Swellable elastomeric HNBR–MgO composite: Magnesium oxide as a novel swelling and reinforcement filler

Abstract In this paper, we introduce a novel reactive rubber composite made by compounding magnesium oxide (MgO) powder with hydrogenated nitrile butadiene rubber (HNBR). This HNBR–MgO composite system initially looks and behaves like rubber, but exposure to water causes it to swell and stiffen. Compared with conventional swellable materials, which lose stiffness significantly upon swelling, the sealing capacity of these novel reactive composites improves significantly with their improved stiffness. Three mixing ratios of HNBR and MgO were examined in this study, and their properties upon reaching equilibrium in water of 82 °C were reported. The elastic modulus value tripled, reaching 80 MPa, while doubling in volume for the rubber filled with 40% by volume of MgO. After drying, modulus of this particular composite increased even further to almost 200 MPa while the volume expansion was largely retained (shrinkage of approximately 10%). In this paper, we will show that the increase in elastic modulus and volume increase are related to the reaction of MgO with water to form magnesium hydroxide, absorbing water molecules into the composite and chemically reacting with it in the process.