A new ultra-fast dry, low VOC, isocyanate-free 2K coating system -- carbon Michael additive system Acure

R. Brinkhuis, J. Schutyser, F. Thys, E. De Wolf, M. Bosma, M. Gessner, T. Buser, J. Kalis, N. Mangnus, A. Bastiaenen, M. Thannhauser, F. G. H. Van Wijk, Qiu Discipline Zhanxin Resin (China) Co., Ltd.
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double-component polyurethane paint has been widely used in coatings. However, there has always been a demand for a coating system that is environmentally friendly and has the health and safety of the person under the premise of increasing production efficiency. This article provides an overview of the latest breakthroughs in low VOC, isocyanate-free and tin-free technologies to meet these needs

. The coating is based on a new type of closed catalyst and a additive package made up of dynamic control aids, and works in synergy with the Michael Additive reaction. The Michael Plus-based coating formulation shows that this coating system can be quickly dried (<15 minutes at room temperature) and low VOC (<250 g/L, or even 50 g/L), and that the drying time and active period (>5 hours) are free of correlation (two-group PU system, drying time and active period are positively related), in the hope that it can lead to a new coating system and construction process solutions. In this paper, the chemical properties of this coating system and the performance of other paint systems are compared. The results show that this coating system can replace the existing two-part system and can be used in the two-part paint system in the marine, anti-corrosion and industrial OEM markets without sacrificing performance.
Introduction
Today's coating technology continues to move towards high-solid content, low-temperature curing and compact coating processes, driven by changes in HSE (Environment, Health, Safety) regulations, lower overall coating application costs, and improved coating performance. It is very challenging to meet these comprehensive requirements using existing chemical crosslinking techniques. Because of the lack of diversity in existing chemical interlinks. The Michael Plus response provides a new perspective for breaking through these limitations.
Michael Plus Reaction (MA) was studied before it was used in coating applications. Maybe it's because it's curing so fast that it's never been used as a mainstream curing technology. The key components of the Michael addition reaction system are electron-deficulated C-C double bonds (e.g. acrylic prepolymers, subjects), acidic C-H bonds (present in acetylacetic acid and propylene diacic acid parts, feeders) and strong enough alkaline catalysts. The catalyst absorbs protons of the C-H bond, producing a pro-nuclear carbon negative ion that can be added to the double bond. A carbon chain is formed between the two parts. Because of its similar reactive nature, the second proton of the supply can continue to react to form a spatial network structure (see Figure 1).
addition reaction between 1/acrylic pre-polymer and propylene polyester resin.
the relevant chemical properties that we can introduce from the Michael addition reaction include:
needs a strong enough base to extract protons from the supply. The pKa of a C-H bond of acetylacetic acid resin is about 10.7, and the pKa of C-H bond of propylene diphate is even higher (>13).
should not contain acids, which can insulate the catalyst.
carbon negative ions with very high reactive activity, especially when propylene-date is used as a feed. In coating formulations, acrylic polyester resins and acrylic monomer pre-polymers complete the Michael addition reaction in minutes. Without the addition of catalysts, propylene polyester resin and acrylic monomers can coexist in the coating system, with good storage stability.
the carbon chain is produced, which does not produce weak chain segments and affects weather resistance.
Michael Plus reaction technology opens a window into the use of low polarity, low equivalent weight cross-linking parts, which can be used to design coating formulations with very low solvent demand and high cross-link density.
we take advantage of the high reactive activity of the propylene polyester-acrylic pre-polymer system and control its reaction speed to create a longer period of activity and development time. These methods are used to expand the application of Michael's add-in reaction. Combined with specially developed resins, with far-reaching results, the Niker-Plus reaction can be considered a completely new curing technology that can be used in many different markets and fields. We refer to this new chemical chemistry below as "AcureTM".
the high reaction activity of the
and drying are difficult to exist at the same time as the longer active period. However, we have found a scheme in the reversible closure reaction between a strong alkaline catalyst and alkyl carbonate, as shown in Scenario 2a. Strong alkalis form alkyl carbonate anion that is so alkaline that it does not trigger a Michael's addition reaction. These carbonate roots are inherently unstable, and by reacting with protonized resins, they can be balanced against free carbon dioxide and alcohol solvents (scenario 2b).
