JP7765895B2 - Epoxy resin, epoxy resin composition and cured product - Google Patents

Description

The present invention relates to an epoxy resin that is easy to handle as a solid at room temperature and useful as an insulating material for highly reliable electrical and electronic components such as semiconductor encapsulation, laminates, and heat dissipation substrates, and that has low viscosity during molding and excellent solvent solubility. It also relates to an epoxy resin composition using the same, and a cured product obtained from the same that has excellent high thermal conductivity.

Epoxy resins have been used in a wide range of industrial applications, but the performance requirements have become increasingly sophisticated in recent years. In the electrical, electronic, and power electronics fields, where electronic circuits are becoming increasingly dense and high-frequency, the heat generated by electronic circuits is increasing, posing a challenge to the heat dissipation capabilities of epoxy resin compositions used in insulating parts. This heat dissipation capability was previously met by the thermal conductivity of the filler, but with the goal of even higher integration, there is now a demand for improved thermal conductivity in the epoxy resin matrix itself.

Epoxy resin compositions with excellent thermal conductivity that use epoxy resins with mesogenic structures are known. For example, Patent Document 1 discloses an epoxy resin composition whose essential components are a biphenol-type epoxy resin and a polyphenol resin curing agent, and discloses that the composition has excellent stability and strength at high temperatures and can be used in a wide range of fields, including adhesion, casting, sealing, molding, and lamination. Patent Document 2 also discloses an epoxy compound whose molecule contains two mesogenic structures linked by a bent chain. Furthermore, Patent Document 3 discloses a resin composition containing an epoxy compound with a mesogenic group.

However, epoxy resins with such mesogenic structures have high melting points, and when mixed, the high-melting component is difficult to dissolve, leaving some undissolved, resulting in problems with reduced curing and heat resistance. Furthermore, high temperatures are required to uniformly mix such epoxy resins with curing agents. At high temperatures, the curing reaction of the epoxy resin proceeds rapidly, shortening the gelation time, severely limiting the mixing process and making handling difficult. Adding a soluble third component to compensate for this drawback lowers the resin's melting point, making uniform mixing easier, but the resulting cured product has a reduced thermal conductivity.

As highly thermally conductive resins that can be melt-mixed, Patent Document 4 discloses an epoxy resin obtained by epoxidizing a mixture of hydroquinone and 4,4'-dihydroxybiphenyl, and Patent Document 5 discloses an epoxy resin obtained by epoxidizing a mixture of 4,4'-dihydroxydiphenylmethane and 4,4'-dihydroxybiphenyl. However, these resins have poor solvent solubility, limiting their applications. Patent Document 6 discloses an epoxy resin composition with a diphenyl ether structure, but the curing agent is limited, and general-purpose curing agents such as phenol novolac do not provide sufficient thermal conductivity and heat resistance.

Japanese Unexamined Patent Publication No. 7-90052 Japanese Patent Application Publication No. 9-118673 Japanese Patent Application Publication No. 11-323162 WO2009/110424 JP 2010-43245 A JP 2012-17405 A

Therefore, the object of the present invention is to solve the above problems and provide a crystalline modified epoxy resin that is easy to handle as a solid at room temperature and has low viscosity during molding and excellent solvent solubility, making it useful as an insulating material for electrical and electronic components such as semiconductor encapsulation, laminates, and heat dissipation substrates, and that has excellent thermal conductivity; an epoxy resin composition using the same; and a cured product obtained from the same that has excellent thermal conductivity.

Through extensive research, the inventors have discovered that the above-mentioned problems can be solved by reacting a mixture of compounds containing specific phenolic hydroxyl groups with epichlorohydrin, and that the resulting cured product exhibits improved thermal conductivity.

That is, the present invention relates to an epoxy resin represented by the following general formula (1), characterized in that the total content of n=0 units is 70 to 95% and the epoxy equivalent is in the range of 150 to 160 g/eq.

(In the formula, each X independently represents a direct bond or —O—, and at least some of them are —O—. n represents a number from 0 to 10, and G represents a glycidyl group.)

In the above general formula (1), the ratio (weight ratio) of the biphenyl structure where X is a direct bond to the diphenyl ether structure where X is -O- is preferably in the range of 0.1 to 10.0.

