DE69215904T2 - Thermal image transfer method - Google Patents

Description

The present invention relates to a method of thermal image transfer in which a scanning exposure source, such as a laser, is used to effect the thermal transfer of colorant from a donor sheet to a receptor for the thermally transferred colorant.

Thermal image transfer involves the image-wise transfer of dye from a donor sheet to a receptor sheet under the action of heat, with the surfaces of the donor and receptor sheets kept in constant, intimate contact. This type of imaging is growing in popularity, mainly because it is "dry" (requiring no chemical development) and therefore compatible with the home or office environment.

The heat required to transfer the colorant is usually supplied by bringing the unitary (but not bonded) donor and receptor sheets into contact with so-called "thermal printheads" comprising rows of miniature electrically heated elements, each of which can be activated in a timed sequence, thereby providing the desired imagewise heat pattern. However, such systems provide rather poor resolution, so there is increasing interest in using radiant or projected energy, particularly infrared radiation, to supply heat, taking advantage of the greater commercial availability of near-infrared emitting laser diodes. This is achieved by enclosing a radiation-absorbing material in one of the donor and receptor sheets, usually the former, and exposing the unitary sheets to an imagewise radiation pattern. When the assembled sheets are irradiated with radiation of an appropriate wavelength, the radiation absorbing material converts the incident energy into heat energy and transfers the heat to colorants in its immediate vicinity, thereby causing image-wise transfer of the colorant to the receptor.

Examples of thermal transfer media include those described in GB Patent No. 1385533, GB-A-2083726, EP-A-403932, 403933, 403934, 404042, 405219, 405296, 407744, 408891, 407907 and 408908, US Patent Nos. 3787210, 3946389, 4541830, 4602263, 4788128, 4904572, 4912083, 4942141, 4948776, 4948777, 4948778, 4950639, 4950640, 4952552 and 4973572, International Patent Publication No. PCT WO 88/04237, JP-A-21-075292 and 30-043294 and Japanese Patent Nos. 51-088016, 56-082293, 63-319191 and 63-319192.

In the majority of conventional systems using radiant or projected energy for thermal transfer of colorant, the donor sheet comprises a support carrying a donor layer containing the colorant, the colorant normally being dissolved or dispersed in a binder and the radiation absorbing material being included either in the same layer as the colorant, as disclosed in, for example, EP-A-403933, or in a separate sublayer between the support and the donor layer, as disclosed in, for example, Japanese Patent Publication No. 63-319191. The donor sheet may be of the diffusion transfer type (sometimes referred to as a "sublimation transfer" material), where the colorant is transferred to the receptor in an amount proportional to the intensity of the absorbed radiation, or of the mass transfer type, where either 0% (zero) or 100% transfer of colorant occurs depending on the attainment of a threshold value for the absorbed energy. In mass transfer materials, both the colorant and the binder are transferred to the receptor sheet.

Two different methods are known in which radiation is used for thermal transfer of a dye. In the first method, a laser scans directly over the unit of donor and receptor sheets while its intensity is modulated according to the digitally stored image information. This method is disclosed, for example, in Research Disclosure No. 142223 (February 1976), Japanese Patent Publication No. 51-88016, US Patent Publication No. 4973572, GB Patent Publication No. 1433025 and GB-A-2083726.

The second method involves exposure by flooding with a short exposure source, such as a xenon flash lamp, through a suitable mask held in contact with the unit of donor and receptor sheets. This method is disclosed, for example, in Research Disclosure No. 142223 (February 1976), U.S. Patent Nos. 3,828,359, 4,123,309, 4,123,578 and 4,157,412 and EP-A-365,222.

U.S. Patent Nos. 4,123,309, 4,123,578 and 4,147,412 disclose a composite release liner for use in the manufacture of fine art prints and the like comprising (a) a receiving tape having a layer of a latent adhesive material and (b) a transfer tape having a donor web bearing a low-adhesion layer of microgranules in surface contact with the adhesive layer. At least one of the microgranule and adhesive layers bears a radiation absorbing pigment. Upon brief exposure to a radiation pattern, the pigment is selectively heated, causing adjacent portions of the adhesive layer to briefly soften and, upon solidification, adhere to the micrograins. The receiving and transferring belts are then separated, with the transferred micrograins adhering to the receiving belt only in the exposed areas.

The pigment is preferably incorporated into the microgranule-containing layer of the transfer ribbon, thereby providing a direct conduction path to the surface of the adhesive layer. The pigment also serves as a coloring material for the microgranules, with dark colored microgranules producing dark graphics. If graphics of a light color are desired, microgranules of that light color can be used in combination with a receiving ribbon comprising an accepting fabric, a pigment coating on the accepting fabric, and a thin layer of an adhesive material adhered to the pigment coating. No advantage is claimed (apart from the visibility of lightly colored graphics) to the placement of the pigment in the receptor sheet, and in fact it is said to result in a reduction in sensitivity.

A preferred source of exposure is a xenon flash lamp which produces a broad spectrum of blue-white light in one flash, the desired image-wise radiation pattern being provided by exposing the composite transfer material through a mask carrying the image information. However, there are some disadvantages associated with this method of imaging. Xenon flash lamps are rather bulky, consume a lot of energy and cause heat scattering problems; more importantly, it is very difficult in practice to obtain large-area, high-quality images using this method without damaging the mask carrying the image information. The reason for this is that under normal circumstances the opaque areas of the mask itself absorb and, due to the irradiation of the entire area of the mask, a large amount of energy is absorbed by the mask without the energy being able to be quickly dissipated. As a result, high temperatures are generated in the mask, leading to melting or deformation. Since the absorbed energy is proportional to the exposed area, the problem becomes more critical for larger images.

