Cobalt is one of the most efficient highly potent and effective drier per unit of metal known today. It is primarily an oxidation catalyst and as such acts as a ‘Surface’ or ‘Top Drier’.
Cobalt provides fastest surface drying properties and is required to be used in small quantities.
Used alone, it may have a tendency to cause surface wrinkling and poor through-dry. Therefore, to provide uniform drying, Cobalt is generally used in combination with other metals, such as manganese, zirconium, lead, calcium and combinations of these metals. Excess use of cobalt may result in premature skin formation. This can prevent further passage of oxygen and then reduce the efficiency of drying, ultimately resulting in formation of soft films.
Lead functions as drier by promoting polymerization of drying oils to hard, insoluble film. In contrast to cobalt and other ‘top driers’, lead causes the film to dry through its entire thickness and is therefore known as a ‘through-drier’.
Lead also improves flexibility, toughness and durability of the film. In addition to its function as through-drier, lead improves the water resistance and salt spray resistance of a film. It also serves as a pigment-dispersing and wetting agent. Lead drier is generally uses in combination with cobalt or manganese and calcium. The use of calcium with lead prevents lead precipitation and hazing.
Lead drier should not be used in aluminum systems because of tendency to de-leaf the aluminum and destroy the metallic luster of the pigmentation. Lead drier will cause darkening of white paints in high sulfide environments. In adverse environmental conditions such as low temperature and high humidity, lead performs the best with respect to any other auxiliary drier. Toxicity of lead drier now precludes their use in many applications.
Calcium is one of the important auxiliary drier. It has little drying action in itself but is very useful in combination with active driers. Calcium has strong surface active properties , hence it is very effective as a dispersing agent. It also helps to certain extent as an anti-adsorption agent to replace part of the lead with a larger amount of calcium to prevent precipitation of the lead and maintain drying efficiency. The most important use of calcium is its ability to prevent haziness or turbidity of lead phthalate observed in compositions containing dibasic acids, like phthalic anhydride.
Zirconium is an important through drier, most widely accepted as replacement for lead drier. Zirconium
Improves through-drying mainly by the formation of co-ordination bonds. Zirconium drier does not undergo any noticeable change even in the presence of strong oxidizing catalyst and does not readily precipitate and disrupt gloss. It often results in more flexible films than lead drier. It is used mainly with cobalt and calcium: Generally one third unit of the quantity of zirconium would replace one unit of lead based on metal. The pigment wetting and dispersing property of zirconium is relatively less than lead drier.
Paint Driers 
