Application Center for Wood Fiber Research HOFZET®

Research focus

Recycling of (bio-) plastics and composite materials

In order to open up recycling options of the highest-possible quality for plastics and composite materials, the properties of residual and waste materials are examined at the HOFZET with a view to developing usages and strategies for component recycling, mechanical recycling and feedstock recycling. In the recycling of (partially) bio-based plastics and composites, we address the particular challenge of separating the heterogeneous material flows and material combinations and of designing and adapting the recycling processes in accordance with the very differently behaving components. The re-use of materials thereby enables the combination of ecological and economic advantages.

 

Component recycling

In component recycling, components are dismantled after their first usage phase, reconditioned and then used again. Their usage in applications which are different from the original is also an option. This type of recycling is, however, associated with considerable challenges, particularly in the case of load-path-suitable components made from fiber composite plastics (FRP). The components must be in a flawless condition in order to be re-used, which places high demands on the non-destructive testing of the components as well as possible repair mechanisms. The possible application areas are also very limited, as the components can only be re-used in applications which are virtually the same. The orientation of the load paths would otherwise be problematic or not possible and a downcycling effect would arise if the corresponding reinforcement structures were not used to the fullest extent.

© Photo Fraunhofer WKI | Manuela Lingnau

From rear to front: Punching waste from yoghurt pots, ground material and tension rods with paper residues

© Photo Fraunhofer WKI | Kathrin Morawietz

Mechanically recycled PLA; left: ground material (flakes), right: regranulate

Mechanical recycling

A very good possibility for re-introducing homogeneous, process-accrued residues or post-consumer waste into the original or new process is mechanical recycling. The entire composite material is hereby re-used in its original composition. At the HOFZET, we recycle both non-reinforced and short-, long- and continuous-fiber-reinforced plastics by first crushing and then re-using them in, amongst others, the processes listed below

In the case of materials with a thermoplastic matrix, the ground material can be processed via extrusion and, if necessary, additives can be applied to enable a particular application. The regranulate can be processed to form new components, for example through injection molding. Depending on the process control, the fibers contained in the recycled material are shortened to differing degrees and damaged by, amongst other things, the grinding step. Depending on the resulting properties, they can nonetheless still be applied as a reinforcing component or filler material. We offer you preparation, process optimization and material optimization from a single source and thereby enable ecologically and economically attractive applications for recyclates. Ground material with thermosetting matrix cannot be melted again but can be used as a filler material for various thermoplastic and thermosetting matrices.

We place a particular focus upon the investigation and optimization of the grinding process as well as the subsequent ensuing processing options. Aspects such as the brittleness of the materials - which can be increased by reducing the temperature during the grinding process - and the size and quality of the ground material are hereby of particular interest. The subsequent fractionation of the ground material enables high-quality further processing in the extrusion. Depending on the properties of the ground material and the individual application, we implement a customized extrusion process and, if necessary, a suitable additive. Following optimization of the process parameters, the recyclates can be used in high-quality application areas. 

Through a holistic approach and the optimization of all the individual steps and parameters, we thereby enable a closed-loop material cycle.

 

Feedstock recycling

One alternative to mechanical recycling is feedstock recycling. For this, the materials are dismantled into individual components before they are further processed. Non-homogeneous materials can hereby also be recycled. At the HOFZET, we are investigating chemical recycling and pyrolysis in this area.

In chemical recycling, the matrix is ​​dissolved in a solvent. This method enables the utilization of both thermoplastic and thermosetting matrices. Thermoplastic materials are subsequently further processed by removing the applied solvent and processing the remaining material via extrusion to form regranulate. For the dissolution of thermoset matrix materials, for example, the resultant solution is fed back into the production process of materials where it is used to impregnate dry semi-finished products from which laminar components can again be produced.

A further form of feedstock recycling is pyrolysis. For this, the materials which are to be re-utilized are subjected to high temperatures under exclusion of oxygen, as a result of which organic bonds are opened and the majority of the constituents pass into the gaseous or liquid phase and only carbon and mineral components remain as solids. If carbon fiber-reinforced materials are processed in this way, very high-quality recyclate carbon fibers can be recovered. When natural fiber-reinforced plastics are applied, a pyrolysis carbon is obtained as recyclate; this is re-used as, amongst other things, filler material. We test the various pyrolysis products (solids, pyrolysis water and pyrolysis gases) for you in order to ensure applications which are as high-quality and sustainable as possible.