In addition, since a xenon lamp emits a wide bandwidth, the use of a xenon flash exposure generally requires the use of carbon black and other materials with a similarly broad absorption as radiation absorbers in order to effectively use the available energy. However, the current trend is to replace carbon black with infrared absorbing dyes, both in the pursuit of higher resolution and to reduce the possibility of image contamination by the radiation absorber, as disclosed e.g. in EP-A-3 12923, 403930, 403931, 403932, 403933, 403934, 404042, 405219, 405296, 407744, 408891, 408907 and 408908. Because of the relatively narrow absorption band of dyes, higher intensity xenon flashes are required, which further exacerbates the heat deformation problems described above.

Japanese Patent Publication No. 3-043294 discloses the use of an infrared radiation absorbing material in a separate sheet held in surface contact with a heat sensitive medium, but no thermal transfer as described therein is disclosed.

JP-A4153087 discloses image-receptive materials for use in thermal sublimation transfer or thermal melt transfer, wherein a light-heat conversion layer and a thermal image-receptive layer are on an image-receptive support.

Thermal transfer donor sheets comprising a layer of an organic or inorganic colorant deposited from the gas phase on a controlled release layer are disclosed in U.S. Patent Application Serial Nos. 07/775,782 (US-A-5139598) and 07/776,602, respectively, filed October 11, 1991. Only imaging with thermal printheads is reported in connection with these materials.

US Patent Nos. 4,599,298 and 4,657,840 disclose radiation-sensitive imaging materials comprising (in sequence): (i) a support, (ii) a vapor-deposited colorant layer, and (iii) a vapor-deposited layer of a metal, metal oxide or metal sulfide. Layer (iii) can be image-wise ablated using a laser and the exposed areas of layer (ii) can be transferred to a receptor by the application of heat, e.g. by direct contact with a hot plate or roller.

EP-A-125086 discloses photoresist elements comprising (in sequence): (i) a support, (ii) a vapor-deposited colorant layer, and (iii) a cover layer of a photoresist. The imagewise exposed elements are subjected to a development step to remove the resist layer in either the exposed or unexposed areas of the element, depending on the positive or negative effect of the resist material, and uniformly heated, e.g. by direct contact with a hot plate, to effect selective transfer of the colorant. A receptor may accept the transferred colorant, or the element from which the dye has been selectively removed may be used as the final image. The colorant may also be transferred without development if the permeability of the resist layer to the colorant is altered by the exposure.

The aim of the present invention is to provide alternative thermal transfer methods and thermal transfer materials.

In one aspect of the present invention, there is provided a method for thermal image transfer comprising the following steps:

(a) bringing into contact a receptor sheet and a donor sheet comprising a donor layer comprising a thermally transferable colorant such that the donor layer is in intimate contact with the receptor sheet, one of the donor and receptor sheets comprising a radiation absorbing material capable of absorbing radiation from an exposure source such that imagewise exposure of the sheets in contact causes heating in the exposed areas, the heating causing imagewise thermal transfer of colorant from the donor sheet to the receptor sheet, and

(b) imagewise exposing the contacting donor and receptor sheets using a scanning exposure source, wherein the donor layer of the donor sheet comprises a layer of a vapor-deposited colorant.

In the process of the invention, a scanning exposure source, such as a laser, is used to effect the thermal transfer of colorant from a donor sheet to a receptor sheet. The term "colorant" is used herein in its broadest sense and includes any material capable of altering the surface of the receptor, whether or not the alteration is visible to the naked eye.

In one aspect of the present invention, the radiation absorbing material is included in the receptor sheet. The inclusion of the radiation absorbing material in the receptor sheet offers significant advantages over conventional thermal transfer materials in which the radiation absorbing material is present in the donor sheet in terms of both higher resolution and higher sensitivity since the heat effect is induced directly in the receptor. In a preferred embodiment, the receptor sheet includes a receptor layer for thermally transferred colorant, with the radiation absorbing material contained in the receptor layer, or more preferably in a conventional adjacent sublayer thereto.

The donor sheets may be of the diffusion transfer type (sublimation transfer type) where dye is transferred to the receptor sheet in an amount proportional to the intensity of absorbed energy (yielding a continuous tone image) but are preferably of the mass transfer type where either 0% (zero) or 100% transfer of dye occurs depending on the achievement of a threshold value for absorbed energy.

Mass transfer donor sheets have several advantages, such as providing matching positive and negative images (on the donor and receptor sheets respectively), rich colors and the ability to print large areas with a uniform optical density and are well suited for halftone imaging. However, poor resolution and high power requirements have hindered their use in conventional thermal image transfer systems. The method of the present invention can produce mass transfer images with unexpectedly high resolution and low power requirements.

The mass transfer materials comprise a support having a vapor-deposited colorant layer, preferably separated by a controlled release layer, as disclosed in U.S. Patent Application Serial Nos. 07/775,782 (US-A-5,139,598) and 07/776,602, filed October 11, 1991. These donor sheets are found to provide images with high resolution, good color saturation, high transparency, and uniform optical density.

In another aspect of the invention there is provided a thermal transfer medium comprising a donor sheet having a donor layer comprising a vapor-deposited thermally transferable colorant and a receptor sheet comprising a radiation absorbing material. The donor support may optionally have a controlled release layer (described below) onto which the colorant is vapor-deposited.

The radiation absorbing material may be contained in a separate, dedicated layer (hereinafter referred to as "radiation absorbing layer"), e.g. in an underlayer to the vapor-deposited colorant layer in the donor sheet, or in each of the receptor layers in the receptor sheet.

