ATM receipts (recently even bus tickets are issued with the same kind of print-outs) 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 print-outs are obtained, but it is also possible to generate red print-outs by controlling the temperature of print heads.
It is common to see these print-outs fade over time even when stored at normal room temperature. More so when heated or brought near a candle flame and when exposed to heat of sunlight. Continuous exposure to sunlight generates a lot of heat, much above the melting point of these coatings, to cause irreversible damage to chemical composition of the coating eventually leading to fading or disappearance of the printed matter.
R. Jagannathan,
CECRI, Karaikudi, Tamil Nadu
This week’s question
Are satellites sent outside the Milky Way to take pictures of it? If not, then how do we get pictures of the Milky Way?
Carolin Sunny, Class VI, Aleppey, Kerala
