Technical Paper
EC Award winning paper
Taming the Michael
Addition reaction
Ultra-fast drying, low VOC, isocyanate-free technology for 2K coatings
Contact:
Dr Fred van Wijk
Nuplex Resins
fred.vanwijk@
nuplex.com
T +31 164 276 593
R. Brinkhuis, J. Schutyser, F. Thys, E. De Wolf, T. Buser,
J. Kalis, N. Mangnus, F. Van Wijk
A new low VOC, isocyanate- and tin-free coating
system is built on a novel blocked catalyst and
kinetic control additive package used in conjunction
with Michael Addition chemistry. Pot life and drying
time can be effectively decoupled, combining fast
cure with an extended pot life. Comparisons with
standard 2K systems are presented.
Figure 1: The Michael Addition reaction between malonate and acryloyl
a)
Figure 2: Blocked
catalyst formation
and activation; HA
presents a proton
donor
b)
Figure 3: Conversion of acrylic double bonds (FT-IR,
809 cm-1) showing
the effect of CO2
expulsion (Figure
2b); green line:
without 1,2,4-triazole, blue line: 1 eq
1,2,4-triazole / mol
cat (50 µeq/g)
34
European Coatings JOURNAL
05 l 2015
D
riven by changes in HSE legislation, competition on
paint application costs and coating performance, today’s paint technology continues to develop in the
direction of higher solids, lower curing temperatures and
faster painting processes. Meeting these combined requirements is very challenging and encounters the limits
of versatility of presently available cure chemistries. Michael Addition chemistry [1] offers a perspective for making steps beyond those limits.
Michael Addition (MA) has been explored before for coating applications [2], although it has never established itself
as a mainstream cure technology yet, most likely because
it is too reactive. The key components of a MA system are
electron deficient C=C double bonds (e.g. an acryloyl, the
acceptor), acidic C-H bonds (as present in acetoacetate and
malonate moieties, the donor), and a base catalyst yielding a nucleophilic carbanion that can add to the double
bond. A carbon-carbon link is formed between the two
moieties. The second proton of the donor species is available for reaction with similar reactivity (Figure 1).
Relevant features derived from the characteristics of MA
chemistry include:
»»The need for a base strong enough to abstract a proton
from the donor species. The pKa of an acetoacetate C-H
is around 10.7, for a malonate it is even higher (>13).
»»The absence of acidic species that will deactivate the
catalyst.
»»The very high reactivity of the carbanions formed,
especially when using malonate species as the donor.
In catalysed paint formulations it is easy to create conditions under which a MA reaction between malonate
and acryloyl species can essentially be completed
within minutes. In this instance, both malonate and
acryloyl may coexist in the uncatalysed paint and present good shelf stability.
»»The nature of the carbon-carbon links formed; not leading to weak spots in durability.
»»Michael addition technology opens a window to using
non-polar, low equivalent weight crosslinking components that can lead to very low solvent demand
formulations capable of creating high crosslink density
polymer networks.
The options of MA chemistry were expanded by focusing
on ways to control the inherent reactivity of a malonateacryloyl system by using its high reactivity potential, whilst
creating both a long potlife and a workable open time as
described below. Combined with specially developed resins, the obtained benefits are so profound that it may be
recognised as a new type of curing technology, finding
use in many different markets and applications. Hereinafter, this new chemistry is refered to as ‘Acure’.
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Technical Paper
EC Award winning paper
Controlling pot life and drying
Very high reactivity cannot easily be combined with
a good pot life. However, a solution was found in the
reversible blocking of a strong base catalyst with a dialkylcarbonate as depicted in Figure 2a. Strong bases will
form alkyl carbonate anions with a basicity low enough
to not initiate the MA reaction. These carbonates are inherently unstable, and through protonated species will
form an equilibrium with free CO2 and alcohol (Figure 2b).
After mixing in a container (with a relatively low surface/
volume ratio), there is no fast CO2 release and long pot
lives are observed (vide infra). Upon applying the paint
however, large surface areas will be created facilitating
easy escape of solvent and especially CO2. This shifts
the equilibria, followed by rapid de-blocking of the basic
species which triggers the full reactivity potential of the
malonate acryloyl system. This de-blocking process will
be accelerated when HA becomes more acidic, shifting
equilibrium 2b to the right (Figure 3). The net result is the
combination of a very long pot life with very fast drying.
Workable pot lives of at least four hours are easily obtained, and if needed can be formulated to be counted in
days. At the same time it can be observed that tack free
times down to 10 minutes after application, and phase 4
(scratch free) drying recorder times not much longer. Figure 4 shows the different pot life / drying speed balance
for Acure versus isocyanate based systems.
The effect of adding alcohols to Acure paints is shifting
the equilibrium of Figure 2b to the left, extending the pot
life without significantly affecting dry times.
When following the acryloyl conversion upon application
with FTIR (809 cm-1), it is observed that conversion can
Results at a glance
Two-component urethane topcoats are well
established in coatings applications. There is a need,
however, for systems with increased productivity
coupled with environmental and health and safety
friendliness. This paper outlines the launch of a new,
low VOC, isocyanate and tin free, breakthrough technology which meets these needs.
Figure 4: Generalised picture of
potlife – drying
balance. Blue area:
the ‘world’ of OH
and NH isocyanate curing; red
area: the field of
operation for Acure
based paints.
easily reach values above 80 % in the first 10 minutes.
This indicates not only a rapid physical dry state of the
film but also a very rapid crosslink density development
with the associated beneficial early coating performance
of both chemical and mechanical robustness. The potential reactivity may also be used at temperatures below
ambient: dry times of less than an hour have been observed at application temperatures as low as 15 ºC. In
terms of its practical use, this system may be used as
a 2K coating with both reactive components (malonate
polymer and acryloyl oligomer) premixed and the catalyst being added as an activator.
Complications arising
from very fast cure
Paint systems with drying times this short may bring
complications not normally recognized for ‘slow’ systems.
Firstly, such drying times become competitive with the
ability of the solvents to escape from the film. Under ambient curing conditions the glass transition temperature
The system is built on a novel blocked catalyst and
kinetic control additive package used in conjunction
with Michael Addition chemistry. Prototype paint
formulae highlight the low VOC capability (<250 g/l)
of this very fast drying 2K system (10 to 50 minutes,
ambient) and the unique decoupling of dry time and
pot life (>5 hours), expected to trigger new paint and
process solutions.
Figure 5: Effect of
blocking the catalyst and adding
an excess (relative
to catalyst) of
kinetic control
additives on the
conversion of
acrylic double
bonds (FT-IR, 809
cm-1) by Michael
addition. Cat conc.:
40 μeq/g resin
solids for all curves,
unless noted
otherwise.
The chemistry of the system and comparative results versus other topcoats are presented. The results
show that the system has significant potential to
displace existing two component topcoats in Marine
and Protective and Industrial OEM market applications, without compromise.
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05 l 2015
European Coatings J OURNAL
35
Technical Paper
EC Award winning paper
1)
2)
Figure 6: Reactivity
control of Acure
chemistry. MA:
Michael Addition.
Reaction (1): potlife
control. Reaction
(2): open time
control. Reactions
(3): crosslinking.
Both malonate
hydrogens react
with acryloyl
3)
Structure
Table 1: Suitable
kinetic control
additives; pKa’s
from [4]
Name
pKa
(in water)
Ethylacetoacetate
10.7
Succinimide
9.5
1H-benzotriazole
8.5
1,2,4-triazole
10.2
Malonate
13
Creating a controlled ‘open time’
(Tg) of the wet coating rises due to solvent release and
reaction. When the Tg reaches room temperature (RT) the
system will vitrify: both solvent diffusion and chemical
reactivity become severely retarded. If such vitrification
in the surface of the coating, becoming tack free, occurs much earlier than in the deeper layers, some solvent
may be retained in those deeper layers which cannot
then easily escape through this closed surface. I.e. further solvent release will be significantly slowed down.
