PAPERmaking! Vol7 Nr2 2021

Cellulose (2020) 27:6149–6162

6159

particle lengths as measured by the fibre analyser. As discussed in the introduction, the zero-span tensile test gives an indication of the frequency of these flaws by forcing fibres to break across their cross section, with a high value indicating a lack of flaws and discontinuities. The zero-span tensile test is not solely a measure of fibre and fibril damage. A good correlation has been found between the microfibril angle and the zero-span tensile strength, at least with fibres that are relatively undamaged by processing (Courchene et al. 2006). This agrees with data and theoretical modelling from El-Hosseiny and Page (1975). However, the microfib- ril angle is not expected to directly influence intrinsic fibril lengths or frequency of fibril damage, and it would therefore be surprising if it influences fibril length in MFC. Although easy to measure in wood, the microfibril angle is difficult to measure accurately in pulped fibres, and so was not attempted in this study, particularly because it is impractical in an industrial setting. This is likely a significant cause of some of the data spread seen in Fig. 4. Figure 4 shows the correlation between the fibre zero-span tensile index and the fibre length of the resultant MFC. There appears to be a linear relation between these two parameters when considering most fibre species, although there are several exceptions

R² = 0.87

0.00 0.01 0.02 0.03 0.04 0.05 0.06 Hemicellulose Content (mass fracon) * MFC Length (mm)

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. 3 Tensile index versus the product of the hemicellulose content and the MFC length

T ¼ B 1 L MFC H þ r 0 ð 6 Þ Figure 3 gives r 0 a value of 4.1 N m/g on a basis of the fit with the twenty-four fibre sources tested. Despite the good fit, using the MFC length in order to shortlist which fibre sources are worth using as a feed source is impractical; the MFC must first be created to obtain this length value, and so this only reduces the need of the tensile test, saving little effort overall. Instead, it is more useful to identify the reasons behind why different fibres give MFC of different MFC particle lengths, and relate this to a parameter that can be measured in the unprocessed fibres.

200

150

100

Infl u ence o f fibre zero-s p an t ensi l e index

It stands to reason that if the fibrils that form the cell wall are long, that once the fibre is disintegrated the liberated fibrils will also be long. Additionally, a greater number of flaws in the fibril structure would make breaking the fibril (and the larger scale fibre) across the cross-section easier. Therefore, a fibre that has intrinsically long fibrils with few discontinuities and cross-sectional flaws would be expected to produce long fibrils when ground down into MFC, and would therefore give a relatively high MFC

0.1

0.2

0.3

0.4

MFC Length (mm)