Printing of reflective sheeting

Abstract: A method of printing a sign including the steps of providing a sign component and an image definition, applying dry toner powder to surface portions corresponding to the image definition, and fusing the applied toner. The image definition is provided as an embossed pattern. A toner composition and a computer program for printing are also disclosed.

Description: The present invention relates to the production of signs, and in particular, to the printing of an image for a weather resistant sign such as a license plate. BACKGROUND OF THE INVENTION Vehicle license plates are a challenging problem. They must provide an overall similarity of styling or appearance to enable rapid recognition of license plates issued by various licensing authorities and to inhibit counterfeiting. At the same time, license plates need to provide a distinct, individualized, and unique identifier code or image for each vehicle. In essence, each vehicle requires a custom, short run manufacturing process to incorporate its unique code or image upon a commonly styled license plate blank. The best known manufacturing process for incorporating the unique image has been stamping or embossing a metallic plate to first provide raised letters or numbers on a plate. Subsequently, the raised letters or numbers are contacted by a liquid ink or paint carried upon a roller or pad. The liquid ink or paint dries to provide a contrasting image which corresponds to the embossed image or raised pattern of the plate. In order to improve the weather resistance, after drying of the ink, the plate is dipped in a liquid clear coating agent (i.e. a solvent born resin or plastic). The resulting plate is relatively durable. However, the embossing and inking process is labor intensive. GB-A-2126389 discloses briefly the use of xerography in conventional printing of vehicle number plates. There is no disclosure of a printing on a raised portion of an embossed plate. Another aspect of the present embossing and liquid inking process is the release of solvent vapor from the drying ink. This solvent release is in addition to solvent released by the dipping and subsequent drying of the protective clear coating layer. The manufacture of large quantities of license plates requires substantial investment in drying ovens and solvent vapor ventilation. Increased scrutiny of possible environmental and health risks associated with various solvent vapors appears to be a trend throughout the world. It would be advantageous to eliminate the generation of solvent vapors from the process of printing license plates. Many traffic signs and license plates have a retroreflective property which is generally considered desirable in the industry. In such cases, inks or paints have been selected for compatibility with the retroreflective materials. A more versatile and efficient printing process would accommodate a variety of colorants and be compatible with retroreflective components used in license plates and traffic signs. SUMMARY OF THE INVENTION According to the present invention there is provided a method of printing an image on a reflective sign comprising the steps of: A. providing a sign component selected from the group consisting of retroreflective sheeting and transparent film, said sign component having a polymeric surface; B. providing an embossed pattern in said sign component, the pattern providing a definition of the image to be printed; C. applying, to the raised portions of the embossed pattern, dry toner powder comprising: (1) colorant; and (2) binding agent which adheres to said polymeric surface and which comprises a polymer; and D. fusing the applied dry toner powder to form a fixed image borne upon the embossed pattern. Preferably the surface, including the fused toner, is covered by a protective coating to seal the fused toner within a weather resistant package. The sign component to be printed may be an opaque or retroreflective sheet or, alternatively, a light transmitting (preferably transparent) cover film for use over an opaque or retroreflective sheet. The image definition is by embossing. The method may be practiced without the generation of solvent vapors and is also suitable for other short runs of signs, such as street name signs. There is also disclosed a license plate printed by the method of this invention. A license plate is a laminate including at least the following layers: a substrate, a retroreflective sheet layer upon the substrate oriented to provide retroreflection of light entering the retroreflective sheet opposite from the side laminated to the substrate, and a discontinuous image layer of fused toner over the retroreflective sheet. A preferred license plate of the present disclosure is a laminate including at least the following layers: a substrate, a retroreflective sheet layer upon the substrate oriented to provide retroreflection of light entering the retroreflective sheet opposite from the side laminated to the substrate, a transparent protective top layer over the retroreflective sheet layer, and a discontinuous image layer of fused toner sandwiched between the retroreflective sheet layer and the transparent top layer. The license plates include an embossed image substantially corresponding to the fused toner. BRIEF DESCRIPTION OF DRAWINGS Figure 1 is a schematic of an embodiment of the present invention; Figure 2 is a partial cross section of a license plate of the present disclosure; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention includes a method of printing a weather resistant sign. The method includes the steps of providing a first sign component having a surface, providing a definition of an image or graphic which is to be printed upon the surface of the first sign component, applying dry toner powder to portions of the surface which correspond to the definition of the image to be printed, and fusing the applied dry toner powder to form a fixed image upon the sign component surface. These steps are explained in greater detail in the following detailed description of the preferred embodiments. Additionally, the invention preferably further includes the step of covering the surface and fused toner with a light-transmitting material to seal or sandwich the fused toner within a weather resistant package. The invention will be explained with respect to the production of an automotive license plate; however, the printing method is also suited to the production of other signs and particularly to highway or outdoor signs which are produced in very limited numbers. One particularly appropriate sign to be printed by the method of the present invention is a street name sign. The method for producing an embossed sign is shown schematically in figure 1. A sign component 22 is provided. Preferably, the sign component 22, is a thin piece of embossable material, for example, a web 24 from a roll 26 of aluminum. A thickness of about 0.8 mm is suitable for aluminum license plates. In a most preferred embodiment, the aluminum web may include a facing layer of retroreflective sheeting. Such retroreflecting sheeting is known in the sign art. Enclosed lens retroreflective sheetings and the use of glass beads to provide for reflexed light reflectors are described in Palmquist, et al ., U.S. 2,407,680; May, U.S. 4,626,127; Tung et al ., U.S. 4,367,920; Tung et al ., U.S. 4,511,210; and Tung et al . U.S. 4,569,920; Bailey et al ., U.S. 4,767,659; Bailey et al. U.S. 4,664,966; and Bailey, U.S. 4,648,932; the disclosures of which are incorporated by reference herein. Generally, enclosed lens retroreflective sheetings include, in order, an adhesive layer for application to a support such as a license plate blank, a specular reflective surface, a light transmitting spacing layer, and a monolayer of glass beads within a light transmitting resin layer. Often, a protective outer layer or top layer is also present. Retroreflective sheeting typically functions in the following manner: Light from an external source, is transmitted to the beads, which serve as lenses and direct the light toward the specular reflective surface, through the spacing. The reflective surface, preferably cupped about each of the glass beads, returns the light to the glass beads which in turn return the light toward the source. The sign component 22 is embossed in an embossing unit 28. Embossing units are well known in the production of license plates and typically include a male 30 and female 32 embossing molds or dies. When the embossing dies 30 and 32 are pressed together with the sign component interposed, a raised portion 34 is formed upon the sign component 22. These die pairs are interchangeable members of a set of dies and generally allow alphanumeric designs or images to be defined. Typically, the raised or embossed portion 34 protrudes about 0.15-0.20 cm relative to the unaltered portions of the surface. If a retroreflective sheeting element is included upon the sign component 22, the retroreflective sheeting would be on the face of the sign component, overlying the raised portion 34 and arranged to provide a retroreflection property to the surface to be printed. Next, a printer unit 36 applies a dry toner powder to the raised portion 34. Sign component 22 may be heated to facilitate transfer of toner. The printer 36 is configured so as to avoid application of the dry toner powder to the unraised portion of the sign component 22. An appropriate application system within the printer unit 36 includes a rotating drum 38 carrying a layer 40 of dry toner powder. The rotating drum 38 contacts or nearly contacts the raised portion 34 of the sign component 22. The contact or near contact between the rotating drum 38 and the raised portion 34 allows a transfer of at least a portion of the layer 40 of dry toner powder onto the raised portion 34, without transferring dry toner onto the unaltered portions (i.e. portions not embossed or in relief) of the surface 35 of the sign component 22. The transfer of dry toner powder may be facilitated by warming of the embossed sign component 22. In the method, when the drum 38 is not an electrostatic drum, a charge carrier is unnecessary in the dry toner powder. For example, a hard rubber roller adequately carries toner powder for transfer to embossed portions of a license plate blank. Next, the applied dry toner powder 42 upon the raised portion 34 is fused. The fusing step may be by means of infrared heating 44 or other suitable means of supplying an amount of heat sufficient to raise the temperature of the dry toner powder to melt a binder component within the dry toner powder. Fusing serves to fix the image on the raised portions. The resulting product may now be cut and used as a license plate or a sign at cutter 60. Preferably, the fused dry toner is covered to protect it from weathering. One method of covering is the well known "dip coating" method currently employed for covering license plates. An alternative means of covering is the lamination of a protective film which may be supplied in roll form. The web of protective film is brought into close proximity to the surface and raised portions of the sign component, for example, by a guide roller. A set of pinch rollers provide a nip for pressing the protective film over the raised portions 34 having fused toner and also onto unraised surface 35. The application of the protective film seals the fused toner between the sign component 22 (optionally having a retroreflective sheet on its face) and protective film. Lamination of a protective film coat, such as an oriented polyalkylmethacrylate coated with a thin layer of pressure-sensitive acrylate adhesive, may also be employed as a combination fusing and covering step. Appropriate temperatures serve to both fuse and laminate but are less than temperatures which alter the orientation of the cover film. The embossing unit 28 in the method serves to define a raised image upon the sign component 22. Traditionally, matching male 30 and female 32 dies must be employed to provide a particular definition of an image. In the schematic of figure 1, such a defined image is represented by the raised portion 34. Because license plates are prepared in extremely small groups (i.e. one or two plates per vehicle) and require distinct images for each vehicle, a change is required in the embossing unit 28 after every one or alternatively two license plates has been embossed. The method has the feature that a license plate may be prepared without the generation of any solvent vapors. The method could be substituted in the traditional manufacture of license plates. Such a substitution, however, would not eliminate the production of a solvent vapor from the clear dipping process. Protective films which may have acceptable amounts of stretch or elongation have been disclosed in Bailey et al . U.S. 4,664,966 and U.S. 4,767,659, the disclosures of which are incorporated by reference herein. The method is also suitable for printing on an embossed sign lacking retroreflective sheeting. License plates with retroreflective sheeting, however, are particularly desired by the industry. A preferred embodiment of a sign of the present disclosure is shown in exploded cross section in figure 2 at 300. The sign 300 includes an optional top protective layer 302 having a first or outermost surface 304 and a second or inner surface 306. The optional top protective layer 302 is located on the side of the sign 300 which is to be viewed and is preferably substantially transparent. Progressing through the sign 300 from the viewing side, the next layer is the discontinuous image layer 308, which is formed of fused dry toner powder. The image layer 308 has a first or outwardly directed surface 310 and a second or inwardly directed surface 312. Progressing through the sign 300, the next layer is a retroreflective sheet layer 314. In figure 2, the retroreflective sheet layer 314 is shown as an enclosed lens type retroreflective sheet and is oriented to be retroreflective to light entering from the direction of the optional top protective layer 302. Such sheets 314 preferably include a lower adhesive layer 316, a specular reflective layer 318, a transparent spacer layer 324, a monolayer of lenses 320, and a transparent upper layer 322. Enclosed lens retroreflective sheets have been previously described in Bailey et al. in U.S. Patent 4,664,966, incorporated by reference herein. Below the retroreflective sheet 314 is a supporting substrate sheet 326, having an upper surface 328 and a lower surface 330. A preferred substate is aluminum, however, steel, wood or plywood, or various plastic sheets may be suitable alternatives. In finished form, the layers 302, 308, 314, and 326 of the sign 300 are laminated together, such that light entering the sign 300 through the top protective layer 302 and not encountering the discontinuous image layer 308 is retroreflected by the retroreflective sheet 314 to pass back out though the top protective layer 302. Because the light is retroreflected, it generally returned along or near to its original pathway. By comparison, light entering the sign 300 through the top protective layer 302 and encountering the discontinuous image layer 308 is affected differently. If the discontinuous image layer is opaque, it will either absorb the light or reflect (but not retroreflect) the light back through the top layer at generally a range of new angles. If the discontinuous image is transmissive to some wavelengths of light and absorptive to other wavelengths, the transmitted wavelengths will pass though to the retroreflective layer 314 for retroreflection back along the original pathway. When viewed, such light transmissive colored images 308 will provide a colored light image suitable for conveying information to an observer.' Preferred solid toners have the following characteristics: resistance to degradation by weathering; and good adhesion to the substrate to which they are applied. A dry toner powder suitable for practicing the present invention requires a colorant and a binding agent. If the toner is to be applied by an electrostatic drum surface, a charge carrier should also be included within the toner. A suitable binding agent may be an alkyl substituted acrylate or methacrylate polymer, with alkyl groups having from 1 to 9 carbon atoms, or mixtures of such acrylates, and especially a copolymer of methyl and butyl methacrylates (such as for example, Acryloid B-66 available from Rohm & Haas Company). Other suitable binding agents are polyvinyl acetals, for