Chloe Zhang
Plastics are used across a multitude of industries due to their ease of manufacture, low cost, and versatility. However, not all plastics are created equal when it comes to heat resistance. Some plastics are highly sensitive to heat and melt easily, while others can withstand extremely high temperatures without losing their structural integrity. This makes them suitable for specialized applications where heat resistance, mechanical strength, and corrosion resistance are crucial. This article will explore the different types of heat-resistant plastics, their unique characteristics, and the factors that contribute to their exceptional performance in high-temperature environments.
| Characteristics | Values |
|---|---|
| Continuous-use temperature | Above 150°C (302°F) |
| Short-term exposure resistance | 250°C (482°F) or more |
| Glass Transition Temperature (Tg) | Varies, can be well below zero degrees Celsius |
| Molecular structure | Aromatic rings, two chemical bonds need to be broken for the structure to break down |
| Additives | Glass fiber, graphite, carbon, flame retardants, antioxidants |
| Types | Thermoplastics, thermosets, photopolymers, amorphous, semicrystalline |
| Examples | PEI, PEEK, PTFE, PAI, PPS, Ultem, UHMW TIVAR H.O.T. |
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What You'll Learn
Thermosets and thermoplastics
Heat-resistant plastics fall into two broad categories: thermosets and thermoplastics. Thermoplastics are a type of resin that is solid at room temperature. When heated, they become soft and eventually turn into fluids due to crystal melting or crossing the glass transition temperature. They can be heated, cooled, and reshaped repeatedly without altering their chemical structure. Thermoplastics are recyclable and are used for a high-quality finish.
Thermosets, on the other hand, are plastics that harden when exposed to heat and undergo a chemical reaction when heated, creating a three-dimensional network of bonded molecules. This process is irreversible, meaning once thermosets have been set, they cannot be melted or reshaped. Thermosets are typically hard and have excellent resistance to heat and chemicals. They are not recyclable.
Thermoplastics gain their heat resistance from their molecular structure. When rigid aromatic rings are added to the resin instead of aliphatic groups, the backbone of the molecular chain is restricted and fortified. With this new structure, a thermoplastic’s chemical and heat resistance can be equal to or better than a thermoset.
A good example of a thermoplastic is PEEK (polyether ether ketone), a semi-crystalline, high-performance engineering thermoplastic that’s resistant to chemicals, wear, fatigue, creep, and heat. It is so strong and adaptable to harsh environments that manufacturers use it as a replacement for metal in many applications, no matter the temperature. PEEK can withstand temperatures as high as 310°C for short periods and has a melting point of over 371°C.
PTFE, commonly known as Teflon, is another example of a thermoplastic. It is a soft, heat-resistant, low-friction plastic with exceptional chemical resistance and high flexural strength. It has a very large operating temperature range and is thermally stable enough to be used anywhere between -200°C and +260°C.
In summary, both thermosets and thermoplastics offer unique advantages in terms of heat resistance, strength, and recyclability. The choice between the two depends on the specific requirements of the application.
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Glass Transition Temperature (Tg)
The Tg value depends on the strain rate and cooling or heating rate, and there is no exact value for Tg. It can be measured using thermal methods such as Differential Scanning Calorimetry (DSC) or Differential Thermal Analysis (DTA), which compare the thermal properties of a sample against a standard reference material.
Tg influences the selection of polymers, formulation of blends, design of processing parameters, and ensuring application-specific performance. By understanding and controlling Tg, manufacturers can fine-tune material properties for optimal performance, durability, and manufacturability. For example, increasing Tg improves handling characteristics, solubility, and reproducibility in the dissolution of solids.
Semi-crystalline thermoplastics, due to their internal structure, have very strong molecular bonds, making them resistant to chemical attacks. They can be used above their Tg due to the crystalline regions retaining their structure until the polymer's melting temperature. Thermoset materials, on the other hand, are typically used below their Tg and are quite rigid. They have excellent chemical resistance, dimensional stability, and heat resistance, and they can be used above or below their Tg without a melting point.
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Amorphous and semicrystalline plastics
There are two main groups of thermoplastics: amorphous and semicrystalline plastics. The key difference between these two groups is their melting behaviour. Amorphous plastics do not have a precise melting point, but instead gradually soften as the temperature increases. They have a range of temperatures over which they melt. This makes them relatively easy to thermoform, as they become more mobile as heat is applied.
Amorphous plastics are more common in applications that require a material that is easier to bend. They also handle paint, glue, and other adhesives much better than semicrystalline plastics. They are also more impact resistant. Amorphous thermoplastics offer excellent resistance to hot water and steam, good chemical resistance, and good stiffness and strength. ULTEM® (polyetherimide), PSU, and PEI are examples of amorphous thermoplastics.
On the other hand, semicrystalline plastics have a very sharp melting point. They remain solid at all temperatures below that point. Once they reach their melting point, their properties immediately begin to drop off. Their molecular chains are tightly packed and organised, with stronger chemical bonds, making them highly durable. Semicrystalline plastics are better for structural, weight-bearing applications. They can handle heavy loads and elevated temperatures. They also have better fatigue performance and wear resistance. Common semicrystalline thermoplastics include acetal, UHMW-PE, PEEK, fluoropolymers, and nylon.
