Towards recyclable
insulation materials
for high voltage cables
I. L. Hosier, A. S. Vaughan and S. G. Swingler
January 2011
Thanks to EPSRC (Supergen AMPerES Project) for financial support
Outline of presentation
• Introduction
• Experimental
• Results - Ethylene based systems
• Results - Propylene based systems
• Conclusions
• Towards the future
2
Introduction
• Paper/oil cables were the first type to be used commercially
but oil leaks are a serious environmental issue
• Nowadays these have been largely superseded by XLPE
cable systems but these also have drawbacks
– At end of life XLPE cannot be easily recycled, although
several re-use schemes have been proposed
– Disposal and environmental issues are getting more and
more important in every area of life
• So, we need a recyclable alternative to XLPE for the next
generation of extruded high voltage cables
3
Possible alternatives to XLPE
• Both PE and PP are already widely recycled through
existing waste channels – ideal replacements......?
• But what is required from a cable material?
– Good flexibility at low temperatures (~-20 oC)
– Good stability at high temperatures (~105 oC or better)
– Sufficiently high melting point (>105 oC)
– Low dielectric loss, high breakdown strength
– Easy and cheap to manufacture
– The same (or better!) than XLPE
Analytical process
• Identify suitable recyclable starting materials
– Various ethylene and propylene based systems
• Make samples and perform tests
– Melting point
– Morphology
– Thermo mechanical stability
– Breakdown strength
• Optimise their performance if required by combining the
properties of more than one material (blending) and through
the control of crystallisation conditions
Experimental
Materials and sample preparation
• Materials
– Ethylene systems – LDPE, HDPE, EVA09, EVA20,
EVA33, EVA40 (EVA systems offer tree retardancy)
– Propylene systems – iPP, sPP, PE02, PE12, PE40, PB12
• Blending
– Solution blends using xylene as solvent
• Samples (melt pressed)
– 60 μm disks for ASTM D149 breakdown tests, ~2 mm
thick plaques for thermo-mechanical/morphology.
Isothermal crystallisation or quenched
Characterisation process
• Thermal - DSC on 2-5 mg samples (Perkin Elmer DSC-7)
• Morphology - Microtome followed by permanganic
etching, gold coating then SEM (Cambridge Instruments
Stereoscan 360 operated at 20 kV, SE mode)
• Mechanical - Tensile testing to 100% strain at room
temperature (Instron 4301)
• Thermo-mechanical - Modulus from -20 to ~150 oC
(Rheometrics RSA II)
• Electrical – ASTM D149 ramp to breakdown test (50 Hz
AC, 50 Vs-1, opposing 6.25 mm ball bearings)
Ethylene based
systems
Thermal properties
Single components
Only LDPE and HDPE
have high enough
melting points for use
in a cable system,
EVA’s have much
lower melting points
Blends (20% HDPE)
Blending HDPE into
LDPE or EVA systems
increases the final
melting points of the
composite material!
Morphology
Single components
LDPE to EVA40 - the
crystalline texture
becomes increasingly
disrupted by the
presence of VA groups
Blends (20% HDPE)
Increasing VA content
leads to increased phase
separation between the
HDPE and EVA
components
Mechanical
Single components
HDPE is brittle (necks), LDPE offers
more ideal (rubbery) behaviour whereas
increasing the VA content offers softer
materials
Blends (20% HDPE)
Adding HDPE increases
the modulus but without
making the composite
system brittle – they
offer us a range of
mechanical properties
Thermo-mechanical
Single components
HDPE offers us good performance at
high temperatures, LDPE is OK up to
~60 oC whereas EVA materials would
be of limited use in distribution
cables
Blends (20% HDPE)
Adding HDPE improves
the high temperature
performance, Blend A
(LDPE based) offers us
properties comparable
to XLPE
Electrical
Single components
HDPE offers the best breakdown
strength, LDPE offers electrical
properties identical to XLPE
whereas the EVA systems are
electrically weak
Blends (20% HDPE)
Adding HDPE improves
the electrical breakdown
performance, Blend A
offers us improved
performance compared
to LDPE or XLPE
Conclusions
• LDPE offers comparable electrical properties to XLPE but
is unsuitable for use above ~60 oC
• HDPE has good high temperature performance, good
breakdown strength but is mechanically brittle
• Various EVA co-polymers show reduced breakdown
strength and reduced high temperature performance
• Adding HDPE to EVA or LDPE improves the breakdown
strength and the high temperature performance
• Blend A (20 % HDPE in LDPE) offers a recyclable
system with comparable properties to XLPE
Propylene based
systems
Thermal properties
• All of the materials melt above ~120oC, sPP has the lowest
melting point and iPP the highest
• Addition of an ethylene or butene co-unit generally reduces
the melting point
• Some systems can behave strangely (PE12)
• Might permit cables to operate at higher temperatures!
