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Alkyd paint and paint driers
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1 Introduction
A brief history of paints and coatings[1-4]

1
...
1

Paints and coatings occupy a prominent place in the cultural history of mankind
...
Archaeologists found pigments and paint grinding
equipment in Zambia (Southern Africa), thought to be between 350,000 and 400,000
years old
...
The ancient paints consisted of animal fat and
colored earth or natural pigments such as ochre
...
Around 3000
BC, Egyptians began using varnishes and enamels made of beeswax, gelatin and clay –
and later protective coatings of pitch and balsam to waterproof their wooden boats
...
Independently, the ancient
Asian cultures developed lacquers and varnishes and by the 2nd century BC, these were
being used as coverings on a variety of buildings, artwork and furnishings in China, Japan
and Korea
...
In India, the secrete
of the lac insect Coccus lacca was used to produce a clear coat to beautify and protect
wooden surfaces and objects
...
It was not until the industrial revolution and ensuing mass production of linseed
oil, however, that the production of modern house-paints began
...
Today, coatings not only protect and beautify the substrate
...
Paints and coatings play an
indispensable role in our modern world and cover virtually everything we use from
household appliances, buildings, cars, ships, airplanes to computers, microchips or printed
circuit boards
...
van Gorkum and E
...
Chem
...
2005, in press

Chapter 1

1
...
2 Alkyd paint
A modern paint is a complex mixture of components (Table 1
...
[5] Oil paint, the
oldest form of modern paint, uses a binder that is
Table 1
...
In alkyd paint, the
Component
Weight-%
binder is a synthetic resin, which is called an
binder
30
alkyd resin
...
Hence, in a chemical
extenders
12
sense the terms alkyd and polyester are
additives
2
synonymous
...
A typical alkyd resin is prepared by heating for example linseed oil, phthalic acid
anhydride and glycerol to obtain a fatty-acid containing polyester, as schematically shown
Fig 1
...
Paints based on alkyd resin binders are usually solvent-borne paints, common
solvents being white spirit (a mixture of saturated aliphatic and alicyclic C7-C12
hydrocarbons with a content of 15-20% (by weight) of aromatic C7-C12 hydrocarbons), or
xylene
...
Examples are: easier application properties, wider application and drying tolerance
under adverse conditions (low temperature, high humidity) and a higher level of
performance on difficult substrates, such as heavily stained or powdery substrates
...
[1] Further important components of
alkyd paint are pigments and extenders
...
Pigments are derived from natural or synthetic materials that have been ground into
fine powders
...

Extenders are also used to adjust the consistency of a paint and to let down colored
pigments of great tinting strength
...
A large variety of coating additives is known, which have
widely differing functions in a coating formulation
...
1: Example of an alkyd resin used as a binder compound in
alkyd paint
...


10

Alkyd Paint and paint driers

catalysts, or driers
...
Without driers, the drying time of
alkyd paint would be over 24 hours, which is clearly undesirable for most applications
...
1
...
The
first process is the physical drying of the paint
...
Then chemical drying
(also called oxidative drying) occurs, a lipid autoxidation process, which means that the
paint dries by oxidation of the binder compound with molecular oxygen from the air
...
During the drying process
four overlapping phases can be discriminated:
•
•
•
•

Induction period
Hydroperoxide formation
Hydroperoxide decomposition into free radicals
Polymerisation / crosslinking

The induction period is the time between application of the paint to a surface and
the start of dioxygen uptake by the paint film
...
Autoxidation of the
unsaturated fatty-acid chains in the alkyd binder then gives rise to hydroperoxides with
uptake of atmospheric oxygen
...
These radicals initiate the polymerisation of
the unsaturated molecules of the binding medium
...
The number of cross-linked sites that are formed determines the
film hardness
...


1
...
2
...
[7] More specifically, autoxidation is described as the
insertion of a molecule of oxygen into a C-H bond of a hydrocarbon chain to give an alkyl
hydroperoxide
...
[8-15] Generally, an
induction time is observed after which the autoxidation reaction abruptly starts and
rapidly attains a limiting, maximum oxidation rate
...
Scheme 1
...

Initiation can occur via several different pathways, either metal mediated or not
...
1 are often proposed as the initiation steps in non-metal-

11

Chapter 1

Initiation
...
1: Radical chain reactions as occuring in metal-catalysed autoxidation
...
[5, 18] Step 1 is the reaction of some arbitrary initiating species with
the substrate, directly forming a carbon-centred radical
...
In metal-mediated initiation, the initiation of radical chains
predominantly occurs through steps 3 to 6
...
[19] This set of reactions is the generally accepted mechanism by which
hydroperoxides are decomposed in polar media by a metal with two stable oxidation
states differing one electron
...
Reaction 4 generally is slower than reaction 3
...
This complex can then
abstract a hydrogen atom from the substrate to form a hydroperoxide complex and a
carbon radical (5b)
...
[21] In
the last initiation reaction in scheme 1
...
[8, 15, 22]
The most important propagation reactions are reactions 7 and 8
...
[10,
18, 23]
Reaction 8 is relatively slow and thus it is the rate-determining step for the
formation of hydroperoxides
...
[23] One initiation event can set off the cycle
between reactions 7 and 8, which is why these reactions are responsible for a large
increase in hydroperoxide concentration during the first stages of autoxidation
...
This
is the main reason for autoxidation to attain a limiting rate
...
These alkoxide
radicals can also propagate radical chains via reaction 9 or various other reactions, see
section 1
...
4 for examples
...
Alkoxide radicals are much
too reactive and hence their concentration is rather low
...
[11] Reaction 11a proceeds according to the
Russell-mechanism,[18, 24] where a tetraoxide intermediate is formed which decomposes to
yield non-radical products: an alcohol, ketone and singlet oxygen
...
This
reaction is proposed to proceed also via a tetraoxide intermediate,[25] some speculation
regarding the validity of that proposal remains, however
...
2 the
tetraoxide intermediate and the course of termination reactions 11a and 11b is shown
...


