Experiments have been performed in order to demonstrate that water is expressed from small cavities in the fibre matrix during the pressing operation.
The gel water* in the fibre was tagged by mixing a fibre suspension with a solution of a polysaccharide (dextran) with a known relative molecular mass, capable of entering only cavities greater than its molecular diameter. When a fibre mat is subjected to an external pressure under these conditions, the water expressed from smaller cavities dilutes the polysaccharide solution. By using solutions of polysaccharides of different relative molecular masses, it was possible to demonstrate from which cavities the water was expressed.
Pressing experiments were performed with different kinds of fibres. Bleached and unbleached fibres, a mechanical pulp and viscose fibres were used.
The effects of beating, drying and de-crilling were investigated.