Abstract:
An ultraviolet radiation curing system is disclosed for treating a substrate, such as fiber optic cable or silicone tubing. The system comprises a processing chamber allowing transport of a continuous piece of substrate to be treated. As the substrate moves through the processing chamber, ultraviolet radiation from a plasma lamp activated by a microwave generator treats the surface of the substrate. The system comprises two elliptical reflectors of different sizes so that larger diameter substrates may be efficiently treated with ultraviolet radiation. The system may also comprise an ultraviolet-transmissive conduit enclosing the substrate and split into a first portion and a second portion, where the second portion is movable from the first portion to open the conduit and allow insertion or alignment of the substrate within the conduit and processing chamber.

Description:
TECHNICAL FIELD 
     The present invention relates generally to ultraviolet lamp systems, and more particularly, to microwave-excited ultraviolet lamp systems configured to irradiate a substrate with ultraviolet radiation. 
     BACKGROUND 
     Ultraviolet lamp systems are commonly used for heating and curing materials including adhesives, sealants, inks, and coatings, as well as optical cables and tubing. Certain ultraviolet lamp systems have electrodeless light sources and operate by exciting an electrodeless plasma lamp in a processing chamber with radiofrequency energy, such as microwave energy. In an electrodeless ultraviolet lamp system that relies upon excitation with radiofrequency energy, the plasma lamp emits a characteristic spectrum isotropically outward along its cylindrical length. Part of the emitted radiation moves directly from the plasma lamp toward a substrate located in the processing chamber. A substantial portion of the emitted radiation must be reflected before reaching the substrate. To efficiently use the ultraviolet radiation emitted by the plasma lamp, various designs of reflectors have been mounted in processing chambers to surround the plasma lamp and the substrate. 
     While conventional reflectors include rectangular box-shaped reflectors and parabolic reflectors, the most efficient reflector system is an elliptical reflector. By placing the plasma lamp along one focus of the elliptical reflector and the substrate at the other focus of the elliptical reflector, all radiation emitted by the plasma lamp reaches the substrate after no more than one reflection. Examples of these conventional systems with elliptical reflectors include U.S. Pat. No. 4,710,638 issued to Wood and U.S. Pat. No. 6,626,561 issued to Carter, et al. One challenge of conventional systems is that the efficiency of irradiation decreases as the diameter of the substrate increases because the substrate moves away from the focal axis. Therefore, the use of a completely elliptical reflector limits the diameter of the substrate being treated. Another shortcoming of conventional systems is that the substrate is often enclosed within a quartz or other ultraviolet-transmissive conduit to protect the substrate or a coating applied to the substrate from contamination. The substrate should not contact this ultraviolet-transmissive conduit, making threading the substrate through the processing chamber difficult. 
     It would be desirable to provide an efficient curing system with reflectors that allow for generally efficient irradiation of elongate, continuous substrates having a wide range of diameters. It would also be desirable to provide a curing system that eases the process of loading such substrates into the processing chamber. 
     SUMMARY OF THE INVENTION 
     An ultraviolet radiation curing system is provided for treating a substrate having a longitudinal axis. The curing system includes a processing chamber having an inlet port and an outlet port to transport the substrate, a plasma lamp mounted within the processing chamber, and a microwave generator coupled to the processing chamber for exciting the plasma lamp to emit ultraviolet radiation. The curing system also includes an ultraviolet-transmissive conduit positioned within the processing space. The curing system further includes a first elliptical reflector defined by a first “a” distance, a first “b” distance, a first focal line collinear with the plasma lamp, and a second focal line collinear with the longitudinal axis of the substrate. Additionally, the curing system includes a second elliptical reflector defined by a second “a” distance larger than the first “a” distance, a second “b” distance larger than the first “b” distance, a third focal line collinear with the longitudinal axis of the substrate, and a fourth focal line collinear with the plasma lamp. The plasma lamp of the curing system emits a first portion of ultraviolet radiation, which directly irradiates the substrate, a second portion of ultraviolet radiation that is reflected by the first reflector before irradiating the substrate, and a third portion of ultraviolet radiation that is reflected by the second reflector before irradiating the substrate. 
