Novel Analytical Techniques for the Assessment of Degradation of Silicone
Elastomers in High Voltage Applications
by
Robert D Sovar BAppSc (QUT)
Presented to the School of Physical and Chemical Sciences, Faculty of Science in
partial fulfilment of the requirements for the degree :
Master of Applied Science by Research and Thesis
Queensland University of Technology
July 2004
ABSTRACT Over the last 20 years “composite” insulators have been increasingly used in high
voltage applications as an alternative traditional materials. More recently,
polydimethylsiloxane (PDMS) have been used as weather sheds on these composite
insulators. The main attraction with PDMS is that the surface hydrophobicity can be
recovered following pollution or surface discharges. Among the possible
mechanisms for recovery the most likely is the migration of low molecular weight
silicone oil (LMWS) from the bulk to the surface encapsulating pollutant particles.
Although it is widely recognised that the migration of LMWS is the cause of this
recovery of hydrophobicity, the mechanism of what actually occurs is not well
understood. It is also not known for how long this process will continue.
The main objective of this study program was to gain improved understanding of the
surface hydrophobic recovery process that is unique to polydimethlysiloxane high-
voltage insulators. Fundamental knowledge of this mechanism has been increased
through the development of the Contact Angle DRIFT Electrostatic Deposition
(CADED) novel analytical technique. This technique enabled study of the
degradation of silicone elastomers subjected to high voltage environments by closely
following LMWS migration from the bulk material to the surface and linking it to the
contact angle measurements. The migration rate data showed that the aged material
recovered faster that the virgin material. Differences in the rate and maximum
surface levels of silicone were seen between materials from different manufacturers.
This has significant implications for the life-time of these materials
A model system has been developed to examine LMWS diffusion through the bulk
material and into the interface of surface and pollutant. This was achieved by
examining theoretical and empirically derived equations and using existing
experimental data to better understand the mechanism of recovery. This diffusion
was Fickian in the initial stages of recovery.
X-ray photoelectron spectroscopy (XPS) and contact angle measurements were used
to substantiate the degree of degradation in in-field silicone insulators by quantifying
ii
the levels of the major degradation products: silica and silica-like material and
alumina.
iii
TABLE OF CONTENTS 1. INTRODUCTION 1
2. BACKGROUND AND THEORY 4 2.1 The Synthesis, Structure and Properties of Silicone Elastomers
2.1.1 Introduction
2.1.2 Synthesis of Silicone Precursors (Cyclic Monomers)
2.1.3 Polymerisation of Silicones
2.1.4 Polymerisation Mechanisms
2.1.5 Thermodynamics and Kinetics
2.1.6 Vulcanisation of Pre-Polymers to Elastomers
2.1.7 Surface Properties of PDMS
2.1.8 Industrial Applications
2.2 Polydimethylsiloxane for Outdoor High-Voltage Insulation
2.2.1 Description of PDMS as an Outdoor Insulator
2.2.2 PDMS as an Outdoor Insulator: A Review
2.3 Fourier Infra-red Spectroscopy and other Characterisation Techniques
2.3.1 Fourier Infra-red Spectroscopy of Polydimethlysiloxane
2.3.2 FTIR Characterisation Methods
3.0 EXPERIMENTAL METHODOLOGY 28 3.1 Diffuse Reflectance Infra-red Spectroscopy Studies
3.1.1. The DRIFT technique
3.1.2 The DRIFT Electrostatic Deposition Method
3.1.3 Calibration Curves
3.2 Contact Angle Studies
3.2.1 Measurement of Contact Angle
3.3 X-ray Photoelectron Spectroscopy
3.4 Soxhlet Extraction of LMWS
3.5 Source of Silicone Elastomer Samples
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4 THE COMBINED DRIFT/CONTACT ANGLE TECHNIQUE 37 4.1 Introduction
4.2 Validation of the DRIFT Electrostatic Deposition Technique
4.3 Contact Angle Measurements
4.4 The Combined Contact Angle/DRIFT/Electrostatic Deposition Technique
(CADED)
5 THE MIGRATION OF LMWS FROM BULK TO SURFACE:
A DIFFUSION MODEL 49 5.1 Molecular Diffusion Theory
5.1.1 Steady State Solution
5.1.2 Diffusion in Polymers
5.2 Diffusion in Polydimethylsiloxane
6.0 OTHER ANALYTICAL CHARACTERISATION
TECHNIQUES. 59 6.1 X-ray Photoelectron Spectroscopy
6.1.1 Introduction
6.1.2 Calibration
6.1.3 Analysis Results
6.1.4 Conclusions
6.2 Contact Angle Measurements on In-Field Silicone Elastomers
6.2.1 Conclusions
7.0 CONCLUSIONS AND FURTHER WORK 66
8.0 BIBLIOGRAPHY 68
v
ACKNOWLEDGEMENTS
The financial support of the Faculty of Science, Queensland University of
Technology is gratefully acknowledged.
My thanks go to my principal supervisor, Professor Graeme George and associate
supervisor Dr Greg Cash for all their support and advice during this time.
Especially, I thank my loving wife Bernadette for her love, patience and
understanding. You have been an inspiration. I could not have done any of this
without you.
And finally my two sons Matthew and Joseph - you have kept me sane throughout
this whole process. I love you all.
Robert
29 July 2004
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1.0 INTRODUCTION For the last 100 years ceramic and glass have been materials of choice for the
insulation of high voltage transmission lines. These materials can provide long life
and the advantages and limitations are well understood. Over the last 20 years
“composite” insulators have been increasingly used in high voltage applications as an
alternative traditional materials (Blackmore, 1998 and Hillborg & Gedde, 2001).
These insulators have a fibreglass reinforced load-bearing core that is covered by a
housing and weather sheds manufactured from synthetic elastomers. Composite
insulators are relatively lightweight, are better resistant to vandalism and offer better
pollution performance as they tend to have a more hydrophobic surface (i.e. water
repellence), than the hydrophilic glass and ceramic (Looms, 1988 and Gorur &
Orbeck, 1991).
Surface hydrophobicity is an extremely important property in outdoor high voltage
(HV) insulation as it reduces surface flashovers that may potentially result in power
outages. This property (surface hydrophobicity) can be lost through deposition of
hydrophilic pollutants and by degradation. EPDM (ethylene propylene diene
monomers), epoxy resins, thermoplastic elastomers and, more recently, polydimethyl
siloxane (PDMS) have been used as weather sheds on the composite insulators.
EPDM with and without added silicone oil has seen wide use but PDMS (or silicone
rubber) is now becoming more popular among electricity utilities worldwide.
The main attraction with PDMS is that the surface hydrophobicity can be recovered
following pollution or surface discharges. This occurs by the migration of low
molecular weight silicone oil (LMWS) from the bulk to the surface encapsulating
pollutant particles. Although it is widely recognised that the migration of LMWS is
the cause of this recovery of hydrophobicity, the mechanism of what actually occurs
is not well understood. It is also not known for how long this process will continue.
Thirdly, there is also some evidence that the actual chemical composition of the
LMWS (the presence of linear and cyclic silicones and their molecular weight range)
may also influence the rate of recovery of the surface after damage (Cash 2001a).
1
It is the objective of this thesis to gain improved understanding of the recovery
process; increasing fundamental knowledge of this mechanism by developing and
using novel analytical techniques in studying the degradation of silicone elastomers
subjected to high voltage environments. A secondary, but as important objective, is
to develop a characterising methodology that allows in-field monitoring of HV
insulators manufactured from silicone rubber.
A number of existing analytical techniques including Diffuse Reflectance Fourier-
Transform Infra-red spectroscopy (DRIFT), contact angle measurements, and Xray
Photoelectron Spectroscopy (XPS) have been used to examine and characterise the
surface silicones. A model system has been developed to examine LMWS diffusion
through the bulk material and into the interface of surface and pollutant with a view
to better understand the mechanism of recovery. This has been achieved by
developing a combined FTIR/contact angle measurement technique that allows
monitoring of the LMW silicone oil as it diffuses from bulk to surface. X-ray
photoelectron spectroscopy (XPS) and contact angle measurements were utilised to
substantiate the degree of degradation in in-field silicone insulators by focusing on
the production of a silica-like species, SiO3-4 as demonstrated by Alexander et al
(1999). The objective of incorporating these techniques is to evaluate whether a
methodology can be developed that allows the assessment of degradation and hence
performance of silicone insulators in the field.
The thesis is organised as follows:
• Chapter 2 contains a background review on the chemical and physical
properties of PDMS; a description of its use in outdoor high voltage
polymers; and a literature review on the past and current research in
the use of PDMS as outdoor insulators;
• Chapter 3 describes the experimental methods used;
• Chapter 4 describes the development of the combined DRIFT/Contact
angle technique and the results;
2
• Chapter 5 assesses the diffusion model developed describing the
migration of the LMW silicone oil from surface to bulk.
• Chapter 6 assesses the significance of the other analytical techniques
used; and
• Chapter 7 summarises the results obtained and their implications on
furthering the understanding of hydrophobic recovery and the
development of an in-field characterising methodology.
3
2.0 BACKGROUND AND THEORY
In this chapter a background review in the area of the synthesis, structure and
properties of silicone elastomers is presented. The areas covered include synthesis of
silicone precursors and polymerisation and vulcanisation of silicones. This review is
not intended to be extensive but illustrates the breadth of the subject. Secondly, a
description of the application of silicone elastomers as a high-voltage material is also
presented. Finally, a literature review focusing on characterisation techniques on the
past and current work will be discussed.
2.1 The Synthesis, Structure and Properties of Silicone Elastomers
2.1.1 Introduction
Silicone polymers have a wide variety of physical properties and hence have been the
most studied of the semi-inorganic polymers. Consequently, these properties have
led to wide range of applications many of which have great commercial importance
(Mark, 2001). The most intensively studied areas are preparative techniques, end-
linking reactions and characterisation of the resulting polymers in terms of their
structure, flexibility, transition temperature, surface properties and permeability.
Applications of these materials have included modification of ceramics for
improving impact strength, high voltage outdoor electrical insulation, and a variety
of medical applications such as contact lens material, burns treatment and artificial
skin.
The purpose of this review is to highlight the variety of silicone elastomers
(including a special focus on polydimethyl siloxane) and to specifically describe their
properties in context of synthesis and structure, and their many industrial
applications aforementioned. The review will focus on the following areas:
• synthesis of silicone precursors (cyclic monomers);
• polymerisation of silicones (including mechanisms, kinetics and
thermodynamics);
4
• vulcanisation of pre-polymers to elastomers (such as room and high temperature
vulcanization denoted as RTV and HTV respectively);
• surface properties of pre-polymers and elastomers; and
• industrial applications of silicone polymers and elastomers.
2.1.2 Synthesis of Silicone Precursors (Cyclic Monomers)
The synthesis of siloxane polymers stems from a precursor reaction in which
methylchlorosilanes (MCS) are produced from elemental silicon and methyl chloride
(Lewis, 2001). The products from this reaction undergo distillation and are then
hydrolysed and condensed to make the various silicone polymers.
Initially, chemical grade silicon is produced from a carbo-electro reduction of sand
(silicon dioxide) by the “Direct Process” under high voltage and high temperature
>1200˚C. The proceeding MCS reaction takes advantage of the many trace elements
in silica being either non-reactive or essential for the completion of the reaction. For
example whilst iron has no effect at normal levels (0.5 %), aluminium is a vital
promoter and titanium and calcium are also considered important. Copper, tin and
zinc are other elements that are encountered at the ppm level. The equation for the
MCS reaction and its respective yields are shown below:
Cu (3-5%) Zn (400-2000 ppm) Sn (5-30 ppm) Al (500-4000 ppm) Si + MeCl Me2SiCl2 (75-90%); MeSiCl3 (5-10%);
290-305˚C Me3SiCl (1-5%); MeHSiCl2 (0.5-3%); Me2HSiCl (0.1-1%); other low volatile compounds (0.1-0.5) and
residue (0.5-5%)
Elemental silicon is treated in a fluidised bed reactor in the presence of copper, zinc,
tin and other promoters (Ward et al, 1986). Important factors for the MCS reaction
include the rate of methylchlorosilane production, selectivity for dimethyl-
dichlorosilane, silicon utilisation and spent metal loss. Polydimethylsiloxane
(PDMS) is the largest volume polymer product that can be produced from the MCS
reaction. It is made from the hydrolysis of dimethyldichlorosilane hence the
5
importance of its selectivity. The MCS reaction occurs in a solid/gas environment
that produces a light product mixture. When the silicon utilisation is high and the
spent metal loss is low optimum economic performance is achieved.
2.1.3 Polymerisation of Silicones
Siloxane polymer formation from methylchlorosilanes can be achieved by subjecting
the product mixture (from the MCS reaction) to several distillation and isolation
steps. For instance the monomers are separated from the residue stream in the
product mixture by distillation. Some monomers can also be recovered from the
residue, which contain siloxanes and disilanes, by various redistribution reactions
(Ritzer, 1994). Individual monomers are separated by distillation, which highlights
difficulties between separating dimethyldichlorosilane and methlytrichlorosilane.
Using dimethyldichlorosilane as an example, the following equation shows the
hydrolysis and condensation to form linear and cyclic polysiloxanes: H2O Me2SiCl2 HOMe-Si-O-SiMe2-(Me2-Si-O)3-Si-MeOH + D4 + H2O (2)
Distillation is used to isolate the main cyclic product from the above reaction, which
is known as octamethylcyclotetrasiloxane (D4).
Another important material is hexamethyldisiloxane which forms from hydrolysis
and condensation of trimethylmonochlorosilane (Me3SiCl):
H2O
Me3SiCl Me3Si-O-SiMe3 + HCl (3)
2.1.4 Polymerisation Mechanisms
Polydimethylsiloxanes (PDMS) as mentioned earlier are important commercial
polymers with a wide range of applications such as being used as adhesives,
surfactants, lubricants, sealants, and release agents etc. To provide an insight into the
mechanisms behind silicone reactions, the preparation of linear and cyclic PDMS is
studied in more depth.
6
The synthesis of linear PDMS by either acid or base catalysed ring opening
polymerisation of D4 can be achieved to virtually any degree of polymerisation with
the necessary concentration of hexamethyldisiloxane added to cap the reactive end
groups. This results in trimethylsilyl end-terminated chains (Rich et al, 1997).
Cyclic PDMS is also synthesised by acid or base catalysed polymerisation of D4 but
is carried under high dilution and without the use of hexamethyldisiloxane as a chain
stopper. In the methods described below a ring/chain equilibrium is established
forming both linear and cyclic PDMS. Upon equilibrium being reached the catalyst
is neutralised and the addition of end groups terminate the linear PDMS allowing the
cyclics to be separated out.
The method of preparing cyclic PDMS as developed by Brown and Slusarczuk
involves an anionic or base catalysed ring-chain equilibrium (Dragger & Semyln,
2000). The mechanism for the anionic polymerisation of D4 is shown below in
Figure 2.1.
Figure 2.1 : Brown and Slusarczuk reaction mechanism. The * denotes a second
siloxane species in the reaction (after Dagger and Semelyn, 2000).
Potassium hydroxide is used as the base in the presence of 2-methoxyethylether
(diglyme). The potassium ions are complexed by the diglyme and promote the
formation of hydroxide ions. The D4 starting material must be dried and distilled
from calcium hydride before use. The reaction is refluxed at 125°C in sodium dried
toluene under a nitrogen atmosphere for 2 weeks. Dry reaction conditions are vital
as the presence of any water terminates the reaction. Once equilibrium is achieved,
7
the reaction is quenched by the addition of glacial acetic acid. The cyclic species can
than be extracted from the reaction mixture.
2.1.5 Thermodynamics and Kinetics
A thermodynamic equilibrium between ring and chain molecules results due to the
acid or base catalysed polymerisation of D4:
-My- = -My-x- + Mx
where -My- and -My-x- represent linear species, Mx represents an x-meric ring species
and M represents one monomer unit.
The ring-chain equilibrium in PDMS was initially investigated to obtain data
attributed to molecular size distribution of ring and chain species in solution and the
undiluted state (Scott, 1946). Kinetic studies of the base catalysed polymerisation of
D4 confirmed that the active species was the silanoate ion (Si-O-K+) (Grubb, 1955).
Brown and Sluzarczuk advanced the understanding of ring-chain equilibria obtaining
evidence for the presence of macrocyclic species in the base catalysed reaction. An
effort to characterise their distribution was made and they demonstrated the existence
of a continuous macrocyclic population extending to D400. Further investigations
have led to the attainment of accurate molar cyclisation equilibrium constants from
the concentrations of the cyclic species. This was achieved over a large range of ring
sizes through calculations based on the cyclisation theory of Jacobsen and
Stockmayer (as quoted by Dagger & Semlyen).
The cyclic populations in ring-chain equilibria can be characterised in terms of molar
cyclisation equilibrium constants Kx for the x-meric ring molecules Mx as follows:
Kx = [-My-x-][ Mx]/[ -My-]
The most probable distribution of chain lengths in the linear part of the equilibria is
characterised by:
8
Kx = [ Mx]/px
where p is the extent of the reaction of functional groups in the linear polymer. As p
is usually close to 1, Kx values are equivalent to the molar concentrations of the
cyclic species apart from the largest ones.
Using Gaussian statistics, the fraction of conformations which are suitable for ring
formation can be calculated (Kuhn, 1934). This implies that a chain undergoing
cyclisation must be long enough and flexible enough to obey the Gaussian
expression:
Wx(r) = (3/2π<rx2>)3/2exp(-3r2/2<rx
2>)
where Wx(r) is the density of the end-to-end vectors (r), and where<rx2> is the mean-
square end-to-end distance of an x-meric chain.
In the Jacobson and Stockmayer cyclisation theory, the molar equilibrium constants
are given by:
Kx = (3/2π<rx2>)3/2 (1/NAσx)
where NA is the Avogadro number and σx is a symmetry number that represents the
number of skeletal bonds of the cyclic that can be opened up by the catalyst.
Thus, the theory gives a simple theoretical expression for the molar cyclisation
equilibrium constants for macrocyclics in ring-chain equilibria. A comparison
between the theoretical and experimental data for the macrocyclisation equilibrium
constants for PDMS (Flory & Semlyen, 1966) is shown in Figure 2.2.
9
Figure 2.2 : Comparison of Molar cyclisation equilibrium constants Kx for cyclics in a
ring-chain equilibrium in toluene solution (dotted line) compared with theoretical values (solid line) (after Dagger and Semyln, 2000)
The ring-opening polymerisation of a trimer or tetramer is one of the most common
preparative techniques to create the silicone polymer of the type [-SiRR’O-]x where
R and R’ can be an alkyl or aryl group and x is the degree of polymerisation. The
product of this reaction is a macrocyclic species to the extent of 10-15 %. Before use
in commercial application, the lower molecular weight species (LMWS) are
generally stripped from the polymer. However, their presence is particularly of
interest in terms of property characterization on a number of fronts. Firstly, the
extent to which they occur can be used as a measure of chain flexibility. Secondly,
the LMWS fraction can be used to test theoretical predictions of the differences
between otherwise identical linear and cyclic oligomers.
2.1.6 Vulcanisation of Pre-Polymers to Elastomers
To produce a suitable material for use in high-voltage applications, the manufactured
pre-polymer must undergo a transformation from a soft, tacky elastomeric material to
a stiff cross-linked elastomer that is temperature stable. Both high and room
temperature vulcanisation techniques are used to achieve this outcome.
10
In high temperature vulcanisation (HTV), cross-linking of the silicone rubber
polymer chains occur through decomposition of organic peroxides above 100°C
(Hillborg, 2001). Depending on the type of peroxide used, decomposition to free
radicals occurs either by the radicals reacting with unsaturated bonds or by
abstraction of hydrogen atoms. Recombination of free radicals then follows allowing
cross-links between the siloxane chains to be formed. Volatile decomposition
products within the HTV silicone elastomer are generally removed by a post-curing
stage involving storage at elevated temperatures.
In room temperature vulcanisation (RTV), two different cross-linking methods are
generally used for outdoor insulation applications. Firstly, a condensation reaction of
silanol groups takes place to form siloxane bonds with the liberation of water:
≡SiOH + ≡SiOH H2O ≡SiOSi≡ + H2O
The reaction is an equilibrium process, involves water and is either catalysed by acid
or base. Secondly, a hydrosilylation reaction involving the addition of a silicon
hydrogen (Si-H) to an unsaturated carbon bond (usually a vinyl group, -CH=CH2)
can be achieved by catalysis by a noble metal (i.e. a platinum complex):
≡SiH + H2C=CHSi≡ “Pt” ≡SiCH2CH2Si≡
This reaction is very specific and the cross-link density can be controlled very
accurately by this method.
2.1.7 Surface Properties of PDMS
PDMS consists of an inorganic backbone of alternating silicon and oxygen atoms.
The repeating polymer unit consists of two methyl groups directly attached to the
silicon atom as shown in Figure 2.3 below:
11
Figure 2.3 : Structure of silicones
The low surface free energy, which accounts for its inherent hydrophobicity, has
been reported between 16-21 mN m-1 depending on molar mass (Hillborg, 2001 &
Owen, 1990).
To understand the unique surface properties attributed to polydimethylsiloxane, it is
necessary to examine the main structural characteristics of the polymer (Hillborg &
Gedde, 2001); these are:
• The unique flexibility of the siloxane backbone (Si-O-Si: 143°);
• The low intermolecular force between methyl groups;
• The high strength of the siloxane bond; and
• The partial ionic nature of the siloxane bond.
PDMS contains the most flexible chain molecules of the silicones in terms of both
dynamic flexibility and equilibrium flexibility (Mark, 2000). Dynamic flexibility
implies the ability of molecules to change its spatial arrangements by rotations
around its skeletal bonds. Thus, PDMS can be cooled much further than others
before its chains lose their flexibility and become glassy. Molecules containing this
kind of dynamic flexibility generally have very low glass transition temperatures, Tg,
e.g. the Tg for PDMS is -128°C (Lide, 1991); one of the lowest of the industrial
polymers. Another consequence of this flexibility is reflected in a very high
permeability and diffusitivity to gases. The oxygen permeability of PDMS at room
temperature has been reported to be 21 times greater than natural rubber and 170
times greater than for low-density polyethylene (Hillborg, 2001 & Pauly, 1989).
12
The polar nature of the siloxane bond is due to the magnitude of the Pauling
electronegativity difference of 1.7 between silicon and oxygen (Noll, 1968). The
resulting 40-50% polar character of this bond makes PDMS very susceptible to
hydrolysis particularly in an acidic or basic environment. The silicon atom, being
positively polarised, is electron withdrawing thus polarising the methyl group
making it less susceptible to radical attack. Consequently, the methyl groups in
PDMS have a high thermal and oxidative stability when compared with a
hydrocarbon polymer such as polypropylene (Hillborg, 2001 & Owen 1990). The
siloxane bond also exhibits a high dissociation energy of 445 kJ/mol giving further
evidence of high thermal stability (Beezer, 1966).
2.1.8 Industrial Applications
The major industrial application of PDMS in the context of this work is in high
voltage insulation and this is outlined in 2.2 below.
2.3 Polydimethylsiloxane for Outdoor High-Voltage Insulation
2.3.1 Description of PDMS as an Outdoor Insulator
For some years HTV silicones have been widely used as a replacement for ceramic
and glass insulators due to their unique properties such as thermal and oxidative
stability, better resistance to vandalism, light weight and preservation of a
hydrophobic surface during extended environmental exposure (Hall, 1993 &
Reynders et al, 1999). In composite or silicone rubber (SiR) insulators, these
silicones are used as housing material on a load-bearing core which is made of a
fibreglass rod. An example is shown in the Figure 2.4 below. The fibreglass rod
(manufactured from E glass and with a polyester or epoxy resin matrix) is used
because of its mechanical strength and high dielectric constant under dry conditions
and the silicone housing is used for its high dielectric constant under wet conditions
(Gorur, 2002 – Symposium for Outdoor Insulators). In addition to protecting the
core from environmental conditions, the main function of the silicone housing is
minimise leakage currents between the high-voltage end and the ground. An
example of an installed long-rod insulator is shown in Figure 2.5:
13
Figure 2.4 : Schematic illustration of a composite insulator
Figure 2.5 : A long-rod insulator installed on a 500kV line. A corona ring is fitted at the “live” end.
2.2.2 PDMS as an Outdoor Insulator: A Review
As these composite insulators are made from polymers, surface current leakage can
lead to “flashovers” and these can damage or even puncture shed and sheath
14
surfaces. This ultimately leads to moisture ingress as a failure mode. Previous
investigations by Cash (2000) with high voltage (HV) insulators have examined
those constructed with ethylene propylene diene rubbers (EPDM) as weather-sheds
and sheaths. This research identified several factors that could be used to determine
end-of-life for these materials (Cash, 2000). The high-temperature-vulcanized (HTV)
and filled silicones used in the current insulators are also elastomers with a low
degree of cross-linking but there the similarity to EPDM ends. Silicones or siloxanes
differ greatly in their chemistry, surface properties and the way they degrade.