R
C

O

CH

O +

OH

+

1

O2

R

R

H
O
H

C

R
R
11b
...
2: Bimolecular peroxyl radical termination via a tetraoxide intermediate
...
In reaction 11’, the tetraoxide
intermediate decomposes to yield two alkoxyl radicals, which recombine in the solvent cage to form a
peroxide cross-link
...
1
...
Common transition metals capable of hydroperoxide
decomposition are Fe, Cu, Co, Mn, Ti, V and Ce
...
A
metal compound has to be able to perform both reaction 3 and 4, or 3 and 6 so that each
change of its oxidation state starts a new radical chain
...
1
...
[14, 15] Particularly for
manganese(II) complexes, reaction 12 is one of the causes for the induction time that is
often observed for metal-catalysed autoxidation reactions
...
[31] Higher valent metals might be
able to inhibit autoxidation (at low oxygen concentration) by scavenging carbon radicals,
reaction 13 in scheme 1
...
Cu2+ and Fe3+ ions have been shown to readily reduce certain
carbon radicals
...
[34]
1
...
2 The autoxidation of alkyd paint
1
...
2
...


These different aspects are highlighted in the following sections
...
2
...
2 Autoxidation in apolar media: metal-hydroperoxide complex formation
Autoxidation drying starts when all solvent is evaporated
...
This has some consequences for the
autoxidation reactions, i
...
ionization processes will be suppressed and metal salts will not
dissociate into ions
...
Hydroperoxides
are decomposed following metal-hydroperoxide complex formation, as shown in scheme
1
...
[9, 10, 14, 35, 36] This has some far-reaching implications for metal catalysis, since
reaction 4b in scheme 1
...
[10]
ROOH + Mn+

º [(ROOH)M]n+

[(ROOH)M]n+ → RO• + [(M(n+1)+(OH–)]n+
ROOH + M(n+1)+

º [(ROOH)M](n+1)+

[(ROOH)M](n+1)+ → ROO• + Mn+ + H+

3a
3b
4a
4b

Scheme 1
...
1)
...
[14]
In apolar media, metal salts will form a complex with a hydroperoxide as long as the
hydroperoxide is available
...
1, and thus sudden inhibition of autoxidation occurs if
the metal concentration becomes higher than the hydroperoxide concentration
...
2
...
3 Fatty acid autoxidation
The fatty acid tail of the alkyd resin (Fig 1
...

Fatty acids are important biomolecules, and are present in lipids as their triester with
glycerol
...
[7, 27]
The fatty acids in an alkyd resin are polyunsaturated fatty acids, commonly
linolenic acid, (α-linolenic acid = 9Z,12Z,15Z-octadecatrienoic acid and γ-linolenic acid =
6Z,9Z,12Z-octadecatrienoic acid) which is a major constituent of linseed oil,[37] or linoleic
acid (9Z,12Z-octadecadienoic acid) which is a major constituent of, for example,
sunflower oil and soya oil
...
2)
monoallylic CH,
88 kcal/mol
R
H

H
C

C

H

bis-allylic CH, 75kcal/mol
H

C

H

R'

H

Alkyl CH, 101 kcal/mol
Figure 1
...
[27, 38, 39] Abstraction of one of the bis-allylic hydrogen atoms results in the
formation of a radical species
...
Molecular oxygen reacts extremely rapid with this
pentanedienyl radical species to form a peroxy radical which has the double bonds
dominantly conjugated, since this is the most stable structure
...
2
...
1, but
in the early stages of autoxidation the dominant reaction will be to abstract a hydrogen
atom from another lipid molecule to form a hydroperoxide and propagate the radical
chain (reaction 8 in scheme 1
...
Scheme 1
...
Figure 1
...
The hydroperoxide concentration will go through a maximum
when hydroperoxide formation is surpassed by hydroperoxide decomposition
...


Scheme 1
...


Figure 1
...
[41]

16

Alkyd Paint and paint driers

1
...
3
...

A general review of earlier work regarding all aspects of the polymerisation of
drying oils was given by Wexler,[40] and several recent reviews are available covering
fatty acid autoxidation
...
[43] Yellowing is caused by over-oxidation of the coating and
is a general problem for alkyd coatings
...

