Patent Application
20080124537
This invention relates generally to improving non-uniform composite surfaces, and more specifically to a modified surfacer coat for improving non-uniform composite surfaces.
Composite materials are engineered materials made from two or more constituent materials that remain macroscopically distinct within the composite material. The constituent materials are synergistically combined such that the composite material possesses characteristics that the constituent materials alone do not possess.
Composite materials contain a matrix material and a reinforcement material, each of which serve a different function. The matrix material surrounds and supports the reinforcement material, and the matrix material transfers forces applied to the composite material to the reinforcement material. The reinforcement material may be stronger than the matrix material, and the reinforcement material may provide the primary load carrying capability of the composite material. Examples of matrix materials include polyester and epoxy materials, and examples of reinforcement materials include glass, carbon, and metal. Reinforcement materials are often formed into fibers which may be woven together.
A traditional example of a composite material is a combination of mud and straw, which was used to fabricate bricks in historic times. The mud, which is the matrix material, has good compression strength but poor tensile strength. The straw, which is the reinforcement material, has good tensile strength but poor compression strength. However, when the mud and the straw are combined together into a composite material for fabricating a brick, the brick has good compression strength as well as tensile strength. Thus, the mud and straw composite material has desirable characteristics of both of its constituent materials.
A more modern example of a composite material is fiberglass. Fiberglass contains a glass reinforcement material, often in the form of a fabric weave, and a plastic matrix material. The glass reinforcement material has good tensile strength but poor shear-strength. However, when the glass reinforcement material is combined with the plastic matrix material, the resulting fiberglass composite material possesses both good shear strength as well as good tensile strength.
Composite materials often possess desirable characteristics which make them suitable for replacing traditional materials in many applications. For example, composite materials may be stronger, stiffer, and/or lighter than traditional materials. Additionally, composite materials may be formed into complex shapes more easily than can be traditional materials. Consequently, composite materials are increasingly used in place of traditional materials in many applications. For example, aircraft tail sections and boat hulls are commonly constructed of composite materials instead of aluminum.
A structure fabricated out of composite materials (“composite structure”) is generally fabricated by forming the composite materials into a desired shape. Such forming may be done by hand or by one or more automated processes. For example, composite materials may be formed into the shape of an aircraft tail by use of a mold having the desired shape. After the composite materials are formed into the desired shape, the structure is cured. Curing may require that the composite structure be exposed to heat and/or pressure, such as by placing the composite structure in an autoclave.
A composite structure often has a rough or non-uniform outer surface resulting from use of a reinforcement material in the composite material. Reinforcement materials frequently have a rough, uneven surface texture, such as the weave pattern present in a reinforcement material of woven fabric. A reinforcement material's non-uniform surface texture often transfers to the composite structure's outer surface. For example, if the reinforcement material consists of a woven fabric, the woven fabric's weave pattern may transfer to the composite structure's outer surface.
A reinforcement material's location within the composite structure in relation to the structure's outer surface may also be a factor contributing to non-uniformity in the structure's outer surface texture, For example, if a woven fabric reinforcement material is located just below a composite structure's outer surface, the woven fabric's weave pattern may transfer to the composite structure's outer surface.
Shrinking of the matrix material may also contribute to surface irregularities in a composite structure's outer surface. Matrix materials generally shrink during curing, and some matrix materials require months to fully cure, As a matrix material shrinks, its thickness decreases. Consequently, a matrix material's ability to conceal a reinforcement material's irregular surface texture diminishes as the matrix material shrinks.
A composite structure having a non-uniform or irregular outer surface may be unacceptable in many applications from a performance perspective, a cosmetic perspective, of both. For example, a composite aircraft panel or a composite boat hull typically must have a relatively smooth, uniform surface to reduce drag. Additionally, customer expectations and aesthetic considerations may mandate that the aircraft or boat have a smooth outer surface. The same may be said with respect to automobile and truck bodies from an aesthetic perspective.
One prior art method of creating a uniform outer surface on a composite structure is to place a shrinkage barrier below the composite structure's outer surface, This method is disclosed in U.S. Pat. No. 5,391,425 to Isley, Jr. et al., entitled “Composite Material With Shrinkage Barrier” (“the '425 patent”). In the '425 patent, a shrinkage barrier, which includes a resin layer having microspheric particles embedded therein, is placed between reinforcement material and the outer surface of the composite structure. The shrinkage barrier blocks the reinforcement material's non-uniform surface from transferring to the composite structure's outer surface. Because the shrinkage barrier is placed within the composite structure, the shrinkage barrier must be placed in the composite structure during its fabrication. Consequently, the shrinkage barrier may not be used to create a uniform outer surface on a preexisting composite structure.
