Low viscosity without VOCs

Along with roller coating and printing, spraying represents the third major application method for UV-curable coatings. Spray application demands the lowest viscosity from the coating formulation. In some applications, this low viscosity can be achieved by diluting UV oligomers with monomers. However, if a high amount of monomer is needed to achieve spray viscosity, this limits the quality achievable.
It is also often more economic to use solvents or water as diluents, accepting the additional processing step of flashing-off before UV cure while achieving a lower coating thickness and less consumption of paint solids for a given coated area.
Price rises in organic solvents and more and more stringent VOC legislation are shifting the industry’s attention to waterborne UV coatings. Dispersions of urethane acrylates (UV-PUDs) often combined with physical drying acrylic dispersions are being used widely in the European furniture coatings industry, and are also finding growing acceptance in the North American market.
Dispersion technology opens up new possibilities in the chemistry of UV coatings. It is no longer necessary to look for the lowest viscosity oligomers since the viscosity of any dispersion is independent of the viscosity of the dispersed substance. High molecular weight urethane acrylates can be synthesised using hydroxy-functional acrylic monomers or oligomers such as polyester or epoxy acrylates, resulting in polymers with high acrylate functionality.
In fact, the ever-important oligomer functionality in 100% solids or solventborne formulations is a parameter of much less relevance in waterborne UV systems. For the same reason, monomers are not really needed in waterborne UV formulations. However, certain monomer types, depending on their compatibility with waterborne systems, can be added either by the manufacturer of the oligomer/polymer or by the formulator to adjust the properties of the cured coating.

The effect of these differences in chemistry on the UV curing parameters of waterborne UV systems has been studied previously [1, 2]. Especially noteworthy is the pronounced influence of the temperature at which UV curing is accomplished on the degree of double bond conversion.

Solventborne UV coatings are still predominantly used for application on plastics such as mobile phones. These and other high-value equipment from the computer, communications and consumer electronics industry are typically coated with physical drying solventborne (or in some cases waterborne) pigmented thermoplastic acrylic (TPA) basecoats and solventborne UV clearcoats. The UV clearcoats are based on high-functionality oligomers and monomers, so that the required high resistance properties are achieved by a high crosslink density.
One of the challenges for waterborne UV systems in this application is to achieve high gloss over basecoats. The physically drying basecoats form a surface with a certain micro-roughness which other physically drying systems are not able to cover. The roughness is thus also found at the surface of the clearcoat and the surface does not show the high gloss and brilliance that can be achieved with the non-physically drying solventborne UV systems.
In wood coatings, the primed wood is usually sanded before application of the topcoat, hence high gloss is possible with physical drying UV systems. This is one of the reasons why the simple transfer of wood coatings to plastic coatings in waterborne UV technology is not usually possible.

Physically drying systems, however, have advantages in pigmented systems. Even if deep curing is incomplete, the combination of partial UV crosslinking and physical drying is sufficient to achieve a high property profile. In furniture coatings, waterborne UV pigmented topcoats are already widely used, e.g. for kitchen cabinets. In most solventborne UV coatings, however, the high amount of monomers used leads if undercuring is caused by the pigmentation to bleeding of uncured monomers from the coating. Single-layer pigmented solventborne UV spray coatings for plastics are consequently rare on the market.

UV-curing polyurethane dispersions were synthesised using polyester or epoxy acrylates by procedures described elsewhere [3] incorporating high-functionality monomers through a proprietary dispersing process. For comparison, a commercial UV-curing solventborne formulation based on a blend of oligomers and monomers for mobile phone coatings was used ("Desmolux B175X" in combination with the urethane hexaacrylate "Desmolux U400", both from Bayer MaterialScience). Commercial photoinitiators and standard additives for waterborne coatings were used.
Flash-off of water and solvents and UV curing was accomplished by a combination of a convection/IR dryer and a UV oven. After application by spray coating, the films were left for five minutes at room temperature. Subsequently, flash-off was accomplished by treatment with hot air at 60°C for two minutes and, where noted, by additional simultaneous irradiation with infrared light.
The surface temperature of the pre-dried coatings could be determined by a temperature sensor immediately before UV cure and was varied between 40 and 80°C. UV cure was accomplished by a
single 120 W/cm mercury or gallium doped lamp. Unless noted otherwise, a dose of 1000 mJ/cm2 was applied.

Glass transition temperatures were determined by differential scanning calorimetry (DSC) after heating the sample briefly to 150°C. Double bond conversion was measured by Fourier transform infrared (FTIR), analysing the C=C-band at 810 cm-1 and using an uncured sample as reference for 0% conversion.

The route for the development of waterborne UV hardcoats is based on UV-PUDs used in wood coatings technology. Two effects are achieved by incorporation or blending of high-functionality UV monomers. The double bond density of the coating formulation is increased to a greater degree than that of solventborne formulations of oligomers and monomers (Figure 1) and secondly, physical drying of the coating is reduced by the low molecular weight monomers, enabling the waterborne system to flow out and yield clearcoats over basecoats with high gloss similar to that of solventborne UV systems (Figure 2).
In further experiments, the influence of the curing parameters on the properties of the cured film of the new waterborne UV hardcoat was investigated. A first screening of available photoinitiators for waterborne UV systems pointed towards bis-HDMAP ("Irgacure 127") as the photoinitiator that yields the best performing coatings in combination with the new UV-PUD. It should be noted that this type of photoinitiator can lead to increased initial yellowing compared to other types. The yellowing, however, bleaches within a few hours.In a statistically designed experiment, photoinitiator concentration, temperature at UV curing and UV dose were varied, studying the effects on film hardness (pencil hardness and pendulum hardness) and the glass transition temperature of the cured film.
At low photoinitiator concentrations, the pendulum hardness increases with temperature at UV cure and with higher UV dose. This dependency is less pronounced at higher photoinitiator concentration, which was thus selected as the optimum concentration. Even at high photoinitiator concentration, the highest pencil hardness of H-2H is only achieved if curing takes place at high surface temperature and high UV dose (Figure 3a).

