COATING COMPOSITIONS, COATINGS WITH FLAT FINISHES PRODUCED THEREFROM AND BLOCKED POLYISOCYANATES

Abstract: The present invention is directed to coating compositions comprising a) polyisocyanates having a content of isocyanate groups blocked by blocking agents of about 10 to 35% by weight and containing about 0.5 to 10.0 % by weight of structural units corresponding to the formula -CO-NH-NH-CO- and b) polyhydroxy polyacrylates in an amount sufficient to provide an equivalent ratio of blocked isocyanate groups to hydroxyl groups of about 0.5:1.0 to 2.0:1.0. The present invention is also directed to the coatings prepared from these coating compositions. Finally, the present invention is directed to blocked polyisocyanates which have a content of isocyanate groups blocked by blocking agents of about 10 to 35% by weight and about 0.5 to 10.0 % by weight (calculated as -CO-NH-NH-CO-, MW 86) of structural units corresponding to the formula -CO-NH-NH-CO-Q- wherein Q represents NH-NH-CO- or the group R-(CO-NH-NH-CO) n R is an n + 1 valent hydrocarbon radical and n has a value from 1 to 3.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention is directed to coating compositions, coatings produced therefrom which have flat finishes and blocked polyisocyanates which are useful in such coating compositions. Description of the Prior Art Polyurethane stoving lacquers are known and described in U.S. Patents 4,055,550; 4,252,923; 4,369,301; and 4,480,008. The coating compositions may be processed as a solution or as a powder and are based on a polyhydroxyl component and a polyisocyanate component wherein the isocyanate groups are completely or partially blocked with a blocking agent to prevent premature reaction of the components. A cross-linking reaction to form the coating only occurs at elevated temperatures which are sufficient to release the blocking agent. Regardless of whether the polyhydroxyl component is a polyester polyol or a polyacrylate polyol, the coatings prepared therefrom generally have a high gloss. In order to produce coatings with a flat or matt finish, it is necessary to add flatting agents to the coating composition. However, the incorporation of these flatting agents often reduces other desirable and necessary properties of the coating. U.S. Patent 4,480,008 is directed to the incorporation of dihydroxy carboxylic acids during the preparation of the blocked polyisocyanate component in order to provide coatings having a flat finish. It an object of the present invention to provide stoving compositions which may be used to prepare coatings having a flat finish. It is an additional object of the present invention to provide stoving compositions which may be used to prepare coatings which retain the desirable and necessary properties of stoving lacquers. These objects may be achieved in accordance with the present invention by the incorporation of certain components during the preparation of the blocked polyisocyanate as described hereinafter. U.S. Patent 4,369,301 is directed to the production of stoving compositions based on a blocked polyisocyanate which has been prepared by reacting an organic polyisocyanate with compounds containing hydroxyl groups and structural units corresponding to the formula -O-CO-NH-NH-CO-O- and/or the formula H₂N-NH-CO-O-. Coatings prepared from the polyisocyanate component disclosed therein possess improved resistance to yellowing. There is no disclosure in this reference that the disclosed structural units provide coatings having a flat finish when the blocked isocyanates are reacted with polyacrylate polyols. SUMMARY OF THE INVENTION The present invention is directed to a coating composition comprising a) a polyisocyanate having a content of isocyanate groups blocked by blocking agents of 10 to 35% by weight and containing 0.5 to 10% by weight of structural units corresponding to the formula -CO-NH-NH-CO-NH-NH-CO- (calculated as -CO-NH-NH-CO-, MW 86) and b) a polyhydroxy polyacrylate having a melting point or melting range within the temperature range of from 40 to 150° C in a amount sufficient to provide an equivalent ratio of blocked isocyanate groups to hydroxyl groups of 0.5:1.0 to 2.0:1.0. The present invention is also directed to the coatings prepared from these coating compositions. Finally, the present invention is directed to a blocked polyisocyanate having a content of isocyanate groups blocked by blocking agents of 10 to 35% by weight and 0.5 to 10% by weight (calculated as -CO-NH-NH-CO-, MW 86) of structural units corresponding to the formula -CO-NH-NH-CO-NH-NH-CO-. DETAILED DESCRIPTION OF THE INVENTION Suitable polyisocyanates for preparing the blocked polyisocyanates include