1
...
2 Studies with drying oils
The fatty acids in natural oils are present predominantly as lipids
...
Drying oils are prone to autoxidation and, as a precursor to
an actual alkyd resin, are often used as a model compound to study paint drying
...
Autoxidation of the
oils is then accomplished thermally or via photo-oxidation
...
[42] Soya oil was dried as a thin
film on glass using cobalt and lead driers (see section 1
...
The oil was converted to
methyl esters prior to analysis with GLC
...
The
soluble part yielded peaks corresponding to oleate, stearate and palmitate
...
[44] The IR spectrum lacked bands
characteristic of unsaturation and showed additional bands typical of OH, COOH and
other, non-ester, carbonyl containing species
...
1
...
[42]
O
CH
(CH2)7
C O
CH3O

n

Figure 1
...
Terminal groups could include OH, COOH and CHO
...
studied the drying and oxidative degradation of linseed oil by
Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG), differential
scanning calorimetry (DSC) and size exclusion chromatography (SEC)
...
Different films were naturally aged, thermo-oxidised at
80 °C, or irradiated with wavelengths > 295 nm
...
in a
study concerning the oxidation of edible oils
...
[45] An insoluble polymer was formed after
extended oxidation times at 80 °C (up to 150 hours)
...
The SEC chromatogram of the soluble part of the
aged sample shows a peak due to the original component, as well as a peak assigned to
dimers and a continuous distribution of higher molecular weight fractions
...
The spectrum of the soluble sample shows signals of lower
intensity for the ester groups relative to the methylene signals in the sample, which would
suggest that a process of fragmentation takes place in which aliphatic chains are
preferentially released from the insoluble network
...
It was thus supposed that
treatment of linseed oil at moderately higher temperatures only accelerated the natural
drying and degradation processes, not altering the type and extension of the reactions
...
sought to gain insight into the long-term behaviour of oil-based
paints by studying the thermo- and photo-oxidation of several different drying oils
...
[47] Samples were spread out as a thin film on a either a KBr or a glass
window, after which they were oxidised in a ventilated oven at 60 °C in the dark
...
[47] Oxidised samples were treated with NO after increasing reaction times to
differentiate between alcohol and hydroperoxide formation
...
[47] Studying the
time course of hydroperoxide formation and alcohol formation, it was concluded that
alcohols are secondary products formed only after hydroperoxides are created
...
Two new peaks were observed after treatment, i
...
at
1843 cm-1, attributed to non-conjugated acid fluorides, and at 1810 cm-1 attributed to
conjugated acid fluorides
...
Carboxylic acid formation was
proposed to occur by oxidation of aldehydes or via hydrogen abstraction from the tertiary
carbon with a hydroperoxide function and subsequent β-scission after hydroperoxide
decomposition, as shown in Fig
...
5
...
5: Carboxylic acid formation via hydrogen atom abstraction and subsequent β-elimination
reactions and oxidation
...
[47] This band was attributed to the formation of
hydroperoxides and also peroxide cross-links, since the band remained present even after
extended drying times (over 100 h), during which period the hydroperoxide concentration
decreased notably
...
concluded that the
oxidation of the fatty acids in linseed and poppyseed oil at 60 °C proceeds according to
the following general sequence: methylene-interrupted cis à trans-conjugated à transnon-conjugated à saturated
...
The decrease in unsaturation was explained by epoxide formation
through addition of peroxyl radicals on conjugated double bonds
...
[48] The hydroperoxide concentration as a function of oxidation time was
determined by iodometric titration
...
For oxidation reactions carried out at higher temperature, PVs
were found to be lower
...
it was found that the
peroxide value determined by iodometric titration was the sum of the ROOH and ROOR
concentrations
...
The ROOR concentration for linseed oil dried at 60 °C is
lower than the ROOH concentration and follows the same trend when monitored over
time: going through a maximum and then decaying
...
[48] The photo-oxidation
of a dried linseed oil film leads to severe degradation of the dried film network, as was
concluded from the observed major changes in the FTIR spectrum, i
...
a rapid decrease of
peaks due to double bonds, ester bonds and even alkyl groups
...
Thus, although the cross-linked network is
quite photo-labile, the thermostability is relatively high
...
3
...
3
...
1 General observations in ethyl linoleate drying
In the last decade, Muizebelt and colleagues made important advancements
concerning knowledge of the oxidative crosslinking of alkyd resins
...
e
...
4
...
Ethyl linoleate
19

Chapter 1

autoxidation was monitored over an extended period of time (up to 1 year) using several
analytical techniques
...
The relative reactivity of ethyl oleate
vs ethyl linoleate was determined using the saturated esters as internal standards
...
[53] Size exclusion
chromatography (SEC) showed relatively fast oligomer formation and the distribution of
oligomers reached after one day did not change substantially upon further reaction
...

Discrete oligomers slowly disappeared, as was evidenced by a smoothing of the SEC
curve after long reaction times
...
From these results it was concluded that cross-linking continues
over prolonged reaction times with concurrent degradation
...
3
...
2 Time evolution of drying and cross-link formation
The cross-linking in the autoxidation of EL (99% purity) was studied in more
detail using 1H NMR spectroscopy, quantitative 13C NMR spectroscopy and 13C NMR
spectroscopy with the so-called POMMIE technique
...
Spectra were recorded at
increasing reaction times and integrated to yield the time dependence of the relative
amounts of double bonds, ether and peroxy cross-links, as well as carboxylic acids,
aldehydes and epoxides
...
The formed conjugated double bonds decreased to low levels after two
months
...

The epoxide concentration also decreased to zero within two months, however
...
Ether and
peroxy cross-links were formed in roughly equal amounts in a period of 133 days
...
Over prolonged reaction times (1 year) the POMMIE 13C-NMR results
showed that the total double bond concentration continues to decrease for the entire
duration of the experiment, the peroxy signal remains more or less constant and the ether
signal increases
...
[50] The amount of dimeric C-C cross-linked material formed in the
autoxidation of EL was determined to be about 5%
...
3
...
3 Oligomerisation in detail
Oligomerisation products of EL dried by cobalt in air were analysed further with
Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), Direct Chemical
Ionisation-Mass Spectroscopy (DCI-MS) and Fast-Atom-Bombardment Mass
Spectroscopy (FAB-MS)
...
[51] After 60 days of drying peaks are smaller and only
dimers remain, with the formation of new compounds with molecular weights between

20

Alkyd Paint and paint driers

that of specific oligomers
...
[51] These observations again indicated the ease of degradation
of the formed oligomers
...
Some important mechanistic conclusions
were drawn from the mass spectra
...
Formation of trimers was attributed to either combination reactions of
(dimeric) radicals, or to radical addition at the double bonds of dimers, related to the
occurrence of peaks with masses of either (3EL-4H), or (3EL-2H), respectively
...
[54] The
observation that chains with conjugated bonds cross-link according to a different
mechanism than chains with non-conjugated bonds was confirmed by drying studies with
methyl ricinoate (MR), which is the methyl ester of a conjugated isomer of linoleic acid
(9,11-octadecadienoic acid, methyl ester)
...
This observation was explained by a cross-link mechanism of radical
addition to a double bond and subsequent disproportionation, which leads to masses of
2M-2 and 2M, as shown in Scheme 1
...
Radical addition to a conjugated double bond

R

+

R2

R1
H1

R

H2

R
H1*

H2
R2

R1

H1

H2

R2

R1

H1

R2

R1

R

R
H1

*

R2

R1 H1

+
H2

*

R2

R1
R

H2

H2*

Disproportionation
Scheme 1
...


generates an allyl radical, which is stabilized by delocalisation and the reason why
conjugated double bonds are more reactive in a radical addition than non-conjugated
double bonds
...
[51]
Modern “high-solid” alkyd paints contain so-called “reactive diluents”
...
Muizebelt et al
...
[55] It was shown by
mass spectroscopy that the reactive diluents do form cross-links with EL
...

In a recent study, Oyman et al
...
[56] The autoxidation was catalysed by a water-emulsifiable
cobalt drier and the reaction was monitored by FTIR and Raman spectroscopy on
emulsion films
...
[56]
1
...
4 Studies with the model compounds (Z,Z)- and (E,E)-3,6-nonadiene
The symmetrical model compounds (Z,Z)- and (E,E)-3,6-nonadiene have been
used to get further insight in the cross-link reactions that occur during the drying of alkyd
resins
...
Similar studies have been carried
out with (Z,Z)-2,5-heptadiene and (Z,Z)-4,7-undecadiene
...
Since the nonadiene isomers are
rather volatile, the reaction was performed in a closed vessel with a sufficient amount of
air present
...
The products that were isolated and characterised and the
proposed reaction paths leading to those products are depicted in Scheme 1
...
Both
isomers were observed to react equally fast and the same products were found when
starting from either the E,E or the Z,Z isomer, due to extensive (Z) à (E) isomerisation
...
[57]
Products 1 through 10 shown in Scheme 1
...
Only very small amounts of compounds 8
and 9 were observed, as was expected
...
Compound 9 is the evidence that some of the oxygen
reacts with C-5, while the C-3 positions are known to be far more reactive
...
The hydroperoxide that is
formed is decomposed, forming alkoxy radical species C
...
[57] Aldehydes and carboxylic acids can be
formed through β-scisson reactions of alkoxy radicals; see Scheme 1
...


22

Alkyd Paint and paint driers

O

10

ROOH
or
O
R
OOH
1

2

4

3

A

5
or

6

7

8

Initiation
9

B

-H

O2

RH

O2

Ox
...


O

O2
RH

RH

OOH

O

HOO O O

3

2

E O
O O

4

O

O

O
Ox
...
6: Products of the cobalt-catalysed autoxidation of (E,E) or (Z,Z)-3,6-nonadiene
...
Species A-F are proposed reaction intermediates
...
[57]

Ketones 3, 7, and 9 can be formed by oxidation of alkoxy radicals and epoxide 4
via an intramolecular addition reaction of the alkoxy radical to an adjacent double bond
...
The most probable routes for direct epoxidation were
proposed to be via peracids and via hydroperoxides
...
A singlet-oxygen pathway was also found to exist, since the
H

β-scission

O

+
O2

Ox
...
7: Example of a β-scission reaction leading to chain breaking and formation of aldehydes and carboxylic
acids
...
This compound must be formed from species G,
which in turn can only be formed by the direct addition of singlet oxygen to a double
bond in 3,6-nonadiene
...
In a separate study, Hubert et al
...
[62]
Two “dimeric” species, 5 and 6 were isolated
...

