Thermal paper-also known as thermal fax paper, thermal recording paper, or thermal copy paper (and referred to as "thermal carbonless paper" in Taiwan)-is a type of processed paper manufactured by coating a high-quality base paper with a "thermal coating" (a heat-sensitive color-changing layer). Although more than a dozen types of chemicals are used in this color-changing layer, they invariably include leuco dyes.
A wide variety of such dyes exists, with fluorescent compounds being the most common. Color developers typically account for less than 20% of the composition, with bisphenols and p-hydroxybenzoic acid being frequently used. Sensitizers make up less than 10% and include benzenesulfonamide compounds. Fillers constitute less than 50%, with calcium carbonate (in particulate form) being a common choice. Binders, such as polyvinyl acetate, account for less than 10%. Other components include stabilizers (e.g., diphenyl terephthalate) and lubricants. Consequently, the manufacturing process is complex and demands high technical standards.