Optimisation of optical coverage of board surfaces with assessment of light scattering and absorption using mineral as a coating component, TAPPICon25

One of the primary functions of mineral inclusion into paper or board is to improve the optical performance of the substrate. A coating may be applied to a sheet in order to cover a dark base, to improve the sheet opacity, to give the correct smoothness and gloss, or to give a suitable surface on which to print.

The brightness of a pigment has long been used as a guide for pigment choice in paper & board. However, the measured paper brightness is a function of colour and light absorption (K) of the coating and base and the light scattering (S) within the sheet resulting from interfaces with different refractive index. The optical performance can be quantified by measuring the S&K coefficients as described by the Kubelka-Munk model/theory in a filled or coated paper sheet. In coating, this is often assessed as a function of coat weight, and the corresponding physical sheet properties are assessed at the same time for example the correct gloss, smoothness, point to point uniformity and printability. The optical performance in the sheet is often not directly related to the pigment brightness, but is largely a function of the particle packing within the sheet and coating layer.

In this work, we initially show how S&K calculations from the Kubelka-Munk equations can be used in coated sheets to determine the optical performance and how this can be used as a predictive tool on the final sheet performance. This is presented for basesheets with different starting brightness. The second part of this work focuses on how mineral combinations in coatings can be used to improve the light scattering and consequently the optical performance of the board. We include theoretical considerations and then share a case study for improvement on the optical properties of recycled board.

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