Powder Enamels Free of Blocking Agents

1. Field of the Invention

The present invention relates to a method for producing polyurethane powder enamels free of blocking agents which have excellent storage stability and which cure above 120.degree. C. into elastic, impact resistant and weather resistant enamel films, as well as the polyurethane powder enamels produced by the process.

2. Description of the Prior Art

In German OS No. 24 20 475 polyurethane powder enamels are described that are produced by the reaction of polyisocyanates having uretidione groups with polyols. However, the examples are limited to the uretidione dimer of toluene diisocyanate and the uretidione dimer of hexamethylene diisocyanate. Also it is necessary in this case to compensate for the higher functional portion of the cross-linking agent with monofunctional reagents, i.e., the uretidione/isocyanurate mixtures must first be linearized.

From German OS No. 25 02 934 it is known to produce PUR-powder enamels, in which polyisocyanates that contain at the same time, latent isocyanate groups in the form of uretidione groups and terminal isocyanate groups blocked with blocking agents, are cured with polyols. Depending on the particular blocking agent used, more or less extensive environmental pollution occurs during the curing process. Furthermore, in this case, the functional groups of the cross-linking component play no part in the production of the enamel.

With the dimerization catalysts belonging to the prior art (tertiary phosphines) it has hitherto not been possible to produce an isocyanurate-free uretidione dimer of isophorone diisocyanate (IPDI). A uretidione dimer of isophorone diisocyanate produced in accordance with German OS No. 16 70 720 or German OS No. 19 34 763 contains an additional ca. 20-40% by weight of IPDI trimer (isocyanurate trimer of isophorone diisocyanate), depending on the reaction conditions.

Thus, it is not surprising that until now no polyurethane powder enamels free of blocking agents were known that were prepared from polyols and linear isophorone diisocyanate polyaddition compounds containing terminal non-blocked NCO groups and uretidione groups, because the prerequisite for producing the above-mentioned polyaddition compounds containing uretidione groups is that the uretidinedione-containing diisocyanate added to extend the chain by reacting with the diols must have an NCO funtionality of 2. When the NCO functionality of the uretidione-containing diisocyanate is >2 at least some gelling must be expected when it is reacted with diols.

The known uretidione dimer of isophorone diisocyanate, which contains isocyanurate, cannot be considered when further reaction with diols is intended in order to prepare useful starting materials for polyurethane chemistry (binding components for PUR powders free of blocking agents).

Production of blocking agent-free polyurethane powder enamels according to the invention was first made possible by the discovery of a catalyst for dimerization of IPDI that permitted the production of a practically isocyanurate-free uretidione dimer of isophorone diisocyanate which can be more than 98% decomposed by heating back into IPDI.

The production of this uretidione-containing isophorone diisocyanate is not the subject of this patent application. It is prepared by dimerizing isophorone diisocyanate, optionally in an inert organic solvent, with the help of a catalyst of the general formula

wherein

m=0, 1 or 2

X=Cl, OR or R,

R=identical or different alkyl radicals, aralkyl radicals or cycloalkyl radicals (optionally alkyl substituted), or two radicals together form a ring,

at temperatures of 0.degree.-80.degree. C., preferably 10.degree.-30.degree. C. After a conversion of 5-70%, preferably 20-50%, the 1,3-diazacyclobutanedione-2,4 may be isolated from the reaction mixture, without prior deactivation of the catalyst, as the residue remaining after thin film distillation. Monomer and catalyst may be isolated in the distillate.

The formation of the polyadduct, which is also not claimed in this application, is carried out by reaction of this pure uretidione-containing dimer of isophorone diisocyanate with diols.

Furthermore, production of the polyurethane powder enamels free of blocking agents according to the invention only became possible with the successful preparation of polyhydroxy compounds which, due to their physical and chemical properties, prevent gelling and, due to their low melt viscosity, assure good flow of the enamel films.

