Isocyanates blocked with 1-amino-alkyl imidazoles as epoxy curing agents

Novel curing agents for heat-curable single package epoxy resins, the agents being adducts of aminoalkyl imidazoles with isocyanates. Cures are effected on heating the epoxy resin for times as short as 5 seconds.

RELATED APPLICATION 
USSN 082,179 filed Aug. 6, 1987, "Aminopropylimidazoles," inventors 
Shiow-Ching Lin and Jennifer M. Quirk, discloses and claims the 
hydrogenation of 1-cyanoethylimidazoles to make the corresponding 
aminopropyl derivatives. 
FIELD OF THE INVENTION 
This invention relates to single package heat-curable epoxy resin systems, 
and more specifically to novel accelerators/curing agents for such 
systems. 
BACKGROUND OF THE INVENTION AND PRIOR ART 
Single package epoxy resin systems conventionally include a latent curing 
agent, typically dicyandiamide. This curing agent requires a long cure 
period, even at high temperatures. For example, it can be demonstrated 
from differential scanning calorimetric studies that the 
dicyandiamide-epoxy system (e.g., Epon 828, Shell Chemical Company), 
without the presence of an accelerator, exhibits an onset cure temperature 
at 193.degree. C. Normally such mixture has to be cured above 180.degree. 
C. for at least 30 minutes to obtain a cured thermoset for practical 
applications (adhesives, coatings, and sealants). To increase the curing 
speed and to reduce the curing temperature, curing accelerators such as 
imidazoles and ureas have been incorporated into epoxy-dicyandiamide 
systems. Due to the basic nature of imidazoles, the prepared one-package 
thermosetting materials normally run into storage stability problems. 
(Note: "Onset cure temperature" is a term peculiar to the thermoset resin 
art. Consider the curing curve. The curve is substantially flat (base 
line) until the resin reaches a temperature at which curing begins. Then 
the curve slopes sharply upward, as a measure of the amount of curing that 
has taken place. "Onset cure temperature" is the intersection of tangents 
drawn respectively to the base line and to the sharp upward curve. A 
consideration of the geometry of the curing curve and this intersection 
will show that curing occurs before onset cure temperature is reached.) 
Various urea derivatives have been reported as epoxy accelerators, with and 
without dicyandiamide, e.g. (all references are to Chemical Abstracts): 
N-phenyl-N',N'-dimethyl urea; 78(10):59299t. 
Phenylurea or 1,3-diphenylurea; 83(22):180415j. 
Substituted urea or thiourea; 85(12):78946n. 
Substituted urea (Monuron); 94(8):48349p. 
3-(substituted phenyl)-1,1-dimethylurea; 102(4): -25735f and 102(4):26023j. 
In other attempts to hasten the cure of epoxy-dicyan-diamide systems, 
certain imidazole-isocyanate reaction products have been tried. European 
patent application 024,119 of July 21, 1980 describes a succinic acid salt 
of imidazole modified by phenylisocyanate which when combined with an 
epoxy resin of polyglycidyl ether, has a shelf life of only 2-3 days. U.S. 
Pat. No. 4,335,228 describes certain isocyanate blocked imidazoles with 
excellent shelf life, but which are useful only with epoxy resins that are 
solid at room temperature, a requirement that rules out use with some of 
the most useful epoxies. The accelerator claimed in U.S. Pat. No. 
4,533,715 is naphthyldiisocyanate blocked with imidazole or certain 
imidazole derivatives. Shelf life of 6-8 months is claimed, along with 
cure times of 5 minutes at 250.degree. F. (with Epon 828). In U.S. Pat. 
No. Pat. No. 4,041,019 the epoxy curing agent is an imidazole capped with 
an isocyanate. 
The instant invention concerns a novel class of imidazole compounds that 
serve as curing agents as well as curing accelerators for single-package 
epoxy thermosetting systems. The invention deals with the preparation and 
the application of these novel imidazoles for curing epoxy resins at an 
extremely rapid speed and at a reduced cure temperature without affecting 
the storage stability of the mixture of epoxy resin plus imidazole 
compound. Details are given below. 
The Invention 
Our epoxy accelerator/curing agents are a new class of compounds, 
EQU X.sub.m Y (I) 
where 
X is 
##STR1## 
m has a value of at least 1 and is preferably 1-3; R and R" are 
independently H, methyl, or ethyl; 
n is 2-5; 
R' is methyl or ethyl; and 
Y has valence m and is an organic radical. 
