Electrostatic Spinning and Properties of Ultrafine Fibers
Veli E. Kalayci, Prabir K. Patra, Steven B. Warner
Introduction:
Electrospinning is not a new technology for polymer fiber production. It has been known since the 1930’s; however, it did not gain significant industrial importance due to the low output of the process, inconsistent and low molecular orientation, poor mechanical properties and high diameter distribution of the electrospun fibers. Although special needs of military, medical and filtration applications have stimulated recent studies and renewed interest in the process, quantitative technical and scientific information regarding process and product characterization are extremely limited.
Traditional methods of polymer fiber production include melt spinning, solution spinning and gel state spinning. These methods rely on mechanical forces to produce fibers by extruding polymer melt or solution through a spinneret and subsequently drawing the resulting filaments as they solidify or coagulate. By using these methods, typical fiber diameters in the range of 5 to 500 microns can be produced. The consistently producible minimum fiber diameter is on the order of a micron.
The principle of electrospinning is to use an electric field to draw a positively charged polymer solution from an orifice to a collector. This creates a jet of solution from the orifice to the grounded collection device. The jet emerges at the base from the nozzle, which has a geometry of a cone (Taylor cone). Then it travels to form a stretched jet, and then divides into many fibers in the splaying region. But splaying may not always be the case. Rutledge et al [1] observed a rapidly-rotating spiral jet, which is indistinguishable from splaying phenomenon to the naked eye. They named this new phenomenon as whipping motion. Whatever the mechanism is, the fibers are eventually collected on a grounded metal screen.
Using electrical forces alone, the electrospinning process can produce fibers with nanometer diameters (Fig. 1). Because of their small diameters, electrospun fibers have larger surface-to-volume ratios, which enable them to absorb more liquids than do fibers having large diameters. Small pore sizes of electrospun fibers make them suitable candidates for military and civilian filtration applications. They may eventually find application in composite materials as reinforcements.
Fig. 1 SEM Photograph of Electrospun Polyacrylonitrile Fiber
Objectives:
- Develop a mechanism for understanding the mechanism of charging and charge loss through the entire electrospinning process and measure charge in electrospun fibers.
- Control the spinning process variables in relation to the production rate and fiber diameter distribution and measure and characterize the electrospun fiber properties in terms of size, orientation and mechanical properties.
- Employ knowledge gained through process characterization to increase the productivity of electrospinning.
Experimental:
We are using a horizontal point-plane electrode configuration as shown in figure 2. The polymer solution is supplied to the charging electrode by a syringe. A syringe pump allows the precise control of feed rate. The collector is a grounded aluminum electrode, which enables the removal of the residual charge on the fibers. An ammeter is connected between the aluminum electrode and ground that provides the measuring of the current flowing from the electrode. For certain experiments we are collecting electrospun fibers onto an insulating film from which we measure the charge to mass ratio by NanoCoulomb Meter. A photograph of the electrospinning apparatus is given in figure. 3.
Fig. 2 Charge Measurement Setup
Different polymer solutions depending upon polymer, solvent, concentration are being used. Our process parameters are applied voltage, feed rate and spinning distance.
We are also interested in fiber diameter distribution and mechanical properties of electrospun fibers.
We believe that the throughput limitation in electrospinning is a limit by the rate of pumping charge into the polymer. For that reason we are trying different charge injection systems to maximize the output of the process.
Fig. 3 Electrospinning Apparatus
References:
- Warner, S. B., Buer, A., Ugbolue, S. C., Rutledge, G. C. and Shin, M. Y., "A Fundamental Investigation of the Formation and Properties of Electrospun Fibers", National Textile Center Annual Report, 1998.
- Buer, A., "Electrostatic Solution Spinning", MSc Thesis, University of Massachusetts, Dartmouth, 2000.
Acknowledgement:
National Textile Center for funding the project.