- Paint usually contains a solvent, a binding medium and a pigment
- The pigment gives the paint its colour
- The binding medium is a liquid that carries the pigment particles and holds them together. When the binding medium goes solid, it sticks the pigments to the painted surface
- The solvent thins the paint and makes it easier to spread
- A colloid consists of small particles of one thing dispersed in another thing
- These particles can be little bits of solid, droplets of liquid, or bubbles of gas
- Colloids don't separate out, because the particles are so small that they don't sink to the bottom
- A paint is a colloid where particles of a pigment that is usually a solid, are dispersed through a liquid
Oil and Water Based Paints
- Water based. The solvent used is water and the binding medium is usually acrylic or vinyl acetate polymer
- A water based emulsion dries when the solvent evaporates, leaving behind the binding medium and pigment as a thin solid film. A thin layer of emulsion paint dries quite quickly
- Emulsion paints are fast drying and don't produce harmful fumes, so they are good for painting inside walls.
Gloss paints and oil paints
- Oil based. The binding medium is oil and the solvent is an organic compund that will dissolve in oil.
- Oil paints dry in 2 stages: first, the solvent evaporates, then the oil is oxidised before it turns solid
- Oil paints are glossy, waterproof and hard wearing, but the solvents used to make them produce harmful fumes. They are often used for painting outside doors and metalwork
- Change colour or become transparent when heated or cooled
- Different pigments change colour at different temperatures, so a mixture of different pigments can be used to make a colour-coded temperature scale. They can be used to make basic thermometers that are stuck onto someones forehead to read the temperature
Uses for thermochromic pigments:
- Can be used in kettles that change colour as the water boils
- Baby products such as spoons and bath toys have them as a safety feature, so you can tell by the items colour if the bath water or food is too hot
- Used on mugs to warn the user if the contents are too hot
Thermochromic pigments can be mixed with acrylic paint to give a wider range of colour changes e.g. mixing a blue thermochromic pigment that loses its colour above 27 degrees mixed with a yellow acrylic paint would give a paint thats green below 27 degrees and yellow above 27 degrees
Special Pigments cont.
- Absorb natural or artificial light and store energy in their molecules, then release energy as light over a period of time. This can range from a few seconds to a couple of hours
- One use is a watch or a clock with glow-in-the-dark hands. Other uses include traffic signs, emergency exit signs, toys and novelty decorations
Glow-in-the-dark watches used to be made with radioactive paints. These paints would glow for years without needing to be "charged up" by putting them in the light. However, many of them weren't safe and could give a high dose of atomic radiation. Phosphorescent pigments were developed as a safer alternative.