GB2080305A

GB2080305A - Carbocyclic Triisocyanates and Powder Coating Compositions Containing Polyisocyanates Derived therefrom

Info

Publication number: GB2080305A
Application number: GB8122531A
Authority: GB
Original assignee: Takeda Chemical Industries Ltd
Current assignee: Takeda Pharmaceutical Co Ltd
Priority date: 1980-07-22
Filing date: 1981-07-22
Publication date: 1982-02-03
Also published as: JPS5790060A; GB2080305B; DE3128743A1; CA1197941A

Abstract

A powder coating composition comprises (i) a blocked polyisocyanate obtained by blocking the free isocyanate groups of a triisocyanate of the formula: <IMAGE> or isocyanate-terminated adduct thereof, with a blocking agent and (ii) a polyol resin having a melting point of not lower than 40 DEG C. The compositions are stable during a long- term storage and some of them can be baked at relatively low temperatures. The compositions give coating films substantially free of yellowing and blowing and having good mechanical properties, chemical resistance and weather resistance. The trisisocyanates are new compounds and methods of preparing them are described.

Description

SPECIFICATION Powder Coating Composition and Method and Triisocyanates This invention relates to powder coating compositions which are suitable for powder coating. In particular, the invention relates to a powder coating composition which comprises a novel triisocyanate or an adduct thereof blocked with a polyol resin.

The process known as powder coating has become popular in recent years, and various compositions for powder coating have been proposed, for example, compositions based on thermoplastic resins such as polyethylene, polyvinyl chloride, nylon, polyester and acrylic resins.

However, coating films made from these compositions have drawbacks in terms of heat-, chemicaland staining resistance, etc. On the other hand, the use in powder coating of thermosetting resins, such as thermosetting acrylics, thermosetting polyesters and thermosetting epoxies has been progressing and the use of isocyanate compounds as crosslinking agents for them is under investigation. It is known that these polyisocyanate compounds can be converted into a component of a powder coating composition by blocking the free isocyanate groups with a blocking agent, the resulting blocked polyisocyanate being combined with a polyol resin. Nevertheless, not all isocyanates are suitable for this purpose, and there are several factors restricting their use in powder compositions.Particularly, in those cases where isocyanates blocked with blocking agents have properties which make powdering difficult, the use of such isocyanates as components for powder compositions is greatly restricted, no matter how good their chemical properties may be. In the case of blocked isocyanates which it is difficult to powder, for example, powdered compositions may be obtained in the form of combinations of such isocyanates with polyol resins which melt only at relatively high temperatures by melting both components and powdering the resulting mixture, but such compositions possess drawbacks such as poor storage stability and the like. A variety of blocking agents that facilitate powdering of polyisocyanate compounds has been sought and -caprolactam has been proposed for use as such a blocking agent.However, such blocking agents in general have high dissociation temperatures, so large quantities of energy are required and difficulties are encountered in the treatment of dissociated blocking agents.

According to one aspect of the present invention, there is provided a powder coating composition which comprises (1) a blocked polyisocyanate obtained by blocking the free isocyanate groups of a triisocyanate of the formula:

isocyanate-terminated adduct thereof with a blocking agent, and (2) a polyol resin having a melting point of not lower than 400 C.

According to another aspect of the present invention there is provided a method of coating a substrate, wherein the powder coating composition just indicated is applied to the substrate and the substrate is subjected to baking.

The isocyanate component of the blocked polyisocyanates useful in the present powder coating composition may be a triisocyanate of the formula (I), namely 1 ,3,5-tris(isocyanatomethyl)-benzene or 1 ,3,5-tris(isocyanatomethyl)cyclohexane, or a modified product of one or other of these compounds, e.g. a dimer, trimer, carbodiimide, or other derivative thereof, an isocyanate-terminated adduct of one of said triisocyanates with a low-molecular-weight compound containing active hydrogen such as ethylene glycol, propylene glycol, dipropylene glycol, butylene glycol, trimethylolpropane, hexanetriol, glycerol, sorbitol, pentarythritol, castor oil, ethylenediamine, hexamethylenediamine, ethanolamine, diethanolamine, triethanolamine, water, ammonia and urea, and isocyanate, terminated adducts of one of said triisocyanates and a high-molecular-weight compound containing active hydrogen such as a polyether polyol, a polyester polyol or an acrylic polyol.

The triisocyanates of the above formula (I) are novel compounds, and accordingly a further aspect of the present invention provides a triisocyanate of the formula where

Yet a further aspect of the present invention provides a method of preparing such a trisisocyanate, wherein a 1 ,3,5-triaminomethyl compound of the formula

C2H2NH2 H2NH2Ch2NH2 where isoris phosgenated.

