New Trichromatic System For Enhanced Dyeing By The Exhaust Process

Introduction‘Right first time’ dyeing with reactive

dyes requires products having very sim-ilar application properties that are littleaffected by small changes in dyeingconditions. The Remazol RR combina-tion is a trichromatic system engineeredto meet all these requirements with highcost effectiveness.

Market situationIn the US cotton represents approxi-

mately one third of the fiber used in tex-tile processing. Reactive dyes are themost important class of dyes for dyeingand printing cotton and other cellulosicblends They represent about 45% (byvalue) of colorants used for coloring cel-lulosic fibers. Reasons for the successof reactive dyes include: flexible appli-cation methods, wide range of shadesavailable (including brilliant shades),their fastness properties, and cost-effi-ciency.

The demands made on reactive dyesare wide-ranging. Requirements dependto a large extent on the specifications‘for particular textile articles and on oper-ating conditions (equipment available,legal constraints, etc.). Economic con-siderations, quality requirements andecological issues are major factors inthe debate concerning the ideal dyeingprocess and optimum dyestuff selec-tion.

Over 60% of reactive dyes areapplied by the exhaust dyeing method.The remainder is applied by padding.The selection of a trichromatic system isof fundamental importance to dyerswhen choosing reactive dyes for theexhaust process. A wide range ofshades can be dyed by combining a sin-gle yellow, red, and blue dye. Suchcombinations can cover over 50% of theshades used for leisure wear.

An important requirement for a

By P. Fowler,DyStar L.P., Charlotte, NC

trichromatic combination is good repro-ducibility. Reliable reproducibility isrequired when recipes are transferredfrom laboratory to plant conditions, aswell as from one batch to another. Thisis the case even if dyeings have to beperformed on different machines anddyeing conditions cannot always beheld constant. Moreover, a trichromaticcombination should be able to satisfy awide range of quality requirements. Thisresults in a minimal number of different

dyes needed in a dyehouse. To sum up, the main demands made on trichromat- ic dye combinations are:

l reliable dyeing resultsl good reproducibility of shade even if

dyeing conditions varyl good levelness*good fastness propertiesl flexible application possibilitiesl high cost effectivenessA reactive dye combination that

meets these demands is DyStar’s new

62 American Dyestuff Reporter q March 1997

Remazol RR trichromatic system. The system is tailor made for exhaust dyeing

and has been used successful lythroughout the world since its introduc-tion late in 1995.

Trichromatic systemState-of-the art production technolo-

gies operating within the framework ofIS0 9001 are used to manufacture thenew dyes. The quality of the intermedi-ates and the production process aremonitored and the finished products arestandardized using standard test meth-ods to ensure constant quality within thespecified tolerances. Factors such asconsistency of dye strength and shadeare important for good reproducibility.

The new trichromatic system com-prises Yellow RR gran, Red RR gran,and Blue RR. gran. These dyes are sup-plied as low dusting granules, which cangreatly improve working conditions inthe dyehouse weighing and mixingarea. The granules are free flowing andare suitable for use with automatic dos-ing units.

These dyes also have very goodbuild-up on cotton and man-made cellu-losic fibers. Only low concentrations ofdye are needed to dye deep shades.

The good build up of these productsmakes them very economical, a proper-ty that is particularly evident when dye-ing deep shades.

These dyes can be used for a widerange of shades. Including a large num-ber of trichromatic fashion shades. Inaddition to being suited for medium anddeep shades, in many instances thenew dyes meet the requirements neces-sary for dyeing pale shades. Thus, wehave come a step closer to achieving

the goal of producing a trichromatic sys-tem for the widest possible shaderange.

Our trichromatic system is the resultof extensive research and development.In this effort a large number of dyeshave been synthesized and tested foruse in combination dyeing and as indi-vidual shades.

The aim was to develop a trichro-matic system for exhaust dyeing at140°F (60°C). It is interesting to note

that research into reactive dyes forexhaust dyeing is increasingly concen-trating on the 120-140°F (50-60°C) tem-perature range because processingcosts and dyeing times are lower thanfor hot dyeing procedures (175”F(8O*C) and above). Almost all fabricqualities can be dyed at 140°F (6O*C),including yarn and fabrics made of mer-cerized cotton and regenerated cellu-losic fibers. However, not all goods canbe dyed at temperatures below 120°F(50°C) because low dyeing tempera-tures can impair the running propertiesof the goods in jets and affect liquor cir-culation in package dyeing units.

