Graduate Thesis Or Dissertation

 

The effects of supercritical CO₂ on the bending properties and treatment defects of four refractory wood species Public Deposited

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  • It is difficult to treat the heartwood of many wood species with liquid preservatives using conventional liquid processes. Supercritical fluids (SCF) used as carriers of preservatives to impregnate refractory wood species may be an attractive alternative to conventional liquid carriers. While SCF impregnation has many potential advantages, some wood species exposed to elevated pressures can be subjected to excessive pressure gradients during treatment. These gradients can lead to treatment defects, such as collapse and splitting, and may also result in significant losses in bending properties. This study examined the effects of three treatment parameters on the bending properties and the development of treatment defects in Douglas-fir (Pseudotsuga menziesii), yellow poplar (Liriodendron tuhpifera), western redcedar (Thuja phcata), and Engelmann spruce (Picea engelmannii). Two rates of pressurization (0.34, 3.44 MPa/min.), two treatment pressures (10.34, 20.69 MPa) and two rates of venting (0.34, 3.44 MPa/min.) were examined on small clear specimens of each species using one treatment temperature (60 °C) and one treatment time (30 min.). SCF treatment of yellow poplar produced no significant reduction in bending properties, while bending properties increased in Douglas-fir. The latter increase appears to have resulted from differences in mean specific gravities between treatments since pressurization rate, treatment pressure, or venting rate did not significantly affect bending properties. Bending properties in western redcedar and Engelmann spruce were significantly reduced when exposed to SCF treatment conditions. Comparisons between treatment and control in western redcedar specimens showed 21.7 to 23.1% reductions in MOR and 26.0 to 31.0% reductions in WML. Properties of Engelmann spruce declined 10.8 to 21.6% in MOR, 16.0 to 21.4% in MOE, and 7.2 to 29.0% in WML following SCF treatment. Rapid venting, at a rate of 3.44 MPa/min., produced the most significant reductions on bending properties for western redcedar, while Engelmann spruce was affected by both the 0.34 and 3.44 MPa/min. pressurization and venting rates. Examination of transverse sections for all species revealed that collapse can occur externally as well as internally within SCF treated samples of Douglas-fir, western redcedar and Engelmann spruce, while the anatomical characteristics of yellow poplar were observed to be unaffected by SCF treatments. The results indicate that SCF treatments, while effective for treating refractory woods, must be used with some caution to avoid inducing negative effects on wood properties.
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