Volume 64, Issue 6

The behaviour of wood components was studied during kraft cooking of Wiley milled Scots pine ( Pinus sylvestris L.) at low temperatures characteristic for the initial phase, but comprising alkali concentrations ranging from 0.31 mol l -1 to 1.55 mol l -1 . The results confirmed the extensive dissolution/degradation rate of galactoglucomannan (GGM) already in the beginning of a kraft cook applying standard conditions. More than 70% of GGM was eliminated along with lignin removal of approximately 30%. A relatively high GGM yield increase of almost 2.5% on o.d. (oven dry) wood has been observed when both the temperature and the alkali concentration was raised to 130°C and to approximately 1.5 mol l -1 , respectively, suggesting that stopping reactions were favoured over peeling reactions. Parallel to the pronounced stabilization of GGM, arabinoxylan experienced an additional loss of approximately 0.5–0.7% on o.d. wood, presumably owing to dissolution rather than alkaline degradation. A comprehensive kinetic model describing the alkaline degradation of GGM along with the lignin removal has been developed based on Wiley milled Scots pine raw material. The proposed kinetic model combines the alkali-dependent equilibria between ionized species of carbohydrate components with the time-dependent rate expressions of peeling, stopping, and alkaline hydrolysis. The model successfully describes the removal of GGM and lignin during initial kraft cooking. The kinetic expressions for the alkaline hydrolysis of carbohydrates were, however, less reliable owing to the limitation of the experiments to low temperatures only.

Changes in supramolecular properties of cellulose I, namely its lateral fibril aggregate dimension (LFAD), in bleached hardwood acid bisulphite pulp during drying was studied using cross-polarization/magic angle spinning carbon-13 nuclear magnetic resonance (CP/MAS 13 C-NMR) in combination with spectral fitting. A significant change in aggregate dimensions was noticed when each of the pulp grades were oven dried. The effect of drying was further investigated with pulp samples subjected to different drying methods. A comparison of a harsh oven drying, mild and rapid air drying, and a very mild and slow condition drying showed that the LFAD of the material decreases in the following order: oven drying > air drying > condition drying. The correlation between the total extractable material S 10 (%) and LFAD and also the LFAD increment (ΔLFAD in %) are presented and shown to be intimately related. This means that the method of drying influences the size of the fibril aggregate dimensions and depends on the presence of extractable material within the fibre cell wall. Reactivity studies were carried out based on the acetylation of cotton linters and commercial 96α pulp. Results indicate that the initial reaction rate is proportional to the specific surface area of the two cellulose pulp samples. Accordingly, the specific surface area is directly related to initial reactivity of the performed acetylation. We demonstrated that it is possible to control the LFAD and hence specific surface area in laboratory-produced pulps 91α, 92α, and 96α by the drying method. Thus controlling LFAD can probably be one viable route for controlling the initial reactivity of dissolving pulp towards acetylation.

The analytical potential of attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy has been tested on the following wood-plastic composites (WPCs): high and low density polyethylene (HDPE and LDPE), polypropylene (PP), polystyrene (PS), and a recycled plastic (rHDPE). The data set of ATR-FTIR spectra has been analyzed by principal component analysis (PCA) and the studied samples could be grouped according to their polymeric matrixes. Additionally, ATR-FTIR spectroscopy proved to be a useful tool for determining the distribution profile of wood and plastic materials within different types of WPCs. Accordingly, the plastic content of the surface layers of HDPE, rHDPE, and PP composites was significantly higher than that of the core layer, whereas homogenous dispersion was observed in the LDPE composite. Among all WPCs, the PS composite displayed the worst dispersion.

Three-part specimens were produced from Norway spruce wood ( Picea abies Karst.) and bonded with the following adhesives: melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF), and a two-component emulsion polymer isocyanate (EPI). The effect of alternating climate conditions on bond strength was studied by tensile tests. The specimens were exposed to a three-step ageing cycle lasting for 7 days [50°C/95% relative humidity (RH), -20°C/65– 70% RH and 75°C/15% RH] which was repeated 24 times. In general, a decrease in internal bond strength of all exposed specimens was observed but it was highest in the case of MUF-bonded joints. Furthermore, a significant decrease of the tensile strength of the wood adherend perpendicular to the grain in the tangential direction was determined after the cyclic climatic changes. The mechanical performance of the different adhesives in the bond line was tested by means of nanoindentation. Reduced values of elastic modulus, hardness, and total indentation were observed after climatic treatment, particularly for the rigid MUF adhesive, whereas the flexible adhesive EPI did not show such changes.

