Abstract:
An epoxy-hardener system is provided having relatively long latency periods combined with relatively short cure times at low cure temperatures. The hardeners of the present invention are ureidoamines and their derivatives, which are chelates of ureido compounds and amines. The ureidoamines are prepared by reacting an amine with the ureido compound and aqueous formaldehyde without a catalyst. Complexes of ureidoamine hardeners with various blocking agents are prepared in the melt. The hardener is prevented from curing the epoxy by the reaction between the hardener and the blocking agent. The blocked hardener is then blended with the epoxy, usually by warming the mixture briefly at about 50-60 degrees C.

Description:
FIELD AND BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to the field of epoxy hardeners and in particular to new and useful hardeners for epoxy compositions.  
         [0002]     Epoxy resins are used extensively in industry for the production of highly diverse articles of manufacture from aerospace structures to sporting goods. Cured epoxies have excellent adhesive strength with a variety of substrates, good to excellent strength and toughness and good resistance to solvents and chemicals. The best properties are obtained from combinations of epoxy resins and various active hydrogen hardeners which produce thermosetting copolymer systems. Epoxy resins are industrial commodities.  
         [0003]     Epoxy hardeners are extremely diverse, including primary and secondary polyamines, tertiary amines, polyphenols, polycarboxylic acids, cyclic anhydrides, acidic polyols and combinations of these.  
         [0004]     Mixtures of epoxy resins and hardeners and various other modifiers, diluents, fillers, etc. are cured either at ambient temperature or at elevated temperature for a time sufficient to convert the initial liquid mixture to a solid copolymer having useful properties. This procedure is referred to as the cure process or “curing.” 
         [0005]     The cure temperature and cure time vary extensively depending on the chemical characteristics of the epoxy-hardener system. The liquid prepolymer mixture will gradually increase in viscosity over a period of time at ambient temperature. Depending on the nature of the manufacturing process, the time for the viscosity of the epoxy-hardener mixture to increase to a predetermined value is referred to as the “latency period”, “pot life”,or “joint open time.” As a practical matter, these times determine the time available for various adhesive bonding processes involving metal or composite adherends or fiber-reinforced composite materials or the cycle times of various thermosetting molding processes.  
         [0006]     Increasing emphasis on reducing manufacturing costs has focused attention on the development of epoxy-hardener systems having relatively long latency periods combined with relatively short cure times at low temperatures. This combination presents some very difficult design problems. In general, cure times and latency periods track together; increasing the latency period by changing a formulation generally results in an increase in the cure time. It is only by changing the chemical characteristics of the hardener system that increased latency combined with shorter cure times can be obtained.  
         [0007]     Polyamine hardeners give relatively short latency periods and will undergo partial curing at ambient temperature. However, properties are improved by a postcure at a temperature above ambient. Various combinations of polyamines and tertiary, amines will cure at moderate temperatures with somewhat longer latency periods and aromatic polyamines and cyclic anhydride curing agents will provide still longer latency periods but good properties require a postcure at relatively high temperatures. New epoxy hardeners which can meet current industry requirements are needed to provide short cures at relatively low temperatures combined with relatively long periods of latency.  
         [0008]     Latent epoxy hardeners consisting of combinations of tertiary amines and acidic polyols have been described in U.S. Pat. Nos. 6,491,845 B1 and 6,743,375 B2. The described tertiary amines were those which were commercially available at the time. However, demands for increases in latency and decreases in cure time have resulted in a search for new amine-type epoxy hardeners which can meet these new performance criteria.  
       SUMMARY OF THE INVENTION  
       [0009]     It is an object of the present invention to provide an epoxy-hardener systems having relatively long latency periods combined with relatively short cure times at low temperatures.  
         [0010]     It is a further object of the present invention to provide hardeners having good inherent latency properties with epoxy resin and which can be blocked efficiently by acidic materials to provide longer latency periods and shorter curing times.  
         [0011]     Accordingly, hardeners for epoxy resins are provided which have the capability of faster curing at lower temperatures than existing hardeners while simultaneously providing longer latency periods than existing hardeners. They are easily handled liquid materials having a range of viscosities and are made from cheap and readily available industrial chemicals. They have low vapor pressure at ambient temperature, have no noxious odors and do not carbonate in air. Curing reactions with epoxy resins exhibit low exotherm and give cured products having exceptionally low cure shrinkage, high tensile strength and high toughness.  
