27.8 Plastics and Recycling

Because plastics are largely non-biodegradable, the huge quantity (one estimate is 10⁸ metric tons per year) of plastic materials produced for consumer and industrial use has created a gigantic problem of what to do with plastic waste.  Plastics are difficult to incinerate safely and threaten to overwhelm the capacity of landfills (Figure 27.8c). An additional consideration is that production of most of the major polymers consumes non-renewable hydrocarbon resources.

Plastics recycling has become a major industry, greatly aided by enlightened trash management policies in the major developed nations. However, it is plagued with some special problems of its own:

• Recycling is only profitable when there is a market for the regenerated material. Such markets vary with the economic cycle.  They practically disappeared during the recession that commenced in 2008.
• The energy-related costs of collecting and transporting plastic waste, and especially of processing it for re-use, are frequently the deciding factor in assessing the practicability of recycling.
• Collection of plastic wastes from diverse sources and locations and their transport to processing centers consumes energy and presents numerous operational problems.
• Most recycling processes are optimized for particular classes of polymers. The diversity of plastic types necessitates their separation into different waste streams — usually requiring manual (i.e., low-cost) labor. This in turn encourages shipment of these wastes to low-wage countries, thus reducing the availability of recycled materials in the countries in which the plastics originated.

In general, plastics are collected and transported to a facility where they are sorted by recycling code. This is one of the major limiting factors of the process. There are several ways the plastics are separated including by hand, through physical property comparison and using technology. The plastics are then shredded, washed, and granulated into pellets.  These pellets are then transported and reformed into future products. Products using recycled pellets or fibers will have different properties than the original plastic.  In addition, there is a limit to the number of times plastics can be recycled and reformed resulting in waste production. (Hein et al., 2014; Brunning, 2018)

Some of the major recycling processes include:

• Thermoplastic polymers can be melted and pelletized, but those of widely differing types must be treated separately to avoid incompatibility problems.
• Thermosetting polymers are usually shredded and used as filler material in recycled thermoplastic polymers.
• Thermal decomposition processes that can accommodate mixed kinds of plastics and render them into fuel oil, but the large inputs of energy they require have been a problem.  Read more about Converting waste plastic bottles into jet fuel on the Chemistry World website
• A small number of condensation polymers can be depolymerized so that the monomers can be recovered and re-used.
• A process to recycle scrap steel has been developed in which many kinds of plastic can be used as a carbon source.
• Mixed recycling of different plastics is possible.  It does not require their separation and is called compatibilization.  It requires use of special chemical bridging agents compatibilizers. Compatibilization can help to keep the quality of recycled material and to skip often expensive and inefficient preliminary scanning of waste plastics streams and their separation/purification.
• Re-use of rubber tires in the construction industry is one method to support the increasing reluctance of landfills to accept used tires.