DE69604636T2 - Laser recording element - Google Patents

Description

This invention relates to a laser recording element and, more particularly, to a single-sheet laser recording element in which the support is a microvoided composite film.

In recent years, thermal transfer systems have been developed to produce prints from images generated electronically by a color video camera. One method of producing such prints involves first subjecting an electronic image to color separation by color filters. The corresponding color-separated images are then converted into electrical signals. These signals are then used to generate cyan, magenta and yellow electrical signals. These signals are then applied to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face-to-face with a dye-receiving element. The two are then inserted between a thermal printer head and a platen roller. A line-type thermal printer head is used to apply heat from the back of the dye-donor sheet. The thermal printer head has many heating elements and is heated in sequence according to the cyan, magenta and yellow signals. This process is then repeated for the other two colors. In this way, a hard color copy is obtained that corresponds to the original image viewed on a screen.

U.S. Patent No. 5,244,861 relates to a receiver element suitable for the thermal dye transfer process described above which comprises a microvoided composite film as a support. There is no indication in this patent that the support would be suitable for other thermal systems.

Another way to obtain a print thermally using the electronic signals described above is to use a laser instead of a thermal printer head. In such a laser transfer system, the donor sheet contains a material that absorbs strongly at the wavelength of the laser. When the donor is irradiated, this absorbing material converts light energy into thermal energy and transfers the heat to the dye in its immediate vicinity, thereby heating the dye to its vaporization temperature for transfer to a receiver. The absorbing material may be present in a layer beneath the dye and/or it may be mixed with the dye. The laser beam is modulated by electronic signals representative of the shape and color of the original image so that each dye is heated with volatilization only in the areas where its presence on the receiver is required to reproduce the color of the original object. Further details of this procedure can be found in GB 2 083 726A.

In another method of imaging using a laser beam, an image is recorded from the dye side on a laser recording element having a dye layer composition comprising an image dye, an infrared absorbing material and a binder coated on the substrate. The energy generated by the laser drives the image dye and other components of the dye layer away at the point where the laser beam strikes the element. In "laser removal" imaging, the laser radiation causes rapid local changes in the image recording layer, thereby removing the material from the layer. The usefulness of such a laser recording element is largely determined by the efficiency with which the image recording dye can be removed upon laser exposure. The transmission Dmin value is a quantitative measure of dye cleanliness: the lower its value on the The larger the recording spot, the more complete the dye removal achieved.

U.S. Patent No. 5,330,876 relates to an ablative dye recording element as described above. The element comprises a support having thereon a dye layer containing an image dye, an IR absorbing dye and binder. The element is imagewise exposed to a laser and portions of the dye layer are ablated to form a dye image. The support for this element is the conventional support used in the art.

In the case of the above ablative dye recording element, a problem exists because the minimum density and the sensitivity are not as good as desired.

It is an object of this invention to provide an ablative dye recording element in which the minimum density and speed are improved over the prior art elements. It is another object of this invention to provide a method of forming an image using this recording element.

These and other objects are achieved according to the invention which relates to a laser recording element comprising a base having thereon a dye layer comprising an image dye dispersed in a polymeric binder, the dye layer having an infrared radiation absorbing material associated therewith, and the base comprising a composite film laminated to at least one side of the support, the dye layer being on the composite film side of the base, and the composite film comprising a microvoided thermoplastic core layer and at least one substantially pore-free thermoplastic surface layer.

Any visible image dye may be used in the laser recording element used in the invention, provided that the image dye can be removed by the action of the laser. Particularly good results have been obtained with dyes such as (magenta) (purple) (blue-green-1) (blue-green-2)

or any of the dyes described in U.S. Patents 4,541,830, 4,698,651, 4,695,287, 4,701,439, 4,757,046, 4,743,582, 4,769,360 and 4,753,922. The above dyes may be used alone or in combination with one another. The dyes may be used at a coverage of from about 0.05 to about 1 g/m² and are preferably hydrophobic.

Another embodiment of the invention relates to a process for producing a dye image comprising imagewise heating the recording element as described above by means of a laser, wherein the laser exposure is through the dye side of the element, and wherein dye is imagewise removed to obtain the dye image in the recording element.

In another preferred embodiment of the invention, the dye is removed imagewise by means of an air stream, a vacuum and filter system.

The laser recording elements of this invention can be used to produce medical images, reprographic masks, printing masks, etc. The image obtained can be a positive or a negative image. The dye removal process can produce either continuous (photographic-like) or halftone images.

