Patent Publication Number: US-5830824-A

Title: Plasticizers for dye-donor element used in thermal dye transfer

Description:
This invention relates to the use of certain plasticizers in dye-donor elements for thermal dye transfer systems. 
     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 one of the cyan, magenta or 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. 
     An important requirement for any thermal dye-donor element is to maintain performance over its useful lifetime without degradation in the quality of the image. The dye layer of a dye-donor element for resistive head thermal dye transfer generally comprises a polymeric binder and diffusible dyes. The percentage of dye in the layer is typically quite high, in the range of 20 to 80%. The dye is usually dissolved in the binder or phase-separated into small domains. During keeping of the dye-donor, the temperature and humidity may be elevated. The dyes in the dye layer, which is in contact with a slipping layer coated on the back side of the dye-donor element when it is wound up in spool form, sometimes become crystallized. Some plasticizers have been proposed in the prior art to be added to such dye-donor elements such as phenol-formaldehyde condensates, or phenolic resins, such as novolacs and resoles. While these materials do not cause the dyes in the dye-donor element to crystallize, they do not enhance dye transfer efficiency, and if used in too great an amount can cause a reduction in dye transfer efficiency. 
     U.S. Pat. No. 4,876,236 relates to the use of nonpolymeric materials or compounds as plasticizers for a dye-donor element. JP 21151485 also relates to the use of plasticizers in a dye-donor element which includes some polymeric compounds such as poly(ethylene glycol) esters and polyester adipate. However, there is a problem with these prior art plasticizers in that they tend to cause dye crystallization or phase separation on storage of the dye-donor element, with subsequent loss of dye density. Further, there is a problem with these prior art plasticizers in that variations in printing density occur when the dye-donors are aged at elevated temperatures. 
     It is an object of this invention to provide a plasticizer for a dye-donor element in which sensitometric changes upon storage and keeping are minimized and dye efficiency during printing is maintained or improved. It is another object of this invention to provide a plasticizer for a dye-donor element which reduces the tendency of the dye to crystallize. 
     These and other objects are achieved in accordance with this invention which relates to a dye-donor element for thermal dye transfer comprising a support having thereon a dye layer comprising an image dye in a polymeric binder, and wherein the dye layer also contains a plasticizer comprising a chlorinated paraffin. 
     By using the plasticizers of the present invention in dye-donor elements, a reduction in sensitometric changes upon keeping occurs along with minimization of dye crystallization. 
     Chlorinated paraffins useful in the invention are long chain hydrocarbons consisting of methylene, methyl, methine and alkene groups, which have chlorine substitution. The length of the long chain hydrocarbon is between about 8 and about 30 carbon atoms, preferably between about 12 and 24. The chains can also be branched. The amount of chlorine in the paraffin can be between about 25 and 75%, preferably between about 40 and 70%. 
     Although the chlorinated paraffin may consist primarily of chains of a single length and chemical structure, minor components of slightly varying chain length or chemical structure may be included. In addition, mixtures of chlorinated paraffins may also be used. 
     In a preferred embodiment of the invention, the chlorinated paraffins that may be used have the following formula: 
     
