A Study for the Statistical Optimization of a High Speed Curtain Coater, 2006 Coating & Graphic Arts Conference

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 its 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. Due to non-impact and non-contact type of coating operation, curtain coating will operate with fewer sheet breaks or the strength requirements of the base sheet can greatly reduced. Being a contour coater, there is no film split patterning, and scratching. 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 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 statistical DOE, enabled us to recognize contribution of variables to the curtain stability and optimized them in a relatively few number of trials. The variables studied were curtain height, steam flow rate of a steam substitution system, measures of 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 the 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.

Higher curtain height and 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. The role of surfactants, although good theoretical understanding exists, was inconclusive.