Ind. Eng. C h e m . Res. 1988,27, 2363-2372
2363
GENERAL RESEARCH
A Macroscopic View of the Melt-Blowing Process for Producing Microfibers Robert L. Shambaugh Department of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019
An energy balance and a dimensional analysis were applied to the melt-blowing process. Actual data from several melt-blowing patents were analyzed; the commercially common Exxon slot die and the Schwarz square and triangular air hole dies were examined. Monodisperse fiber distributions require much less energy to produce than polydisperse fiber distributions. Hence, operation of melt-blown processes in stable region I is inherently more efficient. T h e dominant dimensionless groups in melt blowing are the gas Reynolds number, the polymer Reynolds number, the fiber attenuation, and the ratio of the polymer viscosity to the gas viscosity. Proper selection of these groups will give the best performance for a given die geometry. 1. Introduction In conventional melt spinning, a polymer stream is ejected into a gas, usually air at ambient temperature; see Figure la. The gas performs two functions: it cools the filament, and it exerts a drag force upon the rapidly moving filament. In melt blowing, however, the gas performs a very different task. As illustrated in Figure lb, a high-velocity gas stream impinges upon the polymer as the polymer emerges from the spinneret. The substantial forwarding force obviates the need for the tension provided by a takeup roll. In fact, the fiber speeds in melt blowing are often so high (-30000 m/min) that no mechanical windup is fast enough to take up the fiber. The melt-blowing process can produce fibers that are orders of magnitude smaller than fibers produced by conventional melt spinning. Hence, melt-blown fibers make excellent fdters, have high insulating value, have high cover per unit weight, and have high surface area per unit weight. Also, they potentially have high strength per unit weight. To compare the size of a melt-blown fiber with the size of a conventional melt spun fiber, consider a typical 10 tex (100 denier) polyester apparel yarn (tex, a measure of linear density, is the weight in grams of 1000 m of the yarn). Each of the filaments in this yarn is about 0.3 tex and has a diameter of about 17 pm. In contrast, a typical melt-blown fiber may have a diameter of 2 pm, which is only 0.005 tex, or about 1.4% of the cross-sectional area of the melt-spun fiber. Furthermore, under some conditions, melt-blown fibers of 0.1-pm diameter can be produced. The diameters of these fibers are smaller than the wavelength of visible light (0.4-0.7 pm); fibers of this fineness are nearly invisible to the naked eye or even to a conventional optical microscope. The force of the gas attenuates the fiber much more rapidly than in conventional melt spinning; see Figure 2 for some actual experimental data. For the formation of the smallest, 0.1-pm fibers, the force of the gas attenuates the fiber from the -750-pm diameter of the spinneret down to the final 0.1-pm diameter in about 200 ps over a distance of about 3 cm. This is a 7500X reduction in diameter and a 56000000X reduction in cross-sectional area.
Historically, work on melt blowing dates back to the efforts of V. A. Wente (1954,1956)at the Naval Research Laboratory in the 1950s. Motivation for Wente’s work was the need for microdenier (
