RAPID CURING WEATHER RESISTANT EPOXY COATING

Abstract: The invention is an epoxy resin coating composition. The composition comprises: 1. a cycloaliphatic epoxy resin (hydrogenated bisphenol A resin), and 2. an acrylate ester, reacted with a mixture of 3. a polyether polyamine of the formula: a. NH₂CH(CH₃)CH₂[OCH₂CH(CH₃)] x NH₂ wherein x ranges from 2 to 6, or wherein x + y + z ranges from 4 to 6; and 4. piperazine. The weather resistant formulations cure rapidly to form blemish-free, attractive, high-gloss coatings.

Description: 1. Field of the Invention The invention relates to improved rapid curing, weatherable epoxy coatings. 2. Description of Other Relevant Materials in the Field Cycloaliphatic epoxy resins prepared by the hydrogenation of glycidyl ethers of bisphenol A or other di- or polyphenols may be reacted with selective curatives to form coatings that are resistant to UV degradation. Cycloaliphatic epoxy resins are prepared by the process described in U. S. Patent No. 3,336,241. These resins are much less reactive with conventional epoxy curatives than are the phenolic-based glycidyl ethers. U. S. Patent No. 4,051,195 to W. F. McWhorter teaches curable epoxy resin compositions comprising (1) a blend of an epoxide resin and a polyacrylate or polymethacrylate ester of a polyol wherein the ester contains more than one terminal acrylate or methacrylate and (2) an aliphatic polyamine curing agent. The weight ratio of epoxide resin: ester is 100:5 to 100:100. The aliphatic polyamine is incorporated into the resin composition in a specified amount. The epoxy resin compositions are said to cure rapidly even at low temperature and are useful as coatings and adhesives. U. S. Patent No. 4, 528,345 to H. G. Waddill teaches a method for making weather-resistant epoxy coatings. The method comprises prereacting a cycloaliphatic diepoxide resin with aminoethylpiperazine or a mixture of aminoethylpiperazine and polyoxyalkylenepolyamine in an amount which is balanced to give the maximum level of primary amine reaction with yielding an excessively viscous reaction product. The prereacted product is reacted with a curing amount of a polyoxyalkylene polyamine and an accelerator. U. S. Patent No. 3,875,072 to H. G. Waddill teaches an accelerator for curing epoxy resins. The accelerator comprises piperazine and an alkanolamine in a weight ratio of 1:8 to 1:1. A number of alkanolamines are taught. The product comprising 10-20% piperazine, 5-10% N-aminoethylpiperazine and 60-80% triethanolamine is sold commercially as Accelerator 399 by Texaco Chemical Co. The accelerator is said to be synergistic for accelerating the curing of a polyglycidyl ether of a polyhydric phenol cured with a polyoxyalkylene polyamine at ambient or elevated temperatures. Such amines include polyoxypropylene diamines of the formula: NH₂CH(CH₃)CH₂[OCH₂CH(CH₃)] x NH₂ wherein x ranges from 2 to 40. Such amines also include polyoxypropylene triamines of the formula: wherein R is a lower alkyl and x + y + z ranges from 3 to 40. These diamines and triamines may be synthesized according to U. S. Patent No. 3,654,370 to E. L. Yeakey which teaches a method comprising a nickel, copper and chromium catalyst for aminating polyols. U. S. Patent No. 4,588,788 teaches the esterification products of a mixture of carboxylic acids of the formula CH₂ = CHCO₂(CH₂CH₂CO₂) n H. This product and a mono- or polyhydric compound are used as reactive diluents for polyepoxides which are reacted with an aliphatic amine curing agent. SUMMARY OF THE INVENTION The invention is an epoxy resin coating composition. The epoxy component comprises a mixture of a cycloaliphatic epoxy resin containing at least 1.8 reactive 1,2-epoxy groups per molecule and an acrylate ester in a weight ratio of 2:1 to 10:1. The curative component comprises a curing amount of a polyether polyamine. The curative additionally comprises a piperazine accelerator in molar equivalence with the acrylate ester. The two components are reacted and cured to form an attractive, weatherable, non-yellowing coating free of surface blemishes. DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is an improvement in U. S. Patent No. 4,051,195 to W. R. McWhorter, incorporated herein by reference. An improved epoxy formulation has been developed which cures rapidly to form attractive, blemish-free, high-gloss coatings. The formulation comprises a blend of a cycloaliphatic epoxy resin and an acrylate ester, cured with a mixture of a polyether polyamine and a reactive amine accelerator derived from piperazine. It has been found that