Imidazole Complexes as Hardeners for Epoxy Adhesives R. Dowbenko,' C. C. Anderson, and W.-H. Chang Coatings and Resins Division, PPG Industries, Springdale, Pa. 15144
Complexes of metal salts and imidazoles, easy to make and well-suited for commercial exploitation, are usable as latent hardeners for one-package epoxy adhesives. A large number of such complexes have been prepared and examined as hardeners. Many of these complexes produce a rapid cure of epoxy resins at an elevated temperature to give products with excellent properties and yet are stable on storage at room temperature. This desirable characteristic is accentuated by using dicyandiamide as a cohardener. The mixture of complex and dicyandiamide in general produces faster cures than the complex alone and gives epoxy resin compositions that are more stable to storage at room temperature than the corresponding compositions without dicyandiamide.
E p o x y adhesives find wider appIicatioii in the automotive, electrical, aircraft, and other industries where adhesive bonds are required similar in strength to t,hose obtained by other methods of joining, such as welding. I n addition to bond strength, epoxy adhesives are required to cure quickly (several minutes a t 300-50'F) and handle as one-package composit,ions. The latter requirement means the compounded epoxy adhesive must have stability of a t least several months a t ambient temperature storage. Usually the main ingredients of an epoxy adhesive are ail epoxy resin and a hardener or curing agent. The curing agent causes the resin to harden under the curing condition. If the curing agent reacts with the epoxy resin a t room temperature, the componeiit's niust' be kept separate (two packages) and must be mised before use. When the curing agent does not react with t'he epoxy resin at' room temperature a t sufficient rate, the combination can be kept in one container (onepackage system) and applied wit'hout any modification. The key compoiient in a successful oiie-package eposy adhesive that meets the requirements is the hardener or curing agent'. This compoiieiit' is a catalyst or a coreactaiit (or bot,h) that initiates the polymerizatioii or curing of an epoxy resin, such as Epoii 828 of Shell Chemical Co., aiid converts this inoiiomeric low-molecular-weight material into a high-strength crosslinked adhesive. Numerous types of compounds have been used as hardeners mid have included amines, Lewis acid-amine complexes, carboxylic acids and anhydrides, substituted triazines, amine salts, hydrazides, polyamides, polysulfides, aiid dicyandiamide and imidazoles (13ruiiis, 1968). M o s t of the above materials react with epoxy resin a t room temperat'ures; however, some have been modified chemically or physically to delay their action a t ordinary temperature and to cause hardening of epoxy resins a t elevated temperatures. These are called latent hardeners. We became interested i n imidazoles as hardeners for epoxy resins because of the good mechanical properties these compounds produce n-hell used in adhesives aiid because of their fast catalytic action in curing eposy materials (Hardiiig and Christie, 1964; British Patent, 1967; Christie, 1968). However, the unmodified imidazoles have low stability when 1
To whom correspoiideiice should be addressed.
344 Ind. Eng. Chern. Prod. Res. Develop., Vol. 10, No. 3, 1971
mixed with epoxy resins and as such are useless as latent hardeners. Therefore, we began a program to explore ways in which imidazoles could be st,abilized for use as latent epoxy resin hardeners. The approach most fruitful in stabilizing imidazoles and the subject of this report was utilization of the imidazole complexes with metal salts. Esamiiiation of the literature showed that such complexes were known for many years, but there is no recorded use of these complexes as hardeners for epoxy resins. T h e complexes, formed when a metal salt in a suitable solvent is niised iii an appropriate proportion with an imidazole, represent well-defined compounds of characteristic melting points and colors and are usiially soluble in the more polar organic solvents. Recent references to imidazole complexes are iiumerousfor example, imidazole (Eilbeck et nl., 1967) ; triarylimidazoles (MacDermott, 1966) ; benzimidazole (Goodgame et al., 1967); and various substituted imidazoles (Gariiovskii et al., 1966; Panyushkin et al., 1967). IIowever, most of the complexes reported in this work are new compounds aiid were made from imidazoles available commercially or easily preparable in the laboratory. This report is coiiceriied with the preparation of the complexes, their use as latent hardeners for epoxy adhesives, and the examination of the properties of the resulting adhesives. Experimental
Starting Materials. 13eiiziniidazole was obtained from Aldrich Chemical Co. Imidazole, l-methylimidazole, 2methylimidazole, 1,2-dimethylimidazoleJ and 2-ethylimidazole were obtained from BASF Colors and Chemicals, Iiic. 2Et,hyl-4-methylimidazole ( E l f 1-24) was obtained from Houdry Process and Chemical Co. aiid was used as received. l-Benzyl-2-methylimidazole and 1-vinyl-2-methylimidazole were obtained from Gallard-Schlesinger Chemical hlfg. Co. The remaining compounds were synthesized as described. For convenience the imidazoles and their abbreviations are col1ect)edin a separate chart. Reaction of 2-Ethyl-4-methylimidazole and Ethyl Acrylate. h solutioii of 10.0 grams (0.10 mole) of ethyl acrylate in 30 in1 of methanol was added to 11.0 grams (0.10 mole) of E t l f e I m . KO heat was evolved, and the mixture was heated on the steam bath. l f t e r 2 hr of heating, an addi-
tional 10 grams of ethyl acrylate was added, and heating was continued to remove any unreacted ethyl acrylate and methanol. After a total of 5 hr of heating, the mixture weighed 20 grains and had no odor of ethyl acrylate. I t s infrared spectrum did not show any absorption of the double bond of acrylate. Reaction of Imidazole a n d Acrylamide i n Presence of Acid. '1'0 a solution of 20.5 grams (0.30 mole) of imidazole in 50 rnl of methanol was added 32.5 grams (ca. 0.3 mole) of concentrated hydrochloric acid, and the resulting solution was evaporated to dryness (vacuum). The residue, imidazole hydrochloride, was dissolved in 50 ml of methanol and added
6 N H
Imidazole Im
blowing a stream of air whereupon the residue crystallized. The solid was purified by dissolving it in 300 ml of methanol, filtering from a small amount of insoluble material, adding 200 ml of acetone to the filtrate, and concentrating by boiling on the steam bath. Filtering, washing the solid with acetone, and drying gave 94 grams (68%) of the compound of the title as a white solid, mp 143-5'. Concentration of the filtrate gave a yellow oil which was not examined. The infrared spectrum of the product was ideiitical with that of the compound obtained from the hydrochloride and showed the following characteristic absorption: sharp a t 2.97p, broad, strong at 6 . 0 3 ~ .
0 N
I
bH3 1-Methylimidazole 1-MeIm
I
H 2-Methylimidazole 2-MeIm
CH, 1,2-Dimethylimidazole 1,2-MeJm
CH,
I
CHZCH2CONH2 2-Ethylimidazole EtIm
&H,W l-Benzyl-2methvlimidazole BMeIm
2-Ethyl-4methylimidazole EtMeIm
I
CHsCH, l-Vinyl-2methvlimidazole VMeim
to a solution of 21.3 grams (0.30 mole) of acrylamide in 50 ml of methanol. The resulting solution was boiled on the steam bath for 4 hr whereupon all solvent was removed by blowing a stream of air. The residue was stirred with about 200 ml of acet'one and the resulting mixture filtered. The solid was washed several t'imes with acetone and dried to obt'ain 42 grams (SOYo) of 1-(2-carbamylethyl)iniidazole hydrochloride, white crystals, nip 172" (soft a t 110'). Anal. Calcd. for C685N30.1IC1: N, 23.9; C1, 20.2. Found: N , 23.1; CI, 23.6. To obtain 1-(2-carbamylethyl)imidazole, t,he above hydrochloride was processed as follows. T o a solution of 30.0 grams (0.171 mole) of the hydrochloride in 200 ml of methanol was added methanolic KOH until only alkaline to pH paper (about 11 grams of KOH).The solid was filtered (13 grams, 0.17 mole of KC1) and the filtrate evaporated to dryness to obtain 24 grams of an oil which crystallized on cooling. O n recrystallization from a mixture of ethyl acetate and methanol 15 grams of 1-(2-carbamylethyl)irnidazole was obtained as a white solid, m p 135-7'. I t s infrared spectrum was identical with the material from imidazole and acrylamide. Anal. Calcd. for C6H5X30:N, 30.2. Found: pu', 27.4. Preparation of 1-(2-Carbamylethy1)imidazole. A solution of 68.1 grams (1.00 mole) of imidazole and 71.0 grams (1.00 mole) of acrylamide in a mixture of 200 ml of methanol and 5 drops of concentrated hydrochloric acid was boiled 011 the steam bath for 6 hr. Methanol was then removed b y
1- (2-Carbamylethy1)imidazole CIm
H
Benzimidazole BIm
1-(2-Carbamylethyl)2-ethyl-4-methylimidazole CEtMeIm
I
CH,CH=CH, l-Allyl-2-ethyl-4methylimidazole AlEtMeIm
Preparation of 1-(2-Carbamylethyl)-2-ethyl-4-methylimidazole. A mixture of 213 grams (3.00mole) of acrylamide, 300 grams (3.00 mole) of E t l I e I m , 15 drops of concentrated hydrochloric acid, 0.3 gram of hydroquinone (this later proved unnecessary) , and 300 ml of methanol was boiled on the steam bath (average temperature ea. 75') for 8 hr. All methanol was evaporated whereupon the residue crystallized. T h e solid was stirred with 400 ml of acetone aiid filtered to obtain 405 grams of tali-colored crystals. The crystals were washed with a mixture of hexane and acetone ( 2 : l ) to obtain 355 grams (65%) of the compound of the title as light tan crystals. A 40-gram sample of the materi,al was purified by dissolving in 100 ml of methanol, filtering from a small amount. of insoluble material, d i l u h g with 100 ml of acetone, and concentrating in a stream of air. Filtration and drying gave 26 grams of a white solid, m p 160-2'. I t s infrared spectrum, strong band a t 6 . 0 ~and sharp a t 3 . 0 ~ was , consistent with the assigned structure. Anal. Calcd. for C5HI5N30: N, 23.2. Found: K , 22.2. Preparation of 1-Allyl-2-ethyl-4-methylimidazole. A mixture of 330 grams (3.0 mole) of EtMeIm in 500 ml toluene and 363 grams (3.0 mole) of allyl bromide was allowed to stand at room temperature. After a few minutes an exothermic reaction set in, and when the temperature reached 50', the mixture was cooled and allowed to stand over the weekend. The mixture was refluxed for 7 hr, then stirred with a solution Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1771
345
of 120 grams (3.0 mole) of sodium hydroxide in 150 ml of water, and the layers were separated. The organic layer was evaporated and the residue distilled to obtain two fractions: (1) bp t o 100°/O.l mm, 225 grams and (2) bp 100-30°/0.1 mm, dec, and about 100 grams of a residue (viscous material soluble in wat'er). The infrared spectrum of Fraction 2 was that of impure EtMeIm, and that of Fraction 1 was consistent with the allyl compound but had a weak absorption due to N-H a t 3.0-3.2~. Fraction 1 was refractionated to obtain 161 grams (36%) of the compound of the title as a yellowish liquid, bp 86-90°/0.5 mm. Anal. Calcd. for 11EtMeIm: IY, 18.6. Found: N, 17.2. Imidazole-Metal Complexes. T h e experimental procedures for the several dozens of prepared complexes are very similar; therefore, those for selected compounds only are given to illustrate the general method used aiid any particular problems encountered. Cupric