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The scientific principle of thermal paper

The scientific principle of thermal paper


What we usually call printing is to transfer the ink to a specific area of the paper in a specific way, so as to obtain the text or graphics we want. The printers we use at home or in the office are mainly printers and laser printers. Inkjet printers spray tiny droplets of ink onto the paper, while laser printers use electrostatic attraction to first adsorb the toner on the optical drum and then transfer them to the paper.

However, the printing of shopping receipts is not done in this way. It uses a special paper-thermal paper.

Compared with ordinary paper, the surface of thermal paper has a thin layer of coating, which contains some special chemical substances called leuco dyes. The leuco dye itself is colorless. At room temperature, thermal paper looks no different from ordinary paper. Once the temperature rises, the leuco dye and the acidic substance will melt into liquid one after another, and freely movable molecules will meet. There was an immediate reaction, and the white paper quickly showed the color. This is why the thermal paper gets its name-only when the temperature reaches a certain level will the paper change color. In other words, when we print on thermal paper, the ink is not stored in the printer, but is already covered on the paper.

With thermal paper, if you want to print text or graphics on its surface, a special printer is needed. This is a thermal printer. If you have the opportunity to disassemble the thermal printer, you will find that its internal structure is very simple: without ink cartridges, the main components are the roller and print head. The thermal paper used for printing receipts is usually made into rolls. When a roll of thermal paper is put into the printer, it will be transported forward under the drive of the roller, and will contact the print head. There are many tiny semiconductor components on the surface of the print head, which can heat specific areas of the paper according to the text or graphics we want to print. At the moment the thermal paper comes into contact with the print head, the high temperature generated by the print head causes the dye and acid on the surface of the thermal paper to melt into a liquid and chemically react, so text or graphics appear on the surface of the paper. Driven by the roller, a small shopping ticket was printed out.

Why do businesses use thermal paper and thermal printers to print shopping receipts instead of laser or inkjet printers that we are more familiar with?

First of all, laser or inkjet printers require complicated devices to transfer ink or toner from the printer to the paper. Both printers are bulky and usually require AC power as a power source; merchants often need to be compact The printer, especially when selling goods outdoors or on airplanes, trains, etc., it is obviously unrealistic to carry a heavy printer to print receipts for customers.

Secondly, the replacement of ink cartridges or toner for laser or inkjet printers is often time-consuming and laborious. If this delays the checkout of customers, this is also very unwilling to see for merchants and consumers .

If you use thermal printers and thermal paper instead of laser or inkjet printers, these problems will be solved. Since the ink has been pre-stored on the paper, the thermal printer no longer needs a complicated structure to store and transfer the ink, and can be very compact. And it can be battery-driven, which is very suitable for merchants to carry, especially for printing receipts for customers outdoors or on transportation. Due to the simple structure, the thermal printer is also very easy to maintain. Users do not need to worry about replacing the ink cartridges, as long as the paper is used up quickly to replace a new roll of thermal paper. In addition, the thermal printer has fast printing speed and low noise, which is very suitable for use in shopping malls. Because of these advantages, thermal printing has not only become the preferred method of printing shopping receipts, but also often used to print tickets, labels and even faxes.

Thermal paper also has a relatively large defect, that is, the writing on the documents printed on it will gradually fade after being stored for a period of time. The fading is also due to the unique dyes used in thermal paper. As we mentioned earlier, the leuco dye covering the thermal paper is colorless at room temperature, and becomes another colored structure due to a chemical reaction at high temperature. However, this new structure is not so stable, and it will return to the previous colorless structure under suitable conditions. After a shopping receipt is printed out, it may come into contact with various chemicals in the environment during the preservation process, and may also encounter sunlight or high temperature, which may cause the dye on the thermal paper to return the colorless form and makes the receipt fade.

In order to solve this problem, many thermal paper manufacturers will add an additional protective layer on top of the dye layer to minimize the contact between the dye and other chemicals, so that the documents printed on thermal paper can be stored longer . However, this method will increase the cost of thermal paper, so some businesses will continue to use ordinary thermal paper without a protective layer. If you are worried that the shopping receipt in your hand will fade after being stored for a long time, it is a good way to keep a copy or scan it in time.

Over the years, thermal paper has caused many consumers' concerns due to the presence of bisphenol A.  Bisphenol A is an acidic substance, so it is used in thermal paper to chemically react with leuco dyes at high temperatures to produce colors; in addition, bisphenol A is more used to make certain plastics or coatings Raw materials. Therefore, the main way for bisphenol A to enter the human body is when these containers are used to hold food, a small amount of bisphenol A enters the human body along with the food. However, in recent years, studies have also shown that contact with bills printed on thermal paper may also cause BPA to enter the human body. People worry that it may interfere with the normal endocrine system after entering the body, thereby increasing the risk of many diseases. The concentration of bisphenol A that enters the human body through food and thermal paper is very low. Existing research is still difficult to determine what impact it will have on human health. However, although bisphenol A has not been banned for the production of thermal paper, many manufacturers have begun to use other acidic substances instead.

If you are worried about the small amount of BPA entering the body due to contact with the shopping receipt, the more feasible prevention method is to store it separately as soon as possible after receiving the receipt without excessive contact, and wash clean hands after touching the receipt. 

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