NOVEL MANGANESE AND IRON ACCELERATORS FOR … · novel manganese and iron accelerators for alkyd curing karin maaijen, r&d chemist and dr. ronald hage, cto at catexel

NOVEL MANGANESE AND IRON ACCELERATORS FOR ALKYD CURING

KARIN MAAIJEN, R&D CHEMIST AND DR. RONALD HAGE, CTO AT CATEXEL

ABSTRACT 2

ABSTRACT

The auto-oxidation reaction that hardens alkyd-based paints and inks involves radical-based polymerization reactions.

These can be accelerated by the addition of a siccative or paint drier such as well-known cobalt (2-ethylhexanoate)2. The possible reclassification of cobalt soaps as carcinogens under REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) legislation has initiated the search for cobalt alternatives and resulted in the discovery and increased use of other metal based driers.

Examples of these often involve transition metals like manganese (Mn) or iron (Fe) combined with nitrogen donating ligands. In this paper two recently discovered classes of ligands that, in combination with a transition metal, give good alkyd curing will be outlined.

Alkyd-based resins are frequently used in paints and coatings for a range of applications

and still represent over 20% of the binder systems in use today. An alkyd typically

consists of unsaturated fatty acids, polyols and phthalic anhydride as depicted in

figure 1 [1, 2]. There are continual developments to improve alkyd-based formulations,

especially towards high-solids and water-borne formulations. Furthermore, an

advantage of using alkyd-based resins is that they are made of fatty acid building |

blocks produced from renewable raw materials. Curing of the coating occurs via

an auto-oxidation mechanism, as shown in a simplified scheme overleaf (figure 1).

The fatty acid chain of the alkyd contains allylic hydrogen atoms that are sensitive

towards H* abstraction, which initiates the curing reaction. The radical species that

is formed is stabilised by delocalization. Molecular oxygen reacts rapidly at a diffusion

controlled rate with the CH* group and a conjugated system with peroxyl radicals

is formed. The peroxyl radicals abstract hydrogen atoms from other binder molecules

yielding alkylhydroperoxyl (ROOH) species. The alkylhydroperoxides decompose

in peroxyl (ROO*) and alkoxy (RO*) radicals, which react with other unsaturated

fatty acid groups of the alkyd resin or initiate polymerization forming a network of

alkyd molecules. These reactions are catalysed by transition-metal ion impurities or

transition-metal complexes, as discussed below. Furthermore, side reactions include

the formation of volatile aldehydes giving the typical odour of alkyd resins [1].

INTRODUCTION

INTRODUCTION 3

Cobalt soaps aid in the curing of alkyd resins by initiating the curing reactions and

by accelerating the autoxidation reactions [1, 2]. The most widely used cobalt soap is

cobalt (2-ethylhexanoate)2 (abbr. as Co(2-EH)

2), but cobalt (neodecanoate)

2 or cobalt

(naphtenate)2 are also used. In addition to traditional cobalt soaps, a polymer containing

carboxylic acid groups that binds Co, has also been introduced. By binding dioxygen to

cobalt(II), CoII-O2 or CoIII-superoxide species are formed that in turn react abstract H*

from the allylic CH groups thereby forming the same radical species as described above.

CoII can also activate the alkylhydroperoxide species, which then leads to formation of

alkoxy or peroxyl radicals. In general, cobalt soaps show a robust performance, albeit at

relatively high levels. Typically, levels of 0.02-0.06 wt% of Co is used in the alkyd paints.

Some improved activity is observed if ligands such as 2,2’-bipyridine are added, but

in general cobalt soaps are used as supplied. To enhance the hardness of the coating

layer, improve the through drying and reduce adsorption of Co-soap on solid particles,

other metal soaps are often added. These include zirconium, strontium, calcium, cerium

and barium soaps [3, 4]. Cobalt soaps have also been applied in water-borne (WB)

alkyd paints, however, the stability of the cobalt soaps in water is not very high.

Slow formation of cobalt hexa-aqua species occurs when cobalt soaps in WB alkyd

resin are stored. As the hydrated cobalt species are not very active, the rate of curing

slowly diminishes upon storage, which is known as loss-of-dry.

