Patent Publication Number: US-8536087-B2

Title: Thermographic imaging element

Description:
REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority based upon applicants&#39; provisional application U.S. Ser. No. 61/342,011, filed on Apr. 8, 2010. 
    
    
     FIELD OF THE INVENTION 
     A thermographic substrate comprised of a thermosensitive layer, wherein said thermosensitive layer is comprised of a binder, a multiplicity of hollow sphere organic pigments, and a thermal solvent. 
     BACKGROUND OF THE INVENTION 
     Direct thermal imaging is widely used for printing variable information; for example, this imaging method is commonly used to print facsimiles, receipts, shipping address labels, barcodes and prescription labels. Direct thermal imaging or printing is accomplished by directing heat to specific regions of thermosensitive coated substrate resulting in a change in the color of the region which was heated. Imagewise heating of the thermosensitive substrate is accomplished using a thermal printer such as, for example, the printers provided by Zebra Corporation of 475 Half Day Road, Suite 500, Lincolnshire, Ill. 60069. Such printers contain thermal printheads comprised of linear arrays of individually addressable heating elements, typically containing 60 to 236 such heating elements per linear centimeter of printhead. The thermal printhead is placed in intimate contact with the thermosensitive substrate. As the substrate is caused to move beneath the printhead, the individual heating elements are caused to heat in an imagewise pattern, imaging one complete line across the thermosensitive substrate at a time. Typical printing speeds range from 2.5 centimeters per second cm/s to 30 centimeters per second. 
     Direct thermal imaging has been widely accepted as a fast and efficient digital printing method. However, this printing technology has a significant weakness, i.e., the stability of the printed image to fading from exposure to sunlight. Thus, e.g., U.S. Pat. No. 6,034,704 of Stewart discloses that thermally activated substrates produce images which can be expected to fade. Labels, facsimiles and receipts printed on direct thermal sensitive substrates will fade quickly if they are not stored in a dark environment. Many labeling applications require the printing of variable information onto substrates for outdoors usage and consequently require good resistance to fading induced by exposure to sunlight. 
     It is an object of this invention to provide a thermographic substrate assembly that affords good resistance to fading induced by exposure to sunlight. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, there is provided a thermographic substrate assembly comprised of a colorant and a flexible substrate, wherein said thermographic substrate assembly is further comprised of a thermosensitive layer, and wherein said thermosensitive layer is comprised of a binder, a multiplicity of hollow sphere organic pigments, and a thermal solvent. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 through 5  are schematics of preferred thermographic substrate, and 
         FIG. 6  is a schematic representation of a thermally printed image used to assess the impact of the thermographic substrate on the long term printing performance of the thermal printer, and 
         FIG. 7  is a schematic representation of process used to prepare a thermographic substrate. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In one embodiment of the invention, there are disclosed thermographic materials and, in particular, direct thermal imaging substrates capable of developing sufficient visual contrast such that human and machine readable images may be printed by direct heating of said substrates with a thermal printhead and have sufficient resistance to image fading that they are suitable for outdoors applications. 
     In one embodiment, hollow sphere polymer particles are used in the thermographic imaging element of the invention. 
     Innovative ways to whiten coated layers using microvoids have evolved to overcome the limitation of blushing lacquers. For example, U.S. Pat. No. 4,427,836 of Kowalski, the entire disclosure of which is hereby incorporated by reference into this specification, describes multiple-stage core-sheath polymer dispersions comprised of microvoids. These hollow spheres polymer particles have certain advantages as opacifying agents in aqueous coating solutions either as a supplement to, or replacement of, conventional inorganic pigments. However, these polymer particles have poor solvent resistance, limiting their use almost exclusively to aqueous systems. Marketing literature on hollow organic pigments from Rohm and Haas warns to avoid using solvents and plasticizers with solubility parameters similar to the hollow sphere organic pigments in coating compositions. Such solvents can soften the polymer shell of the pigment, causing collapse of the spheres during film formation. See, e.g., (http://www.rohmhaas.com/assets/attachments/business/architectural_and_functional_coatings/ropaque_opaque_pigment/ropaque_ultra-e/tds/ropaque_ultra-e.pdf). 
     Claim 12 of Kowalski describes: “12. A composition comprising an aqueous dispersion of water-insoluble core/sheath polymer particles having an average diameter of 0.07 to 4.5 microns, having a core polymerized from a monomer system comprising one or more monoethylenically unsaturated monomers having a group of the formula —HC═C&lt;, at least one of said unsaturated monomers having a carboxylic acid group, and having at least one sheath polymerized from at least one different monomer system, at least one of said different monomer systems being hard and producing a polymer (a) having a T.sub.i greater than 50.degree. CELSIUS, (b) being non film-forming at 20.degrees Celsius, (c) being permeable to ammonia and amines, said core being swollen by neutralization with ammonia or amine in the presence of water, said particles having a property such that when subsequently dried a single cavity forms in said core and said particles cause opacity in compositions in which they are contained.” The core/sheath polymer particles described and claimed in Kowalski may be used in the thermographic element of this invention. 
     Claim 15 of U.S. Pat. No. 4,427,836 describes the core/sheath polymer particles as being “ . . . dried core/sheath particles having a single cavity in said core and an average particle size of about 0.07 to 4.5 micron and having a core polymerized from a monomer system comprising one or more monoethylenically unsaturated monomers having a group of the formula —HC═C&lt;, at least one of said unsaturated monomers having a carboxylic acid group, and having at least one sheath polymerized from at least one different monomer system, at least one of said different monomer systems being hard and producing a polymer (a) having a T.sub.i greater than 50 degrees Celsius, (b) being non film-forming at 20 degree Celsius, (c) being permeable to ammonia and amines, said core having been swollen by neutralization with ammonia or amine in the presence of water, and subsequently dried.” 
     Claim 17 of U.S. Pat. No. 4,427,836 describes the core/sheath product as being present “ . . . at a pigment volume concentration of 5 to 50 percent or higher . . . .” 
     Claim 20 of U.S. Pat. No. 4,427,836 describes the core/sheath product as being, prior to swelling, “ . . . essentially impermeable at 20 degrees Celsius to fixed or permanent bases including sodium, potassium, calcium and magnesium hydroxide.” 
     Additional disclosure regarding the hollow sphere particles is presented elsewhere in this specification. 
     In one embodiment of the invention, there are provided thermographic materials and, in particular, direct thermal imaging substrates capable of developing sufficient visual contrast such that human and machine readable images may be printed by direct heating of said substrates with a thermal printhead and have sufficient resistance to image fading that they are suitable for outdoors applications. Such visual contrast preferably is thermally developed by altering the light scattering capability of the layers comprising the direct thermal imaging substrate such that layers of low color saturation may become higher in color saturation or white opaque topcoatings become transparent, revealing underlying colored layers. Optionally and additionally, underlying layers of low color saturation may become higher in color saturation through the application of heat to said substrates, improving contrast of said images further. 
     The thermographic materials of this embodiment are preferably comprised of thermo-sensitive layers applied to thin, flexible substrates suitable for a variety of digital thermal printing applications such as receipts, tickets, labels, tags, bar codes and the like. Said thermo-sensitive layers are comprised of hollow organic pigments, thermal solvents, binders and optionally colorants. The thermal solvent is a solid or liquid substance with solubility characteristics similar to those of the hollow microsphere polymeric pigment. Upon application of heat to said thermosensitive layers, the combination of heat and thermal solvent is able to soften or dissolve the wall of the hollow organic pigment, leading to the collapse of the void contained in the hollow microsphere and resulting in a decrease in opacity of the thermosensitive layer. 
     A further advantage of this embodiment is that it does not rely on leuco dye based thermographic chemistries which are prone to light fade. Additionally, it does not rely upon iron based thermographic chemistries which tend to gain background density upon exposure to light. This preferred embodiment is able to utilize conventional color pigments, either in an underlying substrate or as a part of a thermosensitive layer. Numerous colored pigments are known to those skilled in the art to be resistant to fading from exposure to light and in particular to sun light. This inherent advantage of the instant invention enables direct thermal printable substrates to be prepared which are suitable for outdoor applications. 
     In this embodiment, white opaque thermosensitive layers comprised of hollow particle plastic pigments and thermal solvents may be coated over colored substrates. Heat from a thermal printhead renders the white opaque layer sufficiently transparent that the underlying colored substrate is revealed and sufficient visual contrast is developed between the heated and unheated portions of the substrate such that human and machine readable images can be read. 
     In this embodiment, the thermosensitive layers may additionally comprise pigments capable of absorbing light. Such thermal sensitive layers are opaque and low in color saturation when initially applied to a flexible substrate. However, upon heating with a thermal printhead such layers will become more transparent, enabling the colored pigment to impart color saturation to the layer. Such pigmented thermosensitive layers develop sufficient visual contrast between the heated and unheated portions of the substrate such that human and machine readable images can be read. 
     In a preferred aspect of this embodiment, a flexible substrate is first coated with a pigmented thermally sensitive layer, and then the pigmented layer is overcoated with a white opaque thermally sensitive layer. In this embodiment, heat from the thermal printhead tranparentizes both layers, allowing the underlying pigmented layer&#39;s increased color saturation to be clearly visible. Because the underlying pigmented thermo-sensitive layer is initially low in color saturation, a thinner white opaque thermal sensitive overcoat is required to produce a thermosensitive substrate with low background density or color saturation. A thermosensitive substrate with low background color saturation capable of developing marks of high color saturation with selective application of heat from a thermal printhead has the advantage of being high in visual contrast, making it very suitable for human and machine readable applications. 
     DEFINITIONS OF CERTAIN TERMS 
     In this specification, and in the claims, applicants have used certain terms that are described in the specifications and claims of prior United States patents. These terms are used in the same manner as they have been used in the prior patent(s); and they have the same meaning. 
     Thus, e.g., U.S. Pat. No. 7,182,532, the entire disclosure of which is hereby incorporated by reference into this specification, uses the following terms in its claims: “Knoop hardness” (claim 1), “Sheffield smoothness” (claim 6), “flexible support” (claim 9), “flexible polymeric support” (claim 10), “ . . . poly(ethylene terephthalate), polypropylene, polyolefins, cellophane, polycarbonate, cellulose acetate, polyethylene, polyvinyl chloride, polystyrene, polyimide, polyvinylidene chloride, polyvinyl alcohol, fluororesin, chlorinated resin, ionomer, and mixtures thereof . . . (claim 11), “flexible paper” (claim 12), and “synthetic paper” (claim 36). 
     Thus, e.g., U.S. Pat. No. 7,507,453, the entire disclosure of which is hereby incorporated by reference into this specification, uses the following terms in its claims: “ink layer” (claim 1), “heat activatable layer” (claim 1), “elongation to break” (claim 1), “peel force” (claim 1), “solid carbonaceous binder” (claim 5), “coating weight” (claim 6), “softening point” (claim 7), “synthetic resin” (claim 9), “wax” (claim 14), “colorant” (claim 21), and “opacifier” (claim 24). 
     DETAILED DESCRIPTION OF SOME OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic representation of a thermographic substrate  10  made in accordance with one preferred process of this invention; this Figure is not necessarily drawn to scale. 
     The term “substrate” used in this specification refers to a flexible material or support that is coated with one or more layers of thermosensitive compositions. By way of illustration and not limitation, one may use one or more of the substrates described in U.S. Pat. Nos. 7,182,532, 6,694,885 and 7,507,453, and 5,665,670. The entire disclosure of each of these patents is hereby incorporated by reference into this specification. 
     In the preferred embodiment depicted in  FIG. 1 , the substrate  101  is comprised of a synthetic paper substrate. In one aspect of this embodiment, said substrate  101  is optionally comprised of clay or calcium carbonate treated synthetic papers. These synthetic papers are well known to those skilled in the art. Thus, by way of illustration and not limitation, one may use one or more of the synthetic papers sold by the Hop Industries Corporation of 174 Passaic Street, Garfield, N.J. Thus, e.g., one may use HOP 5.9 microns synthetic paper. Thus, e.g., one may use “HOP-SYN Synthetic Paper,” DLI grade; this paper is clay modified polypropylene, and is a calendared plastic sheet made from a mixture of clay, calcium carbonate and polypropylene resin. Thus, e.g., reference may also be had to U.S. Pat. No. 7,858,161, the entire disclosure of which is hereby incorporated by reference into this specification; claim 8 of this patent refers to, e.g., “paper” and “synthetic paper.” 
     By way of further illustration, one may use one or more of the synthetic papers available (as oriented polypropylene and polyethylene based synthetic papers) as “Yupo synthetic paper” from Oji-Yuka Synthetic Paper Co. of Tokyo, Japan. One may use the “Polyart synthetic paper” obtainable from Arjobex of Paris, France. One may use the “Kimdura synthetic paper” sold by Neenah Paper Corporation of Neenah, Wis. These and other synthetic papers are well known and are disclosed, e.g., in U.S. Pat. Nos. 5,474,966, 6,086,987, and 5,108,834 and in United States published patent application 2003/0089450; the entire disclosure of each of these patent documents is hereby incorporated by reference into this specification. 
     In one preferred embodiment, the substrate  101  either consists essentially of or comprises at least 80 weight percent of a synthetic polymeric material such as, e.g., polyethylene, polyester, nylon, polypropylene, polycarbonate, polyethylene-co-propylene, and the like. 
     In one preferred embodiment, the substrate  101  comprises at least about 90 weight percent of polyethylene or polypropylene or polybutylene, polyvinyl chloride, polyethylene terephthalate, polycarbonate, and mixtures thereof. 
     The substrate  101  preferably has a thickness  106  of from about 25 microns to about 250 microns. It is preferred that the thickness of said support not vary across the substrate  101  by more than about 15 percent. 
     In one embodiment, the support does soften when exposed to solvent(s) or water. 
     In one embodiment, the substrate  101  preferably comprises at least about 80 weight percent of, or consists essentially of, a cellulosic material such as, e.g., paper. 
     When paper is used as substrate  101 , it preferably has a weight per unit area of at least about 50 to about 200 grams per square meter. In one embodiment, the basis weight of the paper is from about 45 to about 65 grams per square meter. 
     In one embodiment, the substrate  101  is a 90 grams per square meter basis paper made from bleached softwood and hardwood fibers. In one aspect of this embodiment, the surface of this paper is sized with starch. 
     In one embodiment, the substrate  101  has a Sheffield smoothness of from about 1 to about 150 Sheffield Units and, more preferably, from about 1 to about 50 Sheffield Units. Means for determining Sheffield smoothness are well known. Reference may be had, e.g., to U.S. Pat. Nos. 5,451,559, 5,271,990, 5,716,900, 6,332,953, 5,985,424, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     In one embodiment the substrate  101  may be comprised of a layered composite of natural and synthetic papers. 
     In one embodiment, the substrate  101  has a surface energy greater than 30 dynes per centimeter. Surface energy, and means for measuring it, are well known to those skilled in the art. Reference may be had, e.g., to U.S. Pat. Nos. 5,121,636; 6,225,409; 6,221,444; 6,075,965; 6,007,918; 5,777,014; and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     In one embodiment, the substrate  101  has a surface energy of more than about 40 dynes per centimeters. 
     In one preferred embodiment, the substrate  101  either consists essentially of or comprises at least 80 weight percent of a synthetic polymeric resins such as, e.g., polyethylene, polyvinyl chloride polyester (such as polyethylene terephthalate), nylon, polyimide, polypropylene, polycarbonate, cellulose acetate, cellulose nitriate, polylactic acid and the like. Such synthetic substrates  101 , comprised of thermoplastics, may preferably be extruded and biaxially oriented to form a film of uniform thickness and high surface smoothness. Multilayer substrates  101  comprised of thermoplastics may be coextruded together such that composite film substrates are prepared. Such multi-layer substrates may differ in composition from core to skin. In one preferred embodiment, the core of a multilayer substrate  101  is microvoided while the surface skins are unvoided. 
     U.S. Pat. No. 5,604,079 of Campbell describes various microvoided substrates in U.S. Pat. No. 5,604,078; the entire disclosure of this patent is hereby incorporated by reference into this specification. As is disclosed in this patent, “Microvoided composite packaging films are conveniently manufactured by coextrusion of the core and surface layers, with subsequent biaxial orientation, whereby voids are formed around void-initiating material contained in the core layer. Such composite films are disclosed in, for example, U.S. Pat. No. 4,377,616 . . . . The core of the composite film should be from 15 to 95% of the total thickness of the film, preferably from 30 to 85% of the total thickness. The nonvoided skin(s) should thus be from 5 to 85% of the film, preferably from 15 to 70% of the thickness. The density (specific gravity) of the composite film should be between 0.2 and 1.0 g/cm.sup.3, preferably between 0.3 and 0.7 g/cm.sup.3. As the core thickness becomes less than 30% or as the specific gravity is increased above 0.7 g/cm.sup.3, the composite film starts to lose useful compressibility and thermal insulating properties. As the core thickness is increased above 85% or as the specific gravity becomes less than 0.3 g/cm.sup.3, the composite film becomes less manufacturable due to a drop in tensile strength and it becomes more susceptible to physical damage. The total thickness of the composite film can range from 20 to 150 .mu.m, preferably from 30 to 70 .mu.m. Below 30 .mu.m, the microvoided films may not be thick enough to minimize any inherent non-planarity in the support and would be more difficult to manufacture. At thicknesses higher than 70 .mu.m, little improvement in either print uniformity or thermal efficiency are seen, and so there is little justification for the further increase in cost for extra materials.” 
     U.S. Pat. No. 4,377,616 discloses that: “Void” is used herein to mean devoid of added solid and liquid matter, although it is likely the “voids” contain gas. The void-initiating particles which remain in the finished packaging film core should be from 0.1 to 10 .mu.m in diameter, preferably round in shape, to produce voids of the desired shape and size. The size of the void is also dependent on the degree of orientation in the machine and transverse directions. Ideally, the void would assume a shape which is defined by two opposed and edge contacting concave disks. In other words, the voids tend to have a lens-like or biconvex shape. The voids are oriented so that the two major dimensions are aligned with the machine and transverse directions of the film. The Z-direction axis is a minor dimension and is roughly the size of the cross diameter of the voiding particle. The voids generally tend to be closed cells, and thus there is virtually no path open from one side of the voided core to the other side through which gas or liquid can traverse.” 
     Referring again to Figure, and in another embodiment, such substrate(s)  101  may be deposited from solvents onto a smooth drum and dried. 
     In a preferred embodiment the substrate  101  is comprised of a renewable material such as cellulose, cellulose derivative(s), polylactic acid, and the like. 
     As the support usable in the present invention, there are preferred polyester films such as polyethylene terephthalate film, polybutylene terephthalate film, polyethylene naphthalate film, polybutylene naphthalate film, polyarylate film, polycarbonate film, polyamide film, aramid film, polyether sulfone film, polysulfone film, polyphenylene sulfide film, polyether ether ketone film, polyether imide film, modified polyphenylene ether film and polyacetal film, and other various plastic films commonly used for the support of recording media of this type. 
     In one preferred embodiment, the substrate  101  either consists essentially of or comprises at least 80 weight percent of a synthetic polymeric resin such as, e.g., polyethylene, polyester, nylon, polypropylene, polycarbonate, cellulose acetate, cellulose nitriate, polylactic acid and the like. 
     In a preferred embodiment, the uncoated substrate  101  (see  FIG. 1 ) has a surface energy greater than 35 dynes per centimeter and a smoothness of from about 10 to about 150 Sheffield Units. 
     Referring again to  FIG. 1 , in one embodiment the substrate  101  is paper that is preferably coated with and contiguous with a surface layer  200  comprised of resin. Thus, e.g., the paper may be extrusion coated with a resin at a coat weight of 10 to 40 grams per square meter. In this embodiment, the resin comprises a polyolefin, such as, e.g., polyethylene, polypropylene, polybutylene, and mixtures thereof. Said resin may be coated means of extrusion, at a temperature of from about 200 to about 300 degrees Celsius. Extrusion coating of a resin is well known. Reference may be had, e.g., to U.S. Pat. Nos. 5,104,722, 4,481,352, 4,389,445, 5,093,306, 5,895,542, and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     In a preferred embodiment, substrate  101  is white in color. In another preferred embodiment, substrate  101  is colored and is comprised of colorant. The term colorant is defined elsewhere in this specification. 
     Surface layer  200  may be comprised of various pigments, clays and latex polymers such as those described in U.S. Pat. No. 4,521,494, the entire disclosure of which is hereby incorporated by reference. 
     In one embodiment the surface layer  200  has a surface energy greater than 30 dynes per centimeter. 
     In one embodiment, the surface layer  200  preferably comprises a material that, when coated upon the substrate  101 , provides a smooth surface with a surface energy of at least 35 dynes per centimeter. Said surface layer  200  may optionally be treated with by flame, plasma or corona to raise said surface energy to at least 40 dynes per cm. 
     In one embodiment, the preferred surface layer coating  200  is substantially smooth. In one aspect of this embodiment, the coated substrate has a Sheffield smoothness of from about 1 to about 150 Sheffield Units and, more preferably, from about 1 to about 50 Sheffield units. 
     Referring again to  FIG. 1 , and in the preferred embodiment depicted therein, the surface layer  200  may be of any composition that will produce the desired surface energy and smoothness upon coating the substrate  101 . Thus, by way of illustration and not limitation, one may utilize cured or uncured polyurethane, polyvinyl chloride, polyacrylate, polyamide, polyester, polyvinyl alcohol, polyimide, polyurea, titinate, ziconate, silane and combinations thereof. 
