Surface Area & Oil Absorption
The surface area is the total area of the solid surface. It is measured in squared
units (m2) and is usually defined for 1 gram of pigment (typical values
for organic pigments are between 10 and 130m2). This surface area is
determined by an accepted measurement technique such as the BET (Brunauer, Emmett,
and Teller) method using nitrogen adsorption. This technique consists in calculating
the adsorption properties of the pigment.
The surface area is closely linked to the pigment's demand for binder. Larger particles
have a smaller surface area and therefore a lower demand for binder. As the size
of particle of pigment is small, the area of surface become large. As a result,
the paint need large amount of binder to wet each of pigment particles during the
dispersion process.
The amount of oil that is required to "wet out" 100 grams of pigment and
to make paint with a pigment is called oil absorption. Oil Absorption is expressed
in number of grams of oil per 100 grams of pigment (or volume relationship from
weight). This value varies depending upon the pigments physical nature and particle
size.
The amount of oil affects the time of dryness. In general, large amount of oil causes
yellowing and delay of dryness.
