Dyestuff Chemistry

Direct Dyes

Introduction

Direct dyes are mainly applied on Cellulosic fibers

Few examples are cotton, viscose rayon Easily applied on cellulosic fibers

They can be directly died from simple solutions in water

That’s why these dyes are called direct dyes They have an affinity for cellulose so can

also be known as substantive dyes

Congo red was the first direct dye which was discovered in 1894

Introduction of reactive dyes was welcomed because its was not expensive to implement.

Direct dyes are marketed under different brand names by different dyestuff manufactures.

Properties of a solution

direct dyes are water soluble They ionize in water That give dye anions (negative ions) and

sodium cations (positive ions) Dyes are manufactured as sulphonic acids

and are converted into their sodium salts

Sulphonic acids are less soluble in water and have less affinity for cellulose than their sodium salts.

Small amount of soda ash is added to the solution to convert sulphonic acid to convert into sodium salt.

Reason to convert sulphonic acid into salt is that then some of the dye will be wasted during their application to the textiles

The dye will not be fixed properly on the fiber

Chemical Structure

DYE SO3H

Direct dyes attaches themselves to the fiber through the formation of large number of weak attractions

These consist of hydrogen bonds and van der waals forces

Cellulose is not ionic in nature so ionic bonds that exist between acid dyes and wool are not possible between direct dyes and cellulose.

Classification of direct dyes

Depending on the effect of salt and temperature on the dyeing , direct dyes are classified into three groups

Group A Group B Group C

Group A Migrate well Have high leveling power When dyeing these dyes, dyeing can be

uneven first but continued dyeing levels the shade

These do not need salt for exhaustion

Group B Poor leveling properties ( not self leveling

dyes) Need controlled addition of salt for the

exhaustion Also known as salt controllable dyes

If these are not taken up uniformly by the fiber in initial stage of dyeing then its very difficult to even out the shade

Group C These are temperature controllable dyes Have poor leveling power ( not self- leveling

dyes) These are highly sensitive to salt

Exhaustion cannot be controlled by the addition of salt alone

Need temperature control

Application of Direct dyes

They can applied by two methods Exhaust method Continuous method

Dyeing Method for Cotton and other Cellulosics

Exhaust Application Application of Azonine and Durantine Dyes to Cellulosic

fiber Azonine dyes are an economical range of direct dyes with

good color values for users where specific fastness properties are not the prime requirement.

Durantine dyes may be used to dye most cellulosic fibres and its blend by exhaust, continuous and printing techniques.

At A add Azonine or Durantine Dye (predissolved)0.1 to 0.5g/l Neutrasol CR (for shades difficult to level)

At B add y g/l Glaubers salt (anhydrous)(1/5th of total amount required).

At C add z g/l Glaubers salt (anhydrous)(4/5th of total amount required).

At D rinse twice cold. Thoroughly rinse after dyeing to remove loose colour

All direct dyes perform rather poorly with respect to wash fastness. Without an appropriate after-treatment, direct dyes bleed a little with every washing, losing their brightness and endangering other clothes washed in the same load. However, there are special after-treatments which may be used to solve this problem

Dyeability of cellulosic fibers

The addition of electrolyte to a solution of direct dye tends to lowers the electrostatic repulsion between the negatively charged dye anions and promote aggregation or exhaustion.

The effect of the presence of an inorganic salt when dyeing cellulosic is to overcome the long-range forces of repulsion between the dye anions and the negatively charged fiber surface

The closer approach then allows the hydrogen bonding and other short-range attractive forces to operate between the dye molecules and the glucoside units of the fibrous polymer

Hydrogen bonding that forms between the hydroxyl groups of cellulose and centers of electronegativity ( nitrogen, oxygen and sulphur atoms) in the dye molecule.

The most hydrogen bonds that a dye can form with the glucosidic polymer, the more readily it can compete with and rupture the fiber-fiber hydrogen bonds in order to penetrate more deeply into the amorphous structure of the polymer

Especially those substituted with hydrogen atoms such as (=N-NH-, NH2, -OH,)

This is widely acknowledged as it contributes to adsorption and the retention of dye molecule.

