FIELD OF THE INVENTION

The invention relates to heat-resistant coatings for use on substrates and, more particularly to a photon-diffusive coating.

BACKGROUND OF THE INVENTION

A solid, opaque substrate (surface) will react to impinging radiation, or heat energy, through the mechanisms of absorption or refection. Energy, if absorbed, may remain in the substrate or leave it through the mechanism of emission.

Many traditional heat control systems rely on the absorption- emission principal - that is, they take up heat and then get rid of it. There are several disadvantages to this mechanism. First, part of such emission occurs from the cold side of the substrate resulting, for example, in undesired heat loss (e.g., to a furnace)

or _h eat gain (e.g., to a building). Second, the substrate is subjected to non-uniform thermal loading which may shorten its useful life.

The reflection mechanism avoids these problems. Because heat energy does not succeed in penetrating the substrate, the substrate is protected from degradation. For example, a reflective lining in a furnace will radiate energy back to the furnace interior where it can do useful work. In the case of protecting a substrate, the reflected heat would reduce the backside heat temperature of the substrate.

In response to the ever-expanding need for heat-efficient materials, dispersive coatings have been developed. The instant coating operates on the principle of photon diffusion. The composition of instant photon diffusive coating is made from klannerite, which is a hydrothermally altered form of silica containing over 55% cristobalite and tridymite. The coating is able to diffuse heat, because of the presence of the minerals cristobalite and tridymite, and they, therefore, are key ingredients in the instant photon diffusive coating formula. The coating has a very low indices of refraction, and thus does not capture and subject rays to internal reflection within the crystal structure. Further, the mineral has a natural microporosity (e.g., with pores smaller than 0.17 μm) . Such PDCs (photon dispersive coatings) exhibit excellent photon reflectance, even at very short

wavelengths. Photon diffusion occurs principally at wavelengths below 5 microns.

Turning briefly to the general "physics" of the situation, dispersive coatings have the ability to reflect electromagnetic radiation with negligible absorption. Their ability to diffuse energy is dependent upon the crystal's size and geometry, and orientation of the coating material. One of ordinary skill in the art to which this invention most nearly pertains will understand that when a solid is heated, it emits light. Emission occurs because surface atoms or molecules are raised to excited states. As the particles spontaneously return to their stable (ground) energy states, energy is emitted in the form of electromagnetic radiation. The emitted radiation, typically referred to as thermal radiation, will be distributed over a range of wavelengths, and may result from changes in the electronic, vibrational, and relational states of the atoms and molecules. The phenomenon of emission is embodied by the well-known formula E _photon = hv/lambda, where h is Planck's constant and v is frequency. Evidently, as the temperature of a body is raised, the radiant energy emitted from the body will always tend towards shorter wavelength (lambda), higher energy photons.

The range of options available in selecting a fire-retardant or fire-proofing material is fairly summarized in U.S. Patent No. 4 , 118 , 325. as follows:

"In selection of a fire-proofing composition, a choice must generally be made in which fire protection is weighed against other considerations which might affect the suitability of a specific composition for a particular use. The characteristics of heavy fire- proofing compositions such as concrete and gunite are well known, but the characteristics of lightweight fire- proofing materials are not so well known and include the ability to provide protection through one or more of the following chemical reactions which take place upon exposure of the compositions to heat from a fire. Calcination of the composition results in a breakdown of the stable materials, generally resulting in absorption of heat which is then unavailable to affect the protected substrate. Ablation is a process of peeling off exhausted surface layers of insulation to expose new protective layers. Intumescence occurs when heat converts a thin coating of a fire-proofing material into a thick insulating barrier. During intumescence, cooling gases are normally released, leaving a reflective multi- cellular foam insulation. Thermal hydrogenation occurs during some calcination reactions when the heat causes release of water of hydration as water vapor."

A number of patents deal with reflecting infra-red energy as an aspect of flame control or resistance to the spread of heat.

