i ABSTRACT KILIC, ALI. Improving Electret Filter Efficiency by Modifying Fibrous Webs with Melt ...

390 Pages·2012·4.45 MB·English

ABSTRACT
KILIC, ALI. Improving Electret Filter Efficiency by Modifying Fibrous Webs with Melt
Additives. (Under the direction of Dr. Behnam Pourdeyhimi and Dr. Eunkyoung Shim).

Having a long history in aerosol filtration technologies, electret filters still have challenging
problems such as low initial efficiency and stability of filtration. One way to modify charging
properties of such polymeric filter media is incorporation of additives. Even though
particularly in industry several polymer additives were used so far, there is not an extensive
study that explains effect of those additives on efficiency and stability of electret filters. This
study addresses the effect of various polymer additives on electret and electrostatic filtration
properties of high efficiency nonwoven electret filters. Undoubtedly incorporation of such
additives will result in changes in chemical structure and morphology of fibers which were
investigated with various techniques such as DSC, XRD, FTIR, EDS and dielectric
spectroscopy. Modified nonwoven webs were also analyzed by means of surface chemistry
and morphology. Relation between those structural data and both their electret and
electrostatic filtration properties were compared. Surface potential and electrostatic filtration
properties were analyzed as resultant characteristics of those webs. The project was not
limited to selection of additives, since the electrostatic filtration efficiency strongly depends
on charging method. Due to chemistry and electronic structure, additives may require
investigation of specific charging conditions, which was also one of the targets of the project.
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Improving Electret Filter Efficiency by Modifying Fibrous Webs with Melt Additives

by
Ali Kilic

A dissertation submitted to the Graduate Faculty of
North Carolina State University
in partial fulfillment of the
requirements for the Degree of
Doctor of Philosophy

Fiber and Polymer Science

Raleigh, North Carolina
2012

APPROVED BY:

____________________ _____________________ _________________
Dr. Richard Spontak Dr. Saad A. Khan Dr. Bong Yeol Yeom
Committee member Committee member Committee member

___________________________ ___________________________
Dr. Behnam Pourdeyhimi Dr. Eunkyoung Shim
Committee Chair Committee Co-Chair
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BIOGRAPHY

Ali Kilic was born on March 14, 1983 in Ankara, Turkey. He received his Bachelor’s degree
in Textile Engineering from Istanbul Technical University, Istanbul, Turkey. After
graduation, he started pursuing his master degree in textile engineering program at ITU in the
fall semester of 2006. In January 2009, he was admitted into the PhD program in Fiber and
Polymer Science and held a research assistantship with the Nonwovens Institute in the
College of Textiles at NCSU. Upon finishing his graduate studies in NC State, he will be
returning to his home country to pursue his career as an academician.
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ACKNOWLEDGMENTS

This dissertation work would have never been accomplished without the valuable guidance
and support of Dr. Behnam Pourdeyhimi, Dr. Eunkyoung Shim, and Dr. Bong Yeol Yeom.
This is almost 4 year with them which was extremely enlightening to draw my future. I have
learned tremendously from them to improve my writing, presentation, and technical skills as
well as methodologies for solving problems. I am extremely thankful to Dr. Richard Spontak
and Dr Saad Khan for valuable discussions and agreeing to be on my graduate committee.
I wish to acknowledge the financial support of this research by the Nonwoven Research
Cooperative Center (NCRC). Special thanks to Dr. Benoît Mazé, Dr. Nagendra
Anantharamaiah, Dr. Yogeshwar Velu (DuPont), Y. van der Zijpp (Colbond), Steve Cox and
Dave Healey (H&V) for their help and fruitful discussions during the research. I would also
like to thank Amy Minton, Birgit Andersen, Roberto Garcia, Chuck Mooney for their
patience during analyzing my samples. I would also like to thank my friends especially
“NCRC student members” who have made my graduate school years bearable even
sometimes enjoyable. I would like to extend my appreciation to our Turkish community in
the Triangle area.
Finally, I would like to thank my dearest family, Professors Demir, Ulcay and Oznergiz for
their support they provided me during my lonely years abroad.