The upside to this picture is that solvents are excellent
plasticizers. This means that the crosslink conversion is
able to rise further while the reaction is not yet impeded
by a high Tg and it is then easier to reach full conversion
before vitrification. However, an excess of entrapped solvent may cause the Tg in deeper layers to remain lower
in which case a lower pendulum hardness may be observed. Kiil modelled the competition between chemical
reaction and solvent escape [3].
36
Secondly, ultrafast drying may lead to reduced appearance as a very narrow time window for levelling remains. One may see such fast drying systems struggle
with releasing entrapped air and picking up overspray.
Under some conditions, a rapid surface cure on top of
a still mobile sublayer can even give rise to wrinkling
phenomena, furthermore telegraphing defects are more
likely to occur.
It is clear that combining a very fast dry profile with very
good pot life is not a guarantee for an application profile
without significant complications. The challenge addressed
was how to save the fast dry time profiles, and simultaneously mitigate the problems described above. The solution
is presented below with the use of special kinetic control
additives to create an induction period in the crosslink reaction, and so create a tuneable ‘open time’ window.
European Coatings JOURNAL
05 l 2015
The concept used revolves around the presence of HA
species in the formulation that can add to the acryloyl
acceptors by a Michael addition after deprotonation, but
differing from malonate in that:
a. HA is significantly more acidic than the malonate C-H;
i.e. the base will deprotonate HA much more readily than
malonate, postponing the fast addition of malonate onto
acryloyl.
b. The conjugate anion (A-) has a significantly lower reactivity towards the acryloyl: the consumption of these species by the MA reaction is slow, i.e. already low concentrations of HA will have a significant effect on early kinetics.
c. Ideally, the generated HA-acryloyl adduct does not significantly increase viscosity of the paint. This implies that
HA is preferably a low molecular weight, mono-functional component; guaranteeing the wet layer’s ability to
wet, level, de-gas, pick-up overspray, etc.
Various components have been identified which meet
the profile above, these are based either on carbon or
nitrogen acids: some examples are listed in Table 1. Each
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Technical Paper
EC Award winning paper
Table 2: The Acure
paint composition
“Acure” paint 30" DIN4-23 °C
Malonate polyester A
Succinimide mod. polyester B
1,2,4-triazole
Acrylate based pigment paste
Paint additives
n-propanol
Butyl acetate
Blocked catalyst
VOC
Solids by weight
PVC
Catalyst
Succinimide / catalyst
1,2,4-triazole / catalyst
Acrylate / malonate-H
1000 g
132.2 g
182.0 g
4.5 g
535.9 g
7.0 g
62.5 g
52.4 g
23.5 g
245 g/l
80.3 %
16.3 %
0.05 meq/g solid resin
1.0 eq/eq
3.0 eq/eq
0.95
Table 3: Appearance and mechanical properties directly after application and
after ageing (DFT: 45 + 4 µm on Q-panel)
Acure
Spray viscosity (Din4)
30
SC at appl.visc. (g/l)
81
NCO wt% on solids
NCO : OH, NH
Cure temperature (°C)
RT
DOI
91
Haze
(HU)
11
Gloss 60°
(GU)
90
Foam limit (mm)
> 240
Haze *
(HU)
34
Gloss retention 60° * (%)
100
PH Persoz
2 hours 108
1 day
169
7 days
189
Reverse impact
> 105
9 mm Crock (mar):
60° gloss retention (%)
75
60° GR after 7 days RT (%)
79
* 24 hrs 120°C
Aspartate
24
83 29.0
1.05
RT
93
10
93
> 240
314
89
194
234
248
4%
4%
4%
OH-NCO A OH-NCO B OH-NCO C
20
20
25
71
72
76 17.4
17.4
22.0
1.1
1.1
1.05
RT
60
80
94
94
94
10
21
16
95
95
97
> 160
~100
~110
63
78
32
99
100
98
45
125
138
137
154
218
207
206
> 105
20
4
> 105
49
51
42
44
49
53
49
62
of these components differs slightly in terms of specific
impact on the application performance. Combined with
the blocked catalyst, warranting a long pot life, an induction (open) time is created; its length is tuneable by
the amount of additive used. Only then a very strong
acceleration of the reaction occurs once the more acidic
HA species are consumed, still reaching high conversion.
