Title of Invention	THERMAL PAPER
Abstract	The present invention provides a thermal paper composite precursor comprising (a) a substrate layer; and (b) a base layer positioned on the substrate layer, the base layer comprising a binder and at least one porosity improver wherein the thermal paper composite precursor has a thermal effusivity that is at least about 2% less than the thermal effusivity of porosity improver-less thermal paper composite precursor. The thermal paper composite precursor is useful in making thermal paper composite.

Title of Invention

THERMAL PAPER

Abstract

The present invention provides a thermal paper composite precursor comprising (a) a substrate layer; and (b) a base layer positioned on the substrate layer, the base layer comprising a binder and at least one porosity improver wherein the thermal paper composite precursor has a thermal effusivity that is at least about 2% less than the thermal effusivity of porosity improver-less thermal paper composite precursor. The thermal paper composite precursor is useful in making thermal paper composite.

Full Text	THERMAL PAPER This patent application claims the priority of pending US patent application Serial No. 60/633,143 flied December 3, 2004, and incorporates ii in its entirety herein by reference. FIELD OF THE INVENTION The present invention generally relates to thermal paper with improved thermal properties, in particular, the present Invention relates to thermal paper containing a base layer that provides improved thermal insulating characteristics that in turn provide numerous advantages to the thermal paper. BACKGROUND OF THE INVENTION Thermal printing systems use a thermal print element energized to heat specific and precise areas of a heat sensitive paper to provide an image of readable characters or graphics on the heat sensitive paper. The heat sensitive paper, also known as thermal paper, includes materlal(s) which is reactive to applied heat. The thermal paper is a self-contained system, referred to as direct thermal, wherein ink need not be applied. This is advantageous in that providing ink or a marking material to the writing instrument is not necessary. Thermal printing systems typically include point of sale (POS) devices, facsimile machines, adding machines, automated teller machines (ATMs), credit card machines, gas pump machines, electronic blackboards, and the like, While the aforementioned thermal printing systems are known and employed extensively in some fields, further exploitation is possible if image quality on thermal paper can be Improved. Some thermal papers produced by thermal printing systems suffer from low resolution of written image, limited time duration of an image (fading), deiicacy of thermal paper before printing (increasing care when handling, shipping, and storing), and the like. SUMMARY OF THE INVENTION The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the Invention. This summary Is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention, Rather, the sole purpose of this summary Is to present some concepts of the Invention, in a simplified form as a prelude to the more detailed description that is presented hereinafter. The present invention provides a thermal paper composite precursor comprising (a) a substrate layer; and (b) a base layer positioned on the substrate layer, the base layer comprising a binder and at least one porosity Improver wherein the thermal paper composite precursor has a thermal tfffuslvlty that is at least about 2% less than the thermal effuslvlty of porosity improver-less thermal paper composite precursor. The present invention provides thermal paper containing a base layer that provides thermal insulating properties which mitigates heat transfer from the active layer to the substrate layer, Mitigating heat transfer results in printing, images of improved quality. The thermal insulating properties of the base layer also permit the use of decreased amounts of active layer materials, which are typically relatively expensive compared to other components of the thermal paper, One aspect of the invention relates to thermal paper containing a substrate layer; an active iayer containing image forming components; and a base layer positioned between the substrate layer and the active layer, the base layer containing a binder and a porosity improver having a specified thermal effuslvity. The specified thermal effusivity dictates, In part, the improved thermal Insulating properties of the thermal paper, The base iayer need not contain Image forming components, which are Included in the active layer, Another aspect of the invention relates to making thermal paper involving forming a base iayer containing a binder and a porosity improver to improve thermal effuslvity over a substrate layer; and forming an active iayer containing Image forming components over the base layer. 3 Yet another aspect of the invention relates to printing thermal paper containing a substrate layer, an active layer, and a base layer positioned between the substrate layer and the active layer, the base layer containing a binder and a porosity improver, involving applying localized heat using a thermal paper printer in the pattern of a desired image to form the desired image In the thermal paper. To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. BRIEF SUMMARY OF THE DRAWINGS Figure 1 is a cross sectional illustration of thermal paper In accordance with an aspect of the subject invention. Figure 2 is 3 cross sectional illustration of thermal paper in accordance with another aspect of the subject invention, Figure 3 is a cross sectional Illustration of a method of forming an image In thermal paper in accordance with an aspect of the subject Invention, DETAILED DESCRIPTION OF THE INVENTION The phrase "porosity Improver-less thermal paper composite precursor means a thermal paper composite precursor that does not contain at least one porosity improver in the base layer thereof, Generally speaking, thermal paper is coated with a base layer and a colorless formula (the active layer) which subsequently develops an image by the application of heat. When passing through an imaging device, precise measures of heat applied by a print head cause a reaction that creates an Image (typically biack or color) on the thermal paper. The base layer of the 4 subject Invention is made so that it possesses a thermal effusivity that improves the quality and/or efficiency of thermal paper printing. Direct thermal imaging technology of the subject invention may employ a print head where heat generated Induces a release of ink in the active layer of thermal paper. This Is also known as direct thermal imaging technology and uses a thermal paper containing ink in a substantially colorless form in an active coating on the surface. Heat generated in the print head element transfers to the thermal paper and activates the ink system to develop an image. Thermal imaging technology may also employ a transfer ribbon in addition to the thermal paper. In this case, heat generated in a print head is transferred to a plastic ribbon, which in turn releases ink for deposition on the thermal paper. This is known as thermal transfer imaging as opposed to the subject of direct thermal imaging. Thermal paper typically has at least three layers: a substrate layer, an active layer for forming an image, and a base layer between the substrate layer and active layer. Thermal paper may optionally have one or more additional layers including a top coating layer (sometimes referred to as a protective layer) over the active layer, a backside barrier adjacent the substrate layer, image enhancing layers, or any other suitable layer to enhance performance and/or handling, The substrate layer is generally in sheet form. Thai is, the substrate layer is in the form of pages, webs, ribbons, tapes, belts, films, cards and the like. Sheet form indicates that the substrate layer has two large surface dimensions and a comparatively small thickness dimension. The substrate layer can be any of opaque, transparent, translucent, colored, and non- colored (white), Examples of substrate layer materials include paper, filamentous synthetic materials, and synthetic films such as cellophane and synthetic polymeric sheets (the synthetic films can be cast, extruded, or otherwise formed). In this sense, the word paper in the term thermal paper is not inherently limiting. The substrate layer is of sufficient basis weight to support at least an active layer and base layer, and optionally of sufficient basis weight to further support additional, optional layers such as a top coating layer and/or a backside barrier. In one embodiment, the substrate layer 'nas a basis weight of abou! 14 g/m2 or more and about 50 g/rn2 or less. In another 'embodiment, the substrate layer has a basis weight of about 30 g/m2 or more and about 148 g/m2 or less. In yet another embodiment, the substrate layer has a thickness of about 40 microns or more and about 130 microns or less, in still yet another embodiment, the substrate layer has a thickness of about 20 microns or more and about 80 microns or less. The active layer contains image forming components that become visible to the human eye or a machine reader after exposure to localized heat. The active layer contains one or more of a dye, chromogenic material, developer, Inert pigment, antioxldants, lubricants, polymeric binder, sensltlzer, stabilizer, wetting agents, and waxes. The active layer is sometimes referred to as a reactive or thermal layer. The components of the active layer are typically uniformly distributed throughout the active layer, Examples of dyes, chromogenlc materials, and Inert pigments include fluorescent, organic and inorganic pigments. These compounds may lead to black-white printing or color printing. Examples of developers include acidic developers such as acidic phenolic compounds and aromatic carboxyllo acids. Examples of sensitizers include ether compounds such as aromatic ether compounds. One or more of any of the active layer components may or may not be microencapsulated, The active layer is of sufficient basis weight to provide a visible, detectable and/or desirable image on the thermal paper for an end user. In one embodiment, the active layer has a basis weight of about 1,5 g/m' or more and about 7,5 g/m2 or less. In another embodiment, the active layer has a basis weight of about 3 g/m2 or more and about 30 g/m2 or less, in yet another embodiment, the active layer has a basis weight of about 5 g/rnz or more and about 15 g/m2 or less, in still yet another embodiment, the active iayer has a thickness of about 1 micron or more and about 30 microns or less. In another embodiment, the active Saver has a thickness of about 5 microns or more and about 20 microns or less. One of the advantages of the subject invention is that a smaller active layer (or less active layer components) is required in thermal paper of the 6 Invention compared to thermal paper that does not contain a base layer having specified thermal effusivity properties as described herein. Since the active layer of thermal paper typically contains the most expensive components of the thermal paper, decreasing the size of the active layer Is a significant advantage associated with making the subject thermal paper. The base layer contains a binder and a porosity improver and has a specified thermal effusivlty as described herein. The base iayer may further and optionally contain a dispersant, wetting agent, and other additives, so long as the thermal effusivlty values are maintained. In one embodiment, the base layer does not contain image forming components; that Is, the base layer does not contain any of a dye, chromogenlc material, and/or organic and inorganic pigments, The base layer contains a sufficient amount of binder to hold the porosity improver. In one embodiment, the base layer contains about 5% by weight or more and about 95% by weight or less of binder. In another embodiment, the base layer contains about 15% by weight or more and about 90% by weight or less of binder, Examples of binders Include water-soluble binders such as starches, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin, casein, polyviny) alcohol, modified polyvinyl alcohol, sodium polyacrylate, acrylic amide/acrylic ester copolymer, acrylic amide/acrylic ester/methacrylic acid terpolymer, alkali salts of styrene/maleic anhydride copolymer, alkali salts of ethylene/maleic anhydride copolymer, polyviny! acetate, polyurethane, polyacryiic esters, styrene/butadlene copolymer, acrylontrile/butadiene copolymer, methyl aorylate/butadiene copolymer, ethylene/viny! acetate copolymer, and the like. Further examples of binders include polyester resin, vinyl chloride resin, polyurethane resin, vinyl chloride-vinyl acetate copolymer, vinyl chlorideacrylonitrile copolymer, epoxy resin, nitrocellulose, and the like. The porosity improver of the subject invention has at least one of high surface area, high pore volume, narrow particle size distribution, and/or high porosity when assembled in a layer (and thus appear to possess a high pore volume), Examples of the porosity improver include one or more of calcined clays such as calcined kaolin, flash calcined kaolin, and calcined bentonlte, 7 acid treated bentonite, high surface area alumina, hydrated alumina, boehmite, flash calcined alumina trlhydrate (ATH), silica, silica gel, zeolites, zeotypes and other molecular sieves, clathrasils, micro-, meso- and macro- poroiis particles, alumina phosphates, metal alumina phosphates, mica, pillared clays and the like. These compounds are commercially available through a number of sources. The base layer may contain at least one porosity improver, at iaasl two porosity improvers, at 'east three porosity impi overs, and so on. The porosity improver contributes to the desirable thermal effuslvlty properties of the base layer. In one embodiment where at least two porosity Improvers are included in the base layer, one porosity Improver Is a calcined clay such as calcined kaolin and the other porosity improver Is one of an acid treated bentonite, high surface area alumina, hydrated alumina, flash calcined kaolin, flash calcined ATH, silica, silica gel, zeolite, micro-, meso- or macro-porous particle, alumina phosphate, molecular sieve, clathrasils, pillared clay, boehmite, mica or metal alumina phosphate. Other useful porosity improvers include zeolites, Zeolites and/or zeotypes, frequently also referred to as molecular sieves, are a ciass of micro- and mesoporous materials with 1, 2 or 3-D pore system and with a variety of compositions including silica, aluminosilicates (natural and traditional synthetic zeolites), alumlno-phosphates (ALPO's), silicon- aiuminophosphates (SAPO's) and many others. One of the key properties of these materials is that they (In many cases) reverslbly adsorb and desorb large quantities of structural water, and if they are stable in their dehydrated state, they will also reversibly adsorb and desorb other gas8s and vapors, This is possibis because of the micro- and mesoporous nature of their structure. The porosity in zeolites can be best described In terms of channels or cages connected by smaller windows. Depending on if and how these intersect, they create 1-, 2- or 3-dimenslona! pore system with pore diameters and pore openings ranging in size from about 2.5 angstroms to more than 100 angstroms. As a result, they contain a non-negligible amount of pore volume In their structures and their densities are lower than those of their 8 non-porous or dense poiymorphs. In some instances they can be at least 50 % less dense, The amount of porosity is most commonly described in terms of pore volume (cc/g), or framework density (FD), The reference FD of dense silica structure (quartz) is approximately 26.5. Table 1 shows examples ot some of the most common structures including their pore characteristics. For the porosity Improvers other than calcined clays, the porosity Improver of the subject invention has one or more of at least about 70% by weight of the particles have a size of 2 microns or less, at least about 50% by weight of the particles have a size of 1 micron or less, a surface area of at least about 10 m2/g, and a pore volume of at least about 0.1 cc/g. in another embodiment, the porosity improver of the subject invention (other than calcined clays) has one or more of at least about 80% by weight of the particles have a size of 2 microns or less, at least about 60% by weight of the particles have a size of 1 micron or less, a surface area of at least about 15 m2/g, and a pore volume of at least about 0.2cc/g. In yet another embodiment, the porosity Improver of the subject invention (other than calcined clays) has one or more of at least about 90% by weight of the particles have a size of 2 microns or less, at least about 70% by weight.of the particles have a size of 1 micron or less, a surface area of at least about 20 m2/g, and a pore volume of at least about 0.3 cc/g. Calcining destroys the crystallinity of hydrous kaolin or bentonlte, and renders the kaolin/clay substantially amorphous. Calcination typically occurs after heating at temperatures In the range from about 700 to about 1200 C. 9 for a sufficient period of time, Commercial vertical and horizontal rotary calciners can be used to produce metakaolin, partially calcined kaolin, and/or calcined kaolin. Acid treatment involves contacting clay with an amount of a mineral acid to render the clay substantially amorphous, in one embodiment, calcined day of the subject invention has one or more of at least about 70% by weight of the particles have a size of 2 microns or less, at least about 50% by weight of the particles have a size of 1 micron or less, a surface area of at least about 5 m2/g, and a pore volume of at least about 0.1 cc/g, In yet another embodiment, calcined clay of the subject Invention has one or more of at least about 80% by weight of the particles have a size of 2 microns or less, at least about 60 % by weight of the particles have a size of 1 micron or less, a surface area of at least about 10 nVVg, and a pore volume of at least about 0.2 cc/g, In still yet another embodiment, calcined day of the subject Invention has one or more of at least about 90% by weight of the particles have a size of 2 microns or less, at ieast about 70% by weight of the particles have a size of 1 micron or less, a surface area of at least about 15 mz/g, and a pore volume of at least about 0,3 cc/g. As noted the non-calcined clay porosity Improver or the calcined clay porosity improver may have a pore volume of at ieast about 0.1 cc/g, at least about 0.2 cc/g, or at least about 0,3 cc/g. Alternatively, the non-calcined clay porosity improver or the calcined clay porosity Improver may have an equivalent pore volume of at least about 0.1 cc/g, at least about 0,2 cc/g, or at least about 0,3 cc/g, in this connection, while the individual porosity Improver particles may not have the required pore volume, when assembled in a layer, the porosity Improver particles may form a resultant structure (base layer) that is porous, and has the porosity as if the layer was made of a porosity Improver having a pore volume of at least about 0.1 cc/g, at least about 0,2 cc/g, or at least about 0.3 cc/g. That is, the base layer may having a pore volume of at ieast about 0,1 cc/g, at ieast about 0,2 cc/g, or at least about 0.3 cc/g. Thus, the porosity Improver may be porous in and of Itself, or It may enhance the porosity of the base layer, Surface area is determined by the art recognized BET method using N2 as the adsorbate. Surface area alternatively is determined using Gardner Coleman Oil Absorption Test and is based on ASTM D-1483-84 which measures grams of oil absorbed per 100 grams of kaolin. Pore volume or porosity Is measured by standard Mercury Porosimetry techniques. Ail particle sizes referred to heteln are determined by a conventional sedimentation technique using a Micromeritics, Inc.'s SEDIGRAPH® 5100 analyzer, The sizes, in microns, are reported as "e.s.d," (equivalent spherical diameter). Particles are slurried in water with a dispersant and pumped through the detector with agitation to disperse loos© agglomerates, Examples of commercially available calcined clay of the subject invention include those under the trade designations such as Ansllex® such as Ansllex® 93, Satlntons®, and Translink®, available from Hngelhard Corporation of lselln, New Jersey. The base layer contains a sufficient amount of a porosity improver to contribute to providing insulating properties, such as a beneficial thermal effusivity, that facilitate high quality image formation in the active layer. In one embodiment, the base layer contains about 5% by weight or more and about 95% by weight or less of a porosity improver, in another embodiment, the base layer contains about 15 % by weight or more and about 90% by weight or less of a porosity improver. In yet another embodiment, the base layer contains about 15% by weight or more and about 40% by weight or less of a porosity improver. Ths base layer is of sufficient basis weight to provide Insulating properties, such as a beneficial thermal effusivity, that facilitate high quality image formation In the active layer. In one embodiment, the base layer has a basis weight of about 1 g/m2 or more and about 50 g/m2 or less, In another embodiment, the base layer has a basis weight of about 3 g/m2 or more and about 40 g/m2 or less, in yet another embodiment, the base layer has a basis weight of about 5 g/m2 or more and about 30 g/mz or less., in still yet another embodiment,, the base layer has a basis weight of about 7 g/m or more and about 20 g/m2 or less, in another embodiment, the base layer has a thickness of about 0.5 microns or more and about 20 microns or less. In yet another embodiment, the base layer has a thickness of about 1 micron or more and about 10 microns or less. In another embodiment, the base layer 11 has a thickness of about 2 microns or more and about 7 microns or less. Another beneficial aspect of the base layer is the thickness uniformity achieved when formed across the substrate layer. In this connection, the thickness of the base layer does not vary by more than about twenty percent when selecting two random locations of the base layer for determining thickness. Each of the layers or coatings is applied to the thermal paper substrate by any suitable method, including coating optionally with a doctor blade, rollers, air knife, spraying, extruding, laminating, printing, pressing, and the like. The thermal paper of the subject invention has one or more of the Improved properties of less active layer material required, enhanced Image intensity, enhanced image density, Improved base layer coating rheology, lower abrasion characteristics, and Improved thermal response, The porosity improver functions as a thermal insulator thereby facilitating reaction between the image forming components of the active layer providing a more intense, crisp image at lowered temperatures and/or faster imaging. That is, the porosity improver functions to Improve the heat Insulating properties In the thermal paper thereby Improving the efficiency of the active layer in forming an image. For thermal paper, thermal sensitivity is defined as the temperature at which the active layer of thermal paper produces an image of satisfactory Intensity. Background is defined as the amount of shade/coloration ot thermal paper before Imaging and/or in the unimaged areas of imaged thermal paper. The ability to maintain the thermal sensitivity of thermal paper while reducing the background shade/coloration is significant advantage of the subject invention, Beneficial increases In thermal response in the active iayer of thermal paper are achieved through the Incorporation of a porosity improver as described herein in the base layer. Comparing thermal papers with similar components, except that one (thermal of the subject invention) has at least one porosity Improver in the base layer, the thermal paper precursor of the subject invention has a thermal effuslvlty value that Is about 2% less than the thermal effusivity of porosity improver-less thermal paper composite precursor. The 2% includes a standard deviation of about 0.5-1 % observed in effusivity measurements of precursor sheets, in another embodiment, the thermal paper precursor of the subject invention has a thermal effusivity value that Is about 5% less than the thermal effusivity of porosity Improver-less thermal paper composite precursor. In another embodiment, the thermal paper precursor of the subject invention has a thermal effusivity value that is about 15% less than the thermal effusivity of porosity improver-less thermal paper composite precursor. Thermal effusivity is a comprehensive measure for heat distribution across a given material. Thermal effusivity characterizes the thermal Impedance of matter (its ability to exchange thermal energy with surroundings). Specifically, thermal effusivity is a function of the density, heat capacity, and thermal conductivity. Thermal effusivity can be calculated by taking the square root of thermal conductivity (W/mK) times the density (kg/m3) times heat capacity (J/kgK). Thermal effusivity is a heat transfer property that dictates the interfacial temperature when two semi-infinite objects at different temperature touch. Thermal effusivity can be determined employing a Mathis Instruments TC-30 Thermal Conductivity Probe using a modified hot wire technique, operating under constant current conditions. The temperature of the heating element is monitored during sample testing, and changes In the temperature at the interface between the probe and sample surface, over the testing time, are continually measured, in one embodiment, the thermal effusivity (Ws1/2/m2K) of the substrate coated with base layer is about 450 or less. In another embodiment, the thermal effusivity of the substrate coated with base layer Is about 370 or less. In yet another embodiment, the thermal effusivity of the substrate coated with base layer is about 330 or less, in still yet another embodiment, the thermal effusivity of the substrate coated with base layer is about 300 or less. The subject invention can be further understood In connection with the drawings. Referring to Figure 1, a cross sectional view of a three layer construction of thermal paper 100 is shown. A substrate layer 102 typically 13 contains a sheet of paper. On one side (the writing side or image side) of the substrate layer 102 Is a base layer 104. The combination of substrate layer 102 and the base layer 104 is an example of the present thermal paper composite precursor. The thermal paper composite precursor car. be combined with an active layer 108 so that the base layer 104 is positioned between the substrate layer 102 and the active layer 106, This combination is an example of a thermal paper composite precursor. The base layer 104 contains a porosity improver in a binder and provides thermal insulating properties and prevents the transfer of thermal energy emanating from a thermal print head through the active layer 106 to the substrate layer 102 during the writing or imaging process, The base layer 104 also prevents the active layer 106 materials from weeping Into the substrate layer 102. The active layer 106 contains components that form an image In specific locations in response to the discrete delivery of heat or Infrared radiation from the thermal print head. Referring to Figure 2, a cross sectional view of a five layer construction of thermal paper 200 is shown, A substrate layer 202 contains a sheet of paper. On one side (the non-writing side or backside) of the substrate layer. 