Flexible Heaters are great examples of how far we’ve come in terms of discrete and efficient ways of heating. Silicone, polyimide, and polyester heaters alike share a common method of heat transfer: conduction. That is the key term for these heaters as they transfer heat by touching the surface of what you want to heat up.
But what is the best method of holding the heater to the desired surface while avoiding heat transfer loss? A thermally conductive layer of adhesive, also known as a pressure-sensitive adhesive (PSA).
In the world of flexible heaters, there is a very common adhesive brand among them all – 3M. The 3M series adhesives are popular for many reasons, and in this blog post, we will discuss which ones are best suited for your project, touching on adhesive options and how they are used as well as W/mK, and the advantages of acrylic adhesives.
What Is An Adhesive?
An adhesive is a substance used to bond two surfaces together, creating a secure attachment without the need for mechanical fasteners. In the case of flexible heaters, adhesives must not only provide strong adhesion but also ensure efficient thermal conductivity to allow proper heat transfer. PSAs are the most common choice for securing flexible heaters to a variety of surfaces due to their ease of application, strong bonding capabilities, and ability to withstand varying temperature conditions.
Polyimide flexible heater with 3M adhesive.
Commonly Used Adhesives and Specs
The most commonly used adhesives for flexible heaters include 3M467, 3M486, 3M966, and 3M VHB. But what differentiates them? And why choose one over the other? We will address these questions using the chart below:
| Adhesive | Thickness | W/mK | Max Temp F | Lowest Temp F |
|---|---|---|---|---|
| 3M 467MP | 0.11mm | 0.158 | 400 | -40 |
| 3M 468MP | 0.11mm | 0.19 | 400 | -40 |
| 3M 966 | 0.09mm | 0.178 | 450 | -40 |
| 3M 8805 | 0.125mm | 0.6 | 212 | -40 |
As you can see, most adhesives are relatively similar to each other. The key differences are how well heat can travel through the adhesive and onto the surface that needs to be heated. Another important difference is the thickness of the adhesive. It may seem like there is not much difference on the chart above but when you have strict clearance tolerance, every 0.01mm matters.
Using PSAs with Flexible Heaters
Pressure-sensitive adhesives are the way to go when you want to stick your heater to a surface of choice. They are favored mostly for two reasons: their thickness and their ability to thermally conduct. But, how thick? Our most used adhesive, 3M 468MP, is actually pretty thin, measuring a nominal 0.005 inches (0.13mm), making it a perfect companion to the polyimide etched foil heater, which measures in at roughly 0.011 inches (0.254mm). This construction is ideal for applications where spacing is critical.
This adhesive is not limited to polyimide heaters, however. Polyester and silicone heaters use this method of adhesion as well. All flexible heater types mentioned can be manufactured to mold onto any surface, and with the help of adhesive, they can be mounted just about anywhere.
Where Does Thermal Conduction Come into Play?
Thermal conduction is the transfer of heat by means of conduction and can be measured in W/mK (Watts per Meter Kelvin). Using our most common adhesive as an example, 3M468MP, it has a thermal conductivity measurement of 0.19W/mK.
Let’s break this down into segments:
- Watts: A unit of power, in this case, the rate at which heat is transferred. 1 watt is equal to 1 joule of energy transferred per second.
- Meter: The distance through which heat is being transferred, length or thickness-wise.
- Kelvin: A unit of temperature differences, defining how heat moves. The greater the difference in temperature between two points, the faster the heat transfer is.
If we have 0.19W/mK, the measurement significance is stating that for each degree Kelvin of temperature difference across that thickness, 0.19 watts of heat energy will transfer through the material. There are, of course, adhesives with better thermal conductivity such 3M’s 8805, which boasts a 0.60W/mK conductivity measurement.
Advantages of Acrylic Adhesives
Heat Resistance
High-temperature acrylic adhesives are an excellent choice. They can support up to 400°F for short periods. Any higher temperature for any acrylic-based adhesive results in degradation and loss of bond strength.
Strong Bonding Performance
Though these adhesives are thin, they can certainly hold on to most surfaces tremendously well, like metals, plastics, glass, and composites. A great way to test these strengths is by doing peel adhesion tests. Per test method ASTM D330, 3M’s 966 held on with a value of 4.8N/cm (Newton Centimeters). This refers to how much force is needed to peel an adhesive from a surface.
Pressure Sensitive
These adhesives are not known as pressure-sensitive adhesives for show. They have a high initial tack or, better put, a high ability to stick on contact. Unlike most adhesives that require a cure time to adhere to a surface efficiently, the pressure placed on the adhesive is what is needed to adhere comfortably making it ideal for a quick and easy application.
Summary
Selecting the right adhesive for flexible heaters is crucial to achieving optimal heat transfer and a secure bond. PSAs, particularly from 3M's lineup, provide reliable adhesion while maintaining efficient thermal conductivity. Factors such as thickness, thermal conductivity (W/mK), and temperature resistance play a major role in determining which adhesive is best suited for your application. Acrylic-based PSAs offer excellent heat resistance, strong bonding performance, and quick application due to their pressure-sensitive nature.
By understanding the different adhesive options available, you can enhance the performance and longevity of your flexible heater setup. Contacting the manufacturer's engineering team, such as the one we have here at Epec, will allow everyone to go over the possibilities for the flexible heater and its adhesives.
Key Takeaways
- Selecting the right adhesive for flexible heaters is crucial to ensure optimal heat transfer and a secure bond, with 3M PSAs being a preferred choice due to their strong adhesion and thermal conductivity.
- Thermal conductivity (W/mK) varies between adhesives, impacting heat transfer efficiency. The higher the thermal conductivity, the easier it is for heat to travel through the object.
- Adhesive thickness plays a key role in heater integration, especially in applications with strict clearance tolerances. Even minor differences in thickness can impact overall performance.
- Acrylic-based pressure-sensitive adhesives (PSAs) offer excellent heat resistance, supporting temperatures up to 400°F, strong bonding to various surfaces, and easy application without curing time.
- Understanding adhesive properties like bonding strength, durability, and compatibility with different heater materials (polyimide, polyester, silicone) helps manufacturers select the best option for their specific application needs.
