Takato Nakano

Contractive force and transformation of microfibril with aqueous sodium hydroxide solution for wood.

Summary The mechanism of longitudinal contraction of Yezo spruce (Picea jezoensis Carr.) during treatment with aqueous NaOH solution is discussed. The contraction of wood samples increased with an increase in the concentration of NaOH solution and in heating temperature. Measurements of the stress relaxation and the twist angle during the alkali treatment showed that the alkali treatment caused the contractive and twist forces of a tracheid cell wall in longitudinal and tangential directions, depending on the components of contractive force. The temperature dependence of the contraction and the thermodynamics led to the conclusion that the longitudinal contraction of samples is due to that of microfibrils via an entropy-elastic force.

Artificial weathering of tropical woods. Part 2: Color change

Abstract This paper describes the change in diffuse reflectance Fourier transform infrared (DRIFT) and ultraviolet-visible (UV-Vis) diffuse reflectance spectra of the following eight tropical woods during artificial weathering up to 600 h in relation to their color changes, especially to yellowing: Amnurana acreana, Acacia auriculiformis, Dipterocarpus spp., Eucalyptus marginata, Eucalyptus robusta, Shorea spp. and Tabebuia spp. with relatively high and low specific gravity. For A. acreana, A. auriculiformis, Dipterocarpus spp. and both Tabebuia spp., Δb* (yellowing) increased with exposure up to 50 h, and decreased above 50 h. For E. marginata, E. robusta and Shorea spp., on the other hand, both Δa* (shift to red) and Δb* decreased with increased exposure time. For woods in which Δb* increased, the Δb* showed a positive dependence on the difference in relative intensity ratio of a band at 1740 cm–1 to that at 2900 cm–1 (ΔD1740/D2900) in DRIFT spectra of specimens before and after exposure. This result indicated that the increased band at 1740 cm–1 played a significant role in the increased Δb*. Then again, also for woods in which Δb* decreased, the ΔD1740/D2900 increased, but was not related to the Db*. The ΔD1740/D2900 for woods in which Δb* decreased had a positive relationship to the difference in remission function at 410 nm (ΔF(R∞)410) in UV-Vis diffuse reflectance spectra for specimens before and after exposure, while woods in which Δb* increased were independent of the ΔF(R∞)410. Therefore, it is suggested that woods in which Δb* increased and decreased differ from one another in the contribution to the increase in the band at 1740 cm–1, resulting in either an increase or decrease of Δb*.

Mechanism of Thermoplasticity for Chemically-Modified Wood

On the basis of a concept of the free volume, the mechanism of thermoplasticity for chemically-modified wood which was treated with trifluoroacetic acid anhydride and the fatty acid (TFAA method), is discussed. A proportional relationship between the Volumetrie swelling with the treatment and the free volume was derived from experimental results and the volume of various introduced side-chains. Moreover, systematic dependence of viscoelastic properties on the amount and the length of the introduced side-chains was due to creation of the free volume in wood substance. From both results and the assumption that TFAA method breaks down the lignin network, the mechanism of thermoplasticity is explained äs follows. The introduction of side-chains into wood substance and breakdown of the lignin network during the reaction cause the Volumetrie swelling which is proportional to the sum of the volume of introduced side-chains and the free volume. Then, the interaction between molecules of wood components decreases because of the creation of the free volume and the modified wood substance has thermoplasticity, when large or many side-chains are introduced with chemical modification.

Artificial weathering of tropical woods. Part 1: Changes in wettability

Abstract Changes in the wettability of eight species of tropical woods during artificial weathering up to 600 h are discussed from the aspect of chemical and structural changes in their surfaces: Amnurana acreana, Acacia auriculiformis, Dipterocarpus spp., Eucalyptus marginata, Eucalyptus robusta, Shorea spp., and Tabebuia spp. with relatively high and low specific gravity. On the whole, the wettability of specimens decreased upon irradiation up to 20 h; above that they increased. Changes in wettability during artificial weathering differed according to wood species. The IR spectra suggest that the specimen surfaces after irradiation for 600 h result in a cellulose-rich layer, and therefore the increase in wettability during artificial weathering can be explained in terms of the increase in hydroxyl groups originating from both the exposed cellulose and adsorbed water. However, the difference in wettability exists between species even after the surface develops a cellulose-rich layer. The stereoscopic micrographs showed the development of cracks for all of the specimens after irradiation for 600 h, and differences in their magnitudes according to species. From these results, the differences in wettability between species were estimated to be due to the structural changes on the surface during artificial weathering, whereas the increase in wettability was due to the chemical changes.

Surface fractal dimensionality and hygroscopicity for heated wood

Summary The relationship between hygroscopicity and the microsurface of heated wood was examined using the fractal surface dimensionality. The hygroscopicity of heated wood decreased with the increase in heating temperature to 250°C, and then decreased again above 350°C after increasing up to 350°C. This change corresponded to chemical changes in the wood, especially a reduction in hydroxyl groups, up to 250°C, and to the temperature dependence of the fractal dimensionality calculated from nitrogen gas adsorption above 250°C. The fractal dimensionality increased gradually from 100 to 250°C, followed by a rapid increase above 250°C with a peak at 350°C, and leveled off above 400°C. From the results, it is concluded that hygroscopicity of heated wood changes at 250°C and that it is dependent upon the chemical properties of wood below 250°C and upon the surface complexity above 250°C.

