||January 1, 2006
Silicone rubber has been used in a variety of fields such as medical instruments and implants, building mateials, electric and electronic parts and automotive parts due to its unique properties which include physiological inertness, durability, good thermal and chemical resistance and a high flexibility due to its low Tg. However, not all of silicone rubber's properties are desirable for every application. For instance, its low surface free energy makes it complicated to paint on/coat silicone rubber. Its low surface free energy, meaning a hydrophobic surface, also makes it unsuitable/unpleasant as an eyewear/contact lens material. During the last decades, extensive efforts have been given to the development of silicone hydrogel contact lenses--a soft contact lens type with high oxygen-permeability as well as hydrophilic properties. This paper states the approach to produce an interpenetrating polymer network (IPN). In this approach, cross-linked silicone is swollen with a mixture of hydrophilic monomer and liquid or supercritical carbon dioxide (scCO2), whereupon the monomer is polymerized and cross-linked. An IPN can be understood as a composite material in which the properties of the individual materials are combined. The silicone network will ensure a high oxygen-permeability, whereas the polymer network will ensure the hydrophilic properties.