Cellulose (2020) 27:6149–6162
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to increase the fibril aggregate dimension size appre- ciably (Hult et al. 2001; Virtanen et al. 2008; Duchesne et al. 2001). This supports the notion that hemicellulose inhibits the spontaneous coalescence of neighbouring microfibrils. Hemicellulose has long been understood to aid in conventional fibre refining for papermaking; chemi- cally removing the hemicellulose content prior to refining reduces the tensile strength of the refined fibres (Bolam 1965). Sorbing hemicellulose onto fibres prior to refining has been found to improve sheet tensile strength, primarily by reducing the ‘kink’ deformations induced in the fibres (Ma¨kela¨ et al. 2010). It is therefore plausible that a higher hemicellulose content would lead to high quality MFC. Existing work in the literature has supported this idea. Iwamoto et al. (2008) showed that drying a pulp after removing hemicellulose by alkali treating results in irreversible microfibril aggregation, inhibiting fibrillation com- pared to an untreated pulp. Numerous authors have found that a high hemicellulose content coincides with a high microfibril yield and better individualisation. This appears true whether comparing fibres from different plant species (Alila et al. 2013; Desmaisons et al. 2017; Chaker et al. 2013) or from the same plant species but with different pulping conditions (Chaker et al. 2013; Petroudy et al. 2015; Spence et al. 2010). There are two major mechanisms thought to explain this. The presence of surface hemicellulose itself improves inter-fibre bonding (or inter-fibril bonding in the case of MFC), since amorphous hemicellulose chains extend out from the microfibrils when immersed in water, and form bridges between neigh- bouring microfibrils when dried (Bolam 1965). There- fore, when disintegrating a high hemicellulose pulp into MFC, the liberated surface area has a higher surface concentration of hemicellulose, and so this strengthening effect is enhanced compared to a low hemicellulose pulp. Arola et al. (2013) removed hemicellulose from nanocellulose with xylanase enzymes, which resulted in poorer tensile properties, even with similar nanocellulose geometry, clearly demonstrating this effect. The second mechanism is that a high hemicellulose pulp produces finer microfibrils with better individu- alisation, as microscopy images in various studies has demonstrated (e.g. Alila et al. 2013; Iwamoto et al. 2008; Chaker et al. 2013). Given similar microfibril
the right-hand side represents the weakness of the individual fibres, whereas the second term represents the weakness of the bonds between fibres. Usually, a sheet of paper fails due to bonds between fibres breaking rather than the fibres themselves breaking, so the second term is limiting. Adding MFC to a fibre furnish greatly increases relative bonded area (Lind- stro¨m et al. 2016), and so tensile index tends to improve considerably.
Hemicellulose
Fibre chemical composition is known to be important in papermaking. Most raw plant material from which cellulose fibres are extracted are rich in hemicellulose. Though pulping and bleaching removes much of the hemicellulose, there is still typically a large residual fraction within the fibre cell wall, with the amount dependent on fibre species and pulping conditions. Hemicellulose is a broad term for a wide variety of polysaccharides with differing monomer sugars, func- tional groups, and degrees of branching, but for woods and many non-woods, there are only two important families; xylans and glucomannans. Xylans are found in the vast majority of plants, and account for almost all the hemicellulose in hardwoods, whereas gluco- mannans are found in large quantities in softwoods (in comparable amounts to xylans) (Ebringerova´ 2005). Compared to cellulose, hemicellulose is always amorphous, whereas cellulose is partly crystalline, and hemicellulose molecules have relatively short chain lengths of 70–200 units (Fengel and Wegener 1983), compared to 300–1700 units typical for cellu- lose (Klemm et al. 2005). Within a fibre cell wall, hemicellulose closely associates with the cellulose microfibril surface, forming a layer separating neighbouring microfibrils. NMR spectroscopy work by Liitia¨ et al. (2003) indicates that both xylan and glucomannan do this, and are comparable in function. Hemicellulose has a branched, amorphous structure, and readily swells in water, as shown by work investigating the change in zeta potential during this process (Uetani and Yano 2012). This hydrophilicity also aids in the plasticity of the fibre to deformation (Bolam 1965), which would be expected to facilitate disintegration into MFC. NMR studies by several authors using fibres that have undergone different pulping conditions have shown that reducing the hemicellulose content appears
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