Various cobalt salts have been found to be toxic and there is a likelihood that cobalt

soaps will become classified as CMR (carcinogenic, mutagenic, reprotoxic) 1B within

REACH. Also, in the USA, a report has been published by the National Toxicology

Programme highlighting the toxicity of cobalt compounds [5]. The potential for

legislative restrictions of cobalt soaps has led paint producers and their suppliers

to develop and introduce alternative alkyd resin siccatives that are based on iron,

manganese or - to a lesser extent - vanadium. As we have recently published a more

extended review article on this topic in an open access journal [2], this paper will

mostly focus on the more recent developments of new siccatives based on Mn and Fe.

INTRODUCTION 4

Figure 1: Simplified scheme of alkyd curing.

NN

NN NNNN NN

NN NN NN

NN

NN

NN

NN

NN

NN NN

NN

NN

NN NN

NN

O

R1 R2

R1 R2

R1

Cross-linking

ß-scission degradation

R2

-H·

+O2

+H·

+

O•

O

CH

n

OO

O

O

O

R1 R2

OHO

+H·

R1 R2

OHO

R1 R2

O•

R1 R2

O•

O•

O

R1 R2

LM(n+¹)+-O2- + H· LM(n+¹)+ -OOH

LM(n+¹)+ + H· LMn+ + H+

RO· + H· ROH

ROO· + H· ROOH

ROOH + LMn+

LMn+ - OOR + H+

- H+

LMn(n+1)+ + ROO. LMn(n+1)+ - OH + RO.

R-RR-O-RR-O-O-R

NN

NN NN

Me3TACN

(a) (b) (c) (d)

Me4DTNE

NN

NN NN

NN

NN NN

NN

Most formulations that contain Co-soaps also contain anti-skinning agents, such

as methylethyl ketoxime (MEKO) [6]. This volatile anti-skinning agent prevents the

fast formation of a layer of skin in a can of paint. After application on the surface,

MEKO evaporates, allowing the radical-based alkyd cross-linking reactions to start,

as explained above. Two mechanisms of how MEKO gives the anti-skinning benefits

have been discussed: the first one is that the oxime group (R2C=N-OH) is a ligand

that reversibly binds to cobalt. So, in the can of paint, most of the cobalt soap is

coordinated by MEKO ligands, preventing it interacting with alkylperoxides

that form the alkoxy and peroxyl radicals needed to obtain curing.

INTRODUCTION 5

After application of the paint on the surface, the excess of MEKO evaporates, shifting the

equilibrium towards non-complexed MEKO and freeing the cobalt species to catalyse

the formation of the radicals. Alternatively, it is also shown that MEKO and other oximes

react with alkoxy radicals also prevents their reaction with the alkyd resin [7]. Based on

model experiments it has been shown that MEKO can, at best, weakly bind to cobalt

ions. This led the authors to suggest that the radical trapping mechanism may be more

relevant than binding of MEKO to the cobalt ions.

As the use of MEKO is under pressure for safety and health reasons, alternative anti-

skinning agents are being investigated. These include ‘classical’ radical trapping

antioxidants, such as phenol- or amine-containing compounds, or other oxime

containing compounds. This paper will not discuss these developments in detail but

preliminary testing of the new Mn and Fe siccatives with MEKO will be highlighted.

INTRODUCTION 6

Manganese carboxylates also show alkyd curing capabilities, but the activity is much

lower than that of cobalt soaps [3]. Therefore, to obtain a reasonable drying behaviour

of manganese soaps, the level of application should be higher than the typical levels

used for cobalt soaps. However, formation of brown coloured MnIII species often

renders the colour of the paint formulation darker than desired (yellowness), which

precludes their use in many paint formulations. It should be noted that sometimes

Mn-soap is applied in combination with Co-soap, so the level of both metals can be

reduced [3]. Improvement of drying using Mn-soaps, and thereby requiring lower

levels of Mn, can be achieved by the addition of ‘accelerators’, i.e. ligands that bind

to the transition metal forming active complexes [4]. A ligand that has been found to

activate Mn-soap is 2,2’-bipyridine, which is sold by various suppliers to the paint and

ink industry. Also, acetylacetonate has been identified to improve the activity of the

Mn-soaps [8]. A more detailed description of the chemistry of these bidentate ligands

can be found in another paper [2].

