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Table 2. Characteristics of FACS Average particle size (μm)
22.52
Specific surface area (m 2 /g)
121 1.63
Pore volume (cm 3 /g)
Pore size ( μm )
0.3 to1
Ignition loss (525 °C; %)
10.5 90.5
Brightness (%ISO)
As shown in Fig. 1, FACS is formed by the accretion of lamellar structures and exhibits a wrinkled, porous surface, which gives it large specific surface area. Additionally, the air voids on the surface of FACS contribute to a lower bulk density. These properties help to produce high-bulk paper but may negatively affect paper strength. In general, pulp fines are defined as the fraction passing through a 200-mesh screen. They are categorized into primary and secondary fines according to their shape (Lee et al. 2011; Hyll 2015). Primary fines (flakes) are present in pulp before refining ( e.g ., vessel elements and ray cells). Secondary fines (fibrils) are produced in the refining process. In this study, most of the fines are secondary as a result of the extensive refining. The fines morphology is shown in Fig. 2.
Fig. 2. Light microscope image of fines Morphology of Composites
The molecular weight and charge density of CPAM are 6.5±0.5×10 6 g/mol and 1015 μequiv/g, respectively. It is generally believed that CPAM , with a high molecular weight and a low charge density, flocculates fillers through a bridging mechanism (Biggs et al. 2000; Blanco et al. 2002; Rasteiro et al. 2008). The CPAM adsorbs on the surface of FACS particles or fines randomly, and its loops and tails extended far beyond the particle surface to interact with other particles, forming FACS-fines composites. As shown in Fig. 3, the FACS particles were entwined with fines fibers, but the composite size and package density were different. All of the composites had larger particle sizes than the FACS flocs. At ratios of 0.15 and 0.3, all the filler and fines formed composites, and no fines or FACS particles existed alone. However, the composites formed at the ratio of 0.3 were larger in size and had a looser structure than those formed at 0.15. At a ratio of 0.75, excess amounts of fines existed alone or self-flocculated to form fines aggregates, which can improve the paper strength but decrease the paper bulk.
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Zhang et al . (2016 ). “ Coflocculated fines & CaSiO 4 ,” B io R esources 11(3), 7406-7415.
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