WO2002009948A2

WO2002009948A2 - Image receptor sheet for use in laser-addressable thermal transfer recording systems

Info

Publication number: WO2002009948A2
Application number: PCT/US2001/023813
Authority: WO
Inventors: Tangyu Wang; Gregory L. Zwadlo
Original assignee: Imation Corp.
Priority date: 2000-07-27
Filing date: 2001-07-27
Publication date: 2002-02-07
Also published as: WO2002009948A3

Abstract

A receptor, and method of forming a half tone image thereon. The receptor includes in order: a support; a thermal absorbing layer having a thickness of at least about 7.5 νm which is capable of dissipating heat from a laser; and a releasable image receiving layer having a coating weight of about 0.54g/m2 to about 4.3 g/m2 and having a textured outer surface comprising a plurality of protrusions projecting above the plane of the outer surface of the receiving layer by an average distance of no greater than about 8 νm and having on average at least 1 of said pluraltiy of protrusions per laser pixel.

Description

IMAGE RECEPTOR FOR USE IN

LASER ADDRESSABLE THERMAL TRANSFER SYSTEMS

FIELD OF THE INVENTION

This invention relates to a process and receptor to be used in a laser addressable thermal transfer imaging system. In particular, the invention relates to a receptor comprising a heat absorbing layer and a surface texture to provide improved image quality when high intensity infrared lasers are utilized as the imaging source.

BACKGROUND OF THE INVENTION

There is continuing interest in imaging by means of laser-induced colorant transfer from a donor element to a receptor. The donor elements typically comprise a support bearing, in one or more coated layers, an absorber for the laser radiation, a transferable colorant (e.g., one or more dyes or pigments) and one or more binder materials. When the donor element is placed in contact with a suitable receptor and subjected to a pattern of laser irradiation, absorption of the laser radiation causes rapid build-up of heat within the donor element, sufficient to cause transfer of colorant to the receptor in irradiated areas. By repeating the transfer process using different donor elements but the same receptor, it is possible to superimpose several monochrome images (e.g., yellow, magenta, cyan and black color separations) on a common receptor, thereby generating a full-color image. The process is ideally suited to the output of digitally stored image information, and has the additional benefits of not requiring chemical processing, and not employing materials that are sensitive to normal white light.

There are two main classes of laser stimulated colorant transfer from a donor to a receptor, namely dye sublimation transfer and mass transfer. In the former, image dyes diffuse or sublime in response to laser-generated heat, but the physical integrity of the donor layer is not affected. In the latter, portions of the donor layer (i.e., binder and colorants and absorber, if present) transfer en masse to the receptor. Different types of image are available by these different mechanisms. Dye sublimation may give a continuous tone image in which the density of the transferred image varies over a significant range with the energy absorbed. Mass transfer typically gives a bi-level image in which either zero or maximum density is transferred, depending on whether the applied energy exceeds a given threshold. Mass transfer systems are therefore ideally suited to the reproduction of half tone images.

This basic distinction between the two classes was recognized since at least the mid 1970's and is disclosed in JP-51-88016. This reference deals primarily with sublimation transfer, and distinguishes the situation in which dye alone is transferred from the situation in which both dye and binder are transferred. The same distinction has been applied consistently in the art, up to and including recently-issued patents, such as, U.S. Pat. No. 5,516,622.

Records of laser-induced mass transfer date from at least the 1960's. For example, JP-46-3710, which was filed in 1966, discloses transfer of colorant from a donor to a receptor by a "sputtering" process mediated by laser exposure. Coatings of printing ink on plastic film are quoted as highly suitable donor sheets.

A paper published in 1970 (Applied Optics. 9 (No. 10), pp. 2260- 2265) distinguishes two modes of laser mass transfer, namely a less energetic mode in which transfer occurs in a fluid state (i.e., melt transfer), and a more energetic mode in which transfer occurs by an explosive force, as a result of generation and rapid expansion of gases at the substrate-coating interface (i.e., ablation transfer). This distinction is also recognized consistently by later authors. For example, U.S. Pat. Nos. 5,156,938, 5,171,650, 5,516,622 and 5,518,861 all refer to ablation transfer as a process distinct from melt transfer, and emphasize its explosive nature. The first two patents in particular emphasize the use of thermally degradable materials to assist with the transfer process. Many other patents, such as U.S. Pat. No. 3,962,513, refer to the use of nitrocellulose or "self-oxidizing" binders in ablation transfer. Conversely, other patents, such as JP-69-319192, EP-A-530018, U.S. Pat. Nos. 5,501,937, 5,580,693, 5,401,606, and 5,019,549, refer to transfer of colorant in a molten or semi-molten (softened) state, with no mention of explosive mechanisms. Many of the known laser-induced melt transfer materials employ one or more waxes as binder materials, in pursuit of a transfer layer which melts sharply to a highly fluid state at moderately elevated temperatures.

However, it has been shown recently that excellent image quality is provided by a melt transfer system in which the colorant layer transfers essentially in the form of a coherent film, and does not apparently achieve a state of high fluidity during the transfer process. Indeed, this transfer mechanism - laser-induced film transfer (LIFT) - is promoted by the inclusion in the transfer layer of compounds which effect at least a partial curing (and ultimately hardening) of that layer during the transfer process, as described in International Publication No. WO 98/07575 (February 26, 1998). Transfer of the colorant layer in the form of a coherent film enables dots or pixels to be transferred to the receptor with excellent edge definition. Furthermore, the curing that occurs in the course of the transfer leads to an image of enhanced durability with excellent overprint characteristics, i.e., it is possible to transfer second and subsequent images to a common receptor without damaging the first image transferred thereto. Both of these factors are important in the successful development of a digital half-tone imaging system based on laser-induced colorant transfer.

Under carefully controlled laboratory conditions, small-area images of excellent quality are formed by the LIFT process using conventional smooth receptors such as resin-coated paper. However, when the imaging is carried out in a less well-controlled environment, the image quality is found to deteriorate markedly, with voids appearing (apparently) at random in the transferred image. The problem may be traced to the presence of dust particles on the surfaces of the donor and receptor. Such particles not only prevent effective contact of the donor with the receptor in their immediate vicinity, but also adversely affect the focusing of the laser by causing bumps or undulations in the absorbing layer, so that even small dust particles can give rise to significant "dropouts." The problem is particularly acute in the case of large area images (e.g., of A2 size), where the likelihood of attracting dust on to the surfaces of the sheets is greater due to the triboelectric charging that inevitably results from the handling of such large sheets. The lamination of smooth sheets in consistent mutual contact, without forming pockets of trapped air, also becomes increasingly difficult as the area of the sheets increases.

