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Wire & Cable ASIA – September/October 2017
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Figure 7
:
Tensile strength retained after air ageing at 135ºC and
150ºC for 168h, 240h and 504h
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Figure 8
:
Elongation at break retained after air ageing at 135ºC
and 150ºC for 168h, 240h and 504h
Focusing only on the MV TPV compounds, as noted
previously for the MFI in section 2.1, by careful balancing
of the components and a correct choice of PP, it is
possible to “tune” the rheological behaviour of the
TPV MV compounds, keeping or even improving the
thermomechanical properties. In this regard, MV TP79 C
exhibits lower stresses, ie viscosity, until very low shear
rates together with the best thermomechanical properties
among the studied TPV MV compounds.
2.4 Mechanical testing
The stress strain properties of the MV insulation
compounds were measured according to the method
ASTM D412, averaging the results of five dumb-bell test
specimens obtained in a Gibitre Tensor Check Profile. The
specimens were die cut along the milling direction from
plaques obtained in a compression moulding machine at
180°C. MV IS79 was pressed ten minutes to complete the
curing process. MV TP79 A, B and C were pressed for one
minute and cooled down under pressure. MV Ref AB and
C were treated identically to the MV TPV compounds to
obtain the test specimens.
Figure 6
illustrates one example
of the stress strain curve for each compound.
At first sight, the analysis of the stress strain curves of the
materials reveals that the MV TPV compounds have similar
performance to the benchmark MV IS79 in terms of TS
and EB, as already pointed out in section 2.1. Besides the
absolute values, the outlined curves follow a similar pattern
with a strong elastic response to the stress applied.
The main difference which can be observed is the higher
Young’s modulus of the MV TPV compounds. This is
caused by the crystallinity of the thermoplastic phase and
is therefore larger for MV TP79 C. The same behaviour is
recognisable in the reference compound MV Ref AB, which
has a Young’s modulus virtually identical to MV TP79 A
and B. Likewise, MV Ref C has a similar Young's modulus
to MV TP79 C.
However, those reference compounds, not being
vulcanised and lacking the elastic character, yield until the
final rupture. In contrast, the MV TPV compounds behave
as crosslinked materials with high elongation
[8-10]
. These
results are in agreement with the rheological studies,
confirming the successful achievement of thermoplastic
vulcanisate compounds. According to CEI 20-86, to
evaluate the performance of the MV TPV compounds at
high temperature, a hot pressure test was carried out
and the longitudinal shrinkage at 130°C summarised in
Table 3
, which is mandatory for thermoplastic insulating
materials rated for 90°C and 105°C. The results show an
improvement of the results going from MV TP79 A to
MV TP79 C. However, this is not a consequence of the
ratio between thermoplastic and elastomeric phase, but
results from the addition of a PP (see
Table 1
), which can
withstand such high temperatures.
2.4.1 Heat ageing resistance
MV insulation compounds were tested at 135°C and
150°C for 168, 240 and 504h, to assess their resistance to
accelerated ageing. Retained TS and EB are graphically
shown in
Figure 7
and
Figure 8
. MV TP79 A and B could
not be tested at 150°C, as the thermoplastic phase
completely melts at this temperature. In this regard,
MV TP79 C, which contains PP with higher melting
temperature, represents the only alternative to MV IS79 at
the test temperature of 150°C.
First, it must be pointed out that all the compounds have
good to excellent resistance at 135°C in terms of retained
TS and EB, which are higher than 70 per cent after 504h.
MV
TP79 A
MV
TP79 B
MV
TP79 C
Hot Pressure
Test
1
[%]
n.a.
2
27
3
Longitudinal
Shrinkage
1
[%]
14
11
2
1
CEI 20-86;
2
Not applicable
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Table 3
:
Hot pressure test and longitudinal shrinkage at 130ºC
of the MV TPV compounds
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Figure 6
:
Stress strain plots of the MV insulation compounds.
Dotted lines: reference compounds
TS [N/mm
2
]
EB [%]