A tour through the world of plastic

Classification and properties of different plastics

The terms “plastic” or “synthetic material” commonly refer to the material out of which a wide variety of everyday objects are made—packaging films, plastic bags, toys, mobile phone covers, Tupperware containers. Plastics are also hidden in quite unexpected places in everyday life—in car paint or kitchen sponges, for instance.

But what do the terms PVC and polyurethane mean? What is the difference between thermosets and elastomers? Read on for more information and an insight into the diverse world of polymers—the umbrella term for all plastics.

All plastics consist of very long and intertwined molecular chains, known as polymers. These polymers are composed of individual building blocks, or monomers. Plastics are so versatile because their properties can be determined almost at will by choosing the starting material, the manufacturing process and the additives. Since the triumphant rise of plastics in the middle of the last century, almost everything is made out of plastic—clothing, park benches or prefabricated houses. Plastics have become an indispensable part of our everyday lives.

A large family

The plastics family can be divided into three large groups: thermoplastics, thermosets and elastomers. Whereas thermoplastics can be melted down through heating and recover their original material properties after cooling, the chemical composition of thermosets changes when they are heated. As a result, these plastics cannot be melted down and reprocessed. The third category is elastomers. Each of these groups has specific properties that make them suitable for everyday or industrial use.

Plastics System

Thermoplastics

Whether at the reverse vending machine, in the stationery shop or in the electronics retail sector—wherever we go we come across synthetic materials that belong to the group of thermoplastics. Anyone who has ever filled a PET bottle with hot water or left a plastic container standing too close to a grill knows that thermoplastics can be shaped by being heated and that they melt when major heat is applied. They retain their new shape after being cooled down but can in theory be reshaped by reheating as often as desired. Most of the plastics produced and used today fall into this group.

Thermoplastics include materials such as polyethylene (PE), which is the most commonly consumed plastic worldwide, low-density PE (LDPE), which is mainly used for films and other thin-layer products, or the somewhat more stable materials of the HDPE group, which are used, for example, for bottles containing household detergents and for larger containers such as waste bins.

Polypropylene (PP) is also often used for packaging. PP’s most important property is its high resistance to fatigue cracking, so that, for example, eyeglass cases with a PP composite between the two shell parts can be bent again and again without breaking.

Polyamide (PA) is used primarily for textiles, but also for the strings of tennis rackets. The material has a very smooth surface and high tensile strength.

Polyethylene terephthalate (PET) well-known due to PET bottles, but it is also used as a textile fibre under the name “polyester” in many common objects such as films or textile fibres.

Other well-known thermoplastics are: polycarbonate (PC), which is used in eyeglass lenses, solar modules and safety helmets; polystyrene (PS), which is commonly used as a foamed insulating material; or polyvinyl chloride (PVC), which is used in pipes and floor surfaces. The entire group of thermoplastics therefore encompasses a multitude of properties and fields of application. If two or more thermoplastics are blended to obtain a material with new properties, the result is called a polyblend.

Thermosets

Thermosets are chemically hardened once during their production process and then retain their shape even when heated. Too much heat, however, degrades the material and makes it brittle. Thermosets are usually used where they can really show their stability even under wildly fluctuating temperatures and so they can be found, for example, in electrical installations or outdoors.

In everyday life they can be found in pedal boat linings or in toys at children’s playgrounds for instance. Well-known varieties of thermosets include polyester resins, and even the first industrially manufactured plastic Bakelite, which was used for the production of vintage telephones, or polyurethane resins in varnishes.

Elastomers

Elastomers can withstand heating without being damaged and they can change their shape under pressure or by stretching before reverting to their original shape—an example of elastomers are rubber bands used in households and offices. The starting material is rubber, a natural product which can be made permanently elastic through, for example, vulcanisation with sulphur. Tyres for cars or bicycles are also made out of elastomers.

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