Thermal dye transfer

DepalmaVA,Sharma R, Tunney SE, Brust DP Slipping layer of polyimide-siloxane copolymer for dye-donor element use in thermal dye transfer. US Patent 5252534 14 pp... 

Two other major technologies, ink-jet and thermal dye transfer (sometimes erroneously referred to as dye sublimation) systems are widely used but no photochemistry is involved so these will not be discussed further. Also omitted for the same reason is the thermal Autochrome system developed by Fuji for small-scale digital imaging. 

For an electron to be transferred from an excited dye molecule to the silver halide conduction band, the excited level should be either above the bottom of the conduction band or near enough to it for thermally assisted transfer to occur during the normal lifetime of the excited state of the dye. If the lowest vacant level of the unexcited dye is below the bottom of the conduction band, transfer of an electron from the conduction band to the unexcited dye molecule becomes possible. This "electron trapping" by the dye could be a factor in the desensitizing action of the dye. 

For Class 2 dyes, absorption of a photon causes photoinjection of a hole into the valence band by transfer of an electron from the valence band to the now vacant Sq level of the excited molecule. The dye molecule retains the electron that has been excited to the level and is in effect a dye radical with an excess electron. In the absence of oxygen or other agent that could react with the radical, a thermally assisted transfer of the electron to the conduction band can occur. The time frame during which this transfer could occur is not limited by the normal lifetime of the excited state, as it is in the direct transfer of an electron in the Class 1 dyes. The time available could be much longer, limited only by the occurrence of some other reaction of the dye radical. 

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

Thermal Printing. Thermal printing is a generic name for methods that mark paper or other media with text and pictures by imagewise heating of special-purpose consumable media. Common technologies are direct thermal thermal, ie, wax, transfer and dye-sublimation, ie, diffusion, transfer. Properties and preferred appHcations are diverse, but apparatus and processes are similar (87—89). 

Transfer occurs by sublimation, condensation, and diffusion (101). Printhead thermal dissipation causes donor dye to travel to the surface of the donor ribbon and convert directiy to a gas. Colorant puffs immediately strike the nearby receptor and soak in, assisted by residual printhead heat. 

Dye sublimation requires more heat dissipation and a longer (>10 ms) heating period to make a dark mark than does thermal transfer. Carehil manipulation of heating time and temperature can proportion mark size and dye content to cover a wide density range (0 to ca 2 optical density). 

In order to develop the dyes for these fields, characteristics of known dyes have been re-examined, and some anthraquinone dyes have been found usable. One example of use is in thermal-transfer recording where the sublimation properties of disperse dyes are appHed. Anthraquinone compounds have also been found to be usehil dichroic dyes for guest-host Hquid crystal displays when the substituents are properly selected to have high order parameters. These dichroic dyes can be used for polarizer films of LCD systems as well. Anthraquinone derivatives that absorb in the near-infrared region have also been discovered, which may be appHcable in semiconductor laser recording. 

Table 8. Examples of Magenta Dyes for Sublimation Thermal-Transfer Printing...

Almost all dyes are quinones or azaquinones of compound 2. The majority are used for thermal transfer processes a recent example is compound 306 (98USP5792587). The second major class are azo dyes, prepared either from aromatic diazonium salts as in compound 307 (95GEP4319296) or from a triazolopyridine 3-diazonium salt as in 308 (81BRP2054630). 

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