A thermoplastic is a class of polymer that can be softened through heating and then processed using methods such as extrusion, injection moulding, thermoforming and blow moulding.
Thermoplastics harden once cooled and do not show any changes in chemical property after being heated and cooled multiple times, making them easily recyclable.
Amorphous and Semi-crystalline Thermoplastics
Thermoplastics are made by joining small molecules, called monomers, together to form long chains using a process called polymerisation. A single polymer chain can be made from many thousands of monomers. The atoms in a polymer chain are joined by strong covalent bonds, whereas the forces between chains are weak.
Depending on the type of monomer, polymer chains may have side branches. If a polymer chain has only a few, short side branches then the chains can form ordered, crystalline regions, called spherulites. However, if the chain has many large side branches, then it is not possible for ordered regions to be formed and the polymer is amorphous. Examples of amorphous polymers are polystyrene (PS), polyvinyl chloride (PVC) and acrylonitrile-butadiene-styrene (ABS). Even for polymers with crystalline regions, there are always some amorphous regions between the crystallites, so these polymers are called semi-crystalline. Examples of semi-crystalline polymers are polyethylene (PE), polyamide (PA) and polypropylene (PP). For semi-crystalline polymers, as the temperature increases, the bonds between the polymer chains weaken to create a pliable solid and then a viscous liquid, which allows the plastic material to be shaped to produce parts.
Amorphous plastics are used for applications where optical clarity is required since light is scattered by crystallites. These amorphous plastics are, however, less resistant to chemical attack and environmental stress cracking due to the lack of crystalline structure.
Before a thermoplastic polymer can be used it is normally mixed with additives, such as stabilisers, plasticisers, lubricants, flame retardants and colourants, to improve the polymer’s functionality, stability or appearance. For example, stabilisers are added to reduce degradation due to sunlight or heat and plasticisers can be added to increase the mobility of amorphous chain segments, lowering the glass transition temperature and decreasing brittleness.
Advantages
The advantages of thermoplastics include:
- Readily recyclable
- Wide range of mechanical properties
- Light weight compared to metals
- Aesthetically-superior surface finish compared to thermosets
- Good chemical resistance
- Energy efficient processing
Disadvantages / Limitations
Despite the many advantages, there are also some limitations associated with thermoplastics. Due to their low melting point compared to metals, thermoplastics are inappropriate for use on some high temperature applications. In addition, some thermoplastics are susceptible to creep when exposed to long-term stress loads.
Examples and applications
Thermoplastics come in a range of types with their own unique applications. Examples of thermoplastic polymers include:
1. Polyethylene
Polyethylene is the most commonly used plastic in the world. It is in fact a family of materials that come with a range of densities and molecular structures, each with their own applications. Trade names include Alathon, Borstar, Dowlex, Eltex, Finathene, Hostalen, Lacqtene, Lupolen, Rigidex and Vestolen A. Strong and resistant to most chemicals, ultra high molecular weight polyethylene (UHMWPE) is used to manufacture moving machine parts, bearings, gears, artificial joints and bulletproof vests. High density polyethylene (HDPE) is used to make items such as chemical tanks, gas and water pipes, toys, shampoo bottles and margarine tubs. Medium density polyethylene (MDPE) is used for gas and water pipes, packaging film and pond liners. Linear low density polyethylene (LLDPE) is used for plastic bags, shrink/stretch films and food packaging. Being the softest and most flexible of these materials, low density polyethylene (LDPE) is used for the manufacture of squeeze bottles, sacks and sheets.
2. Polypropylene
Polypropylene is the second most widely used commodity polymer in the world. It is used across a wide range of industries to create items including reusable food containers, sanitary products, heat resistant medical equipment, ropes, carpets, car batteries, cable insulation, storage bins, and even banknotes! Trade names include Adstif, Clyrell, Hifax, Hostalen, Inspire, Isoplen, Moplen, Novolen, and Vestolen.