IntroductionIncreasing consideration of ecologic consequences of industrial processes aswell as legislation enforcing avoidance of environmental problems have

Supercritical Fluid Dyeing of Textiles

caused a reorientation of thinking & promoted projects for replacement ofconventional technologies. Quality standards to be met by industrial wastewaterswill certainly be raised in future that in turn causes unpredictable increases incosts, particularly thoseincurred by having to disposeofdyehouseeffluents.Dyeing process of fibers in supercritical carbon dioxide (scCO2) is anDyeing process of fibers in supercritical carbon dioxide (scCO2) is anexample of a ‘clean’ process suitable for fulfilling many of requirements ofsustainability. Here, a recyclable process medium(CO2) is used togetherwith an efficient & minimum input of chemicals (only dyes, no auxiliaries)& energy (low dyeing times, fusion of processes, no drying) & with minimal

i i & t d ti Q lit f d d t i l i l hi hemissions & waste production. Qualityof dyed materialsis also very high.What is Supercritical Fluid?Any gas above its critical temperature retains free mobility ofgaseous state but with increasing pressure its density increases towardsthat of a liquid. Supercritical fluids are such high compressed gases &q p g p gas such they combine valuable properties of both liquid & gas.

No distinction between gases & liquids.Properties intermediate between those of gases & liquids.Do not condense or evaporate to form a liquid or a gas.p q g

Unit Gas Liquid Supercritical FluidDensity gm/cm2cm2 cm 1/1000 1 0.6Diffusion Coefficient cm2cm/sec 1/10 5/10000 1/1000Viscosities gm/cm2sec 1/10000 1/100 1/10000

Environmental Compatibility of CO2⇒ No Smog⇒ Non-Toxic⇒ No Damage to Ozone Layer⇒ Non-Carcinogenic 345

380 ppm⇒ Non Carcinogenic⇒ Non-Flammable & Non-Corrosive⇒ Virtually Inexhaustible⇒ No Disposal Problem

315 ppm1960

345 ppm1990

2003

Physiochemical Properties of CO2⇒ At critical point, temperature of CO2 is 31.1oC & pressure 73.8 bar.⇒ Below critical parameters two distinct phases of liquid & gaseous

CO are separated by phase boundaryCO2 are separated by phase boundary.⇒ As temperature & pressure rise along vapour–liquid coexistence

line, liquid CO2 expands & the two phases become less distinctforming a so-called supercritical phase.

⇒ Above critical point, vapour–liquid line completely disappears.⇒ Above critical point, vapour liquid line completely disappears.⇒ Viscosity of supercritical fluids is more gas-like.

Current SCF Dyeing TechnologiesFor significance of PET & cotton, development of supercriticalfluid dyeing technologies worldwide is mainly focused on these fibers.Dyeing of PET works very well in scCO dyeing of polar fibersDyeing of PET works very well in scCO2, dyeing of polar fiberslike cotton is still challenging when high fastness properties &color yields are required.Limitations of dyeing natural fibers in scCO2 arise from inabilityof CO2 to break hydrogen bonds the low degree of fiber swelling &2low reactivity of OH-bonds in cellulose in slightly acidic CO2 medium.Furthermore, disperse dyes only show slight interactions withpolar fibers, leading to unacceptably low fastness data.

Environmental aspects of PET Dyeing in CO2

It is evident that conventional water dyeing is an end-of-pipe process,whereas with scCO2 a quasi-closed loop process can be accomplished. Afterprecipitation of spinning oils & excess dye in a separator, CO2 is recycled& b d ‘Q i’ t ti id f d t ff & i i il& can be reused. ‘Quasi’ means extraction residues of dyestuffs & spinning oilsare not recyclable as well as about10% of CO2 released into atmosphereProcess Steps Water Step:

scCO2 Step:

Dyeing Process

Extraction-IIis started for

Temperaturein plant is Extracted

Process Flow

Extraction Irepresents

partial extraction of spinning oils

Dyeing:

is started for removal of

adhering dye from fabric surface by

rinsing with fresh cold

in plant is decreased as

fast as possible

below Tg of polymer to

avoid

Extracted dyes & spinning oils are precipitated in a separatorfresh cold

scCO2.avoid

extraction of dye from the fiber bulk.

separator.

