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Reactive Dyes – Classification, Properties, Dyeing Method.

Today’s post is about a particular dye, called the reactive dyes, which are the dyes that give the colors to the fabrics.

These dyes are very useful and generally used in the dyeing. They are acid, basic and salts.

When I first started dyeing cloth, I would get a lot of questions about which reactive dyes I used, and how I would react them. I had researched about the basics of reactive dyes and knew they were primarily used for dyeing cotton textiles.

I was surprised to see people asking about the reactive dyes I used for my wool and synthetics. I had read about the different types of reactive dyes, but I had also found a whole other group of reactive dyes.

What is reactive dye?

Reactive dyes are organic compounds that can be used to color fabrics. They are usually applied as an aqueous solution.

Reactive dyes react with the fibers in the fabric at the time of application. There are two main types of reactive dyes: acid dyes,

which contain sulfonate groups, and basic dyes, which contain quaternary ammonium groups. Basic dyes are generally preferred because they have better lightfastness than acid dyes.

Acid dyes are often used for their ability to produce bright colors, but they fade rapidly in sunlight. Acid dyes tend to work best on natural fiber materials such as silk or linen.

On synthetic fibers like polyester, acrylics, nylon, rayon, etc., acid dyes will not penetrate well enough to provide good coverage.

This means you need more dye to cover up the white areas. The result is less vibrant colors.

Basic dyes do not require any pre-treatment before use. You simply add water to make your bath.

Here some other dyeing method and type of dyes:

  • Air dyeing 

Air dyeing is a process where the dyed material is exposed to air while it’s still wet. This causes the dye molecules to bond together and form larger particles.

Air dyeing produces brighter colors and gives off fewer fumes compared to immersion dyeing. It works great for small pieces of clothing, bags, scarves, hats, gloves, socks, underwear, towels, bedding, rugs, curtains, tablecloths, napkins, placemats, aprons, and much more!

  • Cellulosics dyeing

Cellulosics dyeing is a dyeing Mechanism of dying cellulose based products using chemicals. Cellulosic dyeing uses chemical reactions between the dye and the substrate rather than physical absorption into the fibres. Dyeing cellulosics requires special equipment and expertise.

  • Immersion dyeing

In this technique, the textile item being dyed is immersed in a vat containing the appropriate dye. Immersions take longer than air dyeing, but offer excellent results.

  • Cold dyeing

Cold dyeing is a method of dyeing by immersing the items in cold liquid dye baths. Cold dyeing offers many advantages over hot dyeing including faster processing times, lower energy costs, and no heat damage to delicate garments.

  • Hot dyeing

Dying by applying heat to the garment is called “hot dyeing.” Hot dyeing has been around since ancient Egypt when dyers heated pots filled with indigo blue dye until the pot reached boiling point.

  • Dyeing bath

This method of dyeing involves soaking the cloth in a large container full of dye. A dye bath may consist of several containers holding various shades of one color.

The dye bath must be kept warm so that the dye does not cool down too quickly. If the temperature drops below 60°F, the dye will begin to solidify.

  • Textile dyeing

Textile dyeing is a process of coloring textiles through application of an organic compound onto the surface of the fabric. Textile dyeing can be done manually or mechanically.

Manual textile dyeing is usually performed by hand dipping the fabrics into a solution of dye. Mechanical textile dyeing is accomplished by passing the fabric through rollers covered with dye solutions.

  • Uneven dyeing

Uneven dyeing is a way of creating different tones on a single piece of fabric. Uneven dyeing creates interesting patterns and textures.

Properties of a reactive dyes

The properties of a reactive dye are determined by the chemical structure of the dye molecule. Reactive dyes have two primary characteristics:

1. The ability to react with the fiber substrate; and the ability to remain attached to the fibers after reacting.

Reaction with the fiber substrate

When a reactive dye reacts with the fiber substrate, it forms covalent bonds with the fiber. The resulting reaction product absorbs light at wavelengths similar to those absorbed by the original dye.

In addition, some reactive dyes also absorb visible light. These additional absorptions create new hues which do not exist in nature.

2. The ability to form an insoluble colored complex with another substance (called a chromogen). This property allows for easy removal from the finished article.

Classification of reactive dye

There are many types of dye, each with different properties. Some dyes are colorless and transparent, others are opaque and provide color, and still others are intrinsically colored.

The dyes used in cotton fibers are classified according to the type of fiber they are used in, and the type of fiber they are used in depends on the type of dye being used. For example, reactive dyes are used in synthetic fiber such as nylon, acrylic and polyester.

The reactive dyes used in these fibers can be classified according to the type of reactive dye that is used as their base.

  • Moderate reactivity

This kind of reactive dyes contain no more than three functional groups per molecule. They include mono-functional, di-functional and tri-functional dyes.

Mono-functional means there is only one group present on the dye molecule. Di-functional means there are two groups present on the dye molecule, while tri-functional means there are three groups present on the dye molecules.

  • Covalent bond

Covalent bond is the reaction of atoms where electrons are shared between them. Covalent bonding occurs when two elements share four valence electrons.

In this case, the element has lost its outermost electron pair. When the atom gains back the missing electron pair, it becomes positively charged.

It then attracts other negatively charged particles around itself.

A covalent bond is formed when two atoms gain or lose electrons. Electrons are transferred from one atom to another.

  • Vinyl sulphone

This kind of reactive dye that is used in woolen yarns contains vinyl sulphonate groups. It gives bright colors because of the presence of sulfonic acid groups.

Vinyl Sulphone is a monofunctional reactive dye.

  • Appreciable substantivity

This reactivity classification is based upon the degree of permanency of the dyed fabric. A dye having appreciable substantivity will stay permanently bonded to the textile material even if washed several times.

  • Moderate substantivity

A classification of reactivity that is intermediate between substantiality and low substantivity. Dyeing materials containing moderate substantivity have good washfastness but poor fastness to rubbing.

  • Low substantivity

Dyeing materials containing low substantivity have very little tendency to remain attached to fabrics after washing.

  • High substantivity

Reactivity classifications greater than high substantivity are generally considered non-substantive. Reactive dyes with high substantivities tend to migrate out of the dyed fabric during laundering.

Depending on chemical constitution

Chemical composition of reactive dye depends upon its basic skeleton. Basic skeletons include:

  •  Aromatic amines
  • Aniline derivatives
  • Naphthalene derivatives
  • Pyridinium salts
  • Thiazole derivatives
  • Imidazo quinoline
  • Pyrazolo -pyrimidine
  • Indolyl sulfonic acid
  • Benzothiadiazole
  • Azoxyben

Dye classifications depends on reactivity and conditions 

Dyes may be one of the most important materials used in the textile industry. With the help of dyes, fabrics can be dyed to a wide range of colors and achieve different effects.

Dyes are usually classified into four groups:

  1. Direct dyes, which are used directly on the fabric during the dyeing process.
  2. Water-soluble dyes, which are applied to the fabric after it is washed.
  3. Fast-drying dyes, which require a minimum of 30 minutes of exposure to the atmosphere after application.
  4. Slow-drying dyes, which can be left overnight to dry.

Dye conditions

  • Alkaline condition

Alkaline dye condition is defined as pH 8–11. The alkali helps to dissolve the colorant molecules so they can penetrate deep inside the fiber.

Alkaline dyes also provide better light stability for the colored product. However, some types of fibers such as polyester cannot withstand an alkaline environment.

Therefore, acidic dyes should not be used on these kinds of fibers. Acids are added to neutralize the alkalis present in the dyebath.

  • Neutral conditions

This type of dyeing condition has a pH value from 6 to 7.5. Neutral dyes do not need any additional chemicals or additives to make them work properly. They give brighter shades compared to other two categories.

  • Acidic conditions

The pH level of this category ranges from 3 to 5. Acidic dyes are more stable against fading caused by sunlight. But they don’t produce vivid colors like those produced under alkaline conditions.

Reactive dyeing also depends on dyeing temperature

Most reactive dyes require a heat source to activate, and many of the dyes will wash out after a few washes. This means that you can’t dye most fabrics until they are dry.

You will notice that many of the reactive dyes have a melting point, so they can be used at a lower temperature than the rest. This is important because reactive dyes are more likely to fade if the fabric is too stiff.

Reactions to be considered during reactive dyeing

Types of reactions

  • Dye-fiber reaction – reaction between fiber

Dye-fiber is a mechanism of reaction where the dye reacts with the fiber itself. It’s very common when using disperse dyes.

  • Nucleophilic addition reaction

This factor of reaction is based on nucleophilic attack. In this case, the dye molecule attacks another part of the same compound.

For example, the amino group of the dye molecule attaches to the hydroxyl group of the cellulose chain.

  • Nucleophilic substitution reaction

This factor of dyeing reaction is based on electrophilic attack. Here, the dye molecule acts as an electron donor and replaces the hydrogen atom attached to the carbon atom.

  • Electrophilic aromatic substitution reaction

In this kind of reaction, the dye molecule becomes electrically charged due to its positive charge. Then, it attracts electrons from the negative charges of the atoms around it. As a result, the bond between the dye and the fiber breaks down.

  • Reaction of DCT dyes

DCT dyes react with the fiber through covalent bonds. These bonds form between the nitrogen atom of the amine groups and the oxygen atom of the carboxylic acid groups.

  • Cationic ring opening polymerization

A CROP process involves adding monomers into a solution containing a catalyst. Afterward, the mixture undergoes heating and then cooling cycles.

During this time, the monomer chains grow longer and longer. Finally, the resulting material forms a solid film.

Kirsten Carter

Kirsten Carter

Kirsten Carter is a freelance content writer who specialises in writing about travel, technology and health. When she's not traveling between her home of Tanzania and England, she writes for her blog Rightminded Travelling and features on a variety of different travel and technology sites.View Author posts

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