Patent Application
20090096833
The present invention relates generally to a direct thermal imaging method and, more particularly, to a multicolor direct thermal imaging method and member for use therein, wherein a direct thermal imaging member comprising different image-forming compositions is imaged at different speeds by a source of heat to form a multicolored image.
Direct thermal imaging is a technique in which a substrate bearing at least one image-forming layer, which is typically initially colorless, is heated by contact with a thermal printing head to font an image. In direct thermal imaging there is no need for ink, toner, or thermal transfer ribbon. Rather, the chemistry required to form an image is present in the imaging member itself. Direct thermal imaging is commonly used to make black and white images, and is often employed for the printing of, for example, labels and store receipts. There have been described in the prior art numerous attempts to achieve multicolor direct thermal printing. A discussion of various direct thermal color imaging methods is provided in U.S. Pat. No. 6,801,233 B2.
In the method of the present invention, a direct thermal imaging member having more than one image-forming layer is addressed by a thermal printing head to provide a colored image. The imaging member is addressed in more than one pass of a thermal printing head, at least one pass being at a different speed from at least another pass. Optionally, the imaging member is preheated to a different extent in at least one pass than in at least another pass.
In U.S. Pat. No. 6,801,233 B2 there is described and claimed a direct thermal imaging system in which one or more thermal printing heads can form two colors in a single pass on the imaging member. The printer can form these multiple colors by addressing two or more image-forming layers of the imaging member at least partially independently from the same surface so that each color can be formed alone or in selectable proportion with the other color(s). In a preferred embodiment, a printer can form three colors on three image-forming layers, which may be carried by the same surface of a substrate.
Thermal printing devices with variable printing speed are known in the art, as described, for example in U.S. Pat. Nos. 5,319,392 and 6,078,343. These can be direct thermal or thermal transfer printers. In general, the speed of thermal printers depends upon the nature of the image to be printed. Thus, low-quality direct thermal images (such as store receipts) may be printed at speeds of 3 inches/second or more. Thermal transfer printing of photographic quality is typically carried out at speeds of less than 1 inch/second.
Preheating of a thermally activated printing head is described, for example, in U.S. Pat. No. 5,191,357, which describes a recording apparatus for performing recording on a recording medium where the apparatus includes a plurality of recording elements and a control unit for selectively providing energy having a level lower than an actual recording level. It is also known to preheat a thermal transfer ink layer in a thermal transfer imaging method. For example, U.S. Pat. No. 5,529,408 discloses a thermal transfer recording method wherein the thermal transfer ink layer is preheated prior to having energy applied thereto in order to initiate transfer of the ink to a receiving material.
As the state of the thermal imaging art advances, efforts continue to be made to provide thermal imaging materials and thermal imaging methods that can meet new performance requirements.
Described herein are novel methods, devices and reagents for multicolor, direct thermal imaging.
One embodiment is directed to a multicolor thermal imaging method, comprising: (a) using heat to form an image in a first color at a first speed of travel with respect to the source of said heat of a thermal imaging member comprising at least a first image-forming layer forming the first color when heated and a second image-forming layer forming a second color when heated, said first and second colors being different from each other; and (b) using heat to form an image in said second color at a second speed of travel of said thermal imaging member with respect to said source of said heat; wherein said first speed of travel and said second speed of travel are substantially different speeds of travel; whereby a multicolor image is formed in said thermal imaging member. In a particular embodiment, said first speed of travel is greater than 0.5 inches/second and said second speed of travel is less than 0.5 inches/second. In a particular embodiment, said first speed of travel is greater than 0.7 inches/second and said second speed of travel is less than 0.3 inches/second. In a particular embodiment, said source of said heat comprises a thermal printing head. In a particular embodiment, the heat sink of said thermal printing head is maintained at an approximately constant temperature during steps (a) and (b). In a particular embodiment, said approximately constant temperature is at least about 5° C. above ambient temperature. In a particular embodiment, said approximately constant temperature is at least about 20° C. above ambient temperature. In a particular embodiment, the heat sink of said thermal printing head is maintained at a first temperature during step (a) and a second temperature during step (b), said first and said second temperatures differing by at least about 5° C. In a particular embodiment, said source of said heat comprises a laser. In a particular embodiment, said source of said heat comprises more than one heating means, e.g., said source of said heat comprises a first heating means capable of being modulated so as to form an image in said thermal imaging member and a second heating means capable of providing uniform preheating. In a particular embodiment, said first heating means and said second heating means make contact with different points on the same surface of said thermal imaging member at any given instant. In a particular embodiment, said second heating means is maintained at an approximately constant temperature during steps (a) and (b). In a particular embodiment, said approximately constant temperature is at least about 5° C. above ambient temperature. In a particular embodiment, wherein said approximately constant temperature is at least about 20° C. above ambient temperature. In a particular embodiment, said second heating means is maintained at a first temperature during step (a) and at a second temperature during step (b), said first and said second temperatures differing by at least about 5° C. In a particular embodiment, said first image-forming composition has an activating temperature that is higher by at least about 5° C. than that of said second image-forming composition. In a particular embodiment, said first speed of travel is greater than said second speed of travel.