Overall, the choice between amorphous and semicrystalline plastics depends on the specific application requirements. Amorphous plastics are better suited for environments with low-to-zero mechanical abuse or chemical contact, as they offer better dimensional stability and impact strength. In contrast, semicrystalline plastics are ideal for applications that require high dimensional accuracy, stability, and the ability to withstand repeated cyclic loading, chemical contact, or high levels of mechanical abuse.
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PTFE (Teflon)
PTFE, commonly known as Teflon, is a soft, heat-resistant, low-friction plastic with exceptional chemical resistance. It has high flexural strength, adequate weathering resistance, and good electrical insulating power in both hot and wet environments. PTFE is unique because it is almost completely chemically inert and highly insoluble in most solvents, making it ideal for high-temperature applications. PTFE has one of the highest melting points of any thermoplastic at 327°C, and a very large operating temperature range. It is thermally stable enough to be used anywhere between -200°C and +260°C.
PTFE is widely used across many industries due to its unique properties. It is used in components that must withstand extreme temperatures and chemically aggressive environments. It is also used as insulation for wires and cables due to its excellent thermal and electrical resistance. PTFE is commonly used for non-stick coatings for cookware and cooking utensils, taking advantage of its non-stick properties and food safety. PTFE is also used to make body jewellery as it is much safer than other materials like acrylic, which releases toxins into the body at 26.6°C, while PTFE does so only at 650–700°C.
PTFE film is widely used in the production of carbon fibre composites, as well as fibreglass composites, notably in the aerospace industry. PTFE film is used as a barrier between the carbon or fibreglass part being built and, in breather and bagging materials, is used to encapsulate the bondment when debulking and when curing the composite, usually in an autoclave. PTFE is also used as a thread seal tape in plumbing applications, largely replacing paste thread dope.
PTFE's high corrosion resistance makes it useful in laboratory environments, where it is used for lining containers, as a coating for magnetic stirrers, and as tubing for highly corrosive chemicals such as hydrofluoric acid, which will dissolve glass containers. PTFE membrane filters are among the most efficient industrial air filters. PTFE-coated filters are often used in dust collection systems to collect particulate matter from air streams in applications involving high temperatures.
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PEEK (polyether ether ketone)
PEEK, or polyether ether ketone, is a semi-crystalline, high-performance engineering thermoplastic that is revolutionising industries from aerospace to medical. It is known for its exceptional mechanical, thermal, and chemical properties. PEEK is strong, adaptable, and resistant to chemicals, wear, fatigue, creep, and heat. It can withstand temperatures as high as 310°C for short periods and has a melting point of over 371°C.
The material is so strong and adaptable to harsh environments that manufacturers use it as a replacement for metal in many applications, regardless of temperature. Due to its metal-like durability, PEEK is widely used for a variety of medical devices, active components in car transmissions, and aircraft exterior parts. It is also used in the electrical and electronics industries because it insulates well and resists electrical arcing. It is found in connectors, insulators, and circuit breakers, helping to ensure safe and reliable performance in electrical jobs.
PEEK is available in two forms: virgin or containing enhancing additives such as glass, carbon, and graphite. This family of materials is known for its great versatility. It has the added advantage of being easy to machine via injection moulding or extrusion, and solid PEEK is compatible with CNC machining. However, PEEK is susceptible to UV light and certain acids. It is also expensive due to its complex production process, which restricts its use to only the most demanding applications.
PEEK was created in the late 1970s by researchers at Imperial Chemical Industries (ICI) in the UK, who wanted to develop a plastic that could resist harsh chemicals and high temperatures. PEEK received a patent in 1978 and was used commercially in the 1980s. Manufacturers make PEEK by mixing raw materials called aromatic dihalides and aromatic diones, then heating these materials to about 300-400 °C, which causes a polymer reaction.
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Frequently asked questions
Some examples of heat-resistant plastics include ULTEM® (polyetherimide), PEEK (polyether ether ketone), PTFE (Teflon), and UHMW TIVAR H.O.T.
Heat-resistant plastics can be categorized into two main groups: thermoplastics and thermosets. Thermoplastics can be further divided into amorphous and semicrystalline plastics. Amorphous plastics gradually soften when heated, while semicrystalline plastics have a sharp melting point.
The heat resistance of plastics depends on their molecular structure. High-temperature plastics are made up of aromatic rings, which are more difficult to melt compared to aliphatic rings found in low-temperature plastics. Additives such as glass fiber can also improve heat resistance.
Heat-resistant plastics are used in various industries, including automotive, aerospace, medical, and semiconductor. They are suitable for applications requiring high temperatures, mechanical strength, and corrosive resistance. For example, PTFE is used in non-stick cookware, while PEEK is used in medical devices and aircraft parts due to its metal-like durability.
