NEW INVENTION IN TEXTILE DYEING AND FINISHING

Ultra and new modern technologies are always regarded as likely for dyeing and finishing of textile. Usually the process of textile dyeing and finishing are very slower and the process takes almost 8 to 12 hours for dyeing cotton knitted fabric with winch or jet  dyeing machine and almost 5 to 8 hours for finishing that fabric in finishing machine. It takes too much time, costly, labor depended. If we use new technology in textile dyeing and finishing, we will get a very good result.

 

On 16 January, the Hong Kong Productivity Council (HKPC) organized a seminar on dyeing/finishing and functional treatments of textiles. The seminar decision provides new perspectives on industrial upgrade by promoting new technologies in textile dyeing and finishing which both is energy saving and waste reducing. As part of the Cleaner Production Partnership Programme, the seminar aims at helping enterprises achieve green production and cost reduction at the same time.

Electrochemical Process Technology in textile dyeing and finishing:

A modern invention in textile dyeing and finishing is the development of Electrochemical process technology. Electrochemistry means the use of electrical energy in initiating chemical reactions, replacing traditional aid agents in direct chemical reactions. Taking sulphur dyes as example, in traditional technology, sulphides (such as sodium sulphide, Na2S) are used as reducing agents.

What ever the reduction process of dyeing is fast and direct, but it is wasted large amount of chemical energy and Green production has become necessary for enterprises under the update and conversion policy. The Hong Kong Productivity Council (HKPC) promotes new technologies in textile dyeing and finishing, injecting new thoughts to the industry.

New invention in Textile Dyeing and Finishing wastewater with high chemical oxygen demand (COD) value is produced, operation inefficiency become long-term. When electrochemical reduction is adopted, no reducing agents are needed and the

COD value of wastewater can be largely reduced, hence cost of wastewater treatment will be lowering. So using of direct electrochemical reduction is undoubtedly more efficient than the traditional technology, and the underlying chemical principle is also simple. However, as the stability and oxidizing/reducing power of different chemical substances are not the same, dyes can not be directly and effectively reduced by electrodes. Hence the scope of utilizing direct electrochemical reduction is quite narrow.

The system of indirect electrochemical reduction is the same, but in operation another strong oxidizing/reducing agent acts as medium, which makes the technology more applicable to different kinds of dyes. Taking indigo as example, traditional technology takes sodium dithionite (Na2S2O4) as a reducing agent, and the product should be re-oxidized in the air afterwards to fix the colour. Just like traditional reduction of

sulphides, large amount of chemical energy is wasted and wastewater with high COD value is produced.

Enterprises attempt to reduce the amount of sodium dithionite used in order to lower production cost, but such attempt produces other difficulties as well. For example, the oxidation of sodium dithionite can reduce by injecting nitrogen but is very costly. Addition of aldehydes or directly powering with electricity may devloped the reducing power of sodium dithionite, but the wastewater problem remains.

If electrochemical reduction is involved indirectly, sodium dithionite can replace as the reducing agent by the medium. The medium may supply both oxidizing and reducing substances and can regenerate so that both waste and pollution can be removed. Past experiments show that reduction by electrolysis can save about 90 percent of production cost when compared with reduction by sodium dithionite.

For reducing dyes, electrochemical process technology can be utilized in other aspects. Taking bleaching as example, the core principle of electrochemical mercerizing 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 of transportation is greatly reduced, particularly regarding hydrogen peroxide which is explosive.

Another emerging project is the technology of ozone electrolysis. Ozone is strongly oxidizing and can be used in decolorizing and other waterless dye treatments (e.g. ozone jets to prevent wearing out of jeans). As ozone can self decompose, it will not cause pollution problems once carefully treated. New perspectives on industrial upgrade by promoting new technologies which are both energy saving and waste reducing.

In conventional technology of dyeing with water, textiles should involved multiple processes with the help of aid agents, chemical salts, surfactants and reduction clearing agents. In contrast, for the supercritical waterless dyeing technology, only supercritical liquid is needed for dyeing and migration, after which the pressure and temperature can be lowered and the whole process is finished, without producing any wastewater. Also, as carbon dioxide automatically detaches from textiles and remaining dyes, the latter can be reused. More importantly, as operation procedures are reduced, the dyeing cycle is also shortened from several hours to 15 to 60 minutes; energy is also saved due to the lower 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 overall advantage 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 synthetic fibres is high. Overall, the effects of interactions between different textiles with supercritical substances are yet to be fully discovered.

