Note: Descriptions are shown in the official language in which they were submitted.
W 0 92/16702 ~ 3,l.~ PC-r/U591/0762
LIG~T ~DMITTING T~ER~D~ IN8~I~TING 8TRUCTlnRE
BACXGRO~nND OF THE INrVENTION
The present invention relates to an improved light
admitting thermal insulating structure which prevents heat
loss by thermal radiation, convection and conduction,
including controllable transmission and/or reflection and/or
absorption to light, and a convection baffle which transmits
light and thermal radiation or "CBTLTR" to prevent
convection loss.
o Conventional collectors of solar heat include a dark
absorber surface that turns sunlight into heat ~nd a
transparent cover for this surface to prevent the heat from
escaping. The thermal collection efficiency of such a
system is determined by the ratio of the resistance to the
flow of heat of the transparent cover to the resistance of
the rest of the system. By increasing the thermal
resistance of the transparent cover without greatly reducing
its light transmission, the efficiency and/or operating
temperature of the solar heat collector can be greatly
improved. It is estimated that two or three times the
energy consumed in heating a well insulated building falls
on its surface in the form of sunlight. Thus, an insulation
that is transparent when the sun shines would provide most
of the heating for a structure over most of the United
States if it is coupled with a heat storage system for
cloudy weather.
In the past the problems of heat loss through
conduction, convection and thermal radiation (also called
far infrared radiation) and control of sunlight have been
dealt with independently. For example, convection and
conduction losses are reduced using spaces filled with some
fine structural material without consideration of admission
of sunlight. Further,lsunlight has been blocked using
coatings of layers on windows to prevent room overheating
but without consideration of a technique for admitting more
sunlight when it is desired to increase the heat in the
::... .
., , . ,: ... . : . . .: ... ~ . . . ..
W092/16702 ~1 PCT/USg1/07620
~ 2-
room. Thermal radiation loss has been of little concern
since in the latter situation heat loss is desirable and in
the former it is not considered.
Since the heat losses by conduction, convection and -
radiation are in parallel to each other and are of similar
magnitudes, an insulation is ineffective unless all three
are dealt with simultaneously, as heat will leave by the
path of least resistance.
To best understand the invention, reference is made to
the Concise Encyclopedia of Science ~ Technoloqy, Second
Edition, McGraw Hill (1989) for a general definition of
terms. Specifically, reference is made to Solar Optical
Materlals, edited by H. G. Hutchins, Permagon Press (1988);
Transparent Insulation Materials and TransParent Insulation
T2, both edited by L. F. Jesch, Franklin Co. Consultants
Ltd. for the German Solar Energy Society (1986 and 1988,
respectively); Large Area Chromogenics, edited by C. M.
Lampert, SPIE Optical Engineering Press (1988); Spectral
Selective Surface for Heatina and_Cooling_ApPlications, C.
G. Granqvist, SPIE Optical Engineering Press ( ); Optical
Materials TechnololY of Energ~, Efficiency and Solar Enerqy
Conversion, edited by C. G. Granqvist, Vol. 9 (1990), Vol. 8
(1989), Vol. 7 (1988); Material & Optics~for Solar Ener~y
Conversion and Advanced Liq~htinq Technoloqy, edited by C. M.
Lampert, SPIE Optical Engineering Press (1986); Solar
Glazinq, Mid Atlantic Enerqy Association Topical Conference
(1979); and Thermal Shutters and Shades, W. A. Shurcliff,
Brickhouse Press, Andover, Mass. t ). These publications
set forth a comprehensive overview of technology related to
this invention.
A light admitting insulation may be used in conjunction
with an optical shutter to regulate light transmission while
preventing the flow of heat. The optical s~utter may be a
layer or layers covering an aperture. The shutter may be
reversibly activated by: (1) its local temperature
(thermochromic); (2) incident light intensity
(photochromic); (3) both temperature and light
- . . .- ,, .-, . : , , . : ,
:, ' . ~ '., ~ .
W~92/16702 ~lv~ 3 PCT/US91/07620
--3--
(thermophotochromic); or (4) an electric current or field
(electrochromic). The combination of transparent
insulations with a thermochromic optical shutter is the
subject of U.S. Patents 4,085,999 and 3,953,110 by the
applicant herein. Thermochromic and thermophotochromic
shutters, not in combination with a transparent insulation,
are the subject of U.S. Patent 4,307,942 and U.S. Patent
Application Serial No. 06/948,039 by the applicant herein
entitled "structure and Preparation of Automatic Light
Valves," filed on December 31, 1986. See also, "Thinkin~ -
Window switches off the sun When it is Hot", Popular
Science, March, 1984, and my article "contractor Desiqned
Passive Heatina Coolinq, and Davliahtinq", u. s. Passive
Solar Conference (March 1990).
None of the patents or the application mentioned above
provides the important advantages of addressing all forms of
heat losses and heat uses for particular applications
including the combination of a transparent insulation with a
layer of photochromic or thermophotochromic shutter.
SUMMARY OF THE INVENTTON
It is an object of this invention to provide an
improved light admitting thermal insulating structure which
simultaneously substantially reduces heat loss by thermal
radiation, convection and conduction.
It is a further object of this invention to provide a
light admitting thermal structure including a convection
baffle having controllable transmissivity to light.
It is a further object of this invention to provide a
simple, inexpensive light admitting insulating system for
improving the efficiency of collectors for solar space
cooling, water heating and reducing heat losses through
windows as well as improving the efficiency of solar space
heating systems.
According to the linvention, a light admitting thermal
insulating structure is provided having controllable
transmissivity to light including a first layer generally
transparent to light and a second layer transparent to or
" : , ;. , . . . . , - ,. ,,., . ~ ~ ~
WO92/16702 3 PCT~US91/07620
~ ~ U ~j~
absorbing of light and spaced fro~ the first layer. If the
light admitting thermal insulatinc~ structure has a light
transmitting state it is call a "t:ransparent" or "light
transmitting" thermal insulation structure. There is a
partition separating the space bet:ween the layers into
compartments and reducing convection losses. There is a
thermal radiation suppression device for suppressing thermal
radiation transmission and, in so~e embodiments, a variably
transparent control device which controls transmission of
light.
In more detail, the structure may include a convection
baffle which transmits light and thermal radiation and one
or more low emissivity layers, as well as an optical shutter
which controls light transmission. Also, the structure may
include an antireflection coating having low light and
thermal radiation absorption disposed on one or more of the
baffle surfaces. Further, the baffle may be constructed
from a polyolefin selected from the group consisting of
very high crystallinity polyethylene or very low
crystallinity polyethylene thereby defining very low
polyethylene.
Further, the structure of the present invention can be
constructed as a thermal insulating building panel having
controllable light transmission. A panel comprises a
convection baffle which transmits light and thermal-
radiation between the inside and outside of the panel, one
or more low emissivity layers near the outside surface of
the building, and an optical shutter layer near the inside
of the building. It is pointed out that the invention is
not limited to the structure described, but is intended to
broadly cover processes of converting incident light to
other forms of energy by using the inventive light admitting
ther~al insulating structure.
As pointed out in greater detail below, this invention
provides important advantages. Heat losses (by conduction,
convection and radiation) are all dealt with simultaneously
which prevents heat loss by the path of least resistance.
~, . , - -
.
W092/1~702 ` PCT/US91~07620
--5--
Further, the combination of a transparent insulation with a
layer of photochromic or thermophotochromic optical shutters
prevent the flow of heat and heat loss while regulating the
flow of light.
