Visualization of solute diffusion into cell walls in solution-impregnated wood under varying relative humidity using time-of-flight secondary ion mass spectrometry

Peiming Zheng,Masako Seki,Tsunehisa Miki,Soichi Tanaka,Dan Aoki,Kazuhiko Fukushima,Kozo Kanayama,Yasuyuki Matsushita

doi:10.1038/s41598-018-28230-2

Abstract

The purpose of the present study is to clarify the diffusion of non-volatile substances into cell walls during the conditioning procedure under varying relative humidities (RH). In this paper, wood blocks were impregnated using an aqueous solution of melamine formaldehyde (MF), and they were subsequently conditioned under RHs of 11, 43, and 75%. The solute that diffused into the cell walls was visualized using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The volumetric relative swelling of the samples during the conditioning procedure was calculated. The results showed increased cell wall swelling at higher RH, which may have been caused by higher MF diffusion into the cell walls and/or higher moisture content. Cryo-TOF-SIMS measurements showed that more cell cavities were unfilled with MF at higher RH, indicating that most of the MF diffused from the cell cavities into the cell walls. The relative intensity of MF in the cell walls of the cured samples was evaluated from dry-TOF-SIMS images, which showed a higher relative intensity of MF in the cell walls at higher RH. With the ability to visualize and semi-quantitatively evaluate the solute in cell walls, TOF-SIMS will serve as a powerful tool for future studies of solute diffusion mechanisms in solution-impregnated wood.

Highlights

Treatment using non-volatile substances is an effective method for reducing the undesirable properties of wood materials, such as “non-homogeneity, dimensional instability in moist environments, and susceptibility to sunlight, fungi, and insects”[1,2]
Higher Weight percent gain (WPG) was observed at higher relative humidity (RH), which is consistent with previous studies using polyethylene glycol (PEG) as the solute[2,6]
The value of WPG after vacuum drying was significantly different (P < 0.01), i.e., 69.45% ± 0.26%, 66.64% ± 0.08%, and 61.02% ± 0.26% for RHs of 11, 43 and 75%, respectively. These results indicate that melamine formaldehyde (MF) itself degenerated during the conditioning and that the degeneration degree was RH dependent

Summary

Introduction

Treatment using non-volatile substances is an effective method for reducing the undesirable properties of wood materials, such as “non-homogeneity, dimensional instability in moist environments, and susceptibility to sunlight, fungi, and insects”[1,2]. (ii) During conditioning, a greater amount of solvent evaporates from the cell cavities compared with that from the cell walls, which results in higher concentration of solute in the cavities compared with that in the walls. This concentration difference causes the diffusion of the solute from the cavities to the walls.”. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was employed to map the solute distribution in the cell walls of impregnated wood. To avoid movement, draining, or change of the water-soluble chemicals during additional drying process, cryo-TOF-SIMS with frozen samples was recently developed[15,23,24]

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