Fall 2002 Vol. 4, No. 2

The resin system and the additives used have the same major impact on the properties of UV powder coatings that they do in all other coating systems. The composition of the coating formulation of UV powder coatings will affect the flow, chemical resistance and flexibility of the coating. With additives, surface effects like gloss reduction or texture effects are introduced and presented. What is important is the effect of the cure conditions on the network formation. The influence of cure temperature, UV dose and UV spectrum were investigated with different calorimetric methods and are described in this paper.

Introduction

UV-curable powder coatings are now commercially applied at several industrial coating lines. Since the first commercial lines for coatings on preassembled metal substrates (electric motors and automotive radiators) last year, the first furniture manufacturer has started with the use of UV powder coating on medium density fiberboard (MDF). The UV powder system is attractive because it combines the environmental and technical advantages of powder coatings and UV-curing. The manufacturing and application of the UV-curable powder coatings is equivalent to conventional thermal curable powder coatings due to the absence of solvents. The curing of the UV powder coating, however, is initiated by exposure to UV light and, consequently, the curing temperature can be much lower compared to conventional powder coatings.

Similar to conventional powder coatings, the resin system and the additives used affect the final properties of the coating. Also, the parameters of UV curing can have a strong impact on the final coating performance, comparable to the temperature/time curing window of conventional powder coatings. Variables in the curing process of the UV powder formulation are parameters like temperature, amount of UV light energy of UV light and the spectral output of the lamp.

This paper will present how the UV powder coating curing window is optimized for several formulations. After a brief introduction on the chemistry, manufacturing and curing of UV powder coatings, examples will be presented of powder coating formulations with different properties in relation to the curing method used.

UV-curable powder coatings, chemistry and technology

In the literature, several examples have been described of binder systems suitable for UV-curable powder coatings. Examples include unsaturated polyacrylates¹, mixtures of unsaturated polyesters and acrylates², methacrylated polyesters³, acrylated hyper-branched polyesters⁴ and specially formulated solid epoxy resins⁵. DSM Coating Resins has developed a binder system based on a blend of an unsaturated polyester and a vinyl ether urethane component^6,7. The polymerization mechanism of the unsaturated polyester containing maleate or fumarate groups with the vinyl ether crosslinker proceeds via formation of a maleate-vinyl ether (MA-VE) donor-acceptor complex followed by homopolymerization of this complex (Figure 1) yielding a 1:1 alternated MAVE copolymer.

The UV powder coating system of DSM Coating Resins is semicrystalline, in contrast to conventional amorphous thermal-curable powder coatings. Semicrystalline polymers consist of glassy regions, with the corresponding glass transition temperature (T_g), and crystalline regions, with a melting point or range (T_m). Above the melting temperature, the molten crystalline regions cause a strong decrease in viscosity. Consequently, such a semicrystalline UV powder coating system results in much better flow at relatively low temperatures than obtained with amorphous (UV) powder coatings.

Coating formulation and processing technology-In general, the composition of UV powder coating formulations is comparable to conventional powder coatings. The main difference is the use of photoinitiators in the UV powder coating. This photoinitiator will absorb the UV light during the curing process and initiates the polymerisation of the binder system. Typically, solid a-hydroxy-acetophenones (a-HAP) and bis-acylphosphine oxide (BAPO) derivatives are frequently used.

Like conventional powder coatings, the UV powder coating formulation is processed by extrusion. Due to the semi-crystalline nature, the extrusion temperature may not exceed 70°C, and the extrudate temperature may not be higher than 75°C. This temperature is above the glass transition temperature, but below the melting range of the crystalline regions. Above the melting temperature, the melting of the crystalline regions leads to a dramatic reduction in viscosity, which makes extrusion virtually impossible.

Milling and sieving follow extrusion. During this process, the temperature may not become too high because, above approximately 40°C, the powder is above T_g, which may lead to lump formation. At higher temperatures, this may lead to partial melting of the powder.

UV powder application-Tribo charging is very suitable for powder application on MDF. In contrast with corona charging, there is no external high-voltage source. The powder particles are charged by friction in a Teflon tube and transported by airflow. Due to the absence of the electric field, there is no Faraday cage effect and internal surfaces can be coated. Moreover, such an electric field can cause fiber lifting. For poorly conducting materials, the substrate is sometimes preheated to improve the uniformity in the powder layer thickness.⁹

Curing of UV powder-The curing of the applied powder coating is divided into two steps (Figure 2). In the first step, the powder coating particles are heated by IR radiation. This method heats the coating surface very efficiently, while the temperature of the substrate will stay low.

