Recyclable plastics in fiber composites for lightweight construction

Polymeric materials can be used in many ways and are characterized by their lightweight and mechanical properties. To increase the lightweight potential, plastics are reinforced with fibers to form fiber composites. However, due to consumer pressure and regulations, companies are increasingly looking for environmentally friendly and recyclable material substitutes for conventional polymeric materials and composites. For several years, Fraunhofer IFAM has been researching switchable thermosetting materials as a promising alternative. The materials are designed according to a switching principle so that they react to certain external stimuli. The result: The materials are still malleable after curing, self-healing, and can be designed to be both recyclable and biodegradable.

 

What are smart plastics?

The disadvantages of conventional plastics include the fact that they can no longer be formed after curing and are difficult to recycle. They are often shredded for recycling after use because they can no longer be broken down into their original materials. Fraunhofer IFAM is researching so-called switchable or intelligent polymers, which can be deformed or decomposed again by the exposure to certain stimuli.

These polymers function according to a specific switching principle, i.e. they respond to a change in the external stimulus, such as heat or solvent as well as pH value changes, with a change in the material properties. This happens when a critical energy threshold is overcome and chemical equilibria are shifted, exchange reactions are initiated or crystalline domains are melted. Through this principle, the linkage points in thermosets can be selectively switched or influenced. Such switches allow the material to be deformed even after curing.

 

Intelligent thermosets as functional polymers

The incorporation of different crystalline phases as switches makes it possible to address not only the shapability but also other functional properties of the polymer. For example, for several years, Fraunhofer IFAM has been researching polymers that have a shape memory properties. These polymers  store a permanent shape and can be transformed into a temporary shape by a temperature stimulus. After cooling, the polymer has the same mechanical properties as the permanent form.  By heating again, the polymer is autonomously returned to the permanent form. Such shape memory materials are of particular interest as switches of valves or actuators in the automotive industry or as stents or prosthesis materials for medical technology, where metallic materials are already used today.

 

Applications in fiber-reinforced plastics

Why are thermosets so interesting for applications in FRP? Due to their three-dimensionally cross-linked molecular structure, thermosets are characterized by particularly good chemical and thermal resistance and also have very good mechanical properties. However, due to the crosslinking of the polymer chains, it is not possible to subsequently reshape, recycle or repair thermosets, if they are damaged.

In terms of resource efficiency and sustainability, the use of conventional thermosets in FRP is disadvantageous. For example, discarded large components made of FRP are increasingly becoming a problem from an environmental point of view because they cannot be easily returned to the cycle.

Moreover,t is also desirable from an economic perspective to find a new use for the polymers and the valuable fibers. In the case of fiber-reinforced plastics made from thermosets, recycling is nowadays carried out by mechanical shredding of the components or by incineration of the matrix. This converts a high-quality, high-priced material into an inferior material (downcycling), which is also significantly more expensive as a recyclate than conventional fillers. In combustion processes, fibers can be so badly damaged that further use is not possible.

Therefore, so-called covalently adaptable polymer systems have become a strong focus of researchers at Fraunhofer IFAM. They are developing new FRPs based on intelligent thermosets, in which the fiber can be detached from the polymer matrix under the action of a specific stimulus and the FRP can therefore be restored to its original components.

 

Debonding on Demand: Intelligent thermosets make it possible!

Recyclability and biodegradability are also properties that can be achieved with intelligent polymers: With the right setting of the covalently adaptive polymer network and thus of the switch, the polymers are degraded into their individual components and can be reused or recycled.

Thus, intelligent thermosets also play positively on a trend in adhesive bonding technology: debonding on demand. This refers to a secure, yet releasable adhesive bond. In this process, the adhesive can be modified in response to a specific stimulus in such a way that the previously bonded materials can be separated again and, if necessary, even the adhesive can be recovered.

Nowadays, after damage, entire components are mechanically destroyed or sent for recycling instead of disassembling the bonded component into individual parts. Debonding on demand enables bonded components to be dismantled without damaging them, e.g. for repair purposes or for the targeted replacement of individual components.

 

Possible applications for intelligent thermosets

Researchers at Fraunhofer IFAM have been working on developments for use in automotive and shipbuilding for some time. Recently, other areas of application have become the focus of research interest. The table shows some examples.

Industry sector	Possible applications
Adhesive manufacturer Plastics manufacturer	Expanding its portfolio to include materials that are: adaptive, functional, recyclable and biodegradable. Debonding on Demand becomes possible!
Automobile construction Rail vehicle construction Aircraft construction Shipbuilding	Intelligent thermosets are of particular interest for vehicle and aircraft interiors. They allow components made of FRP to be easily dismantled and repaired. Fraunhofer IFAM is also currently researching the use of thermoset based FRP in the structural part of vehicles introducing further features such as reduced flammability and sustainability.
Medical technology	Over the course of their service life, prostheses or orthoses must be repeatedly adapted to the human or, alternatively, completely replaced. Intelligent thermosets enable prostheses and orthoses to be easily adapted over a longer period of time and not have to be completely replaced with new parts.
3D printing	Today, it is already possible to print thermoplastics. Fraunhofer IFAM is investigating ways of printing thermosets directly.

 

The development of polymer materials is a core competence of Fraunhofer IFAM. Dr. Tobias Urbaniak conducts research in the department "Polymer Materials and Structures". His work focuses on researching new polymer systems, which can be plastically formed and recycled despite their three-dimensional network structure.