Study On Drying Characteristic Of Small-diameter Dahurian Larch And Ussuri Poplar

The drying characteristics of small-diameter lumbers of Larch (Larix) and Cathay poplar (Populus cathayana Rehd), located in the Northeast of China, were investigated.First, the anatomical features, physical features and extraction contents of small-diameter lumbers were studied systematically and compared with mature wood. The reason that drying defects occurred easily in small-diameter lumber was discussed based on its properties. Second, the fluctuating-heating and consecution-heating tests were carried out on dry small-diameter lumbers of Larch and Cathay poplar. The effect of heating methods was analyzed based on the development and change law of MC, shrinkage and wood surface drying during drying progress. Third, drying defects and mechanical performance were determined, and the microstructure of the small-diameter lumber was observed with scan electron microscope. The drying quality of lumbers was assessed too. At last, final drying schedules of small-diameter lumbers of Larch and Cathay poplar were developed according to the result of this research, and suggestions were given to improve drying quality.Small diameter lumber of Larch and Cathay poplar has lower wall/diameter ratio, large growth width, low wood density, and different quality between heartwood and sapwood. Some defects including collapse, deforming, checking may occur. Small-diameter Larch has more extraction contents than adult Larch, so its drying rate is quicker, but small diameter Cathay poplar has the opposite properties.Consecution-heating used the most time, fluctuating-heats (6h) in between, and fiuctuating-heats (2h) used the least time. The MC grade, shrinkage and wood surface stress during drying progress was the largest in consecution heating, and was least in fluctuating-heats (6h).The effect of heating methods on dry quantity of small-diameter lumbers was significant. Fluctuating-heating can reduce drying stress, improve drying quality and increase mechanical strength. The longer fluctuating-heating was, the more obvious the effects were. Lumber types (including heart/sap wood and radial/tangential lumber) and lumber size error had different influence on drying quality of small-diameter lumbers.According to the observation of the microstructure of small diameter lumber with scan electron microscope, there are many bordered pits of the radial trancheidal wall in the small-diameter Larch, and most of them have pit film. The resin is not only in resin canal, but also in some trancheid and ray parenchyma cells. The drying defects of small-diameter Larch lumbers were the most severe in consecution heating, fluctuating-heats 2h in between, and drying defects were nearly not observed in fluctuating-heats 6h. There is tylosis in the vessel of the heartwood of small-diameter Cathay poplar. The bordered pits of the radial trancheidal wall are dense, and the pits between vessels are alternate, most of them have pit film. Different collapse happened in all the small-diameter Cathay poplar lumbers. The collapse of small-diameterLarch lumbers was the most severe with consecution heating, fluctuating-heats 2h in between, the least with fluctuating-heating 6h.After each kind of drying quality targets was determined, the drying quantity of small-diameter lumbers with fluctuating-heating 2h and consecution-heating tests got to GB'2; with fluctuating-heating 6h, the quantity reached GB'l. Fluctuating-heating was carried out by automatic conventional drying kiln with top-fan.The final drying schedules of small-diameter lumbers of Larch and Cathay poplar were developed. (Tab.4-1) Some suggestions are given to improve drying quality, such as preliminary picking up before drying, mild schedule, etc.