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

18

(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

19

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

20

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

22

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.