U.S. Pat. No. 5,580,693 describes the problems caused by poor contact between donor and receptor (e.g., due to dust particles) in a melt transfer process, and proposes as a solution the provision, in the donor element, an additional deformable or "cushioning" layer. The addition of such a layer adds significantly to the manufacturing costs.

Since the presence of dust cannot be eliminated under normal working conditions, there is a need for alternative means of ensuring effective contact between a color donor and a receptor that is compatible with transfer of the colorant as a coherent film and also tolerant of the presence of dust particles.

There is a long history of using beads or other inert particles in photographic materials for various purposes, such as for matting purposes, or for spacing purposes, e.g., to prevent adjacent sheets in a stack from sticking together. They have also found widespread use in materials intended for contact exposure, e.g., to promote good vacuum draw-down or to suppress interference fringes resulting from close contact of films. These uses are documented in U.S. Pat. No. 4,711,838 and elsewhere.

In the field of dye sublimation transfer, it has been known since at least the mid 1970's that spacing apart of the donor and receptor is necessary (see, for example, JP-51-88016). Also, U.S. Pat. No. 4,541,830 and 4,777,159 disclose the use of beads or other non-sublimable particles for this purpose. The beads are dispersed within the dye layer, but are of sufficient size to project outwardly from it. U.S. Pat. No. 4,772,582 and 4,876,235 disclose the use of beads coated on top of the dye layer, and the use of beads in the image receiving layer, respectively. U.S. Pat. No. 5,017,547 discloses the use of vacuum hold- down of donor to receptor, one of which contains spacer beads. The provision of a "textured surface" between donor and receptor, the texture being generated by embossing or other suitable means not involving the use of added particulates, is disclosed in U.S. Pat. No. 5,254,514. It is noteworthy that many of these patents describe the primary purpose of the beads or other spacing means as being to prevent adhesion of donor layer to image receiving layer. The APPROVAL digital color proofing system, commercially available from Kodak, is based on dye sublimation transfer, and utilizes a receptor sheet comprising beads in the image receiving layer. Microscopic analysis indicates that the beads are about 20 micrometers in diameter and are present at a coverage of no more than about 100 beads per square millimeter (mm²).

Spacer beads have also been advocated for use in laser mass transfer imaging, but only in the context of ablation transfer. In the field of ablation transfer, it has also long been recognized that spacing apart of donor and receptor is desirable in the interests of improved image quality (see, for example, JP-46-3710 and the 1970 journal article Applied Optics. 9 (No. 10), pp. 2260- 2265). This is confirmed by U.S. Pat. Nos. 5,518,861 and 5,516,622, which disclose, respectively, ablation transfer media having a textured surface (via embossing or similar means) and ablation transfer media comprising spacer beads in the transfer layer. U.S. Pat. No. 5,516,622 cautions against placing the beads in the image receiving layer.

The need for spacing means in dye sublimation transfer and in ablation mass transfer is not surprising. In sublimation transfer, the objective is to transfer dyes in a vaporized state without co-transfer of binder. Preventing the donor layer from actually contacting the image receiving layer over most of its area is therefore a logical approach. In the field of ablation transfer, an explosive expansion of gases propels the colorant towards the receptor, and so a gap between the donor and receptor in no way hinders the transfer process. Indeed, by allowing expansion to proceed preferentially in the forward direction, the tendency to expand sideways (causing image spread) is minimized. In addition to the contact issues discussed above, lasers (typically, infrared lasers) may present an additional problem when used as the imaging source. Infrared lasers expose sources used in imaging applications are typically derived from a single powerful laser (e.g., about 5 watts to about 20 watts or more) out of which is split several hundred beams and used to expose adjacent pixels over a width of a few millimeters. In such cases, a significant amount of heat is generated that can distort the imaging materials and cause unwanted artifacts. These typically show up as small shifts in physical dimension caused by shrinkage or expansion of the materials in localized areas. Separation of the donor from the receiver due to the high temperature can also cause slight differences in transferred materials. If the laser modulation is not carefully controlled, the heat differentials across the imaging area can accentuate the problem.

Current receptors for laser thermal imaging do not include means to moderate the high temperature obtained during and after exposure. Distortion can occur especially where a large area exposure print head is used. Temperature moderation often needs to occur over very short times, in the range of about 3 microseconds to about 30 microseconds. Also energy levels of several hundred millijoules per square centimeter of image need to be dissipated, thus requiring the thermal absorber in intimate proximity to the imaging plane. U.S. Pat. No. 5,077,263 describes a laser thermal receiver construction utilizing a metallic surface to increase dye transfer efficiency and decrease image defects when using a laser energy source. However, this layer is too thin and the dye receiving coatings too thick to provide sufficient heat absorption. The metallic layer is disclosed as being vapor deposited. As such, the layers are typically no greater than about 1 micrometer to avoid defects such as bubbling. U.S. Pat. No. 5,328,885 describes an adiabatic layer or heat absorbing layer between the base film and a light-heat conversion layer to prevent the heat generated by the light-heat conversion layer from being absorbed by the base film. U.S. Pat. No. 5,372,987 describes the incorporation of polymeric beads into an adhesive layer underlying the dye receptive layer to provide better heat insulation from heat loss through the substrate and better heat utilization for the dye diffusion transfer. There is a continuing need for receptors that provide better dissipation of thermal energy in a laser addressed system to prevent image distortion in large image formats.

