L7. Polymer Classification / Stock

< Go back to 1.2 Performance Characteristics

Learning Outcomes

By the end of this unit you should have developed a knowledge and understanding of:

the specific names and classifications of polymers
the processing from source into a usable material
the performance characteristics of different types of polymers
why different types of polymer are suitable for different applications
stock forms of polymers

Thermoplastic and Thermosetting polymers

Polymers can be in three main categories.

Thermoplastic

Polymers that can be reheated/formed

PET, PS, PP, PMMA

Thermosetting

Polymers that cannot be reheated/recycled

Formaldehyde, Resin

elastomers

Soft, flexible polymers with elastic properties

Rubber, elastic, TPE

Sources of Polymers

Polymers are made up of long chain molecules containing carbon, hydrogen and oxygen atoms, along with other chemicals such as chlorine and fluorine.

Polymers occur naturally in nature and can be derived from plant and animal sources.

They include:

Casein (derived from milk) which is still used to make buttons,
Horn,
Resin (from trees) and
Cellulose from plants - In the case of Cellulose it is mixed with acetic acid to make Cellulose acetate which is still used today for OHP slides.

These could be classed as natural polymers as they possess a similar molecular structure to those polymers commercially used today.

However, the main source of synthetic polymers is fossil fuels such as oil, gas and coal.

Synthetic polymers are gained through a process of thermal cracking. As they are derived from fossil fuels (particularly oil) they are carbon based and account for the majority of plastics used today.

In order for it to be useful to us, crude oil is broken down in oil refineries into its component parts, known as fractions, and these can then be used for many different purposes.

Fractions that are produced by the distillation of crude oil can go through a process called cracking. This chemical reaction produces smaller hydrocarbons, including alkanes and alkenes.

Ethene and other alkenes are unsaturated hydrocarbons and can be used to make polymers. Ethene can be used to make ethanol.

Synthetic polymers can be grouped into three types:

Thermoplastics.

A plastic that can be repeatedly reheated and remoulded.

Thermosetting Polymers (Thermosets).

A chemical reaction takes place at the point where they are formed which prevents them from being reheated or reshaped.

Elastomers

These are polymers with good elasticity i.e. they can be distorted under pressure but will return to their original shape.

Polymers are made up of single monomers which form long chain molecules called polymers. How these chains behave when heated define the type of polymer.

Thermoplastic polymers

Polymers have some unique properties which make them particularly useful for modern day applications.

They are good electrical and thermal insulators
They have a good strength to weight ratio. This does not mean they are strong in the same way as steel but that they have good strength compared to their weight.
They generally have good atmospheric and chemical resistance.
They have fairly low melting temperatures (Thermoplastics only)
They are lightweight
They can be self coloured, opaque or transparent and are self finishing.
They are quite hygienic and are water resistant.
They can be enhanced with the addition of fillers, plasticisers and stabilisers to make them more flame retardant or anti static etc.

Recycling

The chart on the right shows the commonly recycled polymers (plastic materials). The order generally signifies how commonly used the material is. For instance PET is used for drinks bottles so many units are sold and disposed of each day.

In the UK only the top three are easily recycled.

Thermoplastic Polymers

PET (1) Polyethylene Terephthalate:

Water bottles

PE (2 and 4) Polyethylene (Polythene):

Comes in two forms: High Density

(HDPE) for buckets and Low Density (LDPE) for carrier bags.

PVC (3) Poly Vinyl Chloride:

Windows and hosepipes.

PP (5) Polypropylene:

Plastic chairs.

PS (6) Polystyrene:

This can come in three forms: Rigid (food packaging), High

Impact (HIPS) (fridge door) and expanded (foam) for packaging.

Polyamide (7+):

Better known as Nylon and used for washers bearings, hair combs.

PMMA (7+) Polymethyl Methacrylate:

Better known as Acrylic and used for car headlights.

PC Polycarbon (7+):

used for safety glasses

ABS Acrylonitrile Butadiene Styrene (7+):

the only thermoplastic that does not begin with ‘poly’. It is a relatively modern plastic used for electrical casings on computers and phones etc.

Thermosetting polymers

Thermosetting plastics are polymers that cannot be reheated and reshaped. When the plastic is formed cross linking takes place across the long chain molecules which cannot be broken down by heat. While this means these types of plastic are good for situations where they will be subjected to heat and electricity, these types of plastics cannot be recycled.