Research method

In situ Imaging of Automotive Metallic Paints

Imaging

Visualization of behavior of aluminum-flake reflective materials in paints

Role of SPring-8Achievements

The formation of coats of automotive metallic paints* was observed in thecross-sectional direction.
It was confirmed through real-time observation that the orientation angle ofaluminum flakes** changed with time after painting as a result of volumetricshrinkage and change in viscosity, although it remained unchanged immediately
after painting.
R&D institution: Daihatsu Motor Co., Ltd.

*Metallic paint: Automotive paint is roughly classified into three types: metallic, pearlescent, and solid. Metallic and pearlescent paints containfine aluminum and mica flakes and give characteristic colors that look different depending on the viewing angle.
**Aluminum flakes: Aluminum flakes are metallic aluminum powder in the form of very thin flakes and one of the reflective materials used inpaints. Aluminum flakes mixed with paints reflect light, and the shininess and color of the paints change with their orientation angle.

BackgroundResults

Recently, resin has been used in the exterior
finish of cars in addition to steel sheets. Color
matching between steel sheets and resin in
metallic paints has been an issue to be addressed.
Because the painting processes and materials used
for steel sheets and resin differ, the orientation
angles of fine aluminum flakes in metallic paints
also differ, causing variation in the brightness andcolor of the paint. Therefore, a scientific approachto analyzing the mechanism underlying thebehavior of aluminum flakes in detail has beenrequired.
The difference in the coats cannot be clarified
from the planar view obtained by optical
microscopy. It has been clarified by scanningelectron microscopy (SEM) that a difference in thedistribution of the orientation angle affects thebrightness of color. However, there have been nomethods to analyze the behavioral changes ofaluminum flakes during the formation of coats.

The behavior of aluminum flakes was observedby X-ray imaging at SPring-8 using high-brilliancesynchrotron radiation with high temporal andspatial resolutions. To observe the time course ofpaints under conditions close to those in theactual process, a setup for actual painting wasinstalled at the experimental hutch.
It was found that the coats shrank with timeafter painting and that the orientation angle ofaluminum flakes varied with the dilution andvolatilization rates of the solvent, although theorientation angle did not vary immediately afterpainting. When the behavior of aluminum flakes
was observed under the condition of multilayer
painting, the viscosity of the paint changed in thepainting process, which was considered to affectthe behavior of the aluminum flakes. Thesefindings are expected to shorten the preparation
time before production in automotive productionlines.

Observation of cross section of paint by
optical microscopy and SEM

Use of resin in car body

SEM image and distribution of orientation angle of aluminum flakes
Clearcoat

Mirror

Painting gun

Aluminum flakes

Synchrotron
radiation
Paintinggun

Coated object
Back door

Cross-section of the coating layerWith the widespread use
Door
Fenderof resin for exterior parts
to reduce the weight ofcar
bodies,multiple
Slightlyattempts at painting andreddish
color matching have beenrequiredfor
paintscontaining aluminum-flakereflective materials (aluminum flakes) used on the steel
sheets and resin of car bodies, as they are manufactured bydifferent production processes. This is one of the causes ofthe long preparation time before automotive production.

1600
1400

Total length

Color matching between door and fender

Detector

1200
1000

Distribution oforientationangle
σ=7.9°

800
600
400

1800
1600
1400
1200
1000

Detector
Specimen

Distribution oforientationangle
σ =11.8°

Coat
Substrate

3050

70

90

0.05
0.04

0.03

0.03

0.02

0.02

0.01

0.01-30-1010

Orientation angle[°]3050

Orientation angle of aluminum flakes
4.8 s after painting

0-50-30-1010

Orientation angle[°]3050

Orientation angle of aluminum flakes
180 s after painting

· The orientation angle of aluminum flakes changes with time.
· The change in the orientation angle depends on the painting conditions.

Formation of coat in the case of using paint with higher
dilution rate

800

Shrinking coat

2000-90 -70 -50 -30 -10 10

Formation of coat in the case of using standard paint0.06

Standard paint
σ=9.1
Paint with higher dilution rate
σ=7.3
Paint with lower dilution rate
σ=8.5
Paint with lower volatilization rate σ=7.9
Paint with higher volatilization rate σ=10.2600
4002000.050-50Specimen
1800

4Synchrotron
radiation

FenderTotal length

Front bumper

Image of optical microscopy

Aluminum flakes

Rear bumper

0.06Standard paint
σ=10.9
Paint with higher dilution rate
σ=10.3
Paint with lower dilution rate
σ=9.9
Paint with lower volatilization rate σ=10.8
Paint with higher volatilization rate σ=10.3

0.04

BasePrimer

Rail cover

Frequency

Experimental setup for painting installedat the experimental hutch of SPring-8

Example of color mismatchin production preparation stage

Frequency

X-ray imaging at SPring-80-90 -70 -50 -30 -10 103050Orientation angle

Orientation angle<Looks bright><Looks dark>

7090

When the painted region was observed by optical microscopy, theoverlapping of aluminum flakes was found but no clear differences in theirorientation angle were seen.
Differences in the orientation angle of aluminum flakes were confirmed bySEM. However, there have been no methods for analyzing the behavioral
changes of aluminum flakes during the formation of coats.

Coat
Substrate

Shrinking coat
Coat
Substrate

An experimental setup for painting was installed at the experimental hutch of SPring-8 BL46XU. To observe the dynamic behavior ofaluminum flakes during the formation of coats, we applied a standard paint as well as paints with different dilution rates and volatilization
rates to the substrate and compared the orientation angles of aluminum flakes 4.8 and 180 s after painting. It was found that the orientation
angle of aluminum flakes during the formation of coats depended on the dilution and volatilization rates of the solvent.

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