Our last blog post “Options When Purchasing Flexible Heaters” provided an overview of a few options when specifying a flexible heater circuit. In this article we would like to continue with that theme, with a focus more on the insulation materials that can be used.
Backing up a bit, we wish to point out that flexible heaters are sub-divided into two primary technology platforms: etched-foil and wire-based technology. Wire based (wire strands woven together or single strand wire) is the more common technology, with multiple customers offering a wide range of products. Etched foil heaters represent a newer technology (developed in the last 40 years) and are used in more advanced thermal management applications with limited thickness installations and multiplanar heated surfaces. All Flex fabricates etched foil heaters, so when discussing materials, we are always referring to etch foil based flexible heaters.
Flexible heaters are commonly grouped into categories based on their dielectric type. All Flex offers three primary types of dielectrics, each have advantages and disadvantages. Heater categories for this discussion include: Standard Polyimide, Silicone Rubber and High Temperature Polyimide.
Polyimide is the primary material type chosen when high thermal control is desired with the thinnest construction possible. The amber colored polyimide is often referred to as “Kapton” a trade name of DuPont. Traditional polyimide heaters are constructed with two layers of polyimide film encapsulating the resistive foil layer. The polyimide film layers are bonded together using high performance thermoset acrylic adhesive. Both flame retardant (FR) grades and non-FR grades of adhesive are available. The polyimide film is rugged and can handle temperatures exceeding 300C. The “weak link” in thee constructions is the adhesive system which can become problematic at temperatures >150C. Polyimide is also excellent for applications requiring low out gassing.
Silicone rubber (SR) is often utilized for general heater applications that require its overall mechanical properties. Silicone rubber can be used for outdoor or wet environments and the overall thickness provides more mechanical protection from damage. All Flex uses silicone rubber that is reinforced with a fiberglass grid which provides excellent dimensional stability. Silicone rubber heaters have a higher thermal rating at 232C. However SR heaters are less flexible and are not well suited for component assembly. Silicone rubber heaters are much heavier and have limits on bonding using typical pressure sensitive adhesives. An attractive bonding option is to mount the silicone rubber heaters directly onto a heat sink using a vulcanizing process. This attachment method uses uncured silicone rubber as the bonding material. Silicone rubber heaters are rugged solutions for industrial and general use applications, but should be avoided when out gassing is critical.
Specialty applications are demanding higher and higher temperature performance from thin and flexible heaters. “Downhole” oil drilling is frequently mentioned as a particularly extreme high temperature environment. Advanced polyimide heater constructions have been recently developed and tested to perform well in applications up to 300C (572F). Elimination of the adhesive layer within the heater construction is a prerequisite to achieve these performance requirements. Combining these specialty “adhesiveless” materials with newly developed manufacturing processes can provide exceptional high temperature heater performance.
The chart below provides a comparison among three flexible heater materials. It should be noted these are “standard” parameters and should be considered safe as guidelines. Exceeding these operating ranges is often possible but consultation with an applications engineer, along with some prototype testing, is recommended.
*Continuous Operating Temperatures. Contact All Flex for short duration peaks. UL certification is not available above 105C
**Application specific and must be validated by customer.
*** Bond data per 3M specification and test protocol. Actual data will vary depending on application.
The following are some excellent sources for learning more about etched foil heater circuits: