International. Bayreuth researchers, led by Prof. Dr. Markus Retsch, a physical chemist, develop a recycling process that gives films composed of aluminum and plastic a second innovative use.
An easy-to-apply coating transforms used APL packaging into versatile, high-performance cooling films that counter another global problem: the high energy demand for cooling systems.
Aluminum-plastic composite films (APLs) are very frequently used for food packaging, but they pose a challenge when it comes to plastic recycling.
Films made of aluminum and plastic laminates (APL) have long been widely used to extend the shelf life of potato chips, roasted and powdered coffee, milk, fruit juices and other foods.
The films consist of several layers of polymer and a layer of aluminum, which protects the products from harmful factors: sunlight, heat, moisture and oxygen. However, recycling such composite films is difficult as a result of this combination of different materials.
The upcycling process of potato chip bags now developed in Bayreuth shows a way to improve the recycling of APL waste and, at the same time, reduce overall energy consumption. The aluminum layer of APL packaging provides a reflective mirror-like surface, like that found on emergency blankets.
If a transparent polymer layer that increases the thermal energy radiation is now applied, a powerful cooling system is completed. A simple laminating film, such as that commonly found in office supply stores, is already sufficient as a material for coating.
The coating creates cooling foils that can be applied to any outdoor surface such as umbrellas, blinds and awnings and thus prevents the heating from dazzling sunlight.
At the same time, ambient heat is diverted into the cool space without the need for an external power supply. These effects are called "passive daytime cooling."
Ideally, they can lead to temperatures below room temperature, even in the case of intense sunlight.
Passive daytime cooling is possible because the materials used meet special optical requirements. They should scatter or reflect as much sunlight as possible, which has a wavelength between 0.3 and 2.5 mm.
On the other hand, in the wavelength range between 8 and 13 mm, the so-called sky window, they must emit as much thermal energy as possible into space in the form of infrared radiation.
According to the researchers, sheets composed of aluminum and plastic meet these requirements very well. Using the example of coated commercial potato chip bags, the Bayreuth researchers have shown that around 87% of sunlight reflects off the aluminium layer.
The additional polymer coating of the new sustainable cooling sheets improves radiation in the wavelength range of the sky window and thus emits heat directly into space.
Prof. Dr. Markus Retsch and his colleague, Dr. Qimeng Song, have tested different ways to turn potato chip bags and other APL packaging into efficient cooling materials. Industrial processes using polydimethylsiloxane (PDMS) as the coating material are possible.
A simple commercial lamination equipment is all that is needed to turn old APL packaging into refreshing materials that can be used as heat shields on patios, balconies, exterior walls or ceilings.
