solutions of natural or artificial bitumens in organic solvents. Bituminous lacquers are made from natural bitumens, which contain a minimum amount of ash and sulfur (asphaltites), or from artificial bitumens obtained during the chemical refining of petroleum. Turpentine, white spirit, solvent, xylol, and so on are used as solvents for bituminous lacquers. Bituminous lacquers are prepared by fusing bitumens with natural resins (for example, rosin) or with so-called artificial copals (phenol-formaldehyde resins modified by rosin) at a temperature of 280° C. The solvents are then introduced into the cooled (170°C) mixture. In producing oily bituminous lacquers, drying oils (such as linseed or tung oil), as well as siccatives (lead, manganese, or cobalt salts of fatty acids), are also added.
Depending on the purpose and conditions of use, bituminous lacquers can be altered significantly to obtain lacquers of varying viscosity, covering quality, and drying properties (from 0.3 to 3 hours). A serious defect of bituminous lacquers is that they thicken when exposed to the oxygen in the air; this is usually eliminated by putting a small amount of turpentine in the lacquer before use.
Bituminous lacquers are applied to objects being covered by methods that are the same for all lacquers (for example, with a brush or by spraying). Oily bituminous lacquers form a nonconvertible (insoluble) coating; those without oil are convertible (soluble). Solutions prepared with bituminous lacquers have good resistance to air, acid, and water and have high electrical insulation properties. Their defects are a “rash” (hard particles on the surface of the coating) and “pockmarks” (circular bare spots that break out on the painted surface).
The use of bituminous lacquers has declined greatly in connection with the development of production of new lacquer materials. However, in a number of industrial sectors, such as electric-motor construction, bituminous lacquers are still important, since they can be relied on as both anticorrosion protection and electrical insulation at low cost.