any aliphatic, cycloaliphatic, araliphatic or aromatic isocyanates, preferably diisocyanates, having a molecular weight of 168 to 300. Examples include 2,4- and/or 2,6-diisocyanatotoluene, 2,4'- and/or 4,4'-diisocyanatodiphenylmethane, the methyl substituted derivatives of the last mentioned diisocyanates, 1,6-hexamethylene diisocyanate, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (isophorone diisocyanate or IPDI), m- and/or p-xylene diisocyanate, α,α,α',α'-tetramethyl-m- and/or -p-xylene diisocyanate, 4,4'-diisocyanatodicyclohexylmethane, 1-methyl-2,4- and/or -2,6-diisocyanatocyclohexane and mixture of the these diisocyanates. Also suitable are reaction products of these diisocyanates with subequivalent quantities of low molecular weight hydroxyl compounds having a molecular weight of 62 to 400 and containing 2 to 4 hydroxyl groups. Examples of these low molecular weight polyols include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, butane diol, hexane diol, 4,4'-dihydroxydicyclohexylmethane, 1,4-bis-(2-hydroxyethoxy)-benzene, trimethylol propane, glycerine, pentaerythritol and mixtures of these polyols. Also suitable are polyisocyanates having biuret groups and prepared by biuretizing the above-mentioned diisocyanates, and in particular 1,6-hexamethylene diisocyanate; polyisocyanates having uretdione groups as described, for example, in German Offenlegungsschrift 2,502,934; and polyisocyanates having isocyanurate groups and obtained by trimerizing the above mentioned diisocyanates, preferably 1,6-hexamethylene diisocyanate, 2,4- and/or 2,6-diisocyanatotoluene, isophorone diisocyanate and "mixed trimers" prepared from mixtures of these diisocyanates, in accordance with European Patent Application No. 10,589, German Offenlegungsschrift Nos. 2,707,656 and 2,806,731 and U.S. Patent 4,324,879. The polyisocyanates generally have an NCO content of 10 to 50 percent by weight, preferably 14 to 35 percent by weight. Preferred polyisocyanates are diisocyanates having aliphatically and/or cycloaliphatically bound isocyanate groups. In order to incorporate groups corresponding to the formula -CO-NH-NH-CO-NH-NH-CO. the previously described polyisocyanates are reacted with carbodihydrazide H₂N-NH-CO-NH-NH₂. Suitable blocking agents which are suitable for preparation of the blocked polyisocyanates in accordance with the present invention are known blocking agents for polyisocyanates. Examples of these blocking agents include monophenols such as phenol, the cresols, the trimethyl phenols and the tert butyl phenols; tertiary alcohols such as tertiary butanol, tertiary amyl alcohol and dimethylphenyl carbonyl; compounds which easily form enols such as acetoacetic ester, acetyl acetone and malonic acid derivatives, e.g., malonic acid diethylester; secondary aromatic amines such N-methylaniline, the N-methyl toluidines, N-phenyl toluidine and N-phenyl xylidene; imides such as succinimide; lactams such as ε-caprolactam and δ-valerolactam; oximes such as acetone oxime, butanone oxime and cyclohexanone oxime; mercaptans such as methylmercaptan, ethyl mercaptan, butyl mercaptan, 2-mercapto-benzothiazole, α-napthyl mercaptan and dodecyl mercaptan; and triazoles such as 1H-1,2,4-triazole. Preferred blocking agents are acetone oxime and especially ε-caprolactam. The reaction between the organic polyisocyanate, carbodihydrazide and blocking agent is generally conducted at a temperature of 50 to 200°C, preferably 100 to 160°C. The reactants are used in such quantities that the blocked polyisocyanates contain 10 to 35%, preferably 15 to 35% by weight of blocked isocyanate groups (calculated as NCO, molecular weight equals 42) and 0.5 to 10%, preferably 1 to 5% by weight of structural units corresponding to the formula -CO-NH-NH-CO-NH-NH-CO- (calculated as -CO-NH-NH-CO; MW 86). The percentages are based on the total weight of the blocked polyisocyanate excluding the blocking agent. The organic polyisocyanate may be reacted with the carbodihydrazide and the blocking agent successively or simultaneously. When the blocking reaction is conducted before the reaction with the carbodihydrazide, care must be taken that all of the isocyanate groups are not blocked by the blocking agent. According to a preferred embodiment of the process, the starting polyisocyanate is reacted simultaneously with the carbodihydrazide and the blocking agent using a solution of the hydrazine adduct in the blocking agent. This reaction is preferably carried out such that the organic polyisocyanate is initially partially blocked with some of the blocking agent and the partly blocked polyisocyanate is then reacted with a solution of the carbodihydrazide in the remainder of the