Equilibrium water retention isotherms have been determined for papermaking fibres through the range of moisture contents that is of greatest papermaking interest.
The experimental method provides not only a measure of the swelling of the fibres and the equilibrium water retention at any force of water removal, but, conversely, the method measures the force with which the water is held and the force with which fibres or fibre elements are held together by residual water.
Factors affecting the mechanism of water retention and the amount of water held by fibres are demonstrated by the isotherms of model and papermaking fibres of widely varying properties and treatments. The part played by the mechanism and extent of water retention on the development of wet web properties during drying is discussed and the need is demonstrated for more information than is provided by the isotherm.
Finally, consideration is given to the role of swelling and water retention in pulp evaluation and some general principles are suggested by which the running and papermaking properties of a pulp might be predicted.