Effects of high-pressure (HP) treatment on densification of poplar sapwood boards and subsequent coatings were evaluated. Tung oil (TO) and epoxy resin (ER)-coated treatments were used to improve the dimensional stability of HP-densified wood. The density of the wood after HP densification increased from 450 ± 50 kg/m 3 for the control to 960 ± 20 kg/m 3 at 125 MPa. This process also resulted in the average thickness of HP-densified boards to reduce significantly from 29.7 ± 0.11 mm for the control to 18.8 ± 0.53 mm after HP densification at 25 MPa and 14.3 ± 0.10 mm after 125 MPa treatment for 30 s. The mechanical strength measured as the hardness of densified wood significantly increased from 35% at 25 MPa to 96% at 125 MPa treatment, compared to untreated wood. As expected both TO and ER-coated treatments significantly reduced set-recovery of densified wood when stored at four relative humidity environments. ER showed better anti-swelling performance than TO, and would be a better choice.

Abstract. Dyeing of poplar wood through high-pressure (HP) processing was evaluated at different pressure levels (40 to 130 MPa) and compared with conventional hot dip treatment. Dyeing performance was evaluated based on external surface color, fractal color dimension, dye uptake, internal color, elemental composition, and subsequent ultraviolet aging of the wood. The internal microstructure of the treated wood was also examined by scanning electron microscopy (SEM). Results showed that the external surface color difference and change in brightness were significantly greater with HP treatment than with hot dip treatment. The hue angle of HP treated wood surfaces was 1.27 to 4.01 higher and the fractal color dimension was significantly (p < 0.05) lower than that of hot dip treated wood, demonstrating more intense and more uniform distribution of color with HP treatment. The internal color of HP treated wood was also more evenly distributed and more intense, while the hot dip treated wood had no internal dye uptake (zero penetration). SEM analysis showed that the wood structure was damaged by HP treatment, which allowed the dye solution to penetrate into the wood more easily. Among the different pressure levels, the higher pressures resulted in better dyeing performance; however, considering the treatment cost, time, and energy efficiency, 100 MPa treatment may be more economical. Keywords: Color, Dyeing, Fractal dimension, High pressure, Hot dip, Poplar wood.

The deformation behavior of high-pressure (HP)-treated hybrid poplar wood, the subsequent swelling, and the equilibrium moisture content properties of HP-densified wood were evaluated using a modified delayed strain/set-recovery method of cyclic humidification-dehumidification at different relative humidity (RH) conditions. The HP treatment resulted in significant compression (densification) of the wood under different treatment conditions. For treated wood samples, the value of delayed elastic strain was relatively small when stored at 20 °C and 65% RH, which indicated that HP-densified wood possessed dimensional stability. The HP-compressed poplar yielded lower equilibrium moisture content than the control at low RH, while major increases were observed at high RH above 76%. Marginal of thickness swelling was observed under the cyclic humidification-dehumidification method in the low RH range while significant swelling occurred at high RH. Conventional methods would only show results that were appropriate for storage at high RH environments. The RH threshold for set-recovery of HP-compressed wood was between 33% and 54% for optimal use, and the extent of set-recovery increased rapidly when RH was between 85% and 95%. Such differences could not be recognized with the conventional methods. In addition, the prolonged holding time significantly decreased the RH threshold value (P < 0.05).

HighlightsHigh-pressure densification (HPD) resulted in more than 100% increase in the density of poplar wood.Density of wood during HPD treatment was up to 300% and 100% higher than observed before and after HPD.A method was developed to measure elastic recovery in the radial and tangential directions under HPD.Abstract. Plastic and elastic strains of poplar wood in the radial and tangential directions were investigated after high-pressure densification (HPD) using a special method that detected the elastic strain. There was a large difference in plastic and elastic strains obtained in compressed wood between the radial and tangential directions. The maximum total strain in the radial and tangential directions reached 26.9% and 66.9%, respectively, when compressed at 150 MPa for 300 s. Longer holding time (300 s) increased the plastic strain, by reducing the elastic strain, when the applied pressure was lower than 60 MPa. In addition, a large difference in compressed wood density prevailed when still under pressure as compared with the density measured after the pressure treatment, and both were nearly 1 to 3 times higher than the density of fresh wood. Further, the maximum density observed under pressure was related to the maximum reported density of dry crystalline cellulose plus the entrapped moisture. The mechanical properties of densified wood were also investigated and found to depend on the pressure treatment conditions. Keywords: Elastic strain, High-pressure densification, Mechanical properties, Plastic strain, Wood density.