5. UREA RESINS
One of the most important series of thermosetting resins is the group made by condensing urea and formaldehyde. As early as 1897 it was discovered that an amorphous condensation product was obtained from the reaction of urea and formaldehyde. The clear glass-like mass obtained led to considerable research work toward the development of a substitute for glass. It was found, however, that the resin obtained absorbed moisture, resulting in a dimming of its luster, and that on standing for a time, the condensation continued producing cracks, fissures, and disfigurements in the molded article. In 1926 a successful commercial product was developed in England by the use of thiourea. Cost of production, however, was high. The addition of thiourea gave the product greater strength and water resistance than that obtained with urea alone but retarded the rate of cure. Also the sulphur present attacked steel molds, which necessitated the use of expensive chromium plated or stainless steel molds.
About 1929 the first successful straight urea product was perfected in the United States. It was found that a filler, such as highly refined alpha cellulose, minimized the stresses. The filler (as much as 30 to 40 percent is usually incorporated), destroys the transparency but permits the manufacture of translucent articles in a wide range of color. Many of the colors possible with the urea resins, particularly the light shades, cannot at present be obtained in molded tar-acid resins.
An interesting fact concerning these resins is that they are produced indirectly from four gases: Ammonia, carbon dioxide, hydrogen, and carbon monoxide. Ammonia and carbon dioxide react to form urea, and hydrogen and carbon monoxide yield methyl alcohol which is converted to formaldehyde.
The urea resins are outstanding largely because of their brilliancy and depth of color, properties not readily obtained in other thermosetting resins. Being odorless and tasteless and completely resistant to oils and greases, they are adapted to use in the manufacture of cosmetic containers. Concentrated acids and alkalies attack the resin. The electrical properties of the urea resins compare favorably with those of the tar-acid resins. They have a lower power factor at high-frequencies than the tar-acid resins, and are replacing, to some extent, established materials in heavy duty electrical equipment where “tracking” causes trouble. Molded articles made from urea resins are resilient but not unbreakable.
Thermostat Case of Molded Urea Resin.Source: Plaskon Company, Inc., 2112 Sylvan Avenue, Toledo, Ohio.
Thermostat Case of Molded Urea Resin.
Source: Plaskon Company, Inc., 2112 Sylvan Avenue, Toledo, Ohio.
Scales Case of Molded Urea Resin.Source: Plaskon Company, Inc., 2112 Sylvan Avenue, Toledo, Ohio.
Scales Case of Molded Urea Resin.
Source: Plaskon Company, Inc., 2112 Sylvan Avenue, Toledo, Ohio.
The important uses of the urea resins are dictated by their pleasing color and appearance. In 1935 the largest outlets were in buttons and buckles, in bottle closures, and in such premium items as biscuit cutters and cereal bowls distributed by a large food manufacturer. Tableware, bathroom fixtures, all sorts of containers and closures, housings for radios, clocks, scales, and other machines for retail stores, and light-colored wall plates and switches, knobs, handles, and trim on dash panels of automobiles, and handles and trimming on gas and electric ranges were among the widespread applications of the urea resins. In 1938 probably the fastest growing outlet for urea resins is in lighting equipment. Use in packaging, in closures, and in housings, is also increasing. Tableware, the principal outlet for a number of years, is declining markedly.
A comparatively new use is in shades and reflectors, replacing opal glass. The unpigmented resin is highly translucent and gives high light transmission and an exceptional degree of light diffusion. These properties, together with low unit manufacturing costs, reduced shipping costs, and resistance to breakage make the urea resins an ideal material for all sorts of shades and reflectors for direct and indirect lighting fixtures. Many of the shades used in railway cars are of this material. The resin is available in degrees of denseness and opacity to give particular ratios of reflection and transmission. Reflectors as large as 28 inches in diameter are on the market.
Although molded articles are the large outlets for the urea resins, other applications are of increasing importance. Sirups used to impregnate paper and cloth are used in laminating and the resulting materials have unusual decorative possibilities. The surface is hard and durable and the wide range of colors possible permits very attractive applications. The urea resins are used both as the principal binding material for laminated sheets or on the surface laminae of sheets where tar-acid resins are used as the chief binder. The latter practice permits a wide color range in decorative materials without loss of strength or other characteristics of the tar-acid resins. In 1937 there were seven makers, and their production of urea resins for laminating accounted for slightly less than 10 percent of the total of all urea resins.
