In recent years, automotive colors have become an important aspect of car design. Market experience shows that car buyers tend to prefer automotive coatings with high chroma shades based on effect pigments because of their visual impact.1 For creation of clearer, more brilliant and more exciting colors for automotive original equipment manufacturers (OEM) finishes the number of pigment types used has therefore increased considerably in the last decade. An end to this trend is not yet in sight.

In addition to conventional "one-dimensional" organic and inorganic pigments, which interact with light by absorption and/or scattering, more "two- and three-dimensional" plate-like pigments are finding their way onto the market.2 The optical impression of these pigments is based on three kinds of interaction of light. First, if they consist only of small metallic flakes with diameters more than five micrometers the incident light is almost all directly reflected. That means these pigments act as small mirrors, which results in a lightness flop with change of the viewing angle. The "dollar" and "corn flake" types of aluminum pigments belong to this "two-dimensional" pigment group.

Second, if the plate-like particles are based on, or contain, selectively absorbing materials the interaction with light is based on absorption and reflection. As a result, both the reflected and the transmitted light are colored. Plate-like iron oxides (e.g., BASF Paliocrom Copper L 3000 and L3101) and plate-like phthalocyanine (BASF Paliochrom Blue Gold L 5000) belong to this group.

If plate-like pigments with diameters of more than five micrometers contain or are based on thin films with a thickness in the range of the visible light, interference and reflection effects are generated. This third interaction of light is well known for the "three-dimensional" plate-like pearlescent pigments based on highly refracting materials like bismuth oxychloride and titanium oxide-coated micas.3

Most interference (pearlescent or luster) pigments now consist of at least three layers of two materials with different refractive indices. Thin flakes (thickness ca. 500 nm) of a material with a low refractive index (mica) are coated with a highly refractive metal oxide (e.g., TiO2, layer thickness ca. 50-150 nm). This results in particles with four interfaces. More complex multilayer pigments containing additional thin (e.g., light absorbing) films have also been produced and can be found in OEM paints.4

Interference (pearlescent or luster) pigments split white light into two complementary colors that depend on the platelet thickness. The interference color dominates under maximum reflection (face angle). The transmitted part dominates at other viewing angles under diffuse viewing conditions provided there is a white (non absorbing) or reflecting background. Variation between face angle and grazing angle for incident rays therefore produces a sharp gloss peak and a color change between two extreme complementary colors (color flop).

Against a black background or in a blend with carbon black (a frequent situation in the basecoats of two-coat paint systems), the transmitted light is absorbed and the reflected interference color is seen as the mass tone of the material. In combination with absorption pigments two-tone effects can be achieved if the absorption color differs from the interference color. The optical impression of plate-like pigments with diameters of more then five microns can be described by the Fresnel equation.

In general the reflection is a function of the refractive index (n) and the absorption constant (k) of the material, the wavelength of incident light (l) and the irradiation angle (e) R=f (n,k,l,e). In a simplified form (e=90?, k=0) the Fresnel equation is:

This equation clearly shows that the greater the difference between the refractive indices of the plate-like pigment (n2) and the binder (n1), the greater the reflection and, accordingly, the gloss of the pigment. This is why the pigment industry prefers highly refractive materials. But the angle-dependent color play observed in thin films of such materials is restricted. In contrast thin films of inorganic materials with low refractive indices like SiO2 or MgF2 (so called dielectric materials) show like soap bubbles strongly angle-dependent color hues. This real color play of such materials can be explained by Snell's Law.

Snell's Law correlates the refractive angles and the refractive indices. In the case of a highly refractive material like TiO2 light rays at a face angle and grazing angle are refracted close to the perpendicular. In the case of SiO2 bending toward the perpendicular is low. As a result the interference conditions for face and grazing angle rays are completely different and strongly angle-dependent colors are obtained. The best materials for optical variable (OV) pigments therefore would be flakes of SiO2.

However, there is one problem. It can be calculated from the simplified Fresnel equation that the reflection of light on a thin solid film with low refractive index is weak.

The solution is to strengthen the reflection - and thus the chroma - by surrounding the non-absorbing dielectric with a very thin semi-transparent layer of highly reflective material.