         [0012]     The hardeners of the present invention are ureidoamines which are chelates of uriedo compounds and amines. Preferred ureido compounds include ethyleneurea, propyleneurea(tetrahydro-2-pyrimidone), and 1,3-dimethylurea. Preferred amines include secondary monoamines, secondary diamines, primary monoamines, mixed primary-secondary amines or primary diamines.  
         [0013]     All of the ureidoamine hardeners are prepared by reacting an amine with either ethyleneurea, 1,3-dimethylurea or propyleneurea and aqueous formaldehyde without a catalyst.  
         [0014]     Blocking agents for ureidoamine hardeners are either hydroxy acids such as 2,2-bis(hydroxymethyl)butyric acid which reacts with the ureidoamine to create an internal acidic acylurea blocking group C(O)NC(O)N or methylenebis(ethyleneurea) which forms a complex with an amine nitrogen atom due to the close proximity of the two carbonyl groups. The hydroxyacid reacts with the ureidoamine in about 15 minutes at 150 degrees C. while methylenebis(ethyleneurea) has only to be dissolved in the ureidoamine at about 50-60 degrees C. A hydroxyacid can also be combined directly with a ureidoamine provided the reaction can be avoided by keeping the temperature well below the reaction temperature of approximately 150 degrees C. The hydroxy acid which is in solution in the hardener mixture forms a complex with the available tertiary amine nitrogen atom. The complex does not catalyze the epoxy curing reactions below the activation temperature and the combination is latent. Methylenebis(ethyleneurea) is an excellent non-reactive solvent for a hydroxyacid such as 2,2-bis(hydroxymethyl)butyricacid and forms a resinous, low-melting complex at 1:1 molar ratio at about 50 degrees C. which is soluble in both the ureidoamine and the epoxy. All of these hardener components react rapidly with the epoxy at the cure temperature while providing excellent latency at ambient temperature.  
         [0015]     The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     Ethyleneurea(2-imidazolidone) is a relatively cheap heterocycle, m.p. 133-135 degrees C. and insoluble in epoxy resins below the melting point. It is a very weak base and reacts with epoxy at an impractically low rate. However, if a primary or secondary amine is combined with one mole ethyleneurea and one mole formaldehyde, a new compound is produced in which the nitrogen atom of the amine and the methylene bridge connecting this nitrogen atom to the ureido nitrogen atom combine to produce a chelate structure in which the amine nitrogen can donate electrons directly to the ureido oxygen atom. This nullifies the internal electron shift on the ureido nitrogen atoms toward the carbonyl group and activates the ureido hydrogen atoms so that they can react with epoxy. A wide variety of epoxy hardeners having useful properties can be produced by applying this principle.  
         [0017]     In addition to ethyleneurea, two other ureido compounds, propyleneurea(tetrahydro-2-pyrimidone), m.p. 264-266 degrees C. and 1,3-dimethylurea, m.p. 101-104 degrees C. are chemically equivalent. Ethyleneurea is the preferred parent compound.  
         [0018]     There are three main types of ureidoamine compounds: The first type consists of the product formed by reacting 1 mole of a primary amine RNH 2  with 2 moles formaldehyde and 2 moles ethyleneurea where R contains carbon atoms and possibly oxygen atoms but does not contain any amine nitrogen atoms. This type of hardener does not have any capacity to catalyze the epoxy-hydroxy crosslinking reactions and is an epoxy modifier and co-curative only. There are also some other hardeners which do not have precisely the same structure but which do not catalyze epoxy crosslinking reactions. The second type of ureidoamine hardener has the same overall structure but the R group contains a tertiary amine or imidazole. For example, R=dialkylaminopropyl- where the alkyl is either methyl or ethyl; or R=imidazolylpropyl-. This type of ureidoamine hardener has both chain extension and crosslinking capabilities. The third type of ureidoamine has the same initial structure as the second type but has been condensed with a hydroxyacid such as 2,2-bis(hydroxymethyl)propionicacid or 2,2-bis(hydroxymethyl)butyricacid or glycolic acid. This converts one of the terminal ureido groups to an acylurea C(O)NC(O)N which causes the tertiary amine or imidazole of the R group to bond internally to this acidic group, thereby rendering the entire molecule non-reactive at temperatures below the activation temperature.  
         [0000]     Hardeners Based on Aminomethylene-ethyleneureas.  
         [0019]     The useful chemical reactions are between ethyleneurea and either secondary monoamines, secondary diamines, primary-monoamines, mixed primary-secondary amines or primary diamines and formaldehyde. Any of these amines may contain additional tertiary amine groups, hydroxyl groups or ether groups.  
         [0020]     Reacting ethyleneurea with two moles of a secondary monoamine RR′NH and two moles formaldehyde produces a catalytic epoxy hardener having no active hydrogen atoms and four potentially active nitrogen atoms, designated by Structure I.  
                         
