The Anisotropic Thermal Conductivity of Plastics

The easiest way to tailor thermal conductivity of plastics is to incorporate some highly thermal conductive filler material into the plastic molding compound. Here the filler particles act as heat carriers in the thermally isolating media. Roughly, if more filler is used, then higher thermal conductivity is achieved. Phenomena related to manufacturability and strength often limit the amount of filler material. In the manufacturing process, the fillers can move so that the particles that were originally randomly oriented become re-oriented in the extrusion direction. As a result, the thermal conductivity parallel to the orientation direction has a higher value than if it were perpendicular to the orientation direction.

However, a more fundamental way to increase the thermal conductivity of plastics is to affect the polymer molecular orientation. In the manufacturing of plastic parts, molecular chains are oriented in the extrusion process, and plastic foils are even drawn to increase their mechanical strength. This drawing also changes the molecular orientation towards the direction of drawing. The anisotropy of the thermal conductivity can even be

3-dimensional and it depends on the structure of the molecules and the drawing ratio. By controlling the orientation one could produce a material that is thermally conductive in one direction but an insulator in the other one.

The thermal conductivity of plastics depends strongly on the degree of crystallinity in polymers. This is because the thermal conductivity in polymers is mostly due to so-called phonon transport that is very efficient along the crystallinity axes but reduced substantially by various scattering processes in other directions. In the case of semicrystalline polymers, like polyethylene, the thermal conductivity parallel to the orientation increases rapidly with increasing orientation, but perpendicular to the orientation it decreases slightly.

It has been shown [1] that for polyethylene with the draw direction ratio of 25, thermal conductivities of 8.5 to 14 W/mK in the direction of molecular orientation have been obtained, corresponding to temperatures of 120 to 320 K respectively. This is comparable to the values of stainless steel. In the direction perpendicular to the molecular chains the thermal conductivity values are 60 times smaller. For amorphous polymers, as for PVC, PMMA, PS, and PC, the anisotropy ratio remains lower, being typically less than 3.

So-called modified Maxwell and Takaynagi models have been developed to estimate theoretically the thermal conductivity ratios. As discussed above and in the previous issue of ElectronicsCooling (Technical Data column, Vol. 7, No. 2, May 2001, P.22), because the thermal conductivity of plastics depends on many factors, no general tabulated values for anisotropic thermal conductivities can be given.

Also, most of the current standard thermal conductivity measurement methods do not take the anisotropy into account: they measure either one-dimensional values or an “average” value based on the assumption that the sample is isotropic. In practice, the values given in the above-mentioned Technical Data column can be used for values in the direction perpendicular to the molecular orientation; the values in the parallel direction could be up to 60 times higher.

Reference

1. Choy C.L., Luk W.H., and Chen F.C., 1978, Thermal Conductivity of Highly Oriented Polyethylene, Polymer, Vol. 19, pp. 155-162