Shear strength and water resistance of the MSF/AMSF adhesives
The wet bonding strength of the MSF/AMSF adhesives is shown in Table 1. The pure soybean-flour-based adhesives without GPE was 0.45 MPa, and the bonding strength increased significantly upon addition of the GPE crosslinking agent, which meets the type II plywood requirement (≥0.7 MPa) of the China National Standard
(GB/T 9846.3-2004). As shown in Table 1, the bonding strength of the plywood bonded with MSF adhesives upon addition of ATP increased by less than 2 wt%. The bonding strength increased by 17.4% and reached a maximum value of 1.01 MPa upon addition of 2 wt% ATP. By increasing the ATP content to 5 wt%, the bonding strength decreased and then levelled off (0.92 MPa). This behaviour was due to the physical rod-like structure of ATP, which acted as a bridge joint between itself and the MSF adhesive. This type of physical attachment may be the main reason for the increase in the wet bonding strength. However, the addition of ATP from 3 to 5 wt% did not further improve the wet shear strength of the MSF adhesive. Therefore, a high solid content and viscosity be detrimental to the flowability of the adhesive and the resulting adhesive would
fail to uniformly coat the veneer surface.
The effect of the ATP addition on the water resistance of three-ply plywood bonded with the MSF/AMSF adhesives is shown in Table 2. For the MSF adhesive (0 wt%), three out of the four specimens delaminated after the first cycle, and all of the specimens delaminated after the second cycle of the boil test. When the ATP content increased from 1 to 3 wt%, the number of three-ply plywood