Innovative Testing Technologies for Effect Finishes
by Gabriele Kigle-Boeckler
Sandra Weixel
February 1, 2010
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 3 to 5 angles. This paper presents
new 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., sunny
sky and cloudy sky). 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 1).
To objectively describe this light-dark flop effect, measurements have
to be done at different angles (Figure 2). 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 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 3).
For the new generation of those pigments the color even travels over a
wider range, i.e., through several quadrants in color space (Figure 4). 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 -15° “behind the gloss” correlation to the visual assessment can be
improved tremendously (Figure 5).
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 6). 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.
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.
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 8 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 Ecmc 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 (25 µm, 34 µm, 54 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 10 - 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
11).
Application
Method Influence on Flake Orientation
In
order to increase paint efficiency, the application method for the basecoat is
being changed to 100% 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 100%
electrostatically and the bumpers were still painted with the traditional
bell/pneumatic application. The total color difference using the color
difference calculation Delta EDIN was acceptable evaluating on the
averaged Delta EDIN value (Figure 12).
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 BYK-mac measurement data reflects the visual impression clearly
evaluating the Sparkle 75° data. The Sparkle 75° measurement evaluates the
aluminum flakes, which are non-parallel oriented; therefore the main changes
can be seen in an increasing sparkle area (Figure 13).
Effect Pigment Type Influence on Color and
Effect
A
black effect finish with a concentration of 0.3% Xirallic was compared to the
same finish with 0.3% Mica. In regard to traditional 5-angle color measurement
the two finishes would be acceptable (Figure 14).
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 15° illumination. The finish with the Xirallic pigments
shows a much higher intensity value than the finish using Mica pigments (Figure
15).
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 16 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 17 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 15°, 45°, 75° 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
thickness 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 18) 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 19).
The measurement signal (Figure 20) is filtered
via mathematical filter functions in different cloud sizes and a “rating” value
calculated for each cloud size (Figure 21). 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 5-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.
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