Introduction of Naphthol Dyes | Dyeing Procedure of Naphthol Dyes |Roles of Different Chemicals in Naphthol Dyeing

Naphthol dyes are insoluble azo dyestuffs that are produced on the fiber by applying a Naphthol to the fiber and then combining it with a diazotized base or salt at a low temperature to produce an insoluble dye molecule within the fiber. Naphthol dyes are classified as fast dyes, usually slightly cheaper than Vat dyeings; the methods of application are complex and the range of colors limited.

Azoic combinations are still the only class of dye that can produce very deep orange, red, scarlet and Bordeaux shades of excellent light and washing fastness.The pigments produced have bright colors, and include navies and blacks, but there are no greens or bright blues. Crocking fastness varies with shades but washing fastness is equal to Vat dyeings, generally with less light fastness than the Vats.

Naphthols :

The Naphthols are phenols, soluble in alkaline solution and substantive to cotton, particularly in the presence of salt. The anilides of BON acid(beta-oxynaphthoic acid or BON acid) are soluble in dilute NaOH solution and form the corresponding naphtholate ion. These relatively small molecules are of only low to moderate substantivity for cotton, but they diffuse rapidly into the fibres. In general, the higher the substantivity the better the rubbing fastness as less azo pigment forms on the fibre surfaces. The naphtholate ions are always coplanar and preferably have elongated molecular structures. They behave essentially as colorless, low molecular weight direct dyes. The substantivity increases with increase in the molecular size of the naphtholate ion, but the diffusion rate in the fibres and solubility in dilute aqueous alkali decrease. Addition of salt promotes better exhaustion of the bath, more being needed for Naphtols of lower substantivity.

Bases:

These are available as the free amine base or as amine salts such as the hydrochloride.Many of the amines used are simple substituted aniline derivatives with no ionic substituents. The so-called Fast Colour Bases require diazotisation. This usually involves reaction of the primary aromatic amine in acidic solution or dispersion with sodium nitrite, at or below room temperature. Successful diazotisation requires careful weighing of all the chemicals and regard for the supplier’s recommendations. Diazotisation of a primary aromatic amine is often difficult and solutions of diazonium ions are inherently unstable. They undergo decomposition even at low temperature and particularly on exposure to light. Storing prepared diazonium ion solutions is not usually possible.

General Dyeing Procedure of Naphthol Dyes
The application of the naphthols is consists of following steps,

1.Dissolution of the naphthol component.
2.Exhaustion of the naphthol dolution onto the substrate or absorption of the naphtholate ion by the cotton;
3.Removal of excess naphthol from the material by squeezing, partial hydroextraction or brine washing.
4.Diazotization of the base component.
5.Development or treatment with the diazonium ion solution to bring about coupling.
6. Neutralisation ,Soaping at the boil to remove superficial pigment, followed by rinsing and drying.

The process can be carried out in almost any type of dyeing machine determined by the form of the goods.

Dyeing Methods

Precautions in Naphthol Dyeing

1.The alkalinity of the naphthol bath shall not drop below the prescribed limit , otherwise the naphthol may presipitate.

2.Formaldehyde shall not be used when working at more than 50 Deg C or when the material is to be dried after naphthol application.

3.Material shall be protected from water spotting,steam,acid and chlorine fumes , and exposure to sunlight after naphthol application.

4.Use of excess salt in naphthol bath may result into precipitation of the bath.

5.The temperature is very important in base preparation stem , otherwise diazotization may not take place.

6.Sodium acetate must be added to the developing bath just before the use , otherwise base will become unstable due to fall in concentration of HCl.

7.Hydroextraction time must not be too long , which may result into light spots after development.

8.Material shall be rinsed without delay after developing , otherwise the mechanically held excess developing liquor will undergo some decomposition and cause deposition of dark colored spots , which will be difficult to remove.

9.It is important to use sufficient amount of alkali binding agents , otherwise it will result into presipitation of developing bath.

