The flexibility available with thermoplastic materials allows the design freedom to create more efficient geometries.  Because of this, applications designed from thermoplastics with a conductivity of 1 to 10 W/m²°K may transfer as much, or even more, total heat than similar parts designed in metals with higher thermal conductivities.

THERMOPLASTIC SOLUTIONS

The following three general classes of fillers can be used to increase the thermal conductivity of thermoplastics:

* Carbon fillers: carbon fibers, carbon powder

* Metallic fillers: copper powder, steel, aluminum powder, aluminum flake

* Ceramic fillers: boron nitride, aluminum nitride, aluminum oxide

Each filler type has the following advantages and disadvantages:

* The carbon fillers and metallic fillers will be electrically conductive as well as thermally conductive. The ceramic fillers will provide materials that are electrical insulators.

* The fiber or flake fillers are generally more efficient fillers, in terms of the loadings needed to achieve conductivity. However, because of their aspect ratio, injection molded parts may have an orientation dependence. Powdered fillers may require higher loadings, but will not show an orientation dependence.

* The metallic fillers often have high specific gravities, which can lead to a weight disadvantage in some applications.

Two thermally conductive materials, using a ceramic filler and the other a carbon fiber filler, are listed in Table 1. These materials contain polyphenylene sulfide (PPS) with 10% fiberglass as the base resin system. Both materials show significant improvements in thermal conductivity, and also illustrate several of the advantages and disadvantages of the different filler types. The carbon fiber system is electrically conductive and exhibits orientation effects: note the difference in the thermal conductivity measured in the plane of the plate versus through the plane. The ceramic filler is a powder, is not electrically conductive, and has the same thermal conductivity in all directions.

The physical properties of these two thermally conductive materials are normal for the type of fillers used. The particulate filler (i.e., the powdered ceramic) has no reinforcing properties and serves to lower the tensile and flexural strength. The carbon fiber filler will add additional reinforcement, which is reflected in the increase in the tensile and flexural strength.

The use of these fillers can be extended to other resin systems. This is demonstrated in Table 2, where examples of a ceramic filler compounded into nylon 6 and polypropylene are shown. Again, there is a five-fold increase in the thermal conductivity versus the base resin, without a significant effect on the physical properties of the resin.

By means of a simple heat transfer model, the thermal performance of materials in air-cooled applications has been outlined. The model shows that convective heat transfer often governs the overall equilibrium temperature gradient. Because convective heat transfer becomes the limiting factor, thermoplastics with a conductivity of I to 10 W/m°K can transfer as much heat as a metal with a higher thermal conductivity. Through the use of thermally conductive fillers, thermoplastic composite materials based on PPS, nylon 6, and polypropylene have been formulated that meet the targeted thermal performance of 1 to 10 W/m°K.