3 BASIC FACTORS IN THE TREATMENT OF WOOD

THE FLOW IN THE process of impregnation of consoldants into wood may be classified as unsteady-state flow, since the conditions for flow change as the void volume is being filled. By using the integrated form of Darcy's law for fluids, and assuming that permeability is constant with length, Siau (1984) derived Equ. 1 to calculate unsteady-state absorption.

Fig. .

where: FVL

= fractional volumetric absorbtion of liquid for flow through both ends of a parallel-sided specimen

= length of specimen in flow direction

= specific permeability

= pressure differential

= time

= porosity of specimen

= viscosity of liquid

Permeability is a basic characteristic of wood determining the flow of fluids during the impregnation process. There are many factors, such as pore size, pit aspiration, and other anatomical features; extractives; moisture content; and specimen flow length, which influence the permeability of wood (Nicholas and Siau, 1973). In general, permeability is higher in sapwood than in heartwood and it is also much higher in the longitudinal than in the transverse direction. Because flow of fluids through softwoods is essentially through the tracheids which are interconnected by bordered pit pairs, the fine pit structure controls the flow rate of fluids through softwood (Siau, 1984). In hardwoods the situation is more complex because of their more intricate structure. According to Siau (1970), permeability is the more significant factor affecting retention compared to pressure and viscosity.

According to Equ. 1, fractional volumetric retention is inversely proportional to the square root of viscosity of the treating solution. Relative void volume within the wood structure will largely determine the maximum retention of the treatment solution. Other properties of fluids, such as the polarity of the solvent, and dissolved gases also affect flow in wood. In general, wood is more permeable to non-polar than to polar solvents.

Equ. 1 indicates that an increased pressure gradient will improve the liquid absorbtion of woods. According to Equ. 1, the fractional volumetric retention is proportional to the square root of time. An initial vacuum applied prior to immersion or pressure can effectively remove the air from wood. Thus, the internal air cushion that might otherwise hinder penetration can be significantly reduced. Furthermore, the removal of air can create a driving force by subsequent release of atmospheric pressure.