Optimizing Coating Formula To Improve The Rrightness Stability Of Hyp-Containing Coated Paper

Recently, the integration of forestry and paper industry developed very quickly in China. The government would like to encourage increasing the proportion of wood fibers in paper making raw materials. But China has virtually no enough wood resources of its own and developing high yield pulping (HYP) may be a good way to effective use of wood resources and decline the pollution in pulping process. As the progress of HYP and bleaching technology, the strength and brightness of HYP was improved so much that it could substitute part of bleached kraft pulp. In addition, HYP could improve some important properties of the paper, such as bulk and opacity. But light-induced yellowing of HYP was still an obstacle of its wild application.In this study, we were going to optimize coating formulation and coating process to inhibit the light-induced yellowing of HYP and increase the usage of HYP in high-grade coated paper. The coating color should have good hiding power, strong UV absorption ability and high brightness. In order to carry out the state concept of producing and using low brightness paper, we chose the base paper with lower brightness and the bright of coated paper was mainly increased by the coating color. So that could relax the limitation of the brightness of base paper. Besides, we studied the impact factors of light-induced yellowing of coated papers, especially had investigated the optical properties of OB A in coating layer. Then we optimized the coating process to improve the brighten efficiency and optical stability of OBA.The result showed that, incorporating UV absorber (UVA), such as TiO2and OBA, into the coating color containing clay and GCC significantly inhibited the color reversion of HYP in base paper. Concentrating UVA to one of the coating layers could increase their efficiency and cut costs in multiple coating. Adding OBA into coating color could improve the brightness and tone of coated paper, but the addition of OBA would increase the color difference and P.C. number in aging process due to the deactivation of OBA itself by tran-cis isomerization. In this research, when the charge of OBA was more than0.6pph in top-coat, the brightness reversion caused by fluorescence loss became the main reason of paper yellowing. And OBA works well only when it is fixed to a carrier. As OBA carriers, PVOH could gain higher initial brightness than CMC and CMC could improve the optical stability of OBA more effectively than PVOH. We found that addition of starch in pre-coat could increase the brighten efficiency and optical stability of OBA. And starch acts in synergy with CMC or PVA for improving optical stability of OBA. In order to further increase the brightness stability of coated paper, we tried to add both TiO2and OBA to the coating color. But when OBA was mixed with TiO2in coating color, T1O2would decrease the brighten efficiency of OBA. Even though we added each of them to different coating layer respectively----OBA in top-coat, TiO2in pre-coat, the immigration of OBA would also effect the OBA brighten efficiency. Fortunately, when we added some water retention agent, such as PVA, in pre-coat, where OBA was added, OBA was able to brighten paper effectively without the influence of TiO2. So the coating layer would have high brightness and high UV absorption ability. At a dosage of1parts of OBA and PVA in top-coat,5parts of TiO2in pre-coat, a30%APMP containing base paper with brightness of74%could used to produce high bright coated paper with brightness over92%,which also had good brightness stability.