Process for Curing Epoxy Resins with a Salt of an Imidazole and Compositions Thereof

United States Patent Office 3,356,615 Patented Dec. 5, 1967 1 3 356 645 PROCESS FOR CURING Epoxy RESINS WITH A SALT or AN IMIDAZOLE AND COMPOSITIONS THEREOF _ Daniel Warren, East Brunswick, N.J., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Apr. 20, 1964, Ser. No. 361,224 7 Claims. (Cl. 260-47) This invention relates to a process for curing poly- epoxides. More particularly, this invention relates to a new process for curing polyepoxides, to novel curing agents used in the process, and to useful products obtained from the process. Specifically, this invention provides a new process for curing polyepoxides, and preferably polyglycidyl poly- ethers of polyhydric phenols and polyhydric alcohols, which comprises mixing and reacting the polyepoxide with a -salt of a heterocyclic nitrogen compound which pos- sesses in the ring (1) a substituted C—N=C group and (2) an —NH— group, and preferably a salt of imidazole compound having the structural formula _C (4)-(3)N JG’... ('£_ \%I’/ such as, 2-ethyl-4-methylimidazole acetate, benzimidazole acetate, and imidazole lactate. This invention further pro- vides cured products obtained by the above process and relates to the novel curing agents described in detail hereinafter. Polyepoxides, such as, for example, those obtained by reacting epichlorohydrin with polyhydric phenols in the presence of caustic, are promising materials for use in many industrial applications as they can be converted with curing agents to form insoluble, infusible products having good chemical resistance. Many conventional poly- epoxide-curing agent systems, however, have certain draw- backs that greatly limit the industrialpuse of the poly- epoxides. For example, the known mixtures comprising the polyepoxides and aliphatic amines set up rather rapidly. This is true even though the mixtures are stored in air- tight containers away from moisture and air. This char- acteristic necessitates mixing of the components just be- fore use and rapid use of the material before cure sets in. Such a procedure places a considerable burden on the individual operators, and in many cases, gives inferior products because of (1) inefficient mixing, and (2) opera- tions are conducted too rapidly. Attempts have been made in the past to solve the above problem by the use of curing agents which are more diflicult to react and would, thus, remain inactive in the polyepoxide composition at lower temperatures. While this action tends to lengthen the pot life or working time of the compositions, it also makes the compositions more ditficult to cure. For example, it is known that the pot life can be extended by the use of aromatic amine curing agents. When aromatic amine curing agents are employed in curing polyepoxides, however, semi—thermoplastic or B-stage resins are rapidly formed during the early stages of cure, that is, before the molecules are all crosslinked. These resins are hard and brittle and consequently, little time is available in which to work with the resins before they set up. It would be desirable, therefore, to have a curing agent or catalyst which would give long pot life and at the same time would form a resin-catalyst system which remains pliable during the early stages of cure, i.e., B-stages very slowly, allowing greater working time in regard to handling of the resins. This is important in 5 10 15 20 25 30 35 40 45 50 2 applications, such as, molding compounds, laminates, castings, etc. It is an object of this invention, therefore, to provide a new class of liquid curing agents for polyepoxides. It is a further object to provide new curing agents for poly- epoxides which give compositions having a relatively long pot life, B-stage very slowly, and remain pliable for a greater period of time. It is a further object to provide new compositions useful for formulating molding com- pounds, laminates, castings, and the like. These and other objects of this invention will be apparent from the fol- lowing detailed description thereof. It has now been discovered that these and other objects may be accomplished by using as curing agents for the polyepoxides, salts of certain heterocyclic compounds which possess in the ring (1) a substituted C—N=C group