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Thermal Expansion

Polymers generally have higher coefficients of thermal expansion (CTE) than most mineral fillers and so, mineral incorporation can significantly reduce the coefficient of expansion of a composite material. This effect is usually beneficial, reducing shrinkage when a part cools after molding and making it easier to match polymers to other materials such as metals in composite structures.

Isotropic particles are best for this, as they produce a uniform effect. Some specialty fillers have been designed to have very low coefficients of thermal expansion (e.g., some glass ceramics). Negative coefficients are also possible (e.g., zirconium tungstate and some zeolites), and these are used for special applications.

Thermal Conductivity

There is a separate chapter on thermally conductive additives, so this is only briefly covered here.

Thermal conductivity is an important topic for some thermoset applications, notably printed circuit boards. These are a significant market for thermoset polymers and require high thermal conductivity to remove heat. As shown in Table 2, metals have the highest thermal conductivity among common materials, but they also have high electrical conductivity, which eliminates them for printed circuit boards. Common mineral fillers, such as calcium carbonates, can significantly improve the performance of polymers, but fillers like aluminum oxide are far superior and widely used for this purpose.

Specialty fillers, such as some naturally occurring aluminosilicates, are used when high thermal conductivity is paramount, for example, in heat sinks for laptop

Table 2 The thermal conductivity of various common materials (the measurement of this property is quite complex and so approximate values only are given here)

Material

Thermal conductivity W/(m K) at 273 K

Common metals

80-400

Aluminum oxide (Corundum)

30

Magnesia

30

Some naturally occurring aluminosilicates

14

Calcium carbonates

~5

Glass

~1

Water

0.6

Polymers

0.2-0.4

Air

0.024

computers where enormous amounts of heat must be managed in a confined space. One would imagine that the higher the thermal conductivity of the filler, the more effective it would be at increasing the thermal conductivity of the composite. This, however, turns out not to be the only factor; the elastic modulus of a composite also has a significant effect, and fillers that significantly increase this can perform better than expected (Weidenfeller et al. 2012).

 
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