Contents
Cover
Half Title page
Title page
Copyright page
Preface
List of Contributors
Part 1: Synthesis Methodologies for Silicones
Chapter 1: Room Temperature Vulcanized Silicone Rubber Coatings: Application in High Voltage Substations
1.1 Introduction
1.2 Pollution of High Voltage Insulators
1.3 Silicone Coatings for High Voltage Ceramic Insulators
1.4 RTV SIR Coatings Formulation
1.5 Hydrophobicity in RTV SIR
1.6 Electrical Performance of RTV SIR Coatings
1.7 Conclusions
References
Chapter 2: Silicone Copolymers: Enzymatic Synthesis and Properties
2.1 Introduction
2.2 Polysiloxanes
2.3 Silicone Aliphatic Polyesters
2.4 Silicone Aliphatic Polyesteramides
2.5 Silicone Fluorinated Aliphatic Polyesteramides
2.6 Silicone Aromatic Polyesters and Polyamides
2.7 Silicone Polycaprolactone
2.8 Silicone Polyethers
2.9 Silicone Sugar Conjugates
2.10 Stereo-Selective Esterification of Organosiloxanes
2.11 Conclusion and Outlook
Acknowledgments
References
Chapter 3: Phosphorus Containing Siliconized Epoxy Resins
3.1 Introduction
3.2 Preparation of Siliconized Epoxy-Bismaleimide Intercrosslinked Matrices
3.3 Phosphorus-Containing Siliconized Epoxy Resin as Thermal and Flame Retardant Coatings
3.4 High Functionality Resins for the Fabrication of Nanocomposites
3.5 Anticorrosive and Antifouling Coating Performance of Siloxane- and Phosphorus-Modified Epoxy Composites
3.6 Summary and Conclusion
Acknowledgement
References
Chapter 4: Nanostructured Silicone Materials
4.1 Introduction
4.2 Solid Particles
4.3 Nanocapsules
4.4 Ultra-Thin Silicone Films
4.5 Conclusion and Outlook
References
Chapter 5: High Refractive Index Silicone
5.1 Introduction
5.2 Theory of RI
5.3 High Refractive Index Silicone
5.4 Applications
5.5 Conclusion and Outlook
References
Chapter 6: Irradiation Induced Chemical and Physical Effects in Silicones
6.1 Introduction
6.2 Sources of Irradiation
6.3 Irradiation-Induced Chemical Effects in Silicones
6.4 Irradiation-Induced Physical Effects in Silicones
6.5 Conclusion and Outlook
References
Chapter 7: Developments and Properties of Reinforced Silicone Rubber Nanocomposites
7.1 Introduction
7.2 Different Types of Nanofillers Used in Silicone Rubber (SR)
7.3 Preparation of Silicone Rubber (SR) Nanocomposites
7.4 Morphology of Silicone Rubber (SR) Nanocomposites
7.5 Properties of Silicone Rubber Nanocomposites
7.6 Conclusion and Outlook
References
Chapter 8: Functionalization of Silicone Rubber Surfaces towards Biomedical Applications
8.1 Introduction
8.2 Silicone Rubber – Material of Excellence for Biomedical Applications?
8.3 Surface Modification of Silicone Rubber
8.4 Conclusion and Outlook
References
Chapter 9: Functionalization of Colloidal Silica Nanoparticles and Their Use in Paint and Coatings
9.1 Introduction to Colloidal Silica
9.2 Chemistry of Silica Surface Functionalization by Organosilanes
9.3 Characterization and Product Properties of Silane-Modified Silica Dispersions
9.4 Applications for Silanized Silica Nanoparticles in Paint and Coatings
9.5 Conclusion and Outlook
References
Chapter 10: Surface Modification of PDMS in Microfluidic Devices
10.1 Introduction
10.2 PDMS Surface Modification Techniques
10.3 Characterization Techniques
10.4 Discussion and Perspectives
References
Part 2: Characterizing the Silicones
Chapter 11: The Development and Application of NMR Methodologies for the Study of Degradation in Complex Silicones
11.1 Introduction
11.2 Applications of NMR for Characterizing Silicones
11.3 Highlights of Recent Advances in NMR Methodology
11.4 Conclusions and Outlook
Acknowledgements
References
Chapter 12: Applications of Some Spectroscopic Techniques on Silicones and Precursor to Silicones
12.1 Introduction
12.2 Fourier Transformation Infrared and Spectroscopy of Silicones
12.3 Raman Spectroscopy of Silicones
12.4 FTIR-Assisted Chemical Component Analysis in Thermal Degradation of Silicones
12.5 X-ray Photoelectron Spectroscopy of Silicones
12.6 Secondary Ion Mass Spectroscopy
12.7 Conclusion and Outlook
Acknowledgement
References
Chapter 13: Degradative Thermal Analysis of Engineering Silicones
13.1 Degradative Thermal Analysis of Engineering Silicones
13.2 Conclusions and Outlook
Acknowledgments
References
Chapter 14: High Frequency Properties and Applications of Elastomeric Silicones
14.1 Introduction
14.2 Silicone Microdevice Fabrication
14.3 Properties of Silicone at Radio Frequencies (1–20 GHz)
14.4 Properties of Silicone at Terahertz Frequencies (0.2 THz – 4.0 THz)
14.5 Conclusion and Outlook
Acknowledgements
References
Chapter 15: Mathematical Modeling of Drug Delivery from Silicone Devices Used in Bovine Estrus Synchronization
15.1 Introduction
