Metallic carboxylates -
Driers for oxidative coating:
Drier metals are traditionally divided into two groups: active (or
primary) and auxiliary (or secondary) though it is an arbitrary
classification. Driers that promote oxygen absorption followed by peroxide
formation and decomposition are termed active; auxiliary driers, while
exhibiting no catalytic action on their own, appear to synergistically
enhance the functioning of the active drier metals. It has been postulated
that secondary driers function by forming complexes with primary drier
Active (Primary) Driers: cobalt, zirconium, lead, cerium, iron etc.
Auxiliary (secondary) Driers: calcium, manganese, barium, zinc,
Cobalt is "the drier" metal and is most extensively used. It
is a powerful oxidation catalyst; and as a result, in coatings containing
cobalt alone, the surface dries preferentially causing surface wrinkling
and poor through dry in the extreme. It is therefore combined with other
metals such as lead, manganese, calcium, zirconium, etc. traditionally
(i.e. in conventional solids coatings) or with aluminum or lithium in
modern high solids coatings. Cobalt has a red-violet purple color :
however the yellow color of oils and resins counter this and resultant
coatings have increased whiteness. Cobalt therefore is invariably
preferred in white coatings.
The wrinkling effect produced by high cobalt levels is taken advantage
of when producing alkyd based wrinkling enamels.
Zirconium is the most widely accepted substitute for lead drier. It
functions mainly by its catalytic activity on drier metals such as cobalt
and manganese. The impetus for increased use of zirconium is environmental
regulations restricting use of lead. Zirconium is effective in both air
dry and bake coating systems. It improves gloss, hardness and through dry
without any adverse effect on other coating properties.
Manganese promotes both 'surface dry' and 'through dry', although it is
less efficient then cobalt and lead in air drying finishes. In baking
finishes manganese is superior to cobalt as it does not cause imbrutement.
Manganese also gives better result than cobalt in low temperature drying
performance and does not suffer from wrinkling under high humidity
conditions. However, manganese is rarely used along but added as a
modifier, with cobalt being use as a primary drier. Manganese generally
imparts a pink/yellow color to white enamels and hence is best avoided in
such finishes. In some systems such as urethane oils, use of manganese in
preference to cobalt results in reduced 'skinning' problems.