Although calcium sulfate dihydrate, commonly referred to as gypsum, has been utilized for some time in fiberglass reinforced polyester (FRP) applications, it does not have the fire retardant filler recognition of ATH. Gypsum is more commonly known for application in cement, soil and water treatment, plaster, and wall-board. Inherent characteristics of gypsum limit its use to low temperature and, until recently, non-aqueous applications.

The cost effectiveness of gypsum over ATH and magnesium hydroxide has been the driving force behind the technical and commercial success in these markets. Characteristics such as lower water content, free moisture content, and water solubility have typically required formulation modification for the application of this hydrated filler at a cost not much higher than calcium carbonate.

Gypsum is a hydrated mineral, which as a fire retardant filler functions as do ATH and magnesium hydroxide (brucite), serving as a heat sink and diluting combustion gases with released molecular water. Although all three are hydrated fillers, they differ with respect to water content and Initial water evolution temperature.

One report cites several reasons for the increased viscosity, including the hydroscopic nature of gypsum, which may introduce more free moisture into the resin compound. Our experience agrees with the findings reported in the Owens-Corning paper with respect to viscosity and that this can and has been compensated for through modification of the compound formulation.

The 50% filler loading using 40% gypsum would be approximately equal to a 25% loading of ATH in conjunction with a 25% loading of calcium carbonate on a water-equivalent basis. Based on water equivalency, with current market prices, the gypsum/calcium carbonate blend represents a considerable savings compared to the ATH/calcium carbonate blend.