Flexible heaters are essential devices to provide heat to specific areas of an application. They are used in a wide range of industries including aerospace, medical, food service, and military. When applying heat to a surface, the right types of materials must be used for the heater. The heater must be flexible enough to bend and wrap around curved surfaces while avoiding working parts. They also have to be able to provide optimal heat transfer without damaging either the application or the heater’s elements and circuitry.
Polyimide and Kapton® are two types of materials used when talking about flexible heaters. They are both made from a polymer film that provides low gassing in vacuum environments and high tensile strength. How hot these two materials can get while being used as flexible heaters will depend on a variety of factors.
Kapton® and Polyimide Materials
When manufacturers mention Kapton® and polyimide, many customers assume that they are both different materials. In actuality, Kapton® is made of polyimide. The company DuPont was the first manufacturer to commercially make this polyimide film back around the 1960s. For branding purposes, they named their polyimide Kapton®.
As more manufacturers began making their own types of polyimides, DuPont began rolling out new grades of their Kapton® films. These grades are designed for specific applications as they may add additional materials to the polyimide to provide certain characteristics, such as heat fusible Fluorinated Ethylene Propylene (FEP) in their Kapton® Film Corona Resistant (FCR).
Flexible polyimide heater with high wattage element design.
Polyimide versus Kapton® Material Differences
Technically, polyimide is a polymeric material and is considered a thermoset polymer. The thermoset is in a liquid form, or a malleable form, as it is cured by using heat, irradiation, or by chemical means. The resin becomes cross-linked to create a strong bond as it holds the polymer material together. Due to the curing process, it cannot be remelted. Also, once the heat is applied to cure the thermoset, it does not become affected by any type of additional heat that may be added. This characteristic makes polyimide suitable for flexible heaters.
Kapton® can be manufactured in different ways. While it is also a polymer material, depending on the additives and other processing, it can be considered both a thermoset polymer and an unfilled thermoplastic. Thermoplastics are types of polymer materials that can be melted and hardened repeatedly. Due to this feature, thermoplastics are not used in the making of polyimide films for flexible heaters. Instead, Kapton® thermoset is used.
Flexible Heating Elements
Polyimide and Kapton® flexible heaters have etched foil elements in their materials. These elements are made from a thin alloy that has corrosion resistance, thermal conductivity, and electrical conductivity. This alloy is acid-etched into the flexible heater's substrate. This substrate becomes sandwiched between two layers of polyimide film. The layers are held together using an acrylic adhesive or an FEP adhesive.
Etched foil elements in polyimide and Kapton® heaters offer flexibility and higher watt densities. They also offer faster, rapid heat up for applications. The etched foil allows the heaters to be manufactured in very thin dimensions as low as 0.007". This thinness allows the heater to be highly flexible to wrap around curved surfaces and fit into very tight spaces. Both types of materials can be manufactured with similar thicknesses and dimensions. The flexible heaters can come as small as 1/2" to as big as 12" x 30" or 16" x 72".
Flexible polyimide heaters with etched foil elements have stability and durability to work in environments with maximum operating temperatures of up to 400°F. The heating elements can obtain maximum operating temperatures of up to 392°F.
Challenges with Polyimide/Kapton® Materials
The challenge of working with the polyimide and Kapton® materials is ensuring there is an optimal seal between the layers of materials that can withstand adverse conditions. The materials need to have acceptable tensile strength as well as resist elongation or shrinkage during the different temperature cycles and working environments.
As time passes with the flexible heater aging in the working conditions, it may begin to experience lesser thermal durability. In addition, if the flexible heater experiences temperature conditions that go above its maximum operating temperatures of over 400°F consistently, the polyimide materials may begin to degrade. The adhesive used to keep the layers together will also degrade, allowing the layers to peel away from each other and from the metal elements as the materials begin to malfunction.
For example, in the case of Kapton® under tensile testing, this polyimide had a minimum peel strength of 700 g/in when the layers were coated on both sides. When only one side was coated, the minimum peel strength dropped to 450 g/in.
Another challenge is to ensure uniform heat output across the entire heater when there may be cut-outs so the heater can fit within certain assemblies. Based on the element pattern along the flexible heater, the polyimide materials may have cold zones and a variation of temperatures when using the same uniform thermal element pattern throughout the surface. The use of analysis tools will be required to understand the heat output in these designs, as the pattern layout may need to be redesigned to prevent heat loss or a build-up of temperatures in specific locations.
Costs for Polyimide/Kapton® Flexible Heaters
Custom manufacturing of polyimide and Kapton® flexible heaters will be based on each customer's specific needs for the application. Costing out the process will require going through initial assessment and concept research phases. The price numbers will be in a ballpark figure to provide the customer a general idea of costs to determine the economic feasibility of the project.
One of the costly phases in the design will be the proofing of the flexible heaters. Many manufacturers provide minimum order quantities in larger sizes of 12" x 18". They may also have high tooling prices when building the parts. To help lower costs, reach out to companies that offer polyimide heater sample kits that have a variety of different polyimide materials in various sizes as well as wattages. You may also look for companies that allow you to order smaller quantities down to 1 piece at a time so you can try out the different configurations with the applications.
Prototyping the flexible heaters is a necessary stage to ensure that the flexible heater has all the specific properties and will be presented with a finished look. Through prototyping, we can test the heater under specific conditions that may be experienced with your application as well as allow for design changes before the final production phase.
Summary
Understanding the thermal limitations of polyimide flexible heaters will be based on the specific manufacturer and the types of materials that will be included during the material's design. Each manufacturer has a different formula, allowing for a wide range of polyimide materials to be made with specific properties.
Determining whether a polyimide from one company or a Kapton® material from another will perform suitably for your application and environmental conditions will require analyzing how the samples of each material will work when placed on the application. Obtaining sample flexible heaters and prototypes is one of the many services that EPEC provides which can help to narrow down the process, so you get a customized flexible heater that is right for your needs.
