, diaminodiphenylmethane, metaphenylenediamine, diaminodiphenylsulfone, dicyandiamide, polyoxypropylenediamine, polyoxypropylenetriamine, N-aminoethylpiperazine, aniline-formaldehyde resin, etc., but are not limited thereto. These may be used alone or in combination of two or more.

As the maleimide compound that can be blended in the curable resin composition of the present invention, a conventionally known maleimide compound can be used. Specific examples of maleimide compounds include 4,4'-diphenylmethane bismaleimide, polyphenylmethane maleimide, m-phenylene bismaleimide, 2,2'-bis[4-(4-maleimidophenoxy)phenyl]propane, 3,3'-dimethyl-5,5'-diethyl-4,4'-diphenylmethane bismaleimide, 4-methyl-1,3-phenylene bismaleimide, 4,4'-diphenylether bismaleimide, 4,4'-diphenylsulfone bismaleimide, 1,3-bis(3-maleimidophenoxy)benzene, 1,3-bis(4-maleimidophenoxy)benzene, etc., but are not limited thereto. These may be used alone or in combination of two or more. The amount of the maleimide compound blended is preferably 5 times or less, more preferably 2 times or less, by weight, of the maleimide resin of the present invention.

As the cyanate ester compound that can be blended in the curable resin composition of the present invention, a conventionally known cyanate ester compound can be used.Specific examples of the cyanate ester compound include, but are not limited to, cyanate ester compounds obtained by reacting polycondensates of phenols and various aldehydes, polymers of phenols and various diene compounds, polycondensates of phenols and ketones, and polycondensates of bisphenols and various aldehydes with cyanogen halide.These compounds may be used alone or in combination of two or more.
Examples of the phenols include phenol, alkyl-substituted phenols, aromatic substituted phenols, naphthol, alkyl-substituted naphthol, dihydroxybenzene, alkyl-substituted dihydroxybenzene, and dihydroxynaphthalene.
Examples of the various aldehydes include formaldehyde, acetaldehyde, alkyl aldehyde, benzaldehyde, alkyl-substituted benzaldehyde, hydroxybenzaldehyde, naphthaldehyde, glutaraldehyde, phthalaldehyde, crotonaldehyde, and cinnamaldehyde.
Examples of the various diene compounds include dicyclopentadiene, terpenes, vinylcyclohexene, norbornadiene, vinylnorbornene, tetrahydroindene, divinylbenzene, divinylbiphenyl, diisopropenylbiphenyl, butadiene, and isoprene.
Examples of the ketones include acetone, methyl ethyl ketone, methyl isobutyl ketone, acetophenone, and benzophenone.
Furthermore, the cyanate ester compound, the synthesis method of which is described in JP-A-2005-264154, is particularly preferred as the cyanate ester compound because it has low moisture absorption, excellent flame retardancy, and excellent dielectric properties.

In the curable resin composition of the present invention, an epoxy resin may be further blended. Any of the conventionally known epoxy resins may be used as the epoxy resin that may be blended. Specific examples of the epoxy resin include polycondensates of phenols and various aldehydes, polymers of phenols and various diene compounds, polycondensates of phenols and ketones, polycondensates of bisphenols and various aldehydes, and glycidyl ether-based epoxy resins obtained by glycidylating alcohols, alicyclic epoxy resins such as 4-vinyl-1-cyclohexene diepoxide and 3,4-epoxycyclohexylmethyl-3,4'-epoxycyclohexanecarboxylate, glycidylamine-based epoxy resins such as tetraglycidyldiaminodiphenylmethane (TGDDM) and triglycidyl-p-aminophenol, and glycidyl ester-based epoxy resins, but are not limited thereto. These may be used alone or in combination of two or more.
In addition, epoxy resins obtained by using a phenol aralkyl resin as a raw material, which is obtained by subjecting a condensation reaction of a phenol with a bishalogenomethylaralkyl derivative or an aralkyl alcohol derivative, and subjecting the resin to a dehydrochlorination reaction with epichlorohydrin are particularly preferred as epoxy resins because they have low moisture absorption, excellent flame retardancy, and excellent dielectric properties.

When epoxy resin is blended, the blending amount is not particularly limited, but is preferably 0.1 to 10 times the weight of the maleimide resin, and more preferably 0.2 to 4 times. If the blending amount of epoxy resin is less than 0.1 times the weight of the maleimide resin, the cured product may become brittle, and if it is more than 10 times, the dielectric properties may deteriorate.

