Combined modeling and experimental studies to optimize the balance between fold crack resistance and stiffness for multilayered paper coatings - Part 2: Pilot coater experimental studies

Fold-crack resistance is a key coated paper quality that influences the functionality and appearance of the final product. Cracking at the fold may lead to strength reduction and the appearance of a visible crack at the folded surface. It is well known that bending stiffness and fold cracking are closely related, with a higher stiffness leading to higher risk of fold cracking. The aim of this study was to optimize the balance of stiffness and fold cracking for multilayered paper coatings. This paper is Part 2 of a combined modeling and experimental study to assess the relationship between fold-crack resistance and bending stiffness in coated papers. The objective of the work was to suggest ways to optimize coated paper to maximize fold-crack resistance as well as bending stiffness. In this experimental part of the study we have worked with double and triple coated paper on the pilot coater scale and studied the influence of the following parameters: the binder stiffness (including the distribution of stiffness between different layers), the thickness of coating layer (including the distribution of thickness) and the coating strength (including distribution of strength between layers). For double coated paper, it was seen that having a strong outer layer is positive. An outer coating layer with an increased amount of a medium stiff binder clearly improved fold crack resistance at similar stiffness. It was also seen that the layer thickness in general has a strong influence, with the thicker layer dominating the results. Therefore, there was no positive effect of making the outer coating stronger if the bottom coating is very thick. A positive effect was seen from making the bottom coating thicker in double coating, and this might be explained by the possible buffering effect on the outer layer as seen in the computer simulations. Further, for double coated paper it was seen that it was positive for the fold crack/stiffness balance to use a relatively soft binder in the outer coating, provided that the strength was high enough. In this study, the best possibilities to optimize the balance between fold cracking and bending stiffness were seen for triple coated paper. As shown in the computer simulations and confirmed by experiments the use of a thin, very stiff bottom layer, combined with a thick medium stiff middle layer and a thin medium stiff outer layer gave the best balance between stiffness and fold-cracking resistance.