Curable composition

Abstract: A curable composition employs the oxazolidinemine ring-forming reaction between an epoxide and a carbodiimide for curing and cures under mild conditions in a short period of time with little side reaction to obtain a polycarbodiimide resin having good strength, heat resistance and processability. The curable composition comprises (a) a polycarbodiimide, (b) a polyepoxide and (c) a zinc halide or a mixture of a zinc halide and an onium salt as a catalyst.

Description: The present invention relates to a curable composition in which a polycarbodiimide is reacted with a polyepoxide in the presence of a specific catalyst to crosslink based on the formation of polyoxazolidine-imine rings. Polyurethane resin or polyurea resin is generally prepared from a curable composition in which a polyisocyanate compound is used as a curing agent. The polyisocyanate compound, however, has a short pot life because of high reactivity and is itself toxic. In order to solve these problems, a blocked polyisocyanate in which the isocyanate groups are blocked with a blocking agent has been proposed. The blocking agent, however, is produced again by unblocking upon curing and volatilized into the air. Also, the resin obtained has poor heat resistance. In order to enhance heat resistance, it has been proposed that the blocked polyisocyanate compound is heated in the presence of a catalyst to condense it with removal of carbon dioxide, thus forming a polycarbodiimide resin. Since the carbodiimide structure has excellent heat resistance, polycarbodiimide resins have now been intensively studied. However, the production of polycarbodiimide resin is difficult because a relatively high temperature is required and it takes a relatively long period of time to accomplish the reaction. Also the resin itself which is obtained has poor processability on molding and the like. In order to improve processability, it is proposed in J. Appl. Polym. Sci., by L.N. Alberine, Vol. 21, p. 1999 (1977) that the molecular weight of the polycarbodiimide is controlled to provide flowability when heated. Japanese Kokai Publication 61-268714 also proposes that a polycarbodiimide having a low degree of polymerization is heat-molded in the presence of a trimerizing agent. Even those improvements, however, require a high temperature treatment to achieve sufficient curing properties and processability is still insufficient for practical use. In relation to the reaction of a carbodiimide and an epoxide, Japanese Kokai Publication 62-1714 discloses that a polycarbodiimide having a low degree of polymerization and an epoxide are heat-molded in the presence of a catalyst of a tertiary amine. It is also proposed in Chem. Ber., by K. Gulbins, Vol. 94, p. 3287 (1961) that a carbodiimide is reacted with an epoxide at 200°C for 7 hours in the presence of a catalyst of lithium chloride or a tertiary amine to form a compound having an imidazolidone ring. In this method, the imidazolidone compound is formed by a thermal-rearrangement reaction of an intermediate compound (i.e. oxazolidineimine). Pankratovv V.V. et al . report that the reaction of a carbodiimide and an epoxide not only forms an oxazolidineimine or an oxazolidone ring, but also causes side-reactions of dimerizing and trimerizing the carbodiimide (see Acta. Polym., Vol. 36 (10), pp. 550-555 (1985), CA:104(20)168970f). Further, Matsuda and Baba report that a carbodiimide is reacted with an epoxide under mild conditions in the presence of a catalyst of an organic tin halide to obtain a compound having an oxazolidineimine (see Synthesis, 1985, No. 12, 1144). These reports, however, neither suggest not teach that the oxazolidineimine ring-forming reaction between an epoxide and a carbodiimide can provide a resin curing reaction. EP-A-0 444 956 of the present Applicants, which is relevant under Article 54(3) and 54(4) EPC, describes a curable composition comprising a polyepoxide, a carbodiimide modified diphenylmethane diisocyanate and an oxazolidone-forming catalyst which may be, inter alia a complex of an organoantimony halide and a zinc halide but which also comprises a radical polymerisable vinyl compound and a radical polymerisation initiator. DE-A-2 542 489 describes a process for producing a compound having an oxazolidone ring comprising reacting an isocyanate compound and an epoxy compound in the presence of as a catalyst a soluble complex of a Lewis acid with at least one Lewis base selected from an ether, a thioether, an amine, a lactam, an N-alkyllactam, an amide, an N-alkylamide, a specified phosphorus compound and a specified sulfur compound. In an Example, a zinc chloride-tetrahydrofuran complex is used as a catalyst. The present invention provides a curable composition which employs the oxazolidineimine ring-forming reaction between an epoxide and a carbodiimide for curing and which cures under mild conditions in a short period of time with little side reaction to obtain a polycarbodiimide resin having good strength, heat resistance and processability. The curable composition of the present invention comprises (a) a polycarbodiimide, (b) a polyepoxide and (c) a zinc halide or a mixture of zinc halide and an onium salt as a catalyst. The polycarbodiimide employed in the present invention is a compound which has at least 2 carbodiimide groups, preferably at least 3 carbodiimide groups in the molecule. The term "poly" of the "polycarbodiimide" refers to the number of the carbodiimide groups and does not mean the size of the molecular weight. The molecular weight of the polycarbodiimide is within the range of 250 to 10,000. The polycarbodiimide is known in the art, and is prepared by polycondensation of an organic polyisocyanate in the presence of a carbodiimide promoting agent. The organic polyisocyanate is known in the art in the polyurethane industry and includes an aromatic polyisocyanate, such as 2,4- or 2,6-tolylene diisocyanate, naphthalene 1,5-diisocyanate and diphenylmethane 4,4'-diisocyanate; and an aliphatic or alicyclic polyisocyanate, such as hexamethylene diisocyanate, hydrogenated diphenylmethane 4,4'-diisocyanate, isophorone diisocyanate, hydrogenated 2,4- or 2,6-tolylene diisocyanate. Another modified polyisocyanate may also be used, for example an isocyanate group-containing prepolymer which is prepared by reacting a polyisocyanate with an active hydrogen containing material (e.g. Isonate 181 (trade name) available from Dow Chemical Co. and Mondur PF (trade name) available from Mobay Chemical Co.), a polyisocyanate having a carbodiimide group or a urethoimine group (e.g. Isonate 1431 (trade name) available from Dow Chemical Co., Mondur CD (trade name) available from Mobay Chemical Co. and Millionate MTL (trade name) available from Nippon Polyurethane Co. Ltd.), a polyisocyanate having an allphanate bond (e.g. Mondur HR (trade name) available from Mobay Chemical Co.), a polyisocyanate having a biuret bond (e.g. Desmodur N (trade name) available from Mobay Chemical Co.), a polyisocyanate having an isocyanurate rate (e.g. Desmodur Z 4270 (trade name) and Desmodur CTS Tabil (trade name) both available from Mobay Chemical Co.) and the like. The carbodiimide promoting agent can be any one known to the art and typically are described in Chemical Review, by Andrew Williams, Ibrahim T. Ibrahim, Vol. 81, No. 4, p. 619 (1981). Among them, 1-phenyl-phosphorene-1-oxide, 1-phenyl-3-methyl-phosphorene-1-oxide, 1-ethyl-phosphorene-1-oxide and mixtures thereof are preferred. In the preparation of the polycarbodiimide, molecular weight can be controlled by terminating the polycondensation with an organic monoisocyanate (see J. Appl. Polym. Sci. by L.N. Alberine, Vol. 21, p. 1999 (1977)). Examples of the organic monoisocyanates are phenyl isocyanate, tolyl isocyanate, cyclohexyl isocyanate and butyl isocyanate. A blocking technique in which a terminal isocyanate group