THERMAL IMAGING MEDIUM

Abstract: A high resolution thermal imaging medium including a support sheet having a surface of a material which may be temporarily liquified by heat and upon which is deposited a particulate or porous layer of an image forming substance which is wettable by the material during its liquified state.

Description: - I - THERMAL IMAGING MEDIUM Reference to Related Applications This is a continuation-in-part of application serial No. 939,854, filed December 9, 1986. * Background of the Invention Field of the Invention The invention relates generally to a heat mode record¬ ing material and, more particularly, to a high resolution thermal imaging material comprising a heat sensitive layer interacting, at an image-wise application of heat, with an image forming substance for producing images of very high resolution. Description of the Prior Art Unlike the image processing of conventional photo¬ graphic materials using silver halide emulsions, thermal imaging materials require neither a dark room nor any other protection from ambient light. Instead, images may be produced with thermal imaging materials by the application of heat patterns corresponding to the image to be produced and, since these materials can provide images by quicker and simpler processes than those applicable to silver halide materials, they are more convenient and economical than conventional photographic imaging materials. Another consideration which contributes to their desirability is that, unlike silver halide materials, thermal imaging media require substantially dry image developing processes and that they are unaffected by sustained periods of elevated ambient temperatures. Moreover, thermal imaging media allow the making of more stable images of higher quality because they do not suffer from the image quality drift resulting from the wet processing and temperature effects of silver halide materials. As thermal imaging media may be used with relative ease and in a potentially wide range of applications, proposals relating to their manufacture and use have not been lacking. One source of heat lately to have become conventional for exposing thermal imaging media are lasers of sufficient power output and appropriately modulated while scanning a medium in an image pattern. The time required for Irradiating the material in this manner is relatively short. Other materials use conventional heat sources such as, for instance, xenon flash tubes. For instance, U.S.Patent 4,123,309 discloses a compo¬ site strip material including an accepting tape comprising a layer of latent adhesive material in face-to-face contact with a layer of microgranules lightly adhered to a donor web. At least one of the layers bears a radiation absorb¬ ing pigment, such as carbon black or iron oxide, which when selectively heated in accordance with a pattern of radia¬ tion, momentarily softens adjacent portions of the adhesive material sufficiently for the latter completely to pene¬ trate through the pigment. Upon separation of the accept¬ ing tape and donor web, microgranules are said to transfer to the accepting tape in the Irradiated areas only. A similar material is disclosed by U.S.Patent 4,123,578. U.4 .Patent 4,157,412 discloses a composite material for forming graphics which includes a layer of latent adhe¬ sive material, a mono-layer of granules lightly adhered to a donor web, and a thin layer of bonding material between and in face-to-face contact with layers of granules and adhesive. The layer of bonding material maintains the adhe¬ sive and granular layers in close proximity and excludes air from therebetween. When the composite material is selectively heated in graphic patterns, corresponding por¬ tions of the bonding layer melt and corresponding portions of the adhesive material and granular layer soften, absorb the melted portions of the bonding layer and adhere together. Upon subsequent separation of the layer of adhesive and the donor web the remaining portions of the layer of bonding material separate, whereas granules transfer to the accepting tape in the heated areas to provide the graphics. In U.S.Patent 4,547,456 a heat mode recording material is described which comprises a support and a heat sensitive layer positioned on the support, in which the heat sensitive layer comprises an ionomer resin obtained by ionically cross-linking with at least one metal ion, a copolymer comprising an alpha- olefin and an alpha methylene aliphatic monocarboxylic acid and a hydrophyllic binder . Other materials are known which instead of using a source of heat to provide an image which may be transferred from one layer to another by locally changing the adhesion of photohardenable image forming substances relative to the layers, rely actinic radiation for forming images. An example of such a material is disclosed in U.S.Patent 4,247,619. None of the thermal imaging materials appear to have found wide acceptance, possibly because of the relatively complicated mechanism of the image-wise transfer of an image-forming substance from a donor layer to a receiving layer as a result of applied heat patterns. Other problems may be involved in the coherence of the image- orming substance which may not consistently yield images of a resolution sufficiently fine to be acceptable to consumers. Still further problems may result from the difficulty of removing microscopical irregularities and air gaps when using two separate donor and receiver webs . It appears that none of the thermal imaging materials cur¬ rently available satisfy the demand for high photographic quality or high resolution required by industry. It is, therefore, desirable to provide a thermal Imaging element of superior performance for forming images of high resolution by a simplified mechanism of image-formatio . Objects and Summary of the Invention It is an object of the Invention to provide an improved high resolution thermal imaging material. It is a further object of the invention to provide a novel high resolution thermal imaging material which requires no transfer of the Imaging-forming material from a donor sheet to a receiving sheet. Another object of the invention resides In the provision of a thermal imaging material yielding images of improved density. A further object of the Invention resides in the provision of a thermal imaging material of improved sensitivity. It is also an object of the invention to provide a thermal imaging material exposable by a source of heat controlled in a binary fashion. Still another object resides in the provision of a thermal imaging material of improved abrasion resistance. In