EPOXY RESIN HARDENER, EPOXY RESIN COMPOSITION, AND EPOXY RESIN HARDENING PROCESS

Abstract: An epoxy resin hardener 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 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; an epoxy resin composition which comprises containing therein the epoxy resin hardener defined above. A process for hardening an epoxy resin which comprises using the epoxy resin hardener defined above. A typical example of the aromatic ester is one which is obtained by esterifying more than 50 % of phenolic hydroxyl groups of a polyhydric phenol with benzoic acid or naphthoic acid optionally having substituted groups and/or atoms.

Description: EPOXY RESIN HARDENER, EPOXY RESIN COMPOSITION, AND EPOXY RESIN HARDENING PROCESS The present invention relates to an epoxy resin hardener, an epoxy resin composition and an epoxy resin hardening process, which are all designed to give a cured product having outstanding moisture resistance. Epoxy resins are used in a broad range of applications, such as adhesion, casting, potting, lamination, moulding and coating, in view of their good handling properties and curing characteristics. On the other hand, a lot of kinds of hardeners are known, which hardeners are used in combination with the epoxy resins to give cured products . The properties of the cured products may vary very much, depending on the hardener used. Therefore appropriate hardeners are selected for specific fields or purposes. Recent technological innovations, which open up new application areas, need resin materials which should meet more stringent requirements than before. Improvement in moisture resistance is an important subject particularly in the fields of adhesion, casting, potting, moulding, lamination, and coating. Unfortunately, no fully satisfactory results are achieved with the hardeners which are generally used at the present moment. In other words, the commonly used hardeners derived from phenol, amine, acid anhydride, or polymerization catalyst do not yield cured products having fully satisfactory moisture resistance. To be more specific, a phenol-based or amine-based hardener forms polar hydroxyl groups due to the reaction of epoxy groups and active hydrogen atoms, which polar hydroxy groups result in an adverse effect on moisture resistance. By contrast, the use of an acid anhydride hardener results in a lower amount of hydroxyl groups but there is an adverse effect on moisture resistance, because after the reaction with an epoxy resin an ester linkage is formed which is poor in moisture resistance. A hardener of polymerization catalyst does not form hydroxyl groups but it aggravates moisture resistance because of its mgn reactivity. It is an object of the present invention to provide an epoxy resin narαener, an epoxy resin composition, ana an epoxy resin hardening process, which are designed to give a cured product having outstanding moisture resistance. In order to solve the above-mentioned problems, the inventors carried out a series of experiments on hardeners which do not have an adverse effect on the moisture resistance of the cured products and which give rise to as few groups as possible which aggravate the moisture resistance of the cured products. As the result, it was found that the object is achieved by using as the hardener an aromatic ester having a carboxylic ester linkage in the molecule which connects the aromatic rings directly to each other. The carboxylic ester linkage reacts with epoxy groups to cure the epoxy resin without giving rise to hydroxyl groups and the like which aggravate the moisture resistance of cured products. The present invention is embodied in an epoxy resin hardener 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 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. The present invention is also embodied in an epoxy resin composition and an epoxy resin hardening process, the former containing and the latter using the epoxy resin hardener defined above. According to the present invention, the epoxy resin hardener is an aromatic ester which has a carboxylic ester linkage which connects the aromatic rings directly to each other (here below referred to briefly as "a direct ester linkage between aromatic rings") . It s this linkage that is involved in the curing reaction mechanism of epoxy resins. The reaction is explained by the following formula (2) with reference to an instance in which the direct ester linkage between aromatic rings is the phenyl benzoate linkage. R'—O—C—R" CHo—CH—R" (Direct ester linkage (Epoxy resin) between aromatic rings) R'-O—CH?