An Overview of Polyester and Polyester Dyeing Part-7

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What is Disperse Dye:
A dye that is almost totally insoluble in water. Disperse dye exist in the dye bath as a suspension or dispersion of microscopic particles, with only a tiny amount in true solution at any time. They are the only dyes that are effective for “Normal” polyester. Some types are used for Nylon and Acetate. Polyester is dyed with disperse dyes by boiling with carrier chemicals or by heating the liquor to about 130°C which requires elevated pressure (Like a pressure cooker).

Where the fabric is padded with dye liquor then dried and heated to about 200°C for about 90 seconds, is also used for polyester and for coloring the polyester component of polycotton blends. Disperse dyes are also used for sublimation printing of synthetic fibres and are the colorant used in crayons and inks sold for making “Iron-ON” transfers.

The first dyes for cellulose acetate fibres were water soluble. The dye molecules contained a methylamino sulphonate group (-NHCH2SO3Na) introduced by reaction of a primary amino group with formaldehyde and sodium bisulphate (Ionamine dyes, 1922). During dyeing, this group hydrolysed to the less soluble parent amine.

Dye-NH-CH2SO3Na (aq) + H2O → Dye-NH2(s) + CH2O(aq) + NaHSO3(aq)

It was soon recognized that it was this compound that the cellulose acetate absorbed. The first true disperse dyes were simple, relatively insoluble azo and anthraquinone compounds dispersed in water using the sodium salt of sulphated ricinoleic acid.

Dye(s) ↔ Dye (aq) ↔ Dye(fiber)

Many of these dyes are obsolete but their development provided the technology for preparing fine aqueous dispersions by grinding the dye with dispersing agents. A fine dispersion is essential for rapid dyeing and avoids deposition of larger dye particles on the material.

Classification of Disperse dye for Polyester:
Disperse dyes for a compound shade on polyester can have quite incompatible dyeing properties. The SDC classification of disperse dyes is based on migration ability during exhaust dyeing, colour build-up, sensitivity to changes in temperature and the rate of dyeing.

This type of dye is often classified on the basis of dyeing rate and sublimation fastness, particularly for polyester dyeing. These two properties are a function of molecular weight and the number of polar groups in the dye molecule. The most common classifying is given bellow :
  1. Low energy.
  2. Medium energy.
  3. High energy.
1. Low Energy Disperse Dye:
Most dyeing and fastness properties change gradually with increase in molecular size. Small dye molecules with low polarity are leveling, rapid dyeing dyes with poor heat resistance. These are called low energy disperse dye.

2. Medium Energy Disperse Dye:
Most of the dyeing and fastness properties change gradually with increase in molecular size. Moderate dye molecules with moderate polarity are leveling, rapid dyeing dyes with moderate heat resistance. These are called medium energy disperse dye.

3. High Energy Disperse Dye:
More polar, higher molecular weight dye has low dyeing rates, poor migration during dyeing but good heat and sublimation fastness. These constitute the high energy disperse dye.

Selection Properties:
Disperse dyes have some general properties which are given bellow –
  1. Solubility: Disperse dyes are insoluble in water or slightly soluble in water. It makes fine dispersion with water with water with dispersing agent. Dissolves in organic solvents like benzene, toluene etc.
  2. Fastness to washing: The fabric dyes with disperse dyes shows moderate to good washing fastness.
  3. Light Fastness: Most of the disperse are very fast to washing. The minimum light fastness rating is 4-5.
  4. Sublime ability: Due to stable electronic arrangement disperse dyes have good sublime ability.
  5. Gas Fading: Fabrics dyed with certn blue & violet disperse dyes conaining anthraquinone structure become fade in presence of nitrous oxide. This nitrous oxide may be made in nature from various sources such as open gas fire, electric heating arrangement. 

