America’s Plastic Makers view plastic as a resource that is too valuable to waste. Recycling is the major strategy for addressing the global plastic waste challenge. Plastic recycling is the reprocessing of scrap plastic wastes into new and useful products. Ever since China imposed restrictions on plastic exports in 2017, plastic recycling investments in the United States have increased significantly. According to the American Chemistry Council, several project investments continue to expand plastic recycling capacity and the use of new recycling technologies, including innovations in mechanical recycling and advanced chemical recycling.
Plastic waste consists of various polymer types: polyolefins make up nearly 50% of all plastic waste, and more than 90% of the waste is made up of polymers which can be remelted. The predominant polymers include: high density polyethylene (HDPE), low density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polyurethane. Scrap plastic waste can be broadly divided into two categories: industrial scrap or post-industrial resin and post-consumer waste. Industrial scrap is typically generated during the production of plastic products and consumer goods, and it can include various categories of waste, for example: flashings, trimmings, purge, and product rejects. In general, the properties and chemical content are different from post-consumer waste.
Currently, the dominant technology for plastic recycling is mechanical recycling. Mechanical recycling uses plastic waste as feedstock to produce recycling resins that can be reprocessed and repurposed in various manufacturing applications. The processes associated with this approach involve an array of technologies, for example: sorting systems for different plastic types, grinding for size reduction, washing, separation of contaminants, drying, extruding, compounding, and re-granulating. Recent innovations with these integrated systems involve more effective sorting technologies, i.e. artificial intelligence, robotics, near-infrared-optics, and tracer-based sorting. Recovered plastics can be substituted for virgin and new plastics without changing the chemical structure of the base polymer. The effectiveness of mechanical recycling depends on the quality of clean baled plastic feedstocks from upstream collection and sorting systems as well as the quality of the recovered resins. These resins are well suited to produce plastics for new durable applications such as HDPE pipes, railroad ties, pallets, plastic lumber, roofing panels, landscape products and a growing number of consumer goods. Plastics that are not recyclable by mechanical recycling can be considered as feedstock for chemical recycling.
Chemical recycling, also referred to advanced recycling and recovery, refers to several different processes that use existing and emerging technologies to convert post-use plastics back into their basic chemical building blocks to create a versatile mix of production outputs, for example: virgin-quality plastics, specialty chemicals, monomers, chemical feedstock (naphtha), fuels and waxes. Unlike mechanical recycling, chemical recycling typically alters the physical form of used plastics. Chemical recycling is increasingly playing a role within the circular economy, especially with mixed plastic scrap, packaging and flexible films that are difficult to recycle or repurpose with mechanical techniques.
There are three primary categories of chemical recycling technologies that process scrap plastics into new plastics and products.
- Solvent-based purification, designed to extract dyes and additives from scrap plastic mixtures to ultimately obtain a purified plastic. Purification processes do not change the polymers at a molecular level. The value of the resulting plastic resin, for example polypropylene is maintained, although there can be yield loss.
- Decomposition, also known as depolymerization, breaks down the molecular bonds that are used to make plastics. These operations result in monomers (molecules) from which the plastics are made.
- Conversion breaks down the molecular bonds that are used to make plastics and recombines them, resulting in hydrocarbons and chemical feedstocks. The processes typically use chemical reactions, such as catalytic cracking and hydrogenation, and/or thermal mechanisms, such as, gasification or pyrolysis. The output products can include a variety of fuels and petrochemical feedstocks along with solids, such as waxes.
With each category, there are acceptance criteria typically used by the chemical recycling facilities to ensure required feedstock specifications are met, including: optimizing required polyolefin content, removing paper, glass and metal contaminants from the plastic feedstock, and limiting incompatible plastics and contaminants adversely impacting process equipment. Tolerance for contamination depends on the desired final output material.
With consumer demands for circularity in the supply chain, on-going investments in mechanical recycling innovations and chemical recycling will continue to play a key role in a circular economy for plastics.