Alternatively, the radiation absorbing material may be included in one of the other layers that are part of the donor or receptor sheets, e.g., the receptor layer of the receptor sheet. If the colorant itself absorbs radiation so that it is considered a radiation absorbing material, no other radiation absorbing material is required.

The radiation absorbing material, which typically absorbs radiation in the wavelength range 600 to 1070 nm, usually in the range 750 to 980 nm, may comprise any suitable material capable of absorbing the radiant energy of the exposure source, converting it into heat energy and transferring that energy to the colorant in its immediate vicinity. Examples of suitable radiation absorbing materials include pigments such as carbon black as disclosed in GB Patent No. 2083726 and dyes including phthalocyanine dyes as disclosed in US Patent No. 4942141; iron complexes as disclosed in US Patent No. 4912083; squarylium dyes as disclosed in US Patent No. 4942141; Chalcogenpyrylo-arylidene dyes, such as disclosed in US Patent No. 4948776; bis(chalcogenpyrylo)polymethine dyes, such as disclosed in US Patent No. 4948777 disclosed; oxyindolizine dyes, for example as disclosed in US Patent No. 4,948,778; bis(aminoaryl)polymethine dyes, for example as disclosed in US Patent No. 4,950,639; merocyanine dyes, for example as disclosed in US Patent No. 4,950,640; tetraarylpolymethine dyes; dyes derived from anthraquinones and naphthoquinones, for example as disclosed in US Patent No. 4,952,552; cyanine dyes, for example as disclosed in US Patent No. 4,973,572; trinuclear cyanine dyes, for example as disclosed in EP-A-403,933; oxonol dyes, for example as disclosed in EP-A-403,934; indene-bridged polymethine dyes, for example as disclosed in EP-A-407,744; nickel dithiolane dye complexes as disclosed in EP-A-408908; and croconium dyes as disclosed in EP-A-568267.

The radiation absorbing material is preferably present in a sufficient amount and distribution so that absorption of the exposing radiation by the material generates sufficient heat locally to enable transfer of colorant from the donor sheet to the receptor sheet. The amount of radiation absorbing material required for effective dye transfer will vary widely depending on the type of material used, etc., but is preferably present in an amount sufficient to provide an optical transmission density of at least 1.0 extinction units, more preferably at least 1.5 extinction units, at the wavelength of the exposing radiation.

The radiation absorbing layer usually comprises a binder layer in which the radiation absorbing material is dissolved or dispersed. If possible, the binder of the radiation absorbing layer may comprise any of a range of suitable materials including poly(vinyl acetals), such as poly(vinyl formal) and poly(vinyl butyral); polycarbonates; poly(styrene acrylonitrile); polysulfones; poly(phenylene oxide); poly(vinylidene chloride-vinyl acetate) copolymers, and mixtures thereof, although binders having a glass transition temperature (Tg) above 100°C are preferred to ensure that the colorant adheres to the receptor sheet/layer and not to the radiation absorbing layer during thermal transfer.

When the radiation absorbing layer comprises a mixture of a dye or pigment and a binder, it is normally applied as a solution or dispersion in a suitable solvent, e.g. lower alcohols, ketones, esters, chlorinated hydrocarbons and mixtures thereof. Any of the known solvent coating processes such as knife coating, roll coating, wire wound rod coating, etc. may be used. The thickness of the radiation absorbing layer must be sufficient to provide the required optical density and will depend on factors such as the extinction coefficient of the dye used or pigment and its solubility in the binder. Relatively thin layers (eg up to 5 µm dry thickness) are preferred.

Alternatively, the radiation absorbing layer may comprise a continuous layer of a solid radiation absorbing pigment or dye without a binder. A particularly suitable pigment in this context is "black alumina", which is a graded mixture of aluminum and aluminum oxide. Layers of this material can be produced by vapor deposition of aluminum metal in the presence of metered amounts of oxygen, as disclosed in U.S. Patent Nos. 4,430,366 and 4,364,995. Very thin (< 1 µm) coatings of this material exhibit high optical density over a wide range of wavelengths, covering the visible and infrared ranges, ensuring compatibility with a wide range of illumination sources.

Receptor sheets for thermally transferred colorants typically comprise a support sheet coated on at least one of its major surfaces with a receptor layer, usually comprising a heat-softenable (low Tg), usually thermoplastic, binder; however, if the radiation absorbing material is present in the receptor layer, the binder may require a higher Tg, typically 100°C or higher. Ideally, the binder should soften during the imaging process to an extent sufficient to transfer the colorant, but not so great as to cause melting, lateral flow or transfer to the donor sheet. This problem is more likely to occur if the radiation absorbing material is present in the receptor layer. In these circumstances, the choice of binder is largely determined by the type of donor sheet used. For example, if the donor sheet comprises a layer of a vapor-deposited dye or pigment, low Tg receptor layers (containing the radiation-absorbing material) are unsuitable for the reasons outlined above, while high Tg layers give good results.

When the radiation absorbing material is present in a separate underlayer, that is, a layer between the support and the normally adjacent receptor layer, it is preferably coated in a high Tg binder, preferably having a Tg of over 90°C, with the receptor (top)layer comprising a lower Tg material, e.g. having a Tg of 40 to 90°C. Preferred high Tg binders include polyesters and polycarbonates, e.g. bisphenol-A polycarbonate.

The receptor layer can be, for example, a polycarbonate, a polyurethane, a polyester, a poly(vinyl chloride), poly(styreneacrylonitrile), poly(ethyleneacrylic acid), poly(caprolactone), Poly(vinylidene chloride-vinyl acetate) or a mixture thereof. The receptor layer may be present in any amount suitable for the intended purpose.