is blocked with an active-hydrogen containing compound (a blocking agent) can also control the molecular weight of the polycarbodiimide. The blocking agent is described in detail in, for example, Progress in Organic Coatings, Vol. 3, p. 73 (1975). Typical examples of blocking agents are alcohols, phenols, lactams, N-hydroxyimide, oximes, imidazoles, triazoles, active methylene compounds (e.g. acetyl acetone and diethyl malonate), aromatic secondary amines, and acidic sodium sulfite. The polyepoxide used in the present invention is a compound having at least two epoxy groups. It should be noted that the term "poly" in "polyepoxide" does not mean high molecular weight but refers to the number of epoxy groups. The molecular weight of the polyepoxide is within the range of 200 to 50,000. Polyepoxides are widely known in the art and typical examples are glycidyl ethers of bisphenols, such as the glycidyl ether of bisphenol A and the glycidyl ether of bisphenol F, such as DER-332 and DER-542 (trade names) available from Dow Chemical Co., YD-128 and YDF-170 (trade names) available from Toto Kasei Company, Epicoat 828 (trade name) available from Shell Chemical Industries Inc., and Epichlone 850 and Epichlone (trade names) available from Dai Nippon Chemical Industries Inc. The polyepoxide can be polynucleo phenol glycidyl ether derivatives or epoxide phenyl novolac resins, for example DEN-431 and DEN-438 (trade names) both available from Dow Chemical Co., and EPN-1139 and ECN-1235 (trade names) both available from Ciba Geigy Co. Another polyepoxide, such as an aromatic glycidyl amine resin obtained from an aromatic amine and epichlorohydrin, can be used. Examples of the polyepoxides are triglycidyl-p-aminophenol, and N,N,N',N'-tetraglycidyl-4,4'-diminodiphenylmethane. The polyepoxide also includes a glycidyl ester of an aromatic aliphatic or alicyclic polycarboxylic acid or a glycidyl ether of a reaction product of the aromatic, aliphatic or alicyclic polycarboxylic acid and a polyol. Examples of such polyepoxides are CY-192 and CY-184 (trade names) both available from Ciba Geigy AG, EPOMIK-508 and EPOMIK-540 (trade names) both available from Mitsui Petrochemical Industries Ltd. Another suitable polyepoxide can be an aliphatic epoxy resin, such as a glycidyl ether of alkylene glycol or polyether polyol. Typical examples of such polyepoxides are DER-736 and DER-732 (trade names) both available from Dow Chemical Co., and EPOMIK R-810 (trade name) available from Mitsui Petrochemical Industries Ltd. Further, an epoxidized compound of a cyclo-olefin can be used as the polyepoxide. Examples of the epoxidized compounds are 3,4-epoxycyclohexylmethyl-3',4'-epoxycyclohexane carboxylate and the like. In addition, an acrylic resin having a glycidyl group or a 3,4-epoxycyclohexylmethyl group derived from glycidyl acrylate, glycidyl methacrylate, 3,4-epoxycyclohexylmethyl acrylate, 3,4-epoxycyclohexylmethyl methacrylate etc. can also be suitable as the polyepoxide. The above polyepoxides can be used alone or in combination. The catalyst employed in the present invention is a compound which promotes the formation of the oxazolidineimine ring obtained from the reaction of the polycarbodiimide and the polyepoxide. In the present invention, the catalyst is zinc halide or a mixture of zinc halide and an onium salt. The halogen atom may include bromine, iodine and chlorine, and the zinc halide can therefore be zinc chloride, zinc bromide, zinc iodide or mixtures thereof. The onium salt may be an ammonium salt or a phosphonium salt which is represented as follows; [wherein Z represents a nitrogen atom or a phosphorus atom, R 1 to R 4 , which are the same or different, represent an organic group and may form a ring by groups in combination, and X represents a halogen atom.] The organic group in the formula (1) includes an aliphatic hydrocarbon group, an alicyclic hydrocarbon group and an aromatic hydrocarbon group. The aliphatic