Commercial (Trade name) Name of Disperse Dyes:
  • Terasil.
  • Foron.
  • Palanil.
  • Resolin.
  • Samaron.
  • Dispersol .
Dispersing Agent:
The actual disperse dye is formed as relatively large particles and in this form it is unsuitable for application on hydrophobic fibers. If these big particles are used in dyeing as such, they produce uneven and specky dyeing and their full colour value is not realized. In order to ensure uniform dyeing, the dye should be present in the dye bath in a uniform and very fine form, which should be stable under dyeing condition. This requires a large amount of suitable dispersing agents followed by grinding. The dispersing agent should be effective under the dyeing conditions and should be stable to hard water, high temperature and other dyeing assistants.
Soap powder, Turkey Red Oil, Alkylsulphates, Alkylarylsulphonates, Fatty Alcholethylene Oxide condensates, Naphthalene-β-sulphonate and formaldehyte etc are the recommended dispersing agent performs many functions. It assists the process of particle size reduction of the dye. It also enables the dye to be formed in the powder form. When the powder is added to the dye bath, it facilitates the recon version of the powder in to a dispersion, it is required for carrying out the dyeing. Finally, it maintains the dispersion in a fine form in the dye bath throughout the dyeing process. Dispersing agents increase the solubility of the disperse dye in water. It is seen that solubility of the dye in water is considerably increased by the dispersing agent and that different dispersing agents affect the solubility to different extents. It can be noted that the dyeing rate increase with increasing solubility the dyeing rate actually decreases. Where the solubility is very high as in the case of direct dyes, practically no dyeing takes place.

Commercial (Trade name) Name of Dispersing agent:
  • Setamol -BASF.
  • Edalon -Sandoz.
  • Calsolene Oil HS –A.C.I.
  • Hipogal –Hoechst.
Point consideration for Textile Coloration:
  • Textile Materials(Fabric/Yarns/Fibres/Garments)
  • Dyes/Pigment
  • Chemicals (Common salt, Caustic, Soda ash etc.)
  • Auxiliaries (Leveling agent, wetting agent, sequestering agent etc.)
  • Machinery
  • Utilities:
  1. Electricity
  2. Water
  3. Steam
  4. Compressed air
  5. Gas
  • Controlling Parameters:
  1. Temperature
  2. Time
  3. Concentration of dyes and chemical
  4. pH
  5. M:L ratio
  6. Pressure
  • Man power
Dyeing Mechanism of Disperse Dye:
The dyeing of hydrophobic fibres like polyester fibres with disperse dyes may be considered as a process of dye transfer from liquid solvent (water) to a solid organic solvent (fibre).Disperse dyes are added to water with a surface active agent to form an aqueous dispersion. The insolubility of disperse dyes enables them to leave the dye liquor as they are more substantive to the organic fibre than to the inorganic dye liquor. The application of heat to the dye liquor increases the energy of dye molecules and accelerates the dyeing of textile fibres.

Heating of dye liquor swells the fibre to some extent and assists the dye to penetrate the fibre polymer system. Thus the dye molecule takes its place in the amorphous regions of the fibre. Once taking place within the fibre polymer system, the dye molecules are held by hydrogen bonds and Van Der Waals’ force.

The dyeing is considered to take place in the following simultaneous steps:

Diffusion of dye in solid phase into water by breaking up into individual molecules. This diffusion depends on dispersibility and solubility of dyestuff and is aided by the presence of dispersing agents and increasing temperature.

Adsorption of the dissolved dye from the solution onto the fibre surface. This dyestuff adsorption by fibre surface is influenced by the solubility of the dye in the dye bath and that in the fibre.

Diffusion of the adsorbed dye from the fibre surface into the interior of the fibre substance towards the centre. In normal condition, the adsorption rate is always higher than the diffusion rate. And this is the governing step of dyeing.