15.2 Bovine Estrous Cycle
15.3 Bovine Estrus Synchronization
15.4 Controlled Release Silicone Devices
15.5 Mathematical Modeling
15.6 Conclusion and Outlook
References
Chapter 16: Safety and Toxicity Aspects of Polysiloxanes (Silicones) Applications
16.1 Introduction
16.2 Business Strategy for Manufacturing and Sale of Polysiloxanes
16.3 Chemical Aspects
16.4 Speciation Analysis
16.5 Application Areas and Direct Human Contact with Polysiloxanes (Silicones)
16.6 Toxicological Aspects
16.7 Conclusion and Outlook
References
Chapter 17: Structure Properties Interrelations of Silicones for Optimal Design in Biomedical Prostheses
17.1 Introduction
17.2 Materials and Methods
17.3 Discussion of Results
17.4 Conclusions and Outlook
References
Part 3: Applications of Silicones
Chapter 18: Silicone-Based Soft Electronics
18.1 Introduction
18.2 Silicone-Based Passive Soft Electronics
18.3 Silicone-Based Integrated Active Soft Electronics
18.4 Conclusion
Acknowledgements
References
Chapter 19: Silicone Hydrogels Materials for Contact Lens Applications
19.1 Introduction
19.2 Synthesis and Development of Materials
19.3 Surface Properties
19.4 Bulk Properties
19.5 Biological Interactions
19.6 Load and Release of Products from Contact Lenses
19.7 Conclusions
Disclosure
References
Chapter 20: Silicone Membranes for Gas, Vapor and Liquid Phase Separations
20.1 Introduction
20.2 Material
20.3 Membrane Type and Configuration
20.4 Membrane Unit Operations Based on Silicones
20.5 Conclusions and Outlook
References
Chapter 21: Polydimethyl Siloxane Elastomers in Maxillofacial Prosthetic Use
21.1 Introduction
21.2 Facial Prostheses
21.3 Polydimethyl Siloxane Elastomers
21.4 Reinforcement
21.5 Biocompatibility and the Microbiological Features
21.6 Future Studies
Acknowledgment
References
Chapter 22: Silicone Films for Fiber-Optic Chemical Sensing
22.1 Introduction
22.2 Silicone Chemistry and Technology Related to Optical Chemical Sensing
22.3 Gas Permeability and Optical Sensing
22.4 Optical Properties of Silicone Thin Films
22.5 Silicone Films for Optical Oxygen Sensing
22.6 Silicone Films for Optical Sensing of Other Species
22.7 Conclusion
Acknowledgements
References
Chapter 23: Surface Design, Fabrication and Properties of Silicone Materials for Use in Tissue Engineering and Regenerative Medicine
23.1 Introduction
23.2 Silicone Biomaterials
23.3 Silicones in Tissue Engineering
23.4 Surface Characterization Techniques
23.5 Conclusion and Outlook
Acknowledgement
References
Chapter 24: Silicones for Microfluidic Systems
24.1 Introduction
24.2 Fabrication of Microfluidic Devices
24.3 Application of PDSM-Based Microfluidic Devices
24.4 Summary and Outlook
References
Chapter 25: Silicone Oil in Biopharmaceutical Containers: Applications and Recent Concerns
25.1 Introduction
25.2 Lubrication of Pharmaceutical Containers and Devices
25.3 Silicone Oil: A Molecular Perspective
25.4 Silicone Oil Coatings in Pharmaceutical Devices
25.5 Protein Adsorption to Hydrophobic Interfaces
25.6 Physical Stability of Biologics in the Presence of Silicone Oil
25.7 Overcoming Silicone Oil-Related Incompatibilities
25.8 Conclusions and Outlook
List of Abbreviations
References
Index
Concise Encyclopedia of High Performance Silicones
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Preface
Mother nature has gifted us with miraculous elements that have been extensively utilized by human beings since ancient times to transform their lifestyle. Biological life, as we know it, is fundamentally dependent on six elements, i.e., oxygen, carbon, hydrogen, calcium, phosphorous and nitrogen. Besides oxygen, silicon is the second most abundant element (25.7%) on the earth and one of the most valuable elements for mankind. Silicon belongs to a class of metalloids and has properties similar to carbon, yet seems to be relegated to a strictly inorganic (originally meaning nonlife) domain. The name silicon was derived from the latin word “silex” (meaning flint) and was discovered by the Swedish scientist Jöns Jacob Berzelius in 1824. In the 19th century, the American scientist Eugene G. Rochow, while working at the general electric company, and the german chemist Richard Müller discovered a synthetic route to produce chloromethylsilane. The production of choloromethylsilane helped the english chemist F.S. Kipping synthesize a large number of si-c compounds that later formed the basis of modern silicone chemistry. The name silicone is a misnomer that stuck, perhaps as a marketing ploy. Silicone was thought to be structurally related to the ketone group, where the carbon was replaced by a silicon atom. This, however, was not the case, but the name stuck.
