A NOVEL APPROACH TO UP-SCALED OPEN-SURFACE ELECTROSPINNING

High Efficiency Production of Nanofibers by Electrospinning.

Muhammad Omer Sahto, Gulbahar Saat, Yusuf Keskin, and Faik Midik, Inovenso Inc., Istanbul, Turkey 

Istanbul Technical University (ITU), Istanbul, Turkey

 

ABSTRACT

Nanofibers can be prepared with several laboratory techniques however electrospinning  remains the most prevalent and versatile method. As of recent, one of the main challenges  for electrospinning is designing a setup that can efficiently upscale the production capacity of  nanofiber membrane media. Generally there are two kinds of electrospinning variants,  needle-based and needle-less. Both approaches have been further developed to maximize  the nanofiber throughput and both can have comparable differences. We explore these  differences by electrospinning five diverse polymers (PVA, PVDF, TPU, PA 6, and PHB) in  both needle-based (nozzle) and needle-less (open-surface) apparatuses. For this  investigation, both Inovenso’s Stream-Spinner 550 and nozzle-based PE 550 pilot-scale  electrospinning machines were used respectively. The open-surface stream-spinning unit  was found to operate on discernably higher flow rates than the nozzle-based unit across all  five polymeric solutions. This resulted in faster production cycles with the open surface unit,  although with differing increased factor rates depending on the polymer. Other common  observations were the overall larger critical voltage requirement for the open surface  technology, most-likely due to the greater surface tension of the solution and an increased  number of electrospinning jet emissions.

 

INTRODUCTION

Electrospinning is a technique where polymeric solutions are incorporated with high voltage to build electrostatic charge and cause jet formation to synthesize fine fibers. These fine fibers or ‘nanofibers’ can be made to have diameters in the sub-micron range with minimal effort. With the growing interest of nanofibers in the research space of materials science, the standard electrospinning method has been modified with several advancements and alterations to cater to a specific requirement. Gas-assisted electrospinning and melt- electrospinning are some examples [1][2]. However, a common obstacle with electrospinning is the difficulty of scaling up the production of the nanofiber media. Standard electrospinning experiments are often a low throughput process where one or two jets are formed from needles, producing small sample-size nanofiber scaffolds. Nanofiber-related media has several applications in industry which is why scalability is a crucial limitation to overcome [3]. There are two major approaches to maximize the output in electrospinning experiments, either increase the number of needles or use specially designed non-needle spinnerets. Both serve to multiply the number of jets streams to have larger amount of materials at a faster rate. In this study, we experiment with both systems to observe the synthesis of polymer nanofibers, understand their morphology, and underline the notable differences.

 

EXPERIMENT

Theory:

The needle-based (NB) and needle-less (NL) electrospinning systems have an identical setup as both consist of a solution pumping mechanism, high voltage supply, and a collector. However, the key difference is the type of spinneret being used to form the electrospun jets. For NB electrospinning, the spinnerets are always needles that can have varying orifice diameter (gauge) sizes. It is a general indication that the smaller the needle diameter the smaller the average fiber diameter will be [4][5]. Polymer solutions are pumped into these needles which are connected to the high voltage, which in turn charges the solution inside, and enables it to dispel from the needle orifice in jet form. The charge accumulation of the solution coupled with the aerodynamic trajectory causes the jets to have an unstable whipping motion [6]. These jets visually resemble aerosol sprays, albeit they are polymers being stretched into fiber form due to the electrostatic field strength. In NL electrospinning, the procedure is the same, but the spinneret can be of various novel shapes such as wires, balls, or rolling cylinders [7][8]. The solution is usually distributed onto the surface of the spinneret. The electrospun jets emerge from this free or ‘open’ surface spinneret and advance towards the collector.

Materials and Method:

For the experimental trials, we utilized two pilot-scale electrospinning machines: the Pilot- Scale Nanospinner 550 (PE 550) (Inovenso, Turkey) and the Stream-Spinner 550 (SS 550) (Inovenso, Turkey). The PE 550 is multi-needle system that has 4 metallic rods, in where each rod has 14 needles installed for a total of 56 needles in the overall machine. In our case, we used conical needles that have an inner diameter of 0.8 mm. The SS 550 has 3 metallic rods with no needles, as each rod contains a gap section that is a thin U-shape groove. This ‘groove’ or thin slot is 2 mm wide and 4 mm deep. A variety of polymers were electrospun with both machines: polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), nylon 6 (PA 6), thermoplastic polyurethane (TPU), and Polyhydroxy butyrate (PHB). Each polymer was dissolved with its own appropriate solvent(s). The standard substrate of choice was Polyethylene terephthalate (PET) non-woven spunbond. SEM images of the nanofiber scaffolds were taken to analyze the morphology of the samples.