     In one embodiment, and referring again to  FIG. 1 , surface layer coating  200  is comprised of voids. Those skilled in the art are well aware of various means for producing a surface layer coating that is comprised of voids. Such voids may be formed through a variety of means. For example, in U.S. Pat. No. 6,402,865, the entire disclosure of which is hereby incorporated by reference into this specification, Handa describes the use of blowing agents to create holes or voids in a layer structure, stating “The above layered morphology development process in a polymer containing dissolved blowing agent is similar to the microcellular foaming process. See, for example, U.S. Pat. No. 4,473,665 issued on Sep. 25, 1984, assigned to J. E. Martini-Vvedensky et al.; U.S. Pat. No. 5,223,545, issued on Jun. 29, 1993, assigned to V. Kumar; U.S. Pat. No. 5,670,102 issued on Sep. 23, 1997, assigned to C. A. Perman et al. However, as mentioned earlier, the layered structure is produced when the temperature of the polymer-blowing agent system is below its T.sub.g whereas in microcellular or conventional foaming, the processing temperature is above the system&#39;s T.sub.g. If the process described here is carried out at a temperature above the system&#39;s T.sub.g, a cellular structure develops in the material and the desired layered morphology can not be achieved.” 
     Voids may also be created within a layer by incorporation of incompatible particles within said layer and then orienting or stretching said layer, see for example U.S. Pat. Nos. 6,958,860, 5,494,735, 6,596,451, 5,462,788, 5,935,904 and 7,762,188. 
     Voids may also be created within a layer by incorporation of expandable particles. Thus, e.g., in U.S. Pat. No. 5,834,526, Wu discloses the use of said expandable particles in various packaging materials, stating that: “Thermoplastic hollow expandable particles having volatile liquid blowing agents encapsulated therein are described in U.S. Pat. No. 3,615,972 to Morehouse, et al. The blowing agents are described as aliphatic hydrocarbons, chlorofluorocarbons, or tetraalkyl silanes. The particles are said to be prepared by combining an oil phase containing monomer and blowing agent with an aqueous phase, and agitating violently. Use of perfluorinated blowing agents or ways to improve encapsulation of such blowing agents are not described. U.S. Pat. No. 4,108,806 to Cohrs, et al., teaches that the expandable synthetic resinous microspheres of U.S. Pat. No. 3,615,972 can be mixed with certain resins and extruded, thereby incorporating the microspheres in the resin and then the microspheres expanded. The only microspheres exemplified are ones made of a copolymer shell of styrene and acrylonitrile having isobutane encapsulated therein. U.S. Pat. No. 5,429,869 to McGregor, et al., describes use of the microspheres of U.S. Pat. No. 3,615,972 in a process for expanding polytetrafluoroethylene. PCT Publication No. WO 93/00390 to 3M Corporation describes composite articles of a fibrillated polyolefin matrix and energy expandable hollow polymeric particles, which upon expansion of the particles provides thermal insulation. This publication teaches that the expandable microspheres are made of a shell of vinyl or vinylidene halides or copolymers of styrene or methylmethacrylate. The blowing agents are described as those disclosed in U.S. Pat. No. 3,615,972 and U.S. Pat. No. 4,483,889. Use of perfluorobutanes, perfluoropentenes and perfluorohexanes is described, but no means of incorporating such compounds inside the microspheres is taught.” 
     Referring again to  FIG. 1 , Voids may also be incorporated into surface layer  200  by the addition of hollow sphere organic pigments which are described elsewhere in this specification. 
     Surface layer  200  preferably has a thickness  206  from about 0.01 micron to about 50 microns. 
     In one preferred embodiment, surface layer  200  has a thickness  206  from about 10 microns to about 30 microns. 
     In another preferred embodiment, surface layer  200  has a thickness  206  from about 0.05 microns to about 5 microns. 
     In another preferred embodiment, surface layer  200  contains voids. Such voids, in one aspect of this embodiment, have an average diameter from 0.1 micron to about 1.5 micron. 
     Referring again to  FIG. 1 , the thermographic substrate  10  contains an optional color layer  300 . This optional color layer  300  preferably comprises one or more thermoplastic binder materials 301 in a concentration of from about 5 to about 95 percent, based upon the dry weight of colorant  302  and binder  301  in such color layer  300 . In one embodiment, the binder  301  is present in a concentration of from about 66 to about 95 percent. In another embodiment, the color layer  300  comprises from about 75 to about 90 weight percent of binder  301 . 
     One may use any of the polymeric binders  301  known to those skilled in the art. Thus, e.g., one may use one or more of the binders disclosed in U.S. Pat. Nos. 6,127,316, 6,124,239, 6,114,088, 6,113,725; 6,083,610, 6,031,556, 6,031,021, 6,013,409, 6,008,157, 5,985,076, and the like. The entire, disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     By way of further illustration, one may use a binder  301  which preferably has a multiplicity of polar moieties such as, e.g., one or more carboxyl groups, hydroxyl groups, chloride groups, carboxylic acid groups, urethane groups, amide groups, amine groups, urea, epoxy resins, mixtures thereof, and the like. Some suitable binders within this class include polyester resins, bisphenol-A polyesters, polyvinyl chloride, copolymers made from terephthalic acid, polymethyl methacrylate, vinylchloride/vinylacetate resins, epoxy resins, nylon resins, urethane-formaldehyde resins, polyurethane, mixtures thereof, and the like. 
     In one embodiment, the binder  301  is comprised of a mixture of two or more synthetic resins. Thus, e.g., one may use a mixture comprising from about 40 to about 60 weight percent of polymethyl methacrylate and from about 40 to about 60 weight percent of vinylchloride/vinylacetate resin. In this embodiment, these materials collectively comprise the binder. 
     In one embodiment, the binder  301  comprises polybutylmethacrylate and polymethylmethacrylate, comprising from 10 to 30 percent of polybutylmethacrylate and from 50 to 80 percent of the polymethyl methacrylate. In one embodiment, this binder comprises cellulose acetate propionate, ethylenevinylacetate, vinyl chloride/vinyl acetate, urethanes, etc. 
     One may obtain these binders  301  from many different commercial sources. Thus, e.g., some of them may be purchased from Dianal America Company of 9675 Bayport Blvd., Pasadena, Tex. 77507; suitable binders available from this source include “Dianal BR 113” and “Dianal BR 106.” Similarly, suitable binders may also be obtained from the Eastman Chemicals Company (Tennessee Eastman Division, Box 511, Kingsport, Tenn.). 
     Referring again to  FIG. 1 , optional color layer  300  is preferably comprised of colorant  302 . Said colorant  302  may comprise one or more dyes or pigments. 
     In one preferred embodiment, one or more of such colorants are light stable pigments that may be organic or inorganic. Herbst and Hunger in their book “Industrial Organic Pigments”, classify colorants as either dyes or pigments. Pigments are defined as “inorganic or organic, colored, white or black materials which are practically insoluble in the medium in which they are incorporated”. While most inorganic pigments are stable against fading due to exposure to sunlight, their colors are often muted and many hues can&#39;t be produced. Organic pigments on the other hand are capable of creating a large number of hues but their resistance to fade is largely dependent upon their chemical structure and crystal form. 
     In one embodiment, the colorants  302  are various organic and inorganic pigments as well as carbon black. Examples of such organic and inorganic pigments include azo pigments (such as monoazo yellow and orange, disazo, beta-napthol, napthol AS reds, azo lake, benzimidazolone, disazo condensation, metal complex azo, isoindolinone, isoindoline), polycyclic pigments (such as quinacridone, perylene, perinone, diketopyrrolo pyrrole and thioindigo), anthraquinone pigments (such as anthrapyrimidine, flavanthrone, pyranthrone, anthanthrone, dioxazine, triarylcarbonium and quinophthalone), phthalocyanine pigments, nitro pigments, nitroso pigments, nigrosine pigments, titanium white, calcium carbonate and barium sulfate. Such pigments may be used in combination with dyes for adjusting the color of the ink layer. The content of the coloring agent in the ink layer is preferably from about 1 percent to about 50 percent more preferably from about 5 percent to about 33 percent. 
     The colorant  302  may take the form of a soluble dye dissolved in the binder  301  color layer  300 . Preferably, the colorant  302  is comprised of a pigment dispersed in the binder  301  of color layer  300 . Additionally, any additive known to those skilled in the art, eg, dispersants, rheology modifiers, defoamers, surfactants, wetting agents, etc, may also be included as needed. 
     In a preferred embodiment, the coating fluid used to prepare color layer  300  is comprised of a surfactant. Surfactants are defined as “surface active agents” in Webster&#39;s Third International Dictionary (Unabridged). The use of surfactants in coating compositions is disclosed by Orem in U.S. Pat. No. 4,370,412, the entire disclosure of which is hereby incorporated by reference into this specification. Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or the interfacial tension between a liquid and a solid. Surfactants are used as detergents, wetting agents, emulsifiers, foaming agents, and dispersants. Surfactants are usually organic compounds that are amphiphilic, meaning they contain both hydrophobic groups (their tails) and hydrophilic groups (their heads). Therefore, a surfactant molecule contains both a water insoluble (oil soluble component) and a water soluble component. Surfactants reduce the surface tension of water by adsorbing at the liquid-gas interface. In addition, they reduce the interfacial tension between oil and water by adsorbing at the liquid-liquid interface. Surfactants are also often classified into four primary groups; anionic, cationic, non-ionic, and zwitterionic (dual charge). 
     For this application, the surfactant is used as a wetting agent. Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. Many technological processes require control of liquid spreading over solid surfaces. By reducing the surface tension of a liquid with surfactants, a non-wetting material can be made to become partially or completely wetting. 
     Adhesive forces between a liquid and solid cause a liquid drop to spread across the surface. Cohesive forces within the liquid cause the drop to ball up and avoid contact with the surface. The contact angle is the angle at which the liquid-vapor interface meets the solid-liquid interface, which is determined by the resultant adhesive and cohesive forces. The tendency of a drop to spread out over a flat, solid surface increases as the contact angle decreases, so the contact angle provides an inverse measure of wettability. A contact angle less than 90° (low contact angle) usually indicates that wetting of the surface is very favorable, and the fluid will spread over a large area of the surface. Contact angles greater than 90° (high contact angle) generally means that wetting of the surface is unfavorable so the fluid will minimize contact with the surface and form a compact liquid droplet. 
     Surfactants are absorbed onto the liquid-vapor, solid-liquid, and solid-vapor interfaces, which modify the wetting behavior of hydrophobic materials to reduce the free energy. As the surfactants are absorbed, the solid-vapor surface tension increases and the edges of the drop become hydrophilic. As a result, the drop spreads. 
     In a preferred embodiment, the color layer  200  will be comprised of a defoamer. The use of defoamers in coated layers is disclosed by Merkel in U.S. Pat. No. 6,331,585, the entire disclosure of which is hereby incorporated by reference into this specification. The action of a defoamer is defined in Webster&#39;s Third International Dictionary (Unabridged) as “to remove the foam from”. Thus, a defoamer or an anti-foaming agent is a chemical additive that reduces and hinders the formation of foam in industrial process liquids. Generally a defoamer is insoluble in the foaming medium and has surface active properties. An essential feature of a defoamer is low viscosity and a facility to spread rapidly on foamy surfaces. Defoamers have an affinity to the air-liquid surface where they destabilizes foam, causing the rupture of air bubbles and breakdown of surface foam. Defoamers also help entrained air bubbles to agglomerate and form larger bubbles which, in turn, can rise to the surface of the bulk liquid more quickly. There are several different types of defoamers, for example oil based, powder, water based, silicone based and alkyl polyacrylates. 
     Defoamers are well known and widely described in the patent literature. Reference may be had, e.g., to U.S. Pat. No. 7,910,633 (see claim 1); the entire disclosure of this patent is hereby incorporated by reference into this specification. 
     In a preferred embodiment, color layer  300  may also comprise dispersants to facilitate the dispersion of pigment based colorants  302 . Dispersants are surface active compounds which help to separate pigment agglomerates into their primary particles in the dispersion process. See, for example, U.S. Pat. No. 4,522,654, the entire disclosure of which is hereby incorporated by reference into this specification. 
     In one preferred embodiment, colorant  302  is comprised of one or more transparent organic pigments such as, e.g., Naphthol Yellow S, Hansa Yellow 5G, Hansa Yellow 3G, Hansa Yellow G, Hansa Yellow GR, Hansa Yellow A, Hansa Yellow RN, Hansa Yellow R, Benzidine Yellow, Benzidine Yellow G, Benzidine Yellow GR, Permanent Yellow NCG and Quinoline Yellow Lake, Permanent Red 4R, Brilliant Fast Scarlet, Brilliant Carmine BS, Permanent Carmine FB, Lithol Red, Permanent Red F5R, Brilliant Carmine 6B, Pigment Scarlet 3B, Rhodamine Lake B, Rhodamine Lake Y and Arizalin Lake, Victoria Blue Lake, metal-free Phthalocyanine Blue, Phthalocyanine Blue and Fast Sky Blue, and dyes such as Rhodamine, Victoria Blue and carbon black. These coloring agents may be used either alone or in combination. 
     As used in this specification, the term “transparent pigment” refers to a pigment which gives a transparently colored ink when dispersed in binder  301  of color layer  300 . 
     In one embodiment, the colorant  302  is comprised of one or more inorganic pigments. Thus, for example, a blue colorant can contain the oxides of a cobalt, chromium, aluminum, copper, manganese, zinc, etc. Thus, e.g., a yellow colorant can contain the oxides of one or more of lead, antimony, zinc, titanium, vanadium, gold, and the like. Thus, e.g., a red colorant can contain the oxides of one or more of chromium, iron (two valence states), zinc, gold, cadmium, selenium, or copper. Thus, e.g., a black colorant can contain the oxides of the metals of copper, chromium, cobalt, iron (plus two valence), nickel, manganese, and the like. Furthermore, in general, one may use colorants comprised of the oxides of calcium, cadmium, zinc, aluminum, silicon, etc. 
     Suitable pigments and colorants are well known to those skilled in the art. See, e.g., U.S. Pat. Nos. 6,120,637, 6,108,456, 6,106,910, 6,103,389, 6,083,872, 6,077,594, 6,075,927, 6,057,028, 6,040,269, 6,040,267; 6,031,021, 6,004,718, 5,977,263, and the like. The disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Color layer  300  preferably has a thickness  306  from about 0.01 micron to about 25 microns. 
     In one preferred embodiment, color layer  200  has a thickness  306  from about 10 microns to about 20 microns. 
     In another preferred embodiment, color layer  200  has a thickness  306  from about 0.1 microns to about 5 microns. 
     Thermosensitive Layer  500   
     Referring again to  FIG. 1 , and in the preferred embodiment depicted therein, thermographic substrate  10  is comprised of a thermosensitive layer  500 . The Webster&#39;s Third New International Dictionary (Unabridged) defines thermosensitive as “relating to or being a material that is in one or more ways sensitive to heat”. 
     Thermosensitive layers are well known to those skilled in the art. Thus, e.g., U.S. Pat. No. 7,671,878, the entire disclosure of which is hereby incorporated by reference into this specification, claims: “1. A thermal printer comprising: a feeding mechanism which feeds one of thermal papers which include a double-sided thermal paper having thermosensitive layers formed on both sides thereof and a single-sided thermal paper having a thermosensitive layer formed on one side thereof; a first thermal head which is so provided as to be brought into contact with a first side of the thermal paper fed by the feeding mechanism and is configured to print an image on the first side of the paper; a second thermal head which is so provided as to be brought into contact with a second side of the thermal paper fed by the feeding mechanism and is configured to print an image on the second side of the paper; a mark determination section which is configured to determine whether a mark has been printed on at least one of the first and second sides of the thermal paper; and a controller which is configured to control print operation based on a determination result from the mark determination section, wherein the controller is configured to control double-sided printing by the first and second thermal heads in the case where the mark determination section has determined that a mark has been printed on both the first and second sides of the thermal paper, the controller is configured to control single-sided printing by the first thermal head in the case where the mark determination section has determined that a mark has been printed on the first side of the thermal paper and control single-sided printing by the second thermal head in the case where the mark determination section has determined that a mark has been printed on one of the second side of the thermal paper.” The thermosensitive layer described in U.S. Pat. No. 7,671,878 may be used in the process of this invention. 
     Thus, e.g., U.S. Pat. No. 7,071,145 of Morita, the entire disclosure of which is hereby incorporated by reference into this specification, claims: “1. A thermosensitive recording material comprising an under layer comprising hollow particles and a thermosensitive layer provided on a substrate in this order, wherein the thermosensitive layer comprises a leuco dye and a color developer, the color developer is an oligomer composition obtained from the reaction of a polyvalent isocyanate compound represented by following Formula (I) with an aromatic amine represented by following Formula (II) . . . in which X represents a tri- or more-valent group, a represents an integer numeral of 3 or more, b and c represent respectively integer numerals in the range of 0 to 5 and they satisfy a relation of b+c=1 to 5, Z represents hydrogen atom, alkyl group, allyl group or aryl group, and the aryl group may include condensed ring structure thereof, and d represents an integer numeral in the range of 0 to 4.” The thermosensitive layer described in U.S. Pat. No. 7,671,878 may be used in the process of this invention. 
     Thus, e.g., U.S. Pat. No. 5,919,729 of Mori, the entire disclosure of which is hereby incorporated by reference into this specification, claims: “1. A thermosensitive recording medium comprising a support, a thermosensitive layer provided on said support and capable of forming a color image when heated imagewise, and a protecting layer provided on said thermosensitive layer and containing a pigment and a core-shell resin obtained by polymerizing a vinyl monomer in an emulsion containing acrylonitrile-containing polymer seeds so that a polymer of said vinyl monomer is copolymerized on each of said seeds to form a shell surrounding a core of said seed, said vinyl monomer containing acrylamide and/or methacrylamide, wherein said core-shell resin meets with the following criteria (a) and (b): (a) the glass transition point Tg of said core is at least 20 degrees Celsius; and (b) the glass transition point Tg of said shell is at least 200 degrees Celsius” The thermosensitive layer described in U.S. Pat. No. 5,919,729 may be used in the process of this invention. 
     In one embodiment of applicants&#39; invention, two different thermosensitive layers are preferably used, one that goes from gray to black, and another that goes from white to clear. It is to be understood that, although applicants have disclosed particular preferred embodiments of each of these individual thermosensitive elements, other “prior art” individual thermosensitive elements also may be used. 
     Referring again to  FIG. 1 , and to the preferred embodiment depicted therein, thermosensitive layer  500  is coated over color layer  300 . Such compositions, when applied to thin, flexible substrates, have a variety of digital thermal printing applications such as, e.g., receipts, tickets, labels, tags, bar codes and the like. 
     Thermosensitive layer  500  enables the direct thermal imaging of substrates with several advantages: (1) no leuco dyes or phenolic compounds are required, (2) no expensive silver-based chemistry is required, (3) no difficult to control blushing lacquers are required, (4) layers may be coated from water, eliminating the need for expensive solvent coating vehicles, (5) a light-stable color dye or pigment may be used, offering a nearly limitless variety of imaged colors, (6) thermal imaging of the substrate is accomplished with a digital thermal printer, (7) images have excellent durability, being resistant to fade, abrasion, water, hand sanitizers, suntan lotion, etc. 
     In one preferred embodiment, the thermosensitive layer  500  is comprised of hollow organic pigments  402 , thermal solvents  404 , binders  501  and optional colorants  502 . Applicants have described particular embodiments of such hollow organic pigments, thermal solvents, binders, and colorants in this specification. It is to be understood, however, that other “prior art” embodiments of such materials also may be used. 
     Thermosensitive layer  500  is preferable coated from water, although such thermosensitive layer may be coated from other solvents or, alternatively, may be a 100 percent solid system that is in a liquid phase when coated and under the conditions of the coating. 
     In one embodiment, water-based compositions are prepared such that many of the components remain separated and not intimately mixed at the time of coating and drying. In one embodiment, thermosensitive layer  500  is comprised of dispersions of solids, emulsions of liquids, microcapsules, hollow sphere polymeric particles and the like. 
     In one preferred embodiment, thermosensitive layer  500  is comprised of a continuous phase binder  501  in which said dispersions, emulsions, microcapsules and hollow sphere polymer particles are dispersed. 
     Thermosensitive layer  500  preferably has a coating weight  506  of 0.5 to 20 grams per square meter, more preferably from 1 to 10 grams per square meter. 
     Hollow Organic Pigments 
     In one preferred embodiment, the thermographic substrate  10  is comprised of hollow organic pigments. Many of such pigments are described in the prior art; and such prior art hollow pigments may be used in the process of this invention. 
     Thus, e.g., U.S. Pat. No. 7,160,608 of Yanagisawa, the entire disclosure of which is hereby incorporated by reference into this specification, claims: “1. A coated paper comprised of a paper substrate, at least one surface of which is provided with at least one coating layer, characterized in that the surface of said coating layer has cracks of a width of 0.2 to 3.0 μm and a length of 3 to 1000 μm in an amount of 1 to 1000 cracks per mm 2 ; wherein said surface of said coating layer has a white paper glossiness of 45 to 85 percent at a light-incident arid receipt angle of 75 degrees in accordance with JIS S 8741 and an Oken type air permeability of not more than 8000 sec. in accordance with the JAPAN TAPPI Pulp and Paper Testing Method No. 5-2:000; said coating layer contains thermoplastic organic microparticles having a glass transition temperature of 20 to 150° Celsius; and, 100 parts by mass of said coating layer contains 40 to 90 parts by mass of an inorganic pigment and 5 to 60 parts by mass of said thermoplastic organic microparticles.” The organic microparticles of this patent may be used in the process of this invention. 
     Thus, e.g., U.S. Pat. No. 7,651,216 of Chen, the entire disclosure of which is hereby incorporated by reference into this specification, claims: “1. A fusible print medium, comprising: a substrate; an ink-receiving layer disposed on the substrate, wherein the ink-receiving layer includes a plurality of hollow beads having a diameter from about 0.3 to 5 μmeters, a void volume of about 20 percent to 70 percent, and a glass transition temperature above 50° Celsius, wherein the hollow beads have substantially the same diameter, and wherein the hollow beads are at least 70 percent of the ink receiving layer.” The hollow beads of this patent may be used in the process of this invention. 
     Thus, e.g., U.S. Pat. No. 7,651,747 of Chen claims: “1. A fusible print medium for use in inkjet printing, consisting of: a substrate; a fusible first ink-receiving layer; and, a second ink-receiving layer disposed between said first layer and said substrate, wherein said fusible first ink-receiving layer includes a mixture comprising: a first plurality of discrete particles comprising at least one hollow organic pigment; and a second plurality of discrete particles comprising at least one solid plasticizer having a melting point between about 40° Celsius and about 150° C. and below a glass transition temperature of the at least one hollow organic pigment; and wherein said medium is formulated to form a fused surface layer approximately 0.2 μm to approximately 10 μm thick on said second ink-receiving layer on said substrate . . . .” The hollow organic pigments of this patent may be used in the process of this invention. 
     Chen &#39;747 discloses hollow organic pigments and that the void volume of such pigments may range from 10 percent to 90 percent. Webster&#39;s Third International Dictionary (Unabridged) defines pigment as “a natural or synthetic inorganic or organic substance that imparts a color including black or white to other materials.” The hollow organic pigments of this patent may be used in the process of this invention. 
