IMAGE RECEIVING ELEMENT FOR THERMAL PRINTERS

Abstract: Image receiving medium comprising a substrate bearing on at least one major surface thereof a coating of heat-­ sensitive material comprising (a) material capable of existing in a supercooled state after melting and subsequent cooling, (b) at least one anti-fouling agent, and (c) optionally, a binder. The anti-fouling agent can be a wax, a silica, a metal silicate, or mixtures thereof.

Description: IMAGE RECEIVING ELEMENT FOR THERMAL PRINTERS BACKGROUND OF THE INVENTION This invention relates to imaging systems, and, more particularly, to receiving element useful in thermal imaging systems. Processes wherein images can be formed by causing a heat-sensitive material to become tacky or fluid in image areas upon imagewise application of heat and then developed by adhering an imaging powder to the tacky image areas are known. An example of such a process is described in U.S. Patent 3,z41,96. Thermal print h-aAs can be used to tackify or fluidize the r--at-sensitixe aterial to form the latent image. A simple thermal print head comprises at least one resistance element between two conductors. The thermal print head may also comprise an array of resistance elements. Thus, for example, there nay be a 5 by 7 element array on the print head. Additionally, the print head away be fixed or moveable with respect to the surface to be imaged. The latent image matter is formed by cortaotln the resistance element to the heat-sensitive material, providing electric current to the element for a ti e sufficient to heat the element and raise its temperature to a level sufficient to melt the material in the area of contact, discontinuing the electric current to the element, and relocating the element with respect to the material. The steps of contacting, heating and relocating are repeated until a sufficient number of melted dot-like areas have been provided to define the desired intent liquid r'age. When the print head has only a single ele --nt, @e steps necessary to form the latent image must be repeated frequently before an image has been defined. then the print head comprises an array (or matrix) of elements, the steps necessary to form the latent image formation need be repeated fewer times. A serious problem frequently encountered with thermal print heads is fouling thereof with the heat-sensitive material of the image receiving surface. Generally, the print head is placed in direct contact with the heat-sensitive material. If even a small amount of material from the heat-sensitive coating transfers to the print head and forms a deposit thereon, resolution or image density, or both, is drastically reduced. In many cases, the thermal print heads are not readily accessible for easy cleaning. Some manufacturers of thermal printers recommend passing coarse bond paper through the printer to abrade the deposits from the print head. It is desirable to increase the interval between recommended cleanings of thermal print heads in order to save time and improve reso ution. SUMMARY OF THE INVENTION The image receiving medium of the present invention comprises a substrate, e.g., a sheet, bearing on at least one major surface thereof a coating of heatsensitive material comprising (a) arterial capable of existing in a supercooled state after melting and st:bseuent cooling, (b) t least one anti-fouling agent selected frorr the group consisting of wax s, silicas, petal silicates, and mixtures thereof, and (c) tioally, a binder. Upon being imagewise heated with a thermal print head, a sheet bearing the aforementioned heat-sensitive coating material becomes tacky in the image areas. Particles of imaging powder can be adhered to these pacified areas. Optionally, the resulting images can be simultaneously or subsequently fixed. The advantage of the h.=at-sensitive sating described is that the thermal print head will avoid being fouled with residue from the coating material, thus assuring formation of images having high resolution for extended periods of use, without the necessity for frequent cleaning of the print head. DETAILED DESCRIPTION The material capable of existing in a supercooled state after melting and subsequent cooling, hereinafter referred to as supercooling material, must have a melting temperature about 10 C above ambient temperature. Ambient temperature, as used herein, refers to the temperature of the environment wherein the imaging process is conducted (e.g., room temperature of about 19 C to 200C). The material of the coating must also form a supercooled melt when cooled to a tempera-ure below its melting temperature, i.e. these materials exit, at least teporarily, as fluid metastable liquids after being melted and then cooled below their melting temperatures. When the latent image has been formed, it should wet the surface of the substrate. Moreover, the image must remain fluid and in place until it is contacted with (i.e., developed by) the dry imaging powder. Alternatively, it -ay be allowed to cool below its melting point to form a supercooled melt before the rage areas are developed. because the supercooled li uid Wes not res ire its solid state, t: r.terial retains sufficient mew wry in the imaged areas to be sevelo?ed and fixed. Once the material regains its solid state in the imaged areas, the latent image ceases to exist as a distinct area. Preferably, the supercooling material melts within the approximate range of 40 C to 1400C. Due to the lack in the available chemical literature of adequate data for defining the supercooling materials useful in the practice of the invention, definitive test procedures have been establish one which will now be described. The melting point or melting range of the supercooling material is determined, for the purposes of this invention, by placing a small amount of the material in paper form on a glass microscope snide, covering the sample with a cover glass, heating the material on a microscope having a hot stage which is provided with temperature measuring means, and observing the temperature at which the particles melt and fuse. A test for determining if a material is a supercooling material suitable for this invention is conveniently accomplished using the same sample as for the melting point test. A Leitz hot stage microscope having an electrically heated stage which may be cooled by circulation of cold water is used for both determinations. After the stage has been heated above the melting point of the sample, it is cooled and the temperature noted at which crystallization or solidification occurs. Both heating and cooling may be accomplished at somewhat higher rates of temperature change than are rdinarily specified where more precise measurements are required. 'materials which when thus treated remain liquid to a temperature well below their melting points, e.g., at least about 600C below their melting points, have been found to be effective as supercooling materials for this invention; materials which crystallize Dr solidify at or near their melting points should not be used for making powder-retaining latent races in accordance with this invention. Some materials solidify to a glassy rather than a visibly crycwalline state, a condition which is easily determined by applying moderate pressure on the cover glass with a spatula; glassy droplets retain their shape, whereas the liquid droplets flow or rapidly crystallize. A nore elaborate test for determination of supercooling materials suitable for this invention is described in U.S. Patent 3,360,367, incorporated herein by reference. A number of supercooling materials are useful in the coatings of the invention. Repr-sentative examples of these materials include dicyclohexyl phthalate, diphenyl phthalate, triphenyl phosphate, dimethyl fumurate, benzotriazole, 2,4-dihydroxy benzophenone, tribenzylamine, benzil, vanillin, and phthalophenone. Another useful material of this type is "Santicizer 9", a mixture of ortho- and para-toluene sulfonamides commercially available from the Monsanto Chemical Company. Mixtures of these materials are also useful. The supercooling material can also consist of two or more materials that are not supercooling by themselves, but are combinable to form a supercooling material. The anti-fouling agent can be selected from the following classes of materials: A. Waxes B. Silicas C. Metal silicates D. flixtures of taxes with silicas or metal silicates or both. As used herein, the term "anti-fouling agent' means a raterial, i.e., a chemical compound or mixture of chemical compounds, that is add to the heat-sens:tive composition that inhibits or prevents foreign substances from being deposited on the thermal print head. The waxes, silicas, and metal silicates that are useful as anti-fouling agents in the composition of this invention have at times been referred to as lubricants and antiblocking agents. axes that are suitable for the compcsiticn o- the present invention Include aliphatic alcohols having at least l; carbon atoms, fatty acids having at least 12 carbon atoms, fatty amides having at least 12 carbon atoms, fatty acid esters having at least 12 carbon atoms, symmetrical ketones derived from fatty acids having at least 12 carbon atoms, metal salts of ratty acids having at least 12 carbon atoms, and fluorocarbon polymers. Aliphatic alcohols that are suitable for the compositions of this invention can be represented by the formula R1-CH20H wherein R1 represents a saturated or unsaturated hydrocarbon radical, e.g. alkyl, alkenyl, having 9 to 21 carbon atoms. Representative examples of such suitable aliphatic alcohols include cetyl, stearyl, lauryl, myristyl, and mixtures thereof. Fatty acids that are suitable for the compositions of this invention can be represented by the formula EMI6.1 wherein R2 represents a saturated or unsaturated hydrocarbon radical, e.g. alkyl, alkenyl, having 11 to 21 carbon atoms. Representative examples of such fatty acids include palmitic, stearic, lauric, myristic, and mixtures thereof. Fatty amides that are suitable for the compositions of this invention can be represented by the formula EMI6.2 wherein R2 is as defined above, and X represents EMI6.3 or H. Representative examples of such fatty amides include stearamide, lauramide, oleamide, ethylene -bls-stearamide and mixtures thereof. Fatty acid esters Wlat are suitable for the compositions of this invention can be represented by the formula EMI7.1 wherein R2 is as defined above, and R3 represents a saturated or unsaturated hydrocarbon radical, e.g., alkyl, alkenyl, having 1 to 22 carbon atoms, said hydrocarbon radical being unsstituted or substituted with hydroxy group. Representative examples of such suitable fatty acid esters include glyceryl stearates, e.g. glyceryl monostearate and diethylene glycol nonostearate, glycol stearates, cetyl palmitate, stearyl stearate, n-butyl stearate, n-octyl stearate. Symmetrical ketones that are suitable for the composition of this invention can be represented by the formula EMI7.2 wherein R2 is as defined above. Representative examples of symmetrical ketones derived -c fatty acids that are useful in compositions of this invention include stearone and laurone. Metal salts of fatty acids that are suitable for the compositions of this invention can be represented by the formula EMI7.3 wherein M represents a petal atom, n represents an integer from 1 to 3, inclusive, and R2 is as defined above. Metal salts of fatty acids that are suitable for the composition of the present invention include octoates, laurates, palmitates, and stearates of aluminum, lead, cadmium, barium, calcium, lithium, magnesium, and zinc. The metal stearates are most preferred. Blends of metal salts of fatty acids, e.g. zinc stearate, and fatty acids, e.g. stearic acid, are also useful as anti-fouling agents in the -composition of the present invention. One or more of the hydrogen atoms of the hydrocarbon radicals R1, R2, R3 can be replaced with other atoms, e.g., halide, or groups of atoms, e.g. hydroxyl, so long as said atoms or groups of atoms do not adversely affect the anti-fouling characteristics of the wax anti-fouling agent. Fluorocarbon polymers that are suitable for the composition of the present invention include polymeric tetrafluoroethylene. Silicas and metal silicates can be used as the anti-fouling agent in the composition of the present invention. Representative examples of these anti-fouling agents include silica gel, fumed silica, precipitated silica, clay, kaolin, and talc. Silicas and metal silicates can be blended with axes such as metal salts of fatty acids, e.g. metal stearates, fluorocarbon polymers, e.g., polytetrafluoroethylene, fatty amides, e.g, stearamide, and the like, to improve their anti-fouling action. Binders can also be included in the heatsensitive composition of the image receiving element. The heat-sensitive composition would tend to flake off under curtain conditions in the absence of inrs, Represen stative examples of organic polysric binders suitable for this invention include water oluble binders such as polyvinyl alcohol, polyvinyl pyrrolidone, hydroxyethyl cellulose, and organic solvent soluble binders such as cellulose acetate, ethyl cellulose, and polyvinyl chloride. Substrates suitable for use in the invention can be selected from any dry, solid material that is compatible with the coating of normally solid, non-tacky material. Examples of materials suitable for the substrate include polymeric films, etal foils, and paper. The preferred substrate is paper. The range of concentration of each ingredient in the heat-sensitive coating material has been found to be important. If too little anti-fouling agent is employed, the thermal print heads will become fouled relatively rapidly. If too much anti-fouling agent is employed, the optical density of the toned image will be too low. The ranges of concentration of each ingredient is also dependent upon the nature of anti-fouling agent e, vFed. When waxes are used as the anti-fouling agent, the concentration ranges for essential ingredients of the ht- sensitive coating material are as follows: Inqredient Percent by weight supercooling material 55 to 99 Anti ruling agent 1 to 16 Binde 0 t7 40 hen silicas or total silicates are rsed as te anti-ouling agent, the concentration ranges for essential ingredients of the heat-sensitive material are as follows: : Ingredient Percent by weight Supercooling material 50 to 95 Anti-fouling agent 5 to 40 Binder 3 to 40 When silicas or metal silicates or both are u-d in combination with waxes as the anti-fouling agent, the concentration ranges for essential ingredients of the heatsensitive material are the same as when silicas alone or metal silicates alone are used as the anti-fouling agent. The coating material can be applied to the surface of a substrate by a variety of techniques, including both solvent coating and dry coating. For example, the heat-sensitive coating material can be dissolved or dispersed in an appropriate solvent (e.g., acetone, or water), the solution or dispersion applied to the substrate, and the solvent allowed to evaporate. The previously dissolved or dispersed solid material is then allowed to crystallize. Evaporation of the solvent can be accelerated, if desired, by heating the coated substrate. However, care should be taken to insure that the substrate does not curl or otherwise suffer adverse effects as a result of the heating. Additionally, crystallization of the dissolved or dispersed solid material can be roelerated by seeding the coated substrate with like solid material Dry coating techniques can also be utilized. The solid form of the heat-sensitive coating material can be brushed or rubbed onto the substrate. Preferably, the solid form of the material is either in the form of a powder or in a form in which it can readily be converted to a powder. The dry coating technique is an efficient means for applying the material to the substrate. Materials applied by the dry orating technique do not soak into the substrate as they do with solvent coating techniques. This is beneficial since it reduces the amount of coating material applied to the substrate while continuing to provide as good an image as that when the coating material is applied by a solvent coating technique. Furthermore, when a plain paper substrate is coated by the dry coating technique, the resultant sheet appears indistingu shhble from an uncoated paper sheet and can be used i ediately after coating. The exact amount of the coating material on the substrate can vary. There should be sufficient coating material to form a latent image but not so much material that the thermal printing means is adversely affected, the article becomes too dielectric, or gives a greasy feel or appearance. A sufficient amount of coating material must be used so that once the latent image has been formed, there will be sufficient adhesion between it and the imaging powder to overcome the triboelectric or magnetic forces, or both, holding the imaging powder to the development roll. It has been found that from about 0.1 to 5 g/m2 provides excellent results. When solvent coating is utilized, the substrate preferably bears from 0.1 to 2 g/m2 of the material, more preferably from about 0.1 to 1.2 g/m2, and most preferably from about 0.2 to 1.0 g/m2 of the material. These relatively small amounts of coating material are sufficient to provide latent images that can be developed and essentially permanently fixed to the substrate. When dry coating techniques are employed, he particulate material is substantially absorbed onto the substrate surface. shewn the substrate is paper1 the material becomes a. ached to the surface of the paper fibers. The imaged area must provide sufficient adhesion to the dry imaging powder. The imaged area may react with the imaging powder; it nav for a solution with the it rav wt the 0W3er; or - nay either absorb or We adsorbed by the powder. Whatever the interaction bet* en the powder and the imaged area is, the image area must hold the powder until the powder is fixed to the substrate. Coatings were evaluat-d by printing a solid bar (26 inches long) with the thermal print head in the T 9140 Facsimile Machine (3M Company). The latent image was then developed with the toner station of a VQC compact copier (3M Company) and toner powder described in U.S. Patent No. 3,925,219, vxam?le 1. The toner particles ranged in size from 10 to 45 micrometers. The printer utilize a 100 styli/inch thick film print head manufactured by Rolm Corporation. Print head residue was evaluated by visually inspecting the head under 5X magnification and rated according to the following criteria: None - No visible residue Trace - Small specks of coating adhering to print head Light - Small amount of residue forming continuous coating on portion of print head, but not interfering with head contact to paper Medium - Residue forms continuous coating over approximately half of the print head Heavy - Large amount of residue on and behind print head and interfering with head contact to paper and heat transfer. Image density after development was measured with a MacBeth TR 924 densitometer in reflection mode. EXAMPLES 1-8 Coating material formulations are set forth in Table I. In the following table the mounts are in parts by weight. TABLE I Amount Example Ingredient 1 2 3 4 5 6 7 8 Supercooling material: Diphenyl phthalate 90 - - 85 75 - - Dicylcohexyl phthalate - 90 90 - - 54 80 65 Binder: Ethyl cellulose 10 - 10 10 10 - 10 10 Cellulose acetate - 10 - - - 10 - Coating Solvent: Acetone 300 300 300 300 300 300 300 300 Anti-fouling agent: Calcium stearate - - - 5 5 - - Aluminum silicatel - - - - 10 30 - Polytetrafluoroethylene2 - - - - - 6 10 Silica gel3 - - - - - - - 25 1 ASP# 101 kaolin from Engelhard Minerals and Chemicals Corp. 2 Fluo HT2 from Micro Powders, Inc. 3 Svloid X-5000 from W. R. Grace and Co. The phthalates ar cel]ulosic binders were dissolved in acetone. The wax ant; fouling agents were dispersed into the phthalate/binder/acetone solution using an ultrasonic bath. The filler anti-fouling agents were disc used into the phthalate/binder/acetone solution using a h*!nogenizer. The dispersions were coated on paper with a 1/2 inch dia ether #8 wire wound rod and air dried, yielding dry coat weight of 0.28 to 0 36 g/ft2. Each coated sheet war evaluated and the results are shown in Table II. TABLE II Printhead residue Optical density 1 Light 1.55 2 Light 1.10 3 Heavy 1.50 4 Trace 1.58 5 Trace 1.60 6 None 1.60 7 None 1.65 8 None 1.20 When no anti-fouling agent was present in the heat-sentitive material, print head residue ranged from light to heavy. When at least one anti-fouling agent was included in the heat-sensitive material, print head residue ranged from none to trace. EXAMPLE 9 This example demonstrates the effect of coating weight on pry thread residue. The following formulation was used to prepare test samples: Ingredient Parts¯bLAeioht Dicyclohexylphthalate 80 Ethyl cellulosel 10 Polytetrafluoroethylene2 10 Acetone 300 1 N-200 grade from Hercules Inc. 2 Fluo HT2 from Micro Powders Inc. Samples of the formulation were coated on paper at coating weights ranging from 0.26 g/m2 to 0.95 g/m2. Coating weight was varied by using different payer rods. The results of the printhead residue evaluation are shown in Table III. TABLE III Dry coating weight Printhead Optical Mayer rod m2) residue density 4 0.26 none 1.40 8 0.35 none 1.35 14 0.62 none 1.58 18 0.74 trace 1.54 22 0.95 light 1.45 From Table III, it can be seen that a dry coating weight of 0.62 g/m2 provided optimum optical density value with no print hcad residue. EXAMPLES 10-16 These examples demonstrate the effect of different waxes in combination with metal silicate (aluminum silicate) in the coating composition. The following formulations were used for the examples. In t e following table, the amounts are in parts bv weight. TABLE IV Amount Example Ingredient 10 11 12 13 14 15 16 Supercooling material: Dicyclohexyl phthalate 60 56 56 56 56 56 56 Binder: Cellulose acetate 20 20 20 20 20 20 20 Anti-fouling agent: Aluminum silicatel 20 17 17 17 17 17 17 Stearic acid - 7 - - - - Stearamide - - 7 - - - Polytetrafluoroettlene2 - - - 7 - - - Polytetrafluoroethylene/ polyethylene3 - - - - 7 - Calcium stearate - - - - - 7 Ethylene-bis-stearamide - - - - - - 7 Coating solvent: Acetone 300 300 300 300 300 300 300 1 ASPD 101 kaolin from Engelhard Minerals and Chemicals Corp. 2 Fluo HT2 from micro Powders Inc. 3 Pslyfluo 540 from Micro Powders Inc. Each coating was evaluated and the results are shown in Table V. TABLE V Printhead Optical Example residue density 10 medium 0.40 11 light 0.22 12 trace 0.15 13 trace 0.75 14 trace 0.65 15 trace 0.60 16 light 0.40 Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.

