Abstract:
Coated objects are recycled by irradiation with a laser beam. The invention is applied to objects (such as documents) that include an entry surface, a surface coated with a coating (such as text or graphics printed with copier or printer toner), and a body interposed between the surface. A laser beam is directed toward a target region of the coated surface, traveling first through the object&#39;s entry surface and body, which are translucent or transparent to the wavelength of light contained in the laser beam. The laser beam generates a plasma shock wave at the target region of the coated surface, thereby discharging particles of the coating located near the target region. The discharged printed matter is discharged away from the object, thereby avoiding undesired impregnation of the coating into the object. Discharged coating may be recycled by collection and subsequent reuse.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to the removal of coating material from various objects. More particularly, the invention concerns a method and apparatus to recycle documents by using a laser to erase toner, ink, or similar material from the documents. 
     2. Description of the Related Art 
     Paper recycling is an important part of the present recycling industry. Recycled paper is used in greeting cards, tissue paper, cardstock, and a variety of other paper-based products. With increasing worldwide concern for the environment, interest in paper recycling is higher than ever. 
     The primary system for recycling paper today involves a &#34;re-pulping&#34; approach, which uses a combination of chemical and mechanical means. In this system, a large volume of used paper is typically ground into a mash, treated with certain known bleaching chemicals, and then reconstituted into sheet form. 
     As an alternative to the &#34;re-pulping&#34; system, many have attempted to recycle paper using various non-destructive whitening methods. Broadly, known non-destructive whitening methods clean printed paper &#34;as is&#34;, without breaking the paper down into pulp and pressing and drying it back into sheet form. Known whitening methods typically employ chemical agents to dissolve toner, bleaching agents to whiten paper, and chemical and/or mechanical pealing agents to remove printing. 
     For some recycling applications, the known whitening methods may be adequate. However, for many users these techniques are not completely satisfactory. These methods, for example, often use dangerous chemicals that may be poisonous to humans. This presents a significant risk of injury to recycling personnel, and also requires comprehensive chemical handling and disposal procedures. Moreover, some users may find non-destructive whitening methods to be too slow and expensive for their applications. Also, these techniques may damage the paper surface, rendering the paper unusable for some purposes. 
     Prior to the present invention, the use of lasers in document recycling has been unknown. In a completely dissimilar field, however, lasers have been used to perform &#34;surface cleaning&#34; of various hard, opaque objects. Several companies, for instance, use lasers to remove paint from metal surfaces of airplanes and ships. Others have applied lasers to clean the surface of stonework, buildings, and other hard opaque objects. 
     As shown in FIG. 1, these prior surface cleaning techniques operate simply by impinging a laser beam 100 onto the undesirable coating 102. The coating 102 is adhered to an underlying object 104. When the laser 100 contacts the coating 102, the laser is absorbed by the coating 102 which then creates a plasma shock wave 200 (FIG. 2). Although the plasma shock wave 200 is oriented toward the object 104, the wave 200 and the underlying object 104 cooperate to discharged a significant portion 202 of the coating proximate the laser beam 100. The discharged material 202 is effectively removed from the object. 
     For many applications, the known laser-based surface cleaning techniques satisfy their user&#39;s expectations. However, the present inventors have discovered that certain characteristics of these methods yield poor performance when applied to the task of paper cleaning. With a rigid surface such as the aluminum skin of an airplane (e.g. FIGS. 1-2), the underlying object 104 effectively repels the coating 202 proximate the laser beam 100. However, the portion of coating 204 directly impinged by the laser beam 100 is not repelled so effectively. In fact, some of the coating 204 may be impregnated into the object 104. In the case of a rigid surface such as a metal substance, this impregnation is minimized due to the difference in hardness between the relatively hard object and the relative soft coating. 
     The inventors have discovered that, unlike hard surfaces such as metal, paper is quite susceptible to coating impregnation during conventional laser-based surface cleaning. As a result, the impinging laser not only does a poor job of removing the coating, but it also causes a significant level of paper damage. As a result, an improved paper recycling technique is still needed. 
     SUMMARY OF THE INVENTION 
     Broadly, the present invention concerns a method and apparatus to clean and recycle objects by using a laser to remove coatings therefrom. More specifically, one embodiment of the invention projects a laser through an entry surface and a body of a document that is translucent to the laser&#39;s wavelength. The document&#39;s body is interposed between the entry surface and a printed surface bearing printed matter such as printed text or graphics. The laser beam generates a plasma shock wave directed at a target region of the printed surface, propelling nearby printed matter from the target region of the document. The propelled printed matter may be collected by a number of different means, for subsequent reuse. 
     In a specific embodiment, the invention may comprise a method or apparatus to erase printer or copier toner from printed sheets of paper by directing a laser through the printed sheets&#39; unprinted undersurface. 
