Urea-superacid salts and their use as a curing catalyst of epoxy resins

Abstract: As a curing catalyst of epoxy resins, superacid salts of an N, N'-di- or N, N', N'-trisubstituted urea are disclosed. The urea component is produced by reacting an organic isocyanate and a primary or secondary amine at least one of which is monofunctional. The catalyst is soluble in conventional organic solvents, well compatible with the epoxy resin and catalizes the cationic polymerization reaction of epoxy resins at room temperature.

Description: BACKGROUND OF THE INVENTION Epoxy resins are known to be resistant to heat and chemicals and to have excellent adhesive, electrical, mechanical and other properties and, therefore, have extensively been used in various fields as adhesives, coatings, sealants, insulating materials, casting and molding materials. Two systems are known to harden or cure the epoxy resin; one using a polyamine or polycarboxylic acid or anhydride hardener and the other being a self-polymerization system containing a cationic or anionic polymerization initiator generally referred to as "curing catalyst". A variety of curing catalysts are known including Bronsted acids, Lewis acids, tertiary amine-superacid salts. These known curing catalysts have certain defects in that they are solid or not soluble in conventional solvents such as toluene or xylene or not fully compatible with the epoxy resin so that they may be uniformly dispersed in the epoxy resin only with difficulty. This may result in incomplete or localized curing of the resin. Furthermore, the prior art curing catalysts are generally capable of curing alicyclic epoxy compounds or resins at room temperature but not non-alicyclic epoxy compounds or resins which are less reactive than the alicyclic epoxy resins. US-A-3,328,444 discloses hexafluoro arsenic and phosphoric acid adducts of urea compounds which are useful as parasiticides and anthelmintics. US-A-4,761,441 discloses acid-curable resin compositions which comprise a mixture of a furan resin, a minor amount of an epoxy resin and an acid catalyst. The latter is an aromatic sulphonic acid or a mixture comprising an aromatic sulphonic acid and the urea salt of said aromatic sulphonic acid. US-A-3,294,749 discloses the use of a substituted urea to accelerate the curing reaction of polyepoxides with polycarboxylic acid anhydrides. DE-A-1 907 243 discloses the use of hexafluoro antimonate-amine-catalysts in the cationic polymerisation of epoxy resins. DE-A-2 507 009 discloses the use of N-acylamine hexafluoroantimonates in the cationic polymerisation of alkylene oxides. A need exists, therefore, for a curing catalyst of epoxy resins which is soluble in conventional organic solvents such as toluene and xylene, highly compatible with epoxy resins and capable of curing of non-alicyclic epoxy compounds or resins at room temperature. Also, a need exists for a resinous composition containing a polyfunctional epoxy compound or resin and such a hardening catalyst. SUMMARY OF THE INVENTION In accordance with the present invention, the above and other needs may be met by using a superacid salt of an N,N'-di- or N,N,N'-trisubstituted urea produced by reacting an isocyanate and a primary or secondary amine, at least one of said isocyanate and said amine being monofunctional, as an initiator of the cationic polymerization of epoxy resins or compounds. The present invention also provides a resinous composition comprising an organic epoxy compound or resin having a plurality of epoxide groups in the molecule and a catalytically effective amount of said urea-superacid salt. DETAILED DESCRIPTION N,N'-di- or N,N,N'-trisubstituted Ureas The reaction of an isocyanate and a primary or secondary amine is well-known in the organic chemistry as a synthesis of di- or trisubstituted ureas. The substituted ureas used in the present invention may easily be synthesized using this well-known method. A typical example of the synthesis includes the reaction of a primary or secondary monoamine and a monoisocyanate to produce the corresponding N, N'-di- or N,N,N'-trisubstituted urea. The present invention includes not only the superacid salt of such monourea but also the salt with a polyurea produced by reacting a polyamine with a monoisocyanate and vice versa. In other