Thermal paper

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Thermal paper is a special fine paper coated with a heat‑sensitive layer. When it passes under a thermal printer’s heated head, tiny heated dots cause the coating to change color where heat is applied. Most coatings turn black, but some turn blue or red, and multicolor options exist. It’s commonly used in receipts, cash registers, card terminals, and small portable printers.

Printing works by moving the paper across a small thermal dot matrix head. The dots heat up very quickly to form an image, then cool rapidly. Because the coating can react to heat, unintended warmth—like a hot cup or even friction from rubbing with a fingernail—can leave marks or blur the print.

Direct thermal papers are the simplest type: they don’t need ink or ribbons, only heat to create the image. Thermal transfer papers use a ribbon to transfer pigment for tougher, longer‑lasting prints. Direct thermal prints can fade over time, while thermal transfer prints stay clearer longer.

Chemistry behind the image involves four components: leuco dyes, developers, sensitizers, and stabilizers. The paper usually comes as rolls (or larger sheets) and may have an adhesive backing for labels. It can be white, colored, or transparent.

Some multicolor thermal papers exist. In the 1990s and 2000s, advanced systems allowed full color in one pass:
- Fuji’s Thermo Autochrome (1993) and Polaroid’s Zink (2007) use layered, colorless pigments that react to heat to form colors. Zink uses heat pulses to activate cyan, magenta, and yellow layers in the right order, producing a full color image without ink cartridges.

Two notable safety notes:
- Some thermal papers are coated with BPA (bisphenol A), which can transfer to skin and may contaminate recycled paper. In 2014, Suffolk County (USA) banned BPA in thermal receipts.
- To reduce BPA exposure, some newer papers use BPA‑free coatings or alternatives like BPS (bisphenol S). BPS can have similar concerns, so many producers are moving to phenol‑free formulations using urea‑based compounds or vitamin C, though these can cost more.

In short, thermal paper lets you print without ink by using heat-sensitive coatings. It’s convenient for receipts and labels, with both simple direct‑thermal and more durable thermal‑transfer options, and there are ongoing innovations to reduce health and recycling concerns.