Heavy build designed for continuous operation at 105 °C, Formvar was one of the first magnet wire insulations based on synthetic materials. Today, Formvar is still widely used in oil-filled transformers as well as demanding cryogenic applications. Proven performance in oil-filled applications. - Rea Formvar heavy build is a film insulation composed of modified polyvinyl acetyl resins. Designed for continuous operation at 105 °C, Formvar was one of the first magnet wire insulations based on synthetic materials.