Resumen de Upcycling of post-consumer plastic waste: identification and mitigation of VOCs in post-consumer polyolefins
- español
Esta tesis estudia el impacto que tienen los COV presentes en los plásticos reciclados para la economía circular. Actualmente, la industria del reciclaje actual no puede cubrir la creciente demanda de plásticos reciclados de alta calidad en los últimos años. Hasta ahora, el reciclaje mecánico es la solución más utilizada a nivel mundial, y su eficacia y fiabilidad han sido demostradas para el tratamiento del residuo plástico post-industrial. Sin embargo, el mismo reciclaje mecánico aplicado a los residuos plásticos post-consumo da lugar a un material reciclado de menor calidad que solamente es apto para aplicaciones de bajo valor añadido, como son las tuberías de riego o cubos de basura. Por ello, esta tesis evalúa la influencia que tienen las sustancias orgánicas que aparecen en los plásticos reciclados post-consumo dentro el sector del plástico.
- English
The demand for high-quality recycled polymers in the European plastic industry is on the increase, likely due to the EU´s Plastic Strategy intended to implement the circular economy model in this sector. The problem is the scarcity of recycled plastic on the market. In terms of volume, post-consumer plastic waste could be key to meeting the current and future demand.
However, post-consumer plastic waste is one of the major issues when it comes to polymer recycling. Particularly, plastic waste from kerbside collection contains a substantial number of impurities from food leftovers, cosmetics, or detergents that contribute to lowering the polymer quality and restrict their applicability to very low-grade applications, specifically for outdoor applications, such as synthetic floors for gardens or plastic coating for wire fencing. Fortunately, this inconvenience could be solved if undesired substances, which appear during the recycling process or are already present in the plastic waste itself, were reduced. In this way, the manufacture of plastic products could operate in a closed-loop system protecting the full material value and greatly contributing to the circular economy. To study and propose new solutions to this issue, this thesis firstly reviews the current technologies dedicated to reducing unwanted substances in polymers and studies the volatile organic compounds identified in different plastics until now, and secondly, addresses the identification of these unwanted substances in polymers and the implementation of new methodologies for their reduction.
Chapter 1 explores the state-of-the-art technologies employed in the mechanical recycling of plastic waste, with particular attention to the methods employed to reduce volatile organic compounds released from recycled materials. Afterward, the analytical techniques that have been applied for the identification of volatile organic compounds are examined. Finally, this review gathers from literature all the volatile organic compounds and odor-active substances identified in virgin and recycled polymers, aiming to evaluate whether there is a notable difference between them based on the chemical structure of the emitted VOCs. As a result, 437 volatiles were gathered that revealed there is a difference between post-consumer recycled plastics and the other plastic waste sources. A larger number of volatile organic compounds, especially flavor and fragrances-based substances as well as oxygen-containing compounds, appear in post-consumer plastic waste compared to virgin and post-industrial plastic scrap.
Chapter 3 depicts the volatile organic compounds present in post-consumer LDPE and HDPE.
These plastics are commonly used in the packaging sector and therefore their recycling is essential to achieving a circular economy in this area. The identification of these unwanted substances in recycled plastics is fundamental for the development of innovative decontamination technologies. In this study, 105 volatile compounds have been identified in recycled LDPE and HDPE from domestic waste. The tentatively identified substances are related to additives, breakdown products from polymeric materials and additives, and contamination from external sources. The results suggest that the VOCs identified in both, recycled LDPE and HDPE, are mostly derived from external contamination from the packaging’s previous use phase.
Some of these VOCs may cause off-odors, being one of the major impediments to introducing recyclates into the market. There are different systems to collect domestic waste whose influence on the formation of odor causative substances in plastics is undiscovered. For this reason, Chapter 4 focuses on studying the differences in odor between separate and nonseparate collection systems. Post-consumer LDPE bags are selected as the case study material.
Furthermore, the effect of hot water washing, commonly applied in the mechanical recycling of plastic waste, on the odor of post-consumer LDPE bags is evaluated. As a result, more than 60 odorants are detected in LDPE bags collected in a separate plastic fraction as well as in LDPE bags from the non-separated collection, and 37 of them are identified. The sensory results reveal that the type of collection system affects the overall odor intensity, the hedonic tone of the odor, and the odor profile. Namely, cheesy and fecal-smelling odorants are predominant in the waste that is not separated at source, whereas odorants with earthy and moldy smells show mostly higher intensity ratings in the waste separated at source. Odorants such as short-chain carboxylic acids, likely originating from microbial spoilage of organic waste, show higher dilution factors in the mixed fraction, and could, accordingly, contribute to the observed differences.
Additionally, the hot washing procedure, applied to the LDPE sample from the separate collection system, reduced the overall odor intensity. However, the washed waste still showed high smell intensity ratings.
Once the undesired substances present in recycled plastics have been identified, this research aims to study novel solutions to reduce the quantity of these compounds in order to increase the market opportunity for recycled plastics. Chapter 5 evaluates the removal of VOCs from post-consumer recycled HDPE through two innovative methods applied for the first time in this field, steam stripping and polyethylene glycol (PEG) extraction. Both methods showed a decrease above 70 % in the VOCs content compared to extrusion-degassing. Moreover, these techniques were compared to hot air stripping, a new technology developed at an industrial scale for the removal of VOCs. As a result, steam stripping improved efficiency in reducing the overall VOCs compared to hot air stripping, and the PEG extraction method lowered the volatile polar compounds further than using hot air stripping. Additionally, none of these technologies modified the HDPE melting flow index.