The trisisocyanates of the above formula can be produced from 1 ,3,5-tricyanobenzene as a starting material, by hydrogenating either its cyano groups or its cyano groups and its benzene ring in the presence of reduction catalyst, followed by phosgenating the resulting 1,3,5tris(aminomethyl)benzene or 1 ,3,5-tris(aminomethyl)cyclohexane by a conventional phosgenation procedure.In producing an isocyanate-terminated adduct of such a triisocyanate, the triisocyanate and a compound containing active hydrogen are subjected to a urethane-, urea- or biuret- converting reaction optionally in the presence of a solvent such as ethyl acetate or toluene under reaction conditions normally employed and with an excess of isocyanate groups, for example by selecting the starting materials in such proportions that the ratio of active hydrogen to free isocyanate groups is within the range of 0.1 to 0.9.

The isocyanates or their adducts as described above, can then be modified into the required blocked polyisocyanates by being blocked with a blocking agent.

Suitable blocking agents which may be used include the known blocking agents recognized to be useful for blocking isocyanates, such as blocking agents based on phenols, lactams, compounds containing active methylene, alcohols, mercaptans, acid amides, imides, amines, imidazoles, ureas, imines, oximes, sulphites, etc. Among these blocking agents phenols, oximes, lactams and alcohols, but especially oximes are preferred.Specific examples of the blocking agents include blocking agents based on phenols such as phenol, cresol, xylenol, nitrophenol, chlorophenol, ethylphenol, phydroxydiphenyl, t-butylphenol, hydroxybenzoic acid, hydroxybenzoic acid esters and 2,5-di-t-butyl-4hydroxytoluene; blocking agents based on lactams such as E-caprolactam, 8-valerolactam, y- butyrolactam and /3-propiolactam; blocking agents based on compounds containing active methylene such as diethyl malonate, dimethyl malonate, ethyl acetoacetate, methyl acetoacetate and acetylacetone; blocking agents based on alcohols such as methanol, ethanoi, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, t-butyl alcohol, n-amyl alcohol, t-amyl alcohol, lauryl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether, methoxymethanol, benzyl alcohol, a-methylbenzyl alcohol, a-ethylbenzyl alcohol, 4-ethyl-benzyl alcohol, furfuryl alcohol, methyl-a-furyl-carbinol, tetrahydrofurfuryl alcohol, ethyltetrahydrofurfuryl alcohol, glycollic acid, glycollates, e.g. methyl glycollate, ethyl glycollate and propyl glycollate, lactic acid, lactates, e.g. methyl lactate, ethyl lactate and butyl lactate, tartronic acid diesters, malic acid diesters, tartaric acid diesters, mandelic acid esters, methylolurea, methylolmelamine, diacetone alcohol, ethylene chlorohydrin, ethylene bromohydrin, 1 ,3-dichloro-2propanol, w-hydroperfluoroalcohol and acetone cyanhydrin; blocking agents based on mercaptans such as butyl mercaptan, hexyl mercaptan, t-butyl mercaptain, t-dodecyl mercaptan. 2mercaptobenzothiazole, thiophenol, methylthiophenol and ethylthiophenol; blocking agents based on acid am ides such as acetaniiide, acetaniside, acetotoluide, acrylamide, methacrylamide, acetamide, stearic acid amide and benzamide; blocking agents based on imides such as succinimide, phthalimide and maleimide; blocking agents based on amines such as diphenylamine, phenylnaphthylamine, xylidine, N-phenylxylidine, carbazole, aniline, naphthylamine, butylamine, dibutylamine and butylphenylamine; blocking agents based on imidazoles such as imidazole and 2-ethylimidazole; blocking agents based on ureas such as urea, thiourea, ethylene urea, ethylene thiourea and 1,3diphenylurea; blocking agents based on carbamates such as N-phenyl carbamate and 2-oxazolidone; blocking agents based on imines such as ethyleneimine; blocking agents based on oximes such as formaldoxime, acetaldoxime, acetoxime, methyl ethyl ketoxime, diacetylmonoxime, benzophenoxime and cyclohexanone oxime; and blocking agents based on sulphites such as sodium bisulphite and potassium bisulphite. These blocking agents may be used singly or mixtures of two or more such blocking agents may be used.

The reaction between the isocyanate and the blocking agent is conducted in an inert solvent having no active hydrogen or in the absence of a solvent by conventional procedures. In conducting the reaction, known reaction catalysts such as tertiary amines and organic metals may be utilized. In those cases in which a solvent is used in the reaction and is required to be removed after the conclusion of the reaction, the solvent can be removed by means of a spray dryer, drum dryer, etc.