A somewhat similar prior art method of creating a uniform outer surface on a composite structure is disclosed in U.S. Pat. No. 7,022,629 to Theriault, entitled “Print Through Elimination in Fiber Reinforced Matrix Composite Mirrors and Method of Construction” (“the '629 patent”). The '629 patent discloses a method of placing a layer of small unbundled fibers between the composite material and an un-reinforced top matrix layer of a composite structure. The layer of small unbundled fibers diffuses and randomizes forces resulting from the non-uniform surface of the reinforcement materials, thereby preventing such non-uniform surface from transferring to the composite structure's outer surface. The layer of small unbundled fibers may consist of random fiber segments, a continuous fiber mat, or a weave of single or finely towed continuous fibers. However, like the method disclosed in the '425 patent, the method taught by the '629 patent can only be used during the fabrication process. Consequently, the method disclosed in the '629 patent may not be used to create a uniform outer surface on a preexisting composite structure.
Another prior art method of creating an uniform outer surface on a composite structure involves the application of one or more layers of a surfacer coat to the outer surface followed by a sanding step which is performed until a sufficiently uniform outer surface is created. A surfacer coat is a material used to prepare a surface, such as a surface of composite structure, to receive a topcoat. A surfacer coat is sometimes referred to as a sanding surfacer, a surfacer, or just a coat.
A surfacer coat may be sanded or otherwise shaped as desired after it has cured. For example, at least one layer of a surfacer coat may be applied to a composite aircraft panel, and the surfacer coat may then be sanded until the panel is sufficiently uniform. Because the surfacer coat is applied to an outer surface of a composite structure, a surfacer coat may be used to create a smooth, uniform outer surface on a preexisting composite structure.
One problem associated with prior art surfacer coats is shrinking during curing. Such shrinking may occur relatively quickly, such as over several days, or may occur relatively slowly, such as over several months. The shrinkage reduces the prior art surfacer coat's thickness and, consequently, the prior art surfacer coat's ability to create a uniform outer surface decreases as the surfacer coat shrinks. Therefore, a reinforcement material's non-uniform outer surface may transfer to a composite structure's outer surface as the prior art surfacer coat shrinks.
Hence, a need exists for a material that exhibits minimal shrinking which may be used to create a smooth, uniform outer surface on an existing composite structure.
The improved surfacer coat and applications thereof herein disclosed advance the art and overcome one or more of the problems articulated above by providing a modified surfacer coat that exhibits minimal shrinking and which may be used to create a uniform outer surface on a preexisting composite structure.
In particular, and by way of example only, a surfacer coat includes a resin material, a plurality of first particles, and a plurality of second particles. The plurality of first particles and the plurality of second particles are uniformly distributed through the resin material. Each first particle is of a size ranging from about 5 microns to about 10 microns, and each second particle is of a size ranging from about 40 microns to about 50 microns.
According to another embodiment, a structure includes at least one layer of composite material. At least one layer of primer is disposed on an outer surface of the composite material, wherein the primer includes a first resin material. At least one layer of surfacer coat is disposed on an outer surface of the primer, wherein the surfacer coat has a thickness of about at least 50 microns. The surfacer coat includes a second resin material, a plurality of first particles, and a plurality of second particles. The first particles and the second particles are uniformly distributed through the second resin material. Each first particle is of a size ranging from about 5 microns to about 10 microns, and each second particle is of a size ranging from about 40 microns to about 50 microns.
In yet another embodiment, a method of preparing an outer surface of a composite structure for receipt of a topcoat includes disposing at least one layer of primer on the outer surface of the composite structure, wherein the primer has a first resin material. At least one layer of surfacer coat is disposed on an outer surface of the primer. The surfacer coat includes a second resin material, a plurality of first particles, and a plurality of second particles, the first particles and the second particles being uniformly distributed through the second resin material. The surfacer coat is then cured.
According to another embodiment, the first and second particles uniformly distributed through the resin materials are substantially spherically shaped.
In yet another embodiment, the first and second particles uniformly distributed through the resin materials are irregularly shaped.
According to still another embodiment, the first and second particles uniformly distributed through the resin materials are substantially elliptically shaped.
In another embodiment, the first and second particles uniformly distributed through the resin materials are a mixture of irregularly shaped, substantially elliptically shaped and substantially spherically shaped particles.
