The Modification of a Curtain Coating Formulation: A Study of Rheology and Surface Tension, and Their Effect on Pitting
Date of Award
Master of Science
Chemical and Paper Engineering
Dr. Sasha Pekarovicova
Dr. Paul D. Fleming
Dr. Charles P. Klass
Curtain, coating, formulation, rheology, surface tension
Masters Thesis-Open Access
When an undisclosed recycled fiber mill installed a “two-slotted” curtain coater to replace their air knife coater, a prominent defect arose known as “pitting,” which is also called pinholing or cratering. Pitting occurs when the coating of the sheet has small holes that mar its surface, which, when clustered together or larger in size, can cause print breakup during the printing process.
Through research, pitting is known to be caused by a boundary layer of air that gets laterally pulled in between the coating and board during their initial contact. Thus, rheological properties and the surface tension of the curtain coating formulations will be evaluated and then manipulated in two different trials in order to promote better wetting capabilities. In one trial, starch was added to the formulation in order to promote rheological healing properties, increase the viscosity, and advance water retention capabilities of the coating. In another, the surfactant content in the coating formulations was doubled from 0.3 parts to 0.6 in order to decrease surface tension.
Statistical results revealed that pitting significantly decreased in size during starch trials, with promoted water retention and rheological properties. The increased surfactant level trial resulted in lower static and dynamic surface tensions, but the pitting remained unaffected. Thus, water retention and rheology seem to be the important factors in alleviating pitting through coating formulation modifications.
Schoenfelder, Samantha Leigh, "The Modification of a Curtain Coating Formulation: A Study of Rheology and Surface Tension, and Their Effect on Pitting" (2017). Masters Theses. 1124.