板材와 單板의 熱板乾燥에 關한 硏究 = Studies on Press-drying of Board and Veneer

다국어 초록 (Multilingual Abstract)

Current rapid increase in the wood demand promotes utilization of fast-growing species and small diameter log as timber resources. These woods have a greater proportion of juvenile wood in their volume, and are unstable in dimension due to anisotropy in shrinkage than mature wood. It is understood that severe drying defects could occur with conventional drying.
The present studies aimed to elucidate characteristics on drying such as the core temperature, current moisture content, moisture gradient, drying rate, drying time, dimensional change and drying defect of board, and drying rate of veneer for an efficient utilization of timber resources. Species selected for the study were poplar(P. euramericana G.) as a fast-growing species and oak(Q. acutissima C.) as refractory wood.
The condition of press-drying adopted was that of platen temperature of 175℃ and pressure of 2.46 kilogram per square centimeter with board thickness of 7, 15 and 24 milimeters. For veneer experiment, the conditions adopted were of temperature of 110℃, 120℃ and 130℃ and pressure of 2.46 kilogram per square centimeter, and veneer thickness of 0.6, 1.2 and 1.8 milimeters. The results of the study are summarized as follows.
1. The core temperature of board thicker than 15 milimeters was divided into three stages of drying : the period of initial heating, the period of the stable core temperature and the period of the slowly rising core temperature. On the other hand, the board with 7 milimeters thick did not show the period of the stable core temperature.
2. The plateau temperature of thicker board was lower and kept longer than that of thinner board. For the board with same thickness, the plateau temperature for end-grained board was lower and kept longer than the temperature for flat-grained board.
3. The average moisture content of thicker board was lower than that of thinner board when the core temperature was reached at 150℃. For the board with same thickness and the same core temperature of 130℃, the average moisture content of end-grained board was lower than that of flat-grained board.
4. The variation of the moisture content in relation to the drying time showed a curvilinear, and that of thinner board changed much faster than that of thicker board. For the board with same thickness the drying rate of end-grained board was greater than that of flat-grained board.
5. The moisture gradient in board was great at the beginning stage of drying, but decreased as the drying was progressed.
6. The drying rate of flat-grained boards showed a positive correlation with the initial moisture content, platen temperature and the pressure applied, but a negative correlation with the board thickness and drying time.
7. The drying time of flat-grained boards showed a positive correlation with the board thickness and initial moisture content, but a negative correlation with platen temperature, pressure and final moisture content.
8. The drying rates of poplar flat-grained boards treating with cold water, hot water and steam were greater than that of controlled(untreated) specimen.
9. The thickness shrinkage of boards for press-drying increased as the drying was progressed. The thickness shrinkage of edge-grained boards was greater than that of flat-grain, and its shrinkage of end-grain was the least. The width of edge-and flat-grained boards expanded as the drying was progressed, but in contrast the end-grained board shrank.
10. The thickness shrinkage of edge-and flat-grained boards increased with pressure applied. The thickness recovery of edge-grained board in relation to pressure variation showed significant difference, but that of flat-grain showed no significant difference.
11. Except for board with high moisture content and drying condition of higher platen temperature and higher pressure, poplar flat-grained boards dried without a serious drying defect such as surface check, honeycomb and collapse. The end-grained board was also showed no significant drying defects.
12. Drying defects of sapwood board was milder than that of heartwood. Drying defects of oak heartwood with steaming treatment reduced the amount of drying defects as compared to those with cold water and hot water treatments.
13. The drying rate of veneer was decreased as drying was progressed. The tinner veneer thickness and higher platen temperature, the greater the drying rate was.