Solid blocked polyisocyanates can be produced by the above-described procedures by employing the aforesaid triisocyanates or their adducts. Such solid blocked polyisocyanates can be easily obtained with the aforesaid triisocyanates or their adducts even with blocking agents which do not or do not readily form blocked products with conventional diisocyanates.For example, the use as a blocking agent of methyl ethyl ketoxime, which is generally regarded as the one capable of easily yielding blocked isocyanate products which dissociate at low temperature, with any of the triisocyanates of the formula (I) or their adducts results in the formation of a solid, which can be crushed to obtain a component suitable for use in the present powder coating composition, while it does not result in the formation of a solid when used to block hexamethylene diisocyanate or hydrogenated xylylene diisocyanate at ambient temperature.

Examples of the polyol resins having a melting point of not lower than 400 C. useful in forming the present powder coating compositions include polyester resins, acrylic polyol resins, hydroxylcontaining resins obtained by hydrolyzing a copolymer of a vinyl ester of a saturated monocarboxylic acid and a vinyl monomer, epoxy resins, polyether resins, polyether ester resins, or mixtures of two or more such resins. Of these resins, polyester resins and acrylic polyol resins are especially preferred.

The polyester resins may be obtained by conventional procedures by subjecting to condensation under an excess of hydroxyl groups, a polybasic acid, such as phthalic acid, isophthalic acid, terephthalic acid, maleic acid, fumaric acid, succinic acid, adipic acid, sebacic acid, azelaic acid or trimellitic acid, and a polyol such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, 1 3-butanediol, 1 ,4-butanediol, 1,5-pentanediol, neopentyl glycol, hexamethylene glycol, decamethylene glycol, hydroquinonebis(hydroxyethyl ether), hydrogenated bisphenol A, trimethylolethane, trimethylolpropane, hexanetriol, glycerol, pentaerythritol, tris(hydroxyethyl)isocyanurate or cyclohexanedimethanol. These acids and polyols can be used in combination in the form of mixtures of two or more of them.In addition, castor oil, higher fatty acids, etc. may be used in combination wih the above to produce oil-modified polyester polyols.

The preferred polyester resins obtained from the above raw materials are those with a molecular weight of 500 to 4000, more preferably 1000 to 3000, a hydroxyl value of 10 to 300, more preferably 20 to 200, an acid value of 1 to 30 and a melting point of 60 to 1200 C. Resins with a melting point of not higher than 400 C. cause blocking of the compositions, while those having a melting point not lower than 1200 C. are susceptible to gelling during melt-blending.

The acrylic polyol resins may be obtained for example by the copolymerization of (1) 2hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, allyl alcohol, cinnamic alcohol, crotonyl alcohol, or a hydroxyl-containing monomer obtained as the reaction product of an unsaturated monocarboxylic acid, e.g. acrylic acid, methacrylic acid, maleic acid, fumaric acid, crotonic acid, itaconic acid, etc. and ethylene glycol, ethylene oxide, propylene glycol, propylene oxide, butylene glycol, 1 ,4-cyclohexyl dimethanol, phenyl glycidyl ether, glycidyl decanoate, etc., and (2) an acrylate such as methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, tert-butyl acrylate and 2-ethylhexyl acrylate, a methacrylate such as methyl methacrylate, ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, tert-butyl methacrylate and 2-ethylhexyl methacrylate, a styrene monomer such as styrene, vinyltoluene and a-methylstyrene, or another copolymerizable a,P-ethylenically unsaturated monomer such as vinyl acetate, vinyl propionate, acrylonitrile, vinyl stearate, acryl acetate, diallyl adipate, dimethyl itaconate, diethyl maleate, vinyl chloride, vinylidene chloride, ethylene, glycidyl methacrylate, N-methylolacrylamide, N-butoxymethylacrylamide, acrylamide and diacetoneacrylamide.

Preferred acrylic polyol resins have a hydroxyl value of 10 to 300, more preferably 20 to 200. Their melting point is preferably 40 to 1 500C., more preferably about 60 to 1 200 C.

Examples of hydroxyl-containing resins obtained by hydrolyzing a copolymer of a vinyl ester of a saturated monocarboxylic acid and a vinyl monomer include a hydrolyzed ethylene/vinyl acetate copolymer with a monomer ratio within the range of 95 wt.% to 50 wt.% of the former and of about 5 wt.% to 50 wt.% of the latter and with a degree of hydrolysis of about 10% to about 98% of the total acetate groups. A preferred resin has a melt index of from 1 g/1 0 min. to 200 g/1 0 min. These hydroxyl-containing resins can be produced by the conventional polymerization and hydrolysis procedures.

