The effect of melamine formaldehyde impregnation and hot-pressing parameters on the density profile of densified poplar wood

The vertical density profile (VDP) of thermo-hydro-mechanically (THM) densified wood is a result of the combined effect of process parameters and properties of unmodified wood. The effect of thermosetting resins on VDP of THM densified wood has not been extensively studied. The aim of this study was to determine the combined effect of methylated melamine–formaldehyde (MF) resin impregnation along with thermal compression at 150 °C on the VDP of poplar wood acquired by X-ray densitometry. Four different stress levels were applied: 0, 10, 20 and 30 kg/cm². Control specimens (without MF) were also impregnated and densified under the same conditions. The results showed that the use of MF increased the density compared to control specimens for the respective CR level. The increase in CR resulted in a linear decrease in WPG values as a result of the MF loss during pressing. Due to the bulking effect of cell walls, thermal compression resulted in higher CR for control than for MF-treated specimens. In the case of uncompressed specimens, MF-treated specimens showed slight density peaks near the surfaces probably due to migration of MF solution from the core to the heated surfaces of the specimens. MF-impregnated specimens generally showed a more uniform density profile since the differences between maximum density and minimum density were smaller than the control ones. Peak width did not seem to be affected by CR for MF-treated specimens. Maximum density was not formed in the outer surface of the specimens but was measured in deeper regions as CR increased. This trend was clearer for control specimens than for MF-impregnated ones.