GB2068000A

GB2068000A - High Solids Polyester Coating Composition

Info

Publication number: GB2068000A
Application number: GB8102556A
Authority: GB
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1980-01-29
Filing date: 1981-01-28
Publication date: 1981-08-05
Also published as: BR8100452A; GB2068000B; ES498908A0; FR2474514A1; AU529963B2; DE3102969A1; NL182319B; MX156962A; ES8300830A1; AU6666281A; FR2474514B1; NL8100402A; CA1245670A

Abstract

A curable, high-solids coating composition has film-forming constituents of a polyester polyol based on hindered glycols, a polyisocyanate or an aminoplast resin, and optionally an epoxy resin or epoxy-resin/acid ester. The composition can be applied at environmentally-acceptable solids levels and cured at ambient conditions or at commercially acceptable temperatures to form a durable film.

Description

SPECIFICATION High Solids Polyester Coating Composition Background of the Invention Field of the Invention This invention relates to a high-solids film-forming composition of a low molecular weight polyester with controlled hydroxyl functionality, a polyisocyanate or aminoplast resin, and optionally an epoxy resin or the ester of an epoxy resin and monocarboxylic acid.

Description of the Prior Art Conventional polyester-based coating compositions are well known in the finishes art, often comprising one or more hydroxyl-functional components which co-react with a suitable curing agent to form a polymeric paint film. For example, U.S. Patent 3,994,851 issued November30, 1976 to Chang shows a specific polyester polyol which is cured with an amine-aldehyde condensation product. U.S.

Patent 3,535,287 issued 0caber 20, 1970 to Wynstrn shows a 3-component polyester oligomer that is cured with a polyisocyanate.

A study of this and related prior art would make it evident, however, that in the field of polyester coatings, it is often necessary to sacrifice one desirable property to enhance another. For example, it is often difficult to obtain a coating composition, applicable at high solids levels, that is also tough, flexible, and durable, or that does not contain a resin which crystallizes out of solution at room temperature.

Therefore, with the current emphasis on reduction of solvent emissions, there is a continued need for coatings that not only can be applied at relatively high solids levels but also can be cured at commercially acceptable temperatures to produce a durable, flexible but hard, weather-resistant finish.

Summary of the Invention There is provided by the present invention, a coating composition of a film-forming blend and a solvent for the blend wherein the blend is at least 40% by weight of the composition and consists essentially of (a) a polyester polyol that is the reaction product of (1) neopentyl glycol and at least one other hindered diol containing two methylol groups wherein each methylol group is attached directly to a cycloaliphatic or aromatic structure or to a tertiary carbon atom, the molar ratio of neopentyl glycol to hindered diol being 2:1 to 6:1, and (2) a mixture of an aromatic and an aliphatic dicarboxylic acid wherein the molar ratio of aromatic acid to aliphatic acid is from 2:1 to 10:1, wherein the molar ratio of (1) to (2) is from 1.3:1 to 1.9::1 and wherein the polyol has a hydroxyl content of about 3.010.0% by weight; (b) a curing agent for the polyol; and (c) 0-50% by weight, based on the weight of (a) plus (b), of an epichlorohydrin-bisphenol-A epoxy resin, or the esterification product of said resin with a monocarboxylic acid, or mixtures of these.

Detailed Description of the Invention The polyester coating composition of the present invention, quite useful as a finish for automobiles, appliances, steel furniture, or even general industrial use, is composed primarily of a filmforming blend and a solvent for the blend. It can, however, also contain pigments, a reaction catalyst to decrease the curing time, and any of the various additives that are advantageously used in coating compositions for industrial or automotive finishes. The film-forming blend consists essentially of a polyester polyol, a polyisocyanate or aminoplast curing agent for the polyol, and optionally, an epoxy resin or epoxy-resin/acid ester. The film-forming blend constitutes 4090%, preferably 5590%, of the combined weight of the blend and the solvent.

The polyester polyol used in the present invention constitutes 5580% by weight of the filmforming blend. This polyol is the condensation-reaction product of neopentyl glycol, at least one other hindered diol, and an aromatic and an aliphatic dicarboxylic acid.

The alcoholic components used to form the relatively specific polyester polyol of the present invention are neopentyl glycol and at least one other hindered, diprimary diol. It has been found that this combination of difunctional alcohols, imparts both stain resistance and hardness to the final coating. The hindered diprimary diols that are usable in this invention are those having two methylol groups, each of which is attached directly to an aromatic or cycloaliphatic hydrocarbon structure or to a tertiary carbon atom. Examples to two preferred such diols are cyclohexane dimethylol and the monoester of neopentyl glycol and hydroxypivalic acid. This ester can be fonned according to U.S.

