Polyamide Resins From Soybeans

John C. Cowan.

Norelac is one of several polyamide resins that can be derived from soybean oil. Its name comes from Northern Regional Lacquer, and it belongs to the same chemical family as nylon. Norelac was first prepared by chemists at the Northern Regional Research Laboratory in 1942. Two industrial' concerns produced it during the Second World War. One large soybean-processing company has been manufacturing it since 1945. Its continued use and production appear to be assured.

The greatest usefulness of Norelac seems to be in the general field of packaging. Its compatibility with resins, waxes, and plasticizers facilitates the formulation of many useful combinations. Its use for protecting K-ration boxes and for filling pores in magnesium castings is primarily based on its cohesiveness and impermeability to moisture, but another property makes it good for packaging. When two coated strips are placed face-to-face and heated under slight pressure, an excellent moisture-vapor-resistant bond is formed. If the resin-coated back of a label is heated and pressed against a cap, bread wrapper, bottle, or paper box, the label adheres firmly to the container. This heat-sealing method of fabrication is being used more and more in packaging. Norelac is now used in some packages sold in the corner grocery. Another application of Norelac in packaging is its use in lamination, where paper, cellophane, and other packaging materials are combined to make attractive and serviceable packages.

Norelac is a hard, transparent, thermoplastic (heat-softening) resin which is useful in lacquers and adhesives. It can be prepared with melting points ranging from 98 to 116 C. (208 to 240 F.). By using some organic di-acids, such as sebacic, the melting-point range can be raised to 188 to 196 C. ( 370 to 384 F.). Its color varies from light yellow to dark brown. It is compatible with a large number of synthetic and natural resins and with plasticizers and waxes.

As a polyamide, Norelac has unusual characteristics of solubility. It dissolves in the alcohols commonly used by the protective-coating industry, such as isopropyl and butyl alcohols, in amines, in fatty acids, and in certain chlorinated hydrocarbons. It is insoluble in many other solvents commonly used by the trade, such as esters, ethers, glycols, ketones, hydrocarbons, and nitrohydrocarbons. While Norelac is insoluble in most petroleum solvents, its alcoholic solutions will tolerate large volumes of these solvents. Consequently, a wide range of inexpensive solvent combination is available.

Usually films are dry "set to touch" within 2 to 5 minutes. Norelac films possess excellent resistance to water, alkalies, and acids. For example, cold water, 4 percent vinegar, and 20 percent sodium hydroxide did not affect the film on 8 days' standing. Sulfuric acid (75 percent) discolors the film but otherwise leaves it unaffected.

Norelac films adhere well to many surfaces, and they have exhibited good outdoor durability on both wood and metal.

A spirit lacquer prepared with zinc chromate (24.8 percent), micaceous lithopone (15.8 percent), asbestine (4.5 percent), and Norelac (55 percent by weight), was coated on cold-roll steel panels. After more than 3 years of exposure at 45 south, the films were still in good condition. Another formulation, in which we used aluminum bronze as the pigment, was exposed for more than 6 years, and the films were still in excellent condition. In another test, Norelac coatings that contained red iron oxide were in good condition after 4 years of exposure.

Furthermore, it was found that the addition of a small percentage of paraffin wax to Norelac imparts moisture impermeability to the resulting films. A thin film cast on paper from a Norelac solution containing 2 percent paraffin had an exceedingly low rate of water vapor transmission the value obtained compared very favorably with that of many organic coating materials.

Norelac should find many applications as a protective coating for wood and metal surfaces. It can be used alone or in combination with other resins as a spirit varnish or lacquer. Norelac in solution can be readily pigmented with a ball or pebble mill, or in the "dry" state in a roller mill. Pigmented solutions of Norelac make excellent rapid-drying enamels of outstanding durability. Norelac solutions make superior vehicles for aluminum and bronzing powders.

Because Norelac is thermoplastic and possesses excellent adhesion, it can also be used as a laminating and heat-sealing agent for paper, glassine, cellophane, vinyl films, and metallic foils. A wide variety of laminates, such as glassine-to-glassine, lead foil-to-sulfate paper, and cellophane-to-cellophane, can be prepared with Norelac. The laminates using Norelac as the bonding agent compare favorably with commercial materials.