To achieve optimum reliability andreproducibility of dyeings, two main cri-teria need to be met:

1. The best possible balance needsto be found between the propertiesof the members of the trichromaticcombination, (i.e. attaining diffu-sion properties, fixation behavior,and washing-off properties).

2. Normal variations in dyeing condi-tions should not affect reproducibil-ity.

All three dyes contain at least onevinyl sulfone group as the reactivegroup. The vinyl sulfone anchor has asignificant effect on the dyeing behaviorof the dyer. In the commercial productthe reactive group IS present in the non-reactive ester form to protect it frombeing destroyed by premature chemicalreactions (Figure 1).

The hydrophilic sulfo groups in themasked react ive group and thehydrophilic groups in the chromophoregive these dyes their excellent solubility.They can therefore also be applied atultra short liquor ratios. The dyes areproduced without additives of solubilityenhancing organic finishing compoundswhich would increase the COD load ofwaste water.

Under standard dyeing conditions ina neutral dye liquor (2% dye, 50 g/l salt,liquor ratio 1O:l. 140*F (60 “C), non-mercerized cotton). these dyes havemedium affinity. They have very gooddiffusion properties on the fiber. Thisensures good penetration of the goodsand thus level dyeings. Neutral exhaus-t ion and di f fusion equi l ibr ia areachieved rapidly so the neutral dyeingphase can be relatively short.

In contrast, “hot-dyeing” reactivedyes have high affinity in neutral dye

liquors. These dyes have to be appliedat elevated temperatures and require

longer dyeing times to ensure adequate

6 4

Figure 4: Automet dyeing process (60°C)

Figure 5: Reproducibility of a dyeing with Remazol RR dyesin fluctuating dyeing conditions (without recipe correction)Figure 6-Washing-off profile of a trichromatic dyeing withRemazol RR dyes.

leveling and diffusion into the fiber. Thisincreases process costs (time) and low-ers productivity.

The new dyes in the free vinyl sul-fone form have higher affinity than in theester form and exhaust more complete-ly. The free vinyl sulfone reactive form isgenerated by addition of alkali. Sincethese dyes are only moderately reac-tive, once the dyes have exhaustedonto the fibers there is sufficient time fordiffusion into the fiber, even under alka-line conditions, before fixation occurs.Progressive dosing of alkali can beused to control the secondary exhaus-tion phase and fixation rate (Figure 2).

Fixation yield of reactive dyesdepends on the dyeing conditions. Thefollowing rules of thumb apply:

l higher yields are obtained at lowerdye concentrations

l higher electrolyte concentrationsresult in higher yields

l shorter liquor ratios result in higheryields

l higher affinity between dyes andgoods results in higher yields.

High fixation yields are obtained withour trichromatic system even in subopti-mal conditions (Figure 3). The relativelylow concentrations of dyestuff requiredto dye specific shades and the compar-atively high fixation yield are positiveecological attributes and help minimize

the cost of waste water treatment.

Dyeing proceduresThe preferred process uses progres-

sively metered addition of the alkali thatallows optimum regulation of the dyeexhaustion. This does not only apply tovinyl sulfone dyes. The advantages ofthe automet system are as follows:

l optimum fixation profilel excellent reproducibilityl excellent levelness*short dyeing timesl automated dyeing processl fewer rejectsl fewer operators required.Figure 5 shows the 140°F (60*C)

automet dyeing process for theRemazol RR system. Following pre-treatment of the goods, the salt andauxiliaries are pre-run in the bath andthe dyes are then added into the liquorAfter a short diffusion phase, the alkalidosing program can be started. Thisshould be followed by a final fixationperiod. As well as cutting out manualerrors and dead time, automation great-ly reduces the number of rejects.By varying dyeing parameters such

as the salt load, type and amount of alkali, addition profile and duration ofindividual steps in the process, the dye-ing method can be adjusted to suit virtu-ally all article qualities and depths of

American Dyestuff Reporter March 1997

shades, including articles made of alltypes of rayon including modal, poly-nosics, and lyocell (Tencell). Switchingthe neutral phase to higher tempera-tures 175/200* F (80/9O*C) is only rec-ommended for articles where penetra-tion is difficult, e.g. high-twist yarns.

The trichromatic system can also bedyed using conventional dyeing proce-dures. Particular advantages are gainedwhen using the automet dyeingprocess.

