Besides film and blade coating, curtain coating is used for the application of coat¬ing media onto paper surfaces. Although curtain coating is a very old application technique, going back to the end of the 19th century, it has only been used for coating of paper since the 1990s. Curtain coating is mainly used for specialty papers. Specialty coats are usually very expensive. Therefore, the application amount must be kept to an absolute minimum.
On the other hand, a certain amount is required to achieve the desired function of the coat. In a curtain coater, a thin film of coating medium is formed which falls by gravity onto the paper surface. The only metering device for the coating color is the nozzle slice. The slot opening is about 20 to 50 times larger than the actual coating color thickness.
Thickness is reduced by gravity forces while the jet is moving towards the web and by the shear forces exerted by the high speed difference between the jet and the running web (Fig. 7.27). Due to the homogeneity of the film, the coverage of the paper by the coating medium is very uniform.
Unlike film and blade coaters, the application is contact-free. This considerably reduces the risk of web breaks and, therefore, improves runnability. It is expected that the significance of curtain coating will increase.