Surface sizing with cationic starch: its effect on paper quality and papermaking process

The characteristics of starches used in surface sizing affect not only paper properties but also the efficiency of broke recycling processes. Oxidized starches commonly used for surface sizing tend to penetrate deeply into the paper structure, and this causes substantial reduction of opacity. Furthermore, oxidized starches act as anionic trash in the broke recycling process. To solve the problems associated with conventional size press starches, cationic starches were prepared and their effects on paper properties and papermaking processes were investigated. Results showed that cationic starches outperformed oxidized starches in opacity, brightness, print gloss, and ink density due to their better holdout property. Effectiveness in surface holdout of cationic starches was verified using a confocal laser scanning microscope (CLSM). Adsorption ratio of the cationic starch onto fiber was substantially larger than that of oxidized starch, and this decreased COD load and increased fines retention and-strength. We observed improvements in stiffness, opacity, brightness, printing quality, and picking resistance when ammonium persulfate starch was substituted with cationic starch in the film transfer size press. Furthermore, diverse beneficial effects of improving white water quality, including the reduction of Cob and cationic demand, were obtained. Surface sizing with cationic starch also gave better picking resistance and ink receptivity for coated paper than ammonium persulfate starch.