example, polyvinyl butryal (such as BUTVAR brand polyvinyl butryals B-90 or B-72 available from the Monsanto Chemical Company); polyolefins; polyesters (such as VITEL brand PE-200D from the Goodyear Tire & Rubber Company or ARAKOTE 3000 brand carboxyl terminated polyester optionally in mixture with ARALDITE PT810 brand polyfunctional epoxy resin (triglycidyl isocyanurate) both available from the Ciba-Geigy Chemical Company); and vinyl resins (such as VINYLITE brand vinyl resin, VAGH copolymer of vinyl chloride and vinyl acetate available from the Union Carbide Corporation). The preferred binding agents are characterized by relatively high transparency and clarity. Additionally, preferred binding agents have glass transition temperatures (T g ) from about -15°C to about 150°C, preferably from about 35°C to about 110°C, and most preferably about 50°C. The most preferred binding agents are chosen based upon their potential strong chemical interactions with the surface to be printed. Specifically envisioned as factors to be considered as providing the potential for strong chemical interactions are the likelihood of formation of bonds such as ionic or covalent bonds, donor-acceptor bonds, as well as secondary bonds such as hydrogen bonds and van der Waals bonds between the binding agent and the surface to be printed. In evaluating the potential, the relevant bond energies may be obtained from textbooks such as Adhesion and Adhesives: Science and Technology by A. J. Kinloch; 1987, University Press Cambridge, Great Britain. Additionally, the most preferred binding agents can be laminated, when incorporated in a dry toner powder, at temperatures of from about 50°C to about 240°C, preferably at temperatures of from about 120°C to about 200°C. For example, the well known REFLECTO-LITE brand retroreflective sheeting available from the Minnesota Mining and Manufacturing Company of St. Paul, Minnesota, has a polyvinyl butyral surface and therefore compatible binding agents, which cause dry toner powders to laminate at temperatures from about 50°C to about 240°C, may be fused during lamination of an ethylene acrylic acid (EAA) copolymer protective film to the retroreflective sheeting. Most preferred are binding agents which may be used at temperatures of about 150°C. Preferred binding agents are also resistant to ultraviolet (UV) light degradation and are adhesive to the surface upon which the toner is printed. Suitable charge carriers may be positive charge control agents designed for use as additives in dry toner formulations such as, for example, copolymers of butyl and methyl methacrylate (such as TRIBLOX PC-100 brand acrylic polymer (available from E. I. DuPont de Nemours Company)). Polyesters and vinyl resins may also be used as charge carriers. A preferred acrylic copolymer charge carrier has the following characteristics: molecular weight of 2000 to 5000; glass transition temperature (Tg) of 53°C to 59°C, onset at about 46°C, nitrogen content of about 1% as measured by NMR. Preferred charge carriers are also relatively light transmissive or transparent materials, and are resistant to UV light degradation. For a black toner, a transparent charge carrier is not essential. For example, an azine dye (Nigrosine Solvent Black 7, CI#50415:1) available from the Orient Chemical Company, port Newark, New Jersey, may be used as a charge carrier for such a toner. The most preferred charge carriers are acrylic polymers (i.e. alkyl acrylates or alkyl methacrylates) having amine functionality (i.e. functional groups including amine nitrogen or quaternary ammonium nitrogen). Suitable colorants may be pigments such as Pigment Red 179 or 224 available from the Harmon-Mobay Chemical Company; Pigment Yellow 110 or Pigment Violet 37 available from the Ciba-Geigy Company; Pigment Green 7 or 36 available from the Sun Chemical Company; Pigment Blue 15;1 or Blue 15;6 available from BASF; and Regal 500R carbon black available from the Cabot Corporation. Suitable colorants may also be dyes such as Amaplast Yellow available from the Color-Chem International Corporation or LATYL Brilliant Blue BGA available from the DuPont Company. Generally, pigments or dyes should be resistant to environmental pollutant chemical degradation and UV light degradation. Preferably, pigments are dispersed in a dispersing resin, for example Red 229 dispersed in Vinylite VAGH resin in a 1:1 weight ratio. Such dispersion helps to maintain the small pigment particle size that is desired for obtaining a light transmittant image. The fused toner image on retroreflective signs is preferably light transmissive for all colors except black. That is at least 10% of light entering the image area passes through the toner, except in the case of carbon black. In the case, however, of black images resulting from the use of carbon black, the fused toner image is preferably opaque. That is, none of the light entering the black image area passes through the toner. Suitable dry powder toners may be prepared by combining from about 64% by weight to about 98% by weight binding agent with about 1% by weight to about 20% by weight charge carrier agent and with about 1% by weight to about 16% by weight colorant; preferably combining from about 76% by weight to about 92% by weight binding agent with about 2% by weight to about 12% by weight charge carrier agent and with about 6% by weight to about 12% by weight colorant; and most preferably combining about 88% by weight binding agent with about 4% by weight charge carrier agent and