Page 342 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.22 CHAPTER 5

5.2.2.5 Silica. Silica is a naturally occurring mineral that is ground down to particle
sizes of 2 to 10 µm and used as a low-cost, stable, white filler.
5.2.2.6 Talc. Talc is a magnesium silicate mineral, often used in polypropylene to im-
prove processing, rigidity, creep resistance, and heat deflection temperature.

5.2.3 Reinforcing Fillers
Fibers increase strength but make melt processing much more difficult. Reinforcing fillers
are fine particles that permit fairly normal melt processing but do increase strength; they
are often referred to as mineral reinforcement. When they are examined under a micro-
scope, they are generally plate-like or fiber-like in appearance. Theoretically, the strength
of reinforced plastics depends on the force required to pull a fiber out of the polymer ma-
trix: if the fiber is embedded far enough into the matrix, it must break before pulling out.
Model calculations often conclude that, when the aspect (L/D) ratio is greater than 20/1,
the fiber will not pull out before it breaks. Many reinforcing fillers appear to have more
than the critical aspect ratio of 20/1.
5.2.3.1 Wood Flour. Wood flour is made by controlled attrition of wood and contains
microscopic cellulose fibers. It was first used in phenolic plastics to increase their strength,
and it remains the basis of general-purpose phenolic moldings. It was occasionally used in
other plastics and is currently gaining popularity in vinyl and other thermoplastic wood/
plastic composites for processability and durability superior to wood alone. At high load-
ings in HDPE and PVC, these “wood/plastics composites” look like wood but are more
durable, and they are finding growing use as “plastic lumber” in outdoor construction and
furniture.

5.2.3.2 Wollastonite. This is a calcium silicate mineral, acicular, with aspect ratios of 3
to 20/1. It is of interest for reinforcement of strength and as a safe replacement for asbes-
tos.

5.2.3.3 Franklin Fiber. This is a calcium sulfate crystal with aspect ratios of 60/2 µm. It
is an easy-processing reinforcement but suffers from water sensitivity.
5.2.3.4 Mica. Mica is a potassium aluminum silicate mineral that occurs as flakes with
aspect ratio up to 50/1. The best grades offer good processability, reinforcement, and im-
permeability.

5.2.3.5 Asbestos. Asbestos is a low-cost magnesium silicate mineral that occurs as very
short, fine fibers of high modulus, strength, and thermal and chemical resistance. It was a
popular filler until it was noticed that it collected in the lungs and caused serious health
problems. Its use has been discontinued except in critical applications such as brake lin-
ings. Since then, a number of other promising short, fine fibers have been abandoned for
fear that they may cause similar problems.

5.2.3.6 Nanofillers. Nanofillers are extremely fine particles, under a micron in size. The
most successful ones have been made by intercalating quaternary ammonium surfactants
between the layers of montmorillonite clay, followed by fluid polymer, to exfoliate them
down to 1-nm platelets with aspect ratio of 1000/1. When these are dispersed in nylon at
low concentrations of 2 to 10 percent, the tremendous numbers of plate-like particles can
produce easy processing, high modulus and strength, heat deflection temperature, trans-

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