Indian Journal of Fibre & Textile Research Vol. 21, March 1996, pp. 41-49. .

New developments in dyeing process control

M DTeli

Department of C hemical Techn9logy, University of Bombay, Matunga . Bombay 400 019, India

Present textile market is buyers' market. The consumer dictates the term as far as the quality, quantity and delivery schedule are concerned. To withstand such competition . the manufacturers/processors have to adopt total quality management approach so that not only they ge t the ~ond~ processed right at the first time, but quality is ensured. I ncreasing environmental polluti on a\l4areness has thrown new challenges before the textile technocrats. The present paper reviews the new developments in the field of dyes manufactured, methods of application and machinery used. It also takes into account the impact of advent of micro fibres and specihc developments in processing of terry-towels, garments. etc. The future trend is also predicted based on all the above factors.

Keywords: Dyei ng. Disperse dyes. Reacti ve dyes. Polyester fibre. Cellulosic fibre. Wool fibre , Silk fibre . Dyeing machinery

1 Introduction The globalization has widen the scope of the textile

business with the inevitable entry of international co­mpetition. The challenges and opportunities are incr­easing and the appropriate response has to be evoked. in terms of technological advancement and on-time delivery . Such "quality and quantity" supply has to be done on the basis of right-first-time, right-every­time, quick response approach. While this is being attempted, it is also incumbent to consider reduction in energy and production cost. The challenges posed by this decade revolve around the conservation of environment, an issue which has aroused the attent­ion at global level and will direct th!! future chemical technological adva ncements predominantly.

The emergence of new materials such as polyester microfibres, increasing applications of computer tec­hnology for recipe prediction as well as for expert systems of dyeing and fini shing, and the increasing use of microprocessor-based process monitoring and control equipment will stimulate the fine tuning of our production efficiency and effectiveness as well ,\seconomy. The opportunities placed before us, of course with a package of challenges, in post-GATT period need to be exploited and it is the technological advances which will be playing major role in shaping the destiny of thi s industry.

When one talks about technological advance­ments in dyeing process control it is natural tha t the majority of these advances are concentrated on dye­ing of woven fabric . However, d yeing of garment, terry-towel, microfibre and hosiery or knit goods

also account for a sizable production and some deve~ lopments in this respect need to be also summarized. The advancements in dyeing processes have bea ring on the developments In dyes and newer applications.

The colouration and finishing are the key stages in imparting fa shion appeal to the textile apparels. The timing and positioning of colouration in the handling of textile are essential factors in ensuring that a chosen ma rket is supplied with the right product at the right time and in right quantities as well as at the right pnce.

The first aspect of Quick Response (QR) to consi­der is the shortening of the traditional pipeline l by eliminating unnecessary inventories and optimiza­tion of process times. Learning to operate with signifi­cantly lower stocks of raw materia ls and finished go­ods is no simple matter because so many new discipli­nes must permeate the organization. One such new discipline is the Right-First-Time concept in wet processing. The objective is to avoid unnecessary act­ivity and complexity- wasted time, wasted energy, wasted material and correction of errors. This in posi­tive terms means quality managementz (QM).

As far as the recent trends in dyeing process contr­ols a re concerned, they are the result of developments in d yes, fibres, application conditions and machine­ry3.

2 Developments in Dyes The technological developments in the dyestuff in­

dust ry are taking place from the point of view of:

42 INDIAN 1. FIBRE TEXT. RES., MARCH 1996

(i) High-performance products Concentrated brands: To minimize neutral cutt· ing agents and to incorporate useful additives. Non-dusting brands: Granulated and liquid brands.

The use of non-dusting grains or powders and liq­uid brands of dyes help in minimizing air/water pollu­tion and in application of automated dye dispensing systems.

(ii) Cost effectiveness Increased automation, advanced instrumentation

and controls, and computerization for the manufact­uring operations with the objectives of high quality standards help in cost reduction and increasing profi­tability with Quality production.

(iii) Health , safety and ecology

It has good fastness to heat-setting/wet treatments too.

Navy Blue··CF: It is an extension to the Palanil CF (Controlled Fastness) series and is suitable for all dye­ing processes.

Brilliant Blue BGM-CF: It can be readily thermo­fixed and has good light fastness even at hi~hte'mpera­ture. It is particularly suitable for automotive textiles.

• Dispersol SF (Super Fast), a new range from ICI which inCludes black, navy, rubine, yellow, blue and brown dyes specifically targeted for dull tern­ary shades, a very difficult area fo r most dyers . They are alkali clearable dyes and exhibit extre­mely low cross-staining.

• The Dispersol XF range complements the ICI Di­spersol SF (Super Fast) dyes which are benzodifu­ranone ba6ed4 .

