Cellulose (2020) 27:6149–6162
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Infl u ence o f f eed fibre geome t r y
what this looks like for different fibre species). These particles are large enough to have some geometric properties inferred from measurement with the fibre analyser. In this study, the length-weighted fibre length of these MFC particles, called from here onwards MFC l eng t h was found to be useful. The fibre analyser interprets the ‘fibre length’ in this case as the longest dimension of an MFC particle. When plotting MFC length against MFC tensile index for all fibre species, there is no general correlation, but for fibres of a similar hemicellulose content, there appears to be a clear positive trend (see Figure D1 in the MFC Corre l a t ion Inves t iga t ion tab in the S uppl emen t ar y Ma t eria l ). Therefore, using the product of hemicellu- lose and MFC length to predict tensile index appeared promising. In order to have some theoretical grounding for this correlation, the Page Equation was used as a starting point. The Page Equation, shown as Eq. (1) in the introduction, was formulated to predict tensile strength for straight, individualised fibres with lengths on the scale of millimetres, when formed into a mineral-free sheet. It is not obvious whether this would be directly applicable to MFC nanopaper, which is in the form of highly entangled and conformable, physically connected networks of fibrils, which in this study forms a composite that is 80% paper filler mineral. However, the Page Equation is conceptually useful since parameters such as fibre length and relative bonded area are expected to be applicable in a similar way to this MFC-mineral composite nanopaper. This equation will be used to justify a semi-empirical model to predict MFC tensile strength using hemicellulose and MFC length. It stands to reason that as relative bonded area increases greatly with MFC production, the bonding term of the Page Equation becomes less limiting, and strength would eventually be dominated by the zero- span term representing fibre or fibril breakage if fibrillation is extensive enough. For chemical-treated nanocellulose (i.e. TEMPO MFC) with very fine microfibrils and very extensive bonding, this may be the case, but at least for the relatively coarse mechan- ically processed FiberLean MFC under consideration, further work not published here shows that this limit is not reached. It was found that the tensile strength of MFC produced using this method varies strongly with mineral content, demonstrating that bonding failure is still important.
Fibre geometry has a strong influence in tensile strength in paper made from cellulose fibres; all other things being equal, longer and thinner fibres result in higher tensile strengths because they maximise aspect ratio. Longer fibres increase the number of connec- tions that each individual fibre can make, and thereby distribute stress over a larger area. Finer fibres (with a thinner cell wall) result in more fibres per unit mass. The fact that a high fibre aspect ratio leads to high sheet tensile strength has been found experimentally for both softwoods (Horn 1974) and hardwoods (Horn 1978). The Page Equation, shown as Eq. (1) earlier in this paper, makes use of such fibre dimensions to model paper tensile strength. Though established as true for fibres, the influence of feed fibre dimensions on MFC properties was an open question. It stands to reason that in the case of partially processed MFC where the initial fibre structure is partly intact that this relationship would hold somewhat, but it is intuitive that as the fibre structure degrades into fully processed MFC, such a correlation would disappear. It was not ruled out, however, that the dimensions of the fibre such as length or cell wall thickness affect how the fibre disintegrates into MFC. Geometric parameters of the fibres were measured using a fibre image analyser, including length and length distributions, widths, and coarseness (a mea- sure of fibre wall cross sectional area). Attempts were made to plot such geometric parameters against tensile index, and no clear correlation was apparent. The geometry of the feed fibres is therefore not thought to have any obvious influence on the quality of fully processed MFC. The geometric parameters measured by the fibre analyser and their near-zero correlations with MFC tensile strength are displayed in the P ulp Geome t r y tab in the S uppl emen t ar y Ma t eria l .
Infl u ence o f MFC l eng t h
Stirred media detritor grinding produces MFC mostly in the form of fibril ‘aggregate’ particles rather than fully individualised fibrils, i.e. the fibrils are liberated from the fibre, but are imperfectly separated from their neighbours and so are physically rooted to other fibrils to form a network (The Microsco py tab in the S uppl emen t ar y Ma t eria l gives several examples of
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