14 Printing with synthetic dyes

S. Karpagam Chinnammal

1. Introduction

Printing is the process of applying colour to fabric in definite pattern or design. Printing involves deposition of viscous paste containing the dyestuff, a thickening agent, a small amount of water and other printing assistants. It is immediately dried to prevent the spreading of the colour beyond the boundaries of the design. The actual transfer of the dyestuff from the thickener film into the fibre of the fabric is carried out by steaming, curing or chemical fixation. In any printed fabric, it is necessary to have excellent color fastness. There are various tests to determine color fastness like washing, rubbing, light etc

2. Learning objectives

•  To get an insight into printing using different synthetic dyes
•  To be acquainted with the after treatments for printed fabrics
•  To be aware of the colour fastness of various dyes

3. Styles of printing

There are various techniques of applying colour on textiles. Broadly they may be classified as

3.1.Direct style- This style is also referred to as ‘steam style’ because in most cases the printed fabric is steamed to fix the colour. The colour is applied directly to the fabric with the required ingredients and after printing and drying the colour is fixed by steaming or thermofixing. Most of the classes of dyestuff are printed on the fabric by the direct style.

3.2. Resist style– In resist printing the bleached goods are printed with a pattern in form of resist paste which cannot be penetrated when the fabric is subsequently immersed in a dye solution. Only those portion which are not covered by the resist paste will be dyed. After washing a white pattern will be formed on a dark back ground in case of white resist. It is possible to obtain colour resist on dark background by using the dyes in the resist paste which can withstand the action of resisting agents.

3.3. Discharge style-in this style of printing, fabric is dyed the ground shade and then printed with a paste containing an alkaline reducing agent such as rangolite C that will destroy the colour (white discharge). Sometimes the ground colour is removed and another colour that can withstand the action of the discharging agent is printed to subsequently obtain  colour design effect. It is then steamed followed by washing to remove the by products of the reaction. The dyes have to be selected properly as not all dyes are dischargeable.

4. Direct style of printing

   4.1.Cotton printing with pigments

In printing, dyes used penetrate inside the fabric and get absorbed in the fibre by electrostatic force or chemical bond. In pigment printing there is no bond formed between fibre and pigment. So, they are not fast to rubbing, washing or light. To improve these properties the printing paste is composed of pigment, a binder, a suitable thickening agent, catalysts, softening agents and auxiliaries.

Pigments are supplied as dispersion called pigment emulsion or pigment paste containing surface active agents to keep them in stable dispersion. The binder is a film forming material of high molecular weight and is the most important ingredient in the printing paste and is used for fixation. A thickener facilitates the properties and it must have no effect on handle of the fabric and must be removed easily after curing. A suitable thickener is an emulsion thickener. Catalysts are acid liberating agents to accelerate the cross linking of binder and to facilitate the reaction between the binder and the substrate. Diammonium phosphate, ammonium sulphate are the most commonly used catalyst. Dimethyl urea and dimethylol ethylene urea are examples of cross linking agents which are not self reacting. Softening agents improve the handle of the fabric. Dispersing agents and poly alcohols are incorporated in the paste to stabilize the emulsion and prevent it from drying

The sequence of process is print-dry-cure. In this process, a binder emulsion (Emulsion is a thick paste which is used as thickening agent for preparing printing paste) is first prepared and is mixed with pigment dispersion, this paste is used then for printing.

Kerosene which is not soluble in water is emulsified by adding emulsifying agent, all three are mixed and stirred at high speed.

The fabric is printed with the printing paste using screen, roller or rotary printing machine. It is dried and cured at 140-150° C for 3-5 min. The paste forms a thin film on the fabric surface and when cured the pigment reacts chemically with the fabric and binds with it. Then the cloth is washed and dried

The advantage of pigments are applicable to all kinds of fibres especially mixtures and blends, high productivity due to elimination of after treatments, high fastness properties, Non staining of white ground and availability of complex range of colours.