202 is a backside barrier 204, The backside barrier 204 in some instances provides additional strength to the substrate layer 202 as well as prevents contamination of the substrate layer 202 that may creep to the writing side. On the other side (the writing side or Image side) of the substrate layer 202 is a base layer 206, an active layer 208, and a protective coat 210. The combination of substrate layer 202 and the base layer 206 is an example of the present thermal paper composite precursor. The base layer 206 is positioned between the substrate iayer 202 and the active layer 208. The base layer 206 contains a porosity improver In a binder and provides thermal insuiatmg properties and prevents the transfer of thermal energy emanating from a thermal print head through the active iayer 208 and protective coat 210 to the substrate layer 202 during the writing or Imaging process. The active layer 208 contains components that form an image in specific locations in response to the discrete delivery of heat or infrared radiation from the thermal print head. The protective coat 210 Is transparent to the subsequently formed 14 image, and prevents loss of active layer 208 components due to abrasion with the thermal paper 200, Although not shown in the figures, the thermal paper structures may contain additional layers, and/or the thermal papor structures may contain additional base and active layers for specific applications. For example, the thermal paper structures may contain a base layer, optionally a backside barrier, three base layers alternating with three active layers, and a protective coating. Referring to Figure 3, a cross sectional view of a method 300 of imaging thermal paper is shown. Thermal paper containing a substrate layer 302, a base layer 304 and an active layer 306 is subjected to a writing process. A thermal print head 308 from a writing machine (not shown) Is positioned near or in close proximity to the side of the thermal paper having the active layer 306. in some instances the thermal print head 308 may contact the thermal paper. Heat 310 is emitted, and the heat generates, induces, or otherwise causes and image 312 to appear in the active layer 308. The temperature of the heat applied or required depends upon a number of factors including the identity of the image forming components In. the active layer. Since the base layer 304 is positioned between the substrate layer 302 and the active layer 306, the base layer 304 mitigates the transfer of thermal energy from the thermal print head 308 through the active layer 306 io the substrate layer 302 owing to its desirable thermal effusivity and thermal insulating properties. Thermal effusivity test method; Thermal properties of materials can be characterized by a number of characteristics, such as thermal conductivity, thermal dlffusivity and thermal effusivity. Thermal conductivity is a measure of the ability of material to conduct heat (W/mK). Thermal diffusMty measures the ability of a material to conduct thermal energy relative to its ability to store energy (mmz/s). Thermal effusivity is defined as the square root of the product of thermal conductivity (k), density (p) and heat capacity (cp) of a material (Ws1/2/m2K). Thermal insulating properties of the pigments of current invention were characterized using Mathls Instruments TC-30 direct thermal conductivity 15 instrument, by measuring thermal effusivities of coated substrates. No active coat was applied. Substrates were typically coated with 5-10 g/m2 of base layer containing the pigment, and then calendered to about the same smoothness of approximately 2 microns as determined by Print-Parker-Surf (PP3) roughness test, A sheet of the coated substrate was then cut into pieces large enough to cover the TC-30 detector. Although the orientation of the base coat with respect to the sensor (If kept constant), is not cruclal for obtaining useful data, orientation "towards the sensor" (as opposed to "away from the sensor") is preferred and was used. To ensure that the heat wave does not penetrate the sample, about 5-10 pieces of coated substrate were layered in the test to increase the useful sample cross section. For each pigment, approximately 100 measurements were performed with optimized test times, regression start times and cool times, and to maximize the base- layer coat area subject to measurement, the bottom piece was removed and placed on top of the stack every 12 measurements. This aiso significantly improved precision of the measurement, Since any air pockets in-between the layers due to non-uniform surface roughness wiii have negative impact on accuracy and precision of the effusivity measurements, calendering is a very Important step in the sample preparation, Any differences In effusivities greater then the standard deviation of respective measurements, typically 0.5- 1 %, can be considered real. As thermal effusivity values of substrates costed with base layer can vary depending on many parameters, Including the base-layer coat weight and Its formulation, nature of the substrate, temperature and humidity during measurement, calendering conditions, smoothness of the tested papers, instrument calibration etc., it is best to evaluate and rank pigments and their thermal properties on a comparative basis vs, control (does not contain porosity improver) rather than by using their absolute measured effusivity values. Inventive Example 1 Two pigments coated as a base coat on a substrate layer and also coated with commercial active layer coat were evaluated for thermal effusivlty and image quality, respectively, to Illustrate the importance of the thermal Insulating properties of the base coat en the Image quality - both optical density and visual quality/uniformity. One of the pigments was a commercially available synthetic pigment - "Synthetic pigment", the other was a 100 % calcined kaolin pigment". Active coats on both papers were developed by- placing 3x3 Inch squares of each paper into an oven sat to 100 C for 2 min. Thermal effustvities of substrate/base coat composites and their corresponding image quality evaluations are summarized in Table 2. The synthetic pigment gave lower effusivity and had higher optical density. Visually, It looked black and had very good image uniformity. Sample coated with calcined kaolin pigment showed higher effusivity and lower optical density. In visual evaluations, this sample looked gray with highly non-uniform appearance. Overall, the data indicate an inverse relationship between the thermal effusivity of the thermal paper precursor and the optical density of the finished thermal paper. Visual evaluation also shows better image quality for lower effusivity pigment. Inventive Example. 2 Two pigments were prepared, coated on a thermal base paper, calendered to about the same PPS roughness of approximately 2μm and evaluated for thermal effusivity, Tnermal effusivities were measured on base paper/base coat composites at about 22 °C and about 40% RH using Mathls Instruments TC-30 thermal conductiviiy/effusivily analyzer. 17 These composite thermal paper precursor sheets were then coated with a commercial active coat and evaluated using Industry standard instrumentation for half energy optical density. The pigments Included commercial standard calcined kaolin and hydrous kaolin treated with sodium silicate (20 ibs/ton clay). Physical characteristics of these pigments and their coatings are summarized in Table 3, The hydrous kaolin treated with sodium silicate is referred to as treated hydrous kaolin In the remainder of this inventive Example 2, Thermal effuslvity of the calcined kaolin containing precursor was more than 5 % lower than that of the treated hydrous kaolin. This lowered effuslvity, as expected, provided improved print quality as measured by higher optical densities. The calcined kaolin showed about 8% Improvement In optical density compared to the treated hydrous kaolin. In the case of treated hydrous kaolin, thermal effuslvity of the thermal paper precursor was higher 18 than that of calcined kaolin, which in turn yielded worse optical density. One can conclude that lower thermal effusivity of the base coat layer, and thus of the thermal paper composite precursor, has a positive effect on the image quality of the final thermal paper. Inventive Example 3 To illustrate the effect of porosity In the bass coat on the thermal effusivity of the thermal paper precursor, four pigments were prepared, coated on a thermal base paper, calendered to about the same PPS roughness of approximately 2μm and evaluated for thermal effusivity using Mathis Instruments TC-30 analyzer. The pigments included commercial calcined kaolin, blend of 80 parts of commercial calcined kaolin and 20 parts of commercially available silica zeolite Y - "80 kaoi!n/20 silicaY", blend of 90 parts of commercial calcined kaolin and 10 parts of Engelhard made zeolite Y - "90 kaolin/10 zsoiiteY" and hydrous keoiin treated with sodium silicate (20 lbs/ton clay) - "treated hydrous kaolin". The effusivities were measured on base paper/base coat composites at about 22°C and about 40% RH; the pore volumes in the base coat layers were obtained from mercury porosimetry. Physical characteristics of these pigments and their coatings are summarized in Table 5. Effusivity measurements of the composite sheets and pore volumes in their respective base coat layers are presented in Table 6. 19 Results show thai the thermal effusivity of the composite precursor is inversely proportional to the pore volume in the base coat layer I.e. that the composite sheet with the highest thermal effusivity has the lowest pore volume, and the composit© with the lowest effusivity contains highest pore volume. This also shows that the presence of a porosity improver In the base coat layer has a positive effect on its thermal properties, such that it reduces the thermal effusivity of the thermal paper composite precursor when compared to the same that does not contain a porosity improver. One can conclude that, a precursor containing a porosity Improver and having an increased pore volume In the base coat will posses lower thermal effusivity and thus will result in Improved image quality of the finished thermal paper. Inventive Example 4 Two pigments were prepared and tested to demonstrate positive benefit of increased base coat layer porosity on thermal effusivity of the thermal paper precursor and on Image quality of the finished thermal paper. One pigment was a hydrous kaolin calcined to muilite Index of 35-55 - "Calcined clay", the second plgmQnt was a blend of 80 parts of commercial calcined kaolin and 20 parts of commercially available silica zeolite Y - "80 kaolln/20 siiicaY". Both pigments were coated on a commercial thermal base paper, calendered to approximately the same PPS roughness of about 2μm, and evaluated for pore volumes and thermal effusivlties. Both effusivities and pore volumes were measured on respective thermal paper precursor sheets. 20 The sheets were also treated with a commercial active coat layer and tested using industry standard Instrumentation (Atlantek 200) for image density. Basic physical characteristics of both pigments and their base coatings are summarized in Table 7. The pore volume of the blended pigment was more than 5 % higher than that of the calcined day. This increased porosity of the blended pigment base coat in turn positively affected thermal effusivity of the full precursor, which was about 5 % lower compared to the calcined clay containing precursor. Most importantly, the image density of the blended pigment containing thermal paper was significantly improved. These results dearly show the benefit of the porosity improver in the base coat, its positive effect on the thermal effusivity of the precursor and Its 6trong positive impact on image quality of the finished thermal paper. 21 While the invention has been explained in relation to certain embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification, Therefore, it Is to be understood that the invention disclosed heroin is Intended to cover such modifications es fall within the scope of the appended claims. 22 CLAIMS What is claimed is: 1. A thermal paper composite precursor comprising (a) a substrate layer; and (b) a base layer positioned on the substrate layer, the base layer comprising a binder and at least one porosity improver wherein said thermal paper composite precursor has a thermal effusivity that Is at least about 2% less than the thermal effusivlty of porosity improver-less thermal paper composite precursor. 2. The thermal paper composite precursor of claim 1 wherein said porosity improver is selected from the group consisting of calcined kaolin, flash calcined kaolin, calcined bentonite, acid treated bentonite, high surface area alumina, hydrated alumina, boehmite, flash calcined alumina trihydrate, silica, silica gel, zeolite, zeotypes, non-zeotype molecular sieves, clathraslls, microporous particles, mesoporous particles, macroporous particles, alumina phosphates, metal alumina phosphates, mica, and pillared days, 3. The thermal paper composite precursor of claim 2 wherein said porosity Improver is selected from the group consisting of calcined kaolin, flash calcined kaolin, and calcined bentonite, 4. The thermal paper composite precursor of claim 2 wherein said porosity improver is selected from the group consisting of acid treated bentonite, high surface area alumina, hydrated alumina, boehmite, flash calcined alumina trihydrate, silica, silica gel, zeolite, zeotypes, non-zeotype molecular sieves, clathrasils, microporous particles, mesoporous particles, macroporous particles, alumina phosphates, metal alumina phosphates, mica, and pillared clays. 5. The thermal paper composite precursor of claim 3 wherein said porosity improver has at least one of: at least about 70% by weight of the particles have a size of 2 microns or less, at least about 50% by weight of the particles have a size of 1 micron or less, a surface area of at least about 5 m2/g, and a pore volume of at least about 01 cc/g. 6. The thermal paper composite precursor of claim A wherein said porosity improver has at least one of. at least about 70% by weight of the particles have a size of 2 microns or less, at least about 50% by weight of the particles have a size of 1 micron or less, a surface area of at least about 10 m2/g, and a pore volume of at least about 0.1 cc/g, 7. The thermal paper composite precursor of claim 1 wherein said thermal effusivity is at least about 5% less. 8. The thermal paper composite precursor of claim 1 wherein said thermal effusivity is at least about 10% less, 9. The thermal paper composite precursor of claim 1 wherein said thermal effusivity is at least about 15% less, 10. A thermal paper composite comprising the thermal paper composite precursor of claim 1 and an active layer comprising image forming components on said base layer (b). The present invention provides a thermal paper composite precursor comprising (a) a substrate layer; and (b) a base layer positioned on the substrate layer, the base layer comprising a binder and at least one porosity improver wherein the thermal paper composite precursor has a thermal effusivity that is at least about 2% less than the thermal effusivity of porosity improver-less thermal paper composite precursor. The thermal paper composite precursor is useful in making thermal paper composite.

