cobalt 2-ethylhexanoate, at least) will be
classified as a carcinogen. Most European
paint manufacturers have already replaced cobalt in their formulations in anticipation of one or all of these concerns
being realized.
Cobalt Replacement
Strategies
Few viable options exist to replace cobalt
in alkyd and other air drying systems. As
noted above, only manganese and iron
complexes can approach cobalt’s activity level. Conventional iron, vanadium
and cerium carboxylate driers are not
effective replacements. “Accelerated”
manganese driers improve the activity
somewhat but may affect the color of
light tinted coatings over time and generate brittle films. However, a new iron-complex drier has been developed to
equal or exceed the drying performance
of cobalt and at reduced levels. The catalytic reaction mechanisms of cobalt and
the iron-complex driers have been shown
to be the same, however the efficiency of
the iron-complex drier is so high that less
oxygen is needed to promote oxidative
cure throughout the film, not just on the
surface as do the cobalt carboxylates.
Iron Complex Drier - FeLT®
The new iron-complex drier is an effective oxidative drier for a wide variety of
binder systems, particularly alkyd emulsions. Figure 4 shows the surface dry
time and Pendulum hardness test results
of several cobalt and manganese-based
driers compared to the iron-complex drier in a long-oil alkyd resin.
The test systems were evaluated for
dry initially and after one week exposure
at 40 ºC. Pendulum hardness tests were
run after one week cure. Note that the
new iron-complex developed faster surface drying at ambient temperatures than
any of the other products tested.
A major advantage of the iron-com-
plex drier is its ability to promote im-
proved oxidative cure under marginal
drying conditions which typically affect
conventional surface driers. In a white,
low-VOC high-solids alkyd-based for-
mula, the iron-complex drier was not
only more effective than cobalt and
accelerated manganese-based driers un-
der standard drying conditions, but also
was virtually unaffected in low tempera-
ture/high humidity environments where
the others experienced decreased activity
(Figure 5).
A major concern about coatings based
on alkyd and drying oil films is their ten-
dency to yellow in the absence of UV
light. Typically a cocktail of through dri-
ers is necessary to reduce yellowing by
forming coordinate bonds with the low-
molecular-weight chromophores that
are generated during the curing process.
The additional through drying property
of the iron-complex drier greatly reduces
the film’s tendency to yellow as seen in
Figure 6.
Reduced yellowing is believed to be
due to the increased level of deep film
oxidative crosslinking from the higher ac-
tivity of the iron-complex drier. However,
as with cobalt, coordinative driers like
zirconium may still be needed to promote
faster hardness values.
Summary of Cobalt
Replacement Driers
No other metal carboxylate drier can approach the oxidative activity of cobalt.
However, the iron-complex drier is a sustainable alternative to cobalt driers and
is not carboxylate-based. This product
is not CMR classified, can provide equal
or superior surface dry times to cobalt at
low levels under standard drying conditions, has improved efficiency in promoting surface curing under low temperature/
high humidity conditions and, due to its
additional through drying properties, can