—CH—R" O—C—R" II o It is apparent from the curing reaction that the hardener (whose main skeleton is connected to the aromatic ring adjacent to the oxygen atom) yields a cured product in which the hardener is connected to the epoxy resin through the carbon-carbon linkage and ether linkage, which do not impair moisture resistance. Moreover, the hardener does not result in the formation of hydroxyl groups which lower the moisture resistance, unlike the conventional phenol- based or amine-based hardener. It follows, therefore, that the hardener yields a cured product having outstanding moisture resistance. The direct ester linkage between aromatic rings in the present invention cannot be replaced by any other ester in which one side or both sides of the ester linkage are not aromatic rings, that is, an aromatic ring and an aliphatic chain are connected directly to each other by an ester linkage, or aliphatic chains are connected directly to each other by an ester linkage. Such an ester, when used as an epoxy resin hardener, does not react with the epoxy group at all, or its reaction rate is too slow to be of practical use even though it undergoes reaction. For reasons mentioned above, the present invention specifies as the epoxy resin hardener an aromatic ester having on average one or more direct ester linkages between aromatic rings in the molecule and also having on average two or more functional groups, which react with epoxy groups, including said direct ester linkage between aromatic rings. This aromatic ester should preferably be one which does not have other functional groups, which react with epoxy groups, than the direct ester linkage between aromatic rings, so that the cured product has the desired moisture resistance. If the aromatic ester does contain other functional groups, their number should be limited such that the number of the ester linkages accounts for more than 50%, preferably more than 70%, of the total number of functional groups. According to the present invention, the epoxy resin hardener may be substituted by groups or atoms on its aromatic rings. Particularly, if the aromatic ester has substituted groups such as phenolic hydroxy groups, primary or secondary ammo groups, carboxylic groups, acid anhydride groups and mercapto groups, such groups will work as functional groups to react with epoxy groups. However, as mentioned before, the number of such functional groups is preferably less than 50% of the total number of functional groups; otherwise, they have an adverse effect on the moisture resistance of the cured product. According to the present invention, the epoxy resin hardener includes various kinds of compounds prepared by various methods. Typical aromatic esters are obtainable by, and are suitably prepared by esterifying more than 50r of phenolic hydroxyl groups of a polyhydric phenol with benzoic acid or naphthoic ac d, said polyhydric phenol optionally being substituted by groups and/or atoms on the aromatic ring, said benzoic acid optionally being substituted by groups and/or atoms on the benzene ring, and said naphthoic acid optionally being substituted by groups and/or atoms on the naphthalene ring. Examples of the polyhydric phenol having optionally substituted groups and/or atoms on the aromatic ring include bisphenol A, bisphenol F, resorcm, hydroquinone, dihydroxynaphthalene, biphenol, tetramethylbiphenol, tetrabromobisphenol A, phenol novolak resin, cresol novolak resin, bisphenol A novolak resin, dicyclopentadiene phenolic resin, terpene phenolic resin, phenol aralkyl resin, naphthalene novolak resin, brominated phenolnovolak resin, and polyhydric phenolic resin (which is a product of condensation reaction of a phenolic compound and an aldehyde such as hydroxybenzaldehyde, crotonaldehyde, and glyoxal) . Esterification of the polyhydric phenol (to produce a benzoate or naphthoate) may be accomplished in several ways. Typically, it employs an esterifying agent represented by the following formula (3) where A represents a benzene ring or naphthalene ring; R^ represents a hydroxyl group, alkoxyl group, substituted or unsubstituted phenoxy group, substituted or unsubstituted naphthoxy group, substituted or unsubstituted benzoate group, substituted or unsubstituted naphthoate group, or halogen atom; R 2 represents a c l-10 al yl group, especially a C^..^ alkyl group, a substituted or unsubstituted phenyl group, a substituted or unsubstituted aralkyl group, an alkoxyl group or a halogen atom, especially a clorine or bromine atom, with two or more of R 2 being the same or different; and m is 0 or an integer of 1 to 5. The epoxy resin hardener of the present invention is obtainable by, and may suitably be prepared by esterifying more than 50% of the phenolic hydroxyl groups of a polyhydric phenol with the above defined esterifying agent. The esterification may be carried out in different ways under varied conditions depending on the kind and combination of the polyhydric phenol and esterifying agent used. Typically, it is carried out by mixing the two components in the presence or absence of an organic solvent in the presence of a catalyst at 0 to 150 °C for 1 to 10 hours. The reaction product is freed of the unreacted esterifying agent, solvent, and by-products. Thus there is obtained the desired aromatic ester. Examples of the catalyst include amines such as trimethylamine, triethyla ine, benzyldimethyla ine, and pyridine; alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; metal alcoholates such as potassium