Patent 3,057,911. A particularly useful polyol is formed when the molar ratio of neopentyl glycol to other hindered diprimary diol is from 2:1 to 6:1.

The dicarboxylic acids useful in the formation of the polyester polyol have the general formula

where R' is aliphatic or aromatic. Of the aliphatic structures, the most useful are where R' is alkylene, vinylene, or cycloaliphatic.

Preferred acids when R' is alkylene are those in which R' has 2-10 carbon atoms. Most preferred of these are succinic acid, glutaric acid, adipic acid, and pimelic acid. When R' is a monounsaturated aliphatic, the most useful acids are those in Which R' has 2-8 carbon atoms with the preferred acids being maleic and itaconic acids. The aromatic dicarboxylic acids that are preferred are phthalic, iso-phthalic, terephthalic, uvitic, and cumidic acids When R' is cycloaliphatic, preferred are cyclohexane or cyclohexene dicarboxylic acids, although other such dicarboxylic acids could also be used.

Mixtures of these aromatic acids and aliphatic acids can be used, but at least one of each kind of acid must be present. Whether mixtures of each kind of acid are used, or whether only one of each kind of acid is used, the molar ratio of aromatic diacids to aliphatic diacids should have a range of about 2:1 to 10:1. A ratio of 2:1 to 6:1 is preferred and a ratio of about 4:1 is most preferred. It is to be further understood that the lower alkyl mono or di-esters of these acids and the anhydrides, where applicable, of these acids can also be used in place of the acids themselves with equivalent results. If the abovementioned ester are used, the alkyl groups preferably have no more than 5 carbon atoms.

A particularly useful polyester polyol is formed when the molar ratio of the alcoholic components to the dicarboxylic acid components is from 1.3:1 to 1.9:1, with about 1.6:1 being preferred.

The polyester polyo! can typically be formed by charging the reactants, a suitable solvent, and optionally a reaction catalyst into a reaction vessel that is usually equipped with a condenser and agitator. Useful solvents are, for example, xylene, toluene, other substituted benzenes, napthalene and substituted naphthalenes. The reaction catalysts can be present in the usual amounts and include, for example, dibutyl tin oxide, dibutyl tin dilaurate, sulfuric acid, or one of the sulfonic acids.

The reaction mixture is heated to its reflux temperature, usually 100--3000C, and there maintained for a period of 1-8 hours. During this period, the esterification by-products are withdrawn.

The reaction product. the polyester polyol, should have a number average molecular weight (determined by gel permeation chromatography based on polystyrene standards) of 300-1 500, preferably 400-700. The reactants should be chosen also so that the polyester polyol has a hydroxyl content of 310% by weight, preferably about 5.57.5% by weight.

As the curing or crosslinking agent for the polyol, an organic polyisocyanate or an aminoplast resin is used. The organic polyisocyanates, present in the coating composition in a stoichiometric amount for the other constituents of the film-forming blend, can be aliphatic, cycloaliphatic, alkaryl, aralkyl, heterocyclic, and aryl di- or triisocyanates. Oligomers of these can also be used.

Typically useful polyisocyanates are, for example, diphenylmethane-4,4'-diisocyanate, diphenylene-4,4'-diisocyanate, toluene-2,4-diisocyanate, toluene-2,6-diisocyanate, 3,3'-dimethoxy-4,4'-diphenylene diisocyanate methylene-bis-(4-cyclohexyl isocyanate) tetramethylene diisocyanate, hexamethylene diisocyanate, decamethylene diisocyanate, ethylene diisocyanate, ethylidene diisocyanate, propylene-1 ,2-diisocyanate, cyclohexylene-1 ,2-diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, 1,5-naphthalene diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanat", 3,3'-dimethoxy-4,4'-biphenylene diisocyanate, 3,3'-diphenyl-4,4'-biphenylene diisocyanate, 4,4'-biphenylene diisocyanate, 3,3'-dichloro-4,4'-biphenylene diisocyanate, furfurylidene diisocyanate, bis-(2-isocyanatoethyl)fu marate, 1,3,5-benzene triisocyanate, para,para',para"-triphenylmethane triisocyanate, 3,3'-diisocyanatodipropyl ether, xylylene diisocyanate, /3"B-diphenyl propane-4,4'-diisocyanate, and isophorone diisocyanate.

Preferred among these are hexamethylene diisocyanate, methylene-bis-(4-cyclohexyl isocyanate), and isophorone diisocyanate. Particularly preferred polyisocyanates are biurets of the formula

where R2 is an aliphatic or aromatic hydrocarbon group having 1-12 carbon atoms. These biurets can be made according to Wagner et al. U.S. Patent 401 5,165, issued September 27, 1977. In a most preferred biuret, R2 is -(CH2)6-. This biuret is a trimer of hexamethylene diisocyanate (HMDI), which is obtained by reacting three moles of HMDI with one mole of water.

The aminoplast resins that are useful are well known as crosslinking or curing agents. Particularly useful are the alkylated products of aminoplast resins, the resins themselves being prepared by the condensation of at least one aldehyde with at least one of urea, N,N-ethyleneurna, dicyandiamide, and aminotriazines such as melamines and guanamines. Among the aldehydes that are suitable are formaldehyde, revertable polymers thereof such as paraformaldehyde, acetaldehyde, crotonaldehyde, and acrolein. Preferred are formaidehyde and revertable polymers thereof. The aminoplast resins can be alkylated with at least one and up to six alkanol molecules containing 1-6 carbon atoms. The alkanols can be straight chain, branched, cyclic, or mixtures of these.Preferred are aminoplast resins that have been alkylated with methanol, butanol, or a mixture of these two. Most preferred are the methylated melamine-formaldehyde resins such as hexamethoxymethylmelamine. In the coating composition, the weight ratio of polyester polyol to aminoplast resin will normally be about 1.5:1 to 4:1.

The film-forming blend of the present invention may also contain up to 50%, preferably 520% and more preferably 510%, by weight, based on the combined weights of the polyol and curing agent, of an epoxy resin or the esterification product of the epoxy resin with a monocarboxylic acid.

The epoxy resin is itself the reaction product of epichlorohydrin and bisphenol-A and has the general formula

where n is sufficiently large to provide an epoxy resin having an epoxide equivalent weight of 4502000, preferably 850-1050. The epoxide equivalent weight is the unit weight of epoxy resin that contains one unit equivalent of epoxide.

The epoxy resin/acid ester, which may be used instead of, or in a mixture with, the epoxy resin, is formed by reacting the above epoxy resin with a monocarboxylic acid. Although the choice of the acid is not critical, most preferred are benzoic acid, para-t-butyl benzoic acid, and the higher fatty acids, those having 1218 carbon atoms.

The esters can be prepared by reacting the acid and epoxy resin together in a closed vessel equipped with agitator, thermocouple, and condenser. The temperature is raised gradually by heat application, with agitation starting as soon as the epoxy resin melts, to about 230-2700C over a 12 hour period. This temperature is maintained until the resultant ester attains the desired functionality, which can be determined by intermittent sampling to measure the acid number of the ester. At this point, the reaction mixture is cooled and can be thinned with an appropriate drganic solvent.

The polyester polyol, epoxy resin, and epoxy-resin/acid ester are normally each in solution and are suitable for direct use to form the coating composition of this invention by blending with each other and with the curing agent. When the curing agent is a polyisocyanate, however, a two-component system should be used. That is, a solution of polyisocyanate is in one package, and a solution of the polyester polyol and, optionally, the epoxy resin or epoxy-resin/acid ester is in a separate package. The two solutions are thoroughly mixed just before applying the coating composition. Separation of the two solutions is usually necessary since the "pot life" of the composition is short-the polyisocyanate reacts with the hydroxyl groups of the polyol at a rapid rate, even at room temperature.

Instead of the two-component, "two-package", system described above, a "one package" coating composition can be prepared if the reactive groups of the polyisocyanate are blocked with a blocking agent such as methyl ethyl ketoxime. This eliminates the need for keeping the polyester polyol apart from the polyisocyanate until just before use. When the coating composition, with the blocked polyisocyanate, is applied and heated to 1500--1600C, the blocking agent is released, permitting the polyisocyanate to react with the polyester.

Regardless of the method by which the final coating composition is mixed, the composition contains 4090% by weight of the film-forming blend and 1060% by weight of a solvent for the blend, these percentages being based on the combined weight of the blend and the solvent. One of the useful aspects of the present invention is that it can be conveniently spray-applied even at these high weight-soiids levels. The solvent of the final composition can be a mixture of the organic solvents in which the constituents of the film-forming blend are each formed.

The composition can also contain mixtures of various diols, preferably hindered, diprimary diols of the kind discussed earlier, as reactive diluents. These materials will advantageously act as a solvent for the film-forming blend, and upon application, will not volatilize but will rather become crosslinked into the final coating.

The coating composition of this invention may contain about 0.012.0% by weight, based on the weight of the film-forming blend, of a curing catalyst. When the curing agent is a polyisocyanate, the catalysts are usually organo metallics such as dibutyl tin dilaurate and zinc octoate, which are preferred, dibutyl tin di-2-ethylhexoate, stannous octoate, stannous oleate, zinc naphthenate, vanadium acetyl acetonate, or zirconium acetyl acetonate. Also useful as catalysts are tertiary amines, such as, for example, triethylene diamine, heptamethyliosbiguanide, triethylamine, pyridine, dimethylaniline, and methyl morpholine. When a two-component system is used, the catalyst can be added to either the polyisocyanate solution or the solution of the other constituents of the film-forming blend.

When the curing agent is an aminoplast, the catalyst can be an acid catalyst such as paratoluenesulfonic acid. Other suitable catalysts include acid phosphates such as methyl and butyl acid phosphate, acid pyrophosphates such as dimethyl acid pyrophosphates, organic acid sulfate esters, and other organic sulfonic acids. The sulfonic acids can be neutralized with an amine, preferably a tertiary amine.

The coating composition of the invention can be pigmented, containing an amount of pigment in a pigment/film-former weight ratio of about .005/1 to 100/1. Useful pigments are, for example, metallic oxides, such as titanium dioxide or zinc oxide; metal hydroxides; metal flakes; sulfides; sulfates; carbonates; carbon black; silica; talc; china clay; and organic dyes.

The pigments can be introduced into the coating composition by first forming a mill base with the polyester polyol. The mill base can be formed, for example, by conventional sand-grinding or bali milling techniques, and then can be blended, by simple stirring or agitation, with the other constituents of the coating composition.

The coating composition can further optionally contain, as a durability enhancer, an ultraviolet light stabilizer, an antioxidant, or both. The ultraviolet light stabilizer can be present in an amount of 120% by weight, based on the weight of the film-forming blend; the antioxidant can be present in an amount of 0.15% by weight, based on the weight of the film-forming blend.

Typical ultraviolet light stabilizers are benzophenones, triazoles, triazines, benzotriazoles, benzoates, lower alkyl thiomethylene-containing phenols, substituted benzenes, organophosphorous sulfides, and substituted methylene malonitriles. Particularly useful are the hindered amines and nickel compounds shown in U.S. Patent 4,061,616 (December 6, 1977).

Typical antioxidants are tetra-kis alkylene (di-alkyl hydroxy aryl) alkyl ester alkanes, reaction product of p-amino diphenylamine and glycidyl methacrylate, and alkyl hydroxyphenyl groups bonded through carboalkoxy linkages to a nitrogen atom of a heterocyclic nucleus containing an imidodicarbonyl group or an imidodithiocarbonyl group.

One preferred combination of ultraviolet light stabilizer and antioxidant is 2-hydroxy-4 dedecyloxy benzophenone or a substituted 2(2'-hydroxyphenyl) benzotriazole and tetra-kis methylene 3(3',5'-dibutyl-4'-hydroxyphenyl) propionate methane.

The coating composition can be applied to a variety of substrates by any of the conventional application methods such as spraying, dipping, brushing, or flow coating. Substrates that can be advantageously coated with the present composition are, for example, metal, steel, wood, glass, or plastics such as polypropylene, polystyrene, copolymers of styrene, and the like. The coating is particularly suited for application over primed or unprimed metal or steel. Typical uses are for coating steel that has been treated with zinc phosphate or iron phosphate, metal substrates pre-coated with conventional alkyd or epoxy primers, and galvanized steel.

When the curing agent is polyisocyanate, the coating can be cured at ambient temperatures.

Also, regardless of which curing agent used, the composition can be dried (cured) by heating at 1 20C2 1 00C for 1 5-30 minutes, although if a blocked isocyanate is used, the temperature should be at least 1 500 C. Either method, however, ultimately produces a coating that is hard, durable, scratch and stain resistant, weather resistant, and chemical resistant. The composition is suitable, for example, for coating automobile or truck bodies, railroad equipnlent, appliances, and any industrial equipment.

In an additional aspect, it is possible to apply the present coating composition as a two-coat system in which a first pigmented coat is applied as previously described over the substrate and is then overlaid with a second, unpigmented coat. This can impart to the finish a gloss or appearance that is improved over that attinabls when a single coat system is used. This is particularly desirable when the composition is used as an automotive coating. When such a two-coat system is employed, however, the first coat should be allowed to cure to a point where it is tack-free before the second coat is applied.This will normally prevent the solvent in the second coat from attacking the first coat. This attack, or strike-in, can cause the polymers of the two coats to combine at their interface, negating the improvement in the gloss or appearance.

Example The follo rvíng 3 ingredients are prepared as follows: 1. . Polyester oiyo eIuion Portion 1 Parts by Weight Monoester of neopentyl glycol and hydroxypivalic acid 652.8 Neopentyl glycol 1331.2 Phthaiic anhydride 592.0 lsophthalic acid 664.0 Adipic acid 292.0 dibutyl tin oxide 3.3 Xylene 125.0 Portion 2 Xylene 78.0 Portion 3 2-Ethyl hexanol 203.0 Portion I is charged into a reaction vessel equipped with an agitator and vapor condenser, and is heated to reflux, approximately 21 OOC. This temperature is maintained until the reaction is completed, determined by monitoring the flow of the water of estedfication from the condenser and by 'intermittent sampling to determine when the acid number reaches 5 (completion). Total water collected is 288 grams. Portion 2 is added to the mixture, which is then allowed to cool to about 125 C, after which portion 3 is added. This mixture is then agitated and filtered. The resulting reaction product, the polyester polyol, has a hydroxyl content of about 6.3% by weight (based on product solids weight) and a number average molecular weight (gel permeation chromatography) of about 570 620.The polyester polyol solution has a Gardner-Holdt viscosity of =-6-1/4 and a solids content of about 87% by weight.

2. Mill Base Parts by Weight Polyester polyol solution (ingredient 1) 34.5 Amyl acetate 18.0 Pigment dispersant (copolymer of methyl methacrylate/ 2-ethylhexyl acrylate in 62.5/37.5 weight ratio in a toluene/methyl isobutyl ketone/methyl ethyl ketone solvent, copolymor/solvent weight ratio of 1/1) 3.0 TiO2 white pigment 100.0 The constituents are added to a mixing vessel and mixed for about 1 hour. The rnixture is then charged into a sand mill and ground at a temperature of about 350C.

A coating composition is then prepared with the following constituents: Portion 1 Parts by Weight Polyester polyol solution (ingredient 1) 40.2 Epoxy resin (60% weight solution of an epichlorohydrin-bisphenol A epoxy resin, having an epoxide equivalent weight of 875-1025, in a methyl ethyl ketone/xylene solvent) 8.3 Mill base (ingredient 2) 155.5 Ethyl acetate 8.2 Portion 2 Dinonylnaphthalene disulfonic acid (40% by weight in isobutanol) 2.0 Portion 3 Hexamethoxymethylmelamine 30.0 Portion 1 is charged into a stainless steel vessel and mixed for 15 minutes after which time portion 2 is added, with mixing continuing for an additional 5 minutes.Portion 3 is then thoroughly mixed into the vessel, giving a coating composition in which the film-forming blend (polyester polyol, epoxy resIn, and hexamethoxymethylmelamine) is about 70% of the combined weights of the filmforming blend and solvent. Including pigments, the coating composition is approximately 83% solids by weight.

The composition is sprayed (airless spray using a pressure of 2400 pounds per square inch) onto "Bonderi:e 1000" panels (cold rolled steel with iron phosphate layer) and the panels thus coated are baked for 30 minutes at 1350C When then tested, the coating has a pencil hardness of 3H, a Tukon hardness of 21.3 at 250C and of 7.0 at 700C.

The coated steel panels of this example have a reverse impact of 80 inch-pounds (Gardner impact tester) and the coating exhibits no visible cracks when the panel is bent 1800 around a conicallyshaped mandrel varying from 1/8 inch to 1-1/2 inches diameter over an 8-inch length.

Several of the coated panels are scored to the metal with a nail and placed in a salt-spray cabinet where they are exposed to a mist of a solution of NaCI (5% by weight) in water. After 300 hours, the coating creepage from the score line is 4 mm. After 500 hours, the creepage is 7 mm.

The coatings on such panels are found to be resistant to stains from such common substances as mustard, lipstick, and orange dye and to be immune from attack by common solvents like toluene, xylene, and methyl ethyl ketone.

