Indian Journal of Fibre & Textile ResearchVol. 22, September 1997, pp. 184-189

Water-free dyeing of textile accessories using supercriticalcarbon dioxide"

Dierk Knittel, Wolfgang Saus & Eckhard SchollmeyerGerman Textile Research Centre North-West e.V., 0-47798 Krefeld, Germany

Received I November 1996; accepted 9 December 1996

Results are given about a new dyeing process for dyeing synthetic fibre material with dispersedyes. The use of supercritical carbon dioxide as dyeing medium completely avoids water pollution andthe need of drying. Laboratory results show excellent levelness and fastnesses on the dyeing ofpoly(ethylene terephthalate) and polyamides. Especially, the good results are achievable for small lotslike textile accessories. The state of technical development is briefly described.

Keywords: Disperse dyes, Dyeing, Supercritical carbon dioxide

1 IntroductionThe costs of using water or of treating waste in

industrial processes like conventional dyeing oftextiles or accessories will increase significantly infuture. Therefore, new concepts have to beevaluated. For dyeing of unmodified poly( ethyleneterephthalate) (PET) or polyamide (PA) fabricsand for some other synthetic material, dispersedyes can be employed. Because of thehydrophobicity of such dyes and of the fibre aconventional aqueous dyeing liquor has to containlarge amounts of dispersing agents and surfactantsto obtain reasonable dyeing rates and usefulshades.

Considering the dissolving power ofsupercritical systems (SC-systems) research hasbeen done at German Textile Research CentreNorth-West e.V., Krefeld (DTNW) , FRG, for theevaluation of those systems as a dyeing mediumusing disperse dyestuffs'".

Textile accessories constitute an important areaof clothing manufacture. In this area, like one mayimagine for zippers", often smaller lots of speciallydyed material but with quick response to themarket are required. Therefore, laboratoryexperiments of dyeing using the new process fortreating preformed textile materials likeaccessories are presented. For those articles even a

'This paper is part of the thesis of W. Saus, University ofDuisburg (1997).

small pilot plant may be satisfactory forproduction. The process has been termed as SFD(Supercritical Fluid Dyeing)'.2 Fundamental

A description of the new dyeing medium isbased on Fig. 1, where the pressure (P),temperature (1) and volume (V) of carbon dioxide(C02) are given". Especially the PoT projectionillustrates the critical point (CP). When surpassingthe conditions of CP the fluid gets solventproperties due to changes in its dielectric constant.Thus, the solvent power, being P- and T-dependent, of pure supercritical CO2 will reach thatof a liquid hydrocarbon like octane, sufficient fordissolving disperse dyes. The transport of dyestuff

p

""""/'

""

Fig. I-Phase diagram of CO2 (ref. 9) [CP-<:ritical point; Tcrt'-31°C; Pcrit -73 bar; g-gaseous; I-liquid; and s-solid]

KNITfEL et al. : WATER-FREE DYEING OF TEXTILE ACCESSORIES 185

dissolving disperse dyes. The transport of dyestuffand heat to the material to be dyed is therebyachieved by the supercritical fluid'.

Table I shows the typical properties ofsupercritical fluids like CO2 which lead toperformance properties well suited for dyeing atlow viscosity and high diffusion rates. Because ofthis, a quick transport and quick penetration, eveninto small pores,will be favoured.

All dyeing systems using disperse dyes andexhaustion techniques may be changed to this newtechnology in future (i.e. even aramids andpolyolefins are dyeable). The new processpromises to be very ecological and to be highlyflexible for quick response to the market. Otherphysicochemical aspects of supercritical fluidapplication and their effect may be found in theliterature":". So, mainly the applications aredescribed.

3 Materials and Methods3.1 Materials

Laces, knobs (PET, PA), velcro tapes, zippers,belting material, and PET standard fabric (forresist patterning) were used.

Dispersant-free disperse dyes (Fa. Novartis AG,Basel) with code nos, and the protonated form ofindicator dye Ethylred (Fa. Merck) were used.

3.2 MethodsFor a systematic investigation of the dyeing

capabilities, a high-pressure laboratory apparatushas been built'". It consists of a heatable autoclaveof 300 ern' capacity fitted with a pressure sealedstirrer. The system is safe to 500 bar and 350°C.Pressure is applied from a CO2 gas cylinder via amembrane compressor.

The sample to be dyed was wrapped around aperforated stainless steel tube or sewed onto thetube (knobs) and mounted inside the autoclave.Dyestuff without auxiliary chemicals was placedon the bottom of the vessel and the apparatus wasclosed and preheated. On reaching the workingtemperature, CO2 was compressed to the workingpressure under constant stirring. Pressure wasmaintained for the dyeing period of 0-60 min andreleased afterwards. Standard conditions for 'PETdyeing are 80-160°C and 250-280 bar for about10-60 min.

Table I-Typical properties of supercritical systemsGas Liquid Supercritical

fluid phaseDensity, g ern? 10-3 0.6

Diffusion coefficient,2 -Icm s 10-1

10-4

5.10-6

10-2Viscosity, g em" S-I

3.2.1 Resist Dyeing (Patterning)PET standard fabric was resist printed with a 8%

paste of alginate thickener (Diagum A8® Diamalt)and dried at 120°C. Afterwards, it was treated likeother samples in CO2, Finally, the thickener anddyestuff, deposited within, were removed byrinsing with water.

4 Results and DiscussionFigs 2-8 show supercritical fluid dyeing results

on preformed materials having different make-up.The examples are mainly from PET and PA butsome mixed materials were included too.

Especially, in the case of zippers one has to statethat simultaneously to the dyeing of the basicfabric, the teeth of the zipper are evenly dyeable.Similar results are obtainable for the hooks ofvelcro tapes.

The laces shown in Fig. 4 consist of elasthanereinforced PET, which require special dyestuffs foreven dyeing. But as shown interesting patterningeffects are yet obtainable.

In most cases the dyeings show excellentrubbing fastness even without any aftertreatment(depending on dyestuff and amount of dye)3.'2,whereas conventionally dyed samples often requiresubsequent reductive cleaning because of adherentdye aggregates. In case of surface contaminationby residual dyestuff this can be removed bylowering the working temperature (below glasstemperature of the sample) after the dyeing periodand flushing the material with supercritical CO2,

Wash and light fastnesses usually orientthemselves on the properties of the dyestuff used.

Fig. 5 shows results on compact accessories,when dyeing PA and PET knobs simultaneously.The PET knob is almost completely dyed all overits cross-section, whereas the PAused reveals aring dyeing after the usual treating time. The red

186 INDIAN 1. FIBRE TEXT. RES., SEPTEMBER 1997

Fig. 2-PA zipper dyed with DTNWI9 at120°C and 260 bar

Fig. 3-PA vclcro tape dyed with DTNW 12 at120°C and 280 bar

Fig. 4-Laees dyed with various disperse dyesill supercritical CO! at 120-130oe and 300 barfor 1 min

KNITTEL et at. : WATER-FREE DYEING OF TEXTILE ACCESSORIES 187

Fig. 5-Dyeing of PET- ,PA- and artificialhorn knobs: blue-black PET with DTNWI9;red-violet PA with DTNW 19; and red artificialhorn with protonated form of cthylrcd

Fig. () ·-O.S 111111 PET wire dyed with FarbstoffDTNW 12 under standard conditions

Fig. 7--PA belt dyed with DTNW 17

Fib. 8-PET standard web dyed withDTNW 12 in supereritical CO2 after resistprinting with alginate thickener

188 INDIAN J. FIBRE TEXT. RES., SEPTEMBER 1997

knob (artificial horn) (Fig. 5) needs attentioninsofar as up to now such a material is not dyeablewith disperse dyes in supercritical fluid dyeing(SFD). In this case, the use of a derivative of anacid like dye (protonated ethylred), which hasproved suitable for polyolefinics too, despite oflow solubility in CO2 gives good results'",

Fig. 6 shows the capability of SFD for dyeingcompact materials like a PET wire of 0.5 mmdiam. Using standard SFD conditions a ring dyeingis obtained, whereas on prolonged treatment aneven dyed cross-section can be obtained.

Fig. 7 shows dyeing on accessories of mixedconstitution (i.e. belting with PA content). Thus, inspecial cases, depending on the blended samplefuture dyeing recipes have to be evaluated f9r evendyeing or otherwise one has a possibility ofpatterning.

Differing from the otherwise water-free SFDprocedure, another way of patterning is describedin Fig. 8. For this sample, a pattern of resist(alginate thickener) has been printed onto thefabric prior to SFD. Since at present, alginate likeother carbohydrate compounds are not dyeable inSFD (using disperse dyestuffs), the resist patternintroduces patterning of the PET -base fabric. Evenif the example only shows weak contrast, this wayof patterning may be improved further. Removingthickener after dyeing causes water consumption.

For the examples presented, shade and dyestuffexhaustion may be adjusted by pressure andtemperature variations. Since pressure is easier tovary, its regulation will be preferred. For the SFDprocess, even the dyestuffs which are normally notclassified as disperse dyes like some pigments (forspinning mass dyeing), and some small molecularionic dyestuffs may be used".

5 ConclusionsAs an illustration for the potential of using

supercritical fluids for textile applications, somecalculations on the water saving in supercriticalfluid dyeing, if widely implemented, for Germansituation can be done. Dyeing of pure PET fibrematerials in FRG (in different kind of preparatione.g. flocks, knitwear, yarns, fabrics, zippers, etc.amounts to about 140.000 tonnes/annum. If all thiscould be done according to the SFD process, asaving in water consumption of about 1.1xl07 m'

could be gained (using a mean value of 80 Lwaterlkg). This saving corresponds to the waterconsumption of the households of a town of about250.000 inhabitants".

For the whole process of SFD for dyeingsynthetic material like PET or PA (and some other)using carbon dioxide as fluid medium the benefitscan be summarized as follows:

- Complete elimination of water pretreatment andof water pollution,

- saving of energy costs for drying textiles,- no need of auxiliary agents,- dyeing occurs with a high degree of levelness

and dye exhaustion,- dyeing in a supercritical system requires little

time, thus giving high flexibility and promoting'just-in-time' delivery,

- in the case of PET and PA, no aftertreatmentlike reductive washing is required if dyeing isdone properly,

-.carbon dioxide is non-toxic, can be gained fromnatural sources and can easily be recycled in adyeing process.

Especially, the short dyeing time needed ascompared to that in conventional dyeingprocedures offers advantages regarding flexibilityof production.

High-pressure apparatus are state of technicalart. Development using SFD has only to deal withtextile specific requirements (i.e. flow in upscaledapparatus, package of the goods, fluid ratio, dosageof dyestuff, etc.). An optimized gamme ofdyestuffs for use in SFD yet exists II •

At present, the SFD technology is used for thedyeing of velcro tapes and pilot plants of 30-100 Lcapacity are being evaluated for their performanceat a yarn dyeing factory and at DTNW in order toobtain data for further scale-up":". But even thisexisting working volume may be now attractive fordyeing small lots of textile accessories.

A study of RIZA on potential of large scale SFDof polyester (yarn or fabric) (based on technicalinformation of 1993), which arrives at lowerenergy and operational costs, states that theprocess will need, depending on research activities,an implementation time of about 5-10 years forbroad acceptance IS. If research promoted by

KNITTEL et al.: WATER-FREE DYEING OF TEXTILE ACCESSORIES 189

dyestuff manufacturers will provide suitabledyestuffs for SFD of polyester/cotton blends, thistechnology will reach general application.

AcknowledgementThe authors are grateful to Novartis AG (Ciba-

Geigy), Switzerland, and to Carbo (FRG) andDiamalt (FRG) for supporting part of this researchand to the country of Nordrhein-Westfalen (FRG)for institutional support.

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