Conceptual Understanding: Most plastics are produced from petrochemicals. Motivated by the finiteness of oil reserves and threat of global warming, bio-plastics are being developed. These plastics degrade upon exposure to sunlight, water or dampness, bacteria, enzymes, wind erosion and in some cases pest or insect attack, but in most cases this does not lead to full breakdown of the plastic. When selecting materials, designers must consider the moral, ethical and environmental implications of their decisions.
Raw materials for plastics
Most modern plastics are derived from natural materials such as crude oil, coal and natural gas with crude oil remaining the most important raw material for their production.
Polymers are substances which are made up from many molecules which are formed into long chains. The differences in the way the chains bond cause the different properties in the different types of polymers.
The terms monomer and polymer are very important in the plastics industry. A monomer is a relatively small molecule that can chemically bond to other monomers, forming a polymer. Remember, all plastics are polymers.
Natural plastics
are naturally occurring materials that can be said to be plastics because they can be shaped and moulded by heat. An example of this is amber, which is a form of fossilised pine tree resin and is often used in jewellery manufacture.
Semi synthetic plastics
are made from naturally occurring materials that have been modified or changed but mixing other materials with them. An example of this is cellulose acetate, which is a reaction of cellulose fibre and acetic acid and is used to make cinema film.
Synthetic plastics
are materials that are derived from breaking down, or ’cracking’ carbon based materials, usually crude oil, coal or gas, so that their molecular structure changes. This is generally done in petrochemical refineries under heat and pressure, and is the first of the manufacturing processes that is required to produce most of our present day, commonly occurring plastics.
Structure of thermoplastics
Thermoplastics are linear chain molecules, sometimes with side bonding of the molecules but with weak secondary bonds between the chains. Between the long chain molecules are secondary bonds which are weak forces of attraction between the molecules.
In a thermoplastic the long chains of molecules are held to each other with weak secondary bonds. When heated the heat energy overcomes these weak bonds and the material softens, allowing it to be reshaped. When cooled the bonds reform. This process can be repeated almost indefinitely and makes thermoplastics relatively easy to recycle.
To summarise: Thermoplastics can be heated and reformed. Their polymer chains do not form cross links. Thus, the chains can move freely each time the plastics are heated.
Material
Polypropylene (PP)
HDPE
HIPS
ABS
PET or
Polyethylene (PE):
PVC
Properties
Light, hard, tough, impact resistant, good chemical resistant, can be sterilised, good resistant to work fatigue
tough, resistant to chemicals, soft and flexible, good electrical insulator
Tough, high impact strength, rigid, good electrical insulator.
High impact strength, tough, scratch-resistant, lightweight, durable, good resistance to chemicals, good electrical insulator
Chemical resistant, high impact resistance, tough, high tensile strength, durable, excellent water and moisture barrier
Good chemical resistance, weather-resistant, lightweight, good electrical insulator, stiff, hard, tough, waterproof, durable
Applications
Used for medical and laboratory equipment, containers, chairs
Detergent Bottles/ Milk bottles
Electrical cases, packaging and trays.
Kitchenware, GO Pro camera cases, Toys (Lego)
Water Bottles
Pipes, Rainwater pipes and guttering,
Window frames and fascias,
Electrical cable insulation
Structure of thermosetting plastics
Thermosets are linear chain molecules but with strong primary bonds between adjacent polymer chains (or cross links). This gives thermosets a rigid 3D structure.
On first heating, the polymer softens and can be moulded into shape under pressure. However, the heat triggers a chemical reaction in which the molecules become permanently locked together. As a result the polymer becomes permanently ‘set’ and cannot be softened again by heating. Examples of thermosetting plastics are polyurethane, urea formaldehyde, melamine resin and epoxy resin.
To summarise: In a thermoset the long chains of molecules are joined to other chains with strong, primary cross-links which gives a rigid 3D structure. When heated these plastics do not soften as there is not sufficient heat energy to overcome covalent bonds. This makes them useful for applications where heat resistance is important i.e. saucepan handles, kitchen work tops and electrical fittings like plugs, sockets and light housings. However too much heat will cause combustion.
Material
Polyurethane
Urea-formaldehyde
Melamine resin
Epoxy resin
Properties
strong electrical insulator, (resistance), good tensile and compressive strength, good thermal resistance, can be fairly hard and tough, can be easily bonded, can be flexible and elastic
high tensile (tension) strength, high heat distortion temperatures, low water absorption, high surface hardness, weight/volume resistance
high electrical resistivity, very low thermal conductivity/ high heat resistance, hard/ solid
scratch resistant, stain resistant, available in a range of thicknesses and sizes
Tough, Chemical resistance (also water), Fatigue and mechanical strength (Tensile strength and compressive strength), Electrical insulation
Temperature resistant (maintains form and strength) (Though some are vulnerable to light), Can be used on metal (The adhesive)
Applications
Wheels, foam, varnish, paint and glue
Tableware
Worktop laminates
Buttons
Electrical casings
kitchen utensils plates, camping bowls (not microwave safe)kitchen utensils and plates, laminated benchtops
Construction of aircraft boats and cars, also are used in electrical circuits and general purpose adhesive and with glass reinforced plastics
Temperature and recycling thermoplastics and thermoset plastics:
-Thermoplastics soften when heated and harden and strengthen after cooling.
-Thermoplastics can be heated, shaped and cooled as often as necessary without causing a chemical change, while thermosetting plastics will burn when heated after the initial molding.
-Non-reversible effect of temperature on a thermoset contributes to it not being able to be recycled. Heating increases the number of permanent cross-links and so hardens the plastic, so therefore cannot be recycled
Thermoplastics:
Heat, Reshape, Cool
Thermosetting
Plastics: Landfill, incinerate
Biodegradable
Plastics: Bury in the ground, landfill
Recovery and disposal of plastics
Nearly all types of plastics can be recycled, however the extent to which they are recycled depends upon technical, economic and logistic factors. As a valuable and finite resource, the optimum recovery route for most plastic items at the ‘end-of-life’ is to be recycled, preferably back into a product that can then be recycled again and again and so on. The UK uses over 5 million tonnes of plastic each year of which an estimated 24% is currently being recovered or recycled.
Recycling: Turning waste into a new substance or product. Includes composting if it meets quality protocols.
Provides a sustainable source of raw materials to industry
Greatly reduces the environmental impact of plastic-rich products which give off harmful pollutants in manufacture and when incinerated
Minimises the amount of plastic being sent to the landfill sites
Avoids the consumption of the Earth’s oil stocks
Consumes less energy than producing new, virgin polymers
Encourages a sustainable lifestyle among children and young-adults
Bioplastics: To reduce the problems of disposing of plastics they can be designed to be biodegradable, known as bioplastics. These are plastics derived from renewable sources, such as vegetable fats and oils, corn starch, pea starch or microbiota.
Production of oil based plastics tends to require more fossil fuels and to produce more greenhouse gases than the production of biobased polymers (bioplastics).
Some, but not all, bioplastics are designed to biodegrade. Biodegradable bioplastics can break down in either anaerobic or aerobic environments, depending on how they are manufactured. Bioplastics can be composed of starches, cellulose, biopolymers, and a variety of other materials.