Thermal paper

Thermal paper (often supplied in roll form, and sometimes referred to as an audit roll) is a special fine paper that is coated with a material formulated to change color locally when exposed to heat. It is used in thermal printers, particularly in inexpensive devices such as adding machines, cash registers, and credit card terminals and small, lightweight portable printers.
The surface of the paper is coated with a substance which changes color when heated above a certain temperature. The printer essentially consists of a transport mechanism which drags the paper across a thermal dot matrix print head. The (very small) dots of the head heat up very quickly to imprint a dot, then cool equally quickly.
Most thermal papers' coatings turn black when heated, but coatings that turn blue or red, and multicolor coatings, are sometimes used. An unintended heat source, such as a coffee cup, can discolour the paper and obscure any printing. A fingernail rubbed quickly across the paper may generate enough heat from friction to produce a mark.
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 HP 9800 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 including Ricoh, Jujo, and Kanzaki, using similar dye-based chemistry, formed partnerships with barcode printer manufacturers including TEC and Sato and entered the emerging global bar code industry, primarily for supermarket receipt printers. U.S. producers including Appleton (NCR's license), Nashua Corporation, and Graphic Controls fought for market share. Users of pressure-sensitive label such as those made by Avery Dennison became major consumers of direct thermal label stock.
In the late 1980s and early 1990s, thermal transfer (distinct from direct thermal, and stable), laser printing, electrophotography, and, to a lesser extent, inkjet printing began to take away industrial and warehouse barcode applications due to better stability and durability of prints. Direct thermal made a strong comeback with point-of-sale receipt printing.
During 1998, Nintendo used thermal paper technology for their Game Boy Printer.
The printer essentially consists of a transport mechanism which drags the paper across a thermal dot matrix print head. The (very small) dots of the head heat up very quickly to imprint a dot, then cool equally quickly.
Four different types of imaging chemicals are used in thermally sensitive papers: leuco dyes, developers, sensitizers and stabilizers.
Papers are supplied either as rolls or (particularly for wider letter-size printers) sheets. They may have a stick-and-peel adhesive backing, for use as labels and similar purposes. Paper may be white, other colors, or transparent.
In 2006, NCR Corporation's Systemedia division introduced two-sided thermal printing technology, called "2ST".
Most direct thermal papers require a protective top-coating to:
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. 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.
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.
From about 2013 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 are available which use either urea-based compounds or vitamin C, and are "phenol free". They can have comparable or even improved print quality, but cost more.