Volume 76, Issue 11-12

Some properties of wood, such as maximum moisture content, cell-wall density and porosity, are not well known, even though they affect the performance of chemical preservatives, glues and coatings on wood. This knowledge gap was addressed in the present study by analysing these physical properties in laurel ( Laurus nobilis L.) wood. Laurel is a common hardwood tree in southern Europe. Seventeen laurel trees were felled for the study, and 300 defect-free specimens were obtained from the trees for analysis. The following mean values were obtained: wood maximum moisture content, 114%; cell-wall oven-dry density, 1198 kg m −3 ; and oven-dry wood porosity, 45%. Significant inter- and intra-tree variations in the three properties were observed. The inter-tree variation was mainly attributed to the tree age, and the trend suggests that wood maximum moisture content and porosity are expected to be lower in older trees than those under study. The values of the three properties were slightly, but statistically significantly, lower at the highest positions in the tree. Harvesting of the basal logs of young trees is therefore advisable only if more porous wood is required, and commercial exploitation of the whole trunk of mature trees is recommended if more compact wood is required. The variation in cell-wall density was not negligible, although this variable is often assumed to be approximately constant for all wood species. Maximum moisture content and wood porosity can be estimated using bulk or apparent density as a predictor variable.

Understanding wood pore geometry and distribution is the basis for studying its macroscopic properties. This research investigated the pore fractal dimension using the pore size distribution of selected softwoods and hardwoods. Mercury intrusion porosimetry explored the detailed structural parameters of wood pore size distributions and assessed their fractal dimension. The results revealed significant variability in pore size distribution, porosity, pore volume, and fractal dimension values. The threshold pressure extracted from porosimetry data can be used as the main parameter to distinguish the pore size distribution regions. Pore sizes ranged from 3 to 35,000 nm, with a corresponding porosity that ranged from 58 to 76%. Three pore size classes were determined and ranged as: macropores (radius 350,000–5000 nm), mesopores (radius 5000–100 nm), and micropores (radius 100–3 nm). The fractal dimension values in the corresponding macropore, mesopore, and micropore size intervals were 2.98–2.998, 2.6–2.92, and 2.53–2.72, respectively, indicating a higher degree of complexity for larger pores.

Solid birch wood ( Betula pendula ) was investigated with a newly developed ultrasonic resonance test set-up enabling accelerated fully reversed fatigue experiments of wood. Ultrasonic fatigue lifetimes were measured at a cycling frequency of about 20 kHz up to 10 9 load cycles. For comparison, servo-hydraulic fatigue tests at 50 Hz cycling frequency up to 5 × 10 6 were performed. Numbers of cycles to failure plotted against stress amplitudes determined at both frequencies show comparable slope and standard deviation in the regime of overlapping lifetimes. Birch does not show a fatigue limit. Failures still occurred up to 10 8 load cycles at stress amplitudes of 30 MPa corresponding to approximately 35% of the ultimate compressive strength. Working principles and control mode of the ultrasonic fatigue test set-up are presented. A modal analysis of excited resonance vibration of the employed load train has been performed by the finite-element-method. Furthermore, the process of determination of applied strain and stress amplitudes was verified by comparative measuring techniques using a contact extensometer.

To investigate the changes in the pore structure of earlywood and latewood (EW and LW) in larch during the heat treatment process, this study applied nitrogen adsorption and mercury intrusion porosimetry (MIP) to measure wood pore characteristics. Wood samples were heat treated within a vacuum atmosphere at temperatures between 180 and 220 °C for 6 h. Analysis conducted on nitrogen adsorption isotherms indicated that the micropores and mesopores in the cell walls were present in both untreated and heat-treated EW and LW, and that the mesopores appeared as slit-shaped pores. More pores were detected in EW compared to LW, and these primarily absorbed in the range of 1.5–4 nm. Compared with the untreated wood, the total pore volume of treated wood decreased with increasing temperatures, with decreased pore volumes at 220 °C of 63 and 42% for EW and LW, respectively. The MIP results showed a greater porosity of EW compared to LW. Compared with the untreated wood, the porosity of heat-treated EW and LW first increased with increasing temperature, and then decreased at 220 °C. For treatment temperatures lower than 200 °C, heat treatment increased the proportion of macropores with pore sizes in the ranges of 100–1000 nm and 10,000–40,000 nm.

Thermal modification or heat treatment can cause the loss of mechanical property of wood. In this study, Poplar ( Populus tomentosa Carr.) and spruce ( Picea obies Mast.) were heat treated at 180, 200, and 220 °C for 2–10 h. Changes of color ( L *, a * and b *) and mechanical strength including modulus of elasticity (MOE), modulus of rupture (MOR) and shear strength after heat treatment were analyzed. Time-temperature superposition methods were used to quantify color and mechanical strength. The prediction models of MOR, MOE and shear strength were assessed with support vector regression model (SVR) based on color parameters. The trends of color change and mechanical strength after heat treatment were highly consistent. The values of apparent activation energy ( E a ) calculated from color parameters (110.6–187.2 kJ/mol) were identical to those from mechanical strengths (103.2–219.2 kJ/mol). Color parameters were used as input variables, and the MOE, MOR, and shear strength were output parameters in the established SVR model. Gaussian radial basis function (RBF) was found to be a kernel function for SRV model. Optimal hyperparameters in SVR model were obtained using cross-validation and grid search. The determination coefficients for MOE, MOR, and shear strength were 0.903, 0.835, and 0.865, respectively for poplar. The high correlation suggested that wood mechanical strength can be predicted non-destructively through measuring color parameters after heat treatment.

By regulating preheating time, effects of water/moisture immigration and distribution in wood on sandwich compression formation were investigated in this study. Kiln-dried poplar wood was first immersed in water to result in high moisture content (MC) layers on wood surfaces. These wood specimens were then conditioned at room temperature in sealed plastic bags and preheated with hot press platens at 180 °C to drive water/moisture into wood. Wood preheated for 10–600 s contained one to two high MC layers all the time. Extended preheating time moved the high MC layers from wood surfaces to the center. Mechanical pressure on preheated wood specimens resulted in sandwich compressed wood with the compressed layer(s) position(s) consistent to that of the high MC layer(s) before compression. The positions of compressed layer(s) and MC peak(s) both increased exponentially as a function of the logarithm of preheating time. Wood preheating by platens led to water/moisture migration and therefore controlled water/moisture distribution, thus promoting the softening of specific wood layer(s), which is mainly responsible for sandwich compression. A logarithmical model that can be used for predicting the position(s) of the compressed layer(s) was developed.

As a renewable biomass material, wood is often deteriorated by decay fungi. Quercetin was here grafted onto poplar ( Populus nigra ) using oxalic acid as a linker through vacuum-pressure treatment and esterification with various molar ratios of quercetin to oxalic acid (Q/O) to enhance the wood decay resistance. Decay resistance, flexural strength, graft mechanism, and microscopic structure of the quercetin-grafted wood were analyzed. Results indicated that the decay resistance of quercetin-grafted wood to white-rot fungi ( Trametes versicolor ) and brown-rot fungi ( Gloephyllum trabeum ) was significantly improved. The modulus of rupture (MOR) of the quercetin-grafted wood was reduced while the modulus of elasticity (MOE) was improved. Quercetin was grafted onto wood through formation of ester groups among oxalic acid, wood, and quercetin. The highest amount of ester groups in the grafted wood was achieved when Q/O was 1:2 and Q/O lower than 1:2 would severely degrade the wood. The thickness of cell walls of the quercetin-grafted wood increased and reached a maximum of 2.2 μm with Q/O of 1:2. Quercetin can be grafted on wood cell wall by oxalic acid as an environment-friendly preservative to enhance the decay resistance of wood.

Improving the quality of cellulose semi-finished products for subsequent chemical processing in combination with the “green” concept is an important technical challenge. The article investigated a number of organic solvents as a delignifying agent. It has been shown that DMSO usage is the most efficient organosolv way to obtain cellulose from birch wood ( B. pendula ). The influence of the concentration of the organic solvent in the cooking liquid, the concentration of the acid catalyst, the duration and temperature of the process has been studied. The conditions for maximum process desirability at which the highest yield of cellulosic semi-finished product (CSP) and degree of delignification were as follows: 2 h treatment time at 156 °C, DMSO/water 75–95% solvent system with solid to liquid ratio of 1:10. At a concentration of DMSO in the cooking liquor of 85–90%, the degree of wood delignification was about 95%. The mechanical properties of the experimental DMSO pulp were similar to those of kraft pulp. DMSO lignin was characterized by Pyr-GC/MS. More than 65% of monomer fragments of phenolic compounds were S-unit derivatives. The results show that organosolv delignification with DMSO/Water can be proposed as an eco-friendly alternative process to the kraft pulping.

Alkali lignin (AL) has attracted great attention as a material for treating dye wastewater due to its low cost and environmental friendliness. However, the unique structure and aggregation characteristics of AL regarding the dye wastewater removal mechanism have not been systematically revealed. Here, the removal process of typical cationic dye contaminants (methylene blue, MB) from dye wastewater by AL at different pH was explored and the unique synergistic effect mechanism of adsorption and flocculation was revealed. With increasing pH, the removal rate initially increased and then decrease. With increasing MB concentration, the optimal pH value corresponding to the maximum adsorption rate increased regularly. Zeta potential and Fourier transform infrared spectroscopy (FTIR) showed that electrostatic and π–π interactions and hydrogen bonding consisted push-pull balance under the influence of pH. In addition, scanning electron microscopy (SEM), ultraviolet and visible spectrum (UV) and particle size analysis showed that the aggregate structure and synergistic mechanism changed with the solution pH and concentration. In the low concentration solution, adsorption dominated. While in the high concentration solution, flocculation dominated. The removal mechanism consisted of the synergy of adsorption and flocculation laying the foundation for the efficient and environmentally friendly treatment of dye wastewater by AL.

Bamboo is a flexible natural material, which has high potential value. However, manufacturing processes for bent bamboo products are highly reliant on manual labor. The main challenge in the industrial processing of raw bamboo is that bamboo’s characteristics have not yet been extracted accurately. This mainly originates from bamboo’s complexity and its response to bending, which depends on chemical composition, structure, and properties of bamboo. In order to understand the response of bamboo to bending, samples from different parts of manual processed bent bamboo culm ring were studied through nanoindentation tests. In this regard, elastic modulus, hardness, irreversible work, and residual depth on different bamboo parts were analyzed. The results show that bending has different effects on the tension side (TS, the convex side) and compression side (CS, the concave side) of the bamboo culm. Moreover, the bending effect was different on the green (GBP) and middle bamboo parts (MBP). It was found that bending improved the elastic modulus of GBP in TS by 101%. Moreover, the hardness of GBP in TS decreased by 21% while that of MBP increased by 12.4%. In CS, the hardness of GBP increased by 10% while that of MBP decreased by 19%. The performed analyses reveal that the ratio of irreversible work to total work increased in the bent bamboo and the difference between GBP and MBP was not large for both TS and CS. The relative residual depth was about 60% for the neutral side (NS) and increased to above 70% in TS and CS. It was proposed that either TS or CS formed a respective complete tension-compression stress system in the bent bamboo specimen.