An important conclusion from the autoxidation experiments with 3,6-nonadiene is
that oxidation proceeds via three different mechanisms: normal radical autoxidation,
photo-sensitised oxidation (reaction with 1O2) and epoxidation
...
[57]

1
...
4
...
Common driers are metal soaps of carboxylic
acids
...
[63] The driers that are used today are based upon synthetic acids, like
2-ethyl hexanoic acid and versatic acid, shown in Fig
...
6
...
6: Carboxylates used in traditional drier metal soaps
...
II: 2-ethylhexanoate, often referred to as “octoate”
...


carbon atom adjacent to the carboxylic acid group resulting in a highly branched
structure
...
The introduction of water-borne and high-solids alkyd paints has led to the
development of new drier systems
...
The metals used for the driers in each category
are listed in Table 1
...
[64, 65]

24

Alkyd Paint and paint driers

Table 1
...
The most important function
for autoxidation catalysts is hydroperoxide decomposition and consequently all primary
drier metals have two accessible valence states that differ by one electron, which allows
for catalytic hydroperoxide decomposition
...

The most widely used metal in primary driers is cobalt
...
[63] The exact structures of the cobalt species present during
alkyd paint drying are not precisely known
...
Cobalt(III) with carboxylate ligands tends to
form polynuclear complexes,[10, 66] and Lombard et al
...
1
...
[67] Also, the
X
St

H
O

Co(III)

St
X

X

Co(III)
O
H

St
St

X

Figure 1
...
St = stearate anion, X = water or alcohol
...
2
...
2)
...
[12]
Manganese soaps are also used as primary driers, although they are almost always
used in combination with cobalt soaps
...
5
...
A disadvantage of the use of manganese is the brown color of its
compounds in the trivalent state, and manganese driers are thus not preferred to be used in
high concentration in light colored or white paints
...
Although
iron complexes are very potent redox catalysts in aqueous solutions, in apolar solvents the
25

Chapter 1

Fe(III) ion in metal soaps is not easily reduced, which prevents the redox cycle necessary
for catalytic hydroperoxide decomposition
...
Iron complexes are also
very strongly colored, which prevents their use for the same reasons as for manganese
complexes
...
[63]
Secondary driers are active in the cross-linking steps of drying, they are
responsible for an overall drying throughout the entire paint layer
...
Zirconium, bismuth, barium and strontium
have all been labelled “lead-replacements”, but zirconium is the most widely accepted
replacement for lead
...
[68, 69] In an account by Ali et al
...
[70]
Aluminium compounds greatly enhance cross-linking
...
[71] Turner et al
...
1
...
[72] The use of aluminium compounds as additional crossR

R

HO

O

Al

Al

O

O

RO

Al
O

O

O

O

R

O
O
Al
O

R

Al
O

R
Figure 1
...


linkers in high-solids alkyd paints has also been studied
...
[73]
Auxiliary driers are added to enhance or alter the activity of the primary drier, and
thereby improve the appearance and quality of the total paint film
...
Zinc, lithium and potassium driers are added to
paints with a cobalt drier to inhibit wrinkling of the paint film, which is caused by
26

Alkyd Paint and paint driers

differential hardening of the surface relative to the remainder of the film
...
[64] Potassium driers combined with cobalt driers
were found to be very effective in the through-drying of especially water-borne alkyd
systems
...
Judged by
volume, calcium is one of the most used driers
...
4
...
[52, 65,
69, 71, 76-81]
Frequently used drier combinations are, for example Co/Zr/Ca, Co/Pb/Ca or
Mn/Zr/Ca
...
have compared the use of cobalt plus
zirconium octoate with cobalt plus lead octoate in the autoxidation of linseed oil at
60 °C
...
Two rate constants could be obtained
from logarithmic plots of viscosity vs time, the increase of the viscosity in a first time
domain (0-12 h) was attributed to oxidation and in a second time domain (14-23 h) to
polymerisation
...
The observation that a Zr catalyst alone (no
cobalt) did not show any activity and the observation that for Co/Zr combinations the
increase in the rate of viscosity change was predominantly in the time domain attributed
to polymerization, led to the conclusion that Zr did not catalyze oxidation-reduction
...
reported two detailed studies examining the effects of
combinations of Co, Zr and Ca on the autoxidation reactions taking place during linseed
oil or alkyd resin drying
...
[80] The
PV maxima for the different drier systems occurred all after the same drying time and the
PVs followed the order: Co alone > Co/Zr > Co/Zr/Ca
...
Peroxide values for linseed oil
dried by Ca/Zr and Zr alone resembled the PV time curve observed for linseed oil dried
without any driers added; Zr and Ca/Zr thus did not catalyse peroxide decomposition, as
was also found by Meneghetti et al
...
[80] This conclusion was further supported by the fact that peroxide decomposition
kept slowly continuing for the drier systems Co/Zr and Co/Zr/Ca even after 3000 hours,
where the linseed oil PV for the mixture with cobalt alone levelled off
...
[80]
The autoxidative cross-linking in an alkyd film, cast from a water-borne alkyd
emulsion and dried by different drier combinations was studied by magnetic resonance
(MR) profiling
...
Alkyd mobility in the film is related to the spin-spin
relaxation time, T2: solids have a low molecular mobility and a short T2 time, whereas
27