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TABLE OF CONTENTS

LIST OF TABLES ................................................................................................................. viii
LIST OF FIGURES ................................................................................................................. ix
1 Introduction ....................................................................................................................... 1
1.1 References .................................................................................................................. 6
2 Literature Review .............................................................................................................. 7
2.1 Electrets ...................................................................................................................... 7
2.2 Electret Filters ............................................................................................................ 9
2.3 Basic Electret Characteristics ................................................................................... 18
2.3.1 Charging and Polarization Phenomena ............................................................. 19
2.3.2 Charge Motions within Electrets ...................................................................... 27
2.4 Methods for Producing Electret Filters .................................................................... 31
2.4.1 Corona Discharge.............................................................................................. 31
2.4.2 Triboelectrification ........................................................................................... 36
2.4.3 Liquid Contact .................................................................................................. 43
2.4.4 Electrospinning ................................................................................................. 45
2.5 Additives for Improving Electret Properties of PP .................................................. 46
2.5.1 High Dielectric Constant Inorganic Additives .................................................. 47
2.5.2 Nucleating Agents ............................................................................................. 55
2.5.3 Antioxidants ...................................................................................................... 70
2.5.4 Oily Mist Resisting Agents ............................................................................... 94
2.6 References ................................................................................................................ 97
3 Optimizing Charging Properties of BaTiO /Polypropylene Composite Filaments ....... 117
3
3.1 Introduction ............................................................................................................ 118
3.2 Materials & Methods .............................................................................................. 120
3.3 Results and Discussion ........................................................................................... 125
3.3.1 Microstructure ................................................................................................. 125
3.3.2 Surface Potential Decay Tests ........................................................................ 131
3.4 Conclusion .............................................................................................................. 136
3.5 References .............................................................................................................. 136
4 Improving Electret Properties of PP Filaments with Barium Titanate .......................... 142
4.1 Introduction ............................................................................................................ 142
4.2 Materials & Methods .............................................................................................. 146
4.2.1 Material and Filament Preparation.................................................................. 146
4.2.2 Charging and Characterization of Charging Property .................................... 147
4.2.3 Fiber Structure ................................................................................................ 149
4.3 Results and Discussions ......................................................................................... 151
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4.3.1 Fiber Structure ................................................................................................ 151
4.3.2 Electret Properties of Barium Titanate Containing PP Filaments .................. 158
4.4 Discussions ............................................................................................................. 162
4.5 Conclusion .............................................................................................................. 165
4.6 References .............................................................................................................. 165
5 Filtration Properties of BaTiO Containing Meltblown Electret Filters........................ 170
3
5.1 Introduction ............................................................................................................ 171
5.2 Materials & Methods .............................................................................................. 174
5.2.1 Materials ......................................................................................................... 174
5.2.2 Methods........................................................................................................... 174
5.3 Results and Discussions ......................................................................................... 179
5.3.1 Basic Web Properties ...................................................................................... 179
5.3.2 Surface Potential and Filtration Properties ..................................................... 181
5.3.3 Filtration Properties after Isothermal Potential Decay .................................... 188
5.3.4 Microstructural Properties of BaTiO /PP Webs ............................................. 190
3
5.4 Conclusion .............................................................................................................. 192
5.5 References .............................................................................................................. 192
6 Effect of Nucleating Agents on Electret Properties of Polypropylene Filaments ......... 198
6.1 Introduction ............................................................................................................ 199
6.2 Experimental .......................................................................................................... 202
6.2.1 Sample preparation ......................................................................................... 202
6.2.2 Charging and Characterization of Charging Property .................................... 204
6.2.3 Fiber microstructure ........................................................................................ 205
6.3 Results .................................................................................................................... 207
6.3.1 Fiber microstructure ........................................................................................ 207
6.3.2 Charging Properties ........................................................................................ 212
6.4 Discussion .............................................................................................................. 219
