Plastics are mostly petrochemicals, made from natural gas or petroleum, a type of oil. Chemical engineers refine the petroleum which goes through a heating process to develop ethylene and propylene, which are the chemical building blocks for many plastics. These chemicals are then combined with other chemicals to produce a polymer.
Chemical recycling is any process that turns plastic polymers back into individual monomers (a molecule that can be bonded to other identical molecules to form a polymer). During the chemical recycling process, the chemical building blocks that make up the plastic are recovered. Once the plastics have been returned to these fundamental building blocks, it can, in some cases, be re-polymerized endlessly, giving them the qualities of brand-new, or virgin, resin.
Chemical recycling is best suited for hard to recycle, multilayered or heavily contaminated plastics. The main benefit of a chemical recycling process is that it is more tolerant of contamination, and it yields polymers that are identical to the original, eliminating downcycling.
Three types of chemical recycling
Pyrolysis: Sometimes called ‘plastics to fuel’, turns non-recycled plastics from municipal solid waste (garbage) into a synthetic crude oil that can be refined into diesel fuel, gasoline, heating oil or waxes. Using pyrolysis to convert non-recycled plastics into ultra-low sulfur diesel (ULSD) fuel reduces greenhouse gas emissions by 14% and water consumption by 58%, and it saves up to 96% in traditional energy use as opposed to ULSD from conventional crude oil. Many plastics include additives to give it specific property
Some polymers include additives during manufacture. Other polymers include additives during processing into their finished parts. Additives are incorporated into polymers to alter and improve basic mechanical, physical or chemical properties. Additives are also used to protect the polymer from the degrading effects of light, heat, or bacteria; to change such polymer processing properties such as melt flow; to provide product color; and to provide special characteristics such as improved surface appearance, reduced friction, and flame retardancy.
Pyrolysis is able to remove these harmful chemicals and additives so any plastics made from this method will be additive free. Mechanical recycling is not able to remove harmful and toxic additives so they are automatically recycled along with the plastic product.
Gasification: Converts non-recycled materials from municipal solid waste into a synthesis gas, or “syngas,” which can be used for electric power generation or converted into fuel or chemical feedstocks, such as ethanol and methanol, some of which can also be used to make new plastics that go into consumer products.
Hydrogenation: Hydrogenation of plastics is a potential alternative for breaking down the polymer chain. Compared to treatments in the absence of hydrogen, hydrogenation leads to the formation of highly saturated products, avoiding the presence of olefins in the liquid fractions, which favours their use as fuels without further treatments. However, hydrogenation suffers several drawbacks, mainly due to the cost of hydrogen and the need to operate under high pressure.
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Depolymerization: (or depolymerisation) is the process of converting a polymer into a monomer or a mixture of monomers.
Olefins: Olefin is a compound comprising hydrogen and carbon, with at least one pair of carbon atoms. Both petrol and diesel contain several different hydrocarbon molecules. Paraffins, olefins and aromatics account form most hydrocarbons in petrol, while diesel is mostly paraffins, aromatics and naphthenes.
Dissolution: Polymers are dissolved in a selected solvent so the polymer can be separated from any contamination before being precipitated back out and re-used as a polymer. Dissolution does not affect the chemical composition of the polymer.
Wikipedia: What is plastic?
Oxford English Dictionery: Monomer
American Chemistry Council: How plastics are made
Americal Chemistry Council: What is chemical recycling?
Chemical & Engineering News: Plastic has a problem; is chemical recycling the solution?
Plastic Recyclers Europe: Chemical recycling
Blue Vision Intelligence: Mechanical, energy or chemical. How different types of recycling work
Plastics Industry Association: Recycling 101: Advanced recycling
‘Recycling of plastic waste into fuels, a review’ by Hayelom Dargo Beyene, International Journal of Science, Technology and Society, 08 December, 2014.
Petro Industry News: An introduction to olefins