It has now been found that extremely high quality polyurethane powder enamels are obtainable by using as cross-linking agents certain isocyanurate-free isophorone diisocyanate polyaddition products which contain uretidione groups, and which are essentially free from isocyanate blocking agents that would split off and become volatile under baking conditions, in combination with polyhydroxy compounds having low melt viscosity and limited hydroxyl functionality.

In addition, the coating media according to the invention have the advantage of providing enameling even in thick layers without film flaws such as bubbles and pits. The fact that no blocking agent is liberated during the curing process constitutes a very substantial advantage. The physiological iritation of workers is avoided, devices to remove offensive odors can be eliminated and there are practically no environmental problems. Furthermore, the economic aspect cannot be ignored, particularly inasmuch as all components are completely incorporated into the enamel film.

Accordingly, the object of the present invention is a process for preparing polyurethane powder enamels free of blocking agents, having good storage stability, which are curable above 120.degree. C., and which are based on linear polyisocyanate addition products containing uretidione groups and terminal non-blocked NCO groups, and polyols, characterized in that,

a. terminally non-blocked polyaddition products of isophorone diisocyanate dimer and diols, which contain uretidione groups, whose uretidione groups derived from isophorone diisocyanate can be more than 98% decomposed back into isophorone diisocyanate by heating, having the following formula ##STR2## n.gtoreq.1, and R'=a divalent aliphatic, cycloaliphatic, araliphatic or aromatic hydrocarbon radical of a diol,

having an NCO/OH ratio of uretidione dimer of isophorone diisocyanate to diol of 1:0.5 to 1:0.9, preferably 1:0.6 to 1:0.8, a uretidione-diol molar ratio of 2:1 to 6:5, preferably 3:2 to 5:4, a free isocyanate content up to 8% by weight and preferably <5% by weight, a molecular weight between 500 and 4000, preferably between 1450 and 2800, and a melting point of .gtoreq.70.degree. C. to .ltoreq.130.degree. C., preferably 85.degree. C. to 120.degree. C.,

b. polyhydroxy compounds having an OH functionality of .gtoreq.2.2 to .ltoreq.3.5, preferably 2.5 to 3.4, an average molecular weight of 2000 to 4500, preferably 2300 to 3900, a hydroxyl number between 30 and 100 mg KOH/g, preferably between 40 and 80 mg KOH/g, a viscosity of less than 80,000 mPa.multidot.s, preferably 30,000 mPa.multidot.s, at 160.degree. C. and a melting point between .gtoreq.65.degree. C. and .ltoreq.120.degree. C., preferably 75.degree. C. to 100.degree. C., and

c. the additives customary in polyurethane chemistry are reacted, wherein the molar ratio of a. and b. is selected so that the NCO/OH ratio is between 0.8:1 and 1.2:1.

The common additives include accelerating catalysts, pigments, dyes, and fillers. The catalysts are added in amounts of 0.05-1.5% by weight, preferably 0.1-0.5% by weight. The proportion of filler is large; it can be up to 60% by weight, preferably up to 50% by weight.

A highly pure uretidione dimer of IPDI is obtained by dimerizing IPDI, optionally in an inert organic solvent, in the presence of certain catalysts at temperatures of 0.degree.-80.degree. C., preferably 10.degree.-30.degree. C., whereby after a conversion of 5-70%, preferably 40-60%, without deactivating the catalyst, uretidione dimer of isophorone diisocyanate is isolated as the residue remaining after thin film distillation, and the monomer and catalyst are recovered in the distillate.

The catalysts are compounds of the following formula:

wherein the symbols have the meaning defined above.

The isocyanurate-free uretidione dimer of isophorone diisocyanate is highly viscous at room temperature (>10.sup.6 mPa.multidot.s; at 60.degree. C., 13.multidot.10.sup.3 mPa.multidot.s; at 80.degree. C., 1.4.multidot.10.sup.3 mPa.multidot.s). Its NCO content is in the range of 16.8-18% NCO; this means that a fairly large proportion of the polyuretidione derived from IPDI must be present in the reaction product. The monomer content is <1%. The NCO content of the reaction product after being heated to 180.degree.-200.degree. C. is 37.1-37.7% NCO.