words, at least one of the amine and isocyanate reactants must be monofunctional. The reaction of a polyisocyanate and a polyamine results in, as is well-known, a polyurea elastomer or polymer and is, therefore, excluded from the urea component of the present invention. Examples of primary and secondary monoamines usable in the synthesis of the above substituted ureas include methylamine, dimethylamine, ethylamine, diethylamine, propylamine, dipropylamine, butylamine, dibutylamine, amylamine, diamylamine, hexylamine, dihexylamine, 2-ethylhexylamine, di-2-ethylhexylamine, octylamine, dioctylamine, laurylamine, dilaurylamine, stearylamine, distearaylamine, benzylamine, allylamine, aniline, o-, m-and p-toluidine, naphthylamine, mono- and diethanolamine and the like. Hybrid secondary monoamines of the above-named primary amines such as benzylmethylamine may also be used. Examples of monoisocyanates include those derived from the above-named primary monoamines by reacting with phosgene. The reaction between the monoamine and the mono-isocyanate gives the corresponding N,N'-di- or N,N,N'-trisubstituted urea. In contrast with this, when one of the amine and isocyanate reactants is polyfunctional, a compound having a plurality of urea linkages will result. A variety of polyfunctional isocyanates are known in the polyurethane and polyurea chemistry and include hexamethylenediisocyanate, tolylenediisocyanate, 4, 4'-diphenylmethanediisocyanate, xylylenediisocyanate, lysin diisocyanate, 4, 4'-methylenebis(cyclohexylisocyanate), methylcyclohexane-2, 4-(or 2, 6-) diisocyanate, 1, 3-bis (isocyanatomethyl)-cyclohexane, isophoronediisocyanate, trimethylhexamethylenediisocyanate, dimer acid diisocyanate, trimers of hexamethylenediisocyanate (biuret or isocyanurate) and the like. Examples of polyfunctional primary and secondary amines usable for the same purpose include ethylene-diamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine and the like. Also included in this class are polyfunctional primary amines corresponding to the above-exemplified organic polyisocyanates. In other words, the very same substituted urea compounds will result starting from a polyfunctional primary amine and a monofunctional primary amine by reacting either one of amines with phosgene and then with the remaining amine. Superacid Salts of Substituted Ureas The term "superacid" as used herein is defined as an acid having an acidity stronger than a 100% sulfuric acid. Urea and substituted ureas exhibit a weak basicity and may bind a superacid in the form of a quarternary ammonium salt. Such salts may be produced by reacting the urea component with an alkali metal salt of a superacid under acidic conditions with hydrochloric acid. The reaction may be carried out , for example, by dissolving the urea component in an suitable organic solvent, acidified with hydrogen chloride on hydrochloric acid and then adding a solution of an alkali metal salt of a superacid. After removing precipitated alkali metal chloride by-product, desired urea-superacid salt may be recovered by evaporation. The product may be further purified by extraction, recrystallization or other conventional methods. Examples of alkali metal salts of superacids include NaSbF₆, NaBF₄, NaAsF₆, NaPF₆, NaCF₃SO₃, KSbF₆, KBF₄, KAsF₆, KPF₆, KCF₃SO₃. The resulting superacid salts are soluble in conventional organic solvents including aromatic hydrocarbons such as toluene and xylene, esters such as butyl acetate and ethyl acetate. They are compatible with a variety of epoxy resins and may initiate their cationic polymerization by releasing a proton at room or elevated temperatures. Their intensity as a cationic polymerization initiator may vary with the hydrocarbon substituent of urea component and also with particular superacids. However, the intensities of urea salts of the above-named superacids are strong enough to initiate the cationic polymerization of non-alicyclic epoxy resins even at room temperature. This has been very difficult with the use of prior art curing catalysts. Curable Resin Compositions The curing catalyst of the present invention may be used in conjunction with a compound or resin having a plurality of epoxy groups. The epoxy group may be part of an alicyclic moiety, such as epoxycyclohexyl, or non-alicyclic moiety