Examples of epoxy resins include those which have at least two hydroxyl groups in one moiecule and which are normally condensation epoxy resins obtained by the reaction of bisphenol A and epichlorohydrin. Such epoxy resins have the following chemical structure:

Epoxy resins of the above formula in which n is 2 to 12, preferably 2 to 1 0, and which have an epoxy equivalent of 425 to 10000, preferably 425 to 4000, are preferred. The melting point of these resins is about 40 to 2000 C., preferably about 60 to 1 500 C. Furthermore, epoxy ester resins obtained, for example, by the reaction of soyabean oil fatty acids and epoxy resins can be utilized in addition to the above epoxy resins.

Preferred polyether resins which may be used in the present composition are polymers made, for example, from paraformaldehyde, a-polyoxymethylene and trioxane, or polytetramethylene ether glycol, polyhexamethylene ether glycol, etc., which have a molecular weight of about 500 to 4000, a hydroxyl value of about 20 to 200 and a melting point of about 60 to 1200 C.

The polyether ester resins may be obtained in conventional manner by the reaction of, for example, (1) a polyether polyol with a hydroxyl value of about 45 to 200, (2) a dibasic acid anhydride, such as phthalic anhydride, maleic anhydride, succinic anhydride, tetrahydrophthalic anhydride, 3,6endomethylenetetrahydrophthalic anhydride, 3,6-endodichloromethylenetetrachlorophthalic anhydride, tetrachlorophthalic an hydride or tetrabromophthalic anhydride, and (3) an oxirane compound, such as ethylene oxide, propylene oxide, butylene oxide, styrene oxide, phenyl glycidyl ether or allyl glycidyl either. The preferred resins are those with a molecular weight of about 500 to 4000, a hydroxyl value of about 20 to 200 and a melting point of about 60 to 1200 C.

The above blocked polyisocyanate and the polyol resin are mixed in amounts such that the equivalent ratio of regenerated free isocyanate groups to hydroxyl groups of the polyol resin is in the range of 0.5 to 2.0, preferably 0.6 to 1.5.

In order to produce compositions suitable for powder coating from blocked polyisocyanates and polyol resins, any of the conventionally known processes may be employed. Thus, both of the components may be dissolved respectively in solvents and mixed, together with, if necessary, pigments, levelling agents, dissociation catalysts, stabilizers, etc. which may be added to the solution, whereafter the mixture is spray dried in order to bring it into a fine powder. Alternatively, both solid components may be heated at 80 to 1300 C. and mixed in the molten state, together with, if necessary pigments and other additives, whereafter the melt is cooled and finely crushed into powder. Other conventional procedures may be employed. The particle size of the powder may be about 5 to 500 micron, preferably about 10 to 300 micron.

The present powder coating composition can be applied to various substrates such as metal, glass, concrete, ceramics, wood, paper and the like. The application of the powder to the surface of a substrate may be effected by conventional procedures known per se. For example, the application can be effected by means of an electrostatic powder applying machine, an electrical-field curtain type powder applying apparatus, etc. Following the application of the powder, the substrate is subjected to baking, the baking conditions normally being a temperature in the range of about 1300 C. to about 3000 C. for a period of about 1 minute to about 60 minutes.

In coating with the present composition, it is desirable to use a dissociation catalyst for blocked polyisocyanates with a specific view to lowering the temperatures required for baking. Such catalysts can be conventional amine-based, metal-based or other known catalysts. The catalyst is conveniently present in an amount of 0.01 to 5 weight % based on the coating composition.

The present compositions obtained by the above procedures are free from agglomeration or gelling, and remain stable during long-term storage. Moreover, they can be baked at relatively low temperatures, and the resultant coating films are substantially free of yellowing and blowing, while exhibiting extremely good mechanical properties, chemical resistance and weather resistance thus making the compositions of high value as powdered paints.

The invention will now be illustrated by the following Examples in which all parts and percentages are by weight unless otherwise indicated.

Example 1 In 25 ml of ethyl acetate there were dissolved 2.49 g of 1 ,3,5-tris(isocyanatomethyl)cyclohexane (H6MTI), and the solution was warmed to about 500 C. A solution of 2.65 g of methyl ethyl ketoxime in 10 ml of ethyl acetate was added dropwise to the solution at about 50 C over a 15-minute period, while stirring. The stirring was continued for 1 hour, while keeping the solution at about 500C, to allow the reaction to go to the completion. The ethyl acetate was distilled off at about 70"C. under reduced pressure, thereby yielding 4.88 g of glass-state, solid product.

