CURABLE COATING COMPOSITIONS

Abstract: The invention relates to an ambient temperature curable coating composition comprising a film-forming epoxy resin, a curing agent for the epoxy resin that contains an average of at least two thiol groups per molecule and a catalyst for the curing reaction, wherein the composition contains a fugitive retarder for the curing reaction which inhibits curing of the composition in the bulk but which leaves the curable composition when the composition is applied as a coating on a substrate. Further, the invention relates to the use of these coating compositions, in particular to their use as primer coatings for steel in shipbuilding or structural steel used in buildings, bridges or oil rigs.

Description: Curable Coating Compositions This invention relates to ambient temperature curable coating compositions. Such compositions generally comprise a film-forming resin, a curing agent for the resin and a catalyst for the curing reaction, optionally together with solvents, pigments and fillers. Usually the resin and the curing agent are packaged separately and are mixed shortly before the coating is to be applied to a substrate, particularly if rapid curing of the coating is required. In general, the pot life, that is the time within which the coating increases in viscosity beyond a sprayable consistency, is required to be as long as possible and the time the coating takes to cure on the substrate is required to be as short as possible. An indication of pot life can also be obtained from the time it takes for the reacting system to double in viscosity or the time to the onset of the reaction exotherm in a bulk reaction. Ambient temperature curing coatings are used in particular for coating large structures which cannot conveniently be heated, for example ships, buildings, bridges, industrial plant and oil production installations, both during construction and for subsequent maintenance. The ambient temperature of application of the coating may vary from 0°C or below to 25°C or above, according to season and location. There is a need for a coating which has a reasonably long pot life such as an hour or more at 25O but which will cure on the substrate to the extent that it can be walked on without substantial damage to the coating within a day or less, preferably 8 hours or less, even at 0°C. An ambient temperature curable coating composition according to the invention comprises a film-forming resin, a curing agent for the resin and a catalyst for the curing reaction, characterised in that the composition contains a fugitive retarder for the curing reaction which inhibits curing of the composition in the bulk but which leaves the curable composition when the composition is applied as a coating on a substrate. By "film-forming" is meant a resin capable of forming a continuous layer when applied to a substrate. Some of the most widely used film-forming resins in ambient temperature curing coatings are epoxy resins. Coatings based on epoxy resins are described for example in US-A-4,835,241 and US-A-4,051,195 and in Chapters 23 and 24 of "Handbook of Epoxy Resins" by H. Lee and K. Neville, published by McGraw-Hill in 1967. Epoxy resins are the preferred resins for use in the coating compositions of the present invention. The epoxy resin can in general be any of the epoxy resins described in the above textbook by Lee and Neville, preferably a glycidyl-type epoxy resin containing glycidyl ether or ester groups. The epoxy resin can for example be a giycidyl ether of a bisphenoi such as the diglycidyl ether of Bisphenoi A or Bisphenoi F or can be a condensed or extended glycidyl ether of a bisphenoi. Such glycidyl ethers derived from a bisphenoi generally have an epoxy functionality of 2 or slightly less, for example 1.5 to 2. The epoxy resin can alternatively be a glycidyl ether of a polyhydric phenol, for example an epoxy novolac resin, or an aliphatic or cycloaliphatic di- or poly- glycidyl ether. Examples of cycloaliphatic glycidyl ethers are hydrogenated glycidyl esters of bisphenols, for example 2,2-bis(4-glycidoxycyclohexyl)propane (hydrogenated Bisphenoi A diglycidyl ether) or hydrogenated condensed glycidyl ethers of bisphenols. Examples of