Selection of filler particle size for maximizing the critical properties of cellulosic paper by filler pre-flocculation

Abstract Filler pre-flocculation has been extensively studied in the past to increase filler content while maintaining the physical strength properties of paper. However, optical properties of paper could be negatively affected due to the filler pre-flocculation. In this work, two kinds of precipitated calcium carbonate (PCC) fillers with different particle sizes, namely PCC1 (4.2 μm) and PCC2 (7.7 μm), were used to investigate the effects of filler pre-flocculation on the strength and optical properties of paper. Results showed that pre-flocculated fillers with smaller particle size (PCC1) could improve the tensile strength of handsheets more effectively, since the tensile index was increased by 22.9 %. Meanwhile, for a given floc size, the handsheets with PCC1 flocs exhibited a better light scattering ability comparing to the ones filled with PCC2 flocs. The improved light scattering ability of handsheets filled with 24 μm PCC1 flocs compared to 24 μm flocs produced with PCC2 can be attributed to the complex micro void structure of flocs. This finding can be used as a guidance for the selection of filler particle size when using filler pre-flocculation to optimize the mechanical and optical properties of paper.