The glass-state, solid product had a softening point of 60 to 700 C. and was easily pulverizable at room temperature. For the purpose of identification, the product was crystallized in ethyl acetate and recrystallized from a mixed solvent of ethyl acetate-cyclohexane (2.5:1), thereby yielding colourless crystals of melting point of 126 to 1 280C. The IR spectrum (KBr) of the crystals is shown in the accompanying drawing which is a plot of transmittance in percent against wave number in cm-', and elemental analysis indicates that the product is in accordance with the H 6MTl having three NCO groups blocked with methyl ethyl ketoxime Analysis Found Calculated for C24H42N606 C: 56.27 56.45 H: 8.29 8.29 N: 16.30 16.46 Reference Example 1 Following the same procedure as in Example 1, except that a solution of 5.04 g of hexamethylene diisocyanate in 50 ml of ethyl acetate and a solution of 5.31 g of methyl ethyl ketoxime in 20 ml of ethyl acetate were used, there were obtained 9.72 g of a slightly yellow, resin formed product. The product was a highly viscous, resinous material at room temperature, and was completely incapable of being powdered.

Reference Example 2 Following the same procedure as in Reference Example 1, except that 5.80 g of hydrogenated xylylene diisocyanate and 5.30 g of methyl ethyl ketoxime were used, there were obtained 10.50 g of a colourless product, which was a highly viscous, resinous material which was completely incapable of being powdered.

Example 2 500 Parts of a polyester resin (OH value 56, acid value 4 and melting point 650 C.) which had been made from terephthalic acid, neopentyl glycol and trimethylolpropane, and 102 parts of H6MTI blocked with methyl ethyl ketoxime (crushed, glass-state solid product) were melt-blended at about 1000C.

and, after cooling, the solidified mixture was crushed into powder. The powder which passed a 150mesh screen was applied to a mild steel plate by electrostatic coating of 60KV and baked at 1 600C for 30 minutes, resulting in a glossy coating film. Results of the evaluation of physical properties, a chemical resistance test and a weather resistance test conducted on the coating film are as follows: Coating-film thickness 60 to 70 Pencil hardness H Mandrel bending test, 10 mm Passed Erichsen cupping test 8 mm Cross hatch adhesion test 100/100 Ethyl-acetate resistance Passed (JIS K-5400) Dew-cycle weatherometer 250 hrs.No change Example 3 In 1.5 1 of ethyl acetate there were dissolved 243 g of 1 ,3,5-tris(isocyanatomethyl)benzene (MTI), and the solution was warmed to about 500 C. A solution of 264 g of methyl ethyl ketoxime in 11 of ethyl acetate was added dropwise to the solution at about 500C over a period of about 60 minutes, while stirring. Stirring was further continued for 1 hour while heating at about 500 C, and the reaction was allowed to go to completion. The ethyl acetate was distilled off at about 50 to 700C under reduced pressure, thereby yielding 490 g of a glass-state solid product. The product had a melting point of 70 to 800C and was easily pulverizable at room temperature.

101 parts of the product thus obtained and comprising MTI blocked with methyl ethyl ketoxime, were melt-blended with 500 parts of the same polyester that was used in Example 2 and, after cooling, the solidified mixture was crushed into a powder. The powder which passed a 150-mesh screen was applied to a mild steel plate by electrostatic coating at 60 KV and baked at 1 600C for 30 minutes, resulting in a glossy coating film.Results of the evaluation of physical properties, a chemical resistance test and a weather resistance test conducted on the coating film are as follows: Coating film thickness 60 to 70 Pencil hardness 2H Mandrel bending test, 10 mm) Passed Erichsen cupping test 8 mm Cross hatch adhesion test 1 00/100 Ethyl-acetate resistance Passed (JIS K-5400) Dew-cycle weatherometer, 250 hrs. No change Example 4 In 1 77 parts of ethyl acetate there were dissolved 249 parts of 1,3,5- tris(isocyanatomethyl)cyclohexane and 282 parts of phenol. To the solution was added 0.1 parts of dibutylin dilaurate and the reaction was continued at 750C until the isocyanate group had disappeared.

After the reaction, ethyl acetate was distilled off from the solution to dryness, thereby yielding solid blocked triisocyanate having a melting point of 151 to 1 540C.

