| During the ongoing pandemic of the novel coronavirus（COVID-19）,the filtration of droplets or aerosols carrying the virus through efficient air filter materials is considered an effective public epidemic prevention method.At the same time,relevant studies have shown that fine particulate matter(PM2.5)in the air can penetrate into the human lungs and even invade the blood.Long-term exposure to polluted air with high PM2.5 content will lead to higher lung cancer incidence and cardiovascular disease,causing serious harm to human health.Wearing masks is considered to be an effective protective measure.In order to achieve efficient filtration of PM2.5 and other harmful particles,we urgently need air filter materials with excellent performance to protect human life and health.The basic morphology,fiber fineness,pore size distribution and porosity of melt-blown nonwovens have an important influence on the filtration performance of air filter materials.At the same time,certain mechanical,air permeability and hydrophobic properties should be taken into account.In this paper,polyvinylidene fluoride（PVDF）with good dielectric and polarity was blended with biodegradable polylactic acid（PLA）by melt blending.An environmentally friendly electret air filter material with fine fiber diameter,small pore size,high filtration efficiency and low filtration resistance was prepared by melt-blown spinning process combined with corona electret.The optimal addition mass ratio of PVDF was explored to be 0.3wt%.The basic properties of PLA and PLA/PVDF melt-blown nonwovens were studied by testing and characterizing the morphology of fiber network,fiber diameter and distribution,thickness and surface density,fiber pore size and distribution,porosity,mechanical properties,air permeability,hydrophobicity and filtration performance.In order to further improve the filtration efficiency of the prepared PLA and PLA/PVDF melt-blown nonwovens,the corona electret process was optimized.It is found that the change of electret voltage has the greatest influence on the filtration performance of PLA/PVDF melt-blown nonwovens,and the change of other process parameters has a secondary influence on the filtration performance.The surface potential of the melt-blown nonwovens generally increases with the increase of the electret voltage,the prolongation of the electret time,the increase of the electret distance and the increase of the number of electrets,and then reaches saturation or slightly decreases.According to the comprehensive process parameters,the electret voltage is selected to be about 30 k V,the electret time can be slightly shorter,and 10 s is selected to save the cost.The number of electrets is twice,the electret distance is 7 cm,and the annealing treatment at a certain temperature before the electret can also improve the filtration efficiency.The electret performance of melt-blown nonwovens after corona electret is closely related to the final filtration performance.In this paper,the crystallization behavior,charge storage performance and the relationship between the mechanism and filtration performance of PVDF on PLA corona electret air filter material were systematically studied.It was found that the introduction of PVDF had an important influence on the crystallization properties of PLA/PVDF melt-blown nonwovens.The introduction of PVDF made the PLA crystal phase more orderly,the crystallization rate increased,and the crystallinity increased.At 30 k V voltage,the initial surface electrostatic potential of PLA/PVDF corona electret melt-blown nonwovens is as high as 3 k V,the peak value of thermally stimulated discharge test is higher,and the charge storage capacity is significantly improved.In particular,at a flow rate of 85L/min,the filtration performance of PLA/PVDF single-layer corona electret melt-blown nonwovens can reach 85%,and the filtration resistance is less than 40 Pa.Compared with the corona electret melt-blown nonwovens without PVDF,the filtration efficiency is increased by more than 20%.Importantly,the charge storage mechanism of PLA/PVDF melt-blown nonwovens after corona electret is proposed,indicating that the charge storage performance of PLA/PVDF melt-blown nonwovens is improved,which ultimately improves the filtration performance.In addition,the electret stability of the corona electret melt-blown nonwovens was studied.After 30 days,the filtration efficiency of the PLA/PVDF melt-blown fabric can still be maintained at 80%,and the electret stability was improved by annealing after corona electret.Finally,it is concluded that the corona electret PLA/PVDF melt-blown nonwovens has a good stability of surface potential and internal stored charge,which ensures the durability of filtration performance. |
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