One embodiment is directed to a multicolor thermal imaging method comprising: (a) using heat to form an image in a first color at a first speed of travel with respect to the source of said heat of a thermal imaging member comprising at least a first image-forming layer forming the first color when heated, a second image-forming layer forming a second color when heated and a third image-forming composition forming a third color when heated, said first, second and third colors being different from each other; (b) using heat to form an image in said second color at a second speed of travel of said thermal imaging member with respect to said source of said heat; and (c) using heat to form an image in said third color at a third speed of travel of said thermal imaging member with respect to said source of said heat; wherein at least two of said first, second and third speeds of travel are substantially different speeds of travel; whereby a multicolor image is formed in said thermal imaging member. In a particular embodiment, two of said first, second and third speeds of travel are the same. In a particular embodiment, an image is formed in at least two of said colors in one pass of said thermal imaging member relative to said source of said heat and an image is formed in at least a third of said colors in another pass of said thermal imaging member relative to said source of said heat. In a particular embodiment, each of said first, second and third speeds of travel are substantially different speeds of travel. In a particular embodiment, said source of said heat comprises a thermal printing head. In a particular embodiment, an image is formed in at least two of said image-forming layers in one printing pass of said thermal printing head and an image is formed in at least a third of said image-forming layers in another printing pass of said thermal printing head, wherein the speeds of travel of said thermal imaging member with respect to said thermal printing head in said printing passes are substantially different speeds of travel. In a particular embodiment, the heat sink of said thermal printing head is maintained at a first temperature during one printing pass and a second temperature during the other printing pass, wherein said first temperature differs from said second temperature by at least about 5° C. In a particular embodiment, the heat sink of said thermal printing head is maintained at a first temperature during one printing pass and a second temperature during the other printing pass, wherein said first temperature differs from said second temperature by less than about 5° C. In a particular embodiment, an image is formed in one of said image-forming layers in a first printing pass of said thermal printing head, an image is formed in another of said image-forming layers in a second printing pass of said thermal printing head, and an image is formed in a third of said image-forming layers in a third printing pass of said thermal printing head, wherein the speeds of travel of said thermal imaging member with respect to said thermal printing head in at least two of said first, second and third printing passes are substantially different speeds of travel. In a particular embodiments the heat sink of said thermal printing head is maintained at a first temperature during the first of said passes, a second temperature during the second of said passes, and a third temperature during the third of said passes, at least one of said first, second, and third temperatures differing from at least another of said first, second and third temperatures by at least about 5° C. In a particular embodiment, the heat sink of said thermal printing head is maintained at a first temperature during the first of said passes, a second temperature during the second of said passes, and a third temperature during the third of said passes, wherein none of said first, second, and third temperatures differ from any other of said first, second and third temperatures by more than about 5° C. In a particular embodiment, said source of said heat comprises more than one heating means. In a particular embodiment, said source of said heat comprises a first heating means capable of being modulated so as to form an image in said thermal imaging member and a second heating means capable of providing uniform preheating. In a particular embodiment, said first heating means and said second heating means make contact with different points on the same surface of said thermal imaging member at any given instant. In a particular embodiment, an image is formed in at least two of said image-forming layers in one pass of said first and second heating means and an image is formed in at least a third of said image-forming layers in another pass of said first and second heating means, the speeds of travel of said thermal imaging member with respect to said first and second heating means in said passes being substantially different speeds of travel. In a particular embodiment, said second heating means is maintained at a first temperature during one pass and at a second temperature during the other pass, wherein said first temperature differs from said second temperature by at least about 5° C. In a particular embodiment, said second heating means is maintained at a first temperature during one pass and at a second temperature during the other pass, wherein said first temperature differs from said second temperature by less than about 5° C. In a particular embodiment, an image is formed in one of said image-forming layers in a first pass of said first and second heating means, an image is formed in another of said image-forming layers in a second pass of said first and second heating means, and an image is formed in a third of said image-forming layers in a third pass of said first and second heating means, wherein the speeds of travel of said thermal imaging member with respect to said first and second heating means in at least two of said first, second and third printing passes are substantially different speeds of travel. In a particular embodiment, said second heating means is maintained at a first temperature during the first of said passes, at a second temperature during the second of said passes, and at a third temperature during the third of said passes, wherein at least two of said first, second, and third temperatures differ from each other by at least about 5° C. In a particular embodiment, said second heating means is maintained at a first temperature during the first of said passes, at a second temperature during the second of said passes, and at a third temperature during the third of said passes, none said first, second, and third temperatures differing from any other of said first, second and third temperatures by more than about 5° C. In a particular embodiment, said first image-forming composition has an activating temperature that is higher than that of said second image-forming composition, and said second image-forming composition has an activating temperature that is higher than that of said third image-forming composition. In a particular embodiment, said first speed of travel is greater than said second speed of travel, and said second speed of travel is greater than said third speed of travel.
For a better understanding of the invention as well as other objects and advantages and further features thereof, reference is made to the following detailed description of various preferred embodiments thereof taken in conjunction with the accompanying drawings wherein:
Specific preferred embodiments of the invention will be described with respect to the drawings, which illustrate thermal imaging members for use with the present thermal imaging method. Referring now to
Each image-forming layer can change color, e.g., from initially colorless to colored, where it is heated to a particular temperature referred to herein as its activating temperature. Any order of the colors of the image-forming layers can be chosen. One preferred color order is as described above. Another preferred order is one in which the three image-forming layers 14, 16, and 18 are yellow, magenta and cyan, respectively.
Spacer layer 20 is preferably thinner than spacer layer 22, provided that the materials comprising both layers have substantially the same thermal diffusivity. The function of the spacer layers is control of thermal diffusion within the imaging member 10. Preferably, spacer layer 22 is at least four times thicker than spacer layer 20.
All the layers disposed on the substrate 12 are substantially transparent before color formation. When the substrate 12 is reflective (e.g., white), the colored image formed on imaging member 10 is viewed through the overcoat 24 against the reflecting background provided by the substrate 12. The transparency of the layers disposed on the substrate ensures that combinations of the colors printed in each of the image-forming layers may be viewed.
In the preferred embodiments of the invention where the thermal imaging member includes at least three image-forming layers, all the image-forming layers may be arranged on the same side of a substrate, or two or more of the image-forming layers may be arranged on one side of a substrate with one or more image-forming layers being arranged on the opposite side of the substrate.
In preferred embodiments of the method of the invention, the image-forming layers are addressed at least partially independently by variation of two adjustable parameters, namely, temperature and time. These parameters can be adjusted in accordance with the invention to obtain the desired results in any particular instance by selecting the temperature of the thermal printing head and the period of time during which heat is applied to the thermal imaging member. Thus, each color of the multicolor imaging member can be printed alone or in selectable proportion with the other colors. As will be described in detail, in these embodiments the temperature-time domain is divided into regions corresponding to the different colors that it is desired to obtain in the final image.
