| Since the outbreak of COVID-19 in 2019,the demand for air filtration products –masks,which can interrupt the spread of the virus,has increased dramatically.The filter layer of the mask is mainly made of meltblown polypropylene(PP)electret nonwovens,but the non-biodegradable characteristics of PP and its extensive use have aggravated the deterioration of a series of environmental problems.Biodegradable polylactic acid(PLA)is widely used in the textile field because of its good thermal processing and mechanical properties,so it is expected to become a suitable alternative to meltblown PP electret nonwovens.At present,the selection of PLA meltblown raw materials,the further development of meltblown PLA electret nonwovens preparation technology,and the stability research of electret nonwovens filtration performance have become the focus of attention of industry and academia.In this paper,the relationship between the melting index of raw materials and the fiber structure,optical purity of raw materials and the crystallization properties of electret nonwovens in meltblown PLA electret nonwovens is first explored,and then two meltblown PLA electret nonwovens with melting indices close to but different light purity are selected for isothermal annealing treatment to study the influence of crystallization on the mechanical properties of electret nonwovens.Finally,based on the relationship between optical purity and the crystallization performance of electret nonwovens,the relationship between crystallinity and the temperature and humidity stability of electret nonwovens was explored under isothermal annealing.The specific conclusions are as follows:First,the average diameter of fibers in meltblown PLA electret nonwovens was highly correlated with the melting index of the raw material,and the average diameter of the fibers in the prepared electret nonwovens decreased from 9.4 um to 2.0 um as the melting index of the raw material increased from 87 g/10 min to 1844 g/10 min.Among them,when the melting index is higher than 400 g/10 min,the average diameter of the fibers in the meltblown PLA electret nonwovens is less than 3.5 um,and the filtration efficiency can reach more than 99%.In addition,with the increase of decay time,the surface voltage and filtration efficiency of meltblown PLA electret nonwovens decrease,and the lowest can be reduced to 30 V and37.0%,respectively.Among them,this performance decline trend is the most obvious within 2h,with a decrease of about 55%,but it tends to be flat after more than 2 h.Second,with the increase of the optical purity of raw materials from 95.2% to 99.2%,the crystallization capacity of meltblown PLA electret nonwovens increased,but the overall crystallinity was low,the highest was about 8.9%.The optical purity of the raw material did not have a significant effect on the average diameter and filtration efficiency of the fibers in the electret nonwoven,that is,it basically remained unchanged with the increase of the optical purity of the raw material.In addition,in the entire optical purity range of the raw materials studied(95.2%-99.2%),the crystallinity and mechanical properties of meltblown PLA electret nonwovens increased with the increase of annealing time.Among them,the mechanical properties of electret nonwovens prepared with high optical purity(99.2%)had the highest increase in mechanical properties,specifically,compared with unannealed electret nonwovens,the increases of tensile strength,tensile modulus and elongation at break were 404%,251%and 564%,respectively.Third,with the increase of ambient temperature and humidity,the surface voltage and filtration efficiency of meltblown PLA electret nonwovens decrease.Among them,when the ambient humidity is constant and the ambient temperature increases from 30 oC to 50 oC,the surface voltage and filtration efficiency of the electret nonwovens are not significantly reduced(≤3%);When the ambient temperature is constant and the ambient humidity exceeds50% RH,the surface voltage and filtration efficiency of the electret nonwovens are significantly reduced(≥7%).The increase of crystallinity contributes to the increase of the initial surface voltage and filtration efficiency of meltblown PLA electret nonwovens(maximum increase of 9%);Increasing the crystallinity can improve the stability of the surface voltage and filtration efficiency of meltblown PLA electret nonwovens to ambient temperature and humidity. |
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