Comparison between a curved soft tip metering element and a conventional blade in the coating of paperboard, PaperCon 2010 Conference

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The use in coating applications of a new metering element with a curved soft tipped polymer recently introduced onto the market has been studied. The basic benefit is extremely good coverage and a uniform mass distribution of the coating layer. It was possible to run the new metering element with a coating colour of high solids content. The coat weight is controlled firstly by the angle between the curved profile and the web and secondarily by the linear pressure. This study gives example of coat weight control with the new metering element and presents a comparison between the new metering element and a conventional blade. Four different cartonboards (white lined chipboards) and one duplex wallpaper were used in the comparison. The properties of pilot-scale samples coated with calcium-carbonate-based coating colours were studied. The optical coverage (White Top Mottle - WTM) was investigated by coating on white line chipboards, and the degree of WTM was quantified by image analysis. The printability was investigated by laboratory offset printing trials (Prufbau), where print mottle was simulated and analyzed by image analysis. The binder distribution was indirectly investigated by analyzing the distribution of Optical Brightener (OBA) by illuminating the coated papers with UV-radiation at a certain angle and photographing with a digital camera followed by image analyses. This determination of UV mottling gives a direct measurement of the distribution of OBA in the xy-plane. It is also assumed that it gives indirect information about the binder distribution as well as the total mass distribution. A good correlation was found between UV mottling and mass unevenness according to burnout tests. Both single- and double-coated cartonboards were investigated. The new metering element gave a better optical coverage (less mottled image), a more uniform distribution of OBA, and less mottled printed areas than the conventional blade.