Products from an Imidazole and Sulfur Dioxide, Compositions Containing Them, and Methods of Preparation

United States Patent [191 Sweeny et al. [541 PRODUCTS FROM AN IMIDAZOLE AND SULFUR DIOXIDE, COMPOSITIONS CONTAINING THEM, AND METHODS OF PREPARATION [75] Inventors: Norman P. Sweeny, North Oaks, Minn.; Karl Friedrich Thom, Cologne, Germany [73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn. [22] Filed: June 14, 1972 [211 App]. No.2 262,557 [52] U.S. Cl. ....... .. 260/47 EN, 260/2 N, 260/18 PF, 260/37 EP, 260/45.8 N, 260/59, 260/78.4 EP, 260/94.7 S, 260/309 [51] Int. Cl ........................................... .. C08g 30/14 [58] Field of Search ....... .. 260/47 EN, 2 N, 309, 59, 260/2, 18, 94.7, 78.4 EP [5 6] References Cited UNITED STATES PATENTS 3,356,645 l2/1967 Warren ............................... .. 260/47 [111 A 3,839,282 [451 Oct. 1,1974 OTHER PUBLICATIONS I Chem. Abst., Vol. 71, Synthetic High Polymers, 1969 - (p. 7l0l2g). Chem. Abst., Vol. 72, Nonferrous Metalsand Alloys, 1970 (p. 1245870). Primary Examirzer—William H. Short Assistant Examiner—T. Pertilla Attorney, Agent. or Firm——Alexander, Sell, Steldt & DeLaHunt [5 7] . ABSTRACT The disclosed compounds are adducts of imidazole or derivatives thereof and sulfur dioxide. The adducts are apparently of the Lewis acid-Lewis base type, and are useful as latent curing agents for epoxide resins. The adducts are prepared by interacting the imidazole and sulfur dioxide under anhydrous conditions. Equimolar imidazole-S02 adducts can serve as a source of sulfur dioxide at room temperature. 5 Claims, No Drawings _ 3,839,282 1 PRODUCTS FROM AN IMIDAZOLE AND SULFUR DIOXIDE, COMPOSITIONS CONTAINING THEM, AND METHODS OF PREPARATION FIELD OF THE. INVENTION This invention relates to sulfur dioxide-imidazole ad- ducts and methods for making them, the “imidazole” portion of the adduct being either imidazole_ itself (C3H,N2) or a derivative thereof. An aspect of this in- vention relates to latent epoxy curing agents and latent curable epoxide resin systems containing sulfur diox- ide-imidazole adducts. A further aspect of this inven- tion relates to a source for an acid stabilizer in a closed system. DESCRIPTION OF THE PRIOR ART It is well known that sulfur dioxide is a Lewis acid, though not nearly so strong a Lewis acid as, for exam- ple, boron trifluoride. It is also known that nitrogen bases (e.g. amines) are Lewis bases and can react with sulfur dioxide to form adducts. The literature relating to such adducts is extensive, representative examples ' being W. C. Fernelius, Ed., Inorganic Synthesis II, McGraw-Hill, N.Y. (1946); W. E. Byrd, Inorganic Chemistry 1, p.‘ 762 (1962); K. R. Hoffman et al, J. Am. Chem. Soc. 68, p. 997 (1946), H. A. Hoffman et "al, J. Am. Chem. Soc. 70, p. 262 (1948). The Byrd arti- cle indicates that the exact structure of aromatic amine-sulfur dioxide complexes is not well understood, since it is possible that the sulfur dioxide could be bound to the adduct via the pi-complex of the aromatic ring. The picture is further complicated by data in the ’ = Byrd article and both Hoffman et al articles indicating that some of these aromatic (or heterocyclic-aromatic) amine—sulfur dioxide adducts or complexes are unsta- ble, though there is little doubt that true adducts, rather than simple mixtures, are formed. According to U.S. Pat. No. 2,270,490 (Wood), is- sued January, 1942, morpholine and sulfur dioxide react to form a compound useful as a photographic de- veloper, local anesthetic, or antioxidant, but in this case it is suggested that the compound can be a true salt, i.e. a salt of the cation-anion type. Some of the aro- matic amine-sulfur dioxide adducts exhibit crystalline character and have sharp melting points, but probably lack this high degree of ionic character. The nature of the amine (aliphatic, aromatic, aromatic heterocyclic, non-aromatic heterocyclic, etc.) appears to have signif- icant effects upon the nature of the sulfur dioxide ad- duct or compound, but these effects have