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et al., 1997; Erra et al., 1999; Dai and Kviz, 2001; Höcker, 2002; Kan et al.,
2003; Sun and Stylios, 2005; Kan and Yuen, 2006; Holme, 2007b; Shishoo,
2007; Thomas, 2007).
Low-temperature plasma (LTP), generated by a strong electrical discharge
at atmospheric pressure (corona discharge), or at low pressure (glow discharge)
has been used for the surface modification of wool fabric for printing and for
conferring shrink-resistance. During the plasma treatment, wool is exposed
to a highly reactive gas containing free radicals, electrons and ions, among
other reactive intermediates. Recent work (Hesse et al., 1995a; Mori and
Inagaki, 2006) has suggested that the plasma treatment not only disrupts the
outermost lipid layer of the epicuticle, but also attacks one-third of the
highly crosslinked disulphide bonds in the exocuticle A-layer. The outer 30–
50 nm of the fibre surface appears to disintegrate as a result of plasma
© 2009 Woodhead Publishing Limited
Advances in wool technology
156
etching. The extent of etching is in the order of gas used: N
2
< air < O
2
. New
hydrophilic groups such as sulphonate and carboxylic acid groups are created
and it is suggested that the disulphide groups are oxidised to intermediate
cystine oxides, i.e. —SO—S— and —SO
2
—S— groups, on the surface of
wool fibres. The shrink-resist effect of the plasma treatment is due to oxidation
and etching reactions on the fibre surface which enhance the fibre wettability
by the formation of polar groups and partial removal of the outermost lipid
layer (Hesse et al., 1995a; Mori and Inagaki, 2006).
Recently the German Wool Research Institute (DWI, Aachen) examined
a continuous atmospheric plasma treatment for wool fabrics over a width of
150 cm at 5–7 m/min. Homogeneous treatment of fabric was achieved by
passing fabric through the grounded barrier layer on both sides for a double-
sided exposure of fabric with energy output up to 3.5 kW (see
Fig. 7.6).
This
treatment demonstrated improved wettability and shrink resistance of the
wool (Thomas et al., 2005).
Plasma modification of the fibre surface is known to cause a significant
increase in the friction coefficients of wool, in both with- and against-scale
directions, resulting in a rather harsh handle. Subsequent polymer application
is necessary to restore fabric softness and fabric tear strength, and to improve
shrinkproofing efficiency. A low level of Synthappret polymer finishing
has been used effectively to coat the scale edges on plasma-treated wool
fibres and achieve a high level of shrink resistance. It was recently reported
that the application of softener Arristan 64 (a reactive polysiloxane-based
softener) improved the handle properties of plasma-treated wool as well as
enhancing wool shrink resistance and colour fastness performance (Thomas
et al., 2005).
Other approaches using solvent media for modifications of the cuticle
scales of wool fibres have previously been investigated. Leeder and Rippon
(1985) studied the reaction of wool with an alkaline reagent (potassium tert-
butoxide) or a reducing agent (sodium sulphite) under anhydrous conditions
using a non-swelling organic solvent. It was reported that the treatment of
wool fabric in 0.1
M
potassium tert-butoxide in tert-butanol at 40
°C can
achieve a high level of shrink resistance with no measurable weight loss. It
appears that alkaline degradation was restricted to the fibre surface, and lipid
materials from the outmost layer of the fibre surface were removed. Julia et
al. (1985) investigated the treatment of wool with sodium sulphite in the
presence of a cationic surfactant (N-cetyl-N,N,N-trimethylammonium bromide)
in a 50:50 isopropanol/water medium. This treatment was found to confer
good shrink resistance on wool fabrics. However, the use of expensive solvent
in place of water is questionable. It is necessary to reduce costs and
environmental impact by recycling the solvents.
© 2009 Woodhead Publishing Limited
W
ool finishing and the development of novel finishes
157
7.6
Schematic setup of the plasma unit with a treatment width of 160–170 cm (Thomas
et al
., 2005).
Generator 1
Generator 2
Generator 4
Generator 3
Ground
electrode
High-voltage electrodes
Batch unwinder with
fabric spreading and
guiding system
Plasma unit
Batch winder
Dielectric coated electrodes
Fabric
© 2009 Woodhead Publishing Limited
Advances in wool technology
158
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