blocking agent. The present process may be carried out in the absence of solvents or in the presence of suitable inert solvents such as ethylene glycol monoethyl ether acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, toluene or xylene. The reaction according to the present invention is generally carried out with heating and within the temperature ranges specified until the homogeneous melt or solution is produced. Auxiliaries or solvents which are optionally used and excess blocking agent which may be present are then removed, for example by distillation. In order to obtain the coating compositions according to the present invention, the blocked polyisocyanates are blended with polyhydroxy polyacrylates. These polyhydroxy polyacrylates are known and may be prepared by known methods such as those described in European Patent Office Publication 68,383, German Patentschrift 2,460,329, British Patent 1,515,868, U.S. Patent 3,002,959, U.S. Patent 3,375,227 or German Auslegeschrift 1,038,754. The polyhydroxy polyacrylates are generally prepared by the radical polymerization or copolymerization of a hydroxyalkyl ester of acrylic or methacrylic acid, with itself or preferably together other hydroxyl-free unsaturated monomers. The polyhydroxy polyacrylates generally have a molecular weight, as determined by vapor pressure osmometry of 232 to 100,000, preferably 1000 to 80,000, more preferably 2000 to 50,000, and a hydroxyl group content of 0.017 to 15% by weight, preferably 0.1 to 10% by weight and more preferably 0.5 to 5% by weight. The polyhydroxy polyacrylates preferably have at least two alcoholic hydroxyl groups per molecule as a statistical average. The molecular weights indicated for the polyacrylates containing hydroxyl groups are number average molecular weights and not weight average molecular weights which may be determined by light scattering. Suitable hydroxylalkyl esters include esters containing 2 to 8, preferably 2 to 4 carbon atoms in the alkyl group and obtained from acrylic or methacrylic acid. Hydroxyalkyl esters of the above-mentioned acids containing ether bridges in the alkyl groups may also be used but are less preferred. The particularly preferred monomers with alcoholic hydroxyl groups include the 2-hydroxyethyl-, 2- and 3-hydroxypropyl-, and 2-, 3- and 4-hydroxybutyl-acrylates and -methacrylates. These monomers containing alcoholic hydroxyl groups may be prepared, for example, by the reaction of the above-mentioned acids with epoxides such as alkylene or propylene oxide. The polyhydroxy polyacrylates which are used may also be prepared by reacting the corresponding polyacrylates containing carboxylic acid groups with alkylene oxides such as propylene oxide and/or ethylene oxide in the presence of suitable alkoxylation catalysts such as tetrabutylammonium bromide. The starting materials for this alkoxylation reaction, i.e., the polyacrylates containing carboxylic acid groups, may be obtained in known manner by the copolymerization of acrylic acid and/or methacrylic acid with unsaturated comonomers which do not contain carboxyl or hydroxyl groups. The preferred method for preparing the polyhydroxy polyacrylates is the copolymerization of the hydroxyalkyl esters. The comonomers used for the above-mentioned hydroxyl group-containing monomers may be any α-β-olefinically unsaturated compounds in the molecular weight range of 28 to 350 which are free from hydroxyl groups such as ethylene, propylene, butene-1, hexene-1, octene-1, styrene, -methylstyrene, divinyl benzene, various isomeric vinyl toluenes, esters of α,β-unsaturated carboxylic acids of the type exemplified above monohydric aliphatic alcohols having 1 to 18, preferably 1 to 10 carbon atoms, in particular the corresponding esters of acrylic or methacrylic acids such as the methyl, ethyl, N-butyl, N-pentyl, N-hexyl, 2-ethylhexyl or octadecyl esters of acrylic or methacrylic acid. Neutral esters of polycarboxylic acids are also suitable comonomers, for example, itaconic, crotonic, maleic or fumaric esters of the monohydric alcohols exemplified above. Acrylic acid, methacrylic acid, vinyl acetate, acrylonitrile, methacrylonitrile and dienes such as isoprene or or butadiene are all suitable comonomers. Vinyl chloride may in principle also be used as a comonomer. Particularly preferred polyhydroxy polyacrylates are obtained from 10 to 50 parts by weight of hydroxyalkyl esters of acrylic or methacrylic acid, 0 to 80 parts by weight of styrene and/or -methylstyrene, 10 to 90 parts by weight of an acrylic and/or methacrylic acid ester free from hydroxyl group of the type exemplified above and 0 to 5 parts by weight of an α,β-unsaturated mono- or dicarboxylic acid