the addition of piperazine to the acrylate ester-epoxy resin system of U. S. Patent No. 4,051,195 shortened drying time and promoted rapid curing of the epoxy resin with the selected polyether polyamines demonstrated in the Example. The cycloaliphatic epoxy resins useful in the invention are those, for example, which are made by the process disclosed in U. S. Patent No. 3,336,241. Preferred cycloaliphatic epoxy resins are the hydrogenated glycidyl ethers of 2,2-bis(4-hydroxyphenyl) propane which is then called 2,2-bis(4-hydroxycyclohexyl) propane. Other cycloaliphatic epoxy resins such as those described in U. S. Patent No. 3,336,241 may also be used by those skilled in the art using the invention described herein to effect weather-resistant epoxy coatings. The acrylate esters useful in this invention are those esters which contain more than one terminal acrylate or methacrylate group. These esters include the acrylic and methacrylic acid esters of aliphatic polyhydric alcohols such as, for example, the di- and polyacrylates and the di- and polymethacrylates of alkylene glycols, alkoxylene glycols, alicyctic glycols and higher polyols, such as ethylene glycol, triethylene glycol, tetraethylene glycol, tetramethylene glycol, hexanediol, trimethylolethane, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol and the like, or mixtures of these with each other or with their partially esterified analogs. Typical compounds include but are not limited to trimethylolpropane triacrylate, trimethylolethane triacrylate, trimethylolpropane trimethacrylate, trimethylolethane trimethacrylate, tetramethylene glycol dimethacrylate, ethylene glycol dimethyacrylate, triethylene glycol dimethacrylate, pentaerythritol triacrylate, pentaerythritol tetraacrylate, 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, dipentaerythritol tetraacrylate, dipentaerythritol pentaacrylate, and the like. Particularly preferred esters are 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, and pentaerythritol tetraacrylate. The procedures for preparing these acrylate and methacrylate esters of epoxide resins is described in U. S. Patent No. 3,377,406 incorporated by reference. The acrylate or methacrylate esters are blended with the epoxide resins in the weight ratio of about 5 to about 100 parts of ester for each 100 parts of epoxide resin. Of the amine curing agents known to be effective in curing a vicinal epoxy resin, preferred curing agents in accordance with the instant invention are the polyoxyalkylene containing amine compounds. A preferred class of polyoxyalkylene polyamines is of the formula: wherein X is hydrogen, methyl or ethyl radical; R is the nucleus of an oxyalkylation susceptible polyhydric alcohol containing 2 to 12 carbon atoms and 2 to 8 hydroxyl groups and; n is a number from 1 to about 15 and r is a number from 2 to 4. The most preferred polyoxyalkylene polyamines are a polyoxypropylene diamine having a molecular weight of about 230 or a triamine of molecular weight about 400. These products are available under the tradename JEFFAMINE® D-230 and JEFFAMINE® T-403 from Texaco Chemical Co. Their use as curing agents is described in U. S. Patent No. 4,189,564. JEFFAMINE® D-230 is represented by the formula: H₂NCH(CH₃)CH₂-[OCH₂CH(CH₃)] x NH₂ wherein x averages 2.6. JEFFAMINE® T-403 is represented by the formula: wherein x + y + z averages 5.3. The reactive amine accelerators of the instant invention are piperazine compounds which are described, for example, in U. S. Patent No. 3,875,072 and U. S. Patent No. 4,528,345 to Waddill incorporated herein by reference. The preferred accelerator is piperazine itself. N-aminoethylpiperazine is shown in the Example to produce good coatings. The piperazine compound is incorporated in an amount which will ensure a maximum level of primary amine reaction but give a reaction product which is not too viscous to handle. The greater the ratio of accelerator to acrylate ester, the less primary amine is reacted. However, a large excess of epoxy resin will result in a very viscous product or even a gel which is undesirable. However, excess piperazine compound is undesirable because of extraneous reactions. Accordingly, the instant coatings are formulated with piperazine compound in an approximate equivalence with the acrylate ester. This invention is shown by way of Example. Coating of cycloaliphatic epoxy and JEFFAMINE® T-403 cured very slowly to form soft, flexible film. Addition of trimethylolpropane (TMP) triacrylate improved drying times was somewhat but tack-free time still lengthy and the