Plastic production (in theory)
Written by Karina Zile Last updated on 25 June 2021
1. Base molecules
Plastics can be either synthetic or biobased. Synthetic plastics, classed as petrochemicals, are derived from fossil fuels, mostly from crude oil and natural gas.
Ingredients for synthetic plastics
Crude oil is a sticky mixture of hydrocarbons (different length chains of carbon atoms with hydrogen atoms stuck on the sides) with some nitrogen, sulphur, oxygen and minerals. Crude oil is refined into different fractions through distillation. Each fraction has a different number of carbon atoms and a different boiling point (in brackets):
- Petroleum gas C1-C4 (<25℃) - butane and lighter products
- Gasoline/petrol C5-C10 (25-60℃) - fuel for cars
- Naphtha C5-C12 (60-180℃) - raw material for the petrochemical industry
- Kerosene C10-C16 (180-220℃) - fuel for aircrafts, paraffin for lighting
- Diesel C14-C20 (220-250℃) - fuel for cars and bigger ground vehicles
- Heavy oils C20+ (>250℃) - fuel for ships and power stations, lubricating oils, bitumen for construction
Natural gas and naphtha refined from crude oil are the main raw material for production of plastics. Both naphtha and natural gas can be cracked (breaking complex hydrocarbons into lower molecular weight molecules) into olefins - C2 (ethylene), C3 (propylene), C4 (butene and butadiene) and C5 (pentene) molecules. Naphtha catalytic reforming is used to produce aromatics (molecules that contain benzene rings) - benzene, toluene, xylene, and naphthalene. Natural gas steam reforming is used to produce syngas, from which methanol and ammonia are produced.
Ingredients for biobased plastics
Biobased plastics are made from renewable products such as:
- Carbohydrates (e.g. sugars from sugar cane)
- Lignin, cellulose and hemicellulose
- Plant fats and oils
- Food waste
The monomers that make up plastics come in different shapes and sizes - some are straight chains, some are branched and some have rings.
Monomers are joined together into polymers, long chain molecules that contain hundreds to thousands of monomers, through a polymerisation reaction. The length of polymer molecules gives plastics their exceptional properties (often described as strength or toughness) that short molecules cannot match. The intermolecular forces within the polymer chains (bonding between the monomers and other forces) determine toughness and melting temperature of a particular plastic.
There are two main types of polymerisation - either the chains grow by combining two shorter chains (step-growth) or by adding one monomer at a time (chain-growth). Both typically require catalysts, heat and pressure.
In step-growth polymerisation, a pair of molecules of any length combine at each step to form a longer polymer molecule. The functional groups of these molecules dictate the exact mechanism of a step-growth reaction. The monomers are consumed very quickly into dimers, trimers and oligomers. The degree of polymerisation increases steadily during the whole polymerisation process; and long chains form only late in the reaction. The process can be controlled to limit the length of the chains and produce a low molecular weight polymer.
Polycondensation is a step-growth polymerisation based on condensation reaction between two molecules with various degrees of polymerisation. At each step a low molecular weight by-product is formed (e.g. water). This is how PET is made.
Polyaddition is a step-growth polymerisation based on addition reaction between two molecules with various degrees of polymerisation. No by-products are formed. This is how PU is made. All monomers that undergo addition polymerization contain double bonds, which allows the carbon atoms to react to form polymers.
In chain-growth polymerisation, a polymer with an active site reacts with a monomer to form a longer polymer molecule. The growth of a chain is initiated by formation of an active site. After chain initiation, the degree of polymerisation increases rapidly and long chains are formed from the beginning of the reaction. This is how PE, PP, PVC and PS are made.