epoxy resins containing glycidyl ester groups are homopolymers or copolymers of a glycidyl ester of an ethylenically unsaturated carboxylic acid such as giycidyl methacrylate or glycidyl acrylate, or diglycidyl esters of dimerised fatty acids or polyglycidyl esters of aromatic polybasic acids. Epoxidised oils, epoxy compounds without additional heteroatoms such as diepoxyoctane, or epoxidised polybutadienes can also be used. The preferred curing agent for use with an epoxy resin in the coating composition of the invention is a compound containing at least two thiol groups per molecule. Examples of epoxy resin coatings cured with polythiols are described in EP-A- 0,129,394, GB-A-1, 513,769 and US-A-3,553,283. The polythiol for use in coating compositions according to the invention can for example contain an average of at least 2, preferably at least 3, thiol groups up to 6 or 8 thiol groups or even more per molecule. In general, either the curable resin or the curing agent should contain more than 2 functional groups per molecule. A polythiol of functionality at least 3 is preferred for use with most epoxy resins, although a dithiol can be used with highly functional epoxy resins such as epoxy novolacs and giycidyl methacrylate polymers. Examples of such polythiols are esters of a thiol-substituted carboxylic acid such as thioglycolic (mercaptoacetic) or mercaptopropionic acid and a polyol such as trimethylolpropane, pentaerythritol, glycerol, trimethyiolethane, ethyleneglycol, 1 ,4-butanediol, dipentaerythritol, sorbitol or mannitol, for example trimethylolpropane tri(thiogiycolate) or pentaerythritol tetra(3-mercaptopropionate) which are available commercially. Further examples of polythiols are cyclohexanedithiol, 1 ,2-ethanedithiol, dipentene dimercaptan, mercaptoethyl-substituted cyclohexanethiol, 1 ,6-hexanedithiol, bis(mercaptoethyl)sulphide, 1 ,8-dimercapto-3-6-dioxaoctane, 1 ,2,3-propanetrithioi, 1 ,3,5-triazine,2,4,6-trithiol, dimercaptothiadiazole or aromatic thiols, such as 1,4- benzenedithiol. Polymeric polythiols can for example be prepared by the reaction of 1 ,2,4-trivinylcyclohexane with a dithiol. Polythiols based on urethanes may be, for example, derived from the reaction of mercaptoalcohols with multifunctional isocyanates. For certain uses where the coating is likely to experience prolonged contact with alkaline or acidic water, for example on the hull of a ship having cathodic protection, polythiols containing no saponifiable carboxylate ester groups may be preferred. The ratio of epoxide to thiol groups in the coating composition is generally in the range 1:5 to 5:1, preferably in the range 1:2 to 2:1, most preferably in the range 0.8:1 to 1.2:1. In some instances it may be desirable to use a blend of polythiols. Alternatively, the thiol functional material(s) may be mixed with a proportion of other types of agents capable of curing epoxides at ambient temperature, as long as the effect on the pot life and drying time are not otherwise compromised. The curing reaction between an epoxy resin and a polythiol generally proceeds slowly at ambient temperature in the absence of a catalyst, particularly if the ambient temperature is below 20°C. The coating compositions of the invention thus contain a catalyst for the curing reaction. The preferred catalyst for the epoxy thiol curing reaction is a tertiary amine, most preferably an amine containing at least one aromatic ring per molecule, for example an aralkyl amine such as 2,4- bis(dimethylaminomethyl)phenol, 2,4,6-tris(dimethylaminomethyl)phenol, N,N-dimethyl benzyiamine, or pyridine, although an aliphatic amine such as dimethyl cocoamine or diazabicyclooctane (triethylene diamine) can be used. The catalyst is generally used at 0.1 to 10% by weight based on the epoxy resin. Secondary or even primary amines are alternative catalysts. They will react with the epoxy resin and so become bound to the crosslinked network. Such type of catalyst may be used at higher levels as long as the pot life and drying time are not compromised. Examples of other catalysts which can be used are other Lewis base catalysts or Lewis acid catalysts or organometallic compounds or organic tin and other metal salts. The fugitive retarder is preferably a volatile compound, generally a volatile liquid organic compound, which leaves the curable composition by evaporating when the composition is applied as a coating on the substrate. The volatile compound generally has a vapour pressure of at least 1 mm Hg at 25°C and is preferably a solvent for the film-forming resin, the curing agent and/or the catalyst. The fugitive retarder can alternatively be a compound whose effect on the curable composition is achieved by means other than evaporation when the composition is applied to a substrate, for example by attachment to the substrate or to some other ingredient of the coating or by phase separation. The volatile solvent suitable as a fugitive retarder can in general be any volatile solvent which is not a H-bond donor. For example a volatile compound suitable as a retarder for a catalysed epoxy resin polythiol coating composition is a volatile hydrocarbon, particularly if it is used in the absence of substantial amounts of a volatile alcohol solvent or of other polar solvents such as primary or secondary amides which are H- bond donors (generally compounds containing active hydrogen, i.e. having H attached to an element other than carbon) having solute hydrogen-bond acidity, ^T α" , greater than 0.1 as defined by M.H. Abraham in Chemical Society Reviews 1993 at pages 73 to 83. The hydrocarbon is preferably an aromatic hydrocarbon such as xylene, ethylbenzene or trimethylbenzene although aliphatic hydrocarbons such as heptane, isooctane and white spirit can be used. Aromatic hydrocarbons have the advantage of being better solvents for the epoxy resin and the curing agent. Blends of such solvents can also be used. Alternative fugitive retarders are volatile solvents which are H-bond acceptors having solute hydrogen bond basicity, ^ β" , greater than 0.2 and solute hydrogen bond acidity less than 0.1 as defined by Abraham (op.cit), such as ethers, esters, ketones or N,N-dialkylamides. The fugitive retarder can for example be a volatile compound containing at least one ether and/or ester group such as butyl acetate, dibutyl ether, diethyl ether, methoxypropyl acetate or butoxyethyl acetate, or a cyclic ether such as dioxan, provided that the ether or ester does not contain a free alcohol group or another H-bond donor. The fugitive retarder can for example be a ketone such as methyl ethyl ketone (2-butanone), methyl isobutyl ketone or methyl isoamyl ketone. The fugitive retarder could also for example be an alkyl carbonate, such as diethylcarbonate, or propylene carbonate. The fugitive retarder can alternatively be a volatile N,N-dialkylamide such as N,N-dimethylformamide or N,N-dimethylacetamide, although amides containing H attached to nitrogen are H-bond donors and reduce rather than extend pot life. The fugitive retarder can be a mixture of different types of fugitive retarder, for example a blend of an aromatic hydrocarbon such as xylene with an ether, ester or ketone solvent such as butyl acetate, methoxypropyl acetate or methyl isobutyl ketone. Nitroalkanes or volatile siioxanes can also be used, provided they show initial compatibility with the curable composition. Whilst epoxy resin coatings are frequently used in solvent blends containing an aromatic hydrocarbon such as xylene, they are not generally used with a hydrocarbon as the only volatile organic solvent. They are often used as solvent blends with an alcohol such as n-butanol or isobutanol or an etheralcohol such as butoxyethanol. Such blends generally contain the xylene and alcohol in volume ratio within the range 1:1 to 3:1. The coating compositions of the invention containing a hydrocarbon such as xylene as fugitive retarder generally contain no more than 20% by volume based on total solvent of any volatile organic compound which is a H-bond donor such as an alcohol or amide. Coating compositions of the invention containing a H-bond acceptor such as an ether, ester or ketone can contain up to 30% or 40% by volume of volatile organic H-bond donor although they preferably contain less than 10% H-bond donor solvent and most preferably none at all. If a typical solvent blend of xylene and n- butanol in volume ratio 3:1 is used in an epoxy resin polythiol coating containing tertiary amine catalyst, the composition is capable of curing within 8 hours at ffC and at 25°C but has an unacceptably short pot life of less than half an hour at 25'C. If the butanol is removed, the composition remains capable of rapid curing at both CC and 25°C but the pot life at 25°C is at least an hour, which is commercially satisfactory for a 2-pack coating which is mixed manually and applied by conventional spray equipment. In general, the viscosity of a coating composition for application by spray should be in the range 0.5 to 10 poise and preferably less than 6 poise. If all the solvent is removed, the pot life becomes unacceptably short, that is to say a solventless composition based on a liquid epoxy resin, liquid polythiol and catalyst gels in bulk in less than half an hour at 25°C. Solvents containing both an H-bond acceptor and H-bond donor group may also be used, but only as a blend with other H-bond accepting solvents, as the H-bond donor property usually overwhelms the H-bond accepting characteristics. Likewise, tautomeric solvents, such as ethyl acetoacetate can be used, as the enol form of the material is in general not the dominant tautomer. It was found that those solvents or mixtures of solvents can be used as fugitive retarder which comply with the following equation: 29.3 π - 272.2 α + 77.9 β > 0 wherein: π is the dipolarity-polarisability parameter of the solvent π" as defined by M.H. Abraham in Chemical Society Reviews 1993 at pages 73 to 83. For a mixture of solvents π equals the sum of the π values for the individual solvents present in the mixture weighted by their volume fraction. The π value for solvents that are not present in the article of Abraham can be estimated by comparison with similar solvents that are listed in the article. α is the hydrogen-bond acidity parameter of the solvent ^ α" as defined by M.H. Abraham in Chemical Society Reviews 1993 at pages 73 to 83. For a mixture of solvents α equals the sum of the α values for the individual solvents present in the mixture weighted by their volume fraction. The α value for solvents that are not present in the article of Abraham can be estimated by comparison with similar solvents that are listed in the article. β is the hydrogen-bond basicity parameter of the solvent ^ β" as defined by M.H. Abraham in Chemical Society Reviews 1993 at pages 73 to 83. For a mixture of solvents β equals the sum of the β values for the individual solvents present in the mixture weighted by their volume fraction. The β value for solvents that are not present in the article of Abraham can be estimated by comparison with similar solvents that are listed in the article. The fugitive retarder can in general be used at 5 to 50% by weight of the coating composition provided it gives the required increase in pot life. For example a volatile hydrocarbon such as xylene used as retarder in a catalysed epoxy resin polythiol coating is preferably used at 10 to 40% by weight, most preferably 15-25%, of the coating composition, and volatile esters, esters or ketones are preferably used in similar amounts. The fugitive retarder can alternatively be a fugitive, for example volatile, acid capable of forming an amine salt with a tertiary amine catalyst. The acid is preferably a carboxylic acid such as acetic acid. The acid is used at a molar ratio of acid to amine catalyst of 0.1:1 to 10:1, most preferably 1:1 to 6:1. However, it was found that not all solvents that comply with the above equation also act as fugitive retarder, e.g. because the presence of these compounds accelerates the reaction since they function as a catalyst. As indicated above, amines can act as a catalyst for the reaction between an epoxy resin and a curing agent having an average of at least two thiol groups per molecule. Therefore, these compounds are not considered as fugitive retarders. Examples of compounds that comply with the above formula but are not considered as fugitive retarders since the accelerate the curing reaction are e.g.: piperidine, N-methylpiperidine, N-ethylpiperidine, N-methyipyrrole, pyrazine, 2-methylpyrazine, and pyrimidine. In principle, any fugitive retarder can be used in the ambient temperature