Stripping Process in Naphthol Dyeing
-Treat the dyed material with Non ionic detergent and 3-5 gpl caustic soda at boil for 15 min. cool to 85 degC
-Add 3-5% sod. hydrosulphite for 30-45 min at 85 deg.
-Rinse hot and cold
-Bleach with 1-2 Gpl Available chlorine for 20 min.
-Antichlore and neutralise.
-Soap at Boil for 15-20 min.
-Cold rinse.

Roles of Different Chemicals in Naphthol Dyeing

T.R. Oil :

Wetting agents for naphthol pasting and dissolution and penetrating agent in fiber in naphthol application.

Caustic Soda:

For solubilising of naphthols and keeping proper alkalinity of naphthol bath.

Formaldehyde:

Protective agent of naphthol impreganated material from effect of air.

Salt:

Electrolyte for exhaustion of naphthol during naphtholation and to prevent the desorption of naphthol in the bath during brine rinsing and development phase.

HCl Acid:

Dissolution of base and to produce nitrous acid in diazotization phase.

Sodium Nitrite:

Producing nitrous acid in diazotization process.

Sodium Acetate:

For neutralization of excess HCl in developing bath.

Acetic Acid:

As an alkali binding agent in developing bath.

Non Ionic Dispersing Agent:

To keep the azoic pigments in fine dispersion phase , which are formed by the coupling of free naphthol in developing bath. Also helps in better color fastness during soaping operation.

Fastness Properties on Cotton
Correctly prepared dyeings with azoic combinations on cotton have fastness properties often comparable, or only slightly inferior, to those produced using quinone vat dyes. They complement the vat dyes because of the wide range of orange, red and Bordeaux shades that they provide.

The fastness to washing of azoic combination dyeings on cotton is usually very good to excellent but only after careful elimination of particles of azo pigment loosely adhering to exposed fiber surfaces. Intermediate drying or rinsing of fabric containing the Naphtol, and the soaping of the final dyeing, are key processes ensuring optimum fastness. The same argument applies to rubbing fastness. Deep dyeing that have not been well soaped easily transfer color onto adjacent white fabric, even under conditions of gentle rubbing.

There are two other problems associated with the fastness properties of azoic combinations on cotton. In pale shades, the dyeings often have much reduced light fastness, particularly under humid conditions. Some sensitive azoic combinations also give dyeings of only fair resistance to chlorine and peroxide bleaching.

Dyeing Process of Jigger Dyeing Machine | Working Process of JiggerDyeing Machine

Jigger Dyeing Machine:

Jigg or jigger dyeing machine is one of the oldest dyeing machine used for cloth dyeing operations. Jigger machine is suitable for dyeing of woven fabrics, up to boiling temperature without any creasing . Jigs exert considerable lengthwise tension on the fabric and are more suitable for the dyeing of woven than knitted fabrics. Since the fabric is handled in open-width, a jig is very suitable for fabrics which crease when dyed in rope form.

Jigger Dyeing Machine

Some wovens are conveniently dyed on jigger are ,

• Taffettas
• Plain wovens
• Satins
• Poplins
• Ducks
• Suiting and Shirting material.
• Sheetings etc.

But have limited application on fabrics which are tension sensitive such as crepes , flat crepes , knits , net fabrics and elastomeric warps etc.
 
Machine Description
The jigger machines have two main rollers which revolve on smooth bearings and are attached to with a suitable driving mechanism , which can be reversed when required. The fabric is wound on one of the main rollers and fed from the other. The fabric move from one roller to the other through the dye liquor trough located at the lower part of the machine . There are various arrangement of guide rollers at the bottom of liquor trough ,and during each passage the cloth passes around these guide rollers .
 
The concentrated dye liquor is usually introduced directly into the dyebath in two equal portions, which are added just before commencing the first and second ends. The liquor is agitated by the movement of the fabric through the dyebath. Several horizontal spray pipes are fitted across the full width of the trough in order to expedite fabric rinsing.
 
