Calcination occurs in two steps.
At 700°C the dehydroxylation of the kaolin is complete forming a poorly crystalline metakaolin. Products produced at this temperature include PoleStar™ 450 which can improve the dielectric properties of PVC cable insulation, and MetaStar™ which acts as a pozzolanic concrete additive.
At 980°C an amorphous defect spinel is formed which undergoes a transformation, with mullite recrystallising in an amorphous glass at temperatures above 1100°C. Products produced within this temperature range provide excellent properties in rubber compounds, especially when coated with silane, improving mechanical properties and chemical resistance. Silane treatment further enhances rigidity, toughness and dimensional stability in polyamide mouldings.