Post on 07-Mar-2021
Cellulose-based electrical insulation materials
Dielectric and mechanical
properties
REBECCA HOLLERTZ
Doctoral thesis
KTH Royal Institute of Technology
School of Chemical Science and Engineering
Department of Fibre and Polymer Technology
ISBN 978-91-7729-327-9
TRITA – CHE-report 2017:21
ISSN 1654-1081
Tryck: US-AB, Stockholm, 2017
The following papers are reprinted with permission from:
Paper I: Springer
Papers II, III, VII and VIII: IEEE Transactions on Dielectric and
Electrical Insulation
AKADEMISK AVHANDLING
Som med tillstånd av Kungliga Tekniska högskolan i Stockholm
framläggs till offentlig granskning för avläggande av teknologie
doktorsexamen fredagen den 12 maj 2017, kl. 10.00 i F3,
Lindstedtsvägen 26, KTH, Stockholm. Avhandlingen försvaras på
engelska. Fakultetsopponent: Professor Markus Biesalski, TU
Darmstadt, Tyskland
Copyright© Rebecca Hollertz, 2017
ABSTRACT
The reliability of the generation and distribution of electricity is highly
dependent on electrical insulation and is essential for the prosperity of our
society and a ubiquitous part of our everyday life. The present study shows
how some important material properties affect the electrical properties of
cellulose-based electrical insulation systems which are used together with
mineral oil in high-voltage transformers. Among other things, the effects
of paper density and of the lignin content of the fibres on the dielectric
response and charge transport of the papers have been studied.
The underlying mechanisms of the inception and propagation of
streamers, responsible for the most costly failures in transformers, at the
oil-solid interface have been investigated and the important role of paper
morphology on streamer propagation has been demonstrated. With papers,
in contrast to films of synthetic polymers and microfibrillated cellulose,
the branching of streamers increased and the length of slow negative
streamers decreased. It was also shown that for polymers with
permittivities close to that of the oil, the inception voltage was higher than
with polymers with higher permittivities. This was explained by a different
distribution of the electric field at the interface, and it means that with a
solid restricting the initiation volume and with a permittivity of the solid
matching the permittivity of the liquid, there is a lower risk for streamer
inception, in negative polarity.
Fibres were also modified prior to paper sheet preparation in attempts
to improve the mechanical and dielectric properties. The properties of
papers containing cellulosic micro- and nanofibrils and SiO2 and ZnO
nanoparticles indicate that these additives can indeed be used to improve
both the mechanical and dielectric properties. For example, the tensile
strength index of papers with periodate-oxidised carboxymethylated CNFs
increased by 56 % and by 89 % with a 2 wt% and 15 wt% addition,
respectively, of the fibrils, when used together with different retention aids.
A three-layered structure with two papers laminated together with a thin
layer of microfibrillated cellulose also showed an increased DC breakdown
strength by 47 % compared to a single-layer paper with a similar thickness.
SAMMANFATTNING
En god elektrisk isolering, i olika utrustningar i distributionskedjan, är i
hög grad väsentligt för en tillförlitlig överföring och distribution av
elektricitet. Detta är i sin tur en förutsättning för dagens välstånd och är av
stor betydelse i vår vardag. Resultaten från doktorandprojektet visar hur
flera materialegenskaper påverkar dielektriska egenskaper hos papper som
används i kombination med mineralolja som elektriskt isoleringsmaterial i
högspänningstransformatorer. Bland annat har inverkan av såväl papperets
densitet som fibrernas ligninhalt på både laddningstransport och elektriska
förluster studerats och kvantifierats. Dessutom har mekanismerna för
uppkomsten och utbredning av streamers, som är den mest kostsamma
orsaken till transformatorhaveri, i gränsytan mellan olja och fast fas
undersökts. Resultaten visar att det finns ett klart samband mellan
papperets morfologi och utbredningen av streamers. För långsamma
negativa streamers på papper, jämfört med på filmer av syntetiska
polymerer och mikrofibrillerad cellulosa, ses en ökad förgrening som
vidare leder till kortare streamers. Genom att jämföra uppkomsten och
utbredningen av streamers på olika polymerer studerades också inverkan
av isoleringsmaterialets permittivitet. Med polymerer med en permittivitet
som matchar oljepermittiviteten krävdes en högre elektrisk
initieringsspänning för streamers. Detta innebär att risken för uppkomst av
streamers i negativ polaritet minskas i gränsytan när permittiviteten hos det
fasta materialet överensstämmer med oljans.
Baserat på dessa resultat genomfördes olika fibermodifieringar för att
förbättra de mekaniska och dielektriska egenskaperna. Resultat för papper
som innehåller fibrer som modifierats med mikro- och nanofibriller och
speciellt utvalda nanopartiklar visar att dessa kan användas som
tillsatsmedel för att förbättra båda dessa egenskaper. Dragstyrkeindex
ökade med 56 % och 89 % med 2 vikt% respektive 15 vikt% nanofibriller
(karboxymetylerade fibriller som perjodat oxiderats) tillsatta i
kombination med lämpliga retentionsmedel. Dessutom ökade DC
genomslagsstyrkan med 47 % med en tre-skikts konstruktion med två
omgivande papper som laminerats med ett skikt av mikrofibrillerad
cellulosa jämfört med ett homogent papper av samma tjocklek.
LIST OF PUBLICATIONS
I. Dielectric properties of lignin and glucomannan as determined by
spectroscopic ellipsometry and Lifshitz estimates of the non-retarded
Hamaker constants
Rebecca Hollertz, Hans Arwin, Bertrand Faure, Yujia Zhang, Lennart
Bergström, Lars Wågberg, Cellulose, 20, p. 639-1648, August 2013
II. Effect of Composition and Morphology on the Dielectric Response of
Cellulose-based Electrical Insulation
Rebecca Hollertz, Claire Pitois, Lars Wågberg, IEEE Transactions on
Dielectrics and Electrical Insulation, 22, p 2239-2248, August 2015
III. First mode negative streamers at mineral oil-solid interfaces
David Ariza, Rebecca Hollertz, Marley Beccera, Claire Pitois, Lars
Wågberg, Accepted for publication in IEEE Transactions on Dielectrics
and Electrical Insulation
IV. Inception of first mode negative streamers at mineral oil-solid
interfaces David Ariza, Marley Beccera, Rebecca Hollertz, Ralf Methling, Lars
Wågberg, Sergey Gortschakow Manuscript
V. Influence of paper properties on streamers creeping in mineral oil
David Ariza, Marley Beccera, Rebecca Hollertz, Lars Wågberg, Ralf
Methling, Sergey Gortschakow, Accepted for publication in the
Proceedings of the 2017 International Conference on Dielectric Liquids
VI. Chemically modified cellulose micro- and nanofibrils as paper-
strength additives Rebecca Hollertz, Verónica López Durán, Per Larsson, Lars Wågberg Submitted
VII. Dielectric Response of Kraft Papers from Fibres Modified by Silica
Nanoparticles
Rebecca Hollertz, David Ariza, Claire Pitois, Lars Wågberg
2015 Annual Report Conference on Electrical Insulation and Dielectric
Phenomena, p 459-462
VIII. Comparison of Oil-impregnated Papers with SiO2 and ZnO
Nanoparticles or High Lignin Content, for the Effect of
Superimposed Impulse Voltage on AC Surface PD
Roya Nikjoo, Hans Edin, Nathaniel Taylor, Rebecca Hollertz, Martin
Wåhlander, Lars Wågberg, Accepted for publication in IEEE
Transactions on Dielectrics and Electrical Insulation
CONTRIBUTIONS TO PUBLICATIONS
I. Main author*. Hollertz prepared the purified lignin and spin-coated
the surfaces of lignin and hemicellulose. The purified hemicellulose
was prepared by Dr Yujia Zhang. The spectroscopic ellipsometry
measurements were performed with the instruction and supervision
of Professor Hans Arwin. The calculations of the non-retarded
Hamaker constants were made by Dr Bertrand Faure. All the co-
authors were engaged in writing the manuscript.
II. Main author*. Hollertz prepared all the samples and performed all
the measurements. Dr Uno Gäfvert assisted in the interpretation of
the data.
III. Co-author. Hollertz took part in planning the experiments and gave
input into the sample preparation. Hollertz prepared the paper
samples and characterized the papers and polymers. The streamer
tests were performed and the data analysed by David Ariza. Hollertz
wrote the descriptions of the materials and took an active part in the
full manuscripts, with analysis and writing.
IV. See III.
V. See III.
VI. Main author*. The fibrils were prepared by PhD-Student Veronica
Lopez Duran. Hollertz prepared all the paper sheets, while the
characterization was performed together with Veronica Lopez
Duran. Although Hollertz had the main responsibility for the analysis
of the data and the paper writing, it was done together with the other
co-authors.
VII. Main author*. Prepared all the paper samples and characterized
them.
VIII. Co-author. Hollertz prepared the paper samples and characterized
the material properties, besides their dielectric response and
behaviour under discharges, and took part in writing the manuscript.
The ZnO nanoparticles were surface-modified and characterized by
Martin Wåhlander.
*Main author embodies the work of formulating hypotheses, planning the
experimental work and having the main responsibility for the manuscript. At
the beginning of the PhD project, these tasks were undertaken together with
Professor Lars Wågberg, but later more independently under his supervision.
Conference papers (peer-reviewed) not appended to this thesis:
Novel cellulose nanomaterials – towards use in electrical insulation
Rebecca Hollertz, Claire Pitois, Lars Wågberg
International Conference on Dielectric Liquids (ICDL, Bled 2014)
Measurements of the charge and velocity of streamers propagating along
transformer oil-pressboard interfaces
David Ariza, Marley Becerra, Rebecca Hollertz, Claire Pitois
International Conference on Dielectric Liquids (ICDL, Bled 2014)
Kraft-Pulp Based Material for Electrical Insulation Rebecca Hollertz, Claire Pitois, Lars Wågberg Nordic Insulation Symposium on Materials Components and Diagnostics (NORD-
IS, Copenhagen 2015)
On the initiation of negative streamers at mineral oil-solid interfaces
David Ariza, Marley Beccera, Rebecca Hollertz, Claire Pitois
IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP, Ann
Arbor 2015), no. 149, session 6B-3
Propagation of streamers along mineral oil-solid interfaces
David Ariza, Marley Beccera, Rebecca Hollertz, Claire Pitois
IEEE Conference on Electrical Insulation and Dielectric Phenomena (CEIDP, Ann
Arbor 2015) no. 362, session 4C-1
Abstracts were also presented at American Chemical Society (ACS) Meeting in
New Orleans in 2013 and in San Diego in 2016.
PREFACE
I would like especially and sincerely to thank my supervisor Professor Lars
Wågberg. Besides being the brain behind the many ideas and hypotheses in the
project, he has also created a welcoming environment built on trust and
responsibility. The level of independence, encouraged and allowed, has also been
of uttermost importance for me, enabling me to pursue my research with a growing
family.
