WO1993006942A1

WO1993006942A1 - Method of printing with viscous high solids, and microencapsulated materials, and resultant product

Info

Publication number: WO1993006942A1
Application number: PCT/US1992/008590
Authority: WO
Inventors: Christopher Tararuj; Stephen Martin
Original assignee: Webcraft Technologies, Inc.
Priority date: 1991-10-11
Filing date: 1992-10-08
Publication date: 1993-04-15

Abstract

A method of raised image printing, such as by letterpress, dry-offset, and particularly flexographic printing, with viscous, high solids, and microencapsulated materials, and the resultant product. In the method, a series of fine lines (41, 42, 43) is used as the raised image or type to impart the image to a substrate. The printed image, which may form a solid image with sufficiently close line spacing, may be formed as a second image over a first image in order to conceal it. Either the first image or second image may be printed with thermochromic or photochromic ink, which upon activation by an external stimulus allows the first image to become visible.

Description

METHOD OF PRINTING WITH VISCOUS HIGH SOLIDS , AND MICROENCAPSULATED MATERIALS, AND RESULTANT PRODUCT

BACKGROUND OP THE INVENTION

Technical Field This invention relates to a method of printing with viscous, high solids, and, particularly, icroencapsulated materials. This invention specifically pertains to any method of printing which utilizes a raised impression to impart a printed image, such as letterpress, dry-offset, and flexographic printing. The viscous or high solids material or microcapsule core material used in the present invention may include thermochromic materials, photochromic materials, dyes, fragrances, fragranced powders, other chemicals, and the like.

Background Information

There are several types of printing which incorporate the use of a raised impression to create a printed image. These printing types include letterpress, dry-offset, and flexographic printing. In letterpress printing, the image or printing area is raised above the non-printing area, and ink is applied to the raised surface which is brought into direct contact with the paper or other surface to be printed on. In dry offset printing, the ink or coating is transferred from the ink fountain to an intermediate series of rollers and then to the raised image. The image is transferred to a blanket and then offset onto paper or another substrate. As a result of the economy, versatility, and adaptability of flexographic printing, such printing is a preferred method of printing in certain applications, particularly on materials such as paper, plastic films, metal or foil, drawn aluminum cans, etc. Flexographic printing utilizes an ink distribution system involving an anilox ink-metering roll, commonly in conjunction with an ink supply and an optional reverse-angle doctor blade assembly. The flexo ink distribution system operates by flooding the surface of the anilox roll with ink, thus filling the cells on the surface of the anilox roll. Anilox rolls typically have from 10 to 500 lines per inch (about 4 to 200 lines per centimeter) . As the anilox roll rotates, the reverse- angle doctor blade shaves the surplus ink flush with the surface of the lands on the surface of the anilox roll, and the resulting uniformly metered ink film is applied to the surface of the printing plate. The printing plate has a raised surface or type which, when inked and impressed upon a substrate, yields a printed image on the substrate. The inks used in letterpress and dry-offset printing can vary somewhat in viscosity, from under 500 cps for a letter press-type news ink to over 5,000 cps for special litho inks formulations. Lower viscosity inks are preferred so as to minimize the need for a multitude of rollers in the ink distribution unit to ensure uniform and adequate transfer of ink to the printing plate and provide for a high quality printed image. The inks used in flexographic printing are commonly even less viscous fluids, typically on the order of 50-100 cps, so as to enable the transfer of ink from the anilox roll to the printing plate and then to the substrate to be printed with a minimum of undesirable striations or "chicken tracks" in the printed image. Generally, the more viscous the ink and/or the fewer the number of cells on the anilox roll, the more pronounced are the striations and chicken tracks in flexographic printing. There are many materials which cannot be satisfactorily applied by way of letterpress, dry- offset, and especially flexographic printing and must, instead, utilize less desirable printing techniques such as silk screen or gravure printing. In particular, such materials include microencapsulated materials wherein, for example, the core materials include thermochromic materials, dyes, photochromic materials, fragrances, other chemicals, and the like. Silk screen and gravure printing have been used with laicroencapsulated materials, but letterpress, dry- offset, and flexographic printing have not yielded satisfactory results with such materials. Since these microencapsulated materials must usually be applied to a substrate in a relatively high quantity (e.g., at least about 2.5 microns in thickness and more typically about 25-75 microns in thickness) , the microencapsulated materials cannot acceptably be diluted to render them useful for application by flexography. Letterpress and dry-offset typically cannot apply a sufficient quantity of ink and maintain adequate print quality. Accordingly, viscous, high solids, and microencapsulated materials continue to be applied to substrates primarily by screen printing and gravure printing, rather than by letterpress, dry- offset, and flexographic printing.

