"There is a consensus that plastics are an unsustainable material [QUB, 2023]"
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].
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).
All technically recyclable
Many plastic types can be recycled, even if the process is not widespread (in fact, all polymers are technically recyclable), however some plastics aren’t recycled simply because they aren’t easily recyclable [SL, 2023].
Physical & chemical recycling
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 & chemical 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.
Recycled plastics may have lower mechanical properties compared to virgin plastics, because each time a plastic is melt & processed, the polymeric chains degrade: these properties can be recovered by mixing it to additives or virgin plastic.
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].
Figure 1: Waste Management Hierarchy
Figure 2: Physical & chemical recycling
Figure 3: Plastic recycling (physical & chemical processes)