Wood Science and Technology

A three-dimensional and unsteady-state mathematical model, which accounts for simultaneous heat and mass transfer taking place during the high temperature treatment of wood, has been developed. It was validated by comparing the predictions with the experimental data. In the model, the coupled heat and mass transfer equations proposed by Luikov are solved, and the temperature and moisture content profiles within wood are predicted as a function of time for different heating rates. For the model validation, an experimental study was carried out with aspen under different operating conditions. The samples were heated to high temperatures using a thermogravimetric system. The weight loss and the temperature distribution within the sample were monitored and registered during the experiment. The model can use constant or variable thermo-physical properties. The temperature and moisture content of the wood predicted by the model using variable properties were compared with those predicted by the same model using constant properties as well as with the experimental data. The experimental and model results are in good agreement, and it was shown that the accuracy of the model depends on the accuracy of the properties. After the model validation was completed, a parametric study was carried out.

The effects on wood of simultaneous mechanical and moisture loading are studied. In order to clarify the mechano-sorptive behaviour of wood, a review of different phenomena presented in the literature is included. Based on this review a constitutive model is proposed for the case of uniaxial stress in the longitudinal direction. The validity of the model is checked independently against test results. The calculations show that the model is capable of describing the response of wood with reasonable accuracy. Simulations indicate that the response of small test specimens is more difficult to describe than that of larger beams. Some differences in behaviour are found to depend on loading mode and nature of moisture cycling. Very large and fast moisture cycles seem to give larger mechano-sorption than smaller variations. The results of the simulations show that there is a significant influence of strain on the shrinkage and swelling response.

The percent explained variation (r2) of tensile strength (T) of dimension lumber can be accounted for primarily by apparent modulus of elasticity (Ea). Tensile strength ratio, a good index of T of structural lumber, and Ea are both dependent on relative knot size, making T a function of Ea. This theoretical relationship helps to increase r2 when T is regressed on Ea and gives the “cause and effect” for some multiple regression analyses. An independent variable was identified on the basis of this theory. Shortening the gage length enhances ability to predict T when flatwise bending Ea or tension Ea is used as an independent variable, but not when edgewise bending Ea is used. Two single variables not previously reported, Ea measured in flatwise bending on 16-inch gage length (EF 16) and Ea measured in tension on 6-inch gage length (ET 6) (based solely on deformation measurements), are individually able to explain approximately as much variation of T (r2=0.75) as the combination of Ea measured flatwise on 48-inch gage length (span length for the existing American grading machines), and ASTM bending strength ratio. The combination of EF 16 and ET6 can explain 85% of the variation of T. The highest r2 (0.87) was obtained when a modified bending strength ratio was added to these new variables.

Bioincising is a promising method for enhancing liquid uptake (e.g. preservatives or wood-modification agents) in refractory wood. Incubation with the white-rot fungus, Physisporinus vitreus, which selectively degrades pit membranes, results in deeper and more homogeneous penetration of liquids. Conventional methods of assessing the degree of fungal discolouration of wood after treatment with preservatives (e.g. European standard EN 152) are partly based on a subjective rating scale, which gives a rough value of the surface colonisation by blue-stain fungi. Hence, an automated image processing (AIP) procedure was developed for standardised quantification of the segmentation thresholds of discolouration and tested against manual segmentation analysis. Using the red filter in the AIP method revealed high correlation (R 2 0.95) and allowed for more user friendly and objective determination of blue staining of wood.

 The objective of this paper is to evaluate the arrangement of wood strands at the surface of oriented strand board (OSB) by image analysis. Two-dimensional image analysis enables the number of strands and the area of each strand to be computed. In addition, the fiber direction of each strand may be measured manually by recording the acute angle between the fiber direction and the longitudinal axis of the specimen. The image analysis results suggest the following: the average strand area is proportional to the reciprocal of the number of strands. Samples containing many smaller strands exhibit a larger variation in strand size. The average strand area does not correlate with the distribution of the strand area represented by the coefficient of variation. However, there is a reasonable correlation between the number of smaller strands in the range 0 to 1 cm2 and the coefficient of variation of strand area. At low average fiber orientations, i.e. better orientation with the principal panel axis, there is smaller variability in orientation. The upper side and lower side of OSB exhibit a different relationship between average fiber orientation and strand area. The upper side of the specimens contains larger strands and exhibits better fiber orientation than the lower side. This is thought to be a function of the production process. The lower side strands fall on a smooth moving substrate, whereas the upper side strands fall on a stable substrate of strands. The number of strands is lower on the upper side of the OSB panel because small size strands tend to migrate to the lower side of the OSB during production. The small particles tend to be vibrated through the strand mat to the lower face before pressing.

The industrial manufacturing of wood-based panels has become a highly technological process, where all parameters have to be perfectly adjusted to manufacture products of high quality. However, variations caused by differing wood characteristics as well as variations of single process parameters can cause out-of-control events. These undesirable events can be diminished by monitoring and controlling the entire manufacturing process using multivariate statistical techniques. Hence, a real-time process adaptation of an industrial scale fibreboard manufacturing process was simulated. Regression results revealed a mean normalised root mean squared error of prediction of 4.6 %, when predicting the internal bond strength of fibreboards. The regression model is regularly validated and, if necessary, recalibrated using the offline determined board properties (feedback control). Consequently, the process can immediately be adapted as soon as the board is produced (feedforward control). The investigations resulted in reliable models and revealed high potential for permanent industrial implementation.

This review describes the treatment of wood with various inorganic silicon compounds. Among these compounds silicofluorides represent one of the long-known classes of wood preservatives based on silicon. Data on an organic fluorosilicon compound (“silafluofen”) are additionally presented. The mode of action of these fluorides is based on their biocidal action. “Water glass”, an alkali silicate, was able to enhance the durability of wood but showed some important drawbacks. Because of its high hygroscopicity and its high pH values, increased moisture absorption and strength loss of wood was frequently observed. Wood treated with tetraalkoxysilanes showed an enhanced dimensional stability, especially when the hydrolysis and the condensation of the silanes was controlled to react within the cell wall. Durability and fire resistance were improved to a certain degree and could be significantly enhanced by addition of boron compounds.

A statistical study of the cell dimensions in a growth ring of spruce along the radial and tangential directions is performed. The data are used to study the variation of the cell vapor permeability in the growth ring. Studying cell rows within one growth ring, the frequency distributions of the cell wall thickness in the radial direction and of the lumen dimension in the tangential direction are found to be both unimodal. In contrast, the frequency distributions of these dimensions in the other directions are bimodal, where the different modes can be attributed to earlywood and latewood. Analysis of the bimodal distributions results in the determination of threshold values of cell wall thickness and the lumen dimension for earlywood and latewood tracheids. The cell dimensions are used to predict cell porosity and water vapor permeability distribution within a growth ring. The bimodal frequency distributions of the tangential cell wall thickness and the radial lumen dimension provide an explanation for the observed bimodal frequency distribution of the cell water vapor permeability both in radial and in tangential directions. Contrary to measured macroscopic vapor permeability results, the tracheid geometry results in lower cell vapor permeability in radial than in tangential direction. This confirms that rays play an important role in the vapor permeability of wood, as they can be considered as pathways for vapor transport in radial direction. The dataset analyzed in this paper leads to a set of parameters characterizing the earlywood and latewood cell dimensions. Such characterization can be used, for example, for producing synthetic data for computational modeling studies.