Note: Descriptions are shown in the official language in which they were submitted.
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A Film having an electrically Conductive Coatin~
Field
This invention relates to a transparent polymeric film composite which is
suitable
for the protection of glazed surfaces which are particularly susceptible to a
build up of
electrostatic charge.
Background of the Invention
It is well known to protect a vehicle windshield by the use of a protective
film cover
that overlies the windscreen and is adhered to the windscreen, see for example
WO
99/2840. The protective cover can be easily removed by peeling from the
windscreen
after use. DE-A-3221 766 discloses a self adhering transparent film that is
used to
protect glass surface on motor vehicles and aeroplanes and which allows the
glass
surface to be cleaned by removal of the film.
A multilayer protective film composite for an automobile windshield is
disclosed in
US-A-5002 326 . The different layers of film are removed successively as each
film
surface becomes dirty to expose a new clean film surface to improve visibility
for a
driver.
The transparent windshield or canopies of aircraft, in particular, helicopters
are
expensive and may become abraded or scratched when the aircraft is used in a
harsh
environment in which the air may filled with dust or sand particles such as
may be
found when operating is deserts.
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The exposed surfaces of helicopter windscreens, in particular, accumulate
large
amounts of static electricity during their operation of the helicopter and
this static
electricity is dissipated to earth when the helicopter touches down.
It has been found that if the windshield or window of a helicopter is
protected from
scratches and abrasions by means of a polymeric film over layer there is an
electrostatic charge build up on the outer surface of the film which
discharges into the
helicopter windscreen on landing and consequently burns holes in the
protective over
layer. Similar problems may arise when the helicopter rotors are running
whilst the
aircraft is sitting on the ground.
EP 1154000 describes a polymeric film having a conductive thin film for static
electricity prevention. The thin film comprises a layer of a resinous binder
containing
metal oxide particles and has a superior transparency with a total light
permeability
of at least 80% and a haze value of no greater than 5%. The conductive thin
film is
applied to glass cases, CRT screens, and as an antistatic material to clean
room floor
and walls.
The present invention provides a film composite which can be used as an
overlay to
protect aircraft windscreen from abrasions and scratches and which also
prevents a
build up of static electricity especially in dry and desert conditions.
Statements of Invention
According to one aspect of the present invention there is provided a
transparent
protective polymeric film composite for laying over a surface, for example
glazing,
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the protective composite comprising a transparent polymeric film having on one
side
an electrically conductive layer formed from a dispersion of electrically
conductive
particles, the layer being coated with a transparent hard coat , said one side
in use
facing away from said surface.
The conductive layer may be formed from a dispersion of electrically
conductive
nanoparticles comprising at least one of carbon, a metal or metal oxide. The
metal
nanoparticles may comprise nanoparticles of aluminium, silver, gold, platinum
or
metal coated nanoparticles. The metal oxide nanoparticles may comprise
nanoparticles of ITO (indium tin oxide), fluorine doped tin oxide, tin oxide,
titanium
oxy nitride, antimony doped zinc oxide and preferably the metallic oxide is
ATO
(antimony tin oxide). The nanoparticle size should be of less than 0.1 microns
diameter.
Alternative conductive layer may be provided by a dispersion of a electrically
conductive polymer such as polythiophene e.g PEDOT-PSS (Baytron) available
from
Bayer.
In general, the transparent composite can be used as an overlay for aircraft
windows,
canopies etc. to combine the advantages of a removable protective film with
static
electricity prevention, without loss of window transparency.
A protective device for an aircraft windows may also comprise a plurality of
sheets of
transparent composite according to the Invention arranged in a stack so that
each sheet
adheres to the adjacent underneath sheet. Such an arrangement is described in
EP
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1489 147. The hard coat may contain a siliconized acrylate resin to assist in
removal
of the adjacent upper sheet.
The polymeric film may comprise one of polycarbonate, acrylic, polypropylene
and
PET, the preferred film being PET. The film is preferably a PET ( polyethylene
terephthalate) film, preferably having a thickness of between 4 mil to 7 mil
(0.1 to 0.175mm) and which may contain a UV absorbing material as is disclosed
in
US 6221 112.