There are several obstacles to printing a satisfactory flexographic coating more than 2.5 microns in thickness, as is usually required of microencapsulated materials. Normally, a fine anilox

(e.g., 200 lines per inch (about 80 lines per centimeter)) is utilized for maximum print quality. Coarser anilox (e.g., 20-110 lines per inch (about 4-

45 lines per centimeter)) are generally utilized for coatings and are not well suited for printing detail. Due to the rheology and relative transparency of microencapsulated materials, particularly those containing thermochromic inks, however, the fine anilox are not desirable to apply such materials since there would be insufficient material deposited on the substrate and the resulting printed image would be too faint. When coarser anilox are utilized, it is difficult to print without considerable distortion to the printed images and/or striations or chicken tracking which renders the printed image aesthetically unappealing. Flooding a fine anilox in order to increase the amount of material transferred to the substrate to be printed allows for a sufficient quantity of material to be imparted to the substrate but similarly impairs the quality of the printed image. Large or bold print areas print poorly, and fine detail areas generally fill in.

The present invention serves to overcome these problems by providing for a method for the letterpress, dry-offset, and flexographic printing of viscous, high solids, and microencapsulated materials which enables an adequate coating of material to be imparted to the substrate to be printed, while maintaining an acceptable print quality. The present invention achieves this result through use of particularly designed printing plates in conjunction with the proper coordination of transfer rolls, anilox type, and material quantities to be ultimately transferred to the substrate to be printed. By use of the present invention, commercial quality printing can be achieved with viscous, high solids, and microencapsulated materials utilizing letterpress, dry-offset, and, particularly, flexographic printing.

Relevant Literature

There are many references which disclose the preparation and/or application to substrates of viscous, high solids, and microencapsulated materials. While some of these references discuss the use of raised image printing, particularly flexographic printing, with such materials, they do not disclose any technique, let alone the present inventive method, for printing high quality images with viscous, high solids, and microencapsulated materials using letterpress, dry-offset, and flexographic printing.

U.S. Patent 4,028,118 discloses microencapsulated thermochromic materials (e.g., col. 9, line 52, through col. 10, line 30, and Examples 30-40) which may be dispersed in a printing ink vehicle to form a printing ink composition suitable for application to substrates in conventional manners, e.g., intaglio, relief, lithographic, or other printing methods (e.g., col. 12, line 31, through col. 13, line 57, and Examples 50-57) . While some of the thus prepared printing ink compositions are purportedly suitable as flexographic ink (e.g., Example 54), there is no discussion of the resulting quality of the printed image or the manner of improving such by use of the present invention.

U.S. Patent 4,086,054 discloses concentrated solutions of alkylbenzenesulfonates of basic dyes in alcohols, glycols, and glycol ethers (e.g., ^"col. 1, lines 8-11) . These solutions are purported to be particularly suitable for producing waterfast prints, colorations, script, and markings and may be used to prepare flexographic inks (e.g., col. 3, lines 39-47). While some of the concentrates were reportedly used to prepare flexographic inks (e.g., Examples), there is no discussion of the application of such inks to prepare printed images or of the quality of printed images attainable with such inks.

U.S. Patent 4,421,560 discloses thermochromic materials that may be included in microcapsules (e.g., col. 3, lines 3-8, and col. 8, line 24, through col. 9, line 25). These thermochromic materials are said to be useful as printing inks which can be applied by letterpress, intaglio, flexographic, and screen printing and so forth (e.g., col. 12, lines 14-18). U.S. Patent 4,425,161 discloses thermochromic materials which may be microencapsulated (e.g., col. 8, line 18, through col. 9, line 46), used to prepare thermochromic printing ink (col. 16, lines 33-38), and applied by letterpress, intaglio, lithographic, and screen printing and so forth (e.g., col. 17, lines 18- 26) . U.S. Patent 4,428,978 discloses a process for preparing concentrated microcapsule suspensions, particularly of encapsulated dye precursors, for use in the production of partly coated systems by the rotogravure and flexographic processes (e.g., col. 1, lines 14-18, and col. 5, lines 12-34). Although the microencapsulated material was applied to a paper substrate by means of a soft rubber sheet in a flexograph machine (e.g., Example 6), the use of the material was for the purpose of preparing carbonless copying paper, and, therefore, there was no direct evaluation of the quality of any printed image.

U.S. Patent 4,717,710 discloses a thermochromic composition which can be used in the form of a solution, emulsion, dispersion, or encapsulated composition and which may further include various other chemicals such as dyes, perfumes, and the like

(e.g., col. 5, lines 10-35). When the thermochromic composition is to be used for printing, it is dissolved or emulsified in a printing ink vehicle or is encapsulated and admixed therewith. The resulting ink is then applied by a known printing method as by gravure, offset, flexso, screen printing, or spraying (e.g., col. 5, lines 36-49). There is no example of microencapsulated materials actually being applied by way of flexographic printing.

U.S. Patent 4,884,504 discloses a flexographic printing method which purportedly is particularly useful with thermochromic ink (e.g., col. 3, lines 50- 56) ; however, there is no discussion of the resulting quality of any printed image formed by thermochromic ink or the manner of improving such through utilization of the present invention.

U.S. Patent 4,936,916 discloses ink compositions containing microencapsulated thermochromic materials, dyes, perfumes, and the like (e.g., col. 1, lines 5- 16, and col. 6, lines 3-11). The ink compositions allegedly can be printed on a substrate by silk screen printing, flexographic printing, gravure printing, or the like (e.g., col. 1, lines 28-39, and col. 8, lines 15-23) . While flexographic capsule ink compositions were reportedly prepared (e.g., Examples), the use of the compositions was to prepare pressure sensitive recording sheets, and, accordingly, the quality of a printed image was not directly evaluated.

BRIEP SUMMARY OF THE INVENTION The present invention pertains to a method of letterpress, dry-offset, and flexographic printing with viscous, high solids, and microencapsulated materials through use of a series of either solid (i.e., continuous) or broken fine lines as the raised image or type used to impart the image to a substrate. More particularly, the present invention is a method of printing an image using a raised image, e.g., letterpress, dry-offset, and flexographic printing, with viscous, high solids, and microencapsulated materials comprising transferring such material from a source of the material to a roll or rolls, transferring the material from the roll(s) to a plate cylinder having a raised image or type, and impressing a substrate onto the plate cylinder such that the material on the raised image or type of the plate cylinder is transferred to the substrate to form an image thereon, wherein the raised image or type predominantly consists of lines of about 1 mm or less in width. Solid printed areas can be obtained by use of such lines spaced about 0.2-1 mm apart such that the material imparted to the substrate fills the intervening spaces between such lines.

An object of the present invention is to provide a method of raised image printing such as letterpress, dry-offset, and flexographic printing with viscous, high solids, and microencapsulated materials to yield printed images of acceptable quality.

Another object of the present invention is to eliminate striations and chicken tracking in images printed with viscous, high solids, and microencapsulated materials using flexography and similarly minimize distortion of the printed image in letterpress and dry-offset printing. A further object of the present invention is to impart a more uniform and aesthetically acceptable solid coating of viscous, high solids, and microencapsulated materials to substrates using letterpress, dry-offset, and flexographic printing. These and other objects and advantages of the present invention will be apparent from the description of the invention provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGURE 1 is a schematic overview of a typical flexographic printing process, and FIG. la is a close- up of a small portion of the surface of the anilox roll depicted in FIG. 1.