1 77 parts of the blocked triisocyanate thus obtained were melt-blended with 303 parts of epoxy resin (epoxy equivalent about 1000, hydroxyl value about 185, melting point 95 to 1 040C.) at 11 00C, followed by cooling. The solidified mixture was crushed into powder The powder passing through a sieve of 1 50-mesh was electrostatically applied to the surface of a mild steel plate at 60 KV and the plate was baked at 1 6O0C for 30 minutes to obtain a tough coating film.Results of the evaluation of physical properties and of a chemical resistance test conducted on the film are as follows: Coating-film thickness 60 to 70 web Pencil hardness H Mandrel bending test, 10 mmS Passed Erichsen cupping test 7 mm Cross hatch adhesion test 100/1 00 Resistance to ethyl acetate Passed (JIS K-5400) Example 5 In 1 91 parts of ethyl acetate there were dissolved 249 parts of 1,3,5- tris(isocyanatomethyl)cyclohexane and 325 parts of benzylalcohol. To the solution was added 0.1 part of dibutylin dilaulate and the reaction was continued at 750C until the isocyanate group had disappeared.After the reaction ethyl acetate was distilled off from the solution to dryness, thereby yielding solid blocked triisocyanate having a melting point of 110 to 11 80C.

191 parts of the blocked triisocyanate were melt-blended at 1 1 OOC with 625 parts of acrylic polyol resin (melting point 78 to 850C, hydroxyl value 86), which was produced by polymerizing 48 parts of styrene, 30 parts of n-butyl acrylate, 20 parts of 2-hydroxyethyl methacrylate and 2 parts of acrylic acid in the conventional manner, followed by cooling. The solidified mixture was crushed into powder.

The powder passing through a sieve of 150-mesh was electrostatically applied to the surface of a mild steel plate at 60 KV and the plate was baked at 220 C for 15 minutes to obtain a tough coating film. Results of the evaluation of physical properties and of a chemical resistance test conducted on the film are as follows: Coating-film thickness 60 to 70 Pencil hardness 2H Mandrel bending test 10 mm Passed Erichsen cupping test 7 mm Cross hatch adhesion test 100/1 0O Resistance to ethyl acetate Passed (JIS K-5400) Dew cycle Weatherometer after 250 hours no change Example 6 In 196 parts of ethyl acetate there were dissolved 249 parts of 1,3,5tris(isocyanatomethyl)cyclohexane and 399 parts of E-caprolactam.To the solution was added 0.78 part of 1,1 ,3,3-tetrabutyl-1,3-diacetoxy distannoxane and the reaction was continued at 750C. under stirring until the isocyanate group had disappeared. After the reaction, ethyl acetate was distilled off from the solution to dryness, thereby yielding solid blocked triisocyanate having a melting point of 70 to 820C.

196 Parts of the blocked triisocyanate thus obtained were melt-blended at 1 0O0C with 395 parts of hydrolyzed ethylene/vinyl acetate resin (hydroxyl value 142, melting point 66 to 690C., ethylene unit content 79.5% vinyl acetate unit content 3.4% vinyl alcohol unit content 11.1 .1%) which was obtained by hydrolyzing the copolymer of 72 parts of ethylene and 28 parts of vinyl acetate, followed by cooling.

The solidified mixture was crushed into powder. The powder which passed a 1 50-mesh sieve was electrostatically applied to a mild steel plate at 60 KV, and the plate was baked at 1 800 C. for 30 minutes to obtain a tough coating film. Results of the evaluation of physical properties and a chemical resistance test conducted on the coating film are as follows: Coating film thickness 70 to 80 Pencil hardness 6B Mandrel bending test, 10 mmX Passed Erichsen pressing test 8 mm Cross hatch adhesion test 1 00/100 Ethyl-acetate resistance Passed (JIS K5400) Dew-cycle weatherometer, 250 hrs.No change Example 7 1 66 Parts of 1 ,3,5-tris(isocyanatomethyl)cyclohexane and 120 parts of hydrogenated bisphenol A were dissolved in 133 parts of ethyl acetate. To the solution was added 0.5 part of dibutyltin dilaurate and the reaction was conducted at 75 to 800 C. for 2 hours. When the isocyanate content in the reaction mixture was 10%, -caprolactam was added to the reaction mixture and the reaction was further continued at 750C. until the isocyanate group had disappeared. After the reaction, the ethyl acetate was distilled off from the solution under reduced pressure to dryness. The procedure provided a solid product comprising blocked isocyanate.

399 parts of the blocked polyisocyanate thus obtained were melt-blended with 1000 parts of the polyester polyol used in Example 2 and 1 50-mesh passed powder was obtained in the same manner as in Example 2. The powder was electrostatically applied to a mild steel plate at 60 KV and the plate was baked at 1 800C. for 30 minutes. Thus, a tough coating film was obtained.Results of the evaluation of physical properties, a chemical resistance test and a weather resistarice test conducted on the coating film are as follows: Coating-film thickness 60 to 70 y Pencil hardness 2H Mandrel bending test, 10 mmb Passed Erichsen cupping test 8 mm Cross hatch adhesion test 1 00/100 Ethyl-acetate resistance Passed (JIS K-5400) Dew-cycle weatherometer, 250 hrs. No change Reference Example 3 Production of 1,3,5-tricyanobenzene To 1 50 parts of a 33% aqueous oxalic acid solution were added 18.2 parts of vanadium pentoxide, and the mixture was heated at about 1000C. on a hot water bath to dissolve the vanadium pentoxide. The resultant solution was referred to as solution A.Similarly, a solution of 20 parts of chromium oxide in 1 50 parts of a 33% aqueous oxalic acid solution was referred to as solution B.

