Latent Curing Agents for Epoxy Resins

The present invention relates to a latent curing agent for epoxy resins. More particularly, it relates to a latent curing agent for epoxy resins which causes rapid curing at moderate elevated temperature and which gives epoxy resin composition having excellent storage stability at room temperature.

One-pack type epoxy resins are preferable to the conventional two-pack type ones because the former are free of misformulation and can be used continuously. One-pack type epoxy resins require a so-called latent curing agent which does not react with epoxy compounds at room temperature but, on heating, reacts with epoxy compounds to effect curing.

Heretofore, several latent curing agents have been proposed. Representative examples are boron trifluoride-amine adduct, dicyandiamide, and dibasic acid dihydrazide. The first one is hard to treat owing to its high hygroscopic property and it affects adversely upon the physical properties of the cured resin. The latter two are useful in formulating epoxy resin compositions having excellent storage stability but full curing by means of these compound could be achieved by heating at higher temperature than 150.degree. C. for a long time. In the case where an epoxy resin is used for sealing electronic components, rapid curing at a low temperature is required not to damage electronic components.

It is known that an amine type curing agent can be improved in workability by converting it into an adduct with an epoxy resin. It has recently been discloed that an adduct obtained by the reaction of the secondary amino group of imidazole compounds or N-methylpiperazine with an epoxy group is a comparatively good latent curing agent. (See U.S. Pat. Nos. 4,066,625 and 4,268,656.) It is known that an adduct is produced by reacting the --NH.sub.2 group or --NH group with an epoxy compound as mentioned above. However, it is not known that a curing agent for epoxy resins is produced by reacting a compound having active hydrogen other than amino groups with an epoxy compound.

We have carried out a series of researches in order to develop a latent curing agent which performs curing at a low temperature and is superior in storage stability. As the result, it has been found that it is possible to obtain a latent curing agent which is equal to or superior to the above-mentioned adduct of an amine type curing agent and an epoxy compound. Such a latent curing agent is an adduct obtained by reacting a compound having active hydrogen other than amino group with an epoxy compound. It is important that the compound having active hydrogen should also have a tertiary amino group in the molecule.

That is, it has been found that an addition product obtained by reacting (a) a polyfunctional epoxy compound and (b) a compound having at least one functional group of OH group, SH group, COOH group and CONHNH.sub.2 group, and a tertiary amino group in the molecule, or an addition product obtained by reacting (a), (b), and (c) an organic compound having two or more active hydrogen atoms in the molecule (excluding a compound having an epoxy group or tertiary amino group) is the latent curing agent that meets the object of this invention. The present invention was completed based on this finding. The one produced from three components (a), (b), and (c) is superior in storage stability to the one produced from two components (a) and (b).

A detailed description will be given in the following with respect to the latent curing agent of this invention.

The polyfunctional epoxy compound as a raw material of the latent curing agent of this invention may be any compound having two or more epoxy groups in the molecule. It includes, for example, polyglycidyl ether obtained by reacting a polyhydric phenol (such as bisphenol A, bisphenol F, catechol, resorcinol) or a polyhydric alcohol (such as glycerin and polyethylene glycol) with epichlorohydrin; glycidyl ether ester obtained by reacting a hydroxycarboxylic acid (such as p-hydroxybenzoic acid and .beta.-hydroxynaphthoic acid) with epichlorohydrin; polyglycidyl ester obtained from a polycarboxylic acid such as terephthalic acid; glycidylamine compounds obtained from 4,4'-diaminodiphenylmethane and m-aminophenol; and epoxidized novolak and epoxidized polyolefin.

The compound having at least one functional group of OH group, SH group, COOH group and CONHNH.sub.2 group together with a tertiary amino group in the molecule, which is reacted with the above-mentioned polyfunctional epoxy compound, may be represented by the following formula. ##STR1## where X is --OH group, --SH group, --COOH group, or --CONHNH.sub.2 group; R.sub.1 and R.sub.2 are C.sub.1-20 alkyl groups, C.sub.2-20 alkenyl groups, and aralkyl groups such as benzyl group, with or without substituent such as oxygen, halogen, and functional group as represented by X mentioned above; and R.sub.3 is a divalent residue of R.sub.1 or R.sub.2. R.sub.1 and R.sub.2 may connect with each other to form a ring, or R.sub.1, R.sub.2, and R.sub.3 may connect with one another to form a ring. The compounds with tertiary amino groups contained in the heterocyclic ring as represented by the following formulas (2) and (3) are also effective. ##STR2## where R.sub.4, R.sub.5, and R.sub.6 are hydrogen atoms, phenyl groups the groups defined by R.sub.1 and R.sub.2 where R.sub.4, R.sub.5, and R.sub.6 are the groups defined by R.sub.1 and R.sub.2 or the functional groups defined by X in the above; R.sub.7 is the same as R.sub.1 or R.sub.2 ; and at least one of R.sub.4, R.sub.5, R.sub.6, and R.sub.7 contains the functional group represented by X. ##STR3## where R.sub.8 is the functional group represented by X, or the same group as R.sub.1 or R.sub.2 that contains a functional group represented by X.

