Pigment Selection for Colored UV-Curable Powder Coatings

Summer 2000 Vol. 2, No. 2



The opening of production lines in the United States and Europe has shown that UV-curable powder coating is now a practical coating technology that can make a significant contribution towards improving production. The ability to coat heat-sensitive substrates in a fast, economical and environmentally clean manner should help bring a rapid expansion of UV-curable powder coatings into applications traditionally dominated by two-part poly- urethane lacquers. Problems such as the treatment of wastewater from spray booths and the delay brought about by the slow cure reaction of polyurethanes are not present in UV-powder.

In UV-curable powder coatings, the powder is electrostatically applied to the sub-strate and then melted at temperatures from 80 to 140°C, just long enough to achieve the degree of flow required (Figure 1). The molten coating is then cured under UV light while hot. In contrast, conventional powder coatings require temperatures from 180 to 200°C to bring about the cross linking reaction, which sets in before the surface flow has completely finished.

UV-curable powder coatings need to be formulated in a variety of surface finishes and colors, and some initial work on this problem has already been reported

(1). When selecting pigments for conventional, thermally cured powder coatings, the thermal stability of the pigment must be considered. This is not a problem with UV-curable powders. As with conventional powder coatings, however, there is a need to achieve optimal pigment dispersion in one extruder pass and to have minimum batch-to-batch variation in color. For these reasons, as well as for cleaner handling, the use of solid pigment dispersions, such as the Microlen range of products, is most advantageous.

Selection of the appropriate photoinitiators is also a matter of great importance. If the photoinitiator lowers the glass transition temperature of the resin (i.e. has a plasticizing effect), the storage stability and ability to recycle the UV-curable powder will be impaired. Other key properties of a suitable photoinitiator for UV-curable powder coatings are (in addition to the spectral absorption): low volatility, low yellowness after cure and absence of volatile scission products that cause odor

(2). For curing pigmented systems, the combination of an alpha-hydroxyketone (AHK) and a bis-acylphosphine oxide (BAPO) is the most suitable. The alpha-hydroxyketone gives the coating good surface cure because this class of compounds is relatively insensitive to oxygen inhibition. The bis-acylphosphine oxide enables a deep cure in pigmented systems because it absorbs in the visible at wavelengths where TiO₂ opacifiers are transparent. Also, the ability of the BAPO class of photoinitiators to photobleach during cure promotes cure through to the base of the coating.

In a previous study, bis(2,4,6-trimethylbenzoyl)-phenyl-phosphine oxide (Irga-cure 819) and 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propan-1-one (Irgacure 2959) were found to fit the requirements for photoinitiators for UV-curable powder and were therefore used in this work (Figure 2).

For good curing performance, it is a prerequisite that the absorption spectrum of the photoinitiator coincides as completely as possible with the emission spectrum of the lamps. It also follows that the pigments used in the coating should interfere as little as possible with the absorption of light by the photoinitiator. When light strikes a pigmented coating, four processes will occur to a greater or lesser extent (Figure 3):

• the light is reflected off the surface of the coating
• the light is scattered by the pigment
• the light is absorbed by the pigment
• the light passes through the coating without being absorbed.

The relative intensities of the light specular reflection, back and forward scattering, absorption and transmission through a pigmented film I_r, I_bs, I_fs, I_a and I_t were measured using a Perkin Elmer Lambda 900 UV/VIS/NIR spectrophotometer fitted with an integrating sphere attachment (Figure 4). A 35- to 40-µm-thick coating (60-µm wet-film thickness) of an alkyd/melamine paint containing 10% pigment on a transparent foil was used for the spectral measurements. The spectra were corrected for the absorbance and reflectance of the substrate. The amount I_t was determined by measuring the transmission through the film at a distance of 180 mm from the opening of the integrating sphere. The sum of I_t + I_fs was determined with the pig mented film placed against the opening of the integrating sphere. The reflected component of the incident light was determined in a similar manner, with the pigmented film placed inside the integrating sphere at an angle to the incident light. From the latter measurement, the total light absorbance of the pigment can be calculated.

The organic pigments used in this study were tested at 4, 2 and 1% concentrations. If inadequate cure was obtained at 4%, the results at 2% were chosen; otherwise, the results were at 1%. Similarly, the inorganic pigments were tested at 20, 10 and 5% concentrations. Where possible, Microlen organic pigment dispersions were used, as this guaranteed the best possible dispersion quality. Two different resin systems were used in this work: the Uracross resin system from DSM Resins was used for coatings onto phosphated steel and the Uvecoat system from UCB Chemicals for coatings onto medium-density fiberboard (MDF) (Figure 5). Whereas the former system produced high-gloss coatings, the latter resulted in coatings with a unique surface structure. The formulations were extruded once using only a Prism 16-mm twin-screw at 70 and 80°C, respectively.

The extrudates were milled to give a powder of mean size at about 45 µm using a Retsch ultracentrifugal mill and sieved through a 125-µm sieve, and 0.25% of silica (Aerosil R972) was added to the crushed extrudate before milling to prevent the finished powders from caking. The powders were sprayed using a Wagner Tribo-star gun. A coating film of exactly 70 µm was achieved by spraying the powder first onto a metal panel with a metal backing plate behind it to prevent powder from adhering to the rear of the panel. In this way, the weight of powder needed to produce a 70-µm film on the metal panel could be determined. The MDF panels could then be sprayed in the same manner until the required weight of powder had been applied.

The coated panels were then melted under an IR lamp until a surface temperature of 140°C was reached and then cured immediately. The panels were cured in an Aetek exposure unit by placing them on a moving belt at different speeds and passing them under 2- by 80-Watt/cm Fusion type H medium-pressure Hg-vapor UV-lamps. The temperature of the panels leaving the conveyor was about 130°C. With the BAPO photoinitiator, it is possible to use normal, undoped mercury lamps.

The physical properties of the steel and MDF panels were tested when they had cooled (30 minutes after curing). The results are as follows:

König pendulum hardness. An average of three determinations according to DIN 53157 was taken.

Acetone double rubs. A 10- by 10- by 5-mm felt pad was used for 100 double rubs, and the result assessed visu ally: 1 = no damage, 2 = slight matting or scratches, 3 = moderate scratching, 4 = heavy damage and 5 = coating fully destroyed. This test main ly indicates the level of surface cure.

The pendulum hardness values for the coatings made with organic pigments in full shade and as 1:20 redu ctions with rutile are shown in Figures 6 and 7, respectively. With a decrease in pigment concentration, an increase in the hardness is seen as expected. Surprisingly, black is a color that is possible to cure with UV light. A more difficult color to cure is gray, because the TiO₂ absorbs the UV-light and the black absorbs the remaining visible light, leaving very little light for the photoinitiator.

If a pendulum hardness of about 100 seconds is taken as the minimum acceptable cure, this value is reached at 4% pigment concentration by Microlen Yellow 2RLTS-UA, Irgazin DPP Orange RA and Irgalite Blue PG. At 2%, this value is reached by Microlen Green GFN-UA; at 0.5%, by Microlen Black B-UA.

It is noteworthy that a reasonable cure was achieved with the isoindolinone pigment Microlen Yellow 2RLTS-UA, which absorbs light at much the same wavelength as the photoinitiator. The hiding power of this pigment, however, is low (Figure 8), which restricts the use of this pigment to combinations with TiO₂. The pendulum hardness results indicate that if allowance is made for experimental error, the pigments that cure well in full shade coatings also cure well as a 1:20 reduction with TiO₂.

Of the inorganic pigments, satisfactory cure of the bismuth vanadate pigment Irgazin Yellow 2093 in full shade could only be achieved at a concentration of 5%, which is a poor hiding power (Figures 9 and 10).

The nickel titanate pigment Irgacolor Yellow 10401 gave noticeably better cure performance than the chromium titanate Irgacolor Yellow 10408, although at a lower hiding power. The two yellow iron-oxide pigments were both difficult to cure even at the low concentration of 5%.

In the case of the iron oxide A, which was the more difficult of the two iron-oxide pigments to cure, the hiding power DE could not be determined because the cured coating was smooth on the white panel but wrinkled (i.e. uncured) on the black panel (Figure 11). This illustrates the importance of the substrate in reflecting the light from the base of the coating on the cure performance. The TiO₂ pigmented coating could be cured without problems and had excellent hiding power. It can be seen that, like the or-ganic pigments, the inorganic pigments that cure easily in full shade formula-tions also cure well in a 1:20 reduction with TiO₂.

The experimental formulation No. 2 was coated onto MDF panels and cured to produce a coating with a pronounced surface relief. For this reason, it was not possible to make pendulum hardness measurements, and the cure was assessed on the basis of the acetone rub resistance. It was found that those pigments that cured well in formulation No. 1 also cured well in formulation No. 2 (Figure 12).

Gallium-doped UV lamps have a spectral output that is greater in the visible region of the spectrum than undoped mercury-vapor UV lamps. It was therefore expected that a better through-cure would be obtained for the yellow pigments, which absorb at the same wavelength as the BAPO photoinitiator (3). When pigmented coatings were cured using either two gallium-doped lamps on their own or the combination of a gallium-doped lamp and an undoped lamp, only minimal improvement of the pendulum hardness was found. This was well below the limits of error (±15 sec.) of the pendulum hardness measurements. It is possible that differences in lamp output and the geometry and construction of the exposure equipment are of importance. Other workers report better cure results with the gallium-doped lamps in white (3) and gray (4) pigmented systems, but this was not found with the equipment used in this study.

The question arises as to why some pigments allow better UV-curing than others, even though the colors of the pigments are nearly identical. For optimal cure, a maximum amount of light in the region of the spectrum at which the photoinitiators absorb must reach the base of the coating. This is a complex problem because the extent of cure depends on:

• spectral output of the lamp
• absorption spectra of the photoinitiators
• quantum yield for radical formation of the photoinitiators--this can be expected to be wavelength dependent
• absorption of light of the wavelength range of interest by all species other than the photoinitiator
• extent of light scattering by the pigment. This increases the path length of the light through the coating and hence the probability of absorption.
• reflectivity of the coating substrate.

Nevertheless, the total integrated transmission I_t + I_fs (Figure 4) is a quantity that can be measured without undue experimental effort. This quantity should be approximately propo rtional to the extent of through-cure. For the two very similar yellow iron oxide pigments A and B, coatings at the same concentration and coating thickness in an alkyd paint were made and examined by microscopy to ensure optimal dispersion. It was found that there is a very significant difference in the total integrated transmission of the coatings in the range 380 to 430 nm (Figure 13), at which BAPO photoinitiator absorbs. This adequately explains the better cure characteristics of the pigment B. If a universally acceptable method of measurement of I_t + I_fs is established, this should enable pigments to be selected for their cure performance, and also predicts how best to formulate a given RAL shade. From knowledge of the spectral output of the lamps, the best type of lamp for the cure of a particular RAL shade might also be predicted.

Summary and Recommendations

In general, the most difficult color to cure is yellow. At present, it is not possible to cure this color if a high color strength is required. In reductions with TiO₂, it is possible to cure yellows at low color strength. The neighboring colors in the color space diagram (orange, brown, some greens and gray) are also difficult to cure (Figure 14).

Reds and blues, however, are relatively easy to cure. Black pigments tend to have good hiding power and can be used in UV-curable powder coatings, but only at concentrations less than 1%. At 0.5% pigment concentration, a pendulum hardness of 123 was achieved. Figure 15 gives recom mendations for pigments for use in powder coatings both in reductions with TiO₂ and at full shade.

In curing colored pigmented powder coatings, it is essential that enough light can be absorbed by the photoinitiator at the bottom of the coating. A higher ratio of BAPO:AHK of 4:1 is therefore recommended, rather than the 1:1 ratio commonly used for white pigmented systems, but at the same total concentration of 2.5%.

This paper was originally presented at Powder Coating Europe 2000, sponsored by Vincentz Verlag.