         C.sub.x H.sub.y C.sub.z 
    
     wherein 
     x is between about 11 and about 24, 
     y is between about 14 and about 43 and 
     z is between about 3 and about 10. 
     Examples of chlorinated paraffins that may be used in the invention include the following: 
     C 24  H 43  Cl 7 , Chlorowax 40®, (Occidental Chemical Corp.), Tg=-50° C., (Tg is the glass transition temperature midpoint determined by differential scanning calorimetry), % Chlorine=41-45 C 20  H 34  Cl 8 , Chlorowax 51-225®, (Occidental Chemical Corp.), Tg=-30° C., % Chlorine=51 Paroil 170HV®, (Dover Chemical, Inc.), Tg=-4 ° C., % Chlorine=70 C 12  H 23  Cl 3 , Chlorowax 45LV®, (Occidental Chemical Corp.), Tg=-92° C., % Chlorine=40-45 C 12  H 17  Cl 9 , Chlorowax 65®, (Occidental Chemical Corp.), Tg=-45° C., % Chlorine=64-65 C 11  H 4  Cl 10 , Chlorowax 70-200®, (Occidental Chemical Corp.), Tg=-17° C., % Chlorine=63-66 Paroil 140HV®, (Dover Chemical, Inc.), Tg=-46° C., % Chlorine=43 and Paroil 1650HV®, (Dover Chemical, Inc.), Tg=-28° C., % Chlorine=63. 
     The plasticizers may be used in the invention in any amount which 3 0 is effective for the intended purpose. In general, good results have been obtained when the plasticizer is present at a concentration of from about 2 to about 12% by weight of the dye layer. In a preferred embodiment, the chlorinated paraffin has a midpoint Tg of from about 0 to about -92° C. 
     Any dye can be used in the dye-donor employed in the invention provided it is transferable to the dye-receiving layer by the action of heat. Especially good results have been obtained with sublimable 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 Sumicaryl Blue 6G® (product of Sumitomo Chemical Co., Ltd.), and Aizen Malachite Green® (product of Hodogaya Chemical Co., Ltd.); ##STR1## 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. 
     A dye-barrier layer may be employed in the dye-donor elements of the invention to improve the density of the transferred dye. Such dye-barrier layer materials include hydrophilic materials such as those described and claimed in U. S. Pat. No. 4,716,144. 
     The dye layer of the dye-donor element 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-donor element of the invention provided it is dimensionally stable and can withstand the heat of the thermal head. Such materials include polyesters such as poly(ethylene terephthalate); polyamides; polycarbonates; cellulose esters such as cellulose acetate; fluorine polymers such as polyvinylidene 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. It may also be coated with a subbing layer, if desired, such as those materials described in U. S. Pat. Nos. 4,695,288 or 4,737,486. 
     The dye in the dye-donor element of the invention is dispersed in a polymeric binder such as a cellulose derivative, e.g., cellulose acetate hydrogen phthalate, cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate, cellulose triacetate or any of the materials described in U. S. Pat. No. 4,700,207; a polycarbonate; poly(vinyl acetate), poly(vinyl acetal), poly(vinyl butyral), poly(styrene-co-acrylonitrile), a polysulfone or a poly(phenylene oxide). The binder may be used at a coverage of from about 0.1 to about 5 g/m 2 . 
     The reverse side of the dye-donor element may be coated with a slipping layer to prevent the printing head from sticking to the dye-donor element. Such a slipping layer would comprise either a solid or liquid lubricating material or mixtures thereof, with or without a polymeric binder or a surface-active agent. Preferred lubricating materials include oils or semicrystalline organic solids that melt below 100° C. such as poly(vinyl stearate), beeswax, perfluorinated alkyl ester polyethers, polycaprolactone, silicone oil, polytetrafluoroethylene, carbowax, poly(ethylene glycols), or any of those materials disclosed in U. S. Pat. nos. 4,717,711; 4,717,712; 4,737,485; and 4,738,950. Suitable polymeric binders for the slipping layer include poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-co-acetal), polystyrene, poly(vinyl acetate), cellulose acetate butyrate, cellulose acetate propionate, cellulose acetate or ethyl cellulose. 
     The amount of the lubricating material to be used in the slipping layer depends largely on the type of lubricating material, but is generally in the range of about 0.001 to about 2 g/m 2 . If a polymeric binder is employed, the lubricating material is present in the range of 0.05 to 50 weight %, preferably 0.5 to 40, of the polymeric binder employed. 
     The dye-receiving element that is used with the dye-donor element of the invention usually comprises a support having thereon a dye image-receiving layer. The support may be a transparent film such as a poly(ether sulfone), a polyimide, a cellulose ester such as cellulose acetate, a poly(vinyl alcohol-co-acetal) or a poly(ethylene terephthalate). The support for the dye-receiving element may also be reflective such as baryta-coated paper, polyethylene-coated paper, an ivory paper, a condenser paper or a synthetic paper such as DuPont Tyvek®. Pigmented supports such as white polyester (transparent polyester with white pigment incorporated therein) may also be used. 
     The dye image-receiving layer may comprise, for example, a polycarbonate, a polyurethane, a polyester, a polyacrylate, poly(vinyl chloride), poly(styrene-co-acrylonitrile), polycaprolactone, a poly(vinyl acetal) such as poly(vinyl alcohol-co-butyral), poly(vinyl alcohol-co-benzal), poly(vinyl alcohol-co-acetal) or mixtures thereof. The dye image-receiving layer may be present in any amount which is effective for the intended purpose. In general, good results have been obtained at a concentration of from about 1 to about 5 g/m 2 . 
     As noted above, the dye-donor elements of the invention are used to form a dye transfer image. Such a process comprises imagewise heating a dye-donor element as described above and transferring a dye image to a dye-receiving element to form the dye transfer image. 
     The dye-donor element of the invention may be used in sheet form or in a continuous roll or ribbon. If a continuous roll or ribbon is employed, it may have alternating areas of dyes such as sublimable cyan and/or magenta and/or yellow and/or black or other dyes. Thus, one-, two-, three- or four-color elements (or higher numbers also) are included within the scope of the invention. 
     In a preferred embodiment of the invention, the dye-donor element comprises a poly(ethylene terephthalate) support coated with sequential repeating areas of cyan, yellow and magenta, and the above process steps are sequentially performed for each color to obtain a three-color dye transfer image. Of course, when the process is only performed for a single color, then a monochrome dye transfer image is obtained. 
     Thermal printing heads which can be used to transfer dye from the dye-donor elements of the invention are available commercially. There can be employed, for example, a Fujitsu Thermal Head (FTP-040 MCSOO1), a TDK Thermal Head F415 HH7-1089 or a Rohm Thermal Head KE 2008-F3. 
     A thermal dye transfer assemblage of the invention comprises 
     a) a dye-donor element as described above, and 
     b) a dye-receiving element as described above, the dye-receiving element being in a superposed relationship with the dye-donor element so that the dye layer of the donor element is in contact with the dye image-receiving layer of the receiving element. 
     The above assemblage comprising these two elements may be preassembled as an integral unit when a monochrome image is to be obtained. This may be done by temporarily adhering the two elements together at their margins. After transfer, the dye-receiving element is then peeled apart to reveal the dye transfer image. 
     When a three-color image is to be obtained, the above assemblage is formed three times using different dye-donor elements. After the first dye is transferred, the elements are peeled apart. A second dye-donor element (or another area of the donor element with a different dye area) is then brought in register with the dye-receiving element and the process repeated. The third color is obtained in the same manner. 
    
