Colorants for Composites

Abstract Colorants and special effect pigments are key ingredients of plastics compounds, offering countless possibilities to designers who want to differentiate their products. Color, simply stated, is light energy, reflected from or passing through an object, and viewed by a human observer or instrument detector. Colorants and the chemicals that may be added to, or naturally occur in, objects that modify the light energy produce the object's color. Colorants contain certain groups of atoms that selectively absorb certain wavelength of visible light and reflect the complementary color. These chemical groups are called chromophores . Other chemical groups, called auxochromes , may be present in a colorant molecule to modify the color and provide undertones. Colorants can be dye, pigment, biological pigment, ink, paint, or colored chemicals. There are two major classifications of colorants used for the coloring of composites. They are dyes and pigments. Dyes are organic chemicals that are soluble in polymers and provide strong, transparent color and only produce color by light absorption. Pigments are essentially insoluble in plastic medium and must be mixed in a fine dispersion to provide uniform coloration. Pigments can be further divided by chemistry into organic and inorganic types. Organic pigments provide strong, translucent or transparent color and have smaller average particle size than inorganic pigments. Conventional organic pigments have thermal stability lower than that of average inorganic pigment, and some can cause part warpage. More new, high heat‐resistant and high performance organic pigments are stable. As with dyes, pigments produce effects by mechanism of light absorption, but since many pigments (inorganic) have refractive indices greater than the plastics in which they are dispersed, pigments produce color effect by light scattering as well. Dyes, because of their fine particle size, tend to be transparent. Pigments have varied opacifying power. Although dyes are gaining increasing acceptance, especially with the transparent plastics such as acrylic, rigid polyvinyl chloride (PVC), polyester, and polystyrene, their general industrial use has been limited. However, it must be stressed that if the plastic application requirements include bright shades, transparency, and good tintorial strength, the application of the dyes should be considered. Because of the limited use of dyes in plastics, the remainder of this discussion is directly to pigment properties and types.