WATER-ABSORPTION, SWELLING, RUPTURE AND SALT RELEASE IN SALT SILICONE-RUBBER COMPOUNDS

Water vapour diffuses rapidly in silicone polydimethylsiloxane (PDMS) rubber. In a PDMS salt compound surrounded by water, the water which has entered the rubber is absorbed by the salt particles, creating an osmotic pressure in the rubber. This pressure leads to internal cracks in the rubber, which eventually percolate and result in a salt exchange between the rubber-salt compound and the surrounding pure water. The absorption and rupture phenomena were derived, measured, and quantitatively analysed. Equations describing the onset and propagation of the microcracks in the silicone were derived as a function of rubber fracture toughness, tensile modulus and osmotic pressure of the salt. A single figure summarizes three possible situations: (1) the salt plus water pockets in the compound reach an equilibrium without cracking, (2) the salt pockets are fully dissolved, and cracking occurs, (3) the salt pockets generate microcracks before full dissolution of the salt. Finally, the salt release has been quantitatively related to the PDMS tensile modulus, fracture surface energy, and salt grain size,