O o d h e a d p u b L i s h I n g L i m I t e d

8.6
Ink-jet printing of wool fabrics
Printing of wool fabrics has always been a small-lot, high-fashion activity;
this has meant that wool printing did not sit easily in print mills equipped
with high productivity rotary screen printing machines capable of printing
speeds of 50 metres per minute. However, even in the cotton fabric printing
industry runs of 500 metres are now common and likely to become even
smaller. Drawbacks of rotary screen printing methods include low machine
utilisation efficiency or downtime; a pattern set-up can take 1 hour and
screen washing and drying processes can take 1–2 hours. Also of concern is
the time delay from print design to screen commissioning and screen delivery;
this can be 3 or 4 weeks. Given the demands for individual designs it could
be that the age of mass production printing to a particular colour-way and
0
4
8
12
16
20
Amount of MAS, g L
–1
80
60
40
20
0
Set, %
pH 3
pH 5
pH 7
8.19 
Percentage set after boiling wool, at pH values 3, 5 and 7, with
sodium maleate.
Wool-SH  +  Na
+
 
–
OOC—CH==CH—COO
–
Na
+
Cysteine residue
⇓
Wool-S—CH(COO
–
Na
+
)—CH
2
—COO
–
Na
+
Reaction of non-absorbed maleate with H
2
S in solution:
H
2
S  +  Na
+
 
–
OOC—CH==CH—COO
–
Na
+
⇓
Na
+
 
–
OOC—CH
2
—CH(—COO
–
Na
+
)—SH
⇓
Na
+
 
–
OOC—CH
2
—CH—CH(—COO
–
Na
+
)—S—CH(—COO
–
Na
+
)—CH—COO
–
Na
+
8.20
 Addition reactions of maleate with cysteinyl residues and H
2
S.
© 2009 Woodhead Publishing Limited

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Advances in wool technology
208
design is coming to an end; the only way to meet this challenge is to develop
digital printing processes for textiles. Successful development of these digital
systems will advantage wool and wool-blend fabric printers with their
traditional small-run production. Digital printing eliminates the screen costs,
allows designers to make pattern and colour changes immediately and
eliminates registration problems when printing.
Two issues are important before digital printing can commence commercially
on wool fabric; the availability of suitable machinery and the formulation of
fabric pre-treatments and ink formulations designed to give maximum colour
yield, eliminate background yellowing and allow production of prints of
high wet- and light-fastness.
8.6.1
Ink-jet printing machinery
Drop on demand ink-jet heads use either thermal or piezoelectric technology
to deliver droplets to the substrate as needed. Thermal ink-jet heads have
been the most popular for paper printing; they operate by thermally producing
a bubble of water-based ink which is jetted out of the nozzle. High-resolution
prints are produced due to the small drop size and up to 1440 drops per inch
(2.5 cm) covering can be obtained.
Piezoelectric ink-jet heads rely on an electric charge pulse to warp the
interior of the ink chamber, thus forcing the drops out of the nozzle. Piezo
heads are seen as more robust and more suited for high-volume printing;
additionally it is envisaged that even pigment and binder compositions could
be printed from such heads.
Continuous ink-jet printing machines are also being trialled; these machines
use a continuous stream of charged ink droplets which passes through a
deflection region where they either are directed onto the substrate or returned
to the reservoir by deflection into a gutter. It seems likely that this system
will be most capable of meeting the production speeds ultimately required to
move ink-jet printing of textiles from a mainly sampling activity into
mainstream production.
Examples of machines currently on the market are the CIBA (now
Huntsman)- Reggiani Dream machine, the Mimaki TX 1600, Aristri (DuPont),
Sophis Silk Express, Chromojet (Zimmer) and Encad TX150; these are all
based on the piezzo system. The most successful full-scale production machine
is the Dream system. In terms of the sampling market the Mimaki systems
seem to dominate; these are based on piezoelectric heads. The Mimaki TX
is mainly used for sampling, giving 720 dots per inch coverage, and the TX2
wide format machine (1.6 m) is used for small production and fast sampling.
The Mimaki machines are sold with a 1 year warranty which requires sole
use of Mimaki ink; replacement ink cartridges cost about £60 for 220 mL of
ink. Each cartridge has a chip that monitors ink level and polices cartridge
© 2009 Woodhead Publishing Limited

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The coloration of wool
209
use. Clearly there is a totally new philosophy of colour purchase being
developed by some printing machine manufacturers which could detract
from rapid development of the textile ink-jet printing field.
8.6.2
Print pre-treatments and ink formulations
Fabric pre-treatment for ink-jet printing is seen by many as requiring a
different approach from that required for screen printing. Thus there is already
an established business activity in which fabric is prepared by simply padding
and drying thickening agent; in the case of cotton ink-jet printing with reactive
dyes an alkali such as sodium carbonate is also included in the preparation.
Wool preparation may be Kroy chlorination but on steaming potentially
unacceptable levels of yellowing are produced. In the case of wool and
cotton fabric ink-jet printing the author holds the strong opinion that reactive
dyes should be the product of choice; they offer the potential of giving prints
of excellent wash-fastness and minimal wash-off providing conditions are
optimised to achieve high fixation. Dye manufacturers offer ranges of high-
purity reactive dyes by desalting commercial products, thus maximising
solubility; for deep shades inks may have to contain up to 200 g/L of dye in
aqueous solution. The latter may be difficult to sustain over a period of time
without aggregation, precipitation and subsequent jet clogging; this problem
was virtually solved by using up to 50% (w/w) N-methylmorpholine oxide
as an ink additive.
79
Again chlorination of wool is the favoured method to pre-treat the fabric
to render it readily printable to give strong vibrant prints, which fix well
following steaming – the environmental downside of chlorination has been
discussed already. There is thus substantial scope for fabric preparation and
ink R&D to allow wool fabrics to take part in the digital printing revolution.
Investment in this activity is vital. Since machinery costs are so low, digital
printers do not have to produce at the same volume as rotary screen printers;
in fact a production speed of 20 m per hour would make the machines, when
networked like looms, competitive on lots of any size.