Question asked by T.S. Karthik, Chennai.
ATM receipts are made by a simple printing method called thermal printing. It is based on the principle of thermochromism, a process of change in colour with heating.
Thermal printing essentially works by creating impressions using print-heads over a special kind of paper roll (found in ATMs, vending machines) coated with organic dyes, waxes. The paper used is a special thermal paper that is impregnated with mixture of a dye and a suitable matrix such as a fluoran leuco dye and an octadecylphosphonic acid. When the print-head made of regular array of minuscule heating elements receives the signal for printing, it raises the temperature to the melting point of the organic coating to cause print impression on the paper roll through the process of thermochromism. Usually black colour printouts are obtained, but it is also possible to generate red printouts by controlling the temperature of print heads.
It is common to see these printouts fade over time, even when stored at normal room temperature. More so when heated or brought near a candle flame or exposed to sunlight. Continuous exposure to sunlight generates a lot of heat, much above the melting point of these coatings, which causes irreversible damage to chemical composition of the coating eventually leading to fading or disappearance of the printed matter.
For preserving the prints for a long time, the original thermal paper with extra coating may be used. The thermal papers may be kept in a safe place and avoid friction on its surface as the friction scratches the coating resulting in damage of the image and fading. While having an extra coating helps reduce fading and adds protection, it is better to keep them away from such elements.
P.A. Chary, Visakhapatnam
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