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It has long been known that heat treatments are an effective method of improving the moisture resistance and dimensional stability of wood. The application of heat treatments to wood fibre composites such as MDF have not been fully evaluated.
Heat treatments can be applied in a number of ways, [Seborg et al., 1953]. For example, treatments involving submerging wood in a molten metal or heating in the presence of a number of chemicals are commonly referred to in the literature. Important treatment parameters include the time and temperature of heat exposure, presence of water or the chemicals used. Heating wood prevents swelling sites in the wood from taking up further moisture and may increases the adhesive properties of lignin [Pizzi, 1994]. Heat may also degrade the more hydrophilic wood constituents, particularly hemicellulose, hence reducing moisture sorption.
Heat treatment of particle-board bonded with phenol formaldehyde (PF)
resin has been shown [Ernst, 1967] to reduce
thickness swelling. The latter study used
temperatures of up to and two hours exposure. A fast heat
treatment was outlined by [Hsu et al., 1989]. They heated phenol-formaldehyde bonded waferboards,
in a hot press. The press parameters for a 7.9 mm thick board were
and 3.45 MPa for 160 seconds. This caused an 18% reduction in
thickness swelling and 43% reduction in EMC 
, and 5% and 15% reductions
in density and MOR respectively, while MOE and IB strength increased
by 15% and 3% respectively. No chemical changes occured. Wood dried
at displays a EMC 3% less than that of air dried
wood [Orman, 1965],[Suematsu, 1980]. Increased dimensional
stability of heat treated composites as a result of heating have been explained by the fact that the
heating temperatures used were higher then the softening points of lignin and
carbohydrate components. Thus plastic flow of lignin during heating may result in a
rearrangement and relaxation of compressive stresses
[Spalt, 1977]. Spalt (1977) found that due to the
relaxation of such compressive stresses, heat treated boards had low
residual compressive stresses and less tendency to expand at high
moisture.
Xu and Suchsland, (1991) found that at high relative humidities, mechanisms other than the swelling of the wood cell wall contribute to the thickness swelling of particleboard. It was suggested that these alternative mechanisms were related to the breakdown of adhesive bonds between particles, which caused permanent modifications of board properties.