Polymers and fibres

What is a polymer?

The word polymer means 'many units' and refers to a group of molecules which react to form a chain of molecules. Each 'unit' is a smaller, individual molecule called a monomer. Monomers become polymers through polymerisation, which is the chemical process by which monomers link together. Polymers contain one type of monomer or link two types in an alternate pattern. A polymer containing more than one monomer is a co-polymer.

Remember that carbon atoms can form up to four bonds with other elements - this means that carbon has the ability to form continuous links with itself, as is required when making a polymer, so carbon tends to form the backbone of many polymers. Molecules from the alkene family, where the carbon atoms form double bonds, are particularly good at making polymers. Instead of the double bond that exists in a self-contained unit, these bonds are broken such that they form links with neighbouring alkenes to make a chain.

An example of this is polyethene (sometimes known as polythene or polyethylene), which is a chain of many ethene molecules that looks like this: See image 1.

There are several types of naturally occurring polymers such as plant-based cellulose and starch, which are both made of a type of sugar called glucose. Similarly, the protein in the human body is created using smaller units called amino acids.

What is plastic?

'Plastic' comes from the Greek word 'plastikos', meaning 'able to be moulded'. In scientific terms, the word 'plastic' describes a material that can be moulded as a characteristic of that material; it does not refer to the substance itself.

The familiar words 'malleable' and 'ductile', are used to describe metal - these are just the plastic properties of metal. Liquids and gases are also 'plastic' as they follow the shape of any container they inhabit.

The two different types of substances that are called 'plastic' are polymers but not all polymers actually fit the definition of 'able to be moulded'.

Thermoplastic polymers

Thermoplastic polymers are truly plastic as they are easily moulded. They feature long thin parallel strands of molecules with weak bonds between the individual strands. Because the bonds are so weak, thermoplastic polymers slip easily over each other to bend and stretch. These polymers also have a low melting point and can be melted and remoulded, and hence recycled. This is the kind of plastic used in packaging, like Cling Wrap and drink bottles, some examples being polystyrene and PVC. See image 2.

Thermoset polymers

Thermoset polymers have strong bonds along and between the strands of molecules, making them harder and less flexible. The bonds between strands are crosslinks, which break and decompose when heated, leaving carbon. This is why thermoset polymers char (blacken) rather than melt. More rigid than thermoplastic polymers, thermoset substances are brittle, which means they will shatter easily. Although known as 'plastic' thermoset polymers are not plastic. Examples of thermoset polymers include Bakelite, which is used for saucepan handles, and laminates, such as those used for kitchen bench tops and cupboards. These polymers need to be made and moulded at the same time as they cannot be melted and remoulded. See image 3.

What is a fibre?

A fibre is a continuous polymer which, when spun into thread, makes fabric. The polymer strands line up close to one another, forming strong bonds between strands. If a fibre is pulled, it tightens the bonds, often making the fibre stronger and harder to break.

Natural fibres, such as cotton or wool, come from plants and animals. Plant-based fibres, including cotton, flax (linen) and hemp, provide natural polymers made of cellulose, which is basically sugar. Animal-based fibres, such as wool, silk and fur, come from protein polymers.

Synthetic fibres, such as nylon and polyester, are chemically constructed from petroleum-based substances by drawing softened plastic through small holes in a machine called a spinneret. These filaments are extremely fine and pliable threads, even when they harden. See image 4.

Fibres that do not fit either category include rayon, a synthetically made fibre derived from wood pulp, and asbestos, which is a mineral fibre not used in the manufacture of fabric. Heat produces fibres such as carbon fibre and fibreglass (glass and resin) from their raw materials.

Properties of fibres

Because of the diversity regarding various fibres, different fibres tend to have different properties depending on the length of their molecules and the type of molecules involved.

Monofilaments are fibres made from molecules that are the same length as the fibre. Characterised by strength and flexibility, monofilaments have no weak spots. Uses include fishing lines (nylon monofilament) and fibre-optic cables (Kevlar). See image 5.

Other types of filaments tend to have polymer strands of various length blended together, where the end of any strand in the filament represents a weak spot. This is characteristic of natural fibres, which are easier to break. The strands make the filament rougher, which in turn increases the surface area of the fibre so that it readily absorbs water and picks up dirt. This is particularly good for clothing that requires breathable fabric, which draws sweat away from the body and has air pockets between the woven fibres.

Conversely, the smooth surface of synthetic fibres makes them water repellent, stain resistant and durable. Woven tightly, synthetic fibres close off air spaces. They are used for raincoats and windbreakers but are not recommended for sweaty activities, as there is no absorption of moisture. Because synthetic fibres are thermoplastic polymers, they will melt if heated. Therefore, many items of clothing use fabric composed of a natural/synthetic blend (e.g. polyester cotton) in order to balance the benefits of natural and synthetic fibres.

Some fibres are elastic, that is, they stretch and return to their previous position. The number of crosslinks in a polymer determines the elasticity of a substance. When there are weak or too few crosslinks, the substance will stretch but not return to its former position. Rigid substances have too many crosslinks and cannot be stretched.

Most fibres have a degree of elasticity because of the molecules of which they are made. Some fibres, such as wool and some forms of nylon, have a coiled structure that will straighten the fibre when it is pulled, and return to a coil when released. Elastomers, such as spandex, are materials developed especially for their elastic properties.