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UBC Theses and Dissertations

Microparticle retention aid systems in mechanical pulp suspensions Wiputri, Yonika


In this thesis, the effectiveness of microparticle retention aid systems comprising of different cationic starches (tapioca and waxy maize), cationic flocculants (flocculant 1, linear with low charge density and flocculant 2, branched with medium charge density polyacrylamides) and anionic colloidal silica in improving retention and drainage of thermomechanical pulp (TMP) suspensions loaded with precipitated calcium carbonate (PCC) is studied. While starch is primarily added as dry strength agent in PCC-filled TMP suspensions, it also has a significant role in improving retention and drainage. Tapioca starch, which has both amylose and amylopectin, is found to be a better retention and drainage aid than waxy maize starch, which only contains amylopectin. In the absence of starch, both flocculants are ineffective in improving retention and drainage. With starch, both flocculant and silica are significant in enhancing retention and drainage further. Increasing the dosage of either flocculant or silica generally increases retention as well as drainage. Flocculant 2 is found to give slightly worse total and filler retention but better drainage compared to flocculant 1. The dosage of flocculant 2 is only half that of flocculant 1 though – for this reason overall flocculant 2 is deemed more effective in improving retention and drainage than flocculant 1. Split starch addition, where a portion of the starch is premixed with PCC and the rest added to the pulp, causes a slight decrease in both retention and drainage. Therefore, should the papermaker decide to use this approach, the starch should be split in a 25:75 ratio between PCC and the pulp to minimize the negative effects. Due to the cost benefits of using increased amounts of PCC, it is desirable to increase PCC content beyond the standard currently used (250 kg/t OD pulp for communications-grade paper). However, at very high dosages of PCC (500 kg/t OD pulp), the best combination identified in this work (tapioca starch, flocculant 2 and silica) is unable to maintain good retention and drainage. Increasing retention aid dosages may help retain more PCC, however this tactic can rapidly become uneconomical. A new approach is thus needed to achieve such highly-filled papers.

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