Mi i Vi f D d S lMicroscopic View of Dyed Sample

Standard SublimationDye does not penetrate fibers but rather sits on

h i il Controlledtop where it can easily wear off. White fiber shows after cutting or needle penetration

Controlled Penetration

Conventional DyesAfter treatment in an aqueous dye-bath & post-dye washing, fibers show complete dye penetration.C l f t & dh

Complete Penetration.

Colorfastness & adherence is low to moderate when exposed to cleaning agents.

Process Conditions

Time: For a dye uptake of 2%, a dyeing time of 40mins in Technicalplant & 60mins in Industrial plant is needed.

Dye uptake of PET in scCO2 is equivalent to a minimum of 4% in waterdyeing, when estimating an amount of auxiliaries in dye formulation of 50%.Temperature & Pressure Programs

Total time is much less as compared to PET dyeing in waterincluding rinsing/washing & drying (6-8hr.)

Energy Consumption

4.53 kWh /kg

Although treatment time in scCO2 is significantly lower comparedwith water dyeing process including drying, energy consumption ofboth processes seems to be similar.

Warm up before dyeing & cooling in Extraction Step-II of thicker steelProbable Reasons

EconomizationWh id i i i l t d ti d d d f

Warm up before dyeing & cooling in Extraction Step II of thicker steelwalls of scCO2 plant, need more energycompared with a waterdyeingplant.Electrical energy consumption in also include cooling energy for liquefactionof CO2 for recycling, was not considered for water-basedprocess.

When considering minimal waste production as demanded forenvironmentally sound processes, scCO2 dyeing of PET is really a‘clean’ finishing process because only dyes are needed.For comparison, environmental impact of analogous water-basedprocess is summarized.Based on significantly shorter process times, in scCO2 theoreticallyup to six process cycles could be accomplished per day.

Product QualityAllshades as in water dyeing but also highcolor yields are obtained.No differences in reflectance spectra at inside, middle, & outsideof fabric pack were found indicating a high levelness of dyeingof fabric pack were found indicating a high levelness of dyeing.Alldyesapplied in thisprocess are approved by Oeko-Tex 100 Standard.Compared with water dyeing process on an industrial scale,wash fastness of PET when dyed in same shade is sometimes upto one grade higher.o o e g a e g e .Stress–strain & viscosmetric molecular weight measurements ofscCO2 dyed PET show that no fiber damage takes place underoptimum dyeing conditions

Other Application in TextilesOther Application in TextilesThe UV stabilizer or even perfumes may be transferred to fiber.New classes of polymeric compounds have been developed with good solubilityin liquid CO2. Thesecompounds are appliedfor non-aqueoussizing.Supercritical NH3 can be used for mercerization.

DisadvantagesHigh Investment Cost.High Pressure required for dye solubility.Impact of dyeing machine weight is related to circulation.Difficult to dye Natural Fibers.

Textile dyes classification:DirectsReactiveReactiveAcids/BasicsSulphurVatMordant

Not Dissolved in Sc- CO2

MordantDispersePigments Disolved in Sc-CO2

Future TrendsDyeing of supercritical fluid was limited to synthetic fibers butdyeing of natural fibers is under progress. Chemical modification ofnatural fabrics before dyeing is one of possible treatments. Reversemicelles provide a stable aqueous micro environment consisting of awater pool in non-aqueous medium. Cellulose material byimpregnating with hydrogen bond breaking chemicals & ormodification of cotton is done & afterwards pressure & temperaturerelease dyestuff is trapped inside fiber.release dyestuff is trapped inside fiber.ConclusionDyeing in scCO2 has been identified as one of the best alternatives towater-based dyeing. But, this favorable concept is waiting for itscommercial implementation. Successful commercialization of above

t d fi it l i i f d i b fconcept definitely improves economics of dyeing by way ofelimination of wastewater discharges.