Latent curing agents for epoxy resins

Abstract: There are disclosed latent curing agents for epoxy resins, which comprise a addition products obtained by reacting (a) a polyfunctional epoxy compound and (b) a compound represented by the formula: wherein R₄, R₅ and R₆ are the same or different and each is a hydrogen atom, a phenyl group, a C₁₋₂₀ alkyl group optionally containing one or more substituents, a C₂₋₂₀ alkenyl group optionally containing one or more substituents, an arlkyl group optionally containing one or more substituents, or a functional group selected from -OH, -SH, -COOH and -CONHNH₂, and wherein R₇ is a C₁₋₂₀ alkyl group optionally containing one or more substituents, a C₂₋₂₀ alkenyl group optionally containing one or more substituents, or an aralkyl group optionally containing one or more substitutents, with the proviso that at least one of R₄, R₅, R₆ and R₇ contains a functional group selected from -OH, -SH, -COOH and -CONHNH₂, in a ratio of 0.8 to 2.5 equivalent of epoxy group in compound (a) for 1 equivalent of active hydrogen in compound (b). The addition products have a softening point of 60 to 180°C.

Description: 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°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 disclosed 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 US-A-4,066,625 and 4,268,656.) It is known that an adduct is produced by reacting the -NH 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. According to the present invention, there is provided a latent curing agent for epoxy resins, which comprises an addition product obtained by reacting: (a) a polyfunctional epoxy compound, (b) a compound represented by the formula: wherein R 4 , R 5 and R 6 are the same or different and each is a hydrogen atom, a phenyl group, a C 1-20 alkyl group optionally containing one or more substituents, a C 2-20 alkenyl group optionally containing one or more substituents, an aralkyl group optionally containing one or more substituents, or a functional group selected from -OH, -SH, -COOH and -CONHNH 2 , and wherein R 7 is a C 1-20 alkyl group optionally containing one or more substituents, a C 2-20 alkenyl group optionally containing one or more substituents, or an aralkyl group optionally containing one or more substituents, with the proviso that at least one of R 4 , R 5 , R 6 and R 7 contains a functional group selected from -OH, -SH, -COOH and -CONHNH 2 , and optionally (c) an organic compound containing two or more active hydrogen atoms (excluding a compound containing an epoxy group or a tertiary amino group), in a ratio of 0.8 to 2.5 equivalent of epoxy group in compound (a) for 1 equivalent of active hydrogen in compound (b) or for 1 equivalent of the total active hydrogen in compounds (b) and (c), the molar ratio of (c) to (b), if (c) is used, being 0.2:1 to 1:1, and said addition product having a softening point of 60 to 180°C. The curing agent produced from the three components (a), (b), and (c) is superior in storage stability to the curing agent produced from the 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 used 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 β-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 (b) used according to the invention is an imidazole compound represented by the general formula given above wherein R 4 , R 5 and R 6 are the same or different and each is a hydrogen atom, a phenyl group, a C 1-20 alkyl group optionally containing one or more substituents (such as oxygen or halogen), a C 2-20 alkenyl group optionally containing one or more substituents (such as oxygen or halogen), an aralkyl group (such as a benzyl group) optionally containing one or more substituents (such as oxygen or halogen), or a functional group selected from -OH, -SH, -COOH and -CONHNH 2 , and wherein R 7 is a C 1-20 alkyl group optionally containing one or more substituents (such as oxygen or halogen), a C 2-20 alkenyl group optionally containing one or more substituents (such as oxygen or halogen) or an aralkyl group (such as a benzyl group) optionally containing one or more substituents (such as oxygen or halogen), with the proviso that at least one of R 4 , R 5 , R 6 , and R 7 contains a functional group selected from -OH, -SH, -COOH and -CONHNH 2 . Representative examples of the imidizole compound (b) are listed below: 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, and methimazole. The active hydrogen in the organic compound (c) 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 2 , -SH, -COOH and -CONHNH 2 . Examples of the organic compound (c) 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-carboxy-ethyl)-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 is 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 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 to 180°C. If the softening point is lower than 60°C, the latent curing agent is poor in storage stability at room temperature; and if it is higher than 180°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 to 150°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 is 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 words "Epon" and "Epiclon" used below are Registered Trade Marks. The abbreviations of the raw materials 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 to 190 Epon 834 (a product of Shell Chemical Co.) Bisphenol A type epoxy resin epoxy equivalent 245 to 260 Epon 1001 (a product of Shell Chemical Co.) Bisphenol A type epoxy resin epoxy equivalent 460 to 480 Epiclon 830 (a product of Dainippon Ink & Chemicals Co.) Bisphenol F type epoxy resin epoxy equivalent 170 to 190 (b) Imidazole compounds 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 BG-MZ 1-(2-Hydroxy-3-butoxypropyl)-2-methylimidazole SMZ 2-Mercapto-1-methylimidazole (c) Organic compounds having two or more of active hydrogen atoms in the molecule BA Bisphenol A HQ Hydroquinone TMP Trimethylolopropane 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 Example 1 Preparation of addition product of PG-MZ, BA and Epon 828 11.6 Grams (0.05 equivalent) of PG-MZ,5.7g (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 19g (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°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°C and referred to as sample No. 18. The sample No. and softening temperature of other addition products prepared by a similar manner as in Example 1 are shown in the following Table. Example 2 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 α-Al 2 O 3 Heating rate 5°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°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°C/min Load 10 g Diameter of needle 1 mm The results obtained are summarized in Table 2. Formulation Epon 828 100 parts by weight Latent curing agent of the present invention 20 ZnO 3 TiO 2 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. Example 3 Reactivity and storage stability of the combination system of the latent curing agent of the present invention and dibasic acid hydrazide were estimated. The formulation is shown in Table 3 and the results are shown in Table 4. Formulation Epon 828 100 parts by weight Latent curing agent of the present invention 20 Adipic dihydrazide 8 ZnO 5 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. Sample No. Reactivity Storage stability at 30°C Glass transition temperature (°C) Ti (°C) Tp (°C) Curing temp. time 4 95 127 100 30 >1 month 109(a) 5 93 134 " " " 104(a) 7 142 158 140 60 " 76(c) 17 96 126 100 30 " 104(a) 18 98 130 " " " 101(a) 19 92 125 " " " 122(a) 21 93 124 100 30 " 110(a) 22 97 122 " " " 125(a) 23 94 122 " " " 120(a) 24 87 126 " " " 122(a) 25 94 126 " " " 113(a) 29 87 124 100 30 " 100(a) 32 86 120 100 30 " 127(a) 33 82 144 " " " 110(a) 34 109 138 100 60 " 104(a) 42 91 130 100 30 " 100(a) 45 100 127 80 60 " 128(a) 47 97 128 100 10 " 130(a) Example 4 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°C for 1 hour. However, the combination system of the latent curing agent and the known latent curing agent caused complete curing at 100°C.

