Abstract:
A light curing structure and related methods are disclosed wherein a framework defining an interior space for containing an object having applied thereon a light-cured material is provided, and means associated with the framework for exposing the object to UV radiation from natural sunlight is used to cure the light-cured material. Preferred embodiments include a booth for applying and curing light-cured materials comprising a housing having a ceiling and walls that define an enclosed interior space for holding an object, means for applying a light-cured material to the object; means for preventing the exposure of the light-cured material to UV radiation during application of the light-cured material to the object, and means for exposing the light-cured material to UV radiation from natural sunlight after application of the light-cured material to the object. Apparatus and methods according to the invention are particularly suited for use with UV-cured paints in the automotive industry.

Description:
BACKGROUND OF THE INVENTION 
     Most metallic and many plastic surfaces of useful articles are painted to prevent weathering and corrosion and for decorative reasons. One common way to paint a large object, e.g. an automobile or automobile part, is by using a housing or booth in which the paint is sprayed on the object. The object then usually remains in the booth while the paint is cured by heating or other means. Such paint spray booths are commonly prefabricated building structures having a ceiling, floor and walls that define an interior space large enough to hold the object to be painted, with sufficient room on all sides for workmen to effectively operate paint spray and other equipment. These booths provide advantages such as reducing particulates, confining paint overspray and evaporated solvents and reducing drying times. 
     One common type of paint spray booth is the downdraft and semi-vertical spray booth that uses a housing positioned over an open floor grate or an exhaust outlet near the bottom of the walls. Air from the ceiling and any entrained paint overspray and solvents are drawn downward over the vehicle during spraying and drying and are then exhausted through the floor grate or exhaust opening. One example of such a spray booth is described in U.S. Pat. No. 6,533,654, incorporated herein by reference. Curing of the paint applied to the object is usually accomplished by using heaters to increase the temperature within the booth. The paint curing time in conventional paint spray booths may take about one hour. 
     Materials that are cured (hardened) by exposure to light have recently shown promise in a variety of industries because such materials have very short cure times without the application of heat, and have several advantages in speed and ease of use. Some of the general benefits of light-cured materials include rapid curing times (in some cases almost instantaneous, allowing for immediate further processing), on demand curing (requiring only exposure to UV light), no solvents (100% solids—no environmental pollution due to solvent evaporation), no heat (low thermal stressing of substrate materials), single component systems (ready-to-use with no mixing, no waste, no cleaning of mixing containers, no problems with pot life or mixed materials, no dangerous isocyanate catalysts), and more efficient use of raw materials and energy. Examples of light-cured materials include structural adhesives for bonding glass, ceramics, ferrites, plastics and metals; UV hot melt pressure sensitive adhesives; UV glob tops, chip coats and conformal coatings; UV potting compounds and encapsulants; UV paints, inks and coatings; and UV hardening polyester resin/glass fiber composite materials. 
     The use of light-cured materials can be found in an extremely wide variety of industries such as, for example, the electronics, printing, furniture, floor covering, medical device, dental, packaging, marine and paper industries. The use of light-cured paints and coatings in the automotive refinish and repair markets has grown considerably in recent years, but to date has typically been limited to primer applications. However, it is likely that light-cured products will be expanded to possibly include basecoats, topcoats, fillers and glazes, adhesives and sealers. 
     Light-cured materials contain chemicals known as photo initiators that comprise certain reactive groups, e.g. acrylate or methacrylate groups, which are sensitive to UV radiation. When photo initiators are illuminated with intense UV light, they initiate a polymerization reaction that hardens the material. If a light-cured material is used as a paint or other coating to cover an object, the hardened material provides a protective finished surface, or one that is ready for sanding and application of other coats in a short period of time, e.g. in about 2-5 minutes. 
