Cellulose
Fig. 10 Air permeability (Gurley)—breaking length relation- ship of the different pulp/primary fines blends (errorbars indicate 95 % confidence interval)
Fig. 12 Apparent density—breaking length relationship of the different pulp/primary fines blends (errorbars indicate 95 % confidence interval)
properties. The significant effect of fines on sheet densification and its consequence for mechanical properties has already been reported in the past (Retulainen et al. 2002). In principle, it should be irrelevant how this densification is achieved, be it by fibre flexibilization, fibre shortening and/or generation of fines. However, a slight increase of breaking length for a given apparent density is generated by the addition of primary fines. An explanation for this observation could be that the increased amount of primary fines in case of less refined and thus less flexible pulp fibres generates additional bonded area due to an aggregation of the fines in the interstices between two crossing fibres during dewatering. The improvement of fibre fibre bonding due to fines acting as a kind of bridging material has already been described in the literature (Ba¨ckstro¨m et al. 2008). At higher refining intensities leading to a higher degree of flexibilization, this effect is less pronounced and no significant difference between the reference and the two blends with additionally added primary fines is evident for denser sheets above 700 [ kg = m 3 ]. An explanation for this behaviour may be that at such high densities with the breaking length approaching 10 kilometers and more the strength of single fibres starts to affect the strength of the sheet and bonded area is not as dominant as it is at lower densities. Overall, an increase in the amount of primary fines shows the potential to reach a desired level of tensile strength in slightly refined pulps at reduced refining energy demand, giving less dense (i.e. more bulky
Fig. 11 Dewatering resistance (Schopper Riegler)—breaking length relationship of the different pulp/primary fines blends (errorbars indicate 95 % confidence interval)
also can be achieved by just refining the pulp. This maximum level of tensile strength is determined by the pulp itself and seems to be a kind of intrinsic pulp property. Several other publications are also demon- strating this behavior for refined pulps and as a consequence of fines addition (Joseleau et al. 2012; Kerekes 2005; Lin et al. 2007; Odabas et al. 2016). Both air permeability and dewatering resistance are affected by sheet densification due to flexibilisation of the fibres during refining on the one hand and by blocking of the pores by presence of additional fines on the other hand. Apparent sheet density is directly related to tensile strenght (breaking length) as it is shown in Fig. 12 . This clearly linear relationship between sheet density and tensile strength is to be expected as higher apparent densitie leads to an increase in relative bonded area and thus tensile
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