Impact of Precoat Structure on Topcoat Performance in Coated Board

Multiple coatings are often employed to give a smooth, highly opaque and even surface on coated board which is suitable for high quality printing. The first coating applied generally contains coarser pigments which give high bulk and good coverage of the fibres. The topcoat is often designed to have finer pigments in order to obtain the required gloss, optical properties and end functionality (laser marking and printing). As well as the impact of the pigment choice in the formulation, the method of application of the coating and the rate that it dries also have a significant influence on the resulting topography and pore structure. The binders, thickeners and additives in the coating formulation will control the rate of water loss during the coating process and this determines the runnability of the coating layer on the board machine as well as its final structure.
This publication is the first in a series of 3 which explore factors which impact the structure of the precoat and its subsequent influence on the topcoat. In this work the same basepaper, pigment and latex binder systems are used throughout and the variables studied are the speed of the coating drying, the weight % solids of the coating colour applied to the base and finally the impact of adding thickener to the precoat. Mercury porosimetry and stain length tests were used to probe the bulk and surface porosity of the layers respectively and in each case the impact on the porosity of the precoat alone and the final top coated board were assessed. A comparison of the bulk porosity of coatings applied onto porous basepaper and onto non porous plastic was also made to determine the impact of dewatering on the dried layer structure. SIMS sectioning and labelling of the precoat and topcoat with markers also allowed assessment of movement of water and starch within the coated board.
The work has proved that the pore structure of the precoat is very dependent upon the substrate to which it has been applied (porous or non-porous), the % solids content and the amount of thickener added as well as the speed of drying. These differences then follow through in influencing the structure of the final topcoat. The work will help provide levers for the paper and board producer to be able to optimise his whole coating formulation.