METHOD FOR USING PULSED OPTICAL ENERGY TO INCREASE THE BONDABILITY OF A SURFACE

Abstract: A method and system for improving the capability of a surface (14) of an organic structure (16) to bond with another material includes irradiating a target area of the surface of a structure with pulsed, incoherent optical energy (18) from an optical energy source (12) having wavelength components which range from 170-5000 nanometers at an intensity sufficient to photodecompose any adventitious organic substances on the surface and to photodecompose a thin layer of molecular bonds forming the surface of the structure; and exposing the target area of the surface (14) to an ionized gas stream (24) from an ionized gas generator (26) that chemically reacts with the target area of the surface to increase the surface free energy of the surface. A similar method may also be employed to improve the bondability of a metal surface by impinging a target area on a metal surface with a stream of particles (36) to preclean and dislodge any inorganic substances from the surface; and then irradiating the target area of the surface with pulsed, incoherent optical energy (18) having wavelength components in the range of 170-5000 nanometers at an intensity sufficient to photodecompose any remaining organic substances present on the surface.

Description: METHOD FOR USING PULSED OPTICAL ENERGY TO INCREASE THE BONDABILITY OF A SURFACE BACKGROUND OF THE INVENTION The present invention relates to optical surface preparation techniques, and more particularly, to improving the capability of the surface of a structure to bond with a material by irradiating the surface with incoherent, pulsed optical energy having a broadband energy spectrum. There are many applications for which it is desirable to bond one material to another. In the aircraft industry, for example, the exterior surface of airplanes must be painted to prevent corrosion that could otherwise weaken the airplane structure. In the automotive industry, metal subpanels must be bonded to polymeric outer body panels. Good bond strength between materials depends to a large extent on appropriate surface preparation. Surface preparation techniques which enhance the capability of the surface of a structure to bond with a material (bondability) include solvent cleaning, abrading, and/or chemical treatment. However, these methods are all characterized by potential toxic chemical hazards, waste disposal problems, and high production costs. All surface preparation techniques generally increase the surface free energy of the surface. Surface free energy refers to the energy required to create a unit area of the surface. In the case of a liquid, the surface free energy is expressed by the surface tension coefficient. Polar liquids, such as water, have high surface tension coefficients (H 2 0 = 73 dynes/cm at 20°C) . For a liquid to wet a solid surface, the surface free energy of the solid surface must therefore be higher tha that of the liquid. As a result, the ability to achieve a water break free surface (i.e., no beading) is often used as a criteria for adequate surface preparation. Surface free energy of a solid surface can be improved by either removing all surface contaminants or by changing the surface chemistry. Removing all surface contaminants improves the surface free energy because, when exposed to the environment, a solid with inherently high surface free energy will attract contaminants as a way to reduce its total energy. As a result, the contaminated surface loses or reduces its ability to bond to other surfaces. Removal of the surface contaminants (which are typically organic materials) will restore the surface's inherent surface free energy. For example, metals such as aluminum can achieve a "water break free" condition when the surfaces are clean. Removing surface contaminants does not always help, however, depending upon the type of material. Most organic materials (particularly those made up large chains of molecules) , for example, usually have low surface free energies regardless of the cleanliness of the surfaces. Hence, to increase the surface free energy, the surface molecular structures of such materials must be modified. For example, the surface affinity for other molecules can be increased by either breaking up the large molecular chains into smaller ones or by the insertion of other atoms into molecular chains at the surface. Optically engineered surface preparation technology is a known alternative to solvent, abrasive, or chemical processes, and avoids some of the aforementioned problems. An example of one optical surface preparation technique is presented in Sowell, R.R. , et al. , "Surface Cleaning By Ultraviolet Radiation," J. Vac. Sci.. Vol. 11, No. 1, Jan./Feb. 1974. The Sowell reference describes a process for removing hydrocarbon contaminants from metal and glass surfaces by irradiating such surfaces with generally steady-state Ultra-Violet (UV) radiation in the presence of a low pressure oxygen atmosphere or in open air. However, the process described by Sowell requires hours to complete due to the limited UV light intensity that can be obtained from a steady-state UV light source. Therefore, the Sowell process is generally not suitable for applications in which it is desirable to increase the bondability of surfaces, some of which may be very large, within a period of time which would make such processing practical. U.S. Patent No. 4,803,021, "Ultraviolet Laser