6.5 Conclusion .............................................................................................................. 220
6.6 References .............................................................................................................. 221
7 Effects of Stabilizers on Charging of PP Filaments ...................................................... 225
7.1 Introduction ............................................................................................................ 226
7.2 Materials & Methods .............................................................................................. 231
7.2.1 Material and Filament Preparation.................................................................. 231
7.2.2 Fiber microstructure ........................................................................................ 232
7.2.3 Charging and Characterization of Charging Property .................................... 232
7.3 Results .................................................................................................................... 234
7.3.1 Fiber Microstructure ....................................................................................... 234
7.3.2 Charging Properties ........................................................................................ 237
7.4 Conclusion .............................................................................................................. 241
7.5 References .............................................................................................................. 242
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8 Effect of Heat Treatment on Charging Properties of Electret Fibers ............................ 247
8.1 Introduction ............................................................................................................ 247
8.2 Experimental .......................................................................................................... 254
8.2.1 Materials and Filament Preparation ................................................................ 254
8.2.2 Charging and Characterization of Charging Property .................................... 255
8.2.3 Surface Crystallinity Analysis ........................................................................ 257
8.3 Results and Discussions ......................................................................................... 258
8.4 Conclusion .............................................................................................................. 264
8.5 References .............................................................................................................. 264
9 Aerosol Filtration Properties of Nucleating Agent Containing Electret Filters ............ 269
9.1 Introduction ............................................................................................................ 269
9.2 Experimental .......................................................................................................... 272
9.3 Results and Discussions ......................................................................................... 277
9.3.1 Analysis of DMDBS/PP Meltblown Webs ..................................................... 277
9.3.2 Analysis of NA11/PP Meltblown Webs ......................................................... 282
9.4 Conclusion .............................................................................................................. 291
9.5 References .............................................................................................................. 291
10 Effect of Stabilizers on Corona Discharged PP Electret Filters ................................ 295
10.1 Introduction ............................................................................................................ 296
10.2 Experimental .......................................................................................................... 297
10.3 Results and Discussions ......................................................................................... 303
10.3.1 Analysis of Irgafos/PP Meltblown Webs ........................................................ 303
10.3.2 Analysis of Tinuvin/PP Meltblown Webs ...................................................... 310
10.4 Conclusion .............................................................................................................. 317
10.5 References .............................................................................................................. 317
11 Effect of Surface Morphology on NA11 containing PP Electret Filters ................... 323
11.1 Introduction ............................................................................................................ 323
11.2 Experimental .......................................................................................................... 326
11.3 Results and Discussions ......................................................................................... 330
11.4 Conclusions ............................................................................................................ 336
11.5 References .............................................................................................................. 336
12 Overall Conclusions ................................................................................................... 343
13 Suggested Future Works ............................................................................................ 345
13.1 References .............................................................................................................. 347
14 Measuring Electrostatic Properties of Fibrous Materials: A Review and a Modified
Surface Potential Decay Technique ...................................................................................... 348
14.1 Introduction ............................................................................................................ 348
14.2 Literature Review ................................................................................................... 349
14.2.1 Direct methods ................................................................................................ 349
14.2.2 Indirect Methods ............................................................................................. 356
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14.3 Experimental .......................................................................................................... 357
14.3.1 Material ........................................................................................................... 357
14.3.2 Corona Discharge............................................................................................ 358
14.3.3 Electrostatic Voltmeter ................................................................................... 359
14.3.4 Sample Preparation for ISPD Test .................................................................. 359
14.4 Results and Discussion ........................................................................................... 360
14.4.1 Results ............................................................................................................. 360
14.4.2 Discussion ....................................................................................................... 366
14.5 Conclusion .............................................................................................................. 368
14.6 References .............................................................................................................. 369