    
     The following examples are provided to illustrate the invention: 
     Example 1 
     A dye-donor element of the invention was prepared by gravure coating a subbing layer of 0.14 g/m 2  of titanium tetrabutoxide, Tyzor TBT® (DuPont Co.) in a propyl acetate/butanol (85:15) solvent mixture onto both sides of a 6 μm poly(ethylene terephthalate) support (DuPont Co.) On one side of this support was coated the following dye layer (the dyes are illustrated above): 
     0.26 g/m 2  CAP 482-20 (20 s viscosity cellulose acetate propionate, Eastman Chemical Co.) 
     0.09 g/m 2  CAP 482-0.5 (0.5 s viscosity cellulose acetate propionate, Eastman Chemical Co.) 
     0.08 g/m 2  M-1 
     0.17 g/m 2  M-2 
     0.08 g/m 2  M-3 
     0.07 g/m 2  2,4,6-trimethylanilide of phenyl-indan-diacid (TMA) 
     0.001 g/m 2  Fluorad FC-430® (a fluorosurfactant from 3M Corp.) 
     0.006 g/m 2  2 μm divinylbenzene beads 
     0.07 g/m 2  Chlorowax 40® chlorinated paraffin 
     On the other side of the above support was coated the following slipping layer: 
     0.39 g/m 2  poly(vinyl acetal) (KS- 1 from Sekisui America Corp.) 
     0.02 g/m 2  candelilla wax 
     5 0.01 g/m 2  PS-513 (an aminopropyl dimethyl-terminated polydimethylsiloxane from Petrarch Systems, Inc.) 
     0.0003 g/m 2  p-toluenesulfonic acid 
     For comparison, another dye-donor element was prepared in the same manner except Chlorowax 40 ® was omitted. 
     Preparation of Receiver Element 
     A dye-receiving element base was prepared employing a support laminated to a packaging film. The support consisted of a paper stock from a blend of Pontiac Maple 51 (a bleached maple hardwood kraft of 0.5 μm length weighted average fiber length) available from Consolidated Pontiac, Inc. and Alpha Hardwood Sulfite (a bleached red-alder hardwood sulfite pulp of 0.69 μm average fiber length) available from Weyerhaeuser Paper Co. This support had a microvoided packaging film of OPPalyte®350 TWK, polypropylene-laminated paper support with a lightly TiO 2  -pigmented polypropylene skin (Mobil Chemical Co.) at a dry coverage of 0.11 g/m 2 , 36 μm thick, laminated on the imaging side. Prior to coating, the support was subjected to a corona discharge treatment at approximately 450 joules/m 2 . 
     A thermal dye-transfer receiving element was prepared from the above receiver support by coating the following layers in order on the top surface of the microvoided packaging film: 
     a) a subbing layer of Prosil® 221 and Prosil® 2210 (PCR, Inc.) (1:1 weight ratio) both are organo-oxysilanes, in an ethanol-methanol-water solvent mixture. The resultant solution (0.10 g/m 2 ) contained approximately 1% of silane component, 1% water, and 98% of 3A alcohol; 
     b) a dye-receiving layer containing Makrolon® KL3-1013 (a polyether-modified bisphenol-A polycarbonate block copolymer) (Bayer AG) (1.78 g/m 2 ), GE Lexan® 141-112 (a bisphenol-A polycarbonate) (General Electric Co.) (1.46 g/m 2 ), Fluorad® FC-431 (perfluorinated alkylsulfonamidoalkyl ester surfactant) (3M Co.) (0.01 g/m 2 ), di-n-butyl phthalate (0.32 g/m 2 ), and diphenyl phthalate (0.32 g/m 2 ) and coated from a solvent mixture of methylene chloride and trichloroethylene (4:1 by weight) (4.1% solids); and 
     c) a dye-receiver overcoat containing a solvent mixture of methylene chloride and trichloroethylene, terpolymer of bisphenol A polycarbonate, diethylene glycol and polydimethylsiloxane (0.65 g/m 2 ), and surfactants DC-510 Silicone Fluid (Dow-Corning Corp.) (0.008 g/m 2 ) and Fluorad® FC-431 (3M Co.) (0.02 g/m 2 ) from dichloromethane. 
     Printing Conditions 
     Assemblies of the dye-donor and dye-receiver test elements as prepared above were subjected to thermal printing in a printer equipped with a TDK thermal print head Model No. LV5416 which had a resolution of 118 dots/cm and an average resistance of 3281 Ω. The printing speed was 5 ms per line. The head voltage was set at 13.75 v to provide a maximum print energy of approximately 5 joule/cm 2  at 36.4° C. A step tablet was used to generate test strips with Status A reflection densities (measured with an X-Rite densitometer, X-Rite Corp., Grandville, Mich.) as follows: 
     