The isocyanurate-free uretidione dimer of isophorone diisocyanate is reacted with diols at an NCO/OH ratio of 1:0.5-1:0.9, preferably 1:0.6-1:0.8, to form the polyaddition product. The process can be conducted so that the diol is added to the uretidione dimer of isophorone diisocyanate all at once or gradually.

Examples of such diols include ethylene glycol, 1,2- and 1,3-propylene glycol, 2-ethyl-1,3-hexanediol, 1,6-hexanediol, 1,8-octanediol, neopentyl glycol, 1,4-bishydroxymethylcyclohexane, 3(4),8(9)-bishydroxymethyltricyclodecane, 2-methyl-1,3-propanediol, 3-methyl-1,5-pentanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol and dibutylene glycol. The preferred linking diol used in the synthesis is 1,4-butylene glycol.

The reagents are mixed together in the given ratios to produce the polyaddition products. In general, one begins with the isocyanate component containing the uretidione and the diol is added. The reaction can be carried out neat or also in the presence of suitable solvents. The following, for example, are suitable solvents: benzene, toluene, methyl or ethyl glycol acetate, dimethylformamide, xylene and other aromatic or aliphatic hydrocarbons; also ketones, such as acetone, methyl butyl ketone, methyl isobutyl ketone, and cyclohexanone and chlorinated aromatic hydrocarbons, as well as any mixtures of these and other inert solvents.

The reaction is generally carried out at temperatures of 50.degree.-120.degree. C., preferably 60.degree.-90.degree. C. The reagents are heated at the given temperatures until all glycol hydroxyl groups have been converted into urethane groups. Depending on the reaction temperature, this requires from 0.5 to 4 hours.

Isocyanate polyaddition accelerating catalysts can also be used. The preferred catalysts used are organic tin compounds such as tin(II) acetate, tin(II) octanoate, tin(II) laurate, dibutyltin diacetate, dibutyltin maleate or dioctyltin diacetate. The catalysts are generally added in amounts between 0.01 and 0.05% by weight, based on the total amount of reactants.

In the workup of the reaction mixture, the polyuretidione-polyurethane is generally freed of any solvent that might have been used. This can be done by simply removing the solvent in a vacuum. Melt extrusion with an evacuated screw is especially suited for removing the solvent.

The total NCO content is determined by titration with di-n-butylamine, while the content of free NCO groups is determined by titration with di-sec-butylamine.

The isophorone diisocyanate-uretidione polyaddition adducts containing uretidione groups which are used in the process of the invention can be ground or pulverized and can be homogeneously dispersed in this form in the reagents. One great advantage is the fact that these uretidione polyaddition adducts are reactive powders that can undergo defined cross-linking reactions with reagents without elimination of any blocking agent. Such reagents are those carrying functional groups that react with isocyanate groups during the curing process, depending on the temperature and the time, e.g., hydroxyl, carboxyl, mercapto, amino, amido, urethane and (thio)urea groups. Polymerizates, polycondensates and polyaddition compounds can be used as polymers.

The preferred compounds are primarily polyethers, polythioethers, polyacetals, polyesteramides, epoxide resins with hydroxyl groups in the molecule, phenol/formaldehyde resins, aminoplasts and their derivatives modified with polyfunctional alcohols, aniline/formaldehyde resins, polyazomethines, polyurethanes, polyureas and polythioureas, polysulfonamides, melamine derivatives, cellulose esters and ethers, partially saponified homopolymers and copolymers of vinyl esters, and especially polyesters and acrylate resins.

