An elaborate investigation was carried out to study the effects of die geometry and process variables on fiber diameter and shot formation of melt blown polypropylene webs. This four part investigation included the study of die orifice di mansions such as diameter and UD ratio, die geometry variables such as nosetip angle, air gap, nosetip setback, face gap, and process variables such as polymer throughput rate, volumetric flow rate of primary air, die-to-collector-distance, resin melt flow rate (MFR), and melt shear viscosity. Also the effects of different process variables on physical and mechanical properties of the webs were studied during phase I. The 6-inch pilot line at UTK was used to melt blow a series of homopolypropylene resins provided by Exxon Chemical Company.
Statistical techniques used to analyze the data in Phase I concluded that polymer throughput was the only variable to have an independent main effect on average fiber diameter when the primary air flow rate was maintained constant while air knife gap and the corresponding nosetip setback, orifice diameter, orifice LID, polymer MFR, and polymer throughput rate were varied. Orifice diameter, resin MFR, air gap, and UD ratio all had a statistically significant effect (interactive) on average fiber diameter and shot formation. It was also found that under the processing conditions employed, 400 MFR was more sensitive to orifice length than 35 MFR. During Phase II with 650 MFR, the air-to-polymer flow rate ratio and air knife gap were maintained constant while polymer throughput rate and nosetip setback were varied. It was learned that the average fiber diameter could be maintained over a range of only 0.46 μm with increasing polymer throughput rate from 0.4 to 2.0 g/hole/min. Under the resin and processing conditions employed, the effect of nosetip setback on fiber diameter was not statistically significant. Orifice UD did not show a statistically significant effect on average fiber diameter. It was also found during phase II that shot notably increased with the increase in polymer throughput rate, nosetip setback and orifice diameter. The general conclusion from phases I and II was that orifice diameter in the range of 15-20 mils had no practical effect on final fiber diameter.
During Phase Ill of this research, 800 MFR webs produced with a 60° nosetip angle were compared to ones made with a 90° tip. The results indicate slightly smaller average fiber diameter for webs obtained with a 60° nosetip. However, the coefficient of variation of fiber diameter was found to be smaller in the case of 90° nosetip angle. The key properties of melt blown webs such as air permeability, bursting strength, and aerosol filtration efficiency to 0.1 μm sodium chloride particles were all higher for the 90° nosetip angle compared to 60° nosetip angle.
Phase IV was carried out to explain the effects of orifice diameter and UD ratio on fiber diameter in terms of viscosity response obtained under different processing conditions. Both the 35 and 800 MFR resins used this part of the research exhibited a linear response between shear stress and shear rate under actual melt blowing conditions with the chosen temperatures, orifice dimensions, and polymer throughput rates
