The present invention relates to a modified epoxy resin, to a process for the preparation thereof, to an epoxy resin composition comprising this modified epoxy resin and a suitable curing agent and to the use of this composition, the latter having good moisture resistance and electrical properties in the cured state.

Since epoxy resins are relatively easy to handle and have excellent properties in the cured state they are widely used in application areas such as adhesion, casting, sealing, laminating, moulding and coating. In particular conventional epoxy resins obtained by reacting an epihalohydrin with a compound having an active hydrogen, such as a phenolic compound, an amine compound or a polycarboxylic acid are often used for the above purposes. Alternatively modified epoxy resins can be used, which can be obtained by reacting the above- described conventional epoxy resin with an active hydrogen containing compound such as an, optionally brominated, polyhydric phenolic compound which modifies the molecular weight and/or imparts additional functionality and optionally flame retardancy.

Recent technological innovations require epoxy resins with improved properties.

For example for electrical applications, the epoxy resin compositions comprising the modified epoxy resins that are currently available do not have the desired level of moisture resistance and electrical properties in the cured state. This is mainly due to the fact that upon reaction of the conventional epoxy resin with the active hydrogen containing compound a hydroxyl group is formed which as a result of its polarity has a negative effect on the moisture resistance and electrical properties of a cured epoxy resin composition comprising this modified epoxy resin.

It is thus an object of the present invention to find a modified epoxy resin which provides excellent moisture resistance and electrical properties to cured epoxy resin compositions comprising said modified epoxy resin. PCT/EP95/00305 relates to an epoxy resin curing agent having in the molecule on average two or more functional groups which may react with epoxy groups, in which on average one or more functional groups is/are aromatic ester groups derived from aromatic carboxylic acids and hydroxy-aromatic compounds, the ester groups directly connecting the aromatic rings of the acid and the hydroxy compound to each other.

It has now been found that this type of ester group containing compounds may suitably be used for the preparation of modified epoxy resins that provide excellent moisture resistance and electrical properties to epoxy resin compositions in the cured state.

The present invention thus relates to a modified epoxy resin obtainable by reacting: a) an epoxy resin having at least two epoxy groups per molecule with b) an ester group containing compound having at least two aromatic ring bonded ester groups represented by the following formulae (1) or (2):

—A (1)

O

wherein A represents a substituted or unsubstituted phenyl- or naphthyl group; —C=C—R (2)

wherein each R may individually be a hydrogen atom, an alkyl group having from 1 to 10 carbon atoms, a substituted or unsubstituted phenyl group, a substituted or unsubstituted aralkyl group, an alkoxy group or a halogen atom;

in such relative amounts that the molar ratio of the aromatic ring bonded ester groups to epoxy groups is from 0.05 to 0.95.

In the reaction between the epoxy resin and the ester group containing compound, the epoxy groups react with the ester groups according to the following reaction scheme. No hydroxyl groups are formed in this reaction and therefore the modified epoxy resin thus produced has a relatively low polarity.

The epoxy resin (a) of the present invention is an epoxy resin having at least two epoxy groups per one molecule. Examples of the epoxy resin used for the preparation of the modified epoxy resin include epoxy resins that can be prepared by reacting epihalohydrin with a phenolic compound, an amine or a carboxylic acid. Suitable phenolic compounds are bisphenol A, bisphenol F, bisphenol AD, hydroquinone, resorcin, methyl resorcin, biphenol, tetramethyl biphenol, dihydroxynaphthalene, tetrabromo- bisphenol A, dihydroxydiphenyl ether, dihydroxydibenzophenone, dihydroxydiphenyl sulfone, phenol novolak resin, cresol novolak resin, bisphenol A novolak resin, dicyclopentadiene phenol resin, terpene phenol resin, phenol aralkyl resin, naphthol novolak resin, brominated ^' phenol novolak resin or polyhydric phenolic resins obtained by condensation reaction of various phenols and various aldehydes such as hydroxybenzaldehyde, crotonaldehyde, or glyoxal. Suitable amines are diaminodiphenyl methane, aminophenol, or xylene diamine. Suitable carboxylic acids are methylhexahydroxyphthalic acid or dimer acid.

tetrabromobisphenol A, phenol novolak resin, cresol novolak resin, bisphenol A novolak resin or brominated phenol novolak resin are particularly preferred since the starting materials are readily available and the epoxy resin provides excellent properties to the cured epoxy resin composition.

Mixtures of the epoxy resins may also be used. Suitable ester group containing compounds used for the preparation of the modified epoxy resin can be represented by the following structural formulae I, II and III: A ¹ - (X ¹ ) n ¹ I wherein A^ represents an n^ valent, substituted or unsubstituted phenol- or naphthyl group, X^ is the same or different active ester group represented by the chemical formula (1) or (2) , and n^ is an integer of from 2 to 6; Y-f-A ² -^ ² ^ ² ]? ¹ II wherein A ² is the same or different (n ² + 1) valent, substituted or unsubstituted phenol- or naphthyl group, X ² is the same or different active ester group represented by the chemical formula (1) or (2), Y represents a p ¹ valent hydrocarbon group having 1 to 10 carbon atoms, n ² is an integer of from 1 to 6 and p is an integer of from 2 to 4; and

wherein A^ which may be the same or different (n^ + 1) valent or (n^ + 2) valent, substituted or unsubstituted phenyl- or naphthyl group, X^ is the same or different active ester group represented by the chemical formula (1) or (2), Z is the same or different hydro¬ carbon group having 1 to 10 carbon atoms, -0-, -CO-, -SO2- or a direct bond, n^ is an integer of from 1 to 6, and p ² is an integer of from 0 to 10.

The ester group-containing compound can be prepared by esterifying all or part of the phenolic hydroxyl groups present in the polyhydric phenolic compound.

The ester group containing compound of the formula I can be prepared by esterification of for example resorcin, hydroquinone, methyl resorcin, trihydroxybenzene, dihydroxynaphthalene or trihydroxynaphthalene. The ester group containing compound of the formula II can be prepared by esterification of for example bisphenol A, bisphenol F, tetrabromobisphenol A, trisphenol methane or tetraphenol ethane.

The ester group containing compound represented by the formula III can be prepared by esterification of for example biphenol, tetramethyl biphenol, phenol novolak resin, cresol novolak resin, bisphenol A novolak resin, dicyclopentadiene phenol resin, terpene phenol resin, phenol aralkyl resin, naphthol novolak resin, brominated phenol novolak resin, dihydroxyphenyl ether, dihydroxybenzophenone or dihydroxyphenyl sulfone. In principle there are no specific requirements as to the process of esterification of the polyhydric phenolic compound, but a suitable method is reacting the polyhydric phenolic compound with a compound of the below formula IV or V.

—B IV

^D, -f

wherein B represents a substituted or unsubstituted phenyl- or naphthyl group and D* represents a hydroxyl group, an alkoxyl group, a substituted or unsubstituted phenoxy group, a substituted or unsubstituted naphthoxy group, a substituted or unsubstituted benzoate group, a substituted or unsubstituted naphthoate group, or a halogen atom.

wherein each R ¹ may be the same or different alkyl group having 1 to 10 carbon atoms, substituted or unsubstituted phenyl group, substituted or unsubstituted aralkyl group, alkoxy group, halogen atom or hydrogen; and wherein D ² represents a hydroxyl group, an

alkoxy group, a substituted or unsubstituted phenoxy group, a substituted or unsubstituted naphthoxy group, a substituted or unsubstituted benzoate group, a substituted or unsubstituted naphthoate group, or a halogen atom. The appropriate reaction method and reaction conditions that are employed in the esterification reaction of the polyhydric phenolic compound depends on the nature of the polyhydric phenolic compound and active esterification agent. A suitable method is the following: The reaction components are mixed optionally in the presence of an organic solvent. The resulting mixture is reacted at a temperature of from 0 to 150 °C for a period of from 1 to 10 hours in the presence of a catalyst. The unreacted active esterification agent, by-products and solvents are removed from the reaction mixture to obtain the ester group-containing compound.

Suitable catalysts are amines such as trimethyl amine, triethyl amine, benzyldimethyl amine or pyridine; alkali metal alcolates such as potassium-t-butoxide or sodium ethoxide; alkyl metals such as butyl lithium or biphenyl sodium; acidic catalysts such as hydrochloric acid, sulphuric acid, oxalic acid, fluoroacetic acid, toluenesulfonic acid; organic acid salts showing an acidity; fluoroboric acid, heteropolysalts; polyphosphoric acids and activated clay.