     Curing of light-cured materials currently requires the use of a UV lamp. These UV lamps typically use a relatively small high-intensity discharge (HID) quartz bulb that is filled with mercury and traces of other elements. A high voltage applied to electrodes in the lamp creates an arc that heats the gaseous atoms to the point at which they emit UV light, visible light and infrared light. Full intensity is usually reached within about 2-5 minutes. The bulb is positioned inside a reflector that collects and focuses the light toward the lamp opening, which is typically designed to filter out more-harmful UV-B and UV-C radiation and allow only UV-A radiation to be emitted by the lamp. 
     UV-curing lamps have certain disadvantages. In addition to being relatively expensive, the HID bulbs slowly decrease in light output over time (thereby causing progressively longer cure times) and should be replaced after about 500 hours of use. It is recommended to turn off the bulb immediately after each use to maximize bulb life, minimize stray UV radiation, heat buildup in the workplace, and make the lamp easier to handle and move. UV-curing lamps are intended to be used only in a restricted area, accessed only by qualified professional operators, due to the dangers of being exposed to the emitted UV radiation and the generation of ozone by the lamp. In addition, UV lamps generally only have a very limited exposure area or footprint. Currently the coverage area of UV-curing lamps may be limited to an area of about 6 square feet, which results in frequent repositioning of the lamp. Although this disadvantage may be limited somewhat by mounting the lamp on a computer guided robot which automatically repositions the lamp to a different exposure area, this solution is very expensive. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to structures and methods of curing light-cured materials without requiring the use of UV-curing lamps. As used herein, a “light-cured” material is intended to include any of the many materials known or to be developed that may be hardened by exposure to light including UV radiation. It is contemplated that the present invention may be useful for any of the known applications for light-cured materials. In particular, the present invention provides structures and methods that are useful for applying and curing light-cured paints in the automotive industry. 
     In accordance with one aspect of the invention, a structure is provided for curing a light-curable material that has been applied to an object, e.g. a UV-curable paint that has been applied to an automobile or automobile part. In its most basic form, the structure comprises a framework that defines an interior space for containing the object to which the light-curable material has been applied, and means associated with the framework for exposing the object to UV radiation obtained from, e.g., natural sunlight and/or UV LEDs. 
     In one embodiment, the framework may comprise a housing wherein the UV radiation-exposing means comprises one or more transparent windows in a ceiling exposed to sunlight and configured to transmit UV radiation from the sunlight to the object contained within the housing. In a preferable embodiment, the ceiling comprises a substantially horizontal opaque middle section connected to side sections on either side thereof which contain the windows. In another embodiment, the side sections may be slanted towards the walls of the housing. The middle section of the ceiling may be used to house desired apparatus, e.g. air circulation apparatus. 
     Since light-cured materials are usually inactive and stable only in the absence of UV light, application of the paint to the object may require conditions void of UV radiation. Accordingly, the application of the light-cured material may be accomplished in a light-tight structure that is separate from the structure in which the light-cured material is cured. One preferred embodiment of the present invention comprises a single structure in which the light-cured material can be both applied to the object and cured. Exemplary spray booths comprise a housing having a ceiling, a floor and one or more walls on the sides, front and/or rear of the housing, any of which may optionally have one or more doors and/or windows therein. Alternatively, in place of walls, the spray booth may have one or more curtains that drape from the periphery of the ceiling and may be drawn to form an enclosure around the object to be painted and cured. Preferred spray booths may have means to introduce filtered air into the interior of the booth from the ceiling and to exhaust the air through an exhaust outlet near the floor, thereby creating an air flow gradient within the interior to facilitate downdraft conditions in the booth. 
     In one spray booth embodiment of the invention, sunlight comprising UV radiation may directed to the coated object during curing by one or more skylight tube systems, each comprising a sunlight collector positioned external to the spray booth, a sunlight exposer positioned within the interior space containing the coated object, and a light transmitting conduit that connects the sunlight collector with the exposer and passes through the spray booth structure. In a preferred embodiment, the conduit(s) passes through a wall of the spray booth, or through a portion of the ceiling that does not interfere with apparatus that is contained in or above the ceiling, e.g. for introducing air into the interior of the booth. 
