ALBERT

Description: In this study, the effect of thermo-mechanical densification on the density, hardness, compression strength, bending strength (MOR), and modulus of elasticity (MOE) of fir and aspen wood pretreated with water repellents was analyzed. Wood specimens were impregnated with paraffin, linseed oil and styrene after pre-vacuum treatment. Then, the impregnated wood specimens were densified with compression ratios of 20 and 40%, and at 120, 150 and 180 °C. The results indicated that the density, hardness and strength properties of the all densified specimens (untreated and impregnated) increased depending on the compression ratio and temperature. For all tested properties, higher increases were obtained in the paraffin and styrene pretreated specimens compared to untreated samples. However, the increase rates in linseed oil pretreated specimens were generally lower than untreated specimens. Regarding water repellents the most successful results in all tested properties were determined in styrene pretreated specimens. The density, hardness and strength properties of all specimens increased with the increase in compression ratio. On the other hand, the increase in the compression temperature negatively affects the properties of untreated and linseed oil pretreated specimens, while having a generally positive effect on the properties of paraffin pretreated specimens. However, all tested properties of styrene pretreated specimens have increased significantly due to the increase in compression temperature. The increasing strength properties of wood as a result of densification have increased much more with paraffin and especially styrene pretreatment. These combinations can be considered as an important potential for applications that require more hardness and strength.

Description: This study aims at identifying the mechanisms of oil and water imbibition in heartwood and sapwood of Douglas-fir through a combination of original experiments with magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) relaxation measurements for oil and free water, and deformation measurements for bound water. Experiments by weighing are performed to verify whether the imbibition process is also consistent with Washburn law. All the results are discussed taking into account the structure of wood (tubular tracheids closed at their tips, but possibly connected to each other via open pits on the side faces) and the preparation of samples. The observation of relatively fast oil flow imbibition confirms that sapwood exhibits a connected hydraulic network through which a liquid can a priori flow and climb along the structure. However, the spontaneous water imbibition is strongly damped by its very poor wetting when in contact with cell-walls only partially saturated with bound water, so that the diffusion of bound water control the uptake dynamics. However, due to preferentially closed pits, the heartwood does not exhibit a continuous hydraulic network and water essentially penetrates into wood by diffusion through the cell walls.

Description: The objective of this work was to provide a rapid and nondestructive imaging method for evaluating the hygroscopic behavior of thermally modified lignocellulosic materials (softwood and hardwood). The difference in the hygroscopic behavior was explained by moisture content (MC) mapping results and molecular association characteristics of absorbed water (i.e. weakly, moderately, and strongly hydrogen-bonded water molecules) with wood at various relative humidities (RH). To achieve this goal, near-infrared (NIR) spectral images in the wavelength range 1816–2130 nm (covering the combination of stretching and deformation vibrations for OH) were used to visualize MC distributions over the surface of Japanese cedar and European beech samples which had been thermally treated at different temperatures. A curve fitting method was utilized to explore changes in water-wood structure characteristics based on shifts to longer wavelength in spectral signals caused by increasing MC. The curve fitting results support the recent nuclear magnetic resonance (NMR) studies that different bound water stabilities may pool in different compartments of the wood cell wall. Furthermore, water was firmly bound to wood at low RHs and H-bonds gained mobility as the number of absorbed molecules increased. It is concluded that NIR hyperspectral imaging also has the potential to be a complementary methodology for studying the transient changes of wood-water interactions before equilibrium.

Description: Density of wood can be increased by filling its porous structure with polymers. Such densification processes aim to increase hardness of wood and are particularly interesting for flooring applications. This study aims to evaluate efficiency of different polymers for chemical densification based on the polymer properties. Yellow birch (Betula alleghaniensis Britt.) was chemically densified with seven monomer mixtures through acrylate monomer impregnation and electron beam in-situ polymerization. Chemical retention and polymer content of densified woods were recorded. Hardness of treated and untreated Yellow birch was measured and compared to hardness of Jatoba (Hymenaea courbaril L.). All densified woods showed higher or comparable hardness to Jatoba. Hardness of densified wood was analyzed in relation to initial density of wood and polymer content of the material using multivariable linear mixed models. Efficiency of polymers for chemical densification was evaluated through effect of polymer content on hardness with interaction coefficients. Polymer films corresponding to monomer impregnating mixtures were prepared through low energy electron beam and characterized by their glass transition temperature, micro hardness, indentation modulus and crosslinking density. Polymers showed statistically significantly different efficiencies and were separated in two main groups. Overall, polymer efficiency increased with increasing glass transition temperature of polyacrylates.

Description: To improve the reactivity of lignin for incorporation into high value polymers, the introduction of amines via Mannich reaction is a commonly used strategy. During this functionalization reaction, intra- as well as intermolecular lignin–lignin crosslinking occurs, which can vastly change the elastic properties of the lignin, and therefore, the properties of the resulting polymer. Therefore, the molecular structure of the amine that is used for such a lignin functionalization may be of great importance. However, the relationship between the molecular structure of the amine and the elastic properties of the lignin-based polymer that is generated thereof, has not been fully understood. Herein, this relationship was investigated in detail and it was observed that the molecular flexibility of the amines plays a predominant role: The use of more flexible amines results in an increase in elasticity and the use of less flexible amines yields more rigid resin material. In addition to the macroscopic 3-point bending flexural tests, the elastic modules of the resins were determined on the nanometer scale by using atomic force microscopy (AFM) in the PeakForce tapping modus. Thus, it could be demonstrated that the intrinsic elasticities of the lignin domains are the main reason for the observed tendency.

Description: The conformation of lignin in solvents is major key factors governing the physicochemical properties of aromatic polymers. However, the conformational changes of lignin in good and poor solvents is not clearly understood at the molecular level. In this study, short- (DP 2.77) and long-chain (DP 4.49) lignin oligomer model compounds composed solely of interunit β–O–4 bonds with a narrow polydispersity were synthesized, and their NMR spectra were recorded to evaluate the molecular structural changes induced by addition of water to an organic solvent. The spectral signals were completely assigned in DMSO-d6 and D2O by applying 2D 1H–13C HSQC, HMBC, and long-range heteronuclear single quantum multiple bond correlation (LR-HSQMBC). The conformation of the long- and short-chain lignin models were analyzed by 2D 1H–1H ROESY. In all the solvent systems, consisting of DMSO-d6 and containing 0–90% volume of acetic acid-d4 buffer in D2O (pD 5.0), the lignin models were found to have folded conformations, but more compact structures were observed in D2O compared with DMSO-d6.

Description: Natural constituents of wood cell-wall layers are affected in various ways by thermal treatment. This study investigated the effect of high-temperature treatment on the properties of cell-wall layers. The properties were studied using PeakForce quantitative nanomechanical mapping and Fourier-transform infrared spectroscopy (FTIR). European beech wood was thermally treated at 200 °C for 1, 3, and 5 h in an oxidizing atmosphere. Modulus of elasticity, adhesion force, and roughness of the secondary S2 layer and the compound middle lamella (CML) were determined using atomic force microscopy (AFM). Results showed that both the S2 layer and CML were affected by thermal treatment. Stiffening of the S2 layer was caused by increased crystallinity of the cellulose-dominated component, having peaked after 1 h of treatment. The degradation thereafter resulted in a decrease of the S2 as well as the CML stiffness. An increase of CML roughness after 3 h of treatment was associated with the effect of thermal degradation on CML integrity. The analysis suggested that the reduction in syringyl lignin is potentially associated with an increase in adhesion of cell-wall layers.

Description: The aim of this investigation was mainly to evaluate the multi-scale distribution of furfuryl alcohol (FA) resin in modified Chinese fir and poplar wood. 13C CP/MAS NMR, Scanning Electron Microscopy (SEM), Confocal Laser Scanning Microscopy (CLSM), Nanoindentation and Imaging Fourier transform infrared microscopy (Imaging FT-IR) were applied to describe the FA resin distribution in wood from bulk to cell wall scale. The results showed that FA resin were mainly located in the cell cavity of Chinese fir tracheids. For poplar, FA resin was mostly deposited in the cavity of fibers and ray cells, while little was found in the adjacent vessels. Lots of pits of wood cells were covered with FA resin which implied a higher risk of drying after wood furfurlation in practical production. Nanoindentation demonstrated that FA resin could easily infiltrate into the wood cell wall because both reduced modulus and hardness of the modified wood cell walls were significantly improved. This conclusion was further supported by the results of imaging FT-IR.

