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Additives, fil lers, and reinforcements
Why they're important
Additives, fillers, and reinforcements are used to change and
improve the physical
and mechanical properties of plastics. In general, reinfo rcing
fibers increase the
mechanical properties of polymer composites while particular fi
llers of various types
increase the modulus.
TABLE 1. Effects of additives, fillers, and reinforcements on
polymer properties
Additive / Filler /Reinforcement Common materials Effects on
polymer properties
Reinforcing fibersBaron, carbon, fibrous minerals,
glass, Kevlar
Increases tensile strength.
Increases flexural modulus.
Increases heat-deflection
temperature (HDT).
Resists shrinkage and
warpage.
Conductive fillers Aluminum powders, carbonfiber, graphite
Improves electrical andthermal conductivity.
Coupling agents Silanes, titanates
Improves interface bonding
between polymer matrix and the
fibers.
Flame retardantsChlorine, bromine, phosphorous,
metallic salts
Reduces the occurrence and
spread of combustion.
Extender fillers Calcium carbonate, silica, clay Reduces
material cost.
PlasticizersMonomeric liquids, low-
molecular-weight materials
Improves melt flow properties.
Enhances flexibility.
Colorants (pigments and dyes)Metal oxides, chromates, carbon
blacks
Provides colorfastness.
Protects from thermal and UV
degradation (with carbon blacks).
Blowing agentsGas, azo compounds, hydrazine
derivatives
Generates a cellular form to
obtain a low-density material.
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Modifying polymer propertiesElectrical properties can be
affected by many fillers. For example, by adding conductive
fillers,
an electromagnetic shielding property can be built into
plastics, which are normally poor
electrical conductors. Anti-static agents can be used to attract
moisture, reducing the build-up of
static charge.
Coupling agents are added to improve the bonding of the plastic
matrix and the reinforcingfibers. Different fillers are used to
lower the cost of materials. Other additives include flame
retardants to reduce the likelihood of combustion, lubricants to
reduce the viscosity of the molten
plastic, plasticizers to increase the flexibility of the
materials, and colorants to provide
colorfastness.
Low-aspect fillersFillers modify the properties and molding of
the compound to which they are added. If the fillers
are characterized with a low aspect ratio between the longest
and the shortest dimensions, thebasic properties will be less
changed from those of the unfilled polymer. Fillers benefit
plastics
parts in the following ways:
q Shrinkage will be less.
q Thermal resistance may be improved.
q Strength, especially compressive strength, will be
improved.
q Impact resistance will often be lower than for the unfilled
polymer.
q Solvent resistance will often be improved.
High-aspect fillers: fibersWhen the aspect ratio between the
longest and the shortest dimension of the filler is large, for
example, greater than 25, the filler can be characterized as a
fiber. Fiber reinforcements will
significantly affect the properties of the compounds to which
they are added.
Fibers impact strength
Assuming good bonding between the fiber and the polymer matrix,
the strength in the fiber
direction will be significantly increased. If many fibers are
oriented in the same direction, large
differences will be noted between the modulus in the orientation
direction and in the direction
perpendicular to the orientation. The latter will be very close
to that for the unfilled polymer.
Fibers affect shrinkage
The fibers will also have a significant effect on the shrinkage
properties of the compound:
shrinkage in the orientation direction will be much less than
the shrinkage in the cross direction.
Importance of predicting fiber orientationBecause the fiber
orientation varies with the flow direction, in the thickness
direction, and at
weld line locations, it is important to be able to predict these
orientations, in order to predict the
properties of the molded article.
