Elke Metzsch-Zilligen
Fraunhofer LBF
Abstract
The stabilization of recyclates is a key approach to improving the quality and performance of recycled materials, making them suitable for demanding applications. Recyclates often exhibit weaknesses caused by the recycling process, such as reduced mechanical strength, lower thermal stability, or increased sensitivity to environmental influences.
To address these deficiencies, Fraunhofer LBF employs various stabilization strategies. One essential approach is the use of additives, where stabilizers, fillers, or reinforcing agents are specifically introduced into the recyclate. These additives can enhance UV resistance, slow thermal aging, or improve mechanical properties such as tensile strength and impact toughness.
For the stabilization of recyclates, various additives are available that can be specifically tailored to the material’s weaknesses and requirements. The selection of additives depends on the type of recyclate, such as polyolefins, PET, or polyamides, as well as the desired properties.
Antioxidants play a central role in preventing thermal and oxidative degradation, which can occur during the recycling process or later use. Primary antioxidants, such as phenols, are used to scavenge free radicals, while secondary antioxidants, such as phosphites or thioesters, decompose peroxides. These additives are particularly important for polyolefins such as polyethylene (PE) and polypropylene (PP).
UV stabilizers are crucial, especially for outdoor applications. They protect the material from UV radiation, which can cause embrittlement, discoloration, and degradation. UV absorbers, such as benzotriazoles or benzophenones, absorb UV radiation, while HALS, or hindered amine light stabilizers, neutralize free radicals generated by UV exposure.
Chain extenders are particularly suitable for repairing degraded polymer material. These additives extend or crosslink polymer chains and are often used for polyesters such as PET or polyamides (PA) to improve viscosity and mechanical properties.
Fillers and reinforcing agents play an important role in enhancing mechanical properties such as stiffness, strength, and impact resistance. Mineral fillers, such as talc or calcium carbonate, as well as glass fibers or carbon fibers, are used particularly for engineering plastics such as PA or PP.
Lubricants and processing aids, such as waxes, silicones, or fatty acid derivatives, reduce friction during processing and improve the material’s flowability, facilitating the processing of recyclates in injection molding or extrusion processes.
Impact modifiers, such as elastomers or copolymers, including ethylene-propylene-diene monomer (EPDM), increase toughness and reduce brittleness. This is particularly beneficial for brittle recyclates such as PS or PP.
Degassing agents and odor absorbers, such as activated carbon-based additives or chemical absorbers, reduce unwanted odors and volatile components that arise during the recycling process. These additives are especially relevant for recyclates from post-consumer waste.
Dyes and pigments, including organic and inorganic pigments, can mask discoloration and improve the optical quality of the recyclate. This is particularly important for applications where aesthetics play a role.
Compatibilizers, such as block copolymers or reactive polymers, improve compatibility between different polymer types in mixed recyclates. They are particularly important for blends such as PE/PP or PET/PA. The selection and dosage of additives must be carefully tailored to the specific recyclate and intended application to achieve optimal results.
Chemical modification also plays a significant role. In this process, the polymer chains in the recyclates are deliberately altered to repair breakages or optimize the molecular structure. This can be achieved through reactions during processing or by using reactive additives.
Another focus is testing and improving long-term stability. Fraunhofer LBF develops methods to simulate the aging processes of recyclates under real-world conditions and derive suitable stabilization measures. Interactions between additives and the recycled material are also studied to ensure optimal performance.
Biography
Division Director Materials | Fraunhofer Institute for Structural Durability and System Reliability LBF
Ms. Metzsch-Zilligen studied chemistry at the University of Cologne and earned her doctorate in physical
chemistry in 2006. After working for several years in the food industry, she joined the German Plastics
Institute (DKI) in 2011, which has been integrated into Fraunhofer LBF since 2012. There, she headed the
“Additivation and Durability” department until April 1, 2024, and subsequently assumed leadership of the
Plastics division at Fraunhofer LBF. As part of a restructuring, the division has belonged to the Research
and Development “Materials” division since January 1, 2026.
Her research focuses on the synthesis of new additives and the selection and evaluation of
optimized additives/additive systems for thermoplastics depending on the specific application. In
addition, she has been working for many years on the sustainability of plastics. This primarily includes
improving the quality of recyclates through tailored post-stabilization to enhance mechanical, sensory,
and optical properties, thereby increasing the amount of recyclates used or making it possible to replace
virgin material with recyclates in the first place.
