New methods for evaluation of tissue creping and the importance of coating, paper and adhesion

Increased competition on the tissue market forces tissue makers to continuously raise their product quality and increase their operating efficiency. The creping process and the conditions on the Yankee cylinder are the key factors in the production process and they therefore need to be kept under good control in order to maintain a high and uniform quality. Due to evaporation, dissolved substances from the wet end and fiber fragments remain on the surface and a natural coating layer always develops on the surface of the Yankee cylinder. However, coating chemicals are also needed and these are continuously sprayed onto the Yankee surface in order to modify the adhesion between the paper and the dryer cylinder. To make it possible to control the process better, on-line measurements of coating thickness as well as of the crepe structure of the tissue paper produced would be very valuable. The fiber furnish affects the adhesion between the paper web and the cylinder dryer. The strength and uniformity of the adhesion of the paper to the cylinder affects the creping process tremendously and more information about parameters affecting the adhesion is of great interest. To perform trials on a full scale, or even on a pilot machine, is very costly and therefore laboratory equipment is sought in order to be able to measure the adhesion force in a cheaper way. In the work described in this thesis, the coating layer was analyzed both chemically and morphologically to obtain information about the coating layer before on-line measurements were started. The chemicals added and the constituents of the pulps are known by the paper producers, but exactly what type of material is left on the cylinder and whether there are different layers of coating material still remains to be investigated. The chemical analysis indicated that the adhesive content was higher in the inner layer than in the outer layer of the coating. The relative amount of polyamide-amine epichlorohydrin resin calculated on the basis of the nitrogen content in the resin was low, indicating that the coating layer consisted of a significant amount of carbohydrates or other substances from the wet end. The coating layer could not be considered transparent. It was observed that the coating was not uniform i.e. it was thick, had a patch-wise appearance and contained fiber fragments. An uneven coating layer affects the adhesion and creping of the paper. Therefore, it was desirable to develop an on-line method to measure the coating thickness. The coating layer contains a lot of fiber fragments and the new method cannot rely on a transparent coating layer. Measurements on a laboratory scale, to be further applied on-line on the tissue machine, have been investigated. The thickness of the