     U.S. Pat. No. 5,162,289 of Betts, the entire disclosure of which is hereby incorporated by reference into this specification, describes hollow microsphere polymer pigment  402 . Betts discloses that: “Plastic pigment particles, including hollow plastic pigment particles, are themselves well-known in the paper industry as constituents of coating compositions. Solid plastic pigments form the subject of Chapter 6 of Tappi Monograph No. 38 entitled “Paper Coating Pigments”, published 1976, and are also the subject of a sub-section on pages 2073 and 2074 of “Pulp &amp;Paper—Chemistry &amp;Chemical Technology” edited by James P. Casey, 3rd Edition, Volume IV, published in 1976 by John Wiley &amp;Sons. Examples of patents on plastic pigments and/or their use in paper coatings are British Patents Nos. 1229503; 1468398 and 1488554. Hollow plastic pigments and their use in paper coatings are disclosed in British Patents Nos. 1270632 and 1389122; in a paper given at the 1984 Tappi Coating Conference by C. P. Hemenway, J. J. Latimer and J. E. Young entitled “Hollow-Sphere Polymer Pigment in Paper Coating” and in an article entitled “Hollow-Sphere Pigment Improves Gloss, Printability of Paper” by W. J. Haskins and D. I. Lunde in “Pulp &amp;Paper”, May 1989 edition. Similar hollow plastic pigments are also the subject of product information literature published by Rohm &amp; Haas Company of Philadelphia, USA in relation to its products sold under the trade mark “Ropaque”.” Claim 1 of Betts describes a “1. Pressure-sensitive copying paper comprising: a paper base; a coating of pressure-rupturable microcapsules on the paper base, the microcapsules containing a solution in an oil solvent of a chromogenic material which develops colour on contact with a colour developer; and a subcoat on the paper base and beneath the microcapsule coating; the subcoat comprising plastic pigment particles and a binder.” The hollow microsphere pigments of this patent may be used in the process of this invention. 
     By way of father illustration, hollow microsphere polymer pigments have also been disclosed in U.S. Pat. No. 4,880,465 of Loria, the entire disclosure of which is hereby incorporated by reference into this specification. Loria claims: “1. A nonpigmented ink suitable for use in ink jet printing comprising: (a) from about 2 to about 20 percent by weight of a resin component; (b) from about 5 to about 25 percent by weight of hollow microspheres; and (c) the remainder being a suitable carrier vehicle comprised of water, ammonium hydroxide, and a volatile solvent; all percentages being based upon the total weight of the ink; the hollow microspheres having permeable walls comprised of a synthetic polymeric material, a central void region filled with water, capable of diffusing through the walls of said microspheres, an inside diameter from about 0.1 to about 0.5 micron and an outside diameter from about 0.4 to about 1 micron; the resin component, the hollow microspheres and the carrier vehicle being chemically nonreactive with each other; and the specific gravity of the ink being about equal to or greater than the specific gravity of the microspheres wherein, the ink has a viscosity at 25 degrees Celsius from about 3 to about 10 centipoises, an electrical resistivity from about 100 to about 3500 ohm-cm, and a sonic velocity from about 1300 to about 1700 meters/second.” The hollow microspheres of this patent may be used in the process of this invention. 
     By way of further illustration, hollow microsphere polymer pigment has been described in U.S. Pat. No. 5,677,043 of Hultman, the entire disclosure of which is hereby incorporated by reference into this specification. Hultman claims: “1. An opaque thermal transfer paper for receiving heated ink from a thermal transfer placed in contact with said opaque thermal transfer paper printer ribbon, said opaque thermal transfer paper comprising in combination: a. a substrate comprising a paper sheet having an outer surface; b. a basecoat coating directly coated on said outer surface of said paper sheet, the coating weight of said basecoat coating ranging between 0.3 g/m.sup.2 to about 10 g/m.sup.2 and the thickness of said basecoat coating ranging between about 1 micron and about 30 microns, said basecoat coating comprising discrete, opaque, plastic, hollow, pigment spheres ranging from about 0.2 microns to about 2 microns in diameter and binders holding together said discrete, opaque, plastic, hollow, pigment spheres, said binders constituting from about 10 percent to about 60 percent by weight of said basecoat coating; and Celsius a topcoat coating directly coated on the basecoat coating, said topcoat coating having a coating weight within the range of from about 1 g/m.sup.2 to about 20 g/m.sup.2 and a coating thickness within the range of from about 1 micron to about 20 microns, said topcoat coating including a plurality of pigment particles having different particle shapes and particle sizes which cooperate to provide a generally open topcoat coating for receiving heated ink from a thermal transfer printing ribbon placed in contact with the topcoat coating, said topcoat coating further including a water holding viscosifying agent, said basecoat coating sandwiched between said substrate and said topcoat coating for attenuating the heat flux from said topcoat coating to said substrate when said topcoat coating receives heated ink from a thermal transfer printer ribbon and for filling in any voids located at said outer surface of said paper sheet.” The hollow microsphere polymer pigments of this patent may be used in the process of this invention. 
     By way of further illustration, hollow microsphere polymer pigment has been described in U.S. Pat. No. 5,071,823 of Matsushita, the entire disclosure of which is hereby incorporated by reference into this specification. Matsushita claims: “1. An image-receiving, transfer recording sheet composed of a substrate and a porous heat insulating layer which, laid on the substrate, comprises 100 parts by weight of macromolecular microspheres and 5-100 parts by weight of a binder, said microspheres consisting mainly of hollow resin particles and/or heterogeneous resin particles.” The hollow microsphere polymer pigments of this patent may be used in the process of this invention. 
     By way of yet further illustration, hollow microsphere polymer pigment has been described in U.S. Pat. No. 7,579,081 of Brown, the entire disclosure of which is hereby incorporated by reference into this specification. Brown claims: “1. An opacifying particle comprising a pigment particle having an average particle diameter of from 0.005 to 5 microns and an index of refraction of at least 1.8; a first polymer attached to the surface of the pigment particle via a silane residue, wherein the first polymer is an addition polymer of at least one ethylenically unsaturated monomer; and a shell comprising a second polymer, the second polymer being formed from at least one ethylenically unsaturated monomer comprising at least one water-soluble monomer selected from the group consisting of an ethylenically unsaturated alcohol-functional monomer, an ethylenically unsaturated inorganic acid-functional monomer, an ethylenically unsaturated carboxylic acid-functional monomer, an ethylenically unsaturated amide-functional monomer, an ethylenically unsaturated amine-functional monomer, and a combination thereof, which substantially encapsulates the pigment particle having the attached first polymer.” The hollow microsphere polymer pigments of this patent may be used in the process of this invention. 
     Referring again to  FIG. 1 , and to the preferred embodiment depicted therein, such hollow organic pigments  402  are typically comprised of an outer wall or sheath of a thermoplastic material (such as, for example, polystyrene, polyacrylates, polycyanoacrylates and various addition polymers and copolymers) surrounding a void  403 . 
     U.S. Pat. No. 7,435,783 of Blankenship, the entire disclosure of which is hereby incorporated by reference into this specification, discloses that “voided latex particles can be prepared by any of several known process, including those described U.S. Pat. Nos. 4,427,836, 4,468,498, 4,594,363, 4,880,842, 5,494,971, 5,521,253, 5,157,084, 5,360,827 among others. Voided latex particles, as described in the references noted above, are prepared by swelling the core of a core-shell emulsion polymer. Some of the processes, such as that described by U.S. Pat. No. 5,360,827 describe the processes whereby, in the latter stages of polymerizing the shell, monomer is added to facilitate diffusion of base into the core of the polymer in order to achieve swelling.” The voided latex particles of this patent may be used in the process of this invention. 
     U.S. Pat. No. 6,720,007 of Walt, the entire disclosure of which is hereby incorporated by reference into this specification, discloses that “the nature of the polymeric shell is varied to accommodate various uses of the hollow microspheres. The microsphere shell typically contains styrene, methacrylate, or any polymer with a high glass-transition temperature (T.sub.g). The shell contains a polymer resulting from the polymerization of one or more monomers selected from the group consisting of acrylonitrile, styrene, benzyl methacrylate, phenyl methacrylate, ethyl methacrylate, divinyl benzene, 2-hydroxyethyl methacrylate, cyclohexyl methacrylate, p-methyl styrene, acrylamide, methacrylamide, methacrylonitrile, hydroxypropyl methacrylate, methoxy styrene, N-acrylylglycinamide, and N-methacrylylglycinamide. Alternatively, the shell contains a co-polymer (random or block) selected from the group consisting of styrene-PMMA, benzyl methacrylate-PMMA, styrene-PHEMA, styrene-PEMA, styrene-methacrylate, and styrene-butylacrylate. The strength and durability of the polymeric shell is increased by crosslinking polymer chains.” The hollow microspheres of this patent may be used in the process of this invention. 
     U.S. Pat. No. 6,235,810 of Pavylyuchenko, the entire disclosure of which is hereby incorporated by reference into this specification, describes “a method for the preparation of latexes with hollow polymer particles useful as opacifying agents. The process comprises the preparation of hollow polymer particle latex by emulsion copolymerization with water soluble initiator and anionic surfactant as follows: a) preparation of seed latex of copolymer containing methyl methacrylate and methacrylic acid; b) preparation of latex of highly carboxylated copolymer containing methyl methacrylate and a cross-linking agent and optionally a vinyl aromatic compound; c) using highly carboxylated latex synthesized at stage b) in the preparation of the intermediate shell comprising a copolymer of methyl methacrylate, acrylic acid ester, a cross-linking agent and optionally vinyl aromatic compound, said intermediate shell copolymer having a glass transition temperature below 80 degrees Celsius; d) swelling the particles prepared at stage c) by addition of volatile basic compound; e) preparation of a hard shell on the swollen expanded particles comprising a copolymer of vinyl aromatic compound, acrylonitrile and/or methyl methacrylate, and cross-linking agent, said hard shell having a glass transition temperature above 80 degrees Celsius; and f) optional preparation of an external shell.” The hollow polymer particles of this patent may be used in the process of this invention. 
     Hollow organic pigment particles often comprise a multi-layer shell of a hydrophilic polymer on the inside of the shell substantially encapsulated by the second polymer. The second polymer presents the outermost surface and typically has a weight average molecular weight, “Mw,” of at least 50,000, preferably of at least 250,000. The second polymer(s) may be a homopolymer or a copolymer. 
     The hollow organic pigment may have a non-spherical shape such as an ellipsoid or a rod-like shape. Preferably, the second polymer forms a spherical shell encapsulating the first polymer. 
     Hollow organic pigment  402  may have a wall thickness of between about 5 nanometers (nm) to 1 micron, preferably in the range of from 50 nanometers to 500 nanometers, most preferably in the range of from 80 nanometers to 150 nanometers. 
     Hollow organic pigment  402  preferably has an average particle size between 0.1 and 10 microns. More preferably, such average particle size is between 0.2 and 1 micron. 
     Hollow sphere polymer pigment  402  preferably has a glass transition temperature higher than 50 degrees Celsius. In one embodiment, such glass transition temperature is from about 60 to about 150 degrees Celsius. 
     In a preferred embodiment, hollow sphere polymer pigment  402  is comprised of a crosslinked polymer shell. 
     In another preferred embodiment, hollow sphere polymer pigment  402  is comprised of monomers which will depolymerize upon application of heat. 
     Hollow organic pigment  402  preferably has a density of less than 0.9 grams per cubic centimeter. In one embodiment, said density is less than 0.7 grams per cubic centimeter and, more preferably, less than 0.55 grams per cubic centimeter. Webster&#39;s Third International Dictionary (Unabridged) defines density as “the mass of a substance per unit volume”. 
     Referring again to  FIG. 1 , and to the preferred embodiment depicted therein, thermosensitive layer  500  preferably comprises 10 percent to 66 percent of hollow organic pigment  402 . In one embodiment, layer  500  is comprised of from about 25 to about 50 percent of such hollow organic pigment  402 . 
     Thermal Solvents  404   
     In one embodiment, the thermosensitive layer  500  is comprised of one or more of thermal solvents  404 . Some of the thermal solvents known to those in the art are described in this specification. It is to be understood that many other prior art thermal solvents also may be used. 
     Webster&#39;s Third International Dictionary (unabridged) defines solvent as “a substance capable of or used in dissolving or dispersing one or more other substances.” Thermal solvents are compounds whose activity is increased with heat and act as solvents for various components of the thermosensitive layer, helping to accelerate the transparentization of said layer at elevated temperatures. 
     In one preferred embodiment, the thermal solvent  404  is a solid or liquid substance with solubility characteristics similar to those of the hollow organic pigment  402 . Upon application of heat to said thermographic substrate  10 , the combination of heat and thermal solvent  404  is able to soften or dissolve the wall of the hollow organic pigment  402 , leading to the collapse of the void  403  contained in the hollow organic pigment and resulting a decrease in opacity of the thermosensitive layer  500 . 
     Thermal solvents are claimed in, e.g., U.S. Pat. No. 5,320,929, the entire disclosure of which is hereby incorporated by reference into this specification. In this patent, thermal solvents are defined as “compounds which are solids at ambient temperature but which melt at the temperature used for processing. The thermal solvent acts as a solvent for various components of the heat-developable materials, it helps to accelerate thermal development and it provides the medium for diffusion of various materials”. 
     Thermal solvents have been extensively used in photo-thermographic imaging elements; see, for example, U.S. Pat. Nos. 5,436,108, 5,328,799, 6,596,470, and 6,790,569; the entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Bailey (U.S. Pat. No. 5,436,109) reviews the use of thermal solvents in photothermographic imaging systems (“Thermal Solvents in Dry Photothermographic Systems”). The thermal solvents described in these patents may be used in the process of this invention. 
     Heat processable photosensitive elements can be constructed so that after exposure, they can be processed in a substantially dry state by applying heat. It is known how to develop latent image in a photographic element not containing silver halide wherein organic silver salts are used as a source of silver for image formation and amplification. Such processes are described in U.S. Pat. Nos. 3,429,706 (Shepard et al.) and 3,442,682 (Fukawa et al.), the entire disclosure of each of which is hereby incorporated by reference into this specification. Other dry processing thermographic systems are described in U.S. Pat. Nos. 3,152,904 (Sorenson et al.) and 3,457,075 (Morgan and Shely); the disclosure of these United States patents are also hereby incorporated by reference into this specification. 
     A variety of compounds have been proposed as “carders” or “thermal solvents” or “heat solvents” for such systems, whereby these additives serve as solvents for incorporated developing agents, or otherwise facilitate the resulting development or silver diffusion processes. Acid amides and carbamates have been proposed as such thermal solvents by Henn and Miller (U.S. Pat. No. 3,347,675) and by Yudelson (U.S. Pat. No. 3,438,776). Bojara and de Mauriac (U.S. Pat. No. 3,667,959) disclose the use of nonaqueous polar solvents containing thione, —SO.sub.2- and —CO— groups as thermal solvents and carders in such photographic elements. Similarly, La Rossa (U.S. Pat. No. 4,168,980) discloses the use of imidazoline-2-thiones as processing addenda in heat developable photographic materials. The disclosure of each of these United States patents is hereby incorporated by reference into this specification; and the thermal solvents described therein may be used in the process of the invention. 
     Thermal solvents for use in substantially dry color photothermographic systems have been disclosed by Komamura et al. (U.S. Pat. No. 4,770,981), Komamura (U.S. Pat. No. 4,948,698), Aomo and Nakamaura (U.S. Pat. No. 4,952,479), and Ohbayashi et al. (U.S. Pat. No. 4,983,502); the entire disclosure of each of these patents is hereby incorporated by reference into this specification. The terms “heat solvent” and “thermal solvent” in these disclosures refer to a non-hydrolyzable organic material which is a liquid at ambient temperature or a solid at an ambient temperature but compatibilizes together with other components at the temperature of heat treatment or below but higher than 40 degrees Celsius. Such solvents may also be solids at temperatures above the thermal processing temperature. Their preferred examples include compounds which can act as a solvent for the developing agent and compounds having a high dielectric constant which accelerate physical development of silver salts. The disclosure of each of these United States patents is hereby incorporated by reference into this specification; and the thermal solvents described therein may be used in the process of the invention. 
     Alkyl and aryl amides are disclosed as “heat solvents” by Komamura et al. (U.S. Pat. No. 4,770,981), and a variety of benzamides have been disclosed as “heat solvents” by Ohbayashi et al. (U.S. Pat. No. 4,983,502). Polyglycols, derivatives of polyethylene oxides, beeswax, monostearin, high dielectric constant compounds having an —SO.sub.2- or —CO— group such as acetamide, ethylcarbamate, urea, methylsulfonamide, polar substances are described in U.S. Pat. No. 3,667,9591 the lactone of 4-hydroxybutanoic acid, methyl anisate, and related compounds are disclosed as thermal solvents in such systems. The role of thermal solvents in these systems is not clear, but it is believed that such thermal solvents promote the diffusion of reactants at the time of thermal development. Masukawa and Koshizuka disclose (U.S. Pat. No. 4,584,267) the use of similar components (such as methyl anisate) as “heat fusers” in thermally developable light-sensitive materials. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification; and the thermal solvents described therein may be used in the process of the invention. 
     Hirai et al. (U.S. Pat. No. 4,590,154), the entire disclosure of which is hereby incorporated by reference into this specification, disclose a heat developable color photographic light-sensitive material comprising silver halide, a hydrophilic binder, dye releasing compounds which release mobile dyes, and a sulfonamide compound. This system requires only heat to develop the latent image and to produce mobile dyes. However, the mobile dyes are affixed to an image receiving material, which must be wetted with water prior to being contacted with the heat developed donor element. The subsequent dye diffusion transfer to the receiver element is therefore of the conventional wet diffusion type. 
     Nakamine et al. (U.S. Pat. No. 5,107,454), the entire disclosure of which is hereby incorporated by reference into this specification, disclose a heat developable photographic chromogenic system that also utilizes diffusion transfer of dyes to an image receiving (fixing) element. The dye diffusion transfer in actuality requires that the image receiving or fixing element be wetted with water prior to being affixed to the dye donor element. The resulting dye transfer, therefore, is a wet diffusion transfer of the conventional type, not dry thermal dye transfer. 
     Such thermal solvents  404  can also facilitate the collapse of the hollow organic pigment at temperature and pressure conditions which can be achieved in a thermal imaging printer. Without the presence of a thermal solvent  404  the temperature at which the hollow organic pigments  402  in the thermosensitive layer  500  will collapse is at least as high as the glass transition temperature of the shell wall of said pigment  402  and typically much higher. While the temperatures which the thermal printer can achieve in a thermosensitive layer may be high (100 to 400 degrees Celsius), the duration of such temperatures in the layer is relatively short (less than 1 ms). Even at temperatures exceeding the glass transition temperature of said shell wall of the hollow organic pigment  402  in a thermal printer there is typically not sufficient time to allow the thermoplastic wall to relax sufficiently to collapse. The inventors have discovered that certain thermal solvents can positively interact with the heat supplied by a thermal printer in the time scale of the heating to facilitate the collapse of the hollow organic pigment  402 ; in particular, they have discovered that the use of one or more appropriate thermal solvents in their system lowers the temperature at which such collapse and resulting transparatization occurs. 
     Thermal solvents  404  may be either liquids or solids. Such agents  404 , incorporated into thermosensitive layer  500  preferably are prevented from mixing with said hollow organic pigments  402  also in said layer  500  in order to maintain the opacity of said layer  500  non-thermally imaged areas. Without wishing to be bound to any particular theory, applicants believe that premature collapse of said pigments  402  would increase the transparency of said layer  500 , defeating the function of the layer as a heat sensitive imaging layer. Separation of the thermal solvents  404  from the hollow polymeric particles  402  can be accomplished in many ways. The agents  404  may be physically separated from the pigments by dispersing them or dissolving them in suitable binder. Alternatively and additionally, said thermal solvents may be encapsulated within said layer  500 . 
     Thermal solvents  404  may be liquids which are emulsified or dissolved in a suitable binder. Such liquid thermal solvents preferable have a boiling point above 100 degrees Celsius and more preferably above 150 degrees Celsius. 
     Thermal solvents  404  may be solids dispersed or dissolved in a suitable binder. The particle size of solid dispersed thermal solvents is preferably less than 10 microns and more preferably less than 5 microns and most preferably less than 1 micron. 
     Solid Thermal solvents  404  may be either amorphous or crystalline or semicrystalline. In the case of crystalline or semicrystalline agents  404 , the melting point is preferably less than 200 degrees Celsius, more preferably less than 150 degrees Celsius, and most preferably less than 100 degrees Celsius. Polar waxes can act as thermal solvents  404 . 
     In one embodiment, the thermal solvents preferably have solubility characteristics similar to those of the hollow organic pigment. The Hildebrand solubility parameter is described in U.S. Pat. No. 7,465,343 of Prasad, the entire disclosure of which is hereby incorporated by reference into this specification. 
     The Hildebrand solubility parameter that is discussed in the Prasad patent also is discussed in Billmeyer&#39;s “Textbook of Polymer Science”, 2 nd  Edition, Wiley-Interscience, New York, 1962. As is disclosed in such text, the Hildebrand solubility parameter is the square root of the cohesive energy density (energy/unit volume) of a material. Thus, the solubility parameter is the square root of calories/cubic cm or in SI units, joule/cubic meter. In SI units a pascal is defined as one joule per cubic meter. Thus, in this specification the units for solubility parameter shall be referred to as the square root of megapascal&#39;s or MPa.sup.1/2. 
     U.S. Pat. No. 5,997,741, the entire disclosure of which is hereby incorporated by reference into this specification, describes a method to determine the solubility parameter using the following formula: solubility parameter=(ΔE v /V) 1/2  wherein ΔE v  is molar evaporation energy, nearly equal to ΔH-RT wherein ΔH is vapor heat, R is the gas constant and T is absolute temperature, and V is the molar volume of a solvent. 
     Solubility parameters are described in many scientific articles and books. By way of illustration, the “Polymer Data Handbook, Basic Edition”, compiled by The Society of Polymer Science, Japan and published by Baifukan Co., Ltd. has tables on solubility parameters by solvent, so that a decision may be made on the choice of the heat stabilizing solvents suitable for the instant invention. Other references giving considerations to solubility parameters include Ind. Chem. Prod. Res. Dev. 8, March 1969, p. 2-11, Chemical Reviews, 75 (1975), p. 731-753, and Encyclopedia of Chemical Technology, 2nd Edition, Supplement Volume (1971), p. 889-910. 
     U.S. Pat. No. 7,041,369 of MacKay, the entire disclosure of which is hereby incorporated by reference into this specification, discloses that “the (.delta.) can be used to estimate the compatibility between materials. Generally, substantial compatibility between two materials can be expected when their solubility parameters are similar. It is known that water has a .delta.sub.water value of 48.0 Megapascals.sup.1/2, which is the highest among common solvents, probably due to the strong hydrogen bonding capacity of water. It is believed that starch typically has a .delta.sub.starch value similar to that of cellulose (about 34 Megapascals.sup.1/2).” 
     Suitable thermal solvents  404  will dissolve, swell, plasticize or mix with hollow organic pigments  402  under conditions of elevated temperature. If such agents  404  are in their liquid state then the rapid heating from the thermal printer must enable said agents to come into direct contact with said pigments  402  by means of diffusion, capillary action, flow and the like. If thermal solvents  404  are in the solid state then they must first melt with the heat provided from the thermal printer and then come into direct contact with said pigments  402  by similar means. 
     According to Mackay (U.S. Pat. No. 7,041,369, the entire disclosure of which is hereby incorporated by reference into this specification), compatible materials have a difference in solubility parameter of less than about 10 Megapascals.sup.1/2. In the instant invention it has been discovered that thermal solvents  404  with solubility parameters from about 16 Megapascals.sup.1/2 to about 28 Megapascals.sup.1/2 will transparentize a polystyrene-co-acrylic acid based polymeric pigment at 150 degrees Celsius. 
     Without wishing to be bound to any particular theory, the inventors believe that the thermal solvent  404  facilitates the thermally induced collapse of the hollow organic pigment  402  if the thermal solvents solubility parameter is within about 10 Megapascals.sup.1/2 of the material making up the outer shell of said pigment  402 . The solubility parameter of polystyrene is reported in the Polymer Handbook, 3 rd  Edition, John Wiley &amp; Sons, NY 1989, pp. VII 554-555, and it ranges from 15.6 to 21. Megapascals.sup.1/2. 
     Suitable thermal solvents  404  may include such plasticizers as those disclosed in U.S. Pat. No. 5,776,280 including, e.g., adipic acid esters, phthalic acid esters, chlorinated biphenyls, cirates, epoxides, glycerols, glycol, hydrocarbons, chlorinated hydrocarbons, phosphates, esters of phthalic acid such as, e.g., di-2-ethylhexylphthalate, phthalic acid esters, polyethylene glycols, esters of citric acid, epoxides, adipic acid esters, and the like. The use of such plasticizing agents is well known and is described, e.g., in U.S. Pat. Nos. 6,121,356; 6,117,572; 6,086,700; 6,060,214; 6,051,171; 6,051,097; 6,045,646; and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. Other suitable plasticizers may be obtained from, e.g., the Eastman Chemical Company. These prior art plasticizers may be used in the process of this invention. 
     Furthermore, thermal solvents useful in the present invention include polar organic compounds such as the polyglycols described in U.S. Pat. No. 3,347,675 and the compounds described in U.S. Pat. No. 3,667,959; urea derivatives, e.g., dimethylurea, diethylurea and phenylurea; amide derivatives, e.g., acetamide, benzamide and p-toluamide; sulfonamide derivatives, e.g., benzenesulfonamide and .alpha.-toluenesulfonamide; and polyhydric alcohols, e.g., 1,2-cyclohexanediol and pentaerythritol. The entire disclosure of each of these patents is hereby incorporated by reference into this specification, and the thermal solvents disclosed therein may be used in the process of the invention. 
     Solid plasticizers are disclosed by Chen in U.S. Pat. No. 7,651,747. Thus, e.g., Chen discloses that: “Solid plasticizers 12 are known in the art and may include a phthalate compound, a terephthalate compound, an isophthalate compound, a benzoate compound, a polymeric adipate compound, or mixtures thereof. Examples of the solid plasticizer 12 include, but are not limited to, sucrose benzoate, 1,4-cyclohexanedimethanol dibenzoate, glyceryl tribenzoate, dicyclohexyl phthalate, benzyl 2-naphthyl ether, dimethyl terephthalate, 2-chloropropionanilide, 4-benzyldiphenyl, dibenzyl oxalate, m-terphenyl, diphenyl phthalate, diphenyl isophthalate, dihexyl phthalate, diactyl phthalate, cumylphenyl isophthalate, dihydroabietyl phthalate, dimethyl isophthalate, ethylene glycol dibenzoate, trimethylolethane tribenzoate, pentaerythritol tetrabenzoate, sucrose octaacetate, tricyclohexyl citrate, N-cyclohexyl-p-toluenesulfonamide, o,p-toluenesulfonamide, N-ethyl-p-toluenesulfonamide, N-butyl-p-toluenesulfonamide, n-tallow-4-toluenesulfonamide, p-toluenesulfonamide-formaldehyde resin, 1,2-di-(3-methylphenoxy)ethane, or mixtures thereof. The solid plasticizer 12 may have an average particle size of less than approximately 5 .mu.m, such as less than approximately 0.5 .mu.m.” These solid plasticizers also may be used in the process of the invention. 
     In one embodiment, the solid plasticizer is selected from the group consisting of “ . . . sucrose benzoate, 1,4-cyclohexanedimethanol dibenzoate, glyceryl tribenzoate, dicyclohexyl phthalate, benzyl 2-naphthyl ether, dimethyl terephthalate, 2-chloropropionanilide, 4-benzyldiphenyl, dibenzyl oxalate, m-terphenyl, diphenyl phthalate, diphenyl isophthalate, o,p-toluenesulfonamide, N-cyclohexyl-p-toluenesulfonamide, 1,2-di-(3-methylphenoxy)ethane, or mixtures thereof.” are incorporated herein by reference. 
     The thermal solvents described by Bailey in U.S. Pat. No. 5,436,109 are hereby incorporated herein by reference; and they may also be used in the process of the invention. 
     Thermal solvents agents  404  may be solid materials, such as the sensitizers described by Metha (U.S. Pat. No. 5,888,283); the entire disclosure of this patent is hereby incorporated by reference into this specification. Mertha discloses that: “Suitable sensitizers for use in the ink composition include diphenoxyethane, aryl or alkyl-substituted biphenyls such as p-benzyl biphenyl, or toluidide phenyl hydroxynaphthoates and aromatic diesters such as dimethyl or dibenzyl terephthalate and dibenzyl oxalate. These materials may be used alone, or they may be combined with waxes or fatty acids. A preferred sensitizer for use is p-benzyl biphenyl. The sensitizer preferably has a softening point of between about 80 degrees Fahrenheit to 85 degrees Fahrenheit (27.degrees. Celsius to 29.degrees Celsius) and a melting point of between about 140 degrees Fahrenheit to 150 degrees Fahrenheit (60 degrees Celsius to 65 degrees Celsius). When the sensitizer is heated to its melting point (such as by the printhead in a direct thermal printer), it melts and lowers the melting point of adjacent color developer and color former particles, causing them to dissolve, react, and form a desired color.” The sensitizers (thermal solvents) of this patent also may be used in the process of this invention. 
     One may use one or more of the sensitizers disclosed in U.S. Pat. No. 5,883,043, the entire disclosure of which is hereby incorporated by reference into this specification. The sensitizers/thermal solvents of this patent also may be used in the process of this invention. 
     In one embodiment, the thermal solvents used include dibenzyl oxalate, propylene carbonate, benzyl alcohol, triethylene glycol, triethylene glycol, dipropylene glycol, dibutyl phthalate, carnauba wax, 1, 2-bis(3-methylphenoxy) ethane and the like. 
     Referring again to  FIG. 1 , thermosensitive layer  500  is preferably comprised of 5 to about 50 weight percent of thermal solvent  404  and, more preferably, from about 10 to about 40 percent of such solvent. In one aspect of this embodiment, the concentration of such thermal solvent is from about 15 to about 30 weight percent. 
     In a preferred embodiment, thermosensitive layer  500  is comprised from about 0.5 to about 1.5 parts by weight of said thermal solvent for each part of said hollow sphere organic pigment. 
     Thermosensitive layer  500  may optionally include a colorant  502 . A preferred colorant is a dye or pigment, such as, e.g., the colorants  302  described in optional colorant layer  300 . Additionally, colorant  502  may be an optical brightener or fluorescent dye or pigment which adds a color effect to the layer without significantly impacting the opacity of the layer. So, for example, light stable dyes, transparent pigment dispersions and the like may be added to the layer to provide a tinting color effect once the layer has been transparentized. Such dyes or pigments may act as a light filter for the underlying color layers. Alternatively or additionally, optical brighteners may be added to thermosensitive layer  501  to improve the apparent whiteness of unimaged areas. 
     Binders for Thermosensitive Layer  500   
     Thermosensitive layer  500  is comprised of a binder  501  wherein said binder preferably constitutes at least 10 percent of said thermosensitive layer, and, in one embodiment, from 15 to 45 percent of said layer. The binders  301  described in optional color layer  300  are suitable for use in thermosensitive layer  500 . 
     Binder  501  may be comprised of resins which are preferably water soluble or water dispersible. Binder resins usable in the thermosensitive layer  401  include, e.g., cellulosic resins such as ethyl cellulose, hydroxyethylcellulose, hydroxypropylcellulose, methylcellulose, cellulose acetate, cellulose acetate buytryate, and nitrocellulose; natural resins and gums such as gelatin, chitan, and various polysaccharides; vinyl resins such as polyvinylalcohol, polyvinylacetate, copolymers of polystyrene-co-butadiene and polyvinylpyrrolidone; acrylic resins such as polyacrylamide, polyacrylonitrile-co-styrene, polybutyl acrylate-co-acrylic acid, polymethylmethacrylate, polymethylmethacrylate-co-butylacrylate, polybutylmethacrylate; polyacetal resins such as polyvinylbutyral, polyvinylacetal, polyvinylformal, polyvinylacetal-co-butyral and the like; water dispersible acrylic resins and the like; polyester, polyurethane, polyamide, polyimide, polycarbonate, polyurea, polyether, polyethylene glycol, polyethylene oxide, polypropylene oxide, epoxy resins and the like; various copolymers and mixtures thereof. 
     In one embodiment, the binder comprises an ionomeric crosslinked resin. In one embodiment, the binder comprises a self crosslinking acrylic resin. 
     In one embodiment, the binder comprises a cross-linked resin. In this case, a resin having several reactive groups, for example, hydroxyl or amine groups, is used in combination with a crosslinking agent, such as a polyisocyanate, glyoxal, epoxies, aldehydes, silanes, titinates, zirconate, aziridine, oxazolin and the like. 
     Referring again to  FIG. 1 , the binder  501  in thermosensitive layer  500  may be comprised of from about 0 to about 100 weight percent of wax and, preferably, from about 5 to about 20 weight percent of such wax. In one embodiment, binder  501  comprises from about 5 to about 10 weight percent of such wax. Suitable waxes which may be used include, e.g., carnuaba wax, rice wax, beeswax, candelilla wax, montan wax, paraffin wax, microcrystalline waxes, synthetic waxes such as oxidized wax, ester wax, low molecular weight polyethylene wax, Fischer-Tropsch wax, and the like. These and other waxes are well known to those skilled in the art and are described, e.g., in U.S. Pat. No. 5,776,280, the entire disclosure of which is hereby incorporated by reference into this specification. One may also use ethoxylated high molecular weight alcohols, long chain high molecular weight linear alcohols, copolymers of alpha olefin and maleic anhydride, polyethylene, polypropylene, and the like. These and other suitable waxes are commercially available from, e.g., the Baker-Hughes Baker Petrolite Company of 12645 West Airport Blvd., Sugarland, Tex. 
     In one preferred embodiment, carnauba wax is used as the wax. As is known to those skilled in the art, carnauba wax is a hard, high-melting lustrous wax which is composed largely of ceryl palmitate; see, e.g., pages 151-152 of George S. Brady et al.&#39;s “Material&#39;s Handbook,” Thirteenth Edition (McGraw-Hill Inc., New York, N.Y., 1991). Reference also may be had, e.g., to U.S. Pat. Nos. 6,024,950; 5,891,476; 5,665,462; 5,569,347; 5,536,627; 5,389,129; 4,873,078; 4,536,218; 4,497,851; 4,4610,490; and the like. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     One may use one or more of the binders disclosed in U.S. Pat. Nos. 6,410,479, the disclosure of which is hereby incorporated by reference into this specification. 
     In one embodiment, carnauba wax is used as both the binder and the thermal solvent. Other comparable polar waxes may be similarly used. 
     Particles for Thermosensitive Layer  500   
     Referring again to  FIG. 1 , thermosensitive layer  500  is comprised of a multiplicity of particle  407 . Particle  407  may be an organic or inorganic particle with an average particle size less than 5 microns and, more preferably, less than 2 microns. Particle  407  is added as filler to the layer with the function of improving the cohesion and thermal stability of layer  500  while providing minimal masking. Particles which mask layer  500  reduce its transparency after thermal imaging by scattering light. For example, particles such as titanium dioxide with a refractive index of 2.49 generate high masking and are not suitable for use in thermosensitive layer  500 . Particles with small differences in refractive index to the binder  501  are preferred. Typically binders such as acrylic and styrene polymers have refractive indices in the range of 1.45 to 1.55. Organic polymeric particles will have refractive indexes in the same range as binder  501 . In addition, many inorganic particles such as clay (Al 2 Si 2 O 5 (OH) 4 ) and microcline (KAlSi 3 O 8 ) have refractive indices in the range of 1.47 to 1.55 and are suitable for use as particle  407  in thermosensitive layer  500 . 
     In a preferred embodiment, particle  407  has a refractive index between 1.0 and 2.0. In another embodiment, particle  407  has a refractive index between 1.25 and 1.75. In yet another embodiment, particle  407  has a refractive index between 1.35 and 1.65. 
     Layer  600  (Optional Barrier Layer) 
     Referring again to  FIG. 1 , thermographic substrate  10  is optionally and preferably comprised of a barrier layer  600 . 
     Barrier layer  600  is optionally applied as contiguous coating over thermosensitive layer  401  and/or  501 . 
     Barrier layer  600  functions to protect underlying thermosensitive layers  400  and/or  500  from attack by agents which could reduce the opacity of such layers. Barrier layer  600  preferably is a water based coating composition. In addition, and in one embodiment, barrier layer  600  helps to maintain distinct layer separation between thermosensitive layer  400  and/or  500  and heat resistant topcoat  700 . 
     Binders described elsewhere in this specification, such as binder  201 ,  301 ,  401  or  501  may be used in layer  600 . 
     Barrier layer  601  is preferably crosslinked, i.e., it is insolubilized after coating so that, when it is contacted with water or organic solvent at ambient temperature it is substantially insoluble in such solvent. In one aspect of this embodiment, thermographic layer  500  also is crosslinked. Consequently, and in this embodiment, the imaged thermographic substrate is substantially insoluble in water and, thus, is resistant to attack by water. Furthermore, unlike prior art thermographic substrates, the substrate of this invention is also resistant to light fading. 
     In one embodiment barrier layer  601  has a density greater than 0.9 grams per cubic centimeter and, preferably, greater than 1 gram per cubic centimeter, In another embodiment, the density of such barrier layer is, greater than 1.1 grams per cubic centimeter. 
     In one embodiment, barrier layer  601  is preferably comprised of polyvinyl alcohol. In a preferred embodiment, barrier layer  601  is comprised of a cross-linked polyvinyl alcohol. 
     In one embodiment, barrier layer  601  is preferably comprised of a chlorine containing polymer. 
     Barrier layer  601  preferably has a coating weight  606  of at least about 0.1 grams per square meter to 10 grams per square meter and, more preferably, from about 0.2 grams per square meter to 5 grams per square meter. In one embodiment, the coating weight is from 0.5 grams per square meter to 2 grams per square meter. 
     In one preferred embodiment, barrier layer  600  is flood coated over the top of thermosensitive layer  500  as a protection to layer  500  from attack by agents which include (but are not limited to) solvents, plasticizers, oils, inks, coating solutions, fingerprint oil, varnishes, adhesives and the like which might come in contact with top most surface  800  of the thermographic substrate  10 . As will be apparent, the combination of a barrier layer and a thermographic layer provides a unique combination of properties, to wit, protection against fading, water, organic solvents, plasticizers, oils, adhesives, etc. 
     Barrier layer  600  is preferably coated from water. In one embodiment, water-based compositions are prepared such that many of the components remain separated and not intimately mixed at the time of coating and drying. In one embodiment, barrier layer  600  is comprised of dispersions of binders  601 , coalescent agents, crosslinkers and the like. 
     In one preferred embodiment, barrier layer  600  is comprised of a continuous phase crosslinked binder  601 . 
     Layer  700 —Heat Resistant Topcoating 
     Referring again to  FIG. 1 , thermographic substrate  100  is optionally comprised of a heat resistant topcoat  700 . The topcoat  700  is the uppermost layer of the thermographic substrate and comes in direct contact with the thermal printhead when the thermographic substrate is imaged in a thermal printer. The thermal print head is comprised of a linear array of individual heating element. As these heating elements are energized in response to image being printed, they may reach temperatures in the range of 200 degrees Celsius to about 400 degrees Celsius. As the thermographic substrate  100  passes beneath the thermal printhead, the topcoat  700  is in turn heated as it comes in contact with energized heating elements of the printhead. Those skilled in the art will understand that many of the binders described in this specification will soften at such temperatures and be inclined to stick or adhere to the hot printing elements. 
     Topcoatings which are heat resistant are able to freely pass beneath an energized thermal printhead without sticking or stalling. Heat resistant layer  700  preferably is resistant to sticking to the thermal printhead and must enable thermographic substrate  10  to pass beneath said printhead with minimal friction, irrespective of the temperature of said printhead. Heat resistant layer  700  preferably should not rub off or build up on the thermal printhead as such material will interfere with the flow of heat from the printhead to the thermographic substrate  10 . 
     In a preferred embodiment, the coefficient of friction of the heat resistant topcoat does not increase by more than 50 percent form 20 degrees Celsius to about 300 degrees Celsius. 
     Heat resistant topcoat  700  is comprised of heat resistant binder  701  and optionally one or more abrasive particles  702  and one or more lubricants  703 . One may use one or more of the binders disclosed elsewhere in this specification. Additionally, the binders disclosed in U.S. Pat. No. 6,410,479, the disclosure of which is hereby incorporated by reference into this specification, may be used. 
     The binder  701  for heat resistant layer  700  may be any composition which does not cause sticking at temperatures of 150 degrees Celsius or higher. Binders described elsewhere in this specification, such as binders  201 ,  301 , and  501 , may be used. In another preferred embodiment, binder  701  has a glass transition temperature of at least 50 degrees Celsius. In yet another preferred embodiment binder,  701  has a glass transition temperature at least 70 degrees Celsius. 
     In a preferred embodiment, binder  701  is a cross-linked polyvinyl alcohol. Crosslinked polyvinyl alcohol base topcoating binders have been described in U.S. Pat. No. 6,410,479, the entire disclosure of which is hereby incorporated by reference into this specification, and such binders may be used in the process of this invention. 
     Preferably, heat resistant layer  700  is comprised of polyvinyl alcohol. The polyvinyl alcohol use in heat resistant layer  700  is preferably fully saponified, partially saponified, or denatured by carboxyl, amide, sulfonic acid or butyl aldehyde. 
     In one embodiment, heat resistant layer  700  is preferably comprised of a crosslinked binder to further improve its heat resistance and its ability not to stick to the thermal printhead. Hydroxyl containing binders, such as polyvinyl alcohol, polyurethane, polyacrylates, polyesters, polyacetals and the like, may preferably be crosslinked with dialdehydes such as glyoxal or polyaldehyde, epoxies such as diglycydil type, dimethylolurea such as glycerindiglycidylether, isocyanates, boron oxides, aziradines, oxazolines; and the like. 
     Heat resistant layer  700  preferably protects the underlying thermosensitive layers  500  from attack by plasticizer, oil, solvents and the like. 
     Heat resistant layer  700  is preferably comprised of a lubricant  703 . The lubricant  703  lowers the friction of said layer  700  against the thermal printhead, particularly at high temperatures. Lubricants  703  may be comprised of the metallic salt of high fatty acid such as zinc stearate, zinc stearyl phosphate, calcium stearate, waxes such as paraffin, polyethylene, carnauba and micro crystalline, silicone compounds, phosphate esters and the like. 
     Heat resistant layer  700  is preferably comprised of one or more abrasive particles  702 . Abrasive particles  702  help to remove any materials which may buildup on the thermal printhead. Preferably, such abrasive particles are comprised of inorganic particles with an average particle size less than 1 micron, more preferably less than 0.5 micron, and most preferably less than 0.1 micron. 
     Abrasive particle  702  preferably have a melting point above 200 degrees Celsius and, more preferably, above 300 degrees Celsius, and, most preferably above 400 degrees Celsius. 
     Abrasive particle  702  is preferably comprised of silica, alumina, Mania, talc, clay and the like. 
     Abrasive particle  702  preferably has a Mohs hardness less that that of the outermost glaze on the thermal printhead. In one embodiment the abrasive particle has a Mohs hardness of less than 7. In another embodiment the abrasive particle  702  has a Mohs hardness of less than 5. 
     The heat resistant layer  700  is preferably comprised of an ultraviolet (UV) cured addition polymer resin. Halbrook discloses the use of UV cured protective coatings in thermosensitive recording materials in U.S. Pat. No. 6,566,752, the entire disclosure of which is hereby incorporated by reference. Halbrook states that: “Suitable UV cured protective overcoats are described in U.S. Pat. No. 4,886,744. Most free radical initiated polymerizations can be suitably cured with the use of a photoinitiator that is responsive in the UV range. These UV overcoats are said to contain additives such as UV absorbers and light stabilizers. Employing the UV cured coating allows for rapid drying. U.S. Pat. No. 4,886,774 discloses the use of a coating comprising the reaction product of acrylated aromatic urethane oligomers as unsaturated oligomer, tetrahydrofural methacrylate, as methacrylate oligomer and trimethylolpropane triacrylate as crosslinking monomer. U.S. Pat. No. 5,158,924 also describes ultraviolet curing resins which are suitable for protective coatings and include urethane resins, epoxy resins, organosiloxane resins, polyfunctional acrylate resins, melamine resins, thermoplastic resins having high softening points such as fluorine plastics, silicone resins, and polycarbonate resins. A specific example of a urethane acrylate-type UV curing resin is UNIDIC C7-157 made by Dainippon Ink &amp; Chemicals Incorporated.” 
     The heat resistant layer  700  is optionally applied over said barrier layer  600 . 
     The heat resistant layer  700  may be applied over said thermosensitive layers  500 . 
     The heat resistant layer  700  preferably has a coating weight  706  of from about 0.05 to about 10.0 grams per square meter, and more preferably from about 0.1 to about 5 grams per square meter; in one embodiment, such coating weight is from 0.5 to 2.0 grams per square meter. 
     Referring again to  FIG. 1 , topcoating  700  protects thermographic substrate  10  from damage as it passes under the printhead of a thermal printer. It must also provide a low friction surface  800  to enable smooth transport of the thermographic substrate beneath the printhead wherein said friction is relatively independent of temperature. 
     Heat resistant topcoatings should protect the thermal printhead from excessive buildup of debris from the printing of said thermographic substrates as this can impede heat flow between the printhead and the substrate. 
     In a preferred embodiment, any build up of debris on a thermal printhead from said thermographic substrate may be easily removed by rubbing said buildup with an alcohol saturated cloth. 
     The abrasive characteristics of heat resistant topcoatings should be great enough to clean any buildup of debris on the thermal printhead and yet low enough to minimize wear of the printhead. 
     In a preferred embodiment, 100,000 inches of thermographic substrate may be printed on a given thermal printhead without noticeable degradation to image quality. 
     In another preferred embodiment, 1,000,000 inches of thermographic substrate may be printed on a given thermal printhead without noticeable degradation to image quality. 
     In yet another preferred embodiment, 5,000,000 inches of thermographic substrate may be printed on a given thermal printhead without noticeable degradation to image quality. 
     Heat resistant topcoating  700  is preferably coated from water. In one embodiment, water-based compositions are prepared such that many of the components remain separated and not intimately mixed at the time of coating and drying. 