The aforesaid new compounds are made by reacting an imidazole of the 
formula 
##STR2## 
with an isocyanate of the formula 
EQU (III) Y(NCO).sub.m (III) 
where m, n, R, R', R" , and Y have the values above given. 
Within (II), 1-aminoethyl-2-methyl imidazole is a known compound. See use 
in Example 1. 
The aminopropylimidazoles in (II) are believed novel, e.g., 
1-aminopropyl-2-ethyl-4-methyl imidazole, as made in Example 6 and used in 
Example 5. 
In general, the compounds in (I) can be readily synthesized by the reaction 
of an amino-imidazole, such as 1-aminoethyl-2-methyl imidazole, with an 
isocyanate, such as toluenediisocyanate. The isocyanate can be mono- or 
multi-functional. The resulting products have the structure set forth in 
(I). 
It will be noted that the compounds in (I) contain complete urea and 
complete imidazole functional groups, i.e., all urea nitrogen is derived 
from aminoalkyl, not (as in the adducts in the aforesaid European patent 
application 24,119, U.S. Pat. No. 4,335,228, 4,041,019, 4,533,715) from 
imidazole nitrogen. The structural difference appears minor, but it 
results in a surprising improvement in cure times. 
A wide variety of isocyanates is useful in the invention, viz.: 
Monoisocyanates such as methyl isocyanate, propyl isocyanate, isopropyl 
isocyanate, n-butyl isocyanate, cyclohexyl isocyanate, octadecyl 
isocyanate, phenyl isocyanate, p-chlorophenyl isocyanate, and 
3,4-dichlorophenyl isocyanate. 
Diisocyanates such as hexamethylene diisocyanate, m-phenylene diisocyanate, 
2,4-toluene diisocyanate, 4,4'-diphenylmethane diisocyanate, dianisidine 
diisocyanate, tolidine isocyanate, isophorone diisocyanate, 
4,4'-dicyclohexylmethane, chlorophenylene-2,4-diisocyanate, 
1,5-naphthalene diisocyanate, ethylene diisocyanate, diethylidene 
diisocyanate, propylene-1,2-diisocyanate, cyclohexylene-1,2-diisocyanate, 
3,3'-dimethyl-4,4'-biphenylene diisocyanate, 
3,3'-dimethoxy-4,4'-bipheylene diisocyanate, 
3,3'-dimethoxy-4,4'-biphenylene diisocyanate, 
3,3'-diphenyl-4,4'-biphenylene diisocyanate, 4,4'-biphenylene 
diisocyanate, 3,3'-dichloro-4,4'-biphenylene diisocyanate, and 
furfurylidene diisocyanate. 
Triisocyanates such as biuret of hexamethylene diisocyanate and 
triphenylmethane triisocyanate. 
Polyisocyanates such as polymeric diphenylmethane diisocyanate. 
The epoxy resins to which our accelerator is applicable include those 
conventionally available. They are those using conventional reactants, 
e.g., as halohydrins: epichlorohydrin, dichlorohydrin, 
1,2-dichloro-3-hydroxypropane, etc. As diepoxies, butadiene dioxide, 
diglycidyl ether, etc. As mononuclear di- and trihydroxy phenols, 
resorcinol, hydroquinone, pyrocatechol, saligenin, phloroglucinol, etc. As 
polynuclear polyhydroxy phenols, Bisphenol A, Bisphenol F, trihydroxyl 
diphenyl dimethyl methane, 4,4'-dihydroxy biphenyl, dihydroxyl diphenyl 
sulfone, etc. 
As is well known, epoxy resins are in resin form both before and after 
being cured, and many (though not all) are initially in liquid form. The 
curing process cross-links the starting resin. When curing is by heating 
(as is the general practice in industry), the effect is thus a 
thermosetting operation. Our invention, as noted, is limited to 
heat-curing and to heat-curable epoxy resins. 
An outstanding advantage of our new compositions is that they combine the 
functions of accelerator and curing agent. This means that conventional 
curing agents such as dicyandiamide may be omitted from the epoxy resin. 
On the other hand epoxies that contain dicyandiamide will not be adversely 
affected by the use of our accelerator/curatives. Besides (in the typical 
case) effecting cure in seconds, our materials are storage stable. (After 
30 days in storage no significant change is detectable.) 
The epoxy resin may be modified by polyols, polyesters, rubber, and the 
like. 
