siloxanen

Siloxanen, also known as siloxanes, are a family of organosilicon compounds characterized by repeating silicon–oxygen–silicon linkages with organic groups attached to silicon. They range from small cyclic oligomers to high-molecular-weight linear polymers. The most familiar member is polydimethylsiloxane (PDMS), a flexible polymer widely used as a lubricant, hydraulic fluid, and in cosmetics and medical devices.

Structure and types: Linear siloxanes form polysiloxane chains such as HO–[SiR2–O]n–SiR2–OH, where R is typically methyl or other hydrocarbon groups. Cyclic siloxanes consist of Si–O rings of defined size, commonly denoted as Dn (for example D4, D5). The properties of siloxanes depend on chain length, branching, and the nature of the R groups.

Synthesis: Siloxanes are produced by hydrolysis and condensation of chlorosilanes or alkoxysilanes, or by controlled polymerization and equilibration of siloxane monomers. Industrial routes include the condensation of silanols and purification to achieve the desired molecular weight and cyclic versus linear balance.

Properties: Siloxanes generally exhibit very low surface energy, high thermal stability, chemical inertness, and good flexibility. Linear siloxanes (polysiloxanes) are typically viscous liquids or elastomeric fluids with low glass-transition temperatures, while cyclic siloxanes have defined volatility and specific ring sizes. They are often immiscible with water and have excellent dielectric and lubricating properties.

Applications: Siloxanes are used as lubricants and fluids, elastomers and sealants, cosmetics and personal care ingredients, medical devices and implants, coatings, and anti-foaming agents. PDMS, in particular, is prized for biocompatibility and versatility.

Safety and environment: PDMS is generally considered low-toxicity and biocompatible. Some cyclic siloxanes have raised environmental concerns due to persistence and potential bioaccumulation, leading to regulatory scrutiny that varies by region and application.