22.06.2014
AP3155 is an adhesion promoter for use with UV and LED curable inks when printing on Glass, Metal, Aluminum, Acrylics, HDPE, Coroplast, Polystyrene and other hard to print substrates.
Acrylic urethane coatings are used in a variety of applications due to their versatility, durability, appearance and superior weatherability compared to other resin systems. UV-curable coatings, on the other hand, are one-component (1K) systems that cure by photoinitiated polymerization of the acrylate monomers and oligomers in the formulation (Figure 2). UV-curable formulations typically contain an acrylated oligomer based on a polyether, polyester or epoxy resin. Several companies have recently reported on dual-cure coatings that combine the benefits of UV and 2K urethane systems and overcome some of their limitations (Figure 31). The final coating properties, however, are achieved by the combination of a UV-curing process and a chemical crosslinking process. We have recently developed a family of acrylic polyols based on allylic alcohols.2 These new acrylic polyols are unique in that they are solvent-free, solid and liquid resins with very low solution viscosities and superior functionality compared to conventional acrylic polyols. The following paragraphs describe the preparation and properties of new acrylic urethane coatings for weatherable applications. Results and Discussion1K Acrylic Urethane UV-Cured Coatings Most acrylated oligomers are based on polyether, polyester and epoxy resins.
Acrylic polyols, on the other hand, are produced by free-radical polymerization and are usually supplied in solution. Synthesis of Acrylated Urethane-Acrylic Oligomers Initial attempts to prepare acrylic-based prepolymers with HDI (hexamethylene diisocyanate), TDI (toluene diisocyanate) or IPDI (isophorone diisocyanate) led to gelling. The liquid acrylic polyol was first treated with excess IPDI to form an isocyanate pre-polymer, which was then capped with HEA. The butyl acetate solvent we used to reduce viscosity can be replaced with acrylate monomers such as IBOA (isobornyl acrylate) to produce solvent-free and VOC-free formulations, albeit with much higher viscosity. Acetone is an excellent viscosity reducer for these resins and a good choice for these systems.
The same chemistry was used to produce low-viscosity, acrylated urethane-acrylic resins with residual OH functionality.
However, instead of using excess IPDI, the acrylic polyol was first treated with two equivalents of isocyanate in the presence of tin catalyst to reduce the hydroxyl functionality to 3-4 per polymer chain, and then treated with two equivalents hydroxyethylacrylate (HEA).
Based on these criteria, we selected IBOA (isobornyl acrylate) and IDEA (isodecyl acrylate) as our mono-functional acrylate monomers and either TMPTA (trimethylolpropane triacrylate), Di-TMPTA or PO6-TMPTA (propoxylated TMPTA) as our multifunctional acrylates.
We chose to add solvent to these formulations because we wanted a sprayable system with good sag resistance and minimal shrinkage during UV cure. Furthermore, a sprayable coating system prepared only with acrylate monomer diluents would be much more likely to sag when applied to vertical surfaces. The choice of photoinitiators was dictated by the UV light source, HALS (hindered amine light stabilizers) and UV-screeners. Film properties were highly dependent on the monomer diluents but were comparable to conventional 2K urethane coatings.
In contrast, the formulations containing PO6-TMPTA showed significant gloss loss and failed after approximately 2000 hours of exposure. UV screeners improved the weatherability of both coatings yet did not interfere with the UV-curing process. 2K, Dual-Cure Coatings Clear coatings were also prepared using the partially acrylated polyol DC60. Because the polyol and isocyanate crosslinker (HDI trimer) react under ambient conditions, they must be mixed shortly before application. Coating hardness was 49 Konig swings immediately after UV cure, and the coatings was tack- and print-free. 2K UV Coatings Without Acrylated Oligomers It is also possible to develop dual-cure coatings using conventional 2K urethane polyol ingredients and acrylate monomers as diluents.
The final coating properties and cure speed are controlled by the formulation components, their relative amounts and their order of addition.