                       TABLE 1                                                     
______________________________________                                    
            Status A Reflection Density                                   
              No Plasticizer                                              
Step No.      (Control)  Invention                                        
______________________________________                                    
1             2.56       2.64                                             
2             1.88       1.94                                             
3             1.26       1.35                                             
5             0.47       0.54                                             
7             0.20       0.30                                             
9             0.07       0.08                                             
______________________________________                                    
 
    
     The above results show that there is an increase in reflection density when a plasticizer in accordance with the invention is used in the dye-donor element. 
     Example 2 
     Example 1 was repeated using several other plasticizers including two comparison plasticizers (ST106P and DPP) and two additional chlorinated paraffins. The densitometric responses of these donor elements are shown in the following Table 2: 
     
                       TABLE 2                                                     
______________________________________                                    
Status A Reflection Density                                               
     No                                                                   
Step Plasticizer                                                          
               ST106P.sup.1                                               
                          DPP.sup.2                                       
                                   Cl-wax                                 
                                         Paroil                           
No.  (Control) (Comparison)                                               
                          (Comparison)                                    
                                   51-225.sup.3                           
                                         170HV.sup.4                      
______________________________________                                    
1    2.56      2.72       2.67     2.61  2.60                             
2    1.88      2.09       2.00     1.92  1.89                             
3    1.26      1.47       1.38     1.32  1.28                             
5    0.47      0.62       0.51     0.50  0.46                             
7    0.20      0.37       0.26     0.26  0.20                             
9    0.07      0.11       0.07     0.07  0.06                             
______________________________________                                    
 .sup.1) ST106P = a plasticizer available from Eastman Kodak Co.          
 .sup.2) DPP = diphenyl phthalate                                         
 .sup.3) Chlorowax 51225 ® = a chloroparaffin available from Occidenta
 Chemical Corp.                                                           
 .sup.4) Paroil 170HV ® = a chloroparaffin available from Dover       
 Chemical, Inc.                                                           
 