The invention itself, together with further ob~ects and
attendant advantages, will best be understood by re~erence
to the following detailed description taken in conjunction ;... .
with the accompanying drawings. :~
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a block diagram showing the functional
components of the light admitting thermal insulating system ..
according to the present invention; :
Figure 2 shows a convection baffle which transmits
light and thermal radiation suitable for suppressing -
convective heat transfer dividing a cavity into compartments
oriented approximately parallel to the aperture it covers
according to the present invention;
Figure 3 shows a CBTLTR suitable for suppressing
convection heat transfer dividing a cavity into compartments
oriented approximately perpendicular to the aperture it `
covers according to the present invention;
Figure 4 shows a CBTLTR suitable for suppressing
convective heat transfer dividing a cavity into compartments ~- :
using parallel sheets within a frame supporting the edges of : -
the sheets according to the present invention;
Figure 5 is a cross section of light admitting thermal
insulating structure using an illustrated modification of :
the CBTLTR of Figure 4 to illustrate the nature and type of :
each surface and layer according to the present invention;
Figure 6 is a transparent insulation'which is another ::
embodiment of the light admitting ther~al insulating
structure of Figure 5 according to the invention;
Figure 7 shows the depen~ence of light transmission on
- both temperature and incident light intensity for a typical
thermophotochromic optical shutter according to the present
invention;
~, . . . , . . , ,, ,, , . , ~ . . . . . .. . .. .. . ... ...
WO92/16702 ,~ PCT/USgl/07620
Figure 8 shows in cross section a light transmitting
building panel such as a window, skylight, or other solar
collector, which is another embodiment of Figure s using an
optical shutter according to the present invention;
Figure 9 is a cut away view of the panel of Figure 8.
DETAILED DESCRIPTION OF' THE INVENTION
Turning now to the drawings, Figur~ 1 is a block
diagram showing the functional components of the light
admitting thermal insulating structure according to the
invention. Figure 1 is a functional block diagram of a
light admitting thermal insulating structure 10 having
controllable transmissivity to light. The insulating
structure 10 includes means for suppressing convection,
conduction and thermal radiation heat losses, e.g.,
convection suppressor 12, conduction suppressor 14, and
thermal radiation suppressor 16 which is a low emissivity
layer. For illustrative purposes, each of the suppressor
12, 14, 16 of thermal transport mechanisms extend over the
whole system but are shown distinctly for convenience in
explanation. ~ctually they are superimposed.
A light control 18, such as an optical shutter, adjusts
the transmissivity of the structure 10 to light in
accordance with the desired illumination level or the
temperature of the area whose environment is to be
controlled. As shown in Figure 1, the incident light 11 may
pass through the light control 18 to provide transmitted
light 13 or may be reflected to become reflected liyht 15.
If it is transmitted, it may be absorbed on light absorber
17 which may or may not be an integral part of the
structure.
The conduction suppressor 14 may be provided through
the use of two or more spaced layers or compartments with a
gas, vacuum or other medium between them to prevent
conduction.
The thermal radiation suppressor 16 may include one or
more coatings or layers of material which reflect and does
not emit thermal radiation to prevent its transmission.
.. . . . . - - . , . ~ . ., . ~ - , ,
W092/16702 ~ 9 73 PCT/US91/07620
-7
These layers are called low emissivity layers. They may be
transparent or may absorb light. The transmitted light 13
may then be absorbed in light absorber 17, where it turns
into heat. If the light absorber 17 is part of the
structure, then it may also be combined with the thermal
radiation suppressor, herein called a "light absorbing low
emissivity layer."
The convection suppressor 12 may include ;
compartmentalizing the space between the layers or providing
a vacuum therein. Another example of a thermal and
convection suppressor is a finely structured, low density
silica or other oxide(s) foam called "aerogel". The
compartmentalizing may be accomplished by baffles or
partitions which extend transversely between the layers
and/or parallel to the layers, such as a CBTLTR, to restrict
convective heat transport.
As illustrated in Figures 2, 3 and 4, a convection
baffle which transmits light and thermal radiation
("CBTLTR") can be made from a thin sheet or film of a light
and thermal radiation transparent material. Its function is
to divide a gas filled cavity into compartments, and to -
thereby suppress convective heat transfer by the gas inside
- the cavity.
In Figure l, the CBTLTR is an example of the convection
suppressor 12. Figures 2, 3 and 4 show three possible
configurations for CBTLTRs which all divide a cavity into
compartments, and thereby suppress convective heat transfer.
Figure 2 shows a CBTLTR made from a polyolefin film 21
(for example, polyethylene) which may be heat sealed 22
together or assembled by another state of the art means.
This embodiment of a CBTLTR has its compartments 23 oriented
approximately parallel to the aperture it covers.
F'igure 3 shows a CBTLTR 30 formed from a honeycomb
whose plurality of compartments 3l are oriented
approximately perpendicular to the aperture it covers. This
example of a CBTLTR may also be made from polyethylene film
32.
- - . ... . . . . .. . . . . .
a- Pcr/US9l/07620
FigurP 4 shows a CBTLTR 40 formed from parallel sheets
41 with a frame 42 supporting the edges of the sheets. The
sheets may be made from polyethylene film.
Figure 5 shows a light admitting thermal insulating
structure where each surface and layer of the structure are
labelled according to the specific function performed, e.g.,
such as CBTLTR 512, 513, 514 of the Figure 4; cover sheet
515, inner surface transparent cover sheet 502, etc. By
changing what the surfaces and layers are, this figure will
be used to illustrate most of the remaining embodiments of
this invention. The scope of these embodiments are not
limited to Figure 5, however, since Figures 2 or 3 or some
other configuration could have been used as easily as Figure
4 as the basis for the figure illustrating the following
lS discussion. For exampie, while the number of layers shown
is five, this number is arbitrary, and is only for the
purposes of illustration only. Further, because of the
general nature of Figure 5 in illustrating numerous spatial
relationships between the various layers and surfaces, other
embodiments as hereinafter described may refer to the
identifying numerals of the layer or surface as different
elements.
The CBTLTR helps keep heat from being transmitted by
not absorbing thermal radiation. If the CBTLTR were made of
a thermal radiation absorbing and emitting material (that is
if it had a high emissivity), then this material would
transfer the heat it absorbs from convection of the gas into
thermal radiation, which would then transfer the heat out.
A CBTLTR can also be used to improve the thermal
resistance and/or reduce the cost of light admitting thermal
insulating structures which use one low emissivity layer.
It also reduces the cost and light transmission losses of
light admitting thermal insulating structures using more
than one low emissivity layer by reducing the number of low
emissivity layers required to achieve a given resistance.
One or more of these low emissivity layer or layers can be
either light transmitting or absorbing.
Wos~/16702 , PCT/US91/07620
t~3
_9_
In a light admitting thermal insulating structure which
uses more than one low emissivity layer in order to increase
its thermal resistance, substituting layers of CBTLTR for
one or more of the low emissivity layers will reduce cost
and improve light transmission with but only slightly
reduced thermal resistance. Thus, it is possible to make
higher light transmission and/or lower cost light admitting
thermal insulating structures by using CBTLTRs.
A CBTLTR can also be used to improve the thermal
resistance of transparent insulations which are used with a
high emissivity layer in order to radiate heat. A CBTLTR on
top of a high emissivity layer can be used to cool a
building when they cover its roof. The high emissivity
layer radiates thermal radiation to the upper atmosphere,
which in dry cli~.ates and at night is as much as 30F cooler
than the temperature of the air at ground level. The CBTLTR
forms an insulation by thermally isolating the cooled
thermal radiation radiator from warm ambient air. At the
same time the CBTLTR is transparent to thermal radiation,
allowing the radiator to operate. For these applications,
the CBTLTR should be weather resistant, inexpensive, easy to
install and creep resistant.
A CBTLTR and a low emissivity layer that absorbs light
can be used to absorb solar heat efficiently. In this case,
Z5 in Figure 1, thermal radiation suppressor 16 and light
absorber 17 are combined in one layer. This layer is called
a "low emissivity layer which absorbs light" or a "selective
black". The CBTLTR 512, 513, 514 of Figure 5 forms a
transparent insulation by preventing convection betwèen the
transparent outer cover sheet 515 of Figure 5 and the low
emissivity, sunlight absorbing inner layer 509 of Figure 5.