The polymers containing hydroxyl groups used according the the invention have an OH functionality of .gtoreq.2.2 to .ltoreq.3.5, preferably 2.5 to 3.4, a hydroxyl number between 30 and 100 mg KOH/g, preferably 40-80 mg KOH/g, a viscosity of <80,000 mPa.multidot.s, preferably <30,000 mPa.multidot.s, at 160.degree. C., and a melting point .gtoreq.65.degree. C. and .ltoreq.130.degree. C., preferably 75.degree. C. to 100.degree. C.

The preferred components are enumerated below:

For the production of polyesters, the preferred carboxylic acids can be aliphatic, cycloaliphatic, aromatic and/or heterocylic in nature and can optionally be substituted, by halogen atoms, for example. They may also be unsaturated. Examples are the following: succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, terephthalic acid, isophthalic acid, trimellitic acid, pyromellitic acid, tetrahydrophthalic acid, hexahydrophthalic acid, di- and tetrachlorophthalic acid, endomethylenetetrahydrophthalic acid and its hexachloro derivative, glutaric acid, maleic acid, fumaric acid or their existing anhydrides, dimers and trimers of fatty acids such as oleic acid, optionally in mixtures with monomeric fatty acids, dimethyl terephthalate, terephthalic acid bis-glycol ester, also cyclic monocarboxylic acids such as benzoic acid, tertbutylbenzoic acid or hexahydrobenzoic acid. Possible polyhydric alcohols are, e.g.: ethylene glycol, 1,2- and 1,3-propylene glycol, 1,4- and 2,3-butylene glycol, di-.beta.-hydroxyethylbutanediol, 1,6-hexanediol, 1,8-octanediol, neopentyl glycol, cyclohexanediol, 1,4-bis(hydroxymethyl)cyclohexane, 2,2-bis(4-(.beta.-hydroxyethoxy)phenyl)propane, 2-methyl-1,3-propanediol, glycerin, trimethylolpropane, 1,2,6-hexanetriol, 1,2,4-butanetriol, tris(.beta.-hydroxyethyl)isocyanurate, trimethylolethane, pentaerythritol, quinitol, mannitol and sorbitol, formoses and their hydroxyalkylation products, methylglycoside, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycols, dipropylene glycol, tripropylene glycol, polypropylene glycols, dibutylene glycol, polybutylene glycols and xylylene glycol. Mono- and polyesters of lactones, e.g., .epsilon.-caprolactam or hydroxycarboxylic acids, such as, e.g., .epsilon.-caprolactam or hydroxycarboxylic acids, such as, e.g., hydroxypivalic acid, .omega.-hydroxydecanoic acid, .omega.-hydroxycaproic acid and thioglycolic acid, can also be used as well as polyesters of the above named polycarboxylic acids or their derivatives and polyphenols such as hydroquinone, bisphenol-A, 4,4'-dihydroxybiphenyl or bis-(4-hydroxyphenyl)sulfone; fatty acid-modified polyesters ("oil alkyds") and naturally occurring saturated or unsaturated polyesters, their decomposition products or transesterification products with polyols such as castor oil, tall oil, soybean oil and linseed oil; polyesters of the carboxylic acids, which can be obtained by conventional procedures from hydroquinone, diphenylolpropane, p-xylylene glycol, ethylene glycol, butanediol or 1,6-hexanediol and other polyols through conventional condensation reactions, e.g., with phosgene or diethyl- or diphenyl carbonate, or from cyclic carbonates such as glycol carbonate or vinylidenecarbonate by conventional polymerization; polyesters of silicic acids; polyesters of phosphonic acids, e.g., of methane-, ethane-, .beta.-chloroethane-, benzene- or styrenephosphonic acid, or -phosphonic acid esters and polyalcohols or polyphenols of the type mentioned above; polyesters of phosphorus-containing acids from phosphorous acid, phosphorous acid esters, -ester amides or -ester chlorides and polyalcohols, polyetheralcohols and polyphenols; polyesters of acid by oxidation or through transesterification of phosphoric acid esters with polyalcohols or polyphenols; polyesters of boric acid; polysiloxanes such as those products obtained by hydrolysis of dialkyldichlorosilanes with water and subsequent treatment with polyalcohols or by addition of polydihydrosilane to olefins such as allyl alcohol or acrylic acid.

