Cellulose (2021) 28:5807–5826
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coarser fiber fractions. The preferential precipitation might reduce the strength-enhancing properties of the fines fraction, although the optical properties would be positively affected. In the present work, PCC was precipitated onto refined and unrefined chemithermomechanical pulp (CTMP) in order to study the role of CTMP compo- sition and the precipitation of PCC onto the CTMP fines. The CTMP pulp, at three different refining levels, was then used for in-situ PCC precipitation and for making PCC-CTMP composite filler. The PCC- CTMP composite fillers were subsequently fraction- ated in order to gain a further understanding of the selectivity and precipitation efficiency of PCC onto each fraction. The effect of the different PCC-CTMP fractions on sheet properties were evaluated. The CTMP fiber was chosen because of its broad fiber size distribution and sufficiently high fines content, while primary wall parts still attached to some of the fibers.
synthetic polyamide fibers (Nylon 66 and Kevlar 29) and reported that the nucleation density was greater on the treated fibers, confirming the importance of charged sites. On the other hand, recent findings have demonstrated that PCC can also be precipitated on essentially uncharged fibers such as bacterial nanocel- lulose (Stoica-Guzun et al. 2012) or polyvinyl alcohol fiber (Park et al. 2017). The formation of the PCC-fiber composite makes it possible to create a novel platform of engineered fillers which may provide new features for the papermaking process and end products. The surface morphology and chemistry of the fibers are altered by the mineral precipitation process, which can be used to enhance the material property space and to control optical properties, fiber-fiber contacts and bonding in sheets. The effects on sheet properties have often been contradictory when filling is done using IS-precipi- tated CaCO 3 . For example, sheet opacity has been reported to increase by Ciobanu et al. (2010), and to decrease by Klungness et al. (2000) and Kumar et al. (2009). The brightness has been reported to increase by Mohamadzadeh-Saghavaz et al. (2014) and to decrease by Klungness et al. (2000). Similarly, the tensile index has been reported to increase by Silenius (2002) and to decrease by Mohamadzadeh-Saghavaz et al. (2014). One possible reason for the contradictory findings and statements is the use of different raw materials. A thorough review of the role of fiber source and particularly of the dependence on size fractions and fiber size in the precipitation mechanism and nucle- ation is lacking. Also identically performed IS- precipitation has been found to result in different PCC-fiber properties depending on the fiber source (Klungness et al. 2000). In general, the presence of fines increases inter-fiber bonding in sheets. Fines affect many strength proper- ties and especially the tensile strength (Odabas et al. 2016), although the strength-enhancing effect is greater with kraft fines than with mechanical fines (Retulainen et al. 1998). Light scattering, important for optical development of sheets, is instead increased by the presence of mechanical fines, while kraft fines tend to reduce it (Odabas et al. 2016). This is explained by the non-bonded surface of the mechanical fines (Leskela¨ 1998). The assumed preferential precipita- tion of PCC onto fines can be expected to alter the properties of the fines fraction more than those of the
Materials and methods
The CTMP was provided by Stora Enso (Kaukopa¨a¨ mill Imatra, Finland) as air dry sheets, which were disintegrated and refined using a Valley beater (SCAN-C 25:96). The Schopper Riegler freeness values (SCAN C 19:65) of the pulps were 23 (0 min refined), 42 (30 min refined) and 58 SR (60 min refined). The fiber length distributions of the CTMP materials are shown in Fig. 1. The calcium hydroxide (Ca(OH) 2 ) was prepared from calcium oxide (CaO, Honeywell reagent, purity [ 96 %).
Sample preparation
PCC-CTMP preparation
The three grades of PCC-CTMP were prepared by beating the pulp to three different levels. Each grade was prepared by reacting Ca(OH) 2 and carbon dioxide (CO 2 (AGA, purity C 99.8 %)) in the presence of CTMP fibers in a 10 l open batch reactor. The reactor height was 50.6 cm and inner diameter 16.2 cm. The reactor was equipped with a gas outlet at the bottom and a stirrer with three impellers, 8.4 cm Roushton which was the lowest and two 8.5 cm pitched blade
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