Sulfonimide catalysts for coatings

Abstract: Sulfonimide cure catalysts are used with aminoplast crosslinking agents and resins capable of reacting with aminoplast crosslinking agents to prepare heat curable compositions capable of crosslinking to form coatings and articles. Liquid or solid curable compositions may be prepared. Solid sulfonimide catalysts used in combination with solid glycoluril type crosslinking agents are well suited for powder coating systems containing hydroxy- and carboxy-functional resins.

Description: This invention relates to the use of sulfonimide cure catalysts in combination with aminoplast crosslinkers and their use in curable compositions. Alkyl and aryl sulfonic acids have found utility in industry as "strong acid" catalysts capable of activating acid-sensitive curable compositions to produce cured coatings, moulded articles or adhesives. U.S. Patent 3,927,141 and U.S. Patent 4,118,437 disclose powder coating compositions containing alkoxyalkyl groups carrying aminoplasts which are cured with in the presence of an acid catalyst including sulfonic acids. U.S. 4,064,191 and EP-A-0 144 810 disclose organic solvent-based compositions thereof, and DE-A-2854440 discloses a water-based composition thereof. It is well known in the art that strong acids such as sulfonic generally favor rapid cure. However, it is suggested in U.S. 4,031,036, that acid strength alone does not always determine effectiveness of a catalyst in complex reactions. In many cases, strong acids produce less than "smooth" surfaces having a "sandpaper-like", appearance. Furthermore, because of the extreme reactivity of sulfonic acid catalysts, curable powders containing them have been unsatisfactory because of poor package stability. The poor package stability is associated with room temperature crosslinking of the curable powders catalyzed by the excessively strong sulfonic acid catalysts. As a result, the low temperature reaction often causes clumping and sintering or fusing of the powdered resin. These are undesirable because they impair fluidization of the powder during application contributing to the production of non-uniform coatings with diminished gloss. As a partial solution for low temperature crosslinking, U.S. 4,510,290 and U.S. 4,281,075 disclose the use of latent-reactive sulfonic acid catalysts such as benzoin tosylates and beta-hydroxy tosylates, respectively. The chemistry of the beta-hydroxy tosylate latent acid catalysts is described in Ind. Eng. Chem. Prod. Res. Dev. , 22 , 440 (1983). Other latent-reactive sulfonic acid catalysts are disclosed in EP-A- 0139609. Even though latent catalyst approach improves the shelf-life of the cure catalyst-containing coating compositions, the overall film properties and appearance of the cured films have been no better than non-latent sulfonic acid catalyzed systems because the active catalyst in both systems is the same: a sulfonic acid. Powder coating technology is dominated by isocyanate crosslinked and epoxide crosslinked reaction systems which are of relatively high cost. Improved crosslinking systems based on aminoplast resins when used in combination with solid components offer cost attractive alternative powder coating formulations. This invention provides sulfonimides as improved catalysts useful in the curing of aminoplast crosslinkable resin compositions with particular utility in powder coatings/formulations. This invention is a curable composition comprising: (i) a sulfonimide cure catalyst, (ii) an aminoplast crosslinking agent containing at least 2 crosslinkable groups, and (iii) a polyfunctional material capable of reacting with aminoplast crosslinking agents. This invention is also a crosslinker/catalyst combination suitable for admixture with an aminoplast crosslinkable resin. The crosslinker/catalyst combination comprises: (a) a sulfonimide cure catalyst, and (b) an aminoplast crosslinking agent. This invention is also an improved method of coating substrates by applying to said substrates the curable coating compositions of the invention. This invention is also a crosslinked article or coating formed from heat-activating the curable compositions of the invention. CURABLE COATING COMPOSITIONS The curable composition of this invention comprises: (i) a sulfonimide cure catalyst of this invention, (ii) a crosslinkingly effective amount of an aminoplast crosslinking agent containing at least 2 crosslinkable groups, (iii) a polyfunctional material capable of reacting with aminoplast crosslinking agents, (iv) optional ingredients. SULFONIMIDE CURE CATALYSTS The novel cure-catalysts of this invention are represented by the Formula: RSO₂NZSO₂R wherein each of the R groups is, independently: (1) a linear, branched, cyclic alkyl, fluoroalkyl, N , N -dialkylamino group of 1 to 20 carbon atoms, or mixtures thereof; or (2) an aryl group of 1 to 20 carbon atoms containing 5 or less aromatic or heterocyclic aromatic rings substituted by R¹, R², R³, R⁴, and R⁵ groups, wherein R¹, R², R³, R⁴, and R⁵ groups are the same or different groups selected from a group consisting of hydrogen, fluoro, alkyl, fluoroalkyl, aryl, haloaryl, carboxy, mercapto, vinyl, chloro, bromo, cyano, nitro, sulfonyl, acyl, alkoxycarbonyl, alkoxy, perfluoroalkoxy, hydroxy, amino, acylamino, alkoxycarbonylamino, carbamoyl, aminocarbonyl, N -alkyl aminocarbonyl, and N , N -dialkylaminocarbonyl groups; or (3) an aralkyl group wherein the alkyl and the aryl groups have the same meaning as the alkyl and aryl groups (1) and (2) above; and wherein the Z group of the formula is: (4) hydrogen, (5) acyl, or (6) an alkyl or an aralkyl group of 1 to 20 carbon atoms, inclusive of sulfonimides as described in DeChristopher, P.J., et. al., J. Amer. Chem. Soc., Vol. 91 (9), p. 2384 (1969) and Curtis, V.A., et. al., Tetrahedron Lett., Vol. 12, p. 199 (1981). Alkyl or aralkyl groups of (6) which are preferred as the substituent Z are those containing at least one hydrogen at the position beta to the point of attachment of the alkyl group to the nitrogen of the sulfonimide such that upon nucleophilic deacylation reaction of the acylated sulfonimide or upon heat-activated or base-catalyzed elimination or nucleophilic displacement reaction of the alkylated sulfonimide, an unsubstituted sulfonimide is obtained which sulfonimide is represented by the following formula: RSO₂NHSO₂R Particularly useful are the sulfonimide cure-catalysts containing both alkyl and aryl groups such as that represented by the formula: wherein the alkyl group imparts solubility and the aryl group contributes to the higher melting point. This combination of substituents produces a solid cure catalyst which is well suited to powder coatings applications. Diarylsulfonimides are similarly well suited for powder coatings applications because of their higher melting point. Particularly useful in this invention are the para-substituted diarylsulfonimide, having Formula: wherein X is selected from a group consisting of hydrogen, chloro, methyl, nitro, amino, cyano, alkoxycarbonyl, carboxyl, acyl, N -alkylaminocarbonyl, and N , N -dialkylaminocarbonyl groups. Sulfonimides containing a plurality of imide groups are also usable as catalysts in the compositions of this invention, and can be of two types: 1. Linear polyfunctional sulfonimides, represented by the formula: (R-SO₂NH-SO₂) n wherein n groups of the (R-SO₂NH-SO₂) units may be the same or different, and the R group in each of the n units is independently selected from the group consisting of: a. a linear, branched, or cyclic alkylene or fluoroalkylene group of 1 to 20 carbon atoms; or b. an arylene group of 1 to 20 carbon atoms containing 5 or less carbocyclic or heterocyclic aromatic rings substituted by R¹, R², R³, R⁴, and R⁵ groups, wherein R¹, R², R³, R⁴, and R⁵ groups are the same or different groups selected from a group consisting of hydrogen, fluoro, alkyl, fluoroalkyl, aryl, haloaryl, carboxy, mercapto vinyl, chloro, bromo, cyano, nitro, sulfonyl, acyl, alkoxycarbonyl, alkoxy, perfluoroalkoxy, hydroxy, amino, acylamino, alkoxycarbonylamino, carbamoyl, aminocarbonyl, N -alkylaminocarbonyl, and N , N -dialkylaminocarbonyl groups; or c. an aralkylene group wherein the alkylene and the arylene groups have the same meaning as the alkylene and arylene groups (a) and (b) above; and wherein n is 2 or greater; and 2. Pendant polyfunctional sulfonimides, represented by the Formula: P(SO₂NHSO₂R) n wherein P is either a polyfunctional group or a polymer backbone of a molecular weight greater than 500, and R is an alkyl or aryl group of 1 to 20 carbon atoms, and n is 2 or greater. The amine