A Product with a History, Used for Modern Applications

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Zein has been examined as a possible raw material for polymer application since the early part of the 20th Century. John Gorham first described in it 1821 after isolating the protein from maize. He named the material “Zeine”. The description that Gorham gave to Zein would easily be recognized today. It resembled bees’ wax and to be soft, ductile, tenacious, and elastic. It had some of the properties of wheat gluten and was analogous to resin. Other early work on Zein placed it in its present classification as a prolamin.

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Research into the possible uses of Zein was well underway early in the 20th Century. Zein’s soft, ductile nature after being precipitated from solvent no doubt lead to it being tried as a plastic, either alone or in mixtures. One of the first to promote Zein as a plastic was Goldsmith (1909). He proposed that Zein could be mixed with compounds that he described as converting agents and then heated. His converting agents were, in all likelihood, plasticizers. Once heated, the Zein softened and other materials such as other resins or fillers could be added. The soft mixture could then be shaped into finished articles. Desvaux and Allaire (1909) showed that Zein could replace part of the nitrocellulose or camphor used in the production of celluloid. The new plastic was less flammable and expensive while retaining the properties of celluloid. Zein was also mentioned for uses in fabric stiffeners (Swett 1920), artificial leather (Satow 1917), lacquer (Satow 1917), filament, and film (Ostenberg 1919) during this same period. These early patents did not establish much use for Zein, but they did set the stage for the research to come in the next two decades by showing what could be produced from Zein.

These patents also demonstrated that chemical and physical methods affected Zein’s properties. The 1930s and 40s saw an explosion in the amount of research done on Zein. This was probably spurred on by the chemurgy movement of the time.

 

A small pilot plant was started in 1938 by Corn Products Refining Company. At the time, the interest in Zein was for use in plastics and coatings. However, after the start of World War II, shellac became in short supply, and Zein was in demand as a replacement in lacquers and coatings. To keep up with Zein demand, Corn Products Refining Company built a much larger plant located in Pekin, IL, in 1943 that had several times the capacity of the original plant. By 1944, more than half of the new plant’s output was going to the paint, varnish, and lacquer industries, principally as a replacement for shellac.

 

Grease and solvent resistance is one of Zein’s most important characteristics. Because of this, Zein found use as protective coatings in packaging resin. Coatings were put onto carton stock for doughnuts, crackers, pies, and cookies. The coatings were a mixture of Zein and rosin along with fatty acid plasticizers. Zein was also used as a protective coating on paperboard salt containers to prevent the penetration of wax that was added for moisture resistance. Zein coatings of fiberboard containers were of interest because of their good grease resistance, printability, and dispersing effect on waxes. Paper coated with Zein was used during World War II to wrap airplane and machine parts for shipment. These coated papers were grease resistant and prevented the escape of oil and grease into packaging. To protect labels on bottles and cans, a combination of Zein and other resins was used in overprint varnishes. Because alcohol was used as the solvent for the overprint varnish, drying took as little as 15 seconds.

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Zein was once used in sizable quantities in the paper industry. The main uses of Zein were for pigment coatings and protective coatings. In pigment coatings, Zein is used as an adhesive to bond the pigment to the base paper. Pigment coatings are usually used in the manufacture of papers for magazine publications. Zein was used in glossy protective coatings for catalogues, children’s books, and “pocket” books.

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In the mid-20th Century, most pharmaceutical tablets were coated with sugar. Sugar coating the tablets was laborious and time-consuming. Because of Zein’s film-forming ability and microbial resistance, it was a natural choice for tablet coating. Coating tablets with Zein was rapid, taking less than 30 minutes. Zein-coated tablets adequately resisted heat, abrasion, and humidity. The coatings also concealed the taste and odors of the original tablets without interfering with solubility.

 

One of the first reports of using Zein in food involved its use as a protective coating in fortified rice. Milled white rice was wetted with thiamin, niacin, and iron in an acidic solution and dried. Then an alcoholic solution of Zein and stearic acid was applied. Zein was also found to have the ability to protect nuts against rancidity. Pecan pieces coated with as little as 1% Zein (w/w) could be stored for nine months without becoming soggy, stale, or rancid. Similar results were obtained with walnuts, almonds, peanuts, and with nuts incorporated into chocolate bars. The Zein coatings were equal or superior to gloss and moisture vapor resistance of shellac used in confectioners glaze for hard gums, sugar-coated confections, and hard candies, and had the added advantage of being edible and nutritious. Coating raw nuts with Zein before roasting reduced the tendency of the skins to flake off.

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For the next 20 years, a number of uses were found for Zein in all fields, including the manufacture of buttons, fiber, adhesives, coatings, and binders, etc. Commercial production of Zein peaked in the late 1950s, approaching 15 million pounds a year in the United States. However, with the introduction of synthetics and other petroleum-based ingredients, Zein became too costly. By 1978 the production of Zein had dropped to one million pounds a year.

 

Renewed interest in Zein as a polymeric material has been stimulated, in part, by the perceived negative impact of plastic on solid waste disposal. Zein offers several potential advantages as a raw material for film, coatings, and plastics applications. It is biodegradable and it is annually renewable. In addition, the cost of using Zein is once again competitive with food shellac and other popular film coatings.