4. ALKYD RESINS
The alkyd resins, used principally in paints, varnishes, and lacquers, are a group of condensation products synthesized by reacting polyhydric alcohols, such as glycerin and the glycols, with dibasic organic acids, such as phthalic, maleic, succinic, and sebacic. The condensation product is almost always modified to give properties to the resin desirable or essential to the specific application contemplated. The modifying agent may be a drying, semidrying, or nondrying oil; the fatty acid of an oil; a natural resin, such as rosin; a synthetic resin of the tar-acid group or of the urea-formaldehyde type; or other substance. Up to the present time unmodified alkyd resins have not been commercially important.
A wide variety of types is obtained by the use of different materials and different modifiers. The variations begin with the dibasic acid used, and with the polyhydric alcohol used. The modifications possible are practically endless, and almost any fixed oil or the correspondingfatty acid, and most of the natural or synthetic resins may be used. The importance of the modifier is shown by the proportion used in most alkyd resins. On the average, approximately 50 percent of the total weight of the drying and semidrying alkyd resin products is modifier, 30 percent dibasic acid, and 20 percent polyhydric alcohol. The proportions will, of course, vary with individual types. Certain types on the market contain only 25 percent modifier while others have as much as 75 percent.
In a new industry such as this, rapid changes in types and applications must be expected. Extensive research is being carried on by various groups. The raw material makers are seeking cheaper products or those with special properties; the resin makers are investigating an endless number of modifications, and the makers of surface coatings are testing most of the new types offered.
Probably the earliest record of research leading to the development of the alkyds was that of van Bemmelen, who reported in a German technical journal in 1856 the sirupy products obtained by heating together succinic acid and glycerin or citric acid and glycerin. The first investigation of the phthalic anhydride-glycerin resins was recorded in 1901.[3]Watson Smith, while engaged in research on phthalein dyes, obtained a transparent, highly refractive resinlike substance when glycerin and phthalic anhydride were heated together. Smith recommended the product as a cement for ceramic wares.
During the period 1910-16 the research laboratories of the General Electric Co., engaged in research on a synthetic resin from glycerin and phthalic anhydride. As a result of these studies numerous patents were granted for this type of resin to which the trade name Glyptal was applied. Intensive research was carried on by several firms, many variations were developed, and literally hundreds of patents were granted.
The paint and varnish industry has been undergoing radical readjustment. Methods and natural products, which for decades or centuries had changed very little, are giving way to synthetic creations of our laboratories. The first important departure from the traditional practices was the development of nitrocellulose lacquers. The commercial application of the alkyd resins followed, and their use is increasing rapidly. Because this development is still comparatively young, the large number of modifications offered has confused the coating manufacturer. It is probable that many of the synthetic products now being marketed have no special technical or economic justification and that they will in time lose out in competition with better products known at present, or still to be developed.
United States Patent No. 1,893,873, dated January 10, 1933, granted to R. H. Kienle and assigned to the General Electric Co., was considered one of the basic patents in this field. Early in 1936 it was declared invalid in a suit claiming infringement brought against the Paramet Chemical Co. of Brooklyn, N. Y. The decision in this case seems to have opened the glycerin-phthalic anhydride resins to a large number of manufacturers.
Among the principal brands of alkyd resins now on the domestic market are Beckosol, Dulux, Esterol, Glyptal, Rezyl, and Teglac. Each of these trade names identifies a series of products.
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 bakedon 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.
Since the alkyd resins are largely used in surface coatings and finishes and since this application in this field is producing great changes in the industry, it is appropriate to consider the effect of their use on other materials.
The average alkyd resin consists of 50 percent glycerol phthalate modified with 50 percent oil, fatty acid, natural resin, or synthetic resin. The alkyd and modifier are dissolved in a solvent, usually a coal-tar light oil such as toluol, or xylol, or a petroleum solvent, and pigmented with titanium dioxide or other pigment. Highly basic pigments such as zinc oxide, carbonate white lead, whiting and aluminum hydrate (all important pigments in the conventional types of finishes) are not used in alkyd finishes.
Prior to 1929, the domestic production of resins from phthalic anhydride was confined largely to one maker. The quantities produced were relatively small. In 1929 there were three producers, the volume of whose production exceeded one million pounds for the first time. Beginning with 1933 the Tariff Commission collected and compiled production and sales statistics for these resins. They are shown in table7.
Table 7.—Alkyd resins from phthalic and maleic anhydride: United States production and sales, 1933-37
1Includes resins from maleic anhydride.Source: Dyes and Other Synthetic Organic Chemicals in the United States, U. S. Tariff Commission.
1Includes resins from maleic anhydride.
Source: Dyes and Other Synthetic Organic Chemicals in the United States, U. S. Tariff Commission.
In 1933 there were 6 makers of resins from phthalic anhydride, in 1935 there were 15, and in 1937 there were 35. The 1937 output of alkyd resins from phthalic anhydride was 58,450,032 pounds net resin, with sales of 32,583,307 pounds valued at $6,446,011. Producing plants are well scattered through northern and eastern United States. In 1936 fewer than one-third of the makers accounted for about 90 percent of the output.
The domestic production of resins from maleic anhydride was reported for the first time in 1933. The output in that year consisted of experimental quantities produced by two firms. A small increase in production occurred in 1934 when another maker began operation. In 1936 there were eight producers and the output was many times that of 1934. In 1937 there were 12 makers of these resins with an output of 2,803,987 pounds and sales of 2,154,988 pounds, valued at $418,183. It is the opinion of some persons in the industry that in volume of production and sales the resins from maleic anhydridewill in the near future approach that obtained from phthalic anhydride.
No imports of alkyd resins have been recorded in official statistics.
Exports of alkyd resin coatings and finishes are not separately shown, but data collected from the several producers show that appreciable quantities were exported in recent years, principally to Central and South American countries.