and (2) an —NH—— group, and preferably a salt of an imidazole compound, such as 2-ethyl-4-methyl- imidazole acetate, benzimidazole acetate, and imidazole lactate. It has been surprisingly found that polyepoxide compositions containing these imidazole salts in low con- centrations have extended pot lives, yet cure rapidly at elevated temperatures. Filled compositions utilizing liquid resins remain putty-like as they advance in cure and have specific advantages in being dust free and extrudable. Thus, the compositions are useful in continuous molding processes. The salts also find utility in applications, such ‘ as laminates and castings, either filled or unfilled. Additional advantage is also found in that the imidazole salts are usually liquids or very low temperature melting solids whereas normally amine salts are solids with rela- tively high melting points. Thus, the imidazole liquid amine salts may be incorporated into the polyepoxide system with case without considera-ble stirring and with- out applying heat to the system. The new curing agents useful in the process of this invention comprise salts of the heterocyclic compounds possessing in the heterocyclic ring (1) a substituted C=N——C group and (2) a secondary amino group, i.e., an =N——H.group. Preferred examples of these hetero- cyclic compounds include, among others, the imidazoles, such as substituted imidazoles and benzimidazoles having the structural formulas If /C R———C———N R—-C \C—-——N H || and I H H R—-C\ /C--R R-—C\ /C-R NH ? N R V respectively, wherein R is selected from hydrogen atoms, 60 65 70 halogen atoms, or an organic radical, such as a hydro- carbon radical or a substituted hydrocarbon radical, for example, the ester, ether, amide, imide, amino, halogen, or mercapto substituted hydrocarbon radicals. The acid portion of the salt is selected from an acid, such as phos- phoric, acetic, lactic, formic, and the like. Especially preferred imidazoles are those wherein the substituent is hydrogen or a hydrocarbon radical, and preferably an alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl, alkaryl or arylalkyl radicals, and particulary those containing no more than 15 carbon atoms and wherein the acid is se- lected from monocarboxylic acids, having from 1 to 8 carbon atoms, lactic, and phosphoric acids. A more detailed description of the chemistry of the imidazoles and benzimidazoles including their properties and structural formulas is found in the book by Klaus Hofmann entitled “Imidazole and Its Derivatives” pub- lished by Interscience Publishers, Inc., New York (1953). Examples of imidazole salts include, among others, ‘the acetate, formate, lactate, and phosphate salts of imidazole. 3,356,645 3 benzimidazole, and substituted imidazoles. Examples of suitable substituted imidazoles include 2-methylimidazole; 2-ethyl-4—methylimidazole; 2-cyclohexyl-4-methylimidaz- ole; 4 - butyl - ethylimidazole; 2-butoxy-4-allylimidazole; 2—carboethoxybutyl - 4 - methylimidazole; 2-octyl-4-hexyl- imidazole; 2-methyl-5-ethylimidazole; 2-ethyl-4-phenyl- imidazole; 2-amide-5-ethylimidazole; 2-ethyl-4-(2-ethyl- amino)-imidazole; 2-methyl - 4- mercaptoethylimidazole; 2-butylacetate-5-methylimidazole;i 2,5-chloro-4-ethylimid- azole; and mixtures thereof. Especially preferred are the alkyl-substituted imidazole acetates and lactates wherein the alkyl groups contain not more than 8 carbon atoms each, or mixtures thereof, and particularly preferred are 2-ethyl-4-methylimidazole acetate, 2-ethyl-4-methylimid- azole lactate, 2-methylimidazole acetate, 2-methylimidaz- ole lactate, imidazole acetate, imidazole lactate, and mix- tures thereof. The above-described imidazole salts can be prepared by reacting the imidazole with the acid to form the cor- responding amine salt. The imidazoles are prepared by conventional techniques of reacting a dialdehyde with am- monia and formaldehyde. The salts are preferably pre- pared by mixing the desired acid with imidazole and main- taining the temperature between 23 ° and 100° C. Solvents are not necessary but may be employed if desired. One should use at least one gram molecular weight of acid to each. gram molecular weight of the imidazole. If greater stability is desired, one should use a larger ratio, i.e., up to a 2:1 ratio of acid to imidazole. The higher