5808
Cellulose (2021) 28:5807–5826
Graphic abstract
Keywords PCC Precipitated calcium carbonate CTMP Improved filler Composite filler In-situ precipitation
pores (Ciobanu et al. 2010; Fortuna et al. 2003). Mechanisms have been proposed to explain the PCC attachment onto the cellulose, but results and theories are controversial and there is still a lack of under- standing of the fixation and adhesion mechanism. Subramanian (2008) suggested that the precipitation proceeds via an amorphous calcium carbonate (ACC) precursor which forms line-like aggregates on the cellulosic particles due to inter-particle interactions (due to polarization of the ACC particles), and is subsequently precipitated as calcite. Silenius (2002), on the other hand, suggested a heterogeneous nucle- ation based on classical crystallization theory. The crystallization on cellulose has been associated with the functional groups capable of binding metal ions (including Ca 2 ? ), which is thought to lead to subcrit- ical nuclei being formed on the cellulosic substrate, and subsequent nucleation (Dalas et al. 2000). Silenius (2002) pointed out that the PCC precipi- tates mainly on the cellulosic fines due to the large surface area of the fines in comparison to that of the larger fibers. ‘Fines’ is usually defined as the material that passes the 200 mesh wire of Bauer McNett classifier (Heinemann and Vehnia¨inen 2009). Sile- nius’ (2002) result has been supported by Seo et al. (2017). Wood pulp fines also contain more carboxyl groups, metal ions and negative charges than the larger fibers (Odabas et al. 2016). This suggests that the preferential precipitation of PCC onto fines may be partially due to the charged, metal-binding groups. The importance of functional groups has also been observed with synthetic polymers. For example, Lakshminarayanan et al. (2003) precipitated PCC in- situ onto both untreated and chemically treated
Introduction
Calcium carbonate, CaCO 3 , is a commonly used filler or pigment in many industrial applications including ceramics, composites, paints, plastics and paper. While the effects of typical fillers and filling mech- anisms on paper properties are fairly well understood and widely discussed in the literature, e.g. by Hubbe and Gill (2016), work on different filler dosing concepts and modified fillers is still in progress and requires more attention. In a review paper by Shen et al. (2009), different approaches for filler modifica- tion were explored and discussed, for example the use of inorganic substances to improve acid tolerance, the use of organic polymers to enhance filler-fiber bond- ing, and filler modification with cellulose and cellulose derivatives. The traditional way of adding fillers has its limitations, and alternative concepts such as in-situ precipitation onto fibers or in-line precipitation have therefore recently gained interest. One approach where CaCO 3 is used as a filler is through in-situ precipitation (IS-precipitation), i.e. the synthesis and precipitation of calcium carbonate (PCC) in the presence of the cellulosic fiber, where the conditions of precipitation can be controlled and adjusted so that the PCC is precipitates directly onto cellulosic material (Rantanen 2016; Silenius 2002; Subramanian 2008) or into the lumen and cell wall
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