The curable resin composition of the present invention may further contain a compound having a phenol resin.
As the phenolic resin that can be blended, any of the conventionally known phenolic resins can be used. Specific examples of the phenolic resin include bisphenols (bisphenol A, bisphenol F, bisphenol S, biphenol, bisphenol AD, etc.), polycondensates of phenols (phenol, alkyl-substituted phenol, aromatic-substituted phenol, naphthol, alkyl-substituted naphthol, dihydroxybenzene, alkyl-substituted dihydroxybenzene, dihydroxynaphthalene, etc.) and various aldehydes (formaldehyde, acetaldehyde, alkyl aldehyde, benzaldehyde, alkyl-substituted benzaldehyde, hydroxybenzaldehyde, naphthaldehyde, glutaraldehyde, phthalaldehyde, crotonaldehyde, cinnamaldehyde, etc.), polycondensates of phenols and various diene compounds (dicyclopentadiene, terpenes, vinylcyclohexene, norbornadiene ... Examples of the condensation products include, but are not limited to, polymers of phenols and ketones (acetone, methyl ethyl ketone, methyl isobutyl ketone, acetophenone, benzophenone, etc.), polycondensates of phenols and aromatic dimethanols (benzene dimethanol, α,α,α',α'-benzene dimethanol, biphenyl dimethanol, α,α,α',α'-biphenyl dimethanol, etc.), polycondensates of phenols and aromatic dichloromethyls (α,α'-dichloroxylene, bischloromethyl biphenyl, etc.), polycondensates of bisphenols and various aldehydes, and modified products thereof. These may be used alone or in combination of two or more.
Furthermore, phenol aralkyl resins obtained by subjecting phenols to a condensation reaction with the above-mentioned bishalogenomethylaralkyl derivatives or aralkyl alcohol derivatives are particularly preferred as phenol resins because they have low moisture absorption, flame retardancy and excellent dielectric properties.
Furthermore, when the above-mentioned phenolic resin has an allyl group or a methallyl group, the reactivity with the maleimide group is higher than that with the hydroxyl group, and therefore the curing speed is increased and the number of crosslinking points is increased, resulting in higher strength and heat resistance, which is preferable.
In addition, it is also possible to compound an allyl ether form in which the hydroxyl group of the above phenolic resin is allylated, or a methallyl ether form in which the hydroxyl group is methallylated, and since the hydroxyl group is etherified, the water absorbency is low.

The curable resin composition of the present invention may further contain a compound having an acid anhydride group.
As the compound having an acid anhydride group that can be blended, any of the conventionally known compounds can be used. Specific examples of the compound having an acid anhydride group include 1,2,3,4-butanetetracarboxylic dianhydride, 1,2,3,4-cyclobutanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, pyromellitic anhydride, 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, and 4-(2,5-dioxotetrahydrofuran-3-yl)-1,2,3,4-tetrahydronaphthalene-1,2-dicarboxylic anhydride.
Compounds having an acid anhydride group can be used alone or in combination of two or more. In addition, the reaction between an acid anhydride group and an amine results in an amic acid, which is further heated at 200°C to 300°C to form an imide structure through a dehydration reaction, resulting in a material with excellent heat resistance.

The curable resin composition of the present invention may contain a curing catalyst (curing accelerator) as necessary. For example, imidazoles such as 2-methylimidazole, 2-ethylimidazole, 2-phenylimidazole, 2-ethyl-4-methylimidazole, 2-undecylimidazole, and 1-cyanoethyl-2-ethyl-4-methylimidazole; amines such as triethylamine, triethylenediamine, 2-(dimethylaminomethyl)phenol, 1,8-diaza-bicyclo(5,4,0)undecene-7, tris(dimethylaminomethyl)phenol, and benzyldimethylamine; triphenylphosphine, tributylphosphine, and tributylphosphine. Examples of the catalyst include phosphines such as octylphosphine, organometallic salts such as tin octylate, zinc octylate, dibutyltin dimaleate, zinc naphthenate, cobalt naphthenate, and tin oleate, metal chlorides such as zinc chloride, aluminum chloride, and tin chloride, organic peroxides such as di-tert-butyl peroxide and dicumyl peroxide, azo compounds such as azobisisobutyronitrile and azobisdimethylvaleronitrile, mineral acids such as hydrochloric acid, sulfuric acid, and phosphoric acid, Lewis acids such as boron trifluoride, and salts such as sodium carbonate and lithium chloride. The amount of the curing catalyst is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, based on 100 parts by weight of the total curable resin composition.