the presence of d-orbital helps silicon to react with a variety of elements and compounds. The silicones also known as polysiloxanes constitute an arrangement of alternating silicon and oxygen atom with organic groups bonded directly to silicon. Although the Si-C bond (307 kJ/mol) is less stable compared to the C-C bond (347–356 kJ/mol), the physical and chemical properties of the organic macromolecules from both these elements are comparatively similar. A great deal of work has been accomplished by scientists in synthesizing a wide array of silicones. Silicones are extremely valuable to the modern world in terms of their ability to change surface tension at an interface. Most of the commercially successful silicones are polysiloxanes with easily understood and established structure-property relationships. Organosilicone finds application in products ranging from biomedical to consumer electronics. For example, in biomedical engineering, silicone rubbers have been used in drug delivery vehicles, respirator tubes, artificial skin, dialysis, dental impressions, catheters, etc. Similarly, silicones are widely used materials in the construction industry in the form of paints and coatings, sealants, connectors, etc. Silicones have also been vastly used in the manufacturing of beauty products such as lipsticks, skin creams, deodorants, facemasks, hair products, etc. In the textile industry, silicones are used as softeners. Silicones are also used as restoring agents for historical paintings, metal and stone sculpture and in architecture. They are widely used materials in automobile and aerospace engineering.
Due to their extreme compatibility with other polymers and their being a scientifically valuable material, an enormous amount of free and patented scientific literature is available on silicones. While working with silicones, we realized that fundamental and essential information was deeply buried in the voluminous literature and extremely difficult to excavate. We have noticed that younger scholars strive hard to find basic information such as FTIR spectral assignments related to organosiloxane. Additionally, there are few if any books available on the applied aspects of silicones. We wanted to publish this concise encyclopedia to collect the vital information that could help the researcher envisage the new commercial potential of silicones. The forehand knowledge of existing application areas will encourage an interdisciplinary research approach in younger students.
The chapters in this concise encyclopedia are grouped in three primary sections, viz., developments, characterization and applications. The first section is dedicated to the synthesis methodologies that are adopted in deployment of new silicone compounds. For example, the development of room-temperature vulcanized silicone rubber coating is reviewed for high voltage insulation applications. The reactions in the synthesis of silicone may include stages that utilize ingredients that are not eco- and human-friendly. Syntheses of biologically safer silicones through the use of enzymes are briefly discussed. A chapter describes the use of silicone-modified epoxy coatings in controlling the corrosion and biofouling activities on metal alloys. Another chapter explains the use of smart silicone nanoparticles in paint and coating systems. Different methods for the development of silicone nanoparticles and nanostructures are reviewed, and their application in ultrathin film formation is explained. Silicone acts as an excellent source of material for the corrosion protection of metal surfaces. The development of high-refractive-index silicone will help in understanding the value of this material in manufacturing of optical guides, optical sensors and intraocular lenses. The effect of irradiation on silicone chemistry is outlined to help in understanding the various physicochemical changes that occur due to such exposure. The use of silicone as a reinforcing agent in rubber is of high industrial curiosity. The development of biocompatible silicones will help readers in adopting synthetic paths that lead to compounds for health care service. The use of polydimethylsiloxane in preparation of microfluidic devices is comprehensively reviewed.
The second section of the book will help readers characterize the silicones. A comprehensive review on the use of nuclear magnetic resonance (NMR) spectroscopy on silicones will assist in analyzing the synthesized silicones with different types of NMR techniques. The use of FTIR techniques on silicones will help readers extract the hard to find spectral assignments. Similarly, the chapters on thermal degradation and behavior of silicones at high frequency will be informative. The knowledge of safety and toxicity aspects, mathematical modeling for drug delivery, and structure-property correlation in silicones will help readers when selecting material for a specific application. The third and final section is dedicated to the applications of silicones. The use of silicones for soft electronics, contact lenses, membranes for separation technology, maxillofacial prosthetics, fiber-optic-based chemical sensing, tissue engineering and regenerative medicine is detailed in individual chapters. Finally, the use of silicone as microfluidic devices and biopharmaceutical containers is described in detail.
We expect to attract a wide readership that will include scholars working in small laboratories with limited facilities and companies with substantial support for research activities. Research chemists could have extensive experience in handling chemical reactions but may have limited in-depth information about the silicones. Similarly, chemical engineers possess little knowledge of silicone chemistry. Young research students who are curious about the different possible applications of silicone compounds will find this book informative. We have tried including most of the potential areas where silicones are being utilized in industries. The material demonstrates excellent biocompatibility so readers may notice a slight inclination towards the biotechnological arena.
We are confident that this book will serve as an excellent reference source for scholars from different disciplines such as physics, chemistry, materials science and engineering, pharmacy, medical science, biotechnology and biomedical engineering. The two editors of this book met in one of the meetings conducted by the Technical Corrosion Collaboration (TCC) program, USA, and mutually agreed to work together on this subject of enormous scientific importance. We are grateful to the organizers of the TCC programs that have motivated and helped scientists to work in collaboration.