such as glycidyl. Specific examples of alicyclic polyepoxy compounds include 3,4-epoxycyclohexyloxirane; 3',4'-epoxycyclohexylmethyl-3,4-epoxycyclohexylcarboxylate; 1,2,5,6-diepoxyperhydroindene; 2-(3,4-epoxycyclohexyl)-3',4'-epoxy-1,3-dioxane-5-spirocyclohexane; 1,2-ethylenedioxybis(3,4-epoxycyclohexylmethane); 1,3-dimethyl-2,3-epoxycyclohexyloxirane; di-(2,3-epoxycyclopentyl)ether; 4',5'-epoxy-2'-methylcyclohexylmethyl-4,5-epoxy-2-methylcyclohexanecarboxylate; 3',4'-epoxy-2'-methylcyclohexylmethyl-3,4-epoxy-2-methylcyclohexanecarboxylate; bis(3,4-epoxycyclohexylmethyl)adipate; bis(4,5-epoxy-2-methylcyclohexylmethyl)adipate; ethylene glycol-bis(3,4-epoxycyclohexanecarboxylate) and the like. Examples of polyfunctional non-alicyclic epoxy resins include polyglycidyl ethers of polyhydric phenols such as bisphenol A, tetrabromobisphenol A, bisphenol S, phenol novolac or cresol novolac; polyglycidyl ethers of polyhydric alcohols such as butanediol, hexanediol, hydrogenated bisphenol A or dimethylolbenzene; polyglycidyl esters of polycarboxylic acids such as terephthalic, isophthalic phthalic or adipic acid; and glycidyl ester-ether of hydroxycarboxylic acid such as p-hydroxybenzoic acid. A further class of epoxy resins include homo- or copolymers of epoxy group-containing acrylic monomers. Typical examples of acrylic monomers having a non-alicyclic group are glycidyl acrylate arid glycidyl methacrylate. Examples of acrylic monomers having an alicyclic epoxy group include 3, 4-epoxycyclohexylmethyl (meth)acrylate (meth)acrylate of the adduct of poly-ε-caprolactone with 3, 4-epoxycylohexylmethanol and reaction products of (meth)acrylic acid and the above-named polyfuctional alicyclic epoxy compounds. Examples of monomers copolymerizable with the epoxy group-containing acrylic monomers include methyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-hydroxyethyl(meth)acrylate, polycaprolactone glycol mono(meth)acrylate, styrene, acrylonitrile, vinyl acetate and the like. The above polyfuctional epoxy compounds and polyfuctional epoxy group-containing resins may be used singly or in combination. The composition of the present invention may contain as a chain-extender or cross-linker a minor amount of a polyhydric alcohol such as ethylene glycol, propylene glycol, 1, 4-butanediol, 1, 3-butanediol, 2, 3-butanediol, diethylene glycol, dipropylene glycol, triethylene glycol, 1, 5-pentanediol, 1, 6-hexanediol, neopentyl glycol, 2, 2, 4-trimethylpentane-1, 3-diol,glycerine trimethylolpropane, trimethylene glycol, polyethylene glycol, polycaprolactone glycol, pentaerythritol, dipentaerythritol and the like. Also included are acryl polyols produced by polymerizing hydroxy group-containing acrylic monomers such as 2-hydroxyethyl (meth)acrylate. The curing catalyst of the present invention may generally be added in an amount of 0.1 to 10% by weight based on the total nonvolatile content of the film-forming components. The exact amount will vary depending upon the type of epoxy resins, properties desired in the cured products, the length of pot life as desired and other parameters. Within the above range it is possible to cure non-alicyclic epoxy resins at room temperature. If necessary, the composition of the present invention may be cured at an elevated temperature. The resinous composition may contain a variety of conventional additives depending upon its intended use. For example, when used for coating purposes, the composition may contain pigments, solvents, light stabilizers, surface conditioners and the like. Other use or application includes adhesives, sealants or potting compounds, casting compounds and other resinous compositions where room temperature-curing is desired. The following examples are intended to further illustrate the present invention without limiting thereto. All parts and percents therein are by weight unless otherwise indicated. Example 1 N,N-di-(2-ethylhexyl)-N'-n-butylurea hexafluoroantimonate A four necked flask equipped with a thermometer, stirrer and drip funnel was charged with 4.82g of di-(2-ethylhexyl)amine and 100g of ethyl acetate. To this was added dropwise a solution of 1.98g of n-butylisocyanate in 30g of ethyl acetate with stirring over 30minutes while maintaining the inner temperature at 10°C and allowed to react until the absorption of isocyanate group at 2240 cm⁻¹ disappeared in the IR spectrum. Then a mixture of 2.086g of 35% hydrochloric acid and 50g of methanol was added dropwise over 30 minutes and allowed to react at room temperature for 20 hours. To this was added dropwise a solution of 5.18g of NaSbF₆ in 50g of methanol over 30 minutes and allowed to react at room temperature for 20 hours. Then the reaction mixture was filtered to remove NaCl by-product and evaporated to remove the solvent. 