In the above general formula (1), the biphenyl structure having two OG groups preferably has a 4,4' configuration.

The present invention also relates to an epoxy resin composition comprising an epoxy resin and a curing agent, characterized in that the epoxy resin is an essential component, either in part or in whole, of the epoxy resin.

Furthermore, the present invention relates to a cured resin product obtained by curing the above-mentioned resin composition.

The epoxy resin and epoxy resin composition of the present invention have excellent solvent solubility, moldability, and reliability, and exhibit excellent thermal conductivity in the cured product. Furthermore, by suppressing crystallinity, yields are improved, which is advantageous from a manufacturing perspective.

1 is a GPC chart of epoxy resin A (Example 1).

The present invention is described in detail below.

The epoxy resin of the present invention is represented by the above general formula (1) and has an epoxy equivalent (g/eq) of 150 to 160. n represents a number from 0 to 10, with an average value (number average) ranging from 0.1 to 5, preferably 0.1 to 2. A mixture of components with different n values is preferred. Examples of compounds with n=0 include compounds in which glycidyl ether groups are bonded to both ends of a biphenyl structure, and compounds in which glycidyl ether groups are bonded to both ends of a diphenyl ether. For structures with excellent thermal conductivity, the glycidyl ether group bonded to the biphenyl structure preferably has a 4,4' bond. Examples of compounds with n=1 or more include mixtures containing homopolymers derived from dihydroxybiphenyl, homopolymers derived from dihydroxydiphenyl ether, and copolymers with a co-reaction structure of dihydroxybiphenyl and dihydroxydiphenyl ether.

The epoxy resin of the present invention can be produced by reacting a mixture of dihydroxybiphenyl and dihydroxydiphenyl ether with epichlorohydrin. This reaction can be carried out in the same manner as a conventional epoxidation reaction. The epoxy resin of the present invention is a mixture that includes epoxidized products of dihydroxybiphenyl and dihydroxydiphenyl ether, as well as epoxidized products having units derived from dihydroxybiphenyl and dihydroxydiphenyl ether in one molecule. Epoxidized products obtained by reacting a dihydroxy compound with epichlorohydrin include epoxidized products with a degree of polymerization of 0 (n=0 form), as well as polymers such as n=1 (di-form) and n=2 (tri-form).

In the epoxy resin of the present invention, the total content of all n=0 isomers derived from dihydroxybiphenyl and dihydroxydiphenyl ether is in the range of 70 to 95 wt %. If it exceeds this range, molding may be impossible due to the crystallinity and high melting point derived from the dihydroxybiphenyl structure. On the other hand, if it is less than this range, crystallinity may be low, which may lead to a decrease in thermal conductivity.
Furthermore, the content of the compound containing the co-reactive structure (n=1 or more) is 5 wt% to 30 wt%, preferably 5 wt% to 15 wt%. If it is less than this range, the melting point and crystallinity may not decrease, making molding impossible. On the other hand, if it is more than this range, there is a risk of reduced thermal conductivity. Here, the co-reactive structure refers to the random reaction of dihydroxybiphenyl and dihydroxydiphenyl ether. For example, when n=1, the resulting epoxy resin has one dihydroxybiphenyl structure and one dihydroxydiphenyl ether structure. By including an epoxy resin with this co-reactive structure, the resulting resin has different properties than when epoxy resins with a single structure consisting of each monomer are simply mixed.

The mixing ratio of dihydroxybiphenyl to dihydroxydiphenyl ether, in weight ratio, is in the range of 0.1 to 10.0 (dihydroxybiphenyl/dihydroxydiphenyl ether), but is preferably in the range of 0.2 to 5.0, more preferably 0.3 to 3.0, and even more preferably 0.5 to 2.0. The structural ratio of the resulting epoxy resin will be roughly the same as the charge ratio, and therefore the range for the epoxy resin will also be similar. If the ratio is lower than this, the high melting point of the 4,4'-dihydroxybiphenyl epoxy compound will make it difficult to handle, and if the ratio is higher than this, the heat resistance, thermal conductivity, and other properties of the cured product will be reduced.

