Stabilization of curtain coater at high speeds.

摘要

High-speed curtain coating is an emerging technology trying to gain commercial acceptance by the paper industry as a pre-metered and 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 non-impact and non-contact type of coating operation, curtain coating can operate with fewer sheet breaks or the strength requirements of the base sheet can be greatly reduced. High speed curtain coating for commodity coated grades like LWC is still a challenge. Relationship between various process, basesheet and coating variables and their mechanism must be understood to realize full potential of curtain coating. This research consists of 4 parts.; The first part deals with understanding process, base sheet and coating variables in curtain coating. Process, base sheet and coating parameters were varied through a Taguchi OA (first phase) and D-optimal (second phase) design of experiments (DOE), to stabilize a pilot curtain coater at high speeds. 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. 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 coating rheology, was found to be very important. Coverage is found to be significantly less dependant on roughness and coat weight. Excellent coating coverage was possible even at low coat weights and high base sheet roughness. Higher curtain height and medium shear thinning coating rheology was favored for obtaining curtain stability at high speeds. 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 with varied hydrophilic lypophilic balance (HLB) used in static and dynamic. Static surface tension was measured using wilhelmy plate in DI water whereas dynamic surface tension was measured using maximum bubble pressure in DI water and Mach angle method for pigmented coatings. Low HLB range surfactants (11--13) were found to be the best for curtain stability.; Third part of research deals with surface and printing of curtain coated papers. Print density and mottle of curtain coated papers were compared that of blade coated papers. Curtain coated paper were found be have better print density and lower print mottle. AFM of surfaces measurements show random alignment of high aspect ratio pigments (clay) and higher amounts of latex on the curtain coated paper. SEM of curtain coated shows an open structure of coating lattice for curtain coated papers.; Fourth part explores the possibility of using curtain coating for barrier application. Contour coating, pinhole free coating layer and virtually 100% coverage at low coat weights makes curtain coating very attractive for barrier coating applications.