Plastics are all around us today and help to make our personal lives cleaner, easier, safer, more convenient and more enjoyable.
Definition: Historically the word plastic was given to all materials that could be moulded, squeezed, cast or squirted into some desired shape. The word plastic is really an adjective meaning “capable of being shaped by pressure”. The use of plastics is increasing all the time as they replace materials such as metal, wood, paper, ceramics and glass in a wide variety of uses. There are also new roles which only plastics can fulfil.
Cars are a good example of a product in which plastics are now extensively used. Over the last 20 years, the use of plastics in cars has increased by 114% and it is estimated that without plastics today’s cars would be at least 200kg heavier. Weight savings achieved through the use of plastics have enabled parts, such as the chassis and drive shaft, to be made lighter. It is estimated that over the average lifespan of a car of 150,000km, this light weighting has contributed to a reduction in fuel consumption of around 750 litres.
But what are plastics? Why are they so useful and so widespread? Why do they behave as they do? What is their chemical structure? Many materials we use every day are made of polymers. These are large, long molecules constructured of smaller, shorter molecules called monomers. Polymers can be synthetic or natural.
Natural polymers are common in animals and plants. Much living tissue is based on polymers – for example proteins in animals and carbohydrates in plants. A lot of our food is based on polymers – for example, fibre, grain and meeat. Plants and animals also produce non-living materials based on polymers. These are usually produced as fibres and then have to be processed to produce materials such as threads and fabrics. These include:
While natural polymers are still part of the plastics world nowadays the word plastic is generally reserved for synthetic plastic materials. Synthetic polymers are made mainly from petroleum (crude oil) or natural gas. This is processed in an oil refinery to produce basic chemicals known as monomers. The monomers are then turned into polymers. Some polymers are turned into solid plastics material, and others into textile fibres. Some can be turned into either, depending on how they are processed. In some cases synthetic plastics are mixed with, or used in combination with, natural materials. Most of this educational resource is based around the synthetic plastics that originate from the petrochemical industry.
The raw material for plastics is crude oil, a complex mixture of thousands of compounds. To become useful, it must be processed. Around 4% of the world’s production is turned into plastics.
Because the compounds in crude oil, have different masses, and therefore boil at different temperatures, it is possible to separate them by a process known as fractional distillation. The mixture is separated into fractions, not into individual compounds. Fractions contain a mixture of compounds whose boiling temperatures are similar.
Cracking breaks large molecules into smaller ones which are more useful - and therefore of greater value. For example, very high boiling point fractions are cracked to produce gasoline and gas oil fractions. Today most cracking uses catalysts, but some heat treatment still occurs.
Monomers and Polymers
A feature central to all plastics is that they are constructed from polymers or macromolecules. This applies to both natural and synthetic plastics. A polymer is a long chain molecule consisting of many (usually thousands) of small units called monomers joined end to end. The relationship between polymers and monomers is that of a chain being made up of many individual links.
Reforming changes the internal structure of molecules to produce different compounds with a greater usefulness – and therefore higher value. By altering conditions – such as temperature, pressures and the catalyst – cracking and reforming techniques can now be controlled to produce exactly the blend of compounds which will be most useful at a particular time.
At the heart of all plastics manufacture is synthesis (or joining together). Monomers are joined or synthesised together to form polymers, this process is called polymerisation.
Curriculum Note: Science students can examine the details of how the molecules are joined together. The focus in Technology is learning about how we might use plastics to satisfy a need or opportunity. Technology students need to be aware that a range of very different synthetic plastics are formed by joining different monomers together.
Eight of the most really important plastic polymers are produced from 3 basic chemicals which come from naptha:
All plastics can be divided into two groups, Thermosets and Thermoplastics. The difference relates to the number of times that the material can be heated and processed.
Thermosets can be moulded and set once only. They cannot be remoulded by reheating.
Thermoplastics are moulded usually under heat and pressure and then set by cooling. They can be reheated and remoulded into different shapes. In theory this can happen indefinitely - although the properties and quality of the plastic may change after a number of reprocessing cycles.
There are advantages and disadvantages to each category. Because they don’t soften readily with heat - Thermoset plastics are good for electrical fittings that get hot. Thermoplastics are suitable for a wider number of processing techniques because of the ease with which they can be melted.
Those which soften on heating and then harden again on cooling.
These polymer molecules consist of long chains which have only weak bonds between the chains.
The bonds between the chains are so weak that they can be broken when the plastic is heated.
The chains can then move to form a different shape.
The weak bonds reform when it is cooled and the thermoplastic material keeps its new shape.
Those which never soften once they have been moulded.
These polymer molecules consist of long chains which have many strong chemical bonds between the chains.
The bonds between the chains are so strong that they cannot be broken when the plastic is heated.
This means that the thermosetting material always keeps its shape.
The bonding process. When thermoplastic polymers are heated they become flexible. There are no cross-links and the molecules can slide over each other. Thermoset polymers do not soften when heated because molecules are cross linked together and remain rigid. It is clear from this that the chemical bonding in a polymer and the shape of the polymer will affect its properties.
There are two ways of producing polymer chains:
Some thermoplastic polymers are condensation polymers. Examples are Nylon and polyethylene teraphthlate (PET). Nylon belongs to a class of polymers called polamides. Nylons are produced by condensation polymerisation.
The family of materials which form plastics has a wide variety of different properties. Some resist high pressure and extremes of temperature, some resist air and moisture. There are different forms of the same plastics type which can be stiff or flexible and so suitable for particular applications. Plastics’ properties can also be tailored by the use of additives.
Polymers are converted into plastics products in seven main ways. Here is a brief description of these methods and the products they are used to make.
Today, plastics provide an environmentally sound and cost-effective solution for many design challenges. Industries, especially the hi-tech ones such as aerospace, medicine, computing and communication, rely on new plastics materials for progress in engineering and design. Plastics are often superior to any other materials in these fields. Many new developments would not in fact be possible without them. Think about the clothes we wear, the houses we live in and how we travel. What about the toys we play with, the televisions we watch, the computers we use and CDs we listen to? Whether we’re shoppping in a supermarket, having major surgery or merely brushing our teeth, plastics are an integral part of everything we do.
Why are plastics so widely used in all our lives? There are a number of basic properties that plastics have and that students should be aware of:
Plastics are a very diverse group of materials. Some are extremely hard - others are very soft They come as:
Almost all plastic items have additives in addition to the polymer resin to modify the properties of the plastic to suit a particular end purpose. The additives used include: