Wood Science and Technology

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Online ISSN: 1432-5225
Print ISSN: 0043-7719
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Heat treatment is admittedly a significant value-adding step in wood processing. However, the decrease in mechanical properties of wood caused by heat treatment has always been an urgent problem to be solved. This study aimed to overcome this barrier by impregnating Balfour spruce (Picea likiangensis var. balfouriana) wood with low-molecular-weight lignin before heat treatment. The low-molecular-weight lignin impregnated more effectively and distributed uniformly in wood cell wall. The equilibrium moisture content of the heat-treated wood with lignin impregnation decreased significantly compared to those without lignin impregnation. The low-molecular-weight lignin impregnation combined with heat treatment improved the dimensional stability and humidity resistance effectively. The impregnated cell walls with lignin were integrated and seldom spoiled during heat treatment. The heat-treated wood impregnated with low-molecular-weight lignin showed an increased MOE, MOR, and compressive strength (CS) of 52, 177, and 25%, respectively, compared with the heat-treated wood without lignin impregnation. The indentation modulus of the secondary walls of heat-treated wood with impregnated low-molecular-weight lignin increased by 300%. These findings provide some insights into the relationship between lignin content and the strength of wood and show a way to enhance the mechanical properties of heat-treated wood without damage to its dimensional stability.
 
The mechanical and flame-retardant properties of sodium silicate-impregnated fast-growing poplar have been greatly improved by a wide variety of methods, which has solved the problem of insufficient supply of natural wood to a certain extent. However, sodium silicate is easily leached and has high hygroscopicity, and sodium silicate-modified poplar (SSMP) has low anti-shrink efficiency (ASE), which make it difficult to maintain its dimensional stability. Dimethylol dihydroxyethylene urea (DMDHEU) contains polyhydroxymethyl active groups that are highly reactive with wood fibers and sodium silicate. Therefore, in this study, sodium silicate and DMDHEU were used as a composite modifier to carry out vacuum-pressure-impregnation modification on fast-growing poplar to form a bridging structure. This treatment fixed the sodium silicate and improved the dimensional stability of poplar. Mechanical properties of poplar wood were improved by modifications with sodium silicate and DMDHEU. The dimensional stability was greatly improved, and the fixation of sodium silicate was improved. Compared with the SSMP, the leaching rate and ASE of SS/DDMP were reduced by 48.82% and 41.79%, respectively. XRD, FTIR, and XPS results showed that C–O–C and Si–O–C bonds were formed between DMDHEU and the wood cell walls and sodium silicate. These bonds closely bound the cellulose crystals, which reduced the number of –OH groups being accessible for water and, thus, the moisture absorption of SS/DDMP. In addition, due to the increase in crystallinity, the heat resistance was further enhanced. The cone calorimetry results showed that SS/DDMP had the lowest heat release rate and total heat release. Compared with SSMP, the mean smoke release rate (mean SPR) and total smoke release decreased by 40.38% and 40.83%, respectively. Moreover, the release of CO and CO2 decreased. In conclusion, compared with other modification methods, the use of SS/DD impregnation to modify poplar has the potential to produce good overall performance of poplar with high-dimensional stability, flame retardancy, and smoke suppression.
 
Wood is an abundant, sustainable, low-carbon emission engineering material. However, fire safety issues and insufficient mechanical strength limit its applications. Herein, a simple and scalable method is developed to prepare a robust and flame-retardant structural wood that endures high-temperature flame attack. Natural wood undergoes partial delignification, flame-retardant modification, and densification to obtain ammonium dihydrogen phosphate (ADP)-densified wood. ADP is uniformly impregnated to the wood cell walls to form P–O–C bonds with a dense and cross-linked structure. ADP-densified wood presents enhanced thermal stability, flame retardancy, and mechanical robustness. Its mechanical performance is well-preserved from high-temperature flame heating. In particular, its tensile strength and flexural strength are 22.6-fold and 17.8-fold higher than natural wood after being heated for 300 s. The improved performance is due to the synergistic effect of ADP flame retardant, which accelerates the insulating char formation, and the dense and cross-linked wood structure, which leads to the condensed and robust char structure. The insulating and condensed char limit the heat and pyrolyzed combustible gases transfer between the wood surface and the interior, which protects the wood interior from high-temperature flame heating. ADP-densified wood, as a robust and flame-retardant structural material, has great potential in green and energy-saving construction applications.
 
In this paper, a method for dimensional stabilization of wood through bulk hydrophobization was investigated using a sol–gel process resulting in in-situ formation of microporous SiO2 aerogel. Two different wood species, beech (Fagus sylvatica) and Scots pine (Pinus sylvestris) were investigated. The incorporation of microporous silica aerogel inside the cell wall and lumen was verified by scanning electron microscopy, energy dispersive spectrometry and Fourier-transform infrared spectroscopy. A leaching test using paper as model material proved the bonding of the aerogel to the cellulose component of the cell wall, which indicates a long-lasting effect of the treatment. The modification of wood with silica aerogel significantly improved its hygroscopicity and dimensional stability, decreased the equilibrium moisture content and water uptake beside a low weight percent gain. Permeability was reduced as a result of the silica aerogel deposition in the macro- and micropores of the modified wood. The treatment resulted in an obvious colour change as well.
 
Bamboo is known to be a sustainable material with fast growth and high mechanical properties, while the high hygroscopicity and low-dimensional stability limit its further utilization in the construction sector. Thus, a novel dual modification method combining maleic anhydride and tung oil heat treatment was firstly applied to bamboo, and an orthogonal experiment was designed to investigate the influence of the processing parameter on the physical properties of bamboo, including weight change rate (WCR), moisture excluding efficiency (MEE) and anti-swelling efficiency (ASE). Furthermore, the morphological characteristics, chemical structure and water status of modified bamboo were also studied. The results showed that a parameter combination of maleic anhydride pretreatment at 80 °C for 3 h and tung oil heat treatment at 140 °C for 3 h was considered as an optimized process. The MEE and ASE (volume) of the modified bamboo could reach approximately 80 and 60%, respectively. Maleic anhydride was deposited on the cell wall and the reaction sites were grafted onto the bamboo by esterification. A portion of tung oil could be impregnated into the bamboo and occluded the water channel, such as vessels and pits, and the chemical linkage between the modified bamboo and tung oil was then established during heat treatment through the Diels–Alder reaction. Additionally, the dual modification method not only changed the water distribution of bamboo, but also shortened the spin–spin relaxation time (T2) of the bound water and free water.
 
FTIR spectra of KL, DKL, and HKL
¹H NMR spectra of KL, DKL, and HKL
Quantitative ³¹P NMR spectra of KL, DKL, and HKL
Molecular weight distribution of AcKL, AcDKL, and AcHKL
In this study, a commercial kraft lignin (KL) was used as the raw material to prepare phenolic resin (PF) based on a detailed analysis of its molecular structure. To further increase the viscosity of the phenolic resins and improve their environmental performance, demethylated and hydroxymethylated KL (DKL and HKL) were also prepared and used to produce DKLPF and HKLPF. Compared with the methoxy content of KL, that of DKL was lower by 39.34%, and the alcoholic hydroxyl contents of HKL were higher by 59.39%. Therefore, the increased active sites on the benzene ring of DKL promote the hydroxymethylation reaction, and HKL adds more methylol groups, which is more conducive to the condensation reaction to produce PF. The synthesis process and properties of the lignin-based PF with high phenol substitution rates were investigated. The results showed that the maximum substitution rates of KL, DKL, and HKL to replace phenol to prepare phenolic resin were 70%, 50%, and 30% (w/w). The viscosity, free formaldehyde content, and other important indices of LPF meet the requirements of the GB/T14732-2017 standard. Besides, as the substitution rate of phenol by lignin increases, the viscosity of DKLPF and HKLPF performance is higher. When the 30% (w/w) phenol was replaced by lignin, the adhesive strengths of KLPF, DKLPF, and HKLPF all reached the maximum, and the trend of adhesive strengths was KLPF < DKLPF < HKLPF.
 