salts of the sulfonimides of this invention are also usable as latent-reactive cure catalysts in the compositions of this invention. They are represented by the formula: RSO₂ N SO₂R HN ⁺RRR wherein the R groups are, independently: a. a linear, branched, or cyclic alkyl, or fluoroalkyl group of 1 to about 10 carbon atoms, or mixture thereof; or b. an aryl group of 1 to 20 carbon atoms containing 5 or less carbocyclic or heterocyclic aromatic rings substituted by R¹, R², R³, R⁴, and R⁵ groups, wherein R¹, R², R³, R⁴, and R⁵ groups are the same or different groups selected from a group consisting of hydrogen, fluoro, alkyl, fluoroalkyl, aryl, haloaryl, carboxy, mercapto, vinyl, chloro, bromo, cyano, nitro, sulfonyl, acyl, alkoxycarbonyl, alkoxy, perfluoroalkoxy, hydroxy, amino, acylamino, alkoxycarbonylamino, carbamoyl, aminocarbonyl, N -alkylaminocarbonyl, and N , N -dialkylaminocarbonyl groups, or c. an aralkyl group wherein the alkyl and the aryl groups have the same meaning as the alkyl and aryl groups in (a) and (b) above. One, two, or all three R groups of the amine part of the sulfonimide salts can be replaced with one, two or three hydrogen atoms to give secondary ammonium, primary ammonium, or ammonium salts, respectively. Like tertiary amine salts, these are usable in the compositions of the invention as latent-reactive cure catalysts. The R group in the amine salts above can further be independently interrupted and substituted with heteroatoms or heteroatom-containing groups to produce amine salts such as those derived from N,N-dimethylethanolamine, triethanolamine, morpholine, 2-amino-2-methyl-1-propanol, N,N-dimethyl ethanolamine, N-methyl diethanolamine, ethanolamine, and diethanolamine. The preferred amine salts of the invention are derived from relatively low molecular weight amines which are sufficiently volatile to facilitate the expulsion of the free amines from the cured coatings either before or during the curing process. Examples of the volatile amines are triethanolamine, diisopropylamine, triisopropylamine, in addition to the heteroatom-containing amines recited above. The sulfonimide cure catalyst of this invention may take a latent reactive form such as an N-sulfonyl-substituted saccharin which is later reacted with a nucleophile such as ammonia, primary or secondary alcohols, mercaptans, or amines. For example: where, R is an alkyl or aryl group such as shown in the reference: Imai, Y. et. a., Nippon Kagaku Kaishi, No. 1, p. 111 (1982). The molecular weight of the sulfonimide cure catalyst used in the crosslinker/catalyst ingredient is preferably high enough to make the sulfonimide catalyst a solid at ambient temperatures, and more preferably a solid at above ambient temperatures (viz., about 40°C) such as those which may be encountered in commercial in-plant storage or transportation environments. The sulfonimide catalysts are generally prepared by the reaction of sulfonyl chlorides with a sulfonamide in the presence of a base. Typical preparatory methods are taught in U.s. Patent Nos. 3,052,656 and 4,510,324, Federal Republic of Germany Patent Nos. 765,789, 1,265,157 and 1,235,300, and DDR Patent No. 9,132. CROSSLINKING AGENTS Crosslinking agents of this invention are aminoplast crosslinkers commonly used in coatings, mouldings, and adhesives. The term "aminoplast" is defined herein as a class of resins prepared by the reaction of an amino group-containing compound and an aldehyde. The reaction product of amino group-containing compounds and aldehydes is often reacted further with an alcohol to produce partially or fully alkylated derivatives. These derivatives are included in the "aminoplast" definition given above. The term "aminoplast" as used in the context of this invention also includes phenoplasts (phenolic resins), specifically, the resole-type resins which are the reaction product of phenol and an aldehyde and have the further property of containing pendant hydroxymethyl or alkoxymethyl groups. Resole resins and resolated Novolak resins are examples of phenoplasts suitable as crosslinking agents (see, Kirk-Othmer, "Encyclopedia of Chemical Technology", Vol. 17, 3'd Edition, pages 384-390 (1985) and "Encyclopedia of Polymer Science and Engineering", Vol. 11, 2'd Edition, edited by Mark, Bikales, Overberger, and Menges, (1985) pages 45-57. Commercially avaiable phenoplasts are Bakelite® Phenolic Baking Resins sold by Union Carbide Company, Danbury, CT, (USA). In the preparation of aminoplasts from amino group-containing compounds and aldehydes and subsequent alkylation, dimeric and oligomeric products resulting from self-condensation reaction are often obtained. These oligomeric self-condensation products are also included in the "aminoplast" definition given above. The aminoplast crosslinking agents of this invention are represented by the formula: A-(R) n wherein the A group in A-(R) n is an n-functional polyamino anchor molecule connected to n(R) groups via the nitrogens and includes the following systems represented by the formulae: (a) wherein R¹ and R² are independently, hydrogen, alkyl or aryl groups of 1 to 10 carbon atoms; (b) wherein R³ is an aliphatic or cycloaliphatic alkyl group, of 1 to 10 carbon atoms, such as methyl, ethyl, butyl, and cyclohexyl ; or R³ is an aromatic group, of 1 to 10 carbon atoms, such as phenyl, tolyl, and naphthyl; or R³ is an aralkyl group, of 1 to 10 carbon atoms, such as benzyl, and cumyl; (c) (d) (e) (f) wherein R is an alkylene or an arylene of 1 to 10 carbon atoms; (g) (h) (i) homopolymers containing acrylamide units of the formula: wherein R is hydrogen or alkyl and m is greater than 2, and copolymers thereof; (j) polycarbamates; (k) polyhydantoins; (l) polyamides; (m) dialkoxyethylene ureas; (n) dihydroxyethylene urea represented by the formula: (o) homopolymers containing carbamate units of the formula: wherein R is hydrogen or alkyl, and m is greater than 2, and copolymers thereof; (p) mixtures of any of (a) through (o); and wherein the R group in A-(R) n is hydrogen, or an alkyl group of 1 to 10 carbon atoms, or a crosslinkable group such as -CH₂OH and -CH₂OR⁴ wherein R⁴ is an alkyl group of 1 to 10 carbon atoms or an aminoplast group-containing oligomeric group provided that the total number of the crosslinkable groups per each aminoplast crosslinking agent is at least 2, and preferably more than 2; and wherein n in A-(R) n is the total functionality of the polyamino anchor molecule A, and is 2 or greater. The preferred crosslinking agents of this invention are substantially fully etherified, substantially fully methylolated, substantially monomeric aminoplast crosslinkers commonly used in coatings industry. They are characterized by having at least 2, preferably more than 2 crosslinkable groups per crosslinker molecule. It is well known in the art that the minimum number of crosslinkable functional groups required to effect crosslinking is 2 provided that the aminoplast-reactive polyfunctional material has more than 2 functional groups per each molecule. If, on the other hand, the aminoplast-reactive polyfunctional material has only 2 functional groups per each molecule, then each aminoplast crosslinking agent is required to have more than two crosslinkable groups per crosslinker molecule to be capable of producing a crosslinked network. The most preferred aminoplast crosslinking agents of the invention are selected from a group consisting of substantially fully etherified, substantially fully methylolated, substantially monomeric glycoluril, melamine, benzoguanamine, cyclohexanecarboguanamine, urea, substituted linear and cyclic ureas, and mixtures thereof. In addition to the substantially fully etherified, substantially monomeric amine-aldehyde aminoplast crosslinking agents described above, the non-etherified or partially etherified substantially fully methylolated or partially methylolated monomeric and oligomeric aminoplasts are also usable in the compositions of this invention. The unetherified or partially etherified aminoplasts generally cure faster than the fully methylolated and etherified aminoplasts in the presence of weaker acid catalyst and may be used advantageously in such cases. Glycoluril Crosslinkers The most preferred glycoluril crosslinkers of this invention are N -substituted glycolurils represented by the formula: wherein at least two of the R groups are selected from a group consisting of methoxymethyl, ethoxymethyl, propoxymethyl, butoxymethyl, pentoxymethyl, hexoxymethyl, heptoxymethyl, octoxymethyl, nonoxymethyl, decoxymethyl and mixtures thereof, and the remaining R groups are selected from