Full Text

THERMAL PAPER
This patent application claims the priority of pending US patent
application Serial No. 60/633,143 flied December 3, 2004, and incorporates ii
in its entirety herein by reference.
FIELD OF THE INVENTION
The present invention generally relates to thermal paper with improved
thermal properties, in particular, the present Invention relates to thermal
paper containing a base layer that provides improved thermal insulating
characteristics that in turn provide numerous advantages to the thermal
paper.
BACKGROUND OF THE INVENTION
Thermal printing systems use a thermal print element energized to
heat specific and precise areas of a heat sensitive paper to provide an image
of readable characters or graphics on the heat sensitive paper. The heat
sensitive paper, also known as thermal paper, includes materlal(s) which is
reactive to applied heat. The thermal paper is a self-contained system,
referred to as direct thermal, wherein ink need not be applied. This is
advantageous in that providing ink or a marking material to the writing
instrument is not necessary.
Thermal printing systems typically include point of sale (POS) devices,
facsimile machines, adding machines, automated teller machines (ATMs),
credit card machines, gas pump machines, electronic blackboards, and the
like, While the aforementioned thermal printing systems are known and
employed extensively in some fields, further exploitation is possible if image
quality on thermal paper can be Improved.
Some thermal papers produced by thermal printing systems suffer
from low resolution of written image, limited time duration of an image
(fading), deiicacy of thermal paper before printing (increasing care when
handling, shipping, and storing), and the like.

SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order
to provide a basic understanding of some aspects of the Invention. This
summary Is not an extensive overview of the invention. It is intended to
neither identify key or critical elements of the invention nor delineate the
scope of the invention, Rather, the sole purpose of this summary Is to
present some concepts of the Invention, in a simplified form as a prelude to
the more detailed description that is presented hereinafter.
The present invention provides a thermal paper composite precursor
comprising (a) a substrate layer; and (b) a base layer positioned on the
substrate layer, the base layer comprising a binder and at least one porosity
Improver wherein the thermal paper composite precursor has a thermal
tfffuslvlty that is at least about 2% less than the thermal effuslvlty of porosity
improver-less thermal paper composite precursor.
The present invention provides thermal paper containing a base layer
that provides thermal insulating properties which mitigates heat transfer from
the active layer to the substrate layer, Mitigating heat transfer results in
printing, images of improved quality. The thermal insulating properties of the
base layer also permit the use of decreased amounts of active layer
materials, which are typically relatively expensive compared to other
components of the thermal paper,
One aspect of the invention relates to thermal paper containing a
substrate layer; an active iayer containing image forming components; and a
base layer positioned between the substrate layer and the active layer, the
base layer containing a binder and a porosity improver having a specified
thermal effuslvity. The specified thermal effusivity dictates, In part, the
improved thermal Insulating properties of the thermal paper, The base iayer
need not contain Image forming components, which are Included in the active
layer,
Another aspect of the invention relates to making thermal paper
involving forming a base iayer containing a binder and a porosity improver to
improve thermal effuslvity over a substrate layer; and forming an active iayer
containing Image forming components over the base layer.

3
Yet another aspect of the invention relates to printing thermal paper
containing a substrate layer, an active layer, and a base layer positioned
between the substrate layer and the active layer, the base layer containing a
binder and a porosity improver, involving applying localized heat using a
thermal paper printer in the pattern of a desired image to form the desired
image In the thermal paper.
To the accomplishment of the foregoing and related ends, the
invention comprises the features hereinafter fully described and particularly
pointed out in the claims. The following description and the annexed
drawings set forth in detail certain illustrative aspects and implementations of
the invention. These are indicative, however, of but a few of the various ways
in which the principles of the invention may be employed. Other objects,
advantages and novel features of the invention will become apparent from the
following detailed description of the invention when considered in conjunction
with the drawings.
BRIEF SUMMARY OF THE DRAWINGS
Figure 1 is a cross sectional illustration of thermal paper In accordance
with an aspect of the subject invention.
Figure 2 is 3 cross sectional illustration of thermal paper in accordance
with another aspect of the subject invention,
Figure 3 is a cross sectional Illustration of a method of forming an
image In thermal paper in accordance with an aspect of the subject Invention,
DETAILED DESCRIPTION OF THE INVENTION
The phrase "porosity Improver-less thermal paper composite
precursor means a thermal paper composite precursor that does not contain
at least one porosity improver in the base layer thereof,
Generally speaking, thermal paper is coated with a base layer and a
colorless formula (the active layer) which subsequently develops an image by
the application of heat. When passing through an imaging device, precise
measures of heat applied by a print head cause a reaction that creates an
Image (typically biack or color) on the thermal paper. The base layer of the

4
subject Invention is made so that it possesses a thermal effusivity that
improves the quality and/or efficiency of thermal paper printing.
Direct thermal imaging technology of the subject invention may employ
a print head where heat generated Induces a release of ink in the active layer
of thermal paper. This Is also known as direct thermal imaging technology
and uses a thermal paper containing ink in a substantially colorless form in an
active coating on the surface. Heat generated in the print head element
transfers to the thermal paper and activates the ink system to develop an
image. Thermal imaging technology may also employ a transfer ribbon in
addition to the thermal paper. In this case, heat generated in a print head is
transferred to a plastic ribbon, which in turn releases ink for deposition on the
thermal paper. This is known as thermal transfer imaging as opposed to the
subject of direct thermal imaging.
Thermal paper typically has at least three layers: a substrate layer, an
active layer for forming an image, and a base layer between the substrate
layer and active layer. Thermal paper may optionally have one or more
additional layers including a top coating layer (sometimes referred to as a
protective layer) over the active layer, a backside barrier adjacent the
substrate layer, image enhancing layers, or any other suitable layer to
enhance performance and/or handling,
The substrate layer is generally in sheet form. Thai is, the substrate
layer is in the form of pages, webs, ribbons, tapes, belts, films, cards and the
like. Sheet form indicates that the substrate layer has two large surface
dimensions and a comparatively small thickness dimension. The substrate
layer can be any of opaque, transparent, translucent, colored, and non-
colored (white), Examples of substrate layer materials include paper,
filamentous synthetic materials, and synthetic films such as cellophane and
synthetic polymeric sheets (the synthetic films can be cast, extruded, or
otherwise formed). In this sense, the word paper in the term thermal paper is
not inherently limiting.
The substrate layer is of sufficient basis weight to support at least an
active layer and base layer, and optionally of sufficient basis weight to further
support additional, optional layers such as a top coating layer and/or a

backside barrier. In one embodiment, the substrate layer 'nas a basis weight
of abou! 14 g/m2 or more and about 50 g/rn2 or less. In another 'embodiment,
the substrate layer has a basis weight of about 30 g/m2 or more and about
148 g/m2 or less. In yet another embodiment, the substrate layer has a
thickness of about 40 microns or more and about 130 microns or less, in still
yet another embodiment, the substrate layer has a thickness of about 20
microns or more and about 80 microns or less.
The active layer contains image forming components that become
visible to the human eye or a machine reader after exposure to localized heat.
The active layer contains one or more of a dye, chromogenic material,
developer, Inert pigment, antioxldants, lubricants, polymeric binder, sensltlzer,
stabilizer, wetting agents, and waxes. The active layer is sometimes referred
to as a reactive or thermal layer. The components of the active layer are
typically uniformly distributed throughout the active layer, Examples of dyes,
chromogenlc materials, and Inert pigments include fluorescent, organic and
inorganic pigments. These compounds may lead to black-white printing or
color printing. Examples of developers include acidic developers such as
acidic phenolic compounds and aromatic carboxyllo acids. Examples of
sensitizers include ether compounds such as aromatic ether compounds.
One or more of any of the active layer components may or may not be
microencapsulated,
The active layer is of sufficient basis weight to provide a visible,
detectable and/or desirable image on the thermal paper for an end user. In
one embodiment, the active layer has a basis weight of about 1,5 g/m' or
more and about 7,5 g/m2 or less. In another embodiment, the active layer
has a basis weight of about 3 g/m2 or more and about 30 g/m2 or less, in yet
another embodiment, the active layer has a basis weight of about 5 g/rnz or
more and about 15 g/m2 or less, in still yet another embodiment, the active
iayer has a thickness of about 1 micron or more and about 30 microns or
less. In another embodiment, the active Saver has a thickness of about 5
microns or more and about 20 microns or less.
One of the advantages of the subject invention is that a smaller active
layer (or less active layer components) is required in thermal paper of the

6
Invention compared to thermal paper that does not contain a base layer
having specified thermal effusivity properties as described herein. Since the
active layer of thermal paper typically contains the most expensive
components of the thermal paper, decreasing the size of the active layer Is a
significant advantage associated with making the subject thermal paper.
The base layer contains a binder and a porosity improver and has a
specified thermal effusivlty as described herein. The base iayer may further
and optionally contain a dispersant, wetting agent, and other additives, so
long as the thermal effusivlty values are maintained. In one embodiment, the
base layer does not contain image forming components; that Is, the base
layer does not contain any of a dye, chromogenlc material, and/or organic
and inorganic pigments,
The base layer contains a sufficient amount of binder to hold the
porosity improver. In one embodiment, the base layer contains about 5% by
weight or more and about 95% by weight or less of binder. In another
embodiment, the base layer contains about 15% by weight or more and about
90% by weight or less of binder,
Examples of binders Include water-soluble binders such as starches,
hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, gelatin,
casein, polyviny) alcohol, modified polyvinyl alcohol, sodium polyacrylate,
acrylic amide/acrylic ester copolymer, acrylic amide/acrylic ester/methacrylic
acid terpolymer, alkali salts of styrene/maleic anhydride copolymer, alkali
salts of ethylene/maleic anhydride copolymer, polyviny! acetate, polyurethane,
polyacryiic esters, styrene/butadlene copolymer, acrylontrile/butadiene
copolymer, methyl aorylate/butadiene copolymer, ethylene/viny! acetate
copolymer, and the like. Further examples of binders include polyester resin,
vinyl chloride resin, polyurethane resin, vinyl chloride-vinyl acetate copolymer,
vinyl chlorideacrylonitrile copolymer, epoxy resin, nitrocellulose, and the like.
The porosity improver of the subject invention has at least one of high
surface area, high pore volume, narrow particle size distribution, and/or high
porosity when assembled in a layer (and thus appear to possess a high pore
volume), Examples of the porosity improver include one or more of calcined
clays such as calcined kaolin, flash calcined kaolin, and calcined bentonlte,

