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2.4 Thermal Conductivity
The thermal conductivity X is defined as
where
Q = heat flow through the surface of area A in a period of time t
(T1-T2) = temperature difference over the length /
(2.14)
Analogous to the specific heat cp and enthalpy h, the thermal conductivity X can be
expressed as [1]
as shown in Figure 2.6.
Thermal conductivity A
mK
polynomial
measured
Temperature T
Figure 2.6 Comparison between measured values of X [6] and polynomial for PP [1 ]
(2.15)
The thermal conductivity increases only slightly with pressure. A pressure increase from
1 bar to 250 bar leads to an increase of only less than 5% of its value at 1 bar.
Within a particular resin category such as LDPE, HDPE, the thermal properties are largely
independent of the molecular structure. Exhaustive measured data of the quantities cp,
hy and X and pressure-volume-temperature diagrams of polymers are given in the VDMA-
Handbook [5].
Approximate values of thermal properties useful for plastics engineers are summarized
in Table 2.1 [4].
0 C