The representative examples of the compound having at least one functional group of --OH, --SH, --COOH and --CONHNH.sub.2 together with a tertiary amino group are listed below.

2-Dimethylaminoethanol, 1-methyl-2-dimethylaminoethanol, 1-phenoxymethyl-2-dimethylaminoethanol, 2-diethylaminoethanol, 1-butoxymethyl-2-dimethylaminoethanol, 1-(2-hydroxy-3-phenoxypropyl)-2-methylimidazole, 1-(2-hydroxy-3-phenoxypropyl)-2-ethyl-4-methylimidazole, 1-(2-hydroxy-3-butoxypropyl)-2-methylimidazole, 1-(2-hydroxy-3-butoxypropyl)-2-ethyl-4-methylimidazole, 1-(2-hydroxy-3-phenoxypropyl)-2-phenylimidazoline, 1-(2-hydroxy-3-butoxypropyl)-2-methylimidazoline, 2-(dimethylaminomethyl)phenol, 2,4,6-tris(dimethylaminomethyl)phenol, N-.beta.-hydroxyethylmorpholine, 2-dimethylaminoethanethiol, 2-mercaptopyridine, 4-mercaptopyridine, N,N-dimethylaminobenzoic acid, N,N-dimethylglycine, N.sup..alpha.,N.sup..alpha. -dimethyl-N.sup..epsilon. -lauroyllysine, nicotinic acid, isonicotinic acid, picolinic acid, N,N-dimethylglycinehydrazide, N,N-dimethylpropionic acid hydrazide, nicotinic acid hydrazide, isonicotinic acid hydrazide and the like.

The active hydrogen in the organic compound having two or more active hydrogen atoms but having no epoxy group and/or tertiary amino group denotes any hydrogen atom connected to oxygen, nitrogen, sulfur, etc. excluding carbon, or hydrogen atoms contained in a functional group such as --OH, --NH, --NH.sub.2, --SH, --COOH, and --CONHNH.sub.2.

The examples of the organic compound having two or more active hydrogen atoms but having no epoxy group and/or tertiary amino group are listed below.

Polyhydric phenols such as bisphenol A, bisphenol F, bisphenol S, hydroquinone, catechol, resorcinol and pyrogallol, phenol-novolak resin; polyhydric alcohols such as trimethylol propane; amine compounds such as piperazine, aniline, and cyclohexylamine; polybasic carboxylic acids such as adipic acid, phthalic acid and 3,9-bis(2-carboxyethyl)-2,4,8,10-tetroxaspiro[5,5]-undecane; polythiols such as 1,2-dimercaptoethane and 2-mercaptoethylether; hydrazides such as phenylacetic acid hydrazide; amino acids such as alanine and valine; compounds having two or more of different functional groups in the molecule such as 2-mercaptoethanol, 1-mercapto-3-phenoxy-2-propanol, mercaptoacetic acid, N-methylethanolamine, diethanolamine, hydroxyaniline, N-methyl-o-aminobenzoic acid, anthranilic acid and lactic acid.

To produce an addition compound which is the latent curing agent of this invention, components (a) and (b) are reacted in a ratio of 0.8 to 2.5 equivalent, preferably 0.9 to 1.5 equivalent of epoxy group in component (a) for 1 equivalent of active hydrogen in component (b). If the epoxy group is less than 0.8 equivalent for 1 equivalent of active hydrogen, the resulting addition product has a low softening point and cannot be readily crushed. Epoxy resins incorporated with such an addition product is poor in storage stability.

If the epoxy group is used in excess of 2.5 equivalent for 1 equivalent of active hydrogen, the resulting addition product will be an insoluble solid with partial three-dimensional networks. Epoxy resins incorporated with such an addition product are slow in curing and the cured product is uneven.

In the case where the latent curing agent is produced from components (a), (b) and (c), the molar ratio of (c) to (b) should be 0.2:1 to 1:1 If component (c) is used in excess of equimolar amount, the resulting latent curing agent is poor in curing performance. Components (a), (b) and (c) should be reacted in a ratio of 0.8 to 2.5 equivalent, preferably 0.9 to 1.5 equivalent of epoxy group in component (a) for 1 equivalent of the total active hydrogen in components (b) and (c), for the reasons mentioned above for the components (a) and (b).

The addition product preferable as the latent curing agent of this invention should have active hydrogen and epoxy group in the above-mentioned ratio, and have a softening point of 60.degree. to 180.degree. C. If the softening point is lower than 60.degree. C., the latent curing agent is poor in storage stability at room temperature; and if it is higher than 180.degree. C., the latent curing agent is poor in curing performance.

Two or more compounds may be mixed for each of components (a), (b), and (c), so long as the active hydrogen and epoxy group are used in quantities as specified above. It is possible to obtain an addition product having a desired softening point by changing the kind and mixing ratio of the compounds of each component.

The latent curing agent of this invention can be easily obtained by thoroughly mixing components (a) and (b), or components (a), (b), and (c), gelling them at room temperature, reacting at 80.degree. to 150.degree. C., cooling, solidifying, and crushing. It can also be prepared by performing addition reaction in a solvent such as tetrahydrofuran, dioxane, and methyl ethyl ketone, removing the solvent, and crushing the solid.

The latent curing agent of this invention can be used in combination with a known curing agent such as acid anhydride, dicyandiamide, dibasic acid hydrazide, guanamine, and melamine. The combined use with dicyandiamide greatly improves the curing performance without the sacrifice of storage stability. The latent curing agent of this invention can be applied to a variety of known epoxy resins having two or more epoxy groups in one molecule. The latent curing agent of this invention should be used in an amount of 0.5 to 50 parts by weight for 100 parts by weight of epoxy resin. If the quantity is less than 0.5 part by weight, satisfactory curing performance is not attained; and using more than 50 parts by weight impairs the performance of the cured product.

The representative examples of epoxy resins to be applied to the latent curing agent of the invention are those based on glycidyl ethers of polyhydric phenols such as 2,2-bis(4-hydroxyphenyl)-propane (Bisphenol A), resorcinol, hydroquinone, pyrocatechol, saligenin, glycidyl ether or Bisphenol F and glycidyl ether of phenolformaldehyde resin.

If necessary, other curing agents, cure accelerator and fillers may be employed in combination with the curing agent of the present invention.

The following examples illustrate the preparation of the latent curing agents and their use as a curing agent for epoxy resin. The abbreviation of raw material employed in the examples are as follows.

(a) Polyfunctional epoxide

Epon 828 (a product of Shell Chemical Co.) Bisphenol A type epoxy resin epoxy equivalent 185.about.190

Epon 834 (a product of Shell Chemical Co.) Bisphenol A type epoxy resin epoxy equivalent 245.about.260

Epon 1001 (a product of Shell Chemical Co.) Bisphenol A type epoxy resin epoxy equivalent 460.about.480

Epiclon 830 (a product of Dainippon Ink & Chemicals Co.) Bisphenol F type epoxy resin epoxy equivalent 170.about.190

(b) Organic compounds having at least one functional group of OH group, SH group, COOH group and CONHNH.sub.2 group together with tertiary amino group in the molecule

DMAE: 2-Dimethylaminoethanol

DMP-30: 2,4,6-Tris(dimethylaminomethyl)phenol

DMP-10: 2-(Dimethylaminomethyl)phenol

PG-MZ: 1-(2-Hydroxy-3-phenoxypropyl)-2-methylimidazole

PG-EMZ: 1-(2-Hydroxy-3-phenoxypropyl)-2-ethyl-4-methylimidazole

PG-PZL: 1-(2-Hydroxy-3-phenoxypropyl)-2-phenylimidazoline

DEAE: 2-Diethylaminoethanol

DMAPA: 2-Dimethylamino-iso-propanol

BG-MZ: 1-(2-Hydroxy-3-butoxypropyl)-2-methylimidazole

SMZ: 2-Mercapto-1-methylimidazole

DMLL: N.sup..alpha.,N.sup..alpha. -Dimethyl-N.sup..epsilon. -lauroyllysine

DMPH: N,N-Dimethyl-.beta.-aminopropionic hydrazide

DMGH: N,N-Dimethylglycine hydrazide

2SPy: 2-Mercaptopyridine

4SPy: 4-Mercaptopyridine

(c) Organic compounds having two or more of the active hydrogen atoms in the molecule

