Inorganic (SR)

Co-Authors and contributors

  • Dr Lee Hitchens, Nexus

Silicone (inorganic) conformal coatings can be very different to the organic coatings.

For example, they tend to have a very wide temperature range of operation compared to the other coatings. This range can be -55°C to +200°C (-67°F to +392°F).

Also, like the organic coatings they have good moisture protection and good chemical resistance to polar solvents.

Curing of silicones occurs through several different mechanisms, depending on the conformal coating, including RTV (Room Temperature Vulcanisation), Heat, UV, Moisture / Condensation and Catalysed Cure.

Some silicone coatings are solvent based, to provide the right viscosity for application. Others are solvent-free, non-volatile chemistries that offer the VOC free alternatives required for some industries.

In production they can be difficult materials to use due to the cure mechanism being difficult to control. Good housekeeping can minimise these effects.

SR coatings can also require different coating equipment or options compared to the organic coatings. This should be considered as part of the wholistic approach.

SR coatings are generally applied at 2-3 times the thickness dry film as organic coatings. This can lead to increased costs but better water repellency properties

The Chemical Structure of a Silicone Conformal Coating

Silicone (SR) is a polymer in which atoms of silicon and oxygen alternate in a chain. Similar to the Organic conformal coatings it is made up of two principal components: the vehicle and a colourant.

The Vehicle

Again, the vehicle is always there. It is made up of the film-forming materials and various other ingredients, which divide into the monomer, solvents, additives and fillers.

The Colourant

In 99% of cases the colourant is generally UV activated. It is not a visible colour in daylight, like a paint in most cases, but visible only under a UV wavelength light. However, there are cases where the colourant is not there.