Effects of temperature on melt electrospinning with auxiliary heating: experiment and simulation study

Abstract

In this study, the effect of the heating temperature of the spinneret on the melt electrospinning process under the condition of application of auxiliary heating was investigated, in a systematical and comprehensive way. The temperature distribution of the melt jet during the melt electrospinning process was simulated by finite element software in order to provide a good deal of insight into the experimental results. In addition, high-speed photography was adopted to capture images of jet formation and jet motion during the melt electrospinning process. The experimental results indicated that the cooling rate of the polypropylene jet decreases obviously under the condition of auxiliary heating; in addition, the higher spinneret temperature leads to greater drafting force, a drawing fiber drafting rate, and greater jet whipping motion, which is conducive to secondary drawing and refinement of the jet.

Keywords

Melt electrospinning spinneret temperature simulation auxiliary heating jet motion fiber diameter

Presently, the electrospinning technique is considered to be the most extensively used, convenient, and straightforward method of preparing ultra-thin fibers.¹ Two main methods used for preparing the nanofibers via electrospinning have been designed and developed at present: solution electrospinning and melt electrospinning.² After years of exploration and research, the technology and theory of solution electrospinning have become relatively integrated.³ In contrast, the research on and development of melt electrospinning show relatively slow progress. Reported in 1936 for the first time, only 0.3% of literature publications related to melt electrospinning have focused on the melt electrospinning up until now.⁴ At present, melt electrospinning has been used to produce fibrous materials in a variety of applications, such as biomedical engineering, clean energy,⁵ filtration,⁶ drug release,⁷ and separation,⁸ due to its unique advantages of a clean and environmentally friendly process, high productivity, and no solvent evaporation.^9,10

So far, a great many novel technologies have been further explored, for instance, multi-temperature control,¹¹ gas-assisted melt electrospinning (GAME),¹² coaxial electrospinning,¹³ bubble electrospinning,¹⁴ needleless electrospinning,¹⁵ etc., intended for more efficient melt electrospinning and mass production. Jason K Lee¹⁶ et al. prepared and characterized ultrafine fibers with encapsulated structures using melt-coaxial electrospinning technique and melt-combined electrospinning technique, respectively, and compared the two. Zhmayev et al.¹⁷ introduced the concept of GAME, which is characterized by simple operation and is capable of reducing the difficulty of controlling the temperature at the spinneret so as to avoid polymer degradation. Based on the experimental observations, it was reported that a tensile force was applied by the turbulent air, resulting in an increase of production and a decrease of the jet diameter. H. Y. Li¹⁸ et al. revealed the advanced nature of the mass production of microfiber by melt electrospinning with an umbrella spinneret by comparing different microfiber production processes.

For the research of melt electrospinning, not only the technical improvement, but also the technological parameters are explored. M. F. Hao¹⁹ et al. revealed that both applied voltage and spinneret temperature had significant effects on the diameter of electrospinning fibers. X. N. Wang et al²⁰. studied the effect of main technological parameters on fiber diameter in melt electrospinning including spinning distance, applied voltage and spinneret temperature. The results show that the applied voltage has the greatest influence on the average diameter compared with the spinning distance and spinneret temperature. Junghyuk Ko²¹ et al. developed a new fiber processing technology based on melt electrospinning to investigate the effect of applied voltage, spinning distance, spinneret temperature and counter electrode on the diameter of melt electrospinning fibers. The experimental results show that the fiber diameter decreases with the increase of the above parameters in a certain range. Our previous study²² shows that higher heating temperature at the spinneret results in shorter jet formation time, smaller fiber diameter, more disordered fiber mat. In this study, we focus on the effect of temperature distribution in the melt electrospinning configuration on the melt electrospinning process and resultant fibers. The jet motion, resultant fiber diameter, fiber mat morphology, inner structures and mechanical properties of the fibers prepared with different temperature distribution via melt electrospinning method with auxiliary heating system were systematic studied. Aimed at in-depth understanding of the temperature distribution in melt jet during the melt electrospinning process, COMSOL Multiphysics software was employed in this study. High-speed photography was adopted to achieve the goal of capturing images of the polymer jet path during the melt electrospinning process in a reliable manner.