hydrogen, alkyl, hydroxymethyl, and glycoluril group-containing oligomeric moieties. While it is preferable to have 4 alkoxymethyl groups per each glycoluril crosslinking agent, under ordinary circumstances it may not be necessary to obtain the pure tetrasubstituted monomeric crosslinker N , N ', N ", N ‴-tetraalkoxymethylglycoluril represented by formula: wherein R is an alkyl group of 1 to 10 carbon atoms. The monomeric tetraalkoxyglycolurils themselves are not considered to be resinous materials since they are, as individual entities, non-polymeric compounds. They are considered, however, to be potential resin-forming compounds when subjected to heat, and particularly when subjected to heat under acidic conditions. As a result of the described resin-forming ability, the substantially monomeric glycoluril crosslinkers of this invention may contain some higher oligomeric components such as dimers, trimers, and tetramers. The presence of modest amounts of these oligomeric forms is permissible and indeed beneficial in cases where a high melting solid crosslinker is desirable as it is the case in powder coatings. An example of glycoluril crosslinker of this invention is POWDERLINK® 1174 Powder Crosslinking Agent, a product of American Cyanamid Company, Wayne, N.J., and has the following properties: Non Volatiles, minimum (% by weight) : 98 Appearance : White to pale yellow granulated flakes Melting Point (°C) : 90-110°C Average Molecular Weight : 350 Equivalent Weight : 90-125 Another example of a glycoluril crosslinker of this invention is CYMEL® 1170 fully butylated glycoluril crosslinker, a product of American Cyanamid Company, Wayne, New Jersey, having the following properties: Urea Crosslinkers An example of a urea crosslinker usable in this invention is BEETLE® 80 butylated urea-formaldehyde resin, a product of American Cyanamid Company, Wayne, New Jersey, having the following properties: Appearance : Clear Liquid Color, Maximum (Gardner 1963) : 1 Non-Volatiles (Weight %) 96 ± 2 Viscosity (Gardner-Holt, 25°C) : X-Z₃ Solvent Tolerance (ASTM D1198-55) : > 500 Melamine Crosslinkers The melamine-based aminoplast crosslinking agents are well known in the art and have been used extensively as effective crosslinkers in coatings. Unlike the tetrafunctional glycolurils, alkoxymethylmelamine functionality can be a maximum of six in a crosslinkingly effective range of 2 to 6 alkoxymethyl groups per each melamine molecule. Like the glycolurils, alkoxymethyl melamines can contain dimers, trimers, tetramers, and higher oligomers, each given combination of monomers and oligomers being preferred for a given application. For example, the lower viscosity monomer-rich compositions are preferred for solvent-based high solids coatings. An example of the substantially fully etherified, substantially fully methylolated, substantially monomeric melamines usable in this invention is CYMEL® 303 melamine crosslinking agent, a product of American Cyanamid Company, Wayne, New Jersey., and has the following properties: Non-Volatiles (% by weight) : 98 Color, Maximum (Gardner 1963) : 1 Viscosity (Gardner-Holt, at 25°C) : X-Z₂ Free Formaldehyde, maximum (weight %) : 0.5 Degree of Polymerization : 1.75 Another example of the substantially fully etherified, substantially fully methylolated, substantially monomeric melamines is CYMEL® 1168 melamine crosslinking agent, a product of American Cyanamid Company, Wayne, New Jersey. The alkyl group in CYMEL® 1168 consists essentially of a mixture of methyl and isobutyl groups. It has the following properties: An example of a substantially methylolated, partially etherified, substantially oligomeric melamine is CYMEL® 370 crosslinking agent, a product of American Cyanamid Company, Wayne, New Jersey. It has the following properties: Non-Volatiles (% by weight) : 88± 2 Solvent : Isobutanol Viscosity (Gardner-Holt, at 25°C) : Z₂-Z₄ Color, maximum (Gardner 1963) : 1 Equivalent Weight : 225-325 Guanamine Crosslinkers As in melamines, the partially or fully methylolated or etherified alkyl and aryl guanamine aminoplasts, both in their monomeric and oligomeric forms, are usable as crosslinking agents in this invention, with the selection depending on the particular application or the properties desired in the product. Benzoguanamine, cyclohexylcarboguanamine and acetoguanamine aminoplasts are especially preferred as crosslinkers in this invention. The benzoguanamine crosslinkers are represented by the formula: wherein R is an alkyl group of 1 to 10 carbon atoms, or a mixture thereof. An example of a benzoguanamine-based crosslinking agent is CYMEL® 1123 Resin as described above, and wherein R is a mixture of methyl and ethyl groups. The acetoguanamine-based crosslinkers are represented by the formula: wherein each R is independently an alkyl group of 1 to 10 carbon atoms. The cyclohexylcarboguanamine-based crosslinkers are represented by the formula: wherein each of the R groups is independently an alkyl group of 1 to 10 carbon atoms, or a mixture thereof. It is evident from the discussions above, that a person skilled in the art, in selecting the most suitable crosslinker for a particular application, may choose a mixture thereof which imparts a balance of properties desired for that particular application. RATIO AND PROPORTIONS OF INGREDIENTS The aminoplast crosslinking agent (ii) is combined with the polyfunctional aminoplast-reactive ingredient (iii) in proportions that give weight ratio of the ingredients (ii) : (iii) ranging from 1:40 to 3:1. The weight ratio of the sulfonimide catalyst (i) to the aminoplast crosslinking agent (ii) is from 1:4 to 1:400. The effective concentration of the sulfonimide cure catalyst is from 0.01 to 10 weight percent based on the total weight of resin solids. It is preferably from 0.3 to 0.8 weight percent of the total resin solids. OPTIONAL INGREDIENTS The optional ingredients present in the curable composition of the invention vary depending on the intended use of the cured article or film. Suitable optional ingredients include fillers, ultraviolet light stabilizers, pigments, flow control agent, plasticizers, mold release agents, and corrosion inhibitors. The composition of the invention may contain as an optional ingredient a liquid medium. such a medium may be used to aid the uniform application and transport of the curable composition. Any or all of the ingredients of the composition may be contacted with the liquid medium. Moreover, the liquid medium may permit formation of a dispersion, emulsion, invert emulsion, or solution of the curable composition ingredients. Particularly preferred is a liquid medium which is a solvent for the curable composition ingredients (i), (ii), and (iii). Suitable solvents are selected from alcohols, ketones, ethers, esters, water, aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons and mixtures thereof. POWDER COATING COMPOSITION The novel curable powder coating composition of this invention comprises: (i) a sulfonimide cure catalyst of this invention, (ii) a polyfunctional material capable of reacting with aminoplast crosslinking agents, (iii) a crosslinkingly effective amount of a aminoplast crosslinking agent containing at least 2 crosslinkable groups, (iv) optional ingredients. It is necessary that at least one of the powder coating ingredients (i), (ii), (iii) or (iv) be a solid, with the proviso that the solid ingredient(s) of the powder coating comopsition be capable of retaining the non-solid ingredient(s) to give a composition that as a whole is a solid. A much preferred embodiment of the invention is a composition wherein the powder coating ingredients (i), (ii), (iii), and (iv) are all solids having the form of a flowable powder. In recent years, powder coating technology has been the subject of considerable interest and commercial growth. Because powder coatings are all solids systems, they do not suffer from the solvent-related disadvantages of the solvent-based coatings systems. Powder coating minimizes the cost of shipping, storing, handling, and solvent waste disposal. In powder coatings the ingredients used are essentially similar to conventional liquid coatings except that the powder coating composition is required to be a solid. Preferred Powder Coating Compositions: Preferred powder coating compositions are those wherein the ingredients (i), (ii), (iii) and (iv) are all solids. More particularly, the individual composition ingredients are as described below: (i) The novel sulfonimide cure catalysts usable in powder coatings are the sulfonimides of this invention having melting points ranging from 40°C to 250°C, and more