Coatings based on polyvinyl formal (Formvar) were first introduced in 1938 as magnet wire insulation, but remain in wide use today. The base polymer is synthesized by partially hydrolyzing polyvinyl acetate and then reacting the free hydroxyl groups of the resulting product with formaldehyde to form the six-membered formal structure. The polymer, therefore, contains three basic functional units: vinyl formal, vinyl alcohol, and vinyl acetate, as in Fig. 4.29.
Polyvinyl formal is thermally stable for extended periods of time at 110°C; its thermal endurance curve is shown in Fig. 4.30. Used alone, Formvar has marginal solvent and abrasion resistance, but its properties are greatly improved by modifying the basic resin through reaction with phenol formaldehyde or cresol formaldehyde resins. Both resins condense with the free hydroxyl groups of the polyvinyl formal to produce coatings that are extremely tough and flexible and that possess good abrasion and solvent resistance. Polyvinyl formal can also be modified with resins such as melamine, urea, epoxy, and urethanes.[59]
The modifications with urethanes are especially favored because of their solder-through properties. Dip-tinning or soldering at high temperatures (up to 350°C) may easily be performed without having to previously strip off the insulation. Three-component coatings consisting of polyvinyl formal, polyurethane, and polyester are also easy to solder through and, in addition, have good flow, flexibility, heat shock properties, and good resistance to solvents and abrasion.[60] In fact, practically all the solderable magnet wire coatings on the market contain urethane modifications.