PET plastic is made from two chemical building blocks: ethylene glycol, a colorless liquid derived from ethylene, and terephthalic acid, a crystalline solid derived from xylene. Both of these starting materials come from crude oil. When heated together with a catalyst, they react to form long chains of polyethylene terephthalate, the full name behind the abbreviation you see stamped on water bottles, food containers, and polyester clothing.
The Two Building Blocks
Ethylene glycol is a simple alcohol with two reactive spots on its molecule (hydroxyl groups). Terephthalic acid is an acid built around a six-sided carbon ring with two reactive spots of its own (carboxyl groups). When these two molecules are heated, the reactive spots link up and release water as a byproduct. The chemical links they form are called ester bonds, and repeating this reaction thousands of times creates the long polymer chains that give PET its strength and flexibility.
Both raw ingredients trace back to petroleum. Ethylene glycol starts as ethylene, a gas produced during oil refining. Terephthalic acid starts as xylene, another petroleum derivative. This fossil fuel origin is a major reason PET production generates significant greenhouse gas emissions and why researchers are working on plant-based alternatives.
How the Reaction Works
PET production happens in two main stages. First, the ethylene glycol and terephthalic acid undergo an esterification reaction at around 260°C. During this step, water is driven off. The reaction is considered complete when the water output reaches about 95% of its theoretical maximum, meaning nearly all the available molecules have linked up.
In the second stage, called polycondensation, a vacuum is applied to pull out any remaining water and force the short chains to keep joining into longer ones. The longer these polymer chains grow, the stronger and more useful the final plastic becomes. A catalyst, typically antimony trioxide, speeds up both stages. The antimony remains in the finished plastic at trace levels, though occupational safety data shows that worker exposure during PET manufacturing is considered negligible.
What Makes PET Useful
The resulting plastic has a combination of properties that explain why it’s everywhere. PET melts at roughly 250 to 280°C, which means it holds up well under heat. It retains its mechanical strength at temperatures up to 175°C. It’s lightweight, chemically stable, and resistant to impact. Its density shifts depending on its internal structure: the crystalline (orderly) regions are denser at 1.455 g/cm³, while the amorphous (disordered) regions are lighter at 1.335 g/cm³. Manufacturers can control this balance to produce either clear, flexible material (more amorphous, good for bottles) or opaque, rigid material (more crystalline, good for trays and fibers).
PET also has a glass transition temperature of about 69°C. Below that point it behaves like a rigid glass; above it, the polymer chains gain enough mobility to become somewhat rubbery. This is why a PET water bottle left in a hot car can start to warp and soften.
PET, Phthalates, and BPA
A common question is whether PET contains the same phthalates that raise health concerns in other plastics. The short answer: not really. The phthalates linked to hormone disruption are ortho-phthalates, which are plasticizers added mainly to PVC to make it soft and flexible. PET doesn’t need plasticizers. Its name includes “terephthalate,” which is a different chemical arrangement (a para isomer rather than an ortho isomer of phthalic acid). PET is rigid on its own and doesn’t require softening agents.
BPA is a separate concern. PET is not manufactured with bisphenol A, and it’s often marketed as BPA-free. However, research has detected small amounts of BPA in beverages stored in PET bottles, with concentrations increasing over longer storage times. How BPA ends up there isn’t entirely clear, since it isn’t an intentional ingredient. The European Commission banned BPA in all food contact materials in December 2024, reflecting growing caution about even low-level exposure.
Plant-Based and Recycled Alternatives
Because conventional PET depends entirely on crude oil, there’s active effort to replace one or both of its building blocks with renewable sources. Bio-based ethylene glycol can be produced from sugarcane. Researchers have investigated first-generation biorefineries using sugarcane molasses and combined first- and second-generation facilities that also use bagasse (the fibrous leftover from sugar extraction) and leaves. The technical goal is a fully bio-based PET that’s chemically identical to petroleum PET but made from plants.
Recycled PET, labeled rPET, takes a different approach. Rather than starting from raw chemicals, mechanical recycling collects used PET bottles and containers, then sorts, crushes, washes, and grinds them into flakes. These flakes can be melted and reformed into new products. The challenge is contamination. Even small amounts of other plastics like PVC, polystyrene, or polypropylene mixed into the waste stream degrade the quality of the recycled material. That’s why sorting accuracy matters so much, and why the recycling number on PET containers (the “1” inside the triangle) exists: to help keep PET separate from other plastic types.
Chemical recycling offers another path, breaking PET back down into its original monomers (ethylene glycol and terephthalic acid) so they can be reassembled into virgin-quality plastic. This handles contamination better than mechanical recycling but requires more energy and infrastructure.