Toshiro Morooka

Stress Relaxation of Sugi (Cryptomeria japonica D.Don) Wood in Radial Compression under High Temperature Steam

Summary To clarify the mechanism of the permanent fixation of compressive deformation of wood by high temperature steaming, stress relaxation and stress-strain relationships in the radial compression for Sugi (Cryptomeria japonica D.Don) wood were measured under steam at temperatures up to 200°C. The stress relaxation curves above 100°C were quite different in shape from those below 100°C, showing a rapid decrease in stress with increasing temperature. In the stress-strain relationships measured above 140°C, the stress reduced as pre-steaming time increased when compared at the same strain. The recovery of compressive deformation (strain recovery) was decreased with steaming time and reached almost 0 in 10 min at 200°C. The relationship between the residual stress and the strain recovery at the end of relaxation measurements could be expressed by a single curve regardless of time and temperature. The permanent fixation of deformation by steaming below 200°C was considered to be due to chain scission of hemicelluloses accompanying a slight cleavage of lignin. In some cases, the increase in regularity of the crystalline lattice space of microfibrils or the formation of crosslinks between the cell wall polymers seemed to play an important role in the permanent fixation of compressive deformation.

Cell wall thickness and tangential Young's modulus in coniferous early wood

To investigate the effect of wall thickening around cell corners on the tangential Youngs modulus of coniferous early wood, tapered beam cell models in which the variation of the cell wall thickness in the axial direction was taken into account were constructed for seven species. Their tangential Youngs moduli were compared with the experimental results. The calculated Youngs moduli of tapered beam cell models were larger than those of the models composed of the cell walls with uniform thickness, although both models showed almost the same density. For some species the calculated Youngs moduli of the models in which the cell wall thickness increased curvilinearly in the axial direction were much closer to the experimental values. The reduction of the radial cell wall deflection due to the increase of the stiffness around cell corners was considered to increase the tangential Youngs modulus of a wood cell.

Softening characteristics of wet wood under quasi static loading

Summary In order to examine the possible influences of temperature on the wood constituents, two groups of wood species namely softwood (needle leaf wood) and hardwood (broad leaf wood) were subjected to heat or steam treatment during large radial compression between 0°C and 200°C. Two well-defined softening regions are observed. Both species showed the glass transition Tg of lignin at ~ 90°C and ~ 60°C for softwood and hardwood respectively and a second transition region at ~ 160°C. The softening behavior between the first and second transition in softwood is in contrast with the softening behavior of hardwood. This difference reflects the difference in the distribution of the relaxation process due to lignin, which suggests the difference in chemical structure of lignin between softwood and hardwood.

Shrinkage stress of wood during drying under superheated steam above 100°C

Abstract In order to reduce the internal stress of wood during drying, an attempt was made to dry wood with superheated steam above 100°C. Sugi (Cryptomeria japonica D. Don) wood specimens were dried with radial restraint using superheated steam at up to 180°C, and the resulting restrained shrinkage stress was examined. A load cell capable of measuring shrinkage stress inside an autoclave was developed and the shrinkage stress was measured at temperatures from 80 to 180°C and a relative humidity of 0% to 80%. It was found that the shrinkage stress decreased dramatically with drying above 140°C and above 60% relative humidity. It was also found that the shrinkage stress above 140°C occurred even at 100% RH and was ascribed to the structural change in cell-wall components.

Compressive creep of wood under high temperature steam.

Summary Creep compliance curves in the radial compression were measured under steam for sugi (Cryptomeria japonica D. Don) at temperatures up to 200°C. The creep compliance curves for 30 min (I) and those with pre-steaming for 30 min (II) were well connected at 30 min when the structural changes due to the degradation of hemicelluloses or the decomposition of lignin as well as hemicelluloses occurred. However, the creep curves (I) at 30 min differed markedly from creep curves (II) when the structural changes due to the increase in regularity of crystalline lattice spacing of the microfibrils or the formation of cross-linkages between the cell wall polymers occurred. It was concluded that two kinds of structural changes could be detected well by creep measurement.

Criterion for estimating humidity control capacity of materials in a room

As a measure of estimating humidity control capacity of materials in an airtight room under sinusoidal temperature variation, we used the Cb value, which is the ratio of the range of variation in relative humidity in a steel box lined with the material of interest to the range in an empty steel box. In order to clarify the factors that affect the Cb value, we focused on both the temperature variation rate and the area that is lined in the box. It was found that changing the rate of temperature variation over one period strongly affected the Cb value, although the amplitude of temperature variation did not. We further noticed that the difference between the time when peak temperature was reached and the time when peak absolute humidity was reached (peak time difference), was found to be useful for estimating the humidity control capacity of materials. Knowing the difference between the phase when peak temperature was reached and the phase when peak absolute humidity was reached (phase angle difference) was also useful. Because the Cb value was affected by both the variation period and the lined area in the box, we can draw a contour diagram of the period and the lined area for materials to give an overview of the humidity control capacity of a material. The materials of primary interest in this study were Japanese cedar and porous ceramics.

