materials feature | Thermal conductivity
Table 1 – Mechanical properties of injection moulded samples Dowlex 2388 625
Tensile modulus (mpa)
Tensile Stress at Yield (mpa) Tensile Strain at Yield (%) Tensile Strain at Break (%) Flexural modulus (mpa) Flexural Stress at 5% (mpa)
18.94 13.76 >300
540 11.74
Dowlex 2388 + 10% C-Therm 001 1119 22.01 10.58 14.15 1187 18.30
particles are strongly aligned along the direction of injection, leading to a very anisotropic thermal conduc- tivity of the sample (anisotropy ~5.8). during compres- sion moulding, the alignment is much weaker, leading to a more isotropic thermal conductivity (anisotropy ~1.3). The extruded pipe has a degree of anisotropy (~2.6)
that is between the injection moulded and compression moulded samples, as shown in Fig. 2. of particular interest for heat exchangers is the through-plane thermal conductivity, which is almost doubled by using just 10% c-Therm 001 compared to virgin dowlex 2388 pipes. Besides thermal conductivity measurements, we
also investigated the mechanical properties. Samples of 10% loaded pE-rT for mechanical testing were produced by mixing virgin polymer with the 20% w/w compound in a 1:1 ratio using a Thermo Haake polylab twin-screw extruder. The results from tensile and flexural tests on injection moulded specimens are summarised in Table 1. compared to virgin polymer, c-Therm compounds
have significantly increased tensile and flexural stress indicating a strong bonding between the polymeric matrix and the filler. This effect increases with filler concentration
Figure 3 - Schematic view of borehole heat exchanger Ground
Backfilling Material
Dowlex 2388 + 20% C-Therm 001 1666 25.95 7.67
14.91 1911 23.78
as expected in such cases [6] . Also tensile and flexural
modulus strongly increase due to a combination of the rigidity of the graphitic structure, the strong anisotropy of the filler and the strong bonding to the polymer. on the other hand, elongation at break is significantly reduced as in most mineral filled composites. in conclusion, c-Therm 001 can strongly increase the thermal conductivity of dowlex 2388. The degree of anisotropy is highly dependent on the moulding process with injection moulded samples being much more anisotropic than compression moulded samples. For extruded pipes, the through-plane thermal conductivity is increased from ~0.4 to ~0.7 w/(m.K) at 10% loading of c-Therm 001. mechanical properties present a positive increase of tensile strength with an associated stiffness increase.
Geothermal pipes geothermal energy can be used for electricity produc- tion or for direct heating purposes. geothermal heat pumps take advantage of the Earth’s relatively constant temperature at small depths and they are one of the most efficient heating and cooling options that can be used almost everywhere in the world. conventional boreholes used as vertical ground heat
exchangers (Fig. 3) are significantly limited in their thermal performance by the low thermal conductivity of the backfilling material and of the plastic pipes. in the case of conventional bentonite grouts (~0.8 w/(m.K)) the main thermal resistance is due to the bentonite itself (~65%) followed by the plastic pipe wall resistance (~35%) [7]
. The overall heat transfer rate can be enhanced by
Plastic Pipe
improving both the bentonite and the pipe thermal conductivity. The decreased thermal resistance of the borehole will reduce the length needed for geothermal heat exchangers and dramatically lower the cost of this type of system. The thermal conductivity of bentonite grouts can be
increased up to 2 w/(m.K) by adding few % natural graphite and up to 5 w/(m.K) by using Timrex c-Therm instead of natural graphite. using backfilling materials with improved thermal
18 compounding world | February 2012
www.compoundingworld.com
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