Fall 2000 Vol. 2, No. 3

Adhesion quality and optical appearance of coatings are determined by film formation. Several factors, including application parameters, rheological behavior and interfacial properties, interact in a complicated manner (1). Simply put, surface tension can be regarded as the driving force and viscosity as the resistance of wetting and leveling (2-5). This article discusses the interfacial properties. The rheological behavior of powder coating melts, as well as the influence of different application parameters (e.g. particle-size distribution, film thickness, etc.) on film formation are not discussed.

Wetting of a substrate and leveling of a fluid film strongly depend on the surface tension of the coating. However, both processes have opposing requirements in terms of surface tension. If the surface tension is too high, poor wetting occurs, which leads to defects such as craters. On the other hand, if the surface tension is too low, the leveling is adversely influenced, which leads to wavy surfaces known as orange peel (6). Moreover, surface flow can cause surface defects due to local surface-tension differences, known as the Marangoni Effect (7,8). These differences can arise from temperature gradients and local inhomgenieties (e.g. contaminations). Therefore, the surface tension of a coating must be controlled and adjusted (9,10).

The surface tension of powder coatings can be adjusted by incorporating leveling additives to a coating formulation. In essence, these additives are surface-active materials that decrease the surface tension of the coating. An optimum range can be adjusted, and local surface tension differences are minimized (9,10). However, most current knowledge regarding the effect and surface activity of leveling additives in powder coatings is based on trial and error. Very little reliable surface tension data are available. A more fundamental and quantitative understanding is necessary to develop new, superior additives for improved powder coating formulations.

Information on this subject is scarce for two reasons. First, surface-tension measurements of polymer melts are not trivial, due to high melt viscosities, high measuring temperatures and limited thermal stability of many polymers. Second, powder coatings are complex multicomponent systems that include binders, curing agents, additives, pigments and fillers. The complexity causes additional difficulties in melt surface-tension measurements. These measurements are even more difficult for reactive systems because of the cross-linking reaction. Therefore, to quantify the effect of leveling additives on melt sur-face tension, sophisticated measuring techniques are needed.

The influence of additive properties such as molecular weight and chemical structures (polyacrylates and polyester-modified polysiloxanes) on surface tension of typical powder coating binders was the subject of a study at the Institute of Polymer Research (Dresden, Germany). The study began with nonreactive powder coating systems (i.e., pure binders) (11,12). A suitable technique for measuring surface tension of polymer melts is Axisymmetric Drop Shape Analysis (ADSA) (13,14). This method was used to systematically study the relationship between chemical structures of selected leveling additives (laboratory products) and their influence on melt surface tension of a powder coating binder (15).

Further insight into the effectiveness of leveling additives was obtained by means of AFM phase imaging and viscosity measurements. AFM phase imaging provides qualitative information about the distribution of the additives at the binder surface (16). Viscosity measurements were taken to determine whether leveling additives would affect the viscosity of the binder. The rather complex rheological behavior of a powder coating melt during film formation is beyond the scope of the present investigation.

This article explains the effect of leveling additives in powder coatings. The results should provide helpful information for choosing appropriate additives and for planning the synthesis of new, improved additives.

The Experiment Materials

DER 664 UE (Dow, Germany), an epoxy resin based on bisphenol-A with a molecular weight of Mw = 2000 g/mol, was chosen as a typical powder coating binder for this study. All additives were obtained from Byk-Chemie (Wesel, Germany).

For a systematic investigation of the effect of molecular weight and molecular structure of the additives on surface tension of the powder coating binder, three different groups of additives were used:

• three homopolymeric n-butylacrylates with different molecular weights
• two copolymeric acrylates based on 2-ethylhexylacrylate with different co-monomers
• two polyester-modified methylalkylpolysiloxanes. Both siloxanes possess the same polyester modification and the same siloxane backbone, but they have different alkyl side chains.

A summary of the laboratory names, the molecular weight and a short description of the additives is given in Table 1. The mixtures of binder and additive (0.1 and 1 weight-percent additive) were made by melt homogenization with a PL 2000 Plasticorder from Brabender Instruments from a master batch at 108°C.

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 proportional 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.