Experimental details

Materials

Pellet-like PP ((C₃H₆) _n , M_w = 210,000 melt index = 16 g/10 min, block copolymer PP) was purchased from the Shandong Yousuo Chemical Technology Co., Ltd, China. Pellet-like PP was extruded into the strip shape by an extruding machine (LME230, Dynisco Ltd, USA) at a 200°C temperature. Subsequently, the molded PP plastic strip was fed into the heater by a propeller on the melt electrospinning machine.

Experimental setup

Figure 1 shows a schematic diagram of self-developed melt electrospinning machine, which consists of a feeding system, a high-voltage system, a collecting system and an auxiliary heating system. Composed of a built-in 0.2 mm inner-diameter nozzle and a heating device, the feeding system is connected to a heating device that can be heated up to 450°C at most. The feeding system and auxiliary heating system can be moved during the spinning process, by placing them onto a horizontal track. The high-voltage power supply (RS60P-20 W, Gamma High Voltage Research, USA) with operating voltage ranging from 0 to 50 kV was applied to the spinneret and the metal roller-grounded collector. A metal drum covered with aluminum foil was used as the collector in this study. The auxiliary heating system consists of an infrared heating lamp (CQ-29P, Chongqing Aerospace Rocket Electronic Technology Co., Ltd., China), placed between the spinning spinneret and the collector, 5 cm away from the central axis to ensure a stable ambient temperature and humidity. The luminous flux and luminance of the lamp is 60 lm/w and 10×10⁶ nt.

A Photron Fastcam High-Speed Camera (Mini AX200, Photron Ltd, Tokyo, Japan) equipped with a Tokina 100 mm and f 2.8 macro lens and capable of recording images at a frame rate of up to 900,000 frames per second (fps) was used to capture images of the Taylor cone and jet path in this study. Two 100 W lamps (Model-100LED TWIN, Shanghai Jinqiaojingyi High-Tech Co., Ltd, China) were used as the light sources.

The feeding rate, applied voltage, working distance, and rotating speed of the collector were set to 2 mm/min, 30 kV, 7 cm, and 30 rpm, respectively, and also, the heating temperatures of the spinneret were set to 220°C, 260°C, 280°C, 300°C, and 320°C, respectively. All experiments were carried out with auxiliary temperature of 75°C and relative humidity of 20 ± 5% in this study.

Characterization

Under different experimental parameters and via the melt index of polypropylene at different temperatures were tested by melt indexer (XNR-400C, Hebei Chengde ). PP fiber morphologies were observed by a metallographic microscope (LV150N, Nikon Co., Ltd, Japan) designed with a 10-times magnification objective lens; in addition, the fiber diameters were measured by ImageJ software. The crystalline structure of the fiber was analyzed by using X-ray diffraction (XRD, XD-3, Beijing Purkay General Instrument Co., Ltd, China) under the circumstances of 40 kV, 150 mA, 2θ range of 5–60°, and 25°C. Differential scanning calorimetry (DSC) measurements were performed with a WKTS-RC500 (Jiangsu Vicometer Instrument Co., Ltd, China) within the temperature scanning range of 30–320°C at 20°C min⁻¹. For each run, 6–8 mg of sample was used. The mechanical properties of the fiber were tested using a single-fiber strength tester (JC503-YG001D, Beijing One-Million Electronic Science and Technology Center, China) at a crosshead speed of 100 mm/min at 20°C. Relevant parameters of the instrument are: grip distance is 5 mm, the force sensitivity is 0.01 cN, elongation measurement accuracy is 0.01 mm. 30 fibers were used as a set of experimental samples. The images of jet motion were captured by a high-speed camera. The temperature distribution results, calculated by the finite element method (FEM), were verified by using an infrared image captured by an infrared camera (E60, FLIR Co., Ltd, USA).

Numerical simulation

The temperature distributions of the jet at different heating temperatures were simulated to help one to deeply understand the effects of temperature on melt electrospinning process. The temperature distribution of the jet was simulated by COMSOL Multiphysics software (Version 5.4, COMSOL Inc., Sweden) using the FEM. The model was determined by referring to the actual dimensions and relative position of the equipment, and also, the parameters of the polymer solution were set according to the actual situation.

