Wood Science and Technology

The effect of alkaline hydrogen peroxide on Douglas fir sapwood and heartwood was investigated by ultraviolet microscopy and reflectance spectrophotometry. It was found that the high absorbance of the heartwood elements around 300 nm which was attributed to the flavanone dihydroquercetin was readily eliminated by the action of peroxide. It is also proposed that the brightening effect of hydrogen peroxide is due in part to the destruction of coniferaldehyde groups which occurs to the same extent in both sapwood and heartwood and in all the morphological elements. A correlation between the concentration of chromophoric groups and the concentration of lignin is demonstrated.

The spiral angle of the elementary cellulose fibril in the wood cell wall, often called microfibril angle, is of primary importance for the mechanical properties of wood. While there are a number of methods to estimate this angle, x-ray diffraction (XRD) techniques have recently obtained a lot of attention because of their ability to provide information averaged over a significantly large specimen volume. Here, we present results from a related method, small-angle x-ray scattering (SAXS). The advantage of SAXS is that, unlike XRD, it does not require any assumption on the orientation of the cellulose crystal axis with respect to the fibril axis. Full three-dimensional scattering patterns were collected using an area detector by rotating the sample around one axis. The distribution of fibrillar orientations was seen to reflect the typical cross-sectional shape of the tracheids (square or circular). In the stem, the spiral angle was found <5° in earlywood and ≈20° in latewood. In branches the angle was ≈30° in the upper part and ≈40° in the lower part, which strongly supports the idea that the spiral angle has primarily mechanical function.

Theoretical models are proposed for predicting the longitudinal and transverse thermal conductivities of wood-polymer composites. The predictions of the models are in good agreement with the measured thermal conductivities of red maple boards impregnated with either polystyrene, polymethyl methacrylate or polyfurfuryl alcohol. The density, heat capacity, transverse thermal conductivity and longitudinal thermal conductivity of the red maple boards were 589 kg/m3,1290 J/kg K, 0.155 W/mK and 0.358 W/mK, respectively. Polymer impregnation moderately altered the thermophysical properties of the boards. The increase in density of the boards ranged from 60% to 79%, the increase in transverse thermal conductivity ranged from 12% to 33%, the increase in longitudinal thermal conductivity ranged from 3% to 13% and the decrease in heat capacity ranged from 3% to 11%. Polystyrene provided the largest increase in density whereas polymethyl methacrylate yielded the greatest increase in thermal conductivity and the largest decrease in heat capacity. Treatment with polyfurfuryl alcohol caused the samples to swell and resulted in the lowest increases in thermal conductivity and density. On average the thermal diffusivity of the composites was 26% smaller than that of the parent wood.

Wood exhibits a pronounced time dependent deformation behavior which is usually split into ‘viscoelastic’ creep at constant moisture content (MC) and ‘mechano-sorptive’ creep in varying MC conditions. Experimental determination of model rheological parameters on a material level remains a serious challenge, and diversity of experimental methods makes published results difficult to compare. In this study, a cantilever experimental setup is proposed for creep tests because of its close analogy with the mechanical behavior of wood during drying. Creep measurements were conducted at different load levels (LL) under controlled temperature and humidity conditions. Radial specimens of white spruce wood [Picea glauca (Moench.) Voss.] with dimensions of 110 mm in length (R), 25 mm in width (T), and 7 mm in thickness (L) were used. The influence of LL and MC on creep behavior of wood was exhibited. In constant MC conditions, no significant difference was observed between creep of tensile and compressive faces of wood cantilever. For load not greater than 50% of the ultimate load, the material exhibited a linear viscoelastic creep behavior at the three equilibrium moisture contents considered in the study. The mechano-sorptive creep after the first sorption phase was several times greater than creep at constant moisture conditions. Experimental data were fitted with numerical simulation of the global rheological model developed by authors for rheological parameter identification.

This study investigated the relationship between the cellulose crystal lattice strain (crystalline region) and the macroscopic surface strain in specimens of Chamaecyparis obtusa wood under repeated uniaxial tension stress in the fiber direction. Changes in the strain of the crystal lattice were measured from the peak of (004) reflection using the transit X-ray method. The macroscopic surface strain of each specimen was measured with a strain gauge. In both loading and unloading, the surface strain changed linearly with changes in stress. However, crystal lattice strain was not linear but exhibited changes along a curve with changing stress. Under stressed conditions, the crystal lattice strain was always less than the surface strain, regardless of the frequency of repetition in the loading and unloading cycle. The ratio of the crystal lattice strain to the surface strain showed a negative correlation for stress in both loading and unloading. That is, the ratio decreased with increasing stress, and finally tend to converge to a specific value. The ratio (I/I 0) between the diffracted intensity (I 0) in the (004) plane in the unloaded condition and the diffracted intensity (I) in the (004) plane in the loaded condition tend to converge on a specific value with increasing frequency of repetition. When the substantial tension Young’s modulus of the wood in the longitudinal direction decreased, the ratio of the strain of the crystal lattice to the surface strain also decreased. Moreover, the ratio decreased with increasing microfibril angle of the specimen.

The deformation and stresses in a circular wood log resulting from an arbitrary radial moisture distribution are examined. In this paper the log is modeled as a layered cylinder, with each layer assumed to be linearly elastic, cylindrically orthotropic, and homogeneous. The general solution to the equations of elasticity for a representative layer is given; constants of integration in the solution are determined through application of appropriate continuity conditions at the layer interfaces. Numerical examples are presented for logs of Scots pine which illustrate the effect of nonuniform moisture content upon the displacement and stress distributions.

Hybrid poplar (Populus deltoides × Populus trichocarpa) and Douglas-fir (Pseudotsuga menziesii) wood specimens were densified with three variations of thermo-hydro-mechanical (THM) treatment. The THM treatments differed in the steam environment, including transient steam (TS), saturated steam (SS), and saturated steam with 1-min post–heat treatment at 200 °C (SS+PHT). The bending properties, FTIR spectra, and colour of the THM wood specimens were studied before and after exposure to two different wood decay fungi, brown rot Gloeophyllum trabeum, and white rot Trametes versicolor. The results showed that the performance of densified hybrid poplar wood was considerably poorer than the performance of Douglas-fir heartwood. The FTIR spectra measurements did not show changes in the densified hybrid poplar wood, while some changes were evident in densified Douglas-fir specimens. After fungal degradation, the most prominent changes were observed on the SS+PHT specimens. Colour is one of the most important parameter predominantly influenced by the wood species and the intensity of the densification process for both wood species, while after fungal exposure, the colour of all densified Douglas-fir specimens obtained more or less the same appearance, and densified hybrid poplar specimens resulted in lighter colour tones, indicating that the pattern of degradation of the densified and non-densified specimens are similar. The 3-point bending test results determined that the THM treatment significantly increased the modulus of rupture (MOR) and modulus of elasticity (MOE) of the densified wood specimens, while fungal exposure decreased the MOE and MOR in hybrid poplar and Douglas-fir specimens.

The wood part of Ulex europæus, a common plant in Galicia (N. W. Spain), has been subjected to an organosolv treatment with dilute hydrochloric acid and phenol. The reaction was carried out at atmospheric pressure and 100 °C. Two simple kinetic models have been used to explain the pre-hydrolysis and delignification processes, studying the influence of the concentration of acid on both processes at three levels: 0.45, 0.90 and 1.80% by weight.