U.S. Patent No. 4,137,198 discloses a foam product intended for use as a construction material with fire-resistant and thermally-insulating properties. At column 12, lines 21 through

29, the following properties of the product are noted:

"A block of polymer-hybrid foam composition of this example was subjected to high temperature combustion conditions at flame temperatures of 1800°F. to 2000°F. in gas fired combustion chamber. It was observed that the surface of the polymer-hybrid foam composition developed infra-red reflective chromophores from decomposition of the hybrid foam, which acted as mirrors, reflecting the infra-red rays from the surface of the composition back to the source of the flame."

U.S. Patent No. 4,174.711 discloses a fire-resistant enclosure

which includes an outer layer intended to reflect infra-red radiation. This reflective layer may be chrome plating or gold galvanization. The reflective layer is then painted with a coating which burns off when heated, thereby exposing the underlying infra¬ red-reflective layer. Other materials, in the form of eutectic mixtures are sandwiched behind the infra-red-reflective layer to provide additional heat-resistant and absorbent properties.

U.S. Patent No. 4,104,427 discloses a laminated fire screen panel to which may be applied an infra-red reflecting coating of either copper, aluminum or a metallic oxide to protect the intumescent fire-retardant material from decomposition and to delay the disposal of the material upon the outbreak of a fire. U.S. Patent No. 4,268.581 discloses the use of similar infra-red reflective coatings.

U.S. Patent No 4,173,668 discloses using an infra-red reflective coating to slow the heating of the intumescent layer, thus prolonging the protection afforded by the fire screen.

U.S. Patent Nos. 3,935,681 and 4,071,649 disclose fire proof glass work. Each of these references teaches the inclusion of a component which is opaque to or reduces the transmission of infra¬ red radiation.

U.S. Patent No. 4,563,843 discloses the use of silver, gold or copper as an infra-red reflective layer for use in a heat insulation window.

U.S. Patent Nos. 3,253.008 and 3.345.132 disclose the use of silicon and silica compounds in fire resistance paints and coatings. In the '008 reference, the use of nitrogen-silicon components as flame-proofing agents (column 2, line 62) is disclosed. In the '132 reference, the use of silicic acids as heat insulation materials (column 3, line 60) is disclosed.

The use of intumescent compounds to afford protection against heat and/or fire is exemplified by the follow group of patents.

U.S. Patent No. 3.535.130 discloses a paint which chars upon being exposed to fire, liberating water vapor and sulfur dioxide to remove hot gasses and flames.

U.S. Patent No. 3,983,082 discloses a fire retardant having a silicon resin base which both intumesces and chars.

U.S. Patent No. 4,097,385 discloses a foaming sealing material contained within a flexible tube which may be placed in openings or cracks. When the tube is heated, the intumescent material contained therewithin expands to close off the opening. The intumescent material includes as aqueous alkali metal silicate

solution or a gel .

U.S. Patent No. 4,118.325 discloses a rigid foamed sodium silicate matrix within which unexpanded particulate fillers such as vermiculite or prelite are dispersed. Fire protection is afforded by the silicate foam matrix and by the expansion of the unexpanded mineral in response to the application of heat.

U.S. Patent No. 4.123,587 discloses an organic foaming agent intended to be included in host materials such as paints, coating compositions and the like. When heated, the composition give off nitrogen gas and/or water. The fire-retardant material as taught in this patent is provided in finely ground form which may be dispersed uniformly throughout the host material.

U.S. Patent No. 4,179.535 discloses a coating composition intended to stabilize and rigidify to prevent the composition from falling away from vertical surfaces during a fire. This patent teaches the use of a slurry of hydrated metal silicate particles and an aqueous alkali metal silicate solution.

U.S. Patent No. 4,265,806 discloses a tumescent foam flame retardant using organic constituents.

U.S. Patent No. 4,297,252 discloses the use of sodium aluminate in connection with a sodium silicate foam in order to

improve the aging characteristics of the foam.

