How does thermal paper printing work? Why does the writing disappear? Can the disappeared writing be restored?

Thermal paper, a special paper that occupies most of the fields of receipts, faxes, and labels, has text that can easily disappear over time. Why is this?
If you need to restore the information on it, is there a way to do it? The secret of thermal paper is hidden in the microscopic world under its thin coating. That key coating is mainly composed of two types of substances: leuco dyes (such as crystal violet lactone) and acidic phenolic developers (bisphenol A was mostly used in the early days). At room temperature, these two types of substances are at peace. However, when the thermal print head of a thermal printer accurately applies high temperature, changes will occur in an instant.

The heat energy melts the developer, and the released protons (H⁺) quickly attack the leuco dye molecules, triggering a violent reorganization of their internal electronic structure – the originally closed molecular ring is forcibly opened in an acidic environment.

This open-ring state drastically changes the ability of the dye molecules to absorb light, so the surface of the paper instantly presents the dark text or pattern we need. However, this seemingly stable color development state cannot withstand the test of time. So why does the text on thermal paper disappear? There are many factors that can cause the above text to change in the daily environment. The first is the disintegration caused by acid-catalyzed hydrolysis. The core of the color state is the open ring structure.

The moisture and acidic substances in the environment (such as carbonic acid formed by the dissolution of carbon dioxide in the air, or organic acids produced by aging of paper) continuously provide a source of protons.
These protons repeatedly attack the open-ring dye molecules, causing their structure to undergo hydrolysis reactions, and eventually irreversibly break and decompose into colorless small molecules. The handwriting disappears like ink drops into water. The second is the invisible erasure of oxidation: oxygen in the air also plays the role of a destroyer. It can oxidize the dye molecules in the open state, change the properties of their color-developing groups, and cause the color intensity to decay or even fade completely, just like the cloth quietly fading under the sun.

Finally, there is the accelerated destruction of light: ultraviolet energy is particularly powerful. It is like a microscopic destructive ray that can directly break the chemical bonds of the dye molecules or intensify the oxidation process, causing the fading to proceed rapidly under light.
If you want to make the disappeared handwriting reappear, is there any way? The first method is to place the faded thermal paper in an extremely low temperature environment (such as below -20°C), and some of the handwriting can temporarily restore clear outlines. This is because the molecular thermal motion slows down at low temperatures, and the decomposition products of the dye molecules may re-form a temporary ordered structure similar to the original open-ring color state. Unfortunately, once the temperature rises, the molecules regain kinetic energy, and this “phantom” disappears in an instant.

The second method is to soak the paper with a solvent (such as ethanol), and then try to locally heat or introduce volatile acids (such as acetic acid vapor) to activate the residual dye or developer to react again.
However, this method is like conducting a micro-chemical experiment on the paper surface, the effect is unpredictable, and it is very easy to damage the paper itself or cause ink to smudge. The third method is to use invisible development using spectral technology. Under the irradiation of infrared light (FTIR) or ultraviolet light of a specific wavelength, the residues of some decomposed dye molecules may show weak but detectable characteristic absorption or fluorescence. Capturing these signals through highly sensitive instruments and reconstructing images with complex algorithms can make the “disappeared” information reappear on the screen. Although this type of method relies on expensive equipment, it provides hope for the rescue of important historical bills.

In fact, instead of waiting for the handwriting on the thermal paper to disappear and then trying to restore it, it is better to protect important information in advance to avoid or delay the fading of thermal paper.
It is most important to store thermal paper documents in a cool, dark and dry place. Low temperature slows down chemical reactions, dryness inhibits hydrolysis, and light avoidance blocks the most destructive ultraviolet rays. Sealed plastic bags or moisture-proof boxes can provide an extra layer of protection. Photocopying or scanning copies while the handwriting is still clear is the most reliable and economical option. Digital copies can be preserved for a longer time and are easy to retrieve. The good news is that the handwriting on thermal paper may not be as perishable as it is now in the future. Research is now underway to use dyes that are more resistant to hydrolysis and oxidation (such as certain fluorane compounds), as well as more environmentally friendly and stable color developers (such as bisphenol S instead of bisphenol A), and add special stabilizers. Although these new generations of paper are slightly more expensive, they have a significantly longer shelf life and are gradually becoming the first choice for important documents.