POLYMER RECYCLING TECHNOLOGIES
Efforts to recycle post-consumer polymer products and factory scraps are gaining momentum worldwide with the growing global emphasis on sustainability. Work in this area includes not only the collection of growing percentages of post-consumer polymer products and factory scraps but also the development of more advanced recycling technologies.
Most of the technologies for recycling post-consumer polymer products and factory scraps fall into one of four major categories; namely, mechanical recycling, chemical recycling, combined mechanical/chemical recycling, and thermolysis.
Mechanical recycling recovers thermoplastic polymers from waste streams while preserving the molecular structure. Its main steps are (1) sorting a waste stream into different recyclable polymers (unless it was pre-sorted during collection), (2) shredding, (3) cleaning (most commonly by washing, but dry cleaning is used with some advanced technologies), (4) melt processing (most commonly via extrusion), (5) filtration (often while melt processing, but sometimes in a separate step), and (6) pelletizing. Use-recycling-reuse cycles often lead to decline in quality, resulting in “downcycling” caused by one or a combination of reduction of the average molecular weight of the polymer chains, other chemical reactions causing undesirable changes in the polymer, and/or accumulation of impurities. Downcycling limits reuse to products of increasingly lower value in successive cycles. It also limits the total possible number of cycles of use–recycling–reuse. The reader is recommended to watch a three-minute video showing a mechanical recycling system (the MAS Double Rotor Disk technology combined with MAS Extruder and Continuous Melt Filtration) in operation for the shredding, dry cleaning, and pelletizing of agricultural film.
Chemical recycling depolymerizes polymers into their constituents to an extent that is controllable by the selection of process conditions. Unlike mechanical recycling whose applications are limited to thermoplastic polymers, chemical recycling can be applied both to thermoplastics and to thermosets. Depolymerization into oligomers is preferred in some implementations while depolymerization all the way to the monomers is preferred in other implementations. The recovered and purified constituents can be used in manufacturing new products comparable in quality to those obtained from virgin monomers. Quality is retained in repeated use-recycling-reuse cycles, placing the target of “closed loop” recycling within reach. “Upcycling” where the recovered constituents are modified during depolymerization into constituents usable in polymers of higher value is also sometimes possible, as in the partial depolymerization of poly(ethylene terephthalate) (PET) combined with auxiliary reactions to obtain valuable aromatic polyols for use in polyurethane manufacturing. These advantages of chemical recycling, however, are offset by the disadvantage that it is usually more expensive than mechanical recycling. The image pasted below illustrates Teijin’s ECO CIRCLE™ technology for the closed-loop chemical recycling of used polyester products to obtain polyester fibers of quality equivalent to virgin polyester fibers, one of the earliest uses of chemical recycling in the textiles industry.
Combined mechanical/chemical recycling combines various chemical reactions (depolymerization, chain extension, functional group attachment, etc.) with a mechanical recycling process. Such reactive processing is usually achieved by using reactive extrusion during combined mechanical/chemical recycling while simple extrusion is used when mechanical processing is performed by itself. The image pasted below, which is reproduced from the graphical portion of the Abstract of Xi Yang, Jocelyn Clénet, Huan Xu, Karin Odelius, and Minna Hakkarainen, Two Step Extrusion Process: From Thermal Recycling of PHB to Plasticized PLA by Reactive Extrusion Grafting of PHB Degradation Products onto PLA Chains, Macromolecules, 2015, 48 (8), pp. 2509–2518, provides an example of combined mechanical/chemical recycling.
Thermolysis, defined as thermochemical decomposition into fuels whose energy content can be recovered, is sometimes the most economically attractive option. This is often the case for “hydrocarbon polymers” consisting of carbon (C) and hydrogen (H) atoms with little or no heteroatom content; such as polyolefins, polystyrene, and some rubbers. It is also the case for some very mixed waste streams, but the economics are unfavorable if components with recoverable energy content are present at very low percentages. Various oils and/or gases are obtained during thermolysis. The composition of these oils and/or gases depends both on the molecular structure of the polymer and on the thermolysis process conditions. There are three types of thermolysis processes. Pyrolysis is defined as thermal cracking in an inert atmosphere. Hydrogenation (or hydrocracking) is defined as cracking by hydrogen (H2) addition via chemical reaction. Gasification, defined as thermooxidative cracking, is performed either in air (cheaper and simpler process) or in oxygen (O2) gas (more expensive but more effective process). The reader is recommended to watch a 90-second video describing the Agilyx plastics-to-oil technology as an example.
The following are the main challenges to the success of polymer recycling efforts:
- Waste stream considerations are of paramount importance.
- Recycling economics are more favorable with increasing quantity and steadiness of the waste stream collected in geographical area, greater social consciousness of residents, and the availability of infrastructure enabling residents to follow best practices.
- The main components of infrastructure enabling residents to follow best practices are simple pre-sorting (i.e., recyclable versus non-recyclable), placing different classes of recyclables into different collection containers, and washing some classes of recyclables before disposal.
- Technology improvements are also needed.
- Improved automated sortation of mixed waste streams, mainly by using methods such as flotation, rely on density differences or methods such as visible and near-infrared spectroscopy rely on optical properties.
- Continued improvements of mechanical and combined mechanical/chemical recycling technologies to obtain product of higher quality so that “downcycling” is slowed and more cycles of use-recycling-reuse become possible.
- Chemical recycling processes and thermolysis processes often need to become more cost-effective to be viable.