Page 369 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.49

5.8.6.3 Color Concentrate (Masterbatch). Expert compounders disperse colorants at
20 to 60 percent concentration in a carrier polymer, using high shear to break down ag-
glomerates and produce uniform dispersion of maximum coloring efficiency. This color
concentrate is used by processors, simply blending it with virgin (natural color) resin (“let-
down with natural”). Typical ratios of concentrate/natural are 1/20 to 1/100. This tech-
nique is low in cost, does not create inventory problems, and is most commonly used with
commodity resins.
5.8.6.4 Liquid Color. The colorant is predispersed in a liquid carrier, hopefully compat-
ible with the resin. It is metered into the base of the hopper or the beginning of the screw
in extrusion or injection molding and blends uniformly with the resin by the time it
reaches the exit from the screw. Although originally billed as a universal technique, it has
rather found applications in certain processes where it is the optimum technique.
5.8.6.5 Color Infusion. This immerses the finished plastic product in a hot aqueous dis-
persion of colorant + dispersant. In several minutes, the color diffuses into the plastic
product, giving it permanent coloration. The length of time determines the depth of the
color.

5.8.7 Special Colorants
Fluorescent colors are used to produce brighter reds and yellows. Phosphorescent colors
are used to produce brighter yellows-greens-blues. Pearlescent colors combine internal
and external reflections; they are made by techniques such as coating titanium dioxide on
mica.
Metallic flakes are added to colorants to give them a metallic sheen. Aluminum flakes
give a silvery sheen and also improve UV stability and impermeability. Bronze flakes can
be formulated into a range of colors from green to red to gold.

5.8.8 Fluorescent Whiteners
Most polymers tend to form conjugated unsaturation during aging, absorbing blue light
from the visible spectrum and therefore turning somewhat yellow. One way to mask this is
to add fluorescent whiteners.
These are primarily bis-benzoxazoles, triazines and triazoles of phenyl coumarins, and
bis-styryl biphenyls (Fig. 5.12). They absorb invisible UV light, dispose of part of the en-

FIGURE 5.12 Fluorescent whiteners.

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