hydrocarbon group preferably has 1 to 16 carbon atoms, for example methyl, ethyl, and octyl. The alicyclic hydrocarbon preferably has 5 to 12 carbon atoms, for example cyclopentane and cyclohexane. The aromatic hydrocarbon group preferably has 6 to 16 carbon atoms, for example phenyl, tolyl and xylyl. Typical examples of the ammonium salts are tetramethylammonium iodide, tetramethylammonium bromide, tetraethylammonium iodide, tetraethylammonium bromide, tetrapropylammonium iodide, tetrapropylammonium bromide, tetrabutylammonium iodide, tetrabutylammonium bromide, tetraisoamylammonium iodide, tetraisoamylammonium bromide, tetrapentylammonium iodide, tetrapentylammonium bromide, methyltriethylammonium iodide, methyltriethylammonium bromide, trimethylphenylammonium iodide, trimethylphenylammonium bromide, trimethylbenzylammonium iodide and trimethylbenzylammonium bromide. Typical examples of the phosphonium salts are tetrabutylphosphonium iodide, tetrabutylphosphonium bromide, tetraphenylphosphonium iodide, tetraphenylphosphonium bromide, butyltriphenylphosphonium iodide, butyltriphenylphosphonium bromide, methyltriphenylphosphonium iodide, methyltriphenylphosphonium bromide, tributylphenylphosphonium iodide and tributylphenylphosphonium bromide. Where the catalyst is the mixture of zinc halide and an onium salt, it is considered that a complex formed from the onium salt and zinc halide actually acts as the catalyst. Accordingly, the complex may be separately prepared from the curable composition and then formulated into the curable composition. The complex can be prepared by reacting zinc halide and the onium salt at a temperature of room temperature to 150°C in a molar ratio of about 1:1 in the presence or absence of an inactive organic solvent. Suitable inactive organic solvents are ketones, esters, ethers, aromatic hydrocarbons and chlorinated hydrocarbons. The curable compositions of the present invention may further contain additives, such as fillers, reinforcing agents, antioxidants, fire-retardants, pigments, surfactants, catalysts, and surface controlling agents. The polyepoxide may be present in the curable composition in an amount of 0.01 to 1.0 equivalent, preferably 0.1 to 1.0 equivalent, as the epoxy group based on one equivalent of the carbodiimide group. The catalyst (c) may be contained in the curable composition in an amount of 0.01 to 10 parts by weight, preferably 0.1 to 5 parts by weight, based on 100 parts by weight of total weight of the polycarbodiimide (a) and the polyepoxide (b). The curable composition easily cures at an elevated temperature of 60 to 200°C, preferably 80 to 160°C for 0.5 to 1.0 hours. The composition therefore can be used for example, for molding, coating, adhesives, and as a sealant. Coating can be conducted by coating the curable composition on a substrate and then curing at an elevated temperature. Examples of substrates are metal, steel, glass and plastics. Coating can be conducted for example by spraying, brushing, dipping and flow coating. it is preferred that the composition is coated in a dried coating amount of 5 to 250 g/m 2 . EXAMPLES The present invention is illustrated by the following Examples which, however, are not to be construed as limiting the present invention to their details. Preparation of polycarbodiimide Polycarbodiimide resin (A) A reaction vessel was charged with 91 g of toluene, to which 25 g of methylenediphenyl diisocyanate, 24 g of phenyl isocyanate and 0.75 g of 3-methyl-1-phenyl-2-phosphorene-1-oxide as the carbodiimide promoting agent were added. The mixture was reacted at about 100°C for 4 hours to obtain a polycarbodiimide resin having a terminal phenyl group (non-volatile content = 35%, number average molecular weight (GPC) = about 400). Polycarbodiimide resin (B) A reaction vessel was charged with 220 g of o-dichlorobenzene, to which 75 g of methylenediphenyl diisocyanate, 18.8 g of phenol (blocking agent) and 0.75 g of 