Irreversible property changes of small loblolly pine specimens heated in air, nitrogen, or oxygen

Abstract

The objective of this research was to characterize the roles played by atmospheric oxygen and wood moisture in the thermal degradation of loblolly pine as measured by selected physical and mechanical properties. These two factors relate, respectively, to the oxidation and hydrolysis reactions that comprise the total degradation reaction. Small clear specimens were individually heated at 150 C from 1 to 16 hours in airtight cylinders, which were flushed with oxygen, nitrogen, or air prior to being to sealed. Measured properties were rellectance, specific gravity, hygroscopicity, and the moduli of rupture and elasticity in bending. Property values generally exhibited a decrease with increasing exposure time. The properties can be ranked from most-to-least degraded as follows: relfectance, modulus of rupture, hygroscopicity, modulus of elasticity, and specific gravity. The degradation of these properties was not adequately described by first-order reaction kinetics, Instead, a nonlinear equation was used, which accurately described the data and still reflected a rate controlled process. The rate of property loss was accelerated by the presence of wood moisture, with the exception of residual hygroscopicity. The greatest decrease in property due to the presence of wood moisture occurred with reflectance. The degradation of modulus of rupture and modulus of elasticity was directly related to the amount of wood moisture present. The apparent role of oxygen in total thermal degradation depended on specimen moisture content. The influence of oxygen on the the degradation process was apparent for specimens heated in the oven-dry condition. When moist specimens were heated, however, losses caused by hydrolysis overshadowed oxygen-dependent degradation. Oxygen-dependent degradation was most visible for moduli of rupture and elasticity, and, to a lesser degree for reflectivity. No effect due to the presence of oxygen could be discerned for residual specific gravity and hygroscopicity.