Theory of Dyeing

Chapter 1

Aggregation of dyes

Dye-dye self association in solution is called dye aggregation, which is important phenomenon where dye molecules or ion takes part. In general, the term aggregation is used for dye-dye interaction and dye association for interaction of dyes with other compounds e.g. polymers.

Generally dye molecules form aggregation in aqueous solution at room temperature and to an extent which depend on

i.       Size of dye molecules

ii.     No of solubilizing groups in the dye molecules

In dye aggregation multiple equilibria need to be considered i.e. diametric, trimetric etc, aggregates are formed

D + DD₂

D₂ + DD₃

D_n-1 + DD_n

Diagrammatical explanation

Dyes generally remain or tend to remain scattered in powder form but in aqueous solutions individual dye molecules stack one on top of other e.g. aggregate

Dye aggregation prevents the dye molecules from diffusion into the fiber pores and hence causes dye wastage as dyes are absorbed in monomeric form which decreases with dye aggregation.

Measurement of dye aggregation

1.    Conductometry

2.    Calorimetry

3.    Polarography

4.    Solubility

5.    Sedimentation

6.    Fluorescence

7.    X-ray diffraction

8.    Measurements of diffusion coefficients

9.    Activity of counter (Sodium)

10.                     Light scattering

11.                     Evaluation of colligative properties

12.                     Visible light adsorption

13.                     1_H and 19_F NMR

Reasons of dye aggregation in dyebath

1.    Dyes are consists of

i.       Hydrophobic aromatic portion

ii.     Polar groups (OH, amino etc.) for water solubility and charged groups (sulfonic or positive charged groups) for rendering molecule water soluble

When dye molecules dissolved in water a new interface is created between the hydrophobic portion and water. Dye can reduce the size of the interfacial water by overlapping of the hydrophobic areas and there will be a tendency to aggregate.

2.    Usually linear and planar dye molecules should tend to stack one molecule upon another with the ionized groups arranged so as to give minimum free energy condition causes aggregation.

3.    Dyes with long aliphatic chains form micelles of a spherical form in which the flexible chains associate in the interior with the sulfonic acid groups exposed on the surface of sphere.

4. Aggregation of dimer is more obvious as aromatic ring system have maximum overlap (van der waals forces) because the distance between the anionic charges is larger (minimum electrostatic repulsion).

As dye concentration increases there will be an increased tendency for trimers, tetramers etc. to be formed.

5.    Aggregation is also expected from the unusual structure of water. When the interface is formed on dissolution of the dye molecule, the water molecules adjacent to the hydrophobic portion form an ‘iceberg’ type structure accompanied by a reduction in entropy. When the dye molecules aggregate not only will energy be gained from the reduction on the interfacial energy but also an increase in energy will rise from the melting of the iceberg structure.

6.    Calculation shows that below concentration of 10^-5 mole/L various higher aggregates appear, giving a polyassociated system.

7.    Higher ionic strength, ionic dye aggregation becomes more dominant.

Prevention of aggregation

1.    By raising the temperature of dyebath

2.    Liberation and existence of monomers by circulations or stirring and keep concentration below 10^-5 mole/L of dye.

Chapter 2

Dyeing Kinetics

The actual dyeing theory mathematically can be obtained from kinetics of dyeing or dyeing equilibria. The dyeing phenomena is found in principle of dyeing curve. The factors for uniform color and optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.

During dyeing process, two methods play a dominant role:

Dyeing kinetics: the rate of transfer of dye in solution (or dispersion) from the dye bath into substrate

Dyeing equilibiria: the position of sorption versus desorption after (theoretically) infinite time. Most of the equilibrium properties of dyeing system depend on three quantities:

·        affinity

·        heat of dyeing

·        entropy change

The kinetic behavior of a dye is graphically depicted by rate of dyeing (or uptake) curve. The transfer consists of:

a. Convectional diffusion to the fiber surface occurring in dyebath.

b. Molecular diffusion through the hydrodynamic boundary layer

c. Adsorption at the outer surface

d. Molecular diffusion into the fiber (Absorption)

e. Anchoring of dye molecule

In the case of disperse dyeing (stage a) is preceded by the dissolution of disperse dye particles. Therefore the particle size distribution may influence the dyeing kinetics of disperse dye. In reactive dyeing, azoic dyeing, metallised dyes, vat and sulfuric acid esters of leucovat dyes, chemical reaction (stage e) takes place.

