TABLE OF CONTENTS Polyurethanes were among the first coatings to be used for printed wiring board electrical insulation and protection from moisture, salt spray, and handling. Polyurethanes are also used as potting compounds for connectors and as vibration-damping fillets for large components. A key attribute of most polyurethanes is the ability to rework defective components after a circuit assembly has been conformally coated. Polyurethanes can be softened and penetrated with a hot solder iron and residues may even act as a flux in resoldering a new component.
Almost all the early work and, in fact, most of the elucidation of the basic chemistry of the polyurethanes was performed by Bayer Corp. in Germany around 1937. Considerable work followed in the United States, primarily by DuPont, around 1940 resulting in a series of patents on the reaction products of polyisocyanates with various glycols, alkyd resins, polyamides, and polyesters. However, it was not until after World War II when much of the practical technology developed in Germany was transferred to the United States, that the commercialization and wide use of polyurethanes occurred.[31]
The urethane technology developed in Germany was based on a series of addition polymers formed from polyisocyanates called desmodurs and polyesters called desmophens. These polymers soon became popular and widely used as coatings because of their flexibility, toughness, excellent electrical insulation properties, and excellent moisture, abrasion, chemical, and corrosion resistance.
The parent compounds for all polyurethane coatings are the di- or polyisocyanates of which the most widely...