Experimental results and discussion

A large number of previously conducted experiments^19–21 have shown that the spinneret temperature has a significant influence on the formation process of the Taylor cone and the movement path of the jet in the process of melt electrospinning. Considered a critical factor, the spinneret temperature can affect the fiber formation and performance in a direct way. In this study, five different spinneret temperatures (220°C, 260°C, 280°C, 300°C, and 320°C) were selected to study the influences of the spinneret temperature on the Taylor cone, jet motion, fiber diameter, internal structure, and mechanical properties with auxiliary heating of the melt-electrospun fiber. The pre-experiment results revealed that under the action of an electrostatic field and auxiliary heating, 220°C and 320°C can be realized as the lowest and highest spinning temperatures of PP, respectively. Specifically speaking, when the spinneret temperature is lower than 220°C, the viscosity of polypropylene in the molten state is large. As a result, the force generated by the electrostatic field cannot overcome its internal friction force and cannot form a jet with spinning forming ability. What is more, a continuous fiber fails to form as well when the spinneret temperature exceeds 320°C. The main reason for the aforesaid phenomenon is that the internal friction generated by critical viscosity of the PP jet is less than the electrostatic field force, and then droplet spray is formed. In addition, higher temperatures are prone to break the intermolecular force of the polymer and, as a consequence, unexpected gaseous PP is formed, which is unfavorable to continuous implementation of the spinning experiment.

Polypropylene Melt Performance

Melt index is a value that indicates the fluidity of a plastic material during processing. Figure 2 shows the melt index of the PP at different temperature. As the temperature increases, the melt index also increases; this is because melt with higher temperature obtain greater fluidity of polymer molecular chains. And the melt index presents a steep rise when the temperature exceeds 300°C

Taylor cone and jet motion

Figure 3 shows high-speed photographic images of the Taylor cones and paths of the melt-electrospun jets at different time points at five different temperatures with the application of auxiliary heating. It is worth noting that the stretching time of the molten polymer at 320°C is significantly shorter than those at other temperatures. A conclusion can be drawn from the experimental phenomenon that the spinning at 320°C in the whipping process provides higher fluidity and better ductility. It can be clearly seen from Figures 3(a1)–(a4) that the polymer melt shows poor fluidity and is ejected from the spinneret in the form of a rubber strip at the temperature of 220°C and at a low speed. Attributed to high melt viscosity and low electric field near the spinneret, the insufficient electric field force fails to overcome the surface tension of the melt and to draft the jet at this time. As the melt slowly falls down onto the collector, a Taylor cone is formed in a gradual manner. In addition, almost no whipping motion happens to the polymer jet during the spinning process.

As can be seen from Figures 3(b1)–(b4), the melt presents in a semicircular shape at the spinneret and slowly drops in the vertical direction when the spinneret temperature rises to 260°C. After a period of time, the melt is drawn and then a Taylor cone is formed under the action of electric field force. Benefiting from the auxiliary heating, the formation process of the Taylor cone at this temperature is basically the same as that at 220°C; however, its formation time is less than that at 220°C.

Figures 3(c1)–(c4) and (d1)–(d4) reveal that a larger droplet of polymer melt is formed at the spinneret and the charge near the spinneret acts on the droplet when the nozzle temperature is further increased to 280°C and 300°C, respectively. In these cases, preliminary drawin on the outermost layer is carried out and the cone is formed, with formation time at 300°C being less than that at 280°C.

What can be apparently viewed from Figures 3(e1)–(e4) is that the melt is quickly wrapped by the charge when it flows out from the spinneret and the Taylor cone is formed in the situation of increasing the temperature to 320°C. At this moment, the formation time is significantly shorter than those at other temperatures. On the other hand, a more obvious whipping jet can be observed during the melt electrospinning process. It can be concluded that the fluidity of the polymer melt and its viscosity caused by different temperatures have a tremendous impact on the initial shape of the Taylor cone and the whipping amplitude of the jet with the application of auxiliary heating, which in turn leads to the differences in resultant fiber diameter and fiber mat morphology. In addition, the temperature during the jet drawing process increases and the stretchable area before solidification becomes larger, so that the secondary drawing on the fiber happens under the action of an electric field, thus increasing the stability and whipping amplitude of the jet in the falling-down process. Based on this, the fiber can be further drawn and refined.

Fiber mat and fiber diameter

Figure 4 demonstrates optical microscope images of fibers mats for fibers and diameters of melt-electrospun fibers prepared at 220°C, 260°C, 280°C, 300°C, and 320°C. With the view of obtaining testing results with higher accuracy, 50 fibers were selected from each fiber membrane to measure diameter. It can be observed that the spinneret temperature has a direct and great influence on the appearance, orientation, and diameter of the fiber with the application of the auxiliary heating.

As shown in Figures 4(a)–(e), the overall appearance of the collected fiber membrane demonstrates that the fiber morphology changes significantly with the increase of temperature. Figures 4(a) and (b) show that the fibers are arranged in straight lines with parallel orientations at the spinneret temperatures of 220°C and 260°C. However, the orientation degrees of the fibers prepared at 220°C are greater than those prepared at 260°C. As can be seen from the figure, the fibers are arranged vertically and the fiber membrane is evenly distributed and in a flat state.

With a relatively loose structure, the fibers begin to be presented in the circular shapes with large radians, which are continuously arranged and evenly distributed in both transverse and longitudinal directions at the spinneret temperature of 280°C. Exhibiting a certain thickness, the fiber membrane is characterized by a smooth surface with a certain quantity of voids formed by the staggered distribution of fibers, as shown in Figure 4(c).

With the further temperature increase of 300°C, the fibers are distributed in small ring shapes with spiral structures. As shown in Figure 4(d), the fibers feature more compact structures, smaller voids, and smoother surfaces. Under the condition of heating temperature reaching up to 320°C, the fibers are arranged in circular patterns with different circle sizes and uneven fiber distribution, which can be further observed in Figure 4(e).

The main reason for the different morphologies of fiber mats is that both viscosity of the polymer melt and the cooling time of the jet change with the variation of temperature, with the application of auxiliary heating. The melt viscosity decreases, the specific surface area of the fiber increases, the stretchable area before solidification becomes larger, in the event of the same applied voltage and increasing spinneret temperature. Due to stronger and stronger effects of the electric field force on the polymer jet, the morphologies of the fiber mats change in a gradual manner and present in various states.

Figures 5(a)–(e) show the average diameters of the electrospun fibers prepared at different spinneret temperatures. It can be seen that when the spinneret temperature increases from 220°C to 320°C with the application of auxiliary heating, the average fiber diameter decreases from 60.97 to 9.06 µm. Furthermore, it also can be observed that the distribution of fiber diameter becomes more narrow as the spinneret temperature increases. The main reason for this is that under the action of the auxiliary heating device, the jet in the falling-down process requires more cooling time, and so the stretching distance of the fiber becomes larger. Under the circumstance of low spinneret temperature, a higher melt viscosity is realized. It follows that the inadequate electric field force fails to overcome the surface tension of the jet and the jet fails to be effectively drawn and refined, thus it takes a long time for the jet to be cooled down and fall onto the collector. In the condition of high spinneret temperature, the melt viscosity decreases. In this case, a sufficient electric field force overcomes the surface tension of the jet followed by the jet whipping motion. Furthermore, the specific surface area and surface charge distribution density of the fiber increase, which make the fiber more refined.

Mechanical properties

The stress–strain curves of the melt-electrospun fibers, fabricated at heating temperatures of 220°C, 260°C, 280°C, 300°C, and 320°C, are displayed in Figure 6. 30 fibers were used as a set of experimental samples and select the most appropriate data as valid data to draw the stress-strain curve. As illustrated in Figure 6, the mechanical strength is significantly affected by the temperature. It is found that the breaking strength dramatically decreases from 13.96 to 0.69 MPa when the temperature increases from 220°C to 320°C. On the one hand, the arrangement of molecular chains becomes more regular correspondingly under the action of the static electric field when changing high-elastic state to a viscous fluid state. On the other hand, the viscous polymer jet becomes into the high-elastic state during the process of cooling and crystallization. The drawing rate of the fiber increases with the increase of spinneret temperature, thus increasing the uneven crystal size in the deposition process. Under the condition of auxiliary heating, the temperature has a significant effect on the formation process of the polymer jet. With the increase of temperature, uneven jet flow is more likely to occur, which leads to fracture caused by the uneven distribution of macromolecular segments in the fiber during the stretching process. The cooling speed of the fiber and spinneret temperature will affect the arrangement of the macromolecular chain, to some extent. With a low spinneret temperature, the slow cooling rate and crystallization rate contribute to the orderly arrangement of molecules. With a high spinneret temperature, the lower melt viscosity and larger stretchable area before solidification enhance the secondary drafting and refinement of the jet, thus reducing the fiber diameter.

Crystallinity

Figure 7 shows the XRD patterns of melt-electrospun fibers fabricated at spinneret temperatures of 220°C, 260°C, 280°C, 300°C, and 320°C. The obtained experimental data are shown in Table 1. It can be concluded from the data in the table that the crystallinity of the fiber mat increases first and then decreases with the increase of temperature with the application of auxiliary heating, reaching its maximum value at 280°C. At 280°C, the appearance scanning results of the fiber mat (as shown in Figure 3) show that the fibers are uniformly distributed in a large circular arc, and jet whipping motion with a large angle is generated under the action of the electric field force. Possessing excellent mechanical properties, the macromolecular chains exhibit relatively regular arrangement, leading to a higher crystallinity than those fabricated at other temperatures.

Simulation and verification of temperature distribution

A two-dimensional (2D) axis-symmetric geometry was generated based on the typical melt electrospinning jet as shown in Figure 8. The geometry of the jet was modeled as a long cylinder. Fluid Heat Transfer interface was selected from the COMSOL software to calculated the temperature distribution of the model. According to the melt electrospinning equipment and process parameters, the inlet and outlet width of the jet was set to 0.2 mm. As shown in Figure 9, the simulation results of temperature distribution of the jet within the range of 20 mm below the spinneret are compared (heating temperature: 75°C; spinneret temperatures: 220°C, 260°C, 280°C, 300°C, and 320°C). What is striking in this figure is that the jet temperature decreases significantly within the range of 20 mm below the spinneret and drops to about 110°C. As the spinneret temperature increases, a more obvious jet temperature decrease can be viewed. The experimental results show that the PP will be presented in a molten state at 220°C, which can be used for the melt electrospinning. In the event of the same melt velocity, the jet requires a longer trajectory for its temperature to drop down to 220°C if the higher spinneret temperature is provided. Therefore, the cooling region of the jet can be effectively extended by means of adopting the auxiliary heating device. Figure 8 shows the phase indicator of jet cooling at different nozzle temperatures with the application of auxiliary heating. As can be seen from the figure, the region for the PP to change from the molten state into the solid state increases significantly with the increase of nozzle temperature. This indicates that the transformation region of PP is greatly affected by the temperature at the same inflow velocity, and the “freezing point” between the molten state and the solid state of PP is positively correlated with the temperature.

Figures 10(a) and (b) show the temperature distribution of infrared thermal imaging and simulation results of the temperature, with the spinneret temperature of 300°C and with the application of auxiliary heating, respectively. As can be seen from Figure 9(a), the temperature distribution near the spinneret is concentrated in the upper part of the spinning area. As shown in Figure 10(b), the environment temperature gradually reduces and the cooling rate with the application of auxiliary heating is slower than that without auxiliary heating, which is beneficial to the secondary stretching of the jet by electric field. It demonstrates that the auxiliary heating is beneficial to slow down the cooling rate of the jet, which makes the jet obtain a greater drawing and refining effect during the falling-down process.

Conclusion

In this study, the effects of spinneret temperature in electrospinning process with the application of auxiliary heating was studied. It was found that under the condition of auxiliary heating, the heating temperature has an enormous influence on the jet formation process, jet motion, fiber diameter, inner structure, and mechanical properties of the resultant fiber and fiber mat morphology. The results indicated that the cooling rate of the PP jet decreases obviously with the application of auxiliary heating spinneret temperature leads to a greater drawing, faster fiber drawing rate, and greater jet whipping motion, which is conducive to secondary drawing and refinement of the jet. The simulation results of temperature distribution within the jet were compared based on the measurement values of infrared camera images, which were in good agreement with each other.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed the receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant No. 11702169).

ORCID iDs

Yuansheng Zheng

Binjie Xin

References

Jiang

Hou

, et al. Polyimide nanofibers by “green” electrospinning via aqueous solution for filtration applications. ACS Sustain Chem Eng 2016; 4: 4797–4804.

Buivydiene

Todea

, et al. Composite micro/nano fibrous air filter by simultaneous melt and solution electrospinning. J Aerosol Sci 2021; 154: 105754.

Molnár

Shear-aided annular needleless electrospinning. Mater Res Expr 2019; 6: 075304.

Hochleitner

Jüngst

Brown

, et al. Additive manufacturing of scaffolds with sub-micron filaments via melt electrospinning writing. Biofabrication 2015; 7: 35002.

Aleksander

Rahul

Velmurugan

, et al. Melt-electrospun fibers for advances in biomedical engineering, clean energy, filtration, and separation. Polym Rev 2011; 51: 265–287.

Ferdows

Caner

Mine

, et al. Biomimicry in bio-manufacturing: developments in melt electrospinning writing technology towards hybrid biomanufacturing. Appl Sci 2019; 9: 3540.

Buivydiene

Edvinas

Darius

, et al. Formation and characterisation of air filter material printed by melt electrospinning. J Aerosol Sci 2019; 131: 48–63.

Midori

Keita

Ryo

, et al . Drug release behavior of a drug-loaded polylactide nanofiber web prepared via laser-electrospinning. J Macromol Sci B 2019; 58: 592–602.

Joseph

Budhavaram

DePolo

, et al. Nanoribbons fabricated by melt electrospinning. Polym J 2021; 53: 493–503.

10.

Chen

Liu

Deng

, et al. Melt differential electrospinning of polyphenylene sulfide nanofibers for flue gas filtration. Polym Eng Sci 2020; 60: 2887–2894.

11.

Zhou

Green

and Joo

YL.

The thermal effects on electrospinning of polylactic acid melts. Polymer 2006; 47: 97–7505.

12.

Zhmayev

Cho

and Joo

YL.

Nanofibers from gas-assisted polymer melt electrospinning. Polymer 2010; 51: 4140–4144.

13.

Mohammadreza

Ali

Richard

, et al. Recent advances in core/shell bicomponent fibers and nanofibers: a review. J Appl Polym Sci 2018; 135: 46265.

14.

Wang

, et al. Bubble melt electrospinning for production of polymer microfibers. Polymers 2018; 10: 1246.

15.

Chen

, et al. Large scaled fabrication of microfibers by air-suction assisted needleless melt electrospinning. Fiber Polym 2016; 17: 576–581.

16.

Lee JK, Ko J, Jun MBG, & Lee PC. Manufacturing and characterization of encapsulated microfibers with different molecular weight poly(?-caprolactone) (PCL) resins using a melt electrospinning technique. Materials Research Express 2016; 3(2): 025301.

17.

Li HY, Bubakir MM, Xia T, Zhong XF, Ding YM, & Yang WM. Mass production of ultra-fine fibre by melt electrospinning method using umbellate spinneret. Materials Research Innovations 2014; 18(sup4), S4 -921 -S4 -925.

18.

Hao MF, Liu Y, He XT, Ding YM, & Yang WM. Factors Influencing Diameter of Polypropylene Fiber in Melt Electrospinning. Advanced Materials Research 2011; 221: 129–134.

19.

Wang XN, Xu Y, Wei QF, Cai YB. Study on Technological Parameters Effecting on Fiber Diameter of Melt Electrospinning. Advanced Materials Research 2011; 1445: 1550–1556.

20.

Ko J, Ahsani V, Yao SX, Mohtaram NK, Lee PC, & Jun MBG. Fabricating and controlling PCL electrospun microfibers using filament feeding melt electrospinning technique. Journal of Micromechanics and Microengineering 2016; 27(2): 025007.

21.

Mu XQ, Zheng YS, Li XQ, et al. Effects of Temperature on Melt Electrospinning: Experiment and Simulation Study. Fibers and Polymers 2021; 22: 964–971.

22.

Shen Y, Liu QS, Deng BY, et al. Experimental study and prediction of the diameter of melt-electrospinning polypropylene fiber. Fibers and Polymers 2016; 17: 1227–1237.

23.

Morikawa K, Vashisth A, Bansala T, et al. Melt Electrospinning Polyethylene Fibers in Inert Atmosphere. Macromolecular Materials and Engineering 2020; 305: 2000106.