Varnishes and Paints From Soybeans

A. J. Lewis.

The use of soybean oil in paints and varnishes is largely an American development and a new one. For more than a century the soybean oil we imported was used for food. Even as late as 1909, soybean oil was practically unknown as a paint oil. At that time, however, some chemists advocated the development of varieties of soybeans that would produce superior drying oils, which, they believed, would stabilize the price of linseed oil.

When soybeans started their climb to become the leading oil crop in this country, they found favor first as a hay crop, then as a source of edible oils, and finally as a source of drying oils. The climb was swift. The paint and varnish industry used 8.5 million pounds of soybean oil in 1933 and 150 million pounds in 1949. Other drying-oil industries used another 100 million pounds to make floor coverings, printing inks, and many other items. The use of soybean oil as a drying oil has thus kept pace with the phenomenal rise in production of soybeans.

Even though the 150 million pounds of soybean oil used in paints and varnishes represents only 11 percent of the total oil produced in 1949, it is 4 percent more than the amount used in 1948 and 57 percent more than the total used from 1943 through 1946. The figures indicate that soybean oil now has attained a definite place in the paint and varnish industry, and that its use no longer depends entirely on fluctuations in the price and supply of linseed oil.

SOYBEANS GENERALLY contain only about 20 percent of oil; the linseed oil in flax amounts to 38 percent.

Almost all the oil is processed by one of three methods solvent extraction, continuous pressing, or hydraulic pressing. The first method is preferred because it produces an oil that is lighter in color and almost free from foreign material and a meal that is practically free from oil and well suited, therefore, for use in water paints, plastics, and glues.

Soybean oil obtained by any of these methods is considered as crude oil, which must subsequently be refined by one of three methods. Mechanical refining consists of emulsifying the oil with hot water or steam and then centrifuging out the foreign material. Acid refining consists of treating the oil with strong sulfuric acid, which chars the foreign material but not the oil, if handled properly. Alkali refining consists of emulsifying the oil at room temperature with a solution containing a slight excess of alkali over that required for neutralizing the free fatty acids of the oil.

The oil obtained by mechanical or acid refining differs from that obtained by alkali refining in that the free fatty acids retained in the oil serve as pigment-wetting agents and make the paint easier to grind. The oil obtained from alkali refining, because of its light color, is preferred for making oil-modified alkyd varnishes, especially those intended for white enamels. However, the oils obtained from any of the refining methods are suitable for protective coatings if they conform to the requirements of Federal Specification JJJ-O-348 for refined soybean oil.

This specification sets standards for specific gravity, iodine number, saponification number, loss on heating at 105 C., unsaponifiable matter percentage of foreign material, and acid number.

Besides those requirements, the oil must be clear and free from sediment and suspended matter when examined by transmitted light at 65 C. (149 F.). Its color must not be darker than that of a solution of 0.38 gram of reagent potassium dichromate in 100 milliliters of sulfuric acid of specific gravity of 1.84, equivalent to the No. 12 tube of the Gardner color scale (1933).

A BODIED OIL is one that has been heated at high temperatures to "body," or thicken, it to a sirup-like consistency by the formation of polymers, which result when molecules combine with one another. The soybean oil used for kettle bodying must be free from foreign, or break, material and should have a high iodine number. The iodine number denotes the amount of iodine that is absorbed by the oil molecules and is the measure of the degree of unsaturation, or capacity of the oil to oxidize and to polymerize. Soybean oil that has an iodine number of 130 takes twice as long to body to a certain viscosity as linseed oil with an iodine number of 175 when heated at the same temperature. The time required for bodying soybean oil can be reduced by heating the oil to as high a temperature as possible without creating a fire hazard or by using high vacuum. Also, a number of chemicals, such as B- methylanthraquinone, phenanthrene, and diphenylcarboxyanthracene, have been used successfully to accelerate the bodying of oils without injuring their quality.

Bodied soybean oils have been used to replace all or part of the oil vehicle of interior and exterior paints with some success in drying and in durability. Bodied soybean oil that has a viscosity of approximately 5 poises (similar to a very heavy lubricating oil) has been mixed with tung oil in proportions of 70 parts to 30 parts by weight and heated to 550 F. to make a processed oil with better drying qualities than linseed oil. This processed oil, known as a copolymerized oil, can be cooked with ester gum and other inexpensive resins to make high-grade varnishes. The polymers of high-viscosity bodied soybean oil are insoluble in acetone and can be readily separated from the unpolymerized portion for use in making good soybean oil-ester gum varnishes.