Morphology
iPP and sPP
Both materials
show spherulitic
textures
Copolymers
Unspherulitic PE02
(left), leads onto an
amorphous texture
in PE40. PE12
shows a two phase
structure (right)
consistent with DSC
Mechanical properties
• Both iPP and PE12 exhibit brittle fracture – not good!
• The remaining materials neck except PE40 which is rubbery
• Despite the advantage of having high melting temperatures
and excellent high temperature stability, all the propylene
materials (except PE40) are too stiff at low temperatures
Electrical properties
• Whilst being the best mechanically, PE40 is the worst
electrically!
• When crystallised isothermally (right) iPP and PE12 also
show very poor breakdown performance
• In conclusion NONE of the propylene materials offer the
right combination of mechanical and electrical properties –
BLENDS
Selected blends I (Thermal)
•
Out of the materials sPP
and PE40 offer the best
(but still not ideal)
mechanical properties
•
Can we add some iPP to
overcome their poor
electrical performance
without introducing
unwanted brittleness?
•
Melting behaviour
indicates that both systems
can still operate to 120 oC
(better than XLPE)
Designation
Composition
Blend 1
20 % iPP in sPP
Blend 2
50 % iPP in sPP
Blend 3
20 % iPP in PE40
Blend 4
50 % iPP in PE40
Selected blends II (Morphology)
Blends 1 and 2 (sPP)
Crystals of iPP in an
sPP “matrix”, organise
into spherulites in
blend 2 (50 % iPP
content)
Blends 3 and 4 (PE40)
Crystals of iPP in a
largely amorphous
PE40 “matrix”
(organise into
spherulites in blend 4)
Selected blends III (Mechanical)
• The sPP based blends 1 and 2 exhibit undesirable necking
and are too stiff at low temperatures for a cable system
• Blends 3 and 4 show a more desirable (rubbery) behaviour
• All systems provide high temperature integrity to ~120 oC
but blends 3 and 4 provide mechanical properties close to
XLPE
Selected blends IV (Electrical)
• Under both quenching and isothermal conditions blends 1
and 2 exhibit dielectric strength better than XLPE!
• However, the current work indicates that blend 3 (20 % iPP
in PE40) offers the best balance of electrical and mechanical
properties
• Increased operating range to 120 oC is possible!
Conclusions
• Unfortunately none of the propylene based materials in
isolation offered good breakdown performance combined
with suitable mechanical properties for a cable
– As in the case of HDPE/LDPE blends, combining a
rubbery (PE40) system with a stiff (iPP) system
combines the rubbery nature of PE40 with the good
breakdown performance afforded by the iPP
– Care must be taken with this system to avoid slow
crystallisation which can lead to a more brittle system
• Advantages of such a blend include better breakdown
strength than XLPE and an improved operating
temperature range – thinner insulation for the same power!
Conclusions
Conclusions
• We have looked at various recyclable ethylene and
propylene based systems
– In isolation none of the single component systems
provides the required balance of properties for a cable
– Blending is a powerful tool for properties optimisation
allowing the advantages of different materials to be
combined effectively to yield a better composite system
– We have identified suitable ethylene and propylene
based blend systems having optimised properties for
future recyclable cable systems
Towards the future
• There has been considerable interest by various cable
manufacturers to commercialising these ideas
– TSB project at Southampton/Surrey is looking at “minicables” (both PE and PP based) – focussing on nonisothermal crystallisation, dielectric and mechanical
properties optimisation
– Papers in press – C. D. Green, A. S. Vaughan et. Al.
– Successfully making the transition from “laboratory
samples” to real extruded cable systems
• The future is indeed bright and we expect implementation of
practical recyclable cable systems within the next decade!
References
•
An investigation of the potential of polypropylene and its blends for use in
recyclable high voltage insulation systems, I. L. Hosier, A. S. Vaughan and S. G.
Swingler,, Submitted to J. Mat. Sci. (December 2010).
•
An investigation of the potential of ethylene vinyl acetate/polyethylene
blends for use in recyclable high voltage cable insulation systems, I. L. Hosier,
A. S. Vaughan and S. G. Swingler, J. Mat. Sci., 45, 10, pp. 2747-2459, 2010.
•
Propylene based systems for high voltage cable insulation applications, I. L.
Hosier, L. Cozzarini, A. S. Vaughan and S. G. Swingler, J. Phys.: Conf. ser., vol. 183,
012015, 2009.
•
Morphology, thermal, mechanical and electrical properties of propylenebased materials for cable applications, I. L. Hosier, S. Reaud, A. S. Vaughan and S.
G. Swingler, In Conf. Rec. of the 2008 International Symposium on Electrical Insulation,
pp. 502-505, 2008.
•
Effect of polyethylene on morphology and dielectric breakdown in EVA
blends, I. L. Hosier, A. S. Vaughan and W. Tseng, In Proc. 2007 International
Conference on Solid Dielectrics, pp. 184 – 187, 2007.