U.S. Patent No. 4,338.374 discloses a foam having a self- curing surface which seals to protect the foam from the evaporation of water therefrom.

U.S. Patent No. 4,442.157 discloses a foaming fire-proofing composition using phenolic resins which decompose to form a stiff foam upon heating.

The following patents relate to heat- or temperature-resistant coatings:

U.S. Patent No. 3.347.684 discloses the use of organic fire- retardant pigments in paints. No silicates are used in the disclosed composition.

U.S. Patent Nos. 3.598.617 and 3,776,741 disclose a composition useful as a coating for resistors in the form of a paint having an orthosilicate base. The inclusion of silicon dioxide in a particular combination of particle sizes and proportions is disclosed.

U.S. Patent No. 4,013,476 discloses a molding material formed from silica and calcium oxide, mineral fibers and other inorganic constituents. The resulting material is non-combustible and may be

molded into a variety of shapes and articles.

U.S. Patent No. 4,097,287 discloses an inorganic film composition useful for the production of non-combustible paints. The composition includes a colloidal silica dispersion, aluminum compounds and powered glass.

U.S. Patent No. 4.137,178 discloses a flame retardant composition form from a phosphorus-nitrogen-silica formula intended to use spent catalyst as part of the formulation. The catalyst involved is a polyphosphoric acid held on a silica support.

U.S. Patent No. 4.114.074 discloses a non-flammable inorganic coating composition, including controlling the shape and size of the aluminum oxide and/or aluminum hydroxide constituents.

U.S. Patent No. 4.168,175 discloses a fire-retardant using borax and powered glass combined with cellulosic fibers.

U.S. Patent No. 4,548,891 discloses a coating formed of a photopolymerised titanium composition.

Other flame-proofing or flame-retardant compositions are exemplified by U.S. Patent Nos. 3.372,040 and 3,502,490 which disclose the manufacture and use of cementitious compositions for

protection from heat and flame.

U.S. Patent No. 2.561.304 discloses the use of silica in paints.

U.S. Patent No. 4,289.952 discloses a process for controlling the size of powder particles using light and heat energy.

As used herein, the terms "heat-resistant", "heat-retardant", "fire-proof" and "flame-resistant" may be used interchangeably.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide an improved heat-resistant coating for substrates exposed to heat.

It is another object to provide an improved heat-resistant coating that provides more even heat distribution as well as energy savings.

It is a further object to provide an improved base coat for intumescent fireproof coatings that has great mechanical resistance and remains active after gas erosion and water blast has removed the intumescent top coat.

It is a further object of the present invention to provide an improved heat-resistant coating and/or base coat for an overlying intumescent fireproof coating, method of making same, and method of applying same, for uses including: boiler interiors and exteriors, gas hoods, hot air ducts, refrigerated tanks and trucks, storage tanks, cryogenic tanks, liquified gas storage tanks, bending furnaces, annealing furnaces, Lehr furnaces, fire hearths, roof coatings, pool deck coatings, corrosion protection (combustion gasses, chlorine, non-ferrous metals) , coating ship superstructures, public transportation vehicles, train exteriors and interiors, rocket engines and spacecraft components, military armaments and systems, and fire-safe corridors and building

components, ferrous and non-ferrous applications to include but not limited to carbox furnaces, reheat furnaces, continuous furnaces, In/Out furnaces, car bottom furnaces, steel lids, batch annealing, box annealing, soaking pits, ladle dryers, ladle melting furnaces, BOF hoods, high heat furnaces, curing furnaces, glazing furnaces, and reheat furnaces, for applications in the steel industry, the ceramics industry, the aluminum industry, the power generation and petro-chemical industry, fireproofing applications, the glass industry, and other applications.

