Polyurethane coatings that are used as wire enamels are generally the reaction products of hydroxylated polyesters or hydroxylated polyethers with tolylene diisocyanate. Admixing these components, however, results in very rapid polymerization—too rapid to be of any practical use. For this reason, polyesters containing excess isocyan-ate groups are first reacted with phenol to yield adducts that are relatively stable at room temperature. After coating the wire and baking at elevated temperatures, the phenol portion of the adduct is released and the resulting free isocyanate groups then react with the hydroxyl groups of the polyester to form the desired polyurethane coating.
This is the ASTM Type 3 polyurethane previously discussed in Ch. 2.
Other polyurethane wire enamels consist of reaction products of
isocyanates with Formvar and, as mentioned in the previous paragraph,
offer good solderability. In fact, the largest share of the solderable
insulation market consists of polyurethane
systems.[61]