SUMMARY OF THE INVENTION Therefore, according to the invention there is provided a receptor comprising in order: (a) a support; (b) a thermal absorbing layer capable of dissipating heat from a laser, wherein the thermal absorbing layer has a thickness of at least about 7.5 micrometers (μm or microns) (and preferably, no greater than about 25 μm); and (c) a releasable image receiving layer having a coating weight of about 50 mg/ft² (0.54 g/m²) to about 400 g/ft² (4.3 g/m²) (preferably, about 100 mg/ft² (1.08 g m²) to about 300 mg/ft² (3.23 g/m²)) and having a textured outer surface comprising a plurality of protrusions projecting above the plane of the outer surface of the receiving layer by an average distance of no greater than about 8 μm (preferably, at least 1 μm), there being on average at least 1 protrusion per pixel (i.e., the smallest area exposed by developing laser). In another embodiment of the present invention, a method of half tone imaging using a laser is provided, the method comprising:

(a) providing a receptor comprising in order: (i) a support; (ii) a thermal absorbing layer capable of dissipating heat from the laser and having a thickness of at least about 7.5 micrometers (μm) (and preferably no greater than about 25 μm) and (iii) a releasable image receiving layer having a coating weight of about 50 mg/ft² (0.54 g/m²) to about 400 g/ft² (4.3 g/m²) (preferably, about 100 mg/ft² (1.08 g/m²) to about 300 mg/ft² (3.23 g/m²)) and having a textured surface comprising a plurality of protrusions projecting above the plane of the outer surface of the receiving layer by an average distance of no greater than about 8 μm (preferably, at least about 1 μm), there being on average at least 1 of such protrusions per pixel;

(b) providing a color donor comprising a support having deposited thereon a color transfer layer comprising a binder, a colorant, and a radiation absorber;

(c) placing in mutual contact the textured surface of the receptor and the color transfer layer of the color donor to form a composite;

(d) exposing the composite to scanned laser radiation of a wavelength absorbed by the radiation absorber, the laser radiation being focused to a spot at the plane of the color transfer layer and being modulated in accordance with digital halftone image information, and thereby causing exposed portions of the color transfer layer to soften or melt and adhere preferentially to the receptor sheet; and

(e) peeling apart the receptor and color donor. Steps (b) to (e) of the imaging method are preferably repeated one or more times using a different color donor comprising a different colorant, but using the same receptor sheet. After steps (b) to (e) have been performed as many times as is necessary, the image-bearing receptor sheet is optionally subjected to a lamination process in which the imaged image receiving layer is transferred to another support.

The outer surface of the receptor refers to the surface of the image receiving layer furthest from the support of the receptor, i.e., the surface of the receptor which is presented to the donor. The reference to the plane of the outer surface of the receptor refers to the plane of surface of the layer between the protrusions. The color transfer layer is one that melts or softens under the action of heat.

It is found that the provision of a surface texture in the receptor, preferably by incorporation of inert particles such as polymer beads, silica and other inorganic oxides such as metal oxides, etc., in a image receiving layer coated on a support, or alternatively by means of embossing or similar techniques, effectively solves the voiding problems caused by dust particles, and permits channeling and bleeding of air which otherwise might become trapped in pockets between the donor and receptor sheets.

Furthermore, it is surprisingly found that the size and spacing of the protrusions in the receptor surface exerts a profound influence on the quality of the dots transferred thereto, and that by controlling the spacing of the protrusions in accordance with the profile of the exposing laser, high-quality film transfer is enabled, even in the case of large-area images.

In addition, the incorporation of a thermal absorbing layer underlying the image receiving layer significantly reduces image distortion caused by localized over heating.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

There is very little teaching in the prior art relevant to the use of spacing means in a laser-induced melt transfer system. In fact, U.S. Pat. No. 5,089,372 teaches that conventional melt-transfer to a rough receptor is not possible. U.S. Pat. No. 5,501,937 mentions the use of beads, but only as a possibility, without teaching any particular benefit or disclosing any information relating to optimum sizes and spacings. Transfer from a roughened donor to a smooth receptor is stated to be equivalent to transfer from a smooth donor to a roughened receptor. U.S. Pat. No. 5,580,693 discloses the incorporation in image receiving layers of matting agents in the form of fine particles, but a maximum particle size of 5 μm at a loading of no more than 10 mg/m² is taught. This is equivalent to about 200 particles per mm². The purpose of the matting agent is to improve the lubricity and handling properties of the sheets, and there is no disclosure of any effect on the image quality.

Consideration of the mechanism of melt transfer, in which the colorant layer must melt or soften and then wet and adhere to the image receiving layer, suggests that spacer beads would be detrimental to the process. The melt transfer process necessarily involves intimate contact of donor layer with image receiving layer. Spacing beads have a long history of use in the imaging art generally, and the field of laser induced colorant transfer in particular, for the precise purpose of preventing such a contact from occurring.

Initial studies lent credence to this pessimistic view. As indicated previously, color donor media of the type disclosed in International Publication No. WO 98/07575 gave excellent results when imaged to a smooth receptor, in that the color layer was seen to transfer as a coherent film, giving transferred dots with sharp edges. However, the images were unacceptable for commercial purposes owing to the presence of random voids caused by dust particles. When the same donor media were imaged under the same conditions to the beaded receptor used in the Kodak APPROVAL color proofing system, the quality of the transferred image was markedly poorer. Microscopic examination of the image revealed that the color layer had fragmented and scattered the pigment particles, giving diffuse dots lacking the sharp edges observed previously. Clearly, the presence of the spacer beads had prevented the color layer from transferring as a coherent film. However, it was also observed that voids caused by dust particles were absent.

Subsequent experiments using receptors comprising larger numbers of smaller-sized beads or inorganic particles revealed that as both the diameter of the particles and the average interparticle distance decreased, the degree of non- uniformity in the transferred image also decreased, until a point was reached where the colorant layer once again transferred as a coherent film. Furthermore, this was achieved without the reappearance of voids caused by dust particles, and thus a solution was found to the problem of exploiting the full benefit of the laser-induced film transfer (LIFT) in the context of large-scale images under normal working conditions.

