In the manufacture of certain paper sheet products, such as tissue and paper towels, one method of drying the sheet after formation and dewatering is to pass heated air through the wet sheet in a process well known in the papermaking art as throughdrying. Throughdrying is advantageous in that it minimizes compaction of the web and thereby produces a bulkier product compared to conventional wet press manufacturing processes, which rely on high levels of compression of the wet web and on a Yankee dryer to dry the web. However, throughdrying has some disadvantages in that it requires a substantial amount of expensive equipment and energy to carry out the drying process. In particular, the drying efficiency of the throughdrying process is in large part dependent upon the air permeability of the wet sheet which permits the hot air to pass therethrough. Air permeability is especially a problem for sheets made from fiber furnishes containing secondary (recycled) fibers, which inherently impart poor air permeability to the wet sheets into which they are incorporated. With the continual efforts to utilize more secondary fibers in paper products for environmental reasons, there is a need to improve the throughdryability of secondary papermaking fibers.
USPTO Backgrounds
USPTO Backgrounds