Liquid Finishes
5
Liquid Finishes
As we all noticed from finished parts, paint is a hard coating. However, we spray a liquid. This change of natureis caused for the most part by elements present in the material, whose function is described below. The composing elements of a paint
Evaporated solvantDry extract Liquid PaintSurface to paint Surface to paint
Wet layerDry layer All paints are generally made of several components diluted in solvent (which may be water), which willeventually go back to solid after they dry out on the painted surface:
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binding materials
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pigments
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charges (additives)The binding materials are generally more or less transparent, like a resin. When diluted alone in solvent, itbecomes a varnish: Binding material + solvent = varnish
The paint is often given the name of the type of binding material it is made from; for example, cellulosicpaints use cellulose as a binding material. To make the film opaque one adds very fine powders high in color
called pigments.Binding material + solvent + pigment = paintTo give the film particular characteristics (mechanical resistance, for example) quite a few charges andadditives are added to the above mixture.
Solvents dissolve the other components of the paint:
There are 3 big families :
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Light solvents: evaporate quite quickly. So much so that the paint drops may dry before they reach thepart, and not overlap correctly. They are never used by themselves, but combined with others.
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Heavy solvents : evaporate rather slowly, allowing the paint to spread well as it hits the surface of the part.They provide the smooth and slick aspect of the film. They are usually added in measured quantities to the light solvents, as they extend the drying time.
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There are medium solvents: they evaporate in a few seconds, allowing the droplets to mix on the surface,and drying quickly enough. To elaborate its paint, the paint manufacturer first considers the list of solvents which will be able to dissolve
the binding materials he wants to use, and then picks up the ones whose volatility matches the type of drying
method requested (air, oven).
Just prior to
spraying, the operator may add a thinner to his paint, to give it the fluidity (viscosity) required for
his spraying operation.
Selection guide - Liquid solutions 2004