Note: Descriptions are shown in the official language in which they were submitted.
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COMPOSITIONS AND METHODS FOR THE PROTECTION OF SUBSTRATES
FROM HEAT FLUX AND FIRE
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority to U.S. Provisional Application Serial No.
60/834,696, which was filed on August 1, 2006, the disclosure of which is
incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
The U.S. Government has a paid-up license in this invention and the right in
limited circumstances to require the patent owner to license others on
reasonable
terms as provided for by the terms of grant W15QKN-06-P-0262 awarded by the
United States Army.
BACKGROUND OF THE INVENTION
Thermal barrier coatings (TBC) insulate and protect a substrate from
prolonged or excessive heat flux and enable the substrate material to retain
its
mechanical property integrity during service. Selection of the type of system
and its
components depends upon the application. Heat may be dissipated away from a
substrate by several methods, including heat sinks, active cooling,
transpiration
cooling, radiation cooling, and intumescence.
A need exists for a coating that is able to protect a substrate from exposure
to
high temperatures and possesses a high strain to failure (i.e. toughness) and
adhesion
capabilities under harsh, cold temperatures while subject to high mechanical
stresses.
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SUMMARY OF THE INVENTION
The present invention is directed to a flame or heat flux protective coating
composition, which includes a fiberglass dispersion in silicone. Also
presented is a
flame or heat flux protective sheet, which includes fiberglass and silicone in
a sheet
form, wherein the fiberglass is dispersed in the silicone or the fiberglass is
a woven
cloth coated with the silicone. A method for coating an article with a flame
or heat
flux protective coating and articles incorporating the flame or heat flux
protective
coating or sheet form are also presented.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a table setting forth descriptions of the tested coatings;
FIG. 2 is a graph of temperature versus time for the flame test; and
FIG. 3 is a graph depicting average flame test results.
DETAILED DESCRIPTION OF THE INVENTION
The fiberglass component imparts high emissivity to the composition of the
present invention. Emissivity is a material's ability to absorb and radiate
energy as a
function of its temperature and is defined herein as the ratio of the total
energy
radiated by a material to a black body at the same temperature. A black body
absorbs
all electromagnetic radiation and is an ideal radiator with an emissivity of
1. The
emissivities of all non-black body objects are less than one and are
determined by the
object's temperature, surface characteristics, geometric shape and size, and
chemical
composition. In order to dissipate heat, high emissivity values close to one
are
desirable. The emissivity of fiberglass ranges from 0.87-0.95.
Fiberglass also provides the coating composition with relatively low heat
conductivity and, thus, a high thermal insulation value. For example, one end
of a
strand of fiberglass is able to radiate heat away from a coated substrate when
subjected to high temperatures, while the other end of the same strand
insulates the
substrate from the radiated heat.
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The fiberglass component is present in an amount suitable to promote
effective radiation cooling when exposed to heat. In one embodiment, the
fiberglass
component is present in an amount from about 4% to about 14% by weight of the
composition. Preferably, the amount of fiberglass is from about 8% to about
14% by
weight of the composition, more preferably from about 8% to about 12% by
weight of
the composition.
The fiberglass component can have any suitable fiber length and diameter.
The fiberglass component can also include fibers having a mixture of suitable
lengths
and diameters. Preferably, the fiber length ranges from about 1mm to about
20mm.
A preferred fiber diameter ranges from about 6 m to about 19 m. Optionally, at
least
a portion of sizing material is removed from the fiberglass component prior to
combining with the silicone component.
The silicone component provides the coating with mechanical flexibility and
thermal stability over a broad temperature range (e.g. -110 - 400 F).
Additionally,
the decomposition of the silicone component at high temperatures (e.g. greater
than
400 F) into silicon dioxide and silicon oxide absorbs a large amount of energy
from
the heat source. Furthermore, as a result of silicone degradation, large
surface areas
of fiberglass are exposed. The matted network of exposed fiberglass increases
the
coating's degree of radiative cooling and serves as insulation by remaining
grounded
in the cooler under layers of silicone near the protected substrate surface.