not been ex- plored fully enough to postulate any general rules for all the possible adducts. Apparently none of these prior art adducts or com- pounds has been investigated for use as a latent catalyst or initiator or curative in epoxy resin technology, though amines per se have been investigated exten- sively. Several different Lewis acid-Lewis base adducts of the amine—boron trifluoride type have been carefully studied by epoxy resin chemists. and various theories have been proposed to explain their potency or latency, as the case may be, in curing diglycidyl ether-bisphenol A epoxides at various cure temperatures. See, for ex- ample, Harris et al, J. Appl. Polymer Science 10, p. 523 (I966). The Harris et al article provides little or no guidance for one attempting to use a Lewis acid-Lewis base adduct derived from sulfur dioxide instead of l0 I5 20 25' 30 35 2 boron trifluoride. The degree of latency of an amine — BF3 adduct appears to be independent of its stability (Harris et al, op cit., pp. 523-525 and 527),. but the sta- bility of amine -—- S02 adducts is so variable that la- tency might very well be a function of the stability of the adduct. lmidazole and its derivatives have been used as non- latent initiators or curative-catalysts for epoxide resins, and various approaches have been used to make latent compounds or complexes containing imidazole nuclei; see, for example, U.S. Pat. No. 3,553,166 (Anderson et al,), issued Jan. 5, I971. Apparently, imidazole-sulfur dioxide adducts have never been reported in the litera- ture and have never been proposed for use as epoxy curing agents or for any other purpose. In view of the variations in the stability of different types of amine- sulfur dioxide adducts and the lack of guidance in the prior art as to their behavior as epoxy initiators, it would be difficult at best to predict the stability and utility of an imidazole-sulfur dioxideadduct. Nor is it even possible to predict with certainty from the avail- able prior art if such an adduct can even be made and what its structure might be. Analogies between imidaz— ole and other heterocyclic and/or aromatic and/or ali- phatic amines are difficult to draw, due to the unique- ness of the imidazole nucleus I ' ( I —i=N_(i=iC)' Accordingly, this invention contemplates synthesiz- _ ing adducts from imidazole or derivatives thereof and ’ sulfur dioxide. This invention further contemplates using these adducts in polymer chemistry and formulat- . ing either curable latentepoxy resin systems or stabi- 40 50 55 60 65 lized acrylate monomers containing the adducts. SUMMARY OF THE INVENTION Briefly summarized, the present invention involves the synthesis of imidazole-sulfur dioxide adducts and the discovery that these adducts are useful both as a source for an acidic stabilizer and in the formulation of latent curable epoxides. These latent curable systems have a shelf life of weeks or months (e.g. at least 6 months) and are rendered non-latent at elevated tem- peratures (e.g. above 50° C., preferably above 130° C.); that is, the typical epoxide curing reactions can be made to occur expeditiously at these elevated tempera- tures. The compounds of this invention can be pre- pared by exposing imidazole or a derivative thereof (optionally dissolved in a solvent) to liquid or gaseous sulfur dioxide, or a saturated solution thereof, under anhydrous conditions for a period of several minutes until the compound precipitates out as a solid sulfur di- oxide-imidazole adduct. The apparent behavior of these solid products in the presence of curable epoxy resins is evidence not only of utility, but also of a stable, adduct-like structure, as opposed to a mere physical mixture. The adducts decompose upon heating into sul- fur dioxide and imidazole (or an imidazole derivative) and thereby serve as a latent source for either the imid- azole or sulfur dioxide. 