Nitrate-Imidazole. A solution of 24.2 grams (0.10 mole) of Cu(N03)2.3HsO in 50 ml of methanol was added to a solution of 27.3 grams (0.40 mole) of imidazole in 50 ml of methanol. Color changed, a large amount of heat was evolved, and crystallization occurred almost immediately. Some solvent was evaporated, the mixture was filtered, and the solid washed twice with acet'one-purple solid, 40 grams (87%). Xnal. Calcd. for Cu(SO&.4Ini: S, 24.6 (not including SOa-). Found: N , 24.8 (by Kjeldahl). Cupric Chloride-2-Methylimidazole. A solution of 17.1 grams (0.10 mole) of CuC12.2HzO in 50 ml methaiiol was added to a solution of 32.8 grams (0.40 mole) of 2-methylimidazole in 40 ml of methanol. Heat was evolved and precipitation of a solid occurred. The mixture was reduced in voliimc and the solid filtered, washed twice with acetone, and dried. Twenty-eight grams (61%) of a dark blue solid was obtained. hnal. Calcd. for CuC12.4(2-MeIm): N , 24.2. Found: N,23.4. Nickel Chloride-2-Methylimidazole. A solution of 23.8 grams (0.10 mole) of XiC12.6H20 in 50 ml of hot methanol was added to a solution of 32.8 grams (0.40 mole) of 2methylimidazole in 40 ml of hot methanol. The resulting niist,ure was reduced in volume and filtered. The solid was washed with a small amount of acetone and dried. Thirtyeight grams (83%) was obtained. Anal. Calcd. for XiC12,4(XIeIni):Xi, 11.4; N, 21.8; C1, 13.8. Found: Si, 12.3; N , 20.5; C1, 12.6. Cupric Bromide-1,2-Dimethylimidazole. ,4 solution of 22.3 grams (0.10 mole) of CuBr2 in 100 ml of hot methanol was added to a solution of 38.4 grams (0.40 mole) of 1,2dimethylimidazole in 50 ml of methanol. Most of the solvent was evaporated, and the mixture was diluted with ca. 250 nil of ethyl acetate and filtered. The solid was washed twice with ethyl acetate and dried to obtain 60 grams (100%) of a dark purple solid. Aiial. Calcd. for CuBr2.4(MenIm): K,18.5. Found: N , 16.0. Cupric Fluoride-2-Ethyl-4-methylimidazole. T o a suspeiision of 20.6 grams (0.15 mole) of CuF2.2H20 in 100 ml of niethaiiol and 100 ml of water was added 74.5 grams (0.675 mole) of 2-ethyl-4-methylimidazole. The mixture became dark blue, and the salt gradually dissolved, finally forming a dark blue solution. Most of the solvents were removed by blowing a stream of air, and the residue was diluted with ca. 300 ml of acetone. The resulting mixture was filtered and the solid washed with acetone and dried to obtain 60 grams (74y0) of a light blue powder. 346
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
Anal. Calcd. for CuF2.4(EtNeIm): Cu, 11.8; N , 20.7; F, 7.0. Found: Cu, 12.0; N , 18.4; F, 7.6. Cupric Sulfate-2-Ethyl-4-methylimidazole.h solutioii of 37.5 grams (0.15 mole) of CuS04.5H20 in 100 m l of water was added to a solution of 74.5 grams (0.675 mole) of 2ethyl-4-niethylimidazole. Precipitation of a solid began immediately. The mixture was allowed to stand a t room temperature for a few hours aiid filtered. The resulting solid was washed three times with 100-nd portions of acetone and dried to obtain dark blue crystals, 78 grams (86%). Anal. Calcd. for CuSO4.4(Et1IeIm): Cu, 10.6; N , 18.8; SO,, 16.3. Found: CU,11.2; S, 16.9; SO,, 16.2. Cupric Chloride-l-(2-Carbamylethyl)imidazole. A solution of 8.55 grams (0.050 mole) of CuC12.2H20 in 30 ml of methanol \vas added to a solution of 29.3 grams (0.21 mole) of 1-(2-carbaniylethyl)imidazolein 100 ml of warm methanol. The crystallized solid was filtered, washed well with acetone, and dried to obtain 34 grams (98%) of a blue-violet solid. Anal. Calcd. for CuC12.4(CIm): Cu, 9.2; N, 24.3; C1, 10.1. Found: Cu, 9.2; N , 22.4; C1,10.5. Cupric Chloride-l-(2-Carbamylethyl)-2-ethyl-4-methylimidazole. A solution of 8.50 grams (0.050 mole) of CuC12.2H20 in 30 ml of methanol was mixed with a solution of 40 grams (0.22 mole) of the imidazole of the title in 100 ml of methanol. The solvent was evaporated, and the residue was dissolved in 300 ml of acetone and allowed to crystallize. Filtration and ~vashiiig with methanol/acetone (1/10) gave 26 grams of a tan-colored solid. Its infrared spectrum was nearly identical with that of the starting compound. Anal. Calcd. for CuCly.l(CIltl\IeIm): S, 13.3. Found: N , 13.7. Cupric Chloride-1-Vinyl-2-methylimidazole. ai solution of 12.8 grams (0.075 mole) of CuC12.2H20 in 40 ml of hot methanol was added to a solution of 32.5 grams (0.30 mole) of 1-vinyl-2-methylimidazole in 100 ml of methanol. A change