FIG. 2 depicts a preferred raised image for use in conjunction with the present invention. FIG. 3 depicts another preferred raised image for use in conjunction with the present invention.

FIG. 4 depicts a preferred raised image with continuous line cross-hatching for use in conjunction with the present invention to yield a solid printed image on a substrate. FIG. 5 depicts a preferred raised image with broken line cross-hatching for use in conjunction with the present invention to yield a solid printed image on a substrate. FIG. 6 depicts a preferred raised image with modified broken line cross-hatching for use in conjunction with the present invention to yield a solid printed image on a substrate.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention may best be understood with reference to the accompanying drawings wherein illustrative embodiments are shown and in the following detailed description of the preferred embodiment. The present invention provides a method of letterpress, dry-offset, and, particularly, flexographic printing with viscous, high solids, and microencapsulated materials utilizing a raised image or type predominately consisting of fine lines. While the present invention is described with an emphasis on flexographic printing, the present invention is also applicable to other raised image printing such as letterpress and dry-offset printing. As illustrated in the schematic overview of flexographic printing depicted in Figure 1, flexographic printing typically involves transferring printing material such as an ink composition from a source 11 of such material, usually in conjunction with a reverse-angle doctor blade assembly 12, to an anilox roll 13. Figure la is a close-up of a small portion of the surface of the anilox roll 13. The anilox roll 13 has on its surface a series of cells 14 formed by crisscrossing ridges 15, which cells 14 are capable of retaining the printing material. Anilox rolls typically have from 10 to 500 lines or ridges per inch (about 4-200 lines or ridges per centimeter) and may employ a variety of cell designs. The surface of the anilox roll 13 is flooded with printing material from the printing material source 11, thus filling the cells 14 on the surface of the anilox roll 13. As the anilox roll 13 rotates, the reverse-angle doctor blade 12 shaves the surplus ink flush with the surface of the lands or ridges 15 on the surface of the anilox roll 13. The anilox roll 13 is in contact with a plate cylinder 16 which has a raised image or type on its surface corresponding to the image to be ultimately printed. The raised image or type is typically made of rubber or synthetic material and preferably has a Durometer hardness of about 60-65. As the anilox roll 13 rotates, it transfers printing material to the raised image or type of the plate cylinder 16. The plate cylinder is also in contact with the substrate 17 to be printed on, such that the printing material on the raised image or type of the plate cylinder 16 is transferred to the substrate 17 to produce the desired image thereon. An impression cylinder 18 is also typically employed to ensure adequate contact between the plate cylinder 16 and substrate 17.

Any usual or modified flexographic printing apparatus or process can be used in the context of the present invention. For example, modified printing material sources, different blade assemblies, and additional transfer rolls can be utilized. In addition, a variety of anilox rolls can be used, such as, for example, the AMS and QCH anilox rolls sold by Consolidated Engravers Corporation (Charlotte, NC) and the ROTO-FLO anilox roll offered by Pamarco Inc. (Summit, NJ) . Other, modified anilox rolls are also described in U.S. Patent 4,301,583. The various modified rolls, which purportedly help reduce striations in printed images, may, as part of the present inventive method, yield further improvements to the quality of the printed image. Similarly, while the present invention has particular usefulness in flexographic printing, the present invention has applicability to other raised image printing processes such as letterpress and dry-offset printing and variations of those processes.