preferably ranging from 80°C to 180°C. (ii) The polyfunctional materials usable in powder coatings are the polyfunctional materials of this invention having a glass transitions (Tg) ranging from 40°C to 150°C, and softening points ranging from 40°C to 150°C, and more preferably ranging from 80°C to 140°C. It is preferable that the powder coating composition as a whole be a solid at ambient temperatures and that it remains solid up to 40°C to preserve the powdery state of the curable composition and to prevent jumping. Upon heating, however, these resins should become fluid to permit formation of low viscosity melts which allow efficient mixing of the components of the curable composition prior to crosslinking. It is desirable that the aminoplast-reactive resins become fluid at temperatures ranging from 80°C to 150°C, and most preferably from 100°C to 140°C. (iii) The aminoplast crosslinking agents usable in powder coatings are the solid aminoplast crosslinking agents of this invention having melting or softening points ranging from 40°C to 150°C, and more preferably from 80°C to 140°C, and most preferably from 100°C to 120°C. Glycoluril type aminoplast crosslinking agents are particularly preferred because they are solids at ambient temperatures and generally have suitable melting points. It is preferable that the solid aminoplast crosslinking agents are solids at ambient temperatures and that they remain solids up to 40°C to preserve the powdery state of the curable composition and to prevent lumping. It is desirable that the aminoplast crosslinking agents become fluid at temperatures ranging from 80°C to 150°C, and more preferably from 100°C to 140°C. (iv) The curable powder coating compositions may include varying amounts of optional ingredients such as benzoin at about 1 to 4 weight percent level. The composition may also include as a flow control agent ingredients such as Modaflow® Powder III Resin Modifier, a product of Monsanto Company, or Resiflow P-67 Flow Control Agent, a product or Estron Chemical, Inc. at 1 to 2 weight percent level. Moreover, fillers and pigments such as titanium dioxide based pigments may be conventionally added to improve the appearance and properties of the coatings and articles formed from the compositions. POLYFUNCTIONAL MATERIALS The polyfunctional material capable of reacting with an aminoplast crosslinking agent is one of three essential components required for the curable compositions of this invention. The other essential components are aminoplast crosslinking agents such as N , N ', N ", N ‴-tetramethoxymethylglycoluril, and sulfonimide cure catalyst such as N -(methanesulfonyl)-p-toluenesulfonamide. The aminoplast-reactive functional groups of the polyfunctional material react, under cure conditions, with crosslinkable groups to produce a three dimensional crosslinked network such as those found in coatings, moulded articles and adhesives. The aminoplast reactive functional groups are selected from a group comprising: amino groups, amido groups, carboxyl groups, mercapto groups, hydroxy groups, and combinations of any of the aforementioned functional groups. Reactive materials having a plurality of carboxy and/or hydroxy groups are particularly preferred in the practice of the invention. Suitable polyfunctional hydroxy reactant materials may be epoxy or polyurethane prepolymers, alkyds, acrylic resins containing pendant or terminal carboxy or hydroxy functionalities, or polyester resins with pendant or terminal hydroxy or carboxy groups. These are described in greater detail below. In cases where resins used in the formulations of this invention are intended for powder coating applications, the resins should be solids at ambient temperatures and should remain solids up to 40°C to preserve the powdery state of the curable composition and prevent lumping. The aminoplast-reactive materials or resins used in the compositions of the invention preferably have a molecular weight of from 100 to 50,000. In the event that the aminoplast reactive material is in the form of an emulsion the molecular weight may be as high as 5,000,000. 1. ACRYLIC RESIN AMINOPLAST-REACTIVE MATERIAL The hydroxyfunctional acrylic resins used in formulating the curable compositions of this invention preferably have molecular weights in the range of from 500 to 50,000 and hydroxyl group equivalent weights of from 200 to 4,000. An examples of a suitable acrylic resin is JONCRYL® SCX-800 A acrylic resin, a product of S.C. Johnson and Sons, Inc., having the following physical and chemical properties: Another example of a suitable acrylic resin is JONCRYL® SCX-800 B acrylic resin, a product of S.C. Johnson and Sons, Inc., having the following physical and chemical properties: Non-Volatiles (weight %) : > 97 Hydroxyl Number : 40 Glass Transition : Temperature (Tg) (°C) : 43 Acid Value : 15-20 Softening Point (°C) : 107 Equivalent Weight : 1402 ICI Viscosity : 200°C, dPa·s, Poise : 45-50 or JONCRYL® 500 acrylic resin, a product of S.C. Johnson and Sons, Inc., having the following properties: Viscosity (mPa·s, centipoise) : 4000 Solids Content (%) : 80 Hydroxyl Number (based on solids) : 140 Equivalent Weight (based on solids) : 400 Molecular Weight (Mn) : 1300 Polydispersity (Mw/Mn) : 1.7 Commercially available acrylic resins are generally applicable for use in preparing the compositions of the invention. Selection of the optimum acrylic resin will depend on the user's desire to impart to the final cured product particular physical properties such as hardness, flexure, chemical and solvent resistance, and etc. 2. POLYESTER RESINS The hydroxy functional polyester resins used in formulating the curable compositions of this invention preferably have molecular weights in the range of from 1,000 to 50,000 and hydroxyl group equivalent weights of from 200 to 4,000. An example of a suitable polyester resin is Arakote® 3109 solid hydroxyl terminated polyester powder coating resin, a product of Ciba-Geigy Corporation, Hawthorne, N.Y., having the following physical and chemical properties: Color Colorless Solid OH Number : 27-32 ICI Viscosity at 200°C (dPa·s, Poise) : 40 Glass Transition Temperature (Tg) (°C) : 66 Equivalent Weight : 1900 Another example of a suitable polyester resin is ALFTALAT® AN 745 hydroxyfunctional polyester resin, a product of Hoechst Corporation, having the following physical and chemical properties: Color, 50% (ASTM D 1544-68) : 3 OH Number : 30 Equivalent Weight : 1870 Acid Value (mg KOH/g) : 8 Glass Transition Temperature (Tg) (°C) : 55 ICI Viscosity (200°C, dPa·s, Poise) : 50-65 Another example of a suitable polyester resin is ARAKOTE® 3010 carboxyl-terminated polyester, a product of Ciba-Geigy Corporation, Ardsley, New York, having the following properties: Color, Maximum (Gardner, ASTM D 2849) : 2 OH Number : ≤ 7 Acid Value (mg KOH/g) : 27-33 ICI Viscosity (200°C, dPa·s, Poise) 35-60 Glass Transition Temperature (Tg) (°C) : 62 Equivalent Weight : 1800 Another example of a suitable polyester resin is RUCOTE® 101 polyester resin, a product of Ruco Polymer Corporation, Hicksville, N.Y., having the following physical and chemical properties: Appearance : granular solid Softening Temperature (°C) : 120 OH Number : 29 Acid Value (mg KOH/g) : 1 Glass Transition Temperature (Tg) (°C) : 53 Equivalent Weight : 2000 ICI Viscosity (200°C, dPa·s, Poise) : 53 Another example of a suitable polyester resin is CARGILL 3000 polyester, a product of Cargill Corporation, having the following physical and chemical properties: OH Number : 50 Equivalent Weight : 1035 Glass Transition Temperature (Tg) (°C) : 56 Acid Value (mg KOH/g) : 15 Upon cure, polyester resins give films of superior properties, as described in detail in the Examples. A wide variety of commercial polyester resins may be used for the aminoplast-reactive ingredient of the invention, provided such resins have the physical properties suitable for the intended application. 