curable coating composition according to the present invention. However, it was found that different problems are encountered when the fugitive retarder consists solely of toluene. It was found that in these compositions toluene is not a suitable solvent for the most desirable types of epoxy resin and is too volatile. The flash point of toluene requires special equipment to handle these coating compositions in a safe way. Solvent mixtures containing toluene as one of the components and a sufficient amount of one or more other components, in general, do not show these drawbacks. The coating compositions of the invention can for example be used as anticorrosive primer coating compositions applied to steel used in shipbuilding or structural steel used in buildings, bridges or oil rigs. The fiim-forming resin and the curing agent are generally stored separately and are mixed shortly before application together as a coating to the substrate. The catalyst also is preferably stored separately from the resin. For example, an epoxy resin component and a polythiol component can be mixed in a container up to 1 or 2 hours before application and the extended pot life conferred by the fugitive retarder is a particular advantage with this system. The resin and curing agent can alternatively be mixed at application in a twin feed spray and applied together; the coatings of the invention have the advantage that the same coating can be mixed and applied by either procedure. The coating compositions of the invention are particularly suitable for curing at ambient temperature, for example below 40°C and particularly below 25°C and including cold climates where the ambient temperature is below 10°C or even below 0°C down to - 5°C or -10°C. The coatings generally harden sufficiently rapidly at 10°C, and usually at 0°C, that a 200 μm coating film can be trodden on 24 hours after application, usually 8 hours after application, without gouging of the film. The compositions of the invention cure and harden more rapidly than epoxy resin compositions based on conventional primary amine curing agents. The coatings of the invention may contain an anticorrosive pigment such as metallic zinc, zinc phosphate, wollastonite or a chromate, molybdate or phosphonate, a barrier pigment such as micaceous iron oxide, glass flake, aluminium flake or mica and/or a filler pigment such as iron oxide, barytes, talc, calcium carbonate or titanium dioxide. The coatings may aiso contain other coating additives such as a wetting agent, a thixotrope, a reactive diluent, for example a monoepoxide or a flow control agent. The pigments and other additives can usually be mixed and stored with the epoxy resin or with the curing agent or with both. Other additives which may be included comprise diluents and plasticisers as long as the properties of these materials do not compromise the pot life and drying properties of the composition. The invention will be elucidated by reference to the following examples. These are intended to illustrate the invention but are not to be construed as limiting in any manner the scope thereof. Example 1 A liquid epoxy resin of epoxide equivalent weight 176 based on a mixture of Bisphenoi A and Bisphenoi F (Epikote 232, ex. Shell Chemicals) was mixed with 2,4,6- tris(dimethyiaminomethyi)phenol (Ancamine K54, ex. Air Products) catalyst in amounts shown in Table 1 and xylene. This composition was mixed with pentaerythritol tetrakis(3-mercaptopropionate) (CAS no. 757-23-7, ex. Dow Chemicals) and xylene to give a 1:1 ratio of epoxide groups to thiol groups; the amount of xylene used was 20% by weight in each component. The xylene was free from hydroxyl or other H-bond donor solvents. Samples of the resulting unpigmented coating compositions were kept in paint cans at different temperatures and the viscosity was measured at various times using a cone and plate viscometer. The measurements of viscosity were made at 25°C. The time taken for the coating composition to double in viscosity was taken as an estimate of the pot life, although in most cases the unpigmented composition still had a low enough viscosity for spray application even after doubling in viscosity. Samples of the unpigmented coating