Another application of urea resins which has grown rapidly in the past 2 years is in combination with alkyd resins in surface coatings. In 1937 there were three makers, and their output of urea resins for coatings amounted to more than 10 percent of the total production of urea. Until recently the use of urea resins in paints and varnishes was discouraged by their insolubility in organic solvents and their instability. On the other hand, their lack of color, their high transparency, their hardness, and their freedom from after-yellowing were desirable characteristics. The development of methods for preparing condensates, which overcome the undesirable properties, has made available resins for this use. They are marketed as water-white viscous solutions in a mixture of organic solvents and are intended for use in baking finishes. They cannot be used alone because the cured resin is extremely hard and brittle and lacks adhesion. When combined with more elastic film-forming materials such as drying or nondrying oil alkyd resins, they produce coatings that are mar-proof, resistant to alcohol, grease, oil, and fruit acids, andavailable in a full range of colors. Applications are in metal furniture finishes, toys, refrigerators, can, and drum coatings.
The value of urea resins as adhesives has been known for many years and one of the first patents issued for such use was United States Patent No. 1,355,834 granted in 1920. Commercial development and application, however, did not take place until the last 2 years. Several brands of urea adhesives are now on the market. These meet the need for a hot-press adhesive which is applied in liquid form, cures rapidly at moderate temperatures, and is economical. For greater economy, the urea adhesive may be mixed with various proportions of flour (up to 50 percent) without affecting its water resistance. Diluted thus it comes within the cost range of animal and vegetable glues and is more durable. At present, it sells for 18 to 20 cents per pound; mixing it with 50 percent flour gives an adhesive for plywood, costing about 10 cents per pound. In 1937 three producers made urea resins for this use.
Other uses are in the treatment of textiles to obtain crease-proof properties and in the impregnation of wood. United States Patent No. 1,951,994 issued on March 20, 1934, reports the preparation of artificial silk from urea resins.
Commercial production of urea resins in the United States was reported for the first time in 1929. Early in that year the American Cyanamid Co. concluded an arrangement with the British Cyanides Co. of England for the American rights to manufacture and sell in the United States a resin made from urea, thiourea, and formaldehyde and marketed as Beetle molding powder. A manufacturing unit was built at Bound Brook, N. J., and in 1930 the output was substantial.
In 1931 another producer, the Toledo Synthetic Products Co., began manufacture of urea resins. Several years prior to that time the Toledo Scale Co. started a search for a material light in weight to replace the heavy porcelain-on-steel used in cases for scales. The search led to the urea resins and to commercial production by their subsidiary. In 1935 the Toledo Synthetic Products Company reached an agreement with the Imperial Chemical Industries of England for the interchange of technical and commercial information and of free patent licenses on urea molding and laminating resins. The name of the domestic firm was later changed to the Plaskon Co.
In 1932 the Unyte Corporation started commercial production of urea resins at Grasselli, N. J. This firm was affiliated with the American I. G. Corporation. Late in 1936 the Plaskon Co. took over the Unyte Corporation.
The output of urea resins increased markedly in 1936 and 1937. Statistics for those years cannot be published without disclosing operations of individual firms. It may be stated, however, that the increase in both years over the previous year was considerably greater than for any earlier period. Most of the production was used in molded articles although appreciable quantities were consumed in laminated articles, in surface coatings, in the impregnation of fabric, and in adhesives.
There were 10 domestic makers of these resins in 1937.
Domestic production and sales of urea resins are shown in table8.
Table 8.—Urea resins: United States production and sales, 1933-37
1Not publishable; figures would reveal operations of individual firms.Source: Dyes and Other Synthetic Organic Chemicals in the United States, U. S. Tariff Commission.
1Not publishable; figures would reveal operations of individual firms.
Source: Dyes and Other Synthetic Organic Chemicals in the United States, U. S. Tariff Commission.
Resins obtained from urea and thiourea, if imported, would probably be classified under paragraph 11 of the Tariff Act of 1930. The present rate of duty under this classification is 4 cents per pound and 30 percent ad valorem.
There has been no importation of these resins. This is due principally to the international licensing arrangements which usually include the allocation of markets.
Exports are not shown separately in official statistics.