PAINTS AND PIGMENTS

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DYES

Pigments are coloured substances used in paint. Paints are a type of mixture called a colloid. They contain several components, including the pigment, a solvent and a binding medium.

Thermochromic and phosphorescent pigments are very useful, and extend the normal properties of paints.

Dyes

Dyes are substances that can be used to colour fabrics. In the past, people usually used natural dyes from animals and plants. For example, Tyrian purplewas extracted from sea snails, and red carthamine from safflowers.

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PAINTS

Paints are used to decorate surfaces or protect them from damage. A pigment is a coloured substance used in paint.

Paints contain these ingredients:

  • a pigment - gives the paint its colour
  • a binding medium – a liquid polymer that hardens to form a continuous layer when the paint dries
  • a solvent - dissolves the binding medium and makes the paint more fluid

Oil paints have a hydrocarbon oil as the binding medium. This is dissolved in a solvent. The pigment is dispersed within this solution.

When the paint dries the solvent evaporates away, leaving behind the pigment and oil binding medium. The oil hardens to form a continuous layer of dry paint.

Emulsion paints are water-based - their solvent is water.

Colloids

Paints are a type of mixture called a colloid. In a colloid, particles of one substance are mixed and dispersed with particles of another substance - but they are not dissolved in it.

In an paint the pigment is dispersed in the liquid formed from the binding medium and solvent solution.

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THERMOCHROMIC AND PHOSPHORESCENT PAINTS

Thermochromic and phosphorescent pigments

Thermochromic pigments are sensitive to temperature. They change colour when they are heated up or cooled down. This property is useful for indicating if the drink in a cup is hot, or if the water in a kettle is hot.

Phosphorescent pigments glow in the dark. They are able to absorb light energy and store it. This stored energy is released as light energy over a period of time.

Phosphorescent pigments are useful for watch faces that glow in the dark. They may also be used in fire safety signs placed near fire extinguishers. In the event of a fire, the position of the fire extinguishers can be seen, even if the lights fail.

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THE DRYING OF PAINTS

The components of a colloid are dispersed throughout the mixture. They do not separate out because their particles are small enough not to settle at the bottom.

When oil paints dry, the oil oxidises to form a hard film. This happens as the solvent evaporates and the oil reacts with oxygen in the air.

In the past, glow-in-the-dark watch faces used radioactive paints. Phosphorescent pigments are much safer, though.

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