"9.2 billion tons are estimated to have been made btw 1950-2017"
Plastics are macromolecules derived from polymerization processes, in which small molecules (monomers) combine chemically to produce a very large chain-like molecule, called a polymer. Monomers may be all alike, or they may represent two, three, or more different compounds [EB, 2023].
The formation of stable covalent chemical bonds btw the monomers sets polymerization apart from other processes, such as crystallization, in which large numbers of molecules aggregate under the influence of weak intermolecular forces.
Plastic consumption has quadrupled over the past 30 years, driven by growth in emerging markets. Global plastics production doubled from 2000 to 2019 to reach 460 million tons [OECD, 2022], and is expected to reach about 600 million tons in 2050 [STAT, 2023].
The most widely utilized plastics are thermoplastics (plastic that becomes moldable at a certain temperature and solidifies upon cooling), and among them the most common are PE (Polyethylene - 29.6%), PP (Polypropylene - 18.9%), PVC (Polyvinyl Chloride - 10.4%), PUR (Polyurethane - 7.4%), PS (Polystyrene - 7.1%), and PET (Polyethylene Terephthalate - 6.9%), respectively [ACS, 2022].
Feedstocks for plastics
Plastics, no matter the source, are mainly made of carbon (about 80% by weight). Polymers are formed by hundreds of thousands of atoms bounded together, so its molecular weights is very high. In a certain polymer, if all the structural units are identical it is called homopolymer; if it comes from two or more monomers, copolymer [ST, 2017].
Ethylene (ethane), a colorless gaseous hydrocarbon (C2H4, the simplest alkene) obtained as a byproduct of the processing of crude oil or NG, is one of the key monomers in plastic industry (95% of its commercial production is based on steam cracking of petroleum hydrocarbons). It is developed into four different compounds, which in turn are developed into several household and other products [AFMP, 2023]:
Polyethylene (common plastic) – used to make food packaging, bottles, bags, and other plastics-based goods.
Ethylene Oxide / Ethylene Glycol – becomes polyester for textiles, as well as antifreeze for airplane engines and wings.
Ethylene Dichloride – this, in turn, becomes a vinyl product used in PVC pipes, siding, medical devices, and clothing.
Styrene – synthetic rubber found in tires, as well as foam insulation.
Ethylene is the most widely produced petrochemical. In 2022, global ethylene production capacity & demand were 205 & 177 million tons, respectively [STAT, 2023]. Figure 1 shows the chemical formula of ethylene & polyethylene.
Plastic chasing arrows
The chasing arrow symbol was adopted by the plastics industry in 1988 for use in the resin identification code system. Figure 2 shows six of them.
#1 - PET (Polyethylene Terephthalate)
One of the most commonly used plastics in consumer products, it is found in most water & pop bottles. PET, which may leach carcinogens, is recyclable (about 25% of PET bottles in the US today are recycled): it is crushed and then shredded into small flakes which are then reprocessed to make new PET bottles, or spun into polyester fiber [LE, 2020].
#2 - HDPE (High-Density Polyethylene)
HDPE is the stiff plastic used to make milk jugs, detergent, oil bottles, and toys. HDPE is the most commonly recycled plastic and is considered one of the safest forms of plastic.
#3 – PVC (Polyvinyl Chloride)
PVC comes in rigid and flexible forms and is used in construction for pipe, doors, and windows. It is also used in making plastic bottles, packaging, and bank cards. PVC, manufactured from either naphtha or ethylene feedstock, is a poison plastic as it contains toxins which it can leach throughout its entire life cycle. Products made using PVC are not recyclable..
#4 – LDPE (Low-Density Polyethylene)
LDPE is often found in shrink wraps, dry cleaner garment bags, squeezable bottles, and bags used to package bread. Plastic grocery bags used in most stores today are made using LDPE plastic. Not commonly recycled, it is considered less toxic than other plastics, and relatively safe for use.
#5 – PP (Polypropylene)
2nd-most widely produced plastic (after PET), PP is partially crystalline, tough, and lightweight plastic, with excellent heat-resistance. It serves as a barrier against moisture, grease, and chemicals. PP is recyclable thru some curbside recycling programs, but only about 3% of PP products are currently being recycled in the US.
#6 – PS (Polystyrene)
PS is an inexpensive, lightweight, and easily-formed plastic, most often used to make disposable foam drinking cups, take-out food containers, and egg cartons. While the tech for recycling polystyrene is available, the market for recycling is small.
Figure 3 shows monomers for common household plastics.
At present, plastic waste accumulation has been observed as one of the most alarming environmental challenges, affecting all forms of life, economy, and natural ecosystems [ACS, 2022]. Recycling plastic saves twice as much energy than burning it in an incinerator.
Figure 1: Chemical formula of polyethylene
Figure 2: Plastic chasing arrows
Figure 3: Monomers for common household plastics