Flashover (and energetic surface discharges) are considered to be more probable
when the rubber has lost its hydrophobicity and a continuous film of water may form
along the insulator length. The loss of hydrophobicity from silicone rubber when
used in high voltage applications has been considered to arise either from
environmental degradation (heat and UV radiation) or corona discharge events
(Hillborg, 2001) around the insulator where the electric field is highest (Phillips,
1999). The effect of corona, thermal and UV exposure on a silicone rubber is to
produce a layer of degradation products on the surface.
During in-field service, degradation due to environmental stress and electrical and
thermal activities may induce changes to electrical and mechanical properties which
can lead to catastrophic and unpredicted failure (Gorur, 1991). In extreme cases,
degradation involving failure of the fibreglass core due to stress corrosion following
the ingress of water has been observed in EPDM clad insulators (Cash, 2000). This
has arisen from the failure of the outer elastomeric layer or shed material to maintain
its integrity. This may arise from cracking of the elastomer due to environmental
degradation or puncture during an electrical event such as flashover. Whilst visual
degradation of the elastomer may be associated with the end of the useful life of the
insulator, this is not always so. Thus, it is highly desirable to be able to monitor the
extent of degradation of in-service SiR insulators and to be able to predict remaining
service life prior to the onset of the catastrophic failure regime.
It has been found that the particular mechanism of degradation of silicone rubber
depends upon the prevailing environmental conditions (Reynders et al, 1999).
15
Generally, the degradation products belong to two categories: silica-like material and
low molecular weight volatile products. Silica-like refers to a structure where the
silicon atom is bonded to more than 2 oxygen atoms. There is good evidence that
silica-like degradation products are produced on the surface of the rubber during
electrical activities such as corona, dry-band arcing, or oxidative plasma (Toth et al,
1994 & Kim et al, 1990). Volatile products are produced during thermal degradation
(Hillborg, 2001 and Grassie, 1978). Hydrophobic recovery depends upon the relative
magnitude of the migration of low molecular weight silicone (LMWS) species to
cover the hydrophilic surface compared to the rate of formation of the hydrophilic
(silica-like) species.
As introduced in Chapter 1, the surface hydrophobicity of SiR is due to its low
surface energy and is a very important property in outdoor HV insulation reducing
the possibility of surface flashover and subsequent power outages. However, the
hydrophobicity may be lost due to degradation or by deposition of a hydrophilic
pollution layer (Reynders et al, 1999 & Gorur, 1991). In in-field applications,
especially in heavily contaminated areas, the latter factor is more pronounced.
Proposed mechanisms of surface hydrophobic recovery of SiR has been postulated
by many researchers but have been summarised by Owen et al (1988):
• Reorientation of polar groups at the surface into the bulk;
• External contamination of the surface;
• Condensation of silanol groups at the surface;
• Loss of volatile oxygen-rich species to the atmosphere; and
• Migration of LMWS from bulk to surface.
PDMS demonstrates large segmental flexibility which enables the fast reorientation
of unoxidised and lightly oxidised material. However, extensively oxidised SiR with
a glassy SiOx surface layer shows very little hysteresis effect, i.e. only small
differences between advancing and receding contact angles (Hillborg and Gedde,
2001). Due to their glassy environment, the polar groups in SiR have very little
possibility of reorientation. It is believed that this mechanism is purely a local
process and cannot fully explain the recovery of oxidized SiR (Morra et. al, 1990).
16
Laboratory experiments have conclusively shown that external contamination of the
surface is not responsible for hydrophobic recovery in SiR. Hydrophobic recovery
occurred at the same rate regardless of a clean room environment or under normal
conditions (Owen et al, 1988). It has also been proposed (Owen, 1990 & Morra et
al, 1990) that silanol groups present in the surface reacted via a condensation
mechanism with the liberation of water. Despite wettability changes caused by
silanol groups forming during water exposure and corona/plasma treatment (Hillborg
and Gedde, 2001), the Si-O-Si cross-links are polar in nature which would not
increase hydrophobicity believed that it less likely to affect the recovery process.
Loss of volatile species would surely be a factor in high-vacuum conditions, but not
under atmospheric pressure (Hillborg and Gedde, 2001).
It has been widely accepted that the low molecular weight silicones in the bulk
polymer migrate to the surface and encapsulate the contaminants, so that the
hydrophobicity of the SiR surface is retained (Reynders et al, 1999; Gorur, 1992;
Toth et al, 1994 & Kim et al, 1990). This unique property of SiR distinguishes it
from ceramics, glass and other polymers used in HV insulation. An explanation for
this behaviour is that the deposition of polar contaminants onto the surface of a
PDMS rubber results in a change of surface energy which disturbs the normal
equilibrium between PDMS and LMWS. This causes the migration of some LMWS
from the bulk to the surface. It has been suggested (Dr R Schue, private
communication) that the polymerisation process that forms PDMS is an equilibrium
reaction and the final product, PDMS, is in equilibrium with the LMWS. The loss of
some LMWS to the surface causes some “unzipping” of the PDMS to produce more
LMWS and restore the equilibrium. It is believed that a study of the diffusion rate of
the LMWS and the factors which affect it including bulk LMWS concentration,
rubber type, aging, thickness of the contaminant layer and temperature, are vital
components into furthering understanding of surface hydrophobic recovery
processes.
17
2.3 Fourier Infra-red Spectroscopy and other Characterisation
Techniques
There are many analytical and chemical techniques that can be applied either
singularly or as a combination to achieve characterisation of silicone polymers.
Depending on the application techniques such as Fourier transform-infrared
spectroscopy (FTIR), 29Si nuclear magnetic resonance (29Si NMR), X-ray
photoelectron spectroscopy (XPS) and gel permeation chromatography (GPC)
amongst others, are commonly used in characterisation work. In this section,
particular emphasis is given to Fourier transform-infrared spectroscopy with
background on the fundamentals of vibrational spectroscopy, as it applies to
silicones, presented. Equally importantly, a review of these characterisation
techniques in the current literature is also given with emphasis on the methods used
in this study.
2.3.1 Fourier Infra-red Spectroscopy of Polydimethlysiloxane
Siloxane compounds show characteristic absorption frequencies in IR spectrum that
enable ready identification. This is due to the nature of the silicon atom; it is large
and heavy, acting as a vibrational buffer between the lighter C-H portions of the
molecule (Lipp and Smith, 1991). It is also important to note the selection rules that
govern the presence or absence of bands in the FTIR spectra. In FTIR, the incident
radiation that has the same frequency as the vibrational molecule is absorbed
provided the dipole moment changes during the vibration. The intensity of the
absorption depends on the rate of change of the dipole moment with the normal
coordinate for the vibration (dμ/dQ)2, where μ is the dipole moment, and Q is the
normal coordinate for the vibration (Griffith and de Haseth, 1986). Therefore
symmetrical vibrations do not show any IR absorption as opposed to the
corresponding antisymmetrical vibrations which are prominent.
For instance the Si-O-Si anti symmetric stretch near 1050 cm-1 has a large dipole
moment and consequently a strong absorption in the infra-red spectrum (Lipp and
Smith, 1991):
18
Figure 2.6 : Si-O-Si antisymmetric stretch (Lipp and Smith, 1991)
However, the corresponding symmetric stretch has a weak dipole moment change
and thus only a weak absorption appears in the IR spectrum:
Figure 2.7 : Si-O-Si symmetric stretch (Lipp and Smith, 1991)
2.3.1.1 Interpretation of Spectra
The infrared absorptions of the bonds containing silicon are quite strong. Under
certain conditions, the position of the Si-R vibrations is affected by other substituents
on the silicon atom. For instance, the characteristic absorption of Si-CH3 at 800 cm-1
is always absent in the substituted CH3-Si-H group. Instead two strong absorption
bands appear at about 893 and 758 cm-1.
Similar displacements of the Si-CH3 vibration in the long-wave region are found
when Si-OC2H5 bonds are present also. In general, the spectra of siloxanes are
relatively simple and easily can be distinguished. Thus substituent molecular groups
such as methyl-, methylphenyl-, methyl-H-, and methylalkoxysiloxanes can be
identified readily.
The determination of molecular size and siloxane functionalities, however, is a much
more difficult process. The siloxane bond is characterised by an absorption in the
region of 1100 to 1000 cm-1. Only in low-molecular weight siloxanes is this
absorption sharp: that is those with a small number of siloxane units. For instance,
hexamethyldisiloxane (Fig.2.8(a).) has a fairly sharp peak at about 1050 cm-1
whereas in cyclic and linear polysiloxanes, (Fig.2.8 (b) and (c)), this same peak is
19
broadened as the number of Si atoms increase above 4. Linear polysiloxanes with
approximately 10 Si atoms already contain an absorption band whose nature no
longer changes when the chain is extended (Noll, 1968).
Figure 2.8 : Infrared spectra of three liquid linear polymethylsiloxanes MDnM: (a)
hexamethyldisiloxane M2; (b) α, ω-bis-(trimethylsiloxy)-polydimethylsiloxane with
n=10; (c) α, ω-bis-(trimethylsiloxy)-polydimethylsiloxane with n=100 (after Noll, 1968).
In heterocyclics, the Si-O-Si grouping is quite sharp and absorbs at higher
wavelengths than in normal siloxanes (980 cm-1). This is shown in figure 2.9 with
six-membered rings of the type.
O
(CH3)2Si Si(CH3)2
H2C CH2
X with X = O, NR, S,
CHR, CR’R’’ Figure 2.9 : Heterocyclic siloxane with substituted groups (after Noll, 1968)
20
The Si-R absorption can be used to deduce the functionality of the siloxane unit only
to a certain extent, whereas mono- and difunctional groups can be more easily
recognised in this way than in trifunctional groups. The Si-H stretching vibration is a
clear indicator of whether the hydrogen is part of a mono-, di- or trifunctional
siloxane unit depending on its relative position in the IR spectrum.
The CH3 deformation mode at 1262 cm-1 is considered to be the single most
characteristic band in the organosilicon infra-red spectrum (Colthup, 1975). This
band is quite sharp and strong and is accompanied by other absorptions around 800-
900 cm-1. SiMe absorbs near 775; SiMe2 near 805; and SiMe3 at 760 and 845 cm-1.
The same pattern occurs if one or more of the methyl groups are replaced by an alkyl
group. The CH3 stretching absorption between 2910 and 2970 cm-1 is weak in the IR
region and is subject to interference by aliphatic materials – making them not
analytically useful. However, in the Raman spectrum the 2910 cm-1 CH3 symmetric
stretch band is very strong and polarised. The Raman overtone of the symmetric
deformation near 2500 cm-1 is also distinctive for thick samples.
2.3.1.2 Sampling Techniques: Gases
Sampling techniques for FTIR for siloxanes are essentially the same as that of
organic materials; only minor modifications are needed to accommodate the
differences inherent in these compounds. The selection of the proper technique is the
most important consideration and depends on the spectral information required and
the physical state of the sample.
The majority of IR measurements on gases are performed by transmission methods
as many monomers and oligomers are volatile enough to permit spectral
measurements. The flexibility in pathlength selection allows analysis over a wide
concentration range where parts per billion levels can be determined if other
atmospheric species do not interfere. This includes trace element measurements
using path lengths over 20 m. GC-IR is a useful technique for gases where a plug of
air (0.5-3 mL) is withdrawn from the sample container and injected directly into the
GC. The large volume of air degrades chromatographic resolution making
21
quantitative applications difficult although spectra of the constituent gases are
generally obtained in minutes.
2.3.1.3 Sampling Techniques: Liquids
Liquid siloxane samples are generally best sampled in transmission either neat or
diluted in the appropriate solvent. A high degree of reproducibility is produced by
this technique which is necessary for quantitative analysis. Neat measurements use
path lengths from 2 micron to several centimetres depending on the region of interest
and absorption strength. In the mid-IR region, silicone samples are usually run as 10
% (w/v) solutions in CCl4 for the regions 4000-1350 cm-1 and 650-200 cm-1, and as 2
% solutions in CS2 for the region 1350-650 cm-1 using 0.1 mm path cells.
Combining path length and concentration gives the most appropriate absorbances for
both qualitative and quantitative analysis of most silicones.