Live steam injected into the bottom of the trough through a perforated pipe across the width of the jig heats the liquor. Some modern jigs also have heat exchangers for indirect heating.
 
Covering the top of the jig minimizes the heat loss to the atmosphere, keeps the temperature uniform on all parts of the fabric and minimizes exposure of the liquor and the cloth to air. Minimizing exposure to air is important when using sulphur or vat dyes since these dyes can be oxidized by atmospheric oxygen.
 
A few meters of leading fabric, similar in construction to the cloth under process, is stitched to each end of the cloth batch , to allow the entire length of the fabric to pass through the dye bath during the dyeing process. When jig processing is completed, the fabric is run onto an A-frame via a nip or suction device to remove extraneous water during unloading.
 
Modern machines such as automatic and jumbo jiggers have full automation in drive , tension regulation and control , fabric speed and metering, smooth and jerk less stop and start , counters for number of turns , gradual and noiseless reversal, automatic temperature regulation and control etc.
 
Dyeing Process by Jigger Dyeing Machine:

The dyeing process on jigger is regarded as a series of intermittent padding operation followed by dwelling periods on the main roller , during which the dyeing action and diffusion takes place. The factors controlling the rate of dye absorption are:

1. The amount of interstitial dye liquor retained in the interstices of the fabric weave.
2. The exhaustion of the interstitial liquor in the dwell period between successive immersions.
3. The degree of interchange of liquor during one immersion (interchange factor).

In the dyeing on jigger machines the cloth revolves on two main rollers , The open-width fabric passes from one roller through the dyebath at the bottom of the machine and then onto a driven take-up roller on the other side. When all the fabric has passed through the bath, the direction is reversed . Each passage is called an end. Dyeing always involves an even number of ends. The dye bath has one or more guide rollers , around which the cloth travels , and during this immersion achieves the desired contact with the dye liquor. During this passage the fabric picks up adequate quantity of dye liquor , excess of which is drained out but still a good quantity is held in the fabric . During rotation of rollers this dye penetrates and diffuse into the fabric. The real dyeing takes place not in the dye liquor but when the cloth is on the rollers, since only a very small length of fabric is in the dyebath and major part is on the rollers . Therefore the speed of cloth during immersion in dye liquor has a very little effect on percentage of shade produced.
 
Some critical problems related to the conventional jigger dyeing machines ( which are minimized in the modern day machines) The major problems are side-to-centre color variations, called listing, and lengthways color variations, called ending.
 
Other problems are

• Temperature control from side-to-side and end-to-end of the roll
• Tension control from end-to-end
• Constant speed control from end-to-end
• Prevention of creases
• Prevention of air

Limitations of Jigger Dyeing

1. Jigs exert considerable lengthwise tension on the fabric and are more suitable for the dyeing of woven than knitted fabrics.
2. In textile preparation due to the swelling and dissolution of size, which makes the fabric slippery and unstable in roll form.
3. The low liquor ratio makes washing-off difficult.
4. There is little mechanical action in a jig machine and it is less suitable where vigorous scouring is required before dyeing.
5. Moiré effects or water marks may arise on some acetate and nylon fabrics because of pressure flattening the structure of the rolled fabric.

Introduction of Direct Dyes | Classification of Direct Dye |Application of Direct Dyes | Aftertreatment of Direct Dye

Direct Dye:
Direct Dye is a class of dyestuffs that are applied directly to the substrate in a neutral or alkaline bath. They produce full shades on cotton and linen without mordanting and can also be applied to rayon, silk, and wool. Direct dyes give bright shades but exhibit poor washfastness. Various aftertreatments are used to improve the washfastness of direct dyes, and such dyes are referred to as “aftertreated direct colors.”Direct Dyes are molecules that adhere to the fabric molecules without help from other chemicals. Direct dyes are defined as anionic dyes with substantivity for cellulosic fibres, normally applied from an aqueous dyebath containing an electrolyte, either sodium chloride (NaCl) or sodium sulfate (Na2SO4)..