For the kinetics of over all dyeing processes stages a, c & d are important. Liquor circulation in the dye bath must be vigorous enough to ensure that stage a is short relative to stage d. The Sorption is faster than the preceding stages. Absorption leads to immbobilisation stage e. In equilibrium dyeing processes a complete immobilisation doesn’t take place.

The dyeing kinetic principle can be shown schematically

Dye in dyebath ^{Convective diffusion} Dye in boundary layer ^{Molecular diffusion} Dye on fiber surface ^{Adsorption + Diffusion} Dye in fiber surface ^Absorption dye

sorption ^Fixation Immobilized

Chapter 3

Dye Fiber Interaction/ Anchoring system

Dye fiber interaction system can be divided into

1.    Nonionic system

2.    Ionic system

3.    Reactive system

4.    Hydrogen bond system

5.    Other interactions

1.    Nonionic system: PET, acrylic, polyamide etc.

2.    Ionic system

o        Fiber which possess charged group:

Anionic and cationic: Acrylic fiber (contain negatively charged sulfonic or –COOH group) and basic dyes

Wool, Silk, Nylons (contain charged –NH4+ groups) & acid dye

o        Fiber which contain no charged groups

Anionic and Anionic: Cellulose is dyed with direct & vat dyes both of which carry negative charges. The dye is absorbed by virtue of its attraction to the fiber & in doing so it is accompanied by other ions of electrolytes e.g. Na+ & Cl-.

3.    Reactive system: cellulose, wool and reactive dye

4.    Hydrogen bond system

5.    Other interactions: Van der Waals force

Role of fiber functional groups in dye fiber interaction systems

·        Cotton: ionic system and covalent bond forces and H-bond

Cotton fiber has –OH groups, which is highly electronegative and is capable of hydrogen bonding. It is also capable of reacting with reactive groups of reactive dyes and form covalent bonds.

·        Protein: ionic system

Wool fiber has –COOH and –NH2 groups which are capable of ionizing and at certain pH are positively or negatively charged. So it can be dyed with basic and acid dyes.

·        Polyester:

contains –COOH, -OH as functional groups but don’t undergo ionisation, so it is not possible to dye them with ionic dyes. So nonionic system and hydrophobic interaction and Van der waals force exist.

·        PAN: ionic and nonionic system

Contains –OSO3H, can be dyed with cationic/basic dyes

·        Rayon: Ionic system

Contains –OH groups, -COOCH3 groups

Forces in dyeing systems

1.    Electrostatic force

2.    Hydrogen Bond

3.    Covalent bond

4.    Van der Waals force

5.    Physical force

6.    Hydrophobic interactions/Entropy factors

·        Electrostatic force:

The forces have a range about 100A°.

Electrostatic force formed when the dye particle and fiber surfaces are oppositely charged. Such force exists in the dyeing of wool, silk, polyamides with anionic dyes (or fibers containing anion with cationic dyes). The polymers of these fibers contain amino and carboxyl groups depending on pH value in water, these groups are either neutral (-COOH, -NH₂), cationic (-NH₃⁺) or anion (-COO^-).

·        Hydrogen Bond:

When hydrogen atoms are united with strongly electronegative group element, the latter by attracting the electron of the hydrogen atom, gives to it a positive bias. This positively charged hydrogen atom may form bond with groups containing unshared pale of electrons. They are of short range 1A° to 5A° (0.1 to 0.5nm)

R^-δ------H^+δ … …. … …ö= C O---^1.5-1.9Aº-----H …^1Aº…… ö

Hydrogen bonds are formed because of extra attraction between such atoms. It is a weak type of bond. This bond may be intermolecular or intramolecular.

·        Covalent bond

The covalent bond between carbon atom in most organic compound is very stable. They are of short range 1A° to 5A° (0.1nm to 0.5nm). Covalent bonds are formed when dyes react chemically with fibers. All reactive dyes form covalent bonds so fastness properties of such dyes are generally good.

·        Van der Waals force

Van der waals forces are only effective for sorption of dyes to fiber molecules if the distance between the dye and fiber is very small. These are weak forces and depend on atoms being at certain relative position.