MANGANESE DRIERS

MANGANESE DRIERS 7

In general, when the denticity of a ligand increases, the binding capabilities of the

ligand to a metal ion improve - especially if the metal-binding atoms are in a ring

structure (macrocyclic ligands) [9]. For example, porphyrin ligands contain four

nitrogen donor atoms in a ring and form very stable complexes with many metal ions.

Mn-porphyrins and Fe-porphyrins are also paint drying catalysts, patented by Dura

Chemicals. Although the colour of the Mn-porphyrin complex itself is very intense, the

examples in the patent show that the colour of the paint layer is not very different from

that of the comparative cobalt drier, suggesting that the level of the Mn-porphyrin in

the paint formulations is quite low [10].

Also, the tridentate 1,4,7-trimethyl-1,4,7-triazacyclononane ligand (Me3TACN)

forms stable manganese complexes [11]. Besides being tridentate, the nature of the

macrocyclic ligand confers an unusual and desirable stability on the complexes.

A wide variety of well-defined Mn complexes is known with this ligand [11, 12].

Originally these Mn complexes were developed to activate hydrogen peroxide for

stain bleaching in detergent products [13], but subsequent work has been published

confirming their performance as catalysts capable of enhancing the curing

of alkyd paint formulations. Oyman et al. was the first to report the

activity of [(MnIV)2(Me

3TACN)

2(μ-O)

3](PF

6)2 towards polymerization

of ethyl linoleate, which was found to be higher than that of

Mn-soap and quite similar to that of Co-soap (based on wt% metal) [14, 15]. Extensive

mechanistic studies were carried out by the same group and it was concluded that

this catalyst also induces radical-based autoxidation processes.

More recently, DSM published patents with a.o. the highly water soluble

[(MnIV)2(Me

3TACN)

2(μ-O)

3](CH

3COO)

2 complex [16]. Somewhat surprisingly it was found

that the complex with the acetate counterion showed a significantly higher activity

towards alkyd paint curing than the PF6 salt, allowing a reduction in the dose rate of the

catalyst. It was also shown that addition of ascorbic acid leads to an increased activity,

which led to the suggestion that lower-valent Mn species with the same ligand could

be involved in, or act as, an intermediate precursor of the species active [16].

Mixtures of Mn-soap and the ligand show good paint drying activity, as a number

of patents filed by Akzo Nobel and Catexel show [17, 18]. Akzo Nobel evidenced

that by increasing the molar ratio between ligand and Mn, the level of Mn in the

paint formulation can be lowered decreasing the colour due to Mn in the paint films.

Using a molar excess of Mn may improve hardness of the paint layer. A detailed study

using high spatial resolution NMR spectroscopy and AFT-FTIR spectroscopy has been

done to compare different driers and their effect on curing alkyd resins [19]. It was

shown that whilst Co-soap clearly gives curing starting from the top layer, the

Mn-soap/Me3TACN mixture induces a much more homogeneous drying behavior.

Related to this are the bis-triazacyclononane ligands (Me4DTNE) which contain a

bridge (e.g. an ethylene bridge) between two triazacyclononane rings (figure 2). The

dinuclear MnIIIMnIV complex of this ligand is also known to activate hydrogen peroxide

in, for example, bleaching reactions. However, the alkyd curing performance of this

complex has been found to be poor and as such, is not applicable in isolation [20].

In contrast, mixing the ligand with MnII-soap or salt leads to good curing behaviour,

similar to that of Me3TACN when tested at equimolar level. This difference between

absence of activity when using the well-defined dinuclear MnIIIMnIV complex and the

high activity of the mixture of MnII salts and Me4DTNE ligand, suggests that the latter

forms different species that are not accessible when starting from the dinuclear

MnIIIMnIV complex.

