In the May 2001 issue, this column discussed the thermal conductivity of unfilled plastics. The interested reader may have noticed that the category of rubbers/elastomers was missing. This was not without reason. The user of these elastic materials should be aware that the final thermal resistance (which is, of course, from an engineering point of view, the more important parameter) might change under pressure and temperature. This occurrence is caused mainly by a decrease in thickness; however, the thermal conductivity might also change. Unfortunately, the extent of the change is not known and depends heavily on the chosen material, especially its density. Hence, the values quoted can only serve as a guideline, and, in any case. the thickness of the rubber in operating conditions should be measured.
Again, all values in the table are defined at room temperature. It is not expected that the rubbers are different in their temperature-dependent behavior, so we may retain the rule-of-thumb of the May issue. Hence, the thermal conductivity increases by a few percent in the range 0 to 100oC.
However, it should be noted that an increase in temperature might result in a decrease in thickness, influencing not the thermal conductivity but certainly the thermal resistance.
|Thermal conductivity of rubbers/elastomers at 25oC (W/mK)|
|Butyl rubber||IIR, CIIR, BIIR||0.09|
|Fluoroelastomer||0.19 – 0.30|
|Silicone rubber||Glass fiber filled||0.35|
|Silicone rubber||For thermal management (Arlon)||0.63 – 2.51|
1 These values were quoted in an overview, but I didn’t succeed in recovering the highest value from the data sheets. Reader comments are most welcome.
Sources (amongst others): www.goodfellow.com, www.efunda.com, www.matls.com.