     In U.S. Pat. No. 6,410,479, the entire disclosure of which is hereby incorporated by reference into this specification, Fukuchi discloses several preferred binders for thermographic topcoatings, stating that: “any composition which does not cause sticking at the higher temperature than 200 degrees Celsius and does not hurt the thermal sensitivity and lustrous property can be used. Concretely, various kinds of polyvinyl alcohol of 200 about 2500 polymerization degree such as fully saponified polyvinyl alcohol, partially saponified polyvinyl alcohol, denatured polyvinyl alcohol e.g. polyvinyl alcohol denatured by carboxyl, polyvinyl alcohol denatured by amide, polyvinyl alcohol denatured by sulfonic acid or polyvinyl alcohol denatured by butylal (butyl aldehyde); water soluble high polymer of cellulose derivatives, such as, hydroxyethyl cellulose, methyl cellulose, ethyl cellulose, carboxymethyl cellulose and acetyl cellulose and (meth)acrylic ester resin such as (meth)acrylic ester copolymer, acrylic ester and/or methacrylic ester, copolymer of styrene and/or vinyl acetate, copolymer of acrylamide/acrylic ester/methacrylic acid, copolymer of colloidal silica complex acrylic ester and copolymer of colloidal silica complex styrene/acrylic ester can be mentioned.” 
     In U.S. Pat. No. 6,410,479, Fukuchi discloses several preferred crosslinking agents for use in thermographic topcoats, stating that: “Concretely, dialdehyde type such as glyoxal or polyaldehyde, polyamine type such as polyethylamine, epoxy type, polyamide resin, melamine resin, diglycydil type, dimethylolurea such as glycerindiglycidylether, further, ammonium persulfate, iron chloride and magnesium chloride can be mentioned, however, the invention is not limited to them. Compared with a three-dimensional bridged type glyoxal cross-linking agent, since a two-dimensional bridged type glyoxal does not deteriorate the glossiness, it is useful for the preparation of excellent lustrous surface. The reason why is unclear, however, it is considered that the light scattering is generated on a micro scale when it is three-dimensionally bridged. The amount of cross-linking agent to be added can be adjusted voluntarily so as to be a fixing composition which does not cause sticking at the temperature higher than 200 degrees Celsius and, for instance, 0.05-0.3 parts can be added to 1 part of water soluble high polymer substance.” 
     In U.S. Pat. No. 6,410,479 Fukuchi discloses several preferred lubricants for use in thermographic topcoats, stating that: “a slipping agent in the glossing layer or the intermediate layer of this invention, for the purpose of improving the thermal head compatibility. As a slipping agent, the slipping agents which are generally used in the conventional thermally sensitive recording medium can be used. As the concrete example, metallic salt of high fatty acid such as zinc stearate or calcium stearate and wax such as paraffin wax, polyethylene wax, carnauba wax, micro crystalline wax and acrylic type wax can be mentioned. Especially, when the thermal head compatibility is concerned, zinc stearate or calcium stearate are desirably used.” 
     Thermographic substrate  10  may be imaged with thermal printers well known to those skilled in the art. One may use one or more of the direct thermal printers disclosed in U.S. Pat. Nos. 6,124,944; 6,118,467; 6,116,709; 6,103,389; 6,102,534; 6,084,623; 6,083,872; 6,082,912; 6,078,346; and the like. The disclosure of each of these United States patents is hereby incorporated by reference into this specification. 
     Digital thermal transfer printers are readily commercially available. Thus, e.g., one may use a printer identified as Zebra 140xilll and various other thermal printers sold by Zebra Corporation of Vernon Hills, Ill. 
     Rd is a measure of the darkness of a given substrate or article. Rd is defined in answers.com as “The common logarithm of the ratio of the luminance of a nonabsorbing perfect diffuser to that of the surface under consideration, when both are illuminated at an angle of 45° to the normal and the direction of measurement is perpendicular to the surface.” 
     Rd is used in claim 1 of U.S. Pat. No. 4,830,503, the entire disclosure of which is hereby incorporated by reference into this specification, Such claim 1 describes: “A Rd measuring system for measuring the Rd of a plurality of objects each having a surface-to-be-measured and being arranged in a common plane on a support means, said system comprising: a light source disposed outside the support means, a measuring head connected to the light source through an optical fiber bundle, the measuring head being movable, under deformation of the optical fiber bundle, to measuring positions which oppose a respective object, the measuring head having a photosensor which receives light which is emitted from the light source and which is subsequently reflected at the surface-to-be-measured, thereby measuring the reflection density of the surface-to-be-measured, said system further comprising means for keeping constant the radius of curvature of the bent portion of the optical fiber bundle in any measuring position of the measuring head.” 
     Preferably, the unimaged color saturation of thermographic substrate  10  is less than 0.8 Rd (Rd), more preferably less than 0.5 Rd and most preferably less than 0.15 Rd. 
     Upon imaging thermographic substrate  10  with a thermal printer, the color saturation of substrate  10  is greater than 0.8 Rd, preferably greater than 1.0. In one embodiment, such color saturation is greater than 1.35. 
       FIG. 2  is a schematic representation of a thermographic substrate  20  made in accordance with one preferred process of this invention; this Figure is not necessarily drawn to scale. Thermographic substrate  20  is comprised of flexible substrate  101 , optional surface layer  200 , thermosensitive layer  500 , optional barrier layer  600  and heat resistant topcoating  700 . In addition, thermographic substrate  20  is comprised of colored thermosensitive layer  400  disposed between substrate  101  and thermosensitive layer  500 . 
     Colored thermosensitive layer  400  is comprised of hollow organic pigments  403 , thermal solvents  404 , binders  40 , and light stable colorants  302  all of which have been described elsewhere in this specifications. 
     Colored thermosensitive layer  400  may preferably be comprised of colored particles  405 . Colored particles  405  are comprised of binders  501  and colorants  302  described elsewhere in this specification. Such colored particles may be in the form of a liquid dispersion which may be incorporated into the fluid used to coat thermosensitive layer  400  of thermographic substrate  20 . 
     Colored particles  405  help to segregate colorants  302  in thermosensitive layer  400 , reducing the light absorption of such colorants and thus contributing to higher lightness value L* for the thermographic substrate  20 . However, upon heating, colored particles  405  readily mix with the binders  401  and thermal solvents  404  in layer  400 , enabling said layer to rapidly increase in color saturation. 
     Colored particles  405  are preferably comprised of wax binders  501 , thermal solvents  404  and colorants  302 . 
     Colored particles  405  preferably have an average particle size less than 10 microns, more preferably less than 5 microns and most preferably less than 2 microns. 
     Colored particles  405  are preferably comprised of from about 1 percent to about 50 percent of colorant  302 . Colored particles  405  are more preferably comprised of from about 10 to about 30 percent of colorant  302 . 
     Thermosensitive layer  400  is preferably comprised for from about 1 to about 50 percent of colored particles  405  and/or colorant  302 . Thermosensitive layer  400  is more preferably comprised for from about 5 to about 40 percent of colored particles  405  and/or colorant  302 . Thermosensitive layer  400  is most preferably comprised for from about 10 to about 33 percent of colored particles  405  and/or colorant  302 . 
     Thermosensitive layer  400  is preferably coated from water. Water based compositions may be prepared such that many of the components remain separated and not intimately mixed at the time of coating and drying. Thermosensitive layer  400  may be comprised of dispersions of solids, emulsions of liquids, microcapsules, hollow organic pigments and the like. 
     Preferably, thermosensitive layer  400  is comprised of a continuous phase binder in which said dispersions, emulsions, microcapsules, pigments, dyes and hollow organic pigments are dispersed. 
     Referring again to  FIG. 2 , thermosensitive layer  400  is comprised of colorants  405  capable of absorbing light. Said thermosensitive layer  400  is opaque and low in color saturation when initially applied to a flexible substrate  101 . However, upon heating with a thermal printhead, said layer  400  will become more transparent, enabling the colorants  405  and/or  302  to impart color saturation to the layer. 
     Thermosensitive layer  400  is comprised of colorants  405  and/or  302  which have sufficient resistance to image fading such that they are suitable for outdoors applications. Such colorants, comprising light stable dyes and/or pigments, are described elsewhere in this specification. Before heating said thermosensitive layer  400 , colorants  405  and/or  302  are partially hidden by hollow organic pigments  402 . Light scattering by said pigments  402  reduces the effectiveness of such colorants to absorb incident light, lowering the color saturation of the layer. In areas printed with a thermal printer, the combination of heat from the thermal printhead and the thermal solvent  404  causes the collapse of the hollow organic pigment  402 . Such collapse of the hollow organic pigment  404  reduces the ability of colored thermosensitive layer  400  to scatter light. The thermally imaged portion of layer  400  thus becomes more transparent, increasing the capability of the colorant  405  and/or  302  to absorb incident light and raising the color saturation of the layer  400 . 
     Referring to  FIG. 2 , thermosensitive layer  400  preferably has a coating weight  406  of 0.5 to 20 grams per square meter, more preferably from 1 to 10 grams per square meter. 
     Referring again to  FIG. 2 , thermosensitive layer  400  is disposed above and contiguous with substrate  101 . 
     Referring again to  FIG. 2 , thermosensitive layer  400  is optionally disposed above and contiguous with optional surface layer  200 . 
     Upon coating, thermosensitive layer  400  is opaque and low in color saturation. Upon heating with a thermal printer, thermosensitive layer  400  decreases in opacity and increases in color saturation. Thermosensitive layer  400  is capable of developing sufficient visual contrast such that human and machine readable images may be printed by direct heating of said layer  400  with a thermal printhead. Such visual contrast is thermally developed by altering the light scattering capability of the layer  400 . 
     In one embodiment, colored thermosensitive layer  400  is disposed between said substrate  101  and said white opaque thermosensitive layer  500 . Upon heating with a thermal printer, thermosensitive layer  500  such becomes transparent, revealing underlying thermosensitive layer  400 . Simultaneously, heating from the thermal printer decreases the opacity of thermosensitive layer  400  and increases its color saturation resulting in a significant increase in visual contrast. 
     Preferably, the unimaged color saturation of thermographic substrate  20  is less than 0.8 Rd, more preferably less than 0.5 Rd and most preferably less than 0.15 Rd. 
     Upon imaging thermographic substrate  10  with a thermal printer the color saturation of thermographic substrate  20  is greater than 0.8 Rd, preferably greater than 1.0. In one aspect of this embodiment, such color saturation is greater than 1.35. 
       FIG. 3  is a schematic representation of a thermographic substrate  30  made in accordance with one preferred process of this invention; this Figure is not necessarily drawn to scale. Thermographic substrate  30  is comprised of flexible substrate  101 , optional surface layer  200 , colored thermosensitive layer  400 , optional barrier layer  600  and optional heat resistant topcoating  700 . 
     Colored thermosensitive layer  400  is comprised of hollow organic pigments  403 , thermal solvents  404 , binders  401 , light stable colorants  302  and colored particles  405  all of which have been described elsewhere in this specification. 
     Thermographic substrate  30  utilizes a high lightness colored thermosensitive layer  400 . Upon coating, colored thermosensitive layer  400  is opaque and low in color saturation. Upon heating with a thermal printer, thermosensitive layer  400  decreases in opacity and increases in color saturation. Thermosensitive layer  400  is capable of developing sufficient visual contrast such that human and machine readable images may be printed by direct heating of said layer  400  with a thermal printhead. Such visual contrast is thermally developed by altering the light scattering capability of the layer  400 . 
     Preferably, the unimaged color saturation of thermographic substrate  30  is less than 0.9 Rd, and more preferably it is less than 0.8 Rd. In one embodiment, such unimaged color saturation is less than 0.3 Rd. 
     Upon imaging thermographic substrate  10  with a thermal printer, the imaged color saturation of thermographic substrate  30  is greater than 1.0 Rd, and, more preferably, greater than 1.2. In one embodiment, the imaged color saturation is greater than 1.3. 
       FIG. 4  is a schematic representation of a thermographic substrate  40  made in accordance with one preferred process of this invention; this Figure is not necessarily drawn to scale. Thermographic substrate  40  is comprised of flexible substrate  101 , optional surface layer  200 , colored thermosensitive layer  400  and a heat resistant thermosensitive layer  800 . 
     Colored thermosensitive layer  400  is comprised of hollow organic pigments  403 , thermal solvents  404 , binders  401 , light stable colorants  302  and colored particles  405 , all of which have been described elsewhere in this specifications. 
     Opaque thermosensitive layer  800  is comprised of hollow organic pigments  403 , thermal solvents,  404 , binders  501 , colorants  502 , abrasive particles  702  and lubricants  703 . 
     Thermographic substrate  40  utilizes two thermosensitive layers,  400  and  800 . Upon coating, thermosensitive layer  400  and  800  are opaque and low in color saturation. Upon heating with a thermal printer, thermosensitive layer  400  decreases in opacity and increases in color saturation while thermosensitive layer  800  decreases in opacity, becoming more transparent. Thermosensitive layers  400  and  800  are capable of developing sufficient visual contrast such that human and machine readable images may be printed by direct heating of said layers with a thermal printhead. Such visual contrast is thermally developed by altering the light scattering capability of both layers  400  and  800 . 
     Thermosensitive layer  800  is the top most layer of thermographic substrate  40  and as such is the layer whose top most surface  802  must come into direct contact with the thermal printhead. Thermosensitive layer  800  must be resistant to sticking to the thermal printhead and must enable thermographic substrate  40  to pass beneath said printhead with minimal friction, irrespective of the temperature of said printhead. Thermosensitive layer  800  should not rub off or build up on the thermal printhead as such material will interfere with the flow of heat from the printhead to the thermographic substrate  40 . 
     Thermosensitive layers with dual functionality as heat resistant topcoatings are known to those skilled in the art. For example, see U.S. Pat. No. 4,675,705 (the entire disclosure of which is hereby incorporated by reference into this specification) in which heat sensitive coating compositions for thermal printing incorporate both lubricants such as wax and zinc stearate and abrasive particles such as calcium carbonate and aluminum hydrate to reduce residue and sticking (on the printhead) and enhance printing. 
     The binder  501  for thermosensitive layer  800  may be any composition which does not cause sticking at temperatures of 150 degrees Celsius or higher. Binders described elsewhere in this specification such as binder  201 ,  301 , and  401 , may be used. In another preferred embodiment, binder  501  has a glass transition temperature of at least 50 degrees Celsius. In yet another preferred embodiment binder  501  has a glass transition temperature at least 70 degrees Celsius. 
     In a preferred embodiment, binder  501  is a cross-linked polyvinyl alcohol. Such crosslinked polyvinyl alcohol base topcoating binders have been described in U.S. Pat. No. 6,410,479, the entire disclosure of which is hereby incorporated by reference into this specification. 
     Preferably, thermosensitive layer  800  is comprised of polyvinyl alcohol. The polyvinyl alcohol use in thermosensitive layer  800  is preferably fully saponified, partially saponified, or denatured by carboxyl, amide, sulfonic acid or butyl aldehyde. 
     Thermosensitive layer  800  is preferably comprised of a crosslinked binder to further improve its heat resistance and ability not to stick to the thermal printhead. Hydroxyl containing binders such as polyvinyl alcohol, polyurethane, polyacrylates, polyesters, polyacetals and the like may preferably be crosslinked with dialdehydes such as glyoxal or polyaldehyde; one may also use epoxies such as diglycydil type, dimethylolurea such as glycerindiglycidylether; isocyanates; boron oxides; aziradines, oxazolines; and the like. 
     Thermosensitive layer  800  is preferably comprised of a lubricants  703  and abrasive particles  702  described, e.g., elsewhere in this specification. 
     Referring to  FIG. 4 , Thermosensitive layer  800  preferably has a coating weight  806  of from 0.5 to 20 grams per square meter, and, more preferably from 1 to 10 grams per square meter. 
     Preferably, the unimaged color saturation of thermographic substrate  40  is less than 0.8 Rd, and more preferably it is less than 0.5 Rd. In one embodiment, such unimaged color saturation is less than 0.15 Rd. 
     Upon imaging thermographic substrate  40  with a thermal printer, the color saturation of thermographic substrate  20  is greater than 0.8 Rd, and it preferably is greater than 1.0. In one embodiment, such color saturation is greater than 1.35. 
       FIG. 5  is a schematic representation of a thermographic substrate  50  made in accordance with one preferred process of this invention; this Figure is not necessarily drawn to scale. Thermographic substrate  50  is comprised of flexible substrate  101 , optional surface layer  200  and a colored, heat resistant thermosensitive layer  900 . 
     Colored, heat resistant thermosensitive layer  900  is comprised of hollow organic pigments  403 , thermal solvents  404 , binders  401 , light stable colorants  302 , colored particles  405 , abrasive particles  702  and lubricants  703 , all of which have been described elsewhere in this specification. 
     Thermographic substrate  50  utilizes thermosensitive layer  900 . Upon coating, thermosensitive layer  900  is opaque and low in color saturation. Upon heating with a thermal printer, thermosensitive layer  900  decreases in opacity and increases in color saturation. Thermosensitive layer  900  is capable of developing sufficient visual contrast such that human and machine readable images may be printed by direct heating of said layers with a thermal printhead. Such visual contrast is thermally developed by altering the light scattering capability of layer  900 . 
     Thermosensitive layer  900  is the top most layer of thermographic substrate  50  and, as such, is the layer whose top most surface  803  must come into direct contact with the thermal printhead. Thermosensitive layer  900  must be resistant to sticking to the thermal printhead and must enable thermographic substrate  50  to pass beneath said printhead with minimal friction, irrespective of the temperature of said printhead. Thermosensitive layer  900  should not rub off or build up on the thermal printhead as such material will interfere with the flow of heat from the printhead to the thermographic substrate  50 . 
     Colored, heat resistant thermosensitive layer  900  is comprised of a binder  401  which preferably has the same attributes as binder  501  of thermosensitive layer  800 . In a preferred embodiment, binder  401  is a cross-linked polyvinyl alcohol. Such crosslinked polyvinyl alcohol base topcoating binders have been described in U.S. Pat. No. 6,410,479, and they may be used in the process of this invention. The entire disclosure of this patent is hereby incorporated by reference into this specification. 
     Thermosensitive layer  900  is preferably comprised of a lubricants  703  and abrasive particles  702  described elsewhere in this specification. 
     Colored, heat resistant thermosensitive layer  900  may preferably be comprised of colorants  302  and colored particles  405  described elsewhere in this specification. 
     Colored, heat resistant thermosensitive layer  900  is preferably comprised of from about 1 to about 50 percent of colored particles  405  and/or colorant  302 . Thermosensitive layer  900  is more preferably comprised of from about 5 to about 40 percent of colored particles  405  and/or colorant  302 . Thermosensitive layer  900  is most preferably comprised of from about 10 to about 33 percent of colored particles  405  and/or colorant  302 . 
     Thermosensitive layer  900  is preferably coated from water. Water based compositions may be prepared such that many of the components remain separated and not intimately mixed at the time of coating and drying. Thermosensitive layer  900  may be comprised of dispersions of solids, emulsions of liquids, microcapsules, hollow organic pigments and the like. 
     Preferably, thermosensitive layer  900  is comprised of a continuous phase binder in which said dispersions, emulsions, microcapsules, pigments, dyes and hollow organic pigments are dispersed. 
     Referring again to  FIG. 5 , thermosensitive layer  900  is opaque and low in color saturation when initially applied to a flexible substrate  101 . However, upon heating with a thermal printhead, said layer  900  will become more transparent, enabling the colorants  405  and/or  302  to impart color saturation to the layer. 
     Thermosensitive layer  900  is comprised of colorants  405  and/or  302  which have sufficient resistance to image fading such that they are suitable for outdoors applications. Such colorants, comprising light stable dyes and/or pigments are described elsewhere in this specification. Before heating said thermosensitive layer  900 , colorants  405  and/or  302  are partially hidden by hollow organic pigments  402 . Light scattering by said pigments  402  reduces the effectiveness of such colorants to absorb incident light, lowering the color saturation of the layer. In areas printed with a thermal printer, the combination of heat from the thermal printhead and the thermal solvent  404  cause the collapse of the hollow organic pigment  402 . Such collapse of the hollow organic pigment  404  reduces the ability of colored, heat resistant thermosensitive layer  900  to scatter light. The thermally imaged portion of layer  900  thus becomes more transparent, increasing the capability of the colorants  405  and/or  302  to absorb incident light and raising the color saturation of the layer  900 . 
     Referring again to  FIG. 5 , thermosensitive layer  900  preferably has a coating weight  906  of 0.5 to 20 grams per square meter and, more preferably, from 1 to 10 grams per square meter. 
     Preferably, the unimaged color saturation of thermographic substrate  50  is less than 0.8 Rd; more preferably it is less than 0.5 Rd; and most preferably it is less than 0.15 Rd. 
     Upon imaging thermographic substrate  50  with a thermal printer, the color saturation of thermographic substrate  50  is greater than 0.8 Rd, preferably greater than 1.0; in one aspect of this embodiment, the color saturation is greater than 1.35. 
       FIG. 6  is a schematic representation of two thermal printing images which may be utilized to assess the performance of thermographic substrates for image quality over the course of extended printing operations. It is known to those skilled in the art that thermographic substrates may damage a thermal printhead after repeated printing either by depositing buildup on the printhead or by excessively wearing the printhead. Such damage can impede the flow of heat between the printhead and the thermographic substrate and in extreme, completely damage one or more heating elements of the printhead. The image  61  in  FIG. 6   b  may be repeatedly printed onto a long length of thermographic substrate to assess the impact of repeated printing on image quality. The printed lines  67  and  69  and printed text  68  parallel to the printing direction shown in the  FIG. 6   b  simulate extended printing under hot conditions while the unprinted areas simulate printing under cold temperatures. Before and after such an extended printing test the image  60  in  FIG. 6   a  may be printed to assess the impact on image quality. Image  60  is comprised on printed lines  62  and  64  and printed text  63  parallel to the printing direction and aligned relative to the print head in the same position as lines  67  and  69  and printed text  68 . If repeated printing of image  61  impacts the image quality of printed thermographic substrate, then a change should be observed between the initial image quality print  60  and the final image quality print  60 . In particular, if there is a degradation in of the image quality of the thermographic print then uniformity of the rectangle  65  will be degraded with streaks parallel to lines  62  and  64  and the continuity of the seven lines  66  perpendicular to the print direction will be broke for image  60  shown in  FIG. 6   a.    