The initiative for this project was taken by Dr Claire Pitois, at the time at ABB
Corporate Research. Pitois contributed greatly and followed the project as co-
supervisor until 2016. At ABB Figeholm, the initiative was led by Dr Torbjörn
Brattberg. The contacts with ABB in Västerås, Figeholm and in Ludvika have been
inspiring and extremely helpful. I would therefore like to acknowledge ABB AB
for their crucial role in the establishment and funding of this project and also for
their professionalism and attentiveness throughout the project: Dr Claire Pitois, Dr
Torbjörn Brattberg, Dr Lars Schmidt, Adjunct Professor Henrik Hillborg, Adjunct
Professor Tord Bengtsson, Dr Uno Gäfvert, Assistant Professor Mikael Unge, Dr
Santanu Singh, Jan Hajek and Anders Ericsson for their contribution to the project
and also for their support over these years. Together with ABB AB, the Swedish
Energy Agency is gratefully acknowledged for funding the project through the
ELEKTRA and ELFORSK programs.
Early we discovered two crucial parameters in the project that were important
to address. Firstly, the dielectric properties stipulated by the type of wood pulp and
the paper quality were not clear to us. Secondly, to characterize the streamer
propagation and to create an understanding of the mechanisms behind streamer
inception a completely new set-up had to be designed and built. To address the
first question two initial projects were initiated to investigate the influence of pulp
and paper composition and processing on the dielectric response. The first paper
was written in collaboration with Professor Lennart Bergström at Stockholm
University and Professor Hans Arwin at Linköping University to whom I express
my gratitude for their expertise, time and effort. In the second paper, we wanted
to characterize the dielectric response of different papers. In connection with this
work I would like to acknowledge my collaborators at the Electromagnetic
Engineering department for their valuable input and for welcoming me to their
laboratory. For the interpretation of the dielectric response data, I had the privilege
on several occasions to meet Dr Uno Gäfvert who gave me a highly valuable input
and introduced me to many new concepts.
To be able to characterize the streamers, a collaboration with Associate
Professor Marley Beccera, working both at KTH Electromagnetic Engineering
Department and ABB Corporate Research, was initiated and resulted in a PhD
project with the PhD-student David Ariza. For the great collaboration and all
excellent and thorough work I would like to thank David Ariza. The
characterization of streamers in positive polarity was also strengthened by
collaboration with the Leibniz Institute for Plasma Science and Technology in
Greifswald, Germany. For DC breakdown tests performed at ABB, I would also
like to acknowledge the work done by Dr Zargham Jabri and Dr Anneli
Jedenmalm.
The addition of chemically modified nanofibrils resulted in a significant
improvement and I would like to acknowledge my co-authors Veronica López
Durán and Dr Per Larsson. The results would not have been obtained nor published
without you!
Considering previous work on dielectrics, we found it attractive to use the
Layer-by-Layer (LbL) technique, well-known in the Fibre Technology group, to
introduce nanoparticles into insulating papers. I would like to acknowledge Eka
chemicals (especially Dr Michael Persson) for suppling silica nanoparticles,
Martin Wåhlander for modifying the zinc oxide nanoparticles and Oruç Köklükaya
for being most generous with both time and knowledge. The partial discharge
measurements were performed by Dr Roya Nikjoo. Unfortunately, the samples
characterized were the last she managed before her dissertation, and additional
complementary measurements were not possible. I would also like to thank co-
authors Dr Nathaniel Taylor, Professors Hans Edin and Eva Malmström.
I would like to direct my thanks to the whole Fibre Technology group; for the
warm and helpful atmosphere. I especially wish to thank my roommates and also
Johan for the pleasant discussions, many times scientific, during early breakfast
and for all the help in the laboratory. I feel very grateful for the possibility to work
with Professor Lars Ödberg who has been involved as co-supervisor and has given
valuable input and support. I would also like to thank Mia Hjertén for all her
support. In summary: This has been a lot of fun!
Finally, I would like to thank my parents and Mikael, Tage and Klara for being
there, and also Elisabeth and Bengt for all your support.
Stockholm, 2017
Rebecca Hollertz
TABLE OF CONTENTS
1. INTRODUCTION ............................................................................... 1
2. BACKGROUND .................................................................................. 2
2.1 Paper and pressboard in oil-filled transformers ................................... 2
2.1.1 Outlook ................................................................................................ 3
2.2 A short introduction to wood fibres ..................................................... 6
2.3 Paper physics and chemistry ................................................................ 8
2.3.1 Cellulose micro- and nanofibrils ........................................................ 10
2.3.2 Layer-by-Layer (LbL) adsorption ...................................................... 11
2.4 A short introduction to dielectric properties ...................................... 12
2.4.1 Dielectric response ............................................................................. 12
2.4.2 Streamers in oil and at oil-solid interfaces ......................................... 15
3. EXPERIMENTAL............................................................................. 17
3.1 Materials ............................................................................................ 17
3.1.1 Wood pulps ........................................................................................ 17
3.1.2 Transformer oil .................................................................................. 17
3.1.3 Polymers............................................................................................. 17
3.1.4 SiO2 and ZnO nanoparticles ............................................................... 18
3.2 Methods –material preparation .......................................................... 18
3.2.1 Lignin and glucomannan model materials (Paper I) ......................... 18
3.2.2 Paper sheets ........................................................................................ 19
3.2.3 Cellulose micro- and nanofibrils ........................................................ 20
3.2.4 Paper sheets with micro- and nanofibrils as strength additives
(Paper VI) ................................................................................................... 21
3.2.5 Films from micro- and nanofibrils ..................................................... 21
3.2.6 Three-layered sandwich structure with microfibrillated cellulose ..... 22
3.2.7 Papers from fibres modified with SiO2 and ZnO nanoparticles
(Papers VII and VIII) ................................................................................. 22
3.3 Methods –material characterization ................................................... 23
3.3.1 Charge density measurements ............................................................ 23
3.3.2 Thickness measurements .................................................................... 24
3.3.3 Mechanical testing ............................................................................. 24
3.3.4 Spectroscopic ellipsometry (Paper I) ................................................ 24
3.3.5 Dielectric spectroscopy (Papers II, VII and VIII) ............................. 25
3.3.6 DC Breakdown test ............................................................................ 26
3.3.7 Streamer measurements (Papers III-V) ............................................. 27
3.3.8 Partial discharge (PD) measurements (Paper VIII) ........................... 27
4. RESULTS AND DISCUSSION ........................................................ 29
4.1 The influence of paper density and morphology on dielectric response
(Paper II) .................................................................................................. 30
4.2 The influence of wood fibre composition on dielectric properties .... 31
4.2.1 Spectroscopic ellipsometry measurements of wood biopolymers
(Paper I) ..................................................................................................... 32
4.2.2 Dielectric response (Papers II, VII and VIII) .................................... 33
4.2.3 Surface partial discharge (PD) resistance (Paper VIII) ..................... 33
4.3 The influence of permittivity, density and morphology on streamer
inception and propagation (Papers III-V) ................................................ 34
4.4 Papers with cellulosic fibres and fibrils combined ............................. 37
4.4.1 DC breakdown strength of a three-layered sandwich with
microfibrillated cellulose ............................................................................ 37
4.4.2 Micro- and nanofibrils as paper strength additives (Paper VI) ......... 39
4.5 Papers from wood fibres with adsorbed SiO2 and ZnO nanoparticles
(Papers VII and VIII) ............................................................................... 42
4.5.1 Adsorption ......................................................................................... 42
4.5.2 Dielectric response ............................................................................. 45
4.5.3 Surface partial discharge (PD) resistance .......................................... 46
5. CONCLUSIONS ................................................................................ 48
6. FUTURE WORK ............................................................................... 51
7. REFERENCES ................................................................................... 52
I have strived, when writing this thesis, to make it digestible for colleagues with a
background in chemistry or in electrical engineering and I hope therefore that both
can acquire some new understanding or even discover an interest into, for them, a
new field. A dielectric material is an electrical insulating material and a summary
of some dielectric properties of materials relevant for this study is presented in
Table A1 in the Appendix on page 60. A list of abbreviations is found on page 59.
INTRODUCTION
1
1. INTRODUCTION
The integration of renewable energy sources into the electric grid, along
with higher voltage ratings and a transition to the use of DC grids are
factors that contribute to a more sustainable electricity production and an
efficient energy transmission and distribution. These processes are already
underway, or planned, and they are necessary for a transition away from
power generation based on the burning of fossil fuels. As a result, there are
new demands on the components of the power transmission system; to
upgrade the power grid requires reliable insulating materials that have low
energy losses and which prevent failures that can lead to costly
maintenance work or even blackouts. A key component in a power
transmission system is the high-voltage power transformer. The insulation
material in these is of paper and pressboard impregnated with and
surrounded by mineral oil. Insulation failure has been identified as the
leading cause of transformer failure in several studies1,2.
Recently, increasing efforts to develop new and also more value-added
products from wood fibres have resulted in new and more sophisticated
modifications of wood fibres, which can potentially be used to improve
cellulose-based dielectric materials.
This project has focused on how the chemistry and the morphology of
the solid insulation material is affected by, and can affect, the electrical
phenomena in a high-voltage transformer. It is, however, difficult to
separate the effects of individual parameters because they can seldom be
independently changed. With a novel experimental set-up, studying
streamer inception and propagation, and an interdisciplinary collaboration,
we have created a solid foundation which has enabled us to investigate the
underlying mechanisms behind this dielectric phenomenon and to begin to
establish connections with the properties of the solid insulation. This is
essential for identifying which and how material properties should be
altered in order to improve their dielectric performance. We also present
novel fibre-engineering techniques that could be explored to alter the
mechanical and electrical insulating properties of the wood-fibre-based
insulation. The aim was to provide new understandings that can inspire and
suggest new strategies to the producers of paper and pressboard used in
transformers.
BACKGROUND
2
2. BACKGROUND
2.1 Paper and pressboard in oil-filled transformers
The combination of oil and paper has been used for more than a century as
electrical insulation in power transformers (Figure 2.1) and it still today
plays an important role in high-voltage transformers and cables.
The liquid insulation is used as coolant and to provide dielectric
strength.
The solid insulation is used as a structural material to create barriers
and thereby subdivide the highly stressed oil.
A good dielectric material has low dielectric losses and a high resistance
to degradation and electrical breakdown both on the surface and in the
bulk. In a transformer, the insulating solid barriers should be perpendicular
to the electrical field to achieve the maximum dielectric strength. The solid
insulation should be impregnable by oil and able to withstand a substantial
mechanical load. The pressboard and paper elements are inserted and then
dried exhaustively and impregnated in the oil-filled transformer tank. The
insulation remains in the transformer tank throughout the lifetime of up to
30-50 years where it is subjected to high mechanical and electrical stress
at elevated temperatures. A useful introduction to transformer insulation is
presented by Oommen and Prevost3,4.
Paper and pressboard are made from a 3D structured network of wood
fibres. High-density pressboard (up to 1200 kg/m3) for transformer
insulation is made by pressing paper sheets together and drying them under
heat and high pressure in a specially designed hydraulic press. To obtain
structural strength, the pressboard parts have a thickness of at least 1 mm.
Structural parts are subsequently made from several sheets of pressboard
that are glued together.
BACKGROUND
3
2.1.1 Outlook
Increasing voltage levels in electrical systems are a clear market
development for both HVAC (high voltage alternating current) and HVDC
(high voltage direct current) networks, demanding new insulating materials
able to withstand higher electrical stress and displaying less losses than
current designs. A predominant part of the research and knowledgebase
regarding dielectrics and the design of electrical equipment is, for historical
reasons, concerned with an AC-field dependence. This provides a great
challenge, as DC applications are becoming increasingly important for the
long-distance transmission needed with the renewable energy sources
located far from populated regions, and it highlights the importance of
adapted diagnostics, design and material properties of electrical insulation
for different applications.