t-butoxide and sodium ethoxide; alkali metal compounds such as butyl lithium and biphenyl sodium; and acidic catalysts such as hydrochloric aciα, sulfuπc acid, oxalic acid, fluoroacetic acid, toluenesulfonic acid, acidic organic acid salts, fluoroboric acid, heteropolyacids, polyphosphoric acid, and activated clay. Examples of the solvent include ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; aromatic hydrocarbons such as benzene, toluene, and xylene; ethers such as dioxane and ethylene glycol dimethyl ether; and aprotic polar solvents such as dimethylsulfoxide and dimethylformamide, which are all inert organic solvents. The thus obtained aromatic ester, which is the epoxy resin hardener of the present invention, is a compound or a mixture of compounds . The above-mentioned reaction should preferably be carried out such a manner that the rate of esterification (or the rate of benzoate or naphthoate) is higher than 50%, preferably higher than 70%. If the rate of esterification is lower than specified above, large amounts of phenolic hydroxyl groups remain unreacted. They react with epoxy groups to contribute to the hardening of epoxy resins but, upon reaction with epoxy groups, they form hydroxyl groups which have an adverse effect on the moisture resistance of the cured product. According to the present invention, the above-mentioned epoxy resin hardener may be incorporated, as an essential component, into an epoxy resin to give the epoxy resm composition of the present invention. There are no restrictions on the epoxy resin. Examples of the epoxy resin include those which are produced from a phenolic compound and an epihalohydrin, those which are produced from an amine and an epihalohydrin, and those which are produced from a carboxylic acid and an epihalohydrin. The phenolic compound includes bisphenol A, bisphenol F, bisphenol AD, hydroquinone, resorcm, methylresorcm, biphenol, tetramethyl- biphenol, dihydroxynaphthalene, phenol novolak resm, cresol novolak resm, bisphenol A novolak resm, dicyclopentadiene phenolic resin, terpene phenolic resm, phenol aralkyl resm, and naphthol novolak resm. The phenolic compound further includes polyhydric phenolic resins which are formed by the condensation reaction of a phenolic compound and an aldehyde such as hydroxybenzaldehyde, croton- aldehyde, and glyoxal. The amme includes diammophenyl-methane, aminophenol, and xylenediamme. The carboxylic acid includes meth- ylhexahydroxyphthal c acid and di er acid. The epoxy resm composition of the present invention may be incorporated with an optional epoxy resin hardener in addition to the epoxy resm hardener of the present invention. Examples of the optional hardener include phenolic resins, polyhydric phenol resins, acid anhydrides, and amines. Phenolic resins include phenol novolak resin, cresol novolak resm, bisphenol A novolak resin, dicyclopentadienephenolic resm, phenolaralkyl resin, terpenephenolic resm, and naphthol novolak resm. Polyhydric phenolic resins include those which are produced by the condensation reaction of a phenolic compound and an aldehyde such as hydroxybenzaldehyde, crotonaldehyde, and glyoxal. Acid anhydrides include methyltetrahydrophthalic anhydride, hexahydrophthalic anhydride, pyromellitic anhydride, and nadic methyl anhydride. Amines include diethylenetπamine, sophoronediamme, diam ophenyl- methane, diammodiphenylsulfone, and dicyandiamide. The optional epoxy resm hardener should preferably be used in an amount less than 100 parts by weight for 100 parts by weight of the epoxy resin hardener of the present invention. It reduces the effect of the present invention if it is used in an excess amount. The epoxy resin hardener should be used in such an amount that it contains 0.5 to 2.0 moles of groups, preferably 0.7 to 1.2 moles of groups, which react with 1 mole of epoxy groups in all the epoxy resm components. As with ordinary epoxy resm composition, the epoxy resin composition of the present invention may be incorporated with a variety of additives, such as cure accelerator, filler, coupling agent, flame retardant, plasticizer, solvent, reactive diluent, and pigment, according to need. Examples of the cure accelerator include lmidazoles such as 2- methylimidazole and 2-ethyl-4-methylιmιdazole; amines such as 2,4,6- tris (dimethylaminomethyl)phenol, benzylmethylamine, DBU, and DCMU, and organic phosphorus compounds such as tributylphosphine, tπ- phenylphosphine, and tn (dιmethoxyphenyl)phosphιne. Examples of the filler include fused silica, crystalline silica, glass powder, alumina, and calcium carbonate. Examples of the flame retardant include antimony trioxide and phosphoric acid. In addition, the epoxy resm composition may be rendered flame retardant by partly replacing the epoxy resin by a brommated epoxy res . The invention also relates to products applied with the cured epoxy res compositions according to the present invention and to products obtained by curing the before mentioned epoxy resin composition. The epoxy resm composition of the present invention