Selection of dyes is necessary from the points of view of: • A fascinating area of disperse dye chemistry has

been the development of high-value speciality co­lours giving brilliant fluorescent dyes. These are complicated, expensive structures involving sop­histicatd organic chemistry yielding brilliant sha­des with acceptable fastness properties s.

Avoiding the use of dyes which can fonn on reduc­tion carcinogenic amines, and Selection of high exhaustion/fixation dyes, parti­cularly among reactives.

Most of the product development has been concen­trated in the field of disperse and reactive dyes. Some of the new dyes of these classes introduced in the mar­ket in recent years are briefly reviewed here .

2.1 Disperse Dyes

Disperse dyes are used to colour polyester. Anthra­quinone dyes have much lower colour values and are more expensive although they give valuable bright red, blue and turquoise shades whereas azo dyes offer a full shade range and high colour values. With the advent of jet dyeing and tHermosol application , the search for new chromophores has led to the develop­ment ofbenzodifuranone dyes of high colour value, brightness of shade, good substantivity and low ther­momigration. Some of the new disperse dyes availa­ble are: • Resoline Brilliant Red F 3GS (Bayer) has good

fastness to dry heat fixation and high brilliance which is required for fashion polyester and poly.e-ster/cotton sports and leisure wear. '

• BASF has introduced a new series of Pal ani! dyes in its Palanil disperse range- Palanil Black G, Red FFB, Navy Blue GN, Navy Blue-CF, Brilli­ant Blue BGFN and Brilliant Blue BGM-CF. These dyes are available as low-dusting powders/ liquids .

Palanil Bue BGFN: Suitable for polyester blend dyeing, it is insensitive to dyebath pH and reduct ion.

• Zeneca (previously ICI) has introduced diester, thiophene and benzod'ifuranone based high fast­ness Dispersol dyes. They obviate the need for reduction clearing and are used in both exhaust and continuous dyeing.

• Marlow-Van Loan Corp. offers a new disperse

• bfack which claims excellent colour yield and ex­haustion properties in dyeing of polyester6 .

Black ASB Extra exhibits superior sublimation, light, wash and perspiration fastness properties and high alkaline stability, thus enabling to acco­mplish single bath dyeing of polyester/cellulosics with minimum staining.

2.2 Reactive O V,,'S

Of all the classes of dyes used for cellulose, reactive dyes account for 21 % of total consumption of dyes in the world and show upward trend in their application for obvious reasons.

Recently, a new generation of bi- and poly­functional dyes have been introduced which consist of homo or hetero reactive systems. As a result, exha­ustion, fixa tion and migration can be controlled, lea­ding to much improved dye fixation and better repro­ducibility. Some of the recent additions are reviewed here . • Sumifix Supra range (bifunctional) has a high de­

gree of exhaustion and fixation, resulting in less wl:lshi ng-off (hence less dye in the effluent), high

TELl: DEVELOPMENTS IN DYEING PROCESS CONTROL 43

fa stness to light , perspiration , peroxide washing and chlorinated water, and high stability to acid hydrolysis?

• Kayacelon reactive dyes are high-temperature neutral -dyeing dyes which a re especially suitable for fast processing o f cotton and o ne-bath dyeing of cotton/nylon , cotton/~crylic and cotton/poly­ester blends.

• Cibacron C rangedyes (Ciba-Geigy) are bifunct­ional reactive dyes which have an aliphatic vinyl sulphone gro up combined with eithe r a monoflu­orotriazine group o r an aromatic vinylsulpho ne group with following features: Suitable for pad application and exhaust d ye ing at low liquor ra tios, Very good solubility a nd high to ve ry high degrees of fixation ,

- Excellent wash-off, and High fastness leve ls8

The environmental benefits include : No urea needed fo r di ssolvin g the d ye,

- Less d ye in effluent due to hi gh fixati o n, a nd - Wate r and energy savings due to easy \vash-ofT.

• Zencca's Procion H-EXLexha ust dyes (hifuncti­o nal) ha ve been engineered to produce hi gh per­formance level d ye ing, mi gration , diffusio n a'nd exceptional build-up pro perties.

Of the reactive d yes available today, new Remazol EF d yes significantl y reduce waste in efflu ent , cla im s Hoechst. These dyes

Require 33-70% less salt than conventional reac­tive d yes, Provide higher fixa tion under low salt conditions as compared to conventional reac ti ve dyes, and Arc available in environrnental safe packaging (low-dust powders or liquid s).

• Rite Industries Inc. has launched the Rite Reac­tive B se ries of d yes which a llow to reduee the amount of salt to be used in the d yeing with similar advantages .

