Preparation And Self-healing Properties Of PUF-SiO₂/DCPD Microcapsule Polyethylene Composites

Polyethylene（PE）cables are widely used in power transmission in power grids due to their good electrical insulation properties,light weight,chemical corrosion resistance,and low price.However,due to process limitations,long-term electrical and thermal stress,excessive stretching and bending and other factors,damage defects such as micro-holes,micro-cracks,and electrical trees can inevitably occur on the surface and inside of the insulation layer of PE cables.These tiny defects can gradually develop under the multiple effects of external stress and internal electrical and thermal stress.If not controlled,the expanding damage defects may cause partial discharges and even cause accidents,seriously threatening the safe operation of the power grid.In this paper,the microcapsule external aid self-healing technology is used to automatically detect and repair the damage defects inside the PE insulating material,fill the damaged structure,restore the material properties,prolong the life of the PE material and reduce the risk factors of the power system.In this paper,poly urea-formaldehyde（PUF）and dicyclopentadiene（DCPD）with more suitable properties were selected as the wall material and core material of the microcapsules,respectively.By means of the combination of simulation and experiment,PUF-SiO₂/DCPD microcapsules with various properties more suitable for the operating environment of PE cables were prepared by doping nano-SiO₂ during the formation of PUF,and PUF-SiO₂/DCPD microcapsules PE composites were also prepared to achieve a good self-healing effect on internal mechanical and electrical damages.The main research work and conclusions are as follows.（1）The effects of particle size,wall thickness and Young’s modulus of PUF microcapsules on the self-healing behavior of PE composites were studied by finite element simulation.The larger the particle size of PUF microcapsules,the smaller the wall thickness,the greater the stress,the easier it is to break,and at the same time,it is more conducive to the development of cracks in the composite material to microcapsules to trigger self-healing behavior;The larger the particle size of PUF microcapsules,the greater the disturbance of the electric field distribution in the composite material,while the wall thickness has almost no effect on the electric field distribution.The crack develops towards the PUF microcapsule during the development process,and the stress on the wall of the microcapsule gradually increases.In order to prevent the premature rupture of the PUF microcapsule,its mechanical strength and Young’s modulus should be increased.Taking the research results into consideration,the parameters of the PUF microcapsules were determined by the fitting curve of the result data.When preparing the PUF microcapsules,the average particle size of the microcapsules should be controlled within the range of 94-107μm,and the wall thickness should be about 3.4μm.At the same time,it is necessary to improve the traditional preparation process of PUF microcapsules and increase the Young’s modulus of microcapsules to 2.1-4 GPa.（2）The preparation process of PUF microcapsules was improved by means of experiments and molecular simulation（MD）,its mechanical and thermal properties were improved,and its parameters within the range of the finite element simulation results.1)MD simulation found that doping nano-SiO₂ is beneficial to the increase of PUF material density,the reduction of free volume fraction and the enhancement of mechanical and thermal properties.2)PUF-SiO₂/DCPD modified microcapsules and traditional PUF/DCPD microcapsules were prepared experimentally,and it was found that the average particle size of both microcapsules was about 103μm;The inner and outer surfaces of the modified microcapsules are attached with more debris of the wall material,thereby increasing the wall thickness and the density of the wall material,and enhancing the mechanical and thermal properties.3)Digging into the internal mechanism,it is found that there are interfacial interactions between nano-SiO₂ and PUF,which can make the cross-linking between PUF chains and nano-SiO₂,and promote the cross-linking between PUF chains,resulting in more PUF wall debris attached to the surface of the capsule,the wall thickness of the microcapsule increases,and the density increases,thereby improving the mechanical strength and thermal stability of the PUF microcapsule.（3）The effects of different concentrations of PUF-SiO₂ microcapsules on the mechanical stress,thermal stress and electric field distribution of PE composites were studied by finite element simulation.When the microcapsule doping concentration is≤5wt%,the composite material can withstand the high temperature environment of70-150°C.The higher the microcapsule doping concentration,the more serious the electric field distortion in the composite material.Using the existing method to predict the self-healing efficiency of PUF-SiO₂/DCPD microcapsule PE composites for crack damage,it was found that the self-healing efficiency of mechanical and dielectric properties in terms of Young’s modulus and dielectric constant gradually increased with the increase of the microcapsule doping volume fraction.Considering the above research results,the appropriate doping concentration range of PUF-SiO₂/DCPD microcapsules was determined by fitting the curve:1-2.5wt%.（4）The PUF-SiO₂/DCPD microcapsule PE composite samples were prepared experimentally,and their basic properties and self-healing properties were tested and analyzed.1)The composite samples were prepared by the process（melting temperature130°C,pressure 0.5 MPa,pressing time 30 min）of this paper,the internal microcapsules are evenly distributed,and the morphology is intact without damage.2)When the doping concentration of microcapsules is≤2.5wt%,the mechanical tensile properties,thermal properties and electrical properties of the PE composite samples are good,which can meet the operation requirements of cables.3)The suitable self-healing temperature and time for the PE composite material is 60℃/30min;4)PUF-SiO₂/DCPD microcapsule-type PE composites have the obvious self-healing properties for scratches and electrical tree damage:filling the damaged structure,restore material properties.For scratch damage,the healing efficiency of the damaged structure of composite with1wt%microcapsules is 84-100%,and the healing efficiency of the breakdown strength is 94.2%.For electrical tree damage,the healing efficiency of the breakdown strength of the composite material is 94.4%.5)Digging into the internal mechanism,it is found that the cracks are generated at the interface between PE and microcapsules,and gradually develop to the PE matrix.The stress on the microcapsule wall can cause the microcapsules to rupture,thereby triggering the self-healing behavior,and there is good chemical compatibility between the healing product and PE matrix.Taking the above results into consideration,it can be seen that when the doping concentration of PUF-SiO₂/DCPD microcapsules is 1-2.5wt%,the PE composites can achieve good self-healing effect on the basis of ensuring good basic properties.