MANGANESE DRIERS 8

NN

NN NNNN NN

NN NN NN

NN

NN

NN

NN

NN

NN NN

NN

NN

NN NN

NN

O

R1 R2

R1 R2

R1

Cross-linking


R2

-H·

+O2

+H·

+

O•

O

CH

n

OO

O

O

O

R1 R2

OHO

+H·

R1 R2

OHO

R1 R2

O•

R1 R2

O•

O•

O

R1 R2

LM(n+¹)+-O2- + H· LM(n+¹)+ -OOH

LM(n+¹)+ + H· LMn+ + H+

RO· + H· ROH

ROO· + H· ROOH

ROOH + LMn+

LMn+ - OOR + H+

- H+


R-RR-O-RR-O-O-R

NN

NN NN

Me3TACN

(a) (b) (c) (d)

Me4DTNE

NN

NN NN

NN

NN NN

NN

Figure 2: Structures of Me3TACN (left) and Me

4DTNE (right)

Without detailed mechanistic studies, one can merely speculate why such different

behaviour is observed, but it is potentially related to the different oxidation states of

the Mn centres and the rigidity of the dinuclear MnIIIMnIV complex enforced by the

ethylene bridge between the two TACN rings.

Examples of paint drying activity using Me3TACN and/or Me

4DTNE ligands, in

combination with manganese(2-ethylhexanoate)2 (abbr. as Mn(2-EH)

2) as an alkyd

drier, is shown in figure 3. The presence of both ligands in combination with Mn(2-EH)2

gives much higher activity than Co(2-EH)2 or Mn(2-EH)

2, resulting in the feasibility to

lower the metal levels even more significantly. Without these ligands added, the activity

of Mn(2-EH)2 is low and much higher levels are needed than Co(2-EH)

2 to obtain

acceptable curing activity. Also, by replacing Mn(2-EH)2 with Mn(acetate)

2 similar drying

is obtained. It should be noted that poor through-drying with Co(2-EH)2 was observed,

which is likely to be due to the absence of secondary driers that are typically used in

combination with Co(2-EH)2 [3, 4]. The dinuclear Mn complex, [(MnIV)

2(Me

3TACN)

2(μ-O)

3]

(CH3COO)

2, also gives good alkyd curing at a much lower level than Co(2-EH)

2.

Also noteworthy is that with the Me4DTNE ligand added to Mn(2-EH)

2 efficient curing

of the alkyd is achieved, which can be improved even further by the addition of a molar

excess of the Me4DTNE ligand. Combining the two ligands, thus the total molar ratio

M:L being 1:2, also gives an improvement in the drying time relative to the 1:1 ratio.

MANGANESE DRIERS 9

Co(2-EH)2 (0.01%)

Mn(2-EH)2 (0.1%)

[Mn2(Me

3TACN)203](CH3COO)

2 (0.025%)

Mn(2-EH)2 + Me

3TACN (0.002%) (1:1)

Mn(Ac)2 + Me

3TACN (0.002%) (1:1)

Mn(2-EH)2 + Me

4DTNE (0.002%) (1:1)

Mn(2-EH)2 + Me

4DTNE (0.002%) (1:5)

Mn(2-EH)2 + Me

3TACN + Me

4TDNE (0.002%) (1:1:1)

0 1 2 3 4 5 6

Drying time (h)Wet Set Surface Through

dry

Mn(2-EH)2 + compound a (0.01%)


Mn(2-EH)2 + compound b (0.01%)


Mn(2-EH)2 +compound b (0.002%)

Mn(2-EH)2 + compound c (0.01%)


0 1 2 3 4 5 6


dry

FeCI2 + compound c (0.01%)(1:1)

FeCI2 + compound d (0.01%)(1:1)



0 1 2 3 4 5 6


dry

Mn(Ac)2 + TPA (0.01%)

Mn(2-EH)2 + TPA (0.01%)

Mn(2-EH)2 + TPA (0.005%)

Fe(napht)2 + TPA (0.01%)

FeCI2 + TPA (0.01%)

0 1 2 3 4 5 6

Drying time (h)Wet Surface Through

dry

Figure 3: Drying of alkyd (70% in white spirits) with [(MnIV)2(Me

3TACN)

2(μ-O)

3](CH

3COO)

2 or

Mn(2-EH)2 and Me

3TACN and Me

4DTNE ligands at room temperature. The molar ratios of the

metal and ligands tested are shown in parentheses (1:1 or 1:5, or 1:1:1 for the mixed-ligand system). Concentrations given in wt% metal based on total formulation. The wet layer thickness of the alkyd was 75 micrometres.