 
         [0021]     This material has no active hydrogens and is therefore an external plasticizer for the polymer system. This limits the concentration to relatively low values and this material is best used as an accelerator. If only one secondary monoamine RR′NH is used, the catalytic hardener is capable of reacting with the epoxy resin and an internal plasticizer is produced, designated by Structure II.  
                         
 
         [0022]     Higher concentrations are practical and a wide range of latency and cure rate characteristics can be obtained depending on the nature of the R and R′ groups.  
         [0023]     Preferably, for both Structures I and II, R and R′ are alkyl groups containing 1-12 carbon atoms, or R is an alkyl group containing 1-12 carbon atoms and R′ is benzyl, or R is methyl or ethyl and R′ is anilino-, or R is methyl and R′ is 2-hydroxyethyl-, or R and R′ are both 2-hydroxyethyl-, or R is methyl and R′ is 2-hydroxyisopropyl-, or R and R′ are both 2-hydroxyisopropyl-, or R and R′ are both 3-dimethylaminopropyl-; or RR′N— represents the imino radical bis(dimethylamino)methaneimino- (from tetramethylguanidine).  
         [0024]     Several heterocyclic groups are also useful, in which case the RR′N— notation may represent piperidyl-, 4-methylpiperidyl-, pyrrolidyl-, 1-methylpiperazinyl-, 1-imidazolyl-, 1-(2-methyl)imidazolyl-, 1-(2-ethyl)imidazolyl-, 1-(2-ethyl-4-methyl)imidazolyl-, or 1-benzimidazolyl-.  
         [0025]     Secondary monoamines RR′NH useful for making the Structures I and II are as follows: a monoamine in which R and R′ are any combination of alkyl groups containing one to twelve carbon atoms; a monoamine with R being any combination of an alkyl group containing from one to twelve carbon atoms and R′=benzyl; piperidine; 4-methylpiperidine; pyrrolidine; 1-methylpiperazine; 3,3′-iminobis(N,N-dimethylpropylamine; tetramethylguanidine; N-alkylaniline where the alkyl is methyl or ethyl; imidazole; 2-methylimidazole; 2-ethylimidazole; 2-ethyl-4-methylimidazole; benzimidazole; N-methylethanolamine; diethanolamine; N-methylisopropanolamine; diisopropanolamine.  
         [0026]     A secondary diamine such as piperazine can react with two moles formaldehyde and two moles ethyleneurea to produce Structure III. Piperazine is the only commercially available secondary diamine at a reasonable price.  
                         
 
         [0027]     The advantage of this structure over that of the original piperazine is that this material has good latency and is therefore useful as a modifier to alter the solvent power of the epoxy, increase viscosity and increase the hydroxyl group concentration. It is not a catalytic hardener.  
         [0028]     One or two moles of a primary monoamine RNH 2  can react with one or two moles formaldehyde and one mole ethyleneurea to produce a wide variety of secondary amines. The monosubstituted version is shown in Structure IVa. The disubstituted version is shown in Structure IVb.  
                         
 
         [0029]     Although this class of epoxy hardeners has considerable utility, latency can be improved by reacting Structure IVb with two additional moles ethyleneurea and two moles formaldehyde, producing Structure V.  
                         
 
         [0030]     A primary monoamine RNH2 can react with two moles formaldehyde and two moles ethyleneurea to produce the Structure VI.  
                         