in color occurred, but not much heat was evolved. Some solvent mas evaporated and a blue solid precipitated, but on addition of ethyl acetate, the color of the solid changed to an intense green. The green solid was filtered, washed with ethyl acetate, and dried to obtain 24 grams of green solid (dissolving with blue color in water). The filtrate did not crystallize. Anal. Calcd. for CuCl2.2(VMeIrn): Cu, 18.1; N, 16.0; C1, 20.2. Found: Cu, 17.9; K,15.2; C1, 18.6. Cupric Sulfate-Benzimidazole. h solution of 12.5 grams (0.050 mole) of CuS04.5H20 in 50 ml of water was added to a solution of 23.6 grams (0.20 mole) of benzimidazole in 500 ml of methanol. A purple solid precipitated. Most of the methanol and the residue diluted with 200 ml of water. Filtration and drying gave 32 grams (9ly0) of a purple solid. Anal. Calcd. for CuS04.4(BIm):N,17.7. Found: K , 16.4. Determination of Gel Times. T h e standardized procedure in this work was use of 5 phr of the substance to be tested with Epon 828. Thus, a mixture of 5.0 grams of Epon 828 (or of a different epoxy resin) and 0.25 gram of the test substance was weighed into an aluminum dish (ca. 2 I/,-in. diameter) making the thickness of the resin casting about l / 8 in. The mixture was heated in an oven maint,ained a t 350-60°F and checked every '/2 to 1 min for gelation. If the mixture did not gel in 15 min, it was checked every 5 min. If the resin did not gel in 10-20 min, it was checked every 30 min. Usually, before the mixture gelled, it turned dark brown and fumed. Gel time was taken when the mixture was completely solid a t the test temperature.
Table I. Complex Imidazole
Im
Salt
CuC12 CuBr? CUF~ KiClr Zn13r2
1-MeIm
-
CUClZ CUFZ 2-1IeIm
-
CUClZ CuBrZ cuso4 ?;iCln
AgXOs MeJm
-
cuc12 CuBr2 cuso, 2-EtIm
-
CUClZ c u (T03) 2 cuso4 GC12 ?;is04 EtMeIm
-
cuc12 NiC12 ZnClz cuso4 ZnBr2 CuBrp CUF~ Cu(BFd2 C'Im
-
CuCla CuBrz CuFz CEtbleIm
BIm
-
cuc12 SiClz BMeIm
-
cue12 cuso4
VRIelm
-
cuc12
h1E t Me1in
-
CuBr2
Gel limes of Salt-Imidazole Complexes Gel time, Epon 828, 35OOF
Stability, Epon 828, 1 OO'F, viscosity, cpr, room temp
I.5 miii 12 min 10 min 1.5hr 5 min, 15 see 2.5hr 1. 5 min 4 min 3 min 1 min 3 . 5 hr 4 . 5 hr 9 hr 7 hr 2.5hr 1.5min 3 min 3 . 5 min 1 . 5 hr 1, 5 min 20 min 50 mill 7 hr 6 hr 2 . 5 hr 2 min 4 miii 4 miii 4 min 7 . 5 min 1 hr, 45 mill 5 . 5 min 1 0 . 5 mill 5 , 5 min 2 , 5 miii 3 hr 3 hr 7 miii 2 miii 1. 5 miii 40 min 40 min 2 . 5 min 4 miii 15 min 6 min 20 min 4 min 7 miii
Gelled in 1 day S o change in 8 days S o change in 8 days X o change in 8 day. S o change in 8 days Initial, 15,000; 8 day., 39,600
The results collected in Table I1 for different epoxy resins were obtained in a similar manner, but, as wit,h other less active compounds, checking for gelation was done every 10-20 mill. Accelerated Stability of Epon 828-Complex Mixtures. The coilcentration of the complex used in the stabilit,y tests was the same as that in the gel time tests-Le., 5 phr. Thus, the viscosity of a 100-gram sample of a well-stirred mixture of Epon 828 and of the complex (at 5 phr) was measured and the sample placed iii a 1 O O O F oveii. The viscosities were measured daily until a large change occurred. The result.;, giving the viscosity of t8hefirst, and the last day of the experimetit, are summarized in Table I. The stabilities of mixtures
Gelled after 1 day Gelled after i days Gelled after 7 days Gelled after 13 days, vise, 445,000 Initial, 19,800; 8 days, 22,600 Initial, 19,600; 8 days, no change Initial, 16,900; 8 days. 89.000 Initial, 12,600; 8 days, 32,400 Initial, 12,300; 8 days, no change Gelled after 2 days Initial, 18,000;8 days, 33,600 Initial, 18,000; 8 days, 27,000
of Epon 828 and some uiicomplesed imidazoles are also gi\-en for comparison. Shear and Peel Strength Tests. T h e epoxy catalysts were tested by mixing wit,h Elion 828, preparing bonds, curing the mixture. and subjecting the bondh to .hear and peel tests. Often. the catalyst wa' formulated n-itli tli AID-201 aluminum powder (Alcaii) , Cab-Oand various other types of el)oxy resitis, as well as Eimii 828 in attempts to optimize the l,erformaiice of tlie adhesive. The aluminum surfaces to be boiided were all depreased with a solution of toluene and methyl ethyl ketone and then immersed in trichloroethylene. The clean metal was then etched by chromic acid for 15 niin a t 165°F (a solutio11 of 66 Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
347
Table II.
