Plastics

The use of plastics is much more recent than that of wood or metals. The first plastic to be used was simply a natural rubber produced by certain plants. In 1839, Charles Goodyear invented the vulcanization process, which made it possible to manufacture artificial rubber from sulfur. Since that time, many plastics have been synthesized in laboratories by polymerization reactions.

Plastics are not found in nature, except in the case of natural rubbers. They must be synthesized in the laboratory. This is done by using substances derived from the refining of fossil fuels (petroleum and natural gas). These substances, called monomers, are assembled into a long chain of molecules, called a polymer, during the polymerization process.

Thermoplastics have the property of softening in heat and hardening in cold. It is therefore possible to reshape them repeatedly without affecting their initial mechanical properties. This characteristic makes it easier to recycle this type of plastic than the other two types. Thermoplastics are by far the most widely used plastics, accounting for more than three-quarters of all plastics produced in the world.

Types of thermoplastics	Mechanical properties	Examples of use
Acrylonitrile-butadiene-styrene (ABS) Black and opaque, easy to recycle	Rigidity, lightness, resilience	Piping, Lego bricks Source
Polyvinyl chloride (PVC) Rigid or flexible, recyclable	Rigidity, hardness, non-flotation, impermeability, acid resistance Source	Line pipe, covers Source
Polycarbonate (PC) Transparent, recyclable	High transparency, impact resistance Source	Automotive headlight bulb, motorcycle helmet, CD and DVD Source
Polypropylene (PP) Shiny appearance, recyclable in a rigid version	Light weight, rigid, or flexible depending on the shape, transparency, flotation capability Source	Dashboard, food packaging Source
Polystyrene (PS) In hard and transparent form or as compact white foam, easy to recycle	Light weight, thermal insulation Source	CD case, plastic cover, food packaging, insulation Source
Polyamide (PA) Also called nylon	Resistance, impermeability, lightness, flexibility	Parachute canopy, clothing Source
Polymethyl methacrylate (PMMA) Also called plexiglass or acrylic	Exceptional optical properties, resistance to corrosion and UV rays, lightness	Manufacture of windows, portholes, aquarium walls Source
Polyethylene (PE) Most used plastic, easy to handle and cost-efficient	Malleability, low rigidity, resistance Source Source	Grocery bags, trash bags, bottles, containers Source
Linear polyesters (PET)	Hardness, resilience Source	Synthetic fiber for clothing, bottles, containers Source

Unlike thermoplastics, which soften under the effect of heat, thermosets have lost this property during their manufacture. Thus, a thermosetting material retains the same rigidity under the action of heat until it reaches its decomposition temperature. Also, once a thermoset has been produced, its shape cannot be changed. Thermosets are difficult to recycle. In Québec, these plastics are not reprocessed. These limitations explain, in part, the fact that they are used less than thermoplastics.

Types of thermosets	Mechanical properties	Examples of use
Aminoplasts (MF and UF)	Resilience, heat, and corrosion resistance	Plastic tableware, flooring, decorative panel
Phenol formaldehyde resin (PF)	Hardness, thermal and electrical insulation	Housing for various objects, pan handle, electrical and aeronautical insulation Source
Unsaturated Polyester (UP)	Hardness, resilience, mechanical resistance, electrical insulation	Boat hull, fishing rod, above-ground pool Source

A distinction is generally made between natural and synthetic elastomers. Natural elastomers come from the latex secreted by certain plants, for instance the rubber tree. They are, however, much less used than synthetic elastomers which are, in turn, produced in the laboratory using the vulcanization process. This process consists of adding sulfur to the rubber, thus making it possible to reduce its elasticity and to improve its resistance. Despite their advantageous mechanical properties, elastomers also tend to be difficult plastics to recycle.

Elastomer type	Mechanical properties	Examples of use
Polychloroprene (CR) (Neoprene)	Elasticity, mechanical resistance, resilience, thermal insulation	Adhesive, wetsuit, neoprene clothing Source
Silicone elastomer	Elasticity, mechanical resistance, resilience, electrical and thermal insulation	Thermal and electrical insulation used in construction
Styrene-butadiene-rubber	Resilience, mechanical resistance, electrical and thermal insulation	Synthetic rubber, tire, elastic Source

As can be seen in the preceding tables, the properties of plastics depend on the different types of plastics considered. However, plastics have several interesting general properties. This explains their great use in the manufacture of technical objects.

• They are light.
• They are resistant to corrosion (rust).
• They can be shaped and moulded by heat or pressure.
• They have excellent durability.
• They are good thermal and electrical insulators.
• They have great resistance.
• They are cost-efficient.

On the other hand, plastics have a major drawback: they are derived from a non-renewable fossil resource, namely petroleum. It is therefore important to recycle plastics in order to ensure the sustainability of the resource.

Plastics are subject to gradual degradation as they age. Their degradation is visible through the appearance of cracks or a change in colour. This process is slow, but it is often irreversible. Various causes can explain the degradation of a plastic. For example, liquid phase substances, such as water, can penetrate some plastics and dissolve some of their chemical additives. Also, the polymers in plastics can deteriorate under the influence of ultraviolet radiation, especially that emitted by the Sun. Finally, some plastics can oxidize upon contact with certain gases.

To slow down, or even prevent, the degradation of plastics, certain means of protection exist. Plastics can be covered with a waterproof coating to prevent liquid from penetrating. Antioxidant substances, such as carbon black, can also be added at the time of manufacture to prevent degradation when exposed to certain gases. Colour pigments can also be added to plastics to absorb ultraviolet rays, thus protecting plastics.