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the bonds between fibres failing as with conventional tensile testing. Since the zero-span tensile test is normalised by weight, the thickness of the fibre cell wall and fibre diameter are accounted for. The notion that zero-span tensile strength is a measure of fibre damage is well-supported. Zeng et al. (2012) damaged fibres by refining, finding that the zero-span tensile strength was inversely proportional to the frequency of fibre kinks induced (i.e. sharp bends in the fibre). Joutsimo et al. (2005) induced cell wall damage (dislocations) by homogenization, which decreased fibre strength; they attributed this to induc- ing non-uniform load distributions across the cross- section. Research has shown that fibril and microfibril-scale damage is also important. Nevell and Nugawela (1987) treated fibres with acids, causing localised damage to microfibrils that substantially reduced zero- span tensile strength. Wathe´n (2006) damaged fibrils homogeneously throughout the fibre by thermal age- ing degradation, and found that the zero-span strength decreased.
lengths, this increases particle aspect ratio, improving tensile strength. Hemicellulose forms an amorphous layer between microfibrils that readily swells in water, and so this would be expected to provide a preferred plane of breakage parallel to the microfibril lengths, thereby facilitating the production of finer microfib- rils. Additionally, xylan develops a surface charge due to carboxyl group dissociation under typical process- ing conditions, causing mutual microfibril repulsion, enhancing this effect to some degree. Due to the importance of hemicellulose indicated so far in the literature, and its expected influence on MFC geometry and bonding, the hemicellulose content was investigated for all fibre species tested in this study.
Zero-span tensile strength
It is later suggested that the fibril lengths of the MFC correlate with a high MFC tensile strength. It is desirable to be able to predict the resultant MFC fibril length from intrinsic fibre properties. It is reasonable to believe that having long fibrils within the fibre structure will, all other things being equal, lead to long liberated fibrils. Also, having fibrils with few pre- existing defects should reduce the degree of fibril length degradation during processing. Both of these factors are expected to affect the individual fibre strength; intrinsically long fibrils result in fewer discontinuities at fibril endpoints, and having undam- aged fibrils means fewer microscopic weak points in the fibre. Both can be seen to influence the ‘quality’ of the fibre cross-sectional area, i.e. having long, undam- aged fibrils should result in the cross-sectional area having few flaws. A measurement that could assess the specific strength of the fibre cross-sectional area could there- fore be useful for indicating the frequency of fibril flaws and intrinsic fibril length; and is therefore expected to correlate with long fibril lengths of the MFC product. The zero-span tensile index of a fibre sheet prior to MFC production is believed to be such a measurement. In the zero-span tensile test, the two clamps are essentially touching (within microns of each other), forcing the vast majority of the fibres between the clamps to be held by both clamps at once, since the separation distance between clamps is a small fraction of typical fibre lengths. When the sample is broken under tensile stress, these clamped fibres will fail by cross-sectional breakage, rather than
Definition of research objectives
MFC is produced by a mechanical process with a large and diverse range of fibre species, and attempts are made to correlate the tensile index of this MFC with the feed fibre properties, including the fibre hemicel- lulose content, and the fibre zero-span tensile index.
Materials and methods
Fibre sources
In total, twenty-four separate fibre species were investigated, from a variety of wood and non-wood sources. These are listed in Table 1 below.
Pulp preparation
Fibres were added to the grinding process as a 30% solids cake. Miscanthus, sorghum, bagasse, enzyme- treated Nordic pine, and Giant Reed were supplied in this form, whereas most other fibre sources were supplied as dry pulp boards. These dry pulps were soaked in water and broken down into individual fibres using a pulp disintegrator for 10 min. Excess water
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