Comparison of Gel Times of Epoxy Resins with Selected Imidazole Complexes. Gel time, 350'F
Complex
Epon 828
Epon
87Ib
Ciba CY 179"
Ciba ECN 1280d
2 hr, 50 min 2 hr, 20 min 2 min 5 hr, 15 min 1 hr, 10 min 16 miii 4 min 3 min 2 hr, 10 min 1 hr 3 . 5 hr 2 hr, 50 miii 5 hr, 45 min 6 min 3 . 5 min 50 mill 2 hr, 50 min 1 hr, 15 min 2 hr, 50 min 5 . 5 hr 1 hr, 50 min 5 hr, 10 min 3 . 5 hr 4 min 2 hr, 50 min 25 min 7 min 5 min 1 hr, 10 min 3 hr, 20 miii 5 hr, 15 miii 40 miii 2 hr, 50 min 4 hr 14 min 4 hr, 15 min 2 hr, 10 min 7 miii 5 hr, 45 min 1 hr, 35 min >45 hre 3 1 , 5 hre 26 hre .i.O grams of epoxy resin, 0.25 gram (,jphr) of complex at 3*5O0F.*Dimer fatty acid diglycidyl ester (Shell Chemical Co.). CuClz-Im CuF2-l-hleIm CuClz-2-hfeIm NiC12-1 ,2-;\Ie21m CuBrz-2-EtIm XiC12-EtlLIeIm CuF2-CIm CuC12-BIm CuSO4-BMeI m CuC12-11Et;\IeIm
a
e
Epoxy resins alone without added hardeners.
part's sodium dichromate, 666 parts 96% H2S04,aiid 1000 parts water). 13ouds for lap shear testing were prepared using Alclad 20241'3 alumilium (Alcoa) and a li2-iii. overlap. The test was run a t a loading rate of 1200 psi/niin on a n Instron Tensile Tester. The bonds for T-peel testing were prcpared with 20 mil alumilium sheeting. The test was run a t a 2 iuiiniii crosshead speed 011 an Iiintron Tensile Tester. Discussion
In geuernl, t,he complexes of metal saks and imidazoles were made by mixing solutioiis, in methaiiol or in water, of the two reactants and filtering the resulting precipitate. Washing wit,li a suitable solvent, such as nietliaiiol, acetone, or ethyl acetat'e, niid drying gave in most cases pure compounds. Dissolution of the reactants did not seem necessary although used for the sake of convenience, since a suspension of a halt-e.g., CuF1-will react t,o form a complex. Many of the complexes have considerable solubility in methanol or water, and eitlier precipitation with a nonsolvent or concentration of the reaction mixture was necessary before a c compound could be isolated. Despite many att'empts, some c'ol~l~~o"nds could llot be crysta et1 and remained viscous oils. 111 some cases iiitrogeii anal! of the complexes differed froin the calculated values, indicatiiig that these compounds contained water or nietliaiiol (or bot'li) as ligands in addition to imidazole. nieirtioiietl most of the compouuds were ohtaiiied by design as 1 : 4 complexes (1 mole of salt : 4 moles of imidazole) -that isj by using the reactants iii this proport,ioii, In some cases wlieii it lvas kiiown that complexes of different composition are formed, proportions ot,lier than 1:4 were used. The relatioiiship between the amount of imidazole present in the complex niid the cure time (with Epon 828) vs. the properties of the resultiiig adhesives was examined for only a few selected compositioiis. S o great differences were observed among the 1 : 2, 1: 4, and 1: 6 complexes. The effectiveness of the complexes as curiiig agents was tested by a miiforin procedure adhered to throughout this work. I3ecause of the difficulty in judging the hardness of the resin ab it cures, particularly with the less active complexes, the gel times give oiily a n estimate of the effectiveness of the 348
Ind. Eng. Chem. Prod. Res. Develop., Vol.
IO, No. 3,
1971
complexes. The gel times of complexes and for uncomplexed imidazoles with Epoii 828 are collected in Table I. Most important in Table I is the unusual degree of stabilization acquired by the mixtures of Epoii 828 aiid imidazole complexes over the mixtures of uncomplexed imidazole arid Epon 828. For example, EtMeIm-Epon 828 mixture cures in 2 mill a t 360" but gels on storage at, 100°F in one day. On the other hand, the mixture of CuC12-EthleIm shows no change in viscosity in 8 days a t 100" but cures in 7l/2 min at 350°F. Similarly, CuCls-Im complex cures in 2 min a t 350°F but shows 110 increase in viscosity in 8 days a t 100°F. Thus, although Epon 828-imidazole mixt,ures gel quickly even a t room temperature, the mixtures of this resin with the imidazole complexes possess considerable stability a t 100°F and yet have fast gel times a t higher temperatures. This property of the imidazole complexes makes them well-suited as curing agents for one-package epoxj- adhesives. h large number of imidazole complexes were prepared after the initial success to find more effective agents aiid t'o examine the scope of the complexation reaction and the act,ivity of the resulting products. Some patterns of curing behavior were hoped to be uncovered and related to the structure of the imidazole moiety and the salt, but no such coiiclusions are possible with the available data. Apparently, more subtle influences than the structure are a t play, aiid all that can be said a t the present time is that 1-subst'ituted (and 1,2-disubstituted) imidazole complexes are in general more reactive with Epoii 828 than the 2-substituted compounds (see Table I). Similarly, benzimidazole complexes seem less reactive than other complexes, although here, as in other cases, precise comparisoii cannot be made, since parts per hundred parts of resin (phr) is the basis of comparison and not the number of equivalents of complex in a given amount of Epon 828. The effectiveness of t,he complexes as curing agents was checked with three resins other than Epoii 828 (Epon 871, Ciba CY 179, and ECN 1280). These resins are dimer fattyacid diglycidyl ester, a cycloaliphatic diepoxide, aiid a novolaktype epoxy resin, respectively. The complexes cure these resins, but in general the gel t,imes are longer than with Epon 828 (Table 11). The gel time depends on the type of epoxy
resin, aiid a given complex is fast or slow only with reference to a particular resin-e.g., r\JiC12.1,2-lle21m and CuF2.1MeIm (Table 11). iZfter the complexes of commercially available imidazoles were prepared and tested, chemical modification of imidazole and its derivatives to maximize the desirable properties of t,he hardeners aiid to define the structural requirements of the imidazole for the successful formation of the complex was coilsidered. I t was particularly interesting to determine if higher substituted imidazoles would form a complex. This exploratory 11-ork was based on simple reactions which would furnish improved curing agents. Although no product could be obtained from the reaction of methacrylic acid with imidazole, ethyl acrylate seemed to react iuider relatively mild conditions to give what appeared from infrared evidence to be 1-(2-carboethoxyethyl)imidazole. This compouiid, an oil, was a n active curing agent, but attention was c1i:inged to acrylamide which gave easily isolated, crystalline products. Initially, the reaction was carried out lietween imidazole and acrylaniide in the presence of one equivalent of hydrochloric acid, similar t,o acrylamidepyridiiie reaction (Dowbeiiko, 1960), and the free base was isolated hy neutralization of the hydrochloride. However, the two compounds later proved to furnish a good yield of CIm on simple heating 011 the steam bath. Both imidazole and E t X e I m yielded crystalline products with acrylamide. Although C h i was reactive with metal salts furiiishing good yields of complexes, CEtMeIm was uiireactive with the salts tried, and in each case the compouiid was recovered unchanged. To ascertain whether all trisubstituted imidazoles are unable to form metal salt complexes, another trisubstituted imidazole was needed, XlEtlIeIm seemed to be an easy compound to synthesize ~vliich\vas undertaken by alkylat’ion of E t h l e I m with allyl bromide. The compound was obtained as an oil aiid in low yield, but it did yield crystalline complexes with CuC12 and C u I h 111 contrast to other compounds, however, the complexes were green instead of the usual blue, and the analyt,ical results showed them to be 1: 2 complexes instead of the usual 1: 4 complexes with cupric salts. 111 retrospect, CEtMeIm could also probably have furnished 1 : 2 complexes if the necessary proportioiis of reactants were used, but, this was not’ rechecked. d i i attempt to find a relationship bet’ween the melting point or decomposition temperature of the complexes and their effectiveness as hardeners was made for a selected group of CuCI? complexes with various imidazoles. D T A aiid TG-4 results are given in Table 111. The relationship bet,weeii the
Table Ill.
Gel Times, Melting and Decomposition Points of Some CuC12-lmidazole Complexes
CuCI* complex
G e l time, Epon 828
Im 2 miii 1,2-Me2Im 3 min 1-RIeIm 4 min E t Me1m 4 min AlEtRIeIm 7 miii ViiiRIeIm 20 min 2-EtIm 20 miii I3Im 40 miii CIm 3 hr 2-5IeIm 3 5hr a By I>TA. By TGA.
