Note: Descriptions are shown in the official language in which they were submitted.
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LOW EMISSIVITY ARTICLE WITH LOW-E FLUOROPOLYMER LAYER
1. Field
The present invention relates to technical membranes that comprise a low
emissivity layer (low-
S a layer) comprising a fluoropolymer having dispersed therein infrared
absorbing particles. The
present invention further relates to a roof, wall or tent that comprises a low-
a layer.
2. Background
In the construction of buildings, the use of membranes is becoming more and
more popular.
Such membranes, also called technical membranes, typically comprise a glass
fiber or polyester
fiber web, e.g. glass fiber textile, that is coated with polyvinyl chloride,
polytetrafluoroethylene
or silicone resin. Such membranes can be used for example as roofs to cover
large areas such as
in football stadiums and airport halls. The membranes are especially suitable
for this purpose
due to their low weight making it possible to make lightweight roof
constructions. For example,
1S the Puchheim city hall, with its multilayer roof skin was made primarily of
noncombustible
lightweight membrane materials and was honored with the third awarding of the
international
prize for textile architecture at TechTextil 1999. Thermal insulation was
implemented by means
of sand integrated between several layers. Similarly the Munich ."airport
Center (MAC) West
with its forum roof made of a combination of a glass fiber membrane coated
with
polytetrafluoroethylene (PTFE), and a multilayer safety glass was awarded a
prize.
Typically, these membranes also have the property of repelling dirt and they
have a high
resistance to rotting. Preferably, these membranes are light transmitting,
i.e. translucent and are
fire proof. Noncombustible materials have been disclosed in DE A 23 1 S 259,
which describes a
2S textile that is coated with a glass bead tetrafluoroethylene polymer
mixture and which is not
combustible. However, this textile does not have a climatizing effect.
According to
DE A 197 40 163, an adhesive layer preferably made of silicone rubber, latex
milk, or a
phthalate resin adhesive is applied to a glass fiber fabric, whereupon glass
beads are pressed into
the adhesive layer. This material is intended to provide high mechanical
tensile and tear strength,
high light reflection, satisfactory thermal insulation and light transmission,
a high degree of
resistance to fire, resistance to wear, weathering, contamination and insect
pests, and an
extremely pleasing aesthetic effect. However, there is an interest in
improving these properties,
particularly the thermal insulation and fire resistance.
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Particularly in areas that experience a hot climate, it is desirable that
technical membranes can
reduce the heat transport between the outside and an interior space. Heat can
be transported by
several ways between a warm and a cool place. Such ways include convection,
heat conduction
as well as transport of heat through radiation.
Heat transport through radiation involves a body at a higher temperature
radiating
electromagnetic radiation against a body at a lower temperature. The intensity
of this radiation
depends on the temperature difference between the two bodies. The emitted
power or emittance
IO is given by the following formula:
W=Ea.ha
wherein W represents the emittance, s represents the emissivity and ~ is the
Stephan-Boltzman
constant. The emissivity is a value between 0 and 1 and is the ratio of the
radiation emitted by a
surface to the radiation emitted by a perfect black body at the same
temperature.
This type of heat transport can amount to 90% of the total heat transport.
Particularly the
radiation in the infrared part of the spectrum will contribute to heating and
is moreover
experienced as unpleasant by human beings. For example, at the same ambient
temperature, a
reduced level of infrared radiation will provide more comfort. Further, it has
been found
through studies that without sacrificing the comfort level, a higher ambient
temperature can be
tolerated if the level of infrared radiation is reduced or minimized.
Accordingly, by reducing the
infrared emission, one can allow a higher temperature for a room, thereby
saving costs in
cooling the room.
EP 1 053 867 discloses a technical membrane that comprises a glass fiber web
that has been
coated with a modified PTFE on which there is provided a so-called low-a
coating. Although
EP 1053867 does not give much detail as to the composition of this low-a
coating, it appears
that this low-a coating is applied through vacuum deposition. This has the
disadvantage
however that the low-a coating is prone to being damaged when constructing for
example a roof
therewith or while cleaning and moreover is prone to corrosion. Accordingly,
it is taught to use
a protective coating on the low-a coating. Unfortunately, this reduces the
effectiveness of the
low-a coating.
