1.7 Process Scalability
One key limitation to implementing electrospinning on a commercial production scale is the low production rate. Conventionally, production rates used with electrospinning are less than 0.3 g/h. To increase productivity, it is possible to increase the number of needles. However, the effect of additional needles/liquid jets on the electric field profile presents a significant challenge. Electrostatic repulsion limits the areal density of jets that can be achieved. There is strong repulsion among the jets which lead to poor fiber quality and reduced fiber production (Persano et al. 2013).
1.7.1 Melt Electrospinning
To avoid the use of hazardous solvents or to process polymers that are difficult to dissolve, e.g. polypropylene and polyethylene, melt electrospinning in which fibers are spun from molten polymer rather than a polymer solution has been considered. Eliminating the solvent increases the mass throughput of electrospinning ~5‐ to 10‐fold. The melt electrospinning process was first described in a patent approved in 1936 for Charles Norton at the Massachusetts Institute of Technology (Norton 1936). Larrondo and Manley published a melt electrospinning method in 1981, demonstrating that micron‐scale filaments could be spun from molten polypropylene and high‐density polyethylene (Larrondo and St. John Manley 1981).
In addition to the high‐voltage power supply and collector, additional elements key to the melt electrospinning apparatus are heating elements to melt the polymer and a temperature control system. Commonly used polymer heating methods may include electrical, heated air, circulating fluid, laser heating, and microwave heating (Zhang et al. 2016). Electrical heating is most widely used due to ease of use, but may result in electrical interference and safety concerns. Electrical interference can be mitigated by reversing the electrodes so that positive voltage is applied to the collector and the spinneret is grounded (Deng et al. 2009; Lyons et al. 2004). The heated air method also offers simplicity, but the polymer melt temperature can be difficult to control using this method (Dalton et al. 2007; Qin et al. 2015). Heated gas has also been used to maintain the polymer temperature in the spinning region and achieve thinner fibers (Zhmayev et al. 2010). Polymers have also been heated with circulating water or oil systems, but the range of temperatures is limited by properties of the heating fluid (Dalton et al. 2007; Detta et al. 2010). Solid polymer rods may also be melted using a collection of lasers, preventing undesirable contact between the voltage and heat sources (Ogata et al. 2007).
Polymer properties (molecular weight and tacticity) play a substantial role in determining the feasibility of electrospinning fibers from a melt and the resulting fiber diameter. Lyons et al. found that the highest molecular weights of polypropylene used in a study produced the largest diameter fibers. Higher polymer tacticity also corresponds to higher crystallinity and larger fiber diameters (Lyons et al. 2004).
When electrospinning from the melt, the conductivity is much lower and the viscosity is much greater than solutions (Hutmacher and Dalton 2011). Semiconducting polymer melts (conductivity of 10−6 to 10−8 S/m) are ideal for forming a stable Taylor cone and jet (Lyons et al. 2004; Brown et al. 2015). Jets formed from highly conductive polymers will break when the voltage is increased and nonconductive polymer melts will not sustain a sufficient surface change to be electrospun. The viscosity is affected by the polymer molecular weight and temperature. Achieving optimal melt viscosity is necessary for polymer flow through the spinneret and for the formation of a stable Taylor cone (Lyons et al. 2004; Brown et al. 2015). Typically, melts with viscosities ~40 to 200 Pa S can be electrospun (Hutmacher and Dalton 2011). As the spinneret temperature increases, polymer viscosity and resulting fiber diameter decrease (Zhou et al. 2006). This phenomenon is due to thermal and mechanical degradation of the polymer (Zhang et al. 2016; Sukigara et al. 2003).
