Polyether Diamide Epoxy Additives

1. Field of the Invention

This invention relates to curable epoxy resins having increased adhesive strength; and, more particularly, to amine cured epoxy resins containing certain polyether diamide additives.

2. Prior Art

Epoxy resins constitute a broad class of polymeric materials having a wide range of physical characteristics. The resins are characterized by epoxide groups which are cured by reaction with certain catalysts or curing agents to provide cured epoxy resin compositions with certain desirable properties. One such class of curing agents are generally the amines. The most commonly used amine curing agents are aliphatic amines such as diethylenetriamine, triethylenetetramine and the like and/or polyoxyalkylene polyamine; such as polyoxypropylenediamines and triamines.

Epoxy resin compositions having improved physical properties are obtained by employing polyoxyalkylenepolyamines, and polyoxyalkylenediamines in particular, as curing agents. It is common to employ with such epoxy resin compositions a co-curing agent such as those described in U.S. Pat. No. 3,549,592.

It is known to use polyamides as epoxy curing agents. Simple amides such as acetamide, benzamide and adipamide have been used, but low activity and/or solubility require use of basic catalysts. The advantages and disadvantages of polyamides as curing agents is discussed in Lee, Henry and Neville, K. Handbook of Epoxy Resins, McGraw Hill Book Co., New York, 1967, but, generally, the hydrogen of the primary or secondary amides are weakly reactive with epoxy groups.

It has now been found that a specific diamide terminated polyoxyalkylene material having a molecular weight of from about 2000 to about 3000, when employed as an additive to amine cured epoxy resins provides cured compositions exhibiting outstanding strength of adhesion. Specifically, epoxy resins incorporating these additives, upon curing with an amine, provide a material with high tensile shear strength and superior adhesion to substrates.

The results of incorporating the additives of the instant invention are unexpectedly superior over similar type compounds of lower molecular weight. The cured epoxy resin compositions of the instant invention are useful as coatings, castings, sealants and especially adhesives.

According to the broad aspect of the instant invention, the adhesive strength of amine-cured epoxy resins is enhanced by the addition of an effective amount of a polyether having terminal amido groups and having a molecular weight of from 2000 to about 3000.

In one aspect, a curable epoxy resin composition having superior adhesive strength comprises a vicinal polyepoxide, a curing amount of an amine curing agent, an accelerator and an effective amount of the polyether diamide additive.

In accordance with a preferred embodiment, a diglycidyl ether of 4,4'-isopropylidene bisphenol, a curing amount of a primary amine-containing curing agent consisting essentially of a polyoxypropylene polyamine having a molecular weight of from about 200 to 500, a piperazine-alkanol amine accelerator and an effective amount of a polyether diamide having terminal amido groups and a molecular weight of about 2000 are utilized to form a resin.

According to the instant invention concept, blends of a polyepoxide, an amine curing agent, the diamido terminated polyether containing compounds, and, optionally, an accelerator are thoroughly admixed and cured in accordance with conventional methods to provide cured epoxy resins having unexpectedly superior adhesive strength.

Generally the vicinal polyepoxide containing compositions which are amine cured are organic materials having an average of at least 1.8 reactive 1,2-epoxy groups per molecular. These polyepoxide materials can be monomeric or polymeric, saturated or unsaturated, aliphatic, cycloaliphatic, aromatic or heterocyclic, and may be substituted if desired with other substituents besides the epoxy groups, e.g., hydroxyl groups, ether radicals, aromatic halogen atoms and the like.

Preferred polyepoxides are those of glycidyl ethers prepared by epoxidizing the corresponding allyl ethers or reacting, by known procedures, a molar excess of epichlorohydrin and an aromatic polyhydroxy compound, i.e., isopropylidene bisphenol, novolac, resorcinol, etc. The epoxy derivatives of methylene or isopropylidene bisphenols are especially preferred.