In practice, dyeing conditions arerarely constant. When switching fromlaboratory equipment to plant equip-ment and if the same shade is dyedrepeatedly either on the same unit or ondifferent units, there are likely to be vari-ations in the following parameters:

*salt loadl temperaturel dyeing time*type and amount of alkalil liquor ratioThe new trichromatic system has two

major advantages:1. Normal variations in dyeing condi-

tions do not materially affect theresults obtained (Figure 5)

2. Extreme differences in dyeing con-ditions tend to affect the strength ofthe dyeing rather than the shadeobtained.

On modem dyeing units, the recom-mended dyeing temperature of 140°F(60°C) can be set within tolerances ofabout +2*C. The new dyes producecomparable results even at significantlylarger temperature differences

“Normal” variations in the salt con-centration are defined as -lO%, i.e. ifthe recommended salt concentration is50 g/l, the concentration can vary from45-55 g/l. Dyeings with the new dyesare relatively insensitive to even largervariations. The diagram shows a rangeof 40-60 g/l.

The same applies to variations in theliquor ratio, i.e. between 7:l and 13:l.High yields are obtained even if dyeingsare produced at higher liquor ratios, asis common for example for polyester/cellulosic blends.

Nearly all major machine manufac-turers now have units for dyeing at shortliquor ratios. Investment in such unitspays off because it cuts operating costs(energy, water, chemicals, dyes, etc.)and raises productivity by reducing pro-cessing times. Consequently, the aimshould be to dye at the shortest possi-ble liquor ratio.

The fixation yield obtained with the

Figure 6: Washing-off profile of a trichromatic dyeing withRemazol RR dyes.

new dyes is roughly the same whenusing either soda ash/caustic solution orsoda ash alone. Dyeings produced withthese dyes do not change even if thedyeing time is prolonged considerably.This is important when adjusting theshade of dyeings or if unplanned deadtimes occur in manual dyeing processesbecause operatorsare busy elsewhere.

These properties show that ourtrichromatic system is a convincing andcost-effective solution. Since success isnot dependent on strict adherence tothe dyeing parameters, the systemenhances the reliability of dyeingprocesses and minimizes rejects.

Post rinsingThe affinity and diffusion behavior

and the amount of unfixed dyestuff arethe main factors affecting the cost of thewashing-off processes. Optimum wet-fastness can only be obtained if allunfixed dyestuff is removed from thef ibers.

The high fixation yield of the newdyes means that only small amounts ofunfixed dye need to be washed off.Figure 6 illustrates the washing-offprocess. Most of the unfixed dye isremoved with the dye liquor when it isdrained out of the bath. The first rinsingbath serves two main purposes: itreduces the salt concentration, therebyreducing the affinity of the unfixeddyestuff, and washes off the alkali. Inthe second bath, the goods are acidifiedto prevent partial hydrolysis of fixeddyes during the subsequent hot rinsingstage. The hot bath brings unfixed dyesto the surface of the fiber so that theycan be washed off and removed whenthe wash liquor is drained from the bath.

Because of the low quantity ofunfixed dye and the good washing-offproperties of these dyes, post rinsing to

obtain the required level of wetfastnesscan be performed quickly and requiresrelatively little energy and water.

In summary, the dyeing and postrinsing processes show that the newtrichromatic system enables dyers toachieve dyeing times and a level of reli-ability that set new standards for reac-tive dyeing.

Fastness propertiesWith these dyes it is possible to obtainfastness properties that exceed thedemands normally made on sports andleisure wear. Two features of the combi-nation’s fastness properties are outlinedbelow.

LightfastnessThe three new dyes fade at essen-

tially the same rate when exposed tolight. The result is a lightening of theshade without a change in shade. Onthe other hand, should the blue dye inthe trichromatic combination bereplaced with a blue dye with higherlightfastness such as C.I. Reactive Blue220, the dyeing will exhibit an objection-able change of shade after exposure tolight.

Stability to acidsThe dyestuff-fiber bond formed by

the new dyes is extremely stable toacids, so these dyes are ideal for dyeingpolyester/cellulosic blends by thereverse dyeing process (pre-dyeing ofthe cellulosic component followed byacidified overdyeing of the polyestercomponent).

Effect of globalizationIn recent years, the globalization of

the market has had a major impact oncompetitive structures. Cost, quality andflexibility are now principal criteria form a r k e t s u c c e s s .

65American Dyestuff Reporter March 1997

Dyeing Unmodified...[Continued from page 18]

dyeings? Is the fastness to light, wash-ing, crocking, dry cleaning, etc. goodenough for commercial consumer prod-ucts? Can the process be extended todyeing polypropylene carpets? At thispoint, no continued work is being sup-ported at The University of Georgia inthis area, and it is unlikely that addition-al work will be done unless there is anexpression of interest in this work andsupport of this work by industry. q

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