The previously reported positive influence of the temperature at UV curing
[1, 2] is obviously also valid for the combination of a high molecular weight PUD with a high functionality acrylic monomer. The same behaviour as for the pencil hardness is also found for the glass transition temperature of the cured film (Figure 3b). Within a range between 60 to above 100°C, the highest values are found for high temperature before UV curing and high UV dose.

Once these optimum curing parameters were established, a formulation of the new UV-PUD hardcoat was subjected to the testing protocols set out by several mobile phone producers. These protocols vary from one OEM to another, but essentially similar characteristics are required by all. The tests were performed on polycarbonate mobile phone shells coated with silver-metallic solventborne thermoplastic acrylic or waterborne acrylic or PU basecoats (10 µm dry film thickness) and WB UV hardcoat (20 µm dry film thickness). The following parameters are included in almost any testing protocol by the mobile phone producers:

- High gloss;
- Pencil hardness H or higher;
- Excellent adhesion in crosshatch/tape test, which must be retained after subjecting the coated part to immersion in boiling water;
- Resistance against prolonged exposure to high temperature and high humidity;
- Chemical resistance against solvents and also against suntan lotion at elevated temperature and high humidity, cosmetics and fluids that simulate human sweat;
- Abrasion resistance in RCA and/or Taber test;
- Low yellowing after irradiation with lamps that emulate sunlight;

- Resistance to abrupt temperature changes (hot-cold-cycles);

If good adhesion of the basecoat-clearcoat build-up could be achieved and the temperature at which UV curing was performed was high, all other tests gave results on the same level as the established solventborne TPA base and solventborne UV clearcoat formulations i.e. the specifications of several mobile phone producers could be passed.

Exploring further the possibilities of the new UV-PUD for hardcoats, a metallic formulation that could be used as single-layer coating was developed. Targets for the development were good gloss and resistance properties as well as excellent adhesion. The question of adhesion is related to the through-cure that is achievable, which in turn depends on the layer thickness and pigmentation level.
A phosphorous-organic treated metallic effect pigment that is stable in waterborne formulations was chosen. Photoinitiators were studied, following the pigment supplier’s formulation recommendations. In this case, a blend of benzophenone and an alpha-hydroxyketone was used for surface cure, while through cure was improved when BAPO was added to the formulation.The influence of curing parameters (IR dose or temperature at UV cure, lamp type, UV dose and layer thickness) on the surface and through cure were studied by FT-IR spectroscopy on the front and reverse side of free coating films.

For surface cure, the variations in double bond conversion were marginal. In all cases, conversion was well above 90% and could thus be considered as complete. [Statistically, for a tetrafunctional monomer, curing of 80% double bond conversion corresponds to 0.24 = 0.16% of wholly unpolymerised molecules, while curing of 90% corresponds to 0.14 = 0.01% of unpolymerised molecules.]

The greatest effect on through-cure is the influence of the layer thickness. Figure 4 illustrates the model statistically derived from the data for two series of dry films, with thicknesses of 13 µm and 26 µm. For the thin films, good through-curing was achieved for large areas of the experimental space. The thicker layer, however, requires optimised conditions. Nonetheless, a conversion of 80% of the double bonds is possible at high temperature and high UV dose. The high temperature (7080°C) at UV curing is best induced by hot air and infrared irradiation. The influence of the lamp type (Hg or Ga) was not statistically significant.
Measurements were carried out on the reverse side of the coatings to determine through cure rather than surface cure. The IR dose affects surface temperature, 0% IR resulting in 50°C and 50% IR in 80°C. Curing was by one Hg lamp at 120 W/cm. The pigmentation level of 19% on formulation solids gave full hiding power at 16 µm dry film thickness.

The findings of good through-cure could be confirmed by application-related testing of the metallic coating on polycarbonate mobile phone shells. The optimised but still basic formulation has good hiding power and gloss of approximately 70% (60° angle), good adhesion, even after immersion in boiling water for one hour, and excellent resistance against solvents. Resistance against suntan lotion is almost at the same level as the non-pigmented formulation.

With a new generation of waterborne UV-curing polyurethane dispersions, it is possible to formulate coatings for plastics with performance characteristics that could hitherto only be achieved by two-component polyurethane systems or solventborne UV systems.
The UV coating of mobile phones and other equipment with similar high demands on the performance properties is for the first time possible without the use of organic solvents. Waterborne UV technology offers the possibility to convert the conventional dual-layer coating system from solventborne to waterborne. The high quality single-layer pigmented systems are a new and very economical option.
Using water as the solvent is the more favourable alternative, economically as well as ecologically. The decision to convert to waterborne UV technology has been taken by many companies in the European furniture industry. It is now up to the manufacturers of high value coated goods made from plastics to look, test, calculate and make their choice.

[1] J. Weikard et al., Proc. of 6th Nürnberg Congress "Creative Advances in Coatings Technology" (2001), 125 144. [2] F. Masson et al., Progress in Organic Coatings 39, (2000), 115 126. [3] EP-B 0753531, Bayer MaterialScience AG.