MANGANESE DRIERS 10

Iron soaps are known to give high temperature curing of coatings such as stoving

enamels [1]. At low temperature, iron soaps show hardly any activity and therefore

initially, less attention has been focused on the possibility of developing iron-based

siccatives. An improvement in drying behaviour has been observed when testing a

specific class of iron complexes called ferrocenes. It was found that ferrocenes give a

similar reactivity on ethyllinoleate as Co(2-EH)2 [21]. Some derivatives were found to

give even higher activity, and it was also shown that synergistic drying behaviour in

conjunction with Co(2-EH)2 could be attained [22].

More recently, very high drying activity of a novel iron catalyst was reported [23].

This catalyst is based on a bispidon ligand that contains five nitrogen donor ligands,

two aliphatic and three pyridine based nitrogens. The rigid bispidon ligand forces the

two aliphatic nitrogen donors to bind to the iron centre in a cis-configuration. The

three pyridine donors bind as well, leaving one coordination site open to interact with

other molecules. In this way, the reactivity of the Fe-bispidon complex towards ROOH

activation is high and the stability towards iron oxide formation greatly enhanced. High

stability was noted in water-borne paint formulations where, unlike the cobalt soap,

good – and highly desirable - storage stability can be achieved. This was predictable to

some extent, as this catalyst was originally developed for laundry detergent products,

including liquid detergents, where storage stability is an essential feature [13]. Due

to the high reactivity observed with the alkyd resins, the amount of catalyst in the

paint formulation is often low, leading to the absence of colour from the iron catalyst.

Various mechanistic studies using infrared and Raman spectroscopy, show that the

reactivity with a model substrate, ethyl linoleate, follows the same pattern as observed

with cobalt soap with CH activation of the alkyd resin and autoxidation processes

involving alkoxy and peroxyl radicals.

Detailed analysis using IR spectroscopy and elasto-viscosity measurements on HS

alkyd resins of Co(2-EH)2 versus the Fe-bispidon drier show that the cobalt soaps react

quicker with the resin, but induce mainly surface curing, whilst the iron catalyst induces

radical curing throughout the resin [24]. The same conclusion was drawn based on

high spatial resolution NMR and AFT-FTIR spectroscopy on this iron catalyst vs

Co(2-EH)2 [19].

IRON DRIERS

IRON DRIERS 11

A new class of diazacycloheptane ligands have been explored and patented by

Catexel [25] for use in alkyd paint systems. The unsubstituted ligands are also

tridentate nitrogen donors similar to the TACN described earlier, however, one

important difference is that one of the nitrogen donors is outside the cycloheptane

ring that contains the two other N donors. As discussed in a paper by Neves and

co-workers, this class of ligands may therefore be easier to synthesise than the

TACN ligands [26].

Figure 4 (compound a) shows the structure of 6-dimethylamino-1,4,6-trimethyl-1,

4-diazacycloheptane, which is the diazacycloheptane-based isomer of the Me3TACN

ligand. Alkyd drying tests show that this compound is not as active as Me3TACN

(figure 5). However, by introducing an additional pyridine group (figure 4, compound b),

the denticity is increased and the improved Mn binding capability results in faster paint

drying activity (figure 5). In this way, by modifying side groups, activity can be tuned to

optimise performance. Several tri-, tetra- and pentadentate ligands have been prepared

and tested. This includes compounds with one or more groups attached to the nitrogen

outside the ring, but also structures where the methyl group on the nitrogen in the ring

is replaced by another group, such as a pyridine (see figure 4, compound c).

