The Yankee coating and creping application is arguably the most important, as well as, the most difficult to control unit operation in the tissue making process. For creped tissue products, this step defines the essential properties of absorbency, bulk, strength, and softness of tissue and towel products. Equally important, is that efficiency and runnability of the creping step controls the efficiency and runnability of the tissue machine as a whole.
A common difficulty with the tissue making process is the non-uniformity in characteristics of the coating on the creping cylinder in the cross direction. The coating is composed of adhesives, modifiers, and release agents applied from the spray boom, as well as, fibers pulled from the web or sheet, organic and inorganic material from evaporated process water, and other chemicals added earlier to the wet end of the tissue manufacturing process. Inhomogeneity in the coating characteristics is often related to variations in temperature, moisture, and regional chemical composition across the face of the dryer. The variation is often quite significant and can result in variable sheet adhesion, deposits of different characteristics and/or a lack of material on the cylinder that can result in excess Yankee/creping cylinder and creping blade-wear. Degradation of final sheet properties, such as absorbency, bulk, strength, and softness can also result from this variation and/or degradation. As a result of these drawbacks, monitoring and control methodologies for the coating on the creping cylinder surface are therefore desired.