BA: Bisphenol A

HQ: Hydroquinone

TMP: Trimethylolpropane

CATU: 3,9-Bis(2-carboxyethyl)-2,4,8,10-tetroxaspiro[5,5]undecane

DME: 1,2-Dimercaptoethane

PG-SH: 1-Mercapto-3-phenoxy-2-propanol

PAAH: Phenylacetylhydrazine

p-HBA: p-Hydroxybenzoic acid

33.44 Grams (0.176 equivalent) of Epon 828 was sufficiently mixed with 9.0 g (0.1 equivalent) of DMAE at room temperature, and the temperature was raised gradually under stirring. The reaction proceeded rapidly when the temperature reached to around 70.degree. C. After heating at 100.degree. C. for about 1 hour, the reaction mixture was cooled to room temperature whereby pale yellowish solid was obtained. This product had a softening temperature of 100.degree. C. and was referred to as sample No. 1.

11.6 Grams (0.05 equivalent) of PG-MZ, 5.7 g (0.05 equivalent) of BA and 50 ml of methyl ethyl ketone as solvent were mixed in a 200 ml three-necked flask equipped with a condenser and a stirrer. To the mixture under stirring was added dropwise 19 g (0.1 equivalent) of Epon 828 dissolved in 30 ml of methyl ethyl ketone over a period of 30 minutes while the temperature was maintained to 80.degree. C. Then the mixture was refluxed for 2 hours under stirring. The reaction mixture was concentrated under reduced pressure to remove methyl ethyl ketone and cooled to room temperature whereby pale yellowish solid was obtained. The product had a softening temperature of 140.degree. C. and referred to as sample No. 13.

The sample No. and softening temperature of other addition product prepared by similar manner as in Examples are shown in the following Table.

Reactivity, storage stability and glass transition temperature of the formulated epoxy resin composition were evaluated.

1. Preparation of the sample

The formulation of the sample is shown in Table 1. The sample was stirred for 1 hour with defoaming under the reduced pressure by using the mixing and grinding machine.

2. Evaluation of the reactivity

(1) Onset temperature (Ti) and peak temperature (Tp) were measured by differential thermal analysis (DTA)

Sample weight: about 10 mg

Standard material: .alpha.-Al.sub.2 O.sub.3

Heating rate: 5.degree. C./min.

(2) The sample was put into the Geer's oven maintained to the prescribed temperature and the resulted cure resin was observed on its appearance.

3. Storage stability

The sample was put into the Geer's oven set to 30.degree. C. and the day required for the sample becoming non-fluidity was measured.

4. Glass transition temperature (Tg)

Tg of the cured resin was measured with a thermal mechanical analysis apparatus (TMA, a product of Rigaku Corporation) by TMA penetration method.

Raising rate of temperature: 10.degree. C./min.

Load: 10 g

Diameter of needle: 1 mm

The results obtained are summarized in Table 2.

As shown in Table 2, the addition product of the present invention is superior latent curing agent because it showed greatly improved storage stability in comparison with compound having an active hydrogen atom and tertiary amino group (control) alone.

Reactivity and storage stability of the combination system of the latent curing agent of the present invention and diabasic acid hydrazide were estimated.

The formulation is shown in Table 3 and the results are shown in Table 4.

It will be understood from the comparison of Table 2 with Table 4 that the latent curing agent of the present invention exerts synergistic effect in reactivity in combination with adipic dihydrazide.

The storage stability and curing of the combination system of the latent curing agent of the present invention and the known latent curing agent were estimated. The formulation and the result are shown in Table 5.

As shown in Table 5, the epoxy system containing the known latent curing agent alone did not entirely cure at 100.degree. C. for 1 hour. However, the combination system of the latent curing agent and the known latent curing agent caused complete curing at 100.degree. C.

The reactivity and the storage stability of the combination system of the latent curing agent and acid anhydride were estimated. The formulation is shown in Table 6 and the results are shown in Table 7.