Multilayer curtain coating technology

Multilayer curtain coating has the potential to revolutionize the art and science of paper and board coating, allowing precise, cost-effective coverage of the base sheet to get the tailor-made functionality desired. The technology is Metso’s new OptiLayer multilayer curtain coating system, which adds several different coating layers to the web at the same time in a non-contact operation with essentially no speed restrictions. This gives extremely high efficiency with regard to coating chemicals, machine runnability, drying efficiency and overall cost of the process. The coating follows the contour of the sheet for complete and even coverage of the web.

A new extra-long durability thermal paper grade was developed using this technology. The process was developed by Metso in close cooperation with the coating formulation supplier Dow Chemical. The two companies have cooperated in the development of this technology for paper and board manufacturing applications. One of the leaders in the use of OptiLayer is Dong Il Paper in Korea, which has succeeded in producing and marketing a new white liner grade based on 100% recycled brown stock. Thus brown simply becomes white in a single, cost-effective coating step. No bleaching, no white fiber.

The OptiLayer technology is based on curtain coating methods that have been around for a fairly long time. What are new and revolutionary are the multilayer aspect and its successful commercial application in installations around the world. While there is a fair amount of secrecy surrounding some of the installations, two of them, Dong Il in Korea and Jujo Thermal in Finland, have been willing to discuss their results. In both of these cases, the companies have used this new technology to develop products, processes and functionality that has helped to keep them a step ahead of the competition.

At its Ansan mill near Seoul, Dong Il rebuilt a brown liner machine with the OptiLayer multilayer curtain coater. It was therefore able to launch two new grades based on 100% recycled brown fibers, which it calls WL (White Liner) and CWL (Coated White Liner), taking a market share on the Korean market. Although Dong Il had no previous experience with coating, the unit easily started up in December 2008 and has been running very well for more than a year.

The multi-layer curtain coater gives us wide flexibility by putting the right properties in the right place at the right cost. Titanium dioxide for light scattering, for example, can be put in the second layer from the top, which frees up the top layer for printability adjusting and fine tuning. This means ratio adjustments for properties like brightness can easily be made on the run. At the same time, the precise curtain coating method gives much lower coating waste and broke.