Cobalt has dominated as the go-to catalyst for curing alkyd coatings for decades, but its likely regulatory reclassification is pushing formulators toward iron and manganese alternatives — and research shows this switch brings a compatibility problem that isn’t obvious until you look closely at how these catalysts actually interact with the rest of the formulation.
What is it?
Driers (siccatives) are metal-carboxylate catalysts added to alkyd and drying-oil-based coatings to accelerate the oxidative curing process after the solvent has evaporated. ASTM D564 provides the standard test methods for determining the metal content of liquid paint driers, covering metals including lead, cobalt, manganese, zinc, and iron. According to a 2019 peer-reviewed review, a primary drier “can reduce the dry-time to hours rather than days,” while secondary driers (such as those based on aluminum, zirconium, or barium) “cannot start cross-linking, but facilitate redox processes throughout the coating.”
Why should I use it?
- A primary drier catalyst is what makes solvent- and oil-based coatings commercially viable, cutting cure time from days down to hours — see “How it’s used.”
- Cobalt’s dominant role is being phased out due to regulatory pressure, but switching to an alternative metal isn’t simply a one-for-one substitution — see Limitations.
How it’s used
Alkyd and drying-oil coatings cure through a radical oxidation process: without a catalyst, this can take days to form a tack-free film and may only fully cure at the surface, where oxygen is most available, leaving a soft coating underneath a hard skin. Primary driers, historically cobalt-based, solve this by initiating the radical cross-linking reaction directly, reducing dry time to hours. Secondary driers don’t start this reaction themselves, but support it by facilitating redox processes throughout the coating film.
Limitations — when it isn’t the right choice
- Switching from cobalt to an iron-based drier can break compatibility with the anti-skinning agent the formulation already relies on. Per the 2019 review, cobalt soaps are “under increased scrutiny because of likely reclassification as carcinogenic under REACH legislation,” a serious concern since these coatings are in regular contact with people — driving development of iron- and manganese-based alternatives. However, one highly active iron catalyst (iron-bispidon) “does not work efficiently with conventional anti-skinning agents like MEKO,” likely because the bulky bispidon ligand sterically blocks MEKO from coordinating to the iron center the way it does with cobalt. This means a formulator can’t simply swap cobalt for iron in an existing recipe and expect the same anti-skinning performance — the whole anti-skinning system may need to be redesigned alongside the catalyst change, since a catalyst that cures faster and more safely can still leave the coating vulnerable to skinning in the can if its supporting chemistry isn’t adapted too.