By doing this, the extent of extinction and enhancement of particular wavelengths is efficiently increased. These OV pigments therefore exhibit deep colors, high gloss and a high color dependence on the viewing angle (color flop). So far, OV pigments of the Fabry-P?t type and OV pigments with inner reflector are on the market. The Fabry-Parot type has a dielectric core of low refractive material, which is surrounded by a semitransparent reflector.5

The inner reflector type has a core of reflecting material, which is surrounded by a symmetrical system of weakly refracting and highly reflecting layers.

This example shows the color flop in the Al/SiO2/Fe2O3-system.

So far, OV pigments are produced only by Flex Products Inc., Merck6-7 and BASF. Recently holographic pigments have added a new dimension to paint. To create these pigments a specific holographic light recording is embossed in 50-100 ?m particles cut from 12 micrometer polyester film.8 PPG Industries is marketing this new product under the trade name Geometric PigmentsTM. BASF Corp. will soon introduce these pigments to the OEM market.

Another concept is the creation of interference pigments on the basis of liquid crystals (Wacker-Chemie AG, BASF AG).9

In addition to these interference pigments micro titanium dioxide must not be forgotten. This ultra fine material (particle size ca. 10-50 nm) is found more frequently in the basecoats of two-coat metallic systems since its introduction in 1987, as it is also possible to obtain a color flop with this product.10

A new generation of effect pigments is represented by aluminum pigments coated with color pigments. The organic or inorganic color pigments are permanently fixed on the flake surface by a layer of silicon dioxide. Compared to the same pigmentation level in conventional metallic coatings ALUCOLOR(r) in blue and green (Eckart-Werke, Germany) provides higher color strength. The blue flop of ALUCOLOR white creates a stainless steel surface finish.

Colored aluminum pigments are also produced by Showa Aluminum Powder K.K. Japan and distributed under the trade name Fireflake. Six colors are available in four particle size ranges. A new class of aluminum pigments are the ALOXAL(r) pigments from Eckhart-Werke Germany. The "champagne-colored" shades of ALOXAL generate warmer and deeper metallic effects with good brilliance and opacity.


Plate-like pigments have visual, physical and chemical advantages. Applications for these pigments include not only cars but also building exteriors, furniture, and cosmetics. Therefore, the forensic scientist is becoming confronted with plate-like pigments more frequently. Several new plate-like pigments will come onto the market soon, including OV pigments of the Fabry-P?t type, inner reflector type, liquid crystal type and holographic pigments.

A concept for classifying and identifying plate-like pigments by applying a combination of analytical methods has been published. 11


1 Novinski, S.J.; Nowak, P.J.; Venturini, M.T. Pearlescent Pigments in High Performance Coatings. Presented at Intertech Conference; 1997 Oct. 27-29, Chicago

2 Esselborn, R.; Franz, K.D.; Hofmeister, F. Color Styling with Lustre Effect Pigments in Multi-Layer Systems. Presented at the Eurocoat 85 Congress; 1985, Stra?burg

3 Franz, K.D.; Emmert, R.; Nitta, K. Interference Pigments. Kontakte (Darmstadt). 1992; 2:3-14. ISSN 0172-8717

4 Teaney, S.; Denne, I. Color Formulations with Iriodin(r) / Afflair(tm) Pigments for Automotive Coatings. Kontakte (Darmstadt). 1992; 2:46-55. ISSN 0172-8717

5 Schmid, R.; Mronga, N.; Radtke, V; Seeger, O. Optically Variable Lustre Pigments - Optisch variable Glanzpigmente. Farbe & Lack. 1998;104:44-48.

6 Sharrock, S.R.; Schuel, N. New effect pigments based on SiO2 and Al2O3 flakes. European Coatings J. 2000;1.

7 Kuntz, R.F.; BrAckner, H.D. Pearl Lustre Pigments with Improved Properties. Presented at the 3rd NArnberg Congress; 1995 March 13-15, NArnberg

8 Anon. Holographic Pigments - Add New Dimensions to Paint. PCI. 1997:48-49.

9 Heinlein, J.; Kasch, M. Pigments Offer Color Effects Matched Only by Nature. PCI. 1999:58-75.

10 Winkler, I.; Proft, B. Vor UV schAtzen und selektiv blaues Licht streuen - Eigenschaften und Anwendungen von nanokristallinem Titandioxid. Farbe & Lack. 2001;107:28-36.

11 Stoecklein, W. The Analysis of New Plate-Like Pigments in Automotive Coatings. PCI. 2001: 48-65.