Tsunehisa Miki

Micropores and mesopores in the cell wall of dry wood

To investigate micropores and mesopores in the cell walls of dry wood, CO2 gas and N2 gas adsorption onto dry wood were measured at ice-water temperature (273 K) and liquid nitrogen temperature (77 K). CO2 gas adsorption isotherms obtained were used for determining micropore volumes smaller than 0.6 nm by the HK method (Horvath-Kawazoe method), and N2 gas adsorption isotherms obtained were used for determining the mesopore volume between 2 nm and 50 nm by the Barrett-Joyner-Halenda (BJH) method. Micropores and mesopores existed in cell walls of dry wood, and the cumulative pore volume was much larger for micropores than for mesopores. Micropores in the cell wall of dry wood decreased with elevating heat treatment temperature, and the decreased micropore was reproducible by wetting and drying. Mesopores did not decrease so much with elevating heat treatment temperature. Micropore volumes for the softwood Hinoki and the hardwood Buna were compared. A larger amount of micropores existed in hardwood Buna than in softwood Hinoki, and this relationship was considered to correspond to the difference in thermal softening properties for lignin in water-swollen Hinoki and Buna. This result probably indicates that micropores in the cell walls of dry wood relate to the structure of lignin.

Preparation of Three Dimensional Products Using Flow Deformability of Wood Treated by Small Molecular Resins

To investigate the effect of the additive agents such as polyethylene glycols (PEGs), melamine formaldehyde resin (MF-resin) and phenol formaldehyde resin (PF-resin) on the flow deformability of solid wood, free compression tests during heating were performed. Various molecular weights ranging from 200 to 20,000 for PEGs and almost similar molecular weight around 380 for MF-resin and PF-resin were applied. It was found from the compression tests that the yield stress indicating wood cell deformation resistance was drastically decreased with smaller molecular PEGs in wood, whereas the initiation of flow behavior, which is derived from detachment/slippage between cells, occurred at lower pressure with larger molecular PEGs. For generating the flow behaviors of solid wood, smaller molecular resin/substance was not always suitable. Thermosetting agents also act as a plasticizer during heating and especially the PF-resin showed better softening effect as well as a promoter of flow behavior than the MF-resin with almost similar molecular weight. This indicates that it is important for generating flow behavior to consider affinity/compatibility of resin to wood constituents. A maximum flow deformation ratio in the tangential direction of wood reached 180 % when using PEG 20,000 and MF-resin as an additive agent. It was also demonstrated that using PF-resin and MF-resin deep cup products shaped by a backward extrusion process had a better size stability against water, steam, and acetone.

溶液含浸木材の養生過程における細胞壁への溶質拡散機構の検証: 相対湿度がポリエチレングリコール水溶液含浸木材の膨潤・収縮挙動に及ぼす影響

To control the amount of solute in cell walls of solution impregnated wood using the conditioning process, the mechanisms of solute diffusion into the cell walls and of solvent evaporation from wood under the process were verified. The effect of relative humidity (RH) on temporal variability of swelling, shrinkage, and mass of wood impregnated with an aqueous solution of polyethylene glycol (PEG1540) was examined. The impregnated wood specimen swelled under the conditioning at the RH over 75%. The specimen was indicated to swell when the amount of the PEG polymers in the cell walls increase in this RH range. On the basis of this indication, the temporal variability of increasing rate of the polymers in the cell walls and of evaporating rate of water from the specimen under the conditioning was well explained by the mechanisms of the solute diffusion and the solvent evaporation, respectively. In the RH range, the increasing amount of the polymers in the cell walls increased with the evaporating amount of the water, which increased with the decrease in the RH. These results were supported by the mechanisms of the solute diffusion and the solvent evaporation, respectively. The diffusion mechanism also supported the effect of the history of the RH on the polymer amount in the cell walls throughout the conditioning and subsequent drying in a vacuum. It was concluded from these findings that the solute diffusion into cell walls is able to be controlled by the surrounding vapor pressure of solvent when the polymers (PEG1540) and water are employed as the solute and solvent, respectively.