    
     The above results show that the chlorinated paraffins had similar effects on increasing the dye transfer efficiency as did the comparison plasticizers. However, the comparison plasticizers had other problems as shown in the following Example. 
     Example 3 
     To evaluate the shelf-keeping stability of the dye-donor elements with respect to dye crystallization and densitometric response, the dye-donor elements were wound on a plastic spool and then placed into an aluminum-lined bag which was then sealed. The sealed bag was placed into an oven kept at 38° C. for 28 days to simulate a longer term of shelf-keeping at ambient conditions or harsh conditions during shipping. The appearance of dye crystallization was noted in the following Table 3: 
     
                       TABLE 3                                                     
______________________________________                                    
Plasticizer      Crystallization                                          
______________________________________                                    
ST106P (comparison)                                                       
                 Yes                                                      
DDP (comparison) Yes                                                      
Chlorowax 40 ®                                                        
                 No                                                       
Chlorowax 51-225 ®                                                    
                 No                                                       
Paroil 170HV ®                                                        
                 No                                                       
______________________________________                                    
 
    
     The above results show that the comparison plasticizers give rise to dye crystallization, while the chlorinated paraffins used according to the present invention do not. 
     Example 4 
     A control yellow dye-donor element was prepared using the subbed substrate of Example 1 by coating on one side the following dye layer. 
     
         ______________________________________                                    
0.26 g/m.sup.2   CAP 482-20                                               
0.13 g/m.sup.2   CAP 482-0.5                                              
0.11 g/m.sup.2   Y-1                                                      
0.13 g/m.sup.2   Y-2                                                      
0.001 g/m.sup.2  FC-430 ®                                             
0.006 g/m.sup.2  2 μm divinylbenzene beads                             
______________________________________                                    
 
    
     The same slipping layer as in Example 1 was coated onto the opposite side of the subbed support. 
     A yellow dye-donor according to the invention was prepared similar to the control but containing an additional 0.07 g/m 2  Chlorowax 40® in the yellow dye layer. 
     Cyan dye-donor elements, both control and invention donors, were prepared in the same way as above using the following dye layers: 
     
         ______________________________________                                    
Control Cyan Dye-Donor                                                    
0.29 g/m.sup.2     CAP 482-20                                             
0.12 g/m.sup.2     C-1                                                    
0.11 g/m.sup.2     C-2                                                    
0.28 g/m.sup.2     C-3                                                    
0.04 g/m.sup.2     TMA                                                    
0.0005 g/m.sup.2   FC-430 ®                                           
0.01 g/m.sup.2     2 μm divinylbenzene beads                           
2) Invention Cyan Dye-Donor                                               
0.29 g/m.sup.2     CAP 482-20                                             
0.12 g/m.sup.2     C-1                                                    
0.11 g/m.sup.2     C-2                                                    
0.28 g/m.sup.2     C-3                                                    
0.04 g/m.sup.2     TMA                                                    
0.0005 g/m.sup.2   FC-430 ®                                           
0.01 g/m.sup.2     2 μm divinylbenzene beads                           
0.03 g/m.sup.2     Chlorowax 40 ®                                     
______________________________________                                    
 
    
     Status A reflection densities were measured as in Example 1 as follows: 
     
                       TABLE 4                                                     
______________________________________                                    
        Reflection Density                                                
        Yellow           Cyan                                             
Step No.  Control Invention  Control                                      
                                   Invention                              
______________________________________                                    
1         2.32    2.33       2.48  2.52                                   
3         1.30    1.43       1.22  1.31                                   
5         0.53    0.63       0.43  0.49                                   
7         0.27    0.39       0.18  0.24                                   
9         0.08    0.12       0.06  0.07                                   
______________________________________                                    
 
    
     The above results show that there was an increase in reflection density for the elements employing the plasticizer in accordance with the invention. 
     In addition, no dye crystallization was observed with the invention dye-donor elements of the above example after 28 days storage at 38° C. 
     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.