Improved Plywood Strength and Lowered Emissions from Soybean Meal/Melamine Urea-Formaldehyde Adhesives

Abstract

The objective of this study was to improve the wet shear strength and reduce the formaldehyde emissions for the soybean meal/melamine urea-formaldehyde (MUF) adhesive–bonded plywood by optimizing the hot pressing parameters in plywood manufacturing. An L(44) experimental design was used. A dynamic mechanical analyzer (DMA) was used to study the curing process of the adhesive during hot pressing. The DMA results showed that the storage modulus (E′) first decreased to a minimum at 72.9°C and then increased as the temperature increased to a maximum at 157.2°C. At temperatures higher than 157.2°C, the storage modulus decreased as the temperature increased. The hot press temperature had a significant effect on both the wet shear strength and the formaldehyde emissions from 120°C to 150°C, while no significant effect was found by varying the hot press time from 50 to 80 s/mm and changing the pressure from 0.8 to 1.2 MPa. The adhesive spread rate had a significant effect on the formaldehyde emissions. No significant effect was found on the wet shear strength of plywood by varying the adhesive spread rate from 145 to 205 g/m2. The optimum hot pressing parameters were found: 150°C hot press temperature, 70 s/mm in hot press time, 1.2 MPa in hot press pressure, and 165 g/m2 in adhesive spread rate. Under such optimum processing conditions, the wet shear strength of the plywood was improved by 24.2 percent from 0.95 to 1.18 MPa, while the formaldehyde emissions was reduced by 21.4 percent from 0.28 to 0.22 mg/liter, which met the type II plywood requirement for wet shear strength and the level E0 requirement for formaldehyde emissions described in the China National Standard GB/T 9846.3-2004.