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Thermal paper is a special paper that changes color when exposed to heat, this reaction is caused by a chemical that the paper is coated with.
Thermal papers are used for a wide variety of thermal printing applications in thermal printers such as adding machines, cash registers, and credit card terminals.
Four chemicals are used in thermal paper production
There are four main chemicals used in thermal paper, each with its own specific properties, and these work together to ensure the best reaction between the chemical mix and heat possible. These chemicals are:
- Leuco Dyes
- Developers
- Sensitisers
- Stabilisers
The leuco dyes used in thermal paper are usually triaryl methane phthalide dyes. These dyes have a colorless leuco form when used on the paper, but become colored when dissolved in a melt and exposed to an acidic environment. These leuco dyes need acid to produce a significant enough colour to be useful, so to achieve this, organic developer acids are also added to the coating mix.
These developer acids are usually organic acids such as phenols (Bisphenol A (BPA) and Bisphenol S (BPS)), sulfonyl ureas and zink salts. These interact with the dyes to become dark enough to be visible.
One problem faced by this reaction, is that when left to their own devices, they do not mix well together. For this reason a third element is added to the mix – sensitiser.
Sensitisers have the added benefit of creating a very specific reaction temperature as well as optimising the mixing of the dyes and developers.
Typically, thermal paper manufacturers make use of 3-methylphenoxy or 2-benzyloxynapthalene which melt at around 100 degrees C.
finally, because the dyes used in the mix are quite unstable, stablisers are required to prevent the dyes returning to an uncolored state by preventing the dyes from separating from the developers, thereby maintaining the integrity of the print.
The chemical bisphenol A (BPA) is used for thermal paper coatings because of its stability and heat-resistance, however there are some who believe that this chemical can have adverse effect on human health. These beliefs are regarded by the scientific and medical community as unfounded and there are many who believe that the recent ban in Suffolk County is a major over reaction and not likely to make a difference to those who handle thermal paper prints on a regular basis.