Examples of inert organic solvents are ketones such as acetone, methyl ethyl ketone, or methyl isobutyl ketone; aromatic hydrocarbons such as benzene, toluene or xylene; ethers such as dioxane, or ethylene glycol dimethyl ether; and non-protonic polar solvents such as dimethyl sulfoxide or dimethyl formamide.

The ester group-containing compound may further contain a functional group that reacts with the epoxy resin other than the ester group. The amount thereof is preferably 50% or less of the total number of the functional groups in order to be able to prepare a modified epoxy resin that provides a cured epoxy resin composition with the desired level of moisture resistance and good electrical

properties. An example of such other functional group is a phenolic hydroxyl group.

The ester group-containing compounds can be used either alone or as a mixture of two or more thereof. The amount of epoxy resin and of ester group containing compound used in the process according to the present invention is such that from 0.05 to 0.95 mol, and preferably from 0.1 to 0.7 mol of ester groups are present per mol of epoxy groups.

If the amount of the ester group containing compound used is too small, the molecular weight of the modified epoxy resin produced is relatively small, and the effect of the modification is decreased. Further, if too large an amount of the ester group containing compound is used, the molecular weight of the modified epoxy resin of the present invention becomes too high and the viscosity thereof increases, which negatively affects its processability.

The epoxy resin and the ester group containing compound may react according to any conventional method.

This is typically by melt-kneading, and reacting the components at a temperature of from 80 to 200 °C for a period of 1 to 10 hours in the presence of a catalyst and optionally in the presence of an inert solvent.

Suitable catalysts include imidazoles such as 2-methyl imidazole or 2-ethyl-4-methyl imidazole; amines such as 2,4,6- tris (dimethylamino-methyl)phenol, benzyl dimethylamine or 1,8- diazabicyclo(5, ,0)-7-undecene; quaternary ammonium salts such as tetramethylammonium chloride, tetramethylammonium hydroxide or benzyl trimethylammonium bromide; phosphines such as tributyl- phosphine, triphenylphosphine or tris (dimethoxyphenyl)phosphine; phosphonium salts such as ethyltriphenylphosphonium iodide, ethyltriphenylphosphonium choride or tetrabutylphosphonium bromide; alkali metal hydroxides such as sodium hydroxide or potassium hydroxide; and alkali metal salts such as sodium bicarbonate, sodium benzoate and lithium chloride. The amount of the reaction catalyst

used is suitably from 50 to 3,000 ppm based on the weight of the epoxy resin.

Suitable inert organic solvents are alcohols such as ethanol or isopropanol; ketones such as acetone, methyl ethyl ketone or methyl isobutyl ketone; aromatic hydrocarbons such as benzene, toluene or xylene; ethers such as dioxane or ethylene glycol dimethyl ether; glycol ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether of propylene glycol monomethyl ether and non- protonic polar solvents such as dimethyl sulfoxide or dimethyl formamid.

The modified epoxy resin produced will have the structural formula (VI) mentioned below when in the process according to the present invention the epoxy resin used is the reaction product of bisphenol A and an epihalohydrin and the active ester group containing compound used is a dibenzoate of bisphenol A. A modified epoxy resin of the structural formula (VII) mentioned below is formed when the epoxy resin used is the reaction product of 3,3',5,5'-tetramethyl-4,4'-biphenol and an epihalohydrin and the ester group containing compound used is the diacrylate of tetrabromobisphenol A.

Structural Formula VI

wherein m is on average a number of from 0.1 to 5. Structural Formula VII

wherein n is on average a number of from 0.1 to 5 in the average value.

The present invention further relates to an epoxy resin composition comprising the modified epoxy resin of the present invention and a curing agent for the modified epoxy resin. The epoxy resin composition may further contain other epoxy resins than the modified epoxy resin if for example, improved processability or specific properties of the cured composition are desired.