3 DETAILED DESCRIPTION Adduct-like compounds made according to the pres- ent invention can be represented by the following for- mula: (Imid),,.SO2_ ' (I ) » wherein lmid is imidazole or a derivative thereof, i.e. a nucleus of the formula ’ 7 (II) The n term represents a small number less than 4, preferably an integer. The exact‘ structure of the vari- ous species encompassed by n is not a known, but avail- able evidence substantiates the formation of Lewis acid-Lewis base adducts wherein n = 1, 2, or 3 or mix- tures of these species. When i1 = 1, these compounds have a tendency to lose sulfur dioxide slowly until the n = 2 species (the moststable) is formed. The terms R‘, R2, R4, and R5 represent suitable ‘sub- stituents or hydrogen, the preferred substituents being non-hindering organic radicals such as alkyl or aryl rad- icals or, particularly in the case of R‘ and R5, fused rings. For convenience,the term “an imidazole” or “an imidazole nucleus” is used in this specification to de- note both imidazole itself (C3N2H,) and the imidazole derivatives of formula (lI),‘set forth previously. The term “curing agent” is used to. denote agents which assist or participate in hardening or crosslinking ’ or polymerization reactions which solidify or increase the viscosity of liquid epoxide monomers or prepoly- mers or convert solid epoxides to tough, durable ther- moset materials. “Curing[agents" are referred to as “hardeners" or.“.‘crosslinkers"- in some contexts, be- 5 l0 15 20 30 3,839,282‘ 4 Adducts of an imidazole and sulfur dioxide can be synthesized by one of the following methods, provided that anhydrous conditions are maintained. ' 1. An imidazole is dissolved in a solvent such as ace- tonitrile and gasoues sulfur ‘dioxide is bubbled through thersolution. An exothermic reaction oc- curs. Upon dilution with the solvent, a white pre- cipitate forms which is the imidazolesulfuridioxide A adduct. 7 ' ' _ _. 2. Gaseous sulfur dioxide can be passed over an imid- azole and the adduct forms. The solid adduct may be recovered fromthe reaction mass by treating it with a solvent. ‘ 2 3. A solution of an imidazole in a solvent can be added to another portion of a similar or the same solvent which is saturated with sulfur dioxide. A precipitate is formed which is the imidazole-sulfur dioxide adduct. . , 4. Liquid sulfur dioxide can be added to an imidazole (either as the pure compound or as a’ solution of the compound) and theadduct forms. The mixture is treated with a solvent to recovergthe solid adduct‘. 5. A solution of an imidazole can beplaced in an au- toclave andthe autoclave pressurized with sulfur dioxide. The adduct precipitates out of solution. The sulfur dioxide gas reactions may utilize, diluent gases to act as carriers and controls forthe exothermic, reaction. V The 2:1 adducts (2 imidazolezl 802) are more stable - than the 1:1 adducts. Sulfur dioxide loss occurs with 35 40' cause of their ability to-convert even the liquid mono- ' _mers or prepolymers to thermoset solids._ It is also com- mon in the art to refer to imidazole as a “catalyst” or “initiator" since it assists in the opening of the oxirane ring. However, it is established that imidazole can make a contribu_tion,to the properties of the cured epoxide- . andis thus more than a simple catalyst. For consistency of terminology, the term “curing agent" is used in this specification.’ _ _’ ‘ I , . ' y , In the art of curing or hardening epoxide,resins,va “la- tent_"_curing_agen't_is orielwhich is effective only under certain specific ‘con’ditions. e.gi temperatures above 50° or 100° C. A‘ latent curing‘. agent can therefore be included in afone-part curable system which is storable forlong’ periods ‘_of.time at normal ambient tempera- tures, and the storable system can be said 'to‘have good . storage stability orfa long “shelf life" or “pot-life,” The shelf lifeof a liquid one-part system can be conve- nientlyrdetermined by‘ observing its viscosity; any ten- ‘ dency toward premature gielationvwill, ofcourse, beev-* idenced by an increase in.viscosity. 50 1:1 adducts at room temperature over a period of three to four days. The equation for this decomposition is: A 2 (lmid)’ 1°C. . T . 