Key Chemicals

flowchart LR
	leuco([Leuco Dye]) ---> pcm[Phase Change Material]
    dev([Developer]) ---> pcm
    
    pcm --> tl[Thermal Layer]

    sen([Sensitizers]) --> tl
    stab([Stabilizers]) --> tl

    tl --> rp((( Receipt Paper)))
    
    style tl stroke-width:4px,stroke-dasharray: 0

Thermal Layer

Within the thermal layer, five chemical components facilitate a reaction when the receipt paper is exposed to heat^{1 2}. The most critical of these are the leuco dye, developer, and phase change materials^{2 3 4 5}.

Leuco Dyes

Leuco Dyes are colourless (or nearly colourless) compounds that can be transformed into a coloured state by a chemical reaction^{2 3}. In thermal paper, leuco dyes develop colour when they react with developers^{2 3}. The colour change typically occurs because of a structural change in the dye molecule, which alters the way it absorbs and reflects light^{2 3}.

Two most common leuco dyes used (click to learn more)^{6 7}:

Fluoran

Fluoran is a broad category of leuco dyes that can produce typically black or blue.

Crystal Violet Lactone (CVL)

CVL is a triphenylmethane leuco dye that is used to produce blue or violet.

Developers

Developers trigger the colour change process by chemically reacting with the leuco dye^{1 8 2}.
Upon the application of heat, the encapsulating medium (see below) melts, releasing the developers to react with the leuco dye^{1 8 2}.

Two most common developers used (click to learn more)^{9 10}:

Bisphenol A (BPA)

BPA is a common and easily produced industrial chemical with acidic properties.

Bisphenol S (BPS)

BPS is a chemical alternative to BPA with similar acidic properties.

Phase Change Materials (PCM)

Phase change materials serve as the medium within which the leuco dye and developer are embedded^{1 2}. When heat is applied, it melts, allowing a reaction between the leuco dye and the developer. A commonly used PCM is paraffin wax^{1 2}.

Next, we will explore the colour-changing reaction that occurs between these critical chemicals.