Preferred polyesters also include, for example, the reaction products of polycarboxylic acids and glycidyl compounds as described, e.g., in German OS No. 24 10 513.

The following are examples of glycidyl compounds that can be used: esters of 2,3-epoxy-1-propanol with monobasic acids having 4-18 carbon atoms such as glycidyl palmitate, glycidyl laurate and glycidyl stearate; alkylene oxides having 4 to 18 carbon atoms such as butylene oxide and glycidyl ethers such as octyl glycidyl ether.

The polycarboxylic acids listed hereinbelow under II can be used in this method as the dicarboxylic acids. Monocarboxylic acids which are listed under III, for example, can also be used.

Monomeric esters are also preferred ingredients, e.g., dicarboxylic acid bis(hydroxyalcohol) esters, monocarboxylic acid esters with more than dihydric polyols and oligoesters that can be produced by condensation from raw materials common in enamel chemistry.

Such compounds are, for example:

I. Alcohols having 2-24, preferably 2-10 C-atoms, and 2-6 OH groups bonded to nonaromatic C-atoms, e.g., ethylene glycol, propylene glycols, diethylene glycol, dipropylene glycol, butandiols, neopentyl glycol, hexanediols, hexanetriols, perhydrobisphenol, dimethylolcyclohexane, glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, dipentaerythritol, mannitol;

II. Di and polycarboxylic acids having 4-36 C-atoms and 2-4 carboxyl groups, as well as their derivatives capable of esterification, such as anhydrides and esters, e.g. phthalic acid (anhydride), isophthalic acid, terephthalic acid hexahydroterephthalic acid, hexahydroisophthalic acid, alkyltetrahydrophthalic acids, endomethylenetetrahydrophthalic acid anhydride, adipic acid, succinic acid, maleic acid, fumaric acid, dimeric fatty acids, trimellitic acid, pyromellitic acid, azelaic acid;

III. Monocarboxylic acids having 6-24 C-atoms, e.g., caprylic acid, 2-ethylhexanoic acid, benzoic acid, p-tert-butylbenzoic acid, hexahydrobenzoic acid, monocarboxylic acid mixtures of natural oils and fats such as coconut oil fatty acid, soybean oil fatty acid, castor oil fatty acid, hydrated and isomerized fatty acids, such as "Konjuvandol"-fatty acid, as well as their mixtures, wherein the fatty acids can also be used as glycerides and can be modified by transesterification and/or dehydration;

IV. Monohydric alcohols having 1-18 C-atoms, e.g., methanol, ethanol, isopropyl alcohol, cyclohexanol, benzyl alcohol, isodecanol, nonanol, octanol and oleyl alcohol.

The polyesters can be obtained in known manner by condensation in an inert gas atmosphere at temperatures of 100.degree.-260.degree. C., preferably 130.degree.-220.degree. C., in the melt or by an azeotropic procedure, as described in, e.g., Methods of Organic Chemistry (Houben-Weyl), Vol 14/2. 1-5, 21-23, 40-44, Georg Thieme Verlag, Stuttgart, 1963, or by C. P. Martens, Alkyd Resins, 51-59, Reinhold Plastics Appl. Series, Reinhold Publishing Co., New York, 1961.

Preferred acrylate resins which can be used as the OH component are homo- or copolymers wherein, for example, the following monomers can be selected as starting materials:esters of acrylic acid and methacrylic acid with dihydric, saturated aliphatic alcohols having 2-4 C-atoms, such as, e.g., 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate and the corresponding methacrylic acid esters; acrylic and methacrylic acid; acrylic acid and methacrylic acid alkyl esters having 1-18, preferably 1-8 C-atoms in the alcohol component, such as methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, tert-butyl acrylate, 2-ethylhexyl acrylate, stearyl acrylate and the corresponding methacrylic acid esters; acrylic acid and methacrylic acid cyclohexyl esters; acrylonitrile and methacrylonitrile; acrylamide and methacrylamide; N-methoxymethyl(meth)acrylamide.