The epoxy resin of the present invention is obtained by reacting a mixture of dihydroxybiphenyl and dihydroxydiphenyl ether (hereinafter referred to as the phenol mixture) with epichlorohydrin. The reaction with epichlorohydrin can be carried out, for example, by dissolving the phenol mixture in an excess amount of epichlorohydrin relative to the phenolic hydroxyl groups, and then reacting the mixture in the presence of an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide at 50 to 150°C, preferably 60 to 100°C, for 1 to 10 hours. The amount of alkali metal hydroxide used is 0.8 to 1.2 mol, preferably 0.9 to 1.1 mol, per mol of hydroxyl groups in the phenol mixture. Epichlorohydrin is used in excess relative to the hydroxyl groups in the phenol mixture, typically 1.5 to 15 mol, preferably 3 to 10 mol, per mol of hydroxyl groups in the phenol mixture. After the reaction is complete, excess epichlorohydrin is distilled off, and the residue is dissolved in a solvent such as toluene or methyl isobutyl ketone, filtered, and washed with water to remove inorganic salts. The solvent is then distilled off to obtain the desired epoxy resin.

When producing the epoxy resin of the present invention, a small amount of a phenolic compound other than the dihydroxybiphenyl and dihydroxydiphenyl ether phenolic compounds can be mixed into the phenol mixture, as long as it does not impair the effects of the present invention. However, in this case, the total amount of the other phenolic compounds should be 50 wt% or less, preferably 30 wt% or less, and more preferably 10 wt% or less of the total phenolic compounds.

The epoxy resin of the present invention is crystalline at room temperature. The manifestation of crystallinity can be confirmed by differential scanning calorimetry (DSC) as an endothermic peak associated with the melting of crystals. In this case, since the epoxy resin of the present invention is a mixture, multiple endothermic peaks are generally observed rather than a single peak. The melting points observed by DSC are endothermic peaks derived from the epoxy resin derived from dihydroxybiphenyl and dihydroxydiphenyl ether, with the lowest endothermic peak being 50°C or higher, preferably 70°C or higher, and the highest endothermic peak being 160°C or lower, preferably 140°C or lower. If the melt viscosity is lower than this, blocking may occur when the resin is made into a powder, reducing its handleability as a solid at room temperature. If the melt viscosity is higher than this, problems such as poor melt-kneadability with curing agents and poor solvent solubility may occur. Furthermore, the lower the melt viscosity at 150°C, the better; it is typically 0.1 Pa·s or lower, preferably 0.01 Pa·s or lower.

The purity of the epoxy resin of the present invention, particularly the amount of hydrolyzable chlorine, should be low in order to improve the reliability of the electronic components to which it is applied. While not particularly limited, it is preferably 1000 ppm or less, and more preferably 500 ppm or less. The hydrolyzable chlorine referred to in this invention refers to a value measured by the following method: 0.5 g of sample is dissolved in 30 ml of dioxane, 10 ml of 1N KOH is added, and the mixture is boiled under reflux for 30 minutes, cooled to room temperature, and 100 ml of 80% acetone water is added. Potentiometric titration with 0.002 N AgNO3 aqueous solution is then performed to obtain the value.

The epoxy resin composition of the present invention, comprising an epoxy resin and a curing agent, is characterized by containing the above-mentioned modified epoxy resin as an essential component, either in part or in whole, of the epoxy resin. Advantageously, the above-mentioned modified epoxy resin accounts for 70 wt% or more, and more preferably 90 wt% or more, of the total epoxy resin. If the proportion of modified epoxy resin used is less than this, the effect of improving the thermal conductivity of the cured product is small.

