Concise Encyclopedia of High Performance Silicones E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 1187

Veröffentlichungsjahr: 2014

Beschreibung

The encyclopedia will be an invaluable source of information for researchers and students from diverse backgrounds including physics, chemistry, materials science and surface engineering, biotechnology, pharmacy, medical science, and biomedical engineering.

Leseprobe

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

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106

Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Copyright © 2014 by Scrivener Publishing LLC. All rights reserved.

Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts. Published simultaneously in Canada.

No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, [phone], fax [phone], or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, [phone], fax [phone], or online at http://www.wiley.com/go/permission.

Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.

For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at [phone], outside the United States at [phone] or fax [phone].

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com.

For more information about Scrivener products please visit www.scrivenerpublishing.com.

Library of Congress Cataloging-in-Publication Data:

ISBN 978-1-118-46965-1

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.

Part 1

SYNTHESIS METHODOLOGIES FOR SILICONES

Chapter 1

Room Temperature Vulcanized Silicone Rubber Coatings: Application in High Voltage Substations

Kiriakos Siderakis* and Dionisios Pylarinos

Department of Electrical Engineering, Technological Educational Institute of Crete, Crete, Greece

*Corresponding author: [email protected]

Abstract

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

1.1 Introduction

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].

1.2 Pollution of High Voltage Insulators

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].

1.3 Silicone Coatings for High Voltage Ceramic Insulators

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.

1.4 RTV SIR Coatings Formulation

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.

1.4.1 The Base Polymer

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:

Low levels of surface tension (energy)

Surface hydrophobicity

Hydrophobicity recovery mechanisms

Good dielectric properties

Resistance to thermal stress

Capable of operating within a wide temperature range, with limited changes of its physical and chemical properties

Increased resistance to the action of various environmental oriented aging mechanisms

Chemically inactive to the action of various oxidative substances

Non-toxic and friendly to the environment

Low glass transition temperature

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].

1.4.1.1 Properties of the Silicone Oxygen Bond

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].

1.4.1.2 Low Intermolecular Forces

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.

1.4.1.3 Chain Mobility and Flexibility of PDMS

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.

1.4.2 Polymerization and Crosslinking Processes

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.

1.4.2.1 High Temperature Vulcanization

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)

1.4.2.2 Room Temperature Vulcanization

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.

The second mechanism (two-part RTV) is very specific. The crosslink density can be controlled very accurately and no byproducts are formed. However, two component vulcanization requires a temperature of 60°C, which is not available in the environment (it is also remarkably lower than the 180°C for the HTV rubber). Therefore the commercially available RTV SIR coatings are vulcanized according to the first crosslinking mechanism (one-part RTV). It is worth mentioning that the byproducts formed in this case (usually alcohol) are volatile and no influence to the material performance is observed after application.

(1.6)

(1.7)

1.4.3 Formulation Additives

Various additives are used depending on the desired material improvements. For RTV SIR coatings used in outdoor insulation a typical formulation includes:

1.4.3.1 Solvent

Coating systems are available in the form of a liquid paint. In order to be applied efficiently, a minimum viscosity level is required. This is achieved with the use of an appropriate solvent, which is either included from the manufacture (ready to use) or it is added at the application site [3, 9, 55].

The solvent becomes the transfer medium to the insulator surface and after application it evaporates, initiating the vulcanization process. Therefore no remains are found in the cured material. However, the rate of evaporation influences the development of the curing process and therefore different curing times can be observed for different solvents. Also, roughness measurements [55] have revealed that the surface quality (appearance, smoothness, etc.) is also affected, thus possible effects on the coating performance can be assumed.

In the case of naphtha and 1,1,1 trichloroethane, solvents frequently used, a superior performance has been observed for the coating using naphtha. H. Deng et al. [55] compared rod insulators coated with the same material but different solvents in a salt fog chamber. For the coatings using naphtha the time to failure was found longer. Of course it must be mentioned that due to the different viscosity (the material was applied by dipping), different thicknesses were achieved, 0.79 ± 0.10 mm and 0.50 ± 0.10 mm for naphtha and 1,1,1 trichloroethane, respectively.

However, the use of naphtha is not always possible. Due to its low flash point naphtha is flammable and therefore the possibility of an accident is increased in the case of an energized application. Consequently the application of naphtha is limited only for non-energized insulators. Otherwise, non-flammable solvents such as 1,1,1 trichloroethane must be used.

