Volume 59, Issue 1

Strength properties and dimensional stability of medium-density fiberboard (MDF) panels made from black spruce ( Picea mariana [Mill.] BSP.) 0–20, 21–40, and over 40 year old fiber were studied. An analysis of covariance (ANCOVA) was performed to examine the differences in modulus of rupture (MOR), modulus of elasticity (MOE), and thickness swell (TS) of the three types of panels, while panel density was treated as a covariate in order to adjust the mean values that were partly attributed to panel density. The results indicate that MOR, internal bond (IB), and water absorption of MDF panels made from 0–20 year old fiber, which contained 100% juvenile wood, were significantly superior to those of panels made from 21–40 and over 40 year old fiber; but linear expansion (LE) of MDF panels made from 0–20 year old fiber was significantly larger than that of panels from the other two age classes. The differences in MOR, IB, water absorption, and LE between panels made from 21–40 and over 40 year old fiber were not significant. The comparisons of panel MOE and TS were relatively dependent on panel density due to existence of interactions among the three age groups.

Aspen ( Populus tremuloides ) is emerging as an important species for laminated veneer lumber (LVL) products in North America. During LVL manufacturing, both veneer stress grades and hot-pressing schedules are vital to product performance. In this study, an experimental design with four three-level variables comprising veneer moisture content (MC), veneer stress grade, platen pressure, and glue spread level was employed to investigate their effects on pressing behavior and stiffness and strength properties of LVL panels. The results show that, within the ranges studied, glue spread level and platen pressure were the two most important variables affecting the hot-pressing time needed for the innermost glueline to reach a target temperature of 105°C. The MC from the glue (mainly in the glueline) affected the rise of the innermost temperature more significantly than the MC in the veneer. Among the four variables studied, veneer stress grade and veneer MC were the two dominant variables that affect LVL stiffness and strength properties. Flat-wise and edge-wise bending stiffness (MOE) and strength (MOR) of LVL panels made from higher stress grade veneers were higher compared to those made from lower stress grade veneers, but there was no direct correlation between LVL shear strength and veneer stress grade. In addition, LVL edge-wise bending stiffness had the highest veneer MC tolerance among all LVL stiffness and strength properties. Further, LVL stiffness enhancement (the ratio of LVL MOE over veneer MOE) was lower with higher stress grade veneers than with lower stress grade veneers. These findings are useful for manufacturing high-stiffness LVL for engineered applications.

An experimental study on the internal stress state perpendicular to grain in glued laminated timber induced by moisture variations along the grain is presented. The stress is evaluated by using measured modulus of elasticity and strain. Different types of tests are performed, in which specimens are exposed to artificial climate change when the specimens are in either a wetting or a drying state, and specimens are exposed to a cyclic climate change. Test results from the specimens that were in a drying state show that the tensile stresses near the end of the specimens at some point exceeds the strength of the wood and cracks. In the opposite test, when the specimens are in a moistening phase, the maximum tensile stresses are lower and in the order of 0.2 MPa during the tested period. The cyclic test, which continued during 20 weeks, showed that there were no accumulated effects regarding strain and stress values. In this test, the average maximum tensile and compression stresses were of the order 0.20 and 0.10 MPa, respectively.

Wood grain orientation differences in the two surfaces to be bonded yield bondlines of different strength in no-adhesives wood welding. Longitudinal wood grain bonding of tangential and radial wood sections yields an approximately 10% difference in strength results of the joint. Cross-grain (±90°) bonding yields instead a much lower strength result, roughly half that observed for pieces bonded with the grain parallel to each other. These differences can be explained by the very marked effect that homogeneity of fibre orientation is known to have on fibre–matrix composites. Oak yields lower results than beech and maple and is more sensitive to welding conditions. Differences in both anatomical and wood constituent composition can account for this difference in performance. Contrary to the other wood species, oak always presents joint bondlines where little or no increase in density at the interface is noticed. This explains its somewhat lower strength results. This is based on the different mode of bonding predominant in this species, while the other species present two different modes of bonding. Thus, two types of bondlines are observed by scanning electron microscopy (SEM): (i) bondlines where entangled fibre–matrix composites are formed at the interface and (ii) bondlines in which direct welding of the cell walls occurs, just by fused intercellular material or cell surface material. In this latter case the cells remain flat, without an entangled fibre–matrix composite being formed. This is the almost exclusively predominant case for oak. Both cases and even hybrid cases between the two have also been observed in beech.

Retification is a heat treatment that decreases the swelling of wood and increases its resistance to fungal attack. In this study, differential scanning calorimetry (DSC) was applied in order to determine the fibre saturation point (FSP) of natural and retified wood. FSP values were used to determine the total swelling of natural and heat-treated wood. The DSC method was compared to the volumetric shrinkage approach. The influence of heat treatment temperature and duration on the swelling of wood was investigated. Relationships between chemical changes and the reduction of swelling were analysed thoroughly. The equivalence of the DSC method and the volumetric shrinkage method is shown. FSP in association with anhydrous density is a good indicator for the evaluation of the overall swelling of heat-treated wood. Reduction of wood swelling with increasing temperature and duration of thermal treatment is often attributed to hemicellulose destruction. This study shows that the reduction of beech wood swelling cannot only be attributed to the disappearance of adsorption sites that goes with hemicellulose destruction. It is suggested that other phenomena such as structural modifications and chemical changes of lignin also play an important role.

The methods so far applied to determine the bound water diffusion coefficient in wood do not provide credible results on this coefficient as well as on the boundary condition. An alternative approach based on the concept of solving the inverse transfer problems was recently applied. Two European species were investigated in the present study. A series of sorption experiments was performed and followed by the numerical identification of the coefficients. Several case studies were carried out for the constant and bound water content dependent diffusion coefficients. The obtained results were validated by comparison to a set of experimental data.

The fluidity of wood remarkably increases during moisture changes. This phenomenon is termed mechano-sorptive creep. The mechanism of mechano-sorptive creep has been studied, including a previous report by our group. Here, creep tests in bending were carried out for wood during and immediately after adsorption of moisture and after a long moisture conditioning. The effects of the rate of moisture adsorption on creep were also examined. The results and conclusions are as follows: (I) Greater creep occurred immediately after the adsorption process as compared with that after a long moisture conditioning, whereas much greater creep occurred during the same adsorption process, similar to the case of drying. Therefore, during the changes in moisture, not only destabilization but also stabilization should occur simultaneously, so that the wood during the changing process is in a remarkably unstable state. (II) Smaller creep occurred immediately after a slower adsorption as compared with that immediately after a more rapid adsorption. This suggests that wood is more stabilized during a slower versus a more rapid adsorption process. However, difference in creep between the final stages of the slower and the more rapid adsorption process was scarcely found. This is considered to result from the difference in degree of stabilization caused by the different duration of both adsorptions. In other words, this result is only an outward appearance. (III) Greater creeps were recognized during larger changes in moisture content (Δ u ) during the adsorption processes corresponding to the drying process. Therefore, mechano-sorptive creep depends not only on Δ u but also on the range of relative humidity (RH). Larger stabilization was found during the changing process of larger Δ u .

Basic physical properties including density, permeability, water sorption characteristics and diffusion coefficients in desorption were measured with specimens taken from for six aspen clone trees harvested in northeastern British Columbia. Results showed that there are significant differences among the clones in their specific permeability and somewhat in the heat of wetting, but very little was found among density, diffusion coefficients and desorption isotherms. These results indicate that the porous structure could be variable among clones whereas the sorption site distribution and availability might not.

The effect of bleaching with a hydrogen peroxide (H 2 O 2 ) solution on coloring of secondary xylem of kiln-dried birch wood ( B. pendula ) was investigated with CIELAB color measurements. Structure of unbleached and bleached wood pieces was studied by light microscopy (LM), environmental scanning electron microscopy (ESEM), and transmission electron microscopy (TEM). In addition, hardness and surface roughness of unbleached and bleached wood pieces were characterized with Brinell hardness and contact angle measurements. The results indicated that surface bleaching with H 2 O 2 solution changed the color of birch wood toward white and less red, simultaneously increasing the porosity and roughness of the uppermost surface as well as decreasing the hardness of bleached wood pieces. With embedding bleaching for 24 h with H 2 O 2 , the color of the wood pieces could be changed further; however, microscale defects were detected in fiber secondary cell walls. Consequently, complementary to results obtained from microscopic studies, lower hardness values supported the degradation of fiber secondary cell wall in bleached wood pieces. In addition, contact angle measurements indicated increased surface roughness of wood pieces after embedding bleaching with H 2 O 2 .

Dielectric-barrier discharge treatment was used to modify fully bleached softwood kraft and unbleached softwood thermomechanical pulp handsheets. The effect of dielectric-barrier discharge treatment on the physical strength properties and dimensional stability of handsheets were analyzed. Increased dielectric-barrier discharge treatment power resulted in significant increases in the wet tensile index of handsheets. Increases in the surface acid content of pulp fibers do not appear to contribute to changes in paper properties, nor do changes to relative bonded area within handsheets. Enhanced wet strength via dielectric-barrier discharge treatment corresponds to increased dimensional stability. This appears to be due to fiber cross-linking in the case of bleached kraft pulp, since the wet-strain decreases as wet tensile and wet Young's modulus increase. In the case of unbleached thermomechanical pulp, the wet-strain shows no decrease, yet the wet Young's modulus increases as the wet tensile index increases.

The survival of two hygienically relevant bacteria, Escherichia coli pIE639 and Enterococcus faecium , was followed on wooden sawdust of seven different European woods (pine, spruce, larch, beech, maple, poplar, and oak) versus polyethylene chips by using cultivation-dependent and molecular-based methods in parallel. The survival of the bacteria on wood was dependent on various factors such as the wood species, the type of the inoculated bacterium, the ambient temperature, and humidity. The bacterial titre decreased fastest on pine followed by oak compared to the other woods and plastic. Cultivation-independent analysis employing DNA extraction, Southern blot hybridisation, and PCR-based detection of marker genes of the test bacteria confirmed this result. The decline in bacterial numbers correlated with the decrease of bacterial DNA in the samples. Amounts of DNA of E. coli and E. faecium recovered from pine and oak-wood sawdust were generally lower compared to the other woods and plastic. The presented study shows that pine and oak exhibit substantially better hygienic performance than plastic and indicates an antibacterial effect caused by a combination of the hygroscopic properties of wood and the effect of wood extractives.

The relationship between copper absorption and density distribution in wood cell walls was investigated in this study. The density distribution on layer level was obtained from two approaches: (1) calculation by using data obtained from literature; (2) microdistribution of carbon and oxygen atoms in the wood cell. The microdistribution of carbon and oxygen in untreated southern yellow pine ( Pinus spp.) sapwood, as well as copper in cell walls of copper-ethanolamine (Cu-EA) treated wood was determined by scanning electron microscopy coupled with energy dispersive X-ray analysis (SEM-EDXA). Both approaches for density distribution led to the same result: the density was higher in the compound middle lamella and cell corners than in the secondary wall. The concentration/intensity of Cu, C and O in the cell wall follow the same trend as the density distribution; suggesting that density may play a major role in SEM-EDXA study of the distribution of metal-containing wood preservatives within the wood cell wall.

Polyphenols in heartwood, sapwood, and knots of Acacia crassicarpa and Acacia mangium were analysed using GC, GC-MS, HPSEC, and NMR. Melacacidin and isomelacacidin dominated in A. crassicarpa and teracacidin and its ketone in A. mangium . Also, bi- and triflavonoids were tentatively identified in the samples. The amounts of polyphenols were larger in the knots than in the heartwood of both species, while the composition within the species was the same.

This paper describes a novel technique to produce cellulose microfibrils through mechanical methods. The technique involved a combination of severe shearing in a refiner, followed by high-impact crushing under liquid nitrogen. Fibers treated in this way were subsequently either freeze-dried or suspended in water. The fibers were characterized using SEM, TEM, AFM, and high-resolution optical microscopy. In the freeze-dried batch, 75% of the fibrils had diameters of 1 μm and below, whereas in the water dispersed batch, 89% of the fibrils had diameters in this range. The aspect ratio of the microfibrils ranged between 15 and 55 for the freeze-dried fibrils, and from 20 to 85 for the fibrils dispersed in water. These measurements suggest that the microfibrils have the potential to produce composites with high strength and stiffness for high-performance applications. The microfibrils in water were compounded with polylactic acid polymer to form a biocomposite. Laser confocal microscopy showed that the microfibrils were well dispersed in the polymer matrix.