Page 339 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.19

5.2.1.8 Coefficient of Thermal Expansion (CTE). CTE is inverse to the attractive
forces holding the molecules together. The weak secondary attractions between polymer
molecules permit a high rate of thermal expansion, whereas the strong primary forces in
inorganic materials restrict them to a much lower rate of thermal expansion. For simple
extender fillers, the expansion rates of polymer and filler are simply additive, so the CTE
simply decreases in proportion to volume fraction of simple extender fillers (Fig. 5.6). Re-
inforcing fillers are more effective, and reinforcing fibers are most effective in reducing
thermal expansion, because they restrict the molecular motion of the polymer molecules.
This brings plastics closer to the performance of metals and ceramics.

Coefficient of Linear Thermal Expansion Reinforcing fiber Simple filler

Reinforcing filler

100% polymer 100% filler
Volume Fraction of Filler

FIGURE 5.6 Effect of fillers on coefficient of thermal expansion.

5.2.1.9 Heat Deflection Temperature. This is increased slightly in amorphous poly-
mers, because the fillers or fibers reduce the mobility of the polymer molecules. It may be
increased tremendously in crystalline polymers, because fillers and especially fibers raise
the plateau of the modulus versus temperature curve just enough to extend the pass/fail
limit of the standard test by hundreds of degrees (Fig. 5.7, Table 5.18). The practical sig-
nificance of this obviously depends on the judgment of the product designer.
5.2.1.10 Thermal Conductivity. The thermal conductivity of inorganic fillers and fibers
is higher than organic polymers, so adding them does increase conductivity in proportion
to volume fraction (Sec. 5.2.5.2).
5.2.1.11 Flame Retardance. Flame retardance is increased somewhat, because fillers
and fibers increase both viscosity and thermal conductivity (Secs. 5.2.5.3 and 5.7).
5.2.1.12 Dielectric Constant and Loss. These are much higher in highly polar inor-
ganic materials, so fillers and fibers generally increase them proportionally in plastics.

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