• BASF hil S introduced five new reac tive d yes into the . Basilen range. These are Basilen Blue E-FRN . Blue E- BGF,.Red F-RM, Red F-3BM and Yell ow f~3RM . Basilen Blue E-FRN is a double anchor type mo nQchl o ro tri zine dye free of heavy metal which produces a unique royal blue shade and ca n be used o n polyester/cellulosic blends by the BASF recommended process. Basi­len Blue E- BGF is a brilliant blue d ye free of he­avy meta ls and stable under hi gh-temperature dyeing conditions (e.g. pol yes ter/cellulose blend d ye ing) . The o ther three Hasi len FM dyes have high exhaustion and easy wash o ut propertics to decrea se water usag.e and wate r contamination.

In Indian market too, a number of companies have come out with high-exhaustion a nd medium-exhau­stion bireact ive d yes which a re finding increasi ng ap­plica tions.

3 Developments in Application Methods

3.1 Pol)"ester Fibres

During the dyeing of po lyester fibre. the conventio­nal dyeing liquor contains la rge a1Tlounts of di spers­ing agents a nd surfactants which are used to ge t rea ­sona ble ra tes of dye ing, ensuri ng levelness. Thi s leads to an effluent loaded with dye and dispersing agents which is to be treated with an ever increasing cost. One radically new approach is to use supercritical CO2 med ium for dyeing PET with disperse d yes9 . Afte r the dyeing o peratio n is conwlete, dry dye powder, which has not been used up. deposits at the bottom . There is no necessit y of dispersing agent or carrier. However. thi s dyeing requires di sperse dyes of special structural features. free from dispersing agents. Also. the cost of eq uipmcnt an d pipings is hi gh. Bl~t the operational costs are very low as compared to that in conventional dyeing method (requires only 1/ 5th of the energy) . There is also no requirement fo r red uction clearing trea tments, washings, dryings, etc., thereby saving time and energy and providing protectio n to environ­ment. Thi s method looks to se t further the cause of QR and hence QM .

Rece ntl y, Hoechst Mit subi shi Kase i (HMK) ha s made poss ible the d ye ing of pol yes ter in an alkalinc medium . thereby making the entire wet processi ng simple and reproducible. The quality of dyed materi­als is also improved. In addition , the problems due to the presence of polyes ter oligomers in acidic d yei ng conditions are also minimized 10.

The HMK system involves both alkali-resistant dyes and a newly developed stabilizer JPH95 for dyes fo r alkaline conditions. The stabilizer has many func­ti o ns such as buffering the dyebath pH to around 9.0-9.5 during dyeing, stabilizing dyes. chelating me­tal io ns and dissolving oligomers. The small variation in pH during the dyeing cycle promotes excellent re­producibility.

3.2 Cellulosic· Fibres

Many ideas surround the concept of blind colorati­o n, ranging from a simple no addition dyeing to the management of all those factors that affect the colora­tion o fa textile, resulting in a right-firs t-time dyeing. The latter definiti o n implies the total control of all factors (including the human ones) that can influence coloration and ca use a shade to deviate fro III the stan­da rd .

44 INDIAN J. FIBRE TEXT. RES .. MARCH 1996

The beneflts of blind dyeing can be summarized as follows: cost saving, improved planning, better qua li ­ty. increased productivity, customer sa tisfaction . etc . I I.

Dyeing with monofunctional reactive dyes is known to be rather sensitive to changes in dyei ng con­ditions such as temperature, liquor ratio, addition of salt and a lkal i. A low sensitivity to dyeing parameters using dyes with two different reactive groups is parti­cu larly advantageous in continuous dyeing operati­ons as it helps to achieve reproducibility of shades, i.e . right-first-time and every time.

The key factors influencing the rates of right-first­time production are dye selection, control of raw mat­eria ls and on-line process control. In itself, improv­ing the rate of right-first-time production can impr­ove productivity by 20% and the ret urn on sa les to a round 13 % (ref. 12).

Right-first-time production will be maximized if these fundamental measures of performance within the reactive dye compatibi lity matrix RCM are set a t:

Substantivity Migration Index LDF Tso, a minimum of 10 min

70-80% > 90% > 70%

A comparison ofRCM of individual dyes will pro­vide a measure of the compatibility of dyes when they are used in combination. Dyes with simi lar performa­nce and with substantivity, migration index, LDF, and Tso values within the target specification will offer robustness to small variations in processing condi­tions, shade reproducibility and level dyeing perfor­mance l2 .

Newer developments controlling add itiona l para­meters relati ng to the physical characteristics of the substrate take into account the influence of me chan i­cal and hydraulic forces exercised by the machine du­ring the process l3

. The control eq uipment a ll ow the hydraulic and mechanical action of the dyeing mach­ine to be adjusted, not only for the entire process but also for each period in the dyeing pcocess. This princi­ple is utilized in the synchron dyeing control system, whic h synchronizes liquor circulation, fabric speed and process cycle l 4

.