 
         [0031]     If R does not contain any additional nitrogen atoms, the single central tertiary nitrogen atom is shared between the two carbonyl groups but is sterically hindered and the material is essentially devoid of catalytic activity. However it is capable of reacting with epoxy at either ambient temperature or elevated temperature depending on the basicity of the amine RN and can be used to modify the epoxy resin, either by adding it to the hardener package or by pre-reacting it with the epoxy. When the modified epoxy is subsequently cured, the increased hydroxyl concentration increases the cure rate markedly. The fastest cures are obtained by pre-reacting this difunctional modifier with the epoxy.  
         [0032]     If R contains a tertiary amine, the hardener molecule is both a chain extender and a catalytic hardener Examples of two primary/tertiary amines are N,N-dialkyl-1,3-propanediamine where alkyl is methyl or ethyl and 1-(3-aminopropyl)imidazole.  
         [0033]     Preferably, for Structures IV, V, and VI, R is an alkyl containing from 1 to 12 carbon atoms, allyl-, benzyl-, 2-hydroxyethyl-, 2-hydroxyisopropyl-, 3-hydroxy-1-propyl-, 3-ethoxypropyl-, 3-propoxypropyl-, 3-isopropoxypropyl-, 2-(2-hydroxyethoxy)ethyl-, 3-(dimethylamino)propyl-, 3-(diethylamino)propyl-, or 3-(1-imidazolyl)propyl-.  
         [0034]     For Structures V and VI, RN═ may be a heterocyclic group in which R represents cyclohexyl-; 3-(pyridyl)methyl-; 2-pyridyl-; 2,4-diethyl-N-anilino-; 2,6-diethyl-N-anilino-; or 2-pyrimidyl-.  
         [0035]     Primary monoamines useful in the preparation of Structures IV, V and VI are as follows: RNH2 is an alkyl monoamine containing from one to twelve carbon atoms in the alkyl group; allylamine; benzylamine; ethanolamine; isopropanolamine; 3-amino-1-propanol; 3-ethoxypropylamine; 3-propoxypropylamine; 3-isopropoxypropylamine; 2-(2-aminoethoxy)ethanol; 3-dimethylamino)propylamine, 3-(diethylamino)propylamine; 1-(3-aminopropyl)imidazole; cyclohexylamine; 3-(aminomethyl)pyridine; 2-aminopyridine; 2,4-diethylaniline; 2,6-diethylaniline; 2-aminopyrimidine.  
         [0036]     Oligomers of higher molecular weight based on Structure VI can be produced by increasing the amount of formaldehyde. For example, Structure VI contains 2 moles ethyleneurea, 2 moles formaldehyde and 1 mole of a primary monoamine. The dimer of VI would be formed from 4 moles ethyleneurea, 5 moles formaldehyde and 2 moles of a primary monoamine. as shown below as Structure VIa.  
                         
 
 Another possibility is 4 moles of ethyleneurea, 4 moles formaldehyde and 1 mole of a primary monoamine, resulting in the oligomer shown below as Structure VIb.  
                         
 
 Such higher molecular weight materials may be useful in improving cured properties and in providing extended latency with tertiary amine type catalytic hardeners. However, it should be expected that cure times will increase with the use of these higher molecular weight materials. 
 
         [0037]     A mixed primary-secondary amine HNR—X—NH2 can be made to react with two moles ethyleneurea and two moles formaldehyde, producing the Structure VII.  
                         
 
         [0038]     While this structure is superficially similar to Structures III and IV, the unreacted hydrogen atom can be employed to attach an additional amine-functional molecule such as imidazole near the center of the molecule which can result in a latent hardener due to the steric hindrance of the added amine group. The backbone of the molecule is also more flexible based on the choice of the connecting group X which can be used to increase flexibility and toughness of the cured polymer.  
         [0039]     For Structure VII, X is a connecting group which can be ethylene for example. R is hydroxyethyl or aminoethyl for example.  
         [0040]     Mixed primary-secondary amines which are useful in the preparation of Structure VII are N-(2-hydroxyethyl)ethylenediamine and diethylenetriamine. Although diethylenetriamine has five active hydrogens, the indicated structure has been successfully prepared using the procedure to be described subsequently. The unreacted amine hydrogens are not epoxy-reactive and can be used to attach side groups such as tertiary amines or imidazole.  
         [0041]     A primary diamine H 2 NXNH 2  can be reacted with two moles formaldehyde and two moles ethyleneurea provided the process is carefully controlled to prevent multiple substitution of the amine groups by formaldehyde. The result is shown in Structure VIII.  
                         
 
         [0042]     Because Structure VIII contains two unreacted amine hydrogen atoms, one or both of these can be used to attach additional amine groups HNRR′ or additional ethyleneurea groups to the molecule via the amine-formaldehyde reaction, forming Structures IX and X. In order to obtain good control over the structure of these molecules, structure IX is obtained from structure VIII by a sequential reaction rather than all at once.  
                         