2-3-—* (Im1d‘)2.SO,,-(,2 + SO74 (gas) (Eq.l) wherein lmid is as defined previously. It is difficult to removethelast traces of solvent from. the compoundstof Formula (I),,and mixture of com- pounds wherein n has more than one value, eg. 2 and 3,Canform.N_ .. ’ « -N » The adduct-like compounds of this invention decom- pose’ or dissociate according to the following equation: ‘ V ‘heat ‘ T 7 (Imid)., -S 02. ———> , n Imid + S 02(ggg) (I) (II) (Eq. 2) For a, given “lmid” moiety, the dissociation tempera- : tures of the compounds of. formula (I) can vary when 55 ‘?n” varies. For example, for ..l,2-dimethylim-idazole, the’ dissociation temperature ..of the .n j-=_ 2 species is higher than that of the n ‘.—' 1 species_..The properties of A cured epoxide‘-systems which have been obtained by‘ 60 heating the curable latent systems containingformula (.1) compounds can also vary with n for a given Irnid moiety. The" dissociation temperatures are not fixed precisely, but generally appear, to be significantly higher than room ’temperature. Undernormal a_mbient temperature and pressure,_'e.g. 23° CA./76_0 mm of Hg, dissociation of the 1:1 (i.e.‘n'= l), 251 (ii-f= 2), ‘and"3i1 ‘ (ri = 3) adducts according to‘_Equation 2 is,§toVfsiay the least, difficult todetect. Experimentation'.with curable epoxide materials such as diglycidyl ethers of bisphenol A A shows that the adducts have little, if any, effect upon 5 the viscosity of the epoxide over a period of months, indicating an epoxide pot life longer than 6 months. This pot life data indicates that little or no free imidaz- ole or imidazole derivative is available for interaction with the vicinal epoxide (oxirane) ring, since free imid- azole can cure typical curable epoxide systems in a manner of minutes at room temperature; see Farkas et al, J. Appl. Polym. Sci. 12, 159 (1968). Even the laten- tizing of imidazole with acetic acid to form imidazole acetate salts extends the pot life of the epoxide system to only about three weeks; see U.S. Pat. No. 3,356,645 (Warren), issued Dec. 5, 1967. Thus, a simple compar- ison with known curable epoxide systems provides fur- ther evidence that the adducts of the present invention are reasonably stable compounds at normal ambient temperature and pressure, at least insofar as loss of the imidazole moiety is concerned. Some clearly detectible loss of imidazole can occur at temperatures above the melting points of the adducts, particularly in the range of 100° — 200° C., as is subsequently shown by the data Table 11. Since the adducts have varying degrees of sta- bility at temperatures above 100° C., the rate or extent of dissociation can be selected in accordance with the desired rate of cure or gel time, as the case may be. To illustrate: given a temperature of 180° C. and a typical latent curable system containing a formula (1) com- pound of this invention, the gel time is much longer for Imid benzimidazole than for Imid = imidazole (C;,N2H,). At 160° C., the benzimidazole adduct ap- pears to have little or no effect upon a diglycidyl ether- bisphenol A epoxide prepolymer; but the imidazole ad- ducts are very effective at this temperature. The 1-alkyl imidazole species of Formula (1) also exhibit longer gel times. The properties of the cured epoxides obtained according to this invention are generally satisfactory. The compounds of Formula (1) are generally solids with fairly small melting ranges, typically not more than 5 Centigrade degrees. These data can be compared to the 80°—85° C. melting range of morpholine-sulfur di- oxide, which is reported to be a salt-like chemical com- pound in the U.S. Pat. No. 2,270,490 referred to previ- ously. The compound (lmid);;.SO2, where Imid = 2- ethyl-4-methylimidazole, appears to be a viscous liquid, but, as shown subsequently by Table I, this is not typi- cal. Formulation of curable epoxide systems containing curing agents of this invention can be carried out along the lines generally laid down by latent imidazole epox- ide