When somebody approaches us looking for a thermally conductive polymer, the starting point is often a target thermal conductivity: 2 W/mK, 5 W/mK, 10 W/mK or more.

It’s an understandable way to specify the material, but it can also be misleading. Thermal conductivity is an important property of the polymer compound, but it doesn’t tell us how effectively a finished component will manage heat.

For that, we need to understand where the heat is coming from, where it needs to go and how the component is going to be manufactured.

Thermal conductivity is only part of the thermal system

Thermal conductivity, normally expressed in W/mK, describes how readily heat passes through a material. In a real component, however, the material forms just one part of the thermal pathway.

Wall thickness, component geometry, the position and size of the heat source, contact area between components, airflow and ambient temperature can all have a substantial influence on performance. Interfaces are particularly important: a highly conductive material cannot compensate indefinitely for poor thermal contact elsewhere in the assembly.

This means that two compounds with quite different published conductivity figures won’t necessarily produce components whose thermal performance differs by the same proportion.

There is another complication with filled polymers: the thermal conductivity of the finished material may not be the same in every direction.

In-plane versus through-plane thermal conductivity

Many of the additives used to increase the thermal conductivity of polymers have a plate-like, fibrous or otherwise non-spherical geometry. As the polymer flows during injection moulding or extrusion, these particles tend to orient themselves.

This can make the finished component anisotropic, meaning that heat travels through it more readily in one direction than another.

For this reason, it is important to distinguish between in-plane conductivity, where heat travels predominantly along the plane of the moulding, and through-plane conductivity, where heat needs to pass through its thickness.

A datasheet showing a high thermal conductivity figure is therefore only genuinely useful if you know how the figure was obtained and whether that direction of heat flow is relevant to your component.

If the objective is to move heat through the wall of an injection-moulded housing, for example, an impressive in-plane result may tell you surprisingly little about how the housing will actually perform.

Why not simply add more conductive filler?

Broadly speaking, increasing the concentration of thermally conductive filler increases thermal conductivity. Unfortunately, most of the other properties of the polymer don’t remain unchanged while you do it.

High filler concentrations can significantly affect melt flow and mouldability, while also influencing impact strength, elongation, density, surface finish and tool wear. Cost usually increases as well.

There is consequently little benefit in chasing the highest possible conductivity if the resulting material is difficult to process or no longer has the mechanical properties required by the component.

The aim is to develop enough conductivity to solve the thermal problem while retaining an acceptable balance of processing and mechanical performance.

Component design can make a bigger difference than expected

One of the most interesting applications for thermally conductive polymers is the replacement of metals such as aluminium. But simply reproducing an aluminium component in plastic doesn’t necessarily make the best use of the material.

Injection moulding provides design freedoms that aren’t available, or aren’t economical, with many metal manufacturing processes. Fins, ribs, mounting features, ducts, clips and housings can potentially be incorporated into a single moulding. Surface area can be increased, heat can be directed towards particular parts of the component and multiple components can sometimes be consolidated into one.

This matters because conductivity only gets the heat to the surface. From there, convection and radiation have to remove it.

A component designed with greater effective surface area or better airflow may therefore outperform a less well-designed component made from a polymer with a higher headline W/mK figure.

Electrical conductivity may be equally important

The choice of conductive additive also influences electrical properties.

Carbon- and metal-based fillers can provide both thermal and electrical conductivity, which can be useful where static dissipation or EMI shielding is required. In other applications — particularly around electronics or live conductors — the material needs to conduct heat while remaining electrically insulating.

These requirements need to be considered at the start of the development process because they significantly influence the fillers and formulation that can be used.

Measure the property that actually matters

At Radical Materials we use laser flash analysis to evaluate thermal diffusivity and calculate the thermal conductivity of polymer compounds. Importantly, planar samples can be assessed in both in-plane and through-plane orientations.

That gives us a much better understanding of how formulation, processing and filler orientation are likely to influence a real component than a single conductivity figure.

Thermal imaging can then be used to visualise heat distribution and compare materials or component designs under representative conditions.

For this reason, when we’re asked for a polymer with a particular thermal conductivity, our next question is usually about the application.

What is generating the heat? Where does it need to go? How large is the component? How will it be manufactured? Does it need to remain electrically insulating? What mechanical properties does it need?

Once those questions are answered, the target W/mK figure often becomes considerably less important than it appeared at the beginning.

KONDUCT thermally conductive polymer compounds can be developed around specific thermal, electrical, mechanical and processing requirements. If you have a thermal management application, send us the component details and we’ll be happy to look at it.