5818
Cellulose (2021) 28:5807–5826
Fig. 9 Unfractionated pulp ( a ) and fractionated pulp ( b ) R30. Both images were taken on PCC-CTMP refined for 60 min
sheets was lower than that of the CTMP sheets. Similarly, fiber-weight-based tensile stiffness index for PCC-CTMP was only slightly lower for R100 and R200. PCC-CTMP R400 sheets instead had a clearly higher tensile stiffness index than the corresponding CTMP R400 sheets.
was 16, 14 and 7 % for samples refined for 0, 30 and 60 min, respectively. The handsheet strength increased with decreasing fiber size and increasing pulp refining for both CTMP and PCC-CTMP. The coarse, stiff fibers, as in R30, are known to give high bulk and a highly porous sheet, but poorer bonding than the more flexible and conformable finer fibers (Retulainen et al. 1998). This result was, therefore, expected. A more relevant comparison between CTMP and PCC-CTMP sheets reveals the effect of the PCC. The increase in density of the PCC-CTMP fraction sheets with increasing refining level was not as pronounced as that of the CTMP sheets. PCC-CTMP sheets from the 60 min refined pulp fractions had lower density than the corresponding CTMP sheets. This was associated with the debonding effect of nanosized particles and agglomerates of PCC on the fiber. The debonding effect is supported by the tensile index and tensile stiffness index results. There was a good linear correlation between tensile index, elastic modulus, tensile stiffness and density for both CTMP and PCC-CTMP sheets made of fractions (Fig. 14). The slopes of the regression lines differed greatly for CTMP and PCC-CTMP sheets in the case of tensile index versus density (0.130 and 0.061, respectively) and tensile stiffness index versus density (0.015 and 0.010, respectively). The difference in slope was significantly less for elastic modulus versus density (0.0075 and 0.0062, respectively). When the fiber-weight-based indices were calcu- lated, the tensile index values of the fractionated PCC- CTMP and the corresponding CTMP sheets were similar, although the tensile index of the PCC-CTMP
Optical properties
Both the brightness (Fig. 15) and the opacity (Fig. 16) of the PCC-CTMP handsheets were greater than those of the CTMP sheets when unfractionated pulps were used. This was not, however, the case for the individual fractions. The opacity of all the fractions increased when PCC was added, but the brightness decreased for fractions R30, R100 and R200 sheets, and increased for R400 sheets. The scattering coefficient ( s r ) and absorption coef- ficient ( k r ) (Table 5), showed that the PCC-CTMP fraction sheets had, on average, a slightly higher s r and k r than the CTMP sheets, except for the R400 fraction sheets. In other words, although the light scattering improved, the change in the opacity of the fractions was due to an increase in light absorption rather than due to an increase in light scattering. The increase in opacity and brightness of the unfractionated PCC- CTMP sheets over those of the unfractionated CTMP
123
Made with FlippingBook Online newsletter maker