yields a cured product having good moisture resistance, which is therefore advantageously used in the fields of adhesion, casting, potting, moulding, lamination, coating, and the like. The invention will be described m more detail with reference to the following examples and comparative examples. Examples 1 to 3 These examples relate to the epoxy resm hardener of the present invention. A 3000-ml three-mouth flask equipped with a thermometer, stirrer, and condenser was charged with phenol novolak resin or phenolsalicylaldehyde novolak resm as a polyhydric phenol; benzoyl chloride or naphthoic acid chloride as an esterifying agent; and pyridine as a catalyst, the quantities of which are shown in Table 1, and held at 30 °C for 2 hours to be reacted. Successively, methyl isobutyl ketone (1000 g) was added to dissolve the reaction product completely. After the solution was washed with water to remove by-products (salts), the solvent was removed by distillation under reduced pressure. Thus the desired epoxy resin hardener (aromatic ester) was obtained. The content of hydroxyl groups of the above obtained hardener, the ratio of esterification (in terms of benzoate or naphthoate) , the average number per molecule of the direct ester linkage between aromatic rings, the average number of phenolic hydroxyl groups, and the softening point are shown in Table 1. Table 1 Example No. 1 2 3 Reactants Phenol novolak resin ^ 200 200 - charged Phenolsalicylaldehyde novolak - - 200 ( g ) res * "■ Benzoyl chloride 273 - 283 Naphthoic acid chloride - 330 Pyridine 200 300 200 Properties Content of hydroxyl groups 0.25 0.62 0.36 of hardener (meq./g) Ratio of esterification (%) * ^ 95 80 93 Average number of ester 3.6 3.0 4.1 linkages (per molecule) ** * Average number of hydroxyl 0.2 0.8 0.3 groups (per molecule) *" * Softening point (°C) 91 95 129 Notes to Table 1 *1 A product of Gunei Kagaku, having a hydroxyl equivalent of 103 g/eq., 3.8 hydroxyl groups (on average) per molecule, and a softening point of 85 °C. + 2 "Epikure YL6065" (a product of Yuka Shell Epoxy K.K.) having a hydroxyl equivalent of 98 g/eq., 4.4 hydroxyl groups (on average) per molecule, and a softening point of 125 °C. *3 Calculated from the content of hydroxyl groups. *4 Calculated from the average number of hydroxyl groups per molecule in the polyhydric phenol as the starting material and also from the rate of esterification. Examples 4 to 8 and Comparative Examples 1 and 2 These examples relate to the epoxy resin composition and the process for curing the same according to the present invention. Epoxy resin compositions were prepared according to the formulation shown in Table 2 from any of resin I (bisphenol A type epoxy resin) , resin II (o-cresolnovolak type epoxy resin) , and resin III (epoxy resin derived from tetramethylbiphenol), any of the epoxy resin hardeners obtained in Examples 1 to 3, hardener I (phenol novolak resin hardener) , and hardener II (methyltetrahydrophthalic anhydride hardener) , and a cure accelerator (2-methylimidazole) . The resulting epoxy resin composition was defoamed and casted into a mould. Upon curing at 180 °C for 8 hours, there were obtained test pieces. They were tested for moisture absorption and glass transition point. The results are shown in Table 2. It is noted that the test pieces in Examples 4 to 8 are much lower in moisture absorption than those in Comparative Examples 1 and 2. Table 2 Example No. Comparative Example No. 4 5 6 7 8 1 2 Formula¬ Epoxy resin Re^in I Resm II Reξjin I Resύi II Resin Re^in I Resin I tion of (parts by weight) III *3 ( 100) epoxy (100) (100) (100) (100) (100) (100) resin Epoxy resin hardener Example Example Example Example Example Hardene composi¬ Hardene (parts by weight) 1 2 3 1 1 r l * 4 tion r ll *5 (109) (166) (105) (100) (109) (55) (80) Cure accelerator (2- 1 1 1 1 1 1 1 methylimidazole) (parts by weight) Properti Moisture absorption ( ) 0.15 0.11 0.20 0.17 0.14 0.53 0.74 es of cured Glass transition point 101 98 120 151 142 104 147 product CO *7 Notes to Table 2 Resin I : bisphenol A type epoxy resin ("Epikote 828" having an epoxy equivalent of 186, from Yuka Shell Epoxy K.K.) Resin II : o-cresol novolak type epoxy resin ("Epikote 180H65" having an epoxy equivalent of 201, from Yuka Shell Epoxy K.K.) Resin III : epoxy resin derived from tetramethylbiphenol ("Epikote YX4000" having an epoxy equivalent of 186, from Yuka Shell Epoxy K.K. ) Hardener I : phenol novolak resin, having a hydroxyl equivalent of 103 g/eq. and a softening point of 85 °C, a product of Gunei Kagaku. *5 Hardener II : methyltetrahydrophthalic anhydride *6 measured after immersion in water at 100 °C for 100 hours. *7 obtained from the transition point of the thermal expansion curve of TMA. The epoxy resin hardeners, the epoxy resin compositions, and epoxy resin curing process described above result in cured products having good moisture resistance. Therefore, they can be used advantageously in a broad range of applications including adhesion, casting, potting, lamination, moulding, and coating.