Examples of such other epoxy resins are an epoxy resin prepared by reacting an epihalohydrin with phenolic compounds such as bisphenol A, bisphenol F, bisphenol AD, hydroquinone, resorcin, methyl resorcin, biphenol, tetramethyl biphenol, dihydroxynaphtha-

lene, tetrabromobisphenol A, dihydroxydiphenyl ether, dihydroxydi- benzophenone, dihydroxydiphenyl fulfone, a phenol novolak resin, a cresol novolak resin, a bisphenol A novolak resin, a dicyclopenta¬ diene phenol resin, a terpene phenol resin, a phenol aralkyl resin, a naphthol novolak resin, or a brominated phenol novolak resin; an epoxy resin prepared by reacting an epihalohydrin with polyhydric phenolic resins, the latter obtained by condensation reaction of a phenolic compound with an aldehyde such as hydroxybenzaldehyde, crotonealdehyde or glyoxal; an epoxy resin prepared by reacting epihalohydrin with an amine compound such as diaminophenylmethane, aminophenol or xylene diamine; an epoxy resin prepared by reacting an epihalohydrin with a carboxylic acid such as methylhexahydroxy- phthalic acid or dimer acid.

The amount of the other epoxy resin that may be used in the epoxy resin composition of the present invention is preferably 100 parts by weight or less per 100 parts by weight of the modified epoxy resin. If the amount of the other epoxy resins used is too high, the properties of the epoxy resin composition according to the present invention in the cured state are unsatisfactory. The type of curing agent used in the epoxy resin composition according to the present invention is not critical and depends on the use and the desired characteristics of the epoxy resin composition.

Suitable curing agents are various phenols; phenolic resins such as a phenol novolak resin, a cresol novolak resin, a bisphenol A novolak resin, a dicyclopentadiene phenol resin, a phenol aralkyl resin or a terpene phenol resin; various phenols; polyhydric phenolic compounds obtained by condensation reaction of various phenols and various aldehydes such as hydroxybenzaldehyde, croton aldehyde and glyoxal; acid anhydrides such as methyltetrahydro- phthalic anhydride, hexahydrophthalic anhydride, pyromellitic anhydride and methylnadic acid; amines such as diethylene triamine, isophorone diamine, diaminodiphenyl methane, diaminodiphenyl sulfone and dicyan diamide.

The epoxy resin composition of the present invention may further comprise various additives such as a cure accelerator, a filler, a coupling agent, a flame retardant, a plasticizer, a solvent, a reactive diluent, and a pigment. Suitable curing accelerators are imidazoles such as 2-methyl imidazole or 2-ethyl-4-methyl imidazole; amines such as 2,4,6- tris (dimethylaminomethyl)phenol, benzyl dimethyl amine or 1,8- diazabicyclo(5,4,0)-7-undecene; organophosphorus compounds such as tributyl phosphine, triphenyl phosphine or tris(dimethoxyphenyl) phosphine.

Suitable fillers are fused silica, crystalline silica, glass powder, alumina and calcium carbonate. Suitable flame retardants are antimony trioxide and phosphoric acid. Alternatively flame retardancy can be introduced by using a brominated epoxy resin as (a part of) the other epoxy resin used.

The epoxy resin compositions according to the present invention have excellent moisture resistance and electrical characteristics in the cured state, and therefore they are particularly suitable for use as coating, adhesive, casting, sealant, moulding and laminating. The present invention is illustrated by means of the following examples. Examples 1 to 4

Preparation of ester group-containing compounds

A 3-litre three-necked flask equipped with a thermometer, a stirrer and a cooling tube was charged with hydroquinone, bisphenol A, tetrabromobisphenol A or a phenol novolak resin as the phenolic compound; benzoyl chloride or methacrylic acid chloride, as the esterification agent, and with pyridine in the amounts as shown in Table 1. The resulting mixture was maintained at 30 °C for 3 hours to be reacted.

Successively, 1,000 g of methyl isobutyl ketone was added to dissolve the reaction product completely. Thereafter the mixture was washed with water to remove by-products such as salts and methyl isobutyl ketone was removed by destination under reduced pressure. The desired ester group-containing compound was thus obtained.

TABLE 1

Preparation Conditions of Ester Group-Containing Compound

Amount used (g) : Example Example Example Example 1 2 3 4

Hydroquinone 110 _ - -

Bisphenol A - 228 - -

Tetrabromobisphenol A _ - 272 —

Phenol novolak resin* - _ — 206

Benzoyl chloride 281 281 141 _

Methacrylic acid chloride - - - 209

Pyridine 500 500 400 400

Manufactured by Gunei Chemical Co., hydroxy equivalent weight: 103 g/eq. softening point: 85 °C.