The composite preferably has a surface resistivity of less than 1 X 109 ohms
/square
at 100volts and when applied to glass the film composite / glass combination
has
optical properties such that it has a%VLT of at least 75%, preferably greater
than
80%, a Haze value of less than 5%.
The conductive nanoparticles may be dispersed in a layer on one side of the
film
with the hard coat being coated onto said layer. Preferably, the conductive
layer
comprises ATO dispersed on the surface of the film with a maximum areal
density of
1.0 gms per m2 and preferably between 0.16-1.0 gms per m2 . Such a layer has a
surface resistivity of 3.3 X 107 ohms /square at 10 volts.
The conductivity of the composite is influenced by the thickness of the
conductive
layer, however the thicker the ATO layer then the lower the adhesion of the
hard
coat to the polymeric film . An increase in thickness of the conductive layer
also
affects the optical properties of the film composite.
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In the preferred embodiment the hard coat is a UV curable acrylate based resin
as is
described in US 4557 980 the contents of which are hereby incorporated into
the
present specification. The hard coat after curing and drying has a thickness
of about
1.8 microns and a pencil harness of about 2H. The composite will have
5 a surface resistivity of about 1.9 X 108 ohms/square at 100 volts.
The other side of the PET film is coated in a suitable adhesive for adhering
the
composite to the glazing, and is preferably a pressure sensitive adhesive such
as the
solvent based adhesives including National Starch 80-1057 . Suitable
releasable
clean peel adhesives may also be used, for example Gelva GMS 3149 (available
from Cytec Inc. Surface Specialities), which in use adhere to the film layer.
A release liner may be laminated over the adhesive coating.
Glazing includes any suitable transparent material which may be used for
vehicle
windscreens, aircraft canopies and windscreen and windows etc. and which
include
glass, acrylic sheet, polyester sheet, polycarbonate sheet.
Such a composite provides a sacrificial layer which protects a windscreen from
damage due to abrasion by dirt etc. and which dissipates static electricity,
and has
good optical properties in the visible and near infra-red . Good IR
transmission
allows for the use of night vision goggles or other night vision instruments
through a
protected screen.
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Another aspect of the invention provides an aircraft window protector
comprising at
least one sheet of transparent composite itself comprising a transparent
polymeric
film having on one side an electrically conductive layer coated with a
transparent
hard coat, said one side in use facing away from said surface, the film having
on its
other side an adhesive for adherence to the window.
The protector may comprise a plurality of sheets of transparent composite
comprising
a transparent polymeric film having on one side an electrically conductive
layer
coated with a transparent hard coat containing a surface energy reducer, said
one
side in use facing away from said surface, the film having on its other side
an
adhesive, the sheets being arranged in a stack so that each sheet adheres to
the
adjacent underneath sheet with the uppermost sheet of each stack being
removable.
Yet another aspect of the Invention provides a method of protecting an
aircraft
windscreen or canopy from damage due to abrasion by dirt etc. wherein in said
method the windscreen is provided with a removable sacrificial layer
comprising a
transparent polymeric film composite which protects a windscreen, has good
optical
properties has optical properties in the visible such that it has a%VLT of at
least
80%, a Haze value of less than 5%, and is sufficiently transmissive in the
visible/near infra-red wavelengths ( 600-1000nm) to allow unimpaired use of
night
vision goggles, and dissipates static electricity.
Also according to yet another aspect of the present invention, there is
provided a
method of manufacture of an anti-static protective transparent film composite
in
which method an aqueous dispersion of an electrically conductive material is
mixed
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with a suitable liquid and applied to a surface a transparent film, the
dispersion is
dried, and then coated with a scratch resistant coating.
The dispersion preferably comprises a nanoparticle dispersion, preferably of
metal or
metal oxide, mixed with an organic solvent.
The metal oxide is preferably ATO and the aqueous dispersion is mixed with
water
miscible solvents such as methanol, isopropanol, and pyrol, to form a liquid
composition having a lower surface tension and increased viscosity thereby
improving the quality of the coating and eliminating "dewets".