A widely used class of polyepoxides which are useful according to the present invention includes the resinous epoxy polyethers obtained by reacting an epihalohydrin, such as epichlorohydrin, and the like, with either a polyhydric phenol or a polyhydric alcohol. An illustrative, but by no means exhaustive, listing of suitable dihydric phenols includes 4,4'-isopropylene bisphenol, 2,4'-dihydroxydiphenylethylmethane, 3,3'-dihydroxydiphenyldiethylmethane, 3,4'-dihydroxydiphenylmethylpropylmethane, 2,3'-dihydroxydiphenylethylphenylmethane, 4,4'-dihydroxydiphenylpropylphenylmethane, 4,4'-dihydroxydiphenylbutylphenylmethane, 2,2'-dihydroxydiphenylditolylmethane, 4,4'-dihydroxydiphenyltolylmethylmethane and the like. Other polyhydric phenols which may also be co-reacted with an epihalohydrin to provide these epoxy polyethers are such compounds are resorcinol, hydroquinone, substituted hydroquinones, e.g., methylhydroquinone, and the like.

Among the polyhydric alcohols which can be co-reacted with an epihalohydrin to provide these resinous epoxy polyethers are such compounds as ethylene glycol, propylene glycols, butylene glycols, pentane diols, bis (4-hydroxycyclohexyl)dimethylmethane, 1,4-dimethylolbenzene, glycerol, 1,2,6-hexanetriol, trimethylolpropane, mannitol, sorbitol, erythritol, pentaerythritol, their dimers, trimers and higher polymers, e.g., polyethylene glycols, polypropylene glycols, triglycerol, dipentaerythritol and the like, polyallyl alcohol, polyhydric thioethers, such as 2,2'-, 3,3'-tetrahydroxydipropylsulfide and the like, mercapto alcohols such as monothioglycerol, dithioglycerol, and the like, polyhydric alcohol, partial esters, such as monostearin, pentaerythritol monoacetate, and the like, and halogenated polyhydric alcohols such as the monochlorohydrins of glycerol, sorbitol, pentaerythritol and the like.

Another class of polymeric polyepoxides which can be amine cured and are in accordance with the instant invention includes the epoxy novolac resins obtained by reacting, preferably in the presence of a basic catalyst, e.g., sodium or potassium hydroxide, an epihalohydrin, such as epichlorohydrin, with the resinous condensate of an aldehyde, e.g., formaldehyde, and either a monohydric phenol, e.g., phenol itself, or a polyhydric phenol. Further details concerning the nature and preparation of these epoxy novolak resins can be obtained in Lee, H. and Neville, K., Handbook of Epoxy Resins, McGraw Hill Book Co., New York, 1967.

It will be appreciated by those skilled in the art that the polyepoxide compositions which are useful according to the practice of the present invention are not limited to those containing the above described polyepoxides, but that these polyepoxides are to be considered merely as being representative of the class of polyepoxides as a whole.

The amine curing agents which can be utilized in accordance with the instant invention are generally any of those amine curing agents which are well known to be useful for the curing of vicinal epoxides. Generally, those curing agents having at least two reactive amine groups are useful.

Exemplary of those amines which can be utilized are alkylene polyamines such as diethylene triamine, triethylene tetramine and the like; oxyalkylene polyamines such as polyoxypropylene, di- and triamine and diamino derivatives of ethylene glycol, such as 1,13-diamino 4,7,10-trioxatridecane.

Additionally, aromatic amine curing agents are useful, such as the alkylene-linked polyphenyl amines, phenylene diamines and polycyclic or fused aromatic primary amine compounds. Further, the corresponding cycloaliphatic compounds are useful.

Likewise, the polyamide curing agents such as the condensation products of polyamines and polycarboxylic acids are useful. Suitable such amide compounds are, for example, the condensation product of a polyamine and a dimerized fatty acid produced in accordance with U.S. Pat. No. 2,379,413.

Of the amine curing agents known to be effective in curing a vicinal epoxy resin, preferred curing agents in accordance with the instant invention are the polyoxyalkylene containing amine compounds. A preferred class of polyoxyalkylene polyamines is depicted by the formula: ##STR1## wherein X is hydrogen, a methyl radical or an ethyl radical; Z is a hydrocarbon radical having 2 to 5 carbon atoms forming from 2 to 4 external ether linkages; n is a number from 1 to about 15 and r is a number from 2 to 4. The most preferred polyoxyalkylene polyamines are the polyoxypropyl diamines wherein X is a methyl radical, n is a number from 1 to 10, Z is a 1,2-propylene radical and r is about 2. These polyoxyalkylene polyamines can be prepared by known methods as disclosed in U.S. Pat. Nos. 3,236,895 and 3,654,370. The most preferred polyoxyalkylene polyamine is a polyoxypropylene diamine having a molecular weight of about 230.

