Abstract:
A process of forming a single color, ablation image comprising imagewise-heating by means of a laser in the absence of a separate receiving element, an ablative recording element comprising a support having thereon, in order, a barrier layer and a colorant layer comprising a colorant dispersed in a polymeric binder, the colorant layer having an infrared-absorbing material associated therewith, the laser exposure taking place through the colorant side of the element, and removing the ablated colorant to obtain the image in the ablative recording element, wherein the colorant layer contains a plasticizer in an amount of up to about 50% by weight of the polymeric binder.

Description:
FIELD OF THE INVENTION 
     This invention relates to a process of forming an ablation image using a laser ablative recording element. 
     BACKGROUND OF THE INVENTION 
     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 substantially all of the image dye and binder at the spot where the laser beam hits 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. The transmission density serves as a measure of the completeness of image dye removal by the laser. 
     U.S. Pat. No. 5,468,591 relates to a recording element for laser ablative imaging. There is a problem with that element, however, in that the ablative colorant layer in the element is sensitive to scratching during processing and handling. 
     It is an object of this invention to provide a single-sheet process of forming a single color, ablation image which does not require a separate receiving element. It is still another object of this invention to provide a single-sheet process of forming a single color, ablation image which has improved scratch and abrasion resistance without sacrificing laser sensitivity, i.e., without increasing the exposure needed to achieve a low Dmin. 
     SUMMARY OF THE INVENTION 
     These and other objects are achieved in accordance with the invention which comprises a process of forming a single color, ablation image comprising imagewise-heating by means of a laser in the absence of a separate receiving element, an ablative recording element comprising a support having thereon, in order, a barrier layer and a colorant layer comprising a colorant dispersed in a polymeric binder, the colorant layer having an infrared-absorbing material associated therewith, the laser exposure taking place through the colorant side of the element, and removing the ablated colorant to obtain the image in the ablative recording element, wherein the colorant layer contains a plasticizer in an amount of up to about 50% by weight of the polymeric binder. 
     By use of the invention, a more scratch-resistant element is obtained that has a practical Dmax and exposure level, i.e., greater efficiency, than the prior art. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Any plasticizers may be used in the colorant layer of the invention provided it has the desired effect. There may be used, for example a solvent having a boiling point greater than about 90° C. In a preferred embodiment, the plasticizer is a dibasic ester, such as dioctyl phthalate, dibutyl phthalate, dimethyl adipate, dimethyl succinate, dimethyl glutarate or dibutylsebacate. Other plasticizers which may be employed in the invention include castor oil, tricresyl phosphate, tributyl phosphate and Hercolyn® (Hercules Corp.). 
     As noted above, the plasticizer is employed in an amount of up to about 50% by weight of the polymeric binder. In a preferred embodiment, the plasticizer is employed in an amount of from about 20% to about 40% by weight of the polymeric binder. 
     The 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 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 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. 
     The dye removal process can be by either continuous (photographic-like) or halftone imaging methods. 
     The higher efficiency achieved in accordance with the invention greatly expands the UV contrast of these ablative elements, which enhances their usefulness when exposing UV-sensitive printing plates with UV radiation. 
     Any polymeric material may be used as the binder in the recording element employed in the process of 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); poly(vinyl halides) such as poly(vinyl chloride) and poly(vinyl chloride)copolymers; poly(vinyl ethers); maleic anhydride copolymers; 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 2 . 
     In a preferred embodiment, the polymeric binder used in the recording element employed in 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 colorant layer of the invention may also contain a hardener to crosslink the polymeric binder or react with itself to form a interpenetrating network. Examples of hardeners that can be employed in the invention fall into several different classes such as the following (including mixtures thereof): 
     a) formaldehyde and compounds that contain two or more aldehyde functional groups such as the homologous series of dialdehydes ranging from glyoxal to adipaldehyde including succinaldehyde and glutaraldehyde; diglycolaldehyde; aromatic dialdehydes, etc.; 
     b) blocked hardeners (substances usually derived from the active hardener that release the active compound under appropriate conditions) such as substances that contain blocked aldehyde functional groups, such as tetrahydro- 4- -hydroxy-5-methyl-2(1H)-pyrimidinone polymers, polymers of the type having a glyoxal polyol reaction product consisting of 1 anhydroglucose unit: 2 glyoxal units, dimethoxylethanal-melamine non-formaldehyde resins, 2,3-dihydroxy-1,4-dioxane, blocked dialdehydes and N-methylol compounds obtained from the condensation of formaldehyde with various aliphatic or cyclic amides, ureas, and nitrogen heterocycles; 
     c) active olefinic compounds having two or more olefinic bonds, especially unsubstituted vinyl groups, activated by adjacent electron withdrawing groups, such as divinyl ketone; resorcinol bis(vinylsulfonate); 4,6-bis(vinylsulfonyl)-m-xylene; bis(vinylsulfonylalkyl)ethers and amines; 1,3,5tris(vinylsulfonyl)hexahydro-s-triazine; diacrylamide; 1,3-bis(acryloyl)urea; N,N′-bismaleimides; bisisomaleimides; bis(2-acetoxyethyl)ketone; 1,3,5-triacryloylhexahydro-s-triazine; and blocked active olefins of the type bis(2-acetoxyethyl)ketone and 3,8-dioxodecane-1,10-bis(pyridinium perchlorate)bis(vinyl sulfonylmethane), bis(vinyl sulfonylmethyl ether), and the like; 
     d) compounds that contain two or more amino groups such as ethylene diamine; and 
     e) inorganic salts such as aluminum sulfate; potassium and ammonium alums of aluminum; ammonium zirconium carbonate; chromium salts 
     In a preferred embodiment, the hardener is a polyfunctional isocyanate, such as Desmodur N 3300® (Bayer Inc.), which is a polyfunctional aliphatic isocyanate resin based on hexamethylene diisocyanate. The hardener may be used in any amount effective for the intended purpose. In general, it may be used from about 0.1% to about 100% by weight of the polymeric binder. 
     To obtain a laser-induced, ablative image using the process of the invention, a 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 ablative recording element, the element must contain an infrared-absorbing material, such as pigments like carbon black, or cyanine infrared-absorbing dyes as described in U.S. Pat. No. 4,973,572, 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 colorant layer and converted to heat by a molecular process known as internal conversion. Thus, the construction of a useful colorant layer will depend not only on the hue, transferability and intensity of the colorant, but also on the ability of the colorant layer to absorb the radiation and convert it to heat. The infrared-absorbing material or dye may be contained in the colorant layer itself or in a separate layer associated therewith, i.e., above or below the colorant layer. As noted above, the laser exposure in the process of the invention takes place through the colorant side of the 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. 
     Any dye can be used in the ablative recording element employed in the invention provided it can be ablated by the action of the laser. Especially good results have been obtained with dyes such as anthraquinone dyes, e.g., Sumikaron Violet RS® (product of Sumitomo Chemical Co., Ltd.), Dianix Fast Violet 3R-FS® (product of Mitsubishi Chemical Industries, Ltd.), and Kayalon Polyol Brilliant Blue N-BGM® and KST Black 146® (products of Nippon Kayaku Co., Ltd.); azo dyes such as Kayalon Polyol Brilliant Blue BM®, Kayalon Polyol Dark Blue 2BM®, and KST Black KR® (products of Nippon Kayaku Co., Ltd.), Sumikaron Diazo Black 5G® (product of Sumitomo Chemical Co., Ltd.), and Miktazol Black 5GH® (product of Mitsui Toatsu Chemicals, Inc.); direct dyes such as Direct Dark Green B® (product of Mitsubishi Chemical Industries, Ltd.) and Direct Brown M® and Direct Fast Black D® (products of Nippon Kayaku Co. Ltd.); acid dyes such as Kayanol Milling Cyanine 5R® (product of Nippon Kayaku Co. Ltd.); basic dyes such as Sumiacryl Blue 6G® (product of Sumitomo Chemical Co., Ltd.), and Aizen Malachite Greens (product of Hodogaya Chemical Co., Ltd.);                           
     or any of the dyes disclosed in U.S. Pat. Nos. 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 disclosures of which are hereby incorporated by reference. The above dyes may be employed singly or in combination. The dyes may be used at a coverage of from about 0.05 to about 1 g/m 2  and are preferably hydrophobic. 
     Pigments which may be used in the colorant layer of the ablative recording layer of the invention include carbon black, graphite, metal phthalocyanines, etc. When a pigment is used in the colorant layer, it may also function as the infrared-absorbing material, so that a separate infrared-absorbing material does not have to be used. 
     The colorant layer of the ablative recording element employed in 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 ablative recording element employed in the invention provided it is dimensionally stable and can withstand the heat of the laser. Such materials include polyesters such as poly(ethylene naphthalate); 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 μm. In a preferred embodiment, the support is transparent. 
     In another preferred embodiment of the invention, the element has an overcoat layer on top of the colorant layer to assist in scratch resistance. Such an overcoat layer will typically contain a polymeric binder, polytetrafluoro-ethylene beads and a surfactant. 
     The following examples are provided to illustrate the invention. 
    
