When we use electronics in commercial settings, one of the major factors that must be taken into consideration is the different temperatures that the circuitry and wiring will experience. Not every electronic device will be used in a stable indoor working environment like an office space.
Aerospace, military, and even warehouse operations can experience extreme heat or cold, as well as varying levels of moisture and humidity. To keep these electronics operating within safe levels, flexible heaters will be adhered onto the device to combat the extreme temperatures and to deal with the moisture. Two of the most used materials for flexible heaters are silicone and polyimide.
Properties of Silicone and Polyimide Materials
When selecting materials for flexible heaters, there are several key characteristics and properties that are required. The material, of course, must be flexible enough to be placed on curved services and should bend at varying degrees. The materials must also be durable enough to maintain the curved and bent shapes for long periods of time, since in some cases the flexible heater will be adhered to electronics that are typically enclosed from the exterior environment. Other desired traits are that the materials should offer low outgassing in vacuum environments, provide optimal thermal transfer, and can resist moisture, fungus, radiation, chemicals, and ozone.
Both silicone and polyimide materials have most of the above properties and characteristics while offering unique advantages based on the specific applications. Silicone is a rubber that has been reinforced with fiberglass to offer increased abrasion resistance and dimensional stability. Polyimide, also called by its DuPont trademark name Kapton®, is a thin, polymer film.
Silicone flexible heaters are most often used for medium and larger applications when several watt densities are desired and when it will be adhered to the application using a vulcanization process. In addition, the thicker rubber provides added protection to working components.
Polyimide flexible heaters are ideal when the application requires thinner and lightweight materials in a smaller size. It is also very resistant to most acids, solvents, and bases.
Temperature Considerations for Flexible Heaters
Extreme heat and coldness in working environments are hurdles that manufacturers must address when selecting the right materials for the flexible heaters. Extremely cold temperatures can cause materials to become brittle and crack, which can impact the operations of the wire-wound or etched foil elements. When selecting a flexible material for electronics that must operate in these conditions, the material should be rugged enough to handle the cold while allowing for even thermal transfer throughout the heater's surface. The material should also stand up to moisture that may be found in certain cold environments, such as condensation that occurs inside airplane instrument panels, cold storage warehouses, and climate regions with an abundance of rain, snow, or ice.
Extremely high temperatures can also cause issues with flexible heaters. One question that is often posed is: “Why would a flexible heater be used in such hot conditions?” Flexible heaters may have to be used in electronic devices that must maintain a certain temperature for the entire duration of operation. The heater can deal with cold spots that may form by providing the right continuous temperature at the optimal level. Also, moisture may be present in the hot environment as the flexible heater provides a "dry heat" that can evaporate condensation from electronic components.
Low Temperature Ranges Tolerated by Flexible Heaters
Both silicone and polyimide can be used in colder environments. However, polyimide is better suited for extremely cold environments when requiring smaller heaters.
Silicone Materials at Low Temperatures
Silicone flexible heaters have a minimum operating temperature of -70°F to -80°F. Any temperature lower than this will cause the heater to malfunction.
In addition, silicone is also a thicker material than polyimide. It can only go down to a thinness of 0.056" when possessing a wire wound element and 0.030" for etched foil elements. As more materials are present, this circumstance increases the amount of insulation between the flexible heater and the application's surface where the electronics are housed. So, limiting this type of heater in colder environments below -80°F is recommended.
Example of silicone materials used in flexible heaters.
One thing to keep in mind when deciding on a flexible heater in a cold environment is the size of the heater that is required. If temperatures will not go below -80°F as the surface space is 36" x 44" or more, silicone flexible heaters with wire wound elements can be used. Etched foil elements can only be crafted into sizes as large as 10" x 70."
Fast Facts: When to Use Silicone Materials at Cold Temperatures
- In cold environments that go down to -70°F to -80°F.
- When requiring medium to large heater sizes.
- If the flexible heaters will require wire wound elements.
- When operations will cause vibrations or abrasions as silicone has excellent stretching and tearing resistance.
Polyimide Materials at Low Temperatures
Polyimide heaters can have a minimum operating temperature of -200°F to -320°F. They are the better choice in subzero environments where they may experience chemical exposure. These heaters also have fast warmups, as their thinness of as little as 0.007" allows for greater thermal contact along the surface for excellent heat transfer.
Example of polyimide flexible heater used in electronic devices.
This type of heater material only uses etched foil resistance elements. So, if the application requires wire would elements, then silicone flexible heaters should be used. There are also size limitations that manufacturers need to be aware about. A polyimide flexible heater can only go up to a size of 10" x 70."
A polyimide heater is an excellent choice for electronics used in satellites and spacecraft. When a low, steady temperature is required, these heaters can handle the work.
Fast Facts: When to Use Polyimide Heaters in Cold Environments
- When minimum operating temperatures are -200°F to -320°F.
- If the application needs a thin flexible heater that won't place on added weight.
- For smaller application sizes of 10" x 70" or lower.
- When using etched foil resistance elements.
High Temperature Ranges Tolerated by Flexible Heaters
On the other end of the spectrum, high temperatures are serious problems with electronics in many industries. Hot outdoor environments can cause electronics to malfunction when they are not protected. While polyimide can be used in these environments, silicone is the clear winner when it comes to tolerating maximum operating temperatures.
Silicone Materials at High Temperatures
Silicone's extra thickness and reinforced fiberglass allows it to have a maximum operating temperature between 400°F to 500°F. It also provides that extra insulation to protect electronics from working components that might cause extra wear and tear caused by abrasion. This material can also absorb thermal and mechanical shocks.
Some high-temperature environments will have high humidity and moisture that is present. In these cases, silicone rubber is the ideal material as it provides a moisture barrier for the electronics. Off-gassing is also another factor that occurs in environments with high temperatures, as the heat can break down the material's properties. Silicone has minimum off-gassing characteristics.
Fast Facts: When to Use Silicone Materials at Hot Temperatures
- For temperatures that can reach as high as 400°F to 500°F.
- When requiring a moisture barrier.
- If off-gassing is an issue, especially in vacuum environments.
Polyimide Materials at High Temperatures
While polyimide materials in flexible heaters are excellent when used in cold temperatures, they are not ideal for high temperatures. The maximum operating temperature for polyimide is from 200°F to 392°F. Higher temperatures will cause the flexible heater to become damaged.
When considering which materials to have for your flexible heater in a hot environment, also take into consideration the material that the flexible heater will be attached on to when operating. If the application's material will become damaged or experience changes to its mechanical or physical properties, then selecting a flexible heater that provides lower operating temperatures may be required.
Fast Facts: When to Use Polyimide Materials in Hot Temperatures
- If outdoor or exterior temperatures do not exceed 392°F.
- When the application's material negatively changes at higher operating temperatures.
- For vacuum environments as it provides low out-gassing.
- When looking for twice the watt density that can't be provided by wire wound elements.
Flexible Heaters Are Ideal for Electronic Applications
Applications that require localized heating to ensure electronics and other components function properly can find immense benefits with attached flexible heaters. Take into consideration the minimum and maximum operating temperatures of the environment as well as the device's operations to select the right materials that can do the work.