The disadvantages are the use of costly and hazardous kerosene and the lack of fastness to rubbing and drycleaning

The present trend is to replace partly or fully the kerosene in the print paste with newer products such as acrylic thickeners.

4.2. Printing of cotton with Reactive dyes

Cellulose fabrics are the most commonly printed substrate, and reactive dyes are the most commonly used dyes in textile printing. The application of reactive dyes leads to formation of covalent bond between dye and fibre resulting in good wash fastness property. The dyes are brighter, easy to apply and gives a full range of colours.

Sodium alginate is the only natural thickener found suitable for reactive printing. Other thickeners like starch and guar gum react with the dye and lowers the colour yield. Sodium alginates produces soft and brilliant prints but it is scarce and expensive. Synthetic thickeners such as polyacrylic acid which do not react have been introduced. Carboxy methylated (Guar gum) is chemically modified and sold as substitute to sodium alginate. It has great economical and environmental advantages and is characterized by high viscosity and good film forming properties, but it is not a good substitute for alginates. An half emulsion thickening in which both sodium alginate and emulsion are present is used.

When selecting reactive dyes for printing the factors of importance to be considered are print paste stability and staining of the ground during washing off. For optimum results the fibre pretreatment is important. Mercerisation is recommended

Reactive dyes are applied by two processes exhausting and fixing. The exhaustion is done under neutral condition and fixation is done in alkaline condition. Reactive dye have low affinity for cellulosic fibres, however they can be exhausted into the fibre using exhausting agent like glaubers salt or common salt. The exhaustion of dye on fibre before the addition of alkali and also the reaction of dye and water, leads to hydrolysis or inactivation of dye. The hydrolysed dye also gets deposited on the fabric surface and gives poor wash fastness.

Printing textile with reactive material causes problem because of hydrolysed dye. When the fabric is printed with reactive dye most of the dye reacts with the printed portion of the fabric but some amount of dye gets hydrolyzed like dyeing. This hydrolyzed dye and the thickener used in the paste has to be removed by soaping. While soaping the hydrolysed dye gets transferred to the soap bath and this transforms into dyebath and tints the fabric with dye and that is clearly seen on white background. Hence reducing white background fabric with reactive dye printing is difficult.

Some reactive dyes get reduced during steaming, so to protect the dye 1 % resist salt /Ludigol on weight of paste is added to the printing paste. Resist salt acts as mild oxidizing agent. The fixation of Reactive dye on the fabric can be brought about by liberation of alkali during steaming process. So alkali liberating agent such as sodium bicarbonate is added

For fixation of reactive dye, one of the following process is used

1.      Steaming

2.      Dry heating

3.      Flash ageing

4.      Wet development

5.      Silicate padding

1. i) In steaming process a stock thickening using alginate or half emulsion thickening with sodium alginate is first prepared as follows

A thickener stock paste is prepared by adding a thickener powder in a small amount of warm de mineralized water and continuously stirred with a mechanical stirrer until a smooth paste is obtained. The prepared paste is stored overnight. Another paste is prepared by mixing urea in warm water and cooled to room temperature. To this, required amount of dye and other auxiliaries except alkali is added and stirred until a smooth paste is obtained. Calculated amount of prepared thickener stock paste is added to this dye paste. Alkali is added and mixed just before printing.

When depth of shade has to be reduced the Printing paste may be diluted by reducing thickener. It can be prepared as follows

Print-dry-steam The sample is printed and printed samples are dried at 90°C for 5 minutes and steamed at 102°C for 15 minutes. During steaming, sodium bicarbonate decomposes to give sufficient alkaline condition for fixation of the dyestuff. The fabric is then washed thoroughly first in cold water then in boiling water for 5 minutes and soaped with 2 gm lissapol D at boil for 15 minutes rinsed and dried

ii) Baking or thermofixing- A high concentration of urea (200 g/kg) has to be used. The alkali used can be sodium bicarbonate or sodium carbonate. After printing and drying the fabric is baked at 150 °C for 5 minutes rinsed soaped with a detergent at boil, rinsed again and dried

iii)Flash ageing: in this process the fabric is printed with a paste which does not contain any alkali and is then padded with an alkaline solution and steamed in the wet condition at 140° C for 30 seconds

iv)Wet development process: The process is similar to the flash ageing process except that the fabric after printing is passed through a hot alkaline solution at 95°Cto 98 °C for 15 sec and is not subjected to steaming

v)Silicate padding or pad batch process: This process is widely used for printing and fixing reactive dyes belonging to vinyl sulphone group (remozols) .The method is suitable for use in the cottage industry since the fixation of the dye is brought about by padding the fabric after printing with sodium silicate solution at ph 11.4 to 12 batched and stored for 12to 14 hours and then rinsed soaped at boil rinsed and dried

 Colour fastness to rubbing in dry condition is very good and in wet good. Colour change due to washing is fair and staining is very good

4.3.Printing silk using Acid dye

Silk is a protein fibre so acid dyes is excellent for printing on silk and Guar gum being acidic in nature is good for printing with acid dyes. Solvent is used as a hygroscopic agent its function is to take sufficient amount of water during steaming to mobilize the dye to get absorbed into the fabric. Urea has dye solubility and hygroscopic property and so is used as an ingredient. Acetic acid is used to give an acidic condition.

Table -6 Preparation of printing paste for silk with acid dye

The printing paste is prepared by mixing all the ingredients and the fabric is printed using block or screen. After printing it is dried, and steamed at 100-120°C for 30-40 minutes. The steamed fabric is thoroughly washed in cold water and dried.

Acid dyes give brilliant shades, good fastness to rubbing and light and moderate fastness to washing on silk.

The materials printed by this recipe are steamed at 100-120°C for 10-15 min. The steamed fabric is thoroughly washed in cold water and then washed in 60° C hot water and again rinsed thoroughly in cold water and dried.

Hot brand reactive dyes when used on silk for printing have excellent brilliant shade and good washing and light fastness. But mild acids are required for fixing these dyes on the fabric.

4.5. Printing of Polyester using disperse dye

Commonly Disperse dyes are used for printing polyester materials. Combination of reactive/disperse dyes or vat/disperse are for blend printing of cotton polyester. Suitable pretreatments for the fabric must be employed to remove sizes and spin finishes before printing. Heat setting is also important pretreatment.

The thickening agents and fixation techniques must be properly selected to produce satisfactory results. Thickeners selected for printing polyester must be taken up by the fabric forming a thin  layer ie locking the pigment on the fabric. And this layer should prevent cracking or dusting away of pigment, it should also give level absorption of pigment and sharp prints. After curing or steaming and during washing the thickener should be easily removable. The most commonly used thickeners for polyester are starch ethers, guar gum (modified), natural gum and indalca.

To prevent the dye from being reduced during steaming or thermofixation resist salt/ludigol is used and sodium chlorate also works well. Disperse dye gets fixed at pH 4.5-5. The printing paste should be slightly acidic in nature, only then disperse dye fixes and produces sharp prints. If the paste is alkaline in nature the dye loses its affinity and produces dull colour print. To make the paste acidic glycolic acid, tartaric acid or ammonium sulphate is added. Levelling agents and carriers could also be added.

Disperse dyes on polyester can be fixed by three methods which are –pressure steaming, high temperature steaming and thermofixation. On the basis of sublimation of dye on fabric the method of fixation is selected. Thermofixation is used for dyes with fast sublimation. Pressure steaming is used for dyes with moderate sublimation.

Pressure steaming (PS)

By adding 30-60 g/kg a carrier and steaming for 20-40 minutes in saturated steam at 100 °C gives acceptable fixation with selected dye

High temperature steaming (HTS)

In this method the steam temperature is raised to 180°C at atmospheric pressure. The printed fabric is continuously passed through steamer for 1-2 minutes depending on the speed of fabric movement. Because of high temperature steam dye gets fixed in short time.

Thermofixaton (TF)

Thermofixation, is a continuous process, in this the printed fabric is carried on stenter and passed through hot air of 200-210 °C temperature for 20-30 seconds. The colour yield of the print are not bright when compared with steaming method.

