Design Of The Wide Coat-Hanger Die And The Fabrication Of Nanofiber In Melt Blowing Process

In this thesis, multi-objective optimization of the coat-hanger die, the design of wide coat-hanger die, fabrication of melt blowing nanofiber, properties of melt blown web structure and mechanism of nanofiber formation in melt blowing were investigated. First, the polymeric fluid flow in coat-hanger die was simulated using the three-dimensional finite element method. The outlet velocity and the residence time distribution were analyzed, which could affect the quality of melt blown nonwoven fabric. A method combining the orthogonal array design and the numerical simulation is used to optimize the geometry parameters of the single coat-hanger die with uniform outlet velocity and minimal residence time. The significant factors were obtained and they were used for optimization of coat-hanger die with the multi-objective genetic algorithm method in the global domain. Then, two different methods were proposed for the design of wide coat-hanger die. A simple analytic method was developed for the double coat-hanger die and an ellipse cavity was inserted in the double coat-hanger die slot. The result showed that the outlet velocity CV value was less than1%for3.4meter width of double coat-hanger die. Meanwhile, multi-manifold width distribution was designed for different polymer melts. The uniform outlet velocity and samll pressure drop were obtained.Melt blowing micronfibers and nanofibers have been produced using different multi-holes dies from available polymers under commercial process conditions. The different properties between microfiber webs and nanofiber webs were compared.The results showed that the smaller pore size, the higher air permeability and the larger hydrohead were achieved for nanofiber webs, which fiber average diameter was less than one micron. Finally, the Rayleigh instability theory of melt blown nanofiber was established, which illustrated the reason of fiber breakup.Both theory and experimental evidenced that surface tension influenced the fiber breakup. Besides, by comparing similarities and differences of melt blowing and electrospinning, this research try to provide a general understanding of limit on barrier of nanofiber from melt blowing technology.This thesis contained6chapters.In chapter1, the research work refering to the theory and experimental of melt blowing technology at home and abroad was reviewed.They are mainly focused on design and optimization of coat-hanger die, airflow field of melt blowing, fiber drawing model of melt blowing and the research of melt blowing process.In chapter2, the multi-objective methods based on the numerical simulation of the polymer flow was proposed to optimize the geometry parameters of the coat-hanger die with uniform velocity and minimal residence time.A method combining the orthogonal array design and the numerical simulation was used and the effects of the manifold angle, the land height and the slot gap on the outlet velocity and the residence time were investigated.The results showed that the effects of all the three parameters were significant for the outlet velocity while manifold angle and slot gap were significant for the residence time.Besides, the interaction between manifold angle and slot gap influenced the outlet velocity. The significant factors were determined and the optimal geometry parameters of coat-hanger die were obtained using orthogonal array design. The CV value of outlet velocity and the residence time decreased to7.7%and292s respectively. Then, the vector evaluated GA (genetic algorithm) method was further used to find the parameter values for uniform outlet velocity and minimal residence time in global domain.The relationship between land height and slot gap was improved.The optimal geometry of coat-hanger die was obtained using goal programming function. The CV value of outlet velocity and the residence time of optimal coat-hanger die were5.1%and169s respectively.In chapter3,the wide coat-hanger die was designed based on the modification of manifold connected,the inserted second manifold and multi-manifold.The uniform outlet velocity was achieved. Based on the previous study, the simple connected manifold for the wide coat-hanger die was not able to satisfy the requirements of melt blowing practical production.In this chapter, a numerical approach was developed for optimal design of double coat-hanger die. The flow convection zone at the center of simple connected double coat-hanger die was improved. And then, the second manifold was inserted in the double coat-hanger die slot for the melt flow distribution. The result of outlet velocity CV value was under1%for3.4meter width of double coat-hanger die and the melt pressure drop was also decreased. In addition, a multi-manifold design method was proposed for the wide coat-hanger die. Both the CV value of outlet velocity and pressure dropt were decreased with the multi-manifold’s distribution. At the same time, the multi-manifold coat-hanger die could reduce drawbacks which might be generated by two metering pump for feeding polymer melt. A variety of polymer distributions were adapted in the multi-manifold wide coat-hanger die.In chapter4, melt blown microfibers and nanofibers have been produced using different multi-holes dies under commercially processing conditions. Mean fiber diameter along with fiber diameters distribution were studied. The properties of melt blown web structure were tested and compared between microfiber webs and nanofiber webs.The results showed that the fiber diameter decreased with the air pressure, air temperature increasing. The fiber diameter distribution which produced by multi-hole dies was normal distribution, which was different from the one observed in a single hole melt blowing process. The average of melt blowing fiber diameter was from600to800nm. The properties of melt blown webs were tested and investigated using instruments, such as density of webs, pore distribution, air permeability, hydrohead and elongation at break. Nanofiber web’s density was lower but the air permeability and elongation at break was similar with microfiber web’s. The web’s pore size decreased and became more uniform with the fiber diameter decreasing, especially for nanofiber webs. Meanwhile, more uniform nanofiber diameter was obtained using the new designed melt blowing die.In chapter5, the nanofiber breakup was observed which was produced by multi-holes dies with different commercially processing conditions.The Rayleigh instability theory of melt blowing was established and illustrated the reason of fiber breakup. The experimental results revealed that polymer viscosity, fiber diameter and the melt blowing process conditions, such as air pressure, air temperature, significantly influenced the fiber breakup.Besides, a high-speed camera was used to capture the fiber path below a single-hole melt blowing slot die and electrospinning process.The air flow field and electric field for fiber drawing were simulated and analyzed.The results showed that melt blowing fiber whipping amplitude was smaller than electrospinning’s.The electric field for electrospining fiber was more uniform while the drawing force for melt blowing attenuated quickly in the same collect distance.Comparing with melt blowing, process and electrospinning process, a general understanding of limit on the nanofiber for commercial melt blowing was explored and the results showed that the fiber diameter of melt blowing was hard to reach500nm under the commercial melt blowing conditions.In chapter6, conclusions and outlooks were presented.Main research findings and insufficiencies of this thesis were summarized. Meanwhile, the further research points involved in this field was described.