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 whenexposed to long-term stress loads.
Examplesand 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.
3.Polyvinyl chloride
Being tough, lightweight, and resistant to acids and bases, polyvinyl chloride (PVC), also known as vinyl, is used by the construction industry for items including water pipes, drainpipes, gutters and roofing sheets. Trade names include Astraglas, Benvic, Vestolit and Vinnolit.
PVC can also be made more flexible with the addition of plasticisers, where it is used for hoses, tubes, electrical insulation, clothing, upholstery and inflatable products such as waterbeds and pool toys. Trade names include, Acvitron and Lifolit.
4.Poly(ethylene terephthalate)
Poly(ethylene terephthalate) (PET) or polyester has a good combination of mechanical and thermal properties, chemical resistance and dimensional stability. It is used for liquid containers, especially carbonated soft drinks, food containers and, in fibre form, for clothing. It is the most recycled polymer worldwide. Trade names include Dacron, Eastapak, Rynite and Terylene.
5.Polyamide
Polyamide (PA) is also known by the trade names Nylon, Akromid, Akulon, Grilamid, Grilon, Rislan and Ultramid. It was originally used as a replacement for silk when making items such as flak vests, parachutes and stockings. Nylon fibres are also used for fabric, carpets, rope and strings for musical instruments. It is also used for machine screws, gear wheels and power tool casings.
6.Polystyrene
Polystyrene (PS), also known by the trade names Styron and Vampstyr, is manufactured in different forms that are suitable for different applications. It is used to make items such as disposable cutlery, cases for CDs and DVDs, and smoke detector housings. Expanded polystyrene (EPS) foam, also called by the trade name Styropor, is used for insulation and packaging materials and extruded polystyrene foam (XPS), also called by the trade name Styrofoam, is used for architectural models and drinking cups. Elsewhere, polystyrene copolymers are used for the manufacture of toys and product casings.
7.Acrylonitrile-butadiene-styrene
ABS, also known by trade names Cycolac and Ensidur, is a lightweight polymer that shows high impact resistance and mechanical toughness compared to most thermoplastics and is widely used in everyday consumer products like toys and telephones.
8.Polycarbonate
Polycarbonate (PC) is also known by trade names including arcoPlus, Lexan, Makroclear and Makrolon. Easy to mould and thermoform, it is used in a range of applications in the medical, construction, electronics, automotive and aerospace industries, including safety glasses, bullet-resistant glass, CDs and DVDs, car headlamp lenses and safety helmets.
9.Poly(methyl methacrylate)
Poly(methyl methacrylate) (PMMA) or acrylic, is also known by the trade names Acrylite, Altuglas, Lucite, Oroglas, Perspex and Plexiglas. It is widely used as a substitute for glass in aquariums, aircraft windows, motorcycle helmet visors and for the lenses on exterior automobile lights. Acrylic is also used for signage, for eye lenses and in bone cement for medical use, and also in paint, where PMMA particles are suspended in water.
10.Polyoxymethylene
Demonstrating a high stiffness, good dimensional stability and low friction, polyoxymethylene (POM), also known as acetal, polyacetal and polyformaldehyde, is used for parts that require high precision, such as bearings, valve parts, gears and electrical components, and is also known by the trade names Celcon, Delrin, Duracon, Hostaform, Kepital, and Ramtal.
11.Poly(lactic acid)
Derived from renewable resources like sugar beet pulp, corn starch, chips, sugarcane and tapioca roots, poly(lactic acid) (PLA) is a compostable thermoplastic. It is used in tableware, food packaging and additive manufacturing (3D printing). Trade names include Bio-Flex, Fozeas and Ingeo.
12.Poly(phenylene oxide)
Poly(phenylene oxide (PPO) offers a range of attractive properties, including high impact strength, heat distortion, and chemical stability against mineral and organic acids. It also offers low water absorption, but can be difficult to use due to the high processing temperature. Commercial resins, such as Noryl, blend PPO with high impact polystyrene (HIPS) to lower the processing temperature, making it easier to process. Applications include electrical components and washing machine parts.
13.Polytetrafluoroethylene
Polytetrafluoroethylene (PTFE) belongs to a class of thermoplastics known as fluoropolymers, and is also known by the trade names Teflon, Dyneon, Fluon and Hostaflon. It has one of the lowest friction coefficients of any known solid and is well-known for its use on non-stick cookware. It is also used as a lubricant to reduce frictional wear between sliding parts like gears, bearings and bushings. Because it is chemically inert, it is also used for pipes and containers that come into contact with reactive chemicals.
14.Poly(vinylidene fluoride)
Poly(vinylidene fluoride) (PVDF) is another member of the fluoropolymer family. It is also known by the trade names Kynar, Hylar and Solef, and is known for its chemical inertness and resistance, used for engineering sheets and pipes as well as to make powders and coatings. PVDF is also widely used in the chemical industry for piping to transport aggressive chemicals and high purity liquids.
15.Polyetheretherketone
Polyetheretherketone (PEEK) is a high-performance thermoplastic used for a range of engineering applications, including bearings, pumps, valves and medical implants, due to its good abrasion resistance and low flammability as well as low emission of smoke or toxic gases. Trade names include Victrex and Vestakeep.
16.Poly(phenylene sulphide)
Poly(phenylene sulphide) (PPS) delivers superb chemical resistance, electrical properties, flame retardance, and transparency to microwave radiation as well as a low coefficient of friction. These properties mean that, when injection or compression moulded at temperatures high enough to create crosslinks, PPS can also be used to make cookware, bearings and pump components suitable for corrosive environments. Trade names include Torelina and Ryton.
17.Polyetherimide
With a high heat distortion temperature, modulus and tensile strength, polyetherimide (PEI) is used in high performance electronic and electrical parts, including for the automotive industry, as well as in consumer items like microwave cookware. Trade names include Ultem.
18.Polyethersulfone
Polyethersulfone (PESU, PES) has high hydrolytic, oxidative, and thermal stability as well as a good resistance to alkalis, salt solutions, acids from aqueous minerals, oils and greases. Application include medical components, gas separation membranes and freezer-to-microwave food containers. Trade names include Ultrason and Veradel.
19.Polybenzimidazole
Polybenzimidazole (PBI), also known by trade names including Celazole and Duratron, has avery high melting point compared with other thermoplastics and shows excellent chemical and thermal stability. PBI’s superb stability, retention, stiffness and toughness at high temperatures has lent it to being used for firefighting clothing, space suits for astronauts, protective gloves, welding apparel, wall fabrics for aircraft and for membranes in fuel cells.
FAQs
Are Thermoplastics Recyclable?
Thermoplastics are easily recyclable as the polymer chain does not degrade when heated. Because the chemical bonds within the chain remain intact while the weaker bonds between polymer chains break down, thermoplastics can be melted and re-used repeatedly.
Are Thermoplastics Safe?
Most types of thermoplastic are safe to use as intended. However, there have been concerns raised over PVC because of the vinyl chloride monomer (VCM) that is used in production. However, modern manufacturing methods mean that the release of VCM is very low while the residual VCM left in the polymer is so low that it can’t be detected.
Are Thermoplastics Biodegradable?
Most thermoplastics are not biodegradable. However, some thermoplastics, such as poly(lactic acid) (PLA), poly (vinyl alcohol) (PVAL, PVOH) and polyhydroxyalkanoates (PHAs) are.
Are Thermoplastics Brittle?
Below their glass transition temperature (Tg), thermoplastics are brittle and deform by elastic deformation. However, when above their Tg, thermoplastics are ductile and deform mainly through plastic deformation. So, in short, thermoplastics go from brittle to ductile as they are heated through their Tg.
Can Thermoplastic be Remoulded?
Thermoplastics can be remoulded repeatedly by heating and then reforming them into new shapes.
Can Thermoplastic Melt?
Semi-crystalline thermoplastics melt at a particular temperature when their crystalline regions transition to a random arrangement. This melting point is different for different thermoplastics. Amorphous thermoplastics do not have an ordered structure and therefore do not melt; they have a glass transition temperature, below which the material is brittle and, as the temperature increases, the material softens and becomes more rubbery.
Can Thermoplastic be Painted?
Thermoplastics can be painted to provide a different surface finish. However, you will need to use the correct type of paint so that it doesn’t react with any polymer coating and cause discolouration and lowering weather resistance. Acrylic based paints, including spray paints, are a good option for painting thermoplastics.
Can Thermoplastics be Welded?
Thermoplastics can be welded using a variety of different techniques. You can find out more about welding thermoplastics here.
Conclusion
Thermoplastics are polymers that can be softened through heating before being processed and then left to cool and harden. Once cooled, they show no changes in chemical properties, meaning they can be re-melted and re-used several times.
There are many types of thermoplastic, each with their own distinct applications and properties, including being non-stick, tough, flexible, and so forth.
Thermoplastics are synthesised from a range of different materials, including renewable and biodegradable resources such as sugar beet, and have uses in industries including construction, aerospace, automotive, electronics, rail, oil and gas, and power, as well as for a huge range of domestic and consumer products.
TWI
TWI provides our Industrial Members with support in using a wide range materials, including thermoplastics. Our expertise includestesting different plastics and compositesas well as materials selection and joining methods for polymeric materials used in different applications.
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