·        Physical force

It is found that although –OH, -NH₂, -N=N- and –CO groups might be responsible for attachment by hydrogen bonds to the fiber but this explanation is to a great extent discounted because the coordinating power of these groups is satisfied by chelation within dye molecules which is due to nonpolar or physical force.

·        Hydrophobic interactions/Entropy factors:

It is found that increasing the no of aromatic rings or unbranched aliphatic chain makes a much greater increase in affinity than does the introduction of potential bond forming groups. This is assumed that the hydrophobic part of unbranched aliphatic chain dissolved in water because of ice-like structure of the water molecules in the immediate vicinity of hydrophobic molecules, which is of completely entropy factors

Chapter 4

Diffusion

Diffusion means penetration/movement of substance owing the existence of conc. gradient i.e. movement of particles between the two surfaces having different density from higher to lower one.

This is very important because it affects the fastness properties and the color yield.

Diffusion depends on

1.    Dye size and nature of fibers and dyes

2.    Structure of fibers (crystallinity and orientation)

3.    Forces of interaction

4.    Environment pH, solvent, temperature etc

Diffusion coefficient

The behavior of dye movement from higher concentration to lower one is described in terms of Fick’s law, which states that the no. of particles which diffuse through a cross-section in the x direction (S in moles/cm²) in a time t (seconds), the so called flux F (g moles/unit area/unit time) is proportional to the gradient of conc. dc/dx (in moles/cm⁴)

ds/dt = F = -DA. dc/dx

Where ds/dt = rate of diffusion

A area of cross section (in cm²)

D= diffusion coefficient/ diffusivity (cm²/sec)

The diffusion coefficient, D indicates dye diffusing through unit time through unit cross section area of the fiber under unit concentration gradient. Fick’s law is applicable only in cases in which the concentration gradient dc/dx is independent of time. Thus D is a measure of the diffusion properties of dyes and permeability of fiber.

Methods of measuring diffusion coefficient

Determination of D is important because, firstly to correlate structure of dye and/or fiber

Secondly, to calculate rates of dyeing or rates of desorption which may asses the practical situation

For determination of D, two methods are available

1.    Non steady method: by varying the concentration gradient

2.    Steady method: by maintaining a steady concentration gradient in the substrate throughout the process

Non steady method:

During the dyeing process, the system is in a non steady state since the concentration gradient in the substrate decreases as the dye concentration at the center of the fiber increases and also the dye concentration in the dyebath decreases. Hence Fick’s second law needs to be applied; D d²c/dx²=dc/dt

Steady State method¹

In dyeing fiber and films, steady state conditions are only present at the very beginning. Thus measurement of this type are possible only a film substrate.

Theory

The film is interposed between two different concentrated solutions (one of them may be zero) and after allowing a sufficient time for a steady concentration gradient to be set up in the film, the rate at which the dye is transferred from concentrated to dilute is measured. Conditions being so arranged that concentration become that constant. D can be calculated, under this conditions, from equation ds/dt=-DA. dc/dx

[fig: Neale’s apparatus for the diffusing dye through a cellulose sheet]

Procedure

The film is clamped between the flanges of two tubes, dye solution is placed on one side and a blank solution is on the other. Two compartments are stirred to avoid any hydrodynamic complications and the rate at which dye is transferred from the dye solution is measured.

After a sufficient period of time had elapsed for the blank solution to contain a measurable amount of dye, it was replaced by blank solution. This procedure was repeated. As dye diffused through the film, the blank solution was removed at intervals and the quantity of dye estimated colorimetrically so that dye concentration in the compartment was maintained approximately zero throughout the experiment.Now as dc/dx is constant, ds/dt can be determined directly, so D can be calculated. A is in cm^2, dc concentration difference between two sides of membrane, x thickness of membrane.

Diffusion Model

There are two models for diffusion

1.    Pore model

2.    Free Volume Model

Pore Model

The pore model was first proposed for the dyeing of cellulosic fibers in 1935

Details:

This model considers the fiber to be network of interconnecting pores. When filled with water, the latter allow the dye molecules to diffuse and be simultaneously absorbed on the wall of the pore.

