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Formaldehyde treatments (formalisation) have been applied to MDF to reduce its thickness swelling, and increase its moisture resistance and mechanical properties. Formalisation which can be successfully performed at high () or low () temperatures [Minato and Seto, 1995] can markedly reduce the EMC of MDF, by blocking the moisture accessible sites and by chemical or physical modifications of hydroxyl groups. One study [Minato, 1993] also found that MDF heated without formaldehyde had more poly-molecularly absorbed water due to thermal degradation of the adhesive resin. This exposed more moisture accessible sites, but the overall effect was a lowering in MC, due to a decrease in macro-molecularly absorbed water.
In another study [Minato et al., 1993] on the effect of formalisation on the properties of MDF, the effect of heat treatments on MDF, with and without formaldehyde, was examined. It was found that the thickness swelling and equilibrium moisture content of MDF treated without formaldehyde, decreased in the low humidity range, but increased in the high humidity range, the reason for this being ``spring back of the remaining strain due to the thermal degradation of adhesive resin and the resultant appearance of moisture adsorption sites''. This study also found that anti-swelling efficiency and moisture excluding efficiency of MDF increased with decreasing relative humidity.
High pressure steam pressing improves the dimensionally stability of wood, but at the expense of mechanical properties. Steam pressing is observed to break down the hemicellulose and alpha cellulose without affecting the lignin. [Okamoto et al., 1994]
The aforementioned studies present conflicting findings regarding the effect of heat treatments on the dimensional stability and mechanical properties of wood composite board materials and there are few if any studies of the effects of heat treatments on the properties of MDF. There is clearly scope for further work in this area. In this thesis the heat treatment outlined by Hsu et al (1989) was adopted.