The inner surface transparent cover sheet 502 of Figure 5
can be either low emissivity or high emissivity layer. For
these applications, the CBTLTR should have high light
transmission and heat resistance as well a~ the performance
characteristics listed above.
. .
33 i ~ -lo- P~r/US91/07620
since chemical degradation processes generally occur as
an exponential function of temperature, it is useful to have
converters of light to heat, such as solar collectors,
become opaque when either the absorber surface or the
outside air exceeds a rertain temperature. Thus, in a
modified embodiment of the invention of Figure 5, a
structure which converts light to heat includes a
thermochromic shutter (at numeral 502), a CBTLTR (at
numerals 512 through 514), and a light absorbing low
emissivity layer (at numeral 515). When the outside
temperature is high and solar energy is not needed, the
shutter is opaque.
Alternately, for use, for example, in a greenhouse
having varieties of plants, another embodiment of the
invention as illustrated in Figure 5 would define the
thermochromic shutter layer (at numeral 509) with a light
absorbing layer behind the shutter (at numeral 510), a
transparent low emissivity layer (at numeral 502), and the
CBTLTR (at numeral 512 through 514). In this case, the
converter of light to heat becomes reflective when its
absorber surface exceeds a preset temperature.
It should be noted that the converter of light to heat
may not be intended as such. Photocells are intended to be
a converter of light to electricity and convert light to
heat to its own detriment. In this case, a CBTLTR and a low
emissivity layer would also be detrimental. The a~ove
structures are only three of many possible examples of
structures which protect converters of light to other forms
of energy with optical shutters.
The CBTLTR and low emissivity layers which transmit
light can be used as a building or other surfaces which
capture solar energy. The sunlight can be used for space
heating, illumination and growing plants. In this case, the
one or more transparent low emissivity layers can be on any
of the CBTLTR surfaces such as shown at numerals 503 through
508 or on the inner surfaces 502, 509 of the transparent
covers tor glazings) 511, 515, which face the CBTLTR. In
WO92/16702 2 ~ V PCT/US91~07620
--11--
these applications, as in the applications described above,
with light absorbing low emissivity layers, the CBTLTR
serves to improve the performance of the low emissivity
layer or layers by suppressing convection, while not
interfering with light transmission.
In another embodiment the Figure 5 representation shows
- an improved transparent insulation using a CBTLTR. In this
case, the outermost layers 511, 515, can be either
transparent, absorptive or reflective (low emissivity) of - -
lo thermal radiation. It is preferred that the layers 511, 515
are opaque to thermal radiation. The inner layers 512, 513,
514 are preferably transparent to thermal radiation so that
they form a CBTLTR. The transparent low emissivity layer
may be on any or all of the surfaces 502 through 509.
~5 Figure 6 shows a window 60 with a high, insulating
value of about 7 square feet hour degree Fahrenheit/BTU
(similar to an opaque insulated wall) and a high light
transmission of about 60%. It is made using one transparent
low emissivity coating and two layers of CBTLTR. For these
applications, the CBTLTR should have all of the performance
characteristics listed above, although heat resistance may
not be as critical. Additionally, the CBTLTR shoul~ not
impair viewing through the windows.
As shown in Figure 6, two light transmitting cover
layers 61 can be made of glass, preferably of low iron
content to prevent absorption and heating by sunlight.
Alternatively they can be made of fiber reinforced polymer
sheets which may be translucent or can be made from polymer
films or sheets. A transparent low emissivity coating 62,
two layers of CBTLTR 63, and spacers and seals 64 form the
window. Alternatively, more than one transparent low
emissivity layer can be used or placed on one or both layers
of the CBTLTR 63. The number of layers of CBTLTR is not
limited to three as s~own in Figure 5 but may be any number
suitable for the particular application.
A light admitting thermal insulating structure may be
used in conjunction with an optical shutter to regulate
WO92/16702 PCT/US91/~762~
~ 3 -12-
light transmission while preventing the flow of heat. The
optical shutter ~ay be a lay~r or layers covering an
aperture. The shutter may be reversibly activated by its
local temperature; incident light intensity (photochromic);
both temperature and light (thermophotochromic as shown in
Figure 7); or an electric current or field (electrochromic).
A CBTLTR, one or more transparent low emissivity
layers, and a thermochromic, photochromic,
thermophotochromic or electrochromic optical shutter layer
lo can be combined to make insulating panels which transmit and
regulate light. In Figure 5, the shutter may be on any of
the layers 511 through 515, but since it is not transparent
to thermal radiation, it is preferably located on the
outermost layers 511, 515. The transparent low emissivity
layer or layers may be located on any of the inner surfaces
502 through 509. The inner layers, 512 through 514, are
preferably CBTLTRs.
These panels can be used as improved collectors of
solar energy which can be used, for example, for space
heating, illumination or plant growth. These panels are an
improvement over existing solar collectors for two reasons.
They prevent the collection of solar energy when it is not
wanted and are more energy efficient because the transparent
insulation prevents unwanted loss or gain of sensible heat.
A thermochromic shutter is used as a light control, as
shown by the light control 18 of Figure 1, when it is
desired to keep the temperature on the "indoor" side of the
panel constant. A photochromic shutter is used when more
constant illumination is desired. A thermophotochromic
shutter is used when constant temperature and illumination
are both desired.
An electrochromic shutter is used when it is desirable
to control the light transmission of the shutter externally,
rather than by some colmbination of the incident light
intensity and the temperature of the shutter. An
electrochromic shutter can be controlled by, f~r example, a
temperature or light sensor, a person or a computer. Unlike
. .
wos2/16702
PCT/US91/07~20
-13-
many thermochromic and thermophotochromic shutters,
electrochromic shutters are usually specularly transmissive
and imaging through them is possib:Le. This is an advantage
for-window applications where a view through the window is
usually desired along with the ill~lmination it provides.
As shown in Figure 7, a thermophotochromic shutter
whose reflectivity response to temperature and light is used
to provide more constant illumination from, for example, a
skylight which has transparent insulation, the shutter layer
should be located on the indoor side of the transparent
insulation. Indoor temperatures are more constant than
outdoor temperatures and variations in the temperature of
the shutter cause the undesired thermal response o~ the
shutter to mask its desired photoresponse.
A light transmitting building panel, for example, a
skylight using an optical shutter, can use a transparent
insulation made from one or more transparent low emissivity
layers and an CBTLTR. Transparent low emissivity surfaces
typically absorb 10% of light. This absorption can make
enough heat to mask a thermochromic or thermophotochromic
shutter's response to heat or incident light intensity,
respectively. Thus, the transparent low emissivity layer or
layers should be located away from the thermochromic or
thermophotochromic shutter near the outside of the
transparent insulation. This location is preferable in some
applications for another reason. It keeps the solar heat
absorbed by the low emissivity layer or layers near the
outside of the building where it can leave without causing
unwanted summer heat loads.
Thus, the preferred structure for solar light and/or
heat collecting panels using: a thermochromic or
thermophotochromic shutter; one or more transparent low
emissivity layers; and a CBTLTR, is: the shutter near (that
is, adjacent to or forlming) the indoor side, the low
emissivity layer or layers near the outdoor side, and the
CBTLTR inbetween.
- . . ..
WO92/16702 1~ PCT/USgl/07620
-14-
As shown in Figure 8, a cross section of a light
transmitting building panel 80 or skylight with this
preferred structure is illustrated and is a special
embodiment of Figure 5. The panel 80 includes light
transmitting cover sheets 81, a low emissivity layer 82, a
CTBLTR ~3, a thermoptical or thermophotoptical shutter 84,
and spacers and seals 85.
Merely by way of example, ~igure 9 illustrates a cut
away view of the light transmitting building panel 80 of
Figure 8. It is pointed out that there can be more than one
transparent low emissivity layer and the low emissivity
layer(s) can be located on the CBTLTR. Also, there can be a
different number of layers in the CBTLTR.