    
     EXAMPLES 
     Example 1 
     The following dyes were used in this Example:                           
     Control Element C-1 (No Plasticizer) 
     A 100 μm poly(ethylene terephthalate) support was coated with a barrier layer containing the following ingredients at the indicated aim dry coverages: 0.38 g/m 2  poly(methyl 2-cyanoacrylate), 0.05 g/m 2  IR Dye-1, and 0.003 g/m 2  surfactant FC-431® (3M Corp.) from acetonitrile. 
     On top of the barrier layer was coated a colorant layer from a methyl isobutyl ketone/ethanol 8:2 solvent mixture at a wet laydown of 32 cc/m 2  containing the following dissolved ingredients at the indicated aim dry coverages: 0.60 g/m 2 cellulose nitrate (1000-15000 cps) (Aqualon Co.), 0.28 g/m 2  UV Dye, 0.13 g/m 2  of Yellow Dye, 0.16 g/m 2  Cyan Dye, and 0.22 g/m 2  IR Dye-1. 
     On top of the colorant layer was coated an overcoat layer applied from an aqueous solution at a wet laydown of 21.6 cc/m 2  and contained the following dissolved ingredients at the indicated aim dry coverages: 0.10 g/m 2  (67/33 mole/mole) of a polymeric binder of a copolymer of ethylacrylate and methacrylic acid, 0.03 g/m 2  of polytetrafluoroethylene beads, Hydrocerf 9174® (Shamrock Technologies, Inc.), 0.05 g/m 2  of polytetrafluoroethylene beads, Fluon AD-1® (ICI America Inc.), and 0.01 g/m 2  of a surfactant, Zonyl FSN® (DuPont Corp.) 
     Element 1 of the Invention 
     This element was the same as Control Element 1 except that the colorant layer contained a plasticizer, Hercolyn D® (Hercules Corp.), in an amount of 20% by weight/weight of binder. 
     Element 2 of the Invention 
     This element was the same as Element 1 of the Invention except that the plasticizer was dimethyladipate. 
     Element 3 of the Invention 
     This element was the same as Element 1 of the Invention except that the plasticizer was dimethylsuccinate. 
     Element 4 of the Invention 
     This element was the same as Element 1 of the Invention except that the plasticizer was dimethylglutarate. 
     Element 5 of the Invention 
     This element was the same as Element 1 of the Invention except that the plasticizer was dibutylsebacate. 
     Element 6 of the Invention 
     This element was the same as Element 1 of the Invention except that the plasticizer was dibutylsebacate in an amount of 40% by weight/weight of binder. 
     Control Element C-2 (Contains More than 50% Plasticizer) 
     This element was the same as Element 6 of the Invention except that the plasticizer was employed in an amount of 60% by weight/weight of binder. 
     Imaging 
     The above recording elements were imaged with a diode laser imaging device as described in U.S. Pat. No. 5,387,496. The laser beam had a wavelength of 830 nm and a nominal power output of 450 m Watts per channel at the end of the optical fiber. Table 1 lists UV transmission density recorded on an X-Rite® densitometer Model 310 (X-Rite Co.). The UV density of the elements before imaging was in the range of 3.6 to 4.6 for all film elements. Speed is reported as the Dmin achieved given 600 mJ/cm 2  exposure. Lower values indicate more efficient (i.e., faster) imaging. 
     Scratch Test 
     Unexposed samples were subjected to surface abrasion with a counterweighted rotating disk and turntable arrangement for a fixed time interval. The UV density of the abraded area (Dscratch) and unabraded area (Dmax) were measured. Scratch is reported as “% Area Lost” calculated using a form of the Murray-Davies equation: 
     
       
         % Area Lost=100−% Area Retained=100(1−(1−10 −D     scratch   )/(1−10 −D     max   )). 
       
     
     The scratch testing is subject to high noise levels. The data reported are derived from averages of eight readings per sample. The following results were obtained: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Plasticizer 
                   
                   
               
               
                 Element 
                 (% of Binder) 
                 Dmin 
                 % Area Lost 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 C-1 
                 None 
                 0.077 
                 1.29 
               
               
                 1 
                 Hercolyn D ® (20) 
                 0.076 
                 0.34 
               
               
                 2 
                 dimethyladipate (20) 
                 0.075 
                 0.18 
               
               
                 3 
                 dimethylsuccinate (20) 
                 0.075 
                 0.30 
               
               
                 4 
                 dimethylglutarate (20) 
                 0.080 
                 0.37 
               
               
                 5 
                 dibutylsebacate (20) 
                 0.076 
                 0.48 
               
               
                 6 
                 dibutylsebacate (40) 
                 0.078 
                 0.57 
               
               
                 C-2 
                 dibutylsebacate (60) 
                 0.082 
                 2.29 
               
               
                   
               
             
          
         
       
     
     The above results show that the elements of the invention had a lower % Area Lost to scratch than the control elements. All of the elements of the invention had Dmin&#39;s comparable to the control elements. 
     Example 2 
     Plasticizer Plus Hardener 
     Element 7 
     This element was prepared the same as Element 1 above, except that the image layer contained 40% (by weight/weight of binder) of a polyfunctional isocyanate hardener, Desmodur N 3300® (Bayer Inc.). 
     Element 8 
     This element was prepared the same as Element 2 above, except that the image layer contained 40% (by weight/weight of binder) of the polyfunctional isocyanate hardener, Desmodur N 3300®. 
     Element 9 
     This element was prepared the same as Element 3 above, except that the image layer contained 40% (by weight/weight of binder) of the polyfunctional isocyanate hardener, Desmodur N 3300®. 
     Element 10 
     This element was prepared the same as Element 4 above, except that the image layer contained 40% (by weight/weight of binder) of the polyfunctional isocyanate hardener, Desmodur N 3300®. 
     Element 11 
     This element was prepared the same as Element 5 above, except that the image layer contained 40% (by weight/weight of binder) of the polyfunctional isocyanate hardener, Desmodur N 3300®. 
     Element 12 
     This element was prepared the same as Element 1 Invention except that the plasticizer was dibutylphthalate and the image layer contained 40% (by weight/weight of binder) of the polyfunctional isocyanate hardener, Desmodur N 3300® 
     Testing 
     The above elements were exposed and tested as in Example 1. The following results were obtained: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Plasticizer 
                   
                   
               
               
                 Element 
                 (% of Binder) 
                 Dmin 
                 % Area Lost 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 C-1 
                 None 
                 0.077 
                 1.29 
               
               
                  7 
                 Hercolyn D ® (20) 
                 0.102 
                 0.15 
               
               
                  8 
                 dimethyladipate (20) 
                 0.087 
                 0.21 
               
               
                  9 
                 dimethylsuccinate (20) 
                 0.083 
                 0.19 
               
               
                 10 
                 dimethylglutarate (20) 
                 0.081 
                 0.19 
               
               
                 11 
                 dibutylsebacate (20) 
                 0.083 
                 0.33 
               
               
                 12 
                 dibutylphthalate (20) 
                 0.081 
                 0.16 
               
               
                   
               
             
          
         
       
     
     The above results show that the elements of the invention with plasticizer and hardener exhibited a lower scratch sensitivity than the control element. 
     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.