Effect of Thermal Treatment on Fracture Properties and Adsorption Properties of Spruce Wood

The effect of thermal treatment on spruce is examined by analyzing the fracture and hygroscopic properties. Specimens were heated at temperatures within the range 120–200 °C for 1 h. Fracture energy was measured using a single-edge notched bending test and the strain-softening index was estimated by dividing the fracture energy by the maximum load. Adsorption properties were estimated using adsorption isotherms. Fiber saturation points (FSPs) were estimated by extrapolating the moisture adsorption isotherm curve. Langmuir’s adsorption coefficient and number of adsorption sites were obtained using Langmuir’s theory and the Hailwood-Horrobin theory, respectively. The fracture energy, FSPs, and specimen weights decreased at temperatures higher than 150 °C, but the critical point for the strain-softening index and the number of adsorption sites was shown to be 180 °C. We hypothesize that the fracture energy and FSP depend on the chemical structure of the cell wall, whereas the strain-softening behavior may be influenced by the number of adsorption sites, and in turn the number of hydrogen bonds in hemicellulose.

Contraction of the microfibrils of wood treated with aqueous NaOH: evidence from changes in the anisotropy of the longitudinal and transverse swelling rates of wood

Previously, we reported that wood samples contract longitudinally, although the detailed mechanism for this contraction was undetermined. Stockmann reported that wood cells contract longitudinally and twist into S-helixes while lignin and hemicelluloses are removed in pulping, and according to Nakano et al., the longitudinal contraction of Yezo spruce (Picea jezoensis) increases with both the concentration of the NaOH solution and temperature. Based on experimental results, Nakano et al. proposed that the longitudinal contraction of samples resulted from the contraction of microfi brils via an entropy–elastic force. For Yezo spruce, Fujimoto and Nakano reported that the fi bril angle increased from 13o to 17o as the concentration of aqueous NaOH solution increased from 10% to 15%, while tracheid length decreased by about 10%. Other than these reports showing indirect evidence that the longitudinal contraction of the microfi brils themselves infl uences the longitudinal contraction of wood samples, no defi nitive evidence has been presented. This study sought to clarify the contribution of microfi bril contraction to the longitudinal contraction of wood samples by examining the basis of the anisotropic changes in the longitudinal and transverse swelling rates of wood samples. Materials and methods

Effects of quenching on relaxation properties of wet wood

A new relaxation property is discussed on the basis of creep behavior of wet wood specimens pretreated with heating at various temperatures followed by quenching. The treated samples showed more marked relaxation than that of an untreated sample. The relationship between relaxation time and heating history was represented by an equation ln(λ) = −(γαf − k1)ΔT + [ln(λg) + k2], where ln(λ) is the logarithmic relaxation time of wet samples after quenching, ΔT is the difference between the heating temperature and the glass transition temperature (Tg), ln(λg) is the logarithmic relaxation time at Tg, γ is a constant, αf is the coefficient of thermal bulk expansion, and k1 and k2 are constants. It was concluded from the analysis of experimental results that the change in the relaxation property caused by heating and the following quenching is due to the temporary free volume created by freezing of molecular chain motion of wood components, most probably lignin, during quenching.

Analysis of the temperature dependence of water sorption for wood on the basis of dual mode theory

Isotherm curves of water sorption for wood at various temperatures were analyzed based on the dual mode theory where the total coverage was represented by a linear combination of the Langmuir and Henry equations. The saturation concentration and affinity constant of the Langmuir equation and the parameter of Henry’s law had a transition point near 60°C. The analysis based on the dual mode theory found that the constants for whole wood were related to those of wood components and depended more on their glass transition temperatures. That is, it was theoretically demonstrated that the characteristic tem-perature dependence of water sorption for wood occurs because wood consists of three components (cellulose, hemicellulose, and lignin) with different glass transition temperatures.

Determination of the change in appearance of lumber surfaces illuminated from various directions.

Abstract Optical appearance is an important aesthetic property of wood, and this unique visual characteristic needs to be described quantitatively for many industrial applications. The objective of this study is to demonstrate the change in appearance of lumber surfaces from the viewpoint of human observation. Ten different specimens of coated and uncoated fancy veneer overlaid plywood of four species and solid and print Sugi (Japanese cedar) boards were prepared. For observation purposes, a simple goniophotometric device was constructed. A series of digital images of each specimen was taken, whereas the surface was illuminated from various lighting azimuths. The images were evaluated by subtraction, correlation, and multiresolution contrast analyses. The first two methods detect reflection anisotropy on the specimen surface through comparison of two images. Subtraction analysis also distinguishes between the coated and uncoated surfaces. However, it is difficult to determine the difference between the solid and print Sugi specimens based on these techniques. By contrast, multiresolution contrast analysis renders possible evaluation of the size and degree of the change in the surface appearances by contrast values. By comparing the contrast values for every lighting azimuth and filter size, this analysis clearly determines the change in appearance peculiar to wood, such as reflection anisotropy, improvement in reflectiveness conferred by coating, and the difference between the solid and print Sugi specimens.