Thermochromic Reaction

Within the receipt printer apparatus, the thermal printhead generates heat in a precise pattern corresponding to the specified text or images^{1 2}. Heat is applied directly to the thermal receipt paper as it passes under the printhead^{1 2}. This localized heating is the trigger for the subsequent chemical reactions in the coated layer of the paper, starting the colour development process in specific areas to create the desired print^{1 2 3}.

PCM Melting

The PCM melts when exposed to the heat from the printhead^{1 2}. As the leuco dye and developer are suspended in the PCM, they cannot interact^{1 2}.

Thermochromic Reaction

The developer, which is commonly BPA or BPS^{4 5}, has acidic properties that trigger a phase change in the leuco dye^{1 2}. his event, called protonation, involves the developer donating hydrogen ions (protons) to the leuco dye^{2 3 6 7}. Adding a hydrogen ion changes the leuco dye’s structure, altering how the molecule interacts with light and thus its colour^{2 3 6 7}. This process is called a thermochromic reaction, as it involves a change in colour triggered by temperature changes^{2 3 6 7}. The exact colour produced depends on the structure of the leuco dye and the nature of the protonation^{2 3}.

Below is an example of the chemical reaction between the leuco dye fluoran and BPA.

Cooling

After moving beyond the printhead, the PCM cools down and solidifies^{1 2}. During this time, the developer and leuco dye are resuspended, albeit in an altered configuration^{2 3}.
Unless heat is reapplied, printed receipt paper will maintain its chemical integrity^{1 2}. Over time, the chemical bonds in the paper start to break down, leading to a gradual fading of the print as its stability decreases^{1 2}.