curing technology, e.g. the technology described in the aforementioned US. Pat. Nos. 3,553,166 and 3,356,645. The U.S. Pat. No. 3,553,166 also describes suitable co-curatives (e.g. of the dicyandiamide type) which can be included in the curable system. It is pre- ferred to introduce at least 0.1 percent by weight, based on the weight of the epoxide monomer or pre- polymer, of a compound of Formula (1) into the cur- able system, and levels of 1-20 percent by weight are particularly useful. Levels higher than 20 percent -- even 50 percent or more — are permissible but exces- sively increase the cost of the system without any signif- icant beneficial result. Typical one-part curable epoxy resin systems formu- lated according to this invention comprise (1) 0.1 — 20 parts by weight of a curing agent of Formula (1), (2) at least 80 parts by weight of a suitable cycloaliphatic, ali- phatic, aromatic, or heterocyclic epoxide, and (3) 0 — 300 parts by weight of suitable fillers, extenders, flex- 3,839,282 10 15 20 25 30 35 40 45 50 55 60 65 6 ibilizers, pigments, and the like, eg. colloidal silica. These one-part systems have sufficient shelf-life at nor- mal ambient temperature to allow for most ordinary shipping and inventory procedures, although the stabil- ity of the system can be further enhanced, if desired, by special precautions such as careful temperature control during storage. It is generally not necessary to formu- late two-part systems (with the epoxide in one con- tainer and the curing agent in another). If a two-part system is made up, however, the benefits of this inven- tion will still be apparent to industrial users who blend the two parts and then carry out one or more coating, molding, laminating, casting, or impregnating steps prior to curing at cure temperatures above 100° C. These steps can be carried out in a leisurely fashion, taking advantage of the long room temperature “pot life” prior to curing at elevated temperatures." Epoxides suitable for use in this invention can be ali- phatic, cycloaliphatic, aromatic or heterocyclic and will typically have an average epoxy equivalency (i.e. the number of epoxy groups contained in the average molecule) of from about 1.7 to 6.0, preferably 2 or 3, this value being the average molecular weight of the ep- oxide divided by the epoxide equivalent weight. The epoxy equivalent weight, which is determined by multi- plying the same weight by 16 and dividing by the num- ber of grams of oxirane oxygen in the sample, is typi- cally greater than 100 for commercially useful curable systems. These materials are variously referred to as. epoxide “monomers” or “prepolymers” and in any event can contain repeating units, e.g. repeating ether units. Typical of such epoxides are the glycidyl-type epoxy resins, e.g. the diglycidyl ethers of polyhydric phenols and of novolak resins, such as described in “Handbook of Epoxy Resins,” by Lee and Neville, McGraw-Hill Book Co., New York (1967). Another useful class of epoxides has a structure of the following type: ~ Ep.— CR2—{— O——R‘—O—CR2—-CROH—CR2 3-, o — R‘ _ o _ CR2 — Ep (in) ' OI‘ _ " E17)" (IV) where Ep is an epoxide ring, R is hydrogen, or a non-hindering aliphatic group (e.g. methyl); R‘ is an aliphatic or aromatic radical; and z is a number from 0 to about 5. In Formula IV, n is a number_from 1 to 6. Typically, these epoxides are glycidyl ethers of poly- hydric phenols obtained by reacting a polyhydric phe- nol or aliphatic polyol with an excess of chlorohydrin, such as epichlorohydrin, e.g. the diglycidyl ether of Bis- phenol-A or of resorcinol, 1,4-butane diol, or the like. Further examples of epoxides of this type which can be used in the practice of this invention are described in U.S. Pat. No. 3,018,262 (Schroeder), _issued Jan. 23, 1962. The preferred cycloaliphatic epoxide monomers or prepolymers preferably contain at least one 5- or 6- 3,839,282 7 membered carbocyclic ring (or heterocyclic ring with equivalent properties) on which is substituted the epox- ide functional