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that deviate significantly, including flax, sisal, and kenaf. Despite this complication, the fit between MFC length and zero-span tensile index for most fibre species appears sufficient for most of the other fibre species to use zero-span tensile index as a replacement for MFC length in Eq. (6). This equation is therefore modified below: T ¼ B 2 Z H þ r 0 ð 7 Þ where Z represents the zero-span tensile index of the fibre and B 2 is a proportionality coefficient. Figure 5 shows the correlation between the product of the fibre zero-span tensile index and hemicellulose content, with the MFC tensile index. As Fig. 4 shows, the assumption that fibre zero-span tensile index correlates with MFC length is not completely accurate, so it is not surprising that the fit shown in Fig. 5 for Eq. (7) (R 2 = 0.78) is poorer than when MFC length was used (R 2 = 0.87). Although this relationship fits well considering the extremes, towards the centre of the graph certain fibre species such as miscanthus, abaca, and sisal deviate consid- erably from the best fit curve.
A particularly notable comparison can be made between Nordic pine and enzyme-treated Nordic pine fibres, which are essentially identical in all ways measured exce pt the zero-span tensile index, which has been greatly degraded in the case of the latter. The enzyme-treated variant consequently has much lower MFC particle length and lower MFC tensile strength. This is evidence that the correlation seen with zero- span tensile index is causal. Despite the increase in data spread compared to when MFC length is used directly, Fig. 5 shows a good fit for most fibre species, and this relationship comes with the important advantage that both predictive parameters are fibre properties that can be measured relatively easily without having to produce the MFC first. This relationship would be a practical tool for shortlisting a large number of fibre species, in order to determine which are worth pursuing to use as a feed for MFC production.
Conclusions
In this work, microfibrillated cellulose was produced from twenty-four fibre species using a stirred media detritor. The product was used in a composite with calcium carbonate mineral to form nanopaper sheets, which were tested to assess the tensile index. The hemicellulose content of the fibres prior to MFC production was measured and was found to correlate moderately with the tensile index of the respective MFC produced. Additionally, SEM images show that higher hemicellulose fibres result in the liberation of microfibrils with finer widths, which is consistent with what others have reported using alternative MFC production methods. Various geometric parameters of the fibres such as length and width were measured with a fibre analyser and did not correlate with the MFC tensile index. However, measurements of the apparent length of the MFC product particles were taken with this equip- ment, and multiplying this length by the hemicellulose content resulted in a parameter that correlated strongly with MFC tensile index. The Page Equation was applied and modified to give this correlation some theoretical basis, with some of the parameters in the bonding term being substituted with hemicellulose content and MFC length. It was found that the addition of a constant r 0 to represent the residual strength in the
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R² = 0.78
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10
8
6
4
2
0
0
10
20
30
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Hemicellulose Content (mass fracon) * Fibre Zero Span Tensile Index (Nm/g)
Nordic Pine
Black Spruce
Radiata Pine Dissolving Pulp
Southern Pine
Enzyme Nordic Pine Douglas Fir
Birch #1
Birch #2
Eucalyptus
Acacia Coon Bagasse
European HW
South Asian HW Tissue Dust
Jeans Kenaf
Abaca
Sisal
Miscanthus
Sorghum
Giant Reed
Flax
Fig. 5 The correlation between the product of the hemicellu- lose content and zero-span tensile index of the unground fibres, with the resultant MFC tensile index when ground to the standard energy input
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