Prediction Of The Morphology And Uniformity Of Melt-blown Fibrous Web

The morphology and uniformity of melt blown fibrous web was an important indicator of the state of melt-blown equipment,due to its direct influence on the quality and property of the final melt blown products.Products with great uniformity exhibited a high-level performance that usually had good appearance and quality,as well as strong mechanical properties along cross direction and machine direction.The prediction of morphology and uniformity provided an insight into the mechanism of fibrous web formation,which could help us not only adjust the processing conditions and optimize the product quality,but also guide the commercial manufacture.With the background described above,the present thesis consisted of four main aspects:(1)The simulation and experimental study of the velocity and temperature distribution for the whole airflow field(involving the airflow near the orifice and the collector).(2)The improvement of the previous 3D numerical model for the fiber drawing during melt blowing and the prediction of physical parameters(involving fiber diameter,fiber temperature,fiber velocity and fiber crystallinity)and the spatial path of fibers in the flow field.(3)The simulation of fibrous web formation according to the improved 3D model and the analyze of the basis weight distribution and uniformity of the fibrous web on the basis of the predicted morphology.(4)The simulation of the influence of processing conditions on the uniformity of fibrous web and the evaluation of the significance level of the above mentioned processing conditions.In detail,the thesis was composed of six chapters.In Chapter 1,the reported references related to this thesis were reviewed,including the airflow filed,the fiber drawing model and the structure of melt-blown fibrous web.The significance and purpose of this study were clarified,the innovations were also pointed out.In Chapter 2,the velocity and temperature distribution of whole airflow field including the regions near orifice and the collection screen were predicted.In order to verify the predicted results,hot-wire anemometer was employed to measure the velocity and temperature distribution of whole airflow field.Combined with the structure of collector,the whole airflow field was deeply analyzed.The velocity and temperature of the airflow near the orifice first violently then gradually decreased when x coordinate increased.The velocity and temperature of the airflow linearly decreased in the z direction.The velocity of airflow under the collector surface was not stable and fluctuated due to the acceleration effect of suction under the collector and obstruction of the wire mesh structure.The temperature and velocity of the airflow on the surface of the collector gradually decreased when x coordinate increased.In Chapter 3,the 3D model of fiber motion and drawing in melt blowing were improved.Giesekus constitutive equation was introduced to simulate the viscoelastic behaviors of melt blown fiber.Crystallization kinetics model was used to describe the crystallization during fiber formation from the orifice to the collection screen.Heat of crystallization and thermal radiation were also considered in the energy conservation equation.In addition,some physical parameters of fiber,such as temperature,velocity,crystallinity were predicted on the basis of the improved model and the obtained velocity and temperature distribution of air jet in Chapter 2.The results provided a good theoretical basement to the mechanism of fibrous web formation and uniformity analysis of fibrous web in melt blowing.The results exhibited that the fiber crystallinity extremely increased in the region cm5~0 in the z direction,and it kept almost stable when the fiber flowed out the region.Our predicted fiber temperature displayed that heat was transferred from the surrounding hot airflow to the molten polymer due to the higher temperature of surrounding hot airflow.Then the airflow attenuated quickly in the z direction,the heat was inversely transferred from the molten polymer to the surrounding hot airflow.Finally,the fiber temperature was identical to the room temperature.Compared with the original model,the predicted temperature of this improved model had a better agreement to the experimental data.In our prediction of fiber velocity,when the molten polymer was extruded by the orifice,it was slower than the airflow and was accelerated by the airflow.But the accelerated velocity of fiber gradually decreased due to the decline of the airflow in the z direction.Therefore,the fiber velocity gradually decreased when it reached maximum value.The fiber diameter extremely decreased once the fiber was extruded from the orifice,then the diameter gradually decreased to 10 ?m,this also had a great agreement with the reported reference.In Chapter 4,the predicted method for the morphology and uniformity of fibrous web was presented.The whipping amplitude of fiber near the orifice was obtained by experiment.Combined with the 3D model improved in the Chapter 3,the fibrous web morphology on the moving collection screen was predicted.Then the basis weight distribution of the fibrous web was calculated and the uniformity of web was analyzed.Our modeling results had a good agreement with the experimental data.The meltblown fibrous web in machine direction was more uniform than that in cross direction.The research in this chapter revealed the formation mechanism of meltblown fibrous web,and provided a good guidance to the commercial manufacture.In Chapter 5,the uniformity of the fibrous webs collected under different processing conditions were studied by prediction and experiment respectively.The processing conditions included the temperature and velocity of air jet,the polymer flow rate,the speed of air from suction,the speed of moving collection screen and die to collector distance.The significance level of these processing conditions was evaluated by variance analysis method.It was also found that the uniformity of fibrous web became worse when die to collector distance(DCD)was increasing.When the air temperature increased,the uniformity of fibrous web was firstly improved until310?,then uniformity gradually became worse.The uniformity of fibrous web was decreasing rapidly when nominal air jet pressure was increasing.The swelling of polymer volume flow rate resulted in the worse uniformity of fibrous web.The fibrous web was improved when collector was accelerated or the suction was amplified.But the improvement was gradually attenuated.The analysis of significance level found that DCD and the air jet had a great influence on the fibrous uniformity.Polymer volume flow rate and the suction also provide the influence on the fibrous uniformity.The speed of collector only play a weak role on the uniformity.Conclusions were presented in Chapter 6.The conclusions and defects of the thesis,as well as further research directions involved in this field were discussed.