Classification of alkyd resins.

A number of classifications of the alkyd resins are possible and practical. Since by far the most important applications are in surface coatings, and their use in molding compositions is relatively unimportant, it seems advisable at this time to emphasize the more important use. For the purpose of this survey the following classification is used:

At least 75 percent of the alkyd resin finishes used at present are of the drying type and about 15 percent of the nondrying type.

Unmodified drying alkyd resins.—This class of alkyd resins consists of a series of compounds made from polyhydric alcohols, polybasic acids, and fatty acids in chemical combination. The alcohol is usually glycerin, and the polybasic acid largely phthalic anhydride or acid, although others, such as maleic anhydride (acid) are increasing rapidly in importance. The fatty acid or oil used may be linseed, tung, perilla, hempseed, soybean, sunflower, safflower, or other drying oil. It is believed that tung oil and perilla oil are the most important at this time.

Unmodified drying alkyd resins are characterized by excellent durability but limited resistance to water in air-dried finishes. Both in air-dried and in baked finishes they are outstanding as to flexibility, quick drying, long luster life, and permanent adhesion. Their principal uses are in finishes for interior walls and woodwork, automobiles, coatings on steel such as for refrigerators, railway equipment, bridges, advertising signs, and lithographed containers. In these applications the products of this type compete with nitrocellulose lacquers and the older types of varnishes and paints. While the initial cost is higher, greater durability is obtained together with faster drying, flexibility, and hardness.

Probably the largest field for surface coatings is outdoor wood finishes. Several attempts have been made to adapt pure alkyd finishes to this use but with limited success because the hard and non-porous finish does not permit the escape of moisture contained in the wood and the pressure developed from vaporization of the moisture by the sun’s rays tends to lift the coating from the wood surface. Recently it has been found practicable to incorporate from 15 to 20 percent alkyd resins in conventional types of outdoor paints for wood. Here the use of alkyds has contributed greater durability and retention of fresh appearance over a longer period. Paints of this type are now on the retail market.

Drying alkyd resins modified with natural materials.—This type of alkyd resin is modified principally with natural resins, such as rosin, damar, mastic, shellac, or copal. The use of these natural resins imparts hardness to the resin but shortens its durability. They make the product less expensive, permit easier incorporation of the drying oil, and in some instances increase the water resistance.

Their principal application is to modify nitrocellulose lacquers and lacquer sealers, in order to impart gloss, hardness, and easy sanding. It has been said that the commercial production of drying alkyds modified with natural resins was as important a development in the surface coating industry as the discovery of the alkyds themselves.

Drying alkyd resins modified with other synthetic resins.—Drying alkyd resins may be modified with tar-acid formaldehyde resins, tar-acid furfural resins, urea-formaldehyde resins, petroleum resins, and the coumarone and indene resins.

Modification with tar-acid resins gives a quicker setting, harder drying finish with a higher gloss. Alkyd resins so modified are adapted to both air-drying and baked undercoats and finishes; they have good durability and adhesion and good resistance to grease, oils, alcohol and abrasion. For some uses the tar-acid resin modification gives better qualities than either component possesses alone, but in light colored finishes it has a tendency to cause the finish to yellow. Coatings made of drying alkyd resins modified with tar acid resins are widely used on automobile chassis, fenders, and bodies, machinery coatings, steel fixtures and toys; they are especially suitable for primers, undercoats, and finishes on metal.

Modification with urea resins produces baked-finish coatings. As much as 40 percent of the urea resin is incorporated. It makes possible coatings with a full range of permanent colors and improves their hardness and mar-proofness, whereas without the ureas the combination of color range with hardness had been difficult to obtain. The urea resin modified alkyds find use on metal surfaces of articles which must stand rough handling, such as toys, furniture, and motors.

Modification with petroleum resins produces air-dried finishes. For industrial use on metal they give coatings with better adhesion, dispersion of pigments, and resistance to acids, alkalies, and moisture at a lower cost than is obtained by ester gum or tar-acid resin modification. The petroleum resin modification minimizes skinning and improves the luster and the flow.

Drying alkyd resins modified with other synthetic resins and oil extended.—Excellent water resistance and versatility are the characteristics of finishes made of alkyd resins modified with other synthetic resins (usually tar-acid) and oil extended. The incorporation of drying oils gives a low cost finish with better compatibility and brushing and with the combined properties of a quick-setting varnish and an alkyd resin. Although not so durable or quick setting as the unmodified finishes, they have better water resistance. These finishes may be brushed or sprayed, air-dried or baked. They have wide industrial and architectural uses.

Semidrying alkyd resins.—Cottonseed oil is the principal modifier in semidrying alkyd resins. Alkyd resins of this type are used in finishes requiring maximum gloss and color retention. When baked on metal at high temperatures they show no tendency to wrinkle. They are used as reinforcing agents to increase flexibility and durability, and to plasticize other finishes.

Nondrying alkyd resins.—The nondrying or nonoxidizing alkyd resins are those containing a nondrying oil, such as castor oil or coconut oil, or the fatty acid of a nondrying oil, such as stearic, palmitic, or oleic acid. Nondrying oils make the resin less sensitive to heat hardening and impart greater flexibility. These resins are used principally as plasticizers in nitrocellulose lacquers. In this use they have the advantage of better retention of plasticizing efficiency than other plasticizers, many of which are lost by evaporation, migration, absorption, or oxidation. These modified nitrocellulose lacquers, either clear or pigmented, are used for coating wood, composition board, cloth, paper, rubber, leather, and similar surfaces.

Miscellaneous modified alkyd resins.—This group includes alkyd resins modified with materials other than those already discussed. To date (1938) there has been little, if any, commercial production of such resins. There are many modifiers which have been suggested and which might be used but for the fact that they are too expensive. Among these are butyl alcohol and benzoic acid.

Alkyd resins in water dispersion.—Emulsions of alkyd resins in water are now available for use in clear and pigmented coatings. These are sold in the form of paste containing 40 to 50 percent solids and are diluted with water at the time of application. They are especially suitable for coating porous surfaces, such as brick, concrete, plaster, stucco, and masonry of all kinds. They are applied by brushing or spraying and they combine the ease of application of water paints with the durability, washability, and hardness of oil paints. They dry quickly, and the dried film cannot again be dissolved or suspended in water; the coating can therefore be washed or, after several weeks, scrubbed with cleansers. Compared with oil paints, they give better coverage, are easier to apply, and cost appreciably less. Compared with other types of water paints, such as kalsomine, they give a glossier coating of greater durability and superior appearance; they seal porous surfaces better; their covering capacity is greater; and their applied cost is slightly less per square yard of surface.

Coatings of this type may be applied directly over fresh plaster without a sizing coat, since they allow the curing of the plaster to continue. The usual paint pigments may be incorporated.

A special use of the water dispersed alkyds is on asphalt or tar since they are nonbleeding in the solvents of these materials. This quality permits their use for traffic and zone markers on streets.

Alkyd resins in molding compositions and other uses.—The alkyd resins are much less important as binders in molded articles than in coatings and finishes. Conversion of the resin to the insoluble infusible form is extremely slow, requiring days as compared with minutes for the tar-acid and urea resins.

The alkyds are used as binders for flake, powder, and split mica to produce insulation material of high electrical strength. Other uses are in the production of linoleums; gaskets; brake linings; laminated fabric, paper, and cardboard sheets; printing inks; and coated paper, textiles, and leathers.