According to the invention, klannerite, a silaceous mineral, (a form of hydrated sodium alumino silicate) , is used as a flame- protective substance (e.g., photon-diffusive) material or coating. The flame-retardant (i.e., beneficial heat-resistant) properties of the coating are achieved by reflecting energy in the infra-red wavelength region, and this reflectance is achieved by chemical adjustment of the particle size and lattice structure of the klannerite molecules.

According to an aspect of the invention, the klannerite is applied in a binder (e.g., latex) and exhibits its infra-red reflective properties after having been subjected to heat. Use of the coating on a heated substrate, such as a steam pipe, will activate the coating's infra-red reflective properties, while use on normally unheated or ambient articles will not activate these reflective properties until heat is applied. The process of

manu _f acturing the photon diffusive coating generally consists of:

1. reacting silica crystals with sodium hydroxide;

2. adding a binder to hold the crystals; and

3. heating the binder and the crystals until infra-red reflectance occurs.

According to an aspect of the invention, a particular distribution of particle sizes and structure is achieved by selectively milling and combining different grinds of silica. Theoretically, the milling/combining process creates a molecular lattice structure within which individual molecules of silica are arranged in a close-packing configuration to provide the desired reflectance of infra-red energy.

Whereas various coatings of the prior art have suggeste _d coatings having infra-red reflective properties responsive to the application of heat, the reflective properties are typically obtained as a result of surface-induced phenomena, rather than _f rom a selected internal crystal structure, as in the present invention.

Further although various coatings of the prior art _h ave suggested the inclusion of silicon dioxide in a particular combination of particle sizes and proportions, it has not heretofore been taught cr suggested that a closely packed lattice, formed according to the techniques cf the present invention, can _b e

employed to reflect infra-red energy.

Further, although various coatings of the prior art have suggested the phenomenon of infra-red reflectance, the use of various silicon compounds, the use of selected range of particle sizes, and heat-responsive deployment or activation of fire- retardant properties, the prior art does not teach the particular combinations of properties or techniques disclosed in the present invention.

The present invention teaches unexpected results resulting from the control of the crystalline size and structure of silica compositions and the use of such structures to reflect infra-red energy as a flame-retardant technique.

Other objects, features and advantages of the invention will become apparent from the description that follows.

DETAILED DESCRIPTION OF THE INVENTION

T _h e present invention contemplates a novel heat-resistant coating and techniques for making and using same.

Generally, klannerite, a silaceous mineral (a form of hydrated sodium alumino silicate), is used as a flame-protective (e.g., photon-diffusive) material or coating. The flame-protective (i.e., beneficial heat-resistant) properties of the coating are achieved by reflecting energy in the infra-red wavelength region, and this reflectance is achieved by chemical adjustment of the particle size and lattice structure of the klannerite molecules.

Generally, the klannerite is applied in a binder and exhibits its infra-red reflective properties after having been subjected to heat. Use of the coating on a heated substrate, such as a steam pipe, will activate the coating's infra-red reflective properties. The coating forms a ceramic bond when heated at a temperature of 500°C for a period of one hour in an oxygen-free environment.

Generally, the process of manufacturing the photon diffusive coating consists of:

1. reacting silica crystals with sodium hydroxide;

2. adding a binder to hold the crystals; and

3. heating the binder and the crystals until infra-red reflectance occurs.

Generally, a particular distribution of particle sizes and structure is achieved by selectively milling and combining different grinds of silica. This milling/combining process creates a molecular lattice structure within which individual molecules of klannerite are arranged in a close-packing configuration to provide the desired reflectance of infra-red energy. In this manner, a closely-packed lattice is formed, which will reflect infra-red energy as a result of the selected internal structure of the coating. Hence, by controlling the crystalline size and structure of the silica composition, the use of such a composition to reflect infra-red energy and as a flame-retardant technique is achieved.