An organic solvent can be added to the curable resin composition of the present invention to form a varnish-like composition (hereinafter simply referred to as varnish). Examples of the solvent that can be used include amide-based solvents such as gamma-butyrolactones, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and N,N-dimethylimidazolidinone, sulfones such as tetramethylene sulfone, ether-based solvents such as diethylene glycol dimethyl ether, diethylene glycol diethyl ether, propylene glycol, propylene glycol monomethyl ether, propylene glycol monomethyl ether monoacetate, and propylene glycol monobutyl ether, ketone-based solvents such as methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, and cyclohexanone, and aromatic solvents such as toluene and xylene. The solvent is used in such a range that the solids concentration excluding the solvent in the obtained varnish is usually 10 to 80% by weight, preferably 20 to 70% by weight.

Furthermore, known additives can be blended into the curable resin composition of the present invention as necessary. Specific examples of additives that can be used include epoxy resin curing agents, polybutadiene and its modified products, modified acrylonitrile copolymers, polyphenylene ether, polystyrene, polyethylene, polyimide, fluororesin, maleimide-based compounds, cyanate ester-based compounds, silicone gel, silicone oil, and inorganic fillers such as silica, alumina, calcium carbonate, quartz powder, aluminum powder, graphite, talc, clay, iron oxide, titanium oxide, aluminum nitride, asbestos, mica, and glass powder, surface treatment agents for fillers such as silane coupling agents, release agents, and colorants such as carbon black, phthalocyanine blue, and phthalocyanine green. The blending amount of these additives is preferably 1,000 parts by weight or less, more preferably 700 parts by weight or less, per 100 parts by weight of the curable resin composition.

The method for preparing the curable resin composition of the present invention is not particularly limited, but each component may be mixed uniformly or may be prepolymerized. For example, the maleimide resin and the cyanate ester compound are prepolymerized by heating in the presence or absence of a catalyst and in the presence or absence of a solvent. Similarly, the maleimide resin of the present invention may be prepolymerized by adding an epoxy resin, an amine compound, a maleimide compound, a cyanate ester compound, a phenolic resin, an acid anhydride compound, and other additives as necessary. The mixing or prepolymerization of each component is carried out using, for example, an extruder, kneader, rolls, etc. in the absence of a solvent, and a reaction kettle with a stirrer, etc. in the presence of a solvent.

The curable resin composition of the present invention can be heated and melted to reduce the viscosity, and then impregnated into reinforcing fibers such as glass fibers, carbon fibers, polyester fibers, polyamide fibers, and alumina fibers to obtain a prepreg.
Moreover, a prepreg can also be obtained by impregnating reinforcing fibers with the varnish and drying the fibers by heating.
The prepreg is cut into a desired shape and laminated with copper foil or the like as necessary. The laminate is then heated and cured while applying pressure thereto by press molding, autoclave molding, sheet winding molding or the like, to obtain an electrical and electronic laminate (printed wiring board) or a carbon fiber reinforced material.

The present invention will be specifically explained below with reference to examples and comparative examples. In the text, "parts" and "%" represent "parts by weight" and "% by weight", respectively. The softening point and melt viscosity were measured by the following methods.

[Analysis method]
Melting point: measured by DSC; GPC (gel permeation chromatography) analysis Manufacturer: Waters
Column: Guard column SHODEX GPC KF-601 (2 columns), KF-602, KF-602.5, KF-603
Flow rate: 0.5 ml/min.
Column temperature: 25°C
Solvent used: THF (tetrahydrofuran)
Detector: RI (differential refractometer)

[Example 1]
A flask equipped with a thermometer, a cooling tube, a Dean-Stark azeotropic distillation trap, and a stirrer was charged with 73.6 parts of maleic anhydride, 100 parts of toluene, 10 parts of N-methyl-2-pyrrolidone, and 1.2 parts of methanesulfonic acid, and heated to reflux. Next, a resin solution in which 63.6 parts of aromatic amine resin (A1) (Kayahard A-A manufactured by Nippon Kayaku Co., Ltd.) represented by formula (1) was dissolved in 50 parts of toluene was dropped over 2 hours while maintaining the reflux state. During this time, the condensed water and toluene that were azeotropically formed under reflux conditions were cooled and separated in the Dean-Stark azeotropic distillation trap, and then the toluene, which is the organic layer, was returned to the system, and the water was discharged outside the system. After the dripping of the resin solution was completed, the reaction was carried out for 4 hours while maintaining the reflux state and performing a dehydration operation.
After the reaction was completed, the mixture was washed three times to remove methanesulfonic acid and excess maleic anhydride, and water was removed from the system by azeotropy of toluene and water under reduced pressure at 70°C or less. Next, toluene was gradually distilled off under reduced pressure at 70°C or less, and crystals were precipitated. The crystals were filtered off and dried under reduced pressure at 70°C or less to obtain 68 parts of maleimide resin (M1) represented by formula (1). The melting point of the obtained maleimide resin (M1) was 212°C. GPC analysis (RI) showed that the bismaleimide of n=1 in formula (1) was 93%. The GPC chart of the maleimide resin (M1) is shown in FIG. 1.

[Example 2]
The reaction was carried out in the same manner as in Example 1. After the reaction was completed, the mixture was washed with water three times to remove methanesulfonic acid and excess maleic anhydride. Toluene was then removed from the oil layer using a rotary evaporator to obtain 101 parts of a resinous maleimide resin (M2) represented by formula (1). GPC analysis (RI) showed that the bismaleimide in formula (1) where n=1 constituted 76% of the mixture.

[Example 3, Comparative Example 1]
Using the maleimide resin (M1) obtained in Example 1 and aniline novolac maleimide resin (M3) (BMI-2300, manufactured by Daiwa Kasei Kogyo Co., Ltd.), various epoxy resins, curing agents, and curing accelerators were mixed in the ratios (parts by weight) shown in Table 1, kneaded with a mixing roll, tableted, and then a resin molded product was prepared by transfer molding and cured for 2 hours at 200° C. The physical properties of the cured product thus obtained were measured for the following items, and the results are shown in Table 1.

[evaluation]
Td5 (5% thermal weight loss temperature): The obtained cured product was pulverized into powder and the thermal decomposition temperature was measured by TG-DTA using a sample with 100 mesh pass and 200 mesh on. The temperature at which the weight decreased by 5% was measured with a sample weight of 10 mg, a heating rate of 10°C/min, and an air volume of 200 ml/hr.
Water absorption rate: The weight increase rate (%) before and after a disk-shaped test piece with a diameter of 5 cm and a thickness of 4 mm was boiled in water at 100° C. for 24 hours.
Moisture absorption rate: Weight increase rate after 24 hours at 85° C./85% and 121° C./100%. The test specimen is a disk with a diameter of 50 mm and a thickness of 4 mm.
Dielectric constant and dielectric loss tangent: (Cavity resonator manufactured by Agilent Technologies) Measured at 1 GHz in accordance with JIS K6991.

E1: NC-3000-L (manufactured by Nippon Kayaku, epoxy equivalent 270 g/eq)
P1: Kayahard GPH-65 (manufactured by Nippon Kayaku Co., Ltd., hydroxyl equivalent: 200 g/eq)
2E4MZ: 2-ethyl-4-methylimidazole (Tokyo Chemical Industry Co., Ltd.)

The results in Table 1 show that Example 3 showed better results in many properties, including heat resistance, low moisture absorption, and dielectric properties, compared to Comparative Example 1.

The maleimide resin of the present invention has excellent solution stability and therefore high workability, and also has excellent heat resistance, low moisture absorption, and dielectric properties, and is therefore suitable for use in electric and electronic parts such as semiconductor encapsulating materials, printed wiring boards, and build-up laminates, as well as lightweight, high-strength materials such as carbon fiber reinforced plastics and glass fiber reinforced plastics.

Claims (6)

A maleimide resin having a melting point of 212° C. and represented by the following formula (1):

(In formula (1), n is the average number of repetitions, and 1<n<5.) A curable resin composition containing the maleimide resin according to claim 1, an epoxy resin, and a curing agent, in which the amount of the maleimide resin added is 0.2 to 4 times the weight of the epoxy resin. The curable resin composition according to claim 2, wherein the dielectric constant of the cured product measured in accordance with JIS K6991 is 3.00 or less at 1 GHz. A cured product obtained by curing the curable resin composition according to claim 2 or 3. An amine resin represented by the following formula (2), which is a precursor of the maleimide resin according to claim 1:

(In formula (2), n is the average number of repetitions, and 1<n<5.) A method for producing a maleimide resin represented by the following formula (1) obtained by the following (Step 1) to (Step 3).
(Step 1) A step of reacting an amine resin represented by the following formula (2) with maleic acid or maleic anhydride in an aprotic polar solvent in the presence of a catalyst. (Step 2) A step of adding water to the reaction solution obtained in the above step 1, separating it into a resin solution layer and a water layer, and removing excess maleic acid, maleic anhydride, aprotic polar solvent, and catalyst by liquid separation. (Step 3) A step of filtering out crystals of a maleimide resin represented by the following formula (1) by distilling off water and aprotic solvent from the resin solution obtained in the above step 2 under heating and reduced pressure.

(In formula (1), n is the average number of repetitions, and 1<n<5.)

(In formula (2), n is the average number of repetitions, and 1<n<5.)

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