The epoxy resins that can be employed with our curing agents in a one 
package system can be either solid or liquid at room temperature, the form 
of the resin dictating the method of incorporating the agent into the 
resin. The lower molecular weight resins are typically viscous liquids at 
room temperatures of 70-78.degree. F. When incorporated into such liquids, 
the agent may be dispersed into the liquid either by conventional milling 
or stirring procedures. When incorporated in a solid resin, the curing 
agent can be incorporated by use of conventional milling process, such as 
a 3-roll mill. 
Suitable resins include 3,4-epoxycyclohexylmethyl-(3,4-epoxy) 
cyclohexanecarboxylate (ERL 4221 by Union Carbide or Araldite CY 179 by 
CIBA Geigy), bis (3,4-epoxy-6-methylcyclohexylmethyl) adipate (ERL 4289 by 
Union Carbide or CY 178 by Ciba), vinylcyclohexenedioxide (ERL 4206 by 
Union Carbide), bis (2,3-epoxycyclopentyl) ether 4205 by Carbide); 
glycidyl ethers of polyphenol epoxy resins such as liquid or solid 
bisphenol A diglycidyl ether epoxy resins (Epon 828, Epon 826, Epon 1001 
and Epon 1002 by Shell and DER 331, DER 332 by Dow Chemical Co.), 
tetraglycidyl methylenedianiline (TGMDA) (MY720 by Ciba), tris 
(hydroxyphenyl) methane based epoxy resins (XD-7142.002 experimental epoxy 
resin by Dow Chemical Co.); flame retardant epoxy resins such as halogen 
containing bisphenol A diglycidyl ether epoxy resins (DER 542, DER 511 by 
Dow); phenol-formaldehyde Novolac polyglycidyl ether epoxy resins (such as 
DEN 438, 431 by Dow), diglycidyl hexahydrophthalate (Araldite CY 183 by 
Ciba and Ed-5662 by Celanese). Other cycloaliphatic epoxies (such as 
Araldite CY 179 and CY 192 by Ciba, ERL 4090, 4205 by Union Carbide), 
2-(3,4-epoxy) cyclohexyl-5,5-spiro (3,4-epoxy)-cyclohexane-m-dioxane, (CY 
175 by Ciba, ERL 4234 by Union Carbide). 
The epoxy resin system of this invention may further include conventional 
fillers and additives such as pigments and colorants. Fillers may be 
generally used to increase performance at high temperatures, reduce the 
coefficient of thermal expansion, increase thermal conductivity, decrease 
shrinkage (by reducing peak exotherm temperature) and alter moisture 
resistance. Suitable fillers include calcium carbonate, talc, aluminum 
oxide, flint powder, silica, mica and metallic powders (Al, Zn, etc.), 
calcium sulfate, glass, and the like. 
Common pigments that may be used include titanium dioxide, carbon black, 
cadmium red, barium sulfate, antimony oxide, phthalocyanine blues and 
greens; red, yellow, black, and brown iron oxides; chrome oxide green, 
etc. Sufficient pigmentation is used to provide an opaque or colored film 
as needed for the desired appearance. 
Leveling agents commonly used in epoxy coatings can be used as desired. For 
example, various leveling agents which can be used to enhance the flow of 
the epoxy under curing conditions are polymeric or monomeric acetals such 
as polyvinylformal, polyvinylacetal, diethyl-2-ethyl hexanol acetal, 
di-2-ethylhexyl acetaldehyde-acetal; and polyglycols and polyglycol ethers 
such as polyethylene glycol, polypropylene glycol, and the like. 
PREFERRED EMBODIMENTS 
The following examples illustrate without limiting the invention.

EXAMPLE 1 
##STR3## 
(Herein, when toluene diisocyanate is used as a reactant, it is a 
commercially available mixture of the 2,4-diisocyanate (80%) and the 
2,6-diisocyanate (20%). Other isomers are possible. The valences may 
therefore appear as dangling, i.e., 
##STR4## 
With vigorous stirring, 35 g of toluenediisocyanate was added dropwise to 
an aqueous solution containing 55 g of 1-aminoethyl-2-methyl imidazole and 
100 ml of water. After the completion of addition, the solution was 
stirred further for 2 hours. A white precipitate of the above product was 
formed. It was filtered, washed with water, and dried at room temperature, 
after which it was pulverized to a fine powder. 
EXAMPLE 2 
Use of Product of Example 1 as Curing Accelerator 
The compound (3.4 parts by weight) from Example 1 was uniformly blended 
with an epoxy resin containing 100 parts of an epoxy resin made from 
diglycidyl ether and Bisphenol A (available commercially as Epon 828 from 
Shell Chemical Company) and 6 parts of dicyandiamide to form a one-package 
epoxy system. The blend was stored in an oven at 40.degree. C. for 10 
days. No noticeable viscosity change was observed. The fresh one-package 
epoxy system was curable at 170.degree. C. in 6 seconds. 