Depending upon the printing time, available printing power, and other factors, various degrees of independence in the addressing of the image-forming layers can be achieved. The term “independently” is used to refer to instances in which the printing of one color-forming layer typically results in a very small, but not generally visible optical density (density<0.05) in the other color-forming layer(s). In the same manner, the term “substantially independent” color printing is used to refer to instances in which inadvertent or unintentional coloration of another image-forming layer or layers results in a visible density that is at a level typical of interimage coloration in multicolor photography (density<0.2). The term “partially independent” addressing of the image-forming layers is used to refer to instances in which the printing of maximum density in the layer being addressed results in the coloration of another image-forming layer or layers at a density higher than 0.2 but not higher than about 0.0. The phrase “at least partially independently” is inclusive of all of the degrees of independence described above.
The image-forming layers of the thermal imaging member undergo a change in color to provide the desired image in the imaging member. The change in color may be from colorless to colored, from colored to colorless, or from one color to another. The term “image-forming layer” as used throughout the application, including in the claims, includes all such embodiments. In the case where the change in color is from colorless to colored, an image having different levels of optical density (i.e., different “gray levels”) of that color may be obtained by varying the amount of color in each pixel of the image from a minimum density, Dmin, which is substantially colorless, to a maximum density, Dmax, in which the maximum amount of color is formed. In the case where the change in color is from colored to colorless, different gray levels are obtained by reducing the amount of color in a given pixel from Dmax to Dmin, where ideally Dmin is substantially colorless.
According to a preferred embodiment of the invention, each of the image-forming layers 14, 16 and 18 is independently addressed by application of heat with a thermal printing head in contact with the topmost layer of the member, optional overcoat layer 24 in the member illustrated in
The heating of the lower image-forming layers, i.e., those closer to the substrate 12 (in this case image-forming layers 16 and 18) is accomplished by maintaining the thermal printing head at temperatures such that the upper image-forming layer(s) remain below their activating temperatures for sufficient periods of time to allow heat to diffuse through them to reach the lower image-forming layer(s). In this way, no image information is provided in the upper image-forming layer(s) when the lower image-forming layer(s) are being imaged. The heating of the image-forming layers according to the method of the invention may be accomplished by two passes of a single thermal printing head, or by a single pass of each of more than one thermal printing head, as is described in detail below.
Although the heating of imaging member 10 is preferably carried out using a thermal printing head, any method providing controlled beating of the thermal imaging member may be used in the practice of the present invention. For example, a modulated source of light (such as a laser) may be used. In this case, as is well known in the art, an absorber for light of a wavelength emitted by the laser must be provided in the thermal imaging member or in contact with the surface of the imaging member.
When a thermal printing head (or other contact heating element) is used to heat the thermal imaging member 10, heat diffuses into the bulk of the thermal imaging member from the layer in contact with the thermal printing head (typically, overcoat layer 24). When a source of light is used for heating, the layer or layers containing an absorber for the light will be heated as light is converted to heat in these layers, and heat will diffuse from these layers throughout the thermal imaging member. It is not necessary that the light-absorbing layers be at the surface of the imaging member, provided that the layers of the thermal imaging member separating the source of light from the absorbing layers are transparent to light of the wavelength to be absorbed. In the discussion below it is assumed that the layer that is directly heated is the overcoat layer 24, and that heat diffuses from this layer into the thermal imaging members but similar arguments apply whichever layer or layers of the thermal imaging member 10 is (or are) heated.
The activating temperatures selected for the image-forming layers are generally in the range of about 90° C. to about 300° C. The activating temperature (Ta1) of the first image-forming layer 18 is preferably as low as possible consistent with thermal stability of the imaging member during shipment and storage and preferably is about 100° C. or more. The activating temperature (Ta3) of the third image-forming layer 14 is preferably as low as possible consistent with allowing the activation of the second and third image-forming layers 16 and 18 by heating through this layer without activating it according to the method of the invention, and preferably is about 200° C. or more. The activating temperature (Ta2) of the second image-forming layer is between Ta1 and Ta3 and is preferably between about 140° C. and about 180° C.
In applications where stability is less important the activating temperature, Ta1, of the first image-forming layer may be as low as about 70° C., the activation temperature of the second image-forming layer, Ta2, is preferably at least about 30° C. above Ta1, and the activating temperature of the third image-forming layer, Ta2, is preferably at least about 30° C. above Ta2.
Thermal printing heads used in the method of the present invention typically include a substantially linear array of resistors that extends across the entire width of the image to be printed. In some embodiments the width of the thermal printing head may be less than that of the image. In such cases the thermal printing head may be translated relative to the thermal imaging member in order to address the entire width of the image, or else more than one thermal printing head may be used. The imaging member is typically imaged while being transported in a direction perpendicular to the line of resistors on the printing head while pulses of heat are provided by supplying electrical current to these resistors. The time period during which heat can be applied to thermal imaging member 10 by a thermal printing head is typically in the range of about 0.001 to about 100 milliseconds per line of the image. The lower limit may be defined by the constraints of the electronic circuitry, while the upper limit is set by the need to print an image in a reasonable length of time. The spacing of the dots that make up the image is generally in the range of 100-600 lines per inch in directions both parallel and transverse to the direction of motion, and is not necessarily the same in each of these directions.
In forming an image using imaging member 10, the thermal printing head may be in contact with the thermal imaging member in a single pass over the surface of the image, and print all three colors in that single pass. There are, however, situations in which the printing is preferably carried out in more than one pass of the thermal printing head. In such cases, two of the image-forming layers may be printed in one pass, and a third in a second pass. Alternatively, three image-forming layers may be printed in three separate passes. One obvious consequence to printing in more than one pass is that the length of time required to obtain an image can be longer than if the image were printed in a single pass. It is an object of the present invention to minimize the time taken to print imaging members such as that illustrated in
It is apparent from
One reason that more than one printing pass may be required is that it may be desirable to preheat the thermal imaging member to a different temperature in one pass than in another. Such selectable preheating allows a greater flexibility in the printing method, and more controlled addressing of the individual image-forming layers.
During the formation of an image, a substantial amount of heat is transferred from the resistors 38 into the printing head glaze, and the temperature of the printing head glaze may rise. Depending upon the speed of printing and the precise area of contact between the thermal imaging member and the printing head glaze, the temperature of the thermal imaging member 10 at the moment of contact with the resistors 38 may not be ambient temperature. Moreover, there may be a gradient of temperature within the thermal imaging member 10 such that the temperatures within each of the image-forming layers are not the same.