If the desired image is the image transferred to the receptor sheet, a radiation absorbing material present in the receptor sheet is preferably colorless to the human eye or photobleachable to avoid "discoloration" of the image. If the final image is the image remaining on the donor, or if the image on the receptor is subsequently transferred to a second receptor, such considerations are immaterial. Examples of radiation absorbers with reduced discoloration properties include phthalocyanines (as disclosed, for example, in U.S. Patent No. 4,788,128), nickel dithiolene complexes (as disclosed, for example, in EP-A-408,908), and croconium dyes (as disclosed, for example, in EP-A-568,267).

If the desired image is the image transferred to the receptor sheet, the receptor layer may be separable from the layer containing the radiation absorbing material following image formation.

The receptor support may be made of any material to which an image-receptive layer can adhere, including smooth or rough, transparent or opaque, flexible or rigid, and continuous or sheet-like materials. The material should be able to withstand the heat required to transfer the colorant without decomposition or deformation. Of course, at least one of the donor and receptor sheets must be transparent to the exposing radiation to allow irradiation of the radiation-absorbing material, the support material being chosen accordingly. Suitable support materials are known in the art, representative examples of which include polyesters, particularly poly(ethylene terephthalate) and poly(ethylene naphthalate); polysulfones; polyolefins such as poly(ethylene), poly(propylene) and poly(styrene); polycarbonates; polyimides; polyamides; cellulose esters such as cellulose acetate and cellulose butyrate; Poly(vinyl chloride) and derivatives thereof, but are not limited to them. A preferred support material is white filled or transparent poly(ethylene terephthalate) or opaque paper. The support may also be reflective, such as baryta paper, ivory paper, capacitor paper or synthetic paper. The support generally has a thickness of 0.05 mm to 5 mm, preferably 0.05 mm to 1 mm.

The receptor support (and if appropriate the donor support) may contain fillers such as carbon black, titanium dioxide, zinc oxide and dyes and may be treated or coated with materials generally used in film formation such as coating aids, lubricants, antioxidants, ultraviolet radiation absorbers, surfactants and catalysts.

According to the present invention, the donor sheet comprises a layer of a vapor-deposited colorant, and either the colorant itself is the radiation-absorbing material so that it is transferred without aid upon irradiation of the unit of donor and receptor sheets, or the donor sheet further comprises a radiation-absorbing material in a separate, usually adjacent, sublayer to the colorant layer, or the receptor comprises a radiation-absorbing material.

The use of a vapor-deposited dye-donor layer offers significant advantages over conventional thermal transfer donor materials in which the dye is dissolved or dispersed in a binder, both in terms of higher resolution and higher sensitivity (speed). A vapor-deposited dye is free of contamination from binder materials and produces a cleaner, more intense image on the receptor sheet. The transferred sheet also exhibits a highly uniform optical density, even when transferring large areas.

Colorants of any chemical nature that can be deposited from the gas phase, i.e., that do not decompose upon heating, can be used. Preferred organic colorants include (but are not limited to) copper phthalocyanine and Pigment Yellow PY17 (commercially available from Sun Chemical Corporation) and Pigment Violet PV19 (commercially available from Ciba Geigy Corporation). Preferred inorganic colorants include (but are not limited to) metals such as aluminum, copper, gold, silver, etc., and metal oxides, particularly "black aluminum oxide" as disclosed in U.S. Patent Nos. 4,430,366 and 4,364,995, which imparts a neutral black color.

The colorant layer deposited from the gas phase is preferably applied in a thickness sufficient to provide an optical transmission density of at least 0.5 extinction units, preferably at least 1.0 extinction units. The thickness of the colorant layer depends on the colorant used and the desired minimum optical density, but it can be several tens of nanometers as well as several micrometers, e.g. 10 to 1000 nm, preferably 50 to 500 nm and more preferably 100 to 400 nm. The colorant is typically pre-purified by sublimation before deposition from the gas phase.

Methods for vapor deposition of colorant layers are known in the art and include resistive heating, radio frequency sputtering, plasma deposition, chemical vapor deposition, epitaxial deposition, and electron beam deposition. Specific examples can be found in, for example, U.S. Patent Specifications Nos. 4430366, 4364995, 4587198, 4599298, and 4657840 and in U.S. patent application Serial Nos. 07/7775782 and 07/776602.

The colorant layer may be continuous or discontinuous, i.e. it may be deposited using suitable masking techniques during vapor deposition in the form of a pattern or in the form of alphanumeric characters. Preferably the colorant layer is continuous.

In many cases, the colorant layer deposited from the gas phase is found to exhibit anisotropic cohesive forces. For example, it may have a columnar microstructure (as disclosed in US-A-5139598) in which the cohesive forces acting between columns are substantially less than the cohesive forces acting within individual columns. Factors believed to affect the microstructure of the deposited layer include support temperature, deposition rate (which is a function of evaporation source temperature, source-to-support distance and support temperature), deposition angles and chamber pressure. (See, e.g., Debe and Poirier, "Effect of Gravity on Copper Phthalocyanine Thin Films III: Microstructure Comparisons of Copper Phthalocyanine Thin Films Grown in Microgravity and Unit Gravity," Thin solid Films, 186, pp. 327-347 (1990); and Zurong et al., Kexue Tongbao, Vol. 29, p. 280 (1984)). While an anisotropic microstructure is not essential to the practice of the present invention, it is strongly preferred because it is believed to contribute significantly to the resolution of the transferred image.

In the embodiment where the colorant layer itself has a suitable radiation absorption and no separate radiation absorbing material is required, the colorant layer is preferably deposited from the gas phase onto a controlled release layer present on the support of the donor sheet. Such a layer provides a controlled adhesion between the colorant and the support so that the colorant is easily transferred to the receptor sheet when required, but is sufficiently resistant to abrasion during normal handling.

Controlled release layers are particularly useful in the case of inorganic colorants such as black alumina which otherwise adhere too strongly to most commonly used donor supports and therefore require uncomfortably high irradiation intensities for transfer. Controlled release layers are described in detail in US patent applications Serial Nos. 07/775782 (US-A-5139598) and 07/776602 and may comprise, for example, mixtures of two or more polymers which differ significantly in their affinity to the donor support, or they may comprise inorganic particles such as boehmite (aluminum monohydrate) particles, hydrophobic silica particles, alumina particles, titanium dioxide particles, etc. The latter type of A controlled release layer is preferred for use with inorganic colorants, and a particularly preferred controlled release layer for use with black alumina comprises a coating of boehmite particles available as an aqueous dispersion under the trade name "CAPTAL D" from Vista Chemical Co., Houston, Texas, USA. The former type of controlled release layer is preferred for use with organic colorants.

The support of the donor sheet usually comprises a transparent support to allow irradiation of the radiation absorbing material by the exposure source. Examples of suitable support materials include polyethersulfones; polyimides such as polyimidamides and polyetherimides; polycarbonates; polyacrylates; polysulfones; cellulose esters such as ethylcellulose, cellulose acetate, cellulose acetate hydrogen phthalate, cellulose acetate butyrate, cellulose acetate propionate, cellulose triacetate, etc.; poly(vinyl alcohol-vinyl acetal) copolymers; polyesters such as poly(ethylene terephthalate), which may be biaxially stabilized, and poly(ethylene naphthalate); fluorinated polymers such as poly(vinylidene fluoride) and poly(tetrafluoroethylene hexafluoropropylene); polyvinyl resins such as poly(vinyl acetate), poly(vinyl chloride); polyethers such as poly(oxyethylene); polyacetals such as poly(vinyl butyral) and poly(vinyl formal); polyolefins such as poly(ethylene), poly(propylene) and poly(styrene); and polyamides. However, if the donor and receptor sheet assembly is exposed through the receptor sheet, the receptor support material may be opaque, contain fillers, etc., since consideration of transparency is important. The donor support may be flexible or rigid, although the former is preferred, or it may be continuous or sheet-shaped.

In another aspect of the present invention there is provided a thermal transfer donor sheet comprising, in sequence, a support, a radiation absorbing layer comprising a dye or a combination of a pigment and a binder, and a layer of a vapor-deposited thermally transferable colorant.

In use, the thermal transfer donor sheet is combined with a receptor sheet and irradiated with radiation of a wavelength suitable for the radiation absorbing layer. In the exposed areas of the donor and receptor sheet assembly, the radiation absorbing layer converts the radiant energy from the exposure source into heat energy and transfers the heat to the colorant, thereby causing imagewise transfer of colorant to the receptor sheet.

The receptor sheet typically comprises a support coated on at least one of its major surfaces with a receptor layer which typically comprises a heat-softenable (ie Tg < 100ºC), usually thermoplastic, binder, although any suitable receptor for thermally transferred dyes may be used.

Any suitable scanning exposure source may be used to thermally transfer the dye from the donor sheet to the receptor sheet, although a laser is preferred as the exposure source, the exposure source and the radiation absorbing material being selected such that the emitted radiation closely matches the wavelength of the absorption maximum of the radiation absorbing material in order to efficiently use the available energy.

A number of different types of lasers can be used for thermal transfer of colorant, including (but not limited to) gas ion lasers such as argon and krypton lasers; metal vapor lasers such as copper, gold and cadmium lasers; solid phase lasers such as ruby or YAG lasers; and diode lasers such as gallium arsenide lasers, but in practice laser diodes are preferred, which offer significant advantages in terms of their small size, low cost, stability, reliability, robustness and ease of modulation consistent with digitally stored information. In general, exposure sources emitting in the infrared range of 750 to 950 nm are preferred, although any source emitting radiation in the range of 600 to 1070 nm can be used in the practice of the present invention.

In one method, the laser scans directly across the donor and receptor sheet assembly while its intensity is modulated in accordance with digitally stored image information. This method is disclosed, for example, in Japanese Patent No. 51-088016, US Patent No. 4973572, GB Patent No. 1433025 and GB-A-2083726 and provides very good resolution.

Another method of image formation entitled "Thermal Image Transfer" is disclosed in EP-0583165 and comprises:

(a) assembling the donor and receptor sheets so that the donor layer of the donor sheet is in intimate contact with the receptor sheet;

(b) bringing a photographic mask into contact with the unit of donor and receptor sheets and

(c) exposing the donor-receptor unit through the photographic mask using a, preferably continuous, scanning exposure source so that the exposure radiation is absorbed by the radiation absorbing material in areas defined by the transparent regions of the mask and converted into thermal energy, thereby causing the thermal transfer of colorant from the donor sheet to the receptor sheet.

By appropriately adjusting the various parameters, such as laser power, spot size, scanning rate and position of the focal point, thermal image transmission is possible without Damage to the photographic mask is possible. This is because only a small area of the mask is irradiated at a time, while the remainder is available as a heat sink. The optimum exposure parameters depend on a number of variables such as the sensitivity of the medium to thermal transfer and the thermal conductivity of both the mask and the radiation absorber. The mask preferably has a thermal conductivity of at least 2 x 10⁻³ Wcm⁻¹ K⁻¹. The unit of donor and receptor sheets preferably constitutes a system sufficiently sensitive to permit thermal transfer of dye at energy levels of less than 4 J/cm².

