Sodium sulphite pulping of Scots pine chips in the initial pH range of 7.5–9 was investigated at 180°C and 165°C (for 120–240 min), using chemical charges between 25 and 50% and anthraquinone (AQ) charges 0–0.2%. As a result, pulps with yields of 52–73% and kappa numbers of 35–106 were produced. Delignification was accelerated by higher Na2SO3 charges and temperatures, and by AQ addition, whereas the effect of initial pH was negligible. The high pulp yields at a given kappa number were attributable to high retention of hemicelluloses, especially that of galactoglucomannan. The relatively low viscosities of pulps were apparently caused by depolymerisation of cellulose by sulphite. The sulphonic acid groups in the pulps correlated well with the residual lignin content, although some sulphonation of polysaccharides may also be possible. The results indicate that pulping of softwood in buffer-free (without Na2CO3 and/or NaOH) sodium sulphite liquor may result in pulps with tailored characteristics.
The potential of neutral sulphite pulps from softwood with different yields (c. 58–84%) and high residual lignin contents (c. 10–25 wt%) was investigated as a raw material for lignin-containing cellulose nanofibrils (LCNFs) by following their fibrillation during grinding. It was found that the lower yield (58–65%) pulps needed two grinding cycles to produce fibrillated fibers with water retention values (WRV) as high as 400 g/g (at the energy consumption level of 1400 kWh/t). In contrast, the high yield (77–84%) pulps fibrillated more slowly, requiring five grinding cycles to reach comparable WRV values. Apparently, higher crosslinking degrees of lignin in the high yield pulps are hampering the fibrillation, although the high hemicellulose contents (21–24 wt%) and the high charge densities (200–350 µmol/g, originating from carboxylic and sulphonic acid groups) of the pulps were expected to enhance the fibrillation. Nevertheless, regardless of the different fibrillation behaviour, most of the pulps formed c. 10–15% of nanosized material below 30 nm and significant amounts of fibrils with size under 100 nm based on the centrifugation method and FE-SEM images. As the pulps were of moderate to high yield and fibrillated easily without any chemical or enzymatic pretreatments, they show promise for cost-efficient production of LCNFs. The nanopapers prepared from the fibrillated pulps showed tensile strengths (73–125 MPa) comparable with the nanopapers from high yield mechanical pulps, whereas the water contact angles (41°–58°) were closer the those of chemical pulps.
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