Another preferred class of polyoxyalkylene polyamines can be depicted by the formula: ##STR2## wherein X, Z, n and r are defined as above and y is 2 or 3. These poly(aminoalkylamino)polyethers are the hydrogenated product of the cyanoalkylated adduct of a polyoxyalkylene polyamine as above described. The preparation of the cyanoalkylated adducts is described in U.S. Pat. No. 3,666,788 issued to Rowton May 30, 1972. Preferred such compounds are the hydrogenated cyanoethylated polyoxypropylene triamines.

The polyether diamide additive can generally be described as polyoxyalkylene containing materials having terminal amido groups and a molecular weight of from about 2000 to about 3000. More specifically, these compounds are polyoxyalkylene compounds having terminal amido groups, of the formula: ##STR3## wherein R is hydrogen; or a branched or straight chain alkyl radical of from 1 to about 10 carbon atoms, and more preferably from 1 to about 6; or a monocyclic aryl, alkaryl or aralkyl having from 6 to about 12 carbon atoms, and more preferably 6 to about 8; or, a branched or straight chain alkenyl or alkadienyl radical of from 2 to about 10 carbon atoms and more preferably 3 to about 8; X is hydrogen, a methyl radical or an ethyl radical; Z is a hydrocarbon alkylene radical having 2 to 5 carbon atoms and n is a number selected such that the molecule of the above formula has a molecular weight of from about 2000 to about 3000. The preferred diamides are of the above formula wherein R is hydrogen or a lower alkyl; and more preferably a lower alkyl of from 1 to 4 carbon atoms; X is a methyl radical; Z is a 1,2-propylene radical; and n is a number from 16 to 19. Preferred lower alkyl groups include methyl, ethyl, n-propyl and n-butyl.

The polyether diamide compounds are formed by the reaction of an amido forming compound with a polyoxyalkylene diamine having a molecular weight of from about 2000 to about 3000 at temperatures in the range from about room temperature to about 200.degree. C in a molar ratio of about 2 moles of amido forming compound for each mole of diamine. There are many known methods for forming each compounds by acylation of the amine reactant.

The diamines that are useful in forming the additives are polyoxyalkylene diamines of the formula: ##STR4## wherein X is a hydrogen, a methyl radical or an ethyl radical; Z is a hydrocarbon alkylene radical having from 2 to 5 carbon atoms; and, n is a number from 15 to about 25. Preferred polyoxypropylene diamines wherein X is a methyl radical, n is a number from 16 to 19, Z is a 1,2-propylene radical. These polyoxyalkylene polyamines can be prepared by known methods as disclosed in U.S. Pat. Nos. 3,236,895 and 3,654,370. It will be realized that the value of n is an average and not an integer.

The amide forming compounds are generally those which supply the acyl ##STR5## radical, wherein R is defined as above. Suitable such compounds include the monocarboxylic acids, the acid chlorides, the esters and the like. Acylation reactions that can be utilized are well known and will not be further herein discussed.

In accordance with these known methods, the reactants are simply admixed in correct molar ratios in a suitable reaction vessel and heated until the reaction occurs.

The functionality of the polyoxyalkylenepolyamine is dependent upon the number of terminal primary amino groups, which, in the instant case, is 2. It will be realized that each mole of amido forming compound will react with a single terminal primary amino group of the polyoxyalkylenepolyamine. It is particularly important that, in forming the additive compounds of the instant invention, a specific molar ratio of reactants be maintained. Specifically, about 1 mole of amido forming compound for each amino group of the polyoxyalkylenepolyamine is required. Thus, with the diamine, about 2 moles of amido forming compound is utilized. Preferably the instant reaction is carried out in the presence of a slight excess of amido forming compound to assure complete conversion of the amino groups.

Optionally, the epoxy resin formulations of the instant invention can include an "accelerator" to speed the amine cure of the epoxy resin, especially at ambient temperatures. In several applications, such acceleration is beneficial, especially when an epoxy resin is used as an adhesive in flammable environment, thus making elevated temperature cure improvement or even hazardous. Lee, H. and Neville, K., Handbook of Epoxy Resins, pp. 7-14 describes the use of certain amine-containing compounds as epoxy curing agent-accelerators.

Many accelerators are known in the art which can be utilized in accordance with the instant invention. Examples include salts of phenols; salicyclic acids; amine salts of fatty acids such as those described in U.S. Pat. No. 2,681,901; and, tertiary amines such as those disclosed in U.S. Pat. No. 2,839,480. A preferred accelerator in accordance with the instant invention is disclosed in U.S. Pat. No. 3,875,072 issued to G. Waddill, Apr. 1, 1975. The accelerator comprises a combination of piperazine and an alkanol amine in a weight ratio of about 1:8 to 1:1.

According to the method of the instant invention, the adhesion properties of prior art amine-cured epoxy resins are enhanced by the addition of an effective amount of a polyether diamide having terminal amido groups and a molecular weight of from about 2000 to about 3000 as hereinbefore described. The amount of additive effective is bringing about the increased adhesive property is somewhat empirical and will depend upon the resin, the amine curing agent, and the use of an accelerator. Generally, the amide additive can be utilized in amounts from about 5 to about 50 parts by weight based on 100 parts by weight of the resin constituent.

Although somewhat empirical, the exact amount of additive used to increase adhesion can readily be determined without undue experimentation owing to the fact that a resin mixture containing an effective amount of the additive will undergo changes which are readily visible as curing proceeds. Specifically, the curing resin takes on an opaque, milky white appearance that becomes more pronounced during curing and results in a product which has a lustrous, white appearance. It will be realized that, advantageously, this optical absorption shift enhances the beauty of cast objects and negates the need to use white pigments or fillers.

Preferably the adhesive properties of prior art amine cured resins are enhanced by addition of an effective amount of the polyoxypropylene diamide additive based upon the condensation of 2 moles formic acid with about 1 mole of a polyoxypropylenediamine having molecular weight of about 2000. The preferred resins comprise polyglycidyl ethers of polyhydric phenols cured by incorporating therein a curing amount of a polyoxyalkylenepolyamine of molecular weight from about 200 to 500 and an accelerator combination of piperazine and an alkanolamine, the combination having a combined weight ratio of between about 1:8 to 1:1. Exemplary preferred prior art compositions which can be enhanced in accordance with the instant invention are disclosed in U.S. Pat. No. 3,943,104 issued Mar. 9, 1976 to G. Waddill.

The curable epoxy resin composition of the instant invention generally comprise a vicinal polyepoxide, a curing amount of an amine curing agent and an effective amount of the polyether diamide additive. Optionally an accelerator can be added.

The amine cured resins having superior adhesion in accordance with the instant invention are prepared in a conventional manner. The amine curing agent is admixed with the polyepoxide composition in amounts according to the amine equivalent weight of the curing agent employed. Generally the number of equivalents of amine groups is from about 0.8 to about 1.2 times the number of epoxide equivalents present in the curable epoxy resin composition, with a stoichiometric amount being preferred. When using an accelerator, amounts from 1 to about 10 parts by weight based on 100 parts by weight of the resin are generally satisfactory. The exact amount of constituents in accordance with the above general requirements will depend primarily on the application for which the cured resin is intended.

The diamide additive is incorporated into the uncured resin by admixing. Preferably, the additive is first admixed with the curing agent and/or the accelerator prior to addition to the resin. The constituents forming the curable material are then intimately admixed by standard methods and degassed in the presence of a commercial defoamer and minute amounts of silicon oils to prevent voids and bubbles.

Although all of the epoxy resins disclosed herein are generally useful in accordance with the instant inventive concept, those based on aliphatic compounds are preferably not used exclusively. The presence of resins containing polyglycidyl ethers of polyhydric phenols in amounts greater than 50% by weight of the resin constituent, and more preferably 80% by weight and more preferably 100% by weight has been shown to greatly enhance the desirable properties of the cured material, especially the adhesive strength.

