ECO FRIENDLY VAT DYEING

By : Karanvir singh

ECO-TEXTILESA new concept is causing the textile industries to rethink their strategies:• Sourcing• Manufacturing/producing• MarketingEco means:-A reduction of every negative impact that

textiles have on the environment In the form of• Pollution• Damage to the planet

TEXTILE PROCESS FLOW

Production process

Products

Waste water

Solid wastes

Air pollutants

Stream

Inputs

Raw materials

Dyes & Chemicals

Water

Energy

Change or modify in all areas.

Vat Dyes: Vat dyes possess C=O group in their structure and

are water insoluble. Non-ionic dyes are reduced to leuco form followed

by conversion to water soluble form in presence of alkali, which in turn exhibit affinity for cellulosic textiles.

Vat dyes have mainly anthraquinone (82%) or indigoid/thioindigoid (9%) structures, with the former having better fastness properties

Vat dyes are easier to reoxidize than sulfur dyes and the oxygen in air is often the agent used. Most vat dyes display high wash fastness

The excess alkali remaining on the cloth is neutralized by scouring.

Representative Anthraquinone vat dye structures (a) Vat Red 13, (b) Vat Black 27, (c) Vat Orange 2, (d) Vat Blue 4, (e) Vat Green 1.

VAT DYES When the ultimate in wash & boil fastness is

required.

Also used to dye over fibre reactive dyes for multi-layered dyeing.

YELLOW GREEN

ORANGE OLIVE BRED BROWN

BLUE NAVYVIOLET BLACK

REDUCTION VAT DYES Leuco potential of vat dyes under standard

conditions lies between -650 and -1000 mV Indigo can be vatted around -750mV, but

anthraquinones require higher reduction potential (around or above -850 mV)

Reduction of vat dyes is a reversible process

CONVENTIONAL REDUCING AGENTS

Sodium hydrosulphite Copperas method Thiourea dioxide Sodium borohydride

SODIUM HYDROSULPHITE Sodium hydrosulphite reduces vat dye at Temp of

30-50c, Resulting in formation of biphenols(leuco-indigo). Leuco indigo,in presence of NaOH gets converted

to mono-phenolate & bi-phenolate forms.

Na2S2O4 decomposes to liberate nascent hydrogen, which in turn reduces dye [3].

Na2S2O4 + 4H2O → 2NaHSO4 + 6H

Na2S2O4 + 2NaOH → 2Na2SO3 + 2H Presence of excess alkali solubilises reduced dye

to its Na- salt.

Na2S2O4 is very unstable. At the time of reduction of dye, it gets decomposed thermally, oxidatively and in various other ways , requiring 2 - 3 times higher amount than required for reduction of dye .

Sulphite, Sulphate, thiosulphate and toxic sulphide heavily contaminate waste water from dyeing plants. An excessively high sulphate concentration in the effluent can cause damage to unprotected concrete pipes.

O

O

OH

OH

ONa

ONa

Na2S2O4 NaOH

COPPERAS METHOD

FeSO4 when reacts with Ca(OH)2 produces Fe(OH)2, which in turn reduces vat dyes and itself gets oxidized to Fe(OH)3 . The overall reaction is

FeSO4 + Ca(OH)2 → Fe(OH)2 + CaSO4

Fe(OH)2 + 2H2O → Fe(OH)3 + 2H ↑

This method is fallen out due to generation of very bulky sediment.

Precipitation of iron on dyed textile makes shade dull & acid sensitive.

THIOUREA DIOXIDE

It chemically inert to many reagents,but an irreversible rearrangement takes place when heated with alkali & water with formation of formamidine sulphinic acid.

Sulphoxylic acid which act as active reducing agent. It having higher reduction potential valve, due to

which over reduction occur. Higher reduction potential value, due to which dyes

prone to over-reduction can not be reduced with it in absence of inhibitors like glucose, nitrite or hydroxylamine .

H2N

C SO2

H2N

H2N

CH S

H2N

OH

O

H2N

C O

H2N

H2SO2H2O

SODIUM BOROHYDRIDE

Sodium borohydride is bicomponent reducing system,mainly developed for pad-steam process.

Pad steam consist of a) Highly alkaline soluntion of Sodium

borohydride b) Solid consisting of hydroxymethane

sulphinic acid & an accelerator based on nickel-cyanide complex.

Actual reducer is sodium salt of hydroxymethane sulphinic acid (Rangolite-c)

Main problem associated with this is over –reduction.

DISADVANTAGE OF CONVENTIONAL REDUCING AGENT

Effluent Costs Liquor Recycling

ECOFRIENDLY REDUCING AGENTS

Hydroxyacetone Glucose Iron(II)Complexs

HYDROXYACETONE

Hydroxyacetone (CH3 – CO – CH2OH) provides reduction potential (~ -810mV).

Advantages Higher quality dyeing, better ring dyeing

effect in dyeing with indigo. Increased productivity. Higher dye uptake, less dyestuff in effluent. Lower chemical consumption.

GLUCOSE – NAOH SYSTEM

A combination of Glucose - NaOH provides reduction potential (–550 to –600mV).

Indigo requires reduction potential of the reducing bath around -700mV which can be achieved with this system at boil.

Produce reduced indigo baths free from sediments and highly stable for several hours.

IRON (II) COMPLEXES

Fe(OH)2 is a strong reducing agent in an alkaline environment , With the increase in the pH, the reducing effect is more.

In alkaline conditions it is less soluble and precipitates so it must be complexed in order to hold the Fe(OH)2 in solution.

A stable complex with reducing power is obtained with weaker ligands , e.g. gluconic acid, diethylene triaminepentacetis acid acid sodium salt (DTPA), 1-hydroxy ethylidene – 1,1 – diphosphonic acid (HEDP), aminotrimethylene phosphonic acid.

There is substantial increase in reduction potential (-765 to -996 mV), even on addition of agent like Triethanolamine (TEA) as low as 1.5 ml/l

CITRIC ACID/TARTARIC ACID

Citric acid/Tartaric acid are also used and observed that the reduction potential is increased in the range of – 873 to -920 mV. It is proposed that for tartaric acid that it has 4 unshared electron donating sites, 2 sites due to –OH groups and 2 sites due to –COOH site takes part in complex formation, 3 molecules of tartaric acid will be required per molecule of Fe(OH)2.

Citric acid as a ligand has 4 unshared electron donating sites, 3 due to –COOH groups and 1 due to –OH groups. •If –COOH groups take part in complex formation, it will behave like a tridentate ligand:

If –OH groups take part, citric acid will behave like a monodentate ligand:

GLUCONIC ACID

Gluconic acid has been found to be effective chelating agent for iron under alkaline conditions

Regarding eco-environmental aspect, gluconic acid can be eliminated in the sewage tank through neutralisation with alkali; free Fe(OH)2 which due to areation gets converted to Fe(OH)3 acts as a flocculent and reduces waste-water load.

DTPA (Diethylene triaminepentacetic acid sodium salt) has a good sequestering action on Fe3+ under alkaline pH.

ELECTROCHEMICAL DYEING This method employs as an

elecrochemical reversible system called a redox mediator , as an electron carrier between electron and the dye.

DYE-BATH CONTAINS