From Chip Bag to Energy-Saving Cooling Foil
Aluminum-plastic composite films (APL) are often used for food packaging. However, they are a challenge for recycling. Researchers led by the Bayreuth physical chemist Prof. Dr. Markus Retsch have now developed an upcycling process that enables such films to have an innovative second use.
Aluminum-plastic composite films, APL (Aluminum-Plastic Laminates) for short, have been used for a long time to extend the shelf life of foods packaged in them, such as chips or coffee. The foils consist of several polymer layers and an aluminum layer, which protects the products from damaging factors – particularly from solar radiation and heat, but also from moisture and oxygen. However, the recycling of such composite films is hardly possible due to the combination of different materials.
However, an upcycling process for potato chip bags developed at the University of Bayreuth in the “physical chemistry” department shows a way to improve the recycling of APL films – and at the same time reduce global energy consumption. After all, according to the university, cooling systems currently account for around 15 percent of global energy consumption. In view of climate change and the resulting frequency of heat waves, this proportion threatens to increase further.
The aluminum layer of APL packaging represents a highly reflective surface, as is known from rescue blankets, for example. If a clear polymer layer is now applied, which promotes the radiation of thermal energy, the scientists believe that a high-performance cooling system is complete. A simple laminating film, such as is commonly used in office supply stores, is sufficient as a material for the coating. The coating creates cooling foils that can be applied to any surface in the open air – such as umbrellas, blinds and awnings – and thus prevent heating up by bright sunlight. At the same time, the ambient heat that is already present is released into cold space without the need for an external energy supply. Researchers refer to these effects as “passive daytime cooling”. Ideally, they can lead to temperatures below the ambient temperature even with intense solar radiation.
According to the scientists, passive daytime cooling is made possible by the fact that the materials used meet special optical requirements. They must scatter or reflect as much of the sunlight which has a wavelength of between 0.3 and 2.5 microns as possible. In the wavelength range between 8 and 13 micrometers, the window of transparency in our atmosphere, they have to emit as much thermal energy as possible into space in the form of infrared radiation. Aluminum-plastic composite films meet these requirements very well.
Using the example of coated commercial potato chip bags, the Bayreuth researchers have demonstrated that around 87 percent of the sunlight is reflected by the aluminum layer. The additional polymer coating of the new sustainable cooling foils improves the radiation in the area of the transparency window of the atmosphere and thus releases heat directly into space.
Prof. Dr. Markus Retsch and his colleague Dr. Qimeng Song have explored different ways to turn potato chip bags and other APL packaging into efficient cooling materials. Industrial processes in which polydimethylsiloxane (PDMS) is used as the coating material are therefore suitable. However, it is also conceivable that the coating will take place in private households in the future. Simple commercially available laminators were sufficient to produce cooling materials from old APL packaging, which can be installed as heat shields on patios, balconies, outside walls or on the roof. An idea that points to the future.