ONE OF THE most interesting war uses for the resins was in hot-dip stripping compounds. Army and Navy equipment and spare parts were dipped in or sprayed with a hot mixture containing Norelac, which solidified to a continuous film that covered the article completely. The film provided protection during shipping, and it was readily stripped off when the article was needed. The polyamide resins had some advantages over other materials used for this purpose. Liquid film could be applied at lower temperatures, thus minimizing the danger of burns. The alkaline polyamide resins absorbed the acids on the metallic surfaces resulting from perspiration from the hands of factory workers. Indeed, it quickly removed fingerprints from accurately machined parts, and thus prevented rusting under the coating during shipping.

THE CHEMISTRY of these polyamide resins is quite simple. The linoleic and linolenic acids (I) present in soybean oil combine with themselves two and three times to give a mixture of dimeric (II) and trimeric (III) acids, which we call polymeric fatty acids.

The acids will combine readily with a large number of chemicals. Experiments at the Northern Laboratory in 1941 and 1942 showed that the polymeric fatty acids can be reacted with ethylene glycol (nonvolatile antifreeze) to give polyesters. The polyesters can be converted into rubberlike materials by mixing rubber fillers and chemicals. In 1942 and 1943, this rubberlike material was manufactured on a commercial scale, but a conservation order curtailed further production. The product was called Norepol (Northern Regional Polymer).

When the ethylene glycol is replaced by a diamine, such as ethylene diamine (IV) or hexamethylene diamine, the polymeric fatty acids react with the diamine to give a polyamide (V).

In the actual preparation, acids are heated to approximately 150 C., and the aqueous ethylene diamine solution is gradually added until all of it has undergone the initial salt formation. Heating is continued as water from the solution and from the salt transformation to amide is distilled. The temperature of the reaction is gradually raised over a period of 90 minutes to 200 C., and continued for at least 1 hour to produce the resin. Chemically, the resin is known as the ethylene diamine polyamide of polymeric fatty acids; for convenience, we have called it Norelac. Other diamines and dibasic acids may be used to give resins of slightly different properties. The polyamides may be modified by the addition of monofunctional derivatives, such as stearic acid or n-monostearylethylene diamine.

The development of a method for suspending polyamide resins in water was announced by industrial researchers in 1950. This development should make it possible to reduce the costs of fabricating paper and other products in which the resins are used. Also, it should extend the utilization of the resins in new products, thus assuring greater industrial use of soybean oil.

The future use of polyamide resins depends somewhat on the price relation of vegetable oils and other resins. Vegetable oils dropped in price in 1949-50; as the supply becomes more abundant, the price of the polyamide resins from these oils should drop and the utilization of the resins increase. A new source of the polymeric fatty acids was announced in 1950, and commercial exploitation of the method should reduce the cost of polyamide resins. The process, developed by a company specializing in fatty acids, can employ any fatty acid stock containing linoleic acid. The linoleic acid is removed from the mixture by polymerization at high temperatures in the presence of water. The polymeric fatty acid will be a byproduct in this process, and it will probably be made available at a reduced cost as compared with other possible sources.

JOHN C. COWAN received a doctor's degree in chemistry from the University of Illinois. He is principal chemist at the Northern Regional Research Laboratory, and is in charge of the oil and protein division. Dr. Cowan has been with the Department since 1940.

WHEN YOU FINISH PAINTING or working on a greasy machinery repair job and want to get cleaned up for dinner, try this:

Mix 5 volumes of turpentine and 1 volume of liquid nonionic emulsifier by shaking them together in a stoppered bottle. This solution will keep. (Don't be frightened by that emulsifier. There are a number of kinds, but one is readily available at the grocery store, where it is known as a liquid dish-washing soap-less soap.) The rest is easy. Pour a little of the solution into your palm and rub it well into the hands to loosen the paint or grease. Wash it off in running water. The paint or grease, turpentine, and emulsifier all wash off easily. You can use the same solution to clean a paint brush right after you finish painting. This time, make the first rinse in a cup or two of water and then follow with running water. Be sure to do it before the paint dries as it will not work on old, dried paint. E. E. Fleck, Bureau of Entomology and Plant Quarantine.