Paper Technology International 2020 - Journal - Page 83

PAPERTECHNOLOGYINTERNATIONAL
Figure 4: Mechanical characterisation of a cellulose fibre along its longitudinal direction.
From top to bottom: CSLM image of the fibre with the local adhesion and dissipation maps along the fibre beneath. The scale bars
are -800 to 0 nN (adhesion) and 1 to 1 x105 J (dissipation).
Beneath: bending behaviour, contact stress (error 30 %) and strain (error 25 %) along the fibre.
The mechanical properties in a) are at 2 % RH, b) at 40 % RH, c) 75 % RH and d) 90 % RH (Auernhammer et al. 2021b).
In figure 4, results of the scanning bending tests are shown.
To detect the local changes in the mechanical properties with
increasing RH, it was increased stepwise from 2 %, to 40 %, to 75 %
and to 90 % RH.
Figure 4 shows at each RH the CLSM image of the
fibre, beneath the local adhesion map, the dissipation map, the
corresponding bending ability and the calculated contact stress and
strain. From the recording of the local mechanical properties via the
scanning bending test, it can be shown that the adhesion and the
dissipation increase with increasing RH. From the bending data in
figure 4, the local bending increased when increasing the RH from
2 % RH to 90 %. Additionally, mechanical effects (clamping) play
a role: the bending of the fibre is reduced at both fixed ends and is
increased in the middle of the fibre. Here, the reduced bending at
both ends is due to attachment to the sample holder. Additionally,
“soft spots” could be identified at an intermediate RH of 40 %.
Increasing the RH, the soft areas became less distinctive when the
entire fibre became softer. This implies that at intermediate RH,
individual soft spots can occur in a cotton linter fibre weakening the
fibre locally, whereas the entire fibre softens at elevated RH. In the
contact stress graphs in figure 4, it is evident that the contact stress
was higher at both ends of the fibre. This is due to the attachment
at the fixed ends and should not be interpreted as a higher strength
of the fibre. The contact stress at both fixed ends decreases as the
RH is increased. Additionally, here, the increased amount of water
molecules inside the cellulose network destroys bonds and act as
a lubricant between the fibrils, which leads to fibre softening. The
strain is linked to the bending properties of the fibre. Thus, the strain
behaviour follows the same trend as the bending behaviour and is
assumed to be interpreted identically.
With the scanning bending test, it is possible to measure
mechanical properties of a free hanging fibre (clamped at both ends)
with colloidal probe atomic force microscopy along the longitudinal
direction of the fibre. The proposed method of scanning along the
longitudinal direction of the fibre provides a more detailed picture
of the mechanical behaviour. To demonstrate the potential of this
approach, the mechanical properties of a single cellulose fibre were
mapped for varying RH. This method can be applied for different
fibre types, plain fibres or coated fibres to test, analyse and detect
differences in their mechanical behaviour.
2.2 Recording quasi-static force-distance curves
PeakForce-Tapping mode
One other method to record mechanical properties of single
cellulosic fibres is the mapping of the local mechanical properties
via quasi static force-distance curves in a small area of the fibre.
This method is described as “PeakForce-Tapping” (B. Pittenger
2012). Here, the sharp tip attached to the cantilever is oscillated
below resonance and is moved (scanning motion) over the fibre
surface. The quasi-static PeakForce-Tapping mode allows for the
simultaneous mapping of the topography and mechanical properties.
Here, cumulative force vs. distance curves were measured at
every pixel in the recorded map. The mechanical properties were
then directly extracted from the force-distance curves (see figure
2). While the adhesion force represents the minimum in the retract
curve, the DMT modulus was fitted with the prediction of the contact
mechanics by Derjaguin, Muller, and Toporov (DMT) (Derjaguin et
al. 1975a).
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