3. OTHER AMINOPLAST-REACTIVE SYSTEMS Other polyhydroxy functional compounds which are useful in the practice of the invention are diols, triols, tetraols, and higher polyhydroxy alcohols. Illustrative polyhydric alcohols are pentaerythritol dipentaerythritol, tripentaerythritol, trimethylolethane, trimethylolpropane, anhydroenneahepititol, 1,2,4-butanetriol, and 1,2,6-hexanetriol. These polyols can also be used to totally or partially replace the acrylic resin or polyester resin aminoplast-reactive ingredient of the composition of the invention. However, the film properties resulting from addition of such non-resinous ingredients sometimes are inferior and lack flexibility in the cured film final product. Polyfunctional epoxy prepolymers, alkyds, and polyurethane prepolymers are also usable as hydroxyfunctional aminoplast-reactive materials. Polyfunctional amides are also suitable as aminoplast-reactive components, provided they have the proper physical properties suitable for the intended use. For example, for powder coatings application, they are solids at up to 40°C to allow the preparation of storage stable powders containing the aminoplast crosslinking agents. The crosslinked films and articles resulting from polyamide-containing curable compositions, however, have physical properties somewhat different (i.e. harder) than polyester and acrylic-based systems described above. THE CATALYST/CROSSLINKER COMBINATION The catalyst/crosslinker combination of the invention is a uniform mixture of the following materials: (a) a sulfonimide cure catalyst, and (b) an aminoplast crosslinking agent containing at least two crosslinker groups. Suitable sulfonimide cure catalysts (a) and aminoplast crosslinking agents (b) are as set out in the preceding sections. The components (a) and (b) can each be either liquids or solids. A particularly preferred catalyst/crosslinker combination is one wherein both the sulfonimide catalyst (a) and the aminoplast crosslinker (b) are both solids at 40°C. THE IMPROVED METHOD OF COATING BY THIS INVENTION: This invention is also an improved method of coating substrates by applying to substrates the coating composition of the invention. A Method of Coating Using Liquid Coating Compositions: This invention is a method of coating substrates using a liquid curable coating composition, where the composition contains the following ingredients: (i) a sulfonimide cure catalyst, (ii) a crosslinkingly effective amount of an aminoplast crosslinking agent containing at least 2 crosslinkable groups, (iii) a polyfunctional material capable of reacting with aminoplast crosslinking agents, and (iv) a liquid medium. The liquid curable coating composition of the invention may be contacted with a substrate by spraying, padding, brushing, rollercoating, curtaincoating, flowcoating, electrostatic spraying, electrocoating or dipping. A Method of Coating Using Powder Coating Compositions: "Powder coating" is an art recognized process and is defined herein as a method of electrostatic powder spraying wherein a finely divided solid coating material is electrostatically attracted to a surface of an article. Alternatively, the powder coating composition may be contacted with the substrate by immersing the substrate in fluidized bed of powder. The article covered with the powder is heated to at least the fusion temperature of the coating composition forcing it to flow out and form a coating which is cured by further application of heat. The prior art powder coating process can be considerably improved by using as the coating material a low temperature heat-curable aminoplast coating composition comprising as essential ingredients: (i) a solid sulfonimide cure catalyst of this invention, (ii) a solid polyfunctional aminoplast-reactive material, and (iii) a solid aminoplast crosslinking agent. The unexpectedly superior properties of the films formed from the powder coating compositions of the invention are their superior appearance manifested in gloss and smoothness, improved impact and corrosion resistance. A typical procedure for using the compositions of the invention is as follows: The substrates are coated with the powder coating composition of the invention and subjected to a temperature of from 90°C to 350°C to melt and spread the composition uniformly onto the substrate surface. Thereafter, the substrate is held at a temperature of from 120°C to 350°C to crosslink the composition and form the permanently cured coating. CROSSLINKED ARTICLES AND COATINGS FORMED BY THE COMPOSITIONS OF THIS INVENTION The curable compositions of this invention containing the sulfonimide cure-catalyst of the invention can be heat cured (typically 90°C to 400°C) to produce crosslinked moulded articles, adhesives and coatings. Because of the polyfunctional nature of the reactants, the resulting product in this case is a crosslinked article in the form of a film such as adhesives and/or coatings or the form of an article such as molded products. The heat-cured compositions of this invention may be employed as coatings for wire, appliances, automotive parts, furniture, pipes, machinery, and the like. Suitable surfaces include metals such as plastic, wood, steel, and aluminum. The heat-cured compositions may also be used to form solid articles such as cases, enclosures, and structural members. The following Examples illustrate various embodiments of the invention. EXAMPLE 1 PREPARATION OF THE SULFONIMIDE CURE CATALYSTS* PART A p-Toluenesulfonyl chloride (38.2 g, 0.20 mole) was added, in five portions over a period of 90 minutes, to a heated (70°C) solution of methanesulfonamide (19.0 g, 0.20 mole) in aqueous 5% sodium hydroxide (176 ml). Throughout the addition, a pH of 12 was maintained by frequent additions of aqueous 5% sodium hydroxide solution. After stirring for about 20 hours at about 70°C, the reaction mixture was cooled to room temperature and acidified with concentrated hydrochloric acid to pH less than 1 and extracted with dichloromethane (3 x 200 ml). The organic layer was separated, dried (sodium sulfate) and concentrated to 150 ml. Addition of n-heptane (200 ml) gave highly pure N -(methanesulfonyl)- p -toluenesulfonamide, a sulfonimide cure catalyst of the invention, as a white precipitate (18.3 g), m.p. 117-120°C. Example 1, Part A illustrates the preparation of a mixed aryl and alkyl group containing cure catalyst of this invention. PART B The procedure of EXAMPLE 1, PART A was followed with the exception that methanesulfonamide was replaced with an equivalent amount of p-toluenesulfonamide (34.2 g, 0.20 mole) and the extraction step was replaced by a purification step to give the symmetrical di-(p-toluene)sulfonimide, a sulfonimide cure catalyst of the invention, as a white solid of 96% purity, m.p. 171-173°C. EXAMPLE 1, PART B illustrates the preparation of the symmetrical di-(p-toluene)sulfonimide, which is another example of the sulfonimide cure catalysts of this invention. PART C A commercially available sulfonimide, N-(4-aminobenzenesulfonyl)benzenesulfonamide, a product of Aldrich Chemicals, Milwaukee, WI, was used as cure catalyst in the preparation of Composition C. PART D The procedure of EXAMPLE 1, PART A, was followed with the exception that methanesulfonamide and p-toluenesulfonyl chloride were replaced, respectively, with equivalent amounts of p-nitrobenzenesulfonamide and a solution of p-nitrobenzenesulfonyl chloride in acetone (50 ml). Also, the extraction step was replaced with a purification step to give the symmetrical di-(4-nitrobenzene)sulfonimide, a sulfonimide cure catalyst of the invention, as an off-white solid of 98% purity, m.p. 238-240°C. EXAMPLE 1, PART D illustrates the preparation of another example of sulfonimide cure catalyst of this invention. EXAMPLE 2 This example illustrates the preparation of powder coatings using the sulfonimide cure catalysts of the invention. The following four curable compositions, Composition A, B, C, and D, were formulated using sulfonimide cure catalysts of this invention and thereafter cured at 190°C (20 min). 1. PREPARATION OF THE CURABLE COMPOSITIONS A. COMPOSITION A Weight (g) Arakote® 3109 Polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 Sulfonimide Cure Catalyst of Example 1, Part A 0.50 Benzoin (a solid additive) 1.40 R960-48® Titanium Dioxide Pigment (product of DuPont Company, Wilmington, Delaware) 40.00 Resiflow P-67 Flow Control Agent (product of Estron Chemical, Inc. Culvert City, Ky) 1.30 B. COMPOSITION B Arakote® 3109 Polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 Sulfonimide Cure Catalyst of Example 1, Part B 0.50 Benzoin (a solid additive) 1.40 R960-48 Titanium Dioxide Pigment 40.00 Resiflow P-67 Flow Control Agent 1.25 C. COMPOSITION C Arakote® 3109 Polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 Sulfonimide Cure Catalyst of Example 1, Part C 0.50 Benzoin (a solid additive) 1.40 R960-48 Titanium Dioxide Pigment 40.00 Resiflow P-67 Flow Control Agent 1.25 D. COMPOSITION D A control composition was prepared as described above in Composition A with the exception that no cure catalyst was used. All components were dry-blended in a Waring blender or a Welex mixer for small and larger scale runs, respectively. The dry-blended powder was then melt-mixed at about 90°C to 135°C for 1-5 minutes in a two roll mill to allow homogeneous mixing without allowing crosslinking to occur. The melt-mixed material was then chopped into smaller pieces in a blender mill or a pelletizer, and fed into a mill classifier where the chips were ground to a fine powder with particle size about 35 microns. The finely ground powder, charged with up to 80 KV power, was then sprayed through an electrostatic spray gun on grounded metal substrates, Bonderite® 1000 (a product of Parker Industries, Inc.) in a spray booth. 