WCA September 2017

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. ❍ ❍ Figure 6 : Stress strain plots of the MV insulation compounds. Dotted lines: reference compounds

❍ ❍ Figure 7 : Tensile strength retained after air ageing at 135ºC and 150ºC for 168h, 240h and 504h

❍ ❍ Figure 8 : Elongation at break retained after air ageing at 135ºC and 150ºC for 168h, 240h and 504h 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.

TS [N/mm 2 ]

EB [%]

❍ ❍ Table 3 : Hot pressure test and longitudinal shrinkage at 130ºC of the MV TPV compounds

MV TP79 A

MV TP79 B

MV TP79 C

Hot Pressure Test 1 [%] Longitudinal Shrinkage 1 [%]

n.a. 2

27

3

14

11

2

1 CEI 20-86; 2 Not applicable

110

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Wire & Cable ASIA – September/October 2017