When equilibrium dyeing is reached, the following equilibria are also established:
  • Dye dispersed in the bath
  • Dye dissolved in the bath
  • Dye adsorbed on the fibre
  • Dye diffused in the fibre
Effect of Various Conditions on Disperse Dyeing:
Effect of Temperature:
In case of dyeing with disperse dye, temperature plays an important role. For the swelling of fibre, temperature above 100°C is required if high temperature dyeing method is applied. Again in case of carrier dyeing method, this swelling occurs at 85-90°C. If it is kept for more time, then dye sublimation and loss of fabric strength may occur.

Effect of pH:
For disperse dyeing the dye bath should be acidic and pH should be in between 4.5-5.5. For maintaining this pH, generally acetic acid is usedAt this pH dye exhaustion is satisfactory. During colour development, correct pH should be maintained otherwise fastness will be inferior and colour will be unstable.

What is Heat Setting?
Heat setting of synthetic fabrics eliminates the internal tensions within the fiber generated during manufacture and the new state can be fixed by rapid cooling. This heat setting fixes the fabrics in the relaxed state and thus avoids subsequent shrinkage or creasing of fabric. Presetting of goods make it possible to use higher temperature for setting without considering the sublimation properties of dyes and also has a favorable effect on dyeing behavior and running properties of goods. On the other hand, post setting can be combined with some other operations such as thermosol dyeing or optical brightening of polyester, post setting as a final finish is useful to get a high dimensional stability along with desired handle.

The application of heat in heat setting can be done by hot air, on a pin stenter at 220c for 20-30 seconds for polyester goods and at a lower temperature range of 190-225C for 15 -20 seconds for polyamides . Acrylics may be heat set partially at 170-190 c for 15-60 seconds to reduce formation of running creases. but higher temperature should be avoided to prevent yellowing.

Hydro setting is so rarely used particularly to get fuller and softer handle on polyamides at 125-135c in autoclaves for 20-30 minutes. It can be combined with dyeing or optical brightening.

Steam setting can be done by saturated or super heated steam. During steaming, uniform treatment can be ensured by initial sequence of alternate short steaming and vacuum application for 20-30 min at 130C under pressure. Super heated steam can be used in stenters and setting time is 25% shorter than for hot air on account of quicker heating up rate. Acrylic fibers have to be protested as some may under go excessive shrinkage or loss of handle. Before the material is heat set, it should be thoroughly washed to remove spin preparations, lubricants, sizing agents and impurities as these are likely to be burned in drying heat setting making their removal difficult.

Method of the Dyeing Synthetic fibres with Disperse Dyes:There are three common method of dyeing with disperse dyes which are as follows:-
  1. Carrier method of dyeing.
  2. High temperature dyeing.
  3. The thermosol process of dyeing.
Carrier Dyeing Method:
Commercial (Trade name) Name of Carrier:
  • Tumescal –A.C.I.
  • Matexil –A.C.I.
  • Levagol –Bayer.
  • Dilatin –Sandoz.
  • Invalon –Ciba.
  • Hisogal –Hoechst.
Recipe:
Dye:
-For light shade<0.5%
-For medium shade 0.5-1.5%
-For deep shade >1.5%
  • Carrier(Phenol) : 3 gm/lit
  • Acetic Acid: 1 gm/lit
  • Dispersing Agent: 2gm/lit
  • Salt: 1-2 gm/lit
  • PH: 4-4.5
  • M:L: 1:10
  • Time: 60 min
  • Tempurature: 90°C
Dyeing sequence of polyester is given below:

