Volume 58, Issue 5

Relationships between basic tree and wood properties, and species, seed source, geographic location, site conditions and management decisions are very complex. The objective of this study was to quantify the effects of forest management practices on wood density, fibre and pulp properties in jack pine, one of the most important commercial species in Northern America. A better understanding of the relationship between initial spacing and wood and end-product quality should help define improved forest management strategies required to produce quality wood and products in the future. On the basis of the oldest jack pine initial spacing trial established in 1941 by the USDA Forest Service, this study examined the impact of four different initial spacing trials on tree growth, wood density, fibre and pulp properties of jack pine. The results clearly show that initial stand spacing has a significant effect on all of these properties, and thus it is possible to improve yield and wood and pulp fibre properties of jack pine through stand density regulation. Additionally, a positive effect of pre-commercial thinning on fibre properties was also demonstrated. As a consequence of these results, basic prescription information for decision-making in the establishment of jack pine plantations with desirable pulp properties can be elucidated.

Residual lignin preparations were isolated from birch, aspen and Eucalyptus grandis kraft pulp by enzymatic hydrolysis of the pulps with cellulase:hemicellulase mixture. Residual lignin preparations were characterized by investigation of nitrogen content, carbohydrate composition and molecular mass distribution. The use of enzyme with high activity and optimization of enzyme charge resulted in significant decrease in protein contaminants in residual lignin preparations as compared to previously published results. A second order law correlation between enzyme mass charge and nitrogen content in birch residual lignin preparations indicates a strong effect of enzyme charge on the amount of protein contaminants. However, the enzyme charge in the range studied does not appreciably affect either the yields of the residual lignin preparations or percentage and composition of carbohydrates in these preparations. The optimal enzyme charge was highest for Eucalyptus grandis pulp and lowest for birch pulp. It has been suggested that a significant part of the hardwood residual lignin in pulps, especially in E. grandis pulp, consists of low molecular mass lignin fragments bonded to pulp carbohydrates. Higher amount of glucose in E. grandis residual lignin preparation compared to those from birch and aspen implied a higher frequency of lignin-cellulose bonds in eucalypt pulp. Different fractions of birch residual lignin have rather similar molecular mass distribution, which was not affected by the charge of the enzyme. Eucalypt residual lignin preparation had higher molecular mass than birch residual lignin.

The microscopic changes in the structure of paper at different equilibrium moisture contents were examined using the novel technique of contrast variation small angle neutron scattering (SANS). Contrast variation, by appropriate selection of the H 2 O:D 2 O ratio, was used to simplify the scattering from hydrated paper to a two-phase approximation. The two phases were paper polymers (cellulose, lignin and hemicelluloses) and a phase consisting of water and voids. The two-phase approximation is justified by consideration of the chemistry and density of each of the polymer species belonging to paper, and by matching the scattering length density of the sorbed moisture to air. The neutron intensity at the higher scattering angles examined was successfully fitted using the Porod law, which is applicable to scattering from a two-phase system at the high scattering angle limit. This limit is related to the specific surface area between the two phases. The limiting scattered intensity of neutrons was found to increase with increasing moisture content for a range of paper materials. It is concluded that the sorption of water increased the limiting intensity because water disrupted polymer-polymer bonding. The Porod law also predicts the slope of the intensity/scattering angle curve at the high scattering angle limit.

In a comparative study, the topochemical distribution of lignin in individual cell wall layers of beech wood tissue was determined by confocal Raman spectroscopy and scanning UV microspectrophotometry. The Raman technique was additionally applied to the determination of cellulose content in individual wall layers. In good agreement, the two methods showed maxima of lignin distribution in middle lamellae and cell corners, along with a minimum of cellulose content.

Spruce and birch earlywood was acetylated to different weight percent gains using three different acetylation procedures. The absorbance spectra of secondary cell wall and compound cell corner middle lamella were determined by means of UV microscopy. Analysis of the spectra showed that the characteristic lignin absorbance peak in the UV spectrum of wood around 280 nm shifted to shorter wavelengths in acetylated samples. A distinct relationship between achieved weight percent gains after acetylation and observed spectral shifts could be established revealing a certain potential to measure acetylation on a cellular level by means of UV microscopy.

