Why does the print disappear on ATM receipts?

ATM, vending machines receipts, bus ticket, use thermal printing. . It is based on the principle of thermochromism, a process of change in colour with heating.  It works by creating impressions using print-heads over a special kind of paper roll coated with organic dyes, waxes. The paper used is a special thermal paper that is impregnated with mixture of a dye, a 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 through the process of thermochromism. Usually black-colour print-outs are taken but  red print-outs can also be taken by controlling the temperature of print heads. However, when the thermochromic paper is exposed to heat or unattended in person’s pockets, it darkens the paper, hence fade the original printing marks on it.