The temperature of an image-forming layer at the moment that the thermal imaging member begins to be heated by the resistors 38 (or other modulated source of heat adapted to form an image in the thermal imaging member) is herein referred to as the “baseline temperature” of that layer. Where a gradient of temperatures exists within the image-forming layer at the time that modulated heating of the thermal imaging member to form an image in the thermal imaging member begins, the baseline temperature of the layer, as that term is used herein, includes the range of temperatures within the gradient. Thus, it should be understood that the term “baseline temperature” is inclusive of a range of temperatures that may be present in different areas of the layer.
Any heating that causes the baseline temperature of an image-forming layer to be greater than ambient temperature is herein referred to as “preheating”. Preheating may be effected by thermal contact of the thermal imaging member with the printing head glaze as described above, or by contact with other preheating means as described in more detail below.
The analysis of time and temperature regions for printing each image-forming layer given above with reference to
For example, referring now to
It will be apparent to one of skill in the art that a given baseline temperature for a particular image-forming layer may be obtained in a variety of different ways, which may result in different gradients of temperature within the imaging member. These gradients, moreover, will change over time. It is also possible that a gradient of temperature may exist across the image-forming layer itself. For these reasons, the analysis given above with reference to
As described above, the rate-limiting layer for forming an image in the thermal imaging member according to the method of the present invention is the most deeply buried image-forming layer, image-forming layer 18 in the imaging member illustrated in
The baseline temperature of any of the image-forming layers within the thermal imaging member as an image is formed therein may be adjusted by a variety of techniques that will be apparent to those skilled in the art. For example, as shown in
Other techniques may be used to adjust the baseline temperature of the image-forming layers of the thermal imaging member during image formation.
Any suitable heat-providing member may be used to preheat the thermal imaging member according to the method of the invention. The preheating element may be a thermally conductive shim that is in thermal contact with the heat sink of a thermal printing head and provides additional area of contact with a thermal imaging member. In some cases, this shim may also serve as the cover for the integrated circuits that supply current to the heating elements of the thermal printing head, or it may be a part of the heat sink of the thermal printing head. Alternatively, the preheating element may include a separate resistive heater, a conduit for a heating fluid, a source of radiation (for example, infra-red radiation), a frictional contact, or other heating means such as are well known to those of skill in the art.
Although
As discussed above, according to a preferred embodiment of the present invention, the first and second applications of heat to the image-forming layers are carried out at different speeds of the imaging member relative to the source of heat used to form the image. In one such step at least a first image-forming layer is at a first baseline temperature when heat is applied to at least a second image-forming layer to form an image therein. Heat is applied to the first image-forming layer to form an image therein when it is at a second baseline temperature. Preheating is used to make the adjustment in the baseline temperature of the first image-forming layer.
The amount of preheating of a particular image-forming layer within the thermal imaging member may itself be affected by the printing speed. As discussed above, preheating may be effected by the printing head glaze of
It is likely, moreover, that a gradient of temperature will be established within the imaging member by its encounter with the preheating element, such that the degree of preheating of a particular image-forming layer will depend also upon the distance separating the particular layer from the preheating element. This is particularly true when the preheating element is a hot object that makes physical contact with the surface of the imaging member.
If two passes of a thermal printing head are used to form an image in an imaging member such as that shown in
A direct thermal imaging method wherein an image is formed in a thermal imaging member having at least two image-forming layers with more than one pass of a thermal printing head, and wherein at least one of the image-forming layers is at a first baseline temperature (T1) when heat is being applied to one or more other image-forming layers in a pass of a printing head and the baseline temperature of that image-forming layer is at a second different temperature (T2) when heat is applied to it is disclosed in co-pending commonly-assigned U.S. patent application Ser. No. 11/400,735, filed on Apr. 6, 2006, now issued as U.S. Pat. No. 7,408,563, the contents of which are incorporated by reference herein in its entirety.
In one preferred embodiment of the present invention, image-forming layers 14 and 16 are imaged in one printing pass while image-forming layer 18 is at a baseline temperature T1 that is substantially equal to the ambient temperature, and image-forming layer 18 is imaged in a second printing pass while it is at a baseline temperature T2 that is substantially above the ambient temperature.
In a particularly preferred embodiment, the preheating element is above ambient temperature and the thermal imaging medium makes contact with the preheating element over a length in the transport direction of at least about 200 microns. Image-forming layers 14 and 16 are imaged in a first pass and image-forming layer 18 is imaged in a second pass, the first pass is preferably carried out at or above a speed of about 0.7 inch/second, and especially preferably at or above a speed of about 1 inch/second, and the second printing pass in which image-forming layer 18 is imaged preferably is carried out at or below a speed of about 0.5 inches/second, and especially preferably at or below a speed of about 0.3 inches/second.
In another particularly preferred embodiment of the method of the invention, the preheating element is above ambient temperature, the thermal imaging member makes contact with the preheating element over a length in the transport direction of at least about 200 microns, and three printing passes are employed. The printing pass or passes in which image-forming layer 14 is imaged is carried out at or above a speed of about 0.7 inch/second, and especially preferably at or above a speed of about 1 inch second, the printing pass or passes in which image-forming layer 16 is imaged is carried out at or above a speed of about 0.7 inch/second, and especially preferably at or above a speed of about 1 inch/second, and the printing pass or passes in which image-forming layer 18 is imaged is carried out at or below a speed of about 0.5 inches/second, and especially preferably at or below a speed of about 0.3 inches/second.
Although the invention has been described with reference to a thermal imaging member having three different image-forming layers, the same principles can be applied to imaging members comprising only two image-forming layers or having more than three such layers. Moreover, the components required for forming each color may be located in the same layer, but separated from each other in some way, for example by microencapsulation. All that is necessary in the practice of the present invention is that the time of heating of a particular layer of the thermal imaging member (typically the surface layer, as mentioned above) that is required for formation of a first color be shorter than the time of heating of that layer required for formation of a second color, and that the activating temperature for the first color be higher than the activating temperature for the second color.
