Can Parylene be used in direct sunlight (UV)?

Parylene can be used outdoors, however, most Parylene variants have one drawback that could limit its suitability in some outdoor applications: sunlight can cause it to turn yellow. With this in mind, product designers specifying a coating for a product to be used where it will be subject to sunlight should carefully consider the coatings pros and cons before specifying it. Frequently, but not always, it remains the best choice.

Parylene and the Great Outdoors

Given it impermeability, Parylene can be an excellent choice for protecting items to be used outdoors. Most of the factors that go into its list of capabilities dovetail nicely with the conditions found outside.

Operating Temperatures

The operating temperature range for Parylene conformal coatings make them compatible with just about every part of the world. Parylene can last at least ten years without any degradation at temperatures of up to 80°C. This means that it will perform in even very hot areas — like Dubai or Death Valley. Its ability to withstand extremely cold temperatures as low as -200°C also means that it can perform in the most extreme winters on Earth, in places like Antarctica or Alaska’s North Slope.

Chemical Resistance

Parylene coated items are able to withstand most of the chemicals that the outdoors can throw at them. It is almost completely impervious to water, resisting 99.94 percent of water after 24 hours — at worst. The coating is also impervious to both the natural gases that exist in the atmosphere — like nitrogen and oxygen — as well as to common pollutants like sulfur dioxide or chlorine gas. Acids, bases and solvents are also unable to penetrate its coating.

Knocks, Scrapes and Bumps

No other coating can offer the same degree of impact resistance as Parylene can at the same thickness. A 3-mil coating of Parylene can resist an impact of over 85 kg per cm. To match this strength, epoxy has to be 375 mils thick — over 100 times thicker. A 250 mil urethane coating resists just 74 kg-cm. Parylene can also handle stretching, thanks to its tensile strength rating of at least 7000 psi.

Dielectric Protection

The outside world is filled with electrical signals and Parylene blocks them out. A 1 mil coating of Parylene C has a dielectric strength of 5,600 volts at 1 mil (0.001”) coating thickness. It also has almost the same dielectric capability regardless of whether it is blocking a 60 hertz AC signal or a 1,000,000 hertz signal from an electronic device. Parylene N has almost no frequency-related performance degradation.

Managing Parylene Yellowing

All of its benefits for outdoor use can be negated by its tendency to yellow in the presence of UV radiation. Interestingly, while it yellows due to UV, it also filters it out, protecting the coated item. Nevertheless, in outdoor situations, this can be a significant challenge when the Parylene coated item is going to be visible. In other words, the yellowing might not matter if it is used to coat a PC board that sits inside of a case, but it could be a deal breaker if it gets used to coat LEDs in outdoor displays.

Different types of Parylene have different abilities to withstand UV. Both the C and N variants are UV-sensitive, although the N is much more sensitive than C since oxygen is better able to penetrate its surface, offering an additional chemical to help in the process of breaking down and yellowing the coating. One solution is to use Parylene HT^®, which is much more UV resistant. Parylene HT has passed a 2,000 hour accelerated UV test, which correlates to ~100,000 hours of outdoor UV exposure.
Deploying Parylene for outdoor applications can be very beneficial for products in a wide range of environments. Working with a team of experts that understand the coating’s characteristics and how to maximize protection for customer designs can lead to increased product quality and longevity in use.