Introduction:

Dyeing of natural fibers such as cotton, wool, silk etc, which are hydrophilic in nature, are done by direct, acid, vat, sulphur etc. Dyes which are water soluble or made soluble by reduction. When hydrophobic fibers made their appearance, soon after first world war, faced a problem in dyeing as the ─OH group has been blocked by ─COOCH3 group. Therefore affinity for dyes has been checked. Scientists took attemps of creating new dyes and developed acetate dyes or disperse dye. The term disperse dyes means free from ionizing groups, low water solubility and are suitable for dyeing hydrophobic fibers from colloidal dispersion.

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), Therm sol dyeing, 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 (on figure-01).

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

Figure-01

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 (on figure-02).

Dye(s) ↔ Dye (aq) ↔ Dye(fiber)

Figure-02

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 :

01.Low energy.

02.Medium energy.

03.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 –

• 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.
• Fastness to washing: The fabric dyes with disperse dyes shows moderate to good washing fastness.
• Light Fastness: Most of the disperse are very fast to washing. The minimum light fastness rating is 4-5.
• Sublime ability: Due to stable electronic arrangement disperse dyes have good sublime ability.
• 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: ####

1. Setamol -BASF
2. Edalon -Sandoz.
3. Calsolene Oil HS –A.C.I.
4. Hipogal –Hoechst.

Commercial (Trade name) Name of Carrier:

1. Tumescal –A.C.I.
2. Matexil –A.C.I.
3. Levagol –Bayer.
4. Dilatin –Sandoz.
5. Invalon –Ciba.
6. Hisogal –Hoechst.

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.

1. Carrier method of dyeing:

Polyester shrinks about 7% in boiling water and even more at higher temperature. To avoid this it is heat set. As a general rule it can be stated that a material (Synthetic) will be dimensionally stable if set at a temperature 30°-40°C higher than that to which it will been subjected during use. Fabrics are usually heat-set on pin stenters over temperature ranges 180°C and 200°C but garments can be set in steam autoclave machines at steam pressures between 20-25 pcs (pound per square inch). Terylene is highly crystalline and highly hydrophobic. Therefore, dyes with large molecules can not easily penetrate in to this fibre. It has no chemically active group and can not combine with dye anions and cat ions. In practice, polyester fibres are dyed with disperse dye. A considerable advance in the dyeing of polyester fibres was made when the carrier method was introduced. It was discovered that quite a number of organic compounds such as phenols, amines or aromatic hydrocarbons, when either dissolved or suspended in the dye bath, accelerated the absorption of disperse dyes by the fibre. The way in which carriers produce the effect is not clearly understood but they do cause some swelling of the fibre. It seems that they can enter into the fine structure of the polyester and push adjacent long-chain molecules apart. This loosens up the molecular pattern and facilitates the entry of the large dyestuff molecules. The water insoluble carriers also appear to from a surface film on the fibre in which the disperse dye is highly soluble. The transfer of dye, in such circumstances, does not take place between the aqueous phase and the fibre but between dyestuff dissolved in the carrier and fibre. But it is found that there is a maximum concentration of the carrier above which the take-up of the dye by the fibre decreases. This optimum carrier concentration corresponds approximately with the amount necessary to saturate both fibre and dye bath phase of the system. Excess will introduce a third phase, namely undissolved carrier, which will compete with the fibre for the dye. Two carriers which have proved success are diphenyl (Matexil CA-DP) and O-Pheny Pheny (Matexil CA-OPE). Diphenyl is a cream-coloured powder, insoluble in water, but in a readily- dispersable stat. Mateil CA-DP is a disphenyl carrier supplied in self-emulsifiable flakes. The emulsion i9s prepared by stirring the flakes into water at 80°C or hooter than this if desired and added to the dye bath at 60°C. The recommended concentration of carrier in the dye bath is 4 to 6 parts per liter. Matexil CA-OPE shouldbe used in the dye bath as 7.5 parts per litter and is added directly to the dye bath at 50°C t o 60°C. The addition of dispersing agent should precede that of Matexil CA-OPE. There are many other carriers which can be used in the carrier dyeing methods. A typical recipe of carrier dyeing is given bellow:

Recipe:

• Dyestuff – 3% (20-30g/liter) on the weight of material
• Carrier – 15g/liter
• Dispersing Agent – 2 to 3g/liter
• Acetic Acid – 5cc/liter
• Material liquor Ratio – 1:20
• Temp – Boil
• Time – 1hour to 1.5 hour

The dye bath is made up with 1/2 - 2 kg (.5 – 1g/liter) of an anionic surface active agent and 3-4kg carrier per 1000 liters. The temperature should be 40°C and after dye has been added, the liquor is slowly brought up to 100°C. After 15 minutes at this temp 5cc acetic acid is added. Boiling is continued for a further 1 hr after the addition of acetic acid. After dyeing the goods are washed out with a detergent and some NaOH to ensure the complete removal of carrier.

