Drying of paints 4

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Level 2 Diploma in Painting and Decorating

PowerPoint

Drying of paints

presentation

Unit 216: Applying paint systems by brush and roller to

complex areas



Learning outcome •  Understand the drying processes and stages of paint coatings.



Drying of paints Coatings dry by various methods:

•  Evaporation

•  Oxidation

•  Polymerisation

•  Coalescence



Evaporation •  The solvent or thinner is dispersed into the air leaving the solid

constituents to form a dry paint film. •  Coatings that dry by this method can be reactivated by the addition of

solvent, returning them to a liquid. These are known as reversible coatings.

•  Two examples of this type of coating are shellac and bitumen.


Level 2 Diploma in Painting and Decorating Oxidisation •  Oxidisation is process of drying whereby the air combines

chemically with the drying oils and resins present in the paint. This has the effect of holding the pigment in suspension.

•  Oxygen is drawn into the paint film by drying the oils and is in turn passed on to the resins (oxidising the coating internally).

•  Once set the paint cannot be reversed – the paint goes through a chemical change.



Polymerisation •  A drying process of some resins used in paints and adhesives,

which will not dry normally by absorbing air from the atmosphere or by evaporation of solvents.

•  The resins have to be mixed with hardeners. When this occurs a chemical reaction takes place and particles of resin join together to make larger particles. They eventually become hard and dry (non- reversible coatings).



Coalescence

•  A drying process whereby the resin content of the paint or adhesive is separated by globules of water.

•  When the solvent evaporates (water) the resin joins together to form a solid film.

•  The gaps left by the water are not fully used up by the resin and the result is a honeycomb effect.

•  This is the characteristic of PVA or acrylic emulsion paints.



Questions

1.  List the three main methods by which paint dries.

2.  How do emulsion paints dry?

3.  Explain oxidisation.

Image reproductions courtesy of Decorating Warehouse and Axminster.



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Drying of paints 4