SubscribeIn the Paper Industry, similar to almost all other producing industries, the requirement to manufacture products with increased performance at a lower cost level is becoming more and more dominant.
For coated products, as made in some fields in the paper industry, this means that coatings shall be produced exactly with the amount of coating liquid needed to maintain the function and distributed evenly both in machine and in cross-machine direction.
New products requiring often multilayer structures are able to improve the function of the entire coating itself. The multilayer possibility may also help to manage the production process. Curtain Coating is the attractive method for achieving both, excellent profiles in cross-web and down-web direction and the possibility to coat multiple layers simultaneously in one pass.
Curtain coating belongs to the class of the so called premetered coating methods, characterized by the following main advantages:
- Simple calculation of coat weights ("conservation of mass").
- Minimum deviation of application profiles (<±0.5% in MD, <±1% in CD).
- Film thickness depends neither on physical properties of the liquid nor on geometry.
- Film thickness accuracy in CD is determined mainly by the design, dimensions and manufacturing tolerances of dies.
- No excess coating liquid.
- Short product changeover time possible.
- Contact-less application process.
- Multiple layer coating possible within just one pass.
- Possibilities for two-component applications (e.g. coating colour and cross-linking agent).
Curtain Coating in particular is attractive in this class of coating methods. The process creates an even coating also on uneven substrates ("contour coating") and coating defects in terms of sharp lines and streaks (which can be observed on coating methods using narrow gaps such as blade coaters or slot die coaters) are virtually absent.
Curtain Coating may be rather unknown and new in the paper industry, but is has been proven as a valuable method to coat single and multiple layer products in the photographic industry since the nineteen seventies. In the meantime curtain coating is used in the paper industry in several specialty applications. This technology in principle is also suitable for the coating of printing papers or cardboard packaging materials, applications which are currently being tested.
Like all other coating methods, curtain coating has its own window of application.
The table below shows the application limits of the technology. The values in the table are just ballpark figures - more detailed figures are necessary to consider a multitude of further variables.
| Parameter | Minimum | Maximum |
| Web speed [m/s] | < 1 | > 35 |
| Wet film thickness (total) [μm] | > 10 | - |
| Dry film thickness (single layer) [μm] | < 1 | - |
| Viscosity [Pas] | 0.010 | 10 |
| No. of layers [-] | > 10 | - |
| Surface tension [mN/m] | - | < 40 |
| Specific flow rate [cm3/cm*s] | > 1 | - |
The specific flow rate is an important parameter to achieve a stable process and produce a minimum film thickness at a given web speed. To bring this to a point, it is possible to achieve low to high coat weights at high speeds. Thin layers at low speed often fail due to insufficient curtain stability. This limit is valid for the total flow rate necessary in the curtain In a multilayer structure it is possible to coat very thin individual layers.
For multilayer curtain coating normally dies in the slide configuration are used, as shown in the scheme. On slide type dies the different coating liquids for the multilayer structure are exiting the upwards orientated metering slots individually. On the top surface ("slide") the second layer flows on top of the first, the third on top of #1 and #2 and so on.
This setup allows the use in a single layer and in a multilayer mode (up to more than 10 layers), as long as the process runs in the window of application. It is also possible to split one very thick layer to two or more slots to improve the performance of the entire process. It is also possible is to combine layers with different thicknesses. In the following chapter some examples for multilayer products in the paper industry shall be presented.
One application is inkjet products, namely the high quality grades. For these products multi-layer curtain coating is already proven. The example shown in the following figure consists of one thin layer (primer) on the top of the paper to improve the adhesion of the coating to the substrate, one thick ink absorption layer and one thin top coat to achieve the desired surface properties such as gloss and protection. In coating facilities using well known single layer coating methods, the product has to be coated in three individual passes.
All these have the risk of certain loss during each individual run. The thick absorption layer could probably be coated individually in a single layer curtain mode, but most likely for the two thin layers (primer and top coat) curtain coating can not be used because the flow rate at the usual machine speeds is too small to establish a stable curtain.
When a multilayer slide curtain die can be used the two thin layers can be coated together with the thick absorption layer (maybe split to two or three slots) in the curtain mode because the requirements for the low flow limit are fulfilled.
The second example shows a thermal paper, consisting of one barrier layer on the substrate and one thermo-sensitive layer on top of it.
Another example based in the adhesive industry shows possibilities, which as well could be transferred into the paper industry. One rather thick layer of adhesive for labels could be split into one thin layer with high adhesion (high functionality) on top of the product, and one thick layer with minor demands on the raw materials for achieving the required coated thickness.
The photographs below show some examples of multi-layer curtain coating dies: