Effects of the Partial Replacement of SB latex with Dextrin Starch on the Thickness Distribution of Coating Layers, 2008 Advanced Coating Fundamentals Symposium

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A pilot coating trial has been carried out where the synthetic binder in the coating colour was partially replaced with dextrin starch. The coating thickness and the coating thickness distribution were investigated by means of burnout tests and mercury porosity measurements. The coating thickness distribution is governed by the surface profile of the base paper underneath the blade; this surface profile is controlled by the compressibility of the base paper and the blade pressure. Dewatering of the coating colours plasticises the paper and increases its compressibility; a linear relationship between dewatering and the standard deviation in the coating thickness was found for a given blade pressure. The characterization of the coating layers showed a small but measurable increase in the thickness variation for increasing starch content, which was caused by the higher water retention of the starch colours. The surface profile of the uncoated and coated paper was investigated with an optical profilometer. The data from those measurements and the coating thickness distribution investigation were used analytically to calculate the roughness change of the base paper during coating. The roughness change was found to be inversely proportional to the original base paper roughness, indicating that the rougher base paper was more compressible.