Considerations for the Selection of Binder in Double and Triple Coated Systems • Effect of Under Layer Starch Migration on End Use Performance, PaperCon 2010 Conference

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In Part 1, pilot trials were run to investigate the impact of binder choice in the precoat and middle layers of triple coated paper. Results are shared on how the choice of starch versus latex as a binder in the under layers impact the performance of the final sheet. For the “Glossy” triple coated papers, using latex in the middle layer gave 1.7 to 2.0 times the binding power versus a derivatized potato (DP) starch. For the precoat of the calendered papers, latex gave 2.1 to 2.6 times the binding power of starch. For the “silk” papers or uncalendered papers, latex had 1.2 times the binding power of starch. When higher levels of starch are used in the middle layer, the ink setting speed of the triple coated paper is reduced significantly. It was also shown that supercalendering increases the binding power of the latex whereas for starch the binding power is actually decreased.

In part 2, analytical investigation validated the hypothesis of starch migration from the middle into the top layer. Starch contents in the top layer were measured at up to 1.8 parts for papers that had 7 parts of starch in the middle layer. This represents about 30 to 35% of the starch initially present in the middle layer which migrated into the top layer.

In Part 3 of this evaluation, the findings from Part 1 were confirmed through additional pilot trials using a double coated system. Thermally modified (TM) and hydroxyethylated (HE) corn starch were used. It was shown that the final paper quality was impacted when running moderate to high levels of starch in the precoat. Lower dry pick strength was observed in the supercalendered glossy grade. Significant increases in print mottle were observed. Additionally, the papers containing moderate to high levels of starch in the precoat required significantly more ink to reach ink density targets typical in sheet offset grades. The end result was a sheet that prints poorer and required more ink to reach the same ink coverage. Further analytical investigation suggested 45-60% of the TM starch and 17-25% of the HE starch migrates into topcoats comprising fine pigments. When introducing coarse pigments for matte grade in the topcoat, lower migration percentages of 26-34% of the TM starch and 0 to 11% of the HE starch were observed. The HE starch is far less prone to migration then the TM starch used in this study.