What chemical components are present in transformer oil?

Most transformer oils are refined hydrocarbons, mainly paraffinic and naphthenic mineral oil fractions, sometimes with aromatic content kept within limits. Many products include oxidation inhibitors such as DBPC, metal passivators to reduce corrosive sulfur effects, and antifoam agents. Natural and synthetic ester oils are based on ester molecules rather than mineral hydrocarbons. Silicone transformer fluids are based on polydimethylsiloxane polymers. Exact composition is typically described as proprietary in SDS, but the chemical family and additive function are disclosed.