Decomposition, OCb
205 82 90 180 162 156 142 141 184 213
207 Ca. 75’ 80 minor, 150 major 182 175 162 120 minor, 185 major 90 minor, 245 major 241 20’7
gel t,imes and the temperatures of decomposition and melting points is far from simple, but there appears to be general increase in the gel times with increasing decompositioii temperatures and melting points of the complexes. I3ased on the available data, several possibilities might be considered regarding how the epoxy resiiis are cured by the imidazole-metal salt comp!exes. First, a reasonable assumption is that the act,ive species initiating the polymerization of the epoxy resin is the imidazole itself, and the problem becomes one of establishing the mode by which the imidazole is released from the complex. The complexed imidazole bound as a ligand t’o the central metal ion, compared with the uncomplexed imidazole, is probably a less likely choice of the active initiating species and in any case could not be distinguished from the data. Solubility of the complexes in the epoxy resin is not a significant factor in the degree of effectiveness of cure since many insoluble complexes cause rapid gelatioii. Conversely, some soluble complexes are quite slow in curiiig the epoxy resins. Melting points and decomposition temperatures of the complexes are probably not important although a reasonably posit’ive correlation exists between these properties a i d the speed of cure (Table 111). The most probable mode of the imidazole release is based on the stability of the complexes aiid their temperature dependence. Accordingly, the release of t’he free imidazole, the active curing agent,, from the complex is goveriied b y the stability constant of the complex. At a given temperature the gel t,inies of epoxy-complex mixtures would be longest for t,he complexes with highest stabilit,y constants, providing the speed of cure of imidazoles of different structures ~vould be approximately t’hesame. The latter assumptioil is not quite warranted because of the known variation of speed of cure depending on the structure of the imidazole and thus would not give a linear relationship between the gel time and the stability constant but would probably hold as a good approximat ion. T h e determining role of the stabilit,y coiistant for the cure of epoxy resiiis also explains the fact that there are “good” and Lipoor’’complexes as hardeners. For t,he present dis-
Table IV. Synergism of Imidazole Complex. Dicyandiamideh on Cure. of Epoxy Resins
Complex
CuSO4-EtXIeIm CuS04-EtMeIm CuSO4-EtMeIm h’iCI2-2-hleIm iYiC12-2-hleIm n’iC1?-2-hIeIm CuS04-2-MeIm CuSO4-2-hleIm CuS04-2-3leIm NiC12-Im NiCl,-I m ?;iC12-I m ZnC12-EtMeIm ZnC12-EtRIeIm
Amt complex, grams
1 0.5 0.1 1 0.5 0.1 1 0.5 0.1 1 0.5 0.1 0.5 0.5
Amt dicyondiamide, grams
0 0 2 0 0
2 0 0 2 0 0 2 0 2
and
t a p shear strength, psi
1780 1980 2800 860 200 3750 1720 1085 3000 1310 1522 3200 590 2180
a All complexes contain four ligands. * Ilicyandiamide does not cure epoxy resin at conditions used. Ten grams of Epon 828 and 2 grams of aluminum powder were mixed with the above and cured for 8 min at 350°F.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
349
Table V.
Bond Strength and Gel limes of Epoxies Cured with Salt-Imidazole Complexes and Dicyandiamide Complex
Imidazole
Im
Salt
G e l timea, Epon 828
G e l timeb, Epon 828 and dicyandiamide, 3OO0F, min
G e l timec, Epon 828 and dicyandiamide, 1 2OoF, days
Lap shear psi
1itrength,
CuClz CuBrz CuFz NiC12 coc12 ZnBr2
12 min 10 3050 10 min 12.3 2600 1 . 5 hrs 4 .. 2300 5.5 .. 5 min, 15 sec 3200 2.5 5 min 20 3250 14.2 2 . 5 hrs ... cuso4 7 . 5 hrs 9.3 2510 ZnCrO, 22 hrs 18 83 2000 7 2 , 5 hrs .. 2320 Cu(NOd2 1-MeIm CUClZ 4.5 4 min 2 2650 CuBr2 11 3 hrs 2550 4 CUFZ 3 min 10 2750 5 CuSO4 5 min 24 2840 2-MeIm 4.5 CUClZ 3 . 5 hrs 28 2800 4.5 CuBrz 4 . 5 hrs 36 3150 cuso4 3.5 .. 9 hrs 2550 5 50 rnin 2180 CU(Nod 2 NiSOc 2 . 5 hrs 2460 IO+ NiC12 5.5 7 hrs 83 3750 3 . 5 hrs 5.75 COClZ 2550 29 4 2 . 5 hrs .. ... AgN03 l,2-MezIm 3 miii cuc12 3.5 6 2600 3 CuBr2 3 . 5 mill 24 2400 5.25 cusoc 1.5hr 49 3120 3 NiC12 3 . 5 min 2 2840 2-EtIm 7.25 CUClZ ca. 20 mill 13 2800 3.25 CuBr2 5 hrs, 30 miii 15 2740 3.75 CuSO4 7 hrs 52 3000 5 , . NiC12 6 hrs 3220 8 NiS04 2 . 5 hr 3010 EtlIeIm KiC12 8 4 rnin 2760 c us04 9.5 15 7 . 5 min 2800 ZnBr2 15 1 hr, 45 min .. 2640 4 CuBr2 .. 5 . 5 min 2940 CuFz 3.3 1 0 . 5 rnin 2920 CIm 1120 CUClZ 42 3 hrs 10 5 CuBr2 28 3 hrs 2860 CuFz 7 min COClZ 5.5 1250 45 2 hrs, 30 miii BIm 10 .. 2620 CuClz 40 rnin 15 CuBr2 850 87 2 hrs, 30 mill .. 13.5 2650 cuso4 3 hrs coc12 1550 1 hr, 45 rnin 45 20+ 14.5 .. NiCl2 3500 40 miii IJMeIni 10 45 cuso4 685 15 rnin VMeIm 6 2760 CUClZ 16 20 mill 10 cuso4 29 2600 5 hrs -4lEt Me1in 14 cuc12 2600 7 miii IO+ 14 7 rnin Cu13rz 2380 IO+ a Test was run on mixture of 10 grams Epon 828 and 0.5 gram complex at 330'F. * Test was run on 3-5 gram-sample at 300'F on platinum press. Formulation contained 10 grams Epon 826, 2 grams dicyandiamide, 0.2 gram complex. Formulation contained 10 grams Epon 828, 2 grams AI powder, 2 grams dicyandiamide, and 0.1 gram complex. d Test was run at room temperature. Formulation containing 10 grams Epon 828,Z grams A1 powder, 2 grams dicyandiamide, and 0.1 gram complex. . . I
