siloxanes

Siloxanes are a class of organosilicon compounds characterized by a silicon-oxygen backbone. They span from low-molecular-weight cyclics to high-molecular-weight polymers known as silicones. The repeating Si-O unit with organic substituents attached to silicon gives diverse physical properties.

In polymers, the general formula is (R2SiO)n, where R is an organic group such as methyl or phenyl. The most common polymer is poly(dimethylsiloxane), PDMS, consisting of repeating units [Si(CH3)2O]. The backbone Si–O bonds confer flexibility; cyclic siloxanes (for example D4, D5) and branched or linear siloxanes vary by ring size or substitution.

Siloxanes are typically highly thermally stable, chemically inert, have low surface energy, and display a wide range of viscosities from fluids to elastomers. They are produced by hydrolysis and condensation of chlorosilanes or related precursors, followed by polymerization and equilibration to give linear or crosslinked materials.

They are widely used as lubricants, silicone oils, elastomers, sealants, adhesives, cosmetics ingredients, medical implants, and in electronics and optics due to electrical insulation and optical clarity.

Health and environmental aspects vary. PDMS itself is considered relatively inert, but some cyclic siloxanes have raised concerns about environmental persistence and aquatic toxicity. Regulatory scrutiny exists in cosmetics and industrial applications, with various restrictions or phase-outs in some regions.