Attenuated total reflectance (ATR) is useful for infrared measurements particularly
for neat liquid samples. Sampling is convenient as no sample preparation is required
and path length control is achieved by the number of reflections through the ATR
crystal. The spectral results achieved show smaller intensities at short wavelengths
compared to transmission spectra because the depth of penetration of light into the
sample is around a tenth of a wavelength. The spectra are also susceptible to
distortions related to polarisation and refractive index effects, which vary with the
crystal material and angle of incidence of the radiation. It can be illustrated that a
polydimethylsiloxane (PDMS) spectrum with the relatively thick sample causes
intensity changes in the region between 1100-1000 cm-1 and a prominent shift to
lower wavenumbers of the 800cm-1 band. Thus ATR spectra do not match
transmission spectra in most cases as identification of subtle spectral details may be
more difficult. However, measurements made on aqueous solutions are feasible with
ATR cells because very small path lengths are possible.
ATR is also useful for high viscosity silicone samples as sample thickness is not
applicable and sample handling is minimised. Controlling the pressure of the sample
against the ATR crystal can also increase the reproducibility of spectra. For optimal
results, the same region of the ATR crystal should be used and the sample should
22
cover the whole width of the crystal. PDMS is occasionally difficult to remove
completely from KRS-5 crystals (Lipp and Smith, 1991).
2.3.1.4 Sampling Techniques: Solids
The most difficult phase to study for IR measurements is the solid phase. Thus a
variety of techniques have been developed for sampling silicone solids.
Transmission measurements are preferred when information about the bulk of a
material (rather than surface characteristics) is desired. The preferred sampling
mode is to use a solution in IR-transmitting solvents similarly to liquids. Other
options for transmission measurements include films cast from solvent, thin slices
cut from the bulk, and use of a diamond anvil cell to compress the sample down to a
reasonable thickness.
For samples that exist as powders, KBr mulls or conventional mineral oil are often
used in preparation. A transmission measurement is then performed on the suspended
particles although the spectrum quality is very dependent on particle size. However,
an alternative technique for powders is diffuse reflectance (DRIFT) from the
powdered surface which can be performed in both the mid and near IR range.
Special attachments for holding the sample and collecting the radiation are required;
these are generally integrating spheres in the near-IR and off-axis ellipsoid mirrors in
the mid-IR and either neat or diluted in powdered KBr or KCl in the mid-IR. In
comparison to KBr pellets or mulls, DRIFT has simpler sample preparation,
improved sensitivity and the absence of scattering artefacts. Diffuse reflectance
measurements give primarily surface information if grinding or fragmenting the
sample is avoided.
Photoacoustic (PA) spectroscopy is less commonly used than DRIFT or ATR but has
definite sampling advantages for those samples that are hard, irregularly shaped, and
cannot be ground. Quantitative applications of PA spectroscopy are more difficult
than ATR or DRIFT, however, qualitative applications can be achieved on solids.
Specular reflection spectroscopy is used to analyse silicone coatings on flat metallic
or highly reflective materials. For coatings in the thickness range of 0.1 to 10
23
microns, use of reflectance at near-normal incident angles is a straightforward way of
obtaining a spectrum. The light passes through the sample twice and use of a highly
reflective substrate gives good energy throughput. Such a measurement contains
information from both the bulk and the metal-coating interface.
2.3.2 FTIR Characterisation Methods
As the combined Diffuse Reflectance FTIR (DRIFT)/contact angle measurement
technique is the foundation for the current study, a review highlighting FTIR
techniques and applications is provided in the following paragraphs.
Silicones or siloxanes are quite often used in copolymer systems in polymer analysis
to confer thermal and oxidative stability, chemical inertness, resistance to
environmental factors and chain flexibility amongst others. Copolymer systems
studied by FTIR include a study of a silicone-based oligomer containing pendant
acrylate groups, which has been photo-crosslinked using linear polysilanes as
initiators. Real-time FTIR was employed to measure the curing rate (Zhang et al,
1997). It was found that n-butyl vinyl ether (BVE), contained in a 1:1 blend with the
initiator, copolymerised with the pendant acrylate groups with excellent conversion
rate (95 %). A study on IR+visible sum frequency vibrational spectroscopy was used
to monitor structural changes of a polymer surface in response to alteration of
environment (Fang et al, 2001). The polymer studied was a polyurethane with
PDMS grafted on as end groups. When the polymer was exposed to air, the surface
spectrum shows that hydrophobic PDMS segments cover most of the surface. When
immersed in water, the PDMS component retreats from the surface whereas the
initially “buried” hydrophilic part of the chain appears at the surface. The surface
structural transformation in response to the environmental change from air to water
takes about 25 hours at 300 K. This study shows that the consistency between the
sum-frequency generation (SFG) data and the contact angle measurement in
characterising the hydrophobicity of the polymer surface demonstrates that SFG can
provide insight into how polymer surfaces behave at a molecular level.
FTIR can also be used to study plasma polymerisation of silicones in the presence of
different carrier gases such as H2, He, N2, Ar and O2 (Sherzer and Decker, 1991). In
24
a recent study hexamethyldisiloxane (HMDSO) was plasma polymerised using
inductively coupled electrodeless glow discharge. FTIR was used to differentiate
degrees of fragmentation in plasma polymerisation for different carrier gases and
radio-frequency (RF) powers. For example, FTIR determined a new Si-H bond
appearing at 2150-2100 cm-1 resulting from molecular fragmentation of Si-CH3
(1260 cm-1) breaking to detach the –CH3 group and then combining with hydrogen
formed from the breaking of the C-H bond. For instance with increasing RF power,
the monomer molecules underwent a high fragmentation and produced higher-degree
crosslinking products. Thus, absorption at 1150-1000 (see Figure 2.10) became
broader at a high RF power because of the breaking of the Si-O-Si bond and its
combining with a carbon group to form Si-O-C.
Real–time FTIR (RTIR) spectroscopy has been recently used to monitor UV-initiated
curing processes that proceed within a fraction of a second (Peinado et al, 2001). A
study of the kinetics of photopolymerisation reactions induced by monochromatic
UV light has been performed by RITR on materials such as silicone acrylates. The
infrared spectra were recorded in real time with a Biorad FTS 6000 FTIR
spectrometer equipped with a MCT detector. The spectrometer is able to record 95
spectra per second at a spectral resolution of 16 cm-1. A heatable single reflection
diamond ATR device provided the sampling up to 200°C. Excellent signal-to-noise
ratio is observed for spectra with this set-up (see figure 2.11) even at the highest data
recording rates:
Figure 2.10: IR spectra of monomer HDMSO plasma polymers for Ar carrier gas at RF
powers of: (A) 30 W, (B) 40 W, (C) 50 W, (D) 60 W (Sherzer and Decker, 1991).
25
Figure 2.11 : Experimental setup of Real-time FTIR instrument (Peinado et al, 2001)
In recent studies Raman spectroscopy has been used to develop a non-invasive
technique for identifying intraocular lenses (IOLs) implanted in patients (Erckens et
al, 2001). Confocal Raman spectroscopy was used with a laser power of 95 μW and
exposure time of 1 second. Three IOLs were characterised with distinct spectral
peaks observed for PMMA, acrylic and silicone. The silicone IOL studied showed a
strong characteristic absorption around the 2900 cm-1 mark.
Studies involving preparative techniques via a sol-gel reaction to form imide-
siloxane block copolymer/silica hybrid membranes were reported (Park et al, 2003).
A combination of FTIR, 29Si NMR, XPS and thermogravimetric analysis (TGA) was
incorporated to elucidate structural information. Also studied were the gas
separation properties of these hybrid membranes in terms of PDMS or silica content
at various temperatures. It was found that the addition of the PDMS phase increased
the permeabilities of He, CO2, O2, and N2 indicating that the gas transport occurred
mainly through the rubbery organic matrix.
26
Infrared spectroscopic ellipsometry (IRSE) has been employed as a means of
determining the dependence of the rate of crosslinking in PDMS on the thickness of
coatings (Simpson et al, 2003). The technique was used as an insitu probe into the
reactions between vinyl end groups and SiH groups in a crosslinker and between
unreacted SiH groups and hydroxyl/silanol groups within PDMS coatings, all on
silicon substrates.
A comparison between two hyphenated characterisation methods was recently made
(Liu et al, 2003). On-line GPC electrospray ionisation (ESI) time-of-flight (TOF)
mass spectrometry (MS) was compared to automated GPC matrix assisted laser
desorption ionisation (MALDI) TOF MS for PDMS analysis. It was found that
GPC-ESI-TOF MS effectively reports the low-mass oligomers and underestimates
the high-mass oligomers, while GPC-MALDI-TOF-MS effectively reports the high-
mass oligomers and underestimates the low -mass oligomers.
27
3.2 EXPERIMENTAL METHODOLOGY
3.3 Diffuse Reflectance Infra-red Spectroscopy Studies
3.1.1. The DRIFT technique
Diffuse-Reflectance Infra-red spectroscopy (DRIFT) is the method of choice when
analysing fine powdered samples. Combining ease of use with minimal sample
preparation, it has the ability to produce good quantitative analysis very efficiently.
The main advantage in using DRIFT is that a very small amount of sample is needed
to ensure reliable results as the effective pathlength of the infrared radiation is
increased manyfold by internal scattering. A detection limit of less than 10 ng has
been reported (McKenzie and Koenig).
The quantitative analysis of DRIFT is based on the Kubelka-Munk equation:
f(R∞) = (1- R∞)2/2R = c/k (1)
where R = absolute reflectance of the layer;
c = concentration of the compound; and
k = constant related to the particle size and molar absorptivity of the
sample.
In practice, a perfect diffuse reflection specimen is not possible. Generally the ratio
of the single-beam reflectance spectrum of the sample and the single-beam
reflectance spectrum of a non-absorbing standard (e.g. KBr) is used instead of R∞.
The quantitative aspects of DRIFT have been discussed in studies of the interaction
of silane coupling agents with fillers used to manufacture high-strength reinforced
composite materials and in particular the advantage of applying KBr overlayers to
assist quantitation (Culler). Due to the non-uniform nature of the sample, the
calibration curves were slightly scattered. Applying DRIFT in quantitative analysis
of multiple medicine components in tablets (Park et al), functional groups in acrylic
acid polymer (Juang and Storey) and organic compounds adsorbed on silica gel (Pere
et al) have also been very successful. Excellent calibration curves and reliable
28
results were produced. To ensure quantitative results, it was recommended that KBr
be used to dilute the sample, that the KBr particle size be keep as constant as possible
and that an internal reference band be used to normalise the intensity.
In a recent study (Liu et. al. unpublished), actual pollution layers from insulators
were used to simulate the effect of a pollution layer on the surface of HV SiR
insulators. As the composition of the field pollution will vary depending upon
environment and time of exposure, no compound in the pollutant itself is appropriate
as an internal reference for the measurement of LMWS concentration. Therefore it
was necessary to add a stable compound such as a cyanate ester (CE) as an internal
reference. The reasons for choosing CE, which has characteristic C≡N stretching
bands at 2238 cm-1 and 2275 cm-1, are that materials with CN group or with
functional groups giving similar bands are not likely to be present in the field
pollutants, the CN stretching bands are well separated from the silicone bands and
CE is stable to almost 200oC which is well within the temperature range used in the
analysis. The area of the silicone band at 1262 cm-1 was ratioed against the total area
of bands at 2238 cm-1 and 2275 cm-1. The normalised silicone band intensity was
plotted versus the real weight ratios of PDMS/CE giving a linear relationship with a
correlation coefficient of 0.99.
For closer examination of the diffusion process, i.e the migration of LMWS species
from the bulk to the surface of the SiR insulator, it was deemed necessary to
introduce an artificial contaminant as a “controlled” substitute for an actual pollution
layer. Kaolin was chosen as it was a naturally occurring, thermally stable compound
and contained characteristic IR bands suitable for DRIFT analysis. Most
importantly, controlled deposition of the kaolin can be readily achieved to produce a
uniform surface layer of an artificial contaminant that mimics the actual pollutant on
SiR insulators. This is essential for any meaningful analysis on the LMWS diffusion
process.
3.1.4 The DRIFT Electrostatic Deposition Method
The KBr and linear polydimethylsiloxane (PDMS) were purchased from Aldrich
Chemical Co. at a purity of greater than 99%. The commercial kaolin was used as
received. Eighteen standard samples for the construction of the calibration curve for
29
the assessment of the LMWS in the kaolin coating were prepared by mixing different
amounts of PDMS with kaolin. About 6 mg of each standard was then diluted with
200 mg KBr. Each sample was ground for 3 minutes before the DRIFT spectrum
was taken. For these standards the concentration of PDMS in kaolin varied from
1.04 % to 18.22%. A Perkin-Elmer system 1000 spectrometer and a DRIFT
accessory were used to collect the spectra at a resolution of 4 cm-1 under nitrogen
purging. Pure KBr, which was ground for the same time, was used to collect the
background spectrum.