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WO 99/39060 similarly teaches a technical membrane that comprises a low-a
coating. No
details are given as to the composition of this low-a coating. WO 99!39060
teaches to arrange
the technical membrane on a sound barrier layer so as to additionally provide
for sound
insulation. WO 99/39060 also teaches the desire to protect the low-a coating
with a protective
layer against abrasion during cleaning.
Metallized coatings for textiles have also been used in for example EP 927 328
as
electromagnetic camouflage materials.
On the other hand, the use of fluoropolymer coatings containing metal
particles on textiles has
been practiced in the art for various reasons. For example JP OS-318659
teaches coating a glass
fiber textile with a fluoropolymer coating that contains aluminum particles in
order to provide
for liquid and gas barrier properties and additionally reflection of heat or
light. US 3,709,721
teaches polytetrafluoroethylene (PTFE) coatings that comprise a hard
particulate filler such as
for example aluminum to provide a heat and abrasion resistant material. WO
96105360 teaches a
multi-layer textile composite that has layer of fluoropolymer having aluminum
particles
arranged as an inner layer. The textile composite is taught for use in
conveyor belts that are
used at elevated temperature in for example commercial food cookir:g
processes. However,
none of these teachings have appreciated the low emissivity properties that
may be obtained
with a fluoropolymer layer containing metal particles.
3. Summary of the invention.
The present inventors have thus determined it desirable to find an improved
low-a layer that can
be used in a technical membrane to effectively reduce emission of
electromagnetic radiation, in
particular of infrared radiation. It would furthermore be desirable that such
low-a coating has a
good abrasion resistance and does not require the use of a protective layer.
It would be
furthermore desirable to find low emittance materials that are difficult to
burn, i.e. materials that
can be classified according to DIN 4102 as hardly flammable or non-flammable
material.
According to one of the requirements in order to be classified according to
DIN 4102 as non-
flammable or hardly flammable material, the material should have a caloric
value of less than
4200 J/g as measured according to DIN 51900.
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In accordance with the present invention, it was found that a layer of
fluoropolymer having
dispersed therein infrared absorbing (IR-absorbing) particles can be used as a
low-a Layer, i.e.
such a layer has a low emissivity (0.6 or less, preferably 0.5 or less, more
preferably 0.4 or less)
and can be used to reduce the amount of heating or cooling that is required to
maintain an
interior space at a desired temperature. By interior space is meant a space
enclosed by a roof
and/or walls such as for example a room or hall in a building. The low-a layer
can be used as a
barrier layer for infrared radiation and can be used to reduce the amount of
infrared radiation in
a room.
In a particular aspect of the present invention, the low-a layer is arranged
as the outermost layer
of a low emittance article, e.g. a technical membrane, so as to achieve an
emissivity of not more
than 0.6 for the low emittance article.
In a still further aspect, the present invention provides a low emittance
article comprising a
substrate having on at least one major surface thereof at least two layers,
the outermost layer of
which comprises a fluoropolymer and IR-absorbing particles in the form of
flakes distributed in
the outermost layer. The IR-absorbing particles typically have an average
particle size of less
than 25 pm, typically less than 15 pm, preferably less than 3 pm, more
preferably not more than
0.8 pm. The IR-absorbing particles are preferably distributed in the outermost
layer in an
amount of at least 10%, more preferably at least 16% by weight. The term
"average particle
size", in case the particles have a substantially non-spherical shape,
indicates the average along
the largest dimension of the particles.
In another aspect, the present invention provides a coating composition for
producing a low
emissivity coating, the composition comprising a dispersion of a fluoropolymer
and metal
particles in the form of flakes.
The invention in one of its aspects also provides a roof, wall or tent that
comprises a low
emissivity layer of a fluoropolymer having dispersed therein infrared
absorbing particles.
4. Detailed description of the present invention.
The present invention has recognized that a coating of fluoropolymer having
dispersed therein
IR absorbing particles, can effectively be used as a low emissivity coating,
i.e., a layer that
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provides a barrier against heat transport through radiation, in particular
infrared radiation. The
low-a coating has a high scratch and abrasion resistance. Further, articles
including the low-a
coating such as technical membranes are easy to transport and handle and can
be manufactured
in a convenient and cost effective way. The low-a coating in accordance with
this invention
typically has an emissivity of not more than 0.6, preferably more than 0.5,
more preferably not
more than 0.4. The low-a coating typically emits IR radiation only slowly.
Accordingly, when
arranged towards the innerspace of a room, the low-a coating will emit less IR
radiation to the
room and thereby help cooling the room. Additionally, during the night when
the room may
cool too much, the low emissivity of the low-a coating will help protect the
room against
cooling.
The low-a coating contains a fluoropolymer. Suitable fluoropolymers for use in
the low-a
coating are typically fluoropolymers that have a fluorinated carbon backbone.
Preferably the
fluoropolymer backbone is at least 50% by weight fluorinated. The partially
fluorinated
backbone of the fluoropolymer may in addition to fluorine contain hydrogen or
chlorine. The
fluoropolymer for use in the low-a coating may also include perfluoropolymers,
i.e., polymers
that have a fully fluorinated or perfluorinated backbone. Examples of
fluoropolymers that can
be used in the low-a coating include polytetrafluoroethylene (PTFE) and
polymers comprising
one or more units derived from vinylidene fluoride, chlorotrifluoroethylene,
ethylene, propylene,
hexafluoropropylene, fluorinated vinyl ethers including perfluoro vinyl ethers
such as
perfluoromethyl vinyl ether, perfluoro(methoxyethyl vinyl) ether, perfluoro
(propyl vinyl) ether,
perfluoro (2-(n-propoxy)propyl vinyl) ether and perfluoro(ethoxyethyl vinyl)
ether.
Fluoropolymers for use in the low-a coating further include for example in
addition to PTFE,
PTFE modified with for example hexafluoropropylene or a perfluorovinyl ether
and
thermoplastic melt-processable fluoropolymers such as copolymers
oftetrafluoroethylene and
hexafluoropropylene and/or one or more perfluorovinyl ethers. It will further
be clear to one
skilled in the art that mixtures of fluoropolymers may be used as well such as
for example
mixtures of PTFE and thermoplastic melt-processable fluoropolymers.
The infrared absorbing particles for use in the low-a coating are preferably
metal particles
including particles that have been provided with a metal coating on their
surface such as for
example glass microspheres having been metallized at their surface. The metal
particles may be
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oxidized at their surface. Metal particles are capable of absorbing IR
radiation and have a low
emissivity.
Examples of suitable metal particles include noble metals such as silver or
gold as well as other
metals such as aluminum, copper, zinc and combinations thereof, including
alloys of such
metals. The average particle size of the IR absorbing particles is typically
less than 25 p.m,
preferably less than 3 p.m, more preferably in the order of colloidal
particles, i.e., not more than
800 nm. By using smaller particles, the light transmission of the coating can
be optimized while
maintaining a low emissivity. The geometry of the particles can be spherical
or substantially
spherical such as elliptical. However, in a preferred embodiment, the
particles are in the form of
flakes preferably having an average particle size, measured along the largest
dimension, of not
more than 25 pm, preferably not more than 20 p,m, and preferably having an
average thickness
of between 0.01 p.m and 1 p,m, preferably between 0.05 pm and 0.5 p,m. IR-
absorbing particles
in the form of flakes may provide the advantage that they are capable of
orientation during
coating such that even at low amounts of the particles, an effective low
emissivity can be
obtained.
The amount of the IR-absorbing particles is typically at least about 2% by
weight based on the
total weight of solids, preferably at least 5-6% by weight, more preferably at
least 10% by
weight and most preferably at least about 15-16% by weight. A typical range of
the amount of
IR-absorbing particles is between 2% by weight and 70% by weight, preferably
between 10 and
50% by weight.
