A New Kinetic Modeling Approach for Predicting the Lifetime of ATH-Filled Silane Cross-Linked Polyethylene in a Nuclear Environment

Abstract

This study focuses on the degradation of a silane cross-linked polyethylene (Si-XLPE) matrix filled with three different contents of aluminum tri-hydrate (ATH): 0, 25, and 50 phr. These three materials were subjected to radiochemical ageing at three different dose rates (8.5, 77.8, and 400 Gy·h−1) in air at low temperatures close to ambient (47, 47, and 21 °C, respectively). Changes due to radio-thermal ageing were investigated according to both a multi-scale and a multi-technique approach. In particular, the changes in the chemical composition, the macromolecular network structure, and the crystallinity of the Si-XLPE matrix were monitored by FTIR spectroscopy, swelling measurements in xylene, differential scanning calorimetry, and density measurements. A more pronounced degradation of the Si-XLPE matrix located in the immediate vicinity of the ATH fillers was clearly highlighted by the swelling measurements. A very fast radiolytic decomposition of the covalent bonds initially formed at the ATH/Si-XLPE interface was proposed to explain the higher concentration of chain scissions. If, as expected, the changes in the elastic properties of the three materials under study are mainly driven by the crystallinity of the Si-XLPE matrix, in contrast, the changes in their fracture properties are also significantly impacted by the degradation of the interfacial region. As an example, the lifetime was found to be approximately halved for the two composite materials compared to the unfilled Si-XLPE matrix under the harshest ageing conditions (i.e., under 400 Gy·h−1 at 21 °C). The radio-thermal oxidation kinetic model previously developed for the unfilled Si-XLPE matrix was extended to the two composite materials by taking into account both the diluting effect of the ATH fillers (i.e., the ATH content) and the interfacial degradation.

Thermal aging of electrical cable insulation made of cross‐linked low density polyethylene for under‐hood application in automotive industry

“…It should be recalled that there is a wide variety of carbonyl products formed during the thermal aging of hydrocarbon polymers. In PE, independently of its macromolecular structure (i.e., linear, branched or chemically crosslinked), the resulting carbonyl composite band is generally centered at 1714–1715 cm −1 in the IR spectra [ 20,34–39 ] due to the much high value of the coefficient of molar extinction of carboxylic acids compared to other carbonyl products. [ 37 ] However, due to the presence of several types of metal oxides in the outer surface of the XLPE insulations under study, as soon as they were formed, many carboxylic acids were converted into carboxylates during the thermal exposure, according to the well‐known reactions reported for instance in Reference [40].…”

“…Many kinetic models have been developed to predict the thermal aging of the insulating layer of electric cables since the early 1980s. Some of these models have been improved several times [ 14–19 ] and are now able to predict with a great accuracy the oxidation of polymers and the consequences of this oxidation on their use properties, for example on their mechanical [ 20 ] and dielectric properties. [ 21 ] As sophisticated as they are, these models still do not take into account the effects of antioxidants so that, at this stage, they are unable to predict the long induction period preceding the onset and sharp acceleration of oxidation.…”

Consequences of Radiothermal Ageing on the Crystalline Morphology of Additive-Free Silane-Crosslinked Polyethylene

“…Crosslinked low-density polyethylene (XLPE) is widely used as an insulating material for electrical cables in nuclear power plants because of its excellent dielectric properties (e.g., its dipolar moment is zero, and its dielectric constant is around 2.3) [ 1 , 2 ], in addition to its low cost, easy processability, chemical resistance to many chemical reagents, lightness, and great flexibility [ 3 ]. However, the main weakness of this material is clearly its fairly high sensitivity to oxidation, which causes its embrittlement [ 4 , 5 , 6 , 7 , 8 ] much earlier than the degradation of its insulating properties [ 9 ]. In order to delay the onset of oxidation and, thus, increase its lifetime, antioxidants are commonly incorporated into the polymer matrix during melt processing [ 10 , 11 , 12 , 13 ].…”

“…However, antioxidants can be physically lost through evaporation and chemically consumed by the reactive species involved in the initiation (i.e., hydroperoxides) and propagation stages (i.e., peroxyl radicals) of the oxidation reaction under service conditions [ 4 , 14 , 15 , 16 , 17 ]. When the XLPE matrix is no longer sufficiently protected, the oxidation rate increases suddenly [ 4 , 16 , 17 ], thus inducing macromolecular and morphological changes which ultimately lead to the catastrophic decay in fracture properties [ 4 , 5 , 6 , 7 , 8 ].…”

“…For this reason, this chemical reaction is commonly called “radiothermal oxidation”. According to the literature, the radiothermal oxidation of XLPE leads to a predominance of chain scissions over crosslinking, which progressively destroys the elastically active chains of the macromolecular network [ 5 , 6 , 7 , 8 ] and produces short macromolecular fragments that can rapidly migrate towards crystalline lamellae in order to integrate them, i.e., to initiate chemicrystallisation phenomena [ 6 , 7 , 8 ]. Of course, such migration is greatly favoured in the case of XLPE because its amorphous phase is in a rubbery state at room temperature.…”