Elastomer disks of 25 mm diameter and 5mm thick were cut from the sheds of
different high voltage composite insulators. The surfaces of the disks were first
cleaned with soft tissue plus ethanol and then were electrostatically coated with a
thin layer of kaolin powder. The DC electrostatic generator was constructed within
the University and was able to apply 7000V to the sample discs as shown below:
Figure 3.1: Electrostatic deposition device for coating silicone elastomers with kaolin
This technique produced a uniform coating similar to that of pollution found in “real
world” elastomers. The amount of coating was determined to be 0.4mg/cm2 for all
studies conducted. The samples were then held at 30oC for different days to allow
LMWS to diffuse into the kaolin. The kaolin powder was then taken off with a small
fine brush and diluted with 80 mg KBr. The DRIFT spectra were then recorded at
the same conditions as the calibration standards.
30
3.1.5 Calibration Curves
The calibration standards as used in this study were developed by this group as
mentioned earlier (H Liu et al, unpublished), and have been utilised in an effort to
not only validate but also expand the DRIFT technique.
For the determination of the concentration of LMWS in the kaolin coatings by
DRIFT, kaolin itself was used as the internal standard because of its thermal stability.
Figure 3.2 shows the spectra of standard KS10 (10.60% PDMS) and Table 3.1
highlights the concentrations of each of the standards used. The bands between 3620
cm-1 and 3700 cm-1 correspond to the OH stretching of kaolin.
Fig 3.2: DRIFT spectrum of standard KS10 showing the silicone band at 1260 cm-1 and
kaolin bands at 3500 -3600 cm-1.
Table 3.1: Standard samples of mixtures of PDMS and kaolin Sample KS1 KS2 KS3 KS4 KS5 KS6 KS7 KS8 KS9 KS10
Si/Kao(wt
x10-2)
1.04 2.04 3.20 3.95 5.64 6.48 6.68 7.77 8.88 10.6
31
Fig 3.3: Calibration curve for the measurement of LMWS in kaolin constructed from
standard mixtures of PDMS in kaolin.
These bands are chosen as the internal reference. The area of the silicone band at
1262 cm-1, which corresponds to the symmetric deformation of CH3 in Si-CH3 group,
was ratioed against the total area of the OH reference bands to obtain the normalised
silicone band intensity. The normalised silicone band intensities were plotted against
the actual weight ratios of PDMS/kaolin as shown in Table 3.1. A linear relationship
with a correlation coefficient of 0.99 was obtained and was used as the calibration for
quantitative analysis of LMWS.
3.2 Contact Angle Studies
Static contact angle measurements on slivers (or discs) of silicone elastomers are
undertaken using a microscope fitted with a Sony Cybershot 3.3 mega pixel CCD
digital camera. The sliver is placed on a raised, flat surface and the image directed
into an optical microscope using a small mirror at a 45° angle as shown in Figure 3.1.
32
Figure 3.1 : Schematic layout for measuring contact angles (Priv. Comm. Cash, 2003)
3.2.1 Measurement of Contact Angle
Both advancing and receding contact angles are measured as follows. Six photos are
taken for each measurement and each sample was measured twice:
1. 5μl of pure water was placed on the elastomer close to the edge (Figure
3.2) using a microliter syringe (say 50 μl). The position of the sample and
mirror were adjusted so that the drop can be seen in the microscope. The
magnification was also adjusted. A photo is then taken.
2. Another 5μl was carefully added to the existing drop and a second photo
thus taken
3. A third 5μl portion of water was added to the expanding drop and a third
photo taken. This finishes the advancing angle part of the measurement
4. To measure receding angle, 5μl was carefully removed from the drop
using the syringe and a photo taken
5. Another 5μl was removed and a photo taken
6. A final 2μl was removed and a photo taken.
7. Another sample was measured.
8. After all samples were measured, they were then measured for a second
time.
33
Figure 3.2: Plan view of the layout for measuring contact angles showing the
relationship between the mirror, sample and water drop (Priv. Comm. Cash, 2003).
3.2.2 Calculation of Contact Angle
1. The angle was calculated by measuring the base of the drop in contact
with the substrate and the height of the drop as shown in Figure 3.
2. The angle was calculated from the formula as shown below from simple
trigonometry:
The formula used was : tan (theta/2) = 2h/d
where
theta = contact angle
h = height of drop
d = length of drop in contact with the substrate
Figure 3.3: Measurement of contact angle using the height(h) and length of drop in contact with the substrate (d)
3. An excel spreadsheet was set up and the values for h and d inserted and
angle calculated.
34
4. As mentioned in 8 above, the measurements was repeated on another area
of the sample but was kept close to the edge.
3.3 X-ray Photoelectron Spectroscopy X-ray Photoelectron Spectroscopy (XPS) permits atomic analysis of the surface of a
material down to a depth of 5nm (approximately the top 5 to 30 atomic layers).
Initial XPS spectra were collected on a 1982 model Phi 560 instrument fitted with a
magnesium X-ray source and a cylindrical mirror analyser. Survey spectra were
collected over the range of 1000 to 0eV at 100eV pass energy and multiplex spectra
at 25eV pass energy (for 10 minutes).
From January 2003 a Kratos Ultra instrument was available. This machine used a
monochromated aluminium source (Kα radiation) and was fitted with a
hemispherical analyser containing eight detectors. Data obtained were analysed using
Kratos Vision v2.0 software which includes a linear least squares optimisation with a
simplex peak fitting algorithm for curve fitting. Survey spectra over the range from
1200 to 0 eV binding energy were collected at 160eV pass energy and the quality of
these permitted quantification of carbon, oxygen, silicone and, where present,
aluminium, fluorine, nitrogen and zinc. Narrow band spectra of carbon, oxygen,
silicon (all samples) were collected at 10eV to permit curve fitting and also to
determine more precise atomic concentrations of these elements. The new instrument
possessed greater sensitivity and revealed trace levels of elements not previously
seen.
The ratios of each silicon-oxygen compound were determined by the use of
proprietary curve fitting software. After curve fitting the silicon peaks, a check was
made to ensure that the fit was sensible, that is, there was sufficient oxygen present
to account for the silicon-oxygen species present. In a few cases, a good curve fit
could be obtained but it did not balance to the available oxygen.
3.6 Soxhlet Extraction of LMWS Low molecular weight silicone oil was removed from the silicone elastomers studied
by soxhlet extraction. Approximately 10 to 12 grams of the bulk material from each
35
elastomer sample was prepared by first removing a 0.5 mm layer of surface material.
The remaining bulk material was then sliced into thin sections (approximately 0.5 x 5
x 20 mm), weighed, placed directly in the extractor (the bottom outlet was covered
by stainless steel mesh) and then extracted with AR grade chloroform for 6 hours.
At the end of the extraction process, most of the solvent was removed in a rotary
evaporator and the extract and remaining solvent transferred to a small vial. The
remaining chloroform was allowed to evaporate at room temperature.
3.7 Source of Silicone Elastomer Samples Powerlink Queensland and two manufacturers of HV insulators provided a number
of long-rod and line post silicone rubber insulators used in this project. Some
insulators were new (virgin) while others had been in service for up to 7 years. The
details of the insulators used are given in Table 3.2. Table 3.2: Details of silicone insulator shed material used for kaolin studies
Sample Type Design Voltage Supplier Time in Service
BV Long-rod 275kV manufacturer 0
CV Long-rod 275 kV Powerlink 0
CA7 Long-rod 275 kV Powerlink 7 years
DV Line post 275 kV manufacturer 0
In addition, samples of silicone insulator shed material were received between 2001
and 2003 from a large accelerated ageing study of silicone rubber HV insulators that
was conducted by EPRI in the USA in a large test chamber. In this chamber
insulators were energised at their design voltage in the recommended orientation and
subjected to cycles of intense UVA light, salt fog and water. QUT analysis included
XPS, contact angle measurements, scanning electron microscopy and
thermogravimetric analysis. The samples included specimens from manufacturers B,
C and D including insulators close in design to those in Table 3.2. In addition types
F and VA from other manufacturers were also analysed. Characterisation of these
elastomers has been previously published (Hunt et al, 2002).
36
4.0 THE COMBINED DRIFT/CONTACT ANGLE
TECHNIQUE
4.1 Introduction
This chapter describes the development of a novel analytical technique from its early
stages of validation, through to improving the DRIFT/electrostatic deposition
methodology, and finally to incorporate contact angle measurements.
Initially, preliminary work was undertaken to ensure the validity of a new technique
that monitored the migration of LMWS from bulk to surface which was driven by an
electrostatically deposited artificial contaminant. Once this was possible it became
necessary to refine and improve the experimental technique with a view to provide a
characterising methodology that allowed in-field monitoring of HV silicone
insulators.
Static contact angle measurements were also undertaken as this was known to be a
useful but inconclusive method to monitor surface hydrophobic recovery. As
promising results developed from these measurements, it became a logical extension
to integrate this simple measurement technique with the experimentally intensive
DRIFT/electrostatic deposition methodology. Although some difficulties arose
during the initial experiments, it is believed that under certain experimental
conditions that the combined DRIFT/contact angle method provides a semi-
quantitative assessment of hydrophobic recovery.
Preliminary work using slivers of silicone elastomer taken by QUT’s live-line
sampling tool (Cash et al, 2001b) will also be discussed as this would ultimately lead
to a characterisation methodology that allows in-field monitoring with no disruption
to the high-voltage transmission service.
37
4.2 Validation of the DRIFT Electrostatic Deposition Technique
The development of an electrostatic coating technique became one of the key criteria
providing a characterising methodology that was both reproducible and repeatable.
To experimentally monitor the migration of LMWS from bulk to surface two
important factors need to be considered. Firstly, a constant surface area of kaolin
had to be maintained or at least be able to be quantified for results to be meaningful.
This was achieved by carefully monitoring the small quantities of kaolin and/or KBr
powder every time a transfer took place (i.e. scraping kaolin from silicone surface to
mortar). Secondly, the experimental procedure for the sampling process had to be as
controlled as possible; so as to minimise any potential variability. The results in this
section indicate that this controlled procedure was successfully implemented.
Preliminary results by this group indicated that monitoring diffusive processes via
the DRIFT Electrostatic Deposition technique is viable. The spectra of kaolin
coatings containing the diffused LMWS were recorded and analysed. The area of
silicone band at 1262 cm-1 was ratioed against the area of the O-H stretching bands
of kaolin as per the standard samples. This normalised silicone band area was used
to calculate the LMWS concentration using the calibration curve. Figure 4.1 shows
the LMWS concentration changes in the kaolin coating (0.4 mg/cm2) with time at
33oC for elastomers DV and CV. These samples are both virgin materials but from
different manufacturers. The diffusion rate at the initial stage was high and it
decreased gradually as the LMWS concentration in the coating increased. As the
LMWS concentration in the coating reached a certain value the diffusion rate became
very low and eventually reached a plateau.
There were differences in the diffusion behaviour of DV and CV with the diffusion
rate for CV being faster than DV, especially at the initial stage. It was also noted that
the equilibrium LMWS concentration for CV was higher than that for DV. Previous
studies (Hunt et al, 2002) have shown that DV has a lower bulk LMWS
concentration than CV and this may explain the differences seen. A higher bulk
LMWS concentration means a bigger reservoir and a greater driving force for
diffusion to occur.
38
The effect of ageing on the rate of diffusion is shown in Figure 4.2. Both samples
CV and CA7 were from the same manufacturer and had the same level of kaolin
coating (0.4 mg cm-2). CV was a virgin elastomer while CA7 had been in the field
for 7 years. It can be seen from Figure 4.2 that recovery rate for aged CA7 is faster
than the virgin CV. This result could be due to a number of factors including a
change in the network structure due to ageing in the field (reduction in cross-linking)
facilitating an increased diffusion of the LMWS or a larger reservoir of LWMS due
to degradation of the polymer. Replication of the CA7 experiment with another set
of samples from the same insulator confirmed the initial result.
Fig 4.1: Change in LMWS concentration in kaolin (0.4mg cm-2) with time for virgin insulators DV ( ) and CV ( ) at 33oC.
39
Fig 4.2: Change in LMWS concentration in kaolin (0.4mg cm-2) with time for virgin insulator CV ( ) and aged insulator CA7 ( )
Once replication of a sample set was achieved it was important to refine the DRIFT
Electrostatic Deposition technique using silicone rubber samples from other
manufacturers. The migration rate for virgin BV insulator was studied and for
comparison it has been plotted with other sample sets in Figure 4.3.
Figure 4.3: Comparison of B C and D Virgin Insulators at 33oC.