The fabric is printed, dried and fixed by any one method described earlier.

After treatment: Washing is done to remove thickener and unfixed disperse dye. Thorough rinsing in cold then warm water should be followed by an alkaline reduction treatment at 40-50 °C eg 1g/l surfactant, 2 ml/l sodium hydroxide, 1-2 g/l sodium hydrosulphite.

Finally the goods are rinsed in warm then cold water and dried. A softner or antistatic agent may be applied in the final rinse or by padding before drying Disperse dyes have good fastness to all agents

4.6. Printing of Acrylic fabrics using cationic dyes

For printing acryic fabrics cationic dyes are used and this class of dye gives sharp prints, good washing, rubbing and lightfastness property.

The printed fabric is steamed for 20-30 minutes at 105-115 °C. The steamed fabric is rinsed thoroughly to remove unfixed dye. To prevent the ground fabric from staining it can be rinsed first with cold water and then with hot water with wetting agent (1 gpl) for 25-30 minutes at 60°C. The fabric is removed , rinsed with hot water followed by cold water and lastly dried.

4.7. Khadi printing

In this printing the dyed fabric is printed with a paste containing white pigment (titanium dioxide). A white print is produced on a coloured background. Also coloured khadi can be produced by adding small amount of pigment in the paste and printing on white fabric.

Table-10-Khadi printing paste preparation

Khadi has no affinity for cotton and so a binder has to be used to trap the pigment between the binder and fabric forming thin film. The pigment gets fixed on the fabric by curing the chemical reaction binder film and using a fixing agent. The printed fabric is dried and cured at 130-150 °C for 3-5 minutes.

5. Discharge printing

5.1.White discharge style

The fabric is first dyed with azoic colours (naptholisation and coupling with diazotized base) and is then printed with the white discharge printing paste, dried and steamed at 100-103° C for 5 minutes and immediately treated at the boil 4-5 mins in sodaash (1.5 g/l) to remove the decomposition products as well as the thickener and chemicals. It is followed by soaping treatment.

Table-11 White discharge paste

5.2.Colour discharge style

Fabric dyed in azoic colour is printed with discharging chemicals which during steaming produce alkaline and reducing condition. At this condition azoic colours present at the printed portion reduces and discharges colour and gives white portion. For producing colour discharge effect , another coloured dye can be added to the printing paste. But such dyes which require alkaline reducing condition to fix on the fabric only can be used .

Since vat dyes need alkaline reducing conditions for vatting and fixation in cellulosic fibre fabrics, discharging of azoic colour as well as the vatting of vat dye ( resistant to discharge effect)can be done simultaneously in vat colour discharge printing on azoic ground.

Table-12 Colour discharge paste

Discharge printing could be done using other dyes like reactive and disperse also

6. Resist printing

Since reactive dyes require alkaline medium for fixation it can be prevented by printing with acid ie citric acid or tartaric acid (resisting agent). After this the fabric is washed to remove  unfixed dye from printed area. It is suitable only for dyes with low affinity. Hence thickeners used must be acid resistant. Locust bean gum and tragacanth are suitable for this purpose.

The process is done in two steps firstly white resist paste is printed and secondly padded with paste containing alkali

Table .13 White resist paste preparation

7. Conclusion

Printing is an embellishment on fabric and there are various types of printing like block, screen, stencil roller, rotary etc. In this module we have dealt with printing using various synthetic dyes there after treatments and colourfastness properties.

REFERENCE

1. Parambir Singh Malhi, Aditi saini, Manmeer kaur, Rohit mittal, Psyllium as a thickening agent in printing of cotton with reactive dyes, Colourage, March 2015 , p.33-35
2. Tanvi Kumari, divya Rani Singh, Use of naural binder with acid dye and check the colour fastness, Textile trend June 2017 P 33-35
3. Hemalatha Jain, Techniques of dyeing and printing, Ane Books Pvt.Ltd, New Delhi 2010 p. 193-211