of the type exemplified, in particular acrylic acid or methacrylic acid. For preparing powder coating compositions in accordance with the present invention the polyhydroxy polyacrylates and the blocked polyisocyanates are each selected from those having melting points resp. melting ranges within the temperature range of from 40 to 150°C, preferably 50 to 120°C. In order to prepare powder coating compositions in accordance with the present invention, the polyhydroxy polyacrylates and the blocked polyisocyanates are mixed and optionally provided with pigments, flowing agents, catalysts (tin compounds) and other conventional additives which are mixed in the melt into a homogeneous material in extruders at temperatures of 100°C to 120°C. The polyhydroxy polyacrylates and the blocked polyisocyanates are used in such quantities that preferably 0.6 to 1.2, more preferably 0.8 to 1.0 isocyanate group is provided for each hydroxyl group. The solidified material is ground and adjusted to the required grain size of approximately 0.1 mm by sieving. The resulting powder lacquer may be applied to molded parts to be coated according to the conventional powder application processes, for example, by electrostatic powder spraying. The coatings are hardened by heating to temperatures of 150°C to 220°C, preferably 170°C to 190°C. Hard, flat elastomeric coatings are obtained which have excellent anti-corrosion characteristics. It if is desired to prepare solvent-containing stoving lacquers, solutions of the blocked polyisocyanates are mixed with solutions of the polyhydroxy polyacrylates and other conventional additives described above. Suitable solvents include toluene, xylene, butylacetate, ethylene glycol monoethyl ether acetate, methyl ethyl ketone, methyl isobutyl ketone, or N-methyl pyrrolidone. The solvents are generally present in quantities of 30 to 60% by weight, based on the weight of the coating composition. These lacquers may be applied by spraying or roll coating. The coated parts may be stoved at 120° to 250°C, depending on the blocking agent used and on the residence time provided in the furnace. Stoving lacquers of this type are used, for example, in tube lacquering or for coil coating sheets. The lacquer films exhibit good mechanical properties in addition to a desirable weather resistance. The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise specified. Preparation of Blocked Polyisocyanates. Example 1 A mixture of 211.2 g of bis(4-isocyanatocyclohexyl) methane® (Desmodur W, Mobay Corporation), and 163.9 g of ε-caprolactam were reacted at 80°C until the isocyanate content (by dibutyl amine titration) reached a constant value of 1.65%. To the mixture were then added 3.6 g of trimethylolpropane and 4.0 g of carbodihydrazide. The reaction temperature was raised to 110°C for two hours and then to 135°C for two additional hours at which time the free isocyanate content was 0.45%. While at this temperature, a total of 8 g of stannous octoate (T-9, Air Products) was added in three portions. The product was poured into an aluminum foil tray, allowed to cool and then broken into small pieces for storage. Blocked NCO content (calculated) 26.7%, content of structural units corresponding to the formula, -CO-NH-NH-CO-,1.7%. Example 2 A product was made according to the procedure of Example 1 except that stannous octoate was not added at the end of the reaction. Blocked NCO content (calculated) 27.1%, content of structural units corresponding to the formula, -CO-NH-NH-CO-,1.8% Example 3 A product was prepared according to the procedure of Example 1 using 209.9 g of bis(4-isocyanatocyclohexyl) methane, 149.2 of ε-caprolactam, 8.05 g of trimethylol propane and 4.5 g of carbodihydrazide. No stannous octoate was added to the product. Blocked NCO content (calculated) 21.3%, content of structural units corresponding to the formula, -CO-NH-NH-CO- 1.9%. Example 4 A product was prepared according to the procedure of Example 1 using 179 g of isophorone diisocyanate, 163.9 of ε-caprolactam, 3.6 g trimethylolpropane, 4.0 g of carbodihydrazide and 8.0 g of stannous octoate. Blocked NCO content (calculated) 30.4%, content of structural units corresponding to the formula, -CO-NH-NH-CO-,2.0%. Example 5 (Comparison) A product was prepared according to the procedure of Example 1 from 211.9 g of bis(4-isocyanatocyclohexyl) methane, 163.9 g of ε-caprolactam, 7.6 g of trimethylol propane and 8.0 g of stannous octoate. Blocked NCO content (calculated) 26.7%, content of structural units corresponding to the formula, -CO-NH-NH-CO, 0.0%. Example 6 (Comparison) A product was prepared according to the procedure of Example 1 from 217 g of isophorone diisocyanate, 