A variety of cellulose-based products for electrical insulation are
available, with different geometries, densities and thermal properties, from
different suppliers. The wood pulps used are, according to suppliers,
characterized by a high level of cleanliness. Polyaramid fibres, known
under the trade name NOMEX, have been presented as a heat-resistant
alternative to the cellulose-based materials. However, this synthetic
material is, hitherto, far more expensive and it is therefore only used in
niche applications or to solve local hot-spot problems in the windings. The
Figure 2.1. The world’s
first 1100kV HVDC
converter transformer,
successfully tested by
ABB in 2012. It weighs
800 tons and is 32 m
long. The transformer
tank conceals iron cores,
copper windings and an
insulation system of
paper, pressboard and
oil.
BACKGROUND
4
chemical company Dupont, that developed NOMEX, has also patented an
insulating system based on NOMEX and cyanoethylated nanocellulose5.
Recent advances in giving wood fibres and materials obtained
therefrom novel functionalities may provide an opportunity to obtain new,
relatively cheap cellulose-based electrical insulation with tailored
properties. Although a modification would certainly increase the cost, the
relatively small scale of the production, the high-value application and the
opportunity to decrease the total amount of material needed favours the
possibility for investments. Improved dielectric performance of the solid
insulation could bring several competitive advantages throughout the
value-chain, for example by contributing to:
A reduction in the size, mass and hence cost of the transformer,
A reduction in energy losses and increased reliability,
Increased voltage rating.
In addition to the rapid development of new materials from cellulose,
progress within nanotechnology has also provided new possibilities to
tailor the properties of a material from a bottom-up way of design.
Components made from renewable resources sometimes have properties
that can be drawbacks in applications but one way to overcome these is by
learning to mimic and make use of nature’s brilliance in constructing
durable and functional materials.
An opposition towards new materials is naturally expected and
reasonable due to the risks associated, in a conservative business, with the
introduction of new and maybe more expensive alternatives. The
validation of a new material, which besides dielectric and mechanical
testing, includes investigations of e.g. thermal and ageing properties, also
requires considerable resources and time. Hence any new material must
show substantial improvements in dielectric properties, at a reasonable
price, to motivate investments. The cost factor is naturally of less
importance in basic research but unreasonably expensive solutions should
be avoided.
Dry type transformers, with e.g. polymeric composites, are generally
used indoors and at lower voltages than oil-filled transformers, but epoxy
BACKGROUND
5
resin, commonly filled with micro-sized silica, has for some applications
also substituted oil-paper as insulation in outdoor applications. However,
the cellulose-based insulation has advantages due to its low cost and
compatibility with oil (which is needed as coolant in high voltage
applications). Among the weaknesses of the oil-paper insulation system, in
addition to its relatively low flashpoint, is its hygroscopic behaviour, while
its resistance to the propagation of discharges and electric stress in the
cellulosic bulk is considered to be a major advantage of this system.
In dielectrics produced from synthetic polymers, nanoparticle
reinforcement has shown many promising effects on the insulating
performance6–12. Due to their intrinsic properties (e.g. bandgap or
permittivity), or to their interaction with the matrix, nanoparticles can
provide e.g. a higher breakdown strength and greater resistance to
degradation. The interaction zone, which can affect the polarizability and
function as trap sites and scattering obstacles for charge carriers, is
dependent on and emphasizes the importance of good nanoparticle
dispersion6,13–18. Recent work has also demonstrated similar improvements
(such as improved breakdown strength and mechanical strength, decreased
permittivity and a more uniform distribution of space charges) with
nanoparticles in cellulosic electrical insulation and in transformer oil19–24.
A substantial amount of research has been conducted on the
performance of alternative or improved transformer oil. Synthetic or
natural ester oils as alternatives to mineral oils are already used in some
distribution transformers where the dimensions are small. Some esters
have a greater fire resistance than mineral oil, and natural esters are also
fully or partly biodegradable25. Additives with lower excitation energies
than the base liquid have been shown in streamer tests to increase the
breakdown voltage in dielectric liquids26. The lower excitation energy of
the additives decreases the threshold propagation voltage of the streamer,
and this facilitates both propagation and branching. The large number of
streamer branches produces an electrostatic shielding of the field in front
of the streamer so that when the voltage is increased, new branches are
formed keeping the field in front of the streamer nearly unchanged. This
BACKGROUND
6
mechanism is known as ‘self-stabilization’ and is responsible for delaying
the appearance of the fast streamers responsible for breakdown27.
2.2 A short introduction to wood fibres
Wood fibres consist of three main components; cellulose, hemicellulose
and lignin. The material conventionally used in electrical grade paper is
made from softwood fibres liberated through the kraft pulping process. In
native softwood, the composition is approximately 40–45 % cellulose, 30-
35 % hemicelluloses and 25-35 % lignin28. During kraft pulping
hemicellulose and lignin are removed to optimize the mechanical
properties of the paper product29. The degree of delignification of the fibres
is most frequently quantified by the kappa number of the pulp which is
mainly a measure of the amount of residual lignin.
Cellulose is a linear polymer that consists of two β-glucose molecules
(Figure 2.2) as repeating unit. The degree of polymerization is often very
high; in naturally occurring cellulose, the number of repeating units can be
over 10 000 whereas, after isolation (depending on the method), it is
lowered to an average degree of polymerization in the range of 800 to
300028.
Figure 2.2 Cellobiose, the repeating unit of cellulose.
BACKGROUND
7
Cellulose in wood fibres is found in both crystalline and less ordered
forms. The cellulose chains are organized into square nanofibrils, with a
dimension of around 5 nm which are aggregated into fibril aggregates
which are approximately 20-30 nm wide30,31. The fibril aggregates are
embedded in an amorphous matrix of hemicelluloses and lignin and
arranged in concentric lamellae in the fibre wall, as illustrated in Figure
2.329,32. Each layer can be distinguished by the orientation of the fibrils.
The three major fibre wall layers are called the primary wall (P), the
secondary wall (S) and the middle lamella (M). The secondary wall is
divided into three sub-layers; the outer layer (S1), the middle layer (S2) and
the inner layer (S3). The middle lamella, which keeps the fibres together in
wood, is mainly composed of lignin and pectin. During chemical pulping,
the middle lamella is removed and the wood fibres liberated in the
delignification process.
Figure 2.3 Schematic representation of the hierarchical structure of wood and the
layered structure of the fibre wall with the primary wall (P), the secondary wall
(S) and the middle lamella (M). The secondary wall is divided into three sub-
layers; the outer layer (S1), the middle layer (S2) and the inner layer (S3). (Original
image by Mats Rundlöf)
S3
S2
S1
P
M
~dm ~mm
~30 µm
~20 nm
~5 nm
~1 nm
BACKGROUND
8
Hemicelluloses are combinations of different monosaccharaides. The
structure is more branched than cellulose and the chains are normally much
shorter. They have a linear backbone composed of linked glucose,
mannose, xylose or galactose units, and they often have short side chains
that may include xylose, glucose, arabinose or glucuronic acid. For
example, galactoglucomannan contains galactose, mannose and glucose
while xylan contains xylose units. Galactoglucomannans and
arabinoglycuronoxylan are the principal hemicelluloses (ca 10-15 % and
7-10 % respectively) in softwood species such as spruce and pine29.
Unlike cellulose and hemicellulose, lignin does not have a well-defined
repeating unit. Lignin has a more complex 3D network structure and it
contains both aromatic and aliphatic units. Lignin is an amorphous polymer
and it has a lower density than cellulose, due to its supramolecular
structure32.
2.3 Paper physics and chemistry
The mechanical properties of wood-fibre-based products are determined
by the strength and length of the fibres and the interaction between them,
the density (the amount of fibres interacting in a given volume) and the
formation during production32.
The removal of hemicellulose and lignin during the kraft pulping
process results in water-rich fibres that are more flexible and which can
produce a denser network during sheet preparation with a higher number
of fibre-fibre contacts and a better joined area in the fibre-fibre contact
zones which in turn increases the mechanical strength of the paper.
Chemical pulping is a compromise between obtaining a high degree of
delignification and a high cellulose yield and low cellulose degradation.
Drying of a wood pulp leads to hornification33,34, which means that the
original structure of the fibre is not completely regained after re-wetting.
Hence, the mechanical properties of a dried pulp are poorer than those of a
similar never-dried pulp. Commercial pulps are commonly mechanically
treated (beaten) to improve the conformability and thereby obtain strong,
high-density products. The beating process increases the swelling of the
BACKGROUND
9
fibres, i.e. the amount of water that can be held inside the fibre wall, the
amount of fines (small fibre fragments) and fibre fibrillation which
increases the strength of the final paper but has the drawback that it
impedes the dewatering process in the paper machine. In addition, beating
is energy consuming and, in the case of free drying, results in a higher
degree of shrinkage of the paper web during drying34–37.
Paper chemicals are often added in the paper-making process to e.g. aid
the retention or obtain desired paper properties such as strength or
printability. Fibres are negatively charged, the charge originating both
from the hemicelluloses, with a major contribution from the carboxylic
acid in xylan, and from oxidised lignin structures29. These charges are
frequently taken advantage of when modifying fibres by the physical
adsorption of charged species. Modification of the fibre wall is possible
provided that the modification chemicals can penetrate into its open porous
structure. Dry-strength additives may increase the paper strength by:
Increasing the joint strength,
Increasing the number of active joints in the network,
Consolidating the sheet by stabilizing the meniscus during drying,
Decreasing the build-up stress concentration during drying.
More than one of these may be working at the same time and, since many
additives are cationic, their effect on retention and formation should also
be considered. The fibre-fibre joint is the most interesting to address with
a dry strength additive since it is most often the weak link in the dry paper.
The nature of the contact zone affects the joint strength but ultimately it is
the intermolecular forces and interactions that act to create the strength of
the inter-fibre joints38. The relative importance of these interactions
(hydrogen bonds, ionic bonds, polar interactions, van der Waals forces and
possibly covalent bonds and molecular entanglements) on the paper
strength is not fully understood and depend on chemical additives, the fibre
furnish composition, fibre treatment and the surface tension of the water,
to mention the most important factors.
Latex, gums, chitosan, hemicelluloses and acrylamide-based polymers
are examples of possible dry-strength additives, but cationic starches are
BACKGROUND
10
the commercially most commonly used dry-strength additives. More
recently, the adsorption of polyelectrolyte multilayers has been proposed
as an alternative way of improving the mechanical properties of the paper
produced from treated fibres38.
Chemically modified cellulose, such as cellulose acetate, nitrocellulose
and carboxymethylated cellulose is widely used in the plastics, food,
textile, building and pharmaceutical industries but in these applications the
cellulose has been modified to make it molecularly soluble39.
As the traditional boreal forest industry is suffering increasingly due to
competition from fast growing trees and a decrease in newsprint and
graphic papers in general, the quest to obtain more refined and
differentiated products with new functionalities from fibres has intensified
the development of novel fibre modifications. Examples of recent activities
that have received attention are:
The use of microfibrillated cellulose (MFC) where the fibres
partially or fully have been liberated to cellulose nanofibrils
(CNFs)- to obtain transparent strong lightweight structures.