     The invention affords its users with a number of distinct advantages. First, unlike prior recycling systems, the present invention cleans printed documents with a modicum of time. In this respect, one embodiment of the invention comprises a machine that may be operated like a laser printer or a copier, by sequentially feeding sheets of paper into the machine, with similar processing time. Another advantage is that the invention avoids the use of hazardous chemicals required by prior recycling techniques. And, unlike known &#34;surface cleaning&#34; methods used for removing coatings from hard objects, the present invention neither damages the object nor impregnates the object&#39;s surface with the coating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The nature, objects, and advantages of the invention will become more apparent to those skilled in the art after considering the following detailed description in connection with the accompanying drawings, in which like reference numerals designate like parts throughout, wherein: 
     FIG. 1 is a diagram showing the relationship between a laser beam, coating, and underlying object in known surface cleaning techniques; 
     FIG. 2 is a diagram showing the effects of known surface cleaning techniques; 
     FIG. 3 is a diagram showing the relationship between a laser beam, coating, and underlying object in accordance with recycling technique of the invention; 
     FIG. 4 is a diagram showing the effects of the recycling technique of the invention; 
     FIG. 5 is a diagram illustrating a recycling apparatus in accordance with the invention; 
     FIG. 6 is a diagram illustrating a first embodiment of a print collector of the invention; 
     FIG. 7 is a diagram illustrating a second embodiment of a print collector of the invention; and 
     FIG. 8 is a flowchart illustrating an exemplary sequence of method steps in accordance with the invention 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     General 
     Broadly, the present invention concerns a method and apparatus for removing a coating adhered to a coated surface of an translucent or transparent object by impinging a laser beam upon the coming from the object&#39;s (uncoated) undersurface. The invention is especially effective when used to erase printer, toner, copier toner, or laserjet ink from documents such as sheets of paper. 
     FIG. 3 depicts the relationship between a laser beam 300, coating 302, and an underlying object 304 in accordance with the recycling technique of the invention. The underlying object 304 includes an coated surface 306, an (uncoated) entry surface 308, and a body 310. The laser beam 300 passes through the entry surface 308 and the body 310 prior to impinging upon the coating 302 adhered to the coated surface 306. FIG. 4 illustrates the result; namely, the laser beam 300 creates a plasma shock wave 400 at the coated surface 306. However, unlike known surface cleaning techniques, the plasma shock wave 400 is oriented away from the object 304. As a result, the coating 302 is completely discharged from the surface 306, as shown by the discharge 402. 
     Structure 
     Referring to FIGS. 3-4 and 5, description is made of the hardware components of one embodiment of recycling apparatus 500 embodying the invention. In the illustrated embodiment, the apparatus 500 receives an object 304 and recycles it to provide a cleaned object 502. The object 304 may comprise, for example, a sheet of white or colored paper, or a transparency sheet for use with overhead projectors. The apparatus 500 includes a conveyor 504 to receive and position the object 304. The conveyor 504 may comprise one or more rollers, a conveyor belt, a slidable paper tray, or another suitable device(s) for transporting the object 304 and supporting it in a prescribed position. In one embodiment, the conveyor 504 may be similar to mechanisms used in commercial laser printers to transport paper from a paper tray and pass it to a printing position and subsequently deliver the printed paper to an exit tray. 
     The apparatus 500 also includes a laser 506. Preferably, the laser 506 comprises a Q-switched Nd:YAG laser. However, the laser 506 may comprise a Dye, Ruby, Alexandrite laser, Titanium Sapphire laser, double-frequency laser, Er:Glass Eye Safety laser, or another suitable type of laser. The laser need not by Q-switched and may operate in a multi or free running mode as an alternative. 
     The laser 506 functions under operating parameters selected for the laser to irradiate the sheet while quickly ejecting the coating before the paper is overheated or otherwise damaged. The operating parameters therefore consider the composition of the sheet, the sheet&#39;s density, and the type of coating to be removed. Additionally, the wavelength of laser is selected to ensure that the particular color and composition of the sheet (or other object) is translucent (i.e. not opaque) to the laser. For white paper, any laser wavelength provides satisfactory results. For colored paper, invisible wavelengths are preferable to minimize absorption of the laser beam by the paper. For cleaning an assortment of white and multicolored paper, then, the appropriate wavelength is preferably provided by a combination of several lasers including Nd:YAG operating at both the fundamental wavelength of 1.064 μm and operating at the frequency-tripled UV wavelength of 0.355 μm. Therefore, the laser 506 may comprise multiple lasers, if desired. 
     As an example, for the different coating types as listed, the laser 506 may exhibit parameters such as those of Table 1, below. 
     
                       TABLE 1______________________________________     First    Second     Third     Example  Example    Example     Printer Toner              Copier Toner                         Laserjet Ink______________________________________Pulse Energy       2.2    J/cm.sup.2                      1.5  J/cm.sup.2                                 2.8  J/cm.sup.2DensityPulse Width 50     ns      50   ns    50   nsRepetition Rate       200    Hz      200  Hz    200  HzSpot Size Diameter       5      mm      5    mm    5    mm______________________________________ 
    
     As shown in Table 1, the laser may be operated in the pulsed mode. The pulse repetition rate of the laser does not affect the cleaning process. The pulse repetition rate only affects the time required to clean any given object. 
     As an alternative to the description above, the laser 506 may comprise one or more semiconductor lasers. For instance, multiple semiconductor lasers may be assembled into a &#34;pooled laser cell&#34; or &#34;laser array.&#34; The array may therefore be sufficiently large to process an entire object 304 simultaneously. Or, if the pooled laser cell is smaller than one object 304, the cell may therefore be scanned across the page 502 with a scanner 507, discussed bellow. 
     For more efficient use of the laser 506, the apparatus 500 may include a scanner 507 to selectively shape and scan the laser beam (or laser array) across the object 304. The scanner 507 may comprise, for example, a motor attached to a laser steering mirror, a selectively rotatable multi-faceted diamond mirror, or another suitable device. 
     In an exemplary embodiment, the time required to clean an 81/2&#34;×11&#34; sheet of paper of unwanted toner ink maybe calculated as follows. In this example, the beam is pre-shaped into a cylinder of 50 mm×5 mm and the laser is scanned horizontally from left to right across one line then returned to the left side of the next line. Using a 50% overlap from one step to the next as the laser is scanned over the entire sheet, the step sizes in the horizontal (&#34;S H  &#34;) and vertically (&#34;S V  &#34;) dimensions are defined by Equations 1 and 2, below. ##EQU1## The number of steps in the horizontal (&#34;N H  &#34;) and vertical (&#34;N V  &#34;) directions are shown by Equations 3 and 4, below. ##EQU2## Accordingly, the total number of steps (&#34;n tot  &#34;) is shown by Equations 5, below. 
     
         n.sub.tot =n.sub.H ·n.sub.V =965                  [5] 
    
     For a laser with a pulse repetition rate (&#34;prf&#34;) of 200 Hz, with a number of pulses per step (&#34;N&#34;) of 3, the dwell (&#34;d&#34;) is shown by Equation 6, below. ##EQU3## The total time required therefore appears as shown by Equation 7, below. ##EQU4## 
     Referring again to FIG. 5, another component of the apparatus 500 is a print collector 508, which collects coating material ejected from the object 304 by the laser 506. In one embodiment, the collector 508 may comprise an ionized collector plate 600 (FIG. 6), placed near the coated surface 306 of the sheet 304 attracts and collects particles of coating 402 ejected from the sheet 304. The collector plate 600 may comprise, for example, aluminum, steel, or another suitable substance. 
     Alternatively, the print collector 508 may, comprise a fluid flow device 700 and a pair of charged venturi plates 704 positioned near the coated surface 306 of the sheet 304. The fluid flow device 700 comprises a fluid source 701 and a fluid 702 receiver/pump 702. During times when coating 402 is being discharged from the sheet 304, the fluid source 701 flows a sheet of a fluid vehicle past the coated surface 306 to collect the discharged particles of coating. The fluid vehicle may comprise an inert gas such as Argon, for example. When the fluid flows between the venturi plates 704, and the coating particles are attracted to the ionized surface of the plates 704, as shown by the arrows 706-707. After the venturi plates 704 extract the coating particles from the fluid vehicle, the fluid receiver/pump 702 receives the fluid vehicle and pumps or otherwise returns the fluid to the source 701. 
     The venturi plates 704 comprise an ionized surface, shaped to increase the velocity and decrease the pressure of fluid passing between the venturi plates 704. Particles of coating in the low-pressure area proximate the venturi plates 704 are attracted to the ionized surface, where they are collected. 
     As another alternative, the print collector 508 may comprise a &#34;discharge net,&#34; adapted from such devices used for pollution cleaning in electric power stations. These nets employ an array of charged collector elements to electrically remove particles from the air. 
     If desired, coating particles collected by the print collector 508 may be recycled by re-using the particles to imprint the same or other sheets. If the coating comprises copier toner, for example, the collected toner particles may be replaced into an empty copier toner cartridge. 
