Effects of degree of processing on the viscoelastic and micro-

Clogging of the interaction chamber during high pressure processing is one of the challenges that are frequently encountered during the fibrillation of non-pretreated cellulose (Pääkko et al., 2007). Hence, various mechanical and chemical pre-treatments are employed to ease fibrillation. Morpholine pre-treatment, carboxymethylation and TEMPO-mediated oxidation pre-treatments, effectively swelled the cellulose fibres, thereby facilitating the processing of these fibres into fibrils and eliminating down-time that would otherwise arise due to clogging issues in the interaction chamber. Moreover, the washing cycles effectively removed the morpholine swelling agent as shown in Figure 5.7, leading to a residual morpholine of 0.003 wt. % from the seventh wash. The initial large error at the beginning of the washing process was as a result of the variations in the amount of water and sample within the centrifugation bottle. However, this error diminished with increasing number of wash and became non existent from the seventh wash.

CHAPTER5. CELLULOSE SWELLING AND CNF PRODUCTION USING CHEMICAL PRE-TREATMENTS

Figure 5.7: Residual morpholine per wash using centrifugation

The effects of the number of passes was monitored via changes in storage modulus from amplitude sweeps and by microscope image observations. The value of G’ was obtained from the LVR between 0.4 to 0.6 % strain and is shown in Figure 5.8 for MCNF, CMCNF and TCNF.

The G’ and G” across the full range of the applied strain at various number of passes are shown in Figures A.4, A.5 and A.6 for MCNF, CMCNF and TCNF respectively.

CHAPTER5. CELLULOSE SWELLING AND CNF PRODUCTION USING CHEMICAL PRE-TREATMENTS

Figure 5.8: Effect of number of passes on the G’ of MCNF, CMCNF and TCNF

‘

For all the passes, the G’ of TCNF was greater than those of CMCNF and those of MCNF, with MCNF having the lowest G’. The total observed viscoelastic properties of a material is the sum of all the contributions from the attractive forces and repulsive forces acting on the material (Equation 3.9). For CNF materials, attractive forces arise from the intramolecular forces and intermolecular forces, leading to physical entanglements; while the presence of surface charged groups bring about the repulsive forces. Therefore, the greater G’ of TCNF compared to CMCNF and MCNF can be attributed to the presence of stronger repulsive carboxyl groups, as shown in Table 5.2. More detail on the effect of chemical pre-treatments on linear viscoelastic properties are presented in Section 5.2.9.

Regarding the effect of number of passes on G’, the G’ of MCNF increased linearly with increase in number of passes through the high shear homogeniser. The breakdown of the large micron sized fibres by the high shearing force led to a reduction in fibre dimensions and to the formation of interconnected networks of fibrils, seen as an increase in elastic modulus. It should be noted that the viscoelastic properties of MCNF would arise from intermolecular and intramolecular attractive forces alone as the material has negligible amount of surface groups.

The effect of the number of passes on G’ has not been much studied for surface unmodified

CHAPTER5. CELLULOSE SWELLING AND CNF PRODUCTION USING CHEMICAL PRE-TREATMENTS

CNF based on wood pulp. However, the changes in viscosity (Grüneberger et al., 2014; Naderi et al., 2016) and yield stress (Samyn and Taheri, 2016) with respect to number of passes, has been investigated for solely mechanically fibrillated CNFs and for enzymatically pre-treated CNFs. These two systems show a linear increase in the viscosity or yield stress with increase in number of passes. Although, the viscosity and the yield stress are nonlinear flow measurements, they can provide complimentary rheological information to those of linear viscoelasticity from amplitude sweeps.

It has been established that the modification of cellulose surface leads to a reduction in the number of passes needed to produce a structurally robust material (Nechyporchuk et al., 2016). An increase in G’ was observed from the first pass to the second for both CMCNF and TCNF. This effect can be attributed to the formation of thinner and entangled fibrils with high aspect ratios (Naderi et al., 2015). Therefore, at one and two passes, the linear viscoelasticity of CMCNF and of TCNF is strongly influenced by the sum of the forces from fibril entanglements and from the repulsive surface charges. However, further increases in the number of passes led to a decrease in G’. This effect can be a result of the reduction in fibril dimensions, especially lengthwise, leading to a reduced network entanglement at the 1 wt. % loading studied. A similar reduction in the shear viscosity of carboxymethylated CNF, at 3-5 passes, was also observed by Naderi et al., 2016.

The reduction in fibre dimensions can be easily observed from the optical micrographs of MCNF as shown in Figure 5.9. Although some persistent large fibres still remained after 5 passes. The widths of these persistent large fibres were still less than those obtained from solely mechanically fibrillated CNF after 10 passes (Josset, 2014) and similar to those that were obtained from enzymatic pre-treatment of cellulose after 40-120 grinding process (Nechyporchuk, 2015).

The reduction in fibre dimensions can also be seen from the respective optical micrographs of CMCNF and of TCNF in Figure 5.10 and Figure 5.11. Large micron sized fibres could still be seen at 1-2 passes. However, from 3-5 passes, clear fields of view manifest because the fibres dimensions are now below that identifiable from the microscope.

CHAPTER5. CELLULOSE SWELLING AND CNF PRODUCTION USING CHEMICAL PRE-TREATMENTS

1 2 3

4 5

Figure 5.9: Optical micrographs of MCNF from 1 to 5 pass samples. Scale bar = 100µm

1 2 3

4

Figure 5.10: Optical micrographs of CMCNF from 1 to 5 pass samples. Scale bar = 100µm

CHAPTER5. CELLULOSE SWELLING AND CNF PRODUCTION USING CHEMICAL PRE-TREATMENTS

1 2 3

4 5

Figure 5.11: Optical micrographs of TCNF from 1 to 5 pass samples. Scale bar = 100µm

The following subsections deal with the effects of the pre-treatment method on the proper-ties of the CNF materials on the basis of a study of the fifth pass samples of MCNF, CMCNF and TCNF, at 1 wt. %, shown in Figure 5.12.

Figure 5.12: Photographic images of MCNF, CMCNF and TCNF suspensions after 5 passes at 1 wt. %

5.3.5 Effects of chemical pre-treatments and of mechanical processing