7
acid treated bentonite, high surface area alumina, hydrated alumina,
boehmite, flash calcined alumina trlhydrate (ATH), silica, silica gel, zeolites,
zeotypes and other molecular sieves, clathrasils, micro-, meso- and macro-
poroiis particles, alumina phosphates, metal alumina phosphates, mica,
pillared clays and the like. These compounds are commercially available
through a number of sources.
The base layer may contain at least one porosity improver, at iaasl two
porosity improvers, at 'east three porosity impi overs, and so on. The porosity
improver contributes to the desirable thermal effuslvlty properties of the base
layer. In one embodiment where at least two porosity Improvers are included
in the base layer, one porosity Improver Is a calcined clay such as calcined
kaolin and the other porosity improver Is one of an acid treated bentonite,
high surface area alumina, hydrated alumina, flash calcined kaolin, flash
calcined ATH, silica, silica gel, zeolite, micro-, meso- or macro-porous
particle, alumina phosphate, molecular sieve, clathrasils, pillared clay,
boehmite, mica or metal alumina phosphate.
Other useful porosity improvers include zeolites, Zeolites and/or
zeotypes, frequently also referred to as molecular sieves, are a ciass of
micro- and mesoporous materials with 1, 2 or 3-D pore system and with a
variety of compositions including silica, aluminosilicates (natural and
traditional synthetic zeolites), alumlno-phosphates (ALPO's), silicon-
aiuminophosphates (SAPO's) and many others. One of the key properties of
these materials is that they (In many cases) reverslbly adsorb and desorb
large quantities of structural water, and if they are stable in their dehydrated
state, they will also reversibly adsorb and desorb other gas8s and vapors,
This is possibis because of the micro- and mesoporous nature of their
structure.
The porosity in zeolites can be best described In terms of channels or
cages connected by smaller windows. Depending on if and how these
intersect, they create 1-, 2- or 3-dimenslona! pore system with pore diameters
and pore openings ranging in size from about 2.5 angstroms to more than
100 angstroms. As a result, they contain a non-negligible amount of pore
volume In their structures and their densities are lower than those of their

8
non-porous or dense poiymorphs. In some instances they can be at least 50
% less dense, The amount of porosity is most commonly described in terms
of pore volume (cc/g), or framework density (FD), The reference FD of dense
silica structure (quartz) is approximately 26.5. Table 1 shows examples ot
some of the most common structures including their pore characteristics.

For the porosity Improvers other than calcined clays, the porosity
Improver of the subject invention has one or more of at least about 70% by
weight of the particles have a size of 2 microns or less, at least about 50% by
weight of the particles have a size of 1 micron or less, a surface area of at
least about 10 m2/g, and a pore volume of at least about 0.1 cc/g. in another
embodiment, the porosity improver of the subject invention (other than
calcined clays) has one or more of at least about 80% by weight of the
particles have a size of 2 microns or less, at least about 60% by weight of the
particles have a size of 1 micron or less, a surface area of at least about 15
m2/g, and a pore volume of at least about 0.2cc/g. In yet another
embodiment, the porosity Improver of the subject invention (other than
calcined clays) has one or more of at least about 90% by weight of the
particles have a size of 2 microns or less, at least about 70% by weight.of the
particles have a size of 1 micron or less, a surface area of at least about 20
m2/g, and a pore volume of at least about 0.3 cc/g.
Calcining destroys the crystallinity of hydrous kaolin or bentonlte, and
renders the kaolin/clay substantially amorphous. Calcination typically occurs
after heating at temperatures In the range from about 700 to about 1200 *C.

9
for a sufficient period of time, Commercial vertical and horizontal rotary
calciners can be used to produce metakaolin, partially calcined kaolin, and/or
calcined kaolin. Acid treatment involves contacting clay with an amount of a
mineral acid to render the clay substantially amorphous,
in one embodiment, calcined day of the subject invention has one or
more of at least about 70% by weight of the particles have a size of 2 microns
or less, at least about 50% by weight of the particles have a size of 1 micron
or less, a surface area of at least about 5 m2/g, and a pore volume of at least
about 0.1 cc/g, In yet another embodiment, calcined clay of the subject
Invention has one or more of at least about 80% by weight of the particles
have a size of 2 microns or less, at least about 60 % by weight of the particles
have a size of 1 micron or less, a surface area of at least about 10 nVVg, and
a pore volume of at least about 0.2 cc/g, In still yet another embodiment,
calcined day of the subject Invention has one or more of at least about 90%
by weight of the particles have a size of 2 microns or less, at ieast about 70%
by weight of the particles have a size of 1 micron or less, a surface area of at
least about 15 mz/g, and a pore volume of at least about 0,3 cc/g.
As noted the non-calcined clay porosity Improver or the calcined clay
porosity improver may have a pore volume of at ieast about 0.1 cc/g, at least
about 0.2 cc/g, or at least about 0,3 cc/g. Alternatively, the non-calcined clay
porosity improver or the calcined clay porosity Improver may have an
equivalent pore volume of at least about 0.1 cc/g, at least about 0,2 cc/g, or at
least about 0,3 cc/g, in this connection, while the individual porosity Improver
particles may not have the required pore volume, when assembled in a layer,
the porosity Improver particles may form a resultant structure (base layer) that
is porous, and has the porosity as if the layer was made of a porosity
Improver having a pore volume of at least about 0.1 cc/g, at least about 0,2
cc/g, or at least about 0.3 cc/g. That is, the base layer may having a pore
volume of at ieast about 0,1 cc/g, at ieast about 0,2 cc/g, or at least about
0.3 cc/g. Thus, the porosity Improver may be porous in and of Itself, or It may
enhance the porosity of the base layer,
Surface area is determined by the art recognized BET method using N2

as the adsorbate. Surface area alternatively is determined using Gardner
Coleman Oil Absorption Test and is based on ASTM D-1483-84 which
measures grams of oil absorbed per 100 grams of kaolin. Pore volume or
porosity Is measured by standard Mercury Porosimetry techniques.
Ail particle sizes referred to heteln are determined by a conventional
sedimentation technique using a Micromeritics, Inc.'s SEDIGRAPH® 5100
analyzer, The sizes, in microns, are reported as "e.s.d," (equivalent spherical
diameter). Particles are slurried in water with a dispersant and pumped
through the detector with agitation to disperse loos© agglomerates,
Examples of commercially available calcined clay of the subject
invention include those under the trade designations such as Ansllex® such
as Ansllex® 93, Satlntons®, and Translink®, available from Hngelhard
Corporation of lselln, New Jersey.
The base layer contains a sufficient amount of a porosity improver to
contribute to providing insulating properties, such as a beneficial thermal
effusivity, that facilitate high quality image formation in the active layer. In
one embodiment, the base layer contains about 5% by weight or more and
about 95% by weight or less of a porosity improver, in another embodiment,
the base layer contains about 15 % by weight or more and about 90% by
weight or less of a porosity improver. In yet another embodiment, the base
layer contains about 15% by weight or more and about 40% by weight or less
of a porosity improver. Ths base layer is of sufficient basis weight to provide
Insulating properties, such as a beneficial thermal effusivity, that facilitate high
quality image formation In the active layer. In one embodiment, the base
layer has a basis weight of about 1 g/m2 or more and about 50 g/m2 or less,
In another embodiment, the base layer has a basis weight of about 3 g/m2 or
more and about 40 g/m2 or less, in yet another embodiment, the base layer
has a basis weight of about 5 g/m2 or more and about 30 g/mz or less., in still
yet another embodiment,, the base layer has a basis weight of about 7 g/m or
more and about 20 g/m2 or less, in another embodiment, the base layer has
a thickness of about 0.5 microns or more and about 20 microns or less. In
yet another embodiment, the base layer has a thickness of about 1 micron or
more and about 10 microns or less. In another embodiment, the base layer