compositions were applied to a steel substrate at 200 μm wet thickness and allowed to dry at different temperatures. The time taken for the coating to dry and cure to a hard surface which was non-tacky to the touch was noted ("touch dry"), as was the time for the coating to cure to a condition where it appeared hard throughout the thickness of the coating ("hard dry"). The results achieved are shown in Table 1 below: Table 1 Pot Life Applied Coating Film Catalysts Level T (°C) Initial Time to double Time to Time to (% by weight of Viscosity viscosity Touch Dry Hard Dry epoxy resin) (poise) 2.0 0 0.645 2 h 30 mins 2 h 20 mins 3 h 15 mins 1.14 15 0.625 1 h 44 mins 1 h 25 mins 1 h 55 mins 1.7 25 0.643 55 mins 1 h 35 mins 1 h 45 mins As shown in Table 1 , the coating composition of Example 1 had an adequate pot life for conventional spray application of about 1 hour even at 25°C while it dried and cured to a hard coating in less than 4 hours even at OPC. It could thus be used as a rapid drying ambient temperature curing at any temperature in the range 0-25 ) C without needing special application equipment. By comparison, a coating composition in which a 3:1 by weight mixture of xylene and butanol was used in place of the xylene took only 21 minutes to double in viscosity on storage at 25°C and a coating composition in which a 1:1 mixture of xylene and butanol was used took only 37 minutes to gel on storage at 25'C. A coating composition in which the xylene was simply omitted took only 22 minutes to double in viscosity at 25°C. Example 2 An epoxy resin paint was prepared from the following ingredients: Bisphenoi A epoxy resin (Araldite GY250, ex. Ciba Chemicals) 24.17kg Xylene 10.42kg Pigments (talc, barytes and iron oxide) 48.94kg Thixotrope 0.55kg Tris(dimethylaminomethyl)phenol 0.24kg Pentaerythritol tetrakis(3-mercaptopropionate) (CAS no. 757-23-7, ex. Dow Chemicals) 15.52kg This yielded a paint with viscosity 3.5 poise at 23°C with a dry film PVC of 30% and a solvent content of 20% by volume. A 20 litre sample of this paint was stored at 23C and was still sprayable after 1 hour. Some of the mixture was sprayed as a coating on 12" x 6" (30.48cm x 15.24cm) steel panels, and allowed to cure at CPC. A film of 150μm dry thickness could be walked on without disrupting the film after 7 hours 10 minutes. By comparison, when the epoxy resin paint base of this Example (with a reduced amount of xylene and containing some butanol solvent) is used with a dipnmary amine curing agent as a coating for internal areas of ships, it does not dry and cure sufficiently hard to be walked on without gouging if the ambient temperature remains at 0°C. Example 3 An epoxy novolac resin having an average of 3.6 epoxide groups per molecule (DEN 438, ex. Dow Chemicals) was mixed with 1.8% by weight 2,4,6- tris(dimethylaminomethyi)pheπol (Ancamine K54, ex. Air Products) catalyst and xylene, and then with ethyicyciohexyldithiol curing agent to form a coating composition containing 20% by weight xylene. The coating composition had a pot life at 25'C (time to double in viscosity) of 1 hour 23 minutes. The coating was capable of curing to a hard coating at CPC. Example 4 Example 1 was repeated using a solely Bisphenoi A based epoxy resin of epoxy equivalent weight 190 sold under the trademark Araldite GY250, ex. Ciba Chemicals, in place of the epoxy resin of Example 1. Conditions of testing were as described in Example 1 and the results obtained were shown in Table 2 below: Table 2 Pot Life Applied Coating Film Catalysts Level T (°C) Initial Time to double Time to Time to (% by weight of Viscosity viscosity Touch Dry Hard Dry epoxy resin) (poise) 1.0 0 0.713 2 h 40 mins 4 h 30 mins 5 h 15 mins 1.0 15 0.803 1 h 37 mins 1 h 30 mins 2 h 1.0 25 0.738 1 h 31 mins 1 h 1 h As shown by Table 2, the coating composition of Example 4 had an adequate pot life even at 25°C and cured to a hard coating in less than 6 hours even at QPC. Examples 5 to 14 10.00 g Araldite GY 250 epoxy resin ex. Ciba Chemicals was mixed with 6.42g pentaerythritol tetrakis(3-mercaptopropionate) (CAS no. 757-23-7, ex. Dow Chemicals), 0.17g 2,4,6-tris(dimethylaminomethyl)phenol (Ancamine K54, ex. Air Products) and 4.24g of each of the solvents shown in Table 3 below. Examples A to G use volatile solvents which are H-bond donors and are not fugitive retarders useful in the present invention. Example 5 uses xylene, and Examples 6 to 11 use H-bond acceptors which are useful as fugitive retarders in the present invention. Example 12 shows the effect of mixing an H-bond donating solvent and an H-bond acceptor. Example 13 represents a tautomeric solvent and shows the ketone and ester groups attached to the same methylene group are not as effective as when the groups are remote. Example 14 shows the effect of an H-bond acceptor which has acidic character. The result is a weak acceleration of the reaction. For each Example, the mixture of resin, curing agent, catalyst and volatile solvent was stored in bulk and the time to exotherm onset was measured. Table 3 imple ; Solvent Time to Exotherm π α β Z 1 Onset (minutes) A Formamide 1 1.30 0.62 0.60 -83.9 B Methanol 2 0.44 0.43 0.47 -67.5 C Ethanol 7 0.42 0.37 0.48 -51.0 D 1 -Butanol 8 0.42 0.37 0.48 -51.0 E 2-ethylhexanol 13 0.39 2 0.37 2 0.48 2 -51.9 F 2-methoxypropanol 26 0.58 2 0.40 2 0.80 2 -29.6 G Chloroform 38 0.49 0.15 0.02 -24.9 5 Xylene 67 0.52 0.00 0.16 27.7 6 Diethyl ether 92 0.25 0.00 0.45 42.4 7 Methyl isobutyl ketone 110 0.65 0.00 0.51 58.8 8 Butyl acetate 113 0.60 0.00 0.45 52.6 9 2-Butanone 133 0.70 0.00 0.51 60.2 10 1 ,4-Dioxan 136 0.75 0.00 0.64 71.8 11 N,N-dimethyiformamide 158 1.31 0.00 0.74 96.0 12 Methyl isobutylketone : 73 0.59 0.09 0.50 31.3 1 -butanol (3:1) 13 t-Butylacetoacetate 78 14 Nitroethane 60 0.95 0.02 0.33 48.1 1 ) Z = 29.3 π - 272.2 α + 77.9 β 2 ) estimated values The time to exotherm onset is related to pot life. The solvents of Examples 6 to 13 can be used to replace xylene as fugitive retarder in the compositions of Examples 1 to 6 and will give pot lives at least as long. The time to hard dry of the coatings containing the fugitive retarders of Examples 6 to 13 is similar to that for the corresponding coatings containing the same level of xylene. As a comparison, a solvent-free composition had a time to exotherm onset of 18 minutes. Example 15 A polythiol crosslinker was synthesised by adding 173 g of mercaptoethanol to 434 g Desmodur N3300 ex. Bayer polyisocyanate in 445 ml of butyl acetate. The mixture was heated to 70°C in the presence of an acid catalyst. After 2 hours, the consumption of isocyanate groups had ceased. The product had a thiol equivalent weight of 298. The material was used as a 62% w/w solution in butyl acetate. 3,38 g of the polythiol solution was mixed with 1 ,74 g of a liquid Bisphenoi A epoxy resin of equivalent weight 190 (Araldite GY250, ex. Ciba Chemicals) and 30 mg of 2,4,6-tris(dimethylaminomethyl)phenol (Ancamine K54, ex. Air Products). After application, a 150 μm film was touch dry in 90 minutes at 25'C. 15 g of the polythiol solution was mixed with 14,2 g of the same liquid epoxy resin, 5 g of xylene and 0,28 g of 2,4,6-tris(dimethylaminomethyl)phenol (Ancamine K54, ex. Air Products). Even after 5 hours at 25°C no peak exotherm was observed, which gives a good indication of the pot life of this composition. Example 16 10 g of a liquid Bisphenoi A epoxy resin of equivalent weight 190 (Araldite GY250, ex. Ciba Chemicals) was mixed with 2,29 g ethylcyclohexyldithiol, 3,47 g pentaerythritol tetramercaptopropionate and 0,17 g 2,4,6-tris(dimethylaminomethyl)phenol (Ancamine K54, ex. Air Products). An amount of xylene equivalent to 20% of the weight of each component was also added. This mixture gave a time to onset of exotherm of 115 minutes and a peak exotherm at 123 minutes. Example 17 27,15 g of a liquid Bisphenoi A epoxy resin of equivalent weight 190 (Araldite GY250, ex. Ciba Chemicals) was mixed with 16,97 g pentaerythritol tetramercaptopropionate, 0,49 g m-xylenediamine and 0,46 g 2,4,6-tris(dimethylaminomethyl)phenol (Ancamine K54, ex. Air Products) in a solvent free formulation. When applied as a 150 μm film at 23°C, it was touch dry within 45 minutes. A similar formulation but where half of the thiol equivalents were replaced by a similar number of N-H equivalents from m-xylenediamine to yield an 80% solids solution in methyl isobutyl ketone gave a time to exotherm onset of 88 minutes at 25'C.

