Abstract
Following high temperature (HT) drying, final steaming in a pressurised environment will reduce twist degrade and enhance stability when compared to normal steaming. This improvement is the result of the enhanced relaxation of drying stresses in the hotter, pressurised environment. However, the precise mechanisms for this improvement are, as yet, unclear. A study was undertaken to evaluate the patterns of moisture uptake during pressure steaming of HT dried radiata pine lumber. Samples of radiata pine heartwood and sapwood, which had been previously high-temperature dried at 140/90°C, were pressure steamed for 15 minutes at target conditions of 150°C and 95% relative humidity (4.6 bar). Specimens rapidly attained moisture levels considerably higher than what would have been predicted by commonly applied equilibrium moisture content (EMC) relationships. Marked differences in the sorption patterns of heartwood and sapwood were also evident.
Zusammenfassung
Eine abschließende Druckdampfbehandlung welche anschließend an eine Hochtemperaturtrocknung durchgeführt wird, reduziert im Vergleich zu einer normalen Dampfbehandlung die Abwertung des Holzes durch Verdrehung, und erhöht die Dimensionsstabilität des Holzes. Diese Verbesserung beruht auf einer erhöhten Relaxation der Trocknungsspannungen in einer heisseren und unter Druck stehenden Umgebung. Der genaue Mechanismus für diese Verbesserung ist jedoch bis jetzt noch unklar. Eine Studie wurde durchgeführt, in welcher das Verhaltensmuster der Feuchtigkeitsaufnahme während der Druckbehandlung von hochtemperaturgetrocknetem Pinus radiata Holz untersucht wurde. Proben von Kern- und Splintholz, welche zuvor bei 140°/90°C getrocknet worden waren, wurden für 15 Minuten mit Druckdampf behandelt, wobei Sollwerte von 150°C und 95% relativer Feuchte (4,6 bar) angestrebt wurden. Die Proben nahmen dabei schnell deutlich höhere Feuchtigkeitswerte an, als von den üblich angewendeten Formeln zum Feuchtigkeitsgleichgewicht vorhergesagt worden wäre. Zudem traten herausstechende Unterschiede im Sorptionsverhalten von Kern- und Splintholz zutage.
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The authors would like to acknowledge the contribution of Dr. F.A. Kamke and the Wood Science and Forest Products Dept. at Virginia Tech, Blacksburg, Virginia for their assistance in facilitating this work. The input of drying scientists Shusheng Pang and Steve Riley at Forest Research was also invaluable.
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Lenth, C.A., Haslett, A.N. Moisture uptake patterns in pressure steaming of Radiata Pine. Holz Roh Werkst 61, 444–448 (2003). https://doi.org/10.1007/s00107-003-0419-0
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DOI: https://doi.org/10.1007/s00107-003-0419-0