Any polymeric material can be used as a binder in the Recording element used in the invention. For example, cellulose derivatives can be used, for example cellulose nitrate, cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose triacetate, a hydroxypropyl cellulose ether, an ethyl cellulose ether, etc., polycarbonates; polyurethanes; polyesters; poly(vinyl acetate); polystyrenes; poly-(styrene-co-acrylonitrile); a polysulfone; a poly(phenylene oxide); a poly(ethylene oxide); a poly(vinyl alcohol-co-acetal), such as poly(vinyl acetal), poly(vinyl alcohol-co-butyral) or poly(vinylbenzal); or mixtures of copolymers thereof. The binder can be used in a coating thickness of about 0.1 to about 5 g/m².

In a preferred embodiment, the polymeric binder used in the recording element used in the process of the invention has a polystyrene equivalent molecular weight of at least 100,000 as measured by size exclusion chromatography as described in U.S. Patent 5,330,876.

In the laser recording element of the invention, if desired, a release layer as described in U.S. Patent 5,459,017 may be used.

To obtain a laser-induced image according to the invention, a diode laser is preferably used, such as an infrared diode laser, since this offers significant advantages due to its small size, low cost, stability, reliability, robustness and ease of modulation. In practice, before an infrared laser can be used to heat the recording element, the element must contain an infrared absorbing material, such as infrared absorbing cyanine dyes as described in US Patent 5,401,618, or other materials as described. those in the following U.S. patents: 4,948,777, 4,950,640,4950,639,4948,776,4948,778,4942,141, 4,952,552, 5,036,040 and 4,912,083. The laser radiation is then absorbed in the dye layer and converted to heat by a molecular process known as internal conversion. Thus, the construction of a suitable dye layer depends not only on the hue, transferability and intensity of the image dyes, but also on the ability of the dye layer to absorb the radiation and convert it to heat. The infrared radiation absorbing dye may be contained in the dye layer itself or in a separate layer associated with the dye layer, i.e. above or below the dye layer. Preferably, the laser exposure in the process of the invention is through the dye side of the recording element, thereby enabling this process to be a single sheet process, ie, a separate receiving element is not required.

The dye layer of the laser recording element of the invention can be coated or printed onto the support by a printing technique such as a gravure process or by means of a coating hopper.

Due to their relatively low cost and good appearance, composite films are widely used and are referred to in the trade as "packaging films". The carrier may comprise a cellulose paper, a polymer film or a synthetic paper.

Unlike synthetic paper materials, microvoided packaging films can be laminated to one side of most substrates and still exhibit excellent bending behavior. The bending behavior can be controlled by the beam intensity of the substrate. As the thickness of a substrate decreases, the beam intensity also decreases.

These films can be laminated to one side of the substrates at fairly low thickness/beam intensity ratios and yet exhibit only minimal curvature.

Microvoided packaging composite films are suitably prepared by coextrusion of the core and surface layers, followed by biaxial orientation, thereby creating voids around void-initiating materials present in the core layer. Such composite films are described, for example, in U.S. Patent 5,244,861.

The core of the composite film should comprise 15 to 95% of the total thickness of the film, preferably 30 to 85% of the total thickness. The non-porous skin or skins should thus comprise 5 to 85% of the film, preferably 15 to 70% of the thickness. The density (specific gravity) of the composite film should be between 0.2 and 1.0 g/cm³, preferably between 0.3 and 0.7 g/cm³. If the thickness of the core becomes less than 30% or the specific gravity is increased above 0.7 g/cm³, the composite film begins to lose suitable compressibility and thermal insulating properties. If the core thickness is increased above 85% or the specific gravity becomes less than 0.3 g/cm3, the composite film becomes less processable due to a drop in tensile strength and it becomes susceptible to physical damage. The total thickness of the composite film can be between 20 and 150 µm, preferably between 30 and 70 µm. Below 30 µm, the microvoided films cannot be thick enough to minimize any inherent non-planarity in the substrate and would be more difficult to manufacture. At thicknesses higher than 70 µm, little improvement in either pressure uniformity or thermal efficiency is seen, so there is little justification for further increasing the cost of extra materials.

Suitable classes of thermoplastic polymers for the core matrix polymer of the composite film include polyolefins, polyesters, polyamides, polycarbonates, cellulose esters, polystyrene, polyvinyl resins, polysulfonamides, polyethers, polyimides, poly(vinylidene fluoride), polyurethanes, poly(phenylene sulfides), polytetrafluoroethylene, polyacetals, polysulfonates, polyester ionomers, and polyolefin ionomers. Copolymers and/or blends of these polymers may be used.

