Influence of curing temperature on the bonding strength of heat-treated plywood made with melamine-urea-formaldehyde and phenol–formaldehyde resins

Melamine-urea-formaldehyde resin (MUF) and phenol–formaldehyde resin (PF) were prepared in the laboratory. Their curing behavior was analyzed by differential scanning calorimetry (DSC). MUF resin was then cured at 110, 120, and 130 °C, while PF resin was cured at 135, 150, and 165 °C. The dry and wet bonding strength of plywood made by hot-pressing at different temperatures and heat-treatment at 200 and 250 °C were measured. DSC results show that the PF resin had a more obvious exothermic peak than the MUF resin during heat scanning. Fourier transform infrared spectroscopy analysis shows that MUF and PF resins cured at higher temperature undergo more condensation reactions. Solvent dissolution test shows that PF resin cured at 135 °C has a weight retention similar to that of resins cured at 150 and 165 °C. However, using a temperature of 120 °C was better than 110 and 130 °C for MUF resin. Thermogravimetric analysis results show that PF resin had better heat resistance than MUF resin. The curing temperature did not influence the thermal degradation behavior of cured resins. However, increasing the curing temperature resulted in higher thermal stability. Heat treatment decreased the bonding strength of plywood. However, the bonding strength still met the requirement of the CNS 1349 standard when heat-treated at 200 °C.