Special-effect finishes are used in many applications

Transcription

1 Innovative Testing Technologies for Effect Finishes Special-effect finishes are used in many applications to create new color impressions, pronouncing the design of a product and at the same time making the product appear alive. The color impression of effect finishes can not only change under different viewing angles but can also look different depending on the lighting conditions. Therefore, new generations of special-effect pigments can no longer sufficiently be described with traditional multi-angle color measurement quantifying the diffused light reflection at to angles. This paper presents new FIGURE Metallic pigments. FIGURE Schematic of illumination and sensing geometry for a multi-angle spectrophotometer. º º º º º technologies that were developed to objectively describe the total impression of color multi-angle color and effect changes (sparkle graininess) as well as paint defects such as cloudiness - mottling. Introduction Special-effect coatings play a dominant role in a variety of applications (automotive, appliances, electronics, cosmetics, etc.) as they make an object distinctively appealing. Designers are looking for a new color, which not only makes the product look pretty, but actually underlines its styling resulting in a living color! Pigment manufacturers are developing new colors that not only change their look under different viewing angles, but also look different under different lighting conditions (i.e., sunlight, cloudy). Again the goal is to make the product appear alive and exciting. In contrast to conventional solid colors, metallic finishes change their appearance with viewing and lighting conditions. Interference colors and special-effect colors show not only a lightness change with viewing angle, but also a chroma and hue change. And, in the latest development of special-effect pigments, additional special sparkling effects are created by changing the lighting conditions from cloudy to sunlight. On the other hand, color harmony, i.e., a uniform and consistent color, is essential to achieve the impression of a high-quality finish and avoid customer complaints. Most of the time a final product consists of several components produced by various suppliers, thus color uniformity is becoming more of a challenge to the entire supply chain. Effect Coatings Color Measurement The first types of effect pigments used were aluminium flakes, creating a metallic look. Dependent on the viewing angle they show a light-dark flop. The larger the lightness difference is between the viewing angles, the more the curved profile of an object will be accentuated (Figure ). To objectively describe this light-dark flop effect, measurements have to be done at different angles (Figure ). It was determined that a minimum of three and best five viewing angles are needed to provide sufficient information on the goniophotometric characteristics of a metallic By Sandra Weixel, Product Manager BYK-Gardner GmbH, Wesel, Germany FEBRUARY

2 finish. The measurement geometry for multi-angle measurement is specified by aspecular angles. The aspecular angle is the viewing angle measured from the specular direction in the illuminator plane. The angle is positive when measured from the specular direction toward the normal direction. Later, pearlescent pigments were introduced that show not only a lightness change with viewing angle, but also a chroma and hue change caused by the interference of light (Figure ). For the new generation of those pigments the color even travels over a wider range, i.e., through several quadrants in color space (Figure ). Quite often the color shift can be noticed on the opposite side of the specular reflection (or behind the gloss ). This effect can no longer sufficiently be described with traditional multi-angle color measurement quantifying the diffused light reflection at three or five angles. Research has shown that by adding at least one additional measurement angle at - behind the gloss correlation to the visual assessment can be improved tremendously (Figure ). Effect Measurement of Effect Coatings Metallic and interference pigments not only change their color impression dependent on the viewing angle, but also dependent on the lighting condition direct sunlight versus cloudy sky. This effect cannot be captured with conventional multi-angle color instruments, because they measure the integral of the spectral reflection over the detected area and cannot distinguish between the basecoat color and the reflection of the effect pigments. Under diffused illumination (cloudy sky) a metallic finish can create a light/dark pattern depending on the aluminum flake size from very fine to very coarse (Figure ). Commonly used terms to describe the phenomenon are graininess, coarseness, texture or salt and pepper. The effect is only obvious at a close distance and does not change with viewing angle. Graininess can vary with the flake size, the orientation of the flakes in the coating, and with agglomeration of flakes during the application process. Under direct illumination (sunlight) the same metallic or effect finish can look completely different (Figure 7). Small light flashes can be seen with low to high intensity. This effect is also referred to as sparkle, micro-brightness, glint or diamonds. Sparkle is caused by the reflectivity of the flakes and therefore is influenced by the flake type (aluminum flake, mica, Xirallic ), the concentration level of the effect pigments, flake size or application method (bell/bell versus bell/ pneumatic). In contrast to graininess, the sparkle effect is very dependent on the illumination angle. FIGURE Interference pigments. FIGURE Color travel of Chromaflair pigment. -a* - +b* +a* b* Traditional -angle Behind the gloss, -º FIGURE Measurement of color travel behind the gloss. º º º º º -º FIGURE 7 Sparkle observed under direct sunlight. FIGURE Graininess observed under diffused light. Total Color Impression Measurement To characterize the impression of effect finishes under different viewing angles and illumination conditions, a new instrument is available from BYK-Gardner that combines the following characteristics. Multi-angle color measurement before and behind the specular reflection to give better insights on the true color travel capabilities of an effect color. Effect measurement of sparkle and graininess simulating the effect changes under cloudy sky and bright sunlight. PAINT & COATINGS INDUSTRY 9