Instrumental analyses of nanostructures and interactions with water molecules of biomass constituents of Japanese cypress

Nanostructures consisting of the biomass constituents of the denatured Japanese cypress (Chamaecyparis obtusa) were examined by instrumental analyses at multiple hierarchical levels. Delignification with NaClO2 solution smoothly proceeded to reveal a distorted cell by scanning electron microscopy; however, a trace amount of lignin still remained in the delignified sample according to attenuated total reflection infrared spectra (ATR-IR). Although hemicellulose could be removed by a treatment with NaOH solution, thermogravimetric analysis and 13C cross-polarization/magic angle spinning (CP-MAS) NMR showed a certain amount of hemicellulose remaining. Reaction of the delignified sample with NaOH solution produced a shrunken cell wall that consisted of cellulose with small amounts of lignin and hemicellulose, which were detected by ATR-IR and 13C CP-MAS NMR, respectively. These samples from which lignin and/or hemicellulose had been removed easily released water molecules, producing a decrease in the 1H signal intensity and longer 1H spin–lattice relaxation time (T1H) values in variable temperature 1H MAS NMR. The T1H values provided information about nano-scale molecular interaction difficult to obtain by other instrumental analyses and they greatly changed depending on the water content and ratio of the biomass constituents. The spin–lattice relaxation of all samples occurred via water molecules under humid conditions that provided sufficient water. Under heat-dried conditions, the spin–lattice relaxation mainly occurred via lignin for the samples with lignin remaining while it occurred via cellulose/hemicellulose for the samples without lignin. The variable temperature T1H analysis indicated that predominant spin–lattice relaxation route via lignin was caused by higher molecular mobility of lignin-containing samples compared with lignin-free samples.

Superplastic deformation of solid wood by slipping cells at sub-micrometre intercellular layers

In order to facilitate the generation of a flow phenomenon due to the slipping of wood cells under a specific temperature condition, a phenol formaldehyde resin of low molecular weight was introduced into wood cells. The effects of the presence of phenol formaldehyde molecules in wood cells on the flow behaviour of solid wood were investigated experimentally by means of a free compression test. The effectiveness of using phenol formaldehyde resin as an adsorbent to act as both binding and plasticising agents in the proposed wood flow forming shaping technique was examined, and an application to wood flow forming was demonstrated. The results revealed that the flow phenomenon of solid wood occurred at a certain resin content, even under compression at less than 25 MPa. An increase in the moisture content led to further improvement of the flowability of solid wood, which resulted from weakened facial strength among wood cells and intercellular layers due to a local increase in the volume of nano-level pores. Finally, the effectiveness of introducing phenol resin into wood for wood flow forming through backward extrusion was confirmed.

Integrated analysis of solid-state NMR spectra and nuclear magnetic relaxation times for the phenol formaldehyde (PF) resin impregnation process into soft wood

The effects of phenol formaldehyde (PF) resin impregnation into Japanese cedar plates were studied by the integrated analysis of solid-state NMR spectra and relaxation times. 13C cross-polarization/magic angle spinning (CP-MAS) NMR spectra showed that PF resin permeated near carbohydrate polymers as well as lignin regions, providing hydrophobicity in a woody material. Additionally, 13C pulse saturation transfer/magic angle spinning (PST-MAS) NMR spectra revealed that the molecular mobility of cellulose endocyclic groups was suppressed by the PF resin impregnation. Spin-lattice relaxation times in the laboratory frame, T1H and T1C, for both untreated and impregnated woods decreased in humid conditions while they were increased by the PF resin impregnation. Meanwhile, spin-lattice relaxation in the rotation frame, T1ρH, decreased in humid conditions as well as being a result of the PF resin impregnation. According to the tendencies of the T1H, T1C, and T1ρH values associated with the PF resin impregnation, although faster molecular motions of lignin OCH3 group were increased in the MHz frequency range, slower molecular motions of the cellulose ring were suppressed in the kHz frequency range, which confirmed the result from the 13C PST-MAS NMR spectra.

