DESIGN AND EVALUATION OF MELT-ELECTROSPINNING...

12. - 14. 10. 2010, Olomouc, Czech Republic, EU

DESIGN AND EVALUATION OF MELT-ELECTROSPINNING ELECTRODES

Michal KOMÁREK, Lenka MARTINOVÁ

Technical University of Liberec, Studentská 2, 461 17 Liberec, Czech Republic, [email protected]

Abstract

Nanofiber production without need for solvent recuperation is one of the possible ways to transform the

laboratory scale electrospinning technique to high productivity process. Melt-electrospinning development

and research is currently bringing first encouraging results as well as challenging demands for new

engineering solutions. This paper presents the construction and evaluation of two laboratory scale melt-

electrospinning setups. Besides the construction of the spinning chamber, the main feature affecting the

spinning performance in melt-electrospinning is the design of the spinning and collecting electrode.

Electrodes might be considered with different shapes, different types of heating, moving or stationary. The

shape of the electrodes determines the distribution of the non-homogenous electrostatic field created in the

spinning chamber. As the electrostatic force is the only drawing force stretching the raw polymer solution or

melt to fibers during electrospinning, it is desirable to concentrate the most of the electrostatic field intensity

to the spinning region. This is particularly true on the emitting electrode, where the electrostatic forces must

overcome the capillary forces, represented by the surface tension of the fluid, to start the spinning process.

Non-homogenous electrostatic field intensity can be modeled by the finite element simulation method for

known applied electrostatic potential and dimensions of the electrodes. In this work, the results of the

numerical simulations were compared with experimentally measured critical intensities of electrostatic field

for polymers with known physical properties.

Keywords: melt-electrospinning, electrode design, FEM, non-homogenous electrostatic field modeling

1. Theoretical background

Preparation of the nonwoven fabrics by electrostatic spinning from polymer solutions or melts is one of the

practical examples of the phenomenon known as electrohydrodynamic atomization. In contrast to

atomization of the simple liquids, where electrostatic charge enforces the splitting of the liquid into fine

droplets, the chain-like structure of polymers causes production of the fine fibers. Difference in the behavior

is caused by better resistibility of polymer containing liquids against the Raileigh instability in the stable jet

region. However, despite of the differences in the elongation of the liquid jet, the principle of the formation of

the Taylor cones is believed to be universal. The Taylor cone is formed on the liquid surface if the

electrostatic pressure overcomes the capillary pressure [1]. The electrostatic pressure on the liquid surface is

defined as

�� , where pe is the electrostatic pressure, ε is relative permittivity of the surrounding gas, and E is

the intensity of the electric field. Capillary pressure is defined from the Laplace-Young equation in the form


�� , where pc is the capillary pressure, γ is the surface tension and r is the mean curvature of the

surface. Condition for formation of the Taylor cone can be written as [2]

�� . The most common experimental setup for the electrospinning emitting electrode is the capillary

electrode. In this case the mean curvature is represented by the inner radius of the capillary. Another method

is the electrospinning from the free liquid surface. This method is often called “needle-less” electrospinning.

Advantage of this method is that it allows production of multiple Taylor cones, thus significantly increases the

productivity of the process. An apparatus for needle-less production of electrostatically spun nanofiber layers

was developed and patented by Jirsák et al. [3]. The definition of the stability of the free liquid surface in the

strong external electric field is published by Lukáš et al. [4]. The theory of the self organization of the jets

during electrospinning is based on the analysis of the dispersion law introduced by Landau et al. [5] and

postulates the critical parameters of the needle-less electrospinning such as critical intensity of the

electrostatic field:

��

� , where Ec is the critical intensity, γ denotes surface tension, ρ represents liquid density and ε

is the permittivity of the surrounding gas. Critical intensity is the minimal electrostatic field intensity required

to overcome the cohesive force of the liquid and start the formation of the jets. The role of the mean

curvature is in the case of needle-less electrohydrodynamic atomization played by so called “capillary

length”, which can be expressed in the form:

� � � ��, where a denotes the capillary length. Another important parameter is the inter-jet distance called

the critical wavelength. This parameter allows estimation of the relative productivity of the electrospinning

process. Calculation of the critical wavelength λc can be done using the equation

�� 2�� .

2. Design and construction of electrodes

Electrodes for electrostatic spinning of polymer melts can be categorized according to several criteria.

