The full diaper is thrown away and a fresh one is put on…: Around 100,000 tons of disposable diapers accumulate in children’s rooms in Germany alone every year – and end up in the residual waste. In order to combat this pile of waste and the waste of raw materials, researchers have now developed a practical recycling process for the absorbent pads, which were previously hardly degradable: only through moistening and subsequent UV irradiation can the polymer material be liquefied into a substance that is used to produce materials such as adhesives. and dyes, their study shows.
They are found primarily in disposable diapers, but also in numerous other hygiene and medical products such as sanitary napkins: So-called superabsorbent materials give these products enormous absorbency and storage capacity for liquids. This effect is based on cross-linked polymer structures made from elaborately manufactured sodium polyacrylate. Unfortunately, this material has proven to be a tough nut to crack when it comes to its recycling options: only a 16-hour acid bath at 80 degrees Celsius seemed to be able to transform the superabsorbents back into reusable substances.
Is it easier to crack the stubborn stuff?
However, this process is so complex and expensive that it is not worth it. That’s why disposable diapers and other superabsorbent materials have not yet been recycled sustainably: millions of tons of these raw material-rich products are landfilled or incinerated worldwide. Scientists led by senior author Pavel Levkin from the Karlsruhe Institute of Technology have now explored whether there is a more practical option for recycling. They carried out experiments with absorbent pads that they cut out of conventional disposable diapers.
During their experiments with various processes, they discovered that it has apparently not yet been discovered how intensively sodium polyacrylate reacts to ultraviolet radiation when wet. When they moistened the untreated absorbent pad material with water and then exposed it to radiation from a 1,000-watt UV lamp, it began to drip after just five minutes. The material apparently went from a solid to a liquid state.
The analyzes of the process then clarified what happened: “The chains that connect the polymers to each other are broken by the UV radiation and are then so loose that they float in the water and become liquid fibers,” says Levkin. Ultimately, it was shown that this comparatively simple UV degradation process can effectively break down the stubborn sodium polyacrylate: without chemicals and at room temperature, irradiation breaks the chains 200 times faster than the problematic acid bath process.
Raw material for adhesives etc
According to the scientists, the process can therefore be used in practice and leads to an extremely valuable raw material. “UV photodegradation and esterification of superabsorbents is fast, scalable, safe and economical, and leads to polymers with controllable molecular weight,” the team writes. To demonstrate the potential, the researchers used the recycled substance from the diapers to produce adhesives and dyes. “It was important that the sodium polyacrylate was soluble and therefore easy to process. “You can certainly do a lot more with it,” emphasizes Levkin.
However, when thinking about used diapers, one can still ask a critical question: clean raw material was used for the tests – couldn’t the typical contamination be a problem for the recycling process? Apparently not: the scientists emphasize that it is easy to clean the superabsorbent material. “So nothing stands in the way of a realistic application,” says Levkin. In addition, the recycling process can be optimized cost-neutrally and ecologically using solar power, says the scientist. “We have found a pioneering strategy for reusing superabsorbents. “This could significantly reduce environmental pollution and contribute to a more sustainable use of polymers,” sums up Levkin.
Source: Karlsruhe Institute of Technology, specialist article: ACS Appl. Mater. Interfaces, doi: 10.1021/acsami.3c06999