Wood based panels are commonly produced hot pressed from wood material and adhesive, both containing water. The moisture content of wood might be on average, for instance, 5 % whereas the moisture content of adhesive could be 50 % or even more. However, the adhesive wood ratio might be only 2–3 %. At the beginning of hot-pressing process the water is rapidly evaporated due to rising temperature and pressure and therefore as vapour. This vapour is termed internal vapour pressure within the board. During evaporation the volume and therefore also the pressure of water increase dramatically. Regardless of the fraction size of the wood material (e.g. fibre, chip, strand or veneer) this increase in pressure of water causes problems unless the hot press regime is controlled in a sufficient manner. The vapour tries to escape the panel but this is possible only through the edges. This induces a pressure difference within the board in such a way that the pressure peak is in the middle of the board – assuming a homogenous spread of moisture at the beginning of the hot-pressing process. However, especially in case of plywood the moisture may not be evenly distributed in the veneers. Nonetheless, the pressure difference makes the vapour travel through the panel which could harm the linkages and bonds within the wood material in a case where the pressure differences rises too high. This might defibrate the fibres and in an extreme case the board explodes. To control the internal vapour pressure within a wood based panel the hot press regime includes a rapid pressure increase at the beginning of the process but a stepwise decrease of the pressure at the end of the process to ensure a steady escape of the vapour. This poster reviews internal vapour pressure in wood based panels during hot-pressing process regarding to vapour pressure as a function of time and manners to control it. Moreover, the maximum limits for harmless internal vapour pressure are discussed.
Keywords: evaporation of water, hot press regime, internal vapour pressure, wood based panels
Authors
Ruponen J.
Department of Forest Products Technology, Aalto University, School of Chemical Technology, AALTO
Rautkari L.
Forest Products Research Institute, Joint Research Institute for Civil and Environmental Engineering, School of Engineering and the Built Environment, Edinburgh Napier University
Department of Forest Products Technology, Aalto University, School of Chemical Technology, AALTO
Hughes M.
Department of Forest Products Technology, Aalto University, School of Chemical Technology, AALTO
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