The thickness of the low-a layer is preferably kept minimal to provide for a
higher light
transmission. Typically, the thickness of the low-a layer will be not more
than 0.3 mm,
preferably not more than 0.05 mm.
The low-a coating may contain additional ingredients such as non-flammable
fillers such as
glass spheres, mica pigments, ceramics or titanium dioxide. Such fillers may
for example be
included in the low-a coating in an amount of 2 to 80% by weight.
The low-a coating can be used to provide a low emittance material, in
particular to provide a
technical membrane, e.g., a light translucent membrane. The low emittance
material comprising
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the low-a coating will typically have a emissivity of not more than 0.6,
preferably not more than
0.5 and more preferably not more than 0.4. The desired emissivity can be
selected by one
skilled in the art through routine experimentation and will generally depend
on such factors as
the thickness of the low-a coating, the position of the low-a coating in the
layer package of the
low emittance material, the amount of IR absorbing particles in the low-a
coating and the size
and geometry of the IR absorbing particles. In a low emittance material, the
low-a coating is
preferably provided as close as possible to the surface of the low emittance
material, most
preferably as an outermost layer. Light transmission of the material can be
increased by
bleaching processes such as annealing and UV radiation. The low emittance
material will
preferably have a light transmission of at least 0.5%, preferably at least
0.8%, more preferably at
least 1%. With the low emittance material of the present invention, even a
light transmission of
2% or more, e.g., 9% or more can be achieved. It should be noted here that a
light transmission
of at least 2% may already provide a sufficient amount of supporting light in
a room.
In a particular embodiment, the low emittance material comprises a substrate,
for example a flat
substrate provided with the low-a coating. Preferably, the substrate has a
high temperature
resistance to allow for the use of high temperatures to provide coatings to
the substrate.
Examples of suitable substrates include glass fiber webs or fabrics, e.g.
glass fiber cloth which
are UV-resistant, organic materials such as polyparaphenylene terephthalamide
which is
commercially available under the brand I~EVLAR, metal fiber fabrics, mineral
fiber materials
such as felts and mats of glass wool and rock wool. The fabric substrates may
be woven or non-
woven. Preferably, the low emittance material comprises at least two layers on
the substrate, the
outermost layer of which is the low-a coating. The one or more further layers
of a multi-layer
low emittance material may comprises further layers of fluoropolymer, in
particular of
polytetrafluoroethylene. Such further layers will generally not comprise the
IR absorbing
particles of the low-a coating. Such one or more further layers may for
example be provided to
increase the adhesion of the low-a coating to the substrate of the low
emittance material.
The one or more further layers may contain additional ingredients such as non-
flammable fillers
such as glass spheres, mica pigments, ceramics or titanium dioxide. Such
fillers may for
example be included in a further layer in an amount of 2 to 80% by weight.
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Such a low emittance material may be provided as a translucent technical
membrane with the
low-a coating as an outermost layer arranged towards the interior of a room.
Because of the low
emissivity level and adsorption of infrared radiation, the interior will be
cooled and moreover,
because of the reduced infrared radiation in the room, the climate therein
will feel more
comfortable. Further, at times when the exterior temperature drops, infrared
emission from the
low emittance material contributes to protecting the room against cooling.
Accordingly, the low
emittance material may act as a heat accumulator that may be charged by solar
radiation during
the day and which slowly releases the accumulated energy during the night.
Accordingly, the
low emittance materials are particularly suitable for use in areas that have a
hot climate such as
tropical and desert climates.
The low emittance material may be obtained by coating a substrate, for example
glass fiber
fabric, with a coating composition comprising the fluoropolymer and the IR-
absorbing particles.
Typically, an aqueous dispersion of the fluoropolymer and IR-absorbing
particles will be used as
' the coating composition. The coating composition may contain multimodal
particle
distributions of the fluoropolymer as taught in DE 197 26 802 to provide for
dense coatings and
a smooth surface. A preferred coating composition may contain the IR absorbing
particles, for
example metal particles such as aluminum in an amount of at least 10% by
weight. The low-a
coating composition may be applied for example through dip coating. Further,.
prior to coating
the low-a coating, the substrate may first be coated with one or more
fluoropolymer layers, e.g.,
polytetrafluoroethylene, which do not contain IR absorbing particles. Suitable
glass fiber
coating methods are disclosed in for example DE 23 15 259 and US 2 731 068,
which are
modified such that preferably the last coating is a coating composition used
to provide the low-a
layer.