The polymer solutions were kept as the control for the electrospinning setups. Hence, the exact same polymer recipe was used for both the NB PE 550 and the NL SS 550. For each trial, a polymer was loaded, and several parameters were adjusted until proper electrospinning jets were attainable. With the primary adjustable parameters being the applied high voltage, spinning distance, and flow rates. Various parameters were tried but to have a more coherent comparison, the spinning distance values that were matching for both the needle-based and needle-less setups were chosen. Each process was kept in standard laboratory ambient conditions to ensure no external influence from the temperature and humidity as they can directly impact electrospinning [9]. To compare productivity, the flow rate was considered as the deciding factor as it has a directly proportional effect on the number of electrospun jets due to greater solution availability [10]. The max flow rates that both setups could avail for optimized electrospinning were taken as a ratio of open surface needle-less upon needle-based (NL/NB).

 

Figure 1. Side-by-side display of the electrospinning setups, where the left one is needle- less, and right is needle-based.

RESULTS AND DISCUSSION

The following table depicts all relevant parameters of each polymer for each electrospinning process:

 

The dissimilar shapes of each spinneret type denote the electric field lines possessing a different orientation for both NB and NL. As a direct consequence, electric field strength requirements varied between NB and NL. To obtain stable jets from the needle-based setup the maximum applicable voltage was determined to be 60 kV. Higher voltage values were attempted but provoked electrical arcing between the NB spinneret and collector electrode pair. This could be explained by the unstable electric field due to irregular field line distributions, where charge accumulates from multiple needles in the near vicinity of each other. Which would moreover cause significant jet interference between the needles, thereby tampering the nanofiber synthesis [11]. By contrast, the needle-less system needed substantially higher voltage to allow optimal jet emission. Applied voltage values of 60 kV and lower were fairly weak to overcome the surface tension of each solution on the NL thin- slot spinneret. The solution is spread on top with no sharp focal point, unlike the orifice of a needle, for charge accumulation at a concentrated position. Thus, high applied voltage is required for localizing charge concentration throughout the surface of the solution. Lower voltage values resulted in sub-par jet formation and infrequent emission from the spinneret to the collector. Consequently, the maximum applied voltage range from 90 to 100 kV was found to be ideal for maximum productivity without any interference or irregular behavior of the fiber jets. This could be attributed to the NL spinneret’s smooth and symmetrical structure, which likely helps facilitate uniform field lines [12].

The following are the corresponding SEM images, being the ones on the left needle-based and the ones on the right needle-less:

 

 

Figure 2. PVDF side-by-side SEM.

Figure 3. PHB side-by-side SEM.

Figure 4. PVA side-by-side SEM.

Figure 5. TPU side-by-side SEM.

Figure 6. PA 6 side-by-side SEM.

By analyzing the data from the process and the SEM images, we can make certain deductions. Apart from the NL experiments needing a stronger electric field, we observed that the flow rate ratio is greater than 1 for all five polymers. Indicating that the flow rate and overall productivity for NL is higher than NB electrospinning. All five polymers had increased production but at differing rates. PVA had the greatest difference, with having approximately fifteen times more solution feeding in NL when compared to NB. PVDF had the smallest increase with having more than double (x2.4) the flow rate in NL. These results meant more jets as well as faster and finer homogeneous coatings on the PET substrate. Another keynote is that the nanofibers prepared with the NL setup had wider range of fiber diameters than the NB setup. This may or may not be desirable depending on the application or research purpose. The reason NB setup has better control of fiber diameter uniformity is because the polymer solution is confined to the small orifice of each needle. However, the solution in some of the needles would on occasion get clogged, whereas this issue did not exist in the NL setup as the solution was entirely on the surface.

CONCLUSION

An open surface electrospinning set up incorporating a novel thin slot spinneret was tried and compared with a multi-needle configuration. All five polymers had higher flow rates on the open surface needle-less setup indicating faster feeding and more jet formation. Although NL has faster coating and no clogging issue, it does not render the standard needle-base setup obsolete. The needle-less setup still requires higher voltage due to greater amount of solution. In NB, the solution discharges out as a small droplet from the needles, allowing easy emission with lower voltage values. There is also better fiber diameter uniformity in the NB setup. But the difference in volume output is immediately recognizable by evaluating the flow rate ratio (NL/NB) which is also followed by better homogeneity. With these results it can be proposed that the needle-less configuration is a better option for electrospinning if industrial-level throughput of nanofiber scaffolds is required.

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