Thermoplastic coating for composite structures

A method of thermoplastic coating composite structures includes heating a tool. A thermoplastic layer is deposited onto the heated tool by thermal spraying a thermoplastic on the heated tool. Composite material is applied onto the thermoplastic layer. The thermoplastic layer and the composite material are then cured.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to the field of materials construction and, more specifically, to a method of thermoplastic coating composite structures.

BACKGROUND OF THE INVENTION

Composite structures are desirable in many industries for many applications. The aerospace industry, for example, uses composite structures extensively because, among other desirable attributes, composites have high strength-to-weight ratios. Because of the ever increasing use of composite structures throughout industry, manufacturers are continually searching for better and more economical ways of fabricating composite structures.

Composite structures applied to the exterior of ships and aircraft can experience significant degradation and damage due to their exposure to erosion and environmental attack. In this regard, such structures are constantly subjected to oxidation, moisture, fouling, salt-spray, UV radiation, chemicals, and high and low temperatures, among other things, that can cause such structures to experience significant degradation and damage over time. As a consequence, such structural components are often constantly repaired or replaced to prevent the possibility that a given vessel or aircraft will be damaged permanently, if not destroyed.

To attempt to prevent the damage caused by fatigue and environmental exposure on such composite components, a variety of coating agents and methods of applying the same to such components have been developed to improve their durability. Thermal spraying is one method for applying thermoplastic polymers to a surface. Thermal spraying comprises insertion of feed stock particles, typically of a polymer-like material, into a high-energy heat source that propels the particles, while in a liquid or semi-liquid state, onto the surface of the component sought to be protected. Once propelled onto the surface sought to be protected, the particles fuse together and form a coating on the surface.

High performance thermoplastics offer the potential of a higher wear resistant and lower permeability barrier material as compared to conventional coatings. Thermoplastic coatings on a variety of substrates have been demonstrated using thermal spray techniques. However, when a high melt temperature, high performance thermoplastic such as liquid crystal polymer (LCP) is thermally sprayed on a composite with moderate temperature resistance, out-gassing of moisture and volatiles from the composite substrate can occur which can degrade the structural morphology of the coating. One solution to this problem, described in U.S. Pat. No. 6,174,405 issued to Clarke et al., involves thermally spraying a thermoplastic such as LCP on a metal mold, commonly referred to as a tool. While the coating is still on the tool, a prepreg or wet lay-up of the composite is applied directly on the thermoplastic-coated tool. The composite/coating/tool assembly is then oven/autoclave cured, and, when curing is completed, the thermoplastic-coated composite structure is removed from the tool.

SUMMARY OF THE INVENTION

A method of thermoplastic coating composite structures includes heating a tool. A thermoplastic layer is deposited onto the heated tool by thermal spraying a thermoplastic on the heated tool. Composite material is applied onto the thermoplastic layer. The thermoplastic layer and the composite material are then cured.

A technical advantage of certain embodiments of the present invention is that the heating of the tool may more fully melt and fuse together the thermoplastic particles during deposition, resulting in a coating that may be of a more uniform thickness and morphology and be less porous and/or permeable. In particular embodiments, the occurrence of partially un-melted or heat-damaged thermoplastic may be reduced.

Another technical advantage certain embodiments of the present invention is a reduction in local overheating of the tool caused by the thermal spray gun. This reduction may result in the avoidance of damage or deformation of the tool during thermoplastic deposition. Such reduction may preserve the integrity of any release coating applied to the tool, increasing the ease with which the thermoplastic-coated composite is removed from the tool after the curing step.

Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-1Dare block diagrams illustrating a method for thermoplastic coating of a composite structure in accordance with one embodiment of the present invention.FIG. 1Aillustrates deposition of a thermoplastic9on a working surface11of a tool10in accordance with one embodiment of the present invention.

Tool10reflects the desired shape of the outer surface of the final composite structure and may be formed from a metal material such as aluminum or steel. In a particular embodiment, the tool10is coated with Frekote™, Teflon™, or another suitable release coating before deposition of the thermoplastic9. It will be understood that tool10may be formed from other suitable materials, such as ceramic. In the illustrated embodiment, tool10is heated before thermoplastic deposition by internal heaters built into the tool10, powered by electrical or other suitable methods12to bring the tool10to a temperature sufficient to help in the thermoplastic melt and coating process. It will be understood that tool10may be heated by other suitable methods, such as non-internal heaters coupled to or in proximity to tool10, by covering the tool10with heat blankets before depositing the thermoplastic layer, or by passing heated fluids through the tool10.

A thermoplastic is polymer that reversibly hardens and softens when its temperature is cycled through its melt or glass transition temperature. In a particular embodiment, the thermoplastic layer9may have a melting temperature greater than 300° C., and heating the tool10results in a temperature difference of less that about 200° C. between the temperature of the heated tool10and the temperature of the sprayed thermoplastic. Some high-performance thermoplastics, such as liquid crystal polymers (LCP), described in greater detail below, have a melting temperature of about 288° C. The teachings of the invention recognize a particular advantageous range of tool temperature for LCP deposition of about 177-205° C. However, other temperature ranges of the tool10and thermoplastic9may be used without departing from the scope of the present invention.

In a particular embodiment, the working surface11of the tool10is heated to a substantially uniform temperature. In this way, the uniformity of the thermoplastic9coverage may be further increased. “Substantially uniform temperature” means a temperature difference of less than about 21° C. between any two points on the working surface. One or more thermocouples and/or infrared pyrometers may be used to monitor the temperature and temperature variations of the working surface11.

In the illustrated embodiment, the thermoplastic is deposited using a thermal spray gun8. In a particular embodiment of the present invention, a thermoplastic9deposited on tool10may comprise a liquid crystal polymer (LCP). LCP is characterized by a high melting point relative to other commonly used thermoplastics, on the order of about 288° C. LCP may comprise co-polyesters, co-polyesteramides, or multiple monomer wholly aromatic polyesters. Among the representative LCP products currently available for use in the practice of the present invention include XYDAR-RT-300, XYDAR-SRT-700, and XYDARSRT-900, produced by Amoco; VECTRA A950, VECTRA L950 and VECTRA E950L, produced by Hoechst Celanese, and Zenite100, Zenite600, Zenite700and Zenite800manufactured by Dupont.

In the illustrated embodiment, deposition of the thermoplastic is by a thermal spray gun8; however, thermal spray gun8may comprise a Teradyne 2000 having a GP type nozzle affixed thereto, of another suitable thermal spray gun such as a combustion spray gun. The thermal spray gun may sustain a stable, non-transferred electric arc between a thoriated tungsten cathode and an annular water-cooled cooper anode. A gas, such as argon or other inert gas, complimented by a small portion of an enthalpy enhancing gas, such as hydrogen, may be introduced at the back of the gun's interior such that gas swirls in a vortex and out of the front end of the anode nozzle. The electric arc from the cathode to the anode completes the circuit, thus forming a flame that axially rotates due to the vortex momentum of the gas. The temperature of the flame just outside the nozzle exit is high enough to melt the thermoplastic. The flame temperature drops off rapidly from the exit of the anode and, therefore, causes the thermoplastic, which is typically in powder or other solid particle form, to be introduced at the hottest part of the flame generated by the thermal spray. Thermal spraying may be done with a hand-held sprayer or by an automated (e.g., robotic) process.

FIG. 1Billustrates a thermoplastic layer14deposited as described above on tool10. By heating working surface11of tool10, the temperature differential between the tool and the thermally-sprayed thermoplastic is reduced and the thermoplastic particles may more fully melt and fuse together during thermal spraying, resulting in a coating that may be of a more uniform thickness and morphology and be less porous and/or permeable than with prior methods. In particular embodiments, the occurrence of partially un-melted or heat-damaged thermoplastic may be reduced.

In addition, local overheating of the tool from the thermal spray gun may be reduced, resulting in the avoidance of damage or deformation of the tool during thermoplastic deposition. Such reduction may preserve the integrity of any release coating applied to the tool, increasing the case with which the thermoplastic-coated composite is removed from the tool after the curing step (described below).

In a particular embodiment, layer14may have a thickness of about 0.005 inches. However, the thickness of the layer14may suitably vary in accordance with various embodiments of the present invention.

As illustrated inFIG. 1C, a layer16of composite material is applied as a prepreg, wet lay-up, or other suitable method onto the thermoplastic layer14. The composite material may comprise fiber reinforced polymers or other suitable materials. The composite/coating/tool assembly is then oven/autoclave cured using conventional process conditions. When curing is completed, the thermoplastic-coated composite structure may be removed from the tool10.

As illustrated inFIG. 1D, the layer16of composite material and thermoplastic layer14are removed from the tool10. Thermoplastic layer14may then form a protective outer surface of the composite layer16, and the composite layer16together with thermoplastic layer14may then be suitably affixed to the outer surface of a vessel or aircraft or other vise suitably employed. In particular embodiments, an adhesive (not explicitly shown) may be applied onto the thermoplastic layer14before applying the layer16of composite material. The adhesive may increase a bond between the thermoplastic layer14and the composite material, thereby, in particular embodiments, increasing the strength of the structure comprising layers14and16and further increasing the ease with which the thermoplastic-covered and cured composite material may be removed from the tool10.

FIG. 2is a flowchart illustrating a method for thermoplastic coating of a composite structure in accordance with one embodiment of the present invention. Beginning with step100, a release agent such as Teflon™ is applied to the working surface11of tool10. At step102, the tool10is heated. In a particular embodiment wherein the thermoplastic to be deposited onto the tool10comprises LCP, tool10may be heated to a temperature of approximately 190° C.

Proceeding to step104, LCP or other suitable thermoplastic is thermally sprayed to deposit a thermoplastic layer14onto the heated tool10. In some embodiments, the melting temperature of the thermoplastic may be around 288° C. The difference between the temperature of the heated tool10and the sprayed thermoplastic can be less than about 100° C. At step106, at layer16of composite material is applied onto the thermoplastic layer14. Thermoplastic layer14and composite material layer16are then cured at step108. Finally, at step110, thermoplastic layer14and composite material layer16are removed from the tool10, resulting in a thermoplastic-coated composite.

Although an embodiment of the invention and its advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the present invention as defined by the appended claims.