Layer-by-Layer (LbL) adsorption, of e.g. electrically conducting
and insulating consecutive layers for energy-storage applications.
The grafting of functionalities which e.g. make wood fibres
compatible with water-insoluble polymers40.
The concepts of MFC, CNFs and LbL adsorption are introduced below.
2.3.1 Cellulose micro- and nanofibrils
Turbak et al. showed in the 1980s that it was possible to liberate
microfibrils from wood fibres via high-pressure homogenization and to
obtain fibrils with a high aspect ratio and high mechanical strength41. With
pre-treatments such as TEMPO-oxidation and carboxymethylation to
introduce charges, less energy is needed to liberate the fibrils and to
achieve higher degree of liberation and a much improved dispersion
stability42. Due to its intriguing properties, this material has received a
dramatically increased interest during recent years, as a competitive
alternative to synthetic polymers, from an abundant and more sustainable
BACKGROUND
11
resource; a possible future for the forest industry as well as a less polluted
planet. CNFs have been successfully used, so far mostly in laboratories, to
produce strong, lightweight films, foams and aerogels, modified to obtain
different functionalities43–47. The large accessible surface area is
advantageous for surface modifications. In the production of paper-based
products, the use of CNFs as an additive is attractive, as it can be
introduced to give an improved or new functionality, as an alternative to
existing paper chemicals, without modifying the entire rawmaterial.
In previous work, the effect of humidity on the dielectric losses has been
compared for nanocellulose from wood fibres and algea and it was
concluded that their crystallinity and moisture sorption has an impact on
the dielectric properties48. For example, it has been suggested that the ion
mobility is higher in nanocellulose from algea (Cladophora cellulose) due
to the state of the water which is less tightly associated than in CNFs from
wood fibres.
Both CNFs and MFC contains less ordered and crystalline cellulose
parts. CNFs have widths between 5-30 nm while MFC have widths
between 10-100 nm (cellulose nanocrystals on the other hand have a higher
concentration of crystalline cellulose and have a width of 3-10 nm), as
defined by the TAPPI WI 3021 standard.
2.3.2 Layer-by-Layer (LbL) adsorption
LbL adsorption is a physical process, in contrast to chemical grafting,
where charged species (e.g. polyelectrolytes) are attracted and adsorbed in
multiple layers onto an oppositely charged substrate. From a
thermodynamic point of view, the main driving force is the entropic gain
upon the release of counter-ions associated with the adsorption49. The
build-up of single- and multilayers can be tailored by an appropriate choice
of adsorption conditions, such as pH and salt concentration. The control of
pH and ionic strength is especially important for weak polyelectrolytes,
where the charge density of the polyelectrolyte is strongly dependent on
the pH. The polyelectrolyte multilayer technique was introduced by
Decher in 199750 and has since achieved great interest for the surface
modification of solid materials. An advantage compared to many chemical
BACKGROUND
12
modifications is that it can be performed in water and hence is easily
incorporated into the already water-based, continuous, papermaking
process.
2.4 A short introduction to dielectric properties
Permittivity and dielectric loss are important dielectric properties which
affect the loss of energy and the build-up of electric fields in an insulator
and at interfaces under normal AC operating conditions. These properties
are normally measured at lower voltages (typically below line voltage),
with an altering current over a chosen frequency range.
While permittivity is an important parameter for the performance under
AC, the electric conductivity and charge accumulation are more important
for under DC conditions.
The dielectric strength, used to describe the maximum electric field a
material can withstand before breaking or short-circuiting, is measured at
higher voltages, usually applied in steps or impulses. For insulating
materials used in high voltage applications the breakdown strength is in the
MV/m range.
Before experiencing breakdown, the insulating material is often
subjected to deteriorating discharges and streamers which can also be
triggered and analysed in a laboratory. Partial discharges (PD) in dielectric
materials or on the surface of a dielectric are caused by locally concentrated
electrical stress and appear as pulses with a duration generally much less
than 1 µs51.
2.4.1 Dielectric response
A dielectric material, such as cellulose, is an electrical insulator. An ideal
dielectric material has no free charges; only bound charges that are
polarized by an applied electric field. Under the influence of an alternating
electric field, the polarization processes and charge transportation that take
place determine the dielectric response. The movement due to polarization
results in a dissipation of energy to the material as heat. The dielectric
constant or permittivity, ε, can be resolved into a real part, the storage part,
BACKGROUND
13
ε´, and an imaginary component, the loss part, ε´´. The dielectric loss
tangent, tan δ = ε´´/ ε´, is a measure of the ratio of the imaginary to the real
part of the dielectric function at a given frequency52.
The frequency dependence of the dielectric response is known as the
dielectric dispersion, shown in Figure 2.4. The origin and nature of the
polarizability depend on the frequency; the higher the frequency, the
shorter is the length scale of polarization. Maxima called “loss peaks”
occur, at characteristic frequencies, in the imaginary part (ε´´) of the
dielectric dispersion, representing different polarization mechanisms. At
high frequencies (in the UV-visible region) the only polarization that is
sufficiently fast to move with the electric field is the electronic
polarization, while at lower frequencies, larger moieties have time to orient
with the electric field. In cellulose, these movements have been shown to
be attributed to the displacement of the (-OH) and (-CH2OH) groups53.
Figure 2.4 The dielectric dispersion; the frequency dependence of ε’ and ε’’. The
origin of the polarization is written above the curves at their characteristic
frequencies52 .
BACKGROUND
14
Even though there are no free charge that can provide conduction in
insulating materials, charge transport can take place and can be studied in
the low frequency part of the dielectric response. For a low frequency
dispersion, which is characteristic of a dielectric such as kraft paper, no
loss peak is observed but both the imaginary and real parts rise steadily
towards lower frequencies54. This is a consequence of hopping charges and
diffusive charge transport. Hopping charges spend most of their time in
localized states but if the localized states form a connected network and the
charges gain energy this can give rise to DC conduction. DC conductivity
is indicated by a steep rise with decreasing frequency in the imaginary part
of the dielectric response (a slope of -1 can be seen in the ε’’ curve)54.
To estimate the permittivity of the oil-impregnated paper, it can be
handled like a composite and the combined dielectric response can be
modelled55. The electric field distribution at a liquid-solid interface will be
affected by the difference in permittivity of the materials on each side52.
Figure 2.5 illustrates the concentration and direction of the field lines at an
interface when the material 2, in this case it could be a cellulosic
pressboard, is either the high or the low permittivity material.
Figure 2.5 Diagram of the concentration and direction of the electric field lines at
an interface when material 2 is a) the high and b) the low permittivity material
a) b)
BACKGROUND
15
The concentration of the electric field can influence the charge
accumulation at the interface. In the case of paper-transformer oil, where
the charges are accumulated in the oil, the mismatch in permittivity results
in a concentration of the electric field at the interface, and the intensity of
the field increases with increasing permittivity of the solid.
2.4.2 Streamers in oil and at the oil-solid interface
Streamers, conducting gaseous channels which can travel at high speed, at
oil-pressboard interfaces have been identified as a significant cause of
transformer failure and as the failure mode with the most dangerous
consequences31.
The streamers responsible for breakdown are those with a filamentary
appearance and supersonic velocity. The streamer filament head, which has
a tip head radius of 2-3 µm, can produce a field of about 15-20 MV/cm57.
With such a field, field-assisted tunnelling and the generation of charge
carriers by impact ionization can occur. Direct ionization of mineral oil
contributes to the formation of the gaseous phase of the streamer and the
generation of hydrocarbon gases. The interested reader is recommended to
turn to the appended Papers III-V and the comprehensive and useful papers
by Lesaint58 and Denant59,60 for more information regarding the mechanism
responsible for streamer inception and propagation.
There are positive and negative streamers. Negative streamers starts at
the cathode and positive streamers at the anode. The initiation voltage for
a slow negative streamer is lower than that for a positive streamer61.
However, since positive streamers propagate faster they are considered
more harmful62.
Branching, the formation of a tree-like structure distributes the electric
field and lowers the potential at the tip of the propagating streamer.
At an interface between gas and solid, gas and liquid or solid and liquid
the streamer may be referred to as electrical creep. The interface dielectric
strength, “creep strength” is affected by the properties of both media. The
mechanism involved in streamer inception and propagation along a solid-
liquid interface is unfortunately not fully understood. As a consequence, it
is not clear which properties of the solid insulation influence the streamer.
BACKGROUND
16
For pressboard it has been shown that the streamer during the fast mode
(>1.4 km/s) moves in close contact with the surface63, as shown in Figure
2.6.
Figure 2.6 A positive streamer propagating at an oil/paper interface (photograph
kindly provided by David Ariza).
EXPERIMENTAL
17
3. EXPERIMENTAL
More information concerning materials and methods can be found in the
respective appended papers.
3.1 Materials
3.1.1 Wood pulps
The pulp used as reference throughout the project was an unbleached dried
electrical grade kraft pulp from Munksjö Aspa Bruk AB, Askersund,
Sweden, with a kappa number of ca 25, corresponding to a lignin content
of ca 3 %. This pulp is used to make paper products used as electrical
insulation and hence needs to meet high customer requirements concerning
cleanliness (evaluated e.g. by conductivity and pH of aqueous extract and
dissipation factor) and mechanical properties. Laboratory-prepared never-
dried unbleached softwood kraft pulps were supplied by SCA Research AB
in Sundsvall, Sweden, and a sulphite softwood dissolving pulp by Aditya
Birla Domsjö Fabriker AB, Domsjö, Sweden.
The pulps were washed and transferred to their sodium form as
described in Paper II prior to further use.
3.1.2 Transformer oil
The mineral oil used in this project was Nytro 10X and 10XN, from Nynäs
AB, Nynäshamn, Sweden. The oil was degassed and heated to 60 ˚C prior
to use for impregnation and measurements.
3.1.3 Polymers
Several different polymer films were investigated in streamer tests (Papers
III-V) to evaluate the influence of permittivity, porosity and surface
roughness on streamer inception and propagation. The polymer films
studied were low density polyethylene (LDPE), polyethylene terephthalate
EXPERIMENTAL
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(PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride
(PVDF), all purchased from Good Fellow Cambridge Ltd, England. More
information can be found in Table A1 in Appendix.
For the adsorption of nanoparticles (Papers VII and VIII) and cellulose
micro- and nanofibrils (Paper VI), several different polyelectrolytes were
used. A negatively charged, polyacrylic acid (PAA) was used to build LbLs
with cationic silica nanoparticles. To construct LbLs of negatively charged
particles, a positively charged polyethyleneimine (PEI) was used. For
paper sheets with cellulose micro- and nanofibrils, polyvinyl amine,
PVAm, and poly(diallyldimethyl-ammonium chloride), pDADMAC, were
used as retention aids. PAA, PEI and PVAm are weak polyelectrolytes
while pDADMAC is a strong polyelectrolyte. More details about the
different polyelectrolytes can be found in the respective papers.
3.1.4 SiO2 and ZnO nanoparticles
Two types of colloidal silica (SiO2), supplied by Eka Chemicals, Sweden,
were used:
Bindzil CAT220, is positively charged, with a primary particle size of
approximately 12 nm. The cationic charge originates from substituted
aluminium ions.
Bindzil 15/500, has a negative charge and a primary particle size of
approximately 5 nm.