In addition to the above-mentioned epoxy resins used as essential components of the present invention, the epoxy resin composition of the present invention may also contain other conventional epoxy resins having two or more epoxy groups in the molecule. Examples of such resins include, as raw materials, bisphenol A, bisphenol F, 3,3',5,5'-tetramethyl-4,4'-dihydroxydiphenylmethane, 4,4'-dihydroxydiphenyl sulfone, 4,4'-dihydroxydiphenyl sulfide, 4,4'-dihydroxydiphenyl ketone, fluorene bisphenol, 4,4'-biphenol, 3,3',5,5'-tetramethyl-4,4'-dihydroxybiphenyl, zorcinol, catechol, hydroquinone, t-butylcatechol, t- Butylhydroquinone, 1,2-dihydroxynaphthalene, 1,3-dihydroxynaphthalene, 1,4-dihydroxynaphthalene, 1,5-dihydroxynaphthalene, 1,6-dihydroxynaphthalene, 1,7-dihydroxynaphthalene, 1,8-dihydroxynaphthalene, 2,3-dihydroxynaphthalene, 2,4-dihydroxynaphthalene, 2,5-dihydroxynaphthalene, 2,6-dihydroxynaphthalene, 2,7-dihydroxynaphthalene, 2,8-dihydroxynaphthalene, and the above dihydroxynaphthalenes. dihydric phenols such as allylated or polyallylated products of hydroxynaphthalene, allylated bisphenol A, allylated bisphenol F, and allylated phenol novolac; or phenol novolac, bisphenol A novolac, o-cresol novolac, m-cresol novolac, p-cresol novolac, xylenol novolac, poly-p-hydroxystyrene, tris-(4-hydroxyphenyl)methane, and 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane Trivalent or higher phenols such as fluoroglycinol, pyrogallol, t-butylpyrogallol, allylated pyrogallol, polyallylated pyrogallol, 1,2,4-benzenetriol, 2,3,4-trihydroxybenzophenone, phenol aralkyl resins, naphthol aralkyl resins, and dicyclopentadiene resins; or halogenated bisphenols such as tetrabromobisphenol A; and glycidyl ethers derived from these raw material phenols are also used. These epoxy resins can be used alone or in combination of two or more.

The curing agent used in the epoxy resin composition of the present invention can be any of those generally known as curing agents for epoxy resins, including dicyandiamide, acid anhydrides, polyhydric phenols, and aromatic and aliphatic amines. Of these, polyhydric phenols are preferred as curing agents in fields requiring high electrical insulation, such as semiconductor encapsulants. Specific examples of curing agents are listed below.

Examples of polyhydric phenols include dihydric phenols such as bisphenol A, bisphenol F, bisphenol S, fluorene bisphenol, 4,4'-biphenol, 2,2'-biphenol, hydroquinone, resorcinol, and naphthalenediol; and trihydric or higher phenols such as tris-(4-hydroxyphenyl)methane, 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane, phenol novolac, o-cresol novolac, naphthol novolac, and polyvinylphenol. Further examples include polyhydric phenolic compounds synthesized from dihydric phenols such as phenols, naphthols, bisphenol A, bisphenol F, bisphenol S, fluorene bisphenol, 4,4'-biphenol, 2,2'-biphenol, hydroquinone, resorcinol, and naphthalenediol, and condensing agents such as formaldehyde, acetaldehyde, benzaldehyde, p-hydroxybenzaldehyde, and p-xylylene glycol.

Examples of acid anhydride curing agents include phthalic anhydride, tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, methylhimic anhydride, dodecynylsuccinic anhydride, nadic anhydride, and trimellitic anhydride.

Amine-based curing agents include aromatic amines such as 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylsulfone, m-phenylenediamine, and p-xylylenediamine; or aliphatic amines such as ethylenediamine, hexamethylenediamine, diethylenetriamine, and triethylenetetramine.

The above epoxy resin composition can contain one or more of these curing agents in combination.

The blending ratio of epoxy resin to curing agent is preferably in the range of 0.8 to 1.5 in terms of equivalent ratio of epoxy groups to functional groups in the curing agent. Outside this range, unreacted epoxy groups or functional groups in the curing agent will remain after curing, reducing the reliability of the sealing function, which is undesirable.

Oligomers or polymeric compounds such as polyester, polyamide, polyimide, polyether, polyurethane, petroleum resin, indene resin, indene-coumarone resin, and phenoxy resin may also be appropriately blended into the resin composition of the present invention as other modifiers. The amount added is typically in the range of 1 to 30 parts by weight per 100 parts by weight of the total resin components.

