A cyanoacrylate adhesive is applied to non-suturable, non-sterile wound surfaces to protect and/or treat such surfaces, to promote wound healing and to retard infection of the wound.

1. Field of the Invention
This invention is directed to methods for treating non-suturable wounds by using cyanoacrylate adhesives. The cyanoacrylate adhesive to be used can be stored in dispensers for single or repeated/inter-mittent use.
2. State of the Art
Cyanoacrylate adhesives have been suggested for a variety of adhesive purposes including glues and surgical adhesives. In particular, cyanoacrylates of formula I:
wherein R is an alkyl or other suitable substituent are disclosed in U.S. Pat. Nos. 3,527,224; 3,591,676; 3,667,472; 3,995,641; 4,035,334; and 4,650,826. Typically, when used as an adhesive for living tissues, the R substituent is alkyl of from 2 to 8 carbon atoms and most often is butyl (e.g., n-butyl).
The suggested medical uses for cyanoacrylate adhesives include surgical environments wherein the cyanoacrylate adhesive is utilized, e.g., as an alternative to sutures or as a hemostat and, as such, are necessarily employed in a sterile environment. See, for example, Halpern, U.S. Pat. No. 3,667,472, and Robertson, et al., U.S. Pat. No. 3,722,599. In such surgical environments, the cyanoacrylate adhesive is applied to the soft tissue under sterile conditions and, in the presence of water or protein found in soft tissue, the adhesive bonds to the tissue as well as polymerizes which, in the case of adhesive sutures, joins the separated sections of soft tissue together.
Contrarily, most small wounds are neither treated in a surgical setting nor in a sterile environment. In a typical home setting, small wounds are generally cleaned and are either left exposed (generally if not bleeding) or are covered with a bandage (generally the case where minor bleeding occurs or where there is irritation). In either case, the wound is usually left to heal on its own over time with the accompanying discomfort during this period.
In any event, the art teaches against the use of alkyl cyanoacrylates on human skin tissue where it will be absorbed internally because of the lack of suitable low toxicity and/or adequate resorption/absorption properties for these cyanoacrylates. See, for example, Robertson, et al., U.S. Pat. No. 3,722,599. Specifically, it has been reported that, when used as suture supplements or internally, polymeric residue of higher homologs of alkyl cyanoacrylate (e.g., n-butyl cyanoacrylate) has been observed by histological examination of the site of application as much as 12 months after application. Apparently, the cyanoacrylate polymeric residue is encased internally and, due to low biological absorption/resorption properties, remains at the site of application for prolonged periods of time. Such poor absorption/resorption properties are contra-indicative for their use as tissue adhesives.
This invention is drawn to methods for treating and/or protecting small superficial wounds by the application of alkyl cyanoacrylate adhesive to the surface of such wounds. Such small superficial wounds are characterized as superficial cuts or abrasions not penetrating through the dermal layer of the skin surface to the subcutaneous layer or tissue.
This invention is based, in part, on the discovery that application of cyanoacrylate adhesive to such small superficial wounds will not result in the undesirable prolonged internalization of polymeric adhesive. Without being limited to any theory, Applicants believe that such internalization of the polymeric residue does not occur because any polymeric residue entrapped within the epidermal and/or dermal layer will be shed as part of the normal shedding of these layers.
Moreover, contrary to typical prior art application methods, Applicants have found that by limiting application to such superficial cuts and abrasions, the cyanoacrylate adhesive can be applied under non-sterile conditions typically employed in consumer usage.
The methods of this invention involve applying a cyanoacrylate adhesive, particularly, n-butyl cyanoacrylate adhesive, onto the wound under non-sterile conditions and allowing the adhesive to polymerize.
In the case of cuts, the cyanoacrylate adhesive is generally applied between the separated skin defining the cut as well as over the cut. The cyanoacrylate adhesive is then allowed to polymerize so as to both bind the separated skin sections and form a polymer layer over the cut. In addition to serving as a protective layer, the polymer layer also serves to promote healing and to retard infection of the cut.
In the case of abrasions, the cyanoacrylate adhesive is generally applied over the abrasion. The cyanoacrylate adhesive is allowed to polymerize so as to form a polymer layer over the abrasion. The polymer layer serves to act as a protective layer which prevents further aggravation to the abrasion while also promoting healing and retarding infection of the abrasion.
