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Plastic is considered an indispensable commodity, but the pollution caused by it is a major global concern due to its rapid accumulation rate, complexity, and lack of management [NCBI, 2021].

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Waste hierarchy

  

The Waste Management Hierarchy (Figure 1) is a conceptual framework designed to guide and rank waste mgmt decisions at both the individual & organizational level [AXIL, 2023].  It places top priority on preventing waste generation (prevention level), lessening the use of virgin raw materials to produce goods & services.  At the other end (disposal level), materials that cannot be reused, recycled, or recovered for energy will be land-filled & incinerated (no energy is obtained here).

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All technically recyclable

  

Most polymers are theoretically recyclable through mechanical or chemical processes; however, recycling is not widely implemented in practice.  Many plastics are not recycled due to technical challenges, material degradation, contamination, and unfavorable economics [SL, 2023].

      

Physical & chemical recycling

 

One of the biggest challenges in recycling plastics is their inert carbon-carbon bonds, which are very stable (ensuring the resistance of plastics to many chemicals) and thus require a significant amount of energy to break [PO, 2023].  The most widespread recycling process is the physical (mechanical) recycling, which involves sorting, shredding, washing, and melting plastic waste to form pellets: in this reaction, no new substance is formed.  Conversely, a chemical recycling produces new compounds, and, different from physical recycling, is irreversible.  Figure 2 shows the essential differences btw physical & cheml recycling.

  

Items made from PET, HDPE, and PP (bottles, milk cartons, yogurt pots, and takeaway trays) are the most common types of plastic that can be recycled [EPW, 2023].  By means of dissolution (a physical recycling process), which is based on using a solvent, different polymers are separated and carry out one or more purification steps.  Depolymerization (a chemical recycling process), a.k.a. chemolysis or solvolysis, uses different combinations of chemistry, solvents & heat to break down polymers into monomers.  Figure 3 shows possible plastic recycling process flows involving physical & chemical reactions.

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Degradation

 

Recycled plastics often exhibit lower mechanical properties than virgin materials, as melting & reprocessing can degrade polymer chains through thermal and mechanical effects.  These properties can be partially restored by adding stabilizing additives or by blending with virgin resin.

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Thermosets vs thermoplastics

 

Thermosets & Thermoplastics sound similar, but their properties & applications differ vastly.  Thermoset plastics contain polymers that form irreversible chemical bonds and cannot be recycled, whereas thermoplastics can be re-melted and re-molded.  Examples of non-recyclable plastics include bioplastics, composite plastic, plastic-coated wrapping paper, and polycarbonate.  Additionally, well known non-recyclable plastics include cling film & blister packaging [EPD, 2023].