Printing of Protein (cellulosic) fibers with Vat and Sulphur Dyes:-

Q. Why we print cellulosic fabric with VAT dyes?

For obtaining excellent all round fastness properties cellulosic fabric are printed with VAT dyes.

Printing Mechanism:-

Various steps are involved with vat dyes.(Reduction/ reduce change the structure, for water soluble, affinity for fibers).

1. Reduction ;-Vat dyes are insoluble in water on treating with reducing agent in presence of alkali; vat dyes are converted into water soluble Sodium salt of Leuco compounds.

2. Printing or Dyeing stuff penetration:-In this reduce form vat dyes have affinity towards cellulosic fibers and are capable of being absorbed by the fibers thus vat dyes penetrate into the inside of fibers.

3. Oxidation:- After penetration into the inside of fibers. These leuco compounds are converted to the original insoluble vat dyes by oxidations which are firmly held with in the fibers.

4. Soaping or Washing off:-Soaping is done to remove the unfixed dyestuff from the surface of fabric.

Printing Methods:-

One phase or one stage printing method.In this method print paste contains vat dyes, thickening agent , full amounts reducing agent along with alkalis & other textile auxiliaries after printing & drying fixation is carried out by the steaming .(All in method)

Two phase or two stage method:-In two phase printing the print paste contains only dyestuff and thickening agent, After printing & drying the reducing agent & alkali required for fixation are applied by padding. The fabric is then steamed immediately oxidation & washing are then carried out.

Chemicals & Textile Auxiliaries Required for printing:- Vat dyes Thickening agents Solvents Hydroscopic agents Alkalis Reducing agents Dispersing agent & leveling agents.

Vat dyes:-

Available in different forms,

Powdered (double strength, concentrated) Ordinary powder form Micro disperseColloisalLiquidPaste

Thickening Agents:- Those thickening agents can be used which are highly resistant to alkali & reducing agents. Suitable thickening agents are :

British GumMixture of starch & Gum TragoncanthMecrogumsAlginates which are suitable to high concentration of alkalis Starch Ether.

Alkalis:- Which are most commonly used are:-

Potassium carbonate (Potash)Sodium carbonate Sodium hydroxide Potassium hydroxide(Potassium carbonate is preferred over sodium carbonate because of higher solubility &---?)

Reducing agents:-Sodium formaldehydes. Sulph oxylate (CH2OHSO2Na) is largely used as reducing agent in printing with Vat dyes because it is stable at low temperature. It decompose at high temperature above (100⁰C), therefore vat dyes are reduce during steaming. Print pastes prepared with this reducing agent are most stable. This type of reducing agent is available under different trade names such as

Rongalite CFormasul GSodium hydrosulphideSodium dithionite also used as reducing agents in vat printing but on limited scale.

Solvents & Hydroscopic agents:-Glycerin is mostly used as solvent & hydroscopic agent in vat printing due to good solvent hydroscopic agent under dry or super heated steam. It improves the solubility of leuco vat dyes & facilitates proper

penetration of vat dyes into the fabric. Dispersing & leveling agent are used which facilitates leveling of the dye stuff.

One phase printing of cotton with Vat dyes:-a. Preparation of Stock thickener agent (Mecro gum, Guar gum, Starch ether, British gum)b. Preparation of print paste

Vat dyestuff (paste) X grams Glycerin 50-100 grams Dispersing & leveling agent 20-30 grams Stock thickening 500-600 grams Potassium carbonate 80-120 grams Rongalite C 50-100 grams Water / stock thickener Y grams

Total = 1000 grams Stir it till it completely mixed.

c. Printingd. Drying:-

After printing fabric is dried at (100-120⁰C)e. Fixation:-

After printing & drying fixation is carried out by steaming with air free saturated steam for 8 to 10 minutes (101-103⁰C). After printing & drying steaming should be carried out as early as possible to avoid uneven & weaker printing result.

f. Mechanism of fixation during steaming:-When printed and dried fabrics enters steam present in the steamer condenser into the water due to contact of steam with cold fabric. Hygroscopic agents in print paste absorb this condensed water & alkali and rongalite C are dissolves in this water. Now as the temperature raises rongalite C in the presence of alkali reduces and solubilizes vat dyes in this way the dissolved leuco vat dyes are transferred from the thick print paste into the fabric.

g. Oxidation:-Oxidation is normally carried out normally

Hydrogen peroxide 2-4ml Acetic acid 1-2ml/L Temperature 50-60⁰CWashing off after oxidation the cloth is rinsed with cold & then with hot water then soaped with

Detergent 1gm/L Soda ash 1gm/L Temperature 95-100⁰C

Finally the fabric is rinsed with warm & then with cold water.Two phase or two stage print of cotton fabric with vat dyes:- Advantage of two phase printing method Wider range of dyes can be used for printing Unlimited print paste stability Prints are not sensitive to drying Unlimited storage of print before steaming Good levelness of printed fabric obtainedEasier washing off

Disadvantages:-More time is required Colour may bleed & stain the white & other colours in the design.

Stock thickening:-Water 930 partsGuar gum 30gm.Starch ether 40 gm.Total 1000 gm.

Preparation of print paste:-Vat dyestuff paste X gm.Stock thickening agent 800 gm.Stock/water Y gm.Total 1000gm.Never curing Hydrosulphide & soda ashPrinting Drying temperature 100-103⁰C Store Fixation is carried out by padding the printed & dried fabric with Cold water 600mlSodium hydroxide (32.5%) 38⁰BeRongalite C 100 gm.Total 1 liter Fixation steaming (flash ager) temperature 110-115⁰C Time 8-10 minutes