Plastic is mostly polymer, a long chain of repeating chemical units derived from oil, natural gas, or occasionally plant material. But the polymer itself is only part of the story. A typical plastic product also contains additives like plasticizers, stabilizers, flame retardants, pigments, and residual manufacturing chemicals, sometimes making up a third or more of the final product’s weight. These additives are what give plastic its color, flexibility, fire resistance, and durability, and they’re also the source of most health and environmental concerns.
The Base Polymers
Every plastic starts with a polymer, which is a molecule built by linking thousands of smaller units (called monomers) into a chain. The raw material for most polymers is fossil fuel. Crude oil or natural gas is refined into simple chemicals like ethylene or propylene, which are then polymerized into solid resin pellets ready for manufacturing.
There are seven standard categories of plastic resin, each with a different chemical backbone:
- PET (code 1): polyethylene terephthalate, used in water bottles and food containers
- HDPE (code 2): high-density polyethylene, used in milk jugs and detergent bottles
- PVC (code 3): polyvinyl chloride, used in pipes, flooring, and medical tubing
- LDPE (code 4): low-density polyethylene, used in plastic bags and squeeze bottles
- PP (code 5): polypropylene, used in yogurt cups and bottle caps
- PS (code 6): polystyrene, used in foam cups and takeout containers
- Other (code 7): a catch-all for everything else, including polycarbonate, nylon, and bioplastics
These polymers have very different properties. Polyethylene is chemically simple and relatively inert. PVC, on the other hand, is rigid and brittle on its own, which is why it requires heavy doses of additives to become the flexible material used in shower curtains and vinyl flooring. The type of polymer determines which additives are needed and, by extension, which chemicals you’re exposed to when you use the product.
Plasticizers: Making Plastic Soft
Rigid plastic becomes flexible through plasticizers, chemicals that wedge between polymer chains and give them room to slide past each other. This lowers the temperature at which the plastic transitions from stiff to pliable. Phthalates are the most well-known family of plasticizers, and they’re used heavily in PVC products like vinyl flooring, food wrap, and inflatable toys. In flexible PVC, plasticizers can account for 25 to 45 percent of the product by weight.
The concern with plasticizers is that many of them aren’t chemically bonded to the polymer. They’re physically mixed in, which means they can gradually migrate out. Research on plastics in seawater, for example, has found that diisononyl phthalate (a common PVC plasticizer) is one of the primary chemicals that leaches from PVC into the surrounding environment. Phthalates have been linked to hormone disruption, which is why several have been restricted in children’s products in the U.S. and EU.
Stabilizers and Heat Protectors
Polymers degrade when exposed to heat, UV light, or oxygen. Without stabilizers, a plastic lawn chair would become brittle and crack within a single summer. Stabilizers absorb or neutralize the chemical reactions that break polymer chains apart.
PVC is particularly unstable on its own, so it requires dedicated heat stabilizers during processing. Historically, lead salts and organotin compounds were the go-to options because of their high efficiency. Both are toxic, and their use is now restricted in many countries, though they haven’t disappeared entirely from global manufacturing. Modern alternatives include calcium-zinc and barium-zinc stabilizer systems, which perform the same function with a lower toxicity profile. UV stabilizers, which protect outdoor plastics from sunlight, are a separate category and are found in everything from garden furniture to car dashboards.
Flame Retardants
Plastics used in electronics, building materials, and textiles often contain flame retardants to meet fire safety standards. Brominated flame retardants are among the most common. Tetrabromobisphenol A, for instance, is widely used in the plastic housings of computers and other electronics. These compounds work by releasing bromine atoms when heated, which interrupt the chemical reactions that sustain a fire.
The tradeoff is that brominated flame retardants are persistent in the environment and accumulate in dust, soil, and living tissue. Some have been phased out or banned, but replacements aren’t always better understood. Organophosphate flame retardants, which are increasingly used as substitutes, have been detected leaching from polyethylene into seawater and are now showing up as widespread environmental contaminants themselves.
Pigments and Colorants
The color of a plastic product comes from pigments or dyes mixed into the polymer during manufacturing. Inorganic pigments based on metals like titanium (for white), iron oxide (for red and yellow), and chromium (for green) are common in plastics that need to resist fading. Organic dyes provide brighter, more varied colors but can be less stable over time. Cadmium-based pigments, once popular for their vivid yellows and reds, have been largely phased out due to toxicity concerns, though they still appear in some imported goods.
Residual Manufacturing Chemicals
Even after production is complete, plastics retain trace amounts of the chemicals used to make them. PET bottles, for example, are manufactured using antimony trioxide as a catalyst to drive the polymerization reaction. The antimony remains embedded in the finished plastic at concentrations of 100 to 300 milligrams per kilogram. A study comparing PET bottles from Japan and China found that 100 percent of Chinese-manufactured bottles contained antimony above 10 mg/kg, compared to about 30 percent of Japanese bottles. Most of this antimony converts into other chemical forms during manufacturing, so the original antimony trioxide is only a minor fraction of the total, but the element persists in the material.
These residual chemicals can migrate into whatever the plastic contains. The rate of migration depends on temperature, acidity, and time. Hotter conditions and longer storage both increase leaching.
BPA and Hormone-Disrupting Compounds
Bisphenol A (BPA) is a building block of polycarbonate plastic, the hard, clear material used in reusable water bottles, food storage containers, and the linings of some canned foods. Unlike an additive that’s simply mixed in, BPA is part of the polymer’s chemical structure. But the bonds can break down over time, releasing free BPA molecules.
Heat accelerates this process significantly. A study on polycarbonate water jugs found that bottles stored outdoors in sunlight for 30 days reached BPA concentrations of about 9 micrograms per liter, considerably higher than bottles kept at room temperature. Even bottles from different manufacturers leached at different rates, suggesting that production quality plays a role. BPA mimics estrogen in the body, and concerns about its effects on development and reproduction have led to bans in baby bottles across many countries and a broader shift toward “BPA-free” products, though some replacement chemicals face similar questions.
PFAS in Food Packaging
Per- and polyfluoroalkyl substances, known as PFAS or “forever chemicals,” show up in plastic food packaging in two ways. Some are intentionally added. The FDA has authorized PFAS for use as nonstick coatings, grease-proofing agents in paper food packaging, sealing gaskets in food processing equipment, and manufacturing aids in other food-contact plastics. When used as manufacturing aids, the amounts are small enough that migration into food is considered negligible by regulators.
The second route is contamination. PFAS can end up in food packaging as an impurity formed during chemical processing, or because the water used in manufacturing was itself contaminated with PFAS. These unintentional occurrences are not authorized by the FDA. PFAS are called “forever chemicals” because their carbon-fluorine bonds are extraordinarily stable, meaning they don’t break down in the environment or in your body for years to decades.
What About Bioplastics?
Bioplastics like PLA (polylactic acid) start from plant sugars rather than petroleum. Corn, sugarcane, or other starchy crops are fermented by bacteria to produce lactic acid, which is then polymerized into a plastic resin. The process still involves industrial chemicals: tin-based catalysts are used during polymerization and must be removed afterward, and sodium carbonate is added to help separate the catalyst from the finished product.
PLA is chemically distinct from petroleum-based plastics, and it doesn’t contain phthalates or BPA. But “bioplastic” doesn’t automatically mean additive-free. PLA products can still contain plasticizers, colorants, and stabilizers depending on their intended use. The plant-based origin changes the polymer backbone, not necessarily everything else mixed into the final product.