If the colorant layer in the donor sheet is present as a discontinuous layer, e.g. as a pattern or as alphanumeric characters, simple irradiation with a continuous scanning laser is sufficient without the need for a mask.

Regardless of the response method used, the laser preferably has a power of at least 5 mW, the upper power limit depending on the properties of the mask (if one is used) and the thermal transfer medium, as well as the scanning speed and spot size. The focus of the laser is set on the radiation absorbing layer, thereby obtaining an irradiated spot of small but defined dimensions, which is scanned over the entire area to be imaged. The exposure of the unit of donor and receptor sheets can be carried out from either side, i.e. through the support of the donor sheet or through the support of the receptor sheet, provided, of course, that all the layers through which the radiation must penetrate before reaching the radiation absorbing material have a suitable transparency. In the case of exposure through a mask, the emission of the laser must be set to a narrow line via a cylindrical lens, the longer dimension of which is perpendicular to the scanning direction, which allows a larger area to be scanned in one pass. Laser scanning can be performed using any known technique, but typically involves raster scanning with consecutive adjacent or overlapping scans. Two or more lasers can be used to scan different areas of a large image simultaneously.

To ensure good resolution and effective image transfer, it is essential that the donor and receptor sheets and the mask (if one is used) are kept in close contact with each other during image formation. This is achieved by subjecting the assembly of mask (if one is used) and donor and receptor sheets to a pressure of usually at least 10 g/mm2, preferably at least 40 g/mm2 and typically about 100 g/mm2.

By repeating the imaging procedures described above with successive donor sheets of different colors, multicolor images can be formed using the same receptor.

If desired, the final image can be transferred from the original receptor to another support, such as paper or cardboard. This transfer can be carried out by conventional heat lamination techniques, such as disclosed in EP-A-454083. If the receptor support is transparent, radiation-induced transfer is also possible.

The present invention will now be described with reference to the accompanying, but non-limiting, examples in which the following resins are used as binder materials for the various layers of the donor/receptor sheets.

BIS A means bisphenol A polycarbonate of the formula:

with a glass transition temperature (Tg) of 160ºC and is commercially available from Polysciences Inc.

CAB 381-20 is cellulose acetate butyrate with a Tg of 138ºC and is commercially available from Eastman Kodak.

CAB 500 is cellulose acetate butyrate with a Tg of 96ºC and is commercially available from Eastman Kodak.

VINYLITE VYNS is poly(vinylidene chloride-vinyl acetate) copolymer with a Tg of 79ºC and is commercially available from Union Carbide.

BUTVAR B-76 means poly(vinyl butyral) resin with a Tg of 56ºC and is commercially available from Monsanto.

example 1

This example demonstrates the use of a scanning exposure source, such as a laser, to thermally transfer colorant from a donor sheet to a receptor sheet comprising a support having a receptor layer for the thermally transferred colorant, the receptor sheet further comprising a radiation absorbing material either in the receptor layer (receptor sheets 1 to 3) or in a separate sublayer between the support and the receptor layer (receptor sheets 4 to 7).

Receptor sheets 1 to 7 were prepared as follows:

Receptor sheet 1

Carrier: Poly(ethylene terephthalate)polyester (100 µm thick).

Receptor layer: A solution of VINYLITE VYNS (1.5 g) and IR dye I (0.05 g) in a mixture (10 g) of methyl ethyl ketone and toluene (1:1) was coated onto the support at a wet thickness of 37.5 µm. IR dye I

Receptor sheet 2

Carrier: As receptor sheet 1.

Receptor layer: A solution of CAB 500 (1 g) and IR dye I (0.05 g) in a mixture (10 g) of methyl ethyl ketone and toluene (1:1).

Receptor sheet 3

Carrier: As receptor sheet 1.

Receptor layer: A solution of BIS A (3 g) and IR dye I (0.1 g) in a mixture (30 g) of cyclohexanone and dichloromethane (3:2).

Receptor sheet 4

Carrier: As receptor sheet 1.

IR-absorbing layer: A mixture of BIS A (6.7 g) and IR dye I (0.05 g) in dichloromethane (53.2 g) and cyclohexanone (6.7 g) was coated onto the support to a wet thickness of 25 µm.

Receptor layer: A solution of BUTVAR B-76 (1 g) in a mixture (10 g) of methyl ethyl ketone and methanol (1:1) was coated onto the dried IR-absorbing layer using K-rod 1. "K-rods" are wire-wound coating rods commercially available from R.K. Print Coat Instruments Ltd.

Receptor sheet 5

Carrier: As receptor sheet 1.

IR-absorbing layer: Like receptor element 4.

Receptor layer: A solution of VINYLIT VYNS (1.5 g) in a mixture (10 g) of methyl ethyl ketone and toluene (1:1) was applied to the dried, IR-absorbing layer using K-rod 1.

Receptor sheet 6

Carrier: As receptor sheet 1.

IR-absorbing layer: Like receptor element 4.

Receptor layer: A solution of CAB 500 (1 g) in a mixture (10 g) of methyl ethyl ketone and methanol (1:1) was applied to the dried, IR-absorbing layer using K-rod 1.

Receptor sheet 7

Carrier: As receptor sheet 1.

IR-absorbing layer: Like receptor element 4.

Receptor layer: A solution of CAB 381-20 (1 g) in a mixture (10 g) of methyl ethyl ketone and methanol (1:1) was applied to the dried, IR-absorbing layer using K-rod 1.

A sample of each of the receptor sheets 1 to 7 was placed in surface contact with the following donor sheet, with the donor layer of the donor sheet in close contact with the receptor layer of the receptor sheet.

