The thermal printing paper rolls
also have a major drawback, that is, after a period of storage of documents printed with them, the handwriting on them will gradually fade away. And fading occurs because of the unique dyes used in thermal paper. As we mentioned earlier, the latent dyes coated on the thermal paper are colorless at room temperature. At high temperatures, chemical reactions occur and they become another colorful structure. However, this new structure is not so stable, and will return to the previous colorless structure when suitable conditions are met. For example, the crystal violet lactone mentioned earlier will turn colored when it meets acidic substances, but this colored structure will turn colorless again when it meets alkaline substances. When a shopping ticket is printed, it may be stored in contact with various chemicals in the environment, and may also be exposed to sunlight or high temperature, which may lead to the return of dyes on heat-sensitive paper to colorless form and make the ticket fade.
In order to solve this problem, many manufacturers of thermal printing paper rolls will add an additional layer of protective layer on top of the dye layer to minimize the contact between dyes and other chemicals, so that documents printed on thermal paper can be preserved longer. However, this method will increase the cost of thermal paper, so there will be businesses continue to use ordinary thermal paper without protective layer. If you're worried about the fading of your shopping receipts after they've been stored for a long time, it's a good idea to keep copies by copying or scanning in time.