Definition, Principle & Classification of Dye Chemistry

 

Most tissues, specially animal tissues, are colorless. Dyes are colored substances with affinity for specific molecules of tissues, so they get attached to the tissue and provide color. Staining is the process for coloring tissues by using dyes. It allows visualizing cells and extracellular matrix to be studied with light microscopes. Staining is usually done on tissue sections and cell smears. The most common sections for staining are those obtained from paraffin embedding and frozen tissues. Dyes are the main components of the general staining protocols performed in histology labs. 

Principle of Dye Chemistry

A dyeing process is the interaction between a dye and a fiber, as well as the movement of dye into the internal part of the fiber. Generally, a dying process involves adsorption (transfer of dyes from the aqueous solution onto the fiber surface) and diffusion (dyes diffused into the fiber). In addition to direct absorption, dyeing may also involve the precipitation of dyes inside the fiber (vat dyes), or chemical reaction with the fiber (reactive dyes). Unlike most organic compounds, dyes possess color because they

 1) absorb light in the visible spectrum (400–700 nm),

2) have at least one chromophore (color-bearing group),

 3) have a conjugated system, i.e. a structure with alternating double and single bonds, and

4) exhibit resonance of electrons, which is a stabilizing force in organic compounds. When any one of these features is lacking from the molecular structure the color is lost. 

Classification of Dyes

Dyes are organic compounds with two components namely Chromophore, which imparts color and Auxochrome that help in substantively (attraction between a substance (such as dye) in solution and a fiber) of dyes. They are classified into natural dyes and synthetic dyes.

To be of use, dyes must possess the following four properties:

1. Colour

2. Solubility in water

3. Ability to be absorbed and retained by fiber (substantivity) or to be chemically combined with it (reactivity).

4. Ability to withstand washing, dry cleaning and exposure to light.

 

• The dye has a color due to the presence of chromophore and its fixed property to the acid or basic groups.

• The polar auxochrome makes the dye water-soluble and binds the dye to the fabric by interaction with the oppositely charged groups of fabric structure.

Such as. Hydroxide (OH), sulfonic acid (SO3H) etc

Natural Dyes

Natural dyes are color substances obtained from natural sources. Natural dyes are used for all types of textile dyeing and printing until the middle of nineteenth century. The use of natural dyes were reduced due to the advent of synthetic dyes, though they were economical and posses excellent fastness properties. However, the growing consumer awareness on the harmful impact of synthetic dyes, concern for environment worldwide and stringent environmental laws lead to the revival of natural dyes.

 Natural dyes are dyes or colorants derived from plants, invertebrates, or minerals.

• The majority of natural dyes are vegetable dyes from plant sources. E.g. roots, berries, bark, leaves, and wood.

 • Other organic sources include fungi and lichens.

 

1. Advantages of Natural Dyes

· Natural dyes are extracted from natural sources and hence they are eco-friendly

· Produces soft and soothing colors

· These dyes provide excellent protection from UV rays

· Natural dyes like turmeric have anti-microbial properties and hence protect the fabrics and wearers from microbial attack

· Some natural dyes possess mosquito repellent and flame resistant property

· They can be obtained from the natural sources which are abundant in a particular area. Hence supply of raw materials will be continuous and transport charges will be lower

 

 2.  Disadvantages of Natural Dyes

·               Natural dyes are difficult to store

·               Dye extraction is a time consuming process

·               Reproducibility of the same color shade is difficult

·               Impurities in natural dyes fade away the color produced

·               Availability of these dyes depends on the seasons

·               Natural dyeing process is difficult to standardize

Synthetic Dyes

Dyes that are produced chemically are called as synthetic dyes. These are classified based on the chemical composition of the dye.

1. Direct Dyes

When a dye colors the fabric directly without the help of any fixing agent, the dye is said to be a direct dye. Direct dyes are water soluble. They are easy to produce, simple to apply and cheap in cost of production and application.

2. Reactive Dyes

Dyes that react with the fibers. They become an integral part of the fiber. They are water soluble and are used to dye cellulose, protein and polyamide fibers. They produce full range of bright shade across the spectrum. They exhibit excellent wash fastness and good light fastness properties.

3. Basic Dyes

Basic dyes have cationic or basic groups (positively charged) and hence they are also known as cationic dyes. Basic dyes react with the acidic groups present in the fibers and form electrovalent bonds. Basic dyes are soluble in alcohol but not easily soluble in water. Basic dyes exhibit brilliant shades of colour which is not shown by other dye classes. Basic dyes are suitable for dyeing wool, silk and acrylic, but they have no affinity towards cellulosic fabrics. Basic dyes are used along with a mordant for fibres such as cotton, linen, acetate, nylon and polyester. Basic dyes show moderate light and wash fastness.

4. Acid Dyes

Water soluble dyes that require acid (sulphuric, acetic, formic acid etc.,) in dye bath to dye silk or wool are called as acid dyes. These acid dyes are mostly sodium salts of organic acids. When dissolved in water, acid dyes produce negative ions (anions or acidic groups) which react with positive ions of protein fibers and get attached to the fiber through electrovalent bonds. Acid dyes are similar to direct dyes however they cannot be applied to cellulosic fiber due to slight variations in structure. A large color range is available with acid dyes. They are inexpensive.