the acid level, the more stable the resin-catalyst mixture. The re- action is preferably accomplished at temperatures be- tween 23° C. and 150° C. As heat is evolved in the re- action, cooling means may be needed to keep the tem- perature in the desired range. The polyepoxides to be used in preparing the composi- tions of the present invention comprise those materials possessing more than one vicinal epoxy group, i.e., more than one 0 ,-—CL_\.C._ group. These compounds may be saturated or unsatu- rated, aliphatic, -cycloaliphatic, aromatic or heterocyclic . and may be substituted with substituents, such as chlorine, hydroxyl groups, ether radicals and the like. They may . be monomeric or polymeric. For clarity, many of the polyepoxides and particularly those of the polymeric type are described in terms ‘of epoxy equivalent values. The meaning of this expresssion is described in U.S. 2,633,458. The polyepoxides used in the present process are those having an epoxy equivalency greater than 1.0. Various examples of polyoxides that may be used in the process of the invention are given in U.S. 2,633,458 and it is to be understood that so much of the disclosure of that patent relative to examples of polyepoxides is in- corporated by reference into this specification. Other examples include the epoxidized esters of the polyethylenically unsaturated monocarboxylic acids, such’ as epoxidized linseed, soybean, perilla, oiticia, tung, wal- nut and dehydrated castor oil, methyl linoleate, butyl linoleate, ethyl 1,12 — octadecadienoate, butyl, 9,12,15- octadecatrienoate, butyl eleostearate, monoglycerides of tung oil fatty acids, monoglycerides of soybean oil, sun- flower, rapeseed, hempseed, sardine, cottonseed oil, and the like. Another group of the epoxy—containing materials used inthe process of the invention include the epoxidized esters of unsaturated monohydric alcohols and polycar- boxylic acids, such as, for example, di(2,3 - epoxybutyl)” adipate, di(2,3 - epoxybutyl)oxalate, di(2,3-epoxy-hexyl) succinate, di('3,4 - epoxybutyl)maleate, di(2,3 - epoxy- octyl)pimelate, di(2,3 - epoxybutyl)phthalate, di(2,3- epoxyoctyl)tetrahydrophthalate, di(4,5 - epoxydodecyl) maleate, di(2,3 - epoxybutyl)terephthalate, di(2,3-epoxy- pentyl)thiodipropionate, di(5,6 - epoxytetradecyl)di- 10 15 20 25 30 35 40 45 50 55 60 761’ 75 4 phenyldicarboxylate, di(3,4 - epoxyheptyl)sulfonyldi- butyrate, tri(2,3 - epoxybutyl) 1,2,4 - butanetricarboxylate, di(5,6 - epoxypentadecyl)tartarate, di(4,5 - epoxytetra- decyl)maleate, di(2,3 - epoxybutyl)azelate, di(3,4—epoxy- butyl)citrate, di(5,6 - epoxyoctyl)cyclohexane - 1,2-di- carboxylate, di(4,5-epoxyoctadecyl) malonate. Another group of the epoxy-containing materials in- cludes those epoxidized esters of unsaturated alcohols and unsaturated carboxylic acids, such as 2,3 - epoxybutyl- 3,4 - epoxypentanoate, 3,4 - epoxyhexyl, 3,4 - epoxy- pentanoate, 3,4 - epoxycyclohexyl - 3,4 - epoxycyclo- hexanoate, 3,4 - epoxycyclohexyl — 4,5 - epoxyoctanoate, 2,3 - epoxycyclohexylmethyl epoxycyclohexane carboxyl- ate. Still another group of the epoxy-containing materials includes epoxidized derivatives of polyethylenically un- saturated polycarboxylic acids, such as, for example, dimethyl 8,9,12,l3 -diepoxyeicosanedioate, dibutyl 7,8, 11,12 - diepoxyoctadecanedioate, dioctyl 10,11 - diethyl- 8,9,l2,13 - diepoxy-eicosanedioate, dihexyl 6,7,l0,11-di- epoxyhexadecanedioate, didecyl 9 — epoxy-ethyl - 10,11- epoxyoctadecanedioate, dibutyl 3 - butyl - 3,4,5,6-diepoxy- cyclohexane - 1,2 - dicarboxylate, dicyclohexyl 3,4,5,6- diepoxycyclohexane - 1,2 - dicarboxylate, dibenzyl 1,2,4,5- diepoxycyclohexane ,- 1,2 - dicarboxylate and diethyl 5,6,10,l1-diepoxyoctadecyl succinate. Still another group comprises the epoxidized polyesters obtained by reacting an unsaturated polyhydric alcohol and/or unsaturated, polycarboxylic acid or anhydride groups, such as, for example, the polyester obtained by reacting 8,9,12,l3-eicosanedienedioic acid with ethylene glycol, the polyester obtained by reacting diethylene gly- col with 2 - cyclohexene - 1,4 - dicarboxylic acid and the like, and mixtures thereof. Still another group comprises the epoxidized poly- ethylenically unsaturated hydrocarbons, such as epoxi- dized 2,2-bis(2-cyclohexenyl) propane, epoxidized vinyl