High Solids Coating Compositions Containing Liquid Bishydroxy Propyl Ether of a Bisphenol

The present invention relates to essentially solvent free resinous coating compositions which are sufficiently fluid to permit effective application and which cure on baking at relatively low temperature to provide hard, flexible, adherent, and chemically resistant coatings. Coatings which cure to provide excellent stain and detergent resistance are particularly contemplated to provide coatings suited for use as appliance finishes. Container coatings and construction coatings are also contemplated.

The present trend in coating technology is to minimize the proportion of volatile organic solvent in the liquid coating composition which is applied, and one way to do this is to provide the needed fluidity using a high boiling reactive liquid which combines chemically with the more resinous components in order to become a permanent component of the final cured film. To this end, efforts have been made to use dihydric or trihydric alcohols as the reactive liquids for the cure of reactive copolymers, but good stain resistance and good detergent resistance have not been attainable in this fashion. These efforts are illustrated by U.S. Pat. Nos. 3,267,174, 3,352,806 and 3,920,595.

Another difficulty is the hardness and flexibility of the cured coatings. Generally, when dihydric alcohols are used as the reactive liquids, the film hardness falls off and the coatings are softer than desired. With tetrahydric alcohols, the proportion of use is limited because of the tendency toward film brittleness, so proportions must be carefully adjusted as described in my copending application filed of even date herewith.

In this invention, coatings of high solids content are obtained which cure to form hard and exceptionally impact resistant films by combining a reactive copolymer with an aminoplast resin and a liquid bishydroxy propyl ether of a bisphenol. These bishydroxy propyl ethers uniquely combine good hardness with exceptional impact resistance and also provide good stain and detergent resistance.

In the reactive copolymer, monoethylene monomers including from about 5% to 20% by weight of monoethylenic monomer having a functional group reactive with N-methylol functionality and selected from carboxylic acid, hydroxy and amide groups, are copolymerized in an organic solvent solution containing a major proportion of the bishydroxy propyl ether. This bis-hydroxy propyl ether is a liquid formed by reacting propylene oxide with a bisphenol to provide a product which is a dihydric alcohol. The term bisphenol denotes a pair of phenolic groups linked together through an intervening divalent alkylene group, and is preferably illustrated by bisphenol A. This unique type of dihydric alcohol will be illustrated by 1,1'-isopropylidenebis-(p-phenyleneoxy)di-2-propanol, but in commerce polyether formation and condensation increase molecular weight. The product is useful herein so long as liquidity is maintained.

Aminoplast resin is incorporated in the solution, and this provides a liquid coating composition which is essentially solvent free and which is sufficiently fluid to permit easy coating application at room or slightly elevated temperature.

Aminoplast resin is incorporated in the solution, and this provides a liquid coating composition which contains far less volatile organic solvent than is normally needed in a solvent solution coating and which is nonetheless sufficiently fluid to permit ordinary coating application at room or slightly elevated temperature.

Proportions are important to the preferred practice of this invention, and this requires the bishydroxy propyl ether to constitute from about 20% to 40% of the mixture while the reactive copolymers constitute from about 15% to 80% of the weight of the bishydroxy propyl ether. Continuing with the proportions which should be used, the reactive copolymer should constitute from about 4.5% to 35% of the mixture, and the balance of the mixture of the three components will consist essentially of aminoplast resin. It is also preferred to use at least about 0.8 part of aminoplast per part of reactive copolymer, preferably at least 1.5 parts per part of reactive copolymer.

Preferred proportions are 10% to 30% of reactive copolymer combined with 25% to 35% of the bishydroxy propyl ether, and 30% to 70% of the aminoplast resin, there being at least 2 parts of bishydroxy propyl ether per part of reactive copolymer.

In this description, and also in the claims, all parts and proportions are by weight unless otherwise noted.

The reactive copolymer and the bishydroxy propyl ether are both co-reactive with the N-methylol groups of the aminoplast resin, and cures take place at temperatures of about 250.degree. F. and higher. Since the proportion of volatile organic solvent is reduced, there is a corresponding reduction in the amount of volatiles liberated during cure, and this reduces undesirable pollution of the atmosphere, and it also reduces cost and hazard.

The non-reactive monoethylenic monomers which are combined with the reactive monomer in the reactive copolymer are subject to wide variation. Normally, monomers which provide hard homopolymers are relied upon to provide at least about 30% of the copolymer, preferably from 50% to 75%, these being especially illustrated by styrene and vinyl toluene, styrene being particularly preferred to maximize hardness and water resistance and thereby offset some of the softness introduced by the dihydric alcohol which is minimized by the nature of this particular type of dihydric alcohol. Other hardening monomers less desirably present are methyl methacrylate, acrylonitrile, and vinyl acetate. Monomers providing soft homopolymers are also desirably included in amounts of at least about 20% of the copolymer. These are illustrated by ethyl acrylate, n-butyl acrylate, isobutyl acrylate, 2-ethylhexyl acrylate, and similar alkyl esters of monoethylenic carboxylic acids. Isobutyl acrylate is particularly preferred. The specific nature of the monoethylenic monomers constituting the balance of the acid copolymer is of secondary significance to this invention. In preferred practice, the non-reactive monoethylenic monomers contain the single ethylenic group as the sole functional group thereof, and constitute at least about 75% of the copolymer and, more preferably, the entire balance of the copolymer, except for the reactive monomer.