Method for measuring viscoelastic properties of wood under high temperature and high pressure steam conditions

A method for measuring the viscoelastic properties of wood under high temperature and high pressure steam was developed using a testing machine with a built-in autoclave. A newly developed load cell capable of resisting a steam pressure of 16kgf/cm2 and a temperature of 200°C was installed in the autoclave. This load cell could be used to determine precisely the loads while steaming at temperatures from 100°C to 200°C. In addition to load-detection problems, it was necessary to avoid the nonuniform thermal degradation of wood during the measurement process under steaming at high temperatures. This nonuniform degradation could be minimized by shortening the time required for the wood to attain thermal equilibrium using specimens conditioned to the fiber saturation point. According to this method, a stress relaxation curve for sugi (Cryptomeria japonica D. Don) wood being compressed while steaming at 180°C was obtained. The stress was seen to decrease rapidly with time, reaching almost zero at 3000s.

Mechanism of partial fixation of compressed wood based on a matrix non-softening method

Abstract Three different mechanisms to explain the partial fixation of the compressive deformation of wood are postulated: non-softening, cross-linking and stress relaxation. This study attempted to fix the compressive deformation of wood by the non-softening mechanism of the cell-wall matrix using acetylation of the cell wall making it more hydrophobic. In this method, partial recovery of compressive deformation by wetting decreased at room temperature as the acetyl content increased. However, almost complete recovery occurred by boiling the compressed wood in water or soaking in acetone. This is due to the ability of boiling water or acetone to soften the cell-wall matrix of acetylated wood enough to enable recovery from compression. It is, therefore, possible to partially fix the compressive deformation of wood, preventing the resoftening of the cell-wall matrix in water.

Predicting the humidity control capacity of material based on a linear excitation-response relationship

The Cb value is a useful parameter for estimating the humidity control capacity of materials. It is defined as the ratio of the range of variation of relative humidity in a steel box lined with the material of interest to that in an empty steel box, when sinusoidal temperature variation is applied. However, because it takes a long time to obtain the Cb values for materials at each temperature variation period, we developed an easier method based on a linear excitation-response theory to obtain the Cb values without measuring at each period. Japanese cedar was the material used in this study. The temperature excitation, a jump from a constant temperature to another constant temperature, was used to obtain the absolute humidity response. Under the assumption that the temperature excitation-humidity response relationship is linear, we were able to predict humidity variation to sinusoidal temperature variation at any period, and we obtained the Cb value for each temperature variation period. Predicted values agreed well with the experimental values. From this, it was found that the Cb value could be predicted without measuring the Cb value at each period over a long time. In addition, the peak time difference, which is closely related to the Cb value, could also be predicted in a similar manner.

Compressive deformation of wood impregnated with low molecular weight phenol formaldehyde (PF) resin V: effects of steam pretreatment

This study evaluated the potential of steam pre-treatment for making highly compressed phenol-formaldehyde (PF) resin-impregnated wood at a low pressing pressure. Sawn veneers of Japanese cedar (Cryptomeria japonica) were first subjected to saturated steam at different steaming temperatures (140°-200°C), followed by impregnation with a 20% low molecular weight PF resin aqueous solution resulting in a weight gain of around 50%-55%. Four oven-dried treated veneers were laminated and compressed up to a pressing pressure of 1 MPa at a pressing temperature of 150°C and pressing speed of 5 mm/min, and the pressure was held for 30 min. Steam treatment, causing partial hydrolysis of hemicellulose, accelerated the compressibility of Japanese cedar in the PF resin-swollen condition. As a consequence, a discernible increment in density was achieved at a pressing pressure of 1 MPa due to steam pretreatment between 140° and 200°C for 10 min. It was also found that even a short steaming time such as 2 min at 160°C is sufficient for obtaining appreciable compression of PF resin-impregnated wood. The density, Young’s modulus, and bending strength of steam-treated (200°C for 10 min) PF resin-impregnated wood composite reached 1.09 g/cm3, 20 GPa, and 207MPa, respectively. In contrast, the values of untreated PF resin-impregnated wood composite were 0.87 g/cm3, 13 GPa, and 170MPa, respectively.