Donor Sheet A

Carrier: Poly(ethylene terephthalate)polyester base (100 µm thick).

Donor layer: A copper phthalocyanine pigment, commercially available from Sun Chemicals Inc., was purified by vacuum sublimation at 500°C and a pressure of 200 Nm-2 (1.5 Torr) (argon). The purified pigment was charged to a stainless steel heater and the heater was positioned approximately 4 cm below the continuous web in a custom-built 30 cm bell jar vacuum coater with diffusion pump and a 15 cm web transport. The support was fed to the web transport prior to evacuating the vacuum chamber to 6.7 x i0-3 Nm-2 (5 x i0-5 Torr). The heater was heated to 410°C using an AC power source to vaporize and deposit the pigment on the support. The rail transport moved at a speed of 0.25 cm per second.

A UGRA line-dot scaling mask was placed on each of the contacting donor and receptor sheets and the unit was addressed using the imaging unit described below with reference to Figure 1 with a laser diode emitting at 830 nm.

The unit of donor and receptor sheets (with mask) was placed between a transparent printing plate (2) and a carrier roller (4) pressed against the plate (2) by a suitable weight (6) acting via a rocker (8). A mirror (10) and a focusing lens (12) on a carrier (14) focused the beam (16) from the laser diode (18) at the point of maximum pressure generated by the carrier roller (4) on the IR-absorbing layer of the receptor sheet. A linear drive motor (20) moved the carrier (14) forward along the rails (22). The image formation in the unit of donor and receptor sheets occurred at an energy level sufficient to produce maximum effect on the donor sheet but only minimal IR-induced heating in the UGRA halftone mask. The operating conditions were as follows: laser power 10 mW, dot size 20 µm, scanning rate 1.5 cm per second, contact pressure (between carrier roller (4) and printing plate (2)) of over 50 gmm-2. This method of contact exposure of materials to monochromatic radiation using halftone masks is disclosed in EP-0583 165. After exposure, each composite was separated and the percentage (%) of dot transfer and the resolved dot area at a resolution of 60 lines per cm were estimated. The results are shown in Table 1 below. Table 1

Units 1 to 3 show that when the donor sheet comprises a vapor-deposited colorant layer and the radiation absorbing material is present in the receptor layer of the receptor sheet, the binder for the latter should preferably have a glass transition temperature (Tg) typically above 100ºC. The use of Lower Tg materials such as VINYLITE VYNS and CAB 500 result in high binder flow and possible melting, preventing mass transfer from the donor sheet.

Units 4 through 7 demonstrate that providing the radiation absorbing material in an underlayer to the receptor layer allows for clean mass transfer (100% transfer) from the donor sheet to the receptor sheet. The best results were obtained with binders with a Tg of 40 to 90ºC, since materials such as CAB 381-20 and CAB 500 with a Tg above 90ºC do not melt/soften sufficiently to allow mass transfer.

Example 2

This example demonstrates the use of a scanning exposure source to thermally transfer a donor sheet comprising a layer of vapor-deposited colorant, wherein either the colorant is capable of absorbing the exposing radiation (donor sheet D) or a separate radiation absorbing material is present in a sublayer adjacent to the colorant layer (donor sheets B, C and E).

Donor sheets B to E are described below:

Donor Sheet B

Carrier: Poly(ethylene terephthalate)polyester base layer (100 µm thick)

IR absorbing layer: IR dye I (0.05 g) was added to BIS-A (3.3 g) in dichloromethane (26 g) and cyclohexanone (3.3 g) and the resulting mixture was stirred in a drum for 24 hours. The mixture was coated onto the support to a wet thickness of 37.5 µm and dried at room temperature. Care was taken to prevent deposition of dust particles on the coating. The optical transmission density of the IR absorbing layer was determined to be 1.2 extinction units at 830 nm.

Colorant layer: Same as donor layer of donor sheet A in Example 1.

Donor Sheet C

Support: As donor sheet B.

IR-absorbing layer: As donor sheet B.

Colorant layer: Violet pigment PV19, commercially available from Ciba Geigy, was purified by vacuum sublimation at 475ºC and a pressure of 2.7 Nm⁻² (20 mTorr) as described in detail above. The purified pigment was deposited from the gas phase onto the coated carrier under almost the same deposition conditions, but at a heating device temperature of 400ºC.

Donor Sheet D

Carrier: Poly(ethylene terephthalate)polyester base (75 µm thick).

IR Absorbing/Colorant Layer: A boehmite (A10.0H) undercoat (0.4 wt% CATAPAL D, commercially available from Vista Chemical Co.; 10 µm wet thickness) was coated onto the support using the procedure described in U.S. Patent Nos. 4,364,995 and 4,430,366, dried at 80°C, and coated with a vapor-deposited layer of "black alumina" (about 0.15 µm thick). The optical transmission density of the layer was determined to be at least 4.6 absorbance units.

Donor Sheet E

Support: As donor sheet D.

IR-absorbing layer: As donor sheet D.

Dye layer: As donor sheet B.

A sample of each of the donor sheets B to E was brought into surface contact with receptor sheets 8 and 9 (see below) with the donor layer of the donor sheet in intimate contact with the receptor layer of the receptor sheet.

Receptor sheet 8

Carrier: Paper base.

Receptor layer: A layer (1.5 µm thick) of a poly(ethylene acrylic acid) emulsion (Tg = 34ºC), commercially available from Schering, was coated on the support.

Receptor sheet 9

Carrier: Poly(ethylene terephthalate)polyester (100 µm thick).

Receptor layer: A layer (1.5 µm thick) of a poly(vinylidene chloride-vinyl acetate) resin (Tg = 79ºC), commercially available from Union Carbide under the trade name VINYLITE VYNS, was coated on the support.