Likewise, although all of the disclosed amine curing agents are generally useful to cure the resin, it has been found that those resins containing curing agents wherein the amino moieties are separated by large aliphatic or oxyalkylene chains do not demonstrate a substantial increase in adhesive strength in the presence of the diamide terminated additives. The exact reason for this is not fully understood but is believed related to compatibility of constituents. For example, resins cured with polyoxypropylenediamine having molecular weights of above about 400, do not demonstrate greatly enhanced adhesive strengths in the presence of otherwise effective amounts of the additives. Examples of such agents include polyoxypropylenediamines having a molecular weight in the range of 200 to 300 and polyoxypropylenepolyamines having a molecular weight of from about 400 to 600.

It will be realized that those amine curing agents which are less effective in providing the adhesively superior resins can readily be avoided by the skilled artisan without undue experimentation, since such agents do not effectuate the "optical shift".

In accordance with a preferred embodiment, a curable resin comprises a diglycidyl ether of 4,4'-isopropylidene bisphenol; a curing amount of a primary amine-containing curing agent consisting essentially of a polyoxypropylene diamine having a molecular weight of from about 200 to 250, an accelerator of piperazine and an triethanolamine in a weight ratio of 3 to 7; and, an effective amount of a polyether diamide having terminal amido groups and a molecular weight of about 2000.

According to a greatly preferred embodiment, stoichiometric amounts of the resin and curing/accelerator composition are utilized. This amount is calculated by adding together the number of equivalents on the basis of weight per replaceable N-H group in the sum of the polyoxyalkylene polyamine, piperazine and alkanolamine components utilized. More preferably, based on the above calculations, the curing agent/accelerator components are utilized in amounts which provide up to about a 10% excess of the stoichiometric amount required based on the resin.

A preferred ratio of constituents comprises from about 1 to about 5 parts by weight, per one hundred parts by weight of the resin, of a piperazine-alkanolamine accelerator in a weight ratio of between about 1:8 and about 1:1 piperazine to alkanolamine. The above amount of accelerator is admixed with a polyoxyalkylenediamine curing agent in amounts of from about 10 to 50 parts by weight accelerator to 100 parts by weight of the polyoxyalkylene polyamine curing agent.

Generally, the mixture of epoxy resin, the polyether diamide, polyoxyalkylenepropylene polyamine, and the accelerator combination of piperazine and alkanolamine is allowed to self-cure at ambient temperatures of between 0.degree. C. to about 45.degree. C. However, it has been found expeditious that the mixture be cured or post-cured at elevated temperatures of up to about 135.degree. C.

According to a greatly preferred embodiment, resins of the polyglycidyl ether of polyhydric phenol type are cured by incorporating therein about a stoichiometric amount of a polyoxyalkylenepolyamine having a molecular weight of about 230; from about 5 to 40 parts by weight of the polyether diamide terminated polyoxyalkylenepolyamine having a molecular weight of about 2000; and from 1 to 5 weight percent based on 100 parts by weight of the resin of an accelerator comprising a 3:7 weight percent admixture of piperazine:triethanolamine. The composition is cured at room temperature (about 25.degree. C.) to produce products having superior adhesive strength in accordance with the instant invention.

It will further be realized that various conveniently employed additives can be admixed with the polyepoxide containing composition of the instant invention prior to final cure. For example, in certain instances it may be desired to add minor amounts of other polyalkyleneamine co-catalysts as herein described, or hardeners along with various other accelerators and curing agent systems well known in the art.

Additionally, conventional pigments, dyes, fillers, flame retarding agents and the like which are compatible, natural or synthetic resins can be added.

Furthermore, although not preferred, known solvents for polyepoxide materials such as toluene, benzene, xylene, dioxane, ethylene glycol monomethylether and the like can be used. The polyepoxide resins containing the additives of the instant invention can be used in any of the above applications for which polyepoxides are customarily used. One outstanding feature of the instant composition resides in the fact that they are opaque upon curing and give a smooth, white, lustrous surface which may be of particular benefit for certain molding and casting operations. The compositions of the instant invention can be used as impregnants, surface coatings, pottings, capsulating compositions, laminants, and, particularly and most importantly, as adhesives for bonding metallic elements or structures permanently together.