Chapter 1

viscous liquids (such as an oil) have a greater mobility and a longer T2 time
...
Only after two to three hours, the cross-linking near the surface of the film
becomes faster than near the substrate, (in this case, “the substrate” means “the surface
where the film is applied upon”) and drying became non-uniform
...
[81] The difference between T2 values
near the surface and the substrate was observed to follow the order Co > Co/Zr >
Co/Zr/Ca > Co/Ca
...
[81] Conventional drying-time
measurements using a drying recorder showed that the Co/Ca system indeed had the
shortest through-drying time
...
[82]
The fact that drying was found to be uniform throughout the paint film for the first three
hours, led to the conclusion that the improvements in cross-link formation by the addition
of secondary driers should be accomplished by their action in the initial drying stages and
not, as one might assume, during later stages of drying
...
5 Enhancing drier activity and the development of new drier systems
1
...
1 The effect of added ligands
In the earliest reports on paint drying, nitrogen-containing ligands were added to
paint mixtures containing cobalt-drier soaps to prevent so-called loss-of-dry
...
Loss-of-dry can be attributed to
failing of the paint drier, which may have several reasons, such as: adsorption of the drier
on the pigment surface,[85, 86] formation of insoluble drier complexes in the paint mixture
and drier hydrolysis (common in waterborne paints)
...
[83] Nicholson had already shown in a conductometric study of the adsorption of
cobalt acetate and [tris(1,10-phenantroline)cobalt(II)]di(acetate) on titanium dioxide in
acetic acid, that the phenantroline complex is apparently adsorbed to a lesser extent on the
TiO2 surface than cobalt acetate
...
[87] No differences were found in adding the ligands in-situ to the paint, or by
using pre-formed complexes
...

In general, aromatic or heterocyclic nitrogen-containing ligands (except pyridine)
or aromatic and aliphatic primary and secondary (di)amines were found to prolong the
drying time to a considerable extent
...
Wheeler also claimed in several US patents that manganese
carboxylates (naphthenates, oleates, octoates) show a remarkably enhanced drying
activity (produce shorter drying times) when o-phenantroline, 2,2’-bipyridine or other
aromatic nitrogen containing ligands are added to the paint mixture (Fig
...
11)
...
, after studying the drying time of paint mixtures vs the molar ratio of added
1,10-phenantroline for different metal concentrations, proposed that a
bis(phenantroline)cobalt complex is formed at the phenantroline concentration which
produces most rapid drying
...
[91]
NH
N
N
N

N

1
...


C

N

N

5
...


H

CH3
C
N

C

N

3
...


N
N

7
...


O
N
N

N

9
...


N

N

11
...
9: Ligands that accelerate paint drying when using manganese (with 1-11) or cobalt (with 1,2)
carboxylates as driers
...
studied the use of different nitrogen ligands as loss-of-dry
agents with cobalt, manganese, iron, lead, chromium and zinc carboxylate driers
...
In contrast to the findings of Nicholson, Zettlemoyer found that
adding 1,10-phenantroline in-situ to a cobalt containing varnish retarded the drying time
...
[84]
Zettlemoyer et al
...
[92] The ligands were added in-situ in a 2:1 molar
ratio of ligand-to-metal
...
From this and earlier findings it was concluded that although
aromatic heterocyclic didentate nitrogen ligands accelerate cobalt driers in pigmented
systems, this is apparently not the case in unpigmented systems
...
3 shows the results of the manganese, iron and lead driers with the ligands
that produced the highest activities for each drier
...
The manganese drier was
accelerated mainly by bifunctional heterocyclic nitrogen ligands or Schiff-base
compounds and retarded by strongly basic diamines, substituted phenylenediamines,
diphenylamine, diethylenetriamine, l-cysteine, tetrabutylethylenediaminetetraacetate and
sulfur-bearing amines
...
Myers, Wheeler and Canty
Table 1
...

Additivea

N,N’-bis(salicylaldehyde)ethylenediamine

a

Activityb
...
03% Co = 1
...
4

Fe
0
...
70

% Improvementc
more improvement =
higher %
Mn
Fe
Pb
180
210

N,N’-bis(salicylaldehyde)-1,2-phenylenediamine

1
...
35

Dicyclohexylamine

1
...
00

100

Cyclohexylamine

1
...
00

100

Pyridine-2-carboxylic acid

0
...
95

90

0
...
95

Dicyandiamide

0
...
35

80

90

2-Naphtylamine

0
...
90

80

2-Methyl-6-aminopyridine

0
...
80

60

2-(2-Dimethylaminoethylamino)pyridine

0
...
75

50

N-Dicyanoethylbenzenesulfonamide

0
...
70

2-Vinylpyridine

0
...
70

40

Triphenylguanidine

0
...
70

40

dl-alanine

0
...
35

40

0
...
35

130

Allylamine

0
...
35

130

4-bromoacetanilide

0
...
35

0
...
65

40

2-Aminoethyl hydrogen sulfate

0
...
55

20

Nitro-1,10-phenantroline

0
...
55

20

Urea

0
...
50

10

Methylacetylurea

0
...
bActivity = (DtimeCo/Dtimem+l) where DtimeCo = the drying time of a
sample containing 0
...
c((Dtimeold–Dtimem+l)/Dtimem+l)×100%, where Dtimeold = the drying time for that metal
naphthenate without any ligands added and Dtimem+l = the drying time with ligands added
...
[91, 93, 94] They supposed that manganese phenantroline complexes perform
better in the coordination and activation of molecular oxygen, and that oxygen activation
followed by addition of the dioxygen molecule to the paint resin is the primary mode of
action of the complex
...
[20, 34]
The ligand 2,2’-bipyridine (bpy) is often added to cobalt and manganese driers in
water-borne alkyd coatings
...
[95, 96] In the coordinated case, the counterions usually are
carboxylates, phosphonates, sulfonates or sulfates
...