accordance with a preferred embodiment of the invention there is provided a thermal imaging medium for forming Images in response to a brief exposure to Intense image-forming radiation, comprising a support formed of a material transparent to the radiation and having an imaging surface iiquefiable and flowable at a selected elevated temperature , a layer of porous or particulate image forming material uniformly coated on the imaging surface and exhibiting a cohesive strength in excess of the adhesive strength between the imaging materiai and the imaging surface layer. At least one of the materials in confronting portions of said layers is absorptive of the radiation to convert it into thermal energy capable of liquefying the imaging surface of the support. Preferably, the material of the imaging surface is such that it liquefies and thereafter solidifies in a substantially short time. The materials of the surface layer, when liquefied, exhibit capillary flow into adjacent portions of the imaging material, thereby locking substantially the entire layer of imaging material to the support when said imaging surface layer cools. In a preferred embodiment of the invention the material of the imaging surface is such that it has a narrow temperature range between liquefying and solidifying. * Brief Description of the Drawings Fig. 1 is a cross-sectional view of a thermal imaging element in accordance with the invention in its simplest form with a schematic illustration of its image forming mechanism; Fig. 2 is a cross-sectional view of the thermal imaging element of Fig. 1 schematically illustrating the processing of the image to its viewable state; Fig. 3 is a cross-sectional view of a preferred embodiment of the thermal imaging element of the present invention before an exposure; Fig. 3a is a schematic presentation of a colorant particle positioned on an imaging surface before exposure; Fig. 4 is a cross-sectional view of the thermal imaging material of Fig. 3 after exposure; Fig. 4a is a view similar to Fig. 3a showing the -oarticie in relation to the imaging surface after exposure; Fig. 5 is a cross-sectional view of a simplified embodiment of a thermal imaging element in accordance with the invention; Fig. 6 is a cross-sectional view of the element of Fig. 5 after exposure, with its imaging and processing layers partially separated; and Fig * 7 is a cross-sectional view of another preferred embodiment of the ■ thermal imaging element in accordance with the present invention," Figs. 8 - 10 are cross-sectional views of further embodiments of thermal Imaging elements according to the invention; Fig. 11 is a diagram illustrating the relationship between exposure time and temperature for various depths into the layer forming an Image in the material according to the invention; and Fig. 12 is a diagram illustrating the effect of temperature on the imaging surface of the thermal element of the present invention. Description of the Preferred Embodiments As used in this specification, the term thermal imaging Is intended to connote producing an image of a subject by exposing a recording medium to an Image-wise distribution of thermal energy. A method particularly preferred for providing the image-wise distribution involves the use of a laser capable of providing a beam sufficiently fine to yield an image of as fine a resolution as one thousand (4000-10000) dots per cm. As will hereinafter be explained in detail, two steps are required to form an image in the thermal imaging medium in accordance with the present invention: one is proper heat exposure, the other is processing of the latent image by a process of removing from the medium those parts of an image forming substance which have not been exposed. The quality of the image thus obtained is a function of a - / reliably predictable interaction between these two variables . For practical purposes and in accordance with a preferred method of exposing the medium in accordance with the invention, the source of heat utilized is a laser. Thus, in the context of the present specification the source of heat utilized for forming a latent image in the medium will be assumed to be a laser, but it should be understood that the invention is not itself restricted to media for laser imaging. In the event, laser exposures cause very high temperatures to be generated in the medium, preferably at the interface between an imaging surface and an image forming substance, preferably deposited on the image forming surface as a particulate or porous uniform layer, hereinafter referred to as colorant/binder layer. The temperature may be as high as 400° C, but it is achieved for a very brief period only, e.g. 0.1 microsecond. It is achieving such high temperatures which causes the particulate or porous layer to adhere to the surface of the imaging web. Once the exposed particulate layer has adhered to the imaging surface, an image may be formed by removing from the imaging surface those portions of the colorant/binder layer which have not been exposed. In preferred embodiments of the invention this may yield complementary "negative" and "positive" images. Models of the mechanism for connecting exposed portions of the colorant/binder layer to the imaging surface, and of the removal of unexposed portions, may be used, with empirical experimentation, as guides to optimizing the chemistry of the layers to supplement the exposure and processing steps. While no definite reasons have been found explaining the superior performance of the thermal imaging medium of the present invention, electronmicroscopical measurements seem to support the conclusions set forth below. It is believed that the connection of the colorant/ binder layer to the imaging surface may qualitatively be modelled by the Washburn equation for the rate of penetra¬ tion of a liquid into a capillary. On the one hand, the pores of the particulate iaye , i.e. the colorant/binder layer, may be considered to constitute a plurality of capillaries; on the other hand, the imaging surface, when heated by the laser, may be assumed to act like a liquid, for polymeric materials of the kind here under conside¬ ration, when heated to about 400° C are about as viscous as water at room temperature. The Washburn equation is: V = a Gi v cos 6/(4 L) (1) where V is the velocity of the liquid entering an iso¬ thermal capillary of radius a; Gι v and v are, respective¬ ly, the surface tension and viscosity of the liquid; 6 is the contact angle of the liquid with the particulate material; and L is the distance the liquid meniscus has travelled along the capillary. The Washburn equation was derived for isothermal systems. However, the medium of the present invention when treated by a laser is an anisothermal system. Thus, additional factors need be taken Into consideration to arrive at a quantitative model of its behavior. Still, the Washburn equation is believed to be useful for qualitatively explaining the behavior of the imaging system In accordance with the invention. The colorant/binder layer does not adhere to the imaging surface before laser heating because the viscosity of the unheated imaging surface is in excess of lO 1 * poise. During laser heating the viscosity drops to about 0.01 poise. Hence, the velocity of the capillary meniscus moving into the particulate layer is 16 orders of magnitude higher during laser heating than at room temperature. For practical purposes the surface tension of most liquids may be assumed to decrease linearly with increasing temperature. When the medium in accordance with the invention is subjected, at least at the interface between the colorant/binder layer and the Imaging surface, to a temperature of about 400°0 the resultant surface tension of the liquefied imaging surface is probably about zero. As the contact angle normally decreases with increases in temperature it may be assumed that the rise in temperature in the medium significantly reduced the contact angle of the liquefied imaging surface with the particulate layer. Capillary attraction occurs when the tension of adhesion, Gι Cos θ, exceeds zero. This is important. For the adhesion tension determines whether the imaging surface possesses capillary attraction relative to the particulate or porous colorant/binder layer, once the viscosity of the imaging surface has been lowered under the impact of laser heating. While conflicting effects occur with an increase in temperature in that Gi v approaches zero and cos θ approaches one, it is nevertheless possible to generalize that (a) the adhesion tension cannot exceed Gi v and (b) if the adhesion tension is less than zero capillary repulsion results. If the adhesion tension of the medium of the invention is between 0 and 50 dynes/cm, and the viscosity of its imaging surface varies between less than 0.01 poise and 10 14 poise, one may deduce from the Washburn equation that the enormous decrease in viscosity has rather greater an impact on the capillary penetration of the liquefied imaging surface into the particulate layer than the adhesion tension. Once a latent image has been formed in the imaging surface by its capillary penetration into "exposed" portions of the image forming layer, further processing is required to render the image viewable. This processing requires removal of those portions of the particulate or porous colorant/binder layer from the imaging surface which have not been treated or exposed by the laser. While the manner of removal of the unexposed portions is immaterial to the concept of the invention, for reasons to be described removal by a peeling process is currently preferred. The peeling process may qualitatively be modelled on a "plunger" analogy. The balance between the force acting to peel an unexposed spot in the colorant/binder layer off the imaging surface, and the sum of the cohesive and base adhesive forces of the colorant/binder layer determines whether or not removal of a spot will take place. That is to say, an isolated unexposed spot in an exposed area is not removed from the imaging surface if, Fp < Fb+(2L/r)Fc; where Fp, Fb and Fc, respectively, are the force acting to peel the layer off the imaging surface, the force of adhesion of the layer to the imaging surface and the the cohesive force of the layer * L is the thickness of the colorant/binder layer and r is the radius of the spot. For forming Images of high resolution or photographic quality, the radius (r) of the spot must be very small. This produces a cohesive force {(2L/r)Fcl which is very- large, and may prevent removing small unexposed spots from the imaging surface. A colorant/binder layer with lower cohesion (Fc) and a small thickness (L) will reduce the cohesive force and allow removing small unexposed spots. However, low cohesion will result in splitting of the particulate layer, rather than in a clean transfer, during peeling. This prevents producing clean "positive" and "negative" images and makes the density of the obtainable image unpredictable. Therefore, to provide images of high resolution, without splitting of the particulate layer, the cohesion of this layer must exceed either the adhesive or the peeling force (Fc > Fb or Fp) . However, the cohesion and/or thickness of this layer must not exceed specific values determined by the desired resolution of the final image, The peeling force is dependent on the peeling temperature and the rate of peeling. While there may exist an ideal temperature related to an ideal peeling rate, the medium should offer parameters which allow producing satisfactory images under less than ideal circumstances. Exposing the medium by means of a laser is believed to increase Fb and/or decrease Fp . For instance, if the colorant/ binder layer of the medium is covered by a heat activated release layer the heat generated by the laser exposure will decrease Fp, or if the imaging surface is heat activated the heat from the laser will increase Fb . Materials for imaging surfaces and colorant/binder layers may be selected on the basis of the criteria set forth above. In this connection, the great importance of viscosity requires selecting materials that display a catastrophic drop in viscosity with increasing temperature at high frequency or short periods. The frequency dependence of the viscosity at a given temperature is of great importance since the heat of the laser may only be applied for about 10- 7 s (10 7 Hz) . A thermal imaging material, hereinafter referred to as the medium, useful for practicing the invention and identified by reference numeral 10 in Fig. 1 basically comprises a first web 12 of polymeric material pervious to image forming radiation and having a substantially continuous smooth image forming surface 14 upon which there is uniformly deposited a uniformly thin particulate or porous colorant/binder layer 16 for forming images in the surface 14 of the web 12. The web 12 may be present in the form of an integral unit having a thickness of from about 1 to about 1000 um, or it may be laminated, either permanently or temporarily, to a subcoat, such as paper or another polymeric material, as a uniform layer of a thickness sufficient for purposes to be described. The imaging surface 14 of the web 12 is preferably made of a material which when subjected to in¬ tense heat within a defined range of elevated temperatures at about 400° C experiences a catastrophic change in vis¬ cosity, as from about 10 14 poise at room temperature to about 10- 2 poise at the elevated temperature. Further¬ more, lest images formed in it be distorted, the web 12 when subjected to rapid heating for liquefication of its imaging surface 14 followed by a no less rapid cooling for solidifying the surface, preferably is dimensionally stable, i.e. it neither expands nor contracts in any dimension. Materials suitable as webs 12 include polystyrene, polyethylene terephthalate, polyethylene, polypropylene, copolymers of styrene and acrylonitrile, polyvinyl chlor¬ ide, polycarbonate and vinylidene chloride. At present, polyethylene terephthalate as traded by E.I.du Pont de Nemours & Co. under its tradename Mylar or by Eastman Kodak Company under its tradename Kodel is preferred The layer 16 comprises an image forming material deposited on the Imaging surface 14 as a_porous or particulate coating. The layer 16 may preferably be formed from a colorant dispersed in a binder, the colorant being a pigment or dye of any desired color preferably substantial¬ ly inert to the elevated temperatures required for image formation. Carbon black has been found to be of particular advantage. It may preferably have' particles 18 of an average diameter of about. 0.01 to 10 micrometers. Although the description will be substantially restricted to describing the use of carbon black, other optically dense substances, such as graphite, phtalocyanine pigments, and other colored pigments, may be used to equal advantage. It may even be possible to utilize substances which change their optical density when subjected to temperatures as herein described.. The binder provides a matrix to form the pigment particles into a cohesive mass and serves initially physically to adhere the pigment/binder layer 16 in its dry state to the Imaging surface 14 of the web 12. The ratio of pigment to binder may be in the range of from about 40 : 1 to about 1 : 2 on a weight basis. In a preferred embodiment the ratio is about 5 : 1. Advantageously, for ease of uniformly coating the imaging surface 14 with the layer 16, the carbon particles 18 may initially be suspended in a preferably inert liquid for spreading, in their suspended state, over the imaging surface 14. Thereafter, the layer 16 may be dried to adhere to the surface 14. It will be appreciated that to improve its spreading characteristics the carbon may be treated with surfactants such as, for instance, ammonium perfluoroalkyl sulfonate. Other substances, such as emulsifiers may be used or added to improve the uniformity of distribution of the carbon in its suspended and, thereafter, in its spread dry states. The layer may range in thickness from about 0.1 to about 10 micrometers. Thinner layers are preferred because they tend to provide images of higher resolution. If a dye is used, it may be soluble in the solvent of the binder, or it may be insoluble and dispersed in the binder. The quantity of dye is selected to provide the desired density in the finished image. Gelatin, polyvinyl alcohol, hydroxyethylcellulose, gum arabic, methylcellulose , polyvinylpyrrolidone , polyethyloxazoline and polystyrene latex are examples of binder materials suitable for use in the present invention. If desired, submicroscopic particles, such as chitin and/or polyamid may be added to the colorant/binder layer 16 to provide abrasion resistance to the finished image. The particles may be present in amounts of from about 1 : 2 to about 1 : 20 , particles to layer solids, weight / weight basis. Polytetrafluoroethylene particles are particularly useful. To be suited for thermal imaging, the medium must be capable of absorbing energy at the wavelength of the exposing source at or near the interface of the web 12, i.e. the imaging surface 14, and the layer 16. The energy absorption characteristic may be inherent in the materials of either web 12 or layer 16, or it may be provided as a separate heat absorption layer. To form an image in the imaging surface 14 of the web 12 a laser beam, schematically indicated by arrow 20, of a fineness corresponding to the desired high resolution of the image is directed to the interface between the colorant/binder layer 16 and the imaging surface 14, through the web 12. The beam 20 emanates from a laser schematically shown at 22 and is scanned across the Imaging surface 14 in a pattern conforming to the image to be formed. The beam 20 is absorbed at the interface and is converted to heat measuring about 400° C, although depending on the characteristics of the imaging surface 14, lower temperatures may also be effective for the purpose of forming an image. As will be appreciated by those skilled In the art the image-wise scanning may be accomplished by linearly scanning the imaging surface 14 and modulating the laser 22, preferably in a binary fashion, to form the image by way of very fine dots in a manner not unlike half-tone printing. While other lasers may be used for exposing the medium according to the invention, the laser 22 is px-eferably either a semiconductor diode laser or a YAG-laser and may have a power output sufficient to stay within upper and lower exposure threshold values of the imaging material 10. The laser 22 may have a power output in the range of about 40 to about 1000 mW. Exposure threshold value, as used herein, connotes, on the one hand, minimum power required to effect an exposure and, on the other, maximum power output tolerable to the imaging material 10 before a "burn out" occurs. Furthermore, the laser 22 is equipped with focussing apparatus (not shown) for precisely focussing the laser beam. Lasers are particularly suitable for exposing the medium of the invention because the latter is intended as what may conveniently be termed a threshold type film. That is to say, it possesses high contrast and, If exposed beyond a certain threshold value, it will yield maximum density, whereas no density at all is obtained below this threshold. The intensity of a focussed Gaussian laser beam gradually decreases from a maximum in the center of the beam. Thus, if the medium were not capable of threshold or, as it were, binary behavior, dots written by a Gaussian laser beam would display a gradual decrease in density from their center towards their margin. The rate of decrease in density is sometimes referred to as the "gamma" of the medium. A low gamma medium would display spots of soft or gradual edges. By contrast, high gamma media would write sharp