Improvement of the tissue manufacturing process
Abstract: The creping process and the conditions on the Yankee cylinder are the key factors in the production process. They need to be kept under good control in order to maintain a high and uniform quality. A natural coating always develops on the surface of the Yankee cylinder due to evaporation from dissolved and dispersed substances and to fiber fragments that remain on the surface after creping. Nevertheless, coating chemicals are sprayed onto the Yankee surface in order to modify the adhesion between the paper and the dryer cylinder. To be able to control the process, on-line measurements of coating thickness as well as of the crepe structure of the tissue paper produced would be very valuable. In the work described in this thesis, the coating layer was analyzed chemically and morphologically to provide information about the coating layer before starting any on-line measurements. The chemicals added and the constituents of the pulps are known to the paper producers, but what is left on the cylinder and whether there are different layers of coating still remain to be investigated. The chemical analysis indicated that the adhesive content was higher in an inner layer of the coating than in the outer layer. The relative amount of polyamide-amine resin calculated on the basis of the nominal nitrogen content in the resin was low indicating that the coating consisted of a significant amount of carbohydrates or other substances. The coating layer could not be considered transparent. It was observed that the coating was thick and appeared patch-wise and that it contained fiber fragments. Measurements on a laboratory scale, to further be applied on-line on the tissue machine, have been investigated and evaluated in this study. The thickness of the coating layer on a laboratory dryer has been measured. The method was based on fluorescence with an optical brightener added to the coating chemicals sprayed on the Yankee dryer. With a UV-LED (Ultra Violet - Light Emitting Diode) the coating layer was exposed to UV-light and the intensity of the light emitted by the optical brightener in the layer was measured. The intensity in the trials with different thickness of the coating and no coating was scattered and no trend could be seen between the different trial points. Attempts were made to measure the wavelength of the tissue paper produced with an optical fiber perpendicular to and at an angle to the paper surface. The paper was travelling in a slow speed while the measurements were made. The collected signal to noise ratio was low and it was not possible to detect any representative characteristic wavelength of the tissue.
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