Claims: A latent curing agent for epoxy resins, which comprises an addition product obtained by reacting: (a) a polyfunctional epoxy compound, (b) a compound represented by the formula: wherein R 4 , R 5 and R 6 are the same or different and each is a hydrogen atom, a phenyl group, a C 1-20 alkyl group optionally containing one or more substituents, a C 2-20 alkenyl group optionally containing one or more substituents, an aralkyl group optionally containing one or more substituents, or a functional group selected from -OH, -SH, -COOH and -CONHNH 2 , and wherein R 7 is a C 1-20 alkyl group optionally containing one or more substituents, a C 2-20 alkenyl group optionally containing one or more substituents, or an aralkyl group optionally containing one or more substitutents, with the proviso that at least one of R 4 , R 5 , R 6 and R 7 contains a functional group selected from -OH, -SH, -COOH and -CONHNH 2 , and optionally (c) an organic compound containing two or more active hydrogen atoms (excluding a compound containing an epoxy group or a tertiary amino group), in a ratio of 0.8 to 2.5 equivalent of epoxy group in compound (a) for 1 equivalent of active hydrogen in compound (b) or for 1 equivalent of the total active hydrogen in compounds (b) and (c), the molar ratio of (c) to (b), if (c) is used, being 0.2:1 to 1:1, and said addition product having a softening point of 60 to 180°C. A latent curing agent as claimed in claim 1, wherein said polyfunctional epoxy compound is a polyglycidyl ether of a polyhydric phenol. A curable epoxy resin composition comprising (1) an epoxy resin having a average of more than one epoxy group per molecule and (2) from 0.5 to 50 parts by weight, per 100 parts by weight of the epoxy resin (1), of a latent curing agent as claimed in claim 1 or 2.

Inventors: ABE MASHAHIRO, HIRAI KIYOMIKI, ITO NOBUO, TAKEUCHI KOJI, ABE, MASHAHIRO, HIRAI, KIYOMIKI, ITO, NOBUO, TAKEUCHI, KOJI
Applicants: AJINOMOTO KK, AJINOMOTO CO., INC.
Application year: 1983
Main classification: C08G59
IPC codes: C08G59