cyclohexene and epoxidized dimer of cyclopentadiene. Another group comprises the epoxidized polymers and copolymers of diolefins, such as butadiene. Examples of this include, among others, butadiene-acrylontrile copoly- mers (Hycar rubbers), butadiene—styrene copolymers and the like. Another group comprises the glycidyl containing nitro- gen compounds, such as diglycidyl aniline and di- and triglycidylamine. The polyepoxides that are particularly preferred for use inlthe compositions of the invention are the glycidyl ethers and particularly the glycidyl ethers of polyhydric phenols and polyhydric alcohols. The glycidyl ethers of polyhydric phenols are obtained by reacting epichloro— hydrin with the desired polyhydric phenols in the presence . of alkali. Polyether A and Polyether B described in the above-noted U.S. 2,633,458 are good examples of poly- epoxides of this type. Other examples include the poly- glycidyl ether of 1,1,2,2 - tetrakis(4 - hydroxyphenyl) ethane (epoxy value of 0.45 eq./100 g. and melting point 85° C.), polyglycidyl ether of 1,1,5,5 - tetrakis(hydro- xyphenyl)pentane (epoxy value of 0.514 eq./ 100 g.) and the like and mixtures thereof. Other examples include the glycidated novolacs as obtained by reacting epichloro- hydrin with novon resins obtained by condensation of aldehyde with polyhydric phenols. The quantites in which the polyepoxides and the hetero- cyclic curing agents are combined will vary over a wide range. To obtain the best cure, the heterocyclic curing agent is preferably employed in amounts varying from about 0.1% to about 30% by weight of the polyepoxide, and still more preferably from 1 to 15 by weight of the olyepoxide. The heterocyclic curing agent can be used in combina- tion with other compounds such as phenols, mercaptans, triphenyl phosphorus, triphenyl arsenic, triphenyl anti- mony, amines, amine salts or quaternary ammonium salts, etc. Preferred additives include the mercaptans, phenols, 5 triphenyl phosphorus and the amines, such as, for ex- ample, benzyldimethylamine, dicyandiamine, p,p’ - bis(di- methylaminophenyl)methane, pyridine, dimethylaniline, benzyldimethylamine, dimethylethanolamine, methyldi- ethanolamine, morpholine, dimethylaminopropylamine, Adibutylaminopropylamine, stearyldimethylamine, tri-n- butyl amine, triamylamine, tri-n-hexylamine, ethylldi-n- proplyamine, m-phenylenediamine, diethylenetriamine and the like, and mixtures thereof. The salts may be exempli- fied by the inorganic and organic acid salts of the amines, such as, for example, the hydrochloride, sulfate and acetate of each of the above-described tertiary amines. The quaternary ammonium salts may be exemplified by the following: benzyltrimethylammoni-um chloride, phen- yltributylammonium chloride, cyclohexyltributylammo- nium sulfate, benzyltrimethylammonium sulfate, benzyl- trimethylammonium borate, diphenyldioctylammonium chloride, and the like, and mixtures thereof. Other addi- tives include polybasic anhydrides, such as, for example, phthalic anhydride, tetrahydrophthalic anhydride, methyl- 3,6 - endomethylene - 4 - tetrahydrophthalic anhydride, chlorendic anhydride, pyromellitic dianhydride, and the like, and the corresponding acids. The above-noted additives are generally employed in amounts varying from 0.1 part to 25 parts per 100 parts of polyepoxide, and preferably from 1 part to 5 parts per 100 -parts of polyepoxide. The curing of the polyepoxides may be accomplished by mixing the polyepoxides with the heterocyclic amine salt catalyst and heating the resulting composition at elevated temperatures, i.e., 150° C. The temperatures em- ployed during the cure may vary over -a wide range. In general, temperatures ranging from about 60° to 200° C. will give satisfactory results. Preferred temperatures range from about 1-00 to 175° C. Additional materials, etc., pig- ment, stabilizers, plasticizers and diluents may be added. One may also add material to accelerate the reaction of the curing agent such as phenols, amines, mercaptans and the like. The heterocyclic -amine salt catalysts at low levels of concentration impart long term room temperature stabil- ity to epoxy resins, e.