Parylene Conformal Coatings and UV Light

Parylene has numerous outdoor applications. However, a major drawback of most Parylene types is limited resistance to direct contact with UV radiation. Daylight is the most common source of UV light. Prolonged exposure to its high energy radiation can cause objects extensive surface damage and lead to eventual malfunction of coated products.

This is a significant drawback since the objective of conformal coatings is the sustain long-term function. While Parylene is generally fine when not directly exposed to UV – as in the many cases where components are internally situated within a product – the optically clear films can yellow and degrade from prolonged contact with sunlight.

Limited UV Protection Provided by Most Parylenes

This is not the case for all types of radiation, where Parylene’s coating properties are maintained. For instance, in a vacuum, Parylene’s radiation resistance to gamma-ray degradation is consistently impressive. Parylenes C, D, N and Parylene HT^® retain electrical and tensile properties at dosages of 1,000 kilograys (kGy), through a dose rate of 16 kGy/hr. However, outside a vacuum and exposed to air, rapid embrittlement develops. Nevertheless, the mentioned Parylene types do generate rather reliable gamma ray protection in a vacuum.

The same cannot be said for exposure to UV light (direct sunlight). Much has to do with the substances’ chemical composition:

• Parylene N is an unsubstituted hydrocarbon molecule.
• Parylene C has one added chlorine group per repeat unit.
• Parylene D has two added chlorine groups per repeat unit.
• In contrast, Parylene HT replaces the hydrogen atoms on the chemical’s benzene ring with fluorine atoms, significantly enhancing its UV-stability.

Diverse UV Protection, According to Parylene Properties

The hydrocarbon dimer Parylene types have different properties. Although it has the best surface-permeating capacities of the Parylenes, type N is especially susceptible to UV-generated damage. It has a significantly higher oxygen permeability than Parylene C, whose composition has an added chlorine atom; Parylene D adds a second chlorine atom. Nevertheless, the superior impermeability of types C and D declines markedly with UV exposure. Oxygen in UV light causes the coating to decompose into aldehydes and carboxylic acids near the conformal film’s surface, yellowing them and reducing their barrier protection, endangering component function.

Parylene HT

Parylene HT is the only Parylene type that displays a consistent degree of UV resistance, one wherein performance is maintained relatively long-term. Like the other types, Parylene HT also forms an effective, structurally continuous protective film as thin as several hundred angstroms, due to the uniqueness of the chemical vapor deposition (CVD) process used to coat substrate surfaces.

Also called aliphatic fluorinate, Parylene HT replaces the alpha hydrogen atom of the N dimer with fluorine. This compositional variation allows Parylene HT to generate the lowest coefficient of friction and dielectric constant throughout UV exposure, and the highest penetrating ability among the variants. Parylene HT is also useful at high temperatures, protecting applications in environments up to 450°C.

Thus, while Parylenes C, D, and N provide dependably stable conformal coatings indoors, degradation commences shortly after exposure to UV light. Only Parylene HT displays longer-term and consistent resistance to UV light.