spots with crisp edges. The medium in accordance with the present invention is such a high gamma film that edges are attainable which are sharper that of the exposing laser beam. In other words, the written dots may be modulated to be either completely dark or completely clear, so that the density of an image formed in the media of the present invention may be varied by a half-tone technique in which increasing area and/or number of dark dots increase the density of that area. Images may, therefore, be created with the medium of the present invention which in quality resemble photographs. As inferred above, focussed laser beams cannot produce a uniformly intense spot, so that in the manner of the very common Gaussian beam spot, some areas of the film may be considered to be well under and well over its exposure threshold. In the Gaussian beam spot the intensity distribution is given by an exponential decay: intensity=l=l 0 exp( -2 ( r/r 0 ) 2 ) (2) where r 0 is the radius of the beam where the intensity has dropped to 1/e 2 of the peak value and l 0 is the beam intensity at r=0. If the intensity of the film exposure threshold is If , the area of a written spot, provided there is no motion between the medium and the laser beam, is : Accordingly, the optimum use of laser energy for a stationary Gaussian laser occurs when as obtained by maximizing the efficiency of laser power usage: If the intensity of the exposure threshold of the me¬ dium is greater than or equal to I 0 , i.e. Io /If < 1, the area of the spot is zero. Thus, there is no written spot. However, if Io /If = e the area of the spot equals 0 . 5Xr 0 2 , the optimal value. Therefore, a spot can only be written on the medium if the center of the focussed Gaussian laser beam is above the exposure threshold of the medium. Since for focussed laser beams It is generally true that points inside a written spot receive an exposure intensity in excess of the exposure threshold intensity, it is important that the medium does not decompose, burn out or otherwise perform poorly when exposed to intensities higher than the minimum threshold value. When the laser power efficiency is less than optimal, Images of superior quality may nevertheless be obtained provided the center of the written spot withstands an exposure Intensity above the film exposure threshold intensity. For purposes of forming an image in the surface 14 of the medium 10 depicted in Fig. 1, it is necessary that at least one of the web 12 or colorant/binder layer 16 be sub¬ stantially non-absorptive of the wavelength of the laser, so that its beam may penetrate to the interface. In the present embodiment, the energy of the laser 22 is directed and penetrates through the web 12. As will be appreciated by those skilled In the art, birefringence of the support web 12 and of the imaging layer 14 must be taken into consi¬ deration when focussing lasers to small spots * If the spot Is too small, e.g. < 5.u , birefringence of the materials of these elements may cause distortion of the spot shape and loss of resolution and sensitivity. In order to deve¬ lop the heat required at the interface momentarily to liquefy the imaging surface 14 of the web 12, either the surface 14 or the particulate layer 16 must be heat absorp¬ tive or Include a heat absorbing material. For instance, infrared absorbing layers have been found to be useful in this respect. However, carbon black being itself an excel¬ lent heat absorbing material it may not be necessary or economical to provide a special layer. The intense (about 400° C) and locally applied heat developed at the interface between the imaging surface 14 and the particulate layer 16 causes the surface 14, where it is subjected to the heat, to liquefy, i.e. experience a catastrophic drop in viscosity from about 10 14 poise to about 10- 2 poise. As may be seen in FIG. 13, the heat is applied for an extremely short period, preferably in the order of <0. Smicroseconds , and causes liquefication of the material to a depth of about 0.1 micrometer (see FIGS. 13 and 14) . At this low viscosity the liquefied material exhibits capillary action with respect to the carbon black particles 18 of the layer 16 sufficiently to penetrate voids between the particles 18 without totally absorbing them. It is believed that the limited penetration of the liquefied surface material into the voids between the carbon black particles 18 is responsible for the fine resolution of images attainable with media of the present invention. Lest the image to be produced lose its desired high resolution because of excessive flow of liquefied surface material, liquefication and subsequent solidification of the imaging surface 14 must occur within a very small interval, in terms of both time and temperature. For instance, the exposure time span may be < 1 msec and the temperature span may be between about 100° C and about 1000°C. After exposure of the medium In the manner described, a sheet 24 having a surface 26 covered with a pressure sensitive adhesive may be superposed on the particulate layer 16, and may then be removed or peeled off in the manner indicated by an arrow 28 (see FIG. 2) . As the sheet 24 is removed, it carries with it those portions 16 u of the particulate layer 16 which were not subjected to the heat of the laser 22. In the manner described, the portions designated 16* treated by the laser 22 remain firmly attached on the surface 14 in form of what for the sake of convenience may be called a "negative" image, the parts I6u removed with the sheet 24 forming a complemen¬ tary or positive image. To yield sharp images it is neces¬ sary that the particulate layer 16 possess an inherent cohesion greater than its adhesion to the stripping sheet 24 and/or the web 12. The particulate layer 16 spread upon the surface 14 of the web 12 preferably adheres thereto, at least Initial¬ ly, in a manner precluding its accidental dislocation. While, as indicated supra, the particulate layer 16 may be provided with a matrix, it has been found that carbon black applied to the surface 14 In powder form, without any- binding agent, will connect to the surface 14 In the manner of this invention after treatment with a heat source. The untreated carboα black may then be removed by rubbing or washing or the like instead of, as in the above embodiment, by an adhesive strip sheet 24. As shown by the preferred embodiment of Fig. 3, the medium 10a may be a laminate structure comprising a web 12a having an Imaging surface 14a, a porous or particulate image forming layer 16a positioned on the surface 14a, a stripping or peeling sheet 24a, and a release layer 24a' in contact with the particulate layer 16a and deposited on the stripping sheet 24a. In Fig. 3a, the particulate matter 18a forming the colorant/binder layer is positioned on the imaging surface 14a and does not penetrate into it. The thermal imaging medium 10a may be exposed by a laser beam 20a in the manner previously described. Thereafter and as shown in Fig. 4 the stripping sheet 24a may be removed carrying with it those portions 16a tt of the particulate colorant layer 16a which have not been treated by the laser beam 20a. The treated portions 16a* will remain, firmly connected to the imaging surface 14a, on the web 12a. As shown in Fig. 4a the particulate matter 18a is now .slightly recessed into the imaging surface 14a as a result of the capillary attraction between the liquefied surface material and the colorant/binder layer 16a, in the manner explained above. An embodiment of a particularly preferred thermal imaging medium 10b is depicted in Fig. 5. The medium 10b comprises a web 12b preferably made of polyethylene terephthalate (Mylar) with a subcoat 12b' made of polystyrene or styreneacrylomtrile (SAN) . Placed on the subcoat 12b' and in contact with an image forming surface 14b thereof is a particulate or porous colorant/bmder layer 16b comprising carbon black and polyvinylalcohol . A release coat 24b' made of a microcrystallme wax emulsion (Michelman 160) is placed over the colorant/binder layer 16b. The release coat 24b' is m turn covered by a strip¬ ping sheet 24b made of carboxylated ethylenevmylacetate and polyvmylacetate (Airflex 416 and Daratak 61L) . Finally, a web 24b" of paper coated with an emulsion of ethylene-vmylacetate (Airflex 400) is coated over the stripping sheet 24b. The medium 10b is preferably exposed by a laser beam 20b directed through the web 12b to generate heat at the interface between the colorant binder layer 16b and the surface 14b of the web 12b. A heat absorption layer, such as an IR-absorber, (not shown) may additionally be provided to direct the effect of the laser beam to a predetermined location m the laminate structure of the medium 10b. The relative adhesive strengths between the several layers of the laminate medium 10b are such that before exposure separation would occur between the subcoat 12b' and the colorant/bmder layer 16b, whereas after exposure the separation would occur between or within the release coat 24b' and the stripping sheet 24b. This embodiment offers several distinct advantages: a) The microcrystallme wax release coat 24b' provides an effective protection against abrasion of the image created in the surface 14b; b) the wax release coat 24b' appears to improve the sensitivity of the medium because of its hydro- phobic nature which may avoid the necessity of the laser energv "boiling off" water from the coating. Furthermore, the use of a hot melt adhesive the stripping sheet 24b allows a laminate structure which may provide for an improved automatic peeling by a device integrated into the laser printer. Another embodiment of the medium 10c is shown in Fig. 6. This embodiment comprises a web 12c covered by a colorant/binder layer 16c, which in turn is covered by a stripping sheet 24c. Exposure of the medium 10c is accomplished by a laser beam 20c directed through the web 12c to generate heat in the manner described above at the interface between the colorant/binder layer 16c and the web surface 14c, in the preferred method through the web 12c provided on the stripping sheet 24c. Fig. 7 is a cross-sectional view of the embodiment of Fig. 6 and shows the separation of the stripping sheet 24c including unexposed portions 16c u of the colorant/binder layer 16c from the web 12c and the exposed portions 16ct • Fig. 8 depicts an embodiment of the invention In which the stripping sheet 24d on its surface opposite the particulate or porous colorant/ )inder layer 16d is provided with a support layer 24d' made, for instance, of paper. The paper support 24d' may be useful for providing a reflec¬ tion print complementing the image formed in the imaging surface 14d of the web 12ά, i.e. it may be a positive image of a negative image formed in the Imaging surface I4d, or vice versa. Fig. 9 is a rendition of a medium lOe similar to that of Fig. 6 except that it is provided with an adhesive layer 24e f laminated to the stripping sheet 24e. The adhesive layer 24e' Is preferably made from a pressure sensitive adhesive and may be useful for automatic removal of the stripping sheet 24e by means of a rotating drum (not shown) brought into contact with the adhesive layer 24e. Fig. 10 depicts an embodiment having an Infrared absorbing layer 34 interposed between the web 12f and the particulate colorant/binder layer 16f for purposes described above. The following examples illustrate the thermal imaging medium of the present invention. Example I A carbon black solution was prepared from 4..25g carbon black solution (43% solids) (sold under the tradename Flexiverse Black CFD-4343 by Sun Chemical Co . ) ; 21.84g water ; 3.66g polyethyloxazoline (10% aqueous solution) ( sold under the tradename PEOX by Dow Chemical Co. ) ; 0.24g fluorochemical surfactant (25% solids) (sold under the tradename FLUORAD FC-120 by 3M Co. ) and coated onto a polyethylene terephthalate (Mylar) web of 0.1mm thickness with a No.10 wire wound ' rod and air dried to give a dry coverage of about 0.7g/m 2 . The structure was exposed through the web by a laser beam with 0.1J/cm 2 for 1 microsecond. After exposure (the delay until this next step could be for any length of time) the layer was overcoated with a solution of 60. Og gelatin (15% solids) ; 29.3g water ; 0.72g FLUORAD surfactant to give a dry layer of about 7g/m 2 . Pressure sensitive adhesive tape was applied to the gelatin layer. The adhe¬ sive tape was peeled from the element leaving a negative carbon black image firmly connected to the surface of the web in areas of laser exposure. Example II A carbon black solution containing no polymeric binder or FLUORAD surfactant was prepared from 4.07g carbon black solution (45% solids) (sold under the tradename Sunsperse Black LHD-6018 by Sun Chemical Co. ) 23.93g water and coated onto the Mylar web as in Example I, to give a dry coverage of about 0.7g/m 2 . The structure was exposed through the web and developed as in Example I. This example illustrated the the polymeric binder and surfactant present in Example I are not necessary to connect the exposed carbon black firmly to the surface of the web. Example III The unexposed carbon black coated web from Example I was coated with a release layer from a solution consisting of: 2.00g was emulsion (25% solids) (sold under the tradename Michemlube 160 by Michelman Chemicals, Inc.); 7.92g water; 0.08g FLUORAD surfactant with a No.10 wire-wound rod to give a dry layer coverage of about 0.04g/m 2 . This was overcoated with a stripping layer from a solution consisting of 60.00g carboxylated ethylenevinylacetate copolymer emulsion (52% solids) (sold under the tradename Airflex 416 by Air Products and Chemicals, Inc.); and 40.00g polyvinylacetate emulsion (55% solids) (sold under the tradename Daratak 61L by W.R.Grace & Co. ) to give a dry layer coverage of about 20g/m 2 . The structure was exposed through the web by a laser beam with 0. lJ/cm 2 for 1 microsecond. The stripping layer was peeled from the element leaving a negative carbon black image firmly connected to the surface of the web in areas of laser exposure. The stripping layer contained a reverse of this image, i.e. , it was transparent in areas of laser exposure, Another structure was prepared as in Example III but with the wax emulsion replaced by a polyethylene aqueous was emulsion (sold under the tradename Jonwax 26 by S.C.Johnson and Son, Inc.) at the same concentration and coverage. .Another structure was prepared in the manner of Example III, except the polyvinylalcohol was substituted in equal amounts for polyethyloxazoline . Another structure was prepared as in Example III but the Mylar surface was first coated with 2g/m 2 of styrene acrylonitrile copolymer. Example IV The unexposed carbon black coated web of Example III was laminated at about 75° C to a second Mylar web of 0.1mm thickness. The laminated structure was exposed through the carbon black coated web of Example III by a laser beam of 0.1 J/cm 2 for 1 microsecond. After exposure the laminate was peeled apart to produce one negative and one positive image. The negative image consisted of exposed carbon black firmly connected to the surface of the web of Example III. The positive image consisted of unexposed carbon black adhered to the surface of the stripping layer, the latter being adhered to the surface of the second Mylar web. The stripping layer was then peeled from the second Mylar web so the latter could be used again for another lamination and peeling. Example V The second Mylar web of Example VII, prior to lamination, was coated with an adhesive solution consisting of ethylenevinylacetate copolymer emulsion (52% solids) (sold under the tradename Airflex 400 by Air Products and Chemicals, Inc. ) to give a dry coverage of about 5g/m 2 . The unexposed carbon black coated web from Example III was laminated at about 70° C to this second Mylar web with the adhesive coating of this example in face-to-face contact with the stripping layer of Example III. The laminate was exposed and processed as in Example IV. After exposure, the laminate was peeled apart to produce one negative and one positive image. However, because of the adhesive layer in this example the stripping layer could not be peeled from the second Mylar web. This example was repeated with a paper second web instead of Mylar to produce a reflection Image in this web instead of a transparency. The second web of this example was heated after the peeling step to a temperature above the melting point of the wax release layer (about 90°C). This improved the durability of the image by allowing the melted wax to flow into the porous carbon black layer. Samples were prepared as in Example IV and this example but the lamination was performed after the laser exposure instead of before. There was no detectable difference in the image quality. Example VI The stripping layer surface of the unexposed carbon black containing web from Example III was overcoated with a 40% aqueous solution of polyethyloxazoline (as in Example Ii to give a dry coverage of about 10g/m 2 . This dried layer was then overcoated with a solution containing equal amounts of a 20% aqueous solution of polyethyloxazoline and a 27.5% aqueous solution of titanium dioxide to give a dry coverage of about 10g/m 2 . This structure was then exposed and peeled as in Example III to produce two images, the first being a negative carbon black image firmly connected to the surface of the Mylar web in areas of laser exposure. The second image was a positive reflection print image consisting of unexposed carbon black adhered to the surface of the stripping layer. Example VII The unexposed carbon black coated web from Example III was coated with a release layer from a solution of 2.00g wax emulsion (25% solids) (sold under the tradename Michemlube 160 by Michelman Chemicals, Inc.); 7.92g water; and 0.08s FLUORAD surfactant, with a No.10 wire-wound rod to give a dry layer coverage of about 0.4g/m 2 . This was then pressure laminated to transparent adhesive tape (sold under the tradename Book Tape #845 by 3M Co. ) . The laminated structure was exposed through the carbon black coated web by a laser beam with 0.1 J/cm 2 for one microsecond. After exposure the laminate was peeled apart to produce one negative and one positive image. The negative image consisted of exposed carbon black firmly connected to the surface of the web from Example III. The positive image consisted of unexposed carbon black adhered to the surface of the transparent adhesive tape. The positive image was then rubbed with magenta pigment toner (sold under the tradename Spectra Magenta Toner by Sage Co. ) such that it stuck to the adhesive tape in areas not covered by the unexposed carbon black. The toned positive image was then washed with soapy water to remove the unexposed carbon black and leave a negative magenta image on the transparent adhesive tape. Conclusion Certain modifications may be introduced into the medium of this invention without departing from the scope of protection sought. For instance, it would be possible, for purposes of increasing the exposure sensitivity of the medium or of reducing the energy of the laser, to subject the medium to a pre-thermal treatment which would provide for an increased connection between the colorant/binder layer and the imaging surface without exposing the medium. Furthermore, it may be possible to increase the exposure sensitivity of the medium by subjecting it to a blanket pre-heating process. Such a process may reduce the heat load on the laser otherwise required to reach the exposure threshold of the medium. As will be apparent to persons skilled in the art, the medium of the present invention may, by appropriately poling the modulation of the laser beam, be useful in providing either positive or negative images in the imaging surfaces described above.