Treating of Molded Surfaces," is directed to a method for preparing the surfaces of molded products to improve bonding and painting performance. Such method includes irradiating the coated surface of molded products with pulsed laser light that decomposes any mold-release agents present on the surface to yield diverse decomposition fragments within the irradiated zone. This process requires that the surface material be etched deeply enough to remove substantially all of the mold- release agent. A surface may only be subjected to this process a finite number of times in order to limit the amount of surface material removed by etching. Since molded plastic is generally released from a mold only once, the minimal amount of material removed by etching is tolerable and may be considered in the design of the such products. However, this process is not suitable for repeatedly treating surfaces as part of a scheduled maintenance program, as for example, where it is desired to prepare a surface for painting, if preservation of the surface is desired. A significant problem with a laser based system, such as that described by the , 021 patent, is that irradiation of large or topologically complex surfaces with the pinpoint beam of a laser is very difficult to achieve, requiring sophisticated scanning and rastering techniques. Furthermore, the operation of a laser requires laser stops to prevent the laser beam from inadvertently escaping the work area, and the building where the laser is operated. This is because lasers pose a serious danger to humans, who could be seriously injured if irradiated with a laser beam. An even more significant problem with laser illumination, however, is controlling the intensity with which a laser beam irradiates a surface. Because the intensity of a laser beam does not follow the inverse square law, the energy density of the area or "footprint" irradiated by the laser beam is generally so high that the beam causes thermal decomposition of the materials at the surface being irradiated. If the structure being irradiated is made of a polymeric material, laser irradiation breaks up the thermal bonds of the molecules of the material, but then (due to the excessive influx of energy) causes the decomposed polymer bonds to recombine into new, and different polymer molecules. The formation of such new polymer molecules may actually cause a decrease in the surface free energy of the irradiated surface, causing the bondability of the surface to possibly decrease. Thus, it may be appreciated that there is a need for a process which enhances the capability of the surface of one structure to be bonded to another. A further need exists or a method to increase the bondability of large or topologically complex surface areas. " A still further need exists for a method to increase the bondability of a surface which does not require the use of toxic chemicals. SUMMARY OF THE INVENTION The aforementioned needs are met by the present invention which provides a method and system for increasing the capability of the surface of a structure to be bonded to a material. One aspect of the present invention involves a method for improving the capability of the surface of an organic structure to bond with another material. Such method comprises the steps of irradiating a target area of a surface of a structure with pulsed, incoherent optical energy having wavelength components which range from 170-5000 nanometers at an intensity sufficient to photodecompose any adventitious organic substances on the surface and to photodecompose a thin layer of molecular bonds forming the surface of the structure; and exposing the target area of the surface to ionized gas that reacts with the photodecomposed molecules of the target area of the surface so as to increase the surface free energy of the surface. Another aspect of the present invention provides a method for improving the capability of a metal surface to bond to a material. This method comprises the steps of impinging a target area on a metal surface with a stream of particles to dislodge any inorganic substances from the surface; and then irradiating the target area of the surface with pulsed, incoherent optical energy having wavelength components in the range of 170-5000 nanometers at an intensity sufficient to photodecompose any organic substances on the surface. The present invention also provides a system for improving the capability of the surface of a structure manufactured of organic material to bond with a material. Such system includes: (1) an optical energy source for generating pulsed, incoherent, optical energy having wavelength components ranging from about 170-5000 nanometers directed to irradiate a target area on the surface of a structure so as to increase the surface free energy of the surface; (2) a pulse modulator operably coupled to control the output of the optical energy source; (3) an electrical power supply operably coupled to provide electrical energy to the pulse modulator; and (4) a source of ionized gas for bathing the irradiated target area in the ionized gas. Another embodiment of the invention provides a system for improving the capability of a metal or similar surface with inherently high free surface energy to bond with a material. Such system comprises: (1) a particle stream generator for generating a particle stream that impinges a target area of a given surface in order to substantially remove any inorganic adventitious materials therefrom; (2) an