Cellulose (2021) 28:5807–5826
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relationship between the surface charge or the surface concentration of a functional groups and precipitated filler could be useful for controlling the formation of PCC-fiber composite material, if such a correlation were universally established. It was shown that the fiber surface and morphology plays a role in PCC precipitation, and that this affects the homogeneity of the PCC-fiber composite, although the precipitation was carried out in a simplified system free from many susbstances, such as cationic polymers and inorganic salts such as sulphates at present in paper mill waters (Laine 2007). Some of these substances may be designed to alter fiber surfaces (such as cationic polymers) and some, for example sulphonates (Tang et al. 2012), may affect the precipitation of PCC. While the concentration of a single substance in PCC- fiber composite preparation may be below the con- centration it alone would significantly affect the precipitation and steering of particles onto the fiber, the net effect of different substances is difficult to predict.
R² = 0.8487
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Surface charge [μekv/g]
Fig. 17 Ash content of PCC-CTMP fractions plotted against the surface charge of the corresponding CTMP fraction. The error bars represent 95 % confidence values
existing surface. In this case the surface charge would be indicative of an approximately proportional increase in surface area, and the observed correlation with the surface charge density could indicate a correlation with surface area. The changes in the surface area alone do not, however, explain why PCC appeared to discriminate against the areas with intact primary wall, and it does not explain the obvious difference in fiber coverage by PCC between fines and larger fibers. It is tempting to think that the correlation between ash content and surface charge supports a precipitation mechanism similar to that described by Subramanian (2008), i.e. that the precursor of PCC aggregates on the cellulosic particles due to electrostatic interactions, and that it adheres and subsequently crystallizes as CaCO 3 . The precursor was expected to be cationic due to an excess of Ca 2 ? in compared to the CO 3 2 - during precipitation. The ‘‘charge neutralization by CaCO 3 precursor’’ concept may be partially supported by the data, as the PCC-CTMP R30 and R100 fractions refined for 0 and 30 min had essentially no anionic surface charge and that of R200 was also extremely low. On the other hand, the highly anionic surface charge of R400 and the ‘pass’ fraction, although the ‘pass’ fraction had a high mineral content, do not fully support the simple ‘‘charge neutralization by CaCO 3 precursor’’ mechanism. The surface charge of a pulp commonly depends on the production process (Laine 2007). Therefore, the
Papermaking potential of PCC-CTMP: influence on fiber bonding and optical properties
As the fiber morphology remained essentially the same, the differences between the CTMP and PCC- CTMP sheets were associated with the presence of the mineral. The information regarding the role of differ- ent fractions may help to explain why the same precipitation process on different pulps may result in differences in the relative development of optical and strength properties, as was reported by Kumar et al. (2009) to happen. The physical and optical properties of the CTMP and PCC-CTMP sheets made using the fractionated pulps suggested that the enrichment of PCC onto the desired fractions of the pulp (or onto the desired parts of the fiber) is important when tailoring the PCC-fiber composite. The differences observed between CTMP and CTMP-PCC sheets were primarily associated with differences in surface characteristics, i.e. mainly in light scattering but also in debonding as indicated by the tensile strength and density development of the sheets. The presence of a salt such as sodium or calcium chloride has been associated with changes in fiber swelling and tensile index (Fa¨lt and Wa˚gberg 2003), but it is not known whether the PCC precip- itation can alter fiber swelling. Another factor
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