Degree of: Doctor of Philosophy
Department: Paper Engineering, Chemical Engineering, and Imaging
Title: Stabilization of Curtain Coater at High Speeds
Committee: Dr. Margaret Joyce, Chair
Dr. Do Ik Lee
Dr. Paul D. Fleming
Dr. Mashahiro Sugihara
Date: Wednesday, March 23, 2005 9:00 a.m.-11:00 a.m.
Parkview B 231
Abstract: High-speed curtain coating is an emerging technology trying to gain commercial acceptance by the paper industry as a non-impact coating process. Curtain coating could offer enormous economic and process advantages over conventional coating methods due to non-impact and excellent coverage at reduced coat weights. Due to its contour nature, it enables excellent coating coverage, resulting in equal coverage at lower coat weights than needed with contact metering coating methods, i.e., rod and blade. It can be operated with minimal substrate tension leading to lower drive power requirements and fewer sheet breaks. Being a contour coater, there is no film split patterning, scratching, or streaking. This results in the production of a defect-free coated surface. It is a versatile coating process, in that it enables a wide range of coating viscosities and coat weights to be applied with a single coater head.
In the current study, process and material parameters were altered through a Tachugi and D-optimal experimental design approach, to stabilize a pilot curtain coater at high speeds. The statistical DOE method used, enabled variables contributing to the curtain stability to be recognized and optimized in a relatively few number of trails. The variables studied were curtain height, steam flow rate of a steam substitution system, coating rheology, surfactant dosage, coat weight, web speed, base sheet roughness and base sheet sizing. Trials were conducted at Mitsubishi Heavy Industry's state of art coating research center in Hiroshima, Japan.
The role of boundary layer air removal system was found to be critical to the stability of the curtain, especially at high speeds. Base sheet roughness, in combination with the parameters of the coating formulation, was found to be very important. Coating coverage improved with the smoothness of the base sheet and excellent coating coverage was possible at low coat weights. Shear thinning coating rheology was favored for obtaining curtain stability at high speeds, as well as, higher curtain heights. The sizing of the base sheet impacted coverage and curtain stability at high speeds due to its impact on the wettability of the base sheet by the liquid curtain.
Second part of research deals with measurement of dynamic surface tension and role of surfactants in stabilizing curtain. A wide range of surfactants (HLBs) used in static and dynamic (via maximum bubble pressure and mach angle methods). Low HLB range surfactants (11-13 HLB) were found to be the best for curtain stability. Third part of research deals with printing of curtain coated papers. Print density and mottle of curtain coated papers were compared to that of blade coated papers. Curtain coated paper were found to have better print density and lower print mottle. AFM of surfaces measurements confirm higher amounts of latex on the curtain coated paper resulting in high ink thickness on the paper. Pigments on curtain coated papers were found to be less aligned than that of blade coated paper.
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