Recently, Warzeska et al
...
[97] The autoxidation of EL catalysed by
cobalt and manganese 2-ethylhexanoate and manganese 2-ethylhexanoate with bpy was
followed by FTIR (see chapter 2 for more information on the exact methods that were
used)
...
Addition of 0
...
of bpy to the manganese catalyst already drastically shortened the induction time
...
5 equivalents did not change the rate or the induction time any
further
...
This result is in agreement with
the earlier observations by Zettlemoyer et al
...
proposed that
coordination of bpy to the cobalt drier
results in a lowering of the concentration
of the catalytically active species, and
consequently, in a decrease in reaction
rate
...
1
...

The fact that, after addition of 0
...

of bpy the autoxidation rate and induction
time of the manganese catalysed reaction
did not change any further, is well in
agreement with the formation of the figure 1
...
[MnII2MnIII2O2(2-ethylhexanoate)6(bpy)2], determined by
crystal X-ray crystallography
...

it was concluded that these kind of cluster

31

Chapter 1

compounds might well be present as catalytically active species in the autoxidation
reaction
...
used time-resolved FTIR and Size Exclusion Chromatography (SEC)
(see also chapter 2) to study the influence of the ligands 2-aminomethylpyridine (amp, a
ligand already proposed by Wheeler, see Fig
...
9, 10) and 2-hydroxymethylpyridine
(hmp), as compared to bpy, on the autoxidation and crosslink formation rates of ethyl
linoleate catalysed by manganese 2-ethylhexanoate
...
[98] The ligand amp was reported to show a comparable effect on the
rate and induction time as for bpy, while hmp shows a slightly shorter induction time and
a larger rate
...
Here,
formation of dimers was observed after 4 hours for the system containing hmp, after 6
hours for amp and 8 hours for the bpy system
...
5 h) followed by manganese + amp
(9 h) and hmp (13 h)
...

1
...
2 Driers not based on metal carboxylates
Few reports are available of driers for alkyd paints not based on metal
carboxylates
...
did some drying experiments with modified hemin, the
iron(III)-porphyrin complex isolated from hemoglobin
...
The properties of dried ink films were not satisfactory, however, and this combined
with the high cost of hemin made the complex unattractive as a commercial drier
...
Although
they have received much attention as catalysts in the fields of autoxidation,[99-101]
polymerisation [102-106] and hydroperoxide decomposition,[100, 101, 106-108] few studies have
been performed on the use of metal acetylacetonates as possible paint driers
...
and Bear et al
...
[109-111] For each metal acetylacetonate, the viscosity change over time in the
absence and presence of tert-butyl hydroperoxide, the dioxygen uptake of the sample, the
weight change of the sample at different temperatures by thermogravimetry and the
infrared spectra of autoxidised linseed oil samples were studied
...
The
thermogravimetric experiments at 60, 80 and 100 °C showed the highest rates of weight
increase for the acetylacetonates of Co2+/Co3+, Mn2+/Mn3+ and Fe2+
...
In chapter 3 the activity of manganese acetylacetonate in the
autoxidation of EL is evaluated
...
5
...
tested bis(1,1,1,5,5,5-hexafluoroacetylacetonate)manganese(II) as a
catalyst in the autoxidation of EL emulsified in water
...

32

Alkyd Paint and paint driers

The first 60 min of autoxidation could not be followed by FTIR due to the large
absorption of water in the region of interest
...
The complex [Mn(hfac)2] by itself was found not to be active in the
autoxidation, when either one of the ligands hmp or amp (see section 1
...
1) were added,
however, induction times (after water evaporation) dropped below 100 min and the
autoxidation proceeded at a quite acceptable rate
...
4
...
4: The EL autoxidation activity of [Mn(hfac)2] compared to Mn WEB,
with and without added ligands
...

Catalyst

Ligands
3 equiv
...
1
70
37
...
2
>500
80
17
...
5

Real alkyd paintb
Drying time (h)
15
7
7½
n
...

5½
8

a
Relative rate calculated from: -ln([EL]t/[EL]60 × 100), a plot of the natural logarithm of the integral
of the 3010 cm-1 peak in the ftir spectrum, vs time
b
White, water-borne paint based on a medium-oil alkyd emulsion siccativated with 0
...
Drying time measured with a Braive drying recorder[70] at 23 °C and 50% relative
humidity
...
Upon
addition of the ligands amp or hmp, a lower activity than the Mn WEB drier was found
for [Mn(hfac)2] in the model system, in real paint, however, the
hexafluoroacetylacetonate complex showed comparable or better drying times than the
commercial drier
...
, new manganese driers for alkyd paints
are described based on manganese
complexes with substituted salen-type
N
N
R
R
ligands
...
11: Examples of salen type ligands used with
manganese as alkyd paint driers
...
In Fig
...
11 two examples of tert-butyl, or R1 = a dodecyl chain, R2 = H
...