optical energy source for generating pulsed, incoherent, optical energy having wavelength components ranging from about 170-5000 nanometers directed to irradiate the given surface cleaned by the particle stream so as to increase the surface free energy of the given surface; (3) a pulse modulator operably coupled to control the output of the optical energy source; and (4) an electrical power supply operably coupled to provide electrical energy to the pulse modulator. An advantage of the system and method of the present invention is that it provides an economic and high throughput process for enhancing the capability of the surface of one structure to be bonded to another. Another advantage of the present invention is that it provides a system and method for increasing the bondability of large or topologically complex surface areas. Still a further advantage of the present invention is that a system and method are provided for increasing the bondability of a surface which does not require the use of toxic chemicals. DESCRIPTION OF THE DRAWINGS The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof presented in conjunction with the following drawings, wherein: FIG. 1 is a schematic/block diagram of a system for increasing the bondability of a surface which employs a supporting structure to facilitate scanning the target area of the surface with optical energy; and FIG. 2 is a schematic/block diagram of a system for increasing the capability of the surface of a structure to be bonded to a material which employs an X-Y table to facilitate scanning the target area of the surface with optical energy. Throughout the specification and various views of the drawings, like components are referred to with like reference numerals. DESCRIPTION OF THE PREFERRED EMBODIMENTS The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. The invention relates to a novel process for significantly improving the bondability of the surfaces of metal or organic structures. It is to be understood that throughout the description presented herein, the term "metal" refers generically to any materials having an inherently high free surface energy. It is also to be understood that the organic structures may include organic matrix composites, thermoset materials, and thermoplastic materials. The method of the present invention involves irradiating a target area of the surface of interest with pulsed, broadband optical energy, which is preferably incoherent, while exposing the target area to an ionized gas. The broadband optical energy is optical energy having wavelength components ranging from about 170-5000 nanometers (nm) . The pulsed optical energy photodecomposes any organic, adventitious materials present on the surface into gases which are transported away from the surface. In applications where the surface of the structure is made of polymeric materials, the optical energy photodecomposes, or breaks the polymer chains of the molecules comprising the surface of the structure. The ionized gas chemically reacts with the broken polymer chains and modifies the surface by introducing highly polar sites to the otherwise relatively non-polar molecules, thereby increasing the surface free energy of the surface. The broadband optical energy provides electromagnetic spectrum components of which at least some have high probabilities of being adsorbed to photodecompose the chemical bonds of the many different types of organic materials that may be found on a surface being processed, either as contaminants, or comprising the surface itself. In other words, using a source of broadband optical energy increases the probability that such energy will overlap an optical adsorption peak of the material being irradiated. A system 10 for increasing the surface free energy of an organic surface, and particularly a polymeric surface, is described with reference to FIG. 1. As seen in FIG. 1, an optical energy source 12 irradiates a target area on a surface 14 of a substrate 16 with incoherent, pulsed, broadband optical energy 18. The optical energy source 12 is preferably a xenon flashlamp which generates optical energy by conducting electrical current through low pressure xenon gas contained in a fused quartz tube. The pulsed optical energy output of the optical energy source 12 is controlled by a pulse modulator 20 that is energized by a power supply 22. The optical energy 18 has a power density (fluence) at the target area of the surface 14 sufficient to photodecompose any adventitious organic materials present at the surface and to photodecompose the molecular bonds of the organic materials such as polymers, that make up the surface 14. However, the optical power density is not so intense as to more than insignificantly etch the surface 14. The optical power intensity at the surface of the substrate depends on the requirements of a particular application, but generally the fluence (optical power density) is within the range of about 0.01-0.5 J/cm 2 /sec. In general, then, the surfaces treated in accordance with the methods of the present invention are irradiated at an intensity sufficient to photodecompose any organic surface contaminants and to break the molecular bonds of the surface of interest. However, the intensity is maintained at a level that is less than that required to significantly etch the surface of interest. For example, the output of a flashlamp may be adjusted to have a frequency of 20 Hz and a pulse width of 1.0 microsecond when the target zone of an organic surface is to be irradiated with a fluence of 0.02-0.05 J/cm 2 /sec. Alternatively, the flashlamp may be adjusted to have a frequency of 0.1 Hz and a pulse width of 180 microseconds when the target zone is to be irradiated with a fluence of 0.2-0.5 J/cm 2 /sec. Shorter optical energy pulses may be employed when it is desired to shift the output of the flashlamp towards blue light, whereas longer optical energy pulses may be used when it is desired to shift the output of the flashlamp towards red light. The choice of the longer or shorter optical energy pulses is dependent on the surfaces and the associated contaminants. The optimum pulse lengths are best found empirically for a given type of material. By way of example, the optical energy source 12 may be implemented as a xenon flash tube constructed of a 6" long fused quartz tube having an 6 mm outside diameter and a 1 mm wall thickness, filled with xenon gas at a pressure of about 100 Torr. The manufacture and operation of flashlamps is known in the art. See e.g., U.S. Patent 5,126,621. The power supply 22 may be implemented as a Maxwell Laboratories, Inc., Model No. CCDS-825-P, power supply capable of providing 25 kV electrical power at a rate of 8 kJ/sec. An ionized gas stream 24, which may include gaseous ions such as N 2 + , N + , 0 2 + , 0 + , and 0 ~ is directed by a nozzle 25 to bathe the target area on the surface 14 with an ionized gas stream 24 received from an ionized gas generator. The ionized gas generator 26 manufactures the ionized gas stream from dry gas provided by a gas supply 28 which may include dry air, ozone, chlorine, nitrogen, carbon dioxide, or ammonia. However, ozone is preferred because it is relatively easy to manufacture and readily oxidizes any photodecomposed organic molecules at the irradiated surface. The ionized gas generator may be of the type manufactured by Fischer America, Inc. The optical energy source 12 is preferably mounted within a hood 30 that enshrouds the target area on the surface 14 being irradiated with the optical energy 18. A vacuum system 32, in fluid communication with the interior of the hood 30, collects any excess ionized gas 24 and photodecomposed organic materials liberated from the surface 14. The hood 30 also prevents ultraviolet light components generated by the optical energy source 12 from escaping into the surrounding work spaces. In order to facilitate the irradiation of large areas with the optical energy source 12, housing 30 optionally may be supported at the end of a supporting structure 40, such as the end of an arm of a conventional robotic positioning system, so that the flashlamp may be conveniently traversed or scanned in a predetermined path over the surface 14, as would be well known by those skilled in the art. As an alternative to supporting the housing 30 and optical energy source 12 by the supporting structure, scanning the surface 14 of the structure 16 may also be optionally facilitated by maintaining the housing 30 stationary, and by mounting the structure 16 on a translating X-Y table 42, shown in FIG. 2. By selective manipulation of the X-Y table 42, appropriate locations on the surface of the structure 16 may be positioned within the zone of illumination of the flashlamp 12. The translating table 42 may be manually or computer controlled in accordance with well known techniques. The nozzle 25 is also preferably mounted to the hood 30, by means not shown. By way of example, the robotic positioning system may be implemented as a CIMROC 4000 Robot Controller manufactured by CimCorp Precision Systems, Inc., Shoreview, MN. The effectiveness of the method of the present invention for improving the bondability of the surface of an organic structure has been verified by an experiment, described below. Experimental Results: Thirty-two lap shear specimens of polypropylene having dimensions of 3.3" X 1.5" X 0.060" were cut from polypropylene sheet. The specimens were cleaned with a distilled water rinse and patted dry with Kimwipes®. The samples were divided into two groups: Series 120 and Series 121. The samples of Series 121 were exposed to pulses (-=180 microseconds) of high intensity optical light at a rate of about 0.1 Hz from a linear xenon . " ashlamp while their irradiated surfaces were bathed in ized air. The xenon flashlamp had a 4mm bore and a 6 inch arc length and was filled with xenon gas at a pressure of 200 T. The input energy to the flashlamp was approximately 1000 J. The series 121 samples were each irradiated with 3 pulses in each of three zones: (1) left edge of sample below lamp; (2) middle of sample below lamp; and (3) right edge of sample below lamp. The Series 120 samples were not exposed to flashlamp radiation or ionized gas. Pairs of samples from Series 120 were bonded together at a 1 in 2 area with an isocyanate based adhesive (Elmer's Superglue) to form eight test structures. Similarly, the samples of Series 121 were also bonded together to form 8 test structures. All of the samples were wetted 100 percent with adhesive when they were joined. However, it was noted that the untreated specimens of the Series 120 nearly always exhibited only about 80-90 per cent adhesive wetting at the lap joint area. The lap joint areas were placed under a two pound weight and were allowed to cure for 24 hours. Each test structure was placed in a tensile loading fixture and subjected to tensile loading until the lap joint parted. The tension applied to the test structure was observed continuously to the point of failure. The failure tensions presented in TABLE 1 are the tensions observed just prior to lap joint failures. No deformation of the test specimens was observed during any of the tests. TABLE 1 shows a doubling of the mean failure tension between the two series. The zero (0) data point for sample 1 in Series 120 represents failure (separation) of the specimen lap joint even before it was placed in the tension device. Each specimen was visually inspected after failure testing to determine the nature of the failure mechanism. In all cases the failure was observed to be adhesive rather than cohesive in nature. An adhesive failure is one in which the bonding agent ("glue") fails whereas in a cohesive failure, the substrate separates from itself. The test results shown in FIG. 1 demonstrated that the method of the present invention of irradiating a polymeric surface with pulsed, incoherent broadband optical energy in the presence of an ionized gas greatly improves the bonding characteristics of a polymeric surface. TABLE 1 POLYPROPYLENE SHEET LAP SHEAR TESTS Series 120: No Treatment Series 121: Samples irradiated with broadband light in the presence of ionized gas The method of the present invention may also be used to enhance the bondability of metal surfaces, as for example, in applications such as bonding of polymeric auto-body panels to metal subpanels in the automotive industry, or painting the space shuttle fuel tanks in the aerospace industry. The preparation of a metal surface in accordance with the methods of the present invention includes impinging the target area on the surface of a structure with a particle stream to dislodge any inorganic substances present on the surface, and then irradiating the surface with pulsed, broadband optical energy in order to photodecompose any organic adventitious substances present on the surface. The use of a pulsed, broadband light source reduces the processing time significantly when compared to the approach of using steady state UV light, as taught by Sowell, because of the high peak and average intensities achievable with the pulsed source. The method of the present invention may be implemented using the system 10 of FIG. 1 shown to further include a particle stream generator 34 for generating a low kinetic energy particle stream 36. The particle stream 36 is directed by nozzle 38 to impinge the target area of the surface 14 in order to substantially remove any inorganic materials and particulate matter from the surface. The particle stream nozzle 38 is preferably mounted to the housing 30 so that as the structure 16 translates with respect to the housing 30, as for example, in the direction of the arrow 42, the target area is impinged with the particle stream 36 just prior to being irradiated with the optical energy 18. The particle stream 36 is preferably comprised of carbon dioxide pellets. Advantageously, carbon dioxide is relatively inert, nontoxic, and inexpensive. The carbon dioxide pellets may be conveyed by dry, heated air at a mass flow rate of 23 kg/hr. Such pellets are typically shaped as cylinders each having a length of about 0.5 cm and a diameter of 0.3 cm. The particle stream generator may be a carbon dioxide pellet source of the type commercially available from Cold Jet, Inc. of Loveland, OH. When the substrate 16 is a metal, the target area usually does not require exposure to ionized gas. Therefore, the ionized gas generator 26 is generally not enabled and the gas supply 28 is shut-off. However, there may be applications where both the ionized gas and the particle stream generator are used in combination with the pulsed broadband optical energy to improve the bondability of a surface of the substrate. One technique that may be used to confirm that the bondability of a metal substrate has been improved by the process of the present invention is to pour distilled water over an area of the metal surface which has been subjected to the particle stream and irradiated. The distilled water should wet the entire treated surface without any exposure of the metal surface within the perimeter of the wetted area. Such exposure would result if the distilled water were to bead up, indicting that the distilled water had a greater affinity for itself than for the metal surface. A surface which wets in this manner is referred to as a "water break free surface." Such condition occurs when the surface energy of the treated surface is higher than the surface energy of the distilled water. Using the above-described confirmation technique, it has been shown that metal surfaces processed in accordance with the present invention do indeed exhibit an enhanced bondability over metal surfaces not prepared in accordance with the invention. For example, aluminum surfaces have been processed using the above-described method to successfully remove organic oils, tape residue, uncatalyzed RTV (silicone rubber), salt spray and fingerprints. Once such adventitious substances were removed from the aluminum, the aluminum then exhibited a significantly enhanced surface free energy. While the present invention has been described in terms of preferred embodiments, it is to be understood that the invention is not to be limited to the exact form of the apparatus or processes disclosed. Therefore, it is to be understood that the invention may be practiced other than as specifically described without departing from the scope of the claims.