Particularly preferred acrylic resins are copolymers of:

1. 0-50% by weight of monoesters of acrylic or methacrylic acid with di- or polyhydric alcohols, such as 1,4-butanediol monoacrylate, hydroxypropyl (meth)acrylate; also vinyl glycol, vinyl thioethanol, allyl alcohol, 1,4-butanediol monovinyl ether;

2. 5-95% by weight of esters of acrylic acid or methacrylic acid with monohydric alcohols containing 1 to 12 carbon atoms, such as methyl methacrylate, ethyl acrylate, n-butyl acrylate or 2-ethylhexyl acrylate;

3. 0-50% by weight of aromatic vinyl compounds, such as styrene, methylstyrene or vinyl toluene;

4. 0-20% by weight of other monomers having functional groups such as acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaric acid, maleic acid anhydride. maleic acid half ester, acrylamide, methacrylamide, acrylonitrile or N-methylol(meth)acrylamide, as well as glycidyl (meth)acrylate, wherein the proportion of groups 1. and/or 4. amounts to at least 5% by weight.

The acrylate resins can be prepared by customary methods, i.e., solution-, suspension-, emulsion- or precipitation-polymerization, preferably by polymerization in the absence of a solvent with initiation by UV light.

Other polymerization initiators such as the common peroxides or azo-compounds, such as dibenzoyl peroxide, tert-butyl perbenzoate, or azobisisobutyronitrile can be used. The molecular weight can be regulated with sulfur compounds such as tert-dodecyl mercaptan.

Preferred polyethers can be produced by homopolyaddition of epoxides, such as ethylene oxide, 3,3-bis(chloromethyl)oxacyclobutane, tetrahydrofuran, styrene oxide, the bis-2,3-epoxypropyl ether of diphenylol propane or epichlorohydrin, e.g., in the presence of BF.sub.3, or by the addition of these epoxides, either in a mixture or sequentially, to starting materials having reactive hydrogen atoms such as alcohols or amines, e.g., water, ethylene glycol, 1,2- or 1,3-propylene glycol, pentamethylene glycol, hexanediol, decamethylene glycol, trimethylolpropane, 4,4'-dihydroxydiphenylpropane, aniline, ammonia, ethanolamine, ethylenediamine, di(.beta.-hydroxypropyl)methylamine, di(.beta.-hydroxyethyl)aniline, hydrazine, as well as hydroxyalkylated phenols, such as O,O-di(.beta.-hydroxyethyl)resorcinol.

Polyurethanes and/or polyureas containing hydroxyl groups can also be used.

Mixtures of several materials can, of course, be used as polyhydroxy compounds.

Pigments and fillers as well as substances that improve flow and help avoid surface defects, such as silicone oils, p-toluenesulfonamide, liquid acrylate resins, plasticizers and catalysts are among the auxiliary agents that may be used in the powdered binding agents of the invention.

The catalysts disclosed for the preparation of the uretidione polyaddition compounds are suitable catalysts.

The softening points of the polyurethane powder enamel components according to the invention are such that at temperatures between about 80.degree. C. and about 120.degree. C. additives necessary to produce the coating material of the invention can be worked in. On the other hand, the softening points of the mixtures are such that they can be ground into lump-free, free-flowing powders with a particle size of about 20 to about 120 microns.

The powdered coating materials can be produced in suitable mixing devices, e.g., in stirred kettles, or mixing screws (extruders), and formed into powders, which can be applied to substrates by conventional procedures, particularly by electrostatic powder spraying (EPS process).

The enamel films cure quickly when heated to temperatures above 120.degree. C., preferably between about 160.degree. C. and 200.degree. C., to hard and smooth coatings.

