There are few materials as versatile as plastic. It is among the cheapest, lightest and most durable of synthetic materials. It is also easy to mould or modify and has many applications. Why is it so versatile? The answer lies in its composition.
The physical and chemical properties of plastic depend on the molecular structure of its component polymers, which are long chains of organic molecules formed by the repeated joining of single units called monomers. The most common monomers used in the manufacture of plastics are ethylene, propylene, vinyl chloride and styrene.
But what types of polymers make up these monomers?
According to their chemical composition
They are classified as organic and inorganic. Organics contain carbon atoms in the main chain, divided into vinyl (carbon only) and non-vinyl (carbon, oxygen and nitrogen). Inorganics have no carbon, and are based on other elements such as sulphur or silicon. This chemical difference determines the main families of polymers.
Another classification is in relation to what happens when they are heated: elastomers deform easily, thermosets do not flow but decompose, and thermoplastics become liquids when they melt and harden when they cool.
Finally, another common classification is according to their possible uses, based on their properties.
Elastomers are used where high extensibility and shape resilience are required; adhesives are used for bonding materials due to their high adhesion and cohesion; fibres are suitable for textiles due to their strength and low extensibility; plastics are used where permanent deformation is required; and coatings provide protection when applied to surfaces.
Physical properties of polymers
The physical properties of plastic polymers depend on their composition and are as follows:
- Density: plastics have densities between 0.85 and 2 g/cm3. The lightest are polyethylene and polystyrene foam.
- Thermal conductivity: most are poor conductors of heat and this makes them good thermal insulators.
- Solubility: some are soluble in organic solvents depending on their functional groups.
- Optical properties: they can be transparent or opaque. Polymethacrylate is highly transparent (93%).
- Softening temperature: temperature at which they begin to soften. This is important for high temperature applications.
- Chemical resistance: some polymers are resistant to acids, bases, solvents or other chemicals.
On the other hand, the mechanical properties of plastics are determined by their polymer. These include:
- Hardness: they can range from soft and flexible to hard and rigid. For example, PVC is harder than polyethylene.
- Impact resistance: the toughest ones can withstand shocks without breaking easily. ABS is one of the toughest.
- Elasticity: some, such as rubber, are very elastic.
- Tensile strength: this is the force needed to stretch them before they break. Nylon is one of the best at this property.
- Fatigue strength: ability to withstand repeated loads without failure. This is important in moving parts.
- Wear resistance: resistance to abrasion. Ultra high molecular weight polyethylene has high wear resistance.
There are many physical and mechanical properties. That is why there are so many applications for different types of plastics.
Practical applications of polymers
Thanks to the versatility of their polymers, the applications of plastics are very varied:
- Containers and packaging of all kinds: bottles, films, bags, trays, etc. PET and polyethylene are widely used for packaging.
- Car components or parts: bumpers, dashboards, fuel tanks, etc. ABS, PVC and polyurethanes, among others, are used.
- Textile fibres such as nylon or polyester are used for clothing, carpets, ropes, etc.
- To make pipes and piping: mainly PVC, used for water and gas piping.
- Electrical insulation for cables: PVC sheaths, cross-linked polyethylene or polyethylene foam.
- Household appliances and electronic devices: ABS, polystyrene or polypropylene casings.
- Household and kitchen utensils: plates, cutlery, frying pans, pots and pans. Melamine, ABS, polypropylene, polystyrene are used.
- Furniture made of PVC, acrylic or polyethylene.
- Toys and sporting goods: scooters, helmets, rackets, balls. Polyethylene, PVC, ABS, nylon are used.
- Medical prosthetics: biocompatible polymers such as polyether-ether-ketone are used for prosthetics.
- Aeronautical components: carbon fibre reinforced composite parts.
Plastic polymers are present in all areas of our daily lives. Therefore, their study and development is essential to find new applications and also to improve their sustainability.