Plastic is made primarily from fossil fuels, specifically natural gas and crude oil. These raw materials supply the carbon and hydrogen atoms that form the backbone of every plastic product, from water bottles to car bumpers. About 10% of global oil production goes toward making plastics, with roughly 10 million barrels per day dedicated to the industry worldwide.
The Raw Materials Behind Plastic
In the United States, natural gas is actually the dominant source of plastic feedstock, not crude oil. Hydrocarbon gas liquids, which are byproducts of natural gas processing, provide the key building blocks. Crude oil plays a role too: refineries produce a liquid called naphtha, which serves as another major feedstock for plastic production.
Both pathways lead to the same destination. Whether it starts as natural gas or crude oil, the raw material gets broken down into small, simple molecules called monomers. The two most important monomers are ethylene and propylene, both of which are hydrocarbons containing just carbon and hydrogen atoms. These tiny molecules are the Lego bricks that snap together to build plastic.
How Raw Materials Become Monomers
The critical step is called “cracking.” In a steam cracker, naphtha or gas liquids are heated to extreme temperatures, which breaks apart their larger molecules into smaller ones like ethylene and propylene. Cracking naphtha into ethylene is enormously energy-intensive, requiring up to 40 gigajoules of heat per tonne of ethylene produced. The process also generates about 1.8 to 2 kilograms of carbon dioxide for every kilogram of ethylene made.
Once separated and purified, these monomers are ready to be linked into the long molecular chains that give plastic its characteristic properties.
How Monomers Become Plastic
Plastic is essentially a polymer: a very long chain made of thousands of identical smaller units (monomers) bonded together. Think of it like a train where every car is the same. Carbon atoms form the spine of the chain, with hydrogen and sometimes other elements attached along the sides. These chains are also called macromolecules, and their length and arrangement determine everything about how the final plastic looks, feels, and performs.
There are two main ways monomers link up. In addition polymerization, unsaturated molecules (those with double bonds between carbon atoms) open up those bonds and snap together end to end. This is how polyethylene and polypropylene are made. No byproducts are released. In condensation polymerization, monomers join together while releasing a small molecule like water as a byproduct. Polyester (PET, the plastic in most drink bottles) is made this way.
The specific monomer used determines the type of plastic. Ethylene becomes polyethylene, the world’s most common plastic, used in grocery bags and milk jugs. Propylene becomes polypropylene, found in yogurt containers and bottle caps. When ethylene is combined with co-monomers like butene or hexene, the result is high-density polyethylene (HDPE), which has a tighter molecular structure that makes it stronger and more rigid.
Why Different Plastics Have Different Properties
The arrangement of polymer chains explains why some plastics are flexible and others are stiff. In thermoplastics like polyethylene and polypropylene, the chains sit alongside each other without permanent connections between them. Heat loosens them up, which is why these plastics can be melted and reshaped repeatedly. Linear chains with few branches pack tightly together, creating denser, harder materials. Chains with lots of branching stay loosely packed, producing softer, more flexible plastics.
Thermoset plastics work differently. Their chains are locked together by permanent cross-links that form a three-dimensional network. Once set, they can’t be remelted. Epoxy resins and the hard casings on electrical outlets are thermosets. The cross-links don’t soften with increasing temperature, which is why these plastics hold their shape under heat but can’t be recycled by melting.
What Additives Do
The base polymer is rarely used on its own. Manufacturers mix in chemical additives that can make up a significant portion of the final product’s weight. Plasticizers make rigid plastics flexible (this is why a vinyl shower curtain bends while a PVC pipe doesn’t, even though both start from the same base polymer). Heat stabilizers prevent the plastic from degrading during processing. UV stabilizers keep outdoor plastics from becoming brittle in sunlight.
Other common additives include flame retardants, which slow or prevent burning; antioxidants, which prevent the polymer from breaking down over time; colorants; antistatic agents that reduce static cling; and fillers or fiber reinforcements that increase strength and stiffness. Blowing agents create the air pockets in foam products like Styrofoam. Even antimicrobial agents are sometimes added to plastics used in medical or food-contact applications. The combination of base polymer and specific additives is what gives each plastic product its particular set of characteristics.
Plant-Based Plastics
Not all plastic comes from fossil fuels. Polylactic acid (PLA) is a biodegradable plastic made entirely from renewable plant sources like corn starch, rice starch, or sugarcane. The production process converts the plant carbohydrates into dextrose (a simple sugar), which is then fermented by bacteria into lactic acid. That lactic acid is polymerized into PLA, most commonly through a method called ring-opening polymerization.
PLA behaves like a conventional thermoplastic and can be molded into packaging, cups, and cutlery. It breaks down under industrial composting conditions, unlike petroleum-based plastics. However, bioplastics like PLA still represent a small fraction of total plastic production globally. The vast majority of the roughly 400 million tonnes of plastic produced each year still originates from oil and natural gas.