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