1.4.3.2 Tracking and Erosion Filler

Tracking and erosion are phenomena observed on the material surface. Tracking is an irreversible degradation referring to the formation of conductive paths, which remain conductive even under dry conditions [56]. Erosion is also an irreversible degradation, nonconducting, which refers to the loss of material [56]. Both can limit the material performance and lifetime, and therefore fillers are added in the formulation in order to improve the system durability. Alumina Trihydrate (ATH) and Silica are the filers usually adopted, and rarely, magnesium oxide [57].

The cause of surface degradation is the electrical activity observed on the material surface during a period of a hydrophobicity loss. Dry band arcing can supply the material volume with energy in the form of heat, which will support the development of chemical and physical changes [58]. The addition of fillers, ATH and Silica, aims to increase the material heat conductivity [59, 60] and further decrease the amount of energy dissipated within the material volume. In addition, in the case of Alumina Trihydrate, when a material temperature of 220°C is exceeded, water is released in the form of vapor (Reaction 1.8) enhancing the filer action [46].

(1.8)

The filler efficiency and the corresponding material endurance depend on many parameters. The filer type, the amount used, the particle size selected and the degree of bonding to the polymer matrix are some of them to be considered. In the case of ATH, which is the filler selected in many formulations, much research has been done in the direction of the optimum formulation that will conclude in an increased material performance and lifetime.

Kim et al. [61] investigated the influence of the ATH filler level in the coating formulation. The filler addition resulted in a decrease of the silicone fluid diffusion to the surface and, therefore, the hydrophobicity recovery process was decelerated. On the other hand, it was shown that after prolonged dry band arcing, the coating with increased filler content performed better. The results from Gorur et al. [62] also pointed out that increased ATH levels provide higher protection levels, however, coatings with lower filler levels showed faster hydrophobicity recovery. Therefore, depending on the field conditions, an optimum filler level can be set.

An optimum particle size for the ATH is also suggested. Comparisons performed between coatings of the same concentration and different particle sizes, indicated an optimum particle size of 4.5 μm [63]. In the case of smaller size, dispersion problems were observed and, on the other hand, a larger size increases the surface roughness and slows down the LMW migration.

If Silica is selected instead of ATH, the same protection levels can be achieved provided the appropriate formulation. Studies of Meyer et al. [59, 60, 64], Omranipour et al. [65, 66] and Siderakis et al. [12, 67] indicate that at higher concentrations (>50%) both fillers yield similar performance. However, in the case of lower amounts, the ATH filler provides superior performance due to the additional action of water release [12, 67].

It is worth mentioning, however, that even in the case of optimum filler content the possibility of tracking still exists. For the ATH/PDMS system studies performed by Kumagai and Yoshimura, applying an inclined plane test revealed that tracking is also possible at high temperatures (> 1600°C) [68, 69]. The mechanism is different since the amounts of carbon formed on the surface are limited and require the participation of gas byproducts. However, a degradation of the material performance is observed [69].

1.4.3.3 Adhesion Promoter

Adhesion to the ceramic substrate is a very important property for a coating. In the case of a poor application there is a possiblity of failure occurring due to the stress applied from the wind. Additionally the necessity of pressurized water washing must be considered, especially in applications where thick contamination such as cement can accumulate on the coating surface.

Adhesion problems were experienced with the early coating systems applied [3, 9, 20]. The RTV SIR coatings available today have incorporated appropriate fillers that can ensure the necessary adhesion levels, providing that the ceramic surface has been thoroughly cleaned before the coating application.

1.4.3.4 Coating Color

A coloring pigment is used to provide the coating color, usually white, blue, grey or yellow. The selected color does not influence the material performance; however, it is a useful tool in order to evaluate the application coverage and the achieved thickness.

It is important to notice that depending on the selected color, different impressions about the surface condition can be induced. However, in this case no correlation of the contaminants build up with the coating performance should be done, as in the case of uncoated insulators.

1.4.3.5 Other Fillers

In addition to the abovementioned, a number of other fillers are also included in the material formulation. For example, crosslinking agents and condensation catalysts are used in order to enhance the material curing process.

1.5 Hydrophobicity in RTV SIR

Hydrophobicity is a surface property correlated to the material internal structure [49]. It is the result of an energy excess observed due to an imbalance of the intermolecular forces that develop. Molecules that are near the surface are subjected to attraction forces from the molecules inside the material volume, which are not symmetrical as for the molecules that are away from the surface. This imbalance is described by the surface tension which corresponds to the amount of energy per unit surface (Joule/m2) or the amount of force per unit length (N/m).

