Polypropylene Nanofibers: Melt Electrospinning Versus Meltblowing by Rajkishore Nayak

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Polypropylene Nanofibers: Melt Electrospinning Versus Meltblowing
By Rajkishore Nayak
Polypropylene Nanofibers: Melt Electrospinning Versus Meltblowing

Contents
1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Research Concepts and Hypotheses . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Aims and Objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4 Contribution of the Research . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2 Review of Literature: Melt Electrospinning . . . . . . . . . . . . . . . . . . . 9
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Electrospinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 Types of Electrospinning. . . . . . . . . . . . . . . . . . . . . . . . 10
2.2.2 Jet Trajectory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Research on Melt Electrospinning . . . . . . . . . . . . . . . . . . . . . . . 26
2.3.1 Equipment Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.3.2 Polymers Used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.4 Fibre Diameter and Influencing Factors . . . . . . . . . . . . . . . . . . . 28
2.4.1 Effects of Molecular Weight/MFI . . . . . . . . . . . . . . . . . 29
2.4.2 Effect of Viscosity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.4.3 Effect of Applied Voltage . . . . . . . . . . . . . . . . . . . . . . . 29
2.4.4 Effect of Collector Distance and Type. . . . . . . . . . . . . . 30
2.4.5 Effect of Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
2.4.6 Effect of Spinneret Size. . . . . . . . . . . . . . . . . . . . . . . . . 31
2.4.7 Effect of Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.4.8 Effect of Ambient Parameters . . . . . . . . . . . . . . . . . . . . 31
2.4.9 Effect of Electrical Conductivity . . . . . . . . . . . . . . . . . . 32
2.4.10 Other Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.5 Surface Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.6 Properties of Melt Electrospun Fibres. . . . . . . . . . . . . . . . . . . . . 33
2.6.1 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.6.2 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.6.3 Crystalline Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.6.4 Other Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
2.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
3 Experimental: Melt Electrospinning. . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.2 Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.2.1 Material Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.2.2 Material Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.3 Melt Electrospinning Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . 44
3.4 Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.5 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.6 Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.6.1 SEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.6.2 Optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.6.3 EDX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.6.4 NMR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.6.5 FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.6.6 DSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
3.6.7 TGA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.6.8 Molecular Weight or Intrinsic Viscosity . . . . . . . . . . . . 50
3.6.9 XRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.6.10 Shear Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
3.6.11 Electrical Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.6.12 Mechanical Characterisation . . . . . . . . . . . . . . . . . . . . . 52
3.6.13 Surface Wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4 Results and Discussion: Melt Electrospinning . . . . . . . . . . . . . . . . . . 55
4.1 Initial Experiments with Pure Polymers . . . . . . . . . . . . . . . . . . . 55
4.1.1 Establishing the Processing Parameters . . . . . . . . . . . . . 55
4.1.2 Initial Results: Effect of Polymer MFI. . . . . . . . . . . . . . 59
4.1.3 Effect of Viscosity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.1.4 Effect of Melt Flow Rate. . . . . . . . . . . . . . . . . . . . . . . . 63
4.1.5 Other Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
4.1.6 Effect of Die Size and Shape. . . . . . . . . . . . . . . . . . . . . 65
4.2 Experiments with Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.2.1 Effect of Additives on Fibre Diameter. . . . . . . . . . . . . . 71
4.3 Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.3.1 Elemental Composition Analysis by Energy
Dispersive X-ray (EDX) . . . . . . . . . . . . . . . . . . . . . . . . 84
4.3.2 Nuclear Magnetic Resonance (NMR) Results . . . . . . . . 87
4.3.3 Fourier Transform Infrared (FTIR) Results . . . . . . . . . . 88
4.3.4 Differential Scanning Calorimetry (DSC) Results . . . . . 89
4.3.5 Thermo Gravimetric Analysis (TGA) Results . . . . . . . . 96
4.3.6 Molecular Weight or Intrinsic Viscosity . . . . . . . . . . . . 96
4.3.7 X-ray Diffraction (XRD) Results . . . . . . . . . . . . . . . . . . 97
4.3.8 Mechanical Property . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.3.9 Hydrophobic Properties . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.4 Washing of the Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
