THE SORPTION BEHAVIOUR OF CELLULOSE FIBRES - Lenzing

14.1%, lyocell 14.6%, viscose 15.1%). Viscose
fibres show the highest water retention value,
lyocell fibres follow. Water retention value of
modal fibres is 15% lower then that of lyocell
fibres and 32% lower then that of viscose fibres.
As shown by the data in Figure 4, the increase in
fibre width after swelling in water is also decreases
from viscose fibres (36.3%) to lyocell
fibres (22.0%) and is considerably smaller in the
case of modal fibres (17.4%). The lowest value
of water adsorption have the natural cellulose
fibres (cotton), whose hydrophilic character increases
from cotton NaOH cleaned to cotton
mercerised.
When considering water sorption, the new lyocell
fibres are similar to the less dense, low crystalline
and least oriented viscose fibres. Water
vapour adsorption, water retention, swelling in
water places lyocell fibres between the viscose
and modal fibres. Contrary to expectations the
sorption phenomena in the aqueous medium are
not in agreement with the basic structural data
i.e. degree of crystallinity and orientation factor
(Table 1). In the case of cellulose fibres the
structure of voids (especially their diameter and
volume, and less inner surface) the fraction and
orientation of amorphous areas are more important
than other structural characteristics. The
ordered regions (crystalline) do not contribute
significantly to the process of water adsorption.
In spite of the highest degree of crystallinity and
orientation, lyocell fibers have excellent sorption
properties, similar to those of viscose fibers due
to their void system and amorphous region.
ζ [mV]
0
-2
-4
-6
-8
-10
-12
-14
-16
Cotton 2% NaOH
Cotton 24% NaOH
Viscose
Lyocell
Modal
2 4 6 8 10
pH
Figure 5. Zeta potential ζ as a function of the
pH (conc. of electrolyte solution (KCl) = 0,001
mol/l).
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Differences are also observed regarding the zeta
potential - pH functions (Figure 5), between the
very hydrophilic regenerated and more crystalline
cotton materials. The hydrophobic character
of cotton in comparison to the hydrophilic character
of regenerated fibres, is responsible for
their highest negative surface charge. The highest
negative zeta potential in the alkaline region,
has boiled cotton fibres (ζplateau of Cotton
2%NaOH = app. – 16 mV). The primary wall of
cotton (location of non-cellulose compounds) at
the fibre surfaces is destroyed using NaOH
cleaning, causing higher accessibility of the surface
groups (COOH). If cotton fibres are mercerised
a decrease of ζplateau is observed, because of
changed cellulose structure. The transformation
of cellulose fine structure from cell I to cell II
causes enlargement of the amorphous part of the
fibre, which leads to the higher reactivity of the
cellulose. The ζplateau values of structured cotton
fibres become similar to those of the regenerated
ones (ζplateau of Cotton 24%NaOH = app –10
mV). The degree of swelling is increased by decreased
crystallinity (the crystallinity of Cotton
24%NaOH is reduced from 90% to 77%; according
to iodine adsorption values ([23]). This is an
explanation for the reduction of the ζplateau .
The ζplateau value of modal is app. –9.5 mV, the ζ
plateau of viscose is the smallest (app. –5 mV),
probably due to the distinct reactive groups (OH
and COOH) accessibility, which is determined
by different structure of regenerated fibres ([1],
[2]).
Conclusion
The results of the vapour adsorption, water retention
power and swelling of different cellulose
fibres show clear differences between the hydrophilic
(viscos, lyocell) and the hydrophobic fibres
(cotton, cotton-mercerised) which are in
excellent correlation with the – pH function of
cellulose fibres.
The results of fibre reactivity and adsorption
characteristics obtained by conventional methods,
correlate with the electrokinetic character of
materials – the natural fibres are less reactive as
the regenerated one, so the ζmax of cotton is the
highest. The hydrophilicity of cotton is improved
by mercerisation (changed primary structureformation
of cell II structure), but not as high as
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