| A fundamental study was undertaken to assess the process-structure-property relationships as a function of the molecular weight of polypropylene resins, polymer throughput rate, air flow rate, melt blowing die orifice diameter, and extrusion and attenuation air temperatures.; The 35, 300, 600, and 1000 melt flow rate (MFR) resin were melt blown at three throughput rates using two air flow rates and two different orifice diameter die. Generally, all of the resins processed well under the different processing conditions required in this study; however, the optimization of the web properties was more difficult at the higher polymer throughput conditions.; Among the process variables studied, polymer throughput rate had the most notable effects on the physical and morphological properties of melt blown webs. It was hypothesized that the fiber-to-fiber thermal bonding was a dominant factor in controlling the physical properties of melt blown webs; and, the final fiber diameter was dependent on the die swell and the change in polymer-to-air ratio for a given air flow rate. The increase in air flow rate at a given throughput rate appeared to have minimal effect on final diameter.; The average fiber diameter, breaking stress, initial modulus, air permeability, shear stiffness, and web density increased as the polymer throughput rate increased. The breaking strain and bursting strength decreased as the polymer throughput rate increased, indicating a more brittle behavior of the fabric at high throughput rates. The tensile properties and bursting strength decreased with increased MFR. The observed failure modes were different for different processing conditions.; There were minimal differences in molecular weight, percent crystallinity, and x-ray peak intensities between low and high throughput webs. However, the differences in molecular weight, percent crystallinity, and x-ray peak intensities between resin and web was noticeable, especially in low MFR resin webs. The web crystallinity increased with increased MFR; and, the web molecular weight decreased with increased MFR.; The DSC thermograms and x-ray diffractograms of higher throughput webs (especially in 35 and 300 MFR webs) indicated the presence of two crystalline species. |
