Epoxy resins, polyurethanes, and silicone rubber are mainly used as potting compounds for electrical and electronic components. Each material group has its advantages and limitations.
Silicone rubber
Silicone rubber has superior temperature resistance and, at the same time, the best low-temperature properties. Furthermore, silicone rubber has a very low dielectric constant and a very low dielectric loss factor. This means that silicone rubber is not heated by, for example, microwaves. In the event of a fire, only a small amount of toxic gases is developed, and the ash, which consists of silicon dioxide, is electrically insulating, in contrast to the ash from polyurethane and epoxy.
Limitations for silicone rubber include a very high price, high water permeability, and relatively poor mechanical properties compared to epoxy and polyurethanes. The latter, however, does not always have to be a disadvantage; for example, it can facilitate the repair of encapsulated components.
Polyurethane and epoxy
The materials that compete most closely with each other are polyurethane and epoxy. Both have their advantages and limitations; which material is best depends entirely on the application.
Compared to epoxy, however, polyurethane has the following advantages:
- Greater range of variations.
- Lower heat generation during curing.
- Lower shrinkage during curing, resulting in lower shrinkage stresses.
- On average, better dielectric strength.
- Shorter curing time.
Epoxy, however, has lower water absorption, higher volume resistivity, and a lower dielectric constant than polyurethanes.
Important electrical properties
Important electrical properties include volume and surface resistivity, dielectric constant, dielectric strength, dielectric loss factor, arc resistance, and tracking resistance.
Other important properties for potting compounds include low flammability, insulation capacity after fire, water vapor permeability, corona resistance, ozone resistance, moisture absorption, aging resistance, and good mechanical properties. Self-extinguishing polyurethanes corresponding to UL 94 V0 can be obtained by mixing, for example, aluminum trihydrate into the casting compound.
Like mechanical properties, electrical properties change more or less with the surrounding environment, frequency, and aging.