3 Innovative Testing Technologies for Effect Finishes FIGURE BYK-mac measures total color impression. º º FIGURE 9 Schematic for multi-angle color and effect measurement. º º CCD-Chip º º º º º º º Metallux Metallux R 7 Metallux R 7 -º FIGURE BYK-mac multi-angle color data for silver finishes with different flake sizes. Sparkle Intensity º Sparkle Area º Flop Index FIGURE BYK-mac effect data for silver finishes with different flake sizes. Graininess To measure sparkle and graininess, the instrument is equipped with a digital camera, which correlates to the spatial resolution of the human eye. The camera takes pictures under various lighting conditions. Diffused illumination by a white-coated hemisphere is used to simulate a cloudy sky condition for measuring graininess. Direct illumination at three angles is used to measure the sparkle impression under direct sunlight (Figures and 9). In order to obtain numerical values that can be used for daily process control and QC purposes, the camera pictures are analyzed with algorithms that were established based on visual evaluations of a variety of automotive finishes together with several partners from the automotive, pigment and paint industry. To allow a better differentiation, the impression of sparkle is described by a two-dimensional system: sparkle area and sparkle intensity. A sparkle tolerance model was developed, which allows setting a Delta Sparkle value for paint batch or part QC. The calculation of the Delta Sparkle value is related to the color difference calculation of Delta E cmc and was tested by several automotive makers and paint suppliers in visual correlation studies. Graininess is evaluated by measuring the uniformity of light and dark areas and is summarized in one graininess value. A graininess value of zero would indicate a solid color, the higher the value the grainier or coarser the sample looks under diffused light. Color and Effect Applications Aluminum Flake Size Influence Silver finishes with three different aluminum flake sizes ( µm, µm, µm) were compared for color and effect change. In multi-angle color measurement the flake size influence can be mainly seen in lightness changes (Figure - Flop Index). Visually, the silver finish with the coarser aluminum pigments appears much more grainy under diffused lighting conditions and more sparkling under direct illumination. The BYK-mac measurement correlates with the visual judgment: sparkling area, sparkling intensity as well as the graininess are increasing with the flake size (Figure ). FIGURE BYK-mac multi-angle color data: Overall acceptable color match between car body components and add-on parts., L*= 7.; a*=.; b*=. L*=.9; a*=.; b*= -.9 L*=.9; a*=.; b*=. E DIN avg. º º º FuelDoor.9... FuelDoor DPillar_R DPillar_L Door_R. Door_L. L*= 9.; a*=.; b*= -. L*=.7; a*=.; b*= -. º Bumper_R. Fuel Door Fuel Door... DPillar_R DPillar_L Bumper_R.... Door_R Door_L Bumper_F.9 Bump_R Bump_R Bump_F Bump_F Bumper_F Bumper_F.9 FEBRUARY

4 Innovative Testing Technologies for Effect Finishes FIGURE BYK-mac effect data: The car body sparkles more at 7, while Sparkle and Graininess are acceptable g Difference in application method and not paint formulation. Sparkle º Sparkle º Sparkle º º s_i s_i s_i S_G=.7; S_i=.; S_a=. S_G=.7; S_i=.; S_a=. S_G=.9; S_i=.9; S_a=9. º º... _a _a _a FIGURE BYK-mac multi-angle color data. Overall acceptable color match between finish with Mica pigment and panel with Xirallic pigment. º º º FIGURE BYK-mac effect data: The finish with Xirallic pigments sparkles more at than the finish with Mica pigments. Sparkle intensity º S_G=7 S_G= Sparkle area º FIGURE 7 Very good match between add-on parts and car body. Brilliant Red d Sparkle Intensity º - - d Sparkle Area º Car Body Add-on Part d Sparkle Intensity º º Graininess FIGURE Poor match between addon parts and car body. Anthracite d Sparkle Area º FIGURE Silver finish with high amount of cloudiness. Car Body Add-on Part Application Method Influence on Flake Orientation In order to increase paint efficiency, the application method for the basecoat is being changed to % electrostatic application. Especially on metallic finishes containing coarser aluminum flakes the flake orientation will be different more non-parallel oriented flakes. The result can be less of a light-dark flop effect and more sparkling at low grazing illumination angle. In another example the basecoat of the car body was applied % electrostatically and the bumpers were still painted with the traditional bell/pneumatic application. The total color difference using the color difference calculation Delta E DIN was acceptable evaluating on the averaged Delta E DIN value (Figure ). Visually, one could see a difference mainly at a low grazing illumination angle, whereas the car body was sparkling considerably more than the bumper. The BYKmac measurement data reflects the visual impression clearly evaluating the Sparkle 7 data. The Sparkle 7 measurement evaluates the aluminum flakes, which are non-parallel oriented; therefore the main changes can be seen in an increasing sparkle area (Figure ). Effect Pigment Type Influence on Color and Effect A black effect finish with a concentration of.% Xirallic was compared to the same finish with.% Mica. In regard to traditional -angle color measurement the two finishes would be acceptable (Figure ). Visually, there is a big difference when the panels are exposed to direct sunlight. The finish containing Xirallic has a much higher sparkling effect than the finish with Mica pigments. The sparkle measurement shows a clear difference at illumination. The finish with the Xirallic pigments shows a much higher intensity value than the finish using Mica pigments (Figure ). Sparkle and Graininess for Daily QC Sparkle and graininess are essential parameters for automotive applications to achieve a harmonized look over the entire car body including add-on parts. Figure shows an example of an anthracite color with a poor sparkle match between add-on parts and car body: all measurements taken on the add-on parts were outside the sparkle tolerance ellipse. Figure 7 on the other hand shows a very good match of a brilliant pearl red. All readings are within the sparkle tolerance. The differences in Sparkle,, 7 and/or graininess can also assist in trouble shooting to determine whether the cause of a mismatch is due to formulation or process differences. Color Measurement Outlook and Challenges of Tomorrow: Cloudiness An additional factor influencing total color impression is an effect called cloudiness or mottling. Cloudiness is a lightness variation that is most obvious on light metallic finishes. It is a very undesirable effect that is quite obvious on large body panels. It can be caused by formulation as well as application variations. The main influencing parameter is flake disorientation, which can be caused, for example, by formulation incompatibilities or film thick- FEBRUARY