40
For comparison purposes, the CV and CA7 result sets highlight the 18 discs in which
18 similar conditioning times were undertaken ranging from zero time to 22 days.
The BV result set only contained 9 discs spread over zero time to 18 days. However,
the CA7 series contained two sets of results in which early results were repeated
under the same conditions and exact same samples. Figure 4.3 shows the results of
DV, the average duplicate set of CV, and the single set of BV. It is clear that the
variation of data points of the BV set has been minimised and all data points are
quite consistent with no real outliers. It is believed that increasing experience with
the technique coupled with close control on the thickness of the kaolin coating has
significantly improved the reproducibility of the method. The surface area of the
coating of kaolin has been consistently set at 0.4 mg cm-2 which means 2.1 ± 0.1 mg
of kaolin was applied to a 25 mm diameter disc. When a select choice of samples
were repeated for the combined contact angle technique (see section 4.4), variation
of only up to about 1 % LMWS content was observed (Table 4.1):
Table 4.1: Comparison of %LMWS with two different sample sets from same insulator
Type % LMW Silicone Content LMWS Relative
CA7 CA7 CA7ca % difference Error %
R05 (2 hrs) 4.30 3.63 0.67 15.58
R15 (1 day) 13.38 12.25 1.13 8.45
R21 (4 days) 15.96 15.11 0.85 5.33
R16 (7 days) 20.12 19.10 1.02 5.07
The “LMWS % difference” column simply refers to difference between two separate
DRIFT measurements on the same silicone elastomer disc that were made
approximately 3 months apart. These results are in good agreement particularly as
the set “CA7ca” were analysed for % LMWS directly after contact angle
measurements. In other words immediately after the surface was coated with kaolin
a duplicate series of contact angle measurements were made (see section 3.2). After
the designated conditioning time (see Table 4.1 – first column), another duplicate
series of contact angle measurements were taken. Not surprisingly, the sample
conditioned for 2 hours had the poorest agreement (15.58 % relative error) as the
41
surface may not have been completely dry after placing two water droplets for
contact angle measurements.
The driving force for the diffusion process is dependent on the amount of artificial
contaminant (i.e. kaolin) on the surface of the silicone disc. Once kaolin is
deposited, the natural surface energy of the silicone is disturbed thus creating a
gradient for the diffusion to occur. It is believed that the LWMS species on reaching
the silicone/kaolin interface, encapsulates the kaolin particles that are only in contact
with the surface. In current studies by this group (Liu et al, unpublished), it was
shown that a thicker coating of kaolin (1.3 mg cm-2) gave an average equilibrium
concentration of 9% LMWS (See Figure 4.4 below). This was considerably less than
the 13.4 % LMWS attributed to the normal thin coating of 0.4 mg cm-2. However,
on closer examination, it was found that if the uppermost layers of the thick coating
was brushed off and discarded, the analysis of the thinner layer remaining was
determined to be 13 % LMWS. This result suggests that there is a concentration
gradient in the thicker coating thus the value of 9 % above is an average value only.
Fig 4.4: Change in LMWS concentration for different thicknesses of kaolin coating –
0.4mg cm-2 ( ) and 1.3 mg cm-2 ( ) - with time for virgin insulator D at 80oC.
42
Figure 4.5 shows the changes of LMWS concentration in the kaolin coatings (0.4
mg/cm2) with time for insulator DV at three different temperatures, 33 oC, 52 oC and
80 oC. It can be seen that temperature did not have significant effect on the diffusion
of LMWS into the kaolin coating. This may be because the glass transition
temperature (Tg) of polydimethylsiloxane (PDMS) is very low (-128oC, Lide, 1991).
Fig 4.5: Change in LMWS concentration in kaolin (0.4mg cm-2) with time for virgin
insulators D at different temperatures – 80oC ⊕ , 52 oC , 33 oC .
4.3 Contact Angle Measurements
The contact angle measurement is used widely to characterise properties of polymer
surfaces such as wettability, roughness, heterogeneity, deformation and surface
roughness (Kato et al, 2003). Contact angles can be measured by a number of
techniques including capillary rise, sessile drop or captive bubble, Wilhelmy balance
and laser goniometry amongst others (Trettinnikov & Ikada, 1994; Adamson, 1990 &
Uyama et al, 1990). These measurements can be divided as either static or dynamic
contact angle methods. The static measurement is of the solid/liquid interface which
is not in motion. However, the dynamic measurement is where the liquid front is in
motion with respect to the solid surface. For example, the Wilhelmy balance is
generally used as a dynamic measurement. For ease of use and availability of
43
equipment, static contact angle measurements were made via the sessile drop
technique as described in section 3.2.
4.4 The Combined Contact Angle/DRIFT/Electrostatic Deposition
Technique (CADED)
To further understand the migration of LMWS from the bulk of the silicone
elastomer, a combined technique incorporating static contact angle measurements
has been developed. This section will discuss the results obtained from a study
undertaken on the CA7 sample set. Following the usual procedure given in Section
3.2, contact angles were measured immediately after electrostatic deposition of
kaolin and again after the set period of aging. Figures 4.6 and 4.7 shows that there is
an increase in contact angle for both advancing and receding (or increase in surface
hydrophobicity) with time.
This correlates well with increasing concentration of LMWS on the surface as
measured by DRIFT. Clearly, the same trend of subsequent decrease in the rate of
migration of LMWS and contact angle are interrelated. Analysis of contact angle
hysteresis (difference between advancing and receding angles) has often been used in
the literature to quantify the effect of surface roughness in SiR insulators. Generally,
it has been found that a large difference between advancing and receding angles
indicates anomalies attributed to surface roughness heterogeneity (Wu and Odom,
1998). Figure 4.8 shows the differences in both advancing and receding contact
angles before and after kaolin coating:
44
20
40
60
80
100
120
140
R01(0 hrs) R05(2 hrs) R15 (1day) R21(4days) R16 (7days)
PDMS CVca
CA
Adv
anci
ng(d
egre
es)
Initial CAFinal CA
Figure 4.6: Increase in advancing contact angle (CA) with respect to aging (Initial CA
taken at time of electrostatic deposition and Final CA taken after aging period). *Note: error bars denote average error for each individual series.
0
20
40
60
80
100
120
R01 (0 hrs) R05(2 hrs) R15 (1day) R21(4days) R16 (7days)
PDMS CVca
CA
Rec
edin
g (d
egre
es)
Initial CAFinal CA
Figure 4.7: Increase in receding contact angle (CA) with respect to aging (Initial CA
taken at time of electrostatic deposition and Final CA taken after aging period). *Note: error bars denote average error for each individual series.
45
-10
0
10
20
30
40
50
60
0 2 4 6 8
Time (days)
Cha
nge
in C
onta
ct A
ngle
(d
egre
es) Advancing CA
Receding CAAverage Difference
Figure 4.8: Change in contact angle (CA) from that measured immediately after kaolin
coating to that that measured after an ageing period. Both advancing and receding measurements are taken.
This average difference contact angle parameter can be used to directly correlate
contact angle semi-quantitatively with the migration of LMWS from the surface to
the bulk (Figure 4.9). It is interesting to note in the above figure (4.8) that there appears to be evidence of
significant contact angle hysteresis effect in the result given at 7 days. Although it
could be argued that the surface is indeed heterogeneous due to the kaolin/SiR
interface, this did not appear to affect the contact angle measurements when a
water/kaolin-SiR interface was directly compared to the water/SiR surface.
Advancing angles were almost identical (100.8° ± 3.1° to 101.7° ± 1.5°) and
receding angles had a small difference (47.5° ± 6.8° to 51.7° ± 4.2°) when measured
on the exact same aged SiR. This result indicates that the technique described can be
used as a characterising tool of surface hydrophobicity. It is also possible that the
hysteresis observed at 7 days can be explained by the increasing amount of diffused
LMWS as it approaches equilibrium conditions. Adamson (1990) postulates three
types of causes for hysteresis which include surface roughness and surface
immobility. The third cause, surface heterogeneity, is demonstrated by an oily
contamination on the surface which would cause a lower receding angle than the
corresponding advancing angle according to Young’s equation. Thus it would seem
the hysteresis result obtained at 7 days could be explained by the increasing oily
46
contamination (i.e.increasing LMWS content) and thus increasing surface
heterogeneity. This it would appear that contact angle measurements are very useful
for correlation with increasing LMWS content as observed in Figure 4.9 below:
y = 2.2516xR2 = 0.9285
-10
0
10
20
30
40
50
0 5 10 15 20 25
% LMWS
Ave
rage
CA
diff
eren
ce (d
egre
es)
Figure 4.9: Correlation of the change in contact angle (see Figure 4.8) with % LMWS
analysed by DRIFT.
The correlation coefficient of 0.9285 obtained indicates that the CADED method is
quite acceptable particularly at higher values of %LMWS. There is deviation at the
low end as expected as it is difficult to obtain better precision when calculating
contact angles using the sessile drop/digital camera method. The two negative values
occurred when measuring contact angles at conditioning times of zero and two hours.
For the initial zero measurement, both advancing and receding contact angles were
simply taken on a clean silicone rubber disc. For the final zero measurement, the
same disc was wiped clean and the contact measurements were taken immediately
after the disc was electrostatically coated with kaolin. The initial advancing and
receding measurements (101.7° ± 1.5° and 51.7° ± 4.2°) were slightly higher than the
corresponding final measurements (100.8° ± 3.1° and 47.5° ± 6.8°). A similar
difference occurred for the measurements taken at two hours. Theoretically at zero
time, for use as an exact quantitative tool, all advancing and receding measurements
should be equal to each other respectively. However, as the contact angle
measurement has an average relative error of 7.3 %, the measurements are quite
reasonable.
47
4.5 Preliminary Migration Studies using Slivers
Some preliminary migration studies were undertaken using slivers of silicone
elastomers cut from virgin insulator sheds by QUT’s live-line sampling tool (Cash et
al, 2001b). The purpose of these tests was to see if it was possible to do the same
measurements on more difficult to handle specimens as this would possible lead to a
characterisation methodology that allows in-field monitoring with no disruption to
the high-voltage transmission service. The slivers had consistent rectangular
dimensions of 20 x 10 mm and an average thickness of 0.5 mm. The same procedure
as used for the other migration studies were then undertaken using the existing
electrostatic deposition/DRIFT techniques. Contact angle measurements were not
undertaken. These preliminary results are compared with similar disc samples in
Table 4.2:
Table 4.2: Comparison between sliver and disc types with three different
manufacturer silicones
Manufacturer Time % LMWS % LMWS
Type Sliver Disc
CV 1 hour 1.19 4.85
BV 4 days 6.52 8.47
CV 4 days 7.56 11.05
DV 4 days 8.06 11.14 The results indicate, as expected, that the thickness of samples is a key parameter in
the migration of LMWS from the bulk to the surface. There is a quite a large
difference in thickness between disc type (25 mm) and sliver type (0.5 mm) which is
reflected by the significantly lower values obtained for the sliver samples. The
surface area is also more than halved when comparing discs (490 mm2) to slivers
(200 mm2). Thus, the reservoir for the creation of LMWS is much smaller for the
slivers than for the discs. It is expected that the diffusion rate would have a similar
difference although further work is necessary for verification.
48
THE MIGRATION OF LMWS FROM BULK TO
SURFACE: A DIFFUSION MODEL
5.1 Molecular Diffusion Theory
The concept of diffusion especially on the permeation of gases and vapours through
solids has been developed for the last 150 years. When a gas or vapour diffuses
through a polymer membrane several processes are involved:
The gas is sorbed and dissolved at the entering face forming equilibrium
between the two phases; and
The dissolved penetrant molecules then diffuse through the membrane (via
random walk) and desorb at the exit face.
Fick discovered the analogy between mass transfer and heat transfer and thus devised
fundamental equations describing these phenomena. The solutions to these equations
revolve around the constant, D, the Diffusion Coefficient, and vary in form
depending on the boundary conditions imposed and also on the particular geometries
of the experimental situations (Veith, 1991).
5.1.1 Steady State Solution
Fick’s diffusion equation can be simply solved at the steady state condition as
follows:
d/dx [D dc/dx] = 0
or:
J = -D dc/dx = constant
The diffusion coefficient, D, can be evaluated directly if the flux, J, and
concentration gradient can be determined accurately.
49
The solubility of the gas must be determined before the concentration of penetrant
within the gas/membrane interface can be known. In many cases the gas/polymer
system can be simplified in accordance with Henry’s law:
c = kp
where: c = the concentration of gas in the polymer;
p = the partial pressure of penetrant at the interface; and
k = the solubility coefficient.