Suitable polyolefins include polypropylene, polyethylene, polymethylpentene and mixtures thereof. Polyolefin copolymers, including copolymers of ethylene and propylene, are also suitable.

The composite film can be made with skin or skins of the same polymeric material as the core matrix or the composite film can be made with skin or skins of a polymer composition different from that of the core matrix. To achieve compatibility, an auxiliary layer can be used to assist adhesion of the skin layer to the core.

Additives may be incorporated into the core matrix to improve the whiteness of these films using any of the techniques known in the art, including the addition of a white pigment such as titanium dioxide, barium sulfate, clay, or calcium carbonate. It may also include the addition of optical brighteners or fluorescent agents that absorb energy in the UV region and emit light largely in the blue region, or other additives that improve the physical properties of the film or the manufacturability of the film.

The coextrusion, quenching, orientation and heat setting of these composite films can be carried out by any method known in the art for the production of of oriented films are known, including a flat film process or a bubble or tube process. The flat film process relies on extruding the mixture through a slot die and rapidly quenching the extruded web on chilled casting drums such that the core matrix polymer component of the film and the skin component or components are quenched to below their glass transition temperatures (Tg). The quenched film is then biaxially oriented by stretching in mutually perpendicular directions at a temperature above the glass transition temperature of the matrix polymers and the skin polymers. The film may be stretched in one direction and then stretched in a second direction, or stretched in both directions simultaneously. After the film has been stretched, it is heat set by heating to a temperature sufficient to crystallize the polymers while restraining the film to some degree from retraction in both directions of stretch.

The presence of at least one non-porous skin on the microporous core increases the tensile strength of the film and makes the film more manufacturable. It allows the films to be manufactured in larger widths and at higher stretch ratios than when films are manufactured in which all layers are porous. Coextruding the layers also facilitates the manufacturing process.

The support to which the microvoided composite films are laminated to form the base or the recording element of the invention may be a polymeric support, a synthetic paper support, or the support may consist of a cellulosic fiber-paper support or laminates thereof.

Preferred cellulose fiber paper supports include those described in U.S. Patent No. 5,250,496. a cellulose fiber paper backing is used, it has been found advantageous to subject the microvoided composite films to extrusion lamination using a polyolefin resin. During the lamination process, it is desirable to maintain minimal tension in the case of the microvoided packaging film in order to minimize curl in the resulting laminated backing. The back side of the paper backing (i.e., the side opposite the microvoided composite film) may also be extrusion coated with a polyolefin resin layer (e.g., from about 10 to 75 g/m²) and may also have a backing layer as described in U.S. Patents 5,011,814 and 5,096,875. In the case of high humidity applications (> 50% RH), it is desirable to provide a resin coating on the back of about 30 to about 75 g/m², more preferably 35 to 50 g/m², to keep the warpage to a minimum.

In a preferred embodiment, to produce recording elements having a desirable photographic look and feel, it is desirable to use relatively thick paper supports (e.g., having a thickness of at least 120 µm, preferably from 120 to 250 µm) and relatively thin microvoided packaging composite films (e.g., having a thickness of less than 50 µm, preferably having a thickness of from 20 to 50 µm, more preferably having a thickness of from 30 to 50 µm).

In another embodiment of the invention, to produce a recording element that resembles plain paper, for example for inclusion in a printed multi-page document, relatively thin paper or polymer supports can be used (for example having a thickness of less than 80 µm, preferably from 25 to 80 µm) in combination with relatively thin microporous packaging Composite films (for example with a thickness of less than 50 µm, preferably from 20 to 50 µm, particularly preferably with a thickness of from 30 to 50 µm).

The following example is intended to further illustrate the invention.

Example Preparation of the microporous carrier - Carrier A

A commercially available packaging film (OPPalyte® 350 TW, Mobil Chemical Co.) was laminated to a paper carrier. OPPalyte® 350 TW is a composite film (38 µm thick) (d = 0.62) consisting of a microvoided and oriented polypropylene core (approximately 73% of the total film thickness) with a titanium dioxide-pigmented, non-microvoided oriented polypropylene layer on each side; the void-initiating material is poly(butylene terephthalate).

