Requirements of Dyes and Pigments and Their Differences

Introduction:
Dyes and pigments are the most important colorants used to add a color or to change the color of something. They are widely used in the textile, pharmaceutical, food, cosmetics, plastics, paint, ink, photographic and paper industries. The difference between dyes and pigments is their relative solubility; dyes are soluble while the pigments are essentially insoluble in the liquid media in which they are dispersed. Before going to differ dyes and pigments we should know basics of dyes and pigments and also requirements to become a dye and pigment.

Share of colorants from organic and inorganic classes.
Fig: Share of colorants from organic and inorganic classes.

What is Dye or Dyestuff?
Dyes are soluble, colored organic compounds that are usually applied to textiles from a solution in water. They are designed to bond strongly to the polymer molecules that make up the textile fiber. Different classes of dye are used for different types of fiber and at different stages of the textile manufacturing process. Many different dyeing processes are exist in textile industry.

Dyes are used for coloring the fabrics. Dyes are molecules which absorb and reflect light at specific wavelengths to give human eyes the sense of color. There are two major types of dyes: natural and synthetic dyes. The natural dyes are complex mixtures of components derived extracted from natural substances such as plants, animals or minerals. Synthetic dyes are made in a laboratory. Chemicals are synthesized for making synthetic dyes. Some of the synthetic dyes contain metals too. All the dyes are not fixed to the fiber during the process of dyeing.

Dyes and Pigments

Requisites of a True Dye:
All colored substances are not dyes. However, the requisites of a true dye are as follows:

  1. It must have a stable color.
  2. It must have an attractive color i.e., it should not undergo structural changes readily.
  3. It must be able to attach itself to material from solution or to be capable to fix on it. For example, azobenzene is colored but cannot fix itself to a fabric. Therefore, azobenzene is not a dye. Further, a dye may not be able to dye all types of substrates. For example, picric acid is able to dye silk or wool a permanent yellow but not cotton. Thus, a dye either forms a chemical union with the substrate being dyed or it may get associated with it an intimate physical union.
  4. It must be soluble in water or must form a stable and good dispersion in water. Alternatively, it must be soluble in the medium other than water. However, it is to be remembered that the pick up of the dye from the medium should he good.
  5. The substrate to be dyed must have a natural affinity for an appropriate dye and must be able to absorb it from solution or aqueous dispersion. If necessary, in the presence of auxiliary substances under suitable conditions of concentration, temperature and pH.
  6. When a dye is fixed to a substrate. It must be first to washing, dry cleaning, perspiration, light, heat and other agencies. It must be resistant to the action of water, acids or alkalis. Particularly the latter due to the alkaline nature of washing soda and washing soap. There is probably no dye which can be guaranteed not to alter shade under all conditions.
  7. The shade and fastness of a given dye may vary depending on the substrate due to different interactions of the molecular orbitals of the dye with the substrate, and the ease with which the dye may dissipate its absorbed energy to its environment without itself decomposing.

The fixed dye must have fastness properties, e.g.,

  • Fastness to light,
  • Fastness to temperature,
  • Resistance to the action of water, dilute acids, alkalis and various organic solvents and soap solution

What is Pigment?
Pigments are insoluble compounds used in paints, printing inks, ceramics and plastics. They are applied by using dispersion in a suitable medium. Most pigments used are also organic compounds. Pigments are extensively used for coloring fabrics like cotton, wool and other man made fibers due to their excellent light fastness. They do not have any affinity to the fibers and are affixed to the fabric with the help of resins. After dyeing, the fabrics are subjected to high temperatures. Organic pigments in general have lower hiding power but greater tinting strength than inorganic pigments.

Requirements of Pigments:
These are as follows:

  1. Fastness to light: The most important criterion by which pigments are evaluated is fastness to light which depends on the medium in which the pigments are dispersed. In general pigments are less fast to light in pale tints than in dark tints.
  2. Fastness to Heat: A pigment should be stable to relatively high temperatures. This applies especially in surface coatings ‘cured’ or polymerized by heat and in thermosetting coloration. With inferior pigments, chemical decomposition may result from such heat treatment or physical change from one modification to another may occur causing deterioration in hue and other properties.
  3. Insolubility in solvents: An ideal pigment should be insoluble in all media. However, this conditions is not completely fulfilled. Solubility in vehicle or solvent may bring about crystallization of the pigment causing a change in the color properties of the paint.
  4. Fastness of Acid: Pigments must be acid-fast if they are employed in acid media or If they are to be exposed to acid vapors.
  5. Fastness to Alkalis: Pigments must be alkali-fast if they are to be used in the manufacture of distempers or the coloration of plaster surfaces.
  6. Insolubility in Water: Complete insolubility of pigment in water is rated as excellent. Where pigments have a slight degree of solubility in water or other liquid media they are said to show ‘bleeding’.

One particular pigment cannot fulfill all the above mentioned requirements. However, to satisfy all these a large number of organic pigments have been developed. These organic pigments belong to most of the different classes of dyestuffs.

Difference between Dyes and Pigments:

Dyes and pigments are substances that impart color to a material. The term colorant is often used for both dyes (also called dyestuffs) and pigments. Both dyes and pigments are powerful colorants. Difference between dyes and pigments are given in below table.

Dyes Pigments
Dyes arc colored substances which impart color to substrate. Pigments arc discrete colored particles which give color to substrate.
Functional groups bond between dye and substrate. Functional groups do not bond; typically need a binder.
Dyes arc organic compounds or mixtures. Pigments arc organic or inorganic compounds.
Dyes arc soluble in medium. Pigments arc insoluble in medium.
Auxochrome groups are present. Auxochrome groups are absent.
Dyes are applied in solutions or in dispersions. Pigments arc used in suspensions.
The chemical constitution of dyes is important in the application. The physical form, particle size of pigments is important in all applications.
Usually dyes are organic (i.e. carbon-based) compound. While pigments are normally inorganic compounds, often involving heavy toxic metals.
Longevity of dyes is shorter. Longevity of pigments is longer.
Vulnerable to fading. Resistant against fading.
Often combustible. Relatively less combustible.
It is costly. It is comparatively cheaper.
Dyes arc used for coloring textile materials, food stuffs, paper, leather etc. Pigments arc used for printing and used for coloring paints, varnishes.
Example: Tartrazine, Alizarin, Eosin etc. Example: Indigo, Copper phthalocyanine etc.

References:

  1. Synthetic Dyes by Gurdeep Chatwal
  2. Textile dyes by N. N. Mahapatra
  3. Textile dyeing by N. N. Mahapatra
  4. Handbook of natural dyes and pigments by Har Bhajan Singh and Kumar Avinash Bharati
  5. Dyes and Pigments by Ahmet Gürses, Metin Açıkyıldız, Kübra Güneş, M. Sadi Gürses

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