Relationship of Surface Strength and Bulk Strength Properties in Uncoated Woodfree Paper

Various uncoated woodfree papers produced on a pilot paper machine were characterized for picking resistance, using an IGT printability tester, and standard sheet properties. The papers were produced with and without surface sizing or a debonding agent, with different filler types (ground calcium carbonate [GCC] or precipitated calcium carbonate [PCC]) and levels (20% and 25%), and by varying the calendering conditions. The goal was to assess the relationship between surface strength of these sheets and their bulk strength properties, such as tensile strength and internal bond. Variables such as the use of a debonding agent or the application of surface size had equal effect on picking resistance and tensile strength. Increase of filler content in paper or replacement of GCC with PCC reduced picking resistance, tensile strength, and internal bond. Increase of the calendering load, for both GCC and PCC sheets, reduced picking resistance but left tensile strength and internal bond unaffected. Picking resistance of the 25% PCC-containing sheets was affected at a significantly larger extent by an increase in calendering load than picking resistance of the 25% GCC-containing sheets. These results point out that the surface of a highly filled PCC sheet is particularly sensitive to surface damage by calendering.