With liquid acrylic polyols as the base polyol and HDI-trimer as the isocyanate crosslinker, mono-functional acrylates such as IBOA (isobornyl acrylate) do not yield a tack-free state after UV cure, unless the HEA is pre-mixed with the isocyanate, as is illustrated in Figure 12. Replacing a portion of the liquid polyol with a high-Tg polyol was also effective in achieving a tack-free state immediately after UV cure. Conclusions Low-VOC, sprayable acrylic-urethane clearcoats have been developed for weatherable applications. These coatings have excellent appearance, physical properties and weatherability, suggesting that acrylic-urethane UV-curing clearcoat technology could potentially be applied to automotive OEM and refinish applications, as well as wood and plastic parts. Resin Management Concept is a registered trademark of Lyondell Chemical Company.AcknowledgementsThe authors wish to thank Ann Good, Bob Good, Dave Pangburn and Mark Smithson for expert technical assistance, and Steve Harris for helpful discussions.


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These applications include automotive clearcoats, industrial maintenance topcoats, general metal, wood furniture and flooring. These oligomers are more difficult to prepare than the acrylic polyols used in 2K urethanes, which contributes to the high cost of UV-curable formulations. The cured films are usually more flexible and adhere better to the substrate than 100% UV-curable systems, presumably because shrinkage is reduced.
These polyols are also designed to be blended together to achieve coating properties for a wide range of applications and technologies, including high solids, UV- and moisture-curable, and powder coatings (Figure 4).
This is because the functionality of these condensation resins can be precisely controlled to 2 or 3 to prevent gelling and keep viscosities low (Figure 5). IPDI was ultimately chosen for the unequal reactivity of the primary and secondary isocyanate groups, which helps control crosslinking and prevents gelling.3 By modifying the process and reagents slightly, we were able to produce gel-free, low-viscosity, low-color acrylated urethane-acrylic oligomers (Table 1).
Excess IPDI is required to prevent gel formation when preparing the isocyanate prepolymer, because the hydroxyl functionality of the acrylic polyol is greater than 5. Under controlled reaction conditions and catalyst concentrations, the acrylic polyols reacted with the primary isocyanate group of IPDI, leaving the secondary isocyanate group intact for subsequent reaction with HEA. It is inexpensive and evaporates quickly, which allows the coating to be sprayed without a lengthy flash-off time. The residual OH functionality provides a crosslinking site for dark or thermal cure with conventional isocyanates or melamine crosslinkers (Figure 7).
The resulting resin had an average of 2 acrylate and 3.6 hydroxyl groups per polymer chain, making it capable of both UV and chemical crosslinking reactions (Table 2).
Because of the relatively poor solvency of acrylate monomers, we would have had to use large amounts of monomers to reduce the system to spray viscosity, thereby increasing the chance of shrinkage and poor adhesion.
We chose acetone as the solvent because of its rapid evaporation rate, excellent solvency, and non-HAP and VOC-exempt status in the U.S. We chose a D-bulb because of its high output in the visible- and near-UV portions of the spectrum and to avoid interference from the HALS and UV screeners.
These results illustrate that although the base acrylic urethane resin is weatherable, the choice of acrylate diluents can have a very negative impact on the UV resistance of the coating. Unlike the fully acrylated version, DC60 does not yield a tack-free coating by itself when exposed to UV light. The NCO-OH reaction continued for another week, during which the coating hardness increased to 120 swings. This may potentially reduce the cost of dual-cure formulations by eliminating the need for acrylated oligomers.
Since neither the polyol nor the isocyanate crosslinker are involved in the UV-curing reaction, the acrylate monomers, acrylic polyol and isocyanate crosslinker must be carefully selected so that a tack-free state is achieved after UV cure. This, presumably, generates an acrylated polyisocyanate that can now be involved in the UV-curing process. This approach is preferred over the use of TMPTA because the final coating has better impact resistance and flexibility. After 5000 hours of exposure, all coatings have retained over 90% of their gloss, even under harsh QUVB conditions (automotive cycle) (Figure 14).
One-component, UV-curable coatings were prepared using novel acrylated urethane-acrylic resins. The most common coating type is two-component (2K), where an acrylic polyol solution is mixed with a polyisocyanate just before use and applied to the substrate. It is even more difficult to prepare acrylated urethane oligomers based on acrylic polyols because their high functionality often leads to gelling or high viscosities. The UV-curing step is used to obtain tack-free coatings so the parts can be handled, buffed or sanded immediately after UV cure. Also, the dark or thermal cure mechanism provides a means to cure the film in areas not fully exposed to UV light. These coating systems offer instant cure and superior properties compared to conventional UV-curable systems. The resins also contained 20-30% of IPDI diacrylate, which is produced when excess IPDI reacts with hydroxyethylacrylate (HEA).
The resulting product is a mixture of acrylated acrylic-urethane oligomer and acrylated IPDI reactive diluent. This unique property of IPDI allows for easy control of the ratio of acrylated urethane-acrylic oligomer to IPDI diacrylate diluent and broader process latitude.
It is important, however to allow the solvent to flash off to prevent the formation of pinholes and solvent popping during the UV cure.


This resin can be used in dual-cure coatings diluted with acrylate monomers but does not, by itself, give tack-free coatings after UV cure.
Although not VOC-exempt in Europe, acetone has low ozone-forming potential and should have minimal impact on tropospheric ozone compared to other, more reactive solvents. We eventually settled on a blend of a BAPO (bis-acyl phosphine oxide) photoinitiator in a liquid µ-hydroxyketone (AHK).
Since polyethers are known to have poor UV stability, we attribute this loss of weatherability on the propoxylated TMPTA. This reaction also provides a dark-cure mechanism that is not found in conventional UV-cured systems.
Heating the coating to 60 °C for 10 minutes after UV cure accelerated the NCO-OH reaction and hardness development and resulted in a slightly harder final film.
The acrylate monomers help reduce the viscosity of the polyol and isocyanate crosslinker and are polymerized with UV light to achieve a solid state. Some tri-functional TMPTA (timethylolpropane triacrylate) is usually required to build enough molecular weight.
Zero-VOC formulations sprayable at 80% solids and above were achieved using acetone and acrylate monomers (Table 5). Two-component coating systems were developed using a partially acrylated urethane acrylic polyol and conventional 2K urethane components, diluted in acrylate monomers and acetone. The coating then cures by a combination of solvent evaporation and chemical crosslinking to form a durable urethane bond (Figure 1).
This is a significant limitation of 2K urethane coatings since one has to trade productivity for compliance, cost and ease of use. We believe this has contributed to the slow growth of UV-curable coatings in applications where excellent UV stability is required, such as automotive clearcoats. Another benefit of achieving an instantaneous, tack-free state is that the coating appearance is improved.
However, as was the case with conventional UV-curable oligomers, the main polymer backbone is a polyester, not an acrylic resin. These systems may allow UV-cure technologies to be used in the most demanding applications including automotive topcoats.
Consequently, the traditional two-step synthesis shown above typically leads to highly crosslinked, viscous, or even gelled products that are not suitable for coating applications. The final coating hardness is also improved if the solvent is allowed to escape before the coating is UV cured. Because Europe does not consider acetone VOC-exempt, the VOC content for Europe is around 300 grams per liter. We again used IBOA as the monofunctional acrylate and TMPTA as the multifunctional acrylate. Adding a hydroxy-functional acrylate monomer (such as HEA) provides a crosslinking mechanism between the UV-generated acrylic resin and the isocyanate crosslinker (Figure 11). However, the impact properties of the final coatings suffer as is expected from the higher crosslink density (Figure 13).
Other limitations of UV-curable coatings include the absence of a dark cure mechanism, difficulty in spraying and curing 3D parts, and poor adhesion and flexibility due to shrinkage during the curing process. This is because fewer airborne impurities have time to contact the wet film and cause defects. After 5,000 hours of UV-A and -B exposure, the base UV120 resin showed good to excellent gloss and color retention (Figure 9). We also used a different adhesion promoter (SR9008 from Sartomer) and added a tin catalyst and amine synergist (triethylenediamine or TEDA) to accelerate the dark cure reaction between the OH groups and the isocyanate crosslinker (Table 4). However, the UV-cure component provides instantaneous hardness development, as illustrated in Figure 10. Nonetheless, we were able to prepare low-viscosity urethane acrylates based on the new liquid acrylic polyols. Conversion of the acrylate unsaturation was measured by Raman spectroscopy and was typically 80% after the first pass and above 90% after the 12th.



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