Environmental Engineering Reference
In-Depth Information
Table 2
Comparison of electrical and mechanical properties of different types of bipolar plates
[ 160 ]
Type of plate
Bulk
density
(g/cm)
Conductivity
(S/cm)
Mechanical strength
(MPa)
Weight
loss at
200 C
In-plane
Through-plane
Tensile
Flexural
CNC machined graphite
plate (Shunk)
1.9
110
20
50
40
1.5 %
Compression moulded plates
based on PET polymer
(Virgina Tech)
\1.8
230
18-25
36.5
53
NA
Injection moulded plates
(liquid crystalline
polymer-graphite
composite)
\1.8
100
NA
NA
NA
NA
electrical conductivity greater than 100 S cm -1 , the benchmark for good bipolar
plates [ 156 ]. Composite of low crystalline polypropylene with CNTs shows
electrical conductivity more than 100 S cm -1 along with improved mechanical
stability and lower thermal expansion [ 159 ]. Thus, CNTs have helped in
improving the desired features of conducting polymer composites towards bipolar
plate applications.
For an industrial scale of production, the composite material granules which are
obtained from the extruder can be employed to manufacture bipolar plates com-
plete with flow field structures by conventional production methods, such as
compression moulding or injection moulding. For small and intermediate series of
bipolar plates, compression moulding still seems to be the most widely adopted
production technology. Injection moulding, on the contrary, is a true mass pro-
duction technique for large series ([50,000) of bipolar plates. Injection moulding
of highly filled graphite composite materials are much more demanding than that
of conventional plastics due to their high viscosity and high flow resistance.
Injection pressure, injection velocity and nozzle temperature have to be optimized
to the specific composite material. Injection moulding of bipolar plates, mostly
with thermoplastic resins, has been demonstrated by a few companies and research
institutes worldwide [ 160 ] (Table 2 ).
Injection moulding of thermosets and thermoplastics are quite different. For
thermoplastics, the composites are heated above the melting point and the plas-
ticized mixture is then injected into the mould which is kept at a lower temper-
ature. In the case of thermosets, the compounded composite mixture is injection
moulded at temperatures significantly below the melting point of the thermoset
resin. Thermoset-bonded graphite composites are generally more complicated to
process, as they have to be post-cured after injection moulding in order to achieve
sufficient stability. Following the injection moulding of thermosets and thermo-
plastics, a thin polymer-rich skin layer has to be removed from the surface of the
bipolar plates by processes like abrasive blasting in order to reduce the contact
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