The resin composition of the present invention can be blended with additives such as inorganic fillers, pigments, flame retardants, thixotropic agents, coupling agents, and flow improvers. Examples of inorganic fillers include thermally conductive fillers, such as spherical or crushed fused silica, silica powders such as crystalline silica, alumina powder, glass powder, mica, talc, calcium carbonate, alumina, and hydrated alumina. When used in semiconductor encapsulants, the blending amount is preferably 70% by weight or more, and more preferably 80% by weight or more.

Pigments include organic or inorganic extender pigments and flake pigments. Thixotropic agents include silicone-based, castor oil-based, aliphatic amide wax, oxidized polyethylene wax, and organic bentonite-based agents.

Furthermore, if necessary, a curing accelerator can be used in the epoxy resin composition of the present invention. Examples include amines, imidazoles, organic phosphines, Lewis acids, etc., and more specifically, amines such as 1,8-diazabicyclo(5,4,0)undecene-7, triethylenediamine, benzyldimethylamine, triethanolamine, dimethylaminoethanol, and tris(dimethylaminomethyl)phenol; or imidazoles such as 2-methylimidazole, 2-phenylimidazole, 2-ethyl-4-methylimidazole, 2-phenyl-4-methylimidazole, and 2-heptadecylimidazole; or tributylphosphines. Examples include organic phosphines such as sphingosine, methyldiphenylphosphine, triphenylphosphine, diphenylphosphine, and phenylphosphine; or Lewis acids such as tetra-substituted phosphonium and tetra-substituted borates such as tetraphenylphosphonium tetraphenylborate, tetraphenylphosphonium ethyltriphenylborate, and tetrabutylphosphonium tetrabutylborate; and tetraphenylboron salts such as 2-ethyl-4-methylimidazole tetraphenylborate and N-methylmorpholine tetraphenylborate. The amount added is typically in the range of 0.01 to 5 parts by weight per 100 parts by weight of the total resin components.

If necessary, the resin composition of the present invention may further contain release agents such as carnauba wax and OP wax, coupling agents such as γ-glycidoxypropyltrimethoxysilane, colorants such as carbon black, flame retardants such as antimony trioxide, stress reducers such as silicone oil, and lubricants such as calcium stearate.

The resin composition of the present invention can be made into a varnish by dissolving an organic solvent in it, and then impregnating it into a fibrous material such as glass cloth, aramid nonwoven fabric, or polyester nonwoven fabric made from liquid crystal polymer, followed by removing the solvent to form a prepreg. Optionally, it can also be applied to a sheet-like material such as copper foil, stainless steel foil, polyimide film, or polyester film to form a laminate.

The resin composition of the present invention can be heated and cured to produce the cured resin of the present invention. This cured product can be obtained by molding the epoxy resin composition using methods such as casting, compression molding, and transfer molding. The temperature used for this is typically in the range of 120 to 220°C.

The present invention will be specifically explained below using synthesis examples, working examples, and comparative examples. However, the present invention is not limited to these. Unless otherwise specified, "parts" means parts by weight, and "%" means % by weight. Measurements were performed using the following methods.

1) Epoxy Equivalent Using a potentiometric titrator, methyl ethyl ketone was used as a solvent, and a tetraethylammonium bromide acetate solution was added, followed by measurement using a 0.1 mol/L perchloric acid-acetic acid solution in the potentiometric titrator.

2) Melting Point The DSC peak temperature was determined using a differential scanning calorimeter (EXSTAR6000 DSC/6200 manufactured by SII NanoTechnology Inc.) at a temperature rise rate of 5°C/min. That is, this DSC peak temperature was defined as the melting point of the resin.

3) Melt Viscosity: Measured at 150°C using a CAP2000H rotational viscometer manufactured by BROOKFIELD.

4) Softening point: Measured by the ring and ball method in accordance with JIS-K-2207.

5) GPC Measurement A main body (HLC-8320GPC, manufactured by Tosoh Corporation) equipped with columns (three TSKgel SuperMultiporeHZ-NG columns and one TSKgel SuperHZ1000 column, both manufactured by Tosoh Corporation) in series was used, and the column temperature was set to 40°C. Tetrahydrofuran (THF) was used as the eluent at a flow rate of 1 mL/min, and a differential refractive index detector was used as the detector. 0.1 g of sample was dissolved in 10 mL of THF and filtered through a microfilter, and 50 μL of this was used as the measurement sample.

