Abstract
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.
Highlights
Cellulose nanofibrils (CNFs) have been one of the key research areas of cellulose chemistry for more than a decade, having notable potential in for example packaging, food and cosmetics applications
CNFs are most commonly produced from fully bleached chemical pulps
Six different neutral sulphite (NS) pulps selected for this study were produced from Scots pine (Pinus sylvestris L.) chips as described by Hanhikoski et al (2019)
Summary
Cellulose nanofibrils (CNFs) have been one of the key research areas of cellulose chemistry for more than a decade, having notable potential in for example packaging, food and cosmetics applications. Despite the increasing production of CNFs at industrial scale, the high energy need for fibrillation remains one of the main challenges (Nechyporchuk et al 2016). These techniques remain rather expensive for bulk industrial use due to the high cost of the required chemicals and enzymes (Solala et al 2020). CNFs are most commonly produced from fully bleached chemical pulps. With the strong need of lowering the chemical and energy consumptions of CNF production, there has been a growing interest in employing alternative raw materials with higher yields, such as unbleached or partially bleached pulps and even wood sawdust With the strong need of lowering the chemical and energy consumptions of CNF production, there has been a growing interest in employing alternative raw materials with higher yields, such as unbleached or partially bleached pulps and even wood sawdust (Spence et al 2010a; Ferrer et al 2012; Solala et al 2012; Hoeger et al 2014; Lahtinen et al 2014; Rojo et al 2015; Visanko et al 2017; Yousefi et al 2018; Ammalaet al. 2019)
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