a glue based on urea-formaldehyde and melamine-formaldehyde resins (“carbamide resins”) and their mixtures. Large quantities of carbamide glue are used in the wood-product industry, primarily for the manufacture of plywood and furniture; it is also used to join porcelain and metal.
Carbamide glue is an aqueous solution of carbamide resin. It frequently contains a hardener (oxalic, phthalic, or hydrochloric acids or certain salts) and a filler (soybean or cereal flour, starch, wood flour, or gypsum). For example, K-17 glue consists of 100 parts MF-17 resin, 7–22 parts 10-percent aqueous oxalic acid solution, and 6–8 parts wood flour (by weight).
Carbamide glue is prepared by mixing a resin solution with other glue ingredients to form a composition. The glue is sometimes produced in the form of a foamed mass. The shelf life of carbamide glue varies from 0.5 to 48 hr, depending on type. Carbamide glue without a hardener has a shelf life largely dependent on the temperature conditions. MMF glue can be stored for 12 months at 10°C and for only 0.5 months at 40°C. The addition of ammonia liquor, urotropin, urea, or melamine doubles the shelf life of carbamide glue.
Carbamide glue can harden upon heating, as well as during storage at ordinary temperatures (only in the presence of a hardener). The surface to be glued is prepared according to general procedure before application of carbamide glue. The glue is usually applied with a brush, whereas low-viscosity compositions are sprayed on; the glue is allowed to dry slightly (sometimes the drying is omitted), and then the components to be glued are joined at a pressure of 0.15–1.7 meganewtons per sq m (MN/m2), or 1.5–17.0 kilograms-force per sq cm (kgf/cm2). During application of hot-setting carbamide glue, the components are heated in a press.
Carbamide glue forms adhesive compounds with good me-chanical strength (10–13 MN/m2, or 100–130 kgf/cm2) andsatisfactory moisture resistance. Glues based on melamine-for-maldehyde resin have better properties than those based on urea-formaldehyde resin; however, the latter is considerably lessexpensive. As a result, urea-formaldehyde resin is frequentlymixed with a small quantity of melamine-formaldehyde resin tosignificantly improve the quality of the glue.