Abstract
To investigate the effects of changes in biopolymer composition on moisture in acetylated poplar wood (Populus euramericana Cv.), the acetylation of control wood was compared to the acetylation of wood with reduced hemicellulose or lignin content (about 9% reduction of total specimen dry weight in both cases). Time-domain nuclear magnetic resonance relaxometry of water-saturated wood gave spin–spin relaxation times (T2) of water populations, while deuteration in a sorption balance was used to characterize the hydroxyl accessibility of the wood cell walls. As expected, the acetylation of pyridine-swelled wood reduced hydroxyl accessibility and made the cell wall less accessible to water, resulting in a reduction of cell wall moisture content by about 24% compared with control wood. Hemicellulose loss per se increased the spin–spin relaxation time of cell wall water, while delignification had the opposite effect. The combined effect of hemicellulose removal and acetylation caused more than a 30% decrease of cell wall moisture content when compared with control wood. The acetylated and partially delignified wood cell walls contained higher cell wall moisture content than acetylated wood. An approximate theoretical calculation of hydroxyl accessibility for acetylated wood was in the low range, but it agreed rather well with the measured accessibility, while acetylated and partially hemicellulose-depleted and partially delignified wood for unknown reasons resulted in substantially lower hydroxyl accessibilities than the theoretical estimate.
Highlights
Wood is receiving more and more attention as a building material due to its low carbon footprint [1,2,3]
After partial hemicellulose removal and delignification, the hemicellulose loss was about 8.6% of the total dry weight, and the lignin loss was about 8.9% of the total dry weight, respectively
The acetylation led to a mass gain of about 15% for all three treatments, which in reality was somewhat larger, as pyridine alone gave a mass reduction of about 2–4%; see Table 2
Summary
Wood is receiving more and more attention as a building material due to its low carbon footprint [1,2,3]. Wood is a hygroscopic material, and the moisture content changes depending on the relative humidity of the air [4]. Many properties of wood, such as dimensional stability [5], mechanical properties [6], and resistance toward biological degradation [7] are affected by the moisture content in wood [8,9]. The applicability of wood and wood products is challenged by moisture changes. The acetylation of wood is typically carried out by the use of acetic anhydride. The reaction of wood with the acetic anhydride results in the substitution of hydrophilic hydroxyl groups with less hydrophilic acetyl groups [9,12,13]. Acetylation bulks wood cell walls [14,15].
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