From controlling condensation on electronics to keeping food warm, flexible heaters provide many advantages to commercial industries such as medical, electronics, aerospace, food and beverage, and refrigeration. Silicone and Kapton® heaters are the most common heaters used due to their flexibility, great thermal transfer, rapid warmup and varying temperature applications.
Selecting the right type of heater is based on several factors including the type of material that can be used, how it will be adhered to the product, the wattage and heat properties, and the size of the heater. Yet manufacturers should know the challenges and limitations of these two heaters to avoid equipment failure or product issues. Certain materials are susceptible to certain acids that are present in the environment or cannot be designed with the appropriate bend radius for applications where there are irregular curves.
Flexible Heater Overview
Before getting into the limitations and challenges experienced when using silicon and Kapton® heaters, let's give a brief overview regarding these heaters to note their similarities and differences.
Example of a silicone rubber heater (top) and a polyimide heater (bottom).
Silicon heaters are made with layers of silicon rubber with either an etched foil or wound wire resistance element. They are commonly used for applications where low to medium heat temperatures are necessary. These flexible heaters can be attached using a pressure sensitive adhesive, factory vulcanization, silicon room temperature vulcanization (RTV), or mechanical attachments. A silicon heater can reach intermittent maximum operating temperatures of 400°F to 500°F or continuous temperature of 392°F.
Kapton® (Polyimide) heaters are made from a slim, organic polymer film with etched foil resistance element. These heaters may be used for semiconductor wafer processing, aerospace instrument heating, vacuum chambers, and medical instruction and imaging. A Kapton® heater can reach a continuous maximum operating temperature of 392°F. They are often used for low temperature applications where quick thermal heat response times are required. This flexible heater can be attached using pressure sensitive adhesive, self-fusing tapes or mechanical attachments.
Silicon and Kapton® Flex Heater Challenges
The types of materials and resistance elements used in flexible heaters can create size limitations in how large the heater can be, as well as the thinness of the flex heater. These size limitations are extremely important to manufacturers who may need smaller heaters for semiconductor wafer processing, large heaters for thick food and drum tanks, or thin flex heaters that will be located between moving components that could cause abrasion.
Silicon and Kapton® heater sizes will be based on the type of resistance element that is used. For silicon heaters using etched foil elements and for all Kapton® heaters, they can have a maximum size of 10" x 70". They cannot be made into a larger size.
If the silicon heater has a wire wound element, the size can be increased up to 36" x 144". If the application needs medium to large heaters, silicon rubber heaters are typically used.
As for thickness, silicon heaters can only go as thin as 0.030" for etched foil and 0.056" for wire wound. Kapton® heaters can reach a thinness of 0.007". For components that are looking for a lightweight, thin heater, Kapton® flex heaters are a common choice.
Many applications must have a set temperature to avoid malfunctions in products or electronics. Temperature limitations are also commonly found in the food industry. Cooked food that is being kept warm with flexible heaters cannot go below a certain temperature before being served or the food can become unsafe to eat. Also, certain foods and beverages may need to be warmed to a temperature so it can be properly dispensed through machines.
Silicon heaters have a minimum operating temperature of -70°F. If the temperature is any lower, the heater will malfunction. For Kapton® heaters, they can go as low as -320°F, making them ideal for heating satellites and spacecraft applications. On the other hand, Kapton® heaters can only be used for low maximum temperature applications as they can only go up to 392°F. Silicon heaters can reach higher temperatures of up to 400°F.
Heated Material Limitations
Manufacturers need to take into account the characteristics of the materials that will be heated when selecting the suitable flex heater. If the material is susceptible to higher temperatures or will change characteristics when heated, then choosing a heater that provides lower temperatures is recommended.
In addition, the thickness of the materials to be heated can also impact the effectiveness of the heater. For example, if the application requires a liquid to be heated and retain varying temperatures throughout, the manufacturer should look into heaters that offer good thermal transfer and continuous operating temperatures. A thicker material, such as an oil or syrup, should use an intermittent low temperature heater.
Another factor to consider is off-gassing. Certain materials can give off a gas over time. This can impact applications in vacuum environments. Silicon and Kapton® heaters have low out-gassing properties.
There are bend limitations to be aware of when using silicon heaters. Due to the thicker rubber that can only reach a thinness of 0.030" thick, the silicon heater will only have a bend radius of 1.5 inches.
Also, the thicker the silicon is on the heater, the less flexible it will be. So, for applications where there are less curves and more straight areas, silicon heaters are ideal. When there are more curves, a flexible Kapton® heater should be used.
Both Kapton® heaters and silicon heaters have a resistance to moisture and ozone. In addition, Kapton® and silicon heaters also are resistant to fungus. Silicon heaters can also be used in environments where radiation may be present and is perfect for outdoor applications with high moisture and humidity due to its moisture barrier.
As mentioned before, environmental temperatures can impact the operation of heaters. Silicon heaters should not be used in extremely cold environments lower than -70°F. While Kapton® heaters can withstand temperatures as low as -320°F, they should not be used in environments with high temperatures over 392°F.
Chemistries, Corrosives, and Oxidizer Exposure Limitations
Kapton® heaters have great chemical resistance. Silicon heaters are also resistant to many salt chemistries such as sodium carbonate and sodium hydroxide. A silicon heater also will not react badly in environments that have ammonium.
However, certain acids can impact the silicon rubber. A silicon heater should not be used in environments where sulfuric acid or phosphoric acid is present. You should also avoid using silicon heaters where there is a presence of mineral oils, transmission fluids and motor oils that can eat into the rubber surface. Additional fluids where silicon heaters have a poor resistance to include carbon tetrachloride and benzene.
Knowing your design limitations is a crucial step in manufacturing a flexible heater that will work in your application. Here at Epec, we provide custom flexible heaters in silicon rubber and polyimide Kapton® for the electrical and electronics industries. We can work with you to help design a custom flex heater to fit your applications requirements.