Pretreatment Processes of dyeing

 Dyeing Pretreatment :

Pretreatment processes of dyeing consist of cleaning operations to rid the fabric of all soil and additives that have accumulated during the weaving or knitting process. These processes are usually the first treatments a fabric undergoes after leaving the loom or knitting machine, and are required before any dyeing, printing, or finishing can be accomplished. In the strict definition of finishing, pretreatment processes do not quality as textile finishes because they are performed prior to dyeing or printing

Grey goods contain warp starches or other sizing (to add stiffness and strength to warp yarns during weaving), as well as oils, waxes, and other lubricants, plus floor dirt or other soils picked up during processing. Complete removal of all these impurities is necessary before any dyeing; printing, or finishing can be done. The processes for cleaning are varied depending on the fiber, the impurities present, and the fabric construction. In cotton, cotton-blend, silk, and manufactured-fiber fabrics, the processes are generally known as the boil-off. In woolens and worsteds, it is called a scour or scouring. Boil-off or scouring is similar to ordinary laundering. Fabrics are treated with soaps or detergents, rinsed, and then dried. If warp starches are present, the fabric is treated with an additional process known as desizing. Enzyme solutions are used to dissolve the starch.

Woolen and worsted fabrics may undergo an additional pretreatment process known as carbonizing. This process removes leaf particles and bits of grass and other cellulosic impurities that become embedded in the wool white sheep are grazing. The treatment consists of steeping the wool fabric in sulfuric acid, which destroys the cellulose but leaves the wool unharmed.

Another dyeing pretreatment process is an operation called singeing, which involves burning off projecting fibers or filament splinters from the surface of a fabric. Improper singeing or elimination of this operation results in unclear print patterns, mottled fabric surface, or premature pilling of fabrics. . Singeing is accomplished by passing gray goods rapidly over gas flames, usually two burners to a side, at a speed of 100 to 250 yards per minute. Improper singeing or elimination of this operation results in unclear print patterns, mottled fabric surface, or premature pilling of fabrics.

Dyeing pretreatment processes may also include bleaching. Fabrics to be dyed in light to medium shades, as well as most prints, are first bleached. Bleaches are required to obtain pure whites because natural fibers are rarely pure white in their natural state; they are usually slightly yellowish or grayish. Bleaches are chemical agents that react with the color compounds in the fiber and render them colorless. Our eyes see the fabric as pure white.

The effectiveness of optical brighteners after dyeing is dependent on an ultraviolet source of light. Natural sunlight is such a source. Fluorescent lighting is also, but to a small degree. Ultraviolet lamps (also called black-light lamps) are another source. Incandescent lighting (regular household bulbs) contains no ultraviolet, so optical brighteners have no effect on fabrics used under this lighting.

The Dyeing Process

The science of dyeing is highly complex, and the mechanisms of dyeing are not completely understood. This discussion about dyeing is necessarily superficial and does not provide an in-depth exploration of the subject. The medium most often used to dissolve or disperse dyes for application to textiles is water. The dye solution, called the dye liquor, is agitated or circulated to increase the migration of the dye to the fiber surface. The attraction or affinity of the dye for a particular fiber is influenced by several factors. Different dye types are attracted chemically or physically to specific fibers.

Fibers often undergo swelling in aqueous dyeing processes, increasing dye absorption. For hydrophobic fibers that do not swell in water, organic solvents can be used as swelling agents or “carriers” for the dye. Mordants can be added to increase the acceptance of dyes. Dyebath additives such as these are classed as dye auxiliaries. Surface features such as the scales on wool or waxes or finishes on fibers can inhibit attraction of the dye to the fiber. Careful preparation of fabrics for dyeing minimizes this effect by removing waxes and sizes. Further, application of aesthetic and functional finishes usually follows the dyeing process, so that the dyeing finishes do not compete for dye sites on the fiber.