Examples 5 to 8 and Comparative Examples 1 and 2

A 1-litre three-necked flask equipped with a thermometer, a stirrer and a cooling tube was charged with an epoxy resin derived from bisphenol A or an epoxy resin derived from tetramethylbiphenol, with a compound prepared in Examples 1 to 4 and with bisphenol A or a phenol novolak resin as a phenolic compound, in the amounts as shown in Table 2. The resulting mixture was dissolved at 130 ^β C followed by adding tetramethylammonium chloride or 2-methyl imidazole as a catalyst in an amount shown in Table 2 below. Thereafter the mixture was gradually heated to 165 ^C C. The mixture was reacted at 165 °C for 5 hours.

At the end of the reaction, the reaction mixture was cooled and solidified to obtain the desired modified epoxy resin.

The epoxy equivalent weight and softening point of the modified epoxy resins thus obtained were measured, and the results are shown in Table 2.

TABLE 2

Example 5 Example 6 Example 7 Example 8 Comp. Ex. 1 Comp. Ex. 2

Preparation Conditions of Modified Epoxy Resin:

Epoxy Resin Type A A A B A B

Amount (g) 500 500 400 600 500 700

Active Ester Group- Containing Compound Type Example 1 Example 2 Example 3 Example 4 Amount (g) 200 230 370 100

Phenol Compound Type - - - C D Amount (g) 140 85

Reaction Catalyst Type E E E F E F Amount (g) 0.2 0.2 0.2 0.3 0.2 0.3

Analytical data:

Epoxy equivalent weight (g/eq.) 465 451 667 269 442 272

Softening point ( ^β C) 70 69 78 65 67 64

A: Epoxy resin derived from bisphenol A ("EPIKOTE 828", a trade name, manufactured by Yuka Shell

Epoxy K.K.; epoxy equivalent weight 186 g/eq.)

B: Epoxy resin derived from tetramethylbiphenol ("EPIKOTE YX4000", a trade name, manufactured by

Yuka Shell Epoxy K.K.; epoxy equivalent weight 186 g/eq.)

C: Bisphenol A D: Phenol novolak resin (manufactured by Gunei Chemical Co.; hydroxy equivalent weight 103 g/eq.; softening point 85 ^β C)

E: Tetramethylammonium chloride F: 2-Methyl imidazole

Examples 9 to 12 and Comparative examples 3 and 4

Epoxy resin compositions were prepared using each of the modified epoxy resins obtained in Examples 5 to 8 and Comparative

Examples 1 and 2 as the modified epoxy resin, methyltetrahydro- phthalic anhydride, dicyan diamide or a phenol novolak resin as the curing agent and 2-methyl imidazole as cure accelerator.

After defoaming each of the composition was cast into a metallic mould and cured at 180 °C for 8 hours to obtain each of test pieces. A rate of moisture absorption and the dielectric constant of each cured epoxy resin composition was measured.

Table 3 shows that each moulding material of Examples 9 to 12 had a significantly lower rate of moisture absorption and dielectric constant than the moulding materials of Comparative Examples 3 and 4.

TABLE 3

Epoxy Resin Composition Example 9 Example 10 Example 11 Example 12 Comparative Comparative Example 3 Example 4

Modified Epoxy Resin Type Example 5 Example 6 Example 7 Example 8 Comparative Comparative Example 1 Example 2 Amount (g) 100 100 100 100 100 100

Curing Agent for Epoxy G H H I H I Resin Type

Amount (g) 35 2.8 1.9 38 2.8 38

Curing Accelerator*^ 1 0.1 0.1 0.5 0.1 0.5

Properties of Cured Resin

Rate of Moisture Absorption* ² (%) 0.41 0.38 0.36 0.33 0.62 0.56

Dielectric Constant* ³ 2.8 3.0 2.9 2.8 3.9 3.9

G: Methyltetrahydrophthalic anhydride

H: Dicyan diamide

I: Phenol novolak resin (manufactured by Gunei Chemical Co.; hydroxy equivalent weight 103 g/eq.; softening point 85 ^β C)

* ; 2-Methyl imidazole

* ² : Rate of moisture absorption after dipping in water at 110 °C for 100 hours

* ³ : 23 °C, 1MHz