The alka li dosing process is becoming popular as a rationalized dyeing method for reacti ve dyes6 - s. Many types of reactive dye can be used , but the opti­mum pH range will depend on the nature of the dye employed . To control the rate of exhaustion, it is desi­rable to use a dye that reacts over a wide pH range so that the conditions can easily be controlled during dyeing. The Sumifix Supra range of dyes contain both

a vinyl sulphone group a nd a monochlo rotriazi ne rea­ct ive group. They react with cellulose ove r a wide pH range and are, therefore , suitable for use in such opti ­mized dyeing processes I 5 .

Thesc d yes have bee n adap ted both fo r exhaust dyeing and semi -continuous operations. Hi gh fixati­on, low substanti vity and good diffu sio n, effect ing rapid and efTicient rcmoval of small amounts of unfix­cd dye, are the sa lient fea tures o f these dyes. To i ncre­ase the fixation of d ye, the one of the most popular alternat ive energy sources which could he used is rad­io-frequency (RF) energy.

Heating of the batch with radio-frequency energy during the batching stage grea!ly acce lerated the rate of fixa ti on. Fixa tion levels achieved usi ng opti mi zed R F-assisted methods were approximately equival ­ent to those achieved in co nventional pad-batch dye­ing l6

.

Dawson International , in 1978, patented a process and the equipmen t design used for fixing dyes or che­micals with RF energyl6. The process involved conti­nuous conveying of treated fibres through closely co­nfined tube located between flat parallel e lectrodes. By generating RF energy field within the tube, a se lf-­seali ng pressure chamber was created by the format­ion of steam from the wetted fibres. The advantage claimed was an increase in the reaction rate ofthe dye on the fi bre.

The semi-continuo us pad-batch process using RF energy has also been recognized as one o f the most efficient methods ofapplying reactive dyes to cellulo­sics with exce llent reproducibility , leading to right­first-time every time concept.

Laser fixation is a feasible low-heat trea tment met­hod for fine-line dye fixation . A comparison of wash fastness of ha nd-ironed and laser-treated specimens showed that the specimens treated with a n argon-i on laser at 129°C and 23YC maintained color significan­tly bcttcr than thc specimens trcatcd with a lowcr lascr heat (94°C) and hand iron ( 190°C). This was true for 10, 20 and 30 washes. A laser tempera! ure of 129°C was as effective in setting fibre reactive d ye in cotton as was a laser temperature of 23°C, indica ting that a lower energy laser would be as useful in setting dye as higher power models 17 .

Electromagnetic hea ti ng produces d yei ngs of si mi­lar shades to those obtained by conventional heating when pressure and temperature conditions are kept the same for both treatments. However, e lectromag­netic heating allows smoother and more homogene­ous boiling tha n docs conventional heating and can be useful industria ll y 18 .

Lewis and eoworkers l 9 ca rried out fibre pretreat­ments a imed a t dyeing with reactive dyes under neut-

TEll: DEVELOPM ENTS IN DYEING PROCESS CONTROL 45

ral to sli ght Iy acid ic conditi ons in the absence of elec t­rolytes. All of these' treatment s introduce cationic (basic) resid ues in the form of quarternary, terti ary and secondary am ines.

One of the most conveni ent pretreatments was to apply polyamide epichlo rohyd rin res in to the cellulo­sic fab ric usin g pad-dry system. Selection of highl y reac tive dyes gave good co lour yield on thi s type or fabr ic and the fixation of dyes proceeded si mpl y by appl ying them from a dyebath se t a t pH 5 and raising the temperature to boil. Unfortunately. rin g dye ing of tbe fibre and ya rn s was clearly evident and as a result , light fas tness red uced by 1-2 points. These dra­wbacks, however, have been overcome by th e use or low molecular weight species and pretrea tin g the co t­ton fabric using a pad-thermollxa ti on technique . Anot her adva nt age is th e need fo r shorter was hin g­olfp rocess to remove the unfixed dye because of hi gh level or dye-libre fixation achi eved.

In eo loura ti on of cellulosic fibres. vat dyes (includ­ing Indigo) and sulphur dyes ho ld a la rge part of the dyestuO' mark et. Vat dyes give exce ll ent fa stness, whereas sulphur dyes a rc economical with good deg­ree of fastness properties. The red uction process emp­loyed here req uires reducing age i1t s and it has ye t not been possi ble to regenera te these co nven ti ona I ag­ents, except in the standing dyeba th procedure .

In recent inves tigations, an electro lytic process was used to <lchieve dye reel uction ~O. I n eli rect electrolysis, the dye itself has to be rcduced at the surface of the ca th ode, while in indirect elec tro lys is the reducing power of the cathode is transferred to the solution by a soluble redox system (mediator) . This reversible re­dox system is continuously regenera ted at the cath­ode so tha t a renewal of the reducing agent is achiev­ed. Thi s technique offers th e possibility of full dye­bath recyeling, including the reuse of reducing ag­ents.

