Effects of Assembly Time on Wet Shear Strength and Formaldehyde Emission of Plywood Bonded by Urea Formaldehyde Resin

The effects of assembly time on the properties of plywood were investigated in detail in this study. Three-layer plywood was fabricated, and its wet shear strength and formaldehyde emission were measured. The assembly time was varied to create three different assembly conditions (A, B, and C). The assembly condition A consisted of UF resin mixed with NH4Cl and kept for 0 to 8 h before gluing; the assembly condition B consisted of glued veneer kept in the open air for 0 to 8 h; and the assembly condition C consisted of glued veneer pre-pressed for 0 to 8 h before hot-pressing. The thermal behavior of the adhesive applied through varying assembly times was tested by differential scanning calorimetry (DSC). Results showed that the plywood prepared under condition C exhibited the highest wet shear strength, which was 37% and 18% higher than those under conditions A and B, respectively. The plywood prepared under condition C exhibited the lowest formaldehyde emission, which was 32% and 16% lower than those under conditions A and B, respectively. The DSC results indicated that the curing process consisted of three sections and that the rate of the curing reaction was the fastest in the first section and was similar in the latter two sections.