Table 1.

Overview of recycling methods for thermoplastic materials.

Recycling Methods	Short Description	Advantageous (+)/ Disadvantageous (−)	Status of the Technology	References
Primary recycling, i.e., mechanical recycling of production scraps	closed-loop recycling: recovered polymer materials must be cut/ crashed/ grinded/ shredded to small-sized pieces and reprocessed with virgin materials	(+) post-consumer and virgin polymers have same chemical nature (+) post-consumer polymers are separated from contaminants (−) post-consumer polymers must be stabilized against degradation during reprocessing	industrially applicable practice	[11,12]
Secondary recycling, i.e., mechanical recycling of waste	polymers separation techniques and reprocessing: (i) separation techniques by floatation, gravity, electrostatic or magnetic separation, and sensor-based sorting; (ii) cutting/crashing/grinding/shredding to small-sized pieces; (iii) reprocessing by traditional processing techniques, such as extrusion, injection/compression molding; etc.	(+) separations based on different principles: floatation, gravity, electrostatic or magnetic separation, and sensor-based sorting (−) polymers experience different degradation/oxidation levels in service (−) mixed waste contain contaminants and they are incompatible (−) second-life materials show low properties	industrially applicable practices, that continuously evolved, considering the changes of waste stream compositions	[13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31]
Tertiary recycling, i.e., chemical recycling through chemolysis, cracking, and gasification	treatments for bonding scission through chemicals, heat with and without catalytic agents, for mono-/oligomers recovery	(+) second-life materials show excellent properties (−) high-cost technology (−) negative environmental impacts due to use of chemicals, solvents, etc.	under investigations; industrially applicable for PET	[32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72]
Quaternary recycling, i.e., energy recovery	incineration of mixed plastic to recover their embedded energy	(+) energy recovery, which is preferable to landfilling and disposal in the seas and oceans (−) gas emissions must be controlled to minimize their negative environmental impacts	applicable	[73,74]