Internal pressure development in Douglas fir lumber during pressure treatment
P F Schneider, J J Morrell
Preservative treatment of Douglas fir lumber to meet current industry standards poses a major challenge and a variety of methods have been developed to overcome these problems. One aspect for treatment improvement that has received less attention is the relative effect of treatment parameters on changes in pressure within the wood. Previous studies of spruce and radiata pine suggest that internal pressure development lags behind surface pressure, but the relationship between internal and external pressure in Douglas fir remains poorly understood. Pressure development in Douglas fir heartwood boards was investigated by implanting pressure transducers at the center of 25 or 50 mm thick boards. The transducers were isolated in the wood using combinations of metal fittings and epoxy and the samples were treated using either air pressure or a light oil-borne solvent. Pressure changes were generally most rapid when air was used, reflecting the order of magnitude differences between air and liquid permeability. Internal pressure changes in liquid treatments were exceedingly slow. For example, after a 30 minute vacuum, internal pressure in 50 mm thick boards pressed at 0.8 MPa had failed to reach equilibrium with surface pressure after 40 hours of pressing. The results suggest that relatively minor changes in pressure or treatment time will not appreciably affect treatment deeper in the wood of this species. Further trials are underway to better understand the effects of the rate of pressure increase on internal pressure changes.
Keywords: DOUGLAS FIR; PONDEROSA PINE; PRESSURE TREATMENT
Conference: 97-05-25/30 Whistler, British Columbia, Canada