Abstract
Knowledge of the dynamic changes in the water absorption process of heat-treated wood is important for providing a scientific basis for the reasonable application of heat-treated wood, especially for outdoor applications. Nuclear magnetic resonance (NMR) techniques provide detailed information about the moisture components and moisture transport processes in wood, which are not available with other methods. In this work, water absorption of untreated and heat treated Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) heartwood was investigated using various NMR methods. The heat treatment temperatures were varied between 160 °C and 220 °C. According to the spin-spin relaxation time (T2), there were two components of water in the samples heat-treated at 160 °C and 180 °C as well as the untreated sample, while three components of water were found in the samples heat-treated at 200 °C and 220 °C, and the mass of each component was calculated by the integral peak areas of the T2 curve. The amount of bound water and free water in heat-treated samples were less compared to the untreated ones, and the water absorption decreased correspondingly, due to the increasing heat-treated temperature. The results obtained by one dimensional frequency coding indicated that the heat treatment made wood difficult to be accessed by moisture. Besides, NMR images revealed that the free water absorption in latewood was faster than in earlywood, but earlywood could absorb more water than latewood.
Highlights
Wood is a porous material which has a complex hierarchical structure, and the hygroscopic nature makes it sensitive to moist conditions
The content of Whether bound water or free water, the absorption amount in untreated samples was larger than the heat-treated ones, and the water absorption decreased correspondingly due to the increasing heat-treated temperature
The integral peak area and the gravimetrically determined moisture content (MC) had a significant linear correlation relationship, which indicate that it was a feasible method to use the T2 relaxation curve to characterize the dynamic changes of moisture in wood samples during the water absorption process
Summary
Wood is a porous material which has a complex hierarchical structure, and the hygroscopic nature makes it sensitive to moist conditions. Water generally exists in wood in two basic states, namely bound water and free water The former is dissolved or absorbed within the hygroscopic cell walls and associated with them by hydrogen bonding, while the latter occupies the cell lumens and cell cavities and is subject to capillary forces. Of all the methods for wood modification, heat treatment is an environmentally friendly method to improve timber performance by changing the wood-water relationship without using chemicals. This method is to treat wood at a high temperature, for example, about 200 ◦ C, in a nearly oxygen-free environment, inducing chemical and morphological changes of the wood substance [5].
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