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)
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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 (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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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].
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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.
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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,
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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
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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
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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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