Tests were performed to evaluate the effects of thermal and solvent aging on the mechanical and dielectric breakdown properties of four types of polyester resins, namely, the insulation layer of poly(butylene terephthalat) (PBT)- based thermoplastic elastomer (TPE, TPE1), poly(butylene 2,6-naphthalate)-based TPE (TPE2), PBT/poly(ethylene terephthalate) alloy (Alloy), and PBT extruded onto a copper conductor of low voltage electric wire. The tensile specimens used in this series were prepared from the same extruded resins. The prepared electric wires and tensile specimens were thermally aged in air and in toluene, xylene, TCB, and NMP. When Alloy and PBT were thermally aged in toluene, xylene and TCB at 120°C for 6？h, the tensile properties were significantly decreased compared to TPE1 and TPE2 at the same condition. The reduction of elongation at break of Alloy was more discernible than that of PBT. This result indicated that Alloy is more affected by thermal and solvent ageing. Among them, TPE2 showed the highest breakdown voltage (BDV), and it has also the highest BDV after thermal and solvent aging. 1. Introduction The service life of electric wire is determined by the condition of the insulating material. Insulations are commonly used as a flexible polymer coating on electric wire. Defects such as voids, contaminants, and scratches in the insulation structure lead to electrical failure in polymer insulted wires [1–4]. Thermoplastic polyesters, enclosing poly(ethylene terephthalate) (PET), poly(butylene terephthalate) (PBT), copolymers, and blends, are widely used for electrical and electronic applications such as switching transformers, inverters, and information and telecommunications equipment due to their good electrical insulation and dielectric properties. Many manufacturers of transformers get electrical insulating varnishes for secondary insulation and have to apply and transform them using a chemical curing process. The insulating varnish is applied to coils of polyester insulated wire, in the process of which it fills the gaps and micropores, and after curing it forms into a continuous resin film. It is important for mechanical stability and protection of coiled wire against dust, moisture, and chemicals which might cause insulation deterioration. Thereby, leakage currents or short circuits can be avoided so that safe and reliable electrical products can be produced [5]. However, the insulating varnishes, which are provided for this purpose, are highly reactive materials. They are usually included in one or more solvents, such as toluene,
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