​Several factors affecting the brightness of the front and side of metallic paint

0 Preface

Metallic glitter paint is a kind of paint with granular flickering effect prepared from transparent organic pigments and flake effect pigments such as aluminum powder, pearl powder, copper-gold powder, etc.

1 Variation effect and principle of angular discoloration of metallic paint

The effect of angle flop refers to an optical effect in which the color will show different brightness and hue with the different viewing angles.

1.

For the coating system of metallic base paint and overcoat varnish, the incident light will be reflected and refracted in the varnish layer first, and then the refracted light will be reflected in the metallic paint layer; for different incident light, according to the law of refraction, the more the incident angle Small, the smaller the refraction angle, the greater the refracted light intensity; therefore, the 0° incident light parallel to the normal (Fig.

Fig.

1.

The reflectivity of effect pigments to light is high.

Fig.

1.
3 The flop effect produced by nano titanium dioxide and pearlescent pigments

Sol Panush, a color matching expert from BASF, classified pigments similar to flake metals, mainly mica pearlescent, as the second-generation effect pigments, and nano-metal oxides as the third-generation effect pigments
.
Nano-titanium dioxide has a small particle size, at 10-50 nm, although the transmission ability of visible light is very high under this particle size, it can still scatter short-wave blue light in visible light
.
Nano-titanium dioxide can only produce a varicolor effect when combined with other flake effect pigments such as aluminum powder or mica pearlescent powder.
Nano-titanium dioxide scatters short-wave blue light to obtain a blue-phase scattering angle, while the transmitted longer-wavelength Green to red light is reflected by the flake pigments, resulting in a golden frontal viewing angle that enhances the depth and depth of the metallic paint
.

For pearlescent pigments, its surface has similar scattering and reflection effects on incident light as aluminum powder pigments, resulting in changes in light and shade under different viewing angles, and moreover, it has color changes caused by interference in its multi-layer structure
.
The commonly used transparent or translucent mica-titanium pearlescent pigment interference color is mainly produced in the direction of the specular reflection angle.
When observed at other scattering angles, it shows the complementary color of the interference color or the true color of the substrate, and also shows the effect of angle-dependent heterochromia.

.

2 Factors affecting the brightness of the front and side of metallic paint

2.
1 Influence of pigment particle size and particle size distribution

The angular discoloration effect of metallic glitter paint is mainly caused by the reflection of light by the effect pigments arranged in parallel, and the effect pigments have a certain thickness.
In addition to the surface reflected light, the side edges will also reflect the light, and the diffuse reflection is the main
.
Taking aluminum powder as an example, the edge of aluminum powder is not as regular as the surface, there is a greater degree of scattering than the surface, and the side is in a vertical ring, and the direction of the reflection angle of the light irradiating the side of the aluminum powder is also different from that of the surface
.
It is this optical difference between the edge and the surface that allows the edges to be clearly distinguished even when the aluminum powder is superimposed, giving the metallic paint a strong grainy sparkle
.
As shown in Figure 3, the aluminum powder has a large particle size, a larger reflection area on the front, and a stronger sense of flickering, less scattered light on the side, darker visual side, obvious contrast on the front and side, and a stronger sense of metal
.
The finer the particle size of the aluminum powder, the more particles, and the smaller the frontal reflection area
.
In addition, when using aluminum flakes with a narrow particle size distribution, the contrast between the front and side brightness is higher than that of pigments with a wide particle size distribution, that is, the narrower particle size distribution of aluminum powder is more obvious
.

Fig.
3 Light scattering at the edge of aluminum powder with different particle sizes

2.
2 Influence of effect pigment shape

Non-floating flake aluminum powders are most commonly used in metallic glitter paints
.
The most common ones are silver dollar-shaped and cornflake-shaped aluminum powders, as shown in Figure 4
.
Corn flake aluminum powder has irregular shape, uneven edge, relatively rough surface and wide particle size distribution due to the cracking, welding and adhesion of aluminum powder in the ball milling process.
, the specular reflectivity is not high, resulting in more diffuse reflection; silver dollar-shaped aluminum powder is prepared by a special process, some surfaces are polished, and the edges are smoother
.
Therefore, the light and dark contrast of the front and side of the corn flake-shaped aluminum powder is not as good as that of the silver-dollar-shaped aluminum powder, and the whiteness of the side of the silver-dollar-shaped aluminum powder is higher
.

