Quality and Recyclability of Plastic from Household Waste

Research output: Book/ReportPh.D. thesis

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To mitigate growing environmental threats such as climate change and resource depletion, the circular economy concept has gained momentum. Traditionally, materials have been lost through incineration or landfilling; however, in a circular economy, materials are recirculated into society, ideally to the same quality levels as they had originally, so that all demands within the material loop are fulfilled and use of virgin material can be avoided. However, recycling of materials, especially from heterogeneous waste streams such as household waste (HHW), often leads to recycled material of reduced quality, which only leads to a partial closing of the material loop – and thereby only partial circularity. This aspect is currently not part of the environmental assessment of materials and is therefore yet to be quantified.
In the transition towards a circular economy, plastic is highlighted as a focus material, as it is produced in large amounts from fossil resources. Consequently, the European Union (EU) has defined mass-based recycling targets for packaging plastic and placed specific focus on plastic from HHW. However, considerable physical and quality-related losses are related to the recycling of plastic from HHW, due to the potential presence of 1) non-plastic material, 2) plastic made from several polymer types (the most common in HHW plastic are polyethylene terephthalate (PET), polyethylene (PE) and polypropylene (PP)) and 3) many different product types (bottles, trays, etc.) with different purposes and design. Moreover, recycled plastic might be contaminated by potentially harmful chemicals, all of which may limit the quality of recycled plastic and thus the circularity. Consequently, in order to identify the most circular plastic recycling systems, thorough knowledge related to the physical and chemical states of waste and recycled plastic from HHW is necessary.
The aim of this thesis was to quantitatively integrate the quality aspect of waste and recycled plastic into circularity assessment of plastic recycling systems, focusing particularly on plastic from HHW. This was achieved by 1) theoretically relating quantity and quality of recycled materials to the circularity of recycling systems, 2) providing selected chemical, physical and mechanical characteristics of waste and recycled plastic, including a detailed composition of source-separated plastic waste, and 3) quantitatively evaluating the performance of current and potential future plastic recycling systems, and on this basis recommend the most circular options.
From a circularity perspective, the quality of recycled materials is closely related to the applicability, i.e. how well the recycled materials can be turned into different products with different quality levels. This quality, together with recycled quantities and knowledge on the distribution of different applications in the specific material market, can be used to quantity the circularity potential of recycling systems. Between 18% and 57% of European PET, PE and PP markets rely on chemically high-quality material for the production of food packaging. As such, it is crucial for the circularity of plastic recycling systems to have the ability to recycle plastic into material that can be used for food contact applications.
Current plastic recycling practices experience substantial material losses and physical contamination. It is therefore recommended to implement state-of-the-art plastic sorting systems with high recovery efficiencies and low contamination levels, as this is the best way to limit these issues. However, even such best-performing systems achieve a circularity potential of only about 0.40 (1 indicates full circularity), and thus current plastic recycling systems are far from able to close the plastic loop. Due to elevated concentrations of metals in recycled plastic, the circularity might be reduced even further in the future, if plastic products are recycled multiple times. Finally, the composition of rigid source-separated plastic reveals a high degree of heterogeneity in regards to the purpose (food or non-food packaging), type (bottles, trays, etc.) and design of waste products within each of the three dominant polymers, i.e. PET, PE and PP, representing more than 90% of the waste.
In order to mitigate these issues and improve the circularity, it is crucial to increase the quantities of recycled plastic while maintaining the chemical, physical and mechanical quality. “Design for recycling” initiatives, where for example all products are produced in a single polymer, are highly recommended, as they can lead to increases in the quantities of recycled plastic of up to 23%. Moreover, separate recycling of food packaging is recommended, as it allows for the production of recycled plastic suitable for food packaging applications and thereby maintains chemical quality. However, from a mechanical and physical perspective, the high degree of heterogeneity of PP food packaging makes it unsuitable for closed loop recycling into new packaging. Thus, in order to create the conditions necessary for closed-loop plastic recycling from HHW, where all quality aspects are maintained, regulation is needed to limit plastic packaging to the polymer types PET, PE and PP while standardising product types within especially PE and PP.
More research is necessary in order to identify the most appropriate combinations of product design, polymer selection and waste collection systems, achieving the highest possible increases in quantity and quality, and thereby circularity. This includes research into 1) detailed compositions of soft and residual plastic, 2) performance of the sorting process, depending on the type and design of waste products, and 3) how and to what extend mechanical and physical properties limit the applicability of recycled plastic.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages47
Publication statusPublished - 2019

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