Page 365 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.45

The largest use is in thermosets, second in PVC, and third in styrenics, with smaller
amounts in polyolefins and engineering thermoplastics. In terms of applications, electrical
and electronics, transportation, and construction use the largest amounts.

5.8 COLORANTS
One-third of all plastics are used in natural uncolored form, and the other two-thirds are
colored, using on the average about 1 percent of colorant.

5.8.1 Color Theory
When white light shines on a plastic material, it may be absorbed, reflected, and/or trans-
mitted. Absorption removes certain wavelengths; those that remain are the color we see.
Opaque pigments, insoluble in the polymer, reflect an opaque color to the eye. Transparent
organic dyes, soluble in the polymer, transmit a “see-through” color to the eye.
Solar radiation is a normal distribution of wavelengths, from shortwave ultraviolet,
through the visible spectrum (rainbow) from blue (4000 A = 400 nm) to green to yellow to
red (7000 A = 700 nm) and on into the infrared. Colored materials absorb some of these
wavelengths and leave the others that we see; each colored material has a characteristic ab-
sorption spectrum that defines it in this way.
Color scientists often use a color sphere to characterize a particular color (Fig. 5.9).
The horizontal circumference of the sphere is called the hue, and it represents the rainbow
spectrum from purple to blue to green to yellow to orange to red around the rim of the cir-
cle. A vertical axis through the sphere is called the lightness or brightness, and it ranges
from black at the south pole, through dark to light grays through the sphere, to white at the
north pole. A radius from the lightness/brightness axis, out to the hue circumference, is
called the saturation or purity or chroma, and it ranges from pure bright colors near the
circumference to pastel or grayish colors toward the center of the sphere. Thus, any color
may be specified by its location within this three-dimensional color sphere.
More recently, color scientists have developed the much more complex CIE chromaticity
diagram (Fig. 5.10) and computerized it so they can calculate precisely the matching of an
experimental color with a desired model color. More practically, color technologists have as-
sembled thousands of shades of color chips, arranging and organizing them and giving them
code numbers for convenient reference. One of the best known is the Munsell Book of Color.

5.8.2 Chemical Classification
Colorants are generally classified as inorganic or organic. Generally, inorganic colors are
coarser particles of lower coloring efficiency, stabler to heat, light, and chemical environ-
ment, and less expensive per pound. But many of the brightest colors are made from met-
als whose toxicity is causing health concerns, both in manufacturing and in the ultimate
environment. Organic colors are finer particles of higher coloring efficiency, less stable,
and more expensive per pound (although not necessarily in the low concentrations re-
quired to produce the desired color); they are generally nontoxic, but they must be moni-
tored for possible impurities that could cause problems.
Colorants may also be classified as dyes or pigments. Dyes are either soluble in the
polymer, or particles so fine (<2 µm) that they do not scatter and reflect light, so they give
transparent colors. They are mainly organic compounds and very efficient colorants, but
they tend to be unstable and extractable. Pigments are either insoluble particles or, less of-
ten, dyes bonded onto the surface of insoluble particles, producing opaque colors; they

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