## 5 Additives in Melt Electrospinning
### Salt, Viscosity Modifiers, Stabilizers and Dyes  
Isotactic polypropylene (iPP) draws attention both in the fiber industry and research because of its good mechanical properties, excellent chemical resistance, low cost, and polymorphism [64]. Melt electrospinningning of iPP was firstly studied by [PERSON] in the 1980s [65] and thenceforth, different efforts have been focused on improving the morphology and properties of melt electrospun fibers. Probably, these works are relevant to discuss the effects of rheology modifiers to decrease the viscosity of the medium and facilitate the formation ultrafine and continuous fibers. Basically, iPP cannot provide thin fibers at low voltages, while at high voltages the formation of continuous fibers is hindered due to the strong electrostatic force that causes fibers to burst forth [66]. Ionic salts in both solution and melt electrospinning have been proven to improve the electrical conductivity. The effect of additives (e.g., sodium oleate (SO), poly (ethylene glycol) (PEG), and poly (dimethyl siloxane) (PDMS) at different weight percentages) on the fiber diameter of melt-electrospum PP was investigated by [PERSON] et al. PEG and PDMS are well-known polymers for lowering the melt viscosity, while SO has the potential to improve the electrical conductivity during melt electrospinning [66]. Therefore, SO can change melt properties, but can also provide uniform and thin fibers (i.e., down to the nanometric scale). SO (7 wt.%) and NaCl (5 wt.%) have also been incorporated [43] leading to nanofibers with diameters of 371 and 310 nm, respectively. Above these concentrations. At higher salt concentrations, melt-electrospinning became unstable causing large Taylor cones and a significant increase of the fiber diameter. Results showed that the electrical conductivity was higher with the addition of NaCl due to its smaller ionic and higher charge density. This feature increased the mobility under the influence of the external field, increased also the stretching of the melt jet and lead to thinner fibers. Addition of SO influenced on the melting process that became more complex and also decreased the hydrophobic character of PP fibers. Other salts such as stearates of mono and divalent metals including sodium, magnesium, calcium, or zinc were also added to PP to obtain fibers with diameters within the 1-5 \(\upmu\)m range (i.e., a significant reduction compared to the diameters larger than 20 \(\upmu\)  
Figure 6: Schematic illustration of laser melt electrospinningning. Copyright 2012 Elsevier [59]. Low molecular additives, SS, SO and sodium myristate (SM), have also been found as potential additives to improve the properties of polyamide 6 (PA6) melt electrospun fibers. PA6, with excellent physicochemical characteristics, such as good mechanical properties, and heat and chemical resistance for a wide range of chemicals, is used to produce fibrous materials for different applications [73; 74; 75]. Pure PA6 has a melting point of 214 \({}^{\circ}\)C, but even at 345 \({}^{\circ}\)C its high viscosity hinders to get thin fibers by melt electrospinning (i.e., diameters 20 \(\upmu\)m are usually obtained). By adding 10 wt.% either of the indicated surfactants, viscosity drops from 5.2 Pa\(\cdot\)s to 0.5-0.6 Pa\(\cdot\)s independently of the selected salt. Electrical conductivity is also modified in the melt due to their ionospheric character. The increase of the processing temperature is also beneficial since decreases the melt viscosity and increases charge mobility. By controlling and optimizing these parameters, thinner fibers can be obtained (i.e., below 1.5 \(\upmu\)m). These fibers could be potentially applied in the filtration of suspended particles from gases, specifically, air. As the average diameter of PA6 fibers decreases, the efficiency of filtration is found to increase significantly. The highest filtration coefficient is observed in the case of materials produced in the presence of 10 wt.% of sodium stearate [76].