group. In polycyclic cycloaliphatic epox- ides, the two rings are preferably independent and pref- erably joined by a bridging radical containing at least one ester or ether linkage. A plurality of these ester or ether linkages can provide flexibilizing properties in the cured system. Further examples of cycloaliphatic epox- ide compounds are described in U.S. Pat. No. 3,117,099 (Proops et al.), issued Jan. 7, 1964. There are a host of commercially available epoxides which can be used in this invention, including the di- glycidyl ether of Bisphenol-A (e.g. “Epon” 828, “EpiRez” 522-C, “Araldite" 7072, “Epon” 1002 and “DER” 332), mixtures of the diglycidyl ether of Bis- phenol A with an alkyl glycidyl ether (e.g. “ERL” A 2795), vinylcyclohexene dioxide (e.g. “ERL”-4206), 3,4-epoxycyclohexylmethyl-3, 4-epoxycyclohexane carboxylate (e.g. “ERL"-422 1 ), 3,4-epoxy—6- methylcyclohexylmethyl-3,4-epoxy-6- methylcyclohexane carboxylate (e.g. “ERL”-4201 ), bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate (e.g. “ERL"-4289), bis(2,3-epoxycyclopentyl)ether (e.g. “ERLA"-0400), aliphatic epoxy modified with poly- propylene glycol (e.g. “ERLA”-4050 and “ERLA”- 4052), dipentene dioxide (e.g. “ERL”-4269), epoxi- dized polybutadiene (e.g. “Oxiron” 2001), silicone epoxy (e.g. “Syl-Kem” 90), 1,4-butanediol diglycidyl ether (e.g. “araldite” RD-2 ), polyglycidyl ether of phe- nolformaldehyde novolak (e.g. “DEN”-431 and “ DEN"-438) resorcinol diglycidyl ether (e.g. Ciba “ERE"-1359), and epoxidized unsaturated esters of carboxylic acids having more than six carbon atoms, e.g. epoxidized soybean oil. (“Epon" is a trade-mark of Shell Chemical Co.; “EpiRez” is a trademark of Jones- Dabney Co.; “Araldite” is a trademark of Ciba Prod- ucts Co.; the various “DER" and “DEN” designations are trade designations of Dow Chemical Co.; the “ERL" designations are trade designations of Union Carbide Plastics Division; “Syl Kem" is a trade designa- tion of Dow Corning; “Oxiron" is a trademark; and “ERE-1359" is a trade designation of Ciba Products Co.) The compounds of Formula (1) and the nuclei of For- mula (II) have already been described in some detail. As will be apparent from this description, the substitu- ents R ‘, R2, R‘, and R5 can be varied considerably with- out any adverse effect upon the operability of this in- vention. The teachings of the aforementioned U.S. Pat. Nos. 3,356,645 and 3,553,166, and of U.S. Pat. No. 3,361,150 (Green), issued Dec. 28, 1971, are generally applicable here with respect to selection of imidazole substituents, which can also include 5- and 6-member fused or separate heterocyclic or carbocyclic rings. Substitution at the 1- position (i.e. R‘ a6 H) is least preferred. Lower alkyl substituents (including substi- tuted lower alkyl) are generally most preferred, al- though higher alkyl substituents (containing, for exam- ple, 7 — 36 carbons) can be used. Other aliphatic (in- cluding substituted aliphatic) radicals can be substi- tuted, as is conventional. lncluded among these are the alkenyl and alkinyl radicals such as allyl. Fused rings (such as fused benzene or other 6-member carbocyclic rings) are preferably attached to the 4 and 5 positions; thus R‘ and R5 together can comprise the three or four carbons or heterocyclic atoms of a fused ring. Separate aromatic rings can be substituted at the 1-, 2-, 4-, or 5- 5 10 15 20 25 30 35 40 45 50 55 60 65 8 (preferably the 2-, 4-, or 5-) positions and can be monocyclic (e.g. phenyl, tolyl, xylyl, etc.) or polycyclic (preferably di— or tri-cyclic, e.g. naphthyl). As pointed out previously, the compounds of For- mula (I) are generally useful when a latent source or other controlled release of either an imidazole or sulfur dioxide is needed. For example: alpha-cyanoacrylate monomers of the formula CH2-=(C(CN)COOR, wherein R can be alkyl, phenyl, alkoxy, etc., polymerize by an ionic mechanism and are sensitive to contaminants, e.g. moisture. A con- siderable body of patent and scientific literature is available concerning the