Amino Resins and Plastics

Abstract

Amino resins are thermosetting polymers made by combining an aldehyde with a compound containing an amino ( \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}${{\relbar \kern-5pt{\relbar}\kern-7pt{\relbar}}{\rm{NH}}{_{2}}}$\end{document} ) group. Urea–formaldehyde (U/F) accounts for > 80% of amino resins; melamine–formaldehyde accounts for most of the rest. The principal attractions of amino resins and plastics are water solubility before curing; colorlessness; excellent solvent resistance; outstanding hardness and abrasion resistance; and good heat resistance. Amino resins provide a wide variety of useful products. Adhesives, the largest single market, are used to make plywood, chipboard, and sawdust board. Other types are used to make laminated wood beams, parquet flooring, and for furniture assembly. Aminoplasts and other thermosetting plastics are molded by an automatic injection‐molding process similar to that used for thermoplastics. The process is best applied to relatively small moldings. The great advantage of injection molding is that it eliminates manual labor costs, thereby allowing amino resins to better compete with thermoplastics. The future for amino resins and plastics seems secure. New developments will probably be in the areas of highly specialized materials for treating textiles, paper, etc, and for use with other resins in the formulation of surface coatings. Most amino resins are based on the reaction of formaldehyde with urea or melamine. The first step in the formation of resins and plastics from formaldehyde and amino compounds is the addition of formaldehyde to introduce the hydroxymethyl group, known as methylolation or hydroxymethylation. The second step is a condensation reaction that involves the linking together of monomer units with the liberation of water to form a dimer, a polymer chain, or a vast network, usually referred to as methylene bridge formation, polymerization, resinification, or simply cure. Precise control of the course, speed, and extent of the reaction is essential for successful manufacture. Amino resins are usually made by a batch process. Both urea– and melamine–formaldehyde resins are of low toxicity. Melamine–formaldehyde resins may be used in paper that contacts aqueous and fatty foods. However, some mills are looking for alternatives.