Once dyes are attracted to the fiber, they should diffuse through it rather than remaining adsorbed on the surface. Dyes are absorbed into the fiber predominantly in the amorphous areas, and making these areas larger or more accessible enhances the dyeing process. Mercerization of cotton, which moves the polymer chains farther apart, allows more dye in. Higher temperatures, around the glass transition phase of thermoplastic fibers, cause some of the crystalline areas to move and separate more, increasing dye accessibility. Nylon fibers have a low glass transition temperature and can often pick up dyes from other fibers.

Exhaustion is the amount of transfer of dye from the dye bath to the fiber, either by adsorption or absorption of dyes. It is possible for all the dye to be removed from the dyeing bath and for the dye bath to be clear at the end of the dyeing process. Some dye classes exhaust better than others. It is not always prudent or even necessary, to dye to complete exhaustion of the bath.

A concern in dyeing is how evenly distributed the dye is in the fiber, a characteristic known as levelness. Low leveling will result in a streaked appearance of the fabric.
Dyes may be only adsorbed on the fiber surface, or dyes that have been absorbed by fibers may migrate back to the surface when the textile is dried, resulting in uneven dyeing. Leveling agents and retarders are dye-bath auxiliaries than can promote even dye absorption.

Once the dye has entered the amorphous regions of fiber, it must be retained within the fiber not only during the dyeing but also when fabrics are laundered or dry cleaned during use. Several factors make for dye retention. The nature of the dyeing chemical bonds and physical attraction between fiber and dye is a crucial element. Dyes may be physically bound to fibers by forces such as hydrogen bonding or ionic forces. Ionic forces are the attraction between positively and negatively charged ions, one on the dye, and the other on the fiber. The chemical structure of dye and fiber may allow the formation of covalent chemical bonds between fiber and dye. Dyes that are absorbed into swollen fibers can be physically trapped when the fibers are cooled and dried. Dyes that are soluble in water, which may also swell some fibers, can be removed when the textile is wetted during dyeing.


Dyeing methods of Direct dyes

Direct dye is populae and conventional dye. There are some popular dyeing method of direct dyeing with suitable dyeing machine those are describeb bellow:
I. Batch method: With batch dyeing, the dyeing method to be selected in each case depends on the type of dyeing equipment (loading system, winch vat, jig, paddle or jet), the nature of the material to be dyed, as well as the solubility and the affinity of the dye. Before the actual dyeing process, the material is pre-treated with a wetting agent. The dye is mixed into a paste with some warm water, and is then diluted with more water, boiled up, filtered, and then added to the dyebath.

Whilst the bath, with the fabric, heats up to the optimum temperature (usually 80–90°C), the electrolyte is added, gradually if necessary. Dyeing takes 30–60 minutes. After the dyebath has been run off, the dyed material is briefly rinsed with cold water and, in general, subjected to after-treatment. The dyeing process is divided into 3 phases: a) uptake (adsorption) through substantivity; b) penetration of the dye into the fibre (diffusion); c) bonding to the fibre (immobilisation) through van der Waals interactions. Assurance of level take-up through progessive addition of salt, temperature control, sufficiently long dyeing time, and use of levelling agents.

II. High-temperature dyeing method:
With suitable dyes, one can work in a closed system at temperatures over 100°C (up to approx. 130°). Due to the rapid diffusion rate, particularly level dyeings are achieved with short dyeing times, even with fabric with difficult dye penetration. After the high-temperature phase, the dyebath is cooled down to 80–90°C, with dye pick-up continuing, and the result is the same depth of colour as in the normal dyeing method at 80–90°C.