Preferably, the silicone component includes dimethylsiloxane and
polydimethylsiloxane.
Another aspect of the current invention includes a method for applying a flame
or heat flux protective coating composition to at least a portion of an
article, wherein
the composition includes a fiberglass dispersion in silicone. In a preferred
embodiment, the coating is applied by brushing onto a substrate. In another
embodiment, the coating is applied by dipping a substrate into the coating
composition. When applying the coating, an even layer is not critical but the
coating
should be thick enough to obstruct vision of the underlying surface.
Another aspect of the current invention includes an article, wherein at least
a
portion is coated with a composition, which includes a fiberglass dispersion
in
silicone.
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Suitable substrates for the coated article include, for example,
thermoplastics,
thermoplastic composites, polyethylene, wood, stone, metal (e.g. steel),
ceramics,
glass, masonry materials (e.g. brick, marble, granite, travertine, limestone,
concrete
block, glass block, tile, etc.), and the like. For example, U.S. Patent Nos.
6,191,228,
5,951,940, 5,916,932, 5,789,477, and 5,298,214 disclose structural recycled
plastic
lumber composites made from post-consumer and post-industrial plastics, in
which
polyolefins are blended with polystyrene or a thermoplastic coated fiber
material such
as fiberglass. The disclosures of all five patents are incorporated herein by
reference.
The coated article can have any shape or form, for example, a round cross-
section, a rectangular cross-section, an hourglass cross-section, a sheet
form, or a
combination thereof. Exemplary forms for plastic composites are disclosed in
U.S.
Application No. 60/486,205 filed July 8, 2003, U.S. Application No. 60/683,115
filed
May19, 2005, U.S. Application No. 10/563,883 filed January 9, 2006, and
International Application No. PCT/US06/19311 filed May 19, 2006. The
disclosures
of all of which are incorporated herein by reference. In one embodiment, the
article is
an L-Beam, I-Beam, a C-Beam, a T-Beam, or a combination thereof.
Exemplary articles suitable for coating with the composition of the present
invention include, but are not limited to, steel ammunition boxes, railroad
ties, plastic
piping, lumber, sheet piling, boat hulls, pick-up truck beds, gasoline
canisters, fuel
tanks in automobiles, airplanes, ships, and submarines, areas near high
temperature
operating components, such as ignition champers, infrastructure, for example,
building support structures and cables in suspension bridges, high-pressure
storage
tanks, and the like.
The composition of the present invention can also be incorporated into a sheet
form. For example, the silicone and fiberglass components can be combined in
an
extruder and extruded into a sheet die. In another embodiment, a woven
fiberglass
cloth is coated with the silicone component.
Exemplary applications for the sheet forms of the present invention include,
but are not limited to, fabrics, for example, fire protective clothing and
blankets, and
sheets applied to any of the articles mentioned above as being suitable for
coating
with the composition of the present invention.
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The following non-limiting examples set forth herein below illustrate certain
aspects of the invention.
EXAMPLES
Example 1 - Sample preparation
5 Blends of 4, 6, 8, 10, 12, and 14 % by weight fiberglass in silicone, with
trace
amounts of silicone oil, were prepared. The components were blended in a mixer
and
applied to a steel coupon with a putty knife targeting a thickness of 1.6 mm
or less.
The fiberglass/silicone coatings were compared against seven commercial
products
(FIG. 1) in a low temperature flexural test and a direct high temperature
flame test.
The coatings were applied to standard 76 by 152 by 0.735 mm steel coupons.
Three
specimens per sample, or coating type, were tested for both experiments.
Example 2 - Low temperature flexural test
Coated steel plates were annealed in dry ice, approximately -79 C, for at
least
minutes followed by bending around a 0.64 cm mandrel to an angle of 180 .