     The sunlight collector may comprise a clear dome of UV radiation-transmitting material or any other apparatus that captures sunlight rays and directs them toward the light-transmitting conduit. The light-transmitting conduit may comprise a tubular or other shaped lumen having a highly reflective interior surface that directs the collected sunlight toward the exposer at the end of the conduit opposite the sunlight collector. The light-transmitting conduit may also comprise other light-transmitting media such as a fiber optic cable. The exposer may comprise a diffuser that spreads the sunlight throughout the interior of the spray booth or may have associated therewith means for directing the sunlight to a particular location within the booth. 
     Any or all of the components of the skylight tube system may comprise means for modifying the sunlight (and/or UV radiation component thereof) that is collected and transmitted to the coated object. For example, the booth of the present invention may preferably include an optical directing system that directs UV radiation from the exposer(s) over the entire painted surface of the object contained therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of a structure according to the invention. 
         FIG. 2A  is a cross-section view of the structure shown in  FIG. 1  during the application of a light-cured paint to an automobile. 
         FIG. 2B  is a cross-section view of the structure shown in  FIG. 1  during the curing of the light-cured material applied to an automobile. 
         FIG. 3  is a cross-section view of another embodiment of a structure according to present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides apparatus and methods for curing light-cured materials with natural sunlight containing UV radiation. Preferred embodiments of the invention provide apparatus and methods for applying a light-cured material to an object in a structure and curing the material in the same structure. The following detailed description is directed specifically to several preferred embodiments useful for applying light-cured paints and other coatings to the surface of automobiles or automobile parts, but one skilled in the art will readily recognize other applications for the invention. 
     As previously described, light-cured materials such as adhesives, inks, paints and other coatings are known in the art and commercially available. For example, light-cured coatings suitable for the automotive and other industries are currently available from Azko Nobel, BASF, DuPont, Montana Products, PPG, NEXA Autocolor and Ciba Specialty Chemicals. It is contemplated that any of such light-cured materials may be used in the practice of the present invention. 
       FIG. 1  illustrates one embodiment of the present invention. Booth  100  is a free-standing structure suitable for an outside location directly exposed to sunlight, and may be used to cure a light-cured material that has been previously applied to an object at another location, or preferably, may be used for both the application of a light-cured material and for curing the material after application. Booth  100  comprises a housing having a ceiling with a substantially horizontal middle section  101  and slanted side sections  102 . The ceiling is supported by side walls  103 , front wall  104  and rear wall  105 , which together with floor  106  define interior space  107 . Slanted ceiling sections  102  may contain one or more transparent windows  108  that are exposed to direct sunlight and are configured to transmit UV radiation from the direct sunlight to an object located within interior space  107 . Side walls  103  may also contain one or more transparent windows  110  that are exposed to ambient or indirect sunlight, and are configured to transmit UV radiation from the sunlight to an object located within interior space  107 . Exposure to UV radiation in the sunlight entering interior space  107  in the various ways described initiates a hardening reaction in the light-cured material applied to the object, thereby curing the material. 
     If booth  100  is used for application of light-cured material, ceiling windows  108  may be fitted with retractable interior shades  109 , and side windows  110  may fitted with retractable interior shades  111 . Front wall  104  and rear wall  105  may include doors  112 , which may have one or more transparent windows  113 , also fitted with retractable interior shades  114  if required. 
     As an illustrative example, the operation of booth  100  during the application of a UV-curable paint to an automobile (or other object) will be described in connection with  FIG. 2A , and the operation of booth  100  during the curing of that paint after application will be described in connection with  FIG. 2B .  FIG. 2A and 2B  use the same reference numbers as used in  FIG. 1  to refer to the same elements. To begin the paint application process, the automobile is placed within interior space  107  of booth  100  after appropriate surface preparation using conventional procedures known in the art. A conventional UV-curable paint  202  may be applied to the surface of automobile  201  using paint applicator(s)  203 . Paint applicator(s)  203  may comprise any of the manually operated or automated paint spray guns or other applicators known to the art as being suitable for the application of UV-curable paint. For example, paint applicator(s)  203  may comprise known aerosol application equipment that eliminates the need for measuring and mixing and makes preparation and cleaning of a paint spray gun unnecessary. 
