Engineering Chemistry S Datta

478 ENGINEERING CHEMISTRY
litharge at 230°C for 5 hrs. The product is called lead oil. This lead oil is mixed with kerosene
(thinner) and asphaltum. These Japans are applied and baked at 200°C for a few hours.
A comparative study of paints, varnishes, lacquers and enamels is given below:
Paint Varnish Lacquer Enamel
1. A paint is a mixture
of opaque solids
dispersed in a
liquid medium
A varnish is a colloidal
dispersion or solution of
syn-thetic or natural
resins in oils or
thinners
A lacquer is a colloi-dal
dispersion or solution of
film forming compound,
i.e., resin in a vol-atile
solvent
Enamels are pigmented
varnishes or
lacquers
2. Used for protective
and decorative aspect
3. Drying is done by
oxidative polymerisation
Same Same Same
Drying by oxidative
polymerisation
Drying by simple evaporation
Drying by either oxidative
polymerisation
or evaporation or by
baking as per composition
SHORT QUESTIONS AND ANSWERS
Q. 1. What are the functions of pigments in a paint?
Ans. The functions are
(i) To protect the film by reflecting back the destructive UV light
(ii) To strengthen the film
(iii) To impart an aesthetic appeal
Q. 2. Name the main constituents of an oil varnish.
Ans. (i) Synthetic resins
(ii) Drying oil
(iii) Volatile solvents
Q. 3. What is a tonner?
Ans. Tonner is an insoluble inorganic dye used in paints.
Q. 4. How are the paint films formed?
Ans. The paint films are formed by drying an unsaturated oil.
Q. 5. What is ‘drying’ of an oil?
Ans. An oil is a glyceride i.e. ester of glycerol and fatty acids. A drying oil is generally
glyceride of unsaturated fatty acids. The main function of the drying oil in a paint formulation
is to form a protective film by complex chemical reactions. Otherwise, drying is a chemical
change of an oil accompanied by oxidation and polymerisation due to the presence of
unsaturation in the fatty acid portion of the formula of drying oil to form a protective film over
the surface.