Cotton may readil y be modified by pret reatment with N-methylolacrylamide to introd uce pendant­activa ted double bonds. These have been ex ploited in two ways to improve the reacti ve dyeing of cotton. Firstly, by introducing amin o residues at these new sites, exce llent dyeings with reactive dyes can be achi­eved at pH 5-7 in absence of an electro lyte, coupled with a very high degree of fixation. Secondl y, it is possi ble to dye the cellulose, modified onl y with N-meth ylolac rylamide, with dyes co ntaining pend­ant aliphatic amino residues. In thi s case, it is necess­ary to dye in the presence oran elec trolyte under alkal­ine condi ti ons but ve ry high colour yields are obtain­ed, thanks to th e elimina tion of the hyd rolytic side reactions nOrilla ll y associated wit h n:acti vc dyeing.

Such amin o-containing dyes can be readily prepared rrom all eOlllme r~ ial ranges of reactive dyes 2 1

.

Prdrea tmen t or co tton wi th pol yepichlorohydrin dilllethylamine prod uces a modi lied cotton that can he dyed un der neutral conditi ons with selected low­reactivit y dyes wi th ou t sa lt. The dyei ngs of trea ted colton ex hibit improved colour yield and hi gh was h r;' lstness .. Bot h ex haust dye ing and continuous dyeing l)f trea ted cotton give hi gh co lou r yield 2 2 .

Pretreatment of co tton with hi gher molecu lar mass pol yepichlo roh ydrin dimeth ylamine co uld intro­duce more cationic sites onto the modified cotton . This would increase the extent of complexing with the dye and thus improve the dyeability with direct a nd reactive dyes~3.

Eth ylened iami ne tetra(methylene phosphonic) acid when adsorbed by cotton fibres prior to the appli­ca tion of the direc t dye Solamine I- ast Red 4BL causes a marked increase in rate of dyeing. It also improves the wet fastne ss properties of the dyed eotton 24

.

3.3 Wool Fibres

. Due to the increasing enviro nmenta l problems of dyeing with heavy metal containing dyes as well as wi th reac ti ve dyes, the dyeing behaviour of plasma­treated wool towa rds these dyes was recently investi­ga ted by Hocker and eoworkers2 s. Plasma trea tment on wool have shown th at it leads to, apa rt from an improvement in the mechanical properties of the fibre , improved dyeability with acid dyes, reactive dyes and metal-containing dyes, and reduced felting tendency. Low-tel)1perature glow discharge trea tm­ent of wool opens up the following possibilities to the wool dyer:

IInproved utili za tion of the existing plant due to reduced dyeing times, Savings in cost and maintenance of intensive wa ­ter purification plants as well as possibilities for recycling the dye effl uents due to reduced di scha­rge of toxic components, Cost savings by reduction in the amounts of dyes and a uxilia ries, and I mproveel market prospects due to environment­friendly process management.

It has also been obse rved by Fibre Resea rch Insti­tute, Israel, th~t wool on treatment with enzymes loo­ses its felting property. At the same time, libre seems to acquire much enhanced acid dyea bility.

3.4 Silk Fibres

As fa r as the dyeing of silk fibre is concerned, the low-temperature dyeing has been attempted using va ri ous redox systems. These systems ca use an incre­ase in the number of dyeing si tes on the silk, through

46 INDIAN J. FIBRE TEXT. RES. , M ARCH 19<.)6

the ac ti on offree radicals which might be assisting in covalent fi xa ti on of the dye in additi on to usua l ion ic links, hydrogen bonds a nd va n der Waals' fo rces. The use of bufTers like sod ium ace tate-acet ic acid has a lso been found to be quite promisin g as far as the fi xati o n and the fastness properties of th e acid dye on silk a re concerned 26.27 .

4 Developments in Machinery As the tex tile wet processors rea li ze that they must

adapt to the changing needs of the ma rket , the machi­nery manufacturers mu st tak e ap pro priate actions through extensive research and development to deve­lop a new genera tion of dyein ,!! machineries that are able to meet the changing market co nditio ns.

The machine ry deve lo pment s are towards achi ev-ing o ne o r more of the following o bjec tives :

Reducing the labo ur complement, Increas ing the prod uctivit y, Minimizing the req uirements of Iltilities, Simplifying the machine operations, Savin g in machine space, Reducin g the effluen ts, thus minimizing possibl e po llutio n, Recovery of heat energy a nd water, and Economizing the cost s tructure.

In batch piece a nd ya rn d yeing, it is essential that the fi ni sher has machines at hi s di sposa l that operate cos t efTectively. At the same time, the finished prod uct must meet the quality demands of the consumer. The introducti o n of new technology and the modifi ca tion of design that entails are the moves in the right directi­on . M o re economica l d yeing processes can be achie­ved in the piece and yarn dye ho use applying the short and ultni-short liquo r techn o logy, liquor fl ow cont­rol and finishing by the Synchron D yeing Sys tem.