 
         [0043]     For Structures VIII, IX, and X, connecting group X is for example alkyl, ethylene as in ethylenediamine, or polyoxypropylene as in poly(propyleneglycol)bis(2-aminopropylether), or hexamethylene or 2,2,4-trimethylhexamethylene. Cycloaliphatic and aromatic connecting groups are also possible. For example, X can also be cyclohexyl or benzene.  
         [0044]     Preferably, for Structure IX, R and R′ are alkyl groups containing 1-12 carbon; atoms, or R is an alkyl group containing 1-12 carbon atoms and R′ is benzyl, or R is methyl or ethyl and R′ is anilino-, or R is methyl and R′ is 2-hydroxyethyl-, or R and R′ are both 2-hydroxyethyl-, or R is methyl and R′ is 2-hydroxyisopropyl-, or R and R′ are both 2-hydroxyisopropyl- or R and R′ are both 3-dimethylaminopropyl- or RR′N— represents the imino radical bis(dimethylamino)methaneimino- (from tetramethylguanidine).  
         [0045]     Several heterocyclic groups are also useful, in which case the RR′N— notation may represent piperidyl-, 4-methylpiperidyl-, pyrrolidyl-, 1-methylpiperazinyl-, 1-imidazolyl-, 1-(2-methyl) imidazolyl-, 1-(2-ethyl)imidazolyl-, 1-(2-ethyl-4-methyl)imidazolyl-, or 1-benzimidazolyl-.  
         [0046]     Secondary monoamines RR′NH useful for making the Structure IX are as follows: a monoamine in which R and R′ are any combination of alkyl groups containing one to twelve carbon atoms; a monoamine with R being any combination of an alkyl group containing from one to twelve carbon atoms and R′=benzyl; piperidine; 4-methylpiperidine; pyrrolidine; 1-methylpiperazine; tetramethylguanidine; N-alkylaniline where the alkyl is methyl or ethyl; imidazole; 2-methylimidazole; 2-ethylimidazole; 2-ethyl-4-methylimidazole; benzimidazole; N-methylethanolamine; diethanolamine; N-methylisopropanolamine; diisopropanolamine; 3,3′-iminobis(N,N-dimethylpropylamine).  
         [0047]     Primary diamines useful in the preparation of Structures VIII and IX, as well as the tetra(ethyleneurea) product Structure X, are as follows: Ethylenediamine; 1,3-propanediamine; 1,3-diaminopentane; 2-methyl-1,5-pentanediamine; 1,6-diaminohexane; 2,2,4-trimethyl-1,6-hexanediamine; poly(propyleneglycol)bis(2-aminopropylether); 1,3-bis(aminomethyl)cyclohexane; bis(p-aminocyclohexyl)methane; isophoronediamine; m-xylylenediamine; 1,2-phenylenediamine; 1,4-phenylenediamine; 4,4′-methylenedianiline; 2,6-diaminopyridine.  
         [0000]     Formation of Latent Hardeners.  
         [0048]     The ureido NH groups of the aminomethylene-ethyleneureas react vigorously with anhydrides and will also condense with carboxylic acids when heated. This characteristic can be used to modify the structure and increase latency. The condensation reaction of Structure VI with either 2,2-bis(hydroxymethyl)propionicacid or 2,2-bis(hydroxymethyl)butyricacid or glycolic acid produces an acylurea group C(O)NC(O)N, which is acidic, shown in Structure XI. That is, the resulting derivative in Structure XI is an acyl derivative of Structure VI. Nitrogen atoms present in the R group will be bonded internally to this acidic group and will become unavailable to catalyze either the reaction of the remaining ureido NH group with epoxy or the epoxy-hydroxy reaction until the temperature reaches the activation temperature. All of the above hydroxy acids are very effective in extending latency and will condense with Structure V1 at about 150-180 degrees C. in 15 minutes, giving products which are amber, solid resins.  
                         
 
         [0049]     The R group is either dialkylaminopropyl- or imidazolylpropyl-. Latency periods of about one week and cure times of about three hours at 75 degrees C. can be obtained with dialkylaminopropylamine while aminopropylimidazole leads to about a one month latency period with a cure time of about one hour at 120 degrees C.  
         [0050]     The same method may be used to convert iminomethylene-ethyleneurea, Structure IVa, into a latent catalytic hardener. The R group is again either dialkylaminopropyl- or imidazolylpropyl-. Reacting the mono-iminomethylene-ethyleneurea with one mole 2,2-bis(hydroxymethyl)butyric acid converts the imino group into a tertiary amide which is weakly acidic and which provides a bonding site for the tertiary amino nitrogen atom in the R group, shown in Structure XII below.  
                         
