Preparation And Properties Of Hydro-charging Polylactic Acid/Calcium Stearate Melt-blown Nonwoven Air Filter Materials

With the rapid development of modern industry,industrial pollution has led to an increasing number of inhalable particles in the atmosphere.Coupled with the outbreak of novel coronavirus pneumonia,the novel coronavirus virus and fine particles seriously threaten human life and safety.Melt-blown nonwoven materials were widely used in air filtration materials because of their large porosity and small fiber diameter.However,most melt-blown nonwoven materials on the market were made of polypropylene（PP）material,which was widely used due to its properties.Due to the non-degradable property of PP,many discarded PP materials bring huge environmental problems,and threaten human health.Therefore,the development of green and environmental protection melt-blown air filter materials is a research hotspot.Polylactic acid（PLA）has the advantages of excellent processability,biocompatibility,biodegradability,and abundant raw material sources,making PLA a current research hotspot in biodegradable materials.PLA melt-blown nonwoven materials are the main research direction of degradable melt-blown air filter materials.However,it is difficult for PLA melt-blown nonwoven materials without electret treatment to meet the standard of high-quality air filters.The electret process can improve the filtration efficiency without increasing the filtration resistance.Hydro-charging process has been applied with the characteristics of environmental protection,high efficiency,low pressure drops,and good stability,but its electret mechanism is not clear and needs further investigation.The electret additive calcium stearate（CaSt₂）was added into PLA melt-blown nonwoven material to increase its charge content,to improve the filtration performance of PLA melt-blown nonwoven materials.In this paper,PLA/CaSt₂ melt-blown nonwoven materials were prepared by using PLA and CaSt₂ as raw materials for the melt-blown process.Combined with the hydro-charging process,the green,environmental protection,high efficiency,and low pressure drops PLA/CaSt₂ melt-blown air filtration materials were obtained.The blends of PLA/CaSt₂ with mass ratios of 100/0,99.9/0.1,99.8/0.2,99.5/0.5,and 99/1 were prepared by the melt blending method,and the feasibility of the blends in the melt-blown process was investigated.Five kinds of PLA/CaSt₂melt-blown nonwoven materials of different proportions were prepared by the melt-blown process with five different proportions of PLA/CaSt₂ blends,and the effect of CaSt₂ on the basic properties of PLA melt-blown nonwovens was explored.Five different proportions of PLA/CaSt₂melt-blown nonwoven materials were treated with hydro-charging process.The effect of hydro-charging process on the filtration performance of PLA/CaSt₂ melt-blown nonwovens was studied and the electret mechanism was studied.Then,the water electret process was optimized by taking the electret distance,electret water pressure,and mesh screen conveying speed as orthogonal factors.The main research conclusions were as follows:（1）After the introduction of CaSt₂ into PLA,the melt fluidity of the blend will decrease,the initial decomposition temperature of the blend will first increase and then decrease,and the crystallization rate of PLA will be accelerated.Therefore,the introduction of the appropriate amount of CaSt₂ was conducive to improving the spinnability of PLA/CaSt₂ blends.（2）PLA/CaSt₂ melt-blown nonwoven materials were prepared with five proportions of PLA/CaSt₂ blends by the melt-blown process.After testing and characterization of five proportions of PLA/CaSt₂ melt-blown nonwovens,it was found that PLA/CaSt₂（99.8/0.2）melt-blown nonwoven materials had the best comprehensive basic performance.After the introduction of CaSt₂,the fibers of PLA/CaSt₂ melt-blown nonwoven materials gradually changed from curly to straight,and the fiber diameter,average pore size and porosity decreased.At the same time,the transformation of PLA fromα‵crystal toαcrystal was promoted,and the crystallinity of PLA was improved,which was beneficial to improve the basic filtration performance of the melt-blown nonwoven materials and the fracture strength of melt-blown nonwoven materials.But the elongation at break of melt-blown nonwovens will be reduced.CaSt₂ has hydrophilicity,and increasing the mass fraction of CaSt₂ introduced slightly increases the moisture permeability of PLA/CaSt₂ melt blown nonwoven materials.（3）Five kinds of melt-blown nonwoven materials with different proportions of PLA/CaSt₂were treated with hydro-charging under the same process parameters,and their filtration performance and electret effect were characterized.The water-electret PLA/CaSt₂（99.8/0.2）melt-blown nonwoven materials had the best filtration performance,filtration efficiency reaches96.78%,and pressure drops were only 65.20 Pa.The surface electrostatic potential increases from 67.92 V to 1410 V,and there are negative and positive charge regions between 55-65℃and 65-100℃.The hydro-charging PLA/CaSt₂（99.8/0.2）melt-blown nonwoven materials had a filtration efficiency of more than 95%and a surface electrostatic potential of 1200V after storage for two months.Based on the above tests,the hydro-charging mechanism was proposed,that is,before the friction with the melt-blown fiber,pure water frictions with the pipe,sprinkler head and air and carries a small amount of positive and negative charges.After the friction between pure water and the fiber,electrons and ions transfer occur between the two,and then in the drying process,the charge under the action of hot temperature,The charge moves randomly and is trapped by the charge trap between the crystalline and non-crystalline regions.CaSt₂ can increase the number of charge traps and Ca²⁺can absorb the charge.（4）The hydro-charging treatment of PLA/CaSt₂（99.8/0.2）melt-blown nonwoven materials were carried out according to the orthogonal test scheme.The influence of hydro-charging process parameters on its filtration efficiency was analyzed,and the electret process was optimized.Using electret distance,electret water pressure and mesh screen conveying speed as test parameters,the optimal process parameters were obtained by range and variance analysis:electret distance of 9 cm,electret water pressure of 4 MPa,and mesh screen conveying speed of1 m/min.