Structure Property Process Relationships for Meltblown Fibrous Media

Meltblowing (MB) is a unique one-step process for producing self-bonded fibrous nonwoven membranes directly from polymer resins, with average fiber diameter ranging between 2 and 5 microm. Determining routes for making nano- or submicron-fibers using this process are desirable since there are many manufacturing assets that are already in place. We used experimental techniques to investigate the influence of different die configurations and operating conditions on fiber and web characteristics. We also report on strategies for reducing the fiber size below one micron to achieve higher filtration quality at lower basis weight relative to the conventional meltblown fibers. The results show significant promise for the use of nano-meltblown fibers in filtration applications.;Fiber formation during the MB process is critically dependent on the aerodynamics of the process as the drag force due to high-speed air jets is the main cause of fiber attenuation. Typically, the air velocity and temperature decreases rapidly due to the early mixing with the ambient air near the die tip. We used simulation as well as experimental techniques to examine design strategies to control the air flow field below the polymer injection point in order to achieve higher fiber attenuation to fabricate small meltblown fiber. In particular, we examined the use of air constrictors and secondary air jets to maintain and/or increase the maximum centerline air velocity and temperature below the injection point for a longer distance than is possible with the existing systems. Computational fluid dynamics (CFD) simulation for the new die configurations showed that vertical or inclined air constrictors and secondary air jets around the primary air jets would keep the centerline air velocity and temperature at their maximum values for 10-15 mm longer in distance below the die face than in the case of the reference die configuration. Therefore polymers streams are kept at a temperature near the polymer melting temperature (Tm) at higher air velocities for a longer period. Hence higher fiber attenuation leading to meltblown webs with smaller fiber diameters are fabricated. Experimental verification of some of these simulated configurations showed 20-60% reduction in fiber diameter, reduced pore size, and improved the filtration properties for the fabricated nonwovens webs.;The common meltblowing die technology is based on the slot concept. A recent meltblowing die design that is based on the parallel plate concept is promising and currently being used in adhesive meltblowing. This new die design is simple and a low cost option to the expensive conventional meltblowing dies technology. In this study, we also investigated the new technology capabilities, compared it to the conventional MB die performance, and pointed out to the main challenges that may be facing this technology. We showed that the parallel plate MB technology is capable of fabricating meltblown webs with fiber diameter between 5-20 microm by modulating the process conditions, but its performance needed to be enhanced to be capable to compete with the conventional slot MB die technology.