     UV-curable paints are usually applied to an object in the absence of UV-radiation to prevent premature curing. Accordingly, during the paint application process illustrated in  FIG. 2A , retractable interior shades  109  may be drawn over windows  108 , retractable interior shades  111  may be drawn over windows  110 , and retractable interior shades  114  may be drawn over windows  113  ( FIG. 2 ) to prevent sunlight from entering interior space  107 . Retractable shades  109 ,  111  and  114  may be positioned on the interior of the windows so that they can also serve to protect windows  108 ,  110  and  113 , respectively, from being covered by any paint overspray. Illumination of interior space  107  during the application process may be accomplished by using a light source having a wavelength to which the photo initiators in the UV-curable paint are not sensitive, but which is (or can be made) visible to the human eye or suitable video monitoring equipment. 
     Booth  100  may be of the type that is positioned over an open grate in floor  106 , or preferably booth  100  uses exhaust outlets  204  near the bottom of side walls  103  (as shown). An air intake and circulation system  205  may be disposed in ceiling section  101  and is adapted to supply air  206  into the interior space  107 . During application of the paint to automobile  201 , circulation system  205  causes air  206 , as well as any paint overspray, to be drawn downward over automobile  201  and to be exhausted through exhaust outlets  204 . Air circulation system  205  may also be used to enhance air flow over automobile  201  during the curing process, if desired. Examples of preferred downdraft spray booths are described in U.S. Pat. No. 6,533,654, and co-pending applications Ser. No. (019254-001110US) and Ser. No. (019254-001120US), all of which are incorporated by reference herein. 
     The UV curing process will now be described with reference to  FIG. 2B . After the application of the UV-curable paint and any required post-application surface preparation, retractable shades  109 ,  111  and  114  ( FIG. 2 ) may be put in their retracted or open position, as shown in  FIG. 2B . This open position allows direct sunlight  207  comprising UV radiation to pass through windows  108 , and ambient sunlight  208  comprising UV radiation to pass through windows  110 . Optical elements  209  may be optionally positioned in the path of direct sunlight  207  and optical elements  210  may be positioned in the path of ambient sunlight  208 , so as to modify the radiation entering interior space  107  and provide enhanced UV radiation  212  and  213 . 
     As used herein, including the appended claims, the phrase “to modify the radiation”means to direct, filter, magnify, concentrate, diffuse and/or otherwise change the character of the radiation as required by any particular application. One may wish to modify the radiation for various reasons, e.g. to accommodate the size, shape and other characteristics of automobile (or other object)  201  and/or to accommodate the properties of the UV-curable paint being cured. As one example, optical elements  209  and  210  may comprise lenses, reflective devices and/or light diffusers to direct enhanced UV radiation  212  and  213  in a manner that assures complete exposure of all of the coated surfaces of automobile (or other object)  201 . As another example, optical elements  209  and  210  may comprise filters to block out certain wavelengths of radiation from entering interior space  107  and exposing the coated surfaces of automobile  201 , e.g. UV-B and UV-C radiation may be filtered out of sunlight  207  and enhanced UV radiation  212  and  213  may comprise only UV-A radiation. As a still further example, optical elements  209  and  210  may magnify or concentrate sunlight  207  so that enhanced UV radiation  212  and  213  is more intense than the UV radiation naturally present in sunlight  207 . Similar optical devices may also be included within interior space  107 , e.g. reflective devices  211  may be included on floor  106  to direct enhanced UV radiation  212  to the underside of automobile  201 , or placed in other locations so to direct enhanced UV radiation  212  and  213  to various other coated surfaces of automobile  201 . As previously mentioned, air circulation system  205  may optionally be used to enhance air flow over automobile  204  during the curing process, if needed or desirable. 