Of co urse, an increase in productivity a nd qua lity is today inco nceiva ble without a n elec tronica ll y-cont ­rol led process seq uence.

It is, the refo re, important for the fini sher when inv­es tin g in new equipment or improving old machine plant to ge t familiar with the la test advancement in dyeing machi ne ma l1Lifacture a nd the lates t process tech no logy 28 .

T he advent of metering techno logy has made the short liquo r dyeing a re liable process. The modern meteri ng sys tem- M ulti-prod uct II1J ection (M PI)- permits contro lled di spensing of solids o r li­quids such as dyes, sal ts, alkalies, acids o r o ther tex tile a uxili a ri es.

M lIlti-product injection (MPI) meterin g sys tem in conjunction with the application of sho rt and ultra­sho rt liquo r technology has the foll owing ad va ntag­es:

A ut o ma tion or the d yei ng process avoiding in­co rrect a ddi ti o ns of dye so luti o n, auxiliaries a nd chemical s: mo reove r, the re are no idl e times. Reliabilit y of the process, because the re is a lways an o ptimum reac ti o n medium a vailable for th e d ye in g process.

- Increased reproducibility by ex act rcpetiti on of the d ye ing process. Optimi za tion of the leve lness . Cost sa vings by using caustic as fixation alkali in s tead of soda (e.g. in reac ti ve dyeing). Cost savings by using acid (acetic o r fo rmic ) in wool/po lya mide d ye ing.

- Simplil ica tion of the rinsing a nd washing proces­ses in reac ti ve d ye ing, owing to less hydrolyzate fo rmati o n during the d yei.ng process. Orten it is possible to reduce the d ye ing time. Mo re re li able sho rt liquor dyei ng, especially wit h dark colours. By meterin g-in the d ye, the conce n­tra ti o n of d ye in the dyebath is reduced so tha t no agglo meration can occur.

Increasing automation in application o f dyes to the fibre substrate has been o ne mo re guid ing principle in the d eve lo pment of the machine ry as described aho­ve. One of th e typica l examples in exhaust d yeing is ADC-200, a me tering device de ve lo ped by Hoec hst a nd ADCON. The process, mo re known as Ramaza l-automat, me ters the supply o f a lkal i for the li xat io n

o f the reac tive d yes a ut o ma ticall y and progressively in order to o btain the hi ghest leve lness in sho rtes t dye ing time.

The emergence or short liq uo r je ts fo r d ye ing cot­to n and it s blends has been marked over recent yea rs. Not only d ocs thi s process offe r the poss ibilit y of more eco no lllical d ye ing but it also reduces the load inflected o n the environment to the tune or 30% (rd. 29).

On-line con trol s in continuous dye ing are ro und to be elTective in improving the cost elliciency and rep ro­ducibi!ity. Forexample, on- line liqu o r pi ck-up mea­surement wit h level co rrecti o n and co ntro l en:lblcs precise calculation of the vo lume o f li quo r Ileelled and conseq ucntl y savings in the cost of dyes a nd auxi l­iaries. On-line colourimctry orthe p;ldJed rab ri c im­proves the contro l over the prod uc tio n process wit h reliability and imp ro ved quality·'o .

4, I J igg('r Dyeing

Jigge r is the mos t stable mac hine in dyeho use and future d yeingjiggers will have a very modern look and will be charac teri zed with use ful aspec ts vvhich a re sUlllmari zed und er " Future Trend<' in this JXlpc r.

4.2 J(" Dyeing

The utilit y, specific applicati o n and press ure con-

TEll: DEVELOPMENTS IN DYEING PROCESS.CONTROL 47

dition inside the vessel as well as the shape of the vessel have been , for quite some time, under constant exami­nation . Some manufacturers have finalized designs for a ll types of ma terials and dyes, which claim all the important features such as:

Dyeing.temperatures up to 140°C when desired, Soft flow or overflow, Low liquor ra tio up to 1:5, Automatic controls by microprocessors , Suitability for light weight as well as heavy fab­rics, say 50-500 g/m 2

,

High fabric speeds at the lowest possible pressure, Tangle-free operation, High quality dyeing results with reduced tension and surface abrasion, Minimum operator involvement, and Economy of energy, water and chemical auxiliary ingredients3 1

.

In other words, more air, less liquor and gentle han­dling have been the key factors behind the develop­ment of jet dyeing machines.

