Optically variable pigments from dielectric layers and their applications.

Over the last decade, several new pigments made from optical layers of metals and dielectric layers, which fit the general term of optically variable pigments (OVPs), have been commercialized. These pigments generate high chroma and color travel, dramatic changes in color at different angles of illumination, and viewing to produce fascinating color effects in coatings, plastics, and cosmetics. Advances in their development have resulted in high chroma color shifting pigments, with greater utility and impact. This article will focus on dielectric layers for generating optically variable effects, present some basic structure appearance relationships, and cover some examples of application and performance in coatings.

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The mechanisms of color generation and color travel of dielectric optically variable pigments are different from traditional pearlescent pigments that derive their color from the interference of light reflected from thin film of high refractive index materials. (1) These traditional effect pigments generate an intense specular, or mirror reflection color, and at aspecular angles lose their color intensity. Viewing these pigments at different specular angles produces only a small change in their hue and loss of color at near-planar angles. This is demonstrated in Figure 1, which shows changes in a* and b* relating to red-green and yellow-blue colors, respectively, with changes in angle of illumination and viewing. The color travel for a traditional blue interference pigment is minimal, changing from a green shade to a red shade blue as shown in Figure 1.

Optically variable pigments, on the other hand, generate their color through the interference of light rays reflected from interfaces between multiple layers of materials differing in refractive index. Most dielectric optically variable pigments use a high-low-high refractive index layer structure referred to as an etalon or "stack" structure. (2)

Pigments with this structure show the traditional chroma reduction in moving from the specular angle to aspecular angles of illumination and reflection. However, viewing the pigment at different specular angles results in intense hue shifts, or color travel, with high chroma. This is demonstrated in Figure 2 by the high a* and b* values, which change from yellow-green to violet. It is the low refractive index layer that provides the extended color travel for these pigments.

Color generation from these optically variable pigments is dependent on the refractive index of the materials being used as well as the thickness of the layers. The high refractive index materials used are typically metals such as aluminum, chromium, or silver, or metal oxides such as Ti[O.sub.2] or [Fe.sub.2][O.sub.3]. Low refractive index materials used have been Si[O.sub.2] or Mg[F.sub.2]. Control of the layer thicknesses determines the color range and hue, and choice of materials further determines the color travel as well as the pigment chroma. Layer thicknesses must be controlled to tolerances of several nanometers for color consistency.

The optically variable pigments fall into two broad categories: opaque and transparent.

OPAQUE OPTICALLY VARIABLE PIGMENTS

Opaque pigments are those produced using a metallic reflector that acts as a high refractive index layer centered in the pigment (Figure 4). Examples of these are structures with an internal layer of Al coated with Mg[F.sub.2] as the low refractive index material followed by a partial mirror of Cr as the outer high refractive index material.

TRANSPARENT OPTICALLY VARIABLE PIGMENTS

The transparent class of optically variable pigment materials, defined as DOVP, includes only dielectric layers and has some distinct advantages in that they can be used in clearcoats over light colored basecoats without contributing a dirty appearance. Several different structures are used to generate pigments with high color travel. A typical structure consists of a low refractive index silica layer coated with a thin layer of highly refractive Ti[O.sub.2]. The silica layer is an integral component of the optical stack, therefore changes in its thickness contributes to variation in the pigment's color travel (Figure 5).

Another structure consists of a borosilicate substrate with a high-low-high stack of Ti[O.sub.2] and Si[O.sub.2]; these structures are commercially available under the trade-name Firemist[R] Colormotion by BASF. Although the borosilicate has a low refractive index, it does not play a role in the pigment's color development as it is somewhat thick. Precise control of the Si[O.sub.2] layer thickness assures that each platelet is virtually the same, magnifying the reflection intensity of the pigment coating to increase chroma in the application (Figure 6).

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Blends of large sized pigments with high color variation create a confetti-like effect due to spatial color resolution. The substrate-based dielectric optically variable pigments with their large size and inter-platelet color consistency have a uniform color, which changes at different angles of illumination and viewing.

The color travel plot of the samples shown in Figure 8, measured at specular angles using an IsoColor goniospectrophotometer, demonstrates that color travel is a property of the individual pigment particles and cannot be created with blends of different pigment colors. The measured color of the interference pigment blend is the sum of the three colors that tends towards a neutral shade. The DOVP pigment shows a strong color intensity at all specular angles with large changes in hue from green-blue to red.

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APPLICATIONS AND PERFORMANCE

The platelet is thin enough to allow its use in low film build liquid coating applications, such as coatings for teletronics and packaging, yet the particle size is big enough to create a sparkle effect in high film build applications like powder coatings and screen printing. For dramatic effects, the pigment should be used over a dark background to promote only the intense interference reflection color. The transmitted color is absorbed into the background. Application over a light background produces a highlight effect due to the intense reflection color combined with full transparency to show the background color.

The pigment concentration can be varied depending on the application: for both liquid and powder coatings, a sufficient effect can be achieved by as low as 1% pigment concentration based on total solids weight; 2.5-5% will have a very intense color and color travel.

Another attribute of the borosilicate-based optically variable pigment is its ability to maintain color travel when used with absorption pigments instead of becoming lost, as happens to some pigments of this type. An example shown in Figure 11 is a violet-red-gold dielectric optically variable pigment in use with an orange that shows excellent color travel in a coating film.

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The dielectric optically variable pigment's one-micron thickness allows for its use in automotive refinish coatings. The performance of the pigments can be enhanced with a surface treatment to suit exterior application requirements.

Humidity resistance is critical for platy structure material. (3) Most humidity-induced failures are due to whitening and wrinkling of the paint film, especially for waterbased coating systems. Whitening and micro-wrinkling in coating films can be measured by changes following exposure in L* and the dullness of the coating film, respectively. Test results from 96 hr of exposure at 40[degrees]C with 100% humidity can provide a good indication whether or not pigments will meet testing requirements for automotive applications.

Tables 1 and 2 show the color change for a surface-treated dielectric optically variable pigment versus an untreated pigment using a traditional mica-based interference green pigment widely used in automotive OEM and refinish coatings as a control. The dielectric optically variable pigmented film shows excellent color retention when tested with an appropriate treatment.

The gloss retention and appearance can also be improved by putting an exterior treatment on these types of pigments, as Table 2 shows. With the exterior treatment, the coating film will have higher gloss retention and less dullness increase during testing.

SUMMARY

High chroma, color changing pigments have been available for more than a decade. New borosilicate substrate-based pigments generate higher chroma, greater impact, and have good durability for a wide range of applications in high performance coatings, from teletronics to automotive refinishes.

ACKNOWLEDGMENT

The authors would like to thank to J. Bagala, D. Cacace, C. Willard, and the staff of the BASF coatings applications laboratory for their support in this project.

References

(1) Greenstein, L.M., "Nacreous (Pearlescent) Pigments and Interference Pigments," Pigment Handbook, Vol. 1: Properties and Economics, 2nd Ed., Lewis, Peter A. (Ed.), John Wiley and Sons, New York, 1988.

(2) Born, M. and Wolf, E., Principles of Optics, 6th Ed., Cambridge University Press, 1980.

(3) Lavallee, C., Venturini, M., and Liu, W., "Effects of Surface Treatment on Humidity Resistance for Coated Mica Pigments in Waterborne Coatings," PCI, May 2002.

by Wei Liu and James B. Carroll, Jr.

Appearance Performance Technologies, BASF Catalysts LLC*

*Specialty Pigments Application Laboratory, 3 Greentown Rd., Buchanan, NY 10511.

Table 1 -- Color Changes for Effect Pigment Following Humidity Exposure

                             [DELTA]L*  [DELTA]a*  [DELTA]b*  [DELTA]E

Turquoise DOVP                1.5       -0.8       -0.8       1.9
Turquoise DOVP with surface   0.4        0.2       -0.1       0.5
  treatment
Traditional interference      2.1        0          0.6       2.1
  green
Traditional interference     -1.2        0.5       -0.3       1.3
  green with surface
  treatment

Table 2 -- Appearance Changes of Effect Pigments Following Humidity
Exposure

                          20% Gloss      Dullness
                          Retention (%)  (Initial/Final)  Appearance

Turquoise DOVP             60            55/59            Very slight
                                                            wrinkling/
                                                            fading
Turquoise DOVP with        89            57/58            No change
  surface treatment
Traditional interference   86            22/50            Very slight
  green                                                     wrinking
Traditional interference  100            23/21            No change
  green with surface
  treatment