Claims: CLAIMS WHAT IS CLAIMED IS: 1. A method for improving the capability of a surface of an organic structure to bond with another material, comprising the steps of: irradiating a target area of a surface of the organic structure with pulsed, incoherent optical energy having wavelength components which range from 170-5000 nanometers at an optical power density within the range of about 0.01-0.5 J/cm 2 sec, said surface having a surface free energy, said optical power density providing an intensity sufficient to photodecompose any adventitious organic substances on said surface and to photodecompose a thin layer of molecular bonds forming said surface of said structure; and exposing said target area of said surface to ionized gas that chemically reacts with the target area of said surface to increase said surface free energy of the surface. 2. The method of claim 1 wherein the step of irradiating said target area of said surface comprises irradiating the target area with optical energy generated by a xenon flashlamp. 3. The method of claim 2 wherein the step of irradiating said target area includes irradiating the target area with said optical energy such that the optical power density varies no more than 15 percent over said target area. 4. The method of claim 2 further including the step of scanning said surface in a predetermined pattern relative to the irradiating of the target area with said optical energy and the exposing of the target area to said ionizing gas so that the target area effectively moves over a desired portion of said surface. 5. A method for improving the capability of a surface to bond to a material, said material having a surface free energy, said method comprising the steps of: generating a particle stream; impinging a target area of said surface with said particle stream in order to substantially remove any inorganic adventitious materials from said surface, thereby providing a substantially inorganic free surface; generating pulsed, incoherent, optical energy having wavelength components ranging from about 170-5000 nanometers; and directing said pulsed optical energy to irradiate said substantially inorganic free surface after said surface has been impinged by said particle stream, said optical energy causing the surface free energy of said surface to increase. 6. The method of Claim 5 wherein the step of generating pulsed, incoherent, optical energy comprises controlling a flashlamp to produce a prescribed pulse modulation. 7. The method of Claim 6 wherein said prescribed pulse modulation includes a frequency of 0.1 to 20 Hz, and a pulse width of from 1.0 to 180 microseconds. 8. A system for improving the bondability of a surface of a given structure to bond with a material, said surface having a surface free energy, said system comprising: an optical energy source for generating pulsed, incoherent, optical energy having wavelength components ranging from about 170-5000 nanometers, said optical energy source being positioned to irradiate a target area on said surface of the given structure with an optical power density in the range of 0.01-0.5 J/cm 2 /sec so as to increase the surface free energy of said surface; a pulse modulator operably coupled to control said optical energy source; an electrical power supply operably coupled to power said pulse modulator; and a source of ionized gas for bathing said irradiated target area with said ionized gas. 9. The system of Claim 8 further including particle stream generator means for generating a particle stream that impinges the target area of said surface of the given structure prior to irradiating the target area with said pulsed optical energy, said particle stream serving to substantially remove any inorganic adventitious materials from said surface. 10. The system of Claim 9 further including a hood to support said optical energy source and to enshroud the irradiated surface to capture a substantial amount of any photodecomposed molecules liberated from said surface, and a vacuum system coupled to collect said photodecomposed molecules collected by said hood.

Inventors: CATES MICHAEL CHRISTOPHER, HAMM RICHARD ROY, CATES, MICHAEL, CHRISTOPHER, HAMM, RICHARD, ROY
Applicants: MAXWELL LAB, MAXWELL LABORATORIES, INC.
Application year: 1993
Main classification: B32B31
IPC codes: B05D3, B08B7, B29C35, B29C59, C08J7, C09J5