Accordingly, in one of its method aspects, this invention is directed to a method for treating and/or protecting non-suturable superficial wounds which comprises:
applying to the surface of a non-suturable wound, in a non-sterile environment, a sufficient amount of a cyanoacrylate adhesive so as to cover the entire wound area wherein the wound is characterized as superficial cuts and/or abrasions which do not penetrate through the dermal layer of the skin surface to the subcutaneous layer; and
polymerizing the cyanoacrylate adhesive so as to join separated skin sections and/or to form an adhesive coating which adheres to the area where the adhesive was applied,
wherein the cyanoacrylate, in monomeric form, is represented by formula I:
xe2x80x83where R is alkyl of 2 to 10 carbon atoms.
Preferably R is alkyl of from 2 to 8 carbon atoms and more preferably alkyl of from 4 to 8 carbon atoms. Most preferably, R is either n-butyl or octyl.
In another of its method aspects, this invention is directed to a method for treating and/or protecting superficial non-suturable cuts which method comprises:
applying to the surface of a non-suturable cut in a non-sterile environment, a sufficient amount of n-butyl cyanoacrylate adhesive to cover the entire cut area wherein said cut is characterized as a superficial cut which does not penetrate through the dermal layer of the skin surface to the subcutaneous layer; and
polymerizing the cyanoacrylate adhesive so as to join separated skin sections and to form an adhesive coating which adheres to the area where the adhesive was applied
wherein, in monomeric form, the n-butyl cyanoacrylate adhesive is represented by the formula:
In a preferred embodiment, the cyanoacrylate is applied at least about 0.02 milliliter (ml), more preferably from about 0.02 to about 0.2 ml, and still more preferably from about 0.02 to about 0.1 ml, of cyanoacrylate adhesive per square centimeter of skin which is to be covered.
In another preferred embodiment, the cyanoacrylate adhesive to be applied to the skin has a viscosity of from about 2 to about 3000 centipoise at 20xc2x0 C. More preferably, the cyanoacrylate adhesive is in monomeric form and has a viscosity of from about 2 to about 100 centipoise at 20xc2x0 C.
As used herein, the following terms have the following meanings:
the term xe2x80x9ccyanoacrylate adhesivexe2x80x9d refers to adhesive formulations based on cyanoacrylate monomers of formula I:
xe2x80x83where R is selected from the group consisting of alkyl of 2 to 10 carbon atoms.
Preferably, R is an alkyl group of from 2-8 carbon atoms including ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, n-pentyl, iso-pentyl, n-hexyl, iso-hexyl, 2-ethyl-hexyl, n-heptyl, and octyl. More preferably, R is butyl or octyl and most preferably, R is n-butyl.
These cyanoacrylate adhesives are known in the art and are described in, for example, U.S. Pat. Nos. 3,527,224; 3,591,676; 3,667,472; 3,995,641; 4,035,334; and 4,650,826 the disclosures of each are incorporated herein by reference in their entirety.
Preferred cyanoacrylate adhesives for use in this invention are n-butyl-2-cyanoacrylate and octyl-2-cyanoacrylate.
The cyanoacrylate adhesives described herein rapidly polymerize in the presence of water vapor or tissue protein, and the n-butyl-cyanoacrylate is capable of bonding human skin tissue without causing histoxicity or cytotoxicity.
The term xe2x80x9cnon-suturable or small woundsxe2x80x9d means superficial cuts and abrasions characterized as superficial cuts and/or abrasions which do not penetrate through the dermal layer of the skin surface to the subcutaneous layer. Such superficial wounds include cuts where the skin is separated and can be joined together, as well as abrasions such as xe2x80x9cnicksxe2x80x9d or xe2x80x9cscrapesxe2x80x9d where the skin is removed. However, non-suturable wounds do not include puncture wounds.
In view of the above, non-suturable wounds as defined herein include common cuts and scratches which rarely need medical attention unless located in a sensitive area or unless secondary infection occurs. As opposed to the edges of suturable wounds which can be widely separated, the edges of non-suturable wounds can easily be opposed or brought together. One particular example of a non-suturable wound treatable by the methods of this invention is skin tearing adjacent the site of a catheter implant.
This invention relates to methods for treating and/or protecting non-suturable superficial wound surfaces with cyanoacrylate adhesives in an every day, typical non-sterile environment.
