Determination of the Influence of Furnish Content on Formation and Basesheet Profile Stability During Coating, 1991 Coating Conference Proceedings

A description is given of optical macro and micro-imaging techniques used to characterize the formation and profile stability after coating of a series of well-defined basesheets. The effects of fibre furnish on the coating structure, coverage and roughness of the coated sheet have been studied in microscopic cross-section using statistical wave transform analysis. Comparisons have been made between groundwood, TMP and pressure groundwood. Increasing replacement of groundwood with TMP, in a furnish containing 50% chemical pulp, forms a smoother basesheet with a lower optical density. However, application of aqueous coating colour is seen, by direct observation of the coating/base interface, to result in significant profile relaxation of the TMP-containing sheet under the coating layer. This effective roughening of the basesheet causes a tendency for uneven coating distribution and a comparatively rough uncalendered coated paper. Groundwood, although forming a more optically dense, rougher basesheet, does retain an apparent stability during coating, generating a more uniform coating distribution and a relatively smoother coated sheet. Replacement of groundwood with pressure groundwood improves basesheet smoothness with an attendant drop in optical density similar to that of TMP. However, contrary to the TMP-containing sheet, the greater stability of the pressure groundwood sheet profile results in a smooth coated sheet and relatively uniform coating layer thickness distribution. Greater stability of TMP-containing sheets is shown to be achievable by increasing the TMP post-refining energy.