To improve the fundamental understanding of wood composite consolidation, the viscoelastic behavior of aspen wood strands is experimentally investigated for a range of temperatures (20–200°C) and moisture contents (0–15%). The results show that the strand stress relaxation modulus and time follows a linear relationship in a log-log plot. The strand stress relaxation rate is highly dependent on the imposed strain levels and the environmental conditions. A model for predicting the stress relaxation of wood strands is developed and compared with the experimental results. It will be useful to further predict the stress relaxation response of strand-based wood composite mats during hot pressing.

Previous studies were devoted to various applications of digital image correlation (DIC) to wood and wood-based composites. However, the focus of these studies was qualitative strain distribution. Overall, there is a lack of testing protocols of DIC for quantifying the elastic properties of woody materials. The objective of this study was to investigate the effects of different specimen thicknesses and loading rates on measurement of Young's modulus and Poisson's ratio by DIC. Young's modulus from DIC decreased as thickness increased at a loading rate of 0.254 mm min -1 . Comparing the different loading rates at a thickness of 0.794 mm, Young's modulus from DIC was not in agreement with the value obtained by means of the extensometer regardless of loading rate. However, Young's modulus from DIC at a thickness of 0.381 mm and a loading rate of 0.254 mm min -1 was in good agreement with the corresponding Young's modulus obtained by an extensometer. Poisson's ratio measured from different loading rates and specimen thicknesses was not significantly different between the two measurement systems. From the testing arrangement applied for this study, it is recommended that DIC should be applied at a loading rate of 0.254 mm min -1 or slower for strands with a thickness of 0.381 mm or less.

The mode I critical stress intensity factor K Ic for wood and medium-density fiberboard (MDF) was obtained by conducting the single-edge-notched tension (SENT) test, designed for isotropic materials, under various loading conditions, and performing subsequent numerical analysis on the test data. The validity of the SENT test for this purpose was examined by conducting double cantilever beam (DCB) tests and comparing test data with those of the SENT tests. The best values of K Ic are obtained by conducting the SENT test under pin loading and introducing an additional crack length into the equations used for analyzing the test data.

In an effort to advance the dynamic mechanical analysis (DMA) of very small biomass specimens, and/or specimens having poor mechanical integrity, the functional equivalent of pendulum-torsion (tensile-torsion) DMA was compared to parallel-plate compressive-torsion DMA. The solvent-saturated lignin glass transition in yellow-poplar ( Liriodendron tulipifera ) was generally similar determined by both modes; however, direct data comparisons should be avoided or carefully considered. First-heat glass transition temperatures ( T g s) were relatively similar; however, specimen densification elevated subsequent cooling-mode T g s by 5–8°C in compressive-torsion. Both modes revealed a first-heat tan δ shoulder; it was more prominent and had grain dependency in compressive-torsion. Below fiber saturation, subambient tensile-torsion DMA was superior; compressive-torsion resulted in an anomalous response, obscuring subambient secondary relaxations. With these differences and limitations in mind, compressive-torsion offers specific advantages. Solvent-submersion studies are simplified because solvent cups are easily devised for torsional rheometers. Specimens lacking mechanical integrity are more easily analyzed. Heavily biodegraded spruce ( Picea sp.) was analyzed in the solvent-submersion mode as fibrous mats and the different actions of Gloeophyllum trabeum and Postia placenta were revealed. Very small specimens are easily analyzed in compressive-torsion; tissue maturity effects were revealed in minute sections of switchgrass ( Panicum virgatum ) stems. Applied appropriately, parallel-plate compressive-torsion DMA will provide new research opportunities.

Moisture absorption and dimensional distortion are the major drawbacks of wood utilization as building material. In this study, poplar wood coated with a thin layer of titanium dioxide (TiO 2 ) was prepared by the cosolvent-controlled hydrothermal method. Subsequently, its moisture absorption and dimensional stability were examined. Scanning electron microscopy analysis revealed that the wood substrate was closely and entirely covered with the TiO 2 coat. Microscale features were visible despite masking of the ultrastructural features of the cell wall. To explore the effects of TiO 2 coating on the water-repellency and dimensional stability of wood, a 90-day water immersion test was carried out. Results showed that water absorption and thickness swelling of TiO 2 -coated wood increase very slowly and minimally. Weight change after 90 days of water immersion was reduced to 20.5%, nearly one-tenth of untreated control wood, and that maximum cross-sectional relative swelling was only 1.2%. Specimens were conditioned for 3 months at 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% relative humidity (RH) to determine the effects of RH on moisture absorption and dimensional swelling of TiO 2 -coated wood. There was no change in weight after 3 months of being exposed to humidity conditions below 60%, whereas there was linear weight increase above 60% RH, but the maximum change was less than 6%. Cross-sectional relative swelling was less than 0.3% below 60% RH but increased as RH exceeded 60%. The maximum change was approximately 3%. Anisotropic thickness swelling of wood was almost eliminated after coating. It is obvious that TiO 2 coating can act as a moisture barrier for wood and is an exceptionally strong water vapor-inhibiting shield under very humid conditions.

