Preparation And Properties Of Aramid Mica Paper By Coating Method

Aramid mica paper has excellent mechanical strength,flexibility,high-temperature resistance,insulation and corona resistance properties,and displays broad application prospects in high-end electrical insulation fields such as high voltage,variable frequency motors and traction motors.Generally,aramid mica paper is prepared by the wet-forming papermaking method with the pulp of aramid fiber and mica as starting materials.However,due to the differences in surface properties and density between aramid fiber and mica,the current method has several limitations:(1)weak interfacial bonding strength and large two-sidedness;(2)mica is prone to settle,leading to clogging of the fabric and uneven mixing of the slurry;(3)poor insulation properties because of a large number of pores in aramid mica paper.Although the surface modification of fibers and mica can alleviate the above problems to a certain extent,the complex process,preparation efficiency and chemical pollution limit its industrialization and scale-up.Therefore,developing efficient and straightforward methods for preparing aramid mica paper is an urgent need in this field.Based on the above background,this paper proposes to adopt a paper coating method to solve the problems mentioned above of pore regulation,mica sedimentation and fabric clogging.That is,the aramid mica paper was prepared via applying mica-containing coating on the surface of aramid base sheets,and nano-fillers were introduced to improve the electrical insulation performance.Besides,the mechanism of the improvement of mechanical properties and insulation properties was investigated.It is expected to simplify the preparation process and improve the overall performance of the aramid mica paper.The main contents are as follows:First,aramid base paper was prepared from aramid fibrid and aramid chopped fibers with different ratios,and a variety of adhesives were coated onto the surface of aramid base paper to prepare resin-coated aramid paper.The effects of fiber ratio and adhesive type on the mechanical properties,insulation properties,corona resistance,and temperature resistance of resin-coated aramid paper were investigated.The results showed that the adhesive could fill the gaps of the base paper and significantly improve the mechanical properties and insulation properties after coating.When the fiber and adhesive were aramid fibrid and styrene-butadiene latex(SBL),respectively,the resin-coated aramid paper had the best comprehensive performance with high strength(tensile index 40.4 N·m/g),good insulation(39.019 kV/mm),excellent corona resistance(292.12 h)and high-temperature resistance(Td10%=390.1℃).In order to further improve the performance of the coated aramid paper,it was hot-pressed to reduce the internal voids,and the three-factor and four-level orthogonal experiments were conducted to optimize the hot-pressing conditions.When the hot-pressing temperature,pressure,and time are 240℃,15 MPa,and 20 min,the performance of aramid coated paper could be significantly improved again.Secondly,mica with excellent insulation and corona resistance was incorporated into styrene-butadiene latex to prepare mica-containing coating.Then,the mica-containing coating was further used to prepare aramid mica paper via the coating method.The effects of mica particle size and SBL content on the properties of aramid mica paper were studied.The results demonstrated that the performance of the aramid mica paper was optimal when the particle size of the mica was 5 μm,and the corresponding breakdown strength and tensile index of the aramid mica coated paper after hot-pressing are 34.082 kV/mm and 30.706 N·m/g,respectively.Compared with the wet-forming process,the breakdown strength and tensile index of aramid mica paper prepared via the coating method increased by 18.3%and 41.7%,respectively.The SBL content also had a significant effect on the physical properties of aramid mica coated paper.As for a low SBL content(6%),the aramid mica coated paper had the best corona resistance(corona resistance time reached 673.15 h),which was 11.8 times higher than that of aramid paper.As for a high SBL content(20%),the aramid mica coated paper has good mechanical and electrical properties,with a tensile index and breakdown strength of 34.6 N·m/g and 48.59 kV/mm,which was increased by 271%and 73%compared with aramid paper.It is worth noting that the aramid mica paper prepared by the coating method not only kept a similar thermal resistance but also had the following advantages:flexibility,high interfacial bonding strength,surface mechanical stability,no lint or powder,excellent resistance to moisture resistance,abrasion and damage.Finally,nano mica flakes and aramid nanofibers(ANFs)were prepared by intercalation and alkali dissolving methods,respectively.And both of them were added to the coatings of SBL and mica,making the use of the "size and polarization effect" of nano-fillers to decrease the porosity,extend the breakdown path and enhance the insulation performance.The results showed that both nano-fillers could effectively improve the mechanical properties and insulation properties of aramid mica coated paper.When 0.6 wt%nano mica flakes were added,the breakdown strength,tensile index,and internal bonding strength of the aramid mica-coated paper reached the maximum values of 55.96 kV/mm,38.2 N·m/g,and 506.1 J/m2,increased by 15.2%,10.4%,and 8.1%compared with aramid mica coated paper without nano mica,respectively.When the amount of ANFs in the coating was 0.6 wt%,the breakdown strength of the aramid mica coated paper reached the maximum value(56.67 kV/mm).Aramid mica coated paper with nano mica flakes and ANFs could maintain excellent thermal stability(Td10%>390.1℃)and corona resistance(corona resistance time was greater than 500 h).Compared with the traditional mixing method,the coating method could significantly improve the comprehensive performance of aramid mica paper.