Parylene can be used outdoors. However, it has one drawback that could limit its suitability in some outdoor applications: sunlight can yellow it. 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.
The operating temperature range for parylene conformal coatings make them compatible with just about every part of the world. Parylene cannot withstand the heat found inside of an engine, but it can last at least ten years without any degradation at temperatures of up to 80 degrees Celsius. 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 degrees Celsius also means that it can perform in the most extreme winters on Earth in places like Antarctica or Alaska's North Slope.
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 not able 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 6500 psi.
The outside world is filled with electrical signals and parylene blocks them out. A 1 mil coating of parylene C -- the least effective dielectric -- can resist spikes of up to 6,600 volts. 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 get canceled out 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 the AF-4 variant, which is much more UV resistant. Unfortunately, AF-4 is 50X more expensive than other types of parylene which limits its use to only the most critical parts.
If using parylene is a must and the AF-4 variant does not fit your budget, the best way to manage the yellowing is to stop it from happening by protecting the parylene coating. We were able to increase the life span of LED strips by protecting the parylene with a special film. This process can be duplicated with success on other products as well.
Deploying parylene for outdoor applications carries a unique set of tradeoffs. It can also require strategies that go beyond simply depositing the coating. Working with a team of experts that understand the coating's characteristics and how to both maximize the positives and mitigate the negatives can lead to better outcomes.