Cellulose
to internal fibrillation, which has also been reported elsewhere (Gharehkhani et al. 2015; Kerekes 2005; Wang et al. 2007). One of the pulp property related parameters that is expected to be affected due to fines addition is dewaterability. Still, in the unrefined state the effect on dewatering (based on degree of beating SR) is barely noticeable (Fig. 9). For low refining intensity of 1000 PFI revolutions an increase in dewatering resistance is noticeable especially at 12% addition rate of primary fines. This effect becomes even more pronounced at higher refining intensities of 4500 and 6000 revolutions in the PFI mill. For the 12% primary fines/pulp blend refined at 6000 PFI revolutions, the degree of dewatering resistance expressed as SR was more than 50% higher than the reference sample containing 5% of primary fines. Based on the morphological evaluation of the blends it can be stated, that in case of the chosen
refining aggregate barely any fines are created for all the assessed refining levels. The refining treatment almost exclusively affects the properties of the fibre material. Therefore, the technological effect of the dosed primary fines is directly accessible at different refining levels of the given pulp.
Paper properties of handsheets prepared from reference pulp and pulp/primary fines blends
A similar behavior as for the dewaterability of the reference pulp and pulp/primary fines blends is evident for air permeability (Fig. 9) with the effect of fines addition being more significant at 4000 and 6000 PFI revolutions. It seems, that the additional fines present in the pulp tend to affect air permeability and dewatering especially in already denser networks formed of more flexible fibres where they are capable of blocking the already smaller remaining pores. When it comes to mechanical properties there clearly is a positive effect on breaking length (Fig. 9) due to the addition of primary fines. This effect is reduced for higher refining intensities and therefore already den- sified sheets up to the point where no significant difference in breaking length is evident after 6000 revolutions in the PFI mill. However, when it comes to applying a such treated softwood unbleached kraft pulp in paper production the main interest is to reach a certain level of tensile strength, as in industry that kind of pulp is refined to reach a certain strength level. The corresponding dewatering properties are an inevitable secondary effect that is taken into account but not adjusted to a certain target. Looking at the relationship between air permeability and tensile strength (Fig. 10) or dewa- tering resistance and tensile strength (Fig. 11) it becomes evident that the addition of primary fines does not have a significant effect on this relationship. Tensile strength (breaking length) tends to level out at higher dewatering resistance while air resistance still increases further. The blend containing 12% primary fines shows a slightly higher dewatering resistance (SR ) at a given breaking length already at lower refining intensities; at an addition level of 9% such a trend is not observed. In case of air permeability (Fig. 10), no significant difference is evident whether primary fines are added or not. It is also evident that the addition of primary fines does not allow to increase tensile strength above a certain maximum level, which
Fig. 9 Development of dewatering resistence (Schopper Riegler), Gurley air permeability and breaking length due to refining (errorbars indicate 95 % confidence interval)
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