In a patent by Micciché et al
...
[113] As
mentioned in Section 1
...
1, iron compounds are not effective driers at room temperature,
since the reduction of iron(III) to iron(II) by hydroperoxides proceeds slow in apolar
media
...
[113]
1

1

2

2

33

Chapter 1

Oyman et al
...
1
...
[114] The complex was found to have a slightly higher EL oxidation activity than
the commercially available manganese catalyst Nuodex-Web-Mn, but a much lower

N
CH3

N

CH3

Mn

CH3
N
O
O

Mn

N

CH3

(PF6)2

O
N

CH3

N
CH3

Figure 1
...


activity compared to a cobalt drier (Nuodex-Web-Co)
...
These observations together with SEC results, which showed that adding more than 3
equiv of hmteta resulted in a decrease in oligomer formation, led to the conclusion that
hmteta probably only increases the hydroperoxide decomposition rate, not the rate of
oligomerisation
...
[114]

1
...
6
...
[115] Common
polyunsaturated fatty acids, such as linoleic, linolenic and arachidonic acids are the
natural substrates for these enzymes, which are widely distributed among plants and
animals
...

The active site of most lipoxygenases contains iron, which is bound to three
histidine residues, a C-terminal isoleucine, water and a variable sixth ligand
...
In Fig
...
13 a picture of the active site of soybean
lipoxygenase is shown, which is the most studied lipoxygenase
...
When the enzyme is in its catalytically active form iron is present as high-spin
iron(III) in an octahedral environment
...
6 V vs NHE), making lipoxygenase a significantly
stronger oxidising agent than other non-heme iron-containing enzymes
...
It is postulated that the existence of
the two cavities allows separate access of dioxygen and substrate to the active site
34

Alkyd Paint and paint driers

iron
...
[115]
The lipoxygenase enzyme from
the fungus Gäumannomyces graminis
contains manganese instead of iron
...
[120] Incubation of the
enzyme with linoleate led to a decrease
of the Mn(II) signals, supposedly
because of oxidation to the EPR silent
manganese(III) state
...
13: Soybean Lipoxygenase L-1 active site iron
coordination geometry, taken from a crystal structure
lipoxygenase over the part that contains determination at 1
...
[116] The enzyme is in the
the three proximal Fe-ligands, His499, resting state
...
The Fe-OAsn distance is around 3 Å
...
This finding, coupled with
recent EPR results, is an indication that the coordination environments for iron and
manganese lipoxygenase are largely the same
...
The main
products
are
13R-hydroperoxy-(9Z,11E)-octadecadienoic
and
13R-hydroperoxy-(9Z,11E,15Z)-octadecatrienoic acids
...
[120]
The formation of such a bis-allylic hydroperoxide from a polyunsaturated fatty acid has
not yet been observed with iron lipoxygenases
...
6
...

Schemes 1
...
9 show a representation of the enzyme's proposed catalytic
mechanism
...
[115, 117,
121]
Linoleic acid is activated by breaking the C-H bond at carbon-11
...
8)
...


L

Scheme 1
...
E =
enzyme and LH = substrate

35

Chapter 1

peroxyl radical (see Scheme 1
...
Release of the product to the solvent
completes the catalytic cycle
...
For example,

L
...


Product, L-OOH
H+

Scheme 1
...

1
...
3 Lipoxygenase as a paint drier
...
Despite the fact that
most enzymes operate only under very specific temperature and pH conditions, the
function of the lipoxygenase enzyme is to create hydroperoxides, whereas the main
function for a paint drier is to decompose hydroperoxides
...
Consequently, coordination compounds that model the active site of the
enzyme could also show the ability to abstract a hydrogen atom from a substrate, for
example an alkyd resin molecule, thereby initiating autoxidation
...


1
...
The use of cobalt compounds
in paints is a topic of debate, due to the toxicity and suspected carcinogenity of cobalt
...
It was soon found that
manganese complexes gave better results than iron complexes
...

In this chapter an overview has been given of the chemistry that plays a role in the
drying of alkyd paint and the metal driers that are used
...
Chapter 2 is
an account of the analytical methods that have been used to study the activity of potential
new paint driers, using ethyl linoleate (EL) as a model for real alkyd paint
...
The
oxidation mechanism of EL by commercial cobalt and manganese catalysts, [Mn(acac)3]
and [Mn(acac)3] with bipyridine added is discussed
...
Chapter 6 is a break-away from paint chemistry, since it is an
account of the coordination chemistry of [Mn(acac)2(bpy)]
...
DFT
calculations have been used to answer the question of why the complex adopts the
trigonal prismatic geometry
...

Parts of this thesis have been published[97, 126, 127] or have been submitted for
publication[128, 129] and several other manuscripts are in preparation
...
CEPE
...

http://www
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org
...
corrosion-club
...
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...
nycap
...
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