239 / 2018-09-24 14:49:07

Evaluation of Aging Status of Flame-retardant Cross-linked Polyethylene by Measuring Indenter Modulus

complex permittivity,FT-MIR,conduction current,degradation

Electric cables play an irreplaceable role in our daily life. As a part of a cable, insulation is important but vulnerable. In nuclear power plants (NPPs), heat and gamma-rays are two main aging factors that would lead to severe degradation in the insulation. Therefore, it is necessary to understand the aging behavior of an insulating material, which can help us make a precise prediction to the lifetime of a cable.
Carbonyl index, namely the ratio of the infrared absorption at around 1711 cm-1 due to carbonyl groups to a sample-dependent invariant reference band, is often used to predict the lifetime or aging status of polymers. Flame-retardant cross-linked polyethylene (FR-XLPE) is an important insulating material in electric cables. However, due to pre-existing carbonyl groups inside FR-XLPE, it is not appropriate to use the carbonyl index to estimate its aging status. Instead, indenter modulus has come into our sight for showing a good monotonic change with the progress of aging. In this paper, sheet-shaped FR-XLPE samples were aged at 100, 135, and 155 oC for various aging periods from 800 to 2000 h. They were aged thermally or concurrently by heat and gamma-rays with a dose rate of 100 Gy/h. For these samples, various measurements were done to understand the aging behavior of FR-XLPE in different properties, including complex permittivity, conduction current, FT-MIR absorption, and indenter modulus.
First, the two parts of complex permittivity and conduction current show a decrease with the progress of aging under mild aging conditions. However, when the aging condition becomes severe, the change becomes opposite. Therefore, it is likely that several aging stages exist in FR-XLPE.
For understanding the whole aging process of FR-XLPE, we used indenter modulus as an indicator to estimate the aging degree of FR-XLPE. The reason for this is that the indenter modulus exhibits a clear monotonic increase as the aging condition becomes severer. By analyzing the correlations among indenter modulus and other parameters, it has become clear that there are three different aging stages. That is, the correlation between the conduction current and the sample's indenter modulus, obtained for FR-XLPE sheet samples that were degraded to various degrees, is shown in Fig. 1, although other various figures are omitted due to the limited space. The conduction current was measured at 150 oC, with a dc electric field of 10 kV/mm, which could give a significant difference between the samples aged under different conditions. As mentioned above, if we consider the value of indenter modulus as an indicator of the aging degree of FR-XLPE, three different aging stages can be seen clearly in Fig. 1.
At the first stage, some structural changes induced by the additives cause a slight decrease in conduction current and the two parts of complex permittivity (not shown). However, when the aging enters the second stage, oxidative degradation becomes dominant, which gives a completely opposite change in these parameters. As for the last stage, the conduction current becomes stable and the formation of carbonyl groups ends. However, the two parts of complex permittivity are still increasing (not shown). For these reasons, it seems that non-oxidative chain scission becomes a major aging mechanism in the last stage.