Image Dye Combination for Laser Ablative Recording Element

This invention relates to use of certain image dyes in a single-sheet laser dye-ablative recording element.

In recent years, thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from a color video camera. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted 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 printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is hereby incorporated by reference.

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

In one ablative mode of imaging by the action of a laser beam, an element with a dye layer composition comprising an image dye, an infrared-absorbing material, and a binder coated onto a substrate is imaged from the dye side. The energy provided by the laser drives off at least the image dye at the spot where the laser beam impinges upon the element. In ablative imaging, the laser radiation causes rapid local changes in the imaging layer thereby causing the material to be ejected from the layer. This is distinguishable from other material transfer techniques in that some sort of chemical change (e.g., bond-breaking), rather than a completely physical change (e.g., melting, evaporation or sublimation), causes an almost complete transfer of the image dye rather than a partial transfer. Usefulness of such an ablative element is largely determined by the efficiency at which the imaging dye can be removed on laser exposure. The transmission Dmin value is a quantitative measure of dye clean-out: the lower its value at the recording spot, the more complete is the attained dye removal.

In U.S. Serial No. 08/259,588 of DoMinh et al., filed Jun. 14, 1994, a single-sheet laser dye-ablative recording element is described in Example 2 which employs a certain yellow dye known as curcumin, in combination with an azamethine cyan dye. However, there is a problem with this dye combination in that under accelerated light fade conditions, the loss in blue density (from yellow dye loss) is pronounced.

It is an object of this invention to provide a dye combination which will have improved light stability. It is another object of this invention to provide a single-sheet process which does not require a separate receiving element.

These and other objects are achieved in accordance with the invention which comprises a laser dye-ablative recording element comprising a support having thereon a dye layer comprising two or more image dyes dispersed in a polymeric binder, the dye layer having an infrared-absorbing material associated therewith, and wherein the image dyes comprise curcumin yellow dye and a 1,4-diaminoanthraquinone dye.

The yellow dye curcumin, also known as Brilliant Yellow S, is a natural product dye found in the spice turmeric. The structure is large for a molecule intended to be ablated, but surprisingly it was found to be readily decomposed to colorless products when subjected to a laser beam, thereby allowing one to achieve very good dye clean-out at modest laser powers.

The dye curcumin is believed to be 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione. While isomers of this compound are believed to exist in the natural compound, the molecule is believed to have the following structure: ##STR1##

Any 1,4-diaminoanthraquinone dye can be employed in this invention as long as it can be ablated by the action of a laser. In a preferred embodiment of the invention, the anthraquinone dye has the formula: ##STR2## wherein R.sup.1 and R.sup.2 each independently represents hydrogen, alkyl, alkenyl, cycloalkyl, haloalkyl, cyanoalkyl, alkoxyalkyl, alkoxyalkoxyalkyl, hydroxyalkyl, hydroxyalkoxyalkyl, hydroxyalkylthioalkyl, tetrahydrofurfuryl, alkenyloxyalkyl, tetrahydrofurfuryloxyalkyl, alkoxycarbonykalkyl, alkoxycarbonyloxyalkyl or alkoxycarbonyloxyalkyl.

In a preferred embodiment of the invention, R.sup.1 and R.sup.2 each independently represents alkyl or aryl. Further examples of these anthraquinone dyes are disclosed in U.S. Pat. No. 5,070,069, the disclosure of which is hereby incorporated by reference.

The curcumin dye or anthraquinone dye employed in the recording element of the invention may each be used at a coverage of from about 0.01 to about 1 g/m.sup.2.

In another preferred embodiment of the invention, the dye layer also contains an ultraviolet-absorbing dye, such as a benzotriazole, a substituted dicyanobutadiene, an aminodicyanobutadiene, or any of those materials disclosed in Patent Publications JP 58/62651; JP 57/38896; JP 57/132154; JP 61/109049; JP 58/17450; or DE 3,139,156, the disclosures of which are hereby incorporated by reference. They may be used in an amount of from about 0.05 to about 1.0 g/m.sup.2.

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

The invention is especially useful in making reprographic masks which are used in publishing and in the generation of printed circuit boards. The masks are placed over a photosensitive material, such as a printing plate, and exposed to a light source. The photosensitive material usually is activated only by certain wavelengths. For example, the photosensitive material can be a polymer which is crosslinked or hardened upon exposure to ultraviolet or blue light but is not affected by red or green light. For these photosensitive materials, the mask, which is used to block light during exposure, must absorb all wavelengths which activate the photosensitive material in the Dmax regions and absorb little in the Dmin regions. For printing plates, it is therefore important that the mask have high blue and UV Dmax. If it does not do this, the printing plate would not be developable to give regions which take up ink and regions which do not.

By use of this invention, a mask can be obtained which has enhanced light stability for making multiple printing plates or circuit boards without mask degradation.

Any polymeric material may be used as the binder in the recording element employed in the invention. For example, there may be used cellulosic derivatives, e.g., 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); polystyrene; 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(vinyl benzal); or mixtures or copolymers thereof. The binder may be used at a coverage of from about 0.1 to about 5 g/m.sup.2.

In a preferred embodiment, the polymeric binder used in the recording element employed 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. Pat. No. 5,330,876, the disclosure of which is hereby incorporated by reference.

A barrier layer may be employed in the laser ablative recording element of the invention if desired, as described in copending U.S. Ser. No. 321,282, filed Oct. 11, 1994, and entitled BARRIER LAYER FOR LASER ABLATIVE IMAGING, the disclosure of which is hereby incorporated by reference.

To obtain a laser-induced, dye ablative image according to the invention, an infrared diode laser is preferably employed since it offers substantial advantages in terms of its small size, low cost, stability, reliability, ruggedness, and ease of modulation. In practice, before any laser can be used to heat a dye-ablative recording element, the element must contain an infrared-absorbing material, such as cyanine infrared-absorbing dyes as described in U.S. Pat. No. 5,401,618, or other materials as described in the following U.S. Pat. Nos. 4,948,777, 4,950,640, 4,950,639, 4,948,776, 4,948,778, 4,942,141, 4,952,552, 5,036,040, and 4,912,083, the disclosures of which are hereby incorporated by reference. The laser radiation is then absorbed into the dye layer and converted to heat by a molecular process known as internal conversion. Thus, the construction of a useful dye layer will depend 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-absorbing dye may be contained in the dye layer itself or in a separate layer associated therewith, i.e., above or below the dye layer. Preferably, the laser exposure in the process of the invention takes place through the dye side of the dye ablative recording element, which enables this process to be a single-sheet process, i.e., a separate receiving element is not required.

Lasers which can be used in the invention are available commercially. There can be employed, for example, Laser Model SDL-2420-H2 from Spectra Diode Labs, or Laser Model SLD 304 V/W from Sony Corp.

The dye layer of the dye-ablative recording element of the invention may be coated on the support or printed thereon by a printing technique such as a gravure process.

Any material can be used as the support for the dye-ablative recording element of the invention provided it is dimensionally stable and can withstand the heat of the laser. Such materials include polyesters such as poly(ethylene naphthalate); polysulfones; poly(ethylene terephthalate); polyamides; polycarbonates; cellulose esters such as cellulose acetate; fluorine polymers such as poly(vinylidene fluoride) or poly(tetrafluoroethylene-co-hexafluoropropylene); polyethers such as polyoxymethylene; polyacetals; polyolefins such as polystyrene, polyethylene, polypropylene or methylpentene polymers; and polyimides such as polyimide-amides and polyether-imides. The support generally has a thickness of from about 5 to about 200 .mu.m. In a preferred embodiment, the support is transparent.

The following examples are provided to illustrate the invention.

The following materials were employed in this example: ##STR3##

A 100 .mu.m thick poly(ethylene terephthalate) support was coated with 0.65 g/m.sup.2 of a copolymer of 70% ethylcyanolacrylate and 30% methylcyanoacrylate, 0.05 g/m.sup.2 infrared dye IR-1, and 0.005 g/m.sup.2 FC-431 surfactant (3M Corp.) from a 78/20/2 blend of dichloromethane/acetone/1-methyl-2-pyrrolidinone.

Samples of this support were then coated with a laser dye ablation layer consisting of 0.22 g/m.sup.2 infrared dye IR-1, 0.41 g/m.sup.2 ultraviolet dye UV-1, 0.14 g/m.sup.2 yellow dye Curcumin, 0.60 g/m.sup.2 nitrocellulose, and 1.07 mmol/m.sup.2 of the cyan dyes E-1 to E-5, and a control dye coated from tetrahydrofuran. The control dye is the cyan dye disclosed in column 9, lines 25-30 of U.S. Pat. No. 5,401,618 discussed above.

The dye ablation layer was then overcoated with 0.11 g/m.sup.2 Witcobond.RTM. 236 polyurethane (Witco Corporation), 0.03 g/m.sup.2 Hydrocerf.RTM. 9174 polytetrafluoroethylene particles (Shamrock Co.), 0.03 g/m.sup.2 MP-1000 polytetrafluoroethylene particles (DuPont Co.), and 0.008 g/m.sup.2 Zonyl.RTM. FSN surfactant (DuPont Co.) coated from a water/methanol solvent blend.

The stability of the resulting dye layers was measured using an X-Rite Densitometer (Model 820, X-Rite Corp.) by the Status A blue density difference between a covered and uncovered sample after exposure to eight hours of 50 kLux sunshine. The following results were obtained:

The above results show that the anthraquinone dyes of the invention stabilize the curcumin dye to light fade in comparison to the prior art control dye.

The following UV dye was used in this example: ##STR4##

A 100 .mu.m thick poly(ethylene terephthalate) support was coated with a laser dye ablation layer consisting of 0.22 g/m.sup.2 infrared dye IR-1, 0.13 g/m.sup.2 ultraviolet dye UV-2, 0.28 g/m.sup.2 yellow dye Curcumin, 0.60 g/m.sup.2 nitrocellulose, and 0.58 mmol/m.sup.2 of either cyan dye E-1 or the control cyan dye coated from an 80/20 (wt/wt) mixture of 4-methyl-2-pentanone and denatured ethanol.

These elements were then processed as in Example 1 with the following results:

The above results show that the anthraquinone dyes of the invention stabilize the curcumin dye to light fade in comparison to the prior art control dye.

These elements were the same as Example 2 except that UV-2 was deleted.

The elements were then processed as in Example 1 with the following results:

The above results show that the anthraquinone dyes of the invention stabilize the curcumin dye to light fade in comparison to the prior art control dye.

Samples of the above examples were ablation written using a laser diode print head, where each laser beam has a wavelength range of 830-840 nm and a nominal power output of 550 mW at the film plane.

The drum, 53 cm in circumference, was rotated at varying speeds and the imaging electronics were activated to provide adequate exposure. The translation stage was incrementally advanced across the dye ablation element by means of a lead screw turned by a microstepping motor, to give a center-to-center line distance of 10.58 .mu.m (945 lines per centimeter or 2400 lines per inch). An air stream was blown over the dye ablation element surface to remove the ablated dye. The ablated dye and other effluents are collected by suction. The measured total power at the focal plane was 550 mW per channel maximum. A useful ablation image was obtained.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.