Packaging films can be laminated to a paper base in various ways (for example, by extrusion, by printing or other means). In the present case, they were laminated to a paper base by extrusion, as described below, with pigmented polyolefin. The pigmented polyolefin consisted of polyethylene (12 g/m²) containing anatase (titanium dioxide) (12.5% by weight) and a benzoxazole-based optical brightener (0.05% by weight).

The paper stock support was 137 µm thick and was made from a 1:1 blend of Pontiac Maple 51 (a bleached maple kraft hardwood with a 0.5 µm weighted average fiber length) available from Consolidated Pontiac Inc. and Alpha Hardwood Sulfite (a bleached red alder hardwood sulfite with an average fiber length of 0.69 µm) available from Weyerhauser Paper Co. The back of the paper stock support was coated with a layer of made of high density polyethylene (30 g/m²).

Preparation of the carrier without micropores - Carrier B, (comparison)

A non-microvoided support was prepared by extrusion coating a pigmented polyolefin onto a paper stock support. The pigmented polyolefin consisted of polyethylene (12 g/m²) containing anatase (titanium dioxide) (12.5 wt%) and a benzoxazole optical brightener (0.05 wt%). The paper stock support was the same as described above. The back of the paper stock support was coated with high density polyethylene (30 g/m²).

Laser dye ablation layer

The following mixture was prepared and stirred until a dissolution was achieved:

12 g nitrocellulose (Hercules)

0.24g IR-1

0.24 g of the dye described above Blue-2

70 g acetone IR-absorbing dye IR-1

Item 1

The above solution was applied in a coating thickness of 34 g/m², wet coating thickness, applied to the paper carrier A as described above.

Comparison 1

This comparison is similar to Element 1, except that carrier B was used instead of carrier A.

Laser exposure

After drying, the elements were exposed using a Spectra Diode Labs Lasers model SDL-2432 laser with a maximum power of 600 mW per laser beam at 830 nm, 1000 rpm and a spot size of approximately 12 µm x 25 µm using a lathe-type printer with a drum circumference of 53 cm. The diode laser beams were scanned on the surface of the element to achieve 954 lines per cm or 2400 lines per inch. An air stream was blown over the donor surface along with a vacuum and filter system to remove the ablated material.

A step tablet image was printed by decreasing the laser intensity in a linear manner in successive patches from maximum to 0. Reflectance Status A red densities were measured using an X-Rite Model 310 reflection densitometer.

The readings were compared to the uncoated paper support. The results are shown in the following table. The close D-max values indicate that the coating thicknesses were the same within experimental errors, as intended (Step 21). Table

The above results show that the element with the microvoided support (Element 1) was more effective, showing a significantly lower D-min (levels 1-5) than Control Element 1, which used a non-microvoided support. The microvoided support also resulted in a speed improvement, which is determined from the data by defining a speed point as the exposure required to print 0.03 over D-min (level 10 for Element 1 and level 7 for Control Element 1). The speed point in the case of Element 1 was 498 mJ/cm2, whereas the speed point in the case of Control Element 1 was 546 mJ/cm2. This means that Element 1 requires 9% less exposure to reach this speed point.

Claims (10)

1. A laser recording element comprising a base having thereon a dye layer comprising an image dye dispersed in a polymeric binder, the dye layer having associated therewith an infrared radiation absorbing material, the base comprising a composite film laminated to at least one side of a support, the dye layer being on the composite film side of the base, and the composite film comprising a microvoided thermoplastic core layer and at least one substantially pore-free thermoplastic surface layer. 2. An element according to claim 1, wherein the thickness of the composite film is 30 to 70 µm. 3. The element of claim 1, wherein the core layer of the composite film comprises 30 to 85% of the thickness of the composite film. 4. The element of claim 1 wherein the composite film comprises a microvoided thermoplastic core layer with a substantially void-free thermoplastic surface layer on each side thereof. 5. The element of claim 1 wherein the support comprises paper. 6. The element of claim 5 wherein the paper support is 120 to 250 µm thick and the composite film has a thickness of 30 to 50 µm. 7. The element of claim 6 further comprising a polyolefin backing layer on the side of the support opposite the composite film. 8. The element of claim 1, wherein the infrared radiation absorbing material is a dye. 9. A process for producing a dye image comprising imagewise heating the laser recording element of claim 1, wherein the laser exposure is through the dye side of the element, and causing dye to be imagewise removed to obtain the dye image in the recording element. 10. The method of claim 9, wherein the dye is imagewise removed by means of an air stream, vacuum and a filter system and wherein the laser is a diode laser.