11
has a thickness of about 2 microns or more and about 7 microns or less.
Another beneficial aspect of the base layer is the thickness uniformity
achieved when formed across the substrate layer. In this connection, the
thickness of the base layer does not vary by more than about twenty percent
when selecting two random locations of the base layer for determining
thickness.
Each of the layers or coatings is applied to the thermal paper substrate
by any suitable method, including coating optionally with a doctor blade,
rollers, air knife, spraying, extruding, laminating, printing, pressing, and the
like.
The thermal paper of the subject invention has one or more of the
Improved properties of less active layer material required, enhanced Image
intensity, enhanced image density, Improved base layer coating rheology,
lower abrasion characteristics, and Improved thermal response, The porosity
improver functions as a thermal insulator thereby facilitating reaction between
the image forming components of the active layer providing a more intense,
crisp image at lowered temperatures and/or faster imaging. That is, the
porosity improver functions to Improve the heat Insulating properties In the
thermal paper thereby Improving the efficiency of the active layer in forming
an image.
For thermal paper, thermal sensitivity is defined as the temperature at
which the active layer of thermal paper produces an image of satisfactory
Intensity. Background is defined as the amount of shade/coloration ot
thermal paper before Imaging and/or in the unimaged areas of imaged
thermal paper. The ability to maintain the thermal sensitivity of thermal paper
while reducing the background shade/coloration is significant advantage of
the subject invention, Beneficial increases In thermal response in the active
iayer of thermal paper are achieved through the Incorporation of a porosity
improver as described herein in the base layer.
Comparing thermal papers with similar components, except that one
(thermal of the subject invention) has at least one porosity Improver in the
base layer, the thermal paper precursor of the subject invention has a thermal
effuslvlty value that Is about 2% less than the thermal effusivity of porosity

improver-less thermal paper composite precursor. The 2% includes a
standard deviation of about 0.5-1 % observed in effusivity measurements of
precursor sheets, in another embodiment, the thermal paper precursor of the
subject invention has a thermal effusivity value that Is about 5% less than the
thermal effusivity of porosity Improver-less thermal paper composite
precursor. In another embodiment, the thermal paper precursor of the subject
invention has a thermal effusivity value that is about 15% less than the
thermal effusivity of porosity improver-less thermal paper composite
precursor.
Thermal effusivity is a comprehensive measure for heat distribution
across a given material. Thermal effusivity characterizes the thermal
Impedance of matter (its ability to exchange thermal energy with
surroundings). Specifically, thermal effusivity is a function of the density, heat
capacity, and thermal conductivity. Thermal effusivity can be calculated by
taking the square root of thermal conductivity (W/mK) times the density
(kg/m3) times heat capacity (J/kgK). Thermal effusivity is a heat transfer
property that dictates the interfacial temperature when two semi-infinite
objects at different temperature touch.
Thermal effusivity can be determined employing a Mathis Instruments
TC-30 Thermal Conductivity Probe using a modified hot wire technique,
operating under constant current conditions. The temperature of the heating
element is monitored during sample testing, and changes In the temperature
at the interface between the probe and sample surface, over the testing time,
are continually measured,
in one embodiment, the thermal effusivity (Ws1/2/m2K) of the substrate
coated with base layer is about 450 or less. In another embodiment, the
thermal effusivity of the substrate coated with base layer Is about 370 or less.
In yet another embodiment, the thermal effusivity of the substrate coated with
base layer is about 330 or less, in still yet another embodiment, the thermal
effusivity of the substrate coated with base layer is about 300 or less.
The subject invention can be further understood In connection with the
drawings. Referring to Figure 1, a cross sectional view of a three layer
construction of thermal paper 100 is shown. A substrate layer 102 typically

13
contains a sheet of paper. On one side (the writing side or image side) of the
substrate layer 102 Is a base layer 104. The combination of substrate layer
102 and the base layer 104 is an example of the present thermal paper
composite precursor.
The thermal paper composite precursor car. be combined with an
active layer 108 so that the base layer 104 is positioned between the
substrate layer 102 and the active layer 106, This combination is an example
of a thermal paper composite precursor. The base layer 104 contains a
porosity improver in a binder and provides thermal insulating properties and
prevents the transfer of thermal energy emanating from a thermal print head
through the active layer 106 to the substrate layer 102 during the writing or
imaging process, The base layer 104 also prevents the active layer 106
materials from weeping Into the substrate layer 102. The active layer 106
contains components that form an image In specific locations in response to
the discrete delivery of heat or Infrared radiation from the thermal print head.
Referring to Figure 2, a cross sectional view of a five layer construction
of thermal paper 200 is shown, A substrate layer 202 contains a sheet of
paper. On one side (the non-writing side or backside) of the substrate layer.
202 is a backside barrier 204, The backside barrier 204 in some instances
provides additional strength to the substrate layer 202 as well as prevents
contamination of the substrate layer 202 that may creep to the writing side.
On the other side (the writing side or Image side) of the substrate layer 202 is
a base layer 206, an active layer 208, and a protective coat 210. The
combination of substrate layer 202 and the base layer 206 is an example of
the present thermal paper composite precursor. The base layer 206 is
positioned between the substrate iayer 202 and the active layer 208. The
base layer 206 contains a porosity improver In a binder and provides thermal
insuiatmg properties and prevents the transfer of thermal energy emanating
from a thermal print head through the active iayer 208 and protective coat 210
to the substrate layer 202 during the writing or Imaging process. The active
layer 208 contains components that form an image in specific locations in
response to the discrete delivery of heat or infrared radiation from the thermal
print head. The protective coat 210 Is transparent to the subsequently formed

14
image, and prevents loss of active layer 208 components due to abrasion with
the thermal paper 200,
Although not shown in the figures, the thermal paper structures may
contain additional layers, and/or the thermal papor structures may contain
additional base and active layers for specific applications. For example, the
thermal paper structures may contain a base layer, optionally a backside
barrier, three base layers alternating with three active layers, and a protective
coating.
Referring to Figure 3, a cross sectional view of a method 300 of
imaging thermal paper is shown. Thermal paper containing a substrate layer
302, a base layer 304 and an active layer 306 is subjected to a writing
process. A thermal print head 308 from a writing machine (not shown) Is
positioned near or in close proximity to the side of the thermal paper having
the active layer 306. in some instances the thermal print head 308 may
contact the thermal paper. Heat 310 is emitted, and the heat generates,
induces, or otherwise causes and image 312 to appear in the active layer
308. The temperature of the heat applied or required depends upon a
number of factors including the identity of the image forming components In.
the active layer. Since the base layer 304 is positioned between the
substrate layer 302 and the active layer 306, the base layer 304 mitigates the
transfer of thermal energy from the thermal print head 308 through the active
layer 306 io the substrate layer 302 owing to its desirable thermal effusivity
and thermal insulating properties.
Thermal effusivity test method; Thermal properties of materials can be
characterized by a number of characteristics, such as thermal conductivity,
thermal dlffusivity and thermal effusivity. Thermal conductivity is a measure of
the ability of material to conduct heat (W/mK). Thermal diffusMty measures
the ability of a material to conduct thermal energy relative to its ability to store
energy (mmz/s). Thermal effusivity is defined as the square root of the
product of thermal conductivity (k), density (p) and heat capacity (cp) of a
material (Ws1/2/m2K).
Thermal insulating properties of the pigments of current invention were
characterized using Mathls Instruments TC-30 direct thermal conductivity

15
instrument, by measuring thermal effusivities of coated substrates. No active
coat was applied. Substrates were typically coated with 5-10 g/m2 of base
layer containing the pigment, and then calendered to about the same
smoothness of approximately 2 microns as determined by Print-Parker-Surf
(PP3) roughness test, A sheet of the coated substrate was then cut into
pieces large enough to cover the TC-30 detector. Although the orientation of
the base coat with respect to the sensor (If kept constant), is not cruclal for
obtaining useful data, orientation "towards the sensor" (as opposed to "away
from the sensor") is preferred and was used. To ensure that the heat wave
does not penetrate the sample, about 5-10 pieces of coated substrate were
layered in the test to increase the useful sample cross section. For each
pigment, approximately 100 measurements were performed with optimized
test times, regression start times and cool times, and to maximize the base-
layer coat area subject to measurement, the bottom piece was removed and
placed on top of the stack every 12 measurements. This aiso significantly
improved precision of the measurement, Since any air pockets in-between the
layers due to non-uniform surface roughness wiii have negative impact on
accuracy and precision of the effusivity measurements, calendering is a very
Important step in the sample preparation, Any differences In effusivities
greater then the standard deviation of respective measurements, typically 0.5-
1 %, can be considered real.
As thermal effusivity values of substrates costed with base layer can vary
depending on many parameters, Including the base-layer coat weight and Its
formulation, nature of the substrate, temperature and humidity during
measurement, calendering conditions, smoothness of the tested papers,
instrument calibration etc., it is best to evaluate and rank pigments and their
thermal properties on a comparative basis vs, control (does not contain
porosity improver) rather than by using their absolute measured effusivity
values.
Inventive Example 1

Two pigments coated as a base coat on a substrate layer and also
coated with commercial active layer coat were evaluated for thermal effusivlty
and image quality, respectively, to Illustrate the importance of the thermal
Insulating properties of the base coat en the Image quality - both optical
density and visual quality/uniformity. One of the pigments was a commercially
available synthetic pigment - "Synthetic pigment", the other was a 100 %
calcined kaolin pigment". Active coats on both papers were developed by-
placing 3x3 Inch squares of each paper into an oven sat to 100 *C for 2 min.
Thermal effustvities of substrate/base coat composites and their
corresponding image quality evaluations are summarized in Table 2. The
synthetic pigment gave lower effusivity and had higher optical density.
Visually, It looked black and had very good image uniformity. Sample coated
with calcined kaolin pigment showed higher effusivity and lower optical
density. In visual evaluations, this sample looked gray with highly non-uniform
appearance. Overall, the data indicate an inverse relationship between the
thermal effusivity of the thermal paper precursor and the optical density of the
finished thermal paper. Visual evaluation also shows better image quality for
lower effusivity pigment.