Claims

1. A coating composition of a film-forming blend and a solvent for the blend wherein the blend is at least 40% by weight of the composition and consists essentially of (a) a polyester polyol that is the reaction product of (1) neopentyl glycol and at least one other hindered diol containing two methylol groups wherein each methylol group is attached directly to a cycloaliphatic or aromatic structure or to a tertiary carbon atom, the molar ratio of neopentyl glycol to hindered diol being 2:1 to 6:1, and (2) a mixture of an aromatic and an aliphatic dicarboxylic acid wherein the molar ratio of aromatic acid to aliphatic acid is from 2:1 to 10:1, wherein the molar ratio of (1) to (2) is from 1.3:1 to 1.9:1 and wherein the po;yol has a hydroxyl content of about 3.010.0% by weight; (b) a curing agent for the polyol; and (c) 050% by weight, based on the weight (a) plus (b), of an epichlorohydrin-bisphenol-A epoxy resin, or the esterification product of said resin with a monocarboxylic acid, or mixtures of these.

2. The coating composition of claim 1 containing 510% by weight, based on the weight of (a) plus (b), of (c).

3. The coating composition of claim 1 or 2 in which the curing agent is diisocyanate or triisocyanate selected from the group consisting of aliphatic, cycloaliphatic, alkaryl, aralkyl, heterocyclic, and aryl isocyanates.

4. The coating composition of claim 1 or 2 in which the curing agent is an aminoplast resin.

5. The coating composition of any preceding claim in which the hindered diol is the monoester of neopentyl glycol and hydroxypivalic acid.

6. The coating composition of any preceding claim in which the aliphatic dicarboxylic acid is selected from the group consisting of succinic acid, glutaric acid, adipic acid, pimelic acid, maleic acid, itaconic acid, and mixtures of these, and in which the aromatic dicarboxylic acid is selected from the group consisting of phthalic acid, isophthalic acid, terephthalic acid, uritic acid, cumidinic acid, and mixtures of these.

7. The coating composition of claim 6 in which the aliphatic dicarboxylic acid is adipic acid and the aromatic dicarboxylic acid is a mixture of phthalic acid and isophthalic acid.

8. The coating composition of claim 1,2, 3, 4, 5, 6, or 7 additionally containing pigment.

9. The coating composition of claim 1,2, 3, 4, 5, 6, or 7 additionally containing an ultraviolet light stabilizer, an antioxidant, or both.

10. A substrate coated with a first cured coating composition according to claim 1 overlaid with a second cured coating composition according to claim 1 wherein the first composition contains pigment and wherein the second composition is unpigmented.

11. The substrate of claim 10 wherein the first cured coating composition additionally contains an ultraviolet light stabilizer.

12. The substrate of claim 10 or 11 wherein the first cured coating composition additionally contains an antioxidant.

1 3. The substrate of claim 10, 11 or 12 wherein the second cured coating composition contains an ultraviolet light stabilizer and an antioxidant.

14. A polyester polyol that is the reaction product of (1) neopentyl glycol and at least one other hindered diol containing two methylol groups wherein each methylol group is attached directly to a cycloaliphatic or aromatic structure or to a tertiary carbon atom, the molar ratio of neopentyl glycol to hindered diol being 2:1 to 6:1; and (2) a mixture of an aromatic and an aliphatic dicarboxylic acid wherein the molar ratio of aromatic acid to aliphatic acid is from 2:1 to 10:1; wherein the molar ratio of (1) to (2) is from 1.3:1 to 1.9:1 and wherein the polyol has a hydroxyl content of about 3.010.0% by weight.

1 5. The polyol of claim 14 in which the hindered diol is the monoester of neopentyl glycol and hydroxypivalic acid.

1 6. The polyol of claim 14 or 1 5 in which the aliphatic dicarboxylic acid is selected from the group consisting of succinic acid, glutamic acid, adipic acid, pimelic acid, maleic acid, itaconic acid, and mixtures of these, and in which the aromatic dicarboxylic acid is selected from the group consisting of phthalic acid, isophthalic acid, terephthaiic acid, uritic acid, cumidinic acid, and mixtures of these.

1 7. The polyol of claim 1 6 in which the aliphatic dicarboxylic acid is adipic acid and the aromatic dicarboxylic acid is a mixture of phthalic acid and isophthalic acid.

18. The polyol of claim 14, 15, 1 6 or 1 7 in which the molar ratio of aromatic acid to aliphatic acid is from 2:1 to 6:1, and the polyol has a hydroxyl content of about 5.57.5% by weight.

19. The coating composition of any of claims 1 and 3 to 9, in which the weight of (c) is 010% of the weight of (a) plus (b).

20. A coating composition as claimed in claim 2 substantially as described in the foregoing Exampie.