Today’s quality requirements for automotive finishes in regards to color and appearance are very high as it is proven that “the first impression” establishes lasting perceptual quality opinions extending beyond design and paint appearance. Therefore, the exterior finish of a car body and its add-on parts should be perfectly matching in color and appearance.
Uniformity was and still is the most important criteria for perceived quality. For years, automotive manufacturers have assessed “mottling” visually - some have used panel sets with various degrees of mottling to assist their visual evaluation. But only when phenomena can be objectively measured, one can control and optimize it.
Cloudiness also referred to as mottling are lightness or color variations which are most obvious on light metallic finishes. They can be caused by formulation incompatibilities as well as variations in application parameters. For example, film thickness variations of the basecoat or flake disorientation can lead to various cloud sizes resulting in a non-uniform appearance.
Psycho-physical experiments were conducted with automotive makers as well as auto OEM paint suppliers to assess what experts and non-experts perceive as disturbing or even call a paint defect. The knowledge of these experiments was used in the design of a testing instrument, cloud-runner, which objectively measures these variations within large panels by scanning the surface over distances of up to 1 m length. Additionally, it also takes into account how severe we perceive the mottling defect which is highly dependent on the lightness and fineness of the effect finish. Studies were performed on metallic finishes with various degrees of aluminum flake sizes of different colors as well as tri-coat systems.
Internal automotive standards describe the surface appearance and quality requirements for painted vehicle exterior and add-on parts. The requirements are described in terms of appearance attributes and surface blemishes. Mottling is an undesirable paint defect of effect coatings. The total color impression shows irregular areas of lightness variations often described as blotchy, non-uniform appearance. As these light – dark “patches are quite large, people also refer to them as clouds. This phenomenon is especially noticeable on large body panels. In order to facilitate the judgement of mottling some automotive companies created reference panels with different degrees of mottling. In a plant environment the lighting conditions are not always ideal to see mottling. It is not only influenced by the lighting, but also by the viewing conditions and distance. Especially, light effect finishes can show different degrees of mottling depending on the viewing angle.
Mottling can be caused by the coating formulation, as well as variations in the application process. Disorientation of the metallic flakes in the paint film (Fig.1) can be due to instable formulation (rheology additives) or application variations (wet / dry application areas). Film thickness variations of the basecoat (Fig.2 and Fig.3) can also lead to various mottle sizes resulting in a non-uniform appearance.
Therefore, there is a high interest in objectively measuring “mottling”. In R&D applications multi-angle spectrophotometers are used to measure lightness variation by “scanning” the surface point by point, which is very time consuming (fingerprint system analysis). The perception of mottling is dependent on the viewing distance: Large mottles can be seen in far distance evaluation, while small mottles are more noticeable in close up evaluation (Fig. 4).
Figure 1 Orientation clouds: Disorientation of the metal flakes in the paint
Figure 2 Thickness clouds
Figure 3 Partial hiding visible at a grazing angle
Figure 4 Visual perception of mottling dependent on viewing distance
To objectively evaluate mottling, it is necessary to measure lightness variations over a large sample area and under different detection angles (Fig.5 and Fig.6)
The cloud-runner optically scans the surface and measures the lightness variations. The specimen is illuminated with a white light LED at a 15° angle measured from the perpendicular and the lightness variations are detected under three viewing angles to simulate visual evaluation under different observing conditions being 15°, 45° and 60° measured from the specular reflection.
The mottling meter is rolled across the surface for a defined distance. The scan length is variable between 10 to 100 cm. The Lightness variations are measured point by point.
The measurement signal is divided via mathematical filter functions into 6 different size ranges and a rating value is calculated for each angle and mottle size. The higher the value is, the more visible the mottling effect.
|Md||6 - 13 mm|
|Me||11 - 24 mm|
|Mf||19 - 42 mm|
|Mg||33 - 72 mm|
|Mh||57 - 126 mm|
|Mi||100 - 200 mm|
The measured values are displayed in a mottle spectrum, a graph showing the mottle size on the X-axis and the rating value on the Y-axis (Fig 7).
In this example, a light blue metallic finish, the influence of the observing angle is quite significant. That means visually the medium to large size mottles are most obvious at a head-on viewing when the sample appears lighter (15°), while at flatter angles the mottling is no longer visible.
Figure 5 Optical schematic of mottling meter, cloud runner
Figure 6 cloud-runner from BYK-Gardner
Figure 7 Mottle spectrum at 3 different angles
Experts and non-experts mainly recognize light – dark patches with an average size between 50 to 100 mm as typical mottling. Therefore, for visual inspection and instrumental measurement a minimum panel size of approx. 30 x 50 cm is recommended. This was confirmed by several field studies with different colors and effects conducted by automotive OEM manufacturers. The smaller mottle sizes usually do not vary in the process and are paint characteristic, i.e. they are related to the size and distribution of the metallic and effect flakes. Thus, the better term for describing small mottles would be texture. The texture overlays and consequently, diminishes the visibility of large mottling. A mottling defect will be more obvious on fine silver metallic finish as it will be on a more textured silver metallic finish.
In order to define customer relevant limits for typical mottling, visual correlation studies were performed. As a result, a mottling index was developed which summarizes the larger mottle sizes in one number:
M = LM • (1-f • T) with LM = ((0.5Mf + max(Mg,Mh ) )) / 1.5
The small mottle sizes were also added up in a texture value T, based on the following equation:
T = ((Md + 0.5Me )) / 1.5 - 6
As the amount of texture will have an influence on the visual perception, a weighting factor f is applied on the M-indices which will allow comparison of different colors with varying texture. As the influence of the texture decreases for flatter angles the weighting factors decrease respectively.
fT(15°) = 0.05
fT(45°) = 0.04
fT(60°) = 0.03
The mottle index M and Texture T enable to show the measurement results in a Mottle-Chart (fig.8) which can be used for process control of many colors as well as paint development tasks. For quality control purposes a Red – Yellow – Green limit can be defined for the M-indices.
Figure 8 Mottle-Chart
As mentioned, mottling can vary due to process changes or formulation changes. Therefore, the quality of new color releases, as well as batch releases and vehicle production QC needs to be evaluated. In figure 9 measurement data of 100 cars, different colors and texture, are shown. The limit value was set to a specific mottle index M15°. Thus, the user quickly gets an overview on the process stability. In the next step the performance by each color and paint line can be analyzed.
A typical problem which can occur in the paint process is the so called "striping effect", which leads to a pattern of light and dark stripes in accordance with the paint application (Fig. 10).
The mottle spectrum in figure 11 shows such an example of a silver metallic finish. The measurement curve of the hood shows a significant increase in Mh, while the measurements on the door do not show noticeable mottling. (Fig. 12)
To guarantee detection of striping it is necessary to measure on areas large areas enough and use a scan length of minimum 45 cm. The readings have to be taken perpendicular to the application direction. It is recommended to take 10 scans in 1 to 2 cm distance.
Figure 9 Mottle-chart of 100 cars
Figure 10 Striping effect
Figure 11 Mottle-spectrums of silver metallic finish with and without striping effect
Figure 12 Mottle-charts of silver metallic finish with and without striping effect
In order to guarantee uniform color and appearance objective measurement tools are needed. The visual inspection of surface finish quality is highly dependent on lighting and viewing conditions which are not always ideal in production environment. A new innovative measurement technology realized in a portable instrument allows objective control of critical paint defects called mottling. Visual field studies were conducted to establish customer relevant limits.
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