Then's Air-Flow ASF machine uses the liquor ra­tio about 1:2 and it is found to give high productivity and low consumption of water and chemica1. The dye solution is automatized with the air stream which is pro­duced by the blower for application on the fabric. The Italian machinery maker 'Laip' has brought out air­water-flow Mod 800, a latest rope dyeing machine, for both knitted and woven fabrics . The air-injection is not simply used to accelerate the fabric to high speed but in some cases to open the fabric. " Apollon Twin Soft Flow" machine manufactured by a Greek mach­inery manufacturer makes use of micro wash system, using clean hot water which is introduced through the double-overflow of the machine to achieve very high cleani ng efficiency32.

4.3 Continuous Dyeing

The component units of the contin.uous dyeing ran­ges are being modified from time to time to minimize the possible imperfections in the final dyed product. Development work regarding these component units is still .in progress. The points which have been a nd which will still be considered during such work inclu­de:

Composition and formation of pad rollers, Pre-drying by infra -red radiation to avoid possi­ble migration , with vertical fa bric passage, and Well-designed and evenly impinging hot flue or high-temperature dryers for thermosol dyeing.

With the advent of synthetics and blends, the ther­mosol system is bound to achieve a very promising position .

The FD 45 pad-stream machine from Ramisch Kleinewefers for the continuous dyeing of cotton and its blends with viscose rayon or polyester has been reported. In addition to a Bicoflex padding arrange­ment, the machine has system for feeding in single doses of chemicals and storage programmable contr­ol. This control system has a modem whidi permits continuous dialogue between the supplier and the cu­stomer. It could be preferably used for vat and sulp­hur dyes33 .

5 Garment Dyeing . Whil st garment dyeing has an in-built lead time

advantage for producing comparatively sm'all tots, the com bined effect of customers wanting larger lo t sizes quicker and more reprod~cibility and the adva­nces in response time made by the colouration routes has driven development in gannent dyeing technolo­gy.

5.1 Dyes

Proper selection of dyes and finishing chemicals is quite important for dyeing of gaments made of cott­on, wool, nylon, polyester, acrylic and .their blends. The range of articles include socks, sportswear, shirts, T-shirts, etc. Of the dyes available in the mark· et, reactive dyes are the most popular dyes used in garment dyeing. Newly introduced bireactive dyes are finding increasing application and are preferred to direct dyes3,,". Subsequent trea tment with dye fixing agent is also recommended to improve fastness prop­erties. Other dyes used for garment include acid and reactive dyes for woollen gannents, cationic dyes for acrylic, and sulphur and vat dyes for cellulosic gar­men ts. The chlorine fastness of sulphur and reactive dyes is poor whereas vat dyes have the best perfonna­nce properties although they a re quite expensive.

5.2 Dyeing Methods

Exhaust dyeing is carried out using special dyeing 111achines for garments.

5.3 Machinery

The most recent development from Gilwood (Fab­rica tors) Co. Ltd, a British machine maker, is an oval, double side-paddle machine for dyeing readymade knitwear. Machine capacity ranges frum 150 to 6800 litres. Positive dye penetra tion is achieved by speci a­lly designed paddles fitted with synchronised dri ve for enhanced liquor circulation . An innovati ve recir­culation system delivers heated liquor simultane~u­sly to the two-paddle a rea, significantly improving the overall temperature controp5.

48 INDIAN 1. FIBRE TEXT RES .. MARCf-1 1996

The recent innova ti on in convcntional drum dye­ing machine is the one with liquor circu lation through the drum centre. Thi s helps impart ing an extremel y gentle treatment to the goods even at low liquor rati­os. i.e. I :6. Thi s kind of machinery has the advantage of dyeing all kinds of hosiery. The goods dyed can also be cent,:ifuged in the same machi ne which means in ve­stment costs ["or centrifugers a re a voided. The techni­ca l advantages include:

Level dyei ng due to rapid liquor interchange. Reduced water eilluent and less steam and chemi­cal consumption because of operations at low liq­lIor rati os. and

- In-built hydroex traction ·". The newer machinery for garment dyeing are based

on the foll owin g features: (i) Low liq uor rati o, (ii) Microprocessors- for improved lot-t o-I ot reprod­ucibilit y, (iii) Hea t exchangers - for rapid heating uJ1dcooling. (iv) !....int filte rs) (v) Centrifugal hydroex­trac tion , (vi) Easy sa mplin g with out droping dye liq uor. (vii) Tilting- for ease of loading/unl oading, (vi ii ) Cushioned suspension, (ix) Variable drum speed, and (x) Automatic balancing drum .

What is important in garment dyeing is to achieve level dyeing, with excellen t penetration in short dye­ing cycle. The developments arc thus directed tow­ards these aspects .

