Cellulose (2020) 27:6149–6162
6155
Results and discussion
MFC has more of this liberated surface area coated in hemicellulose, which is thought to form bridges between particles upon drying and thereby improve relative bonded area and specific bonding strength. The other mechanism is that the presence of hemicel- lulose, being a relatively mechanically weak amor- phous layer, provides a preferred plane of breakage along the microfibril lengths, thereby facilitating disintegration into finer microfibrils, increasing the aspect ratio. To assess whether the latter mechanism is taking place, SEM images were taken of samples with a range of hemicellulose contents to assess fibril and microfibril diameters. Four of such images are displayed as Fig. 2. Figure 2a shows bagasse MFC, which had the highest hemicellulose content of any fibre tested, at 28%. There appears to be a much higher frequency of finer microfibrils in this case than dissolving pulp and cotton MFC, shown as 2c, d, which have 4% and 0% hemicellulose respectively. Nordic pine MFC with a hemicellulose content of 17% is displayed as 2b, and also shows a high abundance of fine microfibrils, though to a lesser extent than bagasse MFC. A comparison of Fig. 2b, c is important; the dissolving pulp used in the latter is a Scandinavian pine/spruce blend, similar to the Nordic pine used in the former; the major difference is in their processing. The Nordic pine pulp underwent a kraft pulping process, which cannot penetrate well into the fibres to remove hemicellulose, whereas the dissolving pulp was pulped with the sulphite process, which is capable of this, removing the vast majority of the hemicellulose. Consequently, the fact that the liberated microfibrils in Nordic pine MFC tend to be much finer than with dissolving pulp MFC implies that the correlation of hemicellulose content with finer microfibrils is causal. There is also the question of whether the two dominant types of hemicellulose found in these fibres, xylan and glucomannan, are equivalent in terms of enhancing microfibrillation. Correlations between MFC tensile strength and xylan or glucomannan alone, or a weighted sum of the two were investigated, but the best fit was found by treating the two as identical and correlating tensile strength with the sum of the two species. These data are displayed in the P ulp Chemica l Com p osi t ion tab in the S uppl emen t ar y Ma t eria l .
Influence of hemicellulose content
Hemicellulose within bleached chemical pulp resides within the fibre cell wall, where it forms a layer around the microfibrils, separating them from their neigh- bours. It was expected both from mechanistic consid- erations and from results previously reported in the literature that it would positively correlate with MFC quality. The hemicellulose content for each fibre species was measured, and is plotted against the MFC tensile index as Fig. 1. Figure 1 shows that a high hemicellulose content of the fibre correlates with a high MFC tensile index. Fitting the data to a general straight line equation gave anR 2 value of around 0.63, which is moderately good for a single parameter, considering that cellulose is a natural product with a complex hierarchy of structures with different scales and chemistries between plant species. As discussed in the introduction, there are two mechanisms by which hemicellulose is thought to improve MFC tensile index. One mechanism is that a high hemicellulose fibre when disintegrated to form
14
R² = 0.63
12
10
8
6
4
2
0
0% 5% 10% 15% 20% 25% 30% Hemicellulose Content (mass fracon) Nordic Pine Black Spruce Radiata Pine Southern Pine Enzyme Nordic Pine Douglas Fir Dissolving Pulp Birch #1 Birch #2 Eucalyptus Acacia European HW South Asian HW Tissue Dust Coon Jeans Abaca Sisal Bagasse Kenaf Miscanthus Sorghum Giant Reed Flax Fig. 1 Fibre hemicellulose content (wt%) versus tensile index at the selected energy input. For this figure and all subsequent plots, crosses represent softwoods, circles represent hardwoods, diamonds represent cotton fibres, triangles represent leaf fibres, and squares represent other miscellaneous non-wood sources
123
Made with FlippingBook Online newsletter maker