Zink oxide (ZnO) nanoparticles (NanoGuard) with a particle size of 67
nm were purchased from Alfa Aesar.
3.2 Methods - sample preparation
3.2.1 Lignin and glucomannan model materials (Paper I)
Purified lignin was prepared from Indulin AT (MeadWestvaco, Richmond,
VA, USA), originating from cooking liquor, according to Norgren et. al64,
while glucomannan was purified from wood chips according to Zhang et
al65. The pure glucomannan and lignin samples, in NH4 solutions, were
EXPERIMENTAL
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spin-coated onto silicon substrates according to Gustafsson et. al66 to
obtain surfaces for characterization by spectroscopic ellipsometry.
3.2.2 Paper sheets
Paper sheets were made from pulp suspensions with a Rapid Köthen sheet
former (Paper Testing Instruments, Austria). For the sheets produced for
dielectric characterization, deionized water was used throughout the
process. The pulp used in reference papers was beaten 4000 revolutions in
PFI-mill, to achieve properties similar to those of the pulp used in
pressboard. The papers were dried under a negative pressure of 95 kPa in
the dryers of the sheet former at 93 ˚C. To study the effect of beating and
drying pressure on paper properties, the pulp was beaten for different
numbers of revolutions in the PFI-mill and the papers were dried under
different pressures, giving papers with different densities. Figure 3.1 shows
photographs of the PFI-mill and the Rapid Köthen sheet former used and
paper sheets made from unbleached kraft pulp.
Figure 3.1 Photographs of a) PFI-mill, b) Rapid Köthen sheet former and c) paper
sheets made from unbleached kraft pulp.
a) b) c)
EXPERIMENTAL
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3.2.3 Cellulose micro- and nanofibrils
Two types of cellulose fibril (shown in Figure 3.2) were used; one type of
nanofibril produced from sulphite dissolving pulp from Domsjö, after
carboxymethylation, were provided by Innventia AB (now RISE
Bioeconomy). The other type, denoted kraft microfibrils or kraft MFC, was
prepared from the electrical grade unbleached kraft pulp from Munksjö,
without chemical pre-treatment.
The kraft MFC was first prepared by beating the pulp for a total of 6000
revolutions in a PFI- mill. Both the kraft pulp and the carboxymethylated
sulphite pulp were homogenized in a Microfluidizer M-110eh
(Microfluidics Inc., USA) at a pressure of 1600 bar by three passes through
a 400 μm and a 200 μm chamber connected in series followed by three
passes through a 200 μm and a 100 μm chamber connected in series.
In Paper VI, the carboxymethylated CNFs was also used with two types
of chemical modification: With an aldehyde functionality, obtained
through periodate oxidation29,67 and with a dopamine functionality,
attached by a coupling agent (1-Ethyl-3-(3-dimethylaminopropyl)
carbodiimide (EDC)) 68,69. The functionalities as well as possible further
crosslinking can be found in the Appendix, Figure A1-A4.
The kraft MFC in this project was produced without chemical pre-
treatment: To rule out the effects of changes in chemistry on the
investigated dielectric properties and thereby enable investigation of the
effect of the different morphologies provided by the fibrils. From a product
development point of view, the MFC produced from the well-characterized
electrical grade pulp could be an attractive first approach, for the present
application. Changes in chemistry due to pre-treatments could, on the other
hand, influence important properties, such as polarizability and ageing,
when used in an insulating context.
The fibres and fibrils from the electrical grade pulp have a low content
of carboxyl groups available for modifications, so the carboxymethylated
CNFs were chosen for grafting with dopamine. With another grafting
method (not EDC coupling) and for periodate-oxidation, untreated fibrils
could be considered.
EXPERIMENTAL
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Figure. 3.2 SEM-images of a) carboxymethylated CNFs and b) kraft MFC. These
samples were prepared by evaporating the water from dilute dispersions, at room
temperature from materials deposited on silicon surfaces.
3.2.4 Papers with cellulose micro- and nanofibrils as
strength additives (Paper VI)
The fibrils were first diluted to 1g/L and dispersed by mixing in an Ultra
Turrax mixer (IKA, T25 Basic, Staufen, Germany) for 15 minutes at 10
000 rpm. Papers with cellulose fibrils were produced with and without
retention aid, as shown in Figure 3.3. For papers with retention aid, the
polyelectrolyte (PVAm or pDADMAC) was added to a fibre suspension at
pH 8 and stirred for 20 min after which excess polymer was rinsed off
through filtration. The cellulose fibrils were then added to the fibre
suspension and stirred for 30 min at pH 8, and paper sheet were made
according to above-mentioned procedure (section 3.2.2).
3.2.5 Films from cellulose micro- and nanofibrils
The CNFs and MFC suspensions were poured into the cylindrical container
of the Rapid Kothen sheet former (Figure 3.1). Water was removed through
vacuum filtration using a 0.45 μm Durapore membrane filter (Merck
Millipore Ltd.). After the filtration, another membrane was placed on top
of the wet film and the material was dried in the driers of the sheet former
at 93 °C under reduced pressure for 15 minutes.
b) a)
EXPERIMENTAL
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Figure 3.3 a) Schematic which shows the preparation procedure of sheets with
fibrils as additives and b) the additions in wt% of fibrils to the paper sheets.
3.2.6 Three-layer sandwich structure with microfibrillated
cellulose The middle layer consisting of kraft MFC film was produced in the sheet
former according to the method described above (section 3.2.5). After
dewatering for approximately 1h, a gel-like film was left which could be
peeled off from the membrane. Two paper sheets made from the reference
pulp were also produced in the sheet former and the fibril film was inserted
between the two sheets, all three layers being still wet. The layers were
couched together and dried for 30 minutes at a temperature of 93 °C and a
reduced pressure of 95 kPa in the driers of the sheet former.
3.2.7 Papers from fibres modified with SiO2 and ZnO
nanoparticles (Papers VII-VIII)
Modified paper sheets with SiO2 nanoparticles were produced, after
consecutive treatments of the electrical grade fibre with nanoparticles and
EXPERIMENTAL
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polyelectrolytes. The amounts of polyelectrolyte and nanoparticle
adsorbed were regulated by changing the charge density of the adsorbed
species by adjusting the pH with hydrochloric acid and sodium hydroxide.
Papers with ZnO nanoparticles were produced by adsorption of the
nanoparticles at pH 6.5. Prior to adsorption onto the wood fibres, the ZnO
nanoparticles had been surface modified, by silanization with (3-
aminopropyl) triethoxy silane (APTES, 98 %) according to a previously
described procedure70. The final nanoparticle concentration was
determined by thermogravimetric analysis (TGA).
3.3 Methods – sample characterization
This section presents the methods used for the determination of the charge
density and the mechanical and dielectric properties of the paper samples.
Methods used for characterization of morphology, yield, etc. can be found
in the respective papers.
3.3.1 Charge density measurements
The principle of polyelectrolyte titration (PET) used for surface charge
measurements on wood fibres has been described by Wågberg et al71. In
brief, a strong cationic polyelectrolyte with a known charge density and a
high molecular weight (to prevent penetration into the fibre-wall and
access to interior charges) was adsorbed onto the fibres. The excess of
cationic polymer was filtered off and the amount adsorbed was thereafter
determined by titration of the filtrate with an anionic polymer in the
presence of an indicator, such as Orthotoluidine Blue (OTB) which turns
from blue to pink with a charge shift from positive to negative. The
equilibrium concentration and the adsorption characteristics of the
polyelectrolytes onto the wood fibres were studied by adsorption
isotherms, obtained in similar manner71. Due to the application, where free
charge carriers are undesired, the influence of ionic strength was never
studied.
EXPERIMENTAL
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The total charge density of fibres was determined by conductometric
titration according to SCAN-CM 65:02 using a Metrohm 702SM Titrino
titrator.
The surface charge density and stability of the micro- and nanofibril
and nanoparticle dispersions were investigated with PET by streaming
potential measurements using a Stabino particle charge mapping
equipment (ParticleMetrix, Germany).
3.3.2 Thickness measurements
The thickness used to calculate the density of paper sheets was determined
in two different ways. For the samples used in all the studies which
included dielectric measurements, the thickness of a stack of 3 test pieces
was measured with a disc micrometer (Absolute Digimatic Quick 227,
Mitutoyo). This procedure (derived from ISO 534:2005) was used due to
the configuration of the dielectric response cell, where three samples were
stacked in a plate-plate configuration and the distance between the plates
is important for consecutive calculations of the real and imaginary parts of
the permittivity.
For the study with fibrils as additives (Paper VI), the thickness of the
samples was determined according to SCAN-P 88:01, which is more
commonly used for paper products.
3.3.3 Mechanical testing
Tensile testing of paper sheets and CNF films was carried out at 23 ˚C and
50 % RH using an Instron 5944 equipped with a 500 N load cell. For paper
sheets, strips with a width of 15 mm and a free span of 100 mm between
the clamps were strained at a constant rate of 100 mm/min.
For wet tensile strength, the sheets were soaked in water for 1 h prior to
tensile testing according to SCAN-P 20:95. The excess water was removed
before the mechanical testing.
3.3.4 Spectroscopic ellipsometry (Paper I)
Measurements in the ultraviolet-visible-near IR (UV-VIS-NIR) region
245–1690 nm were made, at 23 °C and 48 % RH, using a variable-angle
EXPERIMENTAL
25
ellipsometer with dual rotating compensators (RC2, J. A. Woollam Co.
Inc.). From the measurements, the wavelength-dependence of the
refractive index of films of lignin and glucomannan deposited on silicon
substrates was estimated.
The complex refractive index Ñ is a combination of the refractive index,
n, and the extinction coefficient k as given by:
��(𝜔) = 𝑛(𝜔) ± i𝑘(𝜔) (Equation 1)
The extinction coefficient k is a measure of the absorption which has a
maximum at the absorption wavelength derived from electronic or inter-
band transitions in the material corresponding to an excitation of an
electron from an occupied orbital to an unoccupied or partially unoccupied
orbital. When k = 0 the value of the real part of the permittivity ε’ can be
calculated according to:
𝜀′(𝜔) = 𝑛2(𝜔) (Equation 2)
3.3.5 Dielectric spectroscopy (Papers II, VII and VIII)
Dielectric spectroscopy measurements were performed on paper sheets in
vacuum and in transformer oil to obtain the complex permittivity.
The diameters of the ground electrodes were 50 mm and 90 mm for the
cells used in vacuum and in oil respectively. Diagrams of the test cells are
shown in Figure 3.4. To reduce edge effects, a guard ring separated by a
1mm gap from the ground electrode was included in the test cells. Three
paper discs with thicknesses of 100 to 150 µm each were placed between
the two electrodes. The cell was then inserted into a vacuum oven for
drying for 24h at 110 ˚C, and consecutively (before any measurement in
oil) impregnated with oil at 60 ˚C followed by a measurement at 70 ˚C.
The frequency sweep was performed from 1 mHz to 1 kHz, at 200V in
vacuum and 3V in oil, and the capacitance was measured. Impregnation
with oil was done in a vacuum oven without releasing the vacuum after the
paper samples had been dried to ensure continued dryness. The dielectric
losses and permittivity were monitored during drying and impregnation to
ensure drying and stable measurement conditions. Prior to the first
EXPERIMENTAL
26
measurements in vacuum the frequency sweep was performed at lower
voltages (30 V and 50 V), without any sign of non-linearity.