The cyanoacrylate adhesive to-be-applied to the superficial wound surface can be monomeric or partially polymeric. In general, partially polymerized cyanoacrylate adhesives are liquid polymers having a higher viscosity than that of the corresponding monomer and, therefore, are better suited for those applications which are intended to be specific for a particular skin area. In other words, less viscous materials are more likely to xe2x80x9crunxe2x80x9d (i.e., flow) into areas where application was not intended.
The cyanoacrylate adhesives used herein preferably have a viscosity of from about 2 to about 3000 centipoise and more preferably from about 2 to about 100 centipoise at 20xc2x0 C. It is contemplated, however, that pastes and gels having viscosities of up to 50,000 centipoise at 20xc2x0 C. can also be employed and will make for easier skin application.
The specific viscosity of the formulation depends, in part, on the amount and degree of partially polymerized cyanoacrylate adhesive employed as well as additives which are employed in the formulation to enhance or decrease viscosity. Such factors are readily ascertainable by the skilled artisan. For example, methods for preparing partially polymerized cyanoacrylate adhesives are disclosed, for example, by Rabinowitz, U.S. Pat. No. 3,527,224 which is incorporated herein by reference in its entirety. Additives which can be incorporated into the formulation to enhance its viscosity include polymers such as polymethyl methacrylate (PMMA) and polymerized cyanoacrylate adhesives as disclosed in U.S. Pat. Nos. 3,654,239 and 4,038,345 both of which are incorporated herein by reference in their entirety.
Monomeric forms of cyanoacrylate adhesives are often preferred where application is to be made to a large surface area. This preference results from the fact that these forms are less viscous and, accordingly, will permit more facile large surface area application. Mixtures of monomeric forms of cyanoacrylate adhesive and partially polymerized forms of cyanoacrylate adhesive can also be used to prepare a formulation having intermediate viscosities.
For purposes of this invention, monomeric or partially polymerized n-butyl-2-cyanoacrylate is a particularly preferred adhesive and is capable of effectively bonding human skin tissue without causing histoxicity or cytotoxicity.
Upon contact with skin moisture and tissue protein, the cyanoacrylate adhesives will polymerize or, in the case of partially polymerized cyanoacrylate adhesives, will further polymerize, at ambient conditions (skin temperature) over about 10 seconds to 60 seconds to provide a solid layer which forms over and strongly adheres to the surface of the skin, thus providing a protective layer to the wound area.
The adhesive is applied to provide an effectively thick coating over the surface of the superficial wound. Because the to-be-treated wound is superficial and does not extend beyond the dermal layer, any polymeric residues diffusing into or forming in the wound will be naturally extruded from the skin. Generally, the adhesive provides an adhesive film coating over the wound area which when set is satisfactorily flexible and adherent to the tissue without premature peeling or cracking. Preferably, the adhesive coating has a thickness of less than about 0.5 millimeter (mm), and more preferably the coating has a thickness of less than about 0.3 mm. In a particularly preferred embodiment, the thickness of the adhesive coating is from about 0.1 millimeter to about 0.5 millimeter and even more preferably from about 0.1 millimeter to about 0.3 millimeter.
Adhesive coatings of such thicknesses form a physical barrier layer over superficial wounds which coatings provide protection for the wound in the same manner as a conventional bandage, e.g., Band-Aid(trademark) brand bandages. Specifically, the cyanoacrylate adhesive coating provides an airtight, waterproof seal around the wound which does not need to be replaced when the wound gets wet. Once applied, the coating prevents bacterial and contaminant entry into the wound, thus reducing the rate of secondary infection. Generally, the adhesive coating does not limit dexterity and promotes faster wound healing.
Additionally, unlike conventional bandages, the adhesive coating of this invention naturally sloughs off the skin within 2-3 days after application and, accordingly, avoids the discomfort associated with removal of conventional bandages from the skin. However, if early removal of this polymeric coating is desired, such can be achieved by use of solvents such as acetone.
The adhesive coating can be formed by applying at least about 0.02 ml of cyanoacrylate adhesive per square centimeter of skin, more preferably from about 0.02 to about 0.2 ml, and still more preferably from about 0.02 to about 0.1 ml, of cyanoacrylate adhesive per square centimeter of skin and yet more preferably from about 0.02 to about 0.05 ml of cyanoacrylate adhesive per square centimeter of skin.
The cyanoacrylate adhesive formulations employed herein generally comprise monomeric and/or partially polymerized compounds of formula I described above and are sometimes referred to herein as simply cyanoacrylate adhesives. These formulations are liquid in nature and, upon contact with surface skin proteins and moisture, will polymerize to provide a solid film or layer over the skin surface.
The formulations may additionally contain one or more optional additives such as colorants, plasticizers, perfumes, anti-diffusion agents, modifying agents and stabilizers. In practice, each of these optional additives should be both miscible and compatible with the cyanoacrylate adhesive. Compatible additives are those that do not prevent the use of the cyanoacrylate adhesives in the manner described herein.
In general, colorants are added so that the polymerized film will contain a discrete and discernable color. Perfumes are added to provide a pleasant smell to the formulation. Stabilizers, such as sulfur dioxide, are added to retard in situ polymerization in containers during storage. Plasticizers, such as dioctylphthalate or tri (p-cresyl)phosphate, are added in order to enhance the flexibility of the resulting polymer layer. Each of these additives is conventional. For example, suitable stabilizers are disclosed in U.S. Pat. No. 4,650,826 and suitable plasticizers are disclosed in U.S. Pat. Nos. 2,784,127 and 4,444,933 the disclosures of all of these patents being incorporated herein by reference in their entirety.
The amount of each of these optional additives employed in the cyanoacrylate adhesive is an amount necessary to achieve the desired effect.
The formulation is generally stored in an applicator for use in a single dose application or for use in repeated applications. Single dose applicators include those having breakable or removable seals that prevent moisture, including atmospheric moisture, from contacting the formulation and causing in situ polymerization.
For repeated and intermittent usage, minimal exposure to atmospheric moisture is required. This can be achieved by devices having very narrow outlets and low initial dead space. One applicator for such repeated intermittent use is described in U.S. Pat. No. 4,958,748 which is incorporated herein by reference in its entirety.
Another applicator comprises a conventional spray applicator wherein the cyanoacrylate adhesive is sprayed onto the surface skin area which includes the superficial wound. The spray rate of the applicator can be controlled so that application of a metered quantity of adhesive per unit area of skin surface over a set period of time is controlled.
Still another applicator comprises a brush or solid paddle applicator wherein the cyanoacrylate adhesive is xe2x80x9cpaintedxe2x80x9d onto the surface skin area containing the superficial wound.
A preferred applicator for repeated and intermittent usage is an applicator suitable for the non-sterile storage and metered dispersement of a cyanoacrylate adhesive after opening of the applicator wherein the applicator is characterized as having a resealable opening of no more than about 0.008 square inches (0.0516 square centimeters) so as to permit the metered dispersement of the adhesive from the applicator and which is capable of multiple administrations of the adhesive and is further characterized as having resealing means such as a cap which either tightly mates with the applicator or which screws onto the applicator.
Preferably, the opening of the applicator is about 0.0016 to about 0.003 square inches (about 0.0103 to about 0.0194 square centimeters).
In another preferred embodiment, the walls of the applicator are made of a pliable material, so that upon application of pressure onto the walls, the walls depress sufficiently to force the adhesive contained in the applicator through the opening. Preferably, the applicator is manufactured with its opening covered by a metal foil or other similar construction which closes this opening until the device is ready for use. The opening is then reinstated by use of a pin or similar device which punctures the covering.
In applicators suitable for repeated intermittent uses, the cyanoacrylate adhesive is stored at ambient conditions and is selected to be bacteriostatic. See, for example, Rabinowitz et al., U.S. Pat. No. 3,527,224. When the selected adhesive is bacteriostatic, prolonged storage at ambient conditions is without regard to the sterility of the formulation because there is no adverse buildup of bacteria during storage.
The above-described formulations are applied to a wound area under conditions suitable for polymerizing the adhesive so as to form a protective coating. In general, the wound is usually first cleaned (soap/water and optionally a disinfectant), and then sufficient amounts of cyanoacrylate adhesive are employed to cover or encase the entire scrape, scratch or nick area and is preferably extended by at least about 1 centimeter beyond the wound. No other sterilization methods are required as the methods of this invention are readily practiced in a non-sterile environment.
For cuts, an amount sufficient to join the opposing skin edges is applied, and optionally a sufficient amount is applied to also encase the entire cut area. For scrapes, a sufficient amount is applied to encase the entire wound area.
In either case, the adhesive polymer coating should preferably be maintained in a unbroken manner over the entire wound area. This can be assured by careful application of the adhesive onto the skin/wound. Additionally, the use of a plasticizer will facilitate the maintenance of the polymer coating in an unbroken manner. However, to further ensure that the polymer coating is maintained unbroken, after the initial layer of adhesive has cured to provide for an adhesive polymer coating, a second, preferably thinner, layer is applied over the adhesive polymer coating. Additional amounts of cyanoacrylate adhesive can be applied as needed to maintain an unbroken coating covering over the wound areas.
When the adhesive is applied to cover or encase the wound area, sufficient cyanoacrylate adhesive is preferably employed to form a coating of less than about 0.5 mm thick and more preferably at least about 0.1 mm thick. Such coatings are formed by applying at least about 0.02 ml of cyanoacrylate adhesive per square centimeter of skin surface area.
The amount of cyanoacrylate adhesive applied onto the skin surface area can be controlled by the amount of adhesive packaged in a single dose product or by use of a multiple use dispenser which governs the amount of material applied onto a unit area of surface skin. In this regard, the dispenser described by Otake, U.S. Pat. No. 4,958,748, which is incorporated by reference in its entirety, is particularly advantageous because it dispenses the adhesive in a controlled drop wise manner. Other methods for the controlled dispersement of the cyanoacrylate adhesive are as described above including, by way of example, a conventional spray applicator, a brush or solid paddle applicator, and the like.
Upon application of the cyanoacrylate adhesive, the surface skin moisture, tissue protein, and temperature are sufficient to initiate polymerization of the adhesive upon application. Thereafter, the skin surface is maintained under suitable conditions to allow polymerization to proceed to formation of an adhesive coating.
In general, the particular length of time required for polymerization will vary depending on factors such as the amount of adhesive applied, the temperature of the skin, the moisture content of the skin, the surface area of the wound, and the like. However, in a preferred embodiment, polymerization is generally complete within about 10 seconds to about 60 seconds while the skin is maintained at ambient conditions. During this period, the person to whom application of the cyanoacrylate adhesive has been made merely allows the adhesive to form a coating while minimizing any action to prevent the adhesive from being dislodged from that portion of the skin where it was applied or to adhere to unintended objects. Excess adhesive polymer can be removed with acetone (nail polish remover) which can be readily conducted except in the case where the adhesive polymer binds to a sensitive skin part (e.g., eye lids) where it should be removed by a health care professional. After the adhesive coating has formed, the coating strongly adheres to the skin, is flexible and waterproof, thereby protecting the wound area and promoting healing.
It is important to note that the adhesive coating of the invention can be applied in a non-sterile environment to a non-sterile surface. This is directly contrary to the use of cyanoacrylates as surgical adhesives which requires one-time use in a sterile environment. The invention provides for storage of the adhesive in a dispenser for repeated intermittent uses in a non-sterile environment.
In general, the coating will adhere to the skin for a period of about 2-3 days after which time it sloughs off. Additional applications can be made if desired.
The coating protects non-suturable wounds because the adhesive forms a polymer coating which extends over the entire surface of the wound to protect the wound in much the way a bandage does while, in the case of cuts, also joins together the separated skin surfaces. Because the coating is waterproof, the patient is not prevented from bathing and other activities involving exposure to water during the period the adhesive layer protects the wound.
One particular example of a non-suturable wound treatable by the methods of this invention is skin tearing adjacent the site of a catheter implant. Specifically, it is common after catheter implantation to have the skin adjacent the catheter to tear, partially due to catheter movement relative to the site of catheter implantation due to the patient""s muscle contractions, etc. Such tears are typically non-suturable wounds and can become the site of infection and are prone to further tearing. Moreover, as the skin tears adjacent a catheter implant, it becomes difficult to maintain the catheter in the implant site. Heretofore, there were no acceptable methods to treat such tearing and potentially inadvertent catheter removal other than to reinsert the catheter at another site.
The methods of this invention now provide for a method to treat such tearing without removal of the catheter. Specifically, in this particular aspect of the methods of this invention, the cyanoacrylate adhesive is applied onto these tears and then polymerized as described generally above. This results in joining the separated skin sections of the tear which protects the tear and retards further tearing thereof.
This method can also be employed prophylactically by applying the cyanoacrylate adhesive to the skin areas adjacent the catheter prior to actual skin tearing. In this regard, application is generally made in the manner and amounts described above and is preferably applied to the skin area approximately 1 centimeter and preferably about 0.5 centimeter in diameter around the catheter implant.
