Pressure measurement in wood as a method to understand impregnation processes: Conventional and supercritical carbon dioxide

Two approaches to improving wood treatability include the modification of pressure schedules during conventional liquid treatments and the use of supercritical fluids (SCFs). In both of these techniques, pressure differences are important for transporting materials into wood. The level of pressure and its change during SCF treatments are also important for controlling the solubility of these materials. Empirical pressure measurements may be used to explain minimal or inconsistent improvements resulting from pressure schedule modifications during conventional liquid treatments and the non-uniform distribution of deposited materials and wood defects that have resulted from SCF treatments.; This investigation provided empirical data on pressure development in wood during pressure processes. Measurement techniques were developed and evaluated during conventional and supercritical carbon dioxide (SC-CO ₂) treatments. A technique using probes epoxied into samples and then drilled open at the probe tips was the simplest and most effective method for assessing pressure. However, the hydraulic fluid used to transfer pressure from samples to sensors interfered with measurements during the venting phase of SC-CO₂ treatments. The effects of process parameters and wood characteristics on internal pressure development were investigated by making internal pressure measurements under different treatment conditions. Qualitative comparisons between treatments were made using pressure response quantifiers derived from the pressure measurements. Results from conventional liquid treatments indicated that minor changes in pressure schedules are not derived from the pressure measurements. Results from conventional liquid treatments indicated that minor changes in pressure schedules are not likely to enhance the treatability of wood. Results from SC-CO₂ treatments indicated differing pressure response delays with different pressing and venting rates, wood species, and grain orientations. These delays resulted in surface-to-center pressure differences which could be used to explain preservative retention distribution, wood collapse and fracture, and wood dimensional changes observed by other investigators. Finally, a preliminary investigation showed that average air permeability and anatomical measurements were poorly correlated with pressure response quantifiers from SC-CO₂ treatments when pooled for eight softwood and three hardwood species. The pooled results for softwoods suggested that only resin canal dimensions were correlated with pressure response quantifiers.