3-methyl-1-phenyl-2-phosphorene-1-oxide as the carbodiimide promoting agent were added. The mixture was reacted at about 110°C for 8 hours to obtain a polycarbodiimide resin having a terminal blocked isocyanate group (non-volatile content = 30%, number average molecular weight (GPC) = about 1,000). Polycarbodiimide resin (C) A reaction vessel was charged with 97 g of o-dichlorobenzene, to which 44.4 g of isophorone diisocyanate, 20 g of n-butyl isocyanate and 1.3 g of 3-methyl-1-phenyl-2-phosphorene-1-oxide as the carbodiimide promoting agent were added. The mixture was reacted at about 190°C for 4 hours to obtain a polycarbodiimide resin having a terminal n-butyl group (non-volatile content = 40%, number average molecular weight (GPC) = about 600). Polycarbodiimide resin (D) A reaction vessel was charged with 116 g of o-dichlorobenzene, to which 52.4 g of methylenedicyclohexane diisocyanate, 25 g of cyclohexane isocyanate, and 1.5 g of 3-methyl-1-phenyl-2-phosphorene-1-oxide as the carbodiimide promoting agent were added. The mixture was reacted at about 190°C for 20 hours to obtain a polycarbodiimide resin having a terminal cyclohexane group (non-volatile content = 40%, number average molecular weight (GPC) = about 650). Polycarbodiimide resin (E) A reaction vessel was charged with 120 g of o-dichlorobenzene, to which 78.9 g of methylenedicyclohexane diisocyanate and 1.5 g of 3-methyl-1-phenyl-2-phosphorene-1-oxide as the carbodiimide promoting agent were added. The mixture was reacted at about 190°C for 8 hours, and cooled to 110°C at which 17.3 g of 4-methyl-2-pentaneoxime (blocking agent) was dropwise added and reacted 4 hours. A polycarbodiimide resin having a terminal phenyl group (non-volatile content = 35%, number average molecular weight (GPC) = about 400) was obtained. Preparation of polyepoxide Acryl resin (F) having epoxy groups A reaction vessel was charged with 66.7 g of xylene, to which 50 g of glycidyl methacrylate, 38 g of methyl methacrylate, 12 g of 2-ethylhexyl acrylate and 10 g of Kaya Ester-O (trade name) (t-butyl peroxy 2-ethylhexanoate available from Kayakunuri Co.) were added and reacted at 130°C for 3 hours to obtain an acryl resin having epoxy groups (non-volatile content = 58.8, number average molecular weight = about 2,500). Curable composition and its evaluation Example 1 A dispersion solution was prepared by mixing 57.1 g of the polycarbodiimide resin (A), 66.6 g of a bisphenol A glycidyl ether (DER-671-T75 (trade name) having an epoxy equivalent of 500, available from Dow Chemical Co.) and 45 g of ZnBr 2 which was dissolved in acetone. The solution obtained was coated on a tin plate in a thickness of about 20 micron and heated at 100°C for 30 minutes to obtain a colorless transparent smooth film having no surface stickiness. The film was evaluated by pencil hardness (according to JIS K 5400), scratch resistance (nail scratch test) and solvent resistance (xylene rubbing test), but the results were all good. The cured film was subjected to infrared spectrum to show that the carbodiimide bond (2150 cm -1 ) had disappeared and an oxazolidineimine ring bond had appeared at 1675 cm -1 . Examples 2 to 9 The same experiment was conducted as generally described in Example 1, based on the conditions shown in Table 1. The film evaluations are also indicated in Table 1. The cured film was subjected to infrared spectroscopy to show that the carbodiimide bond had disappeared and an oxazolidineimine ring bond had appeared. Examples 10 to 12 The same experiment was conducted as generally descriBed in Example 1, based on the conditions shown in Table 1. The film evaluations are also indicated in Table 1. The cured film was subjected to infrared spectroscopy to show that the carbodiimide bond had disappeared and an oxazolidineimine ring bond had appeared. Examples Formulation 10 11 12 DEN-431 17.5 54.5 Epoxy resin (F) 54.5 Carbodiimide resin (B) 166.0 (E) 100.0 XL-29SE 82.0 XL-27HS 30.0 