Organic pigments are primarily classified based on two key characteristics: chromatic properties and molecular chemical structure.
Here's a breakdown of these classification methods:
Classification by Chromatography (Color Index)
Organic pigments are broadly categorized by their perceived color. This is the most common and intuitive way to differentiate them. This classification typically follows the Color Index (C.I.) system. Examples include:
- Pigment Yellow: Used for producing yellow hues.
- Pigment Orange: Used for producing orange hues.
- Pigment Red: Used for producing red hues.
- Pigment Violet: Used for producing violet/purple hues.
- Pigment Blue: Used for producing blue hues.
- Pigment Green: Used for producing green hues.
- Pigment Brown: Used for producing brown hues.
- Pigment Black: Used for producing black hues.
Within each color category, pigments are further differentiated by shade (e.g., Pigment Red 170 is a different shade of red than Pigment Red 254).
Classification by Molecular Chemical Structure
This classification is based on the underlying chemical structure of the pigment molecule. This is a more technical classification, but it is important for understanding the pigment's properties and performance. Common structural classes include:
- Azo Pigments: These are the most common type of organic pigments, characterized by the presence of one or more azo groups (-N=N-) linking aromatic rings.
- Phthalo Cyanine Pigments: Known for their intense blue and green shades and excellent lightfastness. They are based on the phthalocyanine ring system.
- Diazo Pigments: Similar to azo pigments but contain two azo groups, often resulting in brighter and stronger colors.
- Polycyclic Pigments: These pigments have complex, fused ring structures, such as quinacridones, dioxazines, and perylenes. They are known for their high performance properties like heat and light resistance.
- Vat Pigments: These are insoluble organic dyes that become soluble during application, then are rendered insoluble again by oxidation. They are often used in textile dyeing.
In summary, while color (chromatography) provides a practical way to categorize pigments, the chemical structure defines the pigment's fundamental properties and characteristics.
