1. Field of invention
This invention pertains to web coating systems and more particularly to a coating system for applying a coating composition onto a paper web.
2. Description of the prior art
According to a known method a moving web is treated with liquid by passing the web tightly over an opening in a container connected to a liquid supply and to a vacuum source so that the vacuum is maintained in said container and the liquid is drawn by said vacuum up over said opening, thus reaching the web. Attempts have also been made to arrange a further opening, which could be divided into one or more rows of holes, in this known container and furthermore a horizontal rod has been placed longitudinally across said row of holes. This arrangement has been used to remove excess liquid from the web when passing said rod due to the vacuum in said container. However this removed excess liquid is recirculated to the lower part of the container containing fresh liquid to be supplied to the web.
This known device may be used for treating paper webs with water, surface size in the form of a starch solution and other liquids having low viscosity. The advantage of this known system is that it is compact and that if the web breaks because wetting is performed under vacuum, only a small quantity of the liquid will be spilled.
In producing coated paper for use as printing paper, at least one side of the paper web is coated with a suspension containing pigments, for example clay suspended in water. Those suspensions for coating compositions generally also contain a number of chemical additives, such as binders, dispersing agents etc. The water content in such coating compositions is partly absorbed during the coating of the web and simultaneously a screen cake of pigment builds up on the surface of the web. After the coating operation the web is passed through suitable drying means so that the absorbed water is caused to evaporate either entirely or partially.
This necessary evaporation step not only affects the ultimate costs of the finish product due to the very nature of the coating process, but also requires that the coating operation be performed with a coating composition which is as dry as possible. It has recently been established that blade-coating systems are highly recommendable for coating compositions having a high dryness content. Such blade-coating systems comprises a thin, flexible blade arranged to press the web against a backing surface, such as a rotating roller, thereby smoothing the applied coating composition evenly over the web and at the same time removing the excess coating composition from the web. These known techniques also have the advantage of enabling very high operating speeds to be used for such systems because, due to the high dryness content of the composition, the relatively minor quantity of water can be evaporated without using large drying equipment. It has been found that coating compositions with dryness contents of between 55-60 percent by weight of dry substance could be used in such blade coating systems, but even higher dryness contents are mainly used.
It might perhaps be thought that the above described means with vacuum-suction of liquid could be used for coating paper, for instance, with coating compositions having high dryness content. However, it has been found that such is not the case. The reason is that the smoothing arrangement in this known process in the form of a horizontal rod, is not suitable as a smoothing and regulating member.
It has also been established that this known arrangement has other drawbacks. When coating with coating compositions having dryness contents above 40-50%, considerable problems arise after only a few moments running. These problems appear to have a tendency to increase the higher the dryness content of the coating composition applied. Thus great variations regarding the final thickness of the coating occur during the coating process. Another problem is that uncoated spots are formed from time to time on the web. Yet another problem is formation of streaks in the coating layer.
These problems have proved to be of such a nature that production of coated paper on an industrial scale using this known method is impossible with coating composition having dryness contents above about 50%, and are particularly serious at web speeds of 150-200 m/min. and more.
A closer study of the problem of variations in the coating thickness indicates that these are probably caused by the removed excess coating composition being obviously drier than the fresh coating composition, since water from said composition is absorbed by the paper. Since the thickened excess coating composition recirculated to the bottom part of the container, will not be completely mixed with the fresh coating composition supplied to the same container, which is not so dry, said variations in coating thickness are obtained. The problem of uncoated spots in the paper web is probably due to foam which, as is known, is easily formed in the moving coating composition. The foam will be collected in the container and is removed from said container from time to time with the coating composition applied to the paper web. The problem of streaks is caused by lumps of fiber, agglomerate of pigment and so on which are formed by said rod scraping against the paper web and thus are collected in the lower part of the container, to be fed out from the container from time to time with the coating composition. Particles of fiber and pigment or the like therefore easily catch under the rod and produce streaks in the coating making the coated paper unusable.