Electrospinning process occupies a special place among all technological processes of fiber manufacturing, employed in industry due to its apparatus simplicity and parameter flexibility. It knows as "dry" method of chemical fibers manufacturing because of its basic apparatus setup behavior. All process stages including: deformation of original polymer solution, solidification, fibers drift from nozzle to collector electrode and nonwoven layer creation, are performed only by electric forces.
Electrohydrodynamical liquid dispersion method used for liquid dispersion by effecting liquid drops emerging from nozzle with high voltage electric field and collecting micro drops on collector electrode served as foundation for the electrospinning process.
The figure roughly represents the main stages by three typical zones. The process stages shown in the left part of the diagram correspond to electrical atomization of liquids, whereas those to the right - to electrospinning of fibrous materials, the process of interest here.
The polymer spinning solution is fed, by gravity or excess gas pressure, at a preset volumetric flowrate from container 2 to capillary injection nozzle 3. A metal electrode connected to regulated DC source 1, that is placed in the container, applies a high, usually negative potential difference to the free jet that is discharged from the nozzle. This causes the jet to steadily accelerate and thin out along an axis aligned with the general direction of the electric field. This is the first, relatively easy to regulate, stage of electrospinning of fibrous materials, the stability and results of which control all its subsequent stages and, in the final analysis, the desired properties of the finished fiber.
The second stage consists of several simultaneously occurring processes. In it, the fluctuations of the electric lines of force, caused by time and space variation of the bulk density of electrical charges and the inevitable concurrent deviation of these lines of force from the axis of the high-inertia accelerated jet, induce the appearance of a hydrodynamic force moment that is exerted on the jet by the viscous gas, further increasing this deviation. This causes the jet to turn transversely to the field direction and to be decelerated by the constantly increasing drag force of the gas. This produces a cloud that is forced into the shape of a cone that expands toward its bottom by the action of same-polarity charges. This cone is delimited by a dashed line and is filled by a coiling jet. At this stage the jet may still undergo a sequence of splitting resulting in the formation of an unsteady bulk fiber-mesh structure.
The following, third stage also consists of two simultaneous processes: the first consisting of random deposition of fibers into a layer with axes parallel to the plane of the collecting electrode and the second - of a gas spark discharge between the collecting electrode and the fiber layer forming on it, that closes the electric circuit. At the same time, the rate of vaporization of the solvent that started already at the first stage of the process is steeply intensified, the jet solidifies and the resulting fibrous cloud drifts in the applied electric field onto collecting electrode 4.
There is a number of different apparatus setups for the electrospinning process. The spinning solution could be supplied not only through nozzles but also from the solution surface provided electron density spots are present: such as sharp needles or materials with different electric currency. Negative charge could be applied either to the solution or to the collector electrode.
Our experimental data indicates that polymer-solvent relationship fully determinates which fiber creation mechanism is involved. Fiber creation stacking velocity may change from several meters to several kilometers per second. Three mechanisms of fiber creation process may be observed: such as stretching of liquid jets in loops, splitting of liquid jets and ejection of jets in bends or from the surface of the solution drop at the nozzle.
Spinning solution may be prepared from wide range of polymers including: PVC, polystyrene, polyamide, polyurethane, fluorocarbon polymers and others.
There are opportunities for creation of micro- and nanofiber materials with different physicochemical properties depending on polymer-solvents system prepared for spinning. It is also possible to create fibers from emulsions of insoluble polymers with binding agents added. And the edition of special agents (such as carbon nanotubes, antiseptic agents, luminescent pigment and metal powder) provides ways to create fibers with desired behavior for many purposes.