2. CURING THE CURABLE COMPOSITIONS The electrostatically coated Bonderite 1000 substrates containing the curable compositions of the invention were then baked in an oven at 190°C to allow the powder to fuse, flow out and crosslink. These processes were analyzed by means of a viscometer and a rheometer. TABLE 1 summarizes the cure conditions, the film properties resulting from curing the curable compositions and particularly the film appearance. It is concluded from the results in EXAMPLE 2 that powder coatings prepared by using the curable compositions of this invention which contain the cure catalysts of this invention have excellent film properties and appearance. The control formulation without catalyst did not cure, showing the necessity of catalyst inclusion in the formulation. EXAMPLE 3 The experiments of EXAMPLE 2 were repeated with Compositions A, B, C, and D at the slightly lower temperature of 175°C. Composition C did not cure well. The cure conditions, film properties and film appearance of the remaining compositions are summarized in TABLE 2. It is concluded from the results in TABLE 2 that the curable compositions of this invention containing the sulfonimide cure catalysts are capable of curing at low temperatures (175°C) to produce powder coatings with excellent film properties and acceptable appearance. Powder Coatings from Curable Compositions of the Invention CURABLE COMPOSITIONS Comp. A Comp. B Comp. D FILM APPEARANCE Orange Peel Orange Peel Orange Peel Microblisters Microblisters Microblisters Craters FILM PROPERTIES Film Thickness (mils) 1.8-2.8 2.5-3.5 1.5-2.5 Film Thickness (mm) 0.046-0.071 0.064-0.089 0.038-0.064 Hardness KHN₂₅ 11.8 12.7 10.5 MEK Rubs 20/200+ 10/200+ 200+ Impact F/R (in-lb) 160/160 100-120/0-20 160/160 Color, Tristimulus -0.06 -0.03 0.05 Gloss, 60° 67.4 68.3 58.9 Gloss, 20° 47.3 38.3 24.5 RESISTANCE PROPERTIES Humidity (60°C), Time (hrs) 456 288 192 INCREASED INCREASED INCREASED Appearance after testing Microblisters, some loss of gloss Microblisters Microblisters Salt Spray, Time (hrs) 1008 1008 1008 Creepage (mm) 2 0 0 Appearance (after testing) 9 9 9 EXAMPLE 4 COMPARATIVE Curable compositions E, F, and G were prepared and cured at 190°C/20 min. by the method described in EXAMPLE 2. COMPOSITION E Weight (g) Arakote® 3109 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 Sulfonimide cure catalyst of Example 1, Part A 0.30 Benzoin (a solid additive) 1.40 R960-48 titanium dioxide pigment 40.00 Resiflow P-67 flow control agent 1.30 COMPOSITION F Weight (g) Arakote® 3109 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 p-Toluenesulfonic Acid Monohydrate (solid) 0.30 Benzoin (a solid additive) 1.40 OR 650 titanium dioxide pigment 40.00 Resiflow P-67 flow control agent 1.25 COMPOSITION G Weight (g) Arakote® 3109 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 p-Toluenesulfonic acid monohydrate (solid) 0.20 Benzoin (a solid additive) 1.40 R960 48 titanium dioxide pigment 40.00 Modaflow III flow control agent 1.25 TABLE 3 summarizes the film properties and appearance characteristics of the cured coatings obtained by curing the compositions E, F, and G. It is concluded from the results in EXAMPLE 4 that the curable composition E of this invention containing the sulfonimide cure catalyst of EXAMPLE 1, PART A produces superior powder coatings when compared with curable compositions of the prior art such as compositions F and G containing p-toluenesulfonic acid catalyst commonly used in prior art. The prior art p-toluenesulfonic acid cured system gave unacceptable appearance, lower gloss, and failed in the salt spray test. EXAMPLE 5 Curable composition H was prepared by the method described in EXAMPLE 2. It was cured at 175 and 190°C by the method described in EXAMPLE 2. COMPOSITION H Weight (g) Alftalat® AN 745 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 Sulfonimide cure catalyst of EXAMPLE 1, PART A 0.50 Benzoin (a solid additive) 2.40 R960-48 titanium dioxide pigment 40.00 Resiflow III flow control agent 1.30 TABLE 4 summarizes the film properties and appearance characteristics of the cured coatings obtained by curing composition H. This example illustrates that a sulfonimide cure catalyst of the invention produces excellent powder coating with Alftalat® AN 745 as well as Arakote® 3109 polyesters at two different temperatures. Powder Coatings with Alftalat® AN 745 and Powderlink® 1174 Cured with a Sulfonimide Cure Catalyst CURABLE COMPOSITION H 175°C Cure 190°C Cure FILM APPEARANCE Orange Peel Orange Peel Microblisters Microblisters FILM PROPERTIES Film Thickness (mils) 2.3-2.8 2.1-2.5 Film Thickness (mm) 0.058-0.071 0.053-0.064 Hardness KHN₂₅ 10.9 10.5 MEK Rubs 200+ 200+ Impact F/R (2.54 cm · 0.454 kg, in-lb) 160/160 160/160 Color, Tristimulus -0.07 -0.06 Gloss, 60° 92.7 91.4 Gloss, 20° 77.1 73.4 RESISTANCE PROPERTIES Humidity (60°C), Time (hrs) 432 504 Appearance (after testing) Increased Microblisters No Change Salt Spray, Time (hrs) 1070 1070 Creepage (mm) 1 1 Appearance (after testing) No Change No Change EXAMPLE 6 Curable compositions I was prepared by the method described in EXAMPLE 2. It was cured at 175 and 190°C by the method described in EXAMPLE 2. COMPOSITION I Weight (g) Rucote® 101 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 Sulfonimide cure catalyst of EXAMPLE 1, PART A 0.50 Benzoin (a solid additive) 2.40 R960-48 titanium dioxide pigment 40.00 Resiflow III flow control agent 1.30 TABLE 5 summarizes the film properties and appearance characteristics of the cured coatings obtained by curing composition H. This example illustrates that a sulfonimide cure catalyst of Example 1, Part A produces excellent powder coatings with Rucote® 101 as well as Alftalat® AN 745 and Arakote® 3109 polyesters at two different temperatures. Powder Coatings with Rucote® 101 and Powderlink® 1174 Cured with a Sulfonimide Cure Catalyst CURABLE COMPOSITION I 175°C Cure 190°C Cure FILM APPEARANCE Orange Peel Orange Peel Microblisters Microblisters FILM PROPERTIES Film Thickness (mils) 2.0-2.6 1.7-2.4 Film Thickness (mm) 0.051-0.066 0.043-0.061 Hardness KHN₂₅ 11.9 10.8 MEK Rubs 200+ 200+ Impact F/R (2.54 cm · 0.454 kg, in-lb) 160/160 160/160 Color, Tristimulus -0.06 -0.07 Gloss, 60° 83.5 91.7 Gloss, 20° 66.7 74.6 RESISTANCE PROPERTIES Humidity (60°C), Time (hrs) 336 504 Appearance (after testing) Some loss of gloss No Change Salt Spray, Time (hrs) 1070 1070 Creepage (mm) 1 3 Appearance (after testing) Some blisters Some blisters EXAMPLE 7 An additional sulfonimide cure catalyst was prepared by the method of EXAMPLE 1, PART A, using the same reaction conditions but different reactants: di-(4-Chlorobenzene)sulfonimide , prepared from 4-chlorobenzenesulfonamide and 4-chlorobenzenesulfonyl chloride. Two additional curable compositions were prepared as follows: COMPOSITION J Weight (g) Arakote® 3109 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 di-(Benzene)sulfonimide cure catalyst 0.30 Benzoin (a solid additive) 1.40 R960-48 titanium dioxide pigment 40.00 Resiflow III flow control agent 1.30 COMPOSITION K Weight (g) Arakote® 3109 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 di-(4-Chlorobenzene)sulfonimide cure catalyst 0.30 Benzoin (a solid additive) 1.40 R960-48 titanium dioxide pigment 40.00 Resiflow P-67 flow control agent 1.25 Compositions J and K, along with Composition E from EXAMPLE 4 were cured at 190°C / 20 min by the method of EXAMPLE 2. The results are summarized in TABLE 6. This example illustrates that both symmetrical and unsymmetrical sulfonimide cure catalyst of the invention are effective catalysts. It further illustrates that N -(methanesulfonyl)-p-toluenesulfonamide sulfonimide cure catalyst produces excellent film properties and acceptable appearance. Effect of Symmetrical and Unsymmetrical Cure Catalyst on Film Properties and Appearance CURABLE COMPOSITIONS Comp. E Comp. J Comp. K FILM APPEARANCE Orange Peel Orange Peel Orange Peel Microblisters Microblisters Microblisters Some Wrinkles Wrinkles FILM PROPERTIES Film Thickness (mils) 2.4-2.9 2.0-3.0 1.8-2.2 Film Thickness (mm) 0.061-0.074 0.051-0.076 0.046-0.056 Hardness KHN₂₅ 12.1 12.4 11.8 MEK