Gaskets with Expandable Graphite Treated with Nitric, Sulphuric, Phosphoric Acids and Ferric Chloride

Abstract

The task was to find out the most appropriate expandable graphite intercalation compound GIC for intumescent seals. The application in fire seals required GICs, which exerted high expansion at low burning rates. Therefore graphite was treated with nitric, sulphuric, phosphoric acids and ferric chloride. Phosphoric acid with its flame retardant properties was the favourite, but graphite treated with it, did not exfoliate, therefore expandable graphite was post treated with phosphoric acid. Elemental analysis and thermogravimetric measurements led to chemical formulas of GICs. Their heats of exfoliation were determined by differential scanning calorimetry, by the work done given by the product of increased specific volume and atmospheric pressure, by the Arrhenius plot of exfoliated specific volume dependent on temperature and were calculated in a complete balance of weights and heats of formation. It turned out that the sum of heats of intercalation and exfoliation corresponded with the heats of decomposition and gasification of the intercalated compounds. When the heat of intercalation was added to the lattice energy of graphite, the lattice energy of GIC was obtained. Electron microscopy indicated that expansion did not happen in monolayers but in nanoplatelets consisting of about 40 atomic layers. The expandable GICs were incorporated into polyvinyl acetate strips and applied as fire gaskets in gaps. In a fire test, the gap protected by the strip comprising the GIC treated with sulphuric and phosphoric acid showed the best performance, which corresponded with the highest observed expansion and with the second highest temperature of maximum speed of combustion.