Upcycling Polystyrene - PMC

The plastic industry is leaving an indelible mark on society, as it is negatively associated with plastic pollution. As the industry grows by producing tons of plastic materials, so are the compounding societal problems in dealing with the enormous waste generated. The future of humanity rests on a more livable environment than is available today, enough to incentivize the search for sustainability and circularity in the use of our planet's resources. Considering plastics as a resource, the recovery or reuse in a circular economy model proved to be the most challenging for scientists, economic managers, and policymakers. For one, the exponential consumer demand for virgin plastic materials outpaced post-consumer plastic recycling in terms of workable and scalable material recovery technologies and environmental and economic policy adoption. In the last decade, wastes from packaging materials, for example, were treated as a critical concern due to the impact of global warming and other environmental issues, as demonstrated by the growth in research publications concerning this topic. A plethora of mechanical or chemical approaches for recycling polymeric plastic materials have been proposed. These studies cover a wide range of commonly used plastics, especially polyethylene (PE), PP, and poly(ethylene terephthalate) (PET). However, several approaches could be adaptable for PS wastes as well, and are discussed further in this review. Unlike PS, the recycling of lightweight packaging waste based on polyolefins (PO) separated at source is already state of the art. Furthermore, while several effective recycling strategies exist for PO-type materials, the extensive applications of PS also create a need for a sustainable methodology for dealing with the waste of PS products. These will be covered in the following sections.

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Remarkably, the rapid growth in the use of EPS compounded by the consumption of package goods outweighs the EPS rate of recycling. The accumulating number of waste EPS entering the landfill is of significant concern, as it takes several decades to break down. This long-term environmental stability of EPS further strains the already limited space for landfills. The gargantuan task of recycling EPS is a misconception, as the most challenging stage boils down to logistics. The waste EPS collection, for example, is uneconomical using most local councils' standard curbside systems. Moreover, the lightweight nature of EPS masks the actual cost of collection and disposal when bulky EPS wastes are disposed of in skips or bins. Thus, sustainable collection programs are called for in the removal and collection of post-consumer EPS.

Challenges in Mechanical Recycling

Mechanical recycling is no longer applicable for complex materials, including composites and multicomponent polymer matrices or contaminated plastic containers such as PS-based food packaging. These materials are very challenging to recycle, and if feasible, they are no longer cost-efficient in economic terms. For technical and economic reasons, unrecycled plastics are intentionally sent to landfills or are incinerated. Waste degradation in landfills typically relies on the anaerobic process. Although the use of a bottom liner and a topsoil cover are standard practices in landfilling, greenhouse gas emissions and hazardous leachate are still of significant concern. Apart from the release of methane, carbon dioxide, other undesirable gases, and aquifer-polluting leachates, landfilling is severely limited by land or space availability.

Sorting plastics as a recycling strategy is cumbersome and can be avoided by introducing plastic blends. Insights into the behavior of waste plastic blends relating to their mechanical properties are essential to avoiding the plastic sorting stage. Recent statistical measures have provided some insights into how the mechanical properties of pure PS are affected by an increased ratio of waste PS and PP injected at different temperatures. The study found a dramatic decrease in cost with an increased PS/PP waste ratio at any temperature. A maximum of 30% waste ratio at 200 °C and 220 °C resulted in better mechanical properties.

Advanced Sorting Technologies

Some countries require the sorting of bioplastics and conventional plastics in their waste management programs. However, because the demand for bioplastics is still tiny, few technologies are available for rapid and reliable identification and separation. Such technologies rely on hyperspectral data from a linear spectrometer and a spectroradiometer tuned at ca. 900 nm, the near-infrared region of the light spectrum. The sensing technology is set on reflectance mode, capable of identifying and sorting out PS, PET, and polylactic acid (PLA) polymer-based materials from each other. The bio-based and biodegradable biopolymer, PLA, could also be sorted.

The viability of the sorting process has been the subject of several studies. Several industrial sorting technologies are available, although developing technologies still have drawbacks. For example, the carbon black pigment limits the broad adoption of Near-Infrared Hyperspectral Imagery (NIR-HIS) in the sorting process. This pigment is primarily incorporated into thermoplastics like PS as a colorant and UV agent. The presence of certain pigments could impair the proper identification of polymers in a mixed waste stream. Mid-Infrared (MIR-HSI) could serve as an adequate alternative to resolve this issue in rare instances. Moreover, the use of more advanced characterization techniques facilitated rapid characterization, lowering misidentification, and enabling specific segregation during industrial sorting.

Mechanical Recycling and Its Limitations

Mechanical recycling generally involves precleaning, sorting plastics by type, shredding, melt extrusion, and remolding plastics, including PS. Interestingly, HIPS is well suited for MR, since little degradation in mechanical properties is observed after multiple cycles of reprocessing. The primary washing and drying encompass the precleaning stage. The recycling of plastics is primarily limited by their sorting because their diverse forms, chemical makeup, and formulation are often strongly incompatible, compromising the mechanical properties. MR necessitates manual or automated segregation at the source of the mixed waste stream to obtain clean, high-purity mono-material streams.

Intuitively, the low environmental footprint of polymer materials with exceptional performance renders them appropriate resources for a circular economy. However, this is not the case in the current highly linear global plastic industry. Recently, there has been a push globally to transition to a circular economy model primarily aiming at the reduction of anthropogenic impact on nature. Realizing a circular economy entails the promotion of the three R's: reduction, reuse, and recycling, while retaining those plastic materials of the highest value.

Thermochemical Methods

Recycling plastic wastes, especially PS waste streams, is essential to mitigating the disastrous impacts of their current state of disposal in the environment. Much like other plastics, PS is recyclable through mechanical reprocessing, thermochemical recycling, and incineration techniques. While MR techniques could be cost-limiting, they often lead to low-value products. However, TCR could serve as the alternative eco-friendly way to obtain higher-value products from the plastic waste stream.

The environmental impact of conventional incineration for municipal sewage wastes is vast compared with other processes because of the high volume of carbon dioxide, pollutants, and particulate matter that are emitted. In comparison with MR, conventional plastic waste incineration generates higher emissions but has the potential to recover the associated energy input. Enhancing the quality of plastic feedstock through pre-sorting or cleaning can minimize the impacts associated with this method.

In conclusion, the challenges related to recycling polystyrene require innovative solutions and collaboration across various sectors to enhance plastic recycling rates. Through advancements in technology, improvements in sorting processes, and the exploration of thermochemical methods, a more sustainable approach can be developed to address polystyrene waste effectively.

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Based on the guidelines of the Waste for Life initiative, composite materials based on recycled HIPS (obtained from yogurt cups) and paper plastic laminates (from disposable paper cups) were developed. The inherent recycling incompatibilities of such materials need to be addressed to improve sustainability. Therefore, developing composite materials offers exciting potential for creating a second life for these materials.

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