hki_03_09_textile_e.pdf

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.

hki_03_09_textile_e.pdf

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