Claims: Claims 1. An ambient temperature curable coating composition comprising a film-forming epoxy resin, a curing agent for the epoxy resin that contains an average of at least two thiol groups per molecule and a catalyst for the curing reaction, characterised in that the composition contains a fugitive retarder for the curing reaction which inhibits curing of the composition in the bulk but which leaves the curable composition when the composition is applied as a coating on a substrate, with the proviso that the fugitive retarder does not consist solely of toluene. 2. A curable coating composition according to claim 1 , characterised in that the catalyst is a tertiary amine. 3. A curable coating composition according to claim 2, characterised in that the tertiary amine contains at least one aromatic ring per molecule. 4. A curable coating composition according to any of claims 1 to 3, characterised in that the fugitive retarder is a volatile solvent for the film-forming resin and/or for the curing agent which is not a H-bond donor and leaves the curable composition by evaporating when the composition is applied as a coating on a substrate. 5. A curable coating composition according to claim 4, characterised in that the fugitive retarder is a hydrocarbon. 6. A curable coating composition according to claim 5, characterised in that the fugitive retarder is an aromatic hydrocarbon. 7. A curable coating composition according to claim 4, characterised in that the fugitive retarder is a H-bond acceptor. 8. A curable coating composition according to claim 7, characterised in that the fugitive retarder contains at least one ether and/or ester group. 9. A curable coating composition according to claim 7, characterised in that the 5 fugitive retarder is a ketone. 10. A curable coating composition according to claim 7, characterised in that the fugitive retarder is a N,N-diaikylamide. o 11. A curable coating composition according to any of claims 1 to 10, characterised in that the solvent complies with the equation 29.3 π - 272.2 α + 77.9 β > 0 wherein: π is the dipolarity-polarisability parameter of the solvent π" as defined by M.H. Abraham in Chemical Society Reviews 1993 at pages 73 to 83 and 5 for a mixture of solvents π equals the sum of the π values for the individual solvents present in the mixture weighted by their volume fraction, α is the hydrogen-bond acidity parameter of the solvent ^ α" as defined by M.H. Abraham in Chemical Society Reviews 1993 at pages 73 to 83 and for a mixture of solvents α equals the sum of the α values for the 0 individual solvents present in the mixture weighted by their volume fraction β is the hydrogen-bond basicity parameter of the solvent T β" as defined by M.H. Abraham in Chemical Society Reviews 1993 at pages 73 to 83 and for a mixture of solvents β equals the sum of the β values for the individual solvents present in the mixture weighted by their volume fraction. 5 The β value for solvents that are not present in the article of Abraham can be estimated by comparison with similar solvents that are listed in the article. 12. A curable coating composition according to any of claims 1 to 11 , characterised 0 in that the fugitive retarder is present at 15-25% by weight of the coating composition. 13. Use of an ambient temperature curable coating composition according to any of the preceeding claims as an anticorrosive coating for steel. 14. Use according to claim 14, characterised in that the composition is used as an anticorrosive primer coating for steel used in shipbuilding or structural steel used in buildings, bridges or oil rigs.

Inventors: CAMERON COLIN, FLETCHER IAN DAVID, GUY ALAN, LINES ROBERT, MARRION ALASTAIR ROBERT, MORTON HEATHER, NIEDOBA STEFAN NORBERT RUEDIGE, CAMERON, COLIN, FLETCHER, IAN, DAVID, GUY, ALAN, LINES, ROBERT, MARRION, ALASTAIR, ROBERT, MORTON, HEATHER, NIEDOBA, STEFAN, NORBERT, RUEDIGER
Applicants: CAMERON COLIN, FLETCHER IAN DAVID, GUY ALAN, INT COATINGS LTD, LINES ROBERT, MARRION ALASTAIR ROBERT, MORTON HEATHER, NIEDOBA STEFAN NORBERT RUEDIGE, CAMERON, COLIN, FLETCHER, IAN, DAVID, GUY, ALAN, INTERNATIONAL COATINGS LIMITED, LINES, ROBERT, MARRION, ALASTAIR, ROBERT, MORTON, HEATHER, NIEDOBA, STEFAN, NORBERT, RUEDIGER
Application year: 1999
Main classification: C08G59
IPC codes: C08G59, C09D163