The printing method of the present invention is designed to provide printed images of acceptable quality and uniform coverage with the use of viscous, high solids, and microencapsulated materials. Some such materials have heretofore been capable of being letterpress, dry-offset, or, particularly, flexographic printed to only form images of unsatisfactory or inferior quality, in particular with considerable striations and chicken tracks, low concealment value, low color density, and non-uniform coatings. Viscous, high solids, and microencapsulated materials suitable for use with the present invention include, for example, inks with a viscosity of 50- 5,000 cps or more and materials such as DATRIM™ (3M, St. Paul, Minnesota), as well as microencapsulated materials wherein the core material comprises thermochromic compounds, photochromic compounds, dyes, fragrances, powders, other chemicals, and the like, wherein the viscosity of such microencapsulated materials ranges from about 50-5,000 cps or more. The microcapsules are generally of a size of about 30 microns diameter or less, preferably about 2-30 microns diameter, and most preferably less than 10 microns diameter. High solids materials may contain up to about 75% or more solids and retain printability with use of the present invention. Suitable viscous, high solids, and microencapsulated printing materials are disclosed in the references discussed above. See also U.S. Patents 2,324,671, 3,251,881, 3,341,464, 3,539,375, 3,560,229, 3,615,749, 3,674,535, 3,705,037, 3,741,628, 4,054,684, 4,180,405, 4,341,565, 4,501,876, 4,502,066, 4,666,949, and 4,826,550. All the references cited herein are incorporated in their entireties herein by reference.

In order to print acceptable quality images via letterpress, dry-offset, and flexography using such viscous, high solids, and microencapsulated materials, it has been found that the raised type on the plate cylinder which forms the printed image should predominately consist of lines of about 1 mm or less in width, preferably about 0.5 mm or less in width, and most preferably between about 0.1 mm and about 0.5 mm in width. The use of lines of greater than 1 mm in width generally results in printed images with decreasing print quality with increasing line width. An illustrative typeface for use with the present invention is depicted in Figure 2, wherein lines 21, 22, and 23 represent raised typeface lines of 0.2 mm width to produce the printed image "A". Larger profile images can be printed through use of outlines, as depicted in Figure 3, wherein lines 31 and 32 represent raised typeface lines of 0.2 mm width to produce the printed outline image "A" with open non- printed area 33.

Solid areas of acceptable quality can be printed through use of the present invention by employing a raised type containing an area of lines, preferably parallel and as described above, which are separated from each other by about 0.15 mm to about 1.5 mm, preferably about 0.2 mm to about 1 mm, and most preferably about 0.2 mm to about 0.5 mm. The printed material, depending on the quantity and viscosity of the printing material, nature of the substrate, transfer roll design, and pressure with which the raised type is contacted with the substrate to be printed, will flow to fill in the spacing between these lines such that the area will result in a solid or substantially solid printed image on the substrate. Such solid coverage also allows for hiding existing images on substrates without the need for a second coating and while maintaining aesthetically appealing print quality.

Examples of such a cross-hatched raised type to produce a solid image are depicted in Figures 4-6, wherein continuous lines 41-43 and broken lines 51 and 61 represent raised typeface lines of about 0.2 mm width and cross-hatch lines 43, 51, and 61 are separated from each other by 0.5 mm, such that a large solid printed "A" (Figure 4) or triangle (Figures 5-6) results. As illustrated in Figures 4-6, the cross- hatch lines may be of any suitable configuration, e.g., solid or continuous lines (Figure 4), broken lines (Figure 5) , or modified broken lines in the shape of circles (Figure 6) . The size of the solid printed area can be controlled by the amount of cross- hatched lines and/or the spacing between the lines. For example, lines 31-32 of Figure 3 can be spaced more closely together, e.g., 0.2 mm apart, so as to fill-in open area 33 and produce a solid printed image "A", additional lines parallel to lines 31-32 of Figure 3 could similarly be added to produce an enlarged solid printed image "A", and lines 41-42 of Figure 4 could be spaced more closely together or further apart, with an appropriate decreased or increased amount of cross-hatched lines 43, to yield a smaller or larger solid printed image "A". The spacing between lines to yield the desired image depends on a number of factors including material viscosity, rheology and solids content, as well as the nature of the transfer roll(s) , e.g., anilox cell configuration in a flexographic printing process.

The present inventive method of printing with viscous, high solids, and microencapsulated materials can be used to print images on any suitable substrate, including, for example, various papers, metals, plastics, foils, and virtually any substrate which can be printed utilizing a raised impression image. The amount of printing material transferred to the plate cylinder, and then ultimately to the substrate to be printed, may vary depending on a number of considerations, including the nature of the transfer roll such as the number of cells on the anilox roll in flexographic printing, the desired coating thickness of the printed image, and the pressure with which the plate cylinder and substrate to be printed are contacted to form an image, as well as the number, thickness, and proximity of the lines of the raised image and the viscosity, rheology, and percent solids of the printing material. Generally, at least 1 cubic millimeter of printing material per square centimeter is transferred to the plate cylinder from the transfer roll, usually about 1-20 cubic millimeters of printing material per square centimeter, and preferably about 3-6 cubic millimeters per square centimeter for line type images and about 6-16 cubic millimeters per square centimeter for solid area images or for images where heavier coatings are desirable such as for hiding or concealment purposes. In flexographic printing, the anilox roll may be of any suitable design and will generally contain about 10-500 lines per inch (about 4-200 lines per centimeter) , more typically about 25-110 lines per inch (about 9-45 lines per centimeter) . Finer anilox rolls of about 200 lines per inch (about 80 lines per centimeter) may also be satisfactorily used provided that the anilox is flooded with printing material. In connection with the present invention, anilox rolls of about 10-350 lines per inch (about 4-140 lines per centimeter) are preferably utilized, most preferably anilox rolls of about 25-165 lines per inch (about 9- 65 lines per centimeter) . As a general proposition, the smaller the number of cells per square centimeter on the anilox roll, the thicker the printed image line formed from a given raised typeface line and the greater the distance between lines which will be filled in to yield a solid printed area. For example, a 25 Quad anilox can be run to transfer about 14 cubic millimeters per square centimeter of viscous or encapsulated material to the plate cylinder and cause an area of 0.2 mm width cross-hatch raised typeface lines spaced 0.5 mm apart to produce a solid printed area.

The non-print area (e.g., space between the raised typeface lines) of the plate cylinder is believed to compensate for the otherwise usual printed image distortion in the form of striations and chicken tracking resulting from the flexographic printing of viscous, high solids, and encapsulated materials by allowing the relatively thick printing material to spread out with minimal overlapping. In addition, the film splits only on the lines, and since the area is small in width, the impact of striations and chicken tracking on visual aesthetics is minimized. In this manner, a relatively large quantity of material can be applied to a substrate while maintaining satisfactory print quality of the printed image. This technique can be used in any application where a uniform coating of a viscous, high solids, or microencapsulated material is to be applied on a substrate. For example, the present inventive printing method can be used for microencapsulated fragrance slurry application to provide for substantially complete coverage of microcapsules in fragrance pull-apart samplers, thereby ensuring satisfactory release of the fragrance by increasing the number of microcapsules burst upon sampler opening, and minimizing fiber tear of the paper sampler caused by nonuniform microcapsule application.

The minimum volume necessary to print a satisfactory pull-apart fragrance sampler is approximately 10-20 mm³/cm². This quantity of material is quite difficult to print using a standard solid or screened flexoplate due to the pronounced non- uniformity exhibited by film split or film separation marks and overall striations. This is a well-known phenomenon in printing but is especially pronounced when applying heavy coatings such as are desirable with microencapsulated fragrance material. Pull-apart fragrance samplers with such striations and film separation marks are commercially unacceptable. Generally, a series of dots are utilized to print fragrance microcapsules in order to provide a semblance of uniformity. The present invention, however, allows for coverage of a greater surface area and consequently increases the number of microcapsules efficiently broken upon pulling apart of the sampler. More efficient breakage of capsules per unit area results in higher "lift" of the fragrance (since more oil is released) . This increased efficiency allows for the application of fewer microcapsules per unit area, and, since fewer unbroken microcapsules remain, a significant savings can be realized, especially when high-priced oils are used. The present invention, therefore, allows for improved fragrance sampler quality as well as reduced cost.

Another useful application for the present invention is in the area of thermochromic and photochromic materials which are typically microencapsulated. In order to apply a sufficient quantity of such material to a substrate, the microcapsule content must be at least about 10-25%, preferably more. The lower the microcapsule content, the fainter the image and the higher the volume of material that must be transferred to the substrate. An application of about 0.5-1 mm³/cm² is generally suitable for a legible image, but about 1.5-3 mm³/cm² is more desirable. It is difficult to maintain image clarity and quality, however, adhering to such specifications. In general, as with the fragrance application described above, the greater the quantity that is transferred, the more pronounced is the desired effect, but the print quality adversely suffers and is aesthetically unacceptable.

With the present invention, much more desirable material volumes of about 18-22 mm³/cm² or more can be printed satisfactorily. Such application rates allow for a wide range of utilization for the thermochromic products not previously possible, particularly with flexography. The present invention allows for the use of a smaller quantity of material to provide the desired coverage, enabling, for example, the use of a much reduced quantity of material to conceal existing images on a substrate, e.g., "secret messages" and the like. Specifically, presently utilized methods have provided for a solid coverage (e.g., black square, circle, etc.) prepared from thermochromic ink, which when heated, disappears and reveals an image, e.g, a message indicating a prize that may have been won in a contest. Such concealed images have only been heretofore possible with screen printing and, to a more limited degree, gravure printing. Generally, a single gravure coating does not provide maximum color density and is insufficient to adequately conceal an image. Solid coverage from both silkscreen and gravure printing requires a higher quantity of material for sufficient coverage of the obscured or hidden image or message.

Through use of the present invention, an improved method of concealment is possible, and a significantly smaller quantity of material is required for the improved concealment value. The lines comprising the raised image used in the present invention carry a quantity of material which yields a dark image, and, as discussed above, when the lines are brought into close proximity to each other, the area between the lines can be forced to fill in on the printed substrate to provide high hiding power. The present invention thus offers significant advantages in connection with concealed images.

Moreover, the resulting contrasting darker and lighter areas made possible by use of the present invention with raised lines of varying proximity provide improved hiding power, by confusing and not allowing the eye of a viewer to focus on a single color or plane, similar to the function of the "Chinese pattern" commonly used in print game applications. Since solid coverage is not necessary, and maximum color density covers only about 50 to 80 percent of the concealment area, a raw material saving is made possible. If a second coating is desirable to conceal, for example, a high color density or black image or four color process picture with a high color density coating, the second coating can be printed at a perpendicular or intersecting angle to the first and provide even greater hiding power, still at a saving compared to two-coat solid maximum color density printing using screen process or gravure printing. Additionally, the second coating can be of a different color than the first coating to further improve concealment, and/or reduce the total quantity of coating material while maintaining a given concealment level, by providing for increased camouflage and increased hindrance of the ability of a viewer to discern the hidden image or message. Of course, the present invention can also be used to provide a tinted or clear glossy or matte image or coating over an existing thermochromic or photochromic image which is thereby rendered undiscernible until an external stimulus reveals the thermochromic or photochromic image. Another example with respect to the use of the present invention is in the context of the use of DATRIM™ (3M, St. Paul, Minnesota) type materials which comprise as much as 80-90% solids by volume. The present invention can be utilized to achieve maximum concealment with a high solid, high viscosity, non- encapsulated material. Although high viscosity and high solids are not necessarily synonymous, in general the higher solids materials are of higher viscosities, and it is advantageous to utilize the highest solids possible for concealment, conductivity, or whatever is desired, while maintaining maximum print quality. The present invention provides a means for doing so with raised image printing, e.g., letterpress, dry-offset, and, particularly, flexographic printing.

While this invention has been described with an emphasis upon a preferred embodiment, it will be obvious to those of ordinary skill in the art that variations in the preferred method may be used and that it is intended that the invention may be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications encompassed within the spirit and scope of the following claims.

Claims

What is claimed is;

1. A method of printing an image with viscous, high solids, and microencapsulated materials comprising transferring printing material from a source of said material to a raised image predominantly consisting of solid or broken lines of about 1 mm or less in width and transferring said material on said raised image to said substrate to form an image thereon.

2. The printing method of claim 1, wherein said raised image predominantly consists of solid or broken lines of about 0.5 mm or less.

3. The printing method of claim 2, wherein said raised image predominantly consists of solid or broken lines of about 0.1 to about 0.5 mm.

4. The printing method of claim 1, wherein said raised image contains an area of solid or broken lines of about 1 mm or less in width which are separated from each other by about 0.15 mm to about 1.5 mm such that said area results in a substantially solid printed image on said substrate.

5. The printing method of claim 4, wherein said raised image contains an area of solid or broken lines of about 1 mm or less in width which are separated from each other by about 0.2 mm to about 1 mm such that said area results in a substantially solid printed image on said substrate.

6. The printing method of claim 5, wherein said raised image contains an area of solid or broken lines of about 1 mm or less in width which are separated from each other by about 0.2 mm to about 0.5 mm such that said area results in a substantially solid printed image on said substrate.

7. The printing method of claim 1, wherein said printing method is a flexographic printing method and said printing material has a viscosity of about 50- 5,000 cps.

8. The printing method of claim 1, wherein said printing method is a flexographic printing method and an anilox roll containing about 10-350 lines per inch is used to transfer about 1 to about 16 mm³/cm² of said printing material to said raised image.

9. The printing method of claim 8, wherein an anilox roll containing about 25-165 lines per inch is used to transfer about 3 to about 16 mm³/cm² of said printing material to said raised image.

10. The printing method of claim 1, wherein said printing material contains microencapsulated material.

11. A printed image formed in accordance with the printing method of claim 1.

12. A method of concealing an image formed on a substrate from thermochromic or photochromic ink comprising covering said image formed on a substrate from thermochromic or photochromic ink which is activated by an external stimulus with a tinted or clear glossy or matte coating such that said image is undiscernible until activated by said external stimulus.

13. The method of claim 12, wherein said coating is a clear glossy or matte coating.

14. The method of claim 12, wherein said coating is a tinted glossy or matte coating.

15. A method of concealing a first image on a substrate by printing a second image thereover comprising transferring a material to a raised image predominately consisting of solid or broken lines of about 1 mm or less in width and transferring said material on said raised image to said substrate to form a second image over said first image which renders said first image undiscernible.

16. The concealment method of claim 15, wherein said raised image predominantly consists of solid or broken lines of about 0.5 mm or less.

17. The concealment method of claim 16, wherein said raised image predominantly consists of solid or broken lines of about 0.1 to about 0.5 mm.

18. The concealment method of claim 17, wherein said raised image contains an area of solid or broken lines of about 1 mm or less in width which are separated from each other by about 0.15 mm to about 1.5 mm such that said area results in a substantially solid printed image on said substrate.

19. The concealment method of claim 18, wherein said raised image contains an area of solid or broken lines of about 1 mm or less in width which are separated from each other by about 0.2 mm to about 1 mm such that said area results in a substantially solid printed image on said substrate.

20. The concealment method of claim 19, wherein said raised type contains an area of solid or broken lines of about 1 mm or less in width which are separated from each other by about 0.2 mm to about 0.5 mm such that said area results in a substantially solid printed image on said substrate.

21. The concealment method of claim 15, wherein said second image is formed of thermochromic material which allows for said first image to be discernible upon the application of an external stimulus to said second image.

22. The concealment method of claim 15, wherein said first image is formed of thermochromic material and said second image is formed of a tinted or clear glossy or matte material such that said first image becomes discernible upon the application of an external stimulus to said first image.

23. The concealment method of claim 15, wherein a third image is printed over said first and second images at a perpendicular or intersecting angle to the second image.

24. The concealment method of claim 23, wherein the printing of said third image comprises transferring a material to a raised^* image predominantly consisting of solid or broken lines of about 1 mm or less in width and transferring said material on said raised image to said substrate to form said third image.

25. The concealment method of claim 24, wherein said third image is of a color different than the color of said second image.