       FIG. 3  illustrates another embodiment of the invention wherein booth  300  is located inside a larger building structure  301 . Booth  300  comprises ceiling  302 , side walls  303 , floor  304  and front and rear walls (not shown), which together define interior space  305  for holding automobile  306  to be painted and cured. Interior space  305  may contain any or all of the same elements as previously described in connection with booth  100 , i.e. UV paint applicators (not shown), air circulation system  308 , exhaust outlets  309  and reflective devices  310  (not shown). 
     One or more skylight tube systems  312  may extend through roof  315  of building  301  and through ceiling  302  of booth  300 , and/or one or more skylight tube systems  313  may extend through roof  315  and through side walls  303  of booth  300 . Each skylight tube system respectively comprises a sunlight collector  316 , a light-transmitting conduit  317  and an exposer  318 . Skylight tube systems contemplated as being suitable for use in the present invention are commercially available from a wide variety of companies. As examples, mention may be made of skylight tube systems sold under the trademarks Solatube, SunPipe, Daylite and SunTunnel. 
     Sunlight collector  316  may be mounted external to booth  300 , e.g. sunlight collector  316  may be conveniently located on roof  315  of building  301  directly exposed to sunlight. Sunlight collector  316  may be mounted on a device that positions it in the best possible location for receiving sunlight throughout the day, e.g. sunlight collector  316  may be mounted on the top of extension  322  that places sunlight collector  316  about 3 feet or more above roof  315 . Sunlight collector  316  may comprise a clear dome of sunlight-transmitting material, and may also optionally comprise reflectors mounted external or within the dome, and/or lenses and other known means for enhancing the capture of sunlight and the transmission thereof towards the light-transmitting conduit  317 . Filters or other optical devices may also be associated with sunlight collector  316  to modify the radiation transmitted down the light-transmitting conduit  317 . 
     Light-transmitting conduit  317  may comprise a tubular or other shaped lumen with a highly reflective interior surface adapted to direct collected sunlight down the lumen and out the diffuser at the opposite end. Light-transmitting conduit  317  is connected at one end to sunlight collector  316  and at the opposite end to exposer  318 , and may comprise a rigid or flexible tube or channel. For example, light-transmitting conduit  317  may be a tube of about 10-21 inches in diameter and made from aluminum. The interior surface of light-transmitting conduit  317  may be coated with a reflective material, for example silver or a material containing silver. Examples of commercially available reflective tube materials include those sold under the trademarks Alcoa Everbrite 95 and 3M Siverlux. It is also contemplated that light-transmitting conduits  317  may comprise a fiber-optic cable or other device. 
     Exposer  318  may be mounted on the interior of wall  303  or ceiling  302  of booth  300 , and may comprise a light diffuser that spreads the light coming through light-transmitting conduit  317  throughout interior space  305 . In addition, one or more exposers  319  may be mounted on the end of a flexible reflective tube  320  extending from light-transmitting conduit  317 , thereby allowing exposer  319  to be moved around automobile  306 , e.g. by robot  321 , to achieve a more complete exposure of hard-to-reach surfaces of automobile  306 . 
     At least one of the components of the skylight tube systems may be fitted with means for blocking UV radiation from exposing automobile  306  during the application of UV-curable paint. As one example, exposer  318  may be fitted with a retractable cover  323  that covers exposer  318  during the application process, but which may be retracted to uncover exposer  318  during the curing process. In addition, one or more of the skylight tube system components may include any of the previously mentioned optical devices used to modify the radiation contained in the sunlight transmitted thereby. 
     In another embodiment, the present invention provides a method of curing a light-cured material which comprises the steps of applying a light-cured material to an object; placing the object in an interior space of a structure having associated therewith means for transmitting UV radiation from natural sunlight into the interior space; and exposing the object to said UV radiation. A preferred method comprises applying the light-cured material to the object after it is located in the interior space of the structure and blocking the UV radiation from being transmitted into the interior space during said application. As previously described, preferred methods comprise transmitting UV radiation into the interior space through one or more windows in the structure, and transmitting UV radiation into the interior space through one or more skylight tube systems. 
     Embodiments of the invention have been described in detail for the purposes of clarity and understanding. However, it will be appreciated that certain changes and modifications may be practiced within the scope of the appended claims.