158.2 of ε-caprolactam, 24.8 g of trimethylol propane and 11.2 g of stannous octoate. Blocked NCO content (by calculation) 24.4%, content of structural units corresponding to the formula, -CO-NH-NH-CO, 0.0%. Application Examples The products of Example 3 was evaluated in a powder coatings formulation as described in Table I. The components were melt extruded at ca. 115°C in a ZDSK 28 extruder (Werner and Pfleiderer). The melt was cooled to room temperature, pulverized and run through a 90 µm sieve to remove large particles. The powder was then electrostatically applied to a metal panel. The bake schedules and film evaluations are shown in Table II. Test Definitions Gloss: Specular gloss measured with a Gardner gloss meter at a 60° angle to the coated plate. Acetone Resistance: A cotton ball saturated with acetone was rubbed back and forth over the film 50 times. After 5 minutes recovery the coating was scratched with a fingernail and judged by the following rating scale: 0 - No Effect 1 - Film softened at surface but not through to substrate 2 - Film completely softened and disbonded from substrate All amounts shown in Table I are in parts by weight. Application Example 9 The blocked polyisocyanates from Examples 1 and 2 and Comparison Example 5 were evaluated as cross-linkers for a hydroxyl functional acrylic. A pigmented dispersion of the polyhydroxy polyacrylate was prepared using the formulation and procedure described in Table IA. Each example was combined with a sufficient quantity of pigmented polyol to formulate a coating material having a NCO:OH equivalent ratio of 1:1. These combination weight ratios are listed on Table 2A. Each prepared coating sample, after thorough agitation, was applied by spray to pretreated steel and aluminum anodized panels. Each panel was air dried for 20-40 minutes and then cured at 200°C x 10 minutes in a forced air oven. These coatings were subsequently tested and the results are listed in Table 3A. Test Definition: 1) Gloss: Coating gloss was measured at both 60° and 20° using a Gardner gloss meter. 2) MEK Resistance: (Double Rubs) A MEK saturated cotton ball rubbed repeatedly (100 times) back and forth over the coatings surface. observations, both visual and physical, are made immediately upon completion of the test. 3) Pendulum Hardness: Coating hardness was measured using an ERICKSEN Pendulum Hardness Model #299/300. Values are reported in seconds. Procedure: The polyhydroxy polyacrylate was reduced to 45% solids in methyl n-amyl ketone (MAK). The flow modifiers and reducing solvents were slowly stirred into the resin solution. The pigment was ground into this solution using a shaker mill. Grinding was stopped when a particle size of 40 µm or less was achieved. Application Example 10 The blocked polyisocyanates of Examples 4 and 6 (Comp) were evaluated in accordance with application Example 9. These two hardeners were blended with the pigmented polyol portion at a ratio (pbw) of 93.2:6.8 and 92.8:7.2, respectively. The coatings products were spray applied over pretreated steel and aluminum anodized panels. After a 20-40 minute flash time, the coatings were cured for 10 minutes at 200°C. Gloss measurements were taken and are listed on Table 1B. Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein within the scope of the claims.

Claims: A coating composition comprising a) a polyisocyanate having a content of isocyanate groups blocked by blocking agents of 10 to 35% by weight and containing 0.5 to 10% by weight of structural units corresponding to the formula -CO-NH-NH-CO-NH-NH-CO- (calculated as -CO-NH-NH-CO-, MW 86) both percentages being based on the total weight of the blocked isocyanate excluding the blocking agent and b) a polyhydroxy polyacrylate having a melting point or melting range within the temperature range of from 40 to 150° C in a amount sufficient to provide an equivalent ratio of blocked isocyanate groups to hydroxyl groups of 0.5: 1.0 to 2.0: 1.0. The coating composition of Claim 1 wherein said polyisocyanate is based on a diisocyanate containing aliphatically- and/or cycloaliphatically-bound isocyanate groups. A coating prepared from a coating composition according to claim 1. A blocked polyisocyanate having a content of isocyanate groups blocked by blocking agents of 10 to 35% by weight and 0.5 to 10% by weight (calculated as -CO-NH-NH-CO-, MW 86) of structural units corresponding to the formula -CO-NH-NH-CO-NH-NH-CO- both percentages being based on the total weight of the blocked isocyanate excluding the blocking agent.

Inventors: GRUBBS, HUGH C., POTTER, TERRY A.
Applicants: MILES INC.
Application year: 1990
Main classification: C08G18
IPC codes: C09D5, C08G18, C09D175