Лако-красочные материалы — производство

Oil absorption value has been used as an important specification for pigments and is very useful to coating formulators. Oil absorp­tion value is defined as the minimum amount (in g) of linseed oil required to wet 100 gm of pigment to form a coherent paste.

To perform an oil absorption test, linseed oil is added dropwise to a weighed amount of pigment on a smooth glass plate while rubbing with a spatula until the mixture forms a coherent pasty mass. When the oil absorption test is completed, the surface of each pigment particle is wetted and saturated with oil to encapsulate pigment particle surfaces with a thin layer of oil. At the endpoint, there is just enough linseed oil to adsorb on the surface of all the pigment particles and to fill the interstices between closely packed particles. Thus, the oil absorption value for a pigment depends on surface area per unit mass, which is a function of the pigment’s particle size, shape, roughness and porosity. Pigments with small particle size have a large surface area per unit mass; hence, a larger amount of oil is absorbed on the surface. Similarly, pigments with acicular, lamellar and fibrous shaped particles have a high oil absorption value compared to spherical, blocky and nodular particles due to the larger surface area of the former types. Pigments with a highly porous structure such as diatomaceous silica also have a high oil absorption value due to the penetration of some oil into the pores. The oil absorption value is a mass-based evaluation, and therefore, pigment density also has a significant effect on the value. Denser pigments generally have lower oil absorption values than less dense pigments, provided that their particle size and shape are similar. Apart from these characteristics of pigments, other factors such as energy input during the test (time and vigor of rubbing), acid value of the oil (should be 3 ± 1 mg KOH/g), and moisture content of the pigment influence the oil absorption value.

Experience has shown that dispersion and the packing value obtained by linseed oil absorption are closely related to the critical PVC and pigment packing factor for the same pigment in paint and ink films.

The precision of oil absorption determinations by the spatula method is not high and may vary with operator, although with experience, deviations by a single operator can be reduced. Despite these devi­ations, the oil absorption value still remains very important to coa­ting formulators, as it helps estimate binder demand for a given pigment in a coating formulation.

More details can be found in ISO 787-5, ASTM D 281 and ASTM D 1483.