Claims: CLAIMS: 1. Image receiving element comprising a substrate bearing on at least one major surface thereof a coating comprising (a) material capable of existing in a supercooled state after melting and subsequent cooling and (b) at least one anti-fouling agent selected from the group consisting of waxes, silicas, metal silicates, and mixtures thereof. 2. The element of claim 1 further comprising a binder. 3. The element of claim 1 wherein said anti-fouling agent comprises a wax. 4. The element of claim 2 wherein 5 id anti-fouling agent comprises a wax. 5. The element of claim 3 wherein said antifouling agent is selected from the group consisting of aliphatic alcohols, fatty acids, fatty amides, fatty acid esters, and symmetrical ketones derived from fatty acids. 6. The element of claim 5 wherein said anti-fouling agent is represented by the formula R1-CH2OH wherein R1 represents a saturated or unsaturated hydrocarbon radical having 9 to 21 carbon atoms. 7. The element of claim 5 wherein said anti-fouling agent is represented by the formula EMI19.1 wherein R2 represents a saturated or unsaturated hydrocarbon radical having 11 to 21 carbon atoms. 8. The element of claim 5 wherein said antifouling agent is represented by the formula EMI19.2 wherein R2 represents a saturated or unsaturated hyirocarbon radical having 11 to 21 carbon atoms, and X represents EMI19.3 or H. 9. The element of claim 5 wherein said antifouling agent is represented by the formula EMI19.4 wherein R2 represents a saturated or unsaturated hydrocarbon radical having 11 to 21 carbon atoms, R3 epresents a hydrocarbon radical having 1 to 21 carbon atoms. 10. The element of claim 5 wherein said anti-fouling agent is represented by the formula EMI20.1 wherein R2 represents a saturated or unsaturated hydrocarbon radical having 11 to 21 carbon atoms. 11. The element of claim 3 wherein said antifouling agent is a metal salt of a fatty acid. 12. The element of claim 11 wherein said antifouling agent is represented by the formula EMI20.2 wherein R2 represents a saturated or unsaturated hydrocarbon radical having 11 to 21 carbon atoms, n represents an integer from 1 to 3, inclusive, M represents a metal atom. 13. The element of claim 3 wherein said antifouling agent is a fluorochemical wax. 14. The element of claim 1 wherein said material capable of existing in a supercooled state after melting aild subsequent cooling comprises 55 weight percent to 99 eight percent of the coating, said wax comprises 1 weight percent to 16 weight percent of the coating, and said binder comprises up to 40 weight percent of the coating. 15. The element of claim 2 wherein said materiat capable of existing in a supercooled state after melting and subsequent cooling comprises 55 weight percent to 99 weight percent of the coating, said wax comprises 1 weight percent to 16 weight percent of the coating, and said binder comprises up to 40 weight percent of the coating. 16. The element of claim 1 wherein said antifouling agent comprises a silica or metal silicate. 17. The element of claim 16 wherein said material capable of existing in a supercooled state after melting and subsequent cooling comprises 50 weight percent to 95 weight percent of the coating1 said anti-fouling agent comprises 5 weight percent to 40 weight percent of the coating, and said binder comprises 3 weight percent to 40 weight percent of the coating. 18. The element of claim 1 wherein said anti-fouling agent comprises a mixture of a wax and either a silica or a metal silicate. 19. The element of claim 2 wherein said anti-f-uling agent corDrise a mixture of a wax and either a silica or a metal silicates 20. The element of claim 18 wherein said material capable of existing in a supercooled state after melting and subsequent cooling comprises 50 weight percent to 95 weight percrt of the coating, said anti-fouling agents comprises 5 weight percent to 40 weight percent of the coating, and said binder comprises 3 weight percent to 40 weight percent of the coating. 21. The element of claim 19 wherein said material capable of existing in a supercooled state after melting and subsequent cooling comprises 50 weight percent to 95 weight percent of the coating1 said anti-fouling agents comprises 5 weight percent to 40 weight percent of the coating, and said binder comprises 3 weight percent to 40 weight percent of the coating.

Inventors: BLACK, TERRENCE A. C/O MINNESOTA MINING AND, GEISLER, THOMAS C. C/O MINNESOTA MINING AND
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1985
Main classification: B41M5
IPC codes: B41M5, G03G5, G03G7