The Moisture Variations Of Poplar With Different Drying Methods Studied By Time-Domain Nuclear Magnetic Resonance

In this research, P.beijingensis and P. platyphylla were studied with high temperature drying, low temperature drying and microwave drying methods respectively. Water states and migration was investigated by time-domain nuclear magnetic resonance (TD-NMR) during drying process through free induction decay(FID) curve and transverse relaxation time(T2). The results showed that:1. In the three methods of wood drying, the wood moisture content calculate by oven-dry weighting method is all linear related to the corresponding FID and T2 signal intensity, which indicate that wood moisture content can be measured by FID and T2 signal intensity.2. In the three methods of wood drying, the exponential fitting area of free water with long relaxation time in heartwood of the two kinds of trees showed a trend of first decrease and then increase and decrease again at last, and this phenomenon did not happen in the sapwood. The main reason is the structure of heartwood and sapwood is different, the density of heartwood is greater than sapwood and with more tylosis in vessels,the water transport channel is not smooth as sapwood.3. In the process of high temperature drying, the moisture percentage decrease faster in sapwood than that in heartwood, the evaporation rate of free water significantly faster than the bound water.4. During the low temperature drying, there is a sharp decrement for bound water percentage contents of two kinds of sapwood when moisture content is about 40% and free water percentage content changes slowly for the time being. It indicates that bound water contents will suddenly accelerate at the moment before moisture content is reaching fiber saturation point.5. During the microwave drying of both sapwood and heartwood of two kinds of wood, the exponential fitting area of water in various states significantly decrease with the drying time, and it is obviously different compare with the other two drying ways and it does not exist the phenomenon that one of the water states increased significantly in a dry period. This is because microwave drying will make the wood large pressure difference between the inside and outside, and increase internal permeability, hence, the moisture transporting ability is improved.6. For microwave drying, heat is directly coming from the inside of wood, and it is timely and integrate, so, the moisture content of whole water, bound water and free water all decrease in linear, and there is no significant difference between free water and bound water of heartwood and sapwood.