Curtain Coating of Today and Tomorrow

The results presented in this paper on curtain coating and its applications for refinement of paper surfaces is based on studies performed at LSTM-Erlangen.

The work was supported by the Bayerische Forschungsstiftung and the Stiftung Industrieforschung and was guided by a consortium of companies active in various branches of the coating industry. The objective of the studies was to extend the new numerical calculation procedure for curtain coating, developed at LSTM-Erlangen, to take the fluid properties into account when designing curtain coating equipment.

In the studies performed at LSTM-Erlangen, all of the components of curtain coaters were considered, but the focus was on the film formation itself and the interaction of the supplied film with the substrate to be coated. The most important results of the studies summarized here can be found in the dissertations by U. Lange [1] and G. Sünderhauf [2], where details of the setup programme are given and of its application, with extensive results described in both dissertations.

Fig. 1: Components of a curtain coater

The studies performed at LSTM-Erlangen on curtain coating consisted mainly of numerical calculations. However, the results were, to a considerable extent, verified by experiments. The majority of the calculations were performed for substrate speeds of Us = 1000 m/min and Us = 1800 m/min. The results showed that these speeds, and especially the higher speed limits aimed for today, required process-technical improvements of the coating equipment itself, but also adaptations of the rheological properties of the coating fluid. Only both together will ensure further increases in coating speeds, together with defect-free coated paper surfaces.

Results of Computations

In coating paper with pigment ink, the quality of the layer applied is assessed by the lack of defects and by the adherence of the fluid. These days, it is required to ensure a predefined quality over the entire length and width of the entire coated paper sheet. Pre-metering of curtain coating, yielding = constant for the entire process, ensures the same film thickness along the moving paper substrate. In this way the coated layer thickness can be controlled. Good coater design ensures equality of the layer thickness in the cross-flow direction.

Likewise, back-dosing of coating liquid is not necessary, as it is still found in today's coating processes. Knife edge coaters take part of the applied coating fluid back by scraping it off the paper substrate. Partly for this reason, curtain coating offers advantages for the entire coating process, yielding high-quality layers in paper coating.

A basic understanding of the flow processes in the coating procedure is of decisive importance in mastering the technique of curtain coating with pigment inks. The numerical investigations performed at LSTM-Erlangen therefore served to obtain the knowledge necessary to permit the technical application of curtain coating processes in the paper coating industry.

Starting with a rheological characterization of the inks to be coated, extensive numerical studies were performed on the influences of the operating parameters and the rheology of the inks on the coating process. Based on the results obtained by numerical simulations, essential requirements for the rheological properties of the inks could be deduced and were forwarded to the paper industry.

From the numerical results, the maximum permissible viscosities coupled with the stress on the ink in the area where the film interacts with the substrate were deduced. The rheological requirements imposed on the coated fluid were computed as a function of the coating speed and the coated layer thickness.

It was found that the maximum possible coating speed is lower when the viscosity of the ink is high and high shear rates are present. However, when the layer thickness is increased, higher viscosities can be selected for the ink.

Results such as those obtained in the calculations at LSTM-Erlangen can be seen in the figures below for the parameters listed in the table.

Fig. 2: Streamlines and velocity in the curtain in the region of film impact on the substrate

Fig. 3: Shear rate and expansion rate in the impact region of the film
(Expansion rate is plotted in the coordinate system of the streamlines)

The study performed at LSTM-Erlangen showed that the viscosity of the ink is a decisive property regarding the way in which the "ink curtain" is applied to the substrate. At low viscosities, the ink is "wiped out of the curtain" by shearing caused by the moving paper sheet, whereas at high viscosity, it is deposited on the substrate using expansion forces acting on the ink. Looking at the streamlines of the film flow, it is apparent that viscosity increases with increasing properties of the solids and this imposes limits on coating good films.

The applications of low ink weight, i.e. small film thicknesses, at high coating speeds is possible to only a limited extent when the solid content is high. Since high solid contents are often necessary to reduce the cost of drying and the follow-up process of coating, and to increase the coated film quality, novel inks have been developed in recent years for curtain coating of paper. These meet the specific requirements encountered at high application speeds and are still delivering the desired high quality of the coated films.

The tendency to obtain high viscosity along with high solid contents can be offset by the suitable selection of the pigment type and the chosen particle distribution of the pigments, and also by the addition of binders and thickeners or by the modification of their properties.

