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The paper describes for the first time the analysis of the structure of compressed wood using computed tomography. The anatomical structures of Douglas-fir and hybrid poplar before and after densification with the viscoelastic thermal compression (VTC) process were described by pore size distributions and mean pore sizes and compared. The compression of Douglas-fir mainly affected earlywood, while the compression of hybrid poplar mainly occurred in the vessels. In both wood species, the densification resulted in a significant decrease in the pore volumes. The porosity decreased to less than half of the original value for Douglas-fir earlywood and to approximately one-quarter for the vessels in hybrid poplar. The relevant mean pore sizes also decreased dramatically to about one-quarter compared to the original values. In contrast, latewood in Douglas-fir and libriform fibers in hybrid poplar are quite stable under compression. Douglas-fir latewood retained its original structure after compression and did not show any reduction in pore size. The results confirmed that the anatomical structure of VTC densified wood can be described by pore size distributions and mean pore sizes. However, in the case of broad or bimodal distributions, the mean pore sizes are of less significance. [ABSTRACT FROM AUTHOR]

In this paper, the compressive deformation of hybrid poplar wood ( Populus deltoides × Populus trichocarpa) at high temperature (150, 160, and 170°C) and under various conditions of steam pressure was studied. Temperature and conditions of steam environment affected the relative density change and creep deformation during compression, as well as properties of the resulting densified material. While the temperature significantly affected the compression deformation of specimens compressed under transient and superheated steam conditions, temperature within the range studied had little effect on the compressive deformation in saturated steam. In all tested conditions, compression deformation was achieved without cell wall fractures. Higher temperature of compression, regardless of steam condition, resulted in lower equilibrium moisture content. In specimens compressed under saturated steam, the modulus of rupture (MOR) and modulus of elasticity (MOE) were increased proportionally to the increase in density, while the compression under superheated steam produced lower increase in the MOE and MOR than expected based on the increase in density. Compression in transient steam conditions at 170°C produced densified wood with higher MOE and MOR than expected based on the increase in density. [ABSTRACT FROM AUTHOR]

A novel wood-based composite has been developed for use in structural applications. The process was designed to utilize rapidly-grown, low density, wood species. Plantation grown radiata pine is particularly well suited to this process. This is a laminated composite, where the lamina may be comprised of various materials, some of which have been treated with the viscoelastic thermal compression (VTC) process. The VTC process increases the density of wood, without causing fractures in the cell wall, thus increasing strength and stiffness of the wood material. The process may be applied to veneer, sawn wood, or strand composites. The VTC lamina is then bonded to other lamina to produce the final product. The strength and stiffness of this VTC composite exceeds any wood-based composite that is currently on the market. For example, modulus of elasticity in bending of over 20 GPa is easily obtainable. [ABSTRACT FROM AUTHOR]

Surface energy of yellow-poplar wood from three wood treatment groups was studied using sessile-drop contact-angle measurements and the Chang acid–base model for surface energy calculation. Water, glycerol, formamide, ethylene glycol and α-bromonaphthalene were used as probe liquids. Wood treatments were control, hygro-thermal treatment and densification. Contact angle trends for densified and hygro-thermally treated wood were found to be the same. Total surface energy decreased with both hygro-thermal treatment and densification. [ABSTRACT FROM AUTHOR]

Bond performance of hygro-thermally compression densified wood was studied using hygro-thermally treated and control yellow-poplar wood (Liriodendron tulipifera). Opening mode double cantilever beam fracture testing and cyclic boiling were used to evaluate bond performance. Phenol–formaldehyde (PF) film and polymeric diphenylmethane diisocyanate (pMDI) adhesives were used to bond specimens for fracture testing. Fracture toughness of hygro-thermal samples bonded with PF film was significantly higher than control samples, while no difference was found for densified samples. Fracture toughness of densified samples bonded with pMDI was significantly higher than control samples, however no change was seen for hygro-thermal samples. Cyclic boiling reduced the fracture toughness of hygro-thermal fracture samples only, irrespective of adhesive type. [ABSTRACT FROM AUTHOR]