The principal thermal properties of importance in particulate-filled polymer composites are specific heat, thermal conductivity, coefficient of expansion, and stability. Thermal properties generally vary smoothly with the volume percentage of filler added and do not show the marked percolation effect seen with electrical conductivity (see next section).

Specific Heat

Specific heat capacity is the amount of energy needed to heat a given amount of material. Most books erroneously state that fillers help in this regard by having lower specific heat capacity than polymers. The misunderstanding comes from the units used. The specific heat capacity per unit weight of polymers is indeed ~3 times higher than that for minerals. However, the density of minerals is also ~3 x higher. So it turns out that the specific heat capacity per unit volume is the same, and fillers do not help lower the amount of energy needed to heat and cool plastics. In fact, as a rule of thumb, it is known that the volume-specific heat capacity of all solid materials is about the same.