use and storage of these monomers; see U.S. Pat. No. 2,776,232 (Shearer et al.), issued Jan. 1, 1957, British Patent No. 1,159,548 (Rice et al.) published July 30, 1969, U.S. Pat. No. 3,483,870 (Coover et al.), issued Dec. 16, 1969, and British Patent No. 1,048,906 (Halpern et al.), pub- lished November 23, 1966. A commercially available example of a cyanoacrylate monomer is “Eastman 910,” trade designation of Eastman Kodak Company. Sulfur dioxide has conventionally been used to stabilize these monomers. It has now been found that a com- pound of Formula (1), preferably one wherein n = 1, can be used as a source of constant sulfur dioxide pres- sure to preserve these monomers in a closed system. The principle and practice of this invention is illus- trated by the following non-limiting Examples, wherein all parts are by weight unless otherwise specified. EXAMPLE 1 Ten grams of imidazole were placed in a 250 ml. Er- lenmeyer flask and exposed to gaseous sulfur dioxide for fifteen minutes. lmmediate reaction takes place as is indicated by an exotherm. A light yellow viscous liq- uid is obtained. An increasein weight of nine grams re- sulted. (This liquid will cure epoxy resins immediately and shows no latency). The viscous liquid is subse- quently treated with 150 ml. Cl-{C13 and stirred rapidly. A fine white precipitate is formed and slow evaporation of the solvent yields 18.5 gms. of product. Drying at room temperature under vacuum for three hours yields 16.5 gms. of material. (m.p. = l0O°- 104°; 19.9% N; 19.8% S; [N]/[S] = 2.30.) The onezone adduct of imidazole and sulfur dioxide is a latent curing agent for epoxy resins. Five parts of the adduct admixed with 95 parts of epoxy resin (Epon 828) shows no increase in viscosity over a period of months. At 100° C., this mixture has a gel time in ex- cess of 30 minutes, whereas a similar mixture of imidaz- ole and epoxy has a gel time of five minutes. The onezone adduct shows a slow decomposition to the twozone adduct. This decomposition is shown by an almost linear decrease in sample weight over a period of about 75 hours. There was no further change in sam- ple weight from the 80th to the 135th hour, and weigh- ings were then discontinued. EXAMPLE 2 Two moles (136 gm) of imidazole were placed in a three-neck flask fitted with a condenser, a gas inlet tube, and a stirrer. The imidazole was dissolved in 200 cc. of chloroform. Gaseous sulfur dioxide (one mole) was bubbled into the solution through the inlet tube as the solution was stirred. After the addition of the sulfur dioxide, stirring was continued for one hour. The solu- tion was transferred to a Rotovapor and the solvent 3,839,282 9 evaporated off at room temperature. A white solid was obtained upon removal of the solvent. (M.p. = 70° C; 21.0% N; 12.9% S; [N]/[S] = 3.71.) The elemental analysis indicated that the compound is lmid2SO2, where lmid =. imidazole. The solid showed no weight loss over a period of months. Five parts of the solid were added to 100 parts of “Epon" 828 (Trademark of Shell Chemical Company) resin, which is a diglyeidyl ether of Bisphenol A. A por- tion of this mixture was immediately heated to 160° C. The sample cured to a hard resin in less than three min- utes. The Barcol hardness of the resulting cured sample was 85. Another portion of the mixture was stored on the laboratory shelf for 3 months, during which time no cl_1_a_nge,i_n the consistency of the resin was noted. The 3-month—old portion was warmed to 160° C.. and cured at this temperature to a hard resin in less than three minutes. The Barcol hardness of this cured resin was 85. (A resin system is considered “cured” when it has reached the most advanced state of hardening for that system.) EXAMPLES 3 TO 11 In Examples 3 to 7, imidazole-sulfur dioxide adducts were prepared in the same manner as Example 2 from sulfur dioxide and the following imidazoles: 1-methyl \/ (‘) .... .. O 10 15 20 25 Adduct N \/ N—H -S02 \/ J 10 imidazole (Example 3); 2-methyl imidazole (Example 4); 1,2-dimetliyl