## 2 Melt Processing of Polymers with Additives  
Pure thermoplastic polymers are seldom processed on their own due to the usual disadvantages that they present including possible degradation, high or low viscosity issues, inability of being processed [18; 19; 20] and final applications (e.g., antimicrobial agents can be added to polypropylene to prevent infections [21]). Therefore, pure polymers are compounded with other materials, typically by mechanical blending or melt state mixing, to produce pellets, powders or flakes that are used in subsequent processing operations. The possible additives for thermoplastic polymers include fillers (to reduce cost), reinforcements [22], other polymers [23], stabilizers (to prevent deterioration from light, heat or other environmental factors) [24], various processing aids [25], drugs in biomedical applications [26], etc.  
Thermoplastics are usually processed in the molten state, being problems mainly associated to their viscosity and shear thinning behavior. As the shear rate increases, the viscosity decreases, owing to alignments and disentanglements of the long molecular chains. The viscosity also decreases with increasing temperature [27]. In addition to the viscous behavior, molten polymers exhibit elasticity and a number of unusual rheological phenomena. As an example, slow stress relaxation is responsible for frozen-in stresses in the processed products. The normal stress differences are responsible for some flow instabilities during processing and also of the extrudate swell, (i.e., the significant increase  
Figure 1: Schematic of solution and melt electrospinningning processes and how the jets form fibers. Copyright 2018 John Wiley & Sons [14].  
in cross-sectional area when a molten material is extruded out of a die). These properties play an important role in fiber spinning and electrospinning, since elongational forces are applied to fabricate the ultrafine fibers [28,29].  
The most important polymer processing operations are extrusion and injection molding, which are material-intensive and labor-intensive processes, respectively. Both these processes involve the following sequence of steps: (a) Heating and melting the polymer, (b) Pumping the polymer to the shaping unit, (c) Forming the melt with the required shape and dimensions, and (d) Cooling and solidification. The suitability of a material for a particular process is usually decided on the basis of the melt flow index (MFI, also called melt flow rate or MFR). This is an inverse measure of viscosity based on a rather crude test involving the extrusion of a polymer through a die of standard dimensions under the action of a prescribed weight. In a simple melt/dry spinning, viscosity or MFI play an important role in both polymer processing ability and final properties of the material [30].  
Blends and composites from existing polymers are the preferred form of developing new materials for polymer scientists and industrialists. Lower costs of development, maximum diversification, and the achievement of a broad spectrum of properties are the main reasons. It is also necessary to use the compounding of some virgin plastic with additives to improve the final properties including the reinforcement, electrical insulation characteristics, extension and even polymer processability. The additives used are normally found in powder or in granular form and have different handling and flow characteristics [31]. Nonetheless, some challenges come from the immiscibility and incompatibility between polymers and additives that complicate the processing conditions [31]. Dry blending is useful to have a homogeneous mixture prior to melt processing and compounding [32]. Additives including plasticizers and viscosity modifiers can be mixed with the main polymer in dry state (not molten or solution state) in a high-speed mixer under high shear stress and controlled temperature. Dry blends are generally more economical than molding powders and pellets made by plasticizing and extrusion.