Claims: ■ 2 i - u l a m s 1. A thermal imaging element or orming images in response to intense image-forming radiation, comprising: a support formed of a material transparent to said radiation, said support having an imaging surface layer liquefiable and flowable at a predetermined elevated temperature range; a layer of porous or particulate imaging material uniformly coated on said surface layer, said layer of imaging material exhibiting a cohesive strength which is greater than the adhesive strength between said imaging material and said surface layer; at least one of the materials in said layers being absorptive of said radiation to convert it to thermal energy capable of liquefying the material of said imaging surface layer; the material of said surface layer, when liquefied, exhibiting capillary flow into adjacent portions of said imaging material, thereby locking substantially the entire layer of said imaging material to said support when said imaging surface layer cools. 2. The thermal imaging element of claim 1, wherein said support comprises a web having thickness from about 1 to about 1000 urn. 3. The thermal imaging element of claim 2, wherein said web comprises a thermoplastic material having a surface structure which, when subjected to temperatures of about 400° C, exhibits a catastrophic drop in viscosity of from about 10 14 poise to about 10" 2 poise. 4. The thermal imaging element of claim 3, wherein said web comprises polyethylene terephthalate 5. The thermal imaging element of claim 3, wherein said web comprises polystyrene. 6. The thermal imaging element of claim 3, wherein said web comprises polypropylene. 7._ The thermal imaging element of claim 3, wherein said web comprises polyethylene. 8. The thermal imaging element of claim 3, wherein said web comprises a copolymer of styrene and acrylonitrile. .- 9. The thermal Imaging element of claim 3, wherein said web comprises polyvinylchlorlde 10. The thermal imaging element of claim 3, wherein said web comprises polycarbonate. 11. The thermal imaging element of claim 3, wherein said web comprises vinylidene chloride. 12. The thermal imaging element of claim 3, wherein said web is provided with a layer of paper. 13. The thermal imaging element of claim 3, wherein said web is provided with a subcoat of polystyrene. 14. The thermal imaging element of claim 3, wherein said web is provided with a subcoat of styrene acrylonitrile. 15. the thermal imaging element of claim 1, wherein said layer of Imaging material comprises a pigment. 16. The thermal imaging element of claim 15, wherein said layer of imaging material has a thickness of from about 0.1 to about 10 micrometers 17. The thermal imaging element of claim 16, wherein said pigment comprises carbon black having a particle size of from about 0.01 to about 10 micrometers. 18. The thermal imaging element of claim 16, wherein said pigment comprises graphite. 19. The thermal imaging layer of claim 16, wherein said pigment comprises phtalocyanine pigment. 20. The thermal imaging element of claim 17, wherein said carbon black includes a surfactant. 21. The thermal imaging element of claim 20, wherein said surfactant comprises ammonium perfluoroalkyl sulfonate. 22. The thermal imaging element of claim 17, wherein said carbon black includes a binder for rendering said layer of Imaging material cohesive. 23. The thermal imaging element of claim 22, wherein said binder is polyethyloxazoline. 24. The thermal imaging element of claim 22, wherein said binder ' is gelatin. 25. The thermal imaging element of claim 22, wherein said binder is polyvinyl alcohol. 26. The thermal imaging element of claim 22, wherein said binder is gum arabic. 27. The thermal imaging element of claim 22, wherein said binder is methylcellulose . 28. The thermal imaging element of claim 22, wherein said binder is polyvinylpyrrolidone . 29 * The thermal imaging element of claim 22, wherein said binder is polystyrene latex. 30. The thermal imaging element of claim 16, wherein said layer of image forming material includes polytetrafluoro¬ ethylene. 31. The thermal imaging layer of claim 30, wherein the ratio of polytetrafluoroethylene to pigment is from about 1:2 to about 1:20 by weight. 32. The thermal imaging element of claim 16, wherein said layer of image forming material includes chitin. 33. The thermal imaging element of claim 16, wherein said layer of image forming material includes polyamid. 34. The thermal imaging element of claim 22, wherein said binder is present in a ratio of from about 40:1 to about 1:2 on a weight basis relative to said pigment. 35. The thermal imaging element of claim 34, wherein said ratio is about 5:1. 36. The thermal Imaging element of claim 1, further comprising a stripping sheet on the layer of thermal imaging material on its surface opposite the support. 37. The thermal imaging element of claim 36, wherein said stripping sheet comprises a polymeric sheet having a surface coated with pressure sensitive adhesive. 38. The thermal imaging element of claim 36, wherein said stripping sheet comprises caboxylated ethylenevinylacetate. 39. The thermal imaging element of claim 36, wherein said stripping sheet comprises polyvinylacetate . 40. The Thermal imaging element of claim 36, wherein said stripping sheet comprises a copolymer of carboxylated ethylenevinyl acetate and polyvinyl acetate. 41. The thermal imaging element of claim 36, wherein said stripping sheet comprises paper coated with ethylene vinyl acetate . 42. The thermal imaging element of claim 36, further comprising a coating for increasing the abrasion resistance of said layer of imaging material provided between said stripping sheet and said layer of imaging material. 43. The thermal imaging element of claim 42, wherein said coating comprises a microcrystalline wax. 44. The thermal imaging element of claim 37, wherein said stripping sheet on its surface opposite said thermoplastic coating is provided with a protective sheet. 45. The thermal imaging element of claim 44, wherein said protective sheet comprises paper. 46. The thermal imaging element of claim 37, wherein said stripping sheet comprises a layer of titanium dioxide. 47. The thermal imaging element of claim 1, further comprising an IR-absorption layer.

Inventors: ETZEL MARK R, ETZEL, MARK, R.
Applicants: POLAROID CORP, POLAROID CORPORATION
Application year: 1987
Main classification: B41M5
IPC codes: B41M5, G03F7