Rubs 20/200+ 200+ 200+ Impact F/R (2.54 cm · 0.454 kg, in-lb) 160/160 160/160 110-120/40-50 Color, Tristimulus -0.06 -0.04 0.05 Gloss, 60° 92.7 93.9 95.1 Gloss, 20° 78.6 75.0 81.8 RESISTANCE PROPERTIES Humidity (60°C), Time (hrs) 456 600 600 Appearance after testing Loss of gloss No change No change Salt Spray, Time (hrs) 1008 1008 1008 Creepage (mm) 1 0 0 Appearance after testing 9 9 9 EXAMPLE 8 This example illustrates that a large variety of non-sulfonimide compounds are unsuitable as catalysts in the composition of this invention, when compared to Composition E of Example 4 under the same conditions. The results are summarized in Table 7. It is concluded from the results in Table 7 that Composition E containing the sulfonimide of Example 1, Part A, has better overall properties when compared with related catalyst systems. EXAMPLE 9 This example illustrates that powder coating compositions of the invention containing sulfonimide cure catalyst of the invention cure faster and produce films with superior overall film properties and appearance characteristics than similar compositions containing benzoin tosylate as the cure catalyst. The following curable compositions, Curable Composition L and Curable Composition M, were formulated and thereafer cured at 175°C (20 min) and at 190°C (20 min.) the procedure of EXAMPLE 2. COMPOSITION L Weight (g) Arakote® 3109 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.00 Sulfonimide Cure Catalyst of EXAMPLE 1, PART A 0.5 Benzoin (a solid additive) 2.4 R960-48 titanium dioxide pigment 40.00 Resiflow P-67 flow control agent 1.3 COMPOSITION M Weight (g) Arakote® 3109 polyester (solid) 94.00 Powderlink® 1174 Crosslinking Agent (solid) 6.0 Benzoin tosylate (solid) 2.0 Benzoin (a solid additive) 1.4 R960-48 titanium dioxide pigment 40.00 Resiflow P-67 flow control agent 1.25 Table 8 summarizes the film properties and appearance characteristics of the cured films obtained by curing the comparative pair (Compositions L and M) at 175°C. Table 9 depicts the same at 190°C. It is concluded from the results in Tables 8 and 9 that films obtained by curing the compositions of the invention have superior film properties and appearance characteristics than similar films cured with benzoin tosylate. It can also be concluded that higher levels of benzoin tosylate catalyst are needed to achieve cure compared to sulfonimide catalysts. Comparison of a Sulfonimide Cure Catalyst of the Invention with Benzoin Tosylate in Powder Coatings Cured at 175°C Sulfonimide Curable Comp. L Benzoin Tosylate Curable Comp. M FILM APPEARANCE Orange Peel Orange Peel Microblisters Microblisters FILM PROPERTIES Film Thickness (mils) 1.5-2.2 2.0-2.2 Film Thickness (mm) 0.038-0.056 0.051-0.059 Hardness, KHN₂₅ 11.3 11.9 MEK Rubs 20/200+ 1/15 Impact F/R (2.54 cm · 0.454 kg, in-lb) 160/160 20-30/0-5 Color, Tristimulus -0.06 -0.06 Gloss, 60° 81.1 92.4 Gloss, 80° 56.5 79.5 RESISTANCE PROPERTIES Humidity (60°C), Time (hrs) 504 96 Appearance after testing No change Loss of Gloss Salt Spray, Time (hrs) 888 528 Creepage (mm) 3 0 Appearance after testing Blisters Blisters Comparison of a Sulfonimide Cure Catalyst of the Invention with Benzoin Tosylate in Powder Coatings Cured at 190°C Sulfonimide Curable Composition L Benzoin Tosylate Curable Composition M FILM APPEARANCE Orange Peel Orange Peel Microblisters Microblisters FILM PROPERTIES Film Thickness (mils) 1.5-2.2 1.6-2.0 Film Thickness (mm) 0.038-0.056 0.041-0.056 Hardness, KHN₂₅ 11.2 11.6 MEK Rubs 200+ 200+ Impact F/R (2.54 cm · 0.454 kg, in-lb) 160/160 150-160/150-160 Color, Tristimulus -0.05 -0.06 Gloss, 60° 93.5 94.4 Gloss, 80° 74.9 83.4 RESISTANCE PROPERTIES Humidity (60°C), Time (hrs) 504 264 Appearance after testing No change Increased Microblisters Salt Spray, Time (hrs) 960 528 Creepage (mm) 4 0 Appearance after testing Blisters Blisters EXAMPLE 10 This example illustrates that solvent-based coating compositions of the invention containing a sulfonimide cure catalyst cure at a rate comparable with sulfonic acid-cured compositions. COMPOSITION N Weight (g) JONCRYL® 500 Acrylic 81.25 Resin (80% Solids) CYMEL® 1168 Crosslinking Agent 35.00 di-Benzenesulfonimide (52% Solution) 1.00 n-Butanol 20.00 Xylene 16.55 Non-Volatiles : 65 Weight % Resin/Crosslinker Weight Ratio : 65/35 Catalyst based on resin solids : 0.5 Weight % COMPOSITION O JONCRYL® 500 Acrylic 81.25 Resin (80% Solids) CYMEL® 1168 Crosslinking Agent 35.00 p-Toluenesulfonic Acid (40 Wt % Solution) 0.75 n-Butanol 20.00 Xylene 16.55 Non-Volatiles : 65 Weight % Resin/Crosslinker Weight Ratio : 65/35 Catalyst based on resin solids : 0.3 Weight % Curing of Compositions N and O The curable compositions N and O were applied on Bonderite 100 Cold Roll Steel (CRS) substrate panels by means of a #46 wire cator applicator and were cured by heating at a cure schedule of 30 minutes at 121°C (250°F). The cured film was evaluated by standard methods widely accepted by the coatings industry. The physical properties of the cured films obtained by curing Composition N and Composition O are listed in Table 10. Table 10 shows that the composition cure rates are comparable. This is unexpected since the acidity of sulfonimide catalysts is lower than p-toluenesulfonic acid for comparable cures. The potlife is also improved and the appearance of the cured film is better. It is concluded from this example that sulfonimide cure catalysts of this invention are effective catalysts in solvent-based coatings, and that they cure at a rate comparable to the widely used sulfonic acid catalysts. Cure Response of Solvent-Based Coatings Comparison of Sulfonimide and Sulfonic Acid Catalysts COMPOSITION N COMPOSITION O Film Thickness Mils 1.65-1.75 1.65-1.75 mm 0.042-0.044 0.042-0.044 121°C/30 min Cure Hardness, KHN₂₅ 8.0 9.0 MEK Rubs 200+ 200+ 135°C/30 min Cure Hardness, KHN₂₅ 9.7 9.6 MEK Rubs 200+ 200+ 149°C/30 min Cure Hardness, KHN₂₅ 10.1 10.0 MEK Rubs 200+ 200+ EXAMPLE 11 This example illustrates lower yellowing of sulfonimide-cured compositions relative to sulfonic acid cured compositions in base coat/clear coat solvent-based coatings under overbake cure conditions. WHITE BASE COAT The base coat was prepared by a procedure similar to the procedure described in Example 10 using the following ingredients and conditions: Base Coat Formulation A Weight (g) JONCRYL® 500 Acrylic Resin Solids (80% Solids) 60.00 OR-650 Titanium Dioxide Pigment 80.00 Methyl Amyl Ketone 18.00 BYK® P 104-S Wetting Agent 0.80 (a product of Byk-Mallinckrodt Company, Wallingford, CT) Base Coat Formulation B JONCRYL® 500 Acrylic Resin Solids (80% Solids) 40.00 CYMEL® 303 Crosslinking Agent 20.00 n-Butanol 15.00 p-Toluenesulfonic Acid (40% Solution) 1.00 After grinding Formulation A in a 3-roll mill, Formulation B was added to ground Formulation A, and applied to ED-3150A primed EP1-P60 panel, available from Advanced Coating Technologies, Hillsdale, MO, using a #34 wire cator applicator. The resulting panels were cured at 121°C (250°F) for 15 minutes, and thereafter the clear topcoat Composition P and Composition Q (infra) were each applied to two separate panels coated with the base coat. CLEAR TOP COAT COMPOSITIONS COMPOSITION P Weight (g) JONCRYL® 500 Acrylic Resin (80% Solids) 81.30 CYMEL® 1170 Crosslinking Agent 35.00 di-Benzenesulfonimide 0.52 n-Butanol 16.00 Xylene 21.50 Non-Volatiles : 65 Wt. % Resin/Crosslinker Weight Ratio : 65/35 Catalyst based on Resin Solids : 0.5 Wt. % COMPOSITION Q Weight (g) JONCRYL® 500 Acrylic Resin (80% Solids) 81.30 CYMEL® 1170 Crosslinking Agent 35.00 p-Toluenesulfonic Acid 0.30 n-Butanol 16.00 Xylene 21.50 Non-Volatiles : 65 Wt. % Resin/Crosslinker Weight Ratio : 65/35 Catalyst based on Resin Solids : 0.3 Wt. % The clear coat Compositions P and Q were each applied onto base-coated panels using a #46 wire cator applicator and thereafter cured; first at 121°C/30 minute cure followed by an additional 149°C/30 minute cure, simulating overbake condition which commonly occur in a coating process. The results are summaried in Table 11. It is concluded from the results in Table 11 that the solvent-based coatings compositions of the invention containing the novel sulfonimide cure catalysts of the invention have better overbake tolerance than sulfonic acid catalyzed compositions when cured under overbake conditions. Yellowing of Solvent-Based Base-Coat/Clear-Coat Coatings Under Overbake Conditions Comparison of Sulfonimide and Sulfonic Acid Catalysts COMPOSITION P COMPOSITION Q Film Thickness mils/mm Base Coat 1.26/0.032 1.28/0.032 Clear Coat 1.59/0.040 1.48/0.038 Hardness, KHN₂₅ (Clear Coat) 121°C/30 min cure 0.8 8.0 121°C/30 min cure + 149°C/30 min overbake 9.3 10.3 Yellow Index 121°C/30 min cure -1.56 -1.28 121°C/30 min cure + 149°C/30 min overbake +2.18 +5.32 Total Color Change (difference of E) 2.71 4.39