5 FIGURE 9 New cloud-runner for measuring cloudiness lightness variations over large surface areas. FIGURE Cloudiness rating data for cloud sizes of different silver finishes º Silver Silver.... Cloud size : -mm Cloud size : -mm Cloud size : 9-mm Cloud size : -7mm Cloud size : 7-mm Cloud size : -mm - mm - mm 9- mm -7 mm FIGURE Measurement signal of lightness variations at two viewing angles (, ). Signal + Signal ness variations during the basecoat application. The result will be small and/or large clouds (lightness variations) resulting in an inhomogeneous appearance. Depending on the viewing distance, small clouds (close up evaluation) or large clouds (far distance evaluation) (Figure ) can be seen. In order to objectively measure the mottling effect it is necessary to measure lightness variations over a large sample area. BYK-Gardner is introducing a new solution to scan the surface over a large area and objectively measure the lightness variations at three different viewing angles to simulate the visual evaluation of cloudiness (Figure 9). The measurement signal (Figure ) is filtered via mathematical filter functions in different cloud sizes and a rating value calculated for each cloud size (Figure ). The higher the rating value the more visible the mottling effect. Currently, automotive companies are working on setting cloudiness values for batch approval and process control. Conclusion The introduction of more new effect pigments requires new innovative measurement technologies to capture the total color impression. It is no longer sufficient to measure the color impression only under different viewing angles, but also the effect of different lighting conditions needs to be evaluated. Colors that agree with each other when measured for -angle color can look very different due to sparkle and graininess differences. The objective measurement of the total color harmony of products with special effects coatings is now possible with the development of new innovative technologies implemented in the BYK-Gardner BYK-mac. This instrument fills a need for effect measurement that has not been available until now. The BYK-mac uses multi-angle color measurement at six angles, and also uses multi-angle camera evaluation of sparkle and graininess to give a complete picture of the visual impression of the effect coating. The new sparkle and graininess measurement data can be used for trouble shooting to determine the cause of a mismatch as well as for daily quality control. n References. Baba, G.; Kondo, A.; and Mori, E. Goniometric Colorimetry, Proceedings of the th Congress of the AIC, Vol. II,, Buenos Aires (99).. Alman, D.H. Directional Color Measurement of Metallic Flake Finishes, Proceedings of the ISCC Williamsburg Conference on Appearance, (97).. Schmelzer, H. Farbmessung und Rezeptberechnung bei Metallic- Automobillacken, Proceedings of the th FATIPEC Congress, Vol. I (B), 7 (9).. Saris, H.J.A.; Gottenbos, R.J.B.; van Houwelingen, H. Correlation between Visual and Instrumental Colour Differences of Metallic Paint Films.. ASTM E 9 -, Multiangle Color Measurement of metal Flake Pigmented Materials.. DIN 7-, Farbtoleranzen für Automobillackierungen, Teil : Effektlackierungen. 7. ASTM Task force E.. WK, practice for Multiangle Color Measurement, Identification, and Characterization of Interference Pigments (draft).. Kirchner, E.J.J.; van den Kieboom, G.J.; Njo, S.L.; Super, R.; Gottenbos, R. The Appearance of Metallic and Pearlescent Materials, COLOR research and applications, Wiley Periodicals, Inc. Filler PAINT & COATINGS INDUSTRY