Study On Correlation Between The Microstructure And The Performance Of Electrets And Filtration Performance Of Melt-blown PLA Electret Air Purification Materials

At present,the vast majority of melt-blown electret air purification materials on the market are made of polypropylene(PP).However,polypropylene is extracted from non renewable petroleum resources,and it is difficult to be degraded.A large amount of waste brought about by large-scale industrial applications will pollute the environment.With the continuous deepening of ecological civilization construction,people's awareness of environmental protection has been continuously improved.It has become an urgent need to develop a recyclable and biodegradable environment-friendly melt-blown electret air purification material.Polylactic acid(PLA)is a biodegradable and biocompatible polymer derived from renewable plant resources.Melt-blown air purification materials using PLA as raw material have extremely high environmental value.Therefore,the development of melt-blown PLA electret air purification materials to replace melt-blown PP electret air purification materials has become a very attractive option.Electret is a kind of dielectric material which can store charge for a long time.In addition to the mechanical barrier mechanism of traditional air filters,electret air purification materials also have a unique electrostatic filtration mechanism.Without increasing the filtration resistance,the filtration efficiency can be significantly improved,and at the same time,the bacteria can be eliminated.How to improve the charge storage performance and stability in the electret is the key to the wide application of melt-blown PLA air purification materials.Previous studies have found that the charge storage capacity of electret materials is determined by the charge trap density in the material.The generation of traps is closely related to the microstructure of the material,especially the crystal structure.Up to now many studies have been done on the preparation process of melt-blown PLA air purification materials.However,the regulation of the crystal structure of the melt-blown PLA material and the correlation between the crystal structure and the electret performance and filtration performance have not been thoroughly studied.In the melt-blown process,how to regulate the crystal structure of PLA material and optimize its electret characteristic is rarely reported.The mechanism of charge storage stability inmelt-blown PLA electret materials is still unclear.In this paper,the relationship between the microstructure of melt-blown PLA electret air purification materials and the performance of electrets and filtration performance is studied.On the micro-meltblown testing machine,the regulation of the crystal structure of melt-blown PLA material and the optimization of the electret performance and filtration performance of melt-blown PLA material are realized by changing the melting process condition,adding salt stearate additive and heat treatment.According to the crystal structure,electret performance and filtration performance of the samples under different conditions,the correlation between microstructure and electret performance and filtration performance was analyzed.And considering the final application of the product,a pilot product was prepared on a large melt-blown test line to demonstrate the correlation between microstructure and electret performance and filtration performance.The main findings obtained are as follows:(1)The melt-blown PLA material can form an electret by corona charging.After the formation of the electret,the filtration efficiency of the melt-blown PLA material will be significantly improved.The increase in filtration efficiency is derived from the electric field of the electret rather than the increase in filtration resistance.The deference in filtration resistance is mainly ascribed to the different fiber diameters and their distributions.More narrow distribution of fiber diameter leads to more compact fabrics and to higher filtration resistance.(2)Different crystalline structures of melt-blown PLA materials will result in differences in their electret charge-trapping feature.The melt-blown PLA materials with semi-crystalline characteristics display a regular charge-trapped level distribution and amorphous structure hints the diversity of charge-trapped levels.(3)The charge storage capacity of melt-blown PLA electret is related to its crystallinity.The higher the crystallinity is,the higher the charge storage capacity is.However,the charge storage stability is related to the proportion of its crystal form.The higher the proportion of crystal form with more ordered structure is,the better the charge storage stability is.(4)In the range of test conditions,with the decrease of receiving distance,the increase of temperature and pressure of hot air,the crystallinity of sample increases,the amount of stored charge increases,and the filtration efficiency increases.The charge storage stability of the sample increases with the decrease of receiving distanceand the increase of hot air pressure.The sample has the best charge storage stability when the hot air temperature is 210?.(5)A small amount of salt stearate can be used as nucleating agent for melt-blown PLA,which is beneficial to improve the crystallinity of the sample.With the increase of salt stearate concentration,the crystallinity of the sample increases,which leads to the increase of the amount of stored charge,and then improves the filtration efficiency,and the increase range decreases with the increase of salt stearate concentration.With the increase of salt stearate concentration,the proportion of crystal phase with low order in the sample increases greatly,which leads to the decrease of trapped energy level and the stability of charge storage.(6)The heat treatment can improve the crystallization property,charge storage performance and filtration performance of the melt-blown PLA air purification material,but the improvement effect at 50? in a short time is weak.