Claims: A curable composition comprising: (i) a cure catalyst (ii) a crosslinkingly effective amount of an aminoplast crosslinking agent containing at least 2 crosslinkable groups, and (iii) a polyfunctional material capable of reacting with aminoplast crosslinking agents, characterized in that the cure catalyst is a sulfonimide. The composition according to claim 1, characterized in that it further comprises (iv) as optional ingredient fillers, ultra-violet light stabilizers, pigments, flow control agent, plasticizers, mold release agents and corrosion inhibitors. The composition of claim 1 or 2, characterized in that the aminoplast crosslinking agent (ii) is represented by the formula: A - (R) n wherein A is an n-functional polyamino anchor molecule connected to n(R) groups via the nitrogens, n is at least 2, and each R group is independently selected from hydroxylmethyl, alkoxymethyl, hydrogen, or alkyl of 1 to 10 carbon atoms; with the proviso that at least two R groups are selected from hydroxymethyl, alkoxymethyl or mixtures thereof. The composition of claim 1 or 2, characterized in that the aminoplast crosslinking agent (ii) is a N-substituted glycoluril represented by the formula wherein at least two of the R groups are selected from the group consisting of hydroxymethyl, methoxymethyl, ethoxymethyl, propoxymethyl, butoxymethyl, pentoxymethyl, hexoxymethyl, heptoxymethyl, octoxymethyl, nonoxymethyl, decoxymethyl and mixtures thereof, and the remaining R groups are hydrogen or alkyl. The composition of claim 1 or 2 characterized in that the sulfonimide is selected from the group consisting of: (a) a sulfonimide represented by the formula RSO₂NZSO₂R wherein each of the R groups is, independently (a1) a linear, branched or cyclic alkyl, fluoroalkyl or N,N-dialkylamino group of 1 to 20 carbon atoms, or mixtures thereof, or (a2) an aryl group of 1 to 20 carbon atoms containing 5 or less aromatic or heterocyclic aromatic rings substituted by R¹, R², R³, R⁴, and R⁵ groups, wherein R¹, R², R³, R⁴, and R⁵ groups are the same or different groups selected from the group consisting of hydrogen, fluoro, alkyl, fluoroalkyl, aryl, haloaryl, carboxy, mercapto, vinyl, chloro, bromo, cyano, nitro, sulfonyl, acyl, alkoxycarbonyl, alkoxy, perfluoroalkoxy, hydroxy, amino, acylamino, akloxycarbonylamino, carbamoyl, aminocarbonyl, N-alkyl aminocarbonyl and N,N-dialkyl aminocarbonyl groups, or (a3) an aralkyl group wherein the alkyl and the aryl groups have the same meaning as the alkyl and aryl groups (a1) and (a2), and wherein the Z group is (a4) hydrogen, (a5) acyl or (a6) an alkyl or aralkyl group of 1 to 20 carbon atoms; (b) a linear polyfunctional sulfonimide represented by the formula -(R'-SO₂NH-SO₂)- n wherein n groups of the -(R'-SO₂NH-SO₂)- units may be the same or different, and the R' group in each of the n units is independently selected from the group consisting of (b1) a linear, branched or cyclic alkylene or fluoroalkylene group of 1 to 20 carbon atoms, (b2) an arylene group of 1 to 20 carbon atoms containing 5 or less carbocyclic or heterocyclic aromatic rings substituted by R¹, R², R³, R⁴, and R⁵ groups, wherein R¹, R², R³, R⁴, and R⁵ groups are the same or different groups selected from the group consisting of hydrogen, fluoro, alkyl, fluoroalkyl, aryl, haloaryl, carboxy mercapto, vinyl, chloro, bromo, cyano, nitro, sulfonyl, acyl, alkoxycarbonyl, alkoxy, perfluoroalkoxy, hydroxy, amino, acylamino, akloxycarbonylamino, carbamoyl, aminocarbonyl, N-alkyl aminocarbonyl and N,N-dialkyl aminocarbonyl groups, and (b3) an aralkylene group wherein the alkylene and the arylene groups have the same meaning as the alkylene and the arylene groups (b1) and (b2); (c) a pendant polyfunctional sulfonimide represented by the formula P-(SO₂NHSO₂R") n , wherein P is a polyfunctional group or a polymer of a molecular weight greater than 500, R" is an alkyl or aryl group of 1 to 20 carbon atoms, and n' is 2 or greater; and (d) a latent reactive form thereof. The composition of claim 5, characterized in that the sulfonimide is represented by the formula: wherein X is independently selected from the group consisting of hydrogen, chloro, methyl, nitro, amino, cyano, alkoxycarbonyl, carboxyl, acyl, N-alkylaminocarbonyl and N,N-dialkylaminocarbonyl groups. The composition of claim 5, characterized in that the sulfonimide is represented by the formula: The composition of claim 1 or 2, characterized in that at least one of the ingredients (i), (ii), (iii) and (iv) is solid at 40°C. The composition of claim 1, characterized in that the weight ratio of (ii):(iii) is in the range of from 1:40 to 3:1, and that the sulfonimide is present in a concentration of from 0.01 to 10 weight percent based on the (ii)+(iii). A catalyst/crosslinker combination which is a mixture comprising: (a) a particulate cure catalyst, (b) a particulate aminoplast crosslinking agent containing at least 2 crosslinkable groups, characterized in that the particulate cure catalyst is a particulate sulfonimide, and the weight ratio of (a):(b) is from 1:4 to 1:400. The combination of claim 10, characterized in that the sulfonimide is selected from the group consisting of: (a) a sulfonimide represented by the formula RSO₂NZSO₂R wherein each of the R groups is, independently (a1) a linear, branched or cyclic alkyl, fluoroalkyl or N,N-dialkylamino group of 1 to 20 carbon atoms, or mixtures thereof, or (a2) an aryl group of 1 to 20 carbon atoms containing 5 or less aromatic or heterocyclic aromatic rings substituted by R¹, R², R³, R⁴, and R⁵ groups, wherein R¹, R², R³, R⁴, and R⁵ groups are the same or different groups selected from the group consisting of hydrogen, fluoro, alkyl, fluoroalkyl, aryl, haloaryl, carboxy mercapto, vinyl, chloro, bromo, cyano, nitro, sulfonyl, acyl, alkoxycarbonyl, alkoxy, perfluoroalkoxy, hydroxy, amino, acylamino, akloxycarbonylamino, carbamoyl, aminocarbonyl, N-alkyl aminocarbonyl and N,N-dialkyl aminocarbonyl groups, or (a3) an aralkyl group wherein the alkyl and the aryl groups have the same meaning as the alkyl and the aryl groups (a1) and (a2), and wherein the Z group is (a4) hydrogen (a5) acyl or (a6) an alkyl or aralkyl group of 1 to 20 carbon atoms; (b) a linear polyfunctional sulfonimide represented by the formula -(R'-SO₂NH-SO₂)- n wherein n groups of the -(R'-SO₂NH-SO₂)- units may be the same or different, and the R' group in each of the n units is independently selected from the group consisting of (b1) a linear, branched or cyclic alkylene or fluoroalkylene group of 1 to 20 carbon atoms, (b2) an arylene group of 1 to 20 carbon atoms containing 5 or less carbocyclic or heterocyclic aromatic rings substituted by R¹, R², R³, R⁴, and R⁵ groups, wherein R¹, R², R³, R⁴, and R⁵ groups are the same or different groups selected from the group consisting of hydrogen, fluoro, alkyl, fluoroalkyl, aryl, haloaryl, carboxy mercapto, vinyl, chloro, bromo, cyano, nitro, sulfonyl, acyl, alkoxycarbonyl, alkoxy, perfluoroalkoxy, hydroxy, amino, acylamino, akloxycarbonylamino, carbamoyl, aminocarbonyl, N-alkyl aminocarbonyl and N,N-dialkyl aminocarbonyl groups, and (b3) an aralkylene group wherein the alkylene and the arylene groups have the same meaning as the alkylene and arylene groups (b1) and (b2); (c) a pendant polyfunctional sulfonimide represented by the formula p-(SO₂NHSO₂R") n , wherein P is a polyfunctional group or a polymer of a molecular weight greater than 500, R" is an alkyl or aryl group of 1 to 20 carbon atoms, and n' is 2 or greater; and (d) a latent reactive form thereof. The combination of claim 10 or 11, characterized in that the sulfonimide is in a latent reactive amine salt form. A liquid coating composition comprising the curable composition of any one of claims 1 to 9 in a liquid medium. A powder coating composition comprising the curable composition of any one of claims 1 to 9 in powder form. A method of coating substrates by contacting a substrate with a coating composition according to either claim 13 or claim 14, then heat curing the so-applied coating at a temperature of 90 to 400°C. A coated substrate produced according to the method of claim 15.

Inventors: BROGAN JOHN COLIN, JACOBS III WILLIAM, RAMESH SUBBAN, SZITA JENO G, BROGAN, JOHN COLIN, JACOBS III, WILLIAM, RAMESH, SUBBAN, SZITA, JENO G.
Applicants: CYTEC TECH CORP, CYTEC TECHNOLOGY CORP.
Application year: 1991
Main classification: C08K5
IPC codes: C08K5, C09D167, C09D7, C09D201, C09D5, C08L67