Aspects of boron diffusion through hardwoods

Abstract

The objective of this study was to determine the diffusion coefficient of boron salts in hardwoods and to determine how boron leaching is reduced by surface
coating and increased by coating weathering.
Radial diffusion coefficients of boric acid through oak (Quercus petraea Liebl.) and ash (Fraxinus excelsior Linn.) were measured with a two-chamber diffusion cell. This type of cell proved to be inadequate for diffusion measurements in hardwoods with low diffusion coefficients. A new diffusion cell was designed
which produced higher rates of boron diffusion.
A curve fitting technique was used to analyse diffusion coefficient variation with time. This method helped to estimate the 'true' diffusion coefficients of boron
through wood. The 'true' radial diffusion coefficients of boron through green wood of oak and ash measured with the two-chamber diffusion cell were 1.3 X 10^-11 and 1.6 X 10^-11 m²s^-1 respectively. With the new diffusion cell, the 'true' radial
diffusion coefficient of boron through green wood of ash was 2.8 X 10^-11 m²s^-1.
The difference in results between the two diffusion cells may be due to: natural
variability of the wood; reduction in boundary layer effects due to modifications
made to the stirring system; and, an increase in diffusion rates due to mechanical mixing of the solution into the wood cells.
Diffusion tests with 2 and 4% borax solutions showed that the radial diffusion coefficient does not depend on solution concentration.
Careful drying and resaturation does not seem to affect the radial diffusion
properties of ash wood.
Diffusion through wood coated with a typical woodstain was measured. Experimental tests gave a boron diffusion coefficient of the coating of 4.2 X 10^-14
m²s^-1. Calculations based on electrical resistance analogy gave a value 33% higher (5.6 X 10^-14 m²s^-1).
The microscopic structure of the woodstain coat and the coat/wood interface
of unweathered and weathered samples were compared using a scanning electron microscope (S.E.M.). Weathering increased the cross-sectional open area of the coat which may increase the diffusion coefficient. After 12 months of weathering, the diffusion coefficient of boron through the coat may increase 700 times.
Computer simulations were used to predict the protection given by the woodstain coat against radial leaching of boron from treated round wood. It was predicted that a 10 μm thick model woodstain coat applied to the tangential surface of the model wood may extend its protected life by a factor of 5 when compared with uncoated wood. Two coats of the same model woodstain may increase this factor to 9. A coat of woodstain weathered for 12 months may not protect the treated wood against boron leaching.

Date of Award	1993
Original language	English
Awarding Institution	University College of North Wales, Bangor
Sponsors	Coordenaçao de Aperfeiçoamento de Pessoal de Nivel Superior (CAPES) & National Institute of Amazonian Research (INPA), Manaus
Supervisor	Alan Bolton (Supervisor)