Scheme 2/Closed catalyst formation and resuscing; HA represents proton supply
after mixing in a tank (with a very low surface area), so a long period of activity can be obtained without the rapid release of carbon dioxide (CO2) (see below). However, after coating construction, a very large surface area is obtained, and solvents, especially carbon dioxide, can be released quickly. This changes the previous balance, followed by the rapid unsealing of the alkaline parts, which triggers the reaction of the acrylic and acrylic pre-polymer systems. The stronger the acidity of the proton supply, the faster the unsealing process, and the balance of the balance reaction 2b will move to the right. The result is a solution that has a long reseration period and is very fast to dry. This Michael addition has a tank-opening activity period of at least 5 hours, which can also be provisioned to a day-to-day basis if necessary. At the same time, we observed that the finger drying time after construction was reduced to 10 minutes, and the drying recording time for stage 4 (no scratches) was not too long (dry recorder). Figure 1 shows the contrast between the active period/drying speed balance of the AcureTM system and the isocyanate-based coating system.
1: A balanced picture of the active period - drying time. Blue area: hydroxyl and N-H and isocyanate curing areas; The overlapping areas represent the "Polytianmendolic acid regions (N-H-isocyanates)."
2 shows the effect of adding alcohol solvents to AcureTM coatings. It shows that alcohols not only extend the active period, but also do not significantly affect the drying time. When the conversion rate of carbon double bonds is detected using Fourier Transform Infrared Spectrum (809cm-1), we can see that the conversion rate of carbon double bonds exceeds 80% in the first 10 minutes (see Figure 4). This not only indicates that the lacquer film is in a fast physical drying state, but also indicates the rapid increase of the cross-link density of the lacquer film, which quickly establishes the chemical and mechanical resistance of the early lacquer film. AcureTM coatings are also highly reactive at low temperatures. We observed a curing time of less than an hour at a construction temperature of 15 degrees C. In its practical application, this system can be designed as a two-component coating system. Mix the two reactive parts (propylene polyester and acrylic pre-polymer) together as part A and the catalyst as an active agent and add (part B).
Figure 2/Volatile alcohols are added to the coating to affect the applicable period (left) and drying time (refers to dry-right), the horizontal axis is the percentage of the weight of the base material (propylene-dioxide polymer and acrylic prepolymer), and the active period is the viscosity doubling time.
paint film ills caused by fast curing
in such a short period of time dry paint system, may bring some paint film ills. These ills are not common in slow-dry coating systems.
, such a fast drying time is the opposite of the time the solvent escapes from the paint film. When cured at room temperature, the glass transition temperature (Tg) of the wet film increases due to the release and reaction of the solvent. As the glass transition temperature rises to room temperature, the paint film gradually changes to a glass state, which is severely hampered by both the diffusion of the solvent and the chemical reaction. If this glass state appears on the surface of the paint film, the paint film will reach the finger dry, and the surface of the glassing will be much earlier than the lacquer film depth of the glassing time, some solvents may be trapped in a deeper coating, these solvents are difficult to escape from the already closed paint film surface. That is, further solvent release will be greatly slowed down. The advantage of this condition is that these solvents are excellent plasticizers. This means that the cross-link conversion rate of the paint film can be further improved, the reaction process has not been hindered by the high glass transition temperature, the lacquer film is therefore easier to achieve full cross-linking. However, an oversized residual solvent may result in a lower glassing temperature of the deep paint film, in which case we can observe a decrease in the hardness of the swing bar. Kiil models the competition between chemical reactions and solvent escape.
, ultra-fast drying can cause the paint film to look worse because the flat time window of the paint film can be very narrow. One can see this ultra-fast dry coating system struggling between controlling the release of trapped air from the paint film and controlling overspouting. In some cases, the bottom of the paint film is still flowing, and the surface of the coating has been quickly cured, resulting in wrinkles, and even can easily lead to transmission of paint film defects.