A UGRA line-dot scaling mask was placed on each of the contacting donor and receptor sheets and the unit was used to image as described in Example 1 but using the following operating conditions: laser power 10 mW, spot size 10 µm, scanning rate 1.5 cm per second, contact pressure (between carrier roll and printing plate) of 50 gmm⁻². After exposure the donor and receptor sheets were separated and the percentage (%) of dot transfer and the resolved dot area were estimated at a resolution of 60 lines per cm. The results are presented in Table 2.

Table 2

* The receptor layer (VINYLITE VYNS) tended to lose adhesion to the support and adhere to the black alumina layer of the donor sheet.

The degree of dot transfer was excellent in the majority of cases (100% transfer) with good resolution, with consistent positive and negative images obtained on the donor and receptor sheets, respectively. The images were also characterized by high uniformity of optical density over large areas.

glossary

"VINYLITE VYNS" (Union Carbide), CATAPAL D" (Vista Chemical Co.), "PV17" and "PV19" (Ciba Geigy), "BUTVAR" (Monsanto), "CAB" (Eastman Kodak), "BIS A" (Polysciences) are all trade names/designations.

Claims (22)

1.A method for thermal image transfer comprising the following steps. (a) contacting a receptor sheet with a donor sheet comprising a donor layer comprising a thermally transferable colorant such that the donor layer is in intimate contact with the receptor sheet, wherein one of the donor and receptor sheets comprises a radiation absorbing material capable of absorbing radiation from an exposure source such that imagewise exposure of the contacting sheets causes heating in the exposed areas, the heating causing imagewise thermal transfer of colorant from the donor sheet to the receptor sheet, and (b) imagewise exposure of the sheets in contact using a scanning exposure source, characterized in that the donor layer comprises a layer of a colorant deposited from the gas phase. 2. A method according to claim 1, wherein the receptor comprises a support having applied thereto a receptor layer for the colorant, and wherein the radiation absorbing material is present either in the receptor layer or in a separate sub-layer thereto. 3. The method of claim 2, wherein the receptor layer comprises a layer of a binder in which the radiation absorbing material is dissolved or dispersed. 4. The method of claim 2, wherein the receptor sheet comprises a support on which a radiation-absorbing layer comprising the radiation-absorbing material and the receptor layer are applied as a cover layer thereon in succession. 5. A method according to claim 1, wherein the thermally transferable colorant deposited from the gas phase can absorb the exposure radiation so that it is transferred without aids when the sheet in contact is exposed. 6. A method according to any preceding claim, wherein the donor sheet comprises a support having a controlled release layer onto which the colorant is deposited from the gas phase. 7. The method of claim 1, wherein the donor sheet comprises a support having deposited thereon sequentially a radiation absorbing layer comprising the radiation absorbing material followed by a layer of the vapor-deposited, thermally transferable colorant. 8. A method according to any one of the preceding claims, wherein the colorant layer is deposited as a film with anisotropic topography from the gas phase. 9. The method according to any one of claims 1 to 4, 6 and 7, wherein the radiation-absorbing material is selected from: carbon black and other pigments, nickel dithiolane dye complexes, iron complexes, cyanine dyes, merocyanine dyes, oxyindolizine dyes, indene-bridged polymethine dyes, trinuclear cyanine dyes, bis(aminoaryl)polymethine dyes, tetraarylpolymethine dyes, chalcogen pyrylo-arylidene dyes, bis(chalcogen pyrylo)polymethine dyes, phthalocyanine dyes, squarylium dyes, dyes derived from anthraquinones and naphthoquinones and croconium dyes. 10. A method according to any one of the preceding claims, wherein the thermally transferable colorant is a dye or a pigment. 11. The method of claim 10, wherein the colorant is an organic colorant or an inorganic pigment selected from metals, metal oxides and mixtures thereof. 12. A method according to any preceding claim, wherein step (b) further comprises bringing a photographic mask into intimate contact with the contacting donor and receptor sheets and exposing the assembly through the photographic mask, the exposing radiation causing thermal transfer of colorant from the donor sheet to the receptor sheet in the areas defined by transparent regions of the mask. 13. A method according to claim 12, wherein a pressure of at least 10 g/mm2 is applied to the assembled unit of photographic mask and donor and receptor sheets. 14. A method according to claim 12 or claim 13, wherein the donor and receptor sheets in contact constitute a system sufficiently sensitive to effect transfer of dye at energy levels of less than 4 J/cm2. 15. Method according to one of the preceding claims, wherein the scanning exposure source is a laser or a laser diode. 16. The method of claim 15, wherein the laser has a power of at least 5 mW. 17. A thermal transfer donor sheet comprising the succession of a support, a radiation absorbing layer comprising a dye or a combination of a pigment and a binder, and a layer of a gas-deposited, thermally transferable colorant. 18. The thermal transfer donor sheet of claim 17, wherein the radiation absorbing material absorbs radiation of a wavelength of 600 to 1070 nm. 19. A thermal transfer donor sheet according to claim 17 or claim 18, wherein the radiation absorbing layer comprises a layer of a binder in which the radiation absorbing material is dissolved or dispersed. 20. A thermal transfer donor sheet according to claim 19, wherein the binder of the radiation absorbing layer has a glass transition temperature (Tg) of at least 100°C. 21. A combination of a thermal transfer donor sheet according to any one of claims 17 to 20 and a receptor for the thermally transferable colorant. 22. A thermal transfer medium comprising a donor sheet having a donor layer comprising a layer of a vapor-deposited thermally transferable colorant and a receptor sheet for the thermally transferred colorant comprising a radiation absorbing material.