The net result is a sigmoidal kinetics profile (Figures 5,
7). Along with the amount of catalyst and alcohol co-solvents controlling the reaction rate, a formulation toolbox
is available for easy optimisation of Acure paints to meet
specific application performance demands.
Curing is rapid after tuneable open time
A beneficial consequence of using a relatively acidic species HA is that reaction b in Figure 2 will shift to the right,
38
European Coatings JOURNAL
05 l 2015
increasing the concentration of weak acid (A-) and in particular CO2. The latter will now diffuse out of the paint layer more readily. Because of its low reactivity (compared
to malonate) the higher concentration of A- does not
neutralize the positive effect of the ‘newly created’ open
time. However, if all HA is consumed and CO2 mostly expelled then the maximum concentration of fast reacting anionic species is present in the paint. The malonate
then becomes deprotonated and the full potential of the
ultra-fast Michael addition is released as if there never
was a blocked catalyst (Figure 5). Acure chemistry can
then be summarized as in Figure 6, where a tetrabutyl
ammonium cation counters the blocked base with succinimide as example kinetic control additive (HA).
Note that the paint components controlling the reactions
1 and 2 in Figure 6 are different species, allowing both
the pot life and open time to be tuned independently,
while accepting only a limited compromise in drying
time. Simultaneously, very significant advantages in
pendulum hardness development and appearance are
obtained (Figures 7 and 8). For layer thicknesses above
40 µm it was observed that both 1,2,4-triazole and succinimide are needed for optimum results.
Good durability and excellent
adhesion on most substrates
In general, accelerated weathering tests on white pigmented topcoats yield results that are very comparable
to high quality (4 % OH) urethane coatings (tested in
UV-B, Xenon [5]) even though it was that found various
commonly used UV absorbers are incompatible with the
Acure technology. HALS could be used with the usual
beneficial effects, however. Gloss retention after two
years of Florida exposure was at least on par with high
end WB 2K, HS and MS 2K urethane references [5].
Adhesion studies of Acure top coatings were carried out
over many different types of commercially available
primers used in a wide field of end use applications including general industry, ACE and protective coatings. The
different epoxy primer types that were tested included
new build, holding, fast-cure, surface tolerant, low temperature cure and zinc rich types. Good to excellent adhesion was found on more than 80 % of these primers,
which can be explained by chemical bond formation between remaining free amine groups on the substrate and
acryloyl groups from the Acure paint. Obtaining chemical
adhesion with Acure paints on substrates not containing
NH-moieties can be more difficult. However, it was found
that the presence of 1,2,4-triazole, apart from its function
as kinetic control additive, also helps as an adhesion promoting agent on some metallic substrates. Well-known [6]
metal pre-treatment methods with silanes also exhibited
excellent adhesion with Acure top coats. Alternatively,
known alkoxysilane adhesion promoters can be used in
combination with Acure, if added shortly before use.
Comparing Acure technology with
isocyanate-containing systems
Comparative data obtained from various TiO2-white paint
formulations is shown below; one Acure and four high
solids isocyanate containing systems. All paint formula-
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Technical Paper
EC Award winning paper
tions were optimised for best performance as a high gloss
industrial top coat. All reference paint components used are
commercially available. 4 % OH-NCO paint A is a “state-ofthe-art” very fast RT drying top coat system, paints B and
C are intended for elevated temperature curing (B: 60 °C,
C: 80 °C). All paints have identical pigment to binder ratios. The Acure paint composition is given in Table 2 and
is based on DTMPTA (di-trimethylolpropane tetra acrylate)
as acryloyl acceptor. The malonate functional resin A is a
specially designed polyester resin with a number average molecular weight of about 1780 Da and a malonate
equivalent weight of 350 g/mol solid resin. The resin is at
85 % solids in butyl acetate and has a viscosity of 5-10 Pa.s.