       FIG. 7  describes a process  1000  used to prepare a thermographic substrate  10 . As is illustrated in  FIG. 7 , and in the preferred embodiment depicted therein, in step  1001  the thermographic substrate  10  is prepared by first selecting a flexible substrate  101 . The flexible substrate  101  is the base onto which the various layers of the thermographic substrate  10  will be applied. 
     The flexible substrate may be any of the flat, flexible supports described elsewhere in this specification including various papers and films. Step  1001  may include the treatment of the substrate  101  with corona discharge, flame, ionization or other such treatments to increase the surface energy of said substrate  101  to better facilitate wetting and adhesion of coated layers onto said substrate  101 . 
     Referring again to  FIG. 7 , in step  1002  fluid  305  is prepared. This color fluid  305  is comprised of the various compounds described elsewhere in this specification for the color layer  300  and may include colorant  302 , binder  301  as well as a fluid coating vehicle  815 . 
     Fluid coating vehicle  815  may be comprised of a liquid such as water or solvent. Alternatively, the fluid coating vehicle  815  may be comprised of a wax or resin which, when heated above its melting point or glass transition temperature, become a fluid. The components of the color layer  300  may be mixed into the fluid coating vehicle  815 . Some of the components of color layer  300  may be soluble in fluid coating vehicle  815 . Such soluble components are dissolved into fluid coating vehicle  815  and such dissolution may be facilitate by heating. 
     In a preferred embodiment, the fluid coating vehicle  815  is heated to a temperature which is lower than its boiling point to facilitate dissolution of a soluble component. 
     Some components, such as particulate matter, may be milled into said fluid coating vehicle  815  to form a particulate dispersion. Those skilled in the art will understand that milling methods may include, but are not limited to, three roll milling, grinding, ball milling, attrition, sonication, homogenization, small media milling and the like. After such milling the particulate components of the color layer  300  should have an average particle size from about 50 nm to about 10 microns. 
     Heated dispersions or solutions in the fluid coating vehicle  815  are preferably cooled prior to mixing with other components of the color layer  300 . 
     In a preferred embodiment, a defoaming additive is added to color fluid  305  to control foaming of said fluid  305 . 
     In a preferred embodiment, a surfactant or wetting additive is added to color fluid  305  to improve wetting of color fluid  305  onto substrate  101 . 
     Once all of the components of the color layer  300  are mixed or milled into the fluid coating vehicle  815 , they are combined together and mixed until they are homogeneously dispersed to form said color fluid  305 . 
     Referring again to  FIG. 7 , in step  1003  of process  1000  the color fluid  305  is applied to flexible substrate  101 . Step  1003  may use any of the commonly used coating processes known to those skilled in the art such as slot die, rotogravure coating, flexographic coating, roll coating, extrusion coating, lithographic coating, curtain coating and the like. In addition, various printing methodologies may be used to apply said color fluid  305  to said substrate  101  including, but not limited to, ink jet printing, flexo printing, letter press printing, gravure printing, stamp printing, pad printing and the like. 
     In a preferred embodiment, the color fluid  305  is printed onto the flexible substrate  101  to form an imaged color layer  300  comprised of text, graphics, codes and the like. 
     In another preferred embodiment, several different color fluids  305  are printed onto different regions of the flexible substrate  101  to form a multicolored color layer  300 . 
     When the fluid coating vehicle  815  used to prepare the color fluid  305  is a liquid, then process  1003  to apply the color fluid  305  to the substrate  101  will include a drying step to remove the fluid coating vehicle  815  from the color fluid  305  after it has been applied to the substrate  101 . Color layer  300  must be sufficiently dry so that it is no longer tacky or sticky and can be wound into a roll and resist blocking or adhering to the backside of substrate  101 . Preferably, color layer  300  should contain less than about 10% of fluid coating vehicle  815  after the application and drying of process  1003 . 
     In a preferred embodiment, the temperature of the drier in step  1003  of process  1000  does not exceed 100 degrees Celsius. 
     When the fluid coating vehicle  815  used to prepare the color fluid  305  is a wax or resin, then process  1003  to apply the color fluid  305  to the substrate  101  will heat the color fluid to a temperature above the melting point or glass transition temperatures of these materials. Typically, such temperatures will be between about 50 and 200 degrees Celsius. Process  1003  will include a cooling step to chill fluid coating vehicle  815  below its melting point or glass transitions temperature after the color fluid  305  has been applied to the substrate  101 . Color layer  300  must be sufficiently cool and solidified so that it is no longer tacky or sticky and can be wound into a roll and resist blocking or adhering to the backside of substrate  101 . Color layer  300  may comprise from about 30 to about 95 weight percent of said wax or resin fluid coating vehicle  815  after the application and cooling of process  1003 . 
     The color fluid  305  may be applied with step  1003  to form a color layer  300  of various thicknesses  306 . The color layer dry thickness  306  preferably has a thickness from about 0.1 micron to about 25 microns. 
     Referring again to  FIG. 7 , a thermosensitive fluid  505  is produced in step  1004 . This thermosensitive fluid  505  is comprised of the various compounds described elsewhere in this specification for the thermosensitive layer  500  and may include hollow sphere organic pigments  402 , thermal solvents  404 , particles  407 , binders  501  and optional colorants  502  as well as a fluid coating vehicle  815 . Fluid coating vehicle  815  may be comprised of a liquid such as water or solvent. In a preferred embodiment the fluid coating vehicle  815  is water. When the fluid coating vehicle is solvent, it must be a solvent which is capable of dispersing the hollow sphere organic pigments  402  without attacking, softening, swelling, collapsing or distorting said pigments  402 . The components of the thermosensitive layer  500  may be mixed into the fluid coating vehicle  815 . Some of the components of thermosensitive layer  500  may be soluble in fluid coating vehicle  815 . Such soluble components are dissolved into fluid coating vehicle  815  and such dissolution may be facilitate by heating. 
     In a preferred embodiment, the fluid coating vehicle  815  is heated to a temperature which is lower than its boiling point to facilitate dissolution of a soluble component. 
     Some components, such as particulate matter, may be milled into said fluid coating vehicle  815  to form a particulate dispersion as described elsewhere in this specification. 
     Heated dispersions or solutions in the fluid coating vehicle  815  are preferably cooled prior to mixing with other components of the thermosensitive layer  500 . 
     In a preferred embodiment, particles  407  are added to fluid coating vehicle  815  and mixed prior to adding other components of thermosensitive layer  500 . 
     In a preferred embodiment, a defoaming additive is added to thermosensitive fluid  505  to control foaming of said fluid  505 . 
     In a preferred embodiment, a surfactant or wetting additive is added to thermosensitive fluid  505  to improve wetting of color fluid  505  onto color layer  300 . 
     Once all of the components of the thermosensitive layer  500  are mixed or milled into the fluid coating vehicle  815 , they are combined together and mixed until they are homogeneously dispersed to form said thermosensitive fluid  505 . 
     Referring again to  FIG. 7 , step  1005  applies the thermosensitive fluid  505  to color layer  300  which in turn is applied to flexible substrate  101 . The application step  1005  may use any of the commonly used coating processes known to those skilled in the art such as slot die, rotogravure coating, flexographic coating, roll coating, extrusion coating, lithographic coating, curtain coating and the like. In addition, various printing methodologies may be used to apply said thermosensitive fluid  505  to said substrate  101  including, but not limited to, ink jet printing, flexo printing, letter press printing, gravure printing, stamp printing, pad printing and the like. 
     In a preferred embodiment, the thermosensitive fluid  505  is printed onto the color layer  300  to form an imaged thermosensitive fluid  505  comprised of text, graphics, codes and the like. 
     In another preferred embodiment, several different thermosensitive fluid  505  are printed onto different regions of the color layer  300  to form a multicolored thermosensitive layer  500 . 
     When the fluid coating vehicle  815  used to prepare the thermosensitive fluid  505  is a liquid, then step  1005  to apply the thermosensitive fluid  505  to the color layer  300  will include a drying step to remove the fluid coating vehicle  815  from the thermosensitive layer  500  after it has been applied to the color layer  300 . Thermosensitive layer  500  must be sufficiently dry so that it is no longer tacky or sticky and can be wound into a roll and resist blocking or adhering to the backside of substrate  101 . Preferably, thermosensitive layer  500  should contain less than about 10% of fluid coating vehicle  815  after the application and drying of process  1005 . 
     In a preferred embodiment, the temperature of the drier in step  1005  of process  1000  does not exceed 100 degrees Celsius. 
     The thermosensitive fluid  505  may be applied with step  1005  to form a thermosensitive layer  500  of various thicknesses  506 . Thermosensitive layer  500  preferably has a coating weight  506  of 0.5 to 20 grams per square meter, more preferably from 1 to 10 grams per square meter. 
     Referring again to  FIG. 7 , step  1006  prepares a barrier fluid  605 . This barrier fluid  605  is comprised of the various compounds described elsewhere in this specification for the barrier layer  600  and may include binders  501  as well as a fluid coating vehicle  815 . Fluid coating vehicle  815  may be comprised of a liquid such as water or solvents. In a preferred embodiment the fluid coating vehicle  815  is water. When the fluid coating vehicle is solvent, it must be a solvent which is capable of dispersing the hollow sphere organic pigments  402  without attacking, softening, swelling, collapsing or distorting said pigments  402 . The components of the barrier layer  600  may be mixed into the fluid coating vehicle  815 . 
     Some of the components of barrier layer  600  may be soluble in fluid coating vehicle  815 . Such soluble components are dissolved into fluid coating vehicle  815  and such dissolution may be facilitate by heating. 
     In a preferred embodiment, the fluid coating vehicle  815  is heated to a temperature which is lower than its boiling point to facilitate dissolution of a soluble component. 
     Alternatively, some components, such as particulate matter, may be milled into said fluid coating vehicle  815  to form a particulate dispersion as described elsewhere in this specification. 
     Heated dispersions or solutions in the fluid coating vehicle  815  are preferably cooled prior to mixing with other components of the barrier layer  600 . 
     In a preferred embodiment, a defoaming additive is added to barrier fluid  605  to control foaming of said fluid  605 . 
     In a preferred embodiment, a surfactant or wetting additive is added to barrier fluid  605  to improve wetting of barrier fluid  605  onto thermosensitive layer  500 . 
     Once all of the components of the barrier layer  600  are mixed or milled into the fluid coating vehicle  815 , they are combined together and mixed until they are homogeneously dispersed to form said barrier fluid  605 . 
     Referring again to  FIG. 7 , step  1007  applies the barrier fluid  605  to thermosensitive layer  500  which in turn is applied to color layer  300  on flexible substrate  101 . The application step  1007  may use any of the commonly used coating processes known to those skilled in the art such as slot die, rotogravure coating, flexographic coating, roll coating, extrusion coating, lithographic coating, curtain coating and the like. In addition, various printing methodologies may be used to apply said barrier fluid  605  to said thermosensitive layer  500  including, but not limited to, ink jet printing, flexo printing, letter press printing, gravure printing, stamp printing, pad printing and the like. 
     When the fluid coating vehicle  815  used to prepare the barrier fluid  605  is a liquid, then step  1007  to apply the barrier fluid  605  to the thermosensitive layer  500  will include a drying step to remove the fluid coating vehicle  815  from the barrier layer  600  after it has been applied to the thermosensitive layer  500 . Barrier layer  600  must be sufficiently dry so that it is no longer tacky or sticky and can be wound into a roll and resist blocking or adhering to the backside of substrate  101 . Preferably, barrier layer  600  should contain less than about 10% of fluid coating vehicle  815  after the application and drying of process  1007 . 
     In a preferred embodiment, the temperature of the drier in step  1007  of process  1000  does not exceed 100 degrees Celsius. 
     The barrier fluid  605  may be applied with step  1007  to form a barrier layer  600  of various thicknesses  606 . Barrier layer  600  preferably has a coating weight  606  of at least about 0.1 grams per square meter to 10 grams per square meter. 
     Referring again to  FIG. 7 , step  1008  prepares a topcoating fluid  705 . This coating fluid  705  is comprised of the various compounds described elsewhere in this specification for the heat resistant topcoat  700  which is comprised of heat resistant binder  701  and optionally one or more abrasive particles  702  and one or more lubricants  703  as well as a fluid coating vehicle  815 . Fluid coating vehicle  815  may be comprised of a liquid such as water or solvents. 
     In a preferred embodiment the fluid coating vehicle  815  is water. 
     When the fluid coating vehicle is solvent, it must be a solvent which is capable of dispersing the hollow sphere organic pigments  402  without attacking, softening, swelling, collapsing or distorting said pigments  402 . The components of the topcoat layer  700  may be mixed into the fluid coating vehicle  815 . 
     Some of the components of topcoat layer  700  may be soluble in fluid coating vehicle  815 . Such soluble components are dissolved into fluid coating vehicle  815  and such dissolution may be facilitated by heating. 
     In a preferred embodiment, the fluid coating vehicle  815  is heated to a temperature which is lower than its boiling point to facilitate dissolution of a soluble component. 
     Alternatively, some components, such as particulate matter, may be milled into said fluid coating vehicle  815  to form a particulate dispersion as described elsewhere in this specification. 
     Heated dispersions or solutions in the fluid coating vehicle  815  are preferably cooled prior to mixing with other components of the topcoat layer  700 . 
     In a preferred embodiment, a defoaming additive is added to topcoat fluid  705  to control foaming of said fluid  705 . 
     In a preferred embodiment, a surfactant or wetting additive is added to topcoating fluid  705  to improve wetting of topcoating fluid  705  onto barrier layer  600 . 
     Once all of the components of the topcoat layer  700  are mixed or milled into the fluid coating vehicle  815 , they are combined together and mixed until they are homogeneously dispersed to form said topcoat fluid  705 . 
     Referring again to  FIG. 7 , step  1009  applies the topcoat fluid  705  to barrier layer  600  which in turn is applied to the thermosensitive layer  500  which is applied on the color layer  300  which is applied on flexible substrate  101 . The application process  1009  may use any of the commonly used coating processes known to those skilled in the art such as slot die, rotogravure coating, flexographic coating, roll coating, extrusion coating, lithographic coating, curtain coating and the like. In addition, various printing methodologies may be used to apply said topcoat fluid  705  to said barrier layer  600  including, but not limited to, ink jet printing, flexo printing, letter press printing, gravure printing, stamp printing, pad printing and the like. 
     When the fluid coating vehicle  815  used to prepare the topcoat fluid  705  is a liquid, then step  1009  to apply the topcoat fluid  705  to the barrier layer  600  will include a drying step to remove the fluid coating vehicle  815  from the topcoat layer  700  after it has been applied to the barrier layer  600 . Topcoat layer  700  must be sufficiently dry so that it is no longer tacky or sticky and can be wound into a roll and resist blocking or adhering to the backside of substrate  101 . Preferably, topcoat layer  700  should contain less than about 10% of fluid coating vehicle  815  after the application and drying of process  1009 . 
     In a preferred embodiment, the temperature of the drier in step  1009  of process  1000  does not exceed 100 degrees Celsius. 
     The topcoat fluid  705  may be applied with process  1009  to form a topcoat layer  700  of various thicknesses  706 . Topcoat layer  700  preferably has a coating weight  706  of at least about 0.05 grams per square meter to 10 grams per square meter. 
     The thermographic substrate  10  assembly is prepared by building up, layer over layer, on a flexible substrate  101  a color layer  300  using color fluid  305  and application step  1003 , a thermographic layer  500  using thermosensitive fluid  505  and application step  1005 , a barrier layer  600  using barrier fluid  605  and application step  1007 , and finally a topcoating layer  700  using topcoating fluid  705  and application step  1009 . 
     Referring again to  FIG. 7 , the thermographic substrate  10  may be thermally annealed in process  810 . Such annealing helps to consolidate the various layers of thermographic substrate  10 , improving interlayer adhesion and thermographic performance. Such annealing is preferably done at temperatures less than about 60 degrees Celsius and for times from at least 1 minute to about 96 hours. 
     EXAMPLES 
     The following examples are presented to illustrate the claimed invention but are not to be deemed limitative thereof. Unless otherwise specified, all temperatures are in degrees Celsius, and all parts are by weight. 
     Example 1 
     This example illustrates the preparation of a thermographic substrate that comprises a base polyester flexible substrate (sold as part number “21940 THERMLfilm SELECT “by (FlexCON Inc of Spenser, Mass.), a color (black) layer, a thermo-sensitive opaque (white) layer, and a heat resistant topcoated layer that together will, when heated to a temperature range produced by a thermal printer printhead, produce a visually contrasting colored (black), image. 
     In the experiment described by this example, a black nitrocellulose gravure color ink was applied to the polyester substrate using a #4 Meyer rod for an average wet coverage of 3.57 grams per square meter, and it was allowed to dry, resulting in a black color layer with a dry coverage of 1.07 grams per square meter. 
     In such experiment, the black color layer coated on the polyester substrate was measured for reflective density (Rd) using a MacBeth (Grandville, Mich.) model RD914 densitometer. The Rd of the black color layer was 1.8. 
     An opaque thermosensitive coating ink having the following composition was prepared: 15.5 of a 30 percent styrene/acrylic hollow organic pigment dispersion in water (known as Ropaque OP-96 and sold by the Rohm and Haas Company, 100 Independence Mall West, Philadelphia, Pa.) was added to a small plastic mixing vessel. To this, 6.25 grams of a 40 percent dispersion in water of dibenzyl oxalate thermal solvent (sold as product HS-2046 by Cytech Products, 906 Peterson Dr., Elizabethtown, Ky.), 3.25 grams of a 40 percent calcium stearate dispersion (Cytech Products), 8 grams of C44 30 percent acrylic resin binder solution (CIBA, Port Arthur, Tex.) and 2.69 grams of tap water were added and stirred until the fluid was homogenous. Using #15 Meyer coating rod, the fluid was coated over the top of the black color layer, and the water was dried out of the layer using room temperature blown air. The white opaque thermosensitive layer had a dry coating weight of 5.5 grams per square meter and an of 0.16. 
     A heat resistant topcoating ink having the following composition was prepared: 4.07 grams of water in a heated vessel was brought to a temperature of 90 degrees Celsius, and 0.7 grams of polyvinyl alcohol (Celvol 103, sold by Sekisui Specialty Chem, Dallas, Tx.) were added and mixed until it was dissolved into the water. The solution temperature was cooled to 25 degrees Celsius. To this mixture the following materials were added: 1.44 grams of heat resistant varnish (sold by XSYS Print Solutions, 2401 Whitehall Park Dr, Charlotte N.Y.), 0.23 grams of calcium stearate dispersion as above, 0.3 grams of 15 percent silica dispersion (sold as Perkasil 660, by (WR Grace, 7500 Grace Dr., Columbia, Md.), 0.05 grams of fluorinated wetting agent (sold as Chemwet 29 by the Chemcor Corporation of 48 Leone Lane, Chester, N.Y.) and 2.29 grams of water; and the resulting mixture was mixed. 
     The protective layer was coated with a #6 drawdown rod over the top of the thermosensitive ink layer and dried. The heat resistant layer had a dry coat weight of 1.5 grams per square meter. The resulting thermographic substrate was subjected to a temperature of 45 degrees Celsius (in an oven) for 12 hours. The system was imaged using a Zebra LP2824-Z thermal printer at 2 inches per second print speed and a printer darkness setting of 30. The reflective density of the imaged area was tested to be 1.35, and the Rd of the unimaged area was 0.16, the resulting contrast ration was 7.43. 
     The imaged sample was subjected to a fading test in a QUV Photostability Chamber (sold by Q-Panel Lab Products of Cleveland, Ohio) for 144 hours of continuous exposure. The sample so tested was remeasured for Rd. The unimaged portion had an Rd of 0.20, and the imaged portion had an Rd of 1.33. Consequently, after this accelerated aging test, the imaged portion had a contrast ratio 5.65, substantially above the desired contrast ratio after exposure of 0.5 described elsewhere in this specification. 
     By comparison, when a prior art leuco-dye based thermographic system (obtained from Kanzaki Corporation of Japan) was similarly tested, it had a contrast ratio of 0.14. The contrast ratio of applicants&#39; device was 4013 percent greater than the contrast ratio of the prior art device. 
     It should be noted that, with the prior art device, the contrast ratio prior to the accelerated aging test was comparable (slightly higher) than the contrast ratio of applicants&#39; device. 
     Example 2 
     This example illustrates the preparation of a thermographic substrate that consists of a polyester flexible substrate (21940 THERMLfilm), a black color layer, an opaque (white) thermosensitive layer, and a heat resistant topcoating that together will, when heated to a temperature range, produce a visually colored (black) image. 
     The black color layer described in Example 1 was coated onto the polyester substrate via drawdown rod #4 and dried, and the reflective density was measured to be 1.8. 
     A white opaque thermosensitive coating ink having the following composition was prepared: 12.45 grams of a 30 percent styrene/acrylic hollow organic pigment dispersion in water known as “Ropaque 0-96” was added to a small plastic mixing vessel. To this, 2.08 grams of a 40 percent dispersion in water of dibenzyl oxalate thermal solvent (HS-2046), 2.78 g of a wax binder (a 30 percent Carnauba wax dispersion in water obtained from Koster Keunen, LLC, 1021 Echo Lake Rd., Watertown, Conn.), and 2.69 grams of tap water were added and stirred until the fluid was homogenous. Using a #15 coating rod, this was coated over the top of the black color layer, and the water was dried out of the layer using room temperature blown air to yield a dry coating weight of 5.72 grams per square meter. This thermographic substrate was white in color and had a measured reflective density of 0.15. 
     Similar to the procedure described in Example 1, a heat resistant topcoating was prepared and coated over the white opaque thermosensitive layer, tempered at 45 degrees Celsius for 12 hours, and imaged via Zebra LP2824-Z printer. The Rd of the imaged area was tested to be 1.65, and the Rd of the unimaged area was 0.15; the resulting contrast ratio was 10.0. 
     Example 3 
     Sample thermosensitive coatings comprised of combinations of five different hollow organic pigments and five different thermal solvents were evaluated for transparentization as a function of temperature. The following hollow organic pigments were used: 
                     TABLE 1                  Hollow Organic Pigments of Example 3                                                     wet   dry       per-       Organic           grams/   grams/   PS   cent       Pigment   Supplier   resin   cc   cc   μ   void               Ropaque   Rohm&amp;Haas   styrene/   1.03   0.63   0.5   42       OP-96       acrylate       Hique 168   Nae Woi   styrene/   1.02       0.5               acrylate       Hique 332   Nae Woi   styrene/   1.03       0.4               acrylate       Hique332L   Nae Woi   styrene/   1.03       0.4               acrylate       Hique2050   Nae Woi   styrene   1.02       1.2                    
The following thermal solvents, ranging in solubility parameter from 15.3 Megapascals 1/2  to 29.9 Megapascals 1/2  in Table 2 were used.
 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Thermal Solvents Used in Example 3 
               