Plasma Treatment Technology in textle dyeing and finishing:

When a substance in its gaseous phase absorbs enough energy, the outermost electrons in the 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. These effects can alter the surface structure of textiles; hence plasma is suitable for surface treatment.

Since only the surface structure of materials is altered by plasma, the substrate characteristics of textiles will not be affected. Also, as small amount of plasma is enough to produce profound effect and one set of equipment can accommodate to different kinds of gaseous chemicals, the equipment is relatively cost effective and user friendly. The kinds of plasma undergoing testing are varied, including silanes (SinH2n+2) (waterproof), freons (increasing surface tension and oil- and dirt-proof effects) and phosphoruscontaining organic monomers (fireproof), etc.

Plasma treatment technology can also improve existing dyeing technology, including the newly developed technology of metallised fabrics. On the other hand, HKPC attempts to integrate plasma treatment technology and supercritical fluid dyeing technology, and replace supercritical fluid with plasma in the dyeing process. The lowpressure 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 to overcome technical difficulties and revive after the financial crisis.

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DYEING WITH DYES

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DYEING

The color of a textile product may be its most important marketing attribute. It is the color of the dyed (or printed) fabric that first attracts and then draws consumers to particular items for sale. It is often the color of a product that sells the product. Dyeing is impregnating fiber, yarn, fabric or garment with a dyestuff. Dyes are colorants that are applied to, or formed in, a textile substrate in a molecularly dispersed form. they derive their color from the conjugated double bonds in their molecular structure.

The definition stresses that for use in textiles, dyes must be soluble or capable of being made soluble in the medium in which they are applied, or they must themselves be molecularly dispersible into the fibers. It is this property that distinguishes them from pigments. When a dye colors a fabric directly, without the aid of a fixing agent its called Direct Dye. In some other dyeing processes, a fixing agent is required to fix dye to fiber; the fixer is called mordent

Fastness of color is its ability to remain unchanged. Color fastness may be affected by such factors such as perspiration, dry cleaning, sunlight, salt water etc. Adding color to textiles, thus making fabrics marketable as fashion component, is a sophisticated and complex area where art and creativity meet with science and technology. Although the chemistry of dyes and dyeing are extremely complex, the development of electronic and computer science applied to the dyeing process has opened the world to rapid global trading and quick response systems. Matching shades of dyeing and the approval of colors may now be executed by phone and fax (without the necessity of seeing visual samples that must be sent by mail or courier) thus saving many weeks in international and domestic trade cycling. Details covering electronic and computer usage are included in this chapter as are explanations of the traditional processes of textile dyeing.

Although color is recognized as the most important element in textile sales and merchandising, it is also the source of most problems that consumers and the textile industry encounter in the production and use of fabrics. Fading, bleeding, color staining and color streaking are typical examples. Understanding the dyes and dyeing processes discussed in this chapter can aid in reducing or eliminating many of these problems.

Colorfastness of dyeing:

A fabric that retains its color during care and use is said to be dyeing colorfast. Fastness that is affected by the factors discussed above is an important concern of consumers. Small aggregates of dye molecules distributed evenly throughout the fiber make for a more satisfactory result than do surface applications of dyes. Fabrics may be more or less colorfast to a variety of different substances or conditions.

The importance of colorfastness of dyeing depends on the use of the fabric. Colorfastness to laundering is, of course, important in those garments and household textiles that must undergo frequent laundering. Some dyes are not fast to laundering but are fast to dry cleaning, or vice versa. Perspiration may cause some color change and/or color transfer, and some colors are may be lost or diminished by hear. Dyeinf colorfastness to sunlight may be important in evaluating the - usefulness of fabrics for curtains, draperies, carpets, and outdoor clothing in case of dyeing.

Light fastness of dyeing is usually a function of the dye structure rather than its retention within the fibers. The molecular structure that provides the color can be interrupted by light, particularly ultraviolet light. Additives or finishes are available to stabilize dyes from this type of action in case of dyeing.

Some dyes tend to crock, or rub off on fabrics or other materials with which they come in contact. Others will bleed into water during laundering and may be picked up by lighter-colored fabrics. Chlorine bleaches will remove color from most dyed fabrics, but some dyes are more sensitive than others to the action of chlorine bleaches of dyeing.

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