The malonate functional resin B is the same resin; however
modified with 1.4 % succinimide on solids.
The emphasis was on leveraging the control of the cure
chemistry in systems with relatively high concentrations of
functional groups. Consequently, Acure coatings with very
high crosslink densities (XLD) of typically 2.5 - 3 mmol/cc
are obtained, as derived from DMTA measurements [5].
These XLDs are higher by a factor of around 3 relative to
high end, e.g. 4 % OH – isocyanate or aspartate based systems, even when using high isocyanate levels (see Table
3 for comparing the isocyanate levels used in this study).
For Acure the XLD is amongst others, tuned by the acryloyl – malonate hydrogen ratio, see Figure 9, showing the
resistance to MEK dramatically improve when A/M > 0.9.
Surprisingly, it was observed that yellowing in the dark of
Acure coatings depended also on the A/M ratio. In order to
avoid this, the A/M ratio should also be > 0.9, in which case
it may even out-perform isocyanate based systems.
High level of appearance
and physical properties
The appearance of all coatings in this comparative test was
good. Focussing on the differences, it was noted that the
aspartate coating deteriorated significantly with time when
exposed to 24 hrs 120 °C. An advantage of Acure chemistry
is its relative insensitivity of appearance as a function of the
applied layer thickness. Unlike OH-isocyanate-containing
systems there are no foam generating reactions with water
or solvent popping phenomena and subsequent pin-holing
risk. In the lab Acure paints have been applied at over 240
µm DFT that still exhibit very good appearance.
Table 3 further shows mechanical test results, again only
focussing on differences between the tested systems. All
tested coatings had excellent flexibility (as tested with the
conical mandrel). Hardness development for the various
coatings is remarkably different. Note that paints B and C
were cured at the recommended elevated temperature (30
min 60 °C and 80 °C respectively). Yet, the early hardness
cannot always compete with Acure and aspartate. The superior mar (scratch) resistance is consistent with the high
crosslink density present in the Acure coating.
Resistance to various chemicals is equal to or better than
the references for Acure coatings. Resistance to acid etch,
was found to out-perform the reference coatings.
Finally, Table 4 shows the comparative potlife – drying balance for these paints at room temperature. Paint Acure II differed from Acure I only in that the former contains half the
amount of succinimide and 20 % more catalyst compared
Figure 7: Pendulum
hardness development of an Acure
coating formulated
for 30 min tackfree
at room temperature (red line) and
a typical 4 % OHNCO coating dried
for 30 min at 60 °C
(blue line)
Figure 8: Influence
of catalyst additives succinimide
and 1,2,4-triazole
on Acure appearance (shortwave)
at 60 µm
Hardeners for
Epoxy Systems
Polyamides
Polyaminoamides
Polyamide/polyamine Adducts
Aliphatic/Cycloaliphatic Amines
Mannich Bases
Waterborne Epoxy Systems
Hot Melt Polyamides
Formulated Epoxy Resins
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European Coatings J OURNAL
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Technical Paper
EC Award winning paper
Figure 9: Hardness (DFT 65 µm)
after one day at RT
(blue), yellowing
after 24 hr 120 °C
(red) and resistance to 4 min. MEK
exposure on a
scale of 1 (bad)
to 5 (no visible
damage)
to the latter to match the cure speed of the aspartate paint.
Note the very long pot lives for both Acure paints.
VOC, health and safety
Acure paints consist of malonate functional polyesters
and acryloyl functional oligomers, both of which are
compounds of low polarity with no appreciable hydrogen
bonding. Consequently, the intrinsic viscosity of the paint
is significantly lower than when using hydroxyl functional binders. The Acure paint of Table 2 contained about
245 g/l VOC; no effort was made to optimise for low VOC.
Acure is base catalysed; it does not require an organotin or any other organometallic catalyst, nor does it release formaldehydes. Depending on the type of selected
acryloyl oligomers the paint can be formulated without
skin sensitising compounds; e.g. di-trimethylol propane
tetraacrylate (DTMPTA). If this is not a requirement more
cost effective alternatives are available (e.g. TMPTA).