6 M icrofibre Dyeing The microfibres are the lates t entrant in textile mar­

ke t and have inherent adva ntages for which their con­sumption is increasing day by day. However, because the microlibre has increased su rface area , the rate of dyeing is tremendously enhanced and it becomes ess­en ti al to control the dyeing of microfibres to avoid uneven dyeing. The dyeings obtained are also appare­ntly lighter as compared to the ones with the normal denier polyester yarn and it is difficult to ge t the deep shades in microdenier varieties. The fastness to wash­ing. rubbing and light is inferior to th at of normal denier polyes ter. This necessita tes the selection of proper range of disperse dyes specially suited for mic­rofi bres.

The Fo ron RD dyes of Sandoz seem to be perform­ing favo urabl y. To improve the ligh t fa stness of the dyed microllbres, use ofFadex RF liquid is recommc­nded since this acts as a kind of UV -absorber. A num­ber of dyes manufacturers are making the selection of dyes from the pont of view of their appl ica tion on micro Ii bre fa bric .

7 D)leing of Hosiery/Knits In dyeing of kn ilted fabrics. co ld bran d as well as

hot brand reacti ve dyes and also the direc t dyes are

used I'or 100% cellulosic fabrics. The dyeing is carried out in tensionless form in winch and since the wi nch dyeing machine doesn' t allow uniform temperature Illaintelwnce. the reactive dyes of HE and ME class arc increasingly being preferred.

8 D)'eing of Terry-Towels The dyeing of terry-t owel is more conlined to dye­

ing of ya rn s which a re then subsequen tly used fo r producing jacqua rd designs. The as r ects of dye ing and the se lec tion of dyes rema in same as in the case of hosiery. However, when no tinting of the' gro und is e.\ pec ted , the va t dyed ya rns a re used in manufact ure of terry-towels rather than azo, sulphur, reactive and direct dyes.

A two-step dyeing technique is availa ble by which cross-dyeing of 100% colton te rry cloth can be ach ie­ved with fibre reactive dyes. The fir st step in volves co ntinu ously dyeing of the fa bric, usi ng an auxil iary that promotes colora tion of the tips of the terry cloth . Subsequent dyeing is carried out in a beck with contr­asting colour fibre-reactive dye. An a tt rac ti ve tow­toned appearance result sJ o.

9 Future Trends The ~ye manufacturers wou.ld co nce ntrate on re­

sea rch acti viti es to deve lop new chrolllophores and dyes offering wider shade gamuts, higher colour fa st­ness, improved environme ntal impact and higher ov­erall cost elTic iency in the coloura ti on process.

I n future , the disperse dyes based on benzodif uran­ones wilLencourage m<1re resea rch to dea l with struc­tural and physical fOnll modifications designed to im­prove build -up, and also encourage resea rch for new ehromogens that are particularly suit able for polyes­l er in genera l and micro fibres in part icular.

The future developments in reac ti ve d yes are likel y to be co ncentrated on: • High application flexibilit y: A broad spectrum of

~hades ,h t) uld be dyea ble by widely dilkringappl­ica tion !/.,'l hods with a small number of dyes.

• Gooel reproducibility and good levelling proper­ties.

• Outstanding protecti on to environment. • Maximum productivity.

Apart from these, the dyeing process with dyes con­taining no reactive group (in the usual sense) but cap­able of being crosslinked to cellulose via a suitable reagent will find its pl ace in industrial applicatiun .

For dyei ng cellulosic fibres, high-fastness direct dyes will be selected ( 0 avoid usc of corper or chrom­ium sa lts. In reac ti ve dyeing, use of urea will be mini­mized . For dyeing with sulphur dyes, the hi ghl y poll­utin g sod iulll sulphide will be replaced by o ther ag-

TELl: DEVELOPMENTS IN DYEING PROCESS CONTROL 49

ents such as reducing byproduct from maize sta rch industry or hydroxy acetone37 •38 • For oxidation of vat and sulphur dyes, hydrogen peroxide, sodium pe­rborate and 1, 3-dinitrobenzene sulphonic acid wi ll be used in place of dichromate.

During po lyester dyeing, chlorinated or phenolic carriers will be avoided. Acetic acid used for pH adju­stment having high BOD will be replaced by other agents .

In case of dyeing machineries, the jiggers oftomor­row will be characterized by the following useful as­pects: • Excellent closing and opening. • Heated hook for drip-proof working. • One way flap-opening chimney for releasing un­

wanted hot and wet flue . • Interna l machine lighting to offer a good view of

the fabric under process. • Pump for recirculating the dye liquo r. • Safety devices and emergency switches for avoid­

ing damage owing to sudden brea kdown. • Automatically controlled drain va lve. • Automatic reversal, delay-relay fabric direction

indicator as well as speed indicato rs. • Effective crease removing expanders and motori­

zed selvedge aligning and continuous centering system.

Developments in winch dyeing machinery with better tempera ture. di stribution a nd liquor movem­ent are ex pected , mainly for dyei ng knitted fabrics.

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