Coating coverage of metered size press pre-coated paper

The positioning of the coating on the base paper is very important for good paper quality. If the surface of the base paper is not well covered in the pre-coating stage, the top coating will be uneven and the result will be a poor quality paper. The easiest way to improve pre-coated coverage is to increase the amount of coating, but this is not always possible. An investigation was made to explore the influence of the coater, the base paper, and the coating properties on pre-coating paper coverage obtained with a metered size press (MSP). The properties investigated were MSP roll hardness, machine speed, nip load, coating color pigment ratio (calcium carbonate to clay), coating color solids content, coatweight and base paper. A multivariate analysis program was used to design the trial parameters and to analyze the results. Coating coverage was evaluated with a scanning electron microscope in the backscatter mode (SEM- BSE) and with a Laser-Induced Plasma Spectroscopy (LIPS). The results of an extensive pilot trial showed that improved coating coverage can be obtained by increasing the filler content in the surface of the base paper, by higher levels of clay in the coating formula, and softer MSP rolls. The impact of these variables became more pronounced as coatweight decreased. Increasing the coater speed or using a higher nip load (in the range studied) had no effect on pre-coating coverage. The structural prosperities of the base paper and the solids content of the coating color had minor effects on pre-coating coverage. Further studies are recommended in order to clarify the role of the base paper properties in film coating.