CoatingsTech Archives
Alkyd Coatings Using High-Performance Catalysts
June 2023
By Joshua Halstead
Alkyd-based coatings cure via both physical and chemical drying processes.
The natural drying time of an alkyd can be weeks to months, which is not
desirable from a practical point of view. In practice, the chemical crosslinking process is
accelerated using catalysts, commonly referred to as driers. Generally, these are transition
metal complexes with organic ligands. The most widely known and commonly used driers
are based on cobalt carboxylates. While cobalt driers lead to highly crosslinked hard films,
cobalt-based siccatives have recently faced reclassification as class 1b carcinogens by the
Cobalt REACH consortium,1 a nonprofit group tasked with preparing the registration dossiers
for cobalt and cobalt compounds. As a carcinogen, cobalt that is used to cure
desirable from a practical point of view. In practice, the chemical crosslinking process is
accelerated using catalysts, commonly referred to as driers. Generally, these are transition
metal complexes with organic ligands. The most widely known and commonly used driers
are based on cobalt carboxylates. While cobalt driers lead to highly crosslinked hard films,
cobalt-based siccatives have recently faced reclassification as class 1b carcinogens by the
Cobalt REACH consortium,1 a nonprofit group tasked with preparing the registration dossiers
for cobalt and cobalt compounds. As a carcinogen, cobalt that is used to cure
coatings and inks can be a risk to human health as humans can be in frequent
contact with these substances (especially when applying paints or scraping off old
paint layers). Many regions recognize that the use of cobalt in this industry must be
reduced as part of a movement toward a sustainable future.
contact with these substances (especially when applying paints or scraping off old
paint layers). Many regions recognize that the use of cobalt in this industry must be
reduced as part of a movement toward a sustainable future.
The two leading technologies to replace cobalt driers are based on either man-
ganese or iron. Manganese carboxylates(Mn3+) have long been known to exhibit
good drying activity, although generally to a somewhat lesser degree than that
observed with cobalt. Furthermore, the formation of Mn3+ species often leads to
a brown coloration of the formulation, which precludes its use at high levels or in
light-colored coatings. Most iron driers show poor activity,
especially in relation to cobalt or manganese analogs at ambient temperatures. Iron
carboxylates work well at high temperatures and are often used in stoving enamels
where curing occurs at 80–250 °C. Iron has a distinct yellow-brown color that can lead
to significant discoloration in light-colored formulations as well.
ganese or iron. Manganese carboxylates(Mn3+) have long been known to exhibit
good drying activity, although generally to a somewhat lesser degree than that
observed with cobalt. Furthermore, the formation of Mn3+ species often leads to
a brown coloration of the formulation, which precludes its use at high levels or in
light-colored coatings. Most iron driers show poor activity,
especially in relation to cobalt or manganese analogs at ambient temperatures. Iron
carboxylates work well at high temperatures and are often used in stoving enamels
where curing occurs at 80–250 °C. Iron has a distinct yellow-brown color that can lead
to significant discoloration in light-colored formulations as well.