Machine calendering at conventional temperatures and line pressures has been compared with calendering at elevated temperatures, and calendering with an induced z-directional gradient in web moisture content . Print uniformity and web strengths are considerably improved by the latter calendering method, while rub off, set off, and print through remain at normal levels.
A moisture gradient is established by calendering the paper web before applied moisture has time to wet the paper’s surface. Study of the wetting time shows that newsprint from thermomechanical pulp wets faster than stone groundwood newsprint, that wetting time is decreased when web moisture content is increased, and that pulp drying history is particularly important-papers from bale pulps exhibiting wetting times orders of magnitude greater than those from slush pulps.
The absorption of mineral oil has been measured for papers calendered under different conditions . The studies show that calendering, and moisture gradient calendering in particular, slows the rate of oil absorption-while increasing web moisture content increases the absorption rate.