about 8% by weight colorant. The binding agent, charge carrier agent and colorant may be mechanically mixed (and the binding agent as well as the charge carrier melted) using a twin screw extruder such as a variable speed twin screw extruder, for example a Baker Perkins gear drive model having a Haake rheocord torque rheometer. Preferably, the twin screw extruder generates a temperature of approximately 150°C to approximately 225°C during extrusion. The extruded product may be hammer milled and then jet milled to generate a mixture having particle sizes ranging from about 5 to about 100 micrometers, preferably from about 5 to about 50 micrometers and most preferably from about 5 to about 20 micrometers. A suitable jet mill is a NPA Supersonic Jetmill model PJM IDS-2 available from the Nippon Pneumatic Manufacturing Company. The resulting material may be used in the toner hopper of a laser type printer. Suitable surfaces of sign components to be printed may be made from materials including polymers selected from the group consisting of polyalkylacrylates, polyalkylmethacrylates, polyesters, vinyl polymers, polyurethanes, cellulose esters, fluoropolymers, polycarbonates, polyolefins, ionomeric copolymers and copolymers of ethylene or propylene with acrylic acid, methacrylic acid, or vinyl acetate. Suitable retroreflective sheetings include SCOTCH-LITE brand HIGH INTENSITY retroreflective sheeting and REFLECTO-LITE brand retroreflective sheeting. The surface layers may be made of polyalkylacrylates or polyalkylmethacrylates (especially polymethyl methacrylate (PMMA)), polyesters, vinyl polymers and polyvinyl acetals such as, for example, polyvinyl butryals. The SCOTCH-LITE brand and REFLECTO-LITE brand retroreflective sheetings are available from the Minnesota Mining and Manufacturing Company of St. Paul, Minnesota. EXAMPLE A mixture was prepared of 88% Acryloid B-66 binder resin (Rohm & Haas Company), 4% TRIBLOX PC-100 charge carrier (DuPont Company) and 8% carbon black (Regal 500R from Cabot Corporation). The components were mixed in a Baker Perkins gear drive variable speed twin screw extruder with a Haake rheocord torque rheometer and extruded as a mixture at a temperature range between 150'C to 225'C. The extruded mixture was hammer milled, and subsequently jet milled in a NPK supersonic jetmill model PJM IDS-2 from Nippon Pneumatic Manufacturing Company. The jet milled sample was then classified to collect material having a particle size range from 5 to 20 micrometers. This material was used as the toner. An embossed aluminum license plate blank including a REFLECTO-LITE retroreflective sheeting layer was printed with this dry toner powder as follows: a portion of the dry toner powder was spread on a surface and a hand-held hard rubber roller was rolled over the toner to load the surface of the roller with toner. Next the roller was rolled over the raised portions of the embossed plate which had been slightly warmed. The powder transferred to the raised portions, adhering to the raised portions of the polyvinyl butyral surface. Next, the plate was heated to fuse the powder. The black printed image was clearly visible on the lighter colored retroreflective sheeting and corresponded to the raised image. The combination of the reflective sheeting surfaces listed above with the solid toners described is unique, as is the use of an electrophotographic printing process to make a sign such as a license plate. The toners described are solid and capable of working in the electrophotographic process, adhering well to the reflective sheeting surface. The inventive process decreases air pollution from solvent vapors which are released from liquid inks and eliminates the investment in drying ovens and solvent recovery equipment that are associated with liquid ink processes.

Claims: A method of printing an image on a reflective sign comprising the steps of: A. providing a sign component selected from the group consisting of retroreflective sheeting and transparent film, said sign component having a polymeric surface; B. providing an embossed pattern in said sign component, the pattern providing a definition of the image to be printed; C. applying, to the raised portions of the embossed pattern, dry toner powder comprising: (1) colorant; and (2) binding agent which adheres to said polymeric surface and which comprises a polymer; and D. fusing the applied dry toner powder to form a fixed image borne upon the embossed pattern. The method of claim 1, wherein the polymeric surface comprises a polymer selected from the group consisting of polyalkylacrylates, polyalkylmethacrylates, polyesters, vinyl polymers, polyurethanes, cellulose esters, fluoropolymers, polycarbonates, polyolefins, polyvinyl acetals, ionomeric copolymers, and copolymers of ethylene or propylene with acrylic acid, methacrylic acid, or vinyl acetate. The method of claim 1 or 2, wherein the binding agent adheres to the polymeric surface, has a glass transition temperature of from about -15° to 150°C, and comprises a polymer selected from the group consisting of polyesters, vinyls, polyolefins, polyvinyl acetals, alkyl substituted acrylate polymers and alkyl substituted methacrylate polymers, wherein the alkyl substituents have from 1 to 9 carbon atoms.

Inventors: MAKO VINCENT JOSEPH, RAJAN J SUNDAR, MAKO, VINCENT JOSEPH, RAJAN, J. SUNDAR
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1991
Main classification: G09F13
IPC codes: G03G8, G09F13, B60R13, G03G7, G02B5, G03G21, B41M1, E01F9, G09F7, G03G15