Study On The Relativity Of Crystal Structure And Charge Storage Properties Of Melt-blown Polypropylene Non-woven Fabric

In recent years,as the haze,PM2.5 and other vocabularies related to air quality are well known,breathing clean air has become an urgent need for people.To breathe clean air,using the air purifiers,masks and other related filtering equipment is the most direct and effective method.Therefore,the low resistance,high filtration efficiency of air filtration materials are more and more people’s attention,the market demand is growing.The traditional air filter material is mainly depended on the inertial deposition,gravity deposition,Brown diffusion and other mechanical blocking effect to achieve the purpose of air filtration.For the filtration of particles smaller than 1μm,it is necessary to provided the density of the material,but the resulting high resistance will affect the comfort when wearing a mask,the impact on the air purifier is high energy consumption.Looking for the low flow resistance and high efficiency of the new filter material has become a top priority.Electret air filter material came into being.Electret is the dielectric material that can stored space charge and dipole charge for a long time.The electret air filter material represented by melt-blown polypropylene non-woven fabric not only has the mechanical filtration mechanism,but also has the special electrostatic filtration mechanism,which can make use of the electrostatic adsorption of the electric field generated by the charge existing in the fiber and achieve the interception of dust.In addition,because of the electrostatic field can stimulate bacterial protein variation,damage bacterial cytoplasm and cell membrane,and destroy the bacterial surface structure,which can kill the bacteria.Melt-blown polypropylene electret filter material with high efficiency,low resistance,sterilization of multi-function makes it the first choice for medical masks material,and in the medical facilities clean and clean public places and other fields has been widely used.The electret charge storage performance of materials is the key to determine whether materials can be widely used.In the previous work,the research group put forward the theory of "interface trap",its the accumulation of charge on the two end faces of the crystal grains,the interfacial polarization between the crystalline and crystalline regions,and between the crystalline and amorphous regions is the main mechanism for charge storage in electret polymers such as polypropylene.Based on this theory,this paper study the relationship between the charge storage properties and crystal structure of the material starting from the crystal structure control of melt-blown polypropylene non-woven materials.The main achievements of this paper are as follows:(1)The mesomorphic crystal melt-blown polypropylene non-woven materials were prepared on a micro melt-blow testing machine,and it transformation from mesomorphic crystal to α-crystalline by heat treatment at 120 ℃,the crystal structure of the materials was analyzed by X-ray diffraction and the electret was formed by a corona-injected method.Based on the relationship between material charging performance and filtration efficiency,the charge storage stability of mesomorphic crystal and α-crystal was studied by measuring the stability of filtration efficiency,and the trap depth of the charge storage was studied by using thermal stimulated measurement technique.The results showed,in the electret melt-blown polypropylene non-woven materials,the charge storage stability of the α crystal is better than that of the mesomorphic crystal,the charge-off temperature of the α-crysta material is 125℃and the mesomorphic crystal material is 85℃,and the crystallinity and grain size of the α-crystalline material are larger than those of the mesomorphic crystal.The correlation between charge storage stability and crystal structure shows that The improvement of the charge storage stability of the α-crystalline material is attributed to the increase of the crystallinity and the grain size.(2)Prepared the mesomorphic crystal and α-crystal of melt-blown polypropylene non-woven mixed crystal materials by controlling the aging temperature.The proportions of mesomorphic crystal and α-crystal in mixed electret material were obtained by X-ray diffraction and correlation analysis.Experimental results showed,the mesomorphic crystal is unstable,it’s easy to changed to the α-crystal under the heating condition,and the conversion rate increases with the increase of temperature.The results of the thermal stimulation discharge of materials at experiment the temperature of 60 ℃,90 ℃,120 ℃ and after a certain period of time to dealt with corona-polar showe that the charge off-trap temperature increases with the increase of temperature and treatment time.Compared the crystal phase structure test results,these changes are closely related to the increase in the ratio ofα-crystals in the material.(3)The melt index of the masterbatch has a great effect on the crystallization properties of polypropylene,prepared two kinds of polypropylene melt index of1200g/10 min and 1500g/10 min of the electret melt-blown polypropylene non-woven fabric in the micro-melt blow testing machine.The results of the measurement of thecrystal structure of the obtained material are showed,compared with the material with melt index of 1200g/10 min,the melt index of 1500g/10 min that its crystallinity of the material is higher,the proportion of α-crystal is lower and the grain size is little difference.After the above materials are formed electret by corona-injected,the filtration performance and thermally stimulated measurement experiment show that the charge storage capacity of the material is stronger,but the charge-off temperature is not changed.This result again shows that the charge storage capacity of the material is closely related to the influence of the concentration of the α-crystal.α-crystal larger scale and the charge storage capacity stronger.(4)To study the effect of additives on the crystallization behavior and charge storage properties of melt blown polypropylene nonwovens,prepared melt blown polypropylene nonwovens with the stearate concentration of 0%,0.5% and 5% in the micro melt-blow testing machine,and its become the electret material by the corona-injected.The crystal structure and charge storage ability of these three materials were tested and analyzed shows,the incorporation of stearate can improve the crystallinity,the proportion of α-crystal and the grain size,the greater the concentration of stearate incorporated,the greater the effect.Not doping the crystallinity,α-crystal scale and grain size of the obtained materials is 35.5%,25.89%,4.13 nm and the stearate concentration of 5% there is 37.02%,35.75%,4.92 nm,and the filtration efficiency increased from 92% to 95%.