## Materials and Methods  
### Materials and Preparation  
#### 2.1.1 Preparation of the Microcapsule for Insulating Material  
Discyclopentadiene (DCPD) was selected as the repairing agent (core material) to meet the needs of repairing damage in the polyethylene insulating material. On the one hand, DCPD has the faster reaction rate, which can repair the damage defect in time, on the other hand, the reaction product of DCPD is polydicyclopentadiene (PDCPD). Its dielectric constant and density are close to those of polyethylene, which can effectively homogenize the local high field of the defect, thus reducing the impact of structural damage on polyethylene [14; 22]. Therefore, the microcapsule system of urea-formaldehyde (UF) resin encapsulated DCPD was prepared.  
In this paper, the UF/DCPD microcapsule were prepared by \"two-step\" in-situ polymerization to improve the controllability of the reaction, and obtain a microcapsule with appropriate morphology and properties, so that the microcapsule could adapt to the electrical insulation environment. The raw materials used were all analytical reagent (AR) level.  
In step one, the urea was dissolved in deionized water, and the formaldehyde solution was added. Then, the pH of solution was adjusted to 8.0-9.0 by triethanola mine (TEA). The UF prepolymer was obtained at 70 \({}^{\circ}\)C for 1 h.  
In step two, the stable DCPD oil-in-water (O/W) emulsion was obtained by mixing a sodium dodecyl benzene sulfonate (SDBS) emulsifier, melted DCPD, and deionized water for 30 min. Subsequently, the UF prepolymer, the UF curing agent ammonium chloride, and the UF water resistant modifier resorcinol were added to the emulsion. After that, the pH of the emulsion was slowly adjusted to about 3.0 by dilute hydrochloric acid. Finally, the microcapsules were obtained by reaction at 60 \({}^{\circ}\)C for 3 h.  
In order to make the performances of microcapsule meet the requirements of the insulating material, some variables needed to be regulated. The basic performances of a microcapsule, such as surface morphology and size, were modified by regulating the wall and morphology of microcapsule.  
In the aspect of the microcapsule wall, above all, the mole ratio of urea to formaldehyde was 1. This ensured a sufficient content of dimethylol urea in the UF prepolymer, which could enhance the net structure of UF wall. Moreover, the high mole ratio could increase the toughness of the UF wall, so that the microcapsule could resist the normal thermal, electrical, and mechanical stresses in polyethylene insulating material. Then, the dosage of ammonium chloride and resorcinol were 1.5% and 2.5% of the prepolymer, respectively, to enhance the stability of the microcapsule wall structure.  
In addition, the acidification time (i.e., reaction rate of the wall material) was controlled at 20-30 min to obtain a better spherical shell structure.  
In the aspect of microcapsule morphology, the speed of emulsification was 400 rpm, to adjust the size of the core droplets (about 100 \(\upmu\)m). Thus, the size of microcapsule suitable for polyethylene could be obtained, which ensured the repair effect and reduced the influence on the performance of the insulating material [31; 32]. Moreover, the dosage of SDBS was 5% of the DCPD mass, to ensure the dispersion and stability of emulsion. Besides this, the mass ratio of UF prepolymer to DCPD was about 2:1, to ensure the appropriate encapsulation effect. Furthermore, the rotational speed was adjusted to 300 rpm after acidification to obtain a microcapsule with intact sphericity and excellent dispersibility.