Circular Economy of Mechanical Recycled Post-Consumer PP

Abstract

Background: The growth of the global population in combination with technological development results in the need to improve the living conditions. This led to a significant and unsustainable increase in production of products. The unstable increase is especially the case for plastics, which have an annual worldwide production of over 288 million tons, that grows exponentially. Sustainable solutions and a circular economy are required to limit the unsustainable increased demand on natural resources. Solutions to this unsustainable situation include: the use of bio-based polymers, increased use of flexible packaging or improved recycling; either chemical or mechanical. Objective: This study focuses on the possibility to create a circular economy of plastics by applying mechanical recycling. Mechanical recycling is a promising solution, especially with the new separation technology that is currently in development. The current separation technology of mechanical recycling separates plastic by type, but does not take any further separation criteria into account. This results in a mixed waste stream of many different grades, which cannot be used in a circular economy. However, the separation technology currently being developed can separate plastic on flake level. In addition to accurate density separation on flake level, the technology is able to separate flakes based on wall thickness and color. Separation based on wall thickness enables the possibility to separate based on manufacturing type. This study investigates whether this new separation technology can realize a circular economy for mechanical recycled post-consumer plastic. Methods: The study is performed on post-consumer polypropylene (PP) from the packaging industry, as this industry counts for majority of plastics production. PP is one of the key plastic types used in packaging and its popularity is expected to increase, as transparent PP already replaces poly(ethylene terephthalate) (PET) in many applications. PP is a thermoplastic semi-crystalline polymer which properties are strongly correlated to its molecular parameters. Tacticity, crystallinity, molecular weight and copolymerization impact the properties of PP. The optical, mechanical and rheological properties of further categorized pure post-consumer PP (based on manufacturing type and color) are studied, as these properties give insight in their possibility to address the market needs. Results: A first separation based on blow mold, thermoformed and injection mold leads to recycled material streams with well defined rheological and mechanical properties. The melt flow indices (MFI) of the categorized mechanical recycled waste streams address the needs of plastic manufacturers. Additionally, the thermoformed and blow mold waste streams show a ductile fracture behavior. However, the injection mold grade shows a brittle fracture mechanism. Categorization based on manufacturing type is correlated with a slight color separation. Further categorization of injection mold into a white and transparent grade leads to mechanical recycled materials streams with even better defined optical, rheological and mechanical properties. However, the injection mold white grade has become brittle, probably related to its low molecular weight. Modifications, such as the addition of virgin grade, might be required to meet the needs of the manufacturing industry. Conclusion: Both results show that more defined categorization of recycled post-consumer can realize a circular economy of post-consumer PP. However, regulations for standardization are required to enable the appropriate use of PP in the food packaging industry and to prevent change in product characteristics over time (e.g. yellowing) things like yellowing. By regulating standards to create a circular economy for mechanical recycled post-consumer plastics, manufacturers have to take their responsibility in recyclability, rather than minimizing the mass of packaging waste.