Thermal paper
Thermal paper (sometimes referred to as an audit roll) is a special fine paper that is coated with a material formulated to change color when exposed to heat. It is used in thermal printers, particularly in inexpensive or lightweight devices such as adding machines, cash registers, and credit card terminals. The surface of the paper is coated with a solid-state mixture of dye and a suitable matrix; a combination of a fluoran leuco dye for example. When the matrix is heated above its melting point, the dye reacts with the acid, shifts to its colored form, and  the changed form is then conserved in a metastable state when the matrix solidifies back quickly enough. The reactant acid in thermal paper is often bisphenol A (BPA). Usually, the coating will turn black when heated, but coatings that turn blue or red are sometimes used. While an open heat source, such as a flame, can discolor the paper, a fingernail swiped quickly across the paper will often generate enough heat from friction to produce a mark. Multicolor thermal paper first became available in 1993 with the introduction of the Fuji Thermo-Autochrome (TA) system. This was followed in 2007  by Polaroid's development of the Zink ("zero-ink") system. Both of these methods rely on multi-layer coatings with three separate colorizing layers, with different methods used for independent activation of each layer. History. The earliest direct thermal papers were developed by NCR Corporation (using dye chemistry) and 3M (using metallic salts). The NCR technology became the market leader over time, although the image would fade rather rapidly compared with the much more expensive, but durable 3M technology. Texas Instruments invented the thermal print head in 1965, and the Silent 700, a computer terminal with a thermal  printer, was released in the market in 1969. The Silent 700 was the first thermal print system that printed on thermal paper. During the 1970s, Hewlett-Packard integrated thermal paper printers into the design of its HP9800 series desktop computers, and integrated it into the top of the 2600-series CRT terminals as well as in plotters. In the 1970s and early 1980s, Japanese producers (such as Ricoh, Jujo, and Kanzaki), using similar dye-based chemistry, formed partnerships with barcode printer manufacturers (such as TEC, Sato, and others) and entered the emerging global bar code industry, primarily in supermarkets. U.S.  producers such as Appleton (NCR's licensee), Nashua Corporation, Graphic Controls, and others fought to gain market share. Leading pressure-sensitive label producers such as Avery Dennison became major consumers of direct thermal paper for label applications. In the late 1980s and early 1990s, thermal transfer, laser printing, electrophotography, and, to a lesser extent, ink jet printing began to take away industrial and warehouse barcode applications due to better durability. Direct thermal made a strong comeback with point of sale receipts (gasoline pumps, cash registers, rental car receipts, etc.). During 1998, Nintendo used thermal paper technology for their Game Boy  Printer. In 2006, NCR Corporation's Systemedia division introduced two-sided thermal printing technology, called "2ST". Chemistry. Four different types of imaging chemicals are used in thermally sensitive papers: leuco dyes, developers, sensitizers and stabilizers. Multicolored papers. In the early 2000s, Polaroid developed the Zink "zero-ink" technology. The paper is used in compact photo printers. It has several layers: a backing layer with optional pressure sensitive adhesive, heat-sensitive layers with cyan, magenta and yellow pigments in colorless form, and overcoat. Zink technology allows the printing of full-color images in a single pass without requiring ink cartridges. The  color addressing is achieved by controlling the heat pulse length and intensity. The color-forming layers contain colorless crystals of amorphochromic dyes. These dyes form microcrystals of their colorless tautomers, which convert to the colored form by melting and retain color after resolidification. The yellow layer is the topmost one, sensitive to short heat pulses of high temperature. The magenta layer is in the middle, sensitive to longer pulses of moderate temperature. The cyan layer is at the bottom, sensitive to long pulses of low temperature. The layers are separated by thin interlayers, acting as heat insulation, moderating the heat  throughput. Protective coating. Most direct thermal papers require a protective top-coating to: Health and environmental concerns. Some thermal papers are coated with BPA, a chemical considered to be an endocrine disruptor. This material can contaminate recycled paper. BPA can transfer readily to the skin in small amounts: The chemical bisphenol A (BPA) is used for thermal paper coatings because of its stability and heat-resistance. This allows inkless printing for receipts from cash registers. People who often are in contact with BPA coated receipts do have a higher level of BPA in their bodies than people with average contact . Therefore, the New York Suffolk County signed a resolution to ban BPA in thermal receipt papers. Violation of this new law, the "Safer Sales Slip Act", involves a US$500 penalty. The law became effective beginning January 3, 2014. Recently, bisphenol S (BPS), an analog of BPA that has been shown to have similar "in vitro" estrogenic activity to BPA, has been used in thermal paper coatings. The recycling of thermal paper coated with BPS can introduce BPS into the cycle of paper production and cause BPS contamination of other types of paper products. Newer formulations can be purchased which  are "phenol free" and use either urea based compounds or vitamin-c. The phenol free variants such as urea based can have comparable or even improved print quality, however it can cost more.