SOYBEAN OIL GAINED POPULARITY in the varnish industry in the Second World War when supplies of tung oil were short. Tung oil had been popular since early in the First World War. Before then, most varnishes were made from linseed oil and natural resins. The coatings from these varnishes dried too slowly to meet the demand for fast production of armaments and war equipment. Soon a new type of varnish, Val-spar, appeared. It was made from ester gum (a resin obtained by neutralizing rosin acids with glycerol) , tung oil, and mineral spirits. It, and others like it, dried rapidly, were waterproof, and made excellent grinding materials for paints and hard-drying enamels.

Oil-modified alkyd varnishes, generally called alkyds, are made commercially in closed vacuum kettles. The process usually consists of heating and reacting a dibasic acid, such as phthalic anhydride, and a polyhydroxy alcohol, such as glycerol, with the fatty acids of vegetable, animal, or marine oils. The oils serve as plasticizers and are required because the resin produced by the reaction of the acid and alcohol is too brittle for use in surface coatings without modification. A unique characteristic of alkyds is that the plasticizer becomes a part of the resin by chemical combination rather than by physical admixture. The first alkyds, known as glyptals, utilized only the fatty acids of linseed oil, but in the early 1930's small amounts of soybean fatty acids began to be used in blends with linseed fatty acids. The production of alkyd varnishes increased rapidly because they could be produced economically and were outstanding for adhesion, toughness, durability, flexibility, and hardness. Also, they could be produced in large volumes in single closed kettles, required little supervision, and utilized the oils then available.

The use of soybean fatty acids has been favored for alkyds because of their availability and low linolenic acid content. The low acid content enables the manufacturer to produce white and light-tinted enamel coatings that do not yellow appreciably when applied to refrigerators, automobiles, and the like. Although the slow-drying properties of soybean acids limited their use in the alkyd field for a long time, improved methods for the forced drying of coatings by heat ( especially infrared lamps) have greatly helped to overcome this limitation. An increasing number of manufacturers now produce alkyds containing 100 percent soybean acids. It is likely that half or more of the soybean oil used in protective coatings is being used in making alkyd varnishes.

Soybean oil-ester gum varnishes of 15- and 20-gallon oil lengths ( gallons of oil to 100 pounds of resin), known as short-oil varnishes in the trade, have been made by cooking ester gum and refined soybean oil for 3 1/2 hours at 600 F. But the coatings from these varnishes soften, or "aftertack," badly in hot, humid weather. Nevertheless, the same varnishes, when partly pigmented with small amounts of calcium oxide, produce coatings that dry fast, hard, and flat, and are durable for interior use.

The best soybean oil-ester gum varnishes have been made from either the copolymer of tung and soybean oils, or the polymers extracted from bodied soybean oils, both of which I have described. Other soybean oil-ester gum varnish coatings, which have good drying qualities and resistance to hot and cold water, acids, and alkalies, have been made from some of the special soybean oils, which are described later.

The hardest and most durable varnishes have been those made from an oil-reactive, unmodified phenolic resin and soybean oil. The varnishes were made by heating 20 gallons of refined soybean oil and 100 pounds of phenolic resin (Bakelite resin No. 254) together in a stainless-steel open kettle at 600 F. until bodied sufficiently to give a 5-inch string when a few drops were tested on a cold plate.

The cook was then removed from the heat, allowed to cool to 200 F., and thinned with 24 gallons of mineral spirits followed by 5 gallons of toluene. Cobalt driers of the naphthenate type containing 6 percent cobalt metal were added at room temperature and three-eighths of a gallon of drier gave satisfactory drying qualities to the coatings. The time of bodying at 600 F. to a 5-inch string was approximately an hour; the speed of bodying depended on the use of an oil-reactive resin.

Phenolic varnishes made by this formula and procedure dried rapidly overnight to hard, glossy coatings, which were durable and marproof when tested on floors, launches, bows and arrows, and such. The coatings were highly resistant to hot and cold water, acids, alkalies, gasoline, and alcohol.

Tested comparatively by outdoor weathering, they proved to be more durable than two high-grade commercial varnishes that contain tung oil. Similar varnishes were made with longer oil lengths, but their coatings did not dry so hard and were less resistant than the coatings of the 20-gallon varnish. However, the material costs for varnishes of long oil length are less, and they are easier to apply by brushing. Norelac is another type of varnish that dries by solvent evaporation instead of by oxidation and polymerization.

LITTLE SOYBEAN OIL was used in paints until 1934, when some farmers' cooperative organizations began to distribute exterior paints that contained small percentages of soybean oil and were made by paint manufacturers in accordance with the formulas furnished by the cooperatives. Many of the paints were durable in service, but some became discolored because of dirt collection, a feature that prejudiced many users against soybean-oil paints.