Visual appearance has a strong influence on customer preferences and perceptions of product quality, particularly in high-end applications. Preferences for effect pigments in the automotive market are highly influenced by emerging fashion trends in textiles and the consumer market. “Special-effect pigments display a wide range of properties such as color travel and the reflection, refraction and diffraction of light, plus they can afford a mirror-like appearance and exhibit multiple color effects. As a result, they allow coatings formulators and automotive color designers to create unique and exciting visual effects,” says Arun LeoRoyan, color development manager for AkzoNobel’s Performance Coatings research center in Bangalore, India.
Commercially significant effect pigments are used to create opaque or interference/transparent effects. They are produced using a variety of methods, including chemical synthesis, deposition, multi-layering, and physical micro-level-shaping techniques, according to LeoRoyan. Natural micas, synthetic micas, aluminum flakes, colored aluminum flakes, and glass flakes are the most commonly used effect pigments. Less common examples include stainless steel, bronze and polymer flakes/platelets. Phosphorescent, thermochromic and photochromic pigments are also available. Newer materials include graphite, micronized titanium-dioxide and cellulose fibers. The particles are often coated with different metal oxides—sometimes multiple layers of different materials—to carefully control the interaction of the entire pigment with light and create unique effects.
Micas add depth and color intensity and can create an appearance that can range from satin to glittery. Aluminum provides a silver appearance with good hiding. Colored aluminums intensify the color of organic dyes and pigments with hiding for golds, oranges, and reds. Vacuum metalized pigments provide a liquid metal, chrome-like appearance. Glass flakes provide color purity, brightness, transparency, and sparkle. A metallic gloss appearance is created with metallic pigments, while luster/pearlescent pigments provide angle-dependent effects resulting from the reflection, interference, and absorption of visible light by pigmented multi-layer systems. The color of gonio-chromatic or color-shifting pigments changes when they are viewed at different angles.
Different applications require different particle sizes and chemical treatments to create the desired effect and meet specific needs. These needs typically relate to durability and coating thicknesses, but can be very application specific. For instance, exterior applications require weather-resistant properties achieved through the use of special surface treatments. Aluminum pigments used in water-based formulations must also be treated/encapsulated to prevent decomposition/gassing due to reaction with the water.
Color designers for automotive manufacturers usually collaborate with paint suppliers to determine which pigments should be used to achieve a particular color and look. Coating formulators provide information on the latest developments in special-effect pigment technology and suggest which effects will afford the look desired for specific models. Body style often determines the type of effect pigment that is used. Pigments that create sparkle tend to be used to add more dimension to models with flat sides, while pigments with more color travel tend to be used on vehicles with curves to accentuate those lines. In typical automotive refinishing, pigments that create pearlescent and color-shifting effects, along with synthetic micas, glass flake, colored aluminums, and deposition flakes that create glittery and chrome-like effects, are all popular.
New pigment technologies under development today include those based on microscopic glass beads, recycled mirror chips, volcanic dust, and processed aluminum flakes that can reflect light and create new types of effects, according to LeoRoyan. Paramagnetic pigments that change color directions upon application of an electric voltage are also being explored, as are colors created using pigment technologies with structural surfaces inspired by nature. This work is driven by the desire for automakers to create novel and striking vehicles that are differentiated from the competition.