Direct Dyes 

The dyeing process with direct dyes is very simple, Direct dyeing is normally carried out in a neutral or slight alkaline dyebath, at or near boiling point , but a separate aftertreatment such as cationic dye fixing , to enhance wet fastness has been necessary for most direct dyeing .

Direct dyes are used on cotton, paper,leather, wool, silk and nylon. They are also used as pH indicators and as biological stains.

Chemicals Nature of Direct Dyes:

Chemically they are salts of complex sulfonic acids. 

Structure:-More than 75% of all direct dyes are unmetallised azo structures, great majority of them are disazo or polyazo types.

Ionic Nature:-Their ionic nature is anionic. 

Solubility:-They are soluble in water .

Affinity:-They have an affinity for a wide variety of fibers such as cotton ,viscose, silk jute ,linen etc.. They do not make any permanent chemical bond with the cellulosic fibers but are attached to it via very week hydrogen bonding as well as vander waals forces. Their flat shape and their length enable them to lie along-side cellulose fibers and maximize the Van-der-Waals, dipole and hydrogen bonds. 

Types of Direct Dyes:

The SDC classification of direct dyes is follows

(1) Class A – dyes that are self-levelling, i.e. dyes of good migration or leveling properties.

(2) Class B – dyes that are not self-levelling, but which can be controlled by addition of salt to give level results; they are described as salt-controllable.

(3) Class C – dyes that are not self-levelling and which are highly sensitive to salt, the exhaustion of these dyes cannot adequately be controlled by addition of salt alone and they require additional control by temperature; they are described as temperature-controllable.

Application of Direct Dyes

Direct dyes are usually applied with the addition of electrolyte at or near the boil in the machines capable of running at atmospheric pressure .But in HTHP dyeing machines it is carried out at temperatures above the boil in case of pure as well as blended yarns. 

An addition of alkali, usually sodium carbonate, may be made with acid-sensitive direct dyes and with hard water as well as to enhance the dye solubilisation. When cellulose is immersed in a solution of a direct dye it absorbs dye from the solution until equilibrium is attained, and at this stage most of the dye is taken up by the fibre. The rate of absorption and equilibrium exhaustion vary from dye to dye. The substantivity of the dye for cellulose is the proportion of the dye absorbed by the fibre compared with that remaining in the dyebath.

Dyeing Method

The color is pasted well and dissolved in boiling water to get a lump free solution .An addition of 0.5–2 g l–1 sodium carbonate may be advantageous when applying dyes of only moderate solubility in full depths.

• The dyebath is set at 40°C,
• Raise to the boil at 2 degC min–1
• Hold at the boil for 30–45 min, 
• During hold add 10–15 g l–1 of sodium chloride or calcined Glauber’s salt. Light shades are dyed without or lesser addition of salt.
• Improved yields can be achieved when applying full depths by cooling to 80°C at the end of the period at the boil, adding a further 5 g l–1 salt and rising to the boil again
• Dye bath variables which must be considered for level dyeing,

1.Temperature of Dyeing and rate of heating

2.Electrolyte concentration and addition

3.Time

4.Dye solubility

5.Use of leveling agent 

After Treatment of Direct Dyed Material

The wet fastness properties (particularly washing, water and perspiration) of virtually all dyeing of direct dyes are inadequate for many end uses but notable improvements can be brought about by after treatments.

• Diazotisation and development 
• Metal salt treatments 
• Cationic fixing agents 
• Formaldehyde treatment 
• Cross linking agents and resin treatments 

Stripping:- Most direct dyes can be stripped of the use of stripping salts (Sodium Hydrosulphite) and/or by using a chlorine bleaching agent such as sodium hypochlorite, without harmful effects on the fibres.

Color fastness properties of Direct Dyed material: Generally these dyes are used where high wash fastness is not required.

Wash Fastness: Poor unless treated with suitable dye fixing agent and/or fastness improving finishing agent.

Light Fastness:-Good

Rubbing Fastness: Moderate to Good

Chemical Wash Fastness:- Poor