A. Isocyanate Components

(1) Preparation of the uretidione dimer of isophorone diisocyanate

100 Parts by weight (p.b.w.) of IPDI were mixed with 0.1 p.b.w. of tris(dimethylamino)phosphine and allowed to stand at room temperature for 20 hours. The NCO content of this mixture after this time was 31% b.w., i.e., about 40% of the IPDI had reacted. This mixture was then subjected to thin film distillation at 130.degree. C. and 0.1 torr. The residue was free of catalyst and had an NCO content of 17.6%. When the residue was heated at 180.degree. C. for 30-60 minutes, the NCO content increased to 37.1-37.7%. This so-called "hot value" was a direct measure of the content of uretidione groups in the reaction product.

(2) Preparation of the IPDI-uretidione adducts

General preparation guidelines

The IPDI-uretidione diisocyanate produced according to A(1) is heated, optionally in a solvent such as acetone, methylene chloride, toluene or xylene, to 50.degree. C.-100.degree. C. The glycol is added to the uretidione diisocyanate with brisk stirring and in an inert gas atmosphere in such a way that the reaction temperature does not exceed 110.degree. C. The reaction, which is monitored by titrimetric determination of NCO, is terminated after 2-5 hours at 110.degree. C. After cooling and optionally evacuation and drying, and crushing, if necessary, the reaction product is obtained as a slightly yellowish, free-flowing powder. The IR-spectra of the cross linked adducts exhibit the characteristic strong bands of the uretidione group at 1760-1780 cm.sup.-1.

B. Polyol components

General Preparation Guidelines

The starting materials, terephthalic acid (TA), dimethyl terephthalate (DMT), 1,6-hexanediol (HD), neopentyl glycol (NPG), 1,4-dimethylolcyclohexane (DMC) and trimethylolpropane (TMP) were placed in a reactor and heated by an oil bath. After the greater portion of the materials had melted, at a temperature of 160.degree. C., 0.05% b.w. of di-n-butyltin oxide was added as a catalyst. The first methanol liberation occurred at a temperature of about 170.degree. C. Over 6-8 hours, the temperature was raised to 220.degree.-230.degree. C. and the reaction was complete in another 12-15 hours. The polyester was cooled to 200.degree. C. and its volatile components were largely removed by applying a vacuum (1 mm Hg) for 30-45 minutes. During the entire reaction time the material in the bottom of the reactor was stirred, and a small N.sub.2 stream was passed through the reaction mixture.

The following tables give the polyester compositions and the corresponding physical and chemical data.

(C) Polyurethane powder enamels free of blocking agents

General Preparation Guidelines

The ground materials, adduct of the uretidione dimer of isophorone diisocyanate, polyester flow modifier masterbatch*, and catalyst masterbatch*, are thoroughly mixed, optionally with white pigment and optional fillers, in an edge mill and then homogenized in an extruder at 100.degree.-130.degree. C. After cooling, the extrudate is crushed and ground in a pin mill to a particle size of <100 microns. The powder produced in this manner is applied with an electrostatic powder sprayer at 60 KV onto degreased, optionally pre-treated, iron plates and baked in a recirculating dryer at temperatures between 160.degree. and 200.degree. C. The IR spectra of the powder enamels exhibit no NCO bands at 2400 cm.sup.-1. The free NCO groups have reacted during the extrusion process.

*Flow Modifier Masterbatch

10% by weight of the flow modifier, a commercially available copolymer of butyl acrylate and 2-ethylhexyl acrylate, was homogenized in the polyester used in the melt, and the mixture was pulverized after it had hardened.

*Catalyst Masterbatch

5-10% by weight of the catalyst, e.g., dibutyltin dilaurate, was homogenized in the polyester used in the melt, and the mixture was pulverized after it had hardened.

