What is a Thermoplastic? (Definition and Examples) - TWI

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.

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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.

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 a very 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.

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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 includes testing different plastics and composites as well as materials selection and joining methods for polymeric materials used in different applications.

TWI is an Industrial Membership based organisation. TWI's experts can provide your company with an extension to your own resources. Our experts are dedicated to helping industry improve safety, quality, efficiency and profitability in all aspects of materials joining technology. Industrial Membership of TWI currently extends to over 600 companies worldwide, embracing all industrial sectors.

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Understanding Thermoplastics: A Beginner's Guide

Before delving into the applications of thermoplastic parts across industries, it's essential to grasp the fundamentals. Thermoplastics are a type of polymer that can be melted and reshaped multiple times without undergoing a chemical change. This characteristic sets them apart from thermosetting plastics, which solidify irreversibly upon curing. Common thermoplastic materials include polyethylene, polypropylene, PVC, and nylon, each offering unique properties and benefits in industrial applications.

The versatility of thermoplastic materials stems from their ability to be easily molded into complex shapes, making them ideal for intricate component designs. The process of injection molding, commonly used in manufacturing thermoplastic parts, allows for high precision and consistency in production. Additionally, thermoplastics exhibit favorable mechanical properties, chemical resistance, and electrical insulation, further enhancing their utility across various industries.

One of the key advantages of thermoplastic components is their recyclability. Unlike thermosetting plastics, thermoplastics can be melted and reformed multiple times, promoting a circular economy approach to manufacturing. This sustainable aspect makes thermoplastic parts environmentally friendly and cost-effective, aligning with the growing focus on eco-conscious production practices.

In summary, thermoplastics offer a wide range of benefits that make them indispensable in modern industrial settings. Their versatility, durability, and recyclability make them a preferred choice for applications ranging from consumer products to high-performance engineering components.

Innovative Applications in Automotive Industry

The automotive industry has embraced thermoplastic parts for their lightweight properties and structural capabilities. Thermoplastic materials such as ABS, polycarbonate, and polypropylene are widely used in vehicle interiors, exteriors, and under-the-hood components. The high impact strength and design flexibility of thermoplastics enable manufacturers to create innovative and aerodynamic automotive parts that meet stringent safety standards.

Moreover, thermoplastic compounds offer enhanced resistance to chemicals, heat, and weathering, ensuring the longevity and performance of automotive components in diverse conditions. From bumpers to fuel systems, thermoplastics play a vital role in reducing vehicle weight, improving fuel efficiency, and enhancing overall sustainability in the automotive sector.

As automotive manufacturers continue to prioritize fuel economy and environmental impact, the demand for thermoplastic parts is expected to rise. The ability of thermoplastic components to withstand complex mechanical stresses while maintaining dimensional stability positions them as the material of choice for the future of lightweight automotive design.

The integration of thermoplastics in electric vehicles further highlights their importance in the automotive industry's shift towards sustainability. As automakers strive to optimize performance and reduce carbon emissions, the unique properties of thermoplastic materials prove instrumental in achieving these goals.

Advantages of Thermoplastic Components in Medical Devices

The healthcare sector benefits immensely from the use of thermoplastic components in medical devices and equipment. Thermoplastics offer a myriad of advantages such as biocompatibility, sterilizability, and impact resistance, making them indispensable in a clinical setting. Materials like polyethylene, PVC, and PEEK are widely utilized in manufacturing medical devices due to their reliability and performance.

The ability of thermoplastic parts to withstand repeated sterilization processes without compromising their integrity ensures the safety and longevity of medical instruments. In addition, thermoplastics can be molded into intricate shapes with tight tolerances, allowing for the production of custom implants and surgical tools tailored to individual patient needs.

From implantable devices to diagnostic equipment, thermoplastic components play a critical role in advancing healthcare technologies. Their lightweight nature, corrosion resistance, and thermal stability make thermoplastics ideal for applications requiring precision, hygiene, and patient comfort in the medical field.

The versatility of thermoplastic materials facilitates the development of cutting-edge medical devices that enhance diagnosis, treatment, and patient care. As medical technology continues to evolve, thermoplastic components are poised to drive innovation and address the complex challenges faced by healthcare professionals worldwide.

Sustainability and Recyclability of Thermoplastics

In an era where environmental consciousness is paramount, the sustainability of thermoplastic parts has become a significant focus for industries worldwide. Thermoplastics offer a greener alternative to traditional materials, thanks to their ability to be melted, reshaped, and reused in manufacturing processes.

The recyclability of thermoplastic components not only reduces waste but also minimizes the carbon footprint of industrial production. By incorporating recycled thermoplastics into new products, manufacturers can promote a circular economy model that conserves resources and reduces environmental impact.

Furthermore, the durability and longevity of thermoplastic materials contribute to sustainable practices by extending the lifecycle of products and reducing the need for frequent replacements. This aspect is particularly valuable in sectors such as construction, packaging, and consumer goods where thermoplastics offer long-term performance and environmental benefits.

As industries strive to achieve greater sustainability and reduce their ecological footprint, the versatile nature of thermoplastics provides a viable solution for developing eco-friendly products and systems. The recyclability, strength, and adaptability of thermoplastic parts position them as key enablers of sustainable practices across a wide range of applications.

In conclusion, the widespread adoption of thermoplastic components underscores their significance in driving innovation, efficiency, and environmental responsibility across industries. The continual evolution of thermoplastic materials promises a future where sustainable practices and high-performance solutions go hand in hand, shaping a more environmentally conscious and technologically advanced world.

The Future of Thermoplastic Parts