11g of the title compound was obtained as a pale reddish liquid. This product is hereinafter referred to as "curing catalyst A". This catalyst is soluble in aromatic hydrocarbon solvents such as toluene and xylene as well as esters such as butyl acetate and ethyl acetate. Assingment of IR spectra: SbF₆⁻ : 663.4cm⁻¹ Urea : 3100-3400cm⁻¹, 1600-1700cm⁻¹ Ammonium : about 2500 cm⁻¹ Example 2 N-n-butyl-N'-octylurea triflate A four necked flask equipped with a thermometer, stirrer and drip funnel was charged with 2.58g of octylamine and 200g of ethyl acetate. To this was added dropwise a solution of 1.98g of n-butylisocyanate in 30g of ethyl acetate with stirring over 30 minutes while maintaining the inner temperature at 10°C and allowed to react until the absorption of isocyanato group (2240cm⁻¹) disappeared in the IR spectrum. After dissolving the solidified reaction product in 50g of methanol, a mixture of 2.086g of 35% hydrochloric acid and 50g of methanol was added dropwise to the solution over 30 minutes and allowed to react at room temperature for 20 hours. Then a solution of 3.44g of sodium triflate in 50g of methanol was added dropwise over 30 minutes and allowed to react at room temperature for 20 hours. The reaction mixture was then filtered to remove NaCl by-product and evaporated to remove methano. 8.9g of the title compound was obtained as a pale yellow liquid. This product is hereinafter referred to as "curing catalyst B". This product is soluble in aromatic hydrocarbon solvents such as toluene and xylene as well as esters such as butyl acetate and ethyl acetate. Assignment of IR spectra: CF₃SO₃⁻ :638.3cm⁻¹, 1031.7cm⁻¹, 1172.5cm⁻¹ Urea : 3100-3400cm⁻¹, 1600-1700cm⁻¹ Ammonium : about 2500cm⁻¹ Example 3 N,N-di-(2-ethylhexyl)-N'-naphthylurea hexafluoroantimonate Example 1 was repeated except that 3.38g of naphthylisocyanate was replaced for 1.98g of n-butylisocyanate. 11.6g of the title compound was obtained as a pale brown liquid. This product is hereinafter referred to as "curing catalyst C" which is soluble in aromatic hydrocarbon solvents such as toluene and xylene as well as ester solvents such as butyl acetate and ethyl acetate. Assignment of IR spectra: SbF₆⁻ : 663.4cm⁻¹ Urea : 3100-3400cm⁻¹, 1600-1700cm⁻¹ Ammonium : about 2500cm⁻¹ Example 4 N-n-butyl-N'-octylurea hexafluoroantimonate Example 2 was repeated except that 5.18g of sodium hexafluoroantimonate was replaced for 3.44g of sodium triflate. 9g of the title compound was obtained as a pale red liquid. This product is hereinafter referred to as "curing catalyst D" which is soluble in aromatic hydrocarbon solvents such as toluene xylene as well as esters such as butyl acetate and ethyl acetate. Assignment of IR spectra: SbF₆⁻ : 663.4cm⁻¹ Urea : 3100-3400cm⁻¹, 1600-1700cm⁻¹ Ammonium : about 2500cm⁻¹ Example 5 N, N-di-(2-ethylhexyl)-N'-n-butylurea triflate Example 1 was repeated except that 3.44g of sodium triflate was replaced for 5.18g of sodium hexafluoroantimonate. 9.2g of the title compound was obtained as a pale red liquid. This product is soluble in aromatic hydrocarbon solvents such as toluene and xylene as well as esters such as butyl acetate and ethyl acetate, and hereinafter referred to as "curing catalyst E". Assignment of IR spectra: CF₃SO₃⁻ :638.3cm⁻¹, 1031.7cm⁻¹, 1172.5cm⁻¹ Urea : 3100-3400cm⁻¹, 1600-1700cm⁻¹ Ammonium : about 2500cm⁻¹ Example 6 Curing catalyst F Example 1 was repeated except that 3.86g of Coronate EH(R)(trifunctional isocyanate produced by Nippon Polyurethane Co.,Ltd.) was replaced for 1.98g of n-butylisocyanate. 12g of curing catalyst F was obtained as a pale red liquid. This product is soluble in ester solvents such as butyl acetate and ethyl acetate. Assignment of IR spectra: SbF₆⁻ : 663.4cm⁻¹ Urea : 3100-3400cm⁻¹, 1600-1700cm⁻¹ Ammonium : about 2500cm⁻¹ Example 7 Curing catalyst G Example 2 was repeated except that 1.7g of isophoronediamine was replaced for 2.58g of octylamine. 6g of curing catalyst G was obtained as a pale yellow liquid. This product is soluble in ester solvent such as butyl acetate and ethyl acetate. Assignment of IR spectra: CF₃SO₃⁻ :638.4cm⁻¹, 1031.7cm⁻¹, 1172.5cm⁻¹ Urea : 3100-3400cm⁻¹, 1600-1700cm⁻¹ Ammonium : about 2500cm⁻¹ Production Example 1 Glycidyl group-containing acrylic resin A flask equipped with a thermometer, stirrer and drip funnel was charged with 500g of xylene and 450g of butyl acetate and heated to 120°C. To this was added dropwise the following monomeric mixture over 3 hours. Styrene 400g Glycidyl methacrylate 354g 2-Hydroxyethyl methacrylate 162g n-Butyl acrylate 84g t-Butylperoxy-2-ethylhexanoate 37g Total 1037g After the addition, the mixture was stirred for additional 30 minutes at 120°C. Then a solution of 5g of t-butylperoxy-2-ethylhexanoate in 50g of ethyl acetate was added dropwise. The mixture was stirred for additional 1.5 hours at 120°C and then cooled down. A colorless, transparent and viscous resin solution having a nonvolatile content of 50% was obtained. Number average molecular weight of the resin measured by the GPC method was 5500. Production Example 2 Alicylic epoxy group-containing acrylic resin Production Example 1 was repeated except that 3,4-epoxycyclohexyl methacrylate was replaced for glycidyl methacrylate. A colorless, transparent and viscous resin solution having a nonvolatile content of 50% was obtained. Number average molecular weight of the resin measured by the GPC method was 5200. Examples 8-14 Various resin compositions were formulated as indicated in Table 1, applied on a tin plate by a bar coater to a dry film thickness of about 20 µm and allowed to stand at room temperature for 24 hours. The resulting specimens were tested for solvent resistance by rubbing the coated surface with xylene-impregnated fabric at 20 reciprocations and for pencil hardness according to JIS K 5400 6.14. All curing catalyst were used as a 50% solution in butyl acetate. The results are also shown in Table 1.

Claims: The use of a superacid salt of an N,N'-di- or N,N,N'-trisubstituted urea produced by reacting an organic isocyanate with a primary or secondary amine, at least one of said isocyanate and said amine being monofunctional, as an initiator of the cationic polymerization of epoxy resins or compounds. The use as claimed in claim 1, wherein said superacid is HSbF₆, HBF₄, HAsF₆, HPF₆ or HCF₃SO₃. The use as claimed in claim 1 or 2, wherein both of said isocyanate and said amine are monofunctional. The use as claimed in claim 1 or 2, wherein said isocyanate is monofunctional and said amine is polyfunctional and vice versa. A resinous composition comprising a compound or resin having a plurality of epoxy groups in the molecule, and a catalytically effective amount of a superacid salt of an N,N'-di- or N,N,N'-trisubstituted urea produced by reacting an organic isocyanate and a primary or secondary amine, at least one of said isocyanate and said amine being monofunctional. The resinous composition as claimed in claim 5, wherein said superacid is HSbF₆, HBF₄, HAsF₆, HPF₆ or HCF₃SO₃. The resinous composition as claimed in claim 5 or 6, wherein both of said isocyanate and said amine are monofunctional. The resinous composition as claimed in claim 5 or 6, wherein said isocyanate is monofunctional, said amine is polyfunctional and vice versa. The resinous composition as claimed in any one of claims 5 to 8, wherein said epoxy group is a non-alicyclic epoxy group. The resinous composition as claimed in claim 9, wherein said non-alicyclic epoxy group is glycidyl. The resinous composition as claimed in any one of claims 5 to 8, wherein said epoxy group is an alicyclic epoxy group. The resinous composition as claimed in claim 11, wherein said alicyclic epoxy group is epoxycyclohexyl. The resinous composition as claimed in any one of claims 5 to 12 comprising in addition a chain extending or cross-linking agent, in particular a minor amount of a polyhydric alcohol.

Inventors: MIYAZONO TADAFUMI, TABUCHI KOJI, MIYAZONO, TADAFUMI, TABUCHI, KOJI
Applicants: NIPPON PAINT CO LTD, NIPPON PAINT CO., LTD.
Application year: 1992
Main classification: C08G59
IPC codes: C08G59, C07C275