By Bhupen Kharawala
(Pigment Consultant- Kay Pigments)
Pigments vs Dyes: The Fundamental Difference
To understand the importance of Crystal Structure in a Pigments first it is important to understand the difference between Dyes and Pigments .
While the term “pigment dyes” might seem interchangeable with “pigments,” it’s important to clarify that pigments are not dyes. First and foremost Pigments are not insoluble substances. Pigments are finely ground, insoluble crystalline particles that adhere to a substrate to impart colour, while dyes are soluble substances that penetrate the material being coloured. Think of pigments as sitting on top of a surface, like paint, and dyes as being soluble substances which can be absorbed into the material, like a stain.
Dyes are made up of a molecule with two key group of atoms:
- A chromophore
- An auxochrome
The chromophore is responsible for the colour of the dye , this is the part of the dye that actually absorbs light and produces colour. It contains delocalised electrons that allows for absorption of specific wavelengths of light within the visible spectrum
While the auxochrome attached to the chromophore influences the colour properties of the molecule, increases solubility in the medium and increases the dyes affinity for the material (substrate or medium ) being dyed.
Observe a Diamond, its brilliance, its clarity its pure colour is because of its perfect crystal structure. Similarly Pigments owe their property to give colour to their crystal structure. Pigments are made up of only chromophores they do not need to interact with their chemical environment in order to exhibit colour.
Once having understood that the colouring ability of Pigments is because of its crystal structure we come next to the properties of the crystal structure.
Composition of Crystals in Pigments
- Primary Crystals : These are the smallest units of pigments that contribute to its colour. If these crystal structure is destroyed than the we no longer have colouring property .
The brilliancy and purity of colour depends on these crystal being uniform . These Crystals are measured under electron microscope. Primary particles consist of single particles. The smaller these particles, the greater their surface energy and therefore there is an increased tendency to clump together during the manufacturing process. However, even if it were possible to supply pigments in the form of primary particles, it would be quite impractical, as they would be more like smoke than a powder. In practice, they only exist as the pigment is synthesized. When the particles clump together during the manufacturing process they form either aggregates or agglomerates.
Aggregates are joined along crystal boundaries during synthesis or drying. They are difficult to separate and therefore the pigment manufacturer tries to avoid their formation during the pigment’s production.
Primary particles have a variety of shapes such as needles, platelets, prisms, cubes etc.
The shape of the primary crystals can be studied by X Ray diffraction and electron microscope.
Crystal Structure of Phthalocyanine pigments were first to be studied. The width of Beta Phthalocyanine crystal is 19 A x 19 A and length could vary from 38A to a much larger distance
The Alfa Phthalocyanine Crystal is cubical with dimensions of 24A by 24A by 24A
- These Aggregates can be referred Secondary Crystal structures.
Aggregates cannot be broken in dispersing operation like grinding.
- Agglomerates are loose clusters of primary particles or Aggregates, and can be broken down by efficient dispersion processes. Agglomerates can also be called Tertiary crystals.
The states of aggregation and agglomeration are key factors in the pigment’s dispersibility. Once dispersed it is still possible for particles to re-agglomerate, into loosely held groups, known as flocculates. It frequently occurs when there is a rapid change of state, such as too rapid a dilution, or the addition of an incompatible substance. Flocculation results in a loss of tinctorial strength. However, flocculates are usually easier to separate than true agglomerates, and even normal shear such as brushing out is sufficient. This results in an uneven increase in tinctorial strength, depending on how much shear has been developed during brushing out. Small particles are more prone to flocculation than larger ones, so pigments most at risk are grades of carbon black and conventional fine organic pigments, such as phthalocyanine and dioxazine violet pigments.
How does Crystal Structure affect the colouristic properties in a dispersion?
As we have already seen that pigments are not soluble, so to colour a substrate , we have to mix the pigment in the substrate and to break down the pigment agglomerates to the minimum size possible, preferably to their aggregate level in the substrate itself. This process is called dispersion. Dispersion requires input of energy.
Dispersions are of three kinds:
- Substrate is Solid called Dry Dispersions – Example of Dry dispersion is Coloured Cement or coloured powders like Detergent powder. This dispersion is done ball mill or such equipment.
- Substrate is Viscous Liquid – Example is OffSet Inks, Plastic Masterbatch. The dispersion is done in a triple roll mill or extruder.
- Substrate is Liquid – Example is Liquid Inks, Paints etc. The Dispersion is done in Sand Mill.
At the particle 200-500 nm( 0.2 microns to 0.5 microns) organic pigments show the optimum tinting property, above this size tinting strength decreases with increased particle size. The colour shade also changes with increasing particle size.
Influence On Particle Size On Pigment Colour
| Colour | Bigger particle size | Small particle size |
| Red & Green Pigments | Bluish | Yellowish |
| Blue Pigments | Greenish | Reddish |
| Yellow Pigments | Reddish | Greenish |
Other Properties That Are Affected By Crystal Structure’s Size, Shape And Ionic Charge
The crystal shape of a pigment, particularly in the case of larger particles, does influence the way they pack in the paint film and consequently can affect the properties of the paint. Thus, properly dispersed acicular(needle like) particles are said to reinforce the paint film like the fibres in glass fibre reinforced plastics. Lamellar particles such as aluminium and mica, form an overlapping laminar structure, not unlike roof tiles, and like roof tiles they offer resistance to the passage of water, allowing the paint film to impart good protection properties to the substrate.
In pigment dispersions, viscosity is a crucial property, along with key optical characteristics like transparency and gloss. These properties are closely linked to the pigment crystal’s size, shape, and the ionic charges present on the crystal surface.
Conclusion: The Art of Pigment Manufacturing
Creating a high-performance pigment is both a science and an art. A pigment manufacturer begins with a primary crystal and carefully constructs a crystal structure tailored for its intended application be it brilliant color, stability, gloss, or ease of dispersion
Pigment manufacturing is therefore rightly called an art form a discipline where chemistry, physics, and creativity converge to produce the perfect splash of color.