Fuels, plastics and fabrics

Fossil fuels and petrochemicals

Coal, crude oil and natural gas are fossil fuels. They formed over millions of years from the remains of ancient plants and sea creatures that were buried and squeezed by heat and pressure. They are non-renewable: we use them far faster than they form.

Oil and gas are mostly hydrocarbons, compounds made only of hydrogen and carbon. Australia extracts natural gas mainly from offshore fields off Western Australia and the Northern Territory and from coal seams in Queensland, and smaller amounts of oil from places such as Bass Strait and the Cooper Basin.

Most crude oil (more than three-quarters) ends up as fuel. Some of the rest is used to make petrochemicals: substances made from oil and gas, including plastics, synthetic fabrics, paints, detergents, medicines and fertilisers.

Separating crude oil

Crude oil (petroleum) is a thick, dark mixture of thousands of hydrocarbons with molecules of different lengths. It is separated at a refinery by fractional distillation, which you met in Year 7. The oil is heated until most of it turns to vapour, which rises up a tall column that is hot at the bottom and cooler at the top. Each part, or fraction, condenses at a different height according to its boiling point.

Separating the fractions is a physical change. But refineries also use chemical reactions: cracking breaks large, less useful molecules into smaller ones such as petrol and ethene, the starting material for many plastics.

Polymers

A polymer is a very long molecule made by joining thousands of small molecules called monomers into a chain ("poly" means many; "mono" means one). Joining them is a chemical reaction called polymerisation.

Polymers aren't only made in factories. Many natural polymers exist: cellulose in wood and cotton, starch in potatoes, proteins such as wool, silk and hair, and DNA. Synthetic polymers such as polyethylene, PVC and nylon are made by chemists, mostly from petrochemicals.

"Plastic" really describes a property: a material that can be moulded into shape. All plastics are polymers, but not all polymers are plastics (wool and silk, for example).

Polymers come in two main types:

• Thermoplastics are made of separate chains that can slide past each other when heated, so they soften and can be remoulded and recycled. Examples: polyethylene, polypropylene, PVC, PET.
• Thermosets have strong cross-links that tie the chains together into a network, so they stay hard when heated and char rather than melt. Examples: the hard plastic in saucepan handles and electrical fittings, and the epoxy resin in surfboards.

Fabrics

Natural fibres such as cotton, wool and silk are natural polymers. Since the 1930s chemists have made synthetic fibres:

• Nylon (1935) is strong and stretchy: stockings, ropes, fishing line and sportswear.
• Polyester is the same polymer (PET) used to make drink bottles. It is strong, resists creasing and dries quickly, and is often blended with cotton. Polar fleece is 100% polyester, and is often made from recycled bottles.
• Rayon (including viscose) is "semi-synthetic": natural cellulose from wood pulp is chemically treated and spun into soft, silky fibres.

Sportswear designers and materials scientists work together to design fabrics that are light, stretchy and that draw sweat away from the skin.

The plastic problem

The properties that make plastics useful, being cheap, light, waterproof and long-lasting, also make them a problem. Most plastics take hundreds of years to break down, and they break into microplastics that are now found in oceans, soil and even in our bodies. Solutions include using less, reusing, recycling (the number in the triangle tells recyclers which polymer it is) and developing bioplastics such as PLA (polylactic acid), which is made from plant starch and can be composted in industrial facilities.