B. Particleboard Adhesives
A very important application for UF adhesives is in the manufacture of particleboard. The glue mix is generally composed of a liquid resin to which water has been added to decrease viscosity and to facilitate spraying, plus small amounts of ammonium chloride or sulfate and small amounts of ammonia solution. Small quantities of insecticides, wax emulsion, and fire-retarding agents (such as ammonium phosphates) are added before spraying the adhesive onto the wood chips. Pressing temperatures and maximum pressures used in the cycle are in the range of 150 to 200 C and 2 to 35 kg/cm2, respectively.
The moisture content of glued furnish chips is 7 to 8% for the board core and 10 to
12% for the surface. The resin contents used (i.e., solids) are 6 to 8% for board core and
10 to 11% for board surfaces, but such proportions might be higher for the weaker low
emission adhesives used today and depending for the application envisaged [i.e., particle-
board or medium density fiberboard (MDF)].
and gypsum. If gypsum is used, it must be free of calcium hydroxide, because this
interferes with the acid curing agent.