, a combination of a very fast drying performance and a very good active period does not guarantee that there are no obvious paint film maladies.
challenge we have to solve is how to mitigate the problems described above while retaining fast drying performance. Our solution is to create an induced period during cross-linking reactions by using special dynamic control aids, creating a regulatable "open time" window.
the concept used to create the "open time"
revolves around the proton supply (HA) variety in the formula, which can be reacted by Michael addition, by deprotonization, and added to the acrylic pre-polyporide, but unlike acrylates:
a) proton feed (HA) is much more acidic than acrylic C-H, which means that alkali makes proton supply (HA) more prone to deprotonization than propylene disproton, thus delaying the rapid addition of propylene disgryption to acrylic precarbons.
b) anions (A-) are significantly less responsive to acrylic precesses: the consumption of these varieties in the Michael addition reaction is slow, that is, low concentrations of proton supply (HA) will have a significant effect on the early dynamics of the addition reaction.
c) Ideally, the resulting proton supply (HA) and acrylic prejudes are added without significantly increasing the viscosity of the coating. This means that proton supply (HA) will preferably be low molecular weight, single-activity components, to ensure wet coating wetting, leveling, exhaust, overs injection and other properties.
we have identified various substances (HA) that meet these characteristics, either carbon-based acids or nitrogen-based acids: Table 1 lists some examples that meet these characteristics. Each of these substances has a slightly different effect on application performance. With the addition of closed catalysts, a long-term active, induced (open) long-term scheme is created, and the length of development time can be regulated according to the amount of additives added. Very fast addition reactions occur immediately and can achieve a high conversion rate only when the more acidic proton supply is consumed. The final result can refer to the dynamic curve (Figures 3, Figure 4). By controlling the reaction rate with the dosage of catalysts and alcohol-assisted solvents, we have a formulation toolbox that can be easily optimized for Acure coatings to meet specific application performance requirements.
beneficial consequence of using a relatively more acidic proton supply is that option 2 (b) will shift to the right, increasing the concentration of weak acids (A-) and, in particular, carbon dioxide (CO2). In this way, the latter will more easily spread out of the coating. Due to its low reactivity (compared to propylene due to propylene esters), the high concentration of A-does not shorten the opening hours of the "newly created". However, if all HA is consumed and most of the carbon dioxide is excreted, the maximum concentration of Michael's reaction-active anions will appear in the coating film. The propylene disprotonic resin is then deprotonized, and the full potential of the ultra-fast Michael addition reaction is released, as if there had never been a closed catalyst (see Figures 3 and 4, and Scenario 3).
table-1. Suitable for dynamic control aids; pKa data from
3. Reactive control of AcureTM reactions. MA: Michael Plus. Reaction (1): Application period control, reaction (2): Open time control. Reactions (3) and (4): Cross-linking reactions. Hydrogen from two acrylics reacts with acrylic pre-polymers.
figure 3/Closed catalyst and the amount of dynamically controlled additives (as opposed to the catalyst) have an effect on the conversion rate of acrylic double bonds (Fourier Transform Infrared Spectrum, 809cm-1) via Michael Additive. Catalyst concentration: All curves are 40 micro-equivalent/g resin solid content, except for purple dotted lines. 40 micro-equivalents/g is for the purpose of normal testing of the Fourier transform infrared spectrum. The dashed arrow represents the control window for open hours (15% of the maximum value for crosslink conversions is taken here). For thicker (≥40 micron) highlight coatings, combine amber acrylamide and 1,2,4-benzene triamcinolone in Acure coatings for optimal performance (orange dashed line, see body).
therefore, the AcureTM reaction can be summed up in Scenario 3, wherein tetbic ammonium cations are closed alkalis, amberamide as a typical dynamic control aid (proton lid HA).
scenario 3, the coating control agents that control reaction 1 and reaction 2 are different kinds, so that we can independently regulate the applicable period and opening hours of the can, while making only limited sacrifices in the drying time. At the same time, there is a significantly beneficial improvement in the hardness and appearance of the swing bar (see Figures 5-8, and Table 2). For coatings with thicknesses of more than 40 microns, we found that for best results, both 1,2,4-benzene and amberamide were needed.
4/i.e. Figure 3 above shows the effect of co2 emissions on acrylic conversion rates