Solution A and B were uniformly mixed.

300 Parts of anatase-type titanium dioxide powder calcined at 8000C were added to the mixed solution, and the water was evaporated while stirring. The resulting pasty substance was wet-extruded to cylindrical-formed mouldings of 4 mm in diameter and 5 mm in length. The resulting mouldings were dried at 1000C. for 1 5 hours and calcined at 5000C. for 4 hours in air to make a catalyst.

About 100 ml of the catalyst thus obtained were placed in a conventional fixed-bed reaction apparatus and, while maintaining reaction tubes of the apparatus at a temperature of 3600C. in a bath, a gas mixture consisting of 0.5 mole % of mesitylene, 7 mole % of ammonia and 92.5 mole % of air was reacted under the conditions of space velocity of 100 hrs (converted in NTP) and atmospheric pressure, resulting in the formation of 1 ,3,5-tricyanobenzene in a yierd of 51.2 mole %.

Reference Example 4 Production of 1 ,3,5-tris(aminomethyl)benzene A 1 5 g portion of 1 ,3,5-tricyanobenzene (MTN) was charged into a closed autoclave which had an electromagnetic-stirrer and a capacity of 300 ml, together with 1 5 g of Raney-nickel-chromium catalyst (atomic ratio of Ni: CR=49:1), 27 ml of methanol, 63 ml of m-xylene and 0.18 g of caustic soda. Hydrogen at an initial pressure of 100 kg/cm2G was introduced under pressure into the autoclave, and the reaction was conducted at 1000C., with 0.59 mole of hydrogen being absorbed in 35 minutes. The catalyst was filtered out, and the solvent was distilled off, followed by vacuum distillation.By the above procedure, there were obtained 12.8 g of 1 ,3,5-tris(aminomethyl)benzene (MTA) as colourless crystals, which had a melting point of 49 to 51 OC. and a boiling point of 136 to 1390C/0.4 mmHg.

Reference Example 5 Production of 1 ,3,5-tris(aminomethyl)cyclohexane A 30 g portion of 1 ,3,5-tris(aminomethyl)benzene (MTA) was charged into a closed autoclave which had an electromagnetic-stirrer and a capacity of 300 ml, together with 3 g of 5% ruthenium alumina catalyst (produced by Nippon Engelhardt Co.), 60 g of water and 0.75 g of caustic soda, and high pressure hydrogen at an initial pressure of 1 20 kg/cm2G was introduced under pressure into the autoclave whereafter a reaction was conducted at 11 50C. for 25 minutes. 0.61 Mole of hydrogen was absorbed.

The catalyst was filtered out, and the solvent was distilled off, followed by the vacuum distillation, thereby yielding 26.8 g of 1 ,3,5-tris(aminomethyl)cyclohexane (H6MTA). The H6MTA obtained was a colourless, clear, liquid with a boiling point of 1 27-80C/1 mmHg.

Reference Example 6 Production of 1,3,5-tris(isocyanatomethyl)benzene A 90.0 g portion of 1 ,3,5-tris(aminomethyl)benzene (MTA) was warmed and dissolved in 1200 ml of o-dichloro-benzene in a four-necked flask with a capacity of 2 litres. Carbon dioxide gas was fed into the resulting amine solution until no increase in weight was observed, thereby yielding a slurry of colourless crystals. Phosgene gas was fed into the slurry with stirring, while maintaining it at a temperature not higher than 1 OOC. for 30 minutes. Then the temperature was increased to 1300 C.

over a 2-hour period and held at 1300 C. for 5 hours. As the phosgenation reaction proceeded, the slurry turned to a solution, finally becoming a uniform, slightly yellowish clear solution.

After the completion of the phosgenation reaction, nitrogen gas was introduced to remove dissolved phosgene, and the o-dichlorobenzene solvent was distilled off under reduced pressure. The resultant crude isocyanate was vacuum-distilled, thereby yielding 11 2.9 g of 1,3,5tris(isocyanatomethyl)benzene (MTI) of boiling point of 173 to 1750C./0.4 mmHg (in a molar yield of 85.2%). The MTI was a liquid substance even at 50C., and was free from isocyanate odour. The amine equivalent as determined was 83.25 (theoretical value 81.1).

Reference Example 7 Production of 1,3,5-tris(isocyanatomethyl)cyclohexane Phosgenation was conducted by the same procedure as described in Reference Example 4, except that 70.0 g of 1 ,3,5-tris(aminomethyl)cyclohexane (H6MTA) was used in place of 1,3,5tris(aminomethyl)benzene (MTA) and that the temperature was increased to 1 200 C. from 1 OOC. over a 6-hour period and held at 1200 C. for 6 hours, thereby yielding 91.8 g of 1,3,5tris(isocyanatomethyl)cyclohexane (H6MTI) of boiling point of 1 70 to 1740cm0.53 mmHg (in a molar yield of 90.10/0). The H6MTI was a liquid even at 50C and odourless. The amine equivalent as determined was 84.71 (theoretical value 83.08).

Claims

1. A powder coating composition which comprises (1) a blocked polyisocyanate obtained by blocking the free isocyanate groups of a triisocyanate of the formula:

Cl H2NCO OCNCH2 \CH2NCO (where ) on an isocyanate terminated adduct thereof with a blocking agent, and (2) a polyol resin having a melting point of not lower than 4O0C.

2. A powder coating composition as claimed in Claim 1, wherein the triisocyanate is 1,3,5- tris(isocyanatomethyl)benzene of the formula:

3. A powder coating composition as claimed in Claim 1, wherein the triisocyanate is 1,3,5tris(isocyanatomethyl)cyclohexane of the formula:

4. A powder coating composition as claimed in any one of Claims 1 to 3, wherein the blocking agent is a phenol, a lactam, a compound containing active methylene, an alcohol, a mercaptan, an acid amide, an imide, an amine, an imidazole, a urea, an imine, an oxime, a sulfite or a mixture of two or more thereof.

5. A powder coating composition as claimed in any one of Claims 1 to 3, wherein the blocking agent is a phenol, oxime, lactam or alcohol.

6. A powder coating composition as claimed in any one of Claims 1 to 3, wherein the blocking agent is an oxime.

7. A powder coating composition as claimed in any one of Claim 1 to 6, wherein the polyol resin is a polyester resin, an acrylic polyol resin, a hydroxyl-containing resin obtained by hydrolyzing a copolymer of a vinyl ester of saturated monocarboxylic acid and a vinyl monomer, an epoxy resin, a polyether resin, a polyether ester resin or a mixture of two or more thereof.

8. A powder coating composition as claimed in any one of Claims 1 to 6, wherein the polyol resin is a polyester resin or an acrylic polyol resin.

9. A powder coating composition as claimed in Claim 7 or 8, wherein the polyester resin has a molecular weight of 500 to 4000, a hydroxyl value of 10 to 300, an acid value of 1 to 30 and a melting point of 60 to 1 200C.

10. A powder coating composition as claimed in Claim 7 or 8 wherein the acrylic polyol resin has a hydroxyl value of about 10 to 300 and a melting point of 40 to 1 500C.

11. A powder coating composition as claimed in any one of Claims 1 to 10, wherein the blocked polyisocyanate and the polyol resin are present in amounts such that the equivalent ratio of regenerated free isocyanate groups are hydroxyl groups of the polyol resin is in the range of about 0.5 to 2.0.

12. A powder coating composition as claimed in any one of Claims 1 to 1 wherein the composition includes a dissociation catalyst for a blocked polyisocyanate.

13. A powder coating composition as claimed in Claim 12, wherein the composition contains from 0.01 to 5 weight % based on the composition of said dissociation catalyst.

14. A powder coating composition substantially as hereinbefore described in any one of the foregoing Examples 1 to 7.

1 5. A method of coating a substrate, wherein the powder coating composition claimed in any preceding claim is applied to the.substrate and the substrate is subjected to baking.

1 6. A method as claimed in Claim 15, wherein the baking is effected at a temperature of 130 to 3000C for a period of 1 minute to 60 minutes.

1 7. A method of coating a substrate substantially as hereinbefore described in any one of the foregoing Examples 1 to 7.

1 8. A triisocyanate of the formula where

1 9. 1 ,3,5-Tris(isocyanatomethyl)benzene.

20. 1 ,3,5-Tris(isocyanatomethyl)cyclohexane.

21. A method of preparing a triisocyanate of the formula defined in Claim 1 8, wherein a 1,3,5triaminomethyl compound of the formula

CH2NH2 H2NCH2 < CH2NH2 where /isgsorís phosgenated.

22. A method of preparing a triisocyanate of the formula defined in Claim 1 8 substantially as hereinbefore described in the foregoing Reference Example 3, 4 and 6 or 3, 5 and 7.