The optimum size and concentration of protrusions (e.g., as formed by beads or other particles) was found to depend on the dimensions of the footprint of exposing laser, i.e., the diameter of the illuminated spot at the plane of the colorant layer, which determines the minimum size of dot or pixel which can be transferred from donor to receptor. This is typically in the range of about 5 μm to about 50 μm, but may be different for different designs of imaging engine. For example, the PEARLSETTER imager (Presstek, Hudson, NH) has a pixel size of about 30 μm diameter, while the TRENDSETTER device (Creo, Vancouver, British Columbia, Canada) has a pixel size of about 8 μm. Preferably, the concentration of protrusions in the image receiving layer should be sufficient to provide on average at least 1 point of contact between the donor and image receiving layers per pixel, preferably at least 2 points of contact. The protrusions may be formed by inert particles such as polymeric beads or other particles, or embossing means, for example. The beads or other particles may be of essentially uniform size (i.e., a monodisperse population), or may vary in size (thus, the protrusions may vary in size). Dispersions of inorganic particles such as silica generally have a range of particle sizes, whereas monodisperse suspensions of polymer beads are readily available. Whichever type of population is used, the protrusions should preferably not project above the plane of the surface of the image receiving layer by more than about 8 μm on average, and more preferably, by more than about 6 μm. Preferably, they should preferably project above the plane by at least about 1 μm, and more preferably, by at least about 2 μm, and most preferably, by at least about 3 μm. The composition of polymeric beads, if used to form the protrusions, is generally chosen such that substantially all of the visible wavelengths (400 nm to 700 nm) are transmitted through the material to provide optical transparency. Non-limiting examples of polymeric beads that have excellent optical transparency include polymethylmethacrylate and polystyrene methacrylate beads, described in U.S. Pat. No. 2,701,245; and beads comprising diol dimethacrylate homopolymers or copolymers of these diol dimethacrylates with long chain fatty alcohol esters of methacrylic acid and/or ethylenically unsaturated co-monomers, such as stearyl methacrylate/hexanediol diacrylate crosslinked beads, as described in U.S. Pat. Nos. 5,238,736 and 5,310,595.

The shape, surface characteristics, concentration, size, and size distribution of the protrusions are selected to optimize performance of the transfer process. For example, the smoothness of the surface and shape of the protrusions may be chosen such that the amount of reflected visible wavelengths (400 nm to 700 nm) of light is kept to a minimum. This may or may not be an issue depending upon the actual substrate used. For example, if the color proof is formed on a transparent substrate, the haze introduced by the presence of particles may be effected by the color. The shape of the particles (and, hence, the protrusions) is preferably spherical, oblong, ovoid, or elliptical. In some constructions, it is advantageous to add two distinct sets of particles with different average sizes. This allows the flexibility to balance haze with slip or separation characteristics.

The optimum protrusion size (e.g., particle size) depends on a number of factors, including the thickness of the image receiving layer, the thickness of the color layer to be transferred, and the number of color layers to be transferred to a given receptor. In the case of transfer of two or more color layers to a single receptor, the projections provided by the particles are preferably great enough not to be obscured by the first layer(s) transferred thereto. If the average projection (i.e., protrusion) is significantly greater than about 8 μm, however, transfer of the color layer as a coherent film becomes difficult, and the quality of the transferred image deteriorates markedly.

In the case of polydisperse populations of particles, such as silica particles, excellent results have been obtained when the largest of the particles project above the plane of the image receiving layer by about 4 μm and provide on average at least 1 point of contact per pixel between the donor and image receiving layers, with preferably at least 2 (more preferably, at least 4) smaller particles also present per pixel. Good results have also been obtained using essentially monodisperse populations of polymer beads projecting about 4 μm above the plane of the image receiving layer and providing on average at least 1 point of contact (i.e., direct physical contact) between the donor and image receiving layers per pixel.

Surprisingly, incorporation of similar particles at similar loadings in the donor layer, with transfer to a smooth receptor, does not give the same benefits. This is in contrast to what is observed for other modes of laser induced color transfer (i.e., ablation and dye sublimation transfer), where the roughened donor/smooth receptor configuration is heavily favored by the published art. Receptor elements used in the invention generally comprise a support sheet bearing a releasable image receiving layer containing the beads or other inert particles and a thermal absorbing layer underlying the image receiving layer. Preferably, the image receiving layer also contains a bleaching agent capable of bleaching the infrared dye which typically co-transfers with the color layer.

The composition of the support sheet is not critical, and essentially any sheet-form material may be used, with flexible materials such as paper or plastic film being preferred. The releasable image receiving layer, which may be formed from one or more layers of the same or different materials, is typically a thermoplastic polymer layer of about 1 μm to about 10 μm, preferably about 1.5 μm to about 5 μm, in thickness. A wide variety of polymers may be employed. Preferably, ones are chosen that provide a clear, colorless, nontacky film. Within these constraints, selection of polymers for use in the image receiving layer is governed largely by compatibility with the colorant intended to be transferred to the receptor, and with a bleaching agent, if used. Preferably, the polymer is a thermoplastic polymer. Vinyl polymers such as polyvinyl butyral (e.g., BUTVAR B-76 supplied by Monsanto, St. Louis, MO), vinyl acetate/vinyl pyrrolidone copolymers (e.g., E735, E535 and E335 supplied by GAF, Wayne, NJ), styrene butadiene polymers (e.g., PLIOLITE S5A supplied by Goodyear, Akron, OH), and acrylic polymer (e.g., ELVACITE 2021 supplied by ICI Acrylics, Wilmington, DE) have been found to be particularly suitable. The image receiving layer may be coated directly on the support sheet, or there may be one or more underlayers separating the image receiving layer from the support sheet. A preferred construction comprises a support having a thermal absorbing layer followed by the releasable image receiving layer. An optional release agent may be added to the image receiving layer to assist the transfer of the image receiving layer to a final substrate. Examples of release agents include fluorochemical surfactants and silicone oils, for example. Alternatively, a release layer may be interposed between the thermal absorbing layer and image receiving layer to assist the release of the image receiving layer from the thermal absorbing layer. The image receiving layer (after an image has been transferred thereto) is transferred to another substrate by a process of lamination followed by peeling of the support sheet. The use of release layers for this purpose is well known in the art. See, for example, U.S. Pat. Nos. 5,053,381, 5,126,760, and 5,278,576. Suitable materials include fluorinated polymers, silicones, polyurethanes, etc.

Preferably, the image receiving layer additionally comprises one or more compounds capable of bleaching the radiation absorber (preferably, infrared absorber) associated with the colorant layer (discussed below), as disclosed in EP-A-0675003. Preferred bleach agents include amines, such as, diphenylguanidine and salts thereof. The bleach agents are typically used at a loading equivalent to about 5 wt% to about 20 wt% of the image receiving layer.

For example, a suitable image receiving layer comprises PLIOLITE S5A containing diphenylguanidine (e.g., sec-butyl DPG) as bleach agent (10 wt% of total solids) and beads of poly(stearyl methacrylate) (8 μm diameter) (about 5 wt% of total solids), coated at about 5.9 g/m². Preferably, the releasable image receiving layer is coated at a thickness of less than 3.2 g/m².

A particularly preferred image receiving layer is obtained by coating the following formulation from methylethyl ketone (18 wt%) to provide a dry coating weight of 400 mg/ft² (4.3 g/m²):

PLIOLITE S5A 87 wt% 8 μm Poly(stearyl methacrylate) beads 1 wt%

Diphenylguanidine 12 wt%

As an alternative to the use of beads or particles the receptor surface may be physically textured to provide the required protrusions. Metal surfaces, such as aluminum, may be textured by graining and anodizing. Other textured surfaces may be obtained by microreplication techniques, such as those disclosed in EP-A-382 420. I

The extent of the protrusions on the receptor surface, whether formed by bead, particles, or texturing, may be measured, for example, by interferometry or by examination of the surface using an optical or electron microscope. The thermal absorbing layer dissipates the heat generated by the laser and infrared-absorber, thus minimizing image distortion. The thermal absorbing layer preferably has sufficient mass and thermal conductivity to limit the temperature rise in the materials during and subsequent to exposure, i.e., the thermal absorbing layer is intended to act as a heat sink dissipating heat from the exposed area. The rate of thermal dissipation is proportional to the mathematical product of the absorbing layer thickness (t) and its specific heat conductivity (k). The quantity kt is preferably at least about 2 millijoules/second/°C (mJ/s/°C), more preferably, at least about 3 mJ/s/°C, and preferably, no greater than about 6 mJ/s/°C, more preferably, no greater than about 4 mJ/s/°C.

The thickness of the thermal absorbing layer is preferably at least about 7.5 μm and more preferably at least about 12.7 μm. The thickness of the thermal absorbing layer is preferably no greater than about 25 μm and more preferably no greater than about 18 μm. Suitable materials for the thermal absorbing layer include metals, such as aluminum, tin, etc., and metal oxides such as TiO₂ and ZnO. Other materials may also be useful so long as the layer is capable of dissipating the heat generated during the laser exposure to reduce the maximum temperature experienced by the materials.

Color donor sheets suitable for use in the invention comprise a support and a thermofusible color transfer layer comprising a binder, a colorant, and a radiation absorber. The binder is chosen from film-forming resins, which are typically soluble in common organic solvents, and which are preferably transparent and melt or soften at moderately elevated temperatures but do not decompose catastrophically when heated at such temperatures that occur during laser address and film transfer. Most preferably, the binder resin contains functional groups (e.g., hydroxy functional groups) which enable it to participate in curing reactions with other constituents of the colorant layer. A wide variety of commonly-available resins are potentially suitable, and a preferred material is BUTVAR B-76, available from Monsanto, St. Louis, MO. The radiation absorber used in the invention is a material which will absorb IR or light and convert it to heat. As absorber (i.e., light-to-heat converter), essentially any dye or pigment may be used, providing it absorbs efficiently at the output wavelength of the intended laser imaging source. Preference is given to infrared-absorbing dyes which are soluble in the binder, and in particular dyes which are bleachable by reaction with nucleophiles, such as primary or secondary amines, contained in the image receiving layer. The preferred class of infrared dyes is that of the tetraarylpolymethine (TAPM) dyes, as disclosed in EP-A-0675003. Such dyes may be represented by the formula:

in which Ar¹ to Ar⁴ are aryl groups which may be the same or different, and X is an anion.

Preferably, from one to three of the said aryl groups bear a tertiary amino substituent, preferably in the 4-position. Most preferably, at least one but no more than two of said aryl groups bear a tertiary amino substituent. Preferably, Ar¹ or Ar² and Ar³ or Ar⁴ bear the tertiary amino substituent. Examples of tertiary amino groups include dialkylamino groups (such as dimethylamino, diethylamino, etc.), diarylamino groups (such as diphenylamino), alkylarylamino groups (such as N-methylanilino), and heterocyclic groups such as pyrrolidino, morpholino or piperidino. The tertiary amino group may form part of a fused ring system.

The aryl groups represented by Ar¹ to Ar⁴ may comprise phenyl, naphthyl, or fused ring systems, but phenyl rings are preferred. In addition to the tertiary amino groups discussed previously, substituent which may be present on the rings include alkyl groups (preferably of up to 10 carbon atoms), halogen atoms (such as Cl, Br, etc.), hydroxy groups, thioether groups and alkoxy groups. Substituents which donate electron density to the conjugated system, such as alkoxy groups, are particularly preferred. Substituents, especially alkyl groups of up to 10 carbon atoms or aryl groups of up to 10 ring atoms, may also be present on the polymethine chain. Preferably, the anion X is derived from a strong acid (e.g., HX should have a pKa of less than 3, preferably less than 1). Suitable identities for X include ClO₄, BF₄, CF₃SO₃, PF₆, AsF₆, SbF₆, and perfluoroethylcyclo-hexylsulphonate. Preferred dyes of this class include:

Dl

D2

D3

The relevant dyes may be synthesized by known methods, e.g., by conversion of the appropriate benzophenones to the corresponding 1,1- diarylethylenes (by the Wittig reaction, for example), followed by reaction with a trialkyl orthoester in the presence of strong acid HX.

The infrared absorber should be present in sufficient quantity to provide a transmission optical density of at least 0.75, preferably at least 1.0. Essentially any colorant may be incorporated in the color transfer layer providing the colorant will not sublime under imaging conditions. Suitable colorants include soluble or insoluble dyes, dispersions of pigment particles, or mixtures of both dyes and pigments, but pigment dispersions are preferred. Pigments or mixtures of pigments may be employed so as to impart a particular color to the transfer layer, or to confer particular properties thereto such as magnetic properties, pearlescence, opalescence, fluorescence, etc. Blends of pigments as commonly used in the proofing industry and in printing inks are particularly preferred (preferably matching the color references provided by the International Prepress Proofing Association, known as the SWOP color references), and are most preferably used in conjunction with a dispersant, such as DISPERBYK-161, available from BYK-Chemie.

In addition, the color transfer layer advantageously may comprise a latent curing agent as disclosed in International Publication No. WO 98/07575. Preferred latent curing agents satisfy the formula:

wherein R¹ is H, an alkyl group, a cycloalkyl group, or an aryl group; each R² is independently an alkyl group or an aryl group; each R³ is independently an alkyl group or an aryl group; and R⁴ is an aryl group.