—g., molding materials, without atfect— ing the cure cycle. Because of its stability, the resin- catalyst system B-stages very slowly and remains pliable as it advances in cure allowing time for the material to be molded. The catalysts therefore, are very useful in formulating molding compounds and can be utilized in continuous molding operations. The compositions of this invention are also useful for preparing laminates. In preparing the laminate, the sheets of fibrous material are first treated with the mixture of polyepoxide and curing agent. This is conveniently accom- plished by spreading the paste or solution containing the above-noted mixture onto the sheets of glass cloth, paper, textiles, etc. The sheets are then superimposed and the as- sembly cured under heat and pressure. The assembly is preferably cured in a heated press under a pressure of about 25 to 500 or more pounds per square inch and tem- peratures of about 150° C. The resulting laminate is ex- tremely strong and resistant against heat and the action of organic- and corrosive solvents. 3,356,645 10 15 20 25 30 35 40 50 55 60 6 The fibrous material used in the preparation of the laminates may be of any suitable materials, such as glass cloth and matting, paper, asbestos paper, mica flakes, cot- ton bats, duck muslin, canvas, synthetic fibers such as nylons, dacrons and the like. It is usually preferred to utilize woven glass cloth that has -been given prior treat- ment with well known finishing or sizing agents therefore, such as chrome methacrylate or vinyl trichlorosilane. The compositions of this invention are further useful in filament windings and in casting applications, such as encapsulation and/ or embedment of electrical devices and preparation of cast foams containing microballoons. To illustrate the manner in which the invention may be carried out, the following examples are given. It is under- stood, however, that the examples are for purposes of illustration and the invention is not to be regarded as limited to any of the specific materials or conditions therein. Polyepoxides referred to by letter are those de- scribed in U.S. 2,633,458. EXAMPLE r This example illustrates the preparation of 2-ethyl-4- methylimidazole acetate and its use as a curing agent for Polyether A, i.e., a glycidyl ether of 2,2-bis(4-hydroxy- phenyl)propane. Preparation 110 g. (1 mol) of 2-ethyl-4-methylimidazole is placed in a reaction vessel fitted with a stirrer. 120‘ g. (2 mols) of glacial acetic acid is added to the vessel over a period of 10 minutes while maintaining continuous stirring. The temperature is maintained between 60° C. and 80° C. for a period of one hour to complete the reaction. Upon cool- ing, the salt is ready for use. The 2-ethyl-4-methylimidaz- ole acetate is a light amber colored liquid which darkens upon aging to a deep red-brown color. Use as curing agent 100 parts of Polyether A and 5 parts of 2-ethyl-4- methylimidazole acetate are thoroughly mixed. The com- position at room temperature has a pot life of 5 days. EXAMPLE II (A) 100 parts of Polyether A and 5 parts of 2-ethyl-4- methylimidazole acetate plus 300 parts of silica filler are thoroughly mixed. The composition at room temperature has a pot life of 3 weeks. When heated at 150° C., the composition cures to form a hard, insoluble, infusible resin having excellent chemical resistance. (B) 100 parts of Polyether A and 10 parts of 2-ethyl-4- methylimidazole acetate plus 300 parts of silica filler are thoroughly mixed. The composition at room tempera- ture has a pot life of 2 weeks. When heated at 150° C., the composition cures to form a hard, insoluble, infusible resin having excellent chemical resistance. (C) (B) above is repeated with the exception that 25 parts of crude methylenedianiline is used in place of 10 parts of 2-ethyl-4-methylimidazole acetate. The -gel times of compositions (A) -and (B) are shown in Table I below in comparison with composition (C). TABLE I Formulation: parts Polyether A/curing agent _________________________________________ __ 100 Filler B ............................................................ __ 300 Curing Agent Type 2-ethyl 4-methyl- imidazole acetate. Gel times in seconds at 150° C. after below-indicated number of storage days—- phr.b Initial 1 2 3 6 8 10 13 17 20 22' 117 62 87 54 59 45 44 35 { 5 39 35 34 33 30 30 10 51 32 30 31 23 25 24 Methylene dianiline (crude) ........... . . 70 25 165 95 44 22 20