The low emittance material typically will have a caloric value according to
DIN 51 900) of not
more than 6000 J/g, preferably less than 4200 J/g. Accordingly, the low
emittance material will
be hardly flammable.
The low-a coating may be used in roofs, wall or tents. Such roofs, wall or
tents have been
disclosed in EP 1 053 867 and WO 99/39060. Typically, such roofs, wall or
tents comprise a
translucent technical membrane comprising a substrate, for example as
disclosed above that is
provided on at least one side with a fluoropolymer coating, e.g.,
polytetrafluoroethylene, and a
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low-a coating, preferably as an outermost layer. The low-a coating is
typically arranged towards
the inner side of a room thereby reducing the amount of energy needed to cool
the room. When
the low-a coating is arranged towards the exterior, the low-a coating will
inhibit loss of heat
through emission towards the outside and thus reduces the amount of heating
that is required.
By providing the low-a coating on both sides, an improved heat insulation
results.
A translucent technical membrane having the low-a coating may further be
combined with other
layers such as for example sound barrier layers as disclosed in for example WO
99!39060 which
is incorporated herein by reference. As disclosed in this publication, a light
transmitting sound
barrier layer is arranged at a distance to the outer layer of the technical
membrane that contains a
low-a coating. As is further disclosed in this publication, the substrate of
the technical
membrane, e.g., glass fiber fabric, preferably has openings in it such that
sound and light can
pass through the technical membrane to the light transmitting sound barrier.
Apart from using the low-a coating in a roof, wall or tent, the low-a coating
may also be used in
other materials that are typically used to cover a room against penetration of
sun rays. Such
materials include in particular shading materials including movable shading
materials such as
blinds, awnings, roll-down shutters, curtains, jealousies and lamella. Such
shading materials
may be used on their own to mitigate temperature conditioning of a room or
they can be used in
combination with a roof or wall having the low-a coating.
EXAMPLES
The following examples serve to illustrate the invention further without
however the intention to
limit the invention thereto.
Test methods:
The emissivity of the materials in the following examples was measured using
an Emissionmeter
Model AE from Devices and Services Co., Dallas, Texas, USA according to the
procedures laid
out by the manufacturer of the machine. The emissionmeter was equipped with a
differential
thermopile as a radiation detector. The radiation detector was heated to
82°C and has a nearly
constant response to the thermal wavelengths (3 to 30 p.m). The device was
first calibrated
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using a standard having high emissivity (0.93) and a standard having a low
emissivity (0.04).
The unknown sample was thereafter measured against the standard having a high
emissivity.
Abbreviations:
PTFE: polytetrafluoroethylene.
FEP: copolymer of tetrafluoroethylene and hexafluoropropylene commercially
available as
DyneonTM FEP X 6300.
PFA: copolymer of tetrafluoroethylene and perfluoro-(n-propyl vinyl) ether
commercially
available as DyneonTM PFA 6900 N.
I0
Comparative Example
A glass cloth in linen weave having a weight per unit area of 442 g/m2 was
coated on both sides
with 659 g/m2 of coating material in four coats. The first coating was applied
using a 50% by
weight dispersion of PTFE (diluted from commercially available PTFE dispersion
DyneonTM
TFX 5060), the second and third coats were made using a 62% by weight PTFE
dispersion
containing glass microspheres and commercially available as DyneonTM TFX 5041.
A fourth
coating was applied at 50 g/m2 of PTFE using a dispersion containing 60% by
weight of PTFE
(commercially available as DyneonTM TFX 5060).
This glass fiber cloth containing only PTFE coatings without IR absorbing
particles has an
emissivity of 0.88.