When a droplet is found on a surface two contradictory mechanisms are observed. The attraction forces between the droplet molecules, which action is in favor of the droplet surface decrease (form of minimum energy) and the surface tension in the direction of the droplet dispersion. The equilibrium usually achieved is described by the Young equation and especially the value of the contact angle θ.

Silicone rubber is a hydrophobic material, demonstrating values of surface tension remarkably lower than the corresponding values of porcelain and glass but also lower in comparison to other polymeric materials, as can be seen in Table 1.2. This is the result of the weak intermolecular attractions observed between the silicone rubber molecules, although the molecule structure (backbone) is remarkably stronger (silicon-oxygen bond) in comparison to the other polymeric materials used for outdoor insulation. In addition, the flexibility of the SIR backbone permits the orientation of the side methyl groups towards the surface, pushing the strong silicon oxygen bond away.

Hydrophobicity is the property that will provide the improved performance of a SIR coating in the case of pollution by interrupting the development of the phenomenon, since surface wetting is not achieved. However, during service, periods of hydrophobicity loss possibly appear [70]. The accumulation of contaminants, for example, in the usual case will result in a hydrophilic layer that will hide the original hydrophobic behavior. Additionally, degradation due to various stresses (electrical and environmental) will also reduce the material capability to withstand wetting. Fortunately SIR has the capability to recover hydrophobicity and restore the initial surface behavior. There are two principal mechanisms responsible for the hydrophobicity recovery [71], the reorientation of the side hydrophilic groups away from the surface, and the migration of low molecular weight PDMS molecules from the material volume to the contaminants’ layer. In Figure 1.5, surface hydrophobicity in a clean and in a contaminated RTV SIR coated surface are illustrated.

Figure 1.5 Surface hydrophobicity in a clean (a) and in a contaminated (b) RTV SIR coated surface.

1.5.1 Reorientation Mechanism

The inherent hydrophobic behavior of SIR and the remarkably low values of surface tension are strongly correlated with the presence of the side methyl groups in the PDMS molecule and the low intermolecular forces that exist between them [73, 74]. However, under service conditions, the substitution of these methyl groups is possible as a result of various mechanisms [58]. A typical example is shown in Reaction 1.9. The substitution of the methyl groups by hydroxyl alters the initial hydrophobic behavior to hydrophilic. This change may result to a rapid loss of hydrophobicity due to the strong hydrogen bonding forces that develop between the chains [73–75].

However, the PDMS molecules demonstrate a remarkable flexibility when compared to hydrocarbon or fluorocarbon polymers. The rotation about siloxane bonds is virtually free, since the energy necessary is almost zero. Therefore the rotation of the side hydrophilic groups to the material interior is possible, allowing the remaining methyl groups to approach the surface and decrease the demonstrated surface tension [71–75].

(1.9)

1.5.2 Migration of LMW Molecules

The migration of the low molecular weight PDMS (6 to 15 siloxane monomers) molecules is considered the primary mechanism responsible for the recovery of hydrophobicity [71, 72, 75, 77]. These molecules are formed during the polymerization process. Due to their small size they are quite flexible and are not included (connected) in the crosslinked network. Consequently, considering the weak intermolecular forces observed, they are free to move within the material volume and diffuse to the surface. When a contamination layer is formed they can penetrate into the layer and change the initial hydrophilic behavior to hydrophobic. Thus, although the surface is covered by a hydrophilic film, eventually a hydrophobic behavior is achieved.

The speed of recovery and the endurance of the migration mechanism are affected from the amount of LMW molecules available. The initial population and type (linear or cyclic) depends on the polymerization process and the vulcanization that follows. Further, during service a possible loss of the LMW molecules can be assumed since they are exposed on the surface. Consequently, a probable depletion of the LMW reservoir, which corresponds to the end of the material lifetime, becomes the primary user concern, and thus the parameters that influence the diffusion process have to be considered.

1.5.3 Silicone Fluid – Material Thickness

The amount of the LMW PDMS molecules can be increased either by the addition of silicone fluid or, in the case of a coating, by increasing the material used, thus the coating thickness.

More silicone fluid corresponds to a larger reservoir of LMW molecules, considering that the fraction of the material mass, characterized by low molecular weights, depends on the production procedure (polymerization and crosslinking). It is worth mentioning that the recovery of hydrophobicity is possible by a part (< 20%) of the LMW molecules initially available [76]. Consequently this will result in a faster hydrophobicity recovery and a longer material service time [77, 78].