            
           
           
               
               
               
               
            
               
                 Thermal Solvent 
                 δ MPa 1/2   
                 Supplier 
                 Location 
               
               
                   
               
               
                 Ethylene Glycol 
                 29.9 
                 Univar Inc. 
                 Tonawana, NY 
               
               
                 Dipropylene Glycol 
                 20.5 
                 Dow Corp. 
                 Midland, MI 
               
               
                 Dibenyl oxalate 
                 16.6 
                 DIC Corp. 
                 Osaka, Japan 
               
               
                 (HS2046) 
                   
                   
                   
               
               
                 Butyl Stearate 
                 15.3 
                 Sigma-Aldrich 
                 St. Louis, MO 
               
               
                   
               
            
           
         
       
     
     Samples were prepared by mixing 3 grams of one of the various hollow organic pigment dispersions, with 3 grams of water and 3 grams of a 15 percent solution of Celvol 103 polyvinyl alcohol binder. To this mixture was added 0.6 grams of Chemwet 29 surfactant and 0.75 grams of one of the aforementioned various thermal solvents. For each of the hollow organic pigments in this example, a sample was also prepared without the addition of any thermal solvent. The total number of samples prepared was 30. The mixture was then gently shaken to mix and coated with a #15 Myer Rod onto a Gardner Opacity Chart substrate (AG-5305/2813, available from BYK/Gardner Instruments of Columbia, Md.). The coating was then dried with a heat gun until dry to the touch. The opacity chart had a large black panel printed on it which can be used to gauge the opacity of the dried sample coating. Reflection densities were taken of the coating over the black panel of the opacity chart with an X-Rite 500 Series Spectrodensitometer (available from X-Rite Inc., Grandville, Mich.). Separate portions of each sample were heated to various temperatures by placing them in direct contact with a metal washer sitting atop a thermostatted hotplate for 10 seconds. After the samples were allowed to cool the Rd&#39;s of the heated portions were measured. 
     The contrast ratio ((Heated Rd−Unheated Rd)/Unheated Rd) is a measure of the change in opacity of the coated sample. The higher the contrast ratio, the more transparent the sample coating becomes upon heating. By transparentizing, the coating is less effective at covering the underlying black panel on the opacity chart. 
     Both the Rd of each unheated sample as well as the contrast ratio of each heated sample is shown in Table 3. For each of the hollow organic pigments, a sample was prepared without the addition of any thermal solvent (samples 3a, 3g, 3m, 3s, and 3y). As can be seen in Table 3, each of these samples, without thermal solvent, showed an increase in contrast ratio at 200 degrees Celsius. It might be noted that samples containing Ropaque 96, Hique 168 and Hique 332 hollow organic pigments all showed good transparentization at 200 degrees Celsius and slight transparentization at 175 degrees Celsius. Samples prepared with Hique 332L and Hique 2050 hollow organic pigments shown only slight transparentization at 200 degrees Celsius and no transparentization at 175 degrees Celsius or below. 
     The addition of thermal solvents to the samples generally lowered the temperature at which the sample coated showed good transparentization (contrast ratio greater than 0.5) and increased the un-imaged Rd. The increase in unimaged Rd was attributed to a dilution in the concentration of hollow organic pigments from the added thermal solvent, which accounted about 36 weight percent of the thermosensitive layer when added. Thermal solvents on the extremes of the solubility parameter range, ethylene glycol (29.9 Megapascals 1/2 ) and butyl stearate (15.3 Megapascals 1/2 ), were least effective at promoting transparentization of the sample coatings. Thermal solvents in the mid range of solubility parameters, dibenzyl oxalate (16.6 Megapascals 1/2 ), dipropylene glycol (20.5 Megapascals 1/2 ) and propylene carbonate (27.2 Megapascals 1/2 ), were most effective at promoting transparentization of the sample coatings. Across all 5 hollow organic bead types evaluated in this experiment, these intermediate solubility thermal solvents were able to reduce the temperature at which good transparentization occurred to 175 degrees Celsius or less. In fact, the dibenzyl oxalate thermal solvent was able to reduce the temperature down to 93 degrees Celsius for all 5 hollow organic pigment types (samples 3e, 3k, 3q, 3w and 3ac). 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Thermosensitive Layers of Example 3 
               
            
           
           
               
               
               
               
               
            
               
                 Sample 
                 Hollow 
                 δ 
                 Rd 
                 Contrast Ratio 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 ID 
                 Pigment 
                 Thermal solvent 
                 MPa 1/2   
                 20° C. 
                 93° C. 
                 150° C. 
                 175° C. 
                 200° C. 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 3a. 
                 Ropaque 96 
                 NONE 
                   
                 0.26 
                 0.0 
                 0.2 
                 0.7 
                 2.2 
               
               
                 3b. 
                 Ropaque 96 
                 ethylene glycol 
                 29.9 
                 0.46 
                 0.1 
                 0.3 
                 0.8 
                 1.4 
               
               
                 3c. 
                 Ropaque 96 
                 propylene carbonate 
                 27.2 
                 0.48 
                 −0.2 
                 0.6 
                 1.2 
                 1.5 
               
               
                 3d. 
                 Ropaque 96 
                 dipropylene glycol 
                 20.5 
                 0.46 
                 −0.1 
                 0.5 
                 1.9 
                 2.2 
               
               
                 3e. 
                 Ropaque 96 
                 dibenzyl oxalate 
                 16.6 
                 0.42 
                 0.8 
                 1.4 
                 1.7 
                 1.8 
               
               
                 3f. 
                 Ropaque 96 
                 butyl stearate 
                 15.3 
                 0.39 
                 0.1 
                 0.4 
                 0.7 
                 0.9 
               
               
                 3g. 
                 Hique 168 
                 NONE 
                   
                 0.29 
                 0.0 
                 0.2 
                 0.5 
                 1.6 
               
               
                 3h. 
                 Hique 168 
                 ethylene glycol 
                 29.9 
                 0.56 
                 −0.2 
                 0.0 
                 0.3 
                 0.7 
               
               
                 3i. 
                 Hique 168 
                 propylene carbonate 
                 27.2 
                 0.55 
                 −0.1 
                 0.3 
                 1.2 
                 1.3 
               
               
                 3j. 
                 Hique 168 
                 dipropylene glycol 
                 20.5 
                 0.52 
                 −0.2 
                 0.1 
                 1.4 
                 1.8 
               
               
                 3k. 
                 Hique 168 
                 dibenzyl oxalate 
                 16.6 
                 0.46 
                 1.2 
                 1.4 
                 1.6 
                 1.7 
               
               
                 3l. 
                 Hique 168 
                 butyl stearate 
                 15.3 
                 0.74 
                 0.1 
                 0.1 
                 0.4 
                 0.4 
               
               
                 3m. 
                 Hique 332 
                 NONE 
                   
                 0.26 
                 0.1 
                 0.0 
                 0.4 
                 1.6 
               
               
                 3n. 
                 Hique 332 
                 ethylene glycol 
                 29.9 
                 0.56 
                 0.1 
                 0.1 
                 0.5 
                 0.7 
               
               
                 3o. 
                 Hique 332 
                 propylene carbonate 
                 27.2 
                 0.6 
                 −0.1 
                 0.3 
                 1.0 
                 1.1 
               
               
                 3p. 
                 Hique 332 
                 dipropylene glycol 
                 20.5 
                 0.6 
                 −0.1 
                 0.0 
                 1.2 
                 1.5 
               
               
                 3q. 
                 Hique 332 
                 dibenzyl oxalate 
                 16.6 
                 0.51 
                 0.9 
                 1.1 
                 1.4 
                 1.4 
               
               
                 3r 
                 Hique 332 
                 butyl stearate 
                 15.3 
                 0.66 
                 0.0 
                 0.2 
                 0.6 
                 0.8 
               
               
                 3s. 
                 Hique 332L 
                 NONE 
                   
                 0.27 
                 0.0 
                 0.1 
                 0.2 
                 0.7 
               
               
                 3t. 
                 Hique 332L 
                 ethylene glycol 
                 29.9 
                 0.58 
                 −0.2 
                 0.1 
                 0.1 
                 0.3 
               
               
                 3u. 
                 Hique 332L 
                 propylene carbonate 
                 27.2 
                 0.49 
                 0.1 
                 0.4 
                 0.8 
                 1.0 
               
               
                 3v. 
                 Hique 332L 
                 dipropylene glycol 
                 20.5 
                 0.57 
                 −0.3 
                 0.0 
                 0.6 
                 1.1 
               
               
                 3w. 
                 Hique 332L 
                 dibenzyl oxalate 
                 16.6 
                 0.43 
                 0.9 
                 1.0 
                 1.5 
                 1.5 
               
               
                 3x. 
                 Hique 332L 
                 butyl stearate 
                 15.3 
                 0.51 
                 0.3 
                 0.4 
                 0.4 
                 0.4 
               
               
                 3y. 
                 Hique 2050 
                 NONE 
                   
                 0.25 
                 0.0 
                 0.1 
                 0.2 
                 0.6 
               
               
                 3z. 
                 Hique 2050 
                 ethylene glycol 
                 29.9 
                 0.4 
                 0.3 
                 0.1 
                 0.7 
                 0.7 
               
               
                 3aa. 
                 Hique 2050 
                 propylene carbonate 
                 27.2 
                 0.55 
                 −0.1 
                 0.2 
                 0.6 
                 0.9 
               
               
                 3ab. 
                 Hique 2050 
                 dipropylene glycol 
                 20.5 
                 0.53 
                 −0.2 
                 0.3 
                 0.5 
                 0.8 
               
               
                 3ac. 
                 Hique 2050 
                 dibenzyl oxalate 
                 16.6 
                 0.4 
                 1.0 
                 1.3 
                 1.6 
                 1.8 
               
               
                 3ad. 
                 Hique 2050 
                 butyl stearate 
                 15.3 
                 0.55 
                 −0.1 
                 0.1 
                 0.2 
                 0.3 
               
               
                   
               
            
           
         
       
     
     The differences in behavior exhibited by the different hollow organic pigments is likely the result of many factors including copolymer composition, pigment wall thickness, pigment Tg, pigment particle size and pigment solubility parameter. 
     Example 4 
     This example illustrates the preparation of a thermographic substrate consisting of a clay coated paper substrate (Fasson TT1C from Avery Corporation of Pasadena, Calif.), a color (black) layer, an opaque (white) thermosensitive layer, and a heat resistant topcoating that together will, when heated to a temperature range, produce a visually colored (black) image by transparency of the opaque coated layer as well as color shift of the colored (black) layer. 
     A thermosensitive coating ink of the following composition was prepared: 15.53 grams of a 30 percent styrene/acrylic hollow organic pigment dispersion in water known as Ropaque 0-96 was added to a small plastic mixing vessel. To this, 2.6 grams of a 40 percent dispersion in water of dibenzyl oxalate thermal solvent (HS-2046) 3.47 grams of a wax dispersion (30 percent Carnauba wax [milk] in water), 3.74 grams of a polyvinyl alcohol binder (Celvol 103, Clariant) solution 15 percent in water of 0.14 grams Cartabond GHF clay (Clariant), 1.15 grams of a carbon black dispersion (Aquasperse 1140 percent pigment in water by Evonik of Parsippany, N.J.), and 3.36 grams of tap water were added and stirred until the fluid was homogenous. Using a #6 coating rod, this thermosensitive ink was coated onto the paper substrate, and the water was dried out of the layer using room temperate blown air to yield a coating weight of 2.05 grams per square meter. This colored thermosensitive coating was gray in color and its measure reflective density was 0.61. 
     A white opaque thermosensitive coating ink having the following composition was prepared: 12.45 grams of 30 percent styrene/acrylic hollow organic pigment dispersion in water known as Ropaque 0-96 were added to a small plastic mixing vessel. To this, 2.08 grams of a 40 percent dispersion in water of dibenzyl oxalate (HS-2046), 2.78 grams of wax dispersion (30 percent Carnauba wax milk), and 2.69 grams of tap water were added and stirred until the fluid was homogenous. Using a #6 coating rod, this was coated over the top of the colored thermosensitive layer, and the water was dried out of the layer using room temperate blown air to yielding a coating weight of 2.90 grams per square meter. This system of coatings was white in color, and its measured reflective density was 0.19. 
     Similar to Example 1, a heat resistant topcoating was prepared and coated with a number 3 rod over the white opaque thermosensitive layer and dried at 45 degrees Celsius for 12 hours to yield a dry coating weight of 0.6 grams per square meter; and it was imaged via Zebra LP2824-Z printer. The reflective density of the imaged area was tested to be 1.05, and the Rd of the unimaged area was 0.19; the resulting contrast ratio was 4.52. 
     Example 5 
     This example illustrates the preparation of a thermographic substrate that comprises a polyester flexible substrate TC-390, a color layer, an opaque (white) thermosensitive layer, and a heat resistant topcoating that together will, when heated to a temperature range, produce a visually colored (black) image. 
     A color layer ink was prepared with pigment dispersion. The dispersion was made by melting together the following in a steel-ball attritor (SZEGVARI Attritor Model 01-HD from Union Process of Akron, Ohio) subjected to 110 degrees Celsius: 120 grams of carnauba wax (Strohmeyer and Arpe, Short Hills, N.J.) and 120 grams of Paraffin wax 05 (Koster Keunen) were charged. 400 cubic centimeters of stainless steel ball media were added and the attritor set to 500 revolutions per minute. To this, 60 grams of Printex 25 carbon black pigment (Evonik) was added and milled for 30 minutes. The resulting dispersion was removed from the attitor and media and kept hot. 
     The attritor was cleaned and set to room temperature. 270 grams of water were added along with 400 cubic centimeters of clean stainless media. To this, 30 grams of polyvinyl alcohol (Celvol 103) were added, and the temperature brought to 90 degrees Celsius while attriting for 30 minutes. The temperature was brought to 80 degrees Celsius, and, while attriting, 75 grams of the hot pigment dispersion was added. The temperature was allowed to cool 1 degree Celsius per minute until the fluid reached 30 degrees Celsius. This dispersion was removed from the attritor. 
     A thermosensitive coating ink having the following composition was prepared: 12.84 grams of Ropaque 0-96 hollow organic pigment dispersion were added to a small plastic mixing vessel. To this, 6.45 grams of a 40 percent dispersion in water of dibenzyl oxalate (HS-2046) and 10.71 grams of the above pigment/wax dispersion were charged and stirred until the fluid was homogenous. Using a #15 coating rod, this was coated onto the base paper and the water was dried out of the layer using room temperate blown air to yield a coating weight of 5.72 grams per square meter. This colored thermographic layer was gray in color and measured 0.68 Rd. 
     A white opaque thermosensitive coating ink having the following composition was prepared: 15.5 of a 30 percent styrene/acrylic hollow organic pigment dispersion in water known as Ropaque 0-96 were added to a small plastic mixing vessel. To this, 6.25 grams of a 40 percent dispersion in water of dibenzyl oxalate thermal solvent (HS-2046), 3.25 grams of a 40 percent calcium stearate dispersion (Cytech Products), 8 grams of a C44 30 percent acrylic resin solution, and 2.69 grams of tap water were charged and stirred until fluid was homogenous. Using a #15 coating rod, this was coated over the top of the color thermosensitive layer, and the water was dried out of the layer using room temperature blown air to yield a coating weight of 6.76 grams per square meter. This system of coatings was white in color and measured 0.19 in reflective density. 
     The samples were heated on a hot plate to 175 degrees Celsius for 10 seconds and thereafter allowed to cool. The Rd&#39;s of the heated portions were measured to be 1.42, the unheated portion was measured to be 0.19 and the resulting contrast ration was 6.47. 
     Example 6 
     This example illustrates the preparation of a thermographic substrate that comprises a synthetic paper (PB1 HG polypropylene film from Granwell Products, West Caldwell, N.J.), a color layer, an opaque (white) thermosensitive layer, and a heat resistant topcoating. When thermally imaged, this thermographic substrate will produce a visually colored image by transparency of the opaque thermosensitive coated layer. In addition, this example illustrates a thermographic substrate that maintains significant image contrast when exposed to temperatures less than 100 degrees Celsius. 
     The black color layer described in Example 1 was coated onto the synthetic paper via Meyer rod #4 and dried, and the reflective density was measured to be 2.0. 
     Ten different thermosensitive coating inks were then prepared, nine of which contained an inorganic particle additive and one without any additive. The details on the inorganic particle additives used in this example may be found in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Inorganic particles of Example 6 
               
