THE SORPTION BEHAVIOUR OF CELLULOSE FIBRES - Lenzing

Introduction
Conventional regenerated cellulose fibres are
produced generally by the indirect viscose process
(viscose fibres), while high-tenacity modal
fibres are produced using a modification of the
basic procedure. The fibre production processes
are based on the derivatisation of cellulose using
carbon disulfide [[4],[5]]. New lyocell fibres are
produced by a more environmentally friendly
procedure, by regenerating cellulose into fibre
form out of a solution of cellulose in Nmethylmorpholine-N-oxide,
without the formation
of derivatives [[6],[7],[8],[9],[10]]. The
natural, as well as the conventional and new regenerated
cellulose fibres, consist more or less of
a chemically clean cellulose. The natural fibres
are always cleaned in order to remove the noncellulose
compounds (as pectinic products, wax,
proteins…). Different production processes of
conventional viscose, modal, and new lyocell
fibres cause differences in the structure and
properties of the fibres in spite of having the
same chemical composition.
Cellulose fibres, natural as well as regenerated,
have a crystalline/amorphous microfibrillar
structure. Elementary fibrils consist of a succession
of crystallites and intermediate less-ordered
amorphous regions. Lateral tie molecules connect
laterally adjacent amorphous regions
[[11],[12]]. The differences between natural and
particular types of regenerated cellulose fibres
are, above all, in the size of crystallites and
amorphous regions, amorphous and crystalline
orientation, size and shape of the voids and the
number of interfibrillar lateral tie molecules. In
order to characterise cellulose accessibility, interaction
with water is often employed, which is
able to destroy weaker hydrogen bonds but cannot
penetrate into the regions of high order
[[13],[14]]). The adsorption properties of fibres
can be obtained also on the basis of various
methods for determining the dye, iodine, surfactant
adsorption [[13],[15]]. Currently some additional
methods especially sensitive to surface
properties (determination of the electrokinetic
properties of fibres) were applied in order to
characterize the reactivity of the fibre surfaces
[[16],[17]].
Analyses of the structure characteristics of new
lyocell fibres and comparison with conventional
viscose and modal fibres were performed. The
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degree of polymerisation DP and the supermolecular
structure (crystallinity index CrI, crystallite
dimension Λ*, molecular orientation f∆n, void
structure Vp, Sp, d) of the regenerated cellulose
fibres were investigated [[2],[3]]. Differences in
molecular and supramolecular structure of natural
and different types of regenerated cellulose
fibres cause different adsorption and electrokinetical
properties of fibres. In addition the structural
parameters (amorphous regions and void
system) which significantly influence the adsorption
properties of fibres (moisture adsorption,
water retention, swelling of fibres in aqueous
medium) were determined. The accessibility of
free adsorption places in the amorphous part of
cellulose fibres was investigated by the determination
of electrokinetic properties, which were
analysed by zeta potential (ζ) measurements.
Experimental
Materials. Three types of the regenerated cellulose
fibres (viscose, modal, lyocell) and one of the natural
cellulose fibres (100% cotton fabric; purified and
NaOH mercerised), produced by Lenzing AG Austria,
were investigated. In previous research work the
analysis of structural parameters (molecular mass,
degree of polymerisation, crystallinity index, orientation
factor, void structure) and mechanical properties
of investigated regenerated cellulose fibres was performed
[[2],[3]]. In Table 1 and Table 2 the specifications
and some of the most important 7fibre structure
characteristics of investigated cellulose fibres were
presented.
The following surface and structure modification
processes for the cotton cellulose fibres were applied:
a) - Purifying treatment (the cellulose structure remains
unchanged - cellulose I); symbolized as –
Cotton 2%NaOH
- Boiling - Removal of non-cellulose compounds -
interfibrillar swelling: (20 g/l NaOH; pH = 11.5;
t = 90 min; T = 95°C).
b) - Treatment causing a structural modifications
(the cellulose structure is changed to cellulose
II); symbolized as – Cotton 24%NaOH
- Mercerisation - interfibrillar and intrafibrillar
swelling: (24% NaOH, pH = 13, t = 60s, T =
15°C)
After each treatment the fibres were washed with
distilled water until a conductivity of less than 3
ms/m was reached. The processed material was airdried.
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