The effect of superheated steam drying on the properties of paper

The effect of drying paper by direct contact with superheated steam was determined with respect to the physical, optical and chemical properties of the dried sheet. The results were compared with those for paper dried in a similar fashion by direct contact with hot air.It was found that the results depended on the type of pulp from which the sheet was made. For thermomechanical pulp sheets, superheated steam drying resulted in improved strength properties; burst index, tensile index and elastic modulus were 20-30% higher relative to the air dried sheets. The increase in strength was found to be due to an increase in bonded area, especially of the fine fraction, caused by the higher sheet temperatures occurring in the constant rate period of steam drying. As a result of the increased bonding but not due to any color change, brightness of the steam dried sheet decreased by 5 points. Steam drying of TMP sheets results in strength and optical properties more characteristic of paper made from CTMP.Superheated steam drying of kraft pulp sheets had a smaller and opposite effect, as strength properties decreased and optical properties increased relative to the properties of sheets dried in air. Unlike mechanical pulp, no increase in bonded area was observed because of the already high bonding potential of kraft fibres; a thermally induced drying stress relaxation is thought to be the cause of the decreased strength properties