Engineering Chemistry S Datta

21
Paints
PAINTS
Paint is a uniformly dispersed mixture having viscosity ranging from a thin liquid to a semisolid
paste and consisting of (i) a drying oil, synthetic resin or other film forming components, (ii) a
solvent and (iii) an organic or inorganic pigment.
A paint is a product formulated to protect and decorate mainly metallic and wooden
surfaces. A paint has three main ingredients.
• Pigments, which scatter and absorb light, so that the paint covers up the surface
underneath and decorates it with colour.
• Polymers, which hold the pigment to the surface by forming a smooth plastic film as the
paint dries and sets.
In gloss paints the film forming polymers are alkyd resins; in emulsion paints they are
latex polymers.
• A vehicle, which is a liquid in which the other ingredients are dissolved or dispersed.
In gloss paints a vehicle is traditionally a hydrocarbon solvent. In emulsion paint it is
water.
In case of synthetic resins a drying oil (linseed oil, tung oil, rapeseed oil, mustard oil)
may be used. Here film forming mechanism is different. The oil contains a long unsaturated
fatty acid chain. Some sort of atmospheric oxidation and polymerisation occur due to the
presence of unsaturation and a film is formed on the surface. The film forming substances are
known as binders. The solvent is known as thinner. The binder and the solvent are collectively
called a vehicle. Pigment extenders are generally called fillers. They not only reduce the cost of
paint but also extends its durability.
CH 2
OCO(CH 2
) 7
CH = CH—CH 2
—CH = CH—(CH 2
) 4
—CH 3
⏐
CHOCO(CH 2
) 7
CH = CH—CH 2
—CH = CH—(CH 2
) 4
—CH 3
⏐
CH 2
OCO(CH 2
) 7
CH = CH—CH 2
—CH = CH—(CH 2
) 4
—CH 3
Drying oil (A glyceride of linolenic acid)
Characteristics of pigments
A pigment is any substance that imparts colour to another substance.
• It should be opaque.
• It should possess a good covering power.
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