Several experiments have shown that when the surface is heated to 100 to 120°C, the temperature 3 mm below the surface is more than 40°C lower.¹⁰ In practice, the IR power is controlled via a pyrometer that measures the temperature of the surface and is set on a maximum of 100°C in the case of MDF. This prohibits overheating of the substrate, which could lead to discoloration or deformation. Sometimes the IR heating section is extended with a second section that heats the substrate with a combination of IR and convection heating.⁹ During the melting of the powder, the particles will flow together to form a continuous and smooth film. Depending on the substrate and the heating power, the whole process of melting and flowing takes one to three minutes.

Following the IR heating section, the molten film cures in the UV section to get the final desired coating properties. The UV light source can be either a mercury vapour arc lamp or mercury vapour microwave-powered lamps. For the latter, several types of bulbs are available that differ in the spectral output-the H-bulb (mercury-type short wavelength UV), the D-bulb (iron-doped mercury-type with medium wavelength UV) and the V-bulb (gallium-doped mercury-type with longer, near-visible wavelength UV). One has to be aware that a portion of the light emitted by the lamps is IR radiation. This may lead to extra unwanted heating up of the surface to be cured.

After the UV curing, which will take only seconds, the substrate is left to cool down. Since only the surface is heated, the time required for cooling is also reduced compared to other application processes. The whole curing process can therefore save between 50 and 80% in time, floor space and cost, depending on the currently used technique and substrate.^9,10

Coating formulations and properties

Coatings on MDF-The furniture industry, which frequently uses MDF as a material, requires coatings that possess good chemical resistance in combination with a high hardness. For this purpose, a special vinyl ether component has been developed (VE 1). In Table 1, examples are given of a clear and a white-pigmented (15% TiO₂) coating formulation. These formulations comprise a stoichiometric ratio of the maleate and vinyl ether groups in order to facilitate the 1:1 copolymerization. The coatings were applied in a layer thickness of approximately 100 µm. After melting the powders at 100°C, a good flow was obtained (determined visually). The chemical resistance (acetone and MEK) of both systems is good. Recently we have reported a study that showed that the clear coating performs as good as or even better than several commercially applied solvent-borne or liquid UV coatings.¹²

For optimal aesthetic properties, furniture coatings are frequently low gloss. In Table 2, examples are given of a smooth semigloss and a textured coating. These surface effects are easily obtained by the use of standard, commercially available additives. The semigloss coating composition is cured at a slightly higher temperature (120°C) to improve the melting of the added wax and to, consequently, decrease the gloss. The other general coating properties, like hardness, adhesion to MDF, and the acetone resistance, remain unchanged. Due to the rough surface structure, a very low gloss is measured (3 and 19 units at 20° and 60°C, respectively).

Coatings for metal-For coatings on flexible substrates, like metal, another vinyl ether (VE 2) has been developed. The coating characteristics of a prototype UV-curable powder coating with this crosslinker are described in Table 3. In this case, a layer thickness of 80 µm was applied on aluminium and the coating was melted at 120°C to obtain a good flow. The pendulum hardness is lower compared to the coatings based on VE 1. However, the adhesion on aluminium is much improved and also the flexibility of the coating is higher. The maximum value for aluminum is obtained in the Erickson Slow Penetration (ESP) test together with a Gardner reverse impact resistance of 40 inch pounds.

Evaluation curing window UV powder coatings

Calorimetric techniques-previously we have reported the use of a modulated differential scanning calorimeter (MDSC) as a very accurate technique to determine the degree of curing of UV powder coatings via the coating glass transition temperature. Variations in the UV dose or output spectrum used for the curing process can affect the network formation and this results in differences in the coating glass transition temperature.

Another useful calorimetric technique is the differential photo-calorimeter (DPC). With this technique, heat effects that are induced by exposure to UV light (i.e., the heat of reaction produced upon photopolymerisation) can be measured. In Figure 3, the equipment is schematically depicted. The sample and an inert reference sample are placed in a differential scanning calorimeter. The apparatus is equipped with an optical bench to expose the sample and the reference cell to an equal amount of UV light from a xenon lamp (450 W). During the measurements, the sample and the reference are kept under iso-thermal conditions at a level representing the cure conditions. Figure 4 represents a typical example of the measurement. The measurements were performed under nitrogen and no inhibition time was found. The cure exotherm is the integrated heat flow from the time of UV exposure until completion of the reaction. The cure exotherm is directly related to the conversion and the amount of photopolymerizable moieties in the sample. The time until the exotherm reaches the maximum value can be taken as a measure of the cure speed.

Both calorimetric techniques have been used to determine the cure window for UV powder coatings. In the following paragraphs, the effect of cure temperature, UV exposure and paint composition will be discussed.

Effect of cure temperature-In an earlier stage of our development of UV powder coatings, we experienced that isolation of the IR/UV equipment between the two sections is important in order to get good, reproducible coating properties.^10a Apparently the temperature of the coating at the time of UV exposure affects the curing process, which can be expected based on the relation between the reaction kinetics and temperature (i.e. the degree of conversion). As described above, DPC is a good tool to use to investigate these kinetic effects. Powder coating formulations have been cured in the DPC equipment at different temperatures, and indeed a strong effect is observed of this temperature on the produced reaction heat, i.e., the conversion (Figure 5). From 60°C to 100°C the produced reaction heat increases two times (DH = 41 and 81 J/g at 60 and 100°C). Above this normally applied curing temperature of 100°C, the reaction energy hardly changes and remains 87 ± 2 J/g. A similar trend is observed in the time until the maximum enthalpy is reached (Figure 6). At 60°C, this requires 19 seconds, which decreases to about 10 seconds between 80 and 100°C, after which the final minimum time of 6 seconds is reached.

MDSC measurements confirm the good curing above 100°C as the coating glass transition temperature remains 64 ± 2°C (Figure 7). Below this limit, the final coating glass transition temperature slowly starts to decrease. An earlier study showed that if the IR exposure time is too short, the molten powder temperature does not pass 80°C.^13a

These results clearly explain the effects observed with the poor, isolated experimental curing equipment and it should be emphasised that, in practice, the molten powder requires a minimum temperature of 90°C at the time of UV exposure. This temperature will ensure relatively quick and efficient network formation, leading to good coating properties.

Effect of UV-dose and UV-lamp spectral output on cure pigmented coatings-The microwave-powered UV sources are available with different spectral output. This opens the possibility to choose the optimal lamp for specific formulations. First, photoinitiators differ in the absorption spectrum. For example, the BAPO photoinitiators absorb light of higher wavelength than the a-HAP photoinitiators. Second, pigments affect the transparency of the coating for UV. Especially the TiO₂ pigments absorb a lot of the UV light up to 400 nm. For this reason, pigmented formulations contain a combination of the a-HAP and BAPO photo-initiators (see Table 1). The a-HAP photoinitiator will result in quick surface cure, whereas the BAPO photoinitiator can ensure good through-cure as it absorbs light that is not absorbed by the TiO₂ pigments.¹⁴

The many different absorption spectra of the components in the pigmented coatings make the output spectrum of the UV lamp very important in curing these coatings, since sufficient energy should be absorbed by the photoinitiators. By comparison, first an unpigmented formulation, which contained the same photoinitiator combination as was found optimal for pigmented coatings, was evaluated. The coatings were cured using the different lamp types at various UV dose levels (Figure 8). The combination of an a-HAP and a BAPO photoinitiator makes the cure more or less independent of the spectral output of the lamp. A linear increase in coating Tg with UV-dose was observed, and sufficient cure is obtained with a dose of >1000 mJ/cm². The addition of 15% TiO₂ pigment changes this situation dramatically (Figure 9). With the H-bulb, which has a similar output spectrum as conventional mercury arc lamps, very poor curing is obtained, even with a dose of 2000 mJ/cm², as is reflected by a coating Tg < 40°C. Also, for the D- a nd the V-bulb a dose below 1000 mJ/cm² is not yet sufficient. However, in contrast with the H-bulb a strong increase is observed in the coating T_g above this dose and much better cure is obtained than with the H-bulb. Both the D- and V-bulb emit more UV light with a wavelength above 400 nm than the H-bulb and consequently are less hindered by the absorption of the pigment.

Conclusions

From this paper, it becomes clear that many variables influence the performance of UV powder coatings. First, the resin system determines aspects like hardness, adhesion, chemical resistance and flexibility. Also, the flow of the coating is affected by the resin system and semi-crystalline resins result in a good flow at relatively low melting temperatures.

Second, many other components are used in a UV powder coating formulation. Pigments can be added for color and hiding power, but can have a strong effect on the effectiveness of cure. It is important to optimize the used type and amount of photoinitiator and the applied UV curing (spectrum, dose) for these pigmented coatings. Generally, the optimal curing is obtained with a combination of a-HAP and BAPO photoinitiators, while using a UV lamp with sufficient emission at higher wavelength. Waxes can be added to the formulation to change the surface structure, i.e., giving smooth semi-gloss coatings, or textured surfaces.

Third, all compositions have a cure window. Besides the above-mentioned UV spectrum and dose, the temperature is also important. Curing below 90°C results in a lower conversion of the photopolymerizable groups and, consequently, the final coating glass transition temperature is lower.