Formvar is a synthetic enamel compound of polyvinyl acetal and phenolic resins.
Formvar's hot oil resistance sets it apart from all other magnet wire insulations. The flexibility, adhesion and abrasion scrape resistance of Formvar film is excellent. Its electrical properties are very good and it is resistant to common solvents. Formvar continues to be the recommended film insulation most commonly used in oil-filled transformers.
It has been in general use in the electrical industry as a Class 105°C wire for over fifty years.