Technologies for plastic recycling

The global production of plastic is continuously increasing, primarily due to its extensive use in the packaging sector. The demand for high-quality materials in packaging, combined with the low cost of virgin polymers compared to recycled materials, has led to a preference for new plastics. Additionally, the low quality of recycled plastics —often resulting from the complexity of the plastic waste stream, especially packaging waste— makes them unsuitable for sensitive applications like food packaging. This situation results in low recycling rates for this type of waste and increased landfilling and incineration rates. These practices significantly contribute to climate change and global warming due to the high gas emissions they generate. Therefore, there is an urgent need to close the loop on plastic waste and achieve circular economy targets. The complexity in the composition and properties of packaging waste poses significant challenges in the mechanical recycling process. Key challenges include sorting and separating the different fractions within the waste stream, dewatering the plastic flakes (especially flexible plastic flakes), and decontaminating the recycled material, as the plastic often arrives at recycling facilities contaminated with various impurities, wood, paper dust, and organic contaminants. Multilayer packaging often consists of various polymeric layers, adhesives, inks, and metallic layers such as aluminum. The properties of these materials, along with the lack of appropriate infrastructure, make recycling through mechanical processes very challenging. Optimizing certain processes and adding additional pretreatment steps, such as delamination and decontamination, are required to address these challenges. The goal of this PhD thesis is to study and gain insights into several processes involved in the mechanical recycling of plastic packaging. The focus is on improving the recyclability of these types of plastic waste by developing new methods that could be promising approaches for closed-loop recycling of plastic packaging waste. The first chapter of this PhD thesis introduces the possibilities of a circular economy for plastic packaging. It provides an overview of the current waste management routes for plastic waste, discussing their limitations and advantages. This chapter also reviews the state-of-the-art of mechanical recycling processes, outlining the main processes used and highlighting the weak VIII points and uncertainties of each process in handling flexible plastic packaging. The second chapter addresses the objectives of this research and discusses the outline of the thesis. Chapter 3 presents the results of an in-depth study on dry separation, a cost-effective technique that needs evaluation for its application in plastic recycling. The study models the air classification process to separate plastic flake mixtures and uses image analysis to characterize particle shape and size. Different drag models and experimental data were employed to investigate the behavior of these particles. The study conducted separations of various plastic flake mixtures, including rigid/rigid, rigid/film, and film/film combinations. Dioguardi et al.'s drag model has proved most effective for predicting film plastic behavior , while Wu and Wang’s model accurately has predicted rigid plastic behavior. Multiple regression models have been tested and validated to predict the separation curve of the plastic mixture. The drying of flexible plastic is a highly energy-consuming process that significantly affects the quality of the recycled product. Despite its importance, this operation has not been extensively studied at the laboratory or pilot plant scale. In Chapter 4, the thermal drying of flexible plastic is assessed using two laboratory thermal drying systems to evaluate convective drying in fixed and fluidized beds. The study examines the influence of the drying system, plastic flake size, air velocity, and initial moisture content to optimize the process. It concludes that fluidized bedthermal drying is the most effective one for removing moisture from plastic flakes, with drying kinetics similar to those of agricultural products and sludge. Heat transfer has been identified as the critical transport phenomenon in drying flexible plastics. Evaluating the safety of recycled plastic packaging is crucial for identifying recycled plastic as a viable upcycling material, particularly for sensitive applications like food-contact materials. Chapter 5 focuses on examining the decontamination process of polyethylene plastic in accordance with the European Food Safety Authority (EFSA) challenge test for food-contact recycled polyethylene. Various washing media were tested, and their decontamination kinetics were studied to determine their effectiveness. The general results from this study indicate that the most effective washing media for removing contaminants from plastic are those with a high affinity for both the plastic material and the contaminants, facilitating easier migration of contaminants from the plastic matrix into the solution. The heterogeneous composition of multilayer packaging and the lack of viable recycling solutions complicate its recyclability. The primary obstacle in recycling multilayer packaging (MLP) waste is the separation of its various polymeric and non-polymeric layers and the removal of the printed layer. Chapters 6 and 7 focus on improving the delamination process of multilayer packaging to enhance recyclability by recycling each plastic layer type separately. An innovative technique involving surface modification has been proposed and demonstrated to enhance the diffusivity of the dissolution agent through the inner layer. Chapter 6 begins with the investigation of the microperforation technique, examining different approaches such as microperforation densi ty, temperature influence, surfactant use, and the presence of a printed layer in the material. In Chapter 7, three different surface modification techniques —mechanical microperforation, laser microperforation, and abrasion— are investigated and compared for their effectiveness in improving the delamination process. The results show that all three surface modification techniques significantly enhance the penetration of the dissolution agent into the inner adhesive layer, thereby improving the delamination process. Overall, this work provides systematic data to improve the mechanical recycling of plasticpackaging waste. It offers a comprehensive study of the key processes involved in mechanical recycling, enhancing the understanding of the challenges faced in recycling plastic packaging. Innovative solutions have been proposed to optimize these processes and increase the recycling rate of plastic packaging.

Author

• Berkane, Imene

• Mechanical recycling
• Plastic packaging
• Delamaintion
• Waste management
• Drying
• Separation
• Decontamiantion

Identifier

hdl:10045/163221

OAI identifier oai:identifier

oai:rua.ua.es:10045/163221