Description: In this study, oriented bamboo scrimber board (OBSB) was manufactured with a synthesized formaldehyde-free tannin adhesive. The chemical properties of the tannin adhesive were analyzed with 13C nuclear magnetic resonance spectroscopy (13C-NMR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). Results revealed that 70% aqueous acetone extractives of Acacia confusa bark (AcBAE) contained abundant phenolic compounds and condensed tannins comprising mainly B-type flavonoid bonds. Moreover, cross polarization magic angle spinning (CP/MAS) 13C-NMR mapping of cured Acacia confusa bark tannin glue (AcBTanGlu) revealed that condensed tannin reacted with hexamethylenetetramine to form a polymer with a cross-link structure through a polymerization mechanism. According to the results obtained from gas chromatography-flame ionization detector (GC-FID) analysis, no formaldehyde was emitted from AcBTanGlu-glued OBSB. Thermogravimetric analysis indicated that AcBTanGlu significantly improved the thermal stability of AcBAE after the curing reaction. Comparison of the DRIFT spectra of bamboos before and after AcBTanGlu-glued treatment indicated AcBTanGlu did not impact the chemical functional properties of the bamboo. Compared to phenol formaldehyde (PF) and urea formaldehyde (UF)-glued OBSBs, AcBTanGlu-glued OBSBs statistically had lower modulus of elasticity (MOE) and modulus of rupture (MOR). As for the nail withdraw resistance, AcBTanGlu-glued OBSBs are statistically comparable to PF-glued OBSBs and higher than UF-glued OBSBs.

Description: Cellulose and hemicelluloses were isolated from birch wood using a dilute alkaline solution and then consolidated into pellets as model compounds of cellulose and hemicelluloses in the wood cell wall. The purity of isolated cellulose and hemicelluloses was examined by Fourier-transform infrared spectroscopy and thermogravimetric analysis. The density, thermal diffusivity, heat capacity, and thermal conductivity were experimentally determined for consolidated birch powder, cellulose, and hemicelluloses in over-dry condition. The thermal degradation kinetic parameters of these materials were successfully calculated using a conversion rate step of 0.01, and the relationship with conversion rate was established. The results show that cellulose and hemicelluloses consolidated under 25 MPa had densities of 1362 kg/m3 and 1464 kg/m3, respectively. The cell wall of birch powder in the oven-dry state was not collapsed under 25 MPa. The thermal diffusivity of consolidated birch powder, cellulose, and hemicelluloses linearly decreased with temperature, with values of 0.08, 0.15, and 0.20 mm2/s at room temperature, respectively. The specific heat capacity (1104, 1209, and 1305 J/(kg·K) at 22 °C, respectively) and thermal conductivity (0.09, 0.24, and 0.38 W/(m·K) at 22 °C, respectively) linearly increased with temperature, except for those for hemicelluloses which exhibited a nonlinear relationship with temperature above 120 °C, and their linear experimental prediction equations were given. Birch cellulose was more thermally stable than hemicelluloses. The thermal degradation kinetic parameters including activation energy and pre-exponential factor of birch powder, cellulose, and hemicelluloses varied with the conversion rate and calculation methods, with average activation energy in a conversion rate range of 0.02–0.15 of 123.2, 159.0, and 147.2 kJ/mol, respectively (using the Flynn–Wall–Ozawa method), for average natural logarithm pre-exponential factors of 25.0, 33.1, and 28.7 min−1, respectively. Linear and quadratic equations were fitted to describe the relationship between the kinetic parameters and conversion rates. These results give comprehensive thermal properties of the densified cellulose and hemicelluloses isolated from a specific wood.