How do we create thermally conductive polymers for electric vehicles?

Increasing the thermal conductivity of polymers

The secret to achieving thermal conductivity in polymers is choosing the right filler. Several different classes of materials are available but in principle, they all work in the same way. Achieving a level of thermal conductivity beyond that of the base polymer requires the physical percolation of carefully selected thermally conductive filler particles. Beyond what is known as the percolation threshold, the thermal conductivity starts to ramp up (see Figure 1). Regardless of the filler, the loading levels of thermally conductive filler required to achieve percolation are very high which is typically detrimental to the processability and mechanical properties of the compound.

Figure 1: Schematic representation of thermal conductivity of a polymer compound in function of filling degree. To achieve a level of thermal conductivity, significant filling degrees are required, beyond the percolation threshold.

Thermally conductive fillers from Quarzwerke

Silatherm® by HPF Quarzwerke consists of a broad range of thermally conductive fillers. It has been specially tailored toward achieving thermal conductivity in polymer compounds whilst maximising processibility and polymer mechanical properties. Whilst the formulations are the results of extensive research and are proprietary, the effect is reached in essentially two ways. First, the thermally conductive fillers are given a suitable surface treatment through silane chemistry. You can find more about the benefits of surface treatment in a previous blog article. In simple terms, surface treatment makes the filler surface more compatible with the polymer matrix and as a result, mechanical properties are improved. The right surface treatment also makes it easier to achieve higher loadings, which in turn maximises the thermal conductivity of the compound. A second method that is employed by HPF Quarzwerke toward optimising thermal conductivity in polymer compounds, is to carefully modulate the particle size distribution to ensure optimal packing density within the compound. By using Silatherm® grades, a thermal conductivity of around 1 - 4 W/mK can typically be achieved.

Figure 2: Thermal imaging of a heatsink created using a polyamide compound filled with 65 wt % Silatherm®. The thermal conductivity of the compound is 1.3 W/mK which is sufficient to transfer heat via convection in this heatsink application.

Thermally conductive fillers from 3M

For applications demanding polymers with even higher thermally conductivity, high-end Boron Nitride Cooling Fillers by 3M are more suitable. Hexagonal boron nitride is a synthetic ceramic material that combines a very high intrinsic thermal conductivity with a layered platelet shape (figure 3). The high aspect ratio of the thin platelet shape helps to lower the percolation threshold which makes the material very effective. Since boron nitride is also lightweight (Density of bulk hBN 2.25 g/cm³), thermally conductive compounds can be achieved at lower addition levels (especially when measured in wt%) compared to mineral-based fillers such as Silatherm. The maximum achievable thermal conductivities are also higher, with in-plane thermal conductivities of up to 15 W/mK.

Figure 3: Boron Nitride Cooling Fillers by 3M have a very high intrinsic thermal conductivity and are platelet-shaped which facilitates high in-plane thermal conductivity in polymer compounds up to 15 W/mK.The image on the left shows an SEM micrograph of grade CFP 0075.

Combining Boron nitride and SILATHERM® for high thermal conductivity in polymers

It is also possible to combine materials such as Silatherm® and Boron Nitride Cooling Fillers to maximise performance whilst minimising cost. Figure 4 demonstrates the effect of adding small amounts of hexagonal Boron Nitride to a compound containing Silatherm®. In this example, the thermal conductivity in the x-direction (in-plane) and z-direction (through-plane) can be nearly doubled by adding 10 wt% of Boron Nitride to the compound. Going beyond 10% of boron nitride, the effect on trough-plane thermal conductivity is limited and this is related to the anisotropic platelet shape of boron nitride. More complex 3M Boron Nitride Fillers are available to resolve this problem for which we refer you to the attached 3M literature.

Figure 4: Thermal conductivity of a compound containing Silatherm®, as a function of adding additional hexagonal Boron Nitride. Combining the two materials offer a cost-effective solution toward achieving high thermal conductivities.

Summary

The advancement of thermally conductive polymers is offering new opportunities for weight reduction in automotive applications and in particular in electric vehicles.  However there is no one-size-fits-all solution when it comes to formulating a thermally conductive compound.

For now, compromises still must be made between thermal conductivity, processability, mechanical properties and material costs when selecting a thermally conductive filler. If you would like to discuss your specific needs and requirements, please don’t hesitate to get in touch to discuss your project or request samples from our product pages.