Coloring Plastics

Colorants

There are two different ways to color plastics: pigments and dyes. Dyes are organic liquids that have very little effect on light transmission. Pigments are inorganic colorants that absorb or scatter light. Carbon black is the most common pigment, giving the plastic UV resistance to provide better weatherability. If a colored plastic part is clear, it was probably colored with a dye.

Color matching is extremely important in some applications. The color can be measured with a device called a Colorimeter, and then compared to a control sample. Color matching can be very difficult, because it not only depends on the dispersion of the coloring agent throughout the plastic, but it can also be affected during processing by heat history, and shearing.

Defining Colors

The most common method of defining a color uses the Munsell System. Colors are defined in a three dimensional polar coordinate system, where the three variables are hue, value, and chroma. Hue is the quality of color described, such as red, blue, greed, yellow, etc. Value is the amount of grayness (addition of black or white) in a color. Chroma describes the amount of difference between a color and a gray of the same value. Figure 1 shows a representation of hue, value, and chroma space. The circular ring represents hue (the quality of the color changes). The bars that are spaced in a radial pattern represents the chroma, which is the amount of color in the sample (deviation from the gray center). The vertical tube represents value, the lightness or darkness of the sample.

Figure 2 shows the actual coordinate system used to define colors. This is a three dimensional rectangular coordinate system. The three variables are a, b, and L. a is the greenness - redness variable, b is the yellowness - blueness variable, and L is the lightness - darkness variable. When using a Colorimeter, the output will be in a, b, and L coordinates.

Figure 3 shows an example of a Munsell Color Tree.