Figure 3.4 Diagrams of the test cells for dielectric spectroscopy measurements in
a) vacuum and b) oil.
The permittivity was calculated from the capacitance, C, using:
𝜀 = 𝐶𝜀0𝑑
𝐴 (Equation 3)
where ԑ0 is the vacuum permittivity, d is the distance between the electrodes
and A is their area.
3.3.6 DC breakdown test
The DC breakdown test was performed on a reference paper and on the
sandwich structure with microfibrillated cellulose. The samples were
vacuum impregnated with transformer oil for one week prior to breakdown
testing. Nine specimens from each material were tested, all with a thickness
of ca 150 µm. Tests were performed in oil using a ramp rate of 0.5 kV/s.
The electrode set-up is shown in Figure 3.5.
a)
b)
Sample
Sample
EXPERIMENTAL
27
Figure 3.5 Sample mounted between electrodes for the DC breakdown tests.
3.3.7 Streamer measurements (Papers III-V)
The solid samples, papers and films, (8 mm x 100 mm) were preheated at
105°C (LDPE at 70°) for 24 h, after which they were impregnated with the
filtered and degassed transformer oil at 60°C and 5 mbar for 24 h. Prior to
testing, the oil was degassed and filtered in the test chamber by pumping it
through a filter with a pore size of 2 µm. After the filtration and degassing
process, the chamber was cooled to room temperature and filled with dry
air. All tests were performed at atmospheric pressure.
The experimental set-up for studying the streamer inception and
propagation in a needle-plane configuration is shown in Figure 3.6. The test
chamber was half-filled with transformer oil and the solid sample was
inserted at an angle of 30° with respect to the plane.
A series of square high voltage pulses with a rise time of 35 ns and a
width of 50 µs was applied to the plane electrode with voltage levels
ranging between 11.5 and 22 kV and steps of about 0.6 kV. A single frame
shadowgraph of each streamer was obtained.
3.3.8 Partial discharge (PD) measurement (Paper VIII)
PD measurements were performed on three layers of oil-impregnated
papers. A diagram of the set-up with a high potential needle electrode and
a grounded bar on the surface of the insulation used is shown in Figure 3.7.
To investigate the effect of impulses on partial discharges a 50 Hz AC
source and a standard lightning impulse generator system were combined
to provide an impulse voltage superimposed on the AC voltage.
EXPERIMENTAL
28
Figure 3.6 Detailed diagram of the experimental set-up used for the study of
streamer propagation in oil.
Figure 3.7 Diagram of the set-up used for partial discharge measurements.
(1) Test chamber (11) Pump
(2) Point and probe electrode (12) Particle filter
(3) Plane electrode (13) Valve (4) Charge measuring system (14) Heating rod
(5) High-speed camera (15) High voltage pulse source (6) Xenon lamp (16) Oscilloscope (7) Xenon lamp power source (17) Signal generator
(8) Optical fibre (18) Data acquisition device (9) Photomultiplier (19) Computer
(10) Valve (20) Vacuum pump
RESULTS AND DISCUSSION
29
4. RESULTS AND DISCUSSION
When wood fibres and papers are modified it is almost impossible to
influence one property without affecting others simultaneously. An
example is the beating of wood pulp, which has been referred to earlier,
which is used to improve mechanical properties of paper products. Beating
leads to increased flexibility of the fibres, fibrillation of the fibre surfaces,
the creation of fines and to an increase in the ability the of fibre wall to
swell in water. In paper, this results in a greater contact area between fibres
partly due to densification. Hence, if the dielectric properties of a paper are
changed when the pulp in beaten, the question that arises is whether this is
an effect solely of densification or fibrillation, or both. The same question
is valid for other fibre treatments. To be able to distinguish the effects of
different additives or chemical or physical treatments of the fibres, and the
mechanisms involved, it is hence desirable to separately investigate the
effect of e.g. densification, beating and pulp composition on permittivity
and other dielectric phenomena. This separation was achieved by
investigating model materials, different paper grades and films of synthetic
polymers and microfibrillated cellulose. The first results are presented and
discussed in this context.
In the second part, the possibility of using novel modification routes for
wood fibres to improve the dielectric and mechanical properties of
cellulose-based electrical insulation is demonstrated and discussed.
RESULTS AND DISCUSSION
30
4.1 The influence of paper density and morphology
on the dielectric response (Paper II)
Dielectric spectroscopy in vacuum was carried out on papers made from
pulps beaten to different degrees or pressed at different pressures to
evaluate the influence of density and mechanical refinement on the
dielectric response. This means that the changes in permittivity and
dielectric losses with changes in the paper/oil volume fraction of the
impregnated paper could be studied. The permittivity can be estimated (as
shown in Figure 4.1 for 50 Hz, which is the standard “power frequency”
found in European grids) from the Wiener equation (Equation 4) if the
volume fraction solid/oil or solid/vacuum and the permittivity of the pure
media are known72,73.
1𝜀𝑜𝑖𝑙𝑖𝑚𝑝𝑟𝑒𝑔𝑛𝑎𝑡𝑒𝑑 𝑝𝑎𝑝𝑒𝑟
⁄ =𝑉𝑝𝑎𝑝𝑒𝑟
𝜀𝑝𝑎𝑝𝑒𝑟⁄ +
𝑉𝑜𝑖𝑙𝜀𝑜𝑖𝑙
⁄ (Equation 4)
Beating the pulp and thereby changing the fibre and paper morphology
did not seem to affect the dielectric properties. Although empirical data
should be collected to assess the permittivity and dielectric losses of a
given paper grade in a given medium, the relations give valuable input
regarding the influence of densification on the dielectric properties of
paper/oil insulation. This also suggests that is relatively easy to tailor the
permittivity of paper and hence the electric field distribution at the oil-solid
interface by producing paper parts of different densities. The densities
could be altered e.g. by changing the raw material, the degree of
delignification, the mechanical refinement or the pressure during the
drying step. The implications of such changes on the manufacturing
process, on the overall dielectric properties and on the mechanical
properties must however be compiled and addressed.
RESULTS AND DISCUSSION
31
Figure 4.1 The real part of the permittivity, ԑ’, (dashed lines are from calculations
using the Wiener equation) and tan delta at 50 Hz: In a) and b) of papers with
different densities and in c) and d) of papers with different chemical compositions.
The kappa numbers (correlated to the lignin content) are listed for the SCA pulps.
SCA 26 has a composition similar to that of the electrical grade pulp while SCA
75 and SCA 107 have higher contents of hemicellulose and lignin.
4.2 The influence of wood fibre composition on the
dielectric properties
The effect of fibre composition on the dielectric response was addressed
by investigating the properties and effects of the most common
biopolymers (cellulose, hemicellulose and lignin).
RESULTS AND DISCUSSION
32
200 400 600 800 1000 1200 1400 16001.0
1.2
1.4
1.6
1.8
Lignin
Glucomannan
Wavelength (nm)
Refr
acti
ve
ind
ex,
n
0.05
0.10
0.15
0.20a
Exti
ncti
on
coeff
icie
nt,
k
200 400 600 800 1000 1200 1400 16001.0
1.2
1.4
1.6
1.8
Lignin
Glucomannan
Wavelength (nm)
Refr
acti
ve
ind
ex,
n
0.05
0.10
0.15
0.20a
Ex
tin
cti
on
coeff
icie
nt,
k
4.2.1 Spectroscopic ellipsometry measurements of wood
biopolymers (Paper I)
The wavelength dependence of the refractive index (Figure 4.2) of films of
lignin and glucomannan deposited on silicon substrates was estimated and
compared to previous measurements on cellulose74. The results indicate
that lignin has the highest electronic polarizability of the three
biopolymers, cellulose, hemicellulose and lignin, considering the
relationship, n2 = ԑ, between the refractive index, n, and the permittivity,
ԑ, (where k ≈ 0 in the investigated wavelength region). At power
frequencies, movements of larger moieties contribute to the dielectric
losses, but the electronic polarizability together with the movement of
dipoles influences the dielectric losses and the electric field distribution
during fast events and transients (in the µs and ns range). The results were
expected from the polarizabilities listed for the embodied bonds of the
respective polymers75, but this is one of the few studies where all three
wood components have been evaluated at the same time.
In addition, the measurements of the spectral parameters and the
mathematical approximations presented in Paper I, was used for
calculations of non-retarded Hamaker constants and also showed that
lignin has the lowest energy for the main electronic transitions compared
to glucomannan and cellulose.
Figure 4.2 The refractive index, n, and the extinction coefficient, k, of model films
of glucomannan and lignin calculated from spectroscopic ellipsometry measurements.
RESULTS AND DISCUSSION
33
4.2.2 Dielectric response (Papers II and VIII) Compared to a paper, made from the electrical grade pulp, with a lignin
content of approximately 3 % papers made from pulps with a higher lignin
content (10-15 %) have a similar permittivity but their dielectric loss at 50
Hz is slightly higher both in vacuum and in oil (Figure 4.1), and the
permittivity of the paper made from pulps with a higher lignin content
increases more rapidly at low frequencies, indicating a greater charge
transport in these materials. In oil, where the overall charge transport is
greater than that in vacuum, these trends are even clearer.
The papers differ in composition (relative contents of cellulose, lignin
and hemicellulose) but, as a consequence of the pulping process, they also
differ in e.g. density, morphology, charge density and interaction with the
transformer oil. The results presented in section 4.1 and Paper II indicate,
however, that the paper morphology is insignificant and that the density
differences, do not contribute to any differences in the magnitude of the
charge transport.
The influence of the total charge density (or the charge of the associated
counter-ions) and the wetting of the papers by transformer oil are
parameters that, besides the polarizability and the interaction of the wood
fibre components with mobile charge carriers, could be of importance and
interesting to investigate further.
4.2.3 Surface partial discharge (PD) resistance (Paper VIII)
Surface PD measurements on a paper with a high lignin content (14 wt%)
resulted in a high PD inception voltage (28kV compared to 23kV for the
reference) and the PD pattern was not affected by impulses. Possible
explanations for this could be a low density and hence low permittivity or
a lower excitation energy due to the conjugated bonds in lignin76. Additives
with low excitation energy have previously been shown to inhibit
discharges in oil26.
RESULTS AND DISCUSSION
34
4.3 The influence of permittivity, density and
morphology of the solid on streamer inception and
propagation (Papers III-VI)
Previous sections have summarized the results concerning the effect of
density and fibre composition on the dielectric response of kraft paper. In
order to separate the effects on streamer inception and propagation of
density and morphology of the solid from that of permittivity (which is also
dependent on density), polymeric films with different permittivities and a
kraft MFC film were investigated with regard to streamer measurements
and the results were compared to those of the kraft paper.
The results obtained with streamers in negative polarity on the kraft
paper show that high surface roughness and porosity are associated with a
low velocity and short propagation of the streamer. The porous paper
morphology provides an oil-rich composite and could possibly promote
branching into the paper surface. The similarity in the behaviour to that of
the oil is supported by a well-defined charge step in the early propagation
of the streamer, similar to a case without solid. A low velocity may imply
that the streamer during e.g. an impulse would be less likely to cause short-
circuiting and failure.
