Sizing mechanism of emulsion rosin size-alum systems: Part 3. Solid-state ¹³C-NMR analysis of handsheets prepared by ¹³C-labeled fatty acid-alum systems

Sizing mechanism of emulsion rosin size-alum systems was studied by model experiments using ¹³C-labeld fatty acids and solid-state ¹³C-NMR analysis of the fatty acid-sized handsheets. Anionic and cationic fatty acid emulsion sizes were prepared, and were used as internal sizes with alum for making handsheets. Also soaking treatments into fatty acid / chloroform solutions were applied to alum-treated handsheets. Sizing behavior of fatty acids in these internal sizing and soaking treatments were first studied, and then the structures of fatty acid components in the handsheets were analyzed in terms of chemical shifts of ¹³C-carbonyl carbons of fatty acids by solid-state ¹³C-NMR. The fatty acid emulsion sizes prepared from mixtures of stearic and palmitic acids gave sizing features to handsheets more efficiently than a commercial emulsion rosin size. Solid-state ¹³C-NMR analysis revealed that most of fatty acid components in handsheets prepared with the fatty acid emulsion sizes and alum were in the free acid form without forming aluminum salts. Therefore, probably also in the case of emulsion rosin size-alum systems, most of rosin size components in the papersheet are in the free acid form, and these free rosin acids contribute to the sizing features. When handsheets prepared with 0-4% alum without any internal sizes were soaked in fatty acid / chloroform solutions, a clear difference in sizing degrees was observed between non-alum handsheets and alum-treated ones: +0 s for the blank (0% alum) handsheet (S1) and 40 s for the 4% alum-treated handsheet (S2). However, both NMR patterns and spin-lattice relaxation time of carbonyl carbons of fatty acid components were almost identical between S1 and S2. Thus, in the soaking treatment, the fatty acid components were also present as free acid structures in handsheets without forming aluminum salts. The distinct effect of aluminum compounds in handsheets after soaking treatments is not yet clarified.