materials feature | Thermal conductivity
Figure 1 – In-plane and through-plane thermal conductivity of virgin and C-Therm 001 filled Dowlex 2388 compounds processed in different ways
PE-RT compounds based on C-Therm dowlex 2388 pE-rT resin (a type ii polyethylene of raised temperature resistance) has been chosen as the polymer for the tests. produced in a proprietary solution process developed by dow chemical, dowlex pE-rT resins are ethylene-octene copolymers that are used for hot and cold water distribution and geothermal pipes. compounds with a 20 wt.% loading of Timrex c-
Therm 001 have been prepared using a leistritz co- rotating twin-screw extruder (ZSE 27mm 40d, 200 rpm, T = 220-230°c). The melt flow rate of the virgin polymer (1.94g/10min@190°c/5kg) was decreased to 0.34g/10min @190°c/5kg, still remaining processable by extrusion. compression moulded samples (10cm x 10cm, 2mm
thickness) have been prepared using a labtech lp-S-20 press (T = 230°c, p = 70 Bar, t = 5 min). injection moulded samples (iSo 527-2 1A) have been prepared using an Engel ES330/80Hl press (T = 200-240°c, T mould = 40°c). Extruded pipes (outer diameter = 20mm, wall thickness = 2mm) have been prepared using a 45 mm weber extruder (grooved barrel) with a three-stage HdpE screw and disc calibration unit after mixing the compound with virgin polymer in a 1:1 ratio (final c- Therm 001 concentration in dowlex 2388 = 10 wt.%, mFr 0.95 g/10min@190°c/5kg). [4] on these samples, thermal conductivities have been
Figure 2 – Thermal conductivity anisotropy of virgin and C-Therm 001 filled Dowlex2388 compounds processed in different ways
Expanded graphite has a platelet morphology with a high aspect ratio. This characteristic can be exploited as an advantage in numerous applications, such as gas barrier, reinforcement or high in-plane thermal conductivity. However, it could be a disadvantage for applications in which high through-plane thermal conductivity is needed, especially if a high shear rate process, such as injection moulding, orients the particles in the polymeric matrix. in the first part of this article, we explore the thermal and mechanical properties attainable with Timrex c- Therm in pE-rT. Special emphasis is given to the thermal conductivity anisotropy as a function of sample preparation (compression moulding, injection moulding, pipe extrusion). in the second part of the article, we assess and
discuss the effect of the increased thermal conductivity on the performance of geothermal pipes and the consequences for the full geothermal system.
16 compounding world | February 2012
measured using netzsch TcT416 (in-plane thermal conductivity) and netzsch lFA447 (through-plane thermal conductivity) instruments. The choice of the equipment was dictated by the sample size. Although the two instruments measure the thermal conductivity in a different way (TcT416: steady state technique, lFA447: laser flash transient technique), we defined the thermal conductivity anisotropy as the ratio between the in-plane thermal conductivity and the through- plane thermal conductivity. As shown in Fig. 1, the injection moulded sample
with 20% c-Therm 001 has the highest in-plane thermal conductivity of ~4 w/(m.K), ten times higher than virgin polymer (~0.4 w/(m.K)) and higher than the compression moulded sample with 20% c-Therm 001 (~2.7 w/(m.K)). on the other side, the compression moulded sample
with 20% c-Therm 001 has a through-plane thermal conductivity of ~2.2 w/(m.K) which is much higher than both the injection moulded sample with 20% c-Therm 001 (~0.7 w/(m.K)) and the virgin polymer (~0.4 w/(m.K)). This observation can be explained by the different
degree of alignment of the graphite particles during sample preparation [5]
. The thermal conductivity of the
graphite is highly anisotropic with the in-plane compo- nent being much higher than the through-plane component. during injection moulding, the graphite
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