Volume 74, Issue 9

Nondestructive testing (NDT) includes several highly efficient techniques for the estimation of the physical and mechanical properties of structural timber. However, NDT results are affected by several factors [moisture content (MC), temperature, specimen dimensions, sensor positioning and grain angle, timber-sensor coupling, etc.], of which wood MC is the most important. Scientific research on MC influence in NDT measurements started in the 19 th century, and 2018 was the first year that a MC adjustment for NDT measurements was published in a European standard (EN14081-2). Although MC influence on NDT has been studied for more than 170 years, MC adjustment factor research is important nowadays because it is widely used in the industry. Currently, NDT device manufacturers develop their own MC adjustment factors adapted to their equipment and species. This paper presents an exhaustive review of MC adjustments for different types of NDT equipment, as proposed by several authors from the 19 th century to date, and a discussion of factors affecting MC adjustments.

Vascular parenchyma cells (VPCs) provide a critical metabolic and energetic link for xylem transport of water and mineral nutrients and phloem transport of photoassimilates. Understanding the morphology of these cells is required to understand their function. This study describes the morphology and classification of VPCs of moso bamboo by light microscopy (LM) and scanning electron microscopy (SEM). The key results revealed that vascular parenchyma could be divided into three categories according to cell geometric morphology: cells with two transverse end walls, cells with one oblique end and one transverse end walls, and cells with two oblique end walls. Additionally, there were two types of thickening patterns of the secondary wall, uniform and reticulate thickening, and both diffuse pitting and opposite-alternate pitting were observed. The average length, width, lumen diameter, double wall thickness, and area of the VPCs were 139.0 μm, 17.3 μm, 10.4 μm, 6.9 μm, and 51.1 μm 2 , respectively. Most VPCs were slender and thin-walled, and growth of the VPCs was not correlated in either the length or the width directions.

Time-domain nuclear magnetic resonance (TD-NMR) is widely used in the investigation of wood-water relationship. However, some ambiguities between the NMR signals and the components in wood remain unresolved, particularly the effect of pine resin on NMR signals. To clarify these ambiguities and increase the use of TD-NMR in wood research, different sample treatment methods were studied, including air-drying, low-temperature vacuum-drying, diethyl ether extraction and moisture isothermal adsorption. The corresponding one-dimensional (1D) T 1 , T 2 and two-dimensional (2D) T 1 -T 2 correlation relaxation time distributions of radiata pine and Douglas fir samples were investigated. The NMR signals accounted for “longer relaxation-time components” below the fiber saturation point (FSP), but overlaped in parts of the 1D relaxation time distributions making it difficult to distinguish between pine resin and moisture. The 2D T 1 -T 2 correlation relaxation time distributions produced a better distinction between pine resin and bound water. This distinction established a quantitative relationship between pine resin, moisture and 2D NMR signal amplitudes.

Mahogany wood ( Swietenia macrophylla King) was thermally modified by heating at various temperatures. The wood-water-related parameters of the heat-treated wood, including fiber saturation point, equilibrium moisture content, moisture excluding efficiency, hygroscopic hysteresis, swelling, anti-swelling efficiency, water adsorption, and surface wettability were determined to clarify the mechanism of heat treatment to reduce wood hygroscopicity. The wood treated at a higher temperature demonstrated a more significant decrease in hygroscopicity. The reduction in hygroscopicity of the heat-treated wood was partially diminished as the moisture excluding efficiency decreased in the absorption and desorption processes caused by the changes in environmental temperature and relative humidity. The fiber saturation point (determined by nuclear magnetic resonance spectroscopy), surface free energy, and the surface wettability of wood were reduced by heat treatment, resulting in the decreased hygroscopicity of the heat-treated wood. Mahogany wood became more insensitive to the influence of moisture due to the heat treatment, and this effect was more distinct for wood treated at a high temperature.