A thermal imaging member having two image-forming layers on one side of a transparent substrate and a third image-forming layer on the reverse side of the substrate is illustrated in
Using the method of the present invention, formation of an image in image-forming layer 54 may be accomplished in a first pass at a first speed of travel of the imaging member 50 as described above, and formation of an image in image-forming layer 58 may be accomplished by a second printing pass at a second different speed of travel of the imaging member as described above.
Formation of an image in the third image-forming layer 60 is accomplished by printing on the reverse side of imaging member 50 with a thermal printing head, as described in U.S. Pat. No. 6,801,233 B2. This step may be performed at a speed of travel of the imaging medium that is either the first, or second or a third different speed.
In the practice of the present invention, the printing pulses supplied by the thermal printing head should be adjusted so as to compensate for the residual heat in the printing head itself and in the thermal imaging member that results from the printing of preceding (and neighboring) pixels in the image. Such thermal history compensation may be carried out as described in U.S. Pat. No. 6,819,347 B2.
It is not necessary that the yellow image be formed with as many gray levels as the images in the other two subtractive primary colors. In one embodiment of the invention, the number of gray levels used in forming yellow is deliberately made less than the number of gray levels used for the other colors. In the extreme, it is possible to use a binary image for the yellow image-forming layers (e.g., one with only Dmin and Dmax values allowed in each pixel). Even with such a small number of gray levels of the yellow sub-image, the human eye cannot easily discern a loss in the quality of the overall, three-color image. As would be well-known to one skilled in the art dithering can be used to increase the apparent number of gray levels while trading off spatial resolution.
As described above herein, the method of the present invention can provide independent formation of each color, e.g. cyan, magenta or yellow. Thus, in this embodiment, one combination of temperature and time will permit the selection of any density of one color while not producing any noticeable amount of the other colors. Another combination of temperature and time will permit the selection of another of the three colors, and so forth. A juxtaposition of temperature-time combinations will allow the selection of any combination of the three subtractive primary colors in any relative amounts.
In other embodiments of the invention, thermal addressing of the image-forming layers, rather than being completely independent, may be substantially independent or only partially independent. Various considerations, including material properties, printing speed, energy consumption, material costs and other system requirements may dictate a system with increased lack of addressing independence, the consequence of which is color “cross-talk”, i.e., the contamination of an intended color by another color. While independent or substantially independent color addressing according to the invention is important for imaging of photographic quality, this requirement may be of less importance in the formation of certain images such as, for example, labels or coupons, and in these cases may be sacrificed for economic considerations such as improved printing speed or lower costs.
In the embodiments of the invention where addressing of the separate image-forming layers of a multicolor thermal imaging member is not completely, but rather only substantially or partially independent, and by design the printing of the first color may produce a certain amount of a second color, the color gamut of the imaging member will be reduced. Since, as described above, the color gamut of the imaging member will be affected by the conditions of imaging, these conditions may be selected so as to optimize the overall system for its intended application with respect to color gamut, speed, cost, etc.
A number of image-forming techniques may be exploited in accordance with the invention including thermal diffusion with buried layers (as described in detail above), chemical diffusion or dissolution in conjunction with timing layers, melting transitions and chemical thresholds. Many such image-forming techniques have been described in detail in U.S. Pat. No. 6,801,233 B2. All such image-forming techniques may be exploited in the imaging members utilized in the method of the invention.
It should be noted here that the image-forming layers of the imaging members utilized in the method of the invention may themselves comprise two or more separate layers or phases. For example, where the image-forming material is a leuco dye that is used in conjunction with a developer material, the leuco dye and the developer material may be disposed in separate layers.
The image-forming layers of an imaging member utilized according to the invention may optionally undergo more than one color change. For example, image-forming layer 14 of imaging member 10 (
Any combination of materials that may be thermally induced to change color may be used in the image-forming layers. The materials may react chemically under the influence of heat, either as a result of being brought together by a physical mechanism, such as melting, or through thermal acceleration of a reaction rate. The reaction may be chemically reversible or irreversible.
The substrate for the thermal imaging member, e.g., substrate 12, may be of any suitable material for use in thermal imaging members, such as polymeric materials or treated papers, and may be transparent or reflective. The substrate may also carry layers such as adhesion-promoting layers, antistatic layers, or gas barrier layers. The face of substrate 12 opposite to that onto which is coated image-forming layer 18 may bear indicia such as a logo, or may comprise an adhesive composition such as a pressure-sensitive adhesive. Such an adhesive may be protected by a peelable liner layer. The substrate 12 may be of any practical thickness, depending upon the application, ranging from about 2 micrometers in thickness to card stock of about 500 micrometers in thickness or more.
In a preferred embodiment at least one, and preferably all the image-forming layers include as an image-providing material a chemical compound in a crystalline form, the crystalline form being capable of being converted to a liquid in the amorphous form, where the amorphous form of the chemical compound intrinsically has a different color from the crystalline form. A color thermal imaging method and thermal imaging member wherein at least one image-forming layer includes such a chemical compound are described and claimed in commonly assigned U.S. patent application Ser. No. 10/789,648, filed Feb. 27, 2004, (U.S. Pat. Pub. No: US2004/0176248 A1).
The image-forming layers of the imaging members used according to the method of the invention, e.g., image-forming layers 14, 16 and 18 of imaging member 10, may comprise any of the image-forming materials described above, or any other thermally-activated colorants, and are typically from about 0.5 to about 4 micrometers in thickness, preferably about 2 micrometers. In the case where the image-forming layers comprise more than one layer, as described above, each of the constituent layers is typically from about 0.1 to about 3 micrometers in thickness. The image-forming layers may comprise dispersions of solid materials, encapsulated liquids, amorphous or solid materials or solutions of active materials in polymeric binders, or any combinations of the above.
The distance from the outer surface of the imaging member, e.g. overcoat layer 24, to the interface between the first image-forming layer, e.g., image-forming layer 14, and a spacer layer, e.g., layer 20, is preferably between about 2 and 5 micrometers; the distance from the outer surface of the imaging member to the interface between a second image-forming layer, e.g., image-forming layer 16 and a spacer layer, e.g., spacer layer 22, is preferably between about 7 and about 12 micrometers, and the distance between the outer surface of the imaging member and the interface between the third image-forming layer, e.g., image-forming layer 18 and a substrate, e.g., substrate 12 is preferably at least about 28 micrometers.
Spacer layers, such as spacer layers 20 and 22, function as thermally insulating layers, and may comprise any suitable material. Typical suitable materials include water-soluble polymers such as poly(vinyl alcohol) or waterborne latex materials such as acrylates or polyurethanes. In addition, spacer layers 20 and 22 may comprise inorganic fillers such as for example calcium carbonate, calcium sulfate, silica or barium sulfate; ultraviolet absorbers such as zinc oxide, titanium dioxide, or organic materials such as benzotriazoles; materials that change phase such as organic crystalline compounds; and so on. In some embodiments, spacer layers may be solvent-soluble polymers such as for example poly(ethyl methacrylate). As mentioned above, if two spacer layers in an imaging member, e.g., spacer layers 20 and 22 comprise materials of substantially the same thermal diffusivity, preferably the spacer layer closer to the surface of the imaging member that is contacted by the thermal printing head, e.g., spacer layer 20, is thinner than the spacer layer remote from the contact surface, e.g., spacer layer 22. In a preferred embodiment, the thinner spacer layer is about 3.5 micrometers thick, and the thicker spacer layer is about 20 micrometers thick.
Spacer layers may be coated from water or an organic solvent, or may be applied as a laminated film. They may be opaque or transparent. In cases where one of the spacer layers, e.g., layers 20 and 22, is opaque, the substrate, e.g., substrate 12, is preferably transparent. In a preferred embodiment, the substrate is opaque and both spacer layers are transparent.
The thermal imaging members utilized in the method of the invention may also comprise an overcoat layer. The overcoat layer may comprise more than one layer. The function of the overcoat includes providing a thermally-resistant surface that is in contact with the thermal printing head, providing gas barrier properties and ultraviolet absorption to protect the image, and providing a suitable surface (for example, matte or glossy) for the surface of the image. Preferably, the overcoat layer is not more than 2 micrometers in thickness.
Alternatively, rather than coating overcoat 24, image-forming layer 14 can be coated onto a thin substrate such as poly(ethylene terephthalate) of less than about 4.5 micrometers in thickness. This structure may be laminated onto the remaining layers of the imaging member. Any combination of coating and lamination may be used to build up the structure of imaging member 10.
A particularly preferred thermal imaging member according to the present invention is constructed as follows.
The substrate is a filled, white poly(ethylene terephthalate) base of thickness about 75 microns, Melinex 339, available from Dupont Teijin Films, Hopewell, Va.
A first layer deposited on the substrate is an optional oxygen barrier layer composed of a fully hydrolyzed poly(vinyl alcohol), for example, Celvol 325, available from Celanese, Dallas, Tex. (96.7% by weight), glyoxal (a crosslinker, 3% by weight) and Zonyl FSN (a coating aid, available from Dupont, Wilmington, Del., 0.3% by weight). This layer, when present, has a coverage of about 1.0 g/m2.
Deposited either directly onto the substrate, or onto the optional oxygen barrier layer, is a cyan image-forming layer composed of a cyan color-former having melting point 210° C.) of the type disclosed in the aforementioned U.S. Pat. No. 7,008,759 (1 part by weight), diphenyl sulfone (a thermal solvent having melting point 125° C., coated as an aqueous dispersion of crystals having average particle size under 1 micron, 3.4 parts by weight), Lowinox WSP (a phenolic antioxidant, available from Great Lakes Chemical Co., West Lafayette, Ind., coated as an aqueous dispersion of crystals having average particle size under 1 micron, 0.75 parts by weight), Chinox 1790 (a second phenolic antioxidant, available from Chitec Chemical, Taiwan, coated as an aqueous dispersion of crystals having average particle size under 1 micron, 1 part by weight), poly(vinyl alcohol) (a binder, Celvol 205, available from Celanese, Dallas, Tex., 2.7 parts by weight), glyoxal (0.084 parts by weight) and Zonyl FSN (0.048 parts by weight). This layer has a coverage of about 2.5 g/m2.
Deposited onto the cyan color-forming layer is a barrier layer that contains a fluorescent brightener. This layer is composed of a fully hydrolyzed poly(vinyl alcohol), for example, the abovementioned Celvol 325, available from Celanese, Dallas, Tex. (3.75 parts by weight), glyoxal (0.08 parts by weight), Leucophor BCF P115 (a fluorescent brightener, available from Clariant Corp., Charlotte, N.C., 0.5 parts by weight), boric acid (0.38 parts by weight) and Zonyl FSN (0.05 parts by weight). This layer has a coverage of about 1.5 g/m2.
Deposited on the barrier layer is a thermally-insulating interlayer composed of Glascol C-44 (a latex available from Ciba Specialty Chemicals Corporation, Tarrytown, N.Y., 18 parts by weight), Joncryl 1601 (a latex available from Johnson Polymer, Sturtevant, Wis., 12 parts by weight) and Zonyl FSN (0.02 parts by weight). This layer has a coverage of about 13 g/m2.
Deposited on the thermally-insulating interlayer is a barrier layer composed of a fully hydrolyzed poly(vinyl alcohol), for example, the abovementioned Celvol 325, available from Celanese, Dallas, Tex. (2.47 parts by weight), glyoxal (0.07 parts by weight), boric acid (0.25 parts by weight) and Zonyl FSN (0.06 parts by weight). This layer has a coverage of about 1.0 g/m2.
Deposited on the barrier layer is a magenta color-forming layer composed of a magenta color-former having melting point 155° C., of the type disclosed in U.S. patent application Ser. No. 10/788,963, filed Feb. 27, 2004, U.S. Pat. App. No: US2004/0191668 A1 (1.19 parts by weight); a phenolic antioxidant (Anox 29, having melting point 161-164° C., available from Great Lakes Chemical Co., West Lafayette Ind., coated as an aqueous dispersion of crystals having average particle size under 1 micron 3.58 parts by weight) Lowinox CA22 (a second phenolic antioxidant available from Great Lakes Chemical Co., West Lafayette Ind., coated as an aqueous dispersion of crystals having average particle size under 1 micron, 0.72 parts by weight), poly(vinyl alcohol) (a binder, Celvol 205, available from Celanese, Dallas, Tex., 2 parts by weight), the potassium salt of Carboset 325 (an acrylic copolymer, available from Noveon, Cleveland, Ohio, 1 part by weight) glyoxal (0.06 parts by weight) and Zonyl FSN (0.06 parts by weight). This layer has a coverage of about 2.7 g/m2.
Deposited on the magenta color-forming layer is a barrier layer composed of a fully hydrolyzed poly(vinyl alcohol), for example, the abovementioned Celvol 325, available from Celanese, Dallas, Tex. (2.47 parts by weight), glyoxal (0.07 parts by weight), boric acid (0.25 parts by weight) and Zonyl FSN (0.06 parts by weight). This layer has a coverage of about 1.0 g/m2.
Deposited on the barrier layer is a second thermally-insulating interlayer composed of Glascol C-44 (1 part by weight), Joncryl 1601 (a latex available from Johnson Polymer, 0.67 parts by weight) and Zonyl FSN (0.004 parts by weight). This layer has a coverage of about 2.5 g/m2.
Deposited on the second interlayer is a yellow color-forming layer composed of Dye XI (having melting point 202-203° C.) described in U.S. patent application Ser. No. 10/789,566, filed Feb. 27, 2004, United States Patent Application Publication No. US2004/0204317 A1 (4.57 parts by weight), poly(vinyl alcohol) (a binder, Celvol 540, available from Celanese, Dallas, Tex., 1.98 parts by weight), a colloidal silica (Snowtex 0-40, available from Nissan Chemical Industries, Ltd Tokyo, Japan, 0.1 parts by weight), glyoxal (0.06 parts by weight) and Zonyl FSN (0.017 parts by weight). This layer has a coverage of about 1.6 g/m2.
Deposited on the yellow color-forming layer is a barrier layer composed of a fully hydrolyzed poly(vinyl alcohol), for example, the abovementioned Celvol 325, available from Celanese, Dallas, Tex. (1 part by weight), glyoxal (0.03 parts by weight), boric acid (0.1 parts by weight) and Zonyl FSN (0.037 parts by weight). This layer has a coverage of about 0.5 g/m2.
Deposited on the barrier layer is an ultra-violet blocking layer composed of a nanoparticulate grade of titanium dioxide (MS-7, available from Kobo Products Inc., South Plainfield, N.J., 1 part by weight), poly(vinyl alcohol) (a binder, Elvanol 40-16, available from DuPont, Wilmington, Del., 0.4 parts by weight), Curesan 199 (a crosslinker, available from BASF Corp., Appleton, Wis., 0.16 parts by weight) and Zonyl FSN (0.027 parts by weight). This layer has a coverage of about 1.56 g/m2.
Deposited on the ultra-violet blocking layer is an overcoat composed of a latex (XK-101, available from NeoResins, Inc., Wilmingtom, Mass., 1 part by weight), a styrene/maleic acid copolymer (SMA 17352H, available from Sartomer Company, Wilmington, Pa., 0.17 parts by weight), a crosslinker (Bayhydur VPLS 2336, available from BayerMaterialScience, Pittsburgh, Pa., 1 part by weight), zinc stearate (Hidorin F-115P, available from Cytech Products Inc., Elizabethtown, Ky., 0.66 parts by weight) and Zonyl FSN (0.04 parts by weight). This layer has a coverage of about 0.75 g/m2.
Optimal conditions for printing a yellow image using the preferred thermal imaging member described above are as follows. Thermal printing head parameters:
The yellow color-forming layer is printed as shown in the table below. The line cycle time is divided into individual pulses of 75% duty cycle. The thermal imaging member is preheated by contact with the thermal printing head glaze at the heat sink temperature over a distance of about 0.3 mm.
Optimal conditions for printing a magenta image using the preferred thermal imaging member described above are as follows. Thermal printing head parameters:
The magenta color-forming layer is printed as shown in the table below. The line cycle time is divided into individual pulses of 7.14% duty cycle. The thermal imaging member is preheated by contact with the thermal printing head glaze at the heat sink temperature over a distance of about 0.3 mm.
Optimal conditions for printing a cyan image using the preferred thermal imaging member described above are as follows. Thermal printing head parameters:
The cyan color-forming layer is printed as shown in the table below. The line cycle time is divided into individual pulses of about 4.5% duty cycle. The thermal imaging member is preheated by contact with the thermal printing head glaze at the heat sink temperature over a distance of about 0.3 mm.
The thermal imaging method of the invention will now be described further with respect to a specific preferred embodiment by way of an example, it being understood that this is intended to be illustrative only and the invention is not limited to the materials, amounts, procedures and process parameters, etc. recited therein. All parts and percentages are by weight unless otherwise specified.
The thermal imaging member used in all the Examples below was prepared as follows.
The following materials were used in preparation of the thermal imaging member:
The imaging member was prepared by successive coatings applied to the substrate, which was RPVC 0.008.
A yellow image-forming layer was applied as follows:
Yellow Color Former (10 g) was dispersed in a mixture comprising Celvol 205 (6.3 g of a 17.6% solution in water), methyl acetate (4 g) and water (43.7 g), using an attritor equipped with glass beads, stirred for 24 hours at room temperature. The total solid content of the resulting dispersion was 18%.
The above dispersion was combined with water and the materials listed in the table below to make the coating fluid for the yellow dye-forming layer in proportions stated. The coating composition thus prepared was coated onto RPVC 0.008 for a dried thickness of 1.9 microns.
An interlayer was next applied as follows:
Water was combined with the materials listed in the table below to provide a coating fluid, which was coated onto the yellow image-forming layer for a dried thickness of 18 microns.
A magenta image-forming layer was applied as follows:
Magenta Color Former (587.50 g) was dispersed in a mixture comprising Surfynol CT-111 (26.88 g of a 83% solution in water), Surfynol CT-131 (20.43 g of a 52% solution in water), methyl acetate (375 g) and water (1490.19 g), using an attritor equipped with glass beads, stirred for 21.5 hours at room temperature. The total solid content of the resulting dispersion was 14.03%.
The thermal solvent (510 g) having melting point 165° C. was dispersed in a mixture comprising Tamol 731 (437.32 g of a 6.86% solution in water, adjusted with sulfuric acid to a pH of 6.7-6.8), Celvol 205 (340.91 g of a 17.6% solution in water), and water (711.77 g), using an attritor equipped with glass beads, stirred for 18.5 hours at room temperature. The total solid content of the resulting dispersion was 23.29%.
The above dispersions were combined with water and the materials listed in the table below to make the coating fluid for the magenta dye-forming layer in proportions stated. The coating composition thus prepared was coated onto the interlayer prepared as described above for a dried thickness of 1.9 microns.
A second interlayer was applied as follows:
Water was combined with the materials listed in the table below to provide a coating fluid, which was coated onto the magenta image-forming layer for a dried thickness of 3.5 microns.
A cyan image-forming layer was prepared as follows:
Cyan Color Former (705.0 g, melting point 207-210 C) was dispersed in a mixture comprising Surfynol CT-131 (14.42 g of a 52% solution in water), Pluronic 25R4 (18.75 g of 100% active), Triton X-100 (18.75 g of 100% active) methyl acetate (437.5 g) and water (1312.5 g), using an attritor equipped with glass beads, stirred for 18.5 hours at room temperature. The total solid content of the resulting dispersion was 26.98%.
The above dispersion was combined with water and the materials listed in the table below to make the coating fluid for the cyan dye-forming layer in proportions stated. The coating composition thus prepared was coated onto the second interlayer prepared as described above for a dried thickness of 2.0 microns.
An overcoat was applied as follows:
Water was combined with the materials listed in the table below to provide a coating fluid, which was coated onto the cyan image-forming layer for a dried thickness of 0.76 microns.
In Examples I, II and III below, the following printing parameters were used:
This comparative example illustrates printing of the thermal imaging member prepared as described above in three printing passes, each at the same speed, and each having the same amount of preheating.
All three colors were printed at a resolution in the direction of transport and a line cycle time as shown in the table below. The line cycle time was divided into individual pulses of 95% duty cycle. Each color was printed in a separate pass using the voltage and the number of pulses shown in the table. The thermal imaging member was preheated by contact with material at the heat sink temperature over a distance of about 0.3 mm. Ten areas of the imaging member were printed for each color, ranging from Dmin (using the lowest number of pulses in the indicated range) for Dmax (using the maximum number of pulses in the indicated range.
Each colored patch was measured using a Gretag SPM50 densitometer manufactured by Gretag Ltd., Switzerland. The measurement conditions were: illumination=D50; observer angle=2°; density standard=DIN; calibrated against white base, without filter. The CIELab colors associated with each patch are shown in
It can be seen from
This example illustrates a method of the present invention, in which the thermal imaging member prepared as described above was imaged in three printing passes, each at a different speed, and each having the same temperature of the preheating element.
All three colors were printed in separate passes as indicated in the table below. The line cycle time was divided into individual pulses of 95% duty cycle. The thermal imaging member was preheated by contact with material at the heat sink temperature over a distance of about 0.3 mm. Ten areas of the imaging member were printed for each color, ranging from Dmin (using the lowest number of pulses in the indicated range) for Dmax (using the maximum number of pulses in the indicated range).
Each colored patch was measured as described in Example 1 above. The CIELab colors associated with each patch are shown in
It can be seen from
It can be seen that the time required to form an image in the imaging member according to method of the invention was reduced significantly, even though the slowest pass (in which the yellow image was formed) was slower than the corresponding yellow printing pass in Example I above. Comparison of
This Example illustrates a method of the present invention, in which the thermal imaging member prepared as described above was imaged in three printing passes, each at a different speed, one of which had a different amount of preheating from the other two.
All three colors were printed in separate passes as indicated in the table below. The line cycle time was divided into individual pulses of 95% duty cycle. The thermal imaging member was preheated by contact with material at the heat sink temperature over a distance of about 0.3 mm. Ten areas of the imaging member were printed for each color, ranging from Dmin (using the lowest number of pulses in the indicated range) for Dmax (using the maximum number of pulses in the indicated range).
Each colored patch was measured as described in Example 1 above. The CIELab colors associated with each patch are shown in
It can be seen from
In Example IV the following printing parameters were used:
This Example illustrates a method of the present invention, in which the thermal imaging member prepared as described above was imaged in two printing passes, each at a different speed. In both printing passes the color-forming layers were addressed with a heat sink temperature of approximately 60° C.
All three colors were printed at 400 dpi in the transport direction. A voltage of 34 V was applied to the thermal printing head. Cyan and magenta were printed in a single pass with a line time of 4.2 milliseconds. This line cycle time was divided into 250 individual pulses of varying duty cycle depending on which of the cyan and magenta image-forming layers was being addressed as indicated in the table below. The yellow layer was printed with a line time of 16.7 milliseconds. The thermal imaging member was preheated by contact with material at the heat sink temperature of 58° C. over a distance of about 0.3 mm. Ten areas of the imaging member were printed for each color, ranging from Dmin (using the lowest number of pulses in the indicated range) to Dmax (using the maximum number of pulses in the indicated range).
Each colored patch was measured as described in Example 1 above. The CIELab colors associated with each patch are shown in
It can be seen from
Although the invention has been described in detail with respect to various preferred embodiments, it is not intended to be limited thereto, but rather those skilled in the art will recognize that variations and modifications are possible that are within the spirit of the invention and the scope of the appended claims.
This application is a continuation application of U.S. patent application Ser. No. 11/400,734, filed Apr. 6, 2006; which claims the benefit of U.S. Provisional Application No: 60/668800, filed Apr. 6, 2005; U.S. Provisional Application No. 60/668,702, filed Apr. 6, 2005. The contents of these applications are herein incorporated by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| 60668800 | Apr 2005 | US | |
| 60668702 | Apr 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11400734 | Apr 2006 | US |
| Child | 12339410 | US |