Solute diffusion into cell walls in solution-impregnated wood under conditioning process III: effect of relative humidity schedule on solute diffusion into shrinking cell walls

This study has focused on solute diffusing into cell walls in solution-impregnated wood under conditioning, process of evaporating solvent. The purpose of this paper was to clarify the RH- (relative humidity-) schedule that promotes the solute diffusion into shrinking cell walls during conditioning. The wood samples impregnated with a 20 mass% aqueous solution of polyethylene glycol (PEG1540) was conditioned with a temperature of 40 °C to the equilibrium point at the RH where the samples swelled maximally. The samples were subsequently conditioned at 40 °C under the schedules including four ways of RH-decrease steps where the cell walls shrunk. The amount of solute (PEGs) diffused into cell walls during the conditioning logarithmically increased with increasing the number of the RH-decrease steps. This was well explained by the theoretical model that describes the solute diffusion into shrinking cell walls. It is clarified from the model that the RH, or moisture content of the sample, should be decreased as gradually as possible to increase the total amount of diffused solute into shrinking cell walls, and that the amount of diffused solute is smaller for the lower moisture content. The model also suggests that effect of change in RH schedule on change in total amount of diffused solute does not depend on solute diffusivity in the sample under drying in a vacuum over phosphorous pentoxide, and that impregnated wood should be conditioned under natural convection rather than forced convection for promoting the diffusion into shrinking cell walls.

Solute diffusion into cell walls in solution-impregnated wood under conditioning process II: effect of solution concentration on solute diffusion

This study focused on solute diffusing into cell walls in solution-impregnated wood during conditioning, process of moderate drying of solvent. To clarify the effect of solution concentration on the diffusion during the conditioning, weight percent gain (WPG) and relative swelling of the wood sample impregnated with an aqueous solution of polyethylene glycol (PEG) polymers at a concentration of 10, 20, 30, 40, or 50 mass% were examined during the conditioning and subsequent drying processes. The relation between the concentration and the relative swelling after all processes, an indicator of the amount of the polymers in cell walls, exhibited a concave-downward curve with a maximum value at 20 mass%. The estimated mass of the polymers in cell walls just before conditioning increased with the concentration. This indicates that the distribution of the polymers changed during conditioning. The estimated mass just before conditioning and the relative swelling after all processes were normalized to the packing ratios of the polymers in cell walls. The ratio after all processes subtracted by that just before conditioning was larger than the ratio just before conditioning, and increased with the concentration up to 20 mass%; after which it decreased. This indicates that the majority of the polymers in cell walls increased during conditioning, and that the amount of the polymers that diffused into cell walls was at the maximum at concentration of 20 mass%. This was explained by two factors: the decrease in the diffusivity into cell walls and in the concentration difference of the polymers between cell walls and cell cavity with the concentration, based on the behavior of WPG during conditioning; and the estimated minimum concentration at which the solution contains the least amount of polymers to fill the cell walls.

Destabilization of wood microstructure caused by drying

Abstract To obtain new information about destabilization of wood microstructure caused by drying, effects of drying history on physical properties of wood were studied using the measurements of dynamic viscoelastic properties and gas adsorption. First, dynamic viscoelastic properties of dry wood in the radial direction were measured between 100°C and 200°C. Unstable states of dry wood still existed after heating at 105°C for 30 min and were modified by activated molecular motion in the first heating process to higher temperatures above 105°C, and dry wood subjected to higher temperatures showed larger dynamic elastic modulus (E′) and smaller loss tangent (tan δ). The phenomena thought to be caused by the unstable states reappeared after wetting and drying again. Secondly, carbon dioxide adsorptions onto dry wood at ice-water temperature (273 K) were measured, and micropore size distributions were obtained using the Horvath–Kawazoe (HK) method. Micropores smaller than 0.6 nm exist in dry wood. They decreased with elevating drying temperatures from 50°C to 160°C and increased again after rewetting and drying. In conclusion, it was confirmed that wood components in the microstructures were destabilized by drying and that physical properties of dry wood changed with drying histories.

Anomalous thermal expansion behaviors of wood under dry and low-temperature conditions

Abstract The thermal expansion behavior of dry solid wood was investigated by dynamic dilatometry and thermal mechanical analysis. Anomalous thermal expansion behavior was observed concerning the displacement change under a constant compression pressure, which was not previously reported. Wood submitted to temperatures below 0°C under dry conditions exhibited a large increment in the linear thermal expansion coefficient (CLTE) and a sudden drop in the CLTE around 50°C as well as above 130°C during heating. In subsequent cooling/heating processes, these anomalous behaviors remained at temperatures below 100°C, although less pronounced, and disappeared at temperatures above 100°C. These behaviors were clearly perceptible in the radial and tangential directions but not in the longitudinal direction. The CLTE depended strongly on the heat and moisture history of the samples and the effects are species-specific.