The fiber is heterogeneous material and the dye must follow a tortuous/zigzag path in order to avoid the impenetrable crystalline regions. In passing through the amorphous region of the substrate, the large dye molecule must weave its way through a network of chains or along the surfaces of crystalline regions and may even encounter voids. In general the diameter of the pores may be expected to vary from one type of fiber to another. For example direct dye is easily sorped by cotton than viscose rayon due to larger pore size.

For this model, the channels or pores are considered to be zigzag and occupy a fraction of α volume of the substrate.

Defects:

1.    The pore model can’t be accepted in its entirety. The main defect is that structural parameters of the polymer, namely the size, shape and tortuiosity of the pores can’t be defined with any confidence.

2.    If the pores are defined, the polymer structure is treated secondary one, the pores merely supplying surfaces for dye adsorption.

3.    An exert correlation between the dye affinity and the diffusion coefficient can only be accepted if the dye molecule in the pore could be located out of range of the adsorption forces i.e. in the interior of the pore and the solution considered to possess the properties of normal water, which is practically impossible

4.    The model assumes that the dye molecules diffuse without hindrance down the pore. Such a situation is unlikely

5.    It is assumed that the channels (pores) are circular in cross sections, but observed dichroism in oriented materials and as dye molecules are linear, planar and rigid structure, a pore with an elliptical cross section is more realistic.

Fig. change in cross section of a fiber after orienting

Free Volume Model

Theory

Increasing the temperature (above T_d)² will result in an increase in the segmental of the polymer, thereby allowing more ‘holes’ to be made available for the diffusing, according to Williams-Landel-Ferry (WLF) equation:

log D_T/D_Td = A (T-T_d) / (B + T – T_d)

D_T, D_Td is measured diffusion coefficients at temperature at T & T_d

A, B semi empirical constant.

Details

The free volume model is that volume in a liquid and solid not occupied by the constituent atoms; in fact it arises from the thermal motion of the actions and hence increases with temperature.

Below T_d, the polymer chains may be regarded frozen into position and they only motions they can undergo a thermal vibrations. When T_d is reached/ sufficient energy is available for bond rotation in the backbone of the polymer chain. An adequate free volume has been created to provide a large energy to accommodate rotating polymer segment. Once this segment has moved, the space it has vacated allows another segment to move. The onset of the segmental motion occurs over a narrow temperature range which includes T_d.

Comparison between pore and free volume model

The controversies between the two models, as which one is more correct, the question is incorrectly formulated as any model is a simplification of reality and is therefore expected to fail in certain cases. Thus it is impossible to improve one is superior to another.

In all dyeing processes on the major fibers used today both models are probably effective simultaneously but in widely varying proportions. This conclusion evolved independently and by different methods and showed that the important factors affecting the diffusion coefficient are:

I.       the degree of swelling

II.     Dye affinity

III.   enthalpy change for the ‘hole’ formation

IV.  the temperature dependence given in WLF equation

The diffusion coefficients on the more porous fibers were found to be below and above T_d. Therefore both models exists every dyeing process and to measure the magnitude of the diffusion coefficient during dyeing depends on:

I.       chemical structure of polymer

II.     degree of crystallinity

III.   percentages of stable pores in the polymer

IV.  segmental mobility

V.    The dimensions of the diffusant molecules or ions.

Thus for most widely used textile fiber, with direct and indirect evidence one can conclude that free volume model is dominant one for polyester dyeing.

1 The steady state methods simply monitor the passage of dye through the material without reference to the internal distribution where non steady states yield a more detailed study of various factors.

2 T_d is glass transition temperature measured under dyeing conditions i.e. in water T_g is measured with dry polymers.

Chapter 5

Dyeing Mechanism

Affinity

It is the difference between the chemical potential of dye in its standard state in the fiber & the corresponding chemical potential in the dye bath i.e. tendency of a dye to move from dye bath into a substance. It is expressed in Joule or cal (per mole) and quantitative expression of substantivity.

Substantivity

The attraction between a substrate and a dye or other substance under the precise condition of test whereby the test is selectively extracted from the application medium of substrate. It is the qualitative expression of affinity. Substantivity depends on temperature, type of fiber, electrolyte concentration. Substantive dyes have affinity and are soluble.

Reproducibility of Shades

The shade of the dyes should be reproducible when required. Certain dyes have ability to overcome the factors like liquor ratio, pH, temperature etc. which affect the reproducibility.