 
         [0051]     A second approach to improving latency is to combine a hardener such as shown in Structure VI which contains a tertiary amine or imidazole in the R group with a blocking agent which will bond to this tertiary amine nitrogen atom at ambient temperature but release it at the cure temperature, which will preferably be a low temperature such as about 75 degrees C. This delicate task is performed well by the compound methylenebis(ethyleneurea) which has a structure which places the two carbonyl groups close together, forming an attractive bonding site for a nitrogen atom. While this material is not commercially available, urea-formaldehyde technology is well established and preparation is simple using 1 pph of a basic catalyst such as dimethylamine which need not be removed from the product. This material is mixed with the other hardener components at a ratio of one mole per nitrogen atom to be blocked. It is fully compatible and cures an epoxy in about 30 minutes at 75 degrees C. when base catalyzed as indicated. Latency periods are about 8 hours.  
         [0052]     A hydroxyacid can also be combined directly with a ureidoamine provided the condensation reaction can be avoided by keeping the mixing temperature well below the reaction temperature of approximately 150 degrees C. The hydroxy acid which is in solution in the hardener mixture forms a complex with the available tertiary amine nitrogen atom. The complex does not catalyze the epoxy curing reactions below the activation temperature and the combination is latent. Methylenebis(ethyleneurea) is an excellent non-reactive solvent for a hydroxyacid such as 2,2-bis(hydroxymethyl)butyricacid and forms a resinous, low-melting complex at 1:1 molar ratio at about 50 degrees C. which is soluble in both the ureidoamine and the epoxy. The use of this ureido compound greatly facilitates the mixing of the various hardener components at a low temperature. All of these hardener components react rapidly with the epoxy at the cure temperature while providing excellent latency at ambient temperature.  
         [0000]     Preparation of Ureidoamine Hardeners.  
         [0053]     The same process is used to prepare all of the ureidoamine compounds disclosed. While the process is a simple one, the temperature conditions are exacting and must be strictly adhered to. Bench scale preparation requires a reaction flask, condenser, stir bar, stirring hot plate, water/ice bath and electronic thermometer with a stainless steel probe. An electronic thermometer is required because it has a fast response and does not require a stem correction.  
         [0054]     The reaction involves an amine, water, formaldehyde and ethyleneurea. The order of addition is amine first, then water of dilution, then aqueous formaldehyde, then ethyleneurea.  
         [0055]     The amine is first weighed into the flask and distilled water is added, the weight of which is equal to the weight of ethyleneurea which is to be added at a later step. The purpose of adding water at this step is to allow the exotherm which occurs when the amine reacts with water to dissipate before adding aqueous formaldehyde, thereby minimizing the temperature rise during formaldehyde addition. The extra mass of water also helps to minimize the temperature rise. The stir bar is added and the flask is clamped into a cold bath of water and ice and stirred until the temperature has fallen to 5-10 degrees C. The required weight of formaldehyde is weighed into a beaker and slow addition to the flask is begun using a dropper while closely monitoring the temperature of the mixture. Formaldehyde addition is stopped before the temperature reaches 15 degrees C. and the temperature is allowed to fall back to 5-10 degrees C. before adding more formaldehyde.  
         [0056]     The temperature must not rise above 15 degrees C. during formaldehyde addition. The methylolamine which is formed at this stage associates through the hydroxy groups which protects the unreacted amine hydrogens until the temperature reaches about 30-35 degrees C. at which point the complex dissociates, the methylols condense with amine and the process fails.  