Summarising Acure
performance and outlook
Acure, based on Michael addition chemistry, extended
with the kinetic toolbox developed and put to work with
dedicated malonate polyesters, has yielded an impressive combination of attractive performance parameters.
These innovations are expected to form the basis for a
next generation of novel high performance coatings capable of providing significant improvements in application cost efficiency. Key features and benefits include:
»»very fast drying, with fast crosslink density development, and a very long pot life
»»application at ambient temperatures, or even below
»»very low solvent content (VOC < 250 g/l)
»»excellent appearance
»»thick layer application (> 150 µm) possible
»»very good chemical resistance
»»very good mar resistance
»»excellent flexibility
»»good outdoor durability
»»isocyanate, formaldehyde and organotin-free cure
chemistry
As such, Acure presents a system that can combine many
premium characteristics through an unprecedented
control over the cure kinetics. Of course, being a base
catalysed system, it also encounters some inherent limitations, especially in terms of sensitivity to acidic components that may interact with the catalyst system. Care
must be taken when selecting additives in order to avoid
those that can potentially cause acid contaminations, e.g.
dispersants, rheology control agents, etc. With a proper
choice of additives, complications can be avoided. As an
alternative to acidic rheology additives, Acure compatible malonated polyesters equipped with urea nanocrystal based rheology agents (SCA’s) have been made, providing thixotropy. Cure inhibition complications can also
arise when applying Acure paints onto substrates containing mobile acid species, such as WB base coatings
used in automotive applications.
In this paper, the first observations based on a limited set
of malonated binders are presented. Considerable room
to decrease the viscosity of the binders and develop lower VOC Acure paints is expected. Likewise variations in
e.g. polarity, malonate equivalent weight, or resin Tg and
the nearly unlimited variations in paint formulation will
further add to the potential for Acure in a broad range
of application fields. E.g. it has already been shown that
‘quasi 1K’ paint systems (pot lives extending over days)
are possible whilst still retaining the cure speed by increasing the amount of alcohols.
References
[1] Michael, A. (1887). "Über die Addition von Natriumacetessig- und
Natriummalonsäureäthern zu den Aethern ungesättigter Säuren".
Journal für Praktische Chemie 35: 349–356.
[2] A. Noomen, Progress in Organic Coatings, 32 (1997), 137-142
[3] S. Kiil, J. Coat.Technol.Res., 7 (5), (2010), 569-586.
[4]http://www2.lsdiv.harvard.edu/pdf/evans_pKa_table.pdf
[5]R. Brinkhuis, J. Schutyser, F. Thys, E. De Wolf, M. Bosma, M.
Gessner, T. Buser, J. Kalis, N. Mangnus, A. Bastiaenen, ‘New, ultrafast drying, low VOC, isocyanate free technology for 2K coating
systems’, Proc. Eur. Coatings Conf, Nuremberg, April 2015, in print.
[6] T.S.N. Sankara Narayanan, Rev. Adv. Mater. Sci. 9 (2005), 130 – 177
Table 4: Comparing potlife – drying balance at room temperature
hh:mm
hh:mm
0:35
0:40
0:14
0:18
0:15
0:25
0:55
3:30
1:20
5:35
4%
OH-NCO
A
4:20
> 7:00
hh:mm
20:00
16:00
0:26
1:25
2:05
1:50
“Acure” “Acure”
4%
4%
Aspartate
I
II
OH-NCO A OH-NCO A
RT curing
Dust dry
Tack free
Pot life
DIN53211
40
European Coatings JOURNAL
05 l 2015
Acknowledgements
The autors wish to express their gratitude for their contributions to this
work to: Mrs A. Bastiaenen, Mrs M. Thannhauser, Mrs L. Taylor,
Mr M. Gessner, Mr P. Dolphijn, Mr Dr M Bosma, Mr Dr. J. Akkerman, and
Mr Dr. D. Mestach.
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