6) Glass transition temperature (Tg)
Tg was determined using a thermomechanical measuring device (EXSTAR6000TMA/6100 manufactured by SII Nanotechnology Inc.) at a temperature rise rate of 10°C/min.

7) 5% Weight Loss Temperature (Td5), Carbon Residual Ratio The 5% weight loss temperature (Td5) was measured under a nitrogen atmosphere at a heating rate of 10°C/min using a thermogravimetric/differential thermal analyzer (EXSTAR6000TG/DTA6200 manufactured by SII Nano Technology). The weight loss at 700°C was also measured and calculated as the carbon residual ratio.

8) Thermal Conductivity: Measured by the transient hot wire method using a thermal conductivity meter LFA447 manufactured by NETZSCH.

Example 1
50.0 g of 4,4'-dihydroxybiphenyl and 27.2 g of 4,4'-dihydroxydiphenyl ether were dissolved in 373 g of epichlorohydrin and 56.0 g of diethylene glycol dimethyl ether, and 73.9 g of 48% aqueous sodium hydroxide solution was added dropwise over 3 hours at 60°C under reduced pressure (approximately 130 Torr). During this time, the water produced was removed from the system by azeotropy with epichlorohydrin, and the distilled epichlorohydrin was returned to the system. After completion of the dropwise addition, the reaction was continued for another 1 hour to dehydrate, after which the epichlorohydrin was distilled off, 460 g of toluene was added, and the salt was removed by washing with water. Thereafter, the water was removed by liquid separation, and the toluene was distilled off under reduced pressure to obtain 83.3 g of a white crystalline epoxy resin (epoxy resin A). The epoxy equivalent was 156, the hydrolyzable chlorine content was 45 ppm, the melting points were 75°C and 137°C, and the viscosity at 150°C was 5 mPa·s. GPC measurement determined that the proportion of n=0 (monomer) in the epoxy resin obtained from 4,4'-dihydroxybiphenyl was 70.6%. The proportion of n=0 (monomer) in the epoxy resin obtained from 4,4'-dihydroxydiphenyl ether was 21.0%. The proportion of n=1 or more was 8.3%.

Example 2
50.0 g of 4,4'-dihydroxybiphenyl and 54.3 g of 4,4'-dihydroxydiphenyl ether were dissolved in 500 g of epichlorohydrin and 75 g of diethylene glycol dimethyl ether, and 98.6 g of 48% aqueous sodium hydroxide solution was added dropwise over 3 hours at 60°C under reduced pressure (approximately 130 Torr). During this time, the water produced was removed from the system by azeotropy with epichlorohydrin, and the distilled epichlorohydrin was returned to the system. After completion of the dropwise addition, the reaction was continued for another 1 hour to dehydrate, after which the epichlorohydrin was distilled off, 620 g of toluene was added, and the salt was removed by washing with water. Thereafter, water was removed by separation, and the toluene was distilled off under reduced pressure, yielding 124 g of a white crystalline epoxy resin (epoxy resin B). The epoxy equivalent was 158, the hydrolyzable chlorine content was 40 ppm, the melting points were 74°C and 123°C, and the viscosity at 150°C was 5 mPa·s. GPC measurement determined that the proportion of n=0 (monomer) in the epoxy resin obtained from 4,4'-dihydroxybiphenyl was 53.8%. The proportion of n=0 (monomer) in the epoxy resin obtained from 4,4'-dihydroxydiphenyl ether was 36.2%. The proportion of n=1 or more was 10.0%.

Comparative Example 1
100.0 g of 4,4'-dihydroxydiphenyl ether was dissolved in 460 g of epichlorohydrin and 70 g of diethylene glycol dimethyl ether, and 90.8 g of 48% aqueous sodium hydroxide solution was added dropwise over 3 hours at 60°C under reduced pressure (approximately 130 Torr). During this time, the water produced was removed from the system by azeotropy with epichlorohydrin, and the distilled epichlorohydrin was returned to the system. After completion of the dropwise addition, the reaction was continued for another hour to dehydrate, after which the epichlorohydrin was distilled off, 580 g of toluene was added, and the salt was removed by washing with water. Water was then removed by separation, and the toluene was distilled off under reduced pressure to obtain 126 g of a white crystalline epoxy resin (epoxy resin C). The epoxy equivalent was 163, the hydrolyzable chlorine content was 150 ppm, the melting point was 83°C, and the viscosity at 150°C was 10 mPa s. GPC measurement revealed that the proportion of n=0 (monomer) in the epoxy resin obtained from 4,4'-dihydroxydiphenyl ether was 91.2%, and the proportion of n=1 or more was 8.8%.

