By Marie-Laure Bertet and Rainer Xalter from BASF Plastic Additives
Recently, the EU Commission adopted the long-awaited proposal for a new EU regulation on end-of-life vehicles (ELVs). The draft regulation, which still has to be approved by the EU Parliament, will mandate all new cars manufactured in or imported into the EU to contain 25% recycled material in relation to the total amount of plastic contained in the car. In addition, as a first step towards a closed-loop economy, a quarter of the recyclates used in the manufacture of new cars will have to come from ELV waste streams.
The EU Commission’s Joint Research Center estimates that 757 kt of post-consumer recyclate (PCR) will be required in 2030 to meet this draft regulation, with polypropylene accounting for the largest share, followed by engineering plastics such as polyamide, polycarbonate, polyesters and polyurethanes. Ensuring the supply of suitable recyclate streams will be a major challenge, particularly from ELV sources. It is expected that a whole value chain will need to be established for the collection, separation, and treatment of ELV plastics.
However, even as suitable solutions to fill the current supply chain gap are identified and implemented, there are still challenges related to the quality of post-consumer recyclate (PCR), which limits the applicability of PCR waste streams for technically demanding automotive parts.
The main factors affecting the quality of PCR-based compounds are the type and concentration of impurities contained in the PCR (such as polymer cross-contamination, metals, inorganics, paint residues, degradation products), the degree of pre-degradation, and the residual stabilizer content. Plastic compounds are typically designed to meet the stability requirements for the required processing steps and a product life cycle for a specific application. It is therefore not surprising that the majority of stabilizers is being consumed during the first product life cycle, and that the rheological properties of recyclates typically have altered from their virgin state. Combined with the presence of impurities that can adversely affect mechanical properties and durability, recyclates are often not suitable for a second life in high-value engineering applications. As a result, PCR materials must be upgraded to meet the requirements of their new target applications.
To understand what happens at the molecular level during the reprocessing of recyclates, it is important to understand that the degradation of polymers, especially polyolefins such as polypropylene, is a self-accelerating process. That is, after an induction period during which only minor changes occur in the polymer, a critical state is reached where most of the stabilizers have been consumed and a certain amount of degradation has already occurred; beyond this tipping point, the further degradation process is very rapid and difficult to stop, leading to significant changes in physical properties and eventually to mechanical failure (see Figure 1).
Given the typical pre-degradation levels of PCR and low residual stabilizer levels, it is essential to maintain a critical level of stabilization throughout the recycling steps by replenishing the depleted stabilizers. To avoid reaching or even exceeding the critical tipping point and to prevent further degradation of the PCR, the re-stabilization is ideally added in the first melt processing step of the mechanical recycling process, which is usually the regranulation process performed in recycler production facilities.
However, due to the molecular changes resulting from pre-degradation in their first lifetime and the self-accelerating behavior of polymer degradation, the aging characteristics of PCR materials differ significantly from their corresponding virgin resins. Using the carbonyl index as a measure of the degree of thermo-oxidative degradation, it is easy to see from Figure 2 that recyclates degrade significantly faster than virgin resins under the same thermo-oxidative stress. This implies that adjusted stabilizer solutions are required to ensure optimal re-stabilization of PCR.
To address this need, BASF has developed a range of enhanced stabilizer solutions under the IrgaCycle® brand name to improve the process stability and long-term aging stability of recyclates. For example, by using IrgaCycle® XT 034, the melt flow index (MFI) of PCR from ELV bumpers with some paint residues can be well maintained over several repeated laboratory-scale extrusion steps simulating industrial processing conditions, whereas the same material without any re-stabilization shows a strong MFI increase, indicating significant polymer chain scission. A conventional stabilizer solution intended for use in virgin resins also shows some advantages in MFI retention, but it is much less effective compared to the dedicated recyclate stabilizer solution. The observed performance differences are not due to the different stabilizer loading levels, as IrgaCycle® XT 34 contains additional functionality for compatibilizing paint residues, which is another advantage of this product for mechanical recycling of automotive waste streams.
Conclusion
The expected significant increase in the use of recycled plastics in automotive applications presents both great opportunities and challenges. Plastic additives are an essential part of the circular economy. Using specially designed additive packages to re-stabilize recyclates is a key element in maintaining recyclate quality. Ideally, the re-stabilization package is added during the regranulation step to prevent further degradation in subsequent processing steps. BASF’s extensive knowledge of polymer degradation and stabilization mechanisms enables the company to design and tailor re-stabilization solutions for different recyclate materials and end-use requirements.
