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

Author

• Soichi TanakaEmail author
• Masako Seki
• Tsunehisa Miki
• Ichinori Shigematsu
• Kozo Kanayama

Abstract

This study focused on solute diffusing into cell walls in solution-impregnated wood under conditioning. The purpose of this paper was to clarify the effect of relative humidity (RH) of the conditioning on solute diffusivity in the impregnated wood. Water evaporation, swelling, and shrinkage of wood samples impregnated with an aqueous solution of polyethylene glycol (PEG) polymer were examined under conditioning at an RH of 11, 32, 55, or 80 % followed by drying under vacuum. Dried samples were observed using a micro-focus X-ray computed tomography instrument. The total amount of PEG polymer diffusing into cell walls during conditioning increased with RH. Theoretical interpretation indicated that this was caused by an increase in polymer (solute) diffusivity as the amount of water (solvent) in samples increased. Temporal variability of the evaporation rate of water and of the swelling rate of the sample also were examined. Solute diffusivity, which was similar at each RH at the beginning of conditioning, decreased during conditioning; this decrease was greater at lower RH values due to a higher evaporation rate.

Acknowledgements

Observations of the samples using micro-focus X-ray CT system were conducted in the Graduate School of Agriculture, Kyoto University. The authors would like to express their gratitude to Mr. Yosuke Matsuda, Dr. Yoshiyuki Yanase, and Dr. Yoshihisa Fujii in the University for their help.

Appendix

The relation of the amount of solute diffusing during Δt, Δn W1, to RH, H, was shown to be a concave-downward curve as follows.

According to the assumption in theory, K 1 and x C1 − x W1 were formulated as follows:

K1=aH,$$K_1=aH,$$

(12)

xC1−xW1=−bH+c,$$x_{\text{C}1}-x_{\text{W}1}=-bH+c,$$

(13)

where a and b represent constant of proportionality and c is intercept (a > 0, b > 0, c > 0). The reason for the absence of the intercept in Eq. (12) is that the polymer completely loses its diffusivity at H = 0, which is supported by the fact that the PEG polymer with molecular weight higher than 400 do not swell dried wood [10]. The relation of Δn W1 to H can be deduced by Eqs. (4), (12), and (13) as follows:

ΔnW1=(−abH2+acH)Δt.$$\mathrm{\Delta }{n}_{\text{W}1}=(-ab{H}^2+acH)\mathrm{\Delta }t.$$

(14)

The slope of the curve, Δn W1(H), can be deduced by taking the derivative of Δn W1 with respect to H as follows:

ddH(ΔnW1)=(−2abH+ac)Δt.$$\frac{\text{d}}{\text{d}H}(\mathrm{\Delta }{n}_{\text{W1}})=(-2abH+ac)\mathrm{\Delta }t.$$

(15)

In the RH range of H < c/2b, d(Δn W1)/dH > 0 consists, and thereby Δn W1 increases with H. In the range of H > c/2b, d(Δn W1)/dH < 0 consists, and thereby Δn W1 decreases with the increase in H. Thus, the relation of Δn W1 to H has concave-downward shape.

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