The dynamic method for monitoring ionisation of the lignin functional groups in a 0.1 N NaOH solution at ambient temperature by means of the SEC technique with multiwave UV detection was performed. An increase in absorbance at 210, 278, 304, 320 and 350 nm was examined to reveal the ionisation dynamics in respect to functional groups’ acidity; that is, carboxyl groups, carbonyl- and double-bond conjugated phenol hydroxyls. A distinctive response of absorbance at 210 and 278 nm on the ionisation of carbonyl- and double-bond-conjugated phenols was found. Simultaneous characterisation of lignin by molecular weight (MW) and chemical heterogeneity was performed. A non-uniform distribution of reactive sites over the MW distribution for commercial kraft lignin (Curan) was found employing the family of chemical composition distribution curves. The redistribution of extended chromophore structures over time along the lignin’s MW distribution causes a misrepresentation of the UV chromatogram pattern and influences the accuracy of MW values.

A three-dimensional network of intercellular spaces, deemed an extracellular diffusion pathway for heartwood substances, was detected in Albizia julibrissin . Electron microscopy revealed a large number of blind pits in ray parenchyma cells, most facing intercellular spaces. Heartwood substances, which are synthesized in the ray parenchyma cells, are released not only into neighboring cells through pit pairs, but also into the intercellular spaces through the blind pits. There were two types of wood fiber in the heartwood region: one with a lumen surface lined with heartwood substances and one that lacked such lining on its lumen surface. This finding supports the observation that the cellular distribution of heartwood substances is not homogenous throughout the heartwood. There was no positive correlation, however, between this uneven distribution and the distance of a wood fiber from the ray.

The results of non-contact measurements of local in-plane elastic anisotropy in wood by monitoring the flexural plate wave velocity as a function of azimuth angle of propagation are reported. The Focused Slanted Transmission of air-coupled ultrasound is used to generate and detect locally the flexural waves in wood as well as to measure their velocities. The analysis presented shows that for thin plates the flexural wave velocity can be readily used to evaluate the anisotropy of Young’s modulus. This conclusion is confirmed by measurements of the plate wave anisotropy factors for veneer laminae and cross-ply laminates of spruce and beech. The technique is sensitive enough to detect the anisotropy variation of earlywood caused by the incorporation of latewood areas in growth rings. A new approach based on dynamic nonlinearity of wood is developed and applied to acoustic imaging of wood structure and detecting of defects in wood. The hysteresis mechanism of the dynamic nonlinearity is shown to dominate in clear wood and to manifest in a primary generation of odd acoustic harmonics. Local maxima of the odd higher harmonic amplitudes in the LR- and LT-planes of softwood are observed in the earlywood area close to latewood/earlywood transition interface that indicates the lower stiffness and strength of wood in those areas. The higher-order even harmonics and subharmonics in the nonlinear vibration spectra of wood are mostly produced by “clapping” in defect areas. Measurements of local amplitudes of these modes are applied to nonlinear acoustic imaging of cracks, delaminations and knots in wood and wood composites.

The chemical modification of maritime pine sapwood ( Pinus pinaster ) with alkoxysilanes was studied according to three different pathways: carbamoylation with 3-isocyanatopropyltriethoxysilane, etherification with 3-glycidoxypropyltrimethoxysilane and alcoholysis of n-propyltrimethoxysilane. Grafting was confirmed by weight percent gain calculations (WPG), infrared spectroscopy (FTIR) as well as 13 C and 29 Si NMR CP MAS analysis. Signals of the grafted groups in the different spectra were assigned and the reactivity of the trialkoxysilane moieties towards wood was discussed. Experiments with model wood blocks showed that the reactions investigated occurred within the wood cell walls. Grafted chemicals were found to be relatively stable with regard to water leaching but only slight dimensional stabilisation was noted after treatment.

Chemically modified wood composites were obtained via the compression moulding of thermoplasticised Aspen ( Populus tremula ) sawdust. This sawdust was previously prepared by esterification with maleic anhydride (MA) and subsequent oligoesterification with maleic anhydride and glycidyl methacrylate (GMA). The thermoplastic properties of the chemically modified wood resulting from different modification procedures were confirmed and compared by compression-moulding experiments leading to preliminary and final products. An SEM study of the resulting products clearly showed that the oligoesterified wood had partially melted under pressure and temperature, such that the overlapping and surface melting of particles ensured adhesive bonding between those particles. A new type of wood/thermoplastic-wood composite was obtained. In these composites, the melted part of the modified wood plays the role of the cohesive matrix whilst none-melted wood remains as a fibrous reinforcing material. FTIR spectra suggested that changes in the chemical structure of the modified wood are possible during the thermal forming process (e.g. polymerisation of C=C double bonds). The final composites were yellowish-brown, glossy, plastic-like products that showed interesting physical, mechanical and biological properties. They are water-resistant and dimensionally stable and display good electrical insulating behaviour. Their mechanical properties (bending strength of ca. 64 MPa and tensile strength of ca. 36 MPa) are in the typical range for plastics and conventional wood-fibre/plastic composites, and are superior to common wood products such as fibreboards and particleboards. Furthermore, the outstandingly high internal bond (ca. 3.0 MPa) highlights the totally different adhesion mechanism operating in these new types of composites. Although the novel composites are much more resistant to decay than the original unmodified wood, they remain ultimately biodegradable plastic-like composites.

Bamboo fiber-filled poly(lactic acid) (PLA) eco-composites were prepared by mechano-chemical compositing with bamboo fiber (BF)-esterified maleic anhydride (MA) (BF-e-MA) in the presence of dicumyl peroxide as a radical initiator. Tensile properties of the composites were improved by adding BF-e-MA and the dicumyl peroxide. A sufficient effect of the addition on the tensile property of the composites was observed even in the presence of the dicumyl peroxide of 0.25%. However, the composites with BF-e-MA usually showed a higher activation energy for thermal flow, indicating that their flow became more difficult, because molecular motions were rather suppressed by cross-linking of BF-e-MA with the PLA matrix resin. The crystallization temperature of PLA became higher by the addition of BF and BF-e-MA in non-isothermal crystallization. It was confirmed by scanning electron microscope and polarizing microscope observation that interfacial properties between BF and PLA were improved after the addition of BF-e-MA. A diffusion coefficient (D) of water in the PLA/BF composites with BF-e-MA was smaller than that in the composites without BF-e-MA. An equilibrium water-sorption amount (M ∞ ) was higher in the order of composites without BF-e-MA>composites with BF-e-MA>pure PLA.

Effects of bamboo fiber (BF)-esterified with maleic anhydride (MA) (BF-e-MA) on the properties of BF-filled poly(butylene succinate) (PBS) bio-composites were investigated. Adding 5% BF-e-MA increased the tensile strength of the PBS/BF (70/30) composite from 21 MPa to 28 MPa in the presence of radical initiator (0.5%). Melt viscosity was also increased by adding BF-e-MA. The Avrami and Ozawa equations were used to analyze the isothermal and nonisothermal crystallization kinetics, respectively. At the same crystallization temperature, all composites showed lower values for the Avrami and Ozawa exponents than pure PBS. Furthermore, the values for the PBS/BF-e-MA composite were smaller than those for the PBS/BF composite. Equilibrium water-sorption amount ( M ∞ ) were highest for the composite without BF-e-MA, followed by the composites with BF-e-MA and finally pure PBS. The diffusion coefficient ( D ) was smaller for the composites with BF-e-MA than without. Scanning electron microscopy revealed that interfacial adhesion between BF and PBS matrix was improved after addition of BF-e-MA.

Helium pycnometry was used to determine the changes in volume of wood samples of Corsican pine following modification with acetic anhydride. Results were compared with changes in volume determined from external dimensions only. Percentage change in volume, as determined from external dimensions of the sample, showed a non-linear relationship with weight percentage gain as a result of acetylation; whereas percentage volume change determined by helium pycnometry showed a linear relationship. The difference in volume as determined by the two methods is explained by considering volume change of the lumens as level of cell wall substitution increases. Results obtained in this study show that volume increases due to chemical modification, when determined by measurement of external dimensions, should not be used to evaluate changes in cell wall volume.

The release of formaldehyde from plywood has been greatly reduced by treatment with microwave radiation. Formaldehyde emission in the air was measured before, during, and after 2.45-GHz microwave treatment. The concentration of formaldehyde released increased with microwave irradiation and, after the microwave treatment, decreased to a level below that of nontreated material. Microwaves were effective in releasing free formaldehyde from the plywood, which is preferable to it being emitted as “potential formaldehyde” via hydrolysis. Microwaves released formaldehyde from plywood samples more effectively compared to samples subjected to thermal energy from external heating. This suggests that microwaves directly activate free formaldehyde molecules, which have a polarity that is susceptible to microwaves. Microwave treatment is expected to be an effective way to reduce free formaldehyde emissions from wood-based materials containing resins.

Changes in the wettability of eight species of tropical woods during artificial weathering up to 600 h are discussed from the aspect of chemical and structural changes in their surfaces: Amnurana acreana , Acacia auriculiformis , Dipterocarpus spp., Eucalyptus marginata , Eucalyptus robusta , Shorea spp., and Tabebuia spp. with relatively high and low specific gravity. On the whole, the wettability of specimens decreased upon irradiation up to 20 h; above that they increased. Changes in wettability during artificial weathering differed according to wood species. The IR spectra suggest that the specimen surfaces after irradiation for 600 h result in a cellulose-rich layer, and therefore the increase in wettability during artificial weathering can be explained in terms of the increase in hydroxyl groups originating from both the exposed cellulose and adsorbed water. However, the difference in wettability exists between species even after the surface develops a cellulose-rich layer. The stereoscopic micrographs showed the development of cracks for all of the specimens after irradiation for 600 h, and differences in their magnitudes according to species. From these results, the differences in wettability between species were estimated to be due to the structural changes on the surface during artificial weathering, whereas the increase in wettability was due to the chemical changes.

This paper describes the change in diffuse reflectance Fourier transform infrared (DRIFT) and ultraviolet-visible (UV-Vis) diffuse reflectance spectra of the following eight tropical woods during artificial weathering up to 600 h in relation to their color changes, especially to yellowing: Amnurana acreana , Acacia auriculiformis , Dipterocarpus spp., Eucalyptus marginata , Eucalyptus robusta , Shorea spp. and Tabebuia spp. with relatively high and low specific gravity. For A. acreana , A. auriculiformis , Dipterocarpus spp. and both Tabebuia spp., Δb* (yellowing) increased with exposure up to 50 h, and decreased above 50 h. For E. marginata , E. robusta and Shorea spp., on the other hand, both Δa* (shift to red) and Δb* decreased with increased exposure time. For woods in which Δb* increased, the Δb* showed a positive dependence on the difference in relative intensity ratio of a band at 1740 cm –1 to that at 2900 cm –1 (ΔD 1740 /D 2900 ) in DRIFT spectra of specimens before and after exposure. This result indicated that the increased band at 1740 cm –1 played a significant role in the increased Δb*. Then again, also for woods in which Δb* decreased, the ΔD 1740 /D 2900 increased, but was not related to the Db*. The ΔD 1740 /D 2900 for woods in which Δb* decreased had a positive relationship to the difference in remission function at 410 nm (Δ F (R ∞ ) 410 ) in UV-Vis diffuse reflectance spectra for specimens before and after exposure, while woods in which Δb* increased were independent of the Δ F (R ∞ ) 410 . Therefore, it is suggested that woods in which Δb* increased and decreased differ from one another in the contribution to the increase in the band at 1740 cm –1 , resulting in either an increase or decrease of Δb*.