imidazole (Example 5); 2-ethyl, 4- methyl imidazole (Example 6); and benzimidazolc (Ex- ample 7). Data on these compounds or adducts is set forth in Table I. The melting point of morpholine-sulfur dioxide is also given in Table I'for comparison. Examples 8 — 11 are tabulated in Table II and illus- trate the use of the compounds of Examples 1, 2, 5, and 7 with a curable epoxide. The curable epoxide mono- mer (sometimes referred to as a “prepolymer”) is “Epon" 828 (trademark of Shell Chemical Corp. for a viscous liquid diglyeidyl ether of bisphenol A having an epoxide equivalent weight slightly greater than the the- oretical I70 and an epoxide functionality of slightly less than 2.0). In Examples 8. 9, lOA, and 11A, 95 percent by weight of the liquid epoxide monomer is combined with 5 percent by weight of the compound of Examples 1, 2, 5, and 7, respectively; in Examples 10B and 11B 90 wt. percent of the epoxide is combined with 10 wt. percent of the compound of Examples 5 and 7, respec- tively. For comparison, data on 5 wt; percent imidaz- ole, benzimidazole, and 1,2-dimethyl imidazole are also shown in Table II; the epoxide is again “Epon” 828. In Table II. the Barcol Hardness test is the standard 935 ASTM test. “Gel time” is a measure of the length oftime a sample of resin may be maintained in a pliable form at a given temperature. TABLE I Adducts of imidazoles and sulfur dioxide Found Calculated, percent N 21. 2 Percent N S 19. 9 19. 8 M.P. 100, 104° V S 24.2 [N]llSl 2. 30 N—H-S02 \/ : is eeeeee ;[.@.-...].2... 65, 70° 21. 0 12. 9 3. 71 16. 0 2 60, 65° 20. 7 4. 73 24. 5 60,65’ 19.3 12.5 3.51 24.5 N—H -S02 CH3 2 138,140° 16.7 15.3 2.50 17.5 N—CH3-S02 CH: C Ha N/ ()/> «S02 \N/ , /_—\ (1) 14.5 5.6 5.9 19.7 N N—H .SO2 C 2H5 H 135,140° 18.3 7.5 5.58 18.7 80, 85° .................................... - _ N——H- S 02 / 1 Viscous liquid. *Patent N 0. 2,270,490. 3,839,282 TABLE 11 Examples 8 — 11 Amount GEL TIME minuteszseconds Al: I Barcol Hardness Ex. Compound of (wt. ‘7r) 100°C. 120°C. 140°C. 160°C. 180°C. 200°C. of Cured Product 8 Example 1 5 30 min: 9 min: 2 min: 1 min: 2! sec. 19 sec. 21 sec. — 41 sec. — 84 9 Example 2 5 41 min: 21 min: 4 min: 2 min: 1 min: 5 sec. 27 sec. 40 sec. 10 sec. 44 sec. — 84 10A Example 5 5 _ -— — >90 min. 93 min: 85 min, .. . .— - 35 sec. 85 10B Example 5 10 — —- — — 17 min: 2 min: 4 sec. 13 sec. 85 11A Example 7 . 5 — — —— >90 min. >90 min. >90 min. 80 11B Example 7 I0 - —— — —— 11 min; 4 min: 30 sec. 1 1 sec. 80 imidazole 5 5 min: 2 min: 1 min. 3 sec. 40 sec. 24 sec. 15 sec. — benzimidazole 5 — — 55 min: 2 min: 1 min: 18 sec. 40 sec. 1 1 sec. 47 sec. 1.2-dimethyl imidazole 5 — -— 49 sec. 41 sec. 27 sec. 18 sec. What is claimed is: 1. A latent, curable composition comprising a. an effective amount of a curing agent of the for- mula (lmid),..SO2 wherein n is a number from 1 to 4, and lmid is a compound of the formula wherein R‘, R“, R‘ and R5 independently represent sub- stituents selected from the group consisting of hydro- gen, aliphatic radicals and aromatic radicals, and to- gether R4 and R5 can be the residue of a fused ring, and b. a curable oz-epoxide resin having an average epoxy equivalency of 1.7 to 6.0. 2. A composition according to claim 1 wherein n of said formula is a number selected from the group con- sisting of 2 or 3 or mixtures thereof. 3. A composition according to claim 1 wherein said 30 composition comprises 0.1 — 20 percent by weight of said curing agent. 4. A composition according to claim, 1 wherein said a-epoxide resin is a compound of the formula . 35 Ep — CH2 [O—R‘—-O—CH2—CHOH—CH2]z o _ R1’ —- O — CH2 — Ep wherein Ep is an oz-epoxide ring, 40 R‘ is a divalent group selected from the group con- sisting of aliphatic and aromatic radicals, and z is a number from about zero to about 5. 5. A latent, curable composition comprising a. an effective amount of a curing agent of the for- 45 mula (lmid),,.SO2 wherein . lmid is imidazole, and 50 n is a number from 1 to 4, and b. diglycidyl ether of Bisphenol-A. * * * * >l< 55 60 65