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LIST OF TABLES

Table 2.1 Capture mechanisms in submicron range ............................................................... 12
Table 2.2 Defective structures, their origins and effects [adapted from17,32].......................... 22
Table 2.3 Liquids for contact electrification83 ........................................................................ 45
Table 2.4 Dielectric properties of ABO perovskite structures 41 ........................................... 49
3
Table 2.5 Electret properties of trisamide based nucleating agents containing PP films117 ... 68
Table 2.6 Change in O/C ratio on surface and contact angle upon corona discharge142 ........ 78
Table 2.7 Antioxidants, general classification ........................................................................ 82
Table 3.1 DSC analysis of reference and modified samples ................................................ 127
Table 4.1 Detailed crystal analyses for various BaTiO concentrations. (Since BaTiO peaks
3 3
for 0.01%, 0.1% concentrations were not clear, they were not included) ............................ 155
Table 5.1 Temperature profile of MB process ...................................................................... 175
Table 6.1 Detailed WAXD analysis for various nucleating agent concentrations ............... 211
Table 9.1 Temperature profile of MB process ...................................................................... 274
Table 10.1 Temperature profile during meltblowing process ............................................... 299
Table 11.1 Temperature profile of MB process .................................................................... 328
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LIST OF FIGURES

Figure 1.1 Effect of electrostatic charging on total filtration efficiency 4 ................................ 2
Figure 2.1 Mechanical capture mechanisms [adapted from 5] ................................................ 11
Figure 2.2 Aerosol capture mechanisms [adapted from 10] .................................................... 12
Figure 2.3 Comparison between mechanical and electrostatic filtration 9 .............................. 13
Figure 2.4 Efficiency for mechanical capture mechanisms12 ................................................. 14
Figure 2.5 Schematic representation of state density within a dielectric [adapted from17] .... 20
Figure 2.6 TSD curve for pure PP obtained by Kravtsov et al 33............................................ 24
Figure 2.7 Frequency dependence of polarization mechanisms in dielectrics [adapted from
41,43] ......................................................................................................................................... 27
Figure 2.8 Hopping and tunneling mechanisms [adapted from 31] ......................................... 29
Figure 2.9 Current distribution on corona region and sample ................................................ 33
Figure 2.10 Triboelectric series of common materials [Adapted from 74] .............................. 38
Figure 2.11 Hydroxide adsorption model for nonionic polymers [adapted from 71] .............. 41
Figure 2.12 Electret charge technique from 82 ........................................................................ 44
Figure 2.13 Atomic structure of BaTiO crystal and crystal structures under various
3
temperatures ............................................................................................................................ 51
Figure 2.14 Dielectric constant of barium titanate depending on grain size and temperature 52
Figure 2.15 Crystal forms within 43-48° diffraction angles 105 .............................................. 55
Figure 2.16 Crystal structure before and after nucleating agent ............................................. 56
Figure 2.17 Changes in crystallization temperature, clarity and crystallization half-time
[adapted from 108,111] ............................................................................................................... 59
Figure 2.18 Elongated cavities formed upon modification with NA11 [adapted from89] ...... 66
Figure 2.19 Schematics for percolation concentration ........................................................... 67
Figure 2.20 Loss of tertiary H and crosslinkage in PP ........................................................... 72
Figure 2.21 Initiation of oxidation reaction for PP130 ............................................................. 73
Figure 2.22 Possible propagation mechanisms during the oxidation of PP130 ........................ 74
Figure 2.23 Termination processes upon oxidation130 ............................................................ 75
Figure 2.24 TSC and SPD test performed by 133 .................................................................... 79
Figure 2.25 Primary antioxidants and H donoring mechanism .............................................. 83
Figure 2.26 Functional groups that have chromophoric effects. ............................................ 86
Figure 2.27 Mechanism of energy quenchers131 ..................................................................... 87
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