Why do cash registers not need a change of printer cartridges? Why does the paper of cash register receipts feel so funny? Why does the writing fade when the receipt is exposed to the sun? Why can cash register receipts not be disposed of as waste paper?
It is all down to the clever chemistry behind it. But let us start at the beginning: The receipt is made of thermosensitive paper, or thermal paper for short.
What is Thermal Paper?
Thermal paper consists of several layers. Sandwiched between two protective layers, there are, from the bottom to the top: a layer of base paper, then a precoat layer, and finally a thermoactive layer—this is the one of interest for the printing process.
In the thermoactive layer, ink- and color-developer molecules are encapsulated by long-chain solvent molecules, for example, tetradecanol. Heat (thermochemical energy) sets chemical reactions in motion in this layer, the dyes change color and the type becomes visible on the paper.
How Does the Writing Get onto the Paper?
A print head consisting of many small heating elements is used to write on thermal paper. These are controlled electronically. Wherever a dot is to be created on the paper, a heating element is turned on.
The heat causes a color reaction that proceeds as follows: The color molecules in the paper are pH-sensitive leuco dyes. Leuco is Greek for white. One example is crystal violet lactone, which is initially colorless. As mentioned, it is encapsulated by long-chain solvent molecules in the thermoactive layer of the paper.
If a spot in the paper is now heated by the print head, the solvent melts there. The color-developer molecules, which are also present in the thermoactive layer, and the dye molecules now come together. They react with each other and the dye changes color. In the case of crystal violet lactone, the color changes from colorless to dark violet. More precisely, the color developer is a weak acid and protonates the ester group of the crystal violet lactone. A cation is formed that is resonance-stabilized by the three aromatic residues. The extended π-electron system shifts the excitation energy of the electronic transition into the visible range.
When the spot on the paper cools down again, the solvent becomes solid again and stabilizes the colored, metastable complex of dye and color developer.
What Is So Great about Thermal Printing?
We know thermal printing not only from cash register receipts, but also from parking tickets, tickets, or—mostly in the past—fax paper. The technology is easy to handle even under extreme conditions, the printers are fast, small, and quiet, require only the paper and no printer cartridge, need little maintenance, and do not consume much energy—perfect for cash registers and parking or ticket machines.
Nevertheless, there are also drawbacks.
Why Does the Writing on the Receipt Fade?
We all know this problem: Old cash register receipts or parking tickets lose their color. Although the durability has increased in recent years, it is better not to rely on it.
If the receipt lies in the sun, it becomes warm and the solvent can melt again. In addition, non-polar substances such as plasticizers from transparent films or tanning agents from wallets can diffuse into the thermal paper. Here, they take over the function of an additional solvent. In both cases, the colored, metastable complex of dye and color developer is then no longer stabilized and the dye and color developer separate again. The dye returns to its colorless leuco form. The writing fades or disappears completely from the paper.
Depending on which solvent is used, this can happen more easily (with lower-melting-point solvents or less non-polar substances) or less easily. Stabilizers can also be added. This varies from manufacturer to manufacturer.
Why Can Cash Register Receipts Not Go into the Waste Paper Bin?
Bisphenol A (BPA), bisphenol S (BPS), or alkyl gallate are frequently used as weakly acidic color developers. BPA is particularly suitable due to its stability and heat resistance.
When using thermal paper, BPA can dissolve out of the paper layer and come into contact with the skin, thus entering the human body. When recycled, BPA is dissolved out of the paper and enters the cycle of waste paper production. Thus, other types of paper can become contaminated with BPA. For example, BPA was found in toilet paper in 2004 and in pizza packaging in 2015 [1,2].
BPA is believed to be an endocrine disruptor, meaning it interferes with the natural biochemical action of hormones, and can lead to heart disease, type II diabetes, breast cancer, prostate cancer, infertility, and neurodevelopmental disorders.
Since January 2020, the use of BPA in concentrations greater than 0.02 % in thermal paper has been banned in the European Union. Thus, BPA is being replaced in thermal papers by other chemicals, such as the chemically related bisphenol BPS. However, this is proving to be virtually as harmful to health as BPA in recent studies. More suitable are alkyl gallates, the esters of gallic acid.
Is There Another Way?
There are suppliers of thermal paper who change the chemical reaction for color development and can, thus, dispense with color developers altogether. Here, a dye is covered by an opaque functional layer in the paper. The functional layer contains bubbles that reflect light diffusely. This makes the paper appear blue. Heat causes the bubbles to collapse and the layer becomes transparent at that point. The color from the dye underneath becomes visible.
The paper is compatible with standard thermal printers and, according to the manufacturer, is the first thermal paper approved for direct food contact . It can also be disposed of in waste paper. Customers can identify the paper by its blue color.
 M. Gehring et al., Bisphenol A Contamination of Wastepaper, Cellulose and Recycled Paper Products, WIT Trans. Ecol. Environ. 2004, 78, 294–300. https://doi.org/10.2495/WM040291
 G. Stieger, Chemicals of concern in pizza boxes, Food Packaging Forum, October 15, 2015. foodpackagingforum.org (accessed October 29, 2021)
 Blue4est®, Koehler Paper SE, Oberkirch, Germany. www.koehlerpaper.com (accessed October 29, 2021)
Also of Interest
- V. Koester, How Does Thermal Paper Work, ChemistryViews 2019. https://doi.org/10.1002/chemv.201900038
- Video: How Does Thermal Paper Work?, ChemistryViews 2020. https://doi.org/10.1002/chemv.202000043
- I. Rubner et al., Wie Thermopapier funktioniert, Nachr. Chem. 2019, 67, 26–28. https://doi.org/10.1002/nadc.20194086861
- A. Jonas et al., Thermochromie und die Funktionsweise von Thermopapier: Das Experiment, Chem. Unserer Zeit 2020, 54, 166–174. https://doi.org/10.1002/ciuz.201900849