From snack bag to cooling technology: The rebirth of aluminum-plastic laminates

Thanks to aluminum-plastic laminates, our favorite snacks stay as crisp as we love them. But what if the shiny insides of snack bags hold much greater potential – one that goes far beyond accompanying a movie night? Passive cooling technology could benefit from upcycling these chip bags, says a research team from the University of Bayreuth.

In an interview with K-MAG, Prof. Dr. Markus Retsch, a physical chemist and head of the project, discusses the innovative use of aluminum-plastic laminates, why they have been a challenge for current recycling processes, and how they could pave the way to a more sustainable, energy-efficient future.

Dr. Retsch, why are aluminum-plastic laminates (APLs) so popular for food packaging such as potato chip bags?

Dr. Markus Retsch: APLs are often used for food packaging because they create a barrier layer. This prevents air and water from passing through the packaging plastic. This keeps the packaged food in the bag fresh and durable for longer.

Why are APLs a challenge for common recycling processes?

Dr. Retsch: The problem with APL packaging is their combination of different material classes. The aluminum used cannot be mechanically or thermally recycled like other plastics and thus leads to contamination of the recycled plastics. In addition, the thin aluminum layer is firmly welded to the surrounding polymer films, so that this foreign material cannot be separated from the plastic film. Overall, this means that such packaging films can hardly be recycled.

How did you come up with the idea of using APL packaging for energy-saving cooling foils?

Dr. Retsch: Through our many years of research into novel materials with passive cooling properties, we had long been on the lookout for a cost-effective but high-quality source of highly reflective materials. So it came about that we took a closer look at the inside of such packaging films. A special property of these aluminum-plastic composite films is that they have a highly reflective surface on their non-printed side, i.e. on the inside. This ultimately gave rise to the idea of recycling these packaging films and making them usable for passive cooling applications.

What features of APLs are particularly useful for this?

Dr. Retsch: The key factor for the required properties is a high reflectivity of the aluminum foil. Ultimately, the aim is to ensure that sunlight in the spectral range from 250 nanometers to 2.5 micrometers is completely reflected and that as little sunlight as possible is absorbed. In this way, the sunlight is not converted into heat. Highly reflective surfaces meet these requirements well, and so it is with the aluminum composite foil.

How exactly does the process work – What has to happen for the chip bag to become a cooling foil?

Dr. Retsch: The reflective properties of the aluminum composite foil alone are not sufficient to produce passive cooling properties. For this, it is additionally necessary to improve the emissivity, i.e. the emission of thermal energy in the form of thermal radiation. This can be achieved by coating the aluminum composite films with a polymer which, on the one hand, does not absorb light in the solar spectral range, but on the other hand, greatly increases emissivity in the mid-infrared range, especially in a wavelength range of 8 to 12 micrometers. This allows the advantages of two worlds to be combined.

While it doesn't absorb heat from solar energy, it continuously emits heat in the form of mid-infrared thermal radiation. This emitted heat is then actually absorbed in space, which serves as a heat sink. In this way, it is possible to achieve that heat is emitted from the earth into cold space and such materials can cool down below the ambient temperature under strong solar radiation. The necessary additional coating can be accomplished by various techniques, e.g., by doctoring polymerizable films or by laminating on another plastic film.

Where can these cooling foils be used or where are they even already in use?

Dr. Retsch: As a first step, our cooling foils have shown how such aluminum composite foils can be recycled. In general, such cooling foils have not yet been used commercially on a large scale. However, there are a few startups that have already been able to demonstrate with similar foils that this can save a lot of energy for cooling requirements in data centers or in supermarkets. I see other possible areas of application, for example, in the field of blinds, shutters, awnings or sunshades, which are also widely used by private customers.

What inspiration can the plastics recycling industry draw from your project?

Dr. Retsch: What I find particularly exciting about our cooling film is the idea that a high-quality high-tech material can be produced from a waste product. As a result, there is an incentive to sort such films in a targeted manner and bring them into a profitable second application. However, a commercial and large-scale implementation of our approach is still pending and requires further efforts to make passive cooling films marketable as yard goods. Overall, I think that bold, creative and innovative solutions are needed to bring polymers into further applications even after their first use as valuable materials.

A personal question to finish: Which type of chips is your favorite?

Dr. Retsch: I generally eat very few chips. If I do, I favor salted nachos or chips with salt and vinegar.