     For enhanced document cleaning, the apparatus 500 may also includes a bleach bath 510. The bleach bath 510 functions to eliminate any glue, binder, or other residue remaining after removal of particles of coating 402 removed from the sheet 304. The bath 510 preferably employs a bleaching agent and an oxidizer, for example. In one embodiment, the bleaching agent comprises a chlorine-based chemical and the oxidizer includes a 30% concentration of hydrogen peroxide in water. Alternatively, the oxidizer may employ a 40-45% concentration of potassium permangarate, and then a solution of hydrogen peroxide. A drying bath 512 may be employed to remove any residual water remaining after exposure of the sheet to the hydrogen peroxide bath or other bleaching substance used in the bleach bath 510. The drying bath 512 may comprise an alcohol such as methyl alcohol, for example. The drying bath 512 preferably includes a set of pinch rollers (not shown) to quickly remove any remaining moisture. Finally, the apparatus 500 may use a fan 514 to air dry the erased, bleached sheet. The apparatus 500 therefore provides a sheet 504 that is fully erased, bleached, and dried. 
     Operation 
     Broadly, the method aspect of the invention concerns the recycling of objects having a coated surface by irradiation from the object&#39;s (uncoated) undersurface with a laser light source. FIG. 8 depicts a specific example of the method of the present invention, in the form of a sequence of tasks 800. For ease, consistency, and specificity of illustration, the tasks 800 address the operation of a specific embodiment, concerning the recycling of paper imprinted with printer toner, copier toner, or laserjet toner. However, as mentioned above, the invention also applies to objects made from materials other than paper, or even to objects having shapes other than sheets or pages. 
     After initiation in task 802, the conveyor 504 receives and positions a printed sheet 304 of paper in task 804. Then, as shown in FIGS. 3 and 5, the laser 506 in task 806 generates a beam 300 that passes through the (uncoated) entry surface 308 and body 310 of the sheet 304 before irradiating the sheet&#39;s coated surface 306. In the illustrated embodiment, the laser beam 300 preferably has a 90° angle of incidence with respect to the sheet 304. The wavelength of the beam 300 is selected in relation to the composition of the sheet 304 so that the sheet 304 appears to be partially or completely transparent to the beam 300. To erase toner from all areas of the sheet 304, the laser beam and the sheet 304 may be moved relative to each other. In one embodiment, the screener 507 selectively scans the laser beam (or laser array) across the sheet 304. Alternatively, the conveyor 504 may move the sheet 304 relative to the laser 506 or a combination of the two may be used. 
     The laser 506 creases a plasma shock wave, oriented outward from the coated surface 306, causing the toner to be discharged away from the sheet, thereby avoiding any impregnation of toner particles into the sheet. In task 808, the print collector 508 collects the discharged toner particles. In the case of the ionized conductive plate 600 (FIG. 6), the plate 600 electrically attracts and collects particles of coating 402 ejected from the sheet 304. Moreover, the collector plate 600 may periodically reverse its charge to expel the collected toner. 
     Alternatively, task 808 may be performed using the fluid source 701 (FIG. 7) may flow a fluid sheet past the coated surface 306 to collect the discharged particles of coating. As the fluid flows into the ionized surface of the venturi plates 704, the toner particles move toward the ionized surface as shown by the arrows 706-707. As the fluid source 701 supplies the fluid sheet, the fluid receiver/pump 702 continually receives the fluid vehicle and returns the fluid to the source 701. 
     If desired, the collected toner may be re-used in task 809, for example by placing the toner into the toner cartridge of a printer or a copier. 
     After task 808, the object 304 in task 810 is placed into a bleach bath 510 for whitening. The object 304 is then dried in task 812. As an example, the drying steps may include submersion in a methyl-alcohol bath 512 to remove water created during the bleach bath, passing the sheet through pinch rollers to remove excess moisture, and then air drying by a fan 514. After task 812, the process ends in task 814. 
     Other Embodiments 
     While there have been shown what are presently considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims. 
     The descriptions above, for example, have been made in the context of documents or objects coated on a single side. The invention nonetheless contemplates the recycling of documents or other objects coated on both sides. However, special care must be taken to avoid impregnating a coating on one side of the object during the process of erasing the coating on the opposite side. In particular, the laser 300 in this implementation preferably has an acute angle of incidence, such as 10°-20°, with respect to the object 304. In this case, the plasma is directed and expelled away from both the paper and the laser beam, minimizing any impregnation of coating material present on the laser&#39;s entry surface. 
     An alternate embodiment of the invention may be constructed to share a single laser among multiple different recycling devices. Multiple units may be constructed, each resembling the recycling apparatus without the laser. Each unit may be placed in different rooms of an office, for example, with each unit sharing a common laser. In this embodiment, the laser&#39;s beam is split or otherwise multiplied into many different beams, each of which is separately directed to a different remotely located unit. To provide the laser beam to the different units, the invention may employ fiber optic cable, waveguides, or another suitable light-conveying means.