Effects of High-Pressure Treatment on Poplar Wood: Density Profile, Mechanical Properties, Strength Potential Index, and Microstructure

Abstract

The density profile, mechanical properties, strength potential index, and microstructure changes of hybrid poplar were investigated before and after high-pressure (HP) treatments. The results of density profile indicated that a high uniform density distribution was developed inside the pressurized wood samples. The mechanical properties results showed that the HP treatments significantly increased (P < 0.05) the modulus of elasticity (MOE), the modulus of rupture (MOR), and the Brinell hardness (BH) of the densified wood at selected conditions. Of all the wood samples, the compressed wood at 150 MPa condition possessed the highest density and strength properties. Considering the variation in strength properties along with density, it can be concluded that the compression destruction degree of HP treatment was comparable with that caused by optimized thermal compression technique based on the strength potential index results. The integrity of wood cells presented in scanning electron microscopy results demonstrated the compression of wood cell wall achieved by HP treatment without causing any fractures, which further indicated that HP treatment is a less destructive compression technology. Based on this research, HP treatment has great potential to be applied in wood densification for commercial use.

Fabrication of densified wood via synergy of chemical pretreatment, hot-pressing and post mechanical fixation

“…The process of softening is known as a pretreatment and combined with following mechanical pressing, achieving better compressing result. The thermo-mechanical densification [3][4][5], thermal-hydromechanical [6][7][8], hygro-mechanical steam treatment [9,10], mechanical-microwave treatment [11], and highpressure treatment [12,13] were developed to manufacture densified wood, in which the thermal-hydro is the most common and simplest pretreatment method currently. The cell walls of wood would be swelled by water although it cannot penetrate into crystalline region of cellulose.…”

High-pressure densification and hydrophobic coating for enhancing the mechanical properties and dimensional stability of soft poplar wood boards

“…As expected, with a decrease of board thickness and increase in CR, the density of the treated board increased significantly (p < 0.05) by 55.6%, 86.7%, 91.1%, 102% and 113% at 25 MPa, 50 MPa, 75 MPa, 100 MPa and 125 MPa, respectively. In our preliminary experiments, density was observed to decrease when pressure level exceeded 150 MPa [21]. These results indicate that HP densification for wood densification improves compression effects up to a certain pressure level gradually compressing the void spaces in the boards after which the degree of densification will be small.…”

“…This advantage makes HP-densified wood attractive for long-term indoor use when the environment relative humidity (RH) is low and the climate is relatively stable. Moreover, results showed that HP densification could make the densities of Chinese fir wood/poplar/paulownia wood to increase 2-3 times within 5 min [20,21,23]. This treatment is shown to be universally effective for various low-density species of wood.…”

“…High-pressure (HP) treatment has been proved to be a useful densification method for fast-growing wood species by quickly reducing the void volume of wood through isostatic hydrostatic pressure, in a high-pressure equipment [20,21]. HP densification works as shown in Fig.…”

“…Poplar wood is targeted in this study because of its broad range of use in China. Yu et al [21] demonstrated the densification effects of poplar wood treated by HP densification in which technological parameters of 50-200 MPa pressure levels for 30 s holding time were applied. Their results show that densities and mechanical properties of treated wood could be significantly improved with HP densification up to 150 MPa, and they tend to stabilize or show a decline beyond 150 MPa.…”