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7/29/2019 HKI_03_09_textile_e.pdf
1/328 03/2009/
Industry Affairs
On 16 January, the Hong Kong Productivity
Co unci l ( H KP C) o rg a ni sed a sem i na r o n
dyeing/inishing and unctional treatments o
textiles. The seminar provides new perspectives
on industrial upgrade by promoting new
technologies which are both energy saving and
waste reducing. As part o the Cleaner Production
Partnership Programme, the seminar aims at
helping enterprises achieve green production
and cost reduction at the same time.
Electrochemical Process Technology
Electrochemistry reers to the use o electrical
energy in initiating chemical reactions, replacing
tradit ional a id agents in direct chemical
reactions. Taking sulphur dyes as example, in
traditional technology, sulphides (such as sodium
sulphide, Na2S) are used as reducing agents.
Although reduction process is ast and direct,large amount o chemical energy is wasted and
Green production has become necessary or enterprises under the upgrade and
transormation policy. The Hong Kong Productivity Council (HKPC) promotes new
technologies in textile dyeing and fnishing, injecting new thoughts to the industry.
New Technologies in Textile
Dyeing and Finishing
wastewater with high chemical oxygen
demand (COD) value is produced, making
long-term operation ineicient. I direct
electrochemical reduction is adopted,
no reducing agents are needed and the
COD value o wastewater can be largely
reduced, hence lowering the cost o
wastewater treatment.
D i rect e lectro chem i ca l reduct i o n
is undoubtedly more eicient than
the traditional technology, and the
underlying chemical principle is also
simple. However, as the stability andoxidising/reducing power o dierent
chemical substances are not the same, dyes
may not be directly and eectively reduced by
electrodes. Hence the scope o utilising direct
electrochemical reduction is quite narrow.
Th e principle o indirect electrochemi cal
reduction is the same, but in operation another
strong oxidising/reducing agent acts as medium,
which makes the technology more applicable to
dierent kinds o dyes. Taking indigo as example,traditional technology takes sodium dithionite
(Na2S2O4) as a reducing agent, and the product
should be re-oxidised in the air aterwards to
ix the colour. Just like traditional reduction o
sulphides, large amount o chemical energy is
wasted and wastewater with high COD value is
produced.
Enterprises attempt to reduce the amount
o sodium dithionite used in order to lower
production cost, but such attempt producesother diiculties as well. For example, injecting
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nitrogen can reduce the oxidation
o sodium dithionite but is too
expensive. Adding aldehydes or
directly powering with electricity
can improve the reducing power o
sodium dithionite, but the problem owastewater remains.
I indirect electrochemical
reduct i o n i s a do pted,
the medium can replace
so di um di thi o ni te a s
the reducing agent. The
medium can provide both oxidising
and reducing substances and can
regenerate so that both waste
and pollution can be reduced. Pastexperiments show that reduction by electrolysis
can save about 90 per cent o production cost
when compared with reduction by sodium
dithionite.
Apart rom reducing dyes, electrochemical
process technology can be utilised in other
aspects. Taking bleaching as example, the
core principle o electrochemical mercerising
and bleaching is that bleaching chemicals
can be produced by electrical energy and can
be regenerated; hence the process is easily
controlled, waste-reducing and energy-saving.
The process can be monitored so that bleaching
occurs evenly. Also, the cost and danger o
transportation is greatly reduced, particularly
regarding hydrogen peroxide which is explosive.
Another emerging project is the technology o
ozone electrolysis. Ozone is strongly oxidising
and can be used in decolourising and other
waterless dye treatments (e.g. ozone jets to
prevent wearing out o jeans). As ozone can sel-
decompose, it will not cause pollution problems
once careully treated.
Supercritical Fluid Dyeing Technology
Supercritical luid reers to the phase o a
substance with both temperature and pressure
higher than the critical point (the point where
liquid and gaseous phases o a substancebecome indistinguishable). This phase o
In traditional water-dyeing technology, textiles
should undergo multiple processes with the
help o aid agents, chemical salts, suractants
and reduction clearing agents. In contrast, or
the supercritical waterless dyeing technology,
only supercritical luid is needed or dyeing
and circulation, ater which the pressure andtemperature can be lowered and the whole
process is inished, without producing any
wastewater. Also, as carbon dioxide automatically
detaches rom textiles and remaining dyes,
the latter can be reused. More importantly, as
operation procedures are reduced, the dyeing
cycle is also shortened rom several hours to 15
to 60 minutes; energy is also saved due to the
lower operational temperature.
Regarding the cost, although the equipmentrequired or the process is quite expensive,
New perspectives on industrial
upgrade by promoting new
technologies which are both
energy saving and waste reducing.
a substance enjoys many advantages and
can replace water in the dyeing process. Thesupercritical fuid normally used is carbon dioxide
(CO2), as the critical temperature and pressure are
easier to achieve than that o other substances.
Moreover, carbon dioxide is also non-fammable
without residues, so it is suitable or industrial
use.
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Industry Affairs
the supercritical substance (carbon dioxide)
is cheap and the technology enjoys an overall
advantage in cost. On the other hand, although
the technology is not mature enough regarding
application in natural ibres, the quality o the
end-product made o synthetic ibres is high.Overall, the eects o interactions between
dierent textiles with supercritical substances are
yet to be ully discovered.
Plasma Treatment Technology
When a substance in its gaseous phase absorbs
enough energy, the outermost electrons in
the atoms will escape the nucleus control and
become ree electrons, while the atoms becomepositively charged. This chemical status o a
substance is called plasma. As it is volatile, it
can discharge electricity under certain physical
conditions and react with other substances
(including textiles), leading to various chemical
usions and issions. These eects can alter the
surace structure o textiles; hence plasma is
suitable or surace treatment.
Since only the surace structure o materials isaltered by plasma, the substrate characteristics o
textiles will not be aected. Also, as small amount
o plasma is enough to produce proound eect
and one set o equipment can accommodate
to dierent kinds o gaseous chemicals, the
equipment is relatively cost eective and
user riendly. The kinds o plasma undergoing
testing are varied, including silanes (Si nH2n+2)
(waterproo), reons (increasing surace tension
and oil- and dirt-proo eects) and phosphorus-
containing organic monomers (reproo), etc.
Plasma treatment technology can also improve
existing dyeing technology, including the newly
developed technology o metallised abrics.
On the other hand, HKPC attempts to integrate
plasma treatment technology and supercritical
fuid dyeing technology, and replace supercritical
fuid with plasma in the dyeing process. The low-
pressure plasma dyeing technology is still being
developed.
The texti le dyei ng and i ni sh in g indust ry is
considered energy-wasting and highly-polluting,
which will be orced to withdraw under the
upgrade and transormation policy. However,
with technological development on a ull swing,
traditional industries are able to overcome
technical diculties and revive ater the nancial
crisis.
Recently, the HKPC is providing consultationservice as well as technical assistance in
pilot demonstration and sample trial or
all interested enterprises. For details o the
above new textile dyeing and inishing
technology, please contact Dr Sam Mo
( Tel : 27886187) o r D r Fra nk L i n ( Tel :
27885510), or visit the website: http://www.
cleanerproduction.hk.
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