Analogous to solution electrospinning, process parameters affect fiber size, with lower conductivity, and higher viscosities, the applied voltages required to electrospin are higher than in solution electrospinning (Zhang et al. 2016). Lyons et al. used between 10 and 15 kV/cm to form polypropylene fibers, which is at least 10 times stronger than the electric field strength required for comparable solution spinning (Lyons et al. 2004). A strong electric field is necessary to overcome surface tension and viscoelastic forces of the polymer melt. Increasing the electric field strength from 10 to 15 kV/cm was shown to substantially decrease diameter of polypropylene fibers (Lyons et al. 2004; Zhou et al. 2006). Flow rate of the polymer melt has been identified as one of the most influential process parameters for tuning fiber diameter. An optimal polymer flow rate is essential for producing a stable Taylor cone (Dalton et al. 2007). Higher flow rates have also been shown to produce larger diameter fibers (Zhang et al. 2016; Detta et al. 2010). The high polymer viscosity associated with melt electrospinning necessitates larger‐diameter spinnerets (Zhang et al. 2016). Zhou et al. found that decreasing the spinneret diameter also decreases PLA fiber diameter (Zhou et al. 2006). Qin et al. found that varying collector distance will also alter fiber diameter. As collector distance increases, fiber diameter decreases to a point. With further increases in distance, the diameter increases due as the electrostatic drawing force weakens (Zhang et al. 2016; Qin et al. 2015).
Low conductivity and high viscosity tend to promote jet stability and suppress whipping/bucking instabilities (Hutmacher and Dalton 2011). Therefore, melt electrospun fibers are generally much larger than solution spun fibers (Dalton et al. 2007; Hutmacher and Dalton 2011; Zhou et al. 2006; Brown et al. 2011; Schaefer et al. 2007). Fiber diameters when melt electrospinning are generally larger than when solution spinning (approximately 100 nm to 500 μm compared to ~50 nm to 10 μm, respectively) (Brown et al. 2015). The jet stability provides greater control over fiber collection (Brown et al. 2011) and fiber uniformity. Fiber uniformity is achieved with the ability to establish and maintain a stable jet, as well as a balance of polymer parameters (Brown et al. 2011). Thus, melt electrospinning is often combined with mechanical drawing to reduce fiber size. For example, using the “gap method of alignment,” Dalton et al. produced 270 ± 100 nm fibers from a blend of poly(ethylene glycol)‐block‐poly(ε‐caprolactone) (PEG47‐b‐PCL95 ) and poly(ε‐caprolactone) (PCL) (Dalton et al. 2007). Submicron diameters were achieved by collecting the fibers in a gap between two collectors. As the gap approached 1 mm, the fibers became thinner and oriented.
Polymers that have been melt electrospun have been recently reviewed (Brown et al. 2015); nonpolymer fibers as small as 100 nm have also been produced through electrospinning from glass melt (Praeger et al. 2012). Boron oxide (B2O3) which forms glass has been electrospun into uniform fibers. The surface tension of boron oxide is slightly higher than water and viscosity can be controlled with temperature allows for electrospinning under low voltages (Praeger et al. 2012). Electrospinning such materials opens new possibilities such as integration into lab‐on‐chip devices. Other small molecules have also been electrospin into fibers from the melt. Trisamides, a class of liquid crystals, can be melt electrospun from the nematic liquid crystal phase or isotropic melt. The ability to form fibers was attributed to supramolecular self‐assembly, i.e. columnar stacking due to hydrogen bonds of the three amide groups. Similarly, perylene bisimides with an extended π‐conjugated structure also formed fibers when melt electrospun due to π–π interactions (Singer et al. 2012, 2015).
1.7.2 Needleless or “Free‐Surface” Electrospinning
Alternatively, multijet electrospinning can be achieved by free surface electrospinning or “needleless electrospinning” (Niu et al. 2011; Guo et al. 2010). In needleless electrospinning, an electric field is applied to a thin layer of polymeric solution in the presence as a field concentrator (e.g. cleft made from metal, rotating cylinder) forming a liquid jet. Similar to conventional electrospinning, the jet whips as the polymer solution travels to the collector. When there is sufficient electric field strength, several jets