Elastica stiffness and low load indentation measurements for the mechanical properties of coated papers

Mechanical properties of the coating layer are important in the printing and converting operations and can be related to several defects, such as coating pick, cracking at the fold and dusting at the slitter. In the latter operation, debris and dust are unwanted. One difficulty has been associated with test methods to characterize and quantify bending and compressive properties, especially for lightweight coated papers. In this paper we introduce two dynamic tests for quantifying coating elasticity: a closed loop elastica test and a low load indenter test. Several coatings on lightweight coated (LWC) paper were produced, with varying compositions and processing parameters. Pigment type, starch level, binder glass transition temperature, binder particle size, binder level, and drying method were all varied. Samples were tested using a novel stiffness test utilizing a closed "elastica" geometry for in-plane tensile testing. In addition, low-load indentation was used to measure near-surface moduli of the coatings. The modulus of a coating layer was extracted from elastica stiffness using a two-layer model proposed by Okomori et al. [1]. The elastic moduli obtained were compared and contrasted to those of Husband et al. [2] picking tests and stand alone coating tests. The shape factor of clay coating pigments had a significant effect: increasing shape factor led to increasing in-plane moduli, but a decreasing elastic moduli measured in compression. The level of modified coating starch addition was a significant parameter, proportionally imparting stiffness. Laboratory drying conditions and latex type had a smaller effect.