Page 294 - Carrahers_Polymer_Chemistry,_Eighth_Edition

8 Composites and Fillers

Today, one of the fastest growing areas is composites. Composites offer good strength, corrosion
resistance, and are light weight so they are continually replacing metals. Much of the replacement
of metals is due to desired weight reductions as the cost of fuel continues to increase. Composites
are generally composed of two phases, one called the continuous or matrix phase that surrounds
the discontinuous or dispersed phase.

While many fillers approach being spherical in geometry, some are fiber like. When the length of

these fibrous materials approach 100 times their thickness many combinations offer great increases

in the strength-related properties of the materials that contain them. If these fibers are contained
within a continuous phase, these materials are generally described as being traditional compos-
ites. Because of their importance, much of this chapter will deal with such traditional “long-fi ber”
composites.
8.1 FILLERS

According to the American Society for Testing and Materials standard (ASTM) D-883, a filler is a

relatively inert material added to a plastic to modify its strength, permanence, working properties,
or other qualities or to lower costs, while a reinforced plastic is one with some strength proper-
ties greatly superior to those of the base resin, resulting from the presence of high-strength fi llers

embedded in the composition. The word extender is sometimes used for fillers. Also, the notion that
fillers simply “fill” without adding some needed property is not always appropriate. Some fi llers are

more expensive than the polymer resin and do contribute positively to the overall properties.
Many materials tend to approach a spherical geometry to reduce the interface the materials has
with its surroundings. Further, a spherical geometry is favored because of the old adage that like-
likes-like-the-best and a spherical geometry offer the best association of a material with itself.

The behavior of many fillers can be treated roughly as though they were spheres. Current the-

ories describing the action of these spherical-acting fillers in polymers are based on the Einstein
equation 8.1. Einstein showed that the viscosity of a viscous Newtonian fl uid (η ) was increased
o
when small, rigid, noninteracting spheres were suspended in a liquid. According to the Einstein
equation, the viscosity of the mixture (η) is related to the fractional volume (c) occupied by the
spheres, and that was independent of the size of the spheres or the polarity of the liquid.
η
η = η (1 + 2.5c) and sp = 2.5 (8.1)
o
c
Providing that c is less than 0.1, good agreement with the Einstein equation is found when glass
spheres are suspended in ethylene glycol. The Einstein equation has been modified by including a

hydrodynamics or crowding factor (β). The modifi ed Mooney equation 8.2 resembles the Einstein
equation when β = 0.

 2.5c 
ηη 0  
=
β
 (1 – c  (8.2)
)

Many other empirical modifications of the Einstein equation have been made to predict actual
viscosities. Since the modulus (M) is related to viscosity, these empirical equations, such as the
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