User:Polymer65

== Thermal Printing Materials == Collection of material for modifications of information about thermal printing. Specifically:
 * 1) Correction of preliminary information of Thermographic Printing page, which is not related to Thermographic Printing, but instead to thermal transfer printing
 * 2) Addition of material to the Thermal Printing page about more recent (post 1995) methods, which include the ability to print in full color with direct thermal printing

Technology
Direct Thermal Printing is a thermal printing process in which the chemicals that form the final image are contained within the paper itself and are not applied to the paper by ink-jets, toner transfer or ink-transfer from ribbons. The paper traditionally contains chemicals that mix upon melting and interact to form a colored dye. This media technology was introduced by NCR and 3M in ,,,  When used with  thermal print heads, which were invented by Texas Instruments in 1965, it has found application in many products (digital typewriter, printers, FAX machines, point-of-sale (POS) store receipts, etc.).

Until 1995, direct thermal prints were limited to a single-color (usually black) printing, or at best to a limited selection of colors. This is because activation by temperature alone affords only a single control parameter for color, and is not able to provide selectivity for the three colors needed for a full-color image. For example, if a paper incorporates three color layers that melt at different temperatures, then it is possible to activate the lowest-melting color without the other two, but not possible to activate either of the higher-melting colors without also activating any lower-melting color. Fig. 1 illustrates a hypothetical paper in which the yellow dye turns on at about 100 C, the magenta at about 150 C and the cyan at about 200 C. With this paper it is possible to print yellow (the lowest melting dye layer) by applying a temperature of 125 C; or  (yellow + magenta = red), the combination of the two lowest melting colors, by applying 175 C; or (yellow + magenta + cyan = black), by applying 225 C. However, it is not possible to print arbitrary combinations of the three, such as (magenta + cyan = blue) or (yellow + cyan = green).

Figure 1: The problem of three-color printing with a direct thermal printer

Fuji Thermo-Autochrome (TA) Process
FujiFilm solved this problem in the early 1990's with the release of the Fuji Autochrome system, which produced full-color direct-thermal prints. The system used successive steps of writing on a color layer and then desensitizing that color using UV light. The color layers were arranged on the media such that the lowest-melting layer was closest to the surface and the highest-melting layer was deepest. Writing on the top, lowest-melting-point, layer was followed by desensitization of that layer with UV light (420 nm wavelength). This allowed the next-lowest-melting-point layer to be activated at a higher temperature without affecting the first. Printing of the second layer was followed by desensitization with a second UV light (365 nm wavelength), and then writing on the highest-temperature layer (see Fig. 1). This invention opened the possibility of three-color printing in a printer that required a special paper, but no ink, ribbon or toner cartridges; but the need for two UV light sources and three separate printing cycles added complexity to the hardware.

Figure 2  Schematic of the Fuji Autochrome method of direct thermal printing

Polaroid Zero-Ink (ZINK) Process
In 2000, Polaroid developed an alternate solution to the problem in which all colors were written in a single pass, and with no UV lamps. In this process, the media again has three color layers, but the highest-melting-point layer is placed closest to the surface and the lowest-melting-point layer was deepest. A very short heat pulse of high temperature activates the top layer, but is not given time to diffuse to the second layer. A medium-length pulse of lower temperature has time to reach and activate the second layer but not the third, and because of the lower temperature it does not affect the top layer through which it passes. Finally, a third pulse that is long and of low temperature, reaches the deepest color layer without affecting either of the two layers above it. This solution to the 3-color direct thermal printing problem is now called the ZINK (or "zero-ink") process. (see Fig. 3)

Figure 3 Schematic of the ZINK process for achieving 3-color images

Chemistry
Four different types of imaging chemicals are used in thermally sensitive papers: leuco dyes, developers, sinsitizers and stabilizers.

Leuco Dyes
The leuco dyes used in direct thermal paper are usually triaryl methane phthalide dyes, such as Yamamoto Blue 4450, or fluoran dyes, such as Pergascript Black 2C. A third widely used leuco dye is Crystal Violet lactone. Red or magenta color can be achieved with dyes such as Yamamoto Red 40. Yellow can be produced  by the protonation of a triaryl pyridine, such as Copikem Yellow 37. These dyes have a colorless leuco form when crystalline or when in a pH neutral environment, but become colored when dissoved in a melt and exposed to an acidic environment.

Developers
Leuco dyes, in general, provide little color when melted unless they are melted in conjunction with one or more organic acids. Examples of organic acids suitable for thermochromic papers are phenols such as bisphenol A and bisphenol S. Other suitable acidic materials are sulfonyl ureas such as BTUM and Pergafast 201. Zinc salts of substituted salicylic acids, such as zinc di-tert-butylsalicylate have also been commercially used as developers.

Sensitizers
A leuco dye and a developer, when melted together, are enough to produce color. However, the thermal threshold of the coated layer containing the colorizing components is is determined by the lowest melting component of the layer. Furthermore, developers and leuco dyes often mix poorly upon melting. To optimize the colorization temperature and to facilitate mixing, a thrid chemical called a sensitizer is commonly added to the imaging layer. Sensitizers are commonly simple ether molecules such as 1,2-bis-(3-methylphenoxy)ethane or 2-benzyloxynapthalene. These two materials melt at approximately 100C, which is a practical lower limit for thermal coloration. These low-cost ethers are excellent low viscosity solvents for leuco dyes and developers, and this facilitates color formation at a well-defined temperature and with minimum energy input.

Stabilizers
Dyes in thermally sensitive paper are often unstable and return to their original colorless, crystalline forms when stored in hot or humid conditions. To stabilize the metastable glass formed by the leuco dye, developer and sensitizer, a fourth type of material calld a stabilizer is often added to thermal papers. Stabilizers often share similarities with developers and are often complex multifunctional phenols that inhibit recrystallization of the dye and developer, thereby stabilizing the printed image.