This assumption allows Fick’s steady state law to be written as:
J = DkΔp/l
and P = D k
where: P = the permeability coefficient;
D = the diffusion coefficient; and
k = the solubility coefficient.
This coefficient is a not a fundamental property as it is dependent on both diffusion
and solubility characteristics. However it is frequently used in practice as it is a
measure of the barrier protection or separation potential offered by the polymer.
5.1.2 Diffusion in Polymers
The physical properties of the polymer network and the interactions between
polymer and the solvent are important in the study of solvent diffusion. Thus a
classification system proposed by Alfrey was based on the solvent diffusion rate and
the polymer relaxation rate (Masaro and Zhu, 1999). The categories are: Fickian
(Case I) and non-Fickian (Case II and anomalous) diffusions.
Diffusion of a Fickian nature (Case I) is often observed in polymer networks when
the temperature is substantially above the glass transition temperature of the polymer
(Tg). As the polymer is in the rubbery state, the chains have a higher mobility that
allows easy penetration of the solvent. (Masaro and Zhu, 1999). Thus Fickian
diffusion is characterised by a solvent diffusion rate, Rdiff, that is slower than the
polymer relaxation rate resulting in a large gradient of solvent penetration in the
system.
50
The two methods commonly used to evaluate molecular diffusion in polymers are:
Transmission through a thin membrane; and
Sorption of gases or vapours by a thick slab (Crank and Park, 1968).
The latter demonstrates that the sorption of a penetrant by any homogeneous medium
must initially be proportional to the square root of the time, provided that a steady
surface equilibrium is established immediately and the diffusion coefficient is a
constant. Initially, the ratio of the amount Mt, absorbed at time t to M∞, that
absorbed at equilibrium, is given by:
Mt/M∞ = 4/π(Dt/h2)1/2 (1)
where D is the diffusion coefficient, t is the time and h the thickness of the sheet.
This method involves the penetrant uptake being evaluated for the specific geometry
of the sample such as spherical, cylindrical, and infinite plane type shapes. The
solution in this case is of a graphical nature with a function of t1/2 as the independent
variable and Mt/M∞ as the dependent variable.
5.2 Diffusion in Polydimethylsiloxane
Previous studies of the diffusion process have used an artificial contaminant (De La,
et al, 1996 & Homma et al, 1999). In one such study, a layer of graphite paste was
applied to the elastomer surface and the diffusion of the LMWS through the graphite
paste was studied by energy dispersive X-Ray (EDX) (De La et al, 1996). ATR FT-
IR was also used to study the LMWS diffusion through a carbon coating on an
insulator (Homma et al, 1999). An attempt was made to replicate some of these
studies but it was found that a uniform carbon coating on the SiR surface could not
be obtained and the carbon tended to form clusters. As the surface was not fully
covered this resulted in a non-zero value for the concentration of LMWS at the
beginning of the experiment. A second objection is that freshly deposited carbon is a
non-polar material and is different from the actual contaminants which are mainly
51
inorganic polar compounds. Quartz powder has also used as a mimic contaminant to
study the wetting properties of the surface using a 5-step criteria (Janssen et al,
1999). However, this method as reported was only qualitative.
The diffusion of four PDMS unreactive chains into two PDMS elastomer networks
was investigated (Mazan, 1995). An example of the second method developed by
Crank and Park (aforementioned in Section 5.1) was the basis for this study using
equation (1). It was found that for small values of t the relationship was quite valid
for values of Mt/M∞ below 0.4. This implied that the shortest chains penetrate more
rapidly into a PDMS network than the longest ones. Importantly, the self-diffusion
of the PDMS chain in a PDMS network is not negligible and can significantly
modify the diffusion of other solutes. Gorur also proposed a model based on the
simplified equation by Crank and Park. The equation is as follows:
Mt/M0 = 4 (Dt/πl2)1/2 (2)
where Mt is the change in mass in the bulk after time t, M0 is the initial mass, t is the
time, D is the diffusion coefficient and l is the PDMS sample thickness. The
equation was verified by the diffusion of LMWS into hexane. However,
liquid/liquid diffusion is unlike the current scenario where a liquid is diffusing
through a solid and then encapsulating another solid. Also, the LMWS solubility in
hexane solution is larger, thus, there is a constant driving force for LMWS to diffuse.
While the LMWS in contaminant will reach an equilibrium, there is not a continuous
driving force. This most likely explains the difference in diffusion coefficients
calculated with respect to the current work. Gorur reported an average diffusion
coefficient for silicone oil to be about 3.5x10-12 m2/s. This is to be compared with
the current work in which 0.25x10-12 m2/s was calculated. In this experiment a low-
molecular weight PDMS oligomer contained within the bulk PDMS diffuses to the
contaminated surface as a result of a concentration gradient. Upon reaching the
surface, the low-molecular weight silicone (LMWS) species encapsulates the
contaminant on the surface. The factor of 10 difference may be accounted for by the
different environments in which diffusion is occurring: liquid/liquid diffusion as
opposed to the liquid/solid/encapsulating solid system.
52
The diffusion co-efficient was calculated for an aged silicone rubber (CA7) using the
method adopted from both Gorur and Mazan.
Figure 5.1: Plot according to Fick’s Law (Equation 1) of diffusion of LMW silicone oil
through silicone surface/kaolin interface (sample CA7).
Initially, some approximations must be made particularly in reference of the ratio of
the amount Mt, absorbed at time t to M∞, that absorbed at equilibrium. The highest
value obtained in the diffusion curve given above in Figure 5.1 was used as the M∞
value. It is surmised that given an infinite period, the highest point in the so-called
equilibrium zone should be designated the M∞ as it would be the best approximation
of equilibrium conditions.
Following on from Mazan, it was found that for small values of t, the Crank and Park
relationship was quite valid for values of Mt/M∞ below 0.7 (Figure 5.2).
53
y = 0.0025xR2 = 0.8432
00.10.20.30.40.50.60.70.80.9
1
0 100 200 300 400
time1/2 (s)
Mt/M
inf
Figure 5.2: Diffusion curve of LMW silicone oil through silicone surface/kaolin
interface for values according to Fick's Law (type CA7).
From this equation, the diffusion co-efficient was calculated according to Fick’s Law
and compared with Mazan’s results in Table 5.1 below.
Table 5.1: Calculated values of Diffusion Coefficient, D, for PDMS samples of different molecular weight (Mazan, 1995) and for CA7 and CV types.
0.25CA7 and CV0.83MD380M
0.69 MD180M 2.9MD75M 7.3MD25M
D (cm2 s-1) x 10-8 PDMS Chain
The same analysis was undertaken on a virgin silicone elastomer (type CV), the
identical slope (and therefore diffusion coefficient of 0.26 x 10-8 cm2 s-1) was
achieved despite an intercept above zero (Figure 5.3).
54
y = 0.0025x + 0.1247R2 = 0.9375
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 100 200 300 400
time1/2 (s)
Mt/M
inf
As it is practically impossible to measure the migration at a strictly zero time, it is
inevitable that migration has already begun instantaneously before any
LMWS/kaolin can be brushed off. In focussing on the Fickian component of the
diffusion curve there seems to be no visible difference in the diffusion rate for both
virgin and aged silicone elastomers.
An empirical model for the diffusion of LMWS has also been postulated by this
group particularly in reference to the thickness of contamination layers (Liu, et. al.,
unpublished). In this model, the following points should be considered: (1) the
equilibrium concentration should be controlled by the LMWS concentration in the
bulk and the diffusion coefficient; (2) the speed to reach the equilibrium
concentration should be controlled by the thickness of the coating; (3) the diffusion
rate may be exponential in nature. Based on the experimental results and the factors
which affect the diffusion, an empirical relation was proposed:
Ct = k1DCb(1 - exp(-k2t/d))n = k (1-exp(-k2t/d))n (3)
where Ct is the LMWS concentration in the contaminant at time t, D is the diffusion
coefficient, Cb the LMWS concentration in the bulk, d is the thickness of the
contaminant and n, k1 and k2 are constants. The diffusion coefficient and hence rate
55
may be different within the bulk, from bulk to surface, from surface to contaminant
and within the contaminant itself.
Using the results obtained in Figure 4.4 comparing the diffusion of LMWS into two
different thicknesses, the fitting parameters calculated from this empirical model are
listed in Table 5-2.
Table 5.2: Fitting parameters using equation 3 for the experimental data
Experiment k1 k2/d n
Series A 9.0 0.16 0.25
Series B 10.5 0.50 0.25
It can be seen that k2/d value for Series B (0.50) is 3.1 times that of Series A (0.16).
As k2 is a constant, the coating thickness for A should be 3.1 times that of B as the
thickness, d, is inversely proportional to the diffusion rate. Since the kaolin coating
for series A is 1.3 mg/cm2 and B is 0.4 mg/cm2, the actual estimated coating
thickness for A is 3.3 times of B. The parameter, n, is 0.25 for both thicknesses.
This indicates identical shapes for both curves, which is consistent as they are both a
type D virgin measured at 80°C. These results indicate good agreement with the
empirical model.
In the current work, a theoretical model based on the Alfrey classification system
was also used to calculate diffusion parameters (Masaro and Zhu, 1999). The
exponent, n, which is a parameter related to the diffusion mechanism is an indicator
for any polymer-penetrant system whatever the temperature and the penetrant
activity. The equation below describes the amount of solvent absorbed per unit area
of polymer at time t, Mt:
Mt = ktn (4)
where k is a constant and n is the aforementioned diffusion parameter which often
lies between 0.5 and 1 for non-Fickian (Case II and anomalous) diffusions.
However, for Fickian diffusion (Case I) it is normally 0.5. In the current study the n,
calculated from the slope, was determined to be 0.26 (see figure 5.1 below).
56
Figure 5.3 : Variation of log (Mt/M∞) vs log (t) used to calculate exponential term, n, for
the aged type CA7.
For the virgin type CV insulator, once again a similar logarithmic analysis was
performed to determine the key exponent, n (Figure 5.4).
y = 0.1941x - 0.8257R2 = 0.9498
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
00 1 2 3 4 5
log (t)
log
(Mt/M
inf)
Figure 5.4 : Variation of log (Mt/M∞) vs log (t) used to calculate exponential term, n, for
the virgin type CV.
In this case, however, the n parameter is decidedly different indicating the difference
between aged (0.26) and virgin (0.19) insulators from the same manufacturer. When
analysing the Fickian or straight-line part of the curve, the results do not seem to
differentiate between virgin and aged insulators. Thus there is no visible difference
in the diffusion rate at these times. However, in analysing the results as a whole, it is
57
possible that the parameter, n, can be used as an index to assess the diffusion of
various insulators. This is because it takes into account the critical separation of
values that occur between different insulators approaching equilibrium conditions.
The diffusion curve of LMWS migration from PDMS bulk into a PDMS
surface/kaolin interface could be described as “semi-Fickian”. That is, the diffusion
is approaching true Fickian diffusion only when the exponent, n, reaches 0.5. It is
believed that the curvature, and hence diffusion, can be characterised by the
exponent, n, as it approaches 0.5. As the two models (one theoretically based and
one empirically based) provide similar analyses of diffusion parameters, it is
believed the LMWS diffusion from bulk into surface can be further understood. The
empirical model has been used to confirm the different rates of diffusion that occur
through different thicknesses of kaolin coating. The theoretically derived model has
been used to postulate an index that may assess the rate of diffusion (and increasing
hydrophobic recovery) of different silicone rubber insulators with the same
thickness.
58
6.0 OTHER ANALYTICAL CHARACTERISATION
TECHNIQUES. This chapter describes some of the other analytical techniques used for analysis on
the EPRI test chamber silicone elastomers as described in Section 3.6. Some field-
aged samples were also analysed using the techniques described here.
6.2 X-ray Photoelectron Spectroscopy
6.1.3 Introduction
XPS was used to study the degradation on the surface of the silicone rubber samples
subjected to exposure in the EPRI High Voltage environmental test chamber as
described on page 36. A number of different Si-O compounds can be quantified by
curve fitting the XPS data.
In an XPS study of the degradation of hexamethyldisiloxane (HMDSO) previously
undertaken (Alexander et al, 1999), it was shown that four silicon-oxygen
compounds could be identified. These were the terminal silicone group – SiO -
(silicon bonded to one oxygen only and three methyl groups and referred to the
diagrams as the “terminal silicone fraction”), in-chain silicone -O-Si-O- (silicon
bound to two oxygen and two methyl groups as in PDMS), silica-like (silicon bound
to three oxygen and one methyl group) and silica -SiO4- (silicon bound to four
oxygen). These are illustrated in Figure 6.1.