III. Continuous and semi-continuous method: With these so-called pad methods, the dyeing material, mostly in the form of woven fabric, is first of all steeped in a concentrated dye solution, passed full-width through a trough filled with the dye solution, and subsequently the excess liquor is removed between the rubber rollers. With high fabric speed, temperatures as low as possible (30–40°C for light shades, 60–80°C for medium to dark shades), and with the minimum amount of pad liquor, the situation can be avoided where the dye already picks up substantively in the padding process. Distinction may be made between the following methods:

1. Pad-jig method: Pad, salt bath develop on jig or winch vat.

2. Pad-roll method: Pad, heat up in an IR zone, roll up and rotate in a dwell chamber for a considerable time under fixation temperature/moisture conditions.

3. Pad-salt method: Pad, pass through salt solution at boil in continuous piece-dyeing machine (light shades only; for darker shades an intermediate steam process is necessary).

4. Pad-steam method: Pad, continuous steam, optional final salt bath.

5. HT-steam method: Pad, HT-steam.
After-treatment: The wetfastness properties of direct dyeings are not adequate for the demands of everyday use, particularly in medium and deep shades.

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Direct dyes

Direct dyes are very conventional dyes. It is not used any more for industrial dyeing. Direct dye is one kind of anionic dye and having substantively for all types of cellulosic fibre like cotton, jute, hemp. They have strong affinity to the fibre which can be applied directly on the fibre surface and do not need any assistance.

Direct dye Also known as direct, substantive, benzidine or salt dyes. Group classification for dye which exhaust substantively on cellulosic fibres, cotton, viscose, cupro and high wet modulus fibres, especially in the presence of salt. Chemically they are polyazo or sulphonated disazo dye.It contains sulphonic acid group that’s why is has good solubility in water. The dye molecule contains a complex bond metal atom that’s why they have high light fastness properties. The color fastness is poor but it is easy to improve by dyeing after treatment with metal salts (Õ Aftercoppering dyes).

Benefit of Direct dye:

Direct dyes represent the main class of dyes for cellulosic fibres in terms of usage:
- Specially selected direct dyes are available for streaky dyeing viscose qualities.
- They are simple to apply.
- Economical or cost effective than any other dyes.
- It has generally good leveling properties
- A wide range of color variation and shade.
- Special types contain better wet fastness. It is possible to couple with readily soluble diazo compound by after treatment.
-Direct dyeing on cellulosic fibres exhibit considerable differences in colour fastness to light which ranges between 1 and 7–8.

Different direct dyes can therefore be used for articles which are only required to have low light fastness as well as goods with high light fastness. In general, direct dyes have poor wet fastness properties and dyeing must be after treated to improve their serviceability.

Properties of direct dye:
- Direct dye has sodium salt of sulphonic acid or carboxylic acid group. So it easily dissolve in water.
- Cheap comparative
- It has strong affinity to cellulose fibre. It is possible to dye protein fibre.
- Easily disperse in water and diffusible in to fibre.
- Wash fastness is not good. The range vary from 2 to 3
- It is possible to use in neutral and alkaline medium
- The dye is common in the practical point of view.
- The tin tropical power of this dye is very good.

Some trade and brand name of direct dyes
Benzo, Benzoform       - F.Bayer (Germany)
Coprantine                  - Ciba-geigy (Switzerland)
Solphenyl                    - Ciba-geigy (Switzerland)
Chlorazol Durazol        - Impartial Chemical Industry (ICI)
Solar                          - Sandoz A.G (Switzerland)

There are some related posts to get deep knowledge of direct dye:

# Classification of Direct dye
# How to dye with direct dyes
# Dyeing aftertreatment of direct dye
# Chemistry of dyeing related to direct dye
# Dyeing auxiliaries required for direct dye

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State the problems occurred in dyeing due to water hardness

Problem occurring in dyeing due to hardness:

Potential dangers of textile items:
a) Potential dangers: 
1) Reduced depth of shade: Dyeing shade is a vital point of industrial coloration process. Shade of a color is depend upon the choromophore or auxochoromophore groups of dyes. The material reserved in hard water can change the choromophore complex ions. In that case it is not possible to get desire shade percentage.