Figure 4: Structures of several diazacycloheptane ligands discussed in this paper.

DIAZACYCLOHEPTANE-BASED LIGANDS WITH Mn AND Fe

NN

NN NNNN NN

NN NN NN

NN

NN

NN

NN

NN

NN NN

NN

NN

NN NN

NN

O

R1 R2

R1 R2

R1

Cross-linking


R2

-H·

+O2

+H·

+

O•

O

CH

n

OO

O

O

O

R1 R2

OHO

+H·

R1 R2

OHO

R1 R2

O•

R1 R2

O•

O•

O

R1 R2

LM(n+¹)+-O2- + H· LM(n+¹)+ -OOH

LM(n+¹)+ + H· LMn+ + H+

RO· + H· ROH

ROO· + H· ROOH

ROOH + LMn+

LMn+ - OOR + H+

- H+


R-RR-O-RR-O-O-R

NN

NN NN

Me3TACN

(a) (b) (c) (d)

Me4DTNE

NN

NN NN

NN

NN NN

NN

DIAZACYCLOHEPTANE-BASED LIGANDS WITH Mn AND Fe 12

Figure 5: Drying of alkyd (70% in white spirits) with manganese 2-ethylhexanoate and diazacycloheptane ligands at room temperature. In all cases an equimolar ratio between the metal and each ligand was used. Details of synthesis and drying tests can be found in reference 23. Concentrations given in wt% Mn based on total formulation. The wet layer thickness of the alkyd was 75 micrometres.

Co(2-EH)2 (0.01%)

Mn(2-EH)2 (0.1%)

[Mn2(Me

3TACN)203](CH3COO)

2 (0.025%)

Mn(2-EH)2 + Me

3TACN (0.002%) (1:1)

Mn(Ac)2 + Me

3TACN (0.002%) (1:1)

Mn(2-EH)2 + Me

4DTNE (0.002%) (1:1)

Mn(2-EH)2 + Me

4DTNE (0.002%) (1:5)

Mn(2-EH)2 + Me

3TACN + Me

4TDNE (0.002%) (1:1:1)

0 1 2 3 4 5 6


dry








0 1 2 3 4 5 6


dry





0 1 2 3 4 5 6


dry

Mn(Ac)2 + TPA (0.01%)

Mn(2-EH)2 + TPA (0.01%)

Mn(2-EH)2 + TPA (0.005%)


FeCI2 + TPA (0.01%)

0 1 2 3 4 5 6


dry

Co(2-EH)2 (0.01%)

Mn(2-EH)2 (0.1%)

[Mn2(Me

3TACN)203](CH3COO)

2 (0.025%)

Mn(2-EH)2 + Me

3TACN (0.002%) (1:1)

Mn(Ac)2 + Me

3TACN (0.002%) (1:1)

Mn(2-EH)2 + Me

4DTNE (0.002%) (1:1)

Mn(2-EH)2 + Me

4DTNE (0.002%) (1:5)

Mn(2-EH)2 + Me

3TACN + Me

4TDNE (0.002%) (1:1:1)

0 1 2 3 4 5 6


dry








0 1 2 3 4 5 6


dry





0 1 2 3 4 5 6


dry

Mn(Ac)2 + TPA (0.01%)

Mn(2-EH)2 + TPA (0.01%)

Mn(2-EH)2 + TPA (0.005%)


FeCI2 + TPA (0.01%)

0 1 2 3 4 5 6


dry

The most active iron-based catalyst based on this class of ligands contains a

pentadentate ligand with three aliphatic and two pyridine-based nitrogen donors

(figure 4, compound d). Another pentadentate ligand from the same class of ligands

was also found to be active, except that now the two pyridine donors are connected

to the two aliphatic nitrogen donors of the cycloheptane ring (figure 4, compound c).

Addition of a molar excess of ligand relative to the metal present significantly improved

curing, as shown in figure 6.

Figure 6: Drying of alkyd (70% in white spirits) with iron chloride and diazacycloheptane ligands at room temperature. The molar ratios of FeCl

2 and the ligands tested were 1:1

or 1:2 (given in parentheses). Concentrations given in wt% Fe based on total formulation. The wet layer thickness of the alkyd was 75 micrometres.