The scope of the invention is not to be limited by any
of these illustrative figures, as many other configurations
for CBTLTRs, building panels and skylights are desirable for
different applications.
If a skylight's top surface is domed or faceted, the
preferable location for the transparent low emissivity
surface may be on the CBTLTR layer closest to the outside of
the huilding because it is difficult to apply a transparent
low emissivity layer to a domed surface.
The light transmission of a CBTLTR can be enhanced with
antireflection layers. For example, the light transmission
of the configurations in Figures 6 and 8 can be increased
from about 60% to about 75%, and preferably to about 85%.
However, these antireflection layers must not absorb much
thermal radiation or they will reduce the CBTLTR's thermal
resistance. Low refractive index materials which do not
absorb much thermal radiation when their thickness is in the
order of 1,000 A ~the approximate thickness of
antireflection layers) include: porous and columnar
aluminum or silicon oxide and other oxide layers, which may
have a graded refractive index and broad band antireflection
properties; magnesium fluoride or perfluoropolymer, such as
polytetrafluoroethylene, quarter wave layers; fluorinated
polymer to unfluorinated polymer graded index layers, etc.
WO92/16702 ~ t~ PCTtVS91/07620
-15-
These antireflection layers may be placed on any and all
surfaces of the CBTLTR and its transparent cover or covers,
e.g., in Figure 5, surfaces 501 to 510. -
Antiabrasion antireflection coatings ~AAR) coatings
should have: refractive index in the range of 1.3 to 1.4,
the lower, the better; thickness of one quarter wavelength
of visible light, or 1,000 A, at least for the top layer if
the AAR is multilayer or has a graded refractive index; hard
surface; low coefficient of friction; and for glazing
applications, weather and pollution resistant.
AAR coatings on both sides of as plastic film can
reduce the film's reflectivity of greater than 8% to 2%. To
maintain their high light transmission during use, they are
scratch resistant, repel dust and are easy to clean. Almost
eliminating reflection makes the film or sheet virtually
invisible, increasing packaging materials market appeal, and
increasing glazing efficiency. Glazing applications for AAR -
coated glass and plastic sheet and film include building and
car windows, skylights, greenhouses, solar cells and solar
collectors.
A polymer surface can be made to have a graded index
antireflection layer composed of low refraction index
perfluorinated polymer on the outside which has a
composition gr~ded to unfluorinated polymer inside. For
best results the polymer should be highly fluorinated (more ~-
than 70%) on the outside, and slightly fluorinated (less
than 30%) on the inside.
For example, the thickness of this film graded index
layer can be controlled such that transmission of visible or
of photosynthetically active light, or of solar heat is
maximum. Such antireflection layers have been made by
exposing a polyethylene film to a gas composition of 99.9%
nitrogen or argon and 0.1% elemental fluorine for a few
minutes at room temperature.
Plastic bottles which are blow molded commercially may
have their inside surfaces fluorinated while they are still
hot in mold (for greater thickness of fluorination than is
WO92/16702 PCT/US91/07620
-16-
required for maximum antireflection) to impart
impermeability to oil by exposure to a similar gas mixture.
An advantage of surface fluorination of polymers beyond
antireflection is imparting durability to the surface of the
polymer where degradation (or weathlering or corrosion) of
the polymer takes place first. One of the primary means of
weather degradation of plastic surfaces is stress crack
corrosion, where the plastic produces volatile products from
weather degradation, and then shrinks and cracks. The tip
of this crack is the site of more rapid corrosion due to the
concentration in its small radius of stress from shrinkage
and thus the crack propagates rapidly into the bulk of the
polymer.
Stability is imparted to the polymer by surface
fluorination by at least three mechanisms. First, the
surface volume increases with fluorination which places the
surface under compression, which forms a prestressed surface
more scratch and abrasion resistant and which resists
cracking when the surface is bent, such as in the creasing
of a plastic film.
Second, fluorination prevents degradation because
fluorinated polymers are the most degradation resistant.
Third, fluorinated polymers have a much lower coefficient of
friction than unfluorinated polymers, which imparts greater -
abrasion resistance. Surface fluorination may also be used
with the hindered amine stabilizers described below, to
thereby impart the many attendant advantages of each to the
selected polymer.
While it has long been known that polyolefins, and
preferably polyethylene, are the only highly thermal
radiation transparent polymeric films, polyethylene has not -~
been usable in transparent insulations such as low
emissivity windows because of: (1) its haze and consequent
poor light transmission; (2) its vulnerability to solar
ultraviolet, oxidation and other degradation; and (3) its
creep. The haze in polyethylene is caused by partial
WO92/16702 PCT/US91/07620
-17-
crystallization, so very low haze polyethylene films can be
made by both very high and very low crystallinity. These
polyolefin films can be stabilized for resistance ta
degradation with polymeric hindered amine, for example,
Cyasorb 3346 made by American Cyanamide, at a loading from
about 0.1~ to about 0.5%. Such fi:Lms have passed
accelerated aging equivalent to 30 years of solar W
filtered through commercial glass. To prevent creep, these
polyolefins films can be cross linked by the conventional
methods for polyethylene: electron curtain, W light, or
heat; each with appropriate cross linking additives, such as
- polysaturated compounds, e.g., octadiene and methylene bis
acrylamide.
Very high crystallinity polyethylene films are
preferably made from highly linear, high molecular weight,
narrow molecular weight distribution polyethylene resin or a
linear medium or low density polyethylene resin. All of
these polyethylene resins are made by a low pressure
polymerization process typically using a Ziegler type
catalyst. Low pressure polymerization also produces fewer
degradation sites. This polyethylene film may be uniaxially
oriented and calendared simultaneously to increase its
crystallinity and light transmission both to greater than
90~. A suitable polyethylene film is available fro~
Tredagar Films, Inc. under the trade name of MONOX. The
linear medium and low density materials may then be
transversely oriented on a tenter to produce more
symmetrical biaxial heat shrink properties. Heat shrinking
at predetermined temperatures may be used to easily assemble
essentially wrinkle free CBTLTRs.
Very low crystallinity polyethylene is preferably made
from low density or ultra low densi~y linear, low pressure
polymerized polyethylene which is quenched rapidly
immediately after ext~usion with a chill drum or water to
prevent crystallization. While various polymers and
combinations of a mixture of dif f erent types of polymers may
WO92/16702 PCTtUS91/07620
~ 1 3 l8
be used, it is preferred to use primarily polyolefin,
meaning a polymer composition of at least 80% polyolefin.
variations on the embodiments described above are
possible. For example, in Figure 1, the light control layer
may be either a photochromic or thermophotochromic shutter
while the combination of the transparent convection,
conduction and thermal radiation suppressor constitutes a
transparent insulation.
In another variation, a transparent insulation with a
photochromic or thermophotochromic shutter can be used to
regulate the transmission of solar light and/or heat into a
building. A skylight can use a photochromic shutter to
reduce the fluctuations in transmitted light caused by
variations in incident light and thereby provide more ~ ~ -
constant illumination to minimize unwanted solar heat gains
in the summer.
In yet another variation, a thermophotochromic shutter
can be used to maximize the growth of plants in a
greenhouse. The plants' growth inhibition from heat stress
is minimized by the thermal response of the shutter while
the greenhouses' cooling costs are minimized by the
photoresponse of the shutter.
Both the skylight and the greenhouse applications would
benefit from using a transparent insulation in conjunction -
with the shutter. The transparent insulation would reduce
heating and cooling costs of both the greenhouse and the
building with the sXylight.
Of course, it should be understood that a wide range of
changes and modifications can be made to the preferred
embodiment described above. It is therefore intended that
the foregoing detailed description be understood that it is
the following claims, including all equivalents, which are
intended to define the scope of this invention.