Atul Tiwari, PhD
Mark D. Soucek, PhD
USA, February 2014
List of Contributors
Zulkifli Ahmad is an Associate Professor at the School of Material and Mineral Resources Engineering, Universiti Sains Malaysia, Pulau Pinang, Malaysia.
M. Alagar is a Professor of Chemistry in the Department of Chemical Engineering, Anna University, Chennai, India.
Paola Bernardo is a researcher at the Institute on Membrane Technology (ITM-CNR), Rende (CS), Italy,
Madhu Bhaskaran is an ARC Australian Post-Doctoral Fellow and Co-Leader, Functional Materials and Mcirosystems Research Group, RMIT University, Melbourne, Australia.
Jing Cao is a PhD candidate in the Fiber and Polymer Science program at North Carolina State University, Raleigh, USA.
Shi Cheng is a Research Engineer at the Hardware Radio Division at Ericsson, Stockholm, Sweden, and he currently also holds a Researcher position at the Department of Integrated Devices and Circuits at the School of Information and Communication Technology (ICT) at KTH Royal Institute of Technology, Stockholm, Sweden.
Gabriele Clarizia is a researcher at the Institute on Membrane Technology (ITM-CNR), Rende (CS), Italy.
Vicente Compañ is a Full Professor of Thermodynamics at the Polytechnic University of Valencia, Spain.
H. Serdar Çötert, D.D.S., Ph.D. is a Clinical Professor in Prosthodontics, Ege University Faculty of Dentistry, Department of Prosthodontics, 
zmir, Turkey.
Nitin Dixit is a Doctoral Fellow, Drug Product Development, Shire, Lexington, MA, USA.
Fernando Dourado is an Assistant Professor at the Institute for Biotechnology and Bioengineering (IBB), Centre of Biological Engineering, University of Minho, Campus de Gualtar, Portugal.
Paulo R.B. Fernandes is an Assistant Professor at the University of Minho, Portugal.
Daniela P. Lopes-Ferreira is a PhD student at the University of Minho, Portugal.
José M. González-Méijome is an Associate Professor at the University of Minho, Portugal.
Javier González-Pérez is an Associate Professor at the University of Santiago de Compostela, Spain.
Peter Greenwood is a Business Development Specialist at Akzo Nobel Pulp and Performance Chemicals, Silica and Paper Chemicals RD&I, Sweden.
Stephen J. Harley is a Staff Scientist and Materials Characterization Expert with the chemical sciences division at Lawrence Livermore National Laboratory, California, USA.
Ignacio M. Helbling is a Researcher at the National Council of Scientific and Technical Research (CONICET), Santa Fe, Argentina.
Robert Huszank is a Research Associate at the Institute for Nuclear Research, Hungarian Academy of Sciences, Debrecen, Hungary.
Juan C.D. Ibarra is a Researcher at the National Council of Scientific and Technical Research (CONICET), Santa Fe, Argentina.
Johannes Carolus Jansen is a researcher at the Institute on Membrane Technology (ITM-CNR), Rende (CS), Italy.
Devendra (Davy) S. Kalonia is a Professor of Pharmaceutics at the School of Pharmacy, University of Connecticut, Storrs, CT, USA.
Anna Kowalewska in an Assistant Professor at Centre of Molecular and Molecular Studies, Polish Academy of Sciences in Lodz, Poland.
Mariusz Kepczynski is an Associate Professor at the Faculty of Chemistry of the Jagiellonian University, Krakow, Poland.
S. Ananda Kumar is an Assistant Professor at the Department of Chemistry, Anna University, Chennai, India.
Joanna Lewandowska-
ańcucka is an Assistant Professor at the Faculty of Chemistry of the Jagiellonian University, Krakow, Poland.
James P. Lewicki is a Staff Scientist and Polymer Subject Matter Expert at Lawrence Livermore National Laboratory, California, USA.
Juan López-Gejo is a Senior Research Associate at the Optical Chemosensors & Applied Photochemistry Group (GSOLFA), Department of Organic Chemistry, Faculty of Chemistry, Complutense University of Madrid, Spain.
Julio A. Luna is a Researcher at the National Council of Scientific and Technical Research (CONICET), Santa Fe, Argentina.
Amitesh Maiti is a computational materials physicist with the chemical sciences division of Lawrence Livermore National Laboratory, California, USA.
S. Mandhakini is Postdoctoral Researcher in the Department of Chemical Engineering, Anna University, Chennai, India.
Robert S. Maxwell is a Senior Scientist at Lawrence Livermore National Laboratory, California, USA.
Brian P. Mayer is a research scientist with the Forensic Science Center at Lawrence Livermore National Laboratory California, USA.
Krystyna Mojsiewicz-Pieńkowska is an Assistant Professor at the Medical University of Gdansk, Faculty of Pharmacy with Subfaculty of Laboratory Medicine, Gdansk, Poland.
Sergio Molla obtained his PhD in Engineering at the Polytechnic University of Valencia, Spain.
