Melamine and urea are the principal commercial thermosetting polymers called aminos. The amino resins are formed by an addition reaction of formaldehyde and compounds containing NH 2 amino groups. They are supplied as liquid or dry resins and filled molding compounds. Applying heat in the presence of a catalyst converts the materials into strong, hard products. Aminos are used as molding compounds, laminating resins, wood adhesives, coatings, wet-strength paper resins, and textile-treating resins.
The base resin used in urea and melamine molding compounds is water-white and transparent. Translucent and opaque colors are produced by adding pigments and opacifying agents. Cellulose fibers improve strength and dimensional stability and reduce light transmission.
Properties: Moldings made from amino compounds are hard, rigid, abrasion resistant, and have high resistance to deformation under load. They do not impart taste or odor to foods, and they have excellent electrical insulation characteristics. Melamines are superior to ureas in resistance to acids, alkalies, heat, boiling water, and for applications involving wet/dry cycling.
Urea and some melamine molding compounds have flammability ratings of 94V-0. Melamines containing alpha cellulose, mineral, or glass fibers have the greatest flame resistance. These materials can be used for appliance parts classified by UL as indirect supports of current-carrying parts.
Below -70 °F, urea moldings become brittle, but electrical properties are not affected. Extended exposure above 170 °F is not recommended for ureas because of the effect on color. At 300 °F, color change and blistering may occur after exposure of less than one hour.
Melamine compounds containing cellulose or flock fillers are stable in the range of -70 to 250 °F. Asbestos or glass-filled formulations are stable to 400 °F. Above 210 °F, however, color changes may occur.
Melamines and ureas are not resistant to strong oxidizing acids or strong alkalies, but they can be used safely with conventional household chemicals such as naphtha and detergents. They are unaffected by organic solvents such as acetone, carbon tetrachloride, ethyl alcohol, heptane, and isopropyl alcohol. Petroleum, paraffin hydrocarbons, gasoline, kerosene, motor oil, aromatic hydrocarbons, and fluorinated hydrocarbons (Freon) have no apparent effect on urea and melamine moldings. Dimensional stability is good, but moldings do swell and shrink slightly in varying moisture conditions. Baking of molded parts accelerates postmold shrinkage and improves dimensional stability, dielectric strength, and dissipation factor.
Applications: Typical applications for cellulose-filled urea resins include circuit breakers, receptacles, and other electrical wiring devices, toaster and other appliance bases, pushbuttons, knobs, handles, piano keys, and camera parts.
Cellulose-filled melamine is used principally for dinnerware. Other applications include utensil handles, food-service trays, and housings for electric shavers and mixers. Industrial melamine compounds are used for meter blocks, connector plugs, automotive and aircraft ignition parts, standoff terminals, coil forms, and switch housings.
In liquid form, both urea and melamine resins are used as baking-enamel coatings, particleboard binders, and paper and textile treatment materials. Both resins are also used in compounding adhesives. Melamine, the more durable of the two, is waterproof, which qualifies melamine-based adhesives for exterior use.
Allyl-based prepregs are used to make lightweight, intricate parts such as radomes, printed-circuit boards, tubing, ducting, and aircraft parts. Another use is in copper-clad laminates for high-performance printed-circuit boards.