Introduction
Air-drying coatings consist of at least three main components, a binder
which forms the final coating, a solvent that aids in application and
uniformity coating, and a drier which catalyzes the conversion of the
binder into the coating (Webster 2020). Ongoing interest in sustainable
and non-toxic coatings has led to an increasing use of seed oils as
binders (Hofland 2012). Seed oils contain unsaturated fatty acids (Fig.
1) which polymerize in the presence of oxygen gas to form solid films.
This process, known as autoxidation, is initiated by H-atom abstraction
from sites adjacent to olefins generating C-centered radicals (Hubert et
al. 1997). While seed oil autoxidation does occur in the presence of
oxygen, it typically takes months for binders of this type to fully dry
in the absence of a drier. Driers, typically oil-soluble transition
metal compounds, catalyze H-atom abstraction allowing solid films to
form within hours of application (Bieleman 2002).
Currently, most driers used commercially are based on cobalt. These
cobalt-based driers are inexpensive and active at dosages as low as 0.01
wt. % (Charamzová et al. 2018). Recent concerns about potential
toxicological and environmental effects of cobalt in driers (Leyssens et
al. 2017) have led to efforts to transition from cobalt-based driers to
less toxic substitutes, including driers based on manganese(Bouwman and
van Gorkum 2007), iron (Křižan et al. 2017), and vanadium (Preininger et
al. 2015). To accomplish this transition, a detailed understanding of
the drying process is highly valuable.
Many mechanisms have been proposed for the role of the drier in
autoxidation (van Gorkum and Bouwman 2005; Soucek et al. 2012; Honzíček
2019). One widely invoked hypothesis involves the formation and
decomposition of organic peroxides through a Haber-Weiss-type mechanism
(Haber and Weiss 1932). In this mechanism low-valent driers reduce
organic peroxides by one electron, forming alkoxy radicals
(RO•) and hydroxide ions (OH-).
Subsequently, oxidized driers interact with a second organic peroxide,
reducing the metal to its original state and forming peroxy radicals
(ROO•) and protons (H+) (Fig. 2a). A
second mechanism which has been proposed to contribute to radical
initiation is peroxide homolysis catalyzed by Co(III) serving as a Lewis
acid (Fig. 2b) (Spier et al. 2013). In addition to this mechanism, it
has also been proposed that low-valent driers can interact directly with
molecular oxygen to generate superoxide
(O2•-) and high valent metal-oxo
species (Fig. 2c) (Kumarathasan et al. 1992). Oxidized driers are then
reduced to their original oxidation state by reacting with the binder.
Radical species formed through either mechanism, as well as high valent
metal species that may be formed, then proceed to abstract H-atoms from
bis-allylic methylene sites (Fig. 3, A ) in the binder.
After H-atom abstraction, the olefins rearrange to a conjugatedcis -trans arrangement (Fig. 3, B ) and the
C-centered radicals react with dissolved oxygen gas or other unsaturated
sites in the binder, propagating the radical chain. Further interaction
between these newly formed cis -trans conjugated species
and propagating radicals leads to the formation of isolatedtrans -alkene groups in the binder (Fig. 3, C ) (Hubert et
al. 1997). Details of these mechanisms, including the relative
contribution of each of the possible initiation reactions, the
importance of dissolved oxygen during propagation, and the role or roles
of the drier in propagation and termination have been the subject of
extensive computational modeling (Oakley et al. 2018). In order to test
the veracity of these proposed mechanisms, more experimental data on the
rates of autoxidation using different drier and oil combinations is
needed (Orlova et al. 2021).
Drying oil autoxidation is typically studied with Fourier transform
infrared spectroscopy (FT-IR) using the method developed by van de Voort
(van de Voort et al. 1994). In a typical experiment, the area of the
peak centered at 3010 cm-1 is measured as a function
of time during the autoxidation process. This peak corresponds to an
isolated cis -alkene stretching mode of the non-conjugated olefins
initially present in the drying oil (Fig. 3, A ). During
autoxidation cis -olefins isomerize, and the area of the peak at
3010 cm-1 decreases in intensity (de Boer et al.
2013). The rate of autoxidation is then determined by performing a
linear regression on the peak area as a function of time (Pirš et al.
2014). While this method is generally reliable for determining the rate
at which cis -alkene groups are consumed, it does not provide
information on the other reactions taking place during autoxidation. In
particular, this method does not provide information on the formation oftrans -alkene species (Fig. 3, C ) related to propagation
and termination reactions.
As an alternative to monitoring a single peak in the IR region, we
propose the use of global analysis to study drying oil autoxidation
rates. Global analysis is a family of mathematical techniques used to
find optimal fitting parameters between a model containing multiple
unknown variables and the entirety of known experimental data. Global
analysis typically begins by applying factor analysis to determine the
number of variables contributing to the experimental data (Malinowski
2002). The results of this factor analysis are then combined with
knowledge of the system to propose a model, and an iterative
optimization algorithm identifies the model parameters which provide the
best agreement between the experimental data and the proposed model. In
chemistry and biochemistry, global analysis has been widely applied to
spectroscopic data to improve the understanding of reaction kinetics in
complex systems (Malinowski 2002; van Stokkum et al. 2004). Global
analysis is particularly powerful in cases where: a) the spectrum of one
or more components cannot be independently measured due to experimental
limitations, b) the measured absorbances of one or more components are
small relative to other components due to a low concentration and/or low
absorptivities, and c) there is significant spectral overlap between the
absorbances of different components in the system. Each of these factors
is present in the time-resolved FT-IR spectra of drying oil
autoxidation, making this process ripe for study using global analysis.
In this manuscript, we report the results of our initial application of
global analysis to linseed oil drying by cobalt driers.