Description of the Drawings
The invention will be described by way of example and with reference to the
accompanying drawings in which :
Fig. l is a cross-section through a first protective film composite
according to the present invention,
Fig. 2 is a cross-section through the composite of Fig.1 shown in situ
on glazing, and
Fig. 3 is a cross-section through a second composite according to the
present invention.
Fig.4 is a graph of static charge retention versus time for a
windscreen and windscreen covered with prior art protective
film, and
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Fig. 5 is a graph of static charge retention for a windscreen and for
windscreen covered with film according to the present
invention.
Detailed Description of the Invention
With reference to Fig 1 there is shown a protective film composite 10
comprising a
suitable transparent polymeric film 11 coated on one side with an electrically
conductive layer 12 , preferably of a conductive metal oxide, which in turn is
over
coated with a scratch resistant hardcoat 13. The other side of the polymeric
film is
coated with a transparent adhesive layer 14 covered with a protective release
liner
15.
Suitable transparent films 11 are polycarbonate film, acrylic film and
polyester film,
preferably a polyethyleneterephthalate (PET) film treated with a UV absorber
as
described in US patent 6221 112B so as to absorb up to 99% of UV radiation. A
suitable PET film is DuPont Teijin Films' Melinex 454. The film has a
thickness of
about 7 mil ( 175 microns).
The electrically conductive layer 12 is formed from nanoparticles of ATO
(antimony doped tin oxide). A 22% aqueous dispersion of ATO (available from
LWB
Einhoven BV, Netherlands) is modified by the addition of water miscible
solvents -for
example, methanol, isopropyl alcohol, and pyrol. The resulting liquid
composition
has a lower surface tension coupled with a higher viscosity allowing the
mixture to be
coated into the PET film 11 using known coating techniques, for example,
roller
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coating, reverse and forward gravure techniques, and slot die coating. In the
present
example the coating was applied by reverse gravure techniques.
The composition typically comprises (% by weight)
100 parts of 22% ATO aqueous dispersion
8.9 parts N-methyl pyrrolidone
22.3 parts methanol
13.2 parts isopropyl alcohol
The coating was dried at 140 F(60 C) and has an areal density of ATO of
between
0.16-1.00 gsm . The surface resistivity was measured at 3.3 x 107 Ohms/square
at
10volts using a Keithley Model 6517A High Resistance Meter connected to a
Model
8009 Resistivity Fixture.
Although the conductivity of the ATO layer may be increased by increasing the
thickness of the layer 12 if the areal density of ATO is greater than 1.00 gsm
the
adhesion of the hard coat 13 becomes unacceptably low and the optical
properties of
the composite 10 are adversely affected.
In order to provide for a good dissipation of static electricity coupled with
good
optical properties, that is a %VLT (Visible light transmission) of better than
80%
with a % haze < 2, and high transmission in range 600-1000 nm, the areal
density of
ATO should be between 0.16-1.00 g/m2 .
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The hard coat 13 is a UV cured acrylate based resin which is formed from a
liquid
composition which is applied over the dried ATO dispersion by any suitable
process.
The coating composition may comprise a resin and solvent base as is described
in US
4557 980. The coating composition used for the hard coat layer 13 is formed
from a
5 liquid composition which is applied to the surface of the PET film by a
reverse
gravure process. The coating composition may comprise a resin and solvent base
as
is described in US 4 557 980 and typically comprise the constituents of Table
1
below.
TABLE 1.
10 Acrylate resin 30-75%
Acrylic Acid 0-45%
Solvent 0-40%
Photoinitiator 2.4-5.0%
The percentages are weight percentages of the coating mixture.
The acrylate resin is preferably a mixture of pentaerythritol tetraacrylate
and
triacrylate. A suitable material is Sartomer SR-295 available from Sartomer
(Total).
Suitable solvents, in addition to the acrylic acid which acts as a solvent,
are isopropyl
alcohol and MEK ( methylethyl ketone).
The ingredients for the coating are mixed together and the stable mixture is
stored for
later use.