Maria Nowakowska is a Full Professor at the Faculty of Chemistry of the Jagiellonian University, Kraków, Poland.
Guillermo Orellana is a Professor at the Optical Chemosensors & Applied Photochemistry Group (GSOLFA), Department of Organic Chemistry, Faculty of Chemistry, Complutense University of Madrid, Spain.
Mikael Östling is a Full Professor in Solid State Electronics, the co-founder of the company TranSiC fully acquired by Fairchild Semiconductor in 2011, and a Fellow of the Institute of Electrical and Electronics Engineers (IEEE); Currently he heads the Department of Integrated Devices and Circuits at the School of Information and Communication Technology (ICT) at KTH Royal Institute of Technology, Stockholm, Sweden.
Bruno Pedras is a Postdoctoral Fellow at the Optical Chemosensors & Applied Photochemistry Group (GSOLFA), Department of Organic Chemistry, Faculty of Chemistry, Complutense University of Madrid, Spain.
Yadagiri Poojari is a Research Scientist, Department of Physics, Ohio State University, Columbus, Ohio, USA.
Bratati Pradhan is an Assistant Professor in the Department of Chemistry, Indian Institute of Technology, Kharagpur, India.
Dionisios Pylarinos is a consultant at the Hellenic Electricity Distribution Network Operator S.A., Heraklion, Crete.
Wenjun Qiu is an Associate Professor in The Angstrom Laboratory, Uppsala University, Sweden.
Lígia R Rodrigues is an Assistant Professor at the Institute for Biotechnology and Bioengineering (IBB), Centre of Biological Engineering, University of Minho, Campus de Gualtar, Portugal.
Charan M. Shah is a Ph.D. Candidate, Functional Materials and Mcirosystems Research Group, RMIT University, Melbourne, Australia.
Kiriakos Siderakis is an Assistant Professor, Department of Electrical Engineering, Technological Educational Institute of Crete, Heraklion.
Sharath Sriram is an ARC Australian Post-Doctoral Fellow and Co-Leader, Functional Materials and Mcirosystems Research Group, RMIT University, Melbourne, Australia.
Suneel Kumar Srivastava is a Professor in the Department of Chemistry, Indian Institute of Technology, Kharagpur, India.
Nisarg Tambe is a PhD candidate in the Fiber and Polymer Science program at North Carolina State University, Raleigh, USA.
Petroula Tarantili is an Associate Professor in Polymer Technology at the School of Chemical Engineering of National Technical University of Athens, Greece.
Atul Tiwari is research faculty member in the Department of Mechanical Engineering at the University of Hawaii at Manoa, USA
Anders Törncrona is a Principal Scientist at Akzo Nobel Pulp and Performance Chemicals, Silica and Paper Chemicals RD&I, Sweden.
Julie Willoughby is an Adjunct Assistant Professor in the Department of Textile Engineering, Chemistry and Science at North Carolina State University, Raleigh, USA.
Withawat Withayachumnankul is an ARC Australian Post-Doctoral Fellow, School of Electrical and Electronic Engineering, University of Adelaide, Adelaide, Australia.
Chaoqun Wu is Associate Professor at Wuhan University of Technology, China.
Zhigang Wu is an Associate Professor in The Angstrom Laboratory, Uppsala University, Sweden.
Kewei Xu is a PhD candidate in the Fiber and Polymer Science program at North Carolina State University, Raleigh, USA.
Silicone rubber has brought a new era in the field of outdoor insulation, providing improved performance in comparison to the ceramic materials that were traditionally employed. Its primary advantage occurs as a result of its surface behavior in respect to water, with silicone rubber being able to maintain hydrophobic characteristics in field conditions, even after the deposition of contaminants on the surface. This improved behavior correlates with the material formulation employed, with the properties and capabilities of the base polymer and the included fillers. Room temperature vulcanized silicone rubber (RTV SIR) is one of the forms of silicone rubber implemented in outdoor insulating systems, usually in order to improve the pollution performance of ceramic insulation. This chapter is a review of the basic features and properties of RTV SIR coatings applied on ceramic insulators in high voltage substations.
Keywords: High voltage insulators, silicone rubber, room temperature vulcanization, pollution, substations
High voltage transmission and distribution systems constitute critical infrastructures for the development and the prosperity of today’s society. Substations and transmission lines form a network responsible for interconnecting the power generation facilities, from conventional and renewable sources, with the power consumption centers. In addition, further requirements are set, demanding the optimized operation of these installations with the highest degree of reliability and, at the same time, with the minimum possible cost.
For the majority of these systems, the primary insulating component is the surrounding atmospheric air. This choice is made while considering that the use of increased voltage levels is necessary in an effort, firstly, to reduce the correlated transmission power losses, and further to improve additional features of the transmission performance, such as system stability [1]. Furthermore, the necessity of adequate insulating systems is evident and the surrounding atmospheric air has to demonstrate considerable advantages, starting from the fact that it is free of charge. A significant disadvantage on the other hand, as for any gas or liquid dielectric, is its incapability to mechanically support the high voltage conductors. Consequently, the use of solid insulators capable of providing the required mechanical features is required [2]. Therefore an insulation system is formed combining a gas dielectric, which is the atmospheric air and solid dielectrics in the form of insulators.
