Thermal paper is a special paper that is used in conjunction with thermal printers to primarily produce receipts for ATM’s, cash registers and other points of sale (POS). The reason that this paper is used so commonly with POS systems is that no ink is involved in the process, which makes the printers small and more manageable than traditional printers that require either toner or ink cartridges to be regularly replaced.
To give you a brief overview of what happens when an item is printed through a thermal printer, the special paper is subjected to heat in specific configurations that turn the paper black and so show the writing/imagery that has been programmed to be shown. The reason that the paper turns black and even other colours when exposed to heat is that it has been impregnated with specific chemical combinations that are designed to react this way.
The base chemical composition of thermal paper consists of:
1. 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 dissolved in a melt and exposed to an acidic environment.
2. 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 (BPA) and Bisphenol S (BPS). 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 .
3. Sensitisers - 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 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 third 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 100 °C, 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.
4. Stabilisers - 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 called 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.
Related Posts
