Current Perspectives on Silicone Rubber Technology

Abstract This perspective embraced many factors that are important in understanding silicone elastomer technology and the reasons for the unique properties that they exhibit. The fact that they possess the lowest transition temperatures for any known polymer qualify them as a viscoelastic polymer that can be converted to an elastomer by crosslinking. Important also is the fact that the polymer molecules are predominantly linear prior to vulcanization, which allows high extensibility when the chemical crosslinks are in the desirable range of 100–400 mer units. Siloxane chemistry is uniquely different from hydrocarbon chemistry primarily because silicon is more electropositive than carbon, and the siloxane bonds possess more ionic character. Intermolecular forces for polydimethylsiloxane based elastomers are much lower than hydrocarbon polymer based elastomers. These polymers exhibit unusually high chain flexibility, easy rotation of pendant organic groups attached to silicon, and more free volume at ambient conditions than other polymers. These properties contribute to the lower Tg values and less change in properties (e.g., viscosity of polymers, elastomer modulus and strength) with temperature, compared with organic elastomers. Additional properties that contribute to the unique properties of silicone elastomers are low surface energy, excellent oxidative and thermal resistance formulatibility to provide excellent resistance to hydrolysis and rearrangement of siloxane bonds, easy modification of solubility characteristics by substitution of different organic groups on silicon, and the use of simple chemistry alterations that allow a range of molecular weights for making high and low consistency compositions that can be crosslinked either at elevated or room temperature conditions by a variety of reactions. The unusually high degree of reinforcement obtained with silica fillers compared with nonreinforced silicone polymers is a result of excellent polymer filler bonding which consists of largely physical bonds with some chemical bonds also being present after exposure to elevated temperature curing conditions. The combination of the above mentioned properties contribute to the fact that silicone elastomers have become important engineering materials that find hundreds of difficult-to-satisfy applications. Based on the excellent potential for continued technology advancement for silicone elastomers, it is believed that substantial future growth of this family of elastomers will occur.