15 During the test, photographs were taken of each specimen at 30 , 90 , and
180 of
bending. Visual observation provided information about a coating's response to
thermal shock when bonded to a steel substrate and indicated the type and
severity of
surface damage incurred due to bending at low temperatures. A successful
coating
did not have surface damage after testing.
During bending, the coating stretches to accommodate the substrate's new,
larger surface area. The surface of the coating is in tension and receives the
highest
percent strain during bending. Thus, crack formation is initiated at the
coating
surface. Failure of the coating is indicated by crack development and
propagation in
the coating and delamination. Common modes of failure included tiny crack
formation parallel to the bending axis in the deformation region, large cracks
that
caused pieces of the coating to detach and expose the substrate, and some
brittle
failure. In some cases, the coating delaminated as well. These types of
surface failure
indicate a coating with low strain to failure at low temperatures that will
detach or
delaminate, expose the substrate, and create a point source of radiative heat.
As indicated in FIG. 1, Products A, B, C, D, E, and G failed the low
temperature flexural test due to crack formation. At more severe bending
angles, the
initial cracks simply propagated, caused pieces of the coating to detach from
the
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substrate, and/or the coating delaminated. In the Product C sample, 2 of 3
specimens
passed, and in the Product G sample, 1 of 3 specimens passed. However, all
specimens per sample must pass the test in order to be considered successful.
Product
H, a silicone-based coating, is the only commercial coating tested that did
not suffer
any surface damage and passed the low temperature flexural test. The
fiberglass/silicone composite coating did not suffer any surface damage,
remained
adhered to the substrate during bending, and passed the low temperature bend
test.
The coating thickness does not appear to significantly affect low temperature
performance. For Product E and the fiberglass/silicone composite, specimens
were
prepared at various thicknesses. All Product E specimens failed while all
fiberglass/silicone composite specimens passed.
Example 3 - Flame test
A flame produced by a propane torch was applied normal to the coated side of
a specimen. An IR sensor (Omega OS550 Series Infrared Industrial Pyrometer)
was
aligned on the same axis as the flame and measured temperature as a function
of time
on the back side of the vertical steel coupon. The inner cone length of the
flame was
adjusted to 3.175 cm, and the tip of the inner cone, the hottest part of the
flame, was
positioned directly on the sample's surface 2.54 cm above the bottom edge and
at the
center across the sample width. This configuration delivered worst case
scenario
results for high temperature direct point heating. The adiabatic flame
temperature of
propane in air is approximately 1,927 C +/- 38 C. The flame was applied for a
total
duration of ten minutes. A coating is considered to fail the flame test if the
maximum
temperature detected by the IR sensor exceeds 316 C. The maximum temperature
reached for each coating was compared against the control specimen, an
uncoated
steel plate, as a point of reference.
The flame test results are presented graphically in FIGS. 2 and 3. The average
temperature versus time data collected during the flame test for each sample
is
presented in FIG. 2, and the average maximum temperature and standard
deviation
per sample in FIG. 3. The 12 % fiberglass/silicone coating maintained the
lowest
maximum temperature of all of the coatings. In FIG. 3, the black horizontal
line
signifies the pass/fail temperature limit of 316 C and delineates the
coatings that
passed the flame test from those that did not (e.g. coatings with a maximum
temperature below the line pass, while those above the line fail).
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Coatings with a maximum temperature below the limit were Products D and E
and the fiberglass/silicone composite coatings (excluding the 6 Io fiberglass
composition) (FIG. 1). The average maximum temperatures of Products A, B, C,
F,
G, and H exceeded the limit, thus failing the test.
The foregoing examples and description of the preferred embodiments should
be taken as illustrating, rather than as limiting the present invention as
defined by the
claims. As will be readily appreciated, numerous variations and combinations
of the
features set forth above can be utilized without departing from the present
invention
as set forth in the claims. Such variations are not regarded as a departure
from the
spirit and script of the invention, and all such variations are intended to be
included
within the scope of the following claims.
