OBSERVATIONS ON THE DRYING OF PAPER: FIVE DRYING METHODS AND THE DRYING PROCESS
JANE E. SUGARMAN, & TIMOTHY J. VITALE
All five drying methods caused change in surface texture. The early restraint 1 and 2 and air-dried–humidification methods were noticeably more successful than the traditional and uncushioned methods in maintaining original surface qualities. Drying methods that hold the paper in contact with a flattening surface through the entire drying process produced the least change in surface texture.
The early restraint method of drying is distinguished from the others tested by the relatively high water content at which the paper is put under restraint. Close observation and quantification of the drying process make it clear that the early restraint defines the shape and texture of the sheet before hydrogen bonding and shrinkage can begin at 63% solids.
Subtle differences in the drying methods that varied the drying surfaces in contact with the sheet, such as the difference between early restraint 1 and 2, could not be statistically distinguished at the 95%-confidence level.
Some papers change more than others when exposed to the wetting and drying treatment. Pressing, sizing, and fiber furnish could not predict how much change would occur in surface texture.
The absence of sizing material increased the amount of water absorbed and elongated the stages of drying. Thus, although type or degree of sizing could not be used to differentiate degree of change in paper texture, it had an influence on the rate of drying. Since sizing affects the rate of drying and sizing did vary among the 12 experimental papers, the rate of drying does not appear to influence the final appearance.
Drying of paper can be seen as a succession of stages: flooded, glossy-dark matte, dark matte, light matte, onset of physical distortion, and white. The onset of physical distortion, which occurred during the light matte stage, coincides with hydrogen bonding and shrinkage of fibers. This point occurred at about 63% solids for all papers tested.
Humidification with water vapor results in the fibers attaining the fiber saturation point, a maximum of 86% solids after 3 hours and 81% after several days, a level insufficient to fully break and reform structural hydrogen bonds in the fiber network.