Such compounds are believed to be oxidized during laser exposure to the corresponding pyridinium salts, which can undergo transesterification reactions with hydroxy-functional resins, leading to crosslinking. The process is facilitated by the use, as laser absorber, of cationic dyes such as the above- described TAPM dyes. Preferred latent curing agents include:

H Ph

In preferred embodiments of the invention, the color transfer layer comprises a fluorochemical additive in addition to a dispersion of pigment particles, as disclosed in EP-A-0602893. The use of such an additive in an amount corresponding to at least one part by weight per 20 parts by weight of pigment, preferably at least one part per 10 parts pigment, provides much improved resolution and sensitivity in the laser thermal transfer process. Preferred fluorochemical additives comprise a perfluoroalkyl chain of at least six carbon atoms attached to a polar group such as carboxylic acid, ester, sulphonamide, etc. Minor amounts of other ingredients may optionally be present in the colorant transfer layer, such as surfactants, coating aids, etc., in accordance with known techniques.

Color transfer layers suitable for use in the invention are formed as a coating on a support. The support may be any sheet-form material of suitable thermal and dimensional stability, and for most applications should be transparent to the exposing laser radiation. Polyester film base, of about 20 μm to about 200 μm thickness, is most commonly used, and if necessary may be surface-treated so as to modify its wettability and adhesion to subsequently applied coatings. Such surface treatments include corona discharge treatment, and the application of subbing layers or release layers. The relative proportions of the components of the color transfer layer may vary widely, depending on the particular choice of ingredients and the type of imaging required. Preferred pigmented color transfer layers for use in the invention have the following approximate composition (in which all percentages are by weight): Hydroxy-functional film-forming 35 to 65% resin (e.g., BUTVAR B76)

Latent curing agent (e.g., Ci, C₂ or C₃) up to 30%

Infrared dye (e.g., Di or D ) 3 to 20%

Pigment 10 to 40%

Pigment dispersant 1 to 6% (e.g., DISPERBYK 161)

Fluorochemical additive (e.g., a perfluoroalkylsulphonamide) 1 to 10%

Thin coatings (e.g., of less than about 3 μm dry thickness) of the above formulation may be transferred to the receptor sheets by laser irradiation in accordance with the invention. Transfer occurs with high sensitivity and resolution. Heating the transferred image for relatively short periods (e.g., one minute or more) at temperatures in excess of about 120°C causes curing and hardening, as well as bleaching of the infrared dye. Hence, an image of enhanced durability, uncontaminated by unwanted absorptions is obtained.

Color donor elements for use in the invention are readily prepared by dissolving or dispersing the various components in a suitable organic solvent and coating the mixture on a film base. Pigmented transfer media are most conveniently prepared by predispersing the pigment in the binder resin in roughly equal proportions by weight in the presence of a suitable dispersing aid, in accordance with standard procedures used in the color proofing industry, thereby providing pigment "chips." Milling the chips with solvent provides a miUbase, to which further resin, solvents, etc., are added as required to give the final coating formulation. Any of the standard coating methods may be employed, such as roller coating, knife coating, gravure coating, bar coating, etc., followed by drying at moderately elevated temperatures.

The procedure for imagewise transfer of color layer from donor to receptor is conventional. The two elements are assembled in intimate face-to- face contact, e.g., by vacuum hold down, and scanned by a suitable laser. The assembly may be imaged by any of the commonly used lasers, depending on the absorber used, but address by near infrared emitting lasers such as diode lasers and YAG lasers, is preferred. Any of the known scanning devices may be used, e.g., flat-bed scanners, external drum scanners or internal drum scanners. In these devices, the assembly to be imaged is secured to the drum or bed, e.g., by vacuum hold-down, and the laser beam is focused to a spot, e.g., of about 5 μm to about 50 μm diameter, on the color transfer layer of the donor/receptor assembly. This spot is scanned over the entire area to be- imaged while the laser output is modulated in accordance with electronically stored image information. Two or more lasers may scan different areas of the donor/receptor assembly simultaneously, and if necessary, the output of two or more lasers may be combined optically into a single spot of higher intensity. Laser address is normally from the donor side, but may be from the receptor side if the receptor is transparent to the laser radiation.

When curing and/or bleaching agents are incorporated in the color transfer layer and/or image receiving layer, the image residing on the receptor after peeling the donor sheet from the receptor may be further cured and/or bleached by subjecting it to heat treatment, preferably at temperatures in excess of about 120°C. This may be carried out by a variety of means, such as storage in an oven, hot air treatment, contact with a heated platen or passage through a heated roller device. In the case of multicolor imaging, where two or more monochrome images are transferred sequentially to a common receptor, it is more convenient to delay the heating step until all the separate colorant transfer steps have been completed, then provide a single heat treatment for the composite image. However, if the individual transferred images are particularly soft or easily damaged in their uncured state, then it may be necessary to cure and harden each monochrome image prior to transfer of the next. In preferred embodiments of the invention, this is not necessary.

In some situations, the receptor element of the invention, to which a color image is initially transferred, is not the final substrate on which the image is viewed. For example, U.S. Pat. No. 5,126,760 discloses thermal transfer of a multicolor image to a first receptor, with subsequent transfer of the composite image to a second receptor for viewing purposes. If this technique is employed in the practice of the present invention, curing and/or bleaching of the image may conveniently be accomplished in the course of the transfer to the second receptor. In this embodiment of the invention, the second receptor may be a flexible sheet-form material such as paper, card, plastic film, etc., and transfer is most readily effected by means of a heated roller laminating device such as a MATCHPRINT laminator (Imation Corp., Oakdale, MN). The support sheet of the first receptor element is then peeled away and discarded, the peeling process being facilitated when a release layer is present between the support sheet and image receiving layer.

Advantages of the invention are illustrated by the following examples. However, the particular materials and amounts thereof recited in these examples, as well as other conditions and details, are to be interpreted to apply broadly in the art and should not be construed to unduly limit the invention.

EXAMPLES The invention will now be illustrated by the following Examples in which the following abbreviations, tradenames etc. are used:

BUTVARB-76 polyvinyl butyral resin supplied by Monsanto, with free hydroxyl content of 7 mole% to 13 mole% DISPERBYK 161 dispersing agent supplied by BYK-Chemie, Wallingford, CT

KF-393 amino-silicone release agent supplied by Shin- Etsu Chemical Co., LTD., Tokyo, Japan

MEK methyl ethyl ketone (butan-2-one) PET polyethyleneterephthalate film, DuPont Films, Wilmington, DE

SCHOELLER color proofing base supplied by Schoeller, (Pulaski, NY), 170M comprising silica particles (4 μm to 10 μm diameter) in a resin coating on paper

VIKING grained and anodized aluminum base printing plate base, obtained by removing the photosensitive coating from VIKING printing plates supplied by Imation Corp., St. Paul, MN

KODAK receptor sheet supplied by Kodak Polychrome Graphics, Norwalk, CT, as APPROVAL base part of the APPROVAL proofing system, Kodak Polychrome Graphics, Norwalk, CT

FC-55/35/10 fluorochemical surfactant supplied by the 3M

Company, St. Paul, MN

Example 1 This Example demonstrates the effect on image quality of varying the surface topography of the image receiving layer.

The color donor sheet used in this Example comprised the following, as a layer on PET base of approximately 1 μm dry thickness in which all percentages are by weight: Magenta pigment 23.2%

BUTVARB-76 48.6%

JRDye Dl 9.0%

Curing Agent Cl 15.2%

N-ethylperfluorooctylsulfonamide 4.0%

Samples of the donor sheet were mounted face-to-face with samples of various receptor sheets with vacuum hold down on an exposure test bed comprising a fibre-coupled laser diode (500 mW, 870 nm) focused to a 30 μm spot. A halftone dot pattern was imaged onto each receptor under identical conditions of laser power and scan rate, and the quality of each of the transferred images assessed both microscopically (for dot quality) and visually (for overall appearance). The following receptor sheets were tested:

(a) Kodak Polychrome Graphics APPROVAL receptor

(b) an ink jet receptor comprising a coating on paper of starch particles (approximately 500/mm², at least 10 μm diameter)

(c) Schoeller 170M base (d) a coating on vesicular polyester of silica particles (4 to 10 micrometers diameter, approximately 1500/mm²) in BUTVAR B76 polyvinyl butyral

(e) VIKING printing plate base (f) a smooth coating on paper of BUTVAR B76 polyvinyl butyral

(g) 0.7 mil (18 μ) Aluminum foil laminated to 5 mil (127 μ) PET

The results obtained are summarized in the following table:

TABLE 1

Receptors (a) and (b) gave diffuse images with poor color saturation, whereas receptors (c) to (g) all gave sharp images with bright, saturated color. Mcroscopic examination revealed that the dots transferred to receptors (a) and (b) had fragmented during the transfer process, with pigment scattered over a wide area, whereas the dots transferred to the other receptors were in the form of coherent films. The dots on receptors (c) and (d) showed some edge distortion, but those on receptors (e), (f), and (g) had sharp edges. However, the image on receptors (f) and (g) suffered from "dropouts" caused by dust particles, whereas none of the other images suffered from this defect. Thus, it was concluded that a roughened receptor showing protrusions of on average of less than 10 μm above the plane of the coating, and providing on average at least 1 point of contact per pixel between donor and receptor, is desirable for good quality images, free from dust artifacts. Receptor (e) illustrates the trend for improved image quality as the surface protrusions of the image receiving layer become smaller and more numerous.

Example 2 Cyan, magenta, yellow and black (CMYK) donor sheets were prepared as in Example 1 with weight percentages of components listed in Table 2 in the thermofusible color transfer layer coated at about 1 μm to SWOP specifications for web off-set printing.

Exposure using Presstek PEARLSETTER 74 running at various scan rates (100 to 500 cm/second) and laser power of 500 mW, 30 micrometer, 870 nm, transfer was effected in the order C, M, Y, K to Schoeller 170M base, the donor/receptor being held in tension together. Blocks of color (10 x 20 mm²) were imaged over a range of scan speeds (100 to 500 cm/second). A second set from a different color were directly overprinted the first at same scan speed.

Successful overprint of C, M, Y, K was achieved with no defects observable over an A2 imaging area, over all scanning speed (100 to 500 cm/second). Millbases:

Red Shade Cyan Mllbase

Red Shade Cyan Pigment 7.77 g

BUTVAR B76 7.77 g

DISPERSBYK 161 0.47 g

MEK 42.0 g

1 -methoxy-2-propanol 42.0 g

Phthalo Green MiUbase

Phthalo Green Pigment 7.86 g

BUTVARB76 7.86 g

DISPERSBYK 161 0.47 g

MEK 41.9 g

1 -methoxy-2-propanol 41.9 g

i Red Shade Magenta Mllbase

Red Shade Magenta Pigment 7.78 g

BUTVARB76 7.78 g

DISPERSBYK 161 0.93 g

MEK 41.8 g

1 -methoxy-2-propanol 41.8 g

Blue Shade Magenta Mllbase

Blue Shade Magenta Pigment 7.36 g

BUTVAR B76 7.36 g

DISPERSBYK 161 0.88 g

MEK 42.2 g

1 -methoxy-2-propanol 42.2 g

Black MiUbase

Carbon Black Pigment 9.88 g

BUTVARB76 9.88 g

DISPERSBYK 161 1.03 g

MEK 39.6 g

1 -Methoxy-2-propanol 39.6 g

Green Shade Yellow Mllbase

Green Shade Yellow Pigment 7.28 g

BUTVARB76 7.28 g

DISPERSBYK 161 0.44 g

MEK 42.5 g

1 -methoxy-2-propanol 42.5 g

Red Shade Yellow Mllbase Red Shade Yellow Pigment 7.28 g

BUTVAR B76 7.28 g

DISPERSBYK 161 0.44 g

MEK 42.5 g

1 -methoxy-2-propanol 42.5 g

TABLE 2

Example 3

A receptor was prepared by coating the following formulation from methylethyl ketone (18 wt%) onto 100 micron PET base to provide a dry- coating weight of 400 mg/ft² (4.3 g/m²): PLIOLITE S5A 87 wt%

8 μm Poly(stearyl methacrylate) beads 1 wt%

Diphenylguanidine 12 wt%

The receptor was imaged under the conditions of Example 2 using the cyan, magenta, yellow and black donor sheets. The resulting image was transferred to opaque MATCHPRINT Low Gain base under heat and pressure by passing the receptor and base in contact through a MATCHPRINT laminator. The sheets were peeled apart and the transferred image inspected. The quality of the transferred image was excellent, having good color rendition with no contamination from the IR dye. No dust artefacts were apparent.

