Polymers and Composites

Introduction to Polymers

Polymers are large molecules made up of many repeating units called monomers. These monomers join together through a process called polymerisation, forming long chains or networks.

There are two main types of polymerisation:

• Addition polymerisation: Monomers with double bonds open up and link together without losing any atoms. For example, ethene monomers join to form polythene.
• Condensation polymerisation: Monomers join together with the loss of small molecules like water.

(For detailed chemistry of addition and condensation polymers, see the Organic Chemistry topics.)

Properties of Polymers

Polymers have a wide range of physical properties depending on their structure and type. Two important groups are:

• Thermoplastics: These polymers soften when heated and can be reshaped multiple times. They are usually flexible and can be recycled easily.
• Thermosetting plastics: These polymers harden permanently when heated and do not soften on reheating. They are usually strong, rigid, and heat resistant.

The properties of polymers determine their uses. For example, flexible thermoplastics like polythene are used for plastic bags, while strong thermosetting plastics like bakelite are used in electrical insulation.

Polymers can be:

• Flexible – useful for packaging films and clothing fibres.
• Strong and rigid – useful for pipes, containers, and car parts.
• Lightweight – useful in applications where metal would be too heavy.

For instance, polythene is a thermoplastic that is flexible and used for plastic bags because it can be stretched without breaking.

Composites

A composite is a material made from two or more different materials combined to produce a material with improved properties.

Components of composites:

• Matrix: The continuous phase that holds the composite together (e.g., plastic, metal, or ceramic).
• Reinforcement: The material embedded in the matrix that provides strength and stiffness (e.g., fibres, particles).

Examples of composites include:

• Fibreglass: Glass fibres embedded in a polymer matrix. Used in boat hulls and sports equipment.
• Concrete: Cement (matrix) with sand and gravel (reinforcement). Used in construction.
• Wood: Natural composite of cellulose fibres in a lignin matrix.

Composites have advantages over pure materials:

• Stronger and tougher than individual components.
• Lighter than metals but can have similar strength.
• Can be designed to have specific properties for particular uses.

Environmental Impact

Plastic waste causes serious environmental problems because many polymers are non-biodegradable and persist in the environment for hundreds of years.

• Problems with plastic waste: Pollution of oceans and landscapes, harm to wildlife, and landfill space issues.
• Biodegradable polymers are designed to break down naturally by microorganisms, reducing long-term pollution.
• Recycling polymers helps reduce waste and saves resources by reprocessing plastics into new products.

Recycling methods include:

• Mechanical recycling: Melting and remoulding plastics.
• Chemical recycling: Breaking polymers back into monomers for reuse.

For example, PET bottles can be recycled into fibres for clothing or new containers.