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Introduction
It has been know for a long time that the insulation value of some materials can
be dramatically increased by maintaining them in an evacuated environment. How much
vacuum is required and how much improvement is gained depends on the material and the
level of vacuum.
The "ultimate" example of vacuum insulation is the Dewar's
Flask, commonly known as a Thermos bottle. In a Dewar's Flask, no insulation
"material" is used at all. Instead, the space between the dual walls of a
cylinder is completely (99.999999%) evacuated. With virtually no molecules of gas
available to transport heat between the two walls, the "R" value is extremely
high - typically R250 or better. Unfortunately, the Dewar's Flask is not very
versatile. Because there is no support structure for the walls (the support
structure itself would transfer heat), the shape of the flask is limited to round, oval or
cylindrical. Additionally, since even a few molecules of gas will destroy its
insulation value, the cylinder walls must be absolutely impermeable to gas and
moisture. This limits the the wall material to either specially treated glass or
metal, both of which have a tendency to conduct significant amounts of heat at areas where
the walls are joined together.
Making A Flat Vacuum Insulation Panel
In an effort to improve insulation technology beyond that of common foams and
fiberglass batt (which rely on "trapped air"), engineers have spent years and
many millions of dollars trying to construct a flat panel that would take advantage of the
superior insulation value afforded by a vacuum. The task has proved much more
difficult than would at first seem apparent. Major technical challenges include:
1. Support of the flat walls.
Atmospheric pressure exerts approximately 15 psi (pound per square inch) of
pressure on the evacuated panel. This means that a vacuum panel which is 20"
square has 3 tons (6,000 lbs.) of force compressing it. Since it is not practical to
make the walls thick enough to support such pressure (remember, the walls themselves will
conduct heat where they join together), an suitable internal support material is needed.
This material (often called a "core" material) has to be strong enough to
take the tremendous pressure without collapsing and yet not transfer too much heat itself.
2. Gas Impermeable Membrane
Since the thermal performance of the panel will be proportionate to the internal
pressure, a membrane (i.e. "wall") material was needed which would minimize the
influx of gases into the evacuated space. Additionally, this material had to be low
in cost, easy to work with and easy to join together in an air-tight seal. Lastly,
the material has to be thin enough so as not to conduct a significant amount of heat
around the edges thus providing a "short circuit" for heat flow.
3. Getters And Desiccants
Getters and desiccants are used to absorb gases (getters) and moisture
(desiccants) within the evacuated envelope and prevent (or at least delay) an elevation of
the internal pressure and the degradation in "R" value that would result.
This gas and moisture may enter the vacuum panel in a number of ways including permeation
of the membrane material, permeation of the sealing seams and outgassing of the core
material and membrane itself.
Materials Used in Today's VIPs
 Vacuum insulation panels which offer excellent performance and long life (15+
years) have been around since the mid 1950's. Unfortunately, the production process
to make them has been both expensive and time-consuming. As such, few companies have
been able to develop more than small niche markets for the material. However, in the
past four years worldwide research on vacuum insulation technology has accelerated
dramatically. The goal of this research is to develop new materials and processes
that will significantly reduce VIP production time and cost. For the winners in this
technological race the potential payoff can be tremendous as VIPs are projected to take
over major portions of the world insulation market in the near future if they succeed.
Core Materials
In VIPs, the core material serves two major functions. First, it provides physical
support to the membrane (or barrier) film envelope so that it does not collapse in on
itself when the vacuum is applied. Secondly, the core material acts to interrupt the
flow (free mean path) of the molecules of gas which still remain in the evacuated space,
thereby reducing their ability to transfer heat between the walls of the VIP.
Established and proven core materials include Perlit, mineral powder, mineral fiber,
fiberglass and silica. While most of these materials are not very expensive in their
raw form, they require considerable handling and "pre-processing" which greatly
increases the cost of the end product. A number of new core materials have recently
been developed. These fall into two broad categories open-cell foam and
carbon/silica aerogels. Dow Chemicals and ICI Chemicals have introduced new foam
core materials consist of specially formulated open-cell polyurethane (ICI) and
polystyrene (Dow) foams. They are designed to allow faster evacuation and easier
handling than the older materials identified above. While these new materials permit
much lower-cost production, they are much more sensitive to the influx of gas and moisture
than were the older materials and have a greater tendency to outgas. As such, they
panels produced with foam cores have an inherently shorter life span than do panels made
with most other materials. How much shorter is a question under much debate.
Ultimately the success or failure of these foams will depend on the progress made towards
improved membrane films and getters.
Glacier Bay's BARRIER Ultra-Rtm is the first commercially
available superinsulation panel to utilize a carbon/silica aerogel core. While
considerably more expensive than other core materials, aerogel achieves extremely high
R-values with less vacuum than would be required with other types of cores.
Additionally, aerogel (like precipitated silica) acts as its own getter and
desiccant. These properties combine to give aerogel-based panels a useful life
unmatched by any other core technology.
Membrane Films
The membrane film is the materials which forms the walls of the VIP. It is the job
of the membrane film to provide an effective barrier against all atmospheric gases and
moisture so that the vacuum can be maintained. The impermeable membrane materials
are glass and metal. Unfortunately glass is far too fragile. Metal can be used
but significantly reduces the average insulation value of finished panel due to the
conductance of heat around the edges where the walls are joined (i.e. "edge
effect"). Additional disadvantages of a pure metal membrane is the high
cost of forming and welding the panel.
Because of these problems, many alternative and compromise solutions have
been tried. In some films, a very thin metal film (usually aluminum) is reinforced
by laminating a plastic film on each side. A special plastic with a low
melting temperature is then added so as to allow the finished laminate to be "heat
sealed" rather than welded. These films can have excellent barrier properties
but can still conduct significant heat around the edges. In an effort to reduce this
"edge effect" even further, some films use a "sputter-coated" thin
film deposition technique to get the metal layer even thinner. When done correctly,
these films offer a good compromise between the solid metal films and pure plastic
laminates. Unfortunately, quality control and good consistency in the metal
deposition can be a problem making extensive post-production panel testing imperative.
Films comprised entirely of plastic laminates can be used in situations
where a great deal of getter/desiccant is available (such as when a silica or aerogel core
is used), or when the required panel life is not too great. These films can consist
of up to nine layers of various plastics, each of which offers good barrier properties
against a particular gas. Because these films have virtually no "edge
effect", they are the focus of much research and have been used widely in the past
with the older core materials. The intolerance of the newer "foam"
type core materials to gas influx (see chart below), combined with their minimal
getter/desiccant capacity, precludes the use of today's plastic laminates with these
cores.
Getters and Desiccants
Getters are chemicals which absorb gases, desiccants are chemicals which absorb moisture.
Getters and desiccants are used to extend the life of VIPs by absorbing unwanted
gases and moisture which prevents a rise in pressure within the evacuated space. To
be effective, the getters and desiccants must be carefully matched to the kind and
quantity of gas/moisture they will be expected to absorb. They must also be capable
of effectively absorbing and holding the gasses and moisture at the low pressures inside
the VIP. It is, therefore, important that the quantity and type used be selected in
consideration of the core material, membrane film and required life expectancy.
It is worthwhile to note that several of the older core materials as well
as aerogels are themselves getters and desiccants when properly pre-treated. For
this reason many early panels did not require additional chemicals to be added.
The Lifespan Of VIPs
The life expectancy of a vacuum insulation panel is determined by a number of
factors. Specifically, these are;
1. The initial vacuum level of the panel.
2. The permeation rate of the membrane film.
3. The outgassing (if any) of the core material and membrane film.
4. The permeation rate of the membrane sealing edge
5. The quantity and effectiveness of the getter and desiccant.
6. The effect of pressure rise on the specific core material.
Initial Vacuum Level
Unlike a Dewar's Flask, flat vacuum panels do not maintain a "perfect
vacuum". Most flat vacuum panels are initially evacuated to an internal
pressure of about .05 torr (.066 mbar or .00097 psi). To evacuate the panels to a lower
level would add significantly to the production cost and, in most cases, does not result
in a higher R value. While this level of evacuation is typically the goal,
variations in the production process can cause some panels to only achieve only partial
evacuation. Panels which start out with a higher internal pressure will have a
proportionately shorter effective lifespan than will an, otherwise identical, panel which
is more thoroughly evacuated.
Membrane Permeation Rate
As discussed earlier in this paper, only welded metal and glass are completely air
tight and these materials have significant disadvantages in VIP applications. All membrane
films in use today permit some molecules of gas and moisture to pass through over time.
How much gas passes through the membrane and, how effectively the core and getter
deal with this gas, will have a major effect on panel life. The amount
of permeation through a particular membrane film will depend on the material of its
construction and the resistance of this material to degradation during handling in the
production process. Some films will handle the stress of folding and processing (a
necessary part of panel production) much better than others. A given film can
perform very well in laboratory permeation tests but begin to permeate too easily when
flexed or folded.
Outgassing
Most materials release gases (outgas) when placed in a low pressure environment.
The kind and quantity of gas released, as well as the length of time the outgassing
will continue, varies from material to material. The released gases can contribute
substantially to the rise in internal pressure (i.e. loss of vacuum) of a VIP. In
some cases, the rate at which gas released from the core and membrane materials exceeds
that at which it permeates through the membrane. A few materials, such as silica and
metal do not outgas at all, while other materials never stop outgassing. The core
and membrane materials used by a particular manufacturer will determine what, if any,
impact outgassing will have on the life of their product.
Sealing Edge Permeation
All VIPs are comprised of membrane films which are sealed at the edges to form an
envelope for the core material. In earlier panels which used 100% metal membrane
films, these edges were welded or soldered. In most of today's membrane films, a
thin layer of low temperature plastic is laminated to the inside of the film so than it
can be sealed using heat and pressure. Unfortunately, these layers of heat-sealing
plastic do not have the same resistance to gas and moisture permeation as does the rest of
the film. To minimize the negative impact of permeation of the sealing layer,
manufacturers use as thin a film layer as possible combined with a wide seal lip. The
compromise here is that it is much more difficult to make a good quality seal using a thin
sealing layer and and wide lip than it is to make one with a thick layer and narrow lip.
A thick plastic layer has the ability to "fill in" small wrinkles and
crevices which a thin layer does not. How thin a layer a particular VIP manufacturer
will need depends on their sealing equipment as well as their process and quality control.
Getters and Desiccants
Continuous absorption of extraneous gases and moisture is a vital consideration in
extending the useful life of any VIP. By trapping and holding these gases (whether
from outgassing or permeation), the getters and desiccants prevent the internal pressure
of the panel from rising, thereby preserving an "as new" insulation value.
In silica-based panels (such as the Glacier Bay BARRIER panels), the core material
also serves as the getter and desiccant. In foam-based panels (such as VacuPanel,
Inc.'s VIPs), the core material has no absorbent capacity at all. It is, therefore,
necessary to add these chemicals into the VIP envelope. These chemicals much be
matched to the types of gases which the foam outgasses. Since the addition of these
getters adds cost and reduces the insulation value of the foam panels, a compromise must
be made with the desire to extend panel life. One obvious benefit of using silica as
a core material is the very large amount of getter which is present.
Effect of Pressure Rise
All vacuum insulation panels rely on high vacuum to give their high "R"
values. As the level of vacuum in the envelope decreases - so does the "R"
value. However, the relationship between internal pressure rise and decreasing
"R" value varies tremendously with different core materials. The graph
below compares the effect of rising internal pressure on Glacier Bay's BARRIER Ultra-R
aerogel-based VIPs and panels produced using other core materials. Note that while
all materials offer satisfactory performance at the highest evacuation levels, there is a
dramatic difference between them with only slight increases in internal pressure.
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