Two Malaysian hardwoods, acacia ( Acacia mangium Wild.) and sesendok ( Endospermum malaccense Bent. ex Müll. Arg.) that had been previously subjected to oleo-thermal modification, were studied to determine their dynamic sorption kinetic behaviour. All specimens were thermally modified in palm oil at 180°C, 200°C, and 220°C and 3 h treatment time. Data were analysed using the parallel exponential kinetics (PEK) model, and excellent fits to the experimental data were obtained. The relation between the monolayer water and polylayer water was also examined by the Hailwood-Horrobin (H-H) model at a range of relative humidity (RH) values. The PEK model divides the sorption kinetics curve into a fast and slow sorption processes. Characteristic times of the two processes at various RH showed sorption hysteresis with all of the specimens only in the slow process. However, with mass change data, sorption hysteresis occurred with both the slow and fast processes. By comparing H-H and PEK models, it was found that the fast and slow sorption process of untreated and treated E. malaccense could possibly be linked, respectively, to monolayer and polylayer formation from 5 to 40% RH. No such correlation was found for A. mangium , however.

To better understand the performance of bonded, coated, and modified wood, knowledge of how these processes alter the dimensional change and mechanical properties of wood at a given moisture content (MC) are important. These localized influences on earlywood (EW) and latewood (LW) properties are not well understood. In the present study, the influence of chemical treatments by hydroxymethylated resorcinol (HMR) and acetylation on moisture-induced dimensional change and longitudinal modulus of elasticity (MOE) of isolated EW and LW specimens of Loblolly pine ( Pinus taeda L.) was evaluated. The dimensional change was not altered by the HMR treatment, whereas acetylation lowered it by ∼50% in EW and LW in both radial and tangential directions. The MOE was not influenced by the two chemical treatments tested. Based on results of swelling, shrinkage, and MOE it can be concluded that chemical treatment does not modify EW selectively compared with LW neither in radial nor in tangential orientation.

Internal stress in wood as a result of kiln drying is an important factor that affects the quality of the final product. Internal stress is inherently difficult to measure and it is usually either assessed through destructive sample evaluation after drying, or is predictively modelled using extensive material property relations which require much effort to obtain. This study describes improvements of the design, accuracy, and precision of a previously constructed stress sensor for the direct measurement of internal stress in wood during drying. The sensor is designed to be placed directly into a wood block and it records online internal uniaxial stress. Lurethane ® was found to be the best material for transmitting internal stress to the sensor because reduced creep was observed, compared with Teflon ® which was originally used. Stress modelling showed that a linear relation could be theoretically obtained between the stress from the sensor and the actual primary axis stress that is likely to occur in an intact wood block. Accordingly, the response of a sensor placed in a certain area could be successfully calibrated. However, rotation of the sensor away from the primary stress axis caused a decrease in recorded stress but did not affect stress trends during drying, provided the angle remained constant. Finally, stress results from drying Pinus radiata D. Don at 70°C are also described and discussed.

Oriented strand board (OSB) panels were manufactured with different mixtures of pine and cypress heartwood and resins based on lignin or tannin to develop an eco-friendly wood composite with a natural durability against termite and fungi. Some physical properties and the major elastic moduli of bulk wood as well as of the manufactured panels were determined using different measurement techniques. In addition, a micromechanical model was adapted and validated with the experimental results. The good agreement obtained between the experimental data and model predictions indicates the proper assessment of the most influential parameters, such as raw material and adhesive properties, strand orientation, layer assembly and density profile. A parameter study, enlightening the effect of strand orientation on several elastic constants, enlarges the scope of experiments. We conclude with an optimal combination of resin and wood species mixture resulting in the best performance from a biological and mechanical standpoint.

The purpose of this study was to develop eco-friendly and high-performance glued-in-rod (GIR) joints using compressed wood (CW), which has higher tensile strength than normal hardwood, instead of conventional wooden dowels. To explore the bonding performance of the dowel and base material, punching shear tests were performed and relationships were established between the density of CW dowel and punching characteristics. Pull-out tests of GIR joints were performed and results were compared with those derived from a mechanical model to evaluate the influence of insertion length of dowel and to define its optimized length. The results indicate that CW dowel has almost the same bonding performance in the density range 330–1000 kg m -3 . Hence, CW as a dowel material in GIR joints has a high application potential. Pull-out tests of GIR joints showed that the insertion length of dowel in GIR joints with the CW-67 dowel is the best if the length is 10 times larger than its diameter. In addition to its satisfactory bonding performance, CW dowels have excellent tensile characteristics.

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