Carrier and vessel washed by hydrose and caustic at 1000 C for 20 min

Load the package in the carrier and feed in the vessel

Add washing agent and run at 800 C for 20 min

Dyeing period

Add leveling agent and acid, run at 600 C for 10 min

PH check

Color mixing at 700 C for 40 min

Color dosing at 600 C for 20 min

Polyester dyeing at 1350 C at 20 gradient for 50 min

Sample check

Ok

Temperature cool down at 780 C and drain

Hot wash for 10 min

Rinse for 15 min for light shade and 25 min for dark shade

Add hydrose, soda ash, run at 800 C for 20 min

Drain

Rinse for 10 min

Neutralization by acetic acid at 500 C 20 min

Finishing


Procedure:
  1. At first, a paste of dye and dispersing agent is prepared and then water is added to it.
  2. Dye bath is kept at 60°C temperature and all the chemicals along with the material are added to it. Then the bath is kept for 15 min without raising the temperature.
  3. pH of bath is controlled by acetic acid at 4-5.5.
  4. Now temperature of dye bath is raised to 90°C and at that temperature the bath is kept for 60 min.
  5. Then temperature is lowered to 60°C and resist and reduction cleaning is done if required. Reduction cleaning is done only to improve the wash fastness.
  6. Material is again rinsed well after reduction cleaning and then dried.
Dyeing Curve:

Dyeing Curve
High Temperature Dyeing Method:
Pretreatment:
Pretreatment of polyester fabric is a must before starting the dyeing operation. The pretreatment is essential to remove the lubrication oils and other auxiliaries used during spinning and weaving or knitting operation. This following simple treatment is enough to remove those impurities.
  • Recipe:
  1. Lissopal D paste: 2 gm/lit
  2. Soda Ash: 2 gm/lit
  3. Treat with the above recipe at 90~95°C for 20 minutes.
  • Drain-Hot wash @ 70°C for 10 minutes> cold wash > Neutralize with 1cc/lit Acetic Acid.
  • Cheak the PH- 5.5-6.0
Dyeing Process:
Polyester Texties require a Heat Setting operation before dyeing. Heat settings eliminates the internal tensions within the fibre generated during manufacture and the new state can be fixed by rapid cooling. This heat settings fixed the fabrics in the relaxed state and thus avoids subsequent shrinkage or creasing of fabric.

Dye bath settings & Dyeing:

Recipe:
  • Lyogen DFT: 0.5 gm/lit
  • Sandozen PES: 1.0 gm/lit
  • Acetic Acid: 1 gm/lit
  • PH: 5.5-6.0
  • Temperature: 130°C
  • Time: 1 hr
Procedure:
  1. At first a paste of dye and dispersing agent is prepared and water is added to it.
  2. PH is controlled by adding acetic acid.
  3. This condition is kept for 15 minutes at temperature 60°C.
  4. Then the dye bath temperature is raised to 130°C and this temperature is maintained for 1 hour. Within this time, dye is diffused in dye bath, adsorbed by the fibre and thus required shade is obtained.
  5. The dye bath is cooled as early as possible after dyeing at 60°C.
  6. The fabric is hot rinsed and reduction cleaning is done if required.
  7. Then the fabric is finally rinsed and dried.
Dyeing Curve:
Dyeing Curve
Dyeing of Polyester Fabric in Thermasol Dyeing Method:
Thermasol dyeing method is continuous methods of dyeing with disperse dye. Here dyeing is performed at high temperature like 180-220°C in a close vessel. Here time of dyeing should be maintained very carefully to get required shade and to retain required fabric strength.

Recipe:
  • Dye: X gm/lit
  • Dispursing Agent: 2 gm/lit
  • Sodium Alginate Thickener: 5-10 gm/lit
  • Citric Acid to get PH: 4-5
Sequence:
Pading- Drying- Thermofixing- Aftertreatment

Procedure:
  1. At first the fabric is padded with dye solution using above recipe in a three bowl padding mangle.
  2. Then the fabric is dried at 100°C temperature in dryer. For dyeing, infra red drying method is an ideal method by which water is evaporated from fabric in vapor form. This eliminates the migration of dye particles.
  3. Then the fabric is passed through thermasol unit where thermo fixing is done at about 205°C temp for 60-90 seconds depending on type of fibre, dye and depth of shade. In thermasol process about 75-90% dye is fixed on fabric.
  4. After thermo fixing the unfixed dyes are washed off along with thickener and other chemicals by warm water.
  5. Then soap wash or reduction cleaning is done if required. And finally the fabric is washed . 
 
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