DIAZACYCLOHEPTANE-BASED LIGANDS WITH Mn AND Fe 13

Most of the above-mentioned Mn-based driers contain ligands with three aliphatic

nitrogen donor groups. However, it was not known whether ligands that contain a

majority of pyridine-based donor sites would also give good curing of alkyd paints.

A well-known ligand in coordination chemistry is tris(pyridin-2-ylmethyl)amine (TPA)

(figure 7), which has been widely studied with a.o. the metals Fe and Mn [27, 28].

This ligand has also been tested with various Mn- and Fe-salts for alkyd resin curing

and the results of these tests are shown in figure 8. Also, with TPA in the presence of

Mn and Fe, a clear acceleration of the curing of the alkyd resin has been observed,

although the Mn-TPA mixtures clearly show higher alkyd curing activities than the

equivalent Fe-TPA mixtures.

TRIS(PYRIDINE-2-YLMETHYL)AMINE LIGANDS WITH Mn AND Fe

NN

NN NNNN NN

NN NN NN

NN

NN

NN

NN

NN

NN NN

NN

NN

NN NN

NN

O

R1 R2

R1 R2

R1

Cross-linking


R2

-H·

+O2

+H·

+

O•

O

CH

n

OO

O

O

O

R1 R2

OHO

+H·

R1 R2

OHO

R1 R2

O•

R1 R2

O•

O•

O

R1 R2

LM(n+¹)+-O2- + H· LM(n+¹)+ -OOH

LM(n+¹)+ + H· LMn+ + H+

RO· + H· ROH

ROO· + H· ROOH

ROOH + LMn+

LMn+ - OOR + H+

- H+


R-RR-O-RR-O-O-R

NN

NN NN

Me3TACN

(a) (b) (c) (d)

Me4DTNE

NN

NN NN

NN

NN NN

NN

Figure 7: Structure of tris(pyridine-2-ylmethyl)amine (TPA)

Figure 8: Drying of alkyd (70% in white spirits) with manganese acetate, manganese 2-ethylhexanoate, iron naphthenate or iron chloride in combination with tris(pyridine-2-ylmethylamine) at room temperature. In all cases an equimolar ratio between the metal and the TPA ligand was used. Concentrations given in wt% Mn or Fe based on total formulation. The wet layer thickness of the alkyd was 75 micrometres.

Co(2-EH)2 (0.01%)

Mn(2-EH)2 (0.1%)

[Mn2(Me

3TACN)203](CH3COO)

2 (0.025%)

Mn(2-EH)2 + Me

3TACN (0.002%) (1:1)

Mn(Ac)2 + Me

3TACN (0.002%) (1:1)

Mn(2-EH)2 + Me

4DTNE (0.002%) (1:1)

Mn(2-EH)2 + Me

4DTNE (0.002%) (1:5)

Mn(2-EH)2 + Me

3TACN + Me

4TDNE (0.002%) (1:1:1)

0 1 2 3 4 5 6


dry








0 1 2 3 4 5 6


dry





0 1 2 3 4 5 6


dry

Mn(Ac)2 + TPA (0.01%)

Mn(2-EH)2 + TPA (0.01%)

Mn(2-EH)2 + TPA (0.005%)


FeCI2 + TPA (0.01%)

0 1 2 3 4 5 6


dry

TRIS(PYRIDINE-2-YLMETHYL)AMINE LIGANDS WITH Mn AND Fe 14

Model tests using an alkyd resin (70% in white spirits) containing only the primary drier

catalysts as discussed above have been tested to determine their skinning behaviour in

the absence and presence of 0.22 wt% MEKO. The level of the manganese-based driers

with Me3TACN, Me

4DTNE and ligand b in figure 4, was 0.005 wt% and the other Mn and

Fe driers tested was 0.01 wt%.

The results show that the use of MEKO retards the formation of skin on the alkyd

resin solution for all manganese drier systems, whilst the skinning behaviour of the

iron-based driers tested is not altered in the presence of MEKO. This latter result is

similar to what was found for the Fe-bispidon siccative, discussed elsewhere [23].

