Scientists have long been able to use wood to make helmets, batteries and even sports cars. Now, they are turning their attention to thermal paper, which almost everyone touches every day but few people care about. This change is expected to quietly significantly reduce environmental and health risks.
Thermal paper is the basic material for various bills and receipts such as supermarket receipts, restaurant bills, movie tickets, boarding passes, etc. Its characteristic is that the surface of the paper is coated with a special chemical coating, which can develop color by heating without the need for ink. It can not only preserve information for a long time, but also facilitates large-scale, low-cost production. However, this coating usually contains toxic chemicals such as bisphenol A (BPA) or bisphenol S (BPS). When thermal paper is recycled as ordinary waste paper, these poisons will spread into water and soil. Related components have also been detected in people who have been in frequent contact with receipts for a long time. Environmental and health risks cannot be ignored.
It is not easy to develop alternative materials. The new thermal paper must have the multiple properties of color development when heated, not easy to fade, low price and environmentally friendly. A research team from the Ecole Polytechnique Fédérale de Lausanne (EPFL) in Switzerland recently proposed a new solution: using lignin in wood combined with a sensitizer derived from plant sugars to prepare a functional coating for thermal paper, thereby significantly reducing toxicity without changing the existing thermal printing process.
Lignin is the "glue" naturally found in wood that holds wood fibers together, but is often discarded as waste in the paper and wood industries. The researchers found that lignin itself contains chemical groups that can act as color-developing agents. Its working mechanism is similar to the color-developing system in traditional receipts and bills, and it can change color when heated. The problem is that untreated lignin has a complex structure and darker color, making it unsuitable as a coating for paper that requires a light background.
To this end, the team used a method called "sequential aldehyde-assisted fractionation" to process lignin to obtain a lignin material with a lighter color and a structure more suitable for use as a polymer. They then combined this lignin polymer with a sensitizer derived from plant sugars to make it more active at the high temperatures required for thermal printing. They then mixed the two into a thin coating and applied it to the surface of the paper, ultimately achieving a new type of paper that can be used directly in standard thermal printers.
Test results show that this lignin-based coating is both sustainable and highly durable: the coating remains stable after being exposed to direct sunlight for several months, and printed words are still legible after a year of storage. In terms of toxicity, although lignin coating is not completely harmless, its toxicity level is two to four orders of magnitude lower than BPA, while sugar-based sensitizers do not show any toxicity, and the overall risk is significantly reduced.
At present, the bio-based thermal paper still has the disadvantage that the contrast is not as good as that of traditional receipts, which means that the black and white contrast of the printed paper is weak, affecting readability. The research team still needs to continue to optimize the formula and process to improve the picture quality and achieve industrial production. Relevant research papers have been published in "Science Advances". Researchers believe that as the technology matures, this method of using wood industry waste to prepare safer thermal paper is expected to be widely used on daily carriers such as bills and receipts, and will significantly reduce the emission and exposure of toxic chemicals without changing user habits.