From shadowgraphs obtained in positive polarity, it is possible to
conclude that the paper in this case also enhances branching into the oil
while the propagation is more concentrated to the surfaces of the smoother
polymeric films and of the film of kraft MFC (Figure 4.3). Even though
the effective voltage gradient is high, the branching of a streamer may lead
to a lower electrical stress in the tip of each branch, and this could limit the
deterioration and evaporation of the insulation.
The probability of inception is plotted against applied voltage for the
investigated materials in negative polarity in Figure 4.4. The inception
voltage for mineral oil without an interface is about 13 kV and the
inception voltage for the kraft paper, kraft MFC film, PET and PVDF
interfaces is almost the same, despite differences in their relative
permittivities. On the other hand, a significantly larger inception voltage
was found in the case of PTFE and LDPE (ca 15.5 kV and ca 14.7 kV
RESULTS AND DISCUSSION
35
respectively). The results reported for PTFE and LDPE also show that
these solids, with permittivities similar to that of the mineral oil, can lower
the stopping voltage condition in negative polarity.
From charge and light intensity recordings, it was found that the oil-
solid interfaces with PVDF, PET and kraft fibril paper, with high
permittivity and low surface roughness, induce a quasi-continuous
injection of charge during the early stage of the streamer propagation. This
quasi-continuous injection of charge contributed to an increase in average
length and velocity of the streamer on these materials. Furthermore, it was
shown that the average streamer charge increases with increasing
permittivity of the interface.
A possible explanation for the higher inception and stopping voltages
with PTFE and LDPE could thus be that solids with a lower permittivity
than e.g. impregnated kraft paper may reduce the electric field strength at
the interface, decreasing the electron emission and accumulation of mobile
charges on the solid surface. The similarity between the inception voltage
conditions of the PVDF and PET films, the kraft fibril and kraft papers and
the case without a solid interface indicates that the conditions in these cases
depend mainly on processes taking place in the liquid phase, including
ionization of the molecules in oil.
A hypothesis put forward before the measurements were made, was that
the high electron affinity of fluorine (present in PTFE and PVDF) could be
beneficial for limiting the propagation of streamers, but although the
inception voltage was higher for PTFE, the inception voltage for PVDF
and the propagation on both PTFE and PVDF were similar to that of the
other polymers in negative polarity. A most surprising observation in our
measurements in positive polarity (unpublished results) was, however, that
the streamer was quenched only in the presence of PVDF. This could not
be explained by any of the mechanisms discussed but we expect that more
observations and data collection will bring clarity.
RESULTS AND DISCUSSION
36
Figure 4.3 Images of the propagating streamer on surfaces of a) an oil-
impregnated kraft paper where the streamer branches into the oil, and b) a paper
made of kraft fibrils where it is more concentrated to the surface.
Figure 4.4 The probability of streamer inception plotted against the applied
voltage measured in negative polarity in oil.
RESULTS AND DISCUSSION
37
4.4 Papers with fibres and fibrils combined
The significance of paper density for the permittivity and streamer
properties as well as the potential to achieve a significant improvement in
mechanical strength at a lower density directed our efforts towards
producing papers containing a combination of pulp fibres and cellulosic
micro- and nanofibrils.
4.4.1 DC breakdown strength of a three-layered sandwich
structure with microfibrillated cellulose
The properties of a sandwich structure and of the reference paper are given
in Table 1. The sandwich structure is a way to tailor the paper to obtain a
porous relatively low-density surface with low permittivity and a high-
density interior with higher permittivity. For the complete three-layered
structure, the density increased by approximately 20 % while the fibril
mass was 33 % of the total mass. If it was produced as a free-standing film,
the dry kraft MFC film would have had a thickness of approximately 30
µm (ca 20 % of the total thickness). The permittivity of an oil-impregnated
kraft MFC film was approximately 4.577.
A laminate with an internal kraft MFC film had DC breakdown strength
47 % higher than that of a reference of similar thickness produced only
from pulp fibres. An increasing breakdown strength with increasing
density was expected, but not to the extent found. In addition, the scatter
in the DC breakdown data (Figure 4.5) was significantly smaller for the
sandwich structure, as shown by the higher shape factor, 20 versus 9 in the
reference paper. The microfibrils as well as the outer layers of the sandwich
were produced from the fibres of the electrical grade pulp. Hence, although
the total density increased by 20 %, the chemical composition of the
fibrous material was basically the same.
RESULTS AND DISCUSSION
38
Table 1. Data and properties and of the “sandwich” and a reference sample (kraft
paper).
Figure 4.5 Weibull plot of the DC breakdown strength.
Sample Reference Sandwich
Diagram
fibres fibres
fibrils
fibres
Thickness (µm) 148 ± 8 153 ± 4
Density (g/cm3) 0.82 0.97
Breakdown strength (kV/mm) 120 ± 15 176 ± 11
• Reference
Shape-factor
9.278
▪ Sandwich
Shape-factor
20.36
RESULTS AND DISCUSSION
39
4.4.2 Micro- and nanofibrils as paper strength additives (Paper
VI)
The aim of the work described in Paper VI was firstly to improve the
mechanical properties of papers made from the electrical grade pulp. The
mechanical properties are important for the transformer application and if
the tensile index were significantly increased it could be possible to the use
a product with a lower density or to reduce the size and thus the cost of the
component. In addition, the above-mentioned DC breakdown
measurements promisingly showed improvements when a layer of
microfibrillated cellulose was introduced and a recent publication reported
an increase in AC and DC breakdown strength by up to ca 20 %, with
micro- and nanofibrillated cellulose used as an additive (with 10 wt%
addition)14.
Previously reported data have shown that the addition of micro- and
nanofibrils led to in an increase in density and that the improvements in the
mechanical properties of the paper sheets corresponded to that obtained by
beating the pulp to a similar density78. Our results have shown that it is
possible with chemically modified CNFs as strength additives to produce
stronger sheets without increasing the density.
The tensile strength index of papers containing fibrils is shown in
Figure 4.6. The addition of carboxymethylated CNFs resulted in a low
retention and a small effect on the tensile properties, dewatering rate and
morphology. The strengthening effect achieved with the kraft MFC
appeared to be solely due to the densification of the paper, in contrast to
the effect achieved with the periodate-oxidised and dopamine-grafted
CNFs, where the papers showed most significant improvements. The
greatest strength-enhancing effect was obtained with the periodate-
oxidised CNFs with retention aid. The aldehyde groups on the periodate-
oxidised CNFs are believed to crosslink (see Appendix) with the hydroxyl
groups on the fibres, thereby strengthening the fibre-fibre joints79. The
highest dry strength, 125 kNm/kg, was achieved with pDADMAC and ca
14 wt% of periodate-oxidised CNFs in the paper sheet, an increase in
strength of 89 % compared to the reference containing only pDADMAC.
With PVAm, the tensile strength index increased by 56 % with less than 1
RESULTS AND DISCUSSION
40
wt% CNFs in the paper sheet. It is suggested that the aldehyde groups of
the periodate-oxidised CNFs also form imine bonds with the amines of the
PVAm80,81, as indicated by the wet strength, which was above 10 kNm/kg
with less than 1 wt% CNFs in the paper sheets and almost 30 kNm/kg with
ca 14 wt%. This corresponds to approximately 10 % and 30 % of the dry
strength, respectively. Dopamine-grafted CNFs gave a high retention at
low additions even without a retention aid and gave a significant increase
in tensile strength and a decrease in dewatering rate, which is suggested to
be due to their high propensity to form films (Figure 4.7).
Figure 4.6 Tensile strength index versus sheet density of sheets with MFC and
CNFs; a) with and b) without retention aid (PVAm or, if indicated, pDADMAC).
RESULTS AND DISCUSSION
41
Figure 4.7 SEM-images of papers a) without fibrils and after the addition of b) 2
wt% and c) 15 wt% of dopamine-modified CNFs. The film formation was most
evident with the dopamine-modified CNFs but could be seen in all the samples.
a)
b)
)
c)
RESULTS AND DISCUSSION
42
4.5 Papers from wood fibres with adsorbed SiO2
and ZnO nanoparticles (Papers VII and VIII)
The modification of wood fibres with ZnO and SiO2 nanoparticles was
investigated due to previously reported improvements in the dielectric
properties of electrical insulation materials with these
nanoparticles6,8,9,82,83. In addition, physical adsorption and the Layer-by-
Layer (LbL) technique were evaluated as possible routes to modify wood
fibres for their intended use in electrical insulation materials.
4.5.1 Adsorption
Papers containing ZnO and SiO2 nanoparticles were obtained by physical
adsorption of charged nanoparticles onto wood fibres in a wet pulp
suspension before the papermaking step.
Multilayers were also successfully constructed by alternating the
adsorption of SiO2 nanoparticles and polyelectrolytes. The charge density
of the nanoparticles is pH-dependent. Both the polyelectrolytes used, PEI
and PAA, are weak polyelectrolytes with charge densities and hence
adsorption characteristics dependent on the pH (Figure 4.8). With physical
adsorption and the LbL method, papers with SiO2 contents up to 15.3 wt%
were produced. The hypothesis was that the 5 and 12 nm diameter particles
would be small enough to penetrate the fibre-wall, but this hypothesis was
rejected when SEM-images of fibre cross sections revealed that the SiO2
particles were only present on the fibre surface (Figure 4.9).
The stability of ZnO nanoparticles in water, as shown in Figure 4.10, is
pH-sensitive, and their charge density and dissolution were therefore
carefully investigated to determine the pH suitable for adsorption. The
charge densities of APTES-modified ZnO nanoparticles are more than one
order of magnitude lower than that of the cationic SiO2 nanoparticles.
APTES-modified ZnO nanoparticles were adsorbed onto wood fibres
at pH 6.5 after which paper samples with approximately 1 wt% of
nanoparticles were produced. SEM-images showed that the ZnO
nanoparticles were distributed on the wood fibre surface (Figure 4.11 a-c).
An attempt was made to covalently bond APTES-modified ZnO particles
RESULTS AND DISCUSSION
43
using EDC (1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide) as coupling
agent, but this led to the formation of rod-like structures of ZnO and these
papers were not further studied (Figure 4.11 d).
For the intended application, possible desorption and migration into the
oil and its implications should definitely be investigated, taking into
consideration the ageing conditions and the long operating times (several
decades) of a transformer.
Figure 4.8 The content of SiO2 nanoparticles in paper samples plotted a) versus
pH for a single layer of cationic (catSiO2) particles and b) and as a function of the
number of layers deposited with the aid of different polyelectrolytes at different
pH’s, measured by TGA.
1 2 3
2
4
6
8
10
12
14
16
5.9
8.2
15.3
4.1
4.9
6.1
2.41.8
10.4
6.01
10.210.8
b anSiO2, PEI pH 7
anSiO2, PEI pH 10
catSiO2, PAA pH 3.5
catSiO2, PAA pH 6
SiO
2 n
an
op
art
icle
co
nte
nt
(wt%
)
Number of SiO2 nanoparticle layers
3 4 5 6 7 8
2
4
6
8
2.4
3.3
5.5
6.01
6.6
SiO
2 n
an
op
art
icle
co
nte
nt
(wt%
)
pH
catSiO2 (1 layer)
a
RESULTS AND DISCUSSION
44
Figure 4.9 Cross section of a wood fibre with cationic SiO2 nanoparticles (6 % of
the total paper weight) covering the surface.
