Coating Solutions for Woodcontaining and Woodfree Paper Grades, 1999 Coating Conference Proceedings

Johan Grön & Pentti Rautiainen

Valmet Corporation

Investments in coating technology have to assure a higher coated paper quality without sacrificing production efficiency. There are many different issues to consider when deciding on a coating concept for a specific paper grade. The layout of the coating-line can consist of C2S and C1S+C1S concepts depending on the space available for the production line. There is also a great demand for reduced investment costs, which underlines the usage of C2S coating concepts.

One of the aims is to prove the quality target of woodcontaining ULWC paper grades achievable with the metered size press (MSP) technique. The MSP technique seems to be beneficial for coating ULWC papers since it provides coating coverage at rather low coat weights. A low mechanical stress is also beneficial in respect to production efficiency. By applying the coating colors at high solids content with the MSP, ULWC quality targets are reached. The production efficiency is also improved for LWC and MWC grades resulting from a low stress coating application. At coat weights below 10 g/m²/side, the quality of the MSP coated paper is competitive with blade. Above 10 g/m²/side, the blade technique generally produces the smoothest surface. It is, however, possible to achieve single coated LWC offset and LWC rotogravure paper qualities with the MSP technique through a sufficient process optimization. Both the coating color composition and the base paper structure need to be optimized to reduce the coating color penetration. When double coating woodfree fine paper grades, an improved visible appearance and base paper coverage were achieved by precoating with the MFP technique. The surface of the base paper is to be closed to provide a good coating coverage. A more open surface allows the coating to sink into the sheet and increases the coat weight amount needed to cover the surface.