The paper contains a thermosensitive layer that contains pigments , binders, color formers, developers and auxiliaries. It is available in different font colors, but the most common use is black, followed by blue. Due to different high printing temperatures, several color levels can be realized on suitable thermal paper in special printers. Due to the higher technical complexity, however, it is not used in typical commercial and industrial applications.
Thermal paper is usually delivered wound on a plastic or cardboard core. It is available in weights from 48 to approx. 240 g / m². For printing receipts and receipts, thermal rolls have a standard paper weight of 48 to 55 g / m². The stronger paper grades are used for admission tickets, vouchers, travel tickets and lottery receipts. Due to the thicker paper, which has a higher tolerance to mechanical stress, the print quality is also retained longer.
Paper grades are certified for the food sector (released in Germany by ISEGA-Forschungs- und Investungsgesellschaft mbH, Aschaffenburg ) and for environmental compatibility (in addition to the Blue Angel , the Nordic Swan certificate is also known, the latter, however, as a collaboration between some paper manufacturers and therefore not independent) .
The thermal paper is heated selectively at the points to be written on by small heating elements that are arranged in rows across the width of the paper to be printed. As a result, color formers and developers placed on the paper react and produce the print image.
Simple thermal printers for private households print the paper with at least 90 ° C, as common household paper types discolour from approx. 70 ° C or a little higher. Paper for parking tickets, which is exposed to high temperatures and high radiation intensity in summer, is printed at up to 120 ° C, since the reaction temperature of the paper must be higher here. It is determined by the "static sensitivity". Inexpensive fax machines can usually only print a little faster than the data rate required for fax transmissions (approx. 10–15 seconds per page), high-quality paper types for barcode label printing can be printed at speeds of up to 400 mm per second. The paper can of course be transported faster in print-free areas.
More precise chemical compositions are kept secret in many cases as a trade secret.
Inexpensive thermal papers age very quickly, also depending on the intensity of the printing (heat during the printing process), so that the writing fades a lot after just a few weeks. There are also thermal papers with a 25-year shelf life guarantee. In Japan, paper types are produced with "almost unlimited archivability". However, the values given relate to approx. 20 ° C and 50% relative humidity, without direct sunlight influence - situations that are not found in private households and in most office spaces.
Thermal paper prints are therefore generally not considered document-proof . This is problematic for invoices and other documents that have to be kept for several years. If necessary, a copy should be made on normal paper, which is partially indicated on the back of such printouts - it is therefore up to the customer to make copies that can be stored if necessary.
In order to keep the typeface as long as possible, thermal paper should not be exposed to direct sunlight or excessive heat. Plasticizers , such as those used e.g. B. in plastic film or tape , can remove the font. Tannins, such as those occasionally found in leather (purses), can also cause the printed image to fade. The printout can also become illegible through contact with disinfectants, solvents, oils and fats.
Often, a faded print can be made visible again by careful warming (hair dryer, heat gun, iron at low temperature and a sheet of paper in between as protection).
Bisphenol A use
In July 2016, the EU Commission approved a proposal by the European Chemicals Agency to limit the occurrence of bisphenol A in thermal paper in the future. The change comes into force in 2020 and says that the concentration of BPA in thermal rolls must not be more than 0.02 percent - which amounts to a ban. From 2020 onwards, BPA may no longer be added to paper production. The permitted residual content is generated from residues from recycled paper. The industry has switched to alternative substances or has developed processes for coating that do not require color developers. The Federal Environment Agency recommends that thermal papers continue to be disposed of as residual waste for precautionary reasons.
Due to the ban on bisphenol A, the expected European Chemicals Agency the use of bisphenol S , in addition Pergafast 201 and D8 . Bisphenol-S also poses a health concern (according to a study by the University of California ). A reassessment of health risks by the European Chemicals Agency has been running since 2014 and is carried out by Belgium .
In recent years the quality of the printers used and the thermal papers have been greatly improved, and progress has been made in the shelf life of printed thermal papers. The current trend is to introduce rewritable thermal films for special applications that can be written on and erased several times.
A number of providers of such papers have been offering BPA-free products since 2010 and are promoting them in accordance with a recommendation by the Federal Environment Agency .
Thermal paper without chemical developers has been on the market since 2017.
- Klaus Baldig: Technical article on the structure, properties and suitability purposes of thermal paper (Baldig is Head of Development and Product Management at GeBE Elektronik und Feinwerktechnik GmbH; PDF; 798 kB)
- Bavarian State Office for the Environment: Bisphenol A in thermal paper (PDF).
- Questions and answers on mandatory receipts. In: Umweltbundesamt.de. Federal Environment Agency , January 10, 2020, accessed on January 12, 2020 (section How should thermal paper be disposed of? ): “As a precautionary measure, the Federal Environment Agency recommends disposing of all thermal paper with the residual waste, since color developers containing phenol are still used and the consumer does not use phenol-containing color developers Can differentiate between thermal papers. "
- Use of bisphenol A and its alternatives in thermal paper in the EU - 2018 update. (PDF) European Chemicals Agency , June 2019, p. 1 , accessed on December 30, 2019 (English).
- Yichang Chen: Exposure to the BPA-Substitute Bisphenol S Causes Unique Alterations of Germline Function. In: journals.plos.org. July 29, 2016, accessed January 25, 2020 .
- Bisphenol A - mass chemical with undesirable side effects. Federal Environment Agency , June 9, 2010, accessed on January 12, 2020 .
- Blue4est® - the blue thermal paper from the Black Forest. In: koehlerpaper.com. Koehler Paper Group, accessed January 12, 2020 .