2) Dulling of shade: This is a big problem in dyeing. Water hardness change the activities of dyes chemical that’s why dyeing machine could not properly dull the shade.

3) Unleveled dyeing: It is one of the common and major problem in dye house. It means the variation of color shade in same amount of fabric or other textile material which are dyed in same dyeing bath and batch. Water hardness is one of the main issue of uneven dyeing.

4) Poor shade reproducibility: Shade reproducibility is not a usual operation on dyeing industry. It used to fix the shade according to buyer requirement if it is not up to slandered. It means refit the dyed fabric according to the required shade due to achieve the proper shade. Usually uneven dyed fabric need to shade reproduce. To reduce the shade stripping is required but increase the shade, usually need to re-treatment with dyes and chemicals.

5) Damaged fiber: Degree of hydrogen (PH) is important issue in dye liquor. Hard water contains organic salt that’s why it can change the acidity or alkality of dye and auxiliary solution. On the other hand every fibre have a tolerant limit of acid alkali. As the hard water change the PH of dye solution so the solution could be harmed the fibre. I we rework with dyed material due to dull or unleveled shade, it could easily damage the textile fibre due to more chemical expose.

b) Problem in boiler:
-Scale formation: Hard water makes scales inside the wall and tubes of boiler. It create a layer of organic salt and it is difficult remove the scale inside the boiler.

-Heat loss: It also the cause of heat loss inside boiler.

c) Wastage of soap: Hard water contains the organic salt and oxides ions. Those ions react with soap and reduce the working capability soap and need more soap to maintain the exact ratio during scouring and bleaching.

d) Problem in machine: It makes corrosion of boiler and looses its life time.

e) Problem in processing:
Desizing, Scouring, Dyeing, Printing, Finishing

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Bleaching used for:

Destruction of color impurities for
1. producing base white for subsequent dyeing
2. Producing permanent white

But it may not always be necessary:
1. For synthetic fiber
2. For deep shade
3. In some cases it may require partial bleach.

Parameter for H2O2 Bleaching:
1. Concentration of H2O2
2. Concentration of alkali
3. pH
4. Temperature
5. Time
6. Nature & quality of goods
7. Water hardness & other impurities
8. Types & concentration of auxiliaries
9. Available machine & stabilizer system employed
10. Desired bleaching effect

Status nascendi: In which point where active oxygen will get.

pH < 10 practically no HOO-
pH 10-11 Low to moderate HOO-
pH > 11 Moderate to high HOO-

Major bleaching agent:

Oxidative bleaches:
1. Hydrogen peroxide H2O2
2. Persulfate (NH4)2 S2O8, K2S2O8, Na2S2O8
3. Peracetic acid CH3CO.O.OH
4. Sodium hypochloride, NaOCl
5. Sodium hypochloride, available in bleaching powder with approximate formula 3Ca(OCL)Cl.Ca(OH)2.5H2O
6. Sodium chloride NaClO2

Reactive bleaches:
1. Sulfur dioxide SO2
2. Sodium hyposulfite NaS2O4

Advantages of H2O2 or why called universal bleaching agent

1. Most widely used for the natural cellulosic fibers( cotton, flax, Linen, jute etc) as well as the protein fibers (wool, Silk etc) regenerated cellulose fiber &natural fiber blends.

2. Extremely versatile bleaching agent, applicable over a very wide range of bleaching temperature , time & with wide range machineries.
3. Environmentally friendly
4. Decomposition products are oxygen & water
5. Compatible with most dyes & optical brightening agent (OBA)
6. Excellent storage stability
7. Produces a stable white fiber with good absorbency
8. Allow saving by combining stages such as desizing &scouring of over 50% in labour, water & energy cost.
9. Small amounts of impurities can acts as stabilizer.
10. Give versatile processing (batch\ continuous, hot\cold, rapid\ long dwell).

pH < 10 – no HOO-
pH - 10-11 -- Low to moderate HOO-
pH >11 -- Moderate to high HOO-

HOO- is very important , without it bleaching is not possible,
Commercially we get H2O2 in %
35% (W/W) weight/weight
50% {W/W}
70% (W/W) – conc. Available active oxygen

Hypochloride bleaching agent:
Calcium hypochlorite or sodium hypochlorite are used a hypochlorite bleaching agent. Calcium hypochlorite\rite has cheap source of bleaching powder. It is cheap but unstable. NaOCl is stable & soluble.

