Coefficient of Thermal Expansion
Coefficient of thermal expansion (CTE) simply means the amount a material expands (or contracts) when heated. Polymers generally have high CTE and metals have much lower values. The coefficients of thermal expansion of most mineral fillers are also considerably less than those of polymers. Thus, 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. On the other hand, high-aspect-ratio fillers, especially when aligned by processing, will often give rise to anisotropic shrinkage, leading to problems of warpage. Some specialty fillers have been designed to have very low coefficients of expansion (e.g., some glass ceramics). Negative coefficients are also possible (e.g., zirconium tungstate and some zeolites).
Particulate fillers need to be stable at the temperature reached during processing of the composites; this can be as high as 350 °C. While most fillers are stable at much higher temperatures, some can release water below this temperature, which can cause problems such as porosity in extrusions and moldings. This water can also come from impurities. Some polymers, notably condensation types, such as polyesters and polyamides are very susceptible to hydrolytic degradation during high temperature processing, and in such cases it is prudent to dry any fillers just before use.
Flame retardant fillers are a notable exception, as their effectiveness depends on an endothermic decomposition, accompanied by water release, and this needs to occur at or close to the temperature at which the polymer itself begins to decompose and release flammable gasses. These fillers decompose at temperatures in the range 200-350 °C, and therefore, processing conditions need to be carefully controlled.