Mechanism Of Fiber Formation In Melt Blowing

The characteristics of air flow field in melt blowing with dual slots and the mechanism of fiber formation in melt blowing are mainly studied in this dissertation. First, through numerical simulation of three-dimensional air flow in melt blowing with dual slots, the distribution characteristic of air pressure field and the pressure difference along the fiber spinline are obtained. Based on the analysis of the characteristic of air pressure field, it is reached that there is melt deformation phenomenon during fiber attenuation in melt blowing with dual slots. Then, the formulae of the strain and the strain recovery are deduced theoretically. The traditional one-dimensional model of melt blowing is improved by introduction of the deformation factor, which is used to describe the deformation phenomenon. The improved model can not only describe the history of fiber deformation, but also improve raise the accuracy in predicting the fiber diameter. By analysis of the melt blowing processing, the hypothesis that the molten polymer flow is a type of shear flow is put forward. The equations about the melt shear flow are established and verified experimentally. Finally, the mechanism is investigated based on the theory of melt shear flow.The dissertation included six chapters.In Chapter1, the literatures relevant to the melt blowing area at domestic and overseas are comprehensively reviewed. They are mainly focused on air flow, equipments and forming theory in melt blowing.In Chapter2, the characteristic of air pressure field is investigated via three-dimensional numerical simulation in melt blowing with dual slots.Through comparison of the melt blowing process with dual slots die and annulus die, it is found that there is the bigger difference between them in air flow field, especially in air pressure field. Through the numerical simulation of air flow field in melt blowing with dual slots, the three dimensional distribution of air flow field is obtained. The conclusions are reached as follows by analysis of air pressure field in melt blowing with dual slots:the air flow field is distributed symmetrically; along the fiber spinline, there are two sections:one section, very near to the spinneret orifice, is called the pressure difference section, where there is the pressure difference between two orthogonal directions. Another section, far away from the spinneret orifice, is called the zero pressure difference section, where there is no pressure difference between two orthogonal directions; in zero pressure difference section, there are also three sub-sections. In sub-sections, the pressure difference value alternates between positive value and negative value. The peak in the second sub-section is the highest and the length of the third sub-section is the longest. In pressure difference section, the absolute value of the pressure difference increases as the distance from the fiber spinline increases.In Chapter3, the deformation phenomenon of the molten polymer is studied in melt blowing with dual slots.In melt blowing processing with dual slots, there exists not only the fiber attenuation, but also the change of fiber cross-sectional shape. First, the formula of strain is obtained through theoretical deducing when the molten polymer is subjected to the non-equilibrium pressure, and the formula of the strain recovery is also obtained after the non-equilibrium pressure disappears. Then, according to the relationship about the strain and its recovery, the history of the change of cross-sectional shape is described in the fiber attenuation. Based on the degree of the defonnation, the whole length of fiber spinline is divided into three districts:the first deformation district, the second deformation district and the third deformation district. The first deformation district is the nearest to the spinneret orifice, where there is a big non-equilibrium pressure value. The second one is farther from the spinneret orifice, where since there is no non-equilibrium pressure the polymer is recovering to its circular section. So, there is small degree of deformation in the second deformation district. The third one is the furthest from the spinneret orifice, in which there is no non-equilibrium pressure, the degree of deformation is zero and the polymer section is circular. Seen from the length, the first district is short, the second one is the longer and the third one is the longest. Afterwards, the traditional one-dimensional model of melt blowing is improved by taking into account the deformation phenomenon of polymer melt. A new parameter, deformation factor, is used to describe the deformation phenomenon of polymer melt quantitatively. The deformation factor is introduced to the part of air drawing in the model to describe accurately the air drawing force to the polymer. The improved model can not only make a distinction between the melt blowing with annulus jet and that with dual slot jets, but also can describe the deformation phenomenon in melt blowing with dual slot jets. Compared with the predicted diameter by Shambaugh’s model, the improved model gives the higher prediction accuracy. Finally, the relation between the deformation factor and its effect on the fiber attenuation is discussed theoretically.In Chapter4, the shear flow field of polymer melt in melt blowing is researched by theory and experiment.Through the comparison between the melt blowing and the melt-spinning process, the theory about the shear flow is put forward by analysis of the process of fiber attenuation in melt blowing. The main content of the theory includes:the flow field of melt polymer is a type of shear flow field; the equations of the shear flow field are made up of two equations:the momentum equation and the continuity equation. The melt blowing experiments in the traditional conditions and the rapid cooling conditions are planned and finished, the results verify that the theory of the shear flow is correct basically. At the same time, distribution of the velocity field of the internal polymer melt and that of the cross-sectional micro-structure of the melt blown fiber are described. The profile distribution of the velocity field of the internal polymer melt is nearly a serial of the inverse parabolic profiles. There are an annulus contour of microstructure and a microstructure gradient in the fiber cross-section, a lower gradient in the center and higher gradient near the surface. The conclusions in this chapter provide the foundation theoretically that are used to further study how the melt blown fibers come into being and what the micro-structures are.In Chapter5, the mechanism of fiber formation is investigated in melt blowing.On basis of the conclusions from the Chapter4, the distribution characteristic of the shear flow and the mechanism of fiber formation are summarized by further analysis and deduction, the distribution characteristic of the shear flow is that the polymer melt flows in the form of the collective motion as a unit of micro-layer and there is a sliding motion between every two adjacent micro-layer units. The attenuation mechanism of the fiber formation involves different micro-layers that are arranged in a longer length than that before the fiber is drawn along the axial direction in terms of their velocities.The melt blowing experiments in the rapid cooling conditions and the traditional conditions are done, the results verify that the principle of shear flow field and the mechanism of fiber formation are correct. In the meanwhile, it is disclosed that there is an "anti-V" shape microstructure in the longitudinal direction of the melt blown fiber. The conclusions in this chapter also provide the supports theoretically that are used to further research how the melt blown fibers come into being in microcosmic.In Chapter6, the conclusion and outlook are summarized.The main research achievements and conclusions of the dissertation are summarized, and the insufficiencies are also stated briefly. Simultaneously, further research points and directions in melt blowing are presented constructively.