| As the air particulate matter pollution became increasingly severe and the people’s demand for air quality continuously increased,people increasingly need new high performance air filtering material and individual protective material.Electret melt-blown polypropylene nonwovens could not only intercept dust in air by mechanical mechanism such as ordinary inertial deposition,gravity sedimentation and diffusion brown,but also clear air and kill common bacteria,viruse by electrostatic effect.Therefore,electret melt-blown polypropylene nonwovens has became the best air filtering materials and got widely concerned because of its features as low resistance,high efficiency and dust removal sterilization.Electret melt-blown polypropylene nonwovens filter efficiency mainly comes from its stored charge.There has studies found that as the growth of polypropylene’s relative degree of crystallinity and/or crystal grain size its charge storage ability enhancement.Therefore,research the correlation of melt-blown polypropylene nonwovens filtering performance and crystal phase structure is very important.Isotactic polypropylene(i-PP)always as raw materials for the preparation of electret melt-blown polypropylene nonwovens.Melt-blown process is a atypical non-isothermal crystallization process,i-PP melt formed melt-blown fiber under joint action of draft by hot air and cooling by room temperature.Melt-blown fiber’s crystallization is mesomorphic’s and alpha’s mixed crystal.Monoclinic system of alpha phase is the most conmmon and stable crystal form,but mesomorphic form always exist as quasi crystal or nearly crystal structure,its very unstable in thermodynamics and hard to got under general process conditions.Although i-PP’s crystallization kinetics already got extensive research so far,but the study of mixed crystal especially reports about correlation of the proportion of alpha/mesomorphic form in mixed crystal and electret charge storage performance are rare.This article uses the self-developed micro of melt-blown experiment machine for the i-PP nonwovens product then carries on the corona polarization to form electret and studied i-PP electret melt-blown nonwovens’ s formation mechanism,alpha/mesomorphic’s proportion control and its affection of crystal structure to the performance of filter.The main research results are as follows:1.Due to the rapid cooling of room temperature and drafting of hot air,i-PP melt formed melt-blown i-PP nonwovens’ s crystal exist as alpha’s and mesomorphic’s under micro melt-blown technology.Mixed crystal’s mesomorphic is a kind of metastable crystal form,its growth under a temperature range which lead easy grow of mesomorphic form.The cooling rate slow down benefit the improvement of the growth of the grain size,crystal orderliness and up proportion of alpha crystal.melt-blown i-PP nonwovens’ s grain size is concerned to the proportion of alpha/mesomorphic,the proportion of alpha crystal form is greater,grain size is larger.2.In order to study the influence of melt-blown technology condition to crystal structure of melt-blown i-PP nonwovens,respectively change micro melt-blown technology’s receive distance,hot air temperature and pressure to produce melt-blown nonwovens.As receive distance change from 10 cm to 18 cm,mixed crystal form Research the influence of process conditions to crystal phase structure and filter performance: receive distance at 10 cm,14cm and 18 cm,the relative crystallinity of nonwovens increases slightly,the proportion of alpha crystal form increased,the density of nonwovens and resistacne decline;As hot air temperature change from 180℃ to 200℃,nonwovens relative crystallinity increased significantly,the proportion of alpha crystal form increases considerably,for the further improvement of temperature,nonwovens’ s relative crystallinity and alpha form proportion hardly increase;As hot air pressure increased from 0.1MPa to 0.4MPa,melt-blown i-PP nonwovens’ s relative crystallinity increased,the proportion of alpha crystal form increase greatly.3.In order to study thermal stability of melt-blown i-PP nonwovens,heat treat nonwovens at different temperatures and study its changes of crystal structure.When heat treatment temperature less than 60℃,melt-blown i-PP nonwovens’ s crystal structure is relatively stable.Further increase of heat treatment’s temperature prompt mixed crystal’s mesomorphic form transform to alpha crystal form,as the heat treatment temperature gradually increased to 130 ℃,nonwovens’ s crystal transformation increaseingly clear,the proportion of alpha crystal form increased significantly and relative crystallinity increased.110℃ is the best heat temperature for that moment melt-blown i-PP nonwovens filter performance is the best.4.In order to study the relation of crystallinity of i-PP nonwovens’ s relative crystallinity and the proportion of alpha/mesomorphic.equipped with different content of stearate,rosin with i-PP raw material and then produce its to melt-blown i-PP nonwovens,carries on the crystal analysis in last.Nucleating agent speed up i-PP melt’s crystallization rate and increase nonwovens’ s relative crystallinity.With the increase of content of nucleating agent,the relative crystallinity of nonwovens increased,grain size and the proportion of alpha crystal form increased significantly.Under the same content of stearate modified i-PP nonwovens’ s relative crystallinity is higher than the corresponding content of rosin modified i-PP nonwovens and the filtering effect is better than the latter.5.As melt-blown i-PP nonwovens’ s relative crystallinity increased,charge storage ability of nonwovens enhancement,filtering efficiency is higher,filter effect is poor when the proportion of mixed crystal’s alpha/mesomorphic is too large or too low,only if the proportion of mixed crystal’s alpha/mesomorphic at medium then melt-blown i-PP nonwovens filter effect is the best. |
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