Example 1

An effective photon-diffusive coating was made by combining the following ingredients:

• 733.04 pounds of water;

• 376 pounds of 50% Sodium Hydroxide;

• 175 pounds of Rutile T102 ; and

• 853 pounds of silica, in a mixing vessel, then washing the walls of the vessel with 8.33 pounds of water. The ingredients are mixed well in the vessel, heating to 185 degrees Fahrenheit while mixing.

It should be understood that all values (e.g., weights of ingredients) set forth hereinabove and hereinbelow are intended to

be approximate (e.g., within 10%, including within 5%). The temperatures set forth hereinabove and hereinbelow are also intended to be approximate (e.g. , +/- 10%) .

It should further be understood that the values set forth hereinabove and hereinbelow relate to mixing a particular batch size, and are relative (i.e., to one another) proportions. The actual values may be scaled up or down, as required, to achieve a particular batch size.

After heating and mixing the ingredients, the resulting "slurry" is covered with water (e.g. , sufficient to form a barrier over the surface of the slurry - approximately 33.32 pounds of water), covered (e.g., with a lid), and maintained ("aged") at a temperature of 185 degrees Fahrenheit for a period of 24 hours.

After the initial aging period of 24 hours, the pH is checked. If the pH is above 12.0, aging is continued by maintaining the slurry at 185 degrees Fahrenheit, and the pH of the slurry is rechecked at 8 hour intervals.

Once the pH is 12.0 or lower, the slurry is cooled to 120 degrees Fahrenheit. Once cooled, 137 pounds of Lorcon JK 270 (a polysaccharide resin) are added to the slurry and mixed into the slurry at a low speed - ensuring good mixing action.

Meanwhile, 33.32 pounds of water and 6.5 pounds of sodium sesquicarbonate are mixed (off line) for approximately five minutes. This mixture is then (after cooling and adding the Lorcon JK 270) added slowly to the batch of slurry.

Then, 657 pounds of Airflex 525BP (a latex binder) are added to the batch of slurry.

Finally, up to 74.97 pounds of water are added to control viscosity.

Example 2:

In use, the photon diffusive coating mixture of Example 1 is diluted with deionized water in a range of 3-4 (three to four) parts water to one part mixture. The diluted mixture is filtered through a suitable filter, such as through a coarse paint strainer, and is then applied to a substrate desired to be coated. The preferred dilution is four parts deionized water to one part photon diffusive coating mixture.

The diluted photon diffusive coating mixture may be applied in any suitable manner to a substrate (surface) to be coated. One suitable application technique is simply depositing the diluted coating onto a surface, and using an adjustable doctor blade to meter the thickness of the coating on the surface.

T _h e diluted coating was observed to have a pH of 11.75, and a viscosity of 57KU (as measured according to ASTM test method D- 562) .

Preferably, the dry film thickness of the coating on the substrate is 9 mils (average thickness) , although thicker or thinner coatings are contemplated as being within the scope of this invention.

The coated refractory substrate should be allowed to condition (at room temperature) for approximately twelve hours, then baked, for example, at 950 degrees Fahrenheit for one hour.

Alternative methods of applying and conditioning the coating are discussed hereinbelow (with respect to Example 3) .

Example 3:

The photon diffusive coating mixture of Example 1, applied according to the techniques of Example 2 (to a 9 mil thickness) , was tested for effectiveness as a heat-resistant coating. Coated test panels of 0.020 inch thick aluminum were coated and cured as follows:

• all test panels were air-dried at 77 degrees Fahrenheit (i.e, room temperature), 50% R.H. (relative humidity), for approximately twelve hours. Then the test panels were subjected to

various additional curing steps A-F, as follows:

A. panels were baked at a temperature of 250 degrees Fahrenheit for a period of one hour;

B. panels were baked at a temperature of 350 degrees Fahrenheit for a period of one hour;

C. panels were baked at a temperature of 500 degrees Fahrenheit for a period of one hour;

D. panels were baked for one hour at a temperature of 120 degrees Fahrenheit, followed by baking for one hour at 250 degrees Fahrenheit;