However, laboratory investigations revealed that the addition of silicone fluid has an adverse effect on the material performance, since the heat stress applied is enhanced [55]. This occurs due to the decrease of the material heat conductivity, since the amount of filler (tracking and erosion) in the material mass is reduced. Consequently more energy, available from the surface electrical activity, is dissipated within the material volume and therefore aging is accelerated.

The same effect is observed in the case of a thickness increase [55, 78], since the heat resistance formed between the surface activity and the substrate becomes greater. Of course, a minimum thickness is required in order to minimize the possibility of exposing the substrate due to erosion. Thus, an optimum thickness of 0.38 mm is suggested [55].

1.5.4 Material Formulation

The molecules migration can be affected by the presence of fillers in the material formulation. Especially in the case of ATH, laboratory studies have revealed that increased filler levels correspond to decreased migration rates, mainly for two reasons [79, 80]. Firstly, the ATH particles attract the PDMS molecules and therefore the movement to the surface is more difficult. This action becomes stronger for smaller ATH particle size [80]. Secondly, for a certain formulation the addition of filler corresponds to lower amounts of silicone fluid and therefore LMW molecules.

1.5.5 Influence of the Crosslinking Degree

The LMW molecules do not participate in the crosslinked network due to their increased mobility. Therefore crosslinking does not seem to influence their number. However, there is a possible influence to the molecules diffusion to the surface.

The electrical activity observed, either during periods of hydrophobicity loss (dry band arcing) or even if the surface is hydrophobic (corona along the water droplets), supplies energy to the material that can support oxidation Reactions 1.10 following 1.9, which finally conclude in a silica layer near the surface [72, 81, 82].

(1.10)

This layer impedes the LMW molecules diffusion and therefore the hydrophobicity recovery process. Additionally it must be mentioned that a possible increase of the surface tension is probable to occur due to the increase of the siloxane bonds near the surface.

1.5.6 Influence of the Contamination Nature

The physical and chemical properties of the contamination layer formed on the surface affect the efficiency of the migration mechanism. The LMW molecules have to penetrate into the layer and, by changing the composition, transform the initial hydrophilic behavior to hydrophobic. In fact, since the substances usually found on the insulator surface are characterized by higher surface energy, the LMW movement occurs more in the form of absorption from the contamination and less as a penetration [83]. This occurs in the direction of energy minimization and therefore the larger the difference of the surface energy (LMW and contaminants), the more intense is the absorption observed. Also the physical structure (for example, a porous material) has a role in the migration development [83, 85].

Additionally, the layer thickness also affects the recovery time. Increased amounts of contamination correspond to slower hydrophobicity recovery [86].

1.5.7 Amount of LMW

The LMW molecules that have reached the surface are exposed to the environment. For example, cleaning mechanisms such as rain and wind can be assumed to be capable of removing the LMW molecules together with the surface contamination. Thus, the influence to the LMW molecules surface population must be examined.

Wind: The possible influence of wind to the amount of LMW molecules on the surface has been investigated by Deng and Hackam [87]. The RTV SIR specimens were subjected to an artificial wind with a speed ranging from 7.2 km/h to 7.7 km/h for a period of 20 months. No effect to the LMW content was found.

Surface wetting: Various wetting mechanisms are observed in the environment (rain, fog, mist, etc.). The degree of influence depends on the cleaning capability, thus the amount of water deposited on the surface. For example, a laboratory comparison between water immersion and condensation wetting [88] revealed a decrease of the mass for the immersed samples, thus a decrease of the LMW molecules amount. This change is enhanced when increasing the immersion time in water. Deng et al. measured a 16.6% mass reduction after 12 months of water immersion in a saline solution [87].

However, the stress applied in the case of water immersion is remarkably higher than that observed in the environment, especially by rain, which appears to be the most alarming wetting mechanism, since it provides the largest amount of water among the environmental mechanisms. Consequently, the influence of the environmental wetting to the LMW molecules population and further hydrophobicity is insignificant.

Temperature: The LMW molecules diffusion is a temperature-dependant procedure. Increased ambient temperature will accelerate the molecules movement to the surface and therefore reduce the necessary recovery time [88–90].

UV Radiation: The PDMS backbone cannot be affected by the sun originated UV radiation, since the Si-O bond energy is higher than the energy supplied through UV radiation. However, a reduction of the LMW molecules number is possible to occur. Chemical changes regarding the side groups can be observed due to the UV influence [91], resulting in the attachment of the LMW molecules to larger chains [57]. Consequently, UV radiation can reduce the amount of LMW molecules and therefore hydrophobicity [88, 91].

1.5.8 Influence of the Electrical Surface Activity