Abstract:

Objective In the melt blowing process, the polymer melt is attenuated and formed into microfiber in the high velocity airflow field, the fiber motion together with the airflow field play an important role in the fiber formation. The complex interplay between air velocity and polymer motion can affect the final fiber quality, and the velocity and distribution of the airflow requires study and analysis. The information of fiber motion is sought to reveal the mechanism of the formation of microfibers during melt blowing process. It is essential to understand and optimize the melt blowing process.
Method The airflow field was simulated with the computational fluid dynamic approach for the melt blowing process. The characteristics of distribution for airflow field were investigated, with attentions paying to the airflow alone zone, airflow confluence zone and airflow merge zone. The fiber was modeled using mixed Euler-Lagrange approach, and the motion was predicted in the melt blowing process. The standard linear solid (SLS) models in the bead-viscoelastic fiber element were proposed for melt blown fiber formation simulation. The simulated fiber motion was compared with the fiber motion in experiment, which was captured with a high-speed camera.
Results The simulation results show that there are two airflow recirculation zones between the converging jets for the melt blowing slot-die. The recirculation zones are in a subtriangular area near the die face and are filled with hot air (Fig.3). It is important for not only rapid fiber attenuation but also energy conservation. The melt blowing airflow field is divided into the airflow alone, the airflow contact fusion zone and the airflow merge zone. The distribution of the airflow field causes the airflow velocity to vary at different positions. At x=0 mm and z<1 mm, the airflow velocity fluctuates because there is a recirculation area of the airflow under the melt blowing die. When z>1 mm, the airflow velocity increases first and then decreases. The maximum air velocity is 165.88 m/s at z=5 mm, indicating that the two airflows merge together (Fig.4). For the melt blowing fiber motion simulation, it is found that the airflow turbulent fluctuations are related to the fiber motion in the melt blowing process. The fiber path shows a small perturbation developing into the whipping in the airflow zone alone. As the fibers continue to move, there is fiber crossing and loop formation in the airflow merging zone. In the airflow contact fusion zone, the fiber whipping increases and the drawing ratio is the largest (Fig.5, Fig.6), and this is the main zone of fiber refinement. The fiber motion is also recorded and observed using high-speed camera. In the beginning, the fiber is a straight segment and fluctuates, and then the trajectory forms the fiber loop in the move path. The fiber loops are drawn and elongated in the airflow field. The elongation of the fiber loop results in the attenuation on fiber diameter (Fig.7). The whipping, folding into loops and trajectories of fiber motion are consistent with the simulation results.
Conclusion The airflow field for the melt blown slot-die is simulated. There are three airflow distribution zones: the airflow alone, the airflow contact fusion zone and the airflow merge zone. The distribution of the airflow field has an important influence on the fiber motion. The motion process of the fiber in the melt blowing airflow field is simulated and analyzed using the Euler-Lagrange method. It is pointed out that the melt blowing fiber was divided into three stages in the drawing process. The polymer jet appears with the whipping motion in the airflow alone and the whipping of fiber increases in the airflow contact fusion zone. The fiber movement appears to cross and fold into loops in the airflow merge zone. The motion of the melt blowing fibers is also captured by high-speed camera experiments. It is found that the continuous semi-annular loops appear during the fiber forming process, and the whipping of fiber has an important effect on fiber attenuation. Using airflow simulation and fiber model, fiber attenuation is known to be strongly dependent on airflow field and fiber motion. This work provides an insight that the melt blowing airflow field and the fiber attenuation which gives a useful understanding for the melt blowing fiber formation.

Key words: melt blowing technology, melt-blown fiber, airflow field, fiber model, motion trajectory, Euler-Lagrange approach