Figure 6.1: Representation of the various silicon-oxygen compounds likely to be
present in degraded silicone rubber.
However, there is a problem in that the band for the in-chain silicone can be split
(Figure 6.2) and the binding energy for the smaller 2p½ spin state, 102.5eV
59
(Beamson and Briggs, 1992), is very close to the quoted binding energy for the peak
from the silica-like degradation product (102.6eV) in which silicon is bound to three
oxygen atoms.
One way around this problem was to calculate the theoretical peak area of the 2p1/2
peak by dividing the area of the 2p3/2 peak by 2 and then comparing that value to that
obtained for the area of the SiO3 peak as calculated from the curve fit. In most cases
the SiO3 component disappeared.
6.1.4 Calibration
To properly calibrate the instrument for the silicone rubber samples, a very thin film
of silicone oil (1000 cSt viscosity and a molecular mass of approximately 16,000)
was placed on a flat, polystyrene substrate and this was analysed. The silicon 2p
band from this material was curve fitted and this produced two peaks with an area
ratio of 2:1 as can be seen from Figure 6.2. The larger peak, at binding energy
101.9eV, represents a spin state of 3/2 and the smaller with spin state of 1/2 is found
at binding energy of 102.5eV. Both arise from a silicon atom bound to two oxygen
atoms as in siloxane and correlate with literature values for PDMS (Beamson and
Briggs, 1992). This splitting is due to small energy differences of the electrons in the
2p orbital.
Figure 6.2: Curve-fit result following XPS of a silicone oil showing the two silicon 2p spin states
After curve fitting the silicon peaks, a check was made to ensure that the fit was
sensible, that is, there was sufficient oxygen present to account for the silicon-
60
oxygen species present. In a few cases, a good curve fit could be obtained but it did
not balance to the available oxygen. There is also the possibility of there being a low
level of silica in the virgin samples analysed. This may be due to it actually being
present in the rubber as a filler or surface contamination with dust. Instrumental
artefact has been eliminated.
6.1.3 Analysis Results
The Kratos instrument permitted analysis with greater sensitivity. Trace levels of
nitrogen as nitrate (possibly from surface discharges), calcium and sodium (perhaps
from the salt fog), zinc and iron (from corrosion of the end fittings), aluminium
(from the ATH) and fluorine (possibly from mould release agent during
manufacture) were detected. It was interesting that the under side of some sheds had
more elements present than the top of the same shed. (These sheds were horizontal).
Thus, this orientation may encourage water from the salt fog and mist to drain to the
underside of the shed where the water evaporates leaving the deposits. This trend
was not seen on insulators that had different orientations (ie sheds close to vertical or
at a 45° angle. The XPS survey spectrum for specimen Q124, IC1 shed 2 bottom, is
given in Figure 6.3.
Figure 6.3: XPS survey spectrum of Q124 (IC1 shed 2 bottom) showing that besides the major
constituents (C, O & Si) aluminium, chlorine, calcium, nitrogen and sodium were also present on the surface. Any peak not marked is another arising from an existing,
labelled element.
61
Table 6.1:XPS analysis results (in insulator and shed order) of samples taken from insulators (horizontal sheds) exposed in the EPRI test chamber. X = trace,
B=underside T = top side
Age years
Shed No C O Si Al F Ca Zn Fe N Na Silica% SiO3
% B1 49.4 25.6 22.0 3.0 0 0.5 4T 42.1 33.1 24.8 18.8 1 4T 43.5 32.0 24.5 29.1 2 4T 39.8 36.2 24.0 24.9 10.31 4B 42.1 33.4 24.5 30.8 2 4B 50.2 29.6 20.3 X X X X 5.7 2 34T 46.0 32.5 21.6 0.7 X 0.3 X 13.7
0.5 34B 42.2 25.6 18.5 7.2 13.8 1 34B 46.4 31.3 22.3 10.8 2 34B 47.1 29.3 23.6 0.3 0.2 9.3
C1 44.4 27.9 21.2 6.5 1 2T 45.0 32.6 22.4 15.9 2 2T 45.9 31.3 22.8 X 17.8 1 2B 45.4 33.0 21.6 14.8 3.92 2B 43.8 41.9 14.3 1.0 2.2 5.9 X 6.6
0.5 13T 47.6 29.2 23.2 0.0 5 1 13T 31.8 41.1 27.1 14.9 2 13T 46.6 30.1 23.3 X 23.1
0.5 13B 46.7 28.9 22.5 1.7 6.4 1 13B 47.4 28.3 23.1 1.1 5.2 2 13B 46.4 32.3 21.3 0.4 0.9 8.7
D2 47.2 29.7 20.9 2.3 2 5T 42.8 34.9 22.3 X X X 24.4 11.6
0.5 5B 45.1 28.5 22.2 4.3 9.2 1 5B 43.2 31.2 20.0 5.6 11.5 2 5B 46.9 31.4 21.6 13.3 10.4
0.5 37T 47.1 30.2 21.8 0.8 20.1 1 37T 49.5 29.1 21.4 nd 2 37T 46.7 37.6 15.7 2.2 0.8 X 3.2 X 15.0 2 37B 48.9 29.9 21.2 3.1 11.2
Examination of the later results obtained using the Kratos instrument (they are in
italics in Table 6.1) shows that between 10 and 25% silica-like material is present
after two years ageing. The broadening of the silicone 2p peak with ageing due to
increasing levels of silica and silica-like material is shown Figure 6.4.
62
Figure 6.4:
Change in shape of the silicon 2p peak (from the XPS spectra) of silicone showing an increase in the silica component with increasing age or location of the specimen. The
large silica component in the core (sheath) is most notable. The spectra have been normalised to permit comparison.
6.1.4 Conclusions
XPS (X-ray photoelectron spectroscopy) permits analysis of the top atomic layers on
surfaces. Curve fitting can determine the relative levels of silica and silicone.
Formation of silica or silica-like material is linked to degradation of the silicone
rubber. Previous work has shown that degraded silicone rubber insulators at end-of-
life have levels of surface silica in excess of 30% along with the presence of
aluminum (as alumina) in excess of 4% atomic concentration. Some shed samples in
the from insulators in the EPRI test chamber approached 25% silica but had little or
no surface aluminum and TGA results. XPS is therefore a very useful tool for the
identification of silicone rubber insulators at close to end-of-life.
6.2 Contact Angle Measurements on In-Field Silicone Elastomers
Contact angles were undertaken as a way of measuring the degree of hydrophobicity
of the samples sent for analysis. The procedure is described in detail in Section 3.2.
Each specimen was tested in two separate locations and the results averaged and in
most cases the results were similar. Occasionally it was necessary to measure contact
angles at three separate sites. The result for Q129 (Table 6.2) was interesting in that
63
some parts of the sample were hydrophobic while another area was hydrophilic. The
results for the whole project are given in Table 6.2 below. It can be seen that there
has been some change with time for the horizontal shed insulators with some areas
becoming partially hydrophilic. This is also the case with the vertical shed B2. The
partial wetting of the sheath sample (Q164) is consistent with some observations of
test chamber staff regarding these areas when in service. However, another sheath
sample, Q157, was very hydrophobic and measurement of contact angles on the
deposit was impossible as the water drop ran off the surface. It is highly likely that
the slight curvature of the specimen and its rough surface interfered. A hydrophilic
surface would be expected from the high level of silica that was detected on these
surfaces. The areas under the surface deposit were more hydrophilic.
6.2.1 Conclusions
This is a quantitative measure of surface hydrophobicity. However, as surfaces age
they become rougher and this has caused anomalies in some results. A second
problem with silicones appears to be that some migration of LWMS occurs after the
samples are taken from the insulator and can therefore give higher values of contact
angles. Nevertheless, the various surface hydrophobicity tests that have been
developed for use in the field can alert the utility staff of insulators at risk as loss of
hydrophobicity is correlated with ageing.
64
Table 6.2: Contact angle measurements on shed surfaces after 0.5 to 2 years ageing.
EPRI# Age
(y) Shed Advancing
angle (o) Receding angle (o)
Comment QUT #
B1 0.5 4 top 129±2 83 ± 1 Q043 1 4 top 124 ± 2 77 ± 4 Q083 2 4 top 124 ± 3 55 ± 10 part wet Q119 2 4 bot 120 ± 10 48 ± 2 part wet Q120 C1 0.5 2 top 125±3 79 ± 5 Q047 1 2 top 121 ± 4 69 ± 6 Q087 2 2 top 124 ± 3 74 ± 9 no change Q123 2 2 bot 37 ± 5 0 wet Q124 D2 0.5 5 top 136±1 80 ± 4 Q051 1 5 top 126 ± 5 72 ± 8 Q091 2 5 top 139 ± 4 91 ± 2 no change Q127 2 37 top 90 ± 20 0 wet Q129 F1 0 25 top 108 ± 4 66 ± 5 Q025 0.5 4 top 90 ± 1 0 wet Q055 1 4 top 110 ± 4 86 ±1 Q095 AUS1 0 51 top 95 ± 4 31 ± 5 Q042 0.5 5 top 134 ± 2 70 ± 6 Q071 1 5 top 122 ± 1 88 ± 1 Q111 VA1 0.15 3 top 118±2 52±6 Q075 0.65 3 top 108 ± 4 66 ± 1 Q115 1.6 3 top 122 ± 5 60 ± 10 no change Q151 1.6 3 bot 108 ± 1 43.5 ± 0.5 part wet Q152 B2 0.5 2 front 123±4 98±5 Q059 1 2 front 110 ± 4 86 ± 1 Q099 2 2 front 100 ± 3 28 ± 1 part wet Q135 2 2 back 115 ± 2 79.5 ± 0.5 Q136 C2 0.5 2 front 113±6 45±7 Q063 1 2 front 115 ± 2 93 ± 2 Q103 2 2 front 108 ± 1 91 ± 1 no change Q139 D3 0.5 2 front 120±2 89±9 Q067 1 2 front 102±3 80 ± 1 Q107 2 2 front 107 ± 1 82.8 ± 0.7 no change Q143 2 2 back 106 ± 1 91 ± 3 Q144 LOAD 1
2 1 top 124.5 ± 0.5 99 ± 5 Q155
2 1 bot 135.5 ± 0.5 103 ± 6 Q156 LOAD 2
2 Sheath 1
117.5 ± 1 58 part wet Q164
65
7.0 CONCLUSIONS AND FURTHER WORK
The main objective of this study program was to gain improved understanding of the
surface hydrophobic recovery process that is unique to polydimethlysiloxane high-
voltage insulators. It involved the study of silicone elastomers from commercial high
voltage insulators both new and aged. These materials are partly cross-linked and
are filled normally with hydrated alumina. Fundamental knowledge of this
mechanism has been increased through the development of the Contact Angle
DRIFT Electrostatic Deposition (CADED) novel analytical technique. This
technique enabled study of the degradation of silicone elastomers subjected to high
voltage environments by closely following LMWS migration from the bulk material
to the surface and linking it to the contact angle measurements. The migration rate
data showed that the aged material recovered faster that the virgin material.
Differences in the rate and maximum surface levels of silicone were seen between
materials from different manufacturers. This has significant implications for the life-
time of these materials
A model system has been developed to examine LMWS diffusion through the bulk
material and into the interface of surface and pollutant. This was achieved by
examining theoretical and empirically derived equations and using existing
experimental data to better understand the mechanism of recovery. This diffusion
was Fickian in the initial stages of recovery.
X-ray photoelectron spectroscopy (XPS) and contact angle measurements were used
to substantiate the degree of degradation in in-field silicone insulators by quantifying
the levels of the major degradation products: silica and silica-like material and
alumina.
Preliminary work regarding the testing of slivers (which can be sampled live-line)
has the potential to deliver a characterising methodology that allows in-field
monitoring of HV insulators manufactured from silicone rubber. These slivers can
be coated with kaolin and the concentration of silicone on the surface contact angle
measured to give a numeric indicator of insulator condition.
66
Recommendations for Future Work
From the study of LMWS migration, it is possible to derive a model system using
experimental data from both theoretical and empirical considerations in an attempt to
explain the diffusion process that exists in a PDMS/LMWS/kaolin system. However,
more work is required in understanding the interfacial processes that are occurring.
There is a need to do further experimental work in characterising the kaolin and its
absorption of the LMWS. There is also a need to follow on from Gorur’s work and
attempt a reverse diffusion experiment to see if LMWS will diffuse back into the
PDMS bulk. These experiments will give more insight into the mechanism of
LMWS diffusion from PDMS bulk to the kaolin surface. Preliminary studies by using thin slivers of rubber instead of the bulk material by
others in the research group have shown similar trends. Further follow-up studies are
required.
67
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