Curative Agent Effects on Fluoroelastomer Products
The type of curative or vulcanization agent used will also influence the physical properties and eventual life expectancy of the fluorocarbon seal in service. Here again, three basic chemical families are used to effect vulcanization of the polymers; the diamines (one type imparts a characteristic cinnamon odor); the bisphenol or contained curative system; and peroxides. When the elastomer was first introduced, the only system available was the diamine curatives. The compression set properties were not very good on these materials, typically from 30 to 50%. The bisphenol cure system was marketed to overcome this deficiency and resulted in very low compression set values (less than 20%), making the elastomer attractive for critical o-rings and other high temperature seal applications. Peroxide cures became possible with the introduction of the GF family of polymers, and together they provide the highest chemical resistance properties available.
The curative chosen must work in combination with an appropriate metallic oxide or “acid acceptor” in order for vulcanization to proceed. The metallic oxide reacts with and neutralizes any acidic byproducts generated during the vulcanization process. The most commonly used metallic oxides are magnesium oxide, calcium oxide, zinc oxide, calcium hydroxide and lead oxide. If the rubber gasket is required to seal against aggressive acids such as nitric, acetic, hydrofluoric or hydrochloric acid, the lead oxide acid acceptor is preferred, due to the fact that it forms an insoluble salt after vulcanization is complete. This salt will not further absorb acid or moisture and thus contribute to degradation of the seal. Most fluoroelastomer compounds will resist chemical attack by sulfuric acid, even without special compounding, and are commonly used to seal against this corrosive chemical.
By discussing with our customers, exactly what service conditions the fluorocarbon seal will need to resist, we can recommend the best available polymer formulation. Mosites Rubber Company stocks a full range of sheet materials, in various thicknesses and widths. We will also work with our customers to develop custom formulations if necessary.