| Polymer ultrafine fibers with the properties of high ratio surfaces and special physical and chemical properties are the high-end new materials,which could be used in many application fields such as in high efficiceny filtration,green energy and tissue engineering.The fiber-spinning methods that fabricate ultrafine fibers from polymer melt have the advantages of environment friendly,and high efficiency,and intrinsic security,which are important approaches to prepare nanofiber in large scales.Melt differential electrospinning,which generates the multiple melt jets due to the self-organization of polymer melt under high electric field not to the small drill holes in traditional melt spinning,could avoid the problems of easy block and high expense of drilling small holes.At present,melt differential electrospinning has the problems such as low efficiency,complex spinneret structure,coarse fiber,and complex collector system for uniform membrane.These problems have hindered the industrialization of melt electrospinning.Therefore,it is of great significance to increase the spining efficiency,simplify the nozzle structure and refine fibers further of melt differential electrospinning for preparing nanofibers greenly and in large scale.First,a melt differential electrospinning basd on a linear slot spinneret was proposed.The proposed linear slot spinneret has the advangtages of simple structure and uniform membrane forming.A micro-flow feeding system,a micro-flow distribution structure and a linear slot melt differential electrospinning device were designed.With interjet distance and fiber diameter as the indexes,the spinning process of linear slot nozzle was comprehensively studied.Under the optimized spinning process conditions,the average interjet distance of linear slots could be as low as 1.9 mm,which is much less than 8 mm of the needle-array electrospinning,which improves the efficiency of melt electrospinning significantly.The linear slot spinneret presents an "edge effect" phenomenon with dense and uniform jet distributed in the middle of spinnerent,and sparse and uneven jet distributed at the spinnerent ends.The dense and uniform jet distribution in the middle provides the basis for the preparation of uniform electrospun fiber membrane.The electric field distribution characteristic of linear slot electrospinning was analyzed.It was concluded that the minimum interjet distance was affected by the maximum electric field intensity at the linear slot nozzle.The uniformity of jet distribution was affected not only by the electric field intensity,but also by the charge moving direction in the spinning space.The electrical model of spinning space was established and was also verified by experiments.The opposite movement direction between ionized ions and jets travelling direction will result in the loss of jet and even the climbing of melt.A method to improve the uniformity of jet distribution was obtained.The effects of melt slot thickness,nozzle width and air knife on the maximum electric field intensity and interjet distance were studied.The influence range of "edge effect" remains unchanged under different width of linear slot nozzle.The addition of air knife would weaken the electric field strength at the nozzle and reduce the density of melt-differential jets.However,the influence of air knife on maximum electric field intensity could be reduced by increasing the recession amount above the die face,or using airflow knife made of electric insulation and high temperature resistant material,or increasing spinning voltage.The research above provided the technical support for the melt jet to refine into nano-scale under the "electricity and wind" coupling field.A V-shaped air duct that could make the high-speed airflow converge at the vertex was designed and the drawing refinement of melt jets in the multi-forces field was realized.The effects of airflow channel width,airflow rate and recession amount of air knife on melt jets and fibers diameter were studied.The experimental results showed that the airflow could not only refine the melt jets,but also improved the efficiency of the jets in the formation stage.Through the numerical simulation results,we obtained that the higher the airflow speed and turbulence intensity were,the finer the electrospun fibers were.The melt electrospun fibers with an average diameter of 825 nm were successfully prepared in large scale.The refining of melt differential jets could be divided into two stages.One was differential refining stage;the other was drawing refining stage.In the differential stage,the relationship between the initial jet diameter and electric field strength,melt flow rate in differential stage was obtained based on the waveform model and force balance.In the drawing refinement stage,the force exerted on melt jets and refining mechanism of various forces under multi-forces field was analyzed.Finally,aiming at the high accuracy problem of air filtration,a special shaped fibers membrane,an oriented ultrafine fibers membrane and a composite fibers membrane,which were prepared by melt differential electrospinning,were designed.Aming at the low dust holding capacity of air filter membrane in high concentration dust environment,a filtering component composing of a granular bed of light-weight foam particles as the primary filter and a high efficiency filter membrane as the two-stage filter was proposed.Finally,a high concentration dust protective mask was designed and field tests were carried out in a coalmine.In conclusion,based on the proposed linear slot spinnerent,this study have systematically studied the electrospinning process parameters,electric field regulation and optimization,fiber refinement by multi-forces field and the application of melt electro-spun fiber membrane in air filtration.This research could give some guiding for the industrialization of melt differential electrospinning untrafine fibers and open up a new application field for the application of melt micro-nano fibers. |
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