If MEKO does trap the radicals without binding to metal ions, as suggested to be

the case for Co in a recent publication [7], then the addition of MEKO would be

independent of the choice of metal drier if corrected for the curing activity.

Since it would appear that, in general, MEKO is an efficient anti-skinning agent for

all Co-based and Mn-based driers but not for Fe-based driers, it would appear to

suggest that MEKO does indeed bind to the metal ion/complex to prevent formation

of the radicals, which leads to curing of the alkyd resin.

SKINNING TESTS WITH METHYLETHYL KETOXIME (MEKO)

SKINNING TESTS WITH METHYLETHYL KETOXIME (MEKO) 15

As shown in this paper, a wealth of different manganese and iron catalysts for alkyd

paint curing have been published by different academic groups and companies.

Each of the ligands studied in combination with Mn and/or Fe show different

characteristics. To further demonstrate the complexity of the mechanism, different paint

drying activity can be obtained for the same ligand (Me3TACN) and the same metal

(Mn) depending on whether one starts with a well-defined complex or as a mixture

of a Mn-soap with the ligand. Even more striking is that the dinuclear MnIIIMnIV

complex with Me4DTNE ligand shows no activity whilst a mixture of the same

ligand with a Mn-soap is very active. Although not published, it is very likely that the

opposite behaviour would be found for the Mn-porphyrin. Kinetically, formation of

the Mn-porphyrin is slow and this mixing of a Mn-salt and the free porphyrin would

be unlikely to lead to species that are actively involved in the curing of alkyd resins.

Based on the development of siccatives described above, novel classes of driers have

been discovered. For example, the high activity of the tridentate triazacylcononane

ligands in combination with Mn, led to the realisation that diazacycloheptane ligands

as the backbone can also lead to active driers, especially if the appropriate ancillary

donor sites are chosen well. A high stability of the active species is clearly an essential

requisite to having the right drying activity. This led to the observation that the well-

known TPA ligand, which forms with many metal ions stable complexes, can also be

used for this application.

These examples show that, only by understanding the coordination chemistry involved,

and by careful modelling and testing, will one be able to select the optimal catalytic

system. It is important to realise that the properties of the metal centre are greatly

altered by coordination of the metal ion to ligands. Binding of a ligand to the metal

ion can affect both the (rate of) reactivity with the alkyd resins or ROOH species and

the stability of the preformed metal complexes. Further, the wider examples given in

this paper evidence that various complexes made or formed in situ can, in many cases,

show a much higher activity than Co(2-EH)2 when using, for example, a medium-oil

alkyd resin.

CONCLUDING REMARKS

CONCLUDING REMARKS 16

To obtain the full benefit of this exciting and growing class of compounds and

complexes, applicability testing should embrace the full gamut of paint formulations,

including those containing alkyd resins of different oil lengths, high solid paints and

water-borne alkyd paints - in addition to conventional solvent based alkyds.

Furthermore, by working in conjunction with additive developers and resin makers,

particularly with new and/or modified alkyds, paint formulators will be better positioned

to both maximise performance criteria such as gloss, hardness, compatibility and colour

retention. Perceived performance issues related to loss-of-dry due to adsorption on

fillers or pigments, discolouration, skinning behaviour and interactions of driers with

current and future anti-skinning agents can then begin to be addressed and resolved.

Tailoring systems to maximise the benefit of these new siccative technologies in the

re-emerging alkyd markets is the key to delivering optimum solutions for this important

segment of the paint industry.

CONCLUDING REMARKS 17

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ACKNOWLEDGEMENTS

The authors are indebted to Dr. Johannes W. de Boer and Dr. Pattama Saisaha for the syntheses of the diazacycloheptane-based compounds.

ACKNOWLEDGEMENTS 18www.catexel.com

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NOVEL MANGANESE AND IRON ACCELERATORS FOR … · novel manganese and iron accelerators for alkyd curing karin maaijen, r&d chemist and dr. ronald hage, cto at catexel