Regarding the intended use of the electrode one may distinguish laboratory, pilot plant and industrial setups.

Important factors are heating of the emitting electrode and polymer melt supply. In contrast to solution

electrospinning there is no need to prevent unwanted evaporation of the solvent, but thermal degradation of

the polymer must be considered. In this paper two types of laboratory emitting electrodes are described,

theoretically and experimentally evaluated.


First of the electrodes is the “rod” type electrode, see Fig. 1.

Electrode is designed for discontinuous laboratory evaluation of the

spinning ability of polymer melts and solution at elevated

temperature. Heating is supplied by cartridge heater and

temperature regulation is controlled by PID regulator with

thermocouple sensor. Polymer is dosed to the top of the electrode in

the form of pellets or powder, molten and spun from the free liquid

surface. Advantage of this type of electrode is a low consumption of

the polymer and simplicity of the process that allows evaluation of

the critical parameters of the process described at chapter 1.

Second type of the emitting electrode is the “cleft” spinner, depicted

in the Fig.2. Heating of the electrode is designed also by cartridge

heater and PID temperature regulator. Polymer melt is supplied

volumetrically to the linear spinning cleft from heated piston

reservoir, in the Fig. 2 illustrated in the purple color. Width of the cleft

is defined by metal strip inserted in the cleft. Advantage of this setup

is the relatively easy polymer feeding and possibility to upscale the

technology to industrial level. For the pilot plant

and industrial machines, piston feeding would

have to be substituted by the heated extruder to

ensure the uniform pre-heating. Linear shape of

the cleft is also advantageous for laboratory

measurement of the inter-jet distance, as have

been described in the previous chapter.

Besides the emitting electrode the laboratory

spinning setup consist of the metal plate

stationary collector with adjustable spinning

distance, high voltage supply and spinning

chamber.

3. Simulation of the electrostatic field distribution

The finite element method simulation was used for determination of the distribution of non-homogenous

electrostatic field intensity at given potential difference. In this paper simulations were done using the FEM

software Comsol Multiphysic. The local intensity of the electrostatic field is related to the electrostatic

spinning ability by equations described in the first chapter. Simulations were done for the comparison in 2D

and 3D geometry. Advantage of 2D simulation is the simplicity of the geometry definition and significantly

more detailed meshing of the model. However, in final comparison 3D geometry results provided more

comprehensive information about the distribution of the electrostatic field

Fig. 1 Emitting electrode, the „rod“ type

Fig. 2 Emitting electrode, the „cleft“ type


The boundary conditions were identical for all of the

simulations and are described in the Table 1. The results

of the 3D model of the electrostatic field distribution for

the “rod” type electrode can be seen in Fig 3. The field

distribution is depicted in the form of iso-surfaces. The

maximal strength of the field is represented by the red

color.

The results of the model are in the good agreement with

experimental observations as the maximal field strength

is observed on the circumference of the top of the

emitting electrode.

The results from the 3D simulation of

the electrostatic field distribution for

the cleft spinner with the metal plate

collector are displayed in Fig. 4. The

coloring and graph type are identical

as in the previously described

model. The qualitative result of the

simulation is also in the good

agreement with the experimental

observations.

The maximal field intensity is

concentrated on the outward edges

of the cleft. However, for the

quantitative evaluation of the model

it is important to comment the fact

that the number value of the

intensity at the edge depends on the

size of the integration meshing

element. Thus, for the infinitely small

meshing element the maximal point

value of the intensity grows to

infinity. In this work the value of the

intensity from the cutting plane

placed 1 mm parallel to the edge of

the emitting electrode was taken for

the calculations.

Emitting electrode Ground

Collector Electrostatic

potential 50kV

Exterior boundary

Zero charge / Symmetry

Spinning distance 10 cm

Table 1

Boundary conditions of the simulations

Fig. 3 Distribution of the electrostatic field, the “rod” emitting electrode, metal plate collector

Fig. 4 Distribution of the electrostatic field, the “cleft” emitting electrode, metal plate collector


4. Experimental measurement of the critical parameters of electrospinning

The value of the critical voltage and inter-jet was measured directly for the electrospinning setups described

in the chapter 2 and 3. The spinning distance was set to 10 cm. As the testing materials the isotactic

Polypropylene with Mn=5 000 and Poly (ε- caprolactone) with Mn=10 000 purchased from Sigma-Aldrich Co.

were used. These polymers were selected to represent the polar and non-polar group of polymers. Low

molecular weight polymers were chosen to satisfy the sufficiently low viscosity requirement. Polymers were

used as received. The measurement with the “rod” type electrode is showed at the Fig. 4

The measurement of the critical voltage provided the values of 20,2 kV for the polypropylene at the melt

temperature 230 °C and 21,8 kV for PCL at 180°C. The critical inter-jet distance can be calculated from the

Fig. 4 knowing the perimeter of the rod is 3,14 cm. Values of the critical inter-jet distance were measured as

6,3 mm for PP and 31,4 mm for PCL. The critical voltage for the cleft spinner was measured as 30kV with PP

and 34kV with PCL. The detailed photos of the inter-jet distances on the cleft spinner are depicted in the Fig.

5.

5. Comparison of the simulation and experiment

The theoretical values of the critical intensity and inter-jet distance can be obtained by calculating the

equations introduced in chapter 1 for known material properties. The relevant values of the material

properties of polymers were measured experimentally and results are showed in the Table 2 together with

the consequential values of the critical intensity, capillary length and critical inter-jet distance.

Fig. 4 The inter-jet distance for the critical and increased voltage, the „rod“ type spinning electrode, PP (left), PCL (right)

Fig. 5 The inter-jet distance for the critical and increased voltage, the „cleft“ type spinning electrode, PP (left), PCL (right)


γ (mNm-1)

ρ (gcm-3)

g (ms-2)

ε (Fm-1.10-12)

E (Vm-1 .106)

a (m.10-3)

λc (m.10-3)

Polypropylene 23,4 0,75 9,81 8,85 1,72 1,78 11,19

Poly(ε- caprolactone) 35,07 0,92 9,81 8,85 2,00 1,97 12,37

Quantitative comparison of the results can be done by introducing the measured critical voltage to the

simulation comparing the field intensities or inversely computing the voltage from known intensity and

comparing the voltage to the measured value. The comparison of the calculated and measured critical

intensities and inter-jet distances for both spinning setups are shown in the table 3.

Polymer

Temperature (°C)

Ec (Vm-1. 106) λc (m. 10-3) Spinning setup Theoretical Experimental

Theoretical Experimental

Polypropylene 230 1,721 1,827 11,19 6,3

“Rod”

1,721 1,958 11,19 5,2 “Cleft”

Poly(ε-caprolactone) 180

2,005 2,123 12,37 31,4 “Rod”

2,005 2,240 12,37 8,2 “Cleft”

6. Conclusions

In this paper a method of evaluation of a newly designed electrospinning setups is introduced. The method is

based on the numerical simulation of the electrostatic field distribution and calculation of critical parameters

of the needleless electrospinning. The method is in this paper tested on two types of melt electrospinning

setups and results are evaluated by the direct comparison to the experimental results. Coherence of the

experimental and theoretically calculated values provides an opportunity for the use of the method in the

electrospinning equipment design and prediction of outputs of the electrospinning process.

Literature

1. Rayleigh L.: On the equilibrium of liquid conducting masses charged with electricity, Philosophical

Magazine, 14 (1882), pp. 184-186.

2. Hartman R.P.A., Marijnissen J.C.M.: Electrohydrodynamic atomization in the cone-jet mode. Physical

model of the liquid cone and jet, Journal of Aerosol Science, (1997)

3. Jirsák O., Sanetrník F., Lukáš D., Kotek V., Martinová L., Chaloupek J.: CZ Patent, 294274 (B6), WO

2005024101 (2005), A Method of nanofibers production from polymer solution using electrostatic

spinning and a device for carrying out the method

4. Lukas D., Sarkar A., Pokorny P.: “Self-organization of jets in electrospinning from free liquid surface: A

generalized approach”, J. Appl. Phys. 103, 084309 (2008); DOI:10.1063/1.2907967

5. Landau L. D., Lifshitz E. M.: Electrodynamics of Continuous Media, 2nd ed. Butterworth-Heinemann,

Oxford, 1984

Table 2

Material properties and critical parameters of the jet formation

Table 3

Comparison of the theoretical and experimental values of the crtitical parameters

DESIGN AND EVALUATION OF MELT-ELECTROSPINNING...

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