This time, the focus is on the thermal conductivity of unfilled plastics. There are hundreds of them, so only a small selection can be presented. From a thermal point of view, plastics are a notoriously difficult family. Different sources show large variations in the thermal conductivity, and handbooks provide a range of values for many materials instead of a single value.
The reasons are manifold. Noteworthy is the variation in density, as is clearly demonstrated by the values for polyethylene in the table below. Another important and often overlooked source of (anisotropic) variation in injection-molded plastics is the speed of injection. Research has shown that it is possible to achieve “metallic” values at extreme speeds due to the stretching of the polymer chains in the direction of flow.
|Polyaramide||Kevlar, Nomex fibers||0.04-0.13|
|Polyethylene terephthalate||PET, Polyester||0.15-0.4|
|Polyethylene L||Low density||0.33|
|Polyethylene HD||High density||0.45-0.52|
|Polymethylmethacrylate||PMMA,Acrylic, Perspex, Plexiglass||0.17-0.19|
|Polyphenylene oxide||PPO, Noryl||0.22|
The table also shows an interesting range in thermal conductivity from a thermal engineering point of view. For example, the difference between polyimide and HD-polyethylene is a factor of five, which corresponds to the difference between natural and forced convection from a heat transfer capability point of view.
All values in the table are defined at room temperature. As a rule-of-thumb, the thermal conductivity increases with a few percent in the range 0-100°C. Only below very low temperatures (typically 40K), plastics show a clear decrease, in sharp contrast with metals that exhibit a very impressive increase (Al: >13,000 W/m2K!).