Schematic procedure detailing the manufacturing process of the densified wood
Schematic procedure of the machine learning workflow using a supervised learning and b semi-supervised learning
Confusion matrices with Cohen-Kappa values (κ) using a RF, b SSL-RF, c SVM, and d SSL-SVM. Binary classification 0 = pass; 1 = fail
Neighborhood component analysis (NCA) using a RF and b SSL-RF. Feature index: 1_Maple; 2_Spruce; 3_Walnut; 4_Fir; 5_Mass [g]; 6_Length [mm]; 7_Width [mm]; 8_Height [mm]; 9_Density [kg/m³]
The quality control of wood products is often only checked at the end of the production process so that countermeasures can only be taken with a time delay in the event of fluctuations in product quality. This often leads to unnecessary and cost-intensive rejects. Furthermore, since quality control often requires additional procedural steps to be performed by a skilled worker, testing is time-consuming and costly. While traditional machine learning (ML) methods based on supervised learning have been used in the field with some success, the limited availability of labeled data is the major hurdle for further improving model performance. In the present study, the potential of enhancing the performance of the ML methods random forest (RF) and support vector machines (SVM) for quality classification by using semi-supervised learning (SSL) was investigated. Labeled and unlabeled data were provided by Swiss Wood Solutions AG, which produces densified wood for high-value wood products such as musical instruments. The developed approach includes labeling of the unlabeled data using SSL, training and 10k cross-validation of the ML algorithms RF and SVM, and determining the generalization ability using the hold-out test set. Based on the evaluation indices such as accuracy, F1-score, recall, false-positive-rate and confusion matrices, it was shown that SSL could enhance the prediction performance of the quality classification of ML models compared to the conventional supervised learning method. Despite having a small dataset, the work paves the way for future applications of SSL for wood quality assessment.
 
This study aimed to evaluate and compare the fracture morphology in hardness test of compression (CW), lateral (LW), and opposite woods (OW) of Pinus merkusii and Agathis loranthifolia growing in Indonesia. The hardness of the transverse, radial, and tangential surfaces was examined using Brinell’s method, according to the Korean standard. The fracture surfaces of hardness tested samples were observed by scanning electron and optical microscopy. On all surfaces from both species, CW showed the highest hardness, and no significant differences were detected between LW and OW. On the transverse surface, CW of P. merkusii showed brittle-fractured cell wall, while that of A. loranthifolia displayed smooth cell wall. CW of both species showed intercellular and intrawall failures and a narrow lumen opening. LW of both species and OW of P. merkusii showed folded earlywood tracheids with a crack, whereas in the latewood of P. merkusii, LW and OW displayed smooth cell wall and a narrow lumen opening. OW of A. loranthifolia showed collapsed cell wall. On the radial surface, CW of both species showed buckling tracheids in the tangential direction. LW and OW of both species exhibited shortening earlywood tracheids with folded cell walls, while LW and OW of P. merkusii exhibited buckling latewood tracheids in the radial direction. On the tangential surface, buckling tracheids occurred near the fractured rays in all parts of both species. In conclusion, CW exhibited distinctive hardness and fracture morphologies compared with LW and OW in P. merkusii and A. loranthifolia; in addition, hardness and fracture morphology differed between the two species.
 
Mechanical friction causes electrical surface charges on wooden surfaces. In this research, triboelectric activation of solid wood surfaces was investigated by using a wood brushing machine. The extent of activation and the potential influence of machine parameters, or the influence of various wood species are questions so far unanswered. The electrical surface field strengths were continuously detected by means of an electric field meter. Machine settings, such as feed rate and brush pressure, have been varied to better understand the effects on the resulting surface charges. Data showed that nylon and tynex brushes lead to strong positive electric surface field strengths while natural fibers lead to less positive surface field strengths. In contrast, steel wire brushes showed negative electrical field strengths for oak wood, slightly positive field strengths for beech wood and stronger positive field strengths for softwoods. Overall, the tendency that a higher brush pressure led to higher recorded electrical surface field strengths while a faster feed rate reduced the field strengths was observed. As these findings were influenced by wood species and brushing materials, a better understanding of specific triboelectric interactions is essential for future applications. Tailoring surface charges can be an asset for new technical applications, such as chemistry-free primer treatments prior to wood coating.
 
Thermal modification is a well-established method to improve the dimensional stability and the durability of wood for outdoor use. In this study, samples were thermally treated at 160–220 °C for 1 h. The differences of viscoelastic properties between the cell wall S2 layer and compound middle lamella (CML) of thermally treated wood were investigated by creep compliance testing and dynamic modulus mapping of nanoindentation. The elastic parameters of the S2 and CML, such as reduced modulus and storage modulus, decrease and then increase as the treatment temperature increases. By contrast, the loss moduli and loss factors of the two layers decrease steadily as the treatment temperature increases. In addition, the creep compliance results indicate that the S2 layers of the untreated and treated samples exhibit greater elasticity than does the CML, but the rheological characteristics of the CML are more obvious. Finally, the loss modulus and loss factor of the S2 layer are also larger than those of the CML. The changes of viscoelastic properties of cell walls of thermally treated wood are mainly related to hemicellulose degradation, the cross-linkage structure between cellulose and matrix breakage, and cellulose microfibrils arrangement.
 
a Pulp yield, b cellulose dissolution rate and c hemicellulose dissolution rate after pretreatment with different acidic DES
Dimer of MA/ChCl
Intrinsic viscosity of pulp after pretreatment with different acidic DES
a X-ray diffraction spectra and b crystallinity of pulps pretreated by different CA/ChCl
Relationship between cellulose removal (a) and hemicellulose removal (b) and α of the DESs
In this study, bleached chemical pulp from hardwood was treated with different carboxylic acid choline chloride-based deep eutectic solvents (CA/ChCl DESs) including formic acid, acetic acid, ethanedioic acid, propanedioic acid and choline chloride. The dissolution and degradation of cellulose and hemicellulose in chemical pulps in these DESs were analyzed, while the relationships between the polarity parameters of DESs and the degradation and dissolution of cellulosic fibers were discussed. The results show that dicarboxylic acids/ChCl were more conducive to the solubility and degradation of cellulosic fibers compared to monocarboxylic acids/ChCl at a higher temperature. In addition, the increase in alkyl of the CA could reduce the solubility to cellulosic fiber. The further study reveals that the dissolution and degradation of cellulosic fibers are generally governed both by temperature and the ability of DESs hydrogen bond acidity (α), while temperature shows little effect on α. However, no remarkable relationships were found between the DESs hydrogen bond basicity (β) and cellulose degradation and dissolution.
 
The application of adhesives in modern timber engineering often introduces moisture into the wood, leading to permanent residual stresses after hardening. This paper proposes a novel approach to assess these residual stresses by using wooden bilayers as a reporter system. For thin bilayers, moisture-induced stresses lead to pronounced visible flexion that can be used to identify the stress-driving parameters of the adhesive’s gelation process. These parameters depend solely on the wood/adhesive combination and are inversely determined by fitting a finite element method model on the experimentally obtained flexion state. In a subsequent step, the determined parameters are used to calculate the residual stresses in the adhesive bondline of cross-laminated timber plates, emphasizing this approach’s scale independence and general applicability to larger scale structures. All combinations of European beech and Norway spruce with the adhesives Melamine–Urea–Formaldehyde (MUF), Phenol–Resorcinol–Formaldehyde (PRF), and Polyurethane (PUR) were investigated.
 
Light micrographs of the woods included in Group 1. X-transverse, T-tangential, and R-radial sections of Pterocarpus erinaceus (A), Pterocarpus dalbergioides (B), Pterocarpus indicus (C), Pterocarpus angolensis (D) and Pterocarpus macrocarpus (E) showing the similarity of the five species concerning wood anatomical features
Discrimination accuracy of four classifiers when seperating 29 species based on five features
Accuracy of the best-performing classifiers to discriminate CITES-listed species from their look-alikes using different combinations of wood anatomical features for Group 1–6. VD:Vessel Diameter; VF: Vessel Frequency; RW: Ray Width; RH:Ray Height; RF: Ray Frequency
Confusion matrices generated by the best-performing classification predictions of the best classifiers. A - E: Multinomial Naïve Bayes; F : Random Forest. Species in blue text are CITES-listed
Due to increasing global trade of timber commodities and illegal logging activities, wood species listed in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) appendices are facing extinction, and their international trade has been banned or is under supervision. Reliable and applicable species-level discrimination methods have become urgent to protect global forest resources and promote the legal trade of timbers. This study aims to discriminate CITES-listed species from their look-alikes in international trade using quantitative wood anatomy (QWA) data coupled with machine learning (ML) analysis. Herein, the QWA data of 14 CITES-listed species and 15 of their look-alike species were collected from microscope slide collection, and four ML classifiers, J48, Multinomial Naïve Bayes, Random Forest, and SMO, were used to analyze the QWA data. The results indicated that ML classifiers exhibited better performance than traditional wood identification methods. Specifically, Multinomial Naïve Bayes outperformed other classifiers, and successfully discriminated CITES-listed Pterocarpus species from their look-alike species with an accuracy of 95.83%. Furthermore, the discrimination accuracy was affected by the combinations of wood anatomical features, and combinations with fewer features included could result in higher accuracy at the species level. In conclusion, the QWA data coupled with ML analysis could unlock the potential of wood anatomy to discriminate CITES species from their look-alikes for forensic applications.
 
