Preparation And Study Of Coated Meta-aramid Paper With High Dielectric Breakdown Strength

Because meta-aramid paper has excellent properties like thermal stability,solvent corrosion resistance,electrical insulation and good flame retardancy,it is always used as a kind of good insulating material in the fields of electronic products and transformers.However,there are few active groups on the surface of meta-aramid chopped fiber,so it shows surface inertia.As a result,meta-aramid paper demonstrates a loose structure.In addition,porous structure is a common characteristic of paper-based materials.However,as an insulating material for the high electrical system,porous and loose structure will adversely affect the breakdown strength of insulating materials.The voids are the weak points of the paper-based materials,resulting in electrical breakdown.What's more,capillary action will occur around the pores to absorb water in the air,which will further reduce the dielectric breakdown strength of insulating materials and limit the applications of paper-based materials in the electrical field.Therefore,how to reduce the voids of meta-aramid paper and improve the compactness of meta-aramid paper is especially significant for the preparation of high-performance insulating materials.Aiming to reduce the surface and internal pores of meta-aramid paper and improve the compactness of meta-aramid paper,this paper creatively proposes to coat the surface of meta-aramid paper with poly(m-phthaloyl-m-phenylenediamine)(PMIA)solution,and then the structure of meta-aramid paper would be improved and the breakdown strength of the paper would also be enhanced.Firstly,poly(m-phthaloyl-m-phenylenediamine)/calcium chloride/N,N-Dimethylacetamide(PMIA/CaCl₂/DMAc)solution with solid content of 18.5%was diluted into solution with different solid content by DMAc,and then meta-aramid paper was coated of PMIA/CaCl₂/DMAc solution with different solid content.It was found that when the solid content of PMIA/CaCl₂/DMAc solution gradually increased,the numbers of pores of meta-aramid paper would be reduced,and finally the structure of the meta-aramid paper would be compact.When the solid content of PMIA/CaCl₂/DMAc solution reached 10%,compared with the pristine meta-aramid paper,the tensile strength in the cross and machine direction of coated meta-aramid paper were increased by 63.8%and 25.0%respectively,and the modulus were increased by103.1%and 74.6%respectively.The dielectric constant and dielectric loss gradually increased with the increase of solid content of PMIA/CaCl₂/DMAc solution.At low frequency,the dielectric constant increased from 1.75 to 2.07,the dielectric loss increased from 0.007 to 0.037,and the conductivity hardly changed.Because the dielectric constant was less than 2.1 and the dielectric loss was less than 0.04,the meta-aramid paper still maintained excellent dielectric properties after coating.Furthermore,the breakdown strength of meta-aramid paper was increased from 16.4k V/mm to 29.8 k V/mm after coating with PMIA/CaCl₂/DMAc solution when the solid content was 10%,which was increased by 81.8%.The thermal properties of coated meta-aramid paper were also studied.It was found that the coated meta-aramid paper still maintained excellent thermal stability and the breakdown strength of meta-aramid paper would almost not be influenced after heat treatment.When the coated meta-aramid paper was treated at 180?for 24 h,it still maintained good optical morphology and the breakdown strength of coated meta-aramid paper which solid content was 10%remained at 27.0 k V/mm.Salt in insulating paper is unfavorable to its dielectric properties.Therefore,the effect of salt-free PMIA/DMAc solution on the breakdown strength of meta-aramid paper was studied.The first thing to do was to manufacture PMIA/DMAc solution.PMIA/DMAc solution was prepared by dissolving meta-aramid spun fibers with solvent DMAc.The effect of dissolution time on the solid content of PMIA/DMAc solution was investigated.It was found that at the beginning of dissolution,the amount of dissolution increased rapidly.When the dissolution time was 480 min,the solid content of the dissolved PMIA/DMAc solution remained practically unchanged.Then,the self-made PMIA/DMAc solution was coated on the surface of meta-aramid paper.The experimental results show that when the solid content of PMIA/DMAc solution was 10%,the tensile strength in the cross direction and machine directions were 129.9%and 120.2%of the pristine meta-aramid paper,and the modulus in both directions were 134.5%and 118.9%of the pristine paper respectively.The dielectric constant,dielectric loss and dielectric conductivity were also improved compared with the pristine paper.However,compared with meta-aramid paper coated with PMIA/CaCl₂/DMAc solution,it could be regarded as no change.In addition,the breakdown strength of coated meta-aramid paper reached at 42.8 k V/mm,which was 2.6 times of pristine meta-aramid paper and 1.4 times of coated meta-aramid paper with the same solid content of PMIA/CaCl₂/DMAc solution.Finally,boron nitride nanosheets(BNNS)were added to PMIA/DMAc solution and meta-aramid paper was coated to further improve the breakdown strength.BNNS was prepared by boron nitride(BN)nanoparticles which was used ultrasound to exfoliate in DMAc.After research in transmission electron microscope and atomic force microscope,it was clear seen that the thickness of BN nanoparticles decreased from 100 nm to less than 10 nm,which was showed that BN nanoparticles were successfully exfoliated into BNNS.Furthermore,the PDA@BNNS was prepared by surface modification of BNNS with dopamine(PDA).The aim was to improve the dispersion and interfacial compatibility of BNNS in PMIA/DMAc solution.Through infrared spectroscopy,thermogravimetric analysis and X-ray photo-electron spectroscopy,it was proved that PDA had been successfully coated on the surface of BNNS.After adding different amount of PDA@BNNS into PMIA/DMAc solution,the elongation at break in the cross and machine directions of the coated meta-aramid paper decreased significantly.When the content of PDA@BNNS was less than 6%,the tensile strength in the cross direction basically remained unchanged,while the tensile strength in the machine direction tended to decrease.When the addition of was swelled to 8%,the tensile strength in the cross direction fell slightly,the tensile strength in the machine direction declined sharply,and the modulus in both directions improved significantly.Anyway,the mechanical properties of meta-aramid paper still remained excellent when the addition of PDA@BNNS was less than 6%.What's more,the amount of PDA@BNNS did not affect the dielectric properties of meta-aramid paper,and the coated meta-aramid paper still maintained excellent thermal stability of pristine paper.The breakdown strength of coated meta-aramid paper was up to 55.0 k V/mm when the addition of PDA@BNNS was 6%,and the breakdown strength would also not be influenced by heat treatment at 180?for 24 h.Besides,the breakdown strength of Du Pont Nomex insulating paper is about 17 k V/mm.Compared to the breakdown strength of Du Pont Nomex,the coated meta-aramid paper in this paper have higher breakdown strength which could be used in harsh environment.