            
           
           
               
               
               
               
            
               
                 Inorganic 
                   
                   
                   
               
               
                 Particle 
                 Supplier 
                 Type 
                 Size 
               
               
                   
               
               
                 Burgess 17 
                 Burgess Pigment Co. 
                 High bright 
                 0.5μ 
               
               
                   
                   
                 hydrous kaolin 
                   
               
               
                   
                   
                 clay 
                   
               
               
                 Burgess 40 
                 Burgess Pigment Co. 
                 Medium bright 
                 1.3μ 
               
               
                   
                   
                 hydrous kaolin 
                   
               
               
                   
                   
                 clay 
                   
               
               
                 Burgess 97 
                 Burgess Pigment Co 
                 Medium bright 
                 0.2μ 
               
               
                   
                   
                 hydrous kaolin 
                   
               
               
                   
                   
                 clay 
                   
               
               
                 Optiwhite 
                 Burgess Pigment Co. 
                 Premium white 
                 1.4μ 
               
               
                   
                   
                 flash calcined 
                   
               
               
                   
                   
                 clay 
                   
               
               
                 Kaobrite 
                 Thiele Kaolin Co. 
                 No. 2 Std. 
                 80-86% &lt; 2μ 
               
               
                   
                   
                 hydrous coating 
                   
               
               
                   
                   
                 clay 
                   
               
               
                 Kaogloss 
                 Thiele Kaolin Co. 
                 No. 1 Std. 
                 90-94% &lt; 2μ 
               
               
                   
                   
                 hydrous coating 
                   
               
               
                   
                   
                 clay 
                   
               
               
                 Minex 10 
                 Unimin Corporation 
                 Nepheline syenite 
                 2.3μ 
               
               
                   
                   
                 microcline 
                   
               
               
                 Minex 12 
                 Unimin Corporation 
                 Nepheline syenite 
                 — 
               
               
                   
                   
                 microcline 
                   
               
               
                 Satintone 5 
                 BASF Corporation 
                 High brightness 
                 0.8μ 
               
               
                   
                   
                 calcined 
                   
               
               
                   
                   
                 aluminosilicate 
                   
               
               
                   
                   
                 clay 
               
               
                   
               
            
           
         
       
     
     All of the inorganic particles used in this example were received as dry powders. They were dispersed in water to make 30 percent (by weight) dispersions by adding 15 grams of powder and 35 grams of tap water to a vessel and stirring until homogeneous. Then an ink base was made with 26.96 grams of C44 binder, 40.84 grams of a 52 percent dispersion in water of 1,2-bis(3-methylphenoxy) ethane thermal solvent, 58.84 grams of a 30 percent styrene/acrylic hollow organic pigment dispersion in water (Hique 168), 1.35 grams of a fluoro-surfactant (Chemwet 29), and 0.31 grams of tap water, which were added to a vessel and stirred. For each different type of inorganic particles, 12.8 grams of this ink premix were added to a smaller vessel with 7.2 grams of one of the inorganic particle dispersions, and each was shaken until homogeneous to form a thermosensitive coating solution. 
     Using a #15 coating rod, each thermosensitive coating ink was coated over the top of a separate sheet of black color layer coated synthetic paper and dried using a heat gun to form a white, opaque thermosensitive layer. 
     Similar to the procedure described in Example 1, a heat resistant topcoating was prepared, coated over each of the white, opaque thermosensitive layers, and dried at 50 degrees Celsius for 12 hours to form a thermosensitive substrate. 
     Each of the 10 thermosensitive substrates prepared in this example were thermally imaged with a Zebra LP2824-Z printer set to a printing speed of 3 inches per second and printer energy of 20. 
     Dry coating weights, and reflective densities of both the background (white) and printed area (black) for each ink are shown in Table 5. While all of the inorganic particle containing thermographic substrates in this example showed lower image Rd than the thermographic substrate without particles, those prepared with calcined clay were significantly lower in image Rd than those containing hydrous clays or microclines. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Data for Example 6 
               
            
           
           
               
               
               
               
               
            
               
                 Inorganic 
                 Coat weight 
                 RD 
                 RD 
                 Contrast 
               
               
                 Particle 
                 g/m 2   
                 (background) 
                 (Image) 
                 Ratio 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 None 
                 8.22 
                 0.21 
                 1.73 
                 7.24 
               
               
                 Burgess 17 
                 10.08 
                 0.18 
                 1.39 
                 6.72 
               
               
                 Burgess 40 
                 9.96 
                 0.18 
                 1.20 
                 5.67 
               
               
                 Burgess 97 
                 9.48 
                 0.18 
                 1.48 
                 7.22 
               
               
                 Optiwhite 
                 10.04 
                 0.16 
                 0.90 
                 4.63 
               
               
                 Kaobrite 
                 9.52 
                 0.18 
                 1.44 
                 7.00 
               
               
                 Kaogloss 
                 10.04 
                 0.18 
                 1.57 
                 7.72 
               
               
                 Minex 10 
                 9.56 
                 0.19 
                 1.50 
                 6.89 
               
               
                 Minex 12 
                 9.72 
                 0.20 
                 1.55 
                 6.75 
               
               
                 Satin Tone 5 
                 9.44 
                 0.14 
                 0.95 
                 5.79 
               
               
                   
               
            
           
         
       
     
     To evaluate the thermal image stability of each of the 10 thermosensitive substrates, thermally imaged samples described in Table 5 were exposed to temperatures of 70 degrees Celsius and 80 degrees Celsius continuously for 4 hours, and then allowed to cool to room temperature. The background and image Reflection densities were then re-measured for each example after this heating and the contrast ratio&#39;s calculated. These data are summarized in Table 6. 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Contrast Ratios for Example 6 
               
            
           
           
               
               
               
               
            
               
                 Inorganic 
                 Contrast Ratio 
                 Contrast Ratio 
                 Contrast Ratio 
               
               
                 Partilce 
                 (Original) 
                 (70 C.) 
                 (80 C.) 
               
               
                   
               
               
                 None 
                 7.24 
                 1.38 
                 0.00 
               
               
                 Burgess 17 
                 6.72 
                 3.97 
                 0.41 
               
               
                 Burgess 40 
                 5.67 
                 2.18 
                 0.07 
               
               
                 Burgess 97 
                 7.22 
                 4.39 
                 1.05 
               
               
                 Burgess Optiwhite 
                 4.63 
                 2.14 
                 0.45 
               
               
                 Kaobrite 
                 7.00 
                 3.81 
                 0.42 
               
               
                 Kaogloss 
                 7.72 
                 3.62 
                 1.00 
               
               
                 Minex 10 
                 6.89 
                 2.21 
                 0.00 
               
               
                 Minex 12 
                 6.75 
                 2.56 
                 0.35 
               
               
                 Satin Tone 5 
                 5.79 
                 3.65 
                 1.00 
               
               
                   
               
            
           
         
       
     
     The effect of different types of clay on resisting transparentization when exposed to heat is more evident at 80 degrees Celsius. Table 6 shows the original contrast ratio, and then the contrast ratios after both 70 degrees Celsius and 80 degrees Celsius exposure. As described earlier, the contrast ratio ((Heated (imaged) Rd−Unheated (un-imaged) Rd)/Unheated (un-imaged) Rd) is a measure of the change in opacity of the thermosensitive substrate. In this example, the higher the contrast ratio, the more opaque the background remains during exposure to heat and thus the greater the thermal image stability. When the contrast ratio is lower, it means the thermosensitive layer is less effective at covering the underlying color layer because it loses opacity due to heat exposure. 
     As can be seen in Table 6, although some inorganic particle types are better than others at increasing the thermal image stability (high contrast ratio) of the thermosensitive layer, overall the examples with particles performed better than the example without particles. In addition, smaller particles appear to offer greater image stability than larger particles. 
     Example 7 
     This example illustrates the function of a barrier layer between the heat resistant topcoat and the thermosensitive layer. Two thermographic substrates were prepared, one with a barrier layer and one without. These thermographic substrate were prepared on a base of 3.2 mil synthetic paper (Sold as FPG-80, Yupo, Chesapeake, Va.), along with a color layer (black), an opaque (white) thermographic layer, a barrier layer and a heat resistant topcoat. These layers together when thermally imaged produced a visually colored (black) image. 
     The black color layer described in Example 1 was coated onto the synthetic paper substrate with a #4 Meyer rod and dried with room temperature blown air; the reflective density was measured to be 1.80. 
     The opaque (white) thermographic layer as described in Example 6 (without inorganic particles) was coated over the color layer with a #15 Meyer rod, and dried with a hot air gun. The dry coating weight of the thermographic layer was 8.50 grams per square meter, and the Rd was 0.15. 
     A barrier layer coating solution was prepared: 29.25 grams of a 50 percent solids binder of ethylene vinyl chloride copolymer emulsion in water (Sold as Airflex 4500, Air Products, Allentown, Pa.) was added to a small plastic mixing vessel. To this, 0.94 grams of 40 percent solids fluoro-surfactant (Sold as Chemwet 29) and 69.81 grams of tap water were added to the vessel and stirred until the fluid was homogeneous. Using a #4 Meyer rod this was coated over the top of the thermographic layer and dried using room temperature blown air. The barrier layer was determined to have a dry coating weight of 1.00 grams per square meter. 
     Similar to the procedure described in Example 1, a heat resistant topcoat was prepared and coated over the barrier layer and oven dried at 45 degrees Celsius for 24 hours and thermally imaged with a Zebra LP2824-Z printer at a speed of 3 inches per second and an energy setting of 20. The reflective density of the imaged area was tested to be 1.65, and the Rd of the un-imaged area was 0.15; the resulting contrast ratio was 10.0. 
     In the case of the no barrier layer sample, the thermographic substrate was prepared in a similar fashion to Example 6 (no inorganic particles). This thermographic substrate was thermally imaged with a Zebra LP2824-Z printer at a speed of 3 inches per second and a print energy of 20. The reflective density of the imaged area was tested to be 1.73, and the Rd of the un-imaged area was 0.21; the resulting contrast ratio was 7.24. 
     The resulting thermographic substrates were thermally imaged with a Zebra 140Xi III plus 203 dpi printer in order to test for print degradation over extended printing lengths. A print was produced at the start of the trial to assess the image quality (see  FIG. 6   a ). The image quality print was 8 inches long and 4 inches wide and consisted of a first 0.0625 inches wide line parallel to the printing direction and 4.6875 inches long. In a similar fashion a second 0.0625 inches inch line was printed parallel to the print direction of the print. The second line was 2.1875 inches  long. In between the first and second line a text string was printed parallel to the first and second lines. The text string was the same length as the second line and was also 0.0625 inches wide. Directly after the text string and first and second lines a large solid fill rectangle was printed. The rectangle was 1.375 inches wide and 3.875 inches long. After printing the rectangle, a set of seven parallel lines, each 0.0625 inches wide and perpendicular to the print direction were printed below the solid fill. The separation between each line was 0.0625 inches. The first image quality print for each of the thermographic substrates of this example showed no signs of print degradation in the lines or solid fill. 
     A test print, see  FIG. 6   b , was the same size as the image quality print. However, the test print did not include either the solid fill rectangle or the seven perpendicular lines. The length of the first line was 6.375 inches and the length of the image was 8 inches. The first line was printed over 80 percent of the length of the image quality print and thus was said to be an 80 percent duty cycle vertical line. The second line was 2.375 inches long and in a similar fashion was a 30 percent duty cycle line. In between the first and second line a text string was printed parallel to the first and second lines. The text string was the same length as 30 percent duty cycle second line and was also 0.0625 inch wide. 
     The test print image was repeatedly printed until a total of 400 meters of the thermosensitive substrate had been imaged by the Zebra 140Xi III printer. After the 400 meters of test prints were completed, the thermal print head of the 140Xi III was opened up and cleaned. Cleaning was conducted using a lint free cloth (Sold as KimWipe, Kimberly Clark) wet with isopropyl alcohol. The alcohol wet cloth was wiped back and forth over the print head 6 times and the print head was allowed to dry. After cleaning, a second image quality print was made onto the thermographic substrates of this example. The image quality of the print was scored as follows: If no print degradation was observed the score was 10; if slight lightening in the solid fill rectangle or the seven perpendicular lines was noticeable in the image quality print the score was 5; if significant lightening in the solid fill rectangle or the seven perpendicular lines was noticeable in the image quality print the score as 3; if a total loss or printing in the solid fill rectangle or the seven perpendicular lines was noticeable in the image quality print of if physical damage to the print was observed, then the score was 0. 
     Table 8 summarizes the image quality extended print results for the thermographic coatings of this example. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 Extended Print Test Results 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Image Quality 
               
               
                   
                   
                   
                 after 400M 
               
               
                 Barrier Binder 
                 Supplier 
                 Chemistry 
                 Thermal Printing 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 No Barrier Layer 
                 — 
                 — 
                 5 
               
               
                 Airflex 4500 
                 Air 
                 Ethylene Vinyl 
                 10 
               
               
                   
                 Products 
                 Chloride 
               
               
                   
               
            
           
         
       
     
     Example 8 
     This example illustrates the preparation of a thermographic substrate that comprises a base of a 3.2 mil synthetic paper (Sold as FPG-80, Yupo, Chesapeake, Va.), a color layer (black), an opaque (white) thermographic layer, a barrier layer and a heat resistant topcoat. These layers together when heated produced a visually colored (black) image. 
     The black color layer described in Example 1 was coated onto the synthetic paper substrate with a #4 Meyer rod and dried, the Rd was measured to be 1.80. 
     The opaque (white) thermographic layer as described in Example 6 (without clay) was coated over the described black layer with a #15 Meyer rod for and let dry for a dry coating weight of 8.50 grams per square meter and a Rd of 0.15. 
     The barrier layer as described in Example 7 was coated over the described opaque thermographic layer. The barrier layer was coated with a #4 Meyer rod and let dry for a coating weight of 1.00 grams per square meter. 
     Five topcoatings comprised of five different binders were prepared in this example, the details of which are summarized in Table 9. 
     
       
         
           
               
             
               
                 TABLE 9 
               
             
            
               
                   
               
               
                 Binders of Example 8 
               
            
           
           
               
               
               
               
            
               
                   
                 Binder 
                 Supplier 
                 Chemistry 
               
               
                   
               
               
                   
                 Joncryl 1982 
                 BASF 
                 Self Crosslinking 
               
               
                   
                   
                   
                 Acrylic 
               
               
                   
                 Neocryl XK-12 
                 DSM 
                 Self Crosslinking 
               
               
                   
                   
                 NeoResins 
                 Acrylic 
               
               
                   
                 Celvol 103 
                 Celanese 
                 Polyvinyl Alcohol 
               
               
                   
                 Bariastar 
                 Mitsui 
                 Acrylic Sphere 
               
               
                   
                 B-2000 
                   
                 Dispersion 
               
               
                   
               
            
           
         
       
     
     Each heat resistant topcoating solution of this example was prepared by mixing 87.30 grams of each such binder, each adjusted to 15 percent solids in water, in a plastic vessel. To each binder 5.63 grams of a 15 percent solids dispersion of precipitated silica (sold as Perkasil SM660, Grace Davison), 5.51 grams of silicone emulsion (sold as SF-48E, Cross Chemical), 0.48 grams of silicone emulsion (sold as HV495, Dow Chemical), and 1.08 grams of fluoro-surfactant (Sold as Chemwet 29, Chemcor) were added and stirred until homogeneous. 
     Each of the four heat resistant topcoating solutions was then coated onto the barrier layer of the thermographic substrate with a #7 Meyer rod and dried with a hot air gun; they each had a dry coating weight of 2.25 grams per square meter. The coated substrates were oven dried at 45 degrees Celsius for 24 hours. The image quality after extended printing was evaluated for each topcoated thermographic substrate in the same fashion as those tested in Example 7 and is summarized in Table 10. 
     
       
         
           
               
             
               
                 TABLE 10 
               
             
            
               
                   
               
               
                 Extended Print Testing of Binders in Topcoat 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Image Quality 
               
               
                   
                   
                   
                 after 400M 
               
               
                 Binder 
                 Supplier 
                 Chemistry 
                 Thermal Printing 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Joncryl 1982 
                 BASF 
                 Self Crosslinking 
                 10 
               
               
                   
                   
                 Acrylic 
                   
               
               
                 Neocryl XK-12 
                 DSM 
                 Self Crosslinking 
                 10 
               
               
                   
                 NeoResins 
                 Acrylic 
                   
               
               
                 Celvol 103 
                 Celanese 
                 Polyvinyl Alcohol 
                 3 
               
               
                 Bariastar 
                 Mitsui 
                 Acrylic Sphere 
                 10 
               
               
                 B-2000 
                   
                 Dispersion 
               
               
                   
               
            
           
         
       
     
     Example 9 
     This example illustrates the preparation of a thermographic substrate that comprises a synthetic paper (PB1HG polypropylene film) layer, an opaque (white) thermosensitive layer, and a heat resistant topcoating that together, when heated to a temperature range, produced a visually colored image by transparency of the opaque thermosensitive layer and that maintained significant contrast when exposed to ultraviolet and visible light illumination. 
     The black color layer described in Example 1 was coated onto the synthetic paper via Meyer rod #4 and dried with a heat gun. The black color layer reflective density was measured to be 2.0. 
     A thermosensitive coating fluid was prepared as described in Example 6 using Burgess 17 clay. Then, using a #15 coating rod, the thermosensitive fluid was coated over the top of the black color layer coated synthetic paper and dried using a heat gun to form a white, opaque thermosensitive layer. 
     Similar to the procedure described in Example 8, a heat resistant topcoating was prepared and coated over the thermosensitive layer and dried at 50 degrees Celsius for 12 hours to form a thermographic substrate. 
     Both the thermographic substrate prepared in this example as well as a conventional leuco dye containing thermographic substrate (PolyPro 4000D 3.8 mil Receipt supplied by Zebra Technologies Lincolnshire, Ill. 60069 USA) were thermally imaged with a Zebra LP2824-Z printer set to a printing speed of 3 inches per second and a printer energy of 20. 
     The Rd&#39;s of the background (white) and printed area (black) for each of the two substrates, as well as the calculated contrast ratios are shown in Table 11. 
     
       
         
           
               
             
               
                 TABLE 11 
               
             
            
               
                   
               
               
                 Reflection Densities and Contrast Ratios for Example 9 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 RD 
                 RD 
                 Contrast 
               
               
                   
                 Substrate 
                 (background) 
                 (Image) 
                 Ratio 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Thermographic 
                 0.18 
                 1.55 
                 7.61 
               
               
                   
                 Example 
                   
                   
                   
               
               
                   
                 PolyPro 4000 D 
                 0.05 
                 1.56 
                 30.2 
               
               
                   
               
            
           
         
       
     
     To evaluate the thermal image stability of each of these substrates, the thermally imaged samples described in Table 11 were placed in the QUV chamber (QUV Accelerated Weathering Tester, Q-Lab, Cleveland, Ohio) containing UVB-313 light bulbs (Q-Lab, Cleveland, Ohio) which was set to 60 degrees Celsius for 24 hours and then allowed to cool to room temperature. The background and image Rd&#39;s were then re-measured after this exposure and the contrast ratios calculated. These data are summarized in Table 12. 
     
       
         
           
               
             
               
                 TABLE 12 
               
             
            
               
                   
               
               
                 Reflection Densities and Contrast Ratios after QUV Exosure 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 Contrast 
                 Contrast 
               
               
                   
                 RD 
                 RD 
                 Ratio 
                 Ratio 
               
               
                 Substrate 
                 (background) 
                 (Image) 
                 (Original) 
                 (QUV) 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Thermographic 
                 0.41 
                 1.59 
                 7.33 
                 2.87 
               
               
                 Example 
                   
                   
                   
                   
               
               
                 PolyPro 4000 D 
                 0.55 
                 1.43 
                 30.6 
                 1.60 
               
               
                   
               
            
           
         
       
     
     As can be seen in Table 12, while the contrast ratio of the PolyPro 4000 D was originally much higher than that of the thermosensitive substrate prepared in this example, the decrease in contrast ratio of the thermosensitive substrate was much less significant after exposure to both heat and UV in the QUV chamber than the leuco dye containing PolyPro 4000D substrate. 
     Applicants have described certain preferred processes and the products produced thereby. Many variations of these processes and products will be apparent to those skilled in the art; and they are intended to be comprehended within the scope of the invention. 
     There are many applications in which applicants&#39; novel thermographic substrate can be advantageously used. These include, by way of illustration, pipe identification labels (the durability of such labels makes them useful for many years after the installation of the pipe), electrical wiring labels (the durability of the labels is similarly advantageous in this use), fiber optic cable labels, nursery tags (the resistance of the tags to weathering makes them advantageous for this use), livestock tags (the labels are inedible and resistant to livestock), receipt printing, VIN # labels (where the fading resistance property is advantageous), shelf labelling (where the fading resistance makes the labels advantageous for both indoor and outdoor uses), part labelling, address labels, bar codes, inventory asset tags, fruit labeling, street and traffic signage, license plates, driver licenses, hunting licenses, identification cards, and the like.