In the range of visible wavelengths, parylene displays relatively low absorption and is the coating having the lowest known rate of distortion amongst the existing coatings technologies. Up to a wavelength of 700 nm parylene has high optical transparency and is without color. In contrast, at wavelengths less than 280 nm (UV radiation).
Parylene has very high resistance to UV radiation, and consequently it is in widespread use for applications on Mars and in other space destintations. However, when parylene is exposed to UV radiation in the presence of oxygen/air or argon, its resistance becomes weaker over time. In a test conducted it was discovered that after 300 hours of exposure parylene loses its transparency and a large part of its flexibility.
It should be noted that exposure of a parylene coating (regardless of the type of parylene) to UV radiation in a vacuum, for example, a vacuum or an inert gas, does not cause any damage whatsoever to the coating, even after hundreds of hours.
We have tables that indicate the decrease in the strength of the tearing force, elongation, and opaqueness, that passes through the parylene when exposed to UV radiation.
Within the range of wavelengths suitable for Gamma rays, all types of parylene display high resistance to exposure to x-rays under vacuum conditions. Parylene coatings that have been exposed to 1000 kGy of radiation at a rate of 16 kGy/hr did not show any change in tensile strength and in their electrical characteristics.
Transparency to RF frequencies
Various applications conduct information transmissions at RF frequencies, including various medical applications of invasive devices that transmit information from inside the body to various kinds of data receptors (camera pills, for example) or in devices in the military industry that are sent for large distances and transmit at encoded RF frequencies. In devices of this kind the developer is required to supply a variety of characteristics integrated in a single coating. The component must be inert to the acids and liquids found in the body, transparent to visible light frequencies/ magnetic resonance frequencies, and have a suitable friction coefficient approved by the FDA, or, alternatively, to be proof against liquids, resistant to tin whiskers damage over the years, possess a high dialectric coefficient at low thicknesses and the capability of resistance to the pressure changes involved in climbing to heights, and also to permit RF transmissions − and parylene excels in all these exactly, and in its capability of providing a multi layer solution to varied requirements.
In the civilian industry also, exposure to conditions of humidity is very widespread. In devices that are required for complex electrical functions and the transmission of information under conditions of humidity, parylene is the most recommended polymeric coating.
In the presence of an alternative coating that combines these different characteristics, it is reasonable that the coating will obligate impossible thicknesses for the application.
Parylene has been tested after being applied with different thicknesses to RF transmitters of various intensities, and as expected it has been found to be the only one amongst the types of polymeric conformal coatings that does not cause nor introduce distortions during the transmission and reception of RF, even when these are done at low power. This behavior has been tested up to a frequency of 55 GB without the entry of material distortions.
