Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2000-03-16
2002-04-23
Barr, Michael (Department: 1762)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C427S379000, C427S376400
Reexamination Certificate
active
06376022
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to coatings that protect and insulate surfaces from high temperatures.
BACKGROUND OF THE INVENTION
At the present time TBC's are used to protect metal components exposed to high temperatures by reducing the temperature of the metal. For example, in gas turbines they are used on combustion hardware, after burners, vanes, blades, fuel nozzles and spray bars. More particularly, these fuel nozzles operate in an envelope of 700° F. to 1300° F. air. The fuel nozzles will become clogged if the temperature of the fuel becomes greater than about 400° F. because the fuel will form deposits at these high temperatures. The fuel is, of course, used to provide the energy to the turbine section of the gas turbine engine in order to produce useful thrust, work and/or heat. The primary cause of the fuel becoming too hot is from the heat of the air surrounding the fuel nozzle. The purpose of the TBC is to reduce the amount of heat flowing from this air into the fuel nozzle, and thence, into the fuel.
The difficulties encountered in making a suitable TBC are based on the fact that such protective coating must have a large number of mechanical and thermal properties. The TBC's must be capable of adhering to the metal and capable of being adhered thereto by a simple and low cost method. Also, the TBC's must protect the metal against heat, corrosion, and environmental damage.
TBC's currently in use for fuel nozzles are made by a plasma spray process. Those used for blades and vanes may also be made by an electron-beam, physical deposition process, but this is a very costly process. The plasma spray processes are inherently chaotic in nature, can be difficult to control and can produce unacceptable coatings while still using nominally the same materials and processing conditions. Such unacceptable coatings are those that readily suffer vibrational and/or thermal cycling damage, detachment from the metal surface and subsequent obstruction by the debris of critical air passages in the fuel nozzles. Plasma spray processes are also relatively expensive and require specialized set-up and long processing times.
SUMMARY OF THE INVENTION
The present invention provides a composition and a method for coating surfaces. The method comprises contacting a surface comprising metal with the coating composition. The coating composition has a polymerizable hydrated silicate powder comprising alkali metal ions, a forming agent comprising polyvalent metal ions, and an amount and form of moisture effective to promote ion exchange between the polyvalent metal ions and the alkali metal ions. The coating composition is cured under first curing conditions and for a time effective to 1) initiate polymerization of the silicate powder, and 2) to promote the ion exchange, producing an intermediate coating composition comprising bound water molecules. The first curing conditions are effective to maintain a sufficient amount of the moisture in the coating composition to promote ion exchange while under the first curing conditions. Sufficient energy is imparted to the coating composition at a rate and under second curing conditions effective to drive the bound water molecules from the intermediate coating composition and to produce a durable, adhesive protective coating.
DETAILED DESCRIPTION OF THE INVENTION
The instant coating composition adheres to metal surfaces and has excellent mechanical, thermal barrier, and environmental protective properties. These properties are achieved using a combination of a polymerizable silicate composition and a forming agent cured under conditions effective to remove substantially all of the water from the resulting coating.
The coating composition is formed using a polymerizable spray-dried hydrated alkali metal silicate powder. Any polymerizable spray-dried hydrated alkali silicate powder may be used in the present invention. Briefly, the powder is prepared by conventionally spray drying a solution of alkali metal silicate under conditions effective to produce a free-flowing powder. One example of a suitable polymerizable spray-dried hydrated alkali silicate powder can be found in U.S. Pat. No. 4,030,939. Sodium silicate powder is preferred due to its wide availability and low cost. The spray-dried hydrated sodium silicate powder preferably has a ratio of SiO
2
/Na
2
O of from about 2 to 1 to about 3.5 to 1, most preferably 2.4:1. Such powders are commercially available under the trademark BRITESIL sold by Philadelphia Quartz Corp.
In order to form the coating, a forming agent capable of forming weak acids is combined with the alkali metal silicate powder. Suitable forming agents include but are not necessarily limited polyvalent metal ions, such as zinc, aluminum, and zirconium, and mixtures thereof Preferably, the forming agent is in a finely ground state, from about minus 200 mesh (Tyler Standard) to about minus 400 mesh, most preferably about 5&mgr; or less. The smallest available particle size is desirable because it minimizes the reaction time and increases the rate at which alkali silicate is converted to polysilicic acid. In a preferred embodiment, from about 5 to about 20 parts by weight forming agent is added.
For optimum strength and resistance against shock, it is preferred to add certain siliceous fillers which can also react in forming the preferred protective coating. When the coating is heated to greater than 1,000° F., the fillers will mineralize and hybridize to the corresponding silicate. Such materials include siliceous sand, silica flour, fly ash clays, other argillaceous materials of high silicate content including, rice hull, diatomaceous silica, volcanic ash or mixtures thereof. Of these, silica flour is preferred due to its high availability and low price. For optimum reaction, finely ground filler materials, such as minus 200 mesh (Tyler Standard) should be used.
The coating composition is mixed with water and it has been found that for optimum strength, integrity and continuity of the coatings; namely, to prevent shrinking, cracks and the like, it is preferred to have a ratio of water to spray-dried hydrated sodium silicate powder of from about 0.9:1 to about 1.1:1. Limiting the amount of water is important to provide low porosity, final strength integrity and the desired application properties, such as trowelable, injectable, and castable rheologies. The composition can vary from Newtonian to thixotropic slurries depending on the application method to be used; i.e., dip coating, injection into fine capillaries and annuli, spray coating, or other conventional coating techniques.
Without being bound to any particular theory, it is believed that a polymerization reaction takes place as the forming agent hydrolyzes in the presence of water to liberate small quantities of ionic metal oxides and/or hydroxides. (e.g. Zn
++
, (OH)
2
−
). The liberated ions induce a steady polymerization of silicic acid hydrogel which is liberated by the neutralization of sodium silicate by, for example, by ion exchange with the polyvalent metal ions. As the ionized metal is consumed it is converted into a silicate polymer of high molecular weight products that solidifies around the filler, binding the material together. The siliceous and uniquely charged colloidal silica hydrogel attacks metal and mineral surfaces to provide the basis for forming a silicate bond to that surface. One example of a divalent metal/SiO
2
polymerization is illustrated by the following formula:
[—(SiO
2
)—O—Zn—O—(SiO
2
)—O—Zn—O—(SiO
2
)—O—]
n
where n is from about 1,000 to about 1,000,000, preferably from about 10,000 to about 1,000,000.
In a preferred embodiment, lightweight ceramic microspheres having insulative properties are used in the coating composition. The microspheres provide heat resistance and reduce the overall density of the resulting coating. Suitable microspheres are made in various diameter size and are commercially available from companies such as Philadelphia Quartz Corp., 3M Corp. and the like. T
Bernstein Henry L.
Mallow William A.
Moses Clifford A.
Barr Michael
Paula D. Morris & Associates P.C.
Southwest Research Institute
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