5 Conclusions: Melt Electrospinning . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
5.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
6 Review of Literature: Meltblowing . . . . . . . . . . . . . . . . . . . . . . . . . . 111
6.1 Meltblowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
6.1.1 Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
6.1.2 Fibre Diameter and Influencing Factors. . . . . . . . . . . . . 112
6.1.3 Properties of Meltblown Fibres . . . . . . . . . . . . . . . . . . . 115
6.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7 Experimental: Meltblowing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
7.2 Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
7.3 Meltblowing Apparatus. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
7.4 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7.5 Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7.5.1 Optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7.5.2 SEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
7.5.3 NMR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
7.5.4 FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
7.5.5 DSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
7.5.6 TGA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
7.5.7 Molecular Weight or Intrinsic Viscosity . . . . . . . . . . . . 125
7.5.8 XRD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
7.5.9 Shear Viscosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
7.5.10 Mechanical Characterisation . . . . . . . . . . . . . . . . . . . . . 127
7.5.11 Surface Wettability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
7.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
8 Results and Discussion: Meltblowing . . . . . . . . . . . . . . . . . . . . . . . . . 131
8.1 Initial Experiments with Pure Polymers . . . . . . . . . . . . . . . . . . . 131
8.1.1 Initial Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
8.2 Effect of Fluids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
8.2.1 Process of Fibre Formation with Fluid Injection . . . . . . 133
8.2.2 Collection Pattern of Fibres . . . . . . . . . . . . . . . . . . . . . . 134
8.2.3 Mechanism of Fibre Formation . . . . . . . . . . . . . . . . . . . 136
8.2.4 Morphology and Diameter of Fibres . . . . . . . . . . . . . . . 139
8.3 Characterisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
8.3.1 NMR Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
8.3.2 FTIR Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
8.3.3 DSC Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
8.3.4 TGA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
8.3.5 Molecular Weight or Intrinsic Viscosity . . . . . . . . . . . . 149
8.3.6 XRD Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
8.3.7 Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 152
8.3.8 Hydrophobic Properties . . . . . . . . . . . . . . . . . . . . . . . . . 156
8.4 Processing Difficulties with 2000 MFI PP . . . . . . . . . . . . . . . . . 158
8.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
9 Conclusions: Meltblowing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
9.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
10 Comparison of Results and Future Suggestions . . . . . . . . . . . . . . . . 165
10.1 Results of the Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
10.2 Comparison of Results for the Fabrication of Nanofibres . . . . . . 166
10.2.1 Differences in the Process of Fibre Formation. . . . . . . . 166
10.2.2 Differences in the Material Performance . . . . . . . . . . . . 166
10.2.3 Differences in the Fibre Properties. . . . . . . . . . . . . . . . . 167
10.3 Future Suggestions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
10.4 Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
10.4.1 Journal Publications. . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
10.4.2 Conference Publications . . . . . . . . . . . . . . . . . . . . . . . . 169
10.4.3 Book Chapter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
Appendix A: Melt Electrospinning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
Appendix B: Meltblowing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Abbreviations
AC Alternating current
AFM Atomic force microscopy
CSA Cross-sectional area
DC Direct current
DSC Differential scanning calorimetry
EDX Energy dispersive X-ray
FESEM Field emission scanning electron microscopy
FTIR Fourier transform infrared
iPP Isotactic polypropylene
IV Intrinsic viscosity
kV Kilo volt
MFI Melt flow index
N2 Nitrogen gas
NaCl Sodium chloride
NMR Nuclear magnetic resonance
OM Optical microscopy
PDMS Polydimethylsiloxane
PE Polyethylene
PEG Polyethylene glycol
PET Polyethylene terephthalate
PMMA Polymethyl methacrylate
PP Polypropylene
RH Relative humidity
SCC Supercritical carbon dioxide
SD Standard deviation
SO Sodium oleate
TGA Thermo gravimetric analysis
XRD X-ray diffraction
Symbols
Mw Molecular weight
Tm Melting point
μm Micrometre
nm Nanometre
η Intrinsic viscosity or shear viscosity

Abstract
This book is based on the Ph.D. thesis of Rajkishore Nayak, defended on the 4th November 2011 at RMIT University, Melbourne (Australia). The thesis committee consisted of Prof. Rajiv Padhye, Dr. Lyndon Arnold, Associate Prof. Lijing Wang, Dr. IIias Louis Kyratzis and Dr. Yen Bach Truong, who have approved the content.