cussion, a good hardener is a complex which has a low activity a t a low temperature (such as room temperature) but a high activity a t temperature of about 300-50°F. Conversely, a poor hardener has nearly the same activity a t both temperatures, high or low. A good hardener has good storage stability with the epoxy resin a t room temperature but cures quickly a t an elevated temperature. The results obtained with good and poor hardeners can be explained by the temperature dependence of the stability constants of the complexes. Thus, for a good hardener the 350
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 3, 1971
stability constant has a high value a t a low temperature and decreases sharply with the temperature. For a poor hardener, on the other hand, the stability constant has a weak temperature dependence, low or high. In the first case, when the stability constant is low, the complex will be a fast hardener at elevated temperature but will have a poor stability a t room temperature; in the second case, the complex will have good stability a t low temperature but will be a slow hardener a t the elevated temperature. This discussion fits well with the experimental results and
Table VI. Relationship of Bond Strength of Epoxy Adhesives with Variations of Complex and Dicyandiamide Curing Agent Experiment no.a
Catalyst, gramsb
Dicyondiamide, grams
Lap shear strength, psi
1 0 1.2 KO cure 2 0 1.2 400. 3 0.5 0 1980 4 1.0 0 1780 5 1.0 1. o 2600 6 1.0 2.0 2400 7 0.5 1.5 2400 8 0.5 2 .o 2920 9 0.1 1.5 2700 10 0.1 2.0 2600 11 0.5 0 510 12 0.5 0 590 13 0.5 2.0 2180 14 0.5 3.0 2160 15 1. o 1.0 2020 16 1.5 1. o 1460 a Formulation contains 10 grams Epon 828 and 2 grams A l l ) 201 aluminum powder. Cure is 8 min at 350°F. CuSOa-EtlleIm is catalyst in Experiments 1-10; ZnClp-EtAIeIni is catalyst in Experiments 11-16. c Cured at 330°F for 15 min. Table VII. Aluminum level, phrb
Shear Strength and Peel Strength of OnePackage Epoxy Formulation. Room temp shear, psi
180°F sheor, psi
90' Peel,
180' Peel,
Ib/in.
Ib/in.
20 4170 5400 7 3 100 4215 4950 9 4 4800 150 5140 12 8.5 a Formulation contains 100 parts Epon 828, 20 parts dicyandiamide, and 1 part CuSOa-EthleIm. MU-201 aluminum powder used.
Table VIII. Formulationa
Lap shear, psi
T-peel, Ib/in.
PPG-12655 5260 17 PPG- 12656 5650 20 PPG-12657 5400 22 a All samples were cured in a 350°F oven for 10 min. These materials are proprietary PPG Industry formulations.
equal importance, the complex-dicyandiamide hardeners produced a desirable increase in the mechanical properties of the adhesive bond. These results aiid the syiiergist'ic effect of dicyandiamide are shown in Table IV. Because of the effectiveness of the complex-dicyandiamide hardeners, a large variety of imidazole complexes was examined with respect to gel times and the lap shear strength of the bonds formed. These results are shown in Table V which s h o w the effectiveness of slow curing imidazole complexes in producing a rapid dicyaiidiamide cure. Slow curing complexes had excelleiit stability a t 120°F eveii though the combination of t,he complex and dicyaiidiamide cured rapidly. Various ratios of catalyst to dicyaiidiamide were used. Table VI shows t'he effect of these changes on the lap zliear strength of the bonds prepared. Twenty phr of dicyaiidiamide and 1 phr imidazole coniplex produced the best combination of shear strength, peel streiigth, and shelf stability. Table VI1 shows the variation of physical properties of an epoxy adhesive-shear aiid peel strengths-dependiiig on the concentration of aluminum powder filler. Finally, Table VI11 shows some of the excellent values obtained in both shear and T-peel of selected epoxy formulations catalyzed with imidazole conipleses.
-
literature Cited
British Patent 1,084,667 (Sept. 27, 1967). Bruins, P. F., "Epoxy Resin Technology," I n t e r ~ k i c e ,Sew York, N.Y., 19Si(, pp 43-109. Christie, S. H., 11. S.Patent 3,304,105, July 23, 1068. Iloaberiko. 11.. J . Ora. Chcrn.. 25. 1123 fIO60~. Eilbeck, W'. J.,'IIolm&F., U;ide;,hill, A: I
