Given the many pearl pigment grades now available, and the many transparent absorption colorants that can be deposited on or blended with them, users can choose from an almost unlimited array of color combinations and special effects. These range from iridescence, luminescence, and luster to the ability to change color as viewing and illumination angles change.
The most widely used pearl pigments consist of mica platelets coated with titanium dioxide or iron oxide that give white and colored effects. The crystalline pearlescent layer is formed by calcining mica coated with a metal oxide at about 1350 F. The heat creates an inert pigment that is insoluble in resins, has a stable color, and withstands the thermal stress of plastics processing.
Color in these pigments develops through interference between light rays reflecting at specular angles from the top and bottom surfaces of the metal-oxide layer. Depending on the coating thickness, these colors can be gold, red, blue, or green. The pigments lose color intensity as viewing angle shifts to non-specular angles. In addition, a second color is transmitted through the pigment platelet that is complementary to the reflected one.
The relative contributions of the transmitted and reflected colors can depend on how the plastic is viewed. When a transparent plastic containing pearl pigment is placed against a white surface, the weaker transmission color is reflected back through the film and supplements the stronger reflection color. Against a dark background, the transmission color is absorbed and only the pigment's reflection color is seen.
Combining absorption and interference colors can create a variety of single- or dual-color effects. The presence of an absorption pigment--either coated onto the interference pigment or used in conjunction with it--produces intense specular reflection colors that give way to the color of the absorption pigment at non-specular angles. There is also a transmission color that combines the effects of the two pigments.
Light reflected from pearl platelets as they lie essentially parallel to each other at different levels in the plastic creates a sense of depth and luster. The best luster, brightness, and color intensity occurs with platelets that are 10 to 40 microns long. Smaller platelets impart a smooth, silky luster, and larger ones confer sparkle and glitter.
Mica-based pearlescents can be used in nearly all thermoplastics and most processes. Their effects are most intense in transparent resins like PS, PP, PE, PVC, acrylic, styrene block copolymers, and silicone.
It is also possible to attain pearlescence and luster in polymers having little or no transparency--such as nylon 6, ABS, and HIPS--but you may need higher pigment loadings. Pearlescents combined with dark absorption pigments in opaque polymers can yield a strong reflection color and produce pearlescent effects of great richness and depth.
Modest amounts of transparent absorption pigments--for example, 0.01 to 0.50% of phthalocyanine blue or green or quinacridone red--can give interesting effects when used with pearlescents. But opaque, inorganic, high-coverage pigments should be limited to very small amounts in order to retain any pearlescence. For products such as white HDPE shampoo bottles, some compounders add up to 5% TiO2 to pearlescent color concentrates in order to enhance opacity.
Highly filled plastics are not good candidates for pearlescents because opaque fillers scatter light, eliminating the pearlescent effect. Most users limit fillers to less than 1% in systems containing pearl pigments.
Pearlescents can also be incorporated in many thermosets, including unsaturated polyester, acrylic, urethane, and epoxy. Bismuth oxychloride pearl pigments are often used in cast polyester buttons for brilliance and luster. Mica-based pearls are added to cast polyester cultured marble and onyx components for countertops, floors, furniture, and fireplace elements. Pearls can also be added to cast acrylic sheet and alloyed or blended materials such as rubber-toughened polycarbonate, nylon/ABS, and PC/ABS.