The performance of the gas solid interface formed is quite critical [3, 4]. It will determine the efficiency of the insulator in respect to the experienced service conditions and, furthermore, the reliability of the high voltage installation, considering that a single insulator failure is sufficient to set an installation such as a transmission line out of service for many hours.
A major concern for the operators of many high voltage installations is the change of the insulator surface behavior due to the deposition of contaminants that are or can become conductive [4, 5]. It is a problem known as “pollution of high voltage insulators,” and is responsible for the majority of power outages in many transmission and distribution systems, especially those that are near the sea coast [5, 6]. Under the influence of pollution, the behavior of an insulator is degraded, resulting to a complete loss of the dielectric capability, although the applied voltage stress remains within the nominal limits.
The surface performance under pollution conditions is the comparative advantage of composite materials, and especially silicone rubber, in respect to the ceramic materials, porcelain and glass, which were traditionally employed for the manufacture of insulators [3, 7, 8]. In fact, the introduction of silicone compounds brought a new era in the field of outdoor transmission and distribution insulating systems. This change occurs mainly due to the surface behavior of silicone rubber, and especially due to a property known as hydrophobicity [6–8].
Hydrophobic surfaces resist wetting, which is necessary in most cases, and especially in coastal systems, for the surface contamination to develop electrical conductivity. Consequently, silicone rubber insulators demonstrate a hydrophobic surface behavior and thus a quite improved performance, in comparison to the ceramic insulators. Porcelain and glass are hydrophilic materials and therefore are vulnerable to the action of the pollution phenomenon [3].
Nevertheless, by exploiting silicone technology, and especially the vulcanization process of silicone rubber, it is also possible to develop improved ceramic insulators, and this solution is Room Temperature Vulcanized Silicone Rubber (RTV SIR) coatings. These coatings can be applied on the surface of a ceramic insulator and ascribe a behavior similar to silicone rubber insulators, and therefore ensure an improved performance in the case of pollution [9, 10]. The coating properties, capabilities and efficiency are correlated with the formulation and the fillers incorporated, to the application conditions and procedures, and certainly with the service conditions experienced [11–16].
Pollution of high voltage insulators is a problem experienced in many outdoor high voltage installations worldwide, and in most cases is the primary cause of power outages. It is usually considered as a six stages mechanism, as shown in Figure 1.1 [5]. The first stage is the deposition of contaminants on the insulation surface, experienced mainly due to the wind but also other mechanisms such as acid rain. The amount accumulated and the electrical behavior of the film formed, are critical for the mechanism development. Usually there are substances within the accumulated contaminants that have or may develop electrical conductivity. The second is true in the case of coastal systems, where the primary source of contamination is the sea and the majority of contaminants are sea salts, which become conductive when diluted in water. The wetting agent is available on the insulation surface as the result of mechanisms such as fog, dew, condensation and light rain. Wetting is the second stage of the mechanism and leads to the third stage, which is the formation of surface conductivity and the flow of current, known as leakage current.
Figure 1.1 Development of the pollution phenomenon in stages.
The flow of current unfolds a counterbalancing mechanism as far as the surface conductivity formation is concerned. The contaminants’ film behaves as a resistance distributed on the insulator surface, with a value determined by the amount of contamination accumulated and the degree of wetting. The flow of current, through the development of joule losses, is capable of changing the degree of surface wetting and thus the conductivity value. This change is not uniform on the surface, but it appears to be relevant to the insulator geometry and in fact is more intense in areas with small radius from the insulator axis of symmetry [17]. Along these areas drying is intense, resulting in zones of increased resistance known as dry bands. The formation of the dry bands is considered stage four.
Consequently, the initial insulating surface, where only a small capacitive current was observed, can now be considered as a series combination of electrolytic resistances, with values that vary and depend on the accumulated contamination, the degree of wetting and the joule losses experienced due to the flow of leakage current. As a result, the voltage distribution along the insulator leakage path is changed and the stress distribution is now dependent on the value of conductivity achieved. This change of the surface behavior, and further the voltage distribution that occurs, result in the application of intense stress along parts of the leakage path. Thus surface discharging appears, known as dry band arcing, and this is stage five.
Dry band arcs bridge parts of the leakage path and not the complete leakage distance. Thus, they are present on the insulator surface, but a flashover is not achieved. Only under favorable conditions, and especially an optimum combination between the conductivity values of the surface film and the gas discharge, will the discharge propagate and a complete flashover occur. These favorable conditions are not always present and usually stages one to five are experienced. Dry band arcing and, finally, a flashover on a 150 kV post porcelain insulator during an artificial pollution test is illustrated in Figure 1.2. The considered test took place at the Talos High Voltage Test Station in Crete, Greece [18].
Figure 1.2 Flashover of a 150kV post-porcelain insulator at Talos Test Station [18].