Figure 4 Silver dollar type and corn flake type aluminum powder

2.
3 Influence of orientation aids

The good appearance of glitter paint depends on the good orientation of effect pigments.
By adding cellulose acetate butyrate or nitrocellulose, a faster solvent release can be obtained.
After spraying, the solvent quickly evaporates, the paint film shrinks, and the effect pigments Under the action of contraction force, it is nearly horizontally arranged, as shown in Figure 5.
At this time, most of the incident light is reflected at the specular reflection angle, the front has the maximum brightness, while the scattered light on the side is very little, and the visual perception is dull, and the angle varies.
The heterochromatic effect is strong
.

Fig.
5 Light path of aluminum powder oriented in parallel

When a large amount of orientation wax or matting agent is used in metallic paint, the wet film has a certain thixotropy, which limits the random movement of the effect pigment in the wet film, thereby helping to achieve the orientation of the effect pigment and obtain a uniform wet film surface
.
Subsequently, due to the different degree of shrinkage between the orientation wax or matting agent and the binder resin, with the volatilization of the solvent in the paint film, the binder resin shrinks, the paint film on the orientation wax or matting agent becomes thinner, and the orientation wax or matting agent gradually convex.
As a result of the paint film, a rough surface is formed, which prevents the effect pigments at the alignment agent from aligning in parallel, as shown in Figure 6
.
When the light hits the surface of the paint film, part of the light is reflected by the non-parallel arrangement of effect pigments to the plane surface, which reduces the contrast of light and shade on the front and side of the paint film; The light reflected to the flat surface gradually increases, and eventually the front and side are flipped
.

Fig.
6 Changes of effect pigment arrangement and optical path by directing agent

2.
4 Influence of high hiding pigments

In order to ensure good metallic texture and sparkling effect in metallic paint, the pigments used are generally organic pigments with high transparency
.
Sometimes a small amount of high-hiding inorganic pigments is added to improve hiding power or to improve side whiteness
.
As shown in Figure 7, since inorganic pigments mainly present color by scattering light, part of the incident light will be scattered by the pigment particles covering the effect pigments, which will increase the side scattered light, increase the side brightness, and present a covering The hue of the pigment; and due to the coverage of the inorganic pigment particles, the visual particle size of the effect pigment is reduced, the reflective area is reduced, the light reflected by the effect pigment is also reduced, the front brightness is reduced, and the overall angular discoloration effect of the coating film weaken
.

Figure 7 Influence of inorganic pigments on the optical path of paint film

2.
5 Influence of pearlescent pigments

Pearlescent powder is a multi-layer composite flake effect pigment composed of materials with different refractive indices.
The optical principle of its coloration is relatively complex, and there are multiple refractions, reflections, interference and transmission of light, as shown in Figure 8
.
Observed from the specular reflection angle, the reflection is the strongest, the brightness is the largest, and the interference color is present; when viewed from the side, due to the transparency of the pearl itself, part of the light will be transmitted and scattered on the background color, showing the color of the background color; in the aluminum powder After adding mica-titanium pearlescent to the paint, compared with pure aluminum powder paint, the side scattered light will be enhanced, and the side brightness will increase
.

Figure 8 Schematic diagram of pearl light path

3 Conclusion

The brightness of the front side of the metallic paint is related to the type, shape, particle size and orientation of the effect pigment
.
The larger the particle size of the effect pigment, the more regular the shape, the stronger the specular reflection, the higher the frontal brightness, the lower the side brightness, and the stronger the angle-dependent color effect
.
The better the parallel orientation of the effect pigments, the higher the frontal brightness; the orientation wax and matting agent will change the arrangement direction of the effect pigments and make the sides brighter; the addition of covering pigments and pearl powder will also make the sides brighter and lighter
.

References (omitted)

Author | Ding Bangyong, Zhao Baohua

(CNOOC Changzhou Coating Chemical Research Institute Co.
, Ltd.
, Changzhou, Jiangsu 213016)