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If asiliconized acrylate resin is to be added to the hardcoat, then 0.04-0.7%
siliconiged acrylate (Ebercryl 1360 available from UCB Chemical Corp) should
be
addeo to the hard coat composition.
The hard coat composition is applied using a reverse gravure process in a
thickness
of about 1- 6 microns and coats evenly and levels smoothly. After application
to
the PET film the coating remains stable until drying, and UV curing after
drying.
The final cured dried hard coat has a thickness of about 2 microns, more
typically
between 1.5- 2.5 microns.
The hard coat has the following typical physical properties:
Haze < 1 %,
Gloss 60 degree gloss 100 gloss units
Scratch resistant to 0000 Steel Wool
Abrasion < 12% change in Tabor haze.
Pencil Hardness 2H-3H
Pencil Hardness is measures according to ASTM D3363-92a
The Gloss was measured using a Byk Gardner Glossmeter.
20= The haze was measured using a Hunter Laboratories Ultrascan XE and
calculated
acqarding to ( Diffuse Transmittance/Total Transmittance) X 100 over a light
range of
38Q-.780 nm.
ThQ. scratch test is a subjective test in which the coating is rubbed with
steel wool and
viewed for scratching.
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The abrasion test uses a Taber Abrader in accordance with ASTM D1044-93 using
a
CS10 wheels each loaded with lkg. The results are quoted in an increase in
haze after
100cycles.
The adhesive layer 14 is a solvent based pressure sensitive adhesive applied
to the
underside ( in use) of the film 11 using slot die coating, or other suitable
techniques
and dried at 60 C. A suitable adhesive is National Starch 80-1057 modified
with
Tinuvin 328 to improve durability. As an alternative, the adhesive 14 could
comprise
an easy peel type adhesive for example Gelva GMS 3149 which adheres
preferentially to the film.
The adhesion of the film composite to any underlying glazing must lie between
particular limits. The adhesion must be sufficient to prevent easy release of
the film
composite during use but must not be so adhesive as to damage the glazing when
the composite is removed.
The release linerl5 may comprise a polyethylene coated paper, or PET film with
a
silicone release coating, which can be peeled from the adhesive leaving the
adhesive
layer on the film 11.
The application of the film composite 10 to a windscreen 20 comprises are
series of
steps. The windscreen is cleaned using a non-hazardous film application
solution
comprising at least a mixture of detergent and water. The film composite sheet
is cut
to size and moulded to the shape of the surface to be protected. The release
liner 15 is
removed from the composite 10 and both the windscreen and adhesive layer 14
are
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sprayed with said solution. The film is placed over the surface and smoothed
into
place, expelling all air pockets. The adhesive layer 14 is then allowed to
cure for 24
hours.
Fig. 2 shows a composite 10 in place on a windscreen shown with the release
liner
removed and the composite 10 adhered to glazing 20 for example a helicopter
windscreen.
In use, the protective film composite 10 may be cleaned using the standard
10 windscreen cleaning techniques. The composite 10 is not harmed by standard
window
cleaning chemicals, for example Windex.
Film Clarity
The optical clarity of the windscreen protected by composite 10 was tested for
15 comparison with a unprotected screen, by means of a subjective test in
which an
observer viewed optical charts through the screens at various distances. There
was no
noticeable difference between the two windscreens.
Compatibility with Night Vision Goggles
The film composite 10 was tested by means of subjective test in which pilots
equipped with night vision goggles flew helicopters having half the windscreen
covered in the composite film. The pilots flew for periods of 1.5 hours in
various
light levels from rural dark to well lit urban environments. The testing
showed that the
composite film 10 did not affect night vision goggle performance.
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Electrostatic Testing
Electrostatic Testing was performed by applying static charge using a high
voltage,
low current device. The induced charge and charge decay characteristics were
measured on the bare windscreen and windscreen covered with prior art none
conductive protective film for a 35 kV induced charge.
