On the impact of water soluble wood components on the fibre-to-fibre bonds in MDF

Thermomechanical pulps from beech were in part washed by water at room temperature to remove water soluble components. MDF bonded with acid curing UF-resin and alkaline curing TF-resin were prepared from unwashed and washed fibres under exactly the same conditions on using the same binder. The equilibrium moisture contents (EMC) of pulps and from MDF were measured. The EMC of pulps was lower than that of the chips used for pulping. Washing of pulps leads to further decrease in the EMC. The EMC of the MDF bonded by alkaline curing TF-resin was higher than that of the fibres. In contrast, UF-bonded MDF had lower EMC than wood fibres. In general, the EMC of MDF produced from washed fibres were higher than that of the MDF produced from unwashed fibres. The UF-MDF has lower EMC than that of TF-MDF. The pH-value of the MDF made from washed fibres was higher than that of the MDF produced from unwashed fibres. The pH-value of UF-MDF is lower than the pH-value of the TF-MDF. The transverse tensile strength of UF-MDF produced from washed fibres were lower than that of the MDF made from unwashed fibres. In contrast, the transverse tensile strength of TF-MDF prepared from washed fibres was higher than from unwashed fibres. The difference in behaviour of unwashed and washed fibres is probably, at least partially, due to the influence of washing on the pH-value of fibres and the different interaction between the fibres and the used resin systems.