The landscape of plastic waste recycling is evolving rapidly, with mechanical and chemical recycling technologies offering complementary approaches to addressing the global plastic waste crisis. Mechanical recycling is by far one of the most common and widely used recycling techniques due to its simplicity, solvent-free process, and cost-effectiveness . The process consists of converting waste thermoplastics into new feedstock through collection, sorting, washing, drying, shredding, and melting . Mechanical recycling has specific advantages in terms of energy efficiency, environmental impact, and retaining polymer properties, as it involves sorting, cleaning, and reprocessing to recover the material without introducing new chemicals .

However, mechanical recycling faces significant challenges regarding the degradation of mechanical properties and contamination. A significant challenge of mechanical recycling is the thermo-mechanical degradation that polymers undergo with each new recycling cycle, which limits the number of times they can be reprocessed . This happens due to several factors, such as oxidation, breakage of long polymer chains, and irregular reconnections during extrusion. Therefore, to achieve the desired properties when recycling, it may be necessary to add a portion of virgin polymer and additives . Additionally, post-consumer plastics are usually harder to recycle, either due to decentralized and sometimes unknown origins or because of insufficient information about their chemical composition and identification, with contamination due to inadequate separation further hindering recycling .

Post-consumer recycled plastics processed through mechanical recycling are frequently downcycled for use in applications of reduced value. To bridge the gap between recyclate properties and end-use requirements in flexible packaging, research has investigated the systematic blending of post-consumer polyethylene flexible film packaging waste streams with virgin PE materials . Results show that key mechanical and optical properties improve with increasing virgin content, though in several cases, up to 80% recyclate content still met or exceeded property requirements for target applications . By enhancing the properties of the recyclate and enabling precise classification for specific applications, this research proposes optimized material formulations and targeted market applications, thereby facilitating the expansion of the scope and value of recycled plastics in the circular economy .

Chemical recycling is widely used when the focus is on recovering high-value components. The process consists of breaking down polymers into their monomers or oligomers and using them to produce new polymers or other chemical products . Methods include solvolysis, gasification, hydrogenolysis, hydrocracking, and pyrolysis, with solvolysis regarded as an advanced recycling method that uses solvents such as water, alcohols, ammonia, and supercritical fluids to depolymerize chemical bonds of polymers . One of the main advantages of solvolysis is the recovery of high-quality components. Although solvolysis is less polluting than other methods such as landfilling, it may still have a negative impact on the environment due to the use of solvents . Pyrolysis, an anaerobic thermochemical process occurring at high temperatures around 300-700°C, generates three types of products: char, bio-oil, and fuel gas, and is typically classified into conventional, fast, and flash pyrolysis based on operating conditions .

Recent advances in chemical recycling and upcycling of plastic waste into valuable materials, chemicals, and energy are expanding the possibilities for sustainable polymer materials. Emerging techniques such as microwave-assisted depolymerization, tandem catalysis, and co-processing approaches are being highlighted for their potential to enhance efficiency under milder conditions . Emphasis is also placed on the production of high-value products such as monomers, naphtha-range hydrocarbons, and syngas, with discussion provided on catalyst stability, contaminant removal, scalability, life cycle effects on the environment, and technoeconomic viability . Breakthroughs in recycling technologies are overcoming traditional quality limitations of recycled resins, with advanced sorting systems using AI and robotics achieving purity levels exceeding 99% for PET flakes and chemical recycling methods now capable of breaking down mixed plastic waste into virgin-quality monomers . Despite this potential, chemical recycling currently accounts for only about 0.1% of all recycled plastics, with high capital costs and energy requirements remaining barriers to widespread adoption .