Ca(OCl)2 + H2O ---- Ca (OH)2 + HOCl
Hypochlorous acid
NaOCl + H2O ------- NaOH + HOCl
HOCl ----- H+ + OCL-
Hypochlorous ion
OCl - + OH -- Cellulose ----------- White cotton

pH is need about 10-11

Most disadvantages:

(AOX) – Absorbable Organic Halogen
If AOX is more pollution is more.

Calcium hypochlorite forms CaCO3 with CO2 & water. To remove this acid is used. Which is known as scouring.

Ca(OCl)2 + H2O +CO2 --- CaCO3 + HOCl
CaCO3 + 2HCl ---- CaCl2 + H2O + CO2
CaCO3 + H2SO4 --- CaSO4 + CO2 + H2O

As CaCl2 is more soluble in water then CaSO4. HCl is preferable.

Antichlor treatment :
Antichlor, process is required
To destroying active chlorine bleach liquor
To remove chloramines from textile

R – NH2 + NaOCl ---- R – NHCl + NaOH

Disadvantage of chloramines:
Corrosive & un hygienic yellowing of fiber
To remove the chloramines & Cl2 fabric is treated with reduce in agent for polyamide & peroxide for cellulosic fibers. This process of removal known as Antichlor treatment.

NaOCl + H2O2 -------- NaCl + H2O + O2
R – NHCl + H2O2 --- R – NH2 + HCl + O2

Test for bleaching:

1. Whiteness Index (WI) : AATCC – 110
2. Physical or chemical damage :

Fluidity test

1. Whiteness is get by reflectance value 72-80 acceptable for cotton

The reflectance values of well bleached samples are:
Cotton – polyester/cotton 80 – 90 %
Wool 50 – 60 %
Linen 60 – 80 %

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Fibre dyeing machine

Machine for Continuous loose cotton fibre dyeing:

-When large amount of loose cotton fibre is to be dyed in standared dyeing shade, continuous dyeing may be carried out.
-The output of the machine can be can be as high as 2000lb per hour
-Vat and sulpher dyes are dyed on cotton fibre in theis dyeing machine.
-The cleaning of the performed drum and the conveyors takes about a weeks time.

Working principle:
-In the machine the dye liquor is kept stationary while the cotton is moved in the liquor

-Cotton fibre are feed in the form of laps, 2 inch thick and 36 inch wide, which are uniformly packed. The lap is passed through a squeeze roller are feed into the machine by conveyor belt.

-The method of circulation of dye liquor through the conveyor and the cotton lap is shown as figure. A perforated cast iron cylinder(A) with closed end plates is made to rotate at the conveyor speed. Its axis is provided by a stationary hollow pipe(B). which is connected to a suction pipe(C). Two conveyors E1 and E2. Both being continuous are made to move contact with each other between two sets of squeezing rollers F1 and F2 with suitable guide rollers G1 and G2. Owing to the cut off (D), the pump only sucks through the lower half of the cylinder.

-There are five units, wetting, dyeing, cold wash, hot wash, drying with this type of cylinder and every unit has a definite liquor.

-Every unit has may has a guide roller, two sets squeezing rollers. Five such unit may be connected in series, with one common conveyer as the lower conveyer and five individual upper conveyor.

-Different options are carried out in this those five units and finally the lap is fed to crightom opener, where the lap is disintegrated into loose fibre.