It is evident from the pollution mechanism analysis, that the demonstrated surface behavior is a key factor regarding the vulnerability of an outdoor insulator to the action of the pollution phenomenon. Therefore, imparting the surface with properties that could postpone the mechanism development is a way to increase the system’s efficiency and reliability. The concept of coatings for high voltage ceramic insulators is the application of an additional layer on the insulator surface, which will interfere with the pollution mechanism development and eliminate the possibility of a flashover. This is possible by reducing either the amount of contaminants on the insulator surface or the degree of wetting [3, 19].
In the first case, coatings in the form of grease have been developed, capable of encapsulating the deposited contamination and thus maintaining a clean surface. These formulations were based on hydrocarbons or silicones, with the second mostly employed due to their increased thermal stability, which allowed them to be implemented in various climatic conditions [3]. However, the amount of contamination that could be encapsulated was limited and dependant on the amount of grease applied. Thus saturation was due to appear, and when it appeared the coating replacement was unavoidable. Considering that in the case of a moderate to heavy pollution environment, the time period from the coating application until saturation was less than six months, the application of this type of coating is time and money consuming. In addition, it must be also noticed that after saturation the behavior of the saturated greased insulator is quite inferior to the ungreased ceramic insulator.
The second type of coatings are RTV SIR coatings. In this case, another path is followed. Instead of encapsulating the accumulated contamination, RTV SIR incorporates a molecule migration mechanism to impart hydrophobic properties to the formed contaminants film. In this way, surface wetting is difficult and thus the development of the pollution mechanism is postponed. Figure 1.3 illustrates a section of a porcelain post insulator where the RTV SIR coating can be seen. The porcelain bulk is white but the surface can be detected due to its brown color, and a 5 mm white coat can be seen.
Figure 1.3 Section of a 150kV post-porcelain coated with a 5mm RTV SIR coat.
It is worth mentioning that RTV SIR coatings are not a new technology. The first application found in the literature is in 1968 in the USA, where a small amount was installed in a high voltage substation of Bonneville Power Administration [3]. The achieved improvement was remarkable, however, tracking and erosion problems very soon appeared. Since then, many applications can be found worldwide [6, 14–16, 19–41] and considerable improvement of the coatings performance has been achieved.
Composite materials employed in outdoor insulating systems are usually compositions of one base polymer and a number of additives, known as fillers. Fillers aim to formulate the composition properties according to the application requirements starting, of course, from the properties of the base polymer. Consequently, for the development of a composite material, both the base polymer and the fillers included have a critical role to play. Further, depending on the anticipated service conditions, an optimum formulation (type and amount of the base polymer and filler) can be determined. In the case of RTV SIR coatings silicone rubber, and especially polythimethylsiloxane, is the base polymer implemented.
Polydimethylsiloxane is the base polymer for silicone formulations employed in outdoor high voltage systems, not only in the case of coatings but also for composite insulators [3, 9–11]. It is a widely applied polymer within the silicone family [42], with remarkable and sometimes unique properties, which enable applications not only in outdoor installations but also in many technical fields [42–47]. The polymer chain is composed of silicone oxygen bonds, accompanied by two side methyl groups connected to the silicone atom, as shown in Figure 1.4, where the monomer unit is illustrated.
Figure 1.4 Representation of the PDMS monomer.
In comparison to polymer materials that have been employed in high voltage outdoor insulation systems like EPM and EPDM, PDMS has to demonstrate considerable advantages such as:
These properties are strongly correlated to the monomer structure and especially to the presence of the silicone oxygen bond, the weak intermolecular forces that appear within the material and the flexibility of the polymer chain [43].
In the PDMS chain, silicone is connected to oxygen through a strong covalent chemical bond, with a bond energy that is the highest among the chemical bonds found in polymers employed in high voltage outdoor insulation, as illustrated in Table 1.1 [43–47]. The bond strength is responsible for polymer properties such as the thermal and environmental durability of the material. In addition, there is an influence to the behavior of the side silicone carbon bonds, which become stronger in the polymer chain. As a result, the attachment of the side methyl groups to the silicone oxygen chain appears to be stronger in comparison to other polymers, and therefore the side methyl groups are less exposed to mechanisms such as replacement reactions [42]. The reason for this is the electropositive silicone behavior, which attracts electrons, polarizing the methyl groups and further enforcing the attachment degree.
Table 1.1 Energy of the chemical bonds usually encountered in polymers employed in outdoor high voltage systems.
| Chemical Bond | Bond Energy (kJ/mol) |
| -Si-O- | 445 |
| -C-H- | 414 |
| -C-O- | 360 |
| -C-C- | 348 |
| -Si-C- | 318 |
| -Si-H- | 318 |
Consequently, the presence of the silicone oxygen bond in the material monomer results in the development of a strong structure when compared to other polymers within the same application field, where the polymer chain is composed of carbon atoms instead of silicone. Nevertheless, it must be noted that the polar nature of the silicone oxygen bond results in a material that is vulnerable to the action of hydrolysis [44], which, however, appears to be notable for pH values less than 2.5 or greater than 11 [48].
The levels of surface tension in a material appear to be strongly correlated to the material structure [49, 50]. Usually materials with strong chemical bonds demonstrate a surface with increased levels of surface tension, which further results in a hydrophilic behavior. In the case of silicone rubber, however, despite the presence of the strong silicone oxygen bond within the PDMS monomer, the levels of surface tension experienced are considerably lower, resulting in a hydrophobic surface. In fact, as illustrated in Table 1.2 [51], the value of surface tension experienced in the case of silicone rubber is the second lowest value after PTFE (TEFLON).
Table 1.2 Typical surface tension values encountered in materials employed in outdoor high voltage systems and water.
| Material | Surface tension (mN/m) |
| Porcelain | 366 |
| Glass | 170 |
| Epoxy Resins | 35 |
| EP Rubbers | 30 |
| PDMS | 23 |
| PTFE | 19 |
| water | 72.3 |
This behavior occurs since the surface tension is primarily correlated to the experienced intermolecular forces that develop between the polymer chains. In the case of PDMS, the presence of the side methyl groups decrease the strength of interaction between the polymer chains and further limit the resulting surface tension [42, 43]. Therefore, the side methyl groups conceal the presence of the relatively strong silicone oxygen bond, resulting in a hydrophobic surface [49, 50]. In this direction the number of the side methyl groups and especially the chain flexibility act in favor of a low energy surface behavior.
The PDMS chains are characterized by increased levels of mobility and flexibility. They are capable of moving within the material volume, and the rotation of the side methyl groups is also possible [42, 43, 51]. This remarkable mobility is very important since it is correlated to properties such as the low glass transmission temperature (–127°C), the low levels of surface tension, and thus hydrophobicity and the capability of hydrophobicity recovery, which is quite critical in the case of outdoor insulators. It is worth noticing that the energy required for the methyl groups rotation is considerably lower, less than 4 kJ/mol [52], indicating that this movement is almost free. For the same rotation 14 kJ/mol [53] are required in the case of polyethylene and 20 kJ/mol [53] in the case of PTFE. Further, for polymer chains that are near the surface, the orientation of the methyl groups towards the surface occurs in the direction of minimizing the surface tension, according to the first law of thermodynamics.
The importance of chain mobility and flexibility can be further realized when comparing the behavior of silicone rubber to the behavior of PTFE. As has already been mentioned, PTFE is the most hydrophobic material among the materials employed in outdoor insulation and is more hydrophobic than silicone rubber. However, it is characterized by a high glass transition temperature at 117°C [51], which indicates that at room temperature the material demonstrates a strong solid form rather than rubber as in the case of silicone rubber. As a result, in the case of a hydrophobicity loss, recovery is not possible, in contrast to silicone rubber, where recovery mechanisms are present. Consequently, although PTFE is more hydrophobic than silicone rubber and has been used for the manufacture of insulators during the 70s, today silicone rubber is the dominant material, mainly due to its capability to maintain surface behavior in field conditions.
The manufacturing procedure of silicone rubber consists of various processes starting with the production of dimethyldichlorosilane from powdered silicon and ending in the rubber formation. Polymerization and crosslinking are two processes of paramount importance in order to understand the material structure. Both are considered to increase the chains size; however, they are two different processes sequentially occurring.
Polymerization is a process where a big molecule is formed by appropriately connecting a number of smaller molecules (monomers). In the case of PDMS the polymerization process starts from the hydrolysis of dimethyldichlorosilane (Reaction 1.1), followed by condensation Reactions 1.2 and 1.3 [45].
The molecular weight of the obtained macromolecules is determined by the polymerization conditions and can be controlled by various methods, such as the addition of active monomers (for example, examethyldisilane). The polymer finally manufactured is a mixture of linear and cyclic macromolecules, which are further used to produce larger molecules by anionic or cationic ring opening polymerization of the cyclic oligomers or by polycondensation of the silanol end-block linear oligomers.
(1.1) 
(1.2) 
(1.3) 
Depending on the molecular weight, the material produced is in gas or liquid form (low intermolecular forces). In order to acquire a solid material, the development of connections between the polymers’ molecules that will stabilize the material structure are required. Crosslinking or vulcanization is the process followed in this direction. Two of the vulcanization mechanisms that are applied in the case of silicone rubber (outdoor insulation) are described below.
In High Temperature Vulcanization (HTV) the connections between the rubber macromolecules are developed between the side methyl groups, with the participation of free radicals that abstract hydrogen atoms [45, 46, 54]. This process, described by Reactions 1.4 and 1.5, takes place at elevated temperatures ranging from 115°C to 173°C depending on the radicals used. In addition, a post-curing process is applied (storage at elevated temperatures) in order to remove the byproducts (volatile) of the curing process.
(1.4) 
(1.5) 
There are two mechanisms for the network formation characterized as RTV [45, 46, 54]. The first is a condensation reaction of silanol groups to form siloxanes with the participation of water and drastic groups, catalyzed by acid or base, as shown in Reaction 1.6. The process is triggered by the atmosphere humidity and the connections are developed with oxygen atoms; also by the byproducts formed, usually alchohol.
The second mechanism involves an addition reaction between a siloxane, containing vinyl groups and a siloxane crosslinking agent with Si-H functional groups (Reaction 1.7). The process is catalyzed by a noble metal.