Before proceeding with the detailed description, it is to be appreciated that the present teaching is by way of example only, not by limitation. The concepts herein are not limited to use or application with a specific type of surfacer coat. Thus, although the instrumentalities described herein are for the convenience of explanation, shown and described with respect to exemplary embodiments, it will be appreciated that the principles herein may be applied equally in other types of surfacer coats.
Deformations 204 may be considered cosmetic defects. However, deformations 202 may contribute to defects in the overall structure, inasmuch as they may cause topcoat 110 to crack or affect a change in its properties. For example, if topcoat 110 is a full gloss paint, deformations 202 may dull topcoat's 110 surface appearance.
In certain applications, deformations 204 may also be considered a structural defect, not just a cosmetic defect. For example, if composite structure 100 is part of an aircraft, the aerodynamic properties of composite structure 100 may have a substantial impact on the overall performance of the aircraft. For example, deformations 204 may disturb the airflow over the outer surface 112 while the aircraft is in flight, thus creating unwanted drag on the aircraft and heat at supersonic speeds.
As shown in
Particles 304 may be of one or more suitable sizes and shapes to effectively eliminate or minimize shrinking of surfacer coat 300 during curing. In at least one embodiment as shown in
In at least one embodiment, resin material 302 is a thermoset material. For example, resin material 302 may be a thermoset material including polycarbonate polyol urethane isocyanate resin. Thermoset materials, which are cross linked materials, resist deformation under pressure, are resistant to chemicals, and do not transform to the liquid phase when heated. In contrast, thermoplastic materials, which are not cross linked materials, deform under pressure and transform to the liquid phase when heated. Consequently, a surfacer coat formed of or containing a thermoset resin material may resist hostile environmental conditions, such as mechanical pressure, excessive heat, and chemical exposure better than a surfacer coat formed of a thermoplastic resin material. For example, when a surfacer coat having a thermoset resin material is exposed to hot sunlight, the surfacer coat will generally remain solid and thereby maintain a uniform surface. In contrast, when a surfacer coat having a thermoplastic resin material is exposed to hot sunlight, the surfacer coat may soften and allow the non-uniform surface texture of an underlying composite material to transfer to the surfacer coat's outer surface.
At least one embodiment of surfacer coat 300 may be required to undergo environmental temperature cycling ranging from negative forty-six (−46) degrees Celsius (−50 degrees Fahrenheit) to sixty-six (66) degrees Celsius (150 degrees Fahrenheit) without experiencing excessive cracking. Additionally, at least one embodiment is compatible with a metallic surface, allowing surfacer coat 300 to be used on both metallic and non-metallic surfaces.
In the embodiments described above, particles 304, 304′ are introduced into resin material 302 in a preselected quantity during production of surfacer coat 300, 300′. If the particles settle non-uniformly within resin material, they may be redistributed uniformly within the resin material by mechanically shaking the surfacer coat 300, 300′ prior to application for a sufficient length of time. Empirical mixing tests suggest that shaking the mixture for a period of approximately ten (10) minutes is sufficient to thoroughly mix and evenly distribute the particles throughout the resin.
The distribution of particles 304, 304′ within resin material 302 may also be maintained by mechanically stirring surfacer coat 300, 300′. For example, the resin material may be stirred with a paddle blade at a low speed during application of surfacer coat 300, 300′ to a surface to insure that particles 304, 304′ are distributed evenly throughout.
Composite structure 100 is constructed of suitable composite materials known in the art. An embodiment of composite structure 100 includes fibers 502 and 504. Fibers 504 are disposed at approximately ninety degree angles to fibers 502, and fibers 502 and 504 are woven together in a grid configuration. Fibers 502 and 504 may be formed from glass, carbon, graphite, or other suitable materials as is known in the art. Deformations 104 depict irregularities in outer surface 102 which may appear due to use of fibers 502, 504.
In an embodiment, primer 506 is applied to outer surface 102 of composite structure 100. Primer 506 may inhibit corrosion of composite structure 100. Because primer 506 has little ability to create a uniform surface, deformations 104 may transfer as deformations 510 to top surface 508 of primer 506.
At least one layer of surfacer coat 300, 300′ is applied to top surface 508 of primer 506. In an embodiment, surfacer coat 300, 300′ has a thickness 514 of at least fifty (50) microns. If primer 506 is not used, surfacer coat 300, 300′ may be applied directly to outer surface 102 of composite structure 100. As stated above with respect to
In an embodiment, topcoat 110 is disposed over surfacer coat 300. In at least one embodiment, topcoat 110 may be epoxy or urethane paint.
As stated above, particles 304, 304′ may cause surfacer coat 300, 300′ to exhibit minimal shrinking during its curing. Consequently, particles 304, 304′ may allow surfacer coat 300, 300′ to maintain a uniform outer surface 512, 512′ during curing of surfacer coat 300, 300′. Because surfacer coat 300, 300′ may be applied to an outer surface of a composite structure, the surfacer coat may be used to create a uniform surface on a preexisting composite structure.
Lightning resistance of structure 500, 500′ may be critical when it is used in an outdoor application. For example, as stated above, structure 500 may be an aircraft panel. An aircraft may be in danger from being struck by lightning either when it is airborne or when it is located on the ground. If an aircraft panel were to experience structural failure as the result of a lightning strike, the consequences could be catastrophic.
Insuring that structure 500, 500′ has sufficient lightning resistance is challenging because composite structure 100 is a relatively a poor conductor of electricity. Consequently, if composite structure 100 is struck by lightning, the energy associated with the lighting strike does not distribute itself over composite structure 100, as the energy would do if composite structure 100 were a metallic structure. Instead, the energy remains in the area of contact by the lightning and may cause composite structure 100 to experience structural failure. In order to prevent such structural failure, it is critical that structure 500, 500′ allow lightning energy to reflect off composite structure 100.
The combined thickness of primer 506, surfacer coat 300, 300′, and topcoat 110 affects whether lightning energy will be able to reflect off composite structure 100. When lightning strikes structure 500, 500′, lightning energy will penetrate topcoat 110, surfacer coat 300, 300′, and primer 506. In order to allow the lightning energy to reflect off composite structure 100, the combined thickness of primer 506, surfacer coat 300, 300′, and topcoat 110 must be sufficiently small. Stated simply, decreasing the combined thickness of primer 506, surfacer coat 300, 300′, and topcoat 110 increases the ability of composite structure 100 to survive a lightning strike.
Empirical tests have shown that a composite structure will have reasonable lightning resistance if a layer of surfacer coat disposed on the composite structure has a thickness of about one hundred seventy-eight (178) microns (7.00 mils) or less. Surfacer coat 300, 300′ may provide an uniform surface when applied in one or more layers having thickness 514 as low as fifty (50) microns (1.97 mils), and in an embodiment, surfacer coat 300, 300′ has thickness ranging from fifty (50) microns (1.97 mils) to one hundred seventy-eight (178) microns (7.00 mils) Additionally, in an embodiment, a combined thickness of primer 506, surfacer coat 300, 300′, and topcoat 110 does not exceed two hundred sixteen (216) microns (8.5 mils). Consequently, surfacer coat 300, 300′ may be used in composite structure applications requiring adequate lightning resistance. Additionally, because surfacer coat 300 may have thickness 514 as low as fifty (50) microns (1.97 mils), surfacer coat 300 may be used in applications that require a relatively thin layer of surfacer coat to minimize its weight. An example of such an application is an aircraft application wherein it is desirable to minimize the weight of a surfacer coat in order to maximize aircraft fuel economy.
In step 704, at least one layer of un-cured surfacer coat 300, 300′ is applied to an outer surface of primer 506. As stated above, surfacer coat 300, 300′ exists in its liquid phase before it is cured. For example, surfacer coat 300, 300′ may be applied to the outer surface of primer 506 via a paint brush or a spray apparatus. Surfacer coat 300 includes a plurality of particles 304 to help prevent shrinking of surfacer coat 300 during its curing, and particles 304 may include particles of two sizes, first particles 306 and second particles 308, as shown in
In step 706, the at least one layer of un-cured surfacer coat 300, 300′ is cured. Surfacer coat 300, 300′ is cured by a method appropriate for resin material 302. For example, surfacer coat 300, 300′ may cured by drying it (e.g. by exposing it to the atmosphere), by exposing it to ultraviolet radiation, by exposing it to pressure, and/or by heating it. Step 708 is an optional step. In step 708, after surfacer coat 300, 300′ has cured, it is sanded until an outer layer of surfacer coat 300, 300′ achieves a predetermined uniformity in surface texture. A topcoat can applied after step 708, or a topcoat can be applied after step 706 if step 708 is not to be executed.
Changes may be made in the above methods, systems and structures without departing from the scope hereof. It should thus be noted that the matter contained in the above description and/or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method, system and structure, which, as a matter of language, might be said to fall therebetween.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/809,885, filed May 31, 2006, entitled “Modified Surfacer Coat for Improving Rough Composite Surfaces”, which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 60809885 | May 2006 | US |