E. panels were baked for one hour at a temperature of 120 degrees Fahrenheit, followed by baking for one hour at 350 degrees Fahrenheit;

F. panels were baked for one hour at a temperature of 120 degrees Fahrenheit.

The test consisted of subjecting 0.020 inch thick aluminum panels to a direct flame of approximately 3500 degrees Fahrenheit (e.g., such as is generated at the apex of the inner cone of a Bernz-O-Matic propane torch) . An un-coated panel burned through within four minutes. A coated panel did not burn through after 30 minutes, with the flame directed at the coated side of the panel. When the reverse side (opposite the flame) of the panel was un- coated, the panel burned through within four minutes.

Generally, the coated panel exhibited the ability to delay the

penetration of a high temperature flame through an aluminum panel by over 700%, as compared with an un-coated panel.

More particularly, the panels exhibited the following test results (see TABLE) , based on the additional steps A-F:

TABLE

Schedule A B C D E F

12hrs 77°F X X X X X X lhr 120°F X X X lhr 250°F X X lhr 350°F X lhr 500°F X

Blistering: VS1 Cons Cons None SI None

Adhesive Loss: None Cons Cons None Mod None

Burn through (mins.) >30 a a >30 >30 >30

Note: uncoated aluminum panel burned through in 4 minutes.

Key. a coating blistered and flaked off panel

V Very

SI Slight

Cons Considerable

Mod Moderate

Based on the results of this testing, it can be observed that in curing an aluminum substrate, the coating of the present invention effectively prevents burn-through of an aluminum panel when the coating is air-dried for twelve hours (e.g., overnight), and then force-dried at a temperature of 120°F for one hour. It is possible that further curing of the coating occurred in-situ, during the test (i.e., during exposure to heat from the torch).

It can also be observed, that force-drying (i.e., baking) of the coating above 250°F can result in undesired blistering of the coating.

Example 4:

The photon diffusive coating mixture of Example 1, applied according to the techniques of Example 2 (to a 9 mil thickness) , can be applied using a doctor blade to control thickness. It can also be mixed with a latex base, and sprayed or painted (brush or roller) onto the surface to be coated. For spraying the coating onto a surface, the use of an airless sprayer is recommended, with an appropriate nozzle size (e.g., 0.040 inch) , and further dilution (with water) may be necessary to control spray characteristics, and multiple coats may be required to achieve the desired coating thickness. It is desirable that a monolithic finish be achieved with the coating.

Additional Comments:

The coating disclosed herein provides superior results to other coatings. For example, high-emissivity coatings absorb heat (energy) and re-radiate it. The photon-diffusive coating of the present invention reflects energy rather than absorbing it, thereby reducing heat flow through the coated surface (e.g. , the wall of a furnace) . For example, in the case of a furnace, heat can be

re _fl ected to the interior of the furnace, thereby increasing the efficiency of the furnace and resulting in realizable energy savings.

The composition of the present invention is made from klannerite mineral, which is a hydrothermically altered form of silica, containing over 55% cristobalite and tridymite. Because of the use of crystobalite and tridymite, the photon diffusive coating has a very low indices of refraction, and thus does not capture and subject rays to internal reflection within the crystal structure. Further, the mineral has a natural microporosity (e.g., with pores smaller than 0.17 μm) . The composition of the present invention has been found to be 85% reflective of radiation in the 0.5 - 5.0 micron (wavelength) range, which is highly effective for reflecting radiant energy at 2200°F (which occurs at approximately 2.0 microns) .

Prior to applying the photon diffusive coating to a surface, the surface should be prepared by removing loose materials (e.g., scaling) , oils and grease. For steel surfaces, latex based primer coat should be applied prior to applying photon diffusive coating. For aluminum surfaces, prior acid etching is recommended.

The photon-diffusive material of the present invention is suitable for use as a heat-resistant coating, and the like, and as a base coat for subsequent intumescent fireproof coatings.