Urea and Melamine Adhesives

Urea resins in the liquid form have shorter storage life than in the powdered form. By raising temperature above 70?F. and adjusting the acidity of the mix inversely with temperature, an optimum cure of urea resins can be obtained. Ammonium chloride is most widely used as a hardener either alone or in combination with other materials. Walnut and pecan shell flour are widely accepted as a filler for urea resins. The maximum amount of filler falls in the range of 5 to 20 percent (based on resin solids). Wheat flour is largely used as an extender. The gluten content of the flour determines whether or not the flour is satisfactory as an extender for urea resins. Low density woods tend to absorb moisture from resins; additional water will improve flow characteristics of urea resins. For dense species with high moisture content it is necessary to increase the solids content of the mix to avoid excessive flow. For each 10?F. increase in mix temperature of urea resins, a 50 percent reduction in working life can be expected. The optimum rate of spread is 25 to 35 pounds per thousand feet of single glue line for unextended mixes and 35 to 45 pounds for highly extended mixes when hot pressing. In cold pressing, 40 to 60 pounds per thousand feet are required. The assembly time of urea resins is zero to 2 hours in hot pressing and 20 minutes in cold pressing; however, this varies with the density and moisture content of the veneer and the spread rate. Unextended urea resins (with 25 percent flour) are recommended for quality interior panels or panels to be used in exterior application. Elevated temperatures in combination with high moisture contents or high humidity will cause deterioration of the bond.

You must be logged in to download any documents. Please login (login accounts are free) or learn how to Become a Member