Proposal to Investigate the Mechanism of Water Treeing and to Develop Methods for Detecting Suscepti

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Proposal to Investigate the Mechanism of Water
Treeing and to Develop Methods for Detecting
Susceptibility of Polymers to Water Treeing

• Tzipi Cohen-Hyams and Thomas M. Devine
• Department of Materials Science and Engineering
• University of California, Berkeley

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Overview

• Background
• Mechanisms of Water Treeing
• Mechanical Fatigue of Polymers
• Proposed Research
• Our approach
• Monitor polymer during pre-treeing
• Specific experiments

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• Two main mechanisms of water treeing
• -Water is necessary for Water Treeing
• Mechanical stresses induced by electric field
  deform water
• droplets so that local stresses increase and
  cause bond rupture,
• which leads to cavities and then a water tree.
• Bond rupture at the tip of a deformed water
  droplet creates radicals, which cause additional
  chain scissions, which lead to additional
  cavities, and thus facilitates growth of water
  tree.
• (2) Chemical reactions, esp oxidation, involving
  water, ions and
• PE, induce local chain degradation.
• Heating due to high cable operating temperatures
  or local heating induced by electric field at
  defects accelerate the process.
• One theory oxidation, esp
  carboxylate formation, is the key factor
  in the initiation and growth of water trees.
• At present, mechanical fatigue is viewed as a
  main component of the
• mechanism of water treeing

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Overview

• Background
• Mechanisms of Water Treeing
• Mechanical Fatigue of Polymers
• Proposed Research
• Our approach
• Monitor polymer during pre-treeing
• Specific experiments

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Mechanical Fatigue of Polymers
Backbone C-C-C- bond scission is the precursor
to microvoid formation from which cracks initiate
leading to macroscopic failure.
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Overstressed chain ruptures producing two primary
free radicals (CH2) and relaxing the stress.
Primary free radicals are highly reactive and
interact with neighboring methylene to produce
more stable secondary free radicals (-CH). The
secondary free radicals lead to additional chain
scissions that produce primary free radicals.
Approx 2000 chain scissions are produced per free
radical. Macroscopic failure at a critical
concentration of ruptures 2x1018/cc for
molecular weight of 4x106
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Macroscopic failure at a critical concentration
of ruptures 2x1018/cc for molecular weight of
4x106 Primary and secondary free radicals have
very short lifetimes. IR can detect changes in
Molecular End Groups, which are directly
related to the number of chain scissions per unit
volume. Molecular end groups are
stable. Microvoid concentration and size deduced
from small angle x-ray scattering. One microvoid
is generated per mechanically induced chain
rupture. The size of the microvoid depends on
the number of chain scissions generated by free
radical propagation reactions.
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Overview

• Background
• Mechanisms of Water Treeing
• Mechanical Fatigue of Polymers
• Proposed Research
• Our approach
• Monitor polymer during pre-treeing
• Specific experiments

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Proposal Investigate Mechanism of Water Treeing
in order to Identify Ways of Increasing
Resistance of Polymers to Water Treeing and to
Identify Methods for Detecting Damage that
Precedes Water Treeing Key Point Propagation
of Water Trees is very fast. Initiation of
Water Trees takes a very long time
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Proposed Approach Apply a combination of
mechanical stresses and AC fields in a manner
that facilitates the investigation of water tree
initiation
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Our Proposed Approach
Cyclic Mechanical Stress
Growth is fast, therefore need to focus on
initiation
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Applied AC Field
-V
In the presence of water/humidity
V
Conduct tests on double-notched samples to
failure at one notch and examine the state of the
polymer in the region ahead of the unfailed
notch. This will indicate conditions needed to
initiate water tree
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Monitor Conditions ahead of the notch 1.Use
MicroRaman to monitor concentrations of molecular
end groups, which is equivalent to the number of
chain fractures. 2. Use ultrasound to detect
changes in modulus and density, which precede
water treeing. 3. Measure Impedance as a
function of frequency (0.01Hz-104 Hz) to monitor
the dielectric character of the polymer. 4.
Periodically pull TEM/AFM surface replicas to
inspect for submicroscopic damages that lead to
failure. 5. Use small angle x-ray scattering
(ALS?) to deduce microvoid concentration and
size.
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Raman Spectrum of LDPE - T. Cohen-Hyams
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Monitor Conditions ahead of the notch 1.Use
MicroRaman to monitor concentrations of molecular
end groups, which is equivalent to the number of
chain fractures. 2. Use ultrasound to detect
changes in modulus and density, which precede
water treeing. 3. Measure Impedance as a
function of frequency (0.01Hz-104 Hz) to monitor
the dielectric character of the polymer. 4.
Periodically pull TEM/AFM surface replicas to
inspect for submicroscopic damages that lead to
failure. 5. Use small angle x-ray scattering
(ALS?) to deduce microvoid concentration and
size.
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Monitor Conditions ahead of the notch 1.Use
MicroRaman to monitor concentrations of molecular
end groups, which is equivalent to the number of
chain fractures. 2. Use ultrasound to detect
changes in modulus and density, which precede
water treeing. 3. Measure Impedance as a
function of frequency (0.01Hz-104 Hz) to monitor
the dielectric character of the polymer. 4.
Periodically pull TEM/AFM surface replicas to
inspect for submicroscopic damages that lead to
failure. 5. Use small angle x-ray scattering
(ALS?) to deduce microvoid concentration and
size.
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Specific Experiments One objective is to use our
sample to duplicate some key earlier results
(establishes the credibility of our sample),
second objective is to acquire new information
relevant to mechanism of Water Treeing
Reproduce earlier results Tree growth rate
f(O2, salt) Determine if there is a critical
O2 and/or critical salt. Determine if water
tree depends on identity of salt Water tree
length, L, is proportional to number of electric
field cycles, N. Is there an influence of
frequency? Is there an influence of wave
shape?
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Specific Experiments Mechanism of Water Treeing
Do tests in water, EC, and DEC to determine the
effect of water. Is it just a high dielectric
that is needed (gt EC H2O), just a liquid that
is needed (gt EC H2O DEC), or is it chemical
reactions caused by water that are needed (gt H2O
gt EC, DEC). Also, consider the use of HF. HF
boils at approx 20C and has approx the same
dielectric constant as water. Run tests in
deoxygenated water as a function of the amount of
oxidizer present (permanganate, and nitrate) and
compare to oxygen to determine if oxidizing power
is important or is oxygen important. Compare
effects of oxidizing anions (permanganate, and
nitrate) to effects of oxidizing metal cations
(e.g., Cu, Fe3) Measure effect of temperature
to possibly explain why (or, determine if) high
temperatures during summer or during times of
heavy power usage accelerate water treeing.
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Specific Experiments Mechanism of Water
Treeing Measure mechanical properties (YS,
Fracture stress) as a function of mechanical
stress cycles and compare to mechanical
properties as a function of electric field
cycles. Investigate the effects of combination
of mechanical stress and electric field. Are the
effects additive? Are the two synergistic? Compa
re effects of residual stress to applied
stress Measure water tree length, L, as a
function of temperature to determine activation
energy, Q, and see if the value of Q is
consistent with diffusion of water.
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Specific Experiments Mechanism of Water
Treeing Determine effects of sine wave vs square
wave (vary period to determine effects of hold
time at max field/stress), vs saw-tooth wave.
Compare effects of electric field to effects of
mechanical stress. If dependences are identical
gt mechanical fatigue is a major component of
water treeing.
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Summary

• Use simple, but novel, sample to investigate the
  mechanism of initiation of water trees.
• In particular,
• identify the polymers structural changes, caused
  by combinations of (synergistic effects of)
  mechanical stress and high AC voltage, that
  initiate water trees
• develop techniques (e.g., Raman and Ultrasound)
  for detecting damage that leads to water treeing