     In an alternative embodiment, the curing system of the invention includes a processing chamber, a plasma lamp, and a microwave generator as explained above. This curing system further includes first and second reflectors for reflecting ultraviolet radiation to irradiate the substrate. The curing system also includes an ultraviolet-transmissive conduit for enclosing the substrate, positioned within the processing chamber, and comprising a first portion and a second portion movable relative to the first portion between an open position and a closed position. The open position of the ultraviolet-transmissive conduit allows for the substrate to be loaded into the conduit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description given above and the detailed description given below, serve to explain various aspects of the invention. 
         FIG. 1  is a perspective view of an ultraviolet radiation curing system in accordance with one embodiment of the invention with the processing chamber opened for insertion or alignment of a substrate. 
         FIG. 2  is a cross-sectional view of the ultraviolet radiation curing system of  FIG. 1 . 
         FIG. 3  is an illustration of the geometry of the elliptical reflectors in the ultraviolet radiation curing system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Although the invention will be described next in connection with certain embodiments, the invention is not limited to practice in any one specific type of ultraviolet curing system. The description of the embodiments of the invention is intended to cover all alternatives, modifications, and equivalent arrangements, as may be included within the spirit and scope of the invention, as defined by the appended claims. In particular, those skilled in the art will recognize that the components of the embodiments of the invention described herein could be arranged in multiple different ways. 
     Referring now to the drawings, and specifically  FIGS. 1 and 2 , one embodiment of an ultraviolet radiation curing system  10  is provided. The curing system  10  includes a processing chamber  12  defined by front wall  14 , back wall  16 , and longitudinal side walls  18 ,  20 . An inlet port  22  located in the front wall  14  is adapted to receive a substrate  26  for ultraviolet curing. An outlet port  24  located in the back wall  16  is adapted to permit the substrate  26  to exit the processing chamber  12 . The substrate  26  has a longitudinal axis  28 , and the substrate  26  travels through the curing system  10  continuously along the longitudinal axis  28 . 
     The curing system  10  also includes a plasma lamp  34 , positioned longitudinally within the processing chamber  12 . The ends of the plasma lamp  34  are attached in a conventional manner to the front wall  14  and the back wall  16 . Plasma lamp  34  comprises a hermetically sealed, longitudinally-extending envelope or tube filled with a gas mixture. Plasma lamp  34  does not require either electrical connections or electrodes for its operation. The plasma lamp  34  is formed of an ultraviolet-transmissive material that is an electrical insulator, such as vitreous silica or quartz, so that the plasma lamp  34  is electrically isolated from other structures in the processing chamber  12 . The curing system  10  includes at least one microwave generator  36  mounted above the processing chamber  12 . When activated, the microwave generator  36  sends microwave energy into the processing chamber  12  through a microwave inlet (not shown) as understood by one skilled in the art. The microwave energy is substantially captured within a mesh screen box  38  surrounding the plasma lamp  34 , and the mesh screen box  38  allows air to freely flow while reflecting most of the microwave energy delivered into the processing chamber  12 . 
     Microwave energy provided by the microwave generator  36  excites atoms in the gas mixture within plasma lamp  34  to initiate and, thereafter, sustain the plasma within. A starter bulb (not shown) is provided to assist in initiating a plasma within plasma lamp  34  as understood by those of ordinary skill in the art. By adjusting the shape of processing chamber  12  and the power level of the microwave generator  36 , the density distribution of the microwave energy is selected to excite atoms in the gas mixture along the entire longitudinal length of plasma lamp  34 . Once the plasma is initiated, the intensity of the radiation output by the plasma lamp  34  depends upon the microwave power provided to the processing chamber  12  by microwave generator  36 . 
     The gas mixture inside plasma lamp  34  has an elemental composition selected to produce photons having a predetermined distribution of wavelengths of radiation when the gas atoms are excited to a plasma state. For ultraviolet treating applications, the gas mixture may comprise a mercury vapor and an inert gas, such as argon, and may include trace amounts of one or more elements such as iron, gallium, or indium. The mercury vapor is provided by the vaporization of a small quantity of mercury that is solid at room temperature. The spectrum of radiation output by a plasma excited from such a gas mixture includes highly intense ultraviolet and infrared spectral components. As used herein, radiation is defined as photons having wavelengths ranging between about 200 nm to about 2000 nm, ultraviolet radiation is defined as photons having wavelengths ranging between about 200 nm to about 400 nm, and infrared radiation is defined as photons having wavelengths ranging between about 750 nm to about 2000 nm. 
