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Cellulose (2020) 27:6149–6162
of conclusions applicable to most fibre sources problematic. Fibres can differ from each other in many ways, so it cannot be taken for granted that differences in MFC quality between two fibre sources are due to a difference in one parameter rather than another. This work therefore analyses twenty-four different fibre species in order to have a large enough data set that such comparisons are more valid, and so the relative importance of a fibre property can be assessed with a greater degree of confidence.
long fibrils with micron-scale diameters, which are similarly made up of microfibrils around 30 nm in diameter. These microfibrils in turn consist of ele- mentary fibrils around 3.5 nm in diameter (Chinga- Carrasco 2011). In 1982, Turbak et al. (1983) and Herrick et al. (1983) reported that cellulose fibres could be disinte- grated to liberate these finer structural components, greatly increasing the specific surface area and hydrogen bonding capability. This product was termed microfibri ll a t ed ce llul ose (MFC), and has been found to have many applications that exploit its hydrophilic- ity, renewability, biodegradability, and high specific strength and surface area (for reviews regarding applications, see Klemm et al. 2011; Shatkin et al. 2014). The original production process involved disinte- grating cellulose pulp fibres into MFC using a homogenizer (Turbak et al. 1983). In the decades since, other equipment has been used as an alternative, such as microfluidizers and friction grinders, and chemical pre-treatment methods including enzymatic degradation and 2,2,6,6-tetramethylpiperidine-1-oxl radical (TEMPO) oxidation have been applied to reduce the energy input (for a review on production methods, see Siro´ and Plackett 2010). FiberLean T echno l ogies has adapted minerals grinding technology to mechanically disintegrate cellulose fibres into MFC cost-effectively at large scales, without requiring cellulose pre-treatment. This method uses stirred media detritor technology (Hus- band et al. 2015), which disintegrates fibres into MFC by agitating grinding media beads. In this process, a paper filler mineral such as calcium carbonate is added as a grinding aid, greatly reducing the energy required. A laboratory scale equivalent of this process is used to produce the MFC that is the subject of this research. MFC has been made from numerous sources including hardwoods (e.g. birch, eucalyptus, and acacia), softwoods (various species of coniferous trees) and non-wood sources (e.g. cotton, abaca, flax, bamboo, and sugar beet (Jonoobi et al. 2015). The fibre source can have a strong influence on MFC quality. For example Alila et al. (2013) found that abaca and sisal fibres produced finer MFC than flax and hemp. Others have investigated the influence of fibre species on MFC quality (e.g. Chaker et al. 2013; Desmaisons et al. 2017) but these are limited to comparisons of a small number of species at a time, making the drawing
Paper tensile strength theory
The MFC produced by the FiberLean process is in the form of a MFC-mineral composite that was originally created to service the paper filler additive market. In this application, the addition of MFC to a paper formulation increases the specific strength, allowing for a disproportionate amount of native fibres to be removed and replaced with paper filler mineral; this reduces the cost of the paper considerably and improves optical properties. The MFC quality is assessed by forming nanopaper sheets and testing their tensile strength; this strength appears propor- tional to the specific strength increase that this MFC would impart on paper when added to a fibre furnish. Page (1969) introduced what is today the most commonly used theory for predicting the tensile strength of paper. This postulates that the failure of a sheet occurs partly by bonds between fibres being broken, and partly by the fibres themselves breaking across their cross-sections, and that the tensile index of the sheet is more dependent on the weakest of these two mechanisms. The Page Equation describing this is stated below: 1 Þ where T is the sheet tensile index (Nm/g), Z is the zero-span tensile index (Nm/g), A is the fibre cross- sectional area (m 2 ), P is the fibre cross-section perimeter (m), q is the fibre density (kg/m 3 ), L is the fibre length (m), s B is the shear bond strength per unit area (Pa), and RBA is the relative bonded area. 1 T ¼ 9 8 Z þ 12 A q s B PL RB A ð Þ ð Zero-span tensile index is a measure of individual fibre strength and is discussed later in this work. RBA is a measure of the fraction of the fibre surface area that is used for inter-fibre bonding. The first term on
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