Example 1
The coating procedure as carried out in the comparative example was repeated,
but in place of
the last coat of DyneonTM TFX 5060 there was applied, a PTFE dispersion
(diluted from
commercially available PTFE dispersion DyneonTM TFX 5060) having containing
10% by
weight of aluminum paste relative to the weight of PTFE solids and having a
total amount of
solids of 62% by weight. The aluminum paste comprised 65% by weight of
aluminum flakes,
having an average size of 13 p,m and a thickness of 0.2 p,m, in water. The
aluminum containing
coating was applied such at an amount of 42.Sg/mz which contained about 5.9%
of aluminum.
The emissivity of the coated material was 0.60 and the light transmission was
O.I %.
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Example 2
The coating procedure as carried out in the comparative example was repeated
to coat a total
weight of coating material of 656g/mz, but in place of the last coat of
DyneonTM TFX 5060 there
was applied, a PTFE dispersion (diluted from commercially available PTFE
dispersion
DyneonTM TFX 5060) having containing 30% by weight of aluminum paste used in
example 1
relative to the weight of solids and having a total amount of solids of 52% by
weight. The
aluminum containing coating was applied such at an amount of 22g/mz which
contained about
18.2% of aluminum. The emissivity of the coated material was 0.50 and the
light transmission
was 0.4%.
Example 3
The material per Example 2 was annealed for I2 hours at 250°C. The
transmission thereby
increased to 1%. The emissivity was unchanged at 0.5.
Example 4
The coating procedure as carried out in the comparative example was repeated
to coat a total
weight of coating material of 663g/mz, but in place of the last coat of
DyneonTM TFX 5060 there
was applied, a PTFE dispersion (diluted from commercially available PTFE
dispersion
DyneonTM TFX 5060) having containing 30% by weight of aluminum paste of
Example 1
relative to the weight of solids and having a total amount of solids of 42% by
weight. The
aluminum containing coating was applied such at an amount of 30g/m2 which
contained about
16.7% of aluminum. The emissivity of the coated material was 0.50 and the
light transmission
was I %.
The caloric value of the low emittance material as measured according to DIN
51900 part 1 was
4041 J/g.
Example 5
The material per Example 4 was annealed for I2 hours at 280°C. The
transmission increased to
1.7%. The emissivity was unchanged at 0.5.
Example 6
The procedure of Example 4 was repeated but instead of the aluminum containing
PTFE
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dispersion, a fluoropolymer dispersion containing a mixture of PTFE and a PFA
in equal
amounts was used. This fluoropolymer dispersion further contained 50% by
weight of the
aluminum paste of Example 1 containing 65% by weight of aluminum in water. The
total
amount of solids in the dispersion was 65% by weight and 54 g/m2 (dry weight)
of this coating
was applied on one side of the low emittance material as a last coating. The
amount of
aluminum in this coating was about 33% by weight. The emissivity was 0.45, the
light
transmission 0.7% and the caloric value according to DIN 51900 was 4015 J/g.
Example 7
The procedure of Example 4 was repeated but instead of the aluminum containing
PTFE
dispersion, a fluoropolymer dispersion containing a mixture of PTFE and FEP in
equal amounts
was used. This fluoropolymer dispersion further contained 100% by weight of
the aluminum
paste containing 65% by weight of aluminum in water. The total amount of
solids in the
dispersion was 30% by weight and 12 g/m2 (dry weight) of this coating was
applied on one side
of the low emittance material as a last coating. The amount of aluminum in
this coating was
about 40% by weight based on solids. The emissivity was 0.33 and the light
transmission 0.7%.
Example 8
A glass cloth in linen weave having a weight per unit area of 100 g/m2 was
coated with 44.9
glm2 of coating material in three coatings using DyneonTM TFX 5060. As a last
coat on one side
there was applied 1.1 g/mz (dry weight) of a dispersion containing a total
solids of 20% by
weight and containing PTFE and FEP in equal amounts and the aluminum paste of
Example 1
containing 65% by weight of aluminum in water. The amount of aluminum in the
dispersion
was about 40% by weight based on solids. The light transmission was 9% and the
emissivity
0.45.
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