ZnBr 2 /Bu 4 NI 0.25/0.37 0.25/0.37 0.25/0.37 Curing Conditions Temp. (°C) 140 120 140 Time (min) 30 30 30 Film qualities Visual appearance Good Good Good Hardness HB HB HB Scratch resistance Good Good Good Solvent resistance Good Good Good Comparative Examples 1-4 The same experiment was conducted as generally described in Example 1, based on the conditions shown in Table 3. The film evaluations are also indicated in Table 3, but sufficient results were not obtained. The cured film was subjected to infrared spectroscopy to show that the carbodiimide bond did not disappear and an oxazolidineimine ring bond did not appear. Comparative Examples Formulation 1 2 3 4 DER-671 66.6 66.6 66.6 66.6 XL-29SE 82.0 82.0 82.0 82.0 Bu 4 NI 0.74 Ph 4 SbI 1.10 2-Ethyl,4-methyl imidazole 0.65 Curing Conditions Temp. (°C) 160 160 160 160 Time (min) 30 30 30 30 Film qualities Visual appearance *2 Colored Scratch resistance Poor Poor Poor Poor Solvent resistance Poor Poor Poor Poor The present invention provides a curable composition which is quickly cured under milder conditions and in which the curing reaction can be controlled without side reaction. The composition provides polycarbodiimide resins having excellent strength, heat resistance and processability.

Claims: A curable composition comprising: (a) a polycarbodiimide having at least 2 diimide groups and a number average molecular weight of 250 to 10,000, prepared by polycondensation of an organic polyisocyanate in the presence of a carbodiimide promoting agent; (b) a polyepoxide having at least 2 epoxy groups and a number average molecular weight of 200 to 50,000; and (c) as a catalyst, a zinc halide or a mixture of a zinc halide and an onium salt represented by wherein Z represents a nitrogen atom or a phosphorus atom, R 1 to R 4 , which are the same or different, each represents an organic group and may form a ring in combination, and X represents a halogen atom. A curable composition as claimed in claim 1 wherein said organic polyisocyanate is an aromatic polyisocyanate, an aliphatic or an alicyclic polyisocyanate. A curable composition as claimed in claim 1 or claim 2 wherein said carbodiimide promoting agent is selected from 1-phenyl-phosphorene-1-oxide, 1-phenyI-3-methyI-phosphorene-1-oxide, 1-ethyl-phosphorene-1-oxide and mixtures thereof. A curable composition as claimed in any one of the preceding claims wherein said polyepoxide is a glycidyl ether of a bisphenol, a glycidyl ether of bisphenol F, a polynucleo phenol glycidyl other derivative or epoxide phenyl novolac resin, an aromatic glycidyl amine resin obtained from an aromatic amine and epichlorohydrin, a glycidyl ester of an aromatic, aliphatic or alicyclic polycarboxylic acid, a glycidyl ether of a reaction product of an aromatic, aliphatic or alicyclic polycarboxylic acid and a polyol, an aliphatic epoxy resin, or a mixture thereof. A curable composition as claimed in any one of the preceding claims wherein a complex which is formed from the onium salt and zinc halide is employed as the catalyst. A curable composition as claimed in any one of the preceding claims further comprising a filler, reinforcing agent, antioxidant, fire-retardant, pigment, surfactant, catalyst and/or surface controlling agent. A curable composition as claimed in any one of the preceding claims wherein said polyepoxide is present in the curable composition in an amount of 0.1 to 1.0 equivalent, as epoxy group based on one equivalent of the carbodiimide group. A curable composition as claimed in any one of the preceding claims wherein said catalyst (c) is contained in the curable composition in an amount of 0.01 to 10 parts by weight, based on 100 parts by weight of total weight of the polycarbodiimide (a) and the polyepoxide (b).

Inventors: IWAMOTO NORIKAZU, IWAMOTO, NORIKAZU
Applicants: NIPPON PAINT CO LTD, NIPPON PAINT CO., LTD.
Application year: 1992
Main classification: C08G59
IPC codes: C08L63, C08L79, C08G18, C08G59