Example 4

The following example illustrates the improvement realized by incorporating a thermal absorbing layer into the receptor construction.

A receptor was prepared by coating the following formulation with a #7 Meyer rod (16 μm wet coating thickness) onto the substrates listed in Table 3 and dried at 100°C for 1 minute.

6 μm polystrylmethacrylate beads 0.60 g MEK 38.5 g

Diphenylguanidine 0.57 g

PLIOLITE S5-A (30% T.S. in toluene) 10.76 g KF-393 Amino-silicone 0.14 g

The receptors listed in Table 3 were evaluated by imaging the color donors described in Example 2 with a Creo TRENDSETTER 3244 with 9 Watts of power across 208 pixels each 8 μm x 8 μm in size for imaging at 3200 dpi (dots per inch). Exposure values were varied by adjusting drum speed between 35 revolutions per minute (rpm) and 125 rpm. The color donor and receptor were held in place with a vacuum contact method.

TABLE 3

Samples A and E showed significant distortion of the solid areas in the center portion of the exposure swath. Samples B, D, F, G, and H showed virtually no distortion within an exposure swath (exposure area) and the sensitivity showed a 35% improvement. Sample C showed similar results to B, D, F, G, and H except mottle from the uneven aluminum surface due to the paper substrate was observed.

Example 5

The following example illustrates the use of incorporating a thermal absorbing layer into the receptor formulation followed by imaging and subsequent transfer to an opaque paper sheet. A receptor formulation was prepared by blending the following ingredients:

Elvacite 2021 12.45 grams

Diphenylguanidine 2.25 grams 8 micron PSMA beads 0.15 grams

7.5% FC 55/35/10 2.0 grams

Methyl ethyl ketone 83.15 grams

The receptor formulation was coated onto the aluminum surface of an aluminum laminate (127 micron PET + 18 micron aluminum) using a #12 Meyer rod and dried at 160°F for 105 seconds. The dry coating weight was 2.55 g/m². The receptor sheet was imaged using the CMYK color donors described in Example 2 with a Creo TRENDSETTER 3244 delivering 9 watts of power across 208 pixels each 8 microns x 8 microns in size for imaging at 3200 dpi. Exposure values were optimized by adjusting drum speed between 35 and 125 rpm. The resultant 4-color image was transferred under heat and pressure to opaque MATCHPRINT Low Gain base primed with an adhesive layer by passing the receptor and base in contact through a MATCHPRINT laminator. The receptor sheet was removed and the image inspected. The quality of the transferred image was excellent having good color rendition, no dust artifacts and no distortion within an exposure swath.

Example 6

The following example illustrates the use of a release layer coated onto the thermal absorbing layer followed by imaging and subsequent transfer to an opaque paper sheet.

A priming layer was prepared by blending the following components:

33% NeoRez R-960 (a water borne 149.6 grams urethane supplied by ZENECA Resins, Wilmington, MA)

62.5% TRITON GR-7M (supplied by Union 1.0 grams Carbide Chemicals & Plastics Technology Corporation, Danbury, CT)

Deionized Water 682.7 grams

This solution was applied to the aluminum side of an aluminum laminate (127 micron PET + 18 micron aluminum) and dried for 3 minutes at 150°F. The preferred dried coating weight is 0.32-0.43 g/m². The primed receptor sheet was then overcoated with the following formulation:

Pliolite S5A 11.7 grams

Diphenylguanidine 1.5 grams sec-Butyldiphenylguanidine 1.5 grams

6 micron PSMA beads 0.15 grams 7.5% FC 55/35/10 2.0 grams

Toluene 49.89 grams

Methyl ethyl ketone 33.26 grams

This coating was applied and dried at 150°F to yield a dry coating weight of 2.96 g/m². In a separate experiment, an unprimed aluminum laminate receptor sheet was overcoated with the above PLIOLITE S5A formulation under the same conditions.

Both receptor sheets were imaged using the CMYK color donors described in Example 2 on the Creo TRENDSETTER 3244. The images had good color rendition, no dust artifacts and no distortion within an exposure swath.

Both images were then laminated to opaque MATCHPRINT Low Gain base under heat and pressure using a MATCHPRINT laminator. The receptor sheets were peeled away and both images examined. The aluminum laminate coated with the NeoRez priming solution transferred 100% of its image to the MATCHPRINT Low Gain base giving good color rendition, no dust artifacts and no swathing distortion while the unprimed laminate produced an incomplete transfer resulting in a very poor image on the MATCHPRINT Low Gain base.

Claims

What is claimed is:

1. A receptor comprising in order:

(a) a support; (b) a thermal absorbing layer having a thickness of at least about 7.5 μm which is capable of dissipating heat from a laser; and (c) a releasable image receiving layer having a coating weight of about 0.54 g/m² to about 4.3 g/m² and having a textured outer surface comprising a plurality of protrusions projecting above the plane of the outer surface of the receiving layer by an average distance of no greater than about 8 μm and having on average at least 1 of said plurality of protrusions per pixel.

2. A method of half tone imaging comprising: (a) providing a receptor according to Claim 1;

(e) peeling apart the receptor and color donor.

3. The method of Claim 2 further comprising repeating steps (b) to (e) one or more times using a different colorant but using the same receptor sheet.

4. The method of Claim 2 wherein the resulting image-bearing receptor sheet is subjected to a lamination process wherein the image receiving layer together with the image residing therein is transferred to another support.

5. The method of Claim 2 wherein the laser radiation is focused to a spot having a size of about 5 μm to about 50 μm.

6. The method of Claim 2 wherein the protrusions project above the plane of the outer surface of the receptor by about 2 μm to about 6 μm.

7. The method of Claim 2 wherein the protrusions are formed by the presence of inert particles or by embossing a image receiving layer.

8. The method of Claim 2 wherein the image receiving layer further comprises a compound capable of bleaching the radiation absorber of the donor sheet.

9. The method of Claim 2 wherein the color donor comprises a hydroxy-functional film-forming resin.

10. The method of Claim 2 wherein the color donor further comprises a latent curing agent of the formula: