Effect of Extending Hot Press Urea Resin with Different Types of Wheat and Rye Flour on the Strength and Water Resistance of the Glue Joints

Plywood shear test specimens were taken from panels glued with 60 different glue mixtures. The flour extensions ranged from zero to 100 percent or resin solids, but the catalyst to resin ratio remained constant. Eight different wheat flours and two different rye flours were used as extenders. The resin used was a urea-formaldehyde-type which contained 70 percent solid resin for each 100 pounds of liquid resin. Additional water was added to the mix to make it spreadable when flour was added. The solids content of the mix was not allowed to drop below 50 percent. Fifteen plywood specimens were obtained from each panel. The dry shear values were usually very good and the amount of wood failure was considerable. Joints made with the lower extensions showed higher values. The water resistance of joints made with high extensions was poor in most cases. Several grades of flour were found suitable for glue extension. The soft wheat flours were easily mixed and usually had a suitable viscosity for spreading. The hard winter wheat flours were quite difficult to mix, especially in the higher extensions. The mixes were easily made in the extensions other than the 100 percent which required more water than was practicable. The white rye flour was easily mixed; however the values were not too satisfactory beyond 50 percent extension. The dark rye was not very suitable in the higher extensions. Long term exposure tests should be of additional value in making the final decision.

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