Accordingly, this aspect of the present invention is directed to a method for retarding skin tearing adjacent a catheter implant which method comprises:
applying a sufficient amount of a cyanoacrylate adhesive so as to cover an untorn skin area adjacent a catheter implant; and
polymerizing the cyanoacrylate adhesive so as to form a polymer film over the skin area which film adheres to the skin area where the adhesive was applied,
wherein the cyanoacrylate adhesive, in monomeric form, is represented by formula I as described above.
In the above prophylactic methods, the polymer film or coating is preferably less than about 0.5 millimeter in thickness and more preferably from about 0.1 to about 0.5 millimeter in thickness and still more preferably, from about 0.1 to about 0.3 millimeter in thickness.
Whether employed prophylactically or to treat existing tears, care should be taken during application of the cyanoacrylate adhesive to the skin areas adjacent catheter implantation to ensure against penetration of the adhesive into the skin puncture defined by the catheter so as to avoid skin irritation. One method for avoiding such penetration of the adhesive is to employ a viscous cyanoacrylate adhesive formulation preferably having a viscosity of from about 40 to about 100 centipoise at 20xc2x0 C. Such viscous formulations tend to remain at the site of application and not run into the puncture wound. Moreover, a colorant can be incorporated into the cyanoacrylate adhesive composition to readily discern where the adhesive has been applied.
Still another particular example of a non-suturable wound treatable by the methods of this invention is the superficial skin tearing remaining after suture removal. Typically, in the case of suturable wounds (e.g., surgical wounds), the separated skin sections are joined by either sutures or by staples (collectively xe2x80x9csuturesxe2x80x9d), and the wound typically heals outward. That is to say that the internal sections of the wound heal first with subsequent healing outward to the skin surface. Accordingly, when the sutures are removed, the remaining wound is typically a superficial non-suturable wound because the non-healed portions of the original wound typically extend only to the dermal layer of the skin. In this regard, application of the cyanoacrylate adhesive to these wounds after suture removal as per the methods described herein provides an effective method to treat these wounds. Additionally, when the adhesive is applied to form an adhesive coating over the wound, the resulting coating is waterproof and flexible thereby allowing the patient significant freedom in bathing, swimming, etc. as compared to current practice after suture removal where the patient is instructed to minimize water contact with the wound for several days.

Methods for treating non-suturable wounds by use of cyanoacrylate adhesives


A gas turbine engine abradeable seal system is provided comprising a seal assembly and a cooperating interacting turbine blade. The turbine blade has a tip portion containing cubic boron nitride abrasive particles and the seal assembly has a superalloy substrate with a bond coat thereon having a surface roughness of at least 300 RA and a porous ceramic abradeable seal material on the bond coat having a porosity of from 5 to 15 volume %.

This invention relates to an abradeable seal system, more particularly to the use of a seal assembly with increased erosion resistance.
The efficiency of modem gas turbine engines depends upon a tight seal between the rotating components (blades) and the stationary component (shroud) in the fan, compressor and turbine. This seal is established by allowing the blades to cut (abrade) a groove in an abradeable seal material which prevents a substantial volume of air from leaking past the blade tip. Traditionally the turbine seal materials have been fabricated from woven metallic fibers or sintered metallic particles and brazed in place. While these materials are easily abraded due to their high internal porosity and low strength, their resistance to particle erosion is poor which results in rapid loss of material. This loss of material degrades the seal and the efficiency of the engine rapidly decreases. Seal materials in the more advanced engine utilize thermal sprayed coatings which perform the same function as the braided abradeable seals, but which are easier to apply and easier to replace when an engine is overhauled.
The use of thermal spray powders to form abradeable seals is known in the art as shown by U.S. Pat. No. 4,291,089. Such powders are used to form a coating on a substrate to provide an abradeable seal, that is to say a coating which seals the space between the substrate and an adjacent surface movable relative thereto, and which is abraded to a controlled extent by relative movement between the substrate and the adjacent surface. Such a seal is initially formed by thermal spraying a powder onto the substrate to form a coating with a slightly greater thickness than the spacing between the substrate and the adjacent surface, so that the coating is abraded by relative movement between the substrate and the adjacent surface to a slightly lesser thickness corresponding to the spacing between the substrate and the adjacent surface so as to provided an efficient seal there between. Such seals are used for example with turbine or compressor blades of gas turbine engines, such as those used in aircraft, to provide a seal between the blades and the turbine or compressor housing.