It is generally assumed that the properties of wood against fatigue are good, but little is known about the properties of adhesively bonded wood, which represents today most of the wood-based products. Lap-shear samples glued with three common wood adhesives [two ductile one-component polyurethane (1C-PUR) systems and one brittle phenol resorcinol formaldehyde (PRF) adhesive] were tested under cyclical loads at three different climates [20°C, 35% − 50% − 85% relative humidity (RH)]. For the analysis of data, an empirical model based on reaction kinetics was developed. In addition, a probabilistic model was used to estimate the endurance limit and the expected run-out lifetime. Both models were combined to accurately model fatigue at high and low relative stress intensity. It was shown that ductile 1C-PUR adhesives perform better than the brittle adhesive system under dry climates (35%–50% RH). However, for higher RH, the brittle PRF adhesive showed better performance, most probably due to a better wood-adhesive adhesion in wet climate. An average endurance limit for tensile shear stresses between 20% and 48% of the mean tensile shear strength (TSS) was estimated for the tested adhesives. It was shown that the model parameters could be linked to fundamental physical constants through the reaction kinetics approach; however, further research is needed to correlate these parameters to specimen-specific quantities.

In this study, the effect of antisolvents on the structure of regenerated microcrystalline cellulose (MCC) obtained from the extraction of 1-butyl-3-methylimidazolium chloride (BmimCl) was investigated; further, the usage of the aqueous N , N -dimethylmethanamide (DMF) solution was proposed as an effective antisolvent for cellulose regeneration. The results denoted that regeneration after dissolution resulted in a looser cellulose texture with a high specific surface area, low degree of polymerization (DP), low crystallinity index (CrI), and decreased thermostability, which are favorable for its downstream processing. Among the studied antisolvents, the DMF solution was superior in cellulose regeneration from BmimCl, as demonstrated by the kinetics of enzymatic hydrolysis. The improved ability of the DMF solution with respect to cellulose regeneration can be attributed to the effective dispersion of H-bonds and the inductive hydrophobic orientation of cellulose chains; correspondingly, a looser H-bond network was observed in the regenerated cellulose. The DMF solution as an antisolvent offers an effective cellulose regeneration method and an optimal structure for subsequent processing and applications.

As pH of leaching medium is an important factor in the leaching of wood preservative components, its effects on leaching should be quantified to ensure environmentally safe use of treated wood. In this study, the effects of pH on leaching of copper from wood treated with copper amine-based preservatives [alkaline copper quat (ACQ)-2, bis-(N-cyclohexyldiazeniumdioxy)-copper (CuHDO)-3, and copper azole (CUAZ)-3] were evaluated in comparison with wood treated with chromated copper arsenate (CCA)-3. Radiata pine sapwood blocks treated with these preservatives were leached at five pH levels (3.0, 3.5, 4.0, 4.5, and 6.5). The leached blocks were subjected to laboratory-scale decay tests using two brown-rot fungi. The blocks treated with copper amine-based preservatives leached significant amounts of copper at pH levels below 4.0. At all pH levels, the CuHDO-3-treated samples generally leached the most copper, followed by the samples treated with ACQ-2, CUAZ-3, and CCA-3. When the treated blocks were leached at pH 3.0, the degradation of hemicelluloses, which can chemically adsorb copper, was confirmed through Fourier transform infrared attenuated total reflectance (FTIR-ATR) analysis. Moreover, X-ray photoelectron spectroscopy (XPS) analysis indicated that the ratio of precipitates of the remaining copper in the treated wood severely decreased after leaching at pH levels below 4.0. Subsequent reduction in the biological effectiveness of wood treated with copper amine-based preservatives was not hardly observed after leaching at pH levels 4.0 or above. These results indicate that copper loss at pH levels 4.0 or above is not great enough to cause public concern about environmental problems and reduction of biological efficacy in practical applications.

Wood exhibits very different behavior and properties at different scales. One important scale is the cell wall (CW) that is commonly tested by nanoindentation. Common nanoindentation provides important insight into the material but has limitations because it does not apply uniaxial stress and provides data from single spots. Therefore, the aim was to examine beech CW using two state-of-the-art techniques: micropillar compression (MCo) and nanoindentation mapping (NIP). The mean strength of the beech CW was found to be about 276 MPa and the mean yield stress was 183 MPa. These values were higher than those in most cited literature, which was attributed to the fact that libriform fibers from beech late wood were measured. Mean E obtained from MCo was about 7.95 GPa, which was lower than the values obtained on a macrolevel and about 61% of the value obtained from NIP. NIP also showed that E of the CW around the middle lamella (ML) was about 64% of the value at the location attributed to the S 2 layer. Lower E from MCo may be caused by sinking of the micropillar into the wood structure under the load. Failure of the micropillars showed gradual collapse into themselves, with debonding at the S 3 layer or the MLs.