     The curing system  10  also includes a longitudinally-extending ultraviolet-transmissive conduit  54  attached to the front wall  14  and the back wall  16  of the processing chamber  12 . The ultraviolet-transmissive conduit  54  is aligned with the inlet port  22  and the outlet port  24 . The conduit  54  encloses the substrate  26  during the longitudinal transfer of the substrate  26  through the processing space  12 . The conduit  54  is formed of an insulating material that is highly transmissive of ultraviolet radiation, such as quartz or a vitreous silica. The conduit  54  prevents extraneous forces from acting on substrate  26 , such as forced cooling air currents that may force the substrate  26  to undesirably contact the conduit  54  or contaminate the substrate  26  with dust or other particles in the processing chamber  12 . This isolation ability is particularly important if the substrate  26  is fragile or otherwise prone to damage. 
     The curing system  10  further comprises a longitudinally-extending first reflector  42  coupled to a plurality of support ribs  44  attached in a conventional way to the processing chamber  12 . The first reflector  42  is oriented toward the plasma lamp  34 . The curing system  10  also comprises a longitudinally-extending second reflector  46  coupled to a plurality of support ribs  48  attached in a conventional way to the processing chamber  12 . The second reflector  46  is oriented towards the first reflector  42  and the substrate  26 . The first reflector  42  has an elliptical first reflective surface  50 , and the second reflector  46  has an elliptical second reflective surface  52 . The first reflector  42  and second reflector  46  are spaced apart from each other to allow a longitudinal gap  56  along the processing chamber  12 . 
     A pressurized air supply (not shown) delivers air into the processing chamber  12  through gaps  51  located in the first reflector  42  to blow directly on the plasma lamp  34  and regulate the temperature of the plasma lamp  34 . The forced air flows through the mesh screen box  38 , around the second reflector  46  through longitudinal gap  56 , and exits out an exhaust outlet  40  located at the bottom of the system  10 . A light-blocking material  39  covers the exhaust outlet  40  to allow forced air through while blocking a substantial portion of ultraviolet and other light radiation from exiting the system  10 . 
     As best illustrated in  FIG. 3 , the geometric arrangement of one embodiment of the first reflector  42  and the second reflector  46  is provided. The first reflector  42  and second reflector  46  are shown solid in  FIG. 3 , The first reflective surface  50  is a partial ellipse defined by a first “a” distance A 1  and a first “b” distance B 1  as well understood in the art. The first reflective surface  50  is further defined by a first focal line  58  and a second focal line  60  each extending longitudinally along the first reflector surface  50 . These first and second focal lines  58 ,  60  are separated by a distance D, defined by the following formula:
 
 D =(2)( C ), where  C   2   =A   2   −B   2  
 
Similarly, the second reflective surface  52  is a partial ellipse defined by a second “a” distance A 2  which is larger than the first “a” distance A 1 , and a second “b” distance B 2  which is larger than the first “b” distance B 1 . The second reflective surface  52  is also defined by a third focal line  62  substantially collinear with the second focal line  60 , and a fourth focal line  64  substantially collinear with the first focal line  58 . The second reflective surface  52  consequently shares a distance D between focal lines  62 ,  64  with the first reflective surface  50 , allowing the second reflective surface  52  to be a larger ellipse than the first reflective surface  50 . Note that in  FIG. 3 , the partial ellipses made by the first reflective surface  50  and the second reflective surface  52  are extended in phantom into half-ellipses in order to clearly show each “a” distance relative to the centerline  65  representing one-half of distance D and each “b” distance relative to the focal lines  58 ,  64  and  60 ,  62 .