EXAMPLE 3 
Use in Epoxy-Anhydride System 
The compound (1.5 parts) from Example 1 was mixed with an epoxy resin 
mixture containing 32.5 parts of Epon 828 and, as curing agent, 17.5 parts 
of a copolymer of styrene and maleic anhydride. The mixture cured at 
170.degree. C. in 5 seconds. At 40.degree. C. the mixture was stable for 
over a month. 
EXAMPLE 4 
Use in Epoxy Resin 
Epon 828 (19 parts) was well mixed with 11 parts of the compound from 
Example 1 at room temperature. No dicyandiamide was added. This mixture 
exhibited a fast curing at 170.degree. C. in 5 seconds. 
EXAMPLE 5 
1-Aminopropyl-2-ethyl-4-methyl Imidazole/Toluenediisocyanate Reaction 
Product 
The process of Example 1 was followed except that the imidazole reactant 
was 1-aminopropyl-2-ethyl-4-methyl imidazole. The corresponding bis-urea 
reaction product was obtained as a white powder. It was used to cure an 
epoxy resin with good results, following the procedure of Example 3. It 
showed excellent storage stability, i.e., in an epoxy resin system for 
over a month at 40.degree. C.. 
The amount of our accelerator is not critical. A workable range is 0.01 to 
20 weight %, preferably 0.01 to 5%, based on the weight of the epoxy 
resin. 
Our accelerators provide curing temperatures suitably over the range 
130.degree.-220.degree. C., at times ranging from about 5 seconds to one 
hour. 
Studies of the mechanism of imidazole-curing of epoxy resins include 
Ricciardi et al., Mechanism of Imidazole Catalysis in the Curing of Epoxy 
Resins, J. Polymer Sci.: Polym. Let. Ed., Vol. 20, 127-133 (1982); and 
Ricciardi et al., Mechanism of Imidazole Catalysis in the Curing of Epoxy 
Resins, J. Polymer Sci.: Polym. Chem. Ed., Vol. 21, 1475-1490 (1983). It 
appears that the imidazoles are regenerated during the curing process, 
indicating a true catalytic function. 
Storage Stability 
Storage stability extending over at least a month is virtually a necessity 
for most industrial bulk uses of completely formulated liquid epoxy 
resins. For example, large vats filled with accelerator-containing resins 
are used for dip-work continuously over many days, and the resin must 
remain fluid and un-cured during this use period. Also, spray work is 
often supplied from tanks containing large volumes of resin, and it is 
essential that the resin not destabilize during use continuing over 
several weeks. Our new accelerators qualify eminently in this regard. 
1-Aminoethylimidazoles 
We used 1-aminoethyl-2-methyl-imidazole ("AMZ'), available commercially 
(Shikoku Company, Japan). 
1-Aminopropylimidazoles 
1-Aminopropylimidazoles can be prepared by hydrogenation of the 
corresponding source cyanoethylimidazole, thus: 
##STR5## 
where R, R', and R" have the meanings in (I) above. 
EXAMPLE 6 
1-Aminopropyl-2-ethyl-4-methyl-imidazole 
##STR6## 
1 g 1-cyanoethyl-2-ethyl-4-methylimidazole was put in 20 ml of ethanol into 
a high-pressure reactor. Raney nickel (50 mg) was added as the catalyst, 
and the reactor was pressured to 1000 psi with hydrogen and heated to 
100.degree. C. for 18 hours. The reactor was cooled down and the excess 
hydrogen was vented. The product was identified as 
1-aminopropyl-2-ethyl-4-methylimidazole by gas chromatography and mass 
spectra. 
Substantially identical results were obtained when two other catalysts were 
used instead of Raney nickel, viz., Example 7, palladium-on-carbon; and 
Example 8, rhodium-on-carbon. 
1-Cyanoethyl-2-ethyl-4-methyl-imidazole is available commercially and can 
be made by reacting 2-ethyl-4-methyl-imidazole with acrylonitrile. 
For preparation of 1-aminoalkyl imidazoles generally see Chem. Abs. 93: 
1145 W, citing U.K. patent application 2,016,452 of 1978 to Iizuka et al.; 
and Chem. Abs. 102: 132036a, citing Ger. Offen DE 3,406,414 of 1984, T. 
Wright et al.