Treatment of Wood with Gaseous Reagents

The primary problem for rapid and homogeneous treatment is to minimize distance crossed by diffusion or to maximize the corresponding rates of diffusion. If large quantities of material are required to penetrate into the wood, liquid phase treatment is more effective as with liquid ammonia; whereas when lesser quantities of reactants are required gas phase is more effective, as with chlorination or ozonization. Transport of a gas within the wood can be markedly increased by fluctuating pressures. Transport of liquid ammonia can be delayed by the presence of an inert gas, by impervious sections, and by wood of higher temperature than the boiling point of liquid ammonia. Heat transfer can be the rate-determining step. Wood drying is analyzed and the process is divided into three levels of heat acceptance, namely: first, up to 100?C; second, a period where the drying is proportional to the square root of time and; third; a period where all the wood substance is above 100?C. There appear to be real advantages to the drying of wood with heated inert gases under fluctuating pressure during the second and third periods. Effective heat transferred is a function of the heat capacity of the gas. Aliphatic hydrocarbons are suggested because of their low cost and potential for re-use as fuel. A good review of studies on wood modification by gas phase treatment is presented. Wood modification by chemical agents is still in its infancy and one of the more novel techniques which deserves further research is the treatment of wood with gaseous reagents.

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