TAGS: Sustainability / Natural Coatings
Bayreuth researchers, led by Prof. Dr. Markus Retsch, physical chemist, develop an upcycling process that gives aluminum-plastic composite films an innovative second use. An easy-to-apply coating transforms used APL packaging into high-performance, versatile cooling films that counteract another global problem - the high energy demand for cooling systems.
Aluminum-plastic composite (APL) films are very often used for food packaging, but they pose a challenge when it comes to plastic recycling.
Upcycling Process of Chips Bags
Films made of aluminum-plastic laminates (APL) have long been used extensively to extend the shelf life of chips, roasted and powdered coffee, milk, fruit juices and other foods. The films consist of several polymer layers and an aluminum layer, which protects the products from damaging 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 chips bags now developed in Bayreuth shows a way to improve the recycling of APL waste and, at the same time, reduce global energy consumption. The aluminum layer of APL packaging provides a mirror-like reflective surface, such as that found on emergency blankets.
If a clear polymer layer is now applied that increases the radiation of thermal energy, a powerful cooling system is complete. A simple laminating film, such as is commonly found in office supply stores, is already 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 from glaring sunlight. At the same time, the ambient heat is diverted to the cool space without the need for an external energy supply. These effects are referred to as ‘passive daytime cooling. Ideally, they can lead to temperatures below the ambient temperature, even in the case of intense sunlight.
Polydimethylsiloxane (PDMS) Coating
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 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.
Aluminum-plastic composite foils fulfil these requirements very well. Using the example of coated commercial potato chips bags, the Bayreuth researchers have demonstrated that around 87 % of the sunlight is reflected by the aluminum layer. The additional polymer coating of the new sustainable cooling foils improves the 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 of turning potato chip bags and other APL packaging into efficient cooling materials. Industrial processes that use polydimethylsiloxane (PDMS) as a coating material are possible. Simple commercial laminating equipment is all that is needed to turn old APL packaging into cooling materials that can be used as heat shields on patios, balconies, exterior walls or roofs.
Source: University of Bayreuth