New Technologies in Textile Dyeing

8/11/2019 New Technologies in Textile Dyeing

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New Technologies in Textile

Dyeing Finishing

Source: New Cloth Market




New Technologies in Textile Dyeing & Finishing

Source: New Cloth Market

The industry is desperately in the need of newer and very efficient dyeing/finishing and functional treatments of textiles. There is growing awareness and readiness to adaptnew perspective on industrial upgradation of Cleaner Production Programme, such newtechnologies help enterprises achieve green production and cost reduction at the sametime. Green Production has become necessary for enterprises under the upgrade andtransformation policy. Therefore there is an urgent need to promote new technologies intextile dyeing and finishing, injecting new thoughts to the industry.

Electrochemical Process Technology

Electrochemistry refers to the use of electrical energy in initiating chemical reactions,

replacing traditional aid agents in direct chemical reactions. Taking sulphur dyes asexample, in traditional technology, sulphides (such as sodium sulphide, Na2S) are used asreducing agents. Although reduction process is fast and direct, large amount of chemicalenergy is wasted and wastewater with high chemical oxygen demand (COD) value isproduced, making long-term operation inefficient. If direct electrochemical reduction isadopted, no reducing agents are needed and the COD value of wastewater can be largelyreduced, hence lowering the cost of wastewater treatment.

Direct electrochemical reduction is undoubtedly more efficient than the traditionaltechnology, and the underlying chemical principle is also simple. However, as the stabilityand oxidising/reducing power of different chemical substances are not the same, dyes maynot be directly and effectively reduced by electrodes. Hence the scope of utilising directelectrochemical reduction is quite narrow. The principle of indirect electrochemicalreduction is the same, but in operation another strong oxidising/reducing agent acts asmedium, which makes the technology more applicable to different kinds of dyes. Takingindigo as example, traditional technology takes sodium dithionite (Na2S2O4) as a reducingagent, and the product should be re-oxidised in the air afterwards to fix the colour. Justlike traditional reduction of sulphides, large amount of chemical energy is wasted and

wastewater with high COD value is produced.

Enterprises attempt toreduce the amount ofsodium dithionite used inorder to lower productioncost, but such attemptproduces other difficultiesas well. For example,injecting nitrogen canreduce the oxidation ofsodium dithionite but is tooexpensive. Addingaldehydes or directlypowering with electricity

can improve the reducing power of sodium dithionite, but the problem of wastewaterremains.




If indirect electrochemical reduction is adopted, the medium can replace sodiumdithionite as the reducing agent. The medium can provide both oxidizing and reducingsubstances and can regenerate so that both waste and pollution can be reduced. Pastexperiments show that reduction by electrolysis can save about 90 per cent of productioncost when compared with reduction by sodium dithionite.

Apart from reducing dyes, electrochemical process technology can be utilised in otheraspects. Taking bleaching as example, the core principle of electrochemical mercerizingand bleaching is that bleaching chemicals can be produced by electrical energy and can beregenerated; 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 oftransportation is greatly reduced, particularly regarding hydrogen peroxide which isexplosive.

Another emerging project is the technology of ozone electrolysis. Ozone is stronglyoxidizing and can be used in decolourising and other waterless dye treatments (e.g. ozone

jets to prevent wearing out of jeans). As ozone can selfdecompose, it will not causepollution problems once carefully treated.

Supercritical Fluid Dyeing Technology

Supercritical fluid refers to the phase of a substance with both temperature and pressurehigher than the critical point (the point where liquid and gaseous phases of a substance

become indistinguishable). This phase of a substance enjoys many advantages and canreplace water in the dyeing process. The supercritical fluid normally used is carbon dioxide(CO2), as the critical temperature and pressure are easier to achieve than that of othersubstances. Moreover, carbon dioxide is also non-flammable without residues, so it issuitable for industrial use.

In traditional water-dyeing technology,textiles should undergo multiple processes

with the help of aid agents, chemical salts,surfactants and reduction clearing agents.In contrast, for the supercritical waterlessdyeing technology, only supercritical fluid isneeded for dyeing and circulation, after

which the pressure and temperature can belowered and the whole process is finished,

without producing any wastewater. Also, ascarbon dioxide automatically detaches fromtextiles and remaining dyes, the latter can

be reused. More importantly, as operationprocedures are reduced, the dyeing cycle isalso shortened from several hours to 15 to 60 minutes; energy is also saved due to thelower operational temperature.

Regarding the cost, although the equipment required for the process is quite expensive,the supercritical substance (carbon dioxide) is cheap and the technology enjoys an overalladvantage in cost. On the other hand, although the technology is not mature enough

regarding application in natural fibres, the quality of the end-product made of syntheticfibres is high. Overall, the effects of interactions between different textiles withsupercritical substances are yet to be fully discovered.




Plasma Treatment Technology

When a substance in its gaseous phase absorbs enough energy, the outermost electrons inthe atoms will escape the nucleus’ control and become free electrons, while the atoms

become positively charged. This chemical status of 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 fusions and fissions. Theseeffects can alter the surface structure of textiles; hence plasma is suitable for surfacetreatment.

Since only the surface structure of materials is altered by plasma, the substrate characteristics of textiles willnot be affected.

Also, as small amount of plasma is enough to produceprofound effect and one set of equipment canaccommodate to different kinds of gaseous chemicals,the equipment is relatively cost effective and userfriendly.

The kinds of plasma undergoing testing are varied,including silanes (SinH2n+2) (waterproof), freons(increasing surface tension and oil- and dirt-proofeffects) and phosphoruscontaining organic monomers(fireproof), etc.

Plasma treatment technology can also improveexisting dyeing technology, including the newlydeveloped technology of metallised fabrics. On theother hand, HKPC attempts to integrate plasmareatment technology and supercritical fluid dyeingtechnology, and replace supercritical fluid with

plasma in the dyeing process.

The low pressure plasma dyeing technology is still being developed.

The textile dyeing and finishing industry is considered energy-wasting and highly-polluting, which will be forced to withdraw under the upgrade and transformation policy.

However, with technological development on a full swing, traditional industries are able toovercome technical difficulties and revive after the financial crisis.

O r i g i n a l l y P u b l i sh ed i n New Cl o t h M a r k e t , J a n - 2 0 11

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New Technologies in Textile Dyeing