Characteristics of highly reproducible dyes are:

·        Highly soluble

·        Medium substantivity

·        Medium reactivity

·        Good wash off properties

·        Highly diffusible

Optimization of Dye

The principle is to carry out dyeing in a manner in which the dyestuffs absorbed by substrate almost uniformly with less dye wastage.

1.    Substrate

1.    Affinity

2.    Circulation speed

3.    Action of chemicals before

2.    Dyestuff

1.    Depth of shade

2.    Optimum quantity/yield

3.    Diffusion ability and regularity

4.    Color fastness

5.    Combination & mixability

6.    Chromphore percentage

3.    Auxiliary Products

1.    Optimum quantity

2.    Compatibility with dyestuff and with each other

3.    Levelness

4.    Control of P^H in final exhaustion

5.    Reproducibility

6.    No adverse effect

4.    Temperature and time

1.    Low initial temperature to avoid rapid absorption of dye

2.    Control of critical temperature zone for maximum exhaustion

3.    Sufficient time for penetration and fixing

5.    Machine

1.    Control of batch

2.    Volume of flow

3.    Temperature regulation

The actual dyeing theory can be obtained mathematically from kinetics of dyeing or dyeing equilibria. The dyeing phenomena found in principle of dyeing curve. The factors for uniform color & optimization of dye all are related to kinetic phenomena. Therefore kinetic dyeing is important in the dyeing process.

Functional Groups of fiber

Cotton: OH-, at higher pH it is ionizable

Wool: -COOH, -NH, -CONH2. At pH 3-4 ionized positively so acid dye is used to dyeing

Acrylic: -COOH, -SO3H, -O SO3H

Silk: -NH2, -CONH

Viscose: -OH, -COOH

Polyester: -OH, -COOH. No ionization effect, high temperature used for dyeing with dispersing.

Diacetate: -OH, -COOCH3

Triacetate: -COOCH3

Dyeing Medium

1.    Aqueous medium

·        Water

·        Solvent

·        Foam

2.    Vapor phase: cationic, anionic, nonionic

Dyeing Mechanism

The sequence of dyeing falls into four stages

1.    Transfer of dye onto fiber surface

2.    Adsorption

3.    Diffusion into the fiber

4.    Interaction

Transfer of dye onto fiber surface

The transfer of dye onto the fiber surface depends on:

i.       Environment of the dyebath: environment of the bath refers to

o        Solvent and its type, nature, quantity: solvent may be water and or any other solvents which may be soft/hard, acidic, alkaline, ionic, nonionic etc.

o        pH

o        Dyeing assistants like electrolytes, leveling agents, carrier, dispersing agents etc.

o        temperature of the dyebath which depends on material type (cotton or polyester), type of dye (hot brand or cold brand), method of dyeing (padding or exhaust) Suitable environment ensures easy transference of dye on fiber surface.

ii.     Substantivity

iii.   Mechanical and physical force

Adsorption

The distribution process is called adsorption, if the substance which is to be distributed is retained by a surface. The assembly of dye molecules at the fiber surface is governed by:

Electropotential forces: All fiber when immersed into water or aqueous solution acquires an electric potential known as ‘zeta potential.’ Cellulosic fiber bears a negative charge while protein fibers at higher pH than its isoelectric point bears are negatively charged and at lower pH than isoelectric point is positively charged.

Temperature: most dyes in solution are either in molecular and partially ionized state or exist in the form of ionic micelles; increase in temperature tends to breakdown micelles into less aggregated units. Increase of temperature promotes vibrational activity accelerates the migration of the surface of the fiber.

Agitation: when a fiber is immersed in the dye a large no of molecules tend to enter the fabric at once, thus creating a layer called ‘Barrier.’ If the dye molecules are to reach the fiber surface then the barrier should be broken which is done by agitation.

Dye adsorption has affect on fastness properties.and Apparel sector. Completed Master of Design in Australia with research titled "Ready-Made Garment industry of Bangladesh: How the industry is affected in post MFA period." Currently lecturing on Primeasia University with an innovative approach. This approcah intended to serve the best interest for the students who will be the ultimate future professionals in Textile and Apparel industry of Bangladesh.

Theory of Dyeing 23