The reactive monomers may be of several types which may be used alone or in admixture with one another. Monoethylenic carboxylic acid monomers are preferred, but other reactive monoethylenic monomers may be used such as amide monomers, for example, acrylamide or methacrylamide, or a hydroxy functional monomer such as hydroxy ethyl acrylate or methacrylate. Hydroxy- or carboxy-functional polyesters may also be included in small amount as taught in my prior U.S. Pat. No. 3,163,615.

The preferred carboxylic monomers are illustrated by acrylic acid, methacrylic acid, crotonic acid, monobutyl fumarate and the like. The reactive monomers will include the reactive groups noted, but no other reactive group.

The aminoplast resins constitute a conventional class of heat hardening condensates of polyamines with formaldehyde, and ethers thereof. Urea-formaldehyde, melamine-formaldehyde, and benzoguanime-formaldehyde resins will illustrate this class of materials. Since fluidity at room temperature is a feature of the invention, liquid methyl ethers are preferred, such as hexamethoxy methyl melamine.

The bishydroxy propyl ethers which are uniquely useful herein are formed by reacting propylene oxide with the bisphenol. The oxide is used in at least a stoichiometric amount to insure that all of the phenolic OH is consumer by the reaction. Polyether formation may be encountered, especially if the oxide is used in excess, and this is permissible. Also, some condensation occurs and the product includes some dimers and trimers. The lowest molecular weight consistent with substantially complete removal of the phenolic OH groups is preferred. The reaction is easily carried out by heating the mixture of bisphenol and propylene oxide under pressure in the presence of a small amount of sodium hydroxide and water. The reaction itself is well known, and the product is available in commerce.

The result (ignoring polyether formation and condensation) is a bishydroxy propyl ether of the starting bisphenol having the formula: ##STR1## in which R is an alkylidene group containing from 1-20 carbon atoms, preferably from 2-6 carbon atoms. Using bisphenol A, R is the propylidene group, and the hydroxy propyl ether groups are in the para,para position.

While minor amounts of other polyhydric alcohols may be present in admixture with the bishydroxy propyl ethers, these are preferably absent because, and with the exception of tetrahydric adducts as in my said copending application, these lead to reduced stain resistance and reduced detergent resistance.

While volatile organic solvents are reduced to a minimum in this invention, small amounts thereof are usually needed to assist in dissolving the monomers during copolymerization, or to reduce viscosity after copolymerization to provide whatever final viscosity is desired at the time of application. Usually some volatile solvent (at least 5%) is needed, either to provide desired coating viscosity, or to assist in carrying out the solution copolymerization. This small amount of solvent evaporates during the cure and provides some limited pollution, but the volatile content is much smaller than is characteristic of solution coating compositions. The product properties obtained in this invention justify the presence of up to 30% of volatile organic solvent in the final coating solution, though in the preferred practice of this invention, the volatile organic solvent content would be less than about 20%.

The copolymerization is carried out by heating the solution of monomers in bishydroxy propyl ether and optional volatile organic solvent in the presence of a free radical generating catalyst, such as a peroxide or equivalent catalyst, illustrated by azobisisobutyronitrile. Suitable temperatures for copolymerization may vary from 60.degree. to 200.degree. C., preferably from 80.degree. to 150.degree. C. The bishydroxy propyl ether should constitute a major portion of the solvent (more than 50%), preferably more than 75%.

Typical baking conditions in this invention are quite low, 275.degree. F. for 20 minutes being typical. Lower temperatures require more time, and higher temperatures of about 500.degree. F. allow the cure to be completed in a half minute to one minute.

The invention is illustrated in the following example.

630 Dihydric alcohol reaction product of propylene oxide and bisphenol A having an average molecular weight of 565.

Charge to reactor, set reflux condenser, and heat to 120.degree. C. with agitation.

Premix monomers and catalyst and add to reactor over 3 hours at 125.degree. C. and then hold for one hour.

3 Tertiary butyl perbenzoate

Add catalyst, and hold for two hours. Then cool to 70.degree. C.

Aminoplast resin (hexamethoxymethyl melamine)

Add aminoplast resin slowly at 70.degree. C. and then cool to 30.degree. C. to provide a mixture having the following final characteristics: