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Accelerated lifetime testing of Accelerated lifetime testing of
vacuum insulation panelsvacuum insulation panelsDouglas Smith, Steve Wallace, Dan Martinez and
Bob Keller
NanoPore Insulation LLC
3721 Spirit SE, Albuquerque, NM 87106 USA
Quentin Shrimpton
NanoPore Insulation Ltd.
Brimfield, UK
What is NanoPore Insulation LLC?What is NanoPore Insulation LLC?
• NanoPore Insulation LLC is a joint venture of NanoPore Inc. and Sealed Air corporation (a 4 billion+ $ packaging company with extensive expertise in high-speed vacuum packaging and barrier films)
• NanoPore Insulation LLC has three US-based VIP production lines including a new automated facility and a UK plant producing VIPs to the same specifications.
• NanoPore is the only producer of nanoporous carbon-silica vacuum insulation panels (VIP’s) and has been producing these for >10 years.
• NanoPore has developed and continues to develop a range of barrier/core materials optimized for various applications
• In ’09, we will produce >1M VIPs and >3M in ‘10.
Lifetime of InsulationLifetime of Insulation
• The thermal performance of all thermal insulation degrades over long time periods because of:– Water adsorption/absorption
– Structural degradation from thermal and barometric cycling, water adsorption, etc.
– Outgassing
– Temporal decomposition for foams
• How to define lifetime for VIP’s? Difficult given the logarithmic change of thermal conductivity with pressure/time
• The lifetime of a VIP is more dependent on both its’ use conditions and dimensions than other insulation
• We normally define predicted lifetime as the time required to lose 10% of the thermal resistance under use conditions
Key issues in VIP lifetime Key issues in VIP lifetime
• Operating temperature and humidity
• VIP design
– Core material
– Barrier film
– Getters/desiccants (if any)
– Thickness (affects the surface area to volume ratio)
– Seal layer (material type, thickness, width)
• Barrier film specifications are only a starting point since they will degrade over time with:
– VIP production
– VIP handling and system integration
– Barometric pressure cycling
– Thermal cycling
• Lifetime performance in a “system” can be very different (better or worse) than that for the VIP only.
Effect of flexing Effect of flexing • We know that the WVTR and OTR (hence lifetime) of metalized
films changes dramatically (>100x worse) with flexing.
• Impossible to relate laboratory flex testing back to the real world of VIP use in production and application!
• EvOH-only does not degrade but not nearly as good to begin with!
• No data on either mEvOH or mPET with an EvOH seal layer
OTR (cc/m2d)
Virgin Flex/Lab 1 Flex/NP 2 cycles Flex/NP 10 cycles
Met #1 <0.01 0.75 1.5 4.3
Met #2 <0.0005 0.15 na na
EvOH <0.2 0.06 na na
WVTR (g/m2d)
Virgin Flex/Lab 1 Flex/NP 2 cycles Flex/NP 10 cycles
Met #1 0.02 0.20 0.12 0.25
Met #2 0.01 0.084 na na
EvOH 2.12 2.43 na na
Temperature dependence on Temperature dependence on
barrier film permeationbarrier film permeation
0.0001
0.001
0.01
0.1
1
20 40 60 80 100 120 140
Temperature (oC)
WV
TR
(g
/m2d
ay
)
B4-coated B1-virginB1-2 twists B1-10 twistsB2-virgin B3-virginF1-virgin
• There is a strong temperature dependence on gas/vapor
permeation rates through VIP barriers. Going from 20 to
80 oC represents a ~20x increase in aging rate
• Results for:
– current virgin films
– current flexed films
– next generation barriers
Water Vapor: Water Vapor: TheThe problem for problem for
nonnon--nanoporous cores!nanoporous cores!
• Typical ambient water
vapor pressure is 5-30 mbar
• In buildings, the VIP’s
coldest side sets the vapor
pressure (<ambient)
• For foams and fiberglass,
the useful capacity of the
desiccant is used up at <5%
relative humidity!
• Foams/fiberglass VIPs are
just not applicable for
building applications,
impossible to add enough
desiccant!
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0.1 1 10 100 1000Pressure (mbar)
Co
nd
uc
tiv
ity
(W
/mK
)
silica-carbon
Foam
Fiberglass
ambient
humidity
Water Vapor in ConstructionWater Vapor in Construction
• During summertime, maximum water vapor pressure is the inner wall (22 oC/100% RH) or ~25 mbar independent of the atmospheric
temperature/humidity.
• For winter, even at 100% RH, the lower outer wall temp. means that water
vapor will actually be leaving the VIP, not entering!
• Because of the high adsorption capacity in the 70-80% RH range, water vapor
pressure in the VIP should never exceed 20 mbar in most geographies (except
tropical)
0
5
10
15
20
25
30
35
40
45
0 5 10 15 20 25 30Temperature (oC)
Vap
or
pre
ssu
re (
mb
ar)
0
4
8
12
16
20
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Relative Humidity RH %
We
igh
t o
f H
2O
(w
t %
)
Water vapor pressure at 100% RH Water adsorption on NanoPore core
Does the core really matter?Does the core really matter?
• Compare a silica-carbon VIP with no desiccant to a commercially available (for
warm applications) fiberglass VIP with a desiccant
• 25 mm thick panels and both have similar metalized barriers
• Store at 80 oC, cool to ambient and measure the panel pressure. Convert the
pressure to the room temperature conductivity
• Fiberglass VIP conductivity doubled in two days!
• Silica VIP reached a pressure plateau and has now been aged for over ~450
days at 80 oC with only 20% loss in thermal performance.
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.008
0.009
0.01
0 20 40 60 80 100
Days at 80 oC
Co
nd
uc
tiv
ity
(W
/mK
) a
t 2
0 o
C
silica-carbon
Fiberglass
Long time 80 Long time 80 ooC accelerated testingC accelerated testing
• Compare 25 mm silica-carbon VIP’s with no desiccant and
different barrier materials
• The two metalized barriers have the same 3xmPET structure!
• Store at 80 oC, cool to ambient and measure the panel pressure.
0
10
20
30
40
50
60
70
80
90
100
110
120
0 50 100 150 200 250 300 350 400 450 500
Time (days)
Pre
ss
ure
(m
ba
r)
EvOH EvOHSiO2 #1 SiO2 #1
metallized #1 metallized #1metallized #2 metallized #2
Thermal cyclingThermal cycling
• In a 50 year service life, a VIP will go through >18,000 thermal cycles
due to diurnal temperature variation!
• Silica CTE is different than plastic barriers and other building
materials.
• Thermal cycling induces repeated stress on the barrier material as the
plastic barrier expands and contracts with ambient temperature
variation.
• Stress from barometric cycling is much lower
Material CTE (ppm /oC)
Silica 0.5
PET 73-92
Aluminum 22
Mild steel 12
Wood (perpendicular to grain) 30-50
Wood (parallel to grain) 3-5
Polyurethane 40-160
Masonry brick 6-9Glass 6-9
Thermal cycling (cont)Thermal cycling (cont)
• In buildings, although the interior side of the VIP is essentially
isothermal, the outer VIP surface temperature can vary significantly
depending upon the climate zone.
• Not normally addressed in VIP lifetime analysis! Not an issue for
many VIP applications such as refrigeration (where temperatures are
essentially constant with time)
• Effect of thermal cycling is system design dependent.
• Depends on CTE and modulus of adjacent layers.
– Best case is a VIP only (just CTE mismatch between core &
barrier)
– Worst case is a VIP bonded directly to a metal with a rigid
adhesive
• Stress relief at the VIP interface is a major design consideration
Thermal Cycle TestingThermal Cycle Testing
• Multiple silica/carbon VIPs in an Aluminum/PU
foam/VIP/PU foam/plastic sandwich
Construction Diagram
Chemlite Liner
Aluminum Shin
(0.050" Thk)
Vacuum
Insulated Panels(½" Thk) (x3)
PU Foam Board
(½" Thk) (x2)
Gap filled with foam
1 ½
"
38"
All mating surfaces
glued with PU glue
Build-up
Thermal Cycle Panels
• Attached Aluminum and FRP skins during second gluing operation
• Mounted heat flux sensor to inside of skins at center of door
• Filled voids at edges with expandable PU foam.
• Built-up core (PU/VIP/PU) first
• Attached surface mounted, Type-T thermocouples to each side of VIP’s (8 total)
• Packing foam attached to side of VIP to prevent wear
• Applied PU glue to all surfaces
Thermal Cycle TestingTest Set-up
• Composite panel
mounted to chamber
• Interior cycled from
50 °C to –30°C (i.e.
much larger swing
than normal building
applications to
accelerate aging
• Exterior~20 °C
• Measurements
– 2x Heat flux sensors
with integrated TC
– 8x Surface TC
– 8x Embedded TC on
VIP
Heat Flux
Sensors (x2)
Thermocouples(x18)
Composite Door
Temperature Profile
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100
110
120
130
0 30 60 90 120 150 180 210 240 270
Elapsed Time (Min)
Te
mp
era
ture
(°F
) .
Environmental
Chamber
8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00
Time
-30
-15
0
15
30
45
60
75
90
105
120
135
Te
mp
era
ture
[F
]
21
Inner Skin Temperature
Outer Skin Temperature
Inner VIP Temperature
Outer VIP Temperature
Single cycle• Because of the low thermal diffusivity of VIP’s, a single cycle takes
~6 hours to stabilize at high and low temperatures
• In climates with large diurnal variation (such as New Mexico!), this
shows that it is not just thermal conductivity that matters but also
thermal diffusivity. With more thermal mass or thicker VIPs, this
effect becomes more pronounced!
Thermal resistance versus time• 647 cycles (>160 days), there was minimal thermal resistance change
• After 647 cycles, a temperature controller malfunction caused the
chamber to spike to >100 oC so the test was stopped and panels were
pressure tested
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0 200 400 600 800
# of thermal cycles (-30 C to 50 oC)
No
rmali
zed
th
erm
al
resis
tan
ce
NV1DT
NV2DT
NV3DT
NV4DT
Post thermal excursion analysis
• Hot side PU showed significant thermal degradation as
compared to cold side
• All VIPs were intact! Pressure was measured and found to
be between 10 and 13 mbar despite the hot side excursion
Cold side PU
Hot side PU
VIPs
Summary• For long-life building applications, only nanoporous core
materials such as silica are appropriate because of the
difficulty of maintaining water vapor pressure below 1 mbar
for cores with larger pores (foam/fiberglass).
• Assuming that water vapor is not an issue (i.e, core yields
good thermal performance in the ~20 mbar range such as
for some silica panels), 80 oC accelerated testing indicates
lifetimes well over 30 years with minimal (<20% thermal
performance degradation depending on the barrier material).
• This study seems to be the first long term, accelerated
testing on thermal cycling of VIPs in an integrated system.
• Thermal cycling means that VIP system design is an
important factor in VIP lifetime.