Page 370 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.50 CHAPTER 5

ergy, and re-emit the rest as visible light at the blue-violet end of the visible spectrum. This
neutralizes the yellowness in the polymer, and emits a brilliant white. They are frequently
used in polyolefins, polystyrene, ABS, PVC, polycarbonate, and polyurethanes. Concen-
trations are typically 0.01 to 0.1 percent.

5.8.9 Reference Texts
The major texts in the field are T. C. Patton’s Pigment Handbook (John Wiley & Sons, now
out of print) and F. W. Billmeyer’s Textbook of Color Science (Wiley-Interscience).

5.9 ANTISTATS

When two materials are in contact with each other, electrons migrate across the interface.
When they are separated, some electrons may be caught on the wrong side, producing a
static charge.
Conventional structural materials are conductive enough to bleed off the charge to
ground. Organic polymers are nonconductors, and may hold the charge for a long time.
The charge on plastics may develop during separation from the mold or roll, from fric-
tion during manufacture or use, or simply from evaporation of water from the surface. This
sometimes causes problems in processing, particularly in handling thin films and fibers. It
causes a much greater range of problems in the use of the product: collection of dust; un-
sightly packaging; cling and discomfort of clothing and upholstery; shock; occasional dust
explosions; oxygen hazard in hospitals; “noise” in sound recordings and photography and
magnetic tapes and discs, computer chips, military electronics; and electromagnetic inter-
ference (EMI) of electronic equipment in general.
These are arranged more or less in order of increasing need for static dissipation. They
12
are generally classified in terms of electrical resistance. For example, over 10 Ω-cm is
nonconductive insulation, 10 10–12 is antistatic, 10 6–10 is statically dissipative, 10 2–6 is
1
slightly conductive, 10 is EMI shielding, 10 0 to –3 is semiconductive, and 10 –3 to –5 is me-
tallic conductivity.
Various techniques are used to minimize these problems. In manufacturing, it is possi-
ble to ionize the air and thus neutralize static charges. In textile manufacturing, it is com-
mon to humidify the air to make fiber surfaces more conductive. Organic additives can
make plastics fairly conductive to dissipate a static charge. In more extreme cases, high
loading with carbon black makes rubber and plasticized PVC fairly conductive. And load-
ing with carbon fibers and metallic fillers (particularly aluminum flakes and fibers) makes
plastics conductive for EMI shielding.

5.9.1 Mechanisms of Antistatic Action
When organic antistats are used to reduce static charge on plastics, several theories are of-
fered to explain their action. Most commonly, it is assumed that the additive is polar
enough to exude to the surface of the plastic, where it absorbs moisture from the air, per-
mitting ionic impurities to conduct current electrolytically. The most effective antistats ac-
tually contain ionic groups that are free to migrate and conduct. Some theorists believe
that simple passage of water vapor over the surface of the plastic may be enough to carry
away the static charge. From a different point of view, static charge is created by friction;
the antistat acts as a surface lubricant, reducing friction and therefore reducing the buildup
of a static charge.

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