This book investigates the scope of fabricating nanofibres of polypropylene (PP) by two different melt processes: melt electrospinning and meltblowing. This study achieved uniform nanofibres of PP by melt electrospinning with the introduction of additives such as sodium oleate (SO) and sodium chloride (NaCl), which increased the electrical conductivity of the polymer melt. Rheology modifiers such as polyethylene glycol (PEG) and polydimethylsiloxane (PDMS) were used to lower the viscosity of the polymer melt, which resulted in the decreasing of the fibre diameter. However, the fibres obtained by reducing the melt viscosity were in larger micrometre or micron (μm) size and less uniform compared to the results obtained with increasing the electrical conductivity. Preliminary studies were undertaken to establish the optimum processing parameters such as temperature, applied voltage, collector distance, melt flow rate and spinneret size prior to the fabrication of nanofibres.

The average fibre diameters achieved by the use of pure polymers of 100, 300, 1000 and 2000 melt flow index (MFI) in melt electrospinning were in the range of 3.01–14.19 μm. The average fibre diameter was decreased with the increase in the polymer MFI, applied voltage and collector distance. The increase in the melt flow rate and the spinneret size resulted in the increase of the average fibre diameter. The optimal amount of SO and NaCl were 7% and 5%, respectively, for the fabrication of nanofibres from 1000 and 2000 MFI PP. The lowest average fibre diameters (achieved from 2000 MFI PP) were 0.371 ± 0.106 μm and 0.310 ± 0.102 μm corresponding to the optimum conditions of SO and NaCl, respectively. This research established the fact that there is no effect of the die shape on the cross-sectional shape of the meltelectrospun fibres. Unlike other melt processes such as meltblowing, and meltspinning; melt electrospinning resulted only in circular fibres by the use of trilobal, tetralobal and multilobal die.

This study demonstrated the scope of fabricating nanofibres by meltblowing with the injection of various fluids such as air, nitrogen (N2) and water. The use of water worked better in terms of fibre morphology and diameter compared to the use of air and N2 for the fabrication of nanofibres. The lowest fibre diameter of 0.438 ± 0.315 μm was achieved from the 300 MFI PP with water supply.

This study also established the fact that the molecular weight was the predominant factor governing the fibre diameter in melt electrospinning. In contrast, the molecular weight was not the predominant factor governing the fibre diameter in meltblowing. The polymers with the lowest molecular weight resulted in the formation of the finest fibre in melt electrospinning. However, the lowest molecular weight polymer was really difficult to process in meltblowing and did not produce good fibres. The effective collector distances for nanofibre fabrication in the case of melt electrospinning were lower compared to the meltblowing. The polymer feed rate was substantially lower in the case of melt electrospinning (i.e. 0.64 g/hr) compared to meltblowing (i.e. 80 g/hr).

The results of energy dispersive X-ray (EDX) and Fourier transform infrared (FTIR) studies established the presence of additives in the melt electrospun fibres. Similar FTIR and nuclear magnetic resonance (NMR) spectra of the polymer and the fibres indicated that there was no chemical change of the fibres fabricated by the application of various fluids and high temperature during meltblowing. The melting points of the fibres fabricated in melt electrospinning and meltblowing shifted to lower values compared to the initial polymer. This change was due to the thermal degradation caused by the high temperature during melt electrospinning and meltblowing.

The degree of thermal degradation was higher in the case of meltblowing due to the higher residual time inside the extruder which was verified by comparing the change in the molecular weights of the fibres fabricated by these processes. X-ray diffraction (XRD) studies showed that all the fibres fabricated by melt electrospinning and meltblowing contained low degree of crystallinity. The crystallinity, tensile strength and modulous of the meltblown fibres were increased with annealing. The fibres fabricated by melt electrospinning and meltblowing showed high values of contact angle indicating the hydrophobic nature. The additives SO, PEG and NaCl were washed away from the fibres fabricated by melt electrospinning. Hence, this research has achieved the goal of fabricating nanofibres and added new knowledge to the field of nanofibres.


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