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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]

Compression creep experiments of hybrid poplar (Populus deltoides×Populus trichocarpa) were performed in a pressurized vessel equipped with a heated hydraulic press. The viscoelastic response at various stress levels (2-7 MPa), a temperature of 170°C and transient steam conditions was studied. Moisture content and oven-dry density of compressed specimens were determined. While some recovery of compression strain occurred, compression resulted in permanent deformation and increased wood density. The influence of stress level on the amount of set recovery of compressive deformation was evaluated after 24 h water soaking. Applied stress level had a significant effect on the compression deformation. The initial strain, as well as creep strain, varied depending on the applied stress level. The highest oven-dry density was obtained at a stress level of 6.9 MPa. Lower stress levels resulted in lower moisture content after the compression process, while the equilibrium moisture content of compressed specimens was not significantly affected by stress level. Set recovery increased from 20% to 65% with increased stress level from 1.7 MPa to 4.1 MPa, then decreased to 53% for specimens compressed at 6.9 MPa. Moisture content after the compression process significantly affected the set recovery. [ABSTRACT FROM AUTHOR]

Abstract: The influence of the viscoelastic thermal compression (VTC) of wood on its surface chemistry, wettability, and surface free energy was evaluated. The changes in surface chemistry were studied by means of Fourier transform infrared spectroscopy (FTIR). Wettability was determined on the basis of the contact angle of water, using the Wilhelmy plate method. Surface free energy was determined according to the Owens Wendt Rabel and Kaelble (OWRK) theory, using the contact angle data of diiodomethane and formamide. The studied material was wood of the low-density hybrid poplar (Populus deltoides × Populus trichocarpa), which was densified to three different degrees of densification (63%, 98%, and 132%) using the VTC process. The results, based on the FTIR spectra, revealed that the VTC process caused minor chemical changes on the wood surface. However, wettability changed significantly. The VTC wood surface exhibits rather hydrophobic behaviour. Evaluation of the surface energetics using the OWRK approach showed that total surface free energy dropped significantly due to the VTC process. [Copyright &y& Elsevier]

A mathematical model of the hot-pressing consolidation process for manufacturing a multi-layered oriented strand composite is developed and tested. Such a process model requires independent physical properties and user-specified structure for each layer of the mat. The model uses two-dimensional (slow variation is assumed in the third dimension) heat- and mass-transfer theory and a viscoelastic compression model. The through-the-thickness (vertical) density profile (VDP) was found to be highly dependent on the initial moisture content gradient, internal mat strand properties, and hydrothermal compressive response of the mat. The VDP predicted by the model showed good agreement with experimental results. The proposed mathematical model thus shows promise as a tool to analyze optimally design-oriented strand composites. [ABSTRACT FROM AUTHOR]

Densification techniques, combined with other treatment processes, helping additionally to improve wood properties, play an important role in wood modification. An innovative example of this combination with other techniques is the viscoelastic thermal compression (VTC) process. In this study, material of poplar wood from fast growing short rotation plantation (SRP), modified by VTC process, was used for investigation and results were compared with non-modified native wood of plantation grown poplar. Selected physical and mechanical properties, i.e., density, equilibrium moisture content (EMC), and modulus of elasticity (MOE) were investigated. The material, used for this investigation, was obtained from plantation grown poplar clone Max-4 (Populus nigra x Populus maximowiczii) from the Czech Republic. Results show that compression and heat treatment of the viscoelastic thermal compression process lead to significant changes in physical and mechanical properties of the treated material. The density of material depends on the degree of densification and has a crucial effect on properties. The goal of this study was to examine the improvement of properties of SRP poplar clone after applying the VTC densification process. Property improvement of plantation grown poplar wood has a huge potential for future use of this low-cost and fast-growing material, which would save high value materials for special uses. [ABSTRACT FROM AUTHOR]