Comparative Example 2
100.0 g of 4,4'-dihydroxybiphenyl was dissolved in 700 g of epichlorohydrin and 105 g of diethylene glycol dimethyl ether. Then, 98.6 g of 48% aqueous sodium hydroxide solution was added dropwise over 3 hours at 60°C under reduced pressure (approximately 130 Torr). During this time, the water produced was removed from the system by azeotropy with epichlorohydrin, and the distilled epichlorohydrin was returned to the system. After the addition was completed, the reaction was continued for another hour to dehydrate the mixture, then cooled to room temperature and filtered to recover the precipitate. The precipitate was then washed with water to remove salts and further dried to obtain 112 g of a crystalline powdery epoxy resin (epoxy resin D). The epoxy equivalent was 163, the hydrolyzable chlorine content was 345 ppm, and the melting point was 172°C. The component ratios of the resulting resin determined by GPC analysis were 93.7% for n=0 and 5.9% for n=1 or greater.

Confirmation of Solvent Solubility Solvent solubility was judged by adding epoxy resins A and B obtained in Examples 1 and 2 and epoxy resins C and D obtained in Comparative Examples 1 and 2 to 5 g of solvent (methyl ethyl ketone, toluene, cyclohexanone) so that the solid concentration was 10 wt %, thoroughly stirring at room temperature, and then visually checking for insoluble matter. The presence of insoluble matter was rated as ×, and the absence of insoluble matter was rated as ◯. Furthermore, when insoluble matter was confirmed to have dissolved when heated to 60°C, the result was rated as △. The results are shown in Table 1.

Examples 3 and 4 and Comparative Examples 3 to 5
Epoxy resins A and B obtained in Examples 1 and 2, epoxy resins C and D obtained in Comparative Examples 1 and 2, and a biphenyl-based epoxy resin (epoxy resin E: YX-4000H manufactured by Japan Epoxy Resins; epoxy equivalent: 195) were used as epoxy resin components, and phenol novolac resin (PN; hydroxyl group equivalent: 105 g/eq., softening point: 67°C) was used as the curing agent. Triphenylphosphine was used as the curing accelerator, and epoxy resin compositions were obtained according to the formulation shown in Table 2. The values in the table indicate parts by weight in the formulation. This epoxy resin composition was molded at 175°C and post-cured at 175°C for 5 hours to obtain cured test specimens, which were then used to measure various physical properties.

As is clear from these results, the epoxy resins obtained in the examples have low melt viscosity, good solvent solubility, and their cured products have excellent thermal conductivity. Furthermore, in terms of heat resistance, they have a high Tg and good thermal stability.

Claims (5)

An epoxy resin having a basic skeleton of a biphenyl structure having two OG groups represented by the following general formula (1), characterized in that the ratio ( former/latter: weight ratio) of biphenyl structures in which X is a direct bond to diphenyl ether structures in which X is -O- is in the range of 0.2 to 5.0, the total amount of n=0 units is 70 to 95 wt %, and the epoxy equivalent is in the range of 150 to 160 g/eq.
(In the formula, each X independently represents a direct bond or —O—, some of which are —O— and the remainder are direct bonds. n represents a number from 0 to 10, and G represents a glycidyl group.) 2. The epoxy resin according to claim 1, wherein, in the general formula (1), the ratio ( former/latter: weight ratio) of the biphenyl structure in which X is a direct bond to the diphenyl ether structure in which X is —O— is in the range of 0.5 to 2.0. The epoxy resin according to claim 1, wherein the biphenyl structure having two OG groups is a 4,4' configuration. An epoxy resin composition comprising, as essential components, the epoxy resin according to any one of claims 1 to 3 and a curing agent. A cured resin product obtained by curing the resin composition described in claim 4.