One of the problems in providing a suitable abradeable seal is to produce a thermally sprayed coating which, on the one hand has sufficient structural strength which nevertheless is low enough to provide abradability, and which, on the other hand, has a sufficiently high resistance to erosion by particles impinging on the abradeable seal coating during use. For example, in the case of gas turbine or compressor blades, the seal coating is subjected to impingement by abrasive particles entrained in the air and ingested by the engine.
An abradeable ceramic seat is shown in U.S. Pat. No.4,936,745 which provides a porous ceramic abradeable layer having a porosity of from about 20 to 35 vol %; however, the high porosity provides decreased erosion resistance which is a disadvantage in the severe environment of the high pressure turbine.
Briefly, this invention provides a gas turbine engine abradeable seal system comprising a seal assembly and a cooperating interacting turbine blade. The turbine blade has a tip portion containing cubic boron nitride abrasive particles for contacting the seal assembly to provide sealing. The seal assembly has a superalloy substrate having an MCrAlY bond coat thereon with a surface roughness of at least 300 RA, and a porous ceramic abradeable seal material on the bond coat having a porosity of from 5 to 15 vol %.
An abradeable seal system for gas turbine engines is provided with increased erosion resistance, while still proving an effective seal between the turbine blade and the stationary component. The seal system comprises the seal assembly and the turbine blade which cooperates and interacts with the seal assembly to cut a path into the seal assembly to create the seal. The turbine blade is a rotating member having an abrasive tip portion disposed in rub relationship to a stationary, abradeable seal assembly such that the abrasive tip portion cuts into the abradeable surface of the seal assembly.
The turbine blade has a tip portion which contains cubic boron nitride (CBN) abrasive particles to cut into the seal assembly. The CBN particles are highly effective in cutting through the abradeable seal material. The tip portion containing CBN abrasive particles may be applied by entrapment plating in an oxidation resistant metal matrix. A method as disclosed in U.S. Pat. No. 5,935,407, which is incorporated herein by reference, may be utilized which applies a bond coat to the turbine tip substrate by low pressure plasma spraying, then anchoring to the bond coat abrasive particles by entrapment plating in metal matrix. This method is preferred because of the increased bond strength of the abrasive tip to the turbine blade.
The seal assembly provides an abradeable seal anchored to a superalloy substrate. Generally, the substrate is a turbine or compressor housing or a liner attached thereto, with the superalloy being a cobalt or nickel based superalloy. To anchor the abradeable seal material to the substrate a bond coat is applied to the substrate surface having a surface roughness of greater than 300 RA; preferably greater than 350 RA. The bond coat is an MCrAlY wherein M is Co and/or Ni, which can be modified with Pt and/or diffusion aluminide coating. The increased environmental resistance of the abradeable material combined with the increased cutting ability of the CBN particles in the blade tip provides increased shear to the seal assembly. The increased surface roughness of the bond coat provides the increased bond strength needed to anchor the abradeable material. The bond coat can be applied by plasma spraying, either low pressure or air, to a thickness of about 4 to 15 mils, preferably about 5 to 10 mils. To achieve the surface roughness an MCrAlY is plasma sprayed with a particle size of up to about 150 microns. The bond coat is heat treated for diffusion bonding, either before or after the ceramic is applied, at a temperature of about 1900-2050xc2x0 F. for 2 to 5 hours, typically 1975xc2x0 F. for 4 hours.
To the bond coat, a porous ceramic abradeable seal material is applied having a porosity of from 5 to 15 vol %, preferably 10 to 15 vol %. The decreased level of porosity of this material provides increased environmental resistance allowing the seal to exhibit a longer useful life in the turbine engine. The increased cutting effectiveness of the CBN particles in the tip combined with the increased bond strength of the bond coat provides an effective seal system with increased seal life.
The ceramic abradeable seal material is a zirconia stabilized with 6 to 9% yttria. To create the porosity, the ceramic material is plasma sprayed with a fugitive material, preferably a polyester. To provide a porosity on the order of 5 to 15% a ceramic particle size of less than about 200 microns, preferably about 20 to 125 microns, can be mixed with up to 1.5% by weight, preferably about 1% to 1.5% by weight, of a polyester having a particle size of 45 to 125 microns. The mixture is then plasma sprayed to a thickness of from about 10 to 80 mils, preferably 20 to 40 mils. Optionally, the polyester is removed by heating at above 1300xc2x0 F.; however, it has been observed that most of the polyester is already removed during the plasma spraying process and the remaining polyester can be tolerated in the system.