Thermo Oxidative Resistance Of Cross Linked Polyethylene

2072 words - 8 pages

1. Introduction
Thermo-oxidation is one of the most frequent and important stress that affects polymers, particularly those used as electrically insulating materials and is one of the main causes contributing to the final failure of insulators. By increasing the thermo-oxidative stability of the polymer, the life-time of the insulating system can be considerably prolonged. As an alternative to the bulk modification by additives or dispersed inorganic phases having micro or nano size, the depositions of thin films on the outer surface of the polymeric device has been proposed [1, 2] (valutare se aggiungene un altro di carattere generale). It has been proved that oxygen permeability, and as a consequence thermo-oxidative degradation, can be significantly decreased through the application of thin (about 1 μm) organic-inorganic hybrid coatings on the external surfaces of the material [3, 4]. Moreover, in the last years graphene has attracted a huge interest since its ability to increase, at very low amounts, different properties, in particular electrical conductivity and mechanical strength, but also thermal stability and gas permeability [5-8]. Graphene oxide, the partially or fully oxidized form of graphene, although having a partially disordered structure compared to pure graphene and consequently lower electrical conductivity, can be more easily dispersed in polar media such as the hydroalcholic solutions generally used in the preparation of hybrid coatings by sol-gel method. Thus, it seemed interesting to investigate the effect of the addition of GO to previously studied coatings [4] where water is one of the co-solvent used. In this paper, we describe the effect on the aging of XLPE of coatings (containing SiO2 deriving from TEOS and a ethylene-ethylene glycol functionalized block copolymer) whose composition has been modified through the addition of small amounts (from 0.2 to 0.8 wt%) of graphene oxide. An accelerated aging process at 120 °C (a temperature slightly above the maximum on-service one) has been carried out on coated and uncoated XLPE. The extent of XLPE oxidation has been evaluated by isothermal DSC and infrared analysis (FTIR) as a function of aging time. For the most performing systems, the increased stability has been validated by determining the mechanical properties (strength and strain at break) of the aged samples.

3. Results and discussion
3.1. Thermo-oxidative resistance
Figure 3 shows the trend of the OIT values for uncoated and coated samples. The trend is similar to that reported in another papers [13] controllare rif. se corretto.

Figure 3: OIT values as a function of aging for the different materials.

It is clear that both the hybrid coating (XL-SiO2) and the GO modified one (XL-SiO2-GO_x) provide a higher resistance to thermo-oxidation per se, that is, before the aging process is carried out. As aging proceeds, curves for coated samples are shifted to higher values of OIT and to longer aging times compared to...

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