Figure 4.10 Charge density of APTES-modified ZnO nanoparticles in deionized
water with increasing pH. The particles dissolved below pH 6 and have their
isoelectric point around pH 8.
-6
-4
-2
0
2
4
6
2 4 6 8 10 12
Ch
arge
den
sity
(µ
eq/g
)
pH
Particles are
agglomerated or
negatively charged
Particles are
dissolving
RESULTS AND DISCUSSION
45
Figure 4.11 SEM-images of a) and b) papers containing 1 wt% APTES-modified
ZnO nanoparticles showing an even distribution of small agglomerates, supported
by c) the detection of backscattered electrons, and d) of a paper with rod-like ZnO
structures after coupling with EDC was attempted.
4.5.2 Dielectric response
Generally, SiO2 nanoparticles lowered the permittivity of the papers, even
after taking into account any change in density. The permittivity of SiO2 is
lower than that of pure kraft pulp but higher than that of the paper-oil
combination. ZnO has a higher permittivity (ca 8) then the kraft paper and
since the density of paper samples with ZnO nanoparticles is similar to the
reference sample, neither of these parameters explains the low permittivity
and loss that was measured for paper sheets with ZnO nanoparticles. A low
mobility in interfacial areas could have decreased the overall polarizability.
The interfacial polarization present in conventionally filled materials has
been demonstrated to be suppressed or even eliminated if the dimensions
of the particles are reduced and the local electric fields are mitigated 15,84.
a) b)
c) d)
RESULTS AND DISCUSSION
46
The presence of traps or scattering sites introduced by the incorporation of
interfacial areas may also affect the charge transport mechanism (hopping
charges) by decreasing the mobility of charge carriers, and this would
explain the low loss and the flat slope of the permittivity at low frequencies
for the nanoparticle-filled papers85,86. For nanocomposites with synthetic
polymers, some investigations have shown that the greatest improvements
in the dielectric and mechanical properties are achieved if the inorganic
nanoparticles are surface modified in order to enhance the interactions or
even create covalent bonds between filler and matrix86. In some cases, the
functional groups grafted to the surfaces are also proposed to function as
traps or scattering sites6,13. With wood fibres, on the other hand, the filler-
matrix interactions are already favoured by the hydrophilic nature of the
cellulose, and with charges on wood fibres and nanoparticles, the
interactions are further enhance by electrostatics. The retained mechanical
performance measured for papers containing SiO2 nanoparticle (Paper
VII), indicates that the incorporation of SiO2 nanoparticles at low
concentrations does not weaken the interactions between fibres. An even
distribution, a moderate nanoparticle concentration and a small particles
size were probably beneficial for both the fibre-fibre interaction and for the
formation of the paper sheets. In addition, a large interfacial area was
created without contact between particles, which has that also previously
been shown to be a criterion for improved dielectric and mechanical
properties11.
In some of the modified papers, remaining charge carriers are believed
to have affected the permittivity and dielectric losses at low frequencies,
especially in oil.
4.5.3 Surface partial discharge (PD) resistance (Paper VIII)
Papers with 2 wt% SiO2 and 6 wt% SiO2 nanoparticles had a greater the
impulse resistance of AC PD than both the reference paper and a paper
with ZnO nanoparticles. The paper with ZnO nanoparticles showed a lower
discharge activity in respect to the amount and magnitude of charges then
the reference only before being exposed to an impulse. The lower
permittivty of the nanoparticle-filled papers could improve the surface
RESULTS AND DISCUSSION
47
discharge resistance by decreasing the mismatch with the oil permittivty.
In addition, it has been suggested that the incorporation of the interfacial
zone also works to increase the breakdown voltage and resistance towards
discharges6,11,15, by mechanisms that have been discussed in previous
section.
CONCLUSIONS
48
5. CONCLUSIONS
The development of new electrical insulating materials can be achieved by
both conventional and novel modifications of wood-based fibres. The
physical processes that occurs in the insulation system of a transformer
with high applied operating voltages, impulses, elevated temperatures and
high mechanical loads need to be understood and addressed if new
materials are to be properly evaluated.
All results are meant to contribute with some pieces to an increased
understanding of the phenomena involved and of how to achieve improved
mechanical and dielectric performance of cellulose-based insulation
materials. The following general conclusions can be drawn:
The paper morphology contributes to the branching of streamers.
This is supported by the shorter stopping length for measurements in
negative polarity and shadowgraphs obtained for measurements of
streamer propagation in positive polarity. Branching and shorter stopping
lengths may result in less deterioration of the material and a lower
probability of failure of the power transformers.
On a low permittivity polymeric film, streamers had a higher inception
voltage and induced less charges than on the materials with higher
permittivities.
Low density polyethylene (LDPE) and polytetrafluoroethylene (PTFE)
have permittivities close to that of oil and in negative polarity, higher
streamer inception voltages were measured with these materials (14.7 kV
with LDPE in oil and 15.5 kV with PTFE in oil, compared to ca 13 kV in
oil and with kraft paper in oil).
The permittivity of cellulose is an inherent material property but the
permittivity of an oil-impregnated paper can be affected by the pulp
composition and is highly dependent on the oil:paper ratio. Even though
this makes it possible to alter the permittivity of an oil-impregnated paper,
CONCLUSIONS
49
it is not evident that it is the combined oil-paper permittivity that is
important.
The construction of a low permittivity surface using e.g. a low-density
cellulosic material or a coating could be beneficial for the dielectric
performance and from a manufacturing viewpoint. To explore this, it
would be interesting to define how thick such a surface layer should be; if
and when the relative influence on streamer properties of the bulk
permittivity can be superseded by that of the surface permittivity.
Fibrils can be used to increase both mechanical strength and DC
breakdown strength.
It was showed that the chemically modified fibrils significantly increased
the tensile strength index of a paper produced from electrical grade kraft
pulp. Microfibrils from electrical grade pulp (kraft MFC) were shown to
increase the DC breakdown strength when incorporated as a film between
two regular papers. Higher mechanical strength and higher breakdown
strength could enable down-sizing of the insulating components. However,
on a kraft MFC film, a propagating streamer was concentrated to the
surface and its velocity and length were greater than those of a streamer
propagating on a normal kraft paper. The results suggest that to improve
the overall dielectric performance it is necessary to distinguish between
material properties optimal for high resistance towards dielectric
phenomena at the surface and in the bulk.
Nanoparticles can decrease the permittivity and the surface discharges
under an applied AC voltage.
SiO2 and ZnO nanoparticles in papers were shown to decrease the
permittivity and also to decrease the surface discharge. A possible
mechanism suggested to be responsible for this is decreased mobility and
trapping and scattering of charges in the interfacial region. Physical
adsorption and the Layer-by-Layer (LbL) technique were used to obtain
layers with evenly distributed nanoparticles. However, the washing to
remove excess ions appeared to be more difficult than with an unmodified
pulp.
CONCLUSIONS
50
It has been showed that fibrils and nanoparticles can be used to upgrade
paper products used as electrical insulation. Hopefully, the investigation
into the mechanisms of streamer inception and propagation and their
relation to material properties can also be useful for future studies.
FUTURE WORK
51
6. FUTURE WORK
The aim of this project was to study cellulose-based electrical insulation
materials and how they can be modified to improve resistance towards
streamer inception and propagation. The specially designed and
constructed experimental set-up for studying streamers was primarily used
to investigate the relevant mechanisms and how they are related to material
properties. Due to the time-consuming nature of the measurements, only a
few cellulosic materials have so far been evaluated. For future work, more
cellulosic samples have been and could be produced to further increase our
understanding of how to control streamer inception and propagation. Paper
samples of different compositions and different surface roughnesses and
densities will be evaluated along with the papers made from fibres with
adsorbed nanoparticles.
It is also suggested that further investigations should be made into how
different cellulosic micro- and nanofibrils can be used as additives or in
laminates to improve dielectric properties. In this context, different
chemistries as well as different fibril contents should be compared, and this
should also provide valuable input to the relative influence of surface and
bulk properties on streamer inception and propagation.
For modified materials, the results should add a valuable contribution
to the fundamental understanding of how to improve the insulating
performance, and of the dielectric phenomena. Modified materials should
also be evaluated with respect to their industrial relevance and overall
dielectric and mechanical performance.
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59
LIST OF ABBREVIATIONS
AC Alternating current
AFM Atomic Force Microscopy
APTES (3-aminopropyl) triethoxy silane
CNFs Cellulose nanofibrils
DC Direct current
EDC 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide
IR Infrared
HV High voltage
LbL Layer-by-Layer
LDPE Low density polyethylene
MFC Microfibrillated cellulose
PAA Polyacrylic acid
pDADMAC Polydiallyldimethylammonium chloride
PEI Polyethylene imine
PET Polyethylene terephthalate
PD Partial discharge
PTFE
PVAm
PVDF
Polytetrafluoroethylene
Polyvinyl amine
Polyvinylidene fluoride
SEM Scanning Electron Microscopy
TGA
UV
Thermogravimetric analysis
Ultra Violet
Symbols
tan δ Dielectric loss tangent, dissipation factor
ε Dielectric constant, permittivity
ε0 Vacuum permittivity
60
APPENDIX
Table A1. Material properties
Material Permittivity
ԑ
Dielectric loss
tan δ
Breakdown
strength
(kV/mm)
Product
name
LDPE1 2.2 0.0001-0.001 27 ET311201
PET1 3.0 0.002* 17 ES301400
PTFE1 2.0-2.1 0.0003-0.0007 50-170 FP301300
PVDF1 8.4 0.06 13 FV301300
Pressboard in
oil2 - 0.004 45-50 Elboard
Transformer oil3 2.2 0.003-0.005 >30 Nytro 10XN
1 Data from Good Fellow Cambridge Ltd, England (at 1MHz, *at 1kHz)
2 Data of high density board from Figeholm Elboard, Figeholm, Sweden
3 Data from Nynäs AB, Nynäshamn, Sweden
61
Diagrams showing the formation of cellulose functionalities and possible
crosslinks
Figure A1. Oxidation of the C2-C3 bond of cellulose (left) into dialdehyde
cellulose (right), with sodium periodate67.
Figure A2. Crosslinking can take place between aldehydes of the oxidised
cellulose (Figure A1) and a) hydroxyls forming hemiacetal bonds87 and b) a
primary amine (PVAm) forming imine bonds (also known as Schiff base
formation)80.
a)
b)
62
Figure A3. Carboxymethylation of cellulose29.
Figure A4. Dopamine can be grafted onto the carboxyls of the carboxymethylated
cellulose (Figure A3), i.e. the carboxymethylated cellulose reacts with EDC, and
dopamine is then grafted onto an intermediate structure69.
63
Figure A5. Infrared (IR) spectra of CNF films made from the different fibrils and
from periodate-oxidised carboxymethylated fibrils with retention aids. The
oxidation and the reaction with dopamine was detected with FTIR (with a
reduction of the carboxyl peak68 at 1593-1, the appearance of the characteristic
peak of dialdehyde cellulose88 at 1740 cm-1 and the appearance of bands attributed
to amide carbonyl stretching vibrations68 at 1680-1630 cm-1 for the film with
dopamine fibrils). It was also shown that the periodate-oxidised fibrils have
reacted further with PVAm (N-H deformation vibrations80 were detected at 1555
cm-1) but not with pDADMAC.