         [0057]     When the formaldehyde addition has been completed, the beaker is rinsed with a little distilled water and added to the flask. The required amount of ethyleneurea is now weighed out, the stirrer speed increased to medium high and the ethyleneurea added to the flask all at once through a plastic funnel. The temperature will immediately fall to about 0-5 degrees C. as the ethyleneurea dissolves. The ice should now be removed from the cold bath, the water level in the bath brought up to the level of the contents of the flask, the condenser attached and heating begun at a rate which brings the temperature up to 90 degrees C. in about half an hour. All of the ethyleneurea will be in solution by the time the bath temperature reaches about 35-40 degrees C. The mixture should be heated for 2 hours at 90 degrees C. at the end of which it can be poured into an evaporating dish and dried to constant weight at a liquid temperature of 60-70 degrees C. The products of this reaction are typically colorless to pale amber viscous liquids or amorphous semi-solids of little or no odor. They do not carbonate in air and are soluble in epoxy resins.  
       EXAMPLES  
       [0058]     In the examples given below, the notation A/B denotes a chemical reaction between the two chemical compounds A and B, using formaldehyde as appropriate. The epoxy resin was Epon828 in all cases.  
       Example 1  
     2EU/DEAPA  
       [0059]     This hardener was prepared by reacting 2 moles ethyleneurea with one mole DEAPA [3-(diethylamino)propylamine] and 2 moles formaldehyde according to the procedure previously described and is an example of Structure VI. It is both a chain extender and a catalytic hardener. The physical form of this material was a highly viscous resin at ambient temperature. The recommended minimum concentration in Epon828 is 7.5 phr. 20 phr of this hardener (a large excess) was blended with Epon828 and maintained at ambient temperature, resulting in a three-fold viscosity increase after 4 hours. The cure time was 1 hour at 75 degrees C.  
       Example 2  
     2EU/EDA  
       [0060]     Two moles ethyleneurea and 1 mole ethylenediamine were reacted with 2 moles formaldehyde according to the standard procedure giving a material shown in Structure VIII. This hardener is capable of chain extension only, having no tertiary amine group and is best described as an epoxy modifier. This material was then blended with Epon828 at a concentration of 10 phr and the mixture heated at 1,00 degrees C. for 1.5 hours, giving a product which was a medium viscosity liquid hot and highly viscous at ambient temperature. After one month at ambient temperature with occasional reheats to temperatures as high as 130 degrees C., the modified epoxy was a tacky, flexible semi-solid at ambient temperature. This system has a tendency to increase viscosity slowly over extended periods of time but does not appear to crosslink and remains fusible.  
       Example 3  
     2EU/API Reacted with DMBA  
       [0061]     Two moles ethyleneurea were reacted with 1 mole 1-(3-aminopropyl)imidazole (API) and 2 moles formaldehyde giving a material as shown in Structure VI. A test sample was then prepared using 7.0 phr of this hardener in Epon828. The pot life at ambient temperature was 4 hours. The cure time was 1.5 hours at 80 degrees C.  
         [0062]     A second sample was then prepared using 6.0 phr of this hardener but adding one mole (2.77 phr) DMBA [2,2-bis(hydroxymethyl)butyricacid] per mole 2EU/API and heating for 15 minutes at 100 degrees C. to fully dissolve the acid in the ureidoamine. 4.0 phr 2EU/EDA was then added as a solubilizer. This sample was cured in 1 hour at 120 degrees C.  
         [0063]     The viscosity increased by 2.5 times during the first 2 days at ambient temperature. This was due to the slow reaction of the non-catalytic epoxy modifier 2EU/EDA with the epoxy and is a “conditioning period”. The viscosity of the epoxy-hardener mixture at the end of 23 additional days at ambient temperature was 3.6 times the value at 4 days after mixing. The mixture was still viscous and flowable after 30 days at ambient temperature.  
       Example 4  
     2EU/API Blocked with MBEU  
       [0064]     This sample contained 15 phr 2EU/API and 8.6 phr MBEU [methylenebis(ethyleneurea)] as blocker for the R-group imidazole as shown in Structure VI. The cure time was 35 minutes at 75 degrees C. After 12 hours at ambient temperature, the viscosity of the epoxy-hardener mixture increased by 1.7 times.  
       Example 5  
     2EU/DEAPA Blocked with DMBA.MBEU  
       [0065]     3.4 phr DMBA [2,2-bis(hydroxymethyl)butyricacid] and 4.22 phr MBEU [methylenebis(ethyleneurea)] were heated at about 50 degrees C. for a brief period; the DMBA dissolved rapidly resulting in a clear solution which was a tacky, resinous, low-melting solid at ambient temperature. 7.5 phr 2EU/DEAPA was then added and blended briefly at about 50 degrees C. followed by blending with the epoxy. This mixture cured in 2¼ hours at 75 degrees C. The viscosity of the epoxy-hardener mixture increased by a factor of 3 after 3 days at ambient temperature.