The following examples illustrate the nature of the instant invention but are not intended to be limitative thereof.

In this example a polyether diamide terminated additive for use in accordance with the instant invention, was prepared. Into a suitable reaction vessel, equipped with stirring apparatus, thermometer, reflux condenser, and Dean Stark trap were added 971 grams (0.5 mole) of a polyoxypropylenepolyamine having a molecular weight of approximately 2000, and an analysis of 1.03 milliequivalents (meq.) primary amine/g sold under the tradename "JEFFAMINE.RTM. D-2000" by Jefferson Chemical Co., Austin, Tex. 78751, 76.5 g (1.5 moles) 90% by weight aqueous formic acid, and 200 ml toluene flushed with nitrogen and stirred under a nitrogen pad for 2 hours at reflux. An aqueous phase was separated in the Dean Stark trap. The crude reaction residue was then stripped in a rotary evaporator at 97.degree. C./0.4 mm Hg to produce a viscous residue which upon analysis showed 1.64% N, 0.07 meq. total amine/g.

To illustrate the advantage of the polyether amide additives of this invention, various epoxy formulations employing diglycidyl ether of 4,4'-isopropylidene bisphenol were cured with various known polyamine curing agents. Where indicated a commercial accelerator was utilized. Three drops of silicone fluid were added to each formulation to prevent formation of voids and bubbles. After degassing under vacuum, the formulations were cured under the conditions indicated. In appropriate examples, the cured products were subjected to standard American Society for Testing Material (ASTM) tests for Izod impact strength (ASTM designation D-256), flexural strength and modulus of elasticity in flexure (ASTM designation D-790-66), tensile strength and elongation at break (ASTM designation D-638-64 T), deflection temperature (ASTM designation D-748-56) and hardness (ASTM designation 2240-64 T) and/or hardness Shore D, and peel strength (ASTM D-903). The tensile shear strength (ASTM D-1002-64) was measured on adhesive bonds. All substrates were aluminum panels (No. 2024-T-3 alloy, 16 gage), degreased, then chromic acid etched prior to bonding. The abbreviations in the tables, pbw, psi and g. stand for parts by weight, pounds per square inch and grams, respectively.

In these examples epoxy resins were prepared wherein diglycidyl ether of 4,4'-isopropylidene bisphenol was cured with a polyoxypropylenediamine curing agent of m.w. 230 having an equivalent weight of 58 to which were added the indicated amounts of the diamide prepared in Example I. The resulting resins were used to bond aluminim to aluminum and the resultant subjected to the ASTM tests herein described. The data, which are for comparative purposes only, is presented in the following Table I.

This example demonstrates the improved adhesion strength of the epoxy formulation when amounts of the bisamide are added to an epoxy formulation cured with a polyoxypropylenediamine of m.w. 230.

In this example, a polyether bis(benzamide) additive was prepared for use in accordance with the instant invention. Using the equipment and procedures of Example I, 1330 g (.696 mols) of a polyoxypropylenepolyamine having a molecular weight of approximately 2000, sold under the tradename "JEFFAMINE D-2000" by Jefferson Chemical Company, Austin, Tex. 78765, 170 g benzoic acid (1.393 moles) and 50 ml benzene were charged to a suitable reaction vessel. The resultant admixture was flushed with nitrogen and stirred under a nitrogen pad at reflux (156.degree.-240.degree. C.) with continuous water removal (85% of theoretical). A vacuum was slowly applied over about a 1 hour period to facilitate the removal of the remainder of the water. The admixture was then stirred under vacuum (185.degree. C./30 mm Hg) for an additional hour. Upon cooling, the light brown, viscous liquid reaction product was shown to consist substantially of the bis(benzamide) material.

This example, shows a substituted amide used as an additive in accordance with the instant invention. Using the bis(benzamide) prepared in Example VI as the additive, an anhydride cured formulation was prepared as shown in Table II.

The opaque appearance of the casting after cure indicates the presence of the improved thermal shock properties.

While the invention has been explained in relation to its preferred embodiment, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification and is intended to cover such modifications as fall within the scope of the appended claims.