Inventive Example. 2
Two pigments were prepared, coated on a thermal base paper,
calendered to about the same PPS roughness of approximately 2μm and
evaluated for thermal effusivity, Tnermal effusivities were measured on base
paper/base coat composites at about 22 °C and about 40% RH using Mathls
Instruments TC-30 thermal conductiviiy/effusivily analyzer.

17
These composite thermal paper precursor sheets were then coated
with a commercial active coat and evaluated using Industry standard
instrumentation for half energy optical density. The pigments Included
commercial standard calcined kaolin and hydrous kaolin treated with sodium
silicate (20 ibs/ton clay). Physical characteristics of these pigments and their
coatings are summarized in Table 3, The hydrous kaolin treated with sodium
silicate is referred to as treated hydrous kaolin In the remainder of this
inventive Example 2,

Thermal effuslvity of the calcined kaolin containing precursor was more
than 5 % lower than that of the treated hydrous kaolin. This lowered effuslvity,
as expected, provided improved print quality as measured by higher optical
densities. The calcined kaolin showed about 8% Improvement In optical
density compared to the treated hydrous kaolin. In the case of treated
hydrous kaolin, thermal effuslvity of the thermal paper precursor was higher

18
than that of calcined kaolin, which in turn yielded worse optical density. One
can conclude that lower thermal effusivity of the base coat layer, and thus of
the thermal paper composite precursor, has a positive effect on the image
quality of the final thermal paper.
Inventive Example 3
To illustrate the effect of porosity In the bass coat on the thermal
effusivity of the thermal paper precursor, four pigments were prepared,
coated on a thermal base paper, calendered to about the same PPS
roughness of approximately 2μm and evaluated for thermal effusivity using
Mathis Instruments TC-30 analyzer. The pigments included commercial
calcined kaolin, blend of 80 parts of commercial calcined kaolin and 20 parts
of commercially available silica zeolite Y - "80 kaoi!n/20 silicaY", blend of 90
parts of commercial calcined kaolin and 10 parts of Engelhard made zeolite Y
- "90 kaolin/10 zsoiiteY" and hydrous keoiin treated with sodium silicate (20
lbs/ton clay) - "treated hydrous kaolin". The effusivities were measured on
base paper/base coat composites at about 22°C and about 40% RH; the pore
volumes in the base coat layers were obtained from mercury porosimetry.
Physical characteristics of these pigments and their coatings are summarized
in Table 5.

Effusivity measurements of the composite sheets and pore volumes in their
respective base coat layers are presented in Table 6.

19
Results show thai the thermal effusivity of the composite precursor is
inversely proportional to the pore volume in the base coat layer I.e. that the
composite sheet with the highest thermal effusivity has the lowest pore
volume, and the composit© with the lowest effusivity contains highest pore
volume. This also shows that the presence of a porosity improver In the base
coat layer has a positive effect on its thermal properties, such that it reduces
the thermal effusivity of the thermal paper composite precursor when
compared to the same that does not contain a porosity improver. One can
conclude that, a precursor containing a porosity Improver and having an
increased pore volume In the base coat will posses lower thermal effusivity
and thus will result in Improved image quality of the finished thermal paper.
Inventive Example 4
Two pigments were prepared and tested to demonstrate positive
benefit of increased base coat layer porosity on thermal effusivity of the
thermal paper precursor and on Image quality of the finished thermal paper.
One pigment was a hydrous kaolin calcined to muilite Index of 35-55 -
"Calcined clay", the second plgmQnt was a blend of 80 parts of commercial
calcined kaolin and 20 parts of commercially available silica zeolite Y - "80
kaolln/20 siiicaY". Both pigments were coated on a commercial thermal base
paper, calendered to approximately the same PPS roughness of about 2μm,
and evaluated for pore volumes and thermal effusivlties. Both effusivities and
pore volumes were measured on respective thermal paper precursor sheets.

20
The sheets were also treated with a commercial active coat layer and tested
using industry standard Instrumentation (Atlantek 200) for image density.
Basic physical characteristics of both pigments and their base coatings are
summarized in Table 7.

The pore volume of the blended pigment was more than 5 % higher
than that of the calcined day. This increased porosity of the blended pigment
base coat in turn positively affected thermal effusivity of the full precursor,
which was about 5 % lower compared to the calcined clay containing
precursor. Most importantly, the image density of the blended pigment
containing thermal paper was significantly improved. These results dearly
show the benefit of the porosity improver in the base coat, its positive effect
on the thermal effusivity of the precursor and Its 6trong positive impact on
image quality of the finished thermal paper.

21
While the invention has been explained in relation to certain
embodiments, it is to be understood that various modifications thereof will
become apparent to those skilled in the art upon reading the specification,
Therefore, it Is to be understood that the invention disclosed heroin is
Intended to cover such modifications es fall within the scope of the appended
claims.

22
CLAIMS
What is claimed is:
1. A thermal paper composite precursor comprising
(a) a substrate layer; and
(b) a base layer positioned on the substrate layer, the base
layer comprising a binder and at least one porosity improver wherein said
thermal paper composite precursor has a thermal effusivity that Is at least
about 2% less than the thermal effusivlty of porosity improver-less thermal
paper composite precursor.
2. The thermal paper composite precursor of claim 1 wherein said
porosity improver is selected from the group consisting of calcined kaolin,
flash calcined kaolin, calcined bentonite, acid treated bentonite, high surface
area alumina, hydrated alumina, boehmite, flash calcined alumina trihydrate,
silica, silica gel, zeolite, zeotypes, non-zeotype molecular sieves, clathraslls,
microporous particles, mesoporous particles, macroporous particles, alumina
phosphates, metal alumina phosphates, mica, and pillared days,
3. The thermal paper composite precursor of claim 2 wherein said
porosity Improver is selected from the group consisting of calcined kaolin,
flash calcined kaolin, and calcined bentonite,
4. The thermal paper composite precursor of claim 2 wherein said
porosity improver is selected from the group consisting of acid treated
bentonite, high surface area alumina, hydrated alumina, boehmite, flash
calcined alumina trihydrate, silica, silica gel, zeolite, zeotypes, non-zeotype
molecular sieves, clathrasils, microporous particles, mesoporous particles,
macroporous particles, alumina phosphates, metal alumina phosphates,
mica, and pillared clays.
5. The thermal paper composite precursor of claim 3 wherein said
porosity improver has at least one of: at least about 70% by weight of the

particles have a size of 2 microns or less, at least about 50% by weight of the
particles have a size of 1 micron or less, a surface area of at least about 5
m2/g, and a pore volume of at least about 01 cc/g.
6. The thermal paper composite precursor of claim A wherein said
porosity improver has at least one of. at least about 70% by weight of the
particles have a size of 2 microns or less, at least about 50% by weight of the
particles have a size of 1 micron or less, a surface area of at least about 10
m2/g, and a pore volume of at least about 0.1 cc/g,
7. The thermal paper composite precursor of claim 1 wherein said
thermal effusivity is at least about 5% less.
8. The thermal paper composite precursor of claim 1 wherein said
thermal effusivity is at least about 10% less,
9. The thermal paper composite precursor of claim 1 wherein said
thermal effusivity is at least about 15% less,
10. A thermal paper composite comprising the thermal paper
composite precursor of claim 1 and an active layer comprising image forming
components on said base layer (b).

The present invention provides a thermal paper composite precursor comprising (a) a
substrate layer; and (b) a base layer positioned on the substrate layer, the base layer
comprising a binder and at least one porosity improver wherein the thermal paper
composite precursor has a thermal effusivity that is at least about 2% less than the
thermal effusivity of porosity improver-less thermal paper composite precursor. The
thermal paper composite precursor is useful in making thermal paper composite.

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Patent Number

272631

Indian Patent Application Number

2002/KOLNP/2007

PG Journal Number

16/2016

Publication Date

15-Apr-2016

Grant Date

13-Apr-2016

Date of Filing

04-Jun-2007

Name of Patentee

ENGELHARD CORPORATION

Applicant Address

101, WOOD AVENUE P.O. BOX 770 ISELIN, NJ

Inventors:

#	Inventor's Name	Inventor's Address
1	MATHUR, SHARAD	602 MILLRUN COURT, MACON, GA 31210
2	PETROVIC, IVAN	8C BROOKLINE COURT PRINCETON, NJ 08540
3	YANG, XIAOLIN, DAVID	48 NETHERWOOD CIRCLE, EDISON, NJ 08820
4	BROYLES, DAVID, A.	1277 DEDRICK ROAD, MCINTYRE, GA 31054
5	FINCH, ERNEST, M	10 GINNY LANE HOPEWELL JUNCTION, NY 12533

PCT International Classification Number

B41M 5/40

PCT International Application Number

PCT/US2005/043496

PCT International Filing date

2005-12-01

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	11/291,224	2005-12-01	U.S.A.
2	60/633,143	2004-12-03	U.S.A.