coatings were soft. Addition of piperazine in an amount equivalent to the acrylate material present in the formulation shortened drying time considerably, particularly when the acrylate was 20-30% of total resin amount. Such coatings exhibited rapid curing, formed initially soft, flexible coatings which hardened rapidly to tough, blemish-free, high gloss flexible coatings that were difficult to mar. A number of quite reactive amines were added to a cycloaliphatic epoxy resin/TMP triacrylate blend cured with JEFFAMINE® T-403. The amount of reactive amine added was equivalent to the amount of triacrylate present in the formulation. Addition of only a few of the reactive amines (AEP, BAPP, TETA, MIBPA) resulted in shortened drying times. Of these, only AEP and TETA produced blemish-free decorative coatings. The formulation containing TETA produced a dull, lusterless, blemished coating when heat cured. Of all of the reactive amines tested, only AEP and piperazine (Example 1) resulted in a totally acceptable coating. The curing of a cycloaliphatic epoxy resin with JEFFAMINE® D-230 resulted in excessively long film drying times. Addition of TMP triacrylate did not improve drying, probably because the reaction product of acrylate and JEFFAMINE® D-230 was quite soft, with a plasticizing effect. Addition of TMP triacrylate did not improve drying. Addition of piperazine to a system containing both epoxy resin and acrylate, even though a solvent was necessary, resulted in rapid gellation and rapid development of acceptable coating properties. Epoxy resin systems based on liquid bisphenol A resin (aromatic; unsaturated) yellowed rapidly and significantly when subjected to accelerated exposure. The presence of an acrylate as a partial replacement of bisphenol A resin reduced yellowing somewhat. A cycloaliphatic epoxy resin system yellowed to a lesser degree than the bisphenol A resin system. Addition of acrylate as a resin replacement had little effect on resin yellowing. Addition of piperazine or AEP produced a rapid curing formulation with even better yellowing (or non-yellowing) characteristics. A number of the reactive amine systems produced clear coatings that were resistant to yellowing. However, many of these were previously ruled out for various reasons; i.e., slow drying, blemished surface, etc., so that only piperazine (in solution) or AEP were considered useful for the intended purpose. TABLE OF TEST METHODS Pencil hardness (cure) ASTM D-3363-74 Gardner impact ASTM D-2794-69 Yellowing index ASTM D-1925 While particular embodiments of the invention have been described, it is well understood that the invention is not limited thereto since modifications may be made. It is therefore contemplated to cover by the appended claims any such modifications as fall within the spirit and scope of the claims.

Claims: An epoxy resin coating composition characterised in that it comprises the cured reaction product of: A. a cycloaliphatic epoxy resin containing at least 1.8 reactive 1,2-epoxy groups per molecule, mixed with an acrylate ester, in a weight ratio of epoxy resin:acrylate ester of 2:1 to 10:1; reacted with B. a curing amount of a polyether polyamine of the formula (a) NH₂CH(CH₃)CH₂[OCH₂CH(CH₃)] x NH₂ wherein x ranges from 2 to 6; or of the formula wherein x + y + z ranges from 4 to 6; said polyether polyamine being mixed with piperazine or N-aminoethylpiperazine in approximately molar equivalence with the acrylate ester. A composition according to claim 1 characterised in that the acrylate ester is trimethylolpropane triacrylate. A composition according to claim 1 or claim 2 characterised in that the cycloaliphatic epoxy resin is a saturated bisphenol A resin. A composition according to any of the preceding claims characterised in that the epoxy resin: acrylate ester weight ratio is 10:3 to 10:2. An epoxy resin coating composition comprising the cured reaction product of: A. a cycloaliphatic epoxy resin containing at least 1.8 reactive 1,2-epoxy groups per molecule, mixed with trimethylolpropane triacrylate in a weight ratio of epoxy resin: triacrylate of 10:3 to 10:2; reacted with B. a curing amount of a polyether polyamine of the formula (a) NH₂CH(CH₃)CH[OCH₂CH(CH₃)] x NH₂ wherein x ranges from 2 to 6; or of the formula wherein x + y + z ranges from 4 to 6; said polyether polyamine being mixed with piperazine in approximate molar equivalence with the triacrylate.

Inventors: WADDILL, HAROLD GEORGE
Applicants: TEXACO DEVELOPMENT CORPORATION
Application year: 1988
Main classification: C09D133
IPC codes: C09D163, C09D4, C08G59