The results of the test showed that the protective film acted as a capacitor,
storing up
charge until a level was reached and the built-up charge would arc to the
nearest
conductive-material, the windscreen. The windscreen dissipated the 35kV-
induced
charge in less than five minutes, whereas the very insulative protective film
effectively held a charge greater than 8k V for more than five minutes, as
seen in
Fig.4. The film was also observed to hold a charge of 8 kV for up to 30
minutes. The
protective film held the charge in pockets until enough was built-up, where it
would
then arc through the film to the windscreen. There was no visible arcing,
however
small burns ranging in size from a pencil tip to an eraser were evident in the
protective film. There were 5 to 10 noticeable burn holes generated on each
protective film after one full charging test. Analysis determined that
locations of the
holes were driven by underlying contaminants, acquired during installation.
Electrostatic Testing of Film with Conducting Protective Coating.
Two 8 inch X 4.5 inch ( 200mm x 112.5 mm) samples of composite 10 according
to the present invention, as well as a sample of the prior art non -
conductive film
were installed on an aircraft windscreen. The non - conductive control film
test data
obtained on the second test exactly matched the data collected during the
first test.
The sample of composite 10 were comparable to the plain windscreen in the
manner
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in which the static electricity discharged, as seen in Figure 5.
The composite 10 has a resistivity less than 1 X 109 ohms /square, preferably
about
2.0 X 108 Ohms/square, and typically 1.9 X 108 Ohms/square in order to
dissipate
5 static electrical charge from the windscreen and prevent damage to the
composite.
Resistance to use of wipers
Wipers of an aircraft fitted with the protective film were operated for 1
minute on
each a dry and moist windscreen while the aircraft was parked on the ground. A
10 follow-on test evaluated the same criteria during in-flight operation of
the dry wipers.
The tests were also repeated under a moist windscreen/wiper condition. The
results of
the test showed that the composite 10 was not affected by the usage of dry or
moist
wipers on the ground or during flight.
15 Resistance to windspeed
A speed sweep was performed on aircraft that had the protective film installed
to
evaluate the films ability to stay adhered to the windscreen. The tested
speeds ranged
from hover to 310 knots. The testing showed that the protective film was not
affected
by the speed of the aircraft. The film remained clear and attached under each
flight
speed.
Resistance to Temperature
The protective film's ability to remain clear and adhered to the windscreen
was
evaluated for low temperatures. The aircraft windshield's operating
temperature range
is from -65-160 F (-55- 70 C).
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A sample of the 7mil protective film was attached to a piece of glass. A
preliminary
test was performed where the sample was cold temperature cycled numerous times
from -15- 70 F (-25 - 21 C) to determine if there was any shrinkage or peeling
in the
protective film and if any discoloration, bubbling, or hazing occurred. The
test results
showed no anomalies and the film adhesive strength was not affected by the
cold
temperature.
Through flight-testing, the protective film was evaluated in the temperature
range of
35 C to -35 C. The results of the temperature evaluation showed that the
protective
film was not affected by temperature. The film remained clear and attached
under
each evaluated temperature.
Durability
The protective film's durability and ability to protect the underlying
windscreen was
evaluated throughout the test program. Sand blasting testing showed that a
film
covered windscreen could last nearly twice as long as the glass windscreen
alone
before needing maintenance. The film was flight tested for more than 100 hours
between the three test aircraft. The protective film has flown in harsh
operating
extremes, such as brown-out and hot/old temperature conditions. The film's
durability
was evaluated during the brown-out condition testing. Two aircraft, were
submitted to
brown-out conditions during landings and take-offs, where the windscreens were
blasted with dust, sand, and rocks. Throughout the 7 days of testing, each
aircraft
logged over 50 take-offs and landings. After each test day the aircraft's
windscreens
were evaluated and a comparison was made between the windscreen with the
protective film and the one without. Throughout the test, the windscreens
began to pit
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and show damage. The windscreen with the protective film was protected and
remained unaffected by the elements, where as the windscreen without began to
become more difficult to see through due to the pitting and other damages.
Another embodiment of the Invention is shown in Fig.3. which shows a plurality
of
sheet of composite 10 stacked one on the other on a windscreen 20. The hard
coat
layers 13 incorporate a siliconized acrylate resin to reduce the surface
energy to
enable upper sheets of composite to be removed from underlying sheets as the
upper
most sheet becomes damaged and difficult to see through.
15
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