To investigate the relationship between strain energy release, crack initiation and propagation during wood damage and fracture, the information entropy and correlation dimension D of acoustic emission (AE) signals were used to define the centralized release times of strain energy and the crack propagation characteristics of wood. In three-point bending tests of Zelkova schneideriana and Pinus sylvestris var. mongolica specimens, the AE event information entropy was calculated at an interval of 2 s. The points where the information entropy was significantly lower than the average information entropy were regarded as the moments when the strain energy approached a centralized release. Then, the correlation dimension (G-P algorithm) was obtained by the relationship between the correlation integral and the distance of the univariate time series in the reconstructed phase space, and D was determined by the linear fitting of the least squares method. According to the variation trend of D, the wood damage process was divided into four stages: microscopic damage, main crack inoculation, macroscopic fracture generation and macroscopic crack propagation. The results showed that the early loading damage stage is dominated by the sprouting of microcracks and the expansion along the original fracture. With continuous loading, the r-value of the stage rises after a sudden decrease and maintains a low value, which is defined as the main crack inoculation stage. The pre-fracture could be identified by the lowest point of D.
 
Near-infrared wavelengths selected by genetic algorithm were used to optimize partial least squares (PLS) regression models for loblolly pine (Pinus taeda L.) from the southeastern United States. Wood properties examined included density (D), microfibril angle, modulus of elasticity and tracheid coarseness (C), radial diameter (R), tangential diameter (T), and wall thickness (w)—measured by SilviScan. The optimization process was run for each property with Agenda 2020 samples utilized for PLS model development and the other sets used for prediction. The number of variables (i.e. wavelengths) varied from 10 to 100 with an optimum number identified by genetic algorithm. When compared to a full data set model (based on 700 wavelengths), calibration and prediction performance of optimized PLS regression models were superior for all properties. Importantly, representative wavelengths for each property were consistently related to recognized bond vibrations observed in specific wood components demonstrating that optimization targets wavelengths directly related to changes in wood chemistry within the examined loblolly pine samples.
 
In a growing context of green and circular economy, gaining knowledge of the composition of every crop is crucial, as this will allow for their full exploitation. Cherry ( Prunus avium L . ) is a widespread tree of particular interest for its fruits and its valuable timber. Its wood is rich in extractives and its characterization will allow to consider other applications for this feedstock. In this study, chipped cherry wood was extracted and chemically analysed to determine its total phenolic content, total condensed tannin, antioxidant capacity, and polysaccharide content through wet chemistry analysis. These investigations were coupled with ¹³ C-NMR and FTIR spectrometry, with HPLC as well as elemental analysis to conduct a comprehensive chemical characterization. Thermogravimetric measurements were also taken to understand the behaviour of the extract when exposed to high temperature. The registered findings were benchmarked against commercial mimosa ( Acacia mearnsii De Wild . ) and chestnut ( Castanea sativa Mill.) tannins which were selected as template for condensed and hydrolysable tannins, respectively. Cherry extract was found to be the poorest in phenolics which are mainly constituted of pyrogallic flavonoids strongly interconnected with significant amounts of polysaccharides.
 
A new criterion for investigating the failure of wooden structures using maximum shear stress (MSS) under mixed mode I/II was presented for two conditions in which the crack is oriented along and across the fibers. The impact of non-singular stress terms in Williams series expansion (T-stress) is also included. This criterion is presented by considering the reinforcing effects of fibers in which the orthotropic material is assumed as an isotropic material that is reinforced with the fibers. The proposed failure criterion includes a damage coefficient that establishes an effective relationship between the fracture toughness of isotropic and orthotropic materials. The assumption is that at the onset of fracture, from the microscopic point of view, the crack makes a small kink in a direction where the shear stress has the critical value at a critical distance based on the MSS criterion. Then, the crack will propagate along the fibers after colliding with them. The results are verified by using available experimental data of different wood species. It was shown that with the proposed criteria both the crack growth path and the moment of crack growth are well predicted.
 
The conversion to climate-stable, resilient and productive forests has resulted in an increasing share of mixed stands. Different growth conditions and silvicultural treatments lead to an increased scatter in strength compared to what is expected from monoculture experience. The study (i) quantified the magnitude of variation in strength of European beech timber from stands of different composition and (ii) showed the impact of grading on the characteristic strength value of timber coming from those stands. Strength grading models and machine settings for hardwood tensile classes on over 900 European beech (Fagus sylvatica L.) boards were derived. One model used only the dynamic modulus of elasticity (Edyn), and a more complex model used a knot value in addition. Afterwards, 407 boards from pure beech stands as well as mixed stands of beech with Douglas fir (Pseudotsuga menziesii (Mirb.) Franco), Norway spruce (Picea abies (L.) Karst.), sessile oak (Quercus petraea (Matt.) Liebl.), and Scots pine (Pinus sylvestris L.) were graded and analyzed for their material properties from tension tests parallel to grain. Although a variance components analysis attributed only 4.2% of the variation to mixture, the ungraded timber showed significant strength differences between the pure and the beech-pine stands (65.2 versus 46.6 MPa). The yield of the material graded to the highest class in a class combination was higher in pure beech stands. The required characteristic strength values were mostly met for boards from the pure stands; while boards from the beech-pine mixed stands hardly ever reached the required values. To reduce strength variation and guarantee reliable timber products, strength grading should consider the various growth situations in forests when sampling material for the derivation of settings.
 
Basalt fiber can be used as a reinforcing phase to improve the mechanical properties of wood thermoplastic composites. The purpose of this study was to use differential scanning calorimetry to study the effect of the addition of basalt fiber and wood flour on the non-isothermal crystallization kinetics of high-density polyethylene (HDPE)-based composites. The Avrami method and Avrami–Ozawa method were adopted to describe the non-isothermal crystallization process, and Gaussian model was used to calculate the surface activation energy. This study showed that the addition of basalt fiber and wood flour significantly changed the crystallinity of wood flour/high-density polyethylene (HDPE)/basalt fiber three-phase composites. The apparent activation energy of the three-phase composites was calculated using Gaussian multi-peak simulations, and the trend of the conversion rate was modeled. In addition, basalt fiber and wood flour, as nucleating agents, had the ability to accelerate the crystallization of wood flour/high-density polyethylene (HDPE)/basalt fiber three-phase composites without changing the crystallization mechanism of these composite materials.
 
The contact force of oval mortise and tenon joints was investigated to get an in-depth understanding of the mechanical mechanism concerning outdoor wooden furniture structure. Experimentally, the contact force under different moisture content (13.50 and 50.00%), interference fit (0.1, 0.2, 0.3 and 0.4 mm) and grain direction (radial and tangential) was studied. A numerical simulation was conducted to examine the stress distribution and feasibility of the finite element method. The theoretical model was proposed for a deep insight into the contact force considering elastic constants, interference fit and geometric parameters of the oval tenon. According to the results, the moisture content and grain direction had significant effects on contact force, while the effect of interference fit depended on the moisture content. The result obtained by FEM was consistent with that of the experiment. The theoretical model can predict the contact force. The phenomenon that stress distribution in the thickness direction of the tenon shows a gradual increase from the sides to the middle part was evidenced both by FEM and the theoretical model. Response surface of contact force on tenon’s geometric parameters was also gained. This study is beneficial to outdoor furniture structure optimization.
 
This study examined the effect of hemicellulose molecular structure on wettability and surface adhesion to urea–formaldehyde resin adhesives to better understand the complex adhesion process of wood biopolymers. Molecular structure of two hemicelluloses, such as arabinogalactan and xylan, was characterized using Fourier transform infrared, one-dimensional, and two-dimensional nuclear magnetic resonances. As a result, arabinogalactan had a hyperbranched structure, whereas xylan was more linear, which caused a distinctive morphology in their films, with the latter having a rougher surface. Further, the surface adhesion between hemicellulose and UF resins with various formaldehyde to urea molar ratios (1.0 and 1.6) was measured. The adhesion force and work of adhesion of arabinogalactan with different UF resins were found to be greater than those of xylan due to the former film’s higher surface free energy, more exposed OH groups, and smoother surface. In addition, 1.6 UF resins exhibited greater adhesion than 1.0 UF resins, regardless of the hemicellulose type, demonstrating that dispersion force was dominant in their molecular interactions.
 
Sample preparation from 6 m Pinus radiata sapwood boards. a Board reduction for kiln-drying and scCO2 treatment; b scCO2 treatments and secondary kiln-drying; c stress-cup testing and d colour measurement
Taper planed surfaces for colour measurement
Variation in ΔL* as a function of drying schedule. Closed ball and bars show raw data mean ± S.E
Typical kiln brown stain on kiln-dried boards (all board sections shown are from the same board). KBS is revealed after taper planing in the planed KBS region. If boards had scCO2 pre-treatment, KBS did not develop on secondary kiln-drying in the deep-planed regions. Left to Right–kiln dried 90 °C, then secondary dried at 70 °C/60 °C, 90 °C/60 °C and 120 °C/80 °C
Mean ± 95% C.I. and distribution of individual raw stress-cup data grouped in three different ways: by: a Drying schedule; b Temperature; c Drying schedule and temperature. Quality classes A, B, C, D and E are based on 90% of boards having stress-cup values below 0.5%, 1%, 2%, 3% and 4%, respectively
High pressure (20 MPa), cyclic, supercritical carbon dioxide (scCO2) treatments can reduce the moisture content of green Pinus radiata sapwood from 150–200% to 35–40%. Such treatments can be used as a dewatering pre-treatment before the kiln-drying of timber. Kiln-drying can utilise various temperature and humidity schedules, targeting around 10% moisture content, with a final stress-relieving steam-conditioning step. After scCO2 treatment and kiln-drying of samples, kiln brown stain was evaluated using the CIE L*a*b* colour space while drying stress was assessed by stress-cup measurements. The most significant results of scCO2 pre-treatment of Pinus radiata sapwood followed by kiln-drying plus steam-conditioning were as follows: Drying from green (36 h from a moisture content (MC) of 164%) using a conventional temperature schedule (90 °C/60 °C) took 2–5 times longer than kiln-drying scCO2 pre-treated boards (37.5% MC) to a target of 10% MC. Colour measurements proved that kiln brown stain does not occur. The use of a steam-conditioning step in reducing internal drying stresses was important irrespective of whether or not there was a scCO2 pre-treatment step. Over all drying schedule combinations, internal drying stress of both green and scCO2 pre-treated timber was similar after kiln-drying plus steam-conditioning. However, using only 90 °C/60 °C schedule data, with steam-conditioning, drying stresses were lower using kiln-drying without the scCO2 pre-treatment. This was surprising since the scCO2 step reduced the moisture content to around 37.5% without significant moisture gradients and so a secondary kiln-drying to 10% moisture content could have been expected to yield lower internal stress levels by preventing large moisture gradients to develop during drying. This result confirms the efficacy of the steam-conditioning step following standard kiln-drying. The colour data demonstrating the prevention of kiln brown stain using kiln-drying schedules offers a path to increasing timber quality for interior applications.
 
Thermal modification is widely applied to improve moisture dynamics of wood, however often decreasing the mechanical strength. It is therefore required to enhance the mechanical strength of thermally modified timber (TMT), for example by impregnation with adhesives. Specimens, cut from Douglas-fir, were thermally modified (TM) either after phenol formaldehyde (PF) resin impregnation (IM-TM) or before (TM-IM). The microstructural and chemical properties were investigated with SEM and FTIR. Compressive stress, as one of the important mechanical properties, was measured using a universal testing machine, while strain distribution was recorded with digital image correlation (DIC). The results show that the compressive stress of TM specimens can be enhanced significantly by PF resin impregnation. Compressive stress differences between IM-TM and TM-IM specimens are small despite of the larger amount of resin in TM-IM specimens. Thermal modification decomposes part of the PF resin in the cell lumens and promotes chemical reaction between the PF resin and wood. PF resin improves the stiffness and ductility of the wood cell wall, resulting in smaller strain and homogenous distribution thereof. These factors lead to high compressive stress of IM-TM and TM-IM specimens. Although PF resin impregnation contributes to narrowing strain accumulation in earlywood of TM specimens, control specimens have the smallest strain ratio between earlywood and latewood. The findings of this study are helpful for optimizing the cost effective thermal-impregnation technology of producing TMT with improved compressive stress.
 
To improve the mechanical performance of wooden engineered composites, more information is needed to understand the relationship between adhesive infiltration and mechanical properties of wood-adhesive interphase. Nanoindentation mapping (NI Mapping) was hereby used to achieve large-scale and high-resolution visualization of mechanical properties of wood-adhesive interphase for establishing the basic understanding of such correlation, and it showed the convincible reliability (variation less than 3% as compared with commonly applied NI methodology) and exceptional efficiency (180 min of total analysis time as compared with more than 3000 min using the commonly applied NI methodology). Successful investigation into the distribution of mechanical properties of wood-phenolic resin (wood-PF) and wood-polyurethane (wood-PUR) interphases using NI Mapping has confirmed that gradient elevation in mechanical properties of cell wall in wood-adhesive interphase is related to adhesive infiltration. PF adhesive exhibits favored infiltration which led to the enlarged wood-PF interphase region (over 60 μm) and improved micro-/macro-mechanical properties. NI Mapping was also applied for revealing mechanical performance of early wood-adhesive interphase which cannot be characterized by commonly applied NI methodology due to its small size. The mechanical properties of cell wall in early wood-adhesive interphase are poor; however, the interphase region would be enlarged due to the large lumen of early wood. NI Mapping can be a powerful tool to provide significant insights into the mechanical properties of wood-adhesive interphase, and thus, to improve the knowledge of the adhesive infiltration and mechanical properties of cell wall.
 
Isolation of cellulose, hemicelluloses, and lignin from birch
Equilibrium moisture content values of a birch sample and its major components, b cellulose, c hemicelluloses, and d lignin
Comparison of a birch sample EMC and b holocellulose EMC of experimental test data and prediction based on the rule of mixture
Sorption site density of birch sample, cellulose, hemicelluloses, and lignin, as calculated according to the SSO model
a Absolute hysteresis and b relative hysteresis of birch sample and its chemical components
Cellulose, hemicelluloses, and lignin in wood have completely different structures and hygroscopicity, which markedly impact the wood moisture adsorption properties. The equilibrium moisture content (EMC), the density of sorption sites, and hysteresis characteristics of cellulose, hemicelluloses, and ball-milled lignin isolated from white birch (Betula platyphylla Suk.) were investigated to determine their influence on the adsorption isotherm and hysteresis of wood. A dynamic vapor sorption apparatus was used to test the sorption isotherm and the rule of mixture was employed to evaluate differences between in-situ and isolated chemical components. The sorption site occupancy (SSO) model was used to calculate the density of sorption sites of birch cellulose, hemicelluloses, and lignin, and the calculated results were then compared with the theoretical hydroxyl content. The results show that the dewaxed birch sample, cellulose, hemicelluloses, and lignin at 95% relative humidity had EMC values of 20.7%, 20.4%, 107.4%, and 11.6%, respectively, their largest relative hysteresis values were 1.60, 1.20, 1.40, and 1.74, respectively. Sorption site density of birch sample, cellulose, hemicelluloses, and lignin calculated by the SSO model were 10.5, 9.2, 17.3, and 6.0 mmol/g, close to that of theoretical hydroxyl content. Lignin had the highest relative hysteresis compared with cellulose and hemicellulose, and interaction and cross-linking between wood chemical components have a great influence on wood hysteresis.
 
In this study, furfural production in a two-stage system was investigated using Eucalyptus wood residues (pinchips) as raw material. Firstly, an autohydrolysis treatment at 170 °C was performed for 40 min to solubilise xylosaccharides from biomass. In a second stage with sulphuric acid, furfural production was evaluated at different temperatures (170–240 °C) and reaction times (2.5–30 min) with two acid concentrations (0.3 and 3.0% w/w). The maximum furfural yield achieved was 77.4% of the theoretical production, considering the solubilised xylosaccharides content in the autohydrolysis liquor. This condition was achieved at 210 °C for 10 min with a sulphuric acid concentration of 0.3% w/w. The biomass pretreatment was a two-stage pretreatment with the above autohydrolysis followed by a soda-pulping stage of the solid fraction at 155 °C and 7.8 atm for 90 min using a 2.4% w/w NaOH solution. These results were added to previous experimental results for bioethanol production by enzymatic hydrolysis and fermentation to develop the process design. A lignocellulosic biorefinery plant co-producing furfural (from the hemicellulosic fraction) and bioethanol (from the cellulosic fraction) was proposed and simulated using Aspen Plus® V11. According to the material balance results, the proposed plant processes 166 tonne/d of Eucalyptus pinchips, with a production capacity of 203 tonne/d of bioethanol and 35 tonne/d of furfural. Finally, 33.0 MW of hot and 15.2 MW of cold utilities are required for the proposed process according to the heat integration analysis.
 
Morphology and structure of different wood samples. Photograph of a NW and its cross-sectional SEM image, showing a honeycomb porous structure and compact cell walls; b WS and its cross-sectional SEM image, showing the delamination of the internal structure of the cellulose nanofibers exposed in the cell wall; c WS and SEM image of its cross-section, showing layered butt structure; dβ-CD-WS and its cross-sectional SEM image, showing the layered stacked structure; e FT-IR spectra of different samples; f XRD patterns between different samples; g Relative content of cellulose, hemicellulose, and lignin in different wood samples obtained by chemical composition analysis; h XPS spectra of β-CD-WS@CQDs samples
Fluorescence properties of CQDs and β-CD-WS@CQDs. a Fluorescence excitation (Ex) and emission spectra (Em) of CQDs solution; b CQDs solution with different ions dropwise showing different fluorescence intensities; b1, b2 CQDs solution and β-CD-WS@CQDs with different ions dripped showing different fluorescence intensity under the same UV lamp irradiation; c Fluorescence spectra of CQDs solutions with different concentrations of Cu²⁺added dropwise. c1, c2 The CQDs solution and β-CD-WS@CQDs with different concentrations of Cu²⁺ solution showing different fluorescence intensities under the same UV lamp irradiation; d Fluorescence spectra of CQDs under excitation at different wavelengths; e UV absorption spectra of CQDs and Cu²⁺
Adsorption of β-CD-WS@CQDs to heavy metal ions. a Quasi-first-order equation and quasi-second-order equation of β-CD-WS@CQDs adsorption kinetics are linearly fitted; b Langmuir model and Freundlich model adsorption isotherms of β-CD-WS@CQDs; cβ-CD-WS@CQDs and WS adsorb different heavy metal ions; d Influence of pH value on the adsorption of Cu²⁺ by β-CD-WS@CQDs; e Effect of temperature on the adsorption of Cu²⁺ by β-CD-WS@CQDs; f Effect of the dosage of β-CD-WS@CQDs on the adsorption of Cu²⁺
As a kind of absorbent material, the porous properties of wood sponge as the heavy metal ion detection and adsorption materials have been widely studied in the field of water pollution treatment. In this study, sodium chlorite and sodium hydroxide were used to remove lignin and hemicellulose from the poplar wood, so that it could become a porous three-dimensional cellulose skeleton structure. Subsequently, β-cyclodextrin was cross-linked with cellulose skeleton through hydroxyl bond which can enhance the heavy metal adsorption capacity of wood sponge. By vacuum impregnation, the wood sponge which was attached to carbon quantum dots was a kind of stable fluorescent solution that can be quenched by heavy metal ions. In this way, the wood sponge became a heavy metal ion detecting sensor and absorber, which can sense and absorb heavy metal ions. The microstructure of the obtained wood sponge was investigated by scanning electron microscopy. The results showed that the wood cell wall became thinner after lignin and hemicellulose were removed, but the cellulose skeleton was retained. The carbon quantum dots and β-cyclodextrin successfully cross-linked on the cellulose skeleton by Fourier transform infrared spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The selectivity and sensitivity of a wood sponge capable of detecting and adsorbing heavy metal ions were explored. Among the ions, the wood sponge had a quenching reaction to Cu²⁺. Moreover, the wood sponge showed lower intensity of brightness with increasing Cu²⁺ concentrations. The wood sponge with cross-linking β-cyclodextrin showed excellent heavy metal ions adsorption performance and the adsorption capacity reached 60 mg/g at the adsorption equilibrium time of 3 h. At a pH value of 5, adsorption temperature of 60 °C, and adsorbent dosage of 3 g, the wood sponge had the best adsorption effect.
 
a–b SEM images of the cross-sectional wood-derived carbon (CWC) with different magnifications, c–d SEM images of the vertical-sectional wood-derived carbon (VWC) and pit channel on the surface. e HR-TEM image of mesopores
a N2 adsorption–desorption isotherms and b pore size distribution of as-prepared CWC and VWC. c XRD patterns, and d Raman spectra of as-prepared CWC and VWC samples
Electrochemical performance of as-prepared CWC and VWC electrodes: a, d CV curves at various scan rates; b, e GCD profiles at different current densities; c, f rate performance, and g electrochemical impedance spectroscopy (EIS) plots and h capacitance retention of CWC and VWC electrodes
a–b CWC and VWC electrodes differentiation of capacitance contribution from CV curves at a scan rate of 5 mV s⁻¹; c–d distribution of the CWC and VWC electrodes capacitance contribution from CV curves with different scan rates
It is an effective way to develop basswood (Tilia americana, hardwood)-derived thick carbon electrode for high energy density supercapacitors. The wood-derived anisotropic structural carbon electrodes have different transfer kinetics of the electrolyte, which will significantly affect the electrochemical performance especially at high current density. Therefore, the relationship between the sectional wood-derived electrodes and the electrochemical performance of supercapacitors were systematically studied. The results show that the high specific surface area (542 m² g⁻¹) of cross-sectional wood-derived carbon contributes to the excellent electrochemical performance (specific/area capacitance ~ 3040 mF cm⁻²/118 F g⁻¹ at 1 mA cm⁻², cycle stability ~ 82.2% after 10,000 cycles at 50 mA cm⁻², and capacitive contribution of 76.64% at 5 mV s⁻¹), which is attributed to the hierarchical porous structure and easily allows electrolytes to enter the interior even at high current density. This work provides a fundamental understanding of the dynamic behaviors of electrolyte at different wood sections, and it is expected to be extended to other wood species and wood-like structures for energy storage applications.
 
Densification is commonly adopted to increase the mechanical performance of wood, but research on the micromechanical behaviour of the material during transverse compression is limited. Robust numerical models will enable better predictions of the performance of wood during compression and optimise the manufacturing process of densified wood minimising experimentation. The densification stress–strain response of wood after chemical treatment is reported via numerical simulations. A 3D finite element model of wood microstructure is studied under transverse compression using ABAQUS/Explicit software. A lower cellulose, hemicellulose and lignin content in the chemically treated wood is considered in the material parameters of the cell wall, and an ideal elastoplastic material model is used to represent the nonlinear stress–strain response. Parametric studies regarding the cell wall thickness, yield stress and chemical treatment are also considered. The numerical predictions agree well with microscopy studies of densified wood, and the nominal stress–strain curve obtained is similar to experimental findings under transverse compression as found in the literature. The cell wall thickness and yield stress are found to significantly affect the compressive stress–strain response of wood.
 
Due to the economic, social, and legal requirements, individual wood identity recognition is a crucial technology required in traceability systems, such as the wood block chain system. Traditionally, labeling is a widely used technology which sticks a code label on the wood entity. It can be used to find out the information related to the entity from the traceability system by scanning the code labels. However, many kinds of labels are easily switched and falsified in illegal activities. This study proposed a new security label method that extracts the innate feature as wood fingerprint to prevent the label falsify problem. This method acquired the cross-section image of log or wood board and extracted the oriented fast and rotated brief (ORB) features to identify the individual entities. It is safe because the feature of the wood texture is innate and cannot be transferred from one to another. Furthermore, this features is unique because of the anisotropy of wood. An image acquisition method was presented. A position was marked as the fingerprint area at the cross section of the wood board, and the image of the fingerprint area was acquired by using a camera with a 20× magnifying glass. The individual wood board tracking method based on the ORB was employed to identify the wood entity. The key point set was described by a invariant rotation descriptor, and it can be used as a fingerprint to recognize the wood board by comparing the marked area images in the wood traceability system. In the experiments, 80 woodblocks were chosen as the tracer objects, and 10 places were marked at each cross section of the board. The results showed that this method has high recognition accuracy and robustness. The recognition accuracy reached 100%, and this method still retained high accuracy even when different cameras were used in different environments to acquire the images. This method can be used in the timber trade to trace wood boards or logs and to prevent illegal trading.
 
This work investigated the relationship between freeze-drying and supercritical drying of cellulosic fibers with different moisture contents based on pore and crystallinity measurements. The results showed that freeze-drying reduced the specific surface area and pore volume of cellulosic fibers by 20–160-fold, compared to supercritical drying. At the same time, freeze-dried fibers had a higher crystallinity index and crystal size than supercritically dried fibers. Freeze-drying also significantly changed the nitrogen adsorption–desorption isotherm and pore size distribution of cellulosic fibers. However, most importantly, freeze-dried fibers had linear positive correlations with supercritically dried fibers in terms of pore parameters and crystallinity, indicating that freeze-dried samples retained the trends and qualitative relationships of supercritically dried fibers. Therefore, freeze-drying can be used as the pretreatment procedure for pore and crystallinity measurements of cellulosic fibers from poplar and eucalyptus when comparing the effects of thermal drying. This work also deepens the understanding of the drying of cellulosic fibers.
 
The intrinsic benefits of the urban greenery are undeniable, but the lack of planning of the urban forestry has created a great liability for its population. To guarantee environmental preservation with risk reduction, one must invest in research that evaluates the accuracy of technological tools that have been used in tree inspection, the results of which are used for the decision-making on urban tree management. One of these tools is ultrasonic tomography, frequently used to infer the internal condition of the stem. This research aimed to evaluate, using metrics (accuracy, precision, and sensitivity) of the confusion matrix, the quality of the image generated by tomography using conventional ultrasound equipment. For the study, disks of the species Cenostigma pluviosum (Sibipiruna) were used. They were analyzed considering different regions (heartwood and sapwood) and conditions (deterioration and cavities). Masks representative of these different regions and conditions of the wood were compared to tomographic images. In addition, average values of density and stiffness obtained in each region and condition were also used to validate the representation indicated in the tomographic image. Although ultrasonic tomography does not represent exactly the position and size of the internal regions of the trunk (cavities, deteriorations, heartwood, and sapwood), the accuracy (greater than 85%) and coherence with density and stiffness were sufficient to indicate its suitability as support for tree risk assessment.
 
To improve the undesirable properties of fast-growing wood, an environmentally friendly modification technology that combined controllable densification and superheated steam treatment was used to process fast-growing wood into a sandwich-densified structural material. The effects of the position of the densified layer(s) and superheated steam pressure on the density, microstructure, color, dimensional stability, and mechanical properties of poplar wood were investigated. The result showed that sandwich-densified wood with two structural modes, namely surface-densified wood and central-densified wood can be formed depending on preheating time. Compared with the control wood, the average density of the densified layer, surface hardness, modulus of rupture, and modulus of elasticity (MOE) of sandwich-densified wood with a compression rate of 20% were increased by more than 87.63%, 52.96%, 24.50%, and 36.44%, respectively. The cell lumen volume in densified layer was significantly reduced compared to that of the transitional layer and un-densified layer. When the superheated steam pressure increased by 0.2 MPa, the moisture excluding efficiency and anti-swelling efficiency of sandwich-densified wood were significantly increased (P < 0.05), but the density, MOE, and surface hardness were not significantly decreased (P ≥ 0.05). The sandwich-densified wood treated with superheated steam showed substantially higher mechanical properties and better dimensional stability, providing a promising method for processing fast-growing wood into solid wood flooring.
 
Natural dyes (ND) are gaining increasing interest due to their outstanding merits of being eco-friendly, biodegradable and non-toxic. Wood dyed with extracts from Dalbergia cochinchinensis residues as ND exhibited desirable color appearance and anti-UV property, while the other potential properties of ND dyed wood in terms of water fastness, mildew resistance along with the penetrability of the ND on wood blocks dyeing have not yet been exploited. The present study was aimed at exploring these aspects for multifunction assessment of the ND dyed wood. The results showed that the total color difference (ΔE*: 4.58) and color intensity reduction (PR: 14.01%) of the ND dyed wood declined slightly after washing fastness test in comparison with that of acid red (ΔE*: 32.82; RP: 76.14%) and reactive red (ΔE*: 26.85; RP: 66.52%) dyed wood, which is indicative of its preferable water fastness, which can be ascribed to the hydrophobicity improvement of the wood surface after ND dyeing. In addition, the ND ameliorated the mildew resistance against Aspergillus niger and Trichoderma viride infection. Interestingly, the wood blocks can be completely impregnated with ND under atmospheric pressure dyeing process without any pretreatment and auxiliary. This study provided a promising approach for multifunctional ND dyed wood preparation.
 
This study investigated the effects of ultrasonic treatment on the pore structure of the Korean paulownia (Paulownia coreana). Cylindrical samples (10 mm thickness and 29 mm in diameter) were taken from heartwood in longitudinal direction and divided into two groups; Group (A) consisted of samples with 2 to 3 annual rings, while the samples of Group (B) had 5 to 6 annual rings. Changes in pore structure, gas permeability, and sound absorption were measured after ultrasonic treatment for between 30 and 90 min. This study confirmed that open-pore porosity significantly increased as a result of ultrasonic treatment. However, gas permeability increased only within the first 30 min of ultrasonic treatment, but then did not increase proportionally with increasing treatment time. This study determined that during the early stage of ultrasonic treatment the number of through pores increased, but as treatment time was extended the number of blind pores increased but not the number of through pores. The sound absorption coefficient increased after ultrasonic treatment, but similar to gas permeability, only up to a certain point, beyond which increased treatment time did not result in further improvements. In addition, paulownia’s tyloses were broken by ultrasound treatment, which was responsible for the increase in blind-pore porosity.
 
The range of application for fast-growing plantation wood is limited by its dimensional instability, susceptibility to biodegradation and low mechanical strength. In this study, a novel method based on in situ reaction of a hydrophilic epoxy monomer (1,4-butanediol diglycidyl ether) in the wood cell walls was proposed to improve the performance of plantation wood. The effects of curing agent on the reaction mechanism and performance of treated wood were discussed in detail. The results demonstrated that after curing using methyl hexahydro phthalic anhydride (MHHPA), an epoxy oligomer with a higher molecular weight and superior water resistance was formed inside the wood cell wall compared to curing with the 2-methylimidazole (2-MI). Moreover, the epoxy group was able to react with alcohol and phenolic hydroxyl in wood through 2-MI catalysis resulting in a higher grafting ratio. After the modification reaction, notable improvements were observed in the dimensional stability (anti-swelling efficiency of more than 65%), fungal (Gloeophyllum trabeum) decay resistance (weight loss of < 6%) and compressive strength (increased by 17%) of the wood.
 
In recent years, there has been an increasing interest of engineers and architects in the use of timber in the construction sector. This worldwide trend can be mainly attributed to the reduced environmental impact of building with timber, its high strength-to-weight ratio and its renewable material nature. Nevertheless, in spite of the advantages of building with timber, one of its major disadvantages is its time-dependent structural response. For instance, typical floors and beams made of timber may show decreasing stress levels under constant deformation over time. This phenomenon is known as stress relaxation and can also be affected heavily by environmental factors, such as temperature and moisture changes. This has inevitably led to discourage the use of timber in the construction industry. Due to the relevance of this subject, the present paper addresses the stress relaxation phenomenon in timber specimens subjected to three-point bending loads. In particular, radiata pine species is chosen for this investigation given its popularity as a building material in countries like Australia, Chile, Spain and New Zealand, among others. To investigate experimentally the influence of temperature and relative humidity on the stress relaxation at different deformation states, an environmental chamber was built. The stress relaxation is assessed indirectly by monitoring the relaxation of the load required to maintain the amount of deformation fixed in time inside the environmental test chamber. The experimental results show that within a period of 7 days, the percentage of load relaxation may reach a value of 35% approximately, for a fixed relative humidity of 60% and constant temperature of 27∘C\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${27}\,^{\circ }\hbox {C}$$\end{document}. The present experimental results provide further insight into the time-dependent mechanisms of timber which are still not well-understood, and particularly of those structures made of radiata pine grown in Chile on which only limited experimental data has been reported to date.
 
Lignin extraction from black liquor (BL) was investigated employing four deep eutectic solvents (DESs) and a control solvent ([Emim][Ac]). A screening study was employed to determine the best DES, lactic acid:ChCl at a 11:1 molar ratio. La:ChCl(11:1) could extract 79.7 ± 2.1% of the lignin content of BL compared to [Emim][Ac] with 79.9 ± 2.3% at 95 °C, time of about 4 h, and a DES:BL(20:1). The DESs selected were regenerated three times, and the extraction efficiency using DES regenerated was statistically similar to the extraction values using DES before regeneration. FTIR spectra for the extracted lignin samples agreed with the peaks for a standard lignin. ¹H-¹³C-HSQC-NMR detected the side chain structure and aromatic linkage peaks in the lignin extracted. Lignin extraction was optimized for La:ChCl using RSM. A BBD was utilized to develop a model to predict the lignin extraction based on operating factors. The Anderson–Darling statistic confirmed a normal distribution of the residuals, which demonstrated a reasonable correlation between predicted and experimental data. The highest extraction of lignin predicted using D-optimality analysis was 83.8% at optimum conditions of time = 4.6 h, temperature = 99 °C, DES:BL(20:1), and La:ChCl(11:1). An experiment to test the extraction under these conditions was performed in triplicate and yielded a value of 79.2 ± 1.86%.
 
Effect of temperature, time, L/S on a acetic acid production, b lignin production, c sugar production, and d furfural production
Hydrodynamic size of hydrolysate constituents under different experimental conditions: a 25 °C, 3.5 pH, b 10 mM KCl, 3.5 pH, c 25 °C, 10 mM KCl, d 25 °C, 3.5 pH, 10 mM KCl, e 25 °C, 3.5 pH, 10 mM KCl
Adsorption analysis using HCl as buffer solution a adsorbed mass change as a function of time, b frequency change as a function of time, c dissipation change as a function of time
Adsorption analysis using acetic acid as buffer solution a adsorbed mass change as a function of time, b frequency change as a function of time, c dissipation change as a function of time
a SEM image for the adsorption of hydrolysate components on stainless-steel coated QCM sensor without rinse step using HCl buffer solution, b EDX image of adsorbed polymers, c EDX image of stainless-steel surface
Autohydrolysis is vastly used in industry to extract hemicellulose from lignocellulosic biomass. Despite their potential end-use applications, lignocelluloses cannot be completely extracted from hydrolysate as they may deposit on the equipment during the hydrolysis process. This study tends to investigate the physicochemical properties and adsorption behavior of the components of hydrolysates produced via the autohydrolysis process. In this work, the autohydrolysis of softwood was conducted under different conditions to produce hydrolysates with different lignin and hemicellulose contents. The results provided evidence for the aggregation of lignocelluloses under acidic conditions in hydrolysates. The hydrolysate with a higher concentration of lignin and hemicelluloses (sample 2) had a larger hydrodynamic size (98.5 nm), and the size decreased to 4 nm in the presence of 40 mM KCl. In the salty system, maintaining the hydrolysate unagitated for 4 h increased the hydrodynamic size of the hydrolysate components to 12 nm. Furthermore, the deposition of lignocelluloses on stainless steel surface was studied using Quartz crystal microbalance in hydrogen chloride and acetic acid buffer solutions. The results confirmed that sample 2 possessed a higher affinity for adsorption 3400 × 10⁻⁸ kg/m² than sample 1 did (1290 × 10⁻⁸ kg/m²) on the stainless-steel surface. The image analysis indirectly confirmed the formation of agglomerates in hydrolysate and their deposition on the stainless-steel surface. The reversibility of lignocellulose deposition depicts the weak interaction between the dissolved lignocelluloses and stainless-steel surfaces.
 
The drying process of waterborne wood coating (WWC) is mainly driven by the moisture gradient, part of the water evaporates to the air, other part enters into the wood structure. The water absorption and conduction rate vary within various wood substrates, which have a great influence on the drying rate of WWC. Herein, the focus of this paper is on studying the influence of water movement behavior on the drying efficiency and film-forming performance of WWC under conventional hot-air drying process. A waterborne acrylic-polyurethane was used as representative coating in the study. It was found that balsa wood had more pores and higher specific surface area than oak wood, allowing a faster water transfer at the wood surface. The shortest surface and hard drying time of 573 s and 1393 s were achieved at 60 °C and 40% relative humidity for WWC on balsa wood. The dynamic drying rate study showed that the water transfer on the surface of oak was slower than that of balsa. Based on the film-forming performance of WWC under different drying conditions, a better film property could be obtained by adjusting the drying conditions to achieve a dynamic balance between water evaporation to air and water transfer into the wood substrate. The drying characteristics of WWC on different wood surfaces and drying conditions were comprehensively evaluated by radar map analysis. The results in this work contributed to the efficient and green processing in furniture and coating industries.
 
To investigate effects of furfurylation on interactions between moisture sorption and humidity conditioning of wood, Pinus spp. wood was impregnated with furfuryl alcohol solution. After oven drying, furfurylated wood and untreated wood were subjected to two environments where initial relative humidity (RH) was, respectively, about 60% and 80% at 25 °C. Dynamic mass and RH were continuously recorded with a self-assembled device. Furfural resin polymerized in cell walls, cell lumina and some pits, which caused mass gain and cell wall bulking. Furfurylation weakened humidity conditioning performance and hygroscopicity of wood, and the RH increased by over 20.7% while moisture content decreased by over 33.6% for furfurylated wood compared to untreated wood after conditioning at initially about 80% RH. Humidity conditioning process was generally divided into four stages according to changing rate, where RH exhibited sharp increase, slow increase, slight decrease and sub-equilibrium, respectively. Moisture content showed a negative linear relation with RH in the first three stages. This was a result of combined effects of the pressure from water vapor gradient and moisture content gradient, the temperature changes during sorption, the hindrance from capillary systems and attraction of other water molecules. The interaction rate between humidity conditioning and moisture sorption was decelerated for blocking of water passages, reduction in sorption sites and hydrophobicity increase due to furfurylation. Over 45.2% decrease in moisture change rate with RH was caused by furfurylation in the first stage. The study can help in understanding the effects of furfurylation on interactions between moisture sorption and humidity conditioning of wood and facilitate better application of modified wood in human settlement practically.
 
Preparation of amine-grafted wood sponges. a Schematic illustration of the sponge fabrication, b schematic description of connecting APS on wood sponges
SEM images of cross-sectional cell walls (a-c) and along the fiber direction (d-i) of balsa wood (a, d, g), D-Wood (b, e, h), and AD-Wood (c, f, i)
Physicochemical properties of wood sponges. a XRD patterns, b FTIR spectra, c N2 adsorption–desorption isotherms, d TGA curves, e DTG data of pristine wood, D-Wood and AD-Wood, f surface wetting properties of AD-Wood
CO2 adsorption performance of AD-Wood sponges. CO2 sorption isotherms (a 0 °C, b 25 °C), c CO2 adsorption cycles, d adsorption mechanism
Wood sponges obtained by structure-retaining delignification of natural wood sheets are promising for CO2 storage. Herein, lignin in balsa wood is selectively removed through chemical treatment, leading to a highly porous structure. To boost the CO2 adsorption capacity, amine agents are grafted on the delignified wood via a condensation reaction. The resulting amine-grafted wood sponges exhibit hierarchical porosity and are mechanically resilient. A relatively high CO2 uptake of 1.22 mmol g⁻¹ (25 °C, 1 bar) is achieved due to the physisorption of hierarchical pores and chemisorption of high amine group loading. Such amine-grafted wood sponges fabricated from top-down strategy hold great potential to serve as renewable CO2 capture materials.
 
Lignin is obtained as a by-product in the manufacturing of cellulose pulp for paper. Usually, it is used as internal fuel in this process and, in general, it is not recovered for use as a chemical product. Lignin is the second more abundant natural polymer, and it represents an enormous renewable raw material source. It is well known that epoxy resin is extensively used thanks to its high versatility and its good thermal and mechanical properties. However, most of the commercial epoxy resins use bisphenol A as a precursor, which has been reported to have estrogenic activity. In this work, up to 50 wt% of lignosulfonic acid sodium salt (lignin) has been added to epoxy resin to reduce the amount of both hardener and resin and make a green epoxy adhesive. The effect of lignin on curing kinetics, glass transition temperature and thermal conductivity was explored. The novelty of this work is the use of this green adhesive for cork. It was found that the addition of lignin improves adhesion with cork thanks to the affinity of these materials, and a high percentage of this component produces modifications on the curing process. Besides, with the addition of lignin, the bisphenol A concentration is reduced.
 
This paper presents an investigation of the effect of wood porosity on surface creation by single-grit scratching method. Single grits with cone geometry were prepared to experimentally simulate wood sanding operation. Five wood species with different porosity ranging from about 30 to 80% were selected as samples. Besides, aluminum (Al 1060) samples were made as a contrast. The surface morphology was evaluated by ultra-depth 3D microscope and SEM observations, and the cross-sectional profiles of scratches were analyzed using a 3D profilometer. The results showed that much less plowed ridges or plastic upheaval occurred on both sides of the scratched grooves for wood samples than of Al 1060. With respect to cutting with grain, visible fibrils protrusion was found only for African sandalwood that had the lowest porosity among the five wood species. The deformation absorption capacity of wood was verified through microscopic evaluation. In the case of cutting across grain, fiber bundles of high-porosity wood species like balsa wood and black cherry were mainly faced with compressive crush while tensile rupture was the general pattern for low-porosity wood like sugar maple and African sandalwood. Corresponding force analysis for wood-grit interaction was made to account for the detailed material deformation and removal.
 
Experimental procedure. Heart rate variability, oxygenated hemoglobin, and core body temperature were continually measured before, during, and after the experiment. Additionally, questionnaires for subjective assessments, the irritation scale, the hedonic scale, and the VAS were administered immediately after inhaling the aroma of the experimental materials. Abbreviations are as follows: HRV, heart rate variability; OxyHb, oxygenated hemoglobin; Temp, core body temperature; Inhale, inhaling; Ques, questionnaires
Illustration of the HS-SPME procedure of analyzing the volatile compounds emitted from a square wooden cup filled with alcoholic beverages
HS-SPME/GC–MS analyses of the volatile compounds emitted from a square wooden cup filled with alcoholic beverages. 1. Isoamyl acetate, 2. Isoamyl alcohol, 3. Ethyl caproate, 4. Ethyl caprylate, 5. Decanal, 6. Ethyl caprate, 7. Phenethyl alcohol, 8. α-Cubebene, 9. β-Caryophyllene, 10. Cadina-1(6),4-diene, 11. α-Muurolene, 12. β-Bisabolene, 13. δ-Cadinene, 14. Calamenene, 15. Cubebol, 16. 1-Epicubebol, 17. Cubenol, 18. 1-Epicubenol, 19. Bornyl acetate, 20. Terpinen-4-ol, 21. α-Terpineol, 22. Borneol, 23. γ-Cadinene, 24. α-Calacorene, 25. t-Cadinol, 26. t-Muurolol, 27. α-Cadinol
Cryptomeria japonica (sugi; Japanese cedar) and Chamaecyparis obtusa (hinoki; Japanese cypress) are woods widely used in Japan for building and/or interior materials, furniture, and tableware. The relaxation effects of these wood-derived aromas on humans have been reported. An example of the use of these materials is the square wooden cups (Masu) used for sake vessels, which are commonly used for sake consumption. However, no studies have investigated the subjective and physiological effects of the aroma emitted from the wooden cups when used to consume alcoholic beverages. This study examined the effects of putting alcohol in wooden cups through physiological measurements of heart rate variability, cerebral blood flow, core body temperature, and subjective assessments. Moreover, this study’s result showed that the aroma of sake in all wooden cups and of shochu in C. japonica cups caused feelings of comfort and deliciousness. It was also found that the aroma of shochu in C. japonica cups and sake in Ch. obtusa cups could generate physiological relaxation. Ethyl caproate, ethyl caprylate, and ethyl caprate in alcoholic beverages and δ-cadinene and α-terpineol in wooden cups were determined as major compounds. The present results suggest that the beneficial effects of using sake vessels made from C. japonica and Ch. obtusa woods for alcoholic beverages increases the beverage’s taste, and their combination improves relaxing effects.
 
The oil palm industry is the backbone of the Malaysian economy. However, this industry is threatened by basal stem rot (BSR) caused by the fungus Ganoderma boninense, which has caused severe losses over the past several decades. The study reported here is part of an ongoing project to reduce Ganoderma inoculum and manage oil palm waste on plantations, which could be a green technology approach to reduce BSR infection pressure. A preliminary search for potential white-rot hymenomycetes to accelerate the degradation of oil palm stumps has been initiated. As a continuity, the present study was designed to comprehensively investigate the changes in wood crystallinity at different stages of oil palm wood degradation after pretreatment with three white-rot hymenomycete species. The findings were used to limit potential white-rot candidate(s) for their secretion of cellulolytic, hemicellulolytic and amylolytic enzymes, selectively or simultaneously, using principal component analysis. The overall per cent crystallinity of healthy wood decayed by Lentinus tigrinus increased as degradation proceeded. Conversely, crystallinity generally increased at 75 d in the diseased wood pretreated with Trametes lactinea, owing to the preferential degradation of noncrystalline lignins and hemicelluloses by this strain. Anatomical characterization revealed penetration of fungal mycelia into the degraded wood vessels, phloem and parenchymal tissues with microscopic cavities. In advanced stages of pretreatment, the loosening of the parenchymal tissue, including the wood rays, was more rapid in the diseased wood than in the healthy wood. This study provides new insights into the biochemical and anatomical changes initiated by white-rot hymenomycetes during the degradation of oil palm wood blocks. The present findings could be efficiently utilized for sustainable biological stump pretreatment, particularly with respect to the polysaccharide components of lignocellulosic materials.
 
High absorption and low reflection electromagnetic interference (EMI) shielding films were urgently required to solve increasingly serious radiation pollution. Micro-/Nano-cellulose (MNC) electroless silver film was prepared to obtain ideal electromagnetic shielding performance. The change of electromagnetic shielding efficiency and the performance of the optimal conditions in the process of electroless Ag were analyzed from the standpoint of different electroless Ag concentration to obtain films with excellent electromagnetic shielding. The results showed that Ag particles formed a dense and uniform coating with gradually increasing AgNO3 concentration from 0.05 to 0.125 mol/L. The surface conductivity of the film increased from 1 to 20 S/cm and the surface roughness was 4 μm. The diameter of Ag microcrystalline particles was 18 nm. The contact angle of films was 99°. Here, the electromagnetic shielding efficiency was up to 41 dB in the frequency range from 0.03 to 3000 MHz, indicating that the electromagnetic wave absorption rate of composite films has reached 99.9%, which demonstrated that the EMI shielding mechanism was attributed to the high absorption.
 
To investigate the combustion behavior of furfurylated wood in the presence of montmorillonite (MMT) and its char properties, original, MMT-treated, furfuryl alcohol (FA)-treated (i.e., furfurylated), and MMT/FA-treated wood were pyrolyzed and carbonized using thermo-gravimetry and cone calorimeter, respectively. Results on thermal stability, gas emission, microstructure, surface area, porosity, and chemical components confirmed that MMT dramatically affected the pyrolysis features of both control and furfurylated wood, and increased the char yield due to the catalytic activity and barrier effect of the clay sheets. Furfurylated wood released massive carbon-containing compounds and heat during combustion, which supported burning. The flame retardancy index (FRI) values of MMT/FA-treated wood were increased by 65.6% compared with the furfurylated wood, suggesting that MMT inhibited the spreading of gases and heat and promoted the formation of protective char layer. MMT uniformly distributed in wood cell lumina, promoted the effective carbonization of cellulose, and restrained the formation of carbon gaseous species, thus exerting a pronounced effect on char formation. The microstructure and chemical composition of the char residue indicated that due to the acid catalysis and barrier property of MMT sheets and the wrap of furfuryl alcohol resin, the coherent surface and porous structure of the char residues with Si–O–C/C–Si structures dominated the char-forming process, which was conducive to the comprehensive utilization of modified wood waste.
 
In this study, chromatographic and spectrophotometric analysis of methanolic extract of heartwood of Pterocarpus santalinus Linn (Red Sanders) collected from trees grown in eight different locations was carried out. Usefulness of these techniques for discrimination of some of the commonly occurring species in Pterocarpus genus was assessed. Thin layer chromatography (TLC) fingerprints of methanolic extract of P. santalinus heartwood revealed distinct spots at retention factor (Rf) 0.94 (light brown), 0.91 (purple), 0.79 (blackish brown), 0.75 (light purple), 0.60 (yellow), 0.51 (dark red) and 0.28 (brownish red) in normal phase. High-performance liquid chromatography (HPLC) analysis indicated at least 11 common peaks at retention time (tR) of 3.1, 3.5, 8.9, 11.1, 12.8, 13.5, 14.9, 15.7, 17.9, 18.3, and 26.5 (minutes). Ultraviolet visible spectrophotometric analysis of methanolic extract of heartwood exhibited a broad absorption spectrum with consistency in major absorbance peaks at ~ 280 nm, ~ 320 nm, ~ 472 nm and ~ 505 nm. Similarities in chromatographic pattern between the P. santalinus samples from different locations were evaluated using principal component analysis. The developed fingerprints of methanolic extract of P. santalinus heartwood were compared with those of Pterocarpus marsupium, Pterocarpus soyauxii, and Pterocarpus macrocarpus. The results demonstrate that HPLC and TLC fingerprints can supplement wood anatomy in accurate identification of above Pterocarpus species. UV visible absorption spectrophotometric analysis can be helpful to differentiate P. santalinus from P. macrocarpus and P. marsupium, but P. santalinus and P. soyauxii could not be distinguished using this technique.
 
Schematic diagram of the preparation process
Chemical composition characterization of samples. a FTIR spectrogram, b relative content of cellulose, hemicellulose and lignin in OW and DW samples, c relative content of three elements in OW and DW samples
Pore diagram of the sample. a SEM images of the cross section of the OW sample, b SEM images of the cross section of the DW sample, c SEM images of the cross section of the TW sample, d SEM images of the cross section of the DCW sample, e SEM images of the cross section of the TCW sample, f differential distribution curve of macroporous pore size (the upper right corner is the mercury advance—retreat curve), g differential distribution curve of mesoporous pore size ( the upper right corner is the isothermal—adsorption desorption curve)
Characterization of optical properties of samples. a Transmittance curves of OW, UV resin, TW and TCW samples, b haze curves of OW, UV resin, TW and TCW samples, c macro effect picture, d light scattering effect picture, e chromatogram of TW and TCW samples compared with that of OW sample. (x-axis is perpendicular to the direction of wood fiber, y-axis is parallel to the direction of wood fiber)
Characterization of mechanical properties and thermal stability of samples. a Stress–strain curves of samples along the fiber direction, b histogram of surface hardness, c thermogravimetric analysis diagram
In this work, the top-down modification method is used to densify wood cell walls. The cell wall was simply compressed. The external force is applied to the delignified wood template by using a press to make the radial thickness of the wood template smaller, and finally, a more compact and uniform wood template with cell wall skeleton is obtained. Then, the resin with similar refractive index to cell wall is vacuum impregnated into the dense wood template, and the transparent compressed wood with high transparency is successfully obtained. The transmittance is increased by 34.1%, the haze decreased by 28.1%, and the transparent effect is more excellent through the cell wall densification of the delignification template. Moreover, the transparent compressed wood has better mechanical properties (maximum tensile strength was 113.75 MPa, Young's modulus was 2.276 GPa) and thermal stability. The method has successfully improved the function and structure of transparent wood, and the highly transparent compressed wood is a new functional material with application potential.
 
Top-cited authors
Peter Niemz
  • ETH Zurich
Holger Militz
  • Georg-August-Universität Göttingen
Samuel L. Zelinka
  • US Forest Service Forest Products Laboratory
Samuel V. Glass
  • US Forest Service
Helena Pereira
  • University of Lisbon