Many direct dyes with high affinity for cellulose are disazo or triazo azo structures

The molecules of almost all direct dyes possess flexible chains of aryl nuclei linked by azo or other groups

Such structures can readily adopt a non-linear spatial conformation

Ability to cover neps in cotton fabric

Pale flecks in dyed cotton fabrics can be caused by immature or dead cotton

This causes poor dyeability Poor dye penetration may leave undyed

areas if the neps of loosely attached immature fiber changes position after dyeing

Direct dyes containing more than one amino or amide group in their structure were found most likely to achieve relatively good coverage of neps.

Dye enters cellulose as single molecules and then aggregates inside the cellulose

A decisive factor in determining the rate of dyeing of many direct dyes from aqueous salt solutions is their tendency to aggregate

Application of direct dyes

Direct dyes can be applied to cellulosic fibers by the following methods

Batchwise Semi and fully continuous methods

Batchwise method

Direct dyes are water soluble dyes A solution can be made by adding them into

cold water and then put hot water while stirring

A wetting agent should be added to the dye bath to assist penetration and level dyeing

They are normally applied at the boil Dye-bath is set at 40C and the temperature

is raised 2C per min and maintained at the boil for 35-45 minutes

During which salt is added according to the recipe

Pastel shades are preferably dyed without the addition of salt

Many direct dyes suitable for application by combined scouring and dyeing of either woven fabrics on jigs or knitted fabrics on jets or winches

In this process usual practice is to employ soda ash and a nonionic detergent.

Combined peroxide bleaching and dyeing with selected direct dyes is another long established process

It offers savings of a process time and energy but more care is necessary to ensure good results

Sodium carbonate is preferred over caustic soda as there is a risk of oxidative degradation of direct dyes at higher PH.

Semi and Fully Continuous method

They are less suitable for continuous application than for batchwise dyeing

Wet fastness of direct dyes may be lowered when they are applied by continuous processes

This can be overcome by adding electrolyte or by increasing the impregnation temperature

The pad-roll process is probably the most suitable semi-continuous method for dyeing cellulosic fibers with direct dyes

The temperature range should be 80-100C

It is very important to control the fabric moisture content and pad-liquor temperature, dye selection for optimum compatibility and the use of appropriate auxiliaries

Its more versatile in production than the steaming methods

After treatment processes

The wet fastness properties of all direct dyes are inadequate for many end uses but it can be improved by different types of after treatments.

Diazotization and development

Many long established direct dyes containing primary amino groups could be diazotized and coupled onto the fiber with a variety of developers, including napthols, diamines and phenols to give larger molecules with improved wet fastness properties.

Metal salt treatments

Treatment with acidified copper salt solution (0.25-2% copper sulphate and 1% acetic acid for 20-30 minutes at 60C) results in a marked improvement in the light fastness of certain direct dyes.

Washing and alkaline treatments removes the copper and light fastness becomes normal

Cationic fixing agents

These compounds interact with the sulphonate groups present in direct dyes, conferring increased wet fastness in all tests at temperatures below 60C

They will also precipitate direct dyes from solution and therefore the dyed material must be cleared of loosely held dye before treatment.

Color changes may occur and in some cases light fastness may be reduced

Formaldehyde treatment

Treatment of certain direct dyes mainly blacks, with 2-3% formaldehyde (30%) 1% acetic acid (30%) for 30 minutes at 70-80C improves the wet fastness to both water and washing.

A drop of light fastness may occur

Cross linking agents and resin treatments

Improvements in wet fastness properties can be ensured by treatment with cellulose reactants or amide-formaldehyde resins

Subsequent removal of resin by acid hydrolysis (formic acid at 90C or HCL at 60C) leaves the unfixed direct dye on the fiber with its originally low level of wet fastness.

Treatment with cross linking agents in resin finishing improves the wet fastness but color and light fastness may be affected

Stripping of direct dyes

It is very easy to strip the direct dyes if these have not been after treated.

The colors can be destroyed by boiling with sodium dithionite Na2S2O4.H2O

Or by bleaching with hypochlorite solution containing 1-2gm/l of available chlorine

If a cationic agent has been used as an after treatment, it is removed by boiling with 1-2% formic acid before destroying color with either reduction or oxidation

For materials after treated with metal salts, the metal is first removed by boiling with a metal sequestering agent like sodium salt of ethylene diamine tetra acetic acid (EDTA) in a concentration of 3g/l and then the dye is decomposed by the usual treatment with hydros or chlorine