 
     As an example of an acceptable size for the first reflective surface  50  and the second reflective surface  52  in this embodiment: the A 1  distance is 3.74 inches, the B 1  distance is 2.97 inches, and the D distance between foci is calculated to be 4.56 inches, using the above formula. Now given any A 2  distance, an appropriate B 2  distance can be calculated using the D distance. Continuing the example, the A 2  distance is set at 4.57 inches, so the B 2  distance is therefore 3.97 inches. This aspect of the curing system  10  is not limited to the foregoing example of reflector sizes, as this is one specific known set of reflector sizes, shown for illustration purposes only. 
     The first reflector  42  and second reflector  46  are preferably formed of a radiation-transmissive material that reflects ultraviolet radiation and transmits other kinds of radiation such as infrared and microwave radiation. An example of such a material is a borosilicate glass, such as Pyrex® glass made by the Corning Corporation in Corning, N.Y. Alternatively, the first reflector  42  and second reflector  46  can be formed with any material having suitable reflective and transmissive properties for ultraviolet curing. The preferential transmission and reflection of radiofrequency energy can also be provided by applying a dichroic coating to the first reflector  42  and the second reflector  46  as well understood by those skilled in the art. 
     The first reflector  42  and second reflector  46  are placed so that the plasma lamp  34  is located substantially on the first focal line  58  and fourth focal line  64 , while the longitudinal axis  28  of the substrate  26  is substantially collinear with the second focal line  60  and the third focal line  62 . As best shown in  FIG. 2 , ultraviolet radiation is delivered from the plasma lamp  34  located at the first focal line  58  to the substrate  26  at the second focal line  60  directly or after at least one reflection off the first reflector  42  or the second reflector  46 . Advantageously, the second reflector  46  being larger than the first reflector  42  allows for a larger ultraviolet-transmissive conduit  54  and therefore, a larger diameter substrate  26  to be treated by the curing system  10 . For example, the curing system  10  illustrated by this embodiment can treat a substrate  26  as small as a fiber optic cable or as large as silicone tubing. The treatment process is used to cure the substrate  26  material itself or a coating applied to the substrate  26  that is sensitive to ultraviolet radiation. As illustrated by arrows  66  in  FIG. 2 , the ultraviolet irradiation is substantially efficient on the substrate  26  thanks to the longitudinal axis  28  being located at one focal line  60 ,  62  of each reflector  42 ,  46 . 
     The plasma lamp  34  emits a first portion, second portion, and third portion of ultraviolet radiation. As best shown in  FIG. 2 , the first portion of ultraviolet radiation directly irradiates the substrate  26  without reflection. The first reflector  42  reflects the second portion of ultraviolet radiation emitted by the plasma lamp  34 , while the second reflector  46  reflects the third portion of ultraviolet radiation emitted by the plasma lamp  34 . Consequently, the ultraviolet radiation is delivered to treat the substrate  26  in an efficient manner on all sides of the substrate  26  upon release from the plasma lamp  34 . 
     Another aspect of a curing system  10  is provided as shown in  FIGS. 1-2  and described below. In this aspect, the ultraviolet-transmissive conduit  54  is split into a first portion  84  and a second portion  86  movable relative to the first portion  84  between an open position illustrated in  FIG. 1  and a closed position illustrated in  FIG. 2 . The open position allows the substrate  26  to be loaded into or aligned properly within the ultraviolet-transmissive conduit  54 . 
     In this aspect, the processing chamber  12  further comprises a first portion  88  and a second portion  90  movably connected by a hinge  92  or other conventional methods to the first portion  88 . The first portion  88  of the processing chamber  12  includes the first reflector  42  and the first portion  84  of the ultraviolet-transmissive conduit  54 . The second portion  90  of the processing chamber  12  includes the second reflector  46  and the second portion  86  of the ultraviolet-transmissive conduit  54 . The first portion  88  and second portion  90  of the processing chamber  12  move from an open position shown in  FIG. 1  to a closed position shown in  FIG. 2 . The open position allows for loading and alignment of a substrate  26  in the processing chamber  12  while the closed position allows for operation of the curing system  10  to treat the substrate  26 . The features of this aspect may be combined with the above-described features of the embodiment, or these features may exist separately in the curing system  10 . 
     While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is not limited to the specific details, representative apparatus, or illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of the applicants&#39; general inventive concept.