The abbreviations in the following tables are:

CT=coating thickness in micrometers

HK=Konig hardness in sec (DIN 53 157)

HB=Buchholz hardness (DIN 53 153)

EC=Erichsen cupping in mm (DIN 53 156)

GT=Incised grid test (DIN 53 151)

GG 60.degree.=Gardner gloss (ASTM-D 523)

Imp.=Reverse impact in inch.multidot.lb

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

628.85 p.b.w. Polyester according to Example B 1

151.15 p.b.w. Uretidione-isocyanate adduct according to A 23

600.00 p.b.w. White pigment (TiO.sub.2)

75.00 p.b.w. Flow modifier masterbatch

45.00 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

640.21 p.b.w. Polyester according to Example B 3

139.79 p.b.w. Uretidione-isocyanate adduct according to A 23

600.00 p.b.w. White pigment (TiO.sub.2)

75.00 p.b.w. Flow modifier masterbatch

45.00 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

620.00 p.b.w. Polyester according to Example B 5

160.00 p.b.w. Uretidione-isocyanate adduct according to A 23

600.00 p.b.w. White pigment (TiO.sub.2)

75.00 p.b.w. Flow modifier masterbatch

45.00 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

635.91 p.b.w. Polyester according to Example B 2

144.09 p.b.w. Uretidione-isocyanate adduct according to A 25

600.00 p.b.w. White pigment (TiO.sub.2)

75.00 p.b.w. Flow modifier masterbatch

45.00 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

652.63 p.b.w. Polyester according to Example B 1

127.37 p.b.w. Uretidione-isocyanate adduct according to A 21

600.00 p.b.w. White pigment (TiO.sub.2)

75.00 p.b.w. Flow modifier masterbatch

45.00 p.b.w. Catalyst masterbatch

According to the described procedure as powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

630.4 p.b.w. Polyester according to Example B 3

149.6 p.b.w. Uretidione-isocyanate adduct according to A 28

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

625.5 p.b.w. Polyester according to Example B 4

154.5 p.b.w. Uretidione-isocyanate adduct according to A 28

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

628.4 p.b.w. Polyester according to Example B 3

151.6 p.b.w. Uretidione-isocyanate adduct according to A 210

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

633.4 p.b.w. Polyester according to Example B 2

146.6 p.b.w. Uretidione-isocyanate adduct according to A 210

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst materbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

621.1 p.b.w. Polyester according to Example B 1

158.9 p.b.w. Uretidione-isocyanate adduct according to A 211

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

611.2 p.b.w. Polyester according to Example B 5

168.8 p.b.w. Uretidione-isocyanate adduct according to A 212

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

644.9 p.b.w. Polyester according to Example B 2

135.1 p.b.w. Uretidione-isocyanate adduct according to A 213

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

602.2 p.b.w. Polyester according to Example B 5

177.8 p.b.w. Uretidione-isocyanate adduct according to A 215

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

624.2 p.b.w. Polyester according to Example B 3

155.8 p.b.w. Uretidione-isocyanate adduct according to A 215

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

638.2 p.b.w. Polyester according to Example B 1

141.8 p.b.w. Uretidione-isocyanate adduct according to A 22

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

According to the described procedure a powder enamel having the following composition was prepared, applied and baked at between 160.degree. C. and 200.degree. C.

643.6 p.b.w. Polyester according to Example B 3

136.4 p.b.w. Uretidione-isocyanate adduct according to A 26

600.0 p.b.w. White pigment (TiO.sub.2)

75.0 p.b.w. Flow modifier masterbatch

45.0 p.b.w. Catalyst masterbatch

The sprayable polyurethane powders free of blocking agents exhibit very good to outstanding pourability after 7 days of storage at 40.degree. C.; after 28 days in storage the pourability is still good.

The enamel films were uniform and free of pits and bubbles. The surfaces show good to very good leveling. Depending on the Oh functionality and the free NCO content of the uretidione-containing adduct a mild to slight orange-peel effect was observed.

Exposure for one year to open weathering in an industrial environment caused no change in flexibility, hardness and adhesion. In testing by the Xenotest 450 LF after up to 3000 hours according to DIN 53 231 the results were excellent. Gloss, hardness and flexibility underwent no change.