Plastics are materials which are softened by heat and set into lasting form when shaped in a mould. Some are natural; some are semi-synthetic, the result of chemical action on a natural substance; some are synthetic, built up from the constituents of oil or coal. All are based on the chemistry of Carbon, with its capacity for forming chains. The molecules that compose them (monomers) link together in the setting or curing process to form chains (polymers), which give plastics their flexible strength. Some plastics retain their ability to be softened and reshaped: like wax, they are thermoplastic. Others set permanently in the shapes they are given by heat and pressure: like eggs, they are thermosetting.

Early Plastics

Whilst the first fully synthetic plastics did not appear until the 1900's, natural plastics which can be softened by heat and pressed into shape have existed for centuries. These include horn, bone and tortoiseshell and have a similar molecular structure to the later synthetic plastics.
Horn is a natural plastic material which came into very early use. It behaves like a typical thermoplastic sheet material when it is heated and split open, softened again in boiling water and then flattened and shaped in a hot press. For centuries the chief use of horn was for making combs. From 1770 to 1880 when towering hairstyles became the fashion, horn was the most popular material for making some of the most magnificent combs ever created.

Semi-synthetics/Synthetics

In 1862 the first semi-synthetic or manufactured plastic, initially called Parkesine and later Celluloid, was formed from natural materials developed from cellulose. Again many of the products created from this material were decorative fashion accessories but also more mundane items such as shoehorns and door handles.
The first truly synthetic plastic was developed by a Belgian chemist called Leo Baekland in 1907. This was marketed under the name Bakelite and it proved superior to natural materials in many electrical applications. Its advantages in mass production were exploited by various electrical goods manufacturers to produce radio cabinets, hair dryers etc.

Synthetics

The explosion in the development of plastics took place between the two world wars and it is in this period that many of the plastics which are widespread today (polystyrene, acrylic, PVC, nylon) and the techniques used to shape them (injection moulding patented in 1926) evolved.
By the end of the second world war there was such a surplus of raw plastics that this once "luxury material" came into widespread use in domestic applications (Tupperware in the 1950's, melamine wipe-clean surfaces), clothing and fabrics (drip-dry, crease-resistant Terylene and nylon tights).
The plastics industry has expanded more than any other materials industry in the last fifty years and the impact of synthetic materials is enormous with plastics replacing other materials in applications where weight corrosion resistance, colour and ease of forming are important design criteria.

Thermoplastics

Thermoplastics can be shaped at high temperatures, cooled, reheated and reshaped many times without permanently changing their structure and properties. They consist of chain molecules (polymers) with weak bonding between adjacent chains. It is these bonds between the molecule chains which weaken to allow the plastic to deform when heated. They also give thermoplastics a certain flexibility at room temperature.

Thermosetting Plastics

Thermosetting Plastics or thermosets have strong cross-linking bonds between polymer chains. This cross-linking can be the result of both a chemical reaction and heat, and the net result is a plastic which actually hardens as it is heated and therefore offers good resistance to heat. For this reason it is often used in applications such as pan handles and electrical fittings.
Because of the strong, cross-linking bonds, thermosets tend to behave like a brittle metal or ceramic, showing high strength, low ductility and poor impact resistance compared with other polymers.

The following handout lists a number of the most common Thermosetting Plastics and Thermoplastics, their forms, properties and common uses. It is important that you are able to discuss, with confidence, a number of these plastics as they are extremely common-place in the area of Household Appliances which will be a topic for the terminal exam.



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