As seen in the studies at LSTM-Erlangen, slight changes in the quantity of added thickeners can modify the viscosity over a wide range. The study also showed that an increase in fluid temperature is also helpful in reducing the viscosity. Hence there are several possibilities of adapting the fluid properties to the requirements of the coating process in attempts to use high speeds in curtain coating.

The statements above apply to the knowledge obtained from the work at LSTM-Erlangen with respect to the impact on the substrate to be coated. If one looks at the formation of gaps in the curtain itself or the behaviour of the left and right hand edges of the curtain, other requirements sometimes arise for the fluid properties.

For these reasons, the surface tension also plays a decisive role in the reported investigations. The surface tension of the coated fluid can be influenced by selecting a suitable surfactant additive and in this way stabilizing the coating flow. A higher fluid temperature also acts to decrease the surface tension. For these reasons, paper should be curtain-coated at elevated temperature.

A sensitivity analysis of the coating film flow, in which the flow behaviour with respect to external disturbances was examined within the investigations at LSTM-Erlangen, showed that fluctuations of volume flow rate in a frequency range up to ca. 30 Hz had no influence on the curtain stability. Curtain disturbances due to air movements, however, had considerable influences on the curtain stability.

As was to be expected, velocity fluctuations in the ambient air which acted on the curtain surface led to large fluctuations in the attempted film thickness. The generation of imperfections in the curtain in the form of waves can also be used to determine the dynamic surface tension caused by the diffusion of the surfactants. Since the residence times of the fluid in the curtain were relatively short (between ca. 0.05 and 0.25 s) and the curtain was stretched during falling, the curtain was characterized over its entire length by the dynamic behaviour of the surface tension.

Fig. 4: Slide Die for a Multi-Layer Curtain Coater

Entry of air between the substrate and the coated ink, especially at high coating speeds, is a limiting factor. For this reason, the investigations at LSTM-Erlangen applied numerical calculations to examine more closely the air flow induced on the back of the curtain by the movement of the sheet and the movement of the falling curtain.

This air flow acts as a negative influence factor at high speeds. Comparisons between the scraping off of the air layer that occurred close to the substrate and the sucking off of the air down through the paper showed that with the removal of only a small quantity of air through the substrate, the pressure of the ambient air acting on the curtain was reduced and thus the danger of air inclusion could be made considerably smaller.

In addition to the simulation results obtained, tests were performed on pilot coating equipment. It was found that good coatings can be obtained with the curtain coating process even on a relatively rough carton surface, but small remnants of the structure of the rough carton were still seen on the coated surface.

This means that a contour coating is created, and a surface quality is created with the curtain coating process in which the indentations on a surface to be coated are not entirely filled in. In order to achieve as even a paper surface as possible, the curtain process should be used in combination with a pre-coating process to smooth the unevenness of the untreated paper surface

Conclusions, final remarks and outlook

The knowledge described above represented the state-of-the-art 5-6 years ago when the study at LSTM-Erlangen was completed. Since then, the investigations performed at that time have been taken up again with close cooperation between TSE Troller AG, Applied Coating GmbH and LSTM-Erlangen and further developed - especially with respect to multilayer applications - based on knowledge obtained from computer simulations.

While the results summarized above were coupled with attempts to provide a well-founded basis for curtain coating at speeds of about 2000 m/min, the companies in the consortium around TSE and Applied Coating GmbH are striving for even higher speed coatings of up to 3000 m/min. To achieve this, experts in coating tools , manufacturers of coating equipment and experts in fluid mechanics will have to work closely together. Each must bring their special knowledge to the common efforts to achieve the limits of curtain coating.

* LSTM-Erlangen, D-91058 Erlangen and TSE Troller Schweizer Engineering AG, CH-4853 Murgenthal
** FMP Technology GmbH, D- 91058 Erlangen

References
[1] Ulrich Lange: Strömungsmechanische Optimierung von Komponenten einer Vorhangbeschichtungsanlage, Technische Fakultät der Universität Erlangen-Nürnerg, 1995. Anschrift: Schott Glaswerke, FES/Lag, Ma./120, Hattenbergstr. 10, D-55014 Mainz.
[2] Gerhard Sünderhauf; Strömungsuntersuchung des Vorhangstreichens von Papier, Technische Fakultät der Universität Erlangen-Nürnberg, 2001. Anschrift: Robert Bosch GmbH, FV/SLE3-Sh, Postfach 10 60 50, D-70049 Stuttgart.