Claims: C A I M S 1. An epoxy resin hardener having m the molecule on average two or more functional groups which may react with epoxy groups, in which on average one or more functional groups 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. 2. An epoxy resin hardener as defined in claim 1, obtainable by esterifying more than 50% of the phenolic hydroxyl groups of a polyhydric phenol with benzoic acid or naphthoic acid, said polyhydric phenol optionally being substituted by groups and/or atoms on the aromatic ring, said benzoic acid optionally being substituted by groups and/or atoms on the benzene ring, and said naphthoic acid optionally being substituted by groups and/or atoms on the naphthalene ring. 3. An epoxy resin hardener as defined in claim 1, which is obtained by esterifying more than 50% of the phenolic hydroxyl groups of a polyhydric phenol with benzoic acid or naphthoic acid, said polyhydric phenol optionally being substituted by groups and/or atoms on the aromatic ring, said benzoic acid optionally being substituted by groups and/or atoms on the benzene ring, and said naphthoic acid optionally being substituted by groups and/or atoms on the naphthalene ring. 4. .An epoxy resin hardener as defined in any of claims 1-3, obtainable by esterifying more than 50% of the phenolic hydroxyl groups of a polyhydric phenol with an esterifying agent represented by the following formula (1) R 1 - C - A -i R 2 ) m (1) II o where A represents a benzene ring or naphthalene ring; R 1 represents a hydroxyl group, alkoxyl group, substituted or unsubstituted phenoxy group, substituted or unsubstituted naphthoxy group, substituted or unsubstituted benzoate group, substituted or unsubstituted naphthoate group, or halogen atom; R ώ represents a c l-10 a l κ yl group, substituted or unsubstituted phenyl group, substituted or unsubstituted aralkyl group, alkoxyl group, or halogen atom, with two or more of R 2 being the same or different; and m is 0 or an integer of 1 to 5, said polyhydric phenol optionally being substituted by groups and/or atoms on the aromatic ring. 5. An epoxy resin hardener as defined in any of claims 1-3, which is obtained by esterifying more than 50% of the phenolic hydroxyl groups of a polyhydric phenol with an esterifying agent represented by the following formula (1) >1 „ C - A —(- -R* ) , (1) 0 where A represents a benzene ring or naphthalene ring; R! represents a hydroxyl group, alkoxyl group, substituted or unsubstituted phenoxy group, substituted or unsubstituted naphthoxy group, substituted or unsubstituted benzoate group, substituted or unsubstituted naphthoate group, or halogen atom; R 2 represents a l-10 alkyl group, substituted or unsubstituted phenyl group, substituted or unsubstituted aralkyl group, alkoxyl group, or halogen atom, with two or more of R 2 being the same or different; and m is 0 or an integer of 1 to 5, said polyhydric phenol optionally being substituted by groups and/or atoms on the aromatic ring. 6. An epoxy resin composition comprising therein the epoxy resin hardener defined in any of claims 1 to 5. 7. A process for hardening an epoxy resin which comprises using the epoxy resin hardener defined in any of claims 1 to 5. 8. A product applied with the cured epoxy resin composition according to claim 6. 9. Cured product obtained by curing the epoxy resin composition according to claim 6.

Inventors: MURATA YASUYUKI, NAKANISHI YOSHINORI, TOHRIIWA NORIO, MURATA, YASUYUKI, NAKANISHI, YOSHINORI, TOHRIIWA, NORIO
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1995
Main classification: C08G59
IPC codes: C08G59, C08L61, C08L63