2. High temperature dyeing:

There are many advantages associated with dyeing polyester at temperature range between 120°C and 130°C. Heavy shade can be dyed pleated because of the permanency of the crease so formed. When dyeing at atmospheric pressure, only dyes of low molecular weight which tends to sublime during pleating can be used, the more satisfactory colours with higher molecular weights can be applied at 120°C -130°C. There is no perceptible loss of elasticity or tensile strength when polyester fibres are dyed under neutral or slightly acid conditions at 130°C. But if any alkali is used there is degradation in strength and elasticity. Any alkali used in scouring must be removed entirely before high temperature dyeing. The fibre should be heat-set before dyeing.

The following disperse dyes are recommended for high temperature dyeing: C.I Disperse Yellow 1, 39 C.I Disperse Orange 13 C.I Disperse Red 11, 13 C.I Disperse Violet 26 C.I Disperse Blue 26 The dye bath is made up with dyestuff, dispersing agent (Matexil DN-VI) or some similar product which is stable up to 130°C.

The dyeing should be started at 70°C, the temperature raised slowly to 120°C-130°C and maintained for a period of 30-60 minutes. When very heavy shades have been dyed it may be necessary to give a “Reduction Clearing” to avoid lack of fastness to rubbing. The goods are treated for 20 minutes at 45°C-50°C with 6kg NaOH(68), 2kg Na2S2O4 and 2kg Matexil Sc-A50 per 1000kg of water. Owing to the hydrophobic nature of polyester fibre only, surface dye will be reduced and the chemicals will not penetrate to react with the absorbed colour. After high temperature dyeing the goods should always have a final wash-off at 70°C for 15-20 minutes with suitable detergent.

#### 3. The thermosol process of dyeing: #### This dyeing process is not suitable for garments dyeing, Because of it is Unhygienic.

The Problem of Dyeing Polyester:

Polyester fibres are essentially undyeable bellow 70-80C, leaving only a 20-30C range for increasing the dyeing rate before recharging the boiling temperature. At any temperature, the rate of dyeing of polyester with a given disperse dye is very much lower than for cellulose acetate or nylon fibres.

The rate of diffusion of disperse dyes into the polyester bellow 100C is so low than that dyeing at the boil does not give reasonable exhaustion.

The rate of dyeing is higher for dyes of small molecular size that have higher diffusion coefficients. Dyeing is faster when using fibre swelling agent called carriers to improve the fibre accessibility, or when dyeing at higher temperatures above 100C increase the dye diffusion rate. Fibres of the most common polyester, polyethylene terephthalate (PET or PES), are quite crystalline and very hydrophobic. Hot water does not swell them and large dye molecules do not easily penetrate into the fibre interior. Polyesters have no ionic groups and are dyed almost exclusively with disperse dye. The better diffusion at the boil of low molecular weight dyes results in moderate migration during dyeing dyeing but then the washing fastness is only fair. Many of the more recent disperse dyes are specifically for dyeing polyester. These are of higher molecular weight to provide adequate fastness to sublimation during heat treatments. Some of these produce a reasonable depth of shade by dyeing at the boil. Most, however, require higher dyeing temperatures or carriers for satisfactory results. Dyeing of polyester with disperse dyes have good light fastness. This does not always correlate with the light fastness on other fibres such as cellulose diacetate.

The disperse dyes provide a full range of colours with adequate to good build-up on PET fibres. Uneven filament texturising or heat setting can lead to barre